Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 460

microsphere rendered a well-dei ned geometry, as shown in Figure 16.2d,
and resulted in a patterned growth of nanowire arrays with long-range peri-
odicity wafer scale [75]. h e SAM-based techniques again have advantages
on quality of nanowire; no local destruction or chemical modii cation
occurred, which is common in top-down approaches [82].

16.2.1.3 Growth of ZnO Nanowire on Various Substrates and

Doping

In high temperature growth processes, oriented growth of ZnO nanowires
are possible when substrates have minimum lattice mismatch. However,
solution-based hydrothermal growth enables oriented growth on any
arbitrary substrate at low temperature with/without assisting seed layer
[83-87]. Various reports have shown oriented growth of ZnO nanowire on
l exible substrate using the hydrothermal approach. A position-controlled
growth of ZnO nanorods on transparent glass, l exible polymer and poly-
ethylene terephthalate (PET) substrates are reported, which could be a suit-
able design for l exible optoelectronic devices [84]. Vertically aligned ZnO
nanowire is also transferred on l exible substrate like PDMS or PMMA,
at er growth on l at Au(111) surface, or Si(100) wafer, or titanium, or alu-
minum substrate by the solution approach [83]. A large-scale wafer-size
growth on plastic or silicon substrate is performed using the hydrothermal
process, which rel ects the industrial scale production capability [86]. GaN
and sapphire have already shown highly oriented nanowire structures due
to minimum lattice mismatch with ZnO. Zinc metal has also been demon-
strated as substrate to grow ZnO nanostructures, and zinc plating results
in an alternative to Zn substrate and enables growth on any arbitrary sub-
strate [87].
Zinc oxide shows variable electrical properties due to intrinsic dopant as
vacancies or interstitial of oxygen and zinc and various unintentional dop-
ants like hydrogen and chlorine [88]. Detailed discussion about intrinsic
doping is given in the next section. Extrinsic dopant is important to tune the
optical and electrical properties of ZnO nanostructures [89]. Usually dop-
ing takes place during growth; dopant is co-dissolved in precursor solution
before reaction. However, doping become a little tricky in aqueous solution
growth due to formation of aqua ions which may not incorporate into the
crystal lattice easily, while nonaqueous growth is relatively more facile for a
good quality doping [90]. h ough doping is not as easy, doped ZnO nanow-
ire by various elements like Al, Sn, Ga, In, Sb, and Cu is successfully grown
by hydrothermal process and shows excellent properties [91]. Mostly all the
dopant shows n-type behavior, however, the Djurisic group has shown that