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; [email protected] y The Johns Hopkins University; [email protected] 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 MRS BULLETIN • VOLUME 37 • SEPTEMBER 2012 • www.mrs.org/bulletin 847