Environmental Engineering Reference
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intentional dopant to create extra defect sites in ZnO, which introduce new
states in the band gap region [112]. Lithium, chlorine, hydrogen and hydro-
gen related complex are generally unintentionally introduced in ZnO lattice
during growth and ef ect electronic states. Hydrogen is considered the most
prominent defect in ZnO which creates shallow donor state in electronic
structure [3]. Besides these, surface-related defects become important when
material reaches nanosize regime. Surface band bending, chemisorptions/
photodesorption of species near surfaces, native surface defects/states,
and surface roughness are mainly considered as surface-related defects
[33, 116-121]. A surface accumulation of electrons due to high surface area
in nanostructure materials promotes high surface conduction and contrib-
utes to optoelectical properties [120]. h us, it can be concluded that defects
introduce new states in electronic band structure and strongly af ect the
optical and electrical properties of ZnO nanowire i lms [33].
16.3.2
Photoluminescence of ZnO Nanowire
h e luminescence properties are characteristics of energy band structure
and lattice dynamics of the materials. h e relative amount of radiative and
non-radiative recombination governs the intensity of photoluminescence
(PL). h e PL measurement process involves excitation of semiconducting
material with photons of higher energy than the band gap of that mate-
rial. Absorption of photon results in creation of electron-hole pair and is
followed by recombination of these excited charge carriers [122]. Excited
electron recombine by interband transition and release photon with
energy corresponding to the band gap. Zinc oxide has high excitonic bind-
ing energy (60 meV) which enables ei cient excitonic emission in ZnO at
room temperature [122, 123]. Good quality zinc oxide crystal only shows
band edge transition and results in intense single peak in PL measurement
[124]. h e presence of defect drastically alters the emission spectra and
one more broad peak in the visible region appears. In the case of nano-
structured ZnO, defect-related emission becomes very prominent due to
high surface ef ect [10]. h us, the emission from ZnO nanostructures can
be categorized as UV emission related to band edge transition and visible
emission corresponds to defect states transitions.
16.3.2.1 UV Emission
Ultraviolet (UV) emission of ZnO is associated with transitions between
the electrons in the conduction band and holes in the valence band, which
is inl uenced by excitonic ef ects due to the coulomb interaction [125, 10,
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