Nowadays, white LEDs are the first choice in solid state lighting thanks to their high efficiency, low cost, compactness and robustness. The optimization of luminescent nanomaterials with tailored emission properties in the visible when illuminated with UV light is crucial for the development of white LEDs with better performances. In this view, metal oxide nanopowders are perfect candidates. On the one hand, in many cases their emission properties meet the requirements needed for white phosphors: high luminescence quantum yield and tailored emission properties as a function of composition, dimensions, shape and thermal treatments. On the other hand, their reduced size, typically less than 100 nm, allows for their incorporation in suitable transparent matrix so as to preserve their photoluminescent characteristics without having strong detrimental scattering losses. Within the many synthetic approaches used for the preparation of metal oxide nanoparticles, the microwave-assisted non-aqueous solution-based benzyl alcohol route is one of the most promising way in terms of easiness, reproducibility, reaction yield, and versatility. Here we present new results on UV-excited light emission properties of many different oxide nanopowders prepared by microwave-assisted non-aqueous solution-based benzyl alcohol route. These systems are characterized by complex emission features ascribable to different species. In a recent work , three main components on the emission bands were identified in Eu doped γ-Ga2O3 nanoparticles (NPs): 1) intrinsic emission from oxide NPs, 2) intrinsic emission of Eu ions and 3) extrinsic emission originated by the presence of an organic capping layer resulting from the peculiar synthesis route. Interestingly, proper thermal treatments, able to remove the organic capping layer without altering oxide nanophase, can be performed to control the related emission. In this work, we investigate the photoluminescence behavior of several oxide, prepared by the same synthesis route, so as to recognize some possible common optical feature of extrinsic origin as well as to study the intrinsic photolumiscence in view of application in UV-pumped white-light emitting devices. The sample set comprises both strong luminescent oxides and reference samples of “PL-silent” materials, their emission properties being evaluated before and after organic capping removal. In particular, we focused our attention on titanium, zirconium, hafnium, tantalum, tungsten, zinc and gallium oxides. As prepared samples were characterized by means of X-ray diffraction patterns, absorption/reflection spectra in the UV-Vis-IR range, Raman spectroscopy, steady state and time-resolved photoluminescence emission and excitation spectra. The same analysis have been conducted on samples without organic capping layer and treated at 420°C for 1 hour. X-ray diffraction patterns confirmed a NP size of less than 10 nm. Raman spectroscopy showed that the mild thermal treatment is able to burn all the residual organic without altering the inorganic core. Finally, photoluminescence spectra provided us basic information on the overall spectroscopic features of the systems as regard both intrinsic and extrinsic contributions. In conclusion, untreated oxide nanopowders show emissions centered at 320-450 nm ascribable to the organic capping layer. Emission spectra after thermal treatment shows sample-dependent features originated mainly from defect species (oxygen and/or metal vacancies) and exciton emission. The opportunity of tuning both intrinsic and extrinsic properties of metal oxide nanoparticles will be discussed and presented.
Azarbod, A., Lauria, A., Paleari, A., Lorenzi, R. (2016). Intrinsic and Extrinsic Light Emission Properties of Metal Oxide Nanopowders Synthesized by Non-Aqueous Sol-Gel Route. Intervento presentato a: MRS Fall Meeting, Boston, USA.
Intrinsic and Extrinsic Light Emission Properties of Metal Oxide Nanopowders Synthesized by Non-Aqueous Sol-Gel Route
Lauria, A;Paleari, A;Lorenzi, R
2016
Abstract
Nowadays, white LEDs are the first choice in solid state lighting thanks to their high efficiency, low cost, compactness and robustness. The optimization of luminescent nanomaterials with tailored emission properties in the visible when illuminated with UV light is crucial for the development of white LEDs with better performances. In this view, metal oxide nanopowders are perfect candidates. On the one hand, in many cases their emission properties meet the requirements needed for white phosphors: high luminescence quantum yield and tailored emission properties as a function of composition, dimensions, shape and thermal treatments. On the other hand, their reduced size, typically less than 100 nm, allows for their incorporation in suitable transparent matrix so as to preserve their photoluminescent characteristics without having strong detrimental scattering losses. Within the many synthetic approaches used for the preparation of metal oxide nanoparticles, the microwave-assisted non-aqueous solution-based benzyl alcohol route is one of the most promising way in terms of easiness, reproducibility, reaction yield, and versatility. Here we present new results on UV-excited light emission properties of many different oxide nanopowders prepared by microwave-assisted non-aqueous solution-based benzyl alcohol route. These systems are characterized by complex emission features ascribable to different species. In a recent work , three main components on the emission bands were identified in Eu doped γ-Ga2O3 nanoparticles (NPs): 1) intrinsic emission from oxide NPs, 2) intrinsic emission of Eu ions and 3) extrinsic emission originated by the presence of an organic capping layer resulting from the peculiar synthesis route. Interestingly, proper thermal treatments, able to remove the organic capping layer without altering oxide nanophase, can be performed to control the related emission. In this work, we investigate the photoluminescence behavior of several oxide, prepared by the same synthesis route, so as to recognize some possible common optical feature of extrinsic origin as well as to study the intrinsic photolumiscence in view of application in UV-pumped white-light emitting devices. The sample set comprises both strong luminescent oxides and reference samples of “PL-silent” materials, their emission properties being evaluated before and after organic capping removal. In particular, we focused our attention on titanium, zirconium, hafnium, tantalum, tungsten, zinc and gallium oxides. As prepared samples were characterized by means of X-ray diffraction patterns, absorption/reflection spectra in the UV-Vis-IR range, Raman spectroscopy, steady state and time-resolved photoluminescence emission and excitation spectra. The same analysis have been conducted on samples without organic capping layer and treated at 420°C for 1 hour. X-ray diffraction patterns confirmed a NP size of less than 10 nm. Raman spectroscopy showed that the mild thermal treatment is able to burn all the residual organic without altering the inorganic core. Finally, photoluminescence spectra provided us basic information on the overall spectroscopic features of the systems as regard both intrinsic and extrinsic contributions. In conclusion, untreated oxide nanopowders show emissions centered at 320-450 nm ascribable to the organic capping layer. Emission spectra after thermal treatment shows sample-dependent features originated mainly from defect species (oxygen and/or metal vacancies) and exciton emission. The opportunity of tuning both intrinsic and extrinsic properties of metal oxide nanoparticles will be discussed and presented.
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