Synthesis and photoluminescence property of silicon carbon nanowires
synthesized by the thermal evaporation method
Enlei Zhang
a
, Yuanhong Tang
a,b,
Ã
, Yong Zhang
a
, Chi Guo
a
a
College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China
b
Powder Metallurgy Research Institute, Central South University, Changsha 410083, People’s Republic of China
article info
Article history:
Received 13 November 2008
Received in revised form
18 November 2008
Accepted 18 November 2008
Available online 27 November 2008
PACS:
81.07.Bc
81.40.Tv
Keywords:
Nanostructures
Crystal growth
Electron microscopy
Optical properties
abstract

been developed, including laser ablation [6,7], chemical vapor
deposition via silicon precursor [8–11], physical evaporation,
hydrothermal method [12,13] and catalyst-assisted vapor liquid
solid mechanism [14]. However, these products are available at
the cost of either high pure and expensive carbon nanotube or the
hazardous and easily explosive silicon (carbon) precursor of SiH
4
or SiCl
4
(CH
4
). In addition, the synthesized products were of low
yield and with much SiC bulk. Thus, large-scale synthesis of pure
b
-SiC nanowires still remains a challenge to be considered for the
above-mentioned disadvantage.
In this work, we have developed a simple method for
synthesizing large-scale pure
b
-SiC nanowires by heat-activated
carbon with SiO powders using anodic aluminum oxide (AAO)
template without any metal catalyst. SiO powders cannot react
with activated carbon directly because of AAO template. The
synthesized SiC nanowires were of high yield without much bulk.
The synthesized nanowires consist about 50 nm diameter core
wrapped with an amorphous SiO
2
sheath. The crystal growth
direction /111S is clearly observed. Photoluminescence spec-
trum centered at 400 nm is referred to the SiC nanowires. Based

2
solution at room temperature. Finally, the
template was rinsed with distilled water and immersed in 5%
phosphoric acid for about 30 min at room temperature to adjust
the pore diameter and remove the barrier layer at the bottom of
nanoholes.
2.2. Synthesis of SiC nanowires
The preparation apparatus for synthesis of SiC nanowires is a
conventional furnace with horizontal alumina tube. Solid SiO
powders (1 g, purity 99.9%) were placed in a graphite crucible and
covered with an AAO template. The activated carbon (2 g) was
placed on the AAO template. Then, the crucible was covered with
a graphite lid, placed in the hot zone inside the alumina tube, as
shown in Fig. 1. The chamber was flushed with high purity of Ar
(40 sccm) to eliminate O
2
by means of rotary vacuum pump for
many times. Afterwards, the furnace was rapidly heated from
room temperature to 1400 1C at a heating rate of 10 1C/min and
maintained for reaction for 2 h in atmosphere pressure. The
sample was taken out when it was cooled down to room
temperature, and the AAO template surface was deposited with
thick layer of light-blue fluffylike products.
Morphology and crystal lattice of the samples were observed
by field-emission transmission electron microscopy (TEM, JEOJ
JSM-5600LV) and high-resolution transmission electron micro-
scopy (HR-TEM, JEOL JEM-3010). The crystalline structure was
analyzed by X-ray diffraction (XRD, Semens D5000). The possible
chemical composition of as-grown products was investigated by
energy-dispersive X-ray spectroscopy (EDS) attached to the TEM.

in Fig. 4. As can be seen from the pattern, the major diffraction
peak can be indexed as the (111), (2 0 0), (2 2 0), (311) and (2 2 2)
reflections of cubic
b
-SiC (unit cell parameter
a
¼ 0.4389 nm).
These values are almost identical to the known values for
b
-SiC
(JCPDS Card no. 73-1665).
The internal structure of SiC nanowires was characterized by
TEM. Fig. 5a displays a typical TEM image of the SiC nanowires,
revealing that the periphery of SiC nanowires is very clean and
straight. It also shows that the SiC nanowires possess a high
density of planar defects, stacking faults which are perpendicular
to the wires axes, similar to the already reported results [17–19].
With regard to energetic consideration, the formation of stacking
faults during the growth of SiC nanowires is favorable due to the
contribution of stacking faults themselves with lower energy. By
HR-TEM image (Fig. 6) observation, we have found that nanowires
have a crystalcore and an amorphous sheath with thickness about
2 nm. The SiO
2
sheath could be easily removed by etching in
hydrofluoric acid (HF). The thickness of the SiO
2
sheath could be
controlled by changing the etching time. Fig. 6 also shows that the
spacing of lattice fringes is 0.25 nm, corresponding to the {111}

To investigate PL properties of the synthesized
b
-SiC nano-
wires, the corresponding measurement was carried out at room
temperature and a PL spectrum (Fig. 8) was obtained. When
excited with light from a xenon source (excitation wavelength
354 nm), the nanowires have an emission band between 330 and
600 nm. It is clear that a strong peak centered at 400 nm is
observed. Compared with previously reported luminescence from
the bulk [21], film [22] and nanowire [23] of SiC, the emission
peak for
b
-SiC nanowires is obviously shifted to the blue. The
emergence of the peak with a blueshift is due to the existence of
oxygen defects in the amorphous layer, the special rough core–
shell interface and the morphology effects such as stacking faults
in the nanowires’ core [24]. It also may be attributed to the
quantum confinement effect because of the small size [23,25].
Clearly, no metal catalyst was employed during the whole
procedure. Thus, the growth mechanism may not follow the
previously reported vapor–liquid–solid (VLS) model. On the basis
of experiments, we suggest a possible growth model for
b
-SiC
nanowires. The chemical reaction equations during the process
can be described as in the following.
ARTICLE IN PRESS
700
600
500

mechanism, silica decomposed from SiO is believed to play an
important role, significantly enhancing the nucleation and one-
dimensional growth of Si nanowires, which are clothed with a
SiO
2
sheath
2SiOðgÞ!SiðsÞþSiO
2
ðsÞ (1)
where s and g in the brackets refer to solid and gas state,
respectively. The reaction temperature being 1400 1C, the Si
nanowires with a SiO
2
sheath as templates would react with
activated carbon to form SiC nanowires according to the following
reactions (2) and (3):
SiðsÞþCðsÞ!SiCðsÞ (2)
SiO
2
ðsÞþ3CðsÞ!SiCðsÞþ2COðgÞ (3)
In fact, reaction (3) proceeds through two stages in which a
gaseous intermediate SiO gas is generated according to the
following reaction (4). Once CO is formed, SiO maybe produced
according to reaction (5):
SiO
2
ðsÞþ2CðsÞ!SiOðgÞþCOðgÞ (4)
SiO
2
ðsÞþCOðgÞ!SiOðgÞþCO

may enclose the crystalline SiC nano-
wires [27,28]. This reaction leads to the decrease in enthalpy and
Gibbs energy at temperature below 900 1C. As compared with
reactions (2) and (3), this reaction is thermodynamically favor-
able, and produces large mounts of SiC/SiO
2
composite nanowires.
ARTICLE IN PRESS
Fig. 6. HR-TEM image of b-SiC nanowire. The inset is the corresponding fast
Fourier transform (FFT).
Fig. 7. IR spectrum of the as-synthesized SiC nanowires sample.
Fig. 8. Room-temperature PL spectrum of b-SiC nanowires.
E.L. Zhang et al. / Physica E 41 (2009) 655–659658
4. Conclusions
In summary, scales of pure crystalline
b
-SiC nanowires with
diameters about 50 nm were synthesized using AAO template by
direct thermal evaporation without any metal catalyst at high
temperature. The as-synthesized products mainly consist of
b
-SiC
nanowires. By means of XRD, SEM, EDS, IR and TEM (HR-TEM),
b
-SiC nanowires have been characterized and discussed in detail.
The growth direction of nanowires lies along the /111S
direction. The tentative growth model according to the SiC
nanowires growth process was suggested. Finally, optical property
is found in the photoluminescence emission from
b

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