Environmental Engineering Reference
In-Depth Information
From the earlier reports, it is found that the deposition of QDs by CBD method
densely covers the mesoporous metal oxide surface and enhances the recombi-
nation resistance [
10
,
63
,
69
,
70
]. Despite the effective coverage on TiO
2
surface,
CBD method generally result in nanocrystalline films pattern rather than particles
coating. Therefore, it is mostly suitable for wide-pore-nanostructured electrodes
(nanotube, nanowire, nanotube, and inverse opal) [
71
]. The nanocrystalline film
coatings may block the narrow pore-channel of conventional nanopaticulate
electrodes having limited pore size (5-7 nm). Also, this method takes several
hours for coating (12-48 h) and high roughness surface requires seed layers, for
example CdS is widely applied as seed layer for CdSe deposition [
64
].
3.3 Successive Ionic Layer Adsorption and Reaction
(SILAR)
The successive ionic layer adsorption and reaction (SILAR) method is an
emerging method for depositing variety of semiconductor quantum dots both as
binary (CdS, CdSe, PbS, CdTe, CuS, Sb
2
S
3
,Sb
2
Se
3
,Bi
2
S
3,
etc.) and ternary
compounds (CdS
x
Se
1-x
, CuInS, CuIn
2
S
3
etc.) [
72
,
73
]. The SILAR method is
inexpensive, simple and convenient for large area deposition. There is no
restriction of coating surface and can be widely applied to all kind of materials
such as insulators, semiconductors, and metals. One of the advantages of SILAR
process is the likelihood of achieving coatings at low temperature which avoids
oxidation and corrosion of the substrate. The following parameters are indispen-
sible in controlling the particle size and bandgap of QDs; concentration of the
precursors, nature of complexing agent, pH of the precursor solutions and
adsorption, reaction, and rinsing time durations etc. [
74
]. The reviews by Mane
et al. [
44
] and Pawar et al. [
75
] provide the detailed picture of semiconducting
chalcogenide films coating by SILAR. The growth mechanism involves four most
important steps: (a) specific adsorption of cationic precursor (b) rinsing of the
nonspecifically adhered chemicals, and (c) the chemical reaction between the most
strongly specific adsorbed cations and less strongly adsorbed anions by the sub-
sequent substrate immersion in the anion solution; and (d) rinsing of the species
that did not react. These four steps constitutes a SILAR cycle, it can be repeated
many times, increasing each time the amount of deposited material. Various stages
of QDs growth (for example CdS QDs) is explained in Fig.
3
. The average QD
size can be controlled by the number of deposition cycles. This method has been
used specifically to prepare metal sulfides, but recently SILAR process is expanded
to prepare a variety of metal selenides and tellurides [
76
].
In SILAR method, the particle size of QDs is controlled by number of coating
cycles. The number of coating cycles depends on the concentration of chemical
bath. As explained in CBD method, increasing the bath concentration above the
critical concentration (R
c
) limits the number of SILAR cycles where the QDs are
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