An Investigation into the Selenization Mechanisms of Wurtzite CZTS Nanorods.

Here we report the first detailed investigation into the selenization mechanism of thin films of wurtzite copper zinc tin sulfide (CZTS) nanorods (NRs), giving particular emphasis to the role of the long-chain organic ligands sur-rounding each NR. During selenization, the NRs undergo a selenium-mediated phase change from wurtzite to kesterite, concurrent with the replacement of sulfur with selenium in the lattice and in-situ grain growth, along with the recrystallization of larger copper zinc tin selenide (CZTSe) kesterite grains on top of the existing film. By utilizing a facile ligand removal technique, we demonstrate that the formation of a large grain overlayer is achievable without the presence of ligands. In addition, we demonstrate an elegant ligand-exchange based method for controlling the thickness of the fine grain layer. This report emphasizes the key role played by ligands in determining the structural evolution of CZTS nanocrystal films during selenization, necessitating the identification of optimal ligand chemistries and processing conditions for desirable grain growth.