Resonantly hybridised excitons in moiré superlattices in van der Waals heterostructures (1904.06214v1)

Abstract: Atomically-thin layers of two-dimensional materials can be assembled in vertical stacks held together by relatively weak van der Waals forces, allowing for coupling between monolayer crystals with incommensurate lattices and arbitrary mutual rotation. A profound consequence of using these degrees of freedom is the emergence of an overarching periodicity in the local atomic registry of the constituent crystal structures, known as a moir\'e superlattice. Its presence in graphene/hexagonal boron nitride (hBN) structures led to the observation of electronic minibands, whereas its effect enhanced by interlayer resonant conditions in twisted graphene bilayers culminated in the observation of the superconductor-insulator transition at magic twist angles. Here, we demonstrate that, in semiconducting heterostructures built of incommensurate MoSe2 and WS2 monolayers, excitonic bands can hybridise, resulting in the resonant enhancement of the moir\'e superlattice effects. MoSe2 and WS2 are specifically chosen for the near degeneracy of their conduction band edges to promote the hybridisation of intra- and interlayer excitons, which manifests itself through a pronounced exciton energy shift as a periodic function of the interlayer rotation angle. This occurs as hybridised excitons (hX) are formed by holes residing in MoSe2 bound to a twist-dependent superposition of electron states in the adjacent monolayers. For heterostructures with almost aligned pairs of monolayer crystals, resonant mixing of the electron states leads to pronounced effects of the heterostructure's geometrical moir\'e pattern on the hX dispersion and optical spectrum. Our findings underpin novel strategies for band-structure engineering in semiconductor devices based on van der Waals heterostructures.

• The paper demonstrates that twist-angle manipulation in MoSe₂/WS₂ heterobilayers enables resonant exciton hybridization with a 60 meV photoluminescence redshift.
• The study employs extensive PL and RC spectroscopy, along with theoretical modeling, to quantify the impact of moiré superlattice effects on exciton behavior.
• The findings advocate for precision band-structure engineering in 2D TMD heterostructures, paving the way for twist-angle-specific optoelectronic and valleytronic devices.

Resonantly Hybridized Excitons in Moiré Superlattices in van der Waals Heterostructures

The paper under discussion presents a detailed paper on resonantly hybridized excitons in moiré superlattices of van der Waals heterostructures, specifically focusing on heterobilayers composed of MoSe$_2$ and WS$_2$ monolayers. This paper highlights the nature of exciton hybridization facilitated by near-degeneracy of conduction band edges in these materials, which results in resonant enhancement of the moiré superlattice effects.

Overview of Findings

The research delivers compelling insights into the hybridization phenomena between intra- and interlayer excitons in MoSe$_2$/WS$_2$ heterobilayers. One key observation is the dependency of hybridization on the rotational alignment between the monolayers, leading to continuous tuning of hybridization strength by adjusting the twist angle. Notably, a significant twist-angle-dependent redshift of up to 60 meV was observed in the photoluminescence (PL) peaks associated with hybridized exciton states.

For heterostructures nearing perfect alignment or anti-alignment, the electronic minibands influenced by the moiré superlattice become evident, and the optical spectra exhibit miniband signatures. The hybridization strength and its optical manifestations are intricately linked to the structural moiré pattern, signifying the potential for twist-angle-specific optoelectronic applications.

Experimental and Theoretical Approaches

The paper involved PL and reflectance contrast (RC) spectroscopy on a comprehensive set of more than 100 chemically vapor-deposited (CVD) and several mechanically exfoliated heterobilayers, analyzed over a spectrum of twist angles. The findings were substantiated by a theoretical model that elucidates the intricate interplay between moiré superlattice effects and exciton hybridization.

Theoretical calculations indicate that, for twist angles near $0^\circ$ and $60^\circ$, the energies of the hybridized excitons are notably influenced by the moiré superlattice, beyond the simpler harmonic moiré potential models previously considered.

Implications and Future Directions

These findings open up avenues for designing novel semiconductor devices based on band-structure engineering via twist-angle manipulation in van der Waals heterostructures. The results underscore the capabilities of 2D material heterostructures, with closely aligned conduction or valence bands, to serve as a platform for advanced optoelectronic and valleytronic devices.

Future exploration is likely to delve into the robustness of such hybridized exciton states against various external perturbations and their behavior in the presence of electric and magnetic fields. Additionally, extending the paper to other TMD heterostructures with similar energy alignments could establish a broader class of materials for practical device engineering, enabling component functionalities modulated by designable moiré patterns.

In summation, this research profoundly enhances the understanding of excitonics within TMD heterobilayers, paving the way for innovative device concepts aligned with the burgeoning interest in moiré physics and van der Waals heterostructures.