  First-principles calculations have been performed to study the effects of
adsorbates (CO molecules and O atoms) and defects on electronic structures and
transport properties of Au nanotubes (Au(5, 3) and Au(5, 5)). For CO
adsorption, various adsorption sites of CO on the Au tubes were considered. The
vibrational frequency of the CO molecule was found to be very different for two
nearly degenerate stable adsorption configurations of Au(5, 3), implying the
possibility of distinguishing these two configurations via measuring the
vibrational frequency of CO in experiments. After CO adsorption, the
conductance of Au(5, 3) decreases by 0.9G0 and the conductance of Au(5, 5)
decreases by approximately 0.5G0. For O-adsorbed Au tubes, O atoms strongly
interact with Au tubes, leading to around 2G0 of drop in conductance for both
Au tubes. These results may have implications for Au-tube-based chemical
sensing. When a monovacancy defect is present, we found that, for both tubes,
the conductance decreases by around 1G0. Another type of defect arising from
the adhesion of one Au atom is also considered. For this case, it is found
that, for the Au(5, 3) tube, the defect decreases the conductance by nearly
1G0, whereas for Au(5, 5), the decrease in conductance is only 0.3G0.
