National Institute Of Materials Physics - Romania

Nanoscale Condensed Matter Laboratory

Publications

articles found
51. Orbital Ordering of the Mobile and Localized Electrons at Oxygen-Deficient LaAlO3/SrTiO3 If Interfaces
Authors: Chikina, A; Lechermann, F; Husanu, MA; Caputo, M; Cancellieri, C; Wang, XQ; Schmitt, T; Radovic, M; Strocov, VN

Published: AUG 2018, DOI: 10.1021/acsnano.8b02335

Abstract: Interfacing different transition-metal oxides opens a route to functionalizing their rich interplay of electron, spin, orbital, and lattice degrees of freedom for electronic and spintronic devices. Electronic and magnetic properties of SrTiO3-based interfaces hosting a mobile two-dimensional electron system (2DES) are strongly influenced by oxygen vacancies, which form an electronic dichotomy, where strongly correlated localized electrons in the in-gap states (IGSs) coexist with noncorrelated delocalized 2DES. Here, we use resonant soft-X-ray photoelectron spectroscopy to prove the e(g) character of the IGSs, as opposed to the t(2g) character of the 2DES in the paradigmatic LaAlO3/SrTiO3 interface. We furthermore separate the d(xy) and d(xz)/d(xz) orbital contributions based on deeper consideration of the resonant photoexcitation process in terms of orbital and momentum selectivity. Supported by a self-consistent combination of density functional theory and dynamical mean field theory calculations, this experiment identifies local orbital reconstruction that goes beyond the conventional e(g)-vs-t(2g) band ordering. A hallmark of oxygen-deficient LaAlO3/SrTiO3 is a significant hybridization of the e(g) and t(2g) orbitals. Our findings provide routes for tuning the electronic and magnetic properties of oxide interfaces through "defect engineering" with oxygen vacancies.
52. k-space imaging of anisotropic 2D electron gas in GaN/GaAlN high-electron-mobility transistor heterostructures
Authors: Lev, LL; Maiboroda, IO; Husanu, MA; Grichuk, ES; Chumakov, NK; Ezubchenko, IS; Chernykh, IA; Wang, X; Tobler, B; Schmitt, T; Zanaveskin, ML; Valeyev, VG; Strocov, VN

Published: JUL 11 2018, DOI: 10.1038/s41467-018-04354-x

Abstract: Nanostructures based on buried interfaces and heterostructures are at the heart of modern semiconductor electronics as well as future devices utilizing spintronics, multiferroics, topological effects, and other novel operational principles. Knowledge of electronic structure of these systems resolved in electron momentum k delivers unprecedented insights into their physics. Here we explore 2D electron gas formed in GaN/AlGaN high-electron-mobility transistor heterostructures with an ultrathin barrier layer, key elements in current high-frequency and high-power electronics. Its electronic structure is accessed with angle-resolved photoelectron spectroscopy whose probing depth is pushed to a few nanometers using soft-X-ray synchrotron radiation. The experiment yields direct k-space images of the electronic structure fundamentals of this system-the Fermi surface, band dispersions and occupancy, and the Fourier composition of wavefunctions encoded in the k-dependent photoemission intensity. We discover significant planar anisotropy of the electron Fermi surface and effective mass connected with relaxation of the interfacial atomic positions, which translates into nonlinear (high-field) transport properties of the GaN/AlGaN heterostructures as an anisotropy of the saturation drift velocity of the 2D electrons.
53. Engineering active sites on reduced graphene oxide by hydrogen plasma irradiation: mimicking bifunctional metal/supported catalysts in hydrogenation reactions
Authors: Primo, A; Franconetti, A; Magureanu, M; Mandache, NB; Bucur, C; Rizescu, C; Cojocaru, B; Parvulescu, VI; Garcia, H

Published: JUN 7 2018, DOI: 10.1039/c7gc03397d

Abstract: H2 plasma has been used to generate carbon vacancies on reduced graphene oxide to increase its catalytic activity as a hydrogenation catalyst. A relationship between the power of the plasma treatment and the exposure time with the activity of the material was observed for CvC double bond hydrogenation. The activity data in the case of 1-octene, showing skeletal isomerization besides hydrogenation, indicate that H2 plasma treatment can introduce hydrogenating and acid sites rendering a bifunctional catalyst that is reminiscent of the activity of noble metals supported on acid supports.
54. Combined use of Mossbauer spectroscopy, XPS, HRTEM, dielectric and anelastic spectroscopy for estimating incipient phase separation in lead titanate-based multiferroics
Authors: Craciun, F; Cordero, F; Vasile, BS; Fruth, V; Zaharescu, M; Atkinson, I; Trusca, R; Diamandescu, L; Tanase, LC; Galizia, P; Cernea, M; Galassi, C

Published: JUN 7 2018, DOI: 10.1039/c8cp01456f

Abstract: The formation of separate phases in crystalline materials is promoted by doping with elements with different valences and ionic radii. Control of the formation of separate phases in multiferroics is extremely important for their magnetic, ferroelectric and elastic properties, which are relevant for multifunctional applications. The ordering of dopants and incipient phase separation were studied in lead titanate-based multiferroics with the formula (Pb0.88Nd0.08)(Ti0.98-xFexMn0.02)O-3 (x = 0.00, 0.03, 0.04, 0.05) by means of a combination of Mossbauer spectroscopy, XPS, HRTEM, dielectric and anelastic spectroscopy. We found that Fe ions are substituted as Fe3+ at Ti sites and preferentially exhibit pentahedral coordination, whereas Ti ions have coexisting valences of Ti4+/Ti3+. Fe3+ ions are preferentially ordered in clusters, and there exists a transition temperature T-C1, below which phase separation occurs between a tetragonal phase T1 free of magnetic clusters and a cubic phase, and a lower transition temperature T-C2, below which the cubic phase rich in magnetic clusters is transformed into a tetragonal phase T2. The phase separation persists at the nanoscale level down to room temperature and is visible in HRTEM images as a mixing of nanodomains with different tetragonality ratios. This phase separation was observed over the whole studied concentration range of x(Fe) values. It occurs progressively with the value of x(Fe), and the transition temperature T-C2 decreases with the concentration from about 620 K (x(Fe) = 0.03) to about 600 K (x(Fe) = 0.05), while T-C1 remains nearly constant.
55. New bio-active, antimicrobial and adherent coatings of nanostructured carbon double-reinforced with silver and silicon by Matrix-Assisted Pulsed Laser Evaporation for medical applications
Authors: Duta, L; Ristoscu, C; Stan, GE; Husanu, MA; Besleaga, C; Chifiriuc, MC; Lazar, V; Bleotu, C; Miculescu, F; Mihailescu, N; Axente, E; Badiceanu, M; Bociaga, D; Mihailescu, IN

Published: MAY 31 2018, DOI: 10.1016/j.apsusc.2018.02.047

Abstract: We report on Matrix-Assisted Pulsed Laser Evaporation (MAPLE) deposition of Carbon thin films, simple or reinforced with intended concentrations of Ag and Si. A KrF* (lambda = 248 nm, tau(FWHM) < 25 ns, v = 10 Hz) excimer laser was used for irradiation. The effect of a post-deposition thermal treatment in vacuum was studied. Besides detailed morphological, compositional, structural and pull-out adherence characterizations, the potential of the carbonaceous films for medical applications was investigated in vitro by anti-biofilm and cytocompatibility assays. The microscopic images evidenced no delaminations. Micro-Raman spectroscopy revealed a graphitization tendency depending on preparation conditions, thermal treatment and reinforcing agents' presence. Adherence values improved considerably after thermal treatment. In vitro biological evaluation showed that the films containing similar to 1.85 at.% Ag were non-cytotoxic for MG63 cells, while eliciting a limited antimicrobial activity. The increase of Ag content to 3.6 at.% results in a significant enhancement of antimicrobial activity, whilst maintaining the cytotoxic action and adherence characteristics at acceptable levels. We propose a new class of metamaterials based on C reinforced with Ag and Si obtained by MAPLE for medical applications, i.e. the prevention and treatment of various infections associated with biofilms developed on implants and other medical equipments. (C) 2018 Elsevier B.V. All rights reserved.
56. Large positive linear magnetoresistance in the two-dimensional t(2g) electron gas at the EuO/SrTiO3 interface
Authors: Kormondy, KJ; Gao, LY; Li, X; Lu, SR; Posadas, AB; Shen, SD; Tsoi, M; McCartney, MR; Smith, DJ; Zhou, JS; Lev, LL; Husanu, MA; Strocov, VN; Demkov, AA

Published: MAY 16 2018, DOI: 10.1038/s41598-018-26017-z

Abstract: The development of novel nano-oxide spintronic devices would benefit greatly from interfacing with emergent phenomena at oxide interfaces. In this paper, we integrate highly spin-split ferromagnetic semiconductor EuO onto perovskite SrTiO3 (001). A careful deposition of Eu metal by molecular beam epitaxy results in EuO growth via oxygen out-diffusion from SrTiO3. This in turn leaves behind a highly conductive interfacial layer through generation of oxygen vacancies. Below the Curie temperature of 70 K of EuO, this spin-polarized two- dimensional t(2g) electron gas at the EuO/SrTiO3 interface displays very large positive linear magnetoresistance (MR). Soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) reveals the t(2g) nature of the carriers. First principles calculations strongly suggest that Zeeman splitting, caused by proximity magnetism and oxygen vacancies in SrTiO3, is responsible for the MR. This system offers an as-yet-unexplored route to pursue proximity-induced effects in the oxide two- dimensional t(2g) electron gas.
57. Photoelectrochemical response of carbon dots (CDs) derived from chitosan and their use in electrochemical imaging
Authors: Zhang, DW; Papaioannou, N; David, NM; Luo, H; Gao, H; Tanase, LC; Degousee, T; Samori, P; Sapelkin, A; Fenwick, O; Titirici, MM; Krause, S

Published: MAY 1 2018, DOI: 10.1039/c7mh00784a

Abstract: We report a direct photoelectrochemical response from low cost carbon dots (CDs) prepared from chitosan via a solvothermal method. The carbon dots were covalently linked to an indium tin oxide (ITO) surface through a self-assembled silane monolayer. We attribute the photocurrent of the ITO-silane-CD surface to a photogenerated electron-transfer process by CDs under illumination with a wavelength of 420 nm to 450 nm. The self-assembled monolayer of CDs was used for ac-photocurrent imaging of the surface with micron scale lateral resolution. This discovery opens up new applications for CDs as biocompatible, light-addressable electrochemical sensors in bioanalytical and bioimaging applications.
58. Hydrogenolysis of lignin over Ru-based catalysts: The role of the ruthenium in a lignin fragmentation process
Authors: Verziu, M; Tirsoaga, A; Cojocaru, B; Bucur, C; Tudora, B; Richel, A; Aguedo, M; Samikannu, A; Mikkola, JP

Published: MAY 2018, DOI: 10.1016/j.mcat.2018.03.004

Abstract: The catalytic performances of two different classes of catalysts containing nickel or/and ruthenium as the active sites were studied in the depolymerisation of lignin isolated from Miscanthus x giganteus. The catalysts were prepared either by coprecipitation (ie, (RuNiMgAlO)x, (RuNiAlO)x, (NiAlO)x, (NiMgAlO)x) or by wet impregnation (ie, Ru/Al2O3) and characterized by nitrogen physisorption (BET), XRD, XPS, NH3-TPD, Raman and H-2-TPR techniques. The experimental results indicate that the presence of ruthenium led to dimers as dominant products.
59. Highly Efficient, Easily Recoverable, and Recyclable Re-SiO2-Fe3O4 Catalyst for the Fragmentation of Lignin
Authors: Tudorache, M; Opris, C; Cojocaru, B; Apostol, NG; Tirsoaga, A; Coman, SM; Parvulescu, VI; Duraki, B; Krumeich, F; van Bokhoven, JA

Published: AUG 2018, DOI: 10.1021/acssuschemeng.7b04294

Abstract: A series of Fe3O4@SiO2@Re catalysts were prepared by deposition of rhenium by means of the precipitation-deposition and impregnation procedures. Characterization of the catalysts confirmed that the deposition of silica protected the magnetite nanoparticles, resulting in a stable SiO(2)pFe(3)O(4) composite, which was not affected by the treatments during the further deposition of rhenium. Rhenium was silent in XRD over the range of concentrations, at which it was deposited. Furthermore, XPS detected rhenium, only in the impregnation series; this may confirm that dispersion was high. As expected, rhenium was not reduced to the metallic state and generated weakly acidic Bronsted-type centers as detected by NH3-TPD. H-2-TPD and chemisorption experiments demonstrate the capacity of these catalysts to chemisorb hydrogen. In line with these properties, rhenium catalyzed both C-C hydrogenolysis and C-O hydrolysis in successive steps. The performance of these catalysts was checked for a series of lignins of different origin and by means of different separation procedures. A very important finding is that these catalysts were highly stable and easy to recover.
60. Graphene growth by molecular beam epitaxy: an interplay between desorption, diffusion and intercalation of elemental C species on islands
Authors: Presel, F; Tetlow, H; Bignardi, L; Lacovig, P; Tache, CA; Lizzit, S; Kantorovich, L; Baraldi, A

Published: APR 28 2018, DOI: 10.1039/c8nr00615f

Abstract: The growth of graphene by molecular beam epitaxy from an elemental carbon precursor is a very promising technique to overcome some of the main limitations of the chemical vapour deposition approach, such as the possibility to synthesize graphene directly on a wide variety of surfaces including semiconductors and insulators. However, while the individual steps of the chemical vapour deposition growth process have been extensively studied for several surfaces, such knowledge is still missing for the case of molecular beam epitaxy, even though it is a key ingredient to optimise its performance and effectiveness. In this work, we have performed a combined experimental and theoretical study comparing the growth rate of the molecular beam epitaxy and chemical vapour deposition processes on the prototypical Ir (111) surface. In particular, by employing high-resolution fast X-ray photoelectron spectroscopy, we were able to follow the growth of both single- and multi-layer graphene in real time, and to identify the spectroscopic fingerprints of the different C layers. Our experiments, supported by density functional theory calculations, highlight the role of the interaction between different C precursor species and the growing graphene flakes on the growth rate of graphene. These results provide an overview of the main differences between chemical vapour deposition and molecular beam epitaxy growth and thus on the main parameters which can be tuned to optimise growth conditions.

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