NIMP| Mihail SECU, Author at NIMP

Projects

1. New Chalcogenide Nanostructures for Information and Communication Technology

Project Type: P3, Start Date: 2018-08-01 End Date: 2019-12-01

2. Infrared-Pumped upconversion glass-ceramic nanorods for photonics applications

Project Type: IDEAS, Start Date: 2011-11-01 End Date: 2016-11-30

Publications

1. 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.

2. Optical characterization of Sm3+doped phosphate glasses for potential orange laser applications

Authors: Bayoudhi, D; Bouzidi, C; Matei, E; Secu, M; Galca, AC

Published: JAN 2024, JOURNAL OF LUMINESCENCE, 265, 120204, DOI: 10.1016/j.jlumin.2023.120204

Undoped and Sm3+doped 45P2O5-45Na2O-2Al2O3-8BaO glasses were synthesized by the melt-quenching technique. The glass structure and luminescence properties were investigated by using Raman spectroscopy, scanning electron microscopy (SEM), spectroscopic ellipsometry, Judd-Ofelt theory and photoluminescence. Electron microscopy showed the homogeneity of samples. Raman spectroscopy revealed that the overall struc-ture of the glass was unaffected by the doping of Sm3+ and ellipsometry was used to measure the optical constants. Judd-Ofelt (JO) analysis was performed on the absorption bands of Sm3+ (4f5) and the three phenomenological parameters (omega 2, omega 4, and omega 6) were computed and then used to determine radiative properties such as the radiative transition probability (Ar), the fluorescent branching ratio (beta r), the stimulated emission cross-section (sigma e) and the radiative lifetime (tau rad). Photoluminescence (PL) spectrum showed the typical four transitions of Sm3+ at wavelengths of 564, 600, 645 and 703 nm corresponding to 4G5/2 -> 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively. The spectroscopic quality factors omega 4/omega 6, the predicted lifetime (tau rad) calculated using the JO method and the experimentally lifetime (tau exp) for the 4G5/2level were calculated and discussed. The glass color purity is as high as 98%, which makes it a potential candidate for laser emission.

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. Defect structures and (ferro)magnetism in Zn_1-xFe_xO nanoparticles with the iron concentration level in the dilute regime (x = 0.001 - 0.01) prepared from acetate precursors

Authors: Mihalache, V; Negrila, C; Secu, M; Mercioniu, I; Iacob, N; Kuncser, V

Published: AUG 2023, RESULTS IN PHYSICS, 51, 106644, DOI: 10.1016/j.rinp.2023.106644

Zn1-xFexO nanoparticles with the iron concentrations level in the dilute regime (x = 0.001---0.01) were produced by a sol-gel route from acetate precursors along with an un-doped and 3 at.% Fe-doped reference. The X-ray diffraction of the un-doped and 0.1-1 at.% Fe-doped samples reveal the reflections for only the ZnO wurtzite structure. Fe doping enhances the a-axis lattice constant, the unit cell volume and the microstrain. Iron doping reduces the average crystallite/particle size (confirmed by Scanning Electron Microscopy), improving the surface-to-volume ratio or the concentration of defective surface sites. XPS identifies the iron in both Fe3+ and Fe2+ states. XPS and Fe-57 Mossbauer spectroscopy indicate a broad distribution (distortion) of Fe3+ sites on the surface of ZnO nanoparticles. The blue shift and broadening of the UV emission, and quenching of defect-related photoluminescence in the Fe-doped samples verify the presence of iron in the ZnO lattice and surface intrinsic defects. 0.1-1 at.% Fe-doped ZnO show room temperature ferromagnetism, RTFM, characteristic of dilute magnetic semiconductors, DMS. The magnetization measurements with temperature evidence an antiferromagnetic alignment and an increase of ferromagnetic contribution with Fe doping up to 1 at.%. Zn0.97Fe0.3O reference is a superparamagnetic ZnO/ZnFe2O4 nanocomposite with a blocking temperature of 20 K; HRTEM shows (ultra)fine ZnFe2O4 particles at the surface of ZnO nanoparticles. The analysis of experimental data of 0.1-1 at.% Fe-doped ZnO was done in terms of iron coupling with intrinsic defects, which can generate surface Fe3+ states with geometries similar to the Fe3+ in inverse spinel ZnFe2O4. The superexchange interaction (resembling that in the inverse spinel ZnFe2O4) between the Fe3+ sites with distorted configuration resulting in ferrimagnetism was hypothesised as a possible mechanism of RTFM. Experimental (structural, local chemical, magnetic, optical) and interpretation results can be used to optimize the processing conditions for Fe-doped ZnO to serve as an effective DMS, e.g. for spintronic applications.

5. 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.

6. 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

7. Columnar Liquid Crystals of Copper(I) Complexes with Ionic Conductivity and Solid State Emission

Authors: Circu, V; Ganea, CP; Secu, M; Manaila-Maximean, D; Marinescu, GC; Popescu, RG; Pasuk, I

Published: MAY 19 2023, MOLECULES, 28, 4196, DOI: 10.3390/molecules28104196

Two neutral copper(I) halide complexes ([Cu(BTU)(2)X], X = Cl, Br) were prepared by the reduction of the corresponding copper(II) halides (chloride or bromide) with a benzoylthiourea (BTU, N-(3,4-diheptyloxybenzoyl)-N '-(4-heptadecafluorooctylphenyl)thiourea) ligand in ethanol. The two copper(I) complexes show a very interesting combination of 2D supramolecular structures, liquid crystalline, emission, and 1D ionic conduction properties. Their chemical structure was ascribed based on ESI-MS, elemental analysis, IR, and NMR spectroscopies (H-1 and C-13), while the mesomorphic behavior was analyzed through a combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X-ray diffraction (XRD). These new copper(I) complexes have mesomorphic properties and exhibit a hexagonal columnar mesophase over a large temperature range, more than 100 K, as evidenced by DSC studies and POM observations. The thermogravimetric analysis (TG) indicated a very good thermal stability of these samples up to the isotropization temperatures and over the whole temperature range of the liquid crystalline phase existence. Both complexes displayed a solid-state emission with quantum yields up to 8% at ambient temperature. The electrical properties of the new metallomesogens were investigated by variable temperature dielectric spectroscopy over the entire temperature range of the liquid crystalline phase. It was found that the liquid crystal phases favoured anhydrous proton conduction provided by the hydrogen-bonding networks formed by the NH horizontal ellipsis X moieties (X = halide or oxygen) of the benzoylthiourea ligand in the copper(I) complexes. A proton conductivity of 2.97 x 10(-7) S.cm(-1) was achieved at 430 K for the chloro-complex and 1.37 x 10(-6) S.cm(-1) at 440K for the related bromo-complex.

8. 'Put variety in White': Multi-analytical investigation of the white pigments inlaid on Early Chalcolithic pottery from Southern Romania

Authors: Opris, V; Velea, A; Secu, M; Rostas, AM; Buruiana, AT; Simion, CA; Mirea, DA; Matei, E; Bartha, C; Dimache, M; Lazar, C

Published: APR 2022, JOURNAL OF ARCHAEOLOGICAL SCIENCE-REPORTS, 42, 103402, DOI: 10.1016/j.jasrep.2022.103402

The current study aims to present and discuss the results obtained by complementary archaeometric methods applied for the first time on white pigments inlaid on excised pottery of the Boian-Vidra tradition (Early Chalcolithic, c. 4900-4600 BCE). The samples came from three settlements located in Southern Romania (Sultana-Ghetarie, Vidra, and Vladiceasca). They were selected considering that the pottery was produced in approximately contemporary sites, located relatively close to each other in the same geographical region, namely the Romanian Plain. The experimental part included the analysis of local samples of carbonate concretions and prehistoric animal bone ash as reference materials. Archaeometric investigations consisted in applying "in-air" PIXE and EDX methods for the chemical composition, XRD and FTIR for mineralogical data, SEM for microstructure observation, and EPR for the characterisation of the paramagnetic centres. Calcite, bone ash, and silica rich sediments were identified as the primary decorating pigments. The mixtures of calcite and bone-ash observed in 13 samples were specific to the sites at Vidra and Vladiceasca. Silica-rich sediments from distant sources were the main whitening materials in two samples from Vladiceasca, while for the samples from Sultana-Ghetarie, calcite was the only whitening mineral. The results show with a high degree of confidence the use of both local (i.e., carbonate neo-formations and bone ash) and exotic (silica-rich sediments) raw materials to obtain the white pigment applied to Boian-Vidra pottery. Thus, the current data show the adaptability of the potters with respect to the surrounding resources and also provide new evidence for a vast trade network of raw materials and/or finished products in the Lower Danube area during the Early Chalcolithic. The deliberate mixing of two whitening materials from different sources could be a technological choice and may highlight complex symbolic behaviours.

9. ZnS stacking order influence on the formation of Zn-poor and Zn-rich Cu2ZnSnS4 phase

Authors: Zaki, MY; El Khouja, O; Nouneh, K; Touhami, ME; Matei, E; Azmi, S; Rusu, MI; Grigorescu, CEA; Briche, S; Boutamart, M; Badica, P; Burdusel, M; Secu, M; Pintilie, L; Galca, AC

Published: MAY 2022, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 33, DOI: 10.1007/s10854-022-08160-6

This paper reports the synthesis and characterization of Cu2ZnSnS4 (CZTS) absorber films, prepared by a two-step electrodeposition of a ZnS (zinc sulfide) binary and a CZT (copper, zinc and tin) ternary precursors on Mo/Ti/Si substrates. The as-electrodeposited ZnS/CZT and CZT/ZnS stacks were thermally treated in a tubular furnace in sulfur environment at 550 degrees C. The role of the ZnS buffer layer is to provide a zinc and sulfur reservoir, needed to complete the formation of kesterite phase. X-ray diffraction and Raman analyses revealed the formation of the CZTS phase. The surface morphology and chemical composition of the films were studied using a scanning electron microscope. The bandgap values inferred from diffuse reflectance data, are discussed with respect to the stoichiometry which is considerably affected by the order of the stacks. Room-temperature photoluminescence of the CZT/ZnS sample showed a board PL band of 1.51 eV. It was found that the film with a ZnS layer on top is preferred for the formation of a Zn-rich single CZTS phase.

10. Luminescent heteroleptic copper(I) complexes with phosphine and N-benzoyl thiourea ligands: Synthesis, structure and emission properties

Authors: Tudor, CA; Ilis, M; Secu, M; Ferbinteanu, M; Circu, V

Published: JAN 1 2022, POLYHEDRON, 211, DOI: 10.1016/j.poly.2021.115542

The reactions between [CuX(PPh3)](4) or [CuX(PPh3)(3)], X = Cl or Br, and N-benzoyl-N'-(4-methoxyphenyl)thio-urea (BTU-OMe), N-benzoyl-N'-(4-methylphenyl)thiourea (BTU-Me) in toluene gave new four-coordinated tetrahedral copper(I) complexes of two types: [CuX(BTU)(PPh3)(2)] (1, 2) and [CuX(BTU)(2)(PPh3)] (3,4) [BTU = N-benzoyl-N'-(4-methoxy/methyl phenyl)thiourea derivatives, X = Cl (a) and Br (b)]. These complexes were fully characterized by several analytical and spectroscopic (IR, UV/VIS, H-1, C-13 and P-31 NMR) techniques. The molecular structure of a representative complex, [Cu(BTU-OMe)(PPh3)(2)Cl] (3a), was determined by the single crystal X-ray diffraction method which reveals a distorted tetrahedral geometry where the Cu(I) ion is surrounded by one BTU ligand molecule, two PPh3 groups and one chloride anion. These eight heteroleptic complexes were found to emit in solid state with lifetimes in the 12-99 mu s range and with the highest quantum yield of 4.0% for 3b. The reaction between [CuBr(PPh3)](4) and N-benzoyl-N',N'-(di-n-butyl)thiourea (BTU-Bu-2) ligand gave the simple [CuBr(BTU-Bu-2)(3)] complex (5), when the PPh3 molecules were completely removed by the BTU derivative. The spectroscopic data indicated a distorted tetrahedral geometry produced by three BTU ligand molecules and one bromide anion. Complex 5 has the highest quantum yield (11%) with the lifetime of 29 mu s in solid-state.

11. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

Authors: AitMellal, O; Oufni, L; Messous, MY; Tahri, M; Neatu, S; Florea, M; Neatu, F; Secu, M

Published: JAN 2022, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, DOI: 10.1007/s10854-021-07615-6

To enhance the spectral response of solar cells, an experimental study on LaPO4:0.01Ce(3+)/xNd(3+) (x = 0, 2, 4 mol%) was carried out, where structural and morphological properties of the prepared samples were well characterized by the means of X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electronic microscope. Additionally, the photoluminescence behavior of phosphors in ultraviolet-visible (UV-VIS) and Near-infrared (NIR) regions were investigated to confirm the energy transfer (ET) from Ce3+ to Nd3+. Moreover, the quantum efficiency of Ce3+/Nd3+-co-doped samples was estimated as high as similar to 172% and the possible ET process was described. Accordingly, the LaPO4:Ce3+/Nd3+ phosphors can convert the UV light (275 nm) into NIR photons (approx. 1059 nm) through the possible two-pathway energy transfer processes from Ce3+ sensitizer ions to Nd3+ activators. Obtained NIR down-conversion emissions are suitable for improving the conversion efficiency of c-Si solar cells.

12. ZnS stacking order influence on the formation of Zn-poor and Zn-rich Cu2ZnSnS4 phase

Authors: Zaki, MY; El Khouja, O; Nouneh, K; Touhami, ME; Matei, E; Azmi, S; Rusu, MI; Grigorescu, CEA; Briche, S; Boutamart, M; Badica, P; Burdusel, M; Secu, M; Pintilie, L; Galca, AC

Published: APR 2022, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, DOI: 10.1007/s10854-022-08160-6

13. Bulk and surface characteristics of co-electrodeposited Cu2FeSnS4 thin films sulfurized at different annealing temperatures

Authors: El Khouja, O; Negrila, CC; Nouneh, K; Secu, M; Touhami, ME; Matei, E; Stancu, V; Enculescu, M; Kuncser, V; Galca, AC

Published: JUN 15 2022, JOURNAL OF ALLOYS AND COMPOUNDS, 906, 164379, DOI: 10.1016/j.jallcom.2022.164379

In this study, stannite, Cu2FeSnS4 (CFTS), absorber films were obtained by electrodeposition on Molybdenum-coated soda-lime substrates, followed by sulfurization treatment at certain temperatures in the 400-550 degrees C range. The purposes of this work were to control the manufacturing of CFTS films with good stoichiometry, high crystallinity and to study the annealing temperature impact on CFTS films properties. The X-ray diffraction and the Raman spectroscopy measurement distinguished the CFTS phase formation, with a presence of SnS2 secondary phase. The energy dispersive spectroscopy results reveal compositional differences between samples as well as the in-depth gradients. The photoluminescence emission band around 1.35-1.40 eV is slightly below the direct bandgap inferred from the conventional spectroscopy (diffuse reflectance). X-ray photoelectron spectroscopy results indicate clearly a high amount of Sn on the surface. The Conversion Electron Mossbauer unveiled the presence of Fe in the chalcogenide unit cell. The electrochemical characteristics of the synthesized films are also given. (c) 2022 Elsevier B.V. All rights reserved.

14. Structural and transport properties of Cu2CoSnS4 films prepared by spray pyrolysis

Authors: El Khouja, O; Assahsahi, I; Nouneh, K; Touhami, ME; Secu, M; Talbi, A; Khaaissa, Y; Matei, E; Stancu, V; Galatanu, A; Galca, AC

Published: NOV 1 2022, CERAMICS INTERNATIONAL, 48, DOI: 10.1016/j.ceramint.2022.07.185

In the present work, stannite Cu2CoSnS4 (CCTS) films were elaborated using spray pyrolysis method on soda-lime glass, at different deposition temperatures (T-d = 250, 300, and 350 degrees C), followed by different chosen sulfurization temperatures (T-s = 450, 500, and 550 degrees C). X-ray diffraction (XRD) revealed the nearly single-phase formation of CCTS films at 300 degrees C deposition temperature. After sulfurization in argon flow, the XRD lines become narrower, the average crystallite size expanding above 70 nm. The Raman spectroscopy analysis confirmed the stannite structure formation, as well as the presence CoS2 secondary phases, which reduces at higher sulfurization temperature (550 degrees C). The energy dispersive spectroscopy results indicated atomic ratios of Cu/Co/Sn/S close to the ideal stoichiometric ratio 2:1:1:4. The room temperature photoluminescence emission is recorded with maximum in the 1.35-1.40 eV range. Thermoelectric properties are measured up to 130 degrees C, the films show poor power factor as a result of small positive Seebeck coefficients 10-45 Of K -1 and low electrical conductivity despite of having relatively high carrier concentration (similar to 10(20) cm(-3)).

15. 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.

16. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

Authors: AitMellal, O; Oufni, L; Messous, MY; Tahri, M; Neatu, S; Florea, M; Neatu, F; Secu, M

Published: MAR 2022, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 33, DOI: 10.1007/s10854-021-07615-6

17. Temperature-induced phase transition and tunable luminescence properties of Ce3+-Mn2+-Zr4+ tri-doped LaPO4 phosphor

Authors: AitMellal, O; Oufni, L; Messous, MY; Matei, E; Rostas, AM; Galca, AC; Secu, M

Published: JUL 2022, OPTICAL MATERIALS, 129, 112567, DOI: 10.1016/j.optmat.2022.112567

Tunable blue-white-orange emitting LaPO4:Ce3+/Mn2+/Zr4+(LPCMZ) phosphors have been synthesized by co precipitation followed by calcination. The X-ray diffraction (XRD) shows the phase transition from hexagonal to monoclinic between 600 and 700 ?C, accompanied by the increase of average nanocrystallites size from 8 to 64 nm. The Raman measurements revealed the vibrational modes associated with the LPCMZ crystalline phases, where the band positions and the full width at half maximum values depend on the structural parameters and nanocrystals size. For low-temperature calcination of 500 ?C, scanning electron microscopy (SEM) revealed nanoclusters composed of thinner nanoneedles, which developed into a rod-like self-assembly shape for higher calcination temperatures at around 900 ?C. Electron paramagnetic resonance (EPR) spectroscopy reveals a broad isotropic EPR signal, assigned to agglomerated/clustered Mn2+ ions, which are dispersed only at high temperatures above 900 ?C. The photoluminescence spectra recorded under UV-excitation of Ce3+ ions showed the Mn2+ green/red (546, 630 nm) emissions due to an energy transfer (ET) between Ce3+ and Mn2+. Depending on the calcination temperature, the Mn2+ emission color can be finely adjusted from blue to white and orange.

18. Potassium-containing triple-cation mixed-halide perovskite materials: Toward efficient and stable solar cells

Authors: Derbali, S; Nouneh, K; Florea, M; Leonat, LN; Stancu, V; Tomulescu, AG; Galca, AC; Secu, M; Pintilie, L; Touhami, ME

Published: MAR 25 2021, JOURNAL OF ALLOYS AND COMPOUNDS, 858, DOI: 10.1016/j.jallcom.2020.158335

In this paper, potassium based triple cation mixed-halide perovskite films were explored in order to enhance the stability and photovoltaic performance of perovskite based solar cells. It was found that adding potassium (K+) to a double cation mixed halide perovskite (FA(0.80)MA(0.20)PbI(2.8)Cl(0.2)), structural, morphological and optoelectronic properties of perovskites are improved. The perovskite films were prepared by one-step spin coating method with and without K+ and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-Vis spectroscopy, X-ray photoelectron spectroscopy (XPS), and photoluminescence spectroscopy (PL). The results indicate that potassium incorporation reduces significantly the yellow non-perovskite delta-phase formation and improves the perovskite film quality, thus contributing to the reduction of hysteresis, improves the stability and increases the PCE up to 12.51%. Furthermore, the doped devices exhibit reduced hysteresis and provide remarkable shelf stability by retaining more than 70% of the initial efficiency with low humidity over 850 h. (C) 2020 Elsevier B.V. All rights reserved.

19. 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.

20. Comparative investigation of structural, EPR, optical and photoluminescence properties of nanostructured LaPO4:Ce/RE/Me and LaPO4:Yb/Er phosphors prepared by co-precipitation method

Authors: AitMellal, O; Oufni, L; Messous, MY; Trandafir, MM; Chirica, IM; Florea, M; Neatu, S; Rostas, AM; Secu, M; Neatu, F

Published: SEP 2021, JOURNAL OF SOLID STATE CHEMISTRY, 301, DOI: 10.1016/j.jssc.2021.122310

In this work, the structural, thermal, vibrational, morphological, magnetic and optical properties of LaPO4:Ce/RE/Me (RE=Nd3+, Tb3+; Me-Cr3+, Mn2+) and LaPO4:Yb3+/Er3+ phosphors prepared by the co-precipitation method are presented. The obtained materials crystallized in monoclinic structure with the P2(1)/n space group and the particles were of nanorod shape with about 200 nm in length and the diameter approximately 19 nm. The presence of dopant ions was confirmed by both electron paramagnetic resonance (EPR) and UV-visible spectroscopies. In addition, the down-conversion (DC) and up-conversion (UC) of the LaPO4 nanophosphors via the 275 and 980 nm excitations, respectively, were considered, and a wide range of electronic transitions was observed. Based on the photoluminescence (PL) spectra, there is an efficient energy transfer (ET) process from Ce3+ donors to Nd3+ and Tb3+ acceptors, and the computed ET efficiency was 70% and 88%, respectively. The Ce3+/Cr3+ and Ce3+/Mn2+ doped LaPO4 showed weak far-red and green luminescence with much smaller ET efficiency of about 3.7 and 0.4%, respectively. LaPO4:Yb3+/Er3+ showed UC luminescence under the 980 nm laser radiation, and the resulted red and green light was attributed to the Er3+ transitions.

21. EPR spectroscopy and structural investigations, of Eu2+-doped chloroborate glass-ceramic

Authors: Secu, M; Kuncser, A; Negrila, C; Rostas, AM

Published: DEC 15 2021, CERAMICS INTERNATIONAL, 47, 35095, DOI: 10.1016/j.ceramint.2021.09.051

Europium doped (0.1% and 1%) chlomborate BaO-B2O3-BaCl2 glass and glass-ceramic with embedded BaCl2 nanocrystals have been synthesised, investigated, and compared with the corresponding nanocrystalline powder. The structural analysis of the glass evidenced ionic bonds characteristic to the glass host structure and compositional changes during the melting. X-ray diffraction and transmission electron microscopy revealed the formation of orthorhombic BaCl2 nanocrystals of about tens on nm in size and a smaller amount of BaB2O4 nanocrystals. Structural analysis of the Eu2+-doped BaCl2 nanocrystals has shown a distortion of the crystalline cell assigned to the growth process, affected by defects and ionic impurities. Photoluminescence spectra of the glass-ceramic revealed Eu3+ and Eu2+ luminescent ion species, but only Eu2+ is incorporated within the BaCl2 nanocrystalline phase. Electron paramagnetic resonance measurements in X-band sustain the presence of Eu2+ ions. The EPR parameters (g values and hyperfine constants) resulting from the Q-band EPR spectra simulations recorded on glass-ceramic are similar to those in Eu2+-doped BaCl2 and confirmed the partial incorporation of Eu2+ ions within the BaCl2:Eu2+ nanocrystals in the glass-ceramics.

22. Structural, electrical properties and photoluminescence analyses of the terbium doped barium titanate

Authors: Cernea, M; Secu, M; Radu, R; Ganea, P; Surdu, VA; Trusca, R; Vasile, ET; Secu, EC

Published: OCT 15 2021, JOURNAL OF ALLOYS AND COMPOUNDS, 878, DOI: 10.1016/j.jallcom.2021.160380

Multifunctional materials integrating optical and electric properties into a composite or into a doped single-phase material have attracted much attention from scientists for future optoelectronic devices. In this work, polycrystalline Ba1-xTbxTiO3, x = 0.00, 0.01 and 0.05 materials were synthesized by hydrothermal route and their photoluminescence and dielectric properties were studied. The as-prepared powders showed a microstructure consisting of grains with the shape of cube; and average particle size ranging from 75 nm to 150 nm. The investigated temperature dependence of dielectric permittivity of Ba1-xTbxTiO3 ceramics revealed a decrease of the Curie temperature (Tc) with increasing terbium content, and a more pronounced degree of diffuseness of phase transition. The resistivity at room temperature was discussed in terms of positive temperature coefficient of resistance (PTRC) effect. UV-vis reflectance spectra of Ba1-xTbxTiO3 samples showed a decrease of the band gap energy with the increase of Tb amount, due to the band-gap narrowing effect. The photoluminescence spectra recorded for Tb doped BaTiO3 materials, at 80 K, evidenced the typical f-f luminescence lines of the Tb3+ accompanied the broad luminescence band at about 425 nm due to self-trapped exciton recombination. These results demonstrated that the terbium enhances the dielectric properties and photoluminescence simultaneously, being suitable for electronic applications. (C) 2021 Elsevier B.V. All rights reserved.

23. Structural and Optical Investigations of Ce3+/Mn2+-Doped LaPO4 Phosphors

Authors: Aitmellal, O; Oufni, L; Messous, MY; Neatu, F; Florea, M; Neatu, S; Rostas, AM; Secu, M

Published: APR 2021, JOURNAL OF ELECTRONIC MATERIALS, 50, 2147, DOI: 10.1007/s11664-020-08678-7

Lanthanum orthophosphate (LaPO4) and La0.95-xCe0.05MnxPO4 (x = 0.00, 0.03, 0.10) phosphors were synthesized by a simple and cost-efficient co-precipitation method and the formation of LaPO4 nanorods with a monoclinic P21/n crystal structure was observed. X-ray diffraction pattern analysis indicated a slight distortion of the LaPO4 crystalline structure and an increase of the lattice strain as a consequence of the Mn2+ and Ce3+ dopants incorporation in the host matrix. Scanning electron microscopy revealed that the microstructure of all powders consists of agglomerations of nanorods, which are around 17 +/- 3 nm in diameter and length ranging from 100 nm to 300 nm. Electron paramagnetic resonance measurements have indicated the presence of Mn2+ in isolated species, but also as agglomerates. Ce3+ and Mn2+ doping of LaPO4 resulted also in a decrease of the band gap up to 4.70 eV compared to the un-doped sample. Because of an energy transfer effect from Ce3+ to Mn2+ ions, green emission of Mn2+ ions at around 550 nm was observed upon 275 nm excitation.

24. Thermal and Emission Properties of a Series of Lanthanides Complexes with N-Biphenyl-Alkylated-4-Pyridone Ligands: Crystal Structure of a Terbium Complex with N-Benzyl-4-Pyridone

Authors: Chiriac, FL; Ilis, M; Madalan, A; Manaila-Maximean, D; Secu, M; Circu, V

Published: APR 2021, MOLECULES, 26, DOI: 10.3390/molecules26072017

This work focuses on the investigation of the liquid crystalline behavior and luminescence properties of the lanthanide complexes of Eu(III), Sm(III) and Tb(III) with N-biphenyl-alkylated-4-pyridone ligands. The organic ligands having a biphenyl group attached via a long flexible spacer with either 9 or 10 carbon atoms were synthesized by the reaction between 4-hydroxypyridine and the corresponding bromide compounds. The chemical structures of the organic and lanthanide complexes were assigned based on elemental analysis, single-crystal X-ray diffraction, H-1, C-13 NMR and IR spectroscopies, and thermogravimetric analysis (TGA). The X-ray diffraction analysis of a parent compound shows that the lanthanide ions are surrounded by three monodentate pyridone ligands and three bidentate nitrate ions, giving a 9-coordinate environment. The mesogenic behavior and the type of liquid crystalline phases exhibited by the new complexes were analyzed by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), and powder X-ray diffraction (XRD) studies. Only the lanthanide complexes with longer spacer (10) display a monotropic SmA phase, typically on a short thermal range (less than 10 degrees C). The complexes with shorter flexible chains (9) show no liquid crystalline properties with melting temperatures lower than their analogs with longer spacers. The emission spectra recorded in solid state at room temperatures show typical emission bands for each lanthanide ion employed (Eu(III), Tb(III) and Sm(III)).

25. Towards phase pure kesterite Cu2ZnSnS4 absorber layers growth via single step free sulfurization electrodeposition under a fix applied potential on Mo substrate

Authors: Azmi, S; Moujib, A; Layachi, OA; Matei, E; Galca, AC; Zaki, MY; Secu, M; Rusu, MI; Grigorescu, CEA; Khoumri, EM

Published: NOV 25 2020, JOURNAL OF ALLOYS AND COMPOUNDS, 842, DOI: 10.1016/j.jallcom.2020.155821

This work presents a new synthesis and characterization of Kesterite Cu2ZnSnS4 films by one step electrodeposition with free sulfurization annealing treatment at different applied potentials. This study highlights the effect of applied potential and annealing treatment on the properties of CZTS deposited films. X-ray Diffraction and Raman spectroscopy were employed to assess the structure and composition of the films elaborated at -1V, -1.1V, and -1.2V vs Saturated Calomel Electrode (SCE). The morphological and optical properties were studied using Scanning Electron microscopy and photoluminescence spectroscopy (PL), respectively. The structural properties are improved by annealing treatment, while -1.1V vs SCE was found to be the optimum applied potential to prepare the Kesterite CZTS thin film with a bandgap around 1.5 eV. (C) 2020 Elsevier B.V. All rights reserved.

26. Exploring the effect of aliovalent substitution of Pb2+ by Eu3+ on structural, morphological and optical properties of CH3NH3PbI3 perovskite films

Authors: Derbali, S; Nouneh, K; Florea, M; Neatu, F; Neatu, S; Leonat, LN; Secu, M; Tomulescu, AG; Stancu, V; Pintilie, L; Touhami, ME; Galca, AC

Published: APR 2020, PHYSICA SCRIPTA, 95, DOI: 10.1088/1402-4896/ab5baa

In this work, the effect of aliovalent substitution of Pb2+ by Eu3+ on structural, morphological and optical properties of CH3NH3PbI3 (MAPbI(3)) was studied, aiming to improve the properties of perovskite films used in solar cells application. The surface morphology, the microstructure and the optical properties of the obtained films containing different Europium (Eu) concentrations were characterized by atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, UV-vis spectroscopy and photoluminescence spectroscopy.

27. Optical spectroscopy and dielectric properties of phosphate-tellurite glasses

Authors: Polosan, S; Nitescu, A; Secu, M

Published: MAR 2020, PHYSICA SCRIPTA, 95, DOI: 10.1088/1402-4896/ab5263

Tellurium metallic colloids were evidence in the phosphate tellurite glasses obtained by the melt quenching method over 1000 degrees C. The concentration and subsequent color of these glasses strongly depend on the preparation conditions due to the significant differences among the melting point of the component oxides. The presence of Te metallic colloids was evidenced by the magneto-optical measurements at different temperatures, by the shift of the emissions spectra with the excitation wavelength and the thermoluminescence spectra as a result of recombination between the hole centers, created during x-ray irradiation, with the quasi coupled electrons from the surface of the metallic colloids. The peak position in the absorption spectra of these metallic colloids was modeled based on generalized Mie light scattering theory on these metallic structures, taking into account the size and shape of colloids but also the dielectric constant of the phosphate tellurite glasses in which these colloids are embedded.

28. Temperature dependence and defect related structure, photoluminescence, (ferro)magnetism and ammonia sensitivity of un-doped nanocrystalline ZnO

Authors: Mihalache, V; Secu, M; Negrila, C; Bercu, V; Mercioniu, I; Leca, A

Published: DEC 2020, MATERIALS SCIENCE AND ENGINEERING B-ADVANCED FUNCTIONAL SOLID-STATE MATERIALS, 262, DOI: 10.1016/j.mseb.2020.114748

ZnO nanostructures with intrinsic and extended defects were prepared by rapid decomposition of zinc-propio-nate and annealing at 400 degrees C-970 degrees C. The correlation between the structure/morphology, type of native defects (photoluminescence (PL), EPR) and ferromagnetism was investigated, together with ammonia adsorption capacity. All the samples show room temperature ferromagnetism (RTFM). Crystallite size increases while the unit cell volume, c-axis constant, microstrain and saturation magnetization relax with increasing temperature; morphology varies from aggregated nanoparticles to frameworks of well-welded crystals. 400 degrees C-800 degrees C annealed samples show a broad visible and/or a prominent violet-blue PL emission and, two narrow g = 2.0065 and g = 1.9632 EPR signals. 800 degrees C-970 degrees C annealed samples exhibit very intense green-yellow photoluminescence. The intrinsic defects in conjunction with a deformed lattice and/or pinned by grain-boundaries appear responsible for RTFM and Curie temperature exceeding 700 degrees C. Tuning the morphology, PL intensity and ferromagnetic signal by choice of annealing temperature can find applications in (gas) sensing, photonic/optoelectronic and spintronic devices.

29. 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%.

30. Third-order optical nonlinearity properties of CdCl2-modifed Ge-Sb-S chalcogenide glasses

Authors: Lu, XS; Li, JH; Yang, L; Zhang, RN; Zhang, YD; Ren, J; Galca, AC; Secu, M; Farrell, G; Wang, PF

Published: JAN 15 2020, JOURNAL OF NON-CRYSTALLINE SOLIDS, 528, DOI: 10.1016/j.jnonerysol.2019.119757

We developed a new type of chalcohalide glasses with physicochemical and nonlinear optical properties that are tunable by composition. It is found that more than 60 mol.% CdCl2 heavy metal halide can be dissolved into the ternary Ge Sb S system and forming stable glasses. The visible-light transparency range is extended to shorter wavelengths with the addition of CdCl2, which is beneficial for the optical quality control and infra-red (IR) system alignment. The third-order optical nonlinearity (TONL) is studied using the femtosecond Z-scan method. The results show that both the nonlinear refractive index and two photon absorption co-efficient decrease with CdCl2. Benefiting from the favorable property-tailoring effects of CdCl2, the TONL figure of merit (FOM) can be improved to meet the requirement (FOM < 1) for all-optical switching and IR photonic applications.

31. 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.

32. 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.

33. 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,

34. Effect of dilute doping and non-equilibrium synthesis on the structural, luminescent and magnetic properties of nanocrystalline Zn1-xNixO (x=0.0025-0.03)

Authors: Mihalache, V; Negrila, C; Bercu, V; Secu, M; Vasile, E; Stan, GE

Published: JUL 2019, MATERIALS RESEARCH BULLETIN, 115, 48, DOI: 10.1016/j.materresbull.2019.03.001

We report on the influence of dilute doping combined with the processing conditions on the morphological, structural, chemical states, photoluminescence and magnetic properties of Zn1-xNixO nanopowders. Ni doping changes the ZnO powder morphology from randomly-aggregated nanocrystals to densely-packed nanocrystals arranged in columnar particles, modifies the high-energy-component of O1 s spectrum and increases the modified Auger parameter in XPS, enhances the blue photoluminescence (PL) emission, suppresses the green PL emission and the intensity of the g = 1.997 EPR signal. Ni-ZnO nanostructures show room-temperature ferro-magnetism (implying they can serve as dilute magnetic semiconductors). The saturation magnetization, crystallite size and microstrain increase with the doping level; the c-axis constant and unit cell volume decrease, however, being unexpectedly higher with respect to a (reference) ZnO powder with a relaxed lattice. We demonstrate that the investigated properties were controlled by both (dilute) doping level and donor native defects produced by non-equilibrium (oxygen-deficiency and high rate of) ZnO formation.

35. Structural and optical properties of ZnO thin films grown by rapid atmospheric mist chemical vapor technique

Authors: Derbali, S; Nouneh, K; Galca, AC; Touhami, ME; Secu, M; Matei, E; Leonat, LN; Pintilie, L; El Harfaoui, N; Fahoume, M

Published: JUL 2019, OPTICAL AND QUANTUM ELECTRONICS, 51, DOI: 10.1007/s11082-019-1937-2

In this work, the effect of deposition time on the structural and optical properties of ZnO films deposited by Ultrasonic Spray Mist-CVD was studied aiming the application in perovskite solar cells, as holes blocking layer. Crystallinity, surface morphology and optical properties of the ZnO films were investigated by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), conventional and Photoluminescence (PL) spectroscopies, respectively. The XRD measurement proves the existence of the hexagonal wurtzite phase and a high degree of crystallinity with [001] preferential orientation. The SEM study shows that the films possess a compact structure. Smooth and homogenous surface was confirmed also by AFM. The obtained results indicate that ZnO films deposited by a simple, safe and cost-effective method present a great potential for application in perovskite solar cells.

36. 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.

37. Effect of high gamma radiations on physical properties of In2S3 thin films grown by chemical bath deposition for buffer layer applications

Authors: Souli, M; Bensalem, Y; Secu, M; Bartha, C; Enculescu, M; Mejri, A; Kamoun-Turki, N; Badica, P

Published: JUN 2019, RESULTS IN PHYSICS, 13, DOI: 10.1016/j.rinp.2019.02.051

Polycrystalline In2S3 thin films have been grown on SnO2/glass substrates by chemical bath deposition technique and irradiated at different high gamma doses 3, 7, 15 and 40 kGy. X-ray diffraction, Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), Spectrophotometer, Photoluminescence and Thermoluminescence were used to investigate physical properties of In2S3 thin films induced by gamma irradiation. After being irradiated, structural properties of In2S3 thin films have shown that preferred orientation has been moved from (4 0 0) plan at 2 theta(1)=33.42 degrees to a new created orientation at 2 theta(2)=38.06 degrees for 40 kGy gamma dose. EDS analysis has shown that atomic percentage (S/In) has been strongly varied for 40 kGy which indicate significant changes in stoichiometry. Thermoluminescence of irradiated In2S3 thin films has revealed a good sensitivity toward absorbed gamma dose. After irradiation, optical transmittance of In2S3 thin films has been increased from 50% to a maximum value of 70% in the visible range for 15 kGy dose. Band gap energy E-g has been slightly decreased. Other optical parameters such absorption and extinction coefficients, refractive index and permittivity have been determined. These experimental results show that gamma radiations can be used for tuning physical properties of In2S3 thin films for photovoltaic applications.

38. Growth of SrTiO3 Single Crystals with a Diameter of about 30 mm by the Verneuil Method

Authors: Tateno, Y; Endo, K; Arisawa, S; Vlaicu, AM; Nedelcu, L; Preda, N; Secu, M; Iordanescu, R; Kuncser, AC; Badica, P

Published: FEB 2019, CRYSTAL GROWTH & DESIGN, 19, 612, DOI: 10.1021/acs.cgd.8b01004

The work demonstrates growth by the Verneuil method of SrTiO3 single crystals of 30 mm in diameter. Experiments are performed under an industrial environment. Growth was for 4.75 h, i.e., within one production shift. The optimum growth conditions for which the length of the region with bubbles D is zero and the effective length EL (i.e., the crystal length of commercial value) is maximized are for the amount of SrCO3 additive of similar to 3 wt % and for H-2 outer flow rate of similar to 35 L/min. These two parameters show the strongest influence on the bubble-free growth, but other growth parameters (H-2 inner flow rate, O-2 flow rate increase, rotation speed) were also optimized. Selected crystals are characterized from the structural, microstructural, optical, and THz spectroscopy viewpoints, and they are compared with a commercial substrate and with crystals reported in the literature. This work opens the possibility for the industrial growth of large SrTiO3 single crystals and commercialization of large area substrates.

39. Photoluminescence and thermoluminescence properties of the Sr3Al2O6:Eu3+/Eu2+,Tb3+ persistent phosphor

Authors: Gingasu, D; Mindru, I; Ianculescu, A; Preda, S; Negrila, C; Secu, M

Published: OCT 2019, JOURNAL OF LUMINESCENCE, 214, DOI: 10.1016/j.jlumin.2019.116540

Eu3+,Tb3+-doped Sr3Al2O6 powder phosphor was synthesized via a precursor route and subjected to a subsequent thermal treatment in reducing atmosphere. Photoluminescence and thermoluminescence properties of Sr3Al2O6:Eu3+/Eu2+,Tb3+ were investigated. The structure and morphology of oxides were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used for the nanocrystals surface composition analysis. X-ray diffraction patterns confirmed the formation of the cubic structure specific to Sr3Al2O6 with space group Pa3 and lattice parameter a = 15.8322 angstrom, while SEM investigations revealed equiaxial, polycrystalline particles, with sizes in the submicronic range, for both Sr3Al2O6:Eu3+,Tb3+ and Sr3Al2O6:Eu3+/Eu2+,Tb3+ samples. The photoluminescence spectra showed the typical f-f luminescence lines of the Tb(3+ )and Eu3+ - ions, accompanied by a broad Eu2+ luminescence band at 510 nm (after calcination in reducing atmosphere). The "after-glow" luminescence signal and the thermoluminescence were assigned to the recombination of close neighbor partners (electron and Eu2+ - hole centers) within the same complex of defects.

40. 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.

41. Physical properties investigation of samarium doped calcium sulfate thin films under high gamma irradiations for space photovoltaic and dosimetric applications

Authors: Souli, M; Reghima, M; Secu, M; Bartha, C; Enculescu, M; Mejri, A; Kamoun-Turki, N; Badica, P

Published: FEB 2019, SUPERLATTICES AND MICROSTRUCTURES, 126, 119, DOI: 10.1016/j.spmi.2018.12.021

This research work is dedicated to study structural, morphological, optical and photoluminescence properties of samarium doped calcium sulfate (CaSO4) thin films after exposure to high gamma radiations. Polycrystalline doped CaSO4 thin films have been grown on glass substrates by spray pyrolysis technique and irradiated at different high gamma doses 3, 7, 15 and 40 kGy. Physical characterization of irradiated thin films has been made by X-ray diffraction, Spectrophotometer, Scanning Electron Microscope, Energy Dispersive Spectroscopy, Fluorescence Spectrometer and Thermoluminescence. The most remarkable result, as shown by structural analysis, is the increase of grain size from 52 to a maximum value of 93 nm for 15 kGy gamma dose which indicates a clear enhancement in crystal structure by gamma irradiation. Moreover, the preferred orientation has been immediately changed from (102) plan to (100) just after the first 3 kGy gamma dose. SEM micrographs of irradiated thin layers show deep modifications in surface morphology. Optical transmission spectra have shown a sharp and intense peak at 350 nm wavelength. Band gap energy has been slightly decreased from 3.9 eV before irradiation to 3.6 eV for 40 kGy. A new and strong energy level noted E r , has been emerged and created due to high gamma irradiations in addition to the principal one relative to band gap energy. Other parameters like absorption and extinction coefficients and refractive index have been determined. Thermoluminescence data show a high sensibility to gamma radiations doses which offer opportunities for dosimetry applications. These experimental results suggest the use of irradiated samarium doped calcium sulfate as optical window for space photovoltaic devices where gamma rays are abundant. These results are also helpful for researchers using CaSO4 thin films near nuclear apparatus (nuclear reactors and particle accelerators) or those interested in studying interaction between radiations and condensed matter.

42. Hybrid layered double hydroxides-curcumin thin films deposited via Matrix Assisted Pulsed Laser Evaporation-MAPLE with photoluminescence properties

Authors: Andrei, F; Vlad, A; Birjega, R; Tozar, T; Secu, M; Urzica, I; Dinescu, M; Zavoianu, R

Published: JUN 1 2019, APPLIED SURFACE SCIENCE, 478, 761, DOI: 10.1016/j.apsusc.2019.02.011

Curcumin (CR) is a natural compound with a well-known antioxidant and therapeutic activity. Its stability may be enhanced when incorporated in different matrices as a layered double hydroxides (LDH) matrix. Curcumin intercalated layered double hydroxide nanohybrid is a potential drug delivery system for effective photodynamic therapy in human breast cancer or skin cancer. The synthesis of the hybrid LDH-CR powder implies the dissolution of CR in water or in another organic solvent which is miscible with water. Since the solubility of curcumin in water is very weak, the aim of this study is to investigate the effect of the solvent employed for its dissolution on the structural and physico-chemicalphotoluminescent properties of the resulting hybrid materials. Four powders of curcumin (CR)-containing Mg2.5Al-LDH hybrids (Mg/Al molar ratio of 2.5) were prepared by co-precipitation (P) and reconstruction (R) using two different solvents for the dissolution of curcumin: (i) an alkaline aqueous solution (A), and (ii) ethanol (E). The reconstruction used the calcinated (460 degrees C for 18 h) form of the parent Mg2.5Al-LDH powder. All the solids were characterized by X-ray diffraction (XRD), and FTIR spectroscopy. The FTIR-ATR spectra of the all the powders except the powder prepared via reconstruction in ethanol exhibit LDH characteristics, consistent with the XRD results. Matrix Assisted Pulsed Laser Evaporation (MAPLE) was employed for the deposition of hybrid LDH-CR thin films. Aqueous solutions of the as prepared hybrid LDH-CR powders were frozen and used as targets for MAPLE depositions. The films were deposited using a nanosecond laser emitting at 266 nm. MAPLE is considered a "soft" deposition technique suitable to conserve the CR stability. XRD, scanning electron microscopy, FT-IR spectroscopy and fluorescence measurements were used to characterize the deposited films in order to evidence the influence of the preparation methods on the structural and photophysical characteristics of the hybrid LDH-CR films.

43. Charge carrier traps in tris-(8-hydroxyquinoline) aluminum

Authors: Secu, M; Polosan, S

Published: FEB 2018, JOURNAL OF LUMINESCENCE, 194, 95, DOI: 10.1016/j.jlumin.2017.10.003

Thermoluminescent measurements between 40 and 300 C revealed two traps in the irradiated Alq(3) powder synthetized from Al(OH)(3), at 0.8 and 1.1 eV below the conduction band. These two traps have been assigned with the Alq(3) facial and meridional isomers and the trapping process stand for a long time after the switching off the irradiation source. The spectra recorded an hour after switching recover 60% from the photoluminescence and also decreasing the TL signals. A theoretical model was proposed in order to explain the trapping and detrapping processes in both meridional and facial Alq(3) isomers based on the XPS measurements for the HOMO, LUMO, ionization potential and optical bandgap, completed with the DFT calculated parameters and similar experimental results concerning the evidences of the triplet states in Alq(3) compound.

44. (Fe, Nd) codoped ZnO micro- and nanostructures with multifunctional characteristics like photocatalytic activity, optical and ferromagnetic properties

Authors: Diamandescu, L; Cernea, M; Tolea, F; Secu, EC; Trusca, R; Secu, M; Enculescu, M

Published: DEC 1 2018, CERAMICS INTERNATIONAL, 44, 21975, DOI: 10.1016/j.ceramint.2018.08.310

Zn1-x-yFexNdyO, x = 0.02 and y = 0.00-0.05 were successfully synthesized via a hydrothermal method at 200 degrees C. XRD and Mossbauer spectroscopy indicated that (Fe, Nd) codoped ZnO, calcined at 700 degrees C, 3 h in air, contains hexagonal ZnO phase as main phase and traces of ZnFe2O4; no phase containing neodymium was observed. In this study, a strong powder morphology dependence on Nd concentration and calcination temperature was evidenced by SEM. The photocatalytic properties were found to depend on the sample morphology and Nd content. Thermoluminescence signal is weak compared to the undoped ZnO and the curves shape was almost independent on the doping level and dependent on the calcination temperature and was related to the structural defects induced by the synthesis and calcination. Photoluminescence spectra showed that the green-yellow band due to the oxygen related defects, decreases by Fe and Nd-codoping. Thermo-magnetization curves measured in low applied fields and the magnetic hysteresis loops recorded in the temperature range 5-380 K, indicate two superimposed magnetic regimes, one predominant above 100 K the other below 100 K. The observed magnetic order is not directly related to the dopant Fe or Nd ions, but to different type of defects.

45. Enhanced near-infrared response of a silicon solar cell by using an up-conversion phosphor film of Yb/Er - co-doped CeO2

Authors: Grigoroscuta, M; Secu, M; Trupina, L; Enculescu, M; Besleaga, C; Pintilie, I; Badica, P

Published: SEP 1 2018, SOLAR ENERGY, 171, 46, DOI: 10.1016/j.solener.2018.06.057

Co-doped CeO2 thin films were grown from a bulk target with starting composition Ce0.95Yb0.04Er0.01O2 by pulsed laser deposition (PLD) on a p(+)-n-n(+) single crystal silicon diode. The PLD laser fluence was varied between 1.7 J/cm(2) and 3.7 J/cm(2). The device with the film grown for a laser fluence of 2.3 J/cm(2) delivers the highest performance taking advantage of the up conversion (UC) effect provided by this film. Namely, the increase in the relative power conversion efficiency of the device is 12.1% and 39.2% for illumination under 1 and 2.1 sun, respectively, and its relative external quantum efficiency is 8.2% when illuminated with 980 nm light. The film grown for the optimum 2.3 J/cm(2) fluence shows good target-film composition transfer and a granular morphology with a low roughness. The UC mechanism consists of efficient energy transfer between spatially separated Yb3+ and Er3+ ions, i.e. the absorption of infrared light photons by the Yb3+ ions (F-2(7/2) -> F-2(5/2) transition) is followed by a two-step energy transfer process to neighboring Er3+ ions and by their characteristic luminescent emissions ((H-2(11/2), S-4(3/2)) -> I-4(15/2)) and (F-4(9/2) -> I-4(15/2)).

46. Defect states and room temperature ferromagnetism in cerium oxide nanopowders prepared by decomposition of Ce-propionate

Authors: Mihalache, V; Secu, M; Grivel, JC

Published: APR 15 2018, MATERIALS CHEMISTRY AND PHYSICS, 209, 133, DOI: 10.1016/j.matchemphys.2018.01.053

Four batches of cerium oxide powders (with nanocrystallite size of 6.9 nm-572 nm) were prepared from four precursor nanopowders by thermal decomposition of Ce-propionate and annealing in air between 250 degrees C-1200 degrees C for 10 min-240 min. Ceria formation reactions, structure, vibrational, luminescence and magnetic properties were investigated by differential scanning calorimetry, x-ray diffraction, electron microscopy, infrared spectroscopy, photoluminescence and SQUID. All the samples exhibit room temperature ferromagnetism, RTFM, (with coercivity, H-c, of 8 Oe - 121 Oe and saturation magnetization, M-s, of up to 6.7*10(-3) emu/g) and a broad defect-related photoluminescence, PL, emission in the visible range. The samples derived from the same precursor show M-s proportional to the peak area of defect-related PL emission whereas this is not valid for the samples derived from the different precursors. An improvement of ferromagnetism and intensity of defect-related PL emission was observed when annealing the products in which nanocrystalline cerium oxide coexists with Ce - oxicarbonate traces, Ce2O2CO3. The experimental results were explained based on the following considerations: room temperature ferromagnetism was induced by the defective ceria with high concentration of oxygen vacancies generated by decomposition of Ce-propionate; oxygen vacancies of the starting precursor nanopowders could be redistributed (at the surfaces/grain boundaries, GBs) upon heating under conditions that promote an inert local environment; the decomposition of Ce2O2CO3 residues can provide an excess of oxygen vacancies at the nanoparticles surfaces or GBs, which can induce or enhance ferromagnetism; surfaces/GBs rather than bulk defects appear responsible for RTFM - this can explain the (often reported in literature) inconsistency between oxygen vacancies concentration and M-s. (C) 2018 Elsevier B.V. All rights reserved.

47. 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.

48. Wide-Range Columnar and Lamellar Photoluminescent Liquid-Crystalline Lanthanide Complexes with Mesogenic 4-Pyridone Derivatives

Authors: Chiriac, LF; Pasuk, I; Secu, M; Micutz, M; Circu, V

Published: SEP 12 2018, CHEMISTRY-A EUROPEAN JOURNAL, 24, 13522, DOI: 10.1002/chem.201801781

A series of liquid crystals with various lanthanide ions (Eu-III, Sm-III, and Tb-III) was designed and prepared starting from the corresponding lanthanide nitrates and N-alkylated 4-pyridone derivatives bearing mesogenic 3,4,5-tris(alkyloxy)-benzyl moieties (alkyl=hexyl, octyl, decyl, dodecyl, tetradecyl, or hexadecyl). These new lanthanidomesogens were investigated for their mesogenic properties by a combination of differential scanning calorimetry, polarizing optical microscopy, and temperature-dependent powder X-ray diffraction (XRD). Their thermal stability was assessed by thermogravimetric analysis. All of these complexes show enantiotropic liquid-crystalline behavior with lamellar (SmA) phases in the case of shorter-chain complexes (C-6 and C-8) or hexagonal columnar phases (Col(h)) for complexes with longer alkyl chains (C-12, C-14, and C-16), which were assigned on the basis of their characteristic textures and XRD studies. For complexes with an intermediate number of carbon atoms in the side chains (C-10), both a lamellar phase at lower temperatures and a Col(h) phase at higher temperatures were evidenced. In the solid state, all these complexes show characteristic emissions assigned to the corresponding lanthanide ion. In addition, the luminescence decay curves showed single-exponential decays with characteristic times in the millisecond range (0.75-0.90 ms for Eu-III, 0.045-0.060 ms for Sm-III, and 0.75-1.05 ms for Tb-III).

49. 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.

50. 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.

51. 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.

52. Structural and optical properties of un-doped and doped Sr3Al2O6 obtained through the tartarate precursor method

Authors: Mindru, I; Gingasu, D; Patron, L; Marinescu, G; Culita, DC; Calderon-Moreno, JM; Preda, S; Secu, M

Published: DEC 15 2017, CERAMICS INTERNATIONAL, 43, 16675, DOI: 10.1016/j.ceramint.2017.09.057

Un-doped and doped strontium aluminates (Sr3Al2O6, Sr3Al2O6:Tb3+, Sr3Al2O6:Tb3+,Eu3+ and Sr3Al2O6:Tb3+,Eu3+/Eu2+) were synthesized through a soft chemical method - the tartarate precursor route. The tartarate precursors were characterized by infrared spectroscopy (IR) and thermal analysis. The strontium aluminates were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectroscopy (IR). The XRD patterns confirmed the formation of single-phase cubic structure Sr3Al2O6 with average crystallite size of 18 nm. SEM micrographs showed porous polycrystalline powder microstructures and EDX analysis confirmed the incorporation of the dopant in the Sr3Al2O6 powders. The photoluminescence spectrum recorded on Sr3Al2O6: Tb3+,Eu3+ (obtained from tartarate precursor calcined in air) showed the typical f-f luminescences of the RE-ions. Subsequent calcination of this sample in reducing atmosphere is accompanied by new photoluminescence and thermoluminescence features associated to the Eu2+ ions.

53. Tb3+-doped alkaline-earth aluminates: Synthesis, characterization and optical properties

Authors: Mindru, I; Gingasu, D; Patron, L; Marinescu, G; Calderon-Moreno, JM; Diamandescu, L; Secu, M; Oprea, O

Published: JAN 2017, MATERIALS RESEARCH BULLETIN, 85, 248, DOI: 10.1016/j.materresbull.2016.09.024

Un-doped and Tb3+-doped calcium and strontium aluminates (CaAl2O4, CaAl2O4:Tb3+, SrAl2O4, SrAl2O4: Tb3+) were synthesized by a new soft chemical method the precursor route via the thermal decomposition of tartarate compounds. The complex precursors have been characterized by elemental chemical analysis, infrared spectroScopy (IR), luminescence spectroscopy and thermal analysis. X-ray diffraction (XRD), scanning electron microscopy (SEM), IR and Raman spectroscopy (RS) were used for the structural and morphological characterization of un-doped and doped alkaline-earth aluminate powders. Photoluminescence (PL) and Thermoluminescence (TL) properties of the aluminates were also investigated. The XRD patterns confirmed the formation of low-temperature structures: hexagonal structure for CaAl2O4 and monoclinic structure for SrAl2O4, with average crystallite size of 64 nm and 46 nm for CaAl2O4 and SrAl2O4, respectively. Microstructural analysis by SEM showed the homogeneous microstructure and fine particle size of the powder obtained by precursor method. (C) 2016 Elsevier Ltd. All rights reserved.

54. UPCONVERSION AVALANCHE IN THE Yb3+/Ho3+ DOPED SiO2-LiYF4 NANO-GLASS CERAMIC

Authors: Secu, M

Published: OCT-DEC 2017, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 12, 1080, DOI:

The "avalanche" up-conversion properties of the glass-ceramics containing (Ho3+, Yb3+)doped LiYF4 nanocrystals have been studied by comparison to the corresponding nanocrystals and polycrystalline powder samples. Under 810 nm laser light pumping, all the samples showed "green" ((H-2(11/2), 4S(3/2)) -> I-4(15/2)) up-conversion luminescence. The pump power threshold for the avalanche mechanism is about 75mW for the nanocrystalline or polycrystalline powder samples indicating a weak influence of the nanosize related effects is but smaller of about 43mW for the nano-glass ceramic due to the high "local" Yb3+ concentrations.

55. 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.

56. STRUCTURAL AND MORPHOLOGICAL EVOLUTIONS INDUCED BY THE ANNEALING OF CaF2 NANOCRYSTALLINE POWDERS BY USING Eu3+ ION PROBE LUMINESCENCE

Authors: Polosan, S; Secu, M; Enculescu, M

Published: APR-JUN 2016, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 11, 624, DOI:

Eu-doped CaF(2)nanocrystalline powders have been prepared by using co-precipitation technique. Structural and morphological modifications induced by annealing are followed by using Eu3+-ion probe. Theluminescence properties are changing gradually together with the morphology of doped CaF2 nanoparticles accompanied by anEu(3+)-> Eu2+ conversion. Cathodoluminescence measurements show uniform distribution of the europium dopant ions in the annealed samples accompanied by a strong increase of the integral photoluminescence signal. During annealing the cubic structure of CaF2 nanoparticles is altered going to the spherical morphology due to the Eu3+ ions incorporation. In the annealed samples Eu3+ ions are incorporated as Eu3+-O dimer centres with the coordination symmetry (C-2 nu); in the sintered samples there are two non-equivalent Eu3+ sites. The thermoluminescence peaks have been assigned to the recombination of the Eu3+ related traps in these sites.

57. 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.

58. Dielectric and photoluminescence properties of Nd and Ga codoped-BaTiO3, prepared by sol-gel method

Authors: Cernea, M; Vasilescu, CA; Secu, M; Aldica, G; Surdu, A; Ganea, P

Published: NOV 2016, JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, 27, 11378, DOI: 10.1007/s10854-016-5262-2

Neodymium and gallium codoped barium titanate (Ba1-x-yNdxGayTiO3, x = 0.03, 0.05, 0.07, 0.1 and y = 0.03) were prepared by sol-gel method in order to investigate its dielectric and photoluminescence properties. The structure and morphology of Ba1-x-yNdxGayTiO3 powders calcined at 1100 A degrees C and sintered ceramics by spark plasma sintering (SPS) technique were analyzed using X-ray diffraction and scanning electron microscope, respectively. The photoluminescence and dielectric properties of Nd doped Ba0.97Ga0.03TiO3, (y = 0.03) as a function of Nd3+ concentration and sintering temperature were also investigated. The influence of Ga3+ and Nd3+ dopants on the crystalline phases, sintering process by SPS and macroscopic properties of Ba1-x-yNdxGayTiO3 ceramics was analyzed, as well as a weak hot emission from the 4F5/2 level. The sintered ceramics showed high dielectric constant and moderate dielectric losses. The highest value of the permittivity obtained for Ba1-x-yNdxGayTiO3 ceramics was E >(r) = 5890 at room temperature, 1 kHz and x = y = 0.03. This study shown that Nd3+ decreases the density of the pellets, increases the dielectric losses and decreases the dielectric constant of Ba0.97Ga0.03TiO3 ceramics. Therefore, the values of dielectric properties, at room temperature, of BaTiO3 doped with 3 at.% Ga and aecurrency sign7 at.% Nd are higher than those of undoped BaTiO3 ceramic.

59. 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.

60. 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.

61. 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.

62. Ceramics and amorphous thin films based on gallium sulphide doped by rare-earth sulphides

Authors: Popescu, M; Sava, F; Lorinczi, A; Velea, A; Simandan, ID; Badica, P; Burdusel, M; Galca, AC; Matei, E; Preda, N; Secu, M; Socol, G; Jipa, F; Zamfirescu, M; Balan, A

Published: APR 2015, SEMICONDUCTOR SCIENCE AND TECHNOLOGY, 30, DOI: 10.1088/0268-1242/30/4/044001

Bulk ceramics of Ga2S3 and rare-earth sulfides (EuS, Gd2S3, Er2S3) as well as combinations thereof have been prepared by spark plasma sintering (SPS). The disk-shaped ceramics were used as targets for pulsed laser deposition (PLD) experiments to obtain amorphous thin films. The properties of these new bulks and amorphous thin films have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), optical transmission spectroscopy, and atomic force microscopy (AFM). In order to test the photoexpansion effect in Gd2S3 and the possibility to create planar arrays of microlenses, the film was irradiated with femtosecond laser pulses at different powers. For low laser power pulses (up to 100mW power per pulse) a photoexpansion effect was observed, which leads to formation of hillocks with a height of 40-50 nm. EuS doped Gd2S3 thin film shows luminescence properties, which recommend them for optoelectronic applications.

63. A new class of thermotropic lanthanidomesogens: Eu(III) nitrate complexes with mesogenic 4-pyridone ligands

Authors: Pana, A; Chiriac, FL; Secu, M; Pasuk, I; Ferbinteanu, M; Micutz, M; Circu, V

Published: 2015, DALTON TRANSACTIONS, 44, 14199, DOI: 10.1039/c5dt01197c

A new class of thermotropic lanthanidomesogens has been designed and prepared. They are based on 4-pyridone ligands that possess mesogenic cyanobiphenyl groups attached to the 4-pyridone unit via a flexible long alkyl spacer and show a very high thermal stability (decomposition temperatures near 300 degrees C). Depending on the alkyl length spacer, these complexes exhibit a SmA phase with transition temperatures influenced by the number of mesogenic groups employed and the spacer length.

64. 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.

65. OPTICAL CHARACTERISTICS OF SULPHUR-PASSIVATED n-GaAs (100) SURFACE

Authors: Ghita, RV; Grigorescu, CEA; Secu, M; Predoi, D; Frumosu, F; Cotirlan, C; Feraru, ID

Published: OCT-DEC 2014, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 9, 1478, DOI:

A genuine GaAs surface is covered with a relatively thick layer (similar to 10 nm) of native oxide pinning the surface Fermi level within the band gap of semiconductor. The method presented in this work is related to the sulphur passivation by treating n-GaAs(100) and (110) in sulphide solutions (e.g pure ammonium sulphide and sulphur monochloride) that combines both chemical electronic passivation by reducing the surface state density. The effects of passivation were put into evidence by Photoluminescence measurements where it was observed the variation of state density. The presence of sulphur dipoles at surface was presented in a diagram of Second Harmonic Generation analysis and the general aspect of sulphur compound was identified by SEM images. The presence of the covalent bond Ga-S and the variation of surface barrier as well as the effect of crystal orientation were put into evidence by micro-Raman Spectroscopy. At the surface of n-GaAs it is developed an adherent layer of sulphur compound as a resul of chemical interaction of sulphur ions with n-GaAs surface.

66. 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.

67. 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.

68. 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.

69. 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.

70. 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.

71. 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.

72. 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.

73. Broadband near-infrared emission of chromium-doped sulfide glass-ceramics containing Ga2S3 nanocrystals

Authors: Ren, J; Li, B; Yang, G; Xu, WN; Zhang, ZH; Secu, M; Bercu, V; Zeng, HD; Chen, GR

Published: DEC 15 2012, OPTICS LETTERS, 37, 5045, DOI: 10.1364/OL.37.005043

Upon 808 nm excitation, an intense broadband near-infrared emission from Cr4+ has been observed in 80GeS(2)-20Ga(2)S(3) chalcogenide glass-ceramics (GCs) containing Ga2S3 nanocrystals. The emission band peaking at 1250 nm covers the O, E, S bands (1000-1500 nm). The formation of Ga2S3 nanocrystals (similar to 20 nm) increases the emission intensity of Cr4+ by more than three times. The quantum efficiency of the present GCs is as great as 36% at room temperature. (C) 2012 Optical Society of America

74. 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.

75. 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.

76. 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.

77. 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.

78. 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.

79. 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.

80. Band-gap correlations in Bi4Ge3O12 amorphous and glass-ceramic materials

Authors: Polosan, S; Galca, AC; Secu, M

Published: JAN 2011, SOLID STATE SCIENCES, 13, 53, DOI: 10.1016/j.solidstatesciences.2010.10.007

The Bi4Ge3O12 (BGO) glass materials were sintered from a mixture of oxides powders, rapidly heated and melted at 1323 K and poured on the graphite plates heated at different temperatures. Their optical properties are compared with BGO single crystals being strongly dependent by the experimental conditions, which influence the further stability of the samples. The dielectric functions of the samples reveal a clear difference between the two phases (amorphous and crystalline). The refractive index of the amorphous phase has lower values which indicate that the density of the material is diminishes. Also, a sign of disorder is seen in optical absorption (extinction coefficient). From the optical band gap determinations of those two phases, it is confirmed the disorder structure and red-shifts of the band gap value compared with crystalline phase. The absorption spectra are in good agreement with the ellipsometric measurements, regarding the transparency and the edge of the band gap. (C) 2010 Elsevier Masson SAS. All rights reserved.

81. 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.

82. Nanoparticles size effects in thermoluminescence of oxyfluoride glass-ceramics containing Sm3+-doped CaF2 nanocrystals

Authors: Secu, M

Published: JUL 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 2732, DOI: 10.1007/s11051-010-0181-2

Oxyfluoride glass-ceramic in the system SiO2-Al2O3-CaF2-SmF3 containing Sm3+-doped CaF2 nanocrystals in the range from 15 to 150 nm size were produced by using the controlled ceramization of the precursor glass. The incorporation of the Sm3+-dopant ion in the glass ceramic creates new electron-trapping centers and thermoluminescence (TL) method has been used in order to trace their evolution during glass ceramization. The 370 A degrees C TL peak observed in precursor glass has been assigned to the recombination of the electrons released from the Sm2+-traps in the amorphous glass network. In the glass-ceramic sample containing nanocrystals with about 15 nm size the new weak TL peaks at 270, 290, and 310 A degrees C were attributed to the recombination of the electrons released from the Sm2+-traps located mainly at the surface of the CaF2 nanocrystals. In the glass-ceramic sample containing nanocrystals with about 150 nm size, the new TL peaks at 232, 270, and 302 A degrees C size have been assigned to the recombination of the electrons released from the Sm2+-traps located inside the CaF2 nanocrystals.

83. Structural changes during the crystallization of the Bi4Ge3O12 glasses

Authors: Polosan, S; Nastase, F; Secu, M

Published: FEB 1 2011, JOURNAL OF NON-CRYSTALLINE SOLIDS, 357, 1113, DOI: 10.1016/j.jnoncrysol.2010.10.016

Bismuth-germanate glass ceramics with the composition 40% Bi2O3-60% GeO2 (in molar percents) were prepared through controlled crystallization of melt-quenched glass. The Raman and FTIR spectra recorded in the as-made glasses show broad bands at 240, 400, 780 cm(-1) and 400, 745 cm(-1) have been assigned Ge-O bonds which appear right after preparation. X-ray diffraction has shown that the as-made glasses are amorphous, but after annealing above the crystallization temperature at 558 degrees C, BGO nano-crystallites with a size of about 50 nm precipitate in the glass matrix. The Raman and FTIR spectra reveal sharp peaks associated to the "internal" and "external vibrations" of GeO4 tetrahedral groups inside the BGO nano-crystallites. In the glass ceramic sample the transparency region is shifted at longer wavelengths compared to as-made glass, due to the Rayleigh scattering on the BGO nano-crystallites. (C) 2010 Elsevier B.V. All rights reserved.

84. 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.

85. Growth and optical characterization of Pb-, TI-doped LiF crystals

Authors: Somma, F; Montereali, RM; Vincenti, MA; Polosan, S; Secu, M

Published: AUG 2010, OPTICAL MATERIALS, 32, 1312, DOI: 10.1016/j.optmat.2010.04.038

Large and transparent LiF:Pb and LiF:TI crystals were successfully grown by using Kyropoulos method. They were characterized using spectroscopic measurements combined with differential thermal analysis and thermogravimetry (TG) analysis, in order to investigate the effect of dopants upon the optical properties of these crystals. The optical absorption peaks at 278 nm (for Pb) and 273 nm (for TI) have been assigned to the A-type transition of these ions. Photoluminescence spectra have shown bands at 350 nm (for TI) and 375 nm (for Pb), which have been associated to the dopant ions in different configurations in the lattice. (C) 2010 Elsevier B.V. All rights reserved.

86. Investigations of the neodymium doped quartz glasses obtained by the hybrid sot-gel method

Authors: Niciu, H; Vasiliu, C; Manea, A; Radu, D; Secu, M; Onose, C; Ghervase, L; Niciu, D; Feraru, ID; Grigorescu, CEA

Published: OCT 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1623, DOI:

Undoped and neodymium doped quartz glasses were obtained by the hybrid sol-gel method. The vitrification was performed in He atmosphere. The Nd(3+) characteristic absorption bands in the UV and VIS were put into evidence. Investigations in the NIR range revealed a small amount (86 ppm) of OH(-) in the vitreous matrix. FTIR and Raman spectra exhibit the SiO(2) glass characteristic vibrations and also the characteristic vibrations of Si-OH and Si-Cl bonds. The photoluminescence peaks at 640, 920 and 1080 nm belonging to the Nd(3+) doped quartz glass were evidenced.

87. A new ZnO photoanode for dye-sensitized solar cell

Authors: Sima, M; Secu, M; Sima, M; Vasile, E

Published: SEP 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1328, DOI:

The present work reports the preparation of nanostructured ZnO nanorods in a hydrothermal-electrochemical process and preliminary attempt to fabricate a dye-sensitized solar cell using this material as photoanode. Synthesis of ZnO nanorods in solutions with and without eosin Y dye, used as nanostructuring agent, was studied. ZnO nanorod arrays were characterized by photoluminescence spectroscopy, scanning electron microscopy and X-ray diffraction analysis. Fabricated dye-sensitized solar cell comprised a photoanode prepared with nanostructured ZnO nanorods and eosin Y dye, a platinum counter electrode and an electrolyte consisting of 0.5M KI + 0.03M I-2 in acetonitrile/ethylene carbonate (v/v=1/4). Performance parameters of dye-sensitized solar cell were extracted from standard current-voltage characteristic.

88. Investigations of the non-isothermal crystallization of CaF2 nanoparticles in Sm-doped oxy-fluoride glasses

Authors: Aldica, G; Secu, M

Published: JUL 15 2010, JOURNAL OF NON-CRYSTALLINE SOLIDS, 356, 1636, DOI: 10.1016/j.jnoncrysol.2010.06.017

Oxyfluoride glass samples in the system SiO2-Al2O3-CaF2-SmF3 were produced by using the melt-quenched technique and the kinetics of non-isothermal crystallization of CaF2 nano-crystals has been investigated. Differential thermal analysis (DTA) has shown an exothermic peak at about 700 degrees C which was assigned to the CaF2 nano-crystals precipitation within the glass matrix as was identified by X-ray diffraction (XRD) analysis of the annealed glass samples. It was shown the Ozawa model is most suitable for describing the behaviour of non-isothermal crystallization of CaF2 nano-crystals within the glass matrix. The mean value of the Avrami exponent was found 1.16 which indicates the diffusion-controlled growth process of nano-particles in the glass system with zero nucleation rate as the single operative mechanism; the activation energy of the crystallization is E-d = 379 +/- 25 kJ/mol (3.93 +/- 0.26 eV). The activation energy of the nano-crystals growth was computed within the same model and agrees very well with the activation energy of the crystallization. (C) 2010 Elsevier B.V. All rights reserved.

89. X-ray excited luminescence and photoluminescence of Bi-4(GeO4)(3) glass-ceramics

Authors: Polosan, S; Secu, M

Published: MAR-JUL 2010, RADIATION MEASUREMENTS, 45, 411, DOI: 10.1016/j.radmeas.2010.01.040

Bi-4(GeO4)(3) glass materials have been characterized by X-ray excited luminescence, photoluminescence and cathodo-luminescence measurements. The materials were obtained by crystallization at different temperatures and their spectroscopic parameters were compared before and after crystallization. Thermoluminescence curves recorded after electron irradiation of BGO glass behave similarly to BGO crystals, showing several peaks between 408 K (135 degrees C) and 610 K (337 degrees C). The differences between the Bi-4(GeO4)(3) crystals and glass materials are believed to result from the random distribution of GeO4 tetrahedra around Bi3+ ions which influences the photoluminescence and TL parameters. The CL images of glass-ceramic samples obtained by partial crystallization at 600 degrees C show luminescent crystalline structures, which are probably responsible for the increase in scintillation efficiency. (C) 2010 Elsevier Ltd. All rights reserved.

90. Optical properties of zinc oxide thin films doped with aluminum and lithium

Authors: Galca, AC; Secu, M; Vlad, A; Pedarnig, JD

Published: JUN 1 2010, THIN SOLID FILMS, 518, 4606, DOI: 10.1016/j.tsf.2009.12.041

Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E(g), and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap. (C) 2009 Elsevier B.V. All rights reserved.

91. Optical and X-ray absorption spectroscopy in lead doped lithium fluoride crystals

Authors: Somma, F; Aloe, P; d'Acapito, F; Montereali, RM; Polosan, S; Secu, M; Vincenti, MA

Published: 2010, 11TH EUROPHYSICAL CONFERENCE ON DEFECTS IN INSULATING MATERIALS (EURODIM 2010), 15, DOI: 10.1088/1757-899X/15/1/012035

LiF:Pb doped crystals were successfully grown by Kyropoulos method, starting with drying powders. The presence of Pb2+ ions in the LiF crystals were evidenced by the absorption band at 278 nm and by 375 nm photoluminescence. The presence of some other Pb structures with oxygen compounds in the as made samples was evidenced, decreasing after some annealing procedures. The local environment and valence state of Pb in LiF were studied by X-ray Absorption Spectroscopy at the Pb L-III and L-I edges. XANES data reveal that Pb is present as Pb2+ whereas EXAFS data show that it is incorporated in the crystal and not forming PbF2 precipitates. Identical spectra are obtained for samples as prepared and after thermal annealing up to 650 degrees C demonstrating the stability of the incorporation site. Also the concentration of Pb in the crystal has no effect on the location site of the metal as the same spectrum is obtained for specimens with different dopant concentrations.

92. Radiation induced defects in Tl+-doped LiF crystals

Authors: Polosan, S; Secu, M; Logofatu, C; Somma, F; Montereali, RM; Vincenti, MA

Published: 2010, 11TH EUROPHYSICAL CONFERENCE ON DEFECTS IN INSULATING MATERIALS (EURODIM 2010), 15, DOI: 10.1088/1757-899X/15/1/012081

Tl+-doped lithium fluoride crystals exhibit very interesting properties concerning the formation of colour centres after X-rays irradiation. The presence of Tl+ ions, even in small concentrations, increases the number of traps which stabilize the H-centre aggregates. These H-centre aggregates become smaller and their number increases, inducing two sets of trapping levels. Photoluminescence measurements after subsequent thermal bleaching of the thermoluminescence peaks show a faster decreasing of the F-3(+) centres, in three steps, compared with the F-2 ones. The F-H recombination processes may induce the excitation-emission transitions of Tl+-ions.

93. 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.

94. 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.

95. Radiation induced defects in Pb2+-doped LiF crystals

Authors: Somma, F; Montereali, RM; Vincenti, MA; Polosan, S; Secu, M

Published: 2009, 2008 INTERNATIONAL CONFERENCE ON LUMINESCENCE AND OPTICAL SPECTROSCOPY OF CONDENSED MATTER, 2, 221, DOI: 10.1016/j.phpro.2009.07.013

The influence of Pb2+ doping on the optical properties of LiF crystals before and after irradiation with X-ray was studied by using optical absorption and photoluminescence measurements. After irradiation it was observed an increase of the absorption due to primary and aggregate electronic defects together with a higher photoluminescence signals associated with the F-2 and F-3(+) colour centres in doped samples compared with the undoped ones.

96. 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.

97. Optical spectroscopy and thermoluminescence of electrolytically colored KCl:Tl+ crystals

Authors: Polosan, S; Enculescu, M; Secu, M; Apostol, E

Published: NOV 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 3011, DOI:

The paper presents the spectroscopic properties of electrolytic colored KCl:Tl and KCI:Tl+Ca crystals and the assignments of the new defects which are formed during electron injection process. The new formed defects Tl-0 and Tl-0+Ca2+ with the metal in the anionic sites, presents very high photoluminescence properties in the near infrared region. Together with those centers, the F-centers, their aggregates and the holes centers are formed during electrolytic coloring process. Their properties were studied by optical absorption (OA), photoluminescence (PL) and thermoluminescence (TL) measurements and compared with those obtained by ionizing irradiation in uncolored KCl:Tl crystals. The results suggest a more efficient Tl+-> Tl-0 process in the case of Ca2+ codoped samples due to the prolonged time of electrons injection, low temperature and high electric field.

98. 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.

99. Luminescence and EPR study of ZnO : Mn : Cu nanowire array

Authors: Sima, M; Enculescu, I; Grecu, MN; Secu, M; Sima, M; Matei, E; Vasile, V

Published: MAY 2008, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 40, 2498, DOI: 10.1016/j.physe.2007.08.075

We report on photoluminescence studies on manganese and copper-doped ZnO nanowire array prepared by template method. The ZnO:Mn:Cu semiconductor grown using the electrochemical technique was characterized by electron paramagnetic resonance (EPR) and optical reflection measurements. Morphology and the structure were observed by SEM and X-ray diffraction. The photoluminescence of this system was correlated with manganese and copper concentration in the ZnO matrix. (C) 2007 Elsevier B.V. All rights reserved.

100. Processes involved in the high-temperature thermoluminescence of a Mn2+-doped MgF2 phosphor

Authors: Secu, M; Jipa, S; Secul, CE; Zaharescu, T; Georgescu, R; Cutrubinis, M

Published: JAN 2008, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 245, 162, DOI: 10.1002/pssb.200743108

The thermoluminescence (TL) glow curve recorded after gamma-irradiation at room temperature of Mn-2-doped MgF2 phosphor shows two intense TL peaks at 364 K and 420 K accompanied by several weaker peaks up to 600 K. The TL spectra have shown Mn2+ luminescence at 560 nm. The 364 K TL peak disappears after one day of storage, and so is probably related to the recombination of carriers from some shallow traps. Based on thermal bleaching and optical bleaching (in the 260 nm F-centre absorption band) experiments, the 420 K TL peak was ascribed to the thermally activated recombination between an F-centre and a hole centre trapped in the neighbourhood of the Mn2+ impurity which results in Mn2+ luminescence.

101. Thermoluminescence and optically stimulated luminescence of gamma-irradiated mineral zircon

Authors: Secu, M; Vainshtein, D; Turkin, A; den Hartog, HW

Published: 2007, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 3, 4, +, DOI: 10.1002/pssc.200673738

Thermoluminescence (TL) manifested by gamma-irradiated mineral zircon has shown a strong TL peak at about 165 degrees C which is due to recombination of electrons and Dy3+ related shallow hole traps. After they have been removed by a short preheat we have observed two TL peaks at 300-320 degrees C and approximate to 420 degrees C, which are mainly due to recombination of electrons and Tb3+ related hole traps centres yielding its characteristic luminescence. The experimental results indicate that optically stimulated luminescence (OSL) is due to luminescent emission of Tb3+ ions and [SiO4](4-) groups. The deep traps related to the 420 degrees C TL peak contribute to the Tb3+ related OSL. The deep traps related to the 300-320 degrees C TL peak contribute to OSL associated with the luminescent emission of [SiO4](4-) groups. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhem.

102. 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.

103. 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.

104. 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.

105. Comparison of the luminescence properties of the x-ray storage phosphors BaCl2 : Ce3+ and BaBr2 : Ce

Authors: Selling, J; Corradi, G; Secu, M; Schweizer, S

Published: DEC 21 2005, JOURNAL OF PHYSICS-CONDENSED MATTER, 17, 8078, DOI: 10.1088/0953-8984/17/50/024

The single-crystalline x-ray storage phosphor BaCl2:Ce3+ has been investigated by photo- and x-ray luminescence, and also by observing the afterglow and the photostimulated luminescence (PSL). A single active Ce3+ site has been observed with a characteristic emission doublet at 349 and 373 nm at room temperature. The PSL stimulation spectrum shows a clear resemblance to F-type absorption spectra in undoped BaCl2, indicating that the PSL-active electron centres are F-type centres. The results are compared with previous photoluminescence, PSL and new x-ray luminescence data on BaBr2:Ce3+. The integral PSL efficiency in the chloride is found to be somewhat smaller than in the bromide, which is still large enough for storage phosphor applications, while the efficiency in the bromide may be even larger or comparable to that of the commercial x-ray storage phosphor BaFBr2:Eu2+.

106. Photostimulable defects in nano-crystallites in fluorozirconate glasses

Authors: Schweizer, S; Spaeth, JM; Secu, M; Edgar, A; Williams, GVM

Published: JAN 2005, PHYSICA STATUS SOLIDI A-APPLIED RESEARCH, 202, 249, DOI: 10.1002/pssa.200460220

The photostimulated luminescence (PSL) from Eu-doped fluorobromo- and fluorochlorozirconate glass-ceramics is reviewed. The PSL is attributed to the characteristic emission of Eu2+ present in BaX2 (X = Br, Cl) crystallites which form in the glass upon thermal processing. Both the photoluminescence intensity and the PSL efficiency increase with the size of the crystallites in a way that is consistent with a PSL-inert surface layer surrounding the crystallites. The increasing crystallite sizes also result in decreased optical transparency at the stimulation wavelength due to scattering. (C) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

107. Photoluminescence of manganese- and copper-doped CdS nanowires

Authors: Ghiordanescu, V; Sima, M; Enculescu, I; Grecu, MN; Mihut, L; Secu, M; Neumann, R

Published: FEB 2005, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 202, 454, DOI: 10.1002/pssa.200406927

Arrays of US: Mn2+ : Cu+ micro- and nanowires grown in polycarbonate ion-track templates exhibit photoluminescence in the spectral domain ranging from 500 to 800 mn at room temperature. A comparison with similar US and CdS:Mn2+ wire arrays is presented. The individual contributions to the emission spectra of Cu+ and Mn2+ ions in the US matrix are explained using their energy level schemes. Also SEM, EDX and EPR data are given or these wires. (c) 2005 WILEY-VCH Verlag GmbH A Co. KGaA, Weinheim.

108. Thermo and photoluminescence of silicate laser glass doped with neodymium

Authors: Grigoras, B; Cerchez, M; Ionescu, L; Secu, M; Matei, L; Trutia, A

Published: JUN 2001, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 3, 562, DOI:

The luminescence and thermoluminescence have been measured in vitreous materials doped with Nd3+. The luminescence spectra show a maximum at 400 nm and a shoulder at about 440 nm. The measured global thermoluminescence shows a broad band with a maximum at 270 degreesC, which suggest a continuous distribution of electron traps induced by ionizing radiation. Thermoluminescence measured through two optical filters which transmit at about 400 nm and 440 nm wavelength shows a structured behavior with two glow peaks at 270 degreesC and 330 degreesC. This apparently contradictory behavior has been explained by the existence in this material of a continuous distribution of tr ap levels and a discrete distribution of recombination levels. The 270 degreesC and 330 degreesC glow peaks have been interpreted as being due to recombination of the electrons released from electron traps with O2-. and Nd3+ hole centers. Due to the high stability of the defects induced by the radiation observed from thermoluminescence measurements, this material could be used as a new radiation detector.

109. Modeling of a photostimulable complex induced by X-ray irradiation of BaFCl : Eu X-ray storage phosphor

Authors: Secu, M; Matei, L; Serban, T; Apostol, E; Silion, C

Published: MAR 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 63, DOI:

Photostimulated luminescence (PSL) and diffuse reflectance measurements on X-ray irradiated BaFCl:Eu2+ microcrystalline powders were performed. We have investigated the nature of radiation induced defects involved in the photostimulation effect and its recombination mechanism. The experimental data sustain that X-ray irradiation gives rise to (F(Cl-) - O-F(-) - EU2+) photostimulable complexes and we have proposed several geometrical configurations of these complexes. (Received January 27, 2000; accepted February 16, 2000).

110. Preparation and optical properties of BaFCl : Eu2+ X-ray storage phosphor

Authors: Secu, M; Matei, L; Serban, T; Apostol, E; Aldica, G; Silion, C

Published: NOV 2000, OPTICAL MATERIALS, 15, 122, DOI: 10.1016/S0925-3467(00)00022-7

A new method for the preparation of BaFCl:Eu2+ has been developed. A coprecipitation chemical reaction between BaCl2 and NaF acidified aqueous solution has been used. Doping with Eu2+ was carried out by adding EuF3 during preparation time. A thermal treatment in vacuum similar to those used in the sintering process of supraconductive ceramics was used in order to accomplish the chemical reaction and to improve the homogeneity of europium ion distribution. Finally, a fine powder consisting of microcrystalline, 4-5 mu m grains was obtained. The product has been checked by X-ray diffractometry and characterised by optical methods. Photoluminescence (PL) measurements attest the europium impurification and oxygen contamination during preparation, which has a great importance for the photostimulability properties of the compound. Photostimulated luminescence (PSL) of the X-irradiated samples shows good performance as X-ray storage phosphor. (C) 2000 Elsevier Science B.V. All rights reserved.

111. Growth of metal structures in quartz crystals by electrodiffusion

Authors: Iliescu, B; Enculescu, I; Vasiliu, F; Secu, M

Published: MAR 1999, JOURNAL OF CRYSTAL GROWTH, 198, 510, DOI: 10.1016/S0022-0248(98)01039-2

Metal structures in quartz crystals were obtained by electrodiffusion in air with different metal sources at the anode. The dynamics and the properties of these structures, for different conditions of temperature, applied electric field and anode material were studied. Current-voltage characteristics, Arrhenius plot, optical microscopy, electronic microscopy and thermoluminescence glow curve were employed. (C) 1999 Elsevier Science B.V. All rights reserved.

112. Impurity-V-K centres' associations in KI crystals containing lighter halogens

Authors: Matei, L; Serban, T; Secu, M; Beica, T

Published: JUN 1997, JOURNAL OF LUMINESCENCE, 72-4, 692, DOI: 10.1016/S0022-2313(97)00039-2

An attempt has been made to put in evidence the modified V-K centres in low temperature UV irradiated Kl:Cl crystals. The interpretation of the results seems to favour a new type of V-K centre localised near an impurity halogen instead of a heteronuclear V-K centre.

113. INFLUENCE OF INITIAL CONDITIONS ON THE OPTICAL AND ELECTRICAL CHARACTERIZATION OF SILLENITE-TYPE CRYSTALS

Authors: HAMRI, A; SECU, M; TOPA, V; BRIAT, B

Published: JAN 1995, OPTICAL MATERIALS, 4, 201, DOI: 10.1016/0925-3467(94)00101-4

We compare a set of results concerning our measurement of the optical absorption (OA), photoconductivity (PC), thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) on the same crystal. We also investigate the effects of various preillumination treatments at room or liquid nitrogen temperature. We conclude that illumination with infrared light (rather than solely red) plays an important role which has been underestimated in the past.

114. Structural and Optical Investigations of Ce3+/Mn2+-Doped LaPO4 Phosphors

Authors: Aitmellal, O; Oufni, L; Messous, MY; Neatu, F; Florea, M; Neatu, S; Rostas, AM; Secu, M

Published: , JOURNAL OF ELECTRONIC MATERIALS, DOI: 10.1007/s11664-020-08678-7

115. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

Authors: AitMellal, O; Oufni, L; Messous, MY; Tahri, M; Neatu, S; Florea, M; Neatu, F; Secu, M

Published: , JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, DOI:

Dr. Mihail SECU

Scientific Researcher I

Projects

1. New Chalcogenide Nanostructures for Information and Communication Technology

2. Infrared-Pumped upconversion glass-ceramic nanorods for photonics applications

Publications

1. Structural and magneto-optical investigations of citrate sol-gel derived barium hexaferrite nanocrystalline powder

2. Optical characterization of Sm3+doped phosphate glasses for potential orange laser applications

3. Up-Conversion Luminescence and Magnetic Properties of Multifunctional Er3+/Yb3+-Doped SiO2-GdF3/LiGdF4 Glass Ceramics

4. Defect structures and (ferro)magnetism in Zn1-xFexO nanoparticles with the iron concentration level in the dilute regime (x = 0.001 - 0.01) prepared from acetate precursors

5. Multi-analytical characterization of the white inlaid decoration on the prehistoric pottery from southern Romania

6. Up-Conversion Luminescence and Magnetic Properties of Multifunctional Er3+/Yb3+-Doped SiO2-GdF3/LiGdF4 Glass Ceramics

7. Columnar Liquid Crystals of Copper(I) Complexes with Ionic Conductivity and Solid State Emission

8. 'Put variety in White': Multi-analytical investigation of the white pigments inlaid on Early Chalcolithic pottery from Southern Romania

9. ZnS stacking order influence on the formation of Zn-poor and Zn-rich Cu2ZnSnS4 phase

10. Luminescent heteroleptic copper(I) complexes with phosphine and N-benzoyl thiourea ligands: Synthesis, structure and emission properties

11. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

12. ZnS stacking order influence on the formation of Zn-poor and Zn-rich Cu2ZnSnS4 phase

13. Bulk and surface characteristics of co-electrodeposited Cu2FeSnS4 thin films sulfurized at different annealing temperatures

14. Structural and transport properties of Cu2CoSnS4 films prepared by spray pyrolysis

15. Structural and Optical Characterization of Silica Nanospheres Embedded with Monodisperse CeO2-Eu3+ Nanocrystals

16. Structural properties and near-infrared light from Ce3+/Nd3+-co-doped LaPO4 nanophosphors for solar cell applications

17. Temperature-induced phase transition and tunable luminescence properties of Ce3+-Mn2+-Zr4+ tri-doped LaPO4 phosphor

18. Potassium-containing triple-cation mixed-halide perovskite materials: Toward efficient and stable solar cells

19. Optical Properties of Transparent Rare-Earth Doped Sol-Gel Derived Nano-Glass Ceramics

20. Comparative investigation of structural, EPR, optical and photoluminescence properties of nanostructured LaPO4:Ce/RE/Me and LaPO4:Yb/Er phosphors prepared by co-precipitation method

21. EPR spectroscopy and structural investigations, of Eu2+-doped chloroborate glass-ceramic

22. Structural, electrical properties and photoluminescence analyses of the terbium doped barium titanate

23. Structural and Optical Investigations of Ce3+/Mn2+-Doped LaPO4 Phosphors

24. Thermal and Emission Properties of a Series of Lanthanides Complexes with N-Biphenyl-Alkylated-4-Pyridone Ligands: Crystal Structure of a Terbium Complex with N-Benzyl-4-Pyridone

25. Towards phase pure kesterite Cu2ZnSnS4 absorber layers growth via single step free sulfurization electrodeposition under a fix applied potential on Mo substrate

26. Exploring the effect of aliovalent substitution of Pb2+ by Eu3+ on structural, morphological and optical properties of CH3NH3PbI3 perovskite films

27. Optical spectroscopy and dielectric properties of phosphate-tellurite glasses

28. Temperature dependence and defect related structure, photoluminescence, (ferro)magnetism and ammonia sensitivity of un-doped nanocrystalline ZnO

29. Towards a Correlation between Structural, Magnetic, and Luminescence Properties of CeF3:Tb(3+)Nanocrystals

30. Third-order optical nonlinearity properties of CdCl2-modifed Ge-Sb-S chalcogenide glasses

31. Sol-Gel Processing of Bismuth Germanate Thin-Films

32. Processing and Optical Properties of Eu-Doped Chloroborate Glass-Ceramic

33. Raman investigations on gamma irradiated iPP-VGCNF nanocomposites: The polymer's tale

34. Effect of dilute doping and non-equilibrium synthesis on the structural, luminescent and magnetic properties of nanocrystalline Zn1-xNixO (x=0.0025-0.03)

35. Structural and optical properties of ZnO thin films grown by rapid atmospheric mist chemical vapor technique

36. The influence of the nanocrystals size and surface on the Yb/Er doped LaF3 luminescence properties

37. Effect of high gamma radiations on physical properties of In2S3 thin films grown by chemical bath deposition for buffer layer applications

38. Growth of SrTiO3 Single Crystals with a Diameter of about 30 mm by the Verneuil Method

39. Photoluminescence and thermoluminescence properties of the Sr3Al2O6:Eu3+/Eu2+,Tb3+ persistent phosphor

40. Gd3+ co-doping influence on the morphological, up-conversion luminescence and magnetic properties of LiYF4:Yb3+/Er3+ nanocrystals

41. Physical properties investigation of samarium doped calcium sulfate thin films under high gamma irradiations for space photovoltaic and dosimetric applications

42. Hybrid layered double hydroxides-curcumin thin films deposited via Matrix Assisted Pulsed Laser Evaporation-MAPLE with photoluminescence properties

43. Charge carrier traps in tris-(8-hydroxyquinoline) aluminum

44. (Fe, Nd) codoped ZnO micro- and nanostructures with multifunctional characteristics like photocatalytic activity, optical and ferromagnetic properties

45. Enhanced near-infrared response of a silicon solar cell by using an up-conversion phosphor film of Yb/Er - co-doped CeO2

46. Defect states and room temperature ferromagnetism in cerium oxide nanopowders prepared by decomposition of Ce-propionate

47. Investigation of sol-gel derived BaCl2:Eu2+ luminescent nanophosphor and the corresponding PVP@BaCl2:Eu2+ polymer nanocomposite

48. Wide-Range Columnar and Lamellar Photoluminescent Liquid-Crystalline Lanthanide Complexes with Mesogenic 4-Pyridone Derivatives

49. Synthesis and up-conversion luminescence properties of BaFBr-Er3+ @SiO2 core/shell heterostructures

50. Laser processing of Yb3+/Er3+ co-doped LiYF4 thin films with up-conversion properties

51. Crystallization kinetics mechanism investigation of sol-gel-derived NaYF4:(Yb,Er) up-converting phosphors

52. Structural and optical properties of un-doped and doped Sr3Al2O6 obtained through the tartarate precursor method

53. Tb3+-doped alkaline-earth aluminates: Synthesis, characterization and optical properties

54. UPCONVERSION AVALANCHE IN THE Yb3+/Ho3+ DOPED SiO2-LiYF4 NANO-GLASS CERAMIC

55. Non-isothermal crystallization kinetics growth of LiYF4(Yb,Er) nanoparticles

56. STRUCTURAL AND MORPHOLOGICAL EVOLUTIONS INDUCED BY THE ANNEALING OF CaF2 NANOCRYSTALLINE POWDERS BY USING Eu3+ ION PROBE LUMINESCENCE

57. Up-conversion luminescence of BaCl2:Er3+ nanocrystals embedded in oxychloride nano-glass ceramic

58. Dielectric and photoluminescence properties of Nd and Ga codoped-BaTiO3, prepared by sol-gel method

59. Crystallization and luminescence properties of a new Eu3+-doped LaOCl nano-glass-ceramic

60. Structural and optical properties of fluorescent BaFBr-Eu2+@SiO2 core/shell phosphor heterostructure

61. Up-conversion luminescence of Er3+/Yb3+ co-doped LiYF4 nanocrystals in sol-gel derived oxyfluoride glass-ceramics

62. Ceramics and amorphous thin films based on gallium sulphide doped by rare-earth sulphides

63. A new class of thermotropic lanthanidomesogens: Eu(III) nitrate complexes with mesogenic 4-pyridone ligands

64. Luminescence properties of Eu3+-doped SiO2-LiYF4 glass-ceramic microrods

65. OPTICAL CHARACTERISTICS OF SULPHUR-PASSIVATED n-GaAs (100) SURFACE

66. Thermally activated conversion of a silicate gel to an oxyfluoride glass ceramic: Optical study using Eu3+ probe ion

67. Photoluminescence of Eu-doped LiYF4 thin films grown by pulsed laser deposition and matrix-assisted pulsed laser evaporation

68. BaFBr:Eu2+ nanophosphor-SiO2 hybrid entrapped in Anodise Alumina membrane pores array

69. Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol-gel derived glass-ceramics

70. Eu3+ probe ion for rare-earth dopant site structure in sol-gel derived LiYF4 oxyfluoride glass-ceramic

71. Structural and optical characterization of sol-gel derived Tm-doped BaTiO3 nanopowders and ceramics

72. Luminescent Eosin Y-SiO2 hybrid nano and microrods prepared by sol-gel template method

73. Broadband near-infrared emission of chromium-doped sulfide glass-ceramics containing Ga2S3 nanocrystals

74. Spectroscopic ellipsometry investigations of Eu-doped oxy-fluoride glass and glass-ceramics

4. Defect structures and (ferro)magnetism in Zn_1-xFe_xO nanoparticles with the iron concentration level in the dilute regime (x = 0.001 - 0.01) prepared from acetate precursors

6. Up-Conversion Luminescence and Magnetic Properties of Multifunctional Er³⁺/Yb³⁺-Doped SiO₂-GdF₃/LiGdF₄ Glass Ceramics