Synthesis, structure, linear and nonlinear optical properties of noncentrosymmetric quaternary diamond-like semiconductors, Cu2ZnGeSe4 (CZGSe) and the novel Cu4ZnGe2Se7

Abstract

In this study, two quaternary diamond-like semiconductors (DLSs) in the Cu-Zn-Ge-Se system, the known Cu2ZnGeSe4 (also referred to as CZGSe) and the new Cu4ZnGe2Se7, are compared in terms of their crystal structures, electronic structures and physicochemical properties. Both compounds were prepared by high-temperature, solid-state synthesis at 800 °C. Single crystal X-ray diffraction was used to determine the structure of Cu4ZnGe2Se7. The structures of both Cu2ZnGeSe4 and Cu4ZnGe2Se7 can be considered as derivatives of cubic diamond. Cu4ZnGe2Se7, with a reduced symmetry due to the cation ordering pattern, adopts the Cu4NiSi2S7 structure type, with space group C2. The corner-sharing tetrahedra in Cu4ZnGe2Se7 are slightly distorted because the charge for some of the S2- anions is not compensated by the first-nearest-neighbor cations. Rietveld refinements using synchrotron X-ray powder diffraction data were used to assess the phase purity of the samples and confirm the bulk structural behavior. Diffuse reflectance UV/Vis/NIR spectroscopy shows that Cu2ZnGeSe4 and Cu4ZnGe2Se7 have direct optical bandgaps of 1.38 and 0.91 eV, respectively. Electronic structure calculations implementing density functional theory confirm the direct bandgap for Cu4ZnGe2Se7, with a calculated value of 0.62 eV. Both compounds are air stable, thermally stable up to relatively high temperatures, undergo phase transitions and have wide windows of optical clarity. The Kurtz-Perry powder technique was used to determine the second harmonic generation (SHG) responses using a commercial AgGaSe2 standard. While Cu4ZnGe2Se7 displays a weak SHG response, Cu2ZnGeSe4 exhibits a response that is greater than several benchmark materials, with a large SHG coefficient, χ(2), of 43 ± 6 pm/V at λ = 2900 nm.