Photosensitive Function of Encapsulated Dye in Carbon Nanotubes

Abstract

Single-wall carbon nanotubes (SWCNTs) exhibit resonant absorption localized in specific spectral regions. To expand the light spectrum that can be utilized by SWCNTs, we have encapsulated squarylium dye into SWCNTs and clarified its microscopic structure and photosensitizing function. X-ray diffraction and polarization-resolved optical absorption measurements revealed that the encapsulated dye molecules are located at an off center position inside the tubes and aligned to the nanotube axis. Efficient energy transfer from the encapsulated dye to SWCNTs was clearly observed in the photoluminescence spectra. Enhancement of transient absorption saturation in the S1 state of the semiconducting SWCNTs was detected after the photoexcitation of the encapsulated dye, which indicates that ultrafast (<190 fs) energy transfer occurred from the dye to the SWCNTs.

Techniques of aligning carbon nanotubes

“…The stretching ratio is defined as the length ratio of a film after/before stretching. This ratio is approximately equal to the ratio of optical absorption or luminescence signals measured along and perpendicular to the stretching direction [41,42]. Some polymers (e.g., polyethylene) do not need softening treatments and can be stretched >30 times [65].…”

“…Interestingly, optical techniques allow monitoring of alignment not only of the CNTs themselves, but also of species encapsulated inside CNTs [41][42][43] or attached to Figure 2. From top to bottom: dependence of the absorption spectra of CNTs, without and with encapsulated β-carotene molecules, on the angle θ between the alignment direction of the CNTs and the polarization of incident linearly polarized light.…”

“…For example, it was demonstrated that linear β-carotene [41] and squarylium [42] molecules encapsulated into CNTs are aligned to the inner CNT walls and do not bend or twist. Polarized optical absorption and Raman spectra of β-carotene/CNT/polymer film aligned by mechanical stretching are shown in Figs.…”

“…This is often considered as a drawback, and thus the catalyst is usually removed by acids [41,42] or annealing [1,21]. However, the catalyst particles can also add new functionalities to CNTs.…”

“…However the method is relatively simple and produces optically homogeneous samples cut from the central part of the stretched film. It is probably the most popular CNT alignment technique in scientific research (see, e.g., [41,42,44,68] and Figs. 2 and 3), but it is hardly suitable for industrial applications.…”