Preparation and photoluminescence of high density SiOx nanowires with Fe
3
O
4
nanoparticles catalyst
X.J. Wang
a,
⁎
, B. Dong
b
, Z. Zhou
a
a
School of Electronics Engineering and Computer Science, Peking University, Beijing 100871, China
b
School of Science, Dalian Nationalities University, Dalian 116600, China
abstractarticle info
Article history:
Received 6 January 2009
Accepted 29 January 2009
Available online 12 February 2009
PACS:
81.05Y
78.55
Keywords:
SiOx nanowires
Photoluminescence
Large scale, high density SiOx nanowires have been synthesized using a novel Fe
3
O
4

inverse spinel structure with oxygen forming a fcc closed packing and
iron cations occupying interstitial tetrahedral sites and octahedral
sites. In the past decades, Fe
3
O
4
is becoming more and more attractive
because of its intrinsic half-metallic ferromagnetic nature, which can
be widely used in catalysts, biological assays, chemical sensors, and
superparamagnets [8,9]. Recently, efforts have been devoted to
preparation of Fe
3
O
4
nanoparticles, nanowires and nanotubes. In
2002, Sun and Zeng [10] prepared the size-controlled monodisperse
Fe
3
O
4
nanoparticles with 4–16 nm by high-temperature solution
phase reaction of iron (III) acetylacetonate (Fe(acac)
3
) in the presence
of alcohol, oleic acid, and oleylamine. The size-controlled mono-
disperse Fe
3
O
4
nanoparticles can be used for a wide range of

procedure, when the temperature of reaction region was increased
to 1000–1200 °C, the sample was transferred to reaction region
Materials Letters 63 (2009) 1149–115 2
⁎ Corresponding author. Tel.: +86 1062882397.
E-mail address: [email protected] (X.J. Wang).
0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2009.01.084
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rapidly again. The growth of SiOx nanowires started. The reaction
lasted for 2 h in this procedure under a constant 50 sccm flow rate of
Ar gas. After the furnace was cooled to room temperature, the white
product was found on the surface of Si(111) substrate. Field-emission
scanning electron microscope (FESEM) (XL-SFEG, FEI Corp.) with
energy-dispersive X-ray spectroscopy (EDS) was used for morpholo-
gical observation and element composition analysis of SiOx nano-
wires. X-ray diffraction (XRD) (D/Max-2400, Rigaku Corp.) with
CuKα radiation was assumed for phase structure of Fe
3
O
4
nanopar-
ticles. Transmission electron microscopy (TEM) (Tecnai-20, PHILIPS
Corp.) and high-resolution transmission electron microscopy
(HRTEM) (Tecnai F20, FEI Corp.) with electron energy loss spectro-
meter (EELS) were employed to perform the microanalysis of SiOx
nanowires. Photoluminescence measurement was performed at a
fluorescence spectrometer (F4500, HITACHI corp.) with a resolution of
1.0 nm.

115 0 X.J. Wang et al. / Materials Letters 63 (2009) 1149–115 2
intensities of Fe
3
O
4
nanoparticles agree well with standard Fe
3
O
4
XRD
card (JCPDS No. 85-1436).
Fig. 2(a)–(c) shows the SEM images of SiOx nanowires deposited
on the Si(111) substrate at the reaction temperature of 1000 °C for 2 h.
Large scale, high density nanowires are uniformly covered on the
surface of Si(111) substrate, as illustrated in Fig. 2(a) and (b). The
lengths of these nanowires are in the range of several tens to hundreds
of micrometers. Fig. 2(c) shows the higher magnification SEM image
of nanowire tops. It can be clearly seen that, at every one top of these
nanowires, there is a droplet whose diameter is much larger than that
of related nanowire. Similar products were also observed at reaction
temperatures of 1100 and 1200 °C. The composition analysis of the
products was achieved using EDS. Fig. 2(d) shows EDS spectrum of the
nanowires, indicating that the products only consist of silicon and
oxygen elements. Quantitative analysis shows that the atomic ratio of
Si:O is 1:(1.4–1.7), suggesting that the SiO
1.4–1.7
nanowires have been
synthesized by the method. The EDS measurement achieved on the
nanowire top displays the existence of a small amount of elemental
iron. Further sample characterization was carried out using transmis-

Moreover, EDS also detected the existence of iron only at the top of
SiOx nanowires. The fact suggested that the Fe
3
O
4
catalysts play a key
role in the formation of SiOx nanowires, and a VLS mechanism is the
most probable growth mechanism. Here, we give a description for the
possible formation of SiOx nanowires. Firstly, with the temperature
increased from 400 °C to 1000 °C, Si vapor is generated at high
temperature by the vaporization of silicon powder. Meanwhile, Fe–Si–
O nanoclusters form on the surface of Si(111) substrate, which act as
nuclei for the formation of SiOx nanowires. As the droplets become
supersaturated, amorphous silicon nanomires are formed by the
reaction between Si and O. Oxygen source may come from two factors,
one is the oxygen in Fe
3
O
4
nanoparticles, the other is the remainder
oxygen in the reaction chamber. The presence of a small amount of O
is not expected to change the Fe–Si phase diagram significantly, but in
the meanwhile it acts as the oxygen source during the silicon oxide
growth.
Fig. 4 shows the room-temperature PL spectrum of SiOx nanowires
under the 250 nm ultraviolet fluorescent light excitation. Two broad
PL emission peaks are clearly observed at the center wavelengths of
about 405 nm (3.06 eV) and 465 nm (2.67 eV). The PL properties of
various silica nanowires have been studied extensively [2]. It has been
suggested that the 2.7 eV band is attributed to the neutral oxygen

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115 2 X.J. Wang et al. / Materials Letters 63 (2009) 1149–115 2