J. Phys. Chem. 1996, 100, 16213-16217
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Ultraviolet Photoemission Spectra of Perchlorate-Doped cis- and trans-Polyacetylene Koji Kamiya,† Takayuki Miyamae,† Makoto Oku,‡,§ Kazuhiko Seki,‡ Hiroo Inokuchi,| Chizuko Tanaka,⊥ and Jiro Tanaka*,⊥ Department of Materials Science, Faculty of Engineering, Chiba UniVersity, Inage-ku, Chiba 263, Japan, Department of Chemistry, Faculty of Science, Nagoya UniVersity, Chikusa, Nagoya 464-01, Japan, Institute for Molecular Science, Okazaki 444, Japan, and Department of Chemistry, Faculty of Science, Kanagawa UniVersity, Hiratsuka 259-12, Japan ReceiVed: December 20, 1995; In Final Form: July 29, 1996X
The UPS spectra of cis- and trans-polyacetylene were measured by careful treatment of fresh samples. This is the first UPS measurement of cis-polyacetylene (cis-transoid). Assignments of the observed spectra are given by comparing with energy levels of cis- and trans-C20H22 obtained by ab initio MO calculation. The change of UPS spectra upon stepwise perchlorate doping was measured; at first the intensity of the C2pπ band decreased and the whole band shifted to higher binding energy. By further doping, a new C2pπ band appeared close to the Fermi level EF when the conductivity reached 11 000 S/cm. These changes show insulator to metal transition accompanied with structural transformation. The UPS spectrum near EF showed a strong correlational effect characteristic of a low-dimensional conductor. The spectra of heavily perchloratedoped and potassium-doped polyacetylene near EF are analyzed by a theory of Kopietz, Meden, and Scho¨nhammer (Phys. ReV. Lett. 1995, 74, 2997).
Introduction
Experimental Section
Polyacetylene is an insulator without doping, while it shows a metallic property by heavy doping.1 The conductivity of iodine- or perchlorate-doped polyacetylene reaches 5 × 104 S/cm,2-6 and the reflection spectra in the far infrared region exhibit metallic characteristics.7-10 Ultraviolet photoemission spectroscopy (UPS) is an important method for studying the electronic structure and electronic interaction in conducting polymers. It has been applied not only to undoped polymers but also to various doped states by both donors and acceptors. In particular, Salaneck et al.11 found that the top of the valence band of poly(3-hexylthiophene) moves to the Fermi level at saturated doping, indicating the formation of a metallic state. In this paper we will report the UPS spectra of cis- and transpolyacetylene and the change of the spectrum of cis-polyacetylene with perchlorate doping. The heavily doped film showed an extended band close to the Fermi level, indicating the transition to a metallic state, but the density of state near EF is significantly depressed. This is the first UPS evidence for the metallic state of p-doped polyacetylene.12 The experimental results on neat polyacetylene are compared with the energy levels calculated on model compounds by ab initio SCF MO methods with Gaussian 92.13 The change of energy levels by doping was compared with the energy band of doped polyacetylene calculated with the P-P-P method.14,15 The change of electronic structure by stepwise doping was explained by referring to the calculated results that a charged soliton chain is formed at light doping and a polson16 or a polaron chain17 is formed in the heavily doped film.
Thin polyacetylene films were synthesized by Naarman’s procedure,2 with the aged catalyst prepared by Tsukamoto’s method.3 The catalyst was spread on thin polypropylene film of 50 µm thickness in the glovebox, in which purified Ar gas was circulated. The film with the catalyst was placed in the reaction vessel. Ultrapure acetylene gas (99.999%) was flowed over the catalyst, and the reaction was continued for 1.5 h at room temperature. Immediately after synthesis, the polyacetylene film was stretched 5 times in length. Thicknesses of the films after elongation were 2-8 µm depending on the reaction time. Since neat polyacetylene in an insulator, very thin films were used for UPS measurements for preventing the sample from charging by ionization. Rather thick films were used in the heavily doped case, since they were conducting and were not affected by the charging. Polyacetylene film immediately after synthesis contains more than 90% cis-form, and it isomerizes to the trans-form by standing at room temperature. To avoid isomerization, the fresh film attached to polypropylene base was kept at liquid nitrogen temperature before measurements and doping. The fresh surface of the cis-film was obtained by peeling it off from the polypropylene base in the spectrometer kept at high vacuum. The UPS spectra of the cis-film were measured for the first time by this procedure. The spectra of the trans-film were measured by heating the cis-film at 200 °C for 1 h in situ in the spectrometer, which was kept below 10-8 Torr. Perchlorate doping was carried out by immersing the film into an acetonitrile solution of cupric perchlorate. Different doping levels were attained by varying the immersion time in the solution. The doping concentrations were determined after UPS experiments by weight uptake. The conductivity of doped film was measured with the Montgomery method,18 and the thickness of the film was determined by taking SEM photographs. All procedures of synthesis, elongation, and doping were carried out in the glovebox filled with purified Ar gas, and the films were sealed in a vessel filled with Ar gas. The vessel was opened in another glovebox attached to the UPS
†
Chiba University. Nagoya University. Present address: Kameyama Works, The Furukawa Electric Co. Ltd., Kameyama, Mie 590-01, Japan. | Institute for Molecular Science. ⊥ Kanagawa University. X Abstract published in AdVance ACS Abstracts, September 1, 1996. ‡ §
S0022-3654(95)03793-2 CCC: $12.00
© 1996 American Chemical Society
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Figure 1. UPS spectra of cis-polyacetylene. The binding energy Eb is shown with reference to the vacuum level (EVAC ) 0). The calculated energy levels of cis-C20H22 are shown at the bottom, and the simulated spectra are shown by the dotted line.
Figure 2. UPS spectra of trans-polyacetylene. The binding energy Eb is shown with reference to the vacuum level (EVAC ) 0). The calculated energy levels of trans-C20H22 are shown at the bottom, and the simulated spectra are shown by the dotted line.
system, and the sample was introduced into the spectrometer without exposing to air. The UPS spectra were measured on an angle-resolving spectrometer at the beamline 8B2 of the UVSOR facility of the Institute for Molecular Science. The incident photon energy was 40 eV, and the incident angle was 50°, and the photoelectrons were detected at 0°, both relative to the surface normal. The Fermi level was determined by the Fermi edge of gold film in situ deposited onto the sample surface. For determining the binding energy relative to the vacuum level, the low-energy cutoff of the spectra was determined with an acceleration voltage of 5 V between the sample and the energy analyzer. The ab initio MO calculations were carried out at the Computer Center of the Institute for Molecular Science using the Gaussian 92 program.13 The basis set was 3-21G.
observed and calculated UPS spectra is satisfactory. Subsequently, we will discuss the UPS spectra on the basis of the energy levels found by the ab initio MO calculations of model compounds. The top of the valence band lies below the Fermi level by 0.9 eV in the cis-film and 0.7 eV in the trans-films, respectively. The band gap of the semiconductor or insulator can be estimated approximately as twice these values, if we assume that the Fermi level is at the center of the band gap. Accordingly, the band gaps are estimated to be 1.8 and 1.4 eV, respectively, for the cis- and trans-polyacetylene films. The peak energies of visible absorption band are at 2.0 and 1.5 eV for the cis- and transfilms, respectively, in reasonable agreement with the estimation of the band gap by UPS spectra. When we regard the first and the second peaks of trans- and cis-polyacetylene spectra as being coincident with the top and the bottom of the C2pπ band, the bandwidths are 5.0 and 4.3 eV, respectively. The calculated C2pπ band spreads more widely in the trans-form than in the cis-form; it spans from 4.5 to 10.0 eV (5.5 eV wide) in the trans-film and 5.3 to 9.8 eV (4.5 eV wide) in the cis-film. The calculated values are in fair agreement with the observed values; however, the calculated peak at 10.6 eV of cis-C20H22 was not clearly correlated to the observed spectrum. Experimentally the upper edge of the C2pπ band of the cisfilm is more conspicuous than the trans-film. This trend may correspond to the higher density of states in the cis-form than in the trans-form. On the other hand, the C2pσ level expands broader in cis- than in trans-C20H22, since the C2s and the C2px + C2py levels mix more extensively in cis-C20H22. In transC20H22 the C2s and C2px + C2py levels are not mixed strongly, forming a sharp peak at 16 eV derived from the top of the C2s band. Correspondingly, the observed spectrum of transpolyacetylene film shows a peak at the predicted position. Therefore this peak is assigned to the top level of the C2s band. The top and bottom of the C2px + C2py bands are calculated to appear at 8.5-14 eV in cis- and 9.6-14 eV in trans-C20H22, and experimentally broad bands are found in the corresponding spectral range. We have also measured the orientational effect of UPS on the stretched film. However, a remarkable dispersion effect was
Results and Discussion UPS of cis- and trans-Polyacetylene. The UPS spectra of cis- and trans-polyacetylene are illustrated in Figures 1 and 2, together with the theoretical results described below. The UPS spectrum of the trans-film is close to those reported on polyacetylene prepared by Shirakawa’s method.19 The transfilm measured without exposure to air did not show any shoulder at the ∼9 eV region, but a small bump appeared at this region when the surface was oxidized by air. Figure 1 illustrates the spectrum without the shoulder. Previously, Bredas et al.20 discussed the band structure and the UPS spectra on the basis of the effective Hamiltonian method. They explained qualitative features of the spectra of trans-polyacetylene, but comparison with the cis-film and details of the band shapes were not explored. The UPS spectrum of the cis-film is reported in this paper for the first time. Accordingly, the comparison of UPS spectra of cis- and trans-film will be made referring to the MO levels of short polyene molecules calculated with the ab initio SCF MO method. The MO calculation was carried out on cis-C20H22 (cistransoid) and trans-C20H22. The simulated UPS spectra were obtained by giving a half-bandwidth of 0.4 eV to each level, and they are given by broken lines in Figures 1 and 2. For each MO level, the character of the composing atomic orbitals is illustrated for assignments. The agreement between the
UPS of Percholate-Doped cis- and trans-Polyacetylene
Figure 3. Change of UPS spectra with increasing conductivity by stepwise perchlorate doping.
Figure 4. Detail of UPS spectra near EF. The bottom panel shows the appearance of the metallic energy level in the heavily doped film. The middle panels show a shift of the C2pπ band to the higher energy side. The top panel illustrates the spectra of gold showing the resolution of the spectrometer.
not found in the observed UPS spectra. Probably the stretching of 5 times was insufficient to find the effect of alignment. UPS of Perchlorate-Doped Polyacetylene. In Figure 3 the change of the UPS spectrum by various amounts of perchlorate doping is illustrated. By a slight doping of the trans-film with cupric perchlorate, a little conductivity was found as 1.4 S/cm. The Fermi level was lowered by 0.4 eV, and the intensity of the C2pπ band was decreased. The change of UPS spectra near EF is shown in Figure 4 with an expanded scale. By the next step of doping, the electrical conductivity was increased to 170 S/cm, and the UPS spectra were changed as shown in Figures 3 and 4. The intensity of the C2pπ band was significantly reduced, and the C2pσ and C2s bands were rigidly shifted by 0.4 and 1.5 eV to the higher binding energy side, respectively. After further doping, the conductivity was increased to 1600 S/cm. Nevertheless, the top part of the C2pπ
J. Phys. Chem., Vol. 100, No. 40, 1996 16215
Figure 5. Calculated density of states of C2pπ energy levels of (a) trans-polyacetylene, (b) charged soliton chain, and (c) polson chain. The abscissa is the orbital energy in electronvolts, and the ordinate shows the density of states in arbitrary units.
band vanished, as shown in the middle panel of Figure 4. These values of conductivity correspond to the intermediate stage of doping, and the structure of the chain is considered to be the charged soliton lattice. The charged soliton is a stable species in the initial stage of doping, for instance, when the perchlorate ion is less than 4%. The characteristic features of these spectra are explained by the P-P-P molecular orbital calculation on the doped model chain in Figure 5.16 For the model of the charged soliton chain, the dopant content was assumed to be 4.3%. When we compare the density of occupied states for undoped (a) and moderately doped (b) states, we see that the appearance of the unoccupied midgap level (LUMO) in the gap region shift the Fermi level downward and the whole C2pπ band shifts to the higher binding energy side. These features are in good agreement with the observed spectra. After further doping, the conductivity was increased to 8100 S/cm, and a new UPS band appeared just below EF, as shown in the lower part of the panels in Figures 3 and 4. Finally the conductivity was increased to 11 000 S/cm by further doping, the Fermi level shifted further to the deeper energy side by 0.2 eV, and the top of the C2pπ band suddenly appeared close to the Fermi level, as shown in the bottom of Figures 3 and 4. This indicates the formation of a metallic energy state in the heavily perchlorate-doped polyacetylene. This is consistent with the results deduced from far-infrared spectra.9 The P-P-P MO calculation on the polson chain, which contains 7% dopant (Figure 5c), shows the one-dimensional metallic density of state, accompanied with lowering of the Fermi level. The polaron chain, which has an odd electron for the unit, will also give a metallic energy band.17 Accordingly the present experiment does not distinguish if either the polaron or polson chains are dominant in the heavily perchlorate-doped polyacetylene. In the top panel of Figure 4, the spectra of Au is illustrated to show the resolution of the spectrometer. Comparing with the bottom chart of heavily doped polyacetylene, it is evident that the spectra of the heavily perchlorate-doped film is significantly suppressed in the range of near EF. Although the spectra showed evolution of a faint density of states near EF, it does not show the Fermi step of a typical metal. The unusual UPS of the low-dimensional conductor has attracted much attention, and a power law dependence of spectra
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near EF is regarded as a characteristic property of strongly correlated low-dimensional metals. In the one-dimensional conductor, the electron correlation is much stronger than in the three-dimensional metal since the motion of electrons is limited within the chain direction. Dardel et al. first reported on anomalous UPS for quasi-onedimensional solids, such as (TMTSF)2PF6,21 K0.3MoO3, and (TaSe4)2I.22 They showed that the intensity near EF is represented by I(ω) ∼ ωR, with R ∼ 1.0 for the range of pω ) 0.01.0 eV in (TMTSF)2PF6 and 0.0-0.3 eV in the latter compounds, where ω is the electron binding energy measured from EF. They proposed that the electron correlation effect has appeared in these spectra. Fujimori et al. studied N,N′-dicyanoquinonediimine-Cu salt23 and BaVS324 showing that R ∼ 0.7(300K)-0.85(130K) in the former and 0.9(295K)-1.1(190K) in the latter crystals. Moreover, alkaline-metal-doped fullerene reported by Takahashi et al.25 showed a similar spectral shape near EF, despite that its dimensionality is not so low. Theoretically, Tomonaga26 and Luttinger27 discussed characteristic behavior of low-dimensional electron systems by neglecting backward scattering and the Umklapp process. In the Tomonaga-Luttinger (TL) model, only forward scattering is considered, and electrons near EF are called TL liquid. Theoretical development for this system has been discussed by So´lyom,28 Emery,29 and Voigt.30 As regards the momentum distribution and density of states near EF, Suzumura showed a power law.31 Schultz32 discussed explicitly the anomalous power law observed in UPS. He showed that R will be as small as 0.125 by the one-dimensional Hubbard model even though on site Coulomb repulsion is very large. However, he mentioned that it may be close to 0.5 if one considers more correlation effects than the Hubbard model. Penc et al.33 discussed the limit of the power law as being |ω - F| ∼ t|2/U. Accordingly, the range of t| and U as correlated quantities can be found from the experimental value of the upper limit. In Figure 6, the spectra of heavily perchlorate-doped and heavily potassium-doped films35 are shown comparatively. In both films, the upper limits of the power law are about ∼0.8 eV. From the values, a correlation of t| and U is found as t| ∼ 0.9xU eV. The shape of these films is apparently different; therefore we analyzed the exponents as follows. Recently Kopietz et al.33 presented a theory including threedimensional long-range Coulomb interaction in anisotropic three-dimensional metals, in which quadratic arrays of onedimensional chains are considered. They showed a figure to correlate R and VF indicating that R exceeds 0.5 when the Fermi velocity (VF) is as slow as 107 cm-1. Moreover, a tiny threedimensional Fermi liquid is predicted to appear near EF by the transverse interaction (t⊥), despite that the TL behavior is found in the high-energy region. In one dimension the Fermi wave vector kF is given by kF ) πN/2L, where L is the chain length and N is the number of carriers. By using the relation VF ) pkF/m, we may be able to estimate the effective mass from VF and the carrier density. On the basis of this theoretical result, we have analyzed the perchlorate- and alkaline-metal-doped polyacetylene by the method of Fujimori et al.23 The spectral intensities are simulated by
I(ω) ) max(BωR,C)
(1)
where B and C are positive constants, with B > C, and C represents a tiny Fermi surface near EF. The parameters for fitting of Figure 6 are as follows: perchlorate-doped film (B )
Figure 6. Details of the spectra of the metallic state near the Fermi level: (a) heavily perchlorate-doped polyacetylene; (b) heavily potassium-doped polyacetylene.35 The ordinate is the intensity in arbitrary units, and the abscissa is the binding energy relative to EF ) 0. The black dots are experimental values, and the solid lines are simulated spectra with the R values shown in the text. The dotted lines represents the C term of eq 1 in the text. The broken line shows the base line. The observed points near EF are broadened by thermal effect and instrumental resolution.
94.8, C ) 11.0, R ) 0.656) and potassium-doped film (B ) 98.6, C ) 10.5, R ) 1.127). By the use of these R values, we estimate VF from Figure 1 of Kopietz et al.33 The dopant concentration of heavily perchlorate-doped film is 6-8% and that of potassium is 12-16%. If we assume that every dopant provides a carrier, then the kF of potassium-doped film is about twice that of perchlorate-doped film. By referring to the above R values, the effective mass of the carrier in potassium-doped film is estimated from VF and kF as 1.46me and that of perchlorate-doped film as 0.33me. The effective mass of the carrier in doped polyacetylene has not been found experimentally up to now; therefore the present analysis is significant. The UPS spectra showed unprecedented important information on doped polyacetylene. In conclusion, the UPS spectra are found to be very effective for elucidating the electronic structure of conducting polymers, in particular the electron correlation effect in low-dimensional solids. Acknowledgment. This research was supported by an international joint research project of NEDO. T.M. thanks JSPS for the fellowship for junior scientist. J.T. thanks Professors W. R. Salaneck, D. Baeriswyl, M. Ogata, and A. Fujimori for helpful information. We thank the computer center of IMS for the use of computer and Gaussian programs. We also thank Mr. T. Noda of Nagoya University for his skillful performance for making the glass vacuum apparatus. This work was
UPS of Percholate-Doped cis- and trans-Polyacetylene performed as a Joint Studies Program of the UVSOR facility at IMS. References and Notes (1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977, 577. Chiang, C. K.; Fincher, C. R., Jr.; Park, W.; Heeger, A. J.; Shirakawa, H;, Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Phys. ReV. Lett. 1977, 39, 1098. (2) Naarmann, H.; Theophilou, N., Synth. Met. 1987, 22, 1. (3) Tsukamoto, J.; Takahashi, A.; Kawasaki, K. Jpn. J. Appl. Phys. 1990, 29, 125. (4) Shirakawa, H. Synth. Met. 1995, 69, 3. (5) Ishiguro, T.; Kaneko, H. Synth. Met. 1994, 65, 141. (6) Miyamae, T.; Mori, T.; Seki, K.; Tanaka, J. Bull. Chem. Soc. Jpn. 1995, 68, 803. (7) Kamiya, K.; Tanaka, J. Synth. Met. 1988, 25, 59. (8) Hasegawa, S.; Oku, M.; Shimizu, M.; Tanaka, J. Synth. Met. 1990, 38, 37. (9) Miyamae, T.; Shimizu, M.; and Tanaka, J. Bull. Chem. Soc. Jpn. 1994, 67, 2407. (10) Epstein, A. J.; Joo, J.; Kohlman, R. S.; Du, G.; MacDiarmid, A. G.; Oh, E. J.; Min, Y.; Tsukamoto, J.; Kaneko, H.; Pouget, J. P. Synth. Met. 1994, 65, 149. (11) Lo¨gdlund, M.; Lazzaroni, R.; Stafstro¨m, S.; Salaneck, W. R.; Bre´das, J.-L. Phys. ReV. Lett. 1989, 63, 1841. (12) Kamiya, K.; Inokuchi, H.; Oku, M.; Hasegawa, S.; Tanaka, C.; Tanaka, J.; Seki, K. Synth. Met. 1991, 41-43, 155. (13) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92; Gaussian Inc.: Pittsburgh, PA, 1992. (14) Pariser, R.; Paar, R. G., J. Chem. Phys. 21, 1953, 466, 767. (15) Pople, J. A. Proc. Phys. Soc. (London) 1955, A68, 81. (16) Tanaka, C.; Tanaka, J. Mater. Res. Soc. Symp. Proc. 1992, 247, 577.
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