Scanning tunneling microscope studies of ultrathin graphitic (graphene) films on an insulating substrate under ambient conditions

In the present study, a scanning tunneling microscope (STM), modified to include active lateral position feedback control, is employed to image single and few layer graphene films placed on a nonconductive substrate under ambient conditions. The return path for tunneling electrons was provided by gold electrodes produced by either electron beam lithography or shadow evaporation techniques. STM images of graphene films with a thickness of two or more layers display topographs that are similar to those obtained from a bulk graphite crystal. For single layer graphene sheets, the ability to obtain atomically resolved images was found to be extremely sensitive to sample preparation methods. Graphene microdevices produced by electron beam lithography with edges covered by gold electrodes show hexagonal patterns similar to those obtained from ultrahigh vacuum STM images reported earlier. Ambient STM measurements of graphene microdevices made by shadow mask evaporation, whose edges were exposed to air, exhibited chaotic topographs caused by instability in the STM feedback control loop due to interactions between tip and sample. STM images recorded on these samples reveal "noisy" topographs that are likely not related to any real surface features.a