Nanooptics and photonics

Optical properties of nanocomposites with CdSe quantum dots doped with Ag impurities
L. Borkovska1, N. Korsunska1, T. Stara1, A. Rachkov 2 º
1V. Lashkaryov Institute of Semiconductor Physics of the NAS of Ukraine, 45 Prospect Nauky, 03028 Kyiv, Ukraine. 
E-mail: l_borkovska@ukr.net
2 Institute of Molecular Biology and Genetics of NAS of Ukraine, 150 Zabolotnogo Str., 03680, Kyiv, Ukraine, 

Doping quantum dots (QDs) with transition metal impurities gives the possibility to tune their emission color without changing the size and composition of the host matrix. Here we present the results of study of the optical properties of composites with CdSe QDs doped with Ag impurities by the photoluminescence (PL), optical absorption and Raman spectroscopy.
The QDs were synthesized at room temperature in aqueous media in the presence of mercaptoacetic acid as a stabilizer. The doping with Ag impurities was carried out in the process of QD synthesis. After dialysis the QDs were transferred to the solution polyvinyl alcohol (PVA) or gelatine and dried on glass slides to produce composite films. The films were subjected to 409 nm LED’s light irradiation at room temperature for several hours. 
The resonant Raman spectra of the composites demonstrated the peak at 200 cm-1 corresponding to longitudinal optical (LO) phonon mode of CdSe QDs. The PL spectra of undoped QDs show an exciton PL band peaked at 490 nm and a wide defect-related PL band at 650 nm. The PL spectra of Ag-doped QDs showed defect-related band only with maximum shifted to 550 nm. The band is ascribed to carrier recombination via the level of shallow acceptor created due to Ag doping. The PL study revealed several ensembles of Ag-doped QDs each of them produced their own PL band characterized by a Stokes shift of about 430 meV. 
Irradiation of undoped QDs produced the increase of exciton PL intensity and the decrease of defect-related one ascribed to reconstruction of defect states at the QD surface. In turn, irradiation of Ag-doped QDs resulted in the decreasing of PL intensity and pronounced darkening of the composite. This was accompanied by the decrease of LO phonon mode intensity without changes of phonon frequency. The effect is ascribed to reduction of Ag containing species at the QD surface to metallic Ag under QD photoexcitation. The effects were found only for the QDs in PVA matrix indicating much higher photo stability of QD-gelatine composites.