Physica E 31 (2006) 218–223
Photoluminescence and growth mechanism of amorphous silica
nanowires by vapor phase transport
Y. Yang
a
, B.K. Tay
a
, X.W. Sun
a,Ã
, H.M. Fan
b
, Z.X. Shen
c
a
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
b
Department of Physics, National University of Singapore, Blk S12, 2 Science Drive 3, Singapore 117542, Singapore
c
School of Physical and Mathematical Sciences, Nanyang Technological University, Block 5, 1 Nanyang Walk, Singapore 637616, Singapore
Received 23 November 2005; received in revised form 13 December 2005; accepted 20 December 2005
Available online 21 February 2006
Abstract
Amorphous silica [SiO
x
ð1oxo2Þ] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of
ZnCl
2
, and VO
2
powders at 1100 1C. The length of SiO
x

because of their potential applications in light-emitting
devices compatible to CMOS technology. Amorphous
silica nanowires (SiONWs) are promising 1D luminescence
materials. The photoluminescence (PL) band of bulk silica
or silica films has a peak within 1.6–7.0 eV [2–4] from both
experimental measurements and theoretical calculations.
Yu et al. [5] have pointed out the potential applications of
silica nanowires in high-resolution optical heads of
scanning near-field optical microscope or nanointerconnec-
tions in future integrated optical devices. Much research
interest has recently been directed to synthesize these
materials by various approaches, to understand their
growth mechanism and to realize their controlled growth
on planar substrates.
Various approaches, for instance, vapor phase trans port
[6], bio-mimetic strategies [7,8], excimer laser ablation [9],
physical and thermal chemical evaporation [10–16],
carbothermal reduction [17], thermal chemical vapor
deposition [18], thermal oxidation [19] and solution
method [20,21] have been emp loyed to fabricate the
nanostructured silica with different morphologies including
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Ã
Corresponding author. Tel.: +65 67905369; fax: +65 67920415.
E-mail address: [email protected] (X.W. Sun).
silica ‘‘nanoflowers’’ [22], radial patterns of carbonated
silica fibers [10], silica nanowire ‘‘braids’’ [6], ‘‘bundles’’

temperature (1090–700 1C) as the source of silicon as well
as substrate. Then the small qua rtz tube was placed into a
bigger quartz tube and pushed into the tube furnace. The
furnace was heated from room temperature to 1100 1C and
kept at this temperature for 30 min. When temperature
reached 975 1C, an extra mechanical pump was used
to collect the white fog due to the hydrolyzation and
oxidation of ZnCl
2
, and maintain a pressure of 2–0.3 Pa in
the tube.
The morphology, size, and crystal structure, of the
SiONWs were determined using a cold cathode field
emission scanning electron microscope (SEM) from Jeol
(model JSM-6340F), and a transmission electron micro-
scope (TEM) from Jeol (model JEM-2010F) too. The
chemical composition analysis was carried out using energy
dispersive X-ray spectroscopy (EDX) which was attached
to the SEM. PL measurements were carried out at room
temperature using a Micro-PL system from Renishaw. The
excitation line used was 325 nm and the average power was
10 mW. Laser beam was focused into a spot diameter
below 1 mm on the specimen in the PL measurement.
3. Results and discussion
Figs. 1(a) and (b) show the SEM images of the nanowires
with high aspect ratio (length/diameter). Fig. 1(c) shows
the photograph of the sample, with positions (a) and (b)
labeled, the SEM images in Figs. 1(a) and (b) were taken,
respectively. It can be seen from Fig. 1(c) that bulk
quantity of cotton-like nanostructures was formed. The

Y. Yang et al. / Physica E 31 (2006) 218–223 219
Fig. 2(a) shows a TEM image of the SiO
x
nanowires
obtained near 1000 1C temperature region. The inset is the
corresponding selected-area electron diffraction (SAED)
pattern recorded from the nanowires. The as-deposited
SiONWs is of amorphous phase, indicated by the highly
diffusive SAED ring pattern. We can see that, the
nanowires are remarkably clean and smooth. Fig. 2(b)
shows the high-magnification TEM image of a catalyst tip,
and Figs. 2(c) and (d) are the corresponding SAED and
high-resolution TEM image of the catalyst, respectively.
The catalyst is crystallized and surrounded by amorphous
silica, indicating that the growth process of amorphous
silica nanowires is catalyst-assisted.
EDX was applied to examine the chemical composition
of the as-grown nanowires. Fig. 3(a) shows the EDX
spectrum of the round tip on top of a nanowire (Fig. 2(b)),
and Fig. 3(b) shows the EDX spectrum from long bundles
of tangled silica nanowires in Fig. 1(b), where the
nanowires are so long that almost no tip was in the area
examined. Analyzing Figs. 3(a) and (b) using the EDX
spectrometer’s own computer program, the chemical
compositions are 48.77 at% of O, 41.87 at% of Si,
8.66 at% of V, 0.74 at% of Zn, and no Cl, for the catalyst
tip in Fig. 2(b), and 60.28 at% of O, 39.40 at% of Si,
0.32 at% of V, and no Zn or Cl, for the long nanowires in
Fig. 1(b). Thus, it is confirmed that, V acted as a catalyst in
the growth of silica nanowires; however, most of Zn (ZnO

2000
3000
4000
5000
6000
7000
8000
E
red
E
green
500 nm
100 nm
50 nm
PL intensity (a.u.)
Wavelen
g
th
(
nm
)
Fig. 4. Photoluminescence spectra recorded at room temperature from
SiONWs with the diameters of $50, $100, and $500 nm, respectively.
Y. Yang et al. / Physica E 31 (2006) 218–223220
emission band) and E
green
(green emission band) can be
obtained, although the fitting was quite subjective. The
results are tabulated in Table 1 . The red emission band has
a relatively constant intensity, and a red-shift of about


H:
The red emission band properties observed in our PL
spectra are similar to those of surface-oxidized silicon
nanocrystals, and mesoporous silica [26], without the
exhibition of green emission band.
The green emission band at 2.25 eV can be attributed to
hydrogen-related species in the composites of SiONWs
[28]. Defect concentration in SiONWs is related primarily
to the high surface area and the complex chemistry that
occurs during growth. Thus the PL intensity is highly
related to surface area and inverse proportional to
nanowire diameters as observed in Fig. 4. Considering
the width of the red ($150 nm full-width half-maximum
(FWHM)) and green ($100 nm FWHM) emissions in
Fig. 4, the maximum peak shifts for red (18 nm) and green
(14 nm) emissions for nanowires with different diameters
are rather small (Table 1 and Fig. 4). Obviously, the large
widths of red and green emissions indicate a large range of
energy transitions, and the emission peak should corre-
spond to the dominant transition [29]. At the moment, we
cannot identify a direct link between the shift and the
nanowire diameter. However, we speculate that the peak
shifts for nanowires with different diameter are due to the
fabrication temperature, at which these nanowires grow.
The temperature directly affects the chemical reactions
(reaction rates) during nanowire formation, resulting in
various defects with varied concentration. According to
Liu et al. [30] , the VO
2

and Cl
2
) were not considered because the VO
2
was not oxidized to higher oxidation state (V
5+
) indicated
by the color of this oxide on substrate where nanowires
could be found. Further investigation using V
2
O
5
instead
of VO
2
revealed that neither catalyst tip nor silica
nanowires could be found; with the absence of VO
2
, ZnO
nanowires were found growing on Si substrates, which is
consistent with our previous work [32]. Thus, the catalyst
should be related to V
4+
compound.
As we know, ZnCl
2
is highly hygroscopic. The powder
used in our experiment was actually ZnCl
2
Á nH

O [35]
2ZnCl
2
ðlÞþH
2
OðlÞ!Zn
2
OCl
2
ðgÞþ2HClðgÞ.
Oxidation of ZnCl
2
in the melt [36]
ZnCl
2
ðlÞþ1=2O
2
ðgÞ!ZnOClðgÞþ1=2Cl
2
ðgÞ and
ZnCl
2
ðlÞþx=2O
2
ðgÞ!ZnO
x
ðlÞþCl
2
ðgÞ.
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xðadsÞ
.
Product desorption
SiCl
xðadsÞ
! SiCl
x
ðgÞ.
Oxidation
SiCl
x
ðgÞþx=2O
2
ðgÞ!SiO
x
ðgÞþx=2Cl
2
,
2ZnðgÞþxO
2
ðgÞ!2ZnO
x
ðgÞ.
Hence, the white fog may contain SiO
x
,Cl
2
, HC l, ZnO
x
,

bonds than the Si–O and Zn–O bonds; i.e. it is more easily
to form SiO
2
than the rest compounds during reactions.
From our experiments, pumping was necessary for synth-
esis of SiONWs with co ntrolled amount of zinc and
chloride residue. By keep pumping the furnace tube, small
liquids of ZnO
x
and zinc oxychloride vapor could be
sucked out with the high substrate temperature ranging
from 1050 to 720 1C. Meanwhile, gases containing Cl
2
,
and HCl could be sucked out as well to avoid excessive
etching of substrate. It is worth mentioning that, although
both ZnO
x
and SiCl
x
are in vapor phase, SiCl
x
is an
intermediate phase of a series of chemical reactions, and it
oxidized quickly into SiO
x
with much lower vapor
pressure. However, ZnO
x
is one of the final products of

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