Graphitic structure formation in ion implanted polyetheretherketone

Applied Surface Science 283 (2013) 154–159

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Graphitic structure formation in ion implanted polyetheretherketone E. Tavenner a,c,∗ , B. Wood b,c , M. Curry d , A. Jankovic d , R. Patel d a

Creative Polymers Pty. Ltd., 41 Wilkinson Street, Toowoomba, Queensland 4350, Australia Centre for Microscopy and Microanalysis, University of Queensland, St. Lucia, Queensland 4072, Australia c Chemical Committee, Surface Chemical Analysis, Standards, Australia d Center for Applied Science and Engineering, Missouri State University, 524 North Boonville Avenue, Springfield, MO 65806, USA b

a r t i c l e

i n f o

Article history: Received 15 February 2013 Received in revised form 11 June 2013 Accepted 12 June 2013 Available online 25 June 2013 Keywords: Ion implantation Polyetheretherketone Ion beam induced graphitization XPS RBS ERD

a b s t r a c t Ion implantation is a technique that is used to change the electrical, optical, hardness and biocompatibility of a wide range of inorganic materials. This technique also imparts similar changes to organic or polymer based materials. With polymers, ion implantation can produce a carbon enriched volume. Knowledge as to the nature of this enrichment and its relative concentration is necessary to produce accurate models of the physical properties of the modified material. One technique that can achieve this is X-ray photoelectron spectroscopy. In this study the formation of graphite like structures in the near surface of polyetheretherketone by ion implantation has been elucidated from detailed analysis of the C 1s and valence band peak structures generated by X-ray photoelectron spectroscopy. Further evidence is given by both Rutherford backscatter spectroscopy and elastic recoil detection. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Polymer based electronic materials and devices are slowly making a presence into the consumer marketplace; organic light emitting diode flat panel displays have already been implemented into devices such as cameras and mobile phones [1,2]. Considering the large number of additional applications where polymer electronics could be applied, for example photovoltaics [3,4], transistors [5], sensors and discreet circuit elements [6–9], it is easy to understand the increasing interest in the field. The advantages that polymer electronic materials have over their inorganic cousins include: mechanical flexibility, mechanical robustness, lower constraints on processing, lower energy requirements, cleaner elemental profile and improved performance in certain applications [1,10]. While there is an increase in the development and use of polymer based electronics, the mainstream materials used in their development, namely chemically produced semiconductive and conductive polymers and plastic-nanoparticulate composites, are limited by their processing techniques. Ion implantation, a physical process, is one technique that is not hampered by the limitations

∗ Corresponding author at: Creative Polymers Pty. Ltd., 41 Wilkinson Street, Toowoomba, Queensland 4350, Australia. Tel.: +61 410 475 447. E-mail addresses: [email protected], [email protected] (E. Tavenner). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.060

of chemically controlled processes, i.e. steady state, equilibrium conditions. Ion implantation is a versatile tool that is used to modify a wide range of materials in non-equilibrium conditions yielding structural, chemical and electrical changes that would not be otherwise attainable. The study of the effects ion implantation have on the properties of polymers started in 1982 by Forrest et al. [11] and the study of ion implanted polymers has been added to by many others since (for example see reviews by Das et al. [12], Lee [13], Calcagno [14] and Kramer [15], books by Fink [16,17], Townsend et al. [18], Kondyurin et al. [19] and Clough et al. [20], and book chapters by Giedd et al. [21]). In addition to the scientific developments in ion implanted polymers there have been several proposed devices that utilize such polymers [22–25]. A significant number of studies have stated that there is an increased carbonization within the implanted region [26–35], however there have been few attempts to determine the nature of this carbonization and its relative concentration. This is due to the random nature of the implantation process, the depth of the modified region and the limitations of the various characterization techniques. It is necessary to understand the nature of this carbonization and the relative amount produced during ion implantation in order to develop better models to explain the changes in the electrical and optical properties produced by the implantation process for eventual implementation into electronic and optical devices. One technique that has the promise of determining the nature and concentration of the carbonization is X-ray photoelectron

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spectroscopy. This technique utilizing a method to determine the nature of the carbonization has been used previously [36], however the details of the analysis were not covered. A similar technique was used by Estrade-Szwarckopf in the analysis of degraded carbonaceous materials [37], however instead of applying this technique to analyze graphitic formation in organic materials the reverse situation was studied – the formation of ‘defect’ structures within graphitic materials. We will endeavor to demonstrate that the changes in the polymer structure due to ion implantation are graphitic in nature and that an estimation as to the relative amount of graphitization can be determined. We aim to do this using both a light, normally unreactive ion, nitrogen, and a heavy, normally reactive ion, tin, and by analyzing the changes in structure with X-ray photoelectron spectroscopy, Rutherford backscatter spectroscopy and elastic recoil detection. 2. Materials and methods 2.1. Sample preparation Sample preparation was accomplished at both the Center for Applied Science and Engineering (CASE) and the Australian Nuclear Science and Technology Organisation (ANSTO). Samples produced at CASE were exposed to a 50 keV N+ ion beam with a beam current density of ∼0.37 ␮A/cm2 to doses of 1 × 1015 and 1 × 1016 ions/cm2 . Ion implantation was accomplished with an IBM Taconic ion implanter with high purity nitrogen gas supply. Two sample groups were produced at ANSTO. Both groups were exposed to an ion beam composed of both singularly (+) and doubly (++) ionized tin (Sn+,++ )1 with a beam current density of ∼0.41 ␮A/cm2 . One group was produced with a beam potential of 10 kV, while the other was produced with a beam potential of 45 kV. Both groups consisted of samples with doses of 1 × 1016 and 1 × 1017 ions/cm2 . Ion implantation was accomplished with a metal vapor vacuum arc system, without an analyzing magnet, utilizing a 10 Hz, 285 ␮s, 8 mA pulsed beam. All samples were prepared using 0.1mm thick polyetheretherketone (PEEK) films obtained from the Goodfellow corporation. The PEEK substrates were first cut to the appropriate size, and then cleaned by washing the substrate with methanol, followed with a surface wipe with a Kimwipe, rinsed with deionized water, and dried with dry nitrogen. 2.2. Characterization X-ray photoelectron spectroscopy (XPS): Samples were analyzed using a Kratos Axis Ultra XPS Surface Analysis System with a monochromatic Al X-ray source. The Al K˛ line (1486.6 eV) was used for all measurements. In addition a charge neutralizer was used as needed. Survey (wide) scans were taken at a pass energy of 80 eV and multiplex (narrow) scans of selected elements and valence band were obtained at 20 eV. Percent atomic concentrations were determined from peak areas calculated by the Vision 2 Processing program with manufacturer supplied elemental sensitivity factors. Multiplex scan fitting was accomplished with 80% Gaussian, 20% Lorentzian peaks with limitations on their positions and full width, half max (FWHM) values. Charge correction was accomplished by referencing to the C 1s C O peak at 286.3 eV. If this peak was not resolved enough then the Sn 3d 5/2 metal peak at 485.0 eV was used for samples containing tin, and for samples

1 The beam was composed of approximately 47% Sn+ ions and 53% Sn++ ions according to Brown and Godechot [38]. Both ion charges were in the beam due to inability of apparatus to separate and select one ion charge from the other.

Fig. 1. RBS spectrum of untreated PEEK. Surface elemental recoil energy for carbon (C) and oxygen (O) are located at channel 132 and 187 respectively. Smooth line is RUMP simulation of unimplanted PEEK.

without tin or if the tin metal peak was not resolved enough then the C 1s aromatic bond peak at 284.7 eV was used for charge correction. Linear baseline subtraction was used for the C 1s and O 1s peaks. Rutherford backscatter spectroscopy (RBS) and elastic recoil detection (ERD): Samples were analyzed at ANSTO using two different beamlines. The earlier group of measurements utilized a 3 MV Van de Graff ion source with an IBM geometry RBS analysis chamber. Experimental parameters were: 2 MeV He+ beam, 10 ␮C total charge per sample, 1.1 msr detector solid angle with the detector at a scattering angle of 11◦ measured from the beam path, and with the beam at normal incidence to the sample surface. Later measurements utilized ANSTO’s newer 2 MV Tandetron Accelerator beamline equipped with a multi-geometry measurement chamber. Both RBS and ERD measurements were performed in this chamber. RBS parameters were: 2 MeV He+ beam, 10 ␮C total charge per sample, 1.6 msr detector solid angle with the detector at a scattering angle of 20◦ measured from the beampath, and with the beam at normal incidence to the sample surface. ERD parameters were: 3 MeV He++ beam, 1.2–1.5 nA current over a 5 mm × 5 mm area, 3.0 msr detector solid angle with the detector at a scattering angle of 150◦ from the beam path, the sample surface normal tilted at a 75◦ angle relative to the beam, and two 5 ␮m thick aluminized Mylar sheets, with an areal density of 0.7043 mg/cm2 , placed between the sample and the detector to prevent the He atoms from reaching the detector. RBS data was analyzed using the RUMP2 program [39]. 3. Results and discussion 3.1. Rutherford backscatter spectroscopy and elastic recoil detection analysis The carbon and oxygen regions of the RBS spectrum for unprocessed PEEK can be seen in Fig. 1 (labels indicate surface elemental recoil energy), with the lower channels representing deeper within the sample. As can be seen in this spectrum, the data agrees with the RUMP simulation of the spectrum for PEEK (smooth curve).

2 RUMP does not stand for an acronym per se, M.O. Thompson does not claim that it means anything in particular. RUMP is an RBS analysis program built on top of the GenPlot program.

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Fig. 2. RBS spectrum of 50 keV N+ implanted PEEK to a dose of 1 × 101 6 ions/cm2 . Surface elemental recoil energy for nitrogen (N) is located at channel 156. Smooth line is RUMP simulation of unimplanted PEEK. Table 1 ERDS hydrogen count of untreated and N+ implanted PEEK. N+ implanted sample dose

Total H count

Untreated 1 × 1015 1 × 1016

12,773 10,426 9057

The RBS spectrum for N+ implanted PEEK to a dose of 1 × 1016 ions/cm2 can be seen in Fig. 2. When compared to the RUMP simulation of pristine PEEK the sample displays oxygen depletion at the surface. No detectable levels of the implanted nitrogen can be seen (nitrogen surface recoil energy is labeled in Fig. 2), indicating that the majority of the nitrogen does not become fixed in the sample, but outgasses away during ion implantation. ERD total hydrogen measurements indicate a decrease in the hydrogen content when compared to the untreated values (Table 1). The hydrogen loss is to be expected because when a hydrogen bond is broken within the polymer chain by a high energy ion, the hydrogen does not recombine with the surrounding structure, but outgasses through the surface.3 This hypothesis is supported by residual gas analysis measurements of the implantation chamber where a significant increase of hydrogen gas is detected during ion implantation. The above process describing hydrogen depletion during ion implantation may also explain the oxygen depletion seen in the RBS spectra. The RBS and ERD data for the Sn+,++ implanted PEEK did not show the same pattern for the depletion of hydrogen and oxygen and appears to be strongly correlated with the presence of the tin. However the reason for this is not within the scope of this study of the graphitization of PEEK via ion implantation and will be discussed in a future work. It should be noted that RUMP simulations of only the pristine PEEK films were accomplished because accurate simulation of the ion implanted polymer was not possible using this package. To accurately simulate any spectra, a model is made of the chemical makeup as a function of defined depth regions. However, it was discovered that widely different models can give the same spectral

3 Though there is the possibility for the hydrogen to recombine shortly after its bond is broken, the liberated hydrogen could be highly mobile within the polymer structure reducing the probability of this occurring.

Fig. 3. XPS C 1s spectrum of untreated PEEK. See text for description of fitted peaks. Insert is the monomer structure of PEEK.

response for the ion implanted samples. In addition, the non-linear nature of the chemical composition throughout the depth of the implanted region makes the construction of a viable model rather complex. However simulation of the unimplanted PEEK structure is possible due to the homogeneity and known composition of the sample. So, as has been suggested by Y.Q. Wang, RBS was only used as a qualitative comparison between samples [40]. 3.2. X-ray photoelectron spectroscopy analysis Figs. 3 and 4 are the C 1s and the valence band spectrum of unimplanted PEEK. The spectra agrees closely with data published in High Resolution XPS of Organic Polymers: The Scienta ESCA300 Database [41]. The main aromatic peak in the C 1s spectrum is located at 284.7 eV, the C O peak is at 286.3 eV and the C O peak is at 287.1 eV. The two broad peaks at 288.9 eV and 291.6 eV are the –* transitions associated with the polymer structure. The peak at 27 eV in the low binding energy region (Fig. 4) is due to the O 2s shell, and the group of peaks below 25 eV is the valence band structure.

Fig. 4. XPS valence band spectrum of untreated PEEK. Note that the peak at 27 eV is the O 2s peak, and the group of peaks below 25 eV are indicative of the valence band structure for PEEK.

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Fig. 5. XPS valence band spectrum of N+ implanted PEEK and graphite. Note that the peak at 27 eV on the N+ implanted spectrum is the O 2s peak. The broad peak below 25 eV for bothe the N+ implanted PEEK and for graphite are associated with the valence electrons.

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Fig. 7. XPS C 1s spectrum of N+ implanted PEEK. Note that the broad peak at 288.5 eV is the –* transition and have not been included in the analysis.

Table 2 Near surface composition of untreated and N+ implanted PEEK. Peak

C 1s

O 1s

Fig. 6. XPS C 1s spectra comparing untreated PEEK and N+ implanted PEEK.

After ion implantation the C 1s and valence band spectra show marked differences. The valence band structure changes in shape to something similar to that of graphite (see Fig. 5 for a comparison between N+ implanted PEEK and pure graphite). In addition, graphite like changes can be seen in the C 1s spectrum where the formation of a high binding energy tail and the shifting of the main peak to a lower binding energy are both present (Fig. 6). Graphite, in comparison to untreated PEEK, have a C 1s spectra located at 284.5 eV, and have an asymmetric lineshape with a high binding energy tail due to the conduction band electrons. The location of the C 1s peak for graphite is considered to be correct for the spectrometer in use due to the lack of a shift in binding energy for the spectra taken with and without the charge neutralizer. It should be noted that a significant portion of the original PEEK structure is still apparently present in the N+ implanted samples, hence the presence of a C O peak at 286.3 eV and the broadening of the main peak. Even though the shift between the aromatic and graphite peaks is only 0.2 eV, this shift is readily apparent, indicating the necessity of including the graphite asymmetric lineshape into the fit.

Bonding

Sample

Type

Energy (eV)

Untreated atom%

50 kV 1016 atom %

Aromatic Graphitic C O C O O C O C

284.7 284.5 286.3 287.1 533.3 531.1

66 – 15 5 9 5

48 30 9 3 6 4

Due to the above arguments, for the fitting of the C 1s spectrum of the N+ implanted PEEK the asymmetric lineshape of graphite is included at 284.5 eV (Fig. 7).4 Quantification of both the C 1s and the O 1s peak structures indicated a reduction of oxygen in the implanted region (Table 2). The same phenomena is apparent for Sn+,++ implanted PEEK. That is: the shifting of the C 1s peak to lower binding energy, the formation of a high binding energy tail, the appearance of a asymmetric lineshape, and the presence of a graphite lineshape in the valence band structure (Figs. 8 and 9). It should be noted that the valence band spectrum for 45 kV Sn+,++ implanted PEEK to a dose of 1 × 1016 ions/cm2 is displayed. This is due to the deeper implant and lower dose does not allow the tin valence band and 4d peak structures from overpowering the carbon valence band structure. Following the same arguments for the N+ implanted PEEK, the lineshape of graphite was included into the analysis of the C 1s peak for Sn+,++ implanted samples. The results of the analysis of the C 1s spectra can be seen in Table 3.5 It should be noted in Table 3 that the Sn+,++ implanted samples also display a reduction of the carbon–oxygen bonds, compared to untreated PEEK, similar to what was observed with the N+ implanted samples.

4 The asymetric graphite lineshape used is taken directly from the C 1s spectra of pure graphite taken under identical acquisition parameters. 5 Only the revelent components of the C 1s spectra are being reported due to the complexity of the entire spectral range (O 1s and Sn 3d) for this group of samples. However, atom percentages for the C O and C O bonding types have been confirmed by the corresponding bonding types in the O 1s peak analysis.

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Table 3 C 1s analysis of untreated and Sn+,++ implanted PEEK. Atom percentages are totaled to 100% for listed bonding types. Bonding

Sample

Type

Energy (eV)

Untreated atom %

10 kV 1016 atom %

10 kV 1017 atom %

45 kV 1016 atom %

45 kV 1017 atom %

Aromatic Graphitic C O C O

284.7 284.5 286.3 287.1

77 – 17 6

48 38 11 3

41 45 12 2

40 52 6 2

46 41 10 3

implantation) resulting in the majority of the signal originating from the unmodified substrate instead of from the modified surface and near surface. 4. Conclusion

Fig. 8. XPS C 1s spectra comparing untreated PEEK and Sn+,++ implanted PEEK.

The reduction of hydrogen and oxygen bonds, as evidenced by the RBS and ERD spectra of N+ implanted PEEK, gives supporting evidence of the ‘graphitization’ of ion implanted polymers. However, such evidence is purely circumstantial, and may not necessarily be present with different polymers under similar implantation conditions nor necessarily with similar polymers implanted with different ions (as will be shown in a future article concerning the full analysis of Sn+,++ implanted PEEK). Further evidence for the ‘graphitization’ of ion implanted polymers is given by XPS. The change in the lineshape of the valence band structure, after ion implantation, from that of untreated PEEK to one similar to that of graphite gives strong evidence to the formation of graphite structures within the polymer matrix. Couple this with the changes seen in the C 1s spectra to something resembling the spectrum of graphite (the formation of a high binding energy tail and the shifting of the main peak to a lower binding energy), and the evidence for the formation of graphite structures in N+ implanted PEEK is compelling. Tieing in with the data from RBS, XPS also shows a decrease in the oxygen content within the polymer structure. It was also shown that the same phenomena occurs with Sn+,++ implanted PEEK. Based upon the aforementioned evidence for the formation of graphite structures within ion implanted PEEK, it follows that the asymmetric lineshape of graphite needs to be included in the fitting of the XPS C 1s spectra. Furthermore, a similar type of analysis of the C 1s peak should be used with ion implanted polymers whenever graphite like structures are evident. Acknowledgements We would like to thank Peter Evans (ret.), David Button and Mahili Ionescu of ANSTO for the assistance given during the production of the tin ion implanted samples and for all the help with the collection and analysis of the RBS and ERD data. Additional thanks to Robert Vogel for assistance with the editing of this article.

Fig. 9. XPS valence band spectra comparing graphite and Sn+,++ implanted PEEK. Note that the peak at 27 eV is the combination of the O 2s and the Sn 4d peaks. The broad peak below 25 eV for bothe the N+ implanted PEEK and for graphite are associated with the valence electrons.

It is interesting to note that attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy measurements of the same samples in this study did not show significant changes in the peak structure between the ion implanted samples and the unimplanted PEEK. This is likely due to the analysis depth of this technique (on the order of microns [42]) compared to the depth of the ion implantation (less than 300 nm for 50 kV N+

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