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.

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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 lt L0 2pR0,
where R0 is the characteristic radius of bending
curvature, and q gt 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.