Thin Films of AuCuAl Shape Memory Alloy for Use in Plasmonic Nano-actuators
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Thin Films of AuCuAl Shape Memory Alloy for Use in Plasmonic Nano-actuators. Vijay Bhatia1, Gordon Thorogood2, Annette Dowd1 and Michael B. Cortie1 1 Institute for Nanoscale Technology, University of Technology Sydney, PO Box 123, Broadway NSW 2007, Australia 2 Institute of Materials Engineering, Australian Nuclear Science and Technology Organisation, PMB 1, Menai NSW, 2234, Australia ABSTRACT We describe the fabrication and structure of nanoscale thin films of ȕ phase shape memory alloys with the nominal atomic stoichiometry Au7Cu5Al4 (corresponding to 5.8 wt% Al). These alloys possess properties that suggest they could be used in nanoscale actuators. The films described here are between 20 and 50 nm thick which is below the thickness at which some other shape memory alloys cease to transform. However, microstructural and X-ray studies confirm that the coatings still exhibit the displacive transformations that are a prerequisite for the shape memory effect. INTRODUCTION Shape memory alloys (SMAs) possess the ability to revert back to an initial shape after being deformed. This is achieved through a reversible, displacive phase change in which the atom positions are shifted in a diffusionless process from an austenitic structure to a martensitic structure, or vice versa. ȕ phase shape-memory alloys with stoichiometries in the vicinity of Au7Cu5Al4 exhibit high X-ray and electron absorption, are reasonably resistant to oxidation, have convenient transformation temperatures in the bulk form, and have a dielectric function suitable for the generation of a moderately strong localized surface plasmon resonance. Therefore, we have sought to fabricate nanoscale opto-mechanical actuators from this material. Here we discuss the deposition of nanoscale films of this material and their characterization. Au7Cu5Al4 (sometimes called ‘Spangold’ [1]) is a less well-known example of a SMA. It has been shown to be relatively resistant to aging compared to its copper-based counterparts and has been previously studied in its bulk form [2-12]. However no work has been done on any thin film or nanostructured properties. On the other hand, the TiNi SMA (‘Nitinol’) has been extensively studied over a wide range of size scales and for use in many different applications. Use of TiNi in nano-actuators has been limited to date, due to problems experienced when producing films of less than 100 nm thickness [13, 14]. Oxidation is one of the factors that limits the shape memory effect (SME) in very thin films of TiNi because it changes the composition of the alloy to the point where it is no longer capable of exerting an SME. In contrast, Au7Cu5Al4 is quite resistant to oxidation due to its high gold content and is therefore a good candidate for the fabrication of nanoscale thin films. The other advantage of Au7Cu5Al4 is its optical properties, as mentioned above. The plasmonic heating that would accompany a localized surface plasmon resonance in a suitably designed nanostructure could be used to drive the actuator through its martensite to aust
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