Particular features of the electronic structure and optical properties of Ag2PbGeS4 as evidenced from first-principles DFT calculations and XPS studies

Abstract

We report on a complex study of the electronic structure of the Ag2PbGeS4 compound using both experimental and theoretical methods. Particularly, we use X-ray photoelectron spectroscopy (XPS) to measure the binding energies (BEs) of core electrons for pristine and Ar+-ion bombarded surfaces of Ag2PbGeS4 single crystal and to evaluate the shape of the valence band in the titled compound. Our XPS data reveal that Ar+-ion bombardment does not cause significant changes in the BE values of main features of the XPS core-level and valence-band spectra, however it induces some nonstoichiometry of the Ag2PbGeS4 single crystal surface as a result of preferential etching of Ag atoms in such case. First-principles band-structure calculations based on density functional theory (DFT) within the augmented plane wave + local orbitals (APW+lo) method as implemented in the WIEN2k package have been performed in the present work to gain peculiarities of total density of states and partial densities of states in the Ag2PbGeS4 compound. The present APW+lo calculations manifest that the Pb 6s and Ag 4d states are the main contributors to the bottom and the central portion of the valence band, respectively, and the upper portion of the band is dominated by contributions of the S 3p states, while the bottom of the conduction band of Ag2PbGeS4 is determined by contributions of mainly the unoccupied Pb 5p states. The DFT calculations reveal that the valence-band maximum and conduction-band minimum are positioned at the T and X point, respectively, indicating that the Ag2PbGeS4 compound is an indirect-gap material. The main optical characteristics of Ag2PbGeS4 have been elucidated based on the first-principles DFT calculations.