Larisa L. Sveshnikova

Optical vibrational modes in (Cd, Pb, Zn)S quantum dots embedded in Langmuir–Blodgett matrices

Abstract Structures with CdS, PbS, ZnS quantum dots (QDs) produced using the Langmuir–Blodgett technique were investigated by infrared (IR), Raman and ultra-violet–visible (UV–Vis) spectroscopies. The QDs size derived from the analysis of UV–Vis spectra and high-resolution transmission electron microscopy images amounts to 2–6 nm. The IR and Raman spectra reveal longitudinal optical phonons localised in QDs and surface vibrational modes. The frequency positions of the surface optical vibrational modes are adequately described taking into account confinement of fundamental optical phonons in the QDs.

Combination of surface- and interference-enhanced Raman scattering by CuS nanocrystals on nanopatterned Au structures

Summary We present the results of a Raman study of optical phonons in CuS nanocrystals (NCs) with a low areal density fabricated through the Langmuir–Blodgett technology on nanopatterned Au nanocluster arrays using a combination of surface- and interference-enhanced Raman scattering (SERS and IERS, respectively). Micro-Raman spectra of one monolayer of CuS NCs deposited on a bare Si substrate reveal only features corresponding to crystalline Si. However, a new relatively strong peak occurs in the Raman spectrum of CuS NCs on Au nanocluster arrays at 474 cm−1. This feature is related to the optical phonon mode in CuS NCs and manifests the SERS effect. For CuS NCs deposited on a SiO2 layer this phonon mode is also observed due to the IERS effect. Its intensity changes periodically with increasing SiO2 layer thickness for different laser excitation lines and is enhanced by a factor of about 30. CuS NCs formed on Au nanocluster arrays fabricated on IERS substrates combine the advantages of SERS and IERS and demonstrate stronger SERS enhancement allowing for the observation of Raman signals from CuS NCs with an ultra-low areal density.

Vibrational spectra of quantum dots formed by Langmuir–Blodgett technique

Phonon spectra of CdS, ZnS, PbS, CuS, Ag2S, and ZnO quantum dots formed by using the Langmuir–Blodgett technology are investigated by Raman and infrared spectroscopies. The Raman spectra of structures show peaks corresponding to the scattering by longitudinal and transverse optical (LO and TO) phonons localized in quantum dots that confirm the formation of nanocrystals. In addition to TO and LO phonon modes, the modes observed in the IR spectra between the frequency positions of LO and TO phonons are attributed to the surface optical phonons in quantum dots.

On the low-temperature onset of molecular flexibility in lipid bilayers seen by Raman scattering.

For dipalmitoylphosphatidylcholine (DPPC) lipid/water bilayers, a detailed temperature dependence of the Raman scattering spectra at the spectral range of the CH 2-stretching modes was investigated. Below 150 K the ratio of intensities of the 2880 cm (-1) antisymmetric vibration line and the 2850 cm (-1) symmetric one was found to be nearly temperature-independent. Between 150 and 230 K it decreases slightly as temperature increases; and above 230 K it decreases remarkably. This decrease is accompanied with broadening of the antisymmetric line, from 4.2 cm (-1) at 100 K to 5.7 cm (-1) at 296 K. According to literature, the decrease of the antisymmetric line may be interpreted in two ways: (i) the appearance of a static conformational disorder (or of a disorder fluctuating at the time scale larger than picoseconds) and (ii) relaxation at the ps time scale, which is induced by coupling with temperature-activated librational-torsional motion of the lipid chain. Both these interpretations imply that obtained data evidence the appearance of molecular flexibility of lipids around approximately 200 K. The observed effect is to be compared with low-temperature dynamical transition found in disordered media with neutron scattering, Mossbauer absorption, molecular dynamics simulations and other techniques. This transition implies that with temperature increase harmonic atomic motions are transformed to large-amplitude anharmonic (or stochastic) ones. The characteristic times of these motions lay at the ps time scale. The closeness of the temperature of the transition and of the time scale of motions with those found in this work by Raman scattering for lipid bilayers supports the dynamic nature of the 2880 cm (-1) antisymmetric vibration line decrease (i.e., that it is induced by coupling with libration-torsion). To prove that the observed onset of flexibility is a property of a disordered state, Langmuir-Blodgett films of behenic acid were studied. These films contain, like lipids, long CH 2-tails, but, in opposite to bilayers, they have a well-ordered crystalline-like structure. The relative intensity of the antisymmetric/symmetric CH 2-stretching lines was found in these films to be temperature-independent in the whole temperature range studied, between 60 and 296 K.

CdZnS quantum dots formed by the Langmuir–Blodgett technique

CdZnS quantum dots (QDs) with systematically varied Zn content (from 0 to 100%) are formed in an organic matrix using the Langmuir–Blodgett technique. Annealing of the QD structures leads to a removal of the organic matrix and an increase in the Zn content for free-standing CdZnS QDs. After annealing, the size of QDs as determined from UV–vis absorption experiments is in good agreement with electron microscopy measurements. Analysis of UV–vis absorption and Raman scattering data demonstrates strong changes in the content of the CdZnS QDs upon annealing. A model of the process of QD formation is developed using the precipitation model and is found to adequately describe the experimental results.

Surface-enhanced Raman scattering by colloidal CdSe nanocrystal submonolayers fabricated by the Langmuir–Blodgett technique

Summary We present the results of an investigation of surface-enhanced Raman scattering (SERS) by optical phonons in colloidal CdSe nanocrystals (NCs) homogeneously deposited on both arrays of Au nanoclusters and Au dimers using the Langmuir–Blodgett technique. The coverage of the deposited NCs was less than one monolayer, as determined by transmission and scanning electron microscopy. SERS by optical phonons in CdSe nanocrystals showed a significant enhancement that depends resonantly on the Au nanocluster and dimer size, and thus on the localized surface plasmon resonance (LSPR) energy. The deposition of CdSe nanocrystals on the Au dimer nanocluster arrays enabled us to study the polarization dependence of SERS. The maximal SERS signal was observed for light polarization parallel to the dimer axis. The polarization ratio of the SERS signal parallel and perpendicular to the dimer axis was 20. The SERS signal intensity was also investigated as a function of the distance between nanoclusters in a dimer. Here the maximal SERS enhancement was observed for the minimal distance studied (about 10 nm), confirming the formation of SERS “hot spots”.

Nanoantenna structures for the detection of phonons in nanocrystals

We report a study of the infrared response by localized surface plasmon resonance (LSPR) modes in gold micro- and nanoantenna arrays with various morphologies and surface-enhanced infrared absorption (SEIRA) by optical phonons of semiconductor nanocrystals (NCs) deposited on the arrays. The arrays of nano- and microantennas fabricated with nano- and photolithography reveal infrared-active LSPR modes of energy ranging from the mid to far-infrared that allow the IR response from very low concentrations of organic and inorganic materials deposited onto the arrays to be analyzed. The Langmuir–Blodgett technology was used for homogeneous deposition of CdSe, CdS, and PbS NC monolayers on the antenna arrays. The structural parameters of the arrays were confirmed by scanning electron microscopy. 3D full-wave electromagnetic simulations of the electromagnetic field distribution around the micro- and nanoantennas were employed to realize the maximal SEIRA enhancement for structural parameters of the arrays whereby the LSPR and the NC optical phonon energies coincide. The SEIRA experiments quantitatively confirmed the computational results. The maximum SEIRA enhancement was observed for linear nanoantennas with optimized structural parameters determined from the electromagnetic simulations. The frequency position of the feature’s absorption seen in the SEIRA response evidences that the NC surface and transverse optical phonons are activated in the infrared spectra.

Raman scattering for probing semiconductor nanostructures: From nanocrystal arrays towards a single nanocrystal

We present a study of resonant and surface enhanced Raman scattering (SERS) by arrays of CdS, CuS, ZnO, and PbSe nanocrystals (NCs) with various areal density. Resonant Raman scattering by optical phonons and their overtones up to 9th order was observed for ZnO NC arrays by adjusting the laser energy to that of the interband transitions. The resonance enhancement allowed a Raman response from arrays of NCs with a low areal density (down to 10 PbSe NCs per 1 μm2) to be measured. An enhancement of Raman scattering by LO modes in CdS NC arrays by a factor of about 730 due to the resonant SERS effect was demonstrated. SERS effect by optical phonons in CuS NCs with ultra-low areal density formed on laterally ordered arrays of Au nanoclusters was observed.

Surface-enhanced Raman scattering by GaN and ZnO nanostructures

Surface enhanced Raman scattering is studied in nanostructures with GaN and ZnO nanocolumns. Features related to optical phonons of nanocolumns are observed in the Raman spectra of both free-standing GaN and ZnO nanostructures. Strong enhancement of the Raman scattering intensity by surface optical phonons in GaN and ZnO nanocolumns is observed by deposition of silver clusters onto the nanostructure surfaces.

Clustering of CdS nanocrystals during evaporation of Langmuir — Blodgett matrix

Clustering of CdS nanocrystals during evaporation of the Langmure—Blodgett matrix and influence of matrix thickness on the parameters of the resulting clusters have been investigated. Information about the size and spatial distribution of nanocrystals and clusters has been obtained using atomic-force microscopy. It has been found that at thickness of the matrix less than 6 nm the free-standing nanocrystals are formed; with increase in thickness of the matrix nanocrystals are collected in clusters. The clusters greatly enlarge with further increase in thickness of the matrix, eventually; at thickness of 60 nm nanocrystals totally coat the substrate.