Three-dimensional (3D) GaN is a III-V compound semiconductor with potential optoelectronic applications. In this paper, starting from 3D GaN in wurtzite and zinc-blende structures, we investigated the mechanical, electronic, and optical properties of the 2D single-layer honeycomb structure of GaN (g-GaN) and its bilayer, trilayer, and multilayer van der Waals solids using density-functional theory. Based on high-temperature ab initio molecular-dynamics calculations, we first showed that g-GaN can remain stable at high temperature. Then we performed a comparative study to reveal how the physical properties vary with dimensionality. While 3D GaN is a direct-band-gap semiconductor, g-GaN in two dimensions has a relatively wider indirect band gap. Moreover, 2D g-GaN displays a higher Poisson ratio and slightly less charge transfer from cation to anion. In two dimensions, the optical-absorption spectra of 3D crystalline phases are modified dramatically, and their absorption onset energy is blueshifted. We also showed that the physical properties predicted for freestanding g-GaN are preserved when g-GaN is grown on metallic as well as semiconducting substrates. In particular, 3D layered blue phosphorus, being nearly lattice-matched to g-GaN, is found to be an excellent substrate for growing g-GaN. Bilayer, trilayer, and van der Waals crystals can be constructed by a special stacking sequence of g-GaN, and they can display electronic and optical properties that can be controlled by the number of g-GaN layers. In particular, their fundamental band gap decreases and changes from indirect to direct with an increasing number of g-GaN layers.

The bandstructure of gold is calculated using ab initio many-body perturbation theory. Different approximations within the GW approach are considered. Standard single shot G0W0 corrections modify the sp-like bands while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the 5d bands. Globally, many-body effects achieve an opening of the 5d-6sp interband gap of ∼0.4 to ∼0.8 eV, reducing the discrepancy with the experiment. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional.

Using first-principles spin-polarized density functional theory, we carried out an analysis on the atomic structure, stability, energetics, and mechanical and electronic properties of single-layer structures of arsenene. These are buckled honeycomb, symmetric, and asymmetric washboard arsenene structures. Our analysis is extended to include layered three-dimensional crystalline phase of arsenic, as well as bilayer and trilayer structures to reveal dimensionality effects. The buckled honeycomb and symmetric washboard structures are shown to maintain their stability at high temperatures even when they are freestanding. As a manifestation of the confinement effect, the large fundamental band gap of single-layer phases decreases with increasing number of layers and eventually is closed. Concomitantly, lattice constants partially increase, while interlayer distances decrease. The effects of hybrid functional or self-energy corrections together with the spin-orbit coupling on the electronic structure of arsenene are crucial. The responses of direct and indirect band gaps to biaxial or uniaxial strain are rather complex and directional; while the fundamental band gap decreases and changes from indirect to direct with the biaxial strain applied to buckled arsenene, these effects are strongly directional for washboard arsenene. The width and the indirect/direct character of the band gap can be tuned by the number of layers, as well as by applied uniaxial/biaxial strain.

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, d-doping, single, and multiple quantum wells, as well as core/ shell structures. Finally, outlooks and possible new research directions are briefly discussed. Published by AIP Publishing. https://doi.

An extensive investigation of the optical properties of single-layer buckled and washboard arsenene and their bilayers was performed, starting from layered three-dimensional crystalline phase of arsenic using density functional and many-body perturbation theories combined with random phase approximation. Electron-hole interactions were taken into account by solving the Bethe-Salpeter equation, suggesting first bound exciton energies on the order of 0.7 eV. Thus, many-body effects were found to be crucial for altering the optical properties of arsenene. The light absorption of single-layer and bilayer arsenene structures in general falls within the visible-ultraviolet spectral regime. Moreover, directional anisotropy, varying the number of layers, and applying homogeneous or uniaxial in-plane tensile strain were found to modify the optical properties of two-dimensional arsenene phases, which could be useful for diverse photovoltaic and optoelectronic applications.

Soon after the synthesis of two-dimensional (2D) ultrathin black phosphorus and fabrication of field effect transistors thereof, theoretical studies have predicted that other group-VA elements (or pnictogens), N, As, Sb, and Bi can also form stable, single-layer (SL) structures. These were nitrogene in a buckled honeycomb structure, arsenene, antimonene, and bismuthene in a buckled honeycomb, as well as washboard and square-octagon structures with unusual mechanical, electronic, and optical properties. Subsequently, theoretical studies are followed by experimental efforts that aim at synthesizing these novel 2D materials. Currently, research on 2D pnictogens has been a rapidly growing field revealing exciting properties, which offers diverse applications in flexible electronics, spintronics, thermoelectrics, and sensors. This review presents an evaluation of the previous experimental and theoretical studies until 2019, in order to provide input for further research attempts in this field. To this end, we first reviewed 2D, SL structures of group-VA elements predicted by theoretical studies with an emphasis placed on their dynamical and thermal stabilities, which are crucial for their use in a device. The mechanical, electronic, magnetic, and optical properties of the stable structures and their nanoribbons are analyzed by examining the effect of external factors, such as strain, electric field, and substrates. The effect of vacancy defects and functionalization by chemical doping through adatom adsorption on the fundamental properties of pnictogens has been a critical subject. Interlayer interactions in bilayer and multilayer structures, their stability, and tuning their physical properties by vertical stacking geometries are also discussed. Finally, our review is concluded by highlighting new research directions and future perspectives on the challenges in this emerging field.

Two-dimensional (2D) materials have attracted growing interest due to their potential use in the next generation of nanoelectronic and optoelectronic applications. On the basis of first-principles calculations based on density functional theory, we first investigate the electronic and mechanical properties of single layer boron phosphide (h-BP). Our calculations show that h-BP is a mechanically stable 2D material with a direct band gap of 0.9 eV at the K-point, promising for both electronic and optoelectronic applications. We next investigate the electron transport properties of a p-n junction constructed from single layer boron phosphide (h-BP) using the non-equilibrium Green's function formalism. The n-and p-type doping of BP are achieved by substitutional doping of B with C and P with Si, respectively. C(Si) substitutional doping creates donor (acceptor) states close to the conduction (valence) band edge of BP, which are essential to construct an efficient p-n junction. By modifying the structure and doping concentration, it is possible to tune the electronic and transport properties of the p-n junction which exhibits not only diode characteristics with a large current rectification but also negative differential resistance (NDR). The degree of NDR can be easily tuned via device engineering.

Following its synthesis, borophene has drawn noticeable attention due to its remarkable intrinsic properties. Understanding and modifying these properties are crucial for implementation of borophene in high-technological applications. In this study, we employed ab initio techniques to examine the variation of the optoelectronic properties of buckled borophene by strain and surface functionalization. We find that the optical response can be tuned by applying compressive/tensile strain and covering the surface with hydrogen and fluorine atoms. It is shown that the variations in optical properties can be correlated with structural deformations and modifications in the electronic band structure. Revealing the tunability of the optical response of borophene can pave the way for its potential uses in various optoelectronic devices.