Coating Growth on Nanofibers: Multi-Scale Modeling, Simulations and Experiments A. Buldum*, C.B. Clemons**, E.A. Evans***, K. Kreider**, G.W. Young** * Dept. of Physics, Buldum@uakron.edu, **Dept. of Theoretical &Applied Mathematics, *** Dept. of Chemical Engineering The University of Akron, Akron, OH 44325 ABSTRACT This investigation focuses on the coating of nano- tubes and nanofibers with conductive materials using plasma enhanced physical vapor deposition. We ex- amine experimental procedures for coating electrospun polymer nanofibers with metallic materials, then dis- solve the inner polymer core to yield a nanotube of the coating material. The interrelationships among process- ing factors is investigated from a detailed modeling ap- proach that describes the salient physical and chemi- cal phenomena. Solution strategies that couple contin- uum and atomistic models are used. At the continuum scale we describe the reactor dynamics and deposition of the coatings on the nanofibers. At the atomic level, we use quantum mechanical (QM) and molecular dynamics (MD) simulations to study the deposition mechanisms and migration of atoms in the coating. Keywords: nanofiber, coating, modeling 1 INTRODUCTION Nanotubes have attracted great academic and indus- trial interest in recent years [4]–[6], [13], [14]. Improve- ment in the ability to synthesize nanotubes of different materials has resulted in the suggestion and develop- ment of novel devices based on the properties of the nanotubes [4]. Possible applications for nanotubes in the areas of filtration [3], composites [7], [9], biomedicine [2], [12], and electronics [5] have been suggested. How- ever, several limitations to the widespread synthesis and use of nanotubes can be identified. First, the ability to produce large quantities of nanotubes with controlled electronic and structural properties is still undeveloped. Second, the nanoscale dimensions of these materials of- ten lead to previously unobserved properties that need to be understood and ultimately controlled. 2 EXPERIMENTS This work addresses some aspects of these issues thr- ough a coordinated experimental and modeling program. From the standpoint of nanotube synthesis, we examine physical vapor deposition techniques for applying con- ductive coatings to electrospun polymer nanofibers. We have successfully coated fibers with carbon, copper, and aluminum films by using a plasma enhanced physical va- por deposition (PEPVD) sputtering process (see Figure 4). The power supply drives a 2 inch diameter electrode which forms the target (or source) material. The nano- fibers are placed on a holder that sits 8 cm above the target. A plasma is formed when electrons emitted from the target create ions in the gas phase. Once a plasma is formed, the ions sputter atoms from the target which are then transported to the nanofibers and deposited. The ions also strike the coated nanofibers and tend to make the deposited coating more uniform through a re- sputtering process. The coating growth rate depends on the rate at which atoms are supplied to the nanofiber surface, the nanofiber temperature, and the ion flux to the nanofiber. The morphology of the coating depends on the mobility of the atoms on the surface and how much time the atoms have to move around before the next atoms hit the surface. The rate at which atoms are supplied to the nanofiber is proportional to the rate at which atoms are sputtered from the target and how far away the nanofiber is from the target. The sputtering rate depends on the ion flux, which is determined by the power applied to the target, the pressure of the system, and the working gas used. The nanofiber temperature is controlled using a heater. The ion flux to the nanofiber is controlled by the potential drop between the plasma and the nanofiber, the working gas used and the pres- sure. To determine the effects of these variables on the film growth rate and morphology we analyze the films using TEM. TEM analysis is used to determine the growth rate on the fibers. We compare average thicknesses of the fibers before and after the coating process to deter- mine an average growth rate of the films. To determine coating morphology, TEM images and diffraction pat- terns are taken. Removing the nanofiber core leaves a polycrystalline nanotube of the coating material. Figure 1 shows an aluminum-coated fiber. The cylin- drical cross-section of a tube is shown in Figure 2, which indicates that the tube did not collapse after the poly- mer inside had been removed. The smallest inner diam- eter of the tubes was around 20 nm. The approximate thickness of the wall of the tubes was controlled by the sputtering process. A tube with different wall thickness NSTI-Nanotech 2004, www.nsti.org, ISBN 0-9728422-9-2 Vol. 3, 2004 346