arXiv subjects

D. W. Boukhvalov

Publications and source records attributed to D. W. Boukhvalov.

At least 37 records · Page 2Linked to original sources

arXiv2016

Pleomorphic structural imperfections caused by pulsed Bi-implantation in the bulk and thin-film morphologies of TiO2

The results of combined experimental and theoretical study of substitutional and clustering effects in Bi-doped TiO2 hosts (bulk and thin-film morphologies) are presented. Bi-doping of the bulk and thin-film titanium dioxide was made with help of pulsed ion-implantation (E(Bi+) = 30 keV, D = 1 * 1017 cm-2) without posterior tempering. The X-ray photoelectron spectroscopy (XPS) qualification (core-levels and valence bands) and Density-Functional Theory (DFT) calculations were employed in order to study the electronic structure of Bi-ion implanted TiO2 samples. According to XPS data obtained and DFT calculations, the Bi -> Ti cation substitution occurs in Bi-implanted bulk TiO2, whereas in the thin-film morphology of TiO2:Bi the Bi-atoms have metal-like clusters segregation tendency. Based on the combined XPS and DFT considerations the possible reasons and mechanism for the observed effects are discussed. It is believed that established peculiarities of bismuth embedding into employed TiO2 hosts are mostly the sequence of pleomorphic origin for the formed "bismuth-oxygen" chemical bonding.

cond-mat.mtrl-sci↗

arXiv2016

Sn-loss effect in a Sn-implanted a-SiO2 host-matrix after thermal annealing: A combined XPS, PL, and DFT study

Amorphous a-SiO2 host-matrices were implanted with Sn-ions with and without posterior thermal tempering at 900 °C for 1 hour in ambient air. X-ray photoelectron spectroscopy analysis (XPS core-levels, XPS valence band mapping), photoluminescence (PL) probing, and density functional calculations (DFT) were employed to enable a detailed electronic structure characterization of these samples. It was experimentally established that the process of Sn-embedding into the a-SiO2 host occurs following two dissimilar trends: the Sn4+ -> Si4+ substitution in a-SiO2:Sn (without tempering), and Sn-metal clustering as interstitials in a-SiO2:Sn (900 °C, 1 hour). Both trends were modeled using calculated formation energies and partial densities of states (PDOS) as well as valence band (VB) simulations, which yielded evidence that substitutional defect generation occurs with the help of ion-implantation stimulated translocation of the host-atoms from their stoichiometric positions to the interstitial void. Experimental and theoretical data obtained coincide in terms of the reported Sn-loss effect in a-SiO2:Sn (900 °C, 1 hour) due to thermally-induced electronic host-structure re-arraignment, which manifests as backward host-atoms translocation into stoichiometric positions and the posterior formation of Sn-metal clusters.

cond-mat.mtrl-sci↗

arXiv2015

Electronic structure and photoluminescence properties of Zn-ion implanted silica glass before and after thermal annealing

The results of XPS core-level and valence band measurements, photoluminescence spectra of a-SiO2 implanted by Zn-ions (E=30 keV, D=1*1017 cm^-2) and Density Functional Theory calculations of electronic structure as well as formation energies of structural defects in silica glass induced by Zn-ion implantation are presented. Both theory and experiment show that it is energetically more favorable for implanted zinc ions to occupy the interstitial positions instead of cation substitution. As a result, the Zn-ions embedded to interstitials, form chemical bonds with the surrounding oxygen atoms, formation ZnO-like nanoparticles and oxygen-deficient SiOx matrix. The subsequent thermal annealing at 900 0C (1 hr) strongly reduces the amount of ZnO nanoparticles and induces the formation of α-Zn2SiO4 phase which markedly enhances the green emission.

cond-mat.mtrl-sci↗

arXiv2015

The characterization of Co-nanoparticles supported on graphene

The results of density functional theory calculations and measurements using X-ray photoelectron spectroscopy of Co-nanoparticles dispersed on graphene/Cu are presented. It is found that for low cobalt thickness (0.02 nm - 0.06 nm) the Co forms islands distributed non-homogeneously which are strongly oxidized under exposure to air to form cobalt oxides. At greater thicknesses up to 2 nm the upper Co-layers are similarly oxidized whereas the lower layers contacting the graphene remain metallic. The measurements indicate a Co2+ oxidation state with no evidence of a 3+ state appearing at any Co thickness, consistent with CoO and Co[OH]2. The results show that thicker Co (2nm) coverage induces the formation of a protective oxide layer while providing the magnetic properties of Co nanoparticles.

cond-mat.mtrl-sci↗

arXiv2015

Structural defects and electronic structure of N-ion implanted TiO2: bulk versus thin film

Systematic investigation of atomic structure of N-ion implanted TiO2 (thin films and bulk ceramics) was performed by XPS measurements (core levels and valence bands) and first-principles density functional theory (DFT) calculations. In bulk samples experiment and theory demonstrate anion N->O substitution. For the thin films case experiments evidence valuable contributions from N2 and NO molecule-like structures and theoretical modeling reveals a possibility of formation of these species as result of the appearance of interstitial nitrogen defects on the various surfaces of TiO2. Energetics of formation of oxygen vacancies and its key role for band gap reduction is also discussed.

cond-mat.mtrl-sci↗

arXiv2015

Octahedral conversion of a-SiO2-host matrix by pulsed ion implantation

This is the abstract. The results of measurements of X-ray photoelectron spectra (XPS) of a-SiO2-host material after pulsed implantation with [Mn+] and [Co+, Mn+]-ions as well as DFT-calculations are presented. The low-energy shift is found in XPS Si 2p and O 1s core-levels of single [Mn+] and dual [Co+, Mn+] pulsed ion-implanted a-SiO2 (E = 30 keV, D = 2*10^17 cm^-2) with respect to those of untreated a-SiO2.The similar changes are found in XPS Si 2p and O 1s of stishovite compared to those of quartz. This means that the pulsed ion-implantation induces the local high pressure effect which leads to an appearance of SiO6-structural units in alpha-SiO2 host, forming "stishovite-like" local atomic structure. This process can be described within electronic bonding transition from the four-fold "quartz-like" to six-fold "stishovite-like" high-pressure phase in SiO2 host-matrix. It is found that such octahedral conversion depends on the fluence and starts with doses higher than D = 3*10^16 cm^-2.

cond-mat.mtrl-sci↗

arXiv2015

XPS and DFT study of Sn incorporation into ZnO and TiO2 host matrices by pulsed ion implantation

Bulk and thin films ZnO and TiO2 samples were doped with Sn by pulsed ion implantation and studied by means of X-ray photoelectron core-level and valence band spectroscopy as well as density functional theory calculations for comprehensive study of the incorporation of Sn. XPS spectral analysis showed that isovalent Sn cation substitution occurs in both zinc oxide (Sn2+ -> Zn2+) and titanium dioxide (Sn4+ -> Ti4+) for bulk and film morphologies. For TiO2 films, the implantation also led to occupation of interstitials by doped ions, which induced the clustering of substituted and embedded Sn atoms; this did not occur in ZnO:Sn film samples. Density functional theory (DFT) formation energies were calculated of various incorporation processes, explaining the prevalence of substitutional defects in both matrices. Possible mechanisms and reasons for the observed trends in Sn incorporation into the ZnO and TiO2 matrices are discussed.

cond-mat.mtrl-sci↗

arXiv2015

Chemical modifications and stability of phosphorene with impurities: A first principles study

We perform a systematic first-principles study of phosphorene in the presence of typical monovalent (hydrogen, fluorine) and divalent (oxygen) impurities. The results of our modeling suggest a decomposition of phosphorene into weakly bonded one-dimensional (1D) chains upon single- and double-side hydrogenation and fluorination. In spite of a sizable quasiparticle band gap (2.29 eV), fully hydrogenated phosphorene found to be dynamically unstable. In contrast, full fluorination of phosphorene gives rise to a stable structure, being an indirect gap semiconductor with the band gap of 2.27 eV. We also show that fluorination of phosphorene from the gas phase is significantly more likely than hydrogenation due to the relatively low energy barrier for the dissociative adsorption of F2 (0.19 eV) compared to H2 (2.54 eV). At low concentrations, monovalent impurities tend to form regular atomic rows phosphorene, though such patterns do not seem to be easily achievable due to high migration barriers (1.09 and 2.81 eV for H2 and F2, respectively). Oxidation of phosphorene is shown to be a qualitatively different process. Particularly, we observe instability of phosphorene upon oxidation, leading to the formation of disordered amorphous-like structures at high concentrations of impurities.

cond-mat.mtrl-sci↗

arXiv2014

Atomic, electronic and magnetic structure of graphene/iron and nickel interfaces: theory and experiment

First-principles calculations of the effect of carbon coverage on the atomic, electronic and magnetic structure of nickel and iron substrates demonstrate insignificant changes in the interatomic distances and magnetic moments on the atoms of the metallic substrates. The coverage of the iron surface by mono- and few-layer graphene induces significant changes in the orbital occupancies and exchange interactions between the layers in contrast to the case of a nickel substrate for which changes in the orbital ordering and exchange interactions are much smaller. Experimental measurements demonstrate the presence of ferromagnetic fcc-iron in Fe@C nanoparticles and the superparamagnetic behavior of Ni@C nanoparticles.

cond-mat.mtrl-sci↗

arXiv2014

Modification of titanium and titanium dioxide surfaces by ion implantation: combined XPS and DFT study

The results of XPS measurements (core levels and valence bands) of P+, Ca+, P+Ca+ and Ca+P+ ion implanted (E=30 keV, D=1x1017 cm-2) commercially pure titanium (cp-Ti) and first-principles density functional theory (DFT) calculations demonstrates formation of various structural defects in titanium dioxide films formed on the surface of implanted materials. We have found that for double implantation (Ti:P+,Ca+ and Ti:Ca+,P+) the outermost surface layer formed mainly by Ca and P, respectively, i.e. the implantation sequence is very important. The DFT calculations show that under P+ and Ca+P+ ion implantation the formation energies for both cation (P-Ti) and anion (P-O) substitutions are comparable which can induce the creation of [PO4]3- and Ti-P species. For Ca+ and P+Ca+-ion implantation the calculated formation energies correspond to Ca2+-Ti4+ cation substitution. This conclusion is in agreement with XPS Ca 2p and Ti 2p core levels and valence band measurements and DFT calculations of electronic structure of related compounds. The conversion of implanted ions to Ca2+ and [PO4]3- species provides a good biocompatibility of cp-Ti for further formation of hydroxyapatite.

cond-mat.mtrl-sci↗

arXiv2014

Oxidation of graphite surface: the role of water

Based on density functional calculations, we demonstrate a significant difference in oxidation patterns between graphene and graphite and the formation of defects after oxidation. Step-by-step modeling demonstrates that oxidation of 80% of the graphite surface is favorable. Oxidation above half of the graphite surface significantly decreases the energy costs of vacancy formation with CO2 production. The presence of water is crucial in the transformation of epoxy groups to hydroxyl, the intercalation with further bundle and exfoliation. In water-rich conditions, water intercalates graphite at the initial stages of oxidation and oxidation, which is similar to the oxidation process of free-standing graphene; in contrast, in water-free conditions, large molecules intercalate graphite only after oxidation occurs on more than half of the surface.

cond-mat.mtrl-sci↗

arXiv2014

Proton transport through one atom thick crystals

Graphene is impermeable to all gases and liquids, and even such a small atom as hydrogen is not expected to penetrate through graphene's dense electronic cloud within billions of years. Here we show that monolayers of graphene and hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons, hydrogen ions under ambient conditions. As a reference, no proton transport could be detected for a monolayer of molybdenum disulfide, bilayer graphene or multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton conductivity with a low activation energy of about 0.3 eV but graphene becomes a better conductor at elevated temperatures such that its resistivity to proton flow is estimated to fall below 10^-3 Ohm per cm2 above 250 C. The proton barriers can be further reduced by decorating monolayers with catalytic nanoparticles. These atomically thin proton conductors could be of interest for many hydrogen-based technologies.

cond-mat.mtrl-sci↗

arXiv2014

Local structure of Fe impurity atoms in ZnO: bulk versus surface

By studying Fe-doped ZnO pellets and thin films with various x-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fe-doping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe2+ -> Zn2+). In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe3+ -> Zn2+). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favourable. Furthermore, it is found that ferromagnetic states are energetically unfavourable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e. ZnFe2O4), or codoping with Cu.

cond-mat.mtrl-sci↗

arXiv2014

Structural defects induced by Fe-ion implantation in TiO2

X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission spectroscopy (RXES) measurements of pellet and thin film forms of TiO$_2$ with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO$_2$ pellet sample induces heterovalent cation substitution (Fe$^{2+}\rightarrow$ Ti$^{4+}$) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe$^{3+}$ are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory (DFT) calculations of formation energies for different configurations of structural defects in the implanted TiO$_2$:Fe system. According to our calculations, the clustering of Fe-atoms in TiO$_2$:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios.

cond-mat.mtrl-sci↗

arXiv2013

Reduction of conductivity and ferromagnetism induced by Ag doping in ZnO:Co

Cobalt and silver co-doping has been undertaken in ZnO thin films grown by pulsed laser deposition in order to investigate the ferromagnetic properties in ZnO-based diluted magnetic materials and to understand the eventual relation between ferromagnetism and charge carriers. Hall transport measurements reveal that Ag doping up to 5% leads to a progressive compensation of the native n-type carriers. The magnetization curves show ferromagnetic contributions for all samples at both 5 K and room temperature, decreasing with increasing the Ag concentration. First principles modeling of the possible configurations of Co-Ag defects suggest the formation of nano-clusters around interstitial Co impurity as the origin of the ferromagnetism. The Ag co-doping results in a decrease of the total spin of these clusters and of the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2013

Electronic Structure and Resonant X-ray Emission Spectra of Carbon Shells of Iron Nanoparticles

The electronic structure of carbon shells of carbon encapsulated iron nanoparticles carbon encapsulated Fe@C has been studied by X-ray resonant emission and X-ray absorption spectroscopy. The recorded spectra have been compared to the density functional calculations of the electronic structure of graphene. It has been shown that an Fe@C carbon shell can be represented in the form of several graphene layers with Stone-Wales defects. The dispersion of energy bands of Fe@C has been examined using the measured C Ka resonant X-ray emission spectra.

cond-mat.mtrl-sci↗

arXiv2013

DFT modeling of the covalent functionalization of graphene: from ideal to realistic models

The production of multiple types of graphene, such as free standing, epitaxial graphene on silicon carbide and metals, graphene in solution, chemically grown graphene-like molecules, various graphene nanoribbons, and graphene oxide with different levels of reduction and various chemical composition, demonstrate the need for additional investigation beyond the basic principles of graphene functionalization for avoidance of occasionally contradictions between the predictions from first-principles simulations and experimental results. Herein, we discuss the current state of modeling of the different types of graphene using density functional theory (DFT) methods. We focus on the static (substrate, shape, curvature, strain and doping) and dynamic (starting point of functionalization, migration barriers and stability of configurations) aspects that provide a more correct and selective modeling of the chemisorption of various chemical species on the graphene scaffold. Based on the recent modeling of experimentally realized functionalization of different types of graphene we can conclude that the formation of uniform one- or two-sided functionalized graphene discussed in earlier studies is an exception to the typical scenarios of graphene chemistry. The presence of different substrates, defects and lattice distortions, such as ripples and strain, results in the formation of clusters or lines from the functional groups. Several configurations of the chemical species on the graphene substrate have been found to exist with ideal models but are only stable for graphene functionalized under special conditions. The role of the technical parameters, such as the functionals and computational methods employed, and the important guidelines for the modeling of the chemical properties of various types of graphene are also discussed.

cond-mat.mtrl-sci↗

arXiv2012

Selective surface functionalization at regions of high local curvature in graphene

Monolayer graphene was deposited on a Si wafer substrate decorated with SiO2 nanoparticles (NPs) and then exposed to aryl radicals that were generated in situ from their diazonium precursors. Using micro-Raman mapping, the aryl radicals were found to selectively react with the regions of graphene that covered the NPs. The enhanced chemical reactivity was attributed to the increased strain energy induced by the local mechanical deformation of the graphene.

cond-mat.mtrl-sci↗