Wurtzite-Chalcopyrite Crystal Polymorphism in CuGaS₂ Nanoparticles Dictates the Localized Surface Plasmon Resonance

Localized surface plasmon resonance (LSPR) is widely applied in photo-functional technologies such as photocatalysis and solar cells. Semiconductor nanoparticles (NPs) have recently become a target for LSPR materials research due to the fact that their composition and structure flexibility allow for the fine-tuning of optical properties. However, due to the difficulty of controlling the crystal structure of NPs, many of the crystallographic factors governing the LSPR of semiconductor NPs are not understood. Here, we report on the crystal structure and size-controlled synthesis of CuGaS₂ (CGS) NPs, which are typical I-III-VI₂ semiconductors. Moreover, we investigate their LSPR properties and reveal the crystal structure dependence of LSPR extinction. We show that chalcopyrite and wurtzite CGS (c-CGS and w-CGS, respectively) dictate LSPR in the NIR region. Moreover, the extinction coefficient of c-CGS was approximately 3 times larger than that of w-CGS, which we explain in terms of the larger polarizability of c-CGS. These results deepen our understanding of semiconductor plasmonics and may serve as a foundation for developing highly efficient energy conversion systems.