First-Principles Study of 2D Materials

Abstract

and magnetic structures of selected two dimensional (2D) materials are studied. To induce magnetism in 2D GaS, which is a non-magnetic and indirect band gap semiconductor, N and F as substitutional dopants and adsorbed atoms are considered. Except forNsubstituting for Ga (NGa), all considered cases are found to possess a local magnetic moment. Fluorine, both in its atomic and molecular forms, undergoes a highly exothermic reaction with GaS and its site preference (FS or FGa) as substitutional dopant depends on Garich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F–Ga bond is formed and p-type conductivity is induced in GaS, whereas FS induces a an impurity band about 0.5 eV below the conduction band edge of GaS. Substitutional doping of N at both the S and Ga sites is exothermic when using N atom, whereas more favourable site can be accessed by the less reactive N2 molecule. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, and the non-magnetic NS impurities are in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. Such modified electronic structures of GaS can be exploited to render monolayer GaS as a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS can lead to mid-gap states with characteristic transition energies that can be used to tune light absorption and emission. The effects of incommensurability on the electronic structures of heterostructures of group-III monochalcogenides (GaS, GaSe, InS and InSe) are investigated. For the two heterostructures, GaS/GaSe and GaSe/InS, the cost of having commensurate structures are computed, and the potential energy landscape of both heterostructures are also examined. The commensurate heterostructure may be realized in GaSe/InS as the interaction energy of this system with the monolayers, assuming the average lattice constant, is smaller than the interaction energy of an incommensurate system in which each layer keeps its own lattice constant. For GaS/GaSe, on the other hand, the incommensurate heterostructure is energetically more favourable than the commensurate one, even when taking into account the energetic cost due to the lack of proper registry between the layers. Since the commensurate condition requires that one (or both) layer(s) is (are) strained, we systematically investigate the effect of strain on the band gaps and band edge positions of the monolayer systems. In all monolayers the conduction band minimum is more than 2 times more sensitive to applied strain than the valence band maximum–this was observed to strongly affect the band alignment of GaS/GaSe, as it can

iv change from type-I to type-II with a small variation in the lattice constant of GaS. GaSe/InS heterostructure is shown to have a type-II alignment, which is robust with respect to strain. The electronic structure of monolayer MoS2 under strain is also investigated, and strain increases the density of states(DOS) at the Fermi energy for Y doping (Y = H, Li, and F) at the S top, and strain driven magnetism develops in agreement with the Stoner mean field model. No saturation of the spinmagnetic moment is observed in Li-doped MoS2 due to less charge transfer from the Mo d electrons and the added atoms do not significantly increase the spin-orbit coupling in MoS2. Half-metallic ferro-magnetism is predicted in H and F-doped MoS2. Fixed magnetic moment calculations are also performed, and the DFT computed data is fitted with the Landau mean field model to investigate the emergence of spontaneous magnetism in Y-doped MoS2. We predict spontaneous magnetism in systems with large (small) magnetic moments for H/F (Li) atoms. The large (small) magnetic moments are attributed to the electro-negativity difference between S and Y atoms. Our results suggest that H and F adsorbed monolayer MoS2 is a good candidate for spin-based electronic devices. To date most of the 2D materials (based on light elements) are non-magne -tic. In this thesis a new 2D material, monolayer Li2N, is also predicted. The spin-polarized calculations reveal that 2D Li2Nis magnetic without intrinsic or impurity defects and shows half-metallic behaviour. The magnetic moment of 1.0 mB in 2D Li2Nis mainly contributed by the pz electrons ofNatom. Dynamic instability in planar Li2N monolayer is observed, but a buckled Li2N monolayer is found to be dynamically stable. To access the exchange field strength the ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling between the N atoms is also investigated. Using the Heisenberg mean field model, the planar (buckled) 2D Li2N is a ferromagnetic material with Curie temperature Tc of 161 (572) K.We believe that buckling not only stabilizes Li2N, but also helps to increase Tc.