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Quaternary chalcogenides based on Cu₂ZnSnS₄ (CZTS) are attracting attention as earth-abundant, low-cost semiconductor materials for thin-film photovoltaics (PV) and thermoelectric (TE) applications. Substituting Zn with transition metals such as Ni, Fe, and Co offers opportunities to modify cation distribution, defect structure and electronic properties.
The “green” aqueous synthesis protocol originally developed for CZTS [1], was used to explore the possibility of synthesising Cu₂MSnS₄ (M = Zn, Ni, Fe, Co) nanocrystals (NCs). Protocol modifications have enabled the synthesis of different compositions, particularly Cu₂NiSnS₄ (CNTS), while the synthesis of Fe- and Co-containing compounds remains challenging. This study demonstrates that CZTS and CNTS NCs can be synthesised and size-selected under the same temperature, pressure and atmospheric conditions.
The combination of X-ray diffraction, Raman spectroscopy, ultraviolet-visible (UV-vis) spectroscopy and photoemission/inverse photoemission (UPS/IPES) spectroscopies facilitates the analysis of the influence of NCs size, crystallinity and secondary phases on the structural and electronic properties of these materials. The differences in the main A₁ Raman mode for CZTS and CNTS are discussed in particular. According to DFTB+ phonon calculations, these differences are likely from the distinct sensitivity of the CZTS and CNTS phonon spectra to defect type and stoichiometry.
The results obtained demonstrate the potential of environmentally friendly synthesis for creating CZTS-like nanomaterials, and highlight the necessity of combining experimental methods with modelling in order to reliably establish the relationship between structure and properties.
[1] Stroyuk, O., Raevskaya, A., Selyshchev, O. et al. Sci Rep 8, 13677 (2018).
Acknowledgment:
DFG Project ZA 146/59-1 and DAAD "SOMMER!"
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