Environmental Engineering Reference
In-Depth Information
depends upon width of depletion layer, which is associated with charged
oxygen vacancies. h e PL data (Figure 16.4) show that annealing in air/
oxygen suppresses green defect intensity, which indicates a decrease in the
concentration of oxygen vacancies and surface traps. h e higher density
of V
o
+
vacancies in as-grown NWs i lm reveals large change in band bend-
ing under UV excitation, and leads to an enhanced photoconductivity.
Further, under UV excitation, neutral oxygen vacancies get converted into
doubly charged oxygen vacancies (V
o
+2
) with the release of two electrons.
h e light-induced V
o
+2
vacancies are unstable shallow-level defects, and are
conducting in nature. As a consequence, we consider that the higher con-
centration of charge oxygen vacancies (V
o
+1
) near NW surfaces and neutral
oxygen vacancies (V
o
0
) in bulk (transition from V
o
0
to V
o
+2
) of ZnO nanow-
ire are responsible for the enhanced photocurrent in as-grown NWs. h e
annealed nanowire exhibited less change in band bending due to a reduced
amount of surface defects, and result in relatively low photosensitivity
compared with as-grown NWs. h e ZnO nanowire annealed in other envi-
ronments have also shown the same trend, UV light photosensitivity was
reduced at er post-growth annealing treatment.
16.5.3 SurfaceModii cation and Optoelectrical Properties of
ZnO Nanowires
Optical and electrical properties strongly depend on the surface of nano-
structures due to high surface-to-volume ratio. Surface modii cation by
dif erent means can be an alternative approach to tune the optoelectrical
properties of ZnO nanowire i lms. In the i rst principle study, Huang
et al.
have shown that surface properties modulate the band structure of
ZnO nanowires as signii cantly as that of quantum coni nement [163].
h ey stated that passivation of nanowire by H, F, Cl, and NH
2
is energeti-
cally favorable; also, electronic states can be redesigned in ZnO NWs and
dif erent band gaps can be realized at er surface passivation. Surface of
ZnO nanowire can be modii ed/decorated by coating of various chemical
species, metals nanoparticles, metal oxide or semiconductor nanopar-
ticles and various core-shell formations. Anthracene, poly(vinyl alcohol)
(PVA), polystyrene sulfate (PSS), poly(styrene-co-maleic acid) (PS-co-
MAc), anionically charged poly(N-isopropylacrylamide) (PNIPAM),
and carboxymethylcellulose (CMC), 3-amino propyltriethoxy silane
(APTES), amines and Zn-Porphyrin via Metal-Ligand are demonstrated
as surface functionalizing agent [163-168]. Chemical functionaliza-
tion resulting in enhancement of electrical conductivity and reduction
Search WWH ::
Custom Search