NIMP| Teodorescu Cristian Mihail, Author at NIMP

Education

1984: Baccalaureate, “Gheorghe Lazăr” High-school Bucharest, 10/10.

1990: M. Sc. (Engineer), University of Bucharest (Romania), Department of Applied Physics, 9.87/10.

Dissertation title: Geometric effects and measurement methods of galvanomagnetic effects on circular samples, advisor Dr. Bogdan Logofătu, presented also at the CAS’90 International Semiconductor Conference, Sinaia, 1990.

1995: Ph. D, Université Paris Sud Orsay (France), Chemical Physics, “très honorable”.

Thesis title: Study of evolution of inner-shell excitations from free atmoms to solids. X-ray photoabsorption of metal and insulating clusters, advisor Dr. Jean-Marc Esteva, 6 papers resulting (J. Phys. B 1993 and 1997, Nucl. Instrum. Meth. Phys. Res. A 1994, J. Electr. Spectrosc. Relat. Phenom. 1999 and 2000, Phys. Rev. B 2001, main author for all these papers)

2015: Habilitation in Physics, University of Bucharest, Faculty of Physics.

Habilitation thesis title: Ferromagnetic and ferroelectric surfaces and interfaces.

Positions

1990–1991: Physicist, Institute of Physics and Technology of Materials, Bucharest-Magurele PO Box MG7, 76900, Romania.

1991–1995: Ph.D student, Laboratoire de Spectroscopie Atomique et Ionique, Bât. 350, Université Paris Sud, 91405 Orsay, France.

1995–1996: post-doctoral scientist, LURE, Bât. 209d, Centre Universitaire Paris Sud, BP 34, 91898 Orsay, France.

1996–1998: research scientist, Universität Osnabrück, Barbarastr. 7, 49069 Osnabrück, Germany.

1999–2001: beamline scientist, LURE, Bât. 209d, Centre Universitaire Paris Sud, BP 34, 91898 Orsay, France.

2001–2005: senior scientist III, National Institute of Material Physics (NIMP), Atomiștilor 405A, 077125 Măgurele–Ilfov, Romania.

2001–2002 : postdoctoral fellowship, Daresbury Laboratory, Warrington Cheshire, WA4 4AD, U.K.

2005–2009: senior scientist II, NIMP.

2009–to date: senior scientist I, NIMP.

2010–2011: Department head, Nanoscale Condensed Matter Physics, NIMP Bucharest (35–40 people).

2011– to date: Group leader, Surfaces and Interfaces, NIMP Bucharest (29 people).

2015– to date: Professor, PhD Advisor, University of Bucharest, Faculty of Physics, Atomiștilor 405, 077125 Măgurele–Ilfov, Romania (8 PhD students, of which 3 graduated).

2018 – to date: President of the Scientific Council of NIMP.

Awards

• Third prize, International Physics Olympiad, Sigtuna, Sweden, 1984.

• “Radu Grigorovici” Prize of the Romanian Academy, awarded in 2015 for results obtained in 2013.

Expertize

Keywords : synchrotron radiation, surface science, clusters and nanoparticles, reactivity, structure, ferroelectrics, magnetism, photocatalysis, catalysis, graphene.

Techniques:

• Photoelectron spectroscopy (PES): X-ray photoelectron spectroscopy (XPS) or electronic structure for chemical analysis (ESCA), core-level and valence-band PES, photoelectron diffraction (PED), Fermi surface mapping, angle-resolved PES, angle-resolved ultraviolet photoelectron spectroscopy (ARUPS), spin-resolved PES.

• Surface science techniques: low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy, secondary ion mass spectroscopy (SIMS), sample preparations in UHV, molecular beam epitaxy (MBE), positron annihilation-induced Auger electron spectroscopy (PAES).

• Photoelectron spectromicroscopy, low energy electron microscopy (LEEM), photoelectron microscopy (PEEM), NanoESCA.

• X-ray absorption: extended X-ray absrption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), near-edge absorption fine structure (NEXAFS), X-ray absorption spectroscopy (XAS), X-ray fluorescence.

• Scanning tunneling microscopy / spectroscopy (STM/STS).

• Coincidence spectroscopy: photoelectron-photoion-photoion coincidences (PEPIPICO), zero electron kinetic energy (ZEKE) spectroscopy, threshold electron spectroscopy.

• Magnetism: magneto-optical Kerr effect (MOKE), vibrating sample magnetometry (VSM), superconducting quantum interference device (SQUID), X-ray magnetic (circular, linear) dichroism (XMCD, XMLD).

• X-ray diffraction.

• Development of various devices apparatuses for UHV, surface science, low temperatures, cluster production and analysis.

• Leader of R&D projects of about 7.5 M€ (2004–2019), including a PCCE project (2010–2013) of about 1.6 M€ and a PCCDI project (2018–2020) of about 1.1 M€.

• Director of an infrastructure project (POS-CCE SMIS 2665, 2009–2011) of about 10.5 M€. Setup of four surface science laboratories. Deputee Director of a second infrastructure project (2014–2015) of similar worth.

• Author and co-author of 147 papers in ISI journals, about 20 papers published in journals with high impact factor (over 5), over 2700 citations, over 1800 citations without self-citations, h-index 27 (Web of Science) or 32 (Google Scholar), 7 book chapters.

• Editorial Boards: Open Physics (De Gruyter), Physics (MDPI), Journal of Optoelectronics and Advanced Materials, Optoelectronics and Advanced Materials – Rapid Communications, Digest Journal of Nanomaterials and Biostructures.

• 1 national (RO) patent awarded, 6 patent applications in various stages of evaluation.

View Full CV

Projects

1. Technologic paradigms in synthesis and characterization of variable dimensionality systems

Project Type: PCCDI, Start Date: 2018-01-03 End Date: 2021-04-30

Publications

1. Degenerated TiO(2)Semiconductor Modified with Ni and Zn as Efficient Photocatalysts for the Water Splitting Reaction

Authors: Neatu, F; Abramiuc, LE; Trandafir, MM; Negrea, RF; Florea, M; Teodorescu, CM; Neatu, S

Published: SEP 17 2020, DOI: 10.1002/cctc.202000691

The development of robust, safe, cost-effective and efficient photocatalytic systems for water splitting should take into account the presence of a proper and powerful photon absorber and an efficient, low-cost and earth-abundant electrocatalyst to perform the reaction at high conversions. In this study, Ni-Zn/TiO(2)ternary composites with high photocatalytic activity for water splitting under UV irradiation were successfully prepared via a simple and low-cost deposition-precipitation route. Thus, different Ni : Zn molar ratios (1 : 0, 1 : 1, 3 : 1, 6 : 1, 9 : 1, and 0 : 1) were deposited on TiO(2)in order to reach a total metal loading of 50 wt. %. The obtained composites were characterized using several techniques, such as: X-ray diffraction, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The most active synthesized composite, namely Ni-Zn/TiO2(9 : 1), exhibits H(2)generation rate above 17 mmols g(-1) h(-1), which is nearly one thousand times higher than that obtained with TiO(2)Evonik P25. Our study demonstrates that TiO(2)becomes a degenerated semiconductor in the presence of Ni and ZnO, with remarkable photocatalytic properties. Thus, the obtained results can open new opportunities in the preparation of very active materials for hydrogen production based on the optimization of three-component structures.

2. The interplay of work function and polarization state at the Schottky barriers height for Cu/BaTiO3 interface

Authors: Popescu, DG; Husanu, MA; Chirila, C; Pintilie, L; Teodorescu, CM

Published: FEB 1 2020, DOI: 10.1016/j.apsusc.2019.144101

The emerging field of electronics based on ferro-functional materials relies on driving effectively and predictably a ferroelectric system between different polarization states through bias applied to metallic contacts. This requires detailed understanding of the growth mechanisms and electronic properties of the interface, including ferroelectric and material - dependent band alignment and Schottky barrier heights. Whether the major contribution at the interface band alignment comes from the work function difference or from the ferroelectric state is still under debate. Here, using X-ray photoemsion and ab-initio calculations, we derive the complex microscopic picture of metal/ferroelectric interface formation, including growth mechanism, valence alteration, ferroelectric-dependent electrostatic potential and thickness - dependent compensation mechanisms of ferroelectricity, starting from the ultrathin growth of Cu up to 100 angstrom on BaTiO3. One establishes the evolution of the band bending and of the build-in potential from the initial probed thickness of the ferroelectric in the range of 3 lambda (lambda - the inelastic mean free path) while gradually approaching the contact region with the metal at higher thickness of the top layer. We find that the well-defined orientation of the ferroelectric polarization lead to a band bending at the interface, which add at the bending expected from the work function difference of the two joining materials.

3. 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, 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.

4. Impact on Ferroelectricity and Band Alignment of Gradually Grown Au on BaTiO3

Authors: Popescu, DG; Husanu, MA; Chirila, C; Pintilie, L; Teodorescu, CM

Published: JUL 2019, DOI: 10.1002/pssr.201900077

The competition between interface barrier in the Schottky-Mott limit and polarization driven mechanism is established during gradual formation of metal (Au) - ferroelectric (BaTiO3) interface. X-ray photoelectron spectroscopy provides core level energies and valence band positions in the contact region, to monitor the band alignment from the very first stages of metal deposition on BaTiO3. The band bending at metal/ferroelectric (FE) interface is extracted from the shift of core levels (Ba 3d, Ti 2p) as a function of the metal thickness. It is shown that the interface band alignment mechanism involves a well-defined polarization orientation washing out the bending expected from the work function difference. The sudden modification of the binding energies within ferroelectric at the first 2 angstrom Au indicates that the ferroelectric compensation mechanism triggered by the metal overlayer initiates already at ultrathin top layer, while subsequent growth contributes only at a gradual correction of the potential in the FE. The emerging picture is confirmed in first-principle calculation indicating the preferences of Au to grow preferentially to different terminated regions and to stabilize distinct ferroelectric polarization.

5. Growth of Ag(111) on Si(111) with nearly flat band and abrupt interface

Authors: Bocirnea, AE; Costescu, RM; Apostol, NG; Teodorescu, CM

Published: APR 15 2019, DOI: 10.1016/j.apsusc.2018.12.167

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 x 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 x 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 x 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.

6. Spirobifluorene-based Porous Organic Polymers as Efficient Porous Supports for Pd and Pt for Selective Hydrogenation

Authors: Trandafir, MM; Pop, L; Hadade, ND; Hristea, I; Teodorescu, CM; Krumeich, F; van Bokhoven, JA; Grosu, I; Parvulescu, VI

Published: JAN 9 2019, DOI: 10.1002/cctc.201801247

Spirobifluorene-based porous organic polymers (POP) were synthesized following two different protocols; the acetylenic coupling reaction conditions and the Sonogashira cross-coupling reaction. These were utilized as support for the hydrogenation of a series of species containing unsaturated C=C and C=O bonds (4-nitrostyrene, 4-bromobenzophenone, acetophenone, 7-nitro-1-tetralone and 1,2-naphtoquinone confirmed their efficiency). POP1 prepared via a copper-catalysis protocol was completely inactive, while POP2-4 containing residual Pd exhibited different activities in accordance to the accessibility of the substrates to the metal. Further deposition of 0.5wt% Pd led to active and stable catalysts. They were easily separated by filtration, and after re-dispersion, afforded the same performances for ten successive cycles. This study also evidenced the specific role of the support in these reactions by comparing the behavior of Pd/POP with that of a Pd/C catalyst with the same loading of palladium. The deposition of Pt on these supports led to sub-nanometric particles and, in accordance, to a different catalytic behavior reflected merely by differences in the selectivity.

7. 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, 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.

8. Polarization Orientation in Lead Zirconate Titanate (001) Thin Films Driven by the Interface with the Substrate

Authors: Tanase, LC; Abramiuc, LE; Popescu, DG; Trandafir, AM; Apostol, NG; Bucur, IC; Hrib, L; Pintilie, L; Pasuk, I; Trupina, L; Teodorescu, CM

Published: SEP 11 2018, DOI: 10.1103/PhysRevApplied.10.034020

We investigate the effect of the nature of the substrate and the bottom interface on the out-of-plane polarization orientation of ultrathin (10-nm) lead zirconate titanate (PZT) thin films of (001) orientation by photoelectron spectroscopy of samples without surface contamination. The substrate nature is varied between insulator (strontium titanate, STO) and semiconductor (Nb-doped STO, STON) and finally to a metal with a work function lower than that of PZT (strontium ruthenate, SRO). Outward polarization is obtained for PZT/STON(001) and inward polarization is obtained for PZT/STO(001) and PZT/SRO(001). Explanations are given for all these typical cases, the main elements being charge accumulation for compensation of the depolarization field, self-doping of PZT films, and the interface electric field driving the orientation of the polarization of the ferroelectric films. We find p-type self-doping is correlated with the inward polarization, and the driving field is formed between a negatively charged region with negatively ionized acceptors near the interface with the substrate and the p-type degenerate region with holes accumulated inside, toward the surface. This mechanism may be reversed under the assumption of n-type self-doping, positively ionized donors near the interface, and accumulated electrons toward the surface in the case of an interface with a substrate with a higher work function, being in line with recent data (PZT/Pt or BaTiO3/SRO).

9. 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, 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.

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, 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. Photoelectron Spectroscopic and Microspectroscopic Probes of Ferroelectrics

Authors: Tanase, LC; Abramiuc, LE; Teodorescu, CM

Published: 2017, DOI: 10.1063/1.5017434

This contribution is a review of recent aspects connected with photoelectron spectroscopy of free ferroelectric surfaces, metals interfaced with these surfaces, graphene-like layers together with some exemplifications concerning molecular adsorption, dissociations and desorptions occurring from ferroelectrics. Standard photoelectron spectroscopy is used nowadays in correlation with other characterization techniques, such as piezoresponse force microscopy, high resolution transmission electron spectroscopy, and ferroelectric hysteresis cycles. In this work we will concentrate mainly on photoelectron spectroscopy and spectro-microscopy characterization of ferroelectric thin films, starting from atomically clean ferroelectric surfaces of lead zirco-titanate, then going towards heterostructures using this material in combination with graphene-like carbon layers or with metals. Concepts involving charge accumulation and depolarization near surface will be revisited by taking into account the newest findings in this area.

12. 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, 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.

13. Band bending at magnetic Ni/Ge(001) interface investigated by X-ray photoelectron spectroscopy

Authors: Bocirnea, AE; Tanase, LC; Costescu, RM; Apostol, NG; Teodorescu, CM

Published: DEC 1 2017, DOI: 10.1016/j.apsusc.2017.04.168

We report the molecular beam epitaxy growth of Ni on a clean Ge(001) surface with an intermediate NiGe layer forming at the interface at room temperature. The crystallinity of the substrate is lost after the deposition of more than 2 Ni monolayers. The Schottky barrier formation is investigated by X-ray photoelectron spectroscopy. The method allows us to infer a 0.39-0.45 eV band bending at the interface between the compound and Ge(001). Magneto-optical Kerr effect measurements were conclusive in detecting the ferromagnetic ordering of Ni outermost layers. (C) 2017 Elsevier B.V. All rights reserved.

14. Aging phenomena and wettability control of plasma deposited carbon nanowall layers

Authors: Vizireanu, S; Ionita, MD; Ionita, ER; Stoica, SD; Teodorescu, CM; Husanu, MA; Apostol, NG; Baibarac, M; Panaitescu, D; Dinescu, G

Published: NOV 2017, DOI: 10.1002/ppap.201700023

This paper reports on the aging of carbon nanowalls (CNW) and modification of their wettability by the storage time, growth conditions, and post-fabrication plasma treatments. The as-deposited CNW initially exhibit marked hydrophilic behavior (fresh CNW), but within a few days they become highly hydrophobic (aged CNW). Their final hydrophobicity is closely related to their topography which is controlled by the deposition parameters. In addition, subsequent fluorinated plasma treatments result in super-hydrophobic CNW layers, irrespective of the hydrophilic or hydrophobic character of the pre-treated samples. To explain this, we show that the CNW edges contain many defects initially, but such defects become passivated in time. As a result, the surfaces become highly hydrophobic after aging or fluorination, having inert stable terminations.

15. 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, 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.

16. Oxygenophilic ionic liquids promote the oxygen reduction reaction in Pt-free carbon electrocatalysts

Authors: Qiao, M; Tang, C; Tanase, LC; Teodorescu, CM; Chen, CM; Zhang, Q; Titirici, MM

Published: SEP 1 2017, DOI: 10.1039/c7mh00298j

We propose a novel idea to improve the surface properties of carbon-based Pt-free electrocatalysts in Polymer Electrolyte Membranes (PEMs) and Alkaline Fuel Cells (AFCs). Our concept is based on the addition of oxygenophilic and hydrophobic ionic liquids (ILs) to form a thin passivating layer at the triple point between the electrocatalyst-electrolyte-gas interface where the Oxygen Reduction Reaction (ORR) takes place.

17. Polarization landscape effects in soft X-ray-induced surface chemical decomposition of lead zirco-titanate, evidenced by photoelectron spectromicroscopy

Authors: Abramiuc, LE; Tanase, LC; Barinov, A; Apostol, NG; Chirila, C; Trupina, L; Pintilie, L; Teodorescu, CM

Published: AUG 21 2017, DOI: 10.1039/c7nr03003g

The stability of thin films of lead zirco-titanate (PZT) under intense soft X-ray beams is investigated by time-resolved photoelectron spectromicroscopy with a lateral resolution below 1 micrometer. Surface dissociation is observed when samples are irradiated with intense (5 x 10(23) photons per s per m(2)) soft X-rays, with promotion of reduced lead on the surface. On areas exhibiting outwards polarization (P(+)), the reduced lead is formed at the expense of P(+)-PZT. On areas presenting co-existing P(+) states with areas without out-of-plane polarization (P-(0)), the reduced lead is formed at the expense of the P-(0)-PZT component, while the P(+)-PZT remains constant. The main dissociation mechanism was found to be triggered by 'hot' electrons in the conduction band, with energies exceeding the surface dissociation energies. Dissociation occurs basically when the electron affinity is larger than the dissociation energy of PbO (for P(+) areas) or PbO- (for P-(0) areas). Such mechanisms may be adapted for dissociation of other molecules on surfaces of ferroelectric thin films or for quantifying the stability of ferroelectric surfaces interacting with other radiation, with applications in the fields of photocatalysis or photovoltaic devices.

18. Sustainable metal-free carbogels as oxygen reduction electrocatalysts

Authors: Preuss, K; Tanase, LC; Teodorescu, CM; Abrahams, I; Titirici, MM

Published: AUG 21 2017, DOI: 10.1039/c7ta02001e

Tuneable nitrogen doped carbogels have been synthesised by a simple one-pot hydrothermal carbonisation, followed by pyrolysis at 1000 degrees C, using highly available and low cost precursors such as glucose and ovalbumin. Different physical activation ratios of nitrogen/oxygen were used to demonstrate a sustainable and easy method for changing surface area, pore size and elemental composition in order to investigate their effect on the oxygen reduction reaction when used as electrocatalysts. A ratio of nitrogen mixed with 2% of oxygen was found to be most beneficial for enhancing the catalytic activity by creating a high surface area of 874 m(2) g(-1) as well as a favourable ratio of pyridinic to graphitic nitrogen. The influence of sulphur doping and/or boron on the carbogel structure was investigated. Incorporation of sulphur does not interfere with the structure formation, but decreases the surface area and nitrogen content resulting in diminished ORR performance. However, boron doping with boric acid results in a different carbogel structure by acting as a catalyst, creating an altered morphology, surface area, pore properties and higher nitrogen content by fully utilising ovalbumin as a nitrogen source instead of as a structure directing/surface stabilising agent. Nitrogen content is found to determine the limiting current, while the oxygen content has a small influence on the onset potential. An assumed synergistic effect between nitrogen and boron generates higher electron transfer numbers and lower hydrogen peroxide yields in boron nitrogen co-doped carbogels than those observed in purely nitrogen doped systems.

19. The combined action of methanolysis and heterogeneous photocatalysis in the decomposition of chemical warfare agents

Authors: Petrea, N; Petre, R; Epure, G; Somoghi, V; Tanase, LC; Teodorescu, CM; Neatu, S

Published: 2016, DOI: 10.1039/c6cc07551g

We report the applicability of a hybrid system comprising a La3+-based catalyst and an Au/TiO2 photocatalyst in the decomposition of chemical weapons. This system is able to perform complete degradation of soman, sarin and VX in less than 1 minute under low basic conditions and visible light irradiation.

20. Local Ordering at the Interface of the TiO2-WO3 Bi-Layers

Authors: Macovei, D; Dascaleanu, V; Teodorescu, CM; Luca, D

Published: 2016, DOI: 10.1007/978-3-319-30198-3_10

Reducing the recombination rate of photo-generated electron-hole pairs in the surface of pristine TiO2 materials can be done, among others, via local spatial charge separation in the photocatalytically active surface region. A local electric field acting for this purpose can be ensured, for instance, in hetero-junction regions formed at the interface between a TiO2 film and a WO3 ultra-thin layer. The option for the WO3-TiO2 semiconductor pair is related to the suitable fit of their band structures. To model the interface, a more in-depth knowledge of local atomic environment is required. Here, we discuss the local atomic ordering and the related effects in the interface region of TiO2/WO3 and WO3/TiO2 structures grown on Si (100) substrates. Materials characterization was done by using XRD, XPS and XAS techniques. We demonstrate that tungsten atoms enter as W6+ or W4+ cations into the rutile-type TiO2 lattice, by substituting the Ti4+ cations. While W6+-Ti4+ substitution leaves the surrounding rutile matrix unchanged, the W4+-Ti4+ substitution induces a local rutile-to-anatase transition. The current results are relevant in designing new applications structures with enhanced photocatalytic performances.

21. An adamantane-based COF: stability, adsorption capability, and behaviour as a catalyst and support for Pd and Au for the hydrogenation of nitrostyrene

Authors: Trandafir, MM; Pop, L; Hadade, ND; Florea, M; Neatu, F; Teodorescu, CM; Duraki, B; van Bokhoven, JA; Grosu, I; Parvulescu, VI; Garcia, H

Published: 2016, DOI: 10.1039/c6cy01631f

A bimodal (micro/mesoporous) COF was synthesized by coupling tetrakis-1,3,5,7-(4'-iodophenyl) adamantane with 4,4'-diethynylbiphenyl following a Sonogashira protocol. The COF preparation strategy led, however, to the incomplete recovery of the palladium catalyst and ICP-OES analysis indicated that around 0.1 wt% palladium remained inside the pores. Noteworthily, the remnant palladium catalyst is still accessible and can be valorised in additional catalytic reactions like the hydrogenation of nitrostyrene. Further deposition of 0.5 wt% active metals (like palladium or gold) enhanced the catalytic activity and tuned the catalyst selectivity with respect to analogous metal catalysts prepared using active carbon as a support. The resulting COF-supported metal NPs are stable and recyclable catalysts. Under normal conditions, this COF is also able to adsorb large amounts of weak electrophilic gases like carbon dioxide.

22. 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, 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.

23. 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, 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.

24. Ferroelectric triggering of carbon monoxide adsorption on lead zirco-titanate (001) surfaces

Authors: Tanase, LC; Apostol, NG; Abramiuc, LE; Tache, CA; Hrib, L; Trupina, L; Pintilie, L; Teodorescu, CM

Published: OCT 14 2016, DOI: 10.1038/srep35301

Atomically clean lead zirco-titanate PbZr0.2Ti0.8O3 (001) layers exhibit a polarization oriented inwards P(-), visible by a band bending of all core levels towards lower binding energies, whereas as introduced layers exhibit P(+) polarization under air or in ultrahigh vacuum. The magnitude of the inwards polarization decreases when the temperature is increased at 700 K. CO adsorption on P(-) polarized surfaces saturates at about one quarter of a monolayer of carbon, and occurs in both molecular (oxidized) and dissociated (reduced) states of carbon, with a large majority of reduced state. The sticking of CO on the surface in ultrahigh vacuum is found to be directly related to the P(-) polarization state of the surface. A simple electrostatic mechanism is proposed to explain these dissociation processes and the sticking of carbon on P(-) polarized areas. Carbon desorbs also when the surface is irradiated with soft X-rays. Carbon desorption when the polarization is lost proceeds most probably in form of CO2. Upon carbon desorption cycles, the ferroelectric surface is depleted in oxygen and at some point reverses its polarization, owing to electrons provided by oxygen vacancies which are able to screen the depolarization field produced by positive fixed charges at the surface.

25. Magnetic Nanoparticles for Hepatocellular Carcinoma Diagnosis and Therapy

Authors: Ungureanu, BS; Teodorescu, CM; Saftoiu, A

Published: SEP 2016, DOI: 10.15403/j.gld.2014.1121.253.nan

Hepatocellular carcinoma (HCC) is the most common primary tumor of the liver, ranking as the second most common cause of death from cancer worldwide. Magnetic nanoparticles (MNPs) have been used so far in tumor diagnosis and treatment, demonstrating great potential and promising results. In principle, three different approaches can be used in the treatment of tumors with superparamagnetic iron oxide nanoparticles: magnetically induced hyperthermia, drug targeting and selective suppression of tumor growth. This review focuses on the use of iron oxide nanoparticles for the diagnosis and treatment of liver cancer and offers a walkthrough from the MNPs imaging applicability to further therapeutic options, including their potential flaws. The MNP unique physical and biochemical properties will be mentioned in close relationship to their subsequent effects on the human body, and, also, their toxic potential will be noted. A presentation of what barriers the MNPs should overcome to be more successful will conclude this review.

26. An adamantane-based COF: stability, adsorption capability, and behaviour as a catalyst and support for Pd and Au for the hydrogenation of nitrostyrene (vol 6, pg 8344, 2016)

Authors: Trandafir, MM; Pop, L; Hadade, ND; Florea, M; Neatu, F; Teodorescu, CM; Duraki, B; van Bokhoven, JA; Grosu, I; Parvulescu, VI; Garcia, H

Published: 2016, DOI: 10.1039/c6cy90113a

27. Band bending at copper and gold interfaces with ferroelectric Pb(Zr,Ti)O-3 investigated by photoelectron spectroscopy

Authors: Apostol, NG; Stoflea, LE; Tanase, LC; Bucur, IC; Chirila, C; Negrea, RF; Teodorescu, CM

Published: NOV 1 2015, DOI: 10.1016/j.apsusc.2015.02.167

Interfaces formed by gold and copper on single crystal layers of (0 0 1) PbZr0.2Ti0.8O3 (PZT) produced by pulsed laser deposition and exhibiting outwards polarization are analyzed by X-ray photoelectron spectroscopy. The stoichiometry of the layers reproduces reasonably that of the PZT target. The band bending occurring at the interface between PZT and the metals is investigated by analyzing the core level shifts as function on the metal deposition. It is found that for Au/PZT(0 0 1) the gold layer is not continuous and the observed band bendings can be attributed to a Schottky mechanism, whereas for Cu/PZT(0 0 1) the copper layer is continuous; in this latter case, the observed band bendings towards higher energies (lower binding energies) can be attributed to a concomitant bending due to the Schottky effect together with the disappearance of the initial bending due to the outwards polarization of the samples. Metal Pb is observed to segregate only in the case of Cu/PZT(0 0 1), therefore the surface self-reduction might also be connected to the presence of a metal with lower work function, which for larger coverage forms a continuous metal layer, able to provide electrons to the surface. High resolution transmission electron spectroscopy yielded the disappearance of the tetragonal distortion in the case of Cu/PZT(0 0 1), in line with the assumption of disappearance of the polarization-induced band bending. (C) 2015 Elsevier B.V. All rights reserved.

28. Photoelectron spectroscopy and spectro-microscopy of Pb(Zr,Ti)O-3 (111) thin layers: Imaging ferroelectric domains with binding energy contrast

Authors: Husanu, MA; Popescu, DG; Tache, CA; Apostol, NG; Barinov, A; Lizzit, S; Lacovig, P; Teodorescu, CM

Published: OCT 15 2015, DOI: 10.1016/j.apsusc.2015.01.153

The ability of photoelectron spectro-microscopy with sub-micrometer lateral resolution to identify ferroelectric domains by analysis of surface band bendings is demonstrated on lead zirco-titanate PZT(1 1 1) thin films grown by pulsed laser deposition. Conventional synchrotron radiation X-ray photoelectron spectroscopy allowed one to derive the surface composition of the sample and evidenced shifts toward higher binding energy when the sample is subject to intense soft X-ray beam. A basic model is developed which supposes that photogenerated carriers reduce the depolarization field, yielding a lower torque applied to the ferroelectric polarization. As a consequence, the out-of-plane component of the polarization increases. Domain migration during irradiation with soft X-ray is inferred from the relative amplitude of the components with different binding energy. When the flux density of soft X-ray is on the order of 1011 photons/(s mu m(2)), metal Pb clusters are formed at the surface on areas with the out-of-plane component of the polarization pointing outwards only. (C) 2015 Elsevier B.V. All rights reserved.

29. Polarization induced self-doping in epitaxial Pb(Zr0.20Ti0.80)O-3 thin films

Authors: Pintilie, L; Ghica, C; Teodorescu, CM; Pintilie, I; Chirila, C; Pasuk, I; Trupina, L; Hrib, L; Boni, AG; Apostol, NG; Abramiuc, LE; Negrea, R; Stefan, M; Ghica, D

Published: OCT 8 2015, DOI: 10.1038/srep14974

The compensation of the depolarization field in ferroelectric layers requires the presence of a suitable amount of charges able to follow any variation of the ferroelectric polarization. These can be free carriers or charged defects located in the ferroelectric material or free carriers coming from the electrodes. Here we show that a self-doping phenomenon occurs in epitaxial, tetragonal ferroelectric films of Pb(Zr0.2Ti0.8)O-3, consisting in generation of point defects (vacancies) acting as donors/acceptors. These are introducing free carriers that partly compensate the depolarization field occurring in the film. It is found that the concentration of the free carriers introduced by selfdoping increases with decreasing the thickness of the ferroelectric layer, reaching values of the order of 10(26) m(-3) for 10 nm thick films. One the other hand, microscopic investigations show that, for thicknesses higher than 50 nm, the 2O/(Ti+Zr+Pb) atomic ratio increases with the thickness of the layers. These results suggest that the ratio between the oxygen and cation vacancies varies with the thickness of the layer in such a way that the net free carrier density is sufficient to efficiently compensate the depolarization field and to preserve the outward direction of the polarization.

30. Image molecular dipoles in surface enhanced Raman scattering

Authors: Teodorescu, CM

Published: 2015, DOI: 10.1039/c4cp05082g

The surface enhanced Raman scattering (SERS) effect is explained using the interaction of a polarized molecule with its instantaneous image dipole in a metal surface. This model explains why SERS is obtained mostly on noble metals (Au, Ag), since these metals usually have lower inherent contamination as compared with other more reactive metals; thus, molecules may be found closer to the metal surface. It is shown how stronger SERS amplifications may be obtained using nanostructured surfaces, once the excited molecules are localized in concave sites. The dependence on the fourth power of the incoming radiation electric field is obtained by taking into account the dynamics of adsorption-desorption processes of molecules. The SERS effect is maximal when the excitation frequency is red-shifted with respect to the bulk plasmon resonance. Also, the SERS amplification factor may be dictated by the polarizability of the investigated molecule, alpha, in a much more critical way than just a power law alpha(2) or even alpha(4). By comparing the dipole induced charge density with the amplitudes of plasma waves, the domain of validity of the present theory is derived to be in the low separation regime, where the distance between molecules and metal substrates is below a few nanometres. Some data from the literature are analyzed in the framework of this model, namely the distance, frequency and temperature dependence of the SERS signal, all confirming the validity of the model.

31. Spectro-microscopic photoemission evidence of charge uncompensated areas in Pb(Zr,Ti)O-3(001) layers

Authors: Popescu, DG; Husanu, MA; Trupina, L; Hrib, L; Pintilie, L; Barinov, A; Lizzit, S; Lacovig, P; Teodorescu, CM

Published: 2015, DOI: 10.1039/c4cp04546g

Photoelectron spectroscopy studies of (001) oriented PbTi0.8Zr0.2O3 (PZT) single crystal layers with submicron resolution revealed areas with different Pb 5d binding energies, attributed to the different charge and polarization states of the film surface. Two novel effects are evidenced by using intense synchrotron radiation beam experiments: (i) the progressive increase of a low binding energy component for the Pb core levels (evidenced for both 5d and 4f, on two different measurement setups), which can be attributed to a partial decomposition of the PZT film at its surface and promoting the growth of metallic Pb during the photoemission process, with the eventuality of the progressive formation of areas with downwards ferroelectric polarization; (ii) for films annealed in oxygen under clean conditions (in an ultrahigh vacuum installation) a huge shift of the Pb 5d core levels (by 8-9 eV) towards higher binding energies is attributed to the formation of areas with depleted mobile charge carriers, whose surface density is insufficient to screen the depolarization field. This shift is attenuated progressively with time, as the sample is irradiated with high flux soft X-rays. The formation of these areas with strong internal electric field promotes these films as good candidates for photocatalysis and solar cells, since in the operation of these devices the ability to perform charge separation and to avoid electron-hole recombination is crucial.

32. Schottky barrier versus surface ferroelectric depolarization at Cu/Pb(Zr, Ti)O-3 interfaces

Authors: Stoflea, LE; Apostol, NG; Chirila, C; Trupina, L; Negrea, R; Pintilie, L; Teodorescu, CM

Published: MAY 2014, DOI: 10.1007/s10853-014-8041-6

The band bending at Cu/PZT(001) interfaces is investigated by X-ray photoelectron spectroscopy (XPS) for a PZT(001) layer which exhibits initial outwards ferroelectric polarization. Two competitive processes are identified: (a) formation of the Schottky barrier between the ferroelectric and unconnected Cu islands, and (b) coalescence of the Cu islands, realisation of an electrical contact to the ground of the system, inducing the apparent loss of the component of the ferroelectric polarization perpendicular to the sample surface, at least as it manifests in band bending. Three mechanisms are proposed to explain this loss of band bending when a full metal layer connected to ground is formed on the surface: (i) over-compensation of depolarization field in the sub-surface region, (ii) formation of domains with in-plane orientation of the polarization vector and (iii) loss of polarization in the near-surface layers of the ferroelectric due to electrons provided by the metal. These result in a non-monotonous variation of binding energies with the amount of Cu deposited. High resolution transmission electron microscopy and piezoresponse force microscopy confirmed these hypotheses. The XPS data allowed also to derive the surface PZT composition, its evolution with the deposition of copper and the formation of surface compounds.

33. SPIN WAVES IN SYSTEMS WITH VARIABLE SIZE, DIMENSIONALITY AND DIFFERENT CRYSTAL STRUCTURES: EFFECTS IN THE MAGNETIZATION DEPENDENCE ON TEMPERATURE

Authors: Radutoiu, N; Teodorescu, CM

Published: APR-JUN 2014, DOI:

Monte-Carlo estimations of the integrals of Bose-Einstein distribution function for magnons are performed, by avoiding number of approximations involved in the usual derivation of the Bloch T-3/2 law, describing the dependence of magnetization with temperature in the low temperature regime, where the main mechanism of magnetization decrease is due to spin waves. More precisely: (i) the magnon dispersion law is considered the exact one (depending on the crystal structure) and not the quadratic approximation epsilon(k) alpha k(2); (ii) the integrals are performed over the first Brillouin zone and the limit of the integral is not extended to infinity; (iii) the dimensionality of the system is taken into account: 1D, 2D and 3D systems are considered; (iv) finite size magnetic systems are treated by rigorous summation of the Bose-Einstein distribution function, without using the continuous limit. The results can be summarized as follows: (a) Infinite three-dimensional (3D) systems exhibit deviations from the Bloch T-3/2 law, namely the deviation from zero Kelvin magnetization vertical bar Delta M vertical bar/M(0) behaves as T-gamma, where gamma is sensibly (by some 10 %) larger than 3/2 and, also, depends on the crystal structure considered. (b) Finite 3D systems exhibit more drastic deviation from the Bloch law, in that the gamma exponent deviates seriously from 3/2, going up to 2.5 for small systems of about 1,000 atoms. (d) Finite 2D systems also show serious deviations from the Bloch-like T law. As described a few years ago [C.M. Teodorescu, Surf. Sci. 601, 4292 (2007)], finite temperature magnetization may be considered only in finite size 2D systems. Several recent examples from literature are analyzed in the framework of the actual theoretical results. It is shown that M(T) analysis in the low temperature regime allows one to extract the average nanoparticle size of the magnetic moieties, and as well the exchange interaction and average spin quantum number.

34. Polarization-Control of the Potential Barrier at the Electrode Interfaces in Epitaxial Ferroelectric Thin Films

Authors: Pintilie, I; Teodorescu, CM; Ghica, C; Chirila, C; Boni, AG; Hrib, L; Pasuk, I; Negrea, R; Apostol, N; Pintilie, L

Published: FEB 26 2014, DOI: 10.1021/am405508k

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O-3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

35. Surface Reactivity and Magnetism at Metal-Semiconductor Interfaces

Authors: Apostol, NG; Teodorescu, CM

Published: 2014, DOI: 10.1007/978-3-662-44479-5_8

The interplay between magnetic properties and structure and interdifussion at interfaces is analyzed mainly by surface science methods and by core level spectroscopies (based on X-ray absorption XAS and X-ray photoelectron spectroscopy XPS) on model systems implying deposition of magnetic metals (Fe, Co, Sm) on usual semiconductors (Si, GaAs, InAs). The Chapters begins with a review of the core level spectroscopies and of derived techniques, such as X-ray photoelectron diffraction (XPD) from XPS and X-ray magnetic circular dichroism (XMCD) from XAS, together with a brief description of other surface science techniques employed, such as low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), magneto-optical Kerr effect (MOKE), and with some basics of the molecular beam epitaxy (MBE) method. The examples are organized in pairs, namely one analyzes comparatively (i) interfaces formed by Fe or by Sm on Si(001), (ii) by Fe on GaAs(001) and on InAs(001), (iii) and by Co on bare GaAs(011) and on GaAs(011) passivated with Sb, in order to outline how small changes of the nature of the metal deposited, of the substrate (though identical from the chemical point of view) and of its initial state may result in strong deviations concerning both the quality of structures obtained and their magnetic properties. For instance, it is shown that Sb/Si(001) yields better properties than Fe/Si(001), Fe/InAs(001) exhibits lower reactivity and enhanced Fe magnetic moments than Fe/GaAs(001), and that passivation with antimony of GaAs(011) substrates yields to a reduced As out-diffusion into the metal layer, yielding lower tetragonal distortion and enhanced Co magnetic moments.

36. Selective adsorption of contaminants on Pb(Zr,Ti)O-3 surfaces shown by X-ray photoelectron spectroscopy

Authors: Stoflea, LE; Apostol, NG; Trupina, L; Teodorescu, CM

Published: 2014, DOI: 10.1039/c4ta02660h

X-ray photoelectron spectroscopy (XPS) analyses of lead zirco-titanate Pb(Zr,Ti)O-3(001) single crystal thin layers as a function of the time spent between sample preparation by pulsed laser deposition and introduction in to a ultrahigh vacuum revealed the fact that freshly prepared samples showed a shift of the C 1s towards a higher binding energy, together with shifts of core levels originating from the substrate (particularly Ti 2p and O 1s) towards a lower binding energy. This behaviour is explained by considering that the molecules of contaminants (fatty acids, alcohols, esters) are adsorbed preferentially on areas exhibiting outwards polarization P(+). Thus, photoelectrons originating from contaminants will have larger binding energies because of the charge state of the P(+) areas, whereas the substrate XPS signals from these P(+) areas are attenuated by the contaminants, with the consequence of a prevalence of XPS substrate signals originating from the P(-) areas, shifted towards lower binding energies. Piezoresponse force microscopy confirmed the assumptions derived from XPS results and suggests the existence of an interplay between the adsorption of contaminants and the surface polarization state.

37. Gold nano-island arrays on silicon as SERS active substrate for organic molecule detection

Authors: Ignat, T; Husanu, MA; Munoz, R; Kusko, M; Danila, M; Teodorescu, CM

Published: JAN 1 2014, DOI: 10.1016/j.tsf.2013.10.151

Gold islands forming highly controlled arrays have been fabricated by two potential step electrochemical deposition method using nanopatterned Si surface templates. In the present work, the Raman scattering studies realized using 11-mercaptoundecanoic probe molecule showed that such structures exhibit an enhanced Raman signal compared with nanostructured physical deposited thin gold film on flat silicon substrate and can be valued as surface-enhanced Raman scattering substrates. Besides the more appropriate management of nano-island arrays distribution, the high ratio of their Raman signals can be explain by the epitaxial-like growth mechanism of themetallic nano-islands, clearly showed by X-ray diffraction studies. Furthermore, the substrates enabled reproducibility and stability detection due to the chemically assembling of organothiol molecules, the X-ray photoelectron spectroscopy studies confirming formation of the thiolate species which corresponds to Au - S bonds, and also, the unwanted 'hot-spots' are missing, which make them suitable for high sensitivity biosensing applications. (C) 2013 Published by Elsevier B.V.

38. 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, 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.

39. X-ray photoelectron diffraction study of relaxation and rumpling of ferroelectric domains in BaTiO3(001)

Authors: Pancotti, A; Wang, JL; Chen, PX; Tortech, L; Teodorescu, CM; Frantzeskakis, E; Barrett, N

Published: MAY 28 2013, DOI: 10.1103/PhysRevB.87.184116

The surface of a ferroelectric BaTiO3(001) single crystal was studied using synchrotron radiation induced x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and low-energy electron diffraction (LEED). AFM, XPS, and LEED show that the surface is BaO terminated with a (1 x 1) reconstruction. The Ba 4d, Ti 2p, and O 1s XPD results were compared with multiple scattering simulations for out-of- (P+, P) and in-plane (P-in) polarizations using a genetic algorithm to determine atomic rumpling and interlayer relaxation. Linear combinations of the XPD simulations of the surface structure of each polarization state allow determination of the domain ordering. The best agreement with experiment is found for 55% P+, 38% P-, and 7% P-in. The rumpling is smaller at the surface than in the bulk, suggesting that both domain ordering and surface structural changes contribute to screening of the polarization.

40. 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, 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.

41. Enhancing Oxidative Dehydrogenation Selectivity of Ceria-Based Catalysts with Phosphorus as Additive

Authors: Trotus, IT; Teodorescu, CM; Parvulescu, VI; Marcu, IC

Published: MAR 2013, DOI: 10.1002/cctc.201200699

Surface- and bulk-phosphated ceria catalysts were prepared and studied in propane oxidative dehydrogenation (ODH) at 823 and 873K. The catalysts were characterized by N2 adsorption at 77K, XRD, TEM, and diffuse reflectance IR Fourier transform (DRIFT), Raman, X-ray photoelectron (XPS), and energy dispersive X-ray (EDX) spectroscopies. Both series of catalysts presented an increase in the ODH selectivity with respect to pure ceria mainly at the expense of total oxidation selectivity. Thus, the selectivity to propene was approximately 74% with surface-phosphated catalysts and 62% with bulk-phosphated catalysts. For the surface-phosphated samples, P is confined to the surface and subsurface regions of the ceria particles, whereas P is also dispersed in the bulk of the oxide in the bulk-phosphated samples. Finally, all the characterization techniques led us to the conclusion that Ce4+ cations that interact with P are responsible for the observed increase in the ODH.

42. 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, 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.

43. 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, 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.

44. A new green, ascorbic acid-assisted method for versatile synthesis of Au-graphene hybrids as efficient surface-enhanced Raman scattering platforms

Authors: Iliut, M; Leordean, C; Canpean, V; Teodorescu, CM; Astilean, S

Published: 2013, DOI: 10.1039/c3tc30177j

A new green method for the synthesis of reduced graphene oxide-gold nanoparticle (rGO-AuNP) hybrids in aqueous solution that exploits the ability of ascorbic acid (AA) to operate as an effective dual agent for both graphene oxide (GO) and gold ion reduction is reported. Through careful investigation of the production of rGO-AuNP hybrids stabilized with polyvinylpyrrolidone (PVP), several versatile routes were devised with the aim of controlling the size, shape and distribution of AuNPs anchored onto the graphene sheets as well as the GO reduction. Particularly, when rGO is used as a platform for Au ion nucleation, a relative sparse distribution of AuNPs of size ranging from 20 nm to 50 nm is noticed. In contrast, when gold ions are added to the solution prior to any GO reduction, the density of large AuNPs is rather low relative to the uniformly packed small sized AuNPs (3-12 nm). The progress of GO reduction is explained by considering the contribution of the catalytic activity of AuNPs, besides the reducing activity of AA. Finally, a plausible mechanism for the nucleation and distribution of AuNPs onto the graphenic surface is assumed, highlighting the significance of oxygen moieties. The green method developed here is promising for the fabrication of gold-graphene nanocomposites with tunable surface "decoration", suitable for surface-enhanced Raman spectroscopy (SERS).

45. 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, 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.

46. Phenylboronic-Acid-Modified Nanoparticles: Potential Antiviral Therapeutics

Authors: Khanal, M; Vausselin, T; Barras, A; Bande, O; Turcheniuk, K; Benazza, M; Zaitsev, V; Teodorescu, CM; Boukherroub, R; Siriwardena, A; Dubuisson, J; Szunerits, S

Published: DEC 11 2013, DOI: 10.1021/am403770q

Phenylboronic-acid-modified nanoparticles (NPs) are attracting considerable attention for biological and biomedical applications. We describe here a convenient and general protocol for attaching multiple copies of para-substituted phenylboronic acid moieties onto either iron-oxide-, silica-or diamond-derived NPs. The boronic acid fiinctionalized NPs are all fabricated by first modifying the surface of each particle type with 4-azidobenzoic ester functions. These azide-terminated nanostructures were then reacted with 4-[1-ozo-4-pentyn-1-yl) amino]phenylboronic acid units via a Cu(I) catalyzed Huisgen cycloaddition to furnish, conveniently, the corresponding boronic-acid modified NPs (or "borono-lectins") targeted in this work. The potential of these novel "borono-lectins" as antiviral inhibitors was investigated against the Hepatitis C virus (HCV) exploiting a bioassay that measures the potential of drugs to interfere with the ability of cell-culture-derived JFH1 virus particles to infect healthy hepatocytes. As far as we are aware, this is the first report that describes NP-derived viral entry inhibitors and thus serves as a "proof-of-concept" study. The novel viral entry activity demonstrated, and the fact that the described boronic-acid-functionalized NPs all display much reduced cellular toxicities compared with alternate NPs, sets the stage for their further investigation. The data supports that NP-derived borono-lectins should be pursued as a potential therapeutic strategy for blocking viral entry of HCV.

47. 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, 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.

48. 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, 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.

49. Riboflavin enhanced fluorescence of highly reduced graphene oxide

Authors: Iliut, M; Gabudean, AM; Leordean, C; Simon, T; Teodorescu, CM; Astilean, S

Published: OCT 24 2013, DOI: 10.1016/j.cplett.2013.09.032

The improvement of graphene derivates' fluorescence properties is a challenging topic and very few ways were reported up to now. In this Letter we propose an easy method to enhance the fluorescence of highly reduced graphene oxide (rGO) through non-covalent binding to a molecular fluorophore, namely the riboflavin (Rb). While the fluorescence of Rb is quenched, the Rb - decorated rGO exhibits strong blue fluorescence and significantly increased fluorescence lifetime, as compared to its pristine form. The data reported here represent a promising start towards tailoring the optical properties of rGOs, having utmost importance in optical applications. (C) 2013 Elsevier B.V. All rights reserved.

50. SATELLITES IN Ce 3d X-RAY PHOTOELECTRON SPECTROSCOPY OF CERIA

Authors: Radutoiu, N; Teodorescu, CM

Published: OCT-DEC 2013, DOI:

Ce 3d X-ray photoelectron spectroscopy (XPS) spectra is characterized by the presence of "shake-up" and "shake-down" satellites, which complicate the analysis of the XPS data and especially the identification of different ionization states. The present paper proposes a new model for interpreting the 3d XPS lines of cerium oxide (CeO2). This model supposes 6 pairs of doublets with spin-orbit splitting, three corresponding of trivalent cerium and three corresponding of tetravalent cerium. The three components used for each ionization state correspond to the "shake-down" process, to the "parent" (Koopman) line, and to the "shake-up" line, in order of increasing binding energy. From the time dependence of Ce 3d and C 1s XPS spectra, a model for the contamination process is derived and also the length of the alkyl chains of the hydrocarbons may be estimated. The present analysis allowed a more accurate computation of the Ce 3d atomic sensitivity factor (ASF), in accord with the values found for other rare earths elements.

51. Interface charge transfer in polypyrrole coated perovskite manganite magnetic nanoparticles

Authors: Pana, O; Soran, ML; Leostean, C; Macavei, S; Gautron, E; Teodorescu, CM; Gheorghe, N; Chauvet, O

Published: FEB 15 2012, DOI: 10.1063/1.3686662

Different hybrid structures were obtained by coating magnetic nanoparticles of perovskite type manganite at optimal doping (La0.67Sr0.33MnO3, LSMO) with different quantities of polypyrrole (PPy). The amorphous layer of polypyrrole surrounding the crystalline magnetic core was observed by high resolution transmission electron microscopy (HRTEM) and analyzed by using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements in near edge structure (XANES) techniques. By analyzing the magnetic behavior of the samples one can observe that the surface modification of magnetic nanoparticles by PPy results in an increase in the saturation magnetization of the composites. The process is ascribed to paired electrons transferred from the delocalized p states of the PPy into the outer disordered layers of the manganite. The analysis of pre-edge peak of the Mn K-edge XANES spectra in the case of PPy coated LSMO nanoparticles indicates that the charge transfer between polymer and nanoparticles is (directed) going to missing or distorted oxygen positions, hence increasing the 3d electrons' mobility and orbital hybridization between the neighboring manganese ion. As a consequence, within the surface layers of LSMO nanoparticles, both energy bands disrupted the structure, and the double exchange process between Mn ions was reestablished determining the saturation magnetizations and pre-edge features increase, respectively. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686662]

52. 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, 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.

53. 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, 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.

54. 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, 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.

55. 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, 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.

56. Manganese-based room temperature ferromagnetism in gallium arsenide

Authors: Vasilache, V; Apostol, NG; Lungu, GA; Macovei, D; Teodorescu, CM

Published: NOV-DEC 2012, 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.

57. BAND FERROMAGNETISM IN SYSTEMS WITH LINEAR DENSITY OF STATES

Authors: Lungu, GA; Apostol, NG; Morariu, M; Teodorescu, CM

Published: OCT-DEC 2012, 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).

58. 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, 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.

59. Matrix assisted pulsed laser evaporation of Mn-12(Propionate) thin films

Authors: Pervolaraki, M; Sima, F; Socol, G; Teodorescu, CM; Gheorghe, NG; Socol, M; Mihailescu, IN; Moushi, EE; Tasiopoulos, AJ; Athanasopoulos, GI; Viskadourakis, Z; Giapintzakis, J

Published: SEP 15 2012, DOI: 10.1016/j.apsusc.2011.10.136

Single molecule magnets are of great interest due to a multitude of potential applications for some of which thin films are required. Traditional physical vapor deposition techniques are not suitable for the deposition of these fragile materials with low decomposition temperatures. Matrix Assisted Pulsed Laser Evaporation technique has been employed for the growth of thin films of the single molecule magnet Mn-12(Propionate) on Si and glass substrates. In this paper we report on the appropriate growth conditions and also the morphology, chemical composition and magnetic behavior of the films. Continuous Mn-12(Propionate) films with properties similar to bulk materials have been obtained. (c) 2011 Elsevier B.V. All rights reserved.

60. The Meyer-Neldel rule in amorphous TiO2 films with different Fe content

Authors: Mardare, D; Yildiz, A; Apetrei, R; Rambu, P; Florea, D; Gheorghe, NG; Macovei, D; Teodorescu, CM; Luca, D

Published: SEP 2012, DOI: 10.1557/jmr.2012.193

Amorphous titania thin films with increasing Fe content have been prepared by RF magnetron sputtering. X-ray absorption spectroscopy revealed modifications of both local structures/environment of the Ti and Fe atoms, with formation of phases containing amorphous material and a magnetite-like phase. The temperature dependence of the electrical conductivity of the films was investigated for temperatures higher than half of the Debye temperature (T > 391 K). It was found that the electrical conductivity in the amorphous Fe/TiO2 films obeys the Meyer-Neldel rule. The origin of this behavior is explained on the basis of the multiphonon-assisted hopping model.

61. 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, 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.

62. Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

Authors: Stavarache, I; Lepadatu, AM; Gheorghe, NG; Costescu, RM; Stan, GE; Marcov, D; Slav, A; Iordache, G; Stoica, TF; Iancu, V; Teodorescu, VS; Teodorescu, CM; Ciurea, ML

Published: JAN 2011, DOI: 10.1007/s11051-010-0021-4

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol-gel and radio frequency magnetron sputtering. After the deposition, the sol-gel films are annealed in either N-2 (at 1 atm and 800 A degrees C) or H-2 (at 2 atm and 500 A degrees C), and the sputtered films in H-2 (at 2 atm and 500 A degrees C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3-7 nm size) are observed in sol-gel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10-50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol-gel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.

63. Surface versus volume effects in luminescent ceria nanocrystals synthesized by an oil-in-water microemulsion method

Authors: Tiseanu, C; Parvulescu, VI; Boutonnet, M; Cojocaru, B; Primus, PA; Teodorescu, CM; Solans, C; Dominguez, MS

Published: 2011, DOI: 10.1039/c1cp21135h

Pure and europium (Eu3+) doped cerium dioxide (CeO2) nanocrystals have been synthesized by a novel oil-in-water microemulsion reaction method under soft conditions. In-situ X-ray diffraction and RAMAN spectroscopy, high-resolution transmission electron microscopy, UV/Vis diffuse-reflectance and Fourier transform infrared spectroscopy as well as time-resolved photoluminescence spectroscopy were used to characterize the nanaocrystals. The as-synthesized powders are nanocrystalline and have a narrow size distribution centered on 3 nm and high surface area of similar to 250 m(2) g(-1). Only a small fraction of the europium ions substitutes for the bulk, cubic Ce4+ sites in the europium-doped ceria nanocrystals. Upon calcination up to 1000 degrees C, a remarkable high surface area of similar to 120 m(2) g (-1) is preserved whereas an enrichment of the surface Ce4+ relative to Ce3+ ions and relative strong europium emission with a lifetime of similar to 1.8 ms and FWHM as narrow as 10 cm(-1) are measured. Under excitation into the UV and visible spectral range, the europium doped ceria nanocrystals display a variable emission spanning the orange-red wavelengths. The tunable emission is explained by the heterogeneous distribution of the europium dopants within the ceria nanocrystals coupled with the progressive diffusion of the europium ions from the surface to the inner ceria sites and the selective participation of the ceria host to the emission sensitization. Effects of the bulk-doping and impregnation with europium on the ceria host structure and optical properties are also discussed.

64. X-ray absorption fine structure investigations on heat-treated Cr-doped titania thin films

Authors: Mardare, D; Nica, V; Pohoata, V; Macovei, D; Gheorghe, N; Luca, D; Teodorescu, CM

Published: DEC 1 2011, DOI: 10.1016/j.tsf.2011.04.124

Chromium-doped titanium oxide thin films were investigated in the as-deposited state and after thermal treatment (723 K for 3 h in air). X-ray diffraction data revealed an improvement in film crystallinity induced by the thermal treatment. Extended X-ray absorption fine structure data revealed similar atomic neighboring around Cr atoms in both as-deposited and annealed samples. A lattice contraction of similar to 2% is observed in the annealed samples. The 67% enhancement of the amplitude of the Cr 1 s X-ray absorption fine structure pre-edge peak after thermal treatment, which is a sign of "dipole-forbidden" 1 s -> 3 d transitions, suggests strong alteration in the number of Cr 3 d vacancies, in spite of similar Cr local environment in the two kinds of investigated samples. We discuss here the Cr+ -> Cr4+ and Cr2+ -> Cr6+ changes induced by thermal treatment, and/or the evolution in local structures without inversion center. Refractive index dispersion spectra in the visible wavelength domain allowed us to compute the values of the dispersion energy, the single-oscillator energy and the coordination number of Ti atoms in both as-deposited and annealed samples. (C) 2011 Elsevier B. V. All rights reserved.

65. 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, 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.

66. 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, 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.

67. Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: chemistry, local atomic structure, morphology and magnetism

Authors: Teodorescu, CM; Socol, G; Negrila, C; Luca, D; Macovei, D

Published: 2010, DOI: 10.1080/17458081003671675

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated from the following points of view: (1) the chemical states by X-ray photoelectron spectroscopy (XPS), (2) the local atomic order by X-ray absorption fine structure at both vanadium and cobalt K-edges, (3) the morphology of the films by atomic force microscopy (AFM) and (4) the magnetic properties by magneto-optical Kerr effect (MOKE). The chemical composition of the host matrix was found to be close to VO2 at the sample surface, with V2O3 in the bulk. Co ions are found near the surface in high ionisation states Co4+ (for the samples synthesised in a high vacuum condition, denoted by VO1), or with Co(4-)+ (for the samples synthesised in an oxygen atmosphere, denoted by VO2), whereas in the bulk, Co1.5+ is obtained for VO1 and Co2+ is obtained for VO2. The AFM revealed nanoparticles with sizes 10-25 nm for VO1 samples, whereas a few bigger nanoparticles are observed for VO2 samples. The VO1 samples presented high coercitive fields with a relatively low saturation magnetisation at room temperature, superposed with a superparamagnetic component attributed to the observed nanoparticles, whereas the VO2 samples presented double-loop hysteresis curves, indicating the co-existence of two kinds of magnetic moieties with antiparallel coupling at zero applied field. The proposed two magnetic phases are Co-doped V2O3 and VO2.

68. Novel Pd heterogeneous catalysts for cycloisomerisation of acetylenic carboxylic acids

Authors: Neatu, F; Protesescu, L; Florea, M; Parvulescu, VI; Teodorescu, CM; Apostol, N; Toullec, PY; Michelet, V

Published: DEC 2010, DOI: 10.1039/c0gc00258e

The Pd-TPPTS complex (TPPTS - trisodium salt of 3,3',3 ''-phosphanetriyl benzenesulfonic acid) and PdCl42- salt heterogenised onto Zn2AlNO3 layered double hydroxide (LDH) using an ion-exchange procedure, have been shown to be efficient green catalysts in the cycloisomerisation reaction of acetylenic carboxylic acids to the corresponding 5-membered heterocycles.

69. Cobalt-doped ZnO prepared by electrochemistry: Chemistry, morphology, and magnetism

Authors: Matei, E; Enculescu, I; Vasilache, V; Teodorescu, CM

Published: NOV 2010, DOI: 10.1002/pssa.200925378

ZnO:Co thin films are prepared by electrochemical deposition at different overpotentials and investigated by scanning electron microscopy, optical spectroscopy, and cathodoluminescence, X-ray photoelectron spectroscopy and Kerr magnetometry (MOKE). An increase of structural disorder and crystallite reorientation are observed for cobalt-doped samples. MOKE magnetometry revealed room-temperature ferromagnetic behavior only for ZnO: Co deposited at -850 mV. These are the samples with the lower cobalt content and lowest structural perturbation of the ZnO matrix by cobalt. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

70. ATOMIC STRUCTURE AND MAGNETIC PROPERTIES OF COBALT DOPED ZnO THIN FILMS PREPARED BY THE SOL-GEL METHOD

Authors: Neamtu, J; Georgescu, G; Malaeru, T; Gheorghe, NG; Costescu, RM; Jitaru, I; Ferre, J; Macovei, D; Teodorescu, CM

Published: OCT-DEC 2010, DOI:

The ability of a sol-gel route for the synthesis of cobalt doped ZnO (Zn 1-xCoxO, x = 0.04-0.11) films grown on Si (100) and glass substrates is investigated. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature magneto-optical Kerr effect (Faraday rotation) measurements of the magnetization versus field give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 50 Oe is superimposed, most probably due to metal cobalt nanoclusters.

71. New analytical approximation of diffraction size broadened peak profile for spherical crystallites with a lognormal distribution

Authors: Popa, NC; Teodorescu, CM; Frunza, S

Published: OCT 2010, DOI: 10.1107/S0021889810024829

A new analytical approximation of the diffraction peak size profile due to the lognormal distribution of spherical crystallites is derived. In comparison with the existing approximation constructed with the condition of conserving the volume-averaged column length of the exact profile [Popa & Balzar (2002). J. Appl. Cryst. 35, 338-346], the new approximation conserves both volume- and area-averaged column lengths. Moreover, it is a better approximation for large distribution dispersions. Like the old approximation, the present one is appropriate for implementation in the whole powder pattern fitting codes, Rietveld included.

72. Chemical Imaging of Catalyst Deactivation during the Conversion of Renewables at the Single Particle Level: Etherification of Biomass-Based Polyols with Alkenes over H-Beta Zeolites

Authors: Parvulescu, AN; Mores, D; Stavitski, E; Teodorescu, CM; Bruijnincx, PCA; Gebbink, RJMK; Weckhuysen, BM

Published: AUG 4 2010, DOI: 10.1021/ja102566b

The etherification of biomass-based alcohols with various linear alpha-olefins under solvent-free conditions was followed in a space- and time-resolved manner on 9 mu m large H-Beta zeolite crystals by confocal fluorescence microscopy. This allowed us to visualize the interaction with the substrate and distribution of the coke products into the catalyst at the level of an individual zeolite crystal during the etherification process. The spectroscopic information obtained on the micrometer-scale zeolite was in line with the results obtained with bulk characterization techniques and further confirmed by the catalytic results obtained both for micrometer-scale and nanoscale zeolites. This allowed us to explain the influence of the substrate type (glycerol, glycols, and alkenes) and zeolite properties (Si/Al ratio and particle size) on the etherification activity. The etherification of the biomass-based alcohols takes place mainly on the external surface of the zeolite particles. The gradual blockage of the external surface of the zeolite results in a partial or total loss of etherification activity. The deactivation could be attributed to olefin oligomerization. The high conversions obtained in the etherification of 1,2-propylene glycol with long linear alkenes (up to 80%) and the pronounced deactivation of the zeolite observed in the etherification of glycerol with long linear alkenes (max. 20% conversion) were explained by the spectroscopic measurements and is due to differences in the adsorption, i.e., in the center of the zeolite particle for glycerol and on the external surface in the case of glycols.

73. Analysis of electron traps at the 4H-SiC/SiO2 interface; influence by nitrogen implantation prior to wet oxidation

Authors: Pintilie, I; Teodorescu, CM; Moscatelli, F; Nipoti, R; Poggi, A; Solmi, S; Lovlie, LS; Svensson, BG

Published: JUL 15 2010, DOI: 10.1063/1.3457906

Electron states at the SiO2/4H-SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of similar to 10(19) cm(-3) and 10(16) cm(-3), respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35-295 K. Three main categories of electron states are disclosed, true interface states (D-it), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of D-it and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstrated to be responsible for the parallel shifts and hysterezis effects occurring in the C-V characteristics. All three categories of electron states are reduced in concentration in the implanted samples. This holds particularly for NIToxfast with a peak at similar to 0.1 eV below the conduction band edge of 4H-SiC that is suppressed by at least two orders of magnitude relative to the nonimplanted samples. The decrease for D-it is also substantial (a factor of similar to 10) while the loss for NIToxslow is considerably smaller (only similar to 30%). The results provide firm evidence that NIToxfast and NIToxslow do not originate from the same kind of defect center. (C) 2010 American Institute of Physics. [doi:10.1063/1.3457906]

74. Mesoporous Tin-Triflate Based Catalysts for Transesterification of Sunflower Oil

Authors: Verziu, M; El Haskouri, J; Beltran, D; Amoros, P; Macovei, D; Gheorghe, NG; Teodorescu, CM; Coman, SM; Parvulescu, VI

Published: JUL 2010, DOI: 10.1007/s11244-010-9468-x

Unimodal porous system (denoted as SnTf-MCM-41) and bimodal porosity (denoted as SnTf-UVM-7) were prepared in a two-step synthesis. The triflic acid was incorporated into previously synthesized Sn-containing porous silicas. Chemical analysis was carried out by electron probe microanalysis (EPMA), textural properties were determined by transmission electron microscopy (TEM) and from nitrogen adsorption-desorption isotherms (77 K), while structural characterization by X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements. The structural characterization provided evidences on the direct interaction of triflate with incorporated tin species. The acidity of these catalysts was determined from the ammonia chemisorption measurements. The catalysts have been tested for transesterification of sunflower oil under both microwave and ultrasound activation. These experiments showed a direct reaction between the acidity of the catalysts and catalytic performances. No leaching of the triflate species or catalyst deactivation have been detected over several runs.

75. PREPARATION AND CHARACTERIZATION OF IRON OXIDES EMBEDDED IN FULLERITE MATRICES

Authors: Lungu, GA; Macovei, D; Teodorescu, CM

Published: MAR 2010, 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.

76. One-Pot Synthesis of Menthol Catalyzed by a Highly Diastereoselective Au/MgF2 Catalyst

Authors: Negoi, A; Wuttke, S; Kemnitz, E; Macovei, D; Parvulescu, VI; Teodorescu, CM; Coman, SM

Published: 2010, DOI: 10.1002/anie.201002090

77. Photo-degradation activity of sputter-deposited nitrogen-doped titania thin films

Authors: Apetrei, R; Catrinescu, C; Mardare, D; Teodorescu, CM; Luca, D

Published: DEC 15 2009, DOI: 10.1016/j.tsf.2009.05.068

Various approaches have been reported during the last decade to enhance the photo-catalytic and super-hydrophilic characteristics of the intrinsic TiO2 materials. Results on the photo-degradation activity of RF-sputtered nitrogen-doped titania thin films are presented here with respect to two environmental pollutants in gas- and liquid phases, as a function of dopant concentration. This study was accomplished by monitoring the time dependence of pollutant concentration in contact with the films, both under UV and solar light illumination. The synergic effects of surface morphology, crystalline and electronic structure of the materials are discussed in relation with the observed experimental facts. The results presented here show a close correlation between the oxidative power of the films and their previously reported wettability features. (C) 2009 Elsevier B.V. All rights reserved.

78. Fe- and Eu-doped TiO2 Photocatalytical Materials Prepared by High Energy Ball Milling

Authors: Vasiliu, F; Diamandescu, L; Macovei, D; Teodorescu, CM; Tarabasanu-Mihaila, D; Vlaicu, AM; Parvulescu, V

Published: JUN 2009, DOI: 10.1007/s11244-009-9192-6

TiO2 nanopowders, doped with Fe3+ and Eu3+ were obtained by high-energy ball milling and their physical properties were investigated as a function of the doping content and ball milling time. A noticeable red shift and high photoactivity in the degradation and catalytic oxidation reactions of styrene and phenol were found for all doped specimens.

79. Band ferromagnetism in systems of variable dimensionality II: the two-dimensional finite-temperature case

Authors: Lungu, GA; Teodorescu, CM

Published: APR 2009, 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.

80. Synthesis, structural characterization, and photocatalytic properties of iron-doped TiO2 aerogels

Authors: Popa, M; Diamandescu, L; Vasiliu, F; Teodorescu, CM; Cosoveanu, V; Baia, M; Feder, M; Baia, L; Danciu, V

Published: JAN 2009, DOI: 10.1007/s10853-008-3147-3

Fe(III)-doped TiO2 aerogels are prepared by acid catalyzed sol-gel method followed by supercritical drying, and then heat treatment. Raman spectra together with X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns of the iron-doped TiO2 aerogel samples revealed the existence of both anatase and brookite crystalline phases. It was found that the brookite phase formation is favored by the increase of the iron content in the dried samples. XRD measurements show that the lattice constant c of anatase phase decreases with the dopant addition, while the value of a remains essentially unchanged. The microstructure of the investigated samples is relatively compact with small mesopores as revealed from transmission electron microscopy (TEM). The most enhanced photocatalytic activity was exhibited by the TiO2 aerogel sample with 1.8 at.% Fe(III) whose apparent rate constant of the salicylic acid photodegradation was found to be of almost six times higher than that of Degussa P25.

81. EXAFS investigation of iron local environment in metal-doped titania photocatalysts prepared by hydrothermal and high-energy ball milling routes

Authors: Vasiliu, F; Diamandescu, L; Macovei, D; Teodorescu, CM; Nicula, R

Published: JAN 2009, DOI: 10.1007/s10854-007-9541-9

Iron local environment was investigated by EXAFS in Fe- and (Fe, Eu)-doped TiO(2) photocatalysts, prepared by hydrothermal and high-energy ball milling (HEBM) routes. In the case of the hydrothermal samples, the substitution of Ti(4+) by Fe(3+) ions was evidenced. For the samples prepared by HEBM, the iron environment corresponds to mixed metallic and oxidized (FeO, alpha-Fe(2)O(3)) configurations, without a clear evidence of iron incorporation into the TiO(2) lattice. This could be related to the catalyst contamination by iron microparticles detached from the balls during milling process.

82. Pulsed laser co-deposition of titania with Fe: chemistry, local atomic structure, and light-dependent magnetic properties

Authors: Negrila, C; Logofatu, C; Socol, G; Macovei, D; Teodorescu, CM

Published: 2009, 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 Xray 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.

83. Fabrication and characterization of nano-structured ferromagnetic Ti1-xFexO2 thin films

Authors: Apetrei, R; Negrila, C; Macovei, D; Dascaleanu, V; Teodorescu, CM; Mardare, D; Luca, D

Published: 2009, DOI:

Nanostructured intrinsic and doped titania materials have long been subject to both basic and applied research, in connection with environment and energy related applications. Apart from beneficial effects. such as widening the wavelength range for surface photocatalytic activation, doping TiO2 materials with 3d ferromagnetic cations is a main route to develop ail important class of diluted semiconductors, with potential applications in spintronics. In all the previous investigations the concentration of the ferromagnetic dopant was limited to a few percents. Here, we initiate ail extended investigation of Ti1-xFexO2 within a, wider iron composition range (x = 0 - 0.55) and discuss the magnetic and optical properties of these materials in thin films in relation with their elemental composition, nano-structure and local atomic ordering.

84. Cobalt doped ZnO prepared by electrochemistry: chemistry, morphology, and magnetism

Authors: Enculescu, I; Matei, E; Vasilache, V; Teodorescu, CM

Published: 2009, DOI:

ZnO:Co thin Films are prepared by electrochemical deposition at different overpotentials and investigated by scanning electron microscopy, X-ray photoelectron spectroscopy and Kerr magnetometry (MOKE). Increse of structural disorder and crystallite re-orientation are observed for cobalt doped samples (especially for those obtained at - 750 and at - 800 mV overpotentials). MOKE magnetometry revealed room temperature ferromagnetic behaviour only for ZnO:Co deposited at - 850 mV. These are the samples with the less cobalt content and lowest structural perturbation of the ZnO matrix by cobalt.

85. Photonic molecular effects associated to the sputtering process in a glow discharge optical emission spectrometer

Authors: Surmeian, A; Groza, A; Diplasu, C; Ganciu, M; Teodorescu, CM; Tempez, A; Chapon, P

Published: JAN 2009, DOI:

Spectral emission of a Xenon discharge in the presence of NaCl powder is strongly modified and dominated by the XeCl excimer band at 308 nm. Materials introduced in gas electrical discharges interact with the reactive species produced in the plasma: electrons, ions and metastable atoms. In this paper, the influence of ions and metastable atoms of a capacitive RF Xenon discharge plasma (20W, 30MHz) 5 Torr on the structure of a NaCl powder introduced in the discharge has been investigated.

86. Band ferromagnetism in systems of variable dimensionality

Authors: Teodorescu, CM; Lungu, GA

Published: NOV 2008, 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.

87. Structural and Magneto-Optical Properties of Co-doped ZnO Thin Films Prepared by Sol-Gel Method

Authors: Neamtu, J; Teodorescu, CM; Georgescu, G; Ferre, J; Malaeru, T; Jitaru, I

Published: 2008, DOI:

Recent experiments evidenced room temperature ferromagnetism in ZnO:Co diluted magnetic semiconductors, however the diluted semiconductor nature and the ferromagnetism strongly depends on the preparation method employed. We have investigated the ability of the sol-gel route for the synthesis of cobalt doped ZnO films grown on Si (100) and glass substrates. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature (1.5 K) Kerr magnetometry give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 200 Oe is superimposed, most probably due to metal cobalt nanoclusters.

88. Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: structure, chemistry, morphology, and magnetism

Authors: Teodorescu, CM; Socol, G; Negrila, C; Luca, D; Macovei, D

Published: 2008, DOI:

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated by (i) X-ray photoelectron spectroscopy, (ii) the local atomic order by X-ray absorption near-edge structure (XANES), (iii) the morphology of the films was investigated by atomic force microscopy (AFM), and (iv) magnetic properties were quantified by magneto-optical Kerr effect (MOKE). In most cases, the chemical composition of the host matrix was found to be the vanadium (5+) oxide V2O5 at the sample surface and lower ionization states (+2 and +3) in the bulk. Co ions are found either in high ionization state Co5+ (for samples synthesized in high vacuum condition, denoted by VO1), or with Co in lower ionization states Co4+ (for samples synthesized in a mixture of argon and oxygen atmosphere, denoted by VO2). Consistent information was obtained from chemical shifts from individual core level scans in XPS, compared with existent data in litterature, and the amplitude of the pre-edge peak in XANES, which is a sign of quadrupole 1s -> 3d dipole-forbidden transition and whose amplitude is proportional to the number of 3d vacancies per atom. AFM revealed big particles with sizes > 100 mu m for VO1 samples, whereas smaller nanoparticles with sizes ranging between 20 and 30 mu m were observed for VO2 samples. VO1 samples presented very high coercitive fields with a relatively low saturation magnetisation at room temperature, whereas VO2 samples presented double-loop hysteresis curves, indicating the coexistence of exchange bias between two kinds of magnetic moieties with strong anisotropy.

89. Photocatalytic and structural properties of mixed titania and zirconia aerogels

Authors: Danciu, V; Baia, L; Cosoveanu, V; Baia, M; Vasiliu, F; Diamandescu, L; Teodorescu, CM; Feder, M; Popp, J

Published: 2008, DOI:

A series of TiO2-ZrO2 aerogels with different ZrO2 contents (3-10 %) were prepared by an acid catalyzed sol-gel method followed by low-temperature supercritical drying and heat treatment. The structure and morphology of mixed aerogels were studied by means of Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), xenon sorption and elemental analysis. XRD spectra refinements show that the calcined TiO2-ZrO2 aerogels consist mainly in anatase phase accompanied by a small fraction of tetragonal ZrO2. The Raman spectra proof that the network of the calcined mixed aerogels mainly preserves an anatase structure similar to that of the calcined TiO2 aerogel. The TEM images reveal that the anatase particles present round shapes with a highly homogeneous size (about 3-8 nm). Areas containing chains of amorphous tetragonal zirconia were also identified by the TEM investigations. The addition of zirconia to titania significantly increased the surface area and stabilized the anatase phase of titania. The best catalytic efficiency for the salicylic acid photodecomposition was obtained for the aerogel with 9.39% zirconia that exhibited an apparent rate constant of about three times higher than that of the pure titania aerogel.

90. Structural and photocatalytic properties of iron- and europium-doped TiO2 nanoparticles obtained under hydrothermal conditions

Authors: Diamandescu, L; Vasiliu, F; Tarabasanu-Mihaila, D; Feder, M; Vlaicu, AM; Teodorescu, CM; Macovei, D; Enculescu, I; Parvulescu, V; Vasile, E

Published: NOV 15 2008, DOI: 10.1016/j.matchemphys.2008.05.023

Iron- and europium-doped (<= 1 at.%)TiO2 nanoparticles powders have been synthesized by a hydrothermal route at 200 degrees C, starting with TiCl4, FeCl3 center dot 6H(2)O and EuCl3 center dot 6H(2)O. The structure, morphology and optical peculiarities were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS), Mossbauer spectroscopy and UV-vis measurements. The photocatalytic performance was analysed in the photodegradation reaction of phenol. Rietveld refinements of XRD patterns reveal that the as-prepared samples consist in iron- and europium-doped TiO2 in the tetragonal anatase structural shape, with particle size as low as 15 nm. By means of Mossbauer spectroscopy on both Fe-57 and Eu-151 isotopes as well as by EXAFS analyses, the presence of Fe3+ and/or Eu3+ ions in the nanosized powders has been evidenced. It was found that iron and europium ions can substitute for titanium in the anatase structure. From the UV-vis reflection spectra, by using the transformed Kubelka-Munk functions, the band gap energy (E-g) of the hydrothermal samples has been determined in comparison with that of Degussa P-25 photocatalyst. A decrease of E-g from 2.9eV found for Degussa photocatalyst to 2.8 eV for the titania doped with 1 at.% Fe has been evidenced, indicating a valuable absorption shift (similar to 20 nm) towards visible light region. However, the best photocatalytic activity in the photodegradation reaction of phenol was evidenced for the hydrothermal sample, TiO2: 1 at.% Fe, 0.5 at.% Eu, in both UV and visible light regions. The photocatalytic activities of iron-doped and iron-europium-codoped samples are high and practically the same only in visible light. The photocatalytic properties in Correlation with the structural and optical peculiarities of the hydrothermal samples are discussed. (C) 2008 Elsevier B.V. All rights reserved.

91. Preparation and characterization of increased-efficiency photocatalytic TiO2-2xNx thin films

Authors: Luca, D; Teodorescu, CM; Apetrei, R; Macovei, D; Mardare, D

Published: OCT 15 2007, DOI: 10.1016/j.tsf.2007.03.115

We report here on the characteristics of RF-sputtered 300 nm thick films of TiO2-2xNx prepared on glass substrates at 350 degrees C, by adjusting the N-2:Ar partial pressure ratio in the deposition chamber between 0.00 and 0.33. XRD, XPS, AFM and contact angle data were used to derive film structure, elemental composition and oxidation state of Ti, surface morphology and hydrophilicity, respectively. The band gap was derived from spectral data in the 350-450 nm range. Film structure and composition were changed by adjusting the partial pressure of the reactive gases during sputtering and by post-deposition annealing at 400 degrees C in air, for 90 min. The values of the contact angle of films' surface with de-ionized water and of surface free energy per unit area show that films are super-hydrophilic for high-nitrogen content. Correlations are made between film structure, elemental composition, electronic and wettability properties. (c) 2007 Elsevier B.V. All rights reserved.

92. Low temperature two-dimensional behaviour of spin and orbital moments in Ni monolayers grown on Cu(001)

Authors: Teodorescu, CM

Published: SEP 15 2007, DOI: 10.1016/j.susc.2007.04.233

The Bloch T-3/2 law is obtained for three-dimensional magnons at low temperatures. When decreasing the dimensionality of the magnetic system, the 3/2 exponent is expected to be equal to one for two-dimensional (2D) systems. I present a theoretical sketch of 2D spin wave theory, in which a long magnon wavelength cutoff is introduced, connected to the lateral size of the 2D system investigated. I report a study by X-ray magnetic circular dichroism at the Ni L-edges on the onset of ferromagnetic, order on 1.5 atomic Ni layers deposited on Cu(1 0 0), with spin and orbital moments' dependence with temperature. A linear dependence is found for both spin and orbital moments with the same initial slope. This temperature dependence can be used to probe the size of the 2D magnetic islands, which is particularly useful for embedded 2D objects. (C) 2007 Elsevier B.V. All rights reserved.

93. Structure, morphology and magnetic properties of Fe-Au core-shell nanoparticles

Authors: Pana, O; Teodorescu, CM; Chauvet, O; Payen, C; Macovei, D; Turcu, R; Soran, ML; Aldea, N; Barbu, L

Published: SEP 15 2007, DOI: 10.1016/j.susc.2007.06.024

The aim of the present work is the investigation of the core-shell Fe@Au nanoparticles obtained by the inverse micelles method. Also hybrid structures between Fe@Au nanoparticles and the well-known conducting polymer polypyrrole (PPY) were obtained and investigated. The properties of the composites were investigated by TEM, HRTEM, X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS) and magnetization measurements. The TEM, HRTEM measurements show that two categories of core-shell nanoparticles with mean diameters of 5 nm and 25 nm, respectively, are formed. The XPS investigations of the core-shell Fe@Au nanoparticles indicate that besides Fe-0 inside the cores, small amounts of Fen,111, located onto the gold surface were also formed during the samples preparation. The magnetization of the investigated nanocomposites measured versus increasing and decreasing magnetic field shows no hysteresis loop, this behavior being consistent with a superparamagnetic behavior. The behavior of the SQUID magnetization vs. temperature under field cooled (FC) and zero field cooled (ZFC) at 0.1 T magnetic field for Fe@Au nanoparticles confirmed the existence of superparamagnetism. The superparamagnetic model was used to fit the experimental magnetization vs. applied magnetic field as it was measured by SQUID up to 5 T. (C) 2007 Elsevier B.V. All rights reserved.

94. Fe-doped TiO2 thin films

Authors: Mardare, D; Nica, V; Teodorescu, CM; Macovei, D

Published: SEP 15 2007, DOI: 10.1016/j.susc.2007.04.139

The reactive sputtering technique was used to obtain undoped and Fe-doped TiO2 thin films deposited on glass substrates. At 250 degrees C substrate temperature, undoped TiO2 films crystallize in a mixed rutile/anatase phase, while Fe-doped films exhibit the rutile phase only. Presence of Fe3+ ions into the TiO2 lattice is suggested by the intensity variation of forbidden 1s -> 3d transitions between the Ti and Fe K-edges. Ti K-edge EXAFS data are assessed to a mixture of the two kinds of surroundings, a rutile-like crystalline phase, identified also by X-ray diffraction, and a nanosized or amorphous anatase-like surrounding. The local atomic order about Fe atoms is quite different and could be related also to an amorphous phase. The Swanepoel method is used to obtain the dispersion of the refractive index below the interband absorption edge. The dispersion energy, the single-oscillator energy and the coordination number of the Ti atoms are evaluated using the single-oscillator model (Wemple-DiDomenico). (C) 2007 Elsevier B.V. All rights reserved.

95. On the hydrophilicity of nitrogen-doped TiO2 thin films

Authors: Mardare, D; Luca, D; Teodorescu, CM; Macovei, D

Published: SEP 15 2007, DOI: 10.1016/j.susc.2007.04.156

Nitrogen doping was proposed recently as a low-cost means to shift the absorption edge of titania towards 450-500 run. Results are reported here on the preparation of nitrogen-doped 250 nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N-2 partial pressure in the discharge from 0.00 to 0.27 Pa. An additional amount of oxygen was added in the deposition chamber (Po-2 = 0.11 Pa) to compensate for oxygen depletion in the films. Surface composition and structure were derived from XPS and XRD data, respectively. XRD data revealed a mixture of amorphous phase and rutile polymorph in the as-deposited films, while an anatase-rich phase occurs in the samples deposited under N2 pressure of 0.05 and 0.16 Pa. Above 0.16 Pa, a rutile-rich phase is present. Surface wettability was investigated from contact angle data of de-ionized water with solid surface. The films deposited under mid-range N2 pressure in the discharge are super-hydrophilic. The photo-induced super-hydrophilicity decays after 36 h. Film wettability is discussed in terms of the synergic effects of surface roughness (measured by AFM) and rutile-anatase TiO2 phase transition, induced by doping. The TiO2 surface super-hydrophilicity (and catalytic efficiency) are related to beneficial properties of anatase phase (E-g = 3.2 eV), associated with a low recombination rate of charge carriers in the surface. (C) 2007 Elsevier B.V. All rights reserved.

96. Comparative study of magnetism and interface composition in Fe/GaAs(100) and Fe/InAs(100)

Authors: Teodorescu, CM; Luca, D

Published: SEP 15 2006, DOI: 10.1016/j.susc.2006.01.167

Thin layers of Fe are deposited at 100 degrees C on GaAs(1 0 0) and InAs(1 0 0) substrates by molecular beam epitaxy. These structures are characterized by photoelectron spectroscopy (PES), in order to derive their chemical composition and interface reactivity, and by X-ray magnetic circular dichroism (XMCD) recorded at room temperature in remanence mode, in order to derive the atomic magnetic moment of Fe. As previously reported, Fe/GaAs(1 0 0) exhibits a strong and complicated interface reactivity, with considerable intermixing of Fe with Ga and As. Unlike the above structure, Fe/InAs(1 0 0) exhibits a much lower interface reactivity, whereas the Fe magnetic moment increase is much more pronounced as function of Fe thickness. The total atomic ferromagnetic moment reaches at room temperature 2.0 Bohr magnetons by about I nm Fe effective thickness, which is almost the Fe bcc bulk value (2.2 Bohr magnetons). The novelty of the present study is a direct comparison of the two interfaces prepared and measured in the same experimental arrangements. (c) 2006 Elsevier B.V. All rights reserved.

97. Characterization of titania thin films prepared by reactive pulsed-laser ablation

Authors: Luca, D; Macovei, D; Teodorescu, CM

Published: SEP 15 2006, DOI: 10.1016/j.susc.2006.01.162

Results are reported on the characterization of 200-250 nm thick TiO(x) transparent films grown at temperatures of 150 degrees C and 500 degrees C by reactive pulsed-laser ablation of a metallic Ti target under 0.13-13.3 Pa oxygen atmosphere. Film structure and composition were investigated by X-ray diffraction and X-ray photoelectron spectroscopy. The samples deposited at 150 degrees C reveal a mixture of amorphous TiO(2) and Ti(2)O(3), irrespective of oxygen pressure. The O:Ti atomic ratio fluctuates in their surface around 1.83 for oxygen pressure ranging between 0.13 Pa and 2.66 Pa. The samples deposited at 500 degrees C feature different characteristics, as a function of oxygen pressure: (a) below 2.66 Pa of O(2) the films contain a mixture of significant amount of anatase and rutile TiO(2) phases, along with titanium suboxides; (b) above this threshold, the rutile phase vanishes, the anatase TiO(2) phase remaining dominant along with small amounts of nanocrystalline suboxides. The size of the anatase nanocrystallites decreases with the increase of oxygen pressures. The samples deposited at 500 degrees C feature a slight improvement of surface stoichiometry, from roughly TiO(1.8) to TiO(1.9). The films deposited at high substrate temperature and oxygen pressure are highly hydrophilic. (c) 2006 Elsevier B.V. All rights reserved.

98. Increasing surface hydrophilicity of titania thin films by doping

Authors: Luca, D; Mardare, D; Iacomi, F; Teodorescu, CM

Published: JUL 15 2006, DOI: 10.1016/j.apsusc.2006.05.011

The hydrophilicity of RF sputtered thin films of: (a) pure TiO2 and (b) TiO2 doped with 0.3% Ce, 0.4% Nb, and 0.4% N (atomic percents) was investigated macroscopically by measurements of the contact angle between water and film surface. The results are discussed in terms of the connection of the hydrophilic and photocatalytic properties of the materials with their structure, morphology and optical characteristics. The 280 nm thick films were deposited on optical glass substrates at 250 degrees C. Film structure and surface morphology were investigated by X-ray diffraction and atomic force microscopy. The surface roughness was derived from atomic force microscopy and ellipsometric data. The contact angle of de-ionized water with film surface was monitored during photo-activation and after irradiating with near-UV light. The surface super-hidrophilicity of all the investigated samples decays, when samples are kept in darkness for 48 h after irradiation. The hydrophilic behavior of the doped TiO2 thin films is discussed in terms of the effects of surface roughness, phase transformations enhanced by doping and charge carrier recombination. (c) 2006 Elsevier B.V All rights reserved.

99. Thickness effect in Pb(Zr0.2Ti0.8)O-3 ferroelectric thin films grown by pulsed laser deposition

Authors: Lisca, M; Pintilie, L; Alexe, M; Teodorescu, CM

Published: APR 30 2006, DOI: 10.1016/j.apsusc.2005.07.149

Epitaxial Pb(Zr,Ti)O-3 (PZT) thin films with thicknesses in the range of 50-200 nin and with 0.2 Zr/(Zr + Ti) ratio, were grown by pulsed laser deposition (PLD). The substrates used for PLD deposition are single crystalline 0.5% Nb-doped (1 0 0)SrTiO3 (STON). SrRuO3 (SRO) thin films were deposited as bottom and top electrodes in order to have minimum structural misfit, to insure on one side high quality growth, and on the other side to minimize the influence of the ext,ended structural defects. Structural and electrical characterization was performed. The epitaxial PZT films are c-axis oriented and have an average roughness of 0.4 nm. The ferroelectric behavior was proved in all investigated films by the presence of the hysteresis loops and by the butterfly shape of the capacitance-voltage (C-V) characteristics. The ferroelectricity was present even in the samples with relative high leakage currents, down to a thickness of 50 nm. These results are essential when small thickness is needed for miniaturization of ferroelectric devices using PZT. (c) 2005 Elsevier B.V. All rights reserved.

100. Physical characterization of CdMnS nanocrystalline thin films grown by vacuum thermal evaporation

Authors: Iacomi, F; Salaoru, I; Apetroaei, N; Vasile, A; Teodorescu, CM; Macovei, D

Published: FEB 2006, DOI:

CdMnS ternary alloys have attracted much attention because they permit to tune the energy gap, the effective mass and the lattice constant by varying the concentration of the magnetic material. Growth and physical characterization of Cd1-xMnxS thin films, prepared in different conditions on glass and silicon substrates by thermal evaporation technique is presented. The starting powders, CdS and Mn, were mixed in a crucible in various molar ratios. The samples were characterized by XRD, AFM and XPS. The blue shift of band gap of thin films might be ascribed to the size effect of the small grains and to the fact that Cd was substituted by Mn in the CdS structure.

101. Structural and magnetic investigations of nickel clusters in C-60 matrices

Authors: Teodorescu, CM; Macovei, D; Lungu, A

Published: DEC 2004, 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.

102. Reactivity and magnetism of Fe/InAs(100) interfaces

Authors: Teodorescu, CM; Chevrier, F; Brochier, R; Richter, C; Ilakovac, V; Heckmann, O; De Padova, P; Hricovini, K

Published: AUG 2002, DOI: 10.1140/epjb/e2002-00234-6

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

103. Reducibility of ruthenium in relation with zeolite structure

Authors: Parvulescu, VI; Coman, S; Palade, P; Macovei, D; Teodorescu, CM; Filoti, G; Molina, R; Poncelet, G; Wagner, FE

Published: MAR 1999, DOI: 10.1016/S0169-4332(98)00619-9

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

104. EXAFS CHARACTERIZATION OF DY AND PD-DY ON ALUMINA CATALYSTS

Authors: MACOVEI, D; PARVULESCU, V; TEODORESCU, CM

Published: APR 1994, DOI: 10.1007/BF02129853

Two Pd-Dy/gamma-Al2O3 catalysts, with 1 wt.% Dy and various Pd loadings (0.1 and 0.5 wt.%) have been investigated by EXAFS at the Dy L(III)-edge, and compared with 1 % Dy/gamma-Al2O3. The Dy-carrier interaction was found to change with the Pd dispersity.

Dr. Teodorescu Cristian Mihail

Senior Scientist 1

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Projects

1. Technologic paradigms in synthesis and characterization of variable dimensionality systems

Publications

1. Degenerated TiO(2)Semiconductor Modified with Ni and Zn as Efficient Photocatalysts for the Water Splitting Reaction

2. The interplay of work function and polarization state at the Schottky barriers height for Cu/BaTiO3 interface

3. 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

4. Impact on Ferroelectricity and Band Alignment of Gradually Grown Au on BaTiO3

5. Growth of Ag(111) on Si(111) with nearly flat band and abrupt interface

6. Spirobifluorene-based Porous Organic Polymers as Efficient Porous Supports for Pd and Pt for Selective Hydrogenation

7. Room temperature ferromagnetism and its correlation to ferroelectricity of manganese embedded in lead zirco-titanate

8. Polarization Orientation in Lead Zirconate Titanate (001) Thin Films Driven by the Interface with the Substrate

9. Triggering surface ferroelectric order in Pb(Zr, Ti)O-3(001) by deposition of platinum

10. Long-range magnetic interaction in MnxGe1-x: structural, spectromicroscopic and magnetic investigations

11. Photoelectron Spectroscopic and Microspectroscopic Probes of Ferroelectrics

12. Structural and magnetic properties of Ni nanofilms on Ge(001) by molecular beam epitaxy

13. Band bending at magnetic Ni/Ge(001) interface investigated by X-ray photoelectron spectroscopy

14. Aging phenomena and wettability control of plasma deposited carbon nanowall layers

15. Low-energy electron diffraction from ferroelectric surfaces: Dead layers and surface dipoles in clean Pb(Zr, Ti)O-3(001)

16. Oxygenophilic ionic liquids promote the oxygen reduction reaction in Pt-free carbon electrocatalysts

17. Polarization landscape effects in soft X-ray-induced surface chemical decomposition of lead zirco-titanate, evidenced by photoelectron spectromicroscopy

18. Sustainable metal-free carbogels as oxygen reduction electrocatalysts

19. The combined action of methanolysis and heterogeneous photocatalysis in the decomposition of chemical warfare agents

20. Local Ordering at the Interface of the TiO2-WO3 Bi-Layers

21. An adamantane-based COF: stability, adsorption capability, and behaviour as a catalyst and support for Pd and Au for the hydrogenation of nitrostyrene

22. Non-interacting, sp(2) carbon on a ferroelectric lead zirco-titanate: towards graphene synthesis on ferroelectrics in ultrahigh vacuum

23. Growth mechanisms and band bending in Cu and Pt on Ge(001) investigated by LEED and photoelectron spectroscopy

24. Ferroelectric triggering of carbon monoxide adsorption on lead zirco-titanate (001) surfaces

25. Magnetic Nanoparticles for Hepatocellular Carcinoma Diagnosis and Therapy

26. An adamantane-based COF: stability, adsorption capability, and behaviour as a catalyst and support for Pd and Au for the hydrogenation of nitrostyrene (vol 6, pg 8344, 2016)

27. Band bending at copper and gold interfaces with ferroelectric Pb(Zr,Ti)O-3 investigated by photoelectron spectroscopy

28. Photoelectron spectroscopy and spectro-microscopy of Pb(Zr,Ti)O-3 (111) thin layers: Imaging ferroelectric domains with binding energy contrast

29. Polarization induced self-doping in epitaxial Pb(Zr0.20Ti0.80)O-3 thin films

30. Image molecular dipoles in surface enhanced Raman scattering

31. Spectro-microscopic photoemission evidence of charge uncompensated areas in Pb(Zr,Ti)O-3(001) layers

32. Schottky barrier versus surface ferroelectric depolarization at Cu/Pb(Zr, Ti)O-3 interfaces

33. SPIN WAVES IN SYSTEMS WITH VARIABLE SIZE, DIMENSIONALITY AND DIFFERENT CRYSTAL STRUCTURES: EFFECTS IN THE MAGNETIZATION DEPENDENCE ON TEMPERATURE

34. Polarization-Control of the Potential Barrier at the Electrode Interfaces in Epitaxial Ferroelectric Thin Films

35. Surface Reactivity and Magnetism at Metal-Semiconductor Interfaces

36. Selective adsorption of contaminants on Pb(Zr,Ti)O-3 surfaces shown by X-ray photoelectron spectroscopy

37. Gold nano-island arrays on silicon as SERS active substrate for organic molecule detection

38. Room Temperature Ferromagnetic Mn:Ge(001)

39. X-ray photoelectron diffraction study of relaxation and rumpling of ferroelectric domains in BaTiO3(001)

40. Charge transfer and band bending at Au/Pb(Zr0.2Ti0.8)O-3 interfaces investigated by photoelectron spectroscopy

41. Enhancing Oxidative Dehydrogenation Selectivity of Ceria-Based Catalysts with Phosphorus as Additive

42. Structure, reactivity, electronic configuration and magnetism of samarium atomic layers deposited on Si(001) by molecular beam epitaxy

43. Room Temperature Ferromagnetic, Anisotropic, Germanium Rich FeGe(001) Alloys

44. A new green, ascorbic acid-assisted method for versatile synthesis of Au-graphene hybrids as efficient surface-enhanced Raman scattering platforms

45. FERROMAGNETISM AND REACTIVITY OF FE DEPOSITED ON GaAs(001) BY MAGNETRON SPUTTERING

46. Phenylboronic-Acid-Modified Nanoparticles: Potential Antiviral Therapeutics

47. Band bending at free Pb(Zr,Ti)O-3 surfaces analyzed spectroscopy by X-ray photoelectron

48. Band bending in Au/Pb(Zr,Ti)O-3 investigated by X-ray photoelectron spectroscopy: Dependence on the initial state of the film

49. Riboflavin enhanced fluorescence of highly reduced graphene oxide

50. SATELLITES IN Ce 3d X-RAY PHOTOELECTRON SPECTROSCOPY OF CERIA

51. Interface charge transfer in polypyrrole coated perovskite manganite magnetic nanoparticles

52. Atomic structure and reactivity of ferromagnetic Fe deposited on Si(001)

53. Interface characterization and atomic intermixing processes in Be/W bilayers deposited on Si(001) substrates with Fe buffer layers

54. ATOMIC STRUCTURE AND MAGNETISM OF PLD DEPOSITED Tio(2):Fe

55. REACTIVITY, MAGNETISM AND LOCAL ATOMIC STRUCTURE IN FERROMAGNETIC FE LAYERS DEPOSITED ON SI(001)

56. Manganese-based room temperature ferromagnetism in gallium arsenide

57. BAND FERROMAGNETISM IN SYSTEMS WITH LINEAR DENSITY OF STATES

58. Epitaxial ferromagnetic samarium and samarium silicide synthesized on Si(001)

59. Matrix assisted pulsed laser evaporation of Mn-12(Propionate) thin films

60. The Meyer-Neldel rule in amorphous TiO2 films with different Fe content

61. X-ray photoelectron spectroscopy of pulsed laser deposited Pb(Zr,Ti)O3-delta

62. Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

63. Surface versus volume effects in luminescent ceria nanocrystals synthesized by an oil-in-water microemulsion method

64. X-ray absorption fine structure investigations on heat-treated Cr-doped titania thin films

65. Significantly different contamination of atomically clean Si(001) when investigated by XPS and AES

66. Enhanced contamination of Si(001) when analyzed by AES with respect to XPS

67. Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: chemistry, local atomic structure, morphology and magnetism

68. Novel Pd heterogeneous catalysts for cycloisomerisation of acetylenic carboxylic acids

69. Cobalt-doped ZnO prepared by electrochemistry: Chemistry, morphology, and magnetism

70. ATOMIC STRUCTURE AND MAGNETIC PROPERTIES OF COBALT DOPED ZnO THIN FILMS PREPARED BY THE SOL-GEL METHOD

71. New analytical approximation of diffraction size broadened peak profile for spherical crystallites with a lognormal distribution