Nanooptics and photonics

The photoluminescence of I-III-VI nanocrystals synthesized under different I to III molar ratios
L. Borkovska1, A. Rachkov2, K. Goncharenko3
1 V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine. 
E-mail: l_borkovska@ukr.net
2Institute of Molecular Biology and Genetics of NASU, 150 Zabolotnogo Str., 03680 Kyiv, Ukraine
 3Institute of High Technologies of Taras Shevchenko National University of Kyiv, 4g Hlushkova Avenue, 03022  Kyiv, Ukraine.
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The CuInS2 and AgInS2 are the low-toxic I-III-VI semiconductor materials with direct band gap and high absorption coefficient in the visible and near-IR regions. The interest to nanocrystals (NCs) of I-III-VI semiconductors is motivated by the searching of highly luminescent materials composed of the elements that are available in large quantities and with reduced toxicity. In fact, a number of successful applications of the CuInS2 and AgInS2 NCs for both in vitro and in vivo imaging has been demonstrated. 
Herein, we present the results of the investigations of peculiarities of the photoluminescence (PL) of the Cu-In-S and Ag-In-S NCs synthesized in aqueous media in the presence of mercaptoacetic acid under different [Cu] : [In] and [Ag] : [In] molar ratios. The pristine colloidal solution of the NCs and the NCs embedded in the polymer films were studied. 
The NCs show an intense PL in the orange-red spectral range ascribed to radiative transitions of carriers via the levels of intrinsic defects in the NCs. The PL band is characterized by large full width at a half maximum (~100-150 nm) and large Stokes shift of several hundreds meV. The PL spectra of Cu-In-S NCs contained one broad band, and that of the Ag-In-S NCs showed two broad bands. The latter was ascribed to radiative recombination in two ensembles of NCs of different sizes. The bimodal distribution of the Ag-In-S NCs was proved by the PL study of the NC fractions separated by ultracentrifugation. An increase in the Ag content in the NCs from Ag0.5In4S7 to Ag1.0In4S7 produced the shift of the PL band from 603 to 667 nm, and an increase of Cu content in the NCs from Cu0.3In4S7 to Cu0.8In4S7 produced the shift of PL band from 650 to 700 nm, respectively. The largest PL intensity was found for the Ag1.0In4S7 and Cu1.0In4S7 NCs. The peculiarities of PL excitation spectra of the NCs are discussed.