ABSTRACT
Methylammonium metal halide perovskites have recently been explored for new uses due to their unique and exciting optoelectronic properties. Their exceptional electronic properties have often been attributed to the overlap between the metal cation s and halogen p states. In this study, density functional theory calculations have been carried out based on the orthorhombic phase of the organometal trihalide perovskite CH3NH3MX3 (M: Cu, Zn, Ga, Ge, Sn, Pb; X: Cl, Br, I) to systematically investigate the effects of the metal cation and halogen anion on the structural, electronic, and optical properties for solar cell applications. The calculated lattice parameters agree well with previously obtained experimental and theoretical results. All of these perovskites are direct band gap compounds at the G symmetry point, except CH3NH3GaX3. The band gap increases from iodide to chloride and also with the metal cation size, from Ge to Pb or Cu to Zn. Furthermore, metal halide perovskites show blue shifts in their optical absorption spectra with an increase in metal cation size. Among the studied examples, CH3NH3GaBr3 and CH3NH3CuCl3 absorb a wide range of light, from UV to the visible region, and possess very unusual high dielectric constants and refractive indices. Our calculations reveal that CH3NH3SnI3, CH3NH3GeI3, and CH3NH3ZnI3 are favorable candidates for lead-free photovoltaic applications.
ABSTRACT
After the successful synthesis of graphene/hexagonal boron nitride (h-BN) heterostructures, research works have been carried out for their plausible real-world device applications. Such 2D nanosheets gain great attention as they have shown promising gas sensing properties due to their high surface-to-volume ratio and unique electronic properties between graphene and h-BN. Herein, we report a first-principles density functional theory investigation of the structural and electronic properties of pristine graphene (PG), pristine BN, and their in-plane heterostructures employing B3LYP and dispersion-corrected van der Waals functional WB97XD with the 6-311G (d, p) basis set. We found that these predicted nanosheets show good structural stability with favorable cohesive energy and the bandgap gradually increases with the increase in the B-N concentration. We have also studied their adsorption properties toward toxic gas molecules (SO2 and CO). Among these heterostructures, G2BN2 exhibits greater adsorption energy of about -0.237 eV and -0.335 eV when exposed to SO2 and CO gas molecules, respectively. The electronic properties such as HOMO and LUMO energies, HOMO-LUMO energy gap, Fermi level, work function, and conductivity significantly changed after the adsorption of SO2 gas on the nanosheets except for PG, whereas these parameters remain almost the same after the adsorption of the CO gas molecule. Mulliken and natural bond orbital (NBO) charge analysis reveals that charge transfer occurs from gas molecules to the nanosheets except when SO2 is adsorbed onto PG. Although the adsorption energies for CO gas are slightly greater than those for SO2 gas for these nanosheets, all other investigations such as electronic properties, charge transfer analysis, molecular electrostatic potential (MEP) map, and global indices predict that these nanosheets are good sensors for SO2 gas than CO gas molecules.
