The Photoluminescence Properties of CuInS2 and AgInS2 Nanocrystals Synthesized in Aqueous Solutions

The CuInS₂ and AgInS₂ are the low-toxic semiconductor materials with direct band gap and high absorption coefficient in the visible and near-infrared regions. The nanocrystals (NCs) based on these materials demonstrate enhanced photoluminescence (PL) tunable over the whole visible spectral range resulting in growing attention for their application in light-emitting devices, solar cells and bio-markers.

Despite the fact that different strategies have been developed for the synthesis of I-III-VI NCs, it still remains a challenge to develop a facile, general, and inexpensive method for the preparation of I-III-VI NCs with controllable luminescence spectrum.

Herein, we report the results of investigations of the photoluminescent, optical and structural properties of CuInS₂ and AgInS₂ NCs synthesized in mild conditions in aqueous solutions in the presence of aliphatic mercapoacids. The pristine colloidal solution of the NCs, the powdered NCs as well as the NC embedded in the polymer films of gelatin or polyvinyl alcohol were studied. The X-ray diffraction patterns and the Raman scattering spectra of powdered NCs indicated their tetragonal phase. The average size of the NCs was estimated to be about 4 nm for the AgInS₂ and about 2 nm for the CuInS₂ ones. The optical absorption spectra of the NCs solution as well as of the nanocomposite films showed the band onset larger than the band gap of the corresponding bulk chalcopyrite materials indicating a quantum confinement effect.

The PL spectra showed a wide band with maximum at about 2.0 eV for the AgInS₂ NCs and 1.8 eV for the CuInS₂ NCs and a Stokes shift in the range of 0.3-0.7 eV. The PL band is caused by carrier recombination via the levels of defects in the NCs. No exciton emission was observed. As the temperature decreased, the PL intensity increased, the PL band broadened and the PL maximum shifted to the blue on about 0.02 eV for the CuInS₂ NCs and on about 0.27 eV for the AgInS₂ NCs. It is shown that the PL spectra of the CuInS₂ NCs can be well fitted with single Gaussian function, while those for the AgInS₂ NCs by two. The PL band spectral position did not change essentially with the excitation power, but varied with the excitation energy (wavelength). The latter is explained by strong dependence of the PL excitation spectra on the emission position in the PL spectrum. For the AgInS₂ NCs this dependence was found to be different for two PL components which allowed supposing that these PL components were connected with two different ensembles of the NCs.

It is shown that the PL quantum yield in the NCs studied can reach around 15% that makes them suitable for future optoelectronic and bio-medical applications.