Gallium Nitride (GaN) is a semiconductor of broad interest due to its wide direct band gap, high chemical stability, non-toxicity, high breakdown voltage, and high refractive index. GaN has been extensively studied as a bulk thin-film and single crystal, but exploration of small spheroidal nanocrystals, which can exhibit quantum confinement, has been limited due to the high melting point of the material, which makes synthesis difficult. We have recently developed a new gas-phase process for the synthesis of free-standing and high-quality III-V semiconductor nanocrystals with narrow size distributions termed Nonequilibrium Plasma Aerotaxy (NPA). High purity is important for realizing photoluminescence in the GaN material system, and our processes achieves that by using precursors of only elemental gallium source aerosol, nitrogen, and argon. The nanocrystal size can be tuned by controlling the residence time in the plasma. When the nanocrystals are suspended in a solvent such as methanol, photoluminescence at the band gap is observed if the size is smaller than 10 nm in diameter. The photoluminescence can be blue-shifted to shorter wavelengths when nanocrystal diameter is controlled to below 8 nm, an effect which is attributed to quantum confinement. The nanocrystals can also be deposited into a porous film by inertial impaction or drop-casting. Photoluminescence quantum yield of porous films comprised of GaN nanocrystals is low compared to colloidal suspensions, possibly due to an inter-particle quenching mechanism in the film. Strategies to enhance photoluminescence of thin films comprised of GaN nanocrystals, for example by surface ligation, will be discussed. Furthermore, efforts to red-shift the photoluminescence, for example by incorporation of another metal (e.g. In) or pnictide (e.g. Sb) will be presented.
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