Nanoscale Energy Transfer

In a 2007 report, “Directing Matter and Energy: Five Challenges for Science and the Imagination,” the Basic Energy Science Advisory Committee stated that obtaining a deep understanding of energy transduction pathways in nanoscale systems is a critical step toward precise control of energy flow in complex, integrated materials. For example, the transfer of energy from electromagnetic modes to electronic modes and vice versa is a nanometer-scale phenomenon which plays an important role in a variety of applications such as nanoscale plasmonic waveguides, surface-enhanced and tip-enhanced spectroscopy, photonic bandgap materials, metamaterials, etc. Without insight into the basic physical processes that couple the components within a nanostructured material, optimizing its functionality is inefficient at best, and quite likely impossible. Of particular interest is the potential to improve the efficiency of solar energy conversion by optimizing the energy transfer efficiency between different functional elements within a composite light-harvesting material. Furthermore, a large amount of effort has been dedicated to optimizing the size and shape of optical nano-antennas to enhance light-matter interactions, thereby altering the rate and angular pattern of an emitter’s radiation.

3D Measurements of Tip-Sample Interactions

3D near-field images

We developed a 3D near-field tomography method to study the nature of various tip-sample interactions, and used 20-nm diameter fluorescent beads to demonstrate its capabilities, as shown in the figure to the left. The figure shows a vertical (x-z) tomographical section (upper panel: gold-coated tip) and the intersection of x-y, x-z, and y-z sections (lower panel: silicon tip). In TEFM, the optical signal is proportional to the local light intensity. As described on the TEFM page, the tip can enhance the local optical intensity very close to its apex. This is clearly visible in the figure as the strong and highly-confined enhancement of the fluorescence signal when the silicon tip is directly above the bead (lower panel: axis dimensions in nm). In addition, superposition of the excitation light and light scattered from the tip can induce an intensity interference pattern at the sample. This is visible in the figure as the dark “halo” surrounding the position of the bead for the silicon tip (lower panel), and the weak bright halo beyond that. The asymmetric pattern visible for the gold-coated tip (upper panel: scale bar = 50 nm) is also due to this effect.

In addition to altering the local intensity, a sharp tip, particularly a metal one, can also quench fluorescence via energy transfer from a photoexcited molecule/particle to the tip (see Bioimaging page). In the figure above, this is revealed by the complete lack of enhancement for the gold tip (upper panel) when it is centered directly above the bead. This demonstrates the dominance of quenching over field enhancement for metal-coated tips due to lack of a surface plasmon polariton. For silicon tips, quenching is minimal and enhancement dominates. All three tip-induced effects, field enhancement, optical interference, and fluorescence quenching, are very sensitive to both tip geometry and material, and also to the optical properties and morphology of the sample. In the future, we will use 3D nanoscale tomography to study the light-matter interactions in more complex materials such as conjugated polymers and semiconductor heterostructures.

Energy Transfer Measurements with Carbon Nanotubes

A detailed understanding of energy transduction is crucial for achieving precise control of energy flow in complex, integrated systems. Recently, hybrid materials composed of quantum dots (QDs) attached to carbon nanotubes (CNTs) have been synthesized for a wide range of applications, including photovoltaics, nanotherapeutics, bioimaging, and photocatalysis. The interfacial area in these materials should be extremely large, so interactions between the QD and CNT components are very important for their overall behavior. If QD-CNT composites are indeed to be pursued for various optoelectronic applications, it is clearly important to understand the energy transduction pathways in more detail.

Energy Transfer

To study the energy transduction mechanisms in such systems, we adopted a single-particle approach whereby we measured individual QD-CNT pairs. Individual carbon nanotubes were attached to gold-coated AFM probes and the phase-sensitive photon counting technique described on the Bioimaging page was then used to record the fluorescence rate while probing an isolated QD with a single CNT. The figure to the right shows an example of an energy transfer measurement; the normalized signal S(z) = C(z)/C0 is plotted as a function of the vertical separation (z) between the CNT terminus and QD surface, where C(z) is the z-dependent fluorescence count rate, and C0 is the far-field (z–›∞) count rate. Also shown is an x-z tomographical section of a QD measured with a CNT tip.

The strong reduction in fluorescence shown in the figure above was observed consistently, independent of the particular QD measured or CNT. Further, the data agree to high precision with a model in which the energy transfer occurs via a Förster dipole-dipole coupling between a photoexcited exciton in the QD and a resonantly excited exciton in the CNT. The CNT exciton can be created anywhere along the length of the tube, in contrast to classical Förster theory where the distance between the donor and acceptor dipoles is well defined. To account for this, a parameter z0 is introduced into the Förster theory, which characterizes the average distance above the CNT terminus at which the exciton is generated. A summary of more than 100 measurements on >50 QDs using six CNTs with different lengths is shown in the figure below. The correlation between the measured values of the Förster radius R0 and z0 is striking. A simple calculation for the average position at which the exciton is produced relative to the CNT terminus yields the solid line shown in the figure, which agrees remarkably well with measurements. This same calculation predicts a peak energy transfer efficiency of ≈0.96, which also agrees with measurements (panel b).

The saturation of the peak energy transfer efficiency at ≈0.96 is a direct result of the 1D nature of CNTs in that the exciton can be created anywhere along its length. Furthermore, this saturation seems to be independent of the CNT chirality as we expect that our sample of six contains both semiconducting and metallic varieties. Finally, the saturation is maintained even when the energy transfer must compete with faster internal relaxation of the QD (i.e., lower quantum yield), which implies that the energy transfer rate also increases to compensate. This counterintuitive result is also due to the 1D nature of CNTs in that excitons are less likely to be created far from the CNT tip when the internal relaxation processes of the QD speed up. These measurements are absolutely fundamental for understanding energy transduction in these systems, and may also have important implications for the performance of composite materials. Although on very rare occasions there is some evidence of charge transfer between a QD and CNT, the overwhelming majority of measurements are consistent with energy transfer only. This also has obvious implications for practical applications.

In the future, we will directly measure the fluorescence lifetime of the QDs as a function of the QD-CNT separation using a pulsed laser. This will provide a direct measurement of the energy transfer rate, and will test the interpretation discussed above. In addition, we will measure more complex systems such as semiconductor heterostructures (e.g., tetrapods), and also other strongly absorbing structures such as plasmonic nanoparticles, which are important in various optoelectronic applications. Finally, we would like to determine the chirality of the tip-attached CNTs using optical spectroscopy in order to correlate observed behavior with the diameter and electronic structure of each CNT.

Prof. Jordan Gerton | James Fletcher Building | Room 314 | 115 South 1400 East | Salt Lake City, UT | 84112
Office: +1-801-585-0068 | Lab: +1-801-581-5078 | Email: