1. Synthesis, structural and photoluminescence properties of SiOx nanospheres
prepared by evaporation of silicon monoxide
Sanjay K. Srivastava ⁎, Vikas Sareen, Mukul Sharma, Vandana, P.K. Singh
National Physical Laboratory, Council of Scientific & Industrial Research, Dr. K.S. Krishnan Road, New Delhi-110012, India
a b s t r a c ta r t i c l e i n f o
Article history:
Received 11 March 2011
Accepted 3 July 2011
Available online xxxx
Keywords:
Amorphous materials
Nanoparticles
Microstructure
Luminescence
Electron microscopy
We report synthesis of amorphous silicon oxide nano-spheres in bulk quantity by using a simple non-catalytic
process based on thermal evaporation of silicon monoxide. Structural characteristics of the nano-spheres
were investigated using high resolution scanning and transmission electron microscopes equipped with
energy dispersive X-ray spectroscopy (EDAX). Selected area electron diffraction and EDAX analyses revealed
that nano-spheres were amorphous and comprised of silicon oxide only with Si and O in an atomic ratio of
~1:2. Photoluminescence measurements showed that the SiOx nano-spheres had strong emission band in the
blue region. Possible growth mechanism based on vapor-solid model has been discussed. The SiO vapors
generated at high temperature react with O2 to form SiOx vapors which subsequently condense on the
substrates (placed in the appropriate temperature zone) to form SiOx nano-spheres.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
There has been increasing interest in the synthesis of nanoscale low-
dimensional materials such as zero-dimensional (0-D) (nanospheres,
nanoparticles) and one-dimensional (1-D) (nanowires, nanotubes)
nanostructures owing to their important applications in optical,
catalytic and sensing devices [1]. Variety of 0-D oxide nanostructures
of materials such as, ZnO [2], TiO2 [3], TeO2 [4], etc. have been
synthesized by different techniques, for example, by hydrothermal and
solvothermal methods, sol–gel method, vapor transport or vapor-solid
method, solid state pyrolytic reaction, etc. [2–6]. Among these, silicon
oxide (SiOx) nano-particles have attracted great interest owing to their
novel catalytic properties [7]. The SiOx nano-particles are used to make
electronic substrates, humidity sensors, electrical and thermal in-
sulators, etc. and in each of these applications their roles are different.
In addition, The SiOx nanostructures show intensive blue light emission,
which may be a candidate material for high-resolution optical heads of
scanning near-field optical microscopes, nano interconnection integrat-
ed optical devices, low-dimensional wave-guides, etc. [8–10]. There are
several methods to synthesize SiOx nano-spheres primarily based on
chemical routes [11,12]. However, these methods are complicated, time
consuming and difficult to scale up. SiOx nano-spheres have also been
fabricated by thermal evaporation of Si/SiO2 mixture [13] but as a
byproduct of silicon nanowires. Therefore, synthesis of SiOx nano-
spheres by using a simple and inexpensive process up-gradable to bulk
production is of great interest. Further, not much attention has been
given to photoluminescence (PL) properties of the SiOx nano-spheres,
though PL of high purity silica glasses with lifetime between 0.1 and
10 ms (depending on the origin of the luminescence bands) has been
reported by Nishikawa et al. [14].
In this letter, we report synthesis of pure SiOx nano-spheres using a
non-catalytic approach based on thermal evaporation of silicon
monoxide (SiO). Key features of this approach are: (i) it is a single
step process, (ii) does not require expensive vacuum equipment, and
(iii) free of metallic impurities neither in pure nor in oxide form. Here,
SiO vapors are transported from hot zone (~1300 °C) to downstream
lower temperature zone where they are allowed to condense on the
substrates. Morphology, structural and PL properties of the as-deposited
material has been investigated and growth mechanism of SiOx nano-
spheres is discussed.
2. Experiments
Growth was carried out in a conventional horizontal alumina tube
furnace (ID: 58 mm, hot zone 200 mm with a total tube length:
600 mm). SiO powder (purity ~99.9%, Pure Tech. Inc. New York, USA)
was kept in an alumina boat at the center of the tube. Cleaned silicon
substrates (size: 2×2 cm2
) were placed downstream (in thedirection of
carrier gas flow) in the tube on an alumina sample holder. The source
SiO and the substrates temperatures were measured to be ~1300 °C and
~1100–1150 °C respectively. The alumina tube was purged with Ar
(99.95% purity with traces of oxygen and moisture), which is also used
as the carrier gas, for three hours prior to heating up to 1300 °C under a
constant flow rate of 50 l per hour. Growth was carried out for 1 h. Then
the tube was cooled down to room temperature under flowing Ar. Thick
Materials Letters 65 (2011) 3202–3204
⁎ Corresponding author. Tel.: +91 11 45608617; fax: +91 11 45609310.
E-mail address: srivassk@nplindia.org (S.K. Srivastava).
0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2011.07.006
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet

2. wool-like white material was deposited on silicon substrates, substrates
holder as well as on the tube inner wall.
Morphology of the as-deposited films was examined by scanning
electron microscope (SEM; model LEO 440 VP). Structural and
compositional analysis of the material was carried out by using
high-resolution transmission electron microscope (HR-TEM; model
FEI, Technai G20-stwin, 200 kV) equipped with energy dispersive
X-ray spectroscopy (EDAX; model EDAX Company, USA). PL mea-
surements of the sample were carried out at room temperature using
double mono-chromator based spectrometer (model Perkin Elmer
LS55) with xenon flash lamp as excitation source.
3. Results and discussion
3.1. Micro-structural analysis
Fig. 1 shows a representative SEM image of wool-like white film
deposited on the silicon substrates. It consists of a high density of
spherical nano-particles. Similar nanostructures were also found in
the material deposited over alumina sample holder as well as in the
powder collected from the alumina tube. No other structures were
observed in the temperature region 1100–1150 °C demonstrating that
purely spherical structure and not a mixture of nanowires, nano-
particles, chains etc. can be grown with this process.
Fig. 2(a) shows representative TEM micrograph of the nano-
spheres. Magnified view of the nano-spheres is shown in the inset of
Fig. 2(a). Most of the nano-spheres have diameter in the range of
40–300 nm with center of the distribution at ~60 nm. Some large
spheres (diameter: 500–850 nm) were also found in the deposits. A
representative magnified view of large spheres is shown in Fig. 2(b).
These micrographs clearly show that the microstructures have perfect
spherical shape with remarkably clean and smooth surface.
3.2. Structural and compositional analysis
Highly diffusive ring pattern in selected area electron diffraction
(SAED) pattern (shown in the inset of Fig. 2(c)) reveals that the
nano-spheres are of a purely amorphous nature. The HR-TEM study
further confirmed that no crystalline structure exists in the nano-
spheres. No lattice fringes could be resolved by the HR-TEM across the
diameter of the nanospheres (Fig. 2(c)). Furthermore, no Si–SiO2
core–shell structure was observed. This reveals that the nano-spheres
have uniform amorphous structure across the diameter. The EDAX
spectrum shown in Fig. 2(d) for a single nano-sphere shows that they
are composed of mainly two elements Si and O. Strong C and Cu
signals are attributed to the carbon coated Cu micro-grid. The
quantitative analysis shows that the atomic ratio of Si:O is about
1:1.93. Based on above observations it can be concluded that the
deposited material comprised of pure and amorphous SiOx nano-
spheres. The exact mechanism of SiOx nano-spheres formation by
vapor transport of SiO is not fully understood. However, our results of
nano-sphere formation could be explained on vapor-solid growth
based mechanism proposed in the following paragraph.
The SiO vapors, generated at ~1300 °C, are transported towards
the substrates by the carrier gas and during traversal they get
converted into SiOx molecules directly by reacting with O2 traces
available in Ar. This is because no special arrangement was used to
remove moisture or residual O2 from the carrier gas or the process
tube (either by creating vacuum, using hydrogen gas or putting O2
traps). However, care was taken to avoid large O2 concentration by
purging the process tube with Ar prior to heating. Therefore, following
reaction could take place:
SiOðgÞ þ O2ðgÞ→SiOxðsÞ
The SiOx molecules so formed condense on the substrates in the
temperature zone (~1150 °C) to form SiOx nano-clusters which then
act as nucleation centers. The SiOx nano-clusters have high surface
energy at such a high temperature and subsequently would aggregate
to induce SiOx nano-spheres to minimize their systemic energy [15]
since for a given volume system energy of a sphere is the minimum.
3.3. Photoluminescence
Fig. 3 shows the room temperature PL spectrum of SiOx nano-
spheres recorded with a 241 nm (~5.1 eV) excitation. A strong blue
emission band is observed with peak centered at ~390 nm (~3.15 eV).
In the past, PL in SiOx nanostructures, mostly in nanowires, have been
reported. The occurrence of a single or two PL bands have been
correlated with material structural properties [8,9]. For example, Yu
et al. [8] observed two broad PL peaks at ~2.65 eV and ~3.0 eV,
whereas Wang et al. [9] reported a single broad PL peak at ~2.78 eV
from the SiOx nanowires. These emissions have been attributed to the
structural defects related to oxygen deficiency in SiOx nanowires
which act as radiative recombination centers. Nishikawa et al. [14]
have observed several luminescence bands in the range of 1.9–4.3 eV
in different high purity silica glasses with PL lifetime between 0.1 and
10 ms (depending on the origin of luminescence bands). For example,
lifetime of PL band at ~3.1 eV has been observed to be ~0.1 ms and
attributed to intrinsic diamagnetic defects centers, such as two-fold
co-ordinated silicon lone pair centers (OSiO) caused by high
oxygen deficiency in the samples [14]. Therefore, the observed blue
emission from SiOx nano-spheres at ~3.15 eV could also have its origin
to the structural defects such as oxygen deficiency, possibility of
which is high in the present experiment as we have tried to exploit
the limited presence of O2/moisture in the carrier gas during the
growth. It is also supported by 1:1.93 atomic ratio of Si:O as
determined by EDAX analysis. However, more precise and controlled
experiments are required to establish this fact and present results
may be a guide for further investigations.
4. Conclusions
Amorphous SiOx nano-spheres of an average diameter ~60 nm
were synthesized by using thermal evaporation of SiO. The nano-
spheres were free from metal contaminations and showed blue
photoluminescence. In-situ SiOx vapors formation via reaction of SiO
vapors with O2 resulted into the formation of SiOx nano-clusters
which subsequently formed nanospheres on the substrates placed in
the appropriate temperature zone. The present simple and low cost
Fig. 1. Representative SEM micrograph of spongy-white film deposited on silicon
substrates showing spherical particles.
3203S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204

3. process for the synthesis of pure SiOx nano-spheres may lead to their
potential applications in catalysis, optical and sensing devices.
Acknowledgments
Authors are thankful to Dr. V.N. Singh for HRTEM and Dr. D.
Haranath for PL measurements of the samples. The work was partially
supported by the CSIR project (SIP-17).
References
[1] Rao CNR, Govindaraj A, Vivekchand SRC. Annu Rep Prog Chem Sect A 2006;102:
20–45.
[2] Meulenkamp EA. J Phys Chem B 1998;102:5566–72.
[3] Yang HG, Zeng HC. J Phys Chem B 2004;108:3492–5.
[4] Qin B, Bai Y, Zhou Y, Liu J, Xie X, Zheng W. Mater Lett 2009;63:1949–51.
[5] Chen X, Mao SS. Chem Rev 2007;107:2891–959.
[6] Wang ZL. J Phys Condens Matter 2004;16:R829–58.
[7] Gole JL, White MG. J Catal 2001;204:249–52.
[8] Yu DP, Hang L, Ding Y, Zhang HZ, Bai ZG, Wang JJ, et al. Appl Phys Lett 1998;73:
3076–8.
[9] Wang YW, Liang CH, Meng GW, Peng XS, Zhang LD. J Mater Chem 2002;12:651–3.
[10] Pang CL, Cui H, Wang CX. Cryst Eng Comm 2011;13:4082–5.
[11] Pei LZ. Mater Charact 2008;59:656–9.
[12] Rao KS, El-Hami K, Kodaki T, Matsushige K, Makino K. J Colloid Interface Sci
2005;289:125–31.
[13] Gole JL, Stout JD, Rauch WL, Wang ZL. Appl Phys Lett 2000;76:2346–8.
[14] Nishikawa H, Shiroyama T, Nakamura R, Ohki Y. Phys Rev B 1992;45:586–91.
[15] Zhang Y, Wang N, He R, Liu J, Zhang X, Zhu J. J Cryst Growth 2001;233:803–8.
300 400 500 600 700
0
50
100
150
200
250
300
350
PLIntensity(a.u.)
Wavelength (nm)
Fig. 3. Room temperature PL spectrum of nano-spheres sample.
830 nm
250 nm
130 nm
100 nm
(b)
5 nm
(c)
500 nm
(a)
50 nm
0 2 4 6 8 10
0
100
200
300
400
500
600
700
800
Cu
Cu
Si
Si
Cu
O
C
Counts(a.u.)
Energy (KeV)
(d)
Fig. 2. (a) TEM micrograph showing uniformly dispersed nano-spheres, magnified view of which is shown in the inset, (b) TEM micrograph of large nano-spheres, (c) HR-TEM
micrograph of nano-spheres showing amorphous nature (inset shows the corresponding SAED pattern), and (d) EDAX spectrum of a nano-sphere (quantitative data given in the
inset).
3204 S.K. Srivastava et al. / Materials Letters 65 (2011) 3202–3204