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

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.

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

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

Phys. Rev. B 58 (1998) R16024.
[8] N. Wang, Y.F. Zhang, Y.H. Tang, C.S. Lee, S.T. Lee,
Appl. Phys. Lett. 73 (1998) 3902.
[9] Y.F. Zhang, Y.H. Tang, N. Wang, C.S. Lee, I. Bello, S.T.
Lee, J. Cryst. Growth 197 (1999) 136.
[10] J.L. Gole, J.D. Stout, W.L. Rauch, Z.L. Wang, Appl.
Phys. Lett. 76 (2000) 2346.
[11] S. Botti, R. Gardi, R. Larciprete, A. Goldoni, L. Gregoratti,
B. Kaulich, M. Kiskinova, Chem. Phys. Lett. 371 (2003) 394.
[12] G. Gundiah, A. Govindaraj, C.N.R. Rao, Chem. Phys.
Lett. 351 (2002) 189.
[13] G. Gundiah, G.V. Madhav, A. Govindaraj, M.M. Seikh,
C.N.R. Rao, J. Mater. Chem. 12 (2002) 1606.
[14] F.L. Deepak, K. Mukhopadhyay, C.P. Vinod, A. Govind-
araj, C.N.R. Rao, Chem. Phys. Lett. 353 (2002) 345.
[15] Y.J. Xing, D.P. Yu, Z.H. Xi, Z.Q. Xue, Appl. Phys. A 76
(2003) 551.
[16] H. Fritzsche (Ed.), Amorphous Silicon and Related Ma-
terials, World Scientific, Singapore, 1989.
Fig. 4. SEM images of SiNWs obtained with a Si substrate by
procedure (iii) (a) in the absence of carbon and (b) with carbon
sputtered on the surface.
G. Gundiah et al. / Chemical Physics Letters 381 (2003) 579–583 583