过渡金属离子自旋态影响稀土掺杂上转化纳米晶发光

The rare-earth nanocrystals containing Er3+ emitters offer very promising tools for imaging applications, as they can not only exhibit up-conversion luminescence but also down-conversion luminescence in the second near-infrared window (NIR II). Doping non-lanthanide cations into host matrix was demonstrated to be an effective measure for improving the luminescence efficiency of Er3+ ions, while still awaiting in-depth investigations on the effects of dopants especially those with high valence states on the optical properties of lanthanide nanocrystals. To address this issue, tetravalent Zr4+ doped hexagonal NaGdF4:Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr4+ doping level on both up-conversion luminescence in the visible window and down-conversion luminescence in NIR II window were investigated, with steady-state and transient luminescence spectroscopies. The key role of the local crystal field distortions around Er3+ emitters was elucidated in combination with the results based on both of Zr4+ and its lower valence counterparts, e.g., Sc3+, Mg2+, Mn2+. Univalent ions such as Li+ was utilized to substitute Na+ ion rather than Gd3+, and the synergistic effects of Zr4+ and Li+ ions by co-doping them into NaGdF4:Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF4:Yb,Er nanocrystals was enhanced by more than one order of magnitude compared with undoped nanocrystals. The current studies thus demonstrate that the local crystal field surrounding emitters is an effective parameter for manipulating the luminescence of lanthanide emitters.

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4-L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triple-stranded helical [LnM(Lk)3]5+ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature), and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Multimode luminescent materials exhibit tunable photon emissions under different excitation or stimuli channels, endowing them high encoding capacity and confidentiality for anti‐counterfeiting and encryption. Achieving multimode luminescence into a stable single material presents a promising but remains a challenge. Here, the downshifting/upconversion emissions, color‐tuning persistent luminescence (PersL), temperature‐dependent multi‐color emissions, and hydrochromism are integrated into Er3+ ions doped Cs2NaYbCl6 nanocrystals (NCs) by leveraging shallow defect levels and directed energy migration. The resulting NCs display strong static and dynamic colorful luminescence in response to ultraviolet, 980‐nm laser, and X‐ray. Additionally, the NCs exhibit distinct luminescent colors as the temperature increases from 330 to 430 K. Surprisingly, it also demonstrates the ability of the reversible emission modal and color in response to water. Theoretical calculations and experimental characterizations reveal that self‐trapped exciton state (STEs), chlorine vacancy defects, and ladderlike 4f energy levels of Er3+ ions contribute to multimodal luminescence. More importantly, it has extremely remarkable environmental stability, which can be stored in the air for more than 18 months, showing promising commercial prospects. This work not only gives new insights into lanthanide‐based metal halide NCs but also provides a new route for developing multimodal luminescent nanomaterials for anti‐counterfeiting and encryption.

Lanthanide‐doped nanocrystals typically exhibit limited time‐gated nonlinear luminescence with microsecond lifetimes because of their parity‐forbidden 4f–4f transitions. Here the direct observation of ultralong‐lived upconversion luminescence by coupling molecular triplet orbitals to lanthanide‐doped nanocrystals is reported. Through careful manipulation of the molecular triplet states, nanohybrids capable of producing ultralong upconversion lifetimes up to 1.37 s and tunable color output are constructed, surpassing the intrinsic lifetime of lanthanide ions by a factor of 4500. In addition to the exciton migration pathway from the 4f levels of lanthanide ions to the molecular singlet and then to triplet states, direct activation of the triplet state by exciton from the 4f levels is observed. This phenomenon significantly reduces the required photon energy (by more than 0.83 eV) for stimulation of the triplet state compared to the process involving intersystem crossing. Taking advantage of the unique properties of these nanohybrids, a pulsed laser‐pumped encryption system with enhanced security is developed by embedding additional passwords that contain information about pulse frequency and width. This work provides a new perspective on time‐resolved nonlinear luminescence with an ultrawide time dimension by engineering molecular triplet states.

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The hexagonal-phase (β) of NaREF4 upconversion nanocrystals (RE = rare earth elements) has been widely employed because of the outstanding luminescence performance, yet less is known about the essence of this superior property. The current understanding of this issue is raised from the advantage of weak electron-vibration interactions in fluoride systems, while the interpretability of this statement is controversial and contradictory results are commonly reported. One feasible way to solve this puzzle is from the aspect of "structure-property" relationship, yet even after decades of investigation, the structural details of β-NaREF4 are still under debate. Herein, the reported results relevant to this topic are reviewed, and the conflicting viewpoints are summarized. The similarities and differences between different lattice templates are assessed, and the reasons underlying the divergence are analysed. Based on these discussions, it is realized that the crystal structure of β-NaREF4 should be more reliably depicted as one flexible lattice framework with complex characteristics, and the structural disorder induced by atom displacements in the lattice is probably the key to supporting the superior luminescence properties of β-NaREF4 nanocrystals.

We demonstrate nearly a microsecond of spin coherence in Er3+ ions doped in cerium dioxide nanocrystal hosts, despite a large gyromagnetic ratio and nanometric proximity of the spin defect to the nanocrystal surface. The long spin coherence is enabled by reducing the dopant density below the instantaneous diffusion limit in a nuclear spin-free host material, reaching the limit of a single erbium spin defect per nanocrystal. We observe a large Orbach energy in a highly symmetric cubic site, further protecting the coherence in a qubit that would otherwise rapidly decohere. Spatially correlated electron spectroscopy measurements reveal the presence of Ce3+ at the nanocrystal surface, which likely acts as extraneous paramagnetic spin noise. Even with these factors, defect-embedded nanocrystal hosts show tremendous promise for quantum sensing and quantum communication applications, with multiple avenues, including core-shell fabrication, redox tuning of oxygen vacancies, and organic surfactant modification, available to further enhance their spin coherence and functionality in the future.

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In this work, we improved the photocurrent of self-powered ultraviolet photodetector via doping manganese in CsPbCl3 perovskite nanocrystals light harvester. The doped manganese in nanocrystals has the following three features to assist electron transfer from CsPbCl3 nanocrystals to titanium dioxide: (i) the fast exciton-to-manganese energy transfer process benefits for competing electrons with perovskite exciton recombination, (ii) the charge carrier lifetime is very long for manganese d-states due to its spin and orbital forbidden transition, and (iii) the electrons can effectively transfer to the titanium dioxide layer from 4T1 of manganese d-states due to the smaller energy barrier. Based on the above, the self-powered photocurrent density of photodetectors has nearly twice enhancement from 0.08 mA·cm−2 to 0.14 mA·cm−2 and a high responsivity up to 7.3 mA·W−1 was achieved at 340 nm.

The NV center in diamonds has been widely employed in quantum sensing, quantum computing, and bioimaging due to its controllable ground-state spin, detectable magnetic resonance, excellent photostability, favorable biocompatibility, and chemical inertness. However, conventional excitation using 532 nm green light still exhibits certain limitations in practical applications. To address this, we propose a novel NV center excitation method based on the upconversion of near-infrared light to green emission. Through the synthesis of molybdenum-doped NaYF4: 20% Yb3+, 1.5% Er3+ upconversion materials, efficient excitation of NV centers has been achieved. Both UC-LED luminescence spectroscopy and ODMR measurements confirm that the green light generated via the upconversion process exhibits sufficient intensity to effectively excite NV centers. Meanwhile, the characteristic sharp emission peaks of rare-earth upconversion materials eliminate the need for optical filters, facilitating device miniaturization, and a miniaturized UC-LED sensor has been developed.

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Lanthanide-doped upconversion nanoparticles enable upconversion stimulated emission depletion microscopy with high photostability and low-intensity near-infrared continuous-wave lasers. Controlling energy transfer dynamics in these nanoparticles is crucial for super-resolution microscopy with minimal laser intensities and high photon budgets. However, traditional methods neglect the spatial distribution of lanthanide ions and its effect on energy transfer dynamics. Here, we introduce topology-driven energy transfer networks in lanthanide-doped upconversion nanoparticles for upconversion stimulated emission depletion microscopy with reduced laser intensities, maintaining a high photon budget. Spatial separation of Yb3+ sensitizers and Tm3+ emitters in 50-nm core-shell nanoparticles enhance energy transfer dynamics for super-resolution microscopy. Topology-dependent energy migration produces strong 450-nm upconversion luminescence under low-power 980-nm excitation. Enhanced cross-relaxation improves optical switching efficiency, achieving a saturation intensity of 0.06 MW cm−2 under excitation at 980 nm and depletion at 808 nm. Super-resolution imaging with a 65-nm lateral resolution is achieved using intensities of 0.03 MW cm−2 for a Gaussian-shaped excitation laser at 980 nm and 1 MW cm−2 for a donut-shaped depletion laser at 808 nm, representing a 10-fold reduction in excitation intensity and a 3-fold reduction in depletion intensity compared to conventional methods. These findings demonstrate the potential of harnessing topology-dependent energy transfer dynamics in upconversion nanoparticles for advancing low-power super-resolution applications. Topology-engineered upconversion nanoparticles enable low-power STED microscopy by optimizing energy transfer and cross-relaxation, achieving sub-diffraction resolution with significantly reduced excitation and depletion intensities for efficient super-resolution imaging.

Energy transfer between Tm³⁺ and Tb³⁺ dependent on the power density of pump laser was investigated in NaYF₄: Tb³⁺,Tm³⁺,Yb³⁺ microcrystals. Under the excitation of a 976-nm near-infrared laser at various power densities, Tb³⁺-Tm³⁺-Yb³⁺ doped samples exhibited intense visible emissions with tunable color between green and blue. The ratio of blue and green emission were determined by energy transfer between Tm³⁺ and Tb³⁺. When the power density of pump laser was low, the energy transfer process from Tm³⁺ (³F₄) to Tb³⁺ (⁷F₀) occurred efficiently. Upconversion processes in Tm³⁺ were inhibited, only visible emissions from Tb³⁺ with green color were observed. When the power density increased, energy transfer from the ³F₄ (Tm³⁺) to ⁷F₀ level (Tb³⁺) was restrained and population on high energy levels of Tm³⁺ was increased. Contribution of upconversion emissions from Tm³⁺ gradually became dominant. The emission color was tuned from green to blue with increasing the power density. Energy transfer processes between low-lying levels of activators, such as Tm³⁺ will greatly reduce the population on certain levels for further high-order upconversion processes. The Tb³⁺-Tm³⁺-Yb³⁺ doped phosphors are promising materials for detecting the condition of power density of the invisible near-infrared laser.

Magic-sized (CdSe)13 clusters (MSCs) represent a material class at the boundary between molecules and quantum dots that exhibit a pronounced and well separated excitonic fine structure. The characteristic photoluminescence is composed of exciton bandgap emission and a spectrally broad mid-gap emission related to surface defects. Here, we report on a thermally activated energy transfer from fine-structure split exciton states to surface states by using temperature dependent photoluminescence excitation spectroscopy. We demonstrate that the broad mid-gap emission can be suppressed by a targeted Mn-doping of the MSC leading to the characteristic orange luminescence of the 4T1 → 6A1 Mn2+ transition. The energy transfer to the Mn2+ states is found to be significantly different than the transfer to the surface defect states, as the activation of the dopant emission requires a spin-conserving charge carrier transfer that only dark excitons can provide.

The development, design and the performance evaluation of rare-earth doped host materials is important for further optical investigation and industrial applications. Herein, we successfully fabricate KLu2F7 upconversion nanoparticles (UCNPs) through hydrothermal synthesis by controlling the fluorine-to-lanthanide-ion molar ratio. The structural and morphological results show that the samples are orthorhombic-phase hexagonal-prisms UCNPs, with average side length of 80 nm and average thickness of 110 nm. The reaction time dependent crystal growth experiment suggests that the phase transformation is a thermo-dynamical process and the increasing F−/Ln3+ ratio favors the formation of the thermo-dynamical stable phase - orthorhombic KLu2F7 structure. The upconversion luminescence (UCL) spectra display that the orthorhombic KLu2F7:Yb/Er UCNPs present stronger UCL as much as 280-fold than their cubic counterparts. The UCNPS also display better UCL performance compared with the popular hexagonal-phase NaREF4 (RE = Y, Gd). Our mechanistic investigation, including Judd-Ofelt analysis and time decay behaviors, suggests that the lanthanide tetrad clusters structure at sublattice level accounts for the saturated luminescence and highly efficient UCL in KLu2F7:Yb/Er UCNPs. Our research demonstrates that the orthorhombic KLu2F7 is a promising host material for UCL and can find potential applications in lasing, photovoltaics and biolabeling techniques.

Novel and high-security anti-counterfeiting technology has always been the focus of attention and research. This work proposes a nanocomposite combination of upconversion nanoparticles (UCNPs) and perovskite quantum dots (PeQDs) to achieve color-adjustable dual-mode luminescence anti-counterfeiting. Firstly, a series of NaGdF4: Yb/Tm UCNPs with different sizes were synthesized, and their thermal-enhanced upconversion luminescence performances were investigated. The upconversion luminescence (UCL) intensity of the samples increases with rising temperature, and the UCL thermal enhancement factor rises as the particle size decreases. This intriguing thermal enhancement phenomenon can be attributed to the mitigation of surface luminescence quenching. Furthermore, CsPbBr3 PeQDs were well adhered to the surfaces and surroundings of the UCNPs. Leveraging energy transfer and the contrasting temperature responses of UCNPs and PeQDs, this nanocomposite was utilized as a dual-mode thermochromic anti-counterfeiting system. As the temperature increases, the color of the composite changes from green to pink under 980 nm excitation, while it displays green to non-luminescence under 365 nm excitation. This new anti-counterfeiting material, with its high security and convenience, has great potential in anti-counterfeiting applications.

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Manipulating topological arrangement is a powerful tool for tuning energy migration in natural photosynthetic proteins and artificial polymers. Here, we report an inorganic optical nanosystem composed of NaErF4 and NaYbF4, in which topological arrangement enhanced upconversion luminescence. Three architectures are designed for considerations pertaining to energy migration and energy transfer within nanoparticles: outside-in, inside-out, and local energy transfer. The outside-in architecture produces the maximum upconversion luminescence, around 6-times brighter than that of the inside-out at the single-particle level. Monte Carlo simulation suggests a topology-dependent energy migration favoring the upconversion luminescence of outside-in structure. The optimized outside-in structure shows more than an order of magnitude enhancement of upconversion brightness compared to the conventional core-shell structure at the single-particle level and is used for long-term single-particle tracking in living cells. Our findings enable rational nanoprobe engineering for single-molecule imaging and also reveal counter-intuitive relationships between upconversion nanoparticle structure and optical properties. Manipulating topological arrangement is a powerful tool for tuning the energy migration in molecular systems and inorganic materials. Here, authors synthesize NaErF4 and NaYbF4 upconversion nanoparticles with different architectures and demonstrate topology-dependent enhancement of upconversion luminescence, with up to an order of magnitude increase in brightness compared to conventional core-shell architectures.

Luminescent nanoparticles with dual-mode long-lived luminescence are of great importance for their attractive applications in biosensing, bioimaging and data encoding. Herein, we report the realization of up- and down-conversion emission of Mn2+ dopants in multilayer nanoparticles of NaGdF4:Yb/Tm@NaGdF4:Ce/Mn@NaYF4 upon excitation at 980 and 254 nm, respectively. The dual-mode emission of the Mn2+ dopants at 531 nm have a long-lived lifetime up to ～ 30 ms as a result of the spin-forbidden optical transition of Mn2+ within the 3d5 configuration. After ceasing steady excitation at the two wavelengths, the long-lived feature of Mn2+ luminescence allows a longer persistent time than lanthanide emissions, thereby enabling the ease of data decoding by a cell phone camera under a burst mode. The long-lived green upconversion emission also permits the generation of a long green tail-emission upon dynamic excitation at 980 nm. These attributes make the as-prepared Mn2+-doped multilayer nanoparticles particularly attractive for multilevel anti-counterfeiting.

The abrupt reorganization of the electronic configuration of spin‐crossover (SCO) compounds upon the transition from the low‐spin (LS) to the high‐spin (HS) state leads to significant changes in their structural, optical, and magnetic properties. As spin‐transitions can be triggered by a variety of external stimuli such as variations in temperature, pressure, or through light irradiation, these systems have attracted considerable attention for the design of multifunctional and stimuli‐responsive molecular materials. In this context, the synergistic coupling of SCO and luminescent properties holds great promise for the development of molecular spintronic devices capable of correlating the luminescence with an external perturbation, or of probing the magnetic state of the metal center through variations in emission intensity. This contribution provides a comprehensive overview of the field of luminescent SCO materials, with particular emphasis on systems that exhibit a genuine interplay between the two properties, both in materials and at the molecular level.

Near-infrared (NIR) dye sensitization has been widely used to enhance the brightness of lanthanide-doped upconversion nanoparticles (UCNPs). However, the stability of these dye-sensitized upconversion nanosystems remains a significant challenge, primarily due to singlet oxygen (1O2)-induced damage to the NIR dyes. In this study, a simple strategy is presented to stabilize these nanosystems by flipping the spin of 1O2 through surface-anchored 4-aminotriphenylamine (4A-TPA). The electron-rich triphenylamine group adsorbs electrophilic 1O2 and flips its spin into 3O2 via intersystem crossing of charge transfer states, while the amino group facilitates coordination with the UCNPs surface in parallel to the NIR sensitizing dyes. It is demonstrated that incorporating 4A-TPA significantly enhances the stability of the commonly used IR806-sensitized UCNPs system, improving its photostability by 27-fold compared to the control without 4A-TPA after 30 min of 808 nm irradiation. This stability improvement is further validated in aqueous environments with amphiphilic polymer coating, enabling stable cellular upconversion luminescence imaging under high laser power density (1 200 W cm-2). This work provides a simple yet powerful method to overcome the instability of dye-sensitized upconversion systems, unlocking their potential for a variety of bioapplications ranging from bioimaging to photodynamic therapy.

We report on surface defects, electronic structure, and visible luminescence in pristine and Co-doped ZnO nanocrystals (NCs) by combining experimental characterization with density functional theory (DFT) calculations. Co doping notably reduced the crystallite size (25–18 nm) and nearly doubled the dislocation density (δ), indicating a decline in crystal quality. X-ray photoelectron spectroscopy (XPS) confirmed the incorporation of Co ions in a high-spin Co2+ (3d7, e4t23) configuration, with no detectable traces of Co3+ or metallic Co0 clusters. Photoluminescence (PL) spectra exhibited a dominant blue emission (2.7–2.8 eV), primarily due to electron transitions from the conduction band to zinc vacancy (VZn) acceptor states, further enhanced by Co-induced defects. A strong correlation between the experiment and DFT (Perdew–Burke–Ernzerhof generalized gradients approximation + U with mBJ corrections) elucidates the defect states responsible for visible emission. Our findings show how local atomic defects tune the optical properties and highlight the potential of Co-doped ZnO nanocrystals for blue light-emitting diode applications.

The developed SnWO4 phosphors incorporated with Ho3+, Yb3+ and Mn4+ ions have been explored under 980 nm laser irradiation. The molar concentration of dopants has been optimized to 0.5 Ho3+, 3.0 Yb3+ and 5.0 Mn4+ in SnWO4 phosphors. The upconversion (UC) emission from the codoped SnWO4 phosphors has been substantially amplified up to 13 times and described based on the energy transfer and charge compensation. On incorporating the Mn4+ ions in the Ho3+/Yb3+ codoped system the sharp green luminescence shifted to reddish broadband emission due to the photon avalanche mechanism. The processes accountable for the concentration quenching have been described based on critical distance. The interaction responsible for the concentration quenching in Yb3+ sensitized Ho3+ and Ho3+/Mn4+:SnWO4 phosphors is considered to be dipole-quadrupole and exchange interaction type, respectively. The activation energy 0.19 eV has been determined, and the phenomenon of thermal quenching is discussed using a configuration coordinate diagram.

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Abstract Upconversion photoluminescence in hetero‐oligonuclear metal complex architectures featuring organic ligands is an interesting but still rarely observed phenomenon, despite its great potential from a basic research and application perspective. In this context, a new photonic material consisting of molecular chromium(III) and ytterbium(III) complex ions was developed that exhibits excitation‐power density‐dependent cooperative sensitization of the chromium‐centered 2E/2T1 phosphorescence at approximately 775 nm after excitation of the ytterbium band 2F7/2→2F5/2 at approximately 980 nm in the solid state at ambient temperature. The upconversion process is insensitive to atmospheric oxygen and can be observed in the presence of water molecules in the crystal lattice.

LaOCl doped with 0–10 mol% Cr was synthesized by thermal decomposition of chlorides. X-ray diffraction (XRD) analysis revealed that incorporation of chromium results in a decrease of the lattice parameter a and a simultaneous increase of the lattice parameter c. The local structure of chromium ions was studied with X-ray photoelectron (XPS), X-ray absorption (XANES), multifrequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy techniques. It was determined that synthesis in oxidizing atmosphere promotes the incorporation of chromium ions predominantly in the 5+ oxidation state. Changes of chromium oxidation state and local environment occur after a subsequent treatment in reducing atmosphere. Spin-Hamiltonian (SH) parameters for a Cr5+ and two types of Cr3+ centers in LaOCl were determined from the EPR spectra simulations.

A good quality (5 at.% Yb:GdScO3) single crystal of Φ30 mm × 37 mm was grown successfully by the Czochralski method. Its structure is studied by the x-ray diffraction (XRD), and its atomic coordinates are obtained by Rietveld refinement. The crystal field energy level splitting of Yb3+ in GdScO3 is determined by employing the absorption and photoluminescence spectra at 8 K. Only 2F7/2(4) is far from the ground state 2F7/2(1) by 710 cm−1 among the crystal field energy levels split from 2F7/2, so it is more easier to realize the laser operation of 2F5/2(1)→2F7/2(4) with wavelength 1060 nm. The spin–orbit coupling parameters and intrinsic crystal field parameters (CFPs). The intrinsic crystal field parameters B¯k (k = 2, 4, 6) of the crystal were fitted by the superposition model. The CFPs evaluated with B¯k and coordination factor are taken as the initial parameters to fit the crystal field energy levels of the crystal, and the crystal field parameters Bqk are obtained finally with the root-mean-square deviation 9 cm−1. It is suggested that the ligand point charge, covalency and overlap interaction are slightly weaker than charge interpenetration and coulomb exchange interaction for Yb3+ in GdScO3. The obtained Hamiltonian parameters can be used to calculate crystal field energy levels and wave functions of Yb:GdScO3 to analyze the mechanism of the luminescence or laser.

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