Mechanical Behavior and Microstructure of Nanoporous Gold Films
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0924-Z01-02
Mechanical Behavior and Microstructure of Nanoporous Gold Films Ye Sun, and T. John Balk Chemical and Materials Engineering, University of Kentucky, Lexington, KY, 40506
ABSTRACT Nanoporous gold (NPG) thin films offer an opportunity to investigate the effects of nanoscale geometric confinement on the mechanical properties of metals. In the present study, NPG films supported by substrates were fabricated by dealloying Au-Ag films on Kapton and silicon. The microstructural evolution of NPG at various stages of dealloying was observed and analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cracking occurred at grain boundaries during dealloying and may be exacerbated by pre-existing tensile stress in the film. Stress evolution of NPG films on silicon substrates during dealloying was measured with the wafer curvature technique and revealed an overall shift toward compressive stress. INTRODUCTION Nanoporous gold can be produced by a selective dissolution process called dealloying, during which the less noble atoms (e.g. Ag) are dissolved from a precursor alloy (e.g. Au-Ag), leaving behind a nanoscale, interconnected network of open pores. Free-standing NPG film can be made by simply floating white gold leaf (Au-75at%Ag) onto nitric acid [1], while bulk Au-Ag can be electrochemically dealloyed in an electrolyte to obtain thicker NPG samples [2-4]. Both forms of NPG have significant disadvantages: free-standing NPG films are extremely difficult to handle or mechanically test, while bulk NPG can suffer from an unknown transitional structure to the underlying Au-Ag if the starting material is excessively thick or not completely dealloyed. Thin film NPG supported by a substrate offers a way of stabilizing the NPG film, while assuring that the dealloyed structure is uniform through the film thickness. Stresses are usually present in thin films on substrates because of differing thermal expansion coefficients for the film and substrate, non-equilibrium growth of the film, or coherent interfaces between them [5]. Also, if a film experiences a volume change during fabrication or subsequent processing, the film has a tendency to shrink or expand, which can cause stresses to develop. During the production of NPG films, silver atoms are removed as part of the dealloying process, leading to a slight volume reduction that can generate stresses in the NPG structure. In the present study, we deposited Au-Ag films onto different substrates, including Si wafers coated with silicon nitride, glass slides and Kapton. A gold interlayer was deposited between AuAg films and their substrates to maintain film adhesion after dealloying. Microstructural and stress evolution of NPG were studied systematically at various stages of the dealloying process.
EXPERIMENTAL DETAILS Au-Ag films (72at%Ag) approximately 130 nm thick were sputtered onto different substrates at room temperature under ultrahigh vacuum conditions (AJA ORION system, base pressure better than 10-8 mbar). Substrates included glas
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