异质结的应用综述以及具体的某种异质结的研究

In this study, we report a localized surface plasmon resonance (LSPR) enhanced optoelectronic device based on a ZnSe:Sb nanoribbon (NR)/Si nano-heterojunction. We experimentally demonstrated that the LSPR peaks of plasmonic Ag nanoparticles (Ag NPs) can be readily tuned by changing their size distribution. Optical analysis reveals that the absorption of ZnSe:Sb NRs was increased after the decoration of the Ag NPs with strong LSPR. Further analysis of the optoelectronic device confirmed the device performance can be promoted: for example, the short-circuit photocurrent density of the ZnSe/Si heterojunction solar cell was improved by 57.6% from 11.75 to 18.52 mA cm(-2) compared to that without Ag NPs. Meanwhile, the responsivity and detectivity of the ZnSe:Sb NRs/Si heterojunction device increased from 117.2 to 184.8 mA W(-1), and from 5.86 × 10(11) to 9.20 × 10(11) cm Hz(1/2) W(-1), respectively.

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Mechanisms of defect-formation in heterolayers compounds II-VI had considered. Influence of isovalent impurity on generated dot point defect, optical properties and radiation stabilizing of investigated layers has discussed. The basic parameters of some devices had considered.

Van der Waals heterojunction devices are of great significance for developing high-performance optoelectronic devices. Here, a strategy has been introduced to effectively improve the performance of devices by designing their structure, and three types of devices based on MoS2/GaSe heterojunctions were designed and fabricated. The results have demonstrated that device-III effectively decreases the recombination of the electrons in GaSe flakes by removing the non-heterojunction region and depleting the GaSe flake, which results in electron-dominated channel current and much better electrical performance than device-I and device-II, such as a larger rectification ratio of 1.6 × 105 and an ideality factor of 1.06. Furthermore, a photodetector based on device-III exhibits high performance for self-driven photodetection under 532 nm light irradiation, including a responsivity of 249 mA/W, a specific detectivity of 3.6 × 1011 Jones, an open-circuit voltage of 0.56 V, and a short response/recovery time of 10.5 μs/7.3 μs. The results introduced here provide a path to significantly improve the electrical properties of optoelectronic devices based on a 2D material heterostructure.

Semiconductor p-n junctions are essential building blocks for electronic and optoelectronic devices. In conventional p-n junctions, regions depleted of free charge carriers form on either side of the junction, generating built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes influenced by the spatial extent of this depletion region. With the advent of atomically thin van der Waals materials and their heterostructures, it is now possible to realize a p-n junction at the ultimate thickness limit. Van der Waals junctions composed of p- and n-type semiconductors--each just one unit cell thick--are predicted to exhibit completely different charge transport characteristics than bulk heterojunctions. Here, we report the characterization of the electronic and optoelectronic properties of atomically thin p-n heterojunctions fabricated using van der Waals assembly of transition-metal dichalcogenides. We observe gate-tunable diode-like current rectification and a photovoltaic response across the p-n interface. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p-n junction between graphene layers enhances the collection of the photoexcited carriers. The atomically scaled van der Waals p-n heterostructures presented here constitute the ultimate functional unit for nanoscale electronic and optoelectronic devices.

Broad spectral response and high photoelectric conversion efficiency are key milestones for realizing multifunctional, low-power optoelectronic devices such as artificial synapse and reconfigurable memory devices. Nevertheless, the wide bandgap and narrow spectral response of metal-oxide semiconductors are problematic for efficient metal-oxide optoelectronic devices such as photonic synapse and optical memory devices. Here, a simple titania (TiO₂ )/indium-gallium-zinc-oxide (IGZO) heterojunction structure is proposed for efficient multifunctional optoelectronic devices, enabling widen spectral response range and high photoresponsivity. By overlaying a TiO₂ film on IGZO, the light absorption range extends to red light, along with enhanced photoresponsivity in the full visible light region. By implementing the TiO₂ /IGZO heterojunction structure, various synaptic behaviors are successfully emulated such as short-term memory/long-term memory and paired pulse facilitation. Also, the TiO₂ /IGZO synaptic transistor exhibits a recognition rate up to 90.3% in recognizing handwritten digit images. Moreover, by regulating the photocarrier dynamics and retention behavior using gate-bias modulation, a reconfigurable multilevel (≥8 states) memory is demonstrated using visible light.

An initial study of losses in n‐Al x Ga 1− x N planar waveguides at λ emission ≈ 280 nm using monolithically integrated Al x Ga 1− x N multiple quantum wells (MQWs)‐based light‐emitting diodes and detectors is presented. The epilayer structure for the integrated devices is grown on an AlN (3.5 μm thick) template over sapphire substrates. Emitter–detector optical coupling and the directional independence of radiation within the epistructure are experimentally established. A model for estimating the attenuation coefficient under these conditions is developed. The attenuation coefficient for a planar n‐Al 0.65 Ga 0.35 N waveguide is measured to be 5–6 cm −1 , and it primarily arises from the free‐carrier absorption rather than surface roughness‐dependent Rayleigh scattering.

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Thin films of ZnO were deposited by thermal decomposition of Zn(C5H7O2)2 on semiconductor substrate, n-type silicon, p-type InP and also on transparent glass substrate. The obtained ZnO/Si and ZnO/InP heterostructures were investigated for optical properties by spectrophotometry and surface morphology by AFM. The measured values of optoelectrical parameters in the visible spectral range and the lateral photovoltage characteristics demonstrate the possibility of using ZnO/n-Si and ZnO/p-InP heterojunctions for photodetection and photovoltaic devices applications.

Nano-heterojunctions will play essential roles in future nano-electronic and nano-optoelectronic devices. However, their extensive applications are impeded by the complicated multi-step growth method involved and the requirements for precise nanowire (NW) positioning/alignment. Here, we develop a facile method to fabricate zinc selenide NW (ZnSeNW)/silicon p-n heterojunctions by transferring the p-type ZnSeNWs onto a SiO₂/Si substrate with pre-defined Si windows; the physical contact of the NWs with the Si substrate via van der Waals force leads to the formation of heterojunctions. Electrical measurements on the heterojunctions reveal their excellent diode characteristics with a relatively small ideality factor of ∼1.95, high rectification ratio of ∼10⁶, and low turn-on voltage of ∼0.9 V. Moreover, heterojunction field-effect transistors are constructed based on the p-ZnSeNWs and show remarkable performance enhancement compared to the device counterparts with a metal-oxide-semiconductor field-effect transistor structure. The enhanced gate coupling between the NW conduction channel and the heterojunction gate is believed to be responsible for the high device performance. Significantly, under AM 1.5G light illumination, the heterojunctions exhibit pronounced photovoltaic behavior, yielding a power conversion efficiency of ∼1.8%. Our results demonstrate the great potential of ZnSeNW/Si p-n heterojunctions in high-performance nano-device applications.

Heterojunctions with gradient energy bands confirmed via Kelvin probe force microscopy (KPFM) are confirmed to be effective in accelerating charge transport and suppressing carrier recombination in Cu 2 O-based-film photodetector and solar cell devices.

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.

Graphene-like layered semiconductors are a new class of materials for next generation electronic and optoelectronic devices due to their unique electrical and optical properties. A p–n junction is an elementary building block for electronics and optoelectronics devices. Here, we demonstrate the fabrication of a lateral p–n heterojunction diode of a thin-film InSe/CuInSe2 nanosheet by simple solid-state reaction. We discover that InSe nanosheets can be easily transformed into CuInSe2 thin film by reacting with elemental copper at a temperature of 300 °C. Photodetectors and photovoltaic devices based on this lateral heterojunction p–n diode show a large photoresponsivity of 4.2 A W–1 and a relatively high light-power conversion efficiency of 3.5%, respectively. This work is a giant step forward in practical applications of two-dimensional materials for next generation optoelectronic devices.

Optoelectronic synapses are considered to be important cornerstones in the construction of neuromorphic computing systems because of their low power consumption, high operating speeds, and high scalability. In this work, we demonstrate an optoelectronic synaptic device based on a ZnO/HfOx heterojunction in which optical potentiation/electrical depression behaviors and nonvolatile high current state can be implemented. The heterojunction device exhibits conductance evolution with high linearity. The excellent optoelectronic memristive behavior of the device can be attributed to the interface barrier between ZnO and HfOx, which hinders the recombination of photo-excited electron–hole pairs to increase the carrier lifetime, and realizes the nonvolatile high current state. More importantly, the artificial vision system based on optoelectronic synaptic devices can achieved a high recognition accuracy of 96.1%. Our work provides a feasible pathway toward the development of optoelectronic synaptic devices for use in high-performance neuromorphic vision systems.

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A novel class of optoelectronic devices utilizing thin films of stable crystalline organic semiconductors layered onto inorganic semiconductor substrates is described. The electrical properties of these devices are determined by the energy barrier at the heterojunction contact between the organic and inorganic materials, and in many ways are similar to those of ideal diffused-junction inorganic semiconductor devices. The organic materials can be layered onto semiconductor substrates without inducing large strains in either material, hence allowing a wide range of material combinations with a similarly broad range of optoelectronic functions to be realized. As examples, high-bandwidth photodetectors and field-effect transistors made using organic/inorganic semiconductor heterojunctions are discussed. Modification of the optical and electronic properties of the organic films by irradiation with energetic electron and ion beams is considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Abstract Two‐dimensional (2D) semiconductor molybdenum disulfide (MoS 2 ) can be used as n‐channel and is considered as a key candidate material to advance the promising development of optoelectronic device. The high thermal conductivity, breakdown voltage, carrier mobility, and high saturation velocity of diamond offer the possibility of making it high‐frequency device material in high‐temperature and high‐power fields. The addition of 2D MoS 2 nanolayers and nanosheets to diamond thin film to form heterojunction can improve the carrier transport performance of the optoelectronic device in harsh environments. In this perspective, the prospects of 2D MoS 2 /diamond heterojunction for challenges and new designs of optoelectronic applications are discussed, including photodetectors, memories, transistors, light emission diodes, and electron field emission devices to further explore the development of 2D material device field in complex environments.

The p-n diodes represent the most fundamental device building blocks for diverse optoelectronic functions, but are difficult to achieve in atomically thin transition metal dichalcogenides (TMDs) due to the challenges in selectively doping them into p- or n-type semiconductors. Here, we demonstrate that an atomically thin and sharp heterojunction p-n diode can be created by vertically stacking p-type monolayer tungsten diselenide (WSe2) and n-type few-layer molybdenum disulfide (MoS2). Electrical measurements of the vertically staked WSe2/MoS2 heterojunctions reveal excellent current rectification behavior with an ideality factor of 1.2. Photocurrent mapping shows rapid photoresponse over the entire overlapping region with a highest external quantum efficiency up to 12%. Electroluminescence studies show prominent band edge excitonic emission and strikingly enhanced hot-electron luminescence. A systematic investigation shows distinct layer-number dependent emission characteristics and reveals important insight about the origin of hot-electron luminescence and the nature of electron-orbital interaction in TMDs. We believe that these atomically thin heterojunction p-n diodes represent an interesting system for probing the fundamental electro-optical properties in TMDs and can open up a new pathway to novel optoelectronic devices such as atomically thin photodetectors, photovoltaics, as well as spin- and valley-polarized light emitting diodes, on-chip lasers.

The importance of silicon based optoelectronic devices is due to the well developed silicon technology and its potential for device integration. ZnO/Si light emitting diodes reported in the literature are based mainly on ZnO films grown by the vapor-phase techniques. Electrodeposition, a cost-effective and simple method, has not been explored adequately for the fabrication of such devices. In this study, ZnO films were electrodeposited on the (100) plane of highly B-doped p-Si substrates. Heterojunction devices (p-n and p-i-n) were constructed and characterized by means of current-voltage, capacitance-voltage, photocurrent spectroscopy, photoluminescence, and electroluminescence measurements. Electrodeposition yields compact films with a native donor density ∼1017 cm−3. Diffusion of boron from Si into ZnO, during an annealing process, yields graded p-n junctions with enhanced electroluminescence. Devices exhibit a reasonably good photoresponse in the ultraviolet-blue range. The absorption of subband gap photons in ZnO shows an Urbach tail with a characteristic energy of 115 meV. The absorption and emission of light involves two prominent defect levels in ZnO, namely, L1 and E1.

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A library of two-dimensional (2D) semiconductors with different band gaps offers the construction of van der Waals (vdWs) heterostructures with different band alignments, providing a new platform for developing high-performance optoelectronic devices. Here, we demonstrate all-2D optoelectronic devices based on type-II p-MoS₂/n-InSe vdWs heterojunctions operating at the near infrared (NIR) wavelength range. The p-n heterojunction diode exhibits a rectification ratio of ∼10² at <i>V</i> _ds = ±2 V and a turn-on voltage of ∼0.8 V. Under a forward bias exceeding the turn-on voltage and a proper positive back-gate voltage, the all-2D vdWs heterojunction diode exhibits an electroluminescence with an emission peak centered at ∼1020 nm. Besides, this p-MoS₂/n-InSe heterojunction shows a photoresponse at zero external bias, indicating that it can serve as a photodiode working without an external power supply. The as-demonstrated all-2D vdWs heterojunction which can work as both a light-emitting diode and a self-powered photodetector may find applications in flexible wear, display, and optical communication fields, <i>etc.</i>

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Artificial optoelectronic synapses (OES) have attracted extensive attention in brain-inspired information processing and neuromorphic computing. However, OES at near-infrared wavelengths have rarely been reported, seriously limiting the application in modern optical communication. Herein, high-performance near-infrared OES devices based on VO₂/MoO₃ heterojunctions are presented. The textured MoO₃ films are deposited on the sputtered VO₂ film by using the glancing-angle deposition technique to form a heterojunction device. Through tuning the oxygen defects in the VO₂ film, the fabricated VO₂/MoO₃ heterojunction exhibits versatile electrical synaptic functions. Benefiting from the highly efficient light harvesting and the unique interface effect, the photonic synaptic characteristics, mainly including the short/long-term plasticity and learning experience behavior are successfully realized at the O (1342 nm) and C (1550 nm) optical communication wavebands. Moreover, a single OES device can output messages accurately by converting light signals of the Morse code to distinct synaptic currents. More importantly, a 3 × 3 artificial OES array is constructed to demonstrate the impressive image perceiving and learning capabilities. This work not only indicates the feasibility of defect and interface engineering in modulating the synaptic plasticity of OES devices, but also provides effective strategies to develop advanced artificial neuromorphic visual systems for next-generation optical communication systems.

Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable, and under appropriate gate bias an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.

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Atomically thin two-dimensional (2D) materials range from semimetallic graphene to insulating hexagonal boron nitride to semiconducting transition-metal dichalcogenides. Recently, metal-insulator-semiconductor field effect transistors built from these 2D elements were studied for flexible and transparent electronics. However, to induce ambipolar characteristics for alternative power-efficient circuitry, ion-gel gating is often employed for high capacitive coupling, limiting stable operation at ambient conditions. Here, we report reconfigurable MoTe₂ optoelectronic transistors with all 2D components, where the device can be reconfigured by both drain and gate voltages. Eight different configurations for each fixed voltage are spatially resolved by scanning photocurrent microscopy. In addition, metal-insulator transitions are observed in both electron and hole carriers under 2 V due to strong Coulomb interaction in the system. Furthermore, the vertical tunneling photocurrent through multiple van der Waals layers between the gate and source contacts is measured. Our reconfigurable devices offer potential building blocks for system-on-a-chip optoelectronics.

Abstract Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe 2 /MoS 2 field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio &gt;10 7 and diode with rectification ratio &gt;10 6 . Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.

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Van der Waals (vdW) heterostructures have drawn much interest over the last decade owing to their absence of dangling bonds and their intriguing low-dimensional properties. The emergence of 2D materials has enabled the achievement of significant progress in both the discovery of physical phenomena and the realization of superior devices. In this work, the group IV metal chalcogenide 2D-layered Ge₄ Se₉ is introduced as a new selection of insulating vdW material. 2D-layered Ge₄ Se₉ is synthesized with a rectangular shape using the metalcorganic chemical vapor deposition system using a liquid germanium precursor at 240 °C. By stacking the Ge₄ Se₉ and MoS₂ , vdW heterostructure devices are fabricated with a giant memory window of 129 V by sweeping back gate range of ±80 V. The gate-independent decay time reveals that the large hysteresis is induced by the interfacial charge transfer, which originates from the low band offset. Moreover, repeatable conductance changes are observed over the 2250 pulses with low non-linearity values of 0.26 and 0.95 for potentiation and depression curves, respectively. The energy consumption of the MoS₂ /Ge₄ Se₉ device is about 15 fJ for operating energy and the learning accuracy of image classification reaches 88.3%, which further proves the great potential of artificial synapses.

The optical and electronic properties of van der Waals (vdW) heterostructures depend strongly on the atomic stacking order of the constituent layers. This is exemplified by periodic variation of the local atomic registry, known as moire patterns, giving rise to superconductivity and ferromagnetism in twisted bilayer graphene and novel exciton states in transition metal dichalcogenides (TMD) heterobilayers. However, the presence of the nanometer-scale moire superlattices is typically deduced indirectly, because conventional imaging techniques, such as transmission electron microscopy (TEM), require special sample preparation that is incompatible with most optical and transport measurements. Here, we demonstrate a method that uses a secondary electron microscope to directly image the local stacking order in fully hexagonal boron nitride (hBN) encapsulated, gated vdW heterostructure devices on standard Si-substrates. Using this method, we demonstrate imaging of reconstructed moire patterns in stacked TMDs, ABC/ABA stacking order in graphene multilayers, and AB/BA boundaries in bilayer graphene. Furthermore, we show that the technique is non-destructive, thus unlocking the possibility of directly correlating local stacking order with optical and electronic properties, crucial to the development of vdW heterostructure devices with precisely controlled functionality.

Uniaxial strain has been widely used as a powerful tool for investigating and controlling the properties of quantum materials. However, existing strain techniques have so far mostly been limited to use with bulk crystals. Although recent progress has been made in extending the application of strain to two-dimensional van der Waals (vdW) heterostructures, these techniques have been limited to optical characterization and extremely simple electrical device geometries. Here, we report a piezoelectric-based in situ uniaxial strain technique enabling simultaneous electrical transport and optical spectroscopy characterization of dual-gated vdW heterostructure devices. Critically, our technique remains compatible with vdW heterostructure devices of arbitrary complexity fabricated on conventional silicon/silicon dioxide wafer substrates. We demonstrate a large and continuously tunable strain of up to −0.15% at millikelvin temperatures, with larger strain values also likely achievable. We quantify the strain transmission from the silicon wafer to the vdW heterostructure, and further demonstrate the ability of strain to modify the electronic properties of twisted bilayer graphene. Our technique provides a highly versatile new method for exploring the effect of uniaxial strain on both the electrical and optical properties of vdW heterostructures and can be easily extended to include additional characterization techniques.

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Although 2D materials are widely explored for data storage and neuromorphic computing, the construction of 2D material-based memory devices with optoelectronic responsivity in the short-wave infrared (SWIR) region for in-sensor reservoir computing (RC) at the optical communication band still remains a big challenge. In this work, an electronic/optoelectronic memory device enabled by tellurium-based 2D van der Waals (vdW) heterostructure is reported, where the ferroelectric CuInP₂ S₆ and tellurium channel endow this device with both the long-term potentiation/depression by voltage pulses and short-term potentiation by 1550 nm laser pulses (a typical wavelength in the conventional fiber optical communication band). Leveraging the rich dynamics, a fully memristive in-sensor RC system that can simultaneously sense, decode, and learn messages transmitted by optical fibers is demonstrated. The reported 2D vdW heterostructure-based memory featuring both the long-term and short-term memory behaviors using electrical and optical pulses in SWIR region has not only complemented the wide spectrum of applications of 2D materials family in electronics/optoelectronics but also paves the way for future smart signal processing systems at the edge.

Atomically thin van der Waals materials provide a highly tunable platform for exploring emergent quantum phenomena in solid state systems. Due to their remarkable mechanical strength, one enticing tuning knob is strain. However, the weak strain transfer of graphite and hBN, which are standard components of high-quality vdW devices, poses fundamental challenges for high-strain experiments. Here, we investigate strain transmission in less-explored orthorhombic crystals and find robust transmission up to several percent at cryogenic temperatures. We further show that strain can be efficiently transferred through these crystals to other 2D materials in traditional heterostructure devices. Using this capability, we demonstrate in situ strain and gate control of the optical properties of monolayer WS₂ utilizing the high-κ dielectric insulator Bi₂SeO₅ as a substrate. These results enable the exploration of combined cryo-strain and gate tuning in a variety of layered systems such as moiré heterostructures, air-sensitive 2D magnets and superconductors, and any gated 2D device.

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Abstract 2D van der Waals (vdWs) heterostructure photodetectors have captured significant interest for their ability to achieve substantial optical conductivity gain and device tunability. However, the light absorption in ultrathin 2D inorganic vdWs devices is generally weak that leads to low detectivity. In addition, the intrinsic defects in 2D semiconductors cause significant carrier trapping and scattering by defect states during the transport process, which seriously restricts the response speed of the device. In this paper, a molybdenum tungsten disulfide (Mo 0.1 W 0.9 S 2 ) is used to replace the conventional 2D semiconductor, while the light absorption efficiency of the device is significantly enhanced by the adoption of N’‐ Dimethyl‐3,4,9,10‐perylenedicarboximide (Me‐PTCDI), thus achieving both fast response and high detectivity. A series of type‐II organic/inorganic hybrid vdWs heterostructure photodetectors is systematically investigated based on Me‐PTCDI and Mo 0.1 W 0.9 S 2 . In particular, the device incorporating monolayer (ML) Me‐PTCDI and few‐layer (FL) Mo 0.1 W 0.9 S 2 demonstrates a detectivity of up to 4.4 × 10 11 Jones and a response time of 24.9 µs. By utilizing the device as a light sensing pixel, a single‐detecting pixel imaging system is demonstrated with high precision, showcasing promising prospects in fast imaging applications.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers.

Abstract 2D materials with atomic‐scale thickness have attracted immense interest owing to their intriguing properties, which can be useful for electronic devices. As ultrathin 2D materials are highly vulnerable to external conditions, passivation of 2D materials is required to maintain the stability of 2D electronic devices. However, 2D channels are embedded in passivation layers, making the formation of suitable contacts in passivated 2D devices challenging. Here, a novel method for fabricating irreversible conductive filament (ICF) contacts on a 2D channel passivated by hexagonal boron nitride (hBN) layers is demonstrated. Defective paths are formed in the top hBN layer of hBN‐encapsulated graphene (or MoS 2 ) using oxygen‐plasma treatment, along which ICFs are fabricated by applying repetitive bias. ICF contacts formed in the combined paths of migrated metal atoms and vacancies are stable during device operation, which is in contrast with that the filaments in hBN memristors are reversible. Field‐effect transistors with ICF contacts exhibit a low contact resistance and high stability. This study shows a new contact method, which has great potential for high‐performance 2D electronics devices.

MoTe2 is an emerging two-dimensional layered material showing ambipolar/p-type conductivity, which makes it an important supplement to n-type two-dimensional layered material like MoS2. However, the properties based on its van der Waals heterostructures have been rarely studied. Here, taking advantage of the strong Fermi level tunability of monolayer graphene (G) and the feature of van der Waals interfaces that is free from Fermi level pinning effect, we fabricate G/MoTe2/G van der Waals heterostructures and systematically study the electronic and optoelectronic properties. We demonstrate the G/MoTe2/G FETs with low Schottky barriers for both holes (55.09 meV) and electrons (122.37 meV). Moreover, the G/MoTe2/G phototransistors show high photoresponse performances with on/off ratio, responsivity, and detectivity of ∼105, 87 A/W, and 1012 Jones, respectively. Finally, we find the response time of the phototransistors is effectively tunable and a mechanism therein is proposed to explain our observation. This work provides an alternative choice of contact for high-performance devices based on p-type and ambipolar two-dimensional layered materials.

The recent emergence of a wide variety of two-dimensional (2D) materials has created new opportunities for device concepts and applications. In particular, the availability of semiconducting transition metal dichalcogenides, in addition to semimetallic graphene and insulating boron nitride, has enabled the fabrication of "all 2D" van der Waals heterostructure devices. Furthermore, the concept of van der Waals heterostructures has the potential to be significantly broadened beyond layered solids. For example, molecular and polymeric organic solids, whose surface atoms possess saturated bonds, are also known to interact via van der Waals forces and thus offer an alternative for scalable integration with 2D materials. Here, we demonstrate the integration of an organic small molecule p-type semiconductor, pentacene, with a 2D n-type semiconductor, MoS2. The resulting p-n heterojunction is gate-tunable and shows asymmetric control over the antiambipolar transfer characteristic. In addition, the pentacene/MoS2 heterojunction exhibits a photovoltaic effect attributable to type II band alignment, which suggests that MoS2 can function as an acceptor in hybrid solar cells.

Monolayer MoS2 is a direct band gap semiconductor with large exciton binding energy, which is a promising candidate for the application of ultrathin optoelectronic devices. However, the optoelectronic performance of monolayer MoS2 is seriously limited to its growth quality and carrier mobility. In this work, we report the direct vapor growth and the optoelectronic device of vertically-stacked MoS2/MoSe2 heterostructure, and further discuss the mechanism of improved device performance. The optical and high-resolution atomic characterizations demonstrate that the heterostructure interface is of high-quality without atomic alloying. Electrical transport measurements indicate that the heterostructure transistor exhibits a high mobility of 28.5 cm2/(V·s) and a high on/off ratio of 107. The optoelectronic characterizations prove that the heterostructure device presents an enhanced photoresponsivity of 36 A/W and a remarkable detectivity of 4.8 × 1011 Jones, which benefited from the interface induced built-in electric field and carrier dependent Coulomb screening effect. This work demonstrates that the construction of two-dimensional (2D) semiconductor heterostructures plays a significant role in modifying the optoelectronic device properties of 2D materials.

Broken-gap (type-III) two-dimensional (2D) van der Waals heterostructures (vdWHs) offer an ideal platform for interband tunneling devices due to their broken-gap band offset and sharp band edge. Here, we demonstrate an efficient control of energy band alignment in a typical type-III vdWH, which is composed of vertically-stacked molybdenum telluride (MoTe2) and tin diselenide (SnSe2), via both electrostatic and optical modulation. By a single electrostatic gating with hexagonal boron nitride (h-BN) as the dielectric, a variety of electrical transport characteristics including forward rectifying, Zener tunneling, and backward rectifying are realized on the same heterojunction at low gate voltages of ±1 V. In particular, the heterostructure can function as an Esaki tunnel diode with a room-temperature negative differential resistance. This great tunability originates from the atomically-flat and inert surface of h-BN that significantly suppresses the interfacial trap scattering and strain effects. Upon the illumination of an 885 nm laser, the band alignment of heterojunction can be further tuned to facilitate the direct tunneling of photogenerated charge carriers, which leads to a high photocurrent on/off ratio of > 105 and a competitive photodetectivity of 1.03 × 1012 Jones at zero bias. Moreover, the open-circuit voltage of irradiated heterojunction can be switched from positive to negative at opposite gate voltages, revealing a transition from accumulation mode to depletion mode. Our findings not only promise a simple strategy to tailor the bands of type-III vdWHs but also provide an in-depth understanding of interlayer tunneling for future low-power electronic and optoelectronic applications.

Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe2-WSe2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ~1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Atomically thin two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS2) hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena such as coupled spin-valley physics and the valley Hall effect. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, particularly at low temperature (T), which has hampered efforts to observe its intrinsic quantum transport behaviors. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering and approach the intrinsic limit, we developed a van der Waals (vdW) heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride (hBN), and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Multi-terminal magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm2/Vs for 6-layer MoS2 at low T. Comparison to theory shows a decrease of 1-2 orders of magnitude in the density of charged impurities, indicating that performance at low T in previous studies was limited by extrinsic factors rather than defects in the MoS2. We also observed Shubnikov-de Haas (SdH) oscillations for the first time in high-mobility monolayer and few-layer MoS2. This novel device platform therefore opens up a new way toward measurements of intrinsic properties and the study of quantum transport phenomena in 2D semiconducting materials.

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The vertically stacked devices based on van der Waals heterostructures (vdWHs) of two-dimensional layered materials (2DLMs) have attracted considerable attention due to their superb properties. As a typical structure, graphene/hexagonal boron nitride (h-BN)/graphene vdWH has been proved possible to make tunneling devices. Compared with graphene, transition metal dichalcogenides possess intrinsic bandgap, leading to high performance of electronic devices. Here, tunneling devices based on graphene/h-BN/MoSe2 vdWHs are designed for multiple functions. On the one hand, the device shows a typical tunneling field-effect transistor behavior. A high on/off ratio of tunneling current (5 × 103) and an ultrahigh current rectification ratio (7 × 105) are achieved, which are attributed to relatively small electronic affinity of MoSe2 and optimized thickness of h-BN. On the other hand, the same structure also realizes 2D non-volatile memory with a high program/erase current ratio (&amp;gt;105), large memory window (∼150 V from ±90 V), and good retention characteristic. These results could enhance the fundamental understanding of tunneling behavior in vdWHs and contribute to the design of ultrathin rectifiers and memory based on 2DLMs.

The assembly of individual two-dimensional materials into van der Waals heterostructures enables the construction of layered three-dimensional materials with desirable electronic and optical properties. A core problem in the fabrication of these structures is the formation of clean interfaces between the individual two-dimensional materials which would affect device performance. We present here a technique for the rapid batch fabrication of van der Waals heterostructures, demonstrated by the controlled production of 22 mono-, bi- and trilayer graphene stacks encapsulated in hexagonal boron nitride with close to 100% yield. For the monolayer devices, we found semiclassical mean-free paths up to 0.9 μm, with the narrowest samples showing clear indications of the transport being affected by boundary scattering. The presented method readily lends itself to fabrication of van der Waals heterostructures in both ambient and controlled atmospheres, while the ability to assemble pre-patterned layers paves the way for complex three-dimensional architectures.

Benefiting from the technique of vertically stacking 2D layered materials (2DLMs), an advanced novel device architecture based on a top-gated MoS₂/WSe₂ van der Waals (vdWs) heterostructure is designed. By adopting a self-aligned metal screening layer (Pd) to the WSe₂ channel, a fixed p-doped state of the WSe₂ as well as an independent doping control of the MoS₂ channel can be achieved, thus guaranteeing an effective energy-band offset modulation and large through current. In such a device, under specific top-gate voltages, a sharp PN junction forms at the edge of the Pd layer and can be effectively manipulated. By varying top-gate voltages, the device can be operated under both quasi-Esaki diode and unipolar-Zener diode modes with tunable current modulations. A maximum gate-coupling efficiency as high as ≈90% and a subthreshold swing smaller than 60 mV dec^-1 can be achieved under the band-to-band tunneling regime. The superiority of the proposed device architecture is also confirmed by comparison with a traditional heterostructure device. This work demonstrates the feasibility of a new device structure based on vdWs heterostructures and its potential in future low-power electronic and optoelectronic device applications.

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Abstract In recent years, bismuth iodide (BiI 3 ), a layered metal halide semiconducting light absorber with a wide bandgap of ≈1.8 eV and strong optical absorption in the visible region, has received greater attention for photovoltaic applications. In this study, ultrasensitive visible‐light photodetectors with graphene/BiI 3 vertical heterostructures are achieved by van der Waals epitaxies. The BiI 3 films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI 3 films on SiO 2 substrates, mainly due to weak van der Waals interactions at the graphene/BiI 3 interface. Hybrid photodetectors with highly crystalline graphene/BiI 3 heterostructures demonstrate an ultrahigh responsivity of 6 × 10 6 A W −1 , shot‐noise‐limited detectivity of 7 × 10 14 Jones, and a relatively short response time of ≈8 ms. Compared to most previously reported graphene‐based hybrid photodetectors, these devices have comparable photosensitivities but a faster response speed and lower operation voltage, which is quite promising for ultralow intensity visible‐light sensors. Moreover, the electronic structure and interfacial chemistry at the graphene/BiI 3 heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI 3 , resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI 3 interface.

Van der Waals (vdW) heterostructures based on inorganic layered materials have been demonstrated as potential candidates for a variety of electronic applications due to their flexibility in energy band engineering. However, the presence of unstable charge-trapping states in atomically thin two-dimensional (2D) materials may limit the performance of devices. Here, we aim to conduct a systematic investigation on hybrid heterostructured memory devices that consist of 2D layered organic and inorganic materials. The objective is to explore the potential of these devices in offering efficient charge-trapping states. Molybdenum disulfide (MoS2) is employed as a channel, while N, N′-Dimethyl-3,4,9,10-perylenedicarboximide (Me-PTCDI) serves as the charge-trapping layer to store charges from MoS2. The hysteresis window of these heterostructured devices can be effectively modified within a range of 13–70 V by manipulating both the thickness of the organic layer and the gate voltages. The largest hysteresis window is found in a combination of a few-layer Me-PTCDI (12.6 nm) and MoS2 (6 nm), showing a high on/off current ratio (&amp;gt;104) and a long retention time (104 s). Furthermore, the endurance test, which lasts for over 1000 cycles, demonstrates an exceptional level of stability and reliability. In addition, multilevel memory effects can be observed when gate pulses with different widths and amplitudes are applied. These 2D hybrid heterostructured devices have the capability to broaden the scope of material systems and present substantial potential for functional neuromorphic applications.

We study the electronic and optoelectronic properties of a broken-gap heterojunction composed of SnSe₂ and MoTe₂ with gate-controlled junction modes. Owing to the interband tunneling current, our device can act as an Esaki diode and a backward diode with a peak-to-valley current ratio approaching 5.7 at room temperature. Furthermore, under an 811 nm laser irradiation the heterostructure exhibits a photodetectivity of up to 7.5 × 10¹² Jones. In addition, to harness the electrostatic gate bias, <i>V</i>_oc can be tuned from negative to positive by switching from the accumulation mode to the depletion mode of the heterojunction. Additionally, a photovoltaic effect with a fill factor exceeding 41% was observed, which highlights the significant potential for optoelectronic applications. This study not only demonstrates high-performance multifunctional optoelectronics based on the SnSe₂/MoTe₂ heterostructure but also provides a comprehensive understanding of broken-band alignment and its applications.

Abstract Few‐layer rhenium disulfide (ReS 2 ) field‐effect transistors with a local floating gate (FG) of monolayer graphene separated by a thin hexagonal boron nitride tunnel layer for application to a non‐volatile memory (NVM) device are designed and investigated. FG‐NVM devices based on two‐dimensional van‐der‐Waals heterostructures have been recently studied as important components to realize digital electronics and multifunctional memory applications. Direct bandgap multilayer ReS 2 satisfies various requirements as a channel material for electronic devices as well as being a strong light‐absorbing layer, which makes it possible to realize light‐assisted optoelectronic applications. The NVM operation with a high ON/OFF current ratio, a large memory window, good endurance (&gt;1000 cycles), and stable retention (&gt;10 4 s) are observed. The successive program and erase states using 10 ms gate pulses of +10 V and −10 V are demonstrated, respectively. Laser pulses along with electrostatic gate pulses provide multibit level memory access via opto‐electrostatic coupling. The devices exhibit the dual functionality of a conventional electronic memory and can store laser‐pulse excited signal information for future all‐optical logic and quantum information processing.

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It is critically important to characterize the band alignment in semiconductor heterojunctions (HJs) because it controls the electronic and optical properties. However, the well‐known Anderson's model usually fails to predict the band alignment in bulk HJ systems due to the presence of charge transfer at the interfacial bonding. Atomically thin 2D transition metal dichalcogenide materials have attracted much attention recently since the ultrathin HJs and devices can be easily built and they are promising for future electronics. The vertical HJs based on 2D materials can be constructed via van der Waals stacking regardless of the lattice mismatch between two materials. Despite the defect‐free characteristics of the junction interface, experimental evidence is still lacking on whether the simple Anderson rule can predict the band alignment of HJs. Here, the validity of Anderson's model is verified for the 2D heterojunction systems and the success of Anderson's model is attributed to the absence of dangling bonds (i.e., interface dipoles) at the van der Waal interface. The results from the work set a foundation allowing the use of powerful Anderson's rule to determine the band alignments of 2D HJs, which is beneficial to future electronic, photonic, and optoelectronic devices.

Abstract Insufficient charge extraction at the interfaces between light‐absorbing perovskites and charge transporting layers is one of the drawbacks of state‐of‐the‐art perovskite solar cells. Surface treatments and/or interface engineering are necessary to approach the Shockley–Queisser limit. In this work, novel 2D layered perovskites, such as CHA 2 PbI 4 (CHAI = cyclohexylammonium iodide) and CHMA 2 PbI 4 (CHMAI = cyclohexylmethylammonium iodide), are introduced in between 3D perovskites and hole transporting layers by a simple solution process and the 2D/3D perovskite heterojunction is formed and confirmed. Spontaneous photoluminescence quenching is observed by efficient hole extraction with a favorable valence band alignment. The charge extraction ability and recombination are directly measured by the transient photocurrent and photovoltage. Moreover, the interface resistance of the devices significantly is decreased to 30% as compared to devices without 2D perovskites. As a result, the devices with 2D/3D perovskite heterojunction exhibit improved power conversion efficiency (PCE) from 20.41% to 23.91% primarily because of the increased open‐circuit voltage (1.079 to 1.143 V) and fill factor (78.22% to 84.25%). The results provide a detailed insight into hole extraction and high PCEs with the formation of a 2D/3D perovskite heterojunction.

Abstract Critical catalysis studies often lack elucidation of the mechanistic role of defect equilibria in solid solubility and charge compensation. This approach is applied to interpret the physicochemical properties and catalytic performance of a free‐standing 2D–3D CeO 2− x scaffold, which is comprised of holey 2D nanosheets, and its heterojunctions with MoO 3− x and RuO 2 . The band gap alignment and structural defects are engineered using density functional theory (DFT) simulations and atomic characterization. Further, the heterojunctions are used in hydrogen evolution reaction (HER) and catalytic ozonation applications, and the impacts of the metal oxide heteroatoms are analyzed. A key outcome is that the principal regulator of the ozonation performance is not oxygen vacancies but the concentration of Ce 3+ and Ce vacancies. Cation vacancy defects are measured to be as high as 8.1 at% for Ru‐CeO 2− x . The homogeneous distribution of chemisorbed, Mo‐oxide, heterojunction nanoparticles on the CeO 2− x holey nanosheets facilitates intervalence charge transfer, resulting in the dominant effect and resultant ≈50% decrease in overpotential for HER. The heterojunctions are tested for aqueous‐catalytic ozonation of salicylic acid, revealing excellent catalytic performance from Mo doping despite the adverse impact of Ce vacancies. The present study highlights the use of defect engineering to leverage experimental and DFT results for band alignment.

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0D/2D heterojunctions, especially quantum dots (QDs)/nanosheets (NSs) have attracted significant attention for use of photoexcited electrons/holes due to their high charge mobility. Herein, unprecedent heterojunctions of vanadate (AgVO₃ , BiVO₄ , InVO₄ and CuV₂ O₆ ) QDs/graphitic carbon nitride (g-C₃ N₄ ) NSs exhibiting multiple unique advances beyond traditional 0D/2D composites have been developed. The photoactive contribution, up-conversion absorption, and nitrogen coordinating sites of g-C₃ N₄ NSs, highly dispersed vanadate nanocrystals, as well as the strong coupling and band alignment between them lead to superior visible-light-driven photoelectrochemical (PEC) and photocatalytic performance, competing with the best reported photocatalysts. This work is expected to provide a new concept to construct multifunctional 0D/2D nanocomposites for a large variety of opto-electronic applications, not limited in photocatalysis.

The high-performance broadband photodetectors have attracted intensive scientific interests due to their potential applications in optoelectronic systems. Despite great achievements in two-dimensional (2D) materials based photodetectors such as graphene and black phosphorus, obvious disadvantages such as low optical absorbance and instability preclude their usage for the broadband photodetectors with the desired performance. An alternative approach is to find promising 2D materials and fabricate heterojunction structures for multifunctional hybrid photodetectors. In this work, 2D WS2/Si heterojunction with a type-II band alignment is formed in situ. This heterojunction device produced a high Ion/Ioff ratio over 10,6 responsivity of 224 mA/W, specific detectivity of 1.5 × 1012 Jones, high polarization sensitivity, and broadband response up to 3043 nm. Furthermore, a 4 × 4 device array of WS2/Si heterojunction device is demonstrated with high stability and reproducibility. These results suggest that the WS2/Si type-II heterojunction is an ideal photodetector in broadband detection and integrated optoelectronic system.

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Two-dimensional materials present a versatile platform for developing steep transistors due to their uniform thickness and sharp band edges. We demonstrate 2D-2D tunneling in a WSe2/SnSe2 van der Waals vertical heterojunction device, where WSe2 is used as the gate controlled p-layer and SnSe2 is the degenerately n-type layer. The van der Waals gap facilitates the regulation of band alignment at the heterojunction, without the necessity of a tunneling barrier. ZrO2 is used as the gate dielectric, allowing the scaling of gate oxide to improve device subthreshold swing. Efficient gate control and clean interfaces yield a subthreshold swing of ∼100 mV/dec for &amp;gt;2 decades of drain current at room temperature, hitherto unobserved in 2D-2D tunneling devices. The subthreshold swing is independent of temperature, which is a clear signature of band-to-band tunneling at the heterojunction. A maximum switching ratio ION/IOFF of 107 is obtained. Negative differential resistance in the forward bias characteristics is observed at 77 K. This work bodes well for the possibilities of two-dimensional materials for the realization of energy-efficient future-generation electronics.

Just as biological synapses provide basic functions for the nervous system, artificial synaptic devices serve as the fundamental building blocks of neuromorphic networks; thus, developing novel artificial synapses is essential for neuromorphic computing. By exploiting the band alignment between 2D inorganic and organic semiconductors, the first multi-functional synaptic transistor based on a molybdenum disulfide (MoS₂ )/perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) hybrid heterojunction, with remarkable short-term plasticity (STP) and long-term plasticity (LTP), is reported. Owing to the elaborate design of the energy band structure, both robust electrical and optical modulation are achieved through carriers transfer at the interface of the heterostructure, which is still a challenging task to this day. In electrical modulation, synaptic inhibition and excitation can be achieved simultaneously in the same device by gate voltage tuning. Notably, a minimum inhibition of 3% and maximum facilitation of 500% can be obtained by increasing the electrical number, and the response to different frequency signals indicates a dynamic filtering characteristic. It exhibits flexible tunability of both STP and LTP and synaptic weight changes of up to 60, far superior to previous work in optical modulation. The fully 2D MoS₂ /PTCDA hybrid heterojunction artificial synapse opens up a whole new path for the urgent need for neuromorphic computation devices.

The nitrogenated porous two-dimensional (2D) material C2N has been successfully synthesized using a simple wet-chemical reaction, which provides a high-performance way to produce such 2D materials with novel electronic and optical properties. In this work, density functional theory (DFT) calculations were performed to investigate the structural, electronic, and optical properties of the layered C2N/MoS2 van der Waals (vdW) heterojunction. The C2N/MoS2 heterojunction was found to have a direct band gap of 1.30 eV and to present the typical type-II heterojunction feature, facilitating the effective separation of photogenerated electrons and holes. The calculated band alignment and enhanced optical absorption suggest that the C2N/MoS2 heterojunction should exhibit good light-harvesting properties. The vertical strain can effectively tune the electronic properties and optical absorption of the C2N/MoS2 heterojunction by changing the interaction between the pz orbital of C2N and the dz2 orbital of MoS2. The moderate band gap, well-separated photogenerated electrons and holes, and enhanced visible-light absorption indicate that the C2N/MoS2 heterojunction is a potential photovoltaic structure for solar energy.

van der Waals (vdW) heterojunctions composed of two-dimensional (2D) layered materials are emerging as a solid-state materials family that exhibits novel physics phenomena that can power a range of electronic and photonic applications. Here, we present the first demonstration of an important building block in vdW solids: room temperature Esaki tunnel diodes. The Esaki diodes were realized in vdW heterostructures made of black phosphorus (BP) and tin diselenide (SnSe2), two layered semiconductors that possess a broken-gap energy band offset. The presence of a thin insulating barrier between BP and SnSe2 enabled the observation of a prominent negative differential resistance (NDR) region in the forward-bias current-voltage characteristics, with a peak to valley ratio of 1.8 at 300 K and 2.8 at 80 K. A weak temperature dependence of the NDR indicates electron tunneling being the dominant transport mechanism, and a theoretical model shows excellent agreement with the experimental results. Furthermore, the broken-gap band alignment is confirmed by the junction photoresponse, and the phosphorus double planes in a single layer of BP are resolved in transmission electron microscopy (TEM) for the first time. Our results represent a significant advance in the fundamental understanding of vdW heterojunctions and broaden the potential applications of 2D layered materials.

High defect density and low stability remain significant obstacles in the development of efficient and stable 3D perovskite solar cells (PSCs), while 3D/2D hybrid PSCs have emerged as promising candidates due to their excellent environmental tolerance. However, the performance of 3D/2D PSCs might be further limited by carrier transport within the films caused by quasi-2D phases with multiple n-values. Herein, precrystallized 2D phenylethylamine lead iodide (PEA₂PbI₄) crystals and methylammonium chloride (MACl) were introduced into the bulk phase of methylammonium lead iodide (MAPbI₃) to improve crystal quality and growth orientation. During the film formation process, 2D PEA₂PbI₄ delayed crystal nucleation and formed a 3D/2D mixed film. The as-formed type-II band alignment in the 2D/3D heterojunction film restricts the movement of charge carriers within the 3D phase lattice, avoiding the risk of being trapped by defects at the grain boundaries. Additionally, MACl facilitates vertical crystal growth, enlarging grain size and the thickness of perovskite films. Under the combined effect of 2D PEA₂PbI₄ and MACl, the power conversion efficiency (PCE) of the carbon-based PSCs increased from 10.53% to 16.07%. The unpackaged devices retained 90% of their initial PCE after storage for 30 days at room temperature and 40% relative humidity. These findings not only provide a facile way for the crystal tuning and defect passivation by low dimensional perovskites, but also give new insights into constructing efficient 3D/2D types of carbon-based PSCs.

The heterojunction between two materials brought into contact, for example, in the form of vertical van der Waals heterostructures, exhibits interesting features offering functionalities to devices stimulated by light. We report in this article an investigation of the optical and electronic properties of the heterojunction formed between Sb2Se3, a material with a promising role in photovoltaics characterized by one-dimensional (1D) topology (ribbons), and an emerging two-dimensional (2D) material, PtSe2, exhibiting unique optical properties for photoelectronics and photonics. The controlled growth of PtSe2 on Sb2Se3 underlayer takes place using a transfer-free process by low-temperature selenization of 1–2 nm Pt films thermally evaporated on Sb2Se3 ultrathin substrates. X-ray photoelectron spectroscopy (XPS) data analyzed in the context of the Kraut method provided an estimate for the band offsets at the interface. The valence band offset and the conduction band offset of the PtSe2/Sb2Se3 heterojunction were found to be −0.25 and 1.0 eV, respectively, indicating a type-II heterojunction. The ultrabroad optical absorption of the heterojunction and the protection offered by PtSe2 to Sb2Se3, against oxidation of the latter, render this particular heterojunction a robust candidate for applications in photovoltaics. Finally, the current study of a heterojunction between materials of different dimensionalities may pave the way for a rational design in the field of trans-dimensional heterostructures.

Semiconductor heterostructures are backbones for solid-state-based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures have enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and a long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe₂-WSe₂ heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller than the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is 2 orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to the opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

Abstract All‐inorganic CsPbBrI 2 perovskite has great advantages in terms of ambient phase stability and suitable band gap (1.91 eV) for photovoltaic applications. However, the typically used structure causes reduced device performance, primarily due to the large recombination at the interface between the perovskite, and the hole‐extraction layer (HEL). In this paper, an efficient CsPbBrI 2 perovskite solar cell (PSC) with a dimensionally graded heterojunction is reported, in which the CsPbBrI 2 material is distributed within bulk–nanosheet–quantum dots or 3D–2D–0D dimension‐profiled interface structure so that the energy alignment is optimized in between the valence and conduction bands of both CsPbBrI 2 and the HEL layers. Specifically, the valence‐/conduction‐band edge is leveraged to bend with synergistic advantages: the graded combination enhances the hole extraction and conduction efficiency with effectively decreased recombination loss during the hole‐transfer process, leading to an enhanced built‐in electric field, hence a high V OC of as much as 1.19 V. The profiled structure induces continuously upshifted energy levels, resulting in a higher J SC of as much as 12.93 mA cm −2 and fill factor as high as 80.5%, and therefore record power conversion efficiency (PCE) of 12.39%. As far as it is known, this is the highest PCE for CsPbBrI 2 perovskite‐based PSC.

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The g-C3N4-based composite structure exhibits excellent photocatalytic performance. However, their photogenerated carrier transfer and photocatalytic reaction mechanism were unclear. In this study, a 2D/2D g-C3N4/SnS2 heterojunction was systematically investigated by a hybrid density functional approach. Results indicated that the g-C3N4/SnS2 heterojunction was a staggered band alignment structure, and band bending occurred at the interface. A built-in electric field from the g-C3N4 surface to the SnS2 surface was formed by interfacial interaction. During visible-light irradiation, excited electrons in the conduction band maximum (CBM) of SnS2 easily recombined with the holes in the VBM of g-C3N4 under the electric field force. As a result, photogenerated electrons and holes naturally accumulate at the CBM of g-C3N4 and the valence band maximum (VBM) of SnS2, respectively. The effective separation of holes and electrons in space was advantageous to them participating in catalytic reactions on a different surface. Consequently, a direct Z-scheme photocatalytic reaction mechanism was established to enhance the photocatalytic activity of the g-C3N4/SnS2 heterojunction. Our results not only reveal the photocatalytic reaction mechanism of the g-C3N4/SnS2 heterojunction but also provide a theoretical guidance for the design and preparation of novel g-C3N4-based composite structures.

Recently, constructing van der Waals (vdW) heterojunctions by stacking different two-dimensional (2D) materials has been considered to be effective strategy to obtain the desired properties. Here, through first-principles calculations, we find theoretically that the 2D $n$-InSe/$p$-GeSe(SnS) vdW heterojunctions are the direct-band-gap semiconductor with typical type-II band alignment, facilitating the effective separation of photogenerated electron and hole pairs. Moreover, they possess the high optical absorption strength ($\ensuremath{\sim}{10}^{5}$), broad spectrum width, and excellent carrier mobility ($\ensuremath{\sim}{10}^{3}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). Interestingly, under the influences of the interlayer coupling and external electric field, the characteristics of type-II band alignment is robust, while the band-gap values and band offset are tunable. These results indicate that 2D $n$-InSe/$p$-GeSe(SnS) heterojunctions possess excellent optoelectronic and transport properties, and thus can become good candidates for next-generation optoelectronic nanodevices.

Abstract Van der Waals heterojunctions made of 2D materials offer competitive opportunities in designing and achieving multifunctional and high‐performance electronic and optoelectronic devices. However, due to the significant reverse tunneling current in such thin p–n junctions, a low rectification ratio along with a large reverse current is often inevitable for the heterojunctions. Here, a vertically stacked van der Waals heterojunction (vdWH) tunneling device is reported consisting of black arsenic phosphorus (AsP) and indium selenide (InSe), which shows a record high reverse rectification ratio exceeding 10 7 along with an unusual ultralow forward current below picoampere and a high current on/off ratio over 10 8 simultaneously at room temperature under the proper band alignment design of both the Schottky junction and the heterojunction. Therefore, the vdWH tunneling device can function as an ultrasensitive photodetector with an ultrahigh light on/off ratio of 1 × 10 7 , a comparable responsivity of around 1 A W −1 , and a high detectivity over 1 × 10 12 Jones in the visible wavelength range. Furthermore, the device exhibits a clear photovoltaic effect and shows a spectral detection capability up to 1550 nm. The work sheds light on developing future electronic and optoelectronic multifunctional devices based on the van der Waals integration of 2D materials with designed band alignment.

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We report the band alignment parameters of the GaN/single-layer (SL) MoS2 heterostructure where the GaN thin layer is grown by molecular beam epitaxy on CVD deposited SL-MoS2/c-sapphire. We confirm that the MoS2 is an SL by measuring the separation and position of room temperature micro-Raman E12g and A1g modes, absorbance, and micro-photoluminescence bandgap studies. This is in good agreement with HRTEM cross-sectional analysis. The determination of band offset parameters at the GaN/SL-MoS2 heterojunction is carried out by high-resolution X-ray photoelectron spectroscopy accompanying with electronic bandgap values of SL-MoS2 and GaN. The valence band and conduction band offset values are, respectively, measured to be 1.86 ± 0.08 and 0.56 ± 0.1 eV with type II band alignment. The determination of these unprecedented band offset parameters opens up a way to integrate 3D group III nitride materials with 2D transition metal dichalcogenide layers for designing and modeling of their heterojunction based electronic and photonic devices.

Van der Waals (vdW) p-n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next-generation of electronics and optoelectronics devices. Here, it is reported that the WSe2/WS2 p-n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n-"anti-bipolar"-p-ambipolar is observed in the WSe2/WS2 heterojunctions owing to the successive work of conducting channels of junctions, p-WSe2 and n-WS2 on the electrical transport of the whole systems. The type-II band alignment obtained from first principle calculations and built-in potential in this vdW heterojunction can also facilitate the efficient electron-hole separation, thus enabling the significant photovoltaic effect and a much enhanced self-driven photoswitching response in this system.

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Photoinduced interfacial charge transfer is at the heart of many applications, including photovoltaics, photocatalysis, and photodetection. With the emergence of a new class of semiconductors, i.e., monolayer two-dimensional transition metal dichalcogenides (2D-TMDs), charge transfer at the 2D/2D heterojunctions has attracted several efforts due to the remarkable optical and electrical properties of 2D-TMDs. Unfortunately, in 2D/2D heterojunctions, for a given combination of two materials, the relative energy band alignment and the charge-transfer efficiency are locked. Due to their large variety and broad size tunability, semiconductor quantum dots (0D-QDs) interfaced with 2D-TMDs may become an attractive heterostructure for optoelectronic applications. Here, we incorporate femtosecond pump-probe spectroscopy to reveal the sub-45 fs charge transfer at a 2D/0D heterostructure composed of tungsten disulfide monolayers (2D-WS₂) and a single layer of cadmium selenide/zinc sulfide core/shell 0D-QDs. Furthermore, ultrafast dynamics and steady-state measurements suggested that, following electron transfer from the 2D to the 0D, hybrid excitons, wherein the electron resides in the 0D and the hole resides in the 2D-TMD monolayer, are formed with a binding energy on the order of ∼140 meV, which is several times lower than that of tightly bound excitons in 2D-TMDs.

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2D Perovskites In article number 2102236, Jong Hyeok Park, Nam-Gyu Park, Hyunjung Shin and co-workers extract photo-generated holes from 3D bulk perovskites of FAPbI3 through cyclohexylamonium-based 2D perovskites. 2D/3D perovskite heterojunction structures are successfully fabricated and the efficient extraction of holes is expressed in this cover picture.

Abstract Atomically thin layers of van der Waals (vdW) crystals offer an ideal material platform to realize tunnel field‐effect transistors (TFETs) that exploit the tunneling of charge carriers across the forbidden gap of a vdW heterojunction. This type of device requires a precise energy band alignment of the different layers of the junction to optimize the tunnel current. Among 2D vdW materials, black phosphorus (BP) and indium selenide (InSe) have a Brillouin zone‐centered conduction and valence bands, and a type II band offset, both ideally suited for band‐to‐band tunneling. TFETs based on BP/InSe heterojunctions with diverse electrical transport characteristics are demonstrated: forward rectifying, Zener tunneling, and backward rectifying characteristics are realized in BP/InSe junctions with different thickness of the BP layer or by electrostatic gating of the junction. Electrostatic gating yields a large on/off current ratio of up to 10 8 and negative differential resistance at low applied voltages ( V ≈ 0.2 V). These findings illustrate versatile functionalities of TFETs based on BP and InSe, offering opportunities for applications of these 2D materials beyond the device architectures reported in the current literature.

We report on solid-state mesoscopic heterojunction solar cells employing nanoparticles (NPs) of methyl ammonium lead iodide (CH(3)NH(3))PbI(3) as light harvesters. The perovskite NPs were produced by reaction of methylammonium iodide with PbI(2) and deposited onto a submicron-thick mesoscopic TiO(2) film, whose pores were infiltrated with the hole-conductor spiro-MeOTAD. Illumination with standard AM-1.5 sunlight generated large photocurrents (J(SC)) exceeding 17 mA/cm(2), an open circuit photovoltage (V(OC)) of 0.888 V and a fill factor (FF) of 0.62 yielding a power conversion efficiency (PCE) of 9.7%, the highest reported to date for such cells. Femto second laser studies combined with photo-induced absorption measurements showed charge separation to proceed via hole injection from the excited (CH(3)NH(3))PbI(3) NPs into the spiro-MeOTAD followed by electron transfer to the mesoscopic TiO(2) film. The use of a solid hole conductor dramatically improved the device stability compared to (CH(3)NH(3))PbI(3) -sensitized liquid junction cells.

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Large-area two-dimensional (2D) heterojunctions are promising building blocks of 2D circuits. Understanding their intriguing electrostatics is pivotal but largely hindered by the lack of direct observations. Here graphene-WS₂ heterojunctions are prepared over large areas using a seedless ambient-pressure chemical vapor deposition technique. Kelvin probe force microscopy, photoluminescence spectroscopy, and scanning tunneling microscopy characterize the doping in graphene-WS₂ heterojunctions as-grown on sapphire and transferred to SiO₂ with and without thermal annealing. Both p-n and n-n junctions are observed, and a flat-band condition (zero Schottky barrier height) is found for lightly n-doped WS₂, promising low-resistance ohmic contacts. This indicates a more favorable band alignment for graphene-WS₂ than has been predicted, likely explaining the low barriers observed in transport experiments on similar heterojunctions. Electrostatic modeling demonstrates that the large depletion width of the graphene-WS₂ junction reflects the electrostatics of the one-dimensional junction between two-dimensional materials.

Two-dimensional-on-three-dimensional (2D/3D) halide perovskite heterostructures have been extensively utilized in optoelectronic devices. However, the labile nature of halide perovskites makes it difficult to form such heterostructures with well-defined compositions, orientations, and interfaces, which inhibits understanding of the carrier transfer properties across these heterostructures. Here, we report solution growth of both horizontally and vertically aligned 2D perovskite (PEA)₂PbBr₄ (PEA = phenylethylammonium) microplates onto 3D CsPbBr₃ single crystal thin films, with well-defined heterojunctions. Time-resolved photoluminescence (TRPL) transients of the heterostructures exhibit the monomolecular and bimolecular dynamics expected from exciton annihilation, dissociation, and recombination, as well as evidence for carrier transfer in these heterostructures. Two kinetic models based on Type-I and Type-II band alignments at the interface of horizontal 2D/3D heterostructures are applied to reveal a shift in balance between carrier transfer and recombination: Type-I band alignment better describes the behaviors of heterostructures with thin 2D perovskite microplates but Type-II band alignment better describes those with thick 2D microplates (>150 nm). TRPL of vertically aligned 2D microplates is dominated by directly excited PL and is independent of the height above the 3D film. Electrical measurements reveal current rectification behaviors in both heterostructures with vertical heterostructures showing better electrical transport. As the first systematic study on comparing models of 2D/3D perovskite heterostructures with controlled orientations and compositions, this work provides insights on the charge transfer mechanisms in these perovskite heterostructures and guidelines for designing better optoelectronic devices.

Accelerating the migration of interfacial carriers in a heterojunction is of paramount importance for driving high-performance photoelectric responses. However, the inferior contact area and large resistance at the interface limit the eventual photoelectric performance. Herein, we fabricated an S-scheme heterojunction involving a 2D/2D dual-metalloporphyrin metal-organic framework with metal-center-regulated CuTCPP(Cu)/CuTCPP(Fe) through electrostatic self-assembly. The ultrathin nanosheet-like architectures reduce the carrier migration distance, while the similar porphyrin backbones promote reasonable interface matching through π-π conjugation, thereby inhibiting the recombination of photogenerated carriers. Furthermore, the metal-center-regulated S-scheme band alignments create a giant built-in electric field, which provides a huge driving force for efficient carrier separation and migration. Coupling with the biomimetic catalytic activity of CuTCPP(Fe), the resultant heterojunction was utilized to construct photoelectrochemical uric acid biosensors. This work provides a general strategy to enhance photoelectric responses by engineering the interfacial structure of heterojunctions.

We study the interface exciton at lateral type II heterojunctions of monolayer transition metal dichalcogenides (TMDs), where the electron and hole prefer to stay at complementary sides of the junction. We find that the 1D interface exciton has giant binding energy in the same order as 2D excitons in pristine monolayer TMDs although the effective radius (electron-hole separation) of interface exciton is much larger than that of 2D excitons. The binding energy, exciton radius, and optical dipole strongly depends on the band offset at the junction. The intervalley coupling induced by the electron-hole Coulomb exchange interaction and the quantum confinement effect at interfaces of a closed triangular shape are also investigated. Small triangles realize 0D quantum dot confinement of excitons, and we find a transition from nondegenerate ground state to degenerate ones when the size of the triangle varies. Our findings may facilitate the implementation of the optoelectronic devices based on the lateral heterojunction structures in monolayer semiconductors.

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By virtue of their excellent solution processibility and flexibility, organic field-effect transistors (OFETs) are considered outstanding candidates for application in low-cost, flexible electronics. Not only does the performance of OFETs depend on the molecular properties of the organic semiconductors involved, but it is also dramatically affected by the nature of the interfaces present. Therefore, interface engineering, a novel approach towards high-performance OFETs, has attracted considerable attention. In this Account, we focus on recent advances in the study of OFET interfaces--including electrode/organic layer interfaces, dielectric/organic layer interfaces, and organic/organic layer interfaces--that have resulted in improved device performance, enhanced stability, and the realization of organic light-emitting transistors. The electrode/organic layer interface, one of the most important interfaces in OFETs, usually determines the carrier injection characteristics. Focusing on OFETs with copper and silver electrodes, we describe effective modification approaches of the electrode/organic layer interfaces. Furthermore, the influence of electrode morphology on device performance is demonstrated. These results provide novel approaches towards high-performance, low-cost OFETs. The dielectric/organic layer interface is a vital interface that dominates carrier transport; modification of this interface therefore offers a general way to improve carrier transport accordingly. The dielectric layer also affects the device stability of OFETs. For example, high-performance pentacene OFETs with excellent stability are obtained by the selection of a dielectric layer with an appropriate surface energy. The organic/organic layer interface is a newly investigated topic in OFETs. Introduction of organic/organic layer interfaces, such as heterojunctions, can improve device performance and afford ambipolar OFETs. By designing laterally arranged heterojunctions made of organic field-effect materials and light-emitting materials, we realized both light emission and field effects simultaneously in a single OFET. The preceding decade has seen great progress in OFETs. Interface engineering provides a simple but effective approach toward creating high-performance OFETs and will continue to make essential contributions in the development of useful OFET-based devices. The exploration of novel interface engineering applications also merits further attention; the design of gas sensors through a more complete understanding of interface phenomena serves as just one example.

Semiconductor heterostructures are fundamental building blocks for many important device applications. The emergence of two-dimensional semiconductors opens up a new realm for creating heterostructures. As the bandgaps of transition metal dichalcogenides thin films have sensitive layer dependence, it is natural to create lateral heterojunctions (HJs) using the same materials with different thicknesses. Here we show the real space image of electronic structures across the bilayer-monolayer interface in MoSe2 and WSe2, using scanning tunnelling microscopy and spectroscopy. Most bilayer-monolayer HJs are found to have a zig-zag-orientated interface, and the band alignment of such atomically sharp HJs is of type-I with a well-defined interface mode that acts as a narrower-gap quantum wire. The ability to utilize such commonly existing thickness terraces as lateral HJs is a crucial addition to the tool set for device applications based on atomically thin transition metal dichalcogenides, with the advantage of easy and flexible implementation.

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p-n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high-efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy-free 2D monolayer WSe₂ -MoS₂ lateral p-n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode-spacing design can lead to environment-independent PV properties. These robust PV properties deriving from the atomically sharp lateral p-n interface can help develop the next-generation photovoltaics.

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Heterostructure nanowires have many potential applications due to the avoidance of interface defects by lateral strain relaxation. However, most heterostructure semiconductor nanowires suffer from persistent interface compositional grading, normally attributed to the dissolution of growth species in the common alloy seed particles. Although progress has been made for some material systems, most binary material combinations remain problematic due to the interaction of growth species in the alloy. In this work we investigate the formation of interfaces in InAs-GaAs heterostructures experimentally and theoretically and demonstrate a technique to attain substantially sharper interfaces. We show that by pulsing the Ga source during heterojunction formation, In is pushed out before GaAs growth initiates, greatly reducing In carry-over. This procedure will be directly applicable to any nanowire system with finite nonideal solubility of growth species in the alloy seed particle and greatly improve the applicability of these structures in future devices.

Efficient waste heat dissipation has become increasingly challenging as transistor size has decreased to nanometers. As governed by universal Umklapp phonon scattering, the thermal conductivity of semiconductors decreases at higher temperatures and causes heat transfer deterioration under high-power conditions. In this study, we realized simultaneous electrical and thermal rectification (TR) in a monolayer MoSe₂-WSe₂ lateral heterostructure. The atomically thin MoSe₂-WSe₂ heterojunction forms an electrical diode with a high ON/OFF ratio up to 10⁴. Meanwhile, a preferred heat dissipation channel was formed from MoSe₂ to WSe₂ in the ON state of the heterojunction diode at high bias voltage with a TR factor as high as 96%. Higher thermal conductivity was achieved at higher temperatures owing to the TR effect caused by the local temperature gradient. Furthermore, the TR factor could be regulated from maximum to zero by rotating the angle of the monolayer heterojunction interface. This result opens a path for designing novel nanoelectronic devices with enhanced thermal dissipation.

With the lateral coplanar heterojunctions of two-dimensional monolayer materials turning into reality, the quantitative understanding of their electronic, electrostatic, doping, and scaling properties becomes imperative. In contrast to traditional bulk 3D junctions where carrier equilibrium is reached through local charge redistribution, a highly nonlocalized charge transfer (trailing off as 1/x away from the interface) is present in lateral 2D junctions, increasing the junction size considerably. The depletion width scales as p(-1), while the differential capacitance varies very little with the doping level p. The properties of lateral 2D junctions are further quantified through numerical analysis of realistic materials, with graphene, MoS2, and their hybrid serving as examples. Careful analysis of the built-in potential profile shows strong reduction of Fermi level pinning, suggesting better control of the barrier in 2D metal-semiconductor junctions.

Abstract Solution processing of polymer semiconductors provides a new paradigm for large‐area electronics manufacturing on flexible substrates, but it also severely restricts the realization of interesting advanced device architectures, such as lateral heterostructures with defined interfaces, which are easily accessible with inorganic materials using photolithography. This is because polymer semiconductors degrade, swell, or dissolve during conventional photoresist processing. Here a versatile, high‐resolution photolithographic method is demonstrated for patterning of polymer semiconductors and exemplify this with high‐performance p‐type and n‐type field‐effect transistors (FETs) in both bottom‐ and top‐gate architectures, as well as ambipolar light‐emitting field‐effect transistors (LEFETs), in which the recombination zone can be pinned at a photolithographically defined lateral heterojunction between two semiconducting polymers. The technique therefore enables the realization of a broad range of novel device architectures while retaining optimum materials performance.

In amorphous/crystalline silicon heterojunction solar cells, an inversion layer is present at the front interface. By combining numerical simulations and experiments, we examine the contribution of the inversion layer to lateral transport and assess whether this layer can be exploited to replace the front transparent conductive oxide (TCO) in devices. For this, heterojunction solar cells of different areas (2 × 2, 4 × 4, and 6 × 6 mm2) with and without TCO layers on the front side were prepared. Laser-beam-induced current measurements are compared with simulation results from the ASPIN2 semiconductor simulator. Current collection is constant across millimeter distances for cells with TCO; however, carriers traveling more than a few hundred microns in cells without TCO recombine before they can be collected. Simulations show that increasing the valence band offset increases the concentration of holes under the surface of n-type crystalline silicon, which increases the conductivity of the inversion layer. Unfortunately, this also impedes transport across the barrier to the emitter. We conclude that the lateral conductivity of the inversion layer may not suffice to fully replace the front TCO in heterojunction devices.

Preface.List of Contributors.PART 1: MATERIAL.1. High-Pressure Crystallization of GaN (I. Grzegory, et al.).2. Epitaxial Lateral Overgrowth of GaN (P. Gibart, et al.).3. Plasma-Assisted Molecular Beam Epitaxy of III-V Nitrides (A. Georgakilas, et al.).4. Growth of Gallium Nitride by Hydride Vapor Phase Epitaxy (A. Trassoudaine, et al.).5. Growth and Properties of InN (V. Davydov, et al.).6. Surface Structure and Adatom Kinetics of Group-III Nitrides (J. Neugebauer).PART 2: DEFECTS AND INTERFACES.7. Topological Analysis of Defects in Nitride Semiconductors (G. Dimitrakopulos, et al.).8. Extended Defects in Wurtzite GaN Layers: Atomic Structure, Formation, and Interaction Mechanisms (P. Ruterana, et al.).9. Stain, Chemical Composition, and Defects Analysis at Atomic Level in GaN-based Epitaxial Layers (S. Kret, et al.).PART 3: PROCESSING AND DEVICES.10. Ohmic Contacts to GaN (P. Hartlieb, et al.). 11. Electroluminescent Diodes and Laser Diodes (H. Amano).12. GaN-Based Modulation-Doped FETs and Heterojunction Bipolar Transistors ( H. Morkoc & L. Liu).13. GaN-Based UV Photodetectors (F. Omnes & E. Monroy).Subject Index.

Monolayer MoS₂ has attracted significant attention owing to its excellent performance as an n-type semiconductor from the transition metal dichalcogenide (TMDC) family. It is however strongly desired to develop controllable synthesis methods for 2D p-type MoS₂ , which is crucial for complementary logic applications but remains difficult. In this work, high-quality NbS₂ -MoS₂ lateral heterostructures are synthesized by one-step metal-organic chemical vapor deposition (MOCVD) together with monolayer MoS₂ substitutionally doped by Nb, resulting in a p-type doped behavior. The heterojunction shows a p-type transfer characteristic with a high on/off current ratio of ≈10⁴ , exceeding previously reported values. The band structure through the NbS₂ -MoS₂ heterojunction is investigated by density functional theory (DFT) and quantum transport simulations. This work provides a scalable approach to synthesize substitutionally doped TMDC materials and provides an insight into the interface between 2D metals and semiconductors in lateral heterostructures, which is imperative for the development of next-generation nanoelectronics and highly integrated devices.

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The edge-to-edge connected metal-semiconductor junction (MSJ) for two-dimensional (2D) transistors has the potential to reduce the contact length while improving the performance of the devices. However, typical 2D materials are thermally and chemically unstable, which impedes the reproducible achievement of high-quality edge contacts. Here we present a scalable synthetic strategy to fabricate low-resistance edge contacts to atomic transistors using a thermally stable 2D metal, PtTe₂. The use of PtTe₂ as an epitaxial template enables the lateral growth of monolayer MoS₂ to achieve a PtTe₂-MoS₂ MSJ with the thinnest possible, seamless atomic interface. The synthesized lateral heterojunction enables the reduced dimensions of Schottky barriers and enhanced carrier injection compared to counterparts composed of a vertical 3D metal contact. Furthermore, facile position-selected growth of PtTe₂-MoS₂ MSJ arrays using conventional lithography can facilitate the design of device layouts with high processability, while providing low contact resistivity and ultrashort transfer length on wafer scales.

Van der Waals (vdW) heterojunctions based on two-dimensional (2D) atomic crystals have been extensively studied in recent years. Herein, we show that both vertical and lateral vdW heterojunctions can be realized with layered molecular crystals using a two-step physical vapor transport (PVT) process. Both types of heterojunctions show clean and sharp interfaces without phase mixing under atomic force microscopy (AFM). They also exhibit a strong interfacial built-in electric field similar to that of their inorganic counterparts. These heterojunctions have greater potential for device applications than individual materials. The lateral heterojunction (LHJ) devices show rectifying characteristics due to the asymmetric energy barrier for holes at the interface, while the vertical heterojunction (VHJ) devices behave like metal–insulator–semiconductor tunnel junctions, with pronounced negative differential conductance (NDC). Our work extends the concept of vdW heterojunctions to molecular materials, which can be generalized to other layered organic semiconductors (OSCs) to obtain new device functionalities.

The massless Dirac electron transport in graphene has led to a variety of unique light-matter interaction phenomena, which promise many novel optoelectronic applications. Most of the effects are only accessible by breaking the spatial symmetry, through introducing edges, p-n junctions, or heterogeneous interfaces. The recent development of direct synthesis of lateral heterostructures offers new opportunities to achieve the desired asymmetry. As a proof of concept, we study the photothermoelectric effect in an asymmetric lateral heterojunction between the Dirac semimetallic monolayer graphene and the parabolic semiconducting monolayer MoS₂. Very different hot-carrier cooling mechanisms on the graphene and the MoS₂ sides allow us to resolve the asymmetric thermalization pathways of photoinduced hot carriers spatially with electrostatic gate tunability. We also demonstrate the potential of graphene-2D semiconductor lateral heterojunctions as broadband infrared photodetectors. The proposed structure shows an extreme in-plane asymmetry and provides a new platform to study light-matter interactions in low-dimensional systems.

Stimuli-responsive hybrid van der Waals heterostructures (vdWHs), composed of organic molecular switches superimposed on inorganic 2D materials (2DMs), can combine the outstanding physical properties of the latter components with the virtually infinite variety of tunable functionality of molecules, thereby offering an efficient protocol for the development of high-performance multifunctional materials and devices. The use of light as a remote control to modulate the properties of semiconducting 2DMs when interfaced with photochromic molecules suffers from both the limitation associated with the persistent photoconductivity characterizing the 2DMs and the finite thermal stability of the photochromic molecule in its different states. Here, we have devised a universal approach toward the fabrication of optically switchable electronic devices comprising a few nanometers thick azobenzene (AZO) layer physisorbed on 2D semiconductors supported on a trap-free polymer dielectric. The joint effect of the improved 2D/dielectric interface, the molecule’s light-modulated dipolar doping, and the high thermal stability of cis-AZO offers the highest control over the reversible and efficient charge carrier tuning in 2D semiconductors with a preserved high performance in 2D field-effect transistors, as quantified in terms of carrier mobility and Ion/Ioff ratio. The device has the potential to operate as an optical memory with four current levels and long retention time (>15 h). Furthermore, by using a CMOS-compatible micropatterning process, the photoswitchable resistor–diode transition has been achieved on hybrid lateral heterojunction devices. Our approach is of general applicability toward the generation of high-performance hybrid vdWHs for the emergence of functional and responsive devices.

Sb₂ S₃ is a promising environmentally friendly semiconductor for high performance solar cells. But, like many other polycrystalline materials, Sb₂ S₃ is limited by nonradiative recombination and carrier scattering by grain boundaries (GBs). This work shows how the GB density in Sb₂ S₃ films can be significantly reduced from 1068 ± 40 to 327 ± 23 nm µm^-2 by incorporating an appropriate amount of Ce³⁺ into the precursor solution for Sb₂ S₃ deposition. Through extensive characterization of structural, morphological, and optoelectronic properties, complemented with computations, it is revealed that a critical factor is the formation of an ultrathin Ce₂ S₃ layer at the CdS/Sb₂ S₃ interface, which can reduce the interfacial energy and increase the adhesion work between Sb₂ S₃ and the substrate to encourage heterogeneous nucleation of Sb₂ S₃ , as well as promote lateral grain growth. Through reductions in nonradiative recombination at GBs and/or the CdS/Sb₂ S₃ heterointerface, as well as improved charge-carrier transport properties at the heterojunction, this work achieves high performance Sb₂ S₃ solar cells with a power conversion efficiency reaching 7.66%. An impressive open-circuit voltage (V_OC ) of 796 mV is achieved, which is the highest reported thus far for Sb₂ S₃ solar cells. This work provides a strategy to simultaneously regulate the nucleation and growth of Sb₂ S₃ absorber films for enhanced device performance.

Controlling the direction of exciton-energy flow in two-dimensional (2D) semiconductors is crucial for developing future high-speed optoelectronic devices using excitons as the information carriers. However, intrinsic exciton diffusion in conventional 2D semiconductors is omnidirectional, and efficient exciton-energy transport in a specific direction is difficult to achieve. Here we demonstrate directional exciton-energy transport across the interface in tungsten diselenide (WSe₂)-molybdenum diselenide (MoSe₂) lateral heterostructures. Unidirectional transport is spontaneously driven by the built-in asymmetry of the exciton-energy landscape with respect to the heterojunction interface. At excitation positions close to the interface, the exciton photoluminescence (PL) intensity was substantially decreased in the WSe₂ region and enhanced in the MoSe₂ region. In PL excitation spectroscopy, it was confirmed that the observed phenomenon arises from lateral exciton-energy transport from WSe₂ to MoSe₂. This directional exciton-energy flow in lateral 2D heterostructures can be exploited in future optoelectronic devices.

The operation of organic diodes in solar cells and light-emitting displays strongly depends on the properties of the interfaces between hole- and electron-carrying organic semiconductors. Such interfaces are difficult to characterize, as they are usually buried under the surface or exist as an irregular "bulk heterojunction." Using a unique fluorinated barrier layer-based lithographic technique, we fabricated a lateral organic p-n junction, allowing the first observation of the potential at an organic p-n interface simultaneously with the charge transport measurements. We find that the diode characteristics of the device (current output and rectification ratio) are consistent with the changes in the surface potentials near the junction, and the current-voltage curves and junction potentials are strongly and self-consistently modulated by a third, gate electrode. The generality of our technique makes this an attractive method to investigate the physics of organic semiconductor junctions. The lithographic technique is applicable to a wide variety of soft material patterns. The observation of built-in potentials makes an important connection between organic junctions and textbook descriptions of inorganic devices. Finally, these kinds of potentials may prove to be controlling factors in charge separation efficiency in organic photovoltaics.

Chemical bonds, including covalent and ionic bonds, endow semiconductors with stable electronic configurations but also impose constraints on their synthesis and lattice-mismatched heteroepitaxy. Here, the unique multi-scale van der Waals (vdWs) interactions are explored in one-dimensional tellurium (Te) systems to overcome these restrictions, enabled by the vdWs bonds between Te atomic chains and the spontaneous misfit relaxation at quasi-vdWs interfaces. Wafer-scale Te vdWs nanomeshes composed of self-welding Te nanowires are laterally vapor grown on arbitrary surfaces at a low temperature of 100 °C, bringing greater integration freedoms for enhanced device functionality and broad applicability. The prepared Te vdWs nanomeshes can be patterned at the microscale and exhibit high field-effect hole mobility of 145 cm²/Vs, ultrafast photoresponse below 3 μs in paper-based infrared photodetectors, as well as controllable electronic structure in mixed-dimensional heterojunctions. All these device metrics of Te vdWs nanomesh electronics are promising to meet emerging technological demands.

Naturally occurring carbon-nanotube heterojunctions provide a unique opportunity for studying nearly one-dimensional metal–semiconductor (M–S) interfaces. Coupling the nanoscale lateral precision of the ultrahigh-vacuum STM with spatially-resolved electronic measurements enables detailed characterization of a single-walled carbon nanotube M–S junction (see figure) and of the metal-induced gap states present at the interface.

Abstract 2D transition metal dichalcogenides heterostructures are driving advancements in next‐generation optoelectronic technologies. Lateral 2D heterojunctions with atomically seamless interfaces play a vital role in modulating charge separation and carrier dynamics, yet underlying transport mechanisms remain inadequately understood, limiting practical deployment. Here, monolayer WS 2 ‐MoS 2 lateral edge‐epitaxial heterostructures synthesized via chemical vapor deposition (CVD), providing critical insights into heterointerface effects on charge distribution and photoresponse are reported. Photodetector fabricated from this heterostructures exhibit broadband spectral response from ultraviolet to near‐infrared, achieving peak responsivity of 1850 mA W −1 and detectivity of 4.36 × 10 11 Jones under 565 nm illumination. This represents ≈200% enhancement compared to individual monolayer MoS 2 or WS 2 devices, directly demonstrating the synergistic benefits of lateral heterostructure engineering. Spatially resolved surface potential mapping and second‐harmonic generation imaging reveal that enhanced performance originates at the epitaxial interface, confirming the critical role of interfacial electric fields and nonlinear optical effects in charge carrier dynamics. The characterization provides direct experimental evidence linking atomically seamless interface properties to macroscopic device performance enhancements. These findings underscore the significant potential of CVD‐grown WS 2 ‐MoS 2 lateral heterostructures for high‐performance photodetectors and establish interface engineering as a powerful strategy for advancing 2D semiconductor device technologies.

Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe₃ layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe₃ on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe₃ are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe₃ MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe₂ and ML CrTe₃ with an atomically sharp and seamless interface. Since ML CrTe₂ is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal-semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe₃ , as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.

We report contact resistances as low as 0.035 Ω mm between the ohmic contact metal and the two-dimensional electron gas channel in an AlGaAs-GaAs modulation-doped field-effect transistor structure. The contact resistances are achieved by transient annealing of AuGe/Ni/Au, and are to our knowledge the smallest values reported to date for these structures at room temperature. Assuming no change in the semiconductor sheet resistivity under the metal contact, the calculated specific contact resistivity would be as low as 5×10−8 Ω cm2, however, the true figure is not known. Details of our contact study include varying temperature-time cycles and heterojunction structures. Current-voltage measurements at 77 K show ohmic behavior, revealing the destruction of the heterojunction barrier in the contact region. Preliminary Auger energy spectroscopy profiles support an intermixing of the metal and semiconductor constituents down through the original AlGaAs-GaAs interface. We, therefore, propose a model where the current flows laterally from grains (formed during annealing) into the two-dimensional electron gas.

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We report a theoretical study of the local interface properties at a graphene-MoSe2 (G-MoSe2) in-plane lateral heterostructure. Using a combination of first-principles density functional theory (DFT) calculations and simulations of X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the C K-edge, we examined different local interface arrangements. The simulated XANES signal from interface carbon atoms showed new features compared to the pristine graphene region, which provides a way of identifying different chemical environments and/or geometries of the local interface in the G-MoSe2 lateral hybrid system. Our results also revealed that the local electronic and magnetic properties are dependent on the interface atomic structure, where metallic, semiconductor or half-metallic character was achieved at the G-MoSe2 interface. These findings indicate the great potential of 2D lateral heterojunctions for nanoelectronic and spintronic applications.

In semiconductor devices that are the backbone of modern technology the conducting electrons are often confined by field effect or doping near an interface potential barrier and form a quasi-two-dimensional electron system. High resolution lithography and refined etching techniques now make it possible to laterally confine such originally two-dimensional electron systems to narrow channels with widths comparable to the de-Broglie wavelengths of the electrons. The transition from two-dimensional to one-dimensional electronic behavior that occurs as lateral confinement lengths are lowered to about 100 nm is directly observed with infrared spectroscopy of the electronic excitations in laterally microstructured metal-oxide-semiconductor and semiconductor-heterojunctions devices.

Discovering better materials for electronic devices is important for technological advancements. One of the most promising materials are transition-metal dichalcogenides (TMDCs) due to their ability to form two-dimensional structures with diverse compositions. Recently, experimental breakthroughs were demonstrated for all two-dimensional transistors that contain a semiconducting TMDC channel and other materials for the metallic source and drain. However, metal/semiconductor interfaces that are made of similar TMDC materials are anticipated to be better candidates because they provide better chemical bonding and thus smaller resistance. Furthermore, smaller resistance is expected to be achieved with a metal and a semiconductor that are joined in direct lateral contact. In this work, we use density functional theory to analyze the electronic structure properties of novel lateral TMDC metal/semiconductor heterojunctions. We discover promising metal/semiconductor heterojunctions, for example, VS2/CrS2, which is characterized by strong covalent bonds with limited metal-induced gap states, high charge density around the Fermi level, and no Schottky barrier. These properties are anticipated to be useful for practical implementation of these material heterojunctions in all two-dimensional transistors.

2D transition metal dichalcogenide (2D-TMD) materials and their van der Waals heterostructures (vdWHs) have inspired worldwide efforts in the fields of electronics and optoelectronics. However, photodetectors based on 2D/2D vdWHs suffer from performance limitations due to the weak optical absorption of their atomically thin nature. In this work, taking advantage of an excellent light absorption coefficient, low-temperature solution-processability, and long charge carrier diffusion length, all-inorganic halides perovskite CsPbI_3- <i>_x</i> Br <i>_x</i> quantum dots are integrated with monolayer MoS₂ for high-performance and low-cost photodetectors. A favorable energy band alignment facilitating interfacial photocarrier separation and efficient carrier injection into the MoS₂ layer inside the 0D-2D mixed-dimensional vdWHs are confirmed by a series of optical characterizations. Owing to the synergistic effect of the photogating mechanism and the modulation of Schottky barriers, the corresponding phototransistor exhibits a high photoresponsivity of 7.7 × 10⁴ A W^-1, a specific detectivity of ≈5.6 × 10¹¹ Jones, and an external quantum efficiency exceeding 10⁷%. The demonstration of such 0D-2D mixed-dimensional heterostructures proposed here would open up a wide realm of opportunities for designing low-cost, flexible transparent, and high-performance optoelectronics.

Sensitization of graphene with inorganic semiconducting nanostructures has been demonstrated as a powerful strategy to boost its optoelectronic performance. However, the limited tunability of optical properties and toxicity of metal cations in the inorganic sensitizers prohibits their widespread applications, and the in-depth understanding of the essential interfacial charge-transfer process within such hybrid systems remains elusive. Here, we design and develop high-quality nanographene (NG) dispersions with a large-scale production using high-shear mixing exfoliation. The physisorption of these NG molecules onto graphene gives rise to the formation of graphene-NG van der Waals heterostructures (VDWHs), characterized by strong interlayer coupling through π-π interactions. As a proof of concept, photodetectors fabricated on the basis of such VDWHs show ultrahigh responsivity up to 4.5 × 10⁷ A/W and a specific detectivity reaching 4.6 × 10¹³ Jones, being competitive with the highest values obtained for graphene-based photodetectors. The outstanding device characteristics are attributed to the efficient transfer of photogenerated holes from NGs to graphene and the long-lived charge separation at graphene-NG interfaces (beyond 1 ns), as elucidated by ultrafast terahertz (THz) spectroscopy. These results demonstrate the great potential of such graphene-NG VDWHs as prototypical building blocks for high-performance, low-toxicity optoelectronics.

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe₂/Bi₂Te_3- _xSe_x vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe₂/Bi₂Te₃) to Type III (WSe₂/Bi₂Se₃). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W^-1 and detectivity of 2.91×10¹² Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

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van der Waals heterojunctions (vdWHs) formed between 2D materials have attracted tremendous attention recently due to their extraordinary properties, which cannot be offered by their individual components or other heterojunctions. Intriguing electronic coupling, lowered energy barrier, intimate charge transfer, and efficient exciton separation occurring at the atomically sharp interface promise their applications in catalysis, which, however, are largely unexplored. Herein, we demonstrate a 0D/2D vdWH between 0D graphene quantum dots (GQDs) and 2D pristine graphene sheets, simply prepared by ultrasonication of graphite powder using GQDs as intercalation surfactant. Such an all-carbon Schottky-diode-like 0D/2D vdWH is employed for the emerging photoelectrochemical catalysis (water splitting) with high performance. The demonstrated low-cost and scalable bottom-up growth of heteroatom-doped GQDs will greatly promote their widespread applications. Moreover, the mechanisms underlying GQD growth and heterojunction-mediated catalysis are revealed both experimentally and theoretically.

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Miniaturized spectrometers are of considerable interest for their portability. Most designs to date employ a photodetector array with distinct spectral responses or require elaborated integration of micro & nano optic modules, typically with a centimeter-scale footprint. Here, we report a design of a micron-sized near-infrared ultra-miniaturized spectrometer based on two-dimensional van der Waals heterostructure (2D-vdWH). By introducing heavy metal atoms with delocalized electronic orbitals between 2D-vdWHs, we greatly enhance the interlayer coupling and realize electrically tunable infrared photoresponse (1.15 to 1.47 μm). Combining the gate-tunable photoresponse and regression algorithm, we achieve spectral reconstruction and spectral imaging in a device with an active footprint < 10 μm. Considering the ultra-small footprint and simple fabrication process, the 2D-vdWHs with designable bandgap energy and enhanced photoresponse offer an attractive solution for on-chip infrared spectroscopy.

van der Waals heterostructures, created by stacking two-dimensional materials, represent a novel and largely unexplored class of materials with very interesting optoelectronic properties. Excitons, strongly bound electron-hole pairs, play a crucial role in determining these properties, especially in 2D materials where the electron-hole binding is strong. However, a complete understanding of excitonic effects in 2D layered materials, i.e., when the electronic system transitions from a 2D geometry to a 3D one, is still missing. Here, the authors present a first-principles based multiscale method that attempts to fill this gap. With the help of their framework, one can predict the optoelectronics properties of van der Waals heterostructures and make a closer connection between the available theoretical models and experimental measurements in these materials.

The optoelectronic properties of van der Waals heterostructures (vdWHs) made of two-dimensional materials depend on charge and energy transfer across their atomically thin layers. However, these competing processes remain poorly understood. A new experimental study of a model vdWH reveals details of these dynamics that will be essential for future designs of devices based on vdWHs.

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• The maximum responsivity and detectivity of the optimized WSe 2 /PdSe 2 photodetector are 42.47 A/W and 1.09 × 10 12 Jones. • Photoelectric conversion mechanism of this WSe 2 /PdSe 2 heterojunction have been discussed using energy band diagrams. • The high carrier mobility of PdSe 2 provides efficient transport pathways for charge carriers, thereby significantly enhancing the photodetection performance of the heterojunction. • This study demonstrates the potential of van der Waals heterojunctions (vdWH) as a platform for advancing high-sensitivity photodetection and next-generation optoelectronic technologies. Two-dimensional (2D) materials exhibit significant potential for photodetection applications due to their superior light absorption properties and outstanding carrier mobility. The construction of heterojunctions by stacking distinct 2D materials enables optimization of interfacial charge transfer and broadband light absorption, thereby substantially enhancing the responsivity and detectivity of detectors. In this work, a high-performance WSe 2 /PdSe 2 heterojunction photodetector was fabricated using chemical vapor deposition and micro-nano processing techniques. This device demonstrates a remarkable photoresponsivity of 42.47 A/W and a specific detectivity of 1.09 × 10 12 Jones under 532 nm laser irradiation. The built-in electric field at the heterojunction interface facilitates effective charge separation and transfer, resulting in improvements in detectivity and responsivity by one and two orders of magnitude, respectively, compared to devices based on individual WSe 2 . These results underscore the pivotal potential of heterojunction engineering in advancing high-performance optoelectronic devices.

Abstract New 2D materials with low‐symmetry structures aroused great interest in developing monolithic polarization‐sensitive photodetectors with small volumes, which can provide a new degree of freedom for more information in night, fog, and smoke environments. However, at least half of them presented a small anisotropy with an anisotropic factor (≈2) of photocurrent up to now. Herein, after systematic investigation of the optical anisotropies of GeSe nanosheets, a novel self‐driven polarization‐sensitive imaging photodetector with excellent performance based on a Top‐MoSe 2 /GeSe/Bottom‐MoSe 2 (T‐MoSe 2 /GeSe/B‐MoSe 2 ) van der Waals dual‐heterojunction is proposed. Benefitting from the effective separation and shortening transmission distance of photocarriers by fully depleted Van der Waals dual‐heterojunction on both sides of in‐plane anisotropy of GeSe, the anisotropic photocurrent ratio ( I max / I min ) of T‐MoSe 2 /GeSe/B‐MoSe 2 photodetector can reach as high as 12.5 (635 nm, 0 V). This value is 3.5‐fold higher than that of MoSe 2 /GeSe photodetector, and 7‐fold higher than that of the pristine GeSe photodetector in this work. The responsivity of the T‐MoSe 2 /GeSe/B‐MoSe 2 photodetector (206 mA W −1 , 0 V) is 5 times higher than that of the MoSe 2 /GeSe photodetector. In addition, the T‐MoSe 2 /GeSe/B‐MoSe 2 photodetector exhibited a high light on/off ratio of 4 × 10 4 at 0 V. This work provides novel insights for developing high‐performance polarization‐sensitive imaging photodetectors.

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Novel 2D materials with low-symmetry structures exhibit great potential applications in developing monolithic polarization-sensitive photodetectors with small volume. However, owing to the fact that at least half of them presented a small anisotropic factor of ≈2, comprehensive performance of present polarization-sensitive photodetectors based on 2D materials is still lower than the practical application requirements. Herein, a self-driven photodetector with high polarization sensitivity using a broken-gap ReSe₂/SnSe₂ van der Waals heterojunction (vdWH) is demonstrated. Anisotropic ratio of the photocurrent (I_max/I_min) could reach 12.26 (635 nm, 179 mW cm^-2). Furthermore, after a facile combination of the ReSe₂/SnSe₂ device with multilayer graphene (MLG), I_max/I_min of the MLG/ReSe₂/SnSe₂ can be further increased up to13.27, which is 4 times more than that of pristine ReSe₂ photodetector (3.1) and other 2D material photodetectors even at a bias voltage. Additionally, benefitting from the synergistic effect of unilateral depletion and photoinduced tunneling mechanism, the MLG/ReSe₂/SnSe₂ device exhibits a fast response speed (752/928 µs) and an ultrahigh light on/off ratio (10⁵). More importantly, MLG/ReSe₂/SnSe₂ device exhibits excellent potential applications in polarized imaging and polarization-coded optical communication with quaternary logic state without any power supply. This work provides a novel feasible avenue for constructing next-generation smart polarization-sensitive photodetector with low energy consumption.

Mix-dimensional van der Waals heterostructures (vdWHs) have inspired worldwide interests and efforts in the field of advanced electronics and optoelectronics. The fundamental understanding of interfacial charge transfer is of vital importance for guiding the design of functional optoelectronic applications. In this work, type-II 0D-2D CdSe/ZnS quantum dots/MoS₂ vdWHs are designed to study the light-triggered interfacial charge behaviors and enhanced optoelectronic performances. From spectral measurements in both steady and transient states, the phenomena of suppressed photoluminescence (PL) emissions, shifted Raman signals and changed PL lifetimes provide strong evidences of efficient charge transfer at the 0D-2D interface. A series of spectral evolutions of heterostructures with various QDs overlapping concentrations at different laser powers are analyzed in details, which clarifies the dynamic competition between exciton and trion during an efficient doping of 3.9×10¹³ cm⁻². The enhanced photoresponses (1.57×10⁴ A·W^-1) and detectivities (2.86×10¹¹ Jones) in 0D/2D phototransistors further demonstrate that the light-induced charge transfer is still a feasible way to optimize the performance of optoelectronic devices. These results are expected to inspire the basic understanding of interfacial physics at 0D/2D interfaces, and shed the light on promoting the development of mixed-dimensional optoelectronic devices in the near future.

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Abstract The combination of the exciting properties of graphene with those of monolayer tungsten disulfide (WS 2 ) makes this heterostack of great interest for electronic, optoelectronic and spintronic applications. The scalable synthesis of graphene/WS 2 heterostructures on technologically attractive substrates like SiO 2 would greatly facilitate the implementation of novel two-dimensional (2D) devices. In this work, we report the direct growth of monolayer WS 2 via chemical vapor deposition (CVD) on single-crystal graphene arrays on SiO 2 . Remarkably, spectroscopic and microscopic characterization reveals that WS 2 grows only on top of the graphene crystals so that the vertical heterostack is selectively obtained in a bottom-up fashion. Spectroscopic characterization indicates that, after WS 2 synthesis, graphene undergoes compressive strain and hole doping. Tailored experiments show that such hole doping is caused by the modification of the SiO 2 stoichiometry at the graphene/SiO 2 interface during the WS 2 growth. Electrical transport measurements reveal that the heterostructure behaves like an electron-blocking layer at large positive gate voltage, which makes it a suitable candidate for the development of unipolar optoelectronic components.

The tunneling heterojunctions made of two-dimensional (2D) materials have been explored to have many intriguing properties, such as ultrahigh rectification and on/off ratio, superior photoresponsivity, and improved photoresponse speed, showing great potential in achieving multifunctional and high-performance electronic and optoelectronic devices. Here, we report a systematic study of the tunneling heterojunctions consisting of 2D tellurium (Te) and Tin disulfide (SnS₂). The Te/SnS₂ heterojunctions possess type-II band alignment and can transfer to type-III one under reverse bias, showing a reverse rectification ratio of about 5000 and a current on/off ratio over 10⁴. The tunneling heterojunctions as photodetectors exhibit an ultrahigh photoresponsivity of 50.5 A W^-1 in the visible range, along with a dramatically enhanced photoresponse speed. Furthermore, due to the reasonable type-II band alignment and negligible band bending at the interface, Te/SnS₂ heterojunctions at zero bias exhibit excellent self-powered performance with a high responsivity of 2.21 A W^-1 and external quantum efficiency of 678%. The proposed heterostructure in this work provides a useful guideline for the rational design of a high-performance self-powered photodetector.

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Remote sensing technology, which conventionally employs spectrometers to capture hyperspectral images, allowing for the classification and unmixing based on the reflectance spectrum, has been extensively applied in diverse fields, including environmental monitoring, land resource management, and agriculture. However, miniaturization of remote sensing systems remains a challenge due to the complicated and dispersive optical components of spectrometers. Here, m-phase GaTe_0.5Se_0.5 with wide-spectral photoresponses (250-1064 nm) and stack it with WSe₂ are utilizes to construct a two-dimensional van der Waals heterojunction (2D-vdWH), enabling the design of a gate-tunable wide-spectral photodetector. By utilizing the multi-photoresponses under varying gate voltages, high accuracy recognition can be achieved aided by deep learning algorithms without the original hyperspectral reflectance data. The proof-of-concept device, featuring dozens of tunable gate voltages, achieves an average classification accuracy of 87.00% on 6 prevalent hyperspectral datasets, which is competitive with the accuracy of 250-1000 nm hyperspectral data (88.72%) and far superior to the accuracy of non-tunable photoresponse (71.17%). Artificially designed gate-tunable wide-spectral 2D-vdWHs GaTe_0.5Se_0.5/WSe₂-based photodetector present a promising pathway for the development of miniaturized and cost-effective remote sensing classification technology.

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We report on the fabrication of a vertical 2D/3D heterojunction diode between gallium selenide (GaSe) and silicon (Si), and describe its photoresponse properties. Kelvin probe force microscopy (KPFM) has been employed to investigate the surface potentials of the GaSe/Si heterostructure, leading to the evaluation of the value of the conduction band offset at the heterostructure interface. The current-voltage measurements on the heterojunction device display a diode-like nature. This diode-like nature is attributed to the type-II band alignment that exists at the p-n interface. The key parameters of a photodetector, such as photoresponsivity, detectivity, and external quantum efficiency, have been calculated for the fabricated device and compared with those of other similar devices. The photodetection measurements of the GaSe/Si heterojunction diode show excellent performance of the device, with high photoresponsivity, detectivity, and EQE values of ∼2.8 × 10³ A W^-1, 6.2 × 10¹² Jones, and 6011, respectively, at a biasing of -5 V. Even at zero biasing, a high photoresponsivity of 6 A W^-1 was obtained, making it a self-powered device. Therefore, the GaSe/Si self-driven heterojunction diode has promising potential in the field of efficient optoelectronic devices.

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Tellurene and TMDs show desirable type II band alignment for constructing highly-efficient heterojunction solar cells with strong charge separation and enhanced sunlight absorption.

Two-dimensional (2D) semiconductors can be promising active materials for solar cells due to their advantageous electrical and optical properties, in addition to their ability to form high-quality van der Waals (vdW) heterojunctions using a simple process. Furthermore, the atomically thin nature of these 2D materials allows them to form lightweight and transparent thin-film solar cells. However, strategies appropriate for optimizing their properties have not been extensively studied yet. In this paper, we propose a method for reducing the electrical loss of 2D vdW solar cells by introducing hexagonal boron nitride (h-BN) as a surface passivation layer. This method allowed us to enhance the photovoltaic performance of a MoS₂/WSe₂ solar cell. In particular, we observed ∼74% improvement of the power conversion efficiency owing to a large increase in both short-circuit current and open-circuit voltage. Such a remarkable performance enhancement was due to the reduction of the recombination rate at the junction and surface of nonoverlapped semiconductor regions, which was confirmed via a time-resolved photoluminescence analysis. Furthermore, the h-BN top layer was found to improve the long-term stability of the tested 2D solar cell under ambient conditions. We observed the evolution of our MoS₂/WSe₂ solar cell for a month and found that h-BN passivation effectively suppressed its degradation speed. In particular, the degradation speed of the passivated cell was twice as low as that of a nonpassivated cell. This work reveals that h-BN can successfully suppress the electrical loss and degradation of 2D vdW heterojunction solar cells under ambient conditions.

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Multicomponent quasi-two-dimensional perovskites (Q-2DPs) have efficient luminescence and improved stability, which are highly desirable for light-emitting diode and perovskite solar cell (PSC). However, the lack of radiative recombination at room temperature is still not well understood and the performance of PSC is not good enough as well. The open-circuit voltage ( V_OC) is even lower than that of three-dimensional (3D) PSC with a narrower band gap. In this work, we study the energy transfer of excitons between their multiple components by time-resolved photoluminescence and find that charge transfer from high-energy states to low-energy state is gradually suppressed during elevating temperature owing to trap-mediated recombination. This may reveal the bottleneck of luminescence at room temperature in Q-2DPs, leading to large photovoltage loss in 2D PSC. Therefore, we develop a p-i-n bulk heterojunction (BHJ) structure to reduce the nonradiative recombination and obtain high V_OC of 1.18 V for (PMA)₂MA₄Pb₅I₁₅Cl (33.3% PMA) BHJ device, much higher than that of the planar counterparts. The enhanced efficiency is attributed to the improved exciton dissociation via BHJ interface. Our results provide an important step toward high V_OC and stable 2D PSCs, which could be used for tandem solar cell and colorful photovoltaic windows.

The performance of interdigitated back contact silicon heterojunction solar cells having overlapped p/i and n/i a-Si:H layers on the back has been investigated by two-dimensional simulation in comparison with the conventional cell structure having a gap between p/i and n/i layers. The results show that narrower overlap width leads to higher short circuit current and conversion efficiency, especially for poor heterojunction interface and thinner silicon substrate of the cells in addition to narrower uncovered width of p/i layer by a metal electrode. This is similar to the gap width dependence in the conventional cells, since both overlap and gap act as dead area for diffused excess carriers in the back contacts.

D–A conjugated polymers based on accessible 2D conjugated (<italic>E</italic>)-1,2-bis(5-alkyl-[2,3′-bithiophen]-2′-yl)ethene units possess low bandgaps, shorter π–π stacking distances, higher mobility and higher photovoltaic performance.

Two-dimensional numerical simulations for interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have been studied with the software package Sentaurus Device. Distribution of trap states and thermionic emission were considered for amorphous-silicon material and amorphous-silicon/crystalline-silicon hetero-interface, respectively. The 2D distribution and current-voltage curve were generated. It was found that the performance of IBC-SHJ solar cell depends on the front surface recombination velocity and the dimensions of P, N contacts, which is also shown in simulated LBIC line scan. The simulations show that after optimization, an efficiency of 22% can be achieved for IBC-SHJ solar cells.

We present a first-principles investigation of the optoelectronic properties of vertically stacked bilayer heterostructures composed of 2D transition-metal dichalcogenides (TMDs). The calculations are performed with density-functional theory as well as many-body perturbation theory within the ${G}_{0}{W}_{0}$--Bethe-Salpeter-equation method. Our aim is to propose these TMD heterostructures for potential applications in solar cells. The TMD monolayers constituting the heterojunctions considered in this research are ${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{WS}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$ monolayers due to their favorable band gaps, high carrier mobility, robust absorption in the visible region, and excellent stability. These four TMD monolayers provide the basis for a total of six potential heterostructures (${\mathrm{WS}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{WS}}_{2}$, ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{Se}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{WS}}_{2}$) whose structural, electronic, and optical properties have been studied in this work. At the density-functional-theory level, all six TMD heterostructures considered meet the essential criterion of type-II band alignment, a critical factor in extending carrier lifetime. However, according to ${G}_{0}{W}_{0}$ results, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{WS}}_{2}$ does not exhibit type-II band alignment; instead it shows type-I band alignment. The significantly large quasiparticle band gaps obtained from the ${G}_{0}{W}_{0}$ approximation suggest the presence of strong electron-correlation effects. The heterostructures studied exhibit superior optoelectronic properties compared with their respective isolated monolayers. Quite-significant values of the intrinsic electric fields that arise due to the asymmetric geometry of the heterostructures are obtained. Additionally, the small and nearly equal electron and hole effective masses obtained indicate high mobility and efficient charge-carrier separation, resulting in low recombination losses. The quality of these heterojunction solar cells is estimated by computing their power-conversion efficiencies (PCEs). The PCEs are calculated at both the HSE06 level and the ${G}_{0}{W}_{0}$ level, and the maximum PCE predicted by HSE06 calculations on our designed solar cells is 19.25% for the ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{WS}}_{2}$ heterojunction. In addition, all six TMD heterostructures are examined for their potential applications in photocatalysis for the hydrogen-evolution reaction, and three of them---namely, ${\mathrm{WS}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$, and ${\mathrm{WS}\mathrm{e}}_{2}$/${\mathrm{Mo}\mathrm{S}}_{2}$ heterostructures---qualify as photocatalysts.

Recent developments in organic solar cells show interesting power conversion efficiencies. However, with the use of organic semiconductors and bulk heterojunction cells, many new concepts have to be introduced to understand their characteristics. Only few models investigate these new concepts, and most of them are one-dimensional only. In this work, we present a two-dimensional model based on solving the drift-diffusion equations. The model describes the generation of excitons in the donor phase of the active layer and their diffusion towards an interface between the two separate acceptor and donor domains. Then, when the exciton reaches the interface, it forms a charge transfer state which can split into free charges due to the internal potential. Finally, these free charges are transported toward the electrodes within their respective domains (electrons in acceptor domain, holes in donor domain) before being extracted. In this model, we can follow the distribution of each species and link it to the physical processes taken into account. Using the finite element method to solve the equations of the model, we simulate the effect of the bulk heterojunction morphology on photocurrent curves. We concentrate on the morphology parameters such as the mean acceptor/donor domain sizes and the roughness of,the interface between the donor and acceptor domains. Results are discussed in relation with experimental observations.

Vertical heterojunctions of two two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention recently. A variety of heterojunctions can be constructed by stacking different TMDs to form fundamental building blocks in different optoelectronic devices such as photodetectors, solar cells, and light-emitting diodes. However, these applications are significantly hampered by the challenges of large-scale production of van der Waals stacks of atomically thin materials. Here, we demonstrate scalable production of periodic patterns of few-layer WS2, MoS2, and their vertical heterojunction arrays by a thermal reduction sulfurization process. In this method, a two-step chemical vapor deposition approach was developed to effectively prevent the phase mixing of TMDs in an unpredicted manner, thus affording a well-defined interface between WS2 and MoS2 in the vertical dimension. As a result, large-scale, periodic arrays of few-layer WS2, MoS2, and their vertical heterojunctions can be produced with desired size and density. Photodetectors based on the as-produced MoS2/WS2 vertical heterojunction arrays were fabricated, and a high photoresponsivity of 2.3 A·W(-1) at an excitation wavelength of 450 nm was demonstrated. Flexible photodetector devices using MoS2/WS2 heterojunction arrays were also demonstrated with reasonable signal/noise ratio. The approach in this work is also applicable to other TMD materials and can open up the possibilities of producing a variety of vertical van der Waals heterojunctions in a large scale toward optoelectronic applications.

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Photovoltaic cells based on the two-dimensional quinoxaline-based polymers,<bold>P2</bold>and<bold>P3</bold>, exhibited PCEs of over 3% compared to the PCE of less than 2% in the<bold>P1</bold>-based device.

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Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our p-n diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

Three donor/acceptor (D/A)-type two-dimensional polythiophenes (PTs; PBTFA13, PBTFA12, PBTFA11) featuring difluorobenzothiadiazole (DFBT) derivatives as the conjugated (acceptor) units in the polymer backbone and tertbutyl-substituted triphenylamine (tTPA)-containing moieties as (donor) pendants have been synthesized and characterized. These PTs exhibited good thermal stabilities, broad absorption spectra, and narrow optical band gaps. The cutoff wavelength of the UV-Vis absorption band was red-shifted upon increasing the content of the DFBT units in the PTs. Bulk heterojunction solar cells having an active layer comprising blends of the PTs and fullerene derivatives Incorporating a suitable content of the DFBT derivative in the polymer backbone enhanced the solar absorption ability and conjugation length of the PTs. The photovoltaic properties of the PBTFA13-based solar cells were superior to those of the PBTFA11-and PBTFA12-based solar cells.

We propose to use edge-modified phosphorene nanoflakes (PNFs) as donor and acceptor materials for heterojunction solar cells. By using density functional theory based calculations, we show that heterojunctions consisting of hydrogen- and fluorine-passivated PNFs have a number of desired optoelectronic properties that are suitable for use in a solar cell. We explain why these properties hold for these types of heterojunctions. Our calculations also predict that the maximum energy conversion efficiency of these type of heterojunctions, which can be easily fabricated, can be as high as 20%, making them extremely competitive with other types of two-dimensional heterojunctions.

Two-dimensional phosphorene with desirable optoelectronic properties (ideal band gap, high carrier mobility, and strong visible light absorption) is a promising metal-free photocatalyst for water splitting. However, the band edge positions of the valence band maximum (VBM) and conduction band maximum (CBM) of phosphorene are higher than the redox potentials in photocatalytic water splitting reactions. Thus, phosphorene can only be used as the photocathode for hydrogen evolution reaction as a low-efficiency visible-light-driven photocatalyst for hydrogen production in solar water splitting cells. Here, we propose a new mechanism to improve the photocatalytic efficiency of phosphorene nanoribbons (PNRs) by modifying their edges for full reactions in photocatalytic water splitting. By employing first-principles density functional theory calculations, we find that pseudohalogen (CN and OCN) passivated PNRs not only show desired VBM and CBM band edge positions induced by edge electric dipole layer, but also possess intrinsic optoelectronic properties of phosphorene, for both water oxidation and hydrogen reduction in photocatalytic water splitting without using extra energy. Furthermore, our calculations also predict that the maximum energy conversion efficiency of heterojunction solar cells consisting of different edge-modified PNRs can be as high as 20% for photocatalytic water splitting.

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Perovskite solar cells based on two-dimensional/three-dimensional (2D/3D) hierarchical structure have attracted significant attention in recent years due to their promising photovoltaic performance and stability. However, obtaining a detailed understanding of interfacial mechanism at the 2D/3D heterojunction, for example, the ligand-chemistry-dependent nature of the 2D/3D heterojunction and its influence on charge collection and the final photovoltaic outcome, is not yet fully developed. Here we demonstrate the underlying 3D phase templates growth of quantum wells (QWs) within a 2D capping layer, which is further influenced by the fluorination of spacers and compositional engineering in terms of thickness distribution and orientation. Better QW alignment and faster dynamics of charge transfer at the 2D/3D heterojunction result in higher charge mobility and lower charge recombination loss, largely explaining the significant improvements in charge collection and open-circuit voltage (<i>V</i>_OC) in complete solar cells. As a result, 2D/3D solar cells with a power-conversion efficiency of 21.15% were achieved, significantly higher than the 3D counterpart (19.02%). This work provides key missing information on how interfacial engineering influences the desirable electronic properties of the 2D/3D hierarchical films and device performance via ligand chemistry and compositional engineering in the QW layer.

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The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

The recently emerged organohalide perovskites (e.g., CH3NH3PbI3) have drawn intense attention for high efficiency solar cells. However, with a considerable solubility in many solvents, these perovskites are not typically compatible with conventional lithography processes for more complicated device fabrications that are important for both fundamental studies and technological applications. Here, we report the creation of novel heterojunction devices based on perovskites and two-dimensional (2D) crystals by taking advantage of the layered characteristic of lead iodide (PbI2) and vapor-phase intercalation. We show that a graphene/perovskite/graphene vertical stack can deliver a highest photoresponsivity of ∼950 A/W and photoconductive gain of ∼2200, and a graphene/WSe2/perovskite/graphene heterojunction can display a high on/off ratio (∼10(6)) transistor behavior with distinct gate-tunable diode characteristics and open-circuit voltages. Such unique perovskite-2D heterostructures have significant potential for future optoelectronic research and can enable broad possibilities with compositional tunability of organohalide perovskites and the versatility offered by diverse 2D materials.

We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation.

A new series of electron-deficient molecules based on a central benzothiadiazole moiety flanked with vinylimides has been synthesized via Heck chemistry and used in solution-processed organic photovoltaics (OPV). Two new compounds, 4,7-bis(4-(N-hexyl-phthalimide)vinyl)benzo[c]1,2,5-thiadiazole (PI-BT) and 4,7-bis(4-(N-hexyl-naphthalimide)vinyl)benzo[c]1,2,5-thiadiazole (NI-BT), show significantly different behaviors in bulk heterojunction (BHJ) solar cells using poly(3-hexylthiophene) (P3HT) as the electron donor. Two-dimensional grazing incidence X-ray scattering (2D GIXS) experiments demonstrate that PI-BT shows significant crystallization in spin-coated thin films, whereas NI-BT does not. Density functional theory (DFT) calculations predict that while PI-BT maintains a planar structure in the ground state, steric interactions cause a twist in the NI-BT molecule, likely preventing significant crystallization. In BHJ solar cells with P3HT as donor, PI-BT devices achieved a large open-circuit voltage of 0.96 V and a maximum device power-conversion efficiency of 2.54%, whereas NI-BT containing devices only achieved 0.1% power-conversion efficiency.

Studies of field-effect control of the high mobility electrons in MBE-grown selectively doped GaAs/n-Al x Ga 1- x As heterojunctions are described. Successful fabrication of a new field-effect transistor, called a high electron mobility transistor (HEMT), with extremely high-speed microwave capabilities is reported.

We report on a GaN metal-oxide-semiconductor high-electron-mobility-transistor (MOS-HEMT) using atomic-layer-deposited (ALD) Al2O3 as the gate dielectric. Compared to a conventional GaN high-electron-mobility-transistor (HEMT) of similar design, the MOS-HEMT exhibits several orders of magnitude lower gate leakage and several times higher breakdown voltage and channel current. This implies that the ALD Al2O3∕AlGaN interface is of high quality and the ALD Al2O3∕AlGaN∕GaN MOS-HEMT is of high potential for high-power rf applications. In addition, the high-quality ALD Al2O3 gate dielectric allows the effective two-dimensional (2D) electron mobility at the AlGaN∕GaN heterojunction to be measured under a high transverse field. The resulting effective 2D electron mobility is much higher than that typical of Si, GaAs or InGaAs metal-oxide-semiconductor field-effect-transistors (MOSFETs).

In this paper, we report state-of-the-art high frequency performance of GaN-based high electron mobility transistors (HEMTs) and Schottky diodes achieved through innovative device scaling technologies such as vertically scaled enhancement and depletion mode (E/D mode) AlN/GaN/AlGaN double-heterojunction HEMT epitaxial structures, a low-resistance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$n^{+}$</tex></formula> -GaN/2DEG ohmic contact regrown by MBE, a manufacturable 20-nm symmetric and asymmetric self-aligned-gate process, and a lateral metal/2DEG Schottky contact. As a result of proportional scaling of intrinsic and parasitic delays, an ultrahigh <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> exceeding 450 GHz (with a simultaneous <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{\rm max}$</tex></formula> of 440 GHz) and a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{\rm max}$</tex></formula> close to 600 GHz (with a simultaneous <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{T}$</tex></formula> of 310 GHz) are obtained in deeply scaled GaN HEMTs while maintaining superior Johnson figure of merit. Because of their extremely low on-resistance and high gain at low drain voltages, the devices exhibited excellent noise performance at low power. 501-stage direct-coupled field-effect transistor logic ring oscillator circuits are successfully fabricated with high yield and high uniformity, demonstrating the feasibility of GaN-based E/D-mode integrated circuits with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${&gt;}{1000}$</tex> </formula> transistors. Furthermore, self-aligned GaN Schottky diodes with a lateral metal/2DEG Schottky contact and a 2DEG/ <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$n^{+}$</tex></formula> -GaN ohmic contact exhibited <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$RC$</tex></formula> -limited cutoff frequencies of up to 2.0 THz.

(Al)GaN recess-free normally OFF technology is developed for fabrication of high-yield lateral GaN-based power devices. The recess-free process is achieved by an ultrathin-barrier (UTB) AlGaN/GaN heterostructure that features a natural pinched-off 2-D electron gas channel. The top-down manufacturing technique overcomes the challenges in etching of AlGaN barrier with well-controlled depth and uniformity, which is especially attractive for fabrication of normally OFF GaN-based high-electron-mobility transistors (HEMTs) and metal-insulator-semiconductor HEMTs (MIS-HEMTs) on large-size Si substrate. With SiNx passivation grown by low-pressure chemical-vapor deposition, on-resistance of the UTB-AlGaN/GaN-based power devices can be significantly reduced. High-uniformity low-hysteresis normally OFF HEMTs and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /AlGaN/GaN MIS-HEMTs are successfully demonstrated on the UTB AlGaN/GaN-on-Si platform. It is also a compelling technology platform for manufacturing high-performance GaN-based lateral power diodes, and normally OFF p-(Al)GaN heterojunction field-effect transistors.

Enhancement-mode high electron mobility transistors (E-HEMTs) with selectively doped GaAs/n-Al x Ga 1- x As heterojunction structures grown by molecular beam epitaxy are described. E-HEMTs with 2- µm long gates have exhibited the square-law drain characteristic. The device has a g m of 409 mSmm -1 of gate width at 77 K and 193 mSmm -1 at 300 K. This value of g m at 77 K is the highest in all field effect devices reported thus far.

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Selectively Si-doped GaAs/ N -AlGaAs heterojunction structures have been grown by MBE. Two-dimensional electron gas accumulating at the interface of the heterojunction showed mobilities as high as 69,000 cm 2 /Vs at 77 K and 100,000 cm 2 /Vs at 4.2 K, with a sheet electron concentration of 5.5×10 11 cm -2 , which are higher than any reported so far. An enhancementmode high electron mobility transistor (E-HEMT), which was first fabricated from the heterojunction material, showed a field effect mobility of 49,300 cm 2 /Vs at 77 K, suggesting that this heterojunction material has potential for application to low-power, high-speed integrated circuits.

A GaN/ultrathin AlN∕GaN heterojunction has been used to introduce a GaN spacer between the GaN channel and the AlGaN barrier in AlGaN∕GaN high electron mobility transistors (HEMTs). In conventional AlGaN∕GaN devices, the alloy scattering of the electrons with the AlGaN barrier degrades the electron velocity at high electric fields. This effect is significantly reduced in GaN-spacer transistors, which therefore have much better high field transport properties. While the dc performance of these transistors is similar to conventional AlGaN∕GaN HEMTs, a 20% increase in the electron velocity has been measured by two different techniques.

A discussion is presented of fabrication technologies for manufacturing GaAs devices. Advantages and drawbacks of heterojunction devices are outlined. Areas of concern in GaAs production lines are also examined. The discussion covers the depletion-mode metal-semiconductor field-effect transistor (D-MESFET), the enhancement-mode MESFET (E-MESFET), and the high-electron-mobility transistor (HEMT).

A low on-resistance normally-off GaN double-channel metal–oxide–semiconductor high-electron-mobility transistor (DC-MOS-HEMT) is proposed and demonstrated in this letter, which features a 1.5-nm AlN insertion layer (ISL) located 6 nm below the conventional barrier/GaN interface, forming a second channel at the interface between the AlN-ISL and the underlying GaN. With gate recess terminated at the upper channel, normally-off operation was obtained with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {th}}$ </tex-math></inline-formula> of +0.5 V at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm {DS}}= 10 ~\mu \text{A}$ </tex-math></inline-formula> /mm or +1.4 V from the linear extrapolation of the transfer curve. The lower heterojunction channel is separated from the etched surface in the gate region, thereby maintaining its high field-effect mobility with a peak value of 1801 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}\cdot \text{s}$ </tex-math></inline-formula> ). The on-resistance is as small as 6.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega \cdot \text {mm}$ </tex-math></inline-formula> for a DC-MOS-HEMT with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{G}/L_{\mathrm {GS}}/L_{\mathrm {GD}} = 1.5/2/15~\mu \text{m}$ </tex-math></inline-formula> , and the maximum drain current is 836 mA/mm. A high breakdown voltage (>700 V) and a steep subthreshold swing of 72 mV/decade are also obtained.

Models are developed for the dc I-V curves and microwave small-signal parameters of the Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As/GaAs heterojunction field-effect transistor, called the high electron mobility transistor (HEMT). An analytic velocity versus field model is used, along with the exact variation with density of the GaAs two-dimensional electron gas Fermi level. A numerical integration is used to obtain the drain voltage for a given gate voltage and source-drain current. The resulting I-V curves are in excellent agreement with the experimental data from four different groups. This model is also used to calculate the transconductance and gate capacitance, and a model is developed for the source resistance. These are used to calculate <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{\max}</tex> , the maximum frequency of oscillation, for a range of values of gate length, of AlGaAs alloy composition and doping, and of the thickness of the undoped A1GaAs spacer layer. The results are compared with measured data for HEMT's as well as for a similar GaAs FET with 0.35-µm gate length.

We report enhancement-mode p-channel heterojunction field-effect transistors (HFETs) without gate recess on a standard p-GaN/AlGaN/GaN high electron mobility transistor (HEMT) platform. The p-GaN in the gate region was partially passivated by a low-power hydrogen plasma treatment process, and the remaining active p-GaN and the underlying AlGaN formed the p-channel. The device showed a record low off-state leakage of <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> A/mm and subthreshold swing (SS) of 123.0 mV/dec with a threshold voltage ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> th</sub> ) of -0.6 V and high-temperature stability up to 200 °C. These results indicate that the hydrogen plasma treatment is beneficial for suppressing leakages and preserving excellent material quality in the p-channel. With the success of the p-HFETs, the p-GaN/AlGaN/GaN platform can enable the monolithic integration of GaN n- and p-channel transistors without the need for regrowth. This work represents a significant step towards the implementation of the GaN CMOS technology.

We have integrated chemical vapor-deposited graphene and GaAs/AlxGa1−xAs heterostructure into a hybrid field effect transistor (FET). Depending on the operation scheme, graphene can be utilized either as a gate electrode for a GaAs-based high electron mobility transistor (HEMT) or as a channel material gated by two dimensional electron gas (2DEG) formed in the interface of a heterojunction. Our studies reveal that 2DEG can function as an effective back-electrode to tune the ambipolar effect of graphene. The performance of graphene FET (GFET) is limited by the interface band bending of the heterojunction associated with the gating voltages and the intrinsic surface morphology of GaAs substrate. Our results bode a way to implement HEMT/GEFT-based bi-FET integrated circuits.

The impacts of static and dynamic gate stress on the threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> ) instability in Schottky-type <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula> -GaN gate AlGaN/GaN heterojunction field-effect transistors are experimentally investigated. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> shifts negatively under large positive bias static stress ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {G}}\_ {\text {stress}} &gt; 5$ </tex-math></inline-formula> V) by adopting conventional quasi-static characterization techniques. In contrast, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> under fast-dynamic-stress exhibits positive shift, and a positive frequency dependence occurs within a wide range of frequency from 10 Hz to 1 MHz. The different <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {TH}}$ </tex-math></inline-formula> instability behavior under static and dynamic stress mainly originates from the time-dependent charges (electrons and holes) storage/release mechanisms in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula> -GaN layer, which is floating in the Schottky-type <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${p}$ </tex-math></inline-formula> -GaN gate HEMT.

In order to evaluate the feasibility of newly developed GaN devices in a cryogenic-cooled converter, this paper characterizes a 650 V enhancement-mode Gallium-Nitride heterojunction field-effect transistor (GaN HFET) at cryogenic temperatures. The characterization includes two parts: static and dynamic characterization. The results show that this GaN HEMT is an excellent device candidate to be applied in cryogenic-cooled applications. For example, transconductance at cryogenic temperature is 2.5 times of one at room temperature, and accordingly, peak di/dt during turn-on transients at cryogenic temperature is around 2 times of that at room temperature. Moreover, the on-resistance of the channel at cryogenic temperature is only one-fifth of that at room temperature.

We propose an AlN/GaN/InGaN/GaN double-heterojunction high electron mobility transistor (DH-HEMT) structure with a 4 nm thin AlN barrier layer. The performance of the DH-HEMT device is investigated by using two-dimensional numerical simulation. The conduction band profile is obtained by using the Poisson’s equation and Fermi–Dirac statistics in combination with the polarization charges. Due to large conduction-band offset of the AlN/GaN interface and strong polarization of AlN, the minor channel at GaN/InGaN interface can be eliminated. Further, the hot electron and self-heating effects on the transport properties of this DH-HEMT are investigated by using hydrodynamic model. In comparison with the AlGaN barrier DH-HEMT and conventional HEMT, this kind of DH-HEMT can effectively reduce the hot electron effect under high voltage. The reason is that the maximum field strength is far below the critical value for the existence of the hot electron effect in the AlGaN barrier DH-HEMTs and conventional HEMTs with the same voltage 6 V. The simulation results also show that the ultrathin AlN barrier layer can significantly reduce thermal impedance, and then lower the self-heating effect. Furthermore, the passivation layer has significant role in the self-heating effect of the ultrathin barrier DH-HEMTs.

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Physics-based 3D simulations were conducted on a GaN high-electron-mobility transistor (HEMT) and a super-heterojunction field-effect transistor (SHJFET) to investigate the single event effect mechanism under heavy ion irradiation. Most of the single event transient current in HEMT was attributed to the punch-through effect in the bulk caused by the local increase in electrostatic potential. With improved E-field management and a more favorable potential profile to suppress source electron injection, the SHJFET had a 70% lower transient current peak value compared to the HEMT.

Current status and recent advances in high electron mobility transistor (HEMT) technology for high performance very large scale integration (VLSI) are presented with the focus on material, self-alignment device fabrication, and HEMT large-scale integration (LSI) implementations. HEMT is a very promising device for ultrahigh speed LSI/VLSI due to the supermobility GaAs/AlGaAs heterojunction structure. The technological challenges for large scale integrations are discussed with refined HEMT with self-aligned gate structure, controllability of device parameters, and molecular beam epitaxy material problems. Master–slave flip–flop, divide-by-two circuits achieved the internal logic delay of 22 ps per gate at 77 K at a fan-out of about 2, roughly three times faster than that of GaAs metal-semiconductor field-effect transistor technology. HEMT has already made it possible to develop 16 kb static random access memory (RAM) and a 1.5-kgate gate array, demonstrating high speed LSI operations. With submicron gates, as well as advanced material technologies, a HEMT 64 kb static RAM with subnanosecond access operation and 10 kgate logic LSI with sub-100-ps logic delays, will be achieved.

Inverted pseudomorphic high electron mobility transistor (HEMT) and inverted HEMT heterostructures are demonstrated by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) for the first time and characterized by transmission electron microscopy (TEM), variable temperature Hall effect, and Shubnikov–de Haas measurements. TEM micrographs of both structures show distinct and sharp heterojunction interfaces without indications of interface roughness at the AlGaAs/channel layer interface. Variable temperature Hall effect measurements reveal a monotonic increase in mobility as the temperature is lowered. For the inverted HEMT, the mobility at 15 K is 90 000 cm2/V s with a sheet density of 8.2×1011 cm−2. The mobility of the inverted pseudomorphic HEMT at 15 K is 73 000 cm2/V s with a sheet density of 1.5×1012 cm−2. Shubnikov–de Haas measurements at 4.2 K in magnetic fields up to 18.5 T show clear magnetoresistance oscillations and plateaus in the quantum Hall effect confirming the existence of a two-dimensional electron gas. Fast Fourier power transform of the magnetoresistance versus magnetic field shows two subband levels with a total sheet density of 8.7×1011 cm−2 for the inverted HEMT and a total sheet density of 1.55×1012 cm−2 for the inverted pseudomorphic HEMT in close agreement to the variable temperature Hall effect measurement results.

We present a contactless method capable of characterizing a high electron mobility transistor (HEMT) heterostructure at the wafer stage, right after its growth, before any production process has been attempted, to provide the equilibrium band structure and the density of charge of the 2-D electron gas in the quantum well (QW). The method can thus evaluate critical transistor parameters and help to screen out low performance wafers before the actual fabrication. To this end, we use a simple optical spectroscopy at room temperature that measures the surface photovoltage band-edge responses in the heterostructure and uses a model that takes into account the effect of the built-in electric fields on optical absorption in the layers and heterojunctions to evaluate bandgaps, band offsets, and built-in fields. The QW charge is then calculated from the built-in fields. The main advantage of the method is in its capability to provide information on all the different layers in the typical heterostructure simultaneously in a single measurement. The method is not limited to the HEMT structure but may be used on any other heterostructure. It opens the door for a new type of characterization methods suitable for the post-silicon multi-layer multi-semiconductor heterostructure device era.

Abstract At present, dual‐channel or even multi‐channel recording is a developing trend in the field of photodetection, which is widely applied in environment protection, security, and space science and technology. This paper proposes a novel MoS 2 /InAlAs/InGaAs n–i–n heterojunction phototransistor by integrating multi‐layered MoS 2 with InGaAs‐based high electron mobility transistors (InGaAs‐HEMTs). Due to the internal photocurrent amplification in the InGaAs channels with a narrow energy bandgap of 0.79 eV, this device exhibits high photoresponsivity ( R ) of over 8 × 10 5 A W –1 under near‐infrared illumination of 1550 nm at 500 pW. Furthermore, with the combination of the photoconductance effect in the vertical MoS 2 /InAlAs/InGaAs n–i–n heterojunction and the photogating effect in the lateral phototransistor, this device possesses a unique characteristic under visible illumination that its photoresponsivity can be tuned by the top gate electrode from 6 × 10 5 A W –1 to ‐4 × 10 5 A W –1 by gate voltage. This may lead to a new application as an optically controlled electronic inverter, which needs further study in depth. This MoS 2 /InAlAs/InGaAs phototransistor builds up a new bridge between 2D materials and conventional ternary compounded semiconductor devices.

Over the last decade, gallium nitride (GaN) has emerged as an excellent material for the fabrication of power devices. Among the semiconductors for which power devices are already available in the market, GaN has the widest energy gap, the largest critical field, and the highest saturation velocity, thus representing an excellent material for the fabrication of high-speed/high-voltage components. The presence of spontaneous and piezoelectric polarization allows us to create a two-dimensional electron gas, with high mobility and large channel density, in the absence of any doping, thanks to the use of AlGaN/GaN heterostructures. This contributes to minimize resistive losses; at the same time, for GaN transistors, switching losses are very low, thanks to the small parasitic capacitances and switching charges. Device scaling and monolithic integration enable a high-frequency operation, with consequent advantages in terms of miniaturization. For high power/high-voltage operation, vertical device architectures are being proposed and investigated, and three-dimensional structures—fin-shaped, trench-structured, nanowire-based—are demonstrating great potential. Contrary to Si, GaN is a relatively young material: trapping and degradation processes must be understood and described in detail, with the aim of optimizing device stability and reliability. This Tutorial describes the physics, technology, and reliability of GaN-based power devices: in the first part of the article, starting from a discussion of the main properties of the material, the characteristics of lateral and vertical GaN transistors are discussed in detail to provide guidance in this complex and interesting field. The second part of the paper focuses on trapping and reliability aspects: the physical origin of traps in GaN and the main degradation mechanisms are discussed in detail. The wide set of referenced papers and the insight into the most relevant aspects gives the reader a comprehensive overview on the present and next-generation GaN electronics.

Digital gallium arsenide (GaAs) integrated circuits offer prospects for high-performance electronics, particularly for increased speed and radiation hardness. Prototype GaAs devices fabricated in technologies ranging from ion-implanted metal semiconductor field-effect transistors (MESFETs) and junction field-effect transistors (JFETs) to epitaxial heterostructures, such as high-electron-mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs), have demonstrated these advantages. While these GaAs technologies share many common fabrication features, the unique characteristics of each and GaAs materials present significant manufacturing challenges. It is argues that to produce real integrated circuits (ICs) for system applications, the disciplines and rigors of a production environment as well as the innovations of research and development are required.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

For the first time, a novel heterojunction bipolar transistor (HBT) active-feedback circuit is employed with a high electron mobility transistor (HEMT) low noise amplifier (LNA) which improves the linearity or third-order intercept point (IP3) and gain-bandwidth performance without significantly impacting noise figure. The HEMT and HBT circuits are monolithically integrated using selective molecular beam epitaxy (MBE). The use of HBT active feedback provides several advantages over field-effect transistor (FET) active feedback such as smaller size, lower dc power consumption, active self-bias, and direct-coupled performance. Applied to a 1-11 GHz HEMT LNA design, the HBT active feedback has resulted in a 50% improvement in gain-bandwidth performance and a 4-10 dB improvement in IP3 without degrading noise figure compared to an equivalent resistive-feedback design. In addition, the HBT active feedback consumes only 15% additional dc power and has provided as much as a 20-dB reduction in third-order (two-tone) intermodulation products (IM3s) over a narrow band. This HBT active-feedback linearization technique is a compact, cost-effective means of improving the linearity of HEMT-based LNA/receiver monolithic microwave/millimeter wave integrated circuits (MMICs) for use in wireless multicarrier communications systems requiring a wide dynamic range.

Abstract Vertical heterojunction field‐effect transistors (VHFETs) utilizing re‐grown AlGaN/GaN two‐dimensional electron gas (2DEG) channels on free‐standing GaN substrates have been developed. The VHFETs exhibited a specific on‐resistance (RonA) of 7.6 mΩcm 2 at a threshold voltage (V th ) of ‐1.1 V and a breakdown voltage (VB) of 672 V. The breakdown voltage and the figure of merit (VB 2 /RonA) are the highest among those of the GaN‐based vertical transistors ever reported. It was demonstrated that the threshold voltage can be controlled by the thickness of AlGaN layers and Al concentration. A normally‐off operation was achieved with a 10‐nm‐thick Al 0.2 Ga 0.8 N layer. The possibility that VHFET with smaller current collapse phenomena than planar HEMT was revealed (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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As the data rate of optical telecommunication systems accelerates beyond several gigabits per second, optoelectronic integrated circuits (OEICs) becomes indispensable. The integration of an optical device with an electronic circuit can improve the performance of a system owing to the reduction of parasitics, which are inevitable in hybrid circuits and adversely affect the performance, particularly at high frequencies. In the past several years gigabit-per-second operations of receiver OEICs have been achieved by using new structures and technologies such as GaAs on InP heteroepitaxy,1 pin-junction field effect transistor (JFET) integration,2 metal-semiconductor-metal (MSM) high-electron-mobility transistor (HEMT) integration,3 pin-heterojunction bipolar transistor (HBT) integration,4 and pin-HEMT integration.5 They have exhibited 1.2-2.4-Gb/s operations, and further improvement of the speed can be expected by refining circuit configurations and/or by improving device performance. Among the technologies mentioned above, HEMT integration is considered the most promising approach because of the great potential of AlInAs/GaInAs HEMTs. We have been developing receiver OEICs consisting of GaInAs pin photodiodes (PDs) and AlInAs/GaInAs HEMTs grown by organometallic vapor-phase epitaxy (OMVPE).5 We report here an ultra-high-speed performance of pin-HEMT receiver OEICs, in which a transimpedance circuit configuration is used.

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Designing heterojunction photocatalysts imitating natural photosynthetic systems has been a promising approach for photocatalytic hydrogen generation. However, in the traditional Z-Scheme artificial photosynthetic systems, the poor charge separation, and rapid recombination of photogenerated carriers remain a huge bottleneck. To rationally design S-Scheme (i.e., Step scheme) heterojunctions by avoiding the futile charge transport routes is therefore seen as an attractive approach to achieving high hydrogen evolution rates. Herein, a twin S-scheme heterojunction is proposed involving graphitic C₃ N₄ nanosheets self-assembled with hydrogen-doped rutile TiO₂ nanorods and anatase TiO₂ nanoparticles. This catalyst shows an excellent photocatalytic hydrogen evolution rate of 62.37 mmol g^-1 h^-1 and high apparent quantum efficiency of 45.9% at 365 nm. The significant enhancement of photocatalytic performance is attributed to the efficient charge separation and transfer induced by the unique twin S-scheme structure. The charge transfer route in the twin S-scheme is confirmed by in situ X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spin-trapping tests. Femtosecond transient absorption (fs-TA) spectroscopy, transient-state surface photovoltage (TPV), and other ex situ characterizations further corroborate the efficient charge transport across the catalyst interface. This work offers a new perspective on constructing artificial photosynthetic systems with S-scheme heterojunctions to enhance photocatalytic performance.

Abstract Gold nanoparticles were deposited on the surface of a g‐C 3 N 4 semiconductor by deposition–precipitation, photodeposition, and impregnation methods to make metal–semiconductor junctions for photocatalytic hydrogen evolution from aqueous solution containing an electron donor with visible light illumination. The samples were characterized by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), UV/Vis, and transmission electron microscopy (TEM). Results show that the Au/g‐C 3 N 4 prepared by the deposition–precipitation method possessed the best photocatalytic activity, due to the formation of tight Au–semiconductor heterojunctions effectively promoting the transfer of charge from light‐excited g‐C 3 N 4. Surface modification of the Au/g‐C 3 N 4 with a second metal further improved the activity of the photocatalytic system, which was explained by simultaneous optimization of electron transfer by the gold and chemical reactivity by the secondary metal.

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The construction of a p-n heterojunction is an efficient strategy to resolve the limited light absorption and serious charge-carrier recombination in semiconductors and enhance the photocatalytic activity. However, the promotion effect is greatly limited by poor interfacial charge transfer efficiency as well as reduced redox ability of charge carriers. In this work, we demonstrate that the embedding of metal Pd into the interface between n-type C3N4 and p-type Cu2O can further enhance the interfacial charge transfer and increase the redox ability of charge carriers through the design of the C3N4-Pd-Cu2O stack nanostructure. The embedded Pd nanocubes in the stack structure not only trap the charge carriers from the semiconductors in promoting the electron-hole separation but also act as a Z-scheme "bridge" in keeping the strong reduction/oxidation ability of the electrons/holes for surface reactions. Furthermore, Pd nanocubes also increase the bonding strength between the two semiconductors. Enabled by this unique design, the hydrogen evolution achieved is dramatically higher than that of its counterpart C3N4-Cu2O structure without Pd embedding. The apparent quantum efficiency (AQE) is 0.9% at 420 nm for the designed C3N4-Pd-Cu2O. This work highlights the rational interfacial design of heterojunctions for enhanced photocatalytic performance.

Abstract The aggravating extreme climate changes and natural disasters stimulate the exploration of low‐carbon/zero‐carbon alternatives to traditional carbon‐based fossil fuels. Solar‐to‐hydrogen (STH) transformation is considered as appealing route to convert renewable solar energy into carbon‐free hydrogen. Restricted by the low efficiency and high cost of noble metal cocatalysts, high‐performance and cost‐effective photocatalysts are required to realize the realistic STH transformation. Herein, the 2D FePS 3 (FPS) nanosheets anchored with TiO 2 nanoparticles (TiO 2 /FePS 3 ) are synthesized and tested for the photocatalytic hydrogen evolution reaction. With the integration of FPS, the photocatalytic H 2 ‐evolution rate on TiO 2 /FePS 3 is radically increased by ≈1686%, much faster than that of TiO 2 alone. The origin of the greatly raised activity is revealed by theoretical calculations and various advanced characterizations, such as transient‐state photoluminescence spectroscopy/surface photovoltage spectroscopy, in situ atomic force microscopy combined with Kelvin probe force microscopy (AFM‐KPFM), in situ X‐ray photoelectron spectroscopy (XPS), and synchrotron‐based X‐ray absorption near edge structure. Especially, the in situ AFM‐KPFM and in situ XPS together confirm the electron transport pathway in TiO 2 /FePS 3 with light illumination, unveiling the efficient separation/transfer of charge carrier in TiO 2 /FePS 3 step‐scheme heterojunction. This work sheds light on designing and fabricating novel 2D material‐based S‐scheme heterojunctions in photocatalysis.

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Photocatalytic hydrogen evolution from water has received enormous attention due to its ability to address a number of global environmental and energy-related issues. Here, we synthesize 2D/2D Ti3C2/g-C3N4 composites by electrostatic self-assembly technique and demonstrate their use as photocatalysts for hydrogen evolution under visible light irradiation. The optimized Ti3C2/g-C3N4 composite exhibited a 10 times higher photocatalytic hydrogen evolution performance (72.3 μmol h-1 gcat-1) than that of pristine g-C3N4 (7.1 μmol h-1 gcat-1). Such enhanced photocatalytic performance was due to the formation of 2D/2D heterojunctions in the Ti3C2/g-C3N4 composites. The intimate contact between the monolayer Ti3C2 and g-C3N4 nanosheets promotes the separation of photogenerated charge carriers at the Ti3C2/g-C3N4 interface. Furthermore, the ultrahigh conductivity of Ti3C2 and the Schottky junction formed between g-C3N4/MXene interfaces facilitate the photoinduced electron transfer and suppress the recombination with photogenerated holes. This work demonstrates that the 2D/2D Ti3C2/g-C3N4 composites are promising photocatalysts thanks to the ultrathin MXenes as efficient co-catalysts for photocatalytic hydrogen production.

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Cu-Graphdiyne and CoNiWO 4 were synthesized by organic and hydrothermal methods , respectively. The establishment of an S-scheme heterojunction between Cu-Graphdiyne and CoNiWO 4 was achieved by interface engineering design. The efficient separation and transfer of photogenerated carriers are facilitated by the synergistic effect of the built-in electric field and band bending, while maintaining the strong redox capacity of the catalysts. The introduction of Cu-Graphdiyne effectively enhances the photo absorption capacity and conductivity of the composite catalyst, and significantly suppresses the recombination of photogenerated carriers. The unique two-dimensional planar network structure of Cu-Graphdiyne provides abundant active sites for photocatalytic processes, thereby facilitating the photocatalytic reaction. Density functional theory (DFT) calculations demonstrate that hot electrons generated by surface plasmon resonance effects of Cu will transfer to Graphdiyne to promote hydrogen evolution reaction . This study offers insights into potential applications of Cu-Graphdiyne and nickel-cobalt based catalysts in photocatalytic hydrogen evolution . The Cu-Graphdiyne/CoNiWO 4 S-Scheme heterojunction was constructed to achieve efficient photocatalytic water splitting for hydrogen evolution.

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Abstract Improving the separation of photogenerated carriers and suppressing the rapid complication of electron–hole pairs are essential ways to improve photocatalytic hydrogen production activity. The high recombination rate of the photogenerated carriers is an issue encountered when developing CdS as a promising photocatalytic material. This work allowed to accelerate the separation of photogenerated electrons and holes by loading monoclinic β‐AgVO 3 on hexagonal CdS nanorods to construct a one‐dimensional (1D)/1D p‐n heterojunction. The introduction of monoclinic β‐AgVO 3 with a narrow band gap effectively improves the light absorption of CdS, which is conducive to improving the use of visible light. The integrated electric field of the p–n heterojunction can effectively transfer electrons and holes in the direction suitable to hydrogen evolution. The photoluminescence and electrochemical characterization of the catalysts showed that the p–n heterojunction formed after loading β‐AgVO 3 greatly improved the separation efficiency of photocarriers. The hydrogen evolution experiments show that the composite catalyst has good photocatalytic hydrogen evolution capability and stability. The composite catalyst with the best photocatalytic performance was obtained by studying β‐AgVO 3 with different loadings. The composite catalyst reached 581.5 μmol of hydrogen amount within 5 h, which is 3.8 times higher than that of CdS alone and its apparent quantum efficiency reaches 8.02%. The present work provides a possible solution for the development of perovskite and the extensiveness of CdS in photocatalytic hydrogen evolution.

Abstract 2D amorphous transition metal oxides (a‐TMOs) heterojunctions that have the synergistic effects of interface (efficiently promoting the separation of electron−hole pairs) and amorphous nature (abundant defects and dangling bonds) have attracted substantial interest as compelling photocatalysts for solar energy conversion. Strategies to facilely construct a‐TMOs‐based 2D/2D heterojunctions is still a big challenge due to the difficulty of preparing individual amorphous counterparts. A generalized synthesis strategy based on supramolecular self‐assembly for bottom–up growth of a‐TMOs‐based 2D heterojunctions is reported, by taking 2D/2D g‐C 3 N 4 (CN)/a‐TMOs heterojunction as a proof‐of‐concept. This strategy primarily depends on controlling the cooperation of the growth of supramolecular precursor and the coordinated covalent bonds arising from the tendency of metal ions to attain the stable configuration of electrons, which is independent on the intrinsic character of individual metal ion, indicating it is universally applicable. As a demonstration, the structure, physical properties, and photocatalytic water‐splitting performance of CN/a‐ZnO heterojunction are systematically studied. The optimized 2D/2D CN/a‐ZnO exhibits enhanced photocatalytic performance, the hydrogen (432.6 µmol h −1 g −1 ) and oxygen (532.4 µmol h −1 g −1 ) evolution rate are 15.5 and 12.2 times than bulk CN, respectively. This synthetic strategy is useful to construct 2D a‐TMOs nanomaterials for applications in energy‐related areas and beyond.

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Abstract CuO as a catalyst has shown promising application prospects in photocatalytic splitting of water into hydrogen (H 2 ). However, the instability of CuO in amine aqueous solution limits the applications of CuO‐based photocatalysts in the photocatalytic H 2 evolution. In this work, a novel dodecahedral nitrogen (N)‐doped carbon (C) coated CuO‐In 2 O 3 p–n heterojunction (DNCPH) is designed and synthesized by directly pyrolyzing benzimidazole‐modified dodecahedral Cu/In‐based metal‐organic frameworks, showing long‐term stability in triethanolamine (TEOA) aqueous solution and excellent photocatalytic H 2 production efficiency. The improved stability of DNCPH in TEOA solution is ascribed to the alleviation of electron deficiency in CuO by forming the p–n heterojunction and the protection with coated N‐doped C layer. Based on detailed theoretical calculations and experimental studies, it is found that the improved separation efficiency of photogenerated electron/hole pairs and the mediated adsorption behavior (|∆ G H * |→0) by coupling N‐doped C layer with CuO‐In 2 O 3 p–n heterojunction lead to the excellent photocatalytic H 2 production efficiency of DNCPH. This work provides a feasible strategy for effectively applying CuO‐based photocatalysts in photocatalytic H 2 production.

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The development of noble metal-free catalysts for hydrogen evolution is required for energy applications. In this regard, ternary heterojunction nanocomposites consisting of ZnO nanoparticles anchored on MoS₂ -RGO (RGO=reduced graphene oxide) nanosheets as heterogeneous catalysts show highly efficient photocatalytic H₂ evolution. In the photocatalytic process, the catalyst dispersed in an electrolytic solution (S^2- and SO₃^2- ions) exhibits an enhanced rate of H₂ evolution, and optimization experiments reveal that ZnO with 4.0 wt % of MoS₂ -RGO nanosheets gives the highest photocatalytic H₂ production of 28.616 mmol h^-1 g_cat^-1 under sunlight irradiation; approximately 56 times higher than that on bare ZnO and several times higher than those of other ternary photocatalysts. The superior catalytic activity can be attributed to the in situ generation of ZnS, which leads to improved interfacial charge transfer to the MoS₂ cocatalyst and RGO, which has plenty of active sites available for photocatalytic reactions. Recycling experiments also proved the stability of the optimized photocatalyst. In addition, the ternary nanocomposite displayed multifunctional properties for hydrogen evolution activity under electrocatalytic and photoelectrocatalytic conditions owing to the high electrode-electrolyte contact area. Thus, the present work provides very useful insights for the development of inexpensive, multifunctional catalysts without noble metal loading to achieve a high rate of H₂ generation.

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