A bifractal nature of reticular patterns induced by oxygen plasma on polymer films.

Junwan Bae,I J Lee

doi:10.1038/srep10126

Abstract

Plasma etching was demonstrated to be a promising tool for generating self-organized nano-patterns on various commercial films. Unfortunately, dynamic scaling approach toward fundamental understanding of the formation and growth of the plasma-induced nano-structure has not always been straightforward. The temporal evolution of self-aligned nano-patterns may often evolve with an additional scale-invariance, which leads to breakdown of the well-established dynamic scaling law. The concept of a bifractal interface is successfully applied to reticular patterns induced by oxygen plasma on the surface of polymer films. The reticular pattern, composed of nano-size self-aligned protuberances and underlying structure, develops two types of anomalous dynamic scaling characterized by super-roughening and intrinsic anomalous scaling, respectively. The diffusion and aggregation of short-cleaved chains under the plasma environment are responsible for the regular distribution of the nano-size protuberances. Remarkably, it is uncovered that the dynamic roughening of the underlying structure is governed by a relaxation mechanism described by the Edwards-Wilkinson universality class with a conservative noise. The evidence for the basic phase, characterized by the negative roughness and growth exponents, has been elusive since its first theoretical consideration more than two decades ago.

Highlights

The roughness exponent, α, the dynamic exponent, z, and the growth exponent characterizing the short time behavior of the surface, β, define the universality class of a system under investigation
While the root-mean-square roughness of the polymer surface determined by Wrms = [h (x) − h ]2 1/2 increases with a small power-law of 0.10 ± 0.01, the static contact angles formed by a drop of 7 μL deionized water on the polymer film are drastically reduced to 20 degree and remain nearly the same above t = 300 s
While the polymer films are etched away in the rate of 1.1 nm/s under the oxygen plasma, the generation and desorption of short polymer chains remain in equilibrium such that the total number of short chains on the surface is conserved, which seems compatible with conservative characteristics of the growth dynamics

Summary

Data and Discussions

The q-dependent qth order scaling exponents displayed in inset of Fig. 5 support the bifractal behavior with a transition occurring near q = 3, which is qualitatively different from the multifractality observed in the native polymer films during the continuous growth regime[23] in which γ(q) is obtained as a continuously varying function of the moment q. The UV energy and oxygen species effectively break the polymer chains and terminate the chain-ends with the carbonyl group, resulting in low molecular weight materials, some of which are eventually desorbed under vacuum Those remaining on the surface produce the nano-sized protuberances rich in polar molecules through diffusion and an aggregation process. While the polymer films are etched away in the rate of 1.1 nm/s under the oxygen plasma, the generation and desorption of short polymer chains remain in equilibrium such that the total number of short chains on the surface is conserved, which seems compatible with conservative characteristics of the growth dynamics

Conclusions

Experimental methods

Additional Information