  Carrying out first-principles calculations, we study N-doped capped carbon
nanotube (CNT) electrodes applied to DNA sequencing. While we obtain for the
face-on nucleobase junction configurations a conventional conductance ordering
where the largest signal results from guanine according to its high highest
occupied molecular orbital (HOMO) level, we extract for the edge-on
counterparts a distinct conductance ordering where the low-HOMO thymine
provides the largest signal. The edge-on mode is shown to operate based on a
novel molecular sensing mechanism that reflects the chemical connectivity
between N-doped CNT caps that can act both as electron donors and electron
acceptors and DNA functional groups that include the hyperconjugated thymine
methyl group.
