Preparation and photoluminescence properties of amorphous silica nanowires

Preparation and photoluminescence properties of amorphous
silica nanowires
X.C. Wu
*
, W.H. Song, K.Y. Wang, T. Hu, B. Zhao, Y.P. Sun, J.J. Du
Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, P.O. Box 1129,
Hefei 230031, People's Republic of China
Received 10 October 2000; in ®nal form 4 January 2001
Abstract
Bulk-quantity amorphous silica nanowires (SiONWs) have been synthesized by carbothermal reduction reaction
between silicon dioxide and active carbons. Transmission electron microscopy (TEM) image shows the formation of the
nanowires at a diameter of 60±110 nm and a length up to hundreds micrometers. Besides most smooth-surface polyp-
shaped nanowires, two other forms of nanowires, named amoeba-shaped and frog-egg-shaped nanowires, have also
been observed. The nanowires can emit stable and high brightness blue light at 435 nm (2.85 eV) under excitation at 260
nm (4.77 eV). The formation of the nanowires into dierent shapes may be explained by the vapor±liquid±solid (VLS)
mechanism. Ó 2001 Published by Elsevier Science B.V.
1. Introduction
One-dimensional quantum wires are of great
scienti®c interest due to their great potential for
testing and understanding fundamental concepts
about the roles of dimensionality in mesoscopic
physics and for applications in nanodevices [1,2].
For instance, nanotweezers made of carbon na-
notubes can be used to manipulate submicron
clusters and nanowires [3]. GaAs and InAs nano-
wires have found applications in developing one-
dimensional high-speed ®eld eect transistor, or
laser working at low-threshold current density and
high gain [4]. GaN nanowires may be fabricated
into one-dimensional nanoscale luminescence di-
odes [5]. With the development of mesoscopic

science and the advances in integrated optical
technology, it is important to synthesize nanowires
with optical properties that can meet the demands
of further applications. Silica as photolumines-
cence (PL) materials has long been concerned for,
and people have continually reported PL bands
with peak energies around 1.9±4.3 eV for bulk
SiO
2
or SiO
2
®lms [6±12]. Recently, Yu et al. [13]
have also reported fabrication and intensive blue
light emission phenomenon of silica nanowires
(SiONWs), and pointed out their potential ap-
plications in high-resolution optical heads of
scanning near-®eld optical microscope or nanoin-
terconnections in future integrated optical devices.
Therefore, SiO
2
nanowires are a promising one-
dimensional luminescence materials, but they have
only been synthesized by using excimer laser ab-
lation. In this Letter, we report the carbothermal
reduction synthesis and characterization and PL
properties of the nanowires. Their growth process
has also been discussed.
9 March 2001
Chemical Physics Letters 336 (2001) 53±56
www.elsevier.nl/locate/cplett

*
Corresponding author. Fax: +86-551-5591434.
E-mail address: (X.C. Wu).
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 063-X
2. Experimental
The mixtures of SiO
2
4g,FeNO
3

3
Á 9H
2
O
(250 mg) and active carbons (4 g) were ball-milled
for 20 h in ethanol media. After desiccated, they
were pressed into several circular pellets
(U1cm 0:5 cm) under 10 Mpa. The pellets were
placed at the center of conventional horizontal
furnace with a sintered alumina tube (U2:5cmÂ
100 cm) and calcined at 1350°C for 3 h in ¯owing
argon (40 ml/min). A white product was found to
deposit on the surface of the pellets and the ther-
malcouple. TEM images of the products were taken
with a JEM-200CX transmission electron micro-
scope. The composition of SiONWs was deter-
mined by the X-ray photoemission spectra (XPS),
which were recorded on a VGESCALAB MKII X-
ray photoelectron spectrometer. XPS data were

collected in the constant analyzer energy (CAE)
mode at 20 eV. Mg Ka (hm  1253:6 eV) radiation
was employed as excitation source with an anode
voltage of 12 KV and an emission current of
20 mA. PL spectrum was measured in a Hitachi 850
¯uorescence spectrophotometer with a Xe lamp at
room temperature. The excitation wavelength was
260 nm, and the ®lter wavelength was 310 nm.
3. Results and discussion
TEM micrography shows the general morphol-
ogy and dimension of SiONWs. The nanowires
shown in Fig. 1(a) look like polyp with trunks and
branches, but their surface is smooth. The trunk
has 110 nm in diameter and hundreds micrometers
in length by scanning throughout the sample. The
brunches have the diameter about 60 nm and a
length up to 5 lm. Electron diraction micro-
graphs (inset) show that the nanowires are amor-
phous. Fig. 1(b) indicates the amoeba-shape of the
nanowires, with the diameter of about 70 nm and a
length up to tens of micrometers. Fig. 1(c) shows
frog-egg morphology of the nanowires, which is
similar to wire-like silica nanosphere agglomerates
[14]. Circular nanoparticles are connected via
necks to form pearl-like chains. The largest parti-
cles have the diameter about 100 nm, while the
smallest particles have the diameter about 60 nm.
Due to low reaction temperature, no silicon car-
bide nanowires were observed. The reactants
were suciently ball-milled to accelerate chemical

reaction. If the time grinding the reactants were
reduced, we could still obtain the SiONWs.
Further evidence for the formation of SiO
2
nanowires can obtained through XPS. The two
strong peaks at 103.35 and 532.65 eV as shown in
Fig. 1. TEM morphologies of SiONWs. (a) polyp-shaped
SiONWs; (b) amoeba-shaped SiONWs; (c) frog-egg-shaped
SiONWs.
54 X.C. Wu et al. / Chemical Physics Letters 336 (2001) 53±56
Fig. 2(b) and (c) correspond to the binding ener-
gies of Si(2p) and O(1s) for SiO
2
, respectively. No
obvious Si peaks (Si2p 98.7 eV in Si) are observed.
The quanti®cation of the peaks reveals that atomic
ratio of Si to O is 1:2.41. Obviously, the observa-
tion of oxygen must be due to adsorption and
surface contamination of the sample. The survey
spectrum in Fig. 2(a) also displays C(1s) (at 284.65
eV) peak, which can be attributed to a small
amount of the residual graphite (284.3 eV for C1s
in graphite).
As is shown in Fig. 3, a stable and strong blue
light emission is revealed at 435 nm (2.85 eV) at
room temperature under excitation at 260 nm
while ultraviolet and blue light emission at 350 nm
(3.54 eV), 420 nm (3.0 eV), and 465 nm (2.7 eV) can
also be observed. Compared with [13], intensive
peak at 420 nm in [13] changes into shoulder in

Fig. 3 while shoulder at 435 nm into intensive peak.
The shoulder at 465 nm in Fig. 3 approaches the
peak at 470 nm in [13]. Ultraviolet light emission at
350 nm was observed in oxidized porous silicon
and annealed SiO
2
[15]. The PL spectrum is also
dierent from that of oxidized Si nanowires [16].
The growth process of the nanowires can be
explained by the vapor±liquid±solid (VLS) mech-
anism, since little droplets can be seen at the tops
of the nanowires [17]. The growth of the nanowires
Fig. 2. XPS of the sample. (a) survey spectrum of the sample;
(b) Si2p binding energy spectrum; (c) O1s binding energy
spectrum.
Fig. 3. PL spectrum of the SiONWs at room temperature under
excitation at 260 nm.
X.C. Wu et al. / Chemical Physics Letters 336 (2001) 53±56 55
could be divided into three steps. The ®rst step is
that silica is reduced by active carbon to silicon
and silicon monoxide, and then silicon reacts with
iron to form FeSi
2
[18]. The second step is that
FeSi
2
is evaporated on the surface of the pellets
and the thermalcouple to become liquid-phase
catalytic growth center. The third step is that the
vapor of silicon and silicon monoxide is trans-

ported to the catalytic center to form SiO and Si
nanowires by VLS mechanism while both silicon
and silicon monoxide are all oxidized to amor-
phous SiO
2
during cooling. In the above-growth
model, the nucleation step can be further divided
into monocentric and polycentric nucleation, and
growth step into periodic stable growth and peri-
odic unstable growth [19]. Thus the combination
of dierent nucleation and growth processes can
give rise to dierent forms of SiONWs, which is
similar to the growth model of Si nanowires [20].
The formation process of polyp-shaped SiONWs
is considered to be due to the coexistence of
monocentric and polycentric nucleation and of the
periodic stable growth on the basis of the trunks
and the branches with even diameters. When
trunks stably grow in monocentric nucleus, some
FeSi
2
nanoparticles deposit on the surface of the
trunks to become many new growth centers,
namely, polycentre, resulting in the formation of
branch-shaped nanowires. The branch-shaped
nanowires can grow stably, but the stability is only
relative. Amoeba-shaped nanowires are attributed
to monocentric nucleation and periodic unstable
growth to exhibit a typical periodic instability of
diameter. The block dots of the polyp-shaped and

amoeba-shaped SiONWs are the sites of nuclei.
However, the periodicity is not strict and diers
for various nanowires. Frog-egg-shaped nanowires
are due to wire-like arrangement of deposited sil-
icon oxide nanoparticles by surface tension, and
the necks are formed between nanoparticles at
high temperatures, so its growth process also be-
longs to that of polycentric nucleation.
4. Conclusions
Amorphous SiONWs have been successfully
synthesized on large scale using a carbothermal
reduction approach at 1350°C in a ¯owing argon
atmosphere. Three types of dierent shapes of
SiONWs have been observed. The periodic stable
growth and unstable growth of the nanowires co-
exist in the product.
Acknowledgements
This work was supported by the Ministry of
Science and Technology of China (NKBRSF-
G19990646), National Science Foundation under
contract NSF 59872043, the Fundamental Science
Bureau, Academia Sinica.
References
[1] S. Frank, P. Poncharai, Z.L. Wang, W.A. de Heer, Science
280 (1998) 1744.
[2] A.P. Alivisatos, Science 271 (1996) 933.
[3] P. Kim, C.M. Lieber, Science 286 (1999) 2148.
[4] K. Hiruma, M. Yazawa, T. Katsuyama, K. Okawa, K.
Haraguchi, M. Kogucchi, H. Kakibayashi, J. Appl. Phys.
77 (1995) 447.

[5] W. Han, S. Fan, Q. Li, Y. Hu, Science 277 (1997) 1287.
[6] A.R. Silin, L.N. Skuja, A.V. Shendrik, Fiz i Khim. Stekla 4
(1978) 405.
[7] L.N. Skuja, A.R. Silin, Physica A 56 (1979) K11.
[8] J.H. Stathis, M.A. Kastner, Phys. Rev. B 35 (1987)
2972.
[9] C. Itoh, T. Suzu, N. Itoh, Phys. Rev. B 41 (1990) 3794.
[10] N. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohiki,
K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586.
[11] T. Kanashima, M. Okuyama, Y. Hamakawa, Appl. Surf.
Sci. 79/80 (1994) 321.
[12] L.S. Liao, X.M. Bao, X.Q. Zheng, N.S. Li, N.B. Min,
Appl. Phys. Lett. 68 (1996) 850.
[13] D.P. Yu, Q.L. Hang, Y. Ding, H.Z. Zhang, Z.G. Bai, J.J.
Wang, Y.H. Zou, W. Qian, G.C. Xiong, S.Q. Feng, Appl.
Phys. Lett. 73 (1998) 3078.
[14] J.L. Gole, J.D. Stout, W.L. Rauch, Z.L. Wang, Appl.
Phys. Lett. 76 (2000) 2346.
[15] G.G. Qin, J. Lin, J.Q. Duan, G.Q. Yao, Appl. Phys. Lett.
69 (1996) 1689.
[16] N. Wang, Y.H. Tang, Y.F. Zhang, C.S. Lee, I. Bello, S.T.
Lee, Chem. Phys. Lett. 299 (1999) 237.
[17] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
[18] S.M. Boyer, A.J. Moulson, J. Mater. Sci. 13 (1978)
1637.
[19] E.I. Givargizov, J. Cryst. Growth 31 (1975) 20.
[20] Y.H. Tang, Y.F. Zhang, N. Wang, C.S. Lee, X.D. Han,
I. Bello, S.T. Lee, J. Appl. Phys. 85 (1999) 7981.
56 X.C. Wu et al. / Chemical Physics Letters 336 (2001) 53±56