Tuning of Electronic Structure of C_(60)@SWCNT and C_(70)@SWCNT (Peapods): In-Situ Raman and Vis-NIR Spectroelectrochemical Study

摘要

Charger-transfer on fullerene peapods (C_(60)@SWCNT and C_(70)@SWCNT) was studied by electrochemistry in 0.2 M LiClO_4 + acetonitrile and in butylmethylimidazolium tetrafluoroborate (ionic liquid). The latter medium offers broader window of accessible electrochemical potentials, good electrical conductivity and favorable optical properties for spectroelectrochemistry. Similar to empty SWCNT, the electrochemistry of peapods is dominated by capacitive double-layer charging. Vis-NIR spectra evidence reversible doping-induced bleaching of the transitions between Van Hove singularities. The bleaching of optical transitions is mirrored by quenching of resonance Raman scattering in the region of tube-related modes. The Raman modes of intratubular C_(60) exhibit considerable intensity increase upon anodic doping of peapods, but these modes are not enhanced at cathodic charging. The "anodic enhancement" of Raman scattering from intratubular C_(60) is not reproduced in C_(70)@SWCNT. All relevant Raman modes of intratubular C_(70) show symmetric charge-transfer bleaching as the RBM/TM lines. A tentative interpretation follows from two arguments: (ⅰ) the LUMO of C_(60) is located unusually close to the Fermi level of SWCNT and (ⅱ) C_(70)@SWCNT (peapods) may change the configuration from lying to standing one. This keeps the LUMO of C_(70) at higher energies even in wide tubes.