Large scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide

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 evaporation
Photoluminescence
abstract
A single step non-catalytic process based on thermal evaporation of silicon monoxide has been
established for large-scale synthesis of silica nanowires. Scanning electron microscopy, high-resolution
transmission electron microscopy equipped with energy dispersive X-ray spectrometry (EDAX), X-ray
diffractometry were used to characterize the morphology and structure of the material. The as-
synthesized nanowires had amorphous structures with diameters in the range 30–100 nm and
hundreds of micrometers in length. The EDAX analysis revealed that the nanowires consisted of mainly
two elements Si and O in an atomic ratio of approximately 1:2 corresponding to silicon dioxide.
Photoluminescence spectra of the silica nanowires showed strong blue emission around 393 nm.
Nucleation and growth of silica nanowires has been discussed on the basis of tiny oxide cluster
formation that act s as nucleation centers for the nanowires growth.
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
During past two decades, a lot of attention has been paid to the
growth and characterization of one-dimensional (1-D) nanos-
tructures such as nanotubes, nanowires, nanobelts because of
their distinctive structure, unique properties and applications
[1–3]. Silicon-based nanostructures have attracted significant
attention due to their potential applications in electronics and
opto-electronic devices [4]. For example, silicon oxide (SiO
x
)
nanowires show intensive blue light emission, which may be a
candidate material for high-resolution optical heads of scanning
near-field optical microscopes, nanointerconnection integrated
optical devices, low-dimensional wave-guides, etc. [5–7]. Several
methods such as laser ablation [5,8], thermal evaporation [7,9],
carbothermal reduction or carbon-assisted growth [6,10,11], direct

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

nanowires growth and proposed an alternative catalyst-
assisted mechanism based on preferential adsorption of SiO
x
on the
catalyst droplet without penetration into it.
In this paper, we report large-scale synthesis of pure silica
(SiO
2
) nanowires by a non-catalytic approach based on thermal
evaporation of SiO under argon ambient with traces of oxygen.
The process is simple yet elegant and involves only single process
step wherein the SiO vapors are transported from hot zone
($1200 1C) to downstream low-temperature zone where they are
allowed to condense on a substrate. The structure and photo-
luminescence (PL) property of the as-deposited material has been
investigated and the growth of SiO
2
nanowires is discussed on the
basis of tiny SiO
2
cluster formation via direct reaction of SiO
vapors with O
2
that subsequently acts as nucleation center for
SiO
2
nanowire growth.
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journal hom epage: www.elsevier.com/locate/physe

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).
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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

1200 1C for 1 h in argon ambient (99.95%) with traces of oxygen
and moisture. Therefore, the growth time was 1 h. After
completion of the growth process the system was cooled down
to room temperature under Ar flow. A thick wool-like spongy,
white-colored material was deposited on the silicon substrates
(both on Ni coated and without Ni film) as well as on the alumina
sample holder. A thick white-colored material was also deposited
on the walls of the quartz tube in low-temperature zone. To
investigate the role of SiO as source, only silicon substrates were
heated at 1000 1C (since the silicon substrates were placed at
1000 1C with source SiO at $1200 1C) for 1 h without SiO granules
at the center zone. No deposition was found on the silicon
substrates in this case.
The as-deposited film on silicon substrates and the material
collected from the quartz tube as well as from alumina sample
holder were examined by scanning electron microscopy (SEM;
LEO 440 VP) operating at 15 kV. Structural analysis of the material
was carried out by X-ray diffractometery (XRD) (Bruker, D-8
ADVANCE diffractometer) with CuK
a
radiation, high-resolution
transmission electron microscopy (HRTEM) (FEI, Technai G20-
stwin; 200 kV) equipped with energy dispersive X-ray spectro-
scopy (EDAX) (EDAX company, USA). A part of the white film was
delaminated from the Si wafer and was ultrasonically dispersed in
ethanol for 10 min. A few drops of the suspension were placed on
carbon-coated copper (Cu) microgrid for TEM and HRTEM
investigations. PL measurement of the material was carried out
at room temperature using double monochromator-based spec-
trometer (PerkinElmer LS55) with xenon flash lamp as excitation

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
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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.
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than 100
m
m. Fig. 2(b) gives an idea of nanowires diameter
distribution which, indeed, is quite uniform. The nanowires
formation is straight, free from wrinkles and chain-like
morphology. It is to be noted that a thick white layer was
deposited over the entire area of the Ni-coated Si wafers even on
the side edges. To study the role of Ni on the growth of nanowires,
we performed identical experiment on silicon wafers without Ni
film as well as on alumina substrate holder. A high density
of nanowires was also observed on Si wafers without Ni film (see
Fig. 2(c)) similar to that observed with Ni film. This clearly
indicates the non-catalytic growth of nanowires, which was
further confirmed by the investigations of the white spongy
material deposited on the inner surface of the quartz tube
and also on the alumina sample holder. Fig. 2(d) shows the
low-magnification SEM image of the substance collected from
the quartz tube, which shows agglomerated clusters consisting
of high density of nanowires. The magnified view of a single
cluster is depicted in the inset of Fig. 2(d). These observations
suggest that growth of nanowires in the present process is
probably not governed by the VLS or SLS mechanism, which
essentially is a catalytic process. The present results are different
from that of Aharonovich et al. [8,22] where metal catalyst
(Au or Ni) was found to be essential for the SiO
x
nanowires
growth and also different from bi-cycle chain-like morphology
of silica nanowires observed by Kar and Chaudhuri [14]

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

core-shell kind of structure as
seen by Park and Yong [13] and Zhang et al. [23] was observed.
This reveals that the nanowires have uniform amorphous
structure across the length and diameter. EDAX spectra of the
nanowires were also recorded during TEM investigation to
examine their chemical composition. The EDAX spectrum shown
in Fig. 4 for a single nanowire reveals presence of only two
elements Si and O with an atomic ratio of approximately 1:2
(strong C and Cu signals are attributed to the carbon-coated Cu
microgrid). Based on the above observations, we may conclude
that the nanowires are amorphous silicon dioxide (SiO
2
).
3.3. Photoluminescence
Fig. 5 shows the PL spectrum of SiO
2
nanowires recorded at
room temperature with 241 nm excitation. A strong blue
luminescence is observed with peak position at 393 nm
($3.15 eV). The PL properties of silica nanowires have been
investigated before showing single or two PL bands depending
on their structural properties [5,7,24]. For example, Yu et al. [5]
observed two broad PL peaks of SiO
2
nanowires at 470 nm
($2.65 eV) and 420 nm ($3.0 eV), whereas Zhu et al. [24]
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100 nm
5 nm
Fig. 3. (a) TEM micrographs of the nanowires showing metal-free ends with

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).
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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

x
vapors into the metal
particles, (ii) formation of liquid droplet of metal and Si/SiO
x
and (iii) the precipitation in the form of solid nanowires after
super-saturation of liquid metal–Si/SiO
x
droplet at the liquid–
solid interface. Thus a metal nanoparticle at one end of the
nanowires is usually considered as the evidence for the operation
of the VLS model [26]. But no metal particles were found at either
end of the nanowires in the present study as confirmed by SEM
and TEM results. Therefore, VLS or SLS mechanism is not pertinent
to explain the present experimental observations. Further, growth
of SiO
2
nanowires on silicon wafers without Ni film, alumina
holder and quartz tube rules out the metal catalyst-assisted
growth mechanism by Aharonovich and Lifshitz [22]. In the
present case, the growth of the SiO
2
nanowires cannot be
explained using the oxide-assisted growth (OAG) process pro-
posed by Lee et al. [26–28] for the crystalline Si nanowires. The
OAG process presumes an oxide cluster rather than a metal
particle after the VLS mechanism to assist the formation of the
nanowires wherein an outer oxide shell formation is essential to
prevent the growth along the lateral direction that results in 1-D
Si-SiO
x

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

towards the substrates by the carrier gas and during traversal they
get converted into SiO
2
molecules directly by reacting with O
2
according to the following equation:
2SiOðgÞþ2O
2
ðgÞ!2SiO
2
ðsÞ
It may be remarked here that in the present experiment no
special arrangement was used to remove residual O
2
from the
carrier gas or from the process chamber (either by vacuum or
hydrogen gas or O
2
traps) consequently, the traces of residual O
2
in the chamber could be present. The SiO
2
molecules so formed
condense on the substrate or quartz tube wall in the low-
temperature zone ($1000 1 C) to form SiO
2
nanoclusters that then
act as nucleation center for the growth of SiO
2
nanowires.

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
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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

Fig. 5. Room temperature PL spectrum of the SiO
2
nanowires recorded with
241 nm excitation.
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clusters and hence into 1-D s ilica nanowires. The pr esent study
shows that no p r eferential sit e or m orphology o f the su bstrat e is
required for the formation of the SiO
2
nanowires. Though ex act
formation mechanism of s ilica nanowires from SiO vapors, factors
controlling the diameters of the nanowires in the present approach,
remains still unclear and needs f u rther detail investigations, the
present work is useful due to its simplicity and the low cost.
4. Conclusions
Amorphous silicon dioxide nanowires of several hundred
microns in length and tens of nanometers in diameter have been
synthesized in bulk by a non-catalytic single step process using
thermal evaporation of silicon monoxide under argon atmosphere
with traces of oxygen. The nanowires were free from metal
contaminations and showed blue photoluminescence at room
temperature. It is proposed that in-situ formation of SiO
2
vapors
via reaction of SiO vapors with O
2
leads to the formation of SiO
2
nanoclusters, which consequently results in the formation of large

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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