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.

Resonant plasmon enhancement of light emission from CdSe/CdS nanoplatelets on Au nanodisk arrays.

Semiconducting nanoplatelets (NPLs) have attracted great attention due to the superior photophysical properties compared to their quantum dot analogs. Understanding and tuning the optical and electronic properties of NPLs in a plasmonic environment is a new paradigm in the field of optoelectronics. Here, we report on the resonant plasmon enhancement of light emission including Raman scattering and photoluminescence from colloidal CdSe/CdS nanoplatelets deposited on arrays of Au nanodisks fabricated by electron beam lithography. The localized surface plasmon resonance (LSPR) of the Au nanodisk arrays can be tuned by varying the diameter of the disks. In the case of surface-enhanced Raman scattering (SERS), the Raman intensity profile follows a symmetric Gaussian shape matching the LSPR of the Au nanodisk arrays. The surface-enhanced photoluminescence (SEPL) profile of NPLs, however, follows an asymmetric Gaussian distribution highlighting a compromise between the excitation and emission enhancement mechanisms originating from energy transfer and Purcell effects. The SERS and SEPL enhancement factors depend on the nanodisk size and reach maximal values at 75 and 7, respectively, for the sizes, for which the LSPR energy of Au nanodisks coincides with interband transition energies in the semiconductor platelets. Finally, to explain the origin of the resonant enhancement behavior of SERS and SEPL, we apply a numerical simulation to calculate plasmon energies in Au nanodisk arrays and emission spectra from NPLs in such a plasmonic environment.

Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates.

Tip-enhanced Raman scattering (TERS) has recently emerged as a powerful technique for studying the local properties of low dimensional materials. Being a plasmon driven system, a dramatic enhancement of the TERS sensitivity can be achieved by an appropriate choice of the plasmonic substrate in the so-called gap-mode configuration. Here, we investigate the phonon properties of CdSe nanocrystals (NCs) utilizing gap-mode TERS. Using the Langmuir-Blodgett technique, we homogeneously deposited submonolayers of colloidal CdSe NCs on two different nanostructured plasmonic substrates. Amplified by resonant gap-mode TERS, the scattering by the optical phonon modes of CdSe NCs is markedly enhanced making it possible to observe up to the third overtone of the LO mode reliably. The home-made plasmonic substrates and TERS tips allow the analysis of the TERS images of CdSe phonon modes with nanometer spatial resolution. The CdSe phonon scattering intensity is strongly correlated with the local electromagnetic field distribution across the plasmonic substrates.

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.

Surface- and tip-enhanced resonant Raman scattering from CdSe nanocrystals.

Surface- and tip-enhanced resonant Raman scattering (resonant SERS and TERS) by optical phonons in a monolayer of CdSe quantum dots (QDs) is demonstrated. The SERS enhancement was achieved by employing plasmonically active substrates consisting of gold arrays with varying nanocluster diameters prepared by electron-beam lithography. The magnitude of the SERS enhancement depends on the localized surface plasmon resonance (LSPR) energy, which is determined by the structural parameters. The LSPR positions as a function of nanocluster diameter were experimentally determined from spectroscopic micro-ellipsometry, and compared to numerical simulations showing good qualitative agreement. The monolayer of CdSe QDs was deposited by the Langmuir-Blodgett-based technique on the SERS substrates. By tuning the excitation energy close to the band gap of the CdSe QDs and to the LSPR energy, resonant SERS by longitudinal optical (LO) phonons of CdSe QDs was realized. A SERS enhancement factor of 2 × 10(3) was achieved. This allowed the detection of higher order LO modes of CdSe QDs, evidencing the high crystalline quality of QDs. The dependence of LO phonon mode intensity on the size of Au nanoclusters reveals a resonant character, suggesting that the electromagnetic mechanism of the SERS enhancement is dominant. Finally, the resonant TERS spectrum from CdSe QDs was obtained using electrochemically etched gold tips providing an enhancement on the order of 10(4). This is an important step towards the detection of the phonon spectrum from a single QD.

Colloidal ZnO nanocrystals in dimethylsulfoxide: a new synthesis, optical, photo- and electroluminescent properties.

Stable colloidal solutions of zinc oxide in dimethylsulfoxide were synthesized via interaction between zinc(II) acetate and tetraalkylammonium hydroxides (alkyl-ethyl, propyl, butyl, and pentyl). Colloids of ZnO emit photoluminescence in a broad band with a maximum at 2.3-2.4 eV with quantum yields of up to 9-10% at room temperature and 15-16% at 80 K. The photoluminescence is supposed to originate from the radiative recombination of conduction band electrons with holes captured by deep traps having corresponding states in the band gap 1.0-1.2 eV above the valence band edge. The size of colloidal ZnO nanocrystals depends on the duration and temperature of the post-synthesis treatment and varies in the range of 3-6 nm. Growth of the ZnO nanocrystals can be terminated at any moment of the thermal treatment by freezing the colloidal solution or by addition of tetraethyl orthosilicate which hydrolyses forming core-shell ZnO@SiO2 particles. ZnO nanocrystals introduced into polyethyleneimine films can be used as an active component of an LED emitting at an applied voltage higher than 13 V.

The influence of shell parameters on phonons in core-shell nanoparticles: a resonant Raman study.

The effect of shells of various thicknesses on the vibrational resonant Raman spectra of CdSe/ZnS core-shell nanoparticles is studied. The dependence of the core-shell structure on the method of shell deposition is derived from a comparison of the vibrational and photoluminescence spectra of nanoparticles. Along with the appearance of peaks attributed to the shell, the phonon spectrum of the core undergoes significant changes upon shell growth. The change of the CdSe LO peak lineshape in core-shell nanoparticles is discussed with respect to possible changes in the spectrum of both optical and acoustical phonons upon shell formation. Based on the observed decrease of the CdSe 2LO/LO peak intensity ratio, a weakening of exciton coupling to the CdSe LO phonon upon ZnS shell deposition is supposed. The change in the carrier localization volumes upon shell formation is discussed as a possible reason for the reduced coupling.

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.

Size effects on Raman spectra of small CdSe nanoparticles in polymer films.

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidal CdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS is demonstrated by analysing the NP ensembles with different average size [Formula: see text] and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of [Formula: see text] and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS on surface optical phonons. From the experimentally observed small variation of the I(2LO)/I(LO) ratio for 2-5 nm NPs a minor effect of [Formula: see text] on the electron-phonon coupling strength in this [Formula: see text] range is supposed.