厚度(层间耦合)对二维材料散射时间和输运的影响

2D elemental layered crystals, such as graphene and black phosphorus (B‐P), have received tremendous attentions due to their rich physical and chemical properties. In the applications of nanoelectronic devices, graphene shows super high electronic mobility, but it lacks bandgap which impedes development in logical devices. As an alternative, B‐P shows high mobility of up to about 1000 cm2 V−1 s−1. However, B‐P is very unstable and degrades rapidly in ambient conditions. Orthorhombic arsenic (black arsenic; b‐As) is the “cousin” of B‐P; theoretical prediction shows that b‐As also has excellent physical and chemical properties, but there is almost no experimental report on b‐As. Herein, it is reported on the unique transport characteristics and stability of monolayer and few‐layer b‐As crystals which are exfoliated from the natural mineral. The properties of field‐effect transistors (FETs) strongly depend on the thickness of crystals. In the monolayer limit, the performance shows relatively high carrier mobilities and large on/off ratios. Moreover, the b‐As crystals exhibit a relatively good ambient stability. The few‐layer arsenic based FET still function after exposure to air for about one month. Therefore, b‐As is expected to be a promising 2D material candidate in nanoelectronic devices.

… However, thickness dependent carrier transport in semimetal MoTe 2 has not been studied. Here, we report the transport and optical properties of 1T′ MoTe 2 films with thickness …

… demonstrated thickness dependent conductivity of multilayer InSe with various thicknesses … The conductivity of multilayer InSe reveals a decrease with the thickness increased from 5 …

… Here, we see a qualitative change — thick PtSe 2 is a metal but thin PtSe 2 is a … controlling the thickness of 2D materials, the possibility of tuning their electrical transport characteristics …

Cr_2Si_2Te_6 (CST) is a van der Waals (vdW) ferromagnetic semiconductor. The unique spin model and temperature-dependent magnetic ordering of CST provide opportunities for the next generation of two-dimensional (2D) spintronic devices. Here, abnormal magneto-transport properties are found in CST nanoflakes with variations in thickness. Interestingly, the thickness-dependent magnetoresistance (MR) effect exhibits a nonlinear change as a function of the magnetic field, temperature, and thickness. At a certain temperature below Curie temperature ( T _ c ), a sign reversal of MR ratio from positive to negative can even be detected with thickness reduction. At the temperature range from T _ c to 60 K, the Hall effect also presents a transformation from nonlinear behavior in thick layer CST to linear behavior in thin layer CST. These distinctive magneto-transport properties are attributed to the variation of spin correlation with thickness in CST nanoflakes. These findings probe the unique magneto-transport properties of CST and associate it with ferromagnetic correlation, which provides a basis for subsequent spintronics device design based on this material. This work also offers new insights into the relationship between sample thickness, transport properties, and spin correlation of other vdW ferromagnets. It lays a foundation for future vdW magnet-based device fabrication and possible spintronic applications.

… channel thicknesses, t > 20 nm, well within its direct band gap regime. Through gate dependent … of the channel thickness, carrier transport in these materials occurs via Arrhenius-type …

The discovery of graphene has stimulated the search for and investigations into other 2D materials because of the rich physics and unusual properties exhibited by many of these layered materials. Transition metal dichalcogenides (TMDs), black phosphorus, and SnSe among many others, have emerged to show highly tunable physical and chemical properties that can be exploited in a whole host of promising applications. Alongside the novel electronic and optical properties of such 2D semiconductors, their thermal transport properties have also attracted substantial attention. Here, a comprehensive review of the unique thermal transport properties of various emerging 2D semiconductors is provided, including TMDs, black‐ and blue‐phosphorene among others, and the different mechanisms underlying their thermal conductivity characteristics. The focus is placed on the phonon‐related phenomena and issues encountered in various applications based on 2D semiconductor materials and their heterostructures, including thermoelectric power generation and electron–phonon coupling effect in photoelectric and thermal transistor devices. A thorough understanding of phonon transport physics in 2D semiconductor materials to inform thermal management of next‐generation nanoelectronic devices is comprehensively presented along with strategies for controlling heat energy transport and conversion.

So far, layered PdSe2 has attracted much attention due to its completely tunable band-gap with varying layer numbers, yet the thickness-dependent transporting properties have been rarely studied. We have systematically studied the electronic structures, phonon and charge transport properties, and thermoelectric properties of few-layered (from 1L to 4L) and bulk PdSe2 by first-principles calculations and Boltzmann transport theory. As the thickness increases, the energy levels of band edges relative to 4s of selenium move oppositely due to their different bonding states, leading to the power-law decrease of the band-gap. Meanwhile, the electron effective mass decreases rapidly while the hole effective mass increases significantly compared with those unperturbed. Calculations on elastic constants reveal that both bulk and few-layered PdSe2 are mechanically stable, and the bulk is ductile with a Poisson's ratio of 0.27. The shifts of Raman active modes with respect to the thickness as well as their Gruneisen parameters are analyzed and the underlying physics is discussed. At room temperature, the thermal conductivities of the bulk are 7.7, 10.1 and 0.9 W m-1 K-1 along the a, b and c axes, respectively. It is found that the low-frequency modes (<2.0 THz) contribute about 80% of in-plane thermal conductivities. Due to the enhanced contribution from the ZA mode, the thermal conductivity of few-layered PdSe2 is much larger than that of the bulk. The ZA mode is mainly scattered by itself and the Umklapp scattering dominates in the process as the thickness increases. Calculations on charge transport reveal that the electron mobility increases from 2.5-13.2 (1L) to 121.9-167.8 (4L) cm2 V-1 s-1 with the decreasing anisotropy μb/μa, while the hole mobility remains to be ∼20 cm2 V-1 s-1, which is in good agreement with the experimental results. Calculations on the thermoelectric properties reveal that the ZT value as well as the power factor increases largely as the thickness increases and it gets to be optimum for the triple layer. Interestingly, the transport of electrons and phonons is decoupled along the out-of-plane direction, which makes bulk PdSe2 exhibit good thermoelectric performance along the c axis.

… material, is one-atom thick,1 with an exclusive topological arrangement of carbon atoms, which provides extraordinary physical properties.Electrons and holes in this 2D material obey a …

With high quality topological insulator Bi(2)Se(3) thin films, we report thickness-independent transport properties over wide thickness ranges. Conductance remained nominally constant as the sample thickness changed from 256 to ∼8  QL (where QL refers to quintuple layer, 1  QL≈1  nm). Two surface channels of very different behaviors were identified. The sheet carrier density of one channel remained constant at ∼3.0×10(13)  cm(-2) down to 2 QL, while the other, which exhibited quantum oscillations, remained constant at ∼8×10(12)  cm(-2) only down to ∼8  QL. The weak antilocalization parameters also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively.

… atomic thickness, 2D materials allow superior electrostatic control and as such, 2D TMDCs … gaseous adsorbates in the range of band-like transport. To be able to compare the sensitivity …

… transport in two-dimensional topological materials over the last five years. Topological materials … This review focuses on key developments in the understanding of transport phenomena …

We investigated the dependence of electron mobility on the thickness of MoS2 nanosheets by fabricating bottom-gate single and few-layer MoS2 thin-film transistors with SiO2 gate dielectrics and Au electrodes. All the fabricated MoS2 transistors showed on/off-current ratio of ∼107 and saturated output characteristics without high-k capping layers. As the MoS2 thickness increased from 1 to 6 layers, the field-effect mobility of the fabricated MoS2 transistors increased from ∼10 to ∼18 cm2V−1s−1. The increased subthreshold swing of the fabricated transistors with MoS2 thickness suggests that the increase of MoS2 mobility with thickness may be related to the dependence of the contact resistance and the dielectric constant of MoS2 layer on its thickness.

… attention on the carrier mobility of few-layer BC 2 N. In … carrier mobility of few-layer BC 2 N from first-principles calculations. Our results show that few-layer BC 2 N possesses high carrier …

We report herein that the carrier mobility of the 2%-La-doped BaSnO3 (LBSO) films on (001) SrTiO3 and (001) MgO substrates strongly depends on the thickness whereas it is unrelated to the lattice mismatch (+5.4% for SrTiO3, -2.3% for MgO). Although we observed large differences in the lattice parameters, the lateral grain size (~85 nm for SrTiO3, ~20 nm for MgO), the surface morphology and the density of misfit dislocations, the mobility increased almost simultaneously with the thickness in both cases and saturated at ~100 cm2 V-1 s-1, together with the approaching to the nominal carrier concentration (=[2% La3+]), clearly indicating that the behavior of mobility depends on the film thickness. The present results would be beneficial to understand the behavior of mobility and fruitful to further enhance the mobility of LBSO films.

… layers was grown on GaInNAs layer. … -dependence of carrier mobility and 2D carrier density over the temperature range of 10–300 K. All scattering-limited expressions of carrier mobility …

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied. The coupling between layers plays an important role in the properties of stacked two-dimensional materials. Here, the authors show that Coulomb interactions between electrons in different layers of graphene induce thermal transport even though all electronic states are confined to individual layers.

Engineering ultrafast interlayer coupling provides access to new quantum phenomena and novel device functionalities in atomically thin van der Waals heterostructures. However, due to all the atoms of a monolayer material being exposed at the interfaces, the interlayer coupling is extremely susceptible to defects, resulting in high energy dissipation through heat and low device performance. The study of how defects affect the interlayer coupling at ultrafast and atomic scales remains a challenge. Here, using femtosecond transient absorption microscopy, a new defect‐induced ultrafast interlayer electron–phonon coupling pathway is identified in a WS2/graphene heterostructure, involving a three‐body collision between electrons in WS2 and both acoustic phonons and defects in graphene. This interaction manifests as the reduced defect‐related Raman resonant activity and the accelerated electron–phonon scattering time from 7.1 to 2.4 ps. Furthermore, the ultrafast interlayer coupling process is directly imaged. These insights will advance the fundamental knowledge of heat dissipation in nanoscale devices, and enable new ways to dynamically manipulate electrons and phonons via defects in van der Waals heterostructures.

… This is a result of the enhanced scattering in the bottom layers close to the … times larger than in the case of multi-layer graphene while the interlayer conductivity is about 20 times …

Layered two-dimensional semiconductors have attracted tremendous attention owing to their demonstrated excellent transistor switching characteristics with a large ratio of on-state to off-state current, Ion/Ioff. However, the depletion-mode nature of the transistors sets a limit on the thickness of the layered semiconductor films primarily determined by a given Ion/Ioff as an acceptable specification. Identifying the optimum thickness range is of significance for material synthesis and device fabrication. Here, we systematically investigate the thickness-dependent switching behavior of transistors with a wide thickness range of multilayer-MoS2 films. A difference in Ion/Ioff by several orders of magnitude is observed when the film thickness, t, approaches a critical depletion width. The decrease in Ion/Ioff is exponential for t between 20 nm and 100 nm, by a factor of 10 for each additional 10 nm. For t larger than 100 nm, Ion/Ioff approaches unity. Simulation using technical computer-aided tools established for silicon technology faithfully reproduces the experimentally determined scaling behavior of Ion/Ioff with t. This excellent agreement confirms that multilayer-MoS2 films can be approximated as a homogeneous semiconductor with high surface conductivity that tends to deteriorate Ion/Ioff. Our findings are helpful in guiding material synthesis and designing advanced field-effect transistors based on the layered semiconductors.

Two-dimensional (2D) van der Waals semiconductors represent the thinnest, air stable semiconducting materials known. Their unique optical, electronic and mechanical properties hold great potential for harnessing them as key components in novel applications for electronics and optoelectronics. However, the charge transport behavior in 2D semiconductors is more susceptible to external surroundings (e.g. gaseous adsorbates from air and trapped charges in substrates) and their electronic performance is generally lower than corresponding bulk materials due to the fact that the surface and bulk coincide. In this article, we review recent progress on the charge transport properties and carrier mobility engineering of 2D transition metal chalcogenides, with a particular focus on the markedly high dependence of carrier mobility on thickness. We unveil the origin of this unique thickness dependence and elaborate the devised strategies to master it for carrier mobility optimization. Specifically, physical and chemical methods towards the optimization of the major factors influencing the extrinsic transport such as electrode/semiconductor contacts, interfacial Coulomb impurities and atomic defects are discussed. In particular, the use of ad hoc molecules makes it possible to engineer the interface with the dielectric and heal the vacancies in such materials. By casting fresh light on the theoretical and experimental studies, we provide a guide for improving the electronic performance of 2D semiconductors, with the ultimate goal of achieving technologically viable atomically thin (opto)electronics.

One of the main limiting factors in the performance of devices based on two-dimensional (2D) materials is Fermi level pinning at the contacts, which creates Schottky barriers (SBs) that increase contact resistance and, for most transition metal dichalcogenides (TMDs), limit hole conduction. A promising method to mitigate these problems is surface charge transfer doping (SCTD), which places fixed charge at the surface of the material and thins the SBs by locally shifting the energy bands. We use a mild O2 plasma to convert the top few layers of a given TMD into a substoichiometric oxide that serves as a p-type SCTD layer. A comprehensive experimental study, backed by TCAD simulations, involving MoS2, MoSe2, MoTe2, WS2, and WSe2 flakes of various thicknesses exposed to different plasma times is used to investigate the underlying mechanisms responsible for SCTD. The surface charge at the top of the channel and the gate-modulated surface potential at the bottom are found to have competing effects on the channel potential, which results in a decrease in the doping-induced threshold shift and an increase in minimum OFF state current with increasing thickness. Additionally, an undoped channel region is shown to mitigate carrier injection issues in sufficiently thin flakes. Notably, the band movements underlying the SCTD effects are independent of the particular semiconductor material, SCTD strategy, and doping polarity. Consequently, our findings provide critical insights for the design of high-performance transistors for a wide range of materials and SCTD mechanisms including TMD devices with strong hole conduction.

Weak interlayer couplings at 2D van der Waals (vdW) interfaces fundamentally distinguish out‐of‐plane charge flow, the information carrier in vdW‐assembled vertical electronic and optical devices, from the in‐plane band transport processes. Here, the out‐of‐plane charge transport behavior in 2D vdW semiconducting transition metal dichalcogenides (SCTMD) is reported. The measurements demonstrate that, in the high electric field regime, especially at low temperatures, either electron or hole carrier Fowler–Nordheim (FN) tunneling becomes the dominant quantum transport process in ultrathin SCTMDs, down to monolayers. For few‐layer SCTMDs, sequential layer‐by‐layer FN tunneling is observed to dominate the charge flow, thus serving as a material characterization probe for addressing the Fermi level positions and the layer numbers of the SCTMD films. Furthermore, it is shown that the physical confinement of the electron or hole carrier wave packets inside the sub‐nm thick semiconducting layers reduces the vertical quantum tunneling probability, leading to an enhanced effective mass of tunneling carriers.

The possibility of tailoring physical properties by changing the number of layers in van der Waals crystals is one of the driving forces behind the emergence of two-dimensional materials. One example is bulk MoS2, which changes from an indirect gap semiconductor to a direct bandgap semiconductor in the monolayer form. Here, we show a much bigger tuning range with a complete switching from a metal to a semiconductor in atomically thin PtSe2 as its thickness is reduced. Crystals with a thickness of ~13 nm show metallic behavior with a contact resistance as low as 70 Ω·µm. As they are thinned down to 2.5 nm and below, we observe semiconducting behavior. In such thin crystals, we demonstrate ambipolar transport with a bandgap smaller than 2.2 eV and an on/off ratio of ~105. Our results demonstrate that PtSe2 possesses an unusual behavior among 2D materials, enabling novel applications in nano and optoelectronics. The electronic band structure of van der Waals crystals is strongly sensitive to the number of layers. Here, the authors observe a thickness-dependent metal-to-semiconductor transition in layered PtSe2 by means of electrical transport measurements.

… In this paper, dissipative quantum transport simulations … of monolayer/multilayer 2D semiconductor-based FETs for sub-10 … because of their atomic scale thickness. However, the path …

… scattering mechanisms influencing the operation of 2D … boundaries may hurt charge transport in 2D materials. There … First, 2D semiconductors with atomic thicknesses are the thinnest …

… Figure 3 shows the ohmic mobility for phonon scattering, versus layer width a, at T = 150 K. The upper curve is the mobility given by (35) and (48); the lower curve includes a correction …

Although progress has been made in the ab initio simulation of lattice dynamics in semiconducting crystals, information about the relaxation of nonequilibrium lattice vibrations remains incomplete. This work studies the relaxation times of room-temperature thermal phonons through measurements of thermal conduction along monocrystalline silicon films of thickness down to 74 nm. A repetitive oxidation and etching process ensures that the purity and crystalline quality of the films are comparable with those of bulk samples. Phonon-interface scattering reduces the thermal conductivity by up to 50% at room temperature. The data indicate that the effective mean-free path of the dominant phonons at room temperature is close to 300 nm and thus much longer than the value of 43 nm predicted when phonon dispersion is neglected. This study indicates that a broad variety of lattice transport characteristics for bulk silicon can be obtained through measurements on carefully prepared silicon nanostructures. The present data are also valuable for the thermal simulation of silicon-on-insulator (SOI) transistors.

… phonons to be scattered within a thick film, their mean free path must be of the order of the film thickness … and thus can explain the phonon scattering we have observed in our thick films. …

… that concern specific electronphonon scattering mechanisms. … scattering, and between different types of intravalley electron-… half width of electronic level as “experimental scattering rates,…

… that the phonon-limited electron mobility is enhanced with increasing layer thicknesses and … The electrostatic control is found to be crucial even for a single-layer MoS2 device. With a …

… quantum wells on the phonon spectra and thus on the electron-phonon scattering rates, which … that scattering of electrons from confined phonons increases with increasing well width for …

… L (lower scale) indicates the lateral layer width(s) and defines the initial energy E… phonon scattering and the associated relaxation properties of two-, one-, and zero-dimensional electron …

Thermal engineering of many nanoscale sensors, actuators, and high-density thermomechanical data storage devices, as well as the self-heating in deep submicron transistors, are largely influenced by thermal conduction in ultrathin silicon layers. The present study measures the lateral thermal conductivity of single-crystal silicon layers of thicknesses 20 and 100 nm at temperatures between 20 and 300 K, using Joule heating and electrical–resistance thermometry in suspended microfabricated structures. The thermal conductivity of the 20 nm thick silicon layer is ∼22 W m−1 K−1, which is nearly an order of magnitude smaller than the bulk value at room temperature. In general, a large reduction in thermal conductivity resulting from phonon–boundary scattering, particularly at low temperatures, is observed. It appears that the classical thermal conductivity theory that accounts for the reduced phonon mean-free paths based on a solution of the Boltzmann transport equation along a layer is able to capture the ballistic, or nonlocal, phonon transport in ultrathin silicon films.

… of electron-phonon … thickness of the metal layer for considering electron-phonon coupling transport. If only one side of the metal layer is in contact with the semiconductor, the electron-…

… In this article, the thickness dependent and anisotropic in-plane thermal transport behavior … Those findings here can give physical insights into the thickness dependent and anisotropic …

The resistivity of nanoscale metallic conductors is orientation dependent, even if the bulk resistivity is isotropic and electron scattering cross-sections are independent of momentum, surface orientation, and transport direction. This is demonstrated using a combination of electron transport measurements on epitaxial tungsten layers in combination with transport simulations based on the ab initio predicted electronic structure, showing that the primary reason for the anisotropic size effect is the non-spherical Fermi surface. Electron surface scattering causes the resistivity of epitaxial W(110) and W(001) layers measured at 295 and 77 K to increase as the layer thickness decreases from 320 to 4.5 nm. However, the resistivity is larger for W(001) than W(110) which, if describing the data with the classical Fuchs-Sondheimer model, yields an effective electron mean free path λ* for bulk electron-phonon scattering that is nearly a factor of two smaller for the 110 vs the 001-oriented layers, with λ(011)*= 18.8 ± 0.3 nm vs λ(001)* = 33 ± 0.4 nm at 295 K. Boltzmann transport simulations are done by integration over real and reciprocal space of the thin film and the Brillouin zone, respectively, describing electron-phonon scattering by momentum-independent constant relaxation-time or mean-free-path approximations, and electron-surface scattering as a boundary condition which is independent of electron momentum and surface orientation. The simulations quantify the resistivity increase at the reduced film thickness and predict a smaller resistivity for W(110) than W(001) layers with a simulated ratio λ(011)*/λ(001)* = 0.59 ± 0.01, in excellent agreement with 0.57 ± 0.01 from the experiment. This agreement suggests that the resistivity anisotropy in thin films of metals with isotropic bulk electron transport is fully explained by the non-spherical Fermi surface and velocity distribution, while electron scattering at phonons and surfaces can be kept isotropic and independent of the surface orientation. The simulations correctly predict the anisotropy of the resistivity size effect, but underestimate its absolute magnitude. Quantitative analyses suggest that this may be due to (i) a two-fold increase in the electron-phonon scattering cross-section as the layer thickness is reduced to 5 nm or (ii) a variable wave-vector dependent relaxation time for electron-phonon scattering.

… -filament anisotropic superconducting … anisotropic transport properties, we reveal a competitive relationship between superconductivity and charge density wave (CDW) in the thickness …

The thickness and temperature dependences of electrical and thermal transport coefficients (e.g. electrical conductivity, thermoelectric power, thermal conductivity) of Bi1–xSbx films are described by a non-degenerated two-band model, considering the anisotropic elliptical band structure (many-valley model) of bulk Bi1–xSbx. The transport coefficients are measured in the temperature range 80 to 400 K on films with thicknesses 20 to 400 nm and the results are interpreted and discussed using the deduced relations. Die Schichtdicken- und Temperatura bhangigkeit elektrischer und thermischer Transportkoeffi-zienten (z. B. der elektrischen Leitfahigkeit, der Thermokraft, der Warmeleitfahigkeit) von Bi1–xSbx-Schichten wird im Rahmen eines nicht entarteten Zweiband-Modells unter Berucksichtigung der anisotropen elliptischen Bandstruktur (many valley model) von massivem Bi1–xSbx beschrieben. Die Transportkoeff izienten werden im Temperaturbereich 80 bis 400 K an Schichten von 20 bis 400 nm Dicke gemessen, und die Ergebnisse werden mit den abgeleiteten Beziehungen interpretiert und diskutiert.

… or wavelength dependent Raman anisotropy can further be … anisotropic thermal conductivity with different thickness is … thermal conductivities and highly anisotropic transport properties, …

… On the basis of the density functional theory coupled with the Boltzmann transport equation … mobility of few-layer InSe. Few-layer InSe has a tunable band gap on thickness. At the same …

… We use the nominal layer thicknesses h = 3.35, 4.20, 4.22, and 4.34 A for graphene, silicene… All the calculations are performed by using the Vienna ab initio simulation package (VASP) …

The monolayer of black phosphorous, or phosphorene, has recently emerged as a new 2D semiconductor with intriguing highly anisotropic transport properties. Existing calculations of its intrinsic phonon-limited electronic transport properties so far rely on the deformation potential approximation, which is in general not directly applicable to anisotropic materials since the deformation along one specific direction can scatter electrons traveling in all directions. We perform a first-principles calculation of the electron-phonon interaction in phosphorene based on density functional perturbation theory and Wannier interpolation. Our calculation reveals that 1) the high anisotropy provides extra phase space for electron-phonon scattering, and 2) optical phonons have appreciable contributions. Both effects cannot be captured by the deformation potential calculations.

… with 1-4 channel layers through ab initio quantum transport simulations, and analyses … It is shown by the results that the band gap of GeS decreases with increasing layer thickness…

Thermoelectric materials attract great attention due to promising applications in refrigeration and waste heat recovery. Strategies based on band engineering have been proposed to identify new thermoelectrics with high electrical...

… interaction of PEO with the V 2 O 5 lattice. This study shows that beyond only interlayer spacing, the nature of interlayer interactions … affect Mg-ion charge transport and storage in layered …

… interlayer properties in 2D materials. Furthermore, we discuss the strategies to modulate the interlayer interaction in … The interlayer interaction in 2D materials can be a unique degree of …

A major challenge in the development of high-performance organic photocatalytic polymers is establishing efficient charge-carrier transport pathways. In this study, we propose a molecular design strategy that addresses this issue by enhancing interlayer interactions in two-dimensional vinyl-linked covalent organic frameworks (VL-COFs). This is achieved by incorporating a rigid, planar triazine unit at the framework vertex center. The vertex-centered design promotes stronger interlayer interaction, resulting in well-aligned π-stacked columns that facilitate efficient charge-carrier transport and markedly improve the photocatalytic activity. The resulting VL-COFs exhibited outstanding hydrogen peroxide (H2O2) production rates and excellent long-term stability in pure water. Moreover, the optimized electronic structure accelerates the rate-limiting O2-to-OOH* step in the two-electron oxygen reduction reaction, thereby improving the catalytic performance in H2O2 synthesis. This work demonstrates a vertex design strategy for tuning interlayer interactions in COFs, offering a promising pathway for developing highly efficient photoactive materials for artificial H2O2 photosynthesis.

We report on the implementation of a versatile projection-operator diabatization approach to calculate electronic coupling integrals in layered periodic systems. The approach is applied to model charge transport across the saturated organic spacers in two-dimensional (2D) lead halide perovskites. The calculations yield out-of-plane charge transfer rates that decay exponentially with the increasing length of the alkyl chain, range from a few nanoseconds to milliseconds, and are supportive of a hopping mechanism. Most importantly, we show that the charge carriers strongly couple to distortions of the Pb-I framework and that accounting for the associated nonlocal dynamic disorder increases the thermally averaged interlayer rates by a few orders of magnitude compared to the frozen-ion 0 K-optimized structure. Our formalism provides the first comprehensive insight into the role of the organic spacer cation on vertical transport in 2D lead halide perovskites and can be readily extended to functional π-conjugated spacers, where we expect the improved energy alignment with the inorganic layout to speed up the charge transfer between the semiconducting layers.

Two-dimensional lead-halide perovskites provide a more robust alternative to three-dimensional perovskites in solar energy and optoelectronic applications due to increased chemical stability afforded by interlayer ligands. At the same time, the ligands create barriers for interlayer charge transport, reducing device performance. Using a recently developed ab initio simulation methodology, we demonstrate that ligand fluorination can enhance both hole and electron mobility by 1–2 orders of magnitude. The simulations show that the enhancement arises primarily from improved structural order and reduced thermal atomic fluctuations in the system rather than increased interlayer electronic coupling. Arising from stronger hydrogen bonding and dipolar interactions, the higher structural stability decreases the reorganization energy that enters the Marcus formula and increases the charge transfer rate. The detailed atomistic insights into the electron and hole transfer in layered perovskites indicate that the use of interlayer ligands that make the overall structure more robust is beneficial simultaneously for chemical stability and charge transport, providing an important guideline for the design of new, efficient materials.

One-dimensional (1D) π-d-conjugated coordination polymers (CCPs) with charge delocalization have attracted significant attention due to their potential application in energy conversion and storage. However, the fundamental understanding of the correlation of their structural parameters with photophysical and photocatalytic properties remains underexplored. Herein, we report three novel Cu-node anthracene-based 1D π-d CCPs with systematic variation of steric groups (Ph > Me > H) at the 9 and 10 position of anthracene (denoted as AnPh, AnMe, and AnH), which is aimed at altering the stacking of the polymer chains and its impact on the inter-chain charge transport property. Using the combination of steady-state X-ray absorption spectroscopy, optical transient absorption spectroscopy, X-ray transient absorption spectroscopy, and electrochemical impedance spectroscopy, we show that the linear ligands (AnPh, AnMe, and AnH) with different degrees of steric groups (Ph > Me > H) introduced at the 9 and 10 position of anthracene can alter the stacking of the polymer chains and thus impact their crystallinity, charge separation, and charge transport property, which in turn impacts their photocatalytic performance for hydrogen evolution reaction.

… HTSs, the charge density of the substrate becomes tunable through interlayer charge transfer. … , the electron loss of the sodium atom, the interlayer charge transfer, and the heights of the …

Multilayer rhenium disulfide (ReS2) has recently attracted significant attention because of the decoupled van der Waals interaction between its adjacent layers that leads to a much higher interlayer resistivity than that in other layered materials. Although the carrier transport in multilayer materials is well described by the interlayer resistance and Thomas–Fermi charge screening length (λ) in theoretical resistor network models, the understanding of the effect of electric field-dependent interlayer tunneling barrier (Eint) on current fluctuation in two-dimensional (2D) multilayer materials is limited. Herein, we report the effects of Eint on carrier transport and charge fluctuation in multilayer ReS2. The electrostatic back-gate (VBG)- and drain bias (VD)-dependent Eint causes channel migration along the c-axis in 2D multilayer systems and consequently results in two plateaus in the transconductance curve, thereby allowing us to determine the top and bottom carrier mobilities of multilayer ReS2 separately. Furthermore, the strong correlation between Eint and the Coulomb scattering parameter in multilayer ReS2 is elucidated via low-frequency noise spectroscopy. The results of our study provide a clear insight into the origins of carrier transport and current fluctuation in 2D multilayer devices.

Stacked two-dimensional (2D) heterostructures are evolving as the 'next-generation' opto-electronic materials due to the possibility of designing atomically-thin devices with outstanding characteristics. Yet, most of the existing 2D heterostructures are governed by weak van der Waals interlayer interactions that, as often is the case, exert limited impact on the resulting properties of heterostructures relative to their constituting components. In this work, we investigate the opto-electronic properties of a novel class of 2D MP3 (M = Ge and Sn) materials featuring strong interlayer interactions, applying a robust theoretical framework combining Density Functional Theory and Many-Body Perturbation Theory. We demonstrate that the remarkable intrinsic vertical strain (of ~40 % relative to the monolayers) promotes the exfoliation of these materials into bilayers and profoundly impacts their electronic structure, charge transport and optical properties. Most strikingly, we observe that the strong interlayer hybridization endorses continuous optical absorption across the entire visible range that, together with high charge carrier mobility, makes these 2D MP3 heterostructures attractive for photoconversion applications.

A combination of different 2D layered materials by van der Waals (vdW) stacking or lateral splicing provides the basic building blocks for dynamic behavior researches of interlayer carriers. Anisotropic materials, recently, have further attracted attentions in this field because of their supply of freedoms for regulating the performance of electro‐optical devices, whereas detailed characteristics and mechanisms of interlayer carrier transportation in these materials need remain to be revealed. Here, by using the photoassisted field effect and scanning photocurrent imaging measurements, it is demonstrated that the photoinduced interlayer carrier transportation in cross‐stacked black phosphorus (BP) vdW junctions is strongly dependent on the crystal orientation and stacking morphology. Type‐I and II band alignments are respectively predicted in the BP junctions with parallel and vertical crystal orientation stacking. The interlayer carrier transportation with both vertical and lateral modes is observed within only one sample. Combined first principle calculation with band theory analyses, the small band offset for holes and tunneling effect play key roles during the interlayer transportation. These results highlight the importance of crystal orientation of materials in vdW junctions and provide insights, both experimentally and theoretically, into engineering and design of orientation‐based nanodevices.