Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 454

to magnetic properties, electrical transport properties of ZnO nanostruc-
ture can be correlated with specii c defects and controlled by manipulating
defect states. h e surface charge states and surface defects alter the shape
of band and band bending between nanowire grains/surface of the nano-
structure and strongly ef ect the electrical transport properties [33, 25].
Solution grown ZnO nanowire consists a high density of surface defects,
hence electrical properties can easily be controlled by various surface mod-
ii cation processes.
In this chapter, at er a brief discussion of low temperature hydrothermal
growth, advancements in the hydrothermal approach and other solution
processes are reviewed. Further, types of defects in ZnO are illustrated and
their impact on optical emission spectra is discussed. h e role of defect
states in electrical conductivity of ZnO is reviewed and the cause of n-type
and p-type states is looked into both from a theoretical as well as exper-
imental point of view. Inl uences of defect states on electrical transport
properties of ZnO nanowire i lms are presented and the ef ect of surface
modii cation is discussed. Finally, the chapter concludes with an illustra-
tion of some applications of ZnO nanowire i lms in energy conversion
devices.

16.2 Solution Growth of ZnO Nanowire Films

Fabrication of one-dimensional nanostructure systems is a popular research
topic among nanomaterial researchers. However, control of density, orien-
tation, shape and size are the major challenges during nanowire fabrication
[18]. h e synthesis of highly crystalline, continuous and oriented array of
nanowire is essential for studying the interesting physical properties of
low-dimensional systems and integration of nanostructured devices. In the
last decade, signii cant research has been carried out on nanowire synthe-
sis, using VLS technique, [23, 24] vapor deposition [34], electrochemical
deposition [35], molecular beam epitaxy [36] and laser ablation [37]. h ese
dif erent methods led to the fabrication of a broad range of one-dimen-
sional nanostructures. Zinc oxide nanowire array growth using the solu-
tion approach was reported for the i rst time by Andres-Vergés et al . [38]
and became more popular through the work of Vayssieres et al . [39]. h e
method comprises controlled synthesis of ZnO nanowires on glass and Si
substrates by the thermal decomposition of methenamine and zinc nitrate
in an autoclave.
Several growth mechanisms for the anisotropic growth of nanocrystals
in solution have been proposed. In solution-liquid-solid (SLS) mechanism
growth is initiated from seed particles. In the SLS reaction monomers are