Self-folding thin-film materials: From nanopolyhedra to graphene

Self-folding thin-film materials: From
nanopolyhedra to graphene origami
Vivek
k B. Shenoy and David H. Gracias
Self-folding off thin films
fi
is a more deterministic form off self-assembly wherein structures
curve or fold up either spontaneously on release from the substrate or in response to specific
fi
stimuli. From an intellectual standpoint, the study off the self-folding off thin films
fi
at small size
scales is motivated by the observation that a large number off naturally occurring materials
such as leaves and tissues show curved, wrinkled, or folded micro- and nanoscale geometries.
From a technological standpoint, such a self-assembly methodology is important since it
can be used to transform the precision of existing planar patterning methods, such as
electron-beam lithography, to the third dimension. Also, the self-folding off graphene promises
a means to create a variety off three-dimensional carbon-based micro- and nanostructures.
Finally, stimuli responsive self-folding can be used to realize chemomechanical and tetherfree actuation at small size scales. Here, we review theoretical and experimental aspects of
the self-folding off metallic, semiconducting, and polymeric films.
Introduction
From the early 1900s, itt has been known thatt stresses induced
during orr postt thin-fi
film
m depositionn can
n cause the films to curve.1
Since thin-film
fi m stress can
n cause delamination, cracking, andd premature failure off multilayerr devices such as integrated
d circuits,
a large effortt has been directedd att developing deposition and
processing methods thatt minimize thin-film
fi stress. However,
it has recently become evident that thin-film
fi stresses can
in fact be engineered to shape three-dimensional patterned
micro- and nanostructures with a variety off material compositions, including metals, semiconductors, andd polymers. These
structures thatt provide new capabilities in electronics, optics,
and medicine can be challenging to fabricate using conventional bottom-up orr top-down techniques. Forr example, Prinz
et al.2 reportedd the formation off nanotubes with innerr diameters as small as 2 nm by releasing heteroepitaxially strained
InGaAs/GaAs ultrathin semiconductor films.
fi
Similarly, so
calledd roll-up structures with micro- to nanoscale radii have
been fabricatedd by stress engineering off a variety off materials,
including other semiconductors such as SiGe;3 metals such
as MoCrr alloys,4 chromium,5,6 andd tin;7 oxides such as SiOx;8
andd polymers such as chitosan/poly(PEGMA-co-PEGDMA),9
polystyrene/poly(4-vinylpyridine), 10 polysuccinimide/
polycaprolactone,11 andd differentiallyy cross-linked
d SU8.12 Roll-up
structures have enabledd new functionalities forr electronics,13
12
optics, 14,15 sensing, 16,17 microfluidics,
fl
energy harvesting
and storage,18,19 drug delivery,11,20 tissue engineering,21 and
robotics.22
Stresses can also be engineered within localized regions of
thin films so that they function like hinges to enable out-ofplane rotation. When these hinges are patterned between rigid
panels, they enable a hands-free origami approach that can be
usedd to create three-dimensional micro- andd nanostructures.23,24
Forr example, Syms et al. described the use off surface tension
forces in molten solder to perform out-of-plane rotation of
polysilicon flaps.25 Here, solderr was lithographically patterned
on a hinge material, such as Au orr a polymer, and liquefied
fi
by heating, causing it to deform to reduce surface energy;
this deformation generated the torque required to rotate the
flap. Building on the body off literature on electromechanical
actuation,26 Smela et al. showed electrochemically controlled
bending andd folding off microstructures, such as spirals orr cubic
boxes using bilayer strips or hinges off Au and polypyrrole
(PPy, doped with sodium dodecyl benzene sulfonate).27,28 In
this case, the Au/PPy strips curved when the PPy thin film
fi
was electrochemically oxidized, causing it to shrink. In addition, a numberr off active and passive mechanisms have been
explored, including the use off electrical, magnetic, electrochemical, optical, pneumatic, thermal, and chemical stimuli
to manipulate the stresses in single orr multilayerr films so that
theyy can
n be curvedd orr foldedd onlyy when
n desired.24 Some off these
stimuli require that the structures are tethered to substrates,
Vivek B. Shenoy, Engineering Department, Brown University; vivek_shenoy@brown.edu
y The Johns Hopkins University; dgracias@jhu.edu
David H. Gracias, Departments of Chemical and Biomolecular Engineering, Chemistry and the Institute for Nanobiotechnology,
DOI: 10.1557/mrs.2012.184
© 2012 Materials Research Society
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