(524c) Fabrication of Double-Gyroid Structure Nanowire Arrays
Highly ordered nanoporous films that provide accessibility to an underlying electrode are a key cornerstone of nanofabrication. With this morphology, the pores can be filled by electrodeposition to yield nanowire composites or free standing nanowires after the removal of the original template. Fabrication technologies based on both anodic oxidation of aluminum and orienting assembled block copolymers have advanced rapidly and are now widely used to generate films with controlled pore sizes below 50 nm that directly access the substrate. However, for pores an order of magnitude smaller, similar milestones have not been reached. This is an important size range though since the surface area increases dramatically as pore size decreases (at constant void fraction) and many quantum size effects are only observed when the length scale is less than the thermal de Broglie wavelength (which is typically less than 10 nm). Here, we will report the synthesis of highly ordered nanoporous films on electrode surfaces and their successful use to fabricate both nanowire composites and free standing nanowire structures with 4 nm diameter. In particular, we will discuss the synthesis of nanowire structures that have a well-defined ?double gyroid? nanostructure, defined by the zero mean curvature G-surface. The mathematical description of this surface was first reported by Schoen , but it was first suggested that it may be a good model of self-assembled materials by Scriven in 1976 . Nanoporous powders with this structure have been synthesized by several techniques, but nanoporous thin films have proved more difficult. Only two reports are known[3, 4]. We developed a new robust synthesis of highly ordered and oriented thermally stable mesoporous silica films that have the topology of the cubic Ia3d double gyroid phase. The electrochemical accessibility of the underlying electrodes have been studied by electrochemical impedance spectroscopy and show that roughly 30% of the electrode area is accessible via the nanopores. Electrochemical deposition of platinum yields platinum nanowire composites. The silica template may be removed to yield free standing platinum nanowire structures with long-range order that is the negative replica of the porous double-gyroid phase . Further, the method is shown to be general as we have replicated the results with several other metal and semiconductor materials. Both the nanoporous template and the resulting nanowire structures have been characterized by high resolution FESEM imaging, TEM imaging and simulation, and GISAXS collection and simulation (using the distorted wave Born approximation) .
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