Few-layer graphene has recently attracted much attention as a new electronic material with interesting electronic transport properties, such as field effects and quantum hall effects [1,2]. There have been many theoretical discussions about the electronic structure of graphene depending on its microscopic geometry. However, it is very difficult to measure the electrical properties of nano-order graphene. Our goal was to measure the local conductance of few-layer graphene with nanometer spatial resolution. We have developed an integrated nanogap probe which consists of two Pt electrodes separated by a nano-order gap fabricated by focused ion beam milling on a Si cantilever. This nanogap probe on a SPM system enables us to measure in-plane conductance at nanometer resolution without lithographic techniques [3]. In this paper, we report for the first time on the spatially-resolved in-plane conductance of few-layer (one or two) graphene grown on a SiC substrate, measured using the nanogap probe.
Few-layer graphene samples were prepared by annealing SiC surfaces at high temperatures in UHV. The morphology and number of layers of the thermally grown graphene were confirmed by in-situ observation using low energy electron microscopy (LEEM) [4]. The coverage of the graphene was controlled to be relatively low, about 15 %, to measure the conductance of an individual graphene island. The island areas of the single- and double-layer graphene ranged from 600 nm2 to 10000 nm2. A typical island width was about 30 nm. The gap current (conductance) images were measured using the integrated nanogap probe with a 30-nm-gap on a conventional SPM system in vacuum at -137°C. The island shapes were clearly observed in the conductance image. The single-layer graphene and bi-layer graphene islands were clearly distinguished. The conductance of bi-layer graphene is about 4 times that of single-layer graphene. The layer number of few-layer graphene has successfully been evaluated with the conductance image measured using the integrated nanogap probe.
[1] K. S. Novoselov et al., Science 306, 666 (2004).
[2] Y. Zhang et al., Nature 438, 201 (2005).
[3] M. Nagase et al., Abstract of ICN+T2006 p.173./ J. of Phys. to be published.
[4] H. Hibino et al., to be submitted.
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