Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers (1710.10278v2)

Abstract: We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moir\'e pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moir\'e Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moir\'e potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moir\'e pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.

Theory of Optical Absorption by Interlayer Excitons in Transition Metal Dichalcogenide Heterobilayers

The paper presents a comprehensive theoretical framework for understanding optical absorption by interlayer excitons in transition metal dichalcogenide (TMD) heterobilayers, accounting for phenomena introduced by the presence of moiré patterns. The formation of these moiré patterns arises due to slight misalignments and twists in the layers, leading to significant modifications in optical and electronic properties.

Interlayer Excitons and Moiré Patterns

In TMD heterobilayers composed of materials such as WX₂/MoX₂ (where X represents chalcogens like S or Se), interlayer excitons manifest due to type-II band alignment. Here, electrons and holes occupy different layers, prompting a spatially indirect excitonic nature. The paper addresses the effects of relative rotations in the layers that lead to momentum displacement between electron and hole bands, relocating optically active interlayer excitons from the center to the corners of the moiré Brillouin zone (MBZ).

The paper demonstrates that these moiré patterns restore circular optical selection rules through interlayer excitons, contrary to previously established elliptical rules when isolated layers are translationally invariant. The twist angle is a tunable parameter that influences the energy states of the excitons, offering opportunities for experimental manipulation.

Excitonic Potential Energy and Quantum Dot Arrays

The paper's theoretical exploration predicts that the excitonic potential energy, resulting from moiré patterns, induces localization of excitons near potential minima. This phenomenon hints at the feasibility of creating two-dimensional arrays of quantum dots with consistent properties in Van der Waals heterostructures.

Implications and Future Research

Theoretical predictions about interlayer excitons challenge current understandings of excitonic interactions in heterostructures, paving the way for further experimental validation. The research implies that altering the twist angle between layers can dynamically influence optical absorptive properties, thereby suggesting potential applications in tunable optoelectronic devices and quantum dot engineering.

The prospects for leveraging moiré patterns to instigate two-dimensional arrays of quantum dots could have broad implications in nanoscale device design, potentially enhancing quantum computing or quantum communication systems.

Conclusion

This theoretical framework lays a foundational understanding of interlayer excitons influenced by moiré patterns in TMD heterobilayers, thereby offering a new perspective to guide future experimental and theoretical explorations in two-dimensional materials. By addressing both the theoretical and practical implications of these excitonic behaviors, the paper provides crucial insights that hold promise for advancements in nanotechnology and materials science.