Large-scale synthesis, characterization and photoluminescence properties of
amorphous silica nanowires by thermal evaporation of silicon monoxide
Sanjay K. Srivastava
a,
Ã
, P.K. Singh
a
, V.N. Singh
b
, K.N. Sood
a
, D. Haranath
a
, Vikram Kumar
a
a
National Physical Laboratory, Dr. K. S. Krishnan Marg, Pusa, New Delhi 110012, India
b
Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India
article info
Article history:
Received 29 March 2009
Received in revised form
27 April 2009
Accepted 27 April 2009
PACS:
61.46.–w
81.07.–b
Keywords:
Silicon monoxide
Silicon oxide nanowires

thermal oxidation of Si wafers [12–14] have been used to
synthesize SiO
x
nanowires. However, most of these methods
employ metal catalysts such as Au [7,8,11,14],Ni[14,15],Fe[5,6],
Co [16],Ga[17,18],Cu[19],Sn[20] to assist the synthesis process
and consequently, the nanowires have significant presence of
embedded residual metallic impurities that may affect their
properties. In the recent past, non-catalytic growth of silica
nanowires via carbothermal reduction of metal oxides such as MgO,
CuO, WO
3
has also been reported. Despite considerable experi-
mental efforts, the gr owth m echanism of silica nano wires is not well
understood and indeed no consensus about the growt h mechanisms
has b een achieved. One school of thoughts b e lieves that vapor–
liquid–solid (VLS) [5,6,2 1] or solid–liq uid–solid (S LS) [13,1 5] pro-
cesses are the possible mechanisms i n c atal y st-assist ed g r owth o f
amorphous SiO
x
(a-SiO
x
) nano w ir es . Other sch ool suggest ed differ-
ent chemical reactions and sequences for the a-SiO
x
nanowires
formation [10,11,17 ] to explain their experimental results. Recentl y,
Aharonovich and Lifshitz [22] found that metal catalyst is essential
for S iO
x

Physica E
1386-9477/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physe.2009.04.032
Ã
Corresponding author. Tel.: +9111 4560 8617; fax: +9111 2572 6938.
E-mail address: (S.K. Srivastava).
Physica E ] (]]]]) ]]]–]]]
Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032
2. Experimental
The growth process was carried out in a conventional three-
zone horizontal quartz tube furnace, the schematic of which is
shown in Fig. 1. The requisite amount of source material, i.e.,
silicon monoxide (SiO) granules (purity $99.9%; Pure Tech Inc.,
New York, USA) was kept in an alumina boat that was placed in
the center of the quartz tube. Ultrasonically cleaned silicon strips
of 2 Â 2cm
2
with and without Ni film were placed downstream in
the lower temperature zone of the furnace on an alumina sample
holder. Thin Ni film (Ni powder, purity 99.99%; CERAC Inc., USA)
was deposited on cleaned silicon wafers by thermal evaporation
technique at a base pressure of 3.0 Â 10
À6
Torr. The source SiO and
the substrates were inserted in the tube and kept at locations at
$1200 1C (center zone) and $10 00 1C (in the direction of carrier
gas flow), respectively, identified earlier by temperature profiling.
The quartz tube system was purged with a carrier gas (argon) flow
for three hours before heating up to 1200 1C under a constant
argon flow at the rate of 40 l/h. The source SiO was then heated at

source.
3. Results and discussion
3.1. Microstructural analysis
A low-magnification SEM micrograph of the as-deposited
wool-like thick white film on Ni-coated silicon wafer and its
magnified view is shown in Fig. 2(a) and (b), respectively, where
high density of 1-D nanostructures in the form of wires having
hundreds of micrometers length is clearly seen. Fig. 2(a) also
reveals that several layers of nanowires were deposited one over
another and total thickness of the film is estimated to be more
ARTICLE IN PRESS
Ar
Outlet
1
23
3- Zone Tube Furnace
Quartz Tube
SiO
Alumina boats
Si wafers
Fig. 1. Schematic diagram of horizontal furnace set up for the synthesis of silica
nanowires.
50 µm
10 µm
10 µm
2 µm
25 µm
Fig. 2. SEM micrographs of (a) nanowires deposited on Ni-coated Si wafers (low magnification), (b) magnified view of (a), (c) nanowires deposited on Si wafers without
catalyst Ni film, (d) product collected from quartz tube magnified view of which is shown in the inset.
S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]]2

during carbon-assisted non-catalytic growth. It is also to be
remarked here that no nanowire growth was observed on
silicon substrates heated at 100 0 1C without SiO source at
center of tube (1200 1C), which clearly indicates that the
nanowires growth in the present process does not take place
due to silicon substrates heating in presence of oxygen traces or
moisture. The growth of thick nanowire film on alumina substrate
holder is also evidence that SiO is the main source for silicon
nanowires growth.
Fig. 3(a) shows a typical TEM micrograph of nanowires
deposited on Ni-coated Si wafers. The nanowires have diameter
in the range 30–100 nm with center of the distribution at $50 nm.
The diameters remain nearly constant throughout the length of
the nanowires. The nanowires have remarkably clean and smooth
surface. It is important to note here that the nanowires have
circular cross-section revealing the cylindrical nature (shown by
circles in Fig. 3(a)) and no metal particles are seen at either end of
the wires. The TEM investigations of the material further confirm
our view that growth of nanowires is essentially non-catalytic in
the present process.
3.2. Structural and compositional analyses
The XRD patterns (not shown here) revealed amorphous
character of the deposited nanowires film, which was further
confirmed by the HRTEM (Fig. 3(b)) study. The selected area
electron diffraction (SAED) pattern (shown in the inset of Fig. 3(b))
recorded from a single nanowire where only diffusive rings reveal
the amorphous nature of the nanowires. No lattice fringes could
be resolved in the HRTEM across the diameter of the nanowires
(Fig. 3(b)). Furthermore, no Si-SiO
2

circular cross-section (indicated by circles), (b) HRTEM micrograph of a nanowire
showing amorphous structure. The SAED pattern of the nanowire is shown in the
inset of (b).
S.K. Srivastava et al. / Physica E ] (]]]]) ]]]–]]] 3
Please cite this article as: S.K. Srivastava, et al., Physica E (2009), doi:10.1016/j.physe.2009.04.032
reported that two broad PL peaks of SiO
x
were at around 570 nm
($2.2 eV) and 430 nm ($2.88 eV). On the other hand, Wang et al.
[7] observed single broad PL peak at $446 nm ($2.78 eV) from
SiO
x
nanowires. These emissions have been attributed to the
structural defects related to oxygen deficiency in the silica
nanowires that act as radiative recombination centers.
Nishikawa et al. [25] have observed several luminescence bands
in the range 1.9–4.3 eV in various types of high-purity silica
glasses where the band at 3.1 eV was attributed to some intrinsic
diamagnetic defect centers, such as twofold coordinated silicon
lone pair centers (O–Si–O) caused by high oxygen deficiency in
the samples. Therefore, the observed blue light emission from the
silica nanowires in the present study could have its origin to the
structural defects such as oxygen deficiency, which might have
been generated during the nanowires growth.
3.4. Growth mechanism
The growth mechanism in 1-D nanostructures has been
explained by the screw dislocation model and the VLS model in
the past. The former is not appropriate in case of amorphous
nanowires whereas the latter is based on the three-step mechan-
ism involving (i) diffusion of Si/SiO

core-shell nanowire [26] formation. However, the nano-
wires grown in the present study are amorphous across the
diameter instead of crystalline Si core and amorphous SiO
2
outer
shell structure as confirmed by HRTEM image and SAED pattern.
On the other hand, the formation of crystalline Si–SiO
2
core-shell
nanowires first and then complete oxidation of the structure to
result SiO
2
nanowires is also not practically possible as discussed
by Buttner and Zacharias [29]. Therefore, what could be the
driving force for the formation of 1-D amorphous SiO
2
nanowires?
Since no crystalline Si embedded in SiO
2
outer shell was
observed unlike some earlier reports on growth of Si nanowires by
SiO evaporation [23], the concept that SiO first disproportionate
into Si and SiO
2
to form Si nanowires with SiO
2
outer layer may
not be applied. Therefore, the following reaction mechanism may
be proposed to explain our observations. The SiO vapors,
generated at temperature $1200 1C, are transported downstream

Subsequently, SiO
2
nanoclusters may aggregate to induce 1-D
SiO
2
nanostructures to minimize its systemic energy [30]. The
proposed mechanism may find theoretical support by Zhang and
Zhang [31] who found that growth of energetically favored
anisotropic 1-D silica nanowires may occur without metal catalyst
template by short-range ordering of building blocks such as
(SiO
2
)
8
clusters. They showed that silica cluster (SiO
2
)
8
is
geometrically highly symmetric structure, energetically highly
stable with high chemically reactive ends of SiQO groups, and
thus make it easy to be assembled into larger linearly extended
ARTICLE IN PRESS
Fig. 4. EDAX spectrum of a nanowire (shown in the inset) showing Si and O as
main detected elements. The quantitative data is also shown in the inset.
300
393 nm
PL Intensity (a.u.)
Wavelength (nm)
350 400 450 500 550 600 650 700

nanowires. The present simple and low-cost process of producing
pure silica nanowires (free from metallic contaminations) in bulk
may lead to potential applications in nanoelectronics and optical
devices.
Acknowledgements
The authors wish to thank Ms. Manisha and Dr. S.K. Halder for
XRD measurements of the samples and the Director, NPL for his
permission to publish this work.
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