6th
International Science, Social Sciences, Engineering and Energy Conference
17-19 December, 2014, Prajaktra Design Hotel, Udon Thani, Thailand
I-SEEC 2014
http//iseec2014.udru.ac.th
Self-catalyst synthesis of ITO tree-like nanostructures by
chemical vapor deposition technique
Phan Van Cuonga,e1, Tran Tien Phucb,e2, Do-Hyung Kimc,e3
b
a
Department of Physics, Nha Trang University, 02 Nguyen Dinh Chieu Street, Nha Trang, Vietnam
Department of Industrial Electricity, Nha Trang University, 02 Nguyen Dinh Chieu Street, Nha Trang, Vietnam.
c
Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea
e1
cuongpv@ntu.edu.vn , e2phuctt@ntu.edu.vn, e3kimdh@knu.ac.kr
Abstract
Indium-tin oxide (ITO) tree-like nanostructures (Ns) were self-catalyst synthesized successfully using the chemical
vapor deposition (CVD) technique. The synthesized products were characterized and analyzed using the scanning
electron microscopy (SEM, Hitachi, S-4300), and the high-resolution transmission electron microscopy (HRTEM,
Philips, CM 200). Characterized results show that the ITO nanostructures have branches with the four-fold symmetry,
the trunk of tree-like Ns is about 100nm and the branches are smaller than 50nm. At the end of the trunk and the
branches remain the sphere nanosized droplets, which play a role as a self-catalyst for nano structure growth. The
chemical components on the top, middle, and bottom of the tree-like Ns were analyzed and shown that the indium
(In) component dominated over other components in the ITO Ns. In this report, we also highlighted and explained the
possible growth mechanisms of ITO Ns.
Keywords: ITO, nanostructure, chemical vapor deposition, CVD
1. Introduction
Synthesis and control of nanostructure growth based on a bottom-up approach is important for
future nanodevice applications. Many oxide nanomaterials have been synthesized and studied in order to
investigate the novel properties and applications [1, 2]. Indium-tin oxide is one of the most widely used
transparent conducting oxides. There have been several reports on the synthesis and the structure
characterization of ITO nanostructures [3, 4]. Here we report on self-catalyst synthesis of ITO tree-like
nanostructures via chemical vapor deposition (CVD) technique.
2
Nomenclature
ITO
Indium-tin oxide
Ns
Nanostructures
CVD
Chemical vapor deposition
SEM
Scanning electron microscopy
TEM
Transmission electron microscopy
HRTEM
High-resolution transmission electron microscopy
In
Indium
Sn
Tin
nm
Nanometer
sccm
Standard cubic centimeters per minute
SAED
Selected area electron diffraction
VLS
Vapor liquid solid
2. Experimental
Equal amounts of commercial ITO powder (10 wt% SnO 2 and 90 wt% In2O3, 99.99% purity)
and graphite powder (99.99%) were mechanically pulverized together and transferred to an alumina boat.
The alumina boat was placed in the center of a quartz tube furnace. Bare silicon substrates were inserted
into a tube positioned about 20cm from the center of the boat under a constant flow of argon [Ar,
99.999%, 50 sccm (Standard cubic centimeters per minute)] as the gas carrier. Oxygen (O2, 99.999%, 2
sccm) was added as the reactive gas. The pressure was maintained at 1.7 Torr. The furnace was then
heated to 900oC and the corresponding substrate temperature was about 600 oC. The schematic
representation of experimental set up is shown in figure 1.
Figure 1. The schematic representation of experimental set up
3. Rerults and Discussion
3
The SEM images of the tree-like Ns are shown in figure 2. Figure 2a was captured from the side, the ITO
Ns were grown densely and look like a “forest of ITO nanostructures”. Figure 2b was taken from the top
of ITO Ns and clearly shown that the ITO Ns have branches with the four-fold symmetry.
(a)
(b)
Figure 2. The SEM images of the tree-like Ns (a and b were captured from the side and on the top, respectively)
Figure 3a shows the TEM image of the ITO tree-like Ns. The diameter of the trunk is about 100nm and
the branch diameter is smaller than 50nm. At the end of the trunk and the branches remain the sphere
nanosized droplets. The chemical components on the top, middle, and bottom of the tree-like Ns are
shown in figure 3b which shows that the indium (In) component dominated (around 77%) over other
components in the ITO Ns. The tin component is 8.9% and 9.5% at the middle and the bottom,
respectively. While at the top the tin ratio is only 2.4%.
(a)
(b)
Figure 3. (a) TEM image of the tree-like Ns, (b) Chemical components on the top, middle, and bottom of the treelike Ns.
4
Figure 4a shows The HRTEM images taken from the trunk of ITO Ns are shown in figure 4a and 4b. The
images reveal that the surface of ITO Ns is quite clean. The inset of figure 4b shows the corresponding
SAED pattern and indicates that ITO Ns are single crystalline. The HRTEM image (4b) exhibits a lattice
spacing of the ITO Ns about 0.25 nm.
(a)
(b)
Figure 4. The HRTEM images taken from the trunk of ITO Ns. The inset of 4b shows the corresponding SAED
pattern.
The key synthesis technique of the ITO Ns follows the chemical vapor deposition technique
(CVD, also called VLS mechanism [5] suggested for the first time by Wagner and Ellis in 1964). Using
this method, Wagner and Ellis synthesized the micro size silicon wires with gold catalyst. The CVD
processes can be summarized as follow: The thin gold film was prepared on the silicon substrate, when
the thin gold film was heated at a suitable temperature; it was melted and formed the nanosize gold
droplets on the silicon substrate for the first state. The silicon vapour source was carried and adsorbed
onto the gold droplets, then deposited onto silicon substrate, these processes were repeated - forming the
micro size silicon wires (see figure 5).
Figure 5. The scheme of the CVD mechanism for growing the micro size silicon wires
5
In our study, bare silicon substrate without thin gold film deposited was used to do experiments.
Base on the images of SEM and TEM, the chemical components which were investigated above, our
suggestion and explanation for the self-catalyst growth mechanism of tree-like Ns following the CVD
mechanism, are shown in figure 6. It is well known that indium metal had a lower melting point (about
156oC) than tin metal (Sn, about 232oC). Moreover, the weight ratio of In2O3 was much larger than that of
SnO2 in source materials and In components dominated (see figure 3b) of the ITO Ns. We conclude that
In metal may play a role as a self-catalyst. At the first stage (I), In formed clusters, then nanozise droplets
of In were formed on the Si substrate, these nanozise droplets play a role as the gold catalyst droplets.
Then the growth processes of the ITO Ns trunk will be followed the CVD method using the ITO vapour
material source. We also know that inside the quartz tube, the condense level of In vapor was much
higher than that of Sn vapor; thus, In nanosize droplets may form on the side walls of the trunk, these In
nanosize droplets will be served as a catalyst for the growing of the ITO Ns branches). Similarly, ITO
vapor from the source material, carried by Ar, adsorbed on the nanosize droplets (formed on the trunk) to
grow the ITO Ns branches (stage II).
Figure 6. The suggestion of growth mechanism of ITO tree-like Ns follows CVD mechanism with a self-catalyst.
4. Conclusion
Tree-like ITO nanostructures were successfully synthesized by the chemical vapor deposition
technique. The products were characterized and analyzed using SEM, TEM. The results show that the
ITO nanostructures have branches with the four-fold symmetry. The ITO Ns diameter of the trunk and the
branch is less than 100nm and 50nm, respectively. At the end of the trunk and the branches remain the
self-catalyst sphere nanosized droplets. The chemical components on the top, middle, and bottom of the
tree-like Ns were analyzed and shown that the indium (In) component dominated over other components
of the ITO Ns. The HRTEM images and SAED pattern indicate that ITO Ns are single crystalline with the
lattice spacing about 0.25 nm. The possible growth mechanisms of ITO Ns are also suggested and
explained in this paper.
6
Acknowledgements
This work has been supported by KRF grant (No. KRF-2006-C00536).
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