Nanomechanical Architecture of Strained Bi-layer Thin Films:

1. Nanomechanical Architecture of Strained Bi-layer Thin Films: • from design principles to experimental fabrication • Feng Liu(1), M. Huang(1), and M. Lagally(2) and R. Blick(2) • University of Utah • University of Wisconsin-Madison • Controlled and consistent fabrication of different classes and shapes of nanostructures (as opposed to simply stochastic self-assembly) will be a requirement if nanotechnology expects to achieve its promised impact on society. Recently, we demonstrate by both theory and computation the design principles of an emerging nanofabrication approach based on the nanomechanical architecture of strained bi-layer thin films, which are further confirmed by experiments through fabrication of a variety of nanostructures, including nanotubes, nanorings, nanodrills, and nanocoils (Advanced Materials, Volume 17, 2005, Pages 2860-2864). This approach offers the possibility of fabricating nanostructures with an unprecedented level of control over their size, geometry, and uniformity, based on a priori designs, possesses an unparallel level of versatility for making nanostructures with combinations of different materials, and is compatible with existing device fabrication technologies suitable for parallel mass production of identical or different nanostructures. Further, by combined multi-scale modeling and simulations from first-principles calculation, to molecular dynamics simulation, and to continuum mechanics modeling, we demonstrate how mechanical bending of nanoscale thin films differs from that of macroscopic thin films. Especially, we show that surface stress will even play a more dominant role than misfit strain in bending a film that is down a few monolayers thick. • This work is supported by DOE-BES, project DE-FG02-03ER46027 and CMSN program. The experiments are done in collaboration with • Prof. Max Lagally at University of Wisconsin-Madison.

2. Fig. 1. Schematic illustration of a strained bi-layer film folding into a nanotube when its width W is large, but into a coil when its width W is small, demonstrating the design principles of nanomechanical architecture of strained bi-layer films. The arrows indicate the folding direction. There exist two critical geometric conditions for coil formation: W < L0 = 2pR0, where R0 is the characteristic radius of bending curvature, and q > qc = sin-1(L/W0). The spiral angle of the coil depends on the folding direction as a = tan-1(d/R0), with the smallest spiral angle of q = tan-1(W/R0). Fig. 2. SEM image of nanoarchitectures fabricated from strained Si/SiGe bi-layer films. (a) Nanorings with a thickness of 80 nm, radius of ~3.0 mm, and width of 3 mm; (b) nanodrill with a thickness of 110 nm, radius of ~2.4 mm, and width of 10 mm; and (c) Nanocoil with a thickness of 110 nm, radius of ~2.4 mm, and width of 2 mm. In general, we can vary the thickness from 10 nm to 200 nm and dimensions from 20 nm to 100 mm.