Image

Dr. Corina SECU

Scientific Researcher III

Laboratory of Optical Processes in Nanostructured Materials

cesecu[AT]infim[DOT]ro

Publications

1. Crystallization processes of rare-earth doped GdF3 nanocrystals in silicate glass matrix: Dimorphism and photoluminescence properties
Authors: Secu, CE; Bartha, C; Radu, C; Secu, M

Published: OCT 15 2024, CERAMICS INTERNATIONAL, 50, DOI: 10.1016/j.ceramint.2024.06.335

Rare-earth doped GdF3 nanocrystals embedded in silica glassy matrix have been prepared by controlled crystallization of the xerogel; the influence of rare-earth ion (Pr, Sm, Eu, Tb, Dy, Er, Yb) and additional Li-codopant on structural and optical properties was discussed. The precipitation of RE-doped GdF3 nanocrystalline phase is the result of Gd-trifluoracetate thermolysis revealed as a strong exothermic peak at around 300 degrees C. Structural analysis of RE-doped SiO2-GdF3 glass-ceramic sample calcined at 525 degrees C showed the occurrence of GdF3 nanocrystals of about 25 nm size, showing hexagonal or orthorombic structure depending on the RE-ion. Under UV-light excitation at 273 nm of Gd3+ ions (S-8(7/2) -> I-6(13/2,15/2) transition), photoluminescence spectra showed characteristic RE3+ luminescence due to the non-radiative energy transfer between the excited Gd3+ and acceptor RE ions; the highest energy transfer (congruent to 80 %) was observed for Tb3+ and lowest (congruent to 21 %) for Sm3+.
2. Structural and magneto-optical investigations of citrate sol-gel derived barium hexaferrite nanocrystalline powder
Authors: Secu, M; Secu, CE; Matei, E; Negrilla, C; Turchenko, V; Radu, C; Polosan, S

Published: MAY 5 2024, JOURNAL OF ALLOYS AND COMPOUNDS, 983, 173897, DOI: 10.1016/j.jallcom.2024.173897

Magneto-optical measurements enable the identification of Fe2+ and Fe3+ in the R and S blocks of BaFe12O19 Mhexaferrite powder obtained by sol -gel processing, followed by thermal annealing at 900 C for 3 hours. The ferromagnetic phase is evidenced by the spin-majority configuration (Fe3+) due to their unpaired electrons between d -d orbitals, while the Fe2+ in the low spin possesses diamagnetic behavior strongly dependent on the surrounded crystal field. Additionally, the irradiation with gamma rays changes the ratio between Fe2+/Fe3+, mainly on the surface of BaFe12O19 nanocrystals. These changes were confirmed by X-ray Photoelectron Spectroscopy measurements, in which the concentration of Fe2+ increased from 69% to 82%, while the one of Fe3+ decreased from 31% to 18%. he thermoluminescent measurements reveal the same changes of Fe3+ in Fe2+ by electron capturing during irradiation, which is released as a red emission after recombination processes. The changes are explained by the increasing of some Fe-O bonds along the c-axis, mainly due to breaking a part of these bonds. The X-ray analysis confirms the changing of the parameters for the BaFe12O19 hexagonal structure.
3. Up-Conversion Luminescence and Magnetic Properties of Multifunctional Er3+/Yb3+-Doped SiO2-GdF3/LiGdF4 Glass Ceramics
Authors: Secu, C; Bartha, C; Radu, C; Secu, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 11, DOI: 10.3390/magnetochemistry9010011

Glassy nanocomposites containing Yb3+/Er3+-doped GdF3 and LiGdF4 nanocrystals have been prepared by controlled crystallization of the xerogel and the structural, up-conversion luminescence, and magnetic properties were analyzed and discussed. Structural and morphological analysis showed uniform distribution of both GdF3 and LiGdF4 nanocrystals (tens of nm size), embedded in silica glass matrix as the result of thermal decomposition of the trifluoracetates, revealed as a strong exothermic peak at about 300 degrees C; the Li-ions co-doping showed a strong influence on the GdF3 and LiGdF4 nanocrystalline fraction. The energy dispersive spectrometry mapping showed Gd, F and Yb, Er within the nanocrystals but not in the silica glass matrix. X-ray diffraction pattern analysis indicated the crystalline lattice distortion consistent with the Yb/Er incorporation in both fluoride nanocrystals. The "green" ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and "red" (F-4(9/2)-> I-4(15/2)) up-conversion luminescences at 525, 545, and 660 nm observed under 980 nm laser light pumping were assigned to the Er3+ ions deexcitation through a two-photon process. The magnetic properties of the nanocomposite are strongly temperature dependent. The magnetization hysteresis loops show a ferromagnetic behavior at low temperatures (5K) related to the rare-earth ions contribution and the saturation magnetization of 39 emu/g. At 300 K a paramagnetic behavior was observed that was ascribed to the non-interacting localized nature of the magnetic moment of the rare-earth ions. Hence, such novel, multifunctional magnetic and optical materials can allow the intertwining between magnetism and photonics and might offer new opportunities for new magneto-optical device development.
4. Multi-analytical characterization of the white inlaid decoration on the prehistoric pottery from southern Romania
Authors: Secu, M; Matei, E; Secu, C; Bartha, C; Buruiana, T; Rostas, AM; Popescu, AD; Boroneant, A; Bajenaru, R

Published: JUN 2023, SOLID STATE SCIENCES, 140, 107193, DOI: 10.1016/j.solidstatesciences.2023.107193

Complex physico-chemical investigations have been performed on white inlaid substance used in the orna-mentation of prehistoric clay artefacts from southern Romania ceramics from the Early Chalcolithic, up to the Middle/Late Bronze Age. Structural and morphological investigations of the white pigments have showed hundreds of nm up to microns size particles with calcite and hydroxyapatite (ash bone) as dominant components. The calcite was found on Early Chalcolithic pigment vessels while those where hydroxyapatite was dominant from the Middle/Late Bronze Age. FTIR spectra revealed the biogenic source of the hydroxyapatite (i.e. cremated animal bone) and the crystallinity degree values agrees with the expected temperatures of firing of the ancient furnaces; the calcite-based pigments were supposed to be filled post firing. The EPR was not able to able to provide a clear assignment of the cremated animal bones but thermoluminescence showed various sources of calcite.
5. Up-Conversion Luminescence and Magnetic Properties of Multifunctional Er3+/Yb3+-Doped SiO2-GdF3/LiGdF4 Glass Ceramics
Authors: Secu, C; Bartha, C; Radu, C; Secu, M

Published: JAN 2023, MAGNETOCHEMISTRY, 9, 11, DOI: 10.3390/magnetochemistry9010011

Glassy nanocomposites containing Yb3+/Er3+-doped GdF3 and LiGdF4 nanocrystals have been prepared by controlled crystallization of the xerogel and the structural, up-conversion luminescence, and magnetic properties were analyzed and discussed. Structural and morphological analysis showed uniform distribution of both GdF3 and LiGdF4 nanocrystals (tens of nm size), embedded in silica glass matrix as the result of thermal decomposition of the trifluoracetates, revealed as a strong exothermic peak at about 300 degrees C; the Li-ions co-doping showed a strong influence on the GdF3 and LiGdF4 nanocrystalline fraction. The energy dispersive spectrometry mapping showed Gd, F and Yb, Er within the nanocrystals but not in the silica glass matrix. X-ray diffraction pattern analysis indicated the crystalline lattice distortion consistent with the Yb/Er incorporation in both fluoride nanocrystals. The "green" ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and "red" (F-4(9/2)-> I-4(15/2)) up-conversion luminescences at 525, 545, and 660 nm observed under 980 nm laser light pumping were assigned to the Er3+ ions deexcitation through a two-photon process. The magnetic properties of the nanocomposite are strongly temperature dependent. The magnetization hysteresis loops show a ferromagnetic behavior at low temperatures (5K) related to the rare-earth ions contribution and the saturation magnetization of 39 emu/g. At 300 K a paramagnetic behavior was observed that was ascribed to the non-interacting localized nature of the magnetic moment of the rare-earth ions. Hence, such novel, multifunctional magnetic and optical materials can allow the intertwining between magnetism and photonics and might offer new opportunities for new magneto-optical device development.
6. Structural and Optical Characterization of Silica Nanospheres Embedded with Monodisperse CeO2-Eu3+ Nanocrystals
Authors: Secu, C; Bartha, C; Matei, E; Radu, C; Secu, M

Published: FEB 2022, MAGNETOCHEMISTRY, 8, 22, DOI: 10.3390/magnetochemistry8020022

Luminescent nanocrystals embedded into silica microspheres were shown to be useful for silica labeling for biological applications, ensuring mechanical and chemical stability, nontoxicity, biocompatibility and optical properties. We used sol-gel technology to prepare silica nanospheres embedded with fluorescent and magnetic Eu3+(1 mol%)-doped CeO2 nanocrystals. The X-ray diffraction pattern analysis and transmission electron microscopy investigations showed CeO2:Eu3+(1 mol%) nanocrystals of about 9 nm size and Ce3+ ions substitution by the Eu3+ ions; the nanocrystals dispersed inside the nanosized silica spheres of about 400 nm diameters. The photoluminescence spectra recorded under UV-light excitation showed Eu3+ ions luminescence peaks (D-5(0)-F-7(J), J = 0-4) accompanied by a weaker 425 nm luminescence due to the silica matrix; the quantum yield was 0.14. The weak hysteresis loop and magnetization curves recorded up to 20,000 Oe showed dominantly paramagnetic behavior associated with the silica matrix; a slight opening of the hysteresis loop to a very small magnetic field (about 0.005 Oe) was due to the presence of the two rare earth ions. The photonic crystal properties of SiO2-CeO2:Eu3+(1 mol%) silica nanospheres deposited as films on quartz plates were revealed by the two weak attenuation peaks at 420 and 500 nm and were associated with the reflection from different planes. The SiO2-CeO2:Eu3+(1 mol%) nanospheres are attractive potential candidates for photonics-related applications or for multifunctional bio-labels by combining the luminescence and magnetic properties of the nanocrystals.
7. Optical Properties of Transparent Rare-Earth Doped Sol-Gel Derived Nano-Glass Ceramics
Authors: Secu, M; Secu, C; Bartha, C

Published: NOV 2021, MATERIALS, 14, DOI: 10.3390/ma14226871

Rare-earth doped oxyfluoride glass ceramics represent a new generation of tailorable optical materials with high potential for optical-related applications such as optical amplifiers, optical waveguides, and white LEDs. Their key features are related to the high transparency and remarkable luminescence properties, while keeping the thermal and chemical advantages of oxide glasses. Sol-gel chemistry offers a flexible synthesis approach with several advantages, such as lower processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional flexibility. The review will be focused on optical properties of sol-gel derived nano-glass ceramics related to the RE-doped luminescent nanocrystals (fluorides, chlorides, oxychlorides, etc.) such as photoluminescence, up-conversion luminescence, thermoluminescence and how these properties are influenced by their specific processing, mostly focusing on the findings from our group and similar ones in the literature, along with a discussion of perspectives, potential challenges, and future development directions.
8. Investigations of BaCl2:Eu2+ nanophosphor using electron paramagnetic resonance, structural analysis and thermoluminescence
Authors: Secu, CE; Rostas, AM

Published: JAN 30 2020, JOURNAL OF ALLOYS AND COMPOUNDS, 815, DOI: 10.1016/j.jallcom.2019.152400

Structural analysis of the Eu2+-doped BaCl2 nanocrystals and the doping process was monitored and characterized via electron paramagnetic resonance spectroscopy. Structural analysis has shown a slight distortion of the cell which is reflected in the low value of microstrain and the Eu2+-doping effect is limited to the first order chlorine ions neighbors of Ba2+. Electron paramagnetic resonance measurements have indicated the presence of the Eu2+-dopant ions in the BaCl2 host matrix with a solubility limit of about 2%. From the spectra simulation, the isotropic g-value g(iso) = 1.9951(7), the isotropic hyperfine coupling constant A(iso)= 42.4 MHz and the high-order zero-field splitting parameters from the crystal field B-2(0) = 21 MHz and B-2(2)= -493 MHz were obtained. During X-ray irradiation, defects are produced and stabilized by the Eu2+ dopant ions. The single dominant thermoluminescence peak at 132 degrees C (activation energy E = 1.1 eV) was assigned to the recombination of the F(Cl)-center with Eu2+ related hole centers. (C) 2019 Elsevier B.V. All rights reserved.
9. Towards a Correlation between Structural, Magnetic, and Luminescence Properties of CeF3:Tb(3+)Nanocrystals
Authors: Bartha, C; Secu, C; Matei, E; Negrila, C; Leca, A; Secu, M

Published: JUL 2020, MATERIALS, 13, DOI: 10.3390/ma13132980

In this study, we report on the structural, magnetic, and optical properties of Tb3+-doped CeF(3)nanocrystals prepared via a polyol-assisted route, followed by calcination. X-ray diffraction analysis and electron microscopy investigations have shown the formation of a dominant Ce(0.75)F(3)nanocrystalline phase (of about 99%), with a relatively uniform distribution of nanocrystals about 15 nm in size. Magnetization curves showed typical paramagnetic properties related to the presence of Ce(3+)and Tb(3+)ions. The magnetic susceptibility showed a weak inflexion at about 150 K, assigned to the cerium ions' crystal field splitting. Under UV light excitation of the Ce(3+)ions, we observed Tb(3+)green luminescence with a quantum yield of about 20%.
10. Sol-Gel Processing of Bismuth Germanate Thin-Films
Authors: Secu, M; Secu, CE; Tite, T; Polosan, S

Published: MAR 2020, COATINGS, 10, DOI: 10.3390/coatings10030255

This study aims to obtain uniform and homogeneous bismuth germanate oxides thin films by spin coating and using the sol-gel technique with different precursors, followed by low-temperature annealing at 560 degrees C. By using Bi(NO3)(3) precursors, we have obtained transparent, yellowish thin films with a 200 nm thickness. The structural analysis of the initial sol-gel powder has shown the presence of two crystalline structures, the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 crystallites, which evolves towards the BGO structure after annealing. The elemental analysis confirmed the composition of the desired compound Bi4Ge3O12 with 60 wt % GeO2 and 40 wt % Bi2O5. On the other hand, by changing the precursor to (Bi(CH3COO)(2), the film thickness increased to 500 nm thicker due to the high viscosity of the sol, and a dominant monoclinic Bi2GeO5 crystalline structure appeared. The elemental analysis revealed a nonstoichiometric composition with 38 wt % GeO2 and 62 wt % Bi2O3. Due to the low GeO2 phase content that reacted with metastable Bi2GeO5, we obtained cubic Bi4Ge3O12 as a secondary phase, with Bi2GeO5 as a dominant crystalline phase. The redshifts of both absorptions and emissions spectra peaks confirmed a different disorder structure as an interplay between the cubic Bi4Ge3O12 (BGO) and monoclinic Bi2GeO5 phases.
11. Processing and Optical Properties of Eu-Doped Chloroborate Glass-Ceramic
Authors: Secu, M; Secu, CE

Published: DEC 2020, CRYSTALS, 10, DOI: 10.3390/cryst10121101

An europium doped BaO-B2O3-BaCl2 chloroborate glass-ceramic containing a BaCl2 nanocrystalline phase was produced by melt-quenching followed by glass crystallization during annealing. Structural and morphological investigations using x-ray diffraction and scanning electron microscopy have shown fvBaCl(2) nanocrystals of about tens of nm size accompanied by a smaller amount of the BaB2O4 crystalline phase. Photoluminescence spectra have indicated the reduction of Eu3+ to Eu2+ during processing in air or a reducing atmosphere. The spectra analysis showed the presence of Eu3+ ions in the borate glass matrix, while the Eu2+ were incorporated in both the BaCl2 nanocrystals and glass matrix. Thermoluminescence properties were due to the recombination of F(Cl) centers and Eu2+ related hole centers produced by irradiation within the BaCl2 nanocrystals. The color impression of the samples and the photoluminescence quantum efficiency were influenced by the glass processing.
12. Raman investigations on gamma irradiated iPP-VGCNF nanocomposites: The polymer's tale
Authors: Chipara, DM; Secu, C; Lozano, K; Secu, M; Chipara, M

Published: DEC 2019, SURFACES AND INTERFACES, 17, DOI: 10.1016/j.surfin.2019.100351

Raman investigations on nanocomposites obtained by loading various amounts of vapor grown carbon nanofibers within an isotactic polypropylene matrix, and gamma irradiated in air, at various integral doses ranging between 0 and 27 kGy, are reported. The analysis is focused on the polymer's answers as revealed by Raman spectroscopy and investigate in detail the effect of ionizing radiation on the position of the Raman line originating from the polymer. The as-obtained data are correlated to the elastic features of the nanocomposites. A competition between gamma irradiation and loading by carbon nanofiber, resulting in the stretching of the polymeric matrix and revealed as a displacement of Raman lines towards smaller wavenumber is reported. It is concluded that side groups (CH3) are less affected by the loading with carbon nanofibers,
13. The influence of the nanocrystals size and surface on the Yb/Er doped LaF3 luminescence properties
Authors: Secu, CE; Matei, E; Negrila, C; Secu, M

Published: JUN 30 2019, JOURNAL OF ALLOYS AND COMPOUNDS, 791, 1104, DOI: 10.1016/j.jallcom.2019.03.267

Up-conversion luminescence and thermoluminescence properties of LaF3 nanocrystals, with sizes of about 20 nm, were studied and discussed in relation to -size and surface related effects. XPS spectra have evidenced the presence of oxidized Er and La ions within a thin layer (of about 1 nm thickness) at the nanocrystals surface, as well as Yb ions bonded with fluorine ions. The green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion emissions of Er3+ ions are influenced by the relative dominance of Erions that reside within the thin oxidized layer. The broad thermoluminescence curves are assigned to the recombination of trap defects associated with surface states and within the oxidized surface layer. (C) 2019 Elsevier B.V. All rights reserved.
14. Gd3+ co-doping influence on the morphological, up-conversion luminescence and magnetic properties of LiYF4:Yb3+/Er3+ nanocrystals
Authors: Secu, CE; Bartha, C; Matei, E; Negrila, C; Crisan, A; Secu, M

Published: JUL 2019, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 130, 241, DOI: 10.1016/j.jpcs.2019.03.003

The influence of the Gd3+ co-dopant on the structure, morphology and up-conversion luminescence/magnetic properties of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals was investigated and compared to those of Gd-free samples. Electron microscopy has indicated an inhibiting effect of the agglomeration and collapsing process of the nanocrystals compared to the Gd-free powder samples. The surface analysis of nanocrystals have not shown oxygen-metal bonds related to the metal oxidation within the surface nanometric layer. The paramagnetic properties are related to the magnetic moment of the Gd3+ ions. The up-conversion luminescence of the LiYF4:Gd3+/Yb3+/Er3+ nanocrystals is about six times stronger than in LiYF4:Yb3+/Er3+ nanocrystals; the enhancement effect is most probably due to the lattice distortion induced by the Gd co-doping.
15. Investigation of sol-gel derived BaCl2:Eu2+ luminescent nanophosphor and the corresponding PVP@BaCl2:Eu2+ polymer nanocomposite
Authors: Secu, CE; Negrila, C; Secu, M

Published: AUG 1 2018, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 51, DOI: 10.1088/1361-6463/aace7c

We have prepared Eu2+ (1%) doped BaCl2 nanopowders by using a hybrid sol-gel/thermal decomposition route using barium acetate and trichloracetic acid as in situ chlorination agent. Structural and thermal analysis data have shown that orthorhombic BaCl2 phase crystallization occurs at 260 degrees C. BaCl2:Eu2+ nanocrystals of about 100-200 nm in size were produced after annealing (in air) at 500 degrees C and they displayed Eu2+ d -> f type luminescence at 396 nm. X-ray photoelectron spectroscopy indicated the presence of both Eu2+, Eu3+ ions species and a BaO passivating layer at the surface of the nanocrystals. A polymer nanocomposite (PVP@BaCl2:Eu2+) was also processed and its optical response under x-ray irradiation was found to be similar to that of the nanopowder. X-ray excited luminescence showed Eu2+ luminescence at 396 nm, whereas the thermoluminescence peak at 130 degrees C was assigned to the F(Cl)-center recombination with Eu2+ related hole centers.
16. Synthesis and up-conversion luminescence properties of BaFBr-Er3+ @SiO2 core/shell heterostructures
Authors: Secu, CE; Secu, M; Cernea, M

Published: AUG 2017, JOURNAL OF LUMINESCENCE, 188, 100, DOI: 10.1016/j.jlumin.2017.04.015

BaFBr:Er3+ @SiO2 core-shell composites were synthesized starting from BaFBr:Er3+ phosphor core grains (BaFBr doped with 1 at% Er3+) prepared by coprecipitation method. A sol-gel process was used to obtain the outer Si02 layer shell from silica sol precursor. The polycrystalline BaFBr:Er3+ @SiO2 heterostructures were formed following calcination at 700 degrees C, in air. The resulting material was characterized in terms of microstructure, photoluminescence and up-conversion luminescence properties. The results indicate that the as-obtained composite has a core-shell structure and shows good luminescence properties. The BaFBr:Er3+ @SiO2 heterostructure is made of BaFBr:Er3+ square sheets encased in a SiO2 shell layer with thickness values between 100 and 400 nm. Under 810 nm laser light pumping, the core-shell heterostructure exhibits Er3+ green up conversion luminescence bands ((H-2(11/2),S-4(3/2)) -> I-4(15/2)) at 525 and 545 nm, ascribed to a two-photon process.
17. Laser processing of Yb3+/Er3+ co-doped LiYF4 thin films with up-conversion properties
Authors: Secu, CE; Secu, M; Stokker-Cheregi, F; Ion, V; Brajnicov, S; Dinescu, M

Published: MAR 1 2017, THIN SOLID FILMS, 625, 10, DOI: 10.1016/j.tsf.2017.01.057

We present a comparative study between the properties of Yb3+/Er3+ co-doped LiYF4 thin films obtained by pulsed laser deposition (PLD) and matrix-assisted pulsed evaporation (MAPLE) growth techniques, respectively. We performed structural, morphological, and optical analyses in order to draw a conclusion about the potential of MAPLE as a growth technique used for the obtaining of RE-doped fluoride thin films having up-conversion properties. For both techniques the surface morphology is typical for laser processed thin films of RE-doped fluorides, with roughness values of the order of few hundreds of nanometers and a higher density of defects in the sample obtained by MAPLE. Up-conversion properties do not exhibit major differences between the thin film samples and the pressed target. Under 980 nm laser light pumping, both Yb3+/Er3+ co-doped LiYF4 thin film samples, obtained by PLD and MAPLE, show green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion luminescence explained by a two-photon processes. Therefore MAPLE is a viable growth technique for the fundamental study of RE-doped fluoride thin films and their respective functional properties. (C) 2017 Elsevier B.V. All rights reserved.
18. Crystallization kinetics mechanism investigation of sol-gel-derived NaYF4:(Yb,Er) up-converting phosphors
Authors: Bartha, C; Secu, CE; Matei, E; Secu, M

Published: SEP 14 2017, CRYSTENGCOMM, 19, 5000, DOI: 10.1039/c7ce01265a

The crystallization mechanism of sol-gel-derived NaYF4:(Yb, Er) up-converting phosphors has been studied by differential scanning calorimetry analysis using both model-free and model fitting approaches. Structural and optical data have shown that the hexagonal NaYF4:(Yb, Er) phase crystallization process occurs at around 315 degrees C as a result of the thermal decomposition of the metal trifluoroacetates. As the annealing temperature increases, sphere-like microcrystals of about 1-2 mu m size (at 300 degrees C) break up into smaller ones (400-500 nm size) and finally collapse at higher temperatures (600 degrees C); the up-conversion luminescence signal intensity increases due to the crystallinity improvement and dehydration process. The crystallization process can be described as an autocatalytic-type reaction where the accompanying cubic NaYF4 phase played a catalytic role by reducing the energy barrier against the crystallization of the hexagonal NaYF4 phase, causing its fast self-accelerated crystallization. The energy resulting from the disintegration process of the initial NaYF4 microcrystals contributed to the growth and agglomeration processes and finally the collapse of the crystalline fragments with increasing temperature.
19. Non-isothermal crystallization kinetics growth of LiYF4(Yb,Er) nanoparticles
Authors: Bartha, C; Secu, CE; Secu, M

Published: DEC 2016, CERAMICS INTERNATIONAL, 42, 18736, DOI: 10.1016/j.ceramint.2016.09.012

The crystallization process of Yb3+/Er3+ codoped LiYF4 nanoparticles obtained by sol-gel method was studied using differential scanning calorimetry and X-ray diffraction. Thermal data obtained from differential scanning calorimetry measurements under non-isothermal conditions at different heating rates were examined using both model-free and model fitting approaches. It was shown that LiYF4 phase crystallization occurs at around 383 degrees C as a result of the thermal decomposition of trifluoracetates and is shifted to higher temperatures as the heating rate increases. Both Friedman and Ozawa-Flynn-Wall analyses have indicated that the activation energy and pre-exponential factor decrease slightly as the crystallization proceeds. According to the model-fitting approach (by using extended Prout-Tompkins nth-order, ath autocatalysis reaction), LiYF4 nanocrystalline phase is the result of an autocatalytic process where a second metastable phase YF3 acts as a catalyst that accelerates the crystallization process of the LiYF4 phase. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
20. SOL-GEL TEMPLATE ASSISTED SYNTHESIS AND UP-CONVERSION PROPERTIES OF (Yb-Er) DOPED SiO2-LiYF4 GLASS CERAMIC RODS
Authors: Secu, CE; Matei, E

Published: APR-JUN 2016, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 11, 494, DOI:

Up-conversion properties of the glass-ceramics microrods containing (Er3+, Yb3+)-doped LiYF4 nanocrystals have been studied. Under 980 nm laser light pumping, both Yb3+/Er3+ co-doped LiYF4 samplesshow green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion luminescences explained by a two-photon processes. The luminescence decay time of the green luminescence is much shorter in the glass-ceramics microrods (0.9 mu s) compared to the bulk powder (15 mu s) indicating the enhancement of the non-radiative relaxation rate due to the cross-relaxation effects between Er3+-ions. The quantum efficiencies were estimated for the green luminescence being much smaller in the glass-ceramic rods (eta = 0.15%) compared the bulk powder (eta = 2%) or pellet (eta = 46%).
21. Up-conversion luminescence of BaCl2:Er3+ nanocrystals embedded in oxychloride nano-glass ceramic
Authors: Secu, M; Secu, CE

Published: JUN 2016, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 36, 1703, DOI: 10.1016/j.jeurceramsoc.2016.01.016

A novel glassy nanocomposite containing Er3+-doped BaCl2 nanocrystals of about tens of nm embedded in a silicate matrix has been prepared by using sol-gel route followed by controlled crystallization of the xerogel. Under 810 nm laser light pumping it shows green ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and red (F-4(9/2) -> I-4(15/2)) up-conversion luminescences ascribed by a two-photon processes. The luminescence signal recorded in the oxychloride nano-glass ceramic is only about 10% compared to the NaYF4:Er3+ (18%). In the nano glass ceramic there are effective non-radiative decay channels, most probably related to hydroxyl ions and surface defects acting as centers for the non-radiatively energy dissipation. (C) 2016 Elsevier Ltd. All rights reserved.
22. Crystallization and luminescence properties of a new Eu3+-doped LaOCl nano-glass-ceramic
Authors: Secu, M; Secu, CE; Bartha, C

Published: JAN 2016, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 36, 207, DOI: 10.1016/j.jeurceramsoc.2015.09.001

Glassy nanocomposites containing Eu3+-doped LaOCl nanocrystals of about tens of nm size embedded in a silica matrix have been prepared by using sol-gel route followed by controlled crystallization of the xerogel. Crystallization behavior of LaOCl nanocristalline phase was discussed on the basis of thermal analysis, X-ray diffraction and photoluminescence measurements. LaOCl nanocrystalline phase precipitation is the result of lanthanum chloride hydrolytic and oxidative reactions. As the annealing temperature increases nanocrystallites size grows up to tens of nm; Eu3+-ions are gradually incorporated during the nanocrystals growth. Structural changes were studied using the Eu3+ as probe ion and the luminescence properties were analysed by using Judd-Ofelt theory. (C) 2015 Elsevier Ltd. All rights reserved.
23. Structural and optical properties of fluorescent BaFBr-Eu2+@SiO2 core/shell phosphor heterostructure
Authors: Secu, M; Cernea, M; Secu, CE; Vasile, BS

Published: FEB 1 2015, MATERIALS CHEMISTRY AND PHYSICS, 151, 86, DOI: 10.1016/j.matchemphys.2014.11.038

Core-shell composites BaFBr:Eu2+@SiO2 were synthesized starting from BaFBr:Eu2+ nanophosphor powder prepared by precipitation method, followed by a silica gel coating process. Electron microscopy investigations have shown an average BaFBr:Eu2+ grains size of about 30 nm and 11 nm SiO2 shell thickness. The core-shell heterostructure exhibits Eu2+ luminescence at 385 nm that is improved during subsequent calcination at high temperatures up to 700 degrees C; the core-shell morphology (size and thickness) structure is preserved during the calcination. (C) 2014 Elsevier B.V. All rights reserved.
24. Up-conversion luminescence of Er3+/Yb3+ co-doped LiYF4 nanocrystals in sol-gel derived oxyfluoride glass-ceramics
Authors: Secu, M; Secu, CE

Published: OCT 15 2015, JOURNAL OF NON-CRYSTALLINE SOLIDS, 426, 82, DOI: 10.1016/j.jnoncrysol.2015.07.010

Up-conversion properties of Yb3+/Er3+ co-doped LiYF4 nanocrystals embedded in a sol-gel derived oxyfluoride glass-ceramic have been studied by comparison to the corresponding polycrystalline sample. Under 980 rim laser light pumping, both Yb3+/Er3+ co-doped LiYF4 samples show green ((H-2(11/2,) S-4(3/2)) I-4(15/2)) and red (F-4(9/2) I-4(15/2)) up-conversion luminescences explained by a two-photon processes. The tendency for the saturation of the red up-conversion luminescence band observed in the glass ceramic was ascribed to a back energy transfer process Er3+-Yb3+ and cross-relaxation processes caused by the high Yb3+ and Er3+ ions Concentrations in the nanocrystals. (C) 2015 Elsevier B.V. All rights reserved.
25. Luminescence properties of Eu3+-doped SiO2-LiYF4 glass-ceramic microrods
Authors: Secu, CE; Secu, M

Published: SEP 2015, OPTICAL MATERIALS, 47, 98, DOI: 10.1016/j.optmat.2015.07.009

Photoluminescence properties of the glass-ceramics microrods containing Eu3+-doped LiYF4 nanocrystals have been studied and characterized. Judd-Ofelt parameters and quantum efficiency has been computed from luminescence spectra and discussed by comparison to the glass ceramic bulk and pellet. The radiative decay rate A(rad) is higher in the glass ceramic rods (221 s(-1)) than in the glass ceramic bulk (130 s(-1)) but the quantum efficiency computed is very low (21%) compared to the glass-ceramic bulk (97%). There are effective non-radiative decay channels that might be related to an influence of the dimensional constraints imposed by the membrane pores during xerogel formation and subsequent glass ceramization. (C) 2015 Elsevier B.V. All rights reserved.
26. Thermally activated conversion of a silicate gel to an oxyfluoride glass ceramic: Optical study using Eu3+ probe ion
Authors: Secu, CE; Bartha, C; Polosan, S; Secu, M

Published: FEB 2014, JOURNAL OF LUMINESCENCE, 146, 543, DOI: 10.1016/j.jlumin.2013.10.013

Sol-gel route using metal alkoxides precursor and trifluoroacetic acid as in situ fluorination reagent has been used to prepare Eu3+ -doped silicate xerogel, followed by thermal annealing to obtain oxyfluoride glass ceramic containing Eu3+ -doped BaF2 nanocrystals. We have used Eu3+ as probe ion and we analyzed its characteristic features (photoluminescence, optical absorption and magnetic circular dichroism) to get information about the local environment around the ion during thermally activated evolution of the sol to xerogel and then glass ceramic. As the drying and annealing proceeds silica network is formed accompanied by precipitation of the nanofluoride crystalline phase; Eu3+ coordination changes gradually from a random and assymetric CF3COO- one (in the sol) to a symmetric one (in the BaF2 nanocrystals) given by the fluorine ions. Glass ceramization is based on a homogenous crystallization mechanism with BaF2 nucleation centres resulted from thermal decomposition of Ba-trifluoacetate at around 300 degrees C followed by subsequent growth into BaF2 nanocrystals above 600 degrees C; Eu3+-ions are incorporated during the nanocrystals growth. (C) 2013 Elsevier B.V. All rights reserved.
27. Photoluminescence of Eu-doped LiYF4 thin films grown by pulsed laser deposition and matrix-assisted pulsed laser evaporation
Authors: Stokker-Cheregi, F; Matei, A; Dinescu, M; Secu, CE; Secu, M

Published: JAN 29 2014, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 47, DOI: 10.1088/0022-3727/47/4/045304

Matrix-assisted pulsed laser evaporation (MAPLE) has been investigated as an alternative to the pulsed laser deposition (PLD) technique for Eu3+-doped crystalline LiYF4 thin-films deposition. MAPLE assumes laser ablation of a frozen target made of the material of interest diluted in a solvent, rather than that of a bulk target, of either pressed powder or single crystal, used in the case of PLD. Our approach stems from the assumption that laser ablation of a frozen dilute target would result in thin films with improved morphology, as compared to PLD. Indeed, we find that roughness values of samples obtained by the MAPLE technique are four times lower than in the case of PLD. A lower transmittance was noticed for PLD obtained layers with respect to those grown by MAPLE due to strong scattering of light by the morphological defects. Photoluminescence spectra are showing characteristic Eu3+-ion luminescence bands at 578, 591, 612, 650 and 698 nm (D-5(0) -> F-7(J)); crystal field splitting of the bands indicates dopant ions incorporation in the host material during transfer by either PLD or MAPLE.
28. BaFBr:Eu2+ nanophosphor-SiO2 hybrid entrapped in Anodise Alumina membrane pores array
Authors: Secu, CE; Matei, E; Secu, M; Damian, V

Published: SEP 2014, RADIATION MEASUREMENTS, 68, 41, DOI: 10.1016/j.radmeas.2014.07.001

Sol-gel template method has been used to prepare BaFBr:Eu2+ nanophosphor-SiO2 hybrid entrapped within the nanopores array (of about 200 nm size) of a comercial anodized alumina (AA) membrane. Structural and morphological measurements using electron microscopy (SEM) and X-ray diffraction (XRD) have shown the presence of the BaFBr:Eu2+ nanophosphor in the silica xerogel entrapped within the nanopores array; photoluminescence and X-ray excited luminescence measurements have shown Eu2+ luminescence at 395 nm accompanied by a broad band due to AA membrane. The method assures a relatively uniform spreading of the BaFBr nanophosphor into the AA membrane pores array without the nanoparticles agglomeration. Preliminary imaging tests have shown a spatial resolution in the micrometer range and even in the submicrometer range can be expected. As BaFBr:Eu2+ is a very efficient Xray phosphor the material might be used as X-ray micro-imaging detector. (C) 2014 Elsevier Ltd. All rights reserved.
29. Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol-gel derived glass-ceramics
Authors: Georgescu, S; Voiculescu, AM; Matei, C; Secu, CE; Negrea, RF; Secu, M

Published: NOV 2013, JOURNAL OF LUMINESCENCE, 143, 156, DOI: 10.1016/j.jlumin.2013.04.002

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650 degrees C of the (Yb3+/Er3+) codoped CaF2-SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanoctystals of about 16 nm size. For pumping at 973 nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at similar to 408 nm) and near-ultraviolet emission (at similar to 380 nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated. (C) 2013 Elsevier B.V. All rights reserved.
30. Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic
Authors: Secu, CE; Negrea, RF; Secu, M

Published: OCT 2013, OPTICAL MATERIALS, 35, 2460, DOI: 10.1016/j.optmat.2013.06.053

Sol-gel route using metal alkoxides and trifluoroacetic acid as precursors has been used to prepare oxyfluoride glass-ceramic containing Eu3+-doped LiYF4 nanocrystals of about tens of nm size embedded in a silica matrix through controlled crystallization at higher temperatures of the xerogel. Photoluminescence spectra and decay curves recorded in the Eu3+-doped LiYF4 polycrystalline pellet and glass ceramic have been discussed using group-theoretical arguments. In the glass-ceramic Eu3+ ions are embedded dominantly inside the LiYF4 nanocrystals most probably as Eu-O center and/or dimer centers in low symmetry (C-2v) sites; oxygen ions were incorporated in their neighborhood during the glass ceramization. (C) 2013 Elsevier B.V. All rights reserved.
31. Structural and optical characterization of sol-gel derived Tm-doped BaTiO3 nanopowders and ceramics
Authors: Cernea, M; Secu, CE; Vasile, BS; Secu, M

Published: JAN 2013, CURRENT APPLIED PHYSICS, 13, 141, DOI: 10.1016/j.cap.2012.06.024

Nanocrystalline Tm3+(5%)-doped BaTiO3 (BT-Tm) has been synthesized by the sol-gel method. The morphology, structure, and optical properties of powders and ceramics were characterized. The average grain size of the gel precursor annealed at 700 and 900 degrees C was 20 nm and 30 nm, respectively. These powders were single phase and crystallized with a cubic structure while the BT-Tm sintered ceramics were crystallized with the tetragonal BaTiO3 structure. The photoluminescence spectra showed typical transitions of Tm3+ ions and a structure consistent with the Tm3+ ions incorporation in the BaTiO3 crystalline lattice. Thermoluminescence peaks recorded at 300 degrees C (for annealed samples) or at 230 degrees C for the ceramic sample were assigned to the recombination of the Tm2+-electron traps located mainly at the surface of the nano-crystals or inside the microcrystals, respectively. (C) 2012 Elsevier B.V. All rights reserved.
32. Luminescent Eosin Y-SiO2 hybrid nano and microrods prepared by sol-gel template method
Authors: Secu, M; Secu, CE; Sima, M; Negrea, RF; Bartha, C; Dinescu, M; Damian, V

Published: NOV 2013, JOURNAL OF LUMINESCENCE, 143, 92, DOI: 10.1016/j.jlumin.2013.04.043

Sol-gel chemistry within the pores of a polycarbonate template membrane was used for the preparation of Eosin Y-SiO2 hybrid nano- and microrods, using tetraethylorthosilicate [TEOS, Si(OC2H5)(4)] as the precursor in the presence of trifluoroacetic acid (TFA) catalyst. The ethanolic solution of Eosin-Y was added to the silica sal to trap dye molecules inside the SiO2 gel network during the gelation. Structural and morphological characterization using scanning electron microscopy (SEM) and luminescence microscopy have shown the formation of rods with 200 nm and 1.2 mu m diameter and about 30 mu m length, exhibiting luminescence properties. Spectroscopic characterization has shown that the luminescence is due to Eosin-Y molecule in the xerogel porous network, surrounded by a solvation shell given mainly by the water. (C) 2013 Elsevier B.V. All rights reserved.
33. Spectroscopic ellipsometry investigations of Eu-doped oxy-fluoride glass and glass-ceramics
Authors: Galca, AC; Preda, N; Secu, CE; Luculescu, CR; Secu, M

Published: JUN 2012, OPTICAL MATERIALS, 34, 1496, DOI: 10.1016/j.optmat.2012.03.015

Oxyfluoride glass-ceramics in the system SiO2-Al2O3-CaF2-EuF3 containing Eu3+-doped CaF2 nanocrystals were produced by using controlled crystallization of melt-quenched glass. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 50 nm size. The Eu3+-dopant ions act as the nucleating agent necessary to initiate the crystallization process. The refractive index is higher in the glass ceramics than in the initial glass and varies as the annealing time increases. Two competitive processes are responsible for this behavior, the crystallization of the CaF2 phase and the decrease of the glass ceramic mass density. (C) 2012 Elsevier B.V. All rights reserved.
34. Sol-gel template synthesis of luminescent glass-ceramic rods
Authors: Secu, M; Secu, CE; Sima, M

Published: APR 2012, JOURNAL OF NANOPARTICLE RESEARCH, 14, DOI: 10.1007/s11051-012-0772-1

We report an original way to prepare luminescent glass-ceramic microrods containing Eu3+ doped BaF2 nanocrystals by sol-gel chemistry within the pores of a polycarbonate template membrane. Structural characterization by scanning electron microscopy and X-ray diffraction has shown the formation of glass-ceramic microrods with 0.8-m diameter of and 10 mu m length in which BaF2 nanocrystals of about 30 nm size are embedded. Photoluminescence measurements have indicated the incorporation of Eu3+ ions inside the BaF2 nanocrystals in a broad range of sites with low coordination symmetry. The comparison made with the bulk glass-ceramic indicated an influence of the dimensional constraints imposed by the membrane pores during xerogel formation and subsequent glass ceramization.
35. Magneto-optical investigations of rare earth doped sol-gel derived silicate xerogels
Authors: Secu, CE; Polosan, S; Secu, M

Published: AUG 2011, JOURNAL OF LUMINESCENCE, 131, 1752, DOI: 10.1016/j.jlumin.2011.04.031

Rare-earth doped xerogels (Eu3+, Sm3+, Ho3+, Pr3+) were prepared by using the sal-gel method and their magneto-optical and optical properties have been studied. The Magnetic Circular Dichroism (MCD) spectra are quite similar to those recorded in the RE-doped fluorozirconate glasses; the fine structures shown by the MCD spectra are better resolved compared to the optical absorption spectra. The MCD technique has been correlated with optical measurements in order to investigate the site symmetry in the particular case of Eu3+-doped xerogel and oxyfluoride glasses. In the xerogel, coordination symmetry around the Eu3+ ions is close to (D) under bar (3h) and is lower in the oxyfluoride glass. (C) 2011 Elsevier B.V. All rights reserved.
36. Structural characterization and photoluminescence of nanocrystalline Ho-doped BaTiO3 derived from sol-gel method
Authors: Secu, M; Cernea, M; Secu, CE; Vasile, BS

Published: AUG 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 3128, DOI: 10.1007/s11051-011-0224-3

Nanocrystalline Ho-doped BaTiO3, with average nanocrystals size of 20 nm, have been prepared using a sol-gel combustion technique. The structural and morphological properties of the powders have been investigated by X-ray powder diffraction and high resolution transmission electron microscopy. Chemical states of the holmium on the Ba0.97Ho0.03TiO3 ceramic surface were analyzed using X-ray photoelectron spectroscopy. Furthermore, their photoluminescence properties were analyzed.
37. Eu3+-doped CaF2 nanocrystals in sol-gel derived glass-ceramics
Authors: Secu, M; Secu, CE; Ghica, C

Published: FEB 2011, OPTICAL MATERIALS, 33, 617, DOI: 10.1016/j.optmat.2010.11.016

Glass-ceramics containing Eu3+-doped CaF2 nanocrystals of about 16 nm size have been made using the controlled crystallization at higher temperatures of the Eu3+-doped CaF2-SiO2 xerogels. In the glass-ceramic material the Eu3+ ions are embedded in both silica network and in the non-centrosymmetric sites of the CaF2 nanocrystals structure. Lower phonon energy of CaF2 and the dehydratation processes reduce the probability of non-radiative de-excitations and as a result a luminescence enhancement and an increase of the PL lifetime is observed. Eu-doping modifies the traps distribution within the glass-ceramics and as a result an additional broad thermoluminescence peak at about 380 degrees C was observed. The peak was assigned to the recombination of the electrons thermal released from the Eu3+-related traps in the glass matrix and in the CaF2 nanocrystals. (C) 2010 Elsevier B.V. All rights reserved.
38. Retrospective dosimetry assessment using the 380 degrees C thermoluminescence peak of tooth enamel
Authors: Secu, CE; Cherestes, M; Secu, M; Cherestes, C; Paraschiva, V; Barca, C

Published: OCT 2011, RADIATION MEASUREMENTS, 46, 1112, DOI: 10.1016/j.radmeas.2011.07.014

The thermoluminescence (TL) response to gamma-ray irradiation of tooth enamel is reported. The tooth enamel was separated from dentine by using mechanical and physico-chemical procedures followed by grinding (grain size 100 urn) and etching. Then was attributed to the recombination of CO(2)(-) radicals incorporated into or attached to the surface of hydroxyapatite crystals. The growth of the 380 degrees C peak with absorbed dose was examined with irradiated tooth enamel samples and reconstructed doses evaluated for tooth enamel samples from four human subjects. (C) 2011 Elsevier Ltd. All rights reserved.
39. Structural and thermoluminescence properties of undoped and Fe-doped-TiO2 nanopowders processed by sol-gel method
Authors: Cernea, M; Secu, M; Secu, CE; Baibarac, M; Vasile, BS

Published: JAN 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 85, DOI: 10.1007/s11051-010-0002-7

In this study, we report on the nanocrystalline powders of TiO2 and Fe-doped TiO2 (anatase and rutile phases) prepared by sol-gel method. The X-ray diffraction and Raman spectroscopy measurements indicated the presence of anatase or rutile phase in nanopowders. TEM micrographs showed 10 and 112 nm average particle sizes for anatase and rutile, respectively. Furthermore, their thermoluminescence properties were analyzed.
40. Rare-earth doped sol-gel derived oxyfluoride glass-ceramics: Structural and optical characterization
Authors: Secu, CE; Secu, M; Ghica, C; Mihut, L

Published: SEP 2011, OPTICAL MATERIALS, 33, 1774, DOI: 10.1016/j.optmat.2011.06.004

Glass-ceramics containing RE3+-doped BaF2 nanocrystals (RE = Eu, Sm, Dy, Ho and Pr) with the size below 10 nm size have been made by using the controlled crystallization at higher temperatures of the RE3+-doped SiO2-BaF2 xerogels. Photoluminescence measurements have indicated the incorporation of the RE3+ ions in both silica network and in the BaF2 nanocrystals. Thermoluminescence measurements have shown a peak whose position depends on the nature of RE3+-dopant as it follows: 140 degrees C (for Ho3+, Dy3+), 340 degrees C (for Sm3+) and 370 degrees C (for Eu3+); in Eu3+-doped SiO2 glass the TL peak is shifted to 383 degrees C. The peaks in glass-ceramics were assigned to the recombination of the electrons thermal released from the RE3+-electron traps located in both glass-matrix and BaF2 nanocrystals. Within the series the trivalent lanthanide ions act as increasingly deeper electron trapping centres. (C) 2011 Elsevier B.V. All rights reserved.
41. Photoluminescence and thermoluminescence of ZnO nano-needle arrays and films
Authors: Secu, CE; Sima, M

Published: APR 2009, OPTICAL MATERIALS, 31, 880, DOI: 10.1016/j.optmat.2008.10.025

ZnO nano-needle arrays and films have been prepared by anodic etching of ZnO foil at constant current in potassium zincate solution and sodium hydroxide alcoholic solution, respectively. Photoluminescence (PL) spectra recorded on ZnO nano-needles have shown a broad band at 585 nm due to the native oxygen defects incorporated during the synthesis processes; in a ZnO film-type sample this band is replaced by another one at 432 nm. Thermoluminescence (TL) glow curves recorded on ZnO nano-needle arrays have shown a broad peak at about 360 degrees C similar to that observed in ZnO polycrystalline powder. A very strong TL peak at about 325 degrees C superimposed on a broad one has been observed in a ZnO film-type sample. TL recorded on ZnO nano-needles has tentatively been attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centres; the TL mechanism is supported by a recovery effect of the PL and TL after storage in air. (C) 2008 Elsevier B.V. All rights reserved.
42. Structural investigations of sol-gel derived silicate gels using Eu3+ ion-probe luminescence
Authors: Secu, CE; Predoi, D; Secu, M; Cernea, M; Aldica, G

Published: SEP 2009, OPTICAL MATERIALS, 31, 1748, DOI: 10.1016/j.optmat.2009.05.009

Undoped and Eu3+-doped CaF2-SiO2 gels were prepared by the sol-gel method and their optical properties have been studied. The UV-VIS-NIR absorption and photoluminescence spectra have shown the bands typical for the Eu3+ ions transitions. When the Eu-doped gel is annealed at temperatures up to 800 degrees C (i.e. above the CaF2 crystallisation peak at similar to 460 degrees C) the photoluminescence spectra intensity increase, the 590 nm (D-5(0) -> F-7(1)) and 620 nm (D-5(0) -> F-7(2)) luminescence bands become comparable and a structuring of the 620 nm band is observed. The phonon sidebands peaks associated with the F-5(0) -> D-7(2) transition of the Eu3+ ion were observed at around 1000 and 620 cm(-1) and have been assigned to the Si-O and Ca-O bonds, respectively. A phonon sideband signal in the range of 3007400 cm(-1) was attributed to Ca-F bonds in the precipitated CaF2 phase. From the optical absorption, photoluminescence and phonon sidebands spectra we have concluded that in the gels annealed at 800 degrees C, the Eu3+ ions are incorporated into the silica network and in the precipitated CaF2 phase. (c) 2009 Elsevier B.V. All rights reserved.
43. Crystallization and spectroscopic properties of Eu-doped CaF2 nanocrystals in transparent oxyfluoride glass-ceramics
Authors: Secu, M; Secu, CE; Polosan, S; Aldica, G; Ghica, C

Published: OCT 1 2009, JOURNAL OF NON-CRYSTALLINE SOLIDS, 355, 1872, DOI: 10.1016/j.jnoncrysol.2009.04.062

Transparent oxyfluoride glass-ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing Eu-doped CaF2 nanocrystals were produced by using the controlled crystallization of melt-quenched glass. X-ray diffraction and transmission electron microscopy data have revealed the formation of the CaF2 nanocrystals of about 65 nm size. Photoluminescence spectra have shown an increase of the splitting of the luminescences associated to the Eu3+ ion along with annealing time which is consistent with the Eu3+ environment evolving from a glassy to a crystalline state. (C) 2009 Elsevier B.V. All rights reserved.
44. Thermoluminescence of ZnF2 : Mn
Authors: Jipa, S; Zaharescu, T; Kappel, W; Secu, M; Secu, CE; Lungulescu, M; Mantsch, A

Published: MAR 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 557, DOI:

The thermoluminescence of undoped and Mn-doped ZnF2 Was investigated at the dopant concentration of 1.5 % mole Mn after gamma - ray irradiation at room temperature. The glow curve from undoped ZnF2 presents two peaks centered at 86 degrees C and 273 degrees C, while the ZnF2:Mn presents two peaks located at higher temperatures (325 degrees C and 365 degrees C), whose height ratio is modified with dose. The dose responses and fading process were also examined. The radiative thermostimulated recombination was found to be of the second kinetic order based on the geometric factor calculation. The trap parameters were obtained as activation energy and frequency factor.
45. High temperature thermoluminescence of Mn2+-doped MgF2 phosphor for personal dosimetry
Authors: Secu, M; Secu, CE; Jipa, S; Zaharescu, T; Cutrubinis, M

Published: FEB-JUN 2008, RADIATION MEASUREMENTS, 43, 386, DOI: 10.1016/j.radmeas.2007.11.047

Thermoluminescence (TL) curve recorded after gamma-irradiation at room temperature of Mn2+-doped MgF2 phosphor shows TL peaks at 91, 147, 200, 235 and 295 degrees C. The 147 degrees C peak was tentatively attributed to the F centres recombination with hole centres trapped in the neighbourhood of the Mn2+ impurity. This peak shows a good stability (low fading) and has been used to characterise the TL phosphor dosimetric properties. TL dose dependence has shown a good linearity in the range from 0.2 to 10 mGy, which makes it suitable for personal and medical dosimetry. The TL curves recorded after gamma rays or particles irradiation are similar, which suggests the possibility to use the phosphor also for particle detection. (C) 2007 Elsevier Ltd. All rights reserved.
46. Optical properties of CaF2 : Eu3+ nanoctystals embedded in transparent oxyfluoride glass ceramic
Authors: Polosan, S; Secu, CE

Published: AUG 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 2137, DOI:

Transparent oxyfluoride glass ceramic in the system SiO2-Al2O3-CaF2-EuF2 containing CaF2:Eu3+ nanocrystals were produced by annealing of the initial glasses slightly above the CaF2 phase crystallisation peak. The relative good transparency of these materials was due to the small 65 nm, nucleated CaF2 nanocrystallites. X-ray diffraction data have shown an increase of the cell parameters for CaF2:Eu3+ compared with CaF2 which was attributed to the substitution of Ca2+ ions with Eu3+ ions in the CaF2 nanoparticles. Optical investigations using UV-VIS-NIR absorption and luminescence spectroscopy have shown an increase of the splitting of the Eu3+-ion associated luminescences and small shifts of the Eu3+ ion characteristic IR absorption peaks. These effects have been attributed to the Eu3+ environment evolving from the glass to the crystalline-like one.
47. Preliminary study on detection of irradiated foodstuffs from the Romanian market
Authors: Cutrubinis, M; Chirita, D; Savu, D; Secu, CE; Mihai, R; Secu, M; Ponta, C

Published: AUG-SEP 2007, RADIATION PHYSICS AND CHEMISTRY, 76, 1454, DOI: 10.1016/j.radphyschem.2007.02.049

In order to fulfil the European task for market survey in food irradiation the first Romanian laboratory for detection of irradiated foodstuffs was established at IRASM Irradiation Centre. In this preliminary study, a wide range of Romanian food samples (spices, vegetables and meat) gamma irradiated at IRASM have been studied using different detection methods: (1) DNA comet assay, (2) thermoluminescence (TL) and (3) electron spin resonance (ESR) for foodstuffs containing bone or cellulose. The results suggest that there is no general available detection method and there is no perfect detection method. In conclusion, in order to carry out a correct identification of radiation treatment of a food sample it is recommended to use at least two standardised detection methods. (c) 2007 Elsevier Ltd. All rights reserved.
48. The colloidal route of the sol-gel process - an alternative to produce Fe3O4-SiO2 nanocomposites
Authors: Raileanu, M; Crisan, M; Ballo, A; Cosma, C; Petre, J; Stan, C; Predoi, D; Valsangiacom, CM; Tolea, F; Secu, C

Published: MAY 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1402, DOI:

The iron oxide-based nanomaterials have a great importance because of their impact on a wide number of industries. They have many properties and applications in domains as: environmental protection, biomedical, catalysis, information displays and electronics. In order to avoid the tendency of nanoparticles to aggregate, they are often included in sol-gel derived silica matrices, thus being also ensured a homogeneous dispersion of the ultra-fine metal oxide particles in the host matrix. The reasons of the attempt are both an economic and a non-pollutant one, tacking into account the fact that it is well known that aqueous silica sol is cheaper and less toxic than TEOS. The Fe3O4 was introduced in the reaction mixture as aqueous suspension. A final iron content related to SiO2 of 3% wt. was chosen for the prepared nanocomposite. Thermal analysis, IR spectroscopy, XRD and TEM methods have been used for the structural characterisation,. Some experiments of water depollution (from As) have been proceeded using the prepared Fe3O4-SiO2 nanocomposite.
49. High temperature thermolumineseence of BaFCl : EU2+ X-ray storage phosphor
Authors: Secu, CE; Secu, M; Predoi, D

Published: 2007, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 3, 4, +, DOI: 10.1002/pssc.200673736

Thermoluminescence (TL) of Eu2+-doped BaFCl polycrystalline powders X- ray irradiated at room temperature show several TL peaks at 364 K, 408 K, 470 K, 524 K and 646 K. Thermally activated recombination processes responsible for TL have been studied using complementary techniques as TL and its bleaching, diffuse reflectance, X-ray excited luminescence (XL) and photostimulated luminescence (PSL). Experimental results indicate that F(Cr) centres are the dominant electron trap centres, the Eu2+-trapped hole aggregate centres playing the role of recombination centres. The 470 K and 646 K TSL glow peaks are assigned to the recombination inside the triple aggregates (Eu2+ - hole centre - F(Cl-) centre). The remaining 408 K and 524 K TL peaks are tentatively assigned to recombination between F centres and double aggregates (Eu2+ - hole centre). (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
50. Time-resolved luminescence spectroscopy of Eu2+ in BaFCl : Eu2+ X-ray storage phosphor
Authors: Secu, M; Secu, CE; Vasile, V; Predoi, D; Gazdaru, DM

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1802, DOI:

Photoluminescence (PL) spectra recorded at room temperature (RT) in polycrystalline BaFCI:Eu2+ X-ray storage phosphor have shown the europium associated luminescences, a broad luminescence band at 385nm (3.22eV) with a 3.3 +/- 0.1 mu s PL lifetime accompanied by a luminescence line at about 368nm (3.37eV). The energy difference Delta EB-L =226cm(-1) between the two excited levels from which the europium associated band and line luminescences took place was computed. An additional broad luminescence band at about 425nm (2.92eV) with 0.7 +/- 0.1 mu s PL lifetime has been attributed to the europium luminescence perturbed by an impurity. Time resolved spectroscopy measurements have shown that no other impurity luminescences (like oxygen-vacancy centres) can be observed at RT except the europium associated ones.
51. Characterisation of magnetic iron oxide in hydroxyapatite
Authors: Birsan, M; Predoi, D; Birsan, C; Secu, CE; Andronescu, F

Published: JUN 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 1832, DOI:

In order to be used for medical applications, a material must exhibit many specific characteristics, of which the most fundamental is related to biocompatibility. Lately, the iron oxide nanoparticles are preferred for biomedical applications because they present a superparamagnetic behaviour at room temperature. The preparation method and conditions of iron oxide nanoparticles represents some of the most important challenges that will determine the particle size and shape, the size distribution, the surface chemistry of the particles and consequently their magnetic properties. The samples were characterized by X-ray diffractions (XRD), transmission electron microscopy (TEM) and infrared spectroscopy.
52. ESR studies on laser beam irradiated polyethyleneterephtalate
Authors: Chipara, M; Romero, JR; Chipara, MD; Georgescu, L; Constantinescu, L; Secu, C; Morosanu, C

Published: SEP 2003, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 211, 121, DOI: 10.1016/S0168-583X(03)01179-0

Electron spin resonance investigations on polyethyleneterephtalate irradiated with a YAG-Nd laser are reported. The dependence of electron spin resonance spectra on the incident laser beam intensity (power) is studied. It is noticed that the concentration of free radicals has a sigmoidal dependence on the crystal current, presenting saturation at high incident powers. The nature of free radicals induced by laser irradiation is discussed. (C) 2003 Elsevier B.V. All rights reserved.
53. ESR studies on collagen irradiated with protons
Authors: Chipara, M; Romero, JR; Ignat, M; Constantinescu, B; Secu, C

Published: APR 2003, POLYMER DEGRADATION AND STABILITY, 80, 49, DOI: 10.1016/S0141-3910(02)00381-6

Electron spin resonance investigations on collagen irradiated with protons are reported. The resonance spectra were assigned to a convolution of lines due to similar toCH(2)-CH2. residues and peroxy radicals. Raising the temperature of the sample (in air), the concentration of similar toCH(2)-CH2. residues decreases while, the concentration of peroxy radicals increases due to the denaturation induced enhancement of the oxidation process. An increase in the apparent intensity of the recorded spectrum has been observed around 100 degreesC and ascribed to the release of adsorbed water molecules. (C) 2003 Elsevier Science Ltd. All rights reserved.
54. Radiation effects on polyoctenamer
Authors: Chipara, MI; Georgescu, L; Oproiu, C; Chipara, MD; Niculescu, A; Galatanu, N; Reyes, JR; Secu, CE

Published: AUG 1997, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 131, 191, DOI: 10.1016/S0168-583X(97)00339-X

The effect of electron beams on polyoctenamer is investigated by viscosimetry, swelling and electron spin resonance spectroscopy. From these experimental investigations it has been deduced that the main degradation process in polyoctenamer is its crosslinking during electron beam irradiation in air, at room temperature. (C) 1997 Elsevier Science B.V.
Load More

Back to top

Copyright © 2024 National Institute of Materials Physics. All Rights Reserved