Singularities and topologically protected states in twisted bilayer graphene

Yao, Qiuyan; Chen, Xingchen; Bremen, Rik van; Sotthewes, Kai; Zandvliet, Henricus J.W.

doi:10.1063/1.5135071

Cited by 14 publications

(12 citation statements)

References 26 publications

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“…For example, selective edge/corner functionalization may be combined with structures of specific topologies/geometries (such as square-shaped quantum dots). This may lead to an interesting interplay of and coexistence of exited states with specific properties controlled by electronic localization. , Additionally, control over the degree of nonplanarity induced in these graphene systems shows promise for further tuning of optical properties and warrants further investigation into effects of strain.…”

Section: Discussionmentioning

confidence: 99%

Tunable Optical Features of Graphene Quantum Dots from Edge Functionalization

et al. 2021

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Graphene quantum dots (GQDs) are promising semiconducting materials for practical applications. Chemical functionalization of GQD edges is a strategy to increase their solubility and processability and to modify their optical properties. Such functionalization has a twofold effect on the electronic structure, both attributed to electronic spatial localization. The first is relevant to the introduction of electronegative species, whereas the second effect modifies the electron wave function spatial distribution through the conformational changes of geometry of the otherwise planar GQD backbone. Our computational study shows that functionalization with a weakly electronegative chlorine functional group induces small red-shifts due to the localization of excitonic states around the defect site. Groups with strong electron inducting ability such as nitro introduce stronger red-shifts. In both cases, the planar geometry of the pristine sheet is preserved when functionalization is performed at only a single site. However, functionalizing at multiple sites around the edge results in warping of the geometry of the GQD, leading to stronger electronic localizations and the largest red-shifts. This observation suggests the utility of controlling the planar geometry as a new strategy for chemically modifying 2-dimensional nanomaterials toward optical tuning.

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Section: Discussionmentioning

confidence: 99%

Tunable Optical Features of Graphene Quantum Dots from Edge Functionalization

et al. 2021

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“…The work function difference between sample and tip is difficult to control experimentally, since it is dependent on the tip geometry; however, a minor difference in work function of a few tens of eV already leads to a substantial electric field, given the small sample-tip distances. The latter implies that the topologically protected states can be observed even at zero sample bias: Scanning tunneling spectroscopy (STS) reveals that the local density of states (LDOS) at the domain walls is higher than the AB/BA domain walls at zero bias voltage [28]. Figure 3(e) shows an atomic resolution image of one of the AA stacked domains from Fig.…”

Section: Resultsmentioning

confidence: 98%

Valley-protected one-dimensional states in small-angle twisted bilayer graphene

et al. 2021

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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.

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“…Lately, the explorations of physical properties for tBLG disclose its high dependence on the twist angle. [ 47 ] However, the manufacturing techniques of tBLG with fine control over twist angle face a great challenge. Inspired by the structure of tBLG, the vertical stacking of two monolayer graphene by manual assembly is the simplest way, and the assembly of 2D van der Waals heterostructures provides the relevant experiences.…”

Section: The Manufacturing Techniques Of Tblgmentioning

confidence: 99%

Fabrication Strategies of Twisted Bilayer Graphenes and Their Unique Properties

Cai

2021

Advanced Materials

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Therefore, graphene becomes a rising star on the horizon of materials science. Since the first report of centimeter-scale single-crystal graphene by Ruoff group showed high electrical quality, [9] great achievements have been made in the fabrication of high-quality graphene for the application in electronic devices, including single-crystalline graphene flakes, [10,11] super-ordered graphene structures, [12,13] super-clean graphene films, [14-16] and ultra-flat graphene films. [17] However, the zero-bandgap feature for monolayer graphene has limited its potential applications in semiconductors. In addition, the negligible mass, low yield, and high production costs of graphene are insurmountable bottlenecks for the practical application in energy conversion and storage. In particular, the absence of killer applications leads to the increased difficulty in graphene commercialization. [18,19] Nevertheless, it is undeniable that the unique structure of graphene has sparked intense interest in condensed matter physics, such as fractional quantum Hall effects, [17] proton permeation, [20] and plasmons. [21] More interestingly, twisted bilayer graphene (tBLG), which is similar to the van der Waals heterostructures, has a dramatic effect on the electronic properties. [22,23] The relative rotating graphene layers become a powerful model to study topological physical properties, including ferromagnetism, [24] Mott insulating behavior and superconducting behaviors, [25,26] topological valley transport, [27] van Hove singularities, [28] and tunable bandgap. [29] Consequently, the twisted structure of tBLG has triggered tremendous enthusiasm, even inspiring the formation of a new field for twisted 2D materials. Currently, production methods of tBLG contain chemical vapor deposition (CVD) on metal catalysts; [30] epitaxial growth on SiC substrate; [31] folding monolayer graphene, [32] and stacking monolayer graphene. [33] These methods are divided into two categories: direct growth and manual assembly. Generally, the direct growth of tBLG undergoes an in situ rearrangement of carbon atoms in the non-equilibrium state at high temperatures, whereas the ex situ vertical overlap of monolayer graphene by transfer process is the basic principle for manual assembly. The completely different preparation processes give rise to the distinctive advantages and disadvantages, but precise control over twist angle and super-clean interface are the ultimate pursuits for any preparation method. Recent progress in Twisted bilayer graphene (tBLG) exhibits a host of innovative physical phenomena owing to the formation of moiré superlattice. Especially, the discovery of superconducting behavior has generated new interest in graphene. The growing studies of tBLG mainly focus on its physical properties, while the fabrication of high-quality tBLG is a prerequisite for achieving the desired properties due to the great dependence on the twist angle and the interfacial contact. Here, the cutting-edge preparation strategies and challenges of tBLG fabrica...

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Singularities and topologically protected states in twisted bilayer graphene

Abstract: Long-range coupling interaction between a non-magnetic transition metal capping layer and a neighboring magnetic layer

Cited by 14 publications

References 26 publications

Tunable Optical Features of Graphene Quantum Dots from Edge Functionalization

Tunable Optical Features of Graphene Quantum Dots from Edge Functionalization

Valley-protected one-dimensional states in small-angle twisted bilayer graphene

Fabrication Strategies of Twisted Bilayer Graphenes and Their Unique Properties

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