Disorder and Non‐Stoichiometry in Cu ₂ ZnSnS ₄ and Cu ₂ NiSnS ₄

ABSTRACT

We report a combined experimental–theoretical study of Raman‐active phonons in Cu ₂ ZnSnS ₄ (CZTS) and Cu ₂ NiSnS ₄ (CNTS) nanocrystals (NCs) of small size, synthesized by a “green” aqueous route. Low‐temperature (88 K) Raman spectra of NC films reveal a disordered kesterite structure (DKS) of the NCs, while X‐ray photoelectron spectroscopy (XPS) analysis reveals a non‐stoichiometric composition of both NC compounds. For the calculation of the vibrational spectra of such NCs, we generate cation‐ordered stoichiometric (kesterite), cation‐disordered stoichiometric, and cation‐disordered non‐stoichiometric supercells and perform geometry optimizations and calculations of Raman‐active phonons using the DFTB+ (GFN1‐XTB) package. For the cation‐disordered stoichiometric structure and cation‐disordered non‐stoichiometric structures, 10 and 31 inequivalent configurations are simulated, respectively. For comparison with experimental spectra, the vibrational patterns of these inequivalent configurations are summed into a single pattern. The calculations qualitatively reproduce the experimentally observed disorder‐induced broadening, merging of closely situated bands and the red shift of the main A ₁ –type band with respect to the stoichiometric single crystal. These results provide a reliable basis for further, more expensive calculations of various cationic disorder and stoichiometric compositions of Cu ₂ XSnS ₄ nanocrystals.