A new method that creates large-area patterns of three-dimensional
nanoshapes from metal sheets represents a potential manufacturing system
to inexpensively mass produce innovations such as "plasmonic
metamaterials" for advanced technologies.
The metamaterials have engineered surfaces that contain features,
patterns or elements on the scale of nanometers that enable
unprecedented control of light and could bring innovations such as
high-speed electronics, advanced sensors and solar cells.
The new method, called laser shock imprinting, creates shapes out of
the crystalline forms of metals, potentially giving them ideal
mechanical and optical properties using a bench-top system capable of
mass producing the shapes inexpensively
Findings are detailed in a research paper appearing Friday (Dec. 12) in the journal Science.
The paper is authored by researchers from Purdue University, Harvard
University, Madrid Institute for Advanced Studies, and the University of
California, San Diego. The research is led by Gary Cheng, an associate
professor of industrial engineering at Purdue.
The shapes, which include nanopyramids, gears, bars, grooves and a
fishnet pattern, are too small to be seen without specialized imaging
instruments and are thousands of times thinner than the width of a human
hair. The researchers used their technique to stamp nanoshapes out of
titanium, aluminum, copper, gold and silver.
A key benefit of the shock-induced forming is sharply defined corners and vertical features, or high-fidelity structures.
"These nanoshapes also have extremely smooth surfaces, which is
potentially very advantageous for commercial applications," Cheng said.
"Traditionally it has been really difficult to deform a crystalline
material into a nanomold much smaller than the grain size of starting
materials, and due to the size effects the materials are super-strong
when grain size has to be reduced to very small sizes. Therefore, it is
very challenging to generate metal flow into nanomolds with
high-fidelity 3-D shaping."
The researchers also created hybrid structures that combine metal
with graphene, an ultrathin sheet of carbon promising for various
technologies. Such a hybrid material could enhance the plasmonic effect
and bring "metamaterial perfect absorbers," or MPAs, which have
potential applications in optoelectronics and wireless communications.
"We can generate nanopatterns on metal-graphene hybrid materials, which opens new ways to pattern 2-D crystals," Cheng said.
The technique works by using a pulsed laser to generate "high strain rate" imprinting of metals into the nanomold.
"We start with a metal thin film, and we can deform it into 3-D
nanoshapes patterned over large areas," Cheng said. "What is more
interesting is that the resulting 3-D nanostructures are still
crystalline after the imprinting process, which provides good
electromagnetic and optical properties."
Whereas other researchers have created nanoshapes out of relatively
soft or amorphous materials, the new research shows how to create
nanoshapes out of hard and crystalline metals.
The silicon nanomolds were fabricated at the Birck Nanotechnology
Center in Purdue's Discovery Park by a research group led by Minghao Qi,
an associate professor of electrical and computer engineering.
"It is counter-intuitive to use silicon for molds because it is a
pretty brittle material compared to metals," Qi said. "However, after we
deposit an ultrathin layer of aluminum oxide on the nanomolds, it
performs extremely well for this purpose. The nanomolds could be reused
many times without obvious damage. Part of the reason is that although
the strain rate is very high, the shock pressure applied is only about
1-2 gigapascals."
The shapes were shown to have an "aspect ratio" as high as 5, meaning
the height is five times greater than the width, an important feature
for the performance of plasmonic metamaterials.
"It is a very challenging task from a fabrication point of view to
create ultra-smooth, high-fidelity nanostructures," Qi says. "Normally
when metals recrystallize they form grains and that makes them more or
less rough. Previous trials to form metal nanostructures have had to
resort to very high pressure imprinting of crystalline metals or
imprinting amorphous metal, which either yields high roughness in
crystalline metals or smooth surfaces in amorphous metals but very high
electrical resistance. For potential applications in nanoelectronics,
optoelectronics and plasmonics you want properties such as high
precision, low electromagnetic loss, high electrical and thermal
conductivity. You also want it to be very high fidelity in terms of the
pattern, sharp corners, vertical sidewalls, and those are very difficult
to obtain. Before Gary's breakthrough, I thought it unlikely to achieve
all of the good qualities together."
The paper was authored by Purdue doctoral students Huang Gao, Yaowu
Hu, Ji Li, and Yingling Yang; researcher Ramses V. Martinez from Harvard
and Madrid Institute for Advanced Studies; Purdue research assistant
professor Yi Xuan, Purdue research associate Chunyu Li; Jian Luo, a
professor at the University of California, San Diego; Qi and Cheng.
Future research may focus on using the technique to create a
roll-to-roll manufacturing system, which is used in many industries
including paper and sheet-metal production and may be important for new
applications such as flexible electronics and solar cells.
The work was supported by the National Science Foundation, National
Institutes of Health, Defense Threat Reduction Agency, Office of Naval
Research and the National Research Council.
No comments:
Post a Comment