NIMP| Mariana STEFAN, Author at NIMP

Education

2002: PhD in Physics (Condensed State Physics), University of Bucharest, Romania

Thesis title: Paramagnetic point defects in ionic crystals with structural phase transitions

PhD supervisor: Prof. Dr. Sergiu V. Nistor

1991: BSc Physics (Electronic and Electrotechnical Materials and Components), Faculty of Physics, University of Bucharest, Romania

Positions

Employer: National Institute of Materials Physics

Scientific Researcher II

CERIC (Central European Research Facility Consortium http://www.ceric-eric.eu/)

Responsible for the EPR facility from the Romanian partner NIMP

Awards

2001: “Dragomir Hurmuzescu” award of the Romanian Academy

Research interests

Characterization of point defects in wide band gap semiconductor and insulating crystalline materials
EPR probing of structural and chemical transformations in insulating and semiconductor materials
EPR investigations of nanostructured materials with applications in electronics, (photo)catalysis and gas sensing

Expertize

Multifrequency EPR and computational analysis of the EPR spectra
Single crystal growth by Czochralski and Bridgman techniques

Books

S. V. Nistor, M. Stefan, L. C. Nistor, D. Ghica and C. D. Mateescu, "Defect assisted localization of Mn2+ activating ions in the core of the ZnS quantum dots". Micro and Nanoengineering vol. 19: Nanomaterials and Nanostructures for Various Applications; eds: Gh. Brezeanu, H. Iovu, C. Cobianu,D. Dascălu, Bucureşti : Ed. Academiei Române, 2012
S. V. Nistor, M. Stefan, L. C. Nistor, D. Ghica, C. D. Mateescu and N. J. Barascu, "Preparation and properties of mesoporous cubic ZnS doped with Mn2+ activating ions". Modern trends in nanoscience; eds.: Maria Bălăşoiu, G. M. Arzumanian, Bucureşti: Editura Academiei Române, 2013
M. Stefan, S. V. Nistor, D. Ghica, "ZnS and ZnO semiconductor nanoparticles doped with Mn2+ ions. Size effects investigated by EPR spectroscopy". SIZE EFFECTS IN NANOSTRUCTURES. Basics and Applications, Springer Series in Materials Science, Vol. 205, eds.: Victor Kuncser and Lucica Miu, Part I; Chapter 1, pp. 3-27; Springer-Verlag Berlin Heidelberg 2014
R. Plugaru, I. Mihalache, R. Gavrila, A. I. Danciu, G. Boldeiu, O.T. Nedelcu, N. Plugaru, V. A. Maraloiu, D. Ghica, M. Stefan, S. V. Nistor, "Zinc oxide thin films for radiation hardened devices by materials engineering". Micro and Nanoengineering vol. 24: Nanomaterials, Nanoparticles, Nanodevices; eds: M. Zaharescu, H. Chiriac, D. Dascălu, Bucureşti : Ed. Academiei Române, 2016

Resume

Researcher ID: G-4068-2011
ORCID ID: 0000-0001-5272-4770

Publications

1. Electron paramagnetic resonance signature of rock-forming blue quartz from the Albesti (Romania) granite

Authors: Joita, AC; Ghica, D; Stefan, M; Bulat, S; Pantia, AI

Published: SEP 2024, MINERALOGY AND PETROLOGY, 118, DOI: 10.1007/s00710-024-00868-z

The ca. 480 Ma Albesti granite (Southern Carpathians, Romania) is characterized by the presence of color zoned blue quartz grains, and is part of the rather extensive European Cambro-Ordovician blue quartz landscape. The color is heat sensitive, fading at temperatures as low as 300degree celsius, inconsistent with the thermally stable, light scattering, nanometric rutile/ilmenite inclusions cited in literature. Extensive X- and Q-band electron paramagnetic resonance (EPR) investigations were carried out, searching for distinctive features of the Albesti quartz that are directly or indirectly involved in the generation of the blue coloration. The analyzed quartz grains were extracted from three granite samples of varying coloration and anisotropy, and the quartz from each rock sample was further separated into colored and colorless fractions. The paramagnetic E' and [AlO4]0 centers, as well as Mn2+ ions localized in traces of amorphous associated minerals at grain boundaries or fissure planes, were observed in all quartz samples. Broad EPR lines associated with the presence of magnetic clusters were observed in the spectra of the white quartz sample and the corresponding colorless one. Isochronal annealing up to 500degree celsius induced the correlated recombination of the E' and [AlO4]0 centers, the strong decrease of the Mn2+ spectrum and the formation of a minority iron oxide phase at the grain boundaries and/or fissure planes. The EPR signature was similar for the colored and the corresponding colorless quartz samples, before and after annealing, showing that the heat sensitive coloration of the Albesti quartz does not directly involve the presence of paramagnetic defects and/or minority magnetic phases.

2. In-depth insight into the structural properties of nanoparticulate NiO for CO sensing

Authors: Mihalcea, CG; Stefan, M; Ghica, C; Florea, OG; Stanoiu, A; Simion, CE; Somacescu, S; Ghica, D

Published: APR 1 2024, APPLIED SURFACE SCIENCE, 651, 159252, DOI: 10.1016/j.apsusc.2023.159252

The outstanding properties exhibited by the p-type NiO nanostructures can be greatly affected by morpho-structural and defect characteristics with constructive or competing effects. We have conducted an in-depth study on NiO nanoparticles obtained by hydrothermal synthesis and submitted to various thermal treatments, to monitor the evolution of their structural properties and the effect of the thermal history on their CO sensing. Correlated electron paramagnetic resonance and analytical transmission electron microscopy investigations evidenced an amount of up to 1 % metallic nickel clusters close to surface in the NiO nanoparticles calcined at 400 degrees C and 500 degrees C for 8 h. Subsequent annealing in vacuum and in air of the sample calcined at 400 degrees C resulted in different size distributions and morphology of the NiO nanoparticles and an increase/decrease of the nickel phase, respectively. Comparative CO sensing tests on the two pristine samples and on the sample calcined at 400 degrees C and further annealed in air at 500 degrees C for 2 h showed an increase in the baseline resistance of the later due to the decrease of free charge carriers induced by the dissolution of the nickel clusters. The overall CO sensing results show a strong dependence on the samples thermal history.

3. Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes

Authors: Vlaicu, ID; Stefan, M; Radu, C; Culita, DC; Radu, D; Ghica, D

Published: APR 6 2023, FRONTIERS IN CHEMISTRY, 11, 1154219, DOI: 10.3389/fchem.2023.1154219

Layered zinc hydroxynitrate (ZHN), with the chemical formula Zn-5 (OH)(8) (NO3)(2)center dot 2H(2)O, exhibits a range of special properties such as anion-exchange and intercalation capacity, as well as biocompatibility, making it attractive for a large variety of applications in fields from nanotechnology to healthcare and agriculture. In this study nanocrystalline ZHN doped with 1,000 ppm Mn2+ was prepared by two synthesis methods (coprecipitation and solid state reaction) using similar environment-friendly precursors. The complex morpho-structural [X-ray diffraction, scanning and transmission electron microscopy, textural analysis] and spectroscopic [Fourier transform infrared and electron paramagnetic resonance (EPR)] characterization of the two ZHN nanopowders showed similar crystalline structures with Mn2+ ions localized in the nanocrystals volume, but with differences in their morphological and textural characteristics, as well as in the doping efficiency. ZHN obtained by coprecipitation consists of larger nanoplatelets with more than two times larger specific surface area and pore volume, as well as a dopant concentration than in the ZHN sample obtained by solid state reaction. The thermal stability and the on-set of the structural phase transformation have been investigated at atomic scale with high accuracy by EPR, using Mn2+ as paramagnetic probes. The on-set of the ZHN structural phase transformation toward ZnO was observed by EPR to take place at 110 degrees C and 130 degrees C for the samples prepared by coprecipitation and solid state reaction, respectively, evidencing a manganese induced local decrease of the transformation temperature. Our results contribute to the selection of the most appropriate ZHN synthesis method for specific applications and in the development of new green, cost-effective synthesis routes for Mn2+ doped nano-ZnO.

4. Revolutionizing n-type Co₃O₄ Nanowire for Hydrogen Gas Sensing

Authors: Kumarage, GWC; Zappa, D; Mihalcea, CG; Maraloiu, VA; Stefan, M; Comini, E

Published: 2023 JUL 27 2023, ADVANCED ENERGY AND SUSTAINABILITY RESEARCH, DOI: 10.1002/aesr.202300067

This study presents conductometric sensors based on Co3O4 nanowires for hydrogen detection at ppb levels. The nanowires are synthesized through thermal oxidation of a 50 nm cobalt layer, exhibiting diameters between 6-50 nm and lengths of 1-5 & mu;m, primarily growing along the (311) direction of spinal Co3O4. Raman investigation reveals five characteristic peaks at 195, 482, 521, 620, and 692 cm(-1), corresponding to symmetric phonon modes of crystalline Co3O4. Electron paramagnetic resonance measurements confirm the presence of a ferromagnetic phase, attributed to incomplete cobalt oxidation, which disappears after 8 h of thermal aging at 400 & DEG;C. Conductometry measurements are performed in the temperature range of 300-500 & DEG;C. At temperatures above 300 & DEG;C, sensors exhibit abnormal n-type semiconducting behavior due to lattice oxygen's involvement in the hydrogen sensing mechanism. Operating at 450 & DEG;C in dry air, the sensor shows a higher 232% response to 100 ppm H-2 compared to ethanol, acetone, methane, carbon monoxide, and nitrogen dioxide. Remarkably, the sensor maintains a consistent conductance baseline even under high humidity (90%) for 25 d, with three-cycle repeatability. This distinctive gas-sensing capability is attributed to the catalytic activity and elevated operating temperature.

5. Characterization of defect structures in nanoscaled W-doped TiO₂ tested as supercapacitor electrode materials

Authors: Ammar, AU; Stefan, M; Macavei, SG; Tripon, S; Pana, O; Leostean, C; Vlaicu, ID; Rostas, AM; Erdem, E

Published: JAN 2023, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 34, 8, DOI: 10.1007/s10854-022-09540-8

In this work, Tungsten(W)-doped TiO2 nanoparticles were synthesized using the sol-gel method and were used as electrode materials in supercapacitor applications. The structural and morphological properties of the prepared samples were analyzed by means of XRD, STEM, TEM, and XPS. The analysis of the defect centers was carried out using EPR spectroscopy. The electrochemical analysis of the assembled supercapacitor was done using cyclic voltammetry, galvanostatic cycling with potential limitation technique, potentiostatic electrochemical impedance spectroscopy, and voltage-holding experiments. All the presented samples showed paramagnetic defects in the EPR analysis, while 0.5% W-doped TiO2 showed a maximum signal intensity. The supercapacitor performance from the synthesized electrode material showed highly encouraging results. The equivalent series resistance (R-s) value for all the designs showed values under "1 omega,' which reflects high conductivity. As the maximum EPR intensity comes from TiO2 doped with 0.5% W, the supercapacitor performance of this sample was tested with a newly designed five-electrode system. This design showed superior performance compared to any other used designs with a specific capacitance of 25.5 F g(-1), with an energy density of 14.16 Wh kg (-1 )at 302 kW kg (-1).

6. Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn²⁺ paramagnetic probes

Authors: Vlaicu, ID; Stefan, M; Radu, C; Culita, DC; Radu, D; Ghica, D

Published: APR 6 2023, FRONTIERS IN CHEMISTRY, 11, 1154219, DOI: 10.3389/fchem.2023.1154219

7. Visible-Light-Active Black TiO2 Nanoparticles with Efficient Photocatalytic Performance for Degradation of Pharmaceuticals

Authors: Andronic, L; Ghica, D; Stefan, M; Mihalcea, CG; Vlaicu, AM; Karazhanov, S

Published: AUG 2022, NANOMATERIALS, 12, 2563, DOI: 10.3390/nano12152563

Special attention has recently been paid to surface-defective titanium dioxide and black TiO2 with advanced optical, electrical, and photocatalytic properties. Synthesis of these materials for photodegradation and mineralization of persistent organic pollutants in water, especially under visible radiation, presents interest from scientific and application points of view. Chemical reduction by heating a TiO2 and NaBH4 mixture at 350 degrees C successfully introduced Ti3+ defects and oxygen vacancies at the surface of TiO2, with an increase in the photocatalytic degradation of amoxicillin-an antibiotic that is present in wastewater due to its intense use in human and animal medicine. Three TiO2 samples were prepared at different annealing temperatures to control the ratio between anatase and rutile and were subjected to chemical reduction. Electron paramagnetic resonance investigations showed that the formation of surface Ti3+ defects in a high concentration occurred mainly in the anatase sample annealed at 400 degrees C, contributing to the bandgap reduction from 3.32 eV to 2.92 eV. The reduced band gap enhances visible light absorption and the efficiency of photocatalysis. The nanoparticles of similar to 90 m(2)/g specific surface area and 12 nm average size exhibit similar to 100% efficiency in the degradation of amoxicillin under simulated solar irradiation compared with pristine TiO2. Mineralization of amoxicillin and by-products was over 75% after 48 h irradiation for the anatase sample, where the Ti3+ defects were present in a higher concentration at the catalyst's surface.

8. Hard/soft effects of multivalence co-dopants in correlation with their location in PZT ceramics

Authors: Amarande, L; Cioangher, MC; Toma, V; Miclea, CF; Stefan, M; Pasuk, I; Iuga, AR; Negrila, C; Matei, E; Palici, AM; Joita, AC

Published: DEC 1 2021, CERAMICS INTERNATIONAL, 47, 33389, DOI: 10.1016/j.ceramint.2021.08.243

Piezoelectric hard/soft effects of multivalence co-dopants (Sb and Mn) in correlation with their location in the lattice, were investigated in PZT ceramics, prepared by conventional ceramic technology, with the following compositions: Pb0.98Sr0.02 ((Ti0.49Zr0.51)(1-0.015-x)Mn0.015Sbx)O-3 with x = 0, 0.005, 0.01, 0.02, 0.03, where antimony was initially assumed to substitute for Ti/Zr ions. The antimony valence state was found to be +3 in all samples by X-ray Photoelectron Spectroscopy investigations. The Electron Paramagnetic Resonance spectra evidenced a steep enhancement of the Mn2+ concentration upon increasing antimony doping level, explained by a charge compensation mechanism, between the Sb3+ ions substituting Pb2+ at the A-sites and the Mn2+ ions, localized at the B-sites. The incorporation of Sb3+ at the A-site of the PZT lattice is also supported by the variation of the lattice parameters, determined by X-ray Diffraction, with the increasing Sb concentration. The investigation of the dielectric, electromechanical and ferroelectric properties evidenced a hard piezoelectric behavior, mainly attributed to the presence of large sized Mn2+ ions, localized at B-sites. Our results prove that the piezoelectric hard/soft response is decisively influenced by the interplay between multiple valence states and locations of the co-dopants, on one hand, and the charge compensation mechanisms, on the other hand. This provides indirect information about the location of some co-dopants which can substitute for both cationic sites in the PZT based ceramics.

9. Influence of surfactant-tailored Mn-doped ZnO nanoparticles on ROS production and DNA damage induced in murine fibroblast cells

Authors: Popescu, T; Matei, CO; Vlaicu, ID; Tivig, I; Kuncser, AC; Stefan, M; Ghica, D; Miclea, LC; Savopol, T; Culita, DC; Moisescu, MG

Published: OCT 22 2020, SCIENTIFIC REPORTS, 10, DOI: 10.1038/s41598-020-74816-0

The present study concerns the in vitro oxidative stress responses of non-malignant murine cells exposed to surfactant-tailored ZnO nanoparticles (NPs) with distinct morphologies and different levels of manganese doping. Two series of Mn-doped ZnO NPs were obtained by coprecipitation synthesis method, in the presence of either polyvinylpyrrolidone (PVP) or sodium hexametaphosphate (SHMTP). The samples were investigated by powder X-ray Diffraction, Transmission Electron Microscopy, Fourier-Transform Infrared and Electron Paramagnetic Resonance spectroscopic methods, and N-2 adsorption-desorption analysis. The observed surfactant-dependent effects concerned: i) particle size and morphology; ii) Mn-doping level; iii) specific surface area and porosity. The relationship between the surfactant dependent characteristics of the Mn-doped ZnO NPs and their in vitro toxicity was assessed by studying the cell viability, intracellular reactive oxygen species (ROS) generation, and DNA fragmentation in NIH3T3 fibroblast cells. The results indicated a positive correlation between the specific surface area and the magnitude of the induced toxicological effects and suggested that Mn-doping exerted a protective effect on cells by diminishing the pro-oxidative action associated with the increase in the specific BET area. The obtained results support the possibility to modulate the in vitro toxicity of ZnO nanomaterials by surfactant-controlled Mn-doping.

10. Multidisciplinary characterization of melanin pigments from the black fungus Cryomyces antarcticus

Authors: Pacelli, C; Cassaro, A; Maturilli, A; Timperio, AM; Gevi, F; Cavalazzi, B; Stefan, M; Ghica, D; Onofri, S

Published: JUL 2020, APPLIED MICROBIOLOGY AND BIOTECHNOLOGY, 104, 6395, DOI: 10.1007/s00253-020-10666-0

Melanin is a natural pigment present in almost all biological groups, and is composed of indolic polymers and characterized by black-brown colorization. Furthermore, it is one of the pigments produced by extremophiles including those living in the Antarctic desert, and is mainly involved in their protection from high UV radiation, desiccation, salinity and oxidation. Previous studies have shown that melanized species have an increased capability to survive high level of radiation compared with the non-melanized counterpart. Understanding the molecular composition of fungal melanin could help to understand this peculiar capability. Here, we aimed to characterize the melanin pigment extracted from the Antarctic black fungus Cryomyces antarcticus, which is a good test model for radioprotection researches, by studying its chemical properties and spectral data. Our results demonstrated that, in spite of having a specific type of melanin as the majority of fungi, the fungus possesses the ability to produce both 1,8-dihydroxynaphthalene (DHN) and l 3-4 dihydroxyphenylalanine (L-DOPA) melanins, opening interesting scenarios for the protection role against radiation. Researches on fungal melanin have a huge application in different fields, including radioprotection, bioremediation, and biomedical applications.

11. Electron Small Polaron and Magnetic Interactions Direct Anisotropic Growth of Silicon-Doped Hematite Nanocrystals

Authors: Allieta, M; Beranova, K; Marelli, M; Coduri, M; Stefan, M; Ghica, D; Morello, G; Malara, F; Naldoni, A

Published: JUL 1 2020, CRYSTAL GROWTH & DESIGN, 20, 4730, DOI: 10.1021/acs.cgd.0c00496

Hematite (alpha-Fe2O3) is a promising and Earth-abundant material for solar fuel production, and Si-doping has been employed as a general strategy to improve its performance. However, an atomistic description that reconciles the modifications that Si-doping induces on the morphology, crystalline lattice, and electronic and magnetic properties of alpha-Fe2O3 has remained elusive. Here we report on the role of electron small polarons in driving the morphological transition from nearly rounded-shaped to nanowire nanocrystals in Si-doped hematite alpha-Fe2O3. Electron small polaron formation is evidenced by the formation of Fe2+ and the increase of FeO6 distortion at increasing Si content. Local analysis via pair distribution function highlights an unreported crossover from small to large polarons, which affects the correlation length of the polaronic distortion from short to average scales. Ferromagnetic double exchange interactions between Fe2+/Fe3+ species are found to be the driving force of the crossover, constraining the chaining of chemical bonds along the [110] crystallographic direction. This promotes the increase in the reticular density of Fe atoms along the hematite basal plane only, which boosts the anisotropic growth of nanocrystals with more extended [110] facets. Our results show that magnetic and electronic interactions drive preferential crystallographic growth in Si-doped alpha-Fe2O3, thus providing new insights for the nanoscale structural design of efficient solar fuel devices.

12. SnSe2-Zn-Porphyrin Nanocomposite Thin Films for Threshold Methane Concentration Detection at Room Temperature

Authors: Lorinczi, A; Fagadar-Cosma, E; Socol, G; Mihailescu, A; Matei, E; Sava, F; Stefan, M

Published: DEC 2020, CHEMOSENSORS, 8, DOI: 10.3390/chemosensors8040134

Nanocomposite thin films, sensitive to methane at the room temperature (25-30 degrees C), have been prepared, starting from SnSe2 powder and Zn(II)-5,10,15,20-tetrakis-(4-aminophenyl)- -porphyrin (ZnTAPP) powder, that were fully characterized by XRD, UV-VIS, FT-IR, Nuclear Magnetic Resonance (H-1-NMR and C-13-NMR), Atomic Force Microscopy (AFM), SEM and Electron Paramagnetic Resonance (EPR) techniques. Film deposition was made by drop casting from a suitable solvent for the two starting materials, after mixing them in an ultrasonic bath. The thickness of these films were estimated from SEM images, and found to be around 1.3 mu m. These thin films proved to be sensitive to a threshold methane (CH4) concentration as low as 1000 ppm, at a room temperature of about 25 degrees C, without the need for heating the sensing element. The nanocomposite material has a prompt and reproducible response to methane in the case of air, with 50% relative humidity (RH) as well. A comparison of the methane sensing performances of our new nanocomposite film with that of other recently reported methane sensitive materials is provided. It is suitable for signaling gas presence before reaching the critical lower explosion limit concentration of methane at 50,000 ppm.

13. One-dimensional coordination polymers constructed from copper(II) ions and chromato bridges: Synthesis, crystal structures and thermal analysis

Authors: Dumbrava, A; Maxim, C; Olar, R; Badea, M; Stefan, M; Grecu, MN; Andruh, M

Published: SEP 1 2020, INORGANICA CHIMICA ACTA, 509, DOI: 10.1016/j.ica.2020.119663

14. Nanoclustered Pd decorated nanocrystalline Zn doped SnO2 for ppb NO2 detection at low temperature

Authors: Somacescu, S; Ghica, C; Simion, CE; Kuncser, AC; Vlaicu, AM; Stefan, M; Ghica, D; Florea, OG; Mercioniu, IF; Stanoiu, A

Published: SEP 1 2019, SENSORS AND ACTUATORS B-CHEMICAL, 294, 156, DOI: 10.1016/j.snb.2019.05.033

Nanoclustered Pd (2 mol%) was used to decorate Zn doped SnO2 (10 mol% Zn) in order to increase its sensing performances. Zn doped SnO2 built from nanoparticles was prepared by a hydrothermal method using a nonionic surfactant -Brij52 and Tripropylamine (TPA) as co-templates. The presence of well-dispersed Zn2+ ions in the SnO2 matrix leads to a nonstoichiometric surface. Pd was deposited by subsequent wet impregnation using hydrazine as reducing agent. The as obtained powders were deposited as thick layers onto commercial substrates, in order to obtain the sensitive structures. The coexistence of a mixture of valence states (Pd-0, Pd2+ and Pd4+) was highlighted on the surface of the as prepared layers. Several aspects have been followed regarding the Zn and Pd dispersion into the SnO2 matrix: the large scale and low scale morphology (SEM and TEM/HRTEM) in relation with the synthesis route, the obtained crystallographic phases (XRD, SAED) and the way in which the Zn2+ ions are inserted into the SnO2 structure (XRD, XPS, EPR), the spatial distribution of the added chemical elements, Zn and Pd (SEM, STEM, EDS). All these morphological and structural aspects, as well as the Pd surface chemistry, have been correlated with the sensing properties of the nanostructured materials under controlled gas atmosphere. Through this study, we could harvest the specific role of the aforementioned loadings towards selective detection of low NO2 concentrations, between 350 ppb to 5 ppm, at low operating temperature of 100 degrees C, for infield conditions.

15. Tailoring the Dopant Distribution in ZnO:Mn Nanocrystals

Authors: Ghica, D; Vlaicu, ID; Stefan, M; Maraloiu, VA; Joita, AC; Ghica, C

Published: MAY 3 2019, SCIENTIFIC REPORTS, 9, DOI: 10.1038/s41598-019-43388-z

The synthesis of semiconductor nanocrystals with controlled doping is highly challenging, as often a significant part of the doping ions are found segregated at nanocrystals surface, even forming secondary phases, rather than incorporated in the core. We have investigated the dopant distribution dynamics under slight changes in the preparation procedure of nanocrystalline ZnO doped with manganese in low concentration by electron paramagnetic resonance spectroscopy, paying attention to the formation of transient secondary phases and their transformation into doped ZnO. The acidification of the starting solution in the co-precipitation synthesis from nitrate precursors lead to the decrease of the Mn2+ ions concentration in the core of the ZnO nanocrystals and their accumulation in minority phases, until similar to 79% of the Mn2+ ions were localized in a thin disordered shell of zinc hydroxynitrate (ZHN). A lower synthesis temperature resulted in polycrystalline Mn-doped ZHN. Under isochronal annealing up to 250 degrees C the bulk ZHN and the minority phases from the ZnO samples decomposed into ZnO. The Mn2+ ions distribution in the annealed nanocrystals was significantly altered, varying from a uniform volume distribution to a preferential localization in the outer layers of the nanocrystals. Our results provide a synthesis strategy for tailoring the dopant distribution in ZnO nanocrystals for applications ranging from surface based to ones involving core properties.

16. X-ray Crystal Structure, Geometric Isomerism, and Antimicrobial Activity of New Copper(II) Carboxylate Complexes with Imidazole Derivatives

Authors: Vlaicu, ID; Borodi, G; Scaeteanu, GV; Chifiriuc, MC; Marutescu, L; Popa, M; Stefan, M; Mercioniu, IF; Maurer, M; Daniliuc, CG; Olar, R; Badea, M

Published: DEC 2018, MOLECULES, 23, DOI: 10.3390/molecules23123253

Five new copper(II) acrylate complexes (acr is the acrylate anion: C3H3O2) with imidazole derivatives (2-methylimidazole/2-MeIm, 5-methylimidazole/5-MeIm, 2-ethylimidazole/2-EtIm) of type: cis-[Cu(2-RIm)(2)(acr)(2)]xH(2)O ((1): R = -CH3, x = 2; (4): R = -CH2-CH3, x = 0), trans-[Cu(2-RIm)(2)(acr)(2)] ((2): R = -CH3; (5): R = -CH2-CH3) and trans-[Cu(5-RIm)(2)(acr)(2)] ((3): R = -CH3) have been prepared and characterized by elemental analysis, Fourier Transform Infrared spectrometry (FTIR), Electron Paramagnetic Resonance (EPR), electronic reflectance spectroscopy, scanning electron microscopy, and mass spectrometry. The single crystal X-ray diffraction study of complexes (2) and (5) reveals that the copper(II) ion is located on an inversion center and show elongated octahedral geometry completed by two coplanar bidentate acrylates and two unidentate imidazole derivatives displayed in trans positions. For complex (4) the single crystal X-ray diffraction shows that the copper(II) ion is in a distorted octahedral environment which can be easily confused with a trigonal prism completed by two bidentate acrylates and two unidentate imidazole derivatives displayed in cis positions. These results indicate the fact that complexes (4) and (5) are the geometric isomers of the same compound bis(acrylate)-bis(2-ethylimidazole)-copper(II). Complexes (1) and (2), as well as (4) and (5), were produced simultaneously in the reaction of the corresponding copper(II) acrylate with imidazole derivatives in methanol solution. Furthermore, in order to be able to formulate potential applications of the obtained compounds, our next goal was to investigate the in vitro antimicrobial activity of the synthesized complexes against Gram-positive and Gram-negative bacteria, as well as fungal strains, of both clinical and ecological importance (biodeterioration of historical buildings). The trans isomers (2) and (5), followed by (4) have shown the broadest range of antimicrobial activity. In case of (1) and (2) isomers, the trans isomer (2) was significantly more active than cis (1), while the cis isomer (4) proved to be more active than trans (5). Taken together, the biological evaluation results indicate that the trans (2) was the most active complex, demonstrating its potential for the development of novel antimicrobial agents, with potential applications in the biomedical and restoration of architectural monuments fields.

17. Limits and Particularities of the Synthesis of Ba1-xCaxTiO3 for Piezoelectric Applications, by Topochemical Conversion from Molten Salt Solutions

Authors: Vlaicu, ID; Maraloiu, AV; Ghica, D; Mercioniu, IF; Stefan, M; Vlaicu, AM; Negrea, RF; Kuncser, AC; Bulat, S; Krzmanc, MM; Ciobanu, R; Plopa, O

Published: 2018, 2018 INTERNATIONAL CONFERENCE AND EXPOSITION ON ELECTRICAL AND POWER ENGINEERING (EPE), 1050, DOI:

Because of the unique properties of the ferroelectric perovskite particles with a well-defined anisotropic form like shape-and size dependent at low dimensions they have all the attention of the scientific world. Extensive morphostructural techniques will be used to characterize the piezoelectric material.

18. Mn2+ ions distribution in doped sol-gel deposited ZnO films

Authors: Stefan, M; Ghica, D; Nistor, SV; Maraloiu, AV; Plugaru, R

Published: FEB 28 2017, APPLIED SURFACE SCIENCE, 396, 1889, DOI: 10.1016/j.apsusc.2016.02.167

The localization and distribution of the Mn2+ ions in two sol-gel deposited ZnO films doped with different manganese concentrations were investigated by electron paramagnetic resonance spectroscopy and analytical transmission electron microscopy. In the lightly doped sample the Mn2+ ions are mainly localized substitutionally at isolated tetrahedrally coordinated Zn2+ sites in both crystalline ZnO nanograins (34%) and surrounding disordered ZnO (52%). In the highly doped ZnO film, a much smaller proportion of manganese substitutes Zn2+ in the crystalline and disordered ZnO (10%). The main amount (85%) of manganese aggregates in a secondary phase as an insular-like distribution between the ZnO nanograins. The remaining Mn2+ ions (14% and 5% at low and high doping levels, respectively) are localized at isolated, six-fold coordinated sites, very likely in the disordered intergrain region. Annealing at 600 degrees C induced changes in the Mn2+ ions distribution, reflecting the increase of the ZnO crystallization degree, better observed in the lightly doped sample. (C) 2016 Elsevier B.V. All rights reserved.

19. Origin and chemical composition of the amorphous material from the intergrain pores of self-assembled cubic ZnS:Mn nanocrystals

Authors: Stefan, M; Vlaicu, ID; Nistor, LC; Ghica, D; Nistor, SV

Published: DEC 31 2017, APPLIED SURFACE SCIENCE, 426, 350, DOI: 10.1016/j.apsusc.2017.07.172

We have shown in previous investigations that the low temperature collective magnetism observed in mesoporous cubic ZnS:Mn nanocrystalline powders prepared by colloidal synthesis, with nominal doping concentrations above 0.2 at.%, is due to the formation of Mn2+ clusters with distributed antifer romagnetic coupling localized in an amorphous phase found between the cubic ZnS:Mn nanocrystals. Here we investigate the composition, origin and thermal annealing behavior of this amorphous phase in such a mesoporous ZnS:Mn sample doped with 5 at.% Mn nominal concentration. Correlated analytical transmission electron microscopy, multifrequency electron paramagnetic resonance and Fourier transform infrared spectroscopy data show that the amorphous nanomaterial consists of unreacted precursor hydrated zinc and manganese acetates trapped inside the pores and on the surface of the cubic ZnS nanocrystals. The decomposition of the acetates under isochronal annealing up to 270 degrees C, where the mesoporous structure is still preserved, lead to changes in the nature and strength of the magnetic inter actions between the aggregated Mn2+ ions. These results strongly suggest the possibility to modulate the magnetic properties of such transition metal ions doped II-VI mesoporous structures by varying the synthesis conditions and/or by post-synthesis thermochemical treatments. (C) 2017 Elsevier B.V. All rights reserved.

20. Aggregates of Mn2+ Ions in Mesoporous Self-Assembled Cubic ZnS:Mn Quantum Dots: Composition, Localization, Structure, and Magnetic Properties

Authors: Nistor, SV; Stefan, M; Nistor, LC; Kuncser, V; Ghica, D; Vlaicu, ID

Published: JUL 7 2016, JOURNAL OF PHYSICAL CHEMISTRY C, 120, 14466, DOI: 10.1021/acs.jpcc.6b04866

The source of collective magnetism in II-VI semiconductor quantum dots (QDs) doped with Mn2+ ions at high nominal impurity levels is still under debate. In the particular case of mesoporous, self-assembled cubic ZnS:Mn QDs, quantitative electron paramagnetic resonance (EPR) studies have shown that the Mn2+ ions incorporated in the core and on the surface of the QDs cannot be responsible for the observed collective magnetism because they remain in a diluted paramagnetic state up to the 50 000 ppm nominal concentration. Here we investigate the composition, localization, structure, and magnetic properties of the aggregates of Mn2+ ions incorporated in the mesoporous cZnS:Mn as a possible source of the observed collective magnetism. Samples of mesoporous cubic ZnS:Mn prepared by coprecipitation at several nominal impurity levels from 200 to 50 000 ppm are investigated by EPR, magnetometry, and analytical high resolution (scanning) transmission electron microscopy. The low temperature magnetic properties of the Mn2+ aggregates change from paramagnetic-like, for samples with nominal impurity levels up to 2000 ppm, to ones specific to larger clusters with distributed antiferromagnetic coupling at higher concentrations, behaving superparamagnetically above a certain temperature. There is also strong evidence that the Mn2+ aggregates responsible for the observed low temperature collective magnetism are incorporated as an amorphous phase of mainly Mn-Zn-O composition, localized in the interstices and pores of the mesoporous structure of the cubic ZnS:Mn QDs.

21. Ferritin surplus in mouse spleen 14 months after intravenous injection of iron oxide nanoparticles at clinical dose

Authors: Tamion, A; Hillenkamp, M; Hillion, A; Maraloiu, VA; Vlaicu, ID; Stefan, M; Ghica, D; Rositi, H; Chauveau, F; Blanchin, MG; Wiart, M; Dupuis, V

Published: AUG 2016, NANO RESEARCH, 9, 2410, DOI: 10.1007/s12274-016-1126-6

In this study, we followed the biodegradation of ultra-small superparamagnetic iron oxide nanoparticles injected intravenously at clinical doses in mice. An advanced fitting procedure for magnetic susceptibility curves and low-temperature hysteresis loops was used to fully characterize the magnetic size distribution as well as the magnetic anisotropy energy of the injected P904 nanoparticles (Guerbet Laboratory). Additional magnetometry measurements and transmission electronic microscopy observations were systematically performed to examine dehydrated samples from the spleen and liver of healthy C57B16 mice after nanoparticle injection, with sacrifice of the mice for up to 14 months. At 3 months after injection, the magnetic properties of the spleen and liver were dramatically different. While the liver showed no magnetic signals other than those also present in the reference species, the spleen showed an increased magnetic signal attributed to ferritin. This surplus of ferritin remained constant up to 14 months after injection.

22. On the agent role of Mn2+ in redirecting the synthesis of Zn(OH)(2) towards nano-ZnO with variable morphology

Authors: Ghica, D; Vlaicu, ID; Stefan, M; Nistor, LC; Nistor, SV

Published: 2016, RSC ADVANCES, 6, 106741, DOI: 10.1039/c6ra23065b

One of the simplest routes to prepare polycrystalline Zn(OH)(2) is by coprecipitation, with zinc nitrate as a cation source. However, the addition of even minute amounts of manganese nitrate to the precursors used to prepare pure Zn(OH)(2) results in Mn2+ doped nanostructured ZnO. The comparison with other Mn2+ doped metal hydroxides prepared by the same coprecipitation method, involving metal nitrates precursors, shows that this behavior is unique, pertaining only to Zn(OH)(2). A systematic study of the samples prepared without and with variable amounts of Mn2+ ions, in the 1 to 5000 ppm nominal concentrations range showed that the re-routing of the reaction takes place even for the lowest nominal dopant concentration of 1 ppm. According to X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy investigations, both crystallite size and morphology of the resulting nanostructured ZnO samples varied with the Mn2+ nominal concentration. Moreover, quantitative electron paramagnetic resonance investigations showed that the incorporation rate of the Mn2+ ions at different sites in the nanostructured ZnO depended on the nominal Mn2+ concentration. The results are discussed in terms of the coordination properties of the Mn2+ and Zn2+ ions and the nature of the reaction precursors.

23. Distribution and interaction of Mn2+ ions incorporated in cubic ZnS quantum dots over a broad concentration range

Authors: Nistor, SV; Stefan, M; Nistor, LC; Ghica, D; Vlaicu, ID

Published: MAR 25 2016, JOURNAL OF ALLOYS AND COMPOUNDS, 662, 199, DOI: 10.1016/j.jallcom.2015.11.203

The distribution and interaction of isolated Mn2+ impurity ions incorporated in 2.9 nm average diameter cubic ZnS quantum dots (QDs), prepared by surfactant-assisted liquid-liquid synthesis with initial impurity concentrations in the 20-50,000 ppm range, has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The well resolved spectra, observed in the whole investigated concentration range, reflect the localization of the Mn2+ ions in the core and on the surface of the cZnS: Mn QDs at isolated sites. According to the analysis of the dependences of the concentration of incorporated isolated Mn2+ ions vs. initial impurity concentration and of the core localized Mn2+ ions spectra line-width vs. actual concentration, the isolated Mn2+ ions remain in the whole concentrations range in a diluted paramagnetic state characterized by dipolar magnetic interactions. Pulse EPR measurements of the spinespin dipolar interaction for the incorporated Mn2+ ions confirm their diluted distribution, which excludes these ions as a possible source of collective magnetism properties. (C) 2015 Elsevier B.V. All rights reserved.

24. Revealing the Cu2+ ions localization at low symmetry Bi sites in photorefractive Bi12GeO20 crystals doped with Cu and V by high frequency EPR

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Ramaz, F; Briat, B

Published: OCT 2015, JOURNAL OF MAGNETIC RESONANCE, 259, 94, DOI: 10.1016/j.jmr.2015.07.009

The sites of incorporation of Cu2+ impurity ions in Bi12GeO20 single crystals co-doped with copper and vanadium have been investigated by electron paramagnetic resonance (EPR). While the X-band EPR spectra consist of a simple broad (Delta B similar to 50 mT) line with anisotropic lineshape, the W-band EPR spectra exhibit well resolved, strongly anisotropic lines, due to transitions within the 3d(9)-D-2 ground manifold of the Cu2+ ions. The most intense group of lines, attributed to the dominant Cu2+(I) center, displays a characteristic four components hyperfine structure for magnetic field orientations close to a < 110 > direction. The g and A tensor main axes are very close to one of the 12 possible sets of orthogonal < 1-10 >, < 00-1 > and < 110 > crystal directions. Several less intense lines, with unresolved hyperfine structure and similar symmetry properties, mostly overlapped by the Cu2+(I) spectrum, were attributed to Cu-2 (+)(II) centers. The two paramagnetic centers are identified as substitutional Cu2+ ions at Bi3+ sites with low C-1 symmetry, very likely resulting from different configurations of neighboring charge compensating defects. (C) 2015 Elsevier Inc. All rights reserved.

25. Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a Broad Nominal Concentration Range

Authors: Nistor, SV; Stefan, M; Nistor, LC; Ghica, D; Vlaicu, ID; Joita, AC

Published: OCT 15 2015, JOURNAL OF PHYSICAL CHEMISTRY C, 119, 23789, DOI: 10.1021/acs.jpcc.5b08113

Although impurity doping of nanocrystals is essential in controlling their physical properties for various applications, the doping mechanism of ultrasmall, colloidal II-VI semiconductor nanocrystals, corresponding to the initial stages of growth, is not yet understood. In this study the concentrations of Mn2+ ions in the core, on the surface, and as an agglomerated separate phase in 2.9 nm cubic ZnS nanocrystals, prepared by a surfactant-assisted liquid liquid synthesis within 20 to 20 000 ppm nominal impurity concentration range, have been determined by quantitative multifrequency electron paramagnetic resonance. The unexpected strong decrease in the core doping efficiency with the nominal concentration increase, in contrast to the small variation of the doping efficiency for the surface-bound Mn2+ ions, and the sizable core doping efficiency observed for 1.8 nm nanocrystals were explained with the extended lattice defect assisted mechanism of incorporation. According to this mechanism, which is not size or shape limited, being active from the initial growth stages, the incorporation of Mn2+ ions takes place at surface sites with high binding energy on dislocation steps formed by the emerging stacking defects. High resolution transmission electron microscopy confirms the presence of such stacking defects in a large proportion of the investigated cubic ZnS nanocrystals, ensuring the operation of the proposed doping mechanism.

26. 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, SCIENTIFIC REPORTS, 5, 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.

27. Pulse annealing electron paramagnetic resonance with probing transition ions

Authors: Nistor, SV; Stefan, M; Ghica, D

Published: NOV 2014, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 118, 1031, DOI: 10.1007/s10973-014-3743-1

The analysis of the sequence of electron paramagnetic resonance (EPR) spectra of trace amounts of substitutional probing paramagnetic ions incorporated in (nano)crystalline samples submitted to isothermal and isochronal pulse annealing treatments can offer a wealth of information on the thermally induced compositional and structural changes of the host material. The potential of this new thermal analysis method is illustrated here with results of such investigations on the thermal decomposition of crystalline zinc hydroxide (Zn(OH)(2)) and anhydrous zinc carbonate basic (Zn-5(CO3)(2)(OH)(6)) precursors containing trace amounts of substitutional Mn2+ probing ions into nanostructured zinc oxide-ZnO. The quantitative analysis of the sequence of isochronal pulse annealing EPR spectra could provide, besides the thermal decomposition curves of the two precursors, additional information about the structure of the resulting nanostructured ZnO, some of it hard to get by standard structural diffraction techniques. The analysis of both isochronal and isothermal pulse annealing EPR data was further used to investigate the crystallization mechanism of the initially formed nanostructured disordered ZnO and to quantitatively describe the further growth of the resulting ZnO nanocrystals with the increasing annealing temperature and duration.

28. ZnS and ZnO Semiconductor Nanoparticles Doped with Mn2+ Ions. Size Effects Investigated by EPR Spectroscopy

Authors: Stefan, M; Nistor, SV; Ghica, D

Published: 2014, SIZE EFFECTS IN NANOSTRUCTURES: BASICS AND APPLICATIONS, 205, 27, DOI: 10.1007/978-3-662-44479-5_1

Electron paramagnetic resonance (EPR) spectroscopy has been extensively employed to investigate the presence, localization, distribution and interaction with the host crystalline lattice of the paramagnetic point defects (intrinsic defects and transition metal ions) in semiconductors. The retrieval of such information for nanostructured semiconductors is considerably more difficult, due to the high disorder level in such systems, reflected in broad, featureless EPR spectra. We show here how, with proper adjustments of the EPR experiments and accurate numerical analysis of the resulting spectra, it was possible to obtain more accurate information regarding the localization and structure of various Mn2+ centers in ZnS and ZnO semiconductor nanoparticles (NPs). This lead to the observation and investigation of size related effects such as the presence of the extended lattice defect assisted incorporation of impurities in small (similar to 3 nm) cubic ZnS NPs, the dominant size induced lattice disorder observed for ZnO NPs, independent of the synthesis procedures, or the three steps decomposition of the epsilon-Zn(OH)(2) disordered shell of ZnS NPs with formation of new oxy-hydrated zinc compounds. These effects can be used to synthesize semiconductor nanoparticles with controlled size distribution, doping level and functionalized surfaces for specific technological applications.

29. Evaluation of the Segregation of Paramagnetic Impurities at Grain Boundaries in Nanostructured ZnO Films

Authors: Ghica, D; Stefan, M; Ghica, C; Stan, GE

Published: AUG 27 2014, ACS APPLIED MATERIALS & INTERFACES, 6, 14238, DOI: 10.1021/am5035329

Magnetic and electrical properties of the nanostructured ZnO films are affected by the nonrandom distribution of impurities in the film due to segregation at grain boundaries (GBs) or extended defects. However, mapping the nature and distribution of the impurities in the film is not trivial. Here we demonstrate a simple, statistically relevant, and nondestructive procedure of quantitative determination of the paramagnetic impurities segregated at the GBs in nanostructured semiconducting and insulating films. From correlated electron paramagnetic resonance and transmission electron microscopy investigations, we determined the localization of trace amounts of Mn2+ ions, present as native impurities in a ZnO film deposited by magnetron sputtering at room temperature. In the as-deposited ZnO film, the Mn2+ ions were all localized in nanosized pockets of highly disordered ZnO dispersed between nanocrystalline columns. After the samples had been annealed in air at >400 degrees C, the size of the intercrystalline region decreased and the diffusion in GBs was activated, resulting in the localization of a portion of the Mn2+ ions in the peripheral atomic layers of the ZnO columns neighboring the GBs. The proportion of Mn2+ ions still localized at the GBs after annealing at 600 degrees C was 37%. The proposed method for the assessment of the presence and nature of impurities and the quantitative evaluation of their distribution in semiconducting and insulating nanostructures is expected to find direct applications in nanotechnology, in the synthesis and quality assurance of thin films for spintronics and opto- and nanoelectronics.

30. Magnetic defects in crystalline Zn(OH)(2) and nanocrystalline ZnO resulting from its thermal decomposition

Authors: Nistor, SV; Ghica, D; Stefan, M; Vlaicu, I; Barascu, JN; Bartha, C

Published: JAN 25 2013, JOURNAL OF ALLOYS AND COMPOUNDS, 548, 227, DOI: 10.1016/j.jallcom.2012.09.016

Trace amounts of substitutional Mn2+ ions and shallow donors magnetic centers were identified by electron paramagnetic resonance (EPR) in crystalline Zn(OH)(2) prepared by precipitation of a Zn-nitrate solution with NaOH. Strong changes in the Mn2+ ions spectrum, as well as a sharp increase in the concentration of the shallow donor centers were observed by EPR in the 110-140 degrees C temperature range, during pulse annealing experiments in air up to 240 degrees C. They reflect the decomposition of the crystalline Zn(OH)(2) host lattice into nanocrystalline ZnO, confirmed by X-ray diffraction and thermal analysis measurements. Accurate spin Hamiltonian parameters of the observed paramagnetic centers were determined by lineshape simulation and fitting of the EPR spectra, to be used as reference data in further studies of nanocrystalline systems involving Zn(OH)(2). (C) 2012 Elsevier B.V. All rights reserved.

31. Nanosize induced effects in luminescent ZnS:Mn2+ quantum dots

Authors: Nistor, SV; Stefan, M; Ghica, D; Nistor, LC

Published: SEP 2013, RADIATION MEASUREMENTS, 56, 43, DOI: 10.1016/j.radmeas.2013.01.046

Nanocrystals (NCs) of II-VI semiconductors of few nanometers average size, called quantum dots (QDs), are now intensely investigated as radiation detectors. Besides the expected quantum confinement and influence of surface states, our electron paramagnetic resonance investigations of cZnS QDs doped with Mn2+ ions, correlated with structural data, underline that other properties should be also taken into consideration in developing the II VI semiconductor QDs as radiation detectors. Thus, the preferential localization of Mn2+ in the core of the cubic ZnS QDs at substitutional Zn2+ cation sites next to a stacking lattice defect is expected to lead, besides changes in the impurity energy levels, to specific aggregation properties. An outer shell of different composition can also influence the structural properties of the QDs core with effects on the optical properties as well. (C) 2013 Elsevier Ltd. All rights reserved.

32. Sequential Thermal Decomposition of the Shell of Cubic ZnS/Zn(OH)(2) Core-Shell Quantum Dots Observed With Mn2+ Probing Ions

Authors: Nistor, SV; Ghica, D; Stefan, M; Nistor, LC

Published: OCT 24 2013, JOURNAL OF PHYSICAL CHEMISTRY C, 117, 22028, DOI: 10.1021/jp4063093

Thermally induced changes in the structure and composition of the shell of tightly aggregated cubic ZnS/Zn(OH)(2) core shell quantum dots of 1.9 nm average core size were investigated by multifrequency electron paramagnetic resonance of Mn2+ probing ions. The observed three-steps temperature induced transformation of the Mn2+ surface centers in the 80-450 degrees C temperature range Zn(OH)(2) shell into ZnO, with the formation of the Zn2O(OH)(2) and Zn4O3(OH)(2) intermediate nanocompounds. The presence of a 0.3 to 1.9 nm thick surface layer of disordered nanomaterial separating the cubic ZnS cores and its shrinking to a few atomic layers by mass loss after annealing up to 350 degrees C was observed by high resolution transmission electron microscopy. Unlike the single step dehydration around 120 degrees C of the bulk epsilon-Zn(OH)(2), the complex decomposition of the epsilon-Zn(OH)(2) shell is attributed to its nanosized, disordered structure.

33. Correlation of Lattice Disorder with Crystallite Size and the Growth Kinetics of Mn2+ Doped ZnO Nanocrystals Probed by Electron Paramagnetic Resonance

Authors: Stefan, M; Nistor, SV; Ghica, D

Published: MAR 2013, CRYSTAL GROWTH & DESIGN, 13, 1359, DOI: 10.1021/cg400037c

The correlation of the lattice disorder with the nanocrystal average size, in ZnO nanocrystals synthesized by several different methods, has been quantitatively monitored by line shape analysis of the multifrequency electron paramagnetic resonance (EPR) spectra of low concentrations of substitutional Mn2+ probing ions. The observed correlation between the line broadening parameter of the spectrum and the average ZnO nanocrystals size, independent of the synthesis procedure of the ZnO nanocrystals, demonstrates the dominance of the size related strain/disorder. On the basis of this result, a new method for determining the average ZnO nanocrystal size from the quantitative analysis of the EPR spectra of the Mn2+ probes was derived. The nanocrystallization of the disordered ZnO formed by the thermal decomposition of hydrozincite was monitored using this procedure. The observed ZnO nanocrystallite growth kinetics at lower temperatures was described by a structural relaxation mechanism consisting of the local ordering by rearrangements of the atoms in the interfaces/grain boundaries, with a growth activation energy of similar to 23 kJ/mol. When the nanostructured ZnO was more than 75% crystallized, another growth mechanism of the nanocrystals was found to occur, driven by the reduction of the total grain boundary energy.

34. Accurate determination of the spin Hamiltonian parameters for Mn2+ ions in cubic ZnS nanocrystals by multifrequency EPR spectra analysis

Authors: Stefan, M; Nistor, SV; Barascu, JN

Published: JUN 2011, JOURNAL OF MAGNETIC RESONANCE, 210, 209, DOI: 10.1016/j.jmr.2011.03.004

Accurate determination of the spin Hamiltonian (SH) parameters, describing the electron paramagnetic resonance (EPR) spectra of paramagnetic impurity ions in wide band gap semiconductor nanocrystals, is essential for determining their localization and quantum properties. Here we present a procedure, based on publicly available software, for determining with higher accuracy the SH parameters of isolated Mn2+ impurity ions in small cubic ZnS nanocrystals. The procedure, which can be applied to other cubic II-VI semiconductor nanocrystals as well, is based on the analysis of both low and high frequency EPR spectra with line shape simulation and fitting computing programs, which include the hyperfine forbidden transitions and line broadening effects. The difficulties, limitations and errors which can affect the accuracy in determining some of the SH parameters are also discussed. (C) 2011 Elsevier Inc. All rights reserved.

35. Substitutional and surface Mn2+ centers in cubic ZnS:Mn nanocrystals. A correlated EPR and photoluminescence study

Authors: Stefan, M; Nistor, SV; Ghica, D; Mateescu, CD; Nikl, M; Kucerkova, R

Published: JAN 20 2011, PHYSICAL REVIEW B, 83, DOI: 10.1103/PhysRevB.83.045301

The EPR, radioluminescence, and photoluminescence of cubic ZnS (cZnS) nanocrystals (NCs) with a narrow size distribution centered at 2 nm, doped with 0.1, 0.2, and 0.5 at.% Mn2+ ions were investigated. Besides the main lines from substitutional Mn2+ ions localized in the core of the NCs next to a stacking defect, the EPR spectra exhibited two broader hyperfine sextets, attributed to the so-called Mn(II)and Mn(III) surface centers, which could be separated by adequate thermal treatments. The contribution to the photoluminescence from the Mn2+ ions at various sites was further determined from the analysis of the steady-state and time-resolved photoluminescence data from cZnS: Mn NCs subjected to thermal treatments and from cZnS: Mn single crystals. Thus, the main emission consisting of two intense overlapping bands peaking at 596 and 630 nm was attributed to the T-4(1)-(6)A(1) transition of the substitutional Mn2+ ions in the core of the cZnS nanocrystals and to residual aggregated Mn2+ ions, respectively, the last ones being responsible for a broad EPR line observed in the X-band spectrum. The Mn(II) and Mn(III) centers, consisting of Mn2+ ions in the oxidized and hydrolyzed surface layer of the NCs, respectively, are only indirectly involved in the energy transfer to the substitutional Mn2+ centers, very likely through pairs interaction.

36. Structural phase transformations in annealed cubic ZnS nanocrystals

Authors: Ghica, D; Nistor, SV; Nistor, LC; Stefan, M; Mateescu, CD

Published: SEP 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 4335, DOI: 10.1007/s11051-011-0379-y

The structural changes of cubic ZnS (cZnS) nanocrystals (NCs) doped with 0.2 at.% Mn2+ pulse annealed in vacuum and in air, up to 500 A degrees C, were investigated by multifrequency electron paramagnetic resonance (EPR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples, prepared by a surfactant (Tween20)-assisted liquid-liquid reaction at pH = 6, consist of NCs with a tight size distribution around 3 nm and high crystallinity self-assembled into a stable mesoporous structure. The EPR spectra of the as prepared samples contain only the characteristic lines of the substitutional Mn2+(I) centers. No spectra from Mn2+ ions localized in (hydro)oxidized regions of the NCs surface were observed. The absence of such a surface layer could explain the stability of the cubic (sphalerite) structure observed by XRD and TEM in the investigated cZnS:Mn NCs annealed in vacuum up to 500 A degrees C. The observation of the cubic-hexagonal transformation for the same NCs annealed in air supports the role of such layer in promoting this structural transformation. The narrowing of the EPR spectral lines above 200 A degrees C with the increase in the average size of the cZnS:Mn crystallites was observed. The effect was more pronounced for the sample annealed in air. EPR also revealed the formation of minute amounts of substitutional Mn2+-type centers in a hexagonal ZnO structure at T similar to 300 A degrees C, corresponding to the early stages of the thermally induced oxidation of the cZnS:Mn NCs.

37. Crystallization of Disordered Nanosized ZnO Formed by Thermal Decomposition of Nanocrystalline Hydrozincite

Authors: Nistor, SV; Nistor, LC; Stefan, M; Ghica, D; Aldica, G; Barascu, JN

Published: NOV 2011, CRYSTAL GROWTH & DESIGN, 11, 5038, DOI: 10.1021/cg2009286

The formation and crystallization of disordered nanosized ZnO resulting from the thermal decomposition of nanocrystalline hydrozincite [Zn-5(CO3)(2)(OH)(6)] has been Observed and investigated during pulse annealing experiments Up to 625 degrees C in air or vacuum by electron paramagnetic resonance of trace amounts of substitutional Mn2+ impurity ions, in correlation with X-ray diffraction and transmission electron microscopy measurements. The mesoporous structure of the disordered ZnO, which initially forms in air and vacuum at 225 and 175 degrees C, respectively, further transforms into nanocrystalline ZnO of increasing particle size and improved lattice quality at higher annealing temperatures. The crystallization process, which does not affect the concentration of the substitutional impurity ions, as well as the simultaneous presence of both disordered and crystalline phases, should be considered in further applications of the resulting nanosized ZnO.

38. Local Structure at Mn2+ Ions in Vacuum Annealed Small Cubic ZnS Nanocrystals Self-Assembled Into a Mesoporous Structure

Authors: Nistor, SV; Ghica, D; Nistor, LC; Stefan, M; Mateescu, CD

Published: OCT 2011, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 11, 9303, DOI: 10.1166/jnn.2011.4306

A mesoporous structure of self-assembled nanocrystals of cubic ZnS doped with Mn2+ ions with a homogeneous distribution of pores of similar size was synthesized at room temperature by a surfactant-assisted liquid-liquid reaction. The component nanocrystals exhibit a high crystallinity and a tight size distribution centered at 2 nm, as well as the narrowest Electron Paramagnetic Resonance (EPA) spectra linewidth and the best resolution reported so-far, effects attributed to self-assembling. The observed EPA spectra consist of lines from the substitutional Mn2+(I) and surface Mn2+(II) and Mn2+(III) centers. Here we show that, in contrast with previous reports, our EPA spectra are highly sensitive to structural changes during pulse annealing in vacuum up to 500 degrees C. The changes are related to the transformation of the surface Mn2+ centers in new Mn2+ centers, attributed to an oxidation process in which the thermal decomposition of the Tween 20 additive, also observed by EPA, seems to be involved. We have also been able to observe, for the first time by EPR spectroscopy, the formation of the ZnO phase and the nanocrystals size increase, which occur during annealing up to 500 degrees C, structural changes confirmed by XRD and TEM observations on the samples previously investigated by EPR.

39. Specificity of defects induced in silicon by RF-plasma hydrogenation

Authors: Ghica, C; Nistor, LC; Stefan, M; Ghica, D; Mironov, B; Vizireanu, S; Moldovan, A; Dinescu, M

Published: MAR 2010, APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING, 98, 785, DOI: 10.1007/s00339-009-5527-1

Silicon wafers have been submitted to hydrogen RF-plasma treatment in various experimental conditions. Hydrogen RF-plasma treatment induced two kinds of effects on Si wafers, depending on the treatment conditions: surface corrugation and formation of structural defects below the free surface. Atomic force microscopy (AFM) investigations showed that the surface roughness significantly increased with the treatment duration, leading to the formation of pyramidal humps on the surface. The structural defects resulting after the plasma treatments were investigated by conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) techniques. The specificity of the induced extended defects due to hydrogen decoration was emphasized. Three types of extended defects were identified and characterized: planar defects in the {111} and {100} planes and nanometric voids. Point defects related to the hydrogenation process were investigated by electron paramagnetic resonance (EPR) in correlation with the electron microscopy results.

40. Study of the Kramers rare earth ions ground multiplet with a large orbital contribution by multifrequency EPR spectroscopy: Ce3+ in PbWO4 scintillator

Authors: Popescu, FF; Bercu, V; Barascu, JN; Martinelli, M; Massa, CA; Pardi, LA; Stefan, M; Nistor, SV; Nikl, M

Published: MAR 2010, OPTICAL MATERIALS, 32, 575, DOI: 10.1016/j.optmat.2009.11.015

A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g(parallel to) and g_, are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g_-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S-4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce3+ ions with tetragonal symmetry S-4 in the PbWO4 scintillator. (c) 2009 Elsevier BM. All rights reserved.

41. IMPROVING THE CUBIC ZnS NANOCRYSTALS QUALITY BY SELF-ASSEMBLING INTO A MESOPOROUS STRUCTURE

Authors: Nistor, SV; Nistor, LC; Stefan, M; Ghica, D; Mateescu, CD; Birjega, R

Published: 2010, ROMANIAN REPORTS IN PHYSICS, 62, 328, DOI:

Nanocrystals of cubic ZnS (cZnS) doped with 02 % mol Mn2+, self-assembled into a mesoporous structure, have been prepared at room temperature by a surfactant-assisted liquid-liquid reaction The X-ray diffraction measurements confirm the formation of a sponge-like mesoporous structure built from ZnS nanocrystals with cubic (sphalerite) structure and pores of similar diameter of (1 8 +/- 0 2) nm. The Transmission Electron Microscopy (TEM) images show that the mesoporous structure consists of nanocrystals of cZnS with a tight size distribution centered around the average diameter value (21 +/- 0 3) nm The analysis of the observed Election Paramagnetic Resonance (EPR) spectrum demonstrates the presence of the Mn2+ activating ions at isolated sites in the mesoporous material, resulting in three types of paramagnetic centers called Mn2+(I), Mn2+(II) and Mn2+(III) The EPR spectrum of the Mn2+(I) center, attributed to substitutional Mn2+ ions at Zn2+ cation sites in the ZnS nanocrystals, exhibits the smallest linewidth values reported so far, reflecting an increased lattice ordering The high quality of the nanocrystals forming the mesoporous cZnS Mn, as reflected in a tight nanocrystallites size distribution and reduced crystallites lattice disorder, is attributed to the restraining effect of the self-assembling

42. Multifrequency ESR Characterization of Paramagnetic Point Defects in Semiconducting Cubic BN Crystals

Authors: Nistor, SV; Stefan, M; Ghica, D; Goovaerts, E

Published: OCT 2010, APPLIED MAGNETIC RESONANCE, 39, 101, DOI: 10.1007/s00723-010-0136-x

Low-frequency (X-band) electron spin resonance (ESR) investigations on commercially available large-grained cubic boron nitride (cBN) superabrasive powders of various coloration, combined with high-frequency (W-band) ESR measurements on oriented submillimeter-size single crystallites selected from the same powder samples, resulted in a clear identification of several types of paramagnetic point defects. The resulting spin Hamiltonian parameters describing the ESR spectra observed in the 3-293 K temperature range and the photosensitivity of the paramagnetic defects observed in amber-colored cBN samples are reported. It is shown that the nature of the paramagnetic centers depends on the color of the investigated samples and that, in many cases, uncontrolled impurities seem to be involved in their structure.

43. Lattice defect assisted incorporation of Mn2+ ions in cubic II-VI semiconductor quantum dots

Authors: Nistor, SV; Stefan, M; Nistor, LC; Ghica, D; Mateescu, CD; Barascu, JN

Published: 2010, 11TH EUROPHYSICAL CONFERENCE ON DEFECTS IN INSULATING MATERIALS (EURODIM 2010), 15, DOI: 10.1088/1757-899X/15/1/012024

Electron paramagnetic resonance spectra from substitutional Mn2+ ions in quantum dots of cubic ZnS with tight size distribution centred at 2 nm were recorded in the 9.8 GHz and 34 GHz frequency bands. Their quantitative analysis with line shape simulation and fitting computer programs accounting for both forbidden transitions and line broadening effects demonstrate the presence of a local axial distortion attributed to a neighbouring extended planar stacking defect. The presence of such extended lattice defects, confirmed from a high resolution transmission electron microscopy study on presently investigated cubic ZnS quantum dots, seems to be essential in the incorporation and localization of Mn2+ activating ions in other cubic II-VI semiconductor quantum dots as well.

44. Incorporation and localization of substitutional Mn2+ ions in cubic ZnS quantum dots

Authors: Nistor, SV; Stefan, M; Nistor, LC; Goovaerts, E; Van Tendeloo, G

Published: JAN 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.035336

Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots-QDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter vertical bar D vertical bar = 41 x 10(-4) cm(-1). The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

45. Localization of Mn2+ Ions in Mesoporous ZnS

Authors: Nistor, SV; Stefan, M; Nistor, LC; Mateescu, CD; Birjega, R

Published: SEP 2010, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10, 6205, DOI: 10.1166/jnn.2010.2599

Nanocrystalline cubic ZnS doped with 0.2% mol manganese, exhibiting a stable mesoporous structure, was synthesized at room temperature by a non toxic surfactant-assisted liquid liquid reaction. The X-ray diffraction measurements demonstrate the formation of a sponge-like mesoporous material built from cubic ZnS nanocrystals of 1.8 nm average sizes, with a tight distribution of pores of 1.8 nm mean diameter. The transmission electron microscopy images confirm the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from cubic ZnS nanocrystallites of 2.1 nm average size. The resulting tight distribution of crystallites and pores yields a well resolved Electron Paramagnetic Resonance spectrum, with the narrowest reported component lines attributed to three types of isolated Mn2+ centers, called Mn2+ (I), Mn2+ (II) and Mn2+ (III). From the analysis of the spin Hamiltonian parameters it is shown that in the Mn2+ (I) centers the paramagnetic ion is situated at substitutional Zn sites in the ZnS nanocrystals, being also subjected to a small axial distortion. The relative concentration changes under thermal treatment experiments strongly suggest that in both Mn2+ (II) and Mn2+ (III) centers the Mn2+ ion is localized on the surface of the ZnS nanocrystallites, being bond to an oxygen ion in the first case and to an additional water molecule in the second case.

46. Synthesis and characterization of Mn2+ doped ZnS nanocrystals self-assembled in a tight mesoporous structure

Authors: Nistor, SV; Nistor, LC; Stefan, M; Mateescu, CD; Birjega, R; Solovieva, N; Nikl, M

Published: JUL-AUG 2009, SUPERLATTICES AND MICROSTRUCTURES, 46, 311, DOI: 10.1016/j.spmi.2008.10.001

We report the synthesis, by a surfactant-assisted liquid-liquid reaction, of nanocrystalline ZnS doped with 0.2 mol% Mn2+ ions self-assembled in a mesoporous structure. The XRD measurements demonstrate the formation of a sponge-like mesoporous material with a tight distribution of pores of 1.8 nm mean diameter built from cubic ZnS nanocrystals of 1.8 nm average size. TEM investigation confirms the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from nanocrystallites of cubic ZnS. The ordering effect of self-assembling, which is reflected in the tight size distribution of crystallites and pores, might be also responsible for the well resolved EPR spectra, attributed to the presence of three types of isolated Mn2+ paramagnetic centers. (C) 2008 Elsevier Ltd. All rights reserved.

47. In-depth investigation of EPR spectra of Mn2+ ions in ZnS single crystals with pure cubic structure

Authors: Nistor, SV; Stefan, M

Published: APR 8 2009, JOURNAL OF PHYSICS-CONDENSED MATTER, 21, DOI: 10.1088/0953-8984/21/14/145408

The X (9.8 GHz)-band electron paramagnetic resonance (EPR) properties of substitutional Mn2+ ions in high quality cubic ZnS single crystals grown from PbCl2 flux have been thoroughly investigated. Accurate spin Hamiltonian (SH) parameters: g = 2.002 25 +/- 0.000 06; a = (7.987 +/- 0.008) x 10(-4) cm(-1) and A = -(63.88 +/- 0.02) x 10(-4) cm(-1) were obtained by simulation and fitting to the experimentally allowed transitions recorded for the magnetic field aligned within +/- 0.25 degrees along the main crystal axes. The normally forbidden hyperfine M = + 1/2 <->-1/2, Delta m = +/- 1 transitions were also observed. Their position was found to be in agreement, within the experimental accuracy of Delta H = +/- 0.01 mT, with calculations using the same SH parameters. The angular variation of the ratios of the intensities of the central forbidden to the allowed transitions could be accounted for only by including an additional constant contribution. The observed line broadening of the M = +/- 1/2 <->+/- 3/2 and +/- 3/2 <->+/- 5/2 fine structure transitions and their line width variation in a (110) plane have been quantitatively described by considering a random distribution of lattice strains at the Mn2+ impurity ions. The influence of the forbidden transitions and line broadening on the EPR spectra line shape of the Mn2+ ions in cubic ZnS crystalline powders is also examined.

48. Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals

Authors: Popescu, FF; Bercu, V; Barascu, JN; Martinelli, M; Massa, CA; Pardi, LA; Stefan, M; Nistor, SV; Nikl, M; Bohacek, P

Published: JUL 21 2009, JOURNAL OF CHEMICAL PHYSICS, 131, DOI: 10.1063/1.3180697

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(-)factor and the hyperfine constant A of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular to)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S-4 site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.

49. Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals (vol 131, 034505, 2009)

Authors: Popescu, FF; Bercu, V; Barascu, JN; Martinelli, M; Massa, CA; Pardi, LA; Stefan, M; Nistor, SV; Nikl, M; Bohacek, P

Published: DEC 28 2009, JOURNAL OF CHEMICAL PHYSICS, 131, DOI: 10.1063/1.3280223

50. Electron and hole trapping in irradiated PbBr2 : Tl single crystals

Authors: Stefan, M; Nistor, SV; Darabont, A; Neamtu, C; Goovaerts, E

Published: JUN 2008, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 266, 2761, DOI: 10.1016/j.nimb.2008.03.179

Several hole trapped centres, namely Tl2+, self-trapped hole and A-type centres, were observed by ESR in PbBr2:Tl single crystals after X-ray irradiation at 77 K. The corresponding spectra are visible up to 200 K for all three defect centres. The STH centres are observed at higher temperatures than in the undoped PbBr2 single crystals, probably due to the stabilising effect of the Tl+ impurities. The ESR parameters of the Tl2+ centre point to stronger covalency effects in PbBr2 than in the isostructural PbCl2 crystals. All hole trapped centres started decaying around 160 K, probably due to recombination with nonparamagnetic electron trapped centres. Besides a small concentration of self-trapped electron centres, no other electron trapped centres were observed. (c) 2008 Elsevier B.V. All rights reserved.

51. Irradiation defects in superhard cubic boron nitride single crystals

Authors: Nistor, SV; Ghica, D; Stefan, M; Nistor, LC; Goovaerts, E; Taniguchi, T

Published: JUN 2008, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 266, 2787, DOI: 10.1016/j.nimb.2008.03.117

Single crystallites of superhard, semiconducting, n-type, amber colored and p-type, blue colored, Be-doped cubic boron nitride have been irradiated either with UV (350 nm) light or with an intense beam of accelerated (1 MeV) electrons. The examination of the irradiated samples at low temperatures by high frequency W (95 GHz)-band electron spin resonance reveals several new, radiation-induced, isotropic paramagnetic centers. The UV irradiation of both types of crystals yields centers involving very likely protons. In the amber c-BN crystals the irradiation with I MeV electrons results in the formation of vacancy associated paramagnetic defects and quasi-free electrons in colloidal particles. (C) 2008 Elsevier B.V. All rights reserved.

52. Electron paramagnetic resonance properties of Gd3+ ions in PbWO4 scintillator crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Nikl, M; Bohacek, P

Published: JAN 18 2006, JOURNAL OF PHYSICS-CONDENSED MATTER, 18, 728, DOI: 10.1088/0953-8984/18/2/025

The properties of the electron paramagnetic resonance spectra of Gd3+ ions in PbWO4 single crystals have been investigated in the X-band microwave frequency region, in the 1.5 to 290 K temperature range. The observed S-4 symmetry of the local crystal field at the Gd3+ impurity ions strongly suggests that the Gd3+ ions substitute for the Pb2+ lattice cations, with charge compensation at a distance. The spin Hamiltonian parameters are comparable with those of the Gd3+ ions in other isomorphous tungstates. The temperature variation of the fine structure parameters B-2(0) and B-4(0) points to an energy transfer between the impurity ions and the host lattice, which takes place mainly through a local vibrational mode of frequency omega = 3.1 x 1013 rad s(-1).

53. Single-ion and molecular contributions to the zero-field splitting in an iron(III)-oxo dimer studied by single crystal W-band EPR

Authors: ter Heerdt, P; Stefan, M; Goovaerts, E; Caneschi, A; Cornia, A

Published: MAR 2006, JOURNAL OF MAGNETIC RESONANCE, 179, 37, DOI: 10.1016/j.jmr.2005.10.016

Detailed knowledge of the type and strength of pair interactions between high-spin metal ions is paramount to the understanding and design of molecular magnetic materials. In this work, the anisotropic magnetic interactions in a beta-diketonate-alkoxide iron(III) dimer compound, [Fe-2(OCH3)(2)(dbM)(4), Hdbm = diberizoyhriethane] (Fe-2) have been investigated by single crystal electron paramagnetic resonance (EPR) in the W-band (at 95 GHz). The diamagnetic Substitution method. was employed using the isomorphous gallium(III)-based compound doped with iron(III) to produce Ga-Fe dimers(GaFe). The single-ion zero-field splitting(ZFS) tensor could be separately determined in GaFe with the iron ion in a local environment quasi-identical to the one in Fe2. Its principal directions are found to point in arbitrary directions, uncorrelated with the Fe-O bonds. The Fe2 EPR spectra consist of transitions within the lowest multiplet states S = 1, 2, 3, which were analyzed using the full Spill Hamiltonian description of all exchange coupled pair of s = 5/2 spins. The anisotropic spin-spin interaction tensor of Fe2 possesses a principal axis close to the Fe-Fe direction and was shown to arise both from through-space (dipolar) and through-bond (anisotropic exchange) contributions. The latter involves ail rhombic component J(E) = (J(X) - J(Y))/2 approximate to 0.093 cm(-1) of magnitude comparable. to the dipolar interaction, and even to the rhombic part of the single-ion ZFS (E = 0.097 cm(-1)). Our results show that the anisotropic exchange, usually neglected for S-type ions, is significant for the anisotropic interactions in exchange-coupled iron(III) clusters, including the Fe-4 and Fe-8 families of single-molecule magnets and the anti Ferrornagnetic iron wheels. (c) 2005 Elsevier Inc. All rights reserved.

54. Temperature dependence of the electron paramagnetic resonance spectra of Mn2+, impurity ions in PbWO4 single crystals

Authors: Stefan, M; Nistor, SV; Goovaerts, E; Nikl, M; Bohacek, P

Published: FEB 2 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 728, DOI: 10.1088/0953-8984/17/4/014

The temperature variation of the fine and hyperfine parameters of Mn2+ in single crystals of PbWO4 in the low temperature range reveals the presence of a resonant mode of frequency omega = 8.8 x 10(12) rad s(-1). Moreover, above 60 K, where the temperature induced broadening becomes dominant, a T-2 variation of the relaxation time was inferred from the analysis of the temperature dependence of the Mn2+ linewidth. This variation is attributed to a Raman relaxation process due to the coupling with the same local resonant mode.

55. EPR characterization of Mn2+ impurity ions in PbWO4 single crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Nikl, M; Bohacek, P

Published: AUG-DEC 2004, RADIATION MEASUREMENTS, 38, 658, DOI: 10.1016/j.radmeas.2003.12.024

The electron paramagnetic resonance (EPR) properties of the Mn2+ ions in PbWO4 single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T = 20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn2+ impurity ions is tetragonal, strongly suggesting that the Mn2+ ions substitute for the Pb2+ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn2+ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains. (C) 2003 Elsevier Ltd. All rights reserved.

56. EPR probing of low temperature structural phases of Rb2ZnCl4 crystals with Tl-0 and Tl2+ centers

Authors: Stefan, M; Nistor, SV; Goovaerts, E; Schoemaker, D

Published: MAR 2004, PHYSICAL REVIEW B, 69, DOI: 10.1103/PhysRevB.69.104107

Thallium related paramagnetic centers of Tl2+ (6s(1)) and Tl-0 (6p(1)) type were produced by low temperature x-ray irradiation in ferroelectric Rb2ZnCl4:Tl single crystals. Extensive EPR investigations have been performed in order to elucidate their intrinsic properties and sensitivity as paramagnetic probes for structural phase transitions studies. Compared to the s-type Tl2+ centers, already used in many such studies, it was found that the p-type Tl-0 centers are much more sensitive to the small variations in the local crystal field associated with these transformations. Their EPR spectra provided information about the unit cell tripling in the P2(1)cn phase, as well as the symmetry lowering and lattice dynamics in the C1c1 phase. The temperature induced, continuous changes observed in the EPR spectra of both Tl-0 and Tl2+ centers were explained by the influence of the soft modes responsible for the 74-K structural phase transition.

57. ESR characterization of point defects in amber colored c-BN super abrasive powders

Authors: Nistor, SV; Ghica, D; Stefan, M; Bouwen, A; Goovaerts, E

Published: SEP 2004, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 201, 2590, DOI: 10.1002/pssa.200405194

Cubic boron nitride (c-BN) crystalline superabrasive powder (Borazon** CBN 400), consisting of 200-300 microns sized amber colored crystallites prepared by HP/HT synthesis, has been examined from 2.1 K to 293 K by X-band ESR spectroscopy. The observed spectrum consists of a component line A1, visible in the whole temperature range, and two component lines A2 and A3, visible at high and low temperatures, respectively. The A1 and A3 lines originate from transitions inside S = 1/2 ground states of distinct paramagnetic species and A2 from transitions inside an excited state of another paramagnetic center. The intensity of the A1 and A3 lines changes differently during in situ low temperature illumination in the UV-VIS range. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

58. EPR study of the low temperature ferroelectic phase transition in Cu2+ doped Rb2ZnCl4 single crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D; Ursu, L

Published: SEP 2003, SOLID STATE COMMUNICATIONS, 127, 698, DOI: 10.1016/S0038-1098(03)00649-5

Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P2(1)cn <----> ClcI structural phase transition that takes place in this compound at 74.6 K. From the two types Of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition. (C) 2003 Elsevier Ltd. All rights reserved.

59. Tl-0 and Tl2+ centers as paramagnetic probes for the 74K phase transition in Rb2ZnCl4

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 2002, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 157, 697, DOI: 10.1080/10420150215809

Tl-0 (6 p(1)) irradiation centers are proposed as new paramagnetic probes for structural phase transition investigations. The P2(1)cn --> C1 c1 phase transition in Rb-2 ZnCl4 single crystals is monitored using the Tl-0 and the well-known Tl2+ (6 s(1)) paramagnetic probes. The anomalous temperature dependence exhibited by the EPR spectra of both types of centers in the C1 c1 phase was analyzed considering a soft mode contribution.

60. Fourth-order zero-field splitting parameters of [Mn(cyclam)Br-2]Br determined by single-crystal W-band EPR

Authors: Mossin, S; Stefan, M; ter Heerdt, P; Bouwen, A; Goovaerts, E; Weihe, H

Published: 2001, APPLIED MAGNETIC RESONANCE, 21, 596, DOI: 10.1007/BF03162431

Single-crystal W-band (95 GHz) electron paramagnetic resonance (EPR) studies have been performed at 20 K and at room temperature on a tetragonal Mn(III) compound with potential application as a building block for high-spin clusters. The observed EPR spectra correspond to an anisotropic high-spin S = 2 ground state and have been attributed to equivalent centers related by fourfold symmetry. Accurate values for the spin Hamiltonian parameters were obtained from the analysis of the data at both temperatures. At 20 K the contribution of fourth-order zero-field splitting terms was shown to be significant, with parameter values B-4(0) = 0.0009(3) cm(-1), B2 = 0.0006(2) cm(-1) and 4 4 B-4(4) = 0.0017(3) cm(-1), to be considered together with the second-order parameters D = -1.1677(7) cm(-1) and E = -0.0135(6) cm(-1).

61. Point defects in cubic boron nitride crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Bouwen, A; Schoemaker, D; Dinca, G

Published: MAR-JUL 2001, DIAMOND AND RELATED MATERIALS, 10, 1411, DOI: 10.1016/S0925-9635(00)00396-4

The results of a low-temperature study by high frequency (94 GHz) EPR on brown-to-dark colored single crystals selected from cBN crystalline powders prepared by the HPHT technique with boron excess, are presented. Previous investigations by low frequency (9.4 GHz) EPR spectroscopy on such dark polycrystalline c-BN powders resulted in the identification of two paramagnetic species D1 and D2 associated with the brown-to-dark coloration, the spectrum of the latter one being observed only above 100 K. The present research identifies the D1 species, studied by EPR at low temperatures, as consisting mainly from anisotropic paramagnetic centers with electron spin S = 1/2, local symmetry axis along one of the crystal [111] axes and principal g values g(parallel to) = 2.0032 +/- 0.0009 and g(perpendicular to) = 2.0094 +/- 0.0005 at T = 10 K. (C) 2001 Elsevier Science B.V. All rights reserved.

62. Atomic 6p-Tl-0 centers in ferroelectric Rb2ZnCl4 crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 155, 377, DOI: 10.1080/10420150108214140

Several Tl-0 (6s(2)6p(1))-type paramagnetic centers, produced by low temperature X-ray irradiation, were observed and studied by electron spin resonance (ESR) in the orthorhombic ferroelectric phase of thallium doped Rb2ZnCl4 crystals. The centers were formed by electron trapping at Tl+ ions localized substitutionally at Rb+ sites. The number and properties of the observed centers account for the tripling of the unit cell in the ferroelectric phase.

63. Multifrequency ESR studies of paramagnetic point defects in cubic boron nitride crystals

Authors: Nistor, SV; Stefan, M; Schoemaker, D; Goovaerts, E; Dinca, G

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 156, 194, DOI: 10.1080/10420150108216892

Dark-brown crystalline powders and selected single crystals of cubic boron nitride grown by the HP-HT method, with boron excess, have been studied in a broad temperature range by electron spin resonance (ESR) in the X and W microwave frequency bands, respectively. The X-band spectra consist of two superimposed lorentzian components attributed to two types of related paramagnetic defects called D1 and D2, respectively. According to the W-band data, the D I centre exhibits local axial < 111 > symmetry and ground spin state S = 1/2, with g(parallel to) = 2,0033 and g(perpendicular to) = 2.0094 at T = 10 K. The broad D2 line, observed only in the X-band, at g = 2.0084, at higher temperatures, seems to result from transitions inside the excited levels of another boron related defect.

64. Single-crystal high-frequency electron paramagnetic resonance investigation of a tetranuclear iron(III) single-molecule magnet

Authors: Bouwen, A; Caneschi, A; Gatteschi, D; Goovaerts, E; Schoemaker, D; Sorace, L; Stefan, M

Published: APR 5 2001, JOURNAL OF PHYSICAL CHEMISTRY B, 105, 2663, DOI: 10.1021/jp003441d

A high-frequency (95 GHz) electron paramagnetic resonance (EPR) study is reported on single crystals of the planar tetranuclear complex Fe-4(OCH3)(6)(dpm)(6) (where Hdpm = dipivaloylmethane), which has been previously shown to present typical single-molecule magnet behavior. The spectra, all originating from the S = 5 ground state, possess quasi-axial symmetry along the normal to the plane defined by the four Fe(m) ions. The measured spectra are shown to belong to three different structural variations of the compound, resulting from disorder in the ligands around two of the Fe(III) ions. Accurate values could be obtained for the second- and fourth-order crystal field parameters related to the parallel EPR spectra, while the other parameters could be determined only for the dominant species. The separation between individual lines is decreasing and vanishing with increasing temperature. This effect is attributed to the contribution of fast relaxing excited states, whose population is varying with temperature.

65. Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H-2 flow rates

Authors: Nistor, SV; Stefan, M; Ralchenko, V; Khomich, AV; Schoemaker, D

Published: JUN 15 2000, JOURNAL OF APPLIED PHYSICS, 87, 8746, DOI: 10.1063/1.373604

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H-2 gas flow rates, keeping a constant [CH4]:[H-2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H-2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects. (C) 2000 American Institute of Physics. [S0021- 8979(00)08611-4].

66. Electron-hole recombination in PbCl2 : T1 crystals

Authors: Nistor, SV; Stefan, M; Goovaerts, E; Schoemaker, D

Published: MAY 2000, JOURNAL OF LUMINESCENCE, 87-9, 551, DOI: 10.1016/S0022-2313(99)00287-2

A correlated ESR and optical emission study on samples doped with different concentrations of Tl+ impurity ions shows the involvement of paramagnetic Pb-2(3+) self-trapped electron centers (STEL) and trapped hole A centers in the electron-hole recombination responsible for the 2.6 eV blue-green luminescence. (C) 2000 Elsevier Science B.V. All rights reserved.

67. EPR observation of first point defects in cubic boron nitride crystalline powders

Authors: Nistor, SV; Stefan, M; Schoemaker, D; Dinca, G

Published: 2000, SOLID STATE COMMUNICATIONS, 115, 44, DOI: 10.1016/S0038-1098(00)00135-6

An X-band electron paramagnetic resonance (EPR) study of nominally pure, diamond-like cubic boron nitride (c-BN) crystalline powders, has led to the first identification of a spectrum attributed to two related paramagnetic species. The composite EPR spectrum can be observed only in dark brown colored powders known to contain excess of boron. It consists of two superimposed lorentzian components, called D1 and D2, centered at g1 = 2.0063 and g2 = 2.0084, with peak-to-peak linewidths of 3.3 and 17.9 mT, respectively. The temperature dependence of the integrated intensities, their linewidths and intensity ratio D2/D1 allows one to conclude that the narrow line D1 originates from EPR transitions inside a S = 1/2 ground doublet and the broad line D2 from transitions inside the excited levels of another center. Evidence suggests that both centers are boron related paramagnetic species. (C) 2000 Elsevier Science Ltd. All rights reserved.

68. ESR of paramagnetic atom defects in CVD-grown diamond

Authors: Nistor, SV; Stefan, M; Ralchenko, V; Goovaerts, E; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 149, 307, DOI: 10.1080/10420159908230172

ESR measurements in the 9 and 94 GHz microwave frequency bands were performed on thick free-standing polycrystalline diamond films grown by microwave plasma enhanced CVD from CH4/H-2 mixtures under variable deposition conditions. An exponential decrease in the concentration of the N-0 centres with increase in the H-2 flow rate was found. Neither H1 nor H2 centres could be detected.

69. ESR of trapped-electron Fe+ centres in chlorinated SrCl2 crystals

Authors: Nistor, SV; Stefan, M; Bouwen, A; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 149, 207, DOI: 10.1080/10420159908230156

Two Fe+ centres have been identified by ESR after X-ray irradiation at 80 K of chlorinated SrCl2 : Fe2+ crystals. Both centres, with tetragonal symmetry, consist of an Fe+ ion with a neighbouring charged defect along the [001] symmetry axis. The substitutional Fe+ ion of the Fe+(I) centre is strongly off-centre displaced in the Fe+(II) centres, in the middle of a square of four nearest neighbour chlorine ligands.

70. ESR of paramagnetic Tl2+-type centres in Rb2ZnCl4 crystals

Authors: Stefan, M; Nistor, SV; Schoemaker, D

Published: 1999, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 150, 368, DOI: 10.1080/10420159908226258

Tl2+(6s(1))-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4 : TICl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

71. Growth of pure and doped Rb2ZnCl4 and K2ZnCl4 single crystals by Czochralski technique

Authors: Stefan, M; Nistor, SV; Mateescu, DC; Abakumov, AM

Published: APR 1999, JOURNAL OF CRYSTAL GROWTH, 200, 154, DOI: 10.1016/S0022-0248(98)01247-0

High-quality single crystals of Rb2ZnCl4 and K2ZnCl4, pure or doped with Cu, Mn, Cd, Tl, Sn, Pb and In cations, were grown by Czochralski technique in argon atmosphere, using an experimental setup that allows direct visual access to the whole growth zone. Slowly cooled crystals exhibit excellent cleavage properties. Fastly cooled crystals do cleave poorly. As shown by X-ray diffraction studies, such K2ZnCl4 samples exhibit inclusions of the high-temperature Pmcn phase with lattice parameters a = 7.263(2) Angstrom, b = 12.562(2) Angstrom and c = 8.960(4) Angstrom in the P2(1) cn room temperature stable phase. ESR and optical spectroscopy studies revealed the localization and valence state of the cation dopants. (C) 1999 Elsevier Science B.V. All rights reserved.

72. Off-center displacement of Fe+ ions in irradiated SrCl2 : Fe crystals grown in chlorine

Authors: Nistor, SV; Stefan, M; Schoemaker, D

Published: AUG 1999, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 214, 236, DOI: 10.1002/(SICI)1521-3951(199908)214:2<229::AID-PSSB229>3.0.CO;2-H

Chlorinated SrCl2:Fe2+ crystals exhibit, after X-ray irradiation, two trapped-electron Fe+(I) and Fe+(II) centers both with axial [001] symmetry, but different EPR spectrum parameters and production properties. The analysis of the experimental data strongly suggests a substitutional eight-fold coordination of the Fe+ ion in the Fe+(I) center and a strong [001] off-center displacement to a fourfold coordinated site in the Fe+(II) center resulting in S = 3/2 and S = 1/2 ground states, respectively.

73. Electron and hole trapping in PbCl2 and PbCl2 : Tl crystals

Authors: Nistor, SV; Goovaerts, E; Stefan, M; Schoemaker, D

Published: MAY 1998, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 141, 541, DOI: 10.1016/S0168-583X(98)00051-2

Formation of primary paramagnetic point defects under low temperature X-ray irradiation have been studied by ESR and optical absorption in pure and thallium doped PbCl2 single crystals. Besides Pb-2(3+) self-trapped electron (STEL) centers the PbCl2 : Tl crystals exhibit trapped-electron (PbTl)(+)-type centers. Based on production properties of paramagnetic centers it is suggested that besides forming Tl-2 divided by centers the holes are self trapped at pairs of neighbouring Cl- anions resulting in V-k type centers with various orientation and length of the Cl-Cl axis. (C) 1998 Published by Elsevier Science B.V. All rights reserved.

74. Localization and charge conversion of copper in Rb2ZnCl4:Cu crystals: An ESR and optical absorption study

Authors: Stefan, M; Nistor, SV; Grecu, NM; Schoemaker, D

Published: AUG 1997, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 202, 1011, DOI: 10.1002/1521-3951(199708)202:2<999::AID-PSSB999>3.0.CO;2-5

Electron spin resonance and optical studies reveal the presence of both Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagnetic Cu2+ centers with different concentrations and production properties have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedron. The less abundant Cu2+(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, part of it being afterwards converted to Cu+ precursor centers. The presence of a neighboring-charge compensating anion vacancy and its departure during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) centers, respectively.

Dr. Mariana STEFAN

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1. Electron paramagnetic resonance signature of rock-forming blue quartz from the Albesti (Romania) granite

2. In-depth insight into the structural properties of nanoparticulate NiO for CO sensing

3. Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes

4. Revolutionizing n-type Co3O4 Nanowire for Hydrogen Gas Sensing

5. Characterization of defect structures in nanoscaled W-doped TiO2 tested as supercapacitor electrode materials

6. Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes

7. Visible-Light-Active Black TiO2 Nanoparticles with Efficient Photocatalytic Performance for Degradation of Pharmaceuticals

8. Hard/soft effects of multivalence co-dopants in correlation with their location in PZT ceramics

9. Influence of surfactant-tailored Mn-doped ZnO nanoparticles on ROS production and DNA damage induced in murine fibroblast cells

10. Multidisciplinary characterization of melanin pigments from the black fungus Cryomyces antarcticus

11. Electron Small Polaron and Magnetic Interactions Direct Anisotropic Growth of Silicon-Doped Hematite Nanocrystals

12. SnSe2-Zn-Porphyrin Nanocomposite Thin Films for Threshold Methane Concentration Detection at Room Temperature

13. One-dimensional coordination polymers constructed from copper(II) ions and chromato bridges: Synthesis, crystal structures and thermal analysis

14. Nanoclustered Pd decorated nanocrystalline Zn doped SnO2 for ppb NO2 detection at low temperature

15. Tailoring the Dopant Distribution in ZnO:Mn Nanocrystals

16. X-ray Crystal Structure, Geometric Isomerism, and Antimicrobial Activity of New Copper(II) Carboxylate Complexes with Imidazole Derivatives

17. Limits and Particularities of the Synthesis of Ba1-xCaxTiO3 for Piezoelectric Applications, by Topochemical Conversion from Molten Salt Solutions

18. Mn2+ ions distribution in doped sol-gel deposited ZnO films

19. Origin and chemical composition of the amorphous material from the intergrain pores of self-assembled cubic ZnS:Mn nanocrystals

20. Aggregates of Mn2+ Ions in Mesoporous Self-Assembled Cubic ZnS:Mn Quantum Dots: Composition, Localization, Structure, and Magnetic Properties

21. Ferritin surplus in mouse spleen 14 months after intravenous injection of iron oxide nanoparticles at clinical dose

22. On the agent role of Mn2+ in redirecting the synthesis of Zn(OH)(2) towards nano-ZnO with variable morphology

23. Distribution and interaction of Mn2+ ions incorporated in cubic ZnS quantum dots over a broad concentration range

24. Revealing the Cu2+ ions localization at low symmetry Bi sites in photorefractive Bi12GeO20 crystals doped with Cu and V by high frequency EPR

25. Doping Ultrasmall Cubic ZnS Nanocrystals with Mn2+ Ions over a Broad Nominal Concentration Range

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

27. Pulse annealing electron paramagnetic resonance with probing transition ions

28. ZnS and ZnO Semiconductor Nanoparticles Doped with Mn2+ Ions. Size Effects Investigated by EPR Spectroscopy

29. Evaluation of the Segregation of Paramagnetic Impurities at Grain Boundaries in Nanostructured ZnO Films

30. Magnetic defects in crystalline Zn(OH)(2) and nanocrystalline ZnO resulting from its thermal decomposition

31. Nanosize induced effects in luminescent ZnS:Mn2+ quantum dots

32. Sequential Thermal Decomposition of the Shell of Cubic ZnS/Zn(OH)(2) Core-Shell Quantum Dots Observed With Mn2+ Probing Ions

33. Correlation of Lattice Disorder with Crystallite Size and the Growth Kinetics of Mn2+ Doped ZnO Nanocrystals Probed by Electron Paramagnetic Resonance

34. Accurate determination of the spin Hamiltonian parameters for Mn2+ ions in cubic ZnS nanocrystals by multifrequency EPR spectra analysis

35. Substitutional and surface Mn2+ centers in cubic ZnS:Mn nanocrystals. A correlated EPR and photoluminescence study

36. Structural phase transformations in annealed cubic ZnS nanocrystals

37. Crystallization of Disordered Nanosized ZnO Formed by Thermal Decomposition of Nanocrystalline Hydrozincite

38. Local Structure at Mn2+ Ions in Vacuum Annealed Small Cubic ZnS Nanocrystals Self-Assembled Into a Mesoporous Structure

39. Specificity of defects induced in silicon by RF-plasma hydrogenation

40. Study of the Kramers rare earth ions ground multiplet with a large orbital contribution by multifrequency EPR spectroscopy: Ce3+ in PbWO4 scintillator

41. IMPROVING THE CUBIC ZnS NANOCRYSTALS QUALITY BY SELF-ASSEMBLING INTO A MESOPOROUS STRUCTURE

42. Multifrequency ESR Characterization of Paramagnetic Point Defects in Semiconducting Cubic BN Crystals

43. Lattice defect assisted incorporation of Mn2+ ions in cubic II-VI semiconductor quantum dots

44. Incorporation and localization of substitutional Mn2+ ions in cubic ZnS quantum dots

45. Localization of Mn2+ Ions in Mesoporous ZnS

46. Synthesis and characterization of Mn2+ doped ZnS nanocrystals self-assembled in a tight mesoporous structure

47. In-depth investigation of EPR spectra of Mn2+ ions in ZnS single crystals with pure cubic structure

48. Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals

49. Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals (vol 131, 034505, 2009)

50. Electron and hole trapping in irradiated PbBr2 : Tl single crystals

51. Irradiation defects in superhard cubic boron nitride single crystals

52. Electron paramagnetic resonance properties of Gd3+ ions in PbWO4 scintillator crystals

53. Single-ion and molecular contributions to the zero-field splitting in an iron(III)-oxo dimer studied by single crystal W-band EPR

54. Temperature dependence of the electron paramagnetic resonance spectra of Mn2+, impurity ions in PbWO4 single crystals

55. EPR characterization of Mn2+ impurity ions in PbWO4 single crystals

56. EPR probing of low temperature structural phases of Rb2ZnCl4 crystals with Tl-0 and Tl2+ centers

57. ESR characterization of point defects in amber colored c-BN super abrasive powders

58. EPR study of the low temperature ferroelectic phase transition in Cu2+ doped Rb2ZnCl4 single crystals

59. Tl-0 and Tl2+ centers as paramagnetic probes for the 74K phase transition in Rb2ZnCl4

60. Fourth-order zero-field splitting parameters of [Mn(cyclam)Br-2]Br determined by single-crystal W-band EPR

61. Point defects in cubic boron nitride crystals

62. Atomic 6p-Tl-0 centers in ferroelectric Rb2ZnCl4 crystals

63. Multifrequency ESR studies of paramagnetic point defects in cubic boron nitride crystals

64. Single-crystal high-frequency electron paramagnetic resonance investigation of a tetranuclear iron(III) single-molecule magnet

65. Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H-2 flow rates

66. Electron-hole recombination in PbCl2 : T1 crystals

67. EPR observation of first point defects in cubic boron nitride crystalline powders

68. ESR of paramagnetic atom defects in CVD-grown diamond

69. ESR of trapped-electron Fe+ centres in chlorinated SrCl2 crystals

70. ESR of paramagnetic Tl2+-type centres in Rb2ZnCl4 crystals

4. Revolutionizing n-type Co₃O₄ Nanowire for Hydrogen Gas Sensing

5. Characterization of defect structures in nanoscaled W-doped TiO₂ tested as supercapacitor electrode materials

6. Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn²⁺ paramagnetic probes