Zigzag Direction Research Articles

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Strain engineering on electronic structure, effective mass and charge carrier mobility in monolayer YBr3

In recent years, two-dimensional materials have significant prospects for applications in nanoelectronic devices due to their unique physical properties. In this paper, the strain effect on the electronic structure, effective mass, and charge carrier mobility of monolayer yttrium bromide (YBr3) is systematically investigated using first-principles calculation based on density functional theory. It is found that the monolayer YBr3 undergoes energy band gap reduction under the increasing compressive strain. The effective mass and charge carrier mobility can be effectively tuned by the applied compressive strain. Under the uniaxial compressive strain along the zigzag direction, the hole effective mass in the zigzag direction (m ao1_h) can decrease from 1.64 m 0 to 0.45 m 0. In addition, when the uniaxial compressive strain is applied, the electron and hole mobility can up to ∼103 cm2 V−1 s−1. The present investigations emphasize that monolayer YBr3 is expected to be a candidate material for the preparation of new high-performance nanoelectronic devices by strain engineering.

Near-Isotropic Local Attosecond Charge Transfer within the Anisotropic Puckered Layers of Black Phosphorus.

Black phosphorus possesses useful two-dimensional (2D) characteristics of van der Waals coupled materials but additionally features an in-plane anisotropic puckered layer structure that deviates from common 2D materials. Three distinct directions exist within the lattice of black phosphorus: the in-plane armchair and zigzag directions and the out-of-plane direction, with each distinct phosphorus 3p partial density of states. This structural anisotropy is imprinted onto various collective long-range properties, while the extent to which local electronic processes are governed by this directionality is unclear. At the P L1 edge, the directional selectivity of the core-hole clock method was used to probe the local charge transfer dynamics of electrons excited into the 3p-derived conduction band on an attosecond time scale. Here we show that the surprisingly small anisotropy of 3p electron transfer times reflects the similarly small differences in the 3p-derived unoccupied density of states caused by the underlying phosphorus bonding angles within the puckered layers.

Azimuthal rotation induced by the Marangoni force makes small Leidenfrost droplets move in random zigzag directions

Azimuthal rotation induced by the Marangoni force makes small Leidenfrost droplets move in random zigzag directions

Intralayer Negative Poisson's Ratio in 2D Black Arsenic by Strain Engineering

Negative Poisson's ratio as the anomalous characteristic generally exists in artificial architectures, such as re‐entrant and honeycomb structures. The structures with negative Poisson's ratio have attracted intensive attention due to their unique auxetic effect and many promising applications in shear‐resistant and energy absorption fields. However, experimental observation of negative Poisson's ratio in natural materials barely happens, although various 2D layered materials are predicted in theory. Herein, the anisotropic Raman response and the intrinsic intralayer negative Poisson's ratio of 2D natural black arsenic (b‐As) via strain engineering strategy are reported. The results are evident by the detailed Raman spectrum of b‐As under uniaxial strain together with density functional theory calculations. It is found that b‐As is softer along the armchair than zigzag direction. The anisotropic mechanical features and van der Waals interactions play essential roles in strain‐dependent Raman shifts and negative Poisson's ratio in the natural b‐As along zigzag direction. This work may shed a light on the mechanical properties and potential applications of 2D puckered materials.

C-doped zigzag blue phosphorene nanoribbons for GMR, rectification behaviour and NDR based devices

ABSTRACT Based on density functional theory (DFT) and nonequilibrium Green's function (NEGF) framework, to study the monolayer (ML) blue phosphorene (BlueP) doped with C along zigzag direction, such as the structural stability, and electronic and transport properties. Interestingly, we found that doping engineering strategies can effectively transform monolayer ML ZBlueP doped with C into a spintronic nanodevice with a few unique transport properties. Firstly, our model showed a magnificently giant magnetoresistance (GMR) up to 10 4 under low bias. Secondly, a rectifier with a large rectification ratio (RR) of 10 5 was observed. Lastly, negative differential resistance with a peak-to-valley current ratio (PVCR) of 10 4 was observed. Our research shows a promising route towards the development of high-performance ML ZBlueP spintronics technology by doping with C atom. Our results indicate that our models have immense potential for application in nanoscale spintronic devices.

Investigation on elastic, optical and thermoelectric properties of 2D MgX (X= O, S, Se, Te) materials under DFT framework

The first principles study of two-dimensional magnesium (Mg) chalcogenides (X = O, S, Se, Te) in square lattice (s-MgX) and hexagonal phases (h-MgX) is investigated for the first time. Except for h-MgSe and h-MgTe, all the square lattice and hexagonal structures in MgX compounds are dynamically stable, agreeing to phonon dispersion calculations. The electronic band structure and projected density of states of s- and h-MgX materials provided insight into the essence of electronic properties of these compounds. All s- and h-MgX compounds are found to be indirect wide band gap semiconductors, according to calculations using PBE and HSE06 functionals. The effective mass, mobility and relaxation time of electrons and holes carriers from the band structure of s- and h-MgX are examined to acquire a better intuition into these materials. Along the zigzag direction, h-MgTe has a largest mobility as well as relaxation time of 78104.92 cm2 V−1s−1 and 64385.56 fs, respectively in the entire MgX series. Additionally, we examined their mechanical stability through elastic characteristics, and the derived elastic parameters and polar graphs of Young's modulus and Poisson's ratio verifying their mechanical stability. In application of parallel and perpendicular field polarizations, the optical properties of s- and h-MgX are examined. The thermoelectric properties of the complete s- and h-MgX series are examined for the temperature range from 50 K to 800 K. The results of the present study reveal that s-MgO and h-MgS are the better thermoelectric materials in the considered series. Finally, since these compounds are mostly UV-active, they may find useful applications in UV-protectant and UV-photodetectors materials.

Molecular dynamics study on the uniaxial tensile behavior of mono-layer MoTe2 film defected by mirror twin boundary

High-density mirror twin boundaries, which exhibit fascinating electronic properties, are revealed in low-dimensional MoTe2 films in recent years. Mechanical behavior plays a key role in the application of MoTe2 with these novel defects. In this paper, a mono-layer MoTe2 crystal was constructed, and the 4|4P mirror twin boundary, which is the most easily observed in the experiment, was embedded in the center of model. The molecular dynamics method combined with Stillinger-Weber potential function was used to simulate the uniaxial tension, the effects of this defect on the fracture stress of monolayer MoTe2 were studied. Results show that the 4|4P mirror twin boundary has little effect on the Young's modulus of MoTe2, but reduces the fracture stress. Larger size of the mirror twin boundary, lower temperature and the armchair loading direction can make the decrease of fracture stress more significant, however the size effect is much weaker when the film is stretched along the zigzag direction. Turning the MoTe2 model with a specified large mirror twin boundary into ternary MoTe2(1-x)Se2x model by replacing some Te atoms with Se impurities randomly and uniformly, then setting the condition of temperature and Se concentration into 12 groups for each loading direction, the ratio of the fracture stress derived from the defected model to the value obtained from ideal model almost remains unchanged. Finally, this phenomena are analyzed based on brittle fracture theory.

Visualized and Nondestructive Quality Identification of Two-Dimensional MoS2 Based on Principal Component Analysis.

To date, the common quality characterizations for MoS2 are inefficient or cause irreversible damage to the samples, which have limited scalability and low throughput. Here, we propose a visualized and nondestructive approach to evaluate the quality of MoS2 based on the PCA machine learning method. Through PCA processing of PL mapping, the CVD grown MoS2 with different edge defect densities can be well distinguished. Furthermore, six twin GBs along the sulfur zigzag direction of the six pointed MoS2 stars are also successfully identified. To verify the correctness of the identification results, we measured the lifetime mapping and thermal expansion coefficient of the synthesized MoS2 samples. It is found that the high quality MoS2 samples have a shorter carrier lifetime (∼0.291 ns) and lower thermal expansion coefficient (∼2.03 × 10-5K-1). Therefore, our work offers a new approach to evaluate the quality of MoS2 to drive their practical application.

WS2 and WSSe bilayer with excellent carrier mobility and power conversion efficiency

The WS2/WSSe and WSSe/WS2 bilayers are constructed using single-layer WS2 and WSSe with a lattice mismatch rate of only 1.85 %, and their structural properties, carrier mobility, electrical properties, and photocatalytic performance are performed using density functional theory. The carrier mobility of WS2/WSSe and WSSe/WS2 bilayers is better than that of single-layer WS2. And the conductivity of the WSSe/WS2 bilayer in the Zigzag direction is superior to that in the Armchair direction. The results of band structure using the HSE06 method demonstrate that the most stable WS2/WSSe and WSSe/WS2 bilayers are type-II band alignment. WS2/WSSe and WSSe/WS2 bilayers can undergo redox reactions at pH = 0 and pH = 7 respectively. And they all emerge a built-in electric field to cause photo-generated carriers to recombine between the layers to upgrade the photocatalytic performance of the bilayer, which is a typical S-scheme heterojunction. And the power conversion efficiency (PCE) of the WS2/WSSe and WSSe/WS2 bilayers are 15.5 % and 15.7 % respectively, so the utilization rate of sunlight is relatively high. In general, WS2/WSSe and WSSe/WS2 bilayers are ideal materials in the field of photocatalytic water splitting.

High-pressure response of vibrational properties of b-As x P1–x : in situ Raman studies

The structural evolution of black arsenic-phosphorous (b-As x P1–x ) alloys with varying arsenic concentrations was investigated under hydrostatic pressure using in situ Raman spectroscopy. High-pressure experiments were conducted using a diamond anvil cell, which revealed pressure-induced shifts in vibrational modes associated with P–P bonds (A 1 g , A 2 g , ), As–As bonds (A 1 g , A 2 g , ), and As–P bonds in b-As x P1–x alloys. Two distinct pressure regimes were observed. In the first regime (region I), all vibrational modes exhibited a monotonic upshift, indicating phonon hardening due to hydrostatic pressure. In the second regime (region II), As0.4P0.6 and As0.6P0.4 alloys displayed a linear blueshift (or negligible change in some modes) at a reduced rate, suggesting local structural reorganization with less compression on the bonds. Notably, the alloy with the highest As concentration, As0.8P0.2, exhibited anomalous behavior in the second pressure regime, with a downward shift observed in all As–As and As–P Raman modes (and some P–P modes). Interestingly, the emergence of new peaks corresponding to the Eg mode and A 1g mode of the gray-As phase was observed in this pressure range, indicating compressive strain-induced structural changes. The anomalous change in region II confirms the formation of a new local structure, characterized by elongation of the P–P, As–As, and As–P bonds along the zigzag direction within the b-As x P1–x phase, possibly near the grain boundary. Additionally, a gray-As phase undergoes compressive structural changes. This study underscores the significance of pressure in inducing structural transformations and exploring novel phases in two-dimensional materials, including b-As x P1–x alloys.

Molecular modelling of graphene nanoribbons on the effect of porosity and oxidation on the mechanical and thermal properties

Graphene is considered as the most promising nanomaterial of the recent decades given the huge amount of studies that have been performed to characterize its outstanding properties and in searching of novel applications. Following this tendency, this study covers the modelling of graphene nanoribbons (GNRs) with the aim of analyzing the effect of porosity and oxidation on the tensile mechanical properties and in-plane thermal conductivity through molecular dynamics (MD). Using quasi-static simulations the mechanical properties were evaluated in first place. A ‘hardening’ mechanism was observed for GNRs at porosities below 1%, i.e. perfect or near-perfect GNRs, by which the GNRs could withstand higher loading levels. This hardening effect was manifested in the carbon network by the generation of dislocation lines formed by pentagon-heptagon pairs (5–7 defects), which acted as a stress reliever. The failure of GNRs was produced as a tearing mechanism with cracks growing along the armchair or zigzag directions. The porosity affected all the analysed tensile mechanical properties (i.e., Young’s modulus, Poisson’s ratio, tensile strength and deformation at break), but with different tendency in the fracture properties due to the presence or absence of hardening behaviour in the GNRs. Nevertheless, the oxidation affected only the tensile modulus and Poisson’s ratio but not to the tensile strength and deformation at break. The thermal conductivity of the GNRs was affected either by the porosity and oxidation. Pores and oxidation groups acted as phonon scatterers since they disrupted the carbon network by the generation of vacancies or out-of-plane carbons, respectively, which decreased the phonon mean free path and thus the thermal conductivity. In conclusion, the porosity and oxidation of GNRs greatly determine the tensile mechanical properties and in-plane thermal conductivity of such materials and must be considered when tuning the synthetic pathways.Graphical abstractThe effect of porosity and oxidation on the tensile mechanical and thermal conductivity properties of graphene nanoribbons are evaluated through molecular dynamics simulations.

N‐Type Doping and Passivation of Black Phosphorus Using Pentacene for Infrared Polarization‐Sensitive Photodetection and Imaging

AbstractFew‐layer black phosphorus (FL‐BP) shows great potential in infrared polarization‐sensitive photodetection due to its narrow bandgap and strong in‐plane anisotropy. However, FL‐BP is easy to be oxidized and degraded, which severely restricts its applications. Herein, a simple and scalable in situ interfacial passivation method based on pentacene film is reported to enhance the ambient stability of FL‐BP as well as achieve n‐type doping for improved polarization‐sensitive photoimaging. High crystal quality FL‐BP flakes with intrinsic anisotropy are prepared by electrochemical exfoliation. The morphology and Raman spectra of FL‐BP/pentacene exhibit excellent stability 28 days after passivation. In addition, the hole mobility is significantly enhanced and dark current is substantially suppressed due to the interfacial passivation and n‐type doping effect. The FL‐BP/Pentacene based photodetector shows a broadband response from visible to near‐infrared with good stability. The photoresponsivity under 915 nm illumination is calculated as 2.2 mA W−1 with an anisotropy ratio of 1.29. Furthermore, the polarization imaging under the irradiation at zigzag and armchair directions directly reveals the strong infrared polarization‐sensitive imaging capability of FL‐BP/pentacene. This work also opens up new avenues to enhance the photoresponse and stability of various optoelectronics based on similarly unstable 2D materials.

Temperature, strain rate, and point vacancy dependent anisotropic mechanical behaviors of titanium carbide (Ti3C2) MXene: A molecular dynamics study

MXenes, one of the recognizable examples of modern 2D materials, are sparking tremendous research interest because of their distinctive structural, electrical, and electrochemical properties. This study utilized classical molecular dynamics simulation to investigate temperature, strain rate, and point vacancy-dependent mechanical behaviors for both armchair and zigzag orientations of the Ti3C2 MXene using Tersoff style bond order potential. The elastic modulus of ∼155.76 GPa and fracture strain of ∼0.263 were obtained along the zigzag direction at 300 K temperature, 5.58 % and 28.29 % greater than those obtained along the armchair alignment. However, a superior tensile strength of ∼34.08 GPa was observed along the armchair orientation as opposed to ∼29.08 GPa in the zigzag direction at 300 K temperature. As the temperature rises, it was observed that fracture strength and strain both exhibit a decreasing tendency, whereas increments in strain rates cause an increasing trend. However, we observed that temperature and strain rates barely affect the elastic modulus, which agrees with the literature data. Radial distribution functions were calculated for all pair interactions to make the temperature effect more comprehensible. It was demonstrated that C, Ti, and random point vacancies have significant adverse effects on elastic modulus, fracture strength, and failure strain because of their powerful symmetry-breaking effect. With increasing concentrations of any point vacancies, rapidly declining elastic modulus, fracture strength, and failure strain were perceived. Titanium vacancies were observed to have a more significant influence on mechanical behavior, even though carbon vacancies were more likely to develop with less formation energy. The outcomes of this work deliver a clear overview of the mechanical behavior of Ti3C2 in various aspects of the environment and will shed light on the potential implementation in electrochemical sensing or energy devices.

High-Pressure Synthesis of Single-Crystalline SnS Nanoribbons.

Two-dimensional tin monosulfide (SnS) is attractive for the development of electronic and optoelectronic devices with anisotropic characteristics. However, its shape-controlled synthesis with an atomic thickness and high quality remains challenging. Here, we show that highly crystalline SnS nanoribbons can be produced via high-pressure (0.5 GPa) and thermal treatment (400 °C). These SnS nanoribbons have a length of several tens of micrometers and a thickness down to 5.8 nm, giving an average aspect ratio of ∼30.6. The crystal orientation along the zigzag direction and the in-plane structural anisotropy of the SnS nanoribbons are identified by transmission electron microscopy and polarized Raman spectroscopy, respectively. An ionic liquid-gated field-effect transistor fabricated using the SnS nanoribbon exhibits an on/off current ratio of >103 and a field-effect mobility of ∼0.7 cm2 V-1 s-1. This work provides a unique way to achieve one-dimensional growth of SnS.

Modulating electronic structure of two-dimensional AsP via external strain and electric field

In this paper, the density functional theory calculations show that the electronic structure of monolayer honeycomb AsP can be effectively tailored by both applied strain and electric field. By applying external strains, the band gap of monolayer honeycomb AsP can be monotonous reduced by bigger strains. More interestingly, by applying 10% tensile strains along biaxial and uniaxial zigzag directions, monolayer honeycomb AsP becomes a direct band gap semiconductor with the band gaps of 1.28 and 1.73 eV, respectively, which are in the visible light region and suggest solar energy conversion applications. This work provides a feasible way of making monolayer β-AsP a direct semiconductor and a good candidate for photocatalysts.

Electronic and Optical Properties of Finite Gallium Sulfide Nano Ribbons: A First-Principles Study

The electronic and optical properties of finite GaS nanoribbons are investigated using density functional theory calculations. The effect of size, edge termination, and chemical modification by doping and edge passivation are taken into account. The dynamical stability is confirmed by the positive vibration frequency from infrared spectra; further, the positive binding energies ensure the stable formation of the considered nanoribbons. Accurate control of the energy gap has been achieved. For instance, in armchair nanoribbons, energy gaps ranging from ~ 1 to 4 eV were obtained in varying sizes. Moreover, the energy gap can be increased by up to 5.98 eV through edge passivation with F-atoms or decreased to 0.98 eV through doping with Si-atoms. The density of states shows that the occupied molecular orbitals are dominated by S-atoms orbitals, while unoccupied ones are mostly contributed to by Ga orbitals. Thus, S-atoms will be the electron donor sites, and Ga-atoms will be the electron acceptors in the interactions that the nanoribbons might undergo. The nature of electron–hole interactions in the excited states was investigated using various indices, such as electron–hole overlapping, charge–transfer length, and hole–electron Coulomb attraction energy. The UV-Vis absorption spectra reveal a redshift by increasing the size in the armchair or the zigzag directions. Chemical functionalization shows a significant influence on the absorption spectra, where a redshift or blueshift can be achieved depending on the dopant or the attached element.

First-principles study of two-dimensional MoP2Si2Z2 (Z=N, P, As) monolayer

Two-dimensional MoP2Si2Z2 (Z = N, P, As) monolayers were designed and studied by first-principles calculations. The layers are semiconductors, of which the MoP2Si2N2 and MoP2Si2P2 monolayers have a direct bandgap of 0.014 (0.522) and 0.673 (1.151) eV, respectively, at PBE (HSE06) level. The MoP2Si2As2 monolayer has almost the same indirect and direct band gaps (0.509/0.903 eV at the PBE/HSE06 level). The calculated optical absorptions (less than 2 × 104 cm−1 in the visible region) show that the MoP2Si2P2 and MoP2Si2As2 monolayers are well transparent materials, while the MoP2Si2N2 monolayer has an absorption peak in the infrared region. The layers have good mechanical properties and high carrier mobility. The Young's moduli (E2D) are 314, 206 and 187 N m−1 for MoP2Si2N2, MoP2Si2P2 and MoP2Si2As2 respectively. The carrier mobility of MoP2Si2N2 in zigzag direction (μzig) reaches 2855.3 (electron) and 1280.2 (hole) cm2V−1s−1, respectively. Transitions between metal and semiconductor, and between direct and indirect semiconductors, can be realized for monolayers by in-plane biaxial strain. The properties resulting from the first-principles calculation motivate to synthetize of these monolayers and use them in microelectronics.

Molecular dynamics simulation study on the friction performance of WS2 coated copper during the scratching process

Tungsten disulfide (WS2) has been assumed to have the excellent tribological property that has the potential to be used in the field of lubricating engineering. In this study, WS2 nanosheet was coated onto the surface of the copper to improve the wear resistance. The tribological properties of WS2 coated copper system were investigated by molecular dynamics (MD) simulation to understand the wear mechanisms from the nanoscale. The mechanical response of the bare and the WS2 coated copper in the indentation process were compared firstly. Then, scratching simulation tests were performed at the various indentation depths from 1.2 Å to 11.2 Å to understand the tribological mechanism from the elastic stage to the fracture stage. The effect of the nanoindentation depth and anisotropy orientation of WS2 on friction behavior were also analyzed during the scratching process. Results show that the load-bearing capacity and wear resistance of the copper substrate were improved by the WS2 coated layer before the fracture during the nanoindentation process, while the protection of the layer failed after rupture that the out-of-plane deformation induced the increase of the friction force. Additionally, the difference of the friction force could be induced by the lattice spacing of WS2 and the energy dissipation during ultralow friction stage. The crack propagation along the Zigzag direction dominated the fracture behavior in both Armchair and Zigzag orientation. This study provides important insights into the friction mechanism of the WS2 when conducting nanoscratching process.

Sub-5 nm Gate-Length Monolayer Selenene Transistors.

Two-dimensional (2D) semiconductors are being considered as alternative channel materials as silicon-based field-effect transistors (FETs) have reached their scaling limits. Recently, air-stable 2D selenium nanosheet FETs with a gate length of 5 µm were experimentally produced. In this study, we used an ab initio quantum transport approach to simulate sub-5 nm gate-length double-gate monolayer (ML) selenene FETs. When considering negative-capacitance technology and underlap, we found that 3 nm gate-length p-type ML selenene FETs can meet the 2013 ITRS standards for high-performance applications along the armchair and zigzag directions in the 2028 horizon. Therefore, ML selenene has the potential to be a channel material that can scale Moore's law down to a gate length of 3 nm.