Structured metal films on silicone elastomers

The ability to control surface morphology at nano- and micrometre length scales is required in many applications. It has previously been demonstrated that surface wave patterns are spontaneously generated when heated, layered metal-on-silicone materials are cooled, because of the stress that builds up between two materials of different modulus and coefficients of thermal expansion. We have examined the behaviour of metals sputtered on silicone elastomer surfaces and found that, unlike previous reports, the patterns are encoded by the first 5 nm of the coating layer. While gold coatings did not induce wave formation, platinum and tungsten led to single wavelength (peak to peak distance between adjacent waves) patterns, while chromium and carbon gave surfaces with two strikingly different wavelengths (0.5 μm to 4 μm). Amplitudes (difference between wave peak and trough, 8 nm to 950 nm) of the wavelengths correlated reasonably well with coating thickness. Key to controlling the patterns is the fact that only a very thin upper layer of the silicone elastomer becomes heated during the sputtering process.