Negative Poisson's Ratios of Layered Materials by First-Principles High-Throughput Calculations

Auxetic two-dimensional(2D)materials,known from their negative Poisson's ratios(NPRs),exhibit the unique property of expanding(contracting)longitudinally while being laterally stretched(compressed),contrary to typical materials.These materials offer improved mechanical characteristics and hold great potential for applications in nanoscale devices such as sensors,electronic skins,and tissue engineering.Despite their promising attributes,the availability of 2D materials with NPRs is limited,as most 2D layered materials possess positive Poisson's ratios.In this study,we employ first-principles high-throughput calculations to systematically explore Poisson's ratios of 40 commonly used 2D monolayer materials,along with various bilayer structures.Our investigation reveals that BP,GeS and GeSe exhibit out-of-plane NPRs due to their hinge-like puckered structures.For 1T-type transition metal dichalcogenides such as M X_(2)(M=Mo,W;X=S,Se,Te)and transition metal selenides/halides the auxetic behavior stems from a combination of geometric and electronic structural factors.Notably,our findings unveil V_(2)O_(5) as a novel material with out-of-plane NPR.This behavior arises primarily from the outward movement of the outermost oxygen atoms triggered by the relaxation of strain energy under uniaxial tensile strain along one of the in-plane directions.Furthermore,our computations demonstrate that Poisson's ratio can be tuned by varying the bilayer structure with distinct stacking modes attributed to interlayer coupling disparities.These results not only furnish valuable insights into designing 2D materials with a controllable NPR but also introduce V_(2)O_(5) as an exciting addition to the realm of auxetic 2D materials,holding promise for diverse nanoscale applications.

Temperature-induced phase transition of two-dimensional semiconductor GaTe

GaTe is a two-dimensional Ⅲ-Ⅵ semiconductor with suitable direct bandgap of~1.65 eV and high photoresponsivity,which makes it a promising candidate for optoelectronic applications.GaTe exists in two crystalline phases:monoclinic(m-GaTe,with space group C2/m) and hexagonal(h-GaTe,with space group P63/mmc).The phase transition between the two phases was reported under temperature-varying conditions,such as annealing,laser irradiation,etc.The explicit phase transition temperature and energy barrier during the temperature-induced phase transition have not been explored.In this work,we present a comprehensive study of the phase transition process by using first-principles energetic and phonon calculations within the quasi-harmonic approximation framework.We predicted that the phase transition from h-GaTe to m-GaTe occurs at the temperature decreasing to 261 K.This is in qualitative agreement with the experimental observations.It is a two-step transition process with energy barriers 199 meV and 288 meV,respectively.The relatively high energy barriers demonstrate the irreversible nature of the phase transition.The electronic and phonon properties of the two phases were further investigated by comparison with available experimental and theoretical results.Our results provide insightful understanding on the process of temperature-induced phase transition of GaTe.

Exploration of B-site alloying in partially reducing Pb toxicity and regulating thermodynamic stability and electronic properties of halide perovskites

Alloying strategies provide a high degree of freedom for reducing lead toxicity,improving thermodynamic stability, tuning the optoelectronic properties of ABX3 halide perovskites by varying the alloying element species and their contents.Given the key role of B-site cations in contributing band edge states and modulating structure factors in halide perovskites,the partial replacement of Pb2+with different B-site metal ions has been proposed.Although several experimental attempts have been made to date,the effect of B-site alloying on the stability and electronic properties of halide perovskites has not been fully explored.Herein,we take cubic CsPbBr3 perovskite as the prototype material and systematically explore the effects of B-site alloying on Pb-containing perovskites.According to the presence or absence of the corresponding perovskite phase,the ten alloying elements investigated are classified into three types(i.e.,Type Ⅰ:Sn Ge,Ca,Sr;Type Ⅱ:Cd,Mg,Mn;Type Ⅲ:Ba,Zn,Cu).Based on the first-principles calculations,we obtain the following conclusions.First,these B-site alloys will exist as disordered solid solutions rather than ordered structures at room temperature throughout the composition space.Second,the alloying of Sn and Ge enhances the thermodynamic stability of the cubic perovskite host,whereas the alloying of the other elements has no remarkable effect on the thermodynamic stability of the cubic perovskite host.Third,the underlying physical mechanism for bandgap tuning can be attributed to the atomic orbital energy mismatch or quantum confinement effect.Fourth,the alloying of different elements demonstrates the diversity in the regulation of crystal structure and electronic properties,indicating potential applications in photovoltaic s and self-trapped exciton-based light-emitting applications.Our work provides theoretical guidance for using alloying strategies to reduce lead toxicity,enhance stability,and optimize the electronic properties of halide perovskites to meet the needs of optoelectronic applications.

光电半导体材料的理论设计

光电半导体作为一种可以将光能和电能相互转换的材料,近几十年来,在能源和电子信息等领域得到广泛应用.随着计算机算力的提升和理论算法的发展,理论设计方法可以在短时间内探索成百上千种材料,相比于采用试错法的实验方式,具有开发周期短且成本低等特点,逐渐成为新材料研发的关键步骤.通过将物理原则与高通量计算、智能优化算法和机器学习等理论设计策略相结合,可以准确高效地探索性能优异的光电半导体材料.本文概述了光电半导体材料的设计策略和研究进展.首先,介绍基于第一性原理计算光电性质的方法,并分析相关物理性质的成因及其在光电半导体设计方面的意义;然后,结合具体的研究成果对高通量材料筛选、物理原则导向的材料设计、基于智能算法的材料搜索和基于机器学习方法的材料发现等不同的光电半导体设计策略进行概述,为该领域的理论设计方向提供指导原则;最后,对光电半导体设计方面的工作进行总结,并对该领域未来的发展进行展望.

Anisotropic phonon thermal transport in two-dimensional layered materials

Two-dimensional layered materials(2DLMs)have attracted growing attention in optoelectronic devices due to their intriguing anisotropic physical properties.Different members of 2DLMs exhibit unique anisotropic electrical,optical,and thermal properties,fundamentally related to their crystal structure.Among them,directional heat transfer plays a vital role in the thermal management of electronic devices.Here,we use density functional theory calculations to investigate the thermal transport properties of representative layered materials:β-InSe,γ-InSe,MoS2,and h-BN.We found that the lattice thermal conductivities ofβ-InSe,γ-InSe,MoS_(2),and h-BN display diverse anisotropic behaviors with anisotropy ratios of 10.4,9.4,64.9,and 107.7,respectively.The analysis of the phonon modes further indicates that the phonon group velocity is responsible for the anisotropy of thermal transport.Furthermore,the low lattice thermal conductivity of the layered InSe mainly comes from low phonon group velocity and atomic masses.Our findings provide a fundamental physical understanding of the anisotropic thermal transport in layered materials.We hope this study could inspire the advancement of 2DLMs thermal management applications in next-generation integrated electronic and optoelectronic devices.

Band structure engineering through van der Waals heterostructing superlattices of two-dimensional transition metal dichalcogenides

The indirect-to-direct band-gap transition in transition metal dichalcogenides(TMDCs)from bulk to monolayer,accompanying with other unique properties of two-dimensional materials,has endowed them great potential in optoelectronic devices.The easy transferability and feasible epitaxial growth pave a promising way to further tune the optical properties by constructing van der Waals heterostructures.Here,we performed a systematic high-throughput first-principles study of electronic structure and optical properties of the layerby-layer stacking TMDCs heterostructing superlattices,with the configuration space of[(MX2)n(M0X02)10−n](M/M0=Cr,Mo,W;X/X0=S,Se,Te;n=0-10).Our calculations involving long-range dispersive interaction show that the indirect-to-direct band-gap transition or even semiconductor-to-metal transition can be realized by changing component compositions of superlattices.Further analysis indicates that the indirect-to-direct band-gap transition can be ascribed to the in-plane strain induced by lattice mismatch.The semiconductor-to-metal transition may be attributed to the band offset among different components that is modified by the in-plane strain.The superlattices with direct band-gap show quite weak band-gap optical transition because of the spacial separation of the electronic states involved.In general,the layers stacking-order of superlattices results in a small up to 0.2 eV band gap fluctuation because of the built-in potential.Our results provide useful guidance for engineering band structure and optical properties in TMDCs heterostructing superlattices.