K. A. Drozdov

Photoconductivity of nanocrystalline SnO2 sensitized with colloidal CdSe quantum dots

A highly reproducible photoresponse is observed in nanocrystalline SnO2 thick films sensitized with CdSe quantum dots. The effect of the SnO2 matrix microstructure on the photoconductivity kinetics and photoresponse amplitude is demonstrated. The photoresponse of the sensitized SnO2 thick films reaches more than two orders of magnitude under illumination with the wavelength of the excitonic transition of the quantum dots. Long-term photoconductivity kinetics and photoresponse dependence on illumination intensity reveal power-law behavior inherent to the disordered nature of SnO2. The photoconductivity of the samples rises with the coarsening of the granular structure of the SnO2 matrix. At the saturation region, the photoresponse amplitude remains stable under 104 pulses of illumination switching, demonstrating a remarkably high stability.

Electron energy relaxation under terahertz excitation in (Cd1−xZnx)3As2 Dirac semimetals

We demonstrate that measurements of the photo-electromagnetic effect using terahertz laser radiation provide an argument for the existence of highly conductive surface electron states with a spin texture in Dirac semimetals (Cd1− xZnx)3As2. We performed a study on a range of (Cd1− xZnx)3As2 mixed crystals undergoing a transition from the Dirac semimetal phase with an inverse electron energy spectrum to trivial a semiconductor with a direct spectrum in the crystal bulk by varying the composition x. We show that for the Dirac semimetal phase, the photo-electromagnetic effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial semiconductor phase, it depends on the laser pulse power, irrespective of wavelength. We assume that such behavior is attributed to a strong damping of the interelectron interaction in the Dirac semimetal phase compared to the trivial semiconductor, which may be due to the formation of surface electron states with a spin texture in Dirac semimetals.

Photoconductivity of structures based on the SnO2 porous matrix coupled with core-shell CdSe/CdS quantum dots

Embedding of quantum dots into porous oxide matrixes is a perspective technique for photosensitization of a structure. We show that the sensitization efficiency may be increased by the use of core-shell quantum dots. It is demonstrated that the photoresponse amplitude in a SnO2 porous matrix with CdSe/CdS quantum dots depends non-monotonously on the number of atomic layers in a shell. The best results are obtained for SnO2 matrixes coupled with the quantum dots with three atomic layers of a shell. Mechanisms responsible for the structure sensitization are discussed.

Photoconductivity of composite structures based on porous SnO2 sensitized with CdSe nanocrystals

The introduction of CdSe nanocrystals (colloidal quantum dots) into a porous SnO2 matrix brings about the appearance of photoconductivity in the structures. Sensitization is a consequence of charge exchange between the quantum dots and the matrix. Photoconductivity spectral measurements show that the nanocrystals embedded into the matrix are responsible for the optical activity of the structure. The photoconductivity of the structures sensitized with different-sized quantum dots is studied in the temperature range from 77 to 300 K. It is shown that the maximum photoconductivity is attained by introducing nanocrystals of the minimum size (2.7 nm). The mechanisms of charge-carrier transport in the matrix and the charge-exchange kinetics are discussed.

Modification of Photoconductivity Spectra in ZnO–CdSe Quantum- Dot Composites upon Exposure to Additional Photoexcitation

The incorporation of CdS nanocrystals into a porous ZnO matrix results in sensitization of the composite in the visible spectral region. Studies of the photoconductivity spectra upon variable external illumination show that the spectra undergo reversible transformations. It is shown that the shape of the peak and the position of the local photoconductivity maximum corresponding to nanocrystals depend on the wavelength distribution of the incident radiation intensity. The mechanisms responsible for the process are discussed.

In situ impedance spectroscopy of filament formation by resistive switches in polymer based structures

It is shown that the impedance spectroscopy allows identification of the resistive switching mechanisms in complex composite structures. This statement was demonstrated on an example of organic based sandwich structures with a modified polymer matrix as an active element. The impedance spectroscopy scanning was performed for a series of intermediate states formed within the switching process. Analysis of the experimentally obtained impedance spectra shows that the electron transport is provided by delocalized charge carriers and proceeds via conducting filaments formed in a highly resistive matrix. The filament configuration changes during the switching. With the shift from isolating to conducting states, single isolated filaments are reorganized into a branched network.

Photosensitive Organic-Inorganic Hybrid Materials for Room Temperature Gas Sensor Applications

In this work, the hybrids based on nanocrystalline SnO2 or In2O3 semiconductor matrixes and heterocyclic Ru(II) complex are studied as materials for gas sensors operating at room temperature under photoactivation with visible light. Nanocrystalline semiconductor oxides are obtained by chemical precipitation with subsequent thermal annealing and characterized by XRD, SEM and single-point BET methods. The heterocyclic Ru(II) complex is synthesized for the first time and investigated by 1H NMR, 13C NMR APT, MALDI-MS analysis, and UV-Vis spectroscopy. The HOMO and LUMO energies of the Ru(II) complex are calculated from cyclic voltammetry data. The hybrid materials are characterized by TGA-MS analysis and EDX mapping. The optical properties of hybrids are studied by UV-Vis spectroscopy in the diffuse reflection mode. The investigation of spectral dependencies of photoconductivity of hybrid samples demonstrates that the role of organic dye consists in shifting the photosensitivity range towards longer wavelengths. Sensor measurements demonstrate that hybrid materials are able to detect NO2 in the concentration range of 0.25–2 ppm without the use of thermal heating under periodic illumination with even low-energy long-wavelength (red) light.

Effect of the tin impurity on the energy spectrum and photoelectric properties of nanostructured In2O3 films

It is shown that the doping of colloidal indium-oxide nanocrystals with tin shifts the absorption edge to shorter wavelengths of the visible spectral region and induces considerable absorption in the infrared region. The shape of the absorption and transmittance spectra in the infrared region is characteristic of the local surface plasmon resonance. According to estimations, the concentration of free charge carriers in In2O3(Sn) reaches 1019 cm−3. The temperature dependences of the photoconductivity in the range 77–300 K are indicative of hopping conduction in nanostructured films based on In2O3(Sn) nanocrystals. The mechanisms responsible for the effect are discussed.