the sensor in the case of doping by Ti, V , Cr, and Mn un-
der standard conditions (room temperature and 1 bar of pres-
sure). Among the remaining metals, we identify Ni as is the
most promising candidate for CO detection. For this system
the change in resistance per active site is generally signiﬁcant
(>1 Ω) for small changes in CO concentration in the relevant
range of around 0.1–10 ppm. Our approach is quite general
and is directly applicable to other nanostructures than CNTs,
other functionalizations than metal doping, and other back-
grounds than atmospheric air.
All total energy calculations and structure optimizations
have been performed with the real-space density functional
theory (DFT) code GPAW [22] which is based on the projector
augmented wave method. We use a grid spacing of 0.2 ˚A for
representing the density and wave functions and the PBE ex-
change correlation functional [23]. Transport calculations for
the optimized structures have been performed using the non-
equilibrium Green’s function method [24] with an electronic
Hamiltonian obtained from the SIESTA code [25] in a dou-
ble zeta polarized (DZP) basis set. Spin polarization has been
taken into account in all calculations.
Metallic doping of a (6,6) CNT has been modeled in a su-
percell containing six repeated minimal unit cells along the
CNT axis (dimensions: 15 ˚A×15 ˚A×14.622 ˚A). For this size
of supercell a Γ-point sampling of the Brillouin zone was
found to be sufﬁcient. The formation energy for creating a
vacancy (VC) occupied by a transition metal atom (M) was
calculated using the relation
Eform[M@VC] = E[M@VC] + nE[C] −E[M@NT] (1)
where E[M@VC] is the total energy of a transition metal
atom occupying a vacancy in the nanotube, n is the number
of carbon atoms removed to form the vacancy,E[C] is the en-
ergy per carbon atom in a pristine nanotube, and E[M@NT]
arXiv:1001.2538v1  [cond-mat.mes-hall]  14 Jan 2010