O. V. Ovchinnikov

Spectroscopic investigation of colloidal CdS quantum dots–methylene blue hybrid associates

Spectral properties of hydrophilic associates of colloidal CdS quantum dots (QDs) with methylene blue molecules (MB) prepared by sol–gel method have been studied. The two basic types of hybrid associates were found using FTIR spectra technique. The first-type associates are characterized by planar location of MB heterocycle on QDs spherical interface. In this case MB dimerization is not observed. Both nitrogen and sulfur heteroatoms and double =N+–(CH3)2 bonds of MB are participants of association which causes to conformation of MB heterocycle and π-conjugation length decrease. In UV–Vis absorption and luminescence spectra blue shift of MB peaks for QD-MB associates in comparison with MB spectra in solutions and gelatin was found. The second type of association mostly involves MB nitrogen heteroatoms and CdS QDs interface atoms. At the same time, peaks of UV–Vis and luminescence spectra are red shifted. In this case, dimerization manifestation of MB in QD-MB associates was found. Using the time correlated single photon counting technique resonance electronic excitation energy transfer from recombination luminescence center of CdS QDs to MB molecules was found. Its efficiency is in the range of 0.36–0.51.

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.

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.

Relationship between structural and optical properties of colloidal CdxZn1−xS quantum dots in gelatin

Abstract. Aqueous synthesis of mixed cadmium and zinc sulfides colloidal quantum dots (QDs) has been successfully realized. Colloidal CdxZn1−xS QDs are formed in a cubic crystal lattice with particle size of ∼2  nm. The blueshift of optical absorption spectra from 420 to 295 nm and recombination photoluminescence from 646 to 483 nm with increasing zinc content in QDs was observed. Optimum photoluminescence intensity occurs for QDs with Cd0.3Zn0.7S composition. With increasing zinc content up to Cd0.3Zn0.7S, luminescence intensity increases and decreases when zinc content is larger than 0.7. The increase in photoluminescence intensity is explained by the increase in the number of point defects, such as complexes of interstitial metal atoms–metal vacancies [Mei−VMe]. Such complexes occur due to displacement of the metal atom at the center of the elementary tetrahedron due to substitution of one of the four sulfur atoms by an impurity atom, such as an oxygen atom.

Absorption of light by colloidal semiconductor quantum dots

Abstract. UV-Vis absorption of colloidal cadmium sulfide quantum dots (QDs) synthesized by an aqueous synthesis in a gelatin matrix was investigated. Using the dipole approximation, taking into account the Coulomb interaction between the electron and hole in a QD and the polarization effects on the spherical boundary of QD and matrix, it was found the change of selection rules for optical transitions. It is shown that the optical absorption edge of QDs is formed by two optical transitions of electron between low-excited levels of size quantization of heavy hole (1S3/2 and 2S3/2), located in QDs valence band and fundamental size-quantized states 1Se of conduction band. These transitions are identical in intensity. Estimations of average values of CdS QDs radius were realized using the developed formalism for UV-Vis absorption spectra. These data were compared with experimental values of this parameter, obtained using transmission electron microscope.

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.