Published Reports:

Manipulating the Size Distribution of Supported Gold Nanostructures

In this report we describe a new processing route which sees substrate-based gold nanoparticle size distributions narrowed and which allows for the fabrication of nanostructured arrays with dimensions on length-scales inaccessible to conventional photo- or e-beam lithography. The route confines gold nanostructures between a metal foil and the oxide substrate upon which they are formed. When heated the surface energy gradient between the oxide and foil results in a net migration of gold atoms from the nanostructure to the foil. With time, the nanostructures show a size reduction and a narrowed size distribution. The narrowing results from the formation of foil contact points with only the largest nanostructures, a characteristic which leaves small nanostructures intact while consuming larger ones. Also demonstrated is the size reduction of arrayed gold structures from sub-micron to sub-50nm length-scales.

	FIG. 1. Schematic of the thermal dewetting procedure used to manipulate the nano-particle size distribution.
	FIG. 2. Evolution in the gold nanoparticle size distribution occurring when a platinum foil is placed over a dewetting film at 1065 °C for various time intervals. The evolution is illustrated through SEM images at time intervals (a) 0 min and (b) 10 min and (c) through the measured size distributions where each is obtained from 1000 nanoparticles binned into 50 nm increments. The line between points is a guide for the eye. Only a small number of nanoparticles were observed for the 20 min time interval.
	FIG. 3. AFM images and cross-sections for (a) a gold particle array with a 12.5 μm periodicity and a particle radius of 350 nm and (b) an identical array after being exposed to a platinum foil at 1065 °C for 2.5 min. Note the dramatic size reduction from 350 to 22 nm which is apparent from a comparison of the cross-sections across two adjacent structures. This corresponds to a thousand-fold reduction in nanoparticle volume.

Reference: A. Sundar, R.A. Hughes, P. Farzinpour, K.D. Gilroy, G.A. Devenyi, J.S. Preston, S. Neretina, “Manipulating the Size Distribution of Supported Gold Nanostructures”, Appl. Phys. Lett. 100, 013111 (2012).

The Templated Assembly of Highly Faceted Three-Dimensional Gold Microstructures into Periodic Arrays

In this report we describe a new templated assembly route which sees the formation of periodic arrays of gold microstructures on oxide substrates. The route, which combines elements of subtractive transfer patterning and templated dewetting, involves the placement of an electroformed nickel mesh in conformal contact with a continuous gold film. When heated, the gold beneath the grid selectively attaches to it due to a surface energy gradient which drives gold from the low surface energy oxide surface to the high surface energy nickel mesh. With this process being confined to areas under and adjacent to the mesh, the underlying gold film eventually ruptures at well-defined locations to form isolated islands of gold which subsequently dewet. Removal of the grid reveals a periodic array of three-dimensional highly-faceted gold microstructures. This templated assembly route is demonstrated for the (100)-, (110)- and (111)-surfaces of MgO and MgAl₂O₄ where it is observed that the faceting is strongly influenced by both the substrate and its crystallographic orientation.

FIG. 1. (a) Schematic of the process used to assemble periodic arrays of intricately shaped gold microstructures. (b) The clamping arrangement used to obtain conformal contact between the gold film and nickel mesh. The molybdenum liner prevented titanium contamination. (c)SEM image of a large-area periodic array of gold microstructures formed on a (100)-MgO substrate.

FIG. 2. SEM images showing the high temperature assembly process induced by a nickel mesh (scale bars = 5μm).

FIG. 3.  SEM images of gold microstructures formed in periodic arrays on (a) MgAl₂O₄and (b) MgO substrates of various orientations (scale bar=1μm). The labeled facets are based on those typically observed in face centered cubic materials.

Reference: A. Sundar, C. Decker, R.A. Hughes, S. Neretina, “The Templated Assembly of Highly Faceted Three-Dimensional Gold Microstructures into Periodic Arrays”, accepted to Mater. Lett.  (2012).