  We investigate from first-principles the change in transport properties of a
two-dimensional azobenzene monolayer sandwiched between two Au electrodes that
undergoes molecular switching. We focus on transport differences between a
chemisorbed and physisorbed top monolayer-electrode contact. The conductance of
the monolayer junction with a chemisorbed top contact is higher in trans
configuration, in agreement with the previous theoretical predictions of
one-dimensional single molecule junctions. However, with a physisorbed top
contact, the "ON" state with larger conductance is associated with the cis
configuration due to a reduced effective tunneling pathway by switching from
trans to cis, which successfully explains recently experimental measurements of
azobenzene monolayer junctions. A simple model is developed to explain electron
transmission across subsystems in the molecular junction. We also discuss the
effects of monolayer packing density, molecule tilt angle, and contact geometry
on the calculated transmission functions. In particular, we find that a
tip-like contact with chemisorption significantly affects the electric current
through the cis monolayer, leading to highly asymmetric current-voltage
characteristics as well as large negative differential resistance behavior.
