22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Dissociation of 3-aminopropyltriethoxysilane in Ar-N 2 afterglow: application to
nanoparticles synthesis
M. Gueye1, T. Gries1, C. Noel1, S. Bulou2, P. Choquet2, E. Lecoq2, S. Migot1 and T. Belmonte1
1
Institut Jean Lamour, Lorraine University, CNRS, Parc de Saurupt, CS 50840, FR-54011 Nancy cedex, France
2
Centre de Recherche Public Gabriel Lippmann, 41 rue du Brill, LU-4422 Belvaux, Luxembourg
Abstract: Optical emission spectroscopy (OES) and FTIR absorption spectroscopy in situ
are used to follow the decomposition of 3-aminopropyltriethoxysilane (APTES) in Ar-N 2
afterglow. Ar-N 2 afterglow is a weakly reactive gas medium, chosen to keep the primary
amines present in the precursor (APTES). The OES analyses show the prevalence of the
first positive system N 2 (B-A) in the Ar-N 2 afterglow and the presence of emissive species
such as CN and CH with peaks at 388 and 431 nm respectively, during the decomposition
of APTES. FTIR analyses on the nanoparticles show that it is possible to highly retain the
primary amino groups (-NH 2 ) present in the precursor.
Keywords: low-pressure plasma, APTES precursor, afterglow, nanoparticles synthesis
1. Introduction
3-Aminopropyltriethoxysilane (APTES) is a coupling
agent commonly used for surface functionalization, to
increase the adhesion of polymeric films on glass [1]. It
is also used to improve the attachment of metallic
nanoparticles on silica substrates because of the strong
interaction induced by the presence of an interlayer of this
precursor [2, 3]. It is a colorless to light yellow
transparent liquid ((C 2 H 5 O) 3 -Si-CH 2 -CH 2 -CH 2 –NH 2 )
from which a vapor is extracted in a controlled manner to
feed reactors in chemical vapor deposition processes for
thin films deposition. The presence in the coating of
primary amine functional groups (-NH 2 ) is usually sought
after. Indeed, grafting primary amines on a surface
enhances, for example, bacteria adhesion [4]. With the
development of lab-on-chip, it is necessary to provide
devices where known volumes of bacteria can be
localized and then perform specific analyses.
Furthermore, the use of polymers becomes more and
more the standard for the lab-on-chip development. It is
therefore necessary to perform localized surface treatment
at low temperature. However, it is pretty difficult to keep
the primary amines, even in pulsed PECVD process,
because of the high reactivity of this precursor.
To circumvent this problem, we have chosen to study
the decomposition of APTES in a weakly-reactive gas
medium such as a late Ar-N 2 microwave afterglow. The
aim of this work is to obtain a high retention level of the
(-NH 2 ) group in the coating and in the synthesized
nanoparticles by choosing the optimal experimental
conditions to find a soft treatment that limits the
dissociation of APTES molecules.
In the mbar pressure range, nanoparticles made from
APTES can be synthesized in the gas phase. In order to
confirm the possibility to keep the primary amine
functional groups, their chemical composition, together
with other features of these particles like their size
P-III-6-33
distribution and their shape, are investigated. A better
understanding of the nanoparticles synthesis is achieved
by comparing material results with in situ
characterizations of the gas phase by Fourier transform
infrared absorption (FTIR) and optical emission
spectroscopy (OES).
2. Experimental set-up
The reactor is shown schematically in Fig. 1. It consists
of a quartz tube (length = 120 cm, internal diameter =
16 mm) connected at one end to an infrared spectrometer
FTIR Agilent 680 and at the other end, to an external
MCT detector. The Ar-8.7vol.%N 2 plasma (total flow
rate: 1150 standard cubic centimeters per minute – sccm)
and the precursor are driven in the reactor through two
lateral smaller tubes (diameters: 5 mm) separated from
each other by a distance of 20 cm.
Fig. 1. Schematic drawing of the experimental set-up.
The precursor is introduced in the reactor thanks to a
30 sccm argon flow bubbling in the liquid precursor at
298 K. The saturated vapor of the precursor is thus
conveyed to the injection zone. In our experimental
conditions (Table 1), the APTES flow rate is 0.03 sccm.
1
Pressure
(Pa)
Power
(W)
Q (Ar)
(sccm)
Q (N 2 )
(sccm)
Q(APTES)
(sccm)
2000
100
1050
100
0.03
From then on, knowing the concentration of N 2
(B, v’=11), we can deduce the concentration of the
emitting states.
70000
Ar-N2-APTES
Ar-N2
60000
1st positive syst.
50000
40000
30000
20000
10000
CNred(523 nm)
Intensity (a.u.)
The plasma is created by a surfatron device fed by a
2.45 GHz microwave power generator. The surfatron is
an applicator designed to launch an electromagnetic
surface wave. Vacuum is made by a primary pump. The
pressure is 2000 Pa.
The in situ FTIR device is used to probe the gas phase
and to evaluate the way the APTES bonds evolve
according to the experimental conditions and to detect the
main by-products. Next, optical emission spectroscopy
(OES) is used to characterize the emissive species in the
afterglow and to determine their concentrations by
absolute calibration.
CNv (388 nm)
∆v = 0
CNv
CNv
∆v = +1 ∆v = −1
CH (431 nm)
2nd positive syst.
+ NOβ
CNv
∆v = −2
CNred(560 nm)
Table 1. Experimental parameters used in this work.
0
200
300
400
500
600
700
800
900
Wavelength (nm)
3. Results and discussions
3.1. OES analyses
In Ar-N 2 afterglow, OES analyses (Fig. 2) show the
prevalence of the first positive system N 2 (B, v’=11 →
A, v’’=7), the presence of some peaks of the second
positive system N 2 (C3Π u → B3Π g ) and some weaker
peaks due to NO β emission that results from the presence
of tiny leak flows. The main reactions leading to the
emission of both radiative systems are as follows:
( )
( )
N 4 S + N 4 S + ( N 2 , Ar ) → N 2 (B, v' = 11) + ( N 2 , Ar )
(1)
N 2 ( A) + N 2 ( A) → N 2 (C ) + N 2 ( X , v = 2 )
(2)
N 2 (B ) → N 2 ( A) + hv
(3)
N 2 (C ) → N 2 ( B) + hv
(4)
Fig. 2. Emission spectra of Ar-N 2 late afterglow
respectively with and without APTES injected (20 mbar).
When 0.03 sccm of APTES is introduced in the Ar-N 2
afterglow (Fig. 2), new emission peaks are observed due
to the decomposition of the APTES precursor in
afterglow. One finds the CN violet system (B2Σ→X2Σ)
with Δv = +1, Δv = 0, Δv = -1, Δv = -2, the red CN system
(A3Π→X3Σ) with Δv = 0 and the Angstrom CH (A2Δ
→X2Π) system with Δv = 0.
After identifying the emissive species in the APTESAr-N 2 afterglow, we determined the spatial evolution of
the concentration of these species (Fig. 3).
N 2 (B, v' = 11) + ( N 2 , Ar ) → products + N 2 (5)
d [N 2 (B, v' = 11)] N 2 ,11 2
2
= k1 [N ] [N 2 ] + k1Ar ,11 [N ] [Ar ]
dt
− k 5N 2 ,11 [N 2 (B )][N 2 ] − k 5Ar ,11 [N 2 (B )][Ar ] − ν 311−v" [N 2 (B )]
(6)
the contribution of process (4) to the overall balance
being negligible. The former processes have the following
rate coefficients [5]:
k1N2 ,11 = 4.4×10-34 cm6 s-1; k1Ar ,11 = 4.0×10-34 cm6 s-1;
k
N 2 ,11
5
ν
11−v "
3
-11
= 3.0×10
3
-1
cm s ;
k
Ar ,11
5
-12
= 5.5×10
3
-1
cm s ;
5 -1
= 2×10 s .
Using the steady-state approximation, one has:
2
2
N ,11
Ar ,11
]
[N 2 (B, v' = 11)] = k1 N ,11[N ] [N 2 ]Ar+,11k1 [N ] 11[Ar
−v "
k 5 [N 2 ] + k 5 [Ar ] + ν 3
2
2
2
(7)
Concentration (cm-3)
From these equations, we can calculate the concentration
of N 2 (B, v’=11):
1.0x109
1.0x109
CN
N2(B) with APTES
N2(B) without APTES
1.0x108
-10
0
10
20
30
40
50
1.0x108
60
70
Position (cm)
Fig. 3. Spatial evolution of the concentrations of
N 2 (B,v’=11) at 580 nm and CN(B,v’=0) at 388 nm.
It appears that the consumption of the N 2 (B, v '= 11)
state is much faster in the presence of APTES. At this
stage, it is impossible to say whether it is atomic nitrogen
and / or another precursor like N 2 (A) that is mainly
involved in the dissociation mechanism of APTES.
The existence of a maximum in the concentration of
P-III-6-33
δ NH
1391
1615
1449
1475
1241
1176 r CH2
δ H-C-H
δ Si-CH2
1077
O
CH2-Si-O
O
1108
Si-O-CH2-CH3
0.005
1300
0.000
1700
1500
-1
Wavenumber (cm )
δ SiO(1060) + ω NH2(878)
n OSi−H
Absorbance (a.u.)
0.0025
0.0020
2260
0.0015
n CO2
2140
1932
0.0002
0.0030
Ar-N2-APTES
Pure APTES
n CO
0.0003
n C=O
0.0010
2346
0.0001
0.0000
0.0005
2264
2116
O-H stretching
C-H asym. stretching
C-H sym. stretching
N-H asym. bending
C-H asym. bending
C-H sym. bending
Si-O asym. stretching
S-O sym. stretching
C-H 3 rocking
958
700
1733
Si-OH
CH 2
CH 3
NH 2
Si-CH 2 -CH 3
SiO 3 -CH 2
Si-O-CH 2 -CH 3
Si-O-CH 2 -CH 3
Si-O-CH 2 -CH 3
1100
785
0.000
500
Absorbance (a.u.)
Vibrating mode
900
- -
Table 2. Vibration modes identified by FTIR.
Functional group
- -
Absorbance (a.u.)
712 δ H-C-N
0.005
0.010
Ar-N2-APTES
Pure APTES
785
3.2. FTIR analyses
The Table 2 gives the list of bonds identified in the
infrared spectrum of pure APTES.
Wavenumber
(cm-1)
3674
2987
2899
1615
1392
1241
1077
1011
958
Si-O-CH2-CH3
O
CH2-Si-O
O
878
ω NH2
0.010
Absorbance (a.u.)
CN(B, v'=0) near 20 cm can be explained by the need of a
reaction intermediate. Indeed, it is known that the purple
system of CN in a nitrogen post-discharge is due to the
three-body reaction (8):
(8)
C + N + ( N 2 , Ar ) → CN (B, v') + ( N 2 , Ar )
The decrease in the concentration of CN(B, v'= 0) after
20 cm is clearly related to the decrease in the
concentration of the nitrogen atoms in the Ar-N 2
afterglow.
Si−O overtones
0.0000
P-III-6-33
1900
2100
2300
2500
n NH
3290
0.010
0.008
0.002
0.005
0.002
3100
3200
3300
n CN (HCN)
3400
3674
0.003
0.006
0.000
3311
0.004
0.001
0.000
3500
n Si−OH
3600
0.004
0.002
Absorbance (a.u.)
ns CH
2987
0.004
3434
0.006
Si−O−(CH2)3−NH2
3370
0.007
2899
0.008
nas CH
Wavenumber (cm−1)
Absorbance (a.u.)
When 0.03 sccm of APTES is injected in the Ar-N 2
afterglow, the emergence of new absorption peaks is
observed (Fig. 4). The absorption band centered around
1750 cm-1 is attributed to the C=O double bond
characteristic of esters [6]. The absorption bands at
712 cm-1 and 3311 cm-1 are due to the CH and CN
stretching of the HCN molecule respectively [7]. Carbon
monoxide CO at 2150 cm-1 and the stretching vibration of
Si-H around 2240 cm-1 [8] are also observed.
We note that the O-Si-O-C groups are weakly
decomposed (Fig. 4) whereas the CH groups of APTES
are strongly reduced. Then, the decomposition of APTES
begins by the external groups, i.e., the CH bonds.
However, internal bonds such as O-Si-O-C are also
broken, but to lesser extent, for we found in the FTIR
analyses the stretching vibration of O-Si-H at 2240 cm-1.
3.3. Nanoparticle synthesis
Nanoparticles were obtained in the same experimental
conditions as those described previously (Table 1). They
are formed in the gas phase and collected on a pure
aluminum substrate at a sufficiently large distance
downstream of the afterglow to avoid deposition on the
surface of the substrate.
TEM analyses (Fig. 5) showed that particles are
spherical and amorphous. They are composed of silicon,
carbon, oxygen and nitrogen. As a reminder, the peak of
the aluminum corresponds to the substrate used for the
analysis. The same elements as those present in APTES
are found.
1700
0.000
2600 2800 3000 3200 3400 3600 3800 4000
Wavenumber (cm−1)
Fig. 4. FTIR spectra of pure APTES and APTES in late
Ar-N 2 afterglow at 20 mbar.
Fig. 6 shows the ATR-FTIR spectrum of the assynthesized nanoparticles. The broad absorption band
between 3000 and 3600 cm-1 corresponds to the OH
stretching vibration. Vibrations at 2960 and 2896 cm-1
correspond respectively to symmetric and asymmetric
elongation of CH in CH 3 . The strong absorption band
centered at 1650 cm-1 is attributed to the asymmetric
bending of NH in NH 2 . Absorption peaks at 1280, 1112
and 950 cm-1 respectively correspond to the asymmetric
3
bending of Si-CH 2 , the asymmetric stretching of Si-O in
Si-O-C and the asymmetric stretching of Si-O-Si [9].
OH 3100-3500 cm−1
Si-O-C 1112 cm−1
CH asym 2851 cm−1
CH sym 2930 cm−1
0.010
CO2 2350 cm−1
0.015
NH bend 1650 cm−1
0.020
NH bend 1465 cm−1
Absorbance (a.u.)
0.025
Si-CH3 1260 cm-1
0.030
Si-O-Si 950 cm−1
Fig. 5. a) EDX spectrum, and b) TEM micrograph of the
as-synthesized nanoparticles.
0.005
5. Acknowledgements
These works were done within the framework of the
LEA LIPES, a structure supported by the CNRS to whom
we convey our deepest gratitude. They were financially
supported by The Region Lorraine and the Institut Carnot
ICEEL that the authors wish to thank here.
6. References
[1] S.H. Choi and B.M.Z. Newby. Surf. Sci., 600, 1391
(2006)
[2] D. Enders, T. Nagao, A. Pucci and T. Nakayama.
Surf. Sci., 600, 71 (2006)
[3] E.S. Kooij, E.A Brouwer, M.H. Wormeester and
B. Poelsema. Langmuir, 18, 7677 (2002)
[4] H. Tang, W. Zhang, P. Geng and Z. Lou. Chim.
Acta, 562, 190 (2006)
[5] A. Ricard. Plasma Sources Sci. Technol., 22,
035009 (2013)
[6] G. Socrates. Infrared and Raman Characteristic
Group Frequencies: Tables and Charts. 3rd edition.
(Chichester: Wiley) (2004)
[7] T. Shimanouchi. Tables of Molecular Vibrational
Frequencies Consolidated. Volume I. (Washiinton
DC: National Bureau of Standards) No. NSRDSNBS-39, 1, 160 (1972)
[8] G. Lucovsky, J. Yang, S.S. Chao, J.E. Tyler and
W. Czubatyj. Phys. Rev. B, 28, 3225 (1983)
[9] L.D White and C.P Tripp. J. Colloid Interface Sci.,
232, 400 (2000)
0.000
-0.005
1000 1500 2000 2500 3000 3500 4000
Wavenumber (cm−1)
Fig. 6.
FTIR
nanoparticles.
spectrum
of
the
as-synthesized
We almost found the same bonds as those in pure
APTES in the gas phase (Fig. 4). This analysis allows us
to establish a strong correlation between the gas phase
(APTES Ar-N 2 afterglow) and the solid phase (SiOCN
nanoparticles). It also allows us to assert that nitrogen
present in the nanoparticles (EDX analysis) is present as
NH and not as CN.
4. Conclusion
OES analyses show the domination of the first positive
system N 2 (B-A) in the Ar-N 2 afterglow. They also
evidence the presence of emissive species coming from
the dissociation of APTES in the afterglow such as CN
and CH with peaks at 388 and 431 nm respectively.
FTIR analyses confirm the presence of these species in
their ground state and specially the presence of NH bonds
at 1650 cm-1 in nanoparticles. Therefore, we can say that
it is possible to retain highly the amino-groups in the
particles synthesized in the gas phase.
4
P-III-6-33