Aggregation Control of alpha-Sexithiophene via Isothermal Encapsulation Inside Single-Walled Carbon Nanotubes
Résumé
Liquid phase encapsulation of α-sexithiophene (6T) molecules inside individualized single-walled carbon nanotubes (SWCNTs) is investigated using Raman
imaging and spectroscopy. By taking advantage of the strong Raman response of this system, we probe the encapsulation isotherms at 30°C and 115°C using a statistical ensemble of SWCNT deposited on a Si/SiO2 substrate. Two distinct and sequential stages of
encapsulation are observed: Stage 1 is a one-dimensional (1D) aggregation of 6T aligned
head-to-tail inside the nanotube and stage 2 proceeds with the assembly of a second row, giving pairs of aligned 6Ts stacked together side-by-side. The experimental data are fitted using both Langmuir (type VI) and Ising models, in which the single-aggregate (stage 1) forms spontaneously whereas the pair-aggregate (stage 2) is endothermic in toluene with formation enthalpy of 8Hpair = 260±20 meV. Tunable Raman spectroscopy for each stage reveals a bathochromic shift of the molecular resonance of the pair-aggregate, which is consistent with strong inter-molecular coupling and suggestive of J-type aggregation. This quantitative Raman approach is sensitive to roughly 10 molecules per nanotube and
provides direct evidence of molecular entry from the nanotube ends. These insights into the encapsulation process guide the preparation of well-defined 1D molecular crystals having tailored optical properties.