Metal sulfides are currently under intense study as alternates to silicon and germanium semi-conductor materials. Metal sulfide nanocrystals (e.g., PbS, CdS, etc.) have typically been synthesized in hot solvent-solution phase systems that bring with them a host of quality and productivity issues. Current work at the University of Minnesota seeks to avoid these problems by utilizing a non-thermal plasma reactor and deposition system for nanocrystal production.
Prior process and issues
Using a hot solvent systems, reactants are injected into the solvent where the nanocrystals form. Once formed, the nanocrystals must be cast onto a surface. The solvent system is prone to inclusion of impurities in the crystals, and it is difficult to scale up to achieve high through-put. In addition, the solvent eventually requires disposal as a hazardous waste, thus adding to the cost and inefficiency overall. In preliminary work at the University of Minnesota, creation of metal sulfide nanocrystals using a non-thermal plasma process, with subsequent crystal deposition, has proven to be a viable alternative for the problematic hot solvent approach.
The technology
The first set of experiments were focused on the production of zinc sulfide, a wide band gap semiconductor that has been used as a buffer layer in thin film solar cells and as a surface passivization shell for semiconductor quantum dots. The experimental set-up utilized a tubular glass chamber surrounded by a Radio Frequency (RF) generator that is used to create the non-thermal plasma.
Argon is used as the carrier gas for the plasma-based reaction. In separate feed systems, the argon carrier was infused with elemental sulfur vapor and diethyl zinc vapor. Elemental sulfur was chosen over hydrogen sulfide for toxicity (of the H2S) reasons. Once formed, the particles are deposited on an outlet filter of silicon.
Preliminary results
The apparatus has been shown to produce single crystal nano particles that are stable for months. Crystalline ZnS particles with 2-10 nm diameters were synthesized. Electron microscopy confirmed that the particles were single crystals. Reaction times of 10ms were observed and production rates of 250mg/hr with 25% mass yield were achieved in the small (8 cc) bench scale reactor.
Because of the nature of the feed system and reaction process, the size and composition of the nanocrystals are controllable. Flow rates (residence times), system pressures and feed ratios, and compositions are all factors, and the system exhibits a wide process window.
The nanocrystals can be deposited on substrate surfaces directly or formed into stable nanocrystal dispersions. It is believed that this is the first production of metal sulfide nanocrystals using a non-thermal plasma process.
Future Directions
Utilization of other single organometallic constituent feeds including iron, tin, and copper have yielded preliminary positive results. Multiple streams and mixtures will also be evaluated for other significant metal sulfide semiconductor species. For additional information, contact Elijah Thimsen.