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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NANOPHOM (Nanophosphor-based photonic materials for next generation light-emitting devices)

Teaser

Summary

A transition towards energy-efficient and environmentally friendly light sources is an essential part of the worldwide strategy to reduce electricity consumption. To this end, a major shift in artificial lighting is taking place driven by the development of highly efficient light-emitting diodes (LEDs): the so-called solid-state lighting (SSL) revolution. Nowadays, (inorganic) LEDs use a mature technology that can outperform traditional light sources due to their low power consumption, long lifetime, fast switching, robustness, and compact size. Despite these advantages, the much-needed transition is being hampered by the limited control over color quality and directionality of LED light emission that standard materials and reflectors and lenses, typically employed as secondary optical elements, provide.

NANOPHOM seeks to surpass the limits imposed by geometrical optics in the radiation shaping of light-emitting devices by means of nanophotonics. We aim at tailoring the emission properties of nano-sources using integrated nanostructured optical components, rather than following the traditional approach of intrinsic material or chemical modification. Based on a profound understanding of the light-matter interaction in the nanoscale, optical design can propel the development of versatile LED color converters, overcoming current critical limitations for the integration of SSL technology. Specifically, NANOPHOM explores the combination of rare-earth nanocrystals with different photonic architectures, i.e. multilayers, surface textures and three-dimensional architectures to improve the performance of the color conversion process in light-emitting devices. The ultimate goal of this project is to develop large-area photonic materials with devised chromaticity and improved conversion and extraction efficiencies, which will enable a conscious use of the generated light. Indeed, these results are expected to find a direct application in the field of artificial lighting, especially in unconventional illumination applications such as roadway, stage or retail lighting, and applications beyond, like horticulture or healthcare, where highly specific and demanding specifications are required. Thus, our approach may have significant economic and environmental impact, reducing energy costs for lighting, lessening carbon dioxide emissions and minimizing light pollution.

Work performed

NANOPHOM seeks to develop novel emitting materials based on the combination of nanometre-size rare-earth nanocrystals, so-called nanophosphors, with photonic architectures. It explores new ways of controlling the emission characteristics of nanophosphor sources through the use of dielectric or metallic nanostructures by design. In order to do so, a series of objectives, which represent relevant achievements per se, have been pursued.

Luminescent properties of nanophosphors arise from the interplay among a variety of factors ranging from composition to crystallinity or surface properties, being hard to foresee their efficiency prior to fabrication (see Figure 1). For this reason, we have worked on the assessment of the applicability of nanophosphors for color conversion in LEDs during the first 18 months of NANOPHOM. Firstly, we have investigated nanophosphors based on tungstate, molybdate or vanadate compounds that enable the effective excitation of rare-earth activators through charge or energy transfer. As a result, we have demonstrated thin nanophosphor films of high optical quality that are both bright and efficient, and that are suitable for the integration with photonic materials. At the same time, we have also worked on the design and fabrication of photonic architectures to control the spontaneous emission of nanophosphors. For this, we have developed a series of theoretical tools to calculate all relevant optical quantities to model the interaction between emitters and resonant photonic structures. Different routes were explored to develop nanohosphor-based photonic materials. Based on a judicious optical design, we have already demonstrated that the interplay between photon resonant modes and the natural emission of nanophosphors integrated in an optical cavity allows a fine control over both chromaticity and direccionality of nanophosphor emission, without modifying the chemical composition of the emitters or degrading their efficiency, which constitutes a landmark in the field (see Figure 2). Also, we have worked on the integration of nanophosphors in the vicinity of the interface of a periodic multilayer and a metal film, which sustain optical Tamm plasmons for the enhanced out-coupling of the nanophosphor emission in well defined directions. From a different perspective, we have worked on the preparation of self-standing flexible optically disordered materials in which colloidal nanoparticles that act as Mie resonators are incorporated in a nanophosphor-based matrix to enhance the fraction of light emitted by the nanophosphors that is efficiently out-coupled (see Figure 3). Results attained so far support our thesis that the combination of nanophosphors and photonic architectures opens the door to develop versatile and cost-effective illumination sources by providing efficient and easy-to-handle conversion layers susceptible to be excited by LEDs emitting at wavelengths in the near UV region. Indeed, a precise control of the emission properties of light-emitting devices beyond efficiency connects with the growing interest of our society for the development of light sources with expanded functionalities that would offer new opportunities for the flourishing of LEDs in applications ranging from visible light communications to horticulture or healthcare, where the emission characteristics of the generated light must be accurately adapted.

Final results

NANOPHOM proposes the integration of nanostructured optical materials into light-emitting devices to attain full control over directionality and chromaticity of the color conversion process. Nanophosphors are central for many applications related to the generation of light because these nanomaterials feature exceptional thermal and chemical stability. However, such stability brings along an intrinsic complexity to alter their emission properties. The most common route to control the luminescence spectrum of nanophosphors is modifying their crystalline structure or their chemical composition at the expense of deteriorating the overall efficiency of the emitter. Indeed, there is a growing interest in the development of highly stable, crystalline, phosphor coatings that can provide both transparency and tailored chromaticity. In spite of the intense research in this field, advances are always limited as a result of the difficulty to combine in a single slab both high optical quality, i.e., low scattering and thus high transparency, with efficient emission, which usually is achieved for large inorganic micron size phosphor crystals that strongly scatter light. None of these approaches deals with the integration of photonic nanostructures within them, as we propose. In this context, NANOPHOM integrates a series of photonic approaches that control lightâˆ’matter interaction at the sub-wavelength scale to tailor the emission of nanomaterials without altering their chemical composition, which represent an innovative approach. Our findings are expected to provide a significant advance in the understanding of nanostructured emitting materials for the development of versatile light sources. We believe our results are already pushing a paradigm shift in materials science applied to light generation in which the emission properties of nanophosphors are tailored using integrated photonic architectures, rather than through the modification of their intrinsic material or chemical properties.

During the next 42 months of the project, we will continue with the original research plan, aiming to realize photonic architectures that enable a fine control over light emission. We aim to prove nanophosphor-based photonic materials in which the out-coupling of the generate light is maximized. Also, for the first time in the project we will tackle tasks related to the combination of nanophosphors and bidimensional periodic arrays of dielectric or metallic scatterers and we will investigate the coupling mechanisms between emitters and arrays of scatterers that support photonic or combined photonic-plasmonic modes. Finally, we will pursue an experimental demonstration of two paradigmatic cases, which will serve as proof of concept: i) directional white-light emission within a narrow angular cone; ii) omnidirectional emission of monochromatic light.