Phosphor Nanostructures: Synthesis and Applications

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Objectives: The over-arching objective of this proposal is the study and development of yttrium aluminum garnet (YAG)--based oxide phosphor nanopowders as fluorescent materials that are much more stable than either organic or quantum dot alternatives. Goals include gaining a fundamental understanding of fluorescent phosphor nanoparticles with a long term view to primary application areas including fluorescent solar concentrators and microLEDs. Significance: A Fluorescent Solar Concentrator (FSC) can enable future photovoltaic (PV) building windows for the Building Integrated PhotoVoltaic (BIPV) industry. Here a building window collects sunlight and waveguides it to the edges of the window where it is converted into electricity by silicon solar cells. The nanosize aspect allows visible light scattering to be minimized. A 25 year lifespan is dictated by BIPV component standards. This may be satisfied by very stable oxide fluorescent materials, the best known materials being versatile YAG hosts containing rare earth or transition metal activators. Organic dyes and quantum dots are unable to meet these lifespan requirements. The MicroLED offers the promise of a new display revolution in which well--developed inorganic solid state p-n junction light emitters are dramatically scaled down to produce red/green/blue (RGB) display pixels small enough for cellphone displays. The required size of a single colour sub-pixel is ?10µm to enable 60µm RGB pixels in a retina screen cellphone display. Benefits of the microLED include wide colour gamut, mechanical flexibility for fold-up/roll-up displays and a 6--fold power consumption decrease compared to current LCD and OLED screens. Optimized white--emitting nano-phosphors are required for these microLEDs. Approach: We will focus on fundamental but industrially relevant aspects of fluorescent nanoparticles. Investigation of energy transfer between emission centers will be carried out via optical spectroscopy, in which the concentrations and associated emission intensities of the sensitizing atom (Ce) and activators (Mn, Nb) are controlled in both core--only and core--shell YAG nanophosphors. We aim to extend the surface oxidation technique we have recently investigated to create Mn2+-or Mn4+ charge--compensated activators.   Yttrium, aluminum, and cerium all occupy 3+ valence states in the YAG host. When Mn occupies an Al site, there may be a local charge imbalance. The charge imbalance provides a non--radiative pathway for relaxation acting as a luminescence quenching site which should be avoided. The YAG host is well suited as an infrared emitter with rare earth dopant Nd where it is used to absorb near-infrared radiation in the 800nm range with fluorescence at 1100nm. In order to absorb visible light, energy transfer between Ce and Nd will be utilized.