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Phosphors and Display Materials
Our general research theme focuses on materials that emit light (UV, visible or IR) when excited by different forms of incoming energy. Some of the current research areas of interest to the group are listed below.
Cathodoluminescent phosphors. These are materials that can be stimulated by electrons to emit light. Of particular current interest are new phosphors compatible with field emission displays, (FEDs), which may offer an alternative to LCD flat panel display technology. Recently we completed a DTI funded project to produce new phosphors for FED usage in collaboration with Printable Field Emitters Ltd, Phosphor technology, North East Wales institute and Surrey University.
FED display image using our red phosphor
Nano-phosphors, although we believe very small phosphors do not give optimised luminous efficiency we have developed methods for preparing highly uniform particle morphology and size. This is important for light extraction and particle deposition.
Cathodoluminescence from 300nm
spherical red emitting FED phosphor particles.
Up-conversion and anti-Stokes phosphors. These materials emit light of a higher energy than they absorb. E.g. they may absorb IR light and emit green light. This is achieved by the absorbance of two or more photons followed by the emission of a photon of higher energy. These materials are of interest in security applications and communications.
Compatibility with modern deposition methods (e.g. ink jet printing). Classical phosphor synthesis produces particles in the micrometer size range. We have successfully mastered synthetic procedures for reducing this size to facilitate the use of precise deposition techniques.
Coatings for optimised lifetime and luminescence. Some emissive materials are intrinsically unstable and we have an interest in developing methods for abating their decay.
Alternating Current Electroluminescence (ACEL). Blue and green emission is possible with commercially available phosphors with a lifetime of about 2000-3000 hours at reasonable brightness in subdued lighting. There is no good method of producing a red ACEL phosphor. Commercial devices are produced by screen printing layers onto ITO coated plastic.
1 top electrode dielectric layer 3 phosphor layer
4 transparent electrode 5 electrode support
Generic ACEL device structure
Our interests lie in the production of new electroluminescent phosphors with improved lifetime and colour gamut.
We are also involved in developing our patented formulation that combines layers 2 and 3 thereby reducing production time and increasing productivity. Another form of powder electroluminescence is direct current EL (DCEL). This technology was pioneered in the group. It manifests high brightness. The emission is bright orange and can be made into simple and X,Y addressed displays.
Photonic phosphors. In this branch of materials engineering 3D periodic structure is given to a material. The one shown below contains regularly ordered comprises spherical holes approximately 300nm in diameter. This introduces an optical band gap and allows light emission from the phosphor to be modified.
Electron micrograph of a photonic phosphor, showing ~300nm diameter voids within the lattice.
In the field of solid state lighting the Centre is using Stokes phosphors (down converting photoluminescent materials) excited by blue light to produce a range of other desired colours. White light can be generated using blue LEDs and yellow emitting phosphors. There is a drive to make LEDs that emit at shorter wavelengths (in the UV) that require the development of phosphors to convert these wavelengths into viable light. Through a TCS programme we have developed a head up display using this approach. This is now being marketed by SemeLab for the automotive industry.
Helmet mounted prototype HUD
Schematic of HUD in a car
Phosphors for new developments in fluorescent lighting. We currently have a DTI /industrially funded programme to develop very bright backlights for LCD TVs.
Current collaborators include:
Kodak, Pelikon Ltd., Forge Europa Ltd., Semelab Ltd., LG Philips Displays, Masonlite Ltd., Patterning Technologies Ltd., Keeling & Walker Ltd., Oxley Developments Company Ltd., Advanced Technology Coatings Ltd., and Elumin8 Ltd. The academic partners involved in these are the Materials Departments of the University of Cambridge and Queen Mary College (London University), the Materials Engineering Dept. of the University of Loughborough, the School of Computing and Informatics at Nottingham Trent University, and the Centre for Water Soluble Polymers at North East Wales Institute of Higher Education.
The Centre offers PhD and MSc research programmes in all areas of Materials processing, phosphor materials and materials characterisation.
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