Moiré Materials’ Intralayer and Charge-Transfer Excitons Origin Resolved with Atomistic Bethe-Salpeter Equation Framework

Moiré materials represent a rapidly developing area of materials science, offering exciting possibilities for advanced optoelectronic devices, and researchers are striving to fully understand the behaviour of excitons within these complex structures. Indrajit Maity from Newcastle University, Johannes Lischner and Arash A. Mostofi from Imperial College London, alongside Ángel Rubio from the Max Planck Institute for the Structure and Dynamics of Matter and the Flatiron Institute, have now resolved longstanding discrepancies between simplified and detailed theoretical models of these excitons. The team’s work demonstrates that accurately modelling the influence of surrounding materials, specifically hexagonal boron nitride used for encapsulation, is crucial for predicting exciton behaviour. Their analysis reveals a surprising competition between different types of excitons, dependent on the material composition and stacking arrangement, and establishes a robust atomistic modelling approach for designing future exciton-based technologies in moiré materials.

DFT Studies of Two-Dimensional Materials

This body of work focuses on computational studies of two-dimensional materials, particularly transition metal dichalcogenides, using advanced techniques like density functional theory. Researchers employ these methods to investigate the electronic, optical, and structural properties of these materials, crucial for developing future optoelectronic devices. Investigations explore the impact of defects and strain on material properties, revealing how these factors influence performance and contribute to a deeper understanding of dielectric properties.

Atomistic Modeling Reveals Moiré Exciton Dielectric Screening

This study introduces a highly detailed, atomistic modeling approach to understand moiré excitons, which arise in layered materials created by twisting or stacking two-dimensional materials. Researchers utilize the Bethe-Salpeter equation with localized Wannier functions to accurately describe these excitons, moving beyond simpler continuum models and resolving discrepancies with previous calculations. This research demonstrates that accurately modeling excitons requires including the effects of dielectric screening from the surrounding hexagonal boron nitride (hBN) encapsulation. To fully understand exciton characteristics, scientists meticulously calculated intralayer exciton wavefunctions, mapping electron density distributions within the moiré cell.

By systematically varying the twist angle between layers, the study revealed a competition between Wannier-like and charge-transfer-like exciton behavior, directly linked to variations in band gaps and electron-hole interactions. The team innovated by employing an adiabatic switching approach to investigate the origins of moiré trapping and charge-transfer. By focusing on high-symmetry stacking regions, scientists gained atomistic insight into the factors governing exciton localization and charge distribution. Calculations demonstrated that the lowest-energy bright excitons are Wannier-like in WS2/WSe2 heterobilayers, but transition to charge-transfer-like behavior in twisted WSe2 homobilayers, a difference directly attributable to hBN encapsulation. Without hBN, the lowest-energy exciton in twisted WSe2 reverts to a Wannier-like character, highlighting the significant influence of the surrounding environment on exciton properties.

Moiré Excitons Reveal Dielectric Screening Importance

Scientists achieved a comprehensive understanding of moiré intralayer excitons in layered materials, resolving discrepancies between continuum models and more detailed atomistic calculations. The research team successfully modeled these excitons within WS₂/WSe₂ heterostructures using an atomistic framework based on the Bethe-Salpeter equation and localized Wannier functions. Results demonstrate that accurately capturing the behavior of these excitons requires inclusion of dielectric screening from hexagonal boron nitride (hBN) encapsulation. Experiments reveal three distinct absorption peaks around 1.

7 eV in nearly aligned WS₂/WSe₂ samples, with the lowest peak redshifted compared to larger twist angles. The team’s calculations, incorporating hBN screening, accurately reproduce these experimentally observed features, showing that the lowest-energy bright exciton localizes at high-symmetry regions within the moiré pattern and exhibits a Wannier-like character. Conversely, the highest bright exciton displays strong charge-transfer character and spatial delocalization when hBN screening is considered. Further analysis reveals a competition between local stacking-dependent band gaps and electron-hole interactions that bind the exciton, influencing its character and localization.

Without hBN screening, the charge-transfer character of the highest bright exciton vanishes, and the localization of the lowest bright exciton weakens significantly. Remarkably, the team demonstrated that the lowest bright exciton in twisted WSe₂ bilayers is charge-transfer-like when encapsulated in hBN, but transitions to a Wannier-like character without hBN. This work establishes atomistic modeling as a powerful approach for designing and controlling excitonic phenomena in moiré materials, paving the way for advancements in optoelectronic devices and quantum information processing.

Dielectric Screening Resolves Exciton Discrepancies

This research establishes a highly accurate, atomistic approach to model excitons within moiré materials, created by twisting or stacking two-dimensional materials. The team demonstrates that discrepancies between previous theoretical models and experimental observations arise from an incomplete accounting of dielectric screening caused by the surrounding hexagonal boron nitride (hBN) encapsulation. By incorporating this environmental effect into their calculations, the researchers accurately reproduce observed exciton behavior. Their analysis reveals a competition between “Wannier” and “charge-transfer” exciton characteristics, influenced by atomic relaxations and electron-hole interactions at specific stacking regions.

Importantly, the lowest-energy excitons are found to be Wannier-like in WS2/WSe2 heterobilayers but become charge-transfer-like in twisted WSe2 homobilayers when encapsulated in hBN, highlighting the crucial role of environmental control. Without hBN encapsulation, the lowest-energy exciton in twisted WSe2 reverts to a Wannier-like character. The authors acknowledge that their method relies on classical interatomic potentials for structural relaxation. This advancement paves the way for the rational design of excitonic states in van der Waals heterostructures.