Scanning tunneling microscopy of monolayer graphite epitaxially grown on a TiC(111) surface (original) (raw)
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Journal of Physics D-applied Physics, 2005
Since its invention in 1981, scanning tunnelling microscopy (STM) is well-known for its supreme imaging resolution enabling one to observe atomic-scale structures, which has led to the flourishing of nanoscience. As successful as it is, there still remain phenomena which are observed using STM but are beyond our understanding. Graphite is one of the surfaces which have been most extensively studied using STM. However, there are a number of unusual properties of graphite surfaces. First reported in the 1980s, superlattices on graphite have since been observed many times and by many groups, but as yet our understanding of this phenomenon is quite limited. Most of the observed superlattice phenomena are widely believed to be the result of a Moiré rotation pattern, arising from the misorientation between two graphite layers, as verified experimentally. A Moiré pattern is a lattice with larger periodicity resulting from the overlap of two lattices with smaller periodicities. As graphite layers are composed of hexagonal lattices with a periodicity of 0.246 nm, as observed using STM, when there are misoriented graphite layers overlapping each other, a Moiré pattern with larger periodicity, depending on the misorientation angle, will be produced and appear as a superperiodic hexagonal structure on top of the graphite atomic lattice of the topmost surface layer. It is important to study graphite superlattices because, firstly, knowledge of this phenomenon will enable us to properly interpret STM images; secondly, it helps us to understand the correlation between electronic structures and atomic-structure rearrangement of graphite which is of tremendous aid for engineering material properties; thirdly, and perhaps most importantly, the observation of the phenomenon exhibits the capability of STM to produce images indicating the nature of internal defects which are below the surface. Over recent years, experimental and modelling techniques have been developed to study this anomalous regime of STM; however, there is a lack of a systematic classification of this scattered information. This review article thus serves the purpose of organizing all these results so as to enable a more comprehensive understanding of this phenomenon. We review the discovery of graphite superlattices, the observation of the associated properties, and the research efforts on this subject. An effort is made to envision the future experimental and theoretical research possibilities to unveil the mystery of this anomaly of STM. Applications of graphite superlattices are also proposed.
Electronic effects in scanning tunneling microscopy: Moiré pattern on a graphite surface
1993
We show that the features of the scanning-tunneling-microscopy STM images of graphite can be understood within a simple tight-binding model of the tip-surface system. Using Green's-function techniques, we are able to go beyond the usual Tersoff and Hamann formalism and include the effect of the bias voltage and tip-surface coupling on the tunneling current. We show that the tunneling current is very sensitive to the different crystallographic stacking arrangements of graphite, due to their influence on the local density of states both at and near the Fermi level. In addition, we find that the relative corrugation of the STM image depends strongly upon the nature of the tip-surface interaction. We conclude with a discussion of the extension of our formalism to include surface defects and adsorbates. ͓S0163-1829͑96͒01539-1͔
Observation and investigation of graphite superlattice boundaries by scanning tunneling microscopy
Surface Science, 2007
In this article, we report on scanning tunneling microscopy (STM) observations of several different kinds of superlattice boundaries on highly-oriented pyrolytic graphite (HOPG) including an array of bead-like structures, a monolayer deep trench, a zigzag shaped termination, and a plain boundary without features. Results of a simulation model show that a top rotated graphite layer with a straight boundary does not necessarily lead to the zigzag shaped boundary of the resulting superlattice as has been previously claimed. The formation of the bead-like, trench, and zigzag shaped boundaries is explained from the energetic point of view. Our study also shows evidence for the superlattice-mediated observation of a low-angle grain boundary with a varying tilt angle. A relationship between the periodicity of the boundary dislocations and the periodicity of the superlattice across the boundary is derived. The result of this work is important for an understanding of superlattices on graphite whose origin is not yet completely understood.
Physical Review B, 1987
Images of the (0001) surface of graphite observed in the scanning-tunneling microscope (STM) show a strong asymmetry in the tunneling current between neighboring carbon atoms in the hexagonal ring. The magnitude of this asymmetry is seen to be almost independent of the polarity and to decrease slightly with increasing amplitude of the bias voltage for voltages below 1 V. A theory is developed that explains this anomaly as a purely electronic effect, arising from the symmetry of the states scanned by the STM, which dominates over the topography of graphite. The predicted trends for the bias-voltage dependence of the asymmetry are confirmed experimentally. (b) (c). , ' FICJ. l. (a) Observed and (b) and (c) calculated STM current densities j(x,y, z =const) for a bias voltage V=Q. I V. In (c), calculated results of (b) have been Fourier analyzed and filtered to mimic the experimental conditions. On the grey scale, white corresponds to large current densities.
A new interpretation of the scanning tunneling microscope image of graphite
Chemical Physics, 2008
In this work, highly-resolved scanning tunneling microscopy images of graphite basal plane are obtained and theoretical computations are performed to explain the resolution of only half the atoms in STM images of graphite. Our experimental and computational findings indicate that the bright elliptical spots observed in trigonal STM images of graphite may not correspond to carbon positions but to π-states localized above alternate carbon–carbon bonds. This interpretation is based on STM experiments that suggest that the elliptical shape of the bright spots may not be a tip artifact and on simulated STM images of a graphite using orthorhombic unit cells that are in excellent agreement with experimentally obtained images.
The Journal of Physical Chemistry C, 2008
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.