Chem. Mater. 1990,2, 293-299 with Vcu as a minor species.
293
avoiding the use of samples with damaged surface regions, and use of wider supplies of high quality crystals.
Conclusion From the photoluminescence data presented here it is apparent that the important defect for diffusion is the copper vacancy. This finding confirms results from the effects of chemical treatments on Dcu in CuInSe2,which we mentioned earlier, and which will be reported on and discussed in a future publication. More complete (and firmer) interpretation of luminescence spectra of CuInSe2 may become possible by wider use of metal in-diffusion experiments and Cu extraction, to control Cu content, working at lower temperatures,
Acknowledgment. We thank K. Bachmann (North Carolina State University), H. W. Schock (IPE, University Stuttgart), and S. Endo (Science University of Tokyo) for samples of CuInSe2. This research is supported, in part, by the US-Israel Binational Science Foundation, Jerusalem, Israel, and by the Israel National Council for Research and Development with the KFA Julich (FRG). At SERI research is supported by the US Department of Energy under Contract DE-AC02-83CH10093. Registry No. CuInSe2,12018-95-0.
X-ray Photoelectron Spectroscopic Studies of Carbon Fiber Surfaces. 11. Differences in the Surface Chemistry and Bulk Structure of Different Carbon Fibers Based on Poly(acrylonitri1e) and Pitch and Comparison with Various Graphite Samples Yaoming Xie and Peter M. A. Sherwood* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received November 28, 1989 X-ray photoelectron spectroscopy has been used to monitor the surface chemistry of untreated carbon fibers based on PAN and pitch, a pitch fiber anodicallyoxidized in 1M HNO,, and three different graphite materials. Differences were found in the core and valence-band region indicating a highly graphitic and almost unoxidized nature for untreated pitch-based fibers. The graphite samples were found to have more reactive surfaces than the pitch fibers. X-ray powder diffraction was used to study the bulk structures of these materials, and the Du Pont E-120 pitch-based fiber was found to have the most graphitic structure of all the carbon fibers examined. The anodic oxidation of the pitch fiber changed both surface and bulk structure, producing mainly one type of oxygen-containing group on the surface.
Introduction The mechanical properties and the surface treatment of carbon fibers are both important in producing composite materials with the desired properties. These composites are now widely used, especially in the aircraft industry. We have carried out a range of studies of different carbon fiber surfaces for both PAN (polyacrylonitrile) and pitch-based carbon fibers using various surface treatments.*-'O Our previous studies had focused on the surface chemistry, mainly measured by X-ray photoelectron spectroscopy (XPS or ESCA), an approach we and other workers (see the review in ref 9) have found to be very valuable. In this paper we compare the surface chemistry as measured by XPS (using both core and valence band X P S ) (1) Proctor, A,; Sherwocd, P. M. A. J. Electron Spectrosc. 1982,27, 39. (2) Proctor, A,; Sherwood, P. M. A. Carbon 1983,21, 53. (3) Proctor, A.; Sherwood, P. M. A. Surf. Interface Anal. 1982,4,212. (4) Kozlowski, C.; Sherwood, P. M. A. J. Chem. SOC.,Faraday Trans. 1 1984,80, 2099. (5) Kozlowski, C.; Sherwood, P. M. A. J. Chem. SOC.,Faraday Trans. 1 1985,81, 2745. ( 6 ) Harvey, J.; Kozlowski, C.; Sherwood,P. M. A. J. Mater. Sci. 1987, 22, 1585. (7) Kozlowski, C.; Sherwood, P. M. A. Carbon 1986, 24, 357. (8)Kozlowski, C.; Sherwood, P. M. A. Carbon 1987, 25, 751. (9) Xie, Y.;Sherwood, P. M. A. Appl. Spectrosc. 1989, 43(7), 1153. (10) Xie, Y . ; Sherwood, P. M. A. Chem. Mater. 1989,1, 427.
0897-4756/90/2802-0293$02.50/0
with the bulk properties as measured by powder X-ray diffraction (XRD). We discuss the two main types of carbon fiber, namely, those prepared from PAN and those prepared from pitch (both untreated and treated), and compare these fibers with three different qualities of graphite.
Experimental Section Five different untreated carbon fiber samples were studied, two (a and b) were type I1 fibers based upon PAN and three (ee) were different modulus pitch-based fibers. Sample j is the same as sample e but was examined by using A1 X-rays. Sample i was
surface treated by electrochemicaloxidation. The three different quality graphite samples were samples f-h. Further details of the samples and sample treatment are given in Table I. The results reflect typical spectra for the samples. We have carefully studied a number of samples. In the case of the fibers we have in all our studies'-'O carefully ensured that initially untreated and unsized samples from the original manufactures were obtained. We have also checked to ensure that there were no significant variations associated with the location of the fiber in the fiber tow. Types of fiber simliar to a-c were discussed in a previous publication,1°and they are reported here to compare and to discuss their XRD. In this paper, these samples represented a different sample from the same type of carbon fiber. The X-ray photoelectron ( X P S or ESCA) spectra were collected on an AEI (Kratos) ES200B X-ray photoelectron spectrometer with a base pressure of about loygTorr (except sample f where the spectra were collected on a VSW HA100 XPS instrument with
0 1990 American Chemical Society
294 Chem. Mater., Vol. 2, No. 3, 1990
sample a b C
d e f g h
XPS anode Mg Mg Mg Mg Mg A1 A1 A1 A1 A1
Xie and Sherwood
Table I. Sample Descriptions descriptions of different carbon fibers and graphite samples untreated type I1 PAN-based carbon fiber, washed in 4-D water, dried, and stored in glass for 18 months untreated AU4 PAN-based carbon fiber, 34-Msi modulus, Hercules product, used as received untreated P55X pitch-based carbon fiber, 55-Msi modulus, Amoco product, used as received untreated E-35 pitch-based carbon fiber, 35-Msi modulus, Du Pont proudct, used as received untreated E-120 pitch-based carbon fiber, 120-Msi modulus, Du Pont product, used as received pyrolytic graphite, high-purity untreated commercial sample graphite, cut from an industrial graphite pipe, polished with sand paper, and cleaned with acetone ultrahigh-purity graphite electrode, Ultro Carbon Corp. product E-120 pitch-based carbon fiber as e, but anodically oxidized in 1 M HNO, at 3 V (SCE) for 10 min untreated E-120 pitch-based carbon fiber as e, but the XPS spectrum is collected with A1 X-rays
a base pressure of Torr. The XPS spectrometers were operated in the FRR model (ratio 1:23 AEI, 1:50 VSW) using Mg Ka12 X-ray radiation (240 W) for samples a-e and Al X-ray radiation (240 W) for samples f-j. Analyzer resolution was of the same order as the X-ray line width (0.7 eV Mg, 0.85 eV Al). Typical data collection times were as follows: Cls, 1 h; Ols, 10 h; Nls, 5-40 h; valence band, 16-24 h. No sample decomposition was observed during data collection with the exception of the electrochemically oxidized fiber. The spectrometer energy scale was calibrated by using copper" and the separation (233 eV) between photoelectron peaks generated by Mg and A Ka12 X-rays. A bundle of carbon fibers was cut to a length of 3.0 cm, with both ends tied with a narrow piece of aluminum foil. One of the ends was mounted in a metal sample holder. After insertion into the XPS spectrometer chamber, the sample was connected to the system ground by the metal sample holder. The position of the sample holder was adjusted so that no photoelectrons were seen from the sample holder and the aluminum foil. The curve fitting of the XPS spectra was carried out using a nonlinear least-squares curve fitting program with a Gaussian/ Lorentzian product f u n c t i ~ n . ~The z ~ ~Gaussian/Lorentzian mix was taken as 0.5, except for the "graphitic" carbon peak, which was taken as 0.84 with an exponential tail. These values for the Gaussian/Lorentzian mix were chosen after many studies were carried out where the Gaussian/Lorentzian mix was varied to ensure the best fit to various types of Cls component peak.1-10J2 The C 1s binding energy of the graphitic peak was taken as 284.6 eV for calibration purposes. XRD studies were carried out by with a Scintag XDS 2000 instrument. The X-ray radiation wavelength of Cu K q (1800 W) is 0.154059 nm. The data were collected under the stepscanning mode with a step size of 0.005O. The electrochemistry was carried out by using a standard three-electrode glass cell. A bundle of Du Pont E-120 pitch-based carbon fiber acted as the working electrode, and a piece of platinum foil was the counterelectrode. The potential of the working electrode was measured with respect to a saturated calomel reference electrode (SCE) that was positioned as close to the working electrode as possible by means of a Luggin capillary. The solution of 1 M HN03was prepared with quadruple-distilled (4-D) water and ACS reagent grade nitric acid. After anodic oxidation, the carbon fiber sample was removed from the oxidation (while still in the circuit), washed thoroughly in 4-D water, and dried in an oven a t 100 "C. A full discussion of the electrochemically oxidized fiber for a range of different potentials and the monitoring of the decomposition of the fiber with exposure to X-rays will reported in a later p~b1ication.l~In this paper we present the data in order to compare the surface chemistry with that of the untreated fibers and the graphite samples.
Results and Discussion All the samples showed an intense Cls peak and a weak C(KLL) Auger peak. No elements other than carbon, (11)Annual Book of ASTM Standards, Vol. 03.06, Surf. Interface Anal. 1988, 1 1 , 112. (12) Sherwood, P. M. A. Practical Surface Analysis by Auger and Photoelectron Spectroscopy; Briggs, D., Seah, M. P., E&.; Wiley: London, 1983; Appendix 3. (13) Ansell, R. 0.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electroanal. Chem. 1979, 98, 79. (14) Xie, Y.; Sherwood, P. M. A., to be published.
nitrogen, and oxygen were found on the fiber surface. The 01s peak is much weaker than the Cls peak in all the samples except for the anodically oxidized pitch fiber (sample i), which gave a very intense 01s peak and a substantial O(KLL) Auger peak. Samples c and e (and j) did not show any visible 01s signal in the overall spectra. The Nls peak was of very low intensity in all cases, being undetectable for all the pitch-based samples, and was only of significant intensity for the two PAN-based fibers (samples a and b) and the coarse graphite (sample g). In Figures 1-3 the left-hand column shows data recorded with Mg X-radiation and the right-hand column shows data recorded with A1 X-radiation. Widths are thus comparable only within each column due to the different X-radiation line widths. Sample e was recorded with both radiation sources for comparison purposes (shown as sample e for Mg X-rays and j for A1 X-rays). The sample decomposition that occurred in the case of the electrochemically treated sample was carefully monitored. Thus the Cls spectrum was recorded before and after the valence-band spectrum were recorded. A sequence (Cls-valence band-Cls-valence band-Cls) of spectra were recorded (corresponding to short (typically 40 min) Cls collection times and long (typically 5 h in the first case, 10 h in the second case) valence band collection times). The Cls region has the greater surface sensitivity due to the lower kinetic energy (approximately 970 eV) of the Cls electrons compared with those of the valence band (approximately 1250 eV). Thus in the sequence ((Clsvalence band-Cls) there was approximately 50% loss in the oxide 1 compared to graphitic peak in the Cls region and an approximately 70% loss in the sequence (Cls-valence band-Cls-valence band-Cls). The type of C/O species as reflected in the C l s and 0 1 s chemical shift remained largely unchanged by decomposition. In the sequence (Cls-valence band-Cls-valence band) there was hardly any loss (less than 10%) in the 02s/C2s components of the valence band, the main features in valenceband region appearing much the same for both spectra. These experiments illustrate how the core XPS regions give us the most surface-sensitive information with the valence XPS region allowing us to probe more deeply into the surface. C l s Region. Figure 1 shows the curve-fitted Cls spectra. We have fitted the spectra to component peaks corresponding to the various functional groups known to be found on carbon fiber surfa~es.'-'~J~J~ The spectra were fitted by using a main "graphitic" peak a t 284.6 eV and four "oxide" peaks. The four types of oxide called oxide 1, 2, 3, and 4 were shifted by 1.5-2, 3, 4-4.5, and 6.1 eV from the graphitic peak, respectively. Oxide 1 is assigned to the carbon atom in alcohol (C-OH) or ether (C-0-C) groups. The Cls signal from C=N type groups also falls (15) Da, Y.; Wang, D.; Sun, M. Compos. Sci. Technol. 1987,30, 119. (16) Takahagi, T.; Shimada, I.; Fukuhara, M.; Morita, K.; Ishitani, A. J. Polym. Scien., Part A: Polym. Chem. E d . 1986, 24, 3101.
Chem. Mater., Vol. 2, No. 3, 1990 295
XPS of Carbon Fiber Surfaces
oxide 4 area, BE,eV % 290.7 0.81 2.53 290.8 3.06 290.8 3.07 290.5 290.8 2.44 2.35 290.6 290.6 1.21 290.8 2.78 290.9 0.63 290.7 3.57
sample a
Table 11. C l s Peaks and Relative Peak Areas Cls peaks in different carbon chemical states oxide 1 oxide 3 oxide 2 area, area, area, BE,eV % BE,eV % BE,eV % 287.6 4.2 286.2 10.6 289.1 3.82 286.1 11.7 3.00 287.6 3.9 289.1 186.1 3.2 287.6 2.7 1.81 289.1 286.1 6.2 287.4 4.1 3.49 288.9 286.1 1.7 287.6 2.4 289.2 1.56 286.1 13.1 2.02 287.6 2.6 289.1 286.1 14.6 289.1 2.25 287.6 2.8 286.1 11.3 287.6 2.9 289.1 2.98 288.0 7.2 286.6 48.5 289.4 2.10 286.1 1.7 287.6 2.3 289.1 2.27
main graphitic peak area, fwhm, BE,eV % eV 284.6 80.5 0.96 78.9 1.24 284.6 89.3 0.74 284.6 284.6 83.2 0.85 91.8 0.72 284.6 79.8 1.42 284.6 284.6 79.1 1.55 284.6 80.0 0.91 1.36 284.6 41.6 284.6 90.2 0.87
total oxide area (1 + 2 +3+4), % 19.5 21.1
10.7 16.8 8.2 20.1 20.8 20.0 58.4 9.8
Scheme I H
0
/I
H\o
/A/\
I
2 T--
0/H
I
0
I
0
0
/c\c/c\
into this range. In all cases except for that of sample i the shift was 1.5 eV for this group. The larger shift for sample i may correspond to the presence of an arrangement intermediate between -C-OH and -C=O groups as suggested earliers (see Scheme I here and in ref 5). Oxide 2 corresponds to the C atom in carbonyl (C=O) type groups. Oxide 3 is the Cls peak of carboxyl (COOH) or ester (COOR) type groups. Oxide 4 is probably 420, type groups, though satellite features also occur in this range.'-1° The conducting nature of the graphitic lattice will also give rise to a plasmon feature about 6.9 eV shift from the graphitic peak, but it is not included in the range of Figure 1. Table I1 gives the peak positions and peak area percentages for Cls spectra. For all the samples, oxide 3 and oxide 4 Cls peaks are very weak and they do not change much from sample to sample. The main differences between samples arose in the contribution from the oxide l peak with lesser differences in the contribution from oxide 2. Samples a-c showed the features that we discussed previously.1° Sample a showed continued oxidation with a storage time as previously reportedlo (a was stored for 2 months more than the sample reported before). The very low level of oxidation on the pitch-based Du Pont samples (d and especially e) should be noted. The fwhm of sample e/j is the smallest that we have seen of a carbon fiber sample having a width that is comparable to that of the X-ray line width. It is also less than that of any of the graphite samples. Sample i is the first electrochemically oxidized pitchbased fiber that we have reported. The conditions used lead to very substantial oxidation, just as we have reported for PAN fibers in nitric acid.4 The relative intensity of the oxide 1peak compared to the graphitic peak is greater for similar electrochemical conditions for pitch-based fibers than PAN-based fibers. The oxide 1 peak in this sample is shifted by 2 eV from the main peak as opposed to a shift of 1.5 eV for all the other spectra. We found a similar shift in our previous studies4 of oxidized PAN fibers. Oxidation of the E-120 fiber leads to an increase in the width of the graphitic peak from 0.87 (j)to 1.36 eV (i), consistent with an overall loss of graphitic ~haracter'~?~' caused by oxidation. XRD results (see below) support this conclusion. ~~
~~~
(17)Santiago, F.;Mansour, A. N.; Lee, R. N. Surf. Interface Anal. 1987, 10, 17.
.*E h
l h
C
a
e
b
3 v
.-P
I
3 E
I
' I e
A
A ' d 9 b ' ' '2k
%x-w
Binding Energy (eV) Figure 1. Cls region XPS spectra of various carbon fiber and graphite samples: (a) untreated type I1 PAN fiber, stored for 18 months, (b) untreated AU4 PAN fiber; (c) untreated P55X pitch fiber; (d) untreated E-35 pitch fiber; (e) untreated E-120 pitch fiber; (0pyrolytic graphite; (9) coarse graphitq (h) ultrahigh-purity graphite; (i) anodically oxidized E-120 pitch fiber; 6)untreated E-120 pitch fiber. (a-e) Mg radiation; (f-j) A1 radiation.
Among the three graphite samples (f-h), h (ultrahighpurity graphite electrode) showed the most graphitic features with a 0.91-eV fwhm of the main graphitic peak, g (coarse graphite cut from a piece of industrial graphite peptide) the worst (1.55 eV fwhm), and f (pyrolytic graphite) in the middle with a 1.42-eV fwhm. Their oxide contents on the surfaces showed a similar increasing sequence from sample h, f, to g (see Table 11). The level of oxidation of the graphite sample is comparable to that of the untreated PAN fibers. This implies that the graphite samples and the PAN fibers are more reactive to surface atmospheric oxidation than the pitch-based fibers. 0 1 s Region. Figure 2 shows the 0 1 s region spectra fitted to up to three component peaks. Peak 1 (ca. 535.5 eV) is of low intensity, and it is probably due to chemisorbed oxygen (and perhaps some adsorbed water). Peak 2 (ca. 533 eV) corresponds t o C-OH (and/or C-0-C) groups and perhaps some adsorbed water, and peak 3 (ca.
296 Chem. Mater., Vol. 2, No. 3, 1990
sample a b C
d e f g
h i j
sample a b
Table 111. 0 1 s Peaks, Relative Peak Areas, and Atomic Ratios 01s peaks in different oxygen chemical states peak area peak 1 peak 2 peak 3 ratio, 7% BE, eV area, % BE, eV area, % BE, eV area, % Ols/Cls 535.6 2.6 533.0 70.4 531.5 27.1 9.1 535.0 8.3 533.1 80.6 531.3 11.1 4.5 534.6 11.1 533.0 88.9 531.4 0 2.9 534.8 10.3 533.0 89.7 531.2 0 7.5 0 1.8 535.6 32.9 533.0 67.1 531.5 534.2 27.7 533.0 72.3 531.5 0 9.7 535.6 4.5 533.1 89.1 531.0 6.4 14.5 535.3 17.6 533.0 82.4 531.4 0 6.3 535.1 3.9 532.1 96.1 75.3 0 2.2 535.2 13.9 533.0 86.1 531.0
peak 1 BE, eV area, % 0 403.7 5.0 403.6
C
d e f
g h 1
j
Xie and Sherwood
403.6 403.4 403.5
0 0 0 0 0 0 0 0
Table IV. Nls Peaks and Relative Peak Areas Nls peaks in different nitrogen - chemical states peak 2 peak 3 peak 4 BE, eV area, % BE, eV area, % BE, eV area, % 402.4 27.9 401.2 51.9 400.0 15.8 402.4 16.3 401.1 58.8 399.6 6.6 0 0 0
402.3 402.5 402.3
26.2 36.3 47.6
0 0 0
401.1 401.2 401.0
0 0
531-532 eV) to C=O groups. Table I11 gives the binding energy and percentage areas of each 01s peak. Only the two PAN samples and the coarse graphite sample could be fitted to three peaks, whereas all the other samples were fitted to only two peaks. The strongly oxidized sample i showed almost a single peak at 532 eV corresponding to -C=O groups (perhaps mixed -C=O and -C-OH groups as discussed above) as found for previous electrochemically oxidized The other samples were fitted to two peaks corresponding to chemisorbed oxygen and C-OH and C-0-C groups. These groups corresponded to the groups assigned from the Cls spectra (above), though the relative ratios of the component features will not be exactly the same due to the different sampling depth of the Cls and 01s regions. Sample a has a larger amount of peak 3 and a smaller amount of peak 2 compared to the other samples. The long storage time for this sample showed an increase in C 4 groups on the surface compared with samples stored for a shorter period.1° Table I11 also lists the ratio of the total 0 1 s peak area to the total Cls peak area and the O/C atomic ratio obtained by using Scofield's cross sectionsls and the instrument transmission function assuming a homogeneous sample. It shows that the untreated pitch fiber sample c and e (or j) had the lowest surface O/C ratios, and the anodic oxidized pitch fiber sample i the highest, with the graphite and PAN fiber samples in the middle. Nls Region. The Nls signal from carbon fiber surfaces is usually much weaker than Cls, or 01s signals. The PAN-based fiber samples a and b gave the most intense Nls peaks; the graphite sample f, g, and h had weaker Nls signals, while none of the pitch-based fiber sample c, d, e, i, and j gave any significant Nls peak. Sample b has the most intense Nls peak with the largest contribution from a single component. The curve fitting of N l s spectra was carried out using the approach discussed previouslyQJOwhere the best fit was obtained by using five peaks. Reliable fits could be obtained only for the PAN-based fiber samples; the other
45.5 50.8 44.9
O/C 3.9 1.9 1.2
atomic amt ratio, % N/C (0 + N)/C 0.7 4.6 2.1 4.0 3.2 0.8 4.5 6.7 3.0 31.3 0.9
0.4 0.7 0.4 0 0
Nls/Cls peak area ratio, % 1.2 3.4
0
0 0 0
0 0
0
16.6 9.9 7.5
1.2
peak 5 BE, eV area, % 398.7 4.4 398.2 13.3
0 0
399.9 400.0 399.7
0 0 0
3.2 0.8 4.0 6.0 2.6 31.3 0.9
398.7 398.6 397.5
11.6 3.0 0
0 0
0.7 1.1
0.7 0
0 0
0
..
537
532
Binding Energy (ev)
Figure 2. 0 1 s region XPS spectra of various carbon fiber and graphite samples; a-j as in Figure 1.
spectra were too weak for reliable curve fitting. Table IV lists all the peak positions, percentage area of each peak, and the total Nls/Cls peak area ratios. The N/C atomic amount ratios calculated from the XPS spectra by using Scofield's cross sectionsls and the instrument transmission function assuming a homogeneous sample are listed in Table 111. Table I11 also gives the total atomic amount (18) Scofield, J. H. J. Electron Spectrosc. 1976, 8,129.
XPS of Carbon Fiber Surfaces
35
20 5 35 20 5 Binding Energy (ev)
Figure 3. Valence-band XPS spectra of various carbon fiber and graphite samples; a-j as in Figure 1.
ratios of (0 + N) to C. This is basically consistent with the ratios of total oxide (1 + 2 + 3 + 4) area percentage in Table I1 if it is remembered that the bulk sensitivity follows the order Cls > Nls > 0 1 s as discussed earlier.gJO Valence-Band Spectra. The valence-band XPS spectra of various carbon materials, such as diamond, graphite, and glassy carbon have been well studied,'*= but there has been little work reported on the valence band of carbon fibers. Recently, we reported our valence-band studies on carbon fibers,1° and we are going to report further experimental and theoretical work on the carbon fiber valence band.=*" In this work, the valence band was used to monitor the surface chemistry of different carbon materials. We find'O~~~ that the valence band is more sensitive to changes in surface chemical environment than the core level. Thus calculations on substituted coronenes, which we will discuss in a later p u b l i ~ a t i o nsuggest ,~~ that the 0 2 s region should show a greater sensitivity to the oxygen environment than the 0 1 s region. In particular the type of grouping shown in Scheme I which would correspond to a hydrogen atom placed equidistant from two equal length C-O bonds (with a length between a C-0 single and a C=O double bond) is predicted to have an 02s-C2s separation intermediate between those of a C 4 and a --C-OH group. Also the 0 2 ~ 4 2 separation s > -C=O is predicted to follow the series -C-0-C> -C-OH with variations in the 0 2 s binding energy being much larger than those found for the 0 1 s core levels. (19) Cavell, R.G.; Kowalczyk, S. P.; Ley, L.; Pollak, R. A.; Mills, B.; Shirley, D. A,; Perry, W. Phys. Reu. E 1973, 7, 5313. (20) McFeely, F. R.;Kowlaczyk, S. P.; Ley, L.; Cavell, R. G.; Pollak, R.A,; Shirley, D. A. Phys. Rev. B 1974, 9,5268. (21) Evans, S.; Riley, C. E. J. Chem. SOC.,Faraday Trans. 2 1986,82, 541. (22) Galueka, A. A.;Madden, H. H.; Allred, R. E. Appl. Surf. Sci. 1988, 32, 253. (23) Xie, Y.;Sherwood, P. M. A., t o be published. (24) Xie, Y.;Sherwood, P. M. A. Appl. Spectrosc., in press.
Chem. Mater., Vol. 2, No. 3, 1990 297
35 20
5 35 20 Binding Energy (ev)
5
Figure 4. Valence-band XPS spectra of various carbon fiber and graphite samples: (0 pyrolytic graphite;(g) coarse graphite; (h) ultrahigh-purity graphite, (i) anodically oxidized E-120pitch fiber; 6)untreated E-120pitch fiber. Left column, smoothed spectra; right column, second derivative from the left column.
The calculations also predict changes in the C 2 s 4 2 p - 0 2 ~ envelope that promise to provide additional structural information. Figure 3 shows the valence band spectra of samples a-j. Figure 4 shows the valence band spectra of samples f-j both as the smoothed spectra and as the second derivative spectra. All the spectra show the intense C2s featured0 around 20 eV together with 0 2 s features at binding energies greater than 25 eV. Significant differences in the 02s region for the graphite samples can be seen in Figure 4. The differences are accentuated in the second-derivative spectra, as expected.12 Samples e and j have the weakest 02s features; b, c, and h have a significant 0 2 s shoulder (the second derivative spectrum for h suggests that this may consist of an overlapping doublet); while a, d, f, g, and i have much more 02s intensity with two 0 2 s components clearly visible in f and g (note the two intense features in the second derivative specctrum). The relative amount of 02s/C2s intensity is consistent with the O/C atomic ratios listed in Table I11 (though it should be noted that the valence band probes more deeply into the surface than the 0 1 s and Cls core regions). It is interesting to notice that sample i has a very symmetric and relatively narrow 0 2 s peak shape (note that the second derivative spectrum consists of only one peak). In our previous reportlo of the valence band of commercially surface-treated PAN fibers, the 02s region showed two clear features in one case. The narrow 0 2 s feature for sample i is consistent with the narrow 0 1 s region and our previous findings that anodic oxidation of PAN4i5carbon fibers leads to a single oxygen environment. The range of 0 2 s features, and in particular the clear separation of different 0 2 s peaks in some of the spectra discussed above, follows the predictions of our calculations that the 02s region should fall over a wide binding energy
Xie and Sherwood
298 Chem. Mater., Vol. 2, No. 3, 1990 Table V. Degrees of Graphitization for Different Carbon Materials
d(002), A graphit degree, %
a
b
C
3.494
3.545 a
3.415 29.07
a
d 3.435 5.81
sample e f 3.368 3.368 83.72 83.72
e
3.435 5.81
h 3.355 98.84
i 3.381 68.60
1
3.368 83.72
Degree of graphitization is very low; calculated value from the equation is negative.
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graphite lattice caused by oxidation leading to amorphous character. The (002) feature is often used as a measurement of the degree of graphitizationzkz8due to its high relative intensity and its sensitivity to structural changes. Table V lists d(002) parameters for all these samples and the degree of graphitization, g, (expressed as a percentage) calculated by using Maire and Mering’s e q u a t i ~ n : ~ ~ t ~ ~ d(002) = 3.354g + 3.440(1 - g) A (1) where g is a measure of the ideal graphitic orientation. The expression is based upon the fact that the ideal graphite structure (g = 1 (100%))has a d(002) spacing of 3.354 A, and the totally misorientated structure (g = 0 (0%)) has a d(002) of 3.440 A. As we know the d(002) from the XRD data, the g value can be calculated by 3.440 - d(002) (2) = 3.440 - 3.354 Bulk and Surface Comparisons. The XPS data are surface sensitive (up to approximately 100 A) with the core levels being more surface sensitive than the valence-band region. The XRD data are bulk sensitive, and so a comparison between these two experiments allows us to identify any distinct surface chemical features of the fibers. XRD will be more surface sensitive in thin-film mode, but in our studies the data were collected in order to be bulk sensitive. The XRD suggests that the high-purity graphite, sample h, has the most perfect graphite crystalline lattice whereas the pitch-based fibers sample e/j and the pyrolytic graphite sample f have a very graphitic nature. I t is interesting to see that the anodically oxidized pitch-based fiber still has considerable graphitic character in the bulk, even though the XPS shows considerable broadening of the graphitic Cls peak. This suggests that anodic oxidation produces the main change in the surface region, but nevertheless there is a significant loss of graphitic nature in the bulk. The other samples have considerably less graphitic character, especially the PAN-based fibers, as found by other workers.28 The untreated pitch-based fibers have a narrower graphitic Cls peak (close to the natural width of the X-rays used) than either of the PAN-based fibers or the graphite samples. This is partly due to the more complete graphitic structure on the surface of the pitch-based fibers. Combining the XPS and XRD results suggests that the surface of the pitch-based fibers me somewhat more graphitic than the bulk, which may explain why they are less susceptible to atmospheric attack than the PAN fibers or the graphite samples. We find that the Cls peak can be considerably broadened if the lattice is disordered by methods such as argon ion etching, electrochemical oxidation, and air autoxidation. Another cause for the narrower line width
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Figure 5. XRD spectra of various carbon fiber and graphite samples: (a) untreated type I1 PAN fiber, stored for 18 months; (b) untreated AU4 PAN fiber; (c) untreated P55X pitch fiber; (d) untreated E-35 pitch fiber; (e) untreated E-120 pitch fiber; (f) pyrolytic graphite; (8) coarse graphite; (h) ultrahigh-purity graphite; (i) anodic oxidized E-120 pitch fiber; 6 ) same spectrum as (e). range for different oxygen functionalities. Also we find that the surface functionality found in the Cls and 01s regions follows the predicted 02s functionality from our calculations. Also the valence-band spectra show significant differences in the region below 20-eV binding energy (the C2s-C2p-O2p region) as predicted by our calculations for systems with differing surface functionality. XRD Studies. Figure 5 is the XRD spectra of samples a-j. For the XRD experiments, sample j is exactly the same as e, so identical spectra for e and j are shown for consistency with the other figures. All these samples gave an intense (002) peak. The ultrahigh-purity graphite (h) has the most intense (relative to the other features in the diffractogram) and narrow (002) peak. The untreated E-120 pitch fiber (e/j) and the pyrolytic graphite (f) have a little less relative intensity and a greater width for the (002) peak than sample h. Then the relative (002) peak intensity decreases further in the order i, c, g, d, a, and b, while the width of (002) peak increases in the same order. All these samples have a (lo*) feature at about 44’ 20, but only h, f, and e have distinguishable (100) and (101) features. Some samples (f-i) also showed a (004) peak at 5 5 O 28. The features at about 1 2 O 20 appeared in all these spectra, being weakest for the graphite samples (f-h). We attribute this feature (marked as “0”)as damage to the
(25) Maire, J.; Mering, J. First Conference on Industrial Carbon and Graphite; Society of Chemical Industry, London, 1958, 204. (26) Mering, J.; Maire, J. J. Chim. Phys. 1960,57, 803. (27) Ruland, W. Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker: New York, 1968; Vol. 4, p 1. (28) Yang, Y.; Wang, J.; Liu, C. J. Shanghai Jiaotong Uniu. 1988, 22(5),20.
Chem. Mater. 1990,2, 299-305 is the lack of nitrogen on the pitch-based fiber, thus eliminating the possibility of chemically shifted C-N groups, which can be found as close as about 0.7 eV from the main “graphitic” peak. PAN-based fibers have a significant residue of nitrogen-containing groups left on the surfaces from their precursors, which may contribute to the disordering of the surface graphitic lattice leading to further broadening of the Cls graphitic peak. Thus the XRD and XPS results suggest differences in the surface and bulk chemistry of the fibers.
Conclusions Pitch-based carbon fibers are clearly more graphitic in both surface and bulk structure, and the higher the modulus of the pitch fiber is, the more graphitic its structure. Pitch-based carbon fibers have a lower level of surafce functionality than PAN-based carbon fibers. The surfaces of the graphite samples are also more active to the environment than the pitch-based carbon fibers. Anodic oxidation of a highly graphitic pitch-based carbon fiber led to similar surface species as those seen on
299
PAN-based fibers under similar oxidation conditions, though the pitch-based fibers showed more oxidation. One type of C/O surface species was produced. Decomposition during data collection leads to significant changes in the core regions but little change in the valence-band region, reflecting the greater surface sensitivity of the former regions. Valence-band spectra provide a more sensitive monitor of changes in suface chemistry than the core region. Thus changes in surface functionality result in significant differences (including in some cases separately resolved peaks) in the 02s region, as well as in the region at binding energies below 20 eV. Calculations on model compounds, to be discussed in a later publication, broadly predict these features.
Acknowledgment. We are grateful to the Fibers Department of the Du Pont Co. for supporting this work and to the Department of Defense for providing funding for the X-ray diffraction equipment. Registry No. Graphite, 7782-42-5.
Acid Formation and Deprotection Reaction by Novel Sulfonates in a Chemical Amplification Positive Photoresist L. Schlegel, T. Ueno,* H. Shiraishi, N. Hayashi, and T. Iwayanagi Central Research Laboratory, Hitachi Ltd., Higashi-Koigakubo, Kokubunji, Tokyo 185, Japan Received December 5, 1989 In a positive deep-UV photoresist composed of tris(methanesulfony1oxy)benzene as a novel photo acid generator, bisphenol A protected with tert-butoxycarbonyl groups as a dissolution inhibitor, and a novolak matrix polymer, the deprotection reaction by the generated methanesulfonic acid was studied by using UV spectroscopy. The results were compared with exposure characteristics obtained with the same resist in lithography. The deprotection degree, the catalytic chain length of the deprotection reaction, and the quantum yield of the acid generation were determined. The amount of photogenerated acid was unexpectedly high. This could be due to a sensitizing effect of the strongly absorbing novolak matrix polymer to enerate the acid with high efficiency. The results show that sulfonic acid esters have very high possibi&ies for application in deep-UV resist materials.
Introduction For the fabrication of VLSI circuits with critical dimensions smaller than 0.5 pm, new resist materials are required that are sensitive to deep-UV light (200-300 nm). Resist systems that may be developed in aqueous alkaline developers are very advantageous, to avoid swelling phenomena during the development process. Moreover, these resists offer a high dry-etch resistance due to their high content of aromatic polymers such as novolak resin. A high photosensitivity is required, as the light intensity of the KrF excimer lasers, operating at 248.3 nm, is not so high. Moreover, a resist with a sensitivity of less than 10 mJ/cm2 is necessary, if the even weaker intensity of a Hg lamp at 254 nm should be used in an exposure system. It has been shown that this photosensitivity can most easily be achieved by using the “chemical amplification” mechanism.’ In this case a photosensitive acid generator is (1)FrBchet, J. M. J.; Ito, H.; Willson, C. G.R o c . Microcircuit Eng. 1982, 260. Ito, H.; Willson, C. G.; FrBchet, J. M. J. Ibid. 1982, 262.
Willson, C. G.; Ito, H.; FrBchet, J. M. J.; Houlihan, F. Proc. IUPAC 28th Mucromol. Symp. 1982,448. Ito, H.; Willaon, C. G. Polym. Eng.SCL1983, 23, 1012.
0897-4756/90/2802-0299$02.50/0
decomposed during the exposure. The following acidcatalyzed thermal reaction renders the exposed parts of the resist soluble, in the m e of positive resists, or insoluble, in the case of negative resists. Positive resists of that type are generally composed of an acid generator and an acidlabile compound that leads to products soluble in an aqueous alkaline solution. I t is also possible to use three-component systems in which the acid-labile compound acts as a dissolution inhibitor in a novolak matrix polymer. Resists of this type have been applied to midUV2 (300-350 nm), deep-UV3p4X-ray5 and electron beam lithography.‘j In these resists, onium salt^,^-^ halogen compounds,5s6and nitrobenzyl esters7v8have been used as (2) O’Brien, M. J.; Crivello, J. V. Adu. Resist Technol. Processing V , Proc. SPZE 1988,920,42. (3) Ito, H.; Flores, E.; Renaldo, A. F. J. Electrochem. SOC.1988,135, 2328. (4) McKean, D. R.; MacDonald, S. A.; Clecak, J.; Willson, C. G.Adu. Resist Technol. Processing V,Proc. SPIE 1988, 920, 60. ( 5 ) Dossel, K. F.; Huber, H.-L.; Oertel, H. Microelectron. Eng. 1986, 5, 97. (6) Menschig, A.; Forchel, A.; Da”e1, R.; Lingau, J.; Scheunemann, U.;Theis, J.; Pongratz, S. Microelectron. Eng. 1989, 9, 571.
0 1990 American Chemical Society
