arXiv subjects

I. A. Weinstein

Publications and source records attributed to I. A. Weinstein.

15 recordsLinked to original sources

arXiv2025

Temperature behavior of optical absorption spectra in HfO2 thin films

Hafnium dioxide, also known as hafnia, is an extremely sought-after material in opto- and nanoelectronics for creating optical coatings and various functional media to have stable performance characteristics under varying thermal operating conditions. In this paper, we have investigated the behavior of the optical properties of hafnia thin films exhibiting an amorphous structure in a wide temperature range of 7-296 K. For the first time we have examined the temperature effects in the energy gap of HfO2 films and estimated the effective phonon energy of 30 meV responsible for observed thermally assisted shift of electronic levels. It has been shown that the electron-phonon interaction in the oxygen subsystem predominantly causes the observed changes. The obtained refractive index values for the tested films are established to be compatible with independent predicted data and to decrease as the temperature drops. The energy structure and electron-phonon interaction features that have been found are critical for forecasting how hafnia-thin-film-based optoelectronic devices will behave across a wide temperature range.

cond-mat.mtrl-sci↗

arXiv2025

Mechanisms of Afterglow and Thermally Stimulated Luminescence in UV-irradiated InP/ZnS Quantum Dots

Indium phosphide-based quantum dots (QDs) are a potential material for designing optoelectronic devices, owing their adjustable spectral parameters over the entire visible range, as well as their high biocompatibility and environmental safety. Concurrently, they exhibit structural defects, the rectification of which is crucial for enhancing their optical properties. The present work explores, for the first time, the low-temperature afterglow (AG) and spectrally resolved thermally stimulated luminescence (TSL) of UV-irradiated colloidal core/shell InP/ZnS QDs in the range of 7-340 K. It is shown that, when localized during irradiation and released after additional stimulation, charge carriers recombine involving defect centers based on indium and phosphorus dangling bonds. The mechanisms of the observed luminescent phenomena can be caused by both thermal activation and tunneling processes. By means of the initial rise method, the formalism of general-order kinetics, and the analytical description using the Lambert W function, we have analyzed the kinetic features of possible thermally stimulated mechanisms. We have also estimated the energy characteristics of appropriate trapping centers. A low rate of charge carriers recapture is revealed for InP/ZnS QDs. Active traps in nanocrystals of different sizes are characterized by close values of activation energy in the 26-31 meV range. The current paper discloses new horizons for exploiting TSL approaches to study the properties of local defective states in the energy structure of colloidal QDs, which can contribute to the development of targeted synthesis of nanocrystals with tunable temperature sensitivity for optoelectronic and sensor applications.

cond-mat.mtrl-sci↗

arXiv2023

Luminescence in anion-deficient hafnia nanotubes

Hafnia-based nanostructures and other high-k dielectrics are promising wide-gap materials for developing new opto- and nanoelectronics devices. They possess a unique combination of physical and chemical properties such as insensitivity to electrical and optical degradation, radiation damage stability, a high specific surface area, and an increased concentration of the appropriate active electron-hole centers. The present paper aims to investigate the structural, optical, and luminescent properties of anodized non-stoichiometric $HfO_2$ nanotubes. As-grown amorphous hafnia nanotubes and nanotubes annealed at 700°C with a monoclinic crystal lattice served as samples. It has been shown that the bandgap $E_g$ for direct allowed transitions amounts to $5.65\pm0.05$ eV for amorphous and $5.51\pm0.05$ eV for monoclinic nanotubes. For the first time, we have studied the features of the intrinsic cathodoluminescence and photoluminescence of the obtained nanotubular $HfO_2$ structures with an atomic deficiency in the anion sublattice at temperatures of 10 and 300 K. A broad emission band with a maximum of 2.3-2.4 eV has been revealed. We have also conducted an analysis of the kinetic dependencies of the observed photoluminescence for synthesized $HfO_2$ samples in the millisecond range at room temperature. It showed that there are several types of optically active capture and emission centers based on vacancy states in the $O_{3f}$ and $O_{4f}$ positions with different coordination numbers and a varied number of localized charge carriers ($V^0$, $V^-$, and $V^{2-}$). The uncovered regularities can be used to optimize the functional characteristics of developed-surface luminescent media based on nanotubular and nanoporous modifications of hafnia.

cond-mat.mtrl-sci↗

arXiv2023

Temperature behavior of the Optical Absorption and Photoluminescence Spectra of InP/ZnS Nanocrystals with a Stabilizing Polyvinylpyrrolidone-based Coating

The present paper deals with the results of a research work on the optical properties of colloidal InP/ZnS nanocrystals stabilized with a heterobifunctional polyvinylpyrrolidone polymer. We have analyzed the absorption and photoluminescence spectra of the samples as solutions with various concentrations and deposited films, as well as the patterns of their temperature changes in the range of 6.5 - 296 K. An inhomogeneous broadening of exciton optical bands has been observed to be related to a wide distribution of nanocrystals in size. A temperature shift of the exciton absorption and emission maxima has been found to be due to the interaction with acoustic phonons. It has been shown that the quenching of defect-related luminescence is carried out through local energy levels of dangling bonds of phosphorus atoms involved at the core-shell interface, and the temperature stability of exciton emission is determined by the thickness of the ZnS shell.

cond-mat.mtrl-sci↗

arXiv2022

Family of prime-representing constants: use of the ceiling function

The analysis of regularities and randomness in the distribution of prime numbers remains at the research frontiers for many generations of mathematicians from different groups and topical fields. In 2019 D. Fridman et al. (Am. Math. Mon. 2019, 126:1, 70-73) have suggested the constant $f_1 = 2.9200509773...$ for generation of the complete sequence of primes with using of a recursive relation for $f_n$ such that the floor function $\lfloor f_n \rfloor = p_n$, where $p_n$ is the nth prime. Here I present the family of constants $h_n (h_1 = 1.2148208055...)$ such that the ceiling function $\lceil h_n \rceil = p_n$. The proposed recursive relation $h_n=\lceil h_n \rceil(h_{n-1}-\lceil h_{n-1} \rceil+2)$ generates the sequence of all known prime numbers. I also show that constants $h_n$ are irrational.

math.GM↗

arXiv2022

Thermally stimulated luminescence of oxygen-deficient zirconia nanotubes

ZrO$_2$ nanotubular arrays with intrinsic defects are a promising solid-state basis for the development of devices for detecting, storing, and converting energy. Layers of the self-ordered zirconia nanotubes of 5 $μ$m length and 30 nm diameter, containing oxygen vacancies and their complexes, have been synthesized by anodic oxidation. The spectrally resolved TSL (thermally stimulated luminescence) above room temperature for the samples exposed by UV-irradiation with an energy of 4.1 eV have been studied. Two emission bands with maxima near 2.5 and 2.8 eV, associated with radiative relaxation of T and F+ centers, respectively, have been found. An analysis of the measured glow curves within the framework of general order kinetics established the presence of four TSL peaks caused by charge carriers traps with activation energies of 0.7 - 0.8 eV. A band diagram is proposed to explain the role of optically active centers based on electron (vacancies in positions of three- and four-coordinated oxygen) and hole (interstitial oxygen ions) traps in observed TSL processes during the irradiation and the followed heating of anion-deficient ZrO$_2$ nanotubes.

cond-mat.mtrl-sci↗

arXiv2021

Temperature dependence of self-trapped exciton luminescence in nanostructured hafnia powder

The intrinsic optical properties and peculiarities of the energy structure of hafnium dioxide largely determine the prospects for applying the latter in new generation devices of optoelectronics and nanoelectronics. In this work, we have studied the diffuse reflectance spectra at room temperature for a nominally pure nanostructured $HfO_2$ powder with a monoclinic crystal structure and, as well its photoluminescence in the temperature range of 40 - 300 K. We have also estimated the bandgap $E_g$ under the assumption made for indirect (5.31 eV) and direct (5.61 eV) allowed transitions. We have detected emission with a 4.2 eV maximum at T < 200 K and conducted an analysis of the experimental dependencies to evaluate the activation energies of thermal quenching (140 meV) and enhancement (3 meV) processes. Accounting for both the temperature behavior of the spectral characteristics and the estimation of the Huang-Rhys factor S >> 1 has shown that radiative decay of self-trapped excitons forms the mechanism of the indicated emission. In this case, the localization is mainly due to the interaction of holes with active vibrational modes of oxygen atoms in non-equivalent ($O_{3f}$ and $O_{4f}$) crystal positions. Thorough study of the discussed excitonic effects can advance development of hafnia-based structures with a controlled optical response.

cond-mat.mtrl-sci↗

arXiv2021

Activation energy distribution in thermal quenching of exciton and defect-related photoluminescence of InP/ZnS quantum dots

Thermal quenching is one of the essential factors in reducing the efficiency of radiative processes in luminophores of various nature. The emission activity of low dimensional structures is influenced also by multiplicity of parameters that are related to synthesis processes, treatment regimes, etc. In the present work, we have investigated the temperature dependence of photoluminescence caused by exciton and defect-related transitions in ensembles of biocompatible InP/ZnS core/shell nanocrystals with an average size of 2.1 and 2.3 nm. The spread in the positions of energy levels is shown to be due to size distribution of quantum dots in the ensembles under study. For a quantitative analysis of the experimental data, we have proposed a band model accounting for the Gaussian distribution of the thermally activated barriers in the photoluminescence quenching processes. The model offers the thermal escape of an electrons from the core into the shell as the main mechanism for non-radiative decay of excitons. In turn, the quenching of defect-related emission is predominantly brought about through the emptying of the hole capture centers based on dangling phosphorus bonds. We have revealed the correlation between size distributions of quantum dots and scatter of the activation energy of exciton luminescence quenching. The developed approach will give further the possibility to optimize technological regimes and methods for band engineering of indium phosphide-based type-I quantum dots.

cond-mat.mtrl-sci↗

arXiv2021

Quantum Conductors Formation and Resistive Switching Memory Effects in Zirconia Nanotubes

The development prospects of memristive elements for non-volatile memory with use of the metal-dielectric-metal sandwich structures with a thin oxide layer are due to the possibility of reliable forming the sustained functional states with quantized resistance. In the paper we study the properties of fabricated memristors based on the non-stoichiometric $ZrO_2$ nanotubes in different resistive switching modes. Anodic oxidation of the $Zr$ foil has been used to synthesize a zirconia layer of $1.7$ $μ$$m$ thickness, consisting of an ordered array of vertically oriented nanotubes with outer diameter of 75 nm. $Zr/ZrO_2/Au$ sandwich structures have been fabricated by mask magnetron deposition. The effects of resistive switching in the $Zr/ZrO_2/Au$ memristors in unipolar and bipolar modes have been investigated. The resistance ratios $\geq3\cdot10^4$ between high-resistance (HRS) and low-resistance (LRS) states have been evaluated. It has been founded the conductivity of LRS is quantized in a wide range with minimum value of $0.5G_0=38.74$ $μ$$S$ due to the formation of quantum conductors based on oxygen vacancies ($V_O$). Resistive switching mechanisms of $Zr/ZrO_2/Au$ memristors with allowing for migration of $V_O$ in an applied electric field have been proposed. It has been shown that the ohmic type and space charge limited conductivities are realized in the LRS and HRS, correspondingly. We present the results which can be used for development of effective memristors based on functional $Zr/ZrO_2/Au$ nanolayered structure with multiple resistive states and high resistance ratio.

cond-mat.mtrl-sci↗

arXiv2020

Spectral characterization of long-lived luminescence in h-BN nanopowder under UV excitation

Photoluminescence (PL) features of nanostructured h-BN powder are studied in the range of 200-600 nm in millisecond time window. It is found that four PL excitation bands of 4.58, 5.01, 5.29, and 5.77 eV are characteristic of the spectral region at hand. It is shown that the PL emission spectra can be quantitatively described through a superposition of three Gaussian-shaped peaks: 2.6, 3.1, and 3.7 eV. The observable luminescence is established to be due to recombination processes involving centers whose energy levels in the bandgap are formed by oxygen and carbon impurities, as well as nitrogen vacancies and related complexes.

cond-mat.mtrl-sci↗

arXiv2020

Temperature effects in luminescence of associated oxygen-carbon pairs in hexagonal boron nitride under direct optical excitation within 7-1100 K range

We have studied the temperature dependencies of the photoluminescence (PL) intensity of 4.1 eV in microcrystalline powder of hexagonal boron nitride in the range of 7-1100 K. The results obtained have been analyzed within the band model of energy levels of associated donor-acceptor pairs based on impurity (ONCN) complexes. Luminescence enhancement processes at T<200 K and two independent channels of external thermal activation quenching are typical of the observable luminescence mechanisms under direct (4.26 eV) excitations of the samples. It has been shown that, at T>220 K, when directly excited, the samples diminish the PL intensity because of the processes of thermal ionization of the donor level of the ON-center (122 meV) and the deep acceptor level of the CN-center (1420 meV) as parts of the (ONCN)-complex. The temperature enhancement region with an activation energy of 15 meV is due to the decay of a bound Wannier-Mott exciton followed by transfer of excitation to the associated donor-acceptor pair.

cond-mat.mtrl-sci↗

arXiv2019

When lost in a multiverse again

The short Communication based on results of notes: Andre Geim "When lost in a multiverse" (Nat. Phys. 13, 1142 (2017)) and Klaus von Klitzing "Metrology in 2019" (Nat. Phys. 13, 198 (2017))

cond-mat.mes-hall↗

arXiv2019

Oxide layer thickness effects on the resistance switching characteristics of Ti/TiO2-NT/Au structure

Self-ordered nanotubular titania TiO2-NT with outer tube diameter of 45 nm are synthesized using the anodic oxidation of titanium foil. Four sets of memristors with 100 $μm$ diameter based on Ti/TiO2-NT/Au sandwich structures with an oxide layer thickness of 80, 120, 160 and 200 nm are fabricated. Current-voltage (CV) characteristics for the obtained samples in the static and dynamic operation modes are studied. Resistance in high and low resistance states is estimated. Basing on the analysis of the CV characteristics in dynamic mode (> 14 000 switchings) a prospective of use for synthesized Ti/TiO2-NT/Au micromemristors with oxide layer thickness of 160 nm in non-volatile memory is shown. Keywords: anodic titania, titanium dioxide nanotubes, nanotubular structure, memristor, resistive switching

cond-mat.mtrl-sci↗

arXiv2019

Unidirectional Synapse-Like Behavior of Zr/ZrO2-NT/Au Layered Structure

Zirconia nanotubular layer with an outer tube diameter 25 nm was synthesized by potentiostatic anodization. The Zr/ZrO2-NT/Au memristive structure is fabricated using stencil mask and magnetron sputtering techniques. Current-voltage characteristics are measured in full cycles of resistive switching with varying parameters of the applied harmonic voltage. An equivalent circuit with unidirectional electrical conductivity for the studied structure is proposed. Estimates of the electrical resistance of memristors in high-and intermediate resistivity states are performed. The high synaptic plasticity of memristors based on the Zr/ZrO2-NT/Au structure is shown.

cond-mat.mtrl-sci↗

arXiv2019

Nonstoichiometric titanium dioxide nanotubes with enhanced catalytical activity under visible light

The catalytic activity of nanotubular titanium dioxide films formed during the oxidation of acetone to carbon dioxide under the action of visible light with a wavelength of 450 nm was found to be approximately 2 times higher compared to standard titanium dioxide (Degussa P25). The nanotubular films were grown by the anodization of titanium foil using an original technique. Diffuse reflectance spectra of the films are attributed to enhanced activity in the visible spectrum by the nonstoichiometry of titanium dioxide near the interface between the nanotubular film and the titanium foil substrate.

physics.app-ph↗