22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Stable amine-rich layers prepared by cyclopropylamine plasma polymerization
A. Manakhov1, E. Makhneva1,2, J. Polčak3, D. Nečas1,2 and L. Zajíčková1,2
1
Plasma Technologies, CEITEC Central European Institute of Technology, Masaryk University, Brno, Czech Republic
2
Department of Physical Electronics, Faculty of Science, Masaryk University, Brno, Czech Republic
3
CEITEC - Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic
Abstract: The preparation stable and amine-rich plasma polymers is still challenging due
to high thickness loss of amine-rich coatings in water, preventing their bio-applications. In
this work, the amine films exhibiting 9 at.% of NH x environment and thickness loss of only
2% were prepared by low pressure cyclopropylamine plasma polymerization. The
influence of plasma power and duty cycle on layer chemistry and stability is reported.
Keywords: amines, plasma polymerization, cyclopropylamine, XPS, AFM
1. Introduction
Amine surface functionalization is a hot topic for
material processing, because such functional groups are
enhancing cell adhesion and proliferation and also can
also be used for the immobilisation of biomolecules [1].
Although plasma polymerization of amine containing
precursors has been studied for more than 50 years the
preparation of stable amine films with high concentration
of primary amines is still challenging [2]. In general, the
stability of amine plasma polymers can be enhanced at the
expense of the density of amine groups [3]. Moreover,
the majority of the monomers employed for the amine
plasma polymerization (ammonia, allylamine) are highly
toxic compounds. Therefore, new monomers for the
deposition of amine plasma polymers are required.
Recently we have shown that amine plasma
polymerization of non-toxic cyclopropylamine (CPA) led
to deposition of amine films containing around 9 at.% of
NH x environment and exhibiting thickness loss of 20%
after 48 hours in water. In this work, the amine-rich
plasma polymer (NH x > 7 at.%) were deposited on Si
wafers by CPA RF CCP polymerization in a vertically
oriented stainless steel plasma reactor.
In this
configuration, the bottom electrode bearing the substrates
was direct-current (DC) negatively self-biased due to an
asymmetric coupling but ion bombardment was
significantly reduced by a relatively high process pressure
of 50 Pa. The deposited coatings exhibited thickness loss
below 2%, which is sufficiently low to employ these
amine plasma polymers for biomedical applications.
2. Experimental
The CPA plasma polymers were prepared in a stainless
steel parallel plate reactor [4]. The bottom electrode, 420
mm in diameter, was capacitively coupled to a radio
frequency (RF) generator working at the frequency of
13.56 MHz. The gases were fed into the chamber through
a grounded upper showerhead electrode, 380 mm in
diameter. The distance between the electrodes was
55 mm. The bottom electrode with substrates was
negatively DC self-biased due to an asymmetric coupling.
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The reactor was pumped down to 10-4 Pa by a
turbomolecular pump with a backing rotary pump. The
deposition was carried out with the rotary pump only.
The leak rate including wall desorption was below
0.1 sccm for all the experiments.
The CPA was polymerized in squared pulsed CPA/Ar
plasma at a power of 30 - 250 W and pressure of 50 Pa in
pulsed or continuous mode. The pulse duty cycle and
repetition frequency were 33% and 500 Hz, respectively.
The flow rate of Ar was set to 28 sccm and regulated by
an electronic flow controller Hastings, whereas the flow
rate of CPA vapors was set to 2 sccm by a needle valve.
The deposition time was adjusted to obtain film thickness
around 200 nm. The substrates were sputter-cleaned by
pulsed Ar plasma for 10 minutes prior to the deposition.
The chemistry of the deposited films was characterized
by Fourier Transform Infrared and X-ray Photoelectron
Spectroscopy, whereas layer morphology was analyzed
by Scanning Electron Microscopy and Atomic Force
Microscopy.
3. Results
The deposited amine films exhibited adhesion to
substrate and were homogenous and smooth, as shown in
Fig. 1a. However, some single nanoparticles with the
diameter in the range from 100 to 300 nm were observed
both by SEM and AFM imaging. The density of
nanoparticles was dependent on plasma power and duty
cycle. Both extremely low and high power led to
suppression of nanoparticles densities and size.
Regarding the CPA plasma layer chemistry, FT-IR
revealed the bands of hydrocarbon (CH 3 at, CH 2 and CH)
and amine groups (N-H stretching at and NH 2 scissoring
at). The intensities of amine bands were decreasing with
the P av , while the hydrocarbon peaks were gaining the
intensity with the average power P av (plasma power
multiplied by duty cycle). XPS analyses revealed that all
layers were composed of carbon, oxygen and nitrogen
(hydrogen cannot be detected by XPS). The C.O:N ration
was dependent on P av and the main trend was that carbon
and oxygen concentrations were increasing with P av at
1
large expenses of nitrogen.
The highest nitrogen
concentration equal to 18 at.% was determined for the
layer deposited at 9.9 W (lowest power).
fitting, the NH x environment is decreasing with P av from
13.0 down to 5.5 at.% , while CH x environment increases
from 50 to 60 at.%.
Intensity (counts)
6000
NHx
4000
2000
402
N-C=O
C=N-
400
398
396
BE (eV)
Fig. 3. N1s XPS curve fitting of CPA plasma polymer
deposited at P av of 9.9 W.
Fig. 1.
The topography of CPA plasma polymer
deposited at P av of 33 W: before (a) and after (b)
immersion in the water for 216 hours.
The XPS N1s and C1s curve fitting was employed to
characterize the functional composition of the CPA
plasma polymers. As shown in Fig. 2, the C1s signal was
fitted with a sum of three components, namely
corresponding to hydrocarbons (CH x ~ 285.0 eV), carbon
bonded to nitrogen or oxygen (C-N/C-O ~ 286.3 ± 0.1
eV) and carbon double bonded to oxygen (N-C=O/-C=O
~ 288 eV).
Intensity (counts)
10000
8000
CHx
6000
4000
N-C=O/C=O
C-NHx
C-O
2000
0
290
288
286
284
282
BE (eV)
Fig.2. C1s XPS curve fitting of CPA plasma polymer
deposited at P av of 9.9 W.
The XPS N1s signal was also fitted with a sum three
components corresponding to corresponding to the amine
group (NH x=1,2 ~ 399.1 eV), amide group (N-C=O
~ 400.3 eV) and nitride / imine (-N=C ~ 398.3 eV) as
depicted in Fig. 3. According to the N1s and C1s curve
2
Regarding the film stability in water, the thickness loss
of the layers deposited at the P av of 9.9W was ~15%
(immersion time 216 h) but it decreased to 2% when the
P av was increased to 33 W. It should be noted that such
layer also exhibited high retention of NH x environment
(7 at.%) and no change of the layer morphology (Fig. 1b).
The increase of P av above 100W led to increase of the
thickness up to 10% and cracking of the layer after
immersion in water for 216 h. Hence, the layers
deposited at high power were swelled after immersion in
water. Therefore, the deposition of the stable amine films
from CPA does not require high power in order to achieve
sufficient film stability.
4. Conclusions
The stability, morphology and chemical composition of
CPA plasma polymers was studied as a function of
average power. The NH x was decreasing with P av from
13 to 5.5 at.%. The stability of the CPA plasma polymers
was also dependent on the discharge parameters. Low
power resulted in the thickness loss of 15%, while high
power led to the layer swallowing. The optimum
condition were as follow: plasma power of 100 W and
duty cycle equal to 33%. In this condition the deposited
layer exhibited 9 at.% of NHx environment, whereas the
thickness loss was only 2%. Such low values of the
thickness loss was rarely observed for functional plasma
polymers. Hence, CPA plasma polymerization is a
method of choice for the deposition of stable amine-rich
films.
5. Acknoledgements
This work was supported by the BioFibPlas project
No. 3SGA5652 financed from the SoMoPro II
Programme that has acquired a financial support from the
People Programme (Marie Curie Action) of the Seventh
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Framework Programme of EU according to the REA
Grant Agreement No. 291782 and was further co-financed
by the South-Moravian Region. The research was also
supported by the projects “CEITEC — Central European
Institute of Technology” (CZ.1.05/1.1.00/02.0068) and
“R&D center for low-cost plasma and nanotechnology
surface modifications” (CZ.1.05/2.1.00/03.0086) from the
European Regional Development Fund and by the
Seventh Framework Programme of EU under the
“Capacities” Specific Programme (Contract No.
286154—SYLICA). This publication reflects only the
author's views and the Union is not liable for any use that
may be made of the information contained therein.
6. References
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[2] A. Manakhov, L. Zajíčková, M. Eliáš, J. Čechal,
J. Polčák, J. Hnilica, et al. Plasma Process. Polym.,
11, 532-544 (2014)
[3] A. Abbas, C. Vivien, B. Bocquet, D. Guillochon and
P. Supiot. Plasma Process. Polym., 6, 593-604
(2009)
[4] A. Manakhov, D. Nečas, J. Čechal, D. Pavliňák,
M. Eliáš and L. Zajíčková. Thin Solid Films. in
press doi:10.1016/j.tsf.2014.09.015 (2014)
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