Observation of the Marcus Inverted Region of Electron Transfer from Asymmetric Chemical Doping of Pristine (n,m) Single-Walled Carbon Nanotubes.

The concept of electrical energy generation based on asymmetric chemical doping of single-walled carbon nanotube (SWNT) papers is presented. We explore 27 small, organic, electron-acceptor molecules that are shown to tune the output open-circuit voltage (V) across three types of pristine SWNT papers with varying (n,m) chirality distributions. A considerable enhancement in the observed V, from 80 to 440 mV, is observed for SWNT/molecule acceptor pairs that have molecular volume below 120 Å and lowest unoccupied molecular orbital (LUMO) energies centered around -0.8 eV. The electron transfer (ET) rate constants driving the V generation are shown to vary with the chirality-associated Marcus theory, suggesting that the energy gaps between SWNT and the LUMO of acceptor molecules dictate the ET process. When the ET rate constants and the maximum V are plotted versus the LUMO energy of the acceptor organic molecule, volcano-shaped dependencies, characteristic of the Marcus inverted region, are apparent for three distinct sources of SWNT papers with modes in diameter distributions of 0.95, 0.83, and 0.75 nm. This observation, where the ET driving force exceeds reorganization energies, allows for an estimation of the outer-sphere reorganization energies with values as low as 100 meV for the (8,7) SWNT, consistent with a proposed image-charge modified Born energy model. These results expand the fundamental understanding of ET transfer processes in SWNT and allow for an accurate calculation of energy generation through asymmetric doping for device applications.