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 zigzag 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 discuss the two important contributions to the work function—the first one being an anisotropic bulk property related to the electron affinity of the material, and the second one being directly related to the surface dipole moment 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 extent be minimized by a judicious choice of adsorbates ~typically, adsorbates that are more electropositive than the surface!. The former face-dependent contribution turns out to be the minimum possible work function achievable for a given surface. Our calculations are based on density-functional theory within the local density approximation using nonlocal pseudopotentials and a plane wave basis set. @S0163-1829~99!03632-2#