CNMS User Research
Assembly of Patterned Thin Films into Nanoparticle Ensembles
Rack, Yinfeng Guan (The University of Tennessee, Knoxville); Anatoli
Melechko (North Carolina State University); Jason D. Fowlkes and
Michael Simpson (CNMS)
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.
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
shapes had an initially larger retraction velocity which was attributed to
an additional in-plane curvature.
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.
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.
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.
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