The photogalvanic effect (PGE) plays a crucial role in spintronic applications. However, the coexistence and controllable generation of pure spin current (PSC) and fully spin-polarized current (FSPC) within a single material system remain largely unexplored. In this work, we investigate the PGE in bilayer VS2 using quantum transport simulations based on the Keldysh nonequilibrium Green's-function formalisms combined with density-functional theory. Bilayer VS2 with 2H- and 3R-type stackings lacks spatial inversion symmetry and exhibits appreciable photocurrents under linearly polarized light. These photocurrents exhibit a cosine dependence on polarization angles, consistent with the phenomenological theory of PGE. The photocurrent in the antiparallel configuration (APC) is significantly enhanced compared with that in the parallel configuration (PC), revealing a prominent spin-valve effect. Notably, FSPC is realized in the PC, whereas PSC is generated in the APC, and both can be effectively controlled by tuning photon energy and polarization angle. Interestingly, the magnetoresistance ratio exceeds 90% for most photon energies, reaching as high as 100%, and the perfect spin polarization efficiency achieves 100% in the PC and 8 & times; 103 in the APC. These findings highlight bilayer VS2 as a promising candidate for high-performance spintronic devices.

This study explores the tunable characteristics of optical Tamm states (OTS) in a metal–distributed Bragg reflector (DBR) structure integrated with a monolayer of molybdenum disulfide (MoS2). Through finite element simulations, we demonstrate that incorporating MoS2 enhances electromagnetic field localization at the metal–DBR interface, facilitating enhanced exciton–photon interaction. As the number of DBR periods increases, the OTS resonance wavelength undergoes a blue shift and eventually stabilizes, which indicates a wavelength-locking behavior. Under external bias, the locking threshold is lowered, and the resonance wavelength exhibits a nearly linear blue shift of approximately ~1 nm/V. Moreover, absorptance varies non-monotonically with the metal thickness, reaching over 99% at a thickness of 25 nm, due to the combined effects of plasmonic confinement and MoS2 excitonic enhancement. These findings demonstrate the potential of this structure for application in tunable photonic devices such as optical filters and modulators.

We present a comprehensive study of vertically stacked beta-AsP/ZrBrCl heterostructures using first-principles calculations. Our results demonstrate that the electronic properties of these heterostructures are highly dependent on stacking configuration, with the AA, AB, and AC configurations exhibiting type II indirect semiconductor behavior. The heterostructure displays excellent broadband optical absorption across the visible and near-ultraviolet regions, effectively combining the light absorption characteristics of both monolayers. Taking advantage of these unique properties, we have developed a dual-probe photodetector model with significant polarization-sensitive responses. Notably, the AB-armchair configuration achieves an impressive extinction ratio of 1060 at a photon energy of 2.8 eV, outperforming most existing two-dimensional material-based photodetectors. This work positions beta-AsP/ZrBrCl heterostructures as promising candidates for high-performance polarization-resolved photodetection applications.

Two-dimensional (2D) materials are attracting significant attention for their potential applications in the post-Moore era. In this work, we systematically investigate the effect of strains on the electronic structure, transport and optoelectronic properties of 2D Indium nitride (InN) monolayer using density functional theory and non-equilibrium Green's function methods. The results show that strains can modulate the electronic properties. Specifically, biaxial strain triggers the transition from semiconductor to metal and indirect to direct band gap. On this basis, the constructed InN-based nanodevice exhibits current switching ratios up to 1010. In addition, the optoelectronic device based on InN monolayer exhibits a robust photoelectric response in the red light. Meanwhile, biaxial strain can improve the optoelectronic performance of InN-based optoelectronic devices. The compressive strains blue-shift the photocurrent peaks of the InN monolayer, which effectively modulates its detection range in the visible light region. These findings underscore the potential applications in nanotechnology, particularly in nano-switches and optoelectronic devices.

Two-dimensional (2D) heterostructure materials, known for their tunable multifunctionality and low-dimensional confinement effects, offer vast potential for diverse applications. This work provides a comprehensive investigation of the electronic structure, transport and optical properties of MoSe2/ZrCl2 heterostructures using density functional theory (DFT) with non-equilibrium Green's function (NEGF) methods. Our results demonstrate that the electronic properties of MoSe2/ZrCl2 heterostructure materials can be precisely tuned by applying biaxial strain. Specifically, biaxial strain triggers a semiconductor-to-metal transition and modulates the bandgap from direct to indirect. In addition, we found that the MoSe2/ZrCl2 heterostructure exhibits remarkable optical properties with pronounced absorption peaks in the visible and near-ultraviolet regions. Photodetector simulations reveal substantial photocurrents in the visible range, particularly within the 1.6-3.6 eV photon energy regions, where distinct photocurrent peaks are observed. In addition, the MoSe2/ZrCl2 heterostructure-based device exhibits exceptional photodetector performance, with a current switching ratio reaching 108 at +6% biaxial strain. In summary, our study highlights the ability to finely tune the electronic and optical properties of MoSe2/ZrCl2 heterostructures through biaxial strain, offering promising prospects for the development of MoSe2/ZrCl2-based nano-switching and optoelectronic devices in practical applications.

The discovery of ferroelectricity in HfO2-based materials with high dielectric constant has inspired tremendous research interest for next-generation electronic devices. Importantly, films structure and strain are key factors in exploration of ferroelectricity in fluorite-type oxide HfO2 films. Here we investigate the structures and strain-induced ferroelectric transition in different phases of few-layer HfO2 films (layer number N = 1-5). It is found that HfO2 films for all phases are more stable with increasing films thickness. Among them, the Pmn2(1) (110)-oriented film is most stable, and the films of N = 4, 5 occur with a P2(1) ferroelectric transition under tensile strain, resulting in polarization about 11.8 mu C/cm(2) along in-plane a-axis. The ferroelectric transition is caused by the strain, which induces the displacement of Hf and O atoms on the surface to non-centrosymmetric positions away from the original paraelectric positions, accompanied by the change of surface Hf-O bond lengths. More importantly, three new stable HfO2 2D structures are discovered, together with analyses of computed electronic structures, mechanical, and dielectric properties. This work provides guidance for theoretical and experimental study of the new structures and strain-tuned ferroelectricity in freestanding HfO2 films.

This contribution discusses the results of an in-situ angular XPS study on the thermal evolution of the native oxide layer on Nb3Sn and pure Nb. XPS data were recorded with conventional spectrometers using an AlK(alpha) X-ray source for spectra collected up to 600 C, and an MgK(Alpha) X-rays source for temperatures above 600 C. The effect of the thickness, composition, and thermal stability of that oxide layer is relevant to understanding the functional properties of superconducting radiofrequency (SRF) cavities used in particle accelerators. There is a consensus that oxide plays a role in surface resistance (Rs). The focus of this study is Nb3Sn, which is a promising material that is used in the manufacturing of superconducting radiofrequency (SRF) cavities as well as in quantum sensing, and pure Nb, which was included in the study for comparison. The thermal evolution of the oxide layer in these two materials is found to be quite different, which is ascribed to the influence of the Sn atom on the reactivity of the Nb atom in Nb3Sn films. Nb and Sn atoms in this intermetallic solid have different electronegativity, and the Sn atom can reduce electron density around neighbouring Nb atoms in the solid, thus reducing their reactivity for oxygen. This is shown in the thickness, composition, and thermal stability of the oxide layer formed on Nb3Sn. The XPS spectra were complemented by grazing incident XRD patterns collected using the ESRF synchrotron radiation facility. The results discussed herein shed light on oxide evolution in the Nb3Sn compound and guide its processing for potential applications of the Nb3Sn-based SRF cavities in accelerators and other superconducting devices.

In a van der Waals heterostructure of two-dimensional materials, electronic properties are tunable by means of stacking orders. Here, we study the spin-dependent quantum transport in the bilayer of ferromag-netic zigzag-edged graphene/hexagonal-BN nanoribbons (ZGr-BNNRs) using density-functional theory combined with the Keldysh nonequilibrium Green's-function method. We reveal a strong odd-even effect of transport across the ZGr-BNNRs and a giant magnetoresistance value observed only in even-width ZGr-BNNRs. More interestingly, this value can be optimized by engineering stacking orders, yielding the perfect spin polarization efficiency of 100% and the magnetoresistance value of over 104 in even -width ZGr-BNNRs. Our results provide a route to design and fabricate high-performance spin filters and magnetic storage devices.

The anomalous photocurrent in perovskite CsGeX3 (X = Cl, Br, I), which is enhanced by spontaneous polarization, has recently attracted the attention of both theoretical and experimental researchers. However, the underlying mechanism of ferroelectric-optical coupling remains an open question that requires further investigation. The polarization displacements in our grown single-crystal perovskite structure in CsGeX3 (X = Cl, Br, I) are from the Ge2+ ion distortion based on the calculation of the synchrotron radiation X-ray pair distribution function. Furthermore, we carry out the simulations of the photocurrent in CsGeX3 halides by means of nonequilibrium Green's function (NEGF) method. To examine the effect of polarized light irradiation on the CsGeX3 structures, we calculated the induced photocurrents with different transport directions. The theoretical results indicated that the photocurrents were influenced by diverse halogens at the X site in perovskite CsGeX3.

Motivated by the existence of superconductivity in pyrite-structure CuS$_2$, we explore the possibility of ionic-liquid-gating-induced superconductivity in the proximal antiferromagnetic Mott insulator NiS$_2$. A clear gating-induced transition from a two-dimensional insulating state to a three-dimensional metallic state is observed at positive gate bias on single crystal surfaces. No evidence for superconductivity is observed down to the lowest measured temperature of 0.45 K, however. Based on transport, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an electrochemical gating mechanism involving a substantial decrease in the S:Ni ratio (over hundreds of nm), which is both non-volatile and irreversible. This is in striking contrast to the reversible, volatile, surface-limited, electrostatic gate effect in pyrite FeS$_2$. We attribute this stark difference in electrochemical vs. electrostatic gating response in NiS$_2$ and FeS$_2$ to the much larger S diffusion coefficient in NiS$_2$, analogous to the different behaviors observed among electrolyte-gated oxides with differing O-vacancy diffusivities. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S; this is in contrast to the better understood oxide case, where electrolysis of atmospheric H$_2$O provides an O reservoir. This study of NiS$_2$ thus provides new insight into electrolyte gating mechanisms in functional materials, in a previously unexplored limit.

Two-dimensional ferroelectrics with moderate band gap enable unprecedented applications in optoelectronics. Here, we report that the photogalvanic effect (PGE) of narrow-band-gap semiconductor bilayer ZrI2 is significantly enhanced by the interlayer sliding which turns out to be ferroelectric with both in-plane and out-of-plane polarizations. The intrinsic ferroelectric field promotes the separation efficiency of photogenerated carriers and reduces the recombination rate of electron–hole pairs, thus improving the photoelectric conversion efficiency and resulting in a larger photoresponse. The magnitude of the maximum photoresponse in the bilayer β-ZrI2 (ferroelectric) is enhanced by about 5 times than in the bilayer s-ZrI2 (paraelectric). A robust broadband photoresponse from mid-infrared to visible can be found from its photodetector. Our results provide great insights to engineering novel optoelectronic applications by ferroelectric slidetronics.

本文采用基于非平衡态格林函数-密度泛函理论的第一性原理方法,计算了单层WSe2的光电性质.计算结果表明:在小偏压下,几乎整个可见光范围内都能产生较强的光响应,且光响应与偏振角θ 呈现完美的余弦关系,与唯象理论相符合.锯齿型和扶手椅型WSe2纳米器件均在光子能量为2.8 eV(443 nm,对应于可见光)时,能产生较大的光响应;利用能带结构和态密度分析了产生较大光响应的原因,其主要来自第一布里渊区高对称点X处的电子受激跃迁.此外,WSe2纳米器件还具有较强的各向异性和较高的偏振灵敏度;这些结果可为WSe2在光电子器件中的应用提供重要的理论参考.

The photogalvanic effect (PGE) enables the generation of photocurrent at zero bias and without the need of the p-n junction, and also offer a high polarization sensitivity in a broadband range, showing potential applications in the low-power 2D optoelectronics, however the PGE photocurrent is generally small. Here we studied the PGE in the 2D WTe2 monolayer using the quantum transport simulations, and proposed the mechanism to effectively enhance the PGE photocurrent by substitution-doping and vacancy-defects. The PGE photocurrent can be generated in the 2D WTe2 monolayer under the vertical illumination of linearly polarized light. The photocurrent can be enhanced evidently by creating the vacancy or Ga-substitution doping, and moreover a higher polarization sensitivity can be obtained. This performance is attributed to the reduced symmetry due to the doping, which increases the device asymmetry and therefore enhance the photocurrent. Our results propose the mechanism to effectively enhance the PGE by substitution doping and also show the promising applications of the 2D WTe2 monolayer in optoelectronics including the photodetections.

Using first-principles calculations, we investigate the photogalvanic effect (PGE) in photodetectors based on tungsten telluride/molybdenum telluride (WTe2/MoTe2) heterostructures. Our calculations reveal that the photocurrent generated by the PGE exhibits a highly asymmetric response along the armchair and zigzag directions, which is sensitive to the light polarization state and direction. Furthermore, a large photocurrent can be obtained for almost the entire visible range without the application of external source-drain voltage, thus avoiding the problem of dark current. The photocurrent in the armchair WTe2/MoTe2 photodetector is significantly enhanced compared to that in a basic WTe2 photodetector. It also exhibits a sinusoidal dependence on the polarization angle and chirality of the circular polarization in agreement with phenomenological models of the second-order photocurrent response to the electric field of the light. Furthermore, the WTe2/MoTe2 photodetector shows a high sensitivity to the polarization direction with an extinction ratio greater than 140 in the armchair direction. These advantageous properties suggest that WTe2/MoTe2 is a promising candidate material for photodetection applications.

材料的禁带宽度是影响光电探测器探测范围的重要因素.单层2H-MoTe2因具有合适的禁带宽度引起了科研人员广泛的研究兴趣.本文基于非平衡态格林函数-密度泛函理论,采用第一性原理方法,研究了单层2H-MoTe2的光电效应.结果表明:在线性偏振光照射下,MoTe2产生的光电流函数与唯象理论相吻合;在光子能量范围1.6～1.8 eV(690～770 nm),对应于红光,能产生较大的光电流.利用能带结构和态密度分析了产生较大光电流的原因主要来自第一布里渊区S点的电子受激跃迁.同时发现在锯齿型方向偏压为0.8 V时,光电流达到峰值;然而在扶手椅型方向偏压为0.4 V时,光电流就达到峰值.这些计算结果可用于指导基于MoTe2光电探测器的设计,尤其是红外光电探测器的设计.

基于非平衡态格林函数-密度泛函理论,采用第一性原理方法,计算了VA族元素(N、P、As或Sb)掺杂单层WS2的光电效应,并解释了掺杂提高光电效应的微观机理.结果表明:在线性极化光照射下,单层WS2中可以产生光电流.由于掺杂降低了单层WS2的空间反演对称性,导致N、P、As或Sb分别掺杂的单层WS2的光照中心区产生的光电流明显提升.其中N掺杂的效果最好,掺杂后的单层WS2在光子能量3.1 eV时获得最大光电流(1.75),并且偏振灵敏度达到最大(18.1),P、As、Sb分别掺杂的单层WS2在光子能量3.9 eV时取得较大的光电流,并且有较高的偏振灵敏度.研究结果表明通过掺杂能够有效增强光电效应,获得更高的偏振灵敏度,揭示了掺杂单层WS2在光电子器件领域潜在的应用前景.

We investigate the electronic structures of intrinsic and doped WTe2 by first-principles calculations based on the density functional theory. We first calculate the phonon dispersion and phonon density of states of WTe2. We found that all the phonon modes have positive energies, and that the optical phonon branches have high eigenvalues, which indicates that the structure of WTe2 is thermodynamically stable. In the range of phonon energy from 0 to 10 meV, the acoustic and optical branches overlap, and thus there is no gap between them. The grouping of acoustic phonons and optical branches has implications for the thermal transport properties of WTe2. We next analyzed the electronic band structures of intrinsic and doped WTe2. We show that doping with other chalcogens in the same group of elements as Te reduces the energy band gap but leaves the overall band structure relatively unchanged. However, doping with elements from other groups, such as C and H, greatly modifies the electronic band structure, especially near the Fermi level. In fact, doping with such elements can elicit a transition of WTe2 from a gapped to a gapless phase. (C) 2020 Elsevier B.V. All rights reserved.

The photogalvanic effect (PGE) enables the generation of photocurrent and also offer a high polarization sensitivity in a broadband range, showing potential applications in the low-power two dimensional (2D) optoelectronics, however the photocurrent of PGE is generally small. Here, we investigated the PGE for the 2D T-d-WTe2 monolayer by employing the quantum transport simulations, and proposed the physical mechanism to effectively enhance the photocurrent of PGE at small bias voltage. The photocurrent of PGE can be generated in the 2D T-d-WTe2 monolayer when the linearly polarized light of vertical illumination was applied. In the whole visible and near-infrared range we find the biggish photocurrent which reach up to saturate for the most photon energies under a small bias. The photocurrent of junction exhibits a cosine dependence with respect to the polarization angle. The magnitude of the largest photocurrent can be evidently enhanced about 1 x 10(4) times for a photon energy of 2.4 eV than the one around 0 eV under the bias of 0.2 V in the zigzag direction, but 7 x 10(2) times at 0.9 V in the armchair direction. Moreover, a higher polarization sensitivity can be obtained. In addition, a strong anisotropy of photocurrent can be displayed between the zigzag and armchair T-d-WTe2, and that the photocurrent of zigzag direction is almost 3 times larger than that one of the armchair direction. These results show that in the visible and near-infrared range the 2D T-d-WTe2 monolayer play a potential candidate for the optoelectronics in future. (C) 2020 Elsevier B.V. All rights reserved.

The electron spin conductance switch effect in a two step-likes quantum wire under the modulation of both the Rashba and Dresselhaus spin-orbit couplings (SOCs) is investigated by using the non-equilibrium Green's function (NEGF) method with tight binding approximation scheme.Results show that a very large spin conductance occurs in the system under consideration when the electrons transport in the forward biased case,while the spin conductance vanishes when the electrons transport in the backward biased case.Moreover,it is found that the spin conductance of the system displays a " circular " profile according to the change of both the Rashba and Dresselhaus SOCs,which demonstrates that the two SOCs have the same effects on inducing spin conductance.Therefore,one can control the spin conductance of the system by tuning either the Rashba or Dresselhaus SOC strength,which indicates that the system can be utilized to make the proposed quantum wire with all-electrical spin conductance diode in the future.