st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Coreshell nanoparticles generated by plasma CVD
and their application to Li ion batteries
M. Shiratani1, Y. Morita1, K. Kamataki2, Y. Kanemitsu1, D. Ichida1, G. Uchida1, H. Seo3, N. Itagaki1,4, and K. Koga1
1
Graduate School of Information Science and Electrical Engineering, Kyushu University, Japan
2
Faculty of Arts and Science, Kyushu University, Japan
3
Center of Plasma Nano-interface Engineering, Kyushu University, Japan
4
Prest, Japan Science and Technology Agency, Japan
Abstract: SiC nanoparticle composite films were successfully produced using double multi-hollow discharge plasma CVD, where generation of Si particles and their
hydrocarbonization were independently performed using SiH4/H2 and CH4 multi-hollow discharge plasmas. We also succeeded in controlling carbon content in SiC nanoparticle composite films by varying a number density of CHx radicals irradiated to the Si nanoparticles.
Keywords: Si and SiC nanoparticle films, plasma CVD, Li ion batteries
1. Introduction
Li ion batteries are developed as a promising power
source for hybrid electrical vehicles and electric vehicle.
Increasing the capacity of Li ion battery anodes is attractive route to lower battery weight and volume [1-14]. Our
interest is in Si nanoparticle composite films because
nano-structured Si is promising for high capacity electrodes of Li ion batteries. The charge-discharge capacity
of Si is an order of magnitude higher than that of conventional graphite anode. However fully lithiated Si has been
reported to cause fracture and pulverization of the electrode, thereby leading to capacity degradation and failure
of the battery cells.
In this paper, we report deposition of SiC nanoparticle
composite films, because the carbon matrixes are used to
buffer the volume expansion of Si nanoarticles in the
films. We have succeeded in producing Si nanoparticles
using a multi-hollow discharge plasma CVD, and in controlling their size and structure by gas pressure and hydrogen dilution ratio in SiH4/H2 discharges [15-22]. Our
CVD method has an advantage of growing Si nanoparticles with narrow size distribution. First, we report characteristics of Si nanoparticle composite films. Then, we
present results of SiC nanoparticle composite films deposited by using SiH4/H2 and CH4 double multi-hollow
discharge plasma CVD.
plying peak-to-peak voltage of 300 V to the powered
electrode with SiH4 and H2 flow rates of 2 and 448 sccm,
respectively. Si nanoparticles were nucleated, grown in
the SiH4/H2 plasma produced inside the small holes, and
then transported downstream by a strong SiH4/H2 neutral
gas flow. Hydrocarbon, which was produced in the CH4
plasma (plasma 2) with CH4 flow rate of 20 sccm, was
irradiated onto the surfaces of the Si nanoparticles during
their transport downstream. The peak-to-peak voltage of
plasma 2 was 400 V. Si nanoparticle composite films were
deposited on substrates located 5 mm downstream from
the multi-hollow electrodes, and the substrate temperature
was maintained at 180oC. Here, a total pressure was kept
constant at 3 Torr.
Film thickness was measured with a scanning electron
microscope (SEM; JEOR JIB-4600F). Crystallinity was
evaluated from Raman spectra measured with a laser Raman spectroscope (JASCO NRS-3100).
0
Z
multi hollow
electrodes
grounded
electrode
powered
electrode
grounded
electrode
5 mm
Plasma 1
for production of
Si nanoparticles
H2 , SiH4 flow
matching
network
RF power
(60MHz)
2. Experimental
The production of Si nanoparticles and surface treatment
by hydrocarbon radicals were carried out using a multi-hollow discharge plasma CVD method, shown in Fig. 1,
wherein two discharge plasmas of SiH4/H2 (plasma 1) and
CH4 (plasma 2) are independently generated in a vacuum
chamber. The multi-hollow electrode consisted of a powered electrode and two grounded electrodes 30 mm in
diameter. The discharges were sustained in eight small
holes of 5 mm diameter. Plasma 1 was generated by ap-
plasma
50 mm
substrate
Plasma 2
for production of
CH4 flow CHx radical
matching
network
RF power
(60MHz)
Fig.1 Experimental setup of double multi-hollow discharge
plasma CVD method.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
4. Results
4.1 Deposition of Si nanoparticle composite films
Figure 2 shows two-dimensional image of Si nanoparticle composite film, where SiH4/H2 discharge plasma
(plasma 1) was produced for Si nanoparticle fabrication
[23,24]. Film morphology drastically changes at substrate
position, where dark brown film at Z = 10 mm has a large
roughness as found in SEM images, while brown film at
Z = 30 mm has smooth surface. Deposition rate was 0.19
and 0.029 nm/s at Z = 10 and 30 mm, respectively.
Figure 3 shows Raman spectra observed for Si
nanoaprticle composite films at Z = 10, 24, and 30 mm,
together with that of reference single-crystalline Si. The
film deposited at Z = 10 mm shows a sharp peak, and this
peak is assigned to Si crystal. As Z increases from 10 to
24 mm, the peak slightly shifts to higher frequency.
The Raman peak frequency strongly depends on the grain
size. From Raman spectra, the average grain size was
estimated using d = 2(B/W)1/2, where W is the shift of
Raman peak of nanoparticle composite film with respect
to that of reference single-crystalline Si, and the parameter B = 2.0 cm-1nm2. Figure 4 shows the dependence of
average grain size on substrate position Z. Grain size was
deduced to be 6-11 nm. We also investigate the crystal
orientation of Si nanoparticle composite films. As can be
seen in Fig. 5, X-ray diffraction pattern indicates a preferential growth of the crystallites in the (220) direction
[25,26] Raman analysis also shows that the film structure
changes from nanocrystalline to amorphous with an increase in Z, where horizontal dotted line in Fig. 2 shows
boundary between nanocrystallie and amorphous.
z
c-Si
a-Si
Z = 24 mm
Z = 30 mm
10 mm
Z = 10 mm
0.1 mm
Fig.2 Image of the film deposited by using SiH4/H2 discharge plasma (plasma 1). Horizontal dotted line shows
boundary between c-Si and a-Si region.
Grain size (nm)
Z = 10 mm
15
10
5
0
10
15
20
Z (mm)
25
Fig.4 Substrate position dependence of average grain size,
which is estimated from peak shift of Raman spectra.
Z = 10 mm
Z = 24 mm
Z = 30 mm
a-Si
Intensity (arb. units)
Intensity (aub. units)
c-Si
Z = 10 mm
(111)
(220)
(311)
c-Si reference
350 400 450 500 550 600 650
-1
Raman shift (cm )
Fig. 3 Raman spectra of Si nanoparticle composite films
showing a transition from crystalline structure to amorphous.
25 30 35 40 45 50 55 60
(degree)
Fig.5 X-ray diffraction patter for the film at Z = 10 mm.
st
Absorbance (aub. units)
Z = 10 mm
Z = 30 mm
Si-H stretch Si-H2 stretch
a-Si
1800
c-Si
1900 2000 2100 2200
-1
Wavenumber (cm )
Fig.6 Fourier transform infrared spectra of Si nanoparticle
films at Z = 10 and 30 mm.
The chemical composition of Si nanoparticle composite
films was determined by Fourier transform infrared spectroscopy (FT/IR-620, JASCO Corp.) with an accumulation of 64 times and resolution of 4 cm-1. Figure 6 shows
films at Z = 10 and 30 mm. SiH2 types of bonds (2100
cm-1) are commonly formed at the grain boundaries. The
contribution from grain boundaries is large in crystalline
Si films at Z = 10 mm, and this is due to large volume
fraction of Si nanoparticles in the films
4.2 Deposition of SiC nanoparticle composite films
Then, two discharge plasmas of SiH4/H2 (plasma 1) and
CH4 (plasma 2) were generated for production of SiC
nanoparticle composite films. The film has smooth surface as found in SEM images. C content in SiC nanoparticle composite films was evaluated by measuring C-Ka
fluorescence intensity obtained from X-ray fluorescence
(XRF) measurements.
z
Z = 10 mm
10 mm
1 mm
Fig.7 Image of the film deposited by using two dis-
charge plasmas of SiH4/H2 (plasma 1) and CH4
(plasma 2).
[C] / ([Si] + [C])
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
(mm)
40
50
Fig.8 Substrate position dependence of carbon content
ratio estimated from X-ray fluorescence intensity.
As is found in Fig. 8, C content ratio increases from 0.35
to 0.6 with approaching to the CH4 discharge region. This
demonstrates that our double multi-hollow discharge
plasma CVD method realizes combinatorial deposition of
SiC nanoparticle composite films.
5. Conclusions
We developed the SiH4/H2 and CH4 double multi-hollow
discharge plasma CVD method. We observed the strong
crystalline peaks of Raman spectra from Si nanoparticle
composite films, and average grain size is estimate to be
6-11 nm. We succeeded in deposition of SiC nanoparticle
composite films by the method for application to Li ion
batteries. C content in the films can be control by the
number density of CHx radicals irradiated to the Si nanoparticles.
6. Acknowledgmet
The present study was supported in part by a
Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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Cairns Convention Centre, Queensland, Australia
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