The London Centre for Nanotechnology and the National institute for standards and technology report a simple methodology to fabricate complex sub-micrometre periodic structures in poly(dimethylsiloxane) over large surface areas (several cm2). Single-frequency, uni- and multi-axial sinusoidal surface modulations, with tunable amplitude and wavelength, in the nano- to micrometre range, are readily demonstrated. The technique builds upon a buckling instability of a stiff layer supported by an elastomeric membrane (reported earlier), induced by surface oxidation of a pre-stretched elastomer coupon followed by removal of the applied mechanical strain. Plasma oxidation yields model surfaces with single wavelengths, sub-micrometre periodicity, achieving a dynamic range from sub-200 nm to 10s of µm, which UV ozonolysis extends to 100s of µm.
We find that a single ‘dose’ parameter (exposure x power) characterizes the surface conversion. The strain control provides unprecedented tunability of surface pattern amplitude and morphology, ranging from lines to complex periodic topologies induced under multi-axial deformation. We introduce a novel multiple strain–exposure and replication approach that extends surface topologies beyond lines, chevron and spinodal patterns (isotropic structures with a dominant wavelength).
The resulting structures exhibit a glass-like surface, which is easily grafted with self-assembled monolayers to enhance functionality. Applications of this inexpensive and fast methodology include stamps for soft lithography, micromolding, templating and surface patterning.
Figure: Optical micrographs and FFT of surface patterns generated by single and multi-axial strain fields (indicated by schematics) coupled with plasma (a,b) or UVO (c) exposure. Patterns (a,b) are generated by a single exposure step and yield isotropic and chevron topologies (in addition to lines in Fig. 1a). Pattern (c) yields a checkerboard pattern consisting of peaks and saddles, which has been produced by two successive exposure–strain steps. Scale bars are 20 µm.