Nanoparticle compositions and methods for synthesizing multinary chalcogenide CZTSSe nanoparticles containing Cu, Zn, and Sn in combination with S, Se or both are described. The nanoparticles may be incorporated into one or more ink solutions alone or in combination with other chalcogenide-based particles to make thin films useful for photovoltaic applications, including thin films from multilayer particle films having a composition profile. The composition and stoichiometry of the thin films may be further modified by subjecting the particle films to gas or liquid phase chalcogen exchange reactions.