Ultrafast energy transfer of one-dimensional excitons between carbon nanotubes: a femtosecond time-resolved luminescence study

Excitation energy transfer has long been an intriguing subject in the fields of photoscience and materials science. Along with the recent progress of photovoltaics, photocatalysis, and photosensors using nanoscale materials, excitation energy transfer between a donor and an acceptor at a short distance (<= 1-10 nm) is of growing importance in both fundamental research and technological applications. This Perspective highlights our recent studies on exciton energy transfer between carbon nanotubes with interwall (surface-to-surface) distances of less than similar to 1 nm, which are equivalent to or shorter than the size of one-dimensional excitons in carbon nanotubes. We show exciton energy transfer in bundles of single-walled carbon nanotubes with the interwall distances of similar to 0.34 and 0.9 nm (center-to-center distances similar to 1.3-1.4 and 1.9 nm). For the interwall distance of similar to 0.34 nm (center-to-center distance similar to 1.3-1.4 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is (1.8-1.9) x 10(12) s(-1), and that to adjacent metallic tubes is 1.1 x 10(12) s(-1). For the interwall distance of similar to 0.9 nm (center-to-center distance similar to 1.9 nm), the transfer rate per tube from a semiconducting tube to adjacent semiconducting tubes is 2.7 x 10(11) s(-1). These transfer rates are much lower than those predicted by the Forster model calculation based on a point dipole approximation, indicating the failure of the conventional Forster model calculations. In double-walled carbon nanotubes, which are equivalent to ideal nanoscale coaxial cylinders, we show exciton energy transfer from the inner to the outer tubes. The transfer rate between the inner and the outer tubes with an interwall distance of similar to 0.38 nm is 6.6 x 10(12) s(-1). Our findings provide an insight into the energy transfer mechanisms of one-dimensional excitons.