Physico-Chemical Nanomaterials Science

Raman Spectroscopy of Ag–In–S Polycrystals, 
Films, and Nanoparticles 
Azhniuk Y.M.1,2, Gomonnai A.V.1,2, Solonenko D.3, Loya V.Y. 1, Voynarovych I.M.1, Lopushanska B.V.2, Roman I.Y.1, 
Lopushansky V.V.1, 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, Chemnitz, Germany
Raman spectroscopy is a versatile tool to study the structure of solids. It enables one to distinguish between crystalline and amorphous materials, provides important information about the type of the structure, chemical composition, and strain. Here we present a comparative Raman study of Ag–In–S polycrystals, thermally evaporated films, and nanoparticles of similar chemical composition prepared by colloidal synthesis.
Polycrystalline AgInS2 and AgIn5S8 samples were synthesised from elemental compounds. Ag–In–S films with different chemical compositions were prepared by thermal evaporation. Ag–In–S nanoparticles with average size of 2–5 nm were synthesised in an exchange reaction between Na2S and a mixture of Ag(I) and In(III) complexes with glutathione in aqueous solutions at mild conditions (96-98C). The colloidal solutions were size-fractioned by repeated centrifuging. The presence of the nanoparticles in the colloidal solutions was confirmed by the size-dependent shift of the optical absorption edge and broadband photoluminescence.
Micro-Raman spectra of the Ag–In–S polycrystals, films, and nanoparticles were measured at room temperature using a Horiba LabRAM HR800 spectrometer with a cooled CCD camera. The excitation was provided by a solid-state lasers (exc = 488 or 514.7 nm). The instrumental resolution was better than 2.5 cm–1. The analysis of the Raman spectra of the Ag–In–S polycrystals and films enabled us to judge upon their structure. The Raman spectra of the Ag–In–S nanoparticles clearly reveal the contributions of surface phonons and confinement-related nonzero-wavevector phonons which typically increase with decreasing nanocrystal size. Spectra of the nanoparticles prepared with various contents of elements in the reaction mixture exhibit a strong similarity determined by the fact that the nanoparticle surface is terminated by sulphur atoms simultaneously belonging to the mercapto group of the glutathione capping ligand.