Localized charge carriers in graphene nanodevices

Abstract

Graphene—two-dimensional carbon—is a material with unique mechanical, optical, chemical, and electronic properties. Its use in a wide range of applications was therefore suggested. From an electronic point of view, nanostructured graphene is of great interest due to the potential opening of a band gap, applications in quantum devices, and investigations of physical phenomena. Narrow graphene stripes called “nanoribbons” show clearly different electronical transport properties than micron-sized graphene devices. The conductivity is generally reduced and around the charge neutrality point, the conductance is nearly completely suppressed. While various mechanisms can lead to this observed suppression of conductance, disordered edges resulting in localized charge carriers are likely the main cause in a large number of experiments. Localized charge carriers manifest themselves in transport experiments by the appearance of Coulomb blockade diamonds. This review focuses on the mechanisms responsible for this charge localization, on interpreting the transport details, and on discussing the consequences for physics and applications. Effects such as multiple coupled sites of localized charge, cotunneling processes, and excited states are discussed. Also, different geometries of quantum devices are compared. Finally, an outlook is provided, where open questions are addressed.

Strained fold-assisted transport in graphene systems

“…Several groups have observed clear signatures of equilibrium properties in strained areas predicted by various models, such as pseudo-Landau levels and sublattice symmetry breaking in STM images 40,[43][44][45] . Recent works have reported transport measurements on ribbon geometries 46,47 , with one study revealing ballistic transport at room temperatures along nanoribbons deposited on terraced SiC sub- strates (thus subject to deformations) 48 . This particular geometry highlights the possibility of creating extended strained fold-like structures with unusual transport properties.…”

Measurement Back-Action in Stacked Graphene Quantum Dots

“…18,19 Quantum dots form spontaneously in both ribbons as a result of disorder. 20 The nanoribbons can therefore be thought of as quantum dots located on top of each other and interacting via the Coulomb interaction, i.e. as a system and a detector.…”

Effect of Edge Roughness on Static Characteristics of Graphene Nanoribbon Field Effect Transistor

“…is still controversial in the literature, such that charge localization due to strong edge roughness has been reported in some works [59,60] as a responsible mechanism for Coulomb blockade and the formation of bottlenecks for carrier transport in GNR channels. Further research is required to understand the exact mechanism for charge localization along the edge.…”

“…However, the GNR length as a channel material is 16 nm in our simulation, which is a typical channel length for emerging transistors and the GNR width is~1.6 nm; thus, the strength of 1D problem corresponding to the ratio of the GNR channel length to GNR width (W/L « 1/10) is not large enough to make electron-electron interaction a considerable portion for most roughness amplitudes. However, the soundness of single-particle calculations with regard to width-to-length ratio of GNR is still controversial in the literature, such that charge localization due to strong edge roughness has been reported in some works [59,60] as a responsible mechanism for Coulomb blockade and the formation of bottlenecks for carrier transport in GNR channels. Further research is required to understand the exact mechanism for charge localization along the edge.…”