Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 452

[1]. Zinc oxide crystal has SP 3 bonding between zinc and oxygen atoms and
partial ionic crystal (iconicity ranges between covalent and ionic semicon-
ductors), which result in wide band gap material [1]. Direct and wide band
gap (3.3eV at 300 K) facilitates higher breakdown voltage, ability to sustain
large electric i eld, lower noise generation, and high temperature and high-
power operation in ZnO- based devices [1]. High excitonic binding energy
(60 meV) is another important feature of ZnO, which reveals an ei cient
excitonic emission at room temperature [2]. Direct band gap is suitable for
emission-based devices such as LEDs and lasers, while large exciton bind-
ing energy is useful for devices based on excitonic ef ect [2]. Zinc oxide
also possess piezoelectric properties, spintronic behavior, high thermal
conductivity and highly sensitive surface, which can of er a variety of
device applications [2, 3]. Besides these excellent properties, low-cost and
facile crystal growth makes ZnO an alternative of GaN for numerous opto-
electronic devices. Nanostructures of ZnO of er various advantages and
demonstrated exceptional properties. Variation in size and shape of the
nanostructures can easily tune the optical, electrical and magnetic proper-
ties of ZnO, provides an opportunity for new material design as per the
application need. A nonmagnetic and insulating material (ZnO) becomes
magnetic and electrically conductive due to the presence of various defects
states/surface states at nanoscale [4-7]. Zinc oxide nanostructured i lms
(coni nement at least in one direction) of er the most promising platform
for fabrication of optoelectronics devices. Necessity of high surface area
for generation and separation of large numbers of photogenerated charge
carriers for ei cient energy devices is also satisi ed by nanowire array and
opens a new pathway for more ei cient device fabrication [8, 9]. Nanowire
i lms (free-standing nanostructures on substrates) are fundamentally the
most suitable structural design for optoelectronic and sensing applications.
h e nanowire geometry provides ultrafast charge separation as well as high
surface area within the same volume of device. h is justii es the potential
utility of nanowire i lms in photovoltaic, light emitting diodes, lasers, water
splitting/hydrogen generation, piezoelectric generators and thermoelectric
energy conversion devices [10-17].
Zinc oxide nanowire i lms can be fabricated by a variety of tech-
niques [18, 19]. h e nanowire growth depends on the enhancement of
the crystal growth rate in one dimension and suppression of growth
in the other dimensions [20]. h e free-standing 1D nanostructures
typically grow outwards away from a single nucleation point, growth con-
i ned in one direction because of dif erent growth rated along dif erent
dimensions. Numerous procedures have been utilized to grow oriented
nanowires, namely physical vapor deposition, chemical vapor deposition,