Ultrafine and uniform silicon nanowires grown with zeolites

Ultrafine and uniform silicon nanowires grown with zeolites
C.P. Li
a
, X.H. Sun
a,b
, N.B. Wong
a,b
, C.S. Lee
a
, S.T. Lee
a,
*
, Boon K. Teo
c,1
a
Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films,
The City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China
b
Department of Biology and Chemistry, The City University of Hong Kong, Hong Kong SAR, China
c
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA
Received 21 June 2002; in final form 21 August 2002
Abstract
Ultrafine and uniform silicon nanowires (SiNWs), with a Si crystalline core of 1–5 nm (average 3 nm) in diameter
and a SiO
2
outer layer of 10–20 nm thick, were synthesized by the oxide-assisted growth method via the dispropor-
tionation of thermally evaporated SiO using zeolite as a template/precursor. From transmission and secondary electron
microscopic characterizations, we deduced that the zeolite acted to limit the lateral growth of the Si crystalline core and
supply the excess oxide to form the thick oxide outer layer. The ultrafine SiNWs exhibited strong photoluminescence
that peaked at 720 nm.

Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
Since the discovery of Si whiskers [1], silicon
nanowires (SiNWs) have attracted much attention
in mesoscopic research and device applications, as
well as in the fundamental research because of
their highly interesting optical and electrical
properties [2–14]. The metal-catalyst vapor–liq-
uid–solid (VLS) reaction has been used to grow
SiNWs of different diameters [2]. Other growth
methods and/or strategies include the oxide-as-
sisted growth [3–7]. It is obvious that the control
of the diameter and uniformity of SiNWs is a
crucial factor in the design and fabrication of
nanoscale devices. In this Letter, we report a new
method for the preparation of very fine (1–5 nm)
and uniform SiNWs using zeolites as templates
and/or precursors.
SiNWs were prepared by thermal evaporation
of pure SiO powder (Aldrich, 325 mesh, 99.9%) at
1250 °C in an evacuated alumina tube. The zeolite
(Zeolite Y, a mixture of SiO
2
,Al
2
O
3
, and Na
2
O)

substrate was packed was held by quartz wool in
an inner alumina tube, through which the carrier
gas exited. The carrier gas consisted of 95% Ar and
5% H
2
with a flow rate of 50 SCCM (standard
cubic cm per min) was forced to flow through the
zeolites and the whole system was kept at a pres-
sure of 400 mbar. The zeolite substrate changed
from a white powder to small green pallets. The
www.elsevier.com/locate/cplett
Chemical Physics Letters 365 (2002) 22–26
*
Corresponding author. Fax: +852-27844696.
E-mail addresses: (S.T. Lee),
(B.K. Teo).
1
Also corresponding author.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 7 5 - 1
products were first examined with a scanning
electron microscope (SEM) (Philips XL 30 FEG),
which was equipped with energy dispersive X-ray
spectroscopy (EDS). The SiNWs samples from the
surface of the beads were dispersed onto ÔholeyÕ
carbon TEM grids. The nanostructure of the
samples were then characterized by high-resolu-
tion transmission electron microscope (HRTEM)
(Philips CM200 at 200 kV). A micro-Raman
spectrometer (Renishaw 2000 micro-Raman spec-

trometer) was used to characterize the PL prop-
erties of the sample at room temperature. The
514.5 nm emission from argon ion laser was used
to excite the luminescence.
Fig. 1. (a) A typical SEM image of the SiNWs. (b) A zoom-out image of (a).
C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26 23
Fig. 1a is an overview of the SEM image of the
SiNWs. A large quantity of SiNWs was found on
the surface of the zeolite pallet. In the zoom-out
image (Fig. 1b), we observed that the SiNWs were
attached to the surface of the zeolite pallet. EDS
results show that SiNWs are composed of mainly
Si, O, and a small amount of Al. The small amount
of Al came from zeolite and provides strong evi-
dence for the proposed growth mechanism to be
described later. Fig. 2 is the TEM image of a single
SiNW. In the TEM results, we found each SiNW
has a very fine crystalline silicon core and a thick
amorphous silicon dioxide outer layer. The diam-
eters of the Si cores range from 1 to 5 nm, with the
dominant diameter of 3 nm. These Si cores are
very fine and uniform in diameter throughout the
entire length (1 micron or longer) of each wire.
The diameter of the amorphous SiO
2
layer of the
SiNWs ranges from 20 to 40 nm and is also quite
uniform throughout the entire length of the wires.
A central Si core of 1.3 nm in diameter and a
relative thick SiO

2
outer layer of 20 nm in diameter
were observed in our samples. Assuming a Si–Si
bond length of 0.235 nm, this fine nanowire of 1.3
nm in diameter contains only six to seven silicon
atoms across the short dimension. To the best of
our knowledge, this is the finest SiNW synthesized
to date. The amorphous oxide surface of this wire
is quite rough. This phenomenon can be found in
other SiNWs with the Si core less than 2 nm in
diameter. Due to the very fine SiNWs, the selected
area electron diffraction (SAED) revealed only the
amorphous structure of the outside SiO
2
layer, as
well as the carbon film in background. The inset of
Fig. 2 shows a HRTEM image of the same SiNW.
It confirms that the core is crystalline silicon with
3.1

AA d-spacing. The SiO
2
layer has a thickness of
12 nm on both sides of the center SiNW. The
overall diameter of the nanowire is 30 nm.
Fig. 3 shows a proposed mechanism for the
formation of these very fine and uniform SiNWs.
The growth of the SiNWs is similar to the previ-
ously described oxide-assisted growth mechanism
[15]. The SiO powders were firstly sublimated at

1250 °C and formed nanoclusters in the vapor
phase. In the present experiment, the zeolites were
positioned downstream from the SiO starting
material where the temperature was about 930 °C.
The SiO nanoclusters in the vapor subsequently
deposited on the surface of zeolites and some dif-
fused into the channels of the zeolites as shown
schematically in Fig. 3a. At that temperature re-
gime, SiO nanoclusters disproportionated to form
Si and SiO
2
and resulted in the precipitation of
silicon nanoparticles (the nuclei of Si nanowires)
surrounded by shells of silicon oxide. In the
channels of zeolites, the nucleation process was
limited by the openings of the channels and a large
quantity of SiO
2
in zeolites retarded the dispro-
portionation of SiO. Therefore, the core of SiNWs
Fig. 2. The TEM image of a typical single SiNW with a Si core
diameter of 3 nm covered with a SiO
2
layer of 28 nm. The inset
is the HRTEM image of the same SiNW.
Fig. 3. Proposed growth mechanism for the very fine and
uniform SiNWs.
24 C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26
was limited to 1–3 nm in diameter at the nucle-
ation stage, while the zeolite supplied additional

silicon oxide to form the shell of the SiNW, re-
sulting in an oxide layer much thicker than that in
normal SiNWs, as depicted in Fig. 3b. The finding
of Al in SiNWs provides strong evidence that the
oxide layer of SiNWs comes partly from the zeo-
lite. Because the silicon oxide outer layer plays a
key role in the growth process of SiNWs, the
thicker oxide layer limited the lateral growth of the
Si nucleation core. At this point, the Ôoxide-as-
sistedÕ growth process became operative, with the
ÔoxideÕ being primarily supplied Ôin situÕ by the
zeolites. The increased SiO
2
local concentration
(from the zeolite) at the Si–SiO
2
interface again
limits the growth of the wires to larger diameter.
The net result is a very fine (1–5 nm in diameter)
and uniform SiNW sheathed by a thick and uni-
form oxide layer (20–40 nm in diameter) and each
SiNW has a ÔrootÕ in the zeolite, as shown in Fig.
3c. This growth process is consistent with the SEM
results (see Fig. 1b) showing that the SiNWs were
attached to the surface of zeolites.
The photoluminescence (PL) of these very fine
SiNWs was measured at room temperature. Very
weak PL intensity was obtained from the normal
SiNW sample of 20–50 nm in diameter (curve a in
Fig. 4). The PL peak centers at around 600 nm.

However, the very fine SiNW samples reported in
this Letter exhibited very strong (at least one order
of magnitude higher) photoluminescence in the PL
measurement (curve b Fig. 4). The PL peak centers
around 720 nm. The strong PL intensity probably
arises from the quantum size effect of ultrafine Si
core (<5 nm in diameter) in association with the
interface between the ultrafine silicon core and the
sheathing silicon oxide layer [16].
In summary, we have demonstrated that zeolite
can be used as a template/precursor to grow very
fine and uniform SiNWs via the disproportiona-
tion reaction of SiO by thermal evaporation. The
diameter of the Si core ranges from 1 to 5 nm with
an average of 3 nm sheathed by a thick and uni-
form oxide layer of 20–40 nm in diameter. The
SiNWs show unusually strong photoluminescence.
Acknowledgements
The authors would like to dedicate this Letter
to Mrs. Anna Lee in her memory. This work was
supported in part by the Research Grants Council
of Hong Kong (CityU 1063/01P) and the Strategic
Research Grants of the City University of Hong
Kong (No. 7001175) as well as by a grant from the
National Science Foundation, USA (to B.K. Teo).
B.K. Teo would like to express his most sincere
gratitude for the kind hospitality Prof. S.T. Lee
and his colleagues at COSDAF extended to him
during his visit to the center in the summer of
2001, during which this work was performed.

References
[1] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89.
[2] A.M. Morales, C.M. Lieber, Science 279 (1998) 208.
[3] Y.F. Zhang, Y.H. Tang, N. Wang, D.P. Yu, C.S. Lee,
I. Bello, S.T. Lee, Appl. Phys. Lett. 72 (1998) 1835.
[4] D.P. Yu, Z.G. Bai, Y. Ding, Q.L. Hang, H.Z. Zhang, J.J.
Wang, Y.H. Zou, W. Qian, G.C. Xiong, H.T. Zhou, S.Q.
Feng, Appl. Phys. Lett. 283 (1998) 3458.
[5] N. Wang, Y.H. Tang, Y.F. Zhang, D.P. Yu, C.S. Lee,
I. Bello, S.T. Lee, Chem. Phys. Lett. 283 (1998) 368.
[6] W.S. Shi, H.Y. Peng, Y.F. Zheng, N. Wang, N.G. Shang,
Z.W. Pan, C.S. Lee, S.T. Lee, Adv. Mater. 12 (2000) 1343.
[7] Y.H. Tang, Y.F. Zhang, N. Wang, C.S. Lee, X.D. Han,
I. Bello, S.T. Lee, J. Appl. Phys. 85 (1999) 7981.
[8] F.C.K. Au, K.W. Wong, Y.H. Tang, Y.F. Zhang, I. Bello,
S.T. Lee, Appl. Phys. Lett. 75 (1999) 1700.
[9] S.G. Volz, G. Chen, Appl. Phys. Lett. 75 (1999) 2056.
Fig. 4. The PL spectra from (a) normal SiNWs of 20–50 nm in
diameters (b) very fine and uniform SiNWs of 1–5 nm in di-
ameters synthesized with zeolites.
C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26 25
[10] S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bull. 36
(1999).
[11] Y. Cui, X. Duan, J. Hu, C.M. Lieber, J. Phys. Chem. B 104
(2000) 5213.
[12] Y. Cui, C.M. Lieber, Science 291 (2001) 851.
[13] Y.F. Zhang, L.S. Liao, W.H. Chan, S.T. Lee, R. Sammy-
naiken, T.K. Sham, Phys. Rev. B 61 (2000) 8296.
[14] X.H. Sun, H.Y. Peng,Y.H. Tang, W.S. Shi, N.B. Wong, C.S.
Lee, S.T. Lee, T.K. Sham, J. Appl. Phys. 89 (2000) 6396.

[15] Y.F. Zhang, Y.H. Tang, C. Lam, N. Wang, C.S. Lee,
I. Bello, S.T. Lee, J. of Cryst. Growth 212 (2000) 115.
[16] In a separate experiment, we measured the photolumines-
cence (PL) of Al doped (5–10%) as-prepared SiNWs of 20
nm in diameter and observed no significant enhancement in
the PL over undoped SiNWs, suggesting that the-order-of
magnitude enhancement in the PL of our sample doesnÕt
arise primarily from the Al in the silicon oxide layer of
SiNWs prepared from zeolites.
26 C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26