Selective Phase Transformation of Wurtzite Cu₂ZnSn(SSe)₄ (CZTSSe) Nanocrystals into Zinc-Blende and Kesterite Phases by Solution and Solid State Transformations

A wide range of physical properties from optoelectronic (e.g., band gap) to structural (e.g., hardness) can be affected without changing the composition of a solid but just by altering the crystal phase. Here, we report a selective structural phase transition of metastable wurtzite CZTSSe nanocrystals into more stable phases such as zinc blende and kesterite using solution and solid state transformations, respectively. The phase transformation pathways are selective, with the wurtzite to zinc-blende transition not occurring because of thermal annealing or the wurtzite to kesterite transition because of solution transformation. We show the importance of both ligand chemistry and temperature to lowering the barrier for rapid conversion from one phase to another in this system. The phase transitions are accompanied by changes in the band gap with values for each calculated by cyclic voltammetry.