My new project is to build on the graduate work of Dr. Lorenzo Mangolini, who successfully developed a plasma synthesis route to silicon nanoparticles that are small enough to exhibit photoluminescence.
A Silicon particle with a size below about 8 nanometers will photoluminesce upon UV excitation, due to quantum confinement of the electrons in the nanoparticle. Lorenzo's non-thermal, radio-frequency plasma reactor provides an especially fast and reliable technique for creating these nanoparticles. Because Silicon is non-toxic and abundant, luminescent Silicon nanoparticles have many uses in science and technology, particularly in biological applications such as tumor targeting. However, the luminescence efficiency of the nanoparticles decays upon exposure to air due to oxidation.
There are several methods to prevent this decay in efficiency, as investigated by Lorenzo, such as liquid-phase passivation of the particles with a reactant molecule, or a similar passivation technique using a second plasma discharge. However, many of these techniques also lead to the inability of the particles to be individually dispersed in water and common organic solvents. Because the luminescence of the particles necessitates a free-standing particle solution, and because biological applications require nanoparticle compatibility with water and the atmosphere, it is imperative to render the surface of each nanoparticle passive with respect to the atmosphere and to water, while maintaining the particles' ability to remain unagglomerated in solution.
The challenge of this project is to discover ways to maximize the photoluminescence yield of the particles as well as to created colloidal solutions of particles. I intend to continue Lorenzo Mangolini's work on the plasma synthesis of Silicon nanoparticles and his work on liquid- and gas-phase passivation of the particles in order to improve their stability, luminescence efficiency, and versatility across the scientific and technological board.
Previous Graduate Work:
Plasma Synthesis of GaN Nanocrystals
Energy-saving devices are in high demand, and one way to improve the energy-efficiency of lighting techniques and other emissive devices is in the use of semiconductor materials. Gallium Nitride is an efficient, direct-bandgap semiconductor that shows great promise for use in high-brightness LEDs that emit in the near-UV range, solid-state lasers, and other optical devices because of the thermal and temporal stability of its emission. Nanoparticles of GaN are interesting because they may show improvements in emission quality; furthermore reduction of GaN particle size makes them useful in emerging nanotechnological devices and possibly in photovoltaics. By doping with Indium, the nanoparticles' emission can be tuned to represent different parts of the spectrum, extending the uses into general lighting purposes. Furthermore, nanoparticlulate GaN may serve as a desirable substrate for growth of larger-scale GaN, since low-cost and lattice-matched substrates for this material are hard to find.
My goal for the first year of my graduate studies was to use a non-thermal plasma synthesis route to fabricate nanocrystals of GaN. This quick and inexpensive method has been shown effective for Silicon nanocrystals1 and may have potential for the synthesis of Gallium Nitride. I made polydisperse nanocrystals in the size range 5-50 nanometers. The challenge came in improving the quality, size dispersion, and photoluminescence crystals. Though these methods showed that GaN crystals were made, the particles didn't exhibit high-quality photoluminescence spectra.
1Mangolini, L. et al. Nano Letters, 5 , 4 (2005) 655-659