Heterostructures

Moir’e superlattices from stacks of van der Waals materials offer an exciting arena in the fields of condensed matter physics and materials science. Typically, these moir’e superlattices consist of materials with identical or similar structures, and the long moir’e period arises from a small twist angle or lattice mismatch. In this article, we discuss that long moir’e period appears in a new moir’e system by stacking two dissimilar van der Waals layers with large lattice mismatch, resulting in couplings between moir’e bands from remote valleys in the momentum space. In this system, the first layer is reconstructed using a $sqrt{3}$ by $sqrt{3}$ supercell that resembles the Kekul’e distortion in graphene, and such reconstruction becomes nearly commensurate with the second layer. This Kekul’e moir’e superlattice is realized in heterostructures of transition metal dichalcogenides and metal phosphorus trichalcogenides such as MoTe$_2$/MnPSe$_3$. By first-principles calculations, we demonstrate that the antiferromagnetic MnPSe$_3$ strongly couples the otherwise degenerate Kramers’ valleys of MoTe$_2$, resulting in valley pseudospin textures that depend on N’eel vector direction, stacking geometry, and external fields. With one hole per moir’e supercell, we predict that the system can become a Chern insulator, of which the topology is tunable by external fields.

Published in: "arXiv Material Science".

The possibility of creating heterostructure of two-dimensional (2D) materials has emerged as a viable route towards realizing novel optoelectronic devices. However, the low light absorption due to their small absorption cross section, limits their realistic application. While light-matter interaction mediated by strong exciton-plasmon coupling has been demonstrated to improve absorbance and spontaneous emission in a coupled TMDC and metallic nanostructures, the fabrication of tunable broadband phototransistor with high quantum yield is still a challenging task. By synthesizing Ag nanoparticles (Ag NPs) capped with a thin layer of polyvinylpyrrolidone (PVP) through chemical route, we report a lithography-free fabrication of a large area broadband superior gate-tunable hybrid phototransistor based on monolayer graphene decorated by WS$_2$-Ag NPs in a three-terminal device configuration. The fabricated device exhibits extremely high photoresponsivity (up to $3.2times 10^4$ A/W) which is more than 5 times higher than the bare graphene/WS$_2$ hybrid device, along with a low noise equivalent power (NEP) (~10$^{-13}$ W/Hz$^{0.5}$, considering 1/f noise) and high specific detectivity ~1010 Jones in the wide (325-730 nm) wavelength region. The additional PVP capping of Ag NPs helps to suppress the direct charge and heat transfer and most importantly, increases the device stability by preventing the degradation of WS$_2$-Ag hybrid system. The enhanced optical properties of the hybrid device are explained via dipole mediated strong exciton-plasmon coupling, corroborated by COMSOL Multiphysics simulation. Our work demonstrates a strategy towards obtaining an environment-friendly, scalable, high-performance broadband phototransistor by tuning the exciton-plasmon coupling for new generation opto-electronic devices.

Published in: "arXiv Material Science".

Moir’e superlattices of tunable wavelengths and the further developed moir’e of moir’e systems, by artificially assembling two-dimensional (2D) van der Waals (vdW) materials as designed, have brought up a versatile toolbox to explore fascinating condensed mater physics and their stimulating physicochemical functionalities. In this Perspective, we briefly review the recent progress in the emerging field of moir’e synergy, highlighting the synergetic effects arising in distinct dual moir’e heterostructures of graphene and transition metal dichalcogenides (TMDCs). A spectrum of moir’e of moir’e configurations, the advanced characterization and the exploitation efforts on the moir’e-moir’e interactions will be discussed. Finally, we look out for urgent challenges to be conquered in the community and some potential research directions in the near future.

Published in: "arXiv Material Science".

Irradiation with light provides a powerful tool to interrogate, control or induce new quantum states of matter out of equilibrium, however a microscopic understanding of light-matter coupling in interacting electron systems remains a profound challenge. Here, we show that light grants a new quantum-geometric handle to steer and probe correlated quantum materials, whereby photons can couple directly to the shape and center of the maximally-localized Wannier functions that comprise the material’s interacting bands, dressing both electronic motion and electronic interactions with light. Notably, this effect is generic to any material and purely geometric in origin, but dominates emergent optical responses in correlated electron systems with poorly localized or obstructed Wannier functions. Spectroscopic consequences are first illustrated for a paradigmatic strongly interacting model with a tunable Wannier obstruction. We then present ramifications for non-equilibrium control of moir’e heterostructures and find that subjecting magic-angle twisted bilayer graphene to weak THz radiation can conspire with a fragile topological obstruction to profoundly alter the material’s competing interactions and tune across boundaries to competing phases.

Published in: "arXiv Material Science".