Made available in DSpace on 2014-10-09T12:27:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:37Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Materiais dopados com o íon trivalente Tb apresentam intensa emissão no verde (5D4 → 7Fj, j = 3, 4, 5 e 6) e fraca na região UV-azul (5D3 → 7Fj , j = 4, 5 e 6). A emissão azul pode ocorrer a partir de excitação direta no UV (emissão Stokes) ou por upconversion (emissão anti-Stokes), após a excitação em 488 nm (excitação ressonante no nível 5D4 a partir do estado fundamental). Nesta dissertação analisamos as propriedades da luminescência do Tb3+ em vidros aluminosilicato de cálcio com baixa concentração de sílica. Para excitação em 325nm, verificou-se que a intensidade da luminescência verde aumenta linearmente com a concentração, enquanto a emissão azul é fortemente reduzida com o aumento da concentração de Tb3+ de 0,5 para 15,0% (% em peso). No caso da excitação em 488 nm, observou-se uma forte saturação das emissões verdes, para intensidades duas ordens de grandeza menores que o esperado. Por meio de um sistema de equações de taxa, mostramos que esse comportamento pode ser explicado pelo mecanismo de absorção do estado excitado (AEE) e, por uma seção de choque de absorção (5D4 → 5D3) aproximadamente duas ordens de magnitude maior do que a do estado fundamental (7F6 → 5D4). Nossos resultados sugerem que a relaxação cruzada envolvendo os níveis 5D4 e 5D3 não é o único mecanismo responsável pela depopulação do nível 5D3 , como tipicamente descrito na literatura. Processos de upconversion por luminescência cooperativa entre íons excitados (5D3: 5D3 ou 5D3: 5D4) e outros mecanismos que envolvem transferência de energia para níveis excitados superiores (4f5d, banda de transferência de carga, matriz hospedeira, defeitos, etc) podem desempenhar um papel significativo neste sistema. Medidas de Z-scan resolvidas no tempo com excitação em 488 nm foram realizadas e não-linearidades de altas ordens foram observadas (n2, n4, etc.). A partir do n2, valores de Δαp e Δσ da ordem de 10-25 cm3 e 10-20cm2 foram calculados, respectivamente. Este valor de Δαp é o mais alto já reportado em materiais dopados com TR, provavelmente devido à baixa energia dos níveis 4f5d para o íon Tb3+. / Trivalent Tb-doped materials exhibit strong emission in the green (5D4 → 7Fj, j = 3, 4, 5 e 6) and weak in the UV-blue (5D3 → 7Fj , j = 4,5 e 6). The blue emission can occur from direct UV excitation (Stokes emission) or by upconversion (anti-Stokes emission) through 488 nm excitation (resonant pump of 5D4 metastable level from the ground state). In this dissertation the luminescence properties of Tb3+ doped low silica calcium aluminosilicate glasses are reported and analyzed. The intensity of green luminescence increases proportionally to the concentration while the blue luminescence is strongly reduced with the increase of Tb3+ concentration from 0.5 up to 15.0 wt%. In the case of 488 nm excitation, a saturation behavior of the green emission is observed at intensities two orders of magnitude smaller than expected for bleaching of the ground state population. Using a rate equation model, we show that this behavior can be explained by an excited state absorption (ESA) from 5D4 to upper levels, with an absorption cross section two orders of magnitude larger than the ground state absorption. The effect of ESA could also be observed by the shortening of the green emission risetime with the increase of pump intensity. We infer that the CR is not the only loss mechanism responsible for the 5D3 depopulation. Cooperative upconversion from a pair of excited ions (5D3:5D3 or 5D3:5D4) and other mechanisms involving upper lying states (4f5d, charge transfer, host matrix, defects, etc.) may play a significant role. Time-resolved Z-scan measurements were also performed at 488 nm high order nonlinearities were observed (n2, n4, etc.). From n2, Δαp and Δσ of the order of 10-25 cm3 and 10-20cm2, respectively, were obtained. This Δαp value is the highest ever reported for a RE doped material, it seems to be due to the low energy value of the 4f5d state in Tb3+.

Made available in DSpace on 2014-10-09T12:27:51Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:48Z (GMT). No. of bitstreams: 0 / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:27:11Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:57:37Z (GMT). No. of bitstreams: 0 / Óxidos perovskita do tipo ABO3 têm estrutura ideal cúbica, mas algumas distorções nesse tipo de estrutura podem produzir mudança da simetria para ortorrômbica ou romboédrica. Na estrutura cúbica, o cátion A (de maior raio atômico) ocupa o centro de cada cubo, os cátions B ocupam os 8 vértices e os ânions situamse na metade de cada uma das 12 arestas de cada cubo. Distorções ocorridas nessas estruturas produzem o surgimento de novas propriedades elétricas, estruturais e magnéticas, que instigam grande interesse ciêntifico e tecnologico. Neste trabalho, foram estudados os compostos RCrO3 (R = Gd, Tb, Dy), conhecidas como ortocromitas. As amostras foram confeccionadas através do processo químico Sol Gel. Após a confecção as amostras foram analisadas pela difração de raios-x na qual foi concluida que as amostras tiveram uma única estrutura pertencente ao grupo espacial Pbnm. Logo foram realizadas medidas de correlação angular gama-gama perturbada utilizando para isso os nucleos de prova 181Hf(181Ta) e 111In(111Cd) que substituam os átomos A ou B. Os núcleos de prova foram introduzidos durante a fase de preparo das amostras. Um dos objetivos desse trabalho foi o estudo sobre variação do campo magnético hiperfino nos sítios A e B como função da temperatura, estrutura cristalográfica e a transição de fase antiferromagnética. As medidas foram feitas nas a temperaturas entre 20 e 300K para R = Gd, Tb e 20 a 800 K para R = Dy. Também foi medido o Gradiente de campo elétrico em função da temperatura. Foi possivel observar a transição e comprovar o alinhamento dos Spins do Cr, conforme consta na literatura. As temperaturas de Néel (TN ) para as amostras são ~170 K, ~164 K, e ~148 K, respectivamente para os compostos GdCrO3, TbCrO3 e DyCrO3. / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Made available in DSpace on 2014-10-09T12:27:51Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:48Z (GMT). No. of bitstreams: 0 / Complexos contendo íons de terras raras são de grande interesse na fabricação de dispositivos eletroluminescentes como é o caso do diodo emissor de luz construído com compostos orgânicos (OLED). Esses dispositivos, utilizando íons de terras raras trivalentes (TR3+) como centros emissores, exibem alta luminescência com bandas espectrais extremamente finas devido à estrutura dos seus níveis de energia, tempos de vida longos e a alta eficiência quântica. Este trabalho relata a preparação de complexos de β-dicetonatos (tta - tenoiltrifluoroacetonato e acac - acetilacetonato) de terras raras (Tb3+, Eu3+ e Gd3+) contendo ligante macrocíclico éter coroa (DB18C6 - dibenzo18coroa6) e de filmes poliméricos de UO22+. Os materiais obtidos foram caracterizados por titulação complexométrica com EDTA, análise elementar de CH, espectroscopia de absorção na região do infravermelho, análise termogravimétrica, difratometria de raios X (método do pó) e espectroscopia de luminescência. Na manufatura do dispositivo OLED construído neste trabalho utilizou-se a técnica de deposição física de filmes finos por fase vapor (PVD, Physical Vapor Deposition). / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Un-doped, Tb3+ as well as Yb3+ doped ZnO nanocrystals with different concentrations of RE3+ (Tb3+, Yb3+) ions were successfully synthesized via sol-gel method to produce rare earth activated zinc oxide nanophosphors. The phosphor powders were produced by drying the precursor gels at 200˚C in ambient air. Based on the X-ray diffraction results, it was found that the pure and RE3+ doped ZnO nanophosphors were highly polycrystalline in nature regardless of the incorporation of Tb3+ or Yb3+ ions. Moreover, the diffraction patterns were all indexed to the ZnO Hexagonal wurtzite structure and belong to P63mc symmetry group. The Raman spectroscopy confirmed the wurtzitic structure of the prepared samples. Elemental mapping conducted on the as prepared samples using Scanning electron microscope (SEM) equipped with energy dispersive X-ray spectrometer (EDX) revealed homogeneous distribution of Zn, O, and RE3+ ions. The high resolution transmission electron microscope (HR-TEM) analyses indicated that the un-doped and RE3+ doped samples were composed of hexagonal homogeneously dispersed particles of high crystallinity with an average size ranging from 4 to 7 nm in diameter, which was in agreement with X-ray diffraction (XRD) analyses. ZnO:Tb3+ PL study showed that among different Tb3+ concentrations, 0.5 mol% Tb3+ doped ZnO nanoparticles showed clear emission from the dopant originating from the 4f-4f intra-ionic transitions of Tb3+ while the broad defects emission was dominating in the 0.15 and 1 mol% Tb3+doped ZnO. Optical band-gap was extrapolated from the Ultraviolet Visible spectroscopy (UV-Vis) absorption spectra using TAUC‟s method and the widening of the optical band-gap for the doped samples as compared to the un-doped sample was observed. The PL study of ZnO:Yb3+ samples was studied using a 325 nm He-Cd laser line. It was observed that the ZnO exciton peak was enhanced as Yb3+ions were incorporated in ZnO matrix. Furthermore, UV-VIS absorption spectroscopic study revealed the widening of the band-gap in Tb3+ doped ZnO and a narrowing in the case of Yb3+ doped ZnO system. X-ray photoelectron spectroscopy demonstrated that the dopant was present in the doped samples and the result was found to be consistent with PL data from which an energy transfer was evidenced. Energy transfer mechanism was evidenced between RE3+ and ZnO nanocrystals and was discussed in detail. / Physics / M.Sc. (Physics)

The objective of this project was to study the influence of increased salinities on interaction processes in the system radionuclide – organics – clay – aquifer. For this purpose, complexation, redox, sorption, and diffusion studies were performed under variation of the ionic strength (up to 4 mol/kg) and the background electrolyte. The U(VI) complexation by propionate was studied in dependence on ionic strength (up to 4 mol/kg NaClO4) by TRLFS, ATR FT-IR spectroscopy, and DFT calculations. An influence of ionic strength on stability constants was detected, depending on the charge of the respective complexes. The conditional stability constants, determined for 1:1, 1:2, and 1:3 complexes at specific ionic strengths, were extrapolated to zero ionic strength. The interaction of the bacteria Sporomusa sp. MT-2.99 and Paenibacillus sp. MT-2.2 cells, isolated from Opalinus Clay, with Pu was studied. The experiments can be divided into such without an electron donor where biosorption is favored and such with addition of Na-pyruvate as an electron donor stimulating also bioreduction processes. Moreover, experiments were performed to study the interactions of the halophilic archaeon Halobacterium noricense DSM-15987 with U(VI), Eu(III), and Cm(III) in 3 M NaCl solutions. Research for improving process understanding with respect to the mobility of multivalent metals in systems containing humic matter was focused on the reversibility of elementary processes and on their interaction. Kinetic stabilization processes in the dynamics of humate complexation equilibria were quantified in isotope exchange studies. The influence of high salinity on the mobilizing potential of humic-like clay organics was systematically investigated and was described by modeling. The sorption of Tc(VII)/Tc(IV) onto the iron(II)-containing minerals magnetite and siderite was studied by means of batch sorption experiments, ATR FT-IR and X-ray absorption spectroscopy. The strong Tc retention at these minerals could be attributed to surface-mediated reduction of Tc(VII) to Tc(IV). An influence of ionic strength was not observed. The influence of ionic strength (up to 3 mol/kg) and background electrolyte (NaCl, CaCl2, MgCl2) on U(VI) sorption onto montmorillonite was studied. The U(VI) sorption is influenced by the background electrolyte, the influence of ionic strength is small. Surface complexation modeling was performed applying the 2SPNE SC/CE model. Surface complexation constants were determined for the NaCl and CaCl2 system and were extrapolated to zero ionic strength. Surface complexation in mixed electrolytes can be modeled applying surface complexation constants derived for pure electrolytes. The influence of citrate on U(VI) diffusion in Opalinus Clay was studied using Opalinus Clay pore water as background electrolyte. The diffusion parameter values obtained for the HTO through-diffusion and the U(VI) in-diffusion in the absence of citric acid were in agreement with literature data. In the presence of citric acid, U(VI) diffusion was significantly retarded, which was attributed to a change in speciation, probably U(VI) was reduced to U(IV). Larger-scale heterogeneous material effects on diffusive transport were investigated with PET. Diffusion parameters were derived by optimum fit of a FEM-model to the measurement. These parameters are in accordance with the results from 1D-through-diffusion experiments. Deviations from the simple transversal-isotropic behavior, which are identified as residuals from the model, are indications for heterogeneous transport on the mm-scale. PET measurements were also conducted in order to display the improvement of the EDZ with waterglass injections. These experiments enable to draw conclusions on the complex reactive transport process and thus an estimation of the achieved improvement of the barrier function. The image reconstruction procedure was largely improved, mainly with the aid of Monte-Carlo simulations, and now allows quantitative analysis and error estimation.

The objective of this project was to study the influence of increased salinities on interaction processes in the system radionuclide – organics – clay – aquifer. For this purpose, complexation, redox, sorption, and diffusion studies were performed under variation of the ionic strength (up to 4 mol/kg) and the background electrolyte. The U(VI) complexation by propionate was studied in dependence on ionic strength (up to 4 mol/kg NaClO4) by TRLFS, ATR FT-IR spectroscopy, and DFT calculations. An influence of ionic strength on stability constants was detected, depending on the charge of the respective complexes. The conditional stability constants, determined for 1:1, 1:2, and 1:3 complexes at specific ionic strengths, were extrapolated to zero ionic strength. The interaction of the bacteria Sporomusa sp. MT-2.99 and Paenibacillus sp. MT-2.2 cells, isolated from Opalinus Clay, with Pu was studied. The experiments can be divided into such without an electron donor where biosorption is favored and such with addition of Na-pyruvate as an electron donor stimulating also bioreduction processes. Moreover, experiments were performed to study the interactions of the halophilic archaeon Halobacterium noricense DSM-15987 with U(VI), Eu(III), and Cm(III) in 3 M NaCl solutions. Research for improving process understanding with respect to the mobility of multivalent metals in systems containing humic matter was focused on the reversibility of elementary processes and on their interaction. Kinetic stabilization processes in the dynamics of humate complexation equilibria were quantified in isotope exchange studies. The influence of high salinity on the mobilizing potential of humic-like clay organics was systematically investigated and was described by modeling. The sorption of Tc(VII)/Tc(IV) onto the iron(II)-containing minerals magnetite and siderite was studied by means of batch sorption experiments, ATR FT-IR and X-ray absorption spectroscopy. The strong Tc retention at these minerals could be attributed to surface-mediated reduction of Tc(VII) to Tc(IV). An influence of ionic strength was not observed. The influence of ionic strength (up to 3 mol/kg) and background electrolyte (NaCl, CaCl2, MgCl2) on U(VI) sorption onto montmorillonite was studied. The U(VI) sorption is influenced by the background electrolyte, the influence of ionic strength is small. Surface complexation modeling was performed applying the 2SPNE SC/CE model. Surface complexation constants were determined for the NaCl and CaCl2 system and were extrapolated to zero ionic strength. Surface complexation in mixed electrolytes can be modeled applying surface complexation constants derived for pure electrolytes. The influence of citrate on U(VI) diffusion in Opalinus Clay was studied using Opalinus Clay pore water as background electrolyte. The diffusion parameter values obtained for the HTO through-diffusion and the U(VI) in-diffusion in the absence of citric acid were in agreement with literature data. In the presence of citric acid, U(VI) diffusion was significantly retarded, which was attributed to a change in speciation, probably U(VI) was reduced to U(IV). Larger-scale heterogeneous material effects on diffusive transport were investigated with PET. Diffusion parameters were derived by optimum fit of a FEM-model to the measurement. These parameters are in accordance with the results from 1D-through-diffusion experiments. Deviations from the simple transversal-isotropic behavior, which are identified as residuals from the model, are indications for heterogeneous transport on the mm-scale. PET measurements were also conducted in order to display the improvement of the EDZ with waterglass injections. These experiments enable to draw conclusions on the complex reactive transport process and thus an estimation of the achieved improvement of the barrier function. The image reconstruction procedure was largely improved, mainly with the aid of Monte-Carlo simulations, and now allows quantitative analysis and error estimation.

Giant magnetostrictive RFe2 type (R represents rare earths) intermetallics form an important class of magnetic materials keeping in view of their potential applications as sensors and/ or actuators. In this thesis, one such mixed rare earth compound (Tb,Dy)Fe2 has been chosen for investigations. Being a technologically important material system, several investigations concerning physical and magnetic properties of the material and effect of processing parameters on magnetic properties have been reported in the available literature. However, existing literature does not provide a clear insight into some important aspects such as phase equilibria, evolution of texture and microstructure of directionally solidified Tb-Dy-Fe alloys. Therefore, the present work was undertaken to bring out tangible process-structure-property correlations with an emphasis to clarify the grey areas in the available literature. The investigation on the nature of ternary phase equilibria of Tb-Dy-Fe was taken up with an aim to understand the effect of Tb/Dy ratio on phase equilibria and magnetic properties of TbxDy1-xFe1.95 (x=0-1) alloys. Microstructural and micro-chemical analysis along with study of lattice parameter has been used to predict the nature of phase equilibria and the deviation from the assumed pseudo-binary behaviour. Further, from the microstructural investigations and study of lattice parameter and Curie temperature, a schematic sketch of a section of the ternary diagram, where (Tb,Dy) / Fe =1.95, was formulated and presented. Directional solidification technique is the most widely adopted method for processing the (Tb,Dy)Fe2, to impart grain orientation for practical applications. Therefore, it was aimed in the present study to understand the evolution of texture and microstructure in directionally solidified Tb0.3Dy0.7Fe1.95 alloy by modified Bridgman and zone melting techniques. The alloy was directionally solidified by modified Bridgman technique with a series of growth rates 5 - 100 cm/h, at a constant temperature gradient of 150oC/ cm. Microstructural investigation revealed formation of island banding at lower growth rate and peritectic coupled growth at higher growth rates. The texture study indicated a transition of growth texture from <113> to <110> and finally to <112> with increase of growth rate. A mechanism based on atomic attachment kinetics is proposed to explain the orientation selection with growth rate. The texture and microstructure have been correlated with magnetostriction and static strain co-efficient (dλ/dH) of the Bridgman solidified alloys. The solidification morphology observed in Bridgman solidified samples was found to be mostly plane front. Therefore, in order to understand the microstructure and texture evolution in cellular/ dendritic regime, directional solidification of Tb0.3Dy0.7Fe1.95 was attempted by zone melting technique with a lesser temperature gradient of 100oC/cm. A detailed texture study indicated a transition in preferred growth direction from <110> to <112> with increase of growth rate. In this case of cellular/ dendritic growth regime, a mechanism based on atomic attachment kinetics has been proposed and the preferred morphologies of the solid-liquid interface for <110> and <112> growth have been modelled. The modelled interfaces have been correlated to the shape of cell/ dendrite cross-section observed for the growth rates adopted in this study. Apart from the investigation carried out on the (Tb,Dy)Fe2 alloys, attempts have been made to understand the role of microstructure, especially the ordered phases on the magnetostriction of an emerging magnetostrictive material Fe-Ga. A series of alloy compositions of Fe-x at % Ga (x=17, 20, 23 and 25) were prepared and subjected to different thermal treatments and characterized for microstructural features and magnetostriction. Microstructure investigation of slow cooled, quenched and quenched + aged alloys reveals formation of ordered DO3 phase from disordered A2 phase by first order transformation in 17 and 20 at% Ga alloys, whereas for 23 and 25 at% alloys, the transformation takes place by continuous ordering. It could be observed that large magnetostriction arises owing to the presence of disordered A2 phase or ordered DO3 phase alone. The magnetostriction however decreases substantially when these two phases are co-existing.

A busca por alternativas viáveis de produção de energia limpa e renovável, utilizando recursos naturais, tem sido um grande desafio. Em especial, o interesse no uso da energia solar para obter energia elétrica tem aumentado. Todavia, as células solares convencionais que são confeccionadas a partir de silício cristalino (Si-c) apresentam uma eficiência de conversão limitada, principalmente, devido às perdas por incompatibilidade espectral. Sendo assim, um dos objetivos dos pesquisadores na área de fotovoltaicos tem sido converter eficientemente fótons da região do visível do espectro solar para a região do infravermelho, onde a célula solar de Si-c possui maior eficiência. A eficiência desta conversão poderia ser melhorada de 28% até 40% usando conversores de energia compostos por íons terras raras. Neste trabalho foram estudadas as propriedades espectroscópicas dos íons terras raras Térbio (Tb3+) e Itérbio (Yb3+), embebidos em três diferentes materiais: vidro aluminosilicato de cálcio com baixa concentração de sílica (7%) (LSCAS), vidro tetraborato de cálcio-lítio (Calibo) e nano-cristais de óxido de zircônia (ZrO2). Sendo assim, nossos estudos visam elucidar os mecanismos geradores dos processos de conversão ascendente e descendente de energia. No processo de conversão ascendente de energia há conversão dos fótons de excitação de baixa energia na região do infravermelho em fótons de alta energia na região do visível. Por outro lado, no processo de conversão descendente de energia ocorre a conversão dos fótons de excitação de alta energia na região do ultravioleta/visível em um ou mais fótons de baixa energia na região do infravermelho. As propriedades espectroscópicas dos materiais estudados foram discutidas a partir das medidas de absorção, luminescência, excitação, evolução temporal da luminescência, curva de potência, difração de raios-X, X-Ray Absorption Near Edge Structure e ressonância paramagnética eletrônica, em função da concentração dos íons Tb3+ e Yb3+. Os resultados mostraram que todas as amostras apresentam conversão ascendente de energia. Assim como, em todas as amostras ocorre o processo de conversão descendente de energia. Em ambos os processos foram identificados os mecanismos geradores dos mesmos. Estes resultados mostram um avanço nos estudos dos processos de conversão ascendente e descendente de energia em sistemas co-dopados com íons Tb3+ e Yb3+. Os resultados sugerem que os materiais estudados podem ser empregados para aumentar a eficiência de conversão da célula solar de Si-c via conversão descendente de energia. / The search for possible alternatives to produce clean and renewable energy using the natural resources has been a great challenge. In particular, the interest to use the solar energy to produce electricity has been increased. However, crystalline silicon-based (c-Si) conventional solar cells have limited conversion efficiency, mainly due to spectral mismatch losses. Thus, one of the goals of some researchers in the photovoltaic field has been to efficiently convert photons in the visible region of the solar spectrum to the infrared region, where the c-Si solar cell has its higher efficiency. The efficiency of this conversion could be enhanced from 28% up to 40% using energy converters based on rare-earths ions. In this work, the spectroscopic properties of rare earth ions, such as Terbium (Tb3+) and Ytterbium (Yb3+) embedded in three different materials were studied: low- silica calcium aluminosilicate glass (7%) (LSCAS), lithium calcium tetraborate glass (Calibo) and zirconium oxide nano-crystals (ZrO2). Thus, our studies aimed to elucidate the mechanisms that generate the Up-conversion and Down-conversion processes. In the Up-conversion process, there is the conversion of the low-energy excitation photons in the near-infrared to high-energy photons in the visible. On the other hand, in the Down-conversion process there is the conversion of the high-energy excitation photons in the ultraviolet/visible region to low-energy photons in the near-infrared region. The optical properties of the studied materials were discussed through absorption, luminescence, excitation, temporal evolution of the luminescence, power curve, X-ray diffraction, X-Ray Absorption Near Edge Structure and electron paramagnetic resonance measurements, as a function of the concentration of Tb3+ and Yb3+ ions. The results showed that all samples exhibit Up-conversion process. In addition, in all samples occurs the Down-conversion process. In both processes were identified the mechanisms that produce them. These results show a breakthrough in the studies of the Up-conversion and Down-conversion processes in Tb3+ and Yb3+ co-doped systems. The results suggest that the materials are applicable in enhancing the conversion efficiency of the Si-c solar cell via NIR Down-conversion.