M. S. Smirnov

Decay of Electronic Excitations in CdS and CdS/ZnS Colloidal Quantum Dots: Spectral and Kinetic Investigations

Using the spectral methods of induced absorption, luminescence, and photostimulated luminescence flash, we have experimentally investigated processes of decay of electronic excitations in CdS colloidal quantum dots and in CdS/ZnS “core-shell” systems synthesized in gelatin by the sol-gel method. It has been shown that the decay of electronic excitations in colloidal quantum dots of this type is predominantly related to a fast localization of nonequilibrium charge carriers on surface defects and their subsequent recombination during times on the order of units and tens of picoseconds. The passage to core-shell systems eliminates, to a large extent, surface defects of the core, some of which are luminescence centers. However, upon using the sol-gel synthesis, a noticeable fraction of luminescence centers are formed in the interior of the CdS quantum dot, which, as well as in the case of CdS/ZnS systems, ensures localization of exciton, blocks its direct annihilation, and maintains recombination radiation.

Optical and structural properties of ensembles of colloidal Ag2S quantum dots in gelatin

The size dependences of the absorption and luminescence spectra of ensembles of hydrophilic colloidal Ag2S quantum dots produced by the sol-gel method and dispersed in gelatin are analyzed. By X-ray diffraction analysis and transmission electron microscopy, the formation of core/shell nanoparticles is detected. The characteristic feature of the nanoparticles is the formation of crystalline cores, 1.5–2.0 nm in dimensions, and shells of gelatin and its complexes with the components of synthesis. The observed slight size dependence of the position of infrared photoluminescence bands (in the range 1000–1400 nm) in the ensembles of hydrophilic colloidal Ag2S quantum dots is explained within the context of the model of the radiative recombination of electrons localized at structural and impurity defects with free holes.

Structural and spectral features of MOCVD AlxGayIn1 − x − yAszP1 − z/GaAs (100) alloys

The study is concerned with MOCVD epitaxial heterostructures grown on the basis of AlxGayIn1 − x − yAszP1 − z quinary alloys in the region of alloy compositions isoperiodic to GaAs. By the X-ray diffraction technique and atomic force microscopy, it is shown that, on the surface of the heterostructures, there are nanometric objects capable of lining up along a certain direction. From calculations of the crystal lattice parameters with consideration for internal strains, it can be inferred that the new compound is a phase based on the AlxGayIn1 − x − yAszP1 − z alloy.

Sensitization of photoprocesses in colloidal Ag2S quantum dots by dye molecules

Abstract. The effect of photosensitization of IR luminescence excitation (1205 nm) of colloidal Ag2S quantum dots (QDs) with average size of 2.5±0.6  nm in gelatin at 600 to 660 nm by molecules of 3,3’-di-(γ-sulfopropyl)-4,4’,5,5’-dibenzo-9-ethylthiacarbocyanine betaine pyridinium salt (Dye1) and thionine dye (Dye2) was registered. Cis-J-aggregates of Dye1 and cations monomer of Dye2 conjugated with Ag2S QDs take part in this process. The photosensitization of luminescence excitation of colloidal Ag2S QDs was interpreted by resonance nonradiation transfer of electronic excitation energy from cis-J-aggregates of Dye1 and cations of Dye2 to centers of recombination luminescence of Ag2S QDs.

Spectral manifestations of hybrid association of CdS colloidal quantum dots with methylene blue molecules

We have studied the spectral properties of mixtures formed by CdS colloidal quantum dots with an average diameter of 2.5 nm and methylene blue molecules and that are dispersed into gelatin. We have revealed that, in the presence of CdS quantum dots, the luminescence intensity of methylene blue increases. We suggest a model of this effect, which is based on electronic excitation energy transfer from luminescence centers of CdS quantum dots to methylene blue molecules.

Organic–inorganic nanostructures for luminescent indication in the near-infrared range

Amplifying and quenching of IR luminescence of colloidal Ag2S quantum dots were revealed to take place when they couple to organic dye molecules of 3,3′-di-(γ-sulfopropyl)-9-ethyl-4,5,4′,5′-dibenzothiacarbocyanine betaine and erytrosine pyridinium salts, respectively. The observed effects are explained as due to the formation of organic–inorganic heterostructures with different mutual arrangement of electronic states of the dyes and the quantum dots.

The nature of the luminescence-flash photostimulation spectra in CdS quantum dots

The photostimulated luminescence-flash effect has been detected in colloidal CdS quantum dots synthesized by the sol–gel method. The photostimulation spectra have been studied in the quantum-energy region from 0.6 to 2.0 eV. It is shown that they are caused by optical transitions from localized states having a surface nature to quantum-size electronic states.

Absorption spectra of TiO2 thin films synthesized by the reactive radio-frequency magnetron sputtering of titanium

The optical absorption spectra of TiO2 thin films with different phase compositions (anatase, rutile-brookite, anatase-rutile-brookite) are studied. The films are produced by the reactive radio-frequency magnetron sputtering of titanium in different atmospheres. It is shown that, on the assumption of the additivity of the contributions of each phase to the general spectrum with the fraction of the phase in the layer known, it is possible to use the method of subtraction of the spectra of individual phases. The fundamental absorption spectra of all of the crystal modifications detected in the multiphase films by structural studies are separated out, and the edges of the optical transitions in all of the phase components are established.

Luminescence amplification of dye molecules in the presence of silver nanoparticles

The influence of silver nanoparticles on the luminescence intensity of dye molecules (methylene blue and acridine yellow) is investigated. It is shown that the fact that the attenuation spectrum of the silver particles coincides with the absorption spectrum of the dye molecules results in a small change of their absorption coefficient. At the same time, the luminescence intensity of the molecules increases in the presence of the silver particles by a sizable factor. The dependence of the luminescence gain on the size of the nanoparticles and the mean spacing between them and the dye molecules is determined.

Spectral and kinetic features of nonradiative energy transfer in hybrid associates of colloidal quantum dots and organic dyes

The features of nonradiative resonance energy transfer in hybrid associates of colloidal CdS quantum dots (QDs) with molecules of a thiazine dye and J-aggregates of a carbocyanine dye are discussed. The case of an inhomogeneously broadened recombination band of donor (CdS QDs) luminescence is considered. We detected spectral-selective quenching of recombination luminescence of colloidal CdS QDs in association with organic dyes. The maximum of quenching occurs in the region of absorption of the acceptor (organic dye molecule). It is shown using the time-correlated single photon counting technique that the lifetime of the QD luminescence decreases synchronously with the quenching of the luminescence, but only at the wavelengths falling in the region of absorption of the dye. Based on the concept of a donor–acceptor nature of emission of CdS QDs, the dependence of the efficiency of nonradiative energy transfer on the luminescence wavelength is analyzed taking into account the Poisson distribution of its quenchers.