Gregory M. Rutter

Observing the quantization of zero mass carriers in graphene.

Resolving Landau Levels in Graphene The charge carriers in a two-dimensional conductor, when placed in a magnetic field, can develop an additional set of quantized energy levels. These Landau levels correspond to the carriers now moving in cyclotron orbits. In graphene, which consists of single-atom-thick sheets of graphite, an unusual set of Landau levels with nonequal energy spacing can develop in graphene layers that have undergone symmetry breaking caused by rotation between adjacent layers. Miller et al. (p. 924) used scanning tunneling microscopy at cryogenic temperatures to map out Landau levels in graphene grown on silicon carbide with high energy and momentum resolution, including the characteristic level in graphene that can occur at zero energy. Scanning tunneling microscopy on graphene reveals non-equally spaced Landau energy levels induced by a magnetic field. Application of a magnetic field to conductors causes the charge carriers to circulate in cyclotron orbits with quantized energies called Landau levels (LLs). These are equally spaced in normal metals and two-dimensional electron gases. In graphene, however, the charge carrier velocity is independent of their energy (like massless photons). Consequently, the LL energies are not equally spaced and include a characteristic zero-energy state (the n = 0 LL). With the use of scanning tunneling spectroscopy of graphene grown on silicon carbide, we directly observed the discrete, non-equally–spaced energy-level spectrum of LLs, including the hallmark zero-energy state of graphene. We also detected characteristic magneto-oscillations in the tunneling conductance and mapped the electrostatic potential of graphene by measuring spatial variations in the energy of the n = 0 LL.

Exposure of Epitaxial Graphene on SiC(0001) to Atomic Hydrogen

Graphene films on SiC exhibit coherent transport properties that suggest the potential for novel carbon-based nanoelectronics applications. Recent studies suggest that the role of the interface between single layer graphene and silicon-terminated SiC can strongly influence the electronic properties of the graphene overlayer. In this study, we have exposed the graphitized SiC to atomic hydrogen in an effort to passivate dangling bonds at the interface, while investigating the results utilizing room temperature scanning tunneling microscopy.

Evolution of microscopic localization in graphene in a magnetic field from scattering resonances to quantum dots

The effects of disorder on the electrical characteristics of graphene are found to change drastically in a magnetic field. At zero field, disorder simply causes charge scattering. But at high fields it induces the formation of a network of quantum dots.

Microscopic polarization in bilayer graphene

Its tunable energy bandgap makes bilayer graphene interesting both from a theoretical perspective and with a view to applications. But exactly how the bandgap is formed is still unclear. A scanning tunnelling spectroscopy study now finds that the microscopic picture of the gap is fundamentally different from what is expected from macroscopic measurements and currently developed theories.

Atomic-scale investigation of graphene formation on 6H-SiC(0001)

The growth of graphene on the silicon-terminated face of 6H-SiC(0001) was investigated by scanning tunneling microscopy (STM) measurements. The initial stages of ultrahigh vacuum graphitization resulted in the growth of individual graphene sheets on random SiC terraces. These initial graphene sheets contained few defects, and the regions of clean SiC were free of contamination, exhibiting a 63×63R30° surface reconstruction. However, graphitization to multilayer thickness resulted in multiple defects, as observed with the STM. A high density of defects was observed, which may be attributed to the initial treatment of the SiC wafer. We characterize these defects, showing that they are located predominantly below the first layer of graphene.

Structural and Electronic Properties of Bilayer Epitaxial Graphene

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) are used to study the structural and electronic properties of bilayer epitaxial graphene on SiC(0001). Topographic images reveal that graphene conforms to the SiC interface morphology and is observed to be continuous across steps separating adjoining terraces. Bilayer epitaxial graphene is shown to be Bernal stacked as is evidenced by bias-dependent topographic imaging. STS maps of the differential conductance show that graphene lattice defects cause scattering of charge carriers near the Fermi level. An analysis of stationary scattering patterns observed in the conductance maps determines the energy-momentum dispersion relation within 100meV of the Fermi level. In contrast to lattice defects, disorder at the SiC interface and at subsurface steps plays a much lesser role in the scattering of charge carriers.

Invited Article: Autonomous assembly of atomically perfect nanostructures using a scanning tunneling microscope

A major goal of nanotechnology is to develop the capability to arrange matter at will by placing individual atoms at desired locations in a predetermined configuration to build a nanostructure with specific properties or function. The scanning tunneling microscope has demonstrated the ability to arrange the basic building blocks of matter, single atoms, in two-dimensional configurations. An array of various nanostructures has been assembled, which display the quantum mechanics of quantum confined geometries. The level of human interaction needed to physically locate the atom and bring it to the desired location limits this atom assembly technology. Here we report the use of autonomous atom assembly via path planning technology; this allows atomically perfect nanostructures to be assembled without the need for human intervention, resulting in precise constructions in shorter times. We demonstrate autonomous assembly by assembling various quantum confinement geometries using atoms and molecules and describe the benefits of this approach.

Making Mn Substitutional Impurities in InAs using a Scanning Tunneling Microscope

We describe in detail an atom-by-atom exchange manipulation technique using a scanning tunneling microscope probe. As-deposited Mn adatoms (Mn(ad)) are exchanged one-by-one with surface In atoms (In(su)) to create a Mn surface-substitutional (Mn(In)) and an exchanged In adatom (In(ad)) by an electron tunneling induced reaction Mn(ad) + In(su) --> Mn(In) + In(ad) on the InAs(110) surface. In combination with density-functional theory and high resolution scanning tunneling microscopy imaging, we have identified the reaction pathway for the Mn and In atom exchange.