Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides (1403.3754v2)

Abstract: Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

• The paper demonstrates that WSe₂/MoS₂ heterostructures exhibit a ~100 meV Stokes-like shift, confirming type II band alignment and robust interlayer coupling.
• It shows that integrating h-BN layers can modulate interlayer interactions, offering a novel approach to tailored band engineering in semiconductor devices.
• Advanced TEM, spectroscopy, and transport studies were used to precisely map charge transfer dynamics and resultant optoelectronic behavior.

Strong Interlayer Coupling in Van der Waals Heterostructures

This paper explores the potential of using van der Waals (vdW) heterostructures, specifically those composed of single-layer transition metal dichalcogenides (TMDCs), for developing advanced semiconductor devices. By focusing on a heterostructure comprised of WSe₂ and MoS₂, the paper explores the strong interlayer coupling and its implications for optoelectronic properties.

The intricate nature of TMDCs allows for the creation of multilayer structures with atomically sharp interfaces, avoiding issues seen in traditional heterostructures, such as atomic interdiffusion and resultant interface roughness. Despite these theoretical promises, such vdW heterostructures' optoelectronic behaviors at the single-layer level had not been experimentally verified until this paper.

Key Findings

• Strong Interlayer Coupling: The WSe₂/MoS₂ heterostructure is shown to exhibit a substantial Stokes-like shift of approximately 100 meV. This shift between the photoluminescence (PL) peak and the lowest absorption peak indicates a type II band alignment with spatially direct absorption but spatially indirect emission. The noteworthy intensity of PL suggests robust interlayer coupling of charge carriers.
• Controllable Interlayer Coupling: By integrating hexagonal boron nitride (h-BN) dielectric layers into the vdW gap, the research demonstrates that the coupling at the hetero-interface can be modulated. This adaptability introduces a new degree of freedom in band engineering, paving the way for tailor-made optoelectronic properties.
• Optoelectronic Properties: The paper reports that both single-layer WSe₂ and MoS₂ exhibit direct bandgaps. In contrast, their heterostructure displays a lower-energy PL peak, resulting from the spatially indirect radiative recombination characteristic of type II heterostructures.

Experimental Approaches

Utilizing advanced characterization techniques such as transmission electron microscopy (TEM), x-ray photoelectron microscopy, electron transport studies, and optical spectroscopy enabled a comprehensive analysis of the band alignments and optoelectronic properties. These methodologies allowed for precise mapping of the charge transfer dynamics and the electronic interaction within the heterostructure.

Implications and Future Directions

1. Device Application: The findings underscore the potential of vdW heterostructures in novel semiconductor devices. The feasibility of such materials in optoelectronic applications, including nanoscale light-emitting and lasing devices, is an area ripe for exploration.
2. Electrical Characteristics: The rectifying behavior observed in devices with the WSe₂/MoS₂ hetero-bilayer provides further evidence of their type II band alignment and charge transfer dynamics, crucial for future electronic applications.
3. Materials Engineering: The ability to tune interlayer interactions could facilitate the development of a new class of materials with customizable properties. This includes future work on the bottom-up construction of heterostructures using varying chemical compositions, spacing, and angular alignments.

Conclusion

This paper contributes a pivotal understanding of vdW heterostructures, offering valuable insights into their electronic and optoelectronic behavior. As these findings are consistent with theoretically predicted models, they pave the path for subsequent experimental investigations focused on vdW heterostructures’ full potential in next-generation optoelectronic and electronic devices. The research opens avenues for the development of materials with precisely engineered properties, which are key for the advancement of semiconductor technologies.