Article pubs.acs.org/JPCC
2.7 μm Emission from Transparent Er3+,Tm3+ Codoped Yttrium Aluminum Garnet (Y3Al5O12) Nanocrystals−Tellurate Glass Composites by Novel Comelting Technology Guanqi Chai, Guoping Dong,* Jianrong Qiu, Qinyuan Zhang, and Zhongmin Yang* State Key Laboratory of Luminescent Materials and Devices, and Institute of Optical Communication Materials, South China University of Technology, Guangzhou 510641, PR China ABSTRACT: Yttrium aluminum garnet Y 3 Al 5 O 12 (YAG):Er3+,Tm3+ phosphor powders with preferable luminescent properties and nanocrystals with better morphology were synthesized by the solid-state reaction method and coprecipitation method, respectively. The composition and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), which showed that the nanocrystals were pseudomonodispersed with the particle size of ∼30 nm. Photoluminescence (PL) spectra indicated the 2.7 μm emission of Er3+ was remarkably enhanced via Tm3+ sensitization, and a novel circulatory energy transfer mechanism was proposed. A novel dehydration method was used to decrease the contents of the hydroxyl group (OH−) confirmed by photoluminescence spectra and Fourier transform infrared spectra (FTIR). YAG crystallites were introduced into tellurate glass and formed to glass composites with good optical performance. These nanocrystals−glass composites open a brand new field for the research of mid-infrared laser materials. Dy3+,15 etc. Among them, Er3+ fluorescence emitting at 2.7 μm (4I11/2 → 4I13/2 transition) plays a key role in the investigations and has been achieved in many kinds of glasses,16−19 glass ceramics,20 and single-crystal21 hosts. It is known that there are many factors that should be considered to obtain higher luminescent efficiency, such as host materials must possess a minimal absorption coefficient in the typical H2O absorption band and have lower phonon energy.22 This is because if the host has higher phonon energy the nonradiative transition between the two energy levels of Er3+ ions will be increased and result in the decrease of radiative transition with the 2.7 μm emission. However, the 2.7 μm fluorescence also cannot be obtained efficiently because the fluorescence lifetime of the lower level, 4I13/2, is longer than that of the upper level, 4I11/2.23 Thus, a codoping sensitizer, such as Yb3+,24 Nd3+,18,25 Pr3+,26 Tm3+,27 or Ho3+,28 has been considered to efficiently quench the lower level and realize the 2.7 μm emission of the upper level. Oxide nanocrystals as luminescent hosts have higher melting point and chemical stability than glass and lower cost than single crystals, so they have been widely studied.29 Cubic phase yttrium aluminum garnet Y3Al5O12 (YAG) free of birefringence
1. INTRODUCTION Recently, mid-infrared lasers operating around 2.7 μm have attracted great attention due to various potential applications, including remote sensing, medical surgery, and eye-safe laser radar, etc.1−5 The requirements of host materials for midinfrared lasers are so strict that the research and development have reached a bottleneck state. Thus, it is extremely important to search for a new solid state host. Compared with conventional glass and glass ceramics, nanocrystals with smaller particle size, higher chemical stability, and more mature synthesis methods have been adopted as the candidate hosts for mid-infrared fluorescence and lasers.6−10 Furthermore, rare earth ion doped nanocrystals that are introduced into glass matrixes as luminescence centers have many unique advantages,11 such as higher luminescent efficiency, stability, and controllability, etc. If the particle size of nanocrystals is small enough and the refractive index of the glass matrix matches with that of nanocrystals, the glass composite would be transparent and achieve excellent optical transmission. Thus, the novel technologies of preparing nanocrystal−glass composites will open a brand new field for the research of mid-infrared laser materials, which is extremely important to achieve the preparation of composite optical fiber preforms and fiber forming. Up to now, many kinds of rare-earth ions have been investigated in mid-infrared laser,12 such as Er3+,13 Ho3+,14 and © 2012 American Chemical Society
Received: May 31, 2012 Revised: August 24, 2012 Published: August 28, 2012 19941
dx.doi.org/10.1021/jp3052906 | J. Phys. Chem. C 2012, 116, 19941−19950
The Journal of Physical Chemistry C
Article
stoichiometry of the final nanocrystals. This mixed aqueous solution was added dropwise into an aqueous solution of ammonium hydrogen carbonate (NH4HCO3, analytical grade) under magnetic stirring at room temperature, and the dropping rate was kept at about 2 mL/min. During the titration process, ammonium hydroxide (NH4·H2O, analytical grade) was used to adjust the pH value of the mixed solution, and the final pH value was in the range of 8−9. Then, the mixed solution was aged for 24 h at room temperature. The precursor powders were centrifugated and washed with water and ethanol several times to remove residual ammonia and nitric ions. After drying at 80 °C for 36 h, the precursor powders were crushed and mixed with different mass ratios of ammonium hydrogen fluoride (NH4HF2, analytical grade) and then calcined in air at different temperatures for 2 h. Finally, the corresponding nanocrystals were obtained. (c). Preparation of the Glass Matrix and Glass Composites. The glass matrix materials used in this study were prepared using the conventional melt-quenching technique. The compositions of the glass matrix are 60TeO2−10ZnO−20Na2O (in mol %). The well-mixed batches were melted at 750 °C for 30 min and inletted high-purity oxygen for 30 min to remove the OH¯ in the glass structure. Then, the glass was crushed into powders and mixed well with different mass ratios of YAG:Er3+,Tm3+ nanocrystals and formed the xYAG-TZN (x = 0, 1.5, 2.5, 5, 10 wt %) mixed powders. The mixture was melt-quenched for the second time at 570 °C for 20 min with stirring to form the glass composites. After annealing at 250 °C for 6 h, the glass composites were cut and polished to 10 × 10 × 1 mm3 for further measurements. 2.2. Characterizations. The crystalline structures of YAG powders were observed using X-ray diffraction (XRD) on a D8 advance X-ray diffractometer (Bruker, Switzerland) with Cu Kα radiation (λ = 1.54056 Å) and a scanning speed of 0.2°/min. The morphology and size distribution of the powders were observed by field emission−scanning electron microscopy (FESEM, Nova NanoSEM430, FEI, Netherlands) and highresolution transmission electron microscopy (HR-TEM, 2100F, JEOL, Japan) equipped with an energy-dispersive Xray spectrometer (EDS). The contents of OH¯ in the nanocrystals were measured from Fourior transform-infrared spectroscopy (FTIR, Vector-33, Bruker, Switzerland). The photoluminescence spectra were measured on a Triax 320 spectrometer (Jobin-Yvon Co., France) with a resolution of 1 nm, which were excited by a 976 nm laser diode (LD) and detected with a PbSe photodetector. Optical transmittance spectra of glass composites were recorded on an ultraviolet/ visible/near-infrared (UV/vis/NIR) spectrophotometer (Lambda-900, Perkin-Elmer, USA). All the measurements were carried out at room temperature.
is one of the most important fluorescence laser materials, as well as a kind of high-melting point material,30 and it has been successfully applied as a host for the Er3+ to achieve ∼2.7 μm lasing.31 This is because YAG crystals are provided with easy preparation, good chemical durability, and higher refractive index.32 Thus, we choose YAG nanocrystals as the host material. At present, there are many wet chemical methods to synthesize YAG nanocrystals, including combustion synthesis,33 sol−gels,34 coprecipitation,35 and hydrothermal.36 The coprecipitation method is a milder chemical reaction, so nanocrystals can be controllably synthesized with smaller particle size (
