Environmental Engineering Reference
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nanowire has been demonstrated, which shows highly ei cient PV device
performance [73].
16.2.1.2
Patterned Growth of ZnO Nanowire
Spatially controlled or patterned growth has become important for many
functional devices. h e high-temperature-based VLS growth process has
shown signii cant potential for patterned and highly oriented growth
using special located seed particles; however, low temperature processing
of devices is challenging for high temperature methods [74, 75]. To over-
come this problem the hydrothermal process can be utilized to get desired
control on nanowire growth and patterning by ZnO seed layer. Pattern
growth can be done by using electron-beam lithography, laser interference
lithography, nanoimprint lithography (NIL) or sot chemical approach by
designing various self-assembled monolayers (SAM) followed by hydro-
thermal growth [75-79]. Electron-beam lithography (EBL) is a very prom-
ising tool, which can fabricate pattern of various shape and size with high
precision in nanometer-scales [76]. h e ZnO seed layer can be patterned
on silicon or GaN substrates using photoresist as polymethylmethacrylate
(PMMA) by spin-coating on as-prepared ZnO seed layer. A desired pattern
can be written on the photoresist by electron-beam lithographic system
and then developed into a desired structure and residue layer of PMMA
pattern removed from the surface [79]. Figure 16.2a represents a top view
SEM image of the double line of the ZnO nanorod grid with 2.5 μm line
width and period of 10 μm prepared by using EBL patterning and hydro-
thermal growth methods. h e i gure also shows the parallel ZnO nanorod
lines with 600 nm width and 1 μm period, and 45
tilt view of individual
ZnO nanorod arrays with diameter of 250 nm and period of 1 μm with 1
μm feature size and 4 μm period. High-aspect ratio of lateral array of ZnO
are fabricated by overgrowth from dot patterns created by electron beam
lithography (EBL) [77].
A patterned growth of vertically aligned ZnO nanowire arrays on non-
epitaxial substrate is shown at low temperature and it is observed that
thin seed layer of ZnO assists the directed growth. Initially nanowires are
vertically grown due to structural coni nement between the resist, but the
NW could grow in both a vertical and lateral fashion at a later stage [78].
Dif erent shaped lattice patterns such as primitive rectangular, centered
rectangular, oblique, and hexagonal are grown via laser interference lithog-
raphy followed by hydrothermal synthesis (Figure 16.2b) [74]. Polymer
templated hydrothermal growth rendered excellent control over the loca-
tion and morphology of the ZnO nanostructures; nanorods, pencil-like,
°
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