Light-harvesting function of $\beta$-carotene inside carbon nanotubes

Single-wall carbon nanotubes (SWCNTs) are attractive components for nanoscale electronics, however their optoelectronic properties have been limited by the optical characteristics of semiconducting SWCNTs. To enhance the functionalities of SWCNTs, $\beta$-carotene was encapsulated in SWCNTs and its light-harvesting function was investigated. The detailed structure of encapsulated $\beta$-carotene was clarified using x-ray diffraction and the polarization dependence of the optical absorption spectra. The photoluminescence spectra revealed excited energy transfer from $\beta$-carotene to the SWCNTs.

Figure 1

(a) Chemical structure of $\beta$-carotene, and dependence of the absorption spectra of SWCNT (b) and Car@SWCNT (c) on the angle $\mathrm{\Theta }$ between the stretching direction of the film samples and the polarization of incident linearly polarized light. The absorption feature associated with the encapsulated $\beta$-carotene is labeled as the carotene band. Absorption intensities of the carotene (squares) and S1 (circles) bands are plotted as functions of $\mathrm{\Theta }$ in (d) and (e), respectively, and simulated (solid lines) by $A_i+B_i\cos {\mathrm{\Theta }}^2$ functions.

Figure 2

(Color online) X-ray diffraction patterns of SWCNT (upper curve) and Car@SWCNT (bottom curve). Circles are experimental observations, and the solid red (blue) line is the simulation result for SWCNT (Car@SWCNT). $R$, $g$, and $t$ are the radius of SWCNT, intertube distance, and thickness of the nanotube bundle, respectively. Dashed black line shows the calculated XRD pattern assuming the loss of crystal order.

Figure 3

(a) Simulation model for XRD pattern of Car@SWCNT. (b) The XRD patterns calculated when the distance $\delta$ from the encapsulated molecule to the nanotube wall is changed from 0 to $0.69\mathrm{nm}$ (center of the tube).

Figure 4

(Color online) (a) Absorption spectra of the samples used in photoluminescence (PL) measurements. Also shown are PL maps of SWCNT (b) and Car@SWCNT (c) dispersions. Dotted lines indicate excitation profiles for the major SWCNT chiralities, identified as (11,9), (12,7), and (13,5).

Figure 5

(Color online) (a) presents the difference absorption spectrum $\mathrm{\Delta }A$ between Car@SWCNT and SWCNT revealing the carotene band. (b), (c), and (d) show the difference excitation spectra $\mathrm{\Delta }PLE$ between Car@SWCNT and SWCNT for the (11,9), (12,7), and (13,5) tubes, respectively. The carotene band is also shown with the dashed red line for comparison.

Figure 6

Schematic illustration of energy transfer processes from $\beta$-carotene to SWCNT which might occur after photoexcitation of the $1{}^{1}B_u^{+}$ excited state of $\beta$-carotene.