Theory predicts that the application of an electric field breaks the inversion symmetry of AB and BA stacked domains in twisted bilayer graphene, resulting in the formation of a triangular network of one-dimensional valley-protected helical states. This two-dimensional network of one-dimensional states has been observed in several studies, but direct experimental evidence that the electronic transport in these one-dimensional states is valley protected is still lacking. In this paper, we report the existence of the network in small-angle twisted bilayer graphene at room temperature. Moreover, by analyzing Fourier transforms of atomically resolved scanning tunneling microscopy images of minimally twisted bilayer graphene, we provide convincing experimental evidence that the electronic transport in the counterpropagating one-dimensional states is indeed valley protected.

Graphene inherently possesses defect sites and grain boundaries that are vulnerable to chemical etching by hydrogen radicals. In this study, an etch-mitigation method is presented to selectively passivate these sites using atomic layer deposition (ALD) of a H etch-resistant material. First, as a reference experiment, pristine exfoliated graphitic layers are exposed to H radicals to determine the lateral etch rate from defect sites. Next, these samples are compared to graphitic layers in which the defects are selectively passivated by Al2O3, in the same exposure conditions, using atomic force microscopy at every step in the experiment. The results show that etching is slowed down by local deposition of Al2O3 ALD at sites vulnerable to H radical etching.

In this report we give a brief introduction on the occurrence of topologically protected one-dimensional electronic states in group IV two-dimensional graphene-like materials. We discuss the effect of spin-orbit coupling on the electronic band structure and show that these materials are potential candidates to exhibit the quantum spin Hall effect. The quantum spin Hall effect is characterized by a gapped interior and metallic counterpropagating spin-polarized topologically protected edges states. We also elaborate on the electric-field induced formation of a hexagonal network of one-dimensional topologically protected electronic states in small-angle twisted bilayer graphene.

Two-dimensional transition metal dichalcogenide (TMDC) materials have shown great potential for usage in optoelectronic devices, especially down to the regime of a few layers to a single layer. However, at these limits, the material properties can be strongly influenced by the interfaces. By using photoconductive atomic force microscopy, we show a local enhancement of photoconductivity at the nanoscale in bilayer molybdenum disulfide on mica, where water is confined between the TMDC and the substrate. We have found that the structural phase of the water influences the doping level and thus the tunneling barrier at the nanojunction. This leads to an increase in photocurrent and enhanced photopower generation.