22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Control over the surface structure of the plasma-deposited polyaniline film
A. Zaitsev1, F. Poncin-Epaillard1, A. Lacoste2 and D. Debarnot1
1
Institut des Molécules et Matériaux du Mans, UMR Université du Maine/CNRS 6283, Avenue Olivier Messiaen, 72085
Le Mans Cedex 9, France
2
Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble Alpes, CNRS/IN2P3, 53 rue des
Martyrs, 38026 Grenoble Cedex, France
Abstract: A texturing of the plasma-deposited polyaniline surface is achieved with
apparition of morphological features. Critical parameters are defined and their influence is
demonstrated. A control over their size and density is possible by step-variation of
discharge power and process duration. Different types of surface are synthesized to
demonstrate the effect of each.
Keywords: nanostructures, polyaniline, plasma polymer, surface morphology
1. State of the art
One-dimensional nanostructures are of great interest
due to their interesting properties such as high shape
factor and thus high specific surface (using as sensors)
[1, 2], high thermal conductivity [3] and field-emitting
properties [4]. A lot of research has been done in the field
of synthesis by PECVD of inorganic nanotubes and
nanorods (carbon, titanium dioxide, metals). These
formations have a crystal lattice and their growth occurs
following crystal thermodynamic laws. Moreover, the
surface defects or previously deposited particles of
catalyst play role of initiation sites [5, 6] controlling the
diameter and the density of the formations. This yields
one-dimensional structures with well-controlled shape
and density on the substrate surface.
With polymer nanorods and nanofibers the synthesis is
more complicated as they do not have any crystal lattice
so their orientation is difficult to control. For the same
reason no specific sites on the surface can be used for
preferential growth. The main path for obtaining onedimensional polymer fibers is plasma etching of the
polymer surface (“top-down” technique). A bulk polymer
as well as thin film (spin- or dip-coated) can be used.
Implicated polymer should be semi-crystalline or have a
capability to crystallize under the heat [7]. Thus,
crystallites act as a mask during etching and amorphous
part is removed. The major drawback of this technology
is the necessity of pre-formed surface.
Another technique applied for production of polymer
nanorods is hard template synthesis [8]. The difficulty is
the need in templates (anodic alumina or polycarbonate
membranes) and its dissolution that can damage polymer.
The most relevant work concerning direct synthesis of
the polymer nanofibers was published by Liang et al.
[9, 10]. Authors used electrochemical polymerization of
aniline with step-variation of current density during the
process. They have obtained a well-defined array of
nanofibers of polyaniline but no explanation of
preferential growth was presented.
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However,
no
synthesis
of
one-dimensional
nanostructures by means of cold plasma process was
reported. Some works [2, 11] have reported a synthesis of
fibrillar polyaniline by cold plasma, however these
structures cannot be considered as one-dimensional
because of their irregularity and random orientation on the
surface.
2. Experimental part
Current research presents a tentative to overcome the
majority of drawbacks mentioned previously by applying
a step-variation method to plasma process. For this work
we have used a reactor with 12 electron cyclotron
resonance (ECR) sources evenly distributed over the
plasma chamber circumference and powered by
microwave generator (Sairem GMP20KED) operated at
2.45 GHz through divider guide (Fig. 1).
Fig. 1. General scheme of PECVD reactor.
The chamber was pumped down to 5×10-5 mbar (as
measured by cold cathode gauge Pfeiffer PKR251) by
turbomolecular pump (Adixen ATH300) backed up by
rotary pump (Adixen ACP28G). Aniline vapour pressure
was fixed at 1.7×10-3 mbar (measured by capacitive gauge
Pfeiffer CCR364).
1
Silicon (100) was used as substrate for thickness
measurements thanks to a contact profilometer (Veeco
Dektak 8, tip diameter 3 µm, scan length 2000 µm, scan
duration 40 s) and for AFM analyses (Bruker Innova with
50 nm aluminium coated silicon tips f = 300 kHz,
k = 40 N/m, scan range 5x5 µm). Pressed KBr pellets
were used for FT-IR spectroscopy (Bruker IFS66 in
transmission mode, measurement range 4000 cm-1 –
400 cm-1, resolution 2 cm-1).
These substrates were placed on the water-cooled
(10 °C) substrate holder situated at the centre and 3.5 cm
from the bottom. The deposition was held at different
microwave power supplies (60 W - 480 W) for a fixed
time of 10 minutes.
3. Results
3.1. Deposition kinetics
Deposition kinetics shows a linear increase of the layer
thickness (measured by Dektak 8 contact profiler) up to
120 W. Then the deposition rate remains constant
between 120 W and 360 W. Further increase of the
discharge power leads to decrease of the layer thickness.
3.2. AFM and FT-IR measurements
The layer synthesized at 60 W shows very smooth
surface comparable to the surface of bare silicon (a point
at P = 0 W on Fig. 2a. It has been found that the
roughness increases rapidly with increase of the discharge
power and presents a maximum at 240 W. However,
further increase of the power leads to decrease of the
roughness.
Fig. 3. AFM image of polyaniline obtained at: a) low
power; b) variable power.
0.5
2.0
750 cm-1, 690 cm-1) exhibit a tendency to disappear while
those corresponding to aliphatic groups increase in
intensity. Overall preservation of the aromaticity can be
determined by the area ratio of the bands corresponding to
aromatic and non-aromatic C=C bonds. This ratio as a
function of discharge power is represented on Fig. 2b.
A conclusion was made that a smooth surface was
formed under low discharge power while a significant
structuring was observed at high powers. But while the
first one presents a good conservation of monomer
structure, the second one does not show any. It has been
shown that in the intermediate power domain
(240 W - 360 W) a drastic change in surface roughness
takes place after 5 minutes of deposition with further
cracking of the layer.
To overcome this problem the deposition was held
following step-by-step procedure. It was started at high
discharge power for a short period of time (1 to
3 minutes) in order to form a “morphological” surface
followed by long (25 min) deposition at low power to
deposit “functional” layer. The AFM images below show
the surface of a polyaniline synthesized at the lowest
power (Fig. 3a) and by combined technique (Fig. 3b).
60 W
0.4
A1500/A1600
Rq (nm)
1.5
1.0
0.5
0.3
0.2
0.1
120 W
0
100
200
300
400
500
0.0
180 W 240 W
Discharge power (nm)
(a)
(b)
Fig. 2. Evolution with discharge power of a) RMS
roughness; b) ratio of C=C (arom.)/C=C (aliph.).
Infrared spectrum of the polymer obtained at minimal
power of 60 W shows typical bands for plasmapolyaniline. The most significant are those corresponding
to aromatic and aliphatic C-H stretch (3030 cm-1 and
2930 cm-1 respectively) as well as aromatic and aliphatic
C=C stretch (1500 cm-1 and 1600 cm-1 respectively). The
intensity of aromatic bands confirms preservation of
aromatic character of monomer in polymer layer. Two
bands at 690 cm-1 (aromatic meta-substitution) and
750 cm-1 (aromatic ortho-substitution) are also important
to prove aromatic character of polymer.
Another
important band is situated at 3300 cm-1 and corresponds
to secondary N-H stretch and shows that monomer units
are connected through amine group. But with the increase
of discharge power aromatic bands (3030 cm-1, 1500 cm-1,
2
4. Discussion
4.1. Growth kinetics
Given curve appearance with three zones is in agree
with those given by Yasuda [12]. Linear increase from
the start shows an energy-deficient zone with the excess
of monomer and deposition rate is determined by power
input. Next part corresponds to monomer-deficient zone
where the deposition rate is determined by monomer
pressure and power input is excessive. The third part
shows total monomer destruction coupled with important
heating factor at the surface that leads to the decrease of
polymer layer thickness.
4.2. Plasma polymer chemical structure
The degradation of chemical structure with the increase
of discharge power is caused by increase of the density of
ionizing species. Indeed, with the increase of power
supply, monomer molecules undergo more important
fragmentation that leads to the degradation of overall
polymer chemical composition. Significant loss of
aromaticity means the destruction of the monomer entity
in chemical structure.
4.3. Plasma polymer morphology
Phenomenon of surface structuring seems to be
connected with the surface heating while operating at high
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discharge power. The higher power heats the surface
more enabling deposited aniline oligomers to migrate and
to self-assemble on the surface. However, excessive
heating (by too high discharge power and too long
deposition) destroys already formed structures leading to
smooth polymer surface. The surface temperature was
measured during deposition process at different discharge
power with non-reversible thermosensitive bands (Fig. 4).
From this figure it is clear that every discharge power has
its equilibrium temperature.
70
420 W
480 W
TS (°C)
60
360 W
50
300 W
240 W
40
different temperatures (42°C for 240 W and 47°C for
300 W).
These points correspond to equilibrium
temperature for respective power discharges. A slight
linear increase of the roughness at low temperatures
(20 °C to 40 °C) is explained by slow heating of the
substrate with time. But once equilibrium temperature
has been reached a dewetting phenomenon takes place
and leads to cracking of the layer. Higher dewetting point
for 300 W could be justified by the fact that with more
power input the polymer becomes more reticulated and so
more heat energy is necessary for dewetting to occur.
Thus, it is necessary to limit deposition power to
240 W - 300 W for 1 minute to 4 minutes in order to
avoid cracking of the layer. Varying the discharge power
and the duration of the first deposition step as well as the
number of steps it is possible to finely tune the surface
morphology with preservation of monomer chemical
composition (Fig. 6).
30
20
0
100
200
300
400
500
600
Deposition time (s)
Fig. 4. Substrate temperature evolution during deposition
process at different discharge power.
2.0
2.0
1.5
1.5
Rq (nm)
Rq (nm)
Moreover, cracking of the layer is probably related to
dewetting of the film [13, 14] at high surface temperature
combined with high layer thickness and high value of
accumulated internal stress [12]. The phenomenon of
dewetting results in previously mentioned drastic increase
of roughness after certain deposition time as shown on
Fig. 5a.
1.0
0.5
0.0
Silicon
0
2
4
6
8
10
1.0
0.5
0.0
20
25
30
35
40
Deposition Time (min)
TS (°C)
(a)
(b)
45
50
Fig. 5. Evolution of the surface roughness for different
discharge power: () – 60 W, () – 240 W, () – 300
W. a) with deposition time; b) with surface temperature.
From this graph it is clear that for 60 W the roughness
is nearly constant with time. For 240 W it increases with
time linearly up to 4 minutes and then the increase slope
changes showing more rapid increase . For 300 W the
evolution is clearly different. It reaches a plateau from
1 minute of deposition time and follows it up to
5 minutes. Starting from 5 minutes the graph slope
becomes the same as for 240 W. This behaviour can be
explained by Fig. 5b. It shows an interesting pattern.
Both curves show a drastic change in roughness but at
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Fig. 6. Different patterns of surface obtained by stepdeposition process.
The height, the diameter as well as the density of the
surface nanostructures can be adjusted in a wide range
from clustered surface to completely smooth one. Also
this gives a potential possibility to obtain one-dimensional
polymer nanostructures without use of any template.
5. Conclusions
Deposition of polyaniline at low discharge power
allows good conservation of monomer chemical structure
and deposited layer is smooth. Contrary, at high power
obtained surface is rough with nanometric features but no
monomer entity is present in the layer. Moreover, these
features are formed at the early stages of deposition (few
minutes) and are quickly destructed with longer duration.
A step-variation of discharge power gives possibility of
direct synthesis of a nanostructured surface with
preservation of monomer chemical composition.
Furthermore, the surface structuring is finely tuneable by
changing the number of steps and their power-time
values.
6. Acknowledgments
Our team thanks La Région Pays de la Loire for the
financial support.
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