Abstract An attempt has been made to analyze the
magnetic-spin quenching property of Co, as a representative
of transition metals, in Co-doped singlewalled
carbon nanotubes (SWCNTs) as well as the
binding property of CO with the side walls of the Codoped
SWCNTs by means of hybrid density functional
theory (DFT) calculations. Four different types of
SWCNTs are considered: semi-conducting (5,0) zigzag,
metallic (5,2) and semi conducting (5,3) chirals,
and metallic (5,5) armchair. The results show that
while the spin states of Co in the whole of the present
Co-doped SWCNTs were preserved, the combined
effects of adsorbate (CO) and substrate (Co-doped
SWCNT) were strong enough to favor the low-spin
states, and to quench the spins in the Co-doped
SWCNTs (5,0) and (5,2). The doped Co atom converts
the endothermic reactions of CO molecules on the
outer surfaces of the pure SWCNTs into exothermic
reactions. The nature of charge transfer between the
d-orbitals of Co, and the p* orbital of the nearby C of
CO is clarified. Natural bond orbital (NBO) analysis
reveals that the electronic configuration of the doped
Co metal represents a qualitative change with respect
to that of the free-metal. The binding of CO precursor
is mostly dominated by the metal E(i)Co..CO pairwise
additive contributions, and the role of the SWCNTs is
not restricted to supporting the metal. The spin
quenched SWCNTs are characterized in terms of
isodensity contours of frontier orbitals. Molecular
electrostatic potentials (MEPs) indicate that SWCNTs
can act as effective gas sensors for nucleophiles. The
results show that Co-doped SWCNTs can be useful in
spintronics applications and sensor technology.