Directed Assembly of Patterned Thin Films into Nanoparticle Ensembles

Philip Rack, Yinfeng Guan (The University of Tennessee, Knoxville); Anatoli Melechko (North Carolina State University); Jason D. Fowlkes and Michael Simpson (CNMS)

Achievement
Predictable and repeatable directed-assembly of thin nickel films into ensembles of nanoscale particles was enabled by using electron beam lithography and pulsed laser heating to define and treat thin nickel films of various shapes. The edges and vertices of the lithographically defined nickel films acted as programmable instabilities that drive assembly via dewetting when the laser energy is above the melting threshold.

The pattern formations were monitored as a function of laser pulse and the retraction process was attributed to liquid dewetting and a subsequent resolidification. The calculated retraction velocity 83 m/s and liquid lifetime, ~12 ns, were consistent with the measured nickel retraction distances. The vertices of the shapes had an initially larger retraction velocity which was attributed to an additional in-plane curvature.

Significance
While the break-up and pattern formation via dewetting of continuous thin metal and polymer films has been studied in detail, less work has been devoted to the dewetting and pattern formation of confined or patterned thin films. The edges of the patterned thin film give rise to programmable instabilities which can be useful for the directed assembly of materials. In this work we demonstrate the directed assembly of patterned thin nickel films via nanosecond pulsed laser processing. The short liquid lifetimes offer a unique way to monitor the time dependence of the dewetting process and the subsequent pattern formation. The laser energy density was beyond the melt threshold for the nickel films thus the liquid fronts have been correlated to the dewetting of the films during the short liquid lifetime. The lateral retraction and pattern formation was correlated to a two step process: 1) initially, the surface tension drives the flow of the melted nickel films and 2) a smaller contraction associated with the density difference between the liquid and solid when the liquid film solidifies.

Nanoscience requires the exploration of new methods for manipulating materials below the limits of conventional lithographic techniques. Directed assembly, enabled by a combination of advanced electron beam lithography and a more thorough understanding of patterned metal film dewetting provides a path for achieving control below those limits.

Publication:
P. D. Rack, Y. Guan, J. D. Fowlkes, A.i V. Melechko, and M. L. Simpson, "Pulsed Laser Dewetting of Patterned Thin Films: A means of Directed Assembly," Appl. Phys. Lett. 92, 223108 (2008).

Research supported by the Division of Materials Sciences and Engineering and through a user project at the Center for Nanophase Materials Sciences which is supported by the Division of Scientific User Facilities, U. S. Department of Energy.

Figure 1 Scanning electron micrographs of pulsed laser treated thin nickel patterns. The top images are the initial thin film circle, square and triangle. Subsequent images in each column are after 1, 2, 3, 5, and 10 pulses. The bottom image is a tilted view of the pattern after 10 laser pulses. The dashed lines on the top square and triangle illustrate an axis of the lateral contraction from the vertices and the solid lines demonstrate the axes from the center of the edges.

Figure 2 Measured edge-to-center retraction distances as a function of the number of laser pulses for the circle, the edge center and vertex of the square, and the edge center and vertex of the triangle see dashed and solid lines the figure above for clarification . Inset is an enhanced contrast image of the circle after 3 pulses demonstrating the two-ring signature for each laser pulse.