Atmospheric Plasma Deposition (APD) is an open air, atmospheric pressure chemical vapor deposition technique. We use an atmospheric pressure plasma discharge to fragment and subsequently polymerize small molecules into highly transparent and functional thin films. We constantly develop new processes for the growth of a wide range of materials, including silica, metal oxides, nitrides, and polymer coatings, onto both organic and inorganic substrates. A combination of plasma diagnostics and thin film characterization allows us to understand the reaction pathways, providing a feedback loop which helps us optimize our chemistries. Our research effort currently takes advantage of two distinct types of plasma discharge, each offering unique advantages.
Capacitively Coupled Plasma (CCP)
The CCP plasma source is sustained at a Radio Frequency (RF) of 27.12 MHz, allowing for the dissociation of helium gas. A small amount of an electronegative gas (oxygen, nitrogen, etc) is added to mitigate the formation of discharge filaments, maintaining the homogeneity of the plasma. Additionally, the secondary gas generates reactive species like atomic oxygen, ozone, and nitrous oxides. When these reactive species interact with small molecules, bond scission and reformation occurs in such a way as to produce a coherent network. The major advantage of this reactor style is the relatively low operating temperature (< 80 ºC) allowing for the process of thermally sensitive substrates.
We have developed a number of materials to functionalize plastic surfaces. By adjusting the plasma power, precursor concentrations, and deposition geometry, we are able to control the fragmentation process. For instance, if there is an ethylene bridge that connects two organosilicate centers, we are able to selectively protect or dissociate this connection. When protected, we can deposit incredibly adhesive and tough silica due to the added plasticity of the carbon bridge. By adjusting the plasma conditions, we break this bridge, and create a very hard and dense network. Taking advantage of this phenomenon, we can synthesize hybrid silica structures which exhibit both high adhesion and hardness.
We have given a considerate amount of attention to developing recipes for metal oxides as well. As we move into the age of flexible electronics, the ability to coat and functionalize plastic substrates is becoming increasingly important. Taking advantage of the low operating temperature, we have successfully grown conductive and transparent oxide coatings, including ZnO and hybrid TiN/TiO systems on plastic substrates. More recently, we have developed routes for the deposition of smooth optical coatings; including both high and low index materials. In particular, we are interested in ways to modulate the refractive index and absorption of a material through the manipulation of plasma conditions.
Dielectric Barrier Discharge (DBD)
We have recently expanded our synthesis capabilities by bringing a DBD plasma reactor on-line. Unlike the CCP plasma which uses an RF voltage, DBD style reactors use a DC pulse to ignite the plasma. The consequence of this difference is that we can use a wider variety of gases when compared to the CCP reactor, including argon, nitrogen and compressed air. There are obvious benefits for scalability if you can use a naturally abundant gas like compressed air to facilitate a coating process. As such, we are currently investigating chemistries which take advantage of the natural abundance of both nitrogen and air. Please stay tuned as we update our progress on our venture.
Current Research Projects
- Atmospheric Plasma Deposition of highly adhesive and hard silica coatings
- Atmospheric Plasma Deposition of metal oxide materials