Zhijie Li,Farsane Tabataba-Vakili,Shen Zhao,Anna Rupp,Ismail Bilgin,Ziria Herdegen,Benjamin März,Kenji Watanabe,Takashi Taniguchi,Gabriel Ravanhani Schleder,Anvar S. Baimuratov,Efthimios Kaxiras,Knut Müller-Caspary,Alexander Högele
Lattice Reconstruction in MoSe2–WSe2 Heterobilayers Synthesized by Chemical Vapor Deposition
two-dimensional semiconductors,MoSe2−WSe2,heterostructures,chemical vapor deposition,lattice reconstruction,atomic registries,interlayer excitons
Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moiré systems with rich correlated electron phases and moiré exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe2–WSe2, however, lattice reconstruction eliminates the canonical moiré pattern 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 MoSe2–WSe2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moiré-type cores and extended moiré-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.
Lattice Reconstruction in MoSe2–WSe2 Heterobilayers Synthesized by Chemical Vapor Deposition

Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moiré systems with rich correlated electron phases and moiré exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe2–WSe2, however, lattice reconstruction eliminates the canonical moiré pattern 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 MoSe2–WSe2 heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods, we identify the coexistence of moiré-type cores and extended moiré-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.

Vertical heterostructures of transition metal dichalcogenide semiconductors manifest in two contrasting regimes. On the one hand, exfoliation-stacked heterobilayers with finite lattice mismatch or twist angle give rise to periodic two-dimensional (2D) moiré patterns, which in turn result in flat moiré minibands of charge carriers with rich phenomena of correlated Hubbard model physics. Moiré potentials also profoundly affect strongly bound electron–hole pairs in the form of intralayer and interlayer excitons formed by Coulomb correlations within or across individual layers. This scenario is contrasted by moiré-free heterobilayers on the other hand, obtained from chemical vapor deposition (CVD) synthesis, where the absence of lateral moiré potentials is signified by one atomic registry extending laterally over large sample areas with simple photoluminescence (PL) spectra or enhanced diffusion of interlayer excitons. Heterostructure systems with diffusive interlayer excitons represent an ideal material platform for integrated dipolar exciton circuits, as demonstrated recently in exfoliation-stacked heterostructures with an additional interfacial layer of hexagonal boron nitride (hBN), which mitigates the exciton-confining moiré potential. Even more promising are CVD-synthesized heterobilayers free of moiré effects, featuring enhanced diffusivity of dipolar interlayer excitons, unaffected by diffusion-inhibiting moiré confinement. Such moiré-free heterostructures are not only ideal for integrated exciton circuits with external control by electrostatic gates, they could also enable deterministically imprinting arbitrary, tunable potential landscapes via patterned gate-electrodes, or dielectric superlattices. Moiré-free domains on micrometer length scales also emerge in heterostructures with small lattice mismatch and marginal twist subject to mesoscopic lattice reconstruction, where the driving mechanism behind atom rearrangement into energetically favorable registries is provided by the competition between intralayer strain and interlayer adhesion energy, yielding mesoscopic 2D domains of only one registry in MoSe2–WSe2 stamping-assembled heterostacks. For practical applications, however, such nondeterministic fabrication methods with resulting spatial inhomogeneities in morphology and optical properties limit the required uniformity and scalability, rendering CVD-based approaches to large-area moiré-free systems a promising alternative. In this work, we present an elaborate study of CVD-synthesized vertical MoSe2–WSe2 heterobilayers with evidence for extended reconstruction into domains of one atomic registry, enclosing a central region of periodically reconstructed nanoscale domains. Our studies cover both high-symmetry stacking configurations with 0° (R-type) and 180° (H-type) twist angle, and employ complementary imaging and optical spectroscopy methods to identify the diversity in local configurations of the reconstructed crystal lattice and the respective signatures of exciton transitions. Our work highlights the potential of CVD synthesis for obtaining both extended moiré-free domains and exciton-confining arrays of periodically reconstructed moiré regions, realizing the limits of dipolar excitons with and without a spatially varying potential landscape. Our samples with MoSe2–WSe2 heterobilayers consist of large monolayers of MoSe2 with monolayers of WSe2 on top, both synthesized by CVD in a two-step growth (see Sections 1 and 2 in the Supporting Information for details). Figure 1a depicts an optical micrograph of an as-grown heterobilayer, showing the formation of both H- and R-type stackings. We examined another heterobilayer sample synthesized in the same growth run by scanning electron microscopy (SEM), with representative images shown in Figure 1b,c. The images of H- and R-type samples were recorded with secondary electron imaging with material- and stacking-sensitive contrast (see Section 3 in the Supporting Information for details), clearly discerning a small triangle in the center of the heterostack. Three high-symmetry atomic registries are distinguished in H- and R-type heterostructures, namely Hhh, HhM, HhX and Rhh, RhM, RhX, with M, X, and h referring to the transition metal, chalcogen, and center of the hexagon, and the superscript and subscript to the MoSe2 and WSe2 layer, respectively (note that the nomenclature is chosen to be consistent with previous work). In Figure 1d,e, we show density functional theory (DFT) calculations of the generalized stacking fault energy (GSFE) for the different atomic registries with local extrema at the three high-symmetry stackings (see Section 4 in the Supporting Information for details) indicated by the respective top-view schematics. Minima in GSFE correspond to the energetically most favorable stackings that dominate the reconstruction. In the H-type case shown in Figure 1d, the Hhh atomic registry is by far the energetically most favorable, consistent with prior work. In contrast, in the R-type case (Figure 1e) both RhM and RhX stackings are close to the optimal energy, with RhM being slightly more favorable (inset of Figure 1e). This implies close competition between the two registries, with a higher likelihood of RhM to form extended domains. Our study of individual heteroflakes with aberration-corrected high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (see Section 5 in the Supporting Information for details) confirms the exclusive dominance of the Hhh atomic registry in H-type heterostacks, as well as the reconstruction of R-type stacks into RhM and RhX registries. To begin with, we establish the orientation of the crystallographic axes in the heterobilayers using the surrounding MoSe2 monolayer, with measurement (left) and simulation (right) of the hexagonal lattice shown in the top row of Figure 2a. For this orientation, the bottom rows in Figure 2a show simulations of the three possible high-symmetry stackings in H- and R-type, which compete for energy minimization in reconstruction. In stacks of H-type, our experiments with results as in Figure 2b identify the Hhh stacking only, in agreement with the theoretical prediction above. In R-type heterostacks, on the other hand, we observe extended areas of RhM registry in some flakes (as in Figure 2c), while other flakes exhibit extended areas of RhX stacking (as in Figure 2d). Given the close similarity of the two registry configurations with regard to optimal energy, it is plausible that some R-type flakes reconstruct into RhM while others take on the RhX stacking and that their distribution is stochastic on the flake-to-flake case. To study the optical properties of CVD-synthesized H- and R-type crystals, we encapsulated the sample shown in Figure 1a in hBN (see Section 6 in the Supporting Information for details) and performed cryogenic confocal PL and differential reflectance (DR) spectroscopy. Each atomic registry uniquely determines the combination of transition energies, optical selection rules, and oscillator strengths of interlayer excitons and thus provides means for spectroscopic characterization. For our sample, the maps of maximum interlayer exciton PL intensity in H- and R-type heterostructures are shown in Figure 3a,b with dashed lines indicating the heterostack boundaries. The H-type sample shows extended areas of bright interlayer exciton PL (as on the spot marked by the brown cross in Figure 3a) and a dark region near the center (blue cross in Figure 3a). The R-type sample features similar local variations in the PL map, yet with nearly 2 orders of magnitude lower intensity. The low intensity of R-type as compared to H-type stacks is surprising, as reconstructed mechanically stacked samples support the opposite trend. This observation suggests that domains of RhM registry with relatively dark interlayer excitons dominate CVD-grown R-type heterostacks. In the areas of maximum PL intensity marked by brown crosses in Figure 3, a and b, both H- and R-stacks feature simple spectra with dominant peaks at 1.40 and 1.45 eV in Figure 3, c and d, respectively. In the H-stack, the PL peak at 1.40 eV with a positive degree of circular polarization Pc (shown in the bottom panel of Figure 3c) corresponds to the triplet interlayer exciton transition in extended Hhh domains of reconstructed mechanically stacked samples, and is accompanied by a weak hot-luminescence singlet at 1.42 eV with negative Pc. The main peak in the R-type stack, on the other hand, is blue-shifted by 120 meV from its counterpart observed on reconstructed RhX domains of mechanically stacked heterostructures at 1.33 eV and exhibits zero Pc in Figure 3d. This finding, combined with the PL energy position and reduced relative brightness, corroborates our assumption about the RhM character of interlayer exciton states with z-polarized transitions. Finally, we observe that areas of maximum PL intensity exhibit simple resonances of intralayer exciton in the DR spectra of H- and R-type stacks (shown in Figure 3e,f for positions indicated by brown crosses in Figure 3a,b) as a hallmark of extended areas of one dominant atomic registry. We also point out the absence of trion-related resonances in the DR spectra of both stacks as a signature of low residual doping, which we also confirmed with PL spectroscopy away from heterostacks on monolayer MoSe2 (data not shown). In the central areas of H- and R-type stacks marked by blue crosses in the areas of dotted triangles in Figure 3a,b and spectra in Figure 3c,d, we observe reduced intensity of the main PL peaks and an additional spectral feature around 1.35 eV with negative Pc (not shown) in the R-type case. The latter is a feature of RhX atomic registry, which coexists with RhM in the core of the heterostack. This conclusion, substantiated by the observation of doublets in the DR spectra of Figure 3e,f near the intralayer exciton resonance of MoSe2 at 1.62 eV, identifies the central triangles as arrays of reconstructed quasi zero-dimensional (0D) domains, which can form as nanoscale hexagons of Hhh registry in H-type and in triangular tiling of RhX and RhM domains in R-type stacks. Although the spatial resolution of the SEM in secondary-electron imaging is insufficient to detect the actual tiling geometry, the difference in the contrast between the outer regions of both types of heterostacks and their triangular cores in Figure 1b,c provides compelling support for the assignment of the triangular centers to reconstructed 0D arrays. To elaborate on the nature of interlayer excitons in extended RhM domains not reported previously for MoSe2–WSe2 heterostacks, we performed angle-resolved PL measurements. Using momentum-space imaging with the 2D k-space profile shown in Figure 4a for the PL from a bright H-type area, we first confirm the in-plane character of the optical dipole orientation for Hhh interlayer excitons for reference. Their Gaussian emission profile, with highest intensity at zero in-plane k-vector in the linecut at kx = 0 (bottom panel of Figure 4a) is contrasted by the R-type case in Figure 4b: The PL emission in the energy range 1.43–1.46 eV assigned to RhM atomic registry is nearly zero at small k-vectors and increases with increasing k as a hallmark of out-of-plane dipole moment orientation. With the detection angle of 54° of our objective, we observe the PL from out-of-plane oriented interlayer excitons in RhM atomic registry, which are dark at normal incidence and exhibit z-polarized emission according to dipolar selection rules. To complete the correspondence between the signatures of interlayer excitons in stamping-assembled and CVD-synthesized heterostacks studied here, we discuss the results of magneto-luminescence experiments in Faraday configuration shown in Figure 4c,d for H- and R-type stacks, respectively. In finite magnetic fields, the interlayer triplet and singlet excitons in domains of Hhh atomic registry feature circularly polarized transitions, with linear valley Zeeman splitting given by Δz = E+ – E– = gμBB (with magnetic field B, Bohr magneton μB, exciton Landé factor g, and E± referring to the dispersion branch measured under σ+ and σ– detection). The triplet and singlet transitions differ characteristically in both sign and magnitude of their exciton Landé g-factors, determined from the slopes of simultaneous linear fits to the data in Figure 4c as −17.0 ± 0.4 and +11.5 ± 0.3, respectively, in agreement with DFT calculations and previous experiments. In the R-type case, the magnetic field dependence of the z-polarized RhM interlayer exciton is given by with zero-field exciton energy without exchange interaction E0 and exchange splitting δ. In Figure 4d, both dispersion branches are observed in both circular polarizations, and respecting the sign convention we thus obtain from the fit to the data with δ = 0.1 meV the absolute value of the exciton Landé factor as |g| = 6.5. The g-factor is in quantitative agreement with DFT, predicting a value of 6.3. In the central triangle (data not shown), we determine for RhX an interlayer exciton g-factor of +7.0 ± 0.2 in agreement with previous experiments and theory. Last, we note that in between extended bright areas in the PL maps of Figure 3a,b, we observed reduced PL intensities, which we ascribe to grain boundaries separating reconstructed 2D domains. Figure 5a shows PL and Pc spectra at such a dark position in the H-type stack with Hhh triplet and singlet exciton characteristics and slightly reduced Pc as compared to a bright spot with data in Figure 3c. The variations in the PL intensity of Hhh interlayer excitons in Figure 5c is consistent with local reduction in emission (dip around 4 μm with width given by the optical spot) in between two bright areas separated by a grain boundary. At the corresponding positions with reduced PL in the R-type stack, we observe additional contributions from RhX and Rhh atomic registries with spectra in Figure 5b and negative and positive Pc, respectively. The spatial intensity profiles of RhM and RhX interlayer exciton PL in Figure 5d are anticorrelated, identifying grain boundaries as mutually exclusive areas of competing registries with additional contribution from Rhh interlayer excitons via hot luminescence. The comparison of spectral characteristics of CVD-grown MoSe2–WSe2 heterostacks with our prior work on mechanically stacked samples yields qualitatively equivalent results anticipated from general theoretical considerations. In H-type stacks, we found domains of Hhh atomic registry to dominate extended areas of heterostacks, encompassing a triangular core of 0D domains. In R-type stacks, RhM and RhX domains coexist in triangular cores of CVD-heteroflakes and compete for mutually exclusive reconstruction in the surrounding domains. In R-type stacks with extended domains of RhM atomic registries, we identify the spectral signatures of interlayer excitons not reported previously from mechanically stacked samples. We anticipate that in CVD-grown heterostacks reconstruction takes place during high-temperature synthesis and thus is robust against postprocessing steps of sample transfer or annealing at much lower temperatures. With extended reconstructed 2D domains and 0D arrays in their inner cores, CVD-grown MoSe2–WSe2 heterostacks realize areas of both moiré-free and moiré-like systems. While the latter are technologically viable as quantum dot arrays of exciton-confining potentials on the nanoscale and the former for dipolar exciton circuitry and extrinsic gate-modulation of potential landscapes, grain boundaries between domains of different atomic registries could host excitons with topological protection.