ZnO nanorods synthesis, characterization and applications 박진호(1208)

ZnO nanorods: synthesis, characterization and applications
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INSTITUTE OF PHYSICS PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGY
Semicond. Sci. Technol. 20 (2005) S22–S34 doi:10.1088/0268-1242/20/4/003
ZnO nanorods: synthesis,
characterization and applications
Gyu-Chul Yi, Chunrui Wang and Won Il Park
National CRI Center for Semiconductor Nanorods and Department of Materials Science and
Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784,
Korea
E-mail:
Received 5 October 2004, in final form 21 October 2004
Published 15 March 2005
Online at stacks.iop.org/SST/20/S22
Abstract
This paper presents a review of current research activities on ZnO nanorods
(or nanowires). We begin this paper with a variety of physical and chemical
methods that have been used to synthesize ZnO nanorods (or nanowires).
There follows a discussion of techniques for fabricating aligned arrays,
heterostructures and doping of ZnO nanorods. At the end of this paper, we
discuss a wide range of interesting properties such as luminescence, field
emission, gas sensing and electron transport, associated with ZnO nanorods,
as well as various intriguing applications. We conclude with personal
remarks on the outlook for research on ZnO nanorods.

1. Introduction
One-dimensional (1D) semiconductor nanostructures such as
rods, wires, belts and tubes have in recent years attracted
much attention due to their many unique properties and
the possibility that they may be used as building blocks
for future electronics and photonics [1–3], as well as for
life-science applications [4]. It is generally accepted that
1D nanostructures are useful materials for investigating the
dependence of electrical and thermal transport or mechanical
properties on dimensionality and size reduction (or quantum
confinement) [5]. They are also expected to play an
important role as both interconnects and functional units in
fabricating electronic, optoelectronic, electrochemical and
electromechanical nanodevices [6, 7]. In the past few
years, much effort has been devoted to developing various
1D semiconductor nanostructures [8–10]. Vapour–liquid–
solid (VLS) [11–13] and vapour–solid (VS) [14] mechanisms
for growth of whiskers or fibres at high temperature
are well recognized, and have been used to synthesize
various group III–V and II–VI compound semiconductor
nanostructures [15–17]. 1D semiconductor nanostructures
have also been obtained via laser ablation-catalytic growth
[18, 19], oxide-assisted growth [20], template-induced growth
[21, 22], solution–liquid–solid growth in organic solvents
[23, 24] and metal-organic chemical vapour deposition
(MOCVD) [25].
Zinc oxide (ZnO) is a direct band-gap (E
g
= 3.37 eV)
semiconductor with a large exciton binding energy (60 meV),

exhibiting near UV emission, transparent conductivity
and piezoelectricity. Furthermore, ZnO is bio-safe and
biocompatible, and may be used for biomedical applications
without coating. Intensive research has been focused on
fabricating 1D ZnO nanostructures and in correlating their
morphologies with their size-related optical and electrical
properties [26–30]. Various kinds of ZnO nanostructures
have been realized, such as nanodots, nanorods, nanowires,
nanobelts, nanotubes, nanobridges and nanonails, nanowalls,
nanohelixes, seamless nanorings, mesoporous single-crystal
nanowires, and polyhedral cages [31–33] (figure 1). Among
the 1D nanostructures, ZnO nanorods and nanowires have been
widely studied because of their easy nanomaterials formation
and device applications.
This paper reviews recent research activities that have
focused on ZnO nanorods (or nanowires). The main text of
this paper is organized into four sections. The next section
(section 2) will introduce several concepts related to the
growth of ZnO nanorods (or nanowires). The following
section highlights a wide range of unique properties associated
with ZnO nanorods (nanowires), as well as their potential
applications in various areas. The final section concludes
with personal remarks on the outlook for research on ZnO
nanorods.
0268-1242/05/040022+13$30.00 © 2005 IOP Publishing Ltd Printed in the UK S22
ZnO nanorods: synthesis, characterization and applications
Figure 1. A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders [31].
2. Growth of ZnO nanorods (or nanowires)
2.1. Growth of ZnO nanorods (nanowires)
from the vapour phase

Vapour phase synthesis is probably the most extensively
explored approach to the formation of 1D nanostructures
such as whiskers, nanorods and nanowires [34]. Although
the exact mechanism for 1D growth in the vapour phase is
still not clear, this route has been used by many research
groups to fabricate ZnO nanorods (or nanowires). In a typical
process, vapour species are first generated by evaporation,
chemical reduction and gaseous reaction. These species are
subsequently transported and condensed onto the surface of
a solid substrate placed in a zone with a temperature lower
than that of the source material. With proper control over
the supersaturation factor, one can obtain 1D nanostructures
in large quantities. For example, Zhang et al [35], Yao et al
[36] and Kong et al [37] have synthesized ZnO nanorods (or
nanowires) by evaporating ZnO commercial powder.
Nanowire-nanoribbon junction arrays of ZnO and
hierarchical ZnO nanostructures have also been fabricated by
Gao and Wang [38] and Lao et al [39] by simply heating
commercial ZnO and SnO
2
(or In
2
O
3
) mixed powder at
elevated temperature, respectively.
Although the thermal evaporation method is simple
experimentally, its detailed mechanisms might involve the
formation of intermediates or precursors due to the use of
relatively high temperature. In many cases, decomposition

and other types of side reactions also need to be taken into
consideration. Gundiah et al reported the synthesis of ZnO
nanorods via a carbothermal reduction process, in which Zn
vapour might first be generated in situ through the reduction
of ZnO by carbon, transported in a flow reactor to the growth
zone, and finally oxidized to form ZnO again [40]. In related
studies, ZnO nanorods and nanowires were synthesized by
oxidizing Zn powder at 500–550
◦
C [41, 42].
The VLS process was originally developed by Wagner and
Ellis to produce micrometre-sized whiskers during the 1960s
[11]. A typical VLS process starts with the dissolution of
gaseous reactants into nanosized liquid droplets of a catalyst
metal, followed by nucleation and growth of single crystalline
rods and then wires (figure 2(a)). The VLS process has been
widely used for preparation of ZnO nanorods or nanowires.
In this process, Au, Cu, Ni and Sn are used as typical metal
catalysts [43–47].
2.2. Growth of ZnO nanorods or nanowires via MOCVD
Metal-organic chemical vapour deposition (MOCVD), a
widely used semiconductor thin film process, has also been
used for ZnO nanorod growth. Although ZnO thin film
and quantum dots have been prepared easily using MOCVD
[48–52], MOCVD of ZnO nanorods was very recently
developed. Yi and coworkers used catalyst-free MOCVD for
growing ZnO nanorod and nanoneedle arrays (figure 2(b))
[53, 54]. In this method, no catalyst is employed for ZnO
nanorod formation, which leads to preparation of high purity
ZnO nanorods and easy fabrications of nanorod quantum

structures and heterostructures as mentioned in sections 2.5,
2.6 and 3.1. After this research, related research of ZnO
nanorod growth via MOCVD has been reported by other
groups [55–61].
The catalyst-free growth mechanism of ZnO nanorods
has not been thoroughly investigated but the main reason for
anisotropic growth is anisotropic surface energy in ZnO, which
depends on the crystal faces of wurtzite ZnO. In addition,
high speed laminar gas flow in a certain growth condition
S23
G-C Yi et al
(
c
)(
d
)
(
a
) VLS process Catalyst free MOCVD(
b
)
(
e
)(
f
)
Metal
catalyst
Figure 2. Schematic diagrams illustrating the growth of ZnO nanorods (nanowires) from (a) the VLS process and (b) catalyst-free
MOCVD. FE-SEM images of (c) and (e) VLS-grown (reprinted with permission from reference 28, Science 292 1897. Copyright [2001]

AAAS) and (d) and ( f ) catalyst-free MOCVD grown ZnO nanorods [53].
may induce turbulent flow between the nanostructures, which
results in adsorption of fresh reactant gases only on nanorod
tips. Since more surface steps exist on nanorod tips, the
nanorod growth rate is higher on nanorod tips than on side
walls. This catalyst-free method may be expanded for one-
dimensional nanostructure growth of cubic crystal structure
materials as well as anisotropic crystal structure materials.
The catalyst-free method excludes possible incorporation
of catalytic impurities [53], which might occur during the
condensation–precipitation process for the metal catalyst-
assisted VLS method [11]. Furthermore the catalyst-free
MOCVD enables us to grow ZnO nanorods at 400−500
◦
C,
much lower than the typical growth temperature of 900
◦
C
required for catalyst-assisted nanowire growth [43]. The
ability to grow high purity ZnO nanorods at a low temperature
is expected to greatly increase the versatility and power of
these building blocks for nanoscale photonic and electronic
device applications.
2.3. Synthesis of ZnO nanorods via a chemical route
Hydrothermal synthesis provides another commonly used
methodology for generating ZnO nanorods or nanowires
[62–65]. Other chemical routes such as reverse micelle, sol–
gel, aqueous solution and biomineralization methods were
used to synthesize ZnO nanorods or nanowires [66–68].
O’Brien and coworkers reported a new synthesis of ZnO

nanorods by thermal decomposition of zinc acetate in organic
solvent in the presence of oleic acid which produces relatively
monodisperse ZnO nanorods with typical diameters of 2 nm
and lengths of 40–50 nm [69].
2.4. Fabrication of ZnO nanorod or nanowire arrays
Vertically aligned nanorod arrays with uniform thickness
and length distributions have attracted considerable interest
because they are highly appropriate for further fabrications of
vertical nanodevice arrays. Aligned growth of ZnO nanorods
S24
ZnO nanorods: synthesis, characterization and applications
(figures 2(c) and (e)) has been successfully achieved on a solid
substrate via a VLS process with the use of metal catalysts
such as gold [28, 43, 70–76]. Other techniques that do not
use any catalyst, such as template-assisted growth [77] and
electrical field alignment [78], have also been employed for the
growth of aligned ZnO nanorods. Recently, Yi and coworkers
[53, 54] have developed a technique for growing vertically
aligned ZnO nanorods (figures 2(d) and ( f )) at a low
temperature using catalyst-free MOCVD, which leads to
fabrication of vertical Schottky nanorod device arrays (see
section 3.4).
2.5. Growth of ZnO nanorod heterostructures
1D nanorod heterostructures are potentially ideal functional
components for nanoscale electronics and optoelectronics.
The ability to fabricate nanoscale heterostructures opens
up many new device applications as already proven in
micrometre-scale electronics and photonics [79–81]. Recently
three groups—Lieber et al [80, 81], Yang et al [82] and
Samuelson et al [83–85]—reported the synthesis of nanowire

superlattice structures exploiting the general concept of metal-
catalysed nanowire growth. In this process, they have grown
superlattice nanowires by modulating reactants during VLS
growth. The essence of this concept was shown with the
growth of a silicon nanowire/carbon nanotube structure [86].
In principle, this approach can be successfully implemented if
a metal catalyst is suitable for two different material syntheses.
Gudiksen et al [80] prepared the nanowire heterostructures of
compositionally modulated superlattices of GaP/GaAs and
modulation doped p–n junction nanowires employing laser-
assisted catalytic growth or chemical vapour deposition. Wu
et al prepared Si/SiGe nanowire superlattices employing
a hybrid pulsed laser ablation/chemical vapour deposition
(PLA-CVD) process [82]. InAs/InP nanowire superlattices
with periods ranging from 100 to just several nanometres
were also fabricated by Bj
¨
ork et al using similar metal-
catalysed nanowire growth techniques in an ultrahigh vacuum
chamber designed for chemical beam epitaxy techniques
[83–85]. Moreover, Bj
¨
ork et al also investigated electrical
transport properties and observed resonant tunnelling and
single electron tunnelling behaviours from InAs/InP nanowire
heterostructures where InAs island segments were sandwiched
between InP barriers.
As an alternative approach, Yi and coworkers have
employed a catalyst-free growth technique to minimize
the formation of a mixed interfacial layer, i.e., by

utilizing direct adsorption of atoms on the top surface of
nanorods which demonstrated metal/semiconductor nanorod
heterostructures [87, 88]. Metal/semiconductor nanorod
heterostructures can be used for metal contacts of many
electronic nanorod devices as mentioned in section 3.4.Yiand
coworkers also reported that thickness-controlled magnetic
metal/semiconductor nanorod heterostructures exhibit the
crossover from ferromagnetism to superparamagnetism by
decreasing Ni layer thickness [88]. Similarly, numerous
nanoscale heterostructures can then be formed by direct
epitaxial growth using techniques already developed for
growth of thin film heterostructures.
As for nanorod heterostructures along the radial direction,
termed coaxial nanorod heterostructures, Kim and coworkers
reported the growth of amorphous Al
2
O
3
layers on ZnO
nanorods by atomic layer deposition [89, 90]. Goldberger
et al and An et al also demonstrated fabrications of
GaN/ZnO coaxial nanorod heterostructures via an ‘epitaxial
casting’ approach [91] and metal-organic vapour phase
epitaxy (MOVPE) [92], respectively. These coaxial nanorod
heterostructures offer opportunities for fabrication of high
performance devices including field-effect transistors and
HEMTs [81]. Moreover, amorphous Al
2
O
3

nanotubes and
single crystalline GaN nanotubes were fabricated by etching
the core ZnO nanorods. These hollow nanotubes may have an
important rolein nanocapillary electrophoresis and nanofluidic
biochemical sensor applications [93].
2.6. Growth of ZnO nanorod quantum structures
Nanorod quantum structures composed of heterostructures
of ultrathin layers in a single nanorod would enable novel
physical properties such as quantum confinement to be
exploited, such as the continuous tuning of spectralwavelength
by varying the well thickness. While there are a number
of well-established techniques including molecular beam
epitaxy (MBE) and MOVPE for generating quantum well
or superlattice thin films, there are only a few papers
reporting nanorod heterostructures that exhibit atomically
abrupt interfaces [94, 95]. Even though the catalyst-
assisted VLS growth process was developed for the synthesis
of compositionally modulated nanowire heterostructures,
relatively broad hetero-interfaces caused by re-alloying
of alternating reactants in the metal catalyst during the
condensation–precipitation process lead to difficulties in the
fabrication of nanowire quantum structures with an ultrathin
quantum well layer [80].
Yi and coworkers fabricated ZnO/Zn
0.8
Mg
0.2
O nanorod
quantum structures using catalyst-free MOVPE [94]. Figure 3
shows transmission electron microscopy (TEM) images and

photoluminescence (PL) spectra of the nanorod multiple
quantum well (MQW) samples with 1.1 and 2.5 nm wells,
respectively. Both images exhibit bright and dark layers in
the MQW, corresponding to the Zn
0.8
Mg
0.2
OandZnOlayers,
respectively. Z-contrast images of the nanorod MQWs are
shown in figures 3(b)–(e). Since lighter elements scatter
less, ZnO layers are brighter than Zn
0.8
Mg
0.2
O layers in a
Z-contrast image. With precise thickness control down to
the monolayer level, these heterostructures show the clear
signature of quantum confinement, an increasing blue shift
with decreasing layer thickness. As shown in figure 3( f ),
PL spectra of the series of ZnO/Zn
0.8
Mg
0.2
O MQW nanorod
arrays exhibit new peaks with emission energies dependent
on well widths, as indicated by arrows. Results from other
samples show that the blue shift decreases with increasing
well width and is almost negligible at a well width of 110
˚
A.

The systematic increase in PL emission energy by reducing
well width is consistent with the quantum confinement effect
as expected from theoretical calculation in ten periods of one-
dimensional square potential wells.
Furthermore, optical properties of individual ZnO/
ZnMgO nanorod single quantum well structures (SQWs)
were investigated using scanning near-field optical microscopy
(SNOM) [95]. Yatsui et al studied spatially- and spectrally-
resolved photoluminescence imaging of individual nanorod
S25
G-C Yi et al
(
a
)(
b
)
(
f
)
(
c
)(
d
)(
e
)
Figure 3. TEM images and PL spectra of ZnO/Zn
0.8
Mg
0.2

O MQW nanorods. (a) Low magnification TEM image of the sample with
2.5 nm wells. (b)–(e) Z-contrast images of the 2.5 nm well-width sample with increasing magnification. ( f ) 10 K PL spectra of
ZnO
/Zn
0.8
Mg
0.2
O heterostructure nanorods and ZnO/Zn
0.8
Mg
0.2
O MQW nanorod arrays with band diagrams shown in inset [94].
SQWs [95] for nanophotonic device applications, such as
switching devices [96]. From the SNOM PL spectra, they
observed band-filling in the ground state and the resultant
first excited state of holes in ZnO/ZnMgO nanorod SQW
(figure 4). This SNOM study clearly confirms the observation
of the quantum confinement effect in nanorod quantum well
structures.
Even though the axial nanorod quantum structures
exhibited a blue shift in the band edge PL peak due to a
quantum confinement effect, no quantum confinement effect
in the radial direction was observed for nanorod diameters
thicker than 20 nm because effective masses of electrons and
holes in ZnO are heavy and exciton Bohr radius is as small as
1.25 nm [97]. In order to observe quantum confinement along
the radial direction of ZnO nanorods, ultrafine ZnO nanorods
with diameters less than 10 nm must be prepared.
Very recently, quantum confinement effects along the
radial direction have been observed in ultrafine 1D ZnO

nanostructures by Wang et al [98] and Park et al [99]. Wang
et al have employed a thin Sn film as a catalyst and synthesized
the ultrafine ZnO nanobelts showing an average mean diameter
of 6 nm with a standard deviation of ±1.5 nm [98]. PL
measurement showed a 14 nm blue shift in the emission peak,
which presumably results from quantum confinement arising
from the reduced width of the nanobelts. Park et al have also
fabricated ultrafine ZnO nanorods with very thin diameters
below 10 nm employing catalyst-free MOCVD [99]. The
high-resolution TEM image shows as-grown ultrafine ZnO
nanorods with diameters as small as 8 nm. Ultrafine ZnO
nanorods exhibited a blue-shifted PL peak due to the quantum
confinement effect along the radial direction in ZnO nanorods
(figure 5). While a dominant PL peak for ZnO nanorods
with a diameter of 35 nm was observed at 3.285 eV, the same
position as that of bulk ZnO, ultrafine ZnO nanorods showed
a systematic blue shift in their PL peak position by decreasing
their diameter.
Moreover, Yi and coworkers have also synthesized the
ZnO/Zn
1−x
Mg
x
O coaxial nanorod quantum structures by
subsequent depositions of a Zn
1−x
Mg
x
O shell layer on core
ZnO nanorods. ZnO/Zn

0.8
Mg
0.2
O coaxial nanorod quantum
structures exhibited significantly increased PL intensity and
greatly reduced thermal quenching. These quantum building
blocks create well-defined potential profiles along the radial
direction in the nanorod heterostructures, useful for nanoscale
high electron mobility transistors and light-emitting devices.
2.7. Alloying and doping of ZnO nanorods
ZnO band gap energy can be tuned via divalent substitution on
the cation site to produce heterostructures. The fundamental
bandgap energy of ZnO-based alloys can increase from 3.4 to
∼4.0 eV and decrease to ∼3.0 eV by doping with Mg and Cd,
respectively [51, 94]. Recently, S-doped ZnO nanowires have
been synthesized via a simple physical evaporation approach
by Geng et al [100] and via chemical vapour deposition by Bae
et al [101]. Wan et al also reported the growth of Cd-doped
ZnO nanowires by evaporating metal zinc and cadmium at
900
◦
C [102].
As for ZnO-based magnetic semiconductors, Chang
et al reported the synthesis of diluted magnetic semiconductor
Zn
1−x
Mn
x
O nanowires via vapour phase growth [103] and Ip
et al demonstrated Mn, Co-doped ZnO nanorods via molecular

beam epitaxy [104].
Conductivity of ZnO can be controlled by doping although
as-grown ZnO is n-type normally. For higher n-type doping
concentration, Ga was used as an n-type dopant, and Ga-doped
ZnO nanorods were prepared using pulsed laser deposition
[105]. Making p-type ZnO is more difficult, presumably
due to high background n-type carrier concentration and
self-compensation caused by easily formed donor defects
[106, 107]. As far as we know, p-type conduction in ZnO
nanorods or nanowires has not been reported yet, although
several papers on p-type ZnO epitaxial thin films have been
published [108].
S26
ZnO nanorods: synthesis, characterization and applications
(
a
) (
b
)
(
c
)(
d
)
I
QW
-1
I
QW
-2

I
QW
-1
I
QW
-2
I
ZnMgO
Excitation power (W/cm
2
)
Photon energy (eV)
3.4 3.5 3.6
5 meV
F
2
F
1
Position (nm)
PL intensity (arb. units)
PL intensity (arb. units)
PL intensity (arb. units)
0.1
0.01
110
3
2
1
0
0 50 100 150

55nm
Near
field
Far
field
10.8
12
W/cm
2
9.6
4.8
1.2
Figure 4. (a) Monochromatic PL image of ZnO/ZnMgO nanorod SQWs obtained at a photon energy of 3.483 eV. (b) Cross-sectional PL
profile through the spot X.(c) Solid curves show the near-field PL spectra of ZnO
/ZnMgO nanorod SQWs at various excitation densities
ranging from 1.2 to 12 W cm
−2
. Dashed curves (F
1
and F
2
) show the far-field PL spectra. All spectra were obtained at 15 K. (d) Excitation
power dependence of PL intensity at 3.483 eV (open circles) and at 3.508 eV (closed circles) [95].
(
b
)(
a
)
Figure 5. (a) High resolution TEM images of ultrafine ZnO nanorods with a mean diameter of 8 nm and (b) room temperature PL spectra
of ZnO nanorods with a different mean diameter (D) of 8, 9, 12 and 35 nm [99].

3. Properties and applications of ZnO nanorods
ZnO semiconductor nanowires and nanorods are attractive
components for nanometre scale electronic and photonic
device applications because of their unique chemical and
physical properties. For example, recently, a wide variety
of nanodevices including ultraviolet photodetectors [26, 78,
109–111], sensors [102, 112, 113], field effect transistors
[27], intramolecular p–n junction diodes [114], Schottky
diodes [115] and light emitting device arrays [116] have been
fabricated utilizing ZnO nanorods (nanowires).
3.1. Luminescence
PL spectra of ZnO bulk single crystals have been investigated
in detail and considerable progress has been achieved in
S27
G-C Yi et al
Figure 6. PL spectrum of high quality ZnO nanorods measured at
10 K. The dominant near band edge emission consists of four
distinct peaks at 3.359, 3.360, 3.364 and 3.376 eV with full width at
half maximum (FWHM) values of 1–3 meV [30].
the last few years in explaining the origins of different
ZnO luminescence peaks [117–119]. With respect to the
investigation of single nanowires, the lateral resolution of
the primary laser beam in PL is unusually limited to about
1 µm minimum due to optical limitations. Therefore,
most ZnO nanowire PL spectra [36, 45, 120–122] were
measured on many randomly oriented or aligned nanowires.
Room temperature luminescence of ZnO nanowire arrays
(
a
)(

b
)
(
c
)
Substrate
c axis
Figure 7. (a) Emission spectra from nanowire arrays below (line a) and above (line b and inset) the lasing threshold. The pump powers for
these spectra are 20, 100 and 150 kW cm
−2
, respectively. (b) Integrated emission intensity from nanowires as a function of optical pumping
energy intensity. (c) Schematic illustration of a nanowire as a resonance cavity with two naturally faceted hexagonal end faces acting as
reflecting mirrors (reprinted with permission from reference 28, Science 292 1897. Copyright [2001] AAAS).
in general shows only one very broad peak structure and
therefore does not give much insight into detailed impurity-
related recombination processes as demonstrated in the low-
temperature luminescence of ZnO bulk single crystals [120–
122]. Only a very few low temperature photoluminescence
experiments on ZnO nanowires have been reported up to now
[30, 123–125]. Yi and coworkers [30] identified the free
exciton and three donor-bound exaction PL peaks at 10 K
from PL spectra of high purity ZnO nanorod arrays (figure 6).
Recently, room temperature UV lasing emission
from a directionally grown ZnO nanoarray (figure 7)
was demonstrated with a threshold power density below
100 kW cm
−2
[28, 126]. Choy et al [127] also reported
high UV-lasing efficiency of ZnO nanorod arrays’ (NRA) on
Si wafer, similar to that of ZnO NRAs on Al

2
O
3
substrate [28].
Meanwhile, Yu et al [128] have observed random laser action
with coherent feedback in ZnO nanorod arrays embedded in
ZnO epilayers.
Cathodoluminescence (CL), in comparison to PL, is an
excellent tool for luminescence mapping and for selective
area investigation of nanometre-sized structures including
nanowires. Lorenz et al [129] presented CL investigation of
selected single ZnO nanowires; the CL spectra corresponded
only partially to the PL spectra of ZnO nanowires grown via
MOVPE [30].
Although ZnO is a promising material for short
wavelength photonic device applications, difficult p-type
doping in ZnO has impeded fabrication of ZnO p–n
homojunction devices. As an alternative approach to
S28
ZnO nanorods: synthesis, characterization and applications
Figure 8. Emission current density from ZnO nanowires grown on
silicon substrate at 550
◦
C. Inset reveals that the field emission
follows Fowler–Nordheim behaviour [29].
homojunction, Yi and coworkers [116] employed p-GaN rather
than p-ZnO since these materials have similar fundamental
band gapenergy and crystal structure, and fabricated n-ZnO/p-
GaN nanorod electroluminescent (EL) devices.
3.2. Field emission

Field-emitting cathodes have attracted much attention because
they can be used in flat panel displays and power devices. In
particular, much effort is being devoted to the fabrication of
arrays of field emitters. Most research in this area focused
on the fabrication of Spindt-type metallic cones before the
discovery of carbon nanotubes (CNTs) [130]. CNTs have a
high aspect ratio a few micrometres in length and a diameter of
several nanometres. This large aspect ratio makes it possible
to achieve a high electric field at the tips of CNTs for electron
emission at moderate applied voltages. Several experiments
have shown that the CNTs have the potential to be excellent
field emitters [130–135]. Nevertheless, the achievement
of vertically well-aligned CNTs arrays for applicable field
emission devices has not been facile and degradation of CNT
field emitters by residual gases including oxygen is one of the
problems to be overcome for high performance field emission
displays (FEDs).
After the discoveries of 1D ZnO nanostructures, many
researchers suggested that these nanostructures have the
potential for use as electron emitting sources because they also
have large aspect ratios such as CNTs and the degradation of
field emission characteristics by residual gas may be reduced
for an oxide surface of ZnO. Some of the first experiments
on field emission from ZnO nanowires were demonstrated by
Lee and coworkers [29]. These experiments used an array of
ZnO nanowires. Although the array of ZnO nanowires was
not vertically well aligned, the turn-on and the threshold field
were 6.0 V µm
−1
at current density of 0.1 µAcm

−2
and
11.0 V µm
−1
at 0.1 mA cm
−2
, respectively (figure 8).
Although these field emission characteristics were lower than
those of CNTs (turn-on field of 1 V µm
−1
and field emission
current of 1.5 mA at 3 V µm
−1
; current density, J =
90 µAcm
−2
[135]) they were good enough to be used as an
electron emitter. Soon after the report of Lee et al, several
groups reported on electron emission of an array of ZnO
nanowires, and nanorods [136–138]. The reported results
demonstrated that the field emission characteristics of 1D ZnO
nanostructures are comparable to those of CNTs.
To realize an electron emission source with 1D ZnO
nanostructures, researchers have carried out in viewpoint
on vertical alignment of ZnO nanoneedle arrays. Highly
oriented vertical alignment has been considered as a factor in
enhancing field emission properties since electrostatic models
in metal cones were investigated by theoretical calculation
[139]. Vertically well-aligned ZnO nanowires were fabricated
successfully by diverse methods and used to conduct field

emission experiments. Li et al showed that field emission
characteristics of ZnO nanowires depend on vertical alignment
[140].
Nanorod tip morphology is also an important factor
because sharper tips increase the effective electric field at
the tips [141]. Generally, ZnO 1D nanostructures have
shown better field emission characteristics for needle-like
structures. Li et al examined the tip surface perturbations
of ZnO nanoneedles as an important factor in enhancing
field emission characteristics [142]. Xu et al reported that
ZnO nanopins also have good field emission characteristics
with a turn-on field of 1.9 V µm
−1
at a current density of
0.1 µAcm
−2
[143]. In particular, ZnO nanoneedle arrays
[54] were regarded as structures with great potential for
electron emitters due to their extremely sharp tips. Later, ZnO
nanoneedle arrays were demonstrated to be one of the most
promising candidates for field emitters [144, 145]. A series
of experiments on field emission of 1D ZnO nanostructures,
such as nanowires, nanorods, nanopins and nanoneedles has
attracted considerable interest.
Other groups have carried out several experiments on
the effect of doping in ZnO 1D nanostructures since several
researchers suggested that increases in electrical conductivity
with doping enhance field emission characteristics of ZnO
1D nanostructures. Xu et al reported that the enhancement
of field emission characteristics of ZnO nanofibre arrays can

be achieved by Ga doping, and Jo et al also demonstrated
that hydrogen annealing can affect the enhancement of ZnO
nanowire field emission properties [137, 138, 146].
Many researchers consider ZnO 1D nanostructures as
good field emitters. Research on ZnO 1D nanostructures as
field emitters has only recently begun, so their field emission
characteristics have not been optimized sufficiently. However,
additional experiments on various aspects including electrical
conductivity, density control and device structures may yield
excellent field emitters based on ZnO 1D nanostructures.
3.3. Gas sensing
ZnO, a multifunctional semiconductor metal oxide, is one of
the most promising materials for gas sensor applications [147–
149]. ZnO nanostructures have also attracted considerable
attention for solid-state gas sensors with great potential for
overcoming fundamental limitations due to their ultrahigh
surface-to-volume ratio. Wang et al have carefully studied
the gas sensing characteristics of ZnO nanowires [102, 112,
113]. They found that the current increased rapidly over
three orders of magnitude when Cd-doped ZnO nanowire
was exposed to the moist air of 95% relative humidity. In
addition, gas sensors based on ZnO nanowires fabricated
with a microelectromechanical system exhibited a very high
sensitivity to ethanol gas and a fast response time (within 10 s)
S29
G-C Yi et al
at 300
◦
C, showing a promising application for ZnO nanowire
humidity sensors.

Yi and coworkers demonstrated ZnO nanorod sensors for
detection of biological molecules [150]. They functionalized
ZnO nanorod surfaces with biotin and developed nanosensors
for real time detection of biological molecules using surface-
modified ZnO nanorods as a conducting channel. For the
fabrication of bimolecule nanosensors, single crystalline ZnO
nanorods were prepared using catalyst-free metal-organic
vapour phase epitaxy. Using an e-beam lithography technique,
metal micropatterns were fabricated on a single ZnO
nanorod. Conductance of the biotin-modified ZnO nanorod
electronic biosensors was drastically increased upon exposure
to streptavidin, indicating that ZnO nanorod biosensors are
a promising candidate for electrical detection of biological
species with high sensitivity.
3.4. Electron transport properties
As the critical dimension of an individual device becomes
smaller and smaller, the electron transport properties of
their components become an important issue for study.
Results from Harnack et al indicate that the current–voltage
(I–V) characteristics of ZnO nanorods are strongly nonlinear
and asymmetrical, showing rectifying, diode-like and an
asymmetry factor up to 25 at a bias voltage of 3 V [78].
Lee and coworkers reported that the average resistivity of ZnO
nanowires in anodic aluminium oxide (AAO) templates was
about one order of magnitude higher than that of naked single
ZnO nanowires [114].
Prototype devices that have been demonstrated include
field effect transistors (FETs), p−n junctions, Schottky
diodes and electroluminescent nanodevices based on ZnO
nanostructures [27, 115, 116]. FETs, one of the most

fundamental and important electronic components, were
fabricated by Arnold et al using a ZnO nanobelt [27]. For
field-effect transistor fabrications, they deposited the ZnO
nanobelts on predefined gold electrode arrays on a 120-nm-
thick-SiO
2
gate dielectric/Si (p
+
) substrate. The ZnO nanobelt
field effect transistor showed a threshold voltage of −15 V, a
switching ratio of nearly 100, and apeak conductivity of 1.25×
10
−3
( cm)
−1
(figure 9). The ZnO nanobelt transistor
performance is analogous to those of carbon nanotubes
deposited on top of Au electrodes or covered by Ti electrodes
[151]. Recently, Park et al have fabricated high performance
n-channel ZnO nanorod FETs which significantly improved
FET characteristics with a high current on/off ratio of 10
5
and a transconductance of 1.8 µS [152]. Furthermore, the
electron mobility estimated from transconductance exhibited
a maximum value of 1000–1200 cm
2
V
−1
s
−1

.High
performance nanoscale FETs obtained show the feasibility of
ZnO nanorods for electronic nanodevice applications.
In addition, Yi and coworkers have taken a different
approach for ZnO based device fabrications. They made
vertically aligned metal/semiconductor (M/SC) nanorod
heterostructures simply by evaporating metal onto ZnO
nanorod tips [87]. Since metal is selectively deposited
on ZnO nanorod top surfaces, the interface between the
metal layer and ZnO nanorod was atomically abrupt as
determined by TEM. The I–V characteristics of metal/ZnO
Figure 9. Source-drain current versus gate bias for a ZnO nanobelt
FET in ambient. (Inset) AFM image of ZnO FET across gold
electrodes [27].
(
a
)
(
b
)
Figure 10. Typical I–V characteristic curves of (a)Au/ZnO and
(b)Au
/Ti/ZnO nanorod heterostructures, indicating Schottky and
ohmic behaviour, respectively. Inset shows a TEM image of a
Au
/ZnO nanorod heterostructure [115].
nanorod heterostructures were measured by placing a
Au-coated conducting tip on individual nanorod top surfaces
S30
ZnO nanorods: synthesis, characterization and applications

using current sensing atomic force microscopy (CSAFM).
Au/ZnO heterostructure nanorods exhibited rectifying I–V
characteristic curves without significant reverse-bias leakage
current up to −8 V (figure 10(a)), resulting presumably
from the Schottky contact formation due to a well-defined
interface between Au and ZnO layers. In addition, Au/Ti/ZnO
nanorod heterostructures exhibited linear I–V characteristics
(figure 10(b)), indicating that ohmic contacts were formed
on ZnO nanorod tips. The origin of the ohmic behaviour
of the Au/Ti/ZnO nanorod heterostructures is explained by
an increase in the carrier concentrations at the interface due to
formation of a titanium oxide layer between ZnO and Ti layers.
This result shows the potential application of ZnO nanorods
for ultra-high density nanodevices and Schottky nanodevice
arrays.
4. Conclusion
This paper provides an overview of a variety of physical and
chemical methods that have been developed for fabricating
ZnO nanorods (or nanowires). Each method has its
specific merits and inevitable weaknesses. This paper also
discussed a range of interesting properties associated with ZnO
nanorods (or nanowires), in the context of various intriguing
applications. There are also some challenges remaining. The
first challenge faced by the current synthesis methods of ZnO
nanorods (or nanowires) is their self-assembly into complex
structures or device architectures. The second challenge is
to grow p-type ZnO nanorods. The third challenge is to
demonstrate radically new applications for ZnO nanorod-
based nanostructures. In our opinion, ZnO could be one
of the most important nanomaterials in future research and

applications.
Acknowledgments
This work was supported by the National Creative Research
Initiative Project and the National R&D Project for Nano
Science and Technology (no M1-0214-00-0115) of the
Ministry of Science and Technology, Government of Korea.
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