Meredith Nevius

A wide-bandgap metal-semiconductor-metal nanostructure made entirely from graphene

The electronic properties of graphene are spatially controlled from metallic to semiconducting by patterning steps into the underlying silicon carbide substrate. This bottom-up approach could be the basis for integrated graphene electronics.

Semiconducting Graphene from Highly Ordered Substrate Interactions

While numerous methods have been proposed to produce semiconducting graphene, a significant band gap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, subnanometer disorder prevents the required symmetry breaking necessary to make graphene semiconducting. In this work, we show for the first time that semiconducting graphene can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a band gap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.

Atomic structure of epitaxial graphene sidewall nanoribbons: flat graphene, miniribbons, and the confinement gap.

Graphene nanoribbons grown on sidewall facets of SiC have demonstrated exceptional quantized ballistic transport up to 15 μm at room temperature. Angular-resolved photoemission spectroscopy (ARPES) has shown that the ribbons have the band structure of charge neutral graphene, while bent regions of the ribbon develop a bandgap. We present scanning tunneling microscopy and transmission electron microscopy of armchair nanoribbons grown on recrystallized sidewall trenches etched in SiC. We show that the nanoribbons consist of a single graphene layer essentially decoupled from the facet surface. The nanoribbons are bordered by 1-2 nm wide bent miniribbons at both the top and bottom edges of the nanoribbons. We establish that nanoscale confinement in the graphene miniribbons is the origin of the local large band gap observed in ARPES. The structural results presented here show how this gap is formed and provide a framework to help understand ballistic transport in sidewall graphene.

Wide-Gap Semiconducting Graphene from Nitrogen-Seeded SiC

All carbon electronics based on graphene have been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of graphene electronics. We demonstrate a new approach to produce semiconducting graphene that uses a submonolayer concentration of nitrogen on SiC sufficient to pin epitaxial graphene to the SiC interface as it grows. The resulting buckled graphene opens a band gap greater than 0.7 eV in the otherwise continuous metallic graphene sheet.

Silicon intercalation into the graphene-SiC interface

In this work we use low-energy electron microscopy, x-ray photoemission electron microscopy, and x-ray photoelectron spectroscopy to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(000 (1) over bar) rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020 degrees C. In a sequence of depositions, we have been able to intercalate similar to 6 ML of Si into the graphene-SiC interface.

The bottom-up growth of edge specific graphene nanoribbons.

The discovery of ballistic transport in graphene grown on SiC(0001) sidewall trenches has sparked an intense effort to uncover the origin of this exceptional conductivity. How a ribbons edge termination, width, and topography influence its transport is not yet understood. This work presents the first structural and electronic comparison of sidewall graphene grown with different edge terminations. We show that armchair and zigzag terminated ribbons, grown from SiC, have very different topographies and interact differently with the substrate, properties that are critical to device architecture in sidewall ribbon electronics.