Novel 2d self assembled arrays of siox nanowire bundles

Novel 2D self-assembled arrays of SiO
x
nanowire bundles
Shi-biao Xiang
a,
⁎
, Xu Xiang
b
a
Department of Technical Physics, Zhengzhou Institute of Light Industry, No.5 Dongfeng Road, Zhengzhou 450002, PR China
b
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P. O. BOX 98, Beijing 100029, PR China
Received 15 May 2006; accepted 7 December 2006
Available online 29 December 2006
Abstract
Novel 2D ordered arrays formed by self-assembly of SiO
x
nanowire bundles have been prepared on a silicon wafer via a facile metal-mediated
gas-phase reaction. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-
area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Two types of arrays
were grown on a substrate surface by adjusting the preparation parameters. The findings suggest that the growth atmosphere, the flow rate of
carrier gas and the relative placement of source and substrate are all responsible for the formation of these unique self-assembled structures. The
intermolecular action is the internal driving force for the self-assembly. The formation mechanism of the arrays was proposed on the basis of the
growing process.
© 2006 Elsevier B.V. All rights reserved.
Keywords: SiO
x
nanowire; Self-assembly; Arrays; Gas-phase growth
1. Introduction
During the past several years, increasingly growing interests
have been focused on nanostructured materials, especially for

quasi-one-dimensional nanostructures such as nanorods, nano-
wires, nanotubes, nanobelts, nanorings, etc. because of not only
their importance in understanding fundamental physical phe-
nomenon e.g. the dependence of properties on size and dimen-
sionality, but also their exciting applications as functional
building units for future optical, electronic, magnetic, and bio-
nanodevices [1–4]. Various nanostructures of different chemical
compositions have been synthesized including metals, semi-
conductoring nitrides, oxides, sulfides, insulating oxides and
ceramics [5–7]. Among these inorganic materials, silica or
silicon oxide nanostructures have been extensively studied by
many a researcher worldwide. And some unique nano-ar-
chitectures of silica have been prepared via the fascinating self-
assembly of ordered or well-aligned silica nanowires or fibers
[8–12]. Recently, elemental gallium (Ga), a unique metal with
extremely low melting point (29.77 °C), has been disclosed to be
an excellent solvent and initiator for the self-assembly of silica or
silicon oxide nanowires [13–16], which can be ascribed to the
good solubility of Si atoms in liquid Ga at high temperature
(∼ 10 at.% at 920 °C) [17]. In this paper, we report a novel
structure of silicon oxide, 2D self-assembled arrays consisting of
SiO
x
nanowire bundles, grow n on a silicon wafer by a simple
gas-phase reaction using GaN powder as reactant. The factors,
influencing the formation of SiO
x
arrays, were discussed based
on the experimental results. The findings reveal that some
unique self-assembled architecture can be prepared by utilizing

the intrinsic characteristic of materials, as well as by controlling
the growth conditions.
2. Experimental
An appropriate amount of GaN powders (from Alfa Aesar,
99.99%) was loaded into an end of an alumina boat and a piece
of cleaned silicon wafer was placed above the GaN powders,
separated with a vertical distance of 5 mm. Then the boat was
transferred into the central zone of a ceramics tube positioned in
a horizontal tube furnace. The ceramics tube was sealed and
then filled with argon gas (99.9% purity). Subsequently, the
Materials Letters 61 (2007) 3662– 3665
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doi:10.1016/j.matlet.2006.12.011
furnace was heated to a preset temperature (1350 °C) at a rate of
8 °C/min under a flow of Ar gas of 200 ml/min. The temperature
was held for 150 min. After the reaction finished, the furnace
was allowed to cool down to room temperature. A white dot-like
deposit appeared on the surface of silicon substrate.
The as-prepared products were characterized by scanning
electron microscopy (SEM, Hitachi S-3500N) equipped with
energy dispersive X-ray spectroscopy (EDS), transmission
electron microscopy (TEM, Hitachi H-800) and selected-area
electron diffraction (SAED). For SEM observation, the products
together with the silicon wafer were put into SEM vacuum
chamber with no destruction of the deposit. For TEM analysis,
some products were scraped from the silicon substrate, and then

ultrasonically dispersed in ethanol and a drop was placed onto
an amorphous carbon-coated copper grid. The surface compo-
sition was analyzed by X-ray photoelectron spectroscopy (XPS)
technique. The XPS spectra were recorded at room temperature
on a V.G. Scientific ESCALAB Mark II system using Mg Kα
(hν =1253.6 eV) as X-ray source. The base pressure is about
2×10
− 6
Pa during analysis. The binding energy was referenced
to C
1s
peak (284.6 eV) with an accuracy of ±0.1 eV. The
intensity of each peak was estimated according to the inte-
gration area after subtracting an L-shaped background.
3. Results and discussion
Fig. 1 displays the surface morphology of the products deposited on
the silicon substrate. One can see that a lot of uniform dome-like
structures with well-ordered alignment in two dimensions stand on the
substrate surface, as shown in Fig. 1a. Fig. 1b exhibits a closer view,
presenting a more detailed observation. Each dome-like structure with
a same lateral size of 1.5 μm consists of self-assembled nanowire
bundles, and some dispersive bundles can be observed from its bottom.
The spacing of each self-assembled structure is 1 μm, and a large
quantity of nanowires derived from the bottom of the bundles forms an
interweaved network among the self-assembled structures on the sub-
strate surface.
The composition analysis of the products was achieved using EDS.
Fig. 2a shows EDX spectrum of the self-assembly of nanowire bundles,
suggesting that the as-grown products consist of silicon and oxygen
elements. The EDS measurement achieved on the spacing region be-

tween the self-assembly units displays the existence of a small amount
of elemental gallium (approximately atomic ratio of 1.08%), shown in
Fig. 2b, indicating that the formation of self-assembled structures is
mostly correlated with metallic Ga. Further composition analysis was
also carried out by XPS (Fig. 3). The two peaks at 103.62 eV and
532.96 eV correspond to Si2p and O1s binding energy, respectively,
indicative of the formation of silicon oxide nanowires [18]. The
quantification of the peaks suggests that the atomic ratio of Si to O is
approximately 1:1.5. There fore, the product is defined as SiO
x
nanowires.
Typical TEM photograph of the sample scraped from the silicon
substrate surface is shown in Fig. 4. A bundle of SiO
x
nanowires with
the length of several micrometers can be easily observed. The SAED
Fig. 2. EDX spectra of SiO
x
nanowire bundles (a) and of the spacing region
between self-assembly units (b), clearly suggesting the existence of a small
amount of Ga on the surface.
Fig. 1. SEM images of 2D ordered arrays self-assembled by SiO
x
nanowire
bundles (a) and (b).
3663S. Xiang, X. Xiang / Materials Letters 61 (2007) 3662–3665
pattern, carried out on the bundles, is displayed in the inset, clearly
indicative of the amorphous nature of the sample. The SEM, TEM and
SAED results indicate that the novel 2D ordered arrays are formed by
the self-assembly of SiO

x
nanowire bundles.
It is known that GaN decomposes above the temperature of 900 °C
to stepwise produce Ga and N species. In the present experiments, GaN
powder source will decompose to provide considerable amount of
gaseous Ga atoms or clusters for the further reaction at high
temperature (1350 °C). These Ga species are transferred by carrier
gas and deposited onto the silicon wafer to form tiny liquid Ga droplets,
uniformly dispersing on the substrate surface, which can solubilize Si
atoms to activate the growth of silica or silicon oxide nanowires owing
to the good solubility of Si atoms in molten Ga at high temperature
[17]. The detection of elemental Ga by EDS confirms the key role of
Ga for the growth of SiO
x
nanowires. Intrinsically, the self-assembled
structures, in our view, could be ascribed to the characteristic nature of
silica nanowire unit, which has a strong tendency to self-assemble into
bundles or other ordered structures by the intermolecular action e.g.
electrostatic forces [13,15]. Although silicon oxide nanowire bundles
and some interesting 1D self-assembly have been prepared in the last
several years, ordered 2D self-assembled arrays haven't been reported
until now. Therefore, the further experiments have been achieved to
understand the formation and the factors influencing the final products.
When the experiments were carried out in an oxygen atmosphere or
high-vacuum conditions, no silicon oxide nanowire bundles were
obtained, indicating that the O
2
-doped Ar atmosphere (99.9% purity) is
of great importance for the growth of these unique structures in view of
the purity of Ar gas and the leakage of the reaction system. Upon

placing a Si substrate in the alumina boat, with a horizontal separation
of 5–20 mm with GaN powders, only some SiO
x
bundles were grown,
while not well-aligned 2D arrays. Hence, the relative placement of Si
Fig. 3. XPS spectra of SiO
x
sample (a) Si2p and (b) O1s.
Fig. 4. Typical TEM photograph of a bundle of SiO
x
nanowires, and the
corresponding SAED pattern shown in the inset, revealing the amorphous nature
of the products.
Fig. 5. SEM images of 2D close-packed arrays of SiO
x
nanowire bundles grown
by adjusting the preparation parameters (a) and (b).
3664 S. Xiang, X. Xiang / Materials Letters 61 (2007) 3662–3665
wafer to GaN source is also a key factor for the formation of the unique
2D ordered arrays. According to the SEM observation and EDS
analysis, an interweaved network was formed on the bottom of the self-
assembled structures and a small amount of elemental Ga was detected
in the spacing region on the substrate surface. Based on these results,
we propose the possible formation mechanism of the 2D arrays. Firstly,
Ga droplets deposit on the Si surface and initiate the growth of SiO
x
nanowires. Then a large quantity of nanowires forms a network on the
surface. With the continuous deposition, the quantity of Ga droplets
increases. Due to the intermolecular action, SiO
x

nanowires form self-
assembly units on the specific sites of the Si surface. The further growth
of these self-assembly units, eventually, results in 2D ordered arrays. It
is evident that the reaction conditions have significant effects on the
formation sites of SiO
x
self-assembly on the substrate surface.
Furthermore, the contrast experiments also reveal the flow rate of
Ar gas has a major effect on the morphology of arrays. 2D ordered
close-packed arrays were grown at a lower flow rate of Ar gas (50 ml/
min) under the same reaction temperature, as exhibited in Fig. 5. The
close-packed arrays consist of similar dome-like structures but with no
distinct spacing, showing a dense alignment. Fig. 5b presents a
magnified view, more clearly revealing the uniform morphology and
size of each dome-like structure, which is formed via self-assembly of
nanowire bundles. It is estimated that the lower gas flow rate results in
the more concentrated deposition of liquid Ga droplets on the substrate
surface, consequently leading to a dense growth and self-assembly of
SiO
x
nanowires.
4. Conclusions
In summary, novel 2D ordered arrays self-assembled b y SiO
x
nanowire bundles have been prepared on a silicon wafer via a
simple gas-phase reaction using GaN powders as reactant. Two
types of arrays, the array with the same spacing and the close-
packed array, were grown on the substrate surface by adjusting
the flow rate of Ar gas. The intermolecular action and the
reaction conditions are both responsible for the formation of 2D

ordered arrays of SiO
x
nanowire bundles. The former is the
internal driving force for the self-assembly. The deposi tion of
tiny Ga droplets on the specific sites of Si surface due to the
intermolecular action leads to the unique 2D array structures.
The 2D ordered arrays could act as ideal building blocks for the
future integrated optoelectronic nanodevices.
Acknowledgement
The authors gratefully acknowledge the financial support
from the Natural Science Foundation of Henan Province
(0611054200).
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