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Strong Inter-layer Coupling in van der Waals Heterostructures Built from Individual Monolayers

Strong Inter-layer Coupling in van der Waals Heterostructures Built from Individual Monolayers
Upper: WSe2/MoS2 hetero-bilayer illustration with respective lattice constants and misalignment angle. Lower: TEM images of boundary region of single layer and heterobilayer

Scientific Achievement
Members of Ali Javey’s Electronic Materials Program built new class of heterostructures consisting of layered transition metal components, by van der Waals stacking of individual monolayers into functional multilayer structures.

Significance and Impact
The nature of interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectric properties through customized composite layers.

Summary
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 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.

Javey and team stacked individual monolayers of transition metal components (WSe2/MoS2) into a functional multilayer component. Characterization by transmission electron microscopy, X-ray photoelectron microscopy, electron transport studies, and optical spectroscopy revealed 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 having spatially direct absorption but spatially indirect emission. 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 van der Waals gap, consisting of hexagonal boron nitride. 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 semi- conductor heterostructures having tunable optoelectronic properties with customized composite layers.

Hui Fang et al., PNAS 111, 17 (2014) doi: 10.1073/pnas.1405435111