Carbon Nanotubes as Optoelectronic Energy Transducers

Abstract

The transduction of energy from one type to another is frequently a limiting factor in the efficiency of optoelectronic devices. Such transduction processes are often necessary for energy to propagate across the interface between materials. As an example, next generation solar cells may require several energy transduction steps: conversion of electromagnetic radiation to internal energy of one component, to internal energy of another component, and finally to electric power. In this scenario, each sequential transduction process must be more rapid than the internal relaxation time, or the energy will be dissipated as radiation or heat. Recently, we demonstrated that when a carbon nanotube (CNT) is brought into close proximity with a fluorophore, the fluorophore's emission is nearly completely quenched [1,2]. This indicates that the fluorophore's internal energy is transduced into electronic modes within the CNT. Nanotubes are particularly intriguing candidates for studying nanoscale energy transduction because on the one hand, they can support ballistic charge transport, and on the other, the detailed electronic properties vary from tube to tube according to its chirality. Furthermore, they have been synthesized as dense vertically-aligned networks, facilitating their incorporation into complex optoelectronic materials, and finally they have been used as nanoscale components in molecular electronics.Carbon nanotubes (CNTs) have been proposed as a charge-transport element in next-generation optoelectronic devices and materials, including photovoltaics [3]. Indeed, nano-composite materials with a strong light absorption component (i.e., fluorophores) coupled to a CNT component have been synthesized via several different methods. The interfacial area between donor and acceptor in a nanocomposite material is extremely large, so nanoscale energy transfer is very important, but difficult to measure in a controlled and precise manner. We used individual CNTs attached to atomic force microscope (AFM) tips to probe isolated quantum dots (QDs) while measuring the fluorescence signal. Figure 1(a) shows an example of a fluorescence approach curve; the far-fieldnormalized fluorescence signal, S(z), is plotted as a function of the vertical separation, z, between the CNT terminus and QD surface; also shown is an x-z tomographical section [4] of a QD. The strong reduction in fluorescence was observed consistently in more than 100 measurements on >50 QDs using 6 different CNTs [1]. All data agree to high precision with energy transfer via a Förster dipoledipole coupling between a photoexcited exciton in the QD and a resonantly-excited exciton in the CNT. However, this agreement is only achieved if the standard Förster model is modified to account for the possibility of creating an exciton above the CNT terminus. This model predicts a strong correlation between the Förster radius and the average position at which the exciton is created within the CNT, in agreement with measurements. The model predicts the peak energy transfer eff...

Resolving Single Fluorophores Within Dense Ensembles: Contrast Limits of Tip-Enhanced Fluorescence Microscopy

“…Signal enhancement factors as large as ~20 have been demonstrated for quantum dots using commercially available Si tips [2]. It should be possible to achieve even larger signal enhancement factors using metal tips in combination with fluorophores with low intrinsic quantum yield [3,4].…”