Effects of boron/phosphorus co-doping in metallic single-walled carbon nanotubes

We studied the effects of boron (B)/phosphorus (P) co-doping in metallic single-walled carbon nanotubes (SWCNTs) using a first-principles method based on density functional theory. Boron and phosphorus atoms tend to form a B-P bond and form a B/P pair when co-doping in SWCNTs. It is easier for the B/P pair to dope at the P-1' site in the SWCNT, which is at an angle of 30 degrees to the tube axis. An energy gap is opened after doping B/P pairs in metallic (9, 0) and (6, 6) SWCNTs, and the energy gap increases with increasing concentration of the B/P pairs. Moreover, the different relative positions of the two B/P pairs in the SWCNT can have different effects on the energy gap of the SWCNT; a large energy gap arises if the spatial distance between two B/P pairs is long.