Optical properties of heat-treated polyparaphenylene
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Optical properties of heat-treated polyparaphenylene M. J. Matthewsa) and S. D. M. Brown Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. Endo, T. Takamuku, and T. Karaki Faculty of Engineering, Shinshu University, 500 Wakasato, Nagano 380, Japan (Received 10 April 1998; accepted 22 July 1998)
The optical properties of heat-treated polyparaphenylene (PPP) were investigated by means of Raman and photoluminescence (PL) spectroscopy. Special attention is given to PPP heat-treated to temperatures (T HT ) near the carbonizing temperature region (THT ø 700 ±C) since polymer-based carbonaceous compounds with low-T HT (,1000 ±C) have been found to exhibit electrochemical properties that strongly contrast both the as-prepared polymer and fully carbonized samples. The Raman spectra show that for T HT in the range 650–725 ±C, several Raman bands near 1300 cm21 can be correlated with both ground-state benzenoid and excited-state quinoid PPP Ag modes. An increase in quinoid character is observed with increasing T HT , which is consistent with the theoretically predicted stabilization of the quinoid form in the presence of a high density of defects. The smaller energy bandgap for p 2 p p transitions in the quinoid conformation relative to that for the benzenoid form allows for a resonance condition to be present for laser excitation wavelengths (lexc ) near the visible (,1–2 eV). We also report a small dispersion effect in the observed quinoid breathing mode band which can be compared to dispersion effects previously reported for the case of trans-PA. The decrease in bandgap for the defect-induced quinoid form is also evidenced in the PL spectra of samples heat-treated up to 650 ±C, which show vibronic structure in the blue-green emission data in the energy range 2.4–3.0 eV, with well-resolved peaks separated by quinoid phonon energies of 0.165 eV. Franck–Condon analysis shows an increase in the Huang–Rhys parameter (S) with increasing T HT which can be related to changes in the electron-phonon coupling of valence and conduction band states.
I. INTRODUCTION
PPP has received considerable attention over the last two decades, primarily because of the interesting physical properties of n- or p-doped PPP and the application of conjugated polymers such as PPP to optical1 or electrochemical2,3 devices. More recently, laddertype PPP derivatives4 – 6 have been aggressively investigated as active components in blue light emitting diodes (LED’s) which can be used, for example, in flat-colordisplay devices.7 As a conducting polymer, the electrical conductivity of PPP has been shown to increase by several orders of magnitude through modest doping by species such as AsF5 , FeCl3 , or alkali metals.8,9 In these doped materials, spinless charge carriers in the form of bipolarons
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