Other Applications - Hierarchical Nanostructures for Energy Devices - page 276

and structures that stretch''. Firstly, each Ag hierarchical nanowire possesses
ideal material properties for a stretchable electrode. This is because bulk Ag
itself is a one of the most ductile metals, so that Ag easily accommodates
tensile stress and deforms. In addition, the pentagonal Ag nanowire is
known to have superior yield strength and a size-dependent elastic property
(higher Young's modulus than bulk Ag) 28 due to the enhanced stiffness
because of the internal twin boundary of the nanowires. 28 Secondly, nano-
welded Ag hierarchical nanowire percolation networks can more effectively
accommodate deformation without any significant conductivity change by
changing the network shapes than the Ag thin film, which easily ruptures
into patches with electrical failure under a large strain. The percolation
network made from longer hierarchical nanowires stretched more easily
than that made from shorter nanowires for effective Ag mesh structures
under the same stress. The percolation network with longer hierarchical
nanowires experienced smaller maximum stress than that with shorter
nanowires. Highly stretchable hierarchical nanowires experience two
stretching regimes. When the strain was smaller than the Ecoflex pre-strain,
the wavy substrate was gradually flattened. When the strain exceeded the
pre-strain, the Ag hierarchical nanowire network structures themselves
started to deform by aligning themselves with the direction of the strain. For
some cases with a high nanowire density, the hierarchical nanowire behaved
like a thin film and partially ruptured. However, the hierarchical nanowire
percolation networks were still moderately good to yield robust high
conductivity. The electrical and mechanical compliance characteristics of
the conductor did not deteriorate over time. Furthermore, no changes in
electrical characteristics or mechanical damages were observed after a lot of
bending and stretching cycles, indicating excellent electrical functionalities
and stability under stretching.

d n 3 r 4 n g | 8

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12.2.3 Modification of Surface Characteristics
(Superhydrophobic Surface, Self-cleaning Surface)
Nature provides impressive examples of nanostructured and micro-
structured systems with excellent optical and mechanical functionalities. In
particular, many biological systems have evolved into multi-scale, hier-
archical structures with sophisticated and smart functions. 76 The strong
adhesion abilities of a gecko's foot, 77 superhydrophobic surface of a lotus
leaf 78 and water strider leg, 79 photonic crystal structures in butterfly wings, 80
biomineralization of organic-inorganic structures 81 are just a few examples
of hierarchical structures found in nature. 74 These unique structure-
function relations have motivated researchers to fabricate hierarchical
microstructures and nanostructures that mimic the unique functionalities
of biological systems. 82,83 The wettability of solid surfaces is a very important
property and is governed by both the chemical composition and the geo-
metrical microstructure of the surface as shown in Figure 12.5. 76