Synthesis of highly non-stoichiometric Cu2ZnSnS4 nanoparticles with tunable bandgaps

Non-stoichiometric Cu2ZnSnS4 nanoparticles with average diameters of 4–15 nm and quasi-polyhedral shape were successfully synthesized by a colloidal method. We found that a non-stoichiometric composition of Zn to Cu in Cu2ZnSnS4 nanoparticles yielded a correlation where Zn content increased with a decrease in Cu content, suggesting formation of lattice defects relating to Cu and Zn, such as a Cu vacancy (VCu), antisite with Zn replacing Cu (ZnCu), and/or defect cluster of VCu and ZnCu. The bandgap energy of Cu2ZnSnS4 nanoparticles systematically varies between 1.56 and 1.83 eV depending on the composition ratios of Cu and Zn, resulting in a wider bandgap for Cu-deficient Cu2ZnSnS4 nanoparticles. These characteristics can be ascribed to the modification in electronic band structures due to formation of VCu and ZnCu on the analogy of ternary copper chalcogenide, chalcopyrite CuInSe2, in which the top of the valence band shifts downward with decreasing Cu contents, because much like the structure of CuInSe2, the top of the valence band is composed of a Cu 3d orbital in Cu2ZnSnS4.