Measurement of a Diameter-Dependent Charge Transfer in Solution-Phase Carbon Nanotubes Using Raman Spectroscopy

Superacids such as chlorosulfonic acid (CSA) and oleum spontaneously dissolve carbon nanotubes (CNT) in high concentrations without damaging their structure. Despite protonation-induced electrostatic repulsion being the driving force in this process, relatively little is known about the actual protonation of individual CNT. Furthermore, numerous claims describing diameter and chiral selectivity (or lack thereof) during the acid-induced doping of CNT are in direct conflict and must be reconciled. Herein, we present direct measurement of charge transfer in solution phase CNT over a broad range of CNT diameters and acid compositions. We find that in CSA, the charge density of commercial CNT ranges from near 0.007 for small diameter semiconductors to more than 0.04 holes per carbon atom for the largest diameter mixed chirality sample. The significant difference in CNT charge transfer is explained according to diameter dependence of the CNT work function, and interfacial p-type doping by the superacid without changes to the CNT density of states. Theoretical results predict a simple diameter and acidity dependent charging behavior, which is experimentally confirmed via Raman spectroscopy. In CSA, the CNT fractional charge, f c,CSA, as a function of RBM, ωRBM in cm–1, is found to be f c,CSA (ωRBM) = (0.268 – (5.77 × 10–4)­ωRBM)2 or, equivalently, f c,CSA ( d t ) = ( 0.268 − ( 0.143 ) d t ) 2 as a function of nanotube diameter, d t in nm. This equation relates the CSA-induced CNT charge to the CNT diameter and indicates that the larger diameter CNTs may be better suited for solution processing.