Lattice Reconstruction in MoSe₂–WSe₂ Heterobilayers Synthesized by Chemical Vapor Deposition

Abstract

Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moirésystems with rich correlated electron phases and moiréexciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe 2 −WSe 2 , however, lattice reconstruction eliminates the canonical moirépattern and instead gives rise to arrays of periodically reconstructed nanoscale domains and mesoscopically extended areas of one atomic registry. Here, we elucidate the role of atomic reconstruction in MoSe 2 −WSe 2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moire-type cores and extended moire-free regions in heterostacks with parallel and antiparallel alignment. Our work highlights the potential of chemical vapor deposition for applications requiring laterally extended heterosystems of one atomic registry or exciton-confining heterostack arrays.

Imaging lattice reconstruction in homobilayers and heterobilayers of transition metal dichalcogenides

“…Remarkably, the inspection of R-type homobilayers as in figure 4(b) indicates that both bright and dark domains of R M h and R X h stackings emerge upon large-scale reconstruction [21,23], whereas heterobilayers tend to reconstruct preferentially into bright R M h domains at the expense of dark R X h domains as in figures 4(d) and (e). This suggests an energetic imbalance in the competition between R M h and R X h registries in heterobilayers that is absent in homobilayers, in accord with DFT calculations for MoSe 2 -WSe 2 heterobilayer with a slightly favored R M h stacking [22,44].…”

“…The mutual exclusion of the latter two, in particular, is a hallmark of the energy-driven competition between two near-optimal stackings in R-type MoSe 2 -WSe 2 heterostacks for large-area domains. The observation of nearly complete reconstruction of CVD-grown WSe 2 -MoSe 2 heterobilayers (except for the inner cores as discussed previously elsewhere [44]) into one exclusive registry is remarkable, and was observed as a robust feature on more than 70 heteroflakes. The distribution of distinct stackings for H-and R-type heterostacks is shown in figure 5(e), featuring H h h as the singular stacking for H-type stacks, and the R M h stacking outcompeting the R X h stacking with a ratio of ∼3.…”

“…The distribution of distinct stackings for H-and R-type heterostacks is shown in figure 5(e), featuring H h h as the singular stacking for H-type stacks, and the R M h stacking outcompeting the R X h stacking with a ratio of ∼3. This distribution provides further support into the energetic preference of MoSe 2 -WSe 2 stacks with regard to reconstruction into R M h domains as predicted by DFT calculations [22,44]. In rare cases (3 cases in contrast to 40 single-registry occurrences in the statistics of figure 5(e) for R-type stacks) the competition between R M h and R X h domains realizes alternating registries on the same heterostack, as exemplified in figures 5(f) and (g) for a CVDheterostack synthesized on another bottom MoSe 2 monolayer in the same growth run.…”

Competition of Moiré Network Sites to Form Electronic Quantum Dots in Reconstructed MoX2/WX2 Heterostructures

“…An almost lattice matching at the interface promotes reconstruction of the moiré superlattice (mSL) in a bilayer into domains where monolayer lattices conform to each other, separated by domain wall networks (DWNs) that absorb hydrostatic and shear (for small angle twisted bilayers) strain. Such reconstruction was observed in bilayers assembled by 2D crystal transfer − and synthesized using chemical vapor deposition (CVD) . For bilayers with a larger twist, lattice reconstruction is less prominent; however, those bilayers also feature moiré superstructures, which have been extensively investigated in the context of localization of charge carriers and interlayer excitons in specific stacking regions of the mSL. − …”

“…In terms of the electronic properties of MoX 2 /WX 2 , which are type II heterostructures, the mapping ,, of conduction/valence band edge variation across the mSL appears to be sensitive to the strain developed upon a bilayer’s lattice reconstruction. In particular, for marginally twisted bilayers, the formation of DWNs absorbs the dilation/compression and torsion required to adjust the two crystalline lattices within domains (in terms of the lattice mismatch of MoX 2 and WX 2 and an interlayer twist). − Typical structures are illustrated in the top panels of Figure by the maps of hydrostatic strain in MoSe 2 : honeycomb for bilayers with the antiparallel (AP) orientation of unit cells and triangular for the parallel (P) orientation. For AP bilayers, hexagonal domains correspond to 2H stacking, while corners of the DWN feature energetically unfavorable XX (chalcogen over chalcogen) and MoW (metal over metal) stacking domains. − P bilayers feature MoX (metal over chalcogen) and XW (metal under chalcogen) stacking domains, with XX stacking DWN nodes. − …”