# 摘要

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 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 heterobilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of $\sim 100 \rm{meV}$ between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having 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. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling 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.

# 意义

A new class of heterostructures consisting of layered transition metal dichalcogenide components can be designed and built by van der Waals (vdW) stacking of individual monolayers into functional multilayer structures. Nonetheless, the optoelectronic properties of this new type of vdW heterostructure are unknown. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe spatially direct absorption but spatially indirect emission in this heterostructure, with strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN dielectric layers into the vdW gap. The generic nature of this interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties through customized composite layers.

# 前言

Two-dimensional layered transition metal dichalcogenide (TMDC) semiconductors such as MoS2 and WSe2 have established themselves as strong contenders for next-generation electronics and optoelectronics[1–6] and are promising building blocks for novel semiconductor heterostructures.[7–11] Conventional heterostructures are mainly based on group IV, III-V, or II-VI semiconductors with covalent bonding between atoms at the hetero-interface. Owing to atomic interdiffusion during growth, the resulting atomic-scale interface roughness and composition variation at the hetero-interface inevitably smear the density of states profile and consequently compromise the performance of these heterostructures, especially as the film thicknesses are reduced toward a single atomic layer. In addition, the choice of material components for conventional heterostructures is strongly dictated by lattice mismatch.

In TMDCs, however, individual layers are held together by van der Waals (vdW) forces, without surface dangling bonds.[12] Semiconductor heterostructures built up from monolayer TMDCs would in principle offer atomically regulated interfaces and thereby sharp band edges. Theoretical studies have predicted different electronic structures and optical properties from TMDC heterobilayers;[13–17] however, to date there have been no experimental results. Whereas previous experimental efforts have focused on graphene-based layered heterostructures,[8–11, 18–26] we present an experimental study on the electronic interlayer interaction in a heterostructure built from two singlelayer TMDC semiconductors, namely, MoS2 and WSe2. The hetero-bilayers are characterized by transmission electron microscopy, X-ray photoelectron microscopy, electron transport studies, and optical spectroscopy to elucidate the band alignments, optoelectronic properties, and the degree of the electronic layer coupling in this novel material system.

# 结论

In summary, we have fabricated and characterized an artificial vdW heterostructure by stacking monolayer TMDC building blocks and achieved electronic coupling between the two 2D semiconductor constituents. Strong PL with a large Stokes-like shift was observed from the WSe2/MoS2 hetero-bilayer, consistent with spatially indirect luminescence from a type II heterostructure. We anticipate that our result will trigger subsequent studies focused on the bottom-up creation of new heterostructures by varying chemical composition, interlayer spacing, and angular alignment. In addition, the focus will be on the fabrication of vdW semiconductor heterostructure devices with tuned optoelectronic properties from customized single-layer components. Particularly, electroluminescene efficiency of vdW heterostructures needs to be explored experimentally to examine their viability for use as nanoscale light-emitting/lasing devices.