0 Copyright 1994 American Chemical Society
DECEMBER 1994 VOLUME 10,NUMBER 12
Letters X-ray Absorption Study of Highly Oriented Poly(tetrafluoroethy1ene) Thin Films Ch. Ziegler,t9* Th. Schedel-Niedrig,§G. Beamson,Il D. T. Clark,'I W. R. Salaneck,* H. Sotobayashi,§and A. M. Bradshaw*9§ Institut fiir Physikalische und Theoretische Chemie, Universitat Tubingen, Auf der Morgenstelle 8,72076 Tubingen, Germany, Department of Physics, Linkoping University, 581 83 Linkoping, Sweden, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6,14195 Berlin (Dahlem), Germany, and ICI Wilton Research Centre, Middlesbrough, Cleveland TS6 8JE,U.K. Received March 18,1994. In Final Form: October 10,1994@ By use of highly oriented films, the polarization dependence of the C 1s and F 1s near-edge absorption spectra of poly(tetrafluoroethy1ene) has been measured. At least at the carbon K edge, good agreement is obtained with the predicted behavior based on a consideration of the one-dimensional band structure and the effect of symmetry breaking. The findings are also relevant for the assignment of the resonances in long chain alkanes, a subject which has recently been somewhat controversial. Certain properties of polymers, such as mechanical stiffness, tensile strength, electrical conductivity, and optical transmission, often depend strongly on the degree of order in the material. The electrical conductivity, for example, can increase by more than 2 orders of magnitude, as experiments on oriented, doped conjugated macromolecules have shown.'-3 Various techniques have been developed in the past to produce oriented polymer materials. Although preferential chain orientation can be produced by physical deformation (stretching or rolling) of bulk samples,* this rarely results in a high degree of order, even in so-called doubly oriented samples. Another approach is provided by the Langmuir-Blodgett technique, in which oriented monolayers of amphiphilic + Universitat Tubingen.
* Linkoping University. @
Fritz-Haber-Institut der Max-Planck-Gesellschaft. IC1 Wilton Research Centre. Abstract published in Aduance ACS Abstracts, November 15,
1994. (1)Gagnon, D. R.; Karasz, F. E.; Thomas, E. L.; Lenz, R. W. Synth. Met. 1987,20,85. (2) Naarmann, H.; Theophilou, N. Synth. Met. 1987,22,1. (3) Oh, S. Y.;Akagi, K.; Shirakawa, H. Synth. Met. 1989,32,245. (4)Wunderlich, B.; Arakawa, T. J. Polym. Sci. 1964,A2, 3967.
molecules at the air-water interface are transferred to a solid ~ u r f a c e .Epitaxial ~,~ growth on oriented substrates, even on the moderately well-ordered surfaces of other polymers, has also been to some extent s u c ~ e s s f u l . ~ ~ ~ Recently, Wittmann and Smithg have developed a new technique for the preparation of highly oriented thin films of poly(tetrafluoroethylene), or PTFE, based on observations made during early tribological studies of polymer surfaces.1° It simply involves sliding a bar of PTFE over the surface of a flat substrate under controlled conditions. Electron diffra~tion,~ scanning tunneling microscopy (STM)," and atomic force microscopy (AFM)12 indicate that the polymer chain axes are highly oriented along the slide direction. In the present paper, we describe the use of such films to measure the polarization-dependent X-ray ( 5 ) Tieke, B.; Lieser, G.; Wegner, G. J . Polym. Sci. 1979,17,1631. ( 6 ) Kakimoto, M.;Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.;
Hino, T. Chem. Lett. 1986,823. (7) Dorset, D. L. J . Electron Microsc. Tech. 1987,7,35. (8)Peterman, J.;Broza, G. J . Mater. Sci. 1987,22,1108. (9)Wittman, J. C.; Smith, P. Nature 1992,352,414. (10)Pooley, C.M.; Tabor, D. Proc. R . SOC.London, 1972,A329,251. (11)Bodo, P.; Ziegler, Ch.; Rasmusson, J. R.; Salaneck, W. R.; Clark, D. T. Synth. Met. 1993,55-57, 329. (12)Dietz, P.; Hansma, P. K.; Ihn, K. J.;Motamedi, F.; Smith, P. To be summitted for publication.
0743-7463/94/2410-4399$04.5QJ0 0 1994 American Chemical Society
Letters
4400 Langmuir, Vol. 10, No. 12, 1994 absorption spectra of PTFE a t the C and F 1s edges. The high degree of orientation and the favorable experimental geometry allow the basic features of the spectra, namely, the number and symmetry ofthe resonances, to be studied. The X-ray absorption spectrum (often referred to as the near-edge X-ray absorption fine structure, or NEXAFS13) of a molecule consists of discrete resonances due to the excitation of a core electron into the unoccupied valence and Rydberg levels. For a molecule or polymer adsorbed on a surface the experiment is not performed in a conventional absorption geometry, but rather using a n electron yield technique.13 Because of the fixed geometry, and the fact that the excitations are governed by dipole selection rules, the resonances are polarized, that is, their intensity varies as a function ofthe direction ofthe electric vector (or E vector) of the incident radiation relative to some symmetry element of the molecule. The technique has already been applied to oriented p~lyethylene,'~ as well as to oriented, adsorbed molecules containing alkyl chains of varying length.15-18 Ohta et aZ.16have reported C 1 s absorption spectra from oriented evaporated films of CF~(CFZ)~& (perfluoroeicosane) F~ and, more recently, Castner et al.l9have described similar polarization effects due to preferred chain orientations at the surface of mechanically treated bulk F'TFE samples. There has been some controversy, however, concerning the assignment of the resonances. Moreover, it is still not clear how the problem of the near-edge structure in extended systems should be addressed, even on a phenomenological level, so that orientation determination remains somewhat diffi~ult.~~-~~ X-ray absorption spectra were measured a t the Berlin synchrotron radiation source BESSY using a n ultrahigh vacuum system equipped with a partial electron yield detector. Monochromatic photons were provided by the SX-700 I11 monochromator which was operated with a resolution of 0.3 eV a t the C 1s edge and of 1.2 eV a t the F 1s edge. The photon energy was calibrated to an accuracy of f 0 . 5 eV by reference to the Cu 2p3,2 level of a clean Cu(ll0) crystal. The samples were mounted on a manipulator which enabled both the polar and azimuthal angles of incidence of the radiation to be varied. For convenience, the angle between the E vector of the light and the surface normal, &, is usually quoted (i.e., 6 E = 90" corresponds to normal incidence); a is defined as the angle between the projection of the E vector onto the surface and the slide direction, t. The partial yield detector was set to a cutoff of 150 eV which, from our previous experience with thin or anic films, corresponds to a sampling depth of 20-30 . The spectra were normalized by dividing by the signal from the Cu(ll0) crystal to give the ratio Ill0 displayed in Figures 1 and 2. The films were prepared using a bar of commercially available PTFE (hard sintered Fluon from IC1 Plc, U.K.) mounted vertically in a drawing stage, with a load of 500 g in order to give reproducible drawing conditions. The
0
I
2
Figure 1. Polarization-dependentC 1s near-edge absorption spectra of highly oriented PTFE at angles of a = O",22.5", 45", 67.5", and 90" between the E vector and the chain axis.
22.5'
p:
0"
(13)Stohr, J. N E W S Spectroscopy; Springer-Verlag: Berlin, 1992. (14) Stohr, J.; Outka, D. A,; Baberschke, K.; Arvanitis, D.; Horsley, J. A. Phys. Reu. 1987,B36, 2976. (15)Outka, D. A.;Stohr, J.; Rabe, J. P.; Swalen, J. D.; Rotermund, H. H. Phys. Reu. Lett. 1987,59,1321. (16)Ohta, T. A.;Seki, T. T.; Yokoyama; Morisada, I.; Edamatsu, K. Phys. Scr. 1990,41, 150. (17) Hahner, G.; Kinder, M.; Wo11, Ch.; Grunze, M.; Scheller, M. K.; Cederbaum, L. S. Phys. Rev. Lett. 1991,67, 851. (18)Hahner, G.; Kinder, M.; Thummler, C.; Wo11, Ch.; Grunze, M. J . Vac. Sci. Technol. 1992,A10, 2758. (19)Castner, D.G.; Lewis, K. B.; Fisher, D. A,; Ratner, B. D.; Gland, J. L. Langmuir 1993,9, 537. (20) Bradshaw, A. M.; Somers, J. Phys. Scr. 1990,2'31, 189. (21)Ishii, T.; McLaren, R.; Hitchcock, A. P.; Jordan, K. D.; Choi, Y.; Robin, M. B. Can. J . Chem. 1988,66, 2104.
34
1 2 I
I
I
5 I
I
I
I
I
I
680 690 700 710 720 730 740 750 760
Photon energy (eV) Figure 2. Polarization-dependentF 1s near-edge absorption spectra of highly oriented PTFE at angles of a = O", 22.5", 45", 67.5", and 90" between the E vector and the chain axis. substrate was a Si(100) wafer with a coating of native oxide held a t a temperature of 495 K, the drawing velocity was 0.3 mm s-l. The cleanness of the films was checked with X-ray photoelectron spectroscopy (XPS)in situ immediately before measuring the absorption spectra. After the film was heated to 425 K to desorb water, only
Letters
Langmuir, Vol. 10,No. 12, 1994 4401
the characteristic C 1s photoelectron line corresponding t o -CF2- was observed. Carbon 1s absorption spectra for five a angles at 6E = 90" are shown in Figure 1. In this geometry the light is incident normal to the surface; the sample is rotated such that the E vector is turned from being parallel to perpendicular to the slide direction. We note immediately that there is a very strong polarization dependence of the resonances. The first resonance a t 292.8 eV, for example, dominates the spectrum a t a = go", but its intensity falls to 12% of this value a t a = 0" (this comparison is made by drawing in a linear background between 290 and 304 eV and determining the integrated intensities). The second resonance a t 296.2 eV is the strongest feature a t a = 0"but is not reduced so strongly in intensity a t go", indicating that there are two features with opposite polarization dependences a t this energy. Resonance 4 is strongest a t a = 90". The same may be true of the sixth feature at 308 eV. The shape of the leading edge of the first resonance suggests that the measured line width is determined by lifetime broadening and/or inhomogeneities, rather than by the instrumental resolution. The spectra are similar to those of Ohta et al. for perfluoroeicosane16and of Castner et a1.19for skived PTFE surfaces, although the polarization dependence observed in the present work is more pronounced, probably indicating that the present samples are better ordered. When 6E is varied between 90" and 20" a t a fixed a of 90" (not shown), the spectra are virtually identical to that obtained a t 6E = 90" (i.e., as shown at the top of Figure 1). There thus is no polarization dependence perpendicular to the slide direction, although one might expect (see symmetry discussion below) that the resonances polarized perpendicular to the chain axis are also polarized in the plane containing the zigzag structure of C atoms or perpendicular to it. That this is not the case may indicate a substantial amount of disorder due to the occurrence of gauche conformations, and the formation of helical chains, as in the bulk.22 Figure 2 shows the corresponding set of spectra at the F Is edge as a function of the angle a between E and t. Note that the edge jump appears to be higher, so that, without prior knowledge of the shape of the background, it is dificult to estimate the relative intensities of the resonances. The first resonance shows the same polarization dependence as the first resonance in the C 1s spectrum of Figure 1 (i.e., perpendicular to the chain direction). It is difficult to assess to what extent the second resonance is reduced in intensity a t a = 90" because of the overlap with the first resonance. An approximate analysis, however, indicates that it has less than 25% ofits intensity at a = 0", which is essentially the same situation as a t the C 1s edge. Resonances 3 and 4 appear to have the same polarization dependence as resonance 2. The fifth resonance (35 eV above the first resonance and thus high up in the continuum) is strongly polarized parallel to the chain direction. Since the degree of alignment ofthe chains in the present system is remarkably high, it is instructive to discuss the data of Figures 1 and 2 from the point of view of the assignment of the resonances. As noted above, for both alkane and fluoroalkane chains, this topic has been somewhat controversial in the p a ~ t . l ~ -The ~ I first question to be asked concerns the number of expected resonances. A single PTFE chain belongs to theD%(line)space Because of the extended molecular structure, the valence levels should be regarded as one-dimensional Bloch states. ~
~
(22) Farmer, B. L.; Eby, R. K. Polymer 1981,22, 1481; 1986,26, 1944; 1990,31,2227. (23) Tobin, M. C. J . Mol. Spectrosc. 1966,4,349.
A (C-F)
2l
I
r
I
X Figure 3. Schematicband structure for the unoccupied states of a PTFE chain, after refs 23-27.
As far as the unoccupied states are concerned, the general features of the band structure of PTFE will be similar to those of a simple alkane as shown in a calculation by Seki et aLZ7A schematic representation of this part of the band structure is shown in Figure 3. (The occupied bands are more complicated because of the additional, but to some extent nonbonding, electrons associated with the replacement of H by F.) Since the space group is nonsymmorphic, the mainly C-C band (lowest A state at the X point) and the two, mainly C-F bands (A and B a t the X point) each split into pairs. At the r point, where the point group is D 2 h , the resulting six bands are designated Bsuand BIU,BzUand BI,, Bsuand 4, respectively. Due to an avoided crossing, the bands which become BSuat r reverse their C-C/C-F character. Core level excitation in systems containing two or more equivalent atoms gives rise to symmetry breakingz8which has to be considered when accounting for the polarization dependence of the resonances. The simplest way of treating this problem is to assume that the broken symmetry final state configuration also pertains in the initial state. (When there are only one-dimensional irreducible representations this approach gives the same result as the formally more correct method of forming linear combinations of the various broken symmetry configurations within the higher symmetry of the groundstate configuration.28) The results are summarized in Table 1for the two broken symmetry configurations, which are given by a core hole on a C atom (CZ,)and a core hole on a n F atom (C8),together with the expected polarization dependences of the resonances. We conclude that altogether six resonances should be observed, both a t the C edge and the F edge, and that all six are dipole-allowed. Moreover, at the carbon edge one resonance is expected to be polarized in thex direction along the axis ofthe C-C chain, two in the z direction perpendicular to the plane containing the C-C zigzag structure and three in t h e y direction in the C-C plane. The results show, however, that there is no polarization dependence perpendicular to (24) McCubbin, W. L. Phys. Status Solidi 1966,16, 289.
(25) McCubbin, W. L.; Manne, R. Chem. Phys. Lett. 1968,2,230. (26) Falk, J. E.; Fleming R. J. J . Phys. C.: Solid State Phys. 1973, 6 , 2954. (27) Seki, K.; Tanaka, H.; Ohta, T.; Aoki, Y.; Imamura, A.; Fujimoto, H.; Yamamoto, H.; Inokuchi, H. Phys. Scr. 1990,41,167. (28) Somers, J.;Robinson, A. W.; Lindner,Th.; Ricken, D.;Bradshaw, A. M. Phys. Rev. 1989,B40,2053.
Letters
4402 Langmuir, Vol. 10, No. 12, 1994 Table 1. Broken Symmetry Final State Configurations for FTFE"
czv
c.3
final state
final state
main character
representation a t r in D2h ( h e )
C-F
Ai
CY)
c-c
B3u Biu BZU B1, B3u
Bi Bi
(2) (2)
C-F
Ag
Ai
c-c C-F C-F
Bz (XI
CY)
A' (Yz) A" ( x ) A' (Yz)
A
(Yz)
A' CYZ)
x , y, and z in parentheses refer to the resulting polarization dependences (see text). a
the chain axis (Le. in the zy plane) so that we would then expect five resonances polarized perpendicular to the chain axis and one polarized along the chain axis. The same result is expected a t the fluorine edge. Figure 1shows that this model is confirmedqualitatively by the results for the C edge. Resonance 1 is clearly polarized perpendicular to the chain axis and we provisionally assign it to the B3u(A1)resonance with antibonding C-F character. According to the calculation of Seki et ~ 1this . corresponds ~ ~ to the lowest unoccupied band. Resonance 2, on the other hand, dominates the spectrum a t a = 0" and is the only resonance strongly polarized along the chain direction. It must therefore be assigned to the B1,(B2) resonance of antibonding C-C character. An assignment of the other four resonances in terms of their exact symmetry character is not possible a t present, but in any case we expect them all to be polarized perpendicular to the chain axis. This is certainly true of resonances 3 and 4. Resonance 5 is rather weak and may only be a satellite of a stronger resonance. Moreover, other similar structures may be obscured by the overlapping resonances below 300 eV. This highlights the problem of the analysis of near-edge data: a curveresolving analysis is only possible when the exact number of structures is known. Resonance 6 creates a small problem in that its polarization dependence is, as predicted, perpendicular to the chain axis, but, relative to a = go", there is considerable residual intensity at a = 0". Overall, however, there is still good agreement between the model and the experimental data. The immediate edge region of the corresponding F spectra (Figure 2) shows a polarization dependence similar
to that in the C spectra. Resonances 1and 2 are polarized in opposite directions, allowing the same assignment to be made. Resonances 3 and 5, on the other hand, are polarized parallel to the chain axis. Accordingto the model only one resonance, not three, should have this property (Table 1). At present, we have no explanation for this behavior. It might be argued that the use of a band structure-based description is not appropriate for the F 1s excitation. However, the so-called building-block modeP is also unable to provide a satisfactory explanation. The latter predicts that the resonances are polarized along individual C-C and C-F bond directions which is hardly compatible with elementary notions of molecular orbital theory. Since resonance 5 is high up in the continuum, it is possible that a scattering description is more appropriate and that we are observing the first EXAFS oscillation associated with the next pair of adjacent F atoms along the chain. The overall agreement with the model is therefore less good for the F edge spectra. Finally, we note that the assignment of the first resonance to a n excitation into a u* (C-F) level with C 2s,2pYand F 2py,p, character is also compatible with our general understanding of the bonding in perfluoroalkanes. The term value (i.e. the energy below the ionization potential) for resonance 1 at the C edge is 5.8 eV,16 compared to 2.7 eV in polyethylene,16 indicating a substantial lowering in energy of the first unoccupied level in the molecular ground state due to a weaker C-F bond.29
Acknowledgment. Financial support from the Federal German Ministry of Research and Technologyunder Contract No. 05 5EBFXB 2 is gratefully acknowledged. Research on conjugated polymers in Linkoping is supported in general by grants from the Swedish Natural Sciences Research Council (NFR), the Swedish National Technical Research Board (TFR), the Swedish National Board for Industrial and Technical Development ("IXK), the Neste Corp. Finland, Philips Research Netherlands, and by the Commission of the European Community within the SCIENCE programme (project 0661 POLYSURF). (29) Ishii, K.; McLaren, R.; HItchcock, A. P.; Robin,M. B. J . Chem. Phys. 1987,87,4344.
