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.
Recent experiments revealed that monolayer α-RuCl 3 can be obtain by chemical exfoliation method and exfoliation or restacking of nanosheets can manipulate the magnetic properties of the materials. In this present paper, the electronic and magnetic properties of α-RuCl 3 monolayer are investigated by combining first-principles calculations and Monte Carlo simulations. From first-principles calculations, we found that the spin configuration FM corresponds to the ground state for α-RuCl 3 , however, the other excited zigzag oriented spin configuration has energy of 5 meV/atom higher than the ground state. Energy band gap has been obtained as 3 meV using PBE functionals. When spin-orbit coupling effect is taken into account, corresponding energy gap is determined to be as 57 meV. We also investigate the effect of Hubbard U energy terms on the electronic band structure of α-RuCl 3 monolayer and revealed band gap increases approximately linear with increasing U value. Moreover, spin-spin coupling terms (J 1 , J 2 , J 3 ) have been obtained using first principles calculations. By benefiting from these terms, Monte Carlo simulations with single site update Metropolis algorithm have been implemented to elucidate magnetic properties of the considered system. Thermal variations of magnetization, susceptibility and also specific heat curves indicate that monolayer α-RuCl 3 exhibits a phase transition between ordered and disordered phases at the Curie temperature 14.21 K. We believe that this study can be utilized to improve two-dimensional magnet materials.
We investigated the effects of chemical/substitutional doping, hydrogenation, and anti-site and vacancy defects on the atomic, optoelectronic and magnetic properties of AlN and GaN monolayers. Upon doping of selected atoms, AlN and GaN monolayers can acquire magnetic properties, and their fundamental band gaps are modified by the localized gap states. Spin-polarized gap states broaden into bands at patterned coverage of adatoms, whereby half-metallic or magnetic semiconducting properties can be attained. Specific adatoms adsorbed to Ga atoms break the nearest vertical Ga-N bonds in the GaN bilayer in the heackelite structure and result in changes in the electronic and atomic structure. While adjacent and distant pairs of anion + cation vacancies induce spin polarization with filled and empty gap states, anti-site defects remain nonmagnetic; but both defects induce dramatic changes in the band gap. Fully hydrogenated monolayers are stable only for specific buckled geometries, where one geometry can also lead to an indirect to direct band gap transition. Also, optical activity shifts to the ultra-violet region upon hydrogenation of the monolayers. While H2 and O2 molecules are readily physisorbed on the surfaces of the monolayers with weak van der Waals attraction, they can be dissociated into constituent atoms at the vacancy site of the cation. Our study performed within density functional theory shows that the electronic, magnetic and optical properties of AlN and GaN monolayers can be tuned by doping and point defect formation in order to acquire diverse functionalities.
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