  Carbon nanotubes (CNTs) are a promising material for high-performance
electronics beyond silicon. But unlike silicon, the nature of the transport
band gap in CNTs is not fully understood. The transport gap in CNTs is
predicted to be strongly driven by electron-electron (e-e) interactions and
correlations, even at room temperature. Here, we use dielectric liquids to
screen e-e interactions in individual suspended ultra-clean CNTs. Using
multiple techniques, the transport gap is measured as dielectric screening is
increased. Changing the dielectric environment from air to isopropanol, we
observe a 25% reduction in the transport gap of semiconducting CNTs, and a 32%
reduction in the band gap of narrow-gap CNTs. Additional measurements are
reported in dielectric oils. Our results elucidate the nature of the transport
gap in CNTs, and show that dielectric environment offers a mechanism for
significant control over the transport band gap.
