Nanofibrillar surfaces via reactive ion etching

The reactive ion etching of poly(ethylene terephthalate) using various gas compositions was examined. A radio frequency power supply operating at 30 kHz was used to produce plasmas from the following gases and 50:50 gas mixtures: Ar, N-2, O-2, Ar-N-2, Ar-O-2, and N-2-O-2. Exposure times were typically 10, 20, and 30 min. The relatively inert gases and gas mixtures (Ar, N-2, Ar-N-2) produced a polygonal pattern of protrusions surrounding shallow cavities. In contrast, the presence of oxygen or oxygen-containing plasmas invariably imposed a fibrillar structure on these polygons in which "nanofibrils" typically originated at the triple points of several neighboring polygons. The dimensions of these fibrils varied with the exposure time but were up to 300 nm in length and approximately 20 nm in diameter. Atomic force microscopy was used to quantify the surface roughness and show that the inert gas compositions (Ar, N-2, Ar-N-2) produced statistically indistinguishable values of R. (14.1 +/- 1.7, 15 +/- 2.5, 14.2 +/- 2.9 nm) significantly larger than those of the as-received film. The pure oxygen-etched films have R-a values approximately twice as large as those of the other gas compositions. Prior work by Nie et al. suggests that films consisting of etching-resistant, low-molecular-weight fragments can form under these conditions. We find that surface physics normally associated with thin polar films on apolar substrates adequately describes the origin of the observed nanofibrils. Prior surface deformation has an additional influence on fibril spacing and density.