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In order to improve the photocatalytic response of TiO2 to UV and visible light for hydrogen photoproduction, low cost M/TiO2 semiconductor catalysts were prepared by the impregnation method of five different first row transition metals (M = Fe, Co, Ni, Cu or Zn) on a commercial titania support. The maximum hydrogen production efficiency was achieved for the Cu/TiO2 photocatalyst, with ∼5000 and ∼220 μmol h−1 g−1 H2 production rates for UV and visible irradiation, respectively. Ni/TiO2 and Co/TiO2 also showed a significant photocatalytic activity when UV light was used. The best performing catalyst, Cu/TiO2, was characterized by TEM and XPS measurements. The data showed that Cu was highly dispersed over the TiO2 support and the copper species existed as both reduced Cu0/Cu+ and oxidized Cu2+ on TiO2. Besides, during the hydrogen production reaction, the reduced Cu was partially oxidized to Cu2+ by the transfer of photogenerated holes under UV or visible light irradiation. With UV and visible lamps, the H2 production rates were higher than those obtained with non-impregnated TiO2 by factors of 16 and 3, respectively. These results demonstrated that a Cu/TiO2 photocatalyst could be considered a promising low-cost alternative to the well-known Pt/TiO2 system for hydrogen production, making the Cu-based catalyst an ideal cost-effective candidate for this reaction The authors would like to thank Ministerio de Economía y Competitividad of Spain (CTQ2015-68951-C3-3-R and MAT2016-80933-R), Ministerio de Ciencia, Innovación y Universidades of Spain (Project RTI2018-099668-B-C22) and FEDER funds

TiO2 nano-particles based photocatalysts were modified with different Pt percentages photodeposited on the surface, optimizing the method in order to obtain the best performance for the production of hydrogen. For testing hydrogen production, a novel reaction setup equipped with an elliptical cylindrical reflector to focus the radiation on the reactor has been used, being possible to control the reaction temperature. The hydrogen production has been measured in continuous by means of mass spectrometry. The effects of the reaction temperature and of the sacrificial reagent (methanol, ethanol or isopropanol) on the photocatalytic activity of Pt-TiO2 were studied. The best results at 25 °C were achieved for 2 wt% Pt-TiO2 (2Pt-TiO2) using methanol as sacrificial reagent, with a large maximum production over 16.0 mmol g−1h−1, which decreases to the half of this value after a long period of 72 h. These H2 production rates are larger, by a factor close to 30, than the ones obtained for the commercial P25 TiO2 in the same experimental conditions. The effect of the reaction temperature was analyzed on 2Pt-TiO2/water and 2Pt-TiO2/water/MeOH systems in the temperature range 5–60 °C. The apparent activation energy was determined for both photocatalysts, obtaining values of 17.5 and 23.3 kJ mol−1, respectively. The hydrogen production increases continuously with temperature in this range, achieving at 60 °C a H2 production of 0.83 and 38.0 mmol g−1h−1 for the photocatalyst in water and water/MeOH, respectively, after 2 h of reaction time.

We present research on Ca3Sc2Si3O12 garnets doped with Ce3+ and Er3+ ions that were synthesized by the freeze-drying precursor method. The structural characterization was performed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) images of the calcined material were studied. Nanocrystals of Ca3Sc2Si3O12 were obtained by calcining the precursor at 1200 degrees C for 4 h with a mean size of about 100 nm. The luminescence features of Ce3+ and Er3+ doped samples indicated that these ions are effectively incorporated into the crystalline phase. In addition, codoping with Ce3+-Er3+ ions results in energy transfer processes from Ce3+ to Er3+ ions which led to the enhancement of near-infrared luminescence at 1.5 mu m. The efficiency of this non-radiative energy transfer process has been studied by means of excited state dynamics as a function of the doping concentration. From these results we have found that near-infrared Er3+ luminescence efficiency under VIS Ce3+ excitation is at maximal for 1Ce(3+)-1Er(3+) codoped Ca3Sc2Si3O12 garnets (in mol%). Luminescence quenching at higher Er3+ ion concentrations is observed.

The most extended Si based conventional photovoltaic cells show low efficiency in the UV region, however this low efficiency can be enhanced by the use of suitable down-shifters or down-converters which transform the wavelength of the incoming radiation into a wavelength for which the Si based cells have high efficiency. In this sense europium(III) luminescent benzoate and phenanthroline (and derivatives) complexes are good candidates for such purposes since they exhibit large absorption at wavelengths below 400 nm and significant emissions at the VIS range. In this work we report the synthesis the crystal structure and the spectroscopic properties of two new Eu3+ and Gd3+ complexes with the ligands 4,7-biphenyl-1,10-phenanthroline (bphen) and benzoate (bz), namely, [Eu2(bphen)2(bz)6] (1) and [Gd2(bphen)2(bz)6] (2). The X-ray single crystal study reveals a dinuclear molecular structure for the Eu3+ complex with two bridging benzoate ligands between the two equivalent Eu3+ ions. The Powder X-ray diffraction study shows that the Eu3+ and Gd3+ compounds are isostructural and allows the use of the Gd3+ compound for the characterization of the excited states of the complexes which were investigated to explain the sensitization process that lead to intense red emission of Eu3+ ions under UV excitation. The photoluminescence study of the lanthanide complexes at 15 K and 297 K revealed very efficient energy transfer processes from the antenna to the emitting Eu3+ ions and a high overall quantum yield. Moreover, the ligands provide rigidity, high stability and solubility to the complex in CH2Cl2, allowing the preparation of poly(methylmethacrylate)-doped films that can be used to cover photovoltaic devices. The analysis of external quantum yield and quantum efficiency (EQE) show its potential application to the solar cell technology with an enhancement EQE in the UV region (280-360 nm) that reaches a 4.5% at 300 nm.

Visible-to-UV/violet upconversion luminescence is found and studied in oxyfl uoride glass ceramics with Er 3+ dopant concentrations of 3.5, 6.0, or 12.0 mol%, covering a broad pump-power range. The glass ceramics with doping levels of 3.5 and 6.0 mol% show the strongest upconversion luminescence intensity when compared to both higher doping levels and the respective precursor glasses. A comprehensive analysis of the involved upconversion mechanisms is presented. These materials could be useful to enhance the effi ciency of the water-splitting systems by using UV/violet light upconverted from the green and red parts of the solar spectrum, which are an alternative to photovoltaic solar cells.

Lithium luminescent ZnO:Li nanocrystals have been synthesized. Atomic absorption and optical spectroscopy confirm the high doping level of Li up to 5 at% can be achieved and indicate that the dopants may form neutral Li–Li dimers. The 100 nm width and several milliseconds lifetime long luminescence of the proposed Li–Li dimers occurs around 770 nm and is excited by energy transfer from the ZnO host when pumped in the range between 250 and 420 nm. These parameters of Li-related luminescence have been explained in terms of the energy level diagram of Li–Li dimers, where the Li luminescence is excited by spin-allowed singlet–singlet transitions and occurs due to spin-forbidden triplet–singlet transitions of Li–Li dimers. The milliseconds lifetime of the emitting level of Li dimers and nanosecond lifetime of the feeding state of the ZnO point out a 3-level lasing system. This Li luminescence may be used for the broad spectral range down-conversion of solar spectrum for enhanced efficiency solar cells. The sites for the Li–Li dimers have been proposed, where Li atoms locate in octahedral voids of Zn and O sub-lattices.

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.

Er 3+ –Yb 3+ co-doped transparent glass-ceramics with varying Er/Yb content and ratio have been prepared. High quantum yields for up- and down-conversion luminescence by energy transfer from Yb 3+ to Er 3+ and from Er 3+ to Yb 3+ , respectively, have been detected and optimized with respect to the Er/Yb content and ratio, and proposed in particular for up- and down-conversion of solar spectrum for enhancement of the efficiency of solar cells. The rise and decay kinetics for the population of the excited levels of Er 3+ and Yb 3+ have been studied and fit. Based on these experimental data, the mechanisms for the energy transfers have been suggested with emphasis on the optimized Er/Yb content and ratio for enhancement of the efficiency of the Er 3+ ↔Yb 3+ energy transfers.

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.

Transparent glass-ceramics containing Ln3+-doped YF3 nanocrystals are successfully obtained under adequate thermal treatment of precursor sol-gel glasses for the first time, to the best of our knowledge. Precipitation of YF3 nanocrystals is confirmed by X-ray diffraction and high-resolution transmission electron microscopy images. An exhaustive structural analysis is carried out using Eu3+ and Sm3+ as probe ions of the final local environment in the nano-structured glass-ceramic. Noticeable changes in luminescence spectra, related to relative intensity and Stark structure of band components, along with remarkably different lifetime values, allow us to discern between ions residing in precipitated YF3 nanocrystals and those remaining in a glassy environment. A large fraction of optically active ions is efficiently partitioned into nanocrystals of small size, around 11 nm. Moreover, bright and efficient up-conversion, including very intense high-energy emissions in the UV range, due to 4- and 5-infrared photon processes, are achieved in Yb3+-Tm3+ co-doped samples. Up-conversion mechanisms are analysed in depth by means of intensity dependence on sensitiser Yb3+ concentration and pump power. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

This work describes the photoluminescence (PL) and morphology of stain etched porous silicon nanostructures (PSN) submitted to a gradual oxidation by immersion in a HNO3 boiling point solution and applied to solar cells. The gradual oxidation passivates the extremely reactant fresh porous surface and softens the porous structure for an adequate placement of the metallic contact on top. The pore size and the porosity of the samples have been evaluated by means of the Brunauer, Emmett and Teller (BET) technique. The surface morphology has been analyzed by atomic forces microscopy. The oxidation process of the layers has been evaluated using a Fourier transform infrared spectrometer (FTIR), analyzing the evolution of the Si–O, Si–H and Si–OH absorption bands. It is found a blueshift in the PL spectra attributed to the oxidation process which increases the photoconversion properties of the nanostructures. An efficiency of 15.2% and fill factor (FF) of 80.5% were found.

A measurement of quantum yield of up-conversion luminescence has been done for the Er(3+)-doped transparent oxyfluoride glass-ceramics 32(SiO,)9(AlO1.5)31.5(CdF2)18.5(PbF2)5.5(ZnF2): 3.5(ErF3) mol%, where most of Er3+ dopants partition in 8 nm diameter nano-crystals Er10Pb25F65. The yield was found by newly proposed method using the pump power dependence of the resonant luminescence. The result of the measurement points out that a theoretical maximum of 50% may be reached for the up-conversion luminescence yield in this material. This high yield is shown to be due to low phonon energy and short inter-dopant distances in the nano-crystals.

A novel low cost process has been developed for application in porous silicon-based photodetectors and solar cells, where conventional doping processes are not affordable because of the high processing cost and technical difficulties. Ytterbium and erbium (Yb3+–Er3+) ions were introduced into luminescent porous silicon stain etched by thermal diffusion. Doping profiles were evaluated by energy-dispersive spectroscopy analysis. The visible and near-infrared photoluminescence of Yb3+–Er3+ co-doped stain-etched porous silicon layers is observed and evaluated under 980 nm pumping. Up-conversion processes that could improve the efficiency of silicon-based solar cells are detected.

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)

In this work a simple erbium doping process applied to stain etched porous silicon layers (PSLs) is proposed. This doping process has been developed for application in porous silicon solar cells, where conventional erbium doping processes are not affordable because of the high processing cost and technical difficulties. The PSLs were formed by immersion in a HF/HNO 3 solution to properly adjust the porosity and pore thickness to an optimal doping of the porous structure. After the formation of the porous structure, the PSLs were analyzed by means of nitrogen BET (Brunauer, Emmett and Teller) area measurements and scanning electron microscopy. Subsequently, the PSLs were immersed in a saturated erbium nitrate solution in order to cover the porous surface. Then, the samples were subjected to a thermal process to activate the Er 3+ ions. Different temperatures and annealing times were used in this process. The photoluminescence of the PSLs was evaluated before and after the doping processes and the composition was analyzed by Fourier transform IR spectroscopy.

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.

Up-conversion luminescence has been studied in the Er 3+ -doped oxyfluoride glass, its daughter nano-glass–ceramics and the polycrystalline ErF 3 when excited at infrared wavelengths of either 800 or 980 nm. The mechanism of the up-conversion luminescence is shown to differ for excitations at 800 and 980 nm since the different excited levels of the Er 3+ are involved, respectively. An order of magnitude increase of red-to-green up-conversion luminescence ratio has been observed with nano-ceramming of the precursor glass when excited at 800 nm and a full dominance of the red up-conversion luminescence has been observed in the ErF 3 at the both excitation wavelengths. Observed changes in the spectra of the up-conversion luminescence from the precursor glass to its daughter nano-glass–ceramics provide a tool for tuning the colour of the up-conversion luminescence by ceramming of the precursor glass. These changes are shown to be due to decrease of the vibration energy of phonons coupled to the Er 3+ ions embedded to the PbF 2 nano-crystals with nano-ceramming. Hence we report for the first time that the red up-conversion luminescence from the Er 3+ -doped material can be due to the radiative transition 4 F 5/2 , 4 F 3/2 → 4 I 13/2 of the Er 3+ ion, which is principally allowed only in such a low-phonon energy host as the PbF 2 nano-crystals, while such red up-conversion luminescence is almost completely non-radiatively quenched in the oxyfluoride precursor glass. The up-conversion emission spectrum of the ErF 3 is dominated by a cross-relaxation mechanism typical of hosts with a very high doping level of the Er 3+ 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.

We have developed transparent 3.5 mol% Ho 3+ and Tm 3+ single-doped oxyfluoride glass–ceramics, where a large fraction of dopant ions is segregated into PbF 2 -based nano-crystals. The absorption and emission cross-sections corresponding to the 5 I 7 ↔ 5 I 8 transitions of the Ho 3+ (at 2.0 μm), and to the 3 H 6 ↔ 3 F 4 transitions of the Tm 3+ (at 1.8 μm), have been obtained and respective gain cross-section spectra have been computed as a function of population inversion. For Ho 3+ -doped samples, the maximum gain cross-section increased by 20% on ceramming of the precursor glass and reached, for the ensuing glass–ceramics, a value 1.5 times higher than that reported for fluoride glasses. The maximum gain cross-section for Tm 3+ -doped samples increased 2.5 times on ceramming of the precursor glasses and reached for the ensuing glass–ceramics a value 2–3 times higher than for other fluoride hosts, such as ZBLAN glass, and 1.5 times higher than for silicate and germanate glass hosts. Hence, these Ho 3+ - and Tm 3+ -doped glass–ceramics have an advantage for application in planar and fibre lasers/amplifiers at about 2.0, and 1.8 μm, respectively.

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.

The room temperature emission spectrum at about 2.7 μm corresponding to the laser transition 4I11/2 → 4I13/2 in Er3+-doped nano-scaled transparent oxyfluoride glass–ceramic has been measured and stimulated emission cross-section for the transition has been calculated. The intensity of the transition has been found to be 40 times stronger and lifetime 50 times longer in the glass–ceramics compared to the precursor glass, which we show to be due to a change of frequency of the phonon involved in non-radiative de-excitation of the 4I11/2 level from 900 cm−1 in the precursor glass to 240 cm−1 in the ensuing glass–ceramics. The absorption cross-section for the excited state absorption 4I13/2 → 4I11/2 has been calculated based on the experimental reciprocal emission spectrum and wavelength dependence of the gain cross-section for the lasing transition 4I11/2 → 4I13/2 vs population inversion has been derived. The lasing/optical amplification gain parameters, such as population inversion, pump saturation intensity and product of emission cross-section and fluorescence lifetime have been obtained at the 2.7 μm wavelength. A noteworthy result is that laser action at 2.7 μm is possible in these Er3+-doped glass–ceramics, already not taking into account energy transfer or up-conversion processes, related to the 4I13/2 level, which favour the population inversion.

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.

We propose a diagram showing the sequence of Stark-split energy levels for the 4I15/2 and 4I13/2 states of Er3+ doped into oxyfluoride, nano-scaled glass-ceramics based on low- and room temperature measurements of respective emission and absorption bands. The room temperature broadest and flattest 1.55 μm telecommunications emission band 4I13/2 → 4I15/2 of Er3+ to date reported earlier for such glass-ceramics is interpreted using this framework. By investigating the low-temperature behaviour, we show that the shape of the 1.55 μm band at room temperature is mostly due to thermal population of higher Stark sublevels in the excited 4I13/2 state. Two sites for Er3+ are proposed which relate, respectively, to the glassy and nano-crystalline phases of the glass-ceramics and their relative contributions to the emission spectrum are identified. The Er3+-site in the nano-crystalline phase is shown to be cubic with nearby charge compensation by one interstitial F−, without appreciable clustering of Er3+ ions.

We have prepared polycrystalline alloys x (ErF 3 ):(100 − x )(PbF 2 ), x = 1, 10. A comparison of their low temperature absorption and emission spectra with the respective spectra of transparent oxyfluoride glass-ceramics, 32(SiO 2 )9(AlO 1.5 )31.5(CdF 2 )18.5(PbF 2 )5.5(ZnF 2 ):3.5(ErF 3 ) mol%, indicates that the nano-crystalline phase in this glass-ceramic is Er 3+ -doped PbF 2 .

Pure and 0.5% and 5mol% Eu{sup 3+} doped GaN nanoparticles have been prepared by ammonolysis of the corresponding freeze-dried precursors. A single hexagonal phase with the wurtzite structure was obtained as determined by X-ray Powder Diffraction. The crystallite size determined by XRD was lower than 10nm. From optical spectroscopy characterization, it is found that the Eu{sub 2}O{sub 3} formation is avoided by using nitrates as starting reagent. Fluorescence line narrowing spectra show excitation wavelength dependence, which is indicative that the Eu{sup 3+} ions are well dispersed in the prepared samples. The environment distribution occupied by the Eu{sup 3+} ions has been analyzed by crystal-field calculation and the results are compared with those for other materials.

The lanthanum molybdate phosphor, La2Mo2O9, is a new fast ionic conductor that has shown potential uses as a nanocrystalline matrix for optically active lanthanide ions. Eu3+-doped La2Mo2O9 phosphors were prepared from freeze–dried powder precursor and the nano-polycrystalline powders obtained were characterized by XRD, DSC, TG/DTA, SEM, impedance spectroscopy, and optical spectroscopy. The substitution of La3+ by Eu3+ in La2−xEuxMo2O9 suppresses partially the phase transition observed in the undoped samples and increases both the density of sintered pellets and the ionic conductivity. Polycrystalline powder and sintered pellets of La2Mo2O9 phosphor exhibits optical transitions in the UV–visible region at low temperature, although there is a luminescence quenching process with temperature. At room temperature orange and red emissions are only observed when doping with Eu3+ ions. Moreover, a nanocrystal-size dependent energy transfer process from the matrix to the Eu3+ ions occurs.

A study of optical properties of Nd3+ doped oxyfluoride glasses and glass ceramics prepared by three different methods has been carried out. These methods start from NdF3, Nd2O3, or a Nd3+ ion solution as doping agent. The alternative preparation method based on a preliminary dissolution of the Nd3+ ions is proposed in order to avoid nonhomogeneous dopant distribution and spontaneous devitrification during glass elaboration. In the frame of the Judd–Ofelt theory, main radiative parameters have been studied: transition probabilities, lifetimes, and stimulated emission cross sections. Fluorescence decay curves have been also analyzed in order to study the final distribution of the Nd3+ ions after the ceramming process, discerning between ions that reside in the fluoride nanocrystals precipitated during heat treatment and those remaining in the glassy phase. The NdF3 based glass ceramics present the best values for spectroscopic parameters as the stimulated cross section of the 4F3/2→4I11/2 laser transition.

The optical properties of the low-field sites of Cr 3+ -doped alkali (Li, Na, K) disilicate glasses have been investigated using the single configurational coordinate model. The assumption of a Gaussian site distribution for the Cr 3+ ions taking as parameter the zero-phonon energy has been considered. For alkali disilicate glasses the inhomogeneous contribution to the broadening of the bands, associated to the site distribution, is lower than the homogeneous one. The electron–lattice coupling S and the mean phonon energy ℏ ω 0 have been obtained with results around 4 and 500 cm −1 , respectively, similar to those obtained by other authors in oxide glasses. The site-resolved study of the emission and excitation spectra and the luminescence decay curves have been carried out as a function of temperature. On the one hand, there is evidence of a non-radiative de-excitation process that becomes important over 140 K. On the other hand, and related to the site dependence of the radiative and non-radiative probabilities, different results involving low values for the quantum efficiencies and blue shifts of the emission bands as temperature increases have been explained. Besides, the non-exponential luminescence decay curves have been fitted to an expression proposed by the authors, which takes into account non-coupled distributions for the radiative and non-radiative de-excitation probabilities for the range of temperature covering from 13 to 300 K. From the fits, the temperature dependence of the non-radiative de-excitation probability is obtained for each disilicate glass, the results are in good agreement with the expression obtained assuming the harmonic approximation in the single configurational coordinate model.

The room temperature up-conversion spectrum of a fluoroindate glass doped with 2.5 mol% of Ho3+ has been detected under excitation at 747 nm. Blue, red and very intense green bands are observed. The green emission has been analysed in detail, in order to investigate the efficient up-conversion mechanism involved in this transition. The low absorption cross-section for the transition from the ground state 5I8 to 5I4, the excitation spectrum of the 5S2, 5F4 → 5I8 green emission, based on a strong excited state absorption (5I7 → 5S2, 5F4) and the temporal dynamics of the process indicate that a photon avalanche mechanism occurs.

Two new fluoromanganates(III) of 1,2-bis(4-pyridyl)ethane (bpa) and trans-1,2-bis(4-pyridyl)ethylene (bpe), LH2[MnF4(H2O)2]2 (L = bpa or bpe), have been prepared and their structure have been solved by single-crystal X-ray diffraction. The [MnF4(H2O)2]− anion displays an octahedral geometry with a strong Jahn–Teller tetragonal distortion along the H2O–Mn–OH2 axis. The equatorial metal–ligand distances (Mn–F 1.827(1)–1.859(2) A) are shorter than the axial ones (Mn–O 2.203(2)–2.234(2) A). Three polarized absorption bands at 22500, 18300 and 14500 cm−1 are observed in the optical spectra of (bpaH2)[MnF4(H2O)2]2. Finally, we present theoretical calculations on the equilibrium bond distances as well as the crystal-field electron structure using density functional methods. The calculated Mn–F bond distances (1.85 A) are in agreement with the experimental data but the obtained Mn–O distances (2.53–2.56 A) are higher than the experimental one as ussually found in similar Jahn–Teller distorted systems. The calculated d–d transition energies are compared with experimental energies derived from the optical spectra. The variation of the HOMO energy and transition energies against the Mn–O distance is also shown.

Optical properties of Eu3+ ions in oxyfluoride glasses and glass ceramics doped with two different concentrations, 0.1 and 1 mol %, have been analyzed and compared with previous results for higher concentrated samples, 2.5 mol %. The Eu3+ ions in the 0.1 mol % doped glass ceramics are diluted into like crystalline environments with higher symmetry and lower coupled phonon energy than in the precursor glasses; meanwhile in the 1 mol % doped glass ceramics the presence of EuF3 clusters has been observed in addition to diluted ions. Fluorescence line narrowing measurements indicate the presence of two main fluoride site distributions for the diluted Eu3+ ions in both glass ceramics.