The photoluminescence of CuInS2 nanocrystals: effect of non-stoichiometry and surface modification

Effects of large copper deficiency and surface modification on the photoluminescence of CuInS2 nanocrystals were explored. The large copper deficiency improved PL intensity due to the enhanced internal defect-related emission. Surface modification of copper deficient nanocrystals by simply refluxing with zinc acetate and fatty acid resulted in more than 10 times improvement in the photoluminescence intensity and a large blueshift of the photoluminescence spectra. The uniformity in size/shape distribution after surface modification was attributed to the origin of shrinkage in Stokes shift from similar to 600 meV to similar to 300 meV in Cu0.2InS2/ZnS nanocrystals. Furthermore, the contribution of lattice strain to this large blueshift in emission wavelength in Cu0.2InS2/ZnS nanocrystals was proposed and tested with the CdS shell layers. The electronic process underlying the large enhancement of PL intensity was studied with time-resolved and temperature-dependent photoluminescence. This drastically enhanced photoluminescence after surface modification was attributed to the efficient reduction of non-radiative recombination originated from surface trap states, the activation energy of which was estimated to be 85 meV.