Spectral characterization and white light generation by yttrium silicate nanopowders undoped and doped with Ytterbium(III) at different concentrations when excited by a laser diode at 975 nm
Introduction
Rare earth (RE) ions embedded in host crystals have been investigated for optical applications such as optical sensing photovoltaics, data storage, nonlinear imaging and biomedicine [1], [2], [3]. In some circumstances, they show upconversion properties and multiphoton absorption luminescence characteristics. Due to the crystal field of the host materials, the energy levels of the rare earth ions split into several Stark levels and selection of host material changes the luminescence effects of the emission. Yttrium silicates (YSO) are suitable host materials for the RE ions because of their high thermal and chemical stability [4], [5].
The NIR photons of the diode used to excite the system can’t bridge the energy gap of the host material. Therefore, the light emitted by the excited nanopowder is of frequency higher than that of the diode excitation, the process of excitation should be considered nonlinear since it requires the absorption of several photons of the emitted light by the diode [4].
In cooperative emission, that is a kind of upconversion, two interacting ions in the excited state return to the ground state simultaneously, and emit one photon with the sum of the energies of the single ion transitions. This upconversion has been observed for Yb3+ pairs in many investigations [6], [7]. Yb3+sensitized phosphors are of interest to researchers for their intense emission in the shorter visible wavelength region [8], [9].
The synthesis techniques of nanostructured yttrium silicates affect the optical and structural properties of the materials. The sol-gel method is the preferable technique used to form Y2O3: SiO2 nanocomposite since it has a lot of advantages; such as needed low temperature, minimal thermal decomposition and easy to control process [10], [11].
In the present study, we used the sol gel method to prepare a series of YSO samples with concentrations of 1%, 2%, 5% and 10%, 20% per mole Yb3+ doped and undoped yttrium silicate nanopowders and studied the details of emission for the different pumping powers in each sample investigated under continuous wave near infrared laser excitation (975 nm).
Section snippets
Synthesis of the undoped and Yb3+ doped Y2O3-SiO2 (YSO)
The nanopowders of Y2O3-SiO2 (YSO) undoped and doped with different concentrations of Yb3+ from 1% to 20% nanopowders were synthesized by the sol-gel method [10], [11]. The ratio of Y2O3-SiO2 was kept at 1.127.
Yttrium(III) nitrate hexahydrate (Y(NO3)3·6H2O – 99.9%), Ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O – 99.9%) salts and tetraethyl orthosilicate (TEOS, 99.9%) were purchased from Sigma–Aldrich company. Ethanol and distillate water were used for solving Y(NO3)3·6H2O and Yb(NO3)3·5H2
Results of the structural analyses
Fig. 1 shows the XRD patterns of 1%, 2% and 5% per mole of Yb3+ doped and undoped yttrium silicate nanopowders annealed at 1250 °C for 12 h. The average particle size is estimated to be ∼89 nm, ∼102 nm and ∼87 nm and ∼86 for the 1%, 2% and 5% per mole Yb3+ doped and undoped powders, respectively, by using the Scherer equation. According to JCPDS (Joint Committee for Powder Diffraction Data), the main structure of samples may be found mostly as α-Y2Si2O7 with the card number 38-0223. The
Conclusions
1%, 2%, 5%, 10%, 20% (per mole) Yb3+ doped and undoped yttrium silicate (YSO) nanopowders were synthesized using the sol-gel method. The luminescence characteristics of 1%, 2%, 5% and 10% Yb3+ doped yttrium silicate samples showed blue–green UC characteristics because of the cooperative emissions of two Yb3+ ions and transitions of unwanted Er3+ impurities at “low” pumping power values 0.38 W–2.94 W, 0.38 W–2.33 W, 0.12 W–0.92 W, 0.12 W–1.06 W for 1%, 2%, 5% and 10% Yb3+ doped samples,
Acknowledgements
The study was supported by The Scientific and Technological Research Council of Turkey (TUBITAK with project number: 114F491) and by Support Branch of Research Projects of Istanbul Technical University (ITU) – BAP with the project number: 38542). One of the authors (Hatun Cinkaya) would also like to thank the TUBITAK for its support (2214-A programme). The authors would like to express their gratitude to Prof. Georges Boulon for his friendship and faithful collaboration throughout the years.
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