International Nano Letters

https://doi.org/10.1007/s40089-021-00349-7

REVIEW

Synthesis, properties and uses of ZnO nanorods: a mini review

Peyman K. Aspoukeh1 · Azeez A. Barzinjy2,3 · Samir M. Hamad1

Received: 26 March 2021 / Accepted: 25 July 2021

© Islamic Azad University 2021

Abstract
Zinc oxide (ZnO) nanorods have been extensively investigated, owing to their extraordinary applications in numerous fields,
spatially microchip technology, solar cells, sensors, photodetectors, photocatalysts and many others. Recently, using ZnO
nanorods, as photocatalysts, are receiving increasing attention in environmental defense applications. This mini review sum-
marizes some remarkable applications for ZnO nanorods. First, the various chemical and physical procedures that were used
to produce ZnO nanorods are identified through symmetric matrices and heterogeneous structures, then the authors explain
how to use these methods to produce ZnO nanorods. This mini review, also, discusses the applications of ZnO nanorods
in many fields, especially in field release, emission properties, and electron transference. Last but not least, the appropriate
conclusions for future research using ZnO nanorods have been successfully explained.

Keywords ZnO nanorods · Vapor phase process · MOCVD process · Luminescence · Field emission · Gas sensor · Field
effect transistors

Introduction powders, dispersals of nanoparticles, packages of nanowires,

and nanotubes as well as multi nano-layers [1].
Nanomaterials are bases of nanoscience and nanotechnology. 1D nanomaterials are lengthened in one precise direction
Based on the number of dimensions nanomaterials can be [2]. These type of nanomaterials are leading the analogs 2D
classified into zero, one, two and three dimensions (Fig. 1). and 3D due to their exclusive optical and electronic proper-
One dimensional (1D) nanomaterials in this sequences are ties. Semiconducting ZnO nanorods are of vital attention for
the most significant materials due to their attractive physical the growth of devices in Nano-electronics, chemical and bio-
characteristics. One dimensional nanomaterials, essentially, logical sensing, energy conversion and storage, photovoltaic
possess three different structures, namely nanorods, nanow- cells, batteries, capacitors, hydrogen-storage devices, light-
ires and nanotube. In 1D, one dimension is exterior form the emitting diodes, catalysis, drug delivery, and piezoelectric
nanoscale. While, in two dimensional nanomaterials (2D), energy nano-generation [3].
two dimensions are exterior from the nanoscale. This type Nanorods exceed the other structures since it can be
displays plate like profiles and comprises graphene, nano- made from most elements, i.e. metals/nonmetals and com-
films, nano-layers, and nano-coatings. Lastly, three dimen- pounds, and the artificial requests for their fabrication are
sional nanomaterials (3D) are materials that are not limited much easier than the other structures [4]. A great number of
to the nanoscale in any lengths. This type can comprise bulk researchers are interested in nanorods owing to their excep-
tional chemical and physical characterization as well as wide
application in electronic devices [5–7]. Essential researches,
on what happens when the size of materials approaches to
* Azeez A. Barzinjy the nanoscale, i.e. 1–100 nm, indicating that they possess,
azeez.azeez@su.edu.krd
considerably, dissimilar characteristics than the equivalent
1
Scientific Research Centre, Soran University, 44008 Soran, materials at bulk scale. Moreover, fabricating electronic nan-
Iraq odevices is not an easy process [8, 9]. Nowadays, there are
2
Department of Physics, College of Education, Salahaddin numerous electronic devices that have been thought to be out
University-Erbil, Erbil, Iraq of reach for the last decades [10, 11]. Among the dissimilar
3
Physics Education Department, Faculty of Education, Tishk methods that have been utilized for synthesizing whiskers or
International University, Erbil, Iraq

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Fig. 1  Classification of nanostructures materials: zero dimension (0D), one dimension (1D), two dimension (2D), and three dimension (3D)

fibers, the vapor–solid [12] and Vapor–liquid–solid [13–15] and mixture nanostructures have been deliberated as a
techniques have been used more than the other forms. These capable nominee for refining the device competence in the
methods are useful in synthesizing semiconducting materials electronic and optoelectronic procedures since these con-
like doped heterojunctions and molecular crystals of group structions can offer the improved electrical and optical char-
II–VI and III–V [16, 17]. In addition to the available meth- acterization through the supportive physical interface among
ods for producing one-dimensional nanostructured materi- ZnO nanostructures and CNT [35].
als, such as template-induced growth [18–20], laser abla- On the other hand, with the speedy growing of electronics
tion-catalytic growth [21, 22], solution–liquid–solid growing and telecommunication activities, electromagnetic contami-
in carbon-based diluters [20, 23, 24], metal carbon-based nation is a dangerous issue to be studied since it not only
chemical-vapor-deposition [25], and oxide based growth touches the sympathy and performance of the devices but
[26, 27], other approaches to produce one-dimensional nano- also disturbs human’s healthiness. At this point, lightweight
structures are also available. cross composite mats has been reported this possess high
The exciton binding energy for ZnO is 60 MeV, and it porosity and it composed from zinc oxide nanorods, amor-
has a bandgap energy of 3.5 eV. The ZnO film is transpar- phous carbon, and nickel zinc ferrite for outstanding electro-
ent to light, absorbs UV rays, and produces a piezoelec- magnetic interference (EMI) shielding in the X-band. This
tric response. Additionally, ZnO is compatible with other combination leads to better magnetic loss of the electromag-
atoms and it has potential uses in medicine. Investigation has netic waves [36].
established that the morphology of one-dimensional ZnO This mini review article provides an overview of recent
nanostructured materials affects their optical and electrical developments of ZnO nanorods. The presentation was
properties [28, 29]. It can be stated that, 1D nanostructured arranged to discuss the existing methods for producing ZnO
ZnO is the most productive among other dimensions, due to nanorods. The novelty of this mini review can be highlighted
its simplicity of fabrication and use in electronics applica- here, since this review provides a brief description of the
tions. As shown in Fig. 2, one-dimensional ZnO possesses most important uses of ZnO nanorods and highlights their
several structures, such as nanobelts, nanonails, nanodots, use in many fields. Also, this review describes the most
nanotubes, nanorods, and nanowires [30–32]. important proposals, future and scientific trends in this
Nowadays, carbon nanotubes (CNT) have been one of the field to illuminate the path of researchers. ZnO nanorods
greatest progressive useful supplies owing to their higher synthesis and applications is a quite broad area of research,
electronic characterization, noble thermal and chemical therefore the limitations of this study is the authors can-
steadiness, extraordinary mechanical power, and outsized not cover all of the details spatially within this mini review.
surface area [34]. In recent times, the ZnO-CNT amalgams Owing to the exceptional properties of ZnO nanorods it has

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Fig. 2  Different forms of ZnO powders using thermal evaporation [33]

numerous potential applications for instance photo-catalysis, with the evaporation of ZnO commercial powder [39,
solar cells, sensors, and generators. Amongst the requests 40]; others synthesized ZnO nanowire-nanoribbon junc-
of ZnO nanorods, photo-catalysis is progressively used for tion crossover arrays [41, 42]. Although the vapor-phase
ecological protection. Additional investigation is desired to procedure is easy and straight, heating the commercial
increase the feature of ZnO nanorods and large scale yield mixture powder ­SnO2 and ZnO, but it has to be done on
ZnO nanorods for applied manufacturing requests. Based extremely high temperature. It is essential to analyses the
on this mini review, ZnO nanorods is likely to be one of the additional reactions and breakdown to this process. ZnO
most significant supplies in photo-catalytic along with oth- nanorods vapor-phase synthesis reported by Gundiah et al.
ers applications. utilizing a carbothermic process [43]. They stated that, as
soon as ZnO is reduced by carbon in a flowing reactor,
Zn vapor will form and it can be conveyed to the growing
Growth of ZnO nanorods region, then will be reduced to Zn once again [43]. Other
authors stated that ZnO nanorods and nanowires can be
The vapor‑phase process prepared by oxidizing Zn at high temperatures i.e. between
500 and 550 °C [44, 45]. Wagner and Ellis, for the first
One of the crucial procedures for creating 1D nanorod time, demonstrated how a catalytic process could be used
structure is the thermal evaporation (vapor phase) method to produce micrometer-scale whiskers by means of gold as
[37]. It is still uncertain for researchers what method is a catalytic-agent [46]. Also, in a typical gas–liquid–solid
best for fabricating ZnO nanorods. Initially, the vapor is reaction system, first the reactants can be dissolved in to
generated by evaporation, chemical reaction, and gas reac- nano size dewdrops of a metallic catalytic-agent then the
tion. The specific vapor gets rapidly heated up and then distinct crystalline-rods are grown. This technique has
gradually cooled on the substrate [38] to provide piles of been utilized, enormously, to produce ZnO nanorods [16,
nanomaterial. Some researchers manufactured nanorods 47].

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The MOCVD process surface energy in wurtzite structures. Moreover, for the
turbulent flow of high-speed laminar gas flow arrays, the
ZnO nanorods are typically, in many cases, grownup by reactant gases are adsorbed onto the needle tip. Since the
metal organic chemical-vapor-deposition (MOCVD). Orig- nanorods tips have a larger surface area, the growth rate
inally, MOCVD has been utilized to produce ZnO quan- will be relatively faster [57].
tum dots and thin films [48–50]. Nevertheless, MOCVD This catalyst-free method is not only applicable for the
progress has been recently investigated to grow zinc oxide fabrication of anisotropic crystals, it can be utilized also
nanorods using the free catalytic-agent MOCVD process for the fabrication of nanostructures [58–60]. The catalyst-
as demonstrated in Fig. 3 [51, 52]. ZnO nanorods cre- free MOCVD is also a promising method for cultivation of
ated in this process were pure and contained no impurities ZnO nanorods with a temperature between 400 and 500 °C
or external catalysts. In addition, the formation of ZnO which is basically far away from that expected in grow-
nanorods and other related topics have been reported in ing catalytic based nanowire, i.e. 900 °C [61]. Fabrication
other studies intensevly [53–56]. Similarly, the catalyst- of ZnO nanorods at a low temperature, will offer greater
free process has been studied, and the main reason for this power and versatility making them a candidate for usage in
type of growth is said to be due to a lack of anisotropic photonic nanoscale and microelectronic procedures [62].

Fig. 3  The growth of ZnO

nanorod is indicated by sche-
matic diagrams a VLS and b
MOCVD free of catalysts. c,
e Are the SEM graph of VLS
grownup ZnO nanorods and d, f
are the SEM graph of MOCVD
grownup ZnO nanorods [63]

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The chemical method and InP nanowire structures which can be produced when
InAs being sandwiched between InP barriers.
Hydrothermal synthesis method is another common tech- In place of the traditional method described by Yi et al.,
nique to produce ZnO nanorods [64–67]. Other chemical Zhang et al. proposed the mixed interfacial layer [89, 90].
methods have been utilized for ZnO nanorods fabrication, Metallic and semiconducting Nanostructured materials play
such as aqueous-solution [68], bio mineralization [69], along essential roles in the electronic contacts in many nanosize
with sol–gel method [70]. O’Brien et al. have described a devices [91]. Yi et al. described how decreasing the thick-
new synthesis technique for ZnO nanorods production [71]. ness of the Ni layer in a nanorod heterostructure leads to
They utilized zinc acetate in the thermal decomposition of the decreasing of the boundary between ferromagnetism and
an organic oleic acid solvent to produce ZnO nanorods with superparamagnetism [90]. In a recent study, scientists dis-
a monodisperse in normal length of 40–50 nm with a diam- covered that amorphous of ­Al2O3 layers have been formed
eter of 2 nm. on ZnO nanorods through atomic-layer deposition (ALD)
[92–94]. In an epitaxial-molding attitude [95] and metalic-
Production of ZnO nanorods collections orgian vapor epitaxy (MOVPE) [96], Goldberger et al. and
Ann et al. have validated the preparation of GaN/coaxial
Perpendicularly high-oriented arrangements of nanorods ZnO nanorod heterostructure. The heterostructures bring
through a constant distribution of dimension and thickness inconceivable tools, containing field-effect transistors along
can be defined as nanorods arrays. Nanorods are among the with HEMTs, that are made of coaxial nano-nanorods [97].
important topics in nanotechnology due to their high poten- These hollow nanotubes can be utilized for electrophoresis
tial applications and they account as a building blocks in of nanocapillaries and biochemical sensor nanosubstances
nanodevices [72]. As illustrated in Fig. 3, ZnO nanorods [98].
arrays were synthesized with a solid substratum by applying
the vapor–liquid–solid (VLS) method and using a metal like Nanorods quantum structures
gold [73, 74]. Another method of manufacturing catalyst-
free was used to grow ZnO nanorods aligned to them [59, In addition to possessing multiple novel physical proper-
75]. Recently, a new low temperature method for cultivat- ties, nanorods structure have some quantum mechanical
ing ZnO nanorods (Fig. 3) with catalyst-free MOCVD is properties. To study these novel properties, a number of
reported by Yi et al. [63, 76]. techniques, such as MBE and MOVPE, have been devel-
oped [4, 99, 100]. The catalytic-assisted vapor–liquid–solid
ZnO heterostructures nanorods growing assisted technique for manufacturing the compositionally
modulated nanowires. However, a relatively broad hetero-
One dimensional heterostructures nanorods is one of the interface leads to the production of a quantum structure
most useful techniques in the production of many new elec- with an ultra-thin quantum-well layer, then the structure
tronic devices, spatially in a micrometer scale [77–79]. The becomes extremely hard to synthesize [101]. Yi et al. have
structures of nano superlattice have recently grown by mul- produced quantum structures, without using catalytic ZnO/
tiple vapor-phase semiconductor reactants [80–86]. The core Zn0.8Mg0.2O, through utilizing MOVPE [102]. Nanorods
of this model was displayed by the growth of a structure of manifold quantum-well (MQW) illustrations with 1.1 nm
a silicone nanowire or carbon nanotube [87]. This method and 2.5 nm wells using TEM and also photoluminescence
can be successfully applied for a metal catalyst with two spectra (PL) spectra, respectively, are shown on Fig. 4.
different material. Superlattices up to 20 sheets of GaAs Figure 4 shows spectra layers in multiple quantum wells
in addition to GaP have been compositionally modeled of ZnO/Zn0.8Mg0.2O layers. Since lighter elements are less
[88]. Also, p–n modulated junction nanometrics made by dispersed in a z-contrast image (Fig. 4), the ZnO layers are
chemical vapor or laser-assisted catalytic growth have been lighter than the ZnO/Zn0.8Mg0.2O. These heterostructures
prepared. Wu et al. reported that Si/SiGe superlattices are display the perfect inscription of the quantum-confinement
being produced utilizing an amalgam pulsated laser removal/ and an increased black column change with a diminishing
chemical-vapor-deposition (PLA-CVD) method [82]. Bjork coating width. Similarly, as illustrated in Fig. 4, the PL
et al. also manufactured a time period of 100 to only sev- bands of sequence of ZnO/Zn0.8Mg0.2O MQW nanorods
eral nanometers of InAs/InP nano-superlattices. The use of arrangements display radiation energies reliant upon the
a high-vacuum chamber for chemical beam epitaxy tech- potential well thicknesses. Other research concludes that
niques was similar to metal-catalyzed nanowire growth tech- widening well widths leads to a drop in the blue-shifting
nologies [83–85]. Moreover, Bjork et al. also investigated value [103]. This change can be ignored at a weighted
single-electron tunneling behaviors and electrical transport threshold of 110 Å. The theoretical calculation in ten peri-
properties. They analyzed resonant tunneling between InAs ods shows that random decreasing width of the potential

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Fig. 4  ZnO/Zn0.8 ­Mg0.2O MQW nanorods TEM pictures and PL 2.5 nm fine width example. f The ZnO/Zn0.8 ­Mg0.2O and ZnO/Zn0.8
spectrums. a Low size TEM graph of the 2.5 nm wells example. b, ­Mg0.2O nanorods band arrays were presented in 10 K PL spectra
e Z-distinction descriptions with an increasing magnification of the (ZnO/Zn0.8 ­Mg0.2O) [102]

well produces an increase in the amount of PL emission. a blue shift when the width of nanobelt has been reduced.
In addition, the optical features of individual quantum-well This shift most probably results from the quantum-con-
structures (SQWs) would give insight into the ZnO/ZnMgO finement. Park et al. through using catalyst-free MOCVD
nanorods structures (SNOM) [104]. The exchanging proce- method synthesized ZnO nanorods with a widths fewer
dures is desirable for the nanophotonic tools [105]. Yatsui than 10 nm [109]. The high resolution transmission electron
et al. calculated space and spectrally resulted photolumines- microscope image shows that the ZnO nanorods through
cence graphing of separate nanorods SQWs [106]. In the widths near to 8 nm, possess a blue shift in the PL peak
case of ZnO/ZnMgO nanorods SQW, the spectrum SNOM owing to the quantum-confinement consequence (Fig. 6).
PL shows the band substantial in the ground-state and the Decreasing the diameter in the nanorods causes a blue shift
resulting excited primary-holes (Fig. 5). in the PL peak position.
These studies approves the remark of the quantum-con- Moreover, Yi et al. have synthesized the Z ­ n1–XMgXO
finement consequence in nanorods quantum wells. A blue coaxial-nanorods quantum assemblies, through multiple
shift takes place, in the nanorods structures within the band- coatings ­Zn1–XMgXO sheets. ZnO/Zn0.8Mg0.2O coaxial-
edge PL, as a result of a quantum-confinement effect. How- nanorods quantum assemblies showed a reduction in ther-
ever, no quantum-confinement consequence was observed mal quenching and growth in PL intensity. The quantum
in the radiating path for widths of nanorods larger than structures permit the formation of specific potential profiles
20 nm. This is more likely due to the large concentration in the heterostructures. This construction has a valuable
of electrons and holes in zinc oxide, and the smaller Bohr advantages in the atomic-scale electronic devices such as
exciton (1.25 nm) [107]. To observe the quantum-confine- field-effect transistors as well as organic LEDs [110].
ment behavior in ZnO nanorods, their nanorods should be
less than 10 nm in diameter. Recently, Wang et al. [108] ZnO nanorods alloys and doping
investigated the quantum-confinement effects in ultra-thin
ZnO nanostructures and Park et al. [109] investigated and It can be comprehend that, adding divalent elements to the
grew ultrafine ZnO nanostructures. Wang et al. used a tinny cation site will modify the ZnO bandgap. Doping with mag-
Sn film in place of a reagent and manufactured the ultra- nesium (Mg) can increase the bandgap energy of ZnO-based
thin ZnO nanobelts with typical and a standard deviation of compounds between 3.4 to about 4.0 eV, besides doping
6 ± 1.5 nm [108]. The emission peak of nanobelts experience with cadmium (Cd) can decrease the bandgap to ∼ 3.0 eV

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Fig. 5  a ZnO/ZnMgO nanorod

SQW Monochromatic PL
picture achieved with 3.483 eV
photon energy. b Spot X
cross-sectional PL profile. c
Solid-curves show ZnO/ZnMgO
nanorod SQWs close-field PL
spectrum at different excitation
thicknesses oscillating between
1 and 12 W ­cm−2. The far-field
PL spectrum is shown by the
dashed-curves (F1 and F2).
Every spectrum was achieved at
15 K. d API of 3.48 eV (open-
circles) and 3.508 eV (open-
circuits) (closed-circles) [105]

Fig. 6  a Extraordinary resolu-

tion ZnO TEM graphs through
an regular width of 8 nm. b and
ZnO spectra with different regu-
lar width (D) of 8, 9, 12 and
4 nm. PL spectra with different
average diameter. Reproduces
from [109]

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[111, 112]. Geng et al. [113], through a modest physical pure ZnO nanorods arrangements at 10 K as shown in Fig. 7.
vaporization method, and Bae et al. [114], through applying On the other hand, as displayed in Fig. 8, room temperature
chemical-vapor-deposition, synthesized the S-doped ZnO UV radiation was produced with a threshold power density
nanowires. Wan et al. [115] described Cd-doped growth at below on hundred kW c­ m2 [157, 158]. Choy et al. [159]
900 °C through vaporizing metallic Zn and Cd. Chang et al. found that ZnO nanorods arrays’ possess high UV radiation
manufactured ­Zn1–X ­MnXO magnetic diluted semiconductor efficiency on silicon wafers and also on an aluminum oxide
nanorodes via vapor-phase growth [116]. On the other hand, substrate [157].
Mn, Co-doped ZnO nanorods by means of molecular-beam- Yu et al. [160] intensively discussed the arbitrary laser
epitaxy was synthesized by Ip et al. [117]. Indeed, ZnO is stroke with intelligible response in ZnO nanorods arrays
naturally an n-type semiconductor, so the conductivity of placed on ZnO layers. Compared to phosphorescence,
ZnO is controllable by doping. To prepare Ga-doped ZnO cathodic-luminescence is a much more valuable method,
nanorods, a pulse laser can be utilized [118]. The synthesiz- especially in the mapping of nanoscale sized constructions.
ing of p-type ZnO is a challenge owing to high n-type carrier Lorenz et al. found that the CL spectrum depended only
concentration [119–121]. Nevertheless, There are several slightly on the PL spectrum of ZnO nanowires [161]. ZnO
efforts regarding the preparation of p-type ZnO epitaxial thin is a good material for narrow wavelength photonic requests,
films, but there is a lack of information about the synthesiz- but p–n dopants in the ZnO make construction of ZnO p–n
ing of p-type ZnO nanorods and wires [122]. homojunction procedures difficult [162].

Field radiation
Properties and applications of ZnO
nanorods Field radiation cathodes is another attracted components
owing to their possible requests in power devices and flat
Due to their distinctive characteristics, ZnO semiconduc- sheet spectacles. Considerably, much research has been done
tor nanorods are desirable apparatuses for nanoscale appli- recently to fabricate nanorods arrangements of field emit-
cations, spatially in electronic and photonic devices. For ters. In this review, spinet type metal pine cone development
instance a number of ZnO nanrodes devices such as ultra- beforehand the discovery of the carbon nanotubes (CNTs)
violet photodetectors [123–126], sensors [127–129], light- will be discussed [163]. Nano-sized carbon fibers have a
emitting device arrays [130–132] field consequence tran-
sistors [133–135], intra-molecular p–n junction diode [136,
137] and Schottky diode [138–140] have been manufactured
recently.

Luminescence

Currently, progress has been done in accommodating the

bulk ZnO single crystal photoluminescence spectrum. Sev-
eral groups of researchers described how different lumines-
cent peaks were produced at a given location [141–145].
Concerning the analysis of single nanowires, the PL’s pri-
mary laser beam’s lateral resolution is limited to approxi-
mately one micrometer as a result of visual restrictions. Con-
sequently, greatest number of ZnO nanoparticle PL ranges
remained acquired on numerous arbitrarily rotated or allied
nanoparticles [147–151].
The room temperature luminescence of ZnO nanorods
exhibits a single wide-ranging peak. Currently, this peak
is too short to provide enough information about the reac-
tive recombination process associated with the low tem-
perature photoluminescence of ZnO bulk single-crystals
[152–154]. Until now, there have been few experiments on
Fig. 7  ZnO nanorods measured at 10 K PL spectrum of high qual-
ZnO nanowires at low temperature of photoluminescence
ity. The dominant near-band emission is four different peaks with a
[145, 155, 156]. Yi et al. [146] studied the permitted exciton maximum width of 3.359, 3.360, 3.364, and 3.376eV with full width
and three donor bound excitation ranges of extraordinary of 1–3 meV (FWHM) [146]

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Fig. 8  a Radiation spectra from

the lasing inception from the
nanowire array underneath (line
a) and overhead (line b and
inclusion). These spectra have
a pump capacity of 20, 100
and 150 kW cm−2. b Integrated
nanowire emitting intensity
in function of the energy
intensity of the visual pushing.
c Representation diagram of the
resonance cavity of a nanowire
through two certainly facing
hexagonal end faces that act as
mirrors

diameter of several hundred nanometers and a few microm-

eters. The ratio between the length of the nanowire and its
width makes it an excellent candidate for synthesizing an
extraordinary electric field at the landfills of carbon nano-
tubes for electron emission at reasonable smeared energies.
Numerous reports have demonstrated that the CNTs emit
easily detectable radiation. The manufacture of vertically
aligned CNT collections for field-emission procedures is
however not simple. One of the challenges to be addressed
is the degradation, in addition to oxygen, of residual gases
from CNT field emitters to prepare high achievements field-
emission displays (FEDs) [164].
Once 1D ZnO nanostructure is synthesized a large num-
ber of investigators suggested that such nanostructures
could be utilized in producing bases as they possess big
aspect-ratios like CNT, which could decrease the squalor Fig. 9  Current ZnO emission density nanoparticles developed on
the substrate of silicone at 550. Inset reports the Fowler–Nordheim
of field-emission appearances caused by remaining gas behavior in the field emission [166]. Instead of a p-ZnO system,
[165]. As shown in Fig. 9, Lee et al. demonstrated the first p-GaN used for homojunction, Yi et al. [167] due to a comparable
field-emission experiments from ZnO nanowires [166]. The basic energy bandgap and the crystal structure of the n-ZnO/pGaN
experiments employed a range of ZnO nanowires. Although electroluminescent (EL) tools
there was little vertical alignment of ZnO nanowires, the
turning and threshold fields is about 6.0 V µm−1 at a current and field-emission current of 1.5 mA at 3 V µm−1; current
density of 0.1 µA c­ m−2 and 11.0 V µm−1 at current density density, J = 90 µA ­cm−2 [168]. Shortly after that, several sci-
0.1 mA ­cm−2 respectively [166]. entists reported that a range of ZnO nanowires and nanorods
Even though these field emissions properties are lesser were being issued on electron emission [169, 170]. The
than CNTs, they are sufficiently acceptable to be used for results have shown that the features of 1D ZnO nanostruc-
the usage of electron emitters, i.e. turn-on field of 1 V µm−1 tures field emission are comparable to CNTs.

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The ZnO nanoneedle arrays’ vertical alignment examined Yi et al. demonstrated that ZnO nanowires can be used
by investigators to the analysis of the electron emission foun- as humidity sensors at high temperatures, they also showed
dation with 1D ZnO nanostructures. ZnO aligned nanonee- that ZnO nanorods displayed promising applications as bio-
dle arrays affect field emissions since electrostatic models logical sensors at low temperatures [189]. Nanosensors and
have been checked in metallic seed case by a hypothetical single crystalline ZnO nanorods can be used in bimolecular
control [171]. Various approaches have been settled for the preparation, catalyst-free metal organic vapor-phase epitaxy
production of perpendicularly aligned nanowires and field- method [190, 191]. Other methods such as lithography for
emission testing. In addition, Li et al. described the vertical e-beams are useful for manufacturing metal micro patterns
alignment effect on field-emission features [172]. on one ZnO nanorods. Once the ZnO electronic biosensors
The morphological effect of the nanorods tip on the with biotin modification were introduced to streptavidin,
electric fields is crucial to consider, since high-pitched tips their conductance will significantly increase. It specifies
growth the efficient electric field at the tips [173]. In gen- that ZnO nanosensors have a high electrical sensitivity to
eral, for the needlelike structures the field-emission features biological species detection [192].
of 1D ZnO nanostructures are better. li et al. have stated
that ZnO nanoneedles’ tip surface disturbances play a vital Electron transport properties
role in improving field-emission features [174]. Xu et al.
also described the good field emitting characterization of For the new research, the electron transport properties of
ZnO nanopins with a 1.9 V µm−1 turn-on field at the cur- microscale devices are vital. Harnock et al. have confirmed
rent density of 0.1 µA ­cm−2 [175]. Specifically, owing to that ZnO nanorods are non-linear and asymmetric in cur-
the very high-pitched tips, ZnO nanoneedle arrangements rent–voltage (I-V) with asymmetry factor of more than 25
could be used for electron emitters [176, 177]. There has at a 3 V bias voltage [193]. Lee et al. examined a medial
been substantial attention in field experiments on 1D ZnO resistivity of ZnO nanowire and showed that the regular
nanostructures emissions. resistivity was around one order of extent above uncovered
Other groups conducted several tests on doping 1D ZnO single ZnO nanowires in anodic aluminum oxide (AAO)
nanostructures and they proposed that raises in electrical templates [194]. In recent years, ZnO-based nanodevice such
conductivity with incapacitating would improve ZnO 1D as diodes, p-n junctions, field-effect transistors (FETs), ZnO
nanostructures’ field-emission features [178]. Xu et al. nanostructure-based electroluminescent and Schottky have
showed that Gallium doping enhances ZnO nanofibre arrays’ been investigated by many researchers [138–140, 195].
field-emission characteristics and Jo et al. also reported that The ZnO nanobelts were deposited in a 120 nm thick-
ZnO nanofibre field-emission characteristics could also be SiO2 gate dielectric/Si substrate on predetermined gold
improved with hydrogen annealing [179–181]. 1D nano- electrode arrays for the production of field-effect transistors
structures ZnO are viewed by many researchers as good field [196]. As inception of the ZnO nanobelt field-effect transis-
emitters. The modern researches on 1D ZnO nanostructures tor presented an onset voltage of 15V , a swapping relation
conducted as field emitters, so that their field-emission fea- of approximately 100, and a peak conductivity of 1.25 × 1­ 03
tures were not sufficiently adjusted [169]. Future testing on cm was shown in Fig. 10. The performance characteristics of
numerous characteristics, such as electrical conduct, density ZnO nanobelt transistor is identical to that of carbon nano-
regulator, and instrument constructions can, however, pro- tubes on Au electrodes or Ti electrodes [197]. Park et al.
duce outstanding field emitters built upon 1D ZnO nano- have recently developed high enactment ZnO nanorods FETs
structures [182, 183]. which have suggestively developed FET features with 105
high current on-/off ratios and 1.8 µS [198]. In addition, a
Gas detecting maximum of 1000–1200 ­cm2V−1 ­s−1 was observed in the
electron mobility calculated from trans conductance. The
Nanostructured ZnO have been very attractive for stationary high enactment nanoscale FETs acquired demonstrate ZnO
gas sensors, whose ultra-high surface-to-volume ratio gives Nanrodes’ feasibility for electronic nanodevices [199].
them great potential to overcome fundamental limitations Yi et al. have also used another method for manufacturing
[149, 184–186]. Some research groups have recently exam- the ZnO-based device. They produced nanorods heterostruc-
ined the gas identifying features of ZnO nanowires [187, tures vertically aligned metal/semiconductor (M/SC) only
188]. Wang et al. observed that the current of Cd-doped ZnO through vaporizing metal onto ZnO nanorods tips [84]. As
nanowire increases when irradiated in humid air to 95% this indicated in Fig. 11, Au/ZnO nanorods showed corrective
can be used for detecting moisture. Zinc oxide based micro- I–V representative curves deprived of substantial oppo-
electromechanical systems demonstrated an extraordinary site biases up to − 8 V. Additionally, Au/Ti/ZnO nanorods
compassion to ethanol gas, with a surprisingly quick reply showed rectilinear I–V characteristics, representing ohms
time of 10 s. in the form of ZnO nanorods. The rise of the carriers at the

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International Nano Letters

Fig. 10  Current source-drain versus ZNO nanobelt FET gate bias in

the environment. AFM ZnO FET image on golden electrodes (inset)
[200]

interface resulted in the ohmic behavior of nanorods het-

erostructures in Au/Ti/ZnO. This result demonstrates that
ultra-high-density nanorods and Schottky nanodevice arrays
can be applied to ZnO nanorods [201].

Conclusion

This study is a mini review on the recent researches on

Fig. 11  Typical I–V curves of the characteristic heterostructures a
ZnO nanorods with distinct concentration on its potential
Au/ZnO, and b Au/Ti/ZnO, indicating Schottky and ohmic conduct.
applications. The authors reviewed the semiconducting Inset displays a TEM of the Au/ZnO [202]
photocatalysts and deliberated a selection of production
approaches of ZnO nanorods and their resultant value
in photocatalysis. The mechanism of synthesizing ZnO manufacturing requests. According to this mini review,
nanorods with their advantages and disadvantages has it is more likely that ZnO nanorods could be a good can-
been investigated intensively. Although, a lot of progress didate for the most necessary materials in photocatalysis
in this field has been made recently, but still some chal- as well as other applications. Thus, extra investigation is
lenges have remained. In fact, the first challenge was in anticipated to rise the feature of ZnO nanorods and large
all applied methods, the ZnO nanorods are assembled ran- scale yield ZnO nanorods for applied manufacturing needs.
domly together to produce complex structures or devices.
The second challenge was the problematical issues in the Acknowledgements The authors thank Salahaddin University-Erbil,
Tishk International University and Soran University for their uncon-
growing p-type ZnO nanorods. As a result of the exclusive ditional supports. A sincere thank goes to Dr David M.W. Waswa at
material characterization, ZnO nanorods are important for Tishk International University for his diligent proofreading of this
several possible applications for instance photocatalysis, manuscript.
solar cells, sensors, and many other devices. Amongst the
uses of ZnO nanorods, photocatalysis is being progres- Funding This research received no external funding.
sively utilized for ecological safety. Additional investiga-
tion is desirable to expand the superiority of ZnO nanorods Declarations
and across-the-board yield ZnO nanorods for applied
Conflicts of interest The authors declare no conflict of interest.

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 International Nano Letters

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