1. Crystal structure, electronic structure, and optical properties of Cu4ZnGe2S7 and Cu4CdGe2S7: Polar diamond-like semiconductors exhibiting second harmonic generation.
- Author
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Kelly, Jordan C., Hoyer, Chad E., Shin, Seung Han, Cho, Jeong Bin, Stoyko, Stanislav S., Bonnoni, Allyson, Craig, Andrew J., Rosello, Kate E., O'Hara, Evan T., Jang, Joon I., and Aitken, Jennifer A.
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SECOND harmonic generation , *ULTRASHORT laser pulses , *CRYSTAL structure , *OPTICAL properties , *COPPER , *FOUR-wave mixing , *ELECTRONIC structure , *MODULATIONAL instability - Abstract
The novel I 4 -II-IV 2 -VI 7 quaternary diamond-like semiconductors (DLSs), Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 , were prepared from the constituent elemental powders at elevated temperatures. Cu 4 ZnGe 2 S 7 crystallizes in the polar, chiral space group C 2 and adopts the Cu 4 NiSi 2 S 7 structure-type, which can be considered a derivative of the cubic diamond structure. The cadmium analog takes on the Cu 5 Si 2 S 7 structure-type, which is related to the hexagonal diamond structure, and displays polar, Cc space group symmetry. Both compounds contain [Ge 2 S 7 ]6- anions created by corner-sharing GeS 4 tetrahedra. The crystal structures were carefully evaluated using: 1) extended connectivity tables, 2) minimum bounding ellipsoid analysis, and 3) bond valence sum (BVS) and global instability index (G) calculations. Both compounds are thermally stable up to temperatures exceeding 950 °C and melt incongruently. As assessed using diffuse reflectance spectroscopy, Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 possess optical bandgaps of 1.65 eV and 1.90 eV, respectively, which are narrower than the analogous I 2 -II-IV-VI 4 DLSs. Electronic structure calculations show that both compounds are direct-bandgap semiconductors with significant contributions of Cu- d and S- p orbitals to the states at the top of the valence band. While Cu 4 CdGe 2 S 7 displays a weak second harmonic generation (SHG) response, the second-order nonlinear optical susceptibility, χ(2), of Cu 4 ZnGe 2 S 7 is 14.8 ± 1.5 pm V-1, assessed using the Kurtz-Perry powder method and an optical-grade AgGaSe 2 (AGSe) reference. Both compounds exhibit laser-induced damage threshold values similar to AGSe, ∼0.1 GW cm-2, for λ = 1.064 µm using picosecond laser pulses. Most importantly, Cu 4 ZnGe 2 S 7 exhibits a phase-matching behavior at λ = 3.1 µm. These results suggest that Cu 4 ZnGe 2 S 7 holds potential for use in low-powered laser applications involving efficient wave mixing in the mid-IR region. [Display omitted] • Cu 4 ZnGe 2 S 7 is a novel diamond-like material with the Cu 4 NiSi 2 S 7 structure type. • Cu 4 CdGe 2 S 7 is a novel diamond-like material with the Cu 5 Si 2 S 7 structure type. • Optical bandgaps of Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 are 1.65 and 1.90 eV, respectively. • Cu 4 ZnGe 2 S 7 has χ (2) value of 14.8 ± 1.5 pm V− 1 and is phase matching for λ = 3.1 µm. • Cu 4 ZnGe 2 S 7 is a candidate for low-powered laser applications in the mid-IR region. [ABSTRACT FROM AUTHOR]
- Published
- 2024
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