Dr. George-Adrian LUNGU
Scientific Researcher III
Laboratory of Surface and Interface Science
adrian[DOT]lungu[AT]infim[DOT]roResume
Dr. George-Adrian LUNGU, Researcher III
Born on December 3, 1978 in Braşov, Romania
June 2002 - B.Sc. in Physics at University of Bucharest, Faculty of Physics
February 2004 - M.Sc. in Solid State Physics and Polymer Physics at University of Bucharest, Faculty of Physics
June 2014 - Ph.D. in Physics at Univesity of Bucharest
Ph.D. Thesis title: “Structural and magnetic properties of certain thin films.”
Present position (2016-): Researcher III in Surfaces and Interfaces Laboratory of [Romanian] National Institute of Materials Physics, Măgurele, Romania
International work stage(s):
· January 2005-May 2006 at [Italian] National Institute of Nuclear Physics, Section of Pisa, within Compact Muon Solenoid (CMS) Collaboration of Large Hadron Collider (LHC) large project, as worker at CMS Detector Tracker buiding.
Interest domains:
· Ferromagnetic thin films
· Ferroelectric thin films
· Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy as structural investigation technique
· X-ray Photoelectron Spectroscopy (XPS) as chemical investigation technique
Total published papers: 47, h-index: 12, total citations without self-citations: 4483.
Publications
1. Independent and complementary bio-functional effects of CuO and Ga2O3 incorporated as therapeutic agents in silica- and phosphate- based bioactive glasses
Authors:
Tite, T; Popa, AC; Stuart, BW; Fernandes, HR; Chirica, IM; Lungu, GA; Macovei, D; Bartha, C; Albulescu, L; Tanase, C; Nita, S; Rusu, N; Grant, DM; Ferreira, JMF; Stan, GE
Published: JUL 2022, JOURNAL OF MATERIOMICS, 8, DOI: 10.1016/j.jmat.2021.12.009
The incorporation of therapeutic-capable ions into bioactive glasses (BGs), either based on silica (SBGs) or phosphate (PBGs), is currently envisaged as a proficient path for facilitating bone regeneration. In conjunction with this view, the single and complementary structural and bio-functional roles of CuO and Ga2O3 (in the 2-5 mol% range) were assessed, by deriving a series of SBG and PBG formulations starting from the parent glass systems, FastOs (R) BG -38.5SiO2-36.1CaO-5.6P2O5-19.2MgO-0.6CaF2, and 50.0P2O5-35.0CaO-10.0Na2O-5.0 Fe2O3 (mol%), respectively, using the process of melt-quenching. The inter-linked physico-chemistry -biological response of BGs was assessed in search of bio-functional triggers. Further light was shed on the structural role -as network former or modifier -of Cu and Ga, immersed in SBG and PBG matrices. The preliminary biological performance was surveyed in vitro by quantification of Cu and Ga ion release under homeostatic conditions, cytocompatibility assays (in fibroblast cell cultures) and antibacterial tests (against Staphylococcus aureus). The similar (Cu) and dissimilar (Ga) structural roles in the SBG and PBG vitreous networks governed their release. Namely, Cu ions were leached in similar concentrations (ranging from 10-35 ppm and 50-110 ppm at BG doses of 5 and 50 mg/mL, respectively) for both type of BGs, while the release of Ga ions was 1-2 orders of magnitude lower in the case of SBGs (i.e., 0.2-6 ppm) compared to PBGs (i.e., 9-135 ppm). This was attributed to the network modifier role of Cu in both types of BGs, and conversely, to the network former (SBGs) and network modifier (PBGs) roles of Ga. All glasses were cytocompatible at a dose of 5 mg/mL, while at the same concentration the antimicrobial efficiency was found to be accentuated by the coupled release of Cu and Ga ions from SBG. By collective assessment, the most prominent candidate material for the further development of implant coatings and bone graft substitutes was delineated as the 38.5SiO2-34.1CaO-5.6P2O5-16.2MgO-0.6CaF2-2.0CuO-3.0Ga2O3 (mol%) SBG system, which yiel-ded moderate Cu and Ga ion release, excellent cytocompatibility and marked antibacterial efficacy. (c) 2021 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2. CO adsorption and oxidation at room temperature on graphene synthesized on atomically clean Pt(001)
Authors:
Apostol, NG; Bucur, IC; Lungu, GA; Tache, CA; Teodorescu, CM
Published: APR 15 2021, CATALYSIS TODAY, 366, DOI: 10.1016/j.cattod.2020.02.006
Carbon monoxide is adsorbed at room temperature on graphene formed on atomically clean Pt(001)-hex by chemical vapor deposition, starting with ethylene, in ultrahigh vacuum. The graphene formation is characterized in situ by high resolution photoelectron spectroscopy (HRPES), by low energy electron diffraction (LEED) and by near-edge X-ray absorption fine structure (NEXAFS). The formation of graphene destroys the hex reconstruction of Pt(001) and graphene exhibits totally in-plane sp(2) bonding. CO adsorption is characterized by HRPES and a rigid shift towards higher binding energies by about 96 meV is experienced by Pt 4f core levels, together with a shift towards lower binding energy by 36 meV of the C 1s level corresponding to graphene, while the amplitude analysis of carbon and platinum peaks suggests the intercalation of carbon oxide between graphene and the metal substrate. The presence of oxidized carbon is evidenced by a separate component in the C 1s spectrum (attributed to carbon bond to oxygen) and by the occurrence of the O 1s signal. The coverage expressed in terms of the ratio of the integral amplitudes of the carbon bond to oxygen to the amplitude of the carbon from graphene approaches 3 %, yielding a CO coverage of Pt(001) of about 0.12 monolayer. The derived atomic ratio (O 1s):(C 1s bond to O) is initially close to 1, then evolves in time towards values close to 2, which means that CO is progressively oxidized upon adsorption and irradiation with soft X-rays. The relative amount of oxygen and oxidized carbon decreases in time under irradiation with soft X-rays. Weakly bound graphene on incommensurate metal surfaces may be used as atomic scale nanoreactors for trapping and immediate oxidation of carbon monoxide.
3. Nicolae C. Popa (1945-2020)
Authors:
Lungu, GA
Published: DEC 1 2020, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 53, DOI: 10.1107/S1600576720015289
4. Resistance hysteresis correlated with synchrotron radiation surface studies in atomic sp(2) layers of carbon synthesized on ferroelectric (001) lead zirconate titanate in an ultrahigh vacuum
Authors:
Apostol, NG; Lizzit, D; Lungu, GA; Lacovig, P; Chirila, CF; Pintilie, L; Lizzit, S; Teodorescu, CM
Published: JAN 8 2020, RSC ADVANCES, 10, DOI: 10.1039/c9ra09131a
Carbon layers are deposited on 100 nm thick atomically clean (001) lead zirconate titanate (PZT) in ultrahigh vacuum, ruling out the presence of any contaminants. X-ray photoelectron spectroscopy is used to assess the substrate surface or interface composition, substrate polarization and the thickness of carbon layers, which ranges from less than one monolayer (1 ML) of graphene to several monolayers. Atomically clean PZT(001) exhibit inwards polarization, and this polarization reverses the sign upon carbon deposition. Cationic vacancies are detected near the PZT surface, consistent with heavy p doping of these films near the surface. The carbon layers exhibited a consistent proportion of atoms forming in-plane sp(2) bonds, as detected by near-edge absorption fine structure (NEXAFS) analysis and confirmed partially by scanning tunneling microscopy (STM). In situ poling with simultaneous in-plane transport measurements revealed the presence of resistance anti-hysteresis versus the polarization orientation for films with less than 1 ML carbon amount, evolving towards 'normal' hysteresis for thicker carbon films. The anti-hysteresis is explained in terms of a mixed screening mechanism, involving charge carriers from the sp(2) carbon layers together with holes or ionized acceptors in PZT(001) near the interface. For thicker films, the compensation mechanism becomes extrinsic, involving mostly electrons and holes from carbon, yielding the expected hysteresis.
5. Room temperature ferromagnetism and its correlation to ferroelectricity of manganese embedded in lead zirco-titanate
Authors:
Bucur, IC; Apostol, NG; Abramiuc, LE; Tanase, LC; Tache, CA; Lungu, GA; Costescu, RM; Chirila, CF; Trupina, L; Pintilie, L; Teodorescu, CM
Published: JAN 1 2019, THIN SOLID FILMS, 669, DOI: 10.1016/j.tsf.2018.11.018
Manganese is deposited at high temperature on (001) oriented ferroelectric lead zirco-titanate prepared in two different ways: sputter-annealed or just simply annealed in ultrahigh vacuum. Room temperature ferromagnetism (FM) is obtained for Mn deposited on sputter-annealed substrates, while for the other sample preparation a paramagnetic behaviour is obtained. Also, for the first case a clear inwards component of the polarization P( - ) is observed by X-ray photoelectron spectroscopy and piezoresponse force microscopy. Composition analysis evidenced formation of Pb vacancies in the case of FM - P(( - ) )sample, consistent with hole formation near the surface, needed both to stabilize the inwards polarization state and to intermediate ferromagnetism between Mn2+ ions. The indirect exchange ferromagnetism mediated by holes is stronger, most probably because the interaction energy is proportional with the carrier effective mass. Also, whereas in the case of unsputtered substrate a stable surface Mn oxide is formed, defect formation by sputtering seems to favor Mn migration inside the sample. This also induces the formation of a thin film where ferromagnetism and the orientation of ferroelectric polarization might have the same origin, i. e. holes accumulated near the outer surface.
6. Triggering surface ferroelectric order in Pb(Zr, Ti)O-3(001) by deposition of platinum
Authors:
Bucur, IC; Tanase, LC; Abramiuc, LE; Lungu, GA; Chirila, C; Trupina, L; Apostol, NG; Costescu, RM; Negrea, RF; Pintilie, L; Teodorescu, CM
Published: FEB 28 2018, APPLIED SURFACE SCIENCE, 432, DOI: 10.1016/j.apsusc.2017.04.238
By platinum deposition on a 150 nm thick film of lead zirco-titanate oriented PZT(001), grown on strontium titanate (001) single crystals with a strontium ruthenate buffer layer, which did not show initial preferential out-of-plane orientation of its ferroelectric polarization, a band bending near the interface towards lower energies is observed using photoelectron spectroscopy, by following all core levels from the substrate (Pb 4f, Zr 3d, Ti 2p, O 1s). This is unexpected given the fact that platinum has a larger work function than PZT and a rectifying contact for electrons is expected to be built at the interface. This observation may have two explanations: (i) platinum forms an alloy with elements from PZT yielding a metal with considerable lower work function; (ii) platinum provides electrons to the substrate which are able to compensate the depolarization field generated by the outwards polarization state. Several arguments are brought in favor of the second hypothesis, especially the attenuation of core levels from the substrate which is well described by exponential functions with reasonable values of the photoelectron inelastic mean free path, suggesting the formation of a sharp interface. High resolution transmission electron microscopy confirmed the sharpness of the interface. (C) 2017 Elsevier B.V. All rights reserved.
7. Structural and magnetic properties of Ni nanofilms on Ge(001) by molecular beam epitaxy
Authors:
Bocirnea, AE; Costescu, RM; Pasuk, I; Lungu, GA; Teodorescu, CM
Published: DEC 1 2017, APPLIED SURFACE SCIENCE, 424, DOI: 10.1016/j.apsusc.2017.03.034
Ni films of 20 nm nominal thickness were grown on Ge(001) substrates by molecular beam epitaxy at several different temperatures from room temperature up to 400 C. X-ray diffraction and X-ray photoelectron spectroscopy reveal the nucleation of Ni-Ge compounds (NiGe, Ni2Ge, Ni5Ge2) as well as a departure from the fcc Ni structure exhibited by the films at and beyond a temperature of 100 C. The binding energy of the Ni 2p peak increases from the RT value (852.7 eV) by 0.51.1 eV for the Ni/Ge(001) samples, while the Ge 2p binding energy changes by 0.60.7 eV after Ni growth compared to a clean Ge(001) substrate (there is only a +/- 0.15 eV shift among the samples grown on substrates at higher temperatures). By increasing substrate temperature, we obtained higher intermixing of Ni and Ge, but rather than both Ni and Ge interdiffusing, we find that Ni diffuses further into the germanium with higher substrate temperature, forming increasingly Ni-rich Ni-Ge compounds diluted into the Ge matrix. Based on Magneto-optic Kerr Effect measurements, Ni/Ge(001) grown on substrates at 100 and 200 C does not exhibit a hysteresis loop, while the samples on 300 and 400 C substrates show magnetic behavior, which we attribute to the magnetic character of hexagonal Ni5Ge2 (which is determined here for the first time to be a ferromagnetic phase). (c) 2017 Elsevier B.V. All rights reserved.
8. Low-energy electron diffraction from ferroelectric surfaces: Dead layers and surface dipoles in clean Pb(Zr, Ti)O-3(001)
Authors:
Teodorescu, CM; Pintilie, L; Apostol, NG; Costescu, RM; Lungu, GA; Hrib, L; Trupina, L; Tanase, LC; Bucur, IC; Bocirnea, AE
Published: SEP 19 2017, PHYSICAL REVIEW B, 96, 115438, DOI: 10.1103/PhysRevB.96.115438
The positions of the low energy electron diffraction (LEED) spots from ferroelectric single crystal films depend on its polarization state, due to electric fields generated outside of the sample. Onemay derive the surface potential energy, yielding the depth where the mobile charge carriers compensating the depolarization field are located (delta). On ferroelectric Pb(Zr, Ti)O-3(001) samples, surface potential energies are between 6.7 and 10.6 eV, and d values are unusually low, in the range of 1.8 +/- 0.4 angstrom. When delta is introduced in the values of the band bending inside the ferroelectric, a considerably lower value of the dielectric constant and/or of the polarization near the surface than their bulk values is obtained, evidencing either that the intrinsic 'dielectric constant' of the material has this lower value or the existence of a 'dead layer' at the free surface of clean ferroelectric films. The inwards polarization of these films is explained in the framework of the present considerations by the formation of an electron sheet on the surface. Possible explanations are suggested for discrepancies between the values found for surface potential energies from LEED experiments and those derived from the transition between mirror electron microscopy and low energy electron microscopy.
9. Significant change of local atomic configurations at surface of reduced activation Eurofer steels induced by hydrogenation treatments
Authors:
Greculeasa, SG; Palade, P; Schinteie, G; Kuncser, A; Stanciu, A; Lungu, GA; Porosnicu, C; Lungu, CP; Kuncser, V
Published: APR 30 2017, APPLIED SURFACE SCIENCE, 402, DOI: 10.1016/j.apsusc.2017.01.056
Reduced-activation steels such as Eurofer alloys are candidates for supporting plasma facing components into kamak-like nuclear fusion reactors. In order to investigate the impact of hydrogen/deuterium insertion in their crystalline lattice, annealing treatments in hydrogen atmosphere have been applied on Eurofer slabs. The resulting samples have been analyzed with respect to local structure and atomic configuration both before and after successive annealing treatments, by X-ray diffractometry (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The corroborated data point out for a bcc type structure of the non-hydrogenated alloy, with an average alloy composition approaching Fe0.9Cr0.1 along a depth of about 100 nm. EDS elemental maps do not indicate surface inhomogeneities in concentration whereas the Mossbauer spectra prove significant deviations from a homogeneous alloying. The hydrogenation increases the expulsion of the Cr atoms toward the surface layer and decreases their oxidation, with considerable influence on the surface properties of the steel. The hydrogenation treatment is therefore proposed as a potential alternative for a convenient engineering of the surface of different Fe-Cr based alloys.(C) 2017 Elsevier B.V. All rights reserved.
10. Long-range magnetic interaction in MnxGe1-x: structural, spectromicroscopic and magnetic investigations
Authors:
Tanase, LC; Lungu, GA; Abramiuc, LE; Bucur, IC; Apostol, NG; Costescu, RM; Tache, CA; Macovei, D; Barinov, A; Teodorescu, CM
Published: MAR 2017, JOURNAL OF MATERIALS SCIENCE, 52, DOI: 10.1007/s10853-016-0618-9
We report the structural, electronic and magnetic investigation using extended X-ray absorption fine structure spectroscopy (EXAFS), photoelectron microscopy, spin-resolved photoemission and magneto-optical Kerr effect on the properties of MnGe systems obtained by molecular beam epitaxy deposition of manganese on Ge(001) wafers annealed on temperatures between 50 and 450 C. Magnetic ordering can be achieved when the substrate temperature is higher than 250 C, when the manganese tends to diffuse into the Ge matrix and segregate in MnGe-like compounds, as proved by EXAFS. High spatial resolution photoelectron spectroscopy reveals Mn inhomogeneities in the 5-10 m range, even though Mn is found mostly in the same chemical state all over the surface.
11. Be/W and W/Be bilayers deposited on Si substrates with hydrogenated Fe-Cr and Fe-Cr-Al interlayers for plasma facing components
Authors:
Greculeasa, SG; Palade, P; Schinteie, G; Lungu, GA; Porosnicu, C; Jepu, I; Lungu, CP; Kuncser, V
Published: DEC 2016, JOURNAL OF NUCLEAR MATERIALS, 481, DOI: 10.1016/j.jnucmat.2016.09.010
Be/W and W/Be bilayers, of interest in regard to the specific behavior of plasma facing components (PFCs) were deposited on Si substrates by thermionic vacuum arc, with Fe, Fe-Cr and Fe-Cr-Al interlayers. The interlayers, with compositions approaching the one of the reduced activation steels used in supporting PFCs, were subsequently annealed in hydrogen atmosphere. The multilayers were characterized with respect to morphologic, structural, diffusional and atomic intermixing aspects via XRD, XRR, X-ray photoemission spectroscopy and Mossbauer spectroscopy. All as-prepared samples present partially amorphous structures. A main alpha-Fe phase is observed, as well as (superparamagnetic) secondary Fe oxides, metallic Fe with Si, Cr, W and Be neighbors, Be-rich Fe-Be and Fe-Si phases. High amounts of tungsten and tungsten oxides were also evidenced in the Fe layer. The strong atomic intermixing of W and Be layers was indirectly supported by the unusual densities of Wand Be layers and Fe-57 Mossbauer spectroscopy results. (C) 2016 Elsevier B.V. All rights reserved.
12. Growth mechanisms and band bending in Cu and Pt on Ge(001) investigated by LEED and photoelectron spectroscopy
Authors:
Tanase, LC; Bocirnea, AE; Serban, AB; Abramiuc, LE; Bucur, IC; Lungu, GA; Costescu, RM; Teodorescu, CM
Published: NOV 2016, SURFACE SCIENCE, 653, DOI: 10.1016/j.susc.2016.06.006
We investigate band bending effects occurring at the interface between atomically clean Ge(001) and molecular beam epitaxy (MBE) deposited copper and platinum. Low energy electron diffraction(LEED) confirmed the crystallinity of the surface, evidenced the formation of (2 x 1) and (1 x 2) reconstructions, and revealed that it is strongly affected with metal deposition. X-ray photoelectron spectroscopy (XPS) data let us assume a Stranski-Krastanov growth mechanism and confirmed that the observed band bending is associated to an ohmic contact in both cases. For the platinum contact, the high values of the apparent inelastic mean free path (IMFP) derived from the evolution of the XPS intensities indicate a prevalence of mixture of Pt with Ge nearby the interface. Pt deposited on Ge(001) does not behave like a Schottky contact, as one may have expected due to the higher work function of platinum. The observed effect is similar to the ease where interfacial Pt had a lower work function by 2.25/1.96 eV than that of metallic Pt. We propose a model to explain this fact by the effective mass variation or to the conduction band broadening due to the strong intermixing of platinum with germanium under the surface. (C) 2016 Elsevier B.V. All rights reserved.
13. Aspects of native oxides etching on n-GaSb(100) surface
Authors:
Cotirlan, C; Ghita, RV; Negrila, CC; Logofatu, C; Frumosu, F; Lungu, GA
Published: FEB 15 2016, APPLIED SURFACE SCIENCE, 363, DOI: 10.1016/j.apsusc.2015.11.181
Gallium antimonide (GaSb) is the basis of the most photovoltaic and thermophotovoltaic (TPV) systems and its innovative technological aspects based on modern ultra-high vacuum techniques are in trend for device achievement. The real surface of GaSb is modified by technological processes that can conduce to problems related to the reproducible control of its surface properties. The GaSb surface is reactive in atmosphere due to oxygen presence and exhibits a native oxide layer. The evolution of native oxides during the ion sputtering, chemical etching and thermal annealing processes for preparing the surface is presented in detailed way. Ratios of surface constituents are obtained by Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS). Moreover, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM) and Low-Energy Electron Diffraction (LEED) are used for characterization. The surface stoichiometry is changed using a specific etchant (e.g. citric acid) at different etching time and is analyzed by ARXPS, SEM, EDS and AFM methods. The experimental results provide useful information regarding surface native oxides characteristics on n-GaSb(100) to be taken into account for development of low resistance contacts for TPV devices based on GaSb alloy. (C) 2015 Elsevier B.V. All rights reserved.
14. Non-interacting, sp(2) carbon on a ferroelectric lead zirco-titanate: towards graphene synthesis on ferroelectrics in ultrahigh vacuum
Authors:
Apostol, NG; Lungu, GA; Bucur, IC; Tache, CA; Hrib, L; Pintilie, L; Macovei, D; Teodorescu, CM
Published: 2016, RSC ADVANCES, 6, DOI: 10.1039/c6ra12910b
Carbon layers grown on lead zirco-titanate (PZT) weakly interact with the substrate and exhibit a nearly two dimensional character, up to a carbon surface density approaching that of graphene. The first feature is evidenced by X-ray photoelectron spectroscopy, and the second by angle resolved near-edge-absorption spectroscopy (NEXAFS). The binding energies and lineshape parameters of C 1s are similar to that of graphene. The dichroism of C K-edge NEXAFS shows the prevalence of in-plane sp(2) bonds for layers whose effective coverage is below the graphene surface density. The polarization state of the substrate, oriented outwards, is preserved upon carbon deposition. The surface Pb content is strongly affected by the carbon ad-layers.
15. Effects of annealing in Be/W and Be/C bilayers deposited on Si(001) substrates with Fe buffer layers
Authors:
Schinteie, G; Greculeasa, SG; Palade, P; Lungu, GA; Porosnicu, C; Jepu, I; Lungu, CP; Filoti, G; Kuncser, V
Published: FEB 2015, JOURNAL OF NUCLEAR MATERIALS, 457, DOI: 10.1016/j.jnucmat.2014.11.029
Atomic intermixing processes in relation to structural aspects and phase formation in Be based thin films subjected to different annealing treatments simulating the case of re-deposited layered structures on plasma facing components in nuclear fusion devices are reported. Accordingly, bilayers of Be/W and Be/C have been deposited on Si(001) substrates with Fe buffer layers. The Fe films have been prepared by radiofrequency sputtering and further processed by annealing in hydrogen atmosphere at 300 degrees C, for 90 min, at a pressure of 10 bars of H-2. After the Be/W and Be/C bilayer deposition by means of thermionic vacuum arc method, annealing in vacuum at 600 degrees C, for 10 min has been applied to the complex structures. The influence of annealing on the phase composition and atomic intermixing processes in the complex structures has been studied by means of X-ray photoelectron spectroscopy (XPS) and conversion electron Mossbauer spectroscopy (CEMS). The layered structures present an oxidation gradient with oxide phases in the uppermost layers and non-oxidized phases in the lower layers, as observed from the XPS data. The CEMS results revealed that the as-deposited structures contain a main metallic Fe phase and secondary superparamagnetic Fe oxide phases at the Fe/Be interface, while annealed samples present a large contribution of Fe-Be and Fe-C mixtures. The annealing treatment induces considerable atomic interdiffusion, strongly dependent on the nature of the upper layer. In the case of Be/W system, the annealing provides a much rougher Be/W interface, while in case of the Be/C structure, the annealing treatment only homogenize the structure over the whole depth. (C) 2014 Elsevier B.V. All rights reserved.
16. Room Temperature Ferromagnetic Mn:Ge(001)
Authors:
Lungu, GA; Stoflea, LE; Tanase, LC; Bucur, IC; Radutoiu, N; Vasiliu, F; Mercioniu, I; Kuncser, V; Teodorescu, CM
Published: JAN 2014, MATERIALS, 7, DOI: 10.3390/ma7010106
We report the synthesis of a room temperature ferromagnetic Mn-Ge system obtained by simple deposition of manganese on Ge(001), heated at relatively high temperature (starting with 250 degrees C). The samples were characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and magneto-optical Kerr effect (MOKE). Samples deposited at relatively elevated temperature (350 degrees C) exhibited the formation of similar to 5-8 nm diameter Mn5Ge3 and Mn11Ge8 agglomerates by HRTEM, while XPS identified at least two Mn-containing phases: the agglomerates, together with a Ge-rich MnGe similar to 2.5 phase, or manganese diluted into the Ge(001) crystal. LEED revealed the persistence of long range order after a relatively high amount of Mn (100 nm) deposited on the single crystal substrate. STM probed the existence of dimer rows on the surface, slightly elongated as compared with Ge-Ge dimers on Ge(001). The films exhibited a clear ferromagnetism at room temperature, opening the possibility of forming a magnetic phase behind a nearly ideally terminated Ge surface, which could find applications in integration of magnetic functionalities on semiconductor bases. SQUID probed the co-existence of a superparamagnetic phase, with one phase which may be attributed to a diluted magnetic semiconductor. The hypothesis that the room temperature ferromagnetic phase might be the one with manganese diluted into the Ge crystal is formulated and discussed.
17. Band bending at free Pb(Zr,Ti)O-3 surfaces analyzed spectroscopy by X-ray photoelectron
Authors:
Apostol, NG; Stoflea, LE; Lungu, GA; Tache, CA; Popescu, DG; Pintilie, L; Teodorescu, CM
Published: NOV 20 2013, MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS, 178, DOI: 10.1016/j.mseb.2013.02.007
This paper analyses in detail the core levels evolution of Pb(Zr,Ti)O-3, i.e. Pb 4f, Zr 3d, Ti 2p, O 1s in various conditions: absolutely freshly prepared sample, sample stored under air, and the effects of in vacuum annealing. The aim of the study is to quantify separately the chemical reactivity at the surface and the band bending effects due to the ferroelectric polarization. It is found that freshly prepared samples present mostly inwards (down arrow) polarization. This phenomenon is mostly revealed by the Ti 2p and O 1s spectra, manifested as a distinct component with 1.8 eV lower binding energy in the O 1s binding energy and by 1.1 eV in the Ti 2p binding energy. Sample aging under air suppresses the inwards polarization, and most signal comes from surfaces not presenting ferroelectric permanent polarization perpendicular to the sample surface. This process conducts also to the formation of Pb(CO3)(2) on the surface. Annealing to temperatures up to 400 degrees C stabilizes a surface composed by a main part of surface without polarization perpendicular to the surface, and with some areas presenting outwards (up arrow) polarization. These areas have, most probably, different terminations, the polarized area being (Ti,Zr)O-2 terminated. (C) 2013 Elsevier B.V. All rights reserved.
18. Band bending in Au/Pb(Zr,Ti)O-3 investigated by X-ray photoelectron spectroscopy: Dependence on the initial state of the film
Authors:
Apostol, NG; Stoflea, LE; Lungu, GA; Tanase, LC; Chirila, C; Frunza, L; Pintilie, L; Teodorescu, CM
Published: OCT 31 2013, THIN SOLID FILMS, 545, DOI: 10.1016/j.tsf.2013.04.092
This work presents a systematic investigation by X-ray photoelectron spectroscopy of the mechanisms of interface formation and band bending for Au/Pb(Zr,Ti)O-3 (PZT) layers grown on SrTiO3(001) with a SrRuO3 buffer layer, as function on the initial state of the PZT surface. After isolating the chemical effects, such as the formation of metal Pb at some surfaces, the evolution of the core levels with Au deposition allows one to simultaneously investigate the Schottky barrier formation and the built-in potential effects (charging induced by the static ferroelectric polarization). Areas of the sample with outwards P(+) and no polarization perpendicular to the surface P-(0) are identified for all samples. Only the freshly prepared sample exhibited inward polarization areas P(-). The built-in potential is on the order of 0.9 eV, while the Schottky band bending ranges from 0.2 to 0.6 eV towards lower absolute energies, therefore indicating that the work function of PZT exceeds that of Au deposited. We report also a chemically differentiate value of the built-in potential, manifested by a preferential distribution of the charge accumulated at the surface on Ti and O atoms. The O 1s and Ti 2p core levels manifest quite strong variations with the Au thickness for freshly prepared samples, resulting in shifts on the order of 2 eV towards lower binding energies. Au deposited on areas with an outward polarization is positively charged by the same potential as atoms from the PZT film (0.8-0.9 eV), whereas Au deposited on areas with an inward polarization forms a continuous grounded layer, which progressively pumps the accumulated charge and removes the polarization of these areas. (C) 2013 Elsevier B. V. All rights reserved.
19. Charge transfer and band bending at Au/Pb(Zr0.2Ti0.8)O-3 interfaces investigated by photoelectron spectroscopy
Authors:
Apostol, NG; Stoflea, LE; Lungu, GA; Chirila, C; Trupina, L; Negrea, RF; Ghica, C; Pintilie, L; Teodorescu, CM
Published: MAY 15 2013, APPLIED SURFACE SCIENCE, 273, DOI: 10.1016/j.apsusc.2013.02.056
The growth of gold layers on Pb(Zr,Ti)O-3 (PZT) deposited on SrTiO3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 angstrom. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by similar to 1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P(+) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Phi(PZT) - Phi(Au) similar to 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, O core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles. (C) 2013 Elsevier B.V. All rights reserved.
20. Co environment and magnetic defects in anatase CoxTi1-O-x(2) nanopowders
Authors:
Grecu, MN; Macovei, D; Ghica, D; Logofatu, C; Valsan, S; Apostol, NG; Lungu, GA; Negrea, RF; Piticescu, RR
Published: APR 22 2013, APPLIED PHYSICS LETTERS, 102, 161909, DOI: 10.1063/1.4802819
Cobalt environment and magnetic defects nature in hydrothermal synthesized anatase CoxTi1-xO2 nanopowders (0 <= x <= 0.1) are investigated by x-ray diffraction and a variety of spectroscopic techniques. One shows that cobalt is partially inserted in the anatase lattice, as Co2+ ions located on substitutional and interstitial sites. The fraction of the diluted Co is limited to 3 at.% for x >= 0.05, while the rest of the Co atoms gather into Co3O4 clusters. As found by electron paramagnetic resonance, the Co doping brings about hole-and electron-excess defects. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802819]
21. Dependence of the strain diffraction line broadening on (hkl) and sample direction in textured polycrystals
Authors:
Popa, NC; Lungu, GA
Published: APR 2013, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 46, DOI: 10.1107/S0021889812051928
A new phenomenological approach describing the dependence of the strain diffraction line breadth on direction in both crystal and sample is presented. For a negligibly small dependence on the direction in the sample, these models reduce to those for anisotropic strain broadening that already exist in the literature and which are implemented in popular Rietveld codes. The new model is appropriate for implementation in the Rietveld programs able to process simultaneously diffraction patterns recorded in multiple directions in a sample.
22. Structure, reactivity, electronic configuration and magnetism of samarium atomic layers deposited on Si(001) by molecular beam epitaxy
Authors:
Gheorghe, NG; Lungu, GA; Husanu, MA; Costescu, RM; Macovei, D; Teodorescu, CM
Published: FEB 15 2013, APPLIED SURFACE SCIENCE, 267, DOI: 10.1016/j.apsusc.2012.08.016
The surface structure, interface reactivity, electron configuration and magnetic properties of Sm layers deposited on Si(0 0 1) at various temperatures are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and magneto-optical Kerr effect (MOKE). It is found that metal Sm is present on samples prepared at low temperature, with an interface layer containing SmSi2 and Sm4Si3. When samples are prepared at high temperature, much less metal Sm is found, with an increasing amount of SmSi2. Room temperature ferromagnetism is observed for all prepared layers, with a decrease of the saturation magnetization when samples are prepared at high temperature. It is found that ferromagnetism implies mostly a compound with approximate stoichiometry Sm4Si3. Also, the decrease in the intensity of the XAS 2p(3/2) -> 3d white lines with the corresponding increasing amount of SmSi2 may be explained by assuming a higher occupancy of Sm 5d orbitals (5d(2) configuration), most probably due to hybridation effects. (C) 2012 Elsevier B. V. All rights reserved.
23. Room Temperature Ferromagnetic, Anisotropic, Germanium Rich FeGe(001) Alloys
Authors:
Lungu, GA; Apostol, NG; Stoflea, LE; Costescu, RM; Popescu, DG; Teodorescu, CM
Published: FEB 2013, MATERIALS, 6, DOI: 10.3390/ma6020612
Ferromagnetic FexGe1-x with x = 2%-9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 degrees C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 x 1) surface structure of Ge(001) with Fe deposition. X-ray photoelectron spectroscopy (XPS) at Ge 3d and Fe 2p core levels evidenced strong Fe diffusion into the Ge substrate and formation of Ge-rich compounds, from FeGe3 to approximately FeGe2, depending on the amount of Fe deposited. Room temperature magneto-optical Kerr effect (MOKE) evidenced ferromagnetic ordering at room temperature, with about 0.1 Bohr magnetons per Fe atom, and also a clear uniaxial magnetic anisotropy with the in-plane [110] easy magnetization axis. This compound is a good candidate for promising applications in the field of semiconductor spintronics.
24. FERROMAGNETISM AND REACTIVITY OF FE DEPOSITED ON GaAs(001) BY MAGNETRON SPUTTERING
Authors:
Vasilache, V; Lungu, GA; Logofatu, C; Medianu, RV; Teodorescu, CM
Published: JAN-MAR 2013, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 8, DOI:
Iron layers are grown on GaAs(001) single crystal wavers by magnetron sputtering and characterized by magneto-optical Kerr effect (MOKE) and X-ray photoelectron spectroscopy (XPS). It is found that all deposited layers, starting with 2 nm thickness, are ferromagnetic at room temperature with a relatively high coercitive field (500 to 750 Oe). The average Fe magnetic moment is larger for the thinnest Fe film deposited, of about 0.52 Bohr magnetons per Fe atom. The average Fe magnetic moment decreases to 0.26 Bohr magnetons, then to 0.17 Bohr magnetons for 4 and 8 nm, resepctively. XPS evidenced that the outermost Fe layers are oxidized; however, the thinnest Fe film presented the lower oxidation state. We may connect this reaction with Fe-As bonds, resulting in a nonferromagnetic interface compound. Indeed, XPS depth profiling of the 8 nm sample evidenced the formation of a very thin (about 1.4 nm) Fe metal layer immediately at the interface with GaAs. In this layer, Fe may reach an atomic magnetic moment as high as 1 Bohr magneton.
25. Manganese-based room temperature ferromagnetism in gallium arsenide
Authors:
Vasilache, V; Apostol, NG; Lungu, GA; Macovei, D; Teodorescu, CM
Published: NOV-DEC 2012, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 6, DOI:
Manganese is deposited onto GaAs(001) substrates in high vacuum conditions (10(-7) hPa), with substrates held at 300 degrees C. It is shown that this procedure yields to the diffusion of magnanese into gallium arsenide and the formation of a layer which exhibits room temperature ferromagnetism, with highly diluted Mn (below 1 atomic percent). X-ray absorption fine structure determinations at the Mn and Ga K-edges evidenced that Mn is not placed into substitutional Ga sites in GaAs. Most probably, Mn forms MnO clusters with rocksalt local structure. These clusters are the origin of the detected ferromagnetism.
26. Epitaxial ferromagnetic samarium and samarium silicide synthesized on Si(001)
Authors:
Costescu, RM; Gheorghe, NG; Husanu, MA; Lungu, GA; Macovei, D; Pintilie, I; Popescu, DG; Teodorescu, CM
Published: OCT 2012, JOURNAL OF MATERIALS SCIENCE, 47, DOI: 10.1007/s10853-012-6672-z
Samarium is deposited on Si(001) at various temperatures (room temperature to 400 A degrees C), and the surface structure, interface reactivity, electron configuration, and magnetic properties are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and magneto-optical Kerr effect (MOKE), respectively. It is found that metal Sm is present on samples prepared at room temperature with an interface layer containing mostly Sm2+ and a lower amount of Sm3+. When samples are prepared at high temperature, much less Sm-0 is found with an increasing amount of Sm2+. Freshly prepared Sm-0 and SmSi2 layers react strongly with oxygen from the residual gas, promoting formation of Sm2O3 at the expense of both metal Sm and SmSi2. Room temperature ferromagnetism is observed for all prepared layers with a decrease of the saturation magnetisation when samples are prepared at high temperature. It is found that ferromagnetism implies mostly Sm3+ and Sm metal. In addition to these findings, this work proposes a new assignment of the Sm 3d chemically shifted components. Also, a noticeable variation of the XPS Sm 3d spin-orbit splitting is found as a function of the Sm ionization state.
27. BAND FERROMAGNETISM IN SYSTEMS WITH LINEAR DENSITY OF STATES
Authors:
Lungu, GA; Apostol, NG; Morariu, M; Teodorescu, CM
Published: OCT-DEC 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, DOI:
The band (or Stoner) ferromagnetism in systems exhibiting a linear density of states, such as a graphene sheet, where the density of states g(e) is proportional to e, is investigated as function of temperature. The calculations are performed by neglecting spin-wave excitations and some arguments are presented for this approximation. It is found that the occurrence of ferromagnetism becomes easier at finite temperature than in the low temperature limit, especially for systems with low values of carrier density. An almost linear dependence of the magnetization on temperature is predicted; the temperature of vanishing magnetization is linear with the value of the equilibrium Fermi energy, irrespective of the value of the interaction (Hubbard) parameter. Also, for low values of the Hubbard energy an increase in the average value of the polarization with the temperature is predicted in the range of low temperatures. The most important application of such systems consists of control over the onset of magnetization by the charge density (the value of the Fermi energy).
28. X-ray photoelectron spectroscopy of pulsed laser deposited Pb(Zr,Ti)O3-delta
Authors:
Dragoi, C; Gheorghe, NG; Lungu, GA; Trupina, L; Ibanescu, AG; Teodorescu, CM
Published: JUN 2012, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 209, DOI: 10.1002/pssa.201127740
High quality pulsed laser deposited lead zirconate titanate (PZT) films are obtained by pulsed laser deposition on SrRuO3(111) single crystal layers and characterized by X-ray photoelectron spectroscopy (XPS), to determine the surface composition. It is found that a minor amount of Pb forms PbO2 at the surface and also some Pb is included into the contamination layer, in form of a Pb(CO3)2 layer of about 1?nm thickness, occupying about one quarter of the PZT surface. The stoichiometry of the outermost 45?nm layers yielded as PbZr0.25Ti0.80O2.5, which suggest the formation of an oxygen depleted, Brownmillerite-like layer at the surface, of at least 5?nm thickness.
29. Atomic structure and reactivity of ferromagnetic Fe deposited on Si(001)
Authors:
Gheorghe, NG; Husanu, MA; Lungu, GA; Costescu, RM; Macovei, D; Teodorescu, CM
Published: FEB 2012, JOURNAL OF MATERIALS SCIENCE, 47, DOI: 10.1007/s10853-011-5963-0
This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 angstrom Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 degrees C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe(3)Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.
30. Interface characterization and atomic intermixing processes in Be/W bilayers deposited on Si(001) substrates with Fe buffer layers
Authors:
Kuncser, V; Palade, P; Schinteie, G; Sandu, SG; Trupina, L; Lungu, GA; Gheorghe, NG; Teodorescu, CM; Porosnicu, C; Jepu, I; Lungu, CP; Filoti, G
Published: JAN 25 2012, JOURNAL OF ALLOYS AND COMPOUNDS, 512, DOI: 10.1016/j.jallcom.2011.09.063
Structural aspects and atomic intermixing processes in Be/W bilayers deposited on Si(0 0 1) substrates with Fe buffer layers enriched in the (57)Fe Mossbauer isotope have been studied via atomic force microscopy, grazing incidence X-ray diffractometry, X-ray reflectometry, X-ray photoelectron spectroscopy and conversion electron Mossbauer spectroscopy. The mentioned investigations allowed a full sequential characterization of the involved interfaces. Various ionic configurations appeared for Fe or W, while an amorphous state was observed in the case of Be. It has been proven that the Be layer has a negative influence on the roughness of the whole structure, which however presents an oxidation gradient from more oxidized elements at the surface towards more reduced elements in deeper layers. A strong diffusion of the W atoms inside the Be layer, induced by the deposition method, as well as of the Fe atoms inside the Be layer, induced by thermal annealing, has been evidenced. (C) 2011 Elsevier B.V. All rights reserved.
31. ATOMIC STRUCTURE AND MAGNETISM OF PLD DEPOSITED Tio(2):Fe
Authors:
Costescu, RM; Lungu, GA; Socol, G; Gheorghe, NG; Macovei, D; Negrila, CC; Logofatu, C; Husanu, MA; Popescu, DG; Tache, CA; Teodorescu, CM
Published: JAN-MAR 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, DOI:
TiO2:Fe thin films prepared by pulsed laser deposition exhibit in some case light dependent saturation magnetization, as determined from Kerr magnetometry measurements performed in dark or by illuminating the sample. This phenomenon is studied in correlation with local atomic structure investigated by extended X-ray absorption fine structure, composition and chemical state analyzed by X-ray photoelectron spectroscopy and by X-ray absorption near-edge structure. It is found that light-controllable magnetism is a property of a mixture of Fe and oxidized Fe clusters embedded in the anatase TiO2 matrix.
32. REACTIVITY, MAGNETISM AND LOCAL ATOMIC STRUCTURE IN FERROMAGNETIC FE LAYERS DEPOSITED ON SI(001)
Authors:
Gheorghe, NG; Husanu, MA; Lungu, GA; Costescu, RM; Macovei, D; Popescu, DG; Teodorescu, CM
Published: JAN-MAR 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, DOI:
This work presents recent studies concerning the synthesis of ultrathin ferromagnetic Fe layers on Si(001) and the correlated follow-up measurement of their structural properties, interface reactivity, and magnetism. This study is undertaken as function of the amount of Fe deposited and of substrate temperature. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction (LEED). The local order of Fe atoms is investigated by X-ray absorption fine structure (XAFS) and the magnetism by magneto-optical Kerr effect (MOKE). A general trend established is that a higher deposition temperature stabilizes a better surface ordering, but also enhances Fe and Si interdiffusion and therefore decreases the magnetism. A surprising effect obtained by Fe deposition at room temperature is that, despite the rapid disappearance of the long range order with Fe deposition (no LEED pattern is observed for Fe coverage exceeding one monolayer), the material exhibits a significant uniaxial in-plane magnetic anisotropy. When the deposition is performed at high temperature (500 degrees C), a weak ferromagnetism is still observed, with saturation magnetization of about 10 % of the value obtained for room temperature deposition. The combined MOKE and EXAFS studies allowed inferring consistent values for the range of Fe thicknesses where the reaction takes place and the main properties of the distinct formed layers.
33. Significantly different contamination of atomically clean Si(001) when investigated by XPS and AES
Authors:
Gheorghe, NG; Lungu, GA; Costescu, RM; Teodorescu, CM
Published: AUG 2011, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 248, DOI: 10.1002/pssb.201147220
Clean Si(001) single crystal surfaces provided different surface reconstructions: p(1 x 2) and c(n x 2) (n = 4, 6) at room temperature. The in situ oxidation of these surfaces was followed by Auger electron spectroscopy and by X-ray photoelectron spectroscopy. It is found that, in similar ultrahigh vacuum conditions, much faster contamination (about 500 times) occurs when the samples are investigated by AES than by XPS, owing to the interaction of the electron beam with the sample surface. With the sample subject to the AES investigation, the contamination occurs by forming >Si2C=O complexes based on the Si dimers. During XPS, reaction with water molecules from the residual gas should also be taken into account.
34. Enhanced contamination of Si(001) when analyzed by AES with respect to XPS
Authors:
Gheorghe, NG; Lungu, GA; Costescu, RM; Popescu, DG; Teodorescu, CM
Published: MAY 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, DOI:
Clean Si(001) single crystal surfaces are obtained by cycles of long (30 mins.) annealings in ultrahigh vacuum (fairly below 1 x 10(-9) mbar). The surface reconstruction is investigated by low energy electron diffraction (LEED). This paper reports, in addition to the well-known p(2 x 1) reconstruction, the first observation of c(4 x 2) at room temperature and also the completely new c(6 x 2) reconstruction. The in situ oxidation of these surfaces was investigated by Auger electron spectroscopy (AES) and by X-ray photoelectron spectroscopy (XPS). It is found that, in similar ultrahigh vacuum conditions (6 x 10(-10) mbar), much faster contamination (about 500 times) occurs when the samples are investigated by AES than by XPS, owing mainly to the interaction of the electron beam with the sample surface. Also, much gentler surface bombardment with electrons, such as in LEED, still enhances sample oxidation. Therefore, XPS proves to be a much more convenient technique for non-destructive assessment of the surface composition. When the surface is subjected to the AES investigation, we found that the contamination occurs by forming >Si2C=O complexes based on the Si dimers.
35. Successful Cleaning and Study of Contamination of Si(001) in Ultrahigh Vacuum
Authors:
Gheorghe, NG; Lungu, GA; Husanu, MA; Costescu, RM
Published: 2011, PHYSICS CONFERENCE (TIM-10), 1387, DOI: 10.1063/1.3647078
This paper presents the very first surface physics experiment performed in ultrahigh vacuum (UHV) in Romania, using a new molecular beam epitaxy (MBE) installation. Cleaning of a Si(001) wafer was achieved by using a very simple technique: sequences of annealing at 900-1000 degrees C in ultrahigh vacuum: low 10(-8) mbar, with a base pressure of 1.5 x 10(-10) mbar. The preparation procedure is quite reproducible and allows repeated cleaning of the Si(001) after contamination in ultrahigh vacuum. The Si(001) single crystal surface is characterized by low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), and Auger electron spectroscopy (AES). The latter technique is utilized in order to investigate the sample contamination by the residual gas in the UHV chamber, as determined by a residual gas analyzer (RGA). Unambiguous assignment of oxidized and unoxidized silicon is provided; also, an important feature is that the LVV Auger peak at 90-92 eV cannot be solely attributed to clean Si (i.e. Si surrounded only by Si), but also to silicon atoms bounded with carbon. Even with a sum of partial pressures of oxygen and carbon containing molecules in the range of 5 x 10(-10) mbar, the sample is contaminated very quickly, having a (1/e) lifetime of about 76 minutes.
36. Fe/Si(001) Ferromagnetic Layers: Reactivity, Local Atomic Structure and Magnetism
Authors:
Lungu, GA; Costescu, RM; Anu, MAH; Gheorghe, NG
Published: 2011, PHYSICS CONFERENCE (TIM-10), 1387, DOI: 10.1063/1.3647075
Ultrathin ferromagnetic Fe layers on Si(001) have recently been synthesized using the molecular beam epitaxy (MBE) technique, and their structural and magnetic properties, as well as their interface reactivity have been investigated. The study was undertaken as function of the amount of Fe deposited and of substrate temperature. The interface reactivity was characterized by Auger electron spectroscopy (AES). The surface structure was characterized by low-energy electron diffraction (LEED). The magnetism was investigated by magneto-optical Kerr effect (MOKE). A higher deposition temperature stabilizes a better surface ordering, but it also enhances Fe and Si interdiffusion and it therefore decreases the magnetism. Despite the rapid disappearance of the long range order with Fe deposition at room temperature, the material exhibits a significant uniaxial in-plane magnetic anisotropy. For the Fe deposition performed at high temperature (500 degrees C), a weak ferromagnetism is still observed, with saturation magnetization of about 10 % of the value obtained previously. MOKE studies allowed inferring the main properties of the distinct formed layers.
37. Thermodynamic destabilization of Li-N-H system by Si addition
Authors:
Palade, P; Lungu, GA; Husanu, AM
Published: AUG 27 2010, JOURNAL OF ALLOYS AND COMPOUNDS, 505, DOI: 10.1016/j.jallcom.2010.03.249
Li-N-H and Li-Si-N-H composites have been synthesized by ball milling starting from lithium amide, lithium hydride and silicon powders. Hydrogen sorption behaviour was investigated for the Si-containing composites and a Li-N-H reference sample. The as-milled Si-containing sample showed the presence of the constituent powders but after few absorption-desorption cycles an important amount of Li(2)SiN(2) appeared in both de-hydrogenated and re-hydrogenated composites. This phase was present in higher amount in the de-hydrogenated sample than in the re-hydrogenated one. The presence of this phase was also confirmed by XPS measurements. Li-N-H and Li-Si-N-H composites in de-hydrogenated state also contained lithium imide. X-ray diffraction data indicated a reversible hydrogen generating reaction between lithium amide, lithium hydride and silicon to produce Li(2)SiN(2). lithium imide and hydrogen. The plateau pressure of Li-N-Si-H system is twice the one for Li-N-H at the same temperature (about 265 degrees C), as shown by pressure-composition isotherms in desorption mode. For the first time it has been observed a thermodynamic destabilization of Li-N-H by mixing it with Si. (C) 2010 Elsevier B.V. All rights reserved.
38. PREPARATION AND CHARACTERIZATION OF IRON OXIDES EMBEDDED IN FULLERITE MATRICES
Authors:
Lungu, GA; Macovei, D; Teodorescu, CM
Published: MAR 2010, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 5, DOI:
Nanoparticles of iron oxide embedded in C(60) matrices are synthesized by co-evaporation of iron and C(60)-fullerene, and analyzed using X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy and MOKE magnetometry. C(60) molecules in the neighborhood of the iron oxide nanoclusters transfer charge to the oxide, and the charge deficit fully delocalizes onto the fullerene molecules. The composition of the nanoclusters of iron oxide is similar to both hematite and magnetite, in comparable proportions. One analyzes the amplitude of the pre-edge XANES peak, yielding the average ionization state of absorbing Fe atoms, which is confirmed by the simulations of XANES spectra.
39. Band ferromagnetism in systems of variable dimensionality II: the two-dimensional finite-temperature case
Authors:
Lungu, GA; Teodorescu, CM
Published: APR 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, DOI:
In a previous paper [C.M. Teodorescu and G.A. Lungu, J. Optoelectron. Adv, Mater. 10, 3058 (2008)] we studied the zero temperature problem of the occurence of band ferromagnetism and of the derivation of the Stoner criterion for systems of variable dimensionality: 1D, 2D, 3D. The dimensionality of the system is reflected by a different shape of the density of states. For ideal 2D systems, the density of states is a constant and this seems to be the simplest case to be modelled. In this paper we integrate to this simplest model of constant density of states the influence of temperature, in order to analyse temperature-dependent ferromagnetism in two-dimensional systems, such as magnetic surfaces. Some surprising results are obtained, namely: (i) in contrast to the common belief, in this case the influence of the temperature is to favour, not to inhibit ferromagnetism, i.e. in some conditions ferromagnetism may be obtained at finite temperature. even for systems where the zero temperature Stoner criterion is not satisfied; (ii) for a careful choice of the ratio between the Hubbard energy parameter U and the equilibrium zero-temperature Fermi level value epsilon(mu) (0), systems nonmagnetic at low temperature which become magnetic at higher temperature may be possible. A short review of the experimental data which may be interpreted within the present formalism is also given.
40. Alignment of the CMS silicon strip tracker during standalone commissioning
Authors:
Adam, W; Bergauer, T; Dragicevic, M; (...) Lungu, GA; (...) Tsang, KV (CMS Tracker Collaboration)
Published: JUL 2009, JOURNAL OF INSTRUMENTATION, 4, T07001, DOI: 10.1088/1748-0221/4/07/T07001
The results of the CMS tracker alignment analysis are presented using the data from cosmic tracks, optical survey information, and the laser alignment system at the Tracker Integration Facility at CERN. During several months of operation in the spring and summer of 2007, about five million cosmic track events were collected with a partially active CMS Tracker. This allowed us to perform first alignment of the active silicon modules with the cosmic tracks using three different statistical approaches; validate the survey and laser alignment system performance; and test the stability of Tracker structures under various stresses and temperatures ranging from +15 degrees C to -15 degrees C. Comparison with simulation shows that the achieved alignment precision in the barrel part of the tracker leads to residual distributions similar to those obtained with a random misalignment of 50 (80) mu m RMS in the outer (inner) part of the barrel.
41. Performance studies of the CMS Strip Tracker before installation
Authors:
Adam, W; Bergauer, T; Dragicevic, M; (...) Lungu, GA; (...) Tsang, KV (CMS Tracker Collaboration)
Published: JUN 2009, JOURNAL OF INSTRUMENTATION, 4, P06009, DOI: 10.1088/1748-0221/4/06/P06009
In March 2007 the assembly of the Silicon Strip Tracker was completed at the Tracker Integration Facility at CERN. Nearly 15% of the detector was instrumented using cables, fiber optics, power supplies, and electronics intended for the operation at the LHC. A local chiller was used to circulate the coolant for low temperature operation. In order to understand the efficiency and alignment of the strip tracker modules, a cosmic ray trigger was implemented. From March through July 4.5 million triggers were recorded. This period, referred to as the Sector Test, provided practical experience with the operation of the Tracker, especially safety, data acquisition, power, and cooling systems. This paper describes the performance of the strip system during the Sector Test, which consisted of five distinct periods defined by the coolant temperature. Significant emphasis is placed on comparisons between the data and results from Monte Carlo studies.
42. Stand-alone cosmic muon reconstruction before installation of the CMS silicon strip tracker
Authors:
Adam, W; Bergauer, T; Dragicevic, M; (...) Lungu, GA; (...) Tsang, KV (CMS Tracker Collaboration)
Published: MAY 2009, JOURNAL OF INSTRUMENTATION, 4, P05004, DOI: 10.1088/1748-0221/4/05/P05004
The subsystems of the CMS silicon strip tracker were integrated and commissioned at the Tracker Integration Facility (TIF) in the period from November 2006 to July 2007. As part of the commissioning, large samples of cosmic ray data were recorded under various running conditions in the absence of a magnetic field. Cosmic rays detected by scintillation counters were used to trigger the readout of up to 15% of the final silicon strip detector, and over 4.7 million events were recorded. This document describes the cosmic track reconstruction and presents results on the performance of track and hit reconstruction as from dedicated analyses.
43. Band ferromagnetism in systems of variable dimensionality
Authors:
Teodorescu, CM; Lungu, GA
Published: NOV 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, DOI:
The Stoner instability of the paramagnetic state, yielding to the occurence of ferromagnetism, is reviewed for electron density of states reflecting changes in the dimensionality of the system. The situations treated are one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) cases and also a special case where the density of states has a parabolic shape near the Fermi level, and is half-filled at equilibrium. We recover the basic results obtained in the original work of E.C. Stoner [Proc. Roy. Soc. London A 165, 372 (1938)]; also we demonstrate that in 1D and 2D case, whenever the Stoner criterion is satisfied, the system evolves spontaneously towards maximum polarization allowed by Hund's rules. For the 3D case, the situation is that: (i) when the Stoner criterion is satisfied, but the ratio between the Hubbard repulsion energy U and the Fermi energy epsilon(F) is between 4/3 and 3/2, the system evolves towards a ferromagnetic state with incomplete polarization (the polarization parameter is between 0 and 1); (ii) when U/epsilon(F), > 3/2, the system evolves towards maximum polarization. This situation was also recognized in the original paper of Stoner, but with no further analysis of the obtained polarizations nor comparison with experimental results. We apply the result of calculation in order to predict the Hubbard interaction energy. Finally, for the case of half-filled parabolic density of states a result quite similar with the case of the free electron 3D DOS is obtained, but here the Hubbard energy has to be compared with the band half-widh 6, whereas the absolute value of the Fermi energy has no relevance in this model. The Stoner criterion is U/delta > 4/3 and for U/delta < 2 the total energy is minimized for incomplete polarization. We compare the results obtained from the present theoretical study with the newest available experimental data.
44. The CMS experiment at the CERN LHC
Authors:
Chatrchyan, S; Hmayakyan, G; Khachatryan, V; (...) Lungu, GA; (...) Onengut, G (CMS Collaboration)
Published: AUG 2008, JOURNAL OF INSTRUMENTATION, 3, S08004, DOI: 10.1088/1748-0221/3/08/S08004
The Compact Muon Solenoid (CMS) detector is described. The detector operates at the Large Hadron Collider (LHC) at CERN. It was conceived to study proton-proton (and lead-lead) collisions at a centre-of-mass energy of 14 TeV (5.5 TeV nucleon-nucleon) and at luminosities up to 10(34)cm(-2)s(-1) (10(27)cm(-2)s(-1)). At the core of the CMS detector sits a high-magnetic-field and large-bore superconducting solenoid surrounding an all-silicon pixel and strip tracker, a lead-tungstate scintillating-crystals electromagnetic calorimeter, and a brass-scintillator sampling hadron calorimeter. The iron yoke of the flux-return is instrumented with four stations of muon detectors covering most of the 4 pi solid angle. Forward sampling calorimeters extend the pseudo-rapidity coverage to high values (vertical bar eta vertical bar <= 5) assuring very good hermeticity. The overall dimensions of the CMS detector are a length of 21.6 m, a diameter of 14.6 m and a total weight of 12500 t.
45. CMS physics technical design report: Addendum on high density QCD with heavy ions
Authors:
Chatrchyan, S; Hmayakyan, G; Sirunyan, AM; (...) Lungu, GA; (...) Zabi, A (CMS Collaboration)
Published: NOV 2007, JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, 34, 2307-2455, DOI: 10.1088/0954-3899/34/11/008
This report presents the capabilities of the CMS experiment to explore the rich heavy-ion physics programme offered by the CERN Large Hadron Collider (LHC). The collisions of lead nuclei at energies root(NN)-N-S = 5(5 TeV, will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction - Quantum Chromodynamics (QCD) - in extreme conditions of temperature, density and parton momentum fraction (low-x). This report covers in detail the potential of CMS to carry out a series of representative Pb-Pb measurements. These include \"bulk\" observables, (charged hadron multiplicity, low p(T) inclusive hadron identified spectra and elliptic flow) which provide information on the collective properties of the system, as well as perturbative probes such as quarkonia, heavy-quarks, jets and high p(T) hadrons which yield \"tomographic\" information of the hottest and densest phases of the reaction.
46. CMS physics technical design report, volume II: Physics performance
Authors:
Bayatian, GL; Chatrchyan, S; Hmayakyan, G; (...) Lungu, GA; (...) Yuldashev, BS (CMS Collaboration)
Published: JUN 2007, JOURNAL OF PHYSICS G-NUCLEAR AND PARTICLE PHYSICS, 34, 995-1579, DOI: 10.1088/0954-3899/34/6/S01
CMS is a general purpose experiment, designed to study the physics of pp collisions at 14 TeV at the Large Hadron Collider ( LHC). It currently involves more than 2000 physicists from more than 150 institutes and 37 countries. The LHC will provide extraordinary opportunities for particle physics based on its unprecedented collision energy and luminosity when it begins operation in 2007. The principal aim of this report is to present the strategy of CMS to explore the rich physics programme offered by the LHC. This volume demonstrates the physics capability of the CMS experiment. The prime goals of CMS are to explore physics at the TeV scale and to study the mechanism of electroweak symmetry breaking - through the discovery of the Higgs particle or otherwise. To carry out this task, CMS must be prepared to search for new particles, such as the Higgs boson or supersymmetric partners of the Standard Model particles, from the start- up of the LHC since new physics at the TeV scale may manifest itself with modest data samples of the order of a few fb(-1) or less. The analysis tools that have been developed are applied to study in great detail and with all the methodology of performing an analysis on CMS data specific benchmark processes upon which to gauge the performance of CMS. These processes cover several Higgs boson decay channels, the production and decay of new particles such as Z\' and supersymmetric particles, B-s production and processes in heavy ion collisions. The simulation of these benchmark processes includes subtle effects such as possible detector miscalibration and misalignment. Besides these benchmark processes, the physics reach of CMS is studied for a large number of signatures arising in the Standard Model and also in theories beyond the Standard Model for integrated luminosities ranging from 1 fb(-1) to 30 fb(-1). The Standard Model processes include QCD, B-physics, diffraction, detailed studies of the top quark properties, and electroweak physics topics such as the W and Z(0) boson properties. The production and decay of the Higgs particle is studied for many observable decays, and the precision with which the Higgs boson properties can be derived is determined. About ten different supersymmetry benchmark points are analysed using full simulation. The CMS discovery reach is evaluated in the SUSY parameter space covering a large variety of decay signatures. Furthermore, the discovery reach for a plethora of alternative models for new physics is explored, notably extra dimensions, new vector boson high mass states, little Higgs models, technicolour and others. Methods to discriminate between models have been investigated. This report is organized as follows. Chapter 1, the Introduction, describes the context of this document. Chapters 2-6 describe examples of full analyses, with photons, electrons, muons, jets, missing E-T, B-mesons and tau\'s, and for quarkonia in heavy ion collisions. Chapters 7-15 describe the physics reach for Standard Model processes, Higgs discovery and searches for new physics beyond the Standard Model.
47. Structural and magnetic investigations of nickel clusters in C-60 matrices
Authors:
Teodorescu, CM; Macovei, D; Lungu, A
Published: DEC 2004, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 6, 1285, DOI:
Nanostructured Ni/C-60 thin films, produced by co-evaporation of Ni and fullerene, have been investigated by both X-ray diffraction (XRD) and X-ray magnetic circular dichroism (XMCD). X-ray diffraction has shown the co-existence of diffraction specific peaks for the fullerene face-centered cubic crystal structure and diffraction peaks of Ni nanoparticles in the face-centered cubic structure, which are broadened by nanoparticule finite dimensions. Analysis of peak widths shows that the Ni nanocrystallite size ranges from 4.7 to 19.2 nm. The XMCD signals for a sample series, preparated on relatively low-temperature substrates during the co-evaporation, can be interpreted as having distinct contributions due to both bulk and surface Ni atoms. These two kind of atoms are antiferromagnetic coupled. The Ni atoms which are at the interface with the fullerite matrix show ail enhanced orbital magnetic moment, while the orbital magnetic moment of Ni atoms in the bulk has very low values, as it can be expected for a crystalline field with a cubic symmetry.
Copyright © 2024 National Institute of Materials Physics. All Rights Reserved