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s–p Hybridization in highly oriented pyrolytic graphite and its change on surface modification, as studied by X-ray photoelectron and Raman spectroscopies
Surface Science 504 (2002) 125–137 www.elsevier.com/locate/susc
s–p Hybridization in highly oriented pyrolytic graphite and its change on surface modification, as studied by X-ray photoelectron and Raman spectroscopies D.-Q. Yang, E. Sacher
*
D epartement de g enie Physique, Ecole Polytechnique, C.P. 6079, Succursale Centre-ville Montr eal, Qu ebec, Canada H3C 3A7 Received 31 August 2001; accepted for publication 19 November 2001
Abstract X-ray photoelectron and confocal Raman spectroscopies have been used to study the highly oriented pyrolytic graphite (HOPG) surface before and after Arþ treatment/Cu evaporation, and Cu sputter deposition. They have clarified the HOPG surface structure and the types and extents of damage on surface treatment. The dynamics of the Cu clusters evaporated onto the untreated and Arþ -treated surfaces, and those of the sputtered Cu, indicate that phenomena such as cluster dimensions and surface densities are related to the extent of Cu/surface interaction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Noble gases; Sputtering; Copper; Clusters; Graphite; Sputter deposition
1. Introduction We are interested in the electronic structures and metal interactions of polymers that undergo extensive electron delocalization. One such polymer we have investigated [1] is Dow Cyclotene, a low permittivity material of interest to the microelectronics industry. The structure of the monomer is seen in Fig. 1. It polymerizes through the thermal cleavage of the cyclobutene moieties and their subsequent Diels–Alder reaction across the vinyl groups of other monomers. Our XPS study of this polymer [1] has shown an unexpectedly extensive electron delocalization, to
* Corresponding author. Tel.: +1-514-3404787; fax: +1-5143403218.
the extent that all the carbon atoms, irrespective of their environments, have the same electron density and, thus, the same C1s binding energy. That is, the C1s XPS spectrum is represented by a single narrow component, as if only one C environment existed. Not only is this true for Cyclotene [1], it is also true for other polysiloxanes [2], indicating a previously unrecognized form of hybridization across the Si–O–Si bridge. The C1s XPS spectrum in Cyclotene is relatively unobscured, and it was possible to note [3] that Arþ treatment of the cured Cyclotene surface produced new peaks, in a region usually associated with Cheteroatom bonds. However, neither photoacoustic FTIR nor O1s, N1s, etc., XPS showed evidence that such bonds were formed. In this sense, the Cyclotene results are similar to those obtained on highly oriented pyrolytic graphite (HOPG) [4–6],
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 2 6 - 4
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Fig. 1. Cyclotene monomer.
which also manifests XPS peaks not attributable to C-heteroatom bonds. The Arþ treatment of both cyclotene [3] and HOPG [4–6] (and other materials, as well) is known to cause bond scission, producing localized states, and leading to increased disorder and the modification of both physical and chemical properties. Because the HOPG structure is simpler than that of cyclotene, and is better understood, we were prompted to investigate it, anticipating that such knowledge would aid our understanding of cyclotene. HOPG has long been used as a model for twodimensional surfaces in studies of both fundamental and practical interest [7–11]. In the past decade, techniques such as XPS, Raman spectroscopy and SPM have been used to characterize the pristine HOPG surface and its modification on Arþ treatment [12–20]. Despite the information obtained, on phenomena such as surface protrusions and hillocks [14,15,18–20] and surface disorder [4,5], several crucial questions have not yet been convincingly answered. These include the degree of surface corrugation [21], and the change in hybridization induced on surface bombardment. Concerning this last point, we have found that the literature on HOPG is unclear on s–p hybridization. Because this hybridization and its change on surface treatment are critical to our understanding of the HOPG structure and its modification, we propose to discuss it. This is done in Section 2 so that it may immediately be used in understanding our results. While it may be argued that this discussion is a resume of what is taught in undergraduate organic chemistry, it is undeniable that this understanding is absent from the present literature on HOPG. In addition, due its well-defined surface structure, HOPG has been extensively used to study the initial growth of metal thin films and nanoparticles deposited onto supported substrates [22–26]. In-
deed, the surface of the substrate plays an important role in processes such as coalescence and surface diffusion. All this requires a detailed understanding HOPG surface and the effect of low energy Arþ treatment. Here, we focus our attention on HOPG surface characterization by XPS and Raman spectroscopy, as well as the effect of the surface on initial Cu growth, as deposited by evaporation and sputtering. Our experimental techniques are given in Section 3. Our results are given in Section 4, and they are discussed in Section 5.
2. s–p Hybridization in carbon The C atom ground state is 1s2 2s2 2p1x 2p1y 2p0z . In order to explain the existence of four tetrahedrally directed bonds, all the same length, in methane (CH4 ), as well as the planar structure of ethylene (H2 C@CH2 ) and the linear structure of acetylene (HCBCH), Pauling [27] proposed that one of the 2s electrons was promoted to the previously empty 2pz orbital, to give 2s1 2p1x 2p1y 2p1z . This structure is then able to undergo various types of hybridization. In one of these, all four of the singly occupied orbitals are hybridized; this is referred to as sp3 hybridization and gives four tetrahedrally oriented bonds of equal length. These bonds, in which the electron density lies along the line connecting the atoms, are called r bonds. The 2s orbital is also capable of hybridizing with only two of the 2p orbitals, and is referred to as sp2 hybridization. This leads to three planar r bonds directed to the corners of an equilateral triangle (i.e., uniformly distributed in the plane), with the unhybridized 2p orbital perpendicular to this plane. Two adjacent p orbitals, as in ethylene, could themselves interact to form a p bond, one that is parallel to the r bond along the line connecting the two adjacent atoms, but whose electron density lies above and below that line [28]. Another type of hybridization exists, which is of no interest here. It is sp hybridization, leading to linear r bonds, as in acetylene. The two unhybridized p orbitals are mutually perpendicular to the line of the r bond. Each may overlap with a
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similar orbital on an adjacent atom to form mutually perpendicular p bonds. In the case of 1,3-butadiene (H2 C@CH–CH@ CH2 ), where two next-neighbor p bonds exist, these bonds overlap, which permits their rehybridization, lifting the sp2 hybridization of the terminal vinyl groups and extending the rehybridization across all four atoms [29, Chapter 2]. A simple extension of this, from ethylene to 1,3-butadiene to 1,3,5-hexatriene, etc., will eventually lead to the structure found in HOPG. This structural extension falls into the category of alternant hydrocarbons [30], which occurs when it is possible to divide all the C atoms into two sets, and no two atoms of the same set are connected by a bond. The properties of alternant hydrocarbons are described by certain theorems, one of the most important, to us, being the following: in a neutral alternant hydrocarbon, the p electron density at each atom is unity. With respect of HOPG, which is certainly a neutral alternant hydrocarbon, this means that the structure cannot be discussed in terms of sp2 and sp3 bonding, as too often appears in the literature. Electron delocalization is uniform, and extends over the whole structure; the hybridization lies somewhere between sp2 and sp3 , depending on the extent of delocalization [30].
3. Experimental Type ZYA HOPG was obtained from SPI, Inc. It was cleaved with adhesive tape just prior to each experiment and immediately inserted into each apparatus. XPS was carried out in a VG ESCALAB 3 Mark II, using non-monochromated Mg Ka Xrays (1253.6 eV). The base pressure in the analysis chamber was <1010 Torr. Spectral peaks were separated using an in-house non-linear least-meansquares program. Arþ treatment took place in the instrument preparation chamber at a pressure <109 Torr, using an ion energy of 2 keV and a current density of 0.5 lA/cm2 . The treated samples were immediately transferred to the analysis chamber without exposure to the atmosphere. Cu was sputtered in the preparation chamber, using the Arþ beam, which was directed against a
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high purity disk of the metal. The deposition rate /min. Cu was also evaporated, as prewas 0.7 A viously described [3], at a deposition rate of 12 /min. A Raman spectra were obtained on a Renishaw RM200 Raman microscope equipped with a 1200 lines/mm grating and a motorized three-axis XYZ mapping stage. A 514.5 nm Arþ laser, with an incident power of about 2.5 mW, served as the excitation source. Spectral calibration used the 520.5 cm1 TO peak of a Si wafer reference. Confocal spectra, from the sample surface, were accumulated in the 200–4000 cm1 range of the Stokes shift. Final spectra used an integration time of 30 s/ data point, with four coadditions to improve S=N.
4. Results 4.1. Cu evaporation onto untreated HOPG 4.1.1. XPS All peak positions and attributions to be discussed are found in Table 1. XPS showed no change in the C1s peak, at 284.6 eV, or its satellite, at 291.4 eV, seen in Fig. 2, other than an intensity diminution with Cu coverage; their intensity ratio remained constant at 43. There was no change in the Cu2p3=2 peak shape, although it shifted to
Table 1 Cls XPS components in HOPG and their attributions Peak numbersa
Binding energies (eV)
Attributions
C1
284.6
C2
285.6
C3 C4
286.5 287.8
C5 –b
283.5 291.4
Extensively delocalized aromatic sp2 bonds Localized aromatic sp2 bonds, due to damage on Arþ treatment Surface free radical sp3 bonds p p shake-up peak of C2, above –C–Cu bonds p p shake-up peak of C1, above
a b
Peaks seen in Figs. 6 and 12. Peak seen in Fig. 2.
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Fig. 2. C1s spectra of HOPG before and after Arþ treatment. Fig. 4. Comparison of Cu cluster coalescence, at a nominal , at different deposition rates. thickness of 4 A
Fig. 3. Comparison of Cu2p3=2 binding energy and average cluster size for evaporated Cu.
4.1.2. Raman spectroscopy The Raman spectrum of untreated HOPG is found in Fig. 5. As found by many others [35–39], peaks are exhibited at 1580, 2440 (asymmetric), 2690 (shoulder), 2725 and 3240 cm1 . Their attributions are given in Table 2, along with the letter symbols used [39]. These will be discussed in the following sections, when their evolutions with surface treatment are considered. On Cu deposition, no new peaks appear. The only change seen is due to an intensity attenuation with increasing Cu thickness. 4.2. Cu evaporation onto Arþ -treated HOPG
slightly lower binding energies with coverage. This peak shift is plotted against the average size of the clusters, determined from XPS peak intensity ratios [31], in Fig. 3. The agreement confirms that the shift is due to cluster size effects [32]. The cluster for nominal size stabilization at just under 100 A thicknesses above 7 A, suggestive of a critical dimension, is in good agreement with recent AFM measurement [33]. Cu clusters on untreated HOPG are highly mobile [34], coalescing quickly even at room temperature. As seen in Fig. 4, the average cluster size ultimately attained is influenced by the rate of deposition.
4.2.1. XPS As seen in Fig. 2, Arþ treatment quickly reduced the intensity of the p p shake-up near 291 eV and produced an asymmetric broadening at the high binding energy side in the existing C1s peak (Fig. 2), which is consistent with previously published results [5,40]. This broadened peak was separated into four components, three of them new. The components are found in Fig. 6 and their evolution with treatment time, in Fig. 7. As seen from the evolution, the components at 285.6 and 288.0 eV increase as the components at 284.6 and 291.4 eV decrease, while the component at 286.5 eV remains unchanged.
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Fig. 6. Components of the C1s spectrum after 10 min of Arþ treatment. The shake-up peak at 291.4 eV is omitted.
Fig. 5. Raman spectra of untreated and Arþ treated HOPG: (a) 0–2000 cm1 and (b) 2000–4000 cm1 . Table 2 Raman peak positions in HOPG and their designations Peaks position (cm1 )a 1327 (shoulder) 1367 1580 1620 (shoulder) 2439 2458 2690 2734 2950 3245 a
Untreated-HOPG
G T þ D1 T þ D2 2D1 2D2
Fig. 7. C1s component evolution as a function of Arþ treatment.
Arþ -treated HOPG D1 D2 G D0 T þ D1 T þ D2 2D1 2D2 DþG 2D0
The peak attributions [39] are discussed in the text.
The area ratio of the peak at 284.6 eV to that at 291.4 eV remains constant with evolution, at 43.
Similarly, the area ratio of the peak at 285.6 eV to that at 288.0 eV remains constant, at 7. This indicates that Arþ treatment reduces the extent of electron delocalization by disruption of the HOPG structure: as the original peak (284.6 eV) and its p p satellite (291.4 eV) decrease on surface treatment, a new peak, representative of the new structure (285.6 eV) and its p p satellite (288.0 eV), replace them. The direct correlation of this disappearance/appearance is seen in Fig. 8, where the intensities of the peaks at 284.6 and 291.4 eV are plotted against those at 285.6 and 288.0 eV. The plot is highly linear, with a correlation coefficient of 0.993 and a slope of 1.1. While the peak
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Fig. 8. Correlation of the disappearance of the peaks at 284.6 and 291.4 eV with the appearance of those at 285.6 and 288.0 eV.
at 286.5 eV appears to be uncorrelated, it is not an artifact, having been seen before [5]. Its source will be discussed in Section 5.1.1. The full widths at half maxima (FWHM) of all components increased rapidly during the initial stage of Arþ irradiation and then stabilized to 1.4 eV, as shown in Fig. 9. Further, angle-resolved XPS determinations of C1:C2 peak area ratios (Fig. 10) show significant changes on Arþ treatment, demonstrating that the C2 contribution is surface related.
Fig. 9. C1s FWHM values as a function of Arþ treatment.
Fig. 10. C2:C1 XPS peak area ratios as a function of take-off angle for untreated and Arþ -treated HOPG.
The deposition of Cu causes no change in the C1s spectrum shape. There are, however, changes in the Cu2p3=2 and Cu3d peak positions and widths with nominal Cu thickness, indicating cluster growth. They are consistent with what has previously been found for this system [41–44]. 4.2.2. Raman spectroscopy As seen in Fig. 5, Arþ treatment causes significant changes in several Raman peaks. These peaks have received letter designations in the literature [39]. New peaks appear at 1327 (shoulder, D1 ), 1367 (D2 ) and 1620 (shoulder, D0 ) cm1 , while the doublet at 2690 (shoulder, 2D1 ) and 2734 (2D2 ) cm1 decreases. The D modes are believed to originate from the onset of disorder [35,36] and the 2D modes are their harmonics [39,45]. The Ramanactive E2g mode [46] at 1581 cm1 is called the G mode. Thus, Arþ treatment is expected to cause a decrease in the G mode intensity and increases in all the D mode intensities. In fact, this is not what is observed. As seen in Fig. 11, the G mode decreases abruptly over the first minute, before leveling out, while the D2 mode increases, and the 2D2 mode decreases, in the same fashion. Certainly, neither the decrease in the 2D2 mode nor the constancy of all the peak intensities after one minute of treatment is expected. The normalization of the D2 and 2D2 modes to the G mode give similar evolutionary results. A com-
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Fig. 11. Changes in the D2 , 2D2 and G Raman modes on Arþ treatment.
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Fig. 12. Components of the C1s spectrum after the deposition of of sputtered Cu. The shake-up peak at 291.4 eV is omitted. 4A
parison of Fig. 7, which contains the XPS results on Arþ treatment, with Fig. 11, shows the same abrupt changes over the first minute of treatment before constant intensities are reached, despite further treatment. As with Cu deposition on untreated HOPG, the only changes observed in the Raman spectrum are those due to attenuation by the depositing Cu. Thus, while Arþ treatment of HOPG reduces the coalescence kinetics of Cu clusters deposited by evaporation [34], this is not due to the formation or modification of chemical bonds between the Cu and the substrate. 4.3. Cu sputtering onto untreated HOPG 4.3.1. XPS As in the case of Arþ treatment, the energetic sputtered Cu atoms produced peaks at 285.6/288.0 and 286.5 eV as the peaks at 284.6/291.4 eV decreased. In addition, as seen in Fig. 12, a new peak (C5), at 283.5 eV, indicating Cu–C bond formation [47], is found. Their evolution with time is found in Fig. 13, and those of the FWHM values of Cu2p3=2 and C1s are found in Fig. 14. The similarities of Figs. 6 and 12, Figs. 7 and 13, and Figs. 9 and 14, are obvious. The loss of intensity at 284.6/291.4 eV while 285.6/288.0 and 283.5 eV increase, during which the component at 286.5 eV, once formed, does not change in intensity. The 284.6:291.4 and 285.6:288.0 eV ratios remain constant as before.
Fig. 13. C1s component evolution as a function of sputtered Cu deposition.
Fig. 13 shows that Cu–C formation is essentially complete at a nominal Cu thickness of 2.5 , equivalent to one monolayer. A comparison of A peak intensity ratios with those on Arþ treatment shows that the peak intensity at 283.5 eV is due to essentially equal decreases in the (284:6 þ 291:4) and (285:6 þ 288:0) eV intensities. Thus, while evaporated Cu does not react with Arþ -treated HOPG, sputtered Cu reacts with untreated HOPG while producing surface damage similar to Arþ treatment. The reason for this may be found in a stopping and range of ions in matter (SRIM) simulation. 1 Under our sputtering conditions, 1
http://www.srim.org.
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Fig. 14. C1s and Cu2p3=2 FWHM values as a function of sputtered Cu deposition.
there is a maximum in the number of sputtered atoms, as a function of their energy, at 4 eV [48– 52]. The SRIM calculation, seen in Fig. 15, shows that the percent energy loss of sputtered Cu in the HOPG, as ionization, is maximized in this range. 4.3.2. Raman spectroscopy As in the case of Arþ treatment, the G mode of sputdecreases before leveling out at 2.5 A tered Cu, the 2D1;2 doublet decreases while the D1;2 doublet and D0 increase. Representative spectra are seen in Fig. 16. The small broad peak near
Fig. 16. Raman spectra of untreated HOPG before and after of sputtered Cu deposition. 4A
2950 cm1 , which appears on Cu deposition, is the D1;2 þ G mode, and was also seen for Arþ -treated HOPG in Fig. 5b. No new peaks appear which are attributable to Cu–C formation: according to Maslowsky the Cu–C stretching vibration is below 650 cm1 , in a region easily accessible to the present study [53]. The absence of such bonds may be due to a lack of instrument sensitivity or to Raman inactivity. It is important to note that no other Raman study of ion implantation showed evidence of M–C bonds.
5. Discussion 5.1. Attributions of surface modifications
Fig. 15. SRIM simulation of the percent energy of sputtered Cu lost to ionization.
5.1.1. XPS No change is found in the C1s peak shape on Cu evaporation, indicating little or no surface modification. This is verified in our Raman studies, where no changes are detected. Arþ treatment and Cu sputtering both damage the HOPG surface and produce identical peaks at binding energies above the main C1s peak. As the main peak/satellite pair, at 284.6/291.4 eV, decreases in intensity, a secondary peak/satellite pair, at 285.6/288.0 eV, increase in intensity. As seen in Fig. 8, there is 1:1 correspondence between the main peak/satellite decrease and secondary peak/ satellite increase. The presence of shake-up satel-
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lites in both cases indicates them to be aromatic. Their correspondence indicates that the loss of a structure with a capacity to extensively delocalize electrons, through surface damage by Arþ treatment and Cu sputtering, leads to a structure with a more limited capacity to delocalize electrons, represented by the secondary pair. In addition, a single peak, at 286.5 eV, appears on initial treatment and does not change in intensity; this peak had previously [5] been suggested to be the satellite of the peak at 285.6 eV. The absence of an associated shake-up shows it is not aromatic, and has no ability to delocalize electron density by that mechanism. That is, while both the primary and secondary peaks indicate, respectively, the existence of more extensive and less extensive alternant hydrocarbon structures (Section 2), the peak at 286.5 eV represents sp3 hybridization (Section 2). A possible source of this peak has recently been identified [54a]. Using a slightly lower grade (i.e., more surface imperfections) of HOPG than that used here, but prepared in the same manner, this study employed TOFSIMS to reveal a spectrum dominated by Cx Hþ y positive ions and the total absence of Cþ 6n (n ¼ 1; 2 . . .). The presence of Cx Hþ y was attributed to hydrocarbon adsorption from the atmosphere on the freshly cleaned surface in the moment before entry into the apparatus and the absence of Cþ 6n , to a low sputter yield [54b]. In the present case, it may be that the free radicals created at the onset of surface damage, by Arþ and Cu sputtering, permit reaction with background hydrocarbons. Its presence in untreated HOPG is due to the existence of free radical surface defects, and is also consistent with our ARXPS results in Fig. 10. Since the new species form under treatment conditions, and on a surface structure, which do not favor heterolytic scission, they must form by homolytic scission. That is, they must be free radicals. One may inquire whether free radicals may have binding energies that differ from that of the primary peak by 1.0 and 1.9 eV, and whether this applies to both aromatic and aliphatic free radicals. The answers to both questions are in the affirmative: the study of radical ionization potentials has a long history [29,55–60]. From these experiments, it is clear that both aliphatic and ar-
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omatic free radicals may exist with ionization potentials (binding energies) which differ among themselves by as much as 3 eV, and are substantially higher in binding energy than their saturated equivalents. This occurs in the following way: the electroncontaining frontier orbital of a free radical is a singly occupied molecular orbital (SOMO). In the case of a free radical created in an alternant hydrocarbon, such as HOPG, the SOMO lies closer to the highest occupied molecular orbital (HOMO) than to the lowest unoccupied molecular orbital (LUMO). SOMO–HOMO interactions in free radicals give rise to electrophilic properties; the higher the ionization potential, the lower the energy of the SOMO [29, Chapter 5] and the greater the interaction. Comparing Figs. 9 and 14, it is seen that the FWHM of C1s abruptly increases with both Arþ treatment time and deposited Cu thickness, which is consistent with the results of Marcus et al. [44] for Cu sputter deposition on HOPG. The core level line widths generally increase with decreasing cluster size or atomic coordination number. The interpretation of this effect is still somewhat controversial [61]: some attribute it to cluster inhomogeneity [62,63] and others, to lifetime broadening or reduced screening [64,65]. The effects appear to be strongly related to the mean coordination number (i.e., the number of identical nearest neighbors) [66,67]. This is because there is no chemical bond formation during sputtering in our case, although aromatic –C–C– bonds are broken. The bond breaking decreases the mean coordination number of surface carbon atoms in the HOPG structure and increases the localized states. In Fig. 7, the most important change in the surface structure by XPS is the decrease in the C1 peak at 284.6 eV and the corresponding increase in the C2 peak at 285.6 eV, due to bond breaking on Arþ treatment. Evidence of such damage is found in the 3 nm hillocks observed by An et al., using STM [15]. It is interesting to note that there is a small C2 component part on the fresh HOPG surface; these may well be defects existing on terrace steps, consistent with our ARXPS data in Fig. 10.
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Some publications [4–6] have proposed that the C2 peak is due to sp3 hybrids, in concentrations as high as 20% of the sp2 hybrid concentrations. However, neither HREELS [68] nor AES [69] found sp3 carbons on Ar-treated HOPG. Indeed they were only found on Hþ - and Cþ -treated HOPG surfaces. The FWHM changes found for Cu2p3=2 on Cu sputter deposition are known to be due to initialstate and final-state effects, associated with the Cu cluster size [3,24,25,32,67]. We consider these effects in a separate paper [67]. 5.1.2. Raman spectroscopy As noted in Section 4.2.2 and Table 2, the Raman peaks have been given identifying letters. The doublets at 1327/1367 (D1 , D2 ) and 2690/2734 (2D1 , 2D2 ) and the peak at 1620 (D0 ) cm1 are thought to be disorder induced [35–39,45,46]. The doublet at 2690/2734 cm1 is identified as an overtone of that at 1327/1367 cm1 , and the peak at 3245 cm1 , as an overtone of that at 1620 cm1 . However, as Fig. 5a and b, and Table 2 make clear, all the overtones (2D1 , 2D2 , 2D0 ) exist before the onset of disorder, whereas D1 , D2 and D0 do not, and so cannot be overtones. Further, as Fig. 11 makes clear, the behaviors of 2D2 and G, at 1581 cm1 (as well as 2D1 and 2D0 , not shown) mimic the behavior of the primary XPS peak, at 284.6 eV, as a function of treatment. On the other hand, D2 (as well as D1 and D0 , not shown) mimics the behavior of the secondary XPS peak, at 285.6 eV. Thus, the G, 2D1 , 2D2 and 2D0 peaks are identified as representing phonon modes associated with the large alternant hydrocarbon of the original HOPG structure, while the D1 , D2 and D0 peaks represent phonon modes associated with the disordered structure obtained on Arþ treatment and Cu sputtering. The time-dependent Raman intensity data are also consistent with the measurements of Nakamura and Kitajima [12,13]. They attributed the initial steep rise of the D:G peak intensity ratio to the reduction of the in-plane phonon correlation length and the formation of a quasi-two-dimension structure. The similarity of the time-dependent XPS and Raman results may suggest that the surface damage steady state induced by the Arþ
Table 3 Arþ irradiation damage depth, and XPS and Raman measurement depths in HOPG Arþ damage depth (nm)a
XPSb
Ramanc
0:8 1:5
2.1
40
a
From [12b], using 3 keV Arþ . b C 1s mean free path (nm). c 514.5 nm-radiation optical skin depth (nm).
treatment is achieved relatively quickly (on the order of 60 s). However, it should be noted that XPS and Raman have different probe depths, found in Table 3. We note that the Arþ damage depth is about the same as the C1s photoelectron mean free path, which is over an order of magnitude less than the Raman radiation optical skin depth. 5.2. Cluster dynamics and adhesion Copper deposits onto many substrates in the form of clusters [3,34,70–78]. Because the interaction between Cu clusters and HOPG is very weak, whole clusters move laterally across the substrate surface and coalesce [34,70,74–76]. The rate of coalescence is related to the interaction between the cluster and the surface [34,70,74–76] and, thus, is a measure of interfacial interaction [34,79,80]. For example, treating a Cyclotene surface with a N2 plasma introduces –NH2 groups capable of interacting with evaporated Cu [79]; this causes the cluster coalescence rate to drop to zero [34,79], giving the strongest interaction measured for any Cyclotene surface treatment [79,80]. We have developed a method [31] that uses XPS intensity ratios to evaluate the average cluster diameter, d, and its surface density, n. When this evaluation is initiated immediately after deposition, without exposure to atmosphere, both d and n may be determined as a function of nominal film thickness, as in Fig. 3. Following the intensity changes as a function of time, as in Fig. 4, permits the evaluation of the process kinetics. The increase in d, as a function of film thickness, rises quickly before leveling out [79], as seen for HOPG in Fig. 3. Similar behavior is found for plots of the Auger parameter ½EAuger ðCuL3 M4;5 M4;5 Þ þ Ebinding
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ðCu2p3=2 Þ , the Cu2p3=2 binding energy shift (DEbinding ) and its FWHM [67]. By extrapolating the linear portions of the curves, one may, from their intersection, determine the Cu film thickness at that point. For any given system (same Cu deposition technique, substrate and treatment), the intersection thickness is the same, within experimental error, no matter which type of plot is used. Our method [31] permits us to evaluate d and the average distance between clusters (l ¼ n1=2 ) at this intersection point, where we find that, within experimental error, d ¼ l; that is, the intersection marks the contact between clusters. The only exception to this is apparent rather than real. It occurs for clusters where d is larger than the probe for Cu. depth of the emitted electrons, 30 A Thus, while d, determined from an intensity ratio, is independent of cluster size, the surface density, n, determined from the amount of Cu deposited, is not and, as a result, is overestimated. This intersection is also found in coalescence kinetics. One often attempts to demonstrate the linearity of a log–log plot of d vs. t [34,75]. This is because the theory of coarsening through dynamic coalescence [56], based on von Smoluchowski’s equation [81], which describes the change in the cluster density on binary encounters of diffusing clusters, predicts that d / ta , where a ¼ 0:25. As we recently demonstrated [34], this plot also
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manifests a deviation from the straight line at d ¼ l, the average distance between clusters, with another when the coalescence terminates. Our present data on cluster size as a function of nominal Cu thickness, summarized in Fig. 17, show that lower d (and higher n) correlate with treatments known to give higher Cu adhesion [79,80]. That is, the greater the Cu/surface interaction, the lower the propensity for Cu surface diffusion and growth.
6. Conclusions Both XPS and confocal Raman spectroscopy have been used to study the HOPG surface before and after Arþ treatment/Cu evaporation, and Cu sputtering. Surface treatment breaks bonds in this alternant hydrocarbon, producing free radicals, thereby reducing the capacity to delocalize electrons. These are reflected in consistencies in the X-ray photoelectron and Raman spectra. Direct correlations between them help to clarify the sources of the Raman peaks. The dynamics of the Cu clusters deposited by evaporation and sputtering have been shown to depend on the surface treatment, with lower cluster diameter and higher surface density being associated with stronger interfacial interaction.
Acknowledgements The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for funding, Don Frye for furnishing the HOPG and Craig Hyett for help with the Raman spectra.
References
Fig. 17. Summary of cluster size data.
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s–p Hybridization in highly oriented pyrolytic graphite and its change on surface modification, as studied by X-ray photoelectron and Raman spectroscopies D.-Q. Yang, E. Sacher
*
D epartement de g enie Physique, Ecole Polytechnique, C.P. 6079, Succursale Centre-ville Montr eal, Qu ebec, Canada H3C 3A7 Received 31 August 2001; accepted for publication 19 November 2001
Abstract X-ray photoelectron and confocal Raman spectroscopies have been used to study the highly oriented pyrolytic graphite (HOPG) surface before and after Arþ treatment/Cu evaporation, and Cu sputter deposition. They have clarified the HOPG surface structure and the types and extents of damage on surface treatment. The dynamics of the Cu clusters evaporated onto the untreated and Arþ -treated surfaces, and those of the sputtered Cu, indicate that phenomena such as cluster dimensions and surface densities are related to the extent of Cu/surface interaction. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Noble gases; Sputtering; Copper; Clusters; Graphite; Sputter deposition
1. Introduction We are interested in the electronic structures and metal interactions of polymers that undergo extensive electron delocalization. One such polymer we have investigated [1] is Dow Cyclotene, a low permittivity material of interest to the microelectronics industry. The structure of the monomer is seen in Fig. 1. It polymerizes through the thermal cleavage of the cyclobutene moieties and their subsequent Diels–Alder reaction across the vinyl groups of other monomers. Our XPS study of this polymer [1] has shown an unexpectedly extensive electron delocalization, to
* Corresponding author. Tel.: +1-514-3404787; fax: +1-5143403218.
the extent that all the carbon atoms, irrespective of their environments, have the same electron density and, thus, the same C1s binding energy. That is, the C1s XPS spectrum is represented by a single narrow component, as if only one C environment existed. Not only is this true for Cyclotene [1], it is also true for other polysiloxanes [2], indicating a previously unrecognized form of hybridization across the Si–O–Si bridge. The C1s XPS spectrum in Cyclotene is relatively unobscured, and it was possible to note [3] that Arþ treatment of the cured Cyclotene surface produced new peaks, in a region usually associated with Cheteroatom bonds. However, neither photoacoustic FTIR nor O1s, N1s, etc., XPS showed evidence that such bonds were formed. In this sense, the Cyclotene results are similar to those obtained on highly oriented pyrolytic graphite (HOPG) [4–6],
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 9 2 6 - 4
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Fig. 1. Cyclotene monomer.
which also manifests XPS peaks not attributable to C-heteroatom bonds. The Arþ treatment of both cyclotene [3] and HOPG [4–6] (and other materials, as well) is known to cause bond scission, producing localized states, and leading to increased disorder and the modification of both physical and chemical properties. Because the HOPG structure is simpler than that of cyclotene, and is better understood, we were prompted to investigate it, anticipating that such knowledge would aid our understanding of cyclotene. HOPG has long been used as a model for twodimensional surfaces in studies of both fundamental and practical interest [7–11]. In the past decade, techniques such as XPS, Raman spectroscopy and SPM have been used to characterize the pristine HOPG surface and its modification on Arþ treatment [12–20]. Despite the information obtained, on phenomena such as surface protrusions and hillocks [14,15,18–20] and surface disorder [4,5], several crucial questions have not yet been convincingly answered. These include the degree of surface corrugation [21], and the change in hybridization induced on surface bombardment. Concerning this last point, we have found that the literature on HOPG is unclear on s–p hybridization. Because this hybridization and its change on surface treatment are critical to our understanding of the HOPG structure and its modification, we propose to discuss it. This is done in Section 2 so that it may immediately be used in understanding our results. While it may be argued that this discussion is a resume of what is taught in undergraduate organic chemistry, it is undeniable that this understanding is absent from the present literature on HOPG. In addition, due its well-defined surface structure, HOPG has been extensively used to study the initial growth of metal thin films and nanoparticles deposited onto supported substrates [22–26]. In-
deed, the surface of the substrate plays an important role in processes such as coalescence and surface diffusion. All this requires a detailed understanding HOPG surface and the effect of low energy Arþ treatment. Here, we focus our attention on HOPG surface characterization by XPS and Raman spectroscopy, as well as the effect of the surface on initial Cu growth, as deposited by evaporation and sputtering. Our experimental techniques are given in Section 3. Our results are given in Section 4, and they are discussed in Section 5.
2. s–p Hybridization in carbon The C atom ground state is 1s2 2s2 2p1x 2p1y 2p0z . In order to explain the existence of four tetrahedrally directed bonds, all the same length, in methane (CH4 ), as well as the planar structure of ethylene (H2 C@CH2 ) and the linear structure of acetylene (HCBCH), Pauling [27] proposed that one of the 2s electrons was promoted to the previously empty 2pz orbital, to give 2s1 2p1x 2p1y 2p1z . This structure is then able to undergo various types of hybridization. In one of these, all four of the singly occupied orbitals are hybridized; this is referred to as sp3 hybridization and gives four tetrahedrally oriented bonds of equal length. These bonds, in which the electron density lies along the line connecting the atoms, are called r bonds. The 2s orbital is also capable of hybridizing with only two of the 2p orbitals, and is referred to as sp2 hybridization. This leads to three planar r bonds directed to the corners of an equilateral triangle (i.e., uniformly distributed in the plane), with the unhybridized 2p orbital perpendicular to this plane. Two adjacent p orbitals, as in ethylene, could themselves interact to form a p bond, one that is parallel to the r bond along the line connecting the two adjacent atoms, but whose electron density lies above and below that line [28]. Another type of hybridization exists, which is of no interest here. It is sp hybridization, leading to linear r bonds, as in acetylene. The two unhybridized p orbitals are mutually perpendicular to the line of the r bond. Each may overlap with a
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similar orbital on an adjacent atom to form mutually perpendicular p bonds. In the case of 1,3-butadiene (H2 C@CH–CH@ CH2 ), where two next-neighbor p bonds exist, these bonds overlap, which permits their rehybridization, lifting the sp2 hybridization of the terminal vinyl groups and extending the rehybridization across all four atoms [29, Chapter 2]. A simple extension of this, from ethylene to 1,3-butadiene to 1,3,5-hexatriene, etc., will eventually lead to the structure found in HOPG. This structural extension falls into the category of alternant hydrocarbons [30], which occurs when it is possible to divide all the C atoms into two sets, and no two atoms of the same set are connected by a bond. The properties of alternant hydrocarbons are described by certain theorems, one of the most important, to us, being the following: in a neutral alternant hydrocarbon, the p electron density at each atom is unity. With respect of HOPG, which is certainly a neutral alternant hydrocarbon, this means that the structure cannot be discussed in terms of sp2 and sp3 bonding, as too often appears in the literature. Electron delocalization is uniform, and extends over the whole structure; the hybridization lies somewhere between sp2 and sp3 , depending on the extent of delocalization [30].
3. Experimental Type ZYA HOPG was obtained from SPI, Inc. It was cleaved with adhesive tape just prior to each experiment and immediately inserted into each apparatus. XPS was carried out in a VG ESCALAB 3 Mark II, using non-monochromated Mg Ka Xrays (1253.6 eV). The base pressure in the analysis chamber was <1010 Torr. Spectral peaks were separated using an in-house non-linear least-meansquares program. Arþ treatment took place in the instrument preparation chamber at a pressure <109 Torr, using an ion energy of 2 keV and a current density of 0.5 lA/cm2 . The treated samples were immediately transferred to the analysis chamber without exposure to the atmosphere. Cu was sputtered in the preparation chamber, using the Arþ beam, which was directed against a
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high purity disk of the metal. The deposition rate /min. Cu was also evaporated, as prewas 0.7 A viously described [3], at a deposition rate of 12 /min. A Raman spectra were obtained on a Renishaw RM200 Raman microscope equipped with a 1200 lines/mm grating and a motorized three-axis XYZ mapping stage. A 514.5 nm Arþ laser, with an incident power of about 2.5 mW, served as the excitation source. Spectral calibration used the 520.5 cm1 TO peak of a Si wafer reference. Confocal spectra, from the sample surface, were accumulated in the 200–4000 cm1 range of the Stokes shift. Final spectra used an integration time of 30 s/ data point, with four coadditions to improve S=N.
4. Results 4.1. Cu evaporation onto untreated HOPG 4.1.1. XPS All peak positions and attributions to be discussed are found in Table 1. XPS showed no change in the C1s peak, at 284.6 eV, or its satellite, at 291.4 eV, seen in Fig. 2, other than an intensity diminution with Cu coverage; their intensity ratio remained constant at 43. There was no change in the Cu2p3=2 peak shape, although it shifted to
Table 1 Cls XPS components in HOPG and their attributions Peak numbersa
Binding energies (eV)
Attributions
C1
284.6
C2
285.6
C3 C4
286.5 287.8
C5 –b
283.5 291.4
Extensively delocalized aromatic sp2 bonds Localized aromatic sp2 bonds, due to damage on Arþ treatment Surface free radical sp3 bonds p p shake-up peak of C2, above –C–Cu bonds p p shake-up peak of C1, above
a b
Peaks seen in Figs. 6 and 12. Peak seen in Fig. 2.
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Fig. 2. C1s spectra of HOPG before and after Arþ treatment. Fig. 4. Comparison of Cu cluster coalescence, at a nominal , at different deposition rates. thickness of 4 A
Fig. 3. Comparison of Cu2p3=2 binding energy and average cluster size for evaporated Cu.
4.1.2. Raman spectroscopy The Raman spectrum of untreated HOPG is found in Fig. 5. As found by many others [35–39], peaks are exhibited at 1580, 2440 (asymmetric), 2690 (shoulder), 2725 and 3240 cm1 . Their attributions are given in Table 2, along with the letter symbols used [39]. These will be discussed in the following sections, when their evolutions with surface treatment are considered. On Cu deposition, no new peaks appear. The only change seen is due to an intensity attenuation with increasing Cu thickness. 4.2. Cu evaporation onto Arþ -treated HOPG
slightly lower binding energies with coverage. This peak shift is plotted against the average size of the clusters, determined from XPS peak intensity ratios [31], in Fig. 3. The agreement confirms that the shift is due to cluster size effects [32]. The cluster for nominal size stabilization at just under 100 A thicknesses above 7 A, suggestive of a critical dimension, is in good agreement with recent AFM measurement [33]. Cu clusters on untreated HOPG are highly mobile [34], coalescing quickly even at room temperature. As seen in Fig. 4, the average cluster size ultimately attained is influenced by the rate of deposition.
4.2.1. XPS As seen in Fig. 2, Arþ treatment quickly reduced the intensity of the p p shake-up near 291 eV and produced an asymmetric broadening at the high binding energy side in the existing C1s peak (Fig. 2), which is consistent with previously published results [5,40]. This broadened peak was separated into four components, three of them new. The components are found in Fig. 6 and their evolution with treatment time, in Fig. 7. As seen from the evolution, the components at 285.6 and 288.0 eV increase as the components at 284.6 and 291.4 eV decrease, while the component at 286.5 eV remains unchanged.
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Fig. 6. Components of the C1s spectrum after 10 min of Arþ treatment. The shake-up peak at 291.4 eV is omitted.
Fig. 5. Raman spectra of untreated and Arþ treated HOPG: (a) 0–2000 cm1 and (b) 2000–4000 cm1 . Table 2 Raman peak positions in HOPG and their designations Peaks position (cm1 )a 1327 (shoulder) 1367 1580 1620 (shoulder) 2439 2458 2690 2734 2950 3245 a
Untreated-HOPG
G T þ D1 T þ D2 2D1 2D2
Fig. 7. C1s component evolution as a function of Arþ treatment.
Arþ -treated HOPG D1 D2 G D0 T þ D1 T þ D2 2D1 2D2 DþG 2D0
The peak attributions [39] are discussed in the text.
The area ratio of the peak at 284.6 eV to that at 291.4 eV remains constant with evolution, at 43.
Similarly, the area ratio of the peak at 285.6 eV to that at 288.0 eV remains constant, at 7. This indicates that Arþ treatment reduces the extent of electron delocalization by disruption of the HOPG structure: as the original peak (284.6 eV) and its p p satellite (291.4 eV) decrease on surface treatment, a new peak, representative of the new structure (285.6 eV) and its p p satellite (288.0 eV), replace them. The direct correlation of this disappearance/appearance is seen in Fig. 8, where the intensities of the peaks at 284.6 and 291.4 eV are plotted against those at 285.6 and 288.0 eV. The plot is highly linear, with a correlation coefficient of 0.993 and a slope of 1.1. While the peak
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Fig. 8. Correlation of the disappearance of the peaks at 284.6 and 291.4 eV with the appearance of those at 285.6 and 288.0 eV.
at 286.5 eV appears to be uncorrelated, it is not an artifact, having been seen before [5]. Its source will be discussed in Section 5.1.1. The full widths at half maxima (FWHM) of all components increased rapidly during the initial stage of Arþ irradiation and then stabilized to 1.4 eV, as shown in Fig. 9. Further, angle-resolved XPS determinations of C1:C2 peak area ratios (Fig. 10) show significant changes on Arþ treatment, demonstrating that the C2 contribution is surface related.
Fig. 9. C1s FWHM values as a function of Arþ treatment.
Fig. 10. C2:C1 XPS peak area ratios as a function of take-off angle for untreated and Arþ -treated HOPG.
The deposition of Cu causes no change in the C1s spectrum shape. There are, however, changes in the Cu2p3=2 and Cu3d peak positions and widths with nominal Cu thickness, indicating cluster growth. They are consistent with what has previously been found for this system [41–44]. 4.2.2. Raman spectroscopy As seen in Fig. 5, Arþ treatment causes significant changes in several Raman peaks. These peaks have received letter designations in the literature [39]. New peaks appear at 1327 (shoulder, D1 ), 1367 (D2 ) and 1620 (shoulder, D0 ) cm1 , while the doublet at 2690 (shoulder, 2D1 ) and 2734 (2D2 ) cm1 decreases. The D modes are believed to originate from the onset of disorder [35,36] and the 2D modes are their harmonics [39,45]. The Ramanactive E2g mode [46] at 1581 cm1 is called the G mode. Thus, Arþ treatment is expected to cause a decrease in the G mode intensity and increases in all the D mode intensities. In fact, this is not what is observed. As seen in Fig. 11, the G mode decreases abruptly over the first minute, before leveling out, while the D2 mode increases, and the 2D2 mode decreases, in the same fashion. Certainly, neither the decrease in the 2D2 mode nor the constancy of all the peak intensities after one minute of treatment is expected. The normalization of the D2 and 2D2 modes to the G mode give similar evolutionary results. A com-
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Fig. 11. Changes in the D2 , 2D2 and G Raman modes on Arþ treatment.
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Fig. 12. Components of the C1s spectrum after the deposition of of sputtered Cu. The shake-up peak at 291.4 eV is omitted. 4A
parison of Fig. 7, which contains the XPS results on Arþ treatment, with Fig. 11, shows the same abrupt changes over the first minute of treatment before constant intensities are reached, despite further treatment. As with Cu deposition on untreated HOPG, the only changes observed in the Raman spectrum are those due to attenuation by the depositing Cu. Thus, while Arþ treatment of HOPG reduces the coalescence kinetics of Cu clusters deposited by evaporation [34], this is not due to the formation or modification of chemical bonds between the Cu and the substrate. 4.3. Cu sputtering onto untreated HOPG 4.3.1. XPS As in the case of Arþ treatment, the energetic sputtered Cu atoms produced peaks at 285.6/288.0 and 286.5 eV as the peaks at 284.6/291.4 eV decreased. In addition, as seen in Fig. 12, a new peak (C5), at 283.5 eV, indicating Cu–C bond formation [47], is found. Their evolution with time is found in Fig. 13, and those of the FWHM values of Cu2p3=2 and C1s are found in Fig. 14. The similarities of Figs. 6 and 12, Figs. 7 and 13, and Figs. 9 and 14, are obvious. The loss of intensity at 284.6/291.4 eV while 285.6/288.0 and 283.5 eV increase, during which the component at 286.5 eV, once formed, does not change in intensity. The 284.6:291.4 and 285.6:288.0 eV ratios remain constant as before.
Fig. 13. C1s component evolution as a function of sputtered Cu deposition.
Fig. 13 shows that Cu–C formation is essentially complete at a nominal Cu thickness of 2.5 , equivalent to one monolayer. A comparison of A peak intensity ratios with those on Arþ treatment shows that the peak intensity at 283.5 eV is due to essentially equal decreases in the (284:6 þ 291:4) and (285:6 þ 288:0) eV intensities. Thus, while evaporated Cu does not react with Arþ -treated HOPG, sputtered Cu reacts with untreated HOPG while producing surface damage similar to Arþ treatment. The reason for this may be found in a stopping and range of ions in matter (SRIM) simulation. 1 Under our sputtering conditions, 1
http://www.srim.org.
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Fig. 14. C1s and Cu2p3=2 FWHM values as a function of sputtered Cu deposition.
there is a maximum in the number of sputtered atoms, as a function of their energy, at 4 eV [48– 52]. The SRIM calculation, seen in Fig. 15, shows that the percent energy loss of sputtered Cu in the HOPG, as ionization, is maximized in this range. 4.3.2. Raman spectroscopy As in the case of Arþ treatment, the G mode of sputdecreases before leveling out at 2.5 A tered Cu, the 2D1;2 doublet decreases while the D1;2 doublet and D0 increase. Representative spectra are seen in Fig. 16. The small broad peak near
Fig. 16. Raman spectra of untreated HOPG before and after of sputtered Cu deposition. 4A
2950 cm1 , which appears on Cu deposition, is the D1;2 þ G mode, and was also seen for Arþ -treated HOPG in Fig. 5b. No new peaks appear which are attributable to Cu–C formation: according to Maslowsky the Cu–C stretching vibration is below 650 cm1 , in a region easily accessible to the present study [53]. The absence of such bonds may be due to a lack of instrument sensitivity or to Raman inactivity. It is important to note that no other Raman study of ion implantation showed evidence of M–C bonds.
5. Discussion 5.1. Attributions of surface modifications
Fig. 15. SRIM simulation of the percent energy of sputtered Cu lost to ionization.
5.1.1. XPS No change is found in the C1s peak shape on Cu evaporation, indicating little or no surface modification. This is verified in our Raman studies, where no changes are detected. Arþ treatment and Cu sputtering both damage the HOPG surface and produce identical peaks at binding energies above the main C1s peak. As the main peak/satellite pair, at 284.6/291.4 eV, decreases in intensity, a secondary peak/satellite pair, at 285.6/288.0 eV, increase in intensity. As seen in Fig. 8, there is 1:1 correspondence between the main peak/satellite decrease and secondary peak/ satellite increase. The presence of shake-up satel-
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lites in both cases indicates them to be aromatic. Their correspondence indicates that the loss of a structure with a capacity to extensively delocalize electrons, through surface damage by Arþ treatment and Cu sputtering, leads to a structure with a more limited capacity to delocalize electrons, represented by the secondary pair. In addition, a single peak, at 286.5 eV, appears on initial treatment and does not change in intensity; this peak had previously [5] been suggested to be the satellite of the peak at 285.6 eV. The absence of an associated shake-up shows it is not aromatic, and has no ability to delocalize electron density by that mechanism. That is, while both the primary and secondary peaks indicate, respectively, the existence of more extensive and less extensive alternant hydrocarbon structures (Section 2), the peak at 286.5 eV represents sp3 hybridization (Section 2). A possible source of this peak has recently been identified [54a]. Using a slightly lower grade (i.e., more surface imperfections) of HOPG than that used here, but prepared in the same manner, this study employed TOFSIMS to reveal a spectrum dominated by Cx Hþ y positive ions and the total absence of Cþ 6n (n ¼ 1; 2 . . .). The presence of Cx Hþ y was attributed to hydrocarbon adsorption from the atmosphere on the freshly cleaned surface in the moment before entry into the apparatus and the absence of Cþ 6n , to a low sputter yield [54b]. In the present case, it may be that the free radicals created at the onset of surface damage, by Arþ and Cu sputtering, permit reaction with background hydrocarbons. Its presence in untreated HOPG is due to the existence of free radical surface defects, and is also consistent with our ARXPS results in Fig. 10. Since the new species form under treatment conditions, and on a surface structure, which do not favor heterolytic scission, they must form by homolytic scission. That is, they must be free radicals. One may inquire whether free radicals may have binding energies that differ from that of the primary peak by 1.0 and 1.9 eV, and whether this applies to both aromatic and aliphatic free radicals. The answers to both questions are in the affirmative: the study of radical ionization potentials has a long history [29,55–60]. From these experiments, it is clear that both aliphatic and ar-
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omatic free radicals may exist with ionization potentials (binding energies) which differ among themselves by as much as 3 eV, and are substantially higher in binding energy than their saturated equivalents. This occurs in the following way: the electroncontaining frontier orbital of a free radical is a singly occupied molecular orbital (SOMO). In the case of a free radical created in an alternant hydrocarbon, such as HOPG, the SOMO lies closer to the highest occupied molecular orbital (HOMO) than to the lowest unoccupied molecular orbital (LUMO). SOMO–HOMO interactions in free radicals give rise to electrophilic properties; the higher the ionization potential, the lower the energy of the SOMO [29, Chapter 5] and the greater the interaction. Comparing Figs. 9 and 14, it is seen that the FWHM of C1s abruptly increases with both Arþ treatment time and deposited Cu thickness, which is consistent with the results of Marcus et al. [44] for Cu sputter deposition on HOPG. The core level line widths generally increase with decreasing cluster size or atomic coordination number. The interpretation of this effect is still somewhat controversial [61]: some attribute it to cluster inhomogeneity [62,63] and others, to lifetime broadening or reduced screening [64,65]. The effects appear to be strongly related to the mean coordination number (i.e., the number of identical nearest neighbors) [66,67]. This is because there is no chemical bond formation during sputtering in our case, although aromatic –C–C– bonds are broken. The bond breaking decreases the mean coordination number of surface carbon atoms in the HOPG structure and increases the localized states. In Fig. 7, the most important change in the surface structure by XPS is the decrease in the C1 peak at 284.6 eV and the corresponding increase in the C2 peak at 285.6 eV, due to bond breaking on Arþ treatment. Evidence of such damage is found in the 3 nm hillocks observed by An et al., using STM [15]. It is interesting to note that there is a small C2 component part on the fresh HOPG surface; these may well be defects existing on terrace steps, consistent with our ARXPS data in Fig. 10.
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Some publications [4–6] have proposed that the C2 peak is due to sp3 hybrids, in concentrations as high as 20% of the sp2 hybrid concentrations. However, neither HREELS [68] nor AES [69] found sp3 carbons on Ar-treated HOPG. Indeed they were only found on Hþ - and Cþ -treated HOPG surfaces. The FWHM changes found for Cu2p3=2 on Cu sputter deposition are known to be due to initialstate and final-state effects, associated with the Cu cluster size [3,24,25,32,67]. We consider these effects in a separate paper [67]. 5.1.2. Raman spectroscopy As noted in Section 4.2.2 and Table 2, the Raman peaks have been given identifying letters. The doublets at 1327/1367 (D1 , D2 ) and 2690/2734 (2D1 , 2D2 ) and the peak at 1620 (D0 ) cm1 are thought to be disorder induced [35–39,45,46]. The doublet at 2690/2734 cm1 is identified as an overtone of that at 1327/1367 cm1 , and the peak at 3245 cm1 , as an overtone of that at 1620 cm1 . However, as Fig. 5a and b, and Table 2 make clear, all the overtones (2D1 , 2D2 , 2D0 ) exist before the onset of disorder, whereas D1 , D2 and D0 do not, and so cannot be overtones. Further, as Fig. 11 makes clear, the behaviors of 2D2 and G, at 1581 cm1 (as well as 2D1 and 2D0 , not shown) mimic the behavior of the primary XPS peak, at 284.6 eV, as a function of treatment. On the other hand, D2 (as well as D1 and D0 , not shown) mimics the behavior of the secondary XPS peak, at 285.6 eV. Thus, the G, 2D1 , 2D2 and 2D0 peaks are identified as representing phonon modes associated with the large alternant hydrocarbon of the original HOPG structure, while the D1 , D2 and D0 peaks represent phonon modes associated with the disordered structure obtained on Arþ treatment and Cu sputtering. The time-dependent Raman intensity data are also consistent with the measurements of Nakamura and Kitajima [12,13]. They attributed the initial steep rise of the D:G peak intensity ratio to the reduction of the in-plane phonon correlation length and the formation of a quasi-two-dimension structure. The similarity of the time-dependent XPS and Raman results may suggest that the surface damage steady state induced by the Arþ
Table 3 Arþ irradiation damage depth, and XPS and Raman measurement depths in HOPG Arþ damage depth (nm)a
XPSb
Ramanc
0:8 1:5
2.1
40
a
From [12b], using 3 keV Arþ . b C 1s mean free path (nm). c 514.5 nm-radiation optical skin depth (nm).
treatment is achieved relatively quickly (on the order of 60 s). However, it should be noted that XPS and Raman have different probe depths, found in Table 3. We note that the Arþ damage depth is about the same as the C1s photoelectron mean free path, which is over an order of magnitude less than the Raman radiation optical skin depth. 5.2. Cluster dynamics and adhesion Copper deposits onto many substrates in the form of clusters [3,34,70–78]. Because the interaction between Cu clusters and HOPG is very weak, whole clusters move laterally across the substrate surface and coalesce [34,70,74–76]. The rate of coalescence is related to the interaction between the cluster and the surface [34,70,74–76] and, thus, is a measure of interfacial interaction [34,79,80]. For example, treating a Cyclotene surface with a N2 plasma introduces –NH2 groups capable of interacting with evaporated Cu [79]; this causes the cluster coalescence rate to drop to zero [34,79], giving the strongest interaction measured for any Cyclotene surface treatment [79,80]. We have developed a method [31] that uses XPS intensity ratios to evaluate the average cluster diameter, d, and its surface density, n. When this evaluation is initiated immediately after deposition, without exposure to atmosphere, both d and n may be determined as a function of nominal film thickness, as in Fig. 3. Following the intensity changes as a function of time, as in Fig. 4, permits the evaluation of the process kinetics. The increase in d, as a function of film thickness, rises quickly before leveling out [79], as seen for HOPG in Fig. 3. Similar behavior is found for plots of the Auger parameter ½EAuger ðCuL3 M4;5 M4;5 Þ þ Ebinding
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ðCu2p3=2 Þ , the Cu2p3=2 binding energy shift (DEbinding ) and its FWHM [67]. By extrapolating the linear portions of the curves, one may, from their intersection, determine the Cu film thickness at that point. For any given system (same Cu deposition technique, substrate and treatment), the intersection thickness is the same, within experimental error, no matter which type of plot is used. Our method [31] permits us to evaluate d and the average distance between clusters (l ¼ n1=2 ) at this intersection point, where we find that, within experimental error, d ¼ l; that is, the intersection marks the contact between clusters. The only exception to this is apparent rather than real. It occurs for clusters where d is larger than the probe for Cu. depth of the emitted electrons, 30 A Thus, while d, determined from an intensity ratio, is independent of cluster size, the surface density, n, determined from the amount of Cu deposited, is not and, as a result, is overestimated. This intersection is also found in coalescence kinetics. One often attempts to demonstrate the linearity of a log–log plot of d vs. t [34,75]. This is because the theory of coarsening through dynamic coalescence [56], based on von Smoluchowski’s equation [81], which describes the change in the cluster density on binary encounters of diffusing clusters, predicts that d / ta , where a ¼ 0:25. As we recently demonstrated [34], this plot also
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manifests a deviation from the straight line at d ¼ l, the average distance between clusters, with another when the coalescence terminates. Our present data on cluster size as a function of nominal Cu thickness, summarized in Fig. 17, show that lower d (and higher n) correlate with treatments known to give higher Cu adhesion [79,80]. That is, the greater the Cu/surface interaction, the lower the propensity for Cu surface diffusion and growth.
6. Conclusions Both XPS and confocal Raman spectroscopy have been used to study the HOPG surface before and after Arþ treatment/Cu evaporation, and Cu sputtering. Surface treatment breaks bonds in this alternant hydrocarbon, producing free radicals, thereby reducing the capacity to delocalize electrons. These are reflected in consistencies in the X-ray photoelectron and Raman spectra. Direct correlations between them help to clarify the sources of the Raman peaks. The dynamics of the Cu clusters deposited by evaporation and sputtering have been shown to depend on the surface treatment, with lower cluster diameter and higher surface density being associated with stronger interfacial interaction.
Acknowledgements The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for funding, Don Frye for furnishing the HOPG and Craig Hyett for help with the Raman spectra.
References
Fig. 17. Summary of cluster size data.
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