Large-area highly-oriented SiC nanowire arrays: Synthesis, Raman, and photoluminescence properties

Article abstract of DOI:10.1021/jp063565b

Large-area highly oriented SiC nanowire arrays have been fabricated by chemical vapor reaction using an ordered nanoporous anodic aluminum oxide (AAO) template and a graphite reaction cell. Their microstructures were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy. The results show that the nanowires are single-crystalline β-SiC's with diameters of about 30-60 nm and lengths of about 8 μm, which are parallel to each other, uniformly distributed, highly oriented, and in agreement with the nanopore diameter of the applied AAO template. The nanowire axes lie along the [111] direction and possess a high density of planar defects. Some unique optical properties are found in the Raman spectroscopy and photoluminescence emission from oriented SiC nanowire arrays, which are different from previous observations of SiC materials. The growth mechanism of oriented SiC nanowire arrays is also analyzed and discussed.

Full text of DOI:10.1021/jp063565b

22382
J. Phys. Chem. B 2006, 110, 22382-22386
Large-Area Highly-Oriented SiC Nanowire Arrays: Synthesis, Raman, and
Photoluminescence Properties
Zhenjiang Li,*,† Jinli Zhang,† Alan Meng,‡ and Jianzhang Guo†
College of Electromechanical Engineering and College of Chemistry and Molecular Engineering,
Qingdao UniVersity of Science and Technology, Qingdao 266061, Shandong, P. R. China
ReceiVed: June 8, 2006; In Final Form: September 5, 2006
Large-area highly oriented SiC nanowire arrays have been fabricated by chemical vapor reaction using an
ordered nanoporous anodic aluminum oxide (AAO) template and a graphite reaction cell. Their microstructures
were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction
and high-resolution transmission electron microscopy. The results show that the nanowires are single-crystalline
â-SiC’s with diameters of about 30-60 nm and lengths of about 8 µm, which are parallel to each other,
uniformly distributed, highly oriented, and in agreement with the nanopore diameter of the applied AAO
template. The nanowire axes lie along the [111] direction and possess a high density of planar defects. Some
unique optical properties are found in the Raman spectroscopy and photoluminescence emission from oriented
SiC nanowire arrays, which are different from previous observations of SiC materials. The growth mechanism
of oriented SiC nanowire arrays is also analyzed and discussed.
Introduction
growth mechanism;28-30 (2) using a carbon nanotube-confined
reaction;27,31,32 (3) using a carbon-thermal reduction.33-35 In our
previous studies, we have successfully synthesized large-scale
SiC nanowires36 and SiC nanowire networks37 by a chemical
vapor reaction method. Herein, we report for the first time the
synthesis of large-area highly oriented SiC nanowire arrays in
an ordered nanoporous anodic aluminum oxide (AAO) template
through a chemical vapor reaction approach. A mixture of milled
Si and SiO2 powder and C3H6 were chosen as the starting
materials. Large-area highly oriented SiC nanowire arrays were
fabricated in the nanopores of an AAO template via a series of
chemical reactions of Si, SiO2, and C3H6.
Recently, there has been increasing interest in oriented
nanostructural systems because of their numerous potential
applications in optoelectronics, sensors, nanogenerators, and
electronic circuits.1,2 Hierarchical assembly of nanowires in a
controlled manner is a critical step toward accomplishing these
goals.3-5 Up to now, much effort has been devoted to the
synthesis of one-dimensional oriented nanostructural materials.6-9
Template-assisted synthesis is an elegant chemical approach for
the fabrication of oriented nanostructures, particularly for
different solid nanowire arrays. Among these templates, the
nanoporous anodic alumina templates have received consider-
able attention in synthetic nanostructure materials due to their
unique structural properties, such as their fully controllable and
extremely narrow size distribution in pore diameters, good
chemical inertness, physical stability, and ideal cylindrical shape
of pores.10-12 They have been extensively used to fabricate one-
dimensional oriented nanomaterials by using a variety of
methods.13-17
Silicon carbide (SiC) is one of the most promising semicon-
ductors because of its high breakdown electric field (3-5 MV/
cm), low coefficient of thermal expansion, and high thermal
conductivity (350-490 W m-1 K-1), high resistance to oxida-
tion, high hardness, high strength, and low mass density.18-20
It can be used to fabricate electronic devices operating at high
temperature, high power, high frequency, and in harsh environ-
ments.21,22 In addition, SiC one-dimensional nanostructures have
displayed unique optical and electron field emission proper-
ties,23-26 especially for oriented SiC nanowire arrays.27 More
recently, many methods to synthesize SiC nanowires (nanorods)
have been developed. These methods roughly fall into the
following three categories: (1) utilizing a vapor-liquid-solid
Experimental Section
Fabrication Process of the AAO Template. The AAO
template was prepared by a two-step aluminum anodic oxidation
process similar to that previously described, to obtain a uniform
pore structure.38 Prior to anodization, the high-purity aluminum
thin sheets (99.99%) were annealed at 450 °C for 2 h, rinsed in
distilled water, then electropolished to achieve a smooth surface.
Subsequently, the aluminum samples were anodized in 0.3 M
oxalic acid (40 V, 17 °C, 4-5 h, Al sheet as an anode). The
first-step anodized layer was removed by etching in a mixture
of phosphoric acid and chromic acid at 60 °C for 8-10 h.
During the second step, the samples rinsed in distilled water
and oxalic acid were anodized again in 0.3 M oxalic acid (40
V, 16 °C, 7-9 h, Al sheet as an anode). After the second-step
anodization, the unwanted aluminum matrix was dissolved in
saturated HgCl2 solution at room temperature. Finally, the
template was rinsed with distilled water and immersed in 5%
phosphoric acid for about 20-40 min at 32 °C to adjust the
pore diameter and remove the barrier layer at the bottom of
nanoholes.39
Synthesis Steps of SiC Nanowire Arrays. First, Si and SiO2
powder of 99% purity were milled for 48 h. The ball-milling
process was carried out in a rolling laboratory ball mill using
hardened agate balls and a resin cell. The containment cell was
* Corresponding author. Phone: +86-532-88958602. Fax: +86-532-
88956118. E-mail: zjli126@126.com.
† College of Electromechanical Engineering.
‡ College of Chemistry and Molecular Engineering.
10.1021/jp063565b CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/21/2006

Analysis of Highly Oriented SiC Nanowire Arrays
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22383
loaded with several tens of grams of Si or SiO2 powder together
with 2 balls of 20 mm diameter, 5 balls of 12 mm diameter, 20
balls of 7 mm diameter, and 50 balls of 2 mm diameter.
Following milling, the as-milled powders were dried at 90 °C
for about 24 h in a constant-temperature oven. A 1.2:1 molar
mixture of ball-milled Si and SiO2 was milled and mixed for
30 min in an agate mortar. The Si-SiO2 mixture and AAO
template were placed in a graphite reaction cell. The AAO
template was situated over the mixture of Si-SiO2, and a piece
of carbon cloth was inserted between the template and the
mixture of Si-SiO2. Then the graphite reaction cell was placed
into a vertical-tube graphite heater furnace. Prior to heating,
the system was flushed three times with high-purity argon (Ar)
by using a rotary vacuum pump to reduce the concentration of
O2 to a negligible level from the furnace chamber. The
temperature of the furnace was increased to 1230 °C from room
temperature at a mean rate of 350-400 °C/h. During the heating,
the furnace was evacuated at 1000 °C to obtain a gas pressure
of 350 Torr. The steady C3H6 flow of 8-10 sccm from the
bottom of the graphite reaction cell was started and maintained
for 3-5 min at 1230 °C under a total gas pressure of 540-650
Torr. Then the temperature of furnace was decreased to 500 °C
at a mean rate of 270-300 °C/h. Finally, the power was
switched off, and the furnace was allowed to cool to room
temperature in an Ar atmosphere.
Characterization. The morphology and elemental analysis
of the nanoporous AAO template and the oriented SiC nanowire
arrays were characterized by a Hitachi (Tokyo, Japan) S-4200
field-emission scanning electron microscope (FE-SEM) equipped
with energy-dispersive X-ray (EDX) spectroscopy. Prior to SEM
observation, the samples were sputter-coated with gold under a
high vacuum for 2 min to increase their conductivity. The
structure of the resulting material was characterized by using a
Rigaku (Japan) D/max-3C X-ray diffractometer with Cu KR
radiation at room temperature. Further detailed structural
information of the oriented SiC nanowire arrays was obtained
by using a Hitachi (Tokyo, Japan) H-8100 transmission electron
microscope (TEM), selected-area electron diffraction (SAED),
Figure 1. (a) FE-SEM image of the nanoporous AAO template surface.
(b) FE-SEM image of the cross-section of the nanoporous AAO
template.
and
a high-resolution transmission electron microscope
(HRTEM) (JEOL-2010). For TEM observation, the samples
were dispersed in alcohol with the aid of ultrasonic agitation
for 30 min, and then a drop of solution was dropped onto a
copper grid covered by porous carbon. The Raman scattering
spectra were measured with a micro-Raman spectrometer
(Renishaw 2000) at room temperature. The 514-nm line of an
Ar+ laser was used as the excitation source. Photoluminescence
(PL) spectral measurements were performed in a Hitachi F-4500
fluorescence spectrophotometer using a xenon lamp at room
temperature.
Figure 2. FE-SEM image of oriented SiC nanowire arrays.
seen that the SiC nanowires, with a length of around 8 µm, are
parallel to each other, uniformly distributed, and highly oriented.
The density of SiC nanowires is about 109-1010 cm-2. The EDX
spectrum (Figure 3a) collected from the AAO template dem-
onstrates the presence of Al and O. The quantitative analysis
indicates their atomic ratio of Al and O is approximately 2:3,
corresponding to the stoichiometric composition of Al2O3. The
EDX spectrum (Figure 3b) collected from oriented SiC nanowire
arrays growing in the nanoporous AAO template shows that
the nanowires are composed of Si, C, O, and Al. O and Al
come from the AAO template. The molecular ratio of Si/C of
the nanowires calculated from the EDX data is about 1:1, within
the experimental error, corresponding to the stoichiometric
composition of SiC. The above SEM and EDX results show
that large-area highly oriented SiC nanowire arrays were formed
in the nanoporous AAO template.
Results and Discussion
A typical FE-SEM image of the nanoporous AAO template
surface is shown in Figure 1a. The image shows the nanopores
on the surface with an average pore diameter of about 30-60
nm and a distance among neighboring pores of about 30-50
nm, which are connected to each other to form the nanonetwork.
Figure 1b is the FE-SEM image of the cross-section of the
nanoporous AAO template, which depicts the AAO template
with pores parallel to each other, uniformly distributed, and
perpendicular to the surface of the template.
After the reaction, the area of the products is about 1.5 cm2,
and the color of the samples is light blue. Figure 2 shows the
characteristic FE-SEM image of oriented SiC nanowire arrays
grown with the assistance of an AAO template. It can be clearly
A typical X-ray diffraction XRD pattern of oriented SiC
nanowire arrays growing in the nanoporous AAO template is
shown in Figure 4. The major diffraction peaks are assigned to

22384 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Li et al.
Figure 5. (a) TEM image of oriented SiC nanowires. (b) The
corresponding SAED pattern taken from several SiC nanowires.
Figure 6. HRTEM image of an oriented SiC nanowire.
of nanosize and surface states, and the diffraction peak bulge
between 20° and 25° is the characteristic amorphous Al2O3 peak.
The above results indicate that large-area highly oriented SiC
nanowire arrays have been obtained at relatively low temper-
atures by a chemical vapor reaction approach.
Figure 3. (a) EDX spectrum collected from the AAO template. (b)
EDX spectrum collected from oriented SiC nanowire arrays growing
in the nanoporous AAO template.
Figure 5a is a characteristic TEM image of several parallel
SiC nanowires, revealing that the periphery of SiC nanowires
is very clean, without any wrapping of amorphous material. The
diameter of these nanowires is about 30-60 nm, corresponding
to the diameter of the nanopores of the AAO template. In
addition, we may find that the nanowires are parallel to each
other after ultrasonic agitation. The reason is that the SiC
nanowires have an oriented arrangement in the AAO template.
The corresponding SAED pattern taken from several SiC
nanowires is illustrated in Figure 5b. It shows three discrete
polycrystalline diffraction rings, which correspond to the (111),
(220), and (311) crystallographic planes of cubic â-SiC.
Furthermore, the SAED pattern exhibits planar defects and
streaks of stacking fault structure. Otherwise, a small number
of single-crystal diffraction spots from an individual SiC
nanowire suggest a single-crystalline â-SiC structure.
A representative HRTEM image (in Figure 6) depicts steplike
streaked lines, which suggest that the SiC nanowires possess a
high density of planar defects and stacking faults. We believe
that these planar defects occur during the growth process due
to thermal stress. The insert in Figure 6 shows that the
interplanar spacing, which is perpendicular to the wire axes, is
about 0.25 nm, which corresponds to the separation between
the (111) lattice planes of â-SiC, suggesting that the axes of
the SiC nanowire lie along the [111] direction.
Figure 4. XRD pattern of oriented SiC nanowire arrays.
the (111), (200), (220), and (311) reflections of cubic â-SiC
(unit cell parameter a ) 0.4358 nm). These values are almost
identical to the known values for â-SiC (JCPDS Card. No. 29-
1129). The peak marked with SF is due to stacking faults. The
stronger intensities of â-SiC diffraction peaks (relative to the
background amorphous) indicate that the resulting products were
well-crystallized â-SiC. In addition, the â-SiC diffraction peaks
are obviously broadened, which maybe the result of the effect
The typical Raman spectra taken from oriented â-SiC
nanowires and the AAO template are shown in Figure 7. It
indicates that no Raman mode is observed from a bare AAO

Analysis of Highly Oriented SiC Nanowire Arrays
J. Phys. Chem. B, Vol. 110, No. 45, 2006 22385
Figure 9. Schematic illustration of the growth process of the highly
oriented SiC nanowire arrays growing in the nanoporous AAO
template: (a) The AAO template at the beginning of the reaction. (b)
Nanosized nucleic SiC deposited on the inside of the pores of the AAO
template. (c) The SiC nanowires in the pores are grown along the axial
direction. (d) The outside nanowires sustain the axial growth direction
of the already formed SiC nanowires in the nanopores of the AAO
template.
Figure 7. Raman scattering spectra of the AAO template and oriented
SiC nanowires. Spectra a and b correspond to the AAO template and
oriented SiC nanowires, respectively.
When the furnace is heated to a high temperature, SiO vapor is
generated by a solid-solid reaction between the milled Si and
SiO2; the reaction equation is Si(s) + SiO2(s) f 2SiO(v) (1).
Subsequently, C3H6 vapor from the bottom of the graphite
reaction cell is introduced into the reaction chamber and
pyrolyzed C. The capillary effect47 of the anodic pores with
large aspect ratios (depth/diameter) in the AAO template is
favorable for the conglomeration of C and SiO vapor in the
nanopores and the nucleation of SiC nanowires. When C enters
into the pores of the AAO template together with SiO vapor, it
quickly reaches the nucleic concentration of the SiC nanowires.
Therefore, nanosized nucleic SiC is deposited on the inner
surface of the pores of the AAO template (see Figure 9b), by a
chemical vapor reaction, which can be expressed as 3SiO(v) +
2C3H6(v) f 3SiC(s) + 3CO(v) + 6H2(v) (2). For the growth
of SiC nanowires, 111 has been demonstrated to be the
preferred growth direction, and the growth rate along the 111
axis direction is much faster than that along other directions.27,48
In addition, the space confinement of the alumina channel is
favorable for the 111 axis direction growth of SiC nanowires
in the pores of the AAO template (see Figure 9c). When the
reactions continue, the subsequently generated SiO and C will
react with each other to grow SiC on the top of the formed SiC
nanowires because the needed energy growing on the formed
SiC nanowire top is far lower than that of nucleic SiC on the
other places of the AAO template. So the outside nanowires
sustain the axial growth direction of the formed SiC nanowires
in the nanopores of the AAO template (see Figure 9d). Withal,
we believe that the fabrication of highly oriented SiC nanowire
arrays has to decrease the reaction rate and shorten the C3H6
reaction time, otherwise the nanosized SiC may be nuclear on
the other places of the AAO template simultaneously. In
addition, it is very important to put the nanoporous AAO
template over the mixture of milled Si-SiO2. To test this, we
put the AAO template under the mixture of milled Si-SiO2 to
repeat the above process, and no SiC nanowire arrays were
found. The reason may be that the oriented flow of the vapor
from the bottom of the reaction cell is favorable for driving the
C and SiO vapor into the pores of the AAO template and
facilitates the oriented growth of SiC nanowires. It should be
pointed out that the exact mechanism for the formation of large-
area highly oriented SiC nanowire arrays by the presented
method should be further investigated, which is now underway
with our work.
Figure 8. Room-temperature PL spectrum of oriented SiC nanowires.
template (Figure 7a), and there are only two strong peaks at
around 794 and 966 cm-1 observed, as shown in Figure 7b,
corresponding to the TO and LO phonons at the Γ point of cubic
SiC. We note that the Raman spectral features are strikingly
different from others published elsewhere.40,41 The TO (Γ)
phonon line is not only much stronger than the LO (Γ) phonon
line, but both of the lines are also shifted. This difference may
be explained by the size confinement effect and stacking faults
of oriented â-SiC nanowires.
Figure 8 displays a room-temperature PL spectrum of the
â-SiC nanowires. When excited with light from a xenon source
(excitation wavelength 270 nm), the nanowires have an emission
band between 337 and 496 nm, which is centered at 385 nm.
Compared with the previously reported luminescence from the
nanospheres,42 nanowires,43,44 films,45 and nanobelts46 of â-SiC,
the emission peak for oriented â-SiC nanowires is obviously
shifted to the blue. The reason for these results is yet unknown,
and a deeper study on this work is clearly needed.
Although the detailed growth mechanism of oriented SiC
nanowire arrays is still not fully understood, the above-
mentioned results allow us to suggest a possible process
responsible for the formation of the large-area highly oriented
SiC nanowire arrays, which is illustrated in Figure 9. The empty
AAO template, as shown in Figure 9a, has a number of
nanopores with diameters of 30-60 nm before the reactions.

22386 J. Phys. Chem. B, Vol. 110, No. 45, 2006
Li et al.
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In summary, large-area highly oriented SiC nanowire arrays
have been synthesized by a chemical vapor reaction using an
ordered nanoporous AAO template and a graphite reaction cell.
SEM, EDX, XRD, and HRTEM investigations on these nano-
wires have been carried out. The results show that the nanowires
are single-crystalline â-SiC’s with diameters of about 30-60
nm and lengths of about 8 µm, which are parallel to each other,
uniformly distributed, highly oriented, and in agreement with
the nanopore diameter of the employed AAO template. The
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