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Available online at www.worldscientificnews.com
( Received 30 June 2020; Accepted 21 July 2020; Date of
Publication 22 July 2020 )
WSN 147 (2020) 140-165 EISSN 2392-2192
Review on Multi-dimensional Zinc Oxide Nanostructures
Sarani Acharyya1, Swarnali Acharyya2, Pijus Kanti Samanta3,*
1Department of Physics, Sahid Matangini Hazra Government College
for Women, Tamluk, Purba Medinipur, West Bengal, India
2Department of Physics, Panskura Banamali College (Autonomous),
Panskura, Purba Medinipur, West Bengal, India
3Department of Physics (PG & UG), Prabhat Kumar College,
Contai - 721404, West Bengal, India
*E-mail address: [email protected]
ABSTRACT
Nanostructured materials are being widely investigated due to
their versatile properties leading to
promising applications in various areas starting from
electronics to environment and medical science.
Amongst the various investigated nanostructures, Zinc Oxide
(ZnO) is very important because of its
versatile properties like high and direct band gap, optical
transparency, room temperature
ferromagnetism, piezoelectric property and gas sensing property.
This mini review article is focused on
the morphological study of various ZnO nanostructures starting
from hierarchical nanostructures to
quantum dots.
Keywords: Zinc Oxide, Semiconductor, Band-gap, Morphology
1. INTRODUCTION
Nanomaterials are in the forefront of modern research because of
their unique properties
compared to their bulk counterpart. In a bulk material the
electrons can move over a large
distance (few hundreds of unit cells) within the material.
Hence, the electrons suffer multiple
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scattering and collisions with the lattice ions, phonons,
impurities and defects of the material
[1, 2]. In nanoscale materials, one or more dimensions being
very small (only few tens of unit
cells) the motion of the electrons is very much restricted. As a
result, the scattering and
collisions of electrons with the lattice ions, phonons,
impurities and defects of the material is
very less. This will result in enormous change in the
electrical, optical, thermal, magnetic and
dielectric properties of the material [3-8].
Amongst the various studied metal-oxides, zinc oxide (ZnO) is
very popular because of
is semiconducting nature with high and direct band gap of 3.4 eV
[9-11]. Besides, it has a very
high exciton binding energy of 60 meV [12-15]. Due to this large
excitonic binding energy ZnO
may generate stable lasing emission upon suitable optical
excitation. Huang et al. had reported
UV lasing from ZnO nanorods grown on GaN substrate [16]. Being a
high band gap
semiconductor, UV emission is the characteristics emission from
ZnO nanostructures and may
reports are available in the literature [17-20]. However, at low
temperature, chemically grown
ZnO nanocrystals contain several defects like Zn-vacancy,
O-vacancy, Zn- and O-interstitials
and ionized oxygen vacancy [21-25]. The energy levels of these
defect states have been
calculated by many researchers [26-31]. It reveals that due to
lower energy, any transition
associated with these defect states lead to visible
photoluminescence from ZnO nanostructures.
Visible photoluminescence (red, yellow, green, blue, and violet)
from various types of ZnO
nanostructures are already reported in literature [32-35]. Mai
et al. had reported the synthesis
of ZnO/PMMA nanocomposite by sol-gel method along with
ultrasound. The synthesized
nanostructure shown enhanced red photoluminescence peaked at ~
600 nm owing to interfacial
band-bending effect [36].
Besides, optical emission property, ZnO exhibit room temperature
ferromagnetism as
reported by many researchers [37, 38]. Optical transparency with
room temperature
ferromagnetism may explore applications of ZnO towards
fabricating various spintronic
devices [39]. ZnO crystal is non-centrosymmetric and thus
exhibit permanent dipole moment
directed along its c-axis. This leads to occurrence of
piezoelectric property in the material.
There is report of excellent piezoelectric properties of ZnO by
Wang et al. [40]. The nanorods
were grown on c-plane–oriented α-Al2O3 substrate, using Au
particles as a catalyst synthesized
ZnO using vapor-liquid-solid (VLS) method. When the nanorods
were bent, a strain field is
generated. Separation of charges across the nanowires ccurs
under the combined atiocn of
piezoelectric and semiconducting properties of ZnO. A current is
also generaed when a
Schottky contact is created between the metal tip and the
nanorods. It was reported that the
efficiency of the nanowire based piezoelectric power generator
varies from 17 to 30%.
Here, in this mini review article we shall focus towards
studying ZnO nanostructures of
different morphological architecture. This will help in
understanding the structure property
relationship of the material which is very much essential in for
fabricating ZnO based devices.
2. CRYSTAL STRUCTURE
ZnO usually crystallizes in wurtzite form with space group p63mc
and lattice parameters
𝑎 = 3.296 Å and 𝑐 = 5.2065Å [41]. The crystal has small
deviation from centro-symmetric structure leading to the polar
nature of the crystal [42]. The wurtzite structure of ZnO is
formed
by alternative stacking of planes along c-axis terminated with
Zn2+ and O2− ion which are
tetrahedrally coordinated [41] (see Figure 1).
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Figure 1. Wurtzite structure of ZnO (yellow spheres are zinc and
grey spheres are oxygen
ions respectively). Reproduced from [43].
Figure 2. Various lattice planes of ZnO (reproduced from
[44]).
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The structure lacks of centre of inversion leading to
pyroelectric and piezoelectric
properties in the material. The basal plane is (0001) which is
polar in nature. One side of this
plane is terminated by the Zn2+ ions while the other side is
terminated with O2− ions. This leads
to the development of dipole directed along c-axis of the
crystal. Due to high surface energy,
the poar surfaces allow massive surface reconstruction to get a
stable form. However, (0001)
surfaces are automatically flat and have high stability without
any surface reconstructions. The
other facets {21̅1̅0} and {011̅0} are non-polar with lower
surface energy than {0001} facets [41]. Various planes of ZnO are
shown in Figure 2.
3. METHODOLOGIES OF SYNTHESIS
Synthesis methodologies of nanomaterials can be broadly
classified into two categories-
(a) Top-down approach and (b) Bottom-up approach. The top down
processes are- mechanical
and ball milling, lithography technique. There are many
processes of growing nanostructures
by bottom up process. These are – (a) Vapour phase
process-thermal evaporation, electron
beam evaporation, sputtering (dc and ac), chcemical vapour
deposition, vapour-liquid-solid
mehod, vacuum arc deposition, pulsed laser deposition and
molecular beam epitaxy; (b) Liquid
phse growth- wet chemical method / sol-gel method, closed bath
deposition, hydrothermal
method, electrochemcial method and laser ablation. Each methods
have its advantages and
limitations. The controll parameters are also different in
different methods. So the researchers
can choose any method of growing nanostructures as per their
convenience and need.
4. HIERARCHICAL THREE-DIMENSIONAL ZnO NANOSTRUCTURES
Figure 3. Hierarchical ZnO nanostructures grown on silicon
substrate. Reproduced from [45].
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In three dimensional (3D) nanostructures all the dimensions of
the nanostructure are much
larger than few hundred nanometer. The quantum confinement of
electrons in these structures
is very weak. There are varieties of hierarchical ZnO
nanostructures synthesized by researchers
and reported in the literature.
Liu et al. have reported the synthesis of hierarchical ZnO
nanostructures by thermo-
evaporation method [45]. The thermo-evaporation unit consists of
a tubular furnace with an
arrangement of flow of carrier gas. Pure zinc powder (99.9%)
without any catalysts was used
as the source material. The material was heated to 550 ◦C at a
rate of heating ~ 25 °C/min.
Oxugen gas was slowly pourges into the furnace to supply oxygen
for the formation of ZnO
from Zn. A Si substrate was places inside the furnace as the
substrate. After the desired growth
of the ZnO nanostructures, the source of oxygen was first cut
off and ultrapure nitrogen gas was
ntroduced into the furnace.
Figure 4. FESEM micrographs of ZnO hierarchical nanostructures
composed of a hexagonal
trunk decorated with two sets of mutually intercalated paddle
blades, each set with its own
threefold rotational symmetry, and being rotated for 60 • with
respect to each other. (a) Top,
(b) side, and (c) bottom views, respectively. (d) High
magnification image showing the flat
hexagonal cross sections of the blades. The dashed lines in (d)
serve as guides to the eye.
Reproduced under CCBY 3 lisence from [45].
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Finally the temperature was reduced gradually. By this method
they were able to obtain
hierarchical nanostructures of ZnO in the form of hexagonal
trunk decorated with two sets of
mutually intercalated paddle blades. Figure 3 and 4 shows the
FESEM images of the
nanostructure reported in [45]. The TEM image of the
nanostructures shown in Figure 5 shows
the orientation of lattice planes.
Figure 5. (Color online) Microstructural analysis of the ZnO
propellers composed of a main
trunk with layers of paddle blades. (a) Bright field TEM image,
with the boxed region further
enlarged in (b). (c) HRTEM image of the boxed Hierarchical ZnO
nanostructures grown on
silicon substrate. Reproduced under CCBY 3 lisence from [45]
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There is also report of hierarchical ZnO nanostructures by Rani
et al. [46]. They used zinc
nitrate as precursor in hydrothermal synthesis method. It was
revealed from their results that
the precursor concentration plays an important role in
determining the morphology of the
nanostructures (see Figure 6).
Kim et al. had also reported porous nanoflowers and hierarchical
ZnO nanostructures (see
Figure 7) by chemical method [47]. In brief, Zn(NO3)2·6H2O
aquash solution of appropriate
stoichiometry was mixed with NaOH solution and the mixture was
heated at 90 °C for 1 h. The
resulting precipitate was then heat treated to get the final
product.
Figure 6. FESEM image of ZnO nanorod prepared using zinc nitrate
with different
concentration annealed at 200 °C: 0.05 M (a), 0.1 M (b) and 0.4
M (c) and annealed at 500 °C:
0.05 M (d), 0.1 M (e) and 0.4 M (f). Reproduced under CCBY
license from [46].
5. TWO-DIMENSIONAL ZnO NANOSTRUCTURES
2-D nanostructures are usually extended in a plane while its
thickness is very small up to
few layers of atoms. This indicates that the motion of electrons
in these structures is restricted
in the plane of the film while they have restricted motion along
the film thickness. This leads
to the quantization of energy along only one direction only. The
degree of freedom in these 2-
D nanostructures is 2. Thin films are good examples of 2-D
nanostructures. The film thickness
may vary from few atomic layers to few hundreds of atomic layers
in case of thick film.
Depending of this thickness the property of the thin film
materials will change.
Two-dimensional thin film nanostructures can be fabricated
easily by both solution phase
methods (hydrothermal, chemical, electrochemical, sol-gel) and
also by vapour phase growth
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processes (chemical and physical vapour deposition, sputtering,
molecular beam epitaxy and
vapour-liquid-solid method).
Figure 7. (a-c) SEM images of as-prepared H-NS precursors; (d-f)
SEM images of heat-treated
H-NS ZnO nanostructures; (g-i) TEM images of heat-treated H-NS
ZnO nanostructures.
Reproduced under CCBY license from [47]
Ultrafine ZnO nanosheet synthesis is reported by Kim et al.
[48]. The fabricated structure
exhibit excellent piezoelectric property. The FESEM and TEM
images of the synthesized
product are shown in Figure 8. In chemical growth process the
morphology of the
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nanostructures depends on the current flowing through the
electrode, type of electrodes,
concentration of the electrolyte, temperature of growth, pH of
the electrolyte and the growth
duration.
Figure 8. (a) Plan-view FE-SEM image of the ZnO nanosheets
network grown on Al.
(b) Cross-sectional TEM image of the ZnO nanosheets network,
formation of LDH at the
interface between the ZnO nanosheets and the Al electrode is
highlighted. Reproduced under
CCBY license from [48].
Pradhan et al. had reported thin ZnO nanosheets assembly by
simple electrochemical
deposition method [49]. The FESEM images of the synthesized
structure are shown in Figure
9.
ZnO nanosheets can also be fabricated by pulsed laser ablation.
Ryu et al had reported a
simple pulsed laser ablation method of synthesizing ZnO
nanoflakes [50]. In brief,
predetermined amount of ZnCl2, NH4OH and hexamethylenetetramine
(HMTA) were dissolved
in de-ionised water and stirred well in a magnetic stirrer. The
solution was then put in a Teflon-
lined stainless-steel autoclave.
The autoclave was heated at 150 °C for 5 h. The precipitate was
then processed and
calcined at 350 °C for 2 h for further use. In the second step
of the process, this ZnO powder
and carbon nanotube were dispersed in 400 mL ethanol with weight
ration 10:1. The mixture
was then undergone ultrasonication. Nd:YAG laser (Q-switch type)
system was used for the
ablation process.
The SEM image of the obtained product is shown in figure 10
[50]. This picture clearly
shows the 2-D ZnO nanosheets of size ~ 1×1 μm2.
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Figure 9. SEM images of (a, b) ZnO nanowalls, (c, d) nanodisks,
(e, f) nanospikes, and (g, h)
nanopillars grown on ITO-glass at 70 °C with 0.1 M KCl solution
and varying Zn(NO3)2·6H2O
concentrations of 0.1, 0.05, 0.01, and 0.001 M, respectively.
Insets show the corresponding
cross-sectional images Reproduced under CCBY license from
[49].
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Figure 10. FESEM image of ZnO Nanosheets. Reproduced under CCBY
license from [50].
6. ONE-DIMENSIONAL ZnO NANOSTRUCTURES
Figure 11. Scanning electron microscope (SEM) (top and side
views, respectively): (a,b) Seed;
(c,d) ZnO nanorods (ZNRs) fabricated using the ACG method; (e,f)
ZNRs fabricated using the
MAG method; (g,h) ZnO debris on the surface of vertical ZNRs.
Reproduced under CCBY
license from [55].
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Now let us consider one dimensional nanostructure where two of
the dimensions are in
nanoscale while the third one in extended to few microns.
Nanorods, nanotubes, Nanopencils
are very good examples of 1-D nanostructures. In these
nanostructures the motion of the
electrons is restricted along one dimension and thus have only
one degree of freedom. The
energy gets quantized along two confined direction while
continuous along the rest one
direction.
Chemical synthesis and microwave assisted growth method are very
cost-effective and
popular method of growing ultra-long nanorods especially of
metal oxides and sulphides. There
are several reports on the synthesis of ZnO nanorods by this
method [51-55]. Figure 11 shows
the SEM images of ZnO nanorods synthesized by chemical and
microwave methods as reported
by Rana et al. [55]. The schematic set-up is also shown in
Figure 12 [55].
Rana et al. had followed a simple chemical and microwave
assisted process of growing
ZnO nanorods [55].
They used aquash solution of zinc nitride hexahydrate
[Zn(NO3)2·6H2O] and
methenamine [C6H12N4] as the precursors. The mixed solution was
undergone a thermal
treatment and microwave treatment respectively.
Figure 12. Heating profile for (a) aqueous chemical growth (ACG)
and (b) microwave-assisted
growth (MAG) methods. Reproduced under CCBY license from
[55].
According to the report of Rana et al., aquash solution of
ammonium hydroxide
[NH4OH], zinc nitride hexahydrate and methenamine solution
reaction produce ZnO nuclides
[55]. A seeded substrate when kept in this solution, the growth
of flower-like ZnO
nanostructures are formed.
A schematic of the growth process and the SEM images of the
product are shown in figure
13. Detail of the methodology is available elsewhere in
[55].
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Figure 13. (a–f) Process flow for ZnO nanoflowers (ZNFs) growth
using the ACG and MAG
methods, (g) SEM image of ZNF using the ACG method, and (h) SEM
image of ZNF using the
MAG method. Reproduced under CCBY license from [55].
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Figure 14. SEM images of ZnO nanotubes (ZNTs) (inset: magnified
image), formed via defect-
centric etching of ZNRs grown with (a) ACG and (b) MAG methods;
(c) Etching mechanism
for ACG and MAG ZNRs. Reproduced under CCBY license from
[55].
Figure 14 (a-b) shows the SEM images of ZnO nanotubes reported
in [55]. The
transformation of the nanorods into nanotubes via etching
mechanism is also shown
schematically in Figure 14 (c-d).
Chemical synthesis of ZnO nanorods is also reported by Samanta
et al. [56]. In brief, zinc
nitrate hexahydrate (Zn(NO3)2·6H2O) sodium hydroxide (NaOH)
aquash solution was put
under constant stirring maintaining the temperature at 34
°C.
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Figure 15. FESEM images of the a as synthesized ZnO nanorods,
and ZnO nanorods annealed
at b 200 °C, c 400 °C, and d 800°C, respectively. Reproduced
under CCBY license from [56].
Figure 16. (a) Plan-view SEM image of a patterned template for
ZnO NR growth. (b)–(d) 30°
tilted SEM images of the growth results on the template with the
growth durations of 10, 30,
and 180 min, respectively, when the concentration of the growth
solution is 0.08 M and the
growth temperature is 80 °C Reproduced under CCBY license from
[57].
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Figure 17. SEM top view of the nanotubes obtained in an etching
solution containing:
(a) 0.75 wt% of NH3(aq) and 1.5 wt% of CTAB; (b) 0.75 wt%
NH3(aq) and 0.25 wt% of CTAB.
Reproduced under CCBY license from [58].
Figure 18. SEM images of ALD TiO2 coated TiO2 nanotube layers
soaked in water for 28 days.
All scale bars are 100 nm. Reproduced under CCBY license from
[59].
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The pH of the solution was maintained at 11. At the end of the
reaction, the precipitate
was filtered, washed using distilled water annealed at 200, 400,
and 800 °C respectively. It was
observed that by varying the growth temperature, the nanorods
transformed into Nanopencils
like structure (see Figure 15) [56]. Similar hexagonal ZnO
nanorods is also reported by Yao et
al. using hydrothermal method (see Figure 16) [57]
Solution phase growth of ZnO nanotubes are also reported
elsewhere in [58, 59]. The
corresponding SEM images are also shown in Figure 17 and Figure
18 respectively.
7. ZnO QUANTUM DOTS
Quantum dots (QDs) are of great interest because of their
efficient and intense
photoluminescence leading their potential from electronic
display to biomedical imaging. As
the name suggests, QDs are very small size nanostructure
consisting of few atoms only. Thus,
the electrons are confined from all three direction. Hence the
energy is completely quantized in
all three directions.
There are several models that describes the relation between the
QDs size and its band
gap L. E. Brus first put forward a theoretical calculation of
band gap in semiconductor
nanoparticles/quantum dots (using CdS and CdSe as examples)
based on “effective mass
approximation” (EMA) [60].
𝐸𝑔𝑄𝐷 = 𝐸𝑔
𝑏𝑢𝑙𝑘 +ℎ2
8𝑅2(
1
𝑚𝑒∗+
1
𝑚ℎ∗ ) −
1.8𝑒2
𝜖𝑜𝜖𝑟𝑅
Here, 𝐸𝑔𝑄𝐷
and 𝐸𝑔𝑏𝑢𝑙𝑘 are the band gap of the semiconductor in the bulk
state and quantum dots
respectively and R is radius of the quantum dots. The 2nd term
in the right-hand side is called
the quantum localization term (kinetic energy term) which varies
with 𝑅−2. The third term appears due to the screened Coulombic
interaction between the electrons and holes and varies
with 𝑅−1. Y. Kayanuma modified Brus equation by considering the
electron-hole spatial correlation
effect and the band gap of the quantum dot is given by [61]
𝐸𝑔𝑄𝐷 = 𝐸𝑔
𝑏𝑢𝑙𝑘 +ℎ2
8𝑚0𝑅2(
1
𝑚𝑒∗+
1
𝑚ℎ∗ ) −
1.8𝑒2
𝜖𝑅− 0.248
4𝜋2𝑒4𝑚0
2(4𝜋𝜖)2ℎ2 (1
𝑚𝑒∗+
1𝑚ℎ
∗ )
The last term arises due to salvation energy loss (spatial
correlation effect) and is
independent of R. this term is significant only when the
semiconductor materials have very low
dielectric constant. Thus, the last term is usually
neglected.
Chemical method is a very popular technique to synthesize
ultra-fine QDs. Bera et al. had
reported the sol-gel synthesis of ZnO QDs using zinc acetate and
NaOH [62]. In their synthesis
process, predetermined amount of Zn acetate and NaOH were
dissolved in ethanol and
maintained at 70 °C. Then, 10 ml of 0.5 M OH solution and 0.08 M
zinc acetate solution was
put under vigorous stirring in an ice bath. ZnO QDs after
formation, get precipitated from the
solution. In this method they have been able to produce ZnO QDs
of size 3-8 nm.
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A modified sol-gel method has been reported by Ding et al. to
synthesized ZnO quantum
dots [63]. In this method polyvinyl alcohol (PVA) was used to
form an ink-absorbing coating
with ultraviolet shielding performance on poly (ethylene
terephthalate) (PET) films. In this
method ethanolic solution of zinc acetate and NaOH were used in
suitable molar ratio. The
reaction was initiated by mixing these two solutions for
different time duration (20 min, 40 min,
60 min, 80 min, 100 min, and 120 min) to obtain ZnO QDs.
Zinc acetate-based sol-gel method of synthesis of ZnO QDs is
also reported by Ye et al.
[64]. In this method predetermined zinc acetate dihydrate
(ZnAc2·2H2O) anhydrous ethanolic
solution was put under vigorous stirring for 40 min. Then,
PEG-400 with n(PEG400): n(Zn2+)
= 1:1 was also added to the acetate solution. This results in
the formation of ZnAc2/PEG-
400/ethanol solution. Then LiOH ethanolic solution was added and
stirred further form 30 min.
On addition of 1 mL Oleic acid (OA) and successive stirring for
1 min yield precipitate of PEG-
400/OA-modified ZnO QDs. After aging for 2 h, the solution was
centrifuged for 5 min at a
rate 4000 rpm. During this centrifuging, ethanol was added to
shatter agglomerated white
precipitate. The procedure was repeated further two times to
obtain PEG-400/OA modified ZnO
QDs. It was then dispersed in n-hexane by ultrasonication for 5
min at 0 °C. This results in the
formation of ZnO QD/n-hexane solution. It was also observed that
in this process, reactants
concentration, material ratio and reaction time play important
role in determining the size of
the QDs.
The growth equation in this process are as follows [64, 65]
The other possible reaction mechanisms are [64, 65]
Spray combustion is also a unique method of synthesizing
ultra-fine ZnO QDs. Mädler
has reported spray combustion of Zn on Si to prepare ZnO QDs of
size ~ 1.5 nm [66]. In those
growth technique Si plays an important role in controlling the
growth and stabilization of ZnO
QDs. It was further observed in their research that the band gap
decreases with increasing size
of the QDs.
Microwave assisted non-aqueous method is also a popular
technique to synthesize ZnO
QDs. In a report by Asok et al., hydrolysis of zinc acetate by
lithium hydroxide in ethanol
medium with the aid of microwave heating produces ZnO QDs [67].
This method offers the
production of ZnO QDs of high stability. As discussed in [67],
Zn(OAc)2 and LiOH were used
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as the precursors in the synthesis process. Absolute ethanol was
used as the solvent. The
alcoholic solution of Zn(OAc)2 and LiOH of predetermined
stoichiometry was mixed in a
sealed vessel in an ice bath. This prevent the reaction during
mixing. Under magnetic stirring,
microwave was irradiated over the system by a CEM Discover
reactor. The sample was heated
up to 75 °C and kept fixed. High magnetic stirring was set up
for one minute after which the
microwave was switched off. The solution was then cooled down to
room temperature
Radio frequency atmospheric pressure micro-plasma technique is a
unique very rarely
reported in literature to synthesize QDs. Jain et al. has
reported the synthesis of ZnO QDs of
size 1.9 nm by this method [68]. Further no ligands are required
in controlling the size of the
QDs. The details of the experimental set up is available
elsewhere in [68]. Figure 19 shows the
schematic of the experimental set up. The plasma chamber
consists of a stainless-steel
cylindrical tube along with a quartz capillary tube of external
diameter 1 mm and internal
diameter 0.7 mm). The tube contains a thin Zn-wire of diameter
~0.25 mm. This Zn-wire acts
as the ground electrode as shown in figure. A copper power
electrode wounds surrounding the
tube. A radio frequency source of frequency 13.56 MHz was used
to supply power of 40 W to
the electrodes. Flow of argon gas at rate of 150 sccm was also
used in the system. On application
of the rf voltage, the plasma is generated between the power
electrode and the Zn-wire. The
collector (which is usually a solid substrate) was kept at a
distance of 1.5 cm from the capillary.
The growth process was continued for 30 min. SEM images of the
resulting quantum dots
are shown in Figure 20.
Figure 19. Experimental set-up of the RF plasma reactor used for
ZnO QDs synthesis.
Reproduced from [68].
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Figure 20. Scanning electron microscope image of particle
deposition at 40 W with 150 sccm
of Ar flow. Reproduced from [68].
The SEM image produced by this method is shown in Figure 21
[68]. The TEM images
of the synthesized ZnO QDs are also shown in Figure 19. The
selected area diffraction and
particle size distribution are shown in Figure 22.
Figure 21. Transmission electron microscope images of ZnO QD.
Reproduced from [68].
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Figure 22. SAED pattern (left) and particle size analysis
(right). Reproduced from [68].
8. CONCLUSIONS
Multi-dimensional ZnO nanostructures have been described in
detail with morphological
features. Various methods like, chemical method, electrochemical
method and microwave
assisted chemical method have been deployed to fabricate these
nanostructures like -
hierarchical nanostructures, nanosheets, nanorods, nanotubes,
nanopencils and quantum dots.
The reports suggest that various experimental parameters like,
precursor concentrations, pH of
the solution, type of solvent, growth duration, type of
electrode (in case of electrochemical
method) plays important role in determining the morphology of
the nanostructures. These ZnO
nanostructures will be very useful in the field of
nanoelectronics, gas and chemical sensors,
conducting electrode and various biological applications.
BIOGRAPHY
Sarani Acharyya is a 6th semester undergraduate student of
Physics
(Honours) from Sahid Matangini Hazra Government College for
Women,
located in the East Midnapore district, West Bengal, India. Her
research
interest includes processing of nanomaterials and their
biological
applications. She is presently engaged in synthesizing various
compound
semiconductor nanostructures including CdS, CdSe, ZnO, CuO,
MgO.
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Swarnali Acharyya is a 2nd semester undergraduate student of
Physics
(Honours) from Panskura Banamali College (Autonomous), located
in the
East Midnapore district, West Bengal, India. Her research
interest includes
processing of nanomaterials for optoelectronic applications. She
is presently
engaged in synthesizing various metal oxide nanostructures
including ZnO,
CuO and TiO2
Dr. Pijus Kanti Samanta dis M.Sc. and Ph.D. in Physics from
Indian Institute
of Technology Kharagpur. His research interest includes
synthesis and
characterization of nanomaterials for opto- and nano-electronic,
and
biomedical applications. He is also working on the toxicological
study of
nanomaterials for their use towards sustainable environmental
development.
He has published more than 60 papers in international journals
of high repute
and 5 monographs. He was affiliate member of IUPAC, USA
(2012-2013),
and life member of International Association of Advanced
Materials, Sweden.
Presently he is Assistant Professor of Physics at Prabhat Kumar
College,
Contai, WB.
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