Work Function of Layered Graphene Prepared by Chemical Vapor Deposition in High Vacuum

Abstract

Graphene prepared on an atomically flat Cu(111) surface by chemical vapor deposition (CVD) process was investigated by noncontact atomic force microscopy (NCAFM) and Kelvin probe force microscopy (KPFM) under ultra-high-vacuum (UHV) conditions to exclude extrinsic effects from the atmosphere. Through NCAFM observations, the growth of characteristic layered structures of graphene was recognized. Individual layers were approximately 0.33 nm thick, in agreement with that of graphene. Meanwhile, KPFM measurements revealed that the work function varied with the number of graphene layers on the Cu(111) surface, and the work function for the monolayer was determined to be approximately 4.23 eV. Specifically, the work function obtained from the top layer of stacked graphene sheets increased monotonically depending on the number of stacking layers, reaching 4.8 eV with seven layers, which is similar to that of graphite (4.6 eV). In contrast, characteristic changes in the contrast of the NCAFM image depended on the applied bias voltage between the NCAFM cantilever and observed sample surface, which reflected the two-dimensional features of electronic structures of graphene.