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.Comment: 15 pages, 8 figures, to appear in Appl. Surf. Sc

Atomic and electronic structure of a copper/graphene interface as prepared and 1.5 years after

We report the results of X-ray spectroscopy and Raman measurements of as-prepared graphene on a high quality copper surface and the same materials after 1.5 years under different conditions (ambient and low humidity). The obtained results were compared with density functional theory calculations of the formation energies and electronic structures of various structural defects in graphene/Cu interfaces. For evaluation of the stability of the carbon cover, we propose a two-step model. The first step is oxidation of the graphene, and the second is perforation of graphene with the removal of carbon atoms as part of the carbon dioxide molecule. Results of the modeling and experimental measurements provide evidence that graphene grown on high-quality copper substrate becomes robust and stable in time (1.5 years). However, the stability of this interface depends on the quality of the graphene and the number of native defects in the graphene and substrate. The effect of the presence of a metallic substrate with defects on the stability and electronic structure of graphene is also discussed.Comment: 18 pages, 6 figures, accepted to Appl. Surf. Sc

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.Comment: 7+ pages, 3 Figure, to appear in J. Appl. Phy

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.Comment: 17 pages, 7 figures, accepted to phys. stat. solidi (b

Predicting the band gap of ternary oxides containing 3d(10) and 3d(0) metals

We present soft x-ray spectroscopy measurements and electronic structure calculations of ZnTiO 3, a ternary oxide that is related to wurtzite ZnO and rutile TiO 2. The electronic structure of ZnTiO 3 was calculated using a variety of exchange-correlation functionals, and we compare the predicted band gaps of this material obtained from each functional with estimates from our experimental data and optical gaps quoted from the literature. We find that the main hybridizations in the electronic structure of ZnTiO 3 can be predicted from the electronic structures of the two binary oxides. We further find that ZnTiO 3 has weaker O 2p-Zn 3d repulsion than in ZnO, resulting in a relatively lower valence band maximum and consequently a larger band gap. Although we find a significant core hole shift in the measured O K XAS of ZnTiO 3, we provide a simple empirical scheme for estimating the band gap that may prove to be applicable for any d10-d0 ternary oxide, and could be useful in finding a ternary oxide with a band gap tailored to a specific energy. © 2012 American Physical Society

Optical transparency and local electronic structure of Yb-doped Y 2 O 3 ceramics with tetravalent additives

The results of optical transmission and X-ray core-level spectra measurements of Yb:Y 2 O 3 ceramics with different tetravalent sintering additives (ZrO 2 , CeO 2 and HfO 2 ) fabricated from nanopowders (produced by the laser ablation method) and then annealed at 1400 °C in air for 2 h are presented. It is found that the transmission values for ZrO 2 - and HfO 2 -doped ceramics at the lasing wavelengths are higher than those of CeO 2 -doped samples. The X-ray photoelectron spectra (XPS) O 1s spectra show that the relative intensity of oxygen defect peak detected for 3Yb:Y 2 O 3 + 5CeO 2 ceramics decreases substantially and consistently compared to that of 5Yb:Y 2 O 3 + 5HfO 2 and 3Yb:Y 2 O 3 + 5ZrO 2 samples. This can be attributed to a more complete filling of oxygen vacancies due to annealing-induced oxygen diffusion into the highly defective sintered ceramics. The measurements of XPS Ce 3d spectra showed that the insufficiently complete filling of the oxygen vacancies in the 3Yb:Y 2 O 3 + 5CeO 2 compound is due to the appreciable presence of trivalent cerium ions. © 2019 by the authors.Ministry of Education and Science of the Russian Federation, Minobrnauka: 3.7270.2017/8.9This study was supported by FASO (Theme "Electron" No. AAAA-A18-118020190098-5). The XPS measurements were supported by the Ministry of Education and Science of the Russian Federation (Project No. 3.7270.2017/8.9) and the Government of the Russian Federation (Act 211, agreement No. 02.A03.21.0006). Fabrication of ceramic samples was performed in the framework of the state task of IEP UB RAS

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.Comment: 18 pages, 6 figures, 3 tables, accepted phys. stat. solidi (b

Atomic and electronic structures of stable linear carbon chains on Ag-nanoparticles

In this work, we report X-ray photoelectron (XPS) and valence band (VB) spectroscopy measurements of surfactant-free silver nanoparticles and silver/linear carbon chains (Ag@LCC) structures prepared by pulse laser ablation (PLA) in water. Our measurements demonstrate significant oxidation only on the surfaces of the silver nanoparticles with many covalent carbon-silver bonds but only negligible traces of carbon-oxygen bonds. Theoretical modeling also provides evidence of the formation of robust carbon-silver bonds between linear carbon chains and pure and partially oxidized silver surfaces. A comparison of theoretical and experimental electronic structures also provides evidence of the presence of non-oxidized linear carbon chains on silver surfaces. To evaluate the chemical stability, we investigated the energetics of the physical adsorption of oxidative species (water and oxygen) and found that this adsorption is much preferrable on oxidized or pristine silver surfaces than the adsorption of linear carbon chains, which makes the initial step in the oxidation of LCC energetically unfavorable. © 2017 Elsevier Lt

Electronic Structure of Transition-Metal Dicyanamides Me[N(CN)2_2]2_2 (Me = Mn, Fe, Co, Ni, Cu)

The electronic structure of Me[N(CN)$_2$]$_2$ (Me=Mn, Fe, Co, Ni, Cu) molecular magnets has been investigated using x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) as well as theoretical density-functional-based methods. Both theory and experiments show that the top of the valence band is dominated by Me 3d bands, while a strong hybridization between C 2p and N 2p states determines the valence band electronic structure away from the top. The 2p contributions from non-equivalent nitrogen sites have been identified using resonant inelastic x-ray scattering spectroscopy with the excitation energy tuned near the N 1s threshold. The binding energy of the Me 3d bands and the hybridization between N 2p and Me 3d states both increase in going across the row from Me = Mn to Me = Cu. Localization of the Cu 3d states also leads to weak screening of Cu 2p and 3s states, which accounts for shifts in the core 2p and 3s spectra of the transition metal atoms. Calculations indicate that the ground-state magnetic ordering, which varies across the series is largely dependent on the occupation of the metal 3d shell and that structural differences in the superexchange pathways for different compounds play a secondary role.Comment: 20 pages, 11 figures, 2 table