Peter S. Parfenov

Chlorin e6–ZnSe/ZnS quantum dots based system as reagent for photodynamic therapy

Stable water-soluble complexes of Cd-free ZnSe/ZnS quantum dots (QDs) and chlorin e6 complexes have been prepared. These complexes have shown approximately 50% intracomplex fluorescence resonance energy transfer from QDs to chlorin e6. The photodynamic therapy (PDT) test of the complexes against the Erlich acsite carcinoma cell culture demonstrated a two-fold enhancement of the cancer cell photodynamic destruction as compared to that of free chlorin e6 molecules. It was shown that the PDT effect was significantly increased due to two factors: the efficient QD-chlorin e6 photoexcitation energy transfer and the improvement of cellular uptake of the photosensitizer in the presence of ZnSe/ZnS QDs.

Self-Organization of Colloidal PbS Quantum Dots into Highly Ordered Superlattices

X-ray structural analysis, together with steady-state and transient optical spectroscopy, is used for studying the morphology and optical properties of quantum dot superlattices (QDSLs) formed on glass substrates by the self-organization of PbS quantum dots with a variety of surface ligands. The diameter of the PbS QDs varies from 2.8 to 8.9 nm. The QDSLs period is proportional to the dot diameter, increasing slightly with dot size due to the increase in ligand layer thickness. Removal of the ligands has a number of effects on the morphology of QDSLs formed from the dots of different sizes: for small QDs the reduction in the amount of ligands obstructs the self-organization process, impairing the ordering of the QDSLs, while for large QDs the ordering of the superlattice structure is improved, with an interdot distance as low as 0.4 nm allowing rapid charge carrier transport through the QDSLs. QDSL formation does not induce significant changes to the absorption and photoluminescence spectra of the QDs. However, the luminescence decay time is reduced dramatically, due to the appearance of nonradiative relaxation channels.

Measurement of the luminescence decay times of PbS quantum dots in the near-IR spectral range

Methods for recording luminescence decay times of semiconductor PbS quantum dots (QDs) with optical transitions in the near-IR spectral range have been analyzed. A measuring complex for spectral and kinetic analysis in the near-IR range (0.8–2.0 μm) in the time interval from several tens of nanoseconds to several tens of microseconds is described. In this complex, a semiconductor picosecond laser is used as an excitation source; luminescence decay times are recorded by a fast InGaAs photodiode, a high-speed amplifier, and a high-frequency oscilloscope; and the measurement results are multiply averaged (up to a million times) by a program. The technical features of this method are discussed and compared with the characteristics of techniques based on photon counting or application of more powerful radiation sources, and the limitations on sensitivity are analyzed. The results of measuring the luminescence decay kinetics of PbS QDs 2.7–7.6 nm in size prepared in the form of solutions and incorporated into matrices are reported.

A complex for the fluorescence analysis of macro- and microsamples in the near-infrared

This paper describes the design features and technical parameters of a complex for characterizing the fluorescence parameters of macro- and microsamples in the near-IR region. Along with the standard 90°setup for exciting and recording the fluorescence, a microfluorimetric technique is used. Two types of InGaAs photodiodes are compared, and the features of the measurements are discussed, using as an example the recording of the IR fluorescence spectra of PbS quantum dots.

Note: Near infrared spectral and transient measurements of PbS quantum dots luminescence

We describe an experimental setup for the characterization of luminescence from nanostructures. The setup is intended for steady-state and time-resolved luminescence measurements in the near-infrared region. The setup allows us to study spectral luminescence properties in the spectral range of 0.8-2.0 μm with high spectral resolution and kinetic luminescence properties between 0.8 and 1.7 μm with a time resolution of 3 ns. The capabilities of the system are illustrated by taking luminescence measurements from PbS quantum dots. We established the size dependencies of the optical properties of the PbS quantum dots over a wide spectral range. Finally, the energy transfer process was studied with a high temporal and spectral resolution.

Optical properties of ordered superstructures formed from cadmium and lead chalcogenide colloidal nanocrystals

The optical properties of three-dimensional ordered superstructures formed on glass substrates by self-assembly of cadmium selenide or lead sulfide nanocrystals (NCs) are investigated and compared to the optical properties of the initial NC colloidal solutions. The formation of the superstructures is strongly correlated to the presence of oleic acid molecules on the surface of the NCs. It is found that the absorption band of the NCs in the superstructures is broadened and shifted to shorter wavelengths in comparison with the absorption band of the NCs in solution. The luminescence spectra of the NCs in the superstructures also differ from the spectra of the NCs in solution. The observed modification of optical properties of superstructures is a manifestation of interactions between the NCs and the chemical environment within the superstructures.

Size-dependent room-temperature luminescence decay from PbS quantum dots

We study size dependence of kinetic and spectral properties of near-infrared luminescence from PbS quantum dots in colloidal solution. Luminescence lifetimes are found to lie between 250 ns for the largest quantum dots and 2:5 μs for the smallest ones, while the Stokes shift is found to increase from 4-5 to 300 meV. These results are explained by the presence of the long-living in-gap state, with the size-dependent energy. Analytical modeling shows that the relaxation from this state is dominant in small quantum dots and negligible in large ones. Biexponential luminescence decay with the size-dependent recombination rates is predicted for quantum dots of all sizes.

Self-organization of lead sulfide quantum dots of different sizes

X-ray structural analysis is used for investigation of structures, obtained by self-organization from nanoparticles - lead sulfide (PbS) quantum dots (QDs) of different sizes - deposited on a glass substrate or embedded in a porous matrix. These nanostructures obtained by both methods represent ordered close-packed structures of the nanoparticles. The configuration of obtained structures does not depend on the nanoparticle size and type of substrate and matrix.

Photophysical manifestations of interactions of quantum dots with ortho-phenanthroline molecules

The interaction of CdSe/ZnS nanocrystals (NCs) (solubilized by trioctylphosphine oxide (TOPO) molecules) with ortho-phenanthroline (OP) molecules has been investigated. It is shown that OP molecules can replace TOPO; in this case, bonding of OP molecules with the nanocrystal surface does not lead to the formation of NC aggregates. It is established that the difference in the evolution of the spectral-luminescence NC characteristics in a mixture with OP is independent of the NC size. A model of the interaction of NCs with OP molecules is proposed.

Reorientation dynamics of a dual-frequency nematic single crystal with quasi-homeotropic structure

This paper discusses the features of the switching dynamics of the phase lag of light by 2π and 4π in cells with a dual-frequency nematic liquid crystal as a result of bend and splay deformation of layers with oblique homeotropic orientation by means of an electric field with frequency 30kHz and as a result of relaxation when a low-frequency field of 1kHz is applied. The slope angle of the director was varied from 36° to 84° by means of cerium oxide layers obliquely deposited in vacuum. Studies of the surface texture of the orienting layers by means of atomic-force microscopy showed that this is achieved as a result of variation of the surface nanotexture.