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Ce3+ single-doped, Ce3+-Tb3+, Ce3+-Eu3+ and Tb3+-Eu3+ co-doped, and Ce3+-Tb3+-Eu3+ triply-doped nano-glass-ceramics (nGCs), containing cubic 10 nm size BaGdF5 nanocrystals, were synthesized by thermal treatment of precursor sol-gel glasses. The structural characterization was carried out by X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy, while the spectroscopic study was performed by excitation and emission spectra, along with lifetime measurements. In the Ce3+ single-doped, and Ce3+-Tb3+, Ce3+-Eu3+ co-doped nGCs, the emission spectra by exciting in the maximum of the UV excitation band of Ce3+ ions (∼ 300 nm) show a UV-violet broad band corresponding to the emission of Ce3+ ions. Additionally, Gd3+ UV, Tb3+ green and Eu3+ red emission peaks are also observed respectively, revealing an energy transfer mechanism from the Ce3+ to these ions. For the Tb3+-Eu3+ co-doped nGCs, efficient energy transfer from Gd3+ host to Tb3+ and Eu3+, and from Tb3+ to Eu3+ ions were also evidenced. Moreover, taking advantage of the Eu3+ properties as structural probe ion, it was found that an important fraction of dopant ions were incorporated to the BaGdF5 nanocrystalline environments. In the Ce3+-Tb3+-Eu3+ triply-doped nGCs, intense and simultaneous UV-violet, green and red emissions were observed under Ce3+ excitation. Moreover, it was shown that the emitted colour can be tuned by varying the Eu3+ doping level and/or the excitation wavelength. After the analysis of nGCs luminescent features, the energy transfer and metal-metal charge transfer mechanisms, responsible of these emissions, are proposed. Results suggest these materials have potential applications as efficient UV to VIS converters.

Sol-gel derived nano-glass-ceramics with composition 95SiO2-5BaY0.78-xGdxYb0.2Tm0.02F5, where x = 0, 0.2, 0.4, 0.6 and 0.78, were obtained after adequate thermal treatments of precursor glasses. X-ray diffraction, transmission electron microscope and energy dispersive X-ray spectroscopy, confirmed the precipitation and distribution of nanocrystals, with sizes around 10–13 nm, in an amorphous silica network. Under excitation at 980 nm, along with blue, red and NIR characteristic up-conversion emissions of Tm3+ ions, intense UV up-conversion emissions coming from 1I6 and 1D2 levels of Tm3+ ions were also observed in all the studied samples. Additionally, when substituting Y3+ by Gd3+ ions, UV up-conversion emissions coming from Gd3+ ions, along with a reduction of the UV up-conversion emissions of Tm3+ ions, were observed, involving an energy transfer process from Yb3+/Tm3+ systems to Gd3+ ions. UC mechanisms and energy transfer processes were confirmed by transient emission and pump power measurements. Moreover, power dependence analysis revealed saturation effects in all up-conversion emissions, even at low pump power. Results suggest that these highly efficient up-conversion nano-glass-ceramics present potential applications as UV-blue solid state laser materials and solid state lighting.

Spectral conversion of light is an emerging route for enhancing the efficiency of solar energy harvesting schemes. We present high intense UV-blue up-conversion emissions in Sr2YbF7:Tm3+@Sr2YF7 core-shell nanocrystals successfully obtained by solvothermal method. Photocatalytic degradation of organic dye methylene blue (MB), as a pollutant model, has been successfully achieved under NIR radiation, by means of upconversion luminescence coming from core-shell nanocrystals. Thus, we probe a solely upconversion photonic effect to degrade the organic pollutant, converting the incident NIR radiation before its interaction with titanium dioxide photocatalyst.

RE-doped (Yb3+, Er3+ and Tm3+) nano-glass-ceramics (nGCs) comprising BaGdF5 nanocrystals have been developed under thermal treatment of precursor sol-gel glasses. Structural analysis by means of X-ray diffraction patterns, high-resolution transmission electron microscopy images, and energy dispersive X-ray spectroscopy measurements, confirmed the precipitation and distribution of cubic BaGdF5 nanocrystals (around 10 nm in size) in the silica glass matrix. Under near-IR excitation at 980 nm, up-conversion (UC) emissions have been studied as a function of selected dopants, the doping level and the pump power. In addition to the characteristic NIR, vis and UV UC emissions of Er3+ and Tm3+ dopant ions, the studied nGCs present intense UV UC emissions of the host fluoride nanocrystal, Gd3+. It was also observed that the relative Yb3+ content tended to improve the UC emission intensities. Corresponding UC mechanisms and energy transfer processes were analysed in terms of their energy level diagrams and confirmed by transient emission and pump power measurements. Moreover, power dependence analysis revealed that these emissions present saturation effects with the increase of Yb3+ content, even at low pump power. Results suggest that these high-efficiency UC nGCs have potential applications in UV solid state laser materials, solid state lighting and photovoltaics.

RE3+ (Ce3+, Gd3+, Tb3+, Eu3+)-doped LaF3 silica nano-glass-ceramics (nGCs) were obtained by thermal treatment of precursor sol-gel glasses. The precipitation and distribution of spherical LaF3 nanocrystals (NCs) with sizes around 8.5 nm in the silica matrix were confirmed by X-ray diffraction patterns, transmission electron microscope images and energy dispersive X-ray spectroscopy measurements. Very efficient energy transfer mechanisms from Ce3+ to Tb3+ or Eu3+ ions, mediated by Gd3+ ions through the Ce3+→(Gd3+)n→(Tb3+/Eu3+) scheme, were observed in Ce3+-Gd3+-Tb3+ and Ce3+-Gd3+-Eu3+ co-doped nGCs, yielding intense green and red emissions, respectively. Moreover, simultaneous and tuneable Ce3+ sensitized Tb3+ and Eu3+ emissions were obtained in Ce3+-Gd3+-Tb3+-Eu3+ co-doped nGCs by varying the content of Tb3+ and Eu3+ ions, and the corresponding energy transfer mechanisms were analysed. Results suggest these nGCs as potential UV to VIS converters in the field of lighting and display.

The use of hydrogen to feed Fuel Cells emerges as an attractive renewable solution to meet the terawatt carbon-neutral energy demand challenge. Rare-earth doped nano-glass-ceramics, presenting outstanding high intense UV-blue up-conversion processes, appear as an interesting approach to assist long wavelength light harvesting of solar irradiation, extending the photo-catalytic response to the red and near-infrared region to overcome the drawback of large band-gap materials used in the splitting of water into oxygen and hydrogen in a photo-electrochemical cell.

Transparent nano-glass-ceramics containing KYF4 nanocrystals were successfully obtained by the sol–gel method, doped with Eu3+ and co-doped with Yb3+ and Tm3+ ions. Precipitation of cubic KYF4 nanocrystals was confirmed by X-ray diffraction and high-resolution transmission electron microscope images. Excitation and emission spectra let us to discern between ions into KYF4 nanocrystals and those remaining in a glassy environment, supplemented with time-resolved photoluminescence decays, that also clearly reveal differences between local environments. Unusual high-energy up-conversion emissions in the UV range were obtained in Yb3+–Tm3+ co-doped samples, and involved mechanisms were discussed. The intensity of these high-energy emissions was analyzed as a function of Yb3+ concentration, heat treatment temperature of precursor sol–gel glasses and pump power, determining the optimum values for potential optical applications as highly efficient UV up-conversion materials in UV solid-state lasers.

Grupo de Física de Materiales, Universidad La Laguna RE-doped (Eu3+, Sm3+, Dy3+ or Tb3+) nano-glass-ceramics containing BaGdF5 nanocrystals, have been successfully obtained under thermal treatment of precursor sol-gel glasses. X-ray diffraction, high-resolution transmission electron microscope images and energy dispersive X-ray spectroscopy measurements confirmed the precipitation and distribution of cubic BaGdF5 nanocrystals (around 10 nm in size) in the glass matrix. The luminescent properties, via excitation and emission spectra along with decay curves, were studied. In order to complete the structural analysis, the luminescent features of Eu3+ and Sm3+ ions were also used as sensitive probes, confirming the distribution of a significant fraction of dopant ions into the fluoride nanocrystalline environment. Intense visible emissions were obtained through efficient energy transfer from Gd3+ to RE ions, which leads to consider these materials as potential emitting phosphors for colour converted UV LEDs. Ministerio de Economía y Competitividad (MAT2014-57551-R (LUNALEDs)

Transparent nano-glass–ceramics comprising Yb 3+ , Er 3+ and Tm 3+ co-doped KYF 4 nanocrystals have been developed from sol–gel method. A structural analysis by means of X-ray diffraction confirmed the precipitation of cubic KYF 4 nanocrystals into a silica matrix. Visible luminescence has been analyzed as function of treatment temperature of precursor sol–gel glasses. Highly efficient up-conversion emissions have been obtained under 980 nm excitation and studied by varying the doping level, processing temperature and pump power. Color tuneability has been quantified in terms of CIE diagram and in particular, a white-balanced overall emission has been achieved for a certain doping level and thermal treatment.

95SiO 2 –5LaF 3 sol–gel derived nano-glass–ceramics single doped with Ce 3+ or Tb 3+ and co-doped with Ce 3+ –Tb 3+ were synthesized by thermal treatment of precursor glasses. Precipitation of LaF 3 nanocrystals during ceramming process was confirmed by X-ray diffraction with mean size ranging from 12 to 15 nm. An exhaustive spectroscopic analysis has been carried out. As a result, it was found that the green emission of Tb 3+ ions was greatly enhanced through down shifting process, due to efficient energy transfer from Ce 3+ to Tb 3+ ions in the glass–ceramics, which is favored by the reduction of the interionic distances when the dopant ions are partitioned into LaF 3 nanocrystals. These results suggest the use of these materials to improve the efficiency of solar cells.

95SiO2–5LaF3 sol-gel derived nano-glass-ceramics single doped with Eu3+ or Sm3+ and codoped with both of them were successfully obtained. XRD measurements confirm the precipitation of LaF3 nanocrystals after the ceramming process, with mean size ranging from 10 to 20 nm which increases with the thermal treatment temperature. The incorporation of rare-earth ions into precipitated LaF3 nanocrystals was confirmed from luminescence spectra. Intense yellow-red emissions were detected under UV and blue light excitation in single and codoped samples. The effect of codoping with Eu3+ and Sm3+ ions and the energy transfer mechanism between them have been analyzed in order to increase the yellow-red emissions.

We report a novel class of nanostructured glass-ceramics comprising two co-existing rare-earth doped nanocrystalline phases, SnO2 semiconductor nanocrystal (quantum dot), and LaF3, presenting sizes at around 4.6 and 9.8 nm, respectively, embedded into a silica glass matrix for an efficient simultaneous UV and IR to visible photon conversion. On one hand, the wide and strong UV absorption by SnO2 quantum dot and subsequent efficient energy transfer to Eu3+ and, on the other hand, the also very efficient IR to visible up-conversion with the pair Yb3+–Er3+ partitioned into low phonon LaF3 nanocrystalline environment, yield to visible emissions with application in improving the spectral response of photovoltaic solar cells. We report a novel class of nanostructured glass-ceramics comprising two co-existing rare-earth doped nanocrystalline phases, SnO2 semiconductor nanocrystal (quantum dot) and LaF3, presenting sizes at around 4.6 and 9.8 nm, respectively, embedded into a silica glass matrix for an efficient simultaneous UV and IR to visible photon conversion. On one hand, the wide and strong UV absorption by SnO2 quantum dot and subsequent efficient energy transfer to Eu3+ and, on the other hand, the also very efficient IR to visible up-conversion with the pair Yb3+–Er3+ partitioned into low phonon LaF3 nanocrystalline environment, yield to visible emissions with application in improving the spectral response of photovoltaic solar cells.

Up-conversion emission processes have been studied in a transparent oxyfluoride nano-structured glass–ceramic co-doped with Yb 3+ and Tm 3+ ions. The decay and rise times, and pump power dependence of the different emission peaks have been analysed. The aim was to achieve a complete picture of the dynamics in this multiphoton excitation system. A method based on balance equations for the involved energy levels has been proposed in order to evaluate the up-conversion and decay rates from these levels. The results are analysed from the point of view of potential application to photovoltaic cells.

Sol–gel derived glass–ceramics containing CeF3 nanocrystals have been developed for the first time, to the best of our knowledge, by adequate heat treatments of precursor bulk glasses with composition 95SiO2–5CeF3 doped with 0.1 Eu3+ or 0.1 Sm3+ and co-doped with 0.3 Yb3+ and 0.1 Er3+ ions (in mol%). X-Ray Diffraction and High Resolution Transmission Electron Microscopy confirm the precipitation of CeF3 nanocrystals. Moreover, this structural analysis is completed using Eu3+ and Sm3+ as probe ions of the different local environments for rare-earth ions in the nano-structured glass–ceramics. Luminescence measurements led us to discern the final environments for the ions, revealing the partition of a large fraction of these ions into like-crystalline environment of the precipitated CeF3 nanocrystals. Near infrared emission at 1.5 μm was observed after excitation at 980 nm in Yb3+–Er3+ co-doped samples for potential applications in telecommunications.

Highly transparent sol–gel derived nano-glass-ceramics containing YF3 nanocrystals doped with Dy3+ or Tb3+ and co-doped with Dy3+–Tb3+ have been successfully developed. A structural analysis and luminescence spectra features confirmed the incorporation of rare-earth ions into precipitated YF3 nanocrystals, where efficient energy transfer from Dy3+ to Tb3+ takes place. Higher efficiency is obtained with increasing Tb3+ concentration, and no quenching effects are observed. Observed luminescence features leads the way to enhance solar cell spectral response by down-shifting of the incident solar spectrum, with an extended absorption range from 300 to 500 nm, yielding to a predominant green emission by co-doping with Dy3+ and Tb3+ ions.

YF 3 nanocrystals triply-doped with Yb 3+ , Ho 3+ and Tm 3+ ions embedded in amorphous silica matrix have been successfully obtained by heat treatment of precursor sol–gel glasses for the first time to our knowledge and confirmed by X-ray diffraction and luminescence measurements. Simultaneous UV and visible efficient up-conversion emissions, with well-resolved Stark structure, under 980 nm infrared pump are observed, indicating the effective partition of rare-earth ions into a crystalline-like environment of the YF 3 nanocrystals. Corresponding energy transfer mechanisms have been analyzed and overall colour emission has been quantified in terms of standard chromaticity diagram. By an adequate doping level and heat treatment temperature of precursor sol–gel glasses, a bright white colour has been accomplished, close to the standard equal energy white light illumination point, with potential applications in photo-electronic devices and information processing.

Sol-gel glasses with composition (100−x)SiO2–xSnO2 doped with 0.4 mol% of Sm3+, with x ranging from 1 to 10, have been successfully synthesized. Transparent doped nano-glass-ceramics were prepared by thermal treatment of the precursor glasses at 900°C during 4 hours, leading to nanocomposites comprising SnO2 nanocrystals embedded into an amorphous SiO2 phase. A structural analysis in terms of X-ray Diffraction and High Resolution Transmission Electron Microscopy confirms the precipitation of SnO2 nanocrystals within the glassy matrix. The mean radius of the obtained SnO2 nanocrystals, ranging from 2.1 to 4.7 nm calculated by the Scherrer and Brus equations, similar to the Bohr’s exciton radius, constitutes a wide band-gap semiconductor quantum-dot system. Energy transfer from SnO2 nanocrystal host to Sm3+ ions is confirmed by luminescence spectra and analyzed as a function of SnO2 concentration, showing an evolution that could be ascribed to selective excitation of nanocrystal sets with predetermined size. Besides, a study of the luminescence as a function of temperature helps to clarify the involved energy transfer mechanisms.

NaYF 4 is an excellent host material for rare-earth ions presenting very high efficiencies in up-conversion processes. Thus, nano-glass–ceramics containing NaYF 4 nano-crystals emerge as promising candidates for general lighting appliances and integrated optical devices. Here we report highly transparent sol–gel derived nano-glass–ceramics comprising Yb 3+ –Ho 3+ and Yb 3+ –Ho 3+ –Tm 3+ co-doped NaYF 4 nano-crystals. A structural analysis by means of X-ray diffraction measurements confirmed the formation of NaYF 4 nano-crystals during thermal treatment. Luminescence features have been related to the crystallinity degree of the samples. Violet, blue, green and red up-conversion emissions were obtained under infrared excitation at 980 nm and corresponding mechanisms involved have been analysed. Additionally, the total visible up-conversion emission has been quantified in terms of the standard chromaticity coordinates. In particular, an overall colour emission, very close to the standard equal energy white-light illumination point of the chromaticity diagram, was obtained in the Yb 3+ –Ho 3+ –Tm 3+ triply-doped samples.

The up-conversion of near infrared photons into the visible range is of major interest for lighting appliances and photonic integrated devices. Here we report up-conversion luminescence in transparent nano-glass-ceramics with composition of 95SiO2–5LaF3 co-doped with 0.1 mol% of Pr3+ and 0.3 mol% of Yb3+, which were synthesized by different thermal treatments of precursor sol–gel glasses. Precipitation of LaF3 nanocrystals during ceramming process was confirmed by means of X-ray diffraction and the partition of rare-earth ions into them has been analyzed. The effect of increasing thermal treatment temperature and adding Yb3+ ions in the degree of crystallinity of the samples has been discussed. Efficient up-conversion emissions were observed under infrared excitation of the Yb3+ ions in the co-doped Yb3+–Pr3+ samples, while it was hardly detected in single Pr3+-doped samples.

A spectroscopic study has been carried out in transparent nano-glass–ceramics containing Eu3+-doped NaYF4 nanocrystals, successfully developed for the first time by thermal treatment of precursor sol–gel derived glasses. X-ray diffraction and high-resolution transmission electron microscopy measurements confirmed the precipitation of NaYF4 nanocrystals during heat treatment and their corresponding sizes have been calculated by using Scherrer’s equation and from electron microscopy measurements. Changes in the luminescence spectra with the temperature of the heat treatment have been analyzed and correlated with the crystallinity degree of the samples. In addition, emission spectra have been obtained for selected excitation wavelengths, observing large excitation selectivity among the fraction of the rare-earth ions located in near crystalline environments and those remaining in the silica glassy phase.

White light can be obtained by up-conversion processes in a cheap and efficient way, being of great interest for lighting, displays, and photonic integrated devices. Here we report a generation of white light under 980 nm laser infrared excitation in Y(b3+)-Er(3+)-Tm3+ triply doped sol-gel derived nano-glass-ceramics containing NaYF4 nanocrystals where co-dopants ions are partitioned. Efficient simultaneous blue, green, and red up-conversion emissions give rise to a balanced white overall emission and corresponding up-conversion mechanisms have been analyzed. Changes in pump power intensity or heat treatment temperature widely vary the ratio of emission bands, resulting in a colour tuneability which has been analyzed and quantified in terms of CIE standard chromaticity diagram.

Transparent oxyfluoride nano-glass–ceramics 90(SiO2)10(PbF2) co-doped with 0.3 Yb3+ and 0.1 Er3+ (mol%) have been prepared by thermal treatment of precursor sol–gel glasses. X-ray diffraction and high resolution transmission electron microscopy analysis pointed out a precipitation of cubic β-PbF2 nanocrystals of certain diameter in nano-glass–ceramics varying from 10 to 20 nm depending on heat treatment conditions. The incorporation of Yb3+ and Er3+ dopants in these nanocrystals has been confirmed by signatures of luminescence spectroscopy. Up-conversion luminescence pumped at 980 nm has been detected. Colour tuneability of up-conversion luminescence varying pump power has been analyzed in terms of standard chromaticity diagram. This tuneability opens applications for up-conversion phosphors and three-dimensional optical recording.

Transparent nano-glass-ceramics containing rare-earth-doped NaYF 4 nanocrystals (Eu 3+ single-doped and Yb 3+ ―Er 3+ co-doped) have been successfully developed for the first time by thermal treatment of precursor bulk glasses obtained by sol―gel technique. X-ray diffraction and high-resolution transmission electron microscopy measurements reveal the precipitation of NaYF 4 nanocrystals during the heat treatment and their corresponding sizes have been calculated. Luminescence measurements confirm the partition of rare-earth ions in NaYF 4 nanocrystals. In Eu 3+ -doped samples, changes in the local environment of these rare-earth ions have been investigated by site-selective emission and excitation spectra. In Yb 3+ ―Er 3+ co-doped samples, efficient visible up-conversion luminescence has been obtained under infrared excitation at 980 nm. The red to green up-conversion emission ratio can be varied as a function of temperature of heat treatment, resulting in colour tuneable up-conversion phosphors with applications in optical integrated devices. Colour tuneability has been analysed and quantified in terms of Comission Internationale d'Eclairage standard chromaticity diagram.

Transparent nano-glass-ceramics containing Yb 3+ –Er 3+ co-doped NaYF 4 nanocrystals have been successfully developed for the first time by thermal treatment of precursor bulk glasses obtained by sol–gel technique. Precipitation of NaYF 4 nanocrystals has been confirmed by X-ray diffraction measurements and their sizes have been determined by Scherrer's equation. Luminescence measurements confirm the partition of rare-earth ions in NaYF 4 nanocrystals. Visible up-conversion luminescence has been obtained under infrared excitation at 980 nm. Up-conversion mechanisms have been discussed and the ratio between red and green up-conversion emission bands can be varied as a function of temperature of heat treatment and pump power resulting in colour tuneable up-conversion phosphors with applications in optical integrated devices. Colour tuneability has been also analyzed and quantified in terms of CIE standard chromaticity diagram.

The present work is devoted to the preparation of transparent nano-glass-ceramics (nGC) with composition 30SiO 2 –15AlO 1.5 –29CdF 2 –22PbF 2 –1.5YF 3 co-doped with 1.5YbF 3 , 0.5ErF 3 and 0.5TmF 3 (in mol%). The co-dopant ions mostly dissolve in the fluoride PbF 2 -based nano-crystals precipitated during thermal treatment of precursor glasses. Visible up-conversion luminescence, obtained under 980 nm infrared excitation, have been discussed and compared in glasses and daughter nGCs. Specifically, the ratio between red, green and blue up-conversion emissions can be varied with nano-ceramming of the precursor glass resulting in large colour tuneability, which has been quantified in terms of CIE standard chromaticity diagram, with applications in optical integrated devices and tuneable up-conversion phosphors.

Transparent nano-glass–ceramics of composition 95SiO 2 –5SnO 2 mol%, single doped with 0.4 mol% of Tb 3+ and co-doped with 0.4 mol% of Eu 3+ and Tb 3+ have been obtained by thermal treatment of precursor sol–gel glasses. The segregated SnO 2 nanocrystals, precipitated in the insulator SiO 2 glass matrix during heat treatment, constitute a wide band gap quantum-dot system with mean radius of nanocrystals around 2.2–2.8 nm, comparable to the bulk exciton Bohr radius. A fraction of the rare earth ions is incorporated to the SnO 2 nanocrystals and can be excited by energy transfer from the semiconductor host. The efficiency of this energy transfer process is strongly dependent on the nanocrystal radius in the case of the Tb 3+ ions, whereas the dependence is not appreciable in the case of Eu 3+ ions. These results can be explained taking into account the variations of the overlap of the SnO 2 broad visible emission due to oxygen vacancies with the rare earth excitation spectra.

Nanostructured transparent glass-ceramics with composition of 95SiO2–5LaF3 co-doped with 0.3Yb3+, 0.1Ho3+ and 0.1Tm3+ (mol%) were synthesized by thermal treatment of precursor sol–gel derived glasses. X-ray diffraction and transmission electron microscopy analysis point out the precipitation of hexagonal LaF3 nanocrystals with diameter ranging from 11 to 20 nm in these nano-glass-ceramics. White light generation by means of efficient blue, green and red up-conversion luminescence under infrared excitation at 980 nm was observed and involved mechanisms were analyzed. Colour tuneability is achieved by varying the up-conversion emission ratios as a function of pump power.

The generation of a wide colour gamut, based on up-conversion of cheap near-infrared photons into the visible range, is of great importance for general lighting appliances and integrated optical devices. Here, we report for the first time on up-conversion luminescence under infrared excitation at 980 nm in Yb3+–Er3+–Tm3+ triply doped sol–gel derived SiO2–LaF3 based nano-glass–ceramics (SOL-YET), containing LaF3 nanocrystals with a size about 13 nm. Efficient simultaneous up-conversion emission of the three primary colours (blue, green and red) gives rise to a balanced white overall emission. The ratio between up-conversion emission bands can be varied by changing pump power intensity resulting in colour tuneable up-conversion phosphor.

Transparent Yb 3+ -Er 3+ -Tm 3+ co-doped nano-glass-ceramics have been prepared, 32(SiO2) 9(AlO1.5) 31.5(CdF2) 18.5(PbF2 )5 .5(ZnF 2): 3.5(Yb-Er-TmF3) mol%, where the co-dopants partition mostly to the fluoride PbF2-based nano-crystals. A comparative study of the up-conversion luminescence in nano-glass-ceramics and its precursor glass indicates that these materials can be used as blue/green/red tuneable up-conversion phosphor, in particular for white light generation. A ratio between blue, green and red emission bands of the Tm 3+ and Er 3+ can be widely varied with nano-ceramming of the precursor glass and with changing a pump power of luminescence. The change in the ratio between the blue, green and red emission bands is explained to be due to substantial lowering phonon energy and shortening of inter-dopant distances with nano-ceramming of the precursor glass and due to change in the ratio of 2- and 3-photon up-conversion processes with pump power.

The room temperature near-infrared emission at 1.06 μm corresponding to the 4 F 3/2 → 4 I 11/2 laser transition in Nd 3+ -doped SiO 2 –LaF 3 transparent glass–ceramics prepared by sol–gel method has been characterized in terms of the product of emission cross-section and fluorescence lifetime. Stimulated emission cross-sections have been calculated from experimental emission spectra using the Fuchtbauer–Ladenburg equation. The precipitation of LaF 3 nanocrystals after heat-treatment, confirmed by X-ray diffraction, induces a significant increment of the 1.06 μm near-infrared emission compared with the as made sample. An enhancement of emission cross-section of about 10% has been achieved with increasing temperature of heat-treatment of the nano-ceramming process from 800 to 1000 °C, which is related to the growth of LaF 3 nanocrystals.

Nanostructured transparent glass-ceramics with composition of 95SiO2–5LaF3 doped with 0.1 mol% of Tm3+ or co-doped with 0.1 mol% of Tm3+ and 0.3 mol% of Yb3+ were synthesized by thermal treatment of precursor sol–gel derived glasses. A comparative luminescence analysis in terms of different temperatures of thermal treatments has been carried out. Segregation of LaF3 nanocrystals in the matrix was con- firmed by X-ray diffraction. Blue and red efficient up-conversion emissions were observed under 980 nm excitation at room temperature. These results indicate partition of a significant fraction of co-dopants into the precipitated LaF3 nanocrystals. The mechanisms involved in the up-conversion emissions have been analyzed and can be ascribed to processes with two, three and four photons. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Undoped and Er3+-doped glass–ceramics of composition (100−x)SiO2–xSnO2, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er3+ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO2 semiconductor quantum-dots embedded in the insulator SiO2 glass are obtained. The mean radius of the SnO2 nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er3+ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er3+ ions by energy transfer from the SnO2 nanocrystal host is observed. The Er3+ ions located in the SnO2 nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er3+ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.

Transparent glass-ceramics with composition of 95SiO 2 -5LaF 3 doped with 0.1 mol% of Ho 3+ or co-doped with 0.1 mol% of Ho 3+ and 0.3 mol% of Yb 3+ were synthesized by thermal treatment of precursor sol-gel glasses. Segregation of LaF 3 nanocrystals in the matrix was confirmed from a structural analysis by means of X-ray diffraction. Blue, green and red efficient up-conversion emissions were observed under 980 nm excitation at room temperature. These results could be attributed to the partition of a fraction of Ho 3+ ions into the precipitated LaF 3 nanocrystals. Moreover, near infrared down-conversion at 1.2 μm is also observed. The mechanisms involved in the up-conversion emissions could be ascribed to a two- and three-photon process.

Glass–ceramics with composition of 89.9SiO 2 –10LaF 3 –0.1EuF 3 (mol%) were prepared by sol–gel method. LaF 3 nanocrystals, precipitated by heat-treatments, were identified by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Besides the well-known red emission of Eu 3+ ions, higher energy emissions coming from upper-lying levels 5 D 1 and 5 D 2 can be observed at room temperature due to the very low phonon energies of the LaF 3 matrix. Moreover, from site selective excitation and emission spectra in the temperature range from 13 to 300 K, it is concluded that the fraction of Eu 3+ ions partitioned into LaF 3 nanocrystals, substituting La 3+ ions, is comparable to the one staying in the SiO 2 glassy phase.

Polycrystalline samples of pure calcium tartrate [CaC 4 H 4 O 6 ·2H 2 O]·2H 2 O (labelled CaT) and CaT doped with divalent cations (Sr, Ba, Co, Zn and Cd) were studied using thermal analysis, impedance measurements and X-ray powder diffraction. Cell parameters were calculated for all compounds at six temperatures, in the temperature range 323 to 398 K, by whole-pattern profile matching. Then microstructural parameters were obtained by refining patterns using a pseudo-Voigt function. The ac conductivity of these samples in the frequency range 400 Hz–200 kHz between 338 K and 373 K has been measured. These results show that the conduction mechanisms, thermal stability and microstructure differ, depending on whether the doping agent is earth alkaline or a transition metal.

Nanostructured silica based ceramic samples of composition (100− x)SiO2–xSnO2, doped with 0.4 mol% of Eu3+ and with x from 1 to 10, have been developed after a thermal treatment of precursor sol–gel glasses. A structural analysis has been performed by x-ray diffraction and high resolution transmission electron microscopy. The mean radius of the obtained SnO2 nanocrystals, ranging from 2 to 5 nm, is comparable to the exciton Bohr radius, corresponding to wide bandgap semiconductor quantum dots in an insulator SiO2 glass. A spectroscopy study in terms of emission and excitation spectra has been carried out as a function of SnO2 concentration. Moreover, time-resolved fluorescence measurements have also been performed in order to discern the emission of ions in glassy and nanocrystalline environments. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations.

Trivalent rare-earth (RE = Eu, Sm) doped transparent nano-glass–ceramics comprising BaYF 5 nanocrystals were successfully obtained by appropriate heat-treatment of the corresponding precursor sol–gel glasses. Their structural and spectroscopic properties were investigated and compared with those for Eu 3+ -doped-BaYF 5 nanocrystals prepared by a solvothermal method. X-ray Diffraction, Transmission Electron Microscopy and Energy Dispersive X-ray Spectroscopy measurements confirmed the distribution of BaYF 5 nanocrystals in the glass matrix, presenting a cubic phase structure with space group Fm - 3m . In order to achieve a further structural characterization, the luminescence properties of the Eu 3+ and Sm 3+ dopants were also used as sensitive probes. The reduction in the emission intensities of hypersensitive transitions 5 D 0 → 7 F 2 and 4 G 5/2 → 6 H 9/2 for Eu 3+ and Sm 3+ ions respectively, along with time-resolved measurements, confirm the distribution of a significant fraction of RE ions into the fluoride nanocrystal environment. These results suggest that BaYF 5 nano-glass–ceramics doped with Eu 3+ or Sm 3+ can be considered as potential red-emitting phosphors for the development of white LEDs under near UV excitation.

The electrical conductivity of polycrystalline samples of calcium tartrate tetrahydrate ([CaC4H4O6 2H2O] 2H2O) in pure form and doped with barium and with strontium were studied in the temperature range (65

Polycristalline samples of cobalt, nickel and manganese tartrates were studied using impedance measurements and x-ray powder diffraction in the temperature range 213-300 K. The dependence of the real part of the dielectric permittivity for manganese tartrate showed a peak at about 250 K, which can be interpreted in favour of phase transition. To support this view we have report the XRD patterns a different temperatures around 250 K.

Time-resolved fluorescence line narrowing technique has been used to analyse the evolution of the 5 D 0 → 7 F 0 emission of Eu 3+ ions in sol–gel derived silica glasses. These samples were doped with 2.5 mol% of Eu 3+ and heat treated at different temperatures. Background fluorescence, coming from Eu 3+ ions excited by energy transfer processes, was observed in these experiments. Using a previously proposed equation model, a study of the multipole character of the interaction between Eu 3+ ions was performed. The annealing process carried out in the samples causes important changes in the efficiency of energy transfer process and in its multipole character, varying from S =6 (in a sample treated at 400°C) to S =10 (in a sample treated at 600°C), indicating that the environment of Eu 3+ ions has been modified.

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70

We report here on the dielectric properties (dielectric constant, ɛ′ and dielectric losses ɛ″), magnetic properties (μ′ and μ″), infrared spectroscopic, thermal studies (thermogravimetric analysis (TGA)), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC) and X-ray diffraction (XRD) for the polycrystalline powdered samples of manganese tartrate MnC4H4O6·2H2O obtained by diffusion of manganese ions through silica gel impregnated with tartaric acid. The aforementioned properties are used as a probe for the detection and study of the different transitions that have been found in this salt. Energy dispersive analysis of X-rays (EDAX) confirmed the presence of manganese. X-ray diffraction data giving 2gq, intensity andd-values are reported for the first time. Infrared wavelengths in the range 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis indicated that the material is thermally unstable. Dielectric and magnetic measurements were determined in the frequency range of 45 MHz to 2 GHz.

Glass ceramics of composition 95SiO2-5SnO2 doped with 0.4mol% Eu3+ have been prepared by thermal treatment of sol-gel glasses. The segregated SnO2 nanocrystals present a mean size comparable to the bulk exciton Bohr radius (about 2.4nm), corresponding to a wide band-gap quantum-dot system in an insulator SiO2 glass. A fraction of the Eu3+ ions is incorporated to the SnO2 nanocrystals in the process. In these strong confinement conditions, the energy gap presents a high dependence on the nanocrystal size. Taking advantage of this effect, it has been possible to excite selectively the Eu3+ ions located in the SnO2 nanocrystals, by energy transfer from the host, obtaining emission spectra that depend on the nanocrystal size. The Eu3+ ions environment in small nanocrystals (radius under 2nm) are very distorted, meanwhile they are like crystalline for nanocrystals with a radius of some nanometers.

Energy dispersive analysis by X-rays (EDAX), X-ray diffraction (XRD), infrared (IR), thermogravimetric (TG), differential thermogravimetric (DTG), differential scanning calorimetric (DSC) and electromagnetic studies carried out on crystalline materials obtained by diffusion of gadolinium and calcium ions through silica gel impregnated with tartaric acid and grown by means of slow cooling between 72°C and 40°C are reported. EDAX confirmed the presence of gadolinium. X-ray diffraction data did not show any appreciable differences between the structures of doped and undoped calcium tartrate. IR in the range of 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis (TG, DTG and DSC) indicated that the material is thermally unstable. Dielectric and magnetic measurements were determined in the frequency range 45 MHz to 2 GHz.

89.5(SiO2)10(PbF2)0.5(REF3) silicate glasses have been prepared using room temperature sol-gel processing of Si(OCH2CH3)(4), Pb(CH3COO)(2)center dot 3H(2)O, RE(CH3COO)(3)center dot nH(2)O and trifluoroacetic acid as a fluorinating agent, where RE stands for rare-earth ions, such as Yb3+, Er3+, Ho3+, Tm3+, or combinations of those ions. On heat treatment of these glasses at about 300-400 A degrees C, the rare-earth doped spherical PbF2 nanocrystals precipitate within SiO2 glass matrix providing transparent nano-structured glass-ceramics, while the diameter of the nanocrystals can be set in the range from 5 to 25 nm by varying time and temperature of the heat treatment. The structural and photoluminescence studies confirm the incorporation of rare-earth ions into the PbF2 nanocrystals and white and tuneable colour up-conversion luminescence has been detected in case of Yb3+-Er3+-Tm3+ and Yb3+-Ho3+-Tm3+ co-doped nanocrystals by varying dopant ratio and pump power. ispartof: Journal of sol-gel science and technology vol:53 issue:3 pages:509-514 status: published

Nano-structured glass-ceramics comprising In2O3 nanocrystals (quantum dots) in a silica glassy matrix have been synthesized for the first time to our knowledge, by thermal treatment of sol-gel precursor glasses with different concentrations of In2O3. Undoped and Eu(3+)-doped samples have been obtained and characterized. By means of X-ray Diffraction and Transmission Electron Microscopy analysis, the precipitation of In2O3 cubic crystalline nanoparticles was confirmed. The mean radii of these nanocrystals, from 1 to 4 nm, are comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots. Under interband UV excitation of the nanocrystals, a broad visible emission is observed in the undoped samples. Meanwhile, the Eu(3+)-doped samples only show emission from these ions which are efficiently excited by energy transfer from the In2O3, nanocrystals. Selective excitation of the Eu3+ ions allow us to discern those remaining in the silica glassy matrix from the ones located in the interface SiO2-In2O3.