Sergey I. Pokutnyi

Biexcitons formed from spatially separated electrons and holes in quasi-zero-dimensional semiconductor nanosystems

A theory of biexcitons (formed from spatially separated electron and holes) in nanosystems that consist of zinc-selenide quantum dots synthesized in borosilicate glassy matrices is developed. The dependences of the total energy and the binding energy of the singlet ground biexciton state in such a system on the spacing between the quantum-dot surfaces and the quantum-dot radius are derived by the variational method. It is shown that biexciton formation is of the threshold character and possible in nanosystems, in which the spacing between the quantum-dot surfaces is larger than a certain critical spacing.

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.

Binding energy of excitons formed from spatially separated electrons and holes in insulating quantum dots

It is found that the binding energy of the ground state of an exciton formed from an electron and a hole spatially separated from each other (the hole is moving within a quantum dot, and the electron is localized above the spherical (quantum dot)–(insulating matrix) interface) in a nanosystem containing insulating Al2O3 quantum dots is substantially increased (by nearly two orders of magnitude) compared to the exciton binding energy in an Al2O3 single crystal. It is established that, in the band gap of an Al2O3 nanoparticle, a band of exciton states (formed from spatially separated electrons and holes) appears. It is shown that there exists the possibility of experimentally detecting the ground and excited exciton states in the band gap of Al2O3 nanoparticles at room temperature from the absorption spectrum of the nanosystem.

Exciton states in quasi-zero-dimensional semiconductor nanosystems

The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.

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.

Local electron states in ellipsoidal nanosystems in homogeneous magnetic field

The influence of a homogeneous magnetic field on the electron states localized over the surface of an ellipsoidal dielectric particle by the electrostatic image forces is studied theoretically. The effects of the resonant interaction of light with such local electron states in presence and in absence of a homogeneous magnetic field are investigated.

Biexciton in nanoheterostructures of dielectric quantum dots

Abstract. It has been found that the formation of a biexciton in a nanoheterostructures (NHS) made up of aluminum oxide quantum dots (QDs) synthesized in a dielectric matrix is of threshold character and can occur in a nanosystem, where the distance D between the surfaces of QD is given by the condition Dc(1)≤D≤Dc(2). We have shown that with such an NHS acting as “exciton molecules” (biexcitons consisting of spatially separated electrons and holes) are the QDs of aluminum oxide with excitons localizing over their surfaces. The position of the biexciton state energy band depends both on the mean radius of the QDs, and the distance between their surfaces, which enables one to purposefully control it by varying these parameters of the nanostructure.

Exciton states and direct interband light absorption in the ensemble of toroidal quantum dots

Abstract. One electron and exciton states in toroidal quantum dot (QD) have been considered. The convenient coordinate system has been defined and the Schrodinger equation has been solved in these coordinates. The electron energy spectrum and wave function dependence on the geometrical parameters of toroidal QD have been obtained. The comparison with the results obtained by numerical methods has been done. Optical absorption has been considered in both cases of “small” toroid when the interaction of an electron and a hole is neglected and cases of relatively “large” toroid when the correlation between the particles has been taken into account. Interband optical transitions have been considered in the ensemble of toroidal QDs. The selection rules for quantum transitions and absorption edge dependence on the geometrical parameters have been obtained.

Strongly absorbing light nanostructures containing metal quantum dots

Abstract. In the framework of dipole approximation, it is shown that the oscillator strengths of transitions as well as the dipole moments for transitions for one-particle electron Coulomb states emerging above the spherical surface quantum dot of metal assume giant values considerably (by two orders of magnitude) exceeding the typical values of the corresponding quantities for dielectrics. It has been established that the giant values of the light absorption cross section in the nanosystems under investigation make it possible to use such nanosystems as strongly absorbing nanomaterials in a wide range of infrared waves with a wavelength that can be varied in a wide range depending on the type of contacting materials.