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A. F. Zatsepin

Publications and source records attributed to A. F. Zatsepin.

At least 19 recordsLinked to original sources
arXiv2024

Structural, electronic, and optical properties of 6H-SiC layers synthesized by implantation of carbon ions into silicon

Systematic studies of the gradual fabrication by means of carbon ion-implantation of high-quality 6H-SiC layers on silicon surfaces have been carried out. The fluence of carbon ions varied from 5*10^15 cm-2 to 10^17 cm-2. Results of first-principle calculations, X-ray diffraction (XRD), and Raman spectroscopy demonstrate the amorphization of silicon substrate without any tendency to the segregation of carbon in the samples synthesized at low fluencies. The formation of a SiO2-like structure at this stage was also detected. X-ray photoelectron spectroscopy (XPS), XRD, and Raman spectroscopy demonstrate that an increase in carbon content at 10^17 cm-2 fluence leads to the growth of 6H-SiC films on the surface of the amorphous silicon substrate. Atomic force microscopy (AFM) data obtained also demonstrates the decreasing of surface roughens after the formation of SiC film. XPS and Raman spectra suggest that excessive carbon content leaves the SiC matrix via the formation of an insignificant amount of partially oxidized carbon nanostructures. Optical measurements also support the claim of high-quality 6H-SiC film formation in the samples synthesized at 10^17 cm-2 fluence and demonstrate the absence of any detectable contribution of nanostructures formed from excessive carbon on the optical properties of the material under study.

cond-mat.mtrl-sci↗
arXiv2024

Formation of yttrium oxalate phase filled by carbon clusters on the surface of yttrium oxide films

In the current paper, we report the results of surface modification of cubic Y2O3 films employing carbon-ion implantation. The characterization results demonstrate the formation of a stable yttrium oxalate-based structure with cavities filled with carbon clusters. Theoretical simulations demonstrate that the incorporation of eighteen-atom carbon clusters into the cavities of Y2(C2O4)3 does not lead to valuable changes in the crystal structure of yttrium oxalate. X-ray diffraction and optical measurements demonstrate that the subsurface bulk area of cubic yttrium oxide remains unperturbed. The oxalate "skin" thickness with embedded carbon clusters is estimated to be approximately 10 nm. The prospective employing the method to manage optical properties and increase the biocompatibility of yttria and lanthanide oxides are discussed.

cond-mat.mtrl-sci↗
arXiv2023

Uncommon 2D Diamond-like Carbon Nanodots Derived from Nanotubes: Atomic Structure, Electronic States and Photonic Properties

In this article, we report the results of relatively facile fabrication of carbon nanodots from single-walled and multi-walled carbon nanotubes (SWCNT and MWCNT). The results of X-ray photoelectron spectroscopy (XPS) and Raman measurements show that the obtained carbon nanodots are quasi-two-dimensional objects with a diamond-like structure. Based on the characterization results, a theoretical model of synthesized carbon nanodots was developed. The measured absorption spectra demonstrate the similarity of the local atomic structure of carbon nanodots synthesized from single-walled and multi-walled carbon nanotubes. However, the photoluminescence (PL) spectra of nanodots synthesized from both sources turned out to be completely different. Carbon dots fabricated from MWCNTs exhibit PL spectra similar to nanoscale carbon systems with sp3 hybridization and a valuable edge contribution. At the same time nanodots synthesized from SWCNTs exhibit PL spectra which are typical for quantum dots with an estimated size of ~0.6-1.3 nm.

cond-mat.mes-hall↗
arXiv2022

Comparative Analysis of the Electronic Energy Structure of Nanocrystalline Polymorphs of Y2O3 Thin Layers: Theory and Experiments

The results of fabrication and characterization of atomic structure of nanocrystalline thin layers of Y2O3 in cubic and monoclinic phases is reported. Experimental data demonstrate crystalline ordering in nanocrystalline films with average grain size of ~10-14 nm both for cubic and monoclinic studied structures. Density Functional Theory (DFT) based simulations demonstrate insignificant differences of electronic structure of these phases in the bulk and on the surfaces. Theoretical modeling also pointed out the significant broadening of valence and conductive bands caused by means of energy levels splitting in agreement with experimental data (X-ray photoelectron and photoluminescence spectra). The presence of various intrinsic and extrinsic defects (including surface adsorption of carbon mono- and dioxide) does not promote visible changes in electronic structure of Y2O3 surface for both studied phases. Optical absorption and luminescence measurements indicate insignificant bandgap reduction of Y2O3 nanocrystalline layers and the very little contribution from defect states. Simulation of extrinsic compression and expanding demonstrate stability of the electronic structure of nanocrystalline Y2O3 even under significant strain. Results of comprehensive studies demonstrate that yttrium oxide based nanocrystalline layers are prospective for various optical applications as a stable material.

cond-mat.mtrl-sci↗
arXiv2021

The GoogLeNet-assisted phase transition detectors

In the presence of the phase fluctuations in superconducting nanowires array, the electrical resistance of the superconducting nanowires is always non-zero unless the system undergoes Berezinskii-Kosterlitz-Thouless (BKT) transition where the superconducting vortices and anti-vortices form pairs. The two-dimensional XY model can mimic the superconducting transition temperature Tc and the BKT transition at a lower critical temperature TBKT by observing the heat capacity anomalies upon cooling. If the Josephson coupling across the nanowires is strong, the heat capacity anomalies almost overlap with each other so that it is difficult to distinguish between the Tc and the TBKT. To solve this issue, we apply an artificial-intelligence technique to split the nearly overlapped heat capacity anomalies. After the GoogLeNet-assisted phase transition detector is built, the GoogLeNet model can learn from the features of the phase transitions and then interpret the Tc and TBKT in the unseen system precisely. Our work opens a path for the GoogLeNet model to enter the world of magnetism and superconductivity.

cond-mat.supr-con↗
arXiv2021

The effectiveness of data augmentation in porous substrate, nanowire, fiber and tip images at the level of deep learning intelligence

To prepare for identifying the composition of nanowire-fiber mixtures in Scanning Electron Microscope (SEM) images, we optimize the performance of image classification between nanowires, fibers and tips due to their geometric similarities. The SEM images are analyzed by deep learning techniques where the validation accuracies of 11 convolutional neural network (CNN) models are compared. By increasing the diversity of data such as reflection, translation and scale factor approaches, the highest validation accuracy of recognizing nanowires, fibers and tips is 97.1%. We proceed to classify the level of porosity in anodized aluminum oxide for the self-assisted nanowire growth where the validation accuracy is optimized at 93%. Our software allow scientists to count the percentage of fibers in any nanowire-fiber composite and design the porous substrate for embedding different sizes of nanowires automatically, which assists the software development in Nanoscience Foundries & Fine Analysis (NFFA) Europe Projects.

cond-mat.mtrl-sci↗
arXiv2020

Spin and charge distributions in Graphene/Nickel (111) substrate under Rashba spin-orbital coupling

To understand the coupling factor between Rashba spin-orbital interaction and ferromagnetic proximity effect, we design a Monte Carlo algorithm to simulate the spin and charge distributions for the room-temperature Rashba material, Graphene/Nickel(111) substrate, at finite temperature. We observe that the rate of exchange fluctuation is a key player to produce giant Rashba spin-orbit splitting in graphene. More importantly, we monitor the Rashba spin-splitting phenomenon where the spin-polarized electrons may be escaped from two opposite edges upon heating. However, the escaped electrons show Gaussian-like distribution in interior area that is important for spintronic engineers to optimize the efficiency of spin-state detection. In addition, we investigate if our Monte Carlo model can explain why room-temperature Rashba effect is observed in Graphene/Nickel(111) substrate experimentally. All results are presented in physical units.

physics.comp-ph↗
arXiv2019

Enormous enhancement of p-orbital magnetism and band gap in the lightly doped carbyne

This paper presents a path to tailor adapted magnetic and optical properties in carbyne. Although p-orbital magnetism is generally much weaker than d-orbital magnetism, we demonstrate that the charge fluctuation of the free radical electrons triggered by a time-varying electric dipole moment leads to enormous p-orbital magnetism. By introducing 25% arsenic and 12.5% fluorine into the monoatomic carbon chain, the magnetic moment of the arsenic atom reaches 2.9 Bohr Magneton, which is ~1.3 times stronger than magnetic moment of bulk Fe. This magnetically optimized carbyne composite carries an exchange-correlation energy of 22meV (~270K). On the other hand, we convert the carbyne (in beta-form) from metallic to a semiconducting state by using anionic dopants. After doping 12.5% nitrogen and 12.5% oxygen into the beta-carbyne, the semiconducting gap of this composite is optimized at 1.6eV, which is 1.4 times larger than the band gap of bulk silicon.

cond-mat.mes-hall↗
arXiv2018

Revisiting the semi-flexible entangled chains of polymer in the carbyne model

The Monte Carlo carbyne model is modified to investigate the glass transition of the semi-flexible entangled polymer chains. The stochastic bombardment between monomers are monitored by Metropolis algorithm with help of the consideration of hard potential while the mobility of monomers is governed by its mass, scattering rate and temperature. Our model is capable to show that the glass transition temperature reduces with decreasing film thickness and the formation of critical voids in the thinner polymer contributing to the glass transition that is much easier than the bulk polymer.

physics.comp-ph↗
arXiv2018

Electron-electron interactions of the multi-Cooper-pairs in the 1D limit and their role in the formation of global phase coherence in quasi-one-dimensional superconducting nanowire arrays

Nanostructuring of superconducting materials to form dense arrays of thin parallel nanowires with significantly large transverse Josephson coupling has proven to be an effective way to increase the upper critical field of superconducting elements by as much as two orders of magnitude as compared to the corresponding bulk materials and, in addition, may cause considerable enhancements in their critical temperatures. Such materials have been realized in the linear pores of mesoporous substrates or exist intrinsically in the form of various quasi-1D crystalline materials. The transverse coupling between the superconducting nanowires is determined by the size-dependent coherence length E0. In order to obtain E0 over the Langer-Ambegaokar- McCumber-Halperin (LAMH) theory, extensive experimental fitting parameters have been required over the last 40 years. We propose a novel Monte Carlo algorithm for determining E0 of the multi-Cooper pair system in the 1D limit. The concepts of uncertainty principle, Pauli-limit, spin flip mechanism, electrostatic interaction, thermal perturbation and co-rotating of electrons are considered in the model. We use Pb nanowires as an example to monitor the size effect of E0 as a result of the modified electron-electron interaction without the need for experimental fitting parameters. We investigate how the coherence length determines the transverse coupling of nanowires in dense arrays. This determines whether or not a global phase-coherent state with zero resistance can be formed in such arrays. Our Monte Carlo results are in very good agreement with experimental data from various types of superconducting nanowire arrays

cond-mat.supr-con↗
arXiv2018

Thermal disorder in finite-length carbon nanowire

Chemisorption is one of the active research areas in carbon materials. The occurrence of the monoatomic carbon chain can be made by surrounding the double walled carbon nanotube and meanwhile worldwide efforts have been made to create the extraction technique for unlashing the carbon chains from the enclosure. Here we report an extensive study of the kink structure in the free standing carbon nanowires. Our Monte Carlo simulation considers the multi-monoatomic carbon chains laterally interacted by the Van der Waal force. Despite the linearity of the carbon nanowires is independent of chain length at low temperatures, the same situation does not hold at high temperatures. Disordered kink structure is observed in the short carbon chains especially above Peierls transition temperature. For instance, the average kink angle of 50-atoms carbon nanowire is as large as 35 degree at 800K. We have provided an important inspection that any physical property of the finite-length carbon chain predicted by ab-initio calculation should reconsider the atomic rearrangement due to the thermal instability. Apart from this, the kink structure in the nanowires likely increases the probability of attaching negatively charged atoms which is an encouragement to find the next generation materials for chemisorption.

cond-mat.mtrl-sci↗
arXiv2018

Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies

Two different types of boron-doped graphene/copper interfaces synthesized using two different flow rates of Ar through the bubbler containing the boron source were studied. X-ray photoelectron spectra (XPS) and optically stimulated electron emission (OSEE) measurements have demonstrated that boron-doped graphene coating provides a high corrosion resistivity of Cu-substrate with the light traces of the oxidation of carbon cover. The density functional theory calculations suggest that for the case of substitutional (graphitic) boron-defect only the oxidation near boron impurity is energetically favorable and creation of the vacancies that can induce the oxidation of copper substrate is energetically unfavorable. In the case of non-graphitic boron defects oxidation of the area, a nearby impurity is metastable that not only prevent oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction demonstrates high catalytic performance of these materials.

cond-mat.mtrl-sci↗
arXiv2017

Electronic structure, charge transfer, and intrinsic luminescence of gadolinium oxide nanoparticles: Experiment and theory

The cubic (c) and monoclinic (m) polymorphs of Gd2O3 were studied using the combined analysis of several materials science techniques - X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Density functional theory (DFT) based calculations for the samples under study were performed as well. The cubic phase of gadolinium oxide (c-Gd2O3) synthesized using a precipitation method exhibits spheroidal-like nanoclusters with well-defined edges assembled from primary nanoparticles with an average size of 50 nm, whereas the monoclinic phase of gadolinium oxide (m-Gd2O3) deposited using explosive pyrolysis has a denser structure compared with natural gadolinia. This phase also has a structure composed of three-dimensional complex agglomerates without clear-edged boundaries that are ~21 nm in size plus a cubic phase admixture of only 2 at. % composed of primary edge-boundary nanoparticles ~15 nm in size. These atomic features appear in the electronic structure as different defects ([Gd...O-OH] and [Gd...O-O]) and have dissimilar contributions to the charge-transfer processes among the appropriate electronic states with ambiguous contributions in the Gd 5p - O 2s core-like levels in the valence band structures. The origin of [Gd...O-OH] defects found by XPS was well-supported by PL analysis. The electronic and atomic structures of the synthesized gadolinias calculated using DFT were compared and discussed on the basis of the well-known joint OKT-van der Laan model, and good agreement was established.

cond-mat.mes-hall↗
arXiv2017

The MRO-accompanied modes of Re-implantation into SiO2-host matrix: XPS and DFT based scenarios

The following scenarios of Re-embedding into SiO2-host by pulsed Re-implantation were derived and discussed after XPS-and-DFT electronic structure qualification: (i) low Re-impurity concentration mode -> the formation of combined substitutional and interstitial impurities with Re2O7-like atomic and electronic structures in the vicinity of oxygen vacancies; (ii) high Re-impurity concentration mode -> the fabrication of interstitial Re-metal clusters with the accompanied formation of ReO2-like atomic structures and (iii) an intermediate transient mode with Re-impurity concentration increase, when the precursors of interstitial defect clusters are appeared and growing in the host-matrix structure occur. An amplification regime of Re-metal contribution majority to the final Valence Band structure was found as one of the sequences of intermediate transient mode. It was shown that most of the qualified and discussed modes were accompanied by the MRO (middle range ordering) distortions in the initial oxygen subnetwork of the a-SiO2 host-matrix because of the appeared mixed defect configurations.

cond-mat.mtrl-sci↗
arXiv2017

Enhanced clustering tendency of Cu-impurities with a number of oxygen vacancies in heavy carbon-loaded TiO2 - the bulk and surface morphologies

The over threshold carbon-loadings (~50 at.%) of initial TiO2-hosts and posterior Cu-sensitization (~7 at.%) was made using pulsed ion-implantation technique in sequential mode with 1 hour vacuum-idle cycle between sequential stages of embedding. The final Cx-TiO2:Cu samples were qualified using XPS wide-scan elemental analysis, core-levels and valence band mappings. The results obtained were discussed on the theoretic background employing DFT-calculations. The combined XPS and DFT analysis allows to establish and prove the final formula of the synthesized samples as Cx-TiO2:[Cu+][Cu2+] for the bulk and Cx-TiO2:[Cu+][Cu0] for thin-films. It was demonstrated the in the mode of heavy carbon-loadings the remaining majority of neutral C-C bonds (sp3-type) is dominating and only a lack of embedded carbon is fabricating the O-C=O clusters. No valence base-band width altering was established after sequential carbon-copper modification of the atomic structure of initial TiO2-hosts except the dominating majority of Cu 3s states after Cu-sensitization. The crucial role of neutral carbon low-dimensional impurities as the precursors for the new phases growth was shown for Cu-sensitized Cx-TiO2 intermediate-state hosts.

cond-mat.mtrl-sci↗
arXiv2017

Superconductivity in ultra-thin carbon nanotubes and carbyne-nanotube composites: an ab-initio approach

The superconductivity of the 4-angstrom single-walled carbon nanotubes (SWCNTs) was discovered more than a decade ago, and marked the breakthrough of finding superconductivity in pure elemental undoped carbon compounds. The van Hove singularities in the electronic density of states at the Fermi level in combination with a large Debye temperature of the SWCNTs are expected to cause an impressively large superconducting gap. We have developed an innovative computational algorithm specially tailored for the investigation of superconductivity in ultrathin SWCNTs. We predict the superconducting transition temperature of various thin carbon nanotubes resulting from electron-phonon coupling by an ab-initio method, taking into account the effect of radial pressure, symmetry, chirality (N,M) and bond lengths. By optimizing the geometry of the carbon nanotubes, a maximum Tc of 60K is found. We also use our method to calculate the Tc of a linear carbon chain embedded in the center of (5,0) SWCNTs. The strong curvature in the (5,0) carbon nanotubes in the presence of the inner carbon chain provides an alternative path to increase the Tc of this carbon composite by a factor of 2.2 with respect to the empty (5,0) SWCNTs.

cond-mat.supr-con↗
arXiv2016

Effect of symmetry on the electronic DOS, charge fluctuations and electron-phonon coupling in carbon chains

A theoretical model is provided to address the parameters influencing the electronic properties of kink-structured carbon chain at 0K. It is studied by the principle of DFT and solving the numerical 1D time-independent Schrödinger equation of electron and phonon simultaneously. Two different lengths of branches A and B, are occupied alternatively to generate the asymmetric carbon chain. The ratio of the asymmetric branch length,RAB=A/B, plays an important role in the electronic density of states DOS around Fermi level Ef . The highest DOS(Ef) occurs if the RAB equals to 2 and while the Fermi level coincides with the Von-Hove singularity at RAB=3. The location of the singularity point relative to the Ef is controllable via branch length interestingly. By comparison with the symmetric case, tuning the branch length asymmetrically shows a stronger impact to shift the Ef to the singularity point. The numerical solution of the 1D time independent Schrodinger equation of phonon indicates that the kink reinforces the charge fluctuations but the fluctuations are minimized when the RAB goes up. Based on the simulation results, the electron phonon coupling of the carbon nanowire decreases with chain length. In comparison to the symmetric structure, the electron phonon coupling of the asymmetric carbon chain is higher. The maximum electron phonon coupling of the asymmetric carbon chain takes place at RAB=2 . However, the weakening of the electron phonon coupling is observed owing to ultrahigh concentration of kinks. The reduction of the electron phonon coupling of the carbon chain is occurred under pressure.

cond-mat.mtrl-sci↗