Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 461

p-type nature can be achieved in undoped ZnO nanorods by adjusting the
properties of the seed layer, and proposed that increased concentration of
zinc vacancies might be responsible for this [78].

16.2.2 Alternative Solution Growth Methods

16.2.2.1 Chemical Bath Deposition

Compound semiconductor i lms can be easily grown from aqueous solu-
tions using this method. h e process demonstrated good control over i lm
thickness, deposition rate and quality of crystallites by changing the solu-
tion pH, temperature and bath concentration, however, reproducibility is a
little dii cult [92]. A simple growth involves zinc salts such as zinc acetate
and amine group such as ethylenediamine. A hot plate is used to heat the
solution with stirring and any arbitrary substrate (glass/TCO/Silicon, etc.)
can be used for growth [92]. Growth of nanowire i lms can be performed at
various temperatures. h e reaction mixture was maintained constant at a
desired temperature for deposition with continuous stirring. Multiple dip-
ping is generally executed and then washed with distilled water and dried
in air [92-95]. Two types of nucleation happened in solution: in homoge-
neous nucleation, the growth process is very fast and results in large-size
particles, while heterogeneous nucleation occurs at the substrate surface
and particles grow slowly [92]. By applying ZnO seeded substrate or two-
step growth process, good quality nanowire i lms can grow. h e pretreated
glass substrates were also utilized to form hydrophilic surface for uniform
growth over the entire substrate surface [95]. Growth of nanowire starts
from small ZnO nuclei and crystalline-size nuclei mainly govern the diam-
eter of nanowire [95]. Size, density and orientation of nanowire are easily
controlled by tuning the Zn ions concentration in solution, growth time
and seed layer [95]. Growth of ZnO at room temperature without using
amine at neutral pH is also reported and it has been found that amine only
controls the saturation index of ZnO [93]. A submillimeter channel was
used to l ow the solution for ZnO nanowire growth on heated glass/silicon
substrate, which can improve yield and decrease waste solvent generation
and result in more uniform nanowire i lm [96].

16.2.2.2 Spray Pyrolysis

In typical spray pyrolysis process, an aqueous solution of zinc precursor
(zinc chloride or nitrate or hydride) with variable concentration is used as
per desire [97-100]. h e depositing substrate is kept at higher temperature