  We theoretically demonstrate that a dipole layer on the electrode can
modulate the transmission properties of nanoscale devices by influencing the
contact properties, through first principles simulations on carbon nanotube
based field effect transistors. The dipole layer is realized by potassium
adsorption on Au electrodes, which shifts the electrostatic potential at the
near contact region significantly. The dipoles parallel to the direction of the
bias voltage may act as a supplement to the effect of gate voltages, while the
perpendicular dipoles may modify the interface barrier of the contacts.
