Other Applications - Hierarchical Nanostructures for Energy Devices - page 269

introduced together with a discussion on how hierarchical nanostructuring
will lead to better functionality in the devices.

12.2 Other Applications of Hierarchical
Nanostructures
Hierarchical nanostructures can enhance the e ciency of electronic and
other devices by expanding the functionality. For example, a transparent
conductor and light emitting diode can enhance electrical functionality
while maintaining the high transparency; a highly flexible or stretchable
electrode can gain high mechanical stability while maintaining good
electrical conductivity under various mechanical deformations; modification
of surface characteristics can tune the surface hydrophobicity with broad
tunability; heat transfer can be increased with large surface area and pool
boiling enhancement; adhesion force can be increased with gecko inspired
adhesives.

d n 3 r 4 n g | 8

12.2.1 Metal Nanowire Percolation Network for Transparent
Conductor Applications
A transparent conductor is a core component in many optoelectronic devices
such as touch screens, LCDs, OLEDs, and solar cells, which have recently
been showing a tremendously rapid growth. Indium tin oxide (ITO) has been
the most dominant transparent conductor material. However, the further
development and application of ITO have receded due to two issues: its
scarcity and fragile ceramic nature. 1,2 These two key issues are actively being
addressed by the emergence of the next generation of flexible transparent
materials, such as conducting polymers, 3,4 carbon nanotubes (CNTs), 5-8
graphene, 9-12 as well as metal nanostructures. 13,14-26 Due to the poor
conductivity and instability of conducting polymers and relatively low
transmittance and electrical conductivity of carbon-based nanomaterials,
metal nanowire based hierarchical nanostructures such as the silver
nanowire percolation network are getting much attention as alternative
transparent conductors. 1,2,16 Metals are amongst the most conductive
materials on earth due to their high free-electron density, which con-
sequently also makes them highly reflective in the visible wavelength range
and not very transparent. However, metals with very small dimensions
(smaller than the visible wavelength) can be highly transparent while
maintaining good electrical conductivity. 1
In flexible and transparent conductors, inherently stretchable materials
such as carbon nanomaterials have been dominant research topics. Despite
the many advantages of using metals, as mentioned earlier, metal nanowires
have shown very limited applications with limited performance in flexible
transparent electrodes. 16-26 This is because current widely used nanowires
are relatively short (1-20 mm), 1,2 which deteriorates the transmittance, and

.