- 4:35 PM
370e

Thin-Film Deposition on Nanoparticles and Nanowires In Low-Pressure Plasma

Anaram Shahravan and Themis Matsoukas. Chemical Engineering, Penn State University, Department of Chemical Engineering, Fenske Lab, Room 210, University Park, PA 16802

Film deposition on particles is of great interest for several potential applications, ranging from catalysts, to nanocomposites, to sensors. Currently, this is mainly accomplished in wet environments using the tools of colloid science. Plasma deposition, although a well-established technology for microelectronics, has not been successfully adapted to particles. This work presents a new process that extends the usage of plasma deposition to finely dispersed materials such as micron- and sub-micron particles, and nanowires. We accomplish this by taking advantage of electrostatic effects that allow us to suspend particles in the plasma for extended periods of time. This trapping is now understood to involve an interplay of forces whose main component is electrostatic interaction and gravity. While the collective behavior of these so-called “dusty” plasmas have generated theoretical interest, we view these systems as an analogue to a "fluidized" bed with the ability to electrostatically confine micro and nano-particles, much smaller than the size that can be fluidized by conventional methods based on fluid drag.

In this talk we present results of film deposition on silica particles and on nanowires of various sizes in a plasma reactor in parallel-plate configuration driven by RF generator. Particles are introduced into the reactor via hollow stem of the top electrode and they become trapped in the sheath, a few mm above the bottom electrode. Vertical confinement is accomplished by force balance between the weight of the particles, and electrostatic repulsion from the electrode. Horizontal confinement is achieved by introducing asymmetry between the top and bottom electrodes. Various organic gases and vapors are introduced in the plasma and supply the coating material. Under the influence of the plasma, these organic molecules fragment and produce amorphous hydrogenated carbon (a-C:H) which is deposited on the surface of the suspended particles. Monodisperse silica particles of various sizes are studied for their ability to remain confined and to receive uniform deposition. We find that the thickness of the films is a linear function of time and is of the order of a few nanometers per minute. Analysis of the distribution of the deposited films further suggests that the sheath region where trapping occurs is an inhomogeneous environment that exhibits a wide, quasi exponential distribution of deposition rates. While the majority of particles appear to be trapped in regions of low deposition, substantially higher deposition rates are also observed. We discuss the implications of these findings on the feasibility of converting this semi-batch coating process into one that is continuous.