Raman study of colloidal Cu2ZnSnS4 nanocrystals obtained by "green" synthesis modified by seed nanocrystals or extra cations in the solution.

The method of affordable colloidal synthesis of nanocrystalline Cu2ZnSnS4 (CZTS) is developed, which is suitable for obtaining bare CZTS nanocrystals (NCs), cation substituted CZTS NCs, and CZTS-based hetero-NCs. For the hetero-NCs, the synthesized in advance NCs of another material are introduced into the reaction solution so that the formation of CZTS takes place preferably on these "seed" NCs. Raman spectroscopy is used as the primary method of structural characterization of the NCs in this work because it is very sensitive to the CZTS structure and allows to probe NCs both in solutions and films. Raman data are corroborated by optical absorption measurements and transmission electron microscopy on selected samples. The CdTe and Ag NCs are found to be good seed NCs, resulting in a comparable or even better quality of the CZTS compound compared to bare CZTS NCs. For Au NCs, on the contrary, no hetero-NCs could be obtained under the given condition. Partial substitution of Zn for Ba during the synthesis of bare CZTS NCs results in a superior structural quality of NCs, while the introduction of Ag for partial substitution of Cu deteriorates the structural quality of the NCs.

Synthesis and downconversion photoluminescence of Erbium-doped chalcohalide glasses of AgCl(I)-Ga2S3-La2S3 systems.

Erbium-doped chalcohalide glasses of the AgCl(I)-Ga2S3-La2S3 systems were synthesized by using the melting-cooling method. The introduction of a halide component leads to a decrease in the absorption coefficient, and the addition of erbium leads to the emergence of narrow absorption bands associated with transitions in the f-shell of Er3+ ions. Five PL bands were detected in erbium-doped samples when excited by a laser with a 532 nm wavelength, the most intense of which at 980 nm and 1540 nm are promising for use in optical amplifiers and fiber optic networks. It was established that the glass-forming matrix AgCl-Ga2S3-La2S3, doped with Er3+ ions, is an effective light-emitting medium in the near-infrared range. At the same time, the concentration quenching of the PL was observed in the AgI-Ga2S3-La2S3 system when erbium content increases.

Fermi resonance in the phonon spectra of quaternary chalcogenides 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.

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.

Band offsets and photocurrent spectroscopy of Si/Ge heterostructures with quantum dots.

Raman and lateral photoconductivity spectra of self-assembled SiGe nanoislands were studied with a height of ∼2 nm and a base of ∼20 nm formed at a temperature of 500 °C. It was estimated that the value of elastic deformation (ε(xx)) was -0.022 (ε(zz) = 0.017), while the germanium content in the islands (x) was 0.66. The obtained values of x and ε were used to calculate band offsets at the interfaces and the energy of interband transitions of structures under study. It was shown that the minimal energy of photocurrent observation is 0.52 eV, which is below the bandgap of the QDs under study. The first photocurrent component which began to contribute at 0.52 eV and had a peak at 0.68 eV is explained by optical transitions of electrons from the QD HH localized states of the valence band to the conduction band Δ(2) valley of the surrounding silicon matrix in which tensile strains are present. The second component with limiting energy of 0.73 eV can be caused by interband electron transitions from the HH valence band of the QDs to the Δ(4) valley of the QD conduction band.