Hydrothermal Synthesis and Electronic and Optical Characterization of Ag₂(NH₄)AsS₄

Multinary chalcogenide semiconductors have the potential for use in various optoelectronic and energy-conversion applications. Understanding how to controllably synthesize these semiconductors is paramount to successful device integration. In this report, we analyze the hydrothermal synthesis technique used to make the quaternary sulfide Ag₂(NH₄)AsS₄, focusing on how solvent volume, synthesis time, sulfur background pressure, and initial cation stoichiometry impact the synthesis result. Achieving a reliable synthesis procedure, we characterize the thermal and air stability, calculate the electronic band structure, and measure the optical absorption of Ag₂(NH₄)AsS₄. The sulfide is found to be relatively stable to air exposure at room temperature but is susceptible to thermal decomposition at temperatures below the typical synthesis point (∼220 °C). Ab initio molecular dynamics simulations show that the NH₄⁺ cation can rotate freely within the structure, and single crystal X-ray analysis of Ag₂(NH₄)AsS₄ shows no structural transitions over the temperature range 135-298 K. Hybrid density functional theory calculations indicate that Ag₂(NH₄)AsS₄ is an indirect band gap semiconductor with dispersive band edges, while optical spectroscopy reveals a 2.05(5) eV band gap. The thorough synthesis and materials characterization studies pursued here lay a foundation for film processing of Ag₂(NH₄)AsS₄ and the exploratory synthesis of related quaternary chalcogenides.