Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Trang 1Deposition of carbon nanotubes on Si nanowires by chemical
vapor deposition
Center of Super-Diamond & Advanced Films (COSDAF) and Department of Physics & Mater Sci., City University of Hong Kong,
83 Tat Chee Avenue, Kowloon, Hong Kong, China
Received 19 July 2000
Abstract
By using a hot ®lament chemical vapor deposition (HFCVD) method, deposition of carbon on Si nanowires (Si NWs) has been studied Multi-walled carbon nanotubes (CNTs) were found to form on the surfaces of Si NWs at 900°C with a good surface coverage and adherence However, as the temperature of deposition increased to 1000°C, Si cores tended to transform into b-SiC cores and the carbon layers grown on b-SiC cores were distorted When the temperature
of deposition was as high as 1100°C, the carbon layers bucked openly to form many feather-like carbon sheets sprouting from the surface of the nanowires A mixture of large carbon sheets and b-SiC nanowires was formed when the temperature was over 1300°C Ó 2000 Elsevier Science B.V All rights reserved
1 Introduction
Carbon nanotubes (CNTs) [1] and Si
nano-wires (SiNWs) [2,3] have attracted considerable
attention in recent years because they have
demonstrated the potential to make a major
con-tribution to a variety of nanotechnological
applications [4±6] It leads to speculations that the
modi®cation and combination of these two kinds
of nanomaterials, such as ®lling CNTs with silicon
or coating Si NWs with CNTs, will be more
ro-bust and result in an even more diverse range of
applications Up to date, a composite of CNTs
with SiNWs in longitudinal has been synthesized
[7] However, a composite of these two materials
in transversal has not been reported Only CNTs
sheathed on other materials including in situ growth in carbon gas mixtures and capillarity-driven ®lling of open nanotubes by liquid reagents have been reported as nanocables [8±11] It should
be noticed that the percentage of ®lled nanocables was practically either very low [9,11] or with very small length to diameter ratios [10,11] More re-cently, we have reported a new oxide-assisted growth method by which high-purity SiNWs can
be synthesized in large scale from a mixture of Si and SiO2 powders or from pure SiO powder [12± 15] This supplied a good as-grown nanoscale materials to synthesize a composite of CNTs and SiNWs in transversal Here, we report the syn-thesis of this composite material by a hot ®lament chemical vapor deposition (HFCVD) method The results provided a way to make CNTs coat
on SiNWs or SiC nanowires for further applica-tions in both nanoscale electronic devices and composite materials
Chemical Physics Letters 330 (2000) 48±52
www.elsevier.nl/locate/cplett
* Corresponding author Fax: +852-2784-4696.
E-mail address: apannale@cityu.edu.hk (S.T Lee).
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 8 4 - 8
Trang 22 Experimental
The as-grown Si NWs used in the present
ex-periment were produced by excimer pulsed laser
ablation of a target made of a mixture of Si and
SiO2powders under 9 104 Pa Ar atmosphere at
1200°C The experimental details were the same
as those reported in our previous paper [3] The
experimental apparatus employed for coating the
Si NWs by CNTs was a bell-jar shaped
RF-plasma HFCVD system One of the RF
elec-trodes made of a Mo sheet parallel to the ®lament
was used as the substrate holder This sheet was
grounded supporter by putting it on a quartz
plate The other RF electrode was placed 4 cm
above The ®lament was placed between these two
RF electrodes The distance between the substrate
and the ®lament could be adjusted The as-grown
SiNWs were put on a silicon sheet as a substrate
100 sccm gas ¯ow rate with 8% of methane in
hydrogen was fed from above the ®lament toward
SiNWs The electrodes were supplied with a 13.56
MHz RF power source through a L±C matching
network First, the amorphous silica outerlayer of
the as-grown SiNWs was removed by RF plasma
for 120 min with 300 W plasma power under 1:5 102 Pa gas pressure at 300°C substrate temperature Then, the pre-carbonized tungsten
®lament above the substrate surface was electri-cally heated to 2100°C The temperature of the substrate was measured to be 900°C The carbon deposition on SiNWs was carried out at 3 103
Pa for 10 h
3 Results and discussions The as-grown SiNWs used in this experiment for carbon coating were of high purity with a whitish yellow color Transmission electron mi-croscope (TEM) image showed the nanowires to have primarily smooth and uniform wire-like structures (Fig 1a) The average diameter was at
15 nm Their lengths could extend up to a few millimeters with nearly the same diameter throughout the length The structure of the nanowires was con®rmed to consist of a crystalline
Si core and a silica outerlayer by using selected-area electron diraction (SAED) pattern (inset of Fig 1a) and high resolution transmission electron microscope (HREM) (Fig 1b)
Fig 1 (a) TEM image of the as-grown SiNWs Inset is a SAED pattern; (b) HREM image of a typical SiNW.
Trang 3After the deposition, the color of the nanowires
changed to black Fig 2a showed the morphology
of carbon coated on SiNWs as observed with
scanning electron microscopy (SEM) It can be
seen that the nanowires remained as a web on the
Si substrate Some carbon particles were also
found to co-exist during the carbon coating
pro-cesses Fig 2b showed a TEM image of the same
sample and its corresponding SAED pattern The
diraction rings of crystalline cubic Si and b-SiC
could be identi®ed
High resolution TEM images revealed that Si
NW with carbon coating consisted of a single
crystalline Si core and a sheath of carbon
muti-layers with an inter-layer spacing of 0.34 nm
(Fig 3a) The multi-walled CNTs showed good
uniformity and adherence to the Si core However,
there were also some disturbed interface areas of
CNTs and SiNWs The original silicon dioxide
layer on the Si core disappeared, presumably due
to the etching eect of atomic hydrogen in the
plasma Fig 3b showed a nanowire with the b-SiC
core and a sheath of only a few carbon layers It
had been noticed that two kinds of nanowires shown in Fig 3a, b were from the same specimen This case was owing to the poor thermal contact between the nanowires themselves and also with the Si substrate So the temperatures of individual nanowires were dicult to be kept uniform during the carbon deposition process
When the substrate temperature was increased
to 1100°C, electron diraction revealed that the cores of the nanowires were transformed to b-SiC completely It could be seen from the electron diraction pattern in the right bottom corner of Fig 4a The carbon layers could not self-organize into nanotubes completely and bucked to form many feather-like sheets on the nanowire surface,
as shown in the upper image in Fig 4a A typical nanowire in high magni®cation was shown more clearly in the left bottom image in Fig 4a As the deposition temperature increased to 1300°C, the coated carbon grew to become large carbon sheets,
as shown in the bottom image in Fig 4b Even the nanowires were completely carbonized and mixed into a lot of carbon sheets to form a mixture of
Fig 2 (a) SEM image of the web-like product; (b) corresponding TEM image and SAED pattern.
Trang 4b-SiC nanowires and graphitic carbon, as shown in
the top image in Fig 4b
The characteristics of carbon deposition on the
crystalline Si NW can be envisaged to be
con-trolled by both a template eect for the formation
of tube-like carbon shell structure and a
carbon-ization eect for the transformation of Si NWs to
SiC nanowires At a relatively low substrate
tem-perature (900°C), the deposition rate of carbon
on the surface of Si NWs was slow and if a carbon
nucleation happened on a nanowire surface, it was
possible that a carbon tube-like network formed
around the nanowire As the diameters of the
nanowires were very small, tube-like carbon
structures were more stable than any other bulk
carbon structures However, if the substrate
tem-perature was high enough (1000°C) so that SiC
can be formed by carbon atoms diusing into the
Si NWs, CNTs could not be formed very well on
the surface of nanowires because of both the
volume change of the nanowires and breaking of the carbon network sheathed on the nanowires When the substrate temperature was relatively high (>1100°C), a mixture of graphitic carbon and SiC nanowires was formed because of both the carbonization of the nanowires and the fast carbon deposition to form the carbon-like structure
4 Conclusions
In summary, CNTs encapsulated crystalline SiNWs were synthesized by the HFCVD method
At 900°C, carbon multilayers were formed on the surface of carbonized Si NWs At a higher reaction temperature (1000°C), the silicon cores reacted with carbon and transformed into b-SiC cores Carbon shells formed on the SiC core were not as uniform as those on the Si core As the substrate temperature increased further to 1100°C,
Fig 3 (a) HRTEM image of a nanowire consisting of a crystalline Si core and a sheath of multi-walled CNTs; (b) a nanowire with a part of the core transformed into b-SiC and coated with a sheath of CNTs.
Trang 5feather-like carbon sheets sprouting from the
sur-face of the nanowires were formed At an even
higher deposition temperature (1300°C), a mixture
of carbon pieces with b-SiC nanowires was
ob-tained
Acknowledgements
The authors wish to thank Prof F.H Li and
Prof X.F Duan for useful discussions This work
is supported by the Research Grants Council of
Hong Kong (Project no 9040365)
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Fig 4 (a) TEM image of the nanowires with carbon deposition at 1100°C, and a higher magni®cation image of a single nanowire in the same sample at the left bottom corner and the corresponding SEAD pattern at the right bottom corner; (b) TEM image of the nanowires with carbon deposition at 1300°C and the higher magni®cation image of a part of the sample in the upper image.