Nanocomposites and nanomaterials

CdS nanocrystals formed in amorphous GeS2:Cd films 
by photoenhanced diffusion
Azhniuk Yu.M.1,2, Lopushansky V.V.1, Solonenko D.3, Kryshenik V.M.2, Loya V.Yu.1, Voynarovych I.M.1, Gomonnai A.V.1,2, Zahn D.R.T.3 
1 Institute of Electron Physics, Ukr. Nat. Acad. Sci., 21 Universytetska Str., 88017 Uzhhorod, Ukraine. 
E-mail: yu.azhniuk@gmail.com
2 Uzhhorod National University, 46 Pidhirna Str., 88000 Uzhhorod, Ukraine.
3 Semiconductor Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany.

Besides well-known methods of formation of matrix-embedded II–VI nanocrystals (NCs) (colloidal synthesis, diffusion-limited growth, Langmuir-Blodgett films, sequential ion implantation), one can attempt to fabricate II–VI NCs embedded in semiconducting films of amorphous chalcogenides. Here we report on CdS NCs formed in amorphous GeS2 films due to enhanced diffusion induced by laser illumination during Raman measurements. 
1–2 m thick GeS2 films doped with Cd (nominal content of 1 to 10%) were prepared by thermal evaporation on silicate glass and Si substrates. The film morphology was checked by atomic force microscopy (Agilent AFM 5420) showing a uniform film surface with average roughness of about 5 nm. Optical absorption spectra of the films were measured using a Cary 50 spectrometer (Varian). X-ray diffraction measurements (Rigaku Smartlab) as well as micro-Raman spectroscopy (Horiba LabRAM, 514.7 nm solid-state laser) confirmed the amorphous structure of the films.
The formation of CdS NCs is revealed in the Raman spectra of the Cd-doped films with Cd content above 5 %. Increasing the laser power density Pexc leads to new sharp peaks related to CdS LO (near 300 cm–1) and 2LO (near 600 cm–1) phonons appearing in addition to the broader maxima of amorphous GeS2. High-Pexc illumination leads to a photoinduced (non-thermal) drastic drop of the film viscosity and enhanced diffusion of Cd and S in the amorphous GeS2 film. This effect results in a radial mass transfer from the laser spot and facilitates the CdS NC formation. The latter is confirmed by the appearance of a PL maximum enabling their average size to be estimated.