  A detailed understanding of energy transduction is crucial for achieving
precise control of energy flow in complex, integrated systems. In this context,
carbon nanotubes (CNTs) are intriguing model systems due to their rich,
chirality-dependent electronic and optical properties. Here, we study the
quenching of fluorescence from isolated quantum dots (QDs) upon approach of
individual CNTs attached to atomic force microscope probes. Precision
measurements of many different CNT/QD pairs reveal behavior consistent with
resonant energy transfer between QD and CNT excitons via a Fohrster-like
dipole-dipole coupling. The data reveal large variations in energy transfer
length scales even though peak efficiencies are narrowly distributed around
96%. This saturation of efficiency is maintained even when energy transfer must
compete with elevated intrinsic non-radiative relaxation rates during QD aging.
These observations suggest that excitons can be created at different locations
along the CNT length, thereby resulting in self-limiting behavior.
