A self-assembled nanopattern with near-perfect order

To pattern ever-smaller microprocessors and memory devices, materials scientists look to exploit molecules that can self-assemble into useful nanostructures with little outside intervention. For example, some diblock copolymers—long-chain molecules made up of two chemically different subunits—naturally phase separate into structures that have precise, nanometer-scale dimensions. A diblock copolymer made up of polyethylene oxide and polystyrene, for instance, may form wormlike, PEO microdomains dispersed in a PS matrix. The PEO can then be chemically etched away and replaced with metal to create nanowires or magnetic storage elements. Normally, the microdomains adopt an irregular pattern such as the one seen in the scanning force microscope image at top. To be useful for many devices, however, their arrangement needs to be regular and controllable. In new experiments, Thomas Russell (University of Massachusetts Amherst) and an international team of colleagues coaxed a PS–PEO diblock copolymer to form the well-ordered pattern at bottom. They did so by coating the film on a ridged sapphire substrate; due to entropic effects, the PEO microdomains spontaneously align orthogonally to the sapphire’s ridges. What’s more, the self-assembly adopts the near-perfect arrangement even when the underlying sapphire is marred by defects. Russell anticipates that his team’s new strategy can be used to fabricate ordered patterns with features as small as 3 nm. (S. W. Hong et al., Proc. Natl. Acad. Sci. USA, in press.)—Ashley G. Smart