Rational growth of highly oriented amorphous
silicon nanowire films
Xihong Chen
1
, Yingjie Xing
1
, Jun Xu, Jie Xiang, Dapeng Yu
*
Department of Physics, School of Physics, State Key Laboratory for Mesoscopic Physics, Electron Microscopy Laboratory,
Peking University, Room 211, Building N, Beijing 100871, China
Received 10 April 2003; in final form 7 May 2003
Abstract
Amorphous silicon nanowire films were rationally synthesized using a simple approach. The films consist of highly
oriented nanowires of 30 lm in length and 20–80 nm in diameter. The morphology, microstructure features, and
chemical composition of the nanowires were analyzed using electron microscopy and Raman spectroscopy. A novel
model concerning solid–liquid–solid phases was proposed to explain the growth mechanism of the nanowires. This
approach should be very useful to direct the controlled growth of nanomaterials.
Ó 2003 Published by Elsevier Science B.V.
1. Introduction
One-dimensional nanomaterials have been a
focused research field since the first pioneering
work of the discovery of carbon nanotubes [1] and
nanowires [2–6]. A diverse variety of semiconduc-
tor nanowires, such as silicon, GaAs, GaN, and
ZnO nanowires, were synthesized using different
approaches. Of those nanowire materials, silicon
nanowires (SiNWs) have great scientific and
technological importance, and have attracted
much research interest [7,8]. For example, the sil-
icon nanowires have been used as the building
blocks to build nano-scale logic and computa-

into the chamber at 20 sccm during the growth to
keep an ambient pressure about 1.5 kPa. The
original shiny surface of the substrate became gray
after cooling down to room temperature. The
morphology of the as-grown product was analyzed
using an scanning electron microscope (SEM,
DB235 FIB,FEI). A Hitachi-9000 NAR high-res-
olution transmission electron microscope (TEM)
equipped with nano-beam energy dispersive spec-
troscopy (EDS) was used to characterize the mi-
crostructure and chemical composition of the
nanowires. Raman spectrum was measured using
a Renishaw 2000 system with a laser source of
514.5 nm.
3. Results and discussions
In a brief view of SEM analysis, the whole
substrate was found to be covered with a thick
layer of wool-like product, as is shown in Fig. 1.
The wool-like layer with homogeneous thickness
can be easily scratched from the substrate, which is
marked with arrow in the SEM image. In the
magnified SEM image in the left inset, one piece of
the wool-like carpet was scratched from the sub-
strate and was folded on top of the film. The right
inset shows the details of the edge of the layer, and
reveals that the film consists of fine free-standing
wires of very high density, and has a thickness of
about 30 lm (also the length of the nanowires).
The growth rate of nanowires is faster than 100
nm/s. The catalyst nanoparticles were found at the

Part of the product was scratched off and used
to measure the Raman spectrum in a micro-beam
mode from different places of the sample. Two
peaks around 300 and 516 cm
À1
were observed, as
is shown in Fig. 3a. It is well known that those two
Raman peaks are characteristic of a silicon struc-
ture, corresponding to the second-order transverse
acoustic phonon mode (2TA), and the first-order
transverse optical phonon mode (TO) of silicon,
respectively. The Raman result confirmed that the
nanowire film is composed of silicon. The possi-
bility of the formation amorphous silicon oxide
Fig. 1. SEM image revealing a wool-like film on large area. The
SEM image in the left inset reveals that one sheet of the film was
scratched off the substrate. The film has a thickness about 30
micrometers, and consists of pure nanowires, as shown in the
right inset. The bright contrast indicated by an arrow shows the
catalyst layer between the substract and nanowire film, which
provides evidence for a base growth.
X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630 627
nanowires can be excluded by the following dis-
cussions. Because the growth was conducted in a
steel CVD chamber, it guarantees a very good
vacuum status, and the hydrogen gas keeps a
reduction atmosphere. On the other hand, the
substrate we used is the commercial microelec-
tronic wafers having very thin native oxide layer
(usually <1 nm), which is not thick enough to

growth of SiNWs. Though the eutectic point of
Fig. 2. SEM images showing the high orientation of the nanowire films. (a) Edge of the film showing a wide-spread orientation of the
nanowires. (b) Details of the highly oriented nanowire film.
Fig. 3. (a) Raman spectrum of the nanowire film scratched off
from the substrate. Two peaks at 300 and 516 cm
À1
, were ob-
served which correspond to 2TA, and TO modes of silicon,
respectively. (b) TEM image showing the morphology of the
silicon nanowires. The inset in the left shows the EDS spectrum
and Si and O are visible, where oxygen comes from the surface
oxidation of the nanowires. The inset in the right shows a Si–Ni
nanoparticle capped at the end of the nanowire.
628 X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630
Si
2
Ni is 993 °C, the small-size-melting effect makes
it possible for the deposited Ni nanoparticles to
react with the Si substrate at temperature above
900 °C, forming Si
2
Ni eutectic liquid droplets. So
the source materials comes from directly the sub-
strate instead from the vapor phase in the present
case. Therefore, we proposed a novel model to
explain the growth of the a-SiNWs, which involves
the SLS phases in the growth process. In the SLS
growth, the catalyst Ni dissolves directly the sili-
con substrate to form Si
2

must be considerable. The temperature gradient
should be the driving force for the silicon substrate
to be dissolved to form low temperature Ni–Si
eutectic liquid phase, forming silicon nanowires at
the cooler side.
The SLS mechanism is in some extent an anal-
ogy to the VLS model. In the SLS controlled
growth of nanowires, the eutectic Ni–Si liquid
droplets have to stay at the surface of the silicon
substrate in order to grow continuously, and the
solidified nanoparticles shall remain at the bottom
of the nanowires, as is shown in the right inset in
the SEM image in Fig. 1. In this image, there exists
a bright-contrasted layer between the substrate
and the nanowires film (marked with an arrow),
and EDS analysis proved that this bright-con-
trasted layer consists of Si and Ni.
Two more questions shall be addressed here.
First, it is not well understood why the final
nanowires are in amorphous state instead of a
crystalline phase. The most possible explanation is
the unusual high growth rate. The estimated
growth rate is about 100 nm/s. Such a high growth
rate may explain why the resulting nanowires are
amorphous instead of crystalline, because the
growth is so rapid that the atoms have no time to
stack themselves into crystalline order. Second, we
think that the crowding effect between the very
dense nanowires plays an important role to keep
the nanowires staying oriented. From the SEM

ventional VLS mechanism, and a novel SLS model
was proposed to explain reasonably the growth of
X. Chen et al. / Chemical Physics Letters 374 (2003) 626–630 629
the amorphous silicon nanowires. The as-grown
a-SiNWs and the proposed growth model should
be useful to direct the controllable growth of other
nanowire structures.
Acknowledgements
This project was financially supported by na-
tional Natural Science Foundation of China
(NSFC, No. 50025206, 20151002), and by the
Research Fund for the Doctoral Program of
Higher Education (RFDP), China.
References
[1] S. Iijima, Nature 354 (1991) 56.
[2] D.P. Yu, C.S. Lee, I. Bello, X.S. Sun, G.W. Zhou, Z.G. Bai,
Z. Zhang, S.Q. Feng, Solid State Commun. 105 (1998) 403.
[3] M. Morales, C.M. Lieber, Science 279 (1998) 208.
[4] X.F. Duan, J.F. Wang, C.M. Lieber, Appl. Phys. Lett. 76
(2000) 1116.
[5] W.Q. Han, S.S. Fan, Q. Li, Y. Hu, Science 277 (1997)
1287.
[6] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl.
Phys. Lett. 78 (2001) 407.
[7] J. Hu, M. Ouyang, P. Yang, C.M. Lieber, Nature 399
(1999) 48.
[8] J. Westwater, D.P. Gosain, S. Tomiya, S. Usui, H. Ruda,
J. Vac. Sci. Technol. B 15 (1997) 554.
[9] Y. Huang, X.F. Duan, Y. Cui, J. Lauhon, K.H. Kim, C.M.
Lieber, Science 294 (2001) 1313;