NIMP| Daniela GHICA, Author at NIMP

Education

2006 - Ph. D in Physics (Solid State Physics), University of Bucharest, Romania

Magnetic resonance study of the radiolysis products in chlorinated SrCl₂:Fe crystals

Supervisor: Prof. Dr. S. V. Nistor

Positions

2017 – present

Senior researcher II

Awards

Ion Agarbiceanu, „1^st place” award for the best contribution to the National Physics Conference CNF 2005, Bucharest, Romania.
“Physica Status Solidi Young Researcher Award” for the best contribution to the 10^th Europhysical Conference on Defects in Insulating Materials - EURODIM 2006, Milano, Italy.
"Stefan Procopiu" award granted by the Romanian Academy in 2008.

Expertize

Production and characterization of the paramagnetic point defects induced by doping and/or irradiation in single crystals and nanostructures
Multi-frequency Electron Paramagnetic Resonance - EPR (X- (9 GHz), Q- (34 GHz) and W- (95 GHz) band) and Electron Nuclear Double Resonance - ENDOR (X- and Q-band) spectroscopy of the point defects in single crystals and nanostructures at variable temperatures: from room temperature down to 2 K
Computational analysis of the EPR and ENDOR spectra
XRD measurements and Rietveld analysis
Pulsed laser deposition and plasma/laser processing of thin films

Books

1. 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.

In: Nanomaterials, Nanoparticles, Nanodevices, Eds: M. Zaharescu, H. Chiriac, D. Dascalu,

Ed. Academiei Romane, Bucuresti, Romania, 2016, 185-208, ISBN 978-973-27-2643-3

2. M. Stefan, S. V. Nistor, D. Ghica

ZnS and ZnO semiconductor nanoparticles doped with Mn²⁺ ions. Size effects investigated by EPR spectroscopy

In: Size Effects in Nanostructures. Basics and Application, V. Kuncser and L. Miu (Eds)

Springer Ser. Mater. Sci., Vol. 205, Part I, Springer-Verlag Berlin Heidelberg 2014, 3-27

ISBN: 978-3-662-44478-8, DOI 10.1007/978-3-662-44479-5_1

This chapter was written through equal contributions of all authors.

3. S. V. Nistor, M. Stefan, L. C. Nistor, D. Ghica, C. D. Mateescu, N. J. Barascu

Preparation and properties of mesoporous cubic ZnS doped with Mn²⁺ activating ions

In: Modern Trends in Nanoscience, Eds. M. Balasoiu, G.M. Arzumanyan,

Ed. Academiei Romane, Bucuresti, Romania, 2013, 143-155, ISBN 978-973-27-2230-5

4. S. V. Nistor, M. Stefan, L. C. Nistor, D. Ghica, C. D. Mateescu

Defect assisted localization of Mn²⁺ activating ions in the core of the ZnS quantum dots

In: Nanomaterials and Nanostructures for Various Applications, Eds. G. Brezeanu, H. Iovu,

C. Cobianu, D. Dascalu, Ed. Academiei Romane, Bucuresti, Romania, 2012, 213-223,

ISBN 978-973-27-2169-8

Projects

1. Distribution and dynamics of paramagnetic impurities in nanostructured ZnO for advanced applications in spintronics, opto- and nanoelectronics

Project Type: TE, Start Date: 2015-10-01 End Date: 2017-11-30

2. Sechestrarea CO2 prin legare chimica in compusi minerali utili in scopul reducerii emisiilor de gaze cu efect de sera

Project Type: PC-CNMP-P3, Start Date: 2011-01-01 End Date: 2011-11-30

3. Noi centri Fe+ neconventionali in SrCl2 cristalin studiati prin tehnici de rezonanta magnetica

Project Type: AT-CNCSIS, Start Date: 2003-01-01 End Date: 2004-12-30

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. 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

5. Influence of relative humidity on CO2 interaction mechanism for Gd-doped SnO2 with respect to pure SnO2 and Gd2O3

Authors: Ghica, C; Mihalcea, CG; Simion, CE; Vlaicu, ID; Ghica, D; Dinu, IV; Florea, OG; Stanoiu, A

Published: OCT 1 2022, SENSORS AND ACTUATORS B-CHEMICAL, 368, 132130, DOI: 10.1016/j.snb.2022.132130

The paper aims to identify the CO2 interaction mechanism for chemical sensors based on Gd-doped SnO2, SnO2 and Gd2O3 powders deposited as thick sensitive layers. The low reactivity of CO2 conferred by the thermodynamic stability and chemical inertia can be offset by the presence of relative humidity. The sensitive powders were prepared by wet chemical co-precipitation method. The Gd concentration was varied from 1% to 20 at% in order to determine the limit for Gd integration as a doping ion prior to chemical segregation as a secondary phase. Analytical transmission electron microscopy points to a homogeneous Gd doping of the nanostructured SnO2 powders for low doping concentrations and the formation of a nanocomposite based on SnO2 as main phase and cubic Gd2O3 as secondary phase for the highly doped samples. The electrical resistance is either influenced by the density of oxygen vacancies, or is the result of compensation for two opposite behaviours into the SnO2- Gd2O3 nanocomposite structures. The CO2 exposure to humid atmosphere determines distinct behaviours cor-responding to SnO2 and Gd2O3 as constitutive elements. The associated CO2 interaction mechanism is based on simultaneous DC electrical resistance and Contact Potential Difference measurements, which allow decoupling the ionosorption from the dipolar processes, thus highlighting specific chemical interactions on the SnO2 and Gd2O3 surfaces.

6. Effects of Calcination Temperature on CO-Sensing Mechanism for NiO-Based Gas Sensors

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

Published: MAY 2022, CHEMOSENSORS, 10, 191, DOI: 10.3390/chemosensors10050191

NiO-sensitive materials have been synthesized via the hydrothermal synthesis route and calcined in air at 400 degrees C and, alternatively, at 500 degrees C. Structural, morphological, and spectroscopic investigations were involved. As such, the XRD patterns showed a higher crystallinity degree for the NiO calcined at 500 degrees C. Such an aspect is in line with the XPS data indicating a lower surface hydroxylation relative to NiO calcined at 400 degrees C. An HRTEM microstructural investigation revealed that the two samples differ essentially at the morphological level, having different sizes of the crystalline nanoparticles, different density of the surface defects, and preferential faceting according to the main crystallographic planes. In order to identify their specific gas-sensing mechanism towards CO exposure under the in-field atmosphere, the simultaneous evaluation of the electrical resistance and contact potential difference was carried out. The results allowed the decoupling of the water physisorption from the chemisorption of the ambient oxygen species. Thus, the specific CO interaction mechanism induced by the calcination temperature of NiO has been highlighted.

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. Insight on Ni(II) and Cu(II) complexes of biguanide derivatives developed as effective antimicrobial and antitumour agents

Authors: Badea, M; Grecu, MN; Chifiriuc, MC; Bleotu, C; Popa, M; Iorgulescu, EE; Avram, S; Uivarosi, V; Munteanu, AC; Ghica, D; Olar, R

Published: APR 2021, APPLIED ORGANOMETALLIC CHEMISTRY, 35, DOI: 10.1002/aoc.6155

Six compounds of the types [M(dmbg)(2)]center dot nH(2)O ((1) M: Ni, n = 0; (4) M: Cu, n = 1; Hdmbg: N,N '-dimethylbiguanide) and, respectively, [ML]center dot nH(2)O (where (2) M: Ni, L: L-1, n = 0; (3) M: Ni, L: L-2, n = 0; (5) M: Cu, L: L-1, n = 0 and (6) M: Cu, L: L-2, n = 3, H2L1: ligand resulted from the condensation of (1) with ammonia and formaldehyde and H2L2: ligand resulted from the condensation of (4) with hydrazine and formaldehyde) were characterized as mononuclear species through information provided by NMR, electronic paramagnetic resonance (EPR) and UV-Vis spectroscopy as well as cyclic voltammetry. All data are consistent with macrocyclic formation by condensation. The complexes adopt a distorted square planar geometry resulting from the chelating behaviour of the corresponding ligands. The EPR spectra recorded after the addition of Cu(II) complexes into the corresponding Ni(II) complexes show a well resolved hyperfine structure with the superhyperfine pattern corresponding to four nitrogen atom donors. The cathodically shift of E-pc2 for both series can be correlated with the increased stability of the M(I) species through macrocyclic ligands. Geometry optimization studies for complexes (2), (3), (5) and (6) have further confirmed the experimental data. The pharmacokinetic computational results indicate that the complexes exhibit medium to low intestinal absorption and slow blood-brain barrier permeability but low toxicity. Their predictive pharmacodynamic profiles show that the compounds present the ability to inhibit protease activity. By corroborating the results of the in silico analysis with the experimental ones, the most promising complexes for antimicrobial applications are (1) and (2) and, respectively, (4) and (6) for the development of novel antitumour strategies.

9. Insights about CO Gas-Sensing Mechanism with NiO-Based Gas Sensors-The Influence of Humidity

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

Published: SEP 2021, CHEMOSENSORS, 9, DOI: 10.3390/chemosensors9090244

Polycrystalline NiO thick film-based gas sensors have been exposed to different test gas atmospheres at 250 degrees C and measured via simultaneous electrical resistance and work function investigations. Accordingly, we decoupled different features manifested toward the potential changes, i.e., work function, band-bending, and electron affinity. The experimental results have shown that the presence of moisture induces an unusual behavior toward carbon monoxide (CO) detection by considering different surface adsorption sites. On this basis, we derived an appropriate detection mechanism capable of explaining the lack of moisture influence over the CO detection with NiO-sensitive materials. As such, CO might have both chemical and dipolar interactions with pre-adsorbed or lattice oxygen species, thus canceling out the effect of moisture. Additionally, morphology, structure, and surface chemistry were addressed, and the results have been linked to the sensing properties envisaging the role played by the porous quasispherical-hollow structures and surface hydration.

10. Sensing Properties of NiO Loaded SnO2 Nanoparticles-Specific Selectivity to H2S

Authors: Stanoiu, A; Kuncser, AC; Ghica, D; Florea, OG; Somacescu, S; Simion, CE

Published: JUN 2021, CHEMOSENSORS, 9, DOI: 10.3390/chemosensors9060125

NiO-loaded SnO2 powders were prepared involving two chemical procedures. The mesoporous SnO2 support was synthesized by a hydrothermal route using Brij 35 non-ionic surfactant as a template. The nickel loadings of 1 and 10 wt.%. NiO were deposited by the wet impregnation method. The H2S sensing properties of xNiO-(1-x)SnO2 (x = 0, 1, 10%) thick layers deposited onto commercial substrates have been investigated with respect to different potential interfering gases (NO2, CO, CO2, CH4, NH3 and SO2) over a wide range of operating temperatures and relative humidity specific for in-field conditions. Following the correlation of the sensing results with the morphological ones, 1wt.% NiO/SnO2 was selected for simultaneous electrical resistance and work function investigations. The purpose was to depict the sensing mechanism by splitting between specific changes over the electron affinity induced by the surface coverage with hydroxyl dipoles and over the band bending induced by the variable surface charge under H2S exposure. Thus, it was found that different gas-interaction partners are dependent upon the amount of H2S, mirrored through the threshold value of 5 ppm H2S, which from an applicative point of view, represents the lower limit of health effects, an eight-hour TWA.

11. 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.

12. Electron paramagnetic resonance and microstructural insights into the thermal behavior of simonkolleite nanoplatelets

Authors: Rostas, AM; Kuncser, AC; Ghica, D; Palici, A; Maraloiu, VA; Vlaicu, ID

Published: MAY 7 2020, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 22, 9512, DOI: 10.1039/d0cp00641f

The aim of this paper is the study of the thermal behavior of the simonkolleite Zn-5(OH)(8)Cl-2 center dot H2O (ZHC) by electron paramagnetic resonance (EPR) spectroscopy, in particular. It is well known that during heating ZHC undergoes a complex transformation which involves several overlapping stages. However, with reference to the data reported on this subject, it can be concluded that there is still an ongoing debate regarding the intermediate stages of this process. The data presented in this study support a simple decomposition process of the ZHC prepared using the precipitation method. The EPR data correlated to the data obtained by other experimental techniques, such as XRD, TEM, SEM and EDX, indicate that during the thermal treatment the ZHC suffers a partial decomposition to ZnO with no intermediate products. After annealing at 500 degrees C for 1 h, a recombination process of ZHC is observed. Moreover, the kinetics associated to these decomposition steps were determined and the evolution of the paramagnetic centers was also followed and studied. This study offers new information related to the thermal behavior of ZHC, especially regarding the EPR data which is reported for the first time on this subject and material.

13. 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.

14. 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.

15. 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.

16. 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.

17. Wet chemical synthesis of ZnO-CdS composites and their photocatalytic activity

Authors: Zgura, I; Preda, N; Socol, G; Ghica, C; Ghica, D; Enculescu, M; Negut, I; Nedelcu, L; Frunza, L; Ganea, CP; Frunza, S

Published: MAR 2018, MATERIALS RESEARCH BULLETIN, 99, 181, DOI: 10.1016/j.materresbull.2017.11.013

The present study is focused on the wet chemical synthesis and the characterization of ZnO-CdS composites. The X-ray diffraction shows that the composites contain ZnO in hexagonal wurtzite structure and CdS in cubic phase. The scanning/transmission electron microscopy images reveal flower-like structures with different sizes depending on the CdS content. The optical investigations on composites reveal that the reflectance spectra disclose two thresholds of similar to 370 nm and similar to 460 nm associated with the ZnO and CdS, respectively. The photocatalytic activity measurements evidenced that the degradation efficiency of RhB in the presence of composites is higher comparatively with pristine ZnO, depending on the catalyst morphology, which varies with CdS content and the pH value of RhB solution. The electron paramagnetic resonance revealed the presence of the paramagnetic point defects in the samples. Thus, the wet chemical approaches are suitable for a large scale production of such ZnO-CdS composites having enhanced photocatalytic activity.

18. 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.

19. 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.

20. 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.

21. C-N cross-coupling on supported copper catalysts: The effect of the support, oxidation state, base and solvent (vol 341, pg 205, 2016)

Authors: Tirsoaga, A; Cojocaru, B; Teodorescu, C; Vasiliu, F; Grecu, MN; Ghica, D; Parvulescu, VI; Garcia, H

Published: JAN 2017, JOURNAL OF CATALYSIS, 345, 329, DOI: 10.1016/j.jcat.2016.11.016

22. 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.

23. 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.

24. 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.

25. High atomic diffusivity during pulsed laser irradiation of TiON quasi-amorphous films

Authors: Teodorescu, VS; Maraloiu, AV; Negrea, RF; Ghica, D; Scarisoreanu, ND; Dinescu, M; Gartner, M; Blanchin, MG

Published: JUN 30 2016, APPLIED SURFACE SCIENCE, 374, 251, DOI: 10.1016/j.apsusc.2015.11.188

Quasi-amorphous titanium oxynitride (TiON) films were obtained by annealing sol-gel anatase TiO2 films in NH3 atmosphere at 600 degrees C. These films were irradiated with 50 laser pulses using the fourth harmonic (266 nm) radiation of the Nd-YAG laser, with an average fluence of 20 mJ/cm(2). HRTEM observations of the pulsed laser irradiated films evidenced the rutile TiO2 nanocrystallites formation. The rutile structure was not present either in the TiON films before the laser irradiation, or in the initial sol-gel anatase TiO2 films. During the laser irradiation, the film structure remains in the solid state phase, as it results from the temperature estimation and microscopic observations. For the rutile nanocrystals formation, the atomic diffusion length of the oxygen and titanium atoms should be in the nanometric range during the laser pulse action, which implies a diffusivity close to the values observed in the liquid phase. We consider that the rutile phase formation is a proof of the fast atomic diffusion in the solid amorphous matrix, during the laser irradiation. (C) 2015 Published by Elsevier B.V.

26. 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.

27. On the role of Fe ions on magnetic properties of doped TiO2 nanoparticles

Authors: Tolea, F; Grecu, MN; Kuncser, V; Constantinescu, SG; Ghica, D

Published: APR 6 2015, APPLIED PHYSICS LETTERS, 106, DOI: 10.1063/1.4917037

The role of iron doping on magnetic properties of hydrothermal anatase TiO2:Fe-57 (0-1 at. %) nanoparticles is investigated by combining superconducting quantum interference device magnetometry with Mossbauer and electron paramagnetic resonance techniques. The results on both as-prepared and thermally treated samples in reduced air atmosphere reveal complexity of magnetic interactions, in connection to certain iron ion electron configurations and defects (oxygen vacancies, F-center, and Ti3+ ions). The distribution of iron ions is predominantly at nanoparticle surface layers. Formation of weak ferromagnetic domains up to 380 K is mainly related to defects, supporting the bound magnetic polaron model. (C) 2015 AIP Publishing LLC.

28. 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.

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

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

Published: OCT 8 2015, 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.

30. New coordination polymers with chromato bridges: (1)(infinity)[Ni(phen)(H2O)(2)(mu-O2CrO2)] and (3)(infinity)[Mn(4,4 '-bipy)(H2O)(mu-O3CrO)]center dot H2O

Authors: Dumbrava, A; Olar, R; Badea, M; Maxim, C; Ghica, D; Andruh, M

Published: FEB 24 2015, INORGANICA CHIMICA ACTA, 426, 54, DOI: 10.1016/j.ica.2014.11.009

Two new coordination polymers have been assembled using the chromato ions as bridging ligands: (1)(infinity)[Ni(phen)(H2O)(2)(mu-O2CrO2)] 1, and (3)(infinity)[Mn(4,4'-bipy)(H2O)(mu-O3CrO)]center dot H2O 2. In crystal 1 the chromate ions act as bridges connecting two nickel ions through two oxygen atoms, resulting in infinite chains. 1,10-Phenanthroline acts as a blocking ligand. In compound 2, each chromato ion is connected to three manganese ions, resulting in layers which are parallel to the ab plane. The layers are constructed from {Mn3Cr3} meshes, each metal atom being shared between three other meshes. The structure is expanded into the third direction by connecting the inorganic layers through 4,4'-bipy spacers. The thermal decomposition of the two compounds has been investigated. (C) 2014 Elsevier B.V. All rights reserved.

31. 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.

32. 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.

33. 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.

34. 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.

35. HIGH THERMAL STABILITY OF THE OFF-CENTER PARAMAGNETIC Fe3+ IONS IN CHLORINATED SrCl2:Fe2+ CRYSTALS

Authors: Ghica, D; Nistor, SV

Published: 2013, ROMANIAN REPORTS IN PHYSICS, 65, 831, DOI:

The thermal stability above 300 K of the hole-trapped Fe3+ centers, formed by X-ray irradiation at 80 K in chlorinated SrCl2:Fe2+ crystals, has been investigated by Electron Paramagnetic Resonance. In the temperature range of 300 - 520 K, the trigonal Fe3+ center completely transforms into the cubic Fe3+ center, which further decays up to 700 K. The formation and transformation of the trigonal Fe3+ center, containing off-center Fe3+ ions, is associated with the presence and thermally activated movement of the interstitial Cl- ions, present in excess in chlorinated SrCl2:Fe2+ crystals.

36. Co environment and magnetic defects in anatase CoxTi1-O-x(2) nanopowders

Authors: Grecu, MN; Macovei, D; Ghica, D; Logofatu, C; Valsan, S; Apostol, NG; Lungu, GA; Negrea, RF; Piticescu, RR

Published: APR 22 2013, APPLIED PHYSICS LETTERS, 102, 161909, DOI: 10.1063/1.4802819

Cobalt environment and magnetic defects nature in hydrothermal synthesized anatase CoxTi1-xO2 nanopowders (0 <= x <= 0.1) are investigated by x-ray diffraction and a variety of spectroscopic techniques. One shows that cobalt is partially inserted in the anatase lattice, as Co2+ ions located on substitutional and interstitial sites. The fraction of the diluted Co is limited to 3 at.% for x >= 0.05, while the rest of the Co atoms gather into Co3O4 clusters. As found by electron paramagnetic resonance, the Co doping brings about hole-and electron-excess defects. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802819]

37. LOW TEMPERATURE TEM INVESTIGATION OF ELECTRON BEAM INDUCED DECOMPOSITION OF NANOCRYSTALLINE HYDROZINCITE INTO ZnO

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

Published: 2013, ROMANIAN REPORTS IN PHYSICS, 65, 192, DOI:

The fast decomposition of hydrozincite [Zn-5(CO3)(2)(OH)(6)] into ZnO observed in the transmission electron microscope could be slowed down and investigated in situ by operating the microscope at very low electron beam current densities and cooling the specimen down to -160 degrees C. Thus, it was possible to observe and pursue the distinct structural steps of the disruption of the hydrozincite lattice due to the energetic emission of H2O and CO2 gases. The initially formed disordered ZnO phase was found to further crystalize resulting in a mesoporous structure of small ZnO nanocrystals.

38. 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.

39. 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.

40. PARAMAGNETIC POINT DEFECTS IN PURE AND C-13 AND O-17 IMPLANTED SILICON FOR HIGH ENERGY PARTICLE DETECTORS

Authors: Nistor, SV; Ghica, D; Pintilie, I; Manaila, E

Published: 2013, ROMANIAN REPORTS IN PHYSICS, 65, 819, DOI:

The presence and structure of paramagnetic point defects in C-13 and O-17 implanted ultrapure Si single-crystal material, two impurities which seem to play a major role in the detectors performance degradation and radiation resistance enhancement, respectively, are reported before and after irradiation with 6 MeV electrons. The investigation, performed by Q-band electron spin resonance spectroscopy in the 300 - 10 K temperature range, included also reference ultrapure and O-16 doped single-crystal Si-platelets. It resulted in the observation of points defects associated with lattice defects as dangling bonds and impurities.

41. 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.

42. Electron magnetic resonance and Mossbauer studies on iron doped SnO2 nanoparticles

Authors: Grecu, MN; Constantinescu, SG; Ghica, D; Tarabasanu-Mihaila, D; Diamandescu, L

Published: MAR 2012, HYPERFINE INTERACTIONS, 205, 115, DOI: 10.1007/s10751-011-0447-9

Iron doped (0.25-7.5% molar) hydrothermal nano-SnO2 was characterized by electron magnetic resonance (EMR) and Mossbauer spectroscopies. Only a small fraction of transition metal ions are in magnetic ordered state, contrary to the similar crystallographic compound TiO2. Temperature dependences of spectra suggest that by increasing iron concentration, or annealing temperature, iron ions migrate to nanoparticles surfaces forming disordered iron oxides.

43. 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.

44. 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.

45. 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.

46. Spin dynamics in 57Fe-doped TiO2 anatase nanoparticles

Authors: Grecu, MN; Constantinescu, S; Tarabasanu-Mihaila, D; Ghica, D; Bibicu, I

Published: DEC 2011, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 248, 2931, DOI: 10.1002/pssb.201147124

In this paper we present electron paramagnetic resonance (EPR) and Mossbauer (transmission method, TMS and measurements in the scattering method, CEMS) experiments on Fe-57 (0.11at.%) doped nanocrystalline anatase-type TiO2, synthesized by a hydrothermal method. Different Fe3+ and Fe2+ ions positions, with various hyperfine interactions evidenced in Mossbauer spectra (MS), confirm a partial magnetic ordering at room temperature. The magnetic hyperfine fields in the TMS spectra, better resolved at lower temperatures, do not change essentially with temperature. The first CEMS measurements, carried out on iron-doped TiO2 nanoparticles, reveal a larger disorder in the surface particles layer. The temperature dependence of the double integral EPR spectral intensity, proportional with the sample susceptibility, shows an anomalous behaviour. It suggests the bound magnetic polaron (BMP) mechanism for the magnetic ordering.

47. 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.

48. 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.

49. 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

50. 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.

51. 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.

52. 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.

53. Localization and movement of native interstitials in chlorinated SrCl2 : Fe crystals

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

Published: MAR 2007, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 204, 698, DOI: 10.1002/pssa.200673878

The formation of the electron trapped Fe+(IV) centre, produced by X-ray irradiation at 80 K and further annealing at temperatures of up to 700 K in chlorinated SrCl2: Fe crystals, has been investigated by Electron Paramagnetic Resonance. Our studies report the transformation of the monoclinic Fe+(III) centre into the axial Fe+(IV) centre above 450 K. The formation of the Fe+(IV) centre is attributed to the presence and thermally activated movement of neighbouring interstitial chlorine and alkali impurity Ne ions. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

54. Electron-trapping centers and interstitials in chlorinated SrCl2 : Fe single crystals

Authors: Ghica, D; Nistor, SV; Goovaerts, E; Schoemaker, D; Vrielinck, H; Callens, F

Published: MAY 2006, PHYSICAL REVIEW B, 73, DOI: 10.1103/PhysRevB.73.174103

Electron-trapped Fe+-type centers, produced by x-ray irradiation at 80 K and further annealing at higher temperatures in iron-doped SrCl2 single crystals grown in chlorine gas, have been investigated by electron paramagnetic resonance. The Fe+(III) and Fe+(IIIa) centers, produced by annealing at temperatures higher than 200 K, exhibit monoclinic local symmetry with the two g down arrow- tensor principal axes situated in the (110) plane slightly tilted away from the [001] and [1-10] directions, respectively. The Fe+(IV) center, observed after several cycles of irradiation and annealing to 700 K, exhibits tetragonal local symmetry around < 100 > and a well-resolved four-component structure, attributed to the superhyperfine interaction with a neighboring monovalent impurity ion. The presence and properties of the low symmetry radiation-induced Fe+ paramagnetic centers are attributed to trapping and the thermally activated movement of chlorine interstitials. Both precursor Fe2+ and resulting Fe+ centers are perturbed by these interstitials, which are introduced in SrCl2:Fe crystals during growth under a chlorine atmosphere.

55. High frequency ESR of native point defects in beryllium doped c-BN single crystals

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

Published: SEP 2004, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 201, 2598, DOI: 10.1002/pssa.200405195

High frequency (95 GHz) ESR measurements have been performed down to 5 K, on three cubic boron nitride (c-BN) single crystals doped with beryllium. The measured samples exhibit at low temperature different ESR spectra, which are sensitive to low temperature in situ illumination using a series of Kr+- and Ar+-laser lines. The analysis of the ESR spectra resulted in the identification of several native paramagnetic centers responsible for the observed component spectra lines. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

56. 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.

57. X- and Q-band ENDOR study of the Fe+(II) center in chlorinated SrCl2 : Fe crystals

Authors: Ghica, D; Nistor, SV; Vrielinck, H; Callens, F; Schoemaker, D

Published: JUL 2004, PHYSICAL REVIEW B, 70, DOI: 10.1103/PhysRevB.70.024105

The <001> axially symmetric Fe+(II) center observed in SrCl2:Fe2+ crystals has been studied by the electron nuclear double resonance (ENDOR) technique in the microwave X and Q bands. This center is produced only in crystals grown in chlorine atmosphere and x-ray irradiated at low temperature (below 100 K). The analysis of the ENDOR spectra unambiguously confirms the structural model earlier suggested by the electron paramagnetic resonance (EPR) study, as a Fe+ ion strongly displaced off center along an <001> axis, almost in the center of the square determined by the four nearest Cl- ligands. The unpaired spin densities f(3s) and f(3p) in the 3s and 3p chlorine orbitals have been determined from the resulting ENDOR parameters.

58. Magnetic resonance study of the Fe+(I) center in SrCl2 single crystals

Authors: Vrielinck, H; Callens, F; Matthys, P; Nistor, SV; Ghica, D; Schoemaker, D

Published: JUL 1 2001, PHYSICAL REVIEW B, 64, DOI: 10.1103/PhysRevB.64.024405

An Fe+-type center with axial (001) symmetry has been studied by electron nuclear double resonance (ENDOR) in the microwave X and Q bands. The Fe+(I) center is produced only after irradiation with x or gamma rays at 80 K of SrCl2:Fe2+ crystals grown in a Cl-2 atmosphere. It has a 3d(7) F-4 Gamma (8) ground state with spin S =3/2. As shown by the correlated analysis of the ENDOR data and electron paramagnetic resonance superhyperfine structure, the Fe+ ion is situated in the center of a tetragonally compressed cube of eight nearest Cl- ions. The simplest model of the Fe+(I) center, compatible with the magnetic resonance results, consists of an interstitial Fe+ ion with two substitutional monovalent cations, very likely K+ ions, in two opposite nearest-neighbor Sr2+ sites along the tetragonal axis of the center.

59. X and Q-band ENDOR study of the Fe+(I) center in chlorinated Srcl(2) single crystals

Authors: Vrielinck, H; Ghica, D; Callens, F; Nistor, SV; Schoemaker, D

Published: 2001, RADIATION EFFECTS AND DEFECTS IN SOLIDS, 155, 111, DOI: 10.1080/10420150108214101

Previous Electron Paramagnetic Resonance (EPR) studies have shown that Fe2+ doped SrCl2 crystals grown in a chlorine atmosphere exhibit new trapped electron Fe+ centers after irradiation with X or gamma-rays at 77 K. Two types of such centers, called Fe+ (I) and Fe+ (II), both exhibiting axial [001] symmetry, have been identified. We present here the results of an Electron Nuclear Double Resonance (ENDOR) study in X and Q-band on the Fe+ (I) center, which has been found to be thermally stable up to 700K. The superhyperfine and quadrupole interactions of the paramagnetic Fe+ ion with the nearest Cl- ions have been determined from the ENDOR angular dependence data. The results strongly suggest that the Fe+ (I) center consists of a Fe+ ion in the center of a cube of eight Cl- ions, compressed along an[001] direction.

60. Multilayer structures deposited by laser ablation

Authors: Dinescu, M; Stanciu, C; Ghica, D; Dinu, R; Sandu, V; Nastase, N; Balucani, M; Bondarenko, V; Frachina, L; Lamedica, G; Ferrari, A

Published: APR 20 1999, SENSORS AND ACTUATORS A-PHYSICAL, 74, 30, DOI: 10.1016/S0924-4247(98)00331-8

TiN/Si structures were deposited on Si wafers by pulsed laser deposition technique. The highly conductive TiN films were grown on heated (100) Si substrates by laser ablation of a high purity Ti target in nitrogen reactive atmosphere. Subsequently, the Si layer was deposited by laser ablation of a Si target in vacuum (down to 10(-6) mbar) or in low pressure inert gas. The nitrogen gas pressure and the substrate temperature were found to strongly influence the TiN film structure and orientation. The degree of crystallinity of the Si layer grown on the TiN film was found to depend on Si/TiN collector temperature. Values below 550 degrees C (the threshold of TiN oxidation activation) were used in the experiments. Techniques as X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), High Resolution Transmission Electron Microscopy (HRTEM), Scanning Force Microscopy (SFM) have been used to characterize the deposited structures. The TiN/Si structure rectifying properties were tested. The obtained Si/TiN/Si structure could be suitable for the building of Permeable Base Transistor (PBT, vertical MESFET) devices. (C) 1999 Elsevier Science S.A. All rights reserved.

61. Insight on Ni(II) and Cu(II) complexes of biguanide derivatives developed as effective antimicrobial and antitumour agents