Non stoichiometry effect and disorder in Cu2ZnSnS4 thin films obtained by flash evaporation Raman scattering investigation

The cation disorder in Cu2ZnSnS4 thin films grown by flash evaporation of ZnS, CuS and SnS binary compounds has been studied by Raman spectroscopy. Process parameters such as the substrate temperature during the evaporation and the Ar pressure in the post-thermal treatment determined the samples' composition and Raman spectra. As a measure of cation disorder, the half-width and relative intensity of the Raman band peaking at 331-332 cm-1 is analysed. Comparison of the spectra for different samples of known composition showed that the relative intensity of the 331 cm-1 defect peak correlates with the previously reported theoretical prediction about enhancement of antisite defect formation in Cu2ZnSnS4 under "Cu-poor, Zn-rich" conditions. For "Cu-rich, Zn-poor" films, further experimental confirmation was obtained of the previously detected effect of the enhancement of cation disorder under intense optical excitationThis research is supported by the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Program FP7/2007-2013/ under REA grant greement 269167 (PVICOKEST), the Spanish MINECO project (KEST- PV, ENE2010-21541-C03) and the OPTEC grant. RC acknowledges financial support from Spanish MINECO within the program Ramon y Cajal (RYC-2011-08521

Nanostructured Silver Substrates With Stable and Universal SERS Properties: Application to Organic Molecules and Semiconductor Nanoparticles

Nanostructured silver films have been prepared by thermal deposition on silicon, and their properties as SERS substrates investigated. The optimal conditions of the post-growth annealing of the substrates were established. Atomic force microscopy study revealed that the silver films with relatively dense and homogeneous arrays of 60–80-nm high pyramidal nanoislands are the most efficient for SERS of both organic dye and inorganic nanoparticles analytes. The noticeable enhancement of the Raman signal from colloidal nanoparticles with the help of silver island films is reported for the first time

Green synthesis of silver nanoparticles using aqueous extract of hot chili pepper fruits and its antimicrobial activity against Pseudomonas aeruginosa

Green synthesis of different nanoparticles using the plants aqueous extracts has several advantages over other methods due to the environmentally favorable nature of plants. Moreover, such approach is also cost effective. This work describes the biosynthesis of silver nanoparticles (Ag-NPs) with the use of the aqueous extract of dry pericarps of hot chili peppers (Capsicum sp. cv. Teja (S-17) and cv. Carolina Reaper) with different levels of pungency and their antibacterial effect on the antibiotic resistant Pseudomonas aeruginosa. Phytochemical screening of pericarp tissues showed great distinction in contents of phenolic compounds and capsaicinoids as potential reducing agents wich correlated with total antiradical activity as analyzed by reduction of DPPH radicals. The biosynthesized Ag-NPs were characterized by UV-vis spectrophotometry and scanning electron microscopy (SEM). The average size of the nanoparticles in both samples was less than 25 nm. Іnitial concentration of both samples of Ag-NPs inhibited P. aeruginosa growth with equal efficiency

Fermi Resonance in Phonon Spectra of Quaternary Chalcohenides of the type Cu2ZnGeS4

The experimental resonant and non-resonant Raman scattering spectra of the kesterite structural modification of Cu2ZnGeS4 single crystals are reported. The results are compared with those calculated theoretically within the density functional perturbation theory. For the majority of lines a good agreement (within 2–5 cm−1) is established between experimental and calculated mode frequencies. However, several dominant spectral lines, in particular the two intense fully symmetric modes, are found to deviate from the calculated values by as much as 20 cm−1. A possible reason for this discrepancy is found to be associated with the Fermi resonant interaction between one and two-phonon vibrational excitations. The modelling of spectra, which takes into account the symmetry of interacting states, allows a qualitative description of the observed experimental findings. Due to the similarity of the vibrational spectra of Cu2A II B IV S4 (A  =  Zn, Mn, Cd; B  =  Sn, Ge, Si) chalcogenides, Fermi resonance is argued to be a general phenomenon for this class of compounds

Phonon Raman spectra of colloidal CdTe nanocrystals: effect of size, non-stoichiometry and ligand exchange

Resonant Raman study reveals the noticeable effect of the ligand exchange on the nanocrystal (NC) surface onto the phonon spectra of colloidal CdTe NC of different size and composition. The oleic acid ligand exchange for pyridine ones was found to change noticeably the position and width of the longitudinal optical (LO) phonon mode, as well as its intensity ratio to overtones. The broad shoulder above the LO peak frequency was enhanced and sharpened after pyridine treatment, as well as with decreasing NC size. The low-frequency mode around 100 cm-1 which is commonly related with the disorder-activated acoustical phonons appears in smaller NCs but is not enhanced after pyridine treatment. Surprisingly, the feature at low-frequency shoulder of the LO peak, commonly assigned to the surface optical phonon mode, was not sensitive to ligand exchange and concomitant close packing of the NCs. An increased structural disorder on the NC surface, strain and modified electron-phonon coupling is discussed as the possible reason of the observed changes in the phonon spectrum of ligand-exchanged CdTe NCs

Near-infrared photoluminescence enhancement in Ge/CdS and Ge/ZnS core/shell nanocrystals: Utilizing IV/II-VI semiconductor epitaxy

Ge nanocrystals have a large Bohr radius and a small, size-tunable band gap that may engender direct character via strain or doping. Colloidal Ge nanocrystals are particularly interesting in the development of near-infrared materials for applications in bioimaging, telecommunications and energy conversion. Epitaxial growth of a passivating shell is a common strategy employed in the synthesis of highly luminescent II-VI, III-V and IV-VI semiconductor quantum dots. Here, we use relatively unexplored IV/II-VI epitaxy as a way to enhance the photoluminescence and improve the optical stability of colloidal Ge nanocrystals. Selected on the basis of their relatively small lattice mismatch compared with crystalline Ge, we explore the growth of epitaxial CdS and ZnS shells using the successive ion layer adsorption and reaction method. Powder X-ray diffraction and electron microscopy techniques, including energy dispersive X-ray spectroscopy and selected area electron diffraction, clearly show the controllable growth of as many as 20 epitaxial monolayers of CdS atop Ge cores. In contrast, Ge etching and/or replacement by ZnS result in relatively small Ge/ZnS nanocrystals. The presence of an epitaxial II-VI shell greatly enhances the near-infrared photoluminescence and improves the photoluminescence stability of Ge. Ge/II-VI nanocrystals are reproducibly 1-3 orders of magnitude brighter than the brightest Ge cores. Ge/4.9CdS core/shells show the highest photoluminescence quantum yield and longest radiative recombination lifetime. Thiol ligand exchange easily results in near-infrared active, water-soluble Ge/II-VI nanocrystals. We expect this synthetic IV/II-VI epitaxial approach will lead to further studies into the optoelectronic behavior and practical applications of Si and Ge-based nanomaterials

Non-stoichiometry effect and disorder in Cu2ZnSnS4 thin films obtained by flash evaporation: Raman scattering investigation

The cation disorder in Cu2ZnSnS4 thin films grown by flash evaporation of ZnS, CuS and SnS binary compounds has been studied by Raman spectroscopy. Process parameters such as the substrate temperature during the evaporation and the Ar pressure in the post-thermal treatment determined the samples' composition and Raman spectra. As a measure of cation disorder, the half-width and relative intensity of the Raman band peaking at 331-332 cm-1 is analysed. Comparison of the spectra for different samples of known composition showed that the relative intensity of the 331 cm-1 defect peak correlates with the previously reported theoretical prediction about enhancement of antisite defect formation in Cu2ZnSnS4 under "Cu-poor, Zn-rich" conditions. For "Cu-rich, Zn-poor" films, further experimental confirmation was obtained of the previously detected effect of the enhancement of cation disorder under intense optical excitationThis research is supported by the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Program FP7/2007-2013/ under REA grant greement 269167 (PVICOKEST), the Spanish MINECO project (KEST- PV, ENE2010-21541-C03) and the OPTEC grant. RC acknowledges financial support from Spanish MINECO within the program Ramon y Cajal (RYC-2011-08521

Theoretical and experimental Raman study of superlattices with GeSi quantum dots

The results of the theoretical and experimental study of Raman scattering in the quantum dot (QD) multilayers are reported. In order to obtain an adequate description of the structure with QDs and correctly explain the experimental Raman spectra, a model is proposed which takes into account the real crystal structure of both the QD and surrounding matrix, as well as the QD-matrix interaction. The secondary quantisation and Green function method were used in the theoretical calculation model. The results obtained show that crystal structures with matrix-embedded QDs can be described as a mixed crystal with specific distribution of "impurities" organised in large "molecules". A qualitative agreement in position and intensity of bands between the calculated and experimental Raman spectra for multilayer Ge/Si QD crystal structure is observed, the doublet character of the bands is explained