Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 463

a control of nucleation step and growth with manipulation of deposition
parameter can also result in variable size nanowires [106, 107]. h e nano-
structured growth rate can easily be controlled by deposition potentials,
current densities and salt concentrations [106]. h e concentration of zinc
precursor directs the size of ZnO nanowire, for example, control of the
dimensions of ZnO nanowires from 25 to 80 nm is reported by control-
ling ZnCl 2 concentration [105]. h e presence of hydroxide ions increase
pH close to the cathode and Zn 2+ and OH - ions react together and form
ZnO [102]. h e nucleation, distribution and growth of the ZnO nano-
structures are determined by the availability of Zn +2 ions [102]. Higher
temperature deposition gives in-homogenous morphology and thicker
ZnO columns. Nonaqueous solvents are also used for NW i lms, which
may allow a large deposition potential window and good crystal quality
with improved adherence on substrate [108]. A large-scale synthesis of
hierarchical ZnO nanorods on a transparent conductive substrate is also
shown by using DMSO-H 2 O solution [109]. Zinc oxide nanorod bundle
growth was observed when zinc chloride and tartaric acid precursor were
used at 90

C [110]. h ough template-free electrodeposition yields high
crystal quality and large-scale production capability, very good control of
nanowire aspect ratio and orientation is still missing.
h e solution growth approach leads to large-scale and facile growth of
ZnO nanowire i lms in any arbitrary substrate at low temperature (< 90

°

C).
Control over nanowire orientation, shape/size, distribution, density, pat-
terning and doping can be easily achieve by the solution approach, which
shows high order of applicability in various device coni guration. However,
this growth process results in low crystal quality and various defects, such as
point defects, voids, surface roughness, and surface species/organic residu-
als at surface are created [18]. In this approach, presence of intense defects
states ef ectively alters the optical and electrical properties of nanowire
and is responsible for low ei cient devices. Hence, to utilize the beauty of
the solution growth process for device fabrication, the control over crystal
defects of ZnO nanowire is essential.

°

16.3 Defects and Photoluminescence Properties of ZnO

16.3.1 Defects in ZnO

Missing of any atoms of oxygen or zinc from their original position or
sitting at another site creates point defect in ZnO crystal. Dif erent types
of native point defects can generate in ZnO during growth; it might be