Selective linear etching of monolayer black phosphorus using electron beams

Point and line defects are of vital importance to the physical and chemical properties of certain two-dimensional(2D)materials.Although electron beams have been demonstrated to be capable of creating single-and multi-atom defects in 2D materials,the products are often random and difficult to predict without theoretical inputs.In this study,the thermal motion of atoms and electron incident angle were additionally considered to study the vacancy evolution in a black phosphorus(BP)monolayer by using an improved first-principles molecular dynamics method.The P atoms in monolayer BP tend to be struck away one by one under an electron beam within the displacement threshold energy range of 8.55-8.79 eV,which ultimately induces the formation of a zigzag-like chain vacancy.The chain vacancy is a thermodynamically metastable state and is difficult to obtain by conventional synthesis methods because the vacancy formation energy of 0.79 eV/edge atom is higher than the typical energy in monolayer BP.Covalent-like quasi-bonds and a charge density wave are formed along the chain vacancy,exhibiting rich electronic properties.This work proposes a theoretical protocol for simulating a complete elastic collision process of electron beams with 2D layers and will facilitate the establishment of detailed theoretical guidelines for experiments on 2D material etching using focused high-energy electron beams.

Atomic tracking of thermally-driven structural evolution in 2D crystals:Case of NbSe_(2)

Advanced atomic tracking techniques play a critical role in characterizing structural evolution,elucidating fundamental mechanisms of exotic phenomena and tailoring delicate properties.Thermally driven structural modulation in 2D crystals,such as the charge density wave(CDW),often leads to intriguing quantum properties,making them a valuable platform for exploring fundamental physics and potential device applications.However,despite their significance,experimental studies addressing atomic tracking of thermally-driven structural evolution in 2D crystals have been limited.Herein,we utilize high-accuracy variable-temperature atomic tracking measurements with scanning tunneling microscopy(STM)to directly observe a series of structural transitions in a model 2D crystal,namely NbSe_(2).With the atomic tracking technique,we confirm the existence of the universal thermally-driven CDW transition hysteresis between the heating and cooling cycles.This transition hysteresis,characterized by a constant temperature offset,represents a new phenomenon of structural evolution.Our findings provide a feasible method to track CDW transitions at the atomic scale in 2D crystals,significantly contributing to a better understanding and the potential modulation of these materials'functions in nanodevices.

Improving the band alignment at PtSe_(2)grain boundaries with selective adsorption of TCNQ

Grain boundaries in two-dimensional(2D)semiconductors generally induce distorted band alignment and interfacial charge,which impair their electronic properties for device applications.Here,we report the improvement of band alignment at the grain boundaries of PtSe_(2),a 2D semiconductor,with selective adsorption of a presentative organic acceptor,tetracyanoquinodimethane(TCNQ).TCNQ molecules show selective adsorption at the PtSe_(2)grain boundary with strong interfacial charge.The adsorption of TCNQ distinctly improves the band alignment at the PtSe_(2)grain boundaries.With the charge transfer between the grain boundary and TCNQ,the local charge is inhibited,and the band bending at the grain boundary is suppressed,as revealed by the scanning tunneling microscopy and spectroscopy(STM/S)results.Our finding provides an effective method for the advancement of the band alignment at the grain boundary by functional molecules,improving the electronic properties of 2D semiconductors for their future applications.

Few-layer Tellurium:one-dimensional-like layered elementary semiconductor with striking physical properties

Few-layer Tellurium, an elementary semiconductor, succeeds most of striking physical properties that black phosphorus(BP) offers and could be feasibly synthesized by simple solution-based methods. It is comprised of non-covalently bound parallel Te chains, among which covalent-like feature appears.This feature is, we believe, another demonstration of the previously found covalent-like quasi-bonding(CLQB) where wavefunction hybridization does occur. The strength of this inter-chain CLQB is comparable with that of intra-chain covalent bonding, leading to closed stability of several Te allotropes. It also introduces a tunable bandgap varying from nearly direct 0.31 eV(bulk) to indirect 1.17 eV(2L) and four(two) complex, highly anisotropic and layer-dependent hole(electron) pockets in the first Brillouin zone.It also exhibits an extraordinarily high hole mobility(~10~5 cm^2/Vs) and strong optical absorption along the non-covalently bound direction, nearly isotropic and layer-dependent optical properties, large ideal strength over 20%, better environmental stability than BP and unusual crossover of force constants for interlayer shear and breathing modes. All these results manifest that the few-layer Te is an extraordinary-high-mobility, high optical absorption, intrinsic-anisotropy, low-cost-fabrication, tunable bandgap, better environmental stability and nearly direct bandgap semiconductor. This ‘‘one-dimen sion-like" few-layer Te, together with other geometrically similar layered materials, may promote the emergence of a new family of layered materials.

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

Density functional and many-body perturbation theories calculations were carried out to investigate fundamental and optical bandgap, exciton binding energy and optical absorption property of normal and strain- and twist-engineered few-layer black phosphorus （BP）. We found that the fundamental bandgaps of few layer BP can be engineered by layer stacking and in-plane strain, with linear relationships to their associated exciton binding energies. The strain-dependent optical absorption behaviors are also anisotropic that the position of the first absorption peak monotonically blue-shifts as the strain applies to either direction for incident light polarized along the armchair direction, but this is not the case for that along the zigzag direction. Given those striking properties, we proposed two prototype devices for building potentially more balanced light absorbers and light filter passes, which promotes further applications and investigations of BP in nanoelectronics and optoelectronics.

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.

Shallowing interfacial carrier trap in transition metal dichalcogenide heterostructures with interlayer hybridization

With the unique properties,layered transition metal dichalcogenide(TMD)and its heterostructures exhibit great potential for applications in electronics.The electrical performance,e.g.,contact barrier and resistance to electrodes,of TMD heterostructure devices can be significantly tailored by employing the functional layers,called interlayer engineering.At the interface between different TMD layers,the dangling-bond states normally exist and act as traps against charge carrier flow.In this study,we propose a technique to suppress such carrier trap that uses enhanced interlayer hybridization to saturate dangling-bond states,as demonstrated in a strongly interlayer-coupled monolayer-bilayer PtSe2 heterostructure.The hybridization between the unsaturated states and the interlayer electronic states of PtSe2 significantly reduces the depth of carrier traps at the interface,as corroborated by our scanning tunnelling spectroscopic measurements and density functional theory calculations.The suppressed interfacial trap demonstrates that interlayer saturation may offer an efficient way to relay the charge flow at the interface of TMD heterostructures.Thus,this technique provides an effective way for optimizing the interface contact,the crucial issue exists in two-dimensional electronic community.

Quasi one-dimensional van der Waals gold selenide with strong interchain interaction and giant magnetoresistance

The atomic structure of quasi one-dimensional(1D) van der Waals materials can be regarded as the stacking of atomic chains to form thin flakes or nanoribbons, which substantially differentiates them from typical two-dimensional(2D) layered materials and 1D nanotube/nanowire array. Here we present our studies on quasi 1D gold selenide(AuSe) that possesses highly anisotropic crystal structure, excellent electrical conductivity, giant magnetoresistance, and unusual reentrant metallic behavior. The low inplane symmetry of AuSe gives rise to its high anisotropy of vibrational behavior. In contrast, quasi 1D AuSe exhibits high in-plane electrical conductivity along the directions of both atomic chains and perpendicular one, which can be understood as a result of strong interchain interaction. We found that AuSe exhibits a near quadratic nonsaturating giant magnetoresistance of 1841% with the magnetic field perpendicular to its in-plane. We also observe unusual reentrant metallic behavior, which is caused by the carrier mismatch in the multiband transport. Our works help to establish fundamental understandings on quasi 1D van der Waals semimetallic AuSe and identify it as a new candidate for exploring giant magnetoresistance and compensated semimetals.