Quantifying Free Carriers in Doped SWCNTs: An Optical and Magnetic Resonance Approach

Abstract

Single-walled carbon nanotubes (SWCNTs) have become increasingly important for opto-electronic devices due to their strong optical absorption, tunable carrier densities, and large carrier mobilities. To produce electricity in applications like photovoltaics and sensors, after light absorption the bound exciton must be separated into free charges carriers. However, the nature of charges in s-SWCNTs is still unclear and a way to quantify them has remained elusive. To better understand free carriers in doped semiconducting SWCNTs we employ a new class of molecular charge-transfer dopants, functionalized dodecaborane (DDB) molecules, to tune the charge carrier density. These DDB dopants form delocalized hole charge carriers on the SWCNTs due to the spatial separation between nanotube surface and the boron cluster containing the negative counterion. Importantly, the fluorine-decorated DDB dopants have strong 19F resonances, characteristic of the negatively charged molecules, that can quantify the absolute concentrations of DDB- and SWCNT+. As such, we use a combined approach of steady state optical spectroscopy and 19F NMR studies. Optical spectroscopy allows to track the excitonic absorption quenching and trion signature due to charge carrier density while correlating it to the concentration of carriers extracted from NMR experiments. This combined methodology allows for quantitative determination of a molar extinction coefficient for free carriers in semiconducting SWCNTs. This molar extinction coefficient can then be used in further studies to determine the yield and mobility of free carriers generated in a variety of samples and applications, for example at a heterojunction in photovoltaic devices, to determine quantitative charge transfer efficiencies and dynamics.