Contributions to the work function: A density-functional study of adsorbates at graphene ribbon edges

In the present computational study, we focus on graphene ribbons with zigza g edge atoms with their unsaturated bonds either dangling or terminated by Various adsorbates (H, O, or Cs). Using this system as a test case, we disc uss the two important contributions to the work function-the first one bein g an anisotropic bulk property related to the electron affinity of the mate rial, and the second one being directly related to the surface dipole momen t caused by the spill over of electronic charge into the vacuum. The latter contribution, which tends to increase the work function, can to a large ex tent be minimized by a judicious choice of adsorbates (typically, adsorbate s that are more electropositive than the surface). The former face-dependen t contribution turns out to be the minimum possible work function achievabl e far a given surface. Our calculations are based on density-functional the ory within the local density approximation using nonlocal pseudopotentials and a plane wave basis set. [S0163-1829(99)03632-2].