Elastic limit of silicane

Two-dimensional ferroelasticity and ferroelastic strain controllable anisotropic transport properties in CuTe monolayer

Two-dimensional ferroelastic (2D-FE) materials where FE strain originates from the lattice deformation associated with spontaneous FE phase transition, hold great promise as miniaturized shape-memory devices. Moreover, the structural anisotropy within the low-symmetry 2D-FE materials can usually lead to intrinsic anisotropy in their electronic or transport properties as well. As a result, the strong coupling of FE strain with the anisotropic electronic structure or electric-/thermoelectric-transport will largely extend the functionality and device applications for 2D-FE materials. In the current work, after performing comprehensive first-principles calculations in combination with transport simulations based on the Boltzmann formalism, we identify the experimentally synthesizable CuTe monolayer as a new 2D-FE material whose anisotropic electric- and thermoelectric-transport properties can be effectively manipulated by FE strain. Typically, CuTe monolayers that can be potentially exfoliated from the synthesized van der Waals (vdW) layered CuTe bulk are predicted to exhibit the room temperature stable ferroelasticity and large axial FE strain (up to 18.4%) created by the in-plane orthorhombic lattice deformation. Owing to the planar orientation dependent metallic vs. nearly semiconducting electronic structure, highly anisotropic electric conductivity and thermopower coefficient can be obtained along the two planar principal axes of the CuTe monolayer. To simulate the more realistic experimental scenarios, coherent formation of FE domain walls and domain-wall motion assisted FE switching have also been evaluated in CuTe multi-domain configurations. Based on the transverse thermoelectric effect inherent in anisotropic CuTe monolayers, the schematic model for obtaining the FE strain controllable electric current within CuTe multi-domain configurations has been proposed, which can be verified experimentally.

A DFT study for silicene quantum dots embedded in silicane: controllable magnetism and tuneable band gap by hydrogen

This paper presents a design for silicene quantum dots (SiQDs) embedded in silicane. The shape and size of an embedded SiQD are managed by hydrogen atoms. A first-principles method was used to evaluate the magnetism as well as the electronic and structural properties of embedded SiQDs of various shapes and sizes. The shape of the embedded SiQD determined its electronic structure as well as the dot size. Moreover, the magnetic properties of SiQDs in silicane were highly shape dependent. The triangular SiQDs were all magnetic, some small parallelogram SiQDs were nonmagnetic, and all others were antiferromagnetic; almost all hexagonal SiQDs were nonmagnetic. An unequal number of bare Si atoms at the A and B sites was identified as a critical factor for establishing magnetism in embedded SiQDs. The tip of a triangular SiQD enhanced the magnetic moment of the dot. The parallelogram SiQD with two tip atoms appeared as a magnetic needle and has potential for use in spintronic applications. SiQDs embedded in silicane can be used in the design of silicon-based nanoelectronic devices and binary logic based on nanoscale magnetism.

This paper presents a design for silicene quantum dots (SiQDs) embedded in silicane.

Functionalization of the electronic and magnetic properties of silicene by halogen atoms unilateral adsorption: a first-principles study

Based on first-principles calculations, the structure, electronic and magnetic properties of unilateral halogenated silicene Si₂X₁ (X  =  F, Cl, Br, I) are investigated. The formation energies of all the configurations of studied Si₂X₁ (X  =  F, Cl, Br, I) are found to be lower than that in pristine silicene, which indicates the strong stability. The band structure of half-fluorinated configuration Si₂F₁ presents metallic property, while other unilateral halogenated silicene Si₂X₁ (X  =  Cl, Br, I) exhibits half-metallic properties. In unilateral halogenated silicene Si₂X₁ (X  =  Cl, Br, I), the unpaired electrons in unsaturated silicon atom produce the localized magnetic moment. However, due to the strong electronegativity in F atom, the half-fluorinated silicene Si₂F₁ is almost non-magnetic. The metallic property of Si₂F₁ configuration can be tuned to half-metallic by applying biaxial tensile strain from 11.95% to 13.51%. Furthermore, applying biaxial tensile strains can tune the half-metallic property of unilateral halogenated silicene Si₂X₁ (X  =  Cl, Br, I) to a semiconductor. This half-metallic property in unilateral halogenated silicene Si₂X₁ (X  =  Cl, Br, I) can be recovered and can even be tuned to metallic if continually increasing the biaxial tensile strains.

Structural stability and the electronic properties of a (SiH)2O-formed siloxene sheet: a computational study

Very recently, a (SiH)₂O-formed siloxene sheet has been synthesized in an experiment [S. Li, et al. J. Mater. Chem. A 2016, 4, 15841]. Utilizing first-principles calculations, we have systematically investigated the structural stability and the physical properties of this (SiH)₂O sheet. We have found a dynamically stable structure because it is a puckered one with bent Si-O-Si connections, which leads to an anisotropic elastic behaviour with negative Poisson ratios. The (SiH)₂O sheet has a strain-tunable direct band gap whose valence and conduction band edges straddle the redox potentials of water even in the presence of stress (either tensile or compressive). The calculated carrier mobilities are prominently anisotropic: in its zigzag direction the electron mobility reaches a high value of 5.39 × 10³ cm² V^-1 s^-1, while the hole mobility remains small. These different carrier mobilities permit fast charge separation and transfer, which is crucial for efficient solar-to-electric energy conversion for photovoltaic and water splitting applications. Similar to phosphorene, a typical linear dichroism phenomenon is also present in the (SiH)₂O sheet. Due to its outstanding electronic properties, the (SiH)₂O sheet is a promising material in the fields of nano-electronics, sustainable energy and fuel generation.

The ideal strength of two-dimensional stanene may reach or exceed the Griffith strength estimate

Ideal strength is the maximum stress a material can withstand, and it is an important intrinsic property for structural applications. A Griffith strength limit of ∼E/9 is the best known upper bound of this property for a material loaded in tension. Here we report that stanene, a recently fabricated two-dimensional material, could approach and possibly exceed this limit from a theoretical perspective. Utilizing first-principles density functional theory, we investigated the nonlinear elastic behavior of stanene and found that its strength could reach ∼E/7.4 under uniaxial tension in both armchair and zigzag directions without incurring phonon instability or mechanical failure. The unique mechanical properties of stanene are appreciated by comparison with other Group-IV 2D materials.

Electrical and mechanical controlling of the kinetic and magnetic properties of hydrogen atoms on free-standing silicene

Effects of strain, charge doping and external electric field on kinetic and magnetic properties of hydrogen atoms on a free-standing silicene layer are investigated by first-principles density functional theory. It was found that the charge doping and strain are the most effective ways of changing the hydrogen-silicene binding energy, but they can only raise its value. The perpendicular external electric field can also lower it albeit in a narrower range. The strain has also the strongest impact on diffusion processes, and the diffusion barrier can be modified up to 50% of its unstrained value. The adsorption of hydrogen atoms results in a locally antiferromagnetic ground state with the effective exchange constant of approximately 1 eV. The system can easily be driven into a nonmagnetic phase by the charge doping and strain. The obtained results are very promising in view of the silicene functionalization and potential applications of silicene in fields of modern nanoelectronics and spintronics.

Rationalizing the Hydrogen and Oxygen Evolution Reaction Activity of Two-Dimensional Hydrogenated Silicene and Germanene

We have undertaken first-principles electronic structure calculations to show that the chemical functionalization of two-dimensional hydrogenated silicene (silicane) and germanene (germanane) can become a powerful tool to increase the photocatalytic water-splitting activity. Spin-polarized density functional theory within the GGA-PBE and HSE06 types of exchange correlation functionals has been used to obtain the structural, electronic, and optical properties of silicane and germanane functionalized with a series of nonmetals (N, P, and S), alkali metals (Li, Na, and K) and alkaline-earth metals (Mg and Ca). The surface-adsorbate interaction between the functionalized systems with H2 and O2 molecules that leads to envisaged hydrogen and oxygen evolution reaction activity has been determined.

First principles study of silicene symmetrically and asymmetrically functionalized with halogen atoms

Using first principles calculations, we predicted that a direct-band-gap between 0.98 and 2.13 eV can be obtained in silicene by symmetrically and asymmetrically (Janus) functionalisation with halogen atoms and applying elastic tensile strain.

High anisotropy of fully hydrogenated borophene

We have studied the mechanical properties and phonon dispersions of fully hydrogenated borophene (borophane) under strains by first principles calculations.

H-Si bonding-induced unusual electronic properties of silicene: a method to identify hydrogen concentration

Hydrogenated silicenes possess peculiar properties owing to the strong H-Si bonds, as revealed by an investigation using first principles calculations. Various charge distributions, bond lengths, energy bands, and densities of states strongly depend on different hydrogen configurations and concentrations. The competition between strong H-Si bonds and weak sp(3) hybridization dominate the electronic properties. Chair configurations belong to semiconductors, while the top configurations show a nearly dispersionless energy band at the Fermi level. Both the systems display H-related partially flat bands at middle energy and the recovery of low-lying π bands during the reduction of concentration. Their densities of states exhibit prominent peaks at middle energy, and the top systems have a delta-funtion-like peak at E = 0. The intensity of these peaks is gradually weakened as the concentration decreases, providing an effective method to identify the H-concentration in scanning tunneling spectroscopy experiments.

Mechanical degradation of graphene by epoxidation: insights from first-principles calculations

Oxidation is a major cause for the degradation of materials including graphene, where epoxidation (forming the C-O-C bond) is very common. In addition, graphene oxide, in which the epoxy group is one of the two major functional groups (the other is hydroxy), is an important precursor material used for the bulk synthesis of graphene sheets. Information about the mechanical stabilities, non-linear elastic properties, and elastic limits under various strain components is invaluable for application of these nanomaterials. Here, we investigate the mechanical properties of the epoxidized graphene in ordered graphene oxide, namely C6O1, C6O2, and C6O3, representing the carbon : oxygen ratios of 6 : 1, 3 : 1, and 2 : 1, respectively, using first-principles calculations within the framework of density functional theory. We predict a reduction of Young's modulus of graphene by a factor of 20%, 23%, and 27% for C6O1, C6O2, and C6O3, respectively, indicating a monotonic degradation with respect to epoxidation. However, there is no clear trend for Poisson's ratio, implying that the local atomic configurations are dominant over oxygen concentrations in determining the Poisson ratio. Our computed high order elastic constants are good for the design of graphene oxide based flexible transparent electronics.

Peculiar pressure effect on Poisson ratio of graphone as a strain damper

Hydrogenation is an effective way to modify the electronic and magnetic properties of graphene. The semi-hydrogenated graphene, known as "graphone", has promising applications in nanoelectronics including field-effect transistors. However, the elastic limit of this two-dimensional material remains unknown despite its importance in applications as well as strain engineering to tailor functions and properties. Here we report using first-principles calculations an abnormal increase in the Poisson ratio of graphone in response to an increase in pressure. This peculiar behavior is proposed to originate from the asymmetry of hydrogenation and could be used to design a nanodevice of strain damper to reduce harmful strains in graphene-based nanoelectronics.

Mechanical properties and stabilities of g-ZnS monolayers

Planar graphene-like ZnS monolayers are mechanically stable under various large strains.

Mechanical properties and stabilities of α-boron monolayers

α-Boron monolayers are mechanically stable under various large strains.

The effect of impurities in ultra-thin hydrogenated silicene and germanene: a first principles study

Spin polarized density functional theory within the GGA–PBE and HSE06 approach for the exchange correlation term has been used to investigate the stability and electronic properties of nitrogen and boron impurities in single layers of silicane and germanane.