Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 470

(glass substrate) is a major constraint. Metal doping (Al, Gd and In), sand-
wich of noble metal (Au, Pt and Ag) particles and surface modii cation by
metal/organic molecules are a few important pathways to overcome this
issue [50]. In the case of nanostructured i lms, the surface defects (grain
boundary, surface adsorbed oxygen and unsaturated dangling bonds) are
the most important factors for controlling the electrical conductivity of
i lm [10, 18, 50, 157, 25, 158]. In this section of the chapter, we will dis-
cuss the atomistic engineering of the defect states which could tailor the
characteristic of zinc oxide from insulating to semiconducting. We will
talk about a few surface modii cation schemes which can alter the nanow-
ire surface and control the defect density; and i nally the impact of defect
concentration on electrical properties of these nanowire i lms. Electrical
conductivity of ZnO nanowire i lms can be measured using two dif erent
approaches: two-probe IV measurement or four-probe IV measurement.
For two-probe measurement, a layer of metal contact can be deposited on
top of ZnO nanowire array and another contact on the surface of the seed
layer. h is design basically helps to measure the IV response of nanowire
array, where electrons moves along the nanowire axis, resulting in very fast
transport [159].
However, for optoelectronic application, we also need highly electrical
conducting i lm, thus it is important to see the electrical conductivity of
nanowire i lms in lateral direction [160]. Similarly, if replacement of TCO
with ZnO nanowire i lms is desired, than these nanowire i lms must have
high electrical conductivity in lateral direction. h us, in order to trace the
nanowire i lm IV characteristic, a four-probe conductivity measurement
can be performed, as demonstrated in Figure 16.4. h e silver contacts are

3

ZnO nanowire Array

2

1 Ag Ag Ag Ag

0

ZnO NWs Array -1

-2

-3

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Glass

Voltage/V

( a ) ( b )

Figure 16.4 (a) Schematic diagram shows the four probe contacts for IV measurement
of ZnO nanowire and (b) current-voltage plot for ZnO nanowire measured in lateral
direction of i lms.