Momentum-space indirect interlayer excitons in transition metal dichalcogenide van der Waals heterostructures

Abstract: Monolayers of transition metal dichalcogenides (TMDCs) feature exceptional optical properties that are dominated by excitons, tightly bound electron-hole pairs. Forming van der Waals heterostructures by deterministically stacking individual monolayers allows to tune various properties via choice of materials and relative orientation of the layers. In these structures, a new type of exciton emerges, where electron and hole are spatially separated. These interlayer excitons allow exploration of many-body quantum phenomena and are ideally suited for valleytronic applications. Mostly, a basic model of fully spatially-separated electron and hole stemming from the $K$ valleys of the monolayer Brillouin zones is applied to describe such excitons. Here, we combine photoluminescence spectroscopy and first principle calculations to expand the concept of interlayer excitons. We identify a partially charge-separated electron-hole pair in MoS$_2$/WSe$_2$ heterostructures residing at the $\Gamma$ and $K$ valleys. We control the emission energy of this new type of momentum-space indirect, yet strongly-bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in TMDC heterostructures and devices.

• The paper reveals a distinct 1.6 eV photoluminescence peak from interlayer excitons that is absent in individual monolayers.
• It demonstrates that adjusting the twist angle tunes exciton emission by altering layer separation, with DFT confirming a 0.07 Å change.
• The study establishes that momentum-space indirect excitons arise from Γ and K valley transitions, enabling tailored optoelectronic applications.

Overview of "Momentum-space indirect interlayer excitons in transition metal dichalcogenide van der Waals heterostructures"

The paper explores the optical and electronic properties of transition metal dichalcogenide (TMDC) van der Waals heterostructures, focusing on momentum-space indirect interlayer excitons. It addresses a pivotal aspect of two-dimensional (2D) materials: the formation and control of interlayer excitons (ILEs) by manipulating van der Waals heterostructures, particularly examining TMDCs such as MoS₂/WSe₂.

Key Findings

The research combines photoluminescence spectroscopy with first-principles calculations to uncover the behavior of partially charge-separated interlayer excitons in TMDC heterostructures. Key observations include:

1. Observation of ILEs: The study confirms the presence of ILEs in MoS₂/WSe₂ heterobilayers (HB), manifested as a distinct photoluminescence (PL) peak at 1.6 eV, which is absent in individual monolayers.
2. Twist Angle Dependence: The research demonstrates that the energy of these excitons can be tuned by adjusting the twist angle between the layers. This twist angle causes variations in layer separation, directly influencing the emission energy of ILEs.
3. Role of Momentum-space: The ILEs observed are momentum-space indirect, residing at the Γ and K valleys, with strong interlayer coupling effects. Notably, as the twist angle changes, the emission energy shifts significantly, contrasting with the relatively stable energies of A excitons.
4. Density Functional Theory (DFT) Validation: DFT calculations corroborate empirical observations, showing the twist angle affects the average layer separation by 0.07 Å. The accurate replication of experimental shifts by DFT models reinforces the identification of the ILE related to the Γ-K transition, as opposed to the typically assumed K-K transition.
5. Influence of Interlayer Hybridization: The study highlights that interlayer hybridization significantly impacts the hole states at the Γ-point, affecting exciton binding energies and thus emission characteristics. This provides an enhanced framework for understanding excitonic behavior in these systems.

Implications and Future Directions

The findings elucidate complex excitonic phenomena in TMDC heterostructures, emphasizing the interplay between structural manipulation and electronic properties. The twist-dependent tuning of ILEs paves the way for engineering optoelectronic devices leveraging valleytronics. This manipulation offers potential applications in areas such as quantum computing and photonics, where control over excitonic properties is pivotal.

Theoretically, this study expands the understanding of exciton binding energies and hybridization effects within TMDC layers. Practically, this opens a pathway towards designing materials with specific electronic interactions by adjusting twist angles, thereby tailoring material properties for desired applications.

Future research could focus on exploring other TMDC combinations, examining mechanical strain as an additional parameter for tuning electronic properties, or integrating these materials into broader heterostructure systems. Additionally, the quantum mechanical insights underline the necessity of advanced computational models like DFT to accurately predict and design new materials with desired excitonic properties.

This paper provides a crucial step in advancing the understanding and utility of 2D materials in next-generation electronic and photonic applications, highlighting the intricate balance of structural and electronic factors that define material performance.