22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
In-situ OES characterization of aniline plasma and correlation with its plasmapolymer
A. Aziz Ndiaye1, A. Lacoste2, A. Bes2, A. Zaitsev1, F. Poncin-Epaillard1 and D. Debarnot1
1
Université du Maine, Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, avenue Olivier
Messiaen, 72085 Le Mans, France
2
Université Grenoble Alpes, LPSC, CNRS/IN2P3, 53 rue des Martyrs, 38026 Grenoble Cedex, France
Abstract: Aniline plasma is generated in a multi-dipolar microwave plasma at very low
pressure. The emission spectrum shows characteristic emissions of electronically excited
fragments such as CN, CH, NH, C 2 , H 2 , N 2 , N 2 +, and H-Balmer. Correlation between the
OES and XPS/TFIR data allowed deducing preponderant reaction pathways such as the
dehydrogenation of CH and NH groups initiating the formation of H-Balmer and the PANI
growth.
Keywords: aniline, polymer, optical emission spectroscopy, non-equilibrium plasma
1. Introduction
Aniline polymeric thin films (PANI) present great
challenge for a wide range of promising applications as
chemical sensors [1-8]. Such films prepared with plasma
technology have structure and sensing properties depend
on discharge power [9,10]. To date, there is only limited
information about the aniline plasma emission itself
[11-13]. Herein, we report the detailed OES study of ANI
plasma created in a distributed multi-dipolar plasma
(DMDP) device [14-15].
OES measurements performed at high resolution allow
detecting all chemically active species resulting from the
fragmentation of ANI monomer unit in the UV-NIR
region. The evolution of the main active species
concentrations was considered by conducting a parametric
study. This enabled us to establish and predict optimal
conditions for obtaining adequate PANI thin films. The
variable parameters are the discharge power determined
by the number of the microwave applicators (MwAs) and
the power applied to it.
Finally, a correlation between the aniline OES data and
the PANI films analyzed by XPS and FTIR
spectroscopies is established in order to enlighten the film
growth mechanism. Reaction routes of CN, CH and C 2
species formation are explored to deduct preponderant
chemical reaction.
Relative contribution of the
dehydrogenation of C-H and N-H groups in the H-Balmer
line intensity is also established for predicting the
dominant initiation reaction for H-Balmer production.
2. Experimental part
Very low-pressure ANI plasma is obtained through
12 MwAs, each of them ending with a dipole magnet that
ensures the electron cyclotron resonance (ECR) coupling
at 2.45 GHz frequency. The applicators are uniformly
distributed on one ring on the wall of the plasma chamber
and individually powered by a microwave generator via a
divider guide. Each applicator is also provided by an
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impedance matcher meant to reduce the reflected power
below 10% of the incident value. The discharge power
(defined as number of MwAs × power/MwA) can be
controlled through the power supplying each applicator as
well as by the number of powered applicators. The
microwave power, which ensures the viability and the
homogeneity of the plasma around each magnetic dipole,
is about 5 W/MwA, i.e., 60 W of discharge power when
12 MwAs are powered. It can be further diminished by a
selective activation of the applicators (e.g., one of two)
while the others are disconnected. In this manner, the
discharge power can be reduced to 30 W (5 W/MwA)
whilst preserving the plasma uniformity thanks to the
magnetic coupling between adjacent applicators. Indeed,
the energetic electrons of the plasma produced by an
activated applicator are trapped in the magnetic lines of
their neighbors, leading thus to a discharge power
redistributed over the set of applicators and to the plasma
homogenization in the useful volume of the process
chamber.
The ANI plasma was analyzed by OES. The radiation
of ANI plasma was then collected through a quartz
window located at mid-height of the plasma chamber and
transmitted through an optical fiber (Ø = 1 mm in
diameter and L = 3 m in length) using Jobin Yvon Horiba
TRIAX 320 scanning monochromator (grating
1200 grooves/mm blazed at 500 nm; focal length of
320 mm; f/4.1 in aperture ratio; high spectral resolution of
0.06 nm in the first order for diffraction; dispersion of
2.64 nm/mm). This monochromator is connected to a
JY Symphonie CCD camera-array 1024x256 pixel. This
spectrometer is coupled with the SynerJY software for the
control and acquisition of the spectral data. The apparatus
function ∆λ app of the optical set-up assimilated to a
Gaussian profile with FWHM, is calculated from the
well-known relation ∆λ app = FWHM x f p x D-1, where
FWHM is expressed in pixels, D-1 represent the grating
dispersion (2.64 nm/mm) and f p defines the pixel size
1
3. Results and discussion
3.1. Active Species Analysis
Optical emission spectrum were collected for
MwAs = 6 and P = 50W/MwA. Although these spectra
are rich in optical emission features, most of atomic lines
or molecular band systems can be identified. The
appearance of these different emission bands with various
intensities clearly shows the fragmentation of ANI
monomer unit and the formation of excited species in the
plasma with principal features: (i) the first three Balmer
line series and (ii) the CN(B2Σ u +→X2Σ g +) violet system in
its ∆v = 0 vibrational band sequence.
The carbon resonant line C(3s 1P 1 →2p2 1S 0 )
(247.86 nm) that is commonly observed in other plasma
sources containing organic precursors [16-18] is not
detected here, probably due to its short residence time in
the plasma. It is important to emphasize that some
vibrational bands with very low intensities are attributed
to NH 2 species at λ = 543 nm, 573 nm, 630 nm, 633 nm
and 664 nm. One can also note the presence of an
unidentified weaker line with a peak at 972.55 nm that is
supposed to be the Meinel band transition
N 2 +(A2Π u -X2Σ g +)(3,2) (973.33 nm).
The ANI spectrum clearly shows the existence of two
important emission bands of H 2 molecule which
correspond respectively to H 2 (G-B) and H 2 Fulcher-α
structures [19]. These hydrogen molecular bands have
much lower intensities probably due to their high
recombination rate. One particularity of the ANI plasma
at very low-pressure is the presence of Balmer lines, not
detected under higher pressure (50 - 100 mTorr) [11-13]
illustrating the important role played by dehydrogenation
of ANI during the polymerization. This could be related
to higher electron temperature (T e ). Nevertheless, since
chemical bond dissociation energies of C-H (4.30 eV) and
N-H (4.04 eV) are close [20], it remains arduous to define
the relative contribution of the dehydrogenation of C-H
and N-H groups in H-Balmer line intensities.
3.2. Effect of Discharge Power
The impact of the discharge power on the emission line
intensities shows no disappearance, neither nor creation of
new species in the discharge. The dependence of intensity
ratios (C 2 /CN, CH/C 2 , H α /NH, H α /CH, CH/CN) on the
power/MwA indicates a monotonic increase with a more
or less pronounced slope break (depending on the type of
the followed species). The overall increasing trend
reveals an increase of species concentrations due to
changes of the main plasma parameters, i.e., the rising of
the electron density and temperature (N e , T e ) when the
2
power/MwA is increased. Indeed, the rise of both
parameters, N e and T e , is usually met in reactive plasmas
in the low and very low pressure range [13].
The behaviour of the intensity ratios makes obvious that
the kinetics of the reactions leading to the fragmentation
of the ANI monomer follows an established reaction
pathway. This, for example (Fig. 1), promotes a more
significant production of CH than that of CN species over
all considered power range. However, the slope break
(observed about 20 W/MwA) suggests a change in the
reaction pathways and a new balance between the creation
and losses of the plasma species. This indicates that there
are two distinct operating regimes according to the value
of power/MwA. Regarding the influence of the number
of MwAs, it can be observed that the ratio of the emission
lines is about the same for a given discharge power, e.g.,
~0.14 at 240 W produced either by 12 applicators
powered at 20 W/MwA or 6 applicators powered at
40 W/MwA. Therefore, the plasma parameters for both
experimental conditions are similar and confirm the
plasma production around all MwAs, activated or not.
The microwave power is therefore redistributed to all
MwAs, and 240 W of discharge power would correspond
to an average power of 20 W per applicator.
0.20
CH/CN Intensity Ratio
(25 µm). The calculated value of ∆λ app gives 0.27 nm for
an entrance slit equal to 0.2 mm. All emission spectra
were collected for a monochromator entrance slit of
0.2 mm. For each experimental condition, a total of
10 spectra are averaged over an integration time of 10 ms,
with a background correction performed by subtracting
the baseline.
Slope 2
0.16
1
pe
Slo
0.12
0.08
 MwA = 6
 MwA = 12
0.04
0.00
0
120
240
360
480
Discharge Power (W)
600
Fig. 1. Dependence of the intensity ratio CH/CN on the
discharge power of ANI plasma.
A similar behavior is also observed for the dependence
of H α line intensity on power/MwA. For the two
configurations, the slope break separating the two
regimes, is clearly distinguished at 20 W/MwA for
MwA = 12 and at 40 W/MwA for MwA = 6. On the
other hand, by following the dependence of the intensity
with the total discharge power the slope break is
positioned at the same value of 240 W for both
configurations, i.e. at an average power of 20 W per
applicator.
3.3. Amine group selectivity in low pressure conditions
In our study, an important observation can be extracted
from the same analysis performed for the (NH/N 2 ) ratio,
following its evolution as a function of the total discharge
power (Fig. 2).
It is noteworthy to indicate that the ratio
between NH(A-X)Q(0,0) and N 2 (C-B)(0,0) intensities is
lower than one (I NH(A-X)Q(0,0) / I N2(C-B)(0,0) ≤ 1) for discharge
power ≤ 240 W (or less than ~20 W of average power per
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1.6
1.4
1.2
INH/IN2
1.0
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
600
Discharge power (W)
Fig. 2. Dependence of the NH(A-X)Q(0,0) /
N 2 (C-B)(0,0) intensity ratios on the discharge power.
applicator) which corresponds to an average power
threshold. This threshold is defined here as the limit over
which PANI films do not reproduce the properties of the
precursor, thus separating the two operating regimes
aforementioned. This indicates that an increasing of the
NH(A-X)Q(0,0) intensity with the discharge power (or
average power per applicator) originates from the loss of
the NH 2 group in the polymerization, and therefore of the
chemical functions for deposited PANI thin films.
To explain this effect, it is just necessary to take
into account both the ANI monomer fragmentation rate
and the hydrogen atomic concentration in the plasma
when the average power per applicator is increased.
The dehydrogenation rate of the monomer in the
discharge becomes more important with increasing the
discharge power. Accordingly, the NH 2 group retention
is achieved in very low pressure ANI plasma for an
average threshold power per applicator less than 20 W (or
240 W of discharge power). These results reflect that for
an adequate PPANI thin film deposition, the microwave
powers must be lower than the average threshold power
per applicator.
3.4. Discussion of reaction mechanism: correlation
plasma properties/thin film characteristics
The mechanism of CN formation from an organic
compound in a pulsed helium atmospheric plasma was
directly linked by the set of chemical recombination
reactions [11, 21-23]:
CH + N 2 → CN + NH
NH + CH → CN + H 2
2C + N 2 → 2CN
2C + 2NH → 2CN + H 2
C + C + M → C2 + M
CN + C → C 2 + N
CH + C → C 2 + H
(1)
(2)
(3)
(4)
(5)
(6)
(7)
Additionally, in the nitrogen-containing organic
compounds such as ANI, CN can be directly produced
from the bombardment with energetic He(2s 3S 1 )
metastable atoms (19.82 eV) and He+ ions (24.6 eV) [11].
P-I-1-12
In our case, at very low pressure conditions, all
previously mentioned pathways for the fragmentation of
ANI and CN production are valid and the prevalence of
one or other mechanism can depend on the plasma
properties like N e and T e . Indeed, the molecular plasma
chemistry involving the formation of radical fragments is
a complex phenomenon, because formation of molecular
species will be either from the direct ejection of radicals
or by recombination reaction processes between the
atomic species present in the plasma. Therefore, it
depends greatly from discharge conditions. To attempt
understanding the pathways of formation of excited
species in the plasma mainly those of CN, CH, NH and
C 2 species formation, a methodology based on the
correlations between these radiating species and the PANI
material structure has been employed.
Correlation between the solid phase analyzed by XPS
and FTIR methods and the plasma phase were
realized.
The drawing of a linear regression
line associated to the point cloud has enabled us
to determine the correlation coefficient factor R2 which
measures the linear correlation degree between these two
phases.
Strong correlation coefficient R2 have been obtained
between CN emission and C-H, N-H, C 2 bond signals in
solid phase. However, CN emission does not correlate
with %C and %N in solid phase. This indicates that in
our discharge conditions, CN radicals cannot be formed
from chemical reaction (3), (4) but probably from reaction
(1), (2) or recombination of C 2 and N 2 [24]. The
chemical reaction (5) is known to be highly endothermic
with activation energy of about 1.8 eV [25]. It then
requires extreme heating which can be obtained from
higher temperature prevailing in our discharge conditions.
These reaction pathways for CN molecule generation will
be confirmed by the correlation between CN emission and
CN(C-N+C≡N) bond signals respectively located at
But unfortunately, the
1310 cm-1 and 2213 cm-1.
determination of peak area of CN in solid phase is very
difficult mainly due on the one hand, to the natural
weakness of the C-N stretch band and on the other hand,
to the possible presence of C-O stretch band
(1000 - 1300 cm-1) which introduces a non-estimable
uncertainty in the integration of C-N + C≡N peak area.
Our results for CN and C 2 production pathways agree
with the recent study relative to some series of organic
polymers [24].
3.5. Discussion of Initiation Reaction for H-Balmer
Emissions
The same approach than the one in the previous section
has been used to illustrate the relative contribution of the
dehydrogenation of C-H and N-H groups in the H-Balmer
line intensities. For this purpose, the emission intensity
ratios of (H α /CH) and (H α /NH) were been firstly
investigated. The CH radical is more emissive than the
NH ones mainly due on the one hand, that CH(A-X;
2.88 eV) system is easier to excite than that of NH(A-X;
3.70 eV) and on the other hand, the line strengths of
3
CH(A-X) are greater than those of NH(A-X) system. The
ANI monomer contains five chemical bonds C-H and two
N-H bonds. Due to their similar dissociation probability,
there are more CH radicals than those of NH ones in the
discharge. Therefore, in order to reliably determine the
contribution of C-H and N-H groups in H-Balmer
emission intensities, correlation between H α emission and
%N-H as well with C-H in solid phase have been carried
out. The (CH/H α ) correlation degree is R2 = 0.68 for
MwA = 6 and R2 = 0.78 for MwA = 12, while for
(%NH/H α ) we found R2 = 0.58 for MwA = 6 and
R2 = 0.75 for MwA = 12. From these correlation data,
one can deduct that C-H and N-H groups quasi participate
similarly to the initiation reaction for H-Balmer line
production.
4. Conclusion
This work shows that the discharge is widely dominated
by the CN(B-X) (∆v = 0), H α and H β radiations. An
important effect relative to the prevalence between the
NH(A-X)Q(0,0) and N 2 (C-B)(0,0) emission intensity
according to the discharge power/source value is observed
in this study.
Indeed, it is found that NH(AX)Q(0,0)/N 2 (C-B)(0,0) ≤ 1 only for discharge
power/source ≤ (20-25) W/MwA. This indicates that
there are two different operational regimes characterized
by two domains of electron population: a “cold” one and a
“hot”. This OES diagnostic pointed out that retention of
NH 2 groups for deposition plasma-PANI films is
achieved at microwave discharge power/source ≤ (20-25
W/MwA).
Correlation between the OES data and the XPS/FTIR
ones allowed us to demonstrate that the insitu characterization of the plasma phase is an
important
predictive
tool
of
physicochemical properties for deposited thin films.
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