We measured the electronic local density of states (LDOS) of graphite surfaces near monoatomic step edges, which consist of either the zigzag or armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. The STM data reveal that the (and honeycomb superstructures coexist over a length scale of 3−4 nm from both the edges. By comparing with density-functional derived nonorthogonal tight-binding calculations, we show that the coexistence is due to a slight admixing of the two types of edges at the graphite surfaces. In the STS measurements, a clear peak in the LDOS at negative bias voltages from −100 to −20 mV was observed near the zigzag edges, while such a peak was not observed near the armchair edges. We concluded that this peak corresponds to the graphite "edge state" theoretically predicted by Fujita et al. [J. Phys. Soc. Jpn. 65, 1920] with a tight-binding model for graphene ribbons. The existence of the edge state only at the zigzag type edge was also confirmed by our first-principles calculations with different edge terminations.

A three-dimensional interaction force mapping experiment was carried out on a muscovite mica surface in an aqueous solution using a high-resolution and low-thermal drift frequency-modulation atomic force microscope. By collecting oscillatory frequency shift versus distance curves at the mica∕solution interface, complicated hydration structures on the mica surface were visualized. Reconstructed two-dimensional frequency shift maps showed dot-like or honeycomb-like patterns at different tip-sample distances with a separation of 0.2 nm with each other, which agree well to the water molecule density maps predicted by a statistical-mechanical theory. Moreover, site-specific force versus distance curves showed a good agreement with theoretically calculated site-specific force curves by a molecular dynamics simulation. It is found that the first and second hydration layers give honeycomb-like and dot-like patterns in the two-dimensional frequency shift images, respectively, corresponding to the lateral distribution function in each layer.

We studied experimentally and theoretically the electronic local density of states (LDOS) near single step edges at the surface of exfoliated graphite. In scanning tunneling microscopy measurements, we observed the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures extending over 3$-$4 nm both from the zigzag and armchair edges. Calculations based on a density-functional derived non-orthogonal tight-binding model show that these superstructures can coexist if the two types of edges admix each other in real graphite step edges. Scanning tunneling spectroscopy measurements near the zigzag edge reveal a clear peak in the LDOS at an energy below the Fermi energy by 20 meV. No such a peak was observed near the armchair edge. We concluded that this peak corresponds to the "edge state" theoretically predicted for graphene ribbons, since a similar prominent LDOS peak due to the edge state is obtained by the first principles calculations.Comment: 4 pages, 6 figures, APF9, Appl. Surf. Sci. \bf{241}, 43 (2005

Scanning tunneling spectroscopy (STS) measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra associated with Landau quantization of the quasi two-dimensional electrons and holes. Comparison with calculated local density of states at the surface layers allows us to identify Kish graphite as bulk graphite and HOPG as graphite with finite thickness of 40 layers. This explains the qualitative difference between the two graphites reported in the recent transport measurements which suggested the quantum Hall effect in HOPG. This work demonstrates how powerful the combined approach between the high quality STS measurement and the first-principles calculation is in material science.PACS numbers: 71.70. Di, 71.20.Tx, 73.61.Wp, 73.43.Fj Graphite is one of the best studied materials concerning its electronic properties both experimentally and theoretically. It is a semimetal or zero gap semiconductor with a unique massless linear dispersion relation for the conduction band. Graphite is also an important mother system for technologically attractive materials such as carbon nanotubes, fullerene, nanographites, etc. Thus, any new insight on this material should have broad impacts.Recently, graphite has been attracting renewed fundamental interests particularly for its transport properties in magnetic fields. Kopelevich and coworkers [1] observed a plateau in the Hall resistance for highly oriented pyrolytic graphite (HOPG) in the quasi quantum limit, which suggests the quantum-Hall effect (QHE) characteristic of the 2D electron systems (2DES) with high mobility. But this behaviour was not observed for a Kish graphite sample [1]. Although the QHE is theoretically predicted for a 2D graphene sheet [2], graphite is a far less ideal 2D conductor in spite of its layered structure. Reentrant field-driven metal-insulator transitions were also reported [1,3] for both types of graphites. Theories [4] predict the electron-hole pairing as a possible mechanism. Even for zero field properties, the gapless spin-1 neutral collective mode has been recently suggested from the view point of the resonating valence bond [5].The aim of this work is to investigate the electronic states of Kish graphite and HOPG in high fields and low temperatures by scanning tunneling spectroscopy (STS). The STS technique, which probes the local density of states (LDOS), should potentially be a powerful tool to study the Landau quantization in 2DES in magnetic fields. This was recently demonstrated for the adsorbateinduced 2DES at the surface of InAs(110) with evaporated Fe submonolayers [6]. For both types of graphites, we observed clear peak structures in the tunnel spectra associated with the Landau levels (LLs). This is the first STS observation of the LLs in bulk materials. Comparisons with our LDOS calculations reveal that HOPG has a rather distinct 2D electronic stat...