Anisotropic crystal orientations dependent mechanical properties and fracture mechanisms in zinc blende ZnTe nanowires.

The orientations of crystal growth significantly affect the operating characteristics of elastic and inelastic deformation in semiconductor nanowires (NWs). This work uses molecular dynamics simulation to extensively investigate the orientation-dependent mechanical properties and fracture mechanisms of zinc blende ZnTe NWs. Three different crystal orientations, including [100], [110], and [111], coupled with temperatures (100 to 600 K) on the fracture stress and elastic modulus, are thoroughly studied. In comparison to the [110] and [100] orientations, the [111]-oriented ZnTe NW exhibits a high fracture stress. The percentage decrease in fracture strength exhibits a pronounced variation with increasing temperature, with the highest magnitude observed in the [100] direction and the lowest magnitude observed in the [110] direction. The elastic modulus dropped by the largest percentage in the [111] direction as compared to the [100] direction. Most notably, the [110]-directed ZnTe NW deforms unusually as the strain rate increases, making it more sensitive to strain rate than other orientations. The strong strain rate sensitivity results from the unusual short-range and long-range order crystals appearing due to dislocation slipping and partial twinning. Moreover, the {111} plane is the principal cleavage plane for all orientations, creating a dislocation slipping mechanism at room temperature. The {100} plane becomes active and acts as another fundamental cleavage plane at increasing temperatures. This in-depth analysis paves the way for advancing efficient and reliable ZnTe NWs-based nanodevices and nanomechanical systems.

Crystal orientation-dependent tensile mechanical behavior and deformation mechanisms of zinc-blende ZnSe nanowires.

Crystal deformation mechanisms and mechanical behaviors in semiconductor nanowires (NWs), in particular ZnSe NWs, exhibit a strong orientation dependence. However, very little is known about tensile deformation mechanisms for different crystal orientations. Here, the dependence of crystal orientations on mechanical properties and deformation mechanisms of zinc-blende ZnSe NWs are explored using molecular dynamics simulations. We find that the fracture strength of [111]-oriented ZnSe NWs shows a higher value than that of [110] and [100]-oriented ZnSe NWs. Square shape ZnSe NWs show greater value in terms of fracture strength and elastic modulus compared to a hexagonal shape at all considered diameters. With increasing temperature, the fracture stress and elastic modulus exhibit a sharp decrease. It is observed that the {111} planes are the deformation planes at lower temperatures for the [100] orientation; conversely, when the temperature is increased, the {100} plane is activated and contributes as the second principal cleavage plane. Most importantly, the [110]-directed ZnSe NWs show the highest strain rate sensitivity compared to the other orientations due to the formation of many different cleavage planes with increasing strain rates. The calculated radial distribution function and potential energy per atom further validates the obtained results. This study is very important for the future development of efficient and reliable ZnSe NWs-based nanodevices and nanomechanical systems.

Atomistic reaction mechanism of CVD grown MoS2 through MoO3 and H2S precursors.

Chemical vapor deposition (CVD) through sulfidation of MoO3 is one of the most important synthesis techniques to obtain large-scale and high-quality two-dimensional (2D) MoS2. Recently, H2S precursor is being used in the CVD technique to synthesize 2D MoS2. Although several studies have been carried out to examine the mechanism of MoS2 growth in the presence of sulfur and MoO3 precursors, the growth of MoS2 in the presence of H2S precursor has largely remained unknown. In this study, we present a Reactive molecular dynamics (RMD) simulation to investigate the reaction mechanism of MoS2 from MoO3 and H2S precursors. The intermediate molecules formation, the reason behind those formations, and the surface compositions of MoOxSyHz during the initial steps of CVD have all been quantified. Surprisingly, a sudden separation of sulfur atoms from the surface was observed in the H2S precursor system due to the substantial oxygen evolution after 1660 K. The sulfur detachments and oxygen evolution from the surface were found to have a linear relationship. In addition, the intermediate molecules and surface bonds of MoS2 synthesized by MoO3 and H2S precursors were compared to those of a system using S2 and MoO3 precursors. The most stable subsidiary formation from the H2S precursor was found to be H2O, whereas in case of S2 precursor it was SO. These results provide a valuable insight in the formation of large-scale and high-quality 2D MoS2 by the CVD technique.

Tensile Mechanical Behavior and the Fracture Mechanism in Monolayer Group-III Nitrides XN (X= Ga, In): Effect of Temperature and Point Vacancies.

In this study, we have thoroughly investigated the tensile mechanical behavior of monolayer XN (X = Ga, In) using molecular dynamics simulations. The effects of temperature (100 to 800 K) and point vacancies (PVs, 0.1 to 1%) on fracture stress, strain, and elastic modulus of GaN and InN are studied. The effects of edge chiralities on the tensile mechanical behavior of monolayer XN are also explored. We find that the elastic modulus, tensile strength, and fracture strain reduce with increasing temperature. The point defects cause the stress to be condensed in the vicinity of the vacancies, resulting in straightforward damage. On the other hand, all the mechanical behaviors such as fracture stress, elastic modulus, and fracture strain show substantial anisotropic nature in these materials. To explain the influence of temperature and PVs, the radial distribution function (RDF) at diverse temperatures and potential energy/atom at different vacancy concentrations are calculated. The intensity of the RDF peaks decreases with increasing temperature, and the presence of PVs leads to an increase in potential energy/atom. The current work provides an insight into adjusting the tensile mechanical behaviors by making vacancy defects in XN (X = Ga, In) and provides a guideline for the applications of XN (X = Ga, In) in flexible nanoelectronic and nanoelectromechanical devices.

Vacancy-Induced Thermal Transport and Tensile Mechanical Behavior of Monolayer Honeycomb BeO.

Because of the rapid shrinking trend of integrated circuits, the performances of nanodevices and nanomechanical systems are greatly affected by the joule heating and mechanical failure dilemma. In addition, structural defects are inevitable during experimental synthesis of nanomaterials, which may alter their physical properties significantly. Investigation of the thermal transport and mechanical behavior of nanostructured materials with structural defects is thus a crucial requirement. In this study, the thermal conductivity (TC) and tensile mechanical behavior of monolayer honeycomb BeO are systematically explored using molecular dynamics simulations. An infinite length bulk TC of ∼277.77 ± 8.93 W/mK was found for the pristine monolayer BeO. However, the insertion of 1% single vacancy (SV) and double vacancy (DV) defects reduces the TC by ∼36.98 and ∼33.52%, respectively. On the other hand, the uniaxial tensile loading produces asymmetrical fracture stress, elastic modulus, and fracture strain behaviors in the armchair and zigzag directions. The elastic modulus was reduced by ∼4.7 and ∼6.6% for 1% SV defects along the armchair and zigzag directions, respectively, whereas the reduction was ∼2.7 and ∼ 5.1% for 1% DV defects. Moreover, because of the strong symmetry-breaking effect, both the TC and mechanical strength were significantly lower for the SV defects than those for the DV defects. The highly softening and decreasing trends of the phonon modes with increasing vacancy concentration and temperature, respectively, were noticed for both types of defects, resulting in a reduction of the TC of the defected structures. These findings will be helpful for the understanding of the heat transport and mechanical characteristics of monolayer BeO as well as provide guidance for the design and control of BeO-based nanoelectronic and nanoelectromechanical devices.

Temperature and interlayer coupling induced thermal transport across graphene/2D-SiC van der Waals heterostructure.

Graphene based two-dimensional (2D) van der Waals (vdW) materials have attracted enormous attention because of their extraordinary physical properties. In this study, we explore the temperature and interlayer coupling induced thermal transport across the graphene/2D-SiC vdW interface using non-equilibrium molecular dynamics and transient pump probe methods. We find that the in-plane thermal conductivity κ deviates slightly from the 1/T law at high temperatures. A tunable κ is found with the variation of the interlayer coupling strength χ. The interlayer thermal resistance R across graphene/2D-SiC interface reaches 2.71 [Formula: see text] 10-7 [Formula: see text] at room temperature and χ = 1, and it reduces steadily with the elevation of system temperature and χ, demonstrating around 41% and 56% reduction with increasing temperature to 700 K and a χ of 25, respectively. We also elucidate the heat transport mechanism by estimating the in-plane and out-of-plane phonon modes. Higher phonon propagation possibility and Umklapp scattering across the interface at high temperatures and increased χ lead to the significant reduction of R. This work unveils the mechanism of heat transfer and interface thermal conductance engineering across the graphene/2D-SiC vdW heterostructure.

Temperature- and Defect-Induced Uniaxial Tensile Mechanical Behaviors and the Fracture Mechanism of Two-Dimensional Silicon Germanide.

Recently, monolayer silicon germanide (SiGe), a newly explored buckled honeycomb configuration of silicon and germanium, is predicted to be a promising nanomaterial for next-generation nanoelectromechanical systems (NEMS) due to its intriguing electronic, optical, and piezoelectric properties. In the NEMS applications, the structure is subjected to uniaxial tensile mechanical loading, and the investigation of the mechanical behaviors is of fundamental importance to ensure structural stability. Here, we systematically explored the uniaxial tensile mechanical properties of 2D-SiGe through molecular dynamics simulations. The effects of temperature ranges from 300 to 1000 K and vacancy defects, for instance, point and bi vacancy, for both armchair and zigzag orientations of 2D-SiGe were investigated. In addition, the influence of system areas and strain rates on the stress-strain performance of 2D-SiGe has also been studied. With the increase in temperature and vacancy concentration, the mechanical properties of 2D-SiGe show decreasing behavior for both orientations and the armchair chirality shows superior mechanical strength to the zigzag direction due to its bonding characteristics. A phase transformation-induced second linearly elastic region was observed at large deformation strain, leading to an anomalous stress-strain behavior in the zigzag direction. At 300 K temperature, we obtained a fracture stress of ∼94.83 GPa and an elastic modulus of ∼388.7 GPa along the armchair direction, which are about ∼3.17 and ∼2.83% higher than the zigzag-oriented fracture strength and elastic modulus. Moreover, because of the strong regularity interruption effect, the point vacancy shows the largest decrease in fracture strength, elastic modulus, and fracture strain compared to the bi vacancy defects for both armchair and zigzag orientations. Area and strain rate investigations reveal that 2D-SiGe is less susceptible to the system area and strain rate. These findings provide a deep insight into controlling the tensile mechanical behavior of 2D-SiGe for its applications in next-generation NEMS and nanodevices.

Superior tunable photocatalytic properties for water splitting in two dimensional GeC/SiC van der Waals heterobilayers.

The photocatalytic characteristics of two-dimensional (2D) GeC-based van der Waals heterobilayers (vdW-HBL) are systematically investigated to determine the amount of hydrogen (H2) fuel generated by water splitting. We propose several vdW-HBL structures consisting of 2D-GeC and 2D-SiC with exceptional and tunable optoelectronic properties. The structures exhibit a negative interlayer binding energy and non-negative phonon frequencies, showing that the structures are dynamically stable. The electronic properties of the HBLs depend on the stacking configuration, where the HBLs exhibit direct bandgap values of 1.978 eV, 2.278 eV, and 2.686 eV. The measured absorption coefficients for the HBLs are over ~ 105 cm-1, surpassing the prevalent conversion efficiency of optoelectronic materials. In the absence of external strain, the absorption coefficient for the HBLs reaches around 1 × 106 cm-1. With applied strain, absorption peaks are increased to ~ 3.5 times greater in value than the unstrained HBLs. Furthermore, the HBLs exhibit dynamically controllable bandgaps via the application of biaxial strain. A decrease in the bandgap occurs for both the HBLs when applied biaxial strain changes from the compressive to tensile strain. For + 4% tensile strain, the structure I become unsuitable for photocatalytic water splitting. However, in the biaxial strain range of - 6% to + 6%, both structure II and structure III have a sufficiently higher kinetic potential for demonstrating photocatalytic water-splitting activity in the region of UV to the visible in the light spectrum. These promising properties obtained for the GeC/SiC vdW heterobilayers suggest an application of the structures could boost H2 fuel production via water splitting.

Thermal transport in monolayer zinc-sulfide: effects of length, temperature and vacancy defects.

Of late, atomically thin two-dimensional zinc-sulfide (2D-ZnS) shows great potential for advanced nanodevices and as a substitute to graphene and transition metal di-chalcogenides owing to its exceptional optical and electronic properties. However, the functional performance of nanodevices significantly depends on the effective heat management of the system. In this paper, we explored the thermal transport properties of 2D-ZnS through molecular dynamics simulations. The impact of length, temperature, and vacancy defects on the thermal properties of 2D-ZnS are systematically investigated. We found that the thermal conductivity (TC) rises monotonically with increasing sheet length, and the bulk TC of ∼30.67 W mK-1is explored for an infinite length ZnS. Beyond room temperature (300 K), the TC differs from the usual 1/Trule and displays an abnormal, slowly declining behavior. The point vacancy (PV) shows the largest decrease in TC compared to the bi vacancy (BV) defects. We calculated phonon modes for various lengths, temperatures, and vacancies to elucidate the TC variation. Conversely, quantum corrections are used to avoid phonon modes' icing effects on the TC at low temperatures. The obtained phonon density of states (PDOS) shows a softening and shrinking nature with increasing temperature, which is responsible for the anomaly in the TC at high temperatures. Owing to the increase of vacancy concentration, the PDOS peaks exhibit a decrease for both types of defects. Moreover, the variation of the specific heat capacity and entropy with BV and PV signify our findings of 2D-ZnS TC at diverse concentrations along with the different forms of vacancies. The results elucidated in this study will be a guide for efficient heat management of ZnS-based optoelectronic and nano-electronic devices.

Vacancy-induced thermal transport in two-dimensional silicon carbide: a reverse non-equilibrium molecular dynamics study.

Because of its impressive electrical, thermal, and mechanical properties, two-dimensional silicon carbide (2D-SiC) has recently gained tremendous attention in the field of nanoelectronics and optoelectronics. Here, we investigated the effects of various types of defects such as bi-, point-, and mixed-vacancies on the thermal conductivity of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. The effects of temperature variation on the thermal conductivity of vacancy-defected 2D-SiC were also studied. A significant reduction of the thermal conductivity was observed when the concentrations of the vacancies were increased. The point vacancy resulted in the thermal conductivity decreasing more quickly as compared to bi vacancy and mixed vacancy defects. Moreover, increasing the temperature of vacancy-defected 2D-SiC further reduced the thermal conductivity due to a strong phonon-vacancy scattering effect. Because of the introduction of vacancy defects in the acoustic phonon density of states (PDOS), a softening behavior in the intensity of the characteristic peaks is perceived, and with increasing temperature, a frequency shrinking is noted in the PDOS curves, both of which contribute to the reduction of the thermal conductivity. Additionally, rapid softening of the phonon transmission spectrum and increase in entropy were obtained for the point vacancy-defected structure, which clearly confirms our findings at different vacancy concentrations as well as for types of vacancies. These findings are very much imperative for realizing heat dissipation in nano- and optoelectronic devices based on 2D-SiC as well as for demonstrating an effective method for modulating 2D-SiC thermal conductivity through defect engineering.

Anomalous temperature dependent thermal conductivity of two-dimensional silicon carbide.

Recently, two-dimensional silicon carbide (2D-SiC) has attracted considerable interest due to its exotic electronic and optical properties. Here, we explore the thermal properties of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. At room temperature, a thermal conductivity of ∼313 W mK-1 is obtained for 2D-SiC which is one order higher than that of silicene. Above room temperature, the thermal conductivity deviates the normal 1/T law and shows an anomalous slowly decreasing behavior. To elucidate the variation of thermal conductivity, the phonon modes at different length and temperature are quantified using Fourier transform of the velocity auto-correlation of atoms. The calculated phonon density of states at high temperature shows a shrinking and softening of the peaks, which induces the anomaly in the thermal conductivity. On the other hand, quantum corrections are applied to avoid the freezing effects of phonon modes on the thermal conductivity at low temperature. In addition, the effect of potential on the thermal conductivity calculation is also studied by employing original and optimized Tersoff potentials. These findings provide a means for better understating as well as designing the efficient thermal management of 2D-SiC based electronics and optoelectronics in near future.