A. N. Latyshev

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.

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.

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.

New method for measuring the IR surface impedance of metals

A method for measuring the IR optical properties of metals, which implies the analysis of the angular distribution of the photons emitted from the edge of a metal plate due to the conversion of thermal surface plasmons, has been theoretically and experimentally shown. In this work an increased accuracy of the proposed method in the IR region is demonstrated in comparison with more conventional methods based on the Fresnel formulas. This conclusion is supported experimentally with data obtained from the surface impedance measurement of copper in the range from 8 to 9 μm at 185°C.

The mechanism of sensitized anti-stokes luminescence in crystals with adsorbed dyes

We have analyzed regular features of sensitization processes of crystals AgCl, AgCl0.95I0.05, and Zn0.6Cd0.4S to processes of a low-threshold (10−3–10−4 W/cm2) two-quantum excitation of anti-Stokes luminescence by adsorbed molecules and by J and H aggregates of dyes of different classes. The excitation centers of this luminescence are complex and consist of dye molecules and few-atom silver clusters adsorbed nearby. Luminescence-excitation processes involve stages of photoexcitation of adsorbed dye molecules; resonance transfer of electronic excitation energy to adsorbed atoms and few-atom silver clusters, levels of which lie near the middle of the band gap; and their subsequent photoionization.

Synthesis of thin p -type rutile films

The structure and electrical and optical properties of heterostructures formed on the surface of single-crystal silicon wafers as a result of the heat treatment and pulsed photon treatment of Ti films in oxygen, air, and nitrogen are investigated. It is shown that a TiO2/Ti5Si3/p-Si heterostructure is formed upon heat treatment in air, whereas a TiO2/TiSi2/p-Si heterostructure is formed upon photon treatment. It is established that rutile films with pronounced n-type conductivity are formed as a result of the heat treatment of Ni-doped Ti films in oxygen. Rutile films with p-type conductivity are formed upon the thermal annealing of Ti films in air with subsequent photon treatment in nitrogen.

Photostimulated Luminescence Flash: from Scientific Photography to Photonics of Nanostructured Materials

We have analyzed the possibilities of using the phenomenon of photostimulated luminescence flash for optical diagnosing of energy levels of structural and impurity defects of semiconductor crystals and nanostructures. New data on the spectra of deep localized states associated with adsorbed few-atom clusters Znn on the surface of ZnS; clusters Cdn, Cun, and Agn on the surface of CdS; and clusters Agn on the surface of AgBr(I) have been presented, as well as results of investigation of photostimulated assembling processes of few-atom clusters on the surface of crystals using this phenomenon. We are the first to show the potential of the luminescence flash technique for studying the mutual arrangement of the levels of dye molecules and the bands of the crystal on the surface of which they are adsorbed, as well as of the spectra of localized states in colloidal CdS semiconductor quantum dots.

Low-threshold power limitation of optical radiation in crystals with sensitized anti-Stokes luminescence

This paper shows that it is possible in principle to implement low-threshold power limitation in ionic-covalent crystals with adsorbed metal-cluster-dye-molecule structures that possess sensitized anti-Stokes luminescence. The power-limitation effect for radiation with λ=660nm has been detected and investigated at a temperature of 77K for AgCl(I) nanocrystals 40-70nm in size with adsorbed dye molecules and few-atom clusters of silver, dispersed in a gelatine matrix. The effect was observed for radiation with a density of 1017-1019quant/cm2sec at a pulse width of 1ms. The Z-scanning technique was used to establish that the main limitation mechanism is self-focusing, which results from variation of the refractive index of the medium when the electron concentration on traps in the band gap of the nanocrystals varies.