Photonic nanostructures for Light-Emitting Devices.

Aiming towards energy-efficient and environmentally friendly light sources, a major shift in artificial lighting is taking place driven by the development of highly efficient light-emitting diodes (LEDs). Nowadays, LEDs use a mature technology that can compete with traditional light sources due to the higher efficiencies, longer lifetimes, fast switching, robustness, and compact size that LEDs feature. It is foreseen that the widespread replacement of traditional light sources within the next 10-20 years will lead to a considerable reduction of the worldwide electricity consumption. In order to facilitate such a transition, LEDs must be integrated in many different applications. It is therefore necessary to attain full control over brightness, color and directionality of the light emitters. Luminescent materials based on rare earth ions play an increasingly important role in a variety of applications because of their various of emission color, typical spectral properties, high stability, etc. However, rare earth phosphors in the nanoscale even have much lower luminescent efficiency due to their low crystallinity and high density of surface defects. In the framework of PHOLED it has been demonstrated that the emission color and efficiency of thin layer made by rare-earth doped nanocrystals can be strongly modulated in tunable spectral ranges using optical resonators specifically designed to this end. The color coordinates of nanoparticles can be tuned from blue red with unprecedented precision. Key to the achievement herein reported is the careful analysis of the structural and optical properties of thin nanophosphor layers with the processing temperature in order to achieve efficient photoluminescence while preserving the transparency of the film. The results prove that the emission color of luminescent materials can be tuned by utilizing an external structure platform other than depending on the chemical management during the synthesis, which opened a new path for fundamental and applied research in solid-state lighting.
The accomplishment of the idea present herein implies the consecution of impacts at the fundamental, technological, environmental, and economic level. Results attained within this project will provide significant advance in the development of versatile SSL devices of optimized efficiency, aiming to solve critical limitations that the current technology presents. Improved LEDs obtained within this project will accelerate the replacement of traditional lighting sources, reducing energy costs for lighting, greenhouse gases emission and eliminating the exposure to mercury found in fluorescent bulbs.
The overall objectives of this project is to develop new optical materials and structures to improve the performance of standard phosphors devised for lighting applications. The strategy was the integration of novel optical nanostructures and emitters, such as colloidal quantum dots or nanophosphors, to yield the next generation of light-emitting devices in which full spectral and angular control over the emission properties will be possible. The main research was focused on exploring efficient nanophosphors and the preparation of optical nanostructures containing nanophosphors which will allow a precise control on the intensity, angular distribution and color quality of light emission. Results achieved within this project will provide significant advance both in the comprehension of fundamental phenomena as well as in the development of versatile solid-state lighting devices of optimized efficiency, aiming to overcome technical barriers and maximize performance.

a. Developed GdVO4:Eu3+/Dy3+/Tm3+, GdVO4:Bi3+,Eu3+, NaGd(MoO4)2:Eu3+/Dy3+/Sm3+, La(MoO4)3:Eu3+, Y3Al5O12:Ce3+ nanophosphors that can be introduced into optical nanostructures to form optical resonator.
b. Prepared GdVO4:Bi3+, Eu3+ nanophosphor pastes with TiO2 scattering centers, which will serve to increase the photoemission intensity by selectively reinforcing the optical absorption at the excitation wavelengths. It also allows an improvement of the outcoupling efficiency.
c. Fabricated optical resonators with different structural parameters by combining layers of GdVO4:Eu3+ or GdVO4:Dy3+ nanophosphors with Bragg mirrors made of alternate coatings of SiO2 and ZrO2 materials which ensured the possibility to excite the photoemission in the whole UV and visible ranges. Controllable emission color, enhanced efficiency and directional emission light have been achieved by this approach.
d. Tamm plasmon structures were employed to control the spontaneous emission of GdVO4:Eu3+ nanophosphors. The emission peak coupled with the mode can be enhanced more than 60-fold respect to the emission intensity of the reference. The experimental results and modeling data proved that a strong angular dependence of the spectral properties of the emission can be achieved.
Dra. Geng is fully integrated and actively participating in the daily life of the group and the Institute.
Her results have been so far published in a prestigious journal in the field of optical materials (http://onlinelibrary.wiley.com/doi/10.1002/adom.201700099/full) and several more publications are being currently finalized.
She has attended the International Conference on OSA Light, Energy and the Environment Congress, 2016, Leipzig, Germany, which is which is one of the most significant international conferences organized by the Optical Society of America and presented a poster. Morever, Dra. Geng has given an invited talk at the Silk Road International Symposium for Distinguished Young Scholars 2016, Xian, China. In 2015 Dra. Geng attended the PoLight Workshop, Seville.
On a different note, Dr. Geng has participated in the Researcher’s Night in Seville, also organized within the framework of the H2020 funded programme and the Science Fair 2017, in Seville, Spain.

For the first time, a nanophosphor based photonics susceptible of being used in solid state lighting has been developed. A new approach to obtain changeable emission spectra from exactly the same rare earth nanophosphors by embedding them in different optical resonators was demonstrated. The emission color can now be tuned from green to blue or yellow by precise control of the spectral features of the resonant modes of the photonic environment. Dr. Gend’s work opens a new path to tailor the photoluminescence properties of phosphor-converted light emitting diodes.
This achievement will facilitate the transition from traditional lighting based on incandescent filaments to a more sophisticated and adaptable one, based on made-to-measure LEDs that can be integrated in many different applications.

Periodic Reporting for period 1 - PhoLED (Photonic nanostructures for Light-Emitting Devices.)

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