Scanning probe analysis of twisted graphene grown on a graphene/silicon carbide template

Overlayer growth of graphene on an epitaxial graphene/silicon carbide (SiC) as a solid template by ethanol chemical vapor deposition is performed over a wide growth temperature range from 900 ºC to 1450 ºC. Structural analysis using atomic force and scanning tunneling microscopies reveal that graphene islands grown at 1300 ºC form hexagonal twisted bilayer graphene as a single crystal. When the growth temperature exceeds 1400 ºC, the grown graphene islands show a circular shape. Moreover, moiré patterns with different periods are observed in a single graphene island. This means that the graphene islands grown at high temperature are composed of several graphene domains with different twist angles. From these results, we conclude that graphene overlayer growth on the epitaxial graphene/SiC solid at 1300 ºC effectively synthesizes the twisted few-layer graphene with a high crystallinity.

Surface Transport Properties of Pb-Intercalated Graphene

Intercalation experiments on epitaxial graphene are attracting a lot of attention at present as a tool to further boost the electronic properties of 2D graphene. In this work, we studied the intercalation of Pb using buffer layers on 6H-SiC(0001) by means of electron diffraction, scanning tunneling microscopy, photoelectron spectroscopy and in situ surface transport. Large-area intercalation of a few Pb monolayers succeeded via surface defects. The intercalated Pb forms a characteristic striped phase and leads to formation of almost charge neutral graphene in proximity to a Pb layer. The Pb intercalated layer consists of 2 ML and shows a strong structural corrugation. The epitaxial heterostructure provides an extremely high conductivity of σ=100 mS/□. However, at low temperatures (70 K), we found a metal-insulator transition that we assign to the formation of minigaps in epitaxial graphene, possibly induced by a static distortion of graphene following the corrugation of the interface layer.

Non-invasive on-skin sensors for brain machine interfaces with epitaxial graphene

Brain-machine interfaces are key components for the development of hands-free, brain -controlled devices. Electroencephalogram (EEG) electrodes are particularly attractive for harvesting the neural signals in a non-invasive fashion. Here, we explore the use of epitaxial graphene grown on silicon carbide on silicon for detecting the electroencephalogram signals with high sensitivity. This dry and non-invasive approach exhibits a markedly improved skin contact impedance when benchmarked to commercial dry electrodes, as well as superior robustness, allowing prolonged and repeated use also in a highly saline environment. In addition, we report the newly -observed phenomenon of surface conditioning of the epitaxial graphene electrodes. The prolonged contact of the epitaxial graphene with the skin electrolytes functionalize the grain boundaries of the graphene, leading to the formation of a thin surface film of water through physisorption and consequently reducing its contact impedance by more than 75%. This effect is primed in highly saline environments, and could be also further tailored as pre-conditioning to enhance the performance and reliability of the epitaxial graphene sensors.

Observation of a flat band and bandgap in millimeter-scale twisted bilayer graphene

Magic-angle twisted bilayer graphene, consisting of two graphene layers stacked at a special angle, exhibits superconductivity due to the maximized density of states at the energy of the flat band. Generally, experiments on twisted bilayer graphene have been performed using micrometer-scale samples. Here we report the fabrication of twisted bilayer graphene with an area exceeding 3 × 5 mm2 by transferring epitaxial graphene onto another epitaxial graphene, and observation of a flat band and large bandgap using angle-resolved photoemission spectroscopy. Our results suggest that the substrate potential induces both the asymmetrical doping in large angle twisted bilayer graphene and the electron doped nature of the flat band in magic-angle twisted bilayer graphene.