Local electrical charac terization of t wo-dimensional mat erials with functional atomic force microscopy

Research about two-dimensional (2D) materials is growing exponentially across various scientific and engineering disciplines due to the wealth of unusual physical phenomena that occur when charge transport is confined to a plane. The applications of 2D materials are highly affected by the electrical properties of these mat erials, including curren t dist ribution, surface pot ential, dielectric response, conductivity, perm计tivity, and piezoelectric response. Hence, it is very crucial to characterize these properties at the nanoscale. The Atomic Force Microscopy (AFM)-based techniques are powerful tools that can simultaneously characterize morphology and electrical properties of 2D materials with high spatial resolution, thus being more and more extensively used in this research field. Here, the principles of these AFM techniques are reviewed in detail. After that, their representative applications are further demonstrated in the local characterization of various 2D materials? elcctrical properties.

Negative differential resistance and quantum oscillations in FeSb_2 with embedded antimony

We present a systematical study on single crystalline FeSb2 using electrical transport and magnetic torque measurements at low temperatures. Nonlinear magnetic field dependence of Hall resistivity demonstrates a multi-carrier transport instinct of the electronic transport. Current-controlled negative differential resistance(CC-NDR) observed in currentvoltage characteristics below ～ 7 K is closely associated with the intrinsic transition ～ 5 K of FeSb2, which is, however,mediated by extrinsic current-induced Joule heating effect. The antimony crystallized in a preferred orientation within the FeSb2 lattice in the high-temperature synthesis process leaves its fingerprint in the de Haas-Van Alphen(dHvA) oscillations, and results in the regular angular dependence of the oscillating frequencies. Nevertheless, possible existence of intrinsic non-trivial states cannot be completely ruled out. Our findings call for further theoretical and experimental studies to explore novel physics on flux-free grown FeSb_2 crystals.

Magneto-Transport Properties of Insulating Bulk States in Bi(111) Films

Magneto-transport properties of insulating bulk states in Bi（111） films are systematically investigated under the parallel field （BⅡ）. We find that the magnetotransport of the Bll field is a more powerful tool to distinguish the bulk states and the surface states. A large magnetoresistance （MR） up to 20% in the BⅡ field is induced by the insulating bulk states for the suppression of the backward scattering. With the increasing thickness, a positive MR（BⅡ） from magnetic induced boundary scattering appears in the semimetal films. As the thickness is reduced to 1Ohm, the positive MR（BⅡ） is induced by weak anti-localization from the surface states.

Geometric stability and electronic structure of infinite and finite phosphorus atomic chains

One-dimensional mono- or few-atomic chains were successfully fabricated in a variety of two-dimensional materials, like graphene, BN, and transition metal dichalcogenides, which exhibit striking transport and mechanical properties. How- ever, atomic chains of black phosphorus （BP）, an emerging electronic and optoelectronic material, is yet to be investigated. Here, we comprehensively considered the geometry stability of six categories of infinite BP atomic chains, transitions among them, and their electronic structures. These categories include mono- and dual-atomic linear, armchair, and zigzag chains. Each zigzag chain was found to be the most stable in each category with the same chain width. The mono-atomic zigzag chain was predicted as a Dirac semi-metal. In addition, we proposed prototype structures of suspended and sup- ported finite atomic chains. It was found that the zigzag chain is, again, the most stable form and could be transferred from mono-atomic armchair chains. An orientation dependence was revealed for supported armchair chains that they prefer an angle of roughly 35°-37° perpendicular to the BP edge, corresponding to the [110] direction of the substrate BP sheet. These results may promote successive research on mono- or few-atomic chains of BP and other two-dimensional materials for unveiling their unexplored physical properties.

Poisoning of MoO_3 Precursor on Monolayer MoS_2 Nanosheets Growth by Tellurium-Assisted Chemical Vapor Deposition

We obtain molybdenum disulfide （MoS2） nanosheets （NSs） with edge sizes of 18μm by direct sulfuration of MoO3 powder spread on the SiO2/Si substrates. However, the undesirable MoO3 nanoparticles （NPs） left on the surface of MoS22 NSs poison the MoO3 precursor. Introducing Te vapors to react with MoS2 to form low melting point intermediate MoSxTe2-x, the evaporations of MoO3 precursor recover and MoO3 NPs disappear. Thus Te vapor is effective to suppress poisoning of the MoO3 precursor. Selecting the appropriate amount of Te vapor, we fabricate monolayer MoS22 NSs up to 70μm in edge length. This finding can be significant to understand the role of Te in the Te-assisted chemical vapor deposition growth process of layered chalcogenide materials.

Multi-carrier transport in ZrTe_5 film

Single-layered zirconium pentatelluride （ZrTes） has been predicted to be a large-gap two-dimensional （2D） topolog- ical insulator, which has attracted particular attention in topological phase transitions and potential device applications. Herein, we investigated the transport properties in ZrTe5 films as a function of thickness, ranging from a few nm to several hundred nm. We determined that the temperature of the resistivity anomaly peak （Tp） tends to increase as the thickness decreases. Moreover, at a critical thickness of ~ 40 rim, the dominating carriers in the films change from n-type to p-type. A comprehensive investigation of Shubnikov-de Hass （SdH） oscillations and Hall resistance at variable temperatures revealed a multi-carrier transport tendency in the thin films. We determined the carrier densities and mobilities of two majority car- riers using the simplified two-carrier model. The electron carriers can be attributed to the Dirac band with a non-trivial Berry phase ~, while the hole carriers may originate from surface chemical reaction or unintentional doping during the microfabrication process. It is necessary to encapsulate the ZrTe5 film in an inert or vacuum environment to potentially achieve a substantial improvement in device quality.

Correlation of interfacial bonding mechanism and equilibrium conductance of molecular junctions

We report theoretical investigations oil tile role of interracial bonding mechanism and its resulting structures to quantum transport in molecular wires. Two bonding mechanisms for the Au-S bond in an Au（111） / 1,4-benzenedithiol（BDT）/Au（111） junction were identified by ab initio calculation, con- firmed by a recent experiment, which, we showed, critically control charge conduction. It was found, for Au/BDT/Aujunctions, the hydrogen atom, bound by a dative bond to the Sulfur, is energetically non-dissociativeafter the interface formation. The calculated conductance and junction breakdown forces of H-non-dissociative Au/BDT/Au devices are consistent with the experimental values, while the H-dissociated devices, with the interface governed by typical covalent bonding, give conduc- tance more than an order of magnitude larger. By examining the scattering states that traverse the junctions, we have revealed that mechanical and electric properties of a junction have strong corre- lation with the bonding configuration. This work clearly demonstrates that the interracial details. rather than previously believed ninny-body effects, is of vital importance for correctly predicting equilibrium conductance of molecular junctions; and manifests that the interfaeial contact must be carefully understood for investigating quantum transport properties of molecular nanoelectronics.

Strain- and twist-engineered optical absorption of few-layer black phosphorus

功能的密度和许多身体不安理论计算被执行调查基本、光的 bandgap，激子绑定精力和正常和设计紧张、设计扭曲的很少层黑人磷(BP ) 的光吸收性质。我们发现很少层 BP 的基本 bandgaps 能被层叠和在里面飞机紧张设计，到他们的联系激子绑定精力的线性关系地。紧张依赖者光吸收行为也是各向异性的象紧张 monotonically 蓝移动的第一座吸收山峰的位置为事件光适用于任何一个方向，这极化了扶手椅方向，而是这沿着之字形方向为那不是事实。给那些惹人注目的性质，我们为更潜在地造建议了二台原型设备平衡轻吸收器和灯过滤通行证，它在 nanoelectronics 和光电子支持 BP 的进一步的应用和调查。

Deriving phosphorus atomic chains from few-layer black phosphorus

Phosphorus atomic chains, the narrowest nanostructures of black phosphorus （BP）, are highly relevant to the in-depth development of BP-based one-dimensional （1D） nano-electronics components. In this study, we report a top-down route for the preparation of phosphorus atomic chains via electron beam sculpturing inside a transmission electron microscope （TEM）. The growth and dynamics （i.e., rupture and edge migration） of 1D phosphorus chains are experimentally captured for the first time. Furthermore, the dynamic behavior and associated energetics of the as-formed phosphorus chains are further investigated by density functional theory （DFT） calculations. It is hoped that these 1D BP structures will serve as a novel platform and inspire further exploration of the versatile properties of BP.

Quantum description of transport phenomena： Recent progress

Over 100 years ago, atoms were believed to be the smallest indivisible particles and no one knew what was inside an electric current. For instance, what was cathode ray made of had puzzled many. In 1897, J. J. Thomson, based on his refined and newly designed experiments, discovered that cathode rays were made of electrons! Fifty years later, William Shockley, John Bardeen, and Walter Brattain invented a device called transistor that operated on the flow of electrons. A revolution of information technology started.

Unusually high electron density in an intermolecular non-bonding region:Role of metal substrate

It has been demonstrated that intermolecular interaction,crucial in a plenty of chemical and physical processes,may vary in the presence of metal surface.However,such modification is yet to be quantitatively revealed.Here,we present a systematical density functional theory study on adsorbed bis（para-pyridyl）acetylene（BPPA） tetramer on Au（111） surface.We observed unusually high electron density between two head-to-head N atoms,an intermolecular ＂non-bonded＂ region,in adsorbed BPPA tetramer.This exceptional electron density originates from the wavefunction hybridization of the two compressed N lone-electron-pair states of two BPPA,as squeezed by a newly revealed N-Au-N threecenter bonding.This bond,together with the minor contribution from N...H-C intermolecular hydrogen bonding,shortens the N-N distance from over 4 A to 3.30 A and offers an attractive lateral interacting energy of 0.60 eV,effectively to a surface-confined in-plane pressure.The overlapped non-bonding vvavefunction hybridization arising from the effective pressure induced by the N-Au-N three-center bonding,as not been fully recognized in earlier studies,was manifested in non-contact Atomic Force Microscopy.