Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe2

Abstract

Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. Twisted bilayer transition metal dichalcogenides (TB-TMDs) can host moiré flat bands over a wide range of twist angles. For twist angle close to 60°, it was predicted that TB-TMDs undergo a lattice reconstruction which causes the formation of ultra-flat bands. Here, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3° of 60°. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice reconstruction. Our work provides a foundation for further study of the exotic correlated phases in TB-TMDs.

Observation of moiré excitons in the twisted WS2/WS2 homostructure

“…1–6,47,49 The vertical stacking of two monolayers of two-dimensional (2D) materials together with a rotational misalignment or lattice mismatch between them can produce nanoscale moiré patterns, which can periodically modulate their electrical and optical properties. 7–9 In recent years, twisted bilayer graphene has led to significant progress in related insulators and superconducting materials because moiré superlattices can generate flat electronic bands that cause strong electron interactions. 1,8,10 Inspired by the discovery of twisted bilayer graphene, researchers have predicted flat bands in other twisted bilayers with moiré superlattices, especially in the twisted bilayer structures of transition metal dichalcogenides (TMDs).…”

Electrically Tunable Localized versus Delocalized Intralayer Moiré Excitons and Trions in a Twisted MoS2 Bilayer

“…Robust control of twist angle through the “tear and stack” technique eases the preparation of reliable homobilayer samples providing access to larger moiré length scales. Recent experiments reveal atomic reconstruction, ultraflat bands, electrically tunable hybrid excitons, and valley dynamics , in twisted TMD homobilayers.…”

Recent advances in the controlled chemical vapor deposition growth of bilayer 2D single crystals

“…However, bilayer TMDs, owning their interlayer coupling and charge screening, generate more intriguing performances, emerging as a promising candidate in the family of 2D materials. 74,75 The development of bilayer TMDs not only stimulates competitive applications in the field of phototransistors, 76 photodetectros, 77 solar cells, 78,79 and light-emitting diodes (LED), 80–82 but also provides a possible platform to study fundamental physical properties. 83–85…”

“…1a). These approaches include the direct artificial stacking method, 15–17 tip folding method 18,19 and in situ growth strategy. 20 Generally, the artificial stacking method allows the consecutive transfer of as-obtained monolayer 2D materials and their construction into bilayers or multilayers with the desired twist angles by means of an alignment transfer system.…”