  Strain dependence of electronic structures and work functions of both
pristine and potassium doped (5,5) (armchair) and (9,0) (zigzag) carbon
nanotubes (CNTs) has been thoroughly studied using first-principles
calculations based on density functional theory (DFT). We found that for
pristine cases, the uniaxial strain has strong effects on work functions of
CNTs, and the responses of work functions of CNT (5,5) and (9,0) to the strain
are distinctly different. When the strain changes from -10% to +10%, the work
function of the CNT (5,5) increases monotonically from 3.95 eV to 4.57 eV, and
the work function of the (9,0) varies between 4.27 eV and 5.24 eV in a
complicated manner. When coated with potassium, for both CNTs, work functions
can be lowered down by more than 2.0 eV, and the strain dependence of work
functions changes drastically. Our studies suggested that the combination of
chemical coating and tuning of strain may be a powerful tool for controlling
work functions of CNTs, which in turn will be useful in future design of
CNT-based electronic and field-emitting devices.
