Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

Abstract

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Control of layer stacking in CVD graphene under quasi-static condition

“…This is due to the complete decoupling of layers with a higher twist angle. 14,29 The shape and intensity of the 2D peak are characteristically different for SLG, AB stacked and twisted BLG. Hence, to ensure the nature of layer stacking in CVD grown BLG, the analysis of curve fitting for the 2D peak was performed.…”

“…When the twist angle between the stacked layers is more than 3 degrees, the charge carrier shows the characteristics of massless Dirac fermions, but with smaller carrier velocity and when this angle exceeds 20 degrees, the layers get completely decoupled, consequently their electronic properties become indistinguishable from the SLG. 14 In recent times, for graphene growth by the CVD technique there has been worldwide interest in understanding the growth mechanism to find out the ways to control it. [15][16][17][18][19][20][21][22] The self-limiting effect of low pressure CVD growth of single layer graphene on copper vanishes when the set growth conditions are outside the optimized window and the percentage growth of SLG, BLG or FLG can vary with growth conditions.…”

Observation of the intrinsic bandgap behaviour in as-grown epitaxial twisted graphene

“…For TG with small twist angles (yo15°), the experimental studies showed 1,3,6,9 that the Dirac cones for each layer persist but van Hove singularities are present due to the interlayer interaction. For large twist angles, the twisted layers effectively de-couple from each other and become indistinguishable from SLG 7 . However, several theoretical reports have recently predicted the existence of coupling between large-angle twisted layers.…”

“…T he rotational stacking of graphene layers alters their electronic characteristics resulting in novel and rich physics [1][2][3][4][5][6][7][8] . Bernal (AB)-stacked graphene rotated by 60°b etween adjacent layers is well understood and its electronic properties are radically different from single-layer graphene (SLG).…”

Two-Dimensional MoS2-Graphene-Based Multilayer van der Waals Heterostructures: Enhanced Charge Transfer and Optical Absorption, and Electric-Field Tunable Dirac Point and Band Gap

“…These heterogeneous stacks have unusual properties that are not present in individual layers and encompass a wide spectrum of physical and chemical phenomena exemplified by new van Hove singularities [4][5][6][7] , Fermi velocity renormalization 8,9 , unconventional quantum Hall effects 10 , Hofstadter's butterfly pattern [11][12][13][14] , and others. Peculiar electronic [15][16][17] and optoelectronic properties 18,19 have been revealed in diverse 2D heterostructures based on layered materials such as GR, semiconducting transition metal dichalcogenides (TMDs) and insulating hexagonal boron nitride (h-BN).…”