Amorphous silica nanowires grown by the
vapor–solid mechanism
Ki-Hong Lee
a,
*
, Seung-Woo Lee
a
, Richard R. Vanfleet
b
, Wolfgang Sigmund
a
a
Materials Science and Engineering Department, University of Florida, 255 Rhines Hall, Gainesville, FL 32611, USA
b
Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA
Received 22 May 2003; in final form 3 June 2003
Published online: 9 July 2003
Abstract
Silica nanowires were synthesized by using silica nanoparticles as a growth catalyst using a gas composed of CH
4
and H
2
at 1050 °C. Silica nanoparticles provide silicon and oxygen atoms for the formation of the nanowires, as well
acting as a growth site. The nanowires nucleated on graphitic carbon layers formed around the seed particles, indicating
that the nanowires grow by the vapor–solid mechanism. Photoluminescence spectra of the nanowires normally showed
strong blue emission peaked at 3.1 and 2.8 eV under 3.8 eV laser excitation. Post-hydrogen annealing resulted in the
appearance of longer wavelength photoluminescence band.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
One-dimensional quantum nanowires are
promising materials for nanoelectronic devices due

E-mail address: khonglee@ufl.edu (K H. Lee).
0009-2614/03/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0009-2614(03)01019-4
nucleated from graphitic carbon layers, which are
formed around the seed silica nanoparticles. This
fact implicates that the silica nanowires grow by
vapor–solid (VS) mechanism, not by vapor–
liquid–solid (VLS) mechanism. The SiONWs show
strong blue emission, and a longer process time
and hydrogen annealing after synthesis of the
nanowires results in the emission of longer wave-
length spectrum.
2. Experimental section
N-type silicon h100i wafers (3 X-cm, 1 Â 1 cm)
were used as substrates for the growth of SiONWs.
After thermally oxidizing the Si substrates; iron
films of 30 nm were deposited on the oxide layer by
sputtering. A droplet of aqueous 20 nm silica
nanoparticles (0.1 wt%, 0.2 ml) solution was
placed on a Fe/SiO
2
/Si substrate. Poor wetting of
the aqueous silica solution on the substrate caused
non-uniform coverage of silica nanoparticle layers.
The substrates were heated up to 100 °C on a hot
plate in order to expedite the drying process. After
annealing in H
2
(200 sccm) for 10 min at 1050 °C,
SiONWs were synthesized in a quartz tube furnace

K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 499
revealed two distinct regions on the substrate sur-
face after synthesis. SiONWs grew up to tens of lm
in length on the silica nanoparticles. Graphite, iron
carbide, and few carbon nanotubes were found in
the regions where no silica particle existed. High
magnification FESEM photographs of these two
regions show the difference (Figs. 1b and c).
Formation of the amorphous SiONW phase
could be identified by high resolution transmission
electron microscopy (HRTEM), electron disper-
sive spectroscopy (EDS), electron energy loss
spectroscopy (EELS) as well as selected area dif-
fraction (SAD) pattern analysis. A low magnifi-
cation TEM photograph, as shown in Fig. 2a,
shows the nanowires grown from the seed parti-
cles. The diameter of the nanowires ranges be-
tween 15 and 35 nm. Process time did not affect the
length and the diameter of the nanowires signifi-
cantly. An EELS spectrum, as shown in Fig. 2b,
reveals that the nanowires have amorphous silicon
oxide phase by comparing to the standard EELS
spectrum for amorphous silica. Figs. 2c and d
show HRTEM photographs and SAD patterns of
a nanowire and a seed particle, respectively. The
SAD pattern and HRTEM photographs from the
seed regions indicate that crystalline graphitic
layers formed around the seed particles. EDS
spectra, as shown in Fig. 3, reveals different com-
positions according to positions in a nanowire. In

oxygen and the SiO vapor diffuse up to top regions
of the silica particle layers to form the nanowires.
In an initial stage of the nanowire growth, iron
vapor forms a Fe–Si–O phase, as shown in Fig. 3b,
with silica at the surface of the silica nanoparti-
cles; and the carbon atoms form graphitic layers
around the nanoparticles, as shown in Fig. 2d. The
existence of the graphite phase surrounding
the seed particles is a reasonable indication that
the nanowires grew by VS mechanism. Carbon
vapor rich environment in the initial stage forms
an amorphous diamond like carbon (DLC) phase
in the nanowire near the seed particles, as shown in
Fig. 3c. Following the initial stage, carbothermal
decomposition of the silica nanoparticles by the
carbon vapor produces a SiO vapor rich environ-
ment to form the silica nanowires by the VS mech-
anism. Increasing the process time up to 120 min
does not change the diameter and the length of
the silica nanowires significantly, indicating the
Fig. 3. EDS spectra according to the positions of the silica nanowires: (b) from seeds; (c) from nanowires near to the seeds; (d) from
nanowires. These positions are illustrated (a). (The peak at 8 eV represents Cu from a TEM grid.)
Fig. 4. (a) A schematic diagram of the growth of SiONWs on
the silica nanoparticle layer. (b) A FESEM photograph shows
that the nanowires grow on the top side of the silica nanopar-
ticle layer.
K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503 501
formation of the nanowires is accomplished in a
short period of after starting the synthesis. Fig. 4b
shows a typical growth pattern of the silica

longer processing time only leads to a composition
change of the nanowires. The decreased PL in-
tensity with longer processing time seems to be
caused by oxygen supplement, resulting in de-
creasing defect density, from the source gases. The
peak wavelength change of the PL by the hydro-
gen annealing is not clear at this time.
4. Conclusions
Silica nanowires were synthesized by VS
mechanism by using silica nanoparticles as a
growth catalyst. Silica nanoparticles provide sili-
con and oxygen atoms for the formation of the
nanowires, as well as act as a growth site. The
photoluminescence spectra of the nanowires
showed strong blue emission peaks at 3.1 and 2.8
eV under 3.8 eV laser excitation. Longer process
time and post-hydrogen annealing resulted in the
appearance of a longer wavelength photolumi-
nescence band and broad PL spectra.
Acknowledgements
The authors thank Dr. Young-Ho Lee, Dr.
Myung-Hyun Lee and Dr. Won-Seon Seo (of
Advanced Materials Analysis and Evaluation
Fig. 5. (a) PL spectra from the SiONWs with processing time (the peak at 650 nm shows the secondary harmonic oscillation of the
laser source). (b) Hydrogen annealing effect on the PL characteristics of the SiONWs.
502 K H. Lee et al. / Chemical Physics Letters 376 (2003) 498–503
Center of the Korea Institute of Ceramic Engi-
neering & Technology) for the HRTEM and the
EDS analysis. This work was supported by
DARPA/Army Research Office under Grant No.