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Valley and spin splitting in monolayer TX2/antiferromagnetic MnO (T=Mo, W; X=S, Se) van der Waals heterostructures
摘要： The electronic structure of monolayer MoS2, WS2, MoSe2 and WSe2 on top of antiferromagnetic MnO(111) is investigated systematically by first-principles calculations. It is found that the time-reversal symmetry is broken by the stacking on MnO that leads to the valley polarization. The valley splitting of MoS2/MnO, WS2/MnO, MoSe2/MnO and WSe2/MnO is 161, 193, 171 and 125 meV for K point, and 18 to 253 meV for K' point. The pattern of stacking also induces p- or n-type doping of MoS2, revealing that the conductivity of the heterostructues could be tuned by stacking on MnO. Besides, we also calculate electronic structures of WS2/MnO, MoSe2/MnO and WSe2/MnO heterostructures in the configuration of Mn-terminated (III), and find the spin splitting at K point is 553, 324 and 481 meV, and 215, 9 and 284 meV for K' point, respectively. Furthermore, the termination of MnO substrate can switch the spin splitting of monolayer MoS2, WS2, MoSe2 and WSe2. The spin splitting of MoS2/MnO for six possible interface configurations is varied from 24 to 291 meV for K point, and 18 to 253 meV for K' point. The results present a new type of novel heterostructure that has potential applications in the spintronic and valleytronic devices.
关键词： Two dimensional materials,Heterostructure,Transition metal dichalcogenides,First-principles calculations
更新于2025-09-23 15:23:52
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Color image encryption method based on 2D-variational mode decomposition
摘要： In order to reduce the correlation between adjacent pixels in a plaintext image, and to solve the small key space problem of image encryption algorithms when a low-dimension chaotic map is used, a new encryption method for color images is proposed based on two-dimensional variational mode decomposition (2D-VMD) combined with eight-dimensional (8D) hyper-chaotic systems. 2D-VMD decomposes R, G and B components of a color image respectively. The 8D hyper-chaotic system is constructed by means of variable coupling. After improving and combining the original sequences obtained by iterating the system, two groups of key sequences associated with the plain-image are obtained. One group is used to scramble each mode image obtained by 2D-VMD; the other is used to replace the pixel values of the scrambled mode images. For different mode images, different key sequences are adopted. The encrypted images whose number is equal to the number of the modes can then be obtained. The experimental results show that, compared with the existing methods, the correlation coefficients between the pixels in the spatial domain of the plain-image can be reduced by 2D-VMD, as it’s more difficult to crack. In addition, the encryption method has better statistical and differential characteristics, as well as large enough key space, and better plain-image sensitivity.
关键词： Encryption method,Image decomposition,Two-dimensional variational mode decomposition,Color image,Hyper-chaotic system
更新于2025-09-23 15:23:52
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Analysis and Modeling of White Graphene Physical Properties for Sensor Applications
摘要： White graphene as a two-dimensional material has honeycomb lattice which is comprised of boron and nitrogen atoms. Its mechanical properties are similar to graphene, but its electrical properties are different because it has a large bandgap about 5.5 eV. Moreover, relaxation time plays a key role in its properties that affects on the electron transfer speed in this material. The white graphene is an insulator, but its dielectric polarization does not depend on the electric ?eld and also any external stress cannot change linear response of relaxation. In this research, some physical properties of white graphene and its relaxation time are investigated and modeled. Finally, acceptable results are reported which bring new hopes for the replacement of silicon oxide with white graphene as a one-monolayer insulator in future sensors applications.
关键词： Honeycomb lattice,Graphene,Bandgap,Two-dimensional materials,White graphene
更新于2025-09-23 15:23:52
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Alternative Type 2D-3D Lead Halide Perovskite with Inorganic Sodium Ions as Spacer for High Performance Light Emitting Diodes
摘要： Two-dimensional (2D) lead halide perovskites with long-chain ammonium halides display high photoluminescence quantum yield (PLQY), due to their size and dielectric confinement, which promise a high efficiency and low-cost light emitting diode (LED). However, the presence of insulating organic long-chain spacer cation (L) dramatically deteriorates the charge transport properties along the out-of-plane nanoplatelet direction or adjacent nanocrystals, which would limit the LED device performance. In order to overcome this issue, we successfully incorporate small alkaline ions such as sodium (Na+) to replace long organic molecule. Grazing incident X-ray diffraction (GIXRD) measurements verify the 2D layered formation with preferential crystallite orientation. In addition, the incorporated sodium salt also generates amorphous sodium lead bromide (NaPbBr3) in perovskite as spacers to form nanocrystal-like halide perovskite film. PLQY is dramatically improved in the sodium incorporated film associating with enhanced PL lifetime. With incorporating small concentration of an organic additive, this 2D-3D perovskite can achieve a compact and uniform film. Therefore, a 2D-3D perovskite achieves a high external quantum efficiency (EQE) of 15.9% with good operational stability. Our work develops a type of 2D-3D halide perovskite with various inorganic ions as spacers for high performance of promising optoelectronic devices.
关键词： two-dimensional,alkaline halide,perovskite,dielectric confinement,light-emitting diode
更新于2025-09-23 15:23:52
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Bonding of Large Substrates by Silver Sintering and Characterization of the Interface Thermal Resistance
摘要： Low-temperature silver sintering technology, which has been proven to be a promising die-attach solution, was extended to bonding large substrates. Strong bonding strengths for substrates greater than 25 mm × 50 mm were achieved by sintering a nanosilver paste at temperatures below 270oC with less than 5 MPa pressure. To characterize the thermal performance of the substrate-attach interface, we applied a transient thermal technique with cumulative structure function analysis. Using self-heating and temperature-sensitive threshold voltage of a power device, we measured the transient thermal responses of the device placed at various locations on the bonded structures. Each transient thermal response was used to determine cumulative structure function, which represents the relationship between cumulative thermal capacitance and cumulative thermal resistance from the device junction to the ambient environment. Two-dimensional maps corresponding to interface thermal resistance were obtained from structure function plots. We found that for well-bonded substrates, the average specific thermal resistance contributed by the sintered silver interface was between 5.20 mm2K/W and 5.78 mm2K/W with a variation of 4.7% to 6.0%.
关键词： silver sintering,Substrate-attach,two-dimensional map of interface thermal resistance,transient thermal characterization
更新于2025-09-23 15:23:52
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Facile synthesis of two-dimensional tailored graphitic carbon nitride with enhanced photoelectrochemical properties through a three-step polycondensation method for photocatalysis and photoelectrochemical immunosensor
摘要： Graphitic carbon nitride (g-C3N4) is an ideal alternative two-dimensional (2D) nanostructure for photocatalysis and photoelectrochemical (PEC) application, while controllably fabricating 2D shaped g-C3N4 nanolayers/nanosheets is still facing challenges. On the basis of temperature-dependent polymorphic characters, herein, a 2D extending g-C3N4 (g-CNS3) is synthesized from dicyandiamide as the precursor by operating the condensation temperature in a continuously three-step thermal polycondensation procedure. The g-CNS3 with film-like morphology showed improved visible-light absorption ability and enhanced PEC performance compared to g-CNS1 synthesized via the traditional one-step thermal polymerization method. Benefiting from its excellent PEC properties, the g-CNS3 exhibited high photocatalytic activity to removal MB with fast kinetics and served as the photoactive layer to construct a PEC immunosensor with high sensitivity and specificity for subgroup J avian leukosis virus detection. A linear range from 102.14 to 103.35 TCID50/mL and a detection limit of 102.08 TCID50/mL were obtained for the PEC immunoassay of the target virus. This work might provide a novel protocol for tailoring shaped 2D g-C3N4 nanosemiconductor with superior properties and shed light on its promising PEC applications.
关键词： Two-dimensional material,Photocurrent response,Graphitic carbon nitride,Photocatalysis,Photoelectrochemical immunosensor
更新于2025-09-23 15:23:52
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Spectrally Resolved Ultrafast Exciton Transfer in Mixed Perovskite Quantum Wells
摘要： Solution-processed perovskite quantum wells have been used to fabricate increasingly efficient and stable optoelectronic devices. Little is known about the dynamics of photogenerated excitons in perovskite quantum wells within the first few hundred femtoseconds – a crucial timescale on which energy and charge transfer processes may compete. Here we use ultrafast transient absorption and two-dimensional electronic spectroscopy to clarify the movement of excitons and charges in reduced-dimensional perovskite solids. We report excitonic funneling from strongly to weakly confined perovskite quantum wells within 150 fs, facilitated by strong spectral overlap and orientational alignment among neighboring wells. This energy transfer happens on timescales orders of magnitude faster than charge transfer, which we find to occur instead over 10 - 100s of picoseconds. Simulations of both F?rster-type interwell exciton transfer and free carrier charge transfer are in agreement with these experimental findings, with theoretical exciton transfer calculated to occur in 100s of fs.
关键词： energy transfer,Carrier,metal halide perovskite,two-dimensional electronic spectroscopy,layered perovskite,dynamics
更新于2025-09-23 15:23:52
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Understanding Spatiotemporal Photocarrier Dynamics in Monolayer and Bulk MoTe <sub/>2</sub> for Optimized Optoelectronic Devices
摘要： Semiconducting molybdenum ditelluride has emerged as a promising transition-metal dichalcogenide with a number of novel properties. In particular, its bandgap in infrared range makes it an attractive candidate for ultrathin and high-performance infrared optoelectronic applications. Dynamical properties of photocarriers play a key role in determining performance of such devices. We report an experimental study on spatiotemporal dynamics of photocarriers in both monolayer and bulk MoTe2. Transient absorption measurement in reflection geometry revealed ultrafast thermalization and relaxation processes of photocarriers and lifetimes of about 60 and 80 ps in monolayer and bulk MoTe2, respectively. By spatially resolved transient absorption measurements on monolayer, we obtained an exciton diffusion coefficient of 20 ± 10 cm2 s?1, a mean free time of 200 fs, a mean free path of 20 nm, and a diffusion length of 350 nm. The corresponding values for the bulk sample are 40 ± 10 cm2 s?1, 400 fs, 40 nm, and 570 nm, respectively. These results provide fundamental information for understanding and optimizing performance of MoTe2-based optoelectronic devices.
关键词： two-dimensional material,exciton,transient absorption,molybdenum ditelluride,transition-metal dichalcogenide,diffusion,photocarrier dynamics
更新于2025-09-23 15:23:52
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Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites
摘要： Hybrid organic–inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (?50 cm?1) optical phonons involving motion in the lead iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight into the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.
关键词： resonant impulsive stimulated Raman spectroscopy,polaronic character,two-dimensional perovskites,density functional theory,excitons,phonon coherences
更新于2025-09-23 15:23:52
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Study on the Statistical Errors in X-Ray Stress Measurement with Two-Dimensional Detector
摘要： The sin2 ψ method [1] is conventionally used well as how to measure non-destructively the residual strain and stress states of polycrystalline materials by X-ray diffraction. In the conventional method, there are D?lle-Hauk method [2] and Winholz-Cohen least squares analysis [3] as the determinations of the strain and stress states for limiting the influence of measurement errors. Many researches are made about the statistical error in those methods. In recent years, use of the X-ray stress measurements with two-dimensional detector from the conventional method is spreading. One of the measurements is called the cos α method. The measurement errors have attracted a great deal of attention for users as the spreads. Therefore, the basic equations and determinations of the strain and stress states are examined. The confidence intervals of measured stress by the cos α method. The research and development is performed for the the cos α method which took the influence of measurement errors into consideration.
关键词： Cos α method,Non-destructive inspection,Statistical errors,Two-dimensional detector,X-ray stress measurement,internal stress
更新于2025-09-23 15:23:52