This is a repository copy of Resonantly hybridized excitons in moiré superlattices in van der Waals heterostructures.

Interlayer excitons were observed at the heterojunctions in van der Waals heterostructures (vdW HSs). However, it is not known how the excitonic phenomena are affected by the stacking order. Here, we report twist-angle-dependent interlayer excitons in MoSe/WSe vdW HSs based on photoluminescence (PL) and vdW-corrected density functional theory calculations. The PL intensity of the interlayer excitons depends primarily on the twist angle: It is enhanced at coherently stacked angles of 0° and 60° (owing to strong interlayer coupling) but disappears at incoherent intermediate angles. The calculations confirm twist-angle-dependent interlayer coupling: The states at the edges of the valence band exhibit a long tail that stretches over the other layer for coherently stacked angles; however, the states are largely confined in the respective layers for intermediate angles. This interlayer hybridization of the band edge states also correlates with the interlayer separation between MoSe and WSe layers. Furthermore, the interlayer coupling becomes insignificant, irrespective of twist angles, by the incorporation of a hexagonal boron nitride monolayer between MoSe and WSe.

van der Waals heterostructures composed of two different monolayer crystals have recently attracted attention as a powerful and versatile platform for studying fundamental physics, as well as having great potential in future functional devices because of the diversity in the band alignments and the unique interlayer coupling that occurs at the heterojunction interface. However, despite these attractive features, a fundamental understanding of the underlying physics accounting for the effect of interlayer coupling on the interactions between electrons, photons, and phonons in the stacked heterobilayer is still lacking. Here, we demonstrate a detailed analysis of the strain-dependent excitonic behavior of an epitaxially grown MoS2/WS2 vertical heterostructure under uniaxial tensile and compressive strain that enables the interlayer interactions to be modulated along with the electronic band structure. We find that the strain-modulated interlayer coupling directly affects the characteristic combined vibrational and excitonic properties of each monolayer in the heterobilayer. It is further revealed that the relative photoluminescence intensity ratio of WS2 to MoS2 in our heterobilayer increases monotonically with tensile strain and decreases with compressive strain. We attribute the strain-dependent emission behavior of the heterobilayer to the modulation of the band structure for each monolayer, which is dictated by the alterations in the band gap transitions. These findings present an important pathway toward designing heterostructures and flexible devices.

Vertically stacked atomic layers from different layered crystals can be held together by van der Waals forces, which can be used for building novel heterostructures, offering a platform for developing a new generation of atomically thin, transparent, and flexible devices. The performance of these devices is critically dependent on the layer thickness and the interlayer electronic coupling, influencing the hybridization of the electronic states as well as charge and energy transfer between the layers. The electronic coupling is affected by the relative orientation of the layers as well as by the cleanliness of their interfaces. Here, we demonstrate an efficient method for monitoring interlayer coupling in heterostructures made from transition metal dichalcogenides using photoluminescence imaging in a bright-field optical microscope. The color and brightness in such images are used here to identify mono- and few-layer crystals and to track changes in the interlayer coupling and the emergence of interlayer excitons after thermal annealing in heterobilayers composed of mechanically exfoliated flakes and as a function of the twist angle in atomic layers grown by chemical vapor deposition. Material and crystal thickness sensitivity of the presented imaging technique makes it a powerful tool for characterization of van der Waals heterostructures assembled by a wide variety of methods, using combinations of materials obtained through mechanical or chemical exfoliation and crystal growth.

Transition metal dichalcogenides (TMDs) have recently received increasing attention because of their potential applications in semiconducting and optoelectronic devices exhibiting large optical absorptions in the visible range. However, some studies have reported that the grain boundaries of TMDs can be easily degraded by the presence of oxygen in water and by UV irradiation, ozone, and heating under ambient conditions. We herein demonstrate the photodegradation of WSe2 and MoSe2 by laser exposure (532 nm) and the subsequent prevention of this photodegradation by encapsulation with hexagonal boron nitride (h-BN) layers. The photodegradation was monitored by variation in peak intensities in the Raman and photoluminescence spectra. The rapid photodegradation of WSe2 under air occurred at a laser power of ≥0.5 mW and was not observed to any extent at ≤0.1 mW. However, in the presence of a water droplet, the photodegradation of WSe2 was accelerated and took place even at 0.1 mW. We examined the encapsulation of WSe2 with h-BN and found that this prevented photodegradation. However, a single layer of h-BN was not sufficient to fully prevent this photodegradation, and so a triple layer of h-BN was employed. We also demonstrated that the photodegradation of MoSe2 was prevented by encapsulation with h-BN layers. On the basis of X-ray photoelectron spectroscopy and scanning photoemission microscopy data, we determined that this degradation was caused by the photoinduced oxidation of TMDs. These results can be used to develop a general strategy for improving the stability of 2D materials in practical applications.