Spectral characteristic of metallic state of polyacetylene

Synthetic Metals, 55-57 (1993) 121-126

121

SPECTRAL CHARACTERISTIC OF METALLIC STATE OF POLYACETYLENE

J. TANAKA, C. TANAKA, T. MIYAMAE, K. KAMIYA, M. SHIMIZU, M. OKU, K. SEKI, J. TSUKAMOTO,* S. HASEGAWA** and H. INOKUCHI** Department of Chemistry, Faculty of Science, Nagoya University, Chikusa, Nagoya, 464-01, *Toray Industries, Otsu, 520, and **Institute for Molecular Science, Okazaki, 444, Japan

ABSTRACT The absorption, reflection and ultraviolet photoemission spectra (UPS) of thin films of polyacetylene doped with perchlorate ion or potassium are studied to find the mechanism of insulator to metal transition. The transition occurs by two steps, first the charged soliton chain showing the mid gap absorption is formed in the lightly doped film, and then the metallic state is found in the heavily doped film, which is composed of poison units. The strong far infrared absorption is spectral characteristic band of poison unit. The poison chain gives a metallic far infrared reflectance along the chain direction in the heavily doped film. In the heavily doped film a genuine metallic Fermi level is found in the UPS spectra. The results are interpretted by the molecular orbital calculation of doped polyacetylene which is presented in the accompanied paper. INTRODUCTION Among variety of conducting polymer, the heavily doped stretched polyacetylene (PA) exhibits the highest conductivity.[1,2] The value of conductivity comparable to normal metal is obtained by doping with iodine. Before doping, the PA is an insulator,-therefore the transition of electronic and molecular structure from insulator to metal is of particular importance to elucidate the conduction mechanism of the polymer. The interconnection of chemical and electronic structure of conducting polymer is still unsolved problem from a theoretical chemical viewpoint. We characterized the molecular and electronic structure of polyacetylene during the doping process by several spectral methods. We considered the transition mechanism based on these experiments and the molecular orbital calculation of model compounds and model polymer. EXPERIMENTAL Thin films of polyacetylene are prepared by Naarmann or Tsukamoto's method.[1],[2] The stretched free standing film of 300 /~, thick is obtained by dissolving the backed polyethylene film, and a TEM photograph showed that the smallest fibril is only 50 A wide. (Fig.l) The alignment of film for the stretched direction is better than 80 percent, which is in accord with the spectral data shown below. The absorption spectra were taken with about 500 .~, thick films and the reflection and ultraviolet photoemission spectra were taken with 1 - 4 micron films.

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122

Fig. 1. TEM photograph of trans polyacetylene stretched film. A single fibril, running perpendicular to the main chain, can be seen in the crevice, the diameter of fibril is only 50A. The thin film was on the stage cooled by liq. He.

RESULTS Change of Absorption Spectra by Doping The absorption spectra of stretched PA are shown in Fig.2a for the parallel and perpendicular directions of stretching; the film is composed of 60 percent cis (peak at 16700 cm -t) and 40 percent trans (peak at 15000 cm -1) and the dichroic intensity ratio showed 80 percent orientation. By a light doping with iodine or alkaline metal, the mid gap band is found in the 2000 - 9000 cm-1 region.(Fig.2b)[3,4] The charged soliton and antisoliton structure of Fig.3 gives the mid gap absorption in the near infrared region. The charged soliton vibrational band is found at 1350 cm -1 irrespective of dopant species and dopant level.(Fig.2b,2c) ,

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Fig.2. Absorption spectra of iodine doped PA. Solid lines are parallel m the strectching direction and dashed lines are perpendicular. (a) pure PA (b) lightly doped PA (c) heavily doped PA.

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D n D doping

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Fig.3. The structure of model doped polyenes.

By further doping a drastic change of spectra occurs; the absorption spectra is increased in the far infrared region. (Fig.2c) Appearance of strong far-infrared band below 800 cm -I is a sign of formation of new charged species responsible for metallic conduction. We proposed that the polson is an essential unit for metallic conduction.[5,6,7] The structures of model doped polyenes are illustrated in Fig.3. The isolated polson-antipolson unit will give the far-infrared absorption band in the 200 - 800 cm -1 region. The dichroic intensity ratio of the far-infrared band is higher than the other bands, accordingly the conducting fibril composed of polson chain may be alligned more parallel along the stretched direction.J8] Reflection Spectra of Doped PA In the heavily doped stage the metallic free carrier is formed, and the reflection spectra give most clearly the formation of metallic state. The reflection spectra of perchlorate doped films are shown in Fig.4 for 3.6 percent (b) and 13 percent (a) films. The Kramers-Kronig transformation of reflectance gives optical conductivity spectra, which shows a high optical conductivity comparable to the dc conductivity of 2xl04S/cm for the film (a). The film of intermediate doping (b) does not show comparable optical conductivity in the far infrared region, neverthless its dc conductivity shows a fairly high value of 4400 S/cm. This result suggests that the charge density wave (CDW) may be formed in the intermediate doping level; the CDW may have smaller scattering rate compared to the single particle scattering and the far infrared reflectance may be weaker than the case of metallic free carrier.[9] The metallic high reflectance in the far-infrared region are found for films of more than 4.5 percent doped films up to 13 percent doping, accordingly the transition to the metallic state occurred at about 5 percent in the perchlorate doping. The dc conductivity shown in Fig.5 illustrates that the conductivity of the perchlorate doped film reaches the highest value above 5 percent and continues to 10 percent doped level. This critical value of transformation is comparable to the value found in the iodine doping where it reaches the maximum value at 16 percent doping of I atom, which corresponds to 5 percent for I3-.[8]

124

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125 UPS of Doped PA The ultraviolet photoemission spectra (UPS) shows a direct evidence for the electronic energy band structure of the polymer. [ 10] The UPS spectra of cis- and trans-PA are correlated well to the calculated orbital energies by the ab initio SCF MO calculation. [11] The change of UPS spectra accompanied with doping is illustrated in Fig.6 for the perchlorate and the potassium doping. In the initial stage of doping, the mid gap band is immediately formed and the Fermi level shifts downward in the perchlorate and upward in the potassium doping. The shifts in the n-doped and p-doped films are correlated to the calculated energy levels discussed in a separate paper.[7] The observed and caluculated energy bands are illustrated in Fig.7 schematically. The filled mid gap band is formed in the n-doping while the vacant mid gap band is

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y:o.o ( -lh Eb • Binding Energy Relative to Vacuum Level Fig.6. Ultraviolet photoemission spectra of doped PA. The vaccum level is taken as the origin of the energy scale. The filled mid gap band of moderately K doped PA is marked by hatched line.

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Fig.7. Energy band structure for the n- and p-doped PA.

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126

expected in the p-doping. Actually the filled mid gap band is found in the moderately K doped PA above the valence band. By further doping the metallic Fermi levels are found in both the n- and p-doped films. The experimental energy shifts are in good agreement with the calculated values by the molecular orbital method.[7] The finding of the Fermi levels in the doped PA in both the n- and p-doping is in accord with a recent finding of Fermi level in poly-3-hexylthiophene by Salaneck's group.[ 12] SUMMARY The far infrared absorption spectra of medium doped polyacetylene showed that the poison unit is formed in advance of metallic state is realized. When the concentration of poison unit is increased sufficient enough to form the metallic energy band, the characteristic band of matallic free carder is found in the far infrared reflection spectra. The metallic Fermi level in the UPS of the doped polyacetylene strongly suggests that a genuine metallic state is formed in both the p- and n-doped films. ACKNOWLEDGEMENTS This research was supported by a grant in aid of NEDO and Ministry of Education, Science and Culture of Japan. We also thank to Dr. Yoshinori Fujiyoshi of Protein Engineering Research Institute for taking TEM of polyacetylene, and Mr. Toshiaki Noda of our department for skillful glass works. REFERENCES 1 N.Naarmann and N.Theophilou, ,qynth Me.t : 22 (1987) 1. 2 J.Tsukamoto, A. Takahashi and K.Kawai, lpn IAppl Phy.~ : 2q (1990) 125. 3 N.Suzuld, M.Ozaki, S.Etemad, A.J.Heeger and A.G.MacDiarmid, Phv.~Rev_l~tt _45 (1980) 1209. 4 J.Tanaka, Y.Saito, M.Shimizu, C.Tanaka and M.Tanaka~ FI,il (?ham Roe. lpn:. 60 (1987) 1595. 5 C.Tanaka and J.Tanaka, .qvnth Me.t :41-4a (1991) 3709. 6 C.Tanaka and J.Tanaka, MatReg_goc.qvmn_Proc__ 247 (1992) 577. 7 C.Tanaka and J.Tanaka, these Proceedings, 8 S.Hasegawa, M.Oku, M.Shimizu and J.Tanaka, .qvnth Mat : 38 (1990) 37. 9 S.Srindhar, D.Reagar and G.Gruner, Phyg RevB: a4 (1986) 2223. 10 K.Seki, in H.Baessler(eds.), Optical Techniques to Characterize Polymer Systems, 1989, Elsevier Science Publishers, Amsterdam, pp. 115-180. 11 K.Kamiya,H.Inokuchi, M.Oku ,S.Hasegawa, C.Tanaka, J.Tanaka and K.Seki, gynth M~.t, 155 (1991) 41. and K.Kamiya et. al. to be published. 12 M.L6gdiund, R.Lazzaroni, S.Stafstr6m, W.R.Salaneck and J.L.Br~das, Phy.~ Ra.v l ~ t t : 6q (1989) 1841