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We acknowledge fruitful discussions with P. Feulner and help with
sample preparation from R. Rivers. The experimental work is
supported by the Office of Assistant Secretary of Defense for
Research and Engineering through a National Security Science and
Engineering Faculty Fellowship (NSSEFF), with additional funding
from the Defense Advanced Research Projects Agency PULSE
program through grant W31P4Q-13-1-0017. The W. M. Keck
Foundation and the Department of Energy under contract
DE-AC03-76SF00098 are acknowledged for additional experimental
equipment. S.R.L. and D.M.N. acknowledge Multidisciplinary
University Research Initiatives from the Army Research Office
(WN911NF-14-1-0383) and the Air Force Office of Scientific
Research. S.A.S and K. Y. performed computations at the Institute
of Solid State Physics, University of Tokyo under Japan Society
for the Promotion of Science KAKENHI grants 23340113 and
25104702. M.S. was supported by a Marie Curie International
Outgoing Fellowship (FP7-PEOPLE-2011-IOF). C.D.P. and D.P.
performed work at the Molecular Foundry, supported by the
Office of Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under contract DE-AC02-05CH11231,
with computing resources at Lawrence Berkeley National
Laboratory and the National Energy Research Scientific
Computing Center (NERSC).
Materials and Methods
Figs. S1 to S16
22 August 2014; accepted 10 November 2014
Large-scale nanoshaping of
ultrasmooth 3D crystalline
Huang Gao,1,3 Yaowu Hu,1,3 Yi Xuan,2,3 Ji Li,1,3 Yingling Yang,1,3
Ramses V. Martinez,4,5 Chunyu Li,3,6 Jian Luo,6,7 Minghao Qi,2,3 Gary J. Cheng1,3,8†
We report a low-cost, high-throughput benchtop method that enables thin layers of metal
to be shaped with nanoscale precision by generating ultrahigh-strain-rate deformations.
Laser shock imprinting can create three-dimensional crystalline metallic structures as
small as 10 nanometers with ultrasmooth surfaces at ambient conditions. This technique
enables the successful fabrications of large-area, uniform nanopatterns with aspect ratios
as high as 5 for plasmonic and sensing applications, as well as mechanically strengthened
nanostructures and metal-graphene hybrid nanodevices.
Nanoscale metallic structures and their pos- sible uses are under investigation in a variety of fields such as plasmonics (1), electronics (2), and biosciences (3). However, the large- scale manufacture of such structures with
high fidelity and quality [e.g., ultrasmooth sur-
faces, sharp corners, three-dimensional (3D) shapes,
and high crystallinity] represents a substantial
challenge. Although nanoimprint lithography is
a useful method of fabricating nanometer-scale
patterns on polymers (4, 5) and has been adapted
to metallic glasses (6), direct nanoimprinting of
crystalline metals is still generally infeasible be-
cause of the limitations on formability arising
from (i) fluctuations of plasticity at the nanoscale
caused by localized dislocation bursts (7, 8),
(ii) size effects in plasticity (9, 10), and (iii) grain
size effects, which generally limit the feature size
to be larger than the grains (11, 12). These limita-
tions can be circumvented by using nanocrystal-
line or amorphous metals (6), heating the sample
close to melting temperature (13), or using a
superhard mold (14). But these methods either
place limits on the materials that can be used, or
operate at high temperatures with serious draw-
backs [e.g., the heating and solidification cycle in
hot embossing (14) or melting (15) leads to high
surface roughness, especially in sub–100-nm me-
tallic structures]. Moreover, the low crystallinity
in metallic glasses limits their functionality be-
cause of electric and magnetic losses (16). Sim-
ilarly, fabrication of metal nanopatterns by direct
nanoimprinting of nanoparticles (17) incurs size
effects of the particles during their deposition
into the nanomolds, and is therefore not a good
choice for ultrafine patterns. Currently, fabrica-
tion of metallic nanopatterns relies on time-
consuming, multistep approaches consisting of
electron beam lithography (18, 19) or template
stripping (20, 21). However, high resolution, high
crystallinity, and low sidewall roughness cannot
be achieved simultaneously even with state-of-
Figure 1A summarizes the most common direct shaping processes, materials, ranges of their
respective processing strain rates, and processing
temperatures. A key barrier that impedes the
fabrication of smooth ultrafine crystalline metallic 3D nanostructures is the high strain rate required to activate superplasticity in metals, due
to the need of ultrafine grain/particle sizes to
generate superplastic flows (22, 23). Unfortunately, the existing imprinting methods cannot provide
enough high strain rates in order to generate
ultrafine 3D metallic crystalline nanostructures.
We demonstrate “laser shock imprinting” (LSI),
a cost-effective direct nanoshaping method for
high-throughput fabrication of smooth 3D crystalline nanostructures at ambient conditions. LSI
uses a laser shock to compress metallic sheets
into a silicon nanomold with a variety of shapes.
We have used the nanomolds for over a hundred
times with no damage or degradation in performance (fig. S1). This technique enables large-scale
direct fabrication of smooth ultrafine metallic nanopatterns as small as 10 nm for mass production
(fig. S2). LSI uses ultrahigh–strain rate deformation at room temperature to overcome the limitations in nanoscale formability of coarse-grained
metals. Additionally, LSI can be applied to a broad
range of metals, including metals as hard as Ti
Figure 1B schematically illustrates the LSI
process. We used a Nd: YAG laser pulse (0.3 to
1.4 GW/cm2, wavelength 1064 nm, pulse duration
~5 ns) to irradiate an ablation coating layer
(10 mm graphite) in direct contact with the sample. The sublimation of the ablation layer by the
laser pulse generates a shockwave that, constrained
by the confinement layer (glass or water), propagates through the metallic sheet and plastically
deform it conformably on the underlying silicon
mold with nanoscale resolution. According to
Fabbro’s model (24), the peak pressure of the
shock wave generated by the laser pulse reaches
0.85 to 1.83 GPa, enabling the generation of strain
rates on the order of 106 to 107 s−1 during the laser
shock, depending on the applied laser intensity
and pulse duration.
We fabricated silicon nanomolds by e-beam
lithography (EBL) and reactive ion etching (RIE)
or wet etching. Atomic layer deposition (ALD)
was used to deposit an ultrathin Al2O3 layer (5 to
1352 12 DECEMBER 2014 • VOL 346 ISSUE 6215 sciencemag.org SCIENCE
1School of Industrial Engineering, Purdue University, West
Lafayette, IN 47907, USA 2School of Electrical and Computer
Engineering, Purdue University, West Lafayette, IN 47907,
USA 3Birck Nanotechnology Center, Purdue University, West
Lafayette, IN 47907, USA 4Department of Chemistry and
Chemical Biology, Harvard University, Cambridge, MA 02138,
USA 5Madrid Institute for Advanced Studies, IMDEA
Nanoscience, Ciudad Universitaria de Cantoblanco, 28049
Madrid, Spain. 6School of Materials Engineering, Purdue
University, West Lafayette, IN 47907, USA. 7Department of
NanoEngineering, University of California, San Diego, La
Jolla, CA 92093, USA. 8School of Mechanical Engineering,
Purdue University, West Lafayette, IN 47907, USA.
*These authors contributed equally to this work. †Corresponding
author. E-mail: firstname.lastname@example.org