Synthesis of coaxial nanowires of silicon nitride sheathed with silicon and silicon oxide

Article abstract of DOI:10.1016/S0038-1098(00)00255-6

Coaxial nanowires, with about 45 nm in diameter and about 15 μm in length, have been synthesized by reaction of silicon dioxide nanoparticles with active carbon at 1450 °C in flowing nitrogen atmosphere. Their structures consist of an α-phase silicon nitride core, an amorphous silicon and a silicon dioxide outer shell, similar to those of coaxial nanocables. The formation mechanism of the coaxial nanowires is discussed.

Full text of DOI:10.1016/S0038-1098(00)00255-6

PERGAMON
Solid State Communications 115 (2000) 683±686
www.elsevier.com/locate/ssc
Synthesis of coaxial nanowires of silicon nitride sheathed with
silicon and silicon oxide
*
X.C. Wu , W.H. Song, B. Zhao, W.D. Huang, M.H. Pu, Y.P. Sun, J.J. Du
Laboratory of Internal Friction and Defects in Solid, Institute of Solid State Physics, Academia Sinica, Hefei 230031,
People's Republic of China
Received 1 March 2000; accepted 21 March 2000 by Z.Z. Gan; received in ®nal form by the Publisher 21 June 2000
Abstract
Coaxial nanowires, with about 45 nm in diameter and about 15 mm in length, have been synthesized by reaction of silicon
dioxide nanoparticles with active carbon at 14508C in ¯owing nitrogen atmosphere. Their structures consist of an a-phase
silicon nitride core, an amorphous silicon and a silicon dioxide outer shell, similar to those of coaxial nanocables. The formation
mechanism of the coaxial nanowires is discussed. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: A. Nanostructures; C. Transmission electron microscopy; D. Electronic transport
1
. Introduction
method and thought that carbon nanotubes were converted
into carbide nanorods through reaction with a volatile metal
oxide. Then Han et al. exploited the method to obtain GaN
The discovery of carbon nanotubes [1] has attracted
intense experimental and theoretical interest in such struc-
tures. Theoretical investigations on carbon nanotubes
suggest that their electronic properties can be tuned from
metal to semiconductor with a small gap, depending on the
tube diameter and chirality [2,3]. The idea of making nano-
devices such as nanometer-scale electronic switches by
connecting pure carbon nanotubes has been ®rst proposed
3 4
[10] and Si N [11] nanorods by the reaction of carbon
nanotubes with a volatile metal oxide in ammonia and nitro-
gen atmosphere, respectively.
In this paper, we report the synthesis of a kind of new
coaxial nanostructure material, i.e. silicon nitride nanowires
sheathed with silicon and silicon oxide layers, by the reac-
tion of silicon oxide nanoparticles and active carbons. The
formation mechanism is also discussed.
[
4]. This idea has also promoted syntheses of heteroatomic
nanotubes and multilayered nanotubes such as BN nano-
tubes [5] and nanotubes in the B±C±N ternary system
[
6,7]. Recently, coaxial nanocables comprising silicon
2
. Experimental
carbide and silicon oxide sheathed with boron nitride and
carbon have been successfully synthesized and a great
potential application in electronic transportation or nanode-
vices has been suggested on the basis of their electronic and
structural properties [8]. Therefore, it is very important to
synthesize coaxial nanocable comprising different kinds of
nanotube in axial or radial direction for making nanometer-
scale electronic devices with a variety of functions.
However, the synthesis of one-dimensional nanoscale struc-
ture materials still remains to be a challenge. As for the
synthesis of nanorods, Dai et al. [9] ®rst succeeded in
preparing carbide nanorods by the nanotube template
The preparation processes were as follows: ®rst, SiO
nanoparticles with average dimension about 20 nm were
prepared according to Ref. [12]; polyethylene glycol 6000
2
(50 g) put in alumina crucible was thermally decomposed
into active carbons in nitrogen atmosphere at 7008C for 3 h.
Then the mixture of SiO nanoparticles (5 g) with active
2
carbon powders (5 g) was ball-milled for 20 h, and pressed
to several circular pellets under pressure of 10 MPa. The
pellets were calcined at 14508C in ¯owing nitrogen atmo-
sphere for 2 h. Wool-like products depositing on the surface
of the pellets were collected for characterizing.
X-ray powder diffraction patterns of the products were
obtained with Philips PW 1700 X-ray diffractometer with
*
Corresponding author.
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(00)00255-6

6
84
X.C. Wu et al. / Solid State Communications 115 (2000) 683±686
Fig. 1. X-ray diffraction pattern of the coaxial nanowires; T shows
high-temperature phase of SiO
2
.
Fig. 3. HREM image of the coaxial nanowire; the sheath is amor-
phous layers. The distance between the parallel fringes is 0.433 nm
along the [101] direction, which shows that the center core of the
3 4
coaxial nanowire is a-Si N .
Cu-Ka radiation. Transmission electron microscopy (TEM)
images of the products were taken with a JEM-200CX trans-
mission electron microscope. High-resolution electron
microscopy (HREM) images of the products were taken
on a JEOL-2010 transmission electron microscope. Energy
dispersive X-ray (EDX) spectra of the products were
recorded by OXFORD-6498 energy dispersive X-ray spec-
trometer attached HREM.
3
. Results and discussion
Fig. 1 shows the X-ray powder diffraction pattern of the
products. It is identi®ed as a mixture of a-Si
As shown in Fig. 1, the peaks assigned to a-Si N are stron-
3
3 4 2
N and SiO .
4
ger than those of SiO
2
labeled as T, which indicates the
. A general overview of
products are dominantly a-Si
3
N
4
the low-magni®cation TEM image of the product shown
in Fig. 2(a) indicates that the products are rod-like structure
with about 15 mm in length and 45 nm in diameter. A
magni®ed TEM image as shown in Fig. 2(b) indicates that
each individual nanowires observed in low-magni®cation
TEM images is actually a composite nanowire consisting
of a crystalline core with about 15 nm in diameter and a
surrounding amorphous sheath with about 15 nm in thick-
ness. Selected area electronic diffraction (SAED) pattern as
Fig. 2. (a) TEM image of the coaxial nanowire. (b) A magni®ed
image of the coaxial nanowire. (c) SAED pattern of the coaxial
Å
3 4
shown in Fig. 2(c) indicates the crystalline core of a-Si N
Å
with [111] axis parallel to the electron beams. A HREM
image as shown in Fig. 3 indicates that lattice interlayer
3 4
nanowire along [111] direction shows a-Si N .

X.C. Wu et al. / Solid State Communications 115 (2000) 683±686
685
Fig. 5. EDX spectra of the coaxial nanowire. (a). The electronic
probe is concentrated on the periphery (a in Fig. 4). (b). The elec-
tronic probe is concentrated on the center region (b in Fig. 4). (c).
The electronic probe is concentrated on the region between a and b
in Fig. 4.
Fig. 4. A schematic illustration of the coaxial nanowire; a and b
solid arrows) indicate the positions of the electronic probe.
(
distance of parallel fringes from the core is 0.433 nm, being
the (101) spacing of a-Si and the sheath is amorphous
3
N
4
of the product may have a potential application in electronic
transportation and nanodevices.
layer. EDX quantitative microanalysis across a nanocable
has been carried out. Fig. 5(a) indicates the percentage of
oxygen (65.82 at.%) and silicon (34.18 at.%) when the elec-
tron probes are concentrated on the outer layer, and Fig. 5(b)
shows that of silicon (65.40 at.%), nitrogen (18.00 at.%) and
oxygen (16.60 at.%) on the center region, and Fig. 5(c)
indicates that of silicon (52.94 at.%), nitrogen (0.00 at.%),
oxygen (65.82 at.%) for the region between a and b as
marked in Fig. 4. Because atomic ratio of the oxygen and
silicon on the outer layer approaches 2:1 from Fig. 5(a), the
Considering that no droplets are observed on their ends of
the nanowires, the growth mechanism of the nanowire can
be explained in terms of the vapor±solid (VS) mechanisms
[13,14]. The chemical reaction is expressed as follows
[15±17]:
C ꢀsolid† 1 SiO ꢀsolid† ˆ SiO ꢀvapor† 1 CO ꢀvapor† ꢀ1†
2
SiO ꢀvapor† 1 C ꢀsolid† ˆ Si ꢀvapor† 1 CO ꢀvapor†
ꢀ2†
ꢀ3†
ꢀ4†
outer layer is SiO
core (Si N ) and the outer layer (SiO ) is subtracted on basis
2
. After the contribution of Si in the center
2
SiO ꢀvapor† ˆ Si ꢀvapor or solid† 1 SiO ꢀsolid†
2
3
4
2
of Fig. 5(b), silicon about 43.6 at.% is still left in the inter-
mediate region. Similarly, as the contribution of Si in the
outer layer (SiO ) from Fig. 5(c) is deducted, silicon about
3
Si ꢀvapor† 1 2N ꢀgas† ˆ a-Si N ꢀsolid†
2
3
4
2
The above growth process can be divided into three steps.
The ®rst step is the formation of nuclei of a-Si N and the
20.03 at.% still remains. The difference of the silicon
3
4
content results from the different thickness of the redundant
silicon region where the electron probes go through. There-
fore, it demonstrates that the sheath has a redundant silicon
region from the outsider to the inside. That is to say, the
one-dimensional growth of the nuclei through reactions (1)±
(4). Both active carbon and nanoscale SiO are suf®ciently
ball-milled to form a great many of interfaces between the
2
reactants, which is bene®cial to the reactions. Si vapor is
produced through reactions (2) and (3). In gas phase, the
nuclei of silicon nitride is synthesized through reaction (4),
and deposits on the surfaces of the reactant nanoparticles to
form the center of the one-dimensional growth of silicon
nitride. The HREM image shown in Fig. 3 suggests that
the nanowires of a-Si N grow in the [101] crystallographic
structure of the products consists of a-Si N4 nanowires
3
sheathed with amorphous Si and SiO layers as shown in
2
2
Fig. 4. Because both Si and SiO are amorphous, resulting in
their small contrast on TEM photograph, it is dif®cult to
identify their clear interface. The lattice of the amorphous
2
Si and SiO lacks in periodicity, so it is also impossible to
3
4
observe their lattice fringe on HREM image. Solid arrows
labeled a and b in Fig. 4 indicate the positions of the electron
probes, which are concentrated on the outer layer and the
center region in EDX analysis, respectively. Because amor-
phous silicon is semiconducting, while a-Si N and SiO are
direction. The reaction forming a and b phase silicon nitride
is kinetically competitive. Since the experimental reaction
takes place in gas phase and silicon (gas) move easily to
combine with nitrogen molecules, the obtained product is
almost pure a-Si N [14]. The second step is the formation
3
4
2
3
4
insulators, the prepared nanowire is of the structure of an
insulator±semiconductor±insulator in the radial direction,
similar to coaxial nanocables. This structure characteristic
3 4
of the coating of Si and SiO sheath on the surface of a-Si N
nanowires. Due to the greater molar mass of SiO (gas)
compared with Si (vapor), the diffusive rate of Si vapor is

6
86
X.C. Wu et al. / Solid State Communications 115 (2000) 683±686
faster than that of SiO vapor in the process of the reaction. Si
gas) molecules arrive on the surface of silicon nitride core
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silicon nitride core, an amorphous silicon and silicon diox-
ide have been successfully synthesized by a reaction of
silicon dioxide nanoparticles with active carbon at 14508C
in ¯owing nitrogen atmosphere. They may have potential
applications in nanometer-scale electronic transportation
and nanodevices due to the semiconducting characteristic
of the amorphous silicon.
3
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This work was supported by National Center for R&D on
Superconductivity under contract No. 863-CD010105,
National Science Foundation under contract NSF
3463.
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