Moiré-Potential-Induced Band Structure Engineering in Graphene and Silicene

Moiré Superlattice Structure in Two-Dimensional Catalysts: Synthesis, Property and Activity

Two-dimensional (2D) layered materials have been widely used as catalysts due to their high specific surface area, large fraction of uncoordinated surface atoms, and high charge carrier mobility. Moiré superlattice emerges in 2D layered materials with twist angle or lattice mismatch. By manipulating the moiré superlattice structure, 2D layered materials present modulated electronic band structure, topological edge states, and unconventional superconductivity which are tightly associated with the performance of catalysts. Hence, engineering moiré superlattice structures are proposed to be an important technology in modifying 2D layered materials for improved catalytic properties. However, currently, the investigation of moiré superlattice structure in a catalytic application is still in its infancy. This perspective starts with the discussion of structural features and fabrication strategy of 2D materials with moiré superlattice structure. Afterward, the catalytic applications, including electrocatalytic and photocatalytic applications, are summarized. In particular, the promotion mechanism of the catalytic performance caused by the moiré superlattice structure is proposed. Finally, the perspective is concluded by outlining the remaining challenges and possible solutions for the future development of 2D materials with moiré superlattice structure towards the catalytic applications.

Moiré Enhanced Potentials in Twisted Transition Metal Dichalcogenide Trilayers Homostructures

The exploration of moiré superlatticesholds promising potential to uncover novel quantum phenomena emerging from the interplay of atomic structure and electronic correlation . However, the impact of the moiré potential modulation on the number of twisted layers has yet to be experimentally explored. Here, this work synthesizes a twisted WSe₂ homotrilayer using a dry-transfer method and investigates the enhancement of the moiré potential with increasing number of twisted layers. The results of the study reveal the presence of multiple exciton resonances with positive or negative circularly polarized emission in the WSe₂ homostructure with small twist angles, which are attributed to the excitonic ground and excited states confined to the moiré potential. The distinct g-factor observed in the magneto-optical spectroscopy is also shown to be a result of the confinement of the exciton in the moiré potential. The moiré potential depths of the twisted bilayer and trilayer homostructures are found to be 111 and 212 meV, respectively, an increase of 91% from the bilayer structure. These findings demonstrate that the depth of the moiré potential can be manipulated by adjusting the number of stacked layers, providing a promising avenue for exploration into highly correlated quantum phenomena.

A primer on twistronics: a massless Dirac fermion's journey to moiré patterns and flat bands in twisted bilayer graphene

The recent discovery of superconductivity in magic-angle twisted bilayer graphene (TBLG) has sparked a renewed interest in the strongly-correlated physics ofsp²carbons, in stark contrast to preliminary investigations which were dominated by the one-body physics of the massless Dirac fermions. We thus provide a self-contained, theoretical perspective of the journey of graphene from its single-particle physics-dominated regime to the strongly-correlated physics of the flat bands. Beginning from the origin of the Dirac points in condensed matter systems, we discuss the effect of the superlattice on the Fermi velocity and Van Hove singularities in graphene and how it leads naturally to investigations of the moiré pattern in van der Waals heterostructures exemplified by graphene-hexagonal boron-nitride and TBLG. Subsequently, we illuminate the origin of flat bands in TBLG at the magic angles by elaborating on a broad range of prominent theoretical works in a pedagogical way while linking them to available experimental support, where appropriate. We conclude by providing a list of topics in the study of the electronic properties of TBLG not covered by this review but may readily be approached with the help of this primer.

Developing Graphene-Based Moiré Heterostructures for Twistronics

Graphene-based moiré heterostructures are strongly correlated materials, and they are considered to be an effective platform to investigate the challenges of condensed matter physics. This is due to the distinct electronic properties that are unique to moiré superlattices and peculiar band structures. The increasing research on strongly correlated physics via graphene-based moiré heterostructures, especially unconventional superconductors, greatly promotes the development of condensed matter physics. Herein, the preparation methods of graphene-based moiré heterostructures on both in situ growth and assembling monolayer 2D materials are discussed. Methods to improve the quality of graphene and optimize the transfer process are presented to mitigate the limitations of low-quality graphene and damage caused by the transfer process during the fabrication of graphene-based moiré heterostructures. Then, the topological properties in various graphene-based moiré heterostructures are reviewed. Furthermore, recent advances regarding the factors that influence physical performances via a changing twist angle, the exertion of strain, and regulation of the dielectric environment are presented. Moreover, various unique physical properties in graphene-based moiré heterostructures are demonstrated. Finally, the challenges faced during the preparation and characterization of graphene-based moiré heterostructures are discussed. An outlook for the further development of moiré heterostructures is also presented.

Stacking-configuration-enriched essential properties of bilayer graphenes and silicenes

First-principles calculations show that the geometric and electronic properties of silicene-related systems have diversified phenomena. Critical factors of group-IV monoelements, like buckled/planar structures, stacking configurations, layer numbers, and van der Waals interactions of bilayer composites, are considered simultaneously. The theoretical framework developed provides a concise physical and chemical picture. Delicate evaluations and analyses have been made on the optimal lattices, energy bands, and orbital-projected van Hove singularities. They provide decisive mechanisms, such as buckled/planar honeycomb lattices, multi-/single-orbital hybridizations, and significant/negligible spin-orbital couplings. We investigate the stacking-configuration-induced dramatic transformations of essential properties by relative shift in bilayer graphenes and silicenes. The lattice constant, interlayer distance, buckling height, and total energy essentially depend on the magnitude and direction of the relative shift: AA → AB → AA' → AA. Apparently, sliding bilayer systems are quite different between silicene and graphene in terms of geometric structures, electronic properties, orbital hybridizations, interlayer hopping integrals, and spin interactions.

Layer-Dependent Electronic Structure Changes in Transition Metal Dichalcogenides: The Microscopic Origin

We have examined the electronic structure evolution in transition metal dichalcogenides MX₂ where M = Mo, W and X = S, Se, and Te. These are generally referred to as van der Waals materials on the one hand, yet one has band gap changes as large as 0.6 eV with thickness in some instances. This does not seem to be consistent with a description where the dominant interactions are van der Waals interactions. Mapping onto a tight binding model allows us to quantify the electronic structure changes, which are found to be dictated solely by interlayer hopping interactions. Different environments that an atom encounters could change the Madelung potential and therefore the onsite energies. This could happen while going from the monolayer to the bilayer as well as in cases where the stackings are different from what is found in 2H structures. These effects are quantitatively found to be negligible, enabling us to quantify the thickness-dependent electronic structure changes as arising from interlayer interactions alone.