Near-zero Poisson's ratio and suppressed mechanical anisotropy in strained black phosphorene/SnSe van der Waals heterostructure:a first-principles study

Black phosphorene(BP)and its analogs have attracted intensive attention due to their unique puckered structures,anisotropic characteristics,and negative Poisson’s ratio.The van der Waals(vdW)heterostructures assembly by stacking different materials show novel physical properties,however,the parent materials do not possess.In this work,the first-principles calculations are performed to study the mechanical properties of the vdW heterostructure.Interestingly,a near-zero Poisson’s ratio ν_(zx)is found in BP/SnSe heterostructure.In addition,compared with the parent materials BP and SnSe with strong in-plane anisotropic mechanical properties,the BP/SnSe heterostructure shows strongly suppressed anisotropy.The results show that the vdW heterostructure has quite different mechanical properties compared with the parent materials,and provides new opportunities for the mechanical applications of the heterostructures.

PbI_(2)/Pb_(5)S_(2)I_(6)van der Waals Heterojunction Photodetector

Investigations of two-dimensional(2D)/one-dimensional(1D)van der Waals(vdW)heterojunctions have attracted significant attention due to their excellent properties such as the smooth heterointerface,the highly gate-tunable bandgap,and the ultrafast carrier transport.However,the complicated method of manufacturing vdW heterojunction represents a major problem that severely limits their practical applications.Herein,we develop one-step hydrothermal method and use it to synthesize 2D PbI_(2)/1D Pb_(5)S_(2)I_(6)vdW heterojunction.The PbI_(2)/Pb_(5)S_(2)I_(6)vdW heterojunction photodetector(PD)displays lower dark current(<20 pA),higher responsivity(up to 134 mA·W-1),self-powered and wider response spectrum in comparison with that of pristine PbI_(2)PD and Pb_(5)S_(2)I_(6)PD.This one-step hydrothermal method provides a new idea for preparing other mixed-dimensional heterojunction.

Electronic structure and phase transition engineering in NbS2: Crucial role of van der Waals interactions

Based on first-principles simulations,we revisit the crystal structures,electronic structures,and structural stability of the layered transition metal dichalcogenides(TMDCs)NbS2,and shed more light on the crucial roles of the van der Waals(vdW)interactions.Theoretically calculated results imply that the vdW corrections are important to reproduce the layered crystal structure,which is significant to correctly describe the electronic structure of NbS2.More interestingly,under hydrostatic pressure or tensile strain in ab plane,an isostructural phase transition from two-dimensional layered structure to three-dimensional bulk in the I4/mmm phase has been uncovered.The abnormal structural transition is closely related to the electronic structure instability and interlayer bonding effects.The interlayer Nb-S distances collapse and the interlayer vdW interactions disappear,concomitant with new covalent bond emerging and increasing coordination number.Present work highlights the significance of the vdW interactions,and provides new insights on the unconventional structural transitions in NbS2,which will attract wide audience working in the hectic field of TMDCs.

Unipolar p-type monolayer WSe_(2) field-effect transistors with high current density and low contact resistance enabled by van der Waals contacts

High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm^(−1)) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe2 grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe2 FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm^(−1), contact resistance of 12 kΩ·µm, on/off ratio over 107, and field-effect hole mobility of ~ 103 cm2·V^(−1)·s^(−1). Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe2 with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.

Tailoring lithium intercalation pathway in 2D van der Waals heterostructure for high-speed edge-contacted floating-gate transistor and artificial synapses

Local phase transition in transition metal dichalcogenides (TMDCs) by lithiumintercalation enables the fabrication of high-quality contact interfaces in twodimensional(2D) electronic devices. However, controlling the intercalation oflithium is hitherto challenging in vertically stacked van der Waalsheterostructures (vdWHs) due to the random diffusion of lithium ions in thehetero-interface, which hinders their application for contact engineering of 2DvdWHs devices. Herein, a strategy to restrict the lithium intercalation pathwayin vdWHs is developed by using surface-permeation assisted intercalationwhile sealing all edges, based on which a high-performance edge-contact MoS_(2)vdWHs floating-gate transistor is demonstrated. Our method avoids intercalationfrom edges that are prone to be random but intentionally promotes lithiumintercalation from the top surface. The derived MoS_(2) floating-gatetransistor exhibits improved interface quality and significantly reduced subthresholdswing (SS) from >600 to 100 mV dec^(–1). In addition, ultrafast program/erase performance together with well-distinguished 32 memory statesare demonstrated, making it a promising candidate for low-power artificialsynapses. The study on controlling the lithium intercalation pathways in 2DvdWHs offers a viable route toward high-performance 2D electronics for memoryand neuromorphic computing purposes.

Single-Molecule Characterization of van der Waals Contact Between Alkane and Gold

Van der Waals(vdW)contact,dominated by weak but ubiquitous vdW interactions,plays a significant role in diverse fields such as supramolecular chemistry,nanotechnology,and surface science.Accurate characterization of vdW contact at the single-molecule level remains challenging.Herein,we combine the scanning tunneling microscope break junction technique with first-principles calculations to study the mechanical and electrical characteristics of the alkane/Au vdW contact in an in-situ solution environment.The step-like conductance plateaus indicate a gradual desorption of alkyl chains in units of two methylene groups under force stretching.Two distinct charge transport channels,through the shortest C–H/Au pathway and the entire adsorbed alkyl chain,are identified.Furthermore,we discover that a higher electric field leads to increased conductance and stronger bonding of the alkane/Au vdW contact.These results unveil the intrinsic properties of vdW contact at the molecular and even atomic levels,which are crucial for exploring noncovalent interactions and advancing molecular sciences.

Facile formation of van der Waals metal contact with III-nitride semiconductors

Metal–semiconductor contacts play a pivotal role in controlling carrier transport in the fabrication of modern electronic devices.The exploration of van der Waals(vdW)metal contacts in semiconductor devices can potentially mitigate Fermi-level pinning at the metal–semiconductor interface,with particular success in two-dimensional layered semiconductors,triggering unprecedented electrical and optical characteristics.In this work,for the first time,we report the direct integration of vdW metal contacts with bulk wide bandgap gallium nitride(GaN)by employing a dry transfer technique.High-angle annular dark-field scanning transmission electron microscopy explicitly illustrates the existence of a vdW gap between the metal electrode and GaN.Strikingly,compared with devices fabricated with electron beam-evaporated metal contacts,the vdW contact device exhibits a responsivity two orders of magnitude higher with a significantly suppressed dark current in the nanoampere range.Furthermore,by leveraging the high responsivity and persistent photoconductivity obtained from vdW contact devices,we demonstrate imaging,wireless optical communication,and neuromorphic computing functionality.The integration of vdW contacts with bulk semiconductors offers a promising architecture to overcome device fabrication challenges,forming nearly ideal metal–semiconductor contacts for future integrated electronics and optoelectronics.

硒化锡/2H相碲化钼异质结的外延生长

具有原子光滑表面和特殊层间范德瓦尔斯耦合的二维层状材料的性质与传统材料不同。由于二维层状材料的清洁表面上没有悬空键,因此在晶圆上集成各种二维材料,可以丰富器件的功能。另外,经由二维材料的添加剂生长而形成异质结构,可以构建具有非常规性质的材料。两者都可以通过材料的转移来实现,但在转移的过程中要避免机械损伤或化学污染。高质量二维材料的直接生长通常需要高温,抑制添加剂的生长,或与不同的二维材料进行整体结合,近年来得到了广泛的研究,并有望在未来的集成电子学和光电子学中发挥关键作用。所使用的范德瓦尔斯积分技术,为将不同的二维材料甚至不同的晶体结构集成到异质结构中提供了一种可行的方法,也为探索具有新性能的新型人工材料提供了一个很有前途的平台。本文首次通过两步化学气相沉积的方法,成功地将二硒化锡纳米片与2H相的碲化钼薄膜结合成了p-n异质结。高分辨率透射电镜表征结果表明,多层二硒化锡纳米片垂直堆叠在具有高结晶度的2H相碲化钼薄膜上。材料的拉曼图谱也验证了SnSe_(2)/2H-MoTe_(2)异质结构的形成。

简化接触模型的月壤离散元数值分析

根据月壤其颗粒级配可归类于粉质砂土。针对真实月壤所处的环境(无水、低重力场、低气压等),将Perko等2001年提出的月壤颗粒间的范德华力植入离散元分析软件PFC2D中,模拟了刚性边界下加入该模型与未加该模型试样的双轴压缩试验,研究了颗粒间范德华力对试样的宏观力学特性与微观颗粒接触的影响。结果表明,颗粒间的范德华力对试样的抗剪强度、体应变以及颗粒平均配位数都有显著的影响。

蛋白质-核酸复合物氢键与范德华力作用位点分析

为了深入了解蛋白质-核酸相互作用模式,对复合物结构中氢键/范德华力作用力位点上的氨基酸和核苷酸的偏好性(即相对使用频率)进行了统计分析.结果表明:氢键/范德华力作用位点上对氨基酸的偏好性差异均极其显著,与蛋白质-核酸特异作用密切相关的残基侧链与核苷酸碱基间相互作用力位点对氨基酸类型的选择特异性更高;单链RNA分子与蛋白质发生相互作用时对氨基酸残基的偏好性和双链RNA分子差异不显著;氨基酸的极性大小及方向在决定其是否与核酸分子形成范德华力作用具有重要贡献,氨基酸侧链形成的空间位阻会阻碍其与核酸分子形成氢键作用;蛋白质-DNA复合物结构氢键/范德华力作用位点上残基间空间协同作用时对氨基酸类型的选择与蛋白质-RNA复合物结构间存在显著差异.

一种简单的微球姿态操作模型

分析了在操作环境中微球受力的情况 ,并结合 Hertz接触理论 ,建立了微球姿态操作的力学模型和数学模型 ;对模型求解 ,得出了对微球姿态操作所需的操作力和操作角之间的关系 ,分析了考虑范德华力和忽略范德华力时的不同 ,说明了范德华力在微球姿态操作中的重要影响 。

蛋白质-核酸复合物中氢键和范德华力作用位点偏好性分析

对蛋白质-核酸复合物结构中氢键/范德华力作用位点氨基酸与核苷酸的偏好性(即相对使用频率)进行了统计分析。发现:(1)在蛋白质-DNA复合物结构中,范德华力作用对与氢键数量相当;而在蛋白质-RNA复合物结构中,范德华力作用对数量要远多于氢键;(2)复合物结构中氢键和范德华力作用位点上对氨基酸的偏好性差异显著;(3)氨基酸的极性大小及方向在决定它是否与DNA/RNA分子形成氢键/范德华力相互作用时起到重要的作用。

考虑范德华力的微型活齿传动系统应力分析

提出了一种微型集成活齿传动系统,介绍了其工作原理,推导了微尺度下任一活齿在不同啮合位置的静力学方程,利用范德华势函数对活齿所受的范德华力进行求解,研究了考虑范德华力时啮合活齿所受波发生器、活齿架、中心轮三者作用力的变化规律;运用Hertz理论,对活齿与波发生器、活齿架、中心轮的接触应力进行了理论推导,并研究了微结构不同几何参数对活齿所受作用力及接触应力规律的影响。

GaN/VSe_(2)范德瓦耳斯异质结电接触特性及调控效应

降低金属-半导体界面的肖特基势垒并实现欧姆接触对于研发高性能肖特基场效应管非常重要.鉴于实验上已成功制备GaN及1T-VSe_(2)单层,本文理论构建GaN/1T-VSe_(2)异质结模型,并利用基于密度泛函理论的第一性原理研究了其稳定性、肖特基势垒特性及其调控效应.计算的形成焓及淬火分子动力学模拟表明构建的异质结是稳定的.研究表明:本征异质结为p型肖特基接触,同时发现施加拉伸或压缩应变,异质结始终保持p型肖特基接触不变,没有出现欧姆接触.而施加外电场则不同,具有明显的调控效应,较高的正向电场能使异质结从肖特基接触转变为欧姆接触,较高的反向电场能导致p型肖特基接触转变为n型肖特基接触.特别是实施化学掺杂,异质结较容易实现由肖特基接触到欧姆接触的转变,例如引入B原子能使GaN/1T-VSe_(2)异质结实现典型的欧姆接触,而C和F原子掺杂,能使GaN/1T-VSe_(2)异质结实现准欧姆接触.这些研究对该异质结的实际应用提供了理论参考,特别是对于研发新型高性能纳米场效应管具有重要意义.

High-throughput screening of phase-engineered atomically thin transition-metal dichalcogenides for van der Waals contacts at the Schottky–Mott limit

A main challenge for the development of two-dimensional devices based on atomically thin transition-metal dichalcogenides(TMDs)is the realization of metal–semiconductor junctions(MSJs)with low contact resistance and high charge transport capability.However,traditional metal–TMD junctions usually suffer from strong Fermi-level pinning(FLP)and chemical disorder at the interfaces,resulting in weak device performance and high energy consump-tion.By means of high-throughput first-principles calculations,we report an attractive solution via the formation of van der Waals(vdW)contacts between metallic and semiconducting TMDs.We apply a phase-engineering strategy to create a monolayer TMD database for achieving a wide range of work func-tions and band gaps,hence offering a large degree of freedom to construct TMD vdW MSJs with desired contact types.The Schottky barrier heights and contact types of 728 MSJs have been identified and they exhibit weak FLP(-0.62 to-0.90)as compared with the traditional metal–TMD junctions.We find that the interfacial interactions of the MSJs bring a delicate competition between the FLP strength and carrier tunneling efficiency,which can be uti-lized to screen high-performance MSJs.Based on a set of screening criteria,four potential TMD vdW MSJs(e.g.,NiTe_(2)/ZrSe_(2),NiTe_(2)/PdSe_(2),HfTe_(2)/PdTe_(2),TaSe_(2)/MoTe_(2))with Ohmic contact,weak FLP,and high carrier tunneling probability have been predicted.This work not only provides a fundamental understanding of contact properties of TMD vdW MSJs but also renders their huge potential for electronics and optoelectronics.

Van der Waals contacted WSe_(2) ambipolar transistor for in-sensor computing

Image sensors with an in-sensor computing architecture have shown great potential in meeting the energy-efficient requirements of emergent data-intensive applications,where images are processed within the photodiode arrays.It demands the composed photodiodes are reconfigurable,which are usually achieved by ambipolar two-dimensional(2D)semiconductors.To improve the ambipolar charges injection,here we report a top-gated field-effect transistor(FET)design that is of bottom van der Waals contact via transferring ambipolar 2D WSe_(2) onto Pd/Cr source/drain electrodes.The devices exhibit nearly negligible effective barrier heights for both holes and electrons based on thermionic emission mode,and show an almost balanced on/off ratio in the p-branch and n-branch.By replacing the top gate with two aligned semi-gates,the devices can effectively function as reconfigurable photodiodes.They can be switched between PIN and NIP configurations via controlling the two semi-gates,exhibiting good linearity in terms of short-circuit current(ISC)and incident light power density.The photodiode arrays are also demonstrated for in-sensor optoelectronic convolutional image processing,showing significant potential for in-sensor computing image processors.

石墨烯/C_(3)N范德瓦耳斯异质结的可调电子特性和界面接触

基于石墨烯的范德瓦耳斯异质结既可以调节石墨烯的电子特性,还可以保留原始单层材料的优越特性.利用第一性原理,本文系统地研究了石墨烯/C_(3)N范德瓦耳斯异质结的结构、电接触类型及光学性质.研究表明,平衡态下异质结中存在仅为0.039 eV的准p型欧姆接触.外加电场能调控异质结界面的接触类型,实现p型肖特基接触到欧姆接触的转变.垂直应变可以同时调控石墨烯和C_(3)N的投影能带,甚至为石墨烯打开了一个不可忽视的带隙(360 meV).外加电场和施加垂直应变这两种物理方法都能对异质结中石墨烯层的载流子掺杂类型和浓度进行有效调制.石墨烯层的载流子掺杂浓度的增大通过电场的调制更显著.与单层石墨烯和C_(3)N相比,两者构成的范德瓦耳斯异质结的光学响应范围和光吸收率均得到了提高.光谱中的主吸收峰高达10^(6) cm^(-1).这些结果不仅为基于石墨烯/C_(3)N范德瓦耳斯异质结器件的设计提供了有价值的理论指导,还为异质结在光电纳米器件和场效应晶体管器件应用提供了新的思路和设计.

织构化涂层表面的黏附接触力学行为研究

目的建立刚性微球与织构化涂层表面黏附接触数值分析模型,探究织构化涂层属性对微观接触副黏附力学性能的影响。方法基于Hamaker求和法以及Lennard–Jones势能定律,考虑织构高度对接触体间距离分布的影响以及涂层、织构材料属性对接触体间黏附力的影响,建立织构化涂层表面黏附接触力学模型并验证所提模型。基于所提模型,研究不同Tabor数下织构形貌、密度、高度以及涂层厚度对接触系统黏附力学性能的影响。结果在相同参数下,圆柱型织构黏附力最大,半椭球型织构次之,四棱锥型织构最小。织构密度从200μm^(‒2)增加到4000μm^(‒2)时,最大黏附力随着织构密度的增加而增加,圆柱型织构增加约5~6倍,四棱锥型织构增加约1.5倍。随着织构高度从1ε_(bs)增加至30εbs,最大黏附力减小,四棱锥型织构减小最多,约为原来的1%,圆柱形织构减小最少,约为原来的90%。涂层厚度能够影响黏附力的大小,但影响规律与织构化涂层的Tabor数及织构高度相关。随着涂层厚度从1ε_(bs)增加至16ε_(bs),大Tabor数时黏附力逐渐增加,小Tabor数时黏附力逐渐减小。Tabor数较大、织构高度较小时,黏附力–接近距离曲线会出现2次峰值。结论揭示了织构形貌、密度、高度及涂层厚度对接触系统黏附力学性能的影响,总结了黏附力与最大黏附力的变化规律,所得规律可为工程实践中的黏附微纳米结构设计与应用提供理论指导。

二维Janus结构的PdSSe/石墨烯接触特性的第一性原理探究

Janus结构由于其两侧的原子不同,存在一个内建电场.在本工作中,将具有Janus结构的六角PdSSe与石墨烯复合,构成范德瓦尔斯异质结构.通过基于密度泛函理论的第一性原理计算对其几何结构和电子结构进行了研究.计算中考虑了两种堆叠方式,即Se侧与石墨烯接触和S侧与石墨烯接触.当S侧与石墨烯接触时,体系具有更小的平衡间距和更大的电荷转移,结合能更低.S侧与石墨烯接触时形成了为n型欧姆接触;Se侧与石墨烯接触时形成了势垒极低的n型肖特基接触.最后,讨论了垂直应变对接触特性的影响.通过施加垂直应变,PdSSe/石墨烯的接触类型具有显著的可调性.

On-chip integrated GeSe_(2)/Si vdW heterojunction for ultravioletenhanced broadband photodetection,imaging,and secure optical communication

Broadband photodetection,spanning from ultraviolet(UV)to infrared(IR),is pivotal in diverse technological domains including astronomy,remote sensing,environmental monitoring,and medical diagnostics.However,current commercially available broadband photodetectors,predominately based on conventional narrow-bandgap semiconductors,exhibit limited sensitivity in the UV region.This limitation,stemming from the significant energy disparity between the semiconductor bandgap and UV photon,narrows their application scope.Herein,we report an innovative approach involving the in-situ van der Waals(vdW)integration of two-dimensional(2D)GeSe_(2)layers onto a Si substrate.This process yields a high-quality GeSe_(2)/Si vdW heterojunction device,which features a broad response range covering from UV to near-IR(NIR)with a greatly-enhanced sensitivity in the UV region.The device possesses high responsivities of 325 and 533.4 mA/W,large detectivities of 1.24×10^(13)and 2.57×10^(13)Jones,and fast response speeds of 20.6/82.1 and 17.7/81.0μs under 360 and 980 nm,respectively.Notably,the broadband image sensing and secure invisible optical communication capabilities of the GeSe_(2)/Si heterojunction device are demonstrated.Our work provides a viable approach for UV-enhanced broadband photodetection technology,opening up new possibilities and applications across various scientific and technological domains.