STM studies of epitaxial graphene
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Introduction Because graphene is two-dimensional,1,2 scanning tunneling microscopy (STM) is ideal for its characterization.3,4 To date, many STM studies have been carried out on graphene.5–9 Because of its atomic resolution, STM is able to probe important local physical and electronic details of both pristine and modified epitaxial graphene that other techniques are unable to access. In this review, we first introduce the basic working principles and setups used for STM and scanning tunneling spectroscopy (STS).The capabilities of STM/STS are then illustrated by instructive examples of epitaxial graphene characterization. Molecular interactions, intercalation, and fundamental studies of epitaxial graphene are also discussed.
Principles of STM When an atomically sharp STM tip is brought within a few nanometers of a surface and a voltage bias (from a few millielectronvolts to a few electronvolts) is applied across the gap, electrons can quantum-mechanically tunnel through the potential barrier presented by the gap. This induced tunneling current is exponentially dependent on the gap distance, enabling STM to be a highly sensitive probe. Spatial variation in surface topography can thus be detected through changes in tunneling current. The electronic charge density distribution on the surface also determines the location and energy from which electrons tunnel,
thereby allowing imaging of the precise atomic and electronic structure of the surface. Tip motion is controlled by a piezoelectric mount that responds mechanically to small changes in the applied voltage. There are two modes of topographical imaging: The first is constant-current mode, in which the tunneling current is kept constant by means of a feedback loop as the tip scans across the surface. This current feedback loop instructs the tip to retract from (approach) the surface when there is an increase (decrease) in the current. The other mode is constant-height mode, in which the current is allowed to vary as the tip is scanned across the surface at a fixed distance above it. Schematics for constantcurrent and constant-height modes are shown in Figure 1a–b, respectively. The resultant variation of the tip height or current with the tip position is recorded as an STM topography image. STM can be performed in a liquid or at pressures ranging from atmospheric pressure to ultrahigh-vacuum (UHV) conditions of less than 10–9 mbar. In this review, we focus on STM in UHV, where the ultimate atomic resolution is routinely achievable because contamination is minimized. The current that an STM tip emits or receives due to quantummechanical tunneling is a combination of three factors: applied voltage, distance between tip and surface, and local density of electronic states (DOS). The voltage determines the difference
Swee Liang Wong, Department of Physics, National University of Singapore; [email protected] Han Huang, Department of Physics, National University of Singapore; [email protected] Wei Chen, Department of Physics, National University of Singapore; [email protected]
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