Mitchell J. Senger: Universal interaction-driven gap in metallic carbon nanotubes

Metallic carbon nanotubes (m-CNTs) exhibit a remarkably large energy gap for electronic excitations. The gap often exceeds 100 meV when the m-CNT is suspended in free space, but the gap disappears when the m-CNT lies on a metal surface. The theoretical description of this gap remains controversial and more experiments are needed. We have built ultra-clean suspended CNT devices from CNTs of known diameter and chiral angle. The CNT chirality is identified with spectrally resolved scanning photocurrent microscopy and the energy gap is determined by measuring thermally-activated electron transport. Our results show that the gap scales exactly as 1/D, with no dependence on chiral angle. We also demonstrate that the gap can be tuned by submerging the suspended m-CNT in dielectric liquids. Our results put new constraints on competing theoretical descriptions of a Mott insulator state versus an excitonic insulator state.

Daniel McCulley: Field-Enhanced Exciton Dissociation in Carbon Nanotube Photodiodes
Low-dimensional materials may be useful for building solar cells that harness carrier multiplication and circumvent the Shockley-Queisser limit. For example, quantum dot solar cells with an internal quantum efficiency (IQE) > 100% have been reported. In this work, we search for carrier multiplication effects in CNTs. We use individually-contacted, ultra-clean, suspended, semiconducting carbon nanotubes of known chiral index. Previous work on this system showed an IQE ~ 30% when the built-in electric field was ~ 4 V/um. Here we report an IQE ~ 80% when the electric field is increased to ~ 15 V/μm. At these high fields, photocurrent spectroscopy reveals extreme broadening of low-energy exciton peaks. We compare our results to theoretical predictions for field-induced exciton dissociation in CNTs, and develop a framework to describe the energy dissipation pathways.