Formation and transport of nanoparticles in magnetically confined acetylene
plasmas produced at very low pressure
G. Al Makdessi 1, J. Margot 1, R. Clergereaux 2
Université de Montréal - Montréal (Canada), 2Université Paul Sabatier - Toulouse (France).
1
Abstract: We investigate magnetically confined low pressure dusty plasmas in
acetylene. Our goal is to examine how the use of magnetic confinement allows
circumventing the low pressure by enabling the formation of nanoparticles in the
plasma volume. Our results show that there exist specific conditions in which
nanoparticles are generated. We will present the dependence of characteristics of
the deposits as a function of the plasma parameters.
Keywords: dusty plasma, acetylene, nanoparticles, magnetic field, magnetic
mirror.
1. Introduction
Dusty plasmas are plasmas containing charged nano-sized
or even micro-sized particles. Known for decades, dusty
plasmas have attracted the interest of the scientific
community in the early 80s, especially in astrophysics
when dusty particles were discovered in the rings of
Saturn [1]. Comets, planetary rings, interstellar dust and
interstellar clouds are some examples of natural objects
formed by dusty plasmas [2]. Far from being limited to
distant objects, dusty particles are also found in
laboratories, in plasmas used for deposition and etching of
thin films. In addition, dusty particles are present in
different systems such as radio frequency capacitive
discharges, inductively coupled discharges and DC
discharges when the plasma gas in chemically reactive
such as methane or acetylene. Dusty plasmas may behave
as weakly coupled environment (such as gases) or
strongly coupled environment (like liquids or solids).
Therefore, they can form coulomb crystals also called
plasma crystals in which aggregates of dust have
spontaneously well-organized structure [3,4]. This
property can be used to create monolayer crystals and to
deposit them on the surface of materials to change their
surface properties. Our goal is to investigate the effect of
the magnetic confinement on the formation of carbon
nanoparticles and on the plasma parameters in dusty
acetylene plasma.
long of 96cm. The plasma is produced in acetylene
diluted by argon in such a way to work with four
mixtures: 10%, 20%, 50% and 80% of C2H2 while
maintaining always the pressure at 2 mTorr. The input
acetylene and argon flows are controlled by mass flow
controllers. The frequency of the surface wave is fixed at
200 MHz and the power injected in the plasma is equal to
350 W. The magnetic field is created by four coils
connected in series and mounted to form a magnetic
mirror. The coil current can reach a value of 200 A which
results in a magnetic field equal to 140 Gauss in the
central region between the extreme coils. Dust particles
were collected and all the measurements were achieved in
the center of the magnetic mirror region.
Fig. 1 Schematic of the surface wave plasma reactor
3. Results and discussion
2. Experimental details
a) Nanoparticle observations
The experiments were carried out in a surface wave
discharge plasma reactor schematically shown in fig. 1.
The plasma is generated in a 15cm-diameter quartz tube
connected to a 20 cm-diameter stainless-steel chamber
Nanoparticles synthesized in the C2H2/Ar plasma in
different conditions were scanned with SEM technique, to
analyze the effect of the magnetic field on the size, the
shape and the formation of these particles. Figure 2
Fig. 2 SEM images of collected particles for 0 Gauss (a,
c) and for 140 Gauss (b, d)
As it can be seen in fig. 2(a), many particles and deposits
are observed in the sample prepared without magnetic
field. In contrast, Fig. 1(b) shows that operating at 140 G
does not results in such particles. Zooming on the surface
of these two samples, we can observe that in absence of
magnetic field (fig. 2(c)), the collected particles have the
shape of fragments, which can be attributed to the result
of sputtering of deposits on the reactor walls
(heterogeneous reactions). On the other hand, the particles
synthesized with a magnetic field equal to 140 Gauss
have a spherical and a well-organized shape, as expected
for particles formed in the plasma volume through
homogeneous reactions [5].
We can therefore conclude that the magnetic field plays
an important role on how the particles are formed in the
C2H2/Ar plasma. The magnetic field enhances the rate of
the homogeneous reactions and consequently increases
the formation of nanoparticles in the plasma volume.
Figures 3(a,b,c) shows some examples of nanoparticles
synthesized in the presence of a magnetic field. This
shows how the particles are formed in the volume of the
plasma through coagulation and homogeneous reactions.
In addition, we see in fig. 3(c) that the synthesized
particles can present a crystalline phase. Studies are
presently performed to characterize more precisely this
phase.
The SEM technique was used to study the effect of the
magnetic field on the average diameter of dust particles
synthesized in the C2H2/Ar plasma as shown in fig. 4.
This average diameter increases linearly with the
magnetic field value. In addition, it increases with the
percentage of acetylene in the plasma. This could be
explained by the fact that the magnetic field increases the
residence time of the charged species forming the
nanoparticles, which yields enhanced absorption of ions
and radicals on their surfaces [6]. As a result, their
volume increases rapidly.
20
Average diameter (micrometer)
illustrates the effect of the magnetic field on the formation
of the dusty particles.
80% C2H2, 20% argon
20% C2H2, 80% argon
Power=350W
Pressure=2mTorr
15
10
5
-20
0
20
40
60
80
100
120
140
160
Magnetic field (Gauss)
Fig. 4 Variation of the average diameter of dusty particles
in function of the magnetic field
To analyze the nature of the particles synthesized in the
C2H2/Ar plasma, a Micro-Raman technique was used. All
the samples analyzed show the same pattern. As we can
see in fig. 5, two peaks are present: one at 1596 cm-1
typically labeled as the G band which is characteristic of
graphite-like carbon [7]; the second peak, at 1307 cm-1,
refers to the D band which is disorder [7]. Therefore, the
dust particles synthesized in the reactor refer to
amorphous carbon.
Fig. 3 SEM images of collected dust particles synthesized
with the application of a magnetic field
c) Positive ions
4000
Mass scanning of the positive ions results in the mass
distribution presented in fig.7.
2000
Intensity
1E7
2mTorr
350W
10% C2H2
B=140G
1000000
0
Intensity
100000
-2000
1000
2000
10000
1000
3000
Raman shift (cm-1)
100
Fig. 5 Raman shift diagram of the synthesized dust
particles
10
20
40
60
80
100
Mass
Fig. 7 Mass spectrometry of the positive ions
b) Negative ions
Using mass spectrometry, we were able to detect only one
negative ion species, namely C2H-. This species is known
to be the precursor of the chain reactions of anions in
C2H2 plasma as represented by the following equations
[8]:
C2 n H
C2 H
C2 H 2
H
C2 n 2 H
The dependence on the magnetic field of the percentages
of dominant positive ions is shown is fig. 8. We observe
that the percentage of all species decreases with
increasing field, which can be explained by the
enhancement of their confinement which favors the
volume reactions consuming these species.
H2
Anions are considered to be the species being the most
likely to form the dust particles in the plasma [9]. As it
can be seen in Fig. 6, the concentration of C2H- ions
increases linearly with the magnetic field due to the
increase of either their confinement in the plasma volume
or their formation.
H2
C2H2
C2H4
C
C2
2mTorr
350W
20% C2H2
t=4min
0.01
Percentage
C2 H2 e
The dominant ions are H2+, C+, C2+, C4+, C2H2+, C2H4+ and
Ar+.
1E-3
4500
1E-4
4000
3500
C2HPressure = 2mTorr
Power = 350W
80% C2H2, 20% argon
-20
0
20
40
60
80
100
120
140
160
Magnetic field (Gauss)
Intensity
3000
Fig. 8 Dependence of the positive ion concentration on
the magnetic field
2500
2000
1500
1000
500
-20
0
20
40
60
80
100
120
140
160
Magnetic field (Gauss)
Fig. 6 Concentration of the anion C2H- in function of the
magnetic field
The percentage of the positive ions was also investigated
as a function of the fraction of C2H2 in plasma. The
results are illustrated in Fig. 9. We see that it increases
clearly with C2H2 percentage due to the increase of C2H2
molecules which represent the source of these species.
7.5
10% C2H2
20% C2H2
50% C2H2
80% C2H2
2mTorr
350W
7.0
H2
C2H2
C2H4
C
C2
2mTorr
350W
B=140G
1E-3
6.5
Electron temperature (eV)
Percentage
0.01
6.0
5.5
5.0
4.5
4.0
3.5
3.0
1E-4
2.5
0
10
20
30
40
50
60
70
80
2.0
-20
90
0
C2H2 percentage in the plasma
d) Plasma density and electron temperature
8.00E+010
10% C2H2
20% C2H2
50% C2H2
80% C2H2
2mTorr
350W
Positive ions density (cm-3)
5.00E+010
3.00E+010
2.00E+010
1.00E+010
0
20
40
60
80
100
120
140
160
Fig. 11 Variation of the electron temperature with the
magnetic field for four C2H2/Ar mixtures
The influence of a magnetic field in the magnetic mirror
configuration on the formation of carbon nanoparticles
and on the plasma parameters was studied. The magnetic
field enhances the formation of dust particles through
homogenous reactions inside the plasma volume. The
average diameter of synthesized nanoparticles increases
with the intensity of the magnetic field and with the
percentage of C2H2 in plasma. In addition, the magnetic
field has a very important effect on the plasma parameters
such as the negative ions, positive ions, electron density
and electron temperature.
References
4.00E+010
0.00E+000
-20
60
4. Conclusion
The dependence on the magnetic field of the total ion
density is shown in fig. 10 at four different C2H2
percentages. The ion density increases with B due to the
confinement of the positive species in the plasma volume.
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6.00E+010
40
Magnetic field (Gauss)
Fig. 9 Evolution of the positive ions in function of the
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7.00E+010
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120
140
160
Magnetic field (Gauss)
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As illustrated in Fig. 11, the electron temperature
increases with both the magnetic field intensity and the
C2H2 percentage. This is required to compensate the
enhanced charged particles losses [11, 12].
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