7 research outputs found
Characterization of Hydrogen Plasma Defined Graphene Edges
We investigate the quality of hydrogen plasma defined graphene edges by Raman
spectroscopy, atomic resolution AFM and low temperature electronic transport
measurements. The exposure of graphite samples to a remote hydrogen plasma
leads to the formation of hexagonal shaped etch pits, reflecting the anisotropy
of the etch. Atomic resolution AFM reveals that the sides of these hexagons are
oriented along the zigzag direction of the graphite crystal lattice and the
absence of the D-peak in the Raman spectrum indicates that the edges are high
quality zigzag edges. In a second step of the experiment, we investigate
hexagon edges created in single layer graphene on hexagonal boron nitride and
find a substantial D-peak intensity. Polarization dependent Raman measurements
reveal that hydrogen plasma defined edges consist of a mixture of zigzag and
armchair segments. Furthermore, electronic transport measurements were
performed on hydrogen plasma defined graphene nanoribbons which indicate a high
quality of the bulk but a relatively low edge quality, in agreement with the
Raman data. These findings are supported by tight-binding transport
simulations. Hence, further optimization of the hydrogen plasma etching
technique is required to obtain pure crystalline graphene edges.Comment: 10 pages, 7 figure
Imaging the Conductance of Integer and Fractional Quantum Hall Edge States
We measure the conductance of a quantum point contact while the biased tip of a scanning probe microscope induces a depleted region in the electron gas underneath. At a finite magnetic field, we find plateaus in the real-space maps of the conductance as a function of tip position at integer (ν=1, 2, 3, 4, 6, 8) and fractional (ν=1/3, 2/3, 5/3, 4/5) values of transmission. They resemble theoretically predicted compressible and incompressible stripes of quantum Hall edge states. The scanning tip allows us to shift the constriction limiting the conductance in real space over distances of many microns. The resulting stripes of integer and fractional filling factors are rugged on scales of a few hundred nanometers, i.e., on a scale much smaller than the zero-field elastic mean free path of the electrons. Our experiments demonstrate that microscopic inhomogeneities are relevant even in high-quality samples and lead to locally strongly fluctuating widths of incompressible regions even down to their complete suppression for certain tip positions. The macroscopic quantization of the Hall resistance measured experimentally in a nonlocal contact configuration survives in the presence of these inhomogeneities, and the relevant local energy scale for the ν=2 state turns out to be independent of tip position