Synthesis and Electronic Structure of Mid-Infrared Absorbing Cu₃SbSe₄ and CuSbSe₄ Nanocrystals

Aliovalent I-V-VI semiconductor nanocrystals are promising candidates for thermoelectric and optoelectronic applications. Famatinite Cu₃SbSe₄ stands out due to its high absorption coefficient and narrow band gap in the mid-infrared spectral range. This paper combines experiment and theory to investigate the synthesis and electronic structure of colloidal CuSbSe₄ nanocrystals. We achieve predictive composition control of size-uniform CuSbSe₄ (x = 1.9-3.4) nanocrystals. Density functional theory (DFT)-parametrized tight-binding simulations on nanocrystals show that the more the Cu-vacancies, the wider the band gap of CuSbSe₄ nanocrystals, a trend which we also confirm experimentally via FTIR spectroscopy. We show that Sb_Cu antisite defects can create mid-gap states, which may give rise to sub-bandgap absorption. This work provides a detailed study of CuSbSe₄ nanocrystals and highlights the potential opportunities as well as challenges for their application in infrared devices.