Saturday, April 12, 2014

Ordered arrays of nanoporous gold nanoparticles

SEM images (false color) at 25° tilt of the perfectly
ordered array of the nanoporous gold nanoparticles
formed from the 15 nm Au/30 nm Ag bilayers.
© 2012 Wang et al; licensee Beilstein-Institut.
A combination of a “top-down” approach (substrate-conformal imprint lithography) and two “bottom-up” approaches (dewetting and dealloying) enables fabrication of perfectly ordered 2-dimensional arrays of nanoporous gold nanoparticles. 

The dewetting of Au/Ag bilayers on the periodically prepatterned substrates leads to the interdiffusion of Au and Ag and the formation of an array of Au–Ag alloy nanoparticles. The array of alloy nanoparticles is transformed into an array of nanoporous gold nanoparticles by a following dealloying step. 

Large areas of this new type of material arrangement can be realized with this technique. In addition, this technique allows for the control of particle size, particle spacing, and ligament size (or pore size) by varying the period of the structure, total metal layer thickness, and the thickness ratio of the as-deposited bilayers.

Metallic nanoparticle arrays are attracting more and more attention due to their potential applications in plasmonics, magnetic memories, DNA detection, and catalytic nanowire growth. Nanoporous gold is very interesting for application in catalysi, for sensors, for actuators, and as electrodes for electrochemical supercapacitors. This is due to the unique structural, mechanical and chemical properties of this material. Nanoporous gold, already synthesized in the form of nanoparticles, possesses a much higher surface-to-volume ratio than bulk nanoporous gold films and gold nanoparticles. These nanoporous gold nanoparticles are expected to broaden the range of applications for both gold nanoparticles and nanoporous gold due to their two-level nanostructures (porosity of around 10 nm and particle size of a few hundreds of nanometers).
Solid-state dewetting of metal films is a simple “bottom-up” approach to fabricate nanoparticles. The dewetting of metal films is driven by reducing the surface energy of the film and the interface energy between the film and the substrate, and occurs by diffusion even well below the melting temperature of the film. In addition, alloy nanoparticles can be fabricated by exploiting the dewetting of metallic bilayers. By combining both, “top-down” approaches (such as lithography) and “bottom-up” approaches, an ordered array of metallic nanoparticles can be fabricated. The surface of the substrate is prepatterned into periodic structures by using laser interference lithography, focused ion beam (FIB), or substrate conformal imprint lithography (SCIL). During the dewetting of metal films onto prepatterned substrates, the periodic structure of the prepatterned substrates modulates the local excess chemical potential by the local curvature or by limiting the diffusion paths. This leads to the formation of 2-D nanoparticle arrays with well-defined particle size and particle spacing. Dealloying is a “bottom-up” approach to fabricate nanoporous gold by selectively removing or leaching the element Ag from the Au–Ag alloy in an Ag-corrosive environment. In this paper, perfectly ordered arrays of nanoporous gold nanoparticles are fabricated by using a combination of a “top-down” approach (SCIL) and two “bottom-up” approaches (dewetting and dealloying).

Full paper: