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Dr. Dana POPESCU

Scientific Researcher II

Laboratory of Surface and Interface Science

dana[DOT]popescu[AT]infim[DOT]ro

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Dr. Popescu Dana-Georgeta (37 years) received her Engineer Diploma  at “Politehnica” University in Bucharest at Faculty of Electronics, Telecommunications and Information Technology in 2009 and her PhD degree in Engineering in 2013 at the same university with a thesis focusing on photonic crystals technology. Starting with 2010 she joined the surface and interface science group led by Dr. Cristian-Mihail Teodorescu at NIMP. Here she had the opportunity to do cutting edge research in the field of semiconducting surfaces, their interface with magnetic metals, by preparing systems with unusual magnetism at room temperature. Pursuing research for 3 months in the laboratory led by Dr. N Barret at CEA in France, Dr. Popescu enlarged her experience in the field of electron spectroscopy techniques, especially applied to complex ferroelectric and multiferroic oxide surfaces and interfaces. Other experiments were led by Dr. Popescu on different equipments at Synchrotron lines from Italy, Switzerland and Poland. Recently, she started a study on ferroelectric Rashba materials.

The main original achievements capitalize in 30 articles (24 ISI publications, 3 under review) – 18 as main/corresponding author, 4 book chapter (3 under review) and 43 conferences (6 invited, 19 oral presentation, 1 best poster prize – SCES2021), 2 young research team projects management (2015; 2021), 1 grant from L’Oréal-UNESCO „For women in science” (2021), 2 other personal grants projects and the membership in the other 10 projects, 2 patents, 2 scholarships (TRENDOXIDE 2015, Brescia, Italy; ICTP, Trieste, Italy), a scientific record of more than 250 citation, H-index = 9 and Guest Editor at Materials Journal.

Projects

1. Platforma de tip Rashba pentru stocarea si procesarea informatiei

Project Type: TE, Start Date: 2022-05-15 End Date: 2024-05-14

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2. Integrated Rashba-type platform for data storage and bit manipulation

Project Type: TE, Start Date: 2022-05-15 End Date: 2024-05-14

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3. Magnetoelectric coupling at interfaces of ferroelectric/ferromagnetic heterostructures (CUMIFF)

Project Type: TE, Start Date: 2015-09-01 End Date: 2017-12-01

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Publications

1. MXenes as Heterogeneous Thermal Catalysts: Regioselective Anti-Markovnikov Hydroamination of Terminal Alkynes with 102 h-1 Turnover Frequencies
Authors: Grau, RR; Garcia-Aznar, P; Sastre, G; Goberna-Ferrón, S; Pavel, O; Tirsoaga, A; Cojocaru, B; Popescu, DG; Parvulescu, VI; Primo, A; García, H

Published: JAN 21 2025, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 147, DOI: 10.1021/jacs.4c13481

Due to their conductive properties and optoelectronic tunability, MXenes have revolutionized the area of electrocatalysis and active materials in supercapacitors. In comparison, there are only a few reports on MXenes as thermal catalysts for general organic reactions. Herein, the unprecedented catalytic activity of Ti3C2 MXene for the hydroamination of alkynes is reported, overcoming the limitations of poor activity, lack of selectivity, and stability, which are generally encountered in the solid catalysts known so far. In the case of Ti3C2, hydroamination exhibits almost complete selectivity for the anti-Markovnikov regioisomer, for both aliphatic amines and less-reactive aromatic amines. Ti3C2 also efficiently catalyzes intramolecular hydroamination, leading to the formation of indol heterocycles. The catalytic hydroamination of C-C multiple bonds is a reaction with complete atom efficiency that may form C-N bonds from convenient reagents. The maximum number of hydroamination sites on the Ti3C2 nanosheets is quantified by thermoprogrammed NH3 desorption. The measured TOF values are on the order of 102 h-1, with the highest TOF value being 350 h-1 for 1-hexyne hydroamination by n -butylamine. Therefore, Ti3C2 is among the few heterogeneous hydroamination catalysts studied, with its activity per site being comparable to the best hydroamination catalysts reported so far. Density functional theory calculations on the models indicate the cooperation of neighboring Ti atoms in the mechanism. Considering the compositional and structural versatility of MXenes, the present findings open the door for further application of MXenes in other general organic reactions.
Open Access
2. Cobalt oxyhydroxide co-catalyst loaded onto Al:SrTiO3 surface to boost photocatalytic performance
Authors: Radu, I; Borhan, AI; Gherca, D; Dirtu, AC; Dirtu, D; Popescu, DG; Husanu, MA; Pui, A

Published: FEB 15 2025, MATERIALS CHEMISTRY AND PHYSICS, 332, 130274, DOI: 10.1016/j.matchemphys.2024.130274

In photocatalysis, a common challenge is the rapid recombination of photogenerated electrons and holes, coupled with low surface reaction efficiency in aqueous environments. This study addresses these issues by improving the photocatalytic performance of Al3+-doped SrTiO3 perovskite through the strategic loading of CoOOH onto its surface. We successfully demonstrated that the combined approach of Al3+ doping and CoOOH co-catalyst functionalization significantly enhances the photocatalytic performance of SrTiO3. The Al3%:SrTiO3 was functionalized with CoOOH using a two-step process. This process involved the oxidation of Co2+ ions to Co3+ ions, followed by the precipitation of cobalt oxyhydroxide (CoOOH) onto the surface of Al3%:SrTiO3, resulting in the formation of an Al3%:SrTiO3@CoOOH composite heterostructure. The UV-Vis data shows an enhanced light absorption capabilities into the visible spectrum with a direct band gap of 1.73 eV in contrast to 3.24 eV for the pristine perovskite. XPS analysis confirms the surface functionalization with CoOOH co-catalyst and the determined 1:1 Sr:Ti stoichiometry, the reduced state of Al, and the absence of oxygen vacancies were identified as beneficial properties for photocatalytic applications, as shown in DFT calculations. The oxacillin photodegradation was tested at three different concentrations of Al3%:SrTiO3@CoOOH photocatalyst (0.25 g/L; 0.5 g/L and 1 g/L) and we observed that the removal efficiency significantly varies from 99 % for 1 g/L Al3%: SrTiO3@CoOOH to 78 % and 44 % for 0.5 g/L and, respectively 0.25 g/L. Additionally, under visible light irradiation, the Al3%:SrTiO3@CoOOH composite achieved an exceptional degradation rate of the (3-lactam antibiotic oxacillin of up to 99 %, with holes identified as key players in the photocatalytic process. This study highlights that effective surface modification using a well-chosen co-catalyst can substantially boost the photocatalytic efficiency of semiconductor-based materials, offering a promising strategy for developing advanced photocatalysts for environmental remediation applications.
Open Access
3. Designing TiO2@FexOy magnetic core-shell catalyst with 3D flower-like surface morphology preservation for enhanced photocatalytic performance
Authors: Gherca, D; Roman, T; Popescu, DG; Borhan, AI; Herea, DD; Stoian, G; Chiriac, H; Ababei, G; Lupu, N

Published: JUL 1 2025, APPLIED SURFACE SCIENCE, 696, 163003, DOI: 10.1016/j.apsusc.2025.163003

A synthetic rational design of core-shell magnetic nanomaterials has garnered significant attention for their potential in photocatalysis and adsorption applications. This study presents the synthesis and characterization of a TiO2-based core-shell photocatalyst functionalized with FexOy co-catalyst for the efficient adsorption and degradation of synthetic methylene blue dye. The chemical synthesis technique involved a three-step process consisting in the preparation of TiO2 nanoparticles followed by surface nanocompartmentalization with a ferrihydrite layer exhibiting a flower-like morphology and lastly the calcination of the resulting composite at 400 degrees C to produce a magnetic core-shell nanomaterial. Comprehensive physicochemical characterization was performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultra-high-resolution transmission electron microscopy (UHR-TEM) to elucidate the structural and morphological properties of the synthesized materials. Photodegradation experiments were conducted under both UV and Visible light irradiation using methylene blue as a model contaminant. The results revealed remarkable photocatalytic performance, with nearly instantaneous adsorption of the dye onto the catalyst surface, followed by efficient photodegradation. Detailed investigations confirmed that the adsorption process occurred at an exceptionally rapid rate, which was attributed to the unique surface functionalization and nanocompartmentalized structure of the core-shell material.
Open Access
4. Considerable spin asymmetry of deep valence states induced by partial neutralization of charged SrTiO3(011) surfaces
Authors: Borcan, LE; Iancu, AC; Popescu, DG; Teodorescu, CM

Published: FEB 7 2025, JOURNAL OF CHEMICAL PHYSICS, 162, 054707, DOI: 10.1063/5.0243885

Recently, spin asymmetry in O 2p related deep valence states was evidenced in SrTiO3(001) [Popescu et al., Phys. Scr. 99(10), 105925 (2024)]. In this work, we report the detection of a much higher (about four times) spin asymmetry in SrTiO3(011) by spin resolved photoelectron spectroscopy, with samples characterized also by core level x-ray photoelectron spectroscopy and low energy electron diffraction. The explanation of a so important spin asymmetry is related to the partial neutralization of O-2(4-) or SrTiO(4+) end layers. Missing electrons from O 2p states in the case of O-2 terminations enable robust atomic spins, according to Hund's rule. The parallel analysis of core level shifts for surface atoms and the amplitude of spin asymmetry suggests that 50% of the oxygens from the surface SrO layer of SrTiO3(001) have a 2p(5) configuration with an unpaired electron (the rest are in a 2p(6) configuration), while in the case of O-2 terminated SrTiO3(011), about 50% of surface oxygens have a 2p(5) configuration and 50% of surface oxygens are neutral (2p(4)), yielding a net charge per O-2 surface unit cell of (-1) instead of (-4). The magnetization is oriented along the rows formed by the (4 x 1) reconstruction in the 011 in-plane direction. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(https://creativecommons.org/licenses/by/4.0/).
5. Investigations on HfO2/n-GaAs(110) interface, in-situ obtained by Oxide-MBE
Authors: Negrila, CC; Cotirlan, C; Iancu, AC; Popescu, DG; Palade, C; Trupina, L

Published: NOV 1 2025, MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING, 198, 109746, DOI: 10.1016/j.mssp.2025.109746

Hafnium oxide (HfO2) thin films were deposited on n-type gallium arsenide (GaAs) substrates by Oxide-Molecular Beam Epitaxy (Oxide-MBE) method using Hafnium (Hf) metallic flow in an oxidizing atmosphere of 10-6 mbar molecular oxygen. The Hf metallic flow was provided by an e-beam evaporator and a deposition rate 10 nm/h was established. Semiconductor surface preparation was done prior to deposition, beginning with chemical wet etching and aggressively adjusted by in treatments until a desired stoichiometry was reached. Heterojunctions with HfO2 thin layers of 1 nm, 3 nm, 10 nm and 20 nm were fabricated. X-Ray Photoelectron Spectroscopy (XPS) and ARXPS(Angle Resolved XPS) in-situ analyses provided a clear picture of the structure of the interfaces, the chemical bonds and composition. The interfaces are chemically stable and abrupt. A small amount of Ga2O3 provides a passivating effect of the semiconductor surface. The electrical properties of the heterostructures were determined using the Kraut method and Reflection Electron Energy Loss Spectroscopy (REELS) technique. Band offsets Delta EC=1.75 eV and Delta EV=2.62 eV confirm a high application potential. Additionally, data on the morphology and continuity of the layers were obtained by Atomic Force Microscopy (AFM) technique while the amorphous growth was monitored by XRD(X-ray Diffraction), GIXRD (Grazing Incidence XRD) and XRR(X-ray Reflectivity) measurements. The dielectric layers showed values of the constant k in the range of 19-22, established by electrical measurements on MOS capacitors.
6. Monophasic Titanate-Based Photocatalyst with Heteroatom Mixed Iso-Aliovalency Enabling Water Oxidation
Authors: Gherca, D; Borhan, AI; Popescu, DG; Husanu, MA; Borca, CN; Stoian, G; Chiriac, H; Ababei, G; Lupu, N

Published: MAY 16 2025, ACS APPLIED MATERIALS & INTERFACES, 17, DOI: 10.1021/acsami.5c03417

Rhodium-doped SrTiO3 perovskite as a monophasic titanate-based catalyst (SrTi1-xRhxO3) showed photocatalytic activity for oxygen evolution reaction (OER) from water under solar light irradiation with an instant induction period, although Rh4+ in SrTiO3 introduces deep trap states thereby diminishing the efficiency of the hydrogen evolution reaction (HER). Despite its potential, the exact crystal structure of Rh:SrTiO3 has not been yet completely investigated. Overcoming these challenges, here, we synthesized a monophasic SrTi0.95Rh0.05O3 (RSTO) perovskite oxide with a precisely determined crystal structure and highlighted an unconsidered pivotal role of the Rh iso-aliovalency reversibility that enables excellent photocatalytic water oxidation. With structural, morphological, optical, and electronic insights from XRD, FE-SEM, HR-TEM, XPS, and advanced XAS measurements in both total electron yield (TEY) and fluorescence yield (TFY), the oxygen evolution reaction (OER) process is attributed to the redox dynamics of Rh4+ <-> Rh3+ synergistic interplay.
Open Access
7. Nb2C Mxene as a bifunctional acid-base and oxidation/hydrogenation catalyst
Authors: Pavel, O; Tirsoaga, A; Cojocaru, B; Popescu, D; Ramírez-Grau, R; González-Durán, P; García-Aznar, P; Tian, L; Sastre, G; Primo, A; Parvulescu, V; Garcia, H

Published: JUL 3 2025, EES CATALYSIS, 3, DOI: 10.1039/d5ey00004a

Nb2C MXene, obtained from Nb2AlC by Al3+ etching and exfoliation, was characterized using XRD, HRTEM and AFM, with the data confirming the crystallinity of the sample and the 2D morphology of the sheets with an average layer thickness of 1.5 nm. Surface analysis using XPS revealed the presence of structural defects, and NH3- and CO2-TPD profiles confirmed the low density of acid and basic sites in the range of tens of mu mol gcatalyst-1 of weak and moderate strengths. The combination of acid and basic sites in close proximity on the solid surface was responsible for the remarkable catalytic activity of Nb2C MXene in promoting aldolic condensation with high turnover frequencies of up to 855 h-1, which was comparable to the values of benchmark catalysts, such as MgO or HZSM-5. Nb2C MXene also catalyzed the aerobic oxidative aniline coupling to azo- and azoxy-benzene and hydrogenation of azoxybenzene to azobenzene.
8. Ferrihydrite surface functionalization of anatase TiO2 nanoparticles as flower-like core-shell heterostructure with enhanced visible-light-driven photocatalytic properties
Authors: Gherca, D; Borhan, AI; Herea, DD; Minuti, AE; Stavila, C; Danceanu, CM; Popescu, DG; Borca, CN; Huthwelker, T; Stoian, G; Chiriac, H; Polo, CG; Ababei, G; Lupu, N

Published: JUL 15 2025, SURFACES AND INTERFACES, 69, 106745, DOI: 10.1016/j.surfin.2025.106745

The development of titania-based hybrid nanostructures with enhanced visible-light photocatalytic activity has been a key research area recently. The present study addresses current limitations of the TiO2 based composite photocatalyst by newly-experimental designing of flower-like multifunctional hybrid nanostructure with visible light capability through Ferrihydrite (Fh) surface TiO2 functionalization. Here, we present a versatile nanocompartimentalization process of which, core anatase TiO2 nanoparticles are emebeded into Fh lamellar shell. Physico-chemical properties related to the chemical structure and morphology of the prepared nanomaterials were comprehensively analysed using complementary analytical techniques, such as powder X-Ray Diffraction (XRD), Field-Emission Scanning Electron Microscopy (FE-SEM), Ultra-High Resolution Transmission Electron Microscopy (UHR-TEM), X-Ray Photoelectron Spectroscopy (XPS) and soft X-Ray Absorption Spectroscopy (XAS). The conducted visible-light-driven photocatalytic water splitting tests highlights significant enhancement in the Oxygen Evolution Reaction (OER) performance for TiO2-Fh core-shell nanoheterostructure of 25.6 mu mol/L of molecular oxygen after 60 min of visible light irradiation (AM1.5G), and a photocatalytic water oxidation activity rate of 341.3 mu mol L-1 g-1h-1. The biocompatibility assessment of the developed core-shell structures combined with their enhanced photocatalytic water oxidation activity under visible light illumination suits them as excellent candidates for the development of sustainable environmental remediation technologies.
9. Enhancement of SrTiO3 photocatalytic efficiency by Al doping: Answers from the structure, morphology and electronic properties contributions
Authors: Radu, I; Borhan, AI; Gherca, D; Popescu, DG; Borca, CN; Huthwelker, T; Bulai, G; Stoian, G; Husanu, MA; Pui, A

Published: JUN 1 2024, CERAMICS INTERNATIONAL, 50, DOI: 10.1016/j.ceramint.2024.03.188

Our study focuses on disclosing the mechanisms standing behind the improved photocatalytic performance of SrTiO3, where Al modification of the perovskite structure boosts the photocatalytic O2 evolution activity of the Al:SrTiO3 system. By adapting the synthesis method that produces well-crystallized materials with low defect density and employing surface modification techniques, we aim to enhance the photocatalytic efficiency of SrTiO3 via Al2O3 nanoceramic oxide doping at concentrations ranging from 0 to 10% and further examining of the relationship between doping process and the changes in the electronic and crystalline structure of SrTiO3. The prepared Al-based SrTiO3 perovskite samples (Al3%:SrTiO3, Al7%:SrTiO3, Al10%:SrTiO3) were thoroughly characterized to understand their structural, electronic, and morphological properties. Complementary X-ray techniques were employed to assess the stoichiometry (X-ray photoelectron spectroscopy - XPS), local environment, and chemical state (X-ray absorption spectroscopy - XAS in both total electron yield (TEY) and fluorescence yield (TFY). The comprehensive characterization enables us to understand the changes in the electronic properties and morphological features of the modified samples elucidating the surface formation mechanism while providing insights into the structural modifications induced by Al doping in the SrTiO3 perovskite lattice. Our findings give new perspectives for the development of Al-modified SrTiO3 perovskite materials with enhanced photocatalytic performance providing rich insights into the optimization of photocatalytic processes for applications in environmental remediation and sustainable energy production.
Open Access
10. Hydrothermal carbonization and pyrolysis in wetland engineering: Carbon sequestration, phosphorus recovery, and structural characterization of willow-based chars with X-ray μ-computed tomography
Authors: Acosta, AC; Arias, CA; Biller, P; Wittig, NK; Baragau, IA; Alhnidi, MJ; Ravenni, G; Sárossy, Z; Benedini, L; Abramiuc, LE; Popescu, DG; Negassa, W; Marulanda, VF; Müller-Stöver, DS; Brix, H

Published: JUL 15 2024, CHEMICAL ENGINEERING JOURNAL, 492, 151916, DOI: 10.1016/j.cej.2024.151916

Willows from engineered wetland systems (EWS) offer a sustainable approach to wastewater treatment and biomass production. Our study assesses their potential for nutrient recovery and carbon sequestration using slow pyrolysis (600 degrees C) and hydrothermal carbonization (250 degrees C). Here, we propose EWS-pyrochars as a ready-to integrate opportunity for soil amendment, as they exhibit a predominant CO2 2 release and the absence of harmful compounds in pyrolysis-chromatograms, indicating higher stability than hydrochars. Using sequential Pextractions, we observed a high bioavailability in the willow-woodchips and a significant P-retention in EWSchars-up to 92 % in pyrochars and near-complete retention in hydrochars, along with a higher labile-P fraction of 21 % in hydrochars than 5 % in pyrochars. Utilizing X-ray-based techniques, Raman spectroscopy, scanning electron microscopy, and gas physisorption, we characterized the EWS-chars' structures. We revealed innovative 3D-visualizations, which transcend previous literature by providing insights into the chars' internal porosity and quantifying, for the first time, their carbonaceous structural thickness via a meshing algorithm and the mean Feret diameter. EWS-pyrochars exhibit remarkable aromaticity with a higher concentration of overall sp2 2 Catoms at 63 % vs. 43 % in hydrochars. Moreover, unlike hydrochars, which depict occluded porosity, EWSpyrochars exhibited 92 % water storage-like pores. Although hydrochars indicated lower carbonization and thermal stability than pyrochars, their higher carbon retention (55 vs. 41 % in pyrochar) suggest superior annual benefits-on a 10 ha EWS scale-of 80-tons of carbon sequestration and 334 kg of phosphorus recovery versus 60-tons of carbon and 298 kg of phosphorus with pyrochars. Our findings suggest innovative materials for resource recovery, advancing the engineered wetland systems field, shifting their traditional use, and opening the opportunity for future integration into biorefineries.
Open Access
11. Molecular adsorption-desorption of carbon monoxide on ferroelectric BaTiO3(001)
Authors: Iancu, AC; Apostol, NG; Nicolaev, A; Abramiuc, LE; Chirila, CF; Popescu, DG; Teodorescu, CM

Published: JUL 15 2024, MATERIALS ADVANCES, 5, DOI: 10.1039/d4ma00389f

Carbon monoxide (CO) is reversibly adsorbed on and desorbed from ferroelectric (001) oriented, BaO-terminated barium titanate. All processes are characterized in real time via photoelectron spectroscopy. Adsorption proceeds on different sites/geometries as a function of substrate temperature. Below room temperature, CO is adsorbed on surface Ba. At room temperature, adsorption proceeds on surface oxygen, whereas at high temperatures, "hollow" site adsorption occurs with carbon coordinated with three oxygens, one oxygen initially belonging to CO and two oxygens from the substrate. The amount of CO adsorbed is about one molecule for 10 surface unit cells, which is slightly increased at low temperatures. CO is desorbed if the substrate is heated above the Curie temperature, which is a sign of the definitory role of ferroelectric polarization. The BaTiO3(001) surface is unaffected by repeated cycles of adsorption-desorption.
Open Access
12. Spin asymmetry of O 2p-related states in SrTiO3(001)
Authors: Popescu, DG; Nicolaev, A; Costescu, RM; Borcan, LE; Lungu, GA; Tache, CA; Husanu, MA; Teodorescu, CM

Published: OCT 1 2024, PHYSICA SCRIPTA, 99, 105925, DOI: 10.1088/1402-4896/ad732c

Atomically clean SrTiO3(001) is characterized by low energy electron diffraction, core level and valence band photoelectron spectroscopy, the latter also with spin resolution. Samples prepared by a sputtering-annealing procedure exhibited in-gap states in the valence band spectra, Ti3+ components in Ti 2p core level spectra and a noticeable spin asymmetry in the 3-9 eV binding energy range, which corresponds to valence states of mainly O 2p character. Upon annealing in oxygen, the spin asymmetry vanishes, accompanied by the intensity decrease of the contribution of titanium low ionization states and of in-gap states, indicating that these three phenomena are mutually connected. The observed spin asymmetry may be generated by indirect exchange mediated by the in-gap states between O 2p orbitals, or by the partial Ti 3d character of these states, which acquire non-zero spin in case of incomplete oxygen coordination.
13. Carbon-based heterostructure from multi-photo-active nanobuilding blocks SrTiO3@NiFe2O4@Fe0@Ni0@CNTs with derived nanoreaction metallic clusters for enhanced solar light-driven photodegradation of harmful antibiotics
Authors: Borhan, AI; Ghemes, AI; Husanu, MA; Popescu, DG; Borca, CN; Huthwelker, T; Radu, I; Dirtu, AC; Dirtu, D; Bulai, G; Lupu, N; Palamaru, MN; Iordan, AR; Gherca, D

Published: JUN 15 2023, APPLIED SURFACE SCIENCE, 622, 156987, DOI: 10.1016/j.apsusc.2023.156987

We reported on the sequential development of a high-operative photocatalyst with penta-component inorganic bulk heterojunction for improved charge trapping characteristics at particle-particle interfaces for enhanced solar light-driven photocatalytic degradation of active tetracycline antibiotic. Structural, morphological, optical, and electronic properties of the synthesized samples were investigated using a series of complementary char-acterization techniques, such as XRD, FE-SEM, HR-TEM, XPS, as well as hard and soft XAS in both total electron yield (TEY) and fluorescence yield (TFY). For the case of the carbon composite material, SrTiO3@NiFe2O4@-Fe0@Ni0@CNTs, a reduced crystallinity when compared to the starting support material was noticed, although this translated into a significant improvement of the morphology and the photocatalytic performance. The SrTiO3@NiFe2O4@Fe0@Ni0@CNTs fibrous photocatalyst can efficiently achieve a high-to-total degradation of tetracycline antibiotic under visible light irradiation in less than two hours, following a non-linear PFO kinetic model with an apparent reaction rate of about 0.0606 min-1 and an 98% photodegradation activity. The XPS and XAS analysis demonstrated unequivocally the appearance of nanoscale zero-valent iron (Fe0) and zero-valent nickel (Ni0) on the photocatalyst surface, which facilitates the separation of photogenerated e+ and h+ pairs, and the appearance of more active sites.
Open Access
14. Carbon-based heterostructure from multi-photo-active nanobuilding blocks SrTiO3@NiFe2O4@Fe0@Ni0@CNTs with derived nanoreaction metallic clusters for enhanced solar light-driven photodegradation of harmful antibiotics (vol 622, 156987, 2023)
Authors: Borhan, AI; Ghemes, IA; Husanu, MA; Popescu, DG; Borca, CN; Huthwelker, T; Radu, I; Dirtu, AC; Dirtu, D; Bulai, G; Lupu, N; Palamaru, MN; Iordan, AR; Gherca, D

Published: JUL 1 2023, APPLIED SURFACE SCIENCE, 624, 157067, DOI: 10.1016/j.apsusc.2023.157067

15. Liquid phase oxidation of alkenes and glycerol with molecular oxygen over mixed-ligand copper(II) complexes grafted on GO as catalysts
Authors: Rizescu, C; El Fergani, M; Eftemie, DI; Cojocaru, B; Popescu, D; Andruh, M; Parvulescu, VI

Published: AUG 5 2023, APPLIED CATALYSIS A-GENERAL, 663, 119302, DOI: 10.1016/j.apcata.2023.119302

[Cu(acac)(phen)(H2O)](ClO4) and [Cu(acac)(bipy)(H2O)](ClO4) catalysts were prepared by the immobilization of mixed-ligand complexes on graphene oxide derivatized with monochloroacetic acid. These catalysts were characterized through an ensemble of techniques including XRD, FTIR, Raman, and XPS while the catalytic behavior has been investigated in the oxidation of cyclohexene, 1-octene and glycerol in the presence of molecular oxygen. Cyclohexanone was the dominant product in the oxidation of cyclohexene resulting in selectivities higher than 50% for a conversion of 14.5%. The turnover frequencies were pretty high, namely, 4.5 h(-1) for [Cu(acac)(phen)(H2O)](ClO4) and 2.3 h(-1) for Cu(acac)(bipy)(H2O)](ClO4). The oxidation of 1-octene also occurred with a pretty high selectivity in 2-octanol as the main product. The role of copper in the oxidative dehydrogenation of glyceric acid towards tartronic acid was as well confirmed. Noteworthy, these catalysts were stable and their recycling occurred with no change in the conversion or selectivity.
16. Straightforward FeOOH nanografting of Al-based SrTiO3 perovskite material as core-shell nanoflower-like heteronanostructure with enhanced solar light-driven photodegradation capability
Authors: Borhan, AI; Herea, DD; Husanu, MA; Popescu, DG; Borca, CN; Huthwelker, T; Bulai, G; Radu, I; Dirtu, AC; Dirtu, D; Mita, C; Stoian, G; Ababei, G; Lupu, N; Pui, A; Gherca, D

Published: MAR 30 2023, APPLIED SURFACE SCIENCE, 614, 156247, DOI: 10.1016/j.apsusc.2022.156247

Extensive attention and considerable efforts have been made to construct efficient heterogeneous nano -particulate systems for surface chemical reactions to be active in solar light-driven photodegradation. This work addresses current deficiencies of the nanoparticles-focused systems intended for visible light photodegradation by developing a newly-formulated innovative chemically-engineered multi-component system that functions as a recyclabe, nontoxic, active and inexpensive catalyst for photodegradation of tetracyclne antibiotic. Here, we show a straightforward FeOOH nanografting of Al-based SrTiO3 perovskite material as core-shell nanoflower-like heteronanostructure with enhanced solar light-driven photodegradation capability over harmful antibi-otics. A persuasive surface formation mechanism is proposed based on systematic investigation of the assembly process. In-depth caracterization of structural, optical and morphological properties of the prepared samples was investigated using a series of complementary analytical techniques, such as XRD, FE-SEM, HR-TEM, synchrotron XPS, as well as hard and soft XAS in both total electron yield (TEY) and fluorescence yield (TFY). The oxygen -deficient nature of core and shell interface indicates its n-doping and the availability of free charges in core which can be either transferred to the shell or create localized absorption levels into the valence band. This study provides a real opportunity to rationally photocatalysts design with very promising performance in water treatment.
Open Access
17. Experimental Band Structure of Pb(Zr,Ti)O3: Mechanism of Ferroelectric Stabilization
Authors: Popescu, DG; Husanu, MA; Constantinou, PC; Filip, LD; Trupina, L; Bucur, CI; Pasuk, I; Chirila, C; Hrib, LM; Stancu, V; Pintilie, L; Schmitt, T; Teodorescu, CM; Strocov, VN

Published: FEB 2023, ADVANCED SCIENCE, 10, DOI: 10.1002/advs.202205476

Pb(Zr,Ti)O-3 (PZT) is the most common ferroelectric (FE) material widely used in solid-state technology. Despite intense studies of PZT over decades, its intrinsic band structure, electron energy depending on 3D momentum k, is still unknown. Here, Pb(Zr0.2Ti0.8)O-3 using soft-X-ray angle-resolved photoelectron spectroscopy (ARPES) is explored. The enhanced photoelectron escape depth in this photon energy range allows sharp intrinsic definition of the out-of-plane momentum k and thereby of the full 3D band structure. Furthermore, the problem of sample charging due to the inherently insulating nature of PZT is solved by using thin-film PZT samples, where a thickness-induced self-doping results in their heavy doping. For the first time, the soft-X-ray ARPES experiments deliver the intrinsic 3D band structure of PZT as well as the FE-polarization dependent electrostatic potential profile across the PZT film deposited on SrTiO3 and LaxSrMn1-xO3 substrates. The negative charges near the surface, required to stabilize the FE state pointing away from the sample (P+), are identified as oxygen vacancies creating localized in-gap states below the Fermi energy. For the opposite polarization state (P-), the positive charges near the surface are identified as cation vacancies resulting from non-ideal stoichiometry of the PZT film as deduced from quantitative XPS measurements.
Open Access
18. Nanometric-Thick Metal-Free h-Boron Nitride/Graphene Films as Base Catalyst for the Synthesis of Benzoxazoles
Authors: Rendon-Patiño, A; Primo, A; Cojocaru, B; Ion, SG; Popescu, DG; Parvulescu, V; García, H

Published: JUN 22 2022, CHEMCATCHEM, 14, e202200356, DOI: 10.1002/cctc.202200356

Catalysts are frequently used as pelletized powdered materials, but rarely as thin films. The present work reports the use of 2D h-boron nitride on few layers defective graphene (h-BN/fl-G) films of about 10 nm thickness as base catalyst for the synthesis of benzoxazoles by coupling a 2-amino-phenol with p-substituted benzaldehydes. The synthesis of benzoxazole derivatives has aroused much interest in the last decades due to their application as potent anticancer agents against MCF-7 and MDA-MB-231 breast cancer cells. The results demonstrate that these nanometer-thick films exhibit three orders of magnitude higher activity than K2CO3 taken as benchmark base catalyst. The activity and selectivity of h-BN/fl-G films depend on the nature of substituents on benzaldehyde, the solvent and reaction temperature.
Open Access
19. Ferroelectricity modulates polaronic coupling at multiferroic interfaces
Authors: Husanu, MA; Popescu, DG; Bisti, F; Hrib, LM; Filip, LD; Pasuk, I; Negrea, R; Istrate, MC; Lev, L; Schmitt, T; Pintilie, L; Mishchenko, A; Teodorescu, CM; Strocov, VN

Published: AUG 15 2022, COMMUNICATIONS PHYSICS, 5, 209, DOI: 10.1038/s42005-022-00983-3

A deeper understanding of the coupling at the interface of multiferroics heterostructures is being achieved by the use of synchrotron radiation techniques. Here, the authors use k-resolved soft X-ray photoemission spectroscopy and first principles calculations to investigate the band structure of several multiferroic heterostructures, isolating the distinct signature of the interface. Physics of the multiferroic interfaces is currently understood mostly within a phenomenological framework based on screening of the polarization field and depolarizing charges. Additional effects still unexplored are the band dependence of the interfacial charge modulation and the associated changes of the electron-phonon interaction, coupling the charge and lattice degrees of freedom. Here, multiferroic heterostructures of the colossal-magnetoresistance manganite La1-xSrxMnO3 buried under ferroelectric BaTiO3 and PbZrxTi1-xO3 are investigated using soft-X-ray angle-resolved photoemission. The experimental band dispersions from the buried La1-xSrxMnO3 identify coexisting two-dimensional hole and three-dimensional electron charge carriers. The ferroelectric polarization modulates their charge density, affecting the coupling of the 2D holes and 3D electrons with the lattice which forms large Frohlich polarons inherently reducing mobility of the charge carriers. Our k-resolved results on the orbital occupancy, band filling and electron-lattice interaction in multiferroic oxide heterostructures modulated by the ferroelectric polarization disclose most fundamental physics of these systems needed for further progress of beyond-CMOS ferro-functional electronics.
20. Polarization-dependent magnetism of the Ni/BaTiO3 interface
Authors: Bocirnea, AE; Popescu, DG; Chirila, C; Costescu, RM; Kuncser, V; Stancu, V; Trupina, L; Pasuk, I; Vlaicu, AM; Husanu, MA

Published: MAR 4 2020, PHYSICAL REVIEW MATERIALS, 4, 034402, DOI: 10.1103/PhysRevMaterials.4.034402

We explore the cross coupling between the ferroelectric and ferromagnetic phases in Ni/BaTiO3(001) heterostructures and demonstrate the modulation of the magnetism and incidence of exchange bias in the ultrathin metallic Ni overlayer, depending on the ferroelectric state of the bottom layer. We establish that 5-nm-thick monocrystalline Ni film deposited on BaTiO3 with ferroelectric polarization pointing towards the surface (P+) favors the organization of Ni into uniform ferromagnetic domains. Ni grown on BaTiO3 with opposite ferroelectric polarization is featured by emerging exchange-bias coupling between the ferromagnetic Ni top layers and the antiferromagnetic reacted interface, as theoretically explained by first-principles calculations. We explicitly obtain the morphology of the magnetic domains of the crystalline Ni layer in atomic and magnetic force microscopy measurements (AFM/MFM). The resemblance of AFM and MFM images indicate that, although with radically different morphologies, in both cases all spins orient in the Ni plane. Consequently, the distinct signature of the ferroelectric-ferromagnetic coupling extracted from the magneto-optical Kerr effect measurements encodes all the information of sample magnetism. The peculiar magnetic coupling depending on the ferroelectric state indicates new ways of engineering the functionality of metal/ferroelectric interfaces.
21. 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, APPLIED SURFACE SCIENCE, 502, 144101, 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.
22. 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, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 13, 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.
23. 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, PHYSICAL REVIEW APPLIED, 10, 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).
24. Theoretical investigation of a Ge 2D photonic crystal by optical reflectivity correlated with band distributions
Authors: Popescu, DG

Published: SEP-OCT 2016, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 18, 774, DOI:

A two dimensional photonic crystal with hexagonal symmetry is investigated by finite-difference time-domain and finite difference frequency-domain theoretical calculations. The system consist in a germanium substrate patterned with holes of different radii, varying from 0.1 alpha to 0.45 alpha, The photonic band gaps (PBGs) and the reflectance spectra allow a complete characterization of the structure. It was shown the presence of PBGs at telecommunication wavelengths which makes the present systems suitable for optoelectronic applications, representing a potential alternative to Si-based technology
25. 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, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 17, 520, 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.
26. Correlation of optical reflectivity with numerical calculations for a two-dimensional photonic crystal designed in Ge
Authors: Husanu, MA; Popescu, DG; Ganea, CP; Anghel, I; Florica, C

Published: DEC 10 2015, EUROPEAN PHYSICAL JOURNAL D, 69, DOI: 10.1140/epjd/e2015-60478-7

A two dimensional photonic crystal (2DPhC) with triangular symmetry is investigated using optical reflectivity measurements and numerical calculations. The system has been obtained by direct laser writing, using a pulsed laser (lambda = 775 nm), perforating an In-doped Ge wafer. A lattice of holes with well-defined symmetry has been designed. Analyzing the spectral signature of PBGs recorded experimentaly with finite difference time domain theoretical calculations one was able to prove the relation between the geometric parameters (hole format, lattice constant) of the system and its ability to trap and guide the radiation in specific energy range. It was shown that at low frequency and telecommunication ranges of transvelsal electric modes photonic band gap occur. This structure may have potential aplications in designing photonic devices with applications in energy storage and conversion as potential alternative to Si-based technology.
27. 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, APPLIED SURFACE SCIENCE, 352, 81, 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.
28. Surface topography to reflectivity mapping in two-dimensional photonic crystals designed in germanium
Authors: Husanu, MA; Ganea, CP; Anghel, I; Florica, C; Rasoga, O; Popescu, DG

Published: NOV 15 2015, APPLIED SURFACE SCIENCE, 355, 1191, DOI: 10.1016/j.apsusc.2015.07.218

Light confinement in a two dimensional photonic crystal (2D PhC) with hexagonal symmetry is studied using infra-red reflectance spectromicroscopy and numerical calculations. The structure has been realized by laser ablation, using a pulsed laser (lambda = 775 nm), perforating an In-doped Ge wafer and creating a lattice of holes with well-defined symmetry. Correlating the spectral signature of the photonic gaps recorded experimentally with the results obtained in the finite difference time domain and finite difference frequency domain calculations, we established the relationship between the geometric parameters of the structure (lattice constants, shape of the hole) and its efficiency in trapping and guiding the radiation in a well-defined frequency range. Besides the gap in the low energy range of transversal electric modes, a second one is identified in the telecommunication range, originating in the localization of the leaky modes within the radiation continuum. The emerging picture is of a device with promising characteristics as an alternative to Si-based technology in photonic device fabrication with special emphasize in energy storage and conversion. (C) 2015 Elsevier B.V. All rights reserved.
29. Influence of hole depletion and depolarizing field on the BaTiO3/La0.6Sr0.4MnO3 interface electronic structure revealed by photoelectron spectroscopy and first-principles calculations
Authors: Popescu, DG; Barrett, N; Chirila, C; Pasuk, I; Husanu, MA

Published: DEC 28 2015, PHYSICAL REVIEW B, 92, DOI: 10.1103/PhysRevB.92.235442

The effects of the bonding mechanism and band alignment in a ferroelectric (FE) BaTiO3/ferromagnetic La0.6Sr0.4MnO3 heterostructure are studied using x-ray photoelectron spectroscopy and first-principles calculations. The band lineup at the interface is determined by a combination of band bending and polarization-induced modification of core-hole screening. A Schottky barrier height for electrons of 1.22 +/- 0.17 eV is obtained in the case of downwards FE polarization of the top layer. The symmetry of the bonding states is emphasized by integrating the local density of states +/- 0.2 eV around the Fermi level, and strong dependence on the FE polarization is found: upwards, polarization stabilizes Ti t(2g) (xy) orbitals, while downwards, polarization favors Ti t(2g) (yz) symmetry. It is predicted that the abrupt (La, Sr)vertical bar TiO2 interface is magnetoelectrically active, leading to a A-type antiferromagnetic coupling of the first TiO2 interface layer with the underlying manganite layer through a superexchange mechanism.
30. Epitaxial growth of Au on Ge(001) surface: Photoelectron spectroscopy measurements and first-principles calculations
Authors: Popescu, DG; Husanu, MA

Published: FEB 3 2014, THIN SOLID FILMS, 552, 249, DOI: 10.1016/j.tsf.2013.12.049

A single atomic Au layer is grown epitaxially on a Ge(001) surface featured by (2 x 1) reconstruction. The low energy electron diffraction pattern of the Au/Ge(001) surface indicates the formation of a square structure with the length of crystalline domains of similar to 3 nm. Ab-initio calculations show that Au growth stabilizes the Ge surface in symmetric dimers and angle-resolved photoelectron spectroscopy measurements reveal its metallic character. The modifications in the electronic properties of the Ge surface as a result of annealing are discussed and the consequences as reflected in X-ray photoelectron spectroscopy (XPS) measurements are underlined. The deformation density indicates the regions with covalent Au-Ge bonds. These bonds are identified from Au 4f and Ge 3d XPS data. (C) 2013 Elsevier B.V. All rights reserved.
31. TWO DIMENSIONAL PHOTONIC CRYSTALS WITH DIFFERENT SYMMETRIES FOR WAVEGUIDES AND RESONANT CAVITIES APPLICATIONS
Authors: Popescu, DG

Published: 2013, UNIVERSITY POLITEHNICA OF BUCHAREST SCIENTIFIC BULLETIN-SERIES A-APPLIED MATHEMATICS AND PHYSICS, 75, 252, DOI:

In this paper we investigate the properties of two-dimensional photonic crystals (2D PhCs) with both hexagonal and square symmetry for applications in optoelectronics and telecommunication. An efficient way of manipulating the light is to make use of the defects introduced within the periodic structure, which consequently break the symmetry of the crystal, resulting in linear waveguides or resonant cavities. We calculate using finite-difference time-domain and finite-difference frequency-domain methods the properties associated with several kinds of defects, conveniently selected, performed in 2D PhCs with square and hexagonal symmetry.
32. Analysis of even and odd modes of a two-dimensional photonic crystal at Si/SiO/Cu interface
Authors: Popescu, DG; Sterian, P

Published: JUL-AUG 2013, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 15, 614, DOI:

We present a careful analysis of field characteristics in two-dimensional photonic crystals with square and hexagonal symmetry designed in a Si/SiO2/Cu heterostructure. A special attention is devoted to frequencies close to photonic band gap. Following the symmetry classification in odd and even modes, their field characteristics are studied. The material prepared in a dedicated deposition chamber in ultra-high vacuum by annealing the Si/SiO2 substrate and subsequent deposition of 10 angstrom copper in order to prevent the sample oxidation, allows the study of the photonic characteristics of the copper/silicon oxide/silicon interface. The chemical state of the Cu/SiO/Si system is addressed by photoelectron spectroscopy which allows us to deduce the amount of crystalline Si and amorphous oxide. In this manner a precise estimation of the dielectric constant of the materials is possible. It is expected that at the interface with copper, a strong confinement of radiation should appear due to high reflectivity of copper.
33. Band bending at free Pb(Zr,Ti)O-3 surfaces analyzed spectroscopy by X-ray photoelectron
Authors: Apostol, NG; Stoflea, LE; Lungu, GA; Tache, CA; Popescu, DG; Pintilie, L; Teodorescu, CM

Published: NOV 20 2013, MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS, 178, DOI: 10.1016/j.mseb.2013.02.007

This paper analyses in detail the core levels evolution of Pb(Zr,Ti)O-3, i.e. Pb 4f, Zr 3d, Ti 2p, O 1s in various conditions: absolutely freshly prepared sample, sample stored under air, and the effects of in vacuum annealing. The aim of the study is to quantify separately the chemical reactivity at the surface and the band bending effects due to the ferroelectric polarization. It is found that freshly prepared samples present mostly inwards (down arrow) polarization. This phenomenon is mostly revealed by the Ti 2p and O 1s spectra, manifested as a distinct component with 1.8 eV lower binding energy in the O 1s binding energy and by 1.1 eV in the Ti 2p binding energy. Sample aging under air suppresses the inwards polarization, and most signal comes from surfaces not presenting ferroelectric permanent polarization perpendicular to the sample surface. This process conducts also to the formation of Pb(CO3)(2) on the surface. Annealing to temperatures up to 400 degrees C stabilizes a surface composed by a main part of surface without polarization perpendicular to the surface, and with some areas presenting outwards (up arrow) polarization. These areas have, most probably, different terminations, the polarized area being (Ti,Zr)O-2 terminated. (C) 2013 Elsevier B.V. All rights reserved.
34. Room Temperature Ferromagnetic, Anisotropic, Germanium Rich FeGe(001) Alloys
Authors: Lungu, GA; Apostol, NG; Stoflea, LE; Costescu, RM; Popescu, DG; Teodorescu, CM

Published: FEB 2013, MATERIALS, 6, DOI: 10.3390/ma6020612

Ferromagnetic FexGe1-x with x = 2%-9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 degrees C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 x 1) surface structure of Ge(001) with Fe deposition. X-ray photoelectron spectroscopy (XPS) at Ge 3d and Fe 2p core levels evidenced strong Fe diffusion into the Ge substrate and formation of Ge-rich compounds, from FeGe3 to approximately FeGe2, depending on the amount of Fe deposited. Room temperature magneto-optical Kerr effect (MOKE) evidenced ferromagnetic ordering at room temperature, with about 0.1 Bohr magnetons per Fe atom, and also a clear uniaxial magnetic anisotropy with the in-plane [110] easy magnetization axis. This compound is a good candidate for promising applications in the field of semiconductor spintronics.
35. Au-Ge bonding on a uniformly Au-covered Ge(001) surface
Authors: Popescu, DG; Husanu, MA

Published: APR 2013, PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, 7, 277, DOI: 10.1002/pssr.201307029

The structure and electronic properties of the system resulted by epitaxial growth of a single atomic Au layer on a heated Ge(001) surface featured by (2 x 1) reconstruction are studied. The deposition at similar to 750 K results in a well-ordered Au surface featured by ripples separated by four times the theoretical distance between two neighboring Au atoms. As revealed by valence-band photoemission studies, the Au/Ge(001) system has metallic character. Correlating X-ray photoelectron spectroscopy results with first-principles calculations we derive the implications on the covalent bonding of Au on the Ge dimer surface. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
36. Epitaxial ferromagnetic samarium and samarium silicide synthesized on Si(001)
Authors: Costescu, RM; Gheorghe, NG; Husanu, MA; Lungu, GA; Macovei, D; Pintilie, I; Popescu, DG; Teodorescu, CM

Published: OCT 2012, JOURNAL OF MATERIALS SCIENCE, 47, DOI: 10.1007/s10853-012-6672-z

Samarium is deposited on Si(001) at various temperatures (room temperature to 400 A degrees C), and the surface structure, interface reactivity, electron configuration, and magnetic properties are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and magneto-optical Kerr effect (MOKE), respectively. It is found that metal Sm is present on samples prepared at room temperature with an interface layer containing mostly Sm2+ and a lower amount of Sm3+. When samples are prepared at high temperature, much less Sm-0 is found with an increasing amount of Sm2+. Freshly prepared Sm-0 and SmSi2 layers react strongly with oxygen from the residual gas, promoting formation of Sm2O3 at the expense of both metal Sm and SmSi2. Room temperature ferromagnetism is observed for all prepared layers with a decrease of the saturation magnetisation when samples are prepared at high temperature. It is found that ferromagnetism implies mostly Sm3+ and Sm metal. In addition to these findings, this work proposes a new assignment of the Sm 3d chemically shifted components. Also, a noticeable variation of the XPS Sm 3d spin-orbit splitting is found as a function of the Sm ionization state.
37. ATOMIC STRUCTURE AND MAGNETISM OF PLD DEPOSITED Tio(2):Fe
Authors: Costescu, RM; Lungu, GA; Socol, G; Gheorghe, NG; Macovei, D; Negrila, CC; Logofatu, C; Husanu, MA; Popescu, DG; Tache, CA; Teodorescu, CM

Published: JAN-MAR 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, DOI:

TiO2:Fe thin films prepared by pulsed laser deposition exhibit in some case light dependent saturation magnetization, as determined from Kerr magnetometry measurements performed in dark or by illuminating the sample. This phenomenon is studied in correlation with local atomic structure investigated by extended X-ray absorption fine structure, composition and chemical state analyzed by X-ray photoelectron spectroscopy and by X-ray absorption near-edge structure. It is found that light-controllable magnetism is a property of a mixture of Fe and oxidized Fe clusters embedded in the anatase TiO2 matrix.
38. REACTIVITY, MAGNETISM AND LOCAL ATOMIC STRUCTURE IN FERROMAGNETIC FE LAYERS DEPOSITED ON SI(001)
Authors: Gheorghe, NG; Husanu, MA; Lungu, GA; Costescu, RM; Macovei, D; Popescu, DG; Teodorescu, CM

Published: JAN-MAR 2012, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 7, DOI:

This work presents recent studies concerning the synthesis of ultrathin ferromagnetic Fe layers on Si(001) and the correlated follow-up measurement of their structural properties, interface reactivity, and magnetism. This study is undertaken as function of the amount of Fe deposited and of substrate temperature. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction (LEED). The local order of Fe atoms is investigated by X-ray absorption fine structure (XAFS) and the magnetism by magneto-optical Kerr effect (MOKE). A general trend established is that a higher deposition temperature stabilizes a better surface ordering, but also enhances Fe and Si interdiffusion and therefore decreases the magnetism. A surprising effect obtained by Fe deposition at room temperature is that, despite the rapid disappearance of the long range order with Fe deposition (no LEED pattern is observed for Fe coverage exceeding one monolayer), the material exhibits a significant uniaxial in-plane magnetic anisotropy. When the deposition is performed at high temperature (500 degrees C), a weak ferromagnetism is still observed, with saturation magnetization of about 10 % of the value obtained for room temperature deposition. The combined MOKE and EXAFS studies allowed inferring consistent values for the range of Fe thicknesses where the reaction takes place and the main properties of the distinct formed layers.
39. Second harmonic generation in photonic crystals: numerical simulation
Authors: Popescu, DG; Sterian, P; Bercia, R; Bostan, C

Published: MAR-APR 2012, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 14, 362, DOI:

In this paper, we want to present a simple and efficient numerical method for SHG analysis in one-dimensional photonic crystals (PhCs) based on full nonlinear system of equations. For solving the nonlinear SHG problem we used a simple method of finite elements coupled with fixed point iteration. Our model does not need additional analytic approximation compared with some existing methods, and it can be easily extended to study the SHG problem in two-dimensional photonic crystals. We used the FlexPDE Professional program to plot the diagrams varying the parameters. At the end we obtained two maximum intensities of the second harmonic wave within each high index layer, that being in contrast to the fundamental wave peak. This result can be found also in the literature. In addition, we have plotted the lattice using the Optiwave FDTD software and we observed the propagation of the field in time.
40. Enhanced contamination of Si(001) when analyzed by AES with respect to XPS
Authors: Gheorghe, NG; Lungu, GA; Costescu, RM; Popescu, DG; Teodorescu, CM

Published: MAY 2011, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 5, DOI:

Clean Si(001) single crystal surfaces are obtained by cycles of long (30 mins.) annealings in ultrahigh vacuum (fairly below 1 x 10(-9) mbar). The surface reconstruction is investigated by low energy electron diffraction (LEED). This paper reports, in addition to the well-known p(2 x 1) reconstruction, the first observation of c(4 x 2) at room temperature and also the completely new c(6 x 2) reconstruction. The in situ oxidation of these surfaces was investigated by Auger electron spectroscopy (AES) and by X-ray photoelectron spectroscopy (XPS). It is found that, in similar ultrahigh vacuum conditions (6 x 10(-10) mbar), much faster contamination (about 500 times) occurs when the samples are investigated by AES than by XPS, owing mainly to the interaction of the electron beam with the sample surface. Also, much gentler surface bombardment with electrons, such as in LEED, still enhances sample oxidation. Therefore, XPS proves to be a much more convenient technique for non-destructive assessment of the surface composition. When the surface is subjected to the AES investigation, we found that the contamination occurs by forming >Si2C=O complexes based on the Si dimers.
41. MULTIPHOTON IONIZATION OF RUBIDIUM
Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; CHELLEHMALZADEH, MA; ANDERSON, JA; POPESCU, D; POPESCU, I

Published: 1976, PHYSICAL REVIEW A, 14, 1671, DOI: 10.1103/PhysRevA.14.1662

42. FINE-STRUCTURE MEASUREMENTS OF 2-PHOTON TRANSITIONS IN ATOMIC CESIUM WITH A TUNABLE DYE LASER
Authors: CURRY, SM; COLLINS, CB; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1976, OPTICS COMMUNICATIONS, 16, 255, DOI: 10.1016/0030-4018(76)90229-7

43. MULTIPHOTON EXCITATION OF ATOMIC RUBIDIUM WITH A TUNABLE DYE LASER
Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1975, BULLETIN OF THE AMERICAN PHYSICAL SOCIETY, 20, 246, DOI:

44. DOUBLET LINE-STRENGTH RATIOS FOR PRINCIPAL SERIES OF CESIUM
Authors: POPESCU, D; POPESCU, I; MAURER, J; COLLINS, CB; JOHNSON, BW

Published: 1975, PHYSICAL REVIEW A, 12, 1431, DOI: 10.1103/PhysRevA.12.1425

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