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

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Introduction From the early 1900s, it has been known that stresses induced during or post thin-film deposition can cause the films to curve.1 Since thin-film stress can cause delamination, cracking, and premature failure of multilayer devices such as integrated circuits, a large effort has been directed at developing deposition and processing methods that minimize thin-film stress. However, it has recently become evident that thin-film stresses can in fact be engineered to shape three-dimensional patterned micro- and nanostructures with a variety of material compositions, including metals, semiconductors, and polymers. These structures that provide new capabilities in electronics, optics, and medicine can be challenging to fabricate using conventional bottom-up or top-down techniques. For example, Prinz et al.2 reported the formation of nanotubes with inner diameters as small as 2 nm by releasing heteroepitaxially strained InGaAs/GaAs ultrathin semiconductor films. Similarly, so called roll-up structures with micro- to nanoscale radii have been fabricated by stress engineering of a variety of materials, including other semiconductors such as SiGe;3 metals such as MoCr alloys,4 chromium,5,6 and tin;7 oxides such as SiOx;8 and polymers such as chitosan/poly(PEGMA-co-PEGDMA),9 polystyrene/poly(4-vinylpyridine), 10 polysuccinimide/ polycaprolactone,11 and differentially cross-linked SU8.12 Roll-up structures have enabled new functionalities for electronics,13 optics, 14,15 sensing, 16,17 microfl uidics, 12 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 used to create three-dimensional micro- and nanostructures.23,24 For example, Syms et al. described the use of surface tension forces in molten solder to perform out-of-plane rotation of polysilicon flaps.25 Here, solder was lithographically patterned on a hinge material, such as Au or a polymer, and liquefied by heating, causing it to deform to reduce surface energy; this deformation generated the torque required to rotate the flap. Building on the body of literature on electromechanical actuation,26 Smela et al. showed electrochemically controlled bending and folding of microstructures, such as spirals or cubic boxes using bilayer strips or hinges of 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 was electrochemically oxidized, causing it to shrink. In addition, a number of active and passive mechanisms have been explored, including the use of electrical, magnetic, electrochemical, optical, pneumatic, thermal, and chemical stimuli to manipulate the stresses in single or multilayer films so that they can be curved or folded only when desired.24 Some of these stimuli require that the structures are tethered t