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near-surface bonding in nuclear grade graphites: A comparison of monatomic and cluster depth-profiling techniques
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An XPS/UPS study of the surface/near-surface bonding in nuclear grade graphites: A comparison of monatomic and cluster depth-profiling techniques Alex Theodosioua, , Ben F. Spencerb, Jonathan Counsellc, Abbie N. Jonesa ⁎
a
The Nuclear Graphite Research Group (NGRG), The University of Manchester, Oxford Road, Manchester M13 9PL, UK Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK c Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, UK b
ARTICLE INFO
ABSTRACT
Keywords: Nuclear graphite sp2/sp3 bonding XPS Monatomic vs cluster ion sources Surface characterisation Depth-profiling
Samples of highly-orientated pyrolytic graphite (HOPG) and nuclear graphite grades, Gilsocarbon and Pile Grade-A (PGA), were examined using x-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and Raman spectroscopy. The photoelectron spectra was used to characterise the surface and sub-surface, particularly with regards to the sp2 and sp3 carbon bonding content. A peak-fitting methodology was applied and the results were in good agreement with those obtained through analysis of the C KLL spectra. Depth-profiling was performed using both monatomic Ar+ ions and cluster Arn+ ions with the former found to cause unwanted damage to the graphite structure with a dramatic increase in sp3 content from ~11% to ~88% in both nuclear grades in the ion bombarded region. Monatomic Ar+ etching was also found to result in ion implantation, leading to a broadening of the C 1s line and an increase in high energy component around the CeO region at ~286.0 eV. These effects were not observed when etching with cluster Arn+ ions. Raman spectroscopy also confirmed the difference in induced damage between Ar+ and Arn+ with measured ID/IG ration, within the damaged region only (R0), values of 1.04 and 0.3 respectively.
1. Introduction In the nuclear industry, graphite has been used extensively for > 70 years as a neutron moderator within the reactor core and also as reflector and structural material [1]. In more recent years graphite has been highlighted as a moderator material for the next generation of nuclear reactors, such as, the high-temperature [2] and molten salt reactor designs [3] currently under development in China and USA. Graphite is desirable for a number of reasons, however, under operational conditions, the graphite lattice is subject to significant damage as a result of prolonged neutron irradiation, which can have a considerable effect on its mechanical and physical properties [4,5]. Exposure to neutron irradiation can lead to the generation of damage within the graphite lattice, the mechanisms of which have been discussed in detail elsewhere [6]. In brief, high energy neutrons can displace carbon atoms from their lattice sites, leading to interstitialvacancy pairs [7]. These interstitials can lead to a variety of complex bonding geometries within the system, leading to more sp3 hybridised bonding and hence a more diamond-like structure [8].
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Since the carbon bonding within an ‘ideal’ graphitic system is purely sp2 in nature, then the sp2/sp3 ratio can be determined in order to understand the nature of the graphite structure on an atomic scale and can be used as a measure of damage within the system, thus providing information on the physical properties [9]. Measuring this ratio can also offer useful insight into the radiation history of the graphite with, for example, a high sp3 content indicating a significant exposure to neutron irradiation [9]. Such information could prove valuable with regards to reactor assessment and decommissioning. In addition, an accurate determination of the sp2/sp3 ratio could help characterise irradiation-induced changes to the microstructure, in particular, the presence of interstitial atoms i.e. displaced carbon [10] or impurities, such as fission products released during accident conditions [11]. An increase in sp3 character can also be used as an indication of the presence of larger, fission product, atoms such as Kr, Cs, Ba, Ag etc. disrupting the graphite lattice structure [12]. An effective method for determining the sp2/sp3 ratio in carbon materials is x-ray photoelectron spectroscopy (XPS), which has been used extensively as a means to characterise the surface and study the
Corresponding author. E-mail address: [email protected] (A. Theodosiou).
https://doi.org/10.1016/j.apsusc.2019.144764 Received 6 August 2019; Received in revised form 7 November 2019; Accepted 17 November 2019 Available online 27 November 2019 0169-4332/ © 2019 Published by Elsevier B.V.
Please cite this article as: Alex Theodosiou, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144764
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bonding present within carbon nanotubes [13], diamond-like carbon films [14–16] and more recently grapheme [17]. This present study uses XPS and associated techniques, such as Auger parameter analysis and ultraviolet photoelectron spectroscopy (UPS), to identify the surface distribution of elements within nuclear graphite specimens and to study the carbon bonding, particularly with respect to the sp2/sp3 character. Depth-profiling will also be carried out using both monatomic Ar+ ions and, a novel ion cluster approach, using Arn+ ions. Comparisons between the two methods will be made and the effectiveness of the cluster ion gun will be discussed in terms of providing a way of minimising ion-induced damage in sensitive materials such as graphite.
2.2. Sample preparation HOPG samples (10 × 10 × 2 mm) were purchased from SPI Supplies. Virgin (unirradiated) samples of PGA and Gilsocarbon graphite were machined to the same dimensions from larger blocks before being treated in an ultra-sonic bath and washed with ethanol. The samples were then left to dry overnight in an oven. 2.3. Instrumentation The XPS analyses were carried out on a Kratos AXIS Supra spectrometer using a monochromatic Al Kα source (10 mA, 15 kV). The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The energy resolution of the instrument and acquisition mode used in this study was determined to be 0.5 eV, as measured using the Ag 3d5/2 FWHM taken from a Ag foil calibration sample. Survey scan analyses were carried out with an analysis area of 300 × 700 µm and a pass energy of 160 eV. High resolution analyses were carried out with an analysis area of 300 × 700 µm and a pass energy of 10–20 eV. The instrument is equipped with a Kratos Gas Cluster Ion Source (GCIS) for sample sputtering using Arn+ cluster ions or monatomic Ar+ and in each case the gun is positioned 45 ⁰ relative to the surface of the sample. Ion clusters are created in the GCIS through the supersonic expansion of high pressure Ar gas through a de Laval nozzle into a medium vacuum region [21]. Nascent clusters are transmitted through a differentially pumped region into an electron impact ionisation source. Following ionisation, Arn+ cluster ions are extracted through the magnetic Wien filter to eliminate all small ions and to determine the transmitted cluster size distribution spread around a chosen median value. The filter also incorporates a bend to eliminate high-energy neutral species; the Arn+ cluster ions are then deflected through 2 ° to eliminate neutral and metastable species from the beam before simultaneous focusing and rastering across the sample. Alternately, Ar gas can be directly introduced into the electron impact ionisation source to allow production of monatomic Ar+ ions. The GCIS can deliver Arn+ cluster ions at energies from 0.5 to 20 keV and cluster sizes of n = 150–5000 and, as such, the beam parameters can be carefully chosen so as to be best suited to each application. Published data using Arn+ cluster ion sputtering has investigated the etching of organic material, where relatively high sputter yields allow the practical use of low beam energies for high resolution depth profiling [22]. Recent data published by Cumpson et al. [23] illustrates the necessity of using Arn+ cluster ions of relatively higher energy per nucleon to achieve practically useful sputtering yields when etching the type of materials of interest here. The sputtering yield may be determined through the semi-empirical relationship in Eq. (1).[24]:
2. Material and methods 2.1. Nuclear graphite The production of nuclear grade graphite is complex and the precise manufacturing process will often vary depending on the requirements and the desired properties. A detailed overview of the general manufacturing process is provided by Eatherly and Piper [18]. In the present study, two nuclear graphite grades, Gilsocarbon and Pile Grade-A (PGA) were analysed, along with a reference sample of highly ordered pyrolytic graphite (HOPG). Both graphite grades have been used extensively in the nuclear industry, particularly in the United Kingdom with PGA used in the Generation I Magnox reactor design and Gilsocarbon used in the Generation II Advanced Gas-Cooled Reactors (AGRs); the manufacture of these two graphite grade, in particular, is detailed by Marsden et al. [19]. Graphite is one of the three allotropes of carbon. Fig. 1 provides an illustration of the orbital arrangement in graphite, and for comparison, diamond. Graphite and diamond have identical elemental content, but differ in terms of structure and bonding. Graphite consists of triganol, sp2, bonding, with overlapping p-orbitals between C atoms in two-dimensions. The additional p-orbital contributes an electron to the delocalised electron cloud, leading to π-bonding and electrical conductivity. Conversely, diamond consists of tetrahedral, sp3, bonding, and hence has a much stronger molecular structure. These inherent differences in atomic structure can be exploited by techniques such as XPS, UPS and Raman spectroscopy in order to obtain information on the microstructure.
Y ( ) = n A 1 + erf
U s
(1)
where Y, ε, A and s represent sputtering yield, cluster energy per nucleon, a constant and the energy range of atoms within the cluster upon impact, respectively; U is the effective atomic sputtering threshold. Values of U for organic materials are typically ≈2 eV, whereas values for inorganic and metallic materials (e.g. SiO2, Au, TiO2) have been found to be an order of magnitude larger at ≈20 eV [24]. The Arn+ cluster ion energy per nucleon, ε, must exceed this effective threshold for sputtering to occur. Consequently, Arn+ cluster ion beams of 5.0 keV with n = 2000 (ε ~ 2.5 eV/nucleon) with a 2 × 2 mm raster area are employed in this study, and, as such, according to Eq. (1), some sputtering will occur however the etch rate will be negligible and limited to the loosely bound adventitious carbon [25]. For irradiation of samples with monatomic Ar+, a beam energy of 5.0 keV is used
Fig. 1. An illustration showing the differences between sp2 and sp3 carbon bonding. Modified from [20]. 2
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Table 1 Quantification of C and O species present on the surface of the three graphite samples through analysis of the C 1s peak.
HOPG Gilsocarbon PGA
C (%)
O (%)
97.1 89.4 77.1
2.9 10.6 22.9
throughout to be consistent with standard practice. Ultraviolet photoelectron spectroscopy (UPS) studies were performed on the same instrument using a helium discharge lamp with emission energies of 21.2 eV and 40.8 eV for He(I) and He(II) emission respectively. Raman spectroscopy was performed on a separate instrument, Horiba Xplora, with a laser wavelength of 532.2 nm. 3. Results and discussion 3.1. XPS analysis Fig. 3. Peak-fit of the High Resolution C 1s scan of HOPG. The same fitting parameters was used for all three samples. Inset is an enlargement of the π-π* region.
Large area XPS was carried out on HOPG, PGA and Gilsocarbon. Elements present on the surface were found to be C and O with trace amounts of contamination from Na, Cl and N which were too low to confidently quantify and thus were ignored. HOPG was found to have higher carbon content in the surface region than the nuclear grade samples, particularly PGA, which had significantly more O present on the surface (c.f. Table 1). This O can be attributed to the presence of adventitious species on the surface possibly carbonates, oxides and adsorbed water. High resolution XP spectra for the C 1s photoelectron lines can be seen for the three graphite samples in Fig. 1. All three exhibit a single thin peak with an asymmetric tail towards higher binding energy. The asymmetry arises from the presence of electrons emitted from the C 1s line which have lost energy to the delocalised π-electrons, a characteristic feature of graphite. The peak position of the main line is 284.5 eV which is also typical of graphitic sp2 bonding [26,27] (see Fig. 2).
system also leads to a broad π-π* satellite at a binding energy of 288–294 eV, generated through a continuum of excitations that occurs as the carbon photoelectrons exit the sample, leading to small energy losses. This satellite peak structure is a signature of sp2 carbon since no other relevant photoelectron signal is expected in this binding energy region. All peaks were a standard Gaussian-Lorentzian product peak shape except for graphitic carbon which exhibits an exponential tail to high binding energy (lower kinetic energy) due to the high conductivity of sp2 carbon [30]. Careful constraint of the peak fitting was required given the close binding energy position of sp2 and sp3 carbon (0.3 eV), and indeed the overlap of carbon-oxygen bonding with the asymmetric sp2 peak, [31]. The result of the peak-fit for HOPG is shown in Fig. 3; the same fitting procedure was employed across all three graphite grades. Subsequent quantification of each component was obtained, via the area under the curve, to give 75.8% sp2, 13.8% sp3, 3.0% CO, 0.2% C]O, 0.7% C(O)2 and 6.5% π-π* for the HOPG sample. This asymmetric peak shape, implemented in CasaXPS (www. casaxps.com) using LA(0.8, 1.4, 100), where 0.8 and 1.4 are the asymmetry factors at lower and higher binding energy to the peak, was determined using the HOPG sample and found to adequately fit all datasets from all samples consistently.
3.1.1. Peak fitting The C 1s spectra were fitted with several peaks associated with different chemical species: graphitic i.e. sp2 carbon at 284.5 eV, hydrocarbon i.e. sp3, at 284.8 eV [28], CeO at ~286.0 eV, C]O at ~287.0 eV and C(O)2 at ~288 eV [29]. The presence of a π-bonding
3.1.2. Measurement of D-parameter Previous studies by Mizokawa et al. have shown the degree of sp2/ 3 sp character in the surface carbon to be linearly related to the difference, D, of the maxima and minima of the first derivative of the C KLL spectra [32]. The ‘D-parameter’, as it is known, has been explored more recently as a way of differentiating carbon states in XPS imaging [33]. The first derivative of the obtained C KLL spectra for the three graphite samples can be seen in Fig. 4. From Fig. 4, D-parameter values were measured and found to be 22.5 for HOPG and 21.0 for both Gilsocarbon and PGA, with an error of ± 1. These values indicate a highly graphitic, sp2, nature. In contrast, Mezzi and Kaciulis [34] also determined the D-parameter for diamond, which is purely sp3 in nature, and found it to be 13.7 eV. A linear relationship was found between the D-parameter and increasing sp2 content, with analysis of a SWCNT found to contain intermediate bonding between graphite (sp2) and diamond (sp3) with a D-parameter of ~17.4 eV [34]. The method of measuring a D-parameter to infer the ratio of sp2/sp3 hybridisation is useful, and removes any subjectivity associated with
Fig. 2. High Resolution C 1s scan of the as-received HOPG, Gilsocarbon and PGA samples. 3
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Table 3 Composition of surface species on HOPG as-received and after sputter etching with Ar+ and Ar2000+. In both cases ion energy was 5.0 keV and sputter time was 60 s. Atomic % HOPG
C
O
Ar
As-Received Monatomic Ar+ Cluster Ar2000+
97.1 97.8 100
2.9 0 0
0 2.2 0
Fig. 4. The first derivative of the Auger C KLL spectra for HOPG, PGA and Gilsocarbon.
traditional peak-fitting methodologies. Additionally, since this technique uses the overall width of the Auger feature, rather than its position, then it is not prone to the effects of sample charging [35]. However, the D-parameter is predominately a guide to understanding the level of graphite-like or diamond-like bonding in the carbon specimen and care should be taken when comparing absolute values, as these have been shown to vary across different spectrometers using differing parameters and acquisition conditions [35]. Nevertheless, confidence in the peak-fit can be obtained through comparing D-parameter values calculated from the sp2 and sp3 components of the fit, and those obtained through the first derivative C KLL spectra. Using the derived values obtained from the peak fitting process described above, the D-parameter can be estimated using the method outlined by Alanazi et al. [36]. This was then compared with the Dparameter obtained through the Auger analysis and the results are shown in Table 2. In order to accurately determine the D-parameter from the peak-fitting, the calculated sp2 component and the π-π* components were combined. The reason for this is that the signal from the π-π* region is generated from electrons in unhybridised p-orbitals perpendicular to the C-C bonding within the graphite plane. In aromatic, sp2, systems such as graphite, these p-orbitals are able to overlap to form a cloud of de-localised electrons above and below the plane. For this reason, the sp2 and π-π* content are intrinsically linked, hence why there is no contribution to signal from π-π* transitions observed in the spectrum for diamond [34]. As such, this should be considered when assessing the total sp2 content derived from analysis of the peak at 284.5 eV. The value of 22.5 obtained for HOPG through the C KLL analysis is in close agreement with that by Merel et al. [37] however, the values measured through the peak-fitting of the C 1s peak are slightly lower than those obtained through Auger analysis across all three samples.
Fig. 5. Evidence of argon implantation as a result of sputtering with monatomic Ar+ ions. This is not observed during sputtering with cluster, Ar+ 2000 ions.
Fig. 6. A comparison of the high-resolution C 1s spectra of HOPG before and after sputter etching with monatomic Ar+ ions and cluster ions, Ar2000+, at 5.0 keV for 60 s.
These differences are small and are likely attributed to small errors either within the measurement or the fitting parameters or due to inhomogeneous distribution of impurities between the surface and nearsurface leading to localised differences in sp2/sp3 bonding. Any such differences in the surface region of the samples will be accentuated by the difference in the information depth between the two methods, with the Auger method being even more surface sensitive; that is, the C KLL electrons are omitted from only ̴ 3.3 nm [38]. Additionally, the Dparameter is also known to be effected by differences in crystallinity and grain size, as well as the presence of oxygen impurities [38]; these factors may vary locally in the polycrystalline nuclear graphite grades leading to small discrepancies in the measurements.
Table 2 Comparison of D-parameter obtained through peak fitting of the C 1s and analysis of the C KLL spectra. D parameter Graphite
Peak-Fit
Auger Analysis
HOPG PGA Gilsocarbon
21.0 19.4 20.0
23.0 21.0 21.0
4
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Fig. 7. UPS spectra of HOPG sample, as-received and before and after etching with monatomic Ar+ and cluster Ar2000+.
occurred even at ion energies as low as 56 eV and attributed the presence of the peak at 241.5 eV to Ar ions trapped on defects on the HOPG surface or ions trapped between two graphene layers. At 5.0 keV the mean implantation depth of Ar in graphite was approximated, using the SRIM 2013 software package [40], to be 7.0 nm with a maximum depth of 17.0 nm. This implies that there will be considerable ion implantation within this interaction volume and further subsequent damage to the sub-surface through collision cascades; such processes could be avoided using a cluster source. Further evidence of Ar implantation can be seen in Fig. 6, which shows the presence of a small Ar 2s peak at ~319.5 eV [41]. Perhaps more interestingly, Fig. 6, shows the effect of the implantation on the C 1s peak. After sputter cleaning with Ar2000+ there is no observable shift in peak position and the signal becomes more intense with two distinct features emerging at 291.3 eV and 294.5 eV. Also, a slight decrease in the FWHM of the peak is seen from 0.63 eV to 0.62 eV. The observed increase in intensity combined with a decrease in FWHM is most likely due to the removal of CeO species and adventitious carbon, exposing the virgin HOPG below. After monatomic etching, the π-π* shake-up feature at ~291.0 eV is no longer present indicating loss of sp2 character. There is also a distinct broadening of the peak to both lower and high binding energy with a considerable loss in intensity. The FWHM increases from 0.62 eV to 1.5 eV. The shift to higher binding energy is likely a result of lattice deformation and a migration from sp2 to sp3 character, with the overall picture suggesting that Ar implantation has induced considerable morphological defects contributing to the re-hybridisation [42].
Table 4 The measured work function, f, for the sample asreceived and before and after etching. f (eV) As-Received 5.0 keV Ar+ 5.0 keV Ar2000+ Literature value [44]
4.85 4.74 4.55 4.6
4. Monatomic vs cluster sources Despite the determination of the elemental makeup and, in particular, the carbon bonding of the as-received graphite surfaces, it is unclear whether composition is unique to the surface or accurately represents the near-surface bulk material. To investigate this, sub-surface analysis of the graphite specimens was carried out by means of depth-profiling techniques, using both a traditional monatomic Ar+, and argon cluster ions Arn+ (in this case n = 2000). In recent times cluster ions have been used in surface analysis due to the limited damage propagated into the bulk of a material post-etching; consequently they are typically used for the analysis of soft organic/polymer surfaces [39] however relatively few studies have been reported where the method is applied to graphitic materials. Comparison of these two methods is necessary when designing future experiments for the analysis of such materials. 4.1. Surface cleaning
4.2. UPS analysis
The surface composition of the HOPG was confirmed through a large area survey scan that found 0.5% O on the surface, suggesting the presence of either CeO, C]O and/or C(O)2 species. The sample was sputter cleaned using the two methods described above and the resultant surface composition is shown in Table 3. After monatomic etching an Ar 2p peak at ~241.5 eV was visible in the survey spectra, as shown in Fig. 5, indicating that argon implantation had occurred; quantification was carried out on this peak to give an approximate concentration of 2.2 at.%. Implantation was not observed from the cluster etch, which simply removed all O revealing the pure C surface below. Ion implantation is not unexpected, however, particularly in graphitic materials where the structure is prone to intercalation. Smith et al. [37] found argon implantation in HOPG
UPS studies were also performed on the graphite samples to further compare the two etching methods. Fig. 7 shows the measured UPS spectra for the HOPG sample, before and after 60 s etching with monatomic Ar+ and cluster Ar2000+ ions. Prior to etching a sharp peak is observed at 13.7 eV and a step feature at ~3 eV, these features have previously been attributed to π-electrons and therefore associated with sp2 bonding [43]. After etching with monatomic Ar+ there is a distinct change in the shape of the spectrum. Neither of the sp2 features are present indicating complete removal of sp2 character in the top most layers of the sample. The UPS results imply that all sp2 character is removed and, when compared to a more gradual change in the XPS, the damage can be considered as being more prominent in the very top 5
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Fig. 8. Concentration profile of chemical species during monatomic Ar+ etching of Gilsocarbon and PGA Samples. C 1s is omitted for clarity.
Fig.9. Concentration profile of chemical species during cluster Ar2000+ etching of Gilsocarbon and PGA Samples. Ar 2p is included for comparison despite no signal being detectable.
layers of the sample. These results agree with previous REELS studies which showed a broadening and loss of the π-peak even at low energy etching conditions [43]. The work function, f, for the material can also be calculated by subtracting the full width of the spectrum form the energy of the light source. The results are shown in Table 4. Here we can see that there is greater agreement between the literature value and the value post cluster etch, than compared to the value post monatomic etch. The literature value of 4.6 eV, represents that of a pristine HOPG surface [44].
etch. Both methods result in a decrease in surface O and an associated increase in the % concentration of C, interestingly however, it can be seen that some O remains as the surface is etched away suggesting that it is present in the near-surface structure. This is more apparent in the PGA sample which had a higher concentration of O on the surface to begin with. As mentioned, the monatomic Ar+ etch results in a steady increase in Ar concentration, which appears to saturate at around 1.4% for both Gilsocarbon and PGA, this observation is not seen in the cluster etch despite the longer etching time required. It is also interesting to note that the amount of Ar implantation is considerably less than the 2.2% observed in HOPG under the same conditions, this implies that the highly ordered nature of the HOPG makes it more susceptible to intercalation of foreign species. The high-resolution C 1s scan obtained after depth-profiling of the Gilsocarbon specimen is shown in Fig. 10 and is peak-fitted using the methodology described in Section 3.1.1. Quantification of the peak-fitting process shows a marked change in the sp2/sp3 fraction as a result of the depth-profiling. The results are shown in Table 5 for both the Gilsocarbon and PGA specimens. The results of the quantification highlight some interesting phenomenon. Firstly, for Gilsocarbon there is a considerable increase in sp3 content accompanied by a corresponding decrease in sp2 character as a result of the depth profiling, this can be clearly seen in Fig. 10. The difference is more remarkable for the monatomic ion beam which gives
4.3. Depth-profiling Etching for longer periods can provide valuable insight into the subsurface structure. The nuclear grade Gilsocarbon and PGA samples were probed by depth profiling using both the monatomic and cluster ion sources. A scan was taken at regular intervals throughout the etch and the concentration of the surface species was monitored with time; the results of which are shown in Figs. 8 and 9. As described through Eq. (1), the etch rate using Ar cluster ions is significantly slower than with monatomic ions [45]. This is illustrated by the increased etch time before the surface concentration becomes steady-state. For monatomic ions, a steady-state is reached after ~80 s etching whereas this is not reached until > 500 s for the cluster ion 6
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by the surface sensitivity of the measurement. XPS is known to have an information depth of approximately 3λ, where λ is the mean free path of the emitted photoelectrons. The value of λ will depend on the excitation energy, but for the setup used here is known to be 2–3 nm [47], therefore giving an information depth of approximately 6–10 nm. This corresponds very well to the penetration depth of the Ar+ ions into the graphite lattice, as calculated from SRIM. From the simulation results we know that this region will be highly disordered due to the irradiation damage caused by the energetic Ar+ ions and there will be a considerable amount of displaced C atoms residing in interstitial positions. These interstitials, combined with other effects of irradiation on the atomic structure, will lead to a large increase in irregular bonding within the graphite, specifically more tetrahedral sp3 bonding [48,49]. The effect is not observed to the same extent in the case of the cluster source due to less irradiation damage and no detectable Ar implantation. Depth-profiling using a monatomic Ar+ beam also gives rise to a significant increase in the FWHM of the main peak, leading to an increase in peak asymmetry towards high-energy. The fitting parameters used in this analysis attribute this to the CeO component at ~286.0 eV. However, as explained by Estrade-Szwarckopf [50], the accurate fitting of the C 1s peak in graphitic materials is highly complicated with no general consensus found in the literature. The increase in the CeO component measured here cannot infact be due to an increase in O content, as the opposite is seen from both the intensity of the O 1s signal in the large area survey scan and in Fig. 8, where the O 1s signal of Gilsocarbon is shown to decrease from 6.7% to 1.5% as a result of the Ar+ etch. Therefore an alternative explanation could be that this peak is a superposition of some smaller CeO component and some larger component associated with, either, a localised decrease in conductivity, leading to a continuous broadening of the main peak to higher binding energy than the known sp3 position [50], or perhaps more applicable, an additional component due to intercalation of implanted Ar species between the surface layers of the graphite. Similar high energy shoulders to the C 1s line have been observed previously in ion implanted graphites [42,51] with some debate over the precise origin. Yamada et al. observed the emergence of a significant peak at 286.0 eV when samples of HOPG were treated with a diamond-dispersed solution [52] suggesting that this peak is indicative of sp3 bonding and independent of O content. Very similar trends were also observed for the PGA specimen due to its comparable microstructure/bonding to that of Gilsocarbon. 4.4. Raman spectroscopy
Fig. 10. A comparison of the high-resolution C 1s spectra of Gilsocarbon before and after sputtering with 5.0 keV monatomic Ar+ for 150 s and cluster 5.0 keV Ar2000+ cluster ions for 600 s. The same fitting parameters were also applied to the PGA graphite.
An effective method for analysing near-surface damage in graphitic systems is through Raman spectroscopy. Fig. 11 shows the Raman spectrum of the Gilsocarbon specimen etched with both monatomic Ar+ and cluster Ar2000+ ions. All three spectra exhibit the same characteristic graphite peaks i.e. the disorder-induced D band at 1350 cm−1, the graphitic G band at 1580 cm−1 and the additional disorder induced D’ band at 1620 cm−1, all of which have been detailed and reported extensively in the literature [53,54]. For graphitic material, it is known that the ratio of the intensity of the D band, with respect to the G band intensity i.e. ID/IG can be used to provide a semi-quantitative understanding of the lattice disorder within the graphite; where, as ID/IG increase, damage in the system can be said to increase leading to a decrease in the crystallite size [55]. Additionally, the prominence of the D’ shoulder feature is also attributed to microstructural damage has been attributed to the creation of C2 molecules [56]. From Fig. 10 the ID/IG ratio is measured via deconvolution of the peaks and found to be 0.27, 0.52 and 0.30 for the sample as-received, after monatomic etching and after cluster etching respectively. The information depth of Raman spectroscopy depends on the target material and also the incident wavelength. For a laser wavelength of
Table 5 Quantification of the peak-fitting of the Gilsocarbon and PGA C 1s peaks before and after depth-profiling with monatomic and cluster ions. Gilsocarbon 2
As-Received Ar+ Ar2000+
PGA 3
sp (%)
sp (%)
sp2 (%)
sp3 (%)
80.2 11.3 63.7
19.8 88.7 36.3
84.2 11.7 81.4
15.8 88.3 18.6
a dramatic increase in the sp3 content to a value as high as 88.7%. This agrees with the discussion so far and indicates a considerable destruction to the sub-surface triganol bonding structure and a move to a more diamond-like tetrahedral, sp3, structure, as reported by Jackson and Nuzzo during 3.0 keV Ar+ irradiation of HOPG [46]. The stark contrast in the change of sp2/sp3 ratio for monatomic Ar+ is likely exasperated 7
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recommended that detailed surface analysis of nuclear grade graphites be carried out using a cluster ion source. CRediT authorship contribution statement Alex Theodosiou: Conceptualization, Methodology, Validation, Investigation, Resources, Data curation, Writing - original draft, Visualization, Supervision. Ben F. Spencer: Methodology, Software, Validation, Formal analysis, Data curation, Writing - review & editing. Jonathan Counsell: Methodology, Software, Validation, Data curation, Writing - review & editing. Abbie N. Jones: Validation, Writing - review & editing, Visualization, Supervision, Funding acquisition. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was funded by the UK Engineering and Physical Sciences Research Council under Grant No. EP/R00577X/1. Access to the XPS/ UPS instrumentation was provided by Kratos Analytical Ltd, for which the authors are grateful. Additional thanks go to Jessica Higgins for help with obtaining the Raman data and Jason Peterson for the artwork.
Fig. 11. A comparison of the Raman spectra of Gilsocarbon before and after sputtering with 5.0 keV monatomic Ar+ for 150 s and cluster 5.0 keV Ar2000+ cluster ions for 600 s.
532.2 nm (E = 2.3 eV), this is known to be approximately 50 nm in graphite [51]. Therefore, it follows that the data should be carefully interpreted and account for any discrepancy between the volume of interest in the sample and the information depth. In this case, the spectra from the monatomic etched sample will exhibit a lower ID/IG ratio than expected since the large majority of the data is from the graphite bulk i.e. graphite that has been unperturbed by the Ar+ bombardment. To account for this, the observed ID/IG ratio (Robs) can be adjusted to give the ‘actual’ ID/IG ratio (R0) using a method described by Theodosiou et al. [57]. For 5.0 keV Ar+ irradiation of graphite this has been calculated to give an approximate factor of 2, that is: R0 = 2 × Robs. Therefore, when concerning the damaged region alone, R0 can be estimated as 1.04 after etching with monatomic Ar+ and therefore, since R0 ≥ 1.0, the system can be considered as considerably damaged with an associated drop in long range order [58,59]. However, this adjustment can be neglected in the case of the cluster ion etch since the penetration depth and range of energy deposition into the sample is practically negligible [60]. The calculation of R0 confirms the damage caused through monatomic Ar+ bombardment during the depth profile and agrees with the results seen from the photoelectron spectroscopy techniques.
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5. Conclusions The surface and near-surface bonding in HOPG and two nuclear graphite grades, Gilsocarbon and PGA, has been investigated through XPS, UPS and Raman spectroscopy. Analysis of the C 1s peak was shown to be complex; a peak-fitting approach was used to determine the sp2/ sp3 bonding ratio and the results obtained agreed well with those obtained through analysis of the C KLL spectra through determination of a D-parameter. Additionally, comparisons were made between monatomic Ar+ and cluster Arn+ etching techniques as methods for surface cleaning and depth-profiling. The cluster source was found to be more effective in removing surface contamination from the graphite, compared to traditional monatomic Ar+, with the additional benefit of occurring no unwanted Ar implantation. Peak-fitting of XPS spectra of nuclear grade graphite, depth profiled with monatomic Ar+, exhibited a dramatic decrease in the sp2/sp3 ratio, with an additional high-energy component at 286.0 eV in the C 1s line attributed to an effect of ion implantation. UPS and Raman data confirmed that monatomic Ar+ bombardment leads to damage to the graphite lattice which could potentially affect the results being obtained; for this reason it is 8
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Full Length Article
An XPS/UPS study of the surface/near-surface bonding in nuclear grade graphites: A comparison of monatomic and cluster depth-profiling techniques Alex Theodosioua, , Ben F. Spencerb, Jonathan Counsellc, Abbie N. Jonesa ⁎
a
The Nuclear Graphite Research Group (NGRG), The University of Manchester, Oxford Road, Manchester M13 9PL, UK Department of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, UK c Kratos Analytical, Wharfside, Trafford Wharf Road, Manchester M17 1GP, UK b
ARTICLE INFO
ABSTRACT
Keywords: Nuclear graphite sp2/sp3 bonding XPS Monatomic vs cluster ion sources Surface characterisation Depth-profiling
Samples of highly-orientated pyrolytic graphite (HOPG) and nuclear graphite grades, Gilsocarbon and Pile Grade-A (PGA), were examined using x-ray photoelectron spectroscopy (XPS), ultra-violet photoelectron spectroscopy (UPS) and Raman spectroscopy. The photoelectron spectra was used to characterise the surface and sub-surface, particularly with regards to the sp2 and sp3 carbon bonding content. A peak-fitting methodology was applied and the results were in good agreement with those obtained through analysis of the C KLL spectra. Depth-profiling was performed using both monatomic Ar+ ions and cluster Arn+ ions with the former found to cause unwanted damage to the graphite structure with a dramatic increase in sp3 content from ~11% to ~88% in both nuclear grades in the ion bombarded region. Monatomic Ar+ etching was also found to result in ion implantation, leading to a broadening of the C 1s line and an increase in high energy component around the CeO region at ~286.0 eV. These effects were not observed when etching with cluster Arn+ ions. Raman spectroscopy also confirmed the difference in induced damage between Ar+ and Arn+ with measured ID/IG ration, within the damaged region only (R0), values of 1.04 and 0.3 respectively.
1. Introduction In the nuclear industry, graphite has been used extensively for > 70 years as a neutron moderator within the reactor core and also as reflector and structural material [1]. In more recent years graphite has been highlighted as a moderator material for the next generation of nuclear reactors, such as, the high-temperature [2] and molten salt reactor designs [3] currently under development in China and USA. Graphite is desirable for a number of reasons, however, under operational conditions, the graphite lattice is subject to significant damage as a result of prolonged neutron irradiation, which can have a considerable effect on its mechanical and physical properties [4,5]. Exposure to neutron irradiation can lead to the generation of damage within the graphite lattice, the mechanisms of which have been discussed in detail elsewhere [6]. In brief, high energy neutrons can displace carbon atoms from their lattice sites, leading to interstitialvacancy pairs [7]. These interstitials can lead to a variety of complex bonding geometries within the system, leading to more sp3 hybridised bonding and hence a more diamond-like structure [8].
⁎
Since the carbon bonding within an ‘ideal’ graphitic system is purely sp2 in nature, then the sp2/sp3 ratio can be determined in order to understand the nature of the graphite structure on an atomic scale and can be used as a measure of damage within the system, thus providing information on the physical properties [9]. Measuring this ratio can also offer useful insight into the radiation history of the graphite with, for example, a high sp3 content indicating a significant exposure to neutron irradiation [9]. Such information could prove valuable with regards to reactor assessment and decommissioning. In addition, an accurate determination of the sp2/sp3 ratio could help characterise irradiation-induced changes to the microstructure, in particular, the presence of interstitial atoms i.e. displaced carbon [10] or impurities, such as fission products released during accident conditions [11]. An increase in sp3 character can also be used as an indication of the presence of larger, fission product, atoms such as Kr, Cs, Ba, Ag etc. disrupting the graphite lattice structure [12]. An effective method for determining the sp2/sp3 ratio in carbon materials is x-ray photoelectron spectroscopy (XPS), which has been used extensively as a means to characterise the surface and study the
Corresponding author. E-mail address: [email protected] (A. Theodosiou).
https://doi.org/10.1016/j.apsusc.2019.144764 Received 6 August 2019; Received in revised form 7 November 2019; Accepted 17 November 2019 Available online 27 November 2019 0169-4332/ © 2019 Published by Elsevier B.V.
Please cite this article as: Alex Theodosiou, et al., Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.144764
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bonding present within carbon nanotubes [13], diamond-like carbon films [14–16] and more recently grapheme [17]. This present study uses XPS and associated techniques, such as Auger parameter analysis and ultraviolet photoelectron spectroscopy (UPS), to identify the surface distribution of elements within nuclear graphite specimens and to study the carbon bonding, particularly with respect to the sp2/sp3 character. Depth-profiling will also be carried out using both monatomic Ar+ ions and, a novel ion cluster approach, using Arn+ ions. Comparisons between the two methods will be made and the effectiveness of the cluster ion gun will be discussed in terms of providing a way of minimising ion-induced damage in sensitive materials such as graphite.
2.2. Sample preparation HOPG samples (10 × 10 × 2 mm) were purchased from SPI Supplies. Virgin (unirradiated) samples of PGA and Gilsocarbon graphite were machined to the same dimensions from larger blocks before being treated in an ultra-sonic bath and washed with ethanol. The samples were then left to dry overnight in an oven. 2.3. Instrumentation The XPS analyses were carried out on a Kratos AXIS Supra spectrometer using a monochromatic Al Kα source (10 mA, 15 kV). The instrument work function was calibrated to give a binding energy (BE) of 83.96 eV for the Au 4f7/2 line for metallic gold and the spectrometer dispersion was adjusted to give a BE of 932.62 eV for the Cu 2p3/2 line of metallic copper. The energy resolution of the instrument and acquisition mode used in this study was determined to be 0.5 eV, as measured using the Ag 3d5/2 FWHM taken from a Ag foil calibration sample. Survey scan analyses were carried out with an analysis area of 300 × 700 µm and a pass energy of 160 eV. High resolution analyses were carried out with an analysis area of 300 × 700 µm and a pass energy of 10–20 eV. The instrument is equipped with a Kratos Gas Cluster Ion Source (GCIS) for sample sputtering using Arn+ cluster ions or monatomic Ar+ and in each case the gun is positioned 45 ⁰ relative to the surface of the sample. Ion clusters are created in the GCIS through the supersonic expansion of high pressure Ar gas through a de Laval nozzle into a medium vacuum region [21]. Nascent clusters are transmitted through a differentially pumped region into an electron impact ionisation source. Following ionisation, Arn+ cluster ions are extracted through the magnetic Wien filter to eliminate all small ions and to determine the transmitted cluster size distribution spread around a chosen median value. The filter also incorporates a bend to eliminate high-energy neutral species; the Arn+ cluster ions are then deflected through 2 ° to eliminate neutral and metastable species from the beam before simultaneous focusing and rastering across the sample. Alternately, Ar gas can be directly introduced into the electron impact ionisation source to allow production of monatomic Ar+ ions. The GCIS can deliver Arn+ cluster ions at energies from 0.5 to 20 keV and cluster sizes of n = 150–5000 and, as such, the beam parameters can be carefully chosen so as to be best suited to each application. Published data using Arn+ cluster ion sputtering has investigated the etching of organic material, where relatively high sputter yields allow the practical use of low beam energies for high resolution depth profiling [22]. Recent data published by Cumpson et al. [23] illustrates the necessity of using Arn+ cluster ions of relatively higher energy per nucleon to achieve practically useful sputtering yields when etching the type of materials of interest here. The sputtering yield may be determined through the semi-empirical relationship in Eq. (1).[24]:
2. Material and methods 2.1. Nuclear graphite The production of nuclear grade graphite is complex and the precise manufacturing process will often vary depending on the requirements and the desired properties. A detailed overview of the general manufacturing process is provided by Eatherly and Piper [18]. In the present study, two nuclear graphite grades, Gilsocarbon and Pile Grade-A (PGA) were analysed, along with a reference sample of highly ordered pyrolytic graphite (HOPG). Both graphite grades have been used extensively in the nuclear industry, particularly in the United Kingdom with PGA used in the Generation I Magnox reactor design and Gilsocarbon used in the Generation II Advanced Gas-Cooled Reactors (AGRs); the manufacture of these two graphite grade, in particular, is detailed by Marsden et al. [19]. Graphite is one of the three allotropes of carbon. Fig. 1 provides an illustration of the orbital arrangement in graphite, and for comparison, diamond. Graphite and diamond have identical elemental content, but differ in terms of structure and bonding. Graphite consists of triganol, sp2, bonding, with overlapping p-orbitals between C atoms in two-dimensions. The additional p-orbital contributes an electron to the delocalised electron cloud, leading to π-bonding and electrical conductivity. Conversely, diamond consists of tetrahedral, sp3, bonding, and hence has a much stronger molecular structure. These inherent differences in atomic structure can be exploited by techniques such as XPS, UPS and Raman spectroscopy in order to obtain information on the microstructure.
Y ( ) = n A 1 + erf
U s
(1)
where Y, ε, A and s represent sputtering yield, cluster energy per nucleon, a constant and the energy range of atoms within the cluster upon impact, respectively; U is the effective atomic sputtering threshold. Values of U for organic materials are typically ≈2 eV, whereas values for inorganic and metallic materials (e.g. SiO2, Au, TiO2) have been found to be an order of magnitude larger at ≈20 eV [24]. The Arn+ cluster ion energy per nucleon, ε, must exceed this effective threshold for sputtering to occur. Consequently, Arn+ cluster ion beams of 5.0 keV with n = 2000 (ε ~ 2.5 eV/nucleon) with a 2 × 2 mm raster area are employed in this study, and, as such, according to Eq. (1), some sputtering will occur however the etch rate will be negligible and limited to the loosely bound adventitious carbon [25]. For irradiation of samples with monatomic Ar+, a beam energy of 5.0 keV is used
Fig. 1. An illustration showing the differences between sp2 and sp3 carbon bonding. Modified from [20]. 2
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Table 1 Quantification of C and O species present on the surface of the three graphite samples through analysis of the C 1s peak.
HOPG Gilsocarbon PGA
C (%)
O (%)
97.1 89.4 77.1
2.9 10.6 22.9
throughout to be consistent with standard practice. Ultraviolet photoelectron spectroscopy (UPS) studies were performed on the same instrument using a helium discharge lamp with emission energies of 21.2 eV and 40.8 eV for He(I) and He(II) emission respectively. Raman spectroscopy was performed on a separate instrument, Horiba Xplora, with a laser wavelength of 532.2 nm. 3. Results and discussion 3.1. XPS analysis Fig. 3. Peak-fit of the High Resolution C 1s scan of HOPG. The same fitting parameters was used for all three samples. Inset is an enlargement of the π-π* region.
Large area XPS was carried out on HOPG, PGA and Gilsocarbon. Elements present on the surface were found to be C and O with trace amounts of contamination from Na, Cl and N which were too low to confidently quantify and thus were ignored. HOPG was found to have higher carbon content in the surface region than the nuclear grade samples, particularly PGA, which had significantly more O present on the surface (c.f. Table 1). This O can be attributed to the presence of adventitious species on the surface possibly carbonates, oxides and adsorbed water. High resolution XP spectra for the C 1s photoelectron lines can be seen for the three graphite samples in Fig. 1. All three exhibit a single thin peak with an asymmetric tail towards higher binding energy. The asymmetry arises from the presence of electrons emitted from the C 1s line which have lost energy to the delocalised π-electrons, a characteristic feature of graphite. The peak position of the main line is 284.5 eV which is also typical of graphitic sp2 bonding [26,27] (see Fig. 2).
system also leads to a broad π-π* satellite at a binding energy of 288–294 eV, generated through a continuum of excitations that occurs as the carbon photoelectrons exit the sample, leading to small energy losses. This satellite peak structure is a signature of sp2 carbon since no other relevant photoelectron signal is expected in this binding energy region. All peaks were a standard Gaussian-Lorentzian product peak shape except for graphitic carbon which exhibits an exponential tail to high binding energy (lower kinetic energy) due to the high conductivity of sp2 carbon [30]. Careful constraint of the peak fitting was required given the close binding energy position of sp2 and sp3 carbon (0.3 eV), and indeed the overlap of carbon-oxygen bonding with the asymmetric sp2 peak, [31]. The result of the peak-fit for HOPG is shown in Fig. 3; the same fitting procedure was employed across all three graphite grades. Subsequent quantification of each component was obtained, via the area under the curve, to give 75.8% sp2, 13.8% sp3, 3.0% CO, 0.2% C]O, 0.7% C(O)2 and 6.5% π-π* for the HOPG sample. This asymmetric peak shape, implemented in CasaXPS (www. casaxps.com) using LA(0.8, 1.4, 100), where 0.8 and 1.4 are the asymmetry factors at lower and higher binding energy to the peak, was determined using the HOPG sample and found to adequately fit all datasets from all samples consistently.
3.1.1. Peak fitting The C 1s spectra were fitted with several peaks associated with different chemical species: graphitic i.e. sp2 carbon at 284.5 eV, hydrocarbon i.e. sp3, at 284.8 eV [28], CeO at ~286.0 eV, C]O at ~287.0 eV and C(O)2 at ~288 eV [29]. The presence of a π-bonding
3.1.2. Measurement of D-parameter Previous studies by Mizokawa et al. have shown the degree of sp2/ 3 sp character in the surface carbon to be linearly related to the difference, D, of the maxima and minima of the first derivative of the C KLL spectra [32]. The ‘D-parameter’, as it is known, has been explored more recently as a way of differentiating carbon states in XPS imaging [33]. The first derivative of the obtained C KLL spectra for the three graphite samples can be seen in Fig. 4. From Fig. 4, D-parameter values were measured and found to be 22.5 for HOPG and 21.0 for both Gilsocarbon and PGA, with an error of ± 1. These values indicate a highly graphitic, sp2, nature. In contrast, Mezzi and Kaciulis [34] also determined the D-parameter for diamond, which is purely sp3 in nature, and found it to be 13.7 eV. A linear relationship was found between the D-parameter and increasing sp2 content, with analysis of a SWCNT found to contain intermediate bonding between graphite (sp2) and diamond (sp3) with a D-parameter of ~17.4 eV [34]. The method of measuring a D-parameter to infer the ratio of sp2/sp3 hybridisation is useful, and removes any subjectivity associated with
Fig. 2. High Resolution C 1s scan of the as-received HOPG, Gilsocarbon and PGA samples. 3
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Table 3 Composition of surface species on HOPG as-received and after sputter etching with Ar+ and Ar2000+. In both cases ion energy was 5.0 keV and sputter time was 60 s. Atomic % HOPG
C
O
Ar
As-Received Monatomic Ar+ Cluster Ar2000+
97.1 97.8 100
2.9 0 0
0 2.2 0
Fig. 4. The first derivative of the Auger C KLL spectra for HOPG, PGA and Gilsocarbon.
traditional peak-fitting methodologies. Additionally, since this technique uses the overall width of the Auger feature, rather than its position, then it is not prone to the effects of sample charging [35]. However, the D-parameter is predominately a guide to understanding the level of graphite-like or diamond-like bonding in the carbon specimen and care should be taken when comparing absolute values, as these have been shown to vary across different spectrometers using differing parameters and acquisition conditions [35]. Nevertheless, confidence in the peak-fit can be obtained through comparing D-parameter values calculated from the sp2 and sp3 components of the fit, and those obtained through the first derivative C KLL spectra. Using the derived values obtained from the peak fitting process described above, the D-parameter can be estimated using the method outlined by Alanazi et al. [36]. This was then compared with the Dparameter obtained through the Auger analysis and the results are shown in Table 2. In order to accurately determine the D-parameter from the peak-fitting, the calculated sp2 component and the π-π* components were combined. The reason for this is that the signal from the π-π* region is generated from electrons in unhybridised p-orbitals perpendicular to the C-C bonding within the graphite plane. In aromatic, sp2, systems such as graphite, these p-orbitals are able to overlap to form a cloud of de-localised electrons above and below the plane. For this reason, the sp2 and π-π* content are intrinsically linked, hence why there is no contribution to signal from π-π* transitions observed in the spectrum for diamond [34]. As such, this should be considered when assessing the total sp2 content derived from analysis of the peak at 284.5 eV. The value of 22.5 obtained for HOPG through the C KLL analysis is in close agreement with that by Merel et al. [37] however, the values measured through the peak-fitting of the C 1s peak are slightly lower than those obtained through Auger analysis across all three samples.
Fig. 5. Evidence of argon implantation as a result of sputtering with monatomic Ar+ ions. This is not observed during sputtering with cluster, Ar+ 2000 ions.
Fig. 6. A comparison of the high-resolution C 1s spectra of HOPG before and after sputter etching with monatomic Ar+ ions and cluster ions, Ar2000+, at 5.0 keV for 60 s.
These differences are small and are likely attributed to small errors either within the measurement or the fitting parameters or due to inhomogeneous distribution of impurities between the surface and nearsurface leading to localised differences in sp2/sp3 bonding. Any such differences in the surface region of the samples will be accentuated by the difference in the information depth between the two methods, with the Auger method being even more surface sensitive; that is, the C KLL electrons are omitted from only ̴ 3.3 nm [38]. Additionally, the Dparameter is also known to be effected by differences in crystallinity and grain size, as well as the presence of oxygen impurities [38]; these factors may vary locally in the polycrystalline nuclear graphite grades leading to small discrepancies in the measurements.
Table 2 Comparison of D-parameter obtained through peak fitting of the C 1s and analysis of the C KLL spectra. D parameter Graphite
Peak-Fit
Auger Analysis
HOPG PGA Gilsocarbon
21.0 19.4 20.0
23.0 21.0 21.0
4
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Fig. 7. UPS spectra of HOPG sample, as-received and before and after etching with monatomic Ar+ and cluster Ar2000+.
occurred even at ion energies as low as 56 eV and attributed the presence of the peak at 241.5 eV to Ar ions trapped on defects on the HOPG surface or ions trapped between two graphene layers. At 5.0 keV the mean implantation depth of Ar in graphite was approximated, using the SRIM 2013 software package [40], to be 7.0 nm with a maximum depth of 17.0 nm. This implies that there will be considerable ion implantation within this interaction volume and further subsequent damage to the sub-surface through collision cascades; such processes could be avoided using a cluster source. Further evidence of Ar implantation can be seen in Fig. 6, which shows the presence of a small Ar 2s peak at ~319.5 eV [41]. Perhaps more interestingly, Fig. 6, shows the effect of the implantation on the C 1s peak. After sputter cleaning with Ar2000+ there is no observable shift in peak position and the signal becomes more intense with two distinct features emerging at 291.3 eV and 294.5 eV. Also, a slight decrease in the FWHM of the peak is seen from 0.63 eV to 0.62 eV. The observed increase in intensity combined with a decrease in FWHM is most likely due to the removal of CeO species and adventitious carbon, exposing the virgin HOPG below. After monatomic etching, the π-π* shake-up feature at ~291.0 eV is no longer present indicating loss of sp2 character. There is also a distinct broadening of the peak to both lower and high binding energy with a considerable loss in intensity. The FWHM increases from 0.62 eV to 1.5 eV. The shift to higher binding energy is likely a result of lattice deformation and a migration from sp2 to sp3 character, with the overall picture suggesting that Ar implantation has induced considerable morphological defects contributing to the re-hybridisation [42].
Table 4 The measured work function, f, for the sample asreceived and before and after etching. f (eV) As-Received 5.0 keV Ar+ 5.0 keV Ar2000+ Literature value [44]
4.85 4.74 4.55 4.6
4. Monatomic vs cluster sources Despite the determination of the elemental makeup and, in particular, the carbon bonding of the as-received graphite surfaces, it is unclear whether composition is unique to the surface or accurately represents the near-surface bulk material. To investigate this, sub-surface analysis of the graphite specimens was carried out by means of depth-profiling techniques, using both a traditional monatomic Ar+, and argon cluster ions Arn+ (in this case n = 2000). In recent times cluster ions have been used in surface analysis due to the limited damage propagated into the bulk of a material post-etching; consequently they are typically used for the analysis of soft organic/polymer surfaces [39] however relatively few studies have been reported where the method is applied to graphitic materials. Comparison of these two methods is necessary when designing future experiments for the analysis of such materials. 4.1. Surface cleaning
4.2. UPS analysis
The surface composition of the HOPG was confirmed through a large area survey scan that found 0.5% O on the surface, suggesting the presence of either CeO, C]O and/or C(O)2 species. The sample was sputter cleaned using the two methods described above and the resultant surface composition is shown in Table 3. After monatomic etching an Ar 2p peak at ~241.5 eV was visible in the survey spectra, as shown in Fig. 5, indicating that argon implantation had occurred; quantification was carried out on this peak to give an approximate concentration of 2.2 at.%. Implantation was not observed from the cluster etch, which simply removed all O revealing the pure C surface below. Ion implantation is not unexpected, however, particularly in graphitic materials where the structure is prone to intercalation. Smith et al. [37] found argon implantation in HOPG
UPS studies were also performed on the graphite samples to further compare the two etching methods. Fig. 7 shows the measured UPS spectra for the HOPG sample, before and after 60 s etching with monatomic Ar+ and cluster Ar2000+ ions. Prior to etching a sharp peak is observed at 13.7 eV and a step feature at ~3 eV, these features have previously been attributed to π-electrons and therefore associated with sp2 bonding [43]. After etching with monatomic Ar+ there is a distinct change in the shape of the spectrum. Neither of the sp2 features are present indicating complete removal of sp2 character in the top most layers of the sample. The UPS results imply that all sp2 character is removed and, when compared to a more gradual change in the XPS, the damage can be considered as being more prominent in the very top 5
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Fig. 8. Concentration profile of chemical species during monatomic Ar+ etching of Gilsocarbon and PGA Samples. C 1s is omitted for clarity.
Fig.9. Concentration profile of chemical species during cluster Ar2000+ etching of Gilsocarbon and PGA Samples. Ar 2p is included for comparison despite no signal being detectable.
layers of the sample. These results agree with previous REELS studies which showed a broadening and loss of the π-peak even at low energy etching conditions [43]. The work function, f, for the material can also be calculated by subtracting the full width of the spectrum form the energy of the light source. The results are shown in Table 4. Here we can see that there is greater agreement between the literature value and the value post cluster etch, than compared to the value post monatomic etch. The literature value of 4.6 eV, represents that of a pristine HOPG surface [44].
etch. Both methods result in a decrease in surface O and an associated increase in the % concentration of C, interestingly however, it can be seen that some O remains as the surface is etched away suggesting that it is present in the near-surface structure. This is more apparent in the PGA sample which had a higher concentration of O on the surface to begin with. As mentioned, the monatomic Ar+ etch results in a steady increase in Ar concentration, which appears to saturate at around 1.4% for both Gilsocarbon and PGA, this observation is not seen in the cluster etch despite the longer etching time required. It is also interesting to note that the amount of Ar implantation is considerably less than the 2.2% observed in HOPG under the same conditions, this implies that the highly ordered nature of the HOPG makes it more susceptible to intercalation of foreign species. The high-resolution C 1s scan obtained after depth-profiling of the Gilsocarbon specimen is shown in Fig. 10 and is peak-fitted using the methodology described in Section 3.1.1. Quantification of the peak-fitting process shows a marked change in the sp2/sp3 fraction as a result of the depth-profiling. The results are shown in Table 5 for both the Gilsocarbon and PGA specimens. The results of the quantification highlight some interesting phenomenon. Firstly, for Gilsocarbon there is a considerable increase in sp3 content accompanied by a corresponding decrease in sp2 character as a result of the depth profiling, this can be clearly seen in Fig. 10. The difference is more remarkable for the monatomic ion beam which gives
4.3. Depth-profiling Etching for longer periods can provide valuable insight into the subsurface structure. The nuclear grade Gilsocarbon and PGA samples were probed by depth profiling using both the monatomic and cluster ion sources. A scan was taken at regular intervals throughout the etch and the concentration of the surface species was monitored with time; the results of which are shown in Figs. 8 and 9. As described through Eq. (1), the etch rate using Ar cluster ions is significantly slower than with monatomic ions [45]. This is illustrated by the increased etch time before the surface concentration becomes steady-state. For monatomic ions, a steady-state is reached after ~80 s etching whereas this is not reached until > 500 s for the cluster ion 6
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by the surface sensitivity of the measurement. XPS is known to have an information depth of approximately 3λ, where λ is the mean free path of the emitted photoelectrons. The value of λ will depend on the excitation energy, but for the setup used here is known to be 2–3 nm [47], therefore giving an information depth of approximately 6–10 nm. This corresponds very well to the penetration depth of the Ar+ ions into the graphite lattice, as calculated from SRIM. From the simulation results we know that this region will be highly disordered due to the irradiation damage caused by the energetic Ar+ ions and there will be a considerable amount of displaced C atoms residing in interstitial positions. These interstitials, combined with other effects of irradiation on the atomic structure, will lead to a large increase in irregular bonding within the graphite, specifically more tetrahedral sp3 bonding [48,49]. The effect is not observed to the same extent in the case of the cluster source due to less irradiation damage and no detectable Ar implantation. Depth-profiling using a monatomic Ar+ beam also gives rise to a significant increase in the FWHM of the main peak, leading to an increase in peak asymmetry towards high-energy. The fitting parameters used in this analysis attribute this to the CeO component at ~286.0 eV. However, as explained by Estrade-Szwarckopf [50], the accurate fitting of the C 1s peak in graphitic materials is highly complicated with no general consensus found in the literature. The increase in the CeO component measured here cannot infact be due to an increase in O content, as the opposite is seen from both the intensity of the O 1s signal in the large area survey scan and in Fig. 8, where the O 1s signal of Gilsocarbon is shown to decrease from 6.7% to 1.5% as a result of the Ar+ etch. Therefore an alternative explanation could be that this peak is a superposition of some smaller CeO component and some larger component associated with, either, a localised decrease in conductivity, leading to a continuous broadening of the main peak to higher binding energy than the known sp3 position [50], or perhaps more applicable, an additional component due to intercalation of implanted Ar species between the surface layers of the graphite. Similar high energy shoulders to the C 1s line have been observed previously in ion implanted graphites [42,51] with some debate over the precise origin. Yamada et al. observed the emergence of a significant peak at 286.0 eV when samples of HOPG were treated with a diamond-dispersed solution [52] suggesting that this peak is indicative of sp3 bonding and independent of O content. Very similar trends were also observed for the PGA specimen due to its comparable microstructure/bonding to that of Gilsocarbon. 4.4. Raman spectroscopy
Fig. 10. A comparison of the high-resolution C 1s spectra of Gilsocarbon before and after sputtering with 5.0 keV monatomic Ar+ for 150 s and cluster 5.0 keV Ar2000+ cluster ions for 600 s. The same fitting parameters were also applied to the PGA graphite.
An effective method for analysing near-surface damage in graphitic systems is through Raman spectroscopy. Fig. 11 shows the Raman spectrum of the Gilsocarbon specimen etched with both monatomic Ar+ and cluster Ar2000+ ions. All three spectra exhibit the same characteristic graphite peaks i.e. the disorder-induced D band at 1350 cm−1, the graphitic G band at 1580 cm−1 and the additional disorder induced D’ band at 1620 cm−1, all of which have been detailed and reported extensively in the literature [53,54]. For graphitic material, it is known that the ratio of the intensity of the D band, with respect to the G band intensity i.e. ID/IG can be used to provide a semi-quantitative understanding of the lattice disorder within the graphite; where, as ID/IG increase, damage in the system can be said to increase leading to a decrease in the crystallite size [55]. Additionally, the prominence of the D’ shoulder feature is also attributed to microstructural damage has been attributed to the creation of C2 molecules [56]. From Fig. 10 the ID/IG ratio is measured via deconvolution of the peaks and found to be 0.27, 0.52 and 0.30 for the sample as-received, after monatomic etching and after cluster etching respectively. The information depth of Raman spectroscopy depends on the target material and also the incident wavelength. For a laser wavelength of
Table 5 Quantification of the peak-fitting of the Gilsocarbon and PGA C 1s peaks before and after depth-profiling with monatomic and cluster ions. Gilsocarbon 2
As-Received Ar+ Ar2000+
PGA 3
sp (%)
sp (%)
sp2 (%)
sp3 (%)
80.2 11.3 63.7
19.8 88.7 36.3
84.2 11.7 81.4
15.8 88.3 18.6
a dramatic increase in the sp3 content to a value as high as 88.7%. This agrees with the discussion so far and indicates a considerable destruction to the sub-surface triganol bonding structure and a move to a more diamond-like tetrahedral, sp3, structure, as reported by Jackson and Nuzzo during 3.0 keV Ar+ irradiation of HOPG [46]. The stark contrast in the change of sp2/sp3 ratio for monatomic Ar+ is likely exasperated 7
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recommended that detailed surface analysis of nuclear grade graphites be carried out using a cluster ion source. CRediT authorship contribution statement Alex Theodosiou: Conceptualization, Methodology, Validation, Investigation, Resources, Data curation, Writing - original draft, Visualization, Supervision. Ben F. Spencer: Methodology, Software, Validation, Formal analysis, Data curation, Writing - review & editing. Jonathan Counsell: Methodology, Software, Validation, Data curation, Writing - review & editing. Abbie N. Jones: Validation, Writing - review & editing, Visualization, Supervision, Funding acquisition. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was funded by the UK Engineering and Physical Sciences Research Council under Grant No. EP/R00577X/1. Access to the XPS/ UPS instrumentation was provided by Kratos Analytical Ltd, for which the authors are grateful. Additional thanks go to Jessica Higgins for help with obtaining the Raman data and Jason Peterson for the artwork.
Fig. 11. A comparison of the Raman spectra of Gilsocarbon before and after sputtering with 5.0 keV monatomic Ar+ for 150 s and cluster 5.0 keV Ar2000+ cluster ions for 600 s.
532.2 nm (E = 2.3 eV), this is known to be approximately 50 nm in graphite [51]. Therefore, it follows that the data should be carefully interpreted and account for any discrepancy between the volume of interest in the sample and the information depth. In this case, the spectra from the monatomic etched sample will exhibit a lower ID/IG ratio than expected since the large majority of the data is from the graphite bulk i.e. graphite that has been unperturbed by the Ar+ bombardment. To account for this, the observed ID/IG ratio (Robs) can be adjusted to give the ‘actual’ ID/IG ratio (R0) using a method described by Theodosiou et al. [57]. For 5.0 keV Ar+ irradiation of graphite this has been calculated to give an approximate factor of 2, that is: R0 = 2 × Robs. Therefore, when concerning the damaged region alone, R0 can be estimated as 1.04 after etching with monatomic Ar+ and therefore, since R0 ≥ 1.0, the system can be considered as considerably damaged with an associated drop in long range order [58,59]. However, this adjustment can be neglected in the case of the cluster ion etch since the penetration depth and range of energy deposition into the sample is practically negligible [60]. The calculation of R0 confirms the damage caused through monatomic Ar+ bombardment during the depth profile and agrees with the results seen from the photoelectron spectroscopy techniques.
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5. Conclusions The surface and near-surface bonding in HOPG and two nuclear graphite grades, Gilsocarbon and PGA, has been investigated through XPS, UPS and Raman spectroscopy. Analysis of the C 1s peak was shown to be complex; a peak-fitting approach was used to determine the sp2/ sp3 bonding ratio and the results obtained agreed well with those obtained through analysis of the C KLL spectra through determination of a D-parameter. Additionally, comparisons were made between monatomic Ar+ and cluster Arn+ etching techniques as methods for surface cleaning and depth-profiling. The cluster source was found to be more effective in removing surface contamination from the graphite, compared to traditional monatomic Ar+, with the additional benefit of occurring no unwanted Ar implantation. Peak-fitting of XPS spectra of nuclear grade graphite, depth profiled with monatomic Ar+, exhibited a dramatic decrease in the sp2/sp3 ratio, with an additional high-energy component at 286.0 eV in the C 1s line attributed to an effect of ion implantation. UPS and Raman data confirmed that monatomic Ar+ bombardment leads to damage to the graphite lattice which could potentially affect the results being obtained; for this reason it is 8
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