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electrochemical deposition and solution or chemical process [18, 20-24].
h ough vapor deposition process (physical methods) result in good struc-
tural control on growth and form high quality NWs i lms, but their inef-
i ciency in economical and large-scale production is a main issue. On the
other hand, the solution growth process of er a cost-ef ective, high yield
production with superior control over the structural and physical proper-
ties of ZnO nanowires [18].
Defects become prominent in nanostructures due to high reactive sur-
face area, which signii cantly af ects the optical and electrical properties
of ZnO. h e presence of defect can be easily observed through photolu-
minescence measurement, an additional broad peak appears in the emis-
sion spectra of ZnO due to defect states. h e unstructured defect band
emission extends from green to red (500-750 nm), depending upon the
type of defects/vacancies created during fabrication [25]. Defect emission
can be categorized into three emissions as green, yellow and red emis-
sion, in which the cause of green emission is not well understood and is
highly controversial. Various hypothesis such as transition of singly ion-
ized oxygen vacancies to photoexcited hole; transition between singly
charged oxygen vacancy to valence band; recombination of electrons close
to conduction band with a doubly trap holes at V
o
+2
; oxygen dei ciencies,
and; surface defects are counted as a cause for green emission in ZnO [25].
Defect states also play a vital role in controlling the electrical and mag-
netic properties of the ZnO nanowires [25]. h e ZnO inherently shows
n-type semiconducting nature [26], but the origin of n-type conductiv-
ity in undoped ZnO is unclear and no conclusive remarks are found in
literature. Presence of native point defects as zinc interstitial/vacancies
or oxygen vacancies/interstitial in ZnO lattice are possible cause for such
intrinsic doping. Oxygen vacancies are the most accepted defects states
for unintentional n-type doping, however, a few theoretical reports have
suggested that oxygen vacancies are deep level donor and cannot contrib-
ute in conductivity [26-28]. Janotti
et al.
suggested that an impurity atom
like hydrogen creates unintentional doping in ZnO during growth, which
renders n-type behavior in ZnO. However, there is no clear experimen-
tal evidence which supports hydrogen doping. Most of the experimen-
tal reports still accept that oxygen vacancies are the shallow level defects
abundant in ZnO crystal due to less formation energy. h e role of defect
states on magnetic properties has been explored quite well and it has been
observed that oxygen vacancies are mainly responsible for magnetism in
ZnO nanostructures at room temperature [29]. Surface defects and various
interactions among defects like RKRY type, itinerant electron and small
polaron models are proposed as causes for magnetism [30-32]. Analogous
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