Electroluminescence from Aligned (6,5) Carbon Nanotube Films: Impact Excitation of Excitons and Trions

Charged excitons, i.e., trions, have attracted much interest due to their nonzero charge and spin. While the trion binding energy, defined relative to the exciton energy, is on the order of a few meV to tens of meV in III-V quantum wells and transition metal dichalcogenides, that in semiconducting single-wall carbon nanotubes (SWCNTs) can be as large as 200 meV due to enhanced Coulomb interactions in one dimension, making them clearly observable even at room temperature. Here, we present electrical creation of excitons and trions in aligned films of single-chirality (6,5) SWCNTs. Highly dense SWCNT films were prepared with minimum surfactant and without polymer wrapping to provide high conductivity and enhanced electron and hole transport inside the SWCNT film. We observed that SWCNTs aligned in the direction of the current flow emit light at a lower threshold voltage than those aligned in the perpendicular direction. This increase can be explained in terms of increased conductance in the direction of alignment. Our simulations help to elaborate the nature of the electroluminescence in the aligned CNT films.