Full Paper
Chemical and Physical Properties of Copper
and Nitrogen Plasma-Implanted
Polyethylene
Wei Zhang, Junhui Ji, Yihe Zhang, Qing Yan, Paul K. Chu*
Plasma immersion ion implantation (PIII) is a new technique used to modify the surface
physical and chemical properties of polymers for antibacterial purposes. Our previous research
work has shown that copper and nitrogen PIII can give rise to excellent antibacterial properties
on polyethylene (PE) substrates. In this paper, the effects of the PIII conditions on the chemical
and physical properties of the Cu and N plasma-implanted PE are investigated. Elemental
depth profiles show that Cu and N PIII results in the formation of C–N, C – N bonds in the
surface region, and that the implanted Cu does not
bond with the polymer matrix elements. A longer PIII
time increases the concentrations of C –
– N, C – N and
C–N bonds formed in the near-surface region, but the
elemental chemical states deeper into the sample
remain about the same for different PIII time
durations. However, if N-PIII and Cu-PIII are not carried
out simultaneously, for example, N-PIII following
Cu-PIII, the retained Cu dose is affected due to nitrogen
sputtering, but the Cu chemical state is not altered. In
addition, more C – N and C –
– N bonds are formed in the
outer surface region. A longer N-PIII implantation time
further enriches these chemical functional groups.
W. Zhang, P. K. Chu
Department of Physics & Materials Science, City University of
Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Fax: (852) 27889549; E-mail: [email protected]
W. Zhang, J. Ji, Q. Yan
Technical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing 100101, China
Y. Zhang
School of Materials Science and Technology, China University of
Geosciences, Beijing 100083, China
W. Zhang
Graduate School of the Chinese Academy of Sciences, Beijing
100039, China
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Introduction
Polymer-based materials are widely used in many fields,
including biomedical engineering and microelectronics.[1–6] In general, their surface properties such as
antibacterial ability, biocompatibility, and hydrophilicity
depend on not only the type of polymer, but also on the
surface structure and chemistry. Therefore, the proper
surface modification techniques can selectively improve
one or more of these properties making them more
suitable for particular applications, while the favorable
bulk attributes of the polymers are not altered.[7–10] For
DOI: 10.1002/ppap.200600196
Chemical and Physical Properties of Copper and . . .
example, an antibacterial metal coating on polymer
surfaces produced by plasma implantation can provide
excellent antibacterial properties. Plasma immersion ion
implantation (PIII) is a burgeoning technology used to
modify the surface of polymeric materials.[11,12] In our
previous research, copper and nitrogen were plasmaimplanted into the near surface region of polyethylene
(PE).[13–15] It was observed that N-PIII regulated the release
of the plasma-implanted Cu, and prolonged the antibacterial effects of the co-implanted PE, and that this regulation mainly stemmed from functional groups on the
surface macromolecules.[14,15] In this work, we investigate
the chemical and physical characteristics of copper and
nitrogen plasma-implanted PE under different PIII conditions in an attempt to further understand the copper
release mechanism.
Experimental Part
Low density polyethylene (PE) samples with dimensions of
2 cm 2 cm 0.2 cm were inserted in the plasma immersion
ion implanter equipped with a copper cathodic arc plasma
source.[16,17] The arc was ignited using a pulse duration of 300 ms,
repetition rate of 30 Hz, and arc current of 1 A. To conduct
simultaneous Cu and N PIII, N2 gas was bled into the vicinity of the
copper arc discharge plume at a flow rate of 10 sccm (standard
cubic centimeter). The dual PIII process was conducted by applying
an in-phase bias voltage of 5 kV with a repetition rate of 30 Hz
and pulse width of 300 ms to the PE samples.[11,12] Another set of PE
samples underwent sequential Cu and N PIII. In this case, Cu PIII
was first conducted by applying an in-phase bias voltage of 5 kV
with a repetition rate of 30 Hz and pulse width of 300 ms to the PE
samples, followed by the N-PIII using the same flow rate of N2 and
conditions as before, but without triggering the Cu cathode arc
source. The working pressure in the vacuum chamber was
1–2 10 4 Torr.[18] The elemental depth profiles and chemical
states were determined by X-ray photoelectron spectroscopy (XPS)
using a Physical Electronics PHI 5802.[19] A monochromatic
aluminum X-ray source was used, and the elemental depth
distributions were determined using an argon ion sputtering. The
sputtering rate of 1 nm/min was approximated using that derived
from a silicon oxide sample under similar conditions.[13]
Results and Discussion
Influence of Simultaneous Cu and N PIII Time on
Chemical and Physical Properties
Figure 1 depicts the in-depth elemental distribution of
the PE samples implanted with Cu and N for 10 min and
30 min. Comparing Figure 1a and b, it can be observed that
the quantities of Cu and N in the surface region increase
with increasing the implantation time. Our TEM results
(not shown here) disclose that the implanted Cu and N are
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Figure 1. XPS elemental depth profiles of copper and nitrogen
acquired from PE samples plasma-implanted simultaneously
with Cu and N ions for different time durations: a) 10 min and
b) 30 min.
located in the top several hundred nanometers.[14] Here,
even though the PIII time changes from 10 min to 30 min,
the amount of implanted Cu does not increase proportionally. This nonlinearity is believed to stem from the
accumulation of positive charges on the PE surface over
the course of PIII due to the poor electrical conductance of
the materials.[11,12,20,21] Meanwhile, it is observed that the
amounts of Cu and N deposited on the surface after 30 min
are larger than those on the 10 min samples by more than
3 times. This is because of the lower energy of the incident
ions as positive charges build up on the PE surface, making
the distribution of the ions skewed towards the surface.
The Cu2p XPS spectra acquired from the Cu and N co-PIII
samples implanted for different time durations provide
information about the chemical states of Cu. Figure 2
shows that the implanted Cu has the elemental (zero
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159
W. Zhang, J. Ji, Y. Zhang, Q. Yan, P. K. Chu
Figure 2. Cu2p XPS spectra as a function of the sputtered depth
obtained from PE samples plasma-implanted simultaneously
with Cu and N ions for different time durations: a) 10 min and
b) 30 min.
valence) state and does not react with elements in the
polymer matrix (S-2 9 in Figure 2), whereas the surface
Cu is oxidized (S-1 in Figure 2).[13,19] The similarity in
Figure 2a and Figure 2b suggests that the chemical state of
Cu is not altered during the implantation process.
Cu ions implanted into the materials cause damage to
the polymer chains. Collisions between the incident Cu
ions and polymer molecules, and subsequent energy
transfer lead to dehydrogenation reactions, dissociation,
as well as formation of different functional groups.[21–23]
The C1s XPS high resolution spectra obtained at different
depths can be used to determine the chemical states of C in
the two samples implanted for 10 min and 30 min. Both
Figure 3a and 3b show that during the Cu and N PIII,
dehydrogenation occurs, and new C –
– C bonds are formed.
The dehydrogenated carbon reacts with the implanted N
to produce new N-containing functional groups such as C –
– N. Comparing the fitted XPS spectra (S-2) in
N, C –
– N, –C –
Figure 3a and 3b, the implantation time is found to affect
the in-depth distribution of these functional groups. Ten
minutes of Cu and N PIII produce C –N and C –
– C bonds in
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ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. C1s XPS spectra obtained from PE samples plasmaimplanted simultaneously with Cu and N ions for different time
durations: a) 10 min and b) 30 min, together with the deconvolution results.
the implantation region. On the other hand, thirty minutes
of Cu and N PIII give rise to more C –N, C –
– N, and C –
– N bonds
in the near-surface region. Figure 3b indicates that, as the
depth increases, the peak gradually moves to a lower
binding energy. The fitted S-2 spectrum imparts that the
quantities of C –N, C –
– N, –C –
– N bonds decrease gradually
with increasing the depths, but the opposite is true for the
C –C and C –
– C bonds. That is to say, the chemical state of the
implanted N in the near-surface region is different from
that at a larger depth in the 10 min Cu and N PIII sample.
The N1s high resolution spectra are obtained from both
samples in order to determine the chemical states of the
DOI: 10.1002/ppap.200600196
Chemical and Physical Properties of Copper and . . .
implanted N. Figure 4a shows no shift in the N1s XPS
spectra throughout the entire implanted depth. The fitted
S-2 spectrum suggests that there are mainly C –N and C –
–N
bonds in the near-surface region, and this result is
consistent with the C1s spectra. On the other hand,
Figure 4b shows a small shift to a lower binding energy in
the N1s XPS spectra, suggesting that the N chemical state
varies with depth. The fitted S-2 spectrum proves that a
Figure 4. N1s XPS spectra obtained from PE samples plasmaimplanted simultaneously with Cu and N ions for different time
durations: a) 10 min and b) 30 min, together with the deconvolution results.
Plasma Process. Polym. 2007, 4, 158–164
ß 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
longer Cu and N co-PIII time produces more C –N, C –
– N,
[19]
–C –
– N bonds in the near-surface region.
Influence of Sequential Cu and N PIII on Chemical
and Physical Properties
Figure 5 shows the elemental depth distributions of the
samples plasma-implanted with copper and nitrogen
sequentially. The depth profiles are quite different from
the simultaneously plasma-implanted samples described
in the previous section. Comparing Figure 5a and b with
Figure 1a and b, the amounts of Cu in the sequentially
implanted samples are less than those in the co-implanted
samples. This is because the nitrogen ions sputter the
Figure 5. Elemental depth profiles obtained from PE samples
sequentially plasma-implanted with Cu and N ions for:
a) 10 min Cu-PIII, followed by 10 min N-PIII; and b) 10 min Cu-PIII,
followed by 30 min N-PIII.
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W. Zhang, J. Ji, Y. Zhang, Q. Yan, P. K. Chu
Figure 6. Cu2p XPS spectra acquired from PE samples sequentially
plasma-implanted with Cu and N ions for: a) 10 min Cu-PIII,
followed by 10 min N-PIII; and b) 10 min Cu-PIII, followed by
30 min N-PIII.
previously introduced Cu ions. The sequentially implanted
samples also have higher concentrations of nitrogen,
because nitrogen is sputtered by incident Cu ions during
the co-PIII. The effect is more apparent when the N-PIII
time is increased to 30 min (Figure 5b). The surface Cu
concentration decreases with N-PIII. The Cu2p XPS spectra
in Figure 6 show similar copper chemical states in both the
sequentially implanted and co-implanted samples. That is
to say, the order of implantation has very little effect on
the chemical state of Cu in the PE substrate.
The C1s XPS spectra series acquired from the two
sequentially implanted samples are shown in Figure 7. The
peaks gradually move to lower binding energies with
increasing the depth. This phenomenon is similar to that of
the longer time co-implanted samples. The shift is a little
larger and more C –N, C –
–N, and C –
– N bonds are formed on
the surface. A longer N-PIII time introduces more dehydro–N
genation, resulting in the formation of more C –
– N and C –
bonds.[19] The end result is that the chemical shift is larger,
as indicated in Figure 2a. Similar to the C1s XPS spectra, the
N1s high resolution spectra series taken from the two
samples shown in Figure 8 also exhibit a larger shift to
lower binding energies. This is because of the more
abundant C –
–N and C –
– N bonds on the surface, especially
on the 30 min N-PIII sample. The degree of damage and
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Plasma Process. Polym. 2007, 4, 158–164
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Figure 7. C1s XPS spectra acquired from PE samples sequentially
plasma-implanted with Cu and N ions for: a) 10 min Cu-PIII,
followed by 10 min N-PIII; and b) 10 min Cu-PIII, followed by
30 min N-PIII.
dehydrogenation depends on the nitrogen ion energy
which decreases with increasing the depth. Hence, when
copper and nitrogen are sequentially plasma-implanted,
– N and C –
more C –
– N bonds are formed in the near-surface
region, and more C –N bonds are formed at greater depths.
Similarly, when copper and nitrogen are co-implanted for a
longer time, the same chemical state trend can be
observed. In comparison, when the implantation time is
short, there is less dehydrogenation. Consequently, only a
slightly larger number of C –N bonds, but little C –
– N bonds
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Chemical and Physical Properties of Copper and . . .
C–
– N bonds, and the implanted Cu atoms exists in the
elemental (zero valence) state. If the co-PIII time is longer,
more C –N and C –
– N bonds are formed in the near-surface
region, but the elemental chemical states at greater depths
do not differ much from those of the shorter time implanted sample. In comparison, when copper and nitrogen are
plasma-implanted sequentially, the chemical state of Cu
is not altered, but the amount of Cu is reduced due to
sputtering by nitrogen ions. More C –
– N and C –
– N bonds are
formed in the near-surface region compared to the
co-implanted sample. A longer N-PIII step increases these
functional groups due to more extensive dehydrogenation.
Better understanding of the chemistry of the plasma-implanted samples provides more information on
how to alter the Cu release rate from Cu/N plasmaimplanted PE, and how optimize the long-term surface
antibacterial effects.
Acknowledgements: This research was supported by Hong Kong
Research Grants Council (RGC) Competitive Earmarked Research
Grant (CERG) No. CityU 112306.
Received: October 19, 2006; Revised: December 28, 2006;
Accepted: January 4, 2007; DOI: 10.1002/ppap.200600196
Keywords: copper; plasma immersion ion implantation; polyethylene; XPS
PACS codes: 82.35.Gh; 82.33.Xj; 87.15.By; 87.64.Gb
Figure 8. N1s XPS spectra obtained from PE samples sequentially
plasma-implanted with Cu and N ions for: a) 10 min Cu-PIII,
followed by 10 min N-PIII; and b) 10 min Cu-PIII, followed by
30 min N-PIII.
can be observed throughout the implanted depth. These
new chemical functional groups are expected to play an
important role in the retention of copper and control of Cu
release with time observed in our previous previous
experiments.
Conclusion
We have investigated the chemical states of Cu and N in
sequentially and simultaneously implanted PE samples
and also the effects of the PIII time. Our results show that
simultaneous copper and nitrogen PIII produces C –N and
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