Carbon assisted synthesis of silicon nanowires

Carbon-assisted synthesis of silicon nanowires
Gautam Gundiah, F.L. Deepak, A. Govindaraj, C.N.R. Rao
*
Chemistry and Physics of Materials Unit and CSIR Centre of Excellence in Chemistry,
Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560 064, India
Received 18 September 2003
Published online: 4 November 2003
Abstract
Carbon-assisted synthesis of silicon nanowires has been accomplished with silicon powders as well as solid sub-
strates. The method involves heating an intimate mixture of silicon powder and activated carbon or a carbon coated
solid substrate in argon at 1200–1350 °C, and yields abundant quantities of crystalline nanowires. Besides being simple,
the method eliminates the use of metal catalysts.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
There has been intense research activity in the
area of inorganic nanowires and nanotubes in the
last few years [1–3]. Thus, nanowires of a variety
of inorganic materials including oxides, nitrides
and chalcogenides have been synthes ized and
characterized. In particular, silicon nanowires
(SiNWs) have received considerable attention and
several methods have been employed for their
synthesis. These include thermal evaporation of Si
powder [4], vapor–liquid–solid method involving
liquid metal solvents with low solubility for Si [5],
laser ablation [6,7], and the use of silicon oxide in
mixture with Si [8,9]. SiO
2
-sheathed crystalline
SiNWs have been obtained by heating Si–SiO
2

mixtures [10]. It has been recently reported that
enhanced yields of SiNWs are obtained by heating
a Si substrate coated with carbon nanoparticles at
1050 °C under vacuum [11]. We consider the role
of carbon to be as in other carbothermal methods
of synthesizing nanowires of oxides, nitrides and
other materials, involving a vapor–solid mecha-
nism wherein carbon reacts with the oxide proba-
bly producing a suboxide-type species. As part of
our program on the carbothermal synthesis of in-
organic nanowires [12–14], we have been investi-
gating carbon-assisted synthesis of SiNWs. In this
article, we report our important findings, which
are of relevance to the vapor–solid and oxide-
assisted growth of SiNWs.
2. Experimental
The synthesis of SiNWs has been carried out by
employing the following procedures. Procedure (i)
Chemical Physics Letters 381 (2003) 579–583
www.elsevier.com/locate/cplett
*
Corresponding author. Fax: +91-80-8462760.
E-mail address: (C.N.R. Rao).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.09.142
involved the solid state synthesis in which silicon
powder (Aldrich Chemicals) was finely ground
with activated carbon, keeping the molar ratio of
Si to C at 1:1 or 1:0.5. The activated carbon was
prepared by decomposing polyethylene glycol (600

units) in argon atmos phere at 700 °C for 3 h. The
finely ground mixture was taken in an alumina
boat and heated at 1200 °C for 3 h in a mixture of
Ar (50 sccm; sccm, standard cubic centimeter per
minute) and H
2
(20 sccm). The reaction was also
carried out under similar conditions in the absence
of carbon to verify whether carbon plays a role in
the formation of the nanowires. Procedure (ii) was
similar to (i), except that the reactants were heated
in an Ar atmosphere (without any H
2
). The
product obtained was grey or white in color and
was collected as fine powders.
In procedure (iii), a silicon substrate was used
as the source of silicon. The Si(1 0 0) substrates
were cleaned by ultrasonication in distilled water.
Amorphous carbon was sputtered on the sub-
strates using a JEOL JEE-400 vacuum evaporator,
with a sputtering time of 0.5–1 min. The carbon-
coated Si substrates were heated to 1350 °C for 3 h
in an atmosphere of Ar/H
2
(25 sccm each). The
product formed as a layer on the substrate was
grey or white in color. A blank run with the sili-
con substrate without any sputtered carbon was
carried out under similar conditions.

X-ray diffraction (XRD) patterns of the prod-
ucts were recorded using a Seifert instrument with
Cu Ka radiation. Scanning electron microscope
(SEM) images were obtained with a Leica S-440-i
microscope. Transmission electron microscopic
(TEM) images were obtained with a JEOL (JEM
3010) operating with an accelerating voltage of 300
kV. Powder samples for TEM were dispersed in
CCl
4
using an ultrasonic bath, and a drop of the
suspension placed on a copper support grid cov-
ered with holey carbon film.
3. Results and discussion
Heating silicon powder at 1200 °C, in the ab-
sence of any activated carbon, yields a small pro-
portion of SiNWs. In Fig. 1a, we show a typical
SEM image of the product of such a reaction to
illustrate the poor yield of SiNWs. When the re-
action was carried out in the presence of activated
carbon (Si:C, 1:1) by procedure (i), we obtained
nanowires in a good yield, as can be visualized in
the SEM image in Fig. 1b. These have diameters
ranging from 75–350 nm, with lengths of a few
microns. The XRD pattern of the product shown
in Fig. 2a matches with that of bulk silicon of
cubic structure (JCPDS file: 27-1702). There is a
minor peak (with asterisk) which is attributed to
the surface silicon oxide, since SiNWs undergo
oxidation upon exposure in air. Due to the high

surface-to-volume ratio of the nanowires, a
prominent surface oxide layer is general ly present.
We, however, see no reflections due to carbide and
other impurity phases. Along with the nanowires,
we also obtain Si nanojunctions, as shown in the
low-magnification TEM image in Fig. 3a. The
junction has a Y-shape, with arms of a uniform
width of 200 nm, and a length of a few microns.
Careful studies of the TEM images and electron
diffraction data may unravel the nature of the
junction.
In Fig. 1c, we show the SEM image of the
SiNWs obtained by procedure (i) with Si:C ratio of
1:0.5. The nanowires have diameters between 75
and 600 nm with lengths up to tens of microns.
The TEM image presented in Fig. 3b reveals that
the nanowires have a crystalline core and an
amorphous sheath. The diameter of the cryst alline
core is 40 nm and the thickness of the sheath is
around 17 nm. The amorphous sheath serves as a
protective layer to the underlying crystalline sili-
con core. The amorphous sheath is of silica,
formed by surface oxidation. The selected area
electron diffraction, given in the inset of Fig. 3b,
indicates the core to be of cubic silicon. The XRD
pattern of the product, given in Fig. 2b, is char-
acteristic of cubic silicon with a small impurity of
silica.
Reaction of silicon powder with activated car-
bon in the absence of H

2
, by procedure (ii), yielded
abundant quantities of SiNWs. The product ob-
tained consisted of grey and white portions. The
grey portion comprised SiNWs with diameters of
$50 nm as shown in the SEM image in Fig. 1d.
The length of the nanowires was several tens of
microns. Shown in the inset of Fig. 1d is the SEM
580 G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583
image of the white portion of the product. These
nanowires have diameters ranging from 50 to 700
nm, with several tens of microns in length. A low-
magnification TEM image of the nanowires is
shown in Fig. 3c. The nanowires are highly crys-
talline as can be seen from the high-resolution
transmission electron microscope (HREM) image
in Fig. 3d. The lattice spacing between the fringes
is 0.31 nm, corresponding to the (1 1 1) planes of
silicon. The crystallinity of the nanowires is con-
siderably higher when only argon was used instead
of a mixture of argon and hydrogen. The role of
hydrogen in promoting the amorphization of sili-
con is well-known [15,16].
In order to show the versatility of this method,
we have investigated the formation of SiNWs by
heating silicon substrates coated with carbon, by
procedure (iii). In the absence of carbon, we ob-
tained very few SiNWs, as shown in the SEM
Fig. 1. SEM images of (a) the product of the reaction of silicon powder obtained by procedure (i) in the absence of carbon, (b) SiNWs
obtained by procedure (i) with a Si:C ratio of 1:1, (c) SiNWs obtained by procedure (i) with Si:C ratio of 1:0.5 and (d) SiNWs obtained

in the grey portion of the sample synthesized by procedure (ii). Inset shows the nanowires obtained in the white portion.
Fig. 2. XRD patterns of SiNWs obtained by procedure (i) with
a Si:C ratio of (a) 1:1 and (b) 1:0.5.
G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583 581
image in Fig. 4a. On carrying out the reaction with
sputtered carbon, the yield of SiNWs impro ves
considerably, as can be seen from the SEM image
in Fig. 4b. The nanowires have diameters in the
range of 50–300 nm.
The formation of SiNWs in the presence of
carbon can be explained as follows. Silicon is
generally covered by an oxide layer. The oxide
layer gets reduced by carbon into silicon monoxide
by the reaction
Si
x
O
2
þ C ! Si
x
O þ CO ðx > 1Þð1Þ
Si
x
O ! Si
xÀ1
þ SiO ð2Þ
2SiO ! Si þ SiO
2
ð3Þ
Crystalline silicon, formed in step (3), nucleates

and grows perpendicular to the (1 1 1) direction to
form the nanowires. Similar reactions have been
proposed for the oxide-assisted synthesis of
SiNWs [7], although the monoxide type species is
generated by other means.
4. Conclusions
SiNWs have been obtained by reacting silicon
powder or silicon substrates with carbon in an
inert atmosphere. Carbothermal reduction of the
silica layer covering Si generates crystalline SiNWs
with high aspect ratios. The method is convenient
and inexpensive for the synthesis of Si nanowires,
devoid of metallic impurities.
Fig. 3. (a) TEM image of a Si nanojunction obtained by procedure (i) with Si:C ratio of 1:1. (b) TEM image of a nanowire obtained by
procedure (i) with Si:C ratio of 1:0.5. Inset is the SAED pattern. (c) TEM image of the white portion of the sample obtained by
procedure (ii). (d) HREM image of a single nanowire obtained in the white portion of the sample synthesized by procedure (ii). The
white arrow indicates the direction of growth of the nanowire.
582 G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583
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Fig. 4. SEM images of SiNWs obtained with a Si substrate by
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G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583 583