Surface Modification of DLC Thin Films with Plasma Processing.
Yukinori Yamauchi1, Masayuki Kuzuya1, Yasushi Sasai2, Shin-ichi Kondo2
1Department of Pharmaceutical Physical Chemistry, College of Pharmaceutical Sciences,
Matsuyama University, Japan
2Laboratory of Pharmaceutical Physical Chemistry, Gifu Pharmaceutical University, Japan
Abstract: We attempt to construct the covalent immobilization of monomer onto
DLC films deposited using plasma-enhanced chemical vapor deposition for
industrial and biological needs. After the DLC films were deposited on glass
plate, gaseous monomers, such as acrylic acid, allylamine, and allyl bromide,
were sprayed on the surface of DLC with various reaction conditions. The
resulted DLC film has shown to be of highly-functionalized surface. In fact, the
covalently graft polymerization underwent on the surface of as-deposited DLC,
based on the surface-sensitive techniques including X-ray photoelectron
spectroscopy (XPS) and water contact angle measurement. These results indicate
that present procedure would be one of the fundamental method to fabricate the
advanced DLC film with long-acting functional surface.
Keywords: Diamond-like carbon (DLC) film, Electron spin resonance (ESR),
Plasma-enhanced chemical vapor deposition (PECVD), functional surface
1. Introduction
The Diamond-like carbon (DLC) thin films have
widely used in a variety of industrial fields due to
the attractive properties such as high hardness, low
friction coefficient, optical transparency, chemical
inertness, and high electrical resistivity [1, 2]. The
DLC is also expected as an excellent candidate for
use as biocompatible coating on biomedical implants,
which is due to not only its excellent properties but
also its chemical composition containing only
carbon and hydrogen, which are biologically
compatible with human cells [3, 4].
The immobilization of biological active species
onto a large variety of materials including DLC is
the crucial step for the fabrication of bio/chemical
micro-electromechanical systems. Several fields are
concerned including biological analysis, chemical
micro-reactors, environmental investigations and
clinical diagnosis. The fixation of biomolecules
through a covalent bound is one of the most
interesting methods.
Recently we reported the detailed-ESR study with
regard to the stability and reactivity of the
immobilized dangling-bond sites (DBS) in DLC
films prepared from several kinds of hydrocarbons
utilizing
plasma-enhanced
chemical
vapor
deposition (PECVD), and demonstrated that the
nature of DBS are closely reflected with the
microstructure of DLC films [5-7].
In this study, we attempt to construct the high
functional surface of DLC thin films for industrial
and biological needs by plasma surface treatment.
2. Experimental
2.1 Preparation of functionalized-DLC Films and
ESR Spectral Measurement
The DLC films were deposited using plasma RF
generator with matching network system. Ethylene
gas was used as the precursor gas.
A neutral glass capillary (1 mm i.d., 65 mm long)
or glass plate (5 x 5 mm) was placed in a specially
designed ampoule with a side branch (30 mm i.d.,
100 mm long) connected to a capillary tube (2 mm
i.d.) at the upmost part of the ampoule. Prior to
depositing the DLC films, the substrates were
cleaned by an Ar etch at the following conditions:
pressure = 0.5 Torr, rf power = 50 w, flow rate = 50
mL/min, time = 5 min. The preparation of DLC film
by PECVD was carried out using inductively
coupled plasma in the region encircled by the
radiofrequency discharge coil at 13.56 MHz. The
plasma state was sustained at 50W of supplied
power for prescribed period of time with the
precursor gases flow rate of 30 mL/min. After the
plasma irradiation was discontinued, gaseous
monomers, such as acrylic acid, allylamine, and allyl
brimide were introduced into the reactor through a
flow controller for prescribed period of time. Then
the ample reactor was kept in vacuo for 5 min to
remove the remaining low-molecular weight
materials and sealed.
The ESR spectral
measurements of DLC films formed on the glass
capillary were performed by turning the ample
upside down. ESR spectra were recorded with a
JES-FA200 spectrometer (JEOL) with X-band and
100 kHz field modulation.
The procedure is
essentially the same as that reported earlier [5-7].
average of a minimum of seven measurements taken
for each examined surface.
3. Results and Discussion
3.1 ESR spectra
DLC films were deposited by PECVD with four
precursor gases such as methane, ethylene, acetylene
and benzene in gas phase. Figure 1 (a) shows the
ESR spectrum of DBS observed in DLC film
prepared with ethylene as representative four
precursor gases. All spectra showed that DBS
observed in all films were characterized by an
isotropic broad single line with ΔHmsl of 1.82-1.91
and g-value of 2.000-2.001 which is consistent with
carbon center radicals and not glass radicals. (data
except ethylene not shown)
Figure 2 (a) shows the progressive changes in
ESR spectral intensity (determined by double
integration) of the DBS in all DLC films in the
course of PECVD. It is clear in all cases that the
spectral intensity increases linearly as the reaction
proceeds, but the rate varies with the precursor gases.
(a)
x 25
2.2 Surface analysis
To confirm the chemical composition of the DLC
films formed on neutral glass plate (5 × 5 mm) by
PECVD, X-ray photoelectron spectrum (XPS)
measurement was carried out using the conventional
photoelectron spectroscopy apparatus, Shimadzu
ESCA-3400. The Mg Kα line (1253.6 eV), used as
X-ray source, was incident at 45° with respect to the
surface normal. The total energy resolution was
approximately 0.5 eV. The base pressure in the
photoelectron analysis chamber was maintained at
least 5 × 10-6 Pa. In order to investigate the
chemical composition of bulk phase in DLC film,
XPS sputter depth profiling was performed using 10
keV argon ion beam. The sputter rate was 10-20
nm/min.
Wetting properties of the resulted DLC thin films
were determined by measuring the wetting contact
angles of water (WCA). The reported values are an
2 mT
ΔHmsl/mT
1.90
g value
2.000
treated acrylic acid
exposed to air
(b)
(c)
2 mT
x 10
2 mT
ΔHmsl/mT
1.19
1.82
g value
2.001
2.000
Figure 1. ESR spectra of DBS of DLC films, (a) as-deposited,
(b) after exposure to air and (c) after treated acrylic acid.
Although the spectral intensity of DBS observed
in all cases is almost unchanged at room temperature
for a long period of time so long as the films are kept
under anaerobic conditions, it observed after
exposed to air is appreciably reduced relative to that
of the spectra before exposed to air as shown in
Relative intensity, It/I0
10
5
0
20
40
60
Plasma duration / min
0.8
N1s
C1s
Br3p, Br3d
(a) acrylic acid
methane
ethylene
benzene
acetylene
1.0
Spectral intensity
(arb. units)
O1s
(b)
(a)
0
important amount of nitrogen and bromide on
surface of films, respectively.
(b) allylamine
Intensity (arb. units)
Figure 1 (b). Figure 2 (b) shows progressive changes
of relative ESR spectral intensity of DBS in DLC
films on standing at room temperature under aerobic
conditions. It is seen that the spectral intensity quite
rapidly decreases with time under aerobic conditions.
These indicate that considerable amount of DBS are
reacted with oxygen and terminated to the stable
diamagnetic molecules at room temperature.
0.6
0.4
(c) allyl bromide
0.2
(d) untreated
0.0
0
1
2
3
Standing time / h
Figure 2. Progressive changes in (a) ESR spectral intensity of
the DBS in the course of PECVD and (b) relative ESR spectral
intensity of the DBS on standing in air at room temperature.
1000
750
500
250
0
Binding energy / eV
Figure 3. XPS survey scan spectra (0–1100 eV) of DLC films
before and after treatment with acrylic acid, allylamine and allyl
bromide. The spectra are offset for clarity.
Intensity (arb. units)
N1s
C1s
XPS has been used to assess the chemical changes
that occur at the as-deposited DLC surface exposed
to the three kinds of monomer gases. Figures 3 and
4 show the XPS survey scan (0–1000 eV), C1s, N1s,
and Br3d core-level spectra of DLC films with or
without functionalizing treatment.
In the case of treatment with acrylic acid, based
on the C 1s peak split by assuming 287 eV for the
C=O bonds, 285.8 eV for the C-O bonds, 289.2 eV
for the O=C-O bonds and 285.0 eV for the C-C
bonds, carboxyl groups are generated on the DLC
film, as shown in Figures 3, 4 and 5.
For treated allylamine and allyl bromide, new
peaks appeared about 401 eV and 72 eV showing the
(d) untreated
(a) acrylic acid
410
Intensity (arb. units)
3.2 Surface Analysis of the DLC films introduced
Functional Groups
(b) allylamine
405
400
395
390
Binding energy / eV
(b) allylamine
Br3d
(c) allyl bromid
(d) untreated
290
285
Binding energy / eV
280
Intensity (arb. units)
After preparation of DLC films with ethylene gas,
acrylic acid in gas phase were sprayed on the
surfaces for 30 min. The ESR spectrum was a broad
single line with ΔHmsl of 1.82 as that of DBS trapped
in DLC, as shown in Figure 1 (c).
(c) allyl bromide
(d) untreated
80
75
70
65
60
Binding energy / eV
Figure 4. XPS C1s, N1s, and Br3d core-level spectra of DLC
films before and after treatment with acrylic acid, allylamine and
allyl bromide. The spectra are offset for clarity.
The chemical compositions determined from the
XPS survey scans of all DLC films are shown in
Table 1. An indication of the approximate amount
of monomer graft polymerized on the DLC surface
can be inferred from the [O]/[C], [N]/[C], and/or
[Br]/[C] ratio. From Table 1, the approximate
amount of monomer graft polymerized on the DLC
surfaces are estimated 57w%, 33w%, and 24w% for
acrylic acid, allylamine, and allyl bromide,
respectively. The small amount of oxygen present in
all samples must be due to the adsorption of this
element during the handling in air of the DLC
samples after its preparation.
The as-deposited DLC film presents a WCA value
of 77˚. After functionalizing treatment with acrylic
acid, the contact angle significantly decreased to less
than 3˚, indicating that the DLC surfaces become
quite hydrophilic. This result is consistent with XPS
data.
Inten sity (arb. un its)
4. Conclusion
290
2 85
280
Bind ing energy / eV
Figure 5 XPS spectra of DLC surface in C 1s binding energy
region after functionalizing treatment with acrylic acid.
Table 1 Chemical Compositions (at.%) Determined by XPS on
The DLC Films before and after Functionalizing Treatment
Elemental content (wt %)
(Theoretical)
Annealing
Conditions
References
C 1s
O 1s
N 1s
Br 3d
DLC (untreated)
89.7
10.3
-
-
Acrylic acid
80.9
(66.7)
19.1
(33.3)
-
-
Allyl amine
84.6
(75)
7.2
(0)
8.2
(25)
-
Allyl bromide
84.4
(75)
9.6
(0)
-
6.0
(25)
Wetting properties of DLC films were determined
by measuring the wetting contact angles of water
(Figure 6).
As-deposited DLC
Treated Acrylic acid
77°
In this study, we attempt to construct the high
functional surface of DLC thin films by use of
reactive surface radicals formed during DLC
preparation. On monomer treatment of as-deposited
DLC films under anaerobic condition, the graft
polymerization underwent on the surface of the films,
based on the surface analysis with XPS and WCA
measurement. And the resulted DLC films have
shown to be of highly- functionalized surface.
These results indicate that present procedure would
be one of the fundamental methods to fabricate the
advanced DLC film with long-acting functional
surface.
< 3°
Figure 6. Results of the contact angle measurements for the
DLC films before and after functionalizing treatment with
acrylic acid.
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