Infrared transmission of heavily doped polyacetylene

Abstract

2014 Dans ce travail nous présentons des résultats de mesures de transmission et réflexion dans le domaine infrarouge (4 03BCm-25 03BCm) effectuées sur des films de polyacétylène. Les mesures ont été faites sur des échantillons de polyacétylène [CH(I3-)Y]x de diverses épaisseurs pour des taux Y de dopage à l'iode allant de 0 à 7 %. La méthode utilisée permet d'obtenir des spectres sur des films libres de tout support. De plus contrairement aux techniques usuelles elle permet de travailler, à fort dopage sur des échantillons assez épais, pour lesquels il est possible de déterminer l'épaisseur avec précision et également le taux de dopage par pesée. Les résultats confirment que, bien que le dopage à l'intérieur des fibrilles ne soit pas uniforme, le système est complètement isomérisé. De plus notre analyse prouve que le mode (03C92 à 900 cm-1 qui apparait sous l'action du dopage ne peut pas être le mode d'ancrage. Abstract-This report presents the results of measurements of transmission and reflexion spectra of polyacetylene films in the infrared range 4-25 03BCm. [CH(I3-)Y]x samples were used with various thickness for iodine doping level Y up to 0.07. We used a high detectivity spectrometer which permitted us to obtain spectra with very highly doped free-standing films. In this way, we could measure both the relatively large thicknesses of samples with accuracy and also directly measure the doping level by weight uptake. The results show that, although intrafibril doping is nonuniform, the material is quite isomerized. In addition, our analysis demonstrates that the 03C92 band (900 cm-1) induced upon doping cannot be the pinned mode.

Infrared Properties of Polyacetylene. Experiments and Theory

“…The 900 cm-' one warps and shifts also towards high frequencies. This result is quite different from that observed with usual (CH), in which upon increasing the dopant level the shift is towards low frequencies [39,47]. The authors think that the observed behaviour could arise from the differences in conjugated length.…”

“…Thus the origin of the enhanced IRAV mode intensities in heavily doped polyacetylene remains an important, unresolved, problem (Kim and Heeger [52]). Following the conclusion of [47] we deduce that the 900 cm-' mode could have a molecular origin which should be determined. At this point we must indicate that an alternative explanation can be proposed.…”

“…An important question concerns the behaviour of the 900 cm-' band when the dopant concentration increases: no significant shift is observed by Kiess et al [50], but Rabolt et al [39] with AsF, the band is shifted from 901 cm-' (y x 0.03) to 832 cm-' (y rz 0.10). The band is found at M 800 cm-' by Benoit et al [47] for 4% I; and by Fujimoto et al [61] for saturation.…”

“…Furthermore, as the conductivity increases, the potential V(x) must be screened by the charge carriers. Consequently, the pinning frequency should drop down to zero (except the effect due to the discrete character of ions) when the doping level increases [47]. This effect is used by Boss and Hicks [123] to explain the large width of the 900 cm-' mode in photoinduced experiments ;…”

Polarized spectra of oriented polyacetylene doped with iodine

“…The logarithm of transmittance is given by where R is the reflection factor and x the sample thickness. Reflectivity measurements on non-oriented doped sample [9] show that R increases with doping level from 0.1 to 0.3 when conductivity varies from 1 0 P to lof1 Q-l cm-l. Thus, there is evidence that the observed very high value of polarization degree is not due to reflection but rather arises from absorption.…”