Beyond van der Waals Interaction: The Case of MoSe2 Epitaxially

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Beyond van der Waals Interaction: The Case of MoSe Epitaxially Grown on Few-Layer Graphene 2

Minh Tuan Dau, Maxime Gay, Daniela Di-Felice, Céline Vergnaud, Alain Marty, Cyrille Beigné, Gilles Renaud, Olivier Renault, Pierre Mallet, Toai Le Quang, JeanYves Veuillen, Loïc Huder, Vincent T Renard, Claude Chapelier, Giovanni Zamborlini, Matteo Jugovac, Vitaliy Feyer, Yannick J. Dappe, Pascal Pochet, and Matthieu Jamet ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07446 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Beyond van der Waals Interaction: The Case of MoSe2 Epitaxially Grown on Few-Layer Graphene Minh Tuan Dau,∗,† Maxime Gay,‡ Daniela Di Felice,¶ Céline Vergnaud,† Alain Marty,† Cyrille Beigné,† Gilles Renaud,§ Olivier Renault,‡ Pierre Mallet,k Toai Le Quang,k Jean-Yves Veuillen,k Lo¨ıc Huder,⊥ Vincent T. Renard,⊥ Claude Chapelier,⊥ Giovanni Zamborlini,# Matteo Jugovac,# Vitaliy Feyer,# Yannick J. Dappe,¶ Pascal Pochet,§ and Matthieu Jamet∗,† E-mail: [email protected]; [email protected]

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To whom correspondence should be addressed Univ. Grenoble Alpes, CEA, CNRS, Grenoble INP, INAC-SPINTEC, 38000 Grenoble, France ‡ Univ. Grenoble Alpes, CEA, LETI, Minatec Campus, F-38054 Grenoble, France ¶ SPEC, CEA, CNRS, Université Paris Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France § Univ. Grenoble Alpes, CEA, INAC, MEM, 38000 Grenoble, France k Univ. Grenoble Alpes, CNRS, Institut Néel, F-38000 Grenoble, France ⊥ Univ. Grenoble Alpes, CEA, INAC, PHELIQS, 38000 Grenoble, France # Peter Gr¨ unberg Institute (PGI-6), Forschungszentrum J¨ ulich GmbH, D-52425, J¨ ulich, Germany †

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Abstract Van der Waals heterojunctions composed of graphene and transition metal dichalcogenides have gain much attention because of the possibility to control and tailor band structure, promising applications in two-dimensional optoelectronics and electronics. In this report, we characterized the van der Waals heterojunction MoSe2 /few-layer graphene with a high quality interface using cutting-edge surface techniques scaling from atomic to microscopic range. These surface analyses gave us a complete picture of the atomic structure and electronic properties of the heterojunction. In particular, we found two important results: the commensurability between the MoSe2 and few-layer graphene lattices and a bandgap opening in the few-layer graphene. The bandgap is as large as 250 meV and we ascribed it to an interface charge transfer which results in an electronic depletion in the few-layer graphene. This conclusion is well supported by electron spectroscopy data and density functional theory calculations. The commensurability between the MoSe2 and graphene lattices as well as the bandgap opening clearly show that the interlayer interaction goes beyond the simple van der Waals interaction. Hence, stacking two-dimensional materials in van der Waals heterojunctions enables us to tailor the atomic and electronic properties of individual layers. It also permits the introduction of a bandgap in few-layer graphene by interface charge transfer.

Keywords: van der Waals interaction, bandgap opening, heterojunction, fewlayer graphene, MoSe2 , commensurability, charge transfer

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Graphene and two-dimentional (2D) materials beyond graphene constitute a 2D flatland which has become one of the active emerging fields in materials research. 1,2 In the route to original functionnalities of 2D materials, three dimensional entities, conceptualized from 2Dlego pieces, have drawn particular attention because of dimensionality effects and their exotic properties. 1,3–6 As 2D layers are held together by a van der Waals (vdW) force, the realization of vertical heterojunctions based on these layers is an approach which offers a fertile platform to study fascinating vdW force-driven properties: commensurate lattice coincidence, electronic structure and band structure alignment, spin-orbit coupling or exchange coupling induced by proximity effect and inversion symmetry breaking. For application perspectives, 2D heterojunctions are very promising for low power consumption and flexible electronics, optoelectronic devices as well as energy harvesting, photocatalysis and biosensors. 7–11 Graphene is commonly used as a template for the overgrowth of 2D crystals because of its versatility and its large density of surface or egde nucleation sites. 12 Ex situ production of vertically stacked layers comprised of graphene and other 2D materials like boron nitride, transition metal dichalcogenides (TMDs) has been studied by using mechanical exfoliation combined with a transfer process. 13–15 Alternatively, direct growth of TMDs on graphene involving in situ fabrication process of both graphene and TMDs using high vacuum chemical vapor deposition (CVD) and ultra-high vacuum molecular beam epitaxy (UHV-MBE) has also been reported. 5,16 Such a fabrication of heterostrutures insures very clean vdW interfaces. In particular, the MBE technique allows for a large-area production of heterojunctions, which scales with the substrate surface. The ability to achieve large-area graphene-based heterojunctions with an uncontaminated vdW interface allows us to investigate their intrinsic properties, vdW interaction, proximity effect as well as the interplay between their structure and electronic bands. For instance, it has been found experimentally that there exists a set of commensurate rotation of 2D layers with respect to graphene. The relative rotation results in a Moiré-pattern registry, leading to outstanding electronic properties of the heterojunction. 17–19 Theoretically, Moiré pattern

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could also be engineered following a recently reported dislocation based framework. 20 In this paper, we combined cutting-edge surface analysis techniques to characterize the 2D MoSe2 /few-layer graphene heterojunction: scanning tunneling microscopy/spectroscopy at low temperature (STM/STS), grazing incidence x-ray diffraction (GIXRD) using synchrotron radiation and photoemission electron microscopy imaging in k -space (k -PEEM). These techniques allowed us to study the atomic (STM, GIXRD) and electronic (STS, k PEEM) structures of the vdW heterojunction. In this work, the large-scale heterojunction was grown by MBE using the vdW epitaxy of MoSe2 21 on graphene/SiC substrate. We found two important characteristics of the heterojunction: a commensurable growth of MoSe2 /few-layer graphene and a bandgap opening in the few-layer graphene. These findings clearly showed that the interaction between the 2D layers is not a simple vdW interaction as previously suggested. Our point-by-point results are outlined as follows. First, point defects and twin boundaries in the MoSe2 layer are imaged with STM. STS measurements show a bandgap separating the valence band maximum (VBM) and conduction band minimum (CBM) and the n-type doping of the MoSe2 layer. The reciprocal space mapping and radial scans obtained with GIXRD reveal a lattice alignment of the MoSe2 layer with respect to graphene layers and the SiC substrate. This finding suggests one configuration of epitaxial registry between MBE grown MoSe2 and graphene. We observed, however, a broadening of all in-plane MoSe2 peaks as measured by rocking scan well fitted by Gaussians with full width at half maximum of 8.0 ± 0.5◦ , which is a measure of the in-plane mosaic spread. The inspection of k -PEEM data shows the direct band gap of the monolayer, based on the relative energy levels of high symmetry K and Γ points. Constant energy maps show an azimuthal matching of the two Brillouin zones of MoSe2 and graphene, which is in good agreement with X-ray diffraction results. Interestingly, we found a gap opening in the vicinity of the Fermi level of few-layer graphene as compared with a bare graphene/SiC substrate. Unlike exfoliated and transferred heterojunctions, we did not observe any Moiré pattern neither in STM images nor in the X-ray radial scans. Thus, we cannot state that a Moiré pattern is at

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the origin of our finding. The origin of the observed gap in few-layer graphene is ascribed to an electronic interaction between MoSe2 and graphene associated with a significant charge transfer. This argument was supported by Density Functional Theory (DFT) calculations that show an enhanced bandgap width of bilayer graphene in MoSe2 /graphene/SiC compared to bare bilayer graphene/SiC. Our results allow us to shed light on advanced features of the structural and electronic properties at the vdW interface: commensurable epitaxial registry and charge transfer between the layers. These results also offer the possibility to control and tailor the band structure of few-layer graphene.

Results and discussion The structural and electronic properties of the heterojunction down to the atomic scale were first examined by STM-STS at 8 K. The substrate with one-, two-monolayer-thick-graphene employed for this study was elaborated by thermal decomposition in ultra-high vacuum (UHV). 22 A low MoSe2 nominal coverage (< 0.4 monolayer) was chosen in order to keep uncovered graphene regions as a reference for STM measurements. In Figure 1a, a large scale (200 nm x 200 nm) constant-current STM-STS image is shown, exhibiting well-defined monolayer MoSe2 (1 ML-MoSe2 ; if not stated further, ML refers to monolayer high) islands supported by the graphitized SiC surface. Here, the sample bias is fixed at a rather high value, +2.2 V, meaning that the tip probes empty states outside the electronic bandgap of the TMD. In this regime and with sufficiently small tunneling current, we avoid the possible severe damages induced by the tip on the TMD islands that we have frequently observed when the tip crosses the edge of a TMD island using small sample bias values or high currents. Most of the 1 ML-MoSe2 flakes shown in Figure 1a lie on graphene terraces. The flakes have sizes ranging between 10 and 100 nm and exhibit edges with average directions following a six-fold symmetry. The nominal coverage of the TMD deduced from Figure 1a and similar large-scale images is close to 0.3 monolayer. Here, small islands of bilayer MoSe2 (2 ML-

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MoSe2 ) are also present on the bottom part of the 1 ML flakes. The apparent heights of the 1 ML- and 2 ML-MoSe2 flakes are deduced from the profile shown in Figure 1c performed along the red line drawn in Figure 1b. The measured step height between 1 ML-MoSe2 and 2 ML-MoSe2 is 0.64 ± 0.02 nm, which corresponded to the distance between adjacent MoSe2 layers in the bulk material. 23 We find a slightly higher value (0.82 ± 0.02 nm) for the step height measured between graphene and 1 ML-MoSe2 , which is ascribed to the different electronic contributions of graphene and MoSe2 to the STM image. We now focus on the inner part of the 1 ML-MoSe2 flakes. As depicted in the upper part of Figure 1d, which is a zoom-in of the boxed area in Figure 1b, the core of the flake exhibits small regions (10-15 nm wide) with a homogeneous crystalline structure. This is confirmed by the bottom part of Figure 1d, a STM image with atomic resolution corresponding to the dashed boxed region of the upper part image: the triangular lattice of the surface (Se) atoms is resolved, with a period of 0.33 ± 0.02 nm. The homogeneous regions of the flake are limited by edges or by inversion domain boundaries (or twin boundaries, labeled TB on the upper image in Figure 1d), which are commonly found in 2D TMD materials grown by MBE, and have been recently studied in MoSe2 by STM. 24,25 Point-like defects are also often found within the flakes, as exemplified in Figure 1d (defects labeled D), that we ascribe to Se vacancies. 26 A strong asset of the STM technique lies in its capability to perform direct measurements of the local electronic density of states of the heterojunction through STS. Here, we measured the electronic bandgap of the monolayer MoSe2 phase by performing STS in defect-free areas of the MoSe2 flake such as those shown in Figure 1a. A typical dI/dV(V) spectrum of STS, displayed in a vertical logarithmic scale, is shown in Figure 1e. The tunneling conductance dI/dV(V) roughly reflects the local density of states of the flake at the tip position at energy EF + eV. As suggested in Refs. [27-29], displaying the dI/dV(V) curves in a vertical logarithmic scale allows for a proper estimation of the onset positions of the VBM and CBM of the TMD. 27–29 Indeed, in the monolayer limit, as the VBM of MoSe2 is located at the

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K point of the Brillouin zone and the out-of-plane spatial extension of the electronic states close to the K point is very weak (compared to the ones at the Γ point), the signal measured by the STM tip at the VBM can be significantly enhanced by using a vertical logarithmic scale. 30 Several spectra (corresponding to different setpoints) were recorded at each position for a quantitative analysis. From Figure 1e, we find the average energy position of the VBM and CBM at 1.85 ± 0.04 eV below EF and 0.32 ± 0.04 eV above EF , respectively. These values correspond to the biases where the signal raises above the maximum noise level within the gap. Consistent values were obtained for spectra with different setpoints. We therefore deduce the value of the electronic bandgap: 2.17 ± 0.08 eV which is similar to the ones reported previously. 27,30 We can also identify n-type doping of the MoSe2 by referrering to the position of the Fermi level in the bandgap. Furthermore, a pronounced dI/dV peak at -2.24 ± 0.02 eV can be seen in the STS spectrum. From the band structure calculations, 28 this peak is ascribed to the dispersion band maximum at the center of the Γ point of the Brillouin zone, suggesting an energy separation of 0.39 eV between the valence band maxima of the K and Γ points. It is noted that the data acquired at different positions in the inner part of the flake remains almost unchanged while the bandgap becomes smaller when approaching the edges. This feature is presumably due to the change of the electronic states induced by dangling bond in the MoSe2 layer at the edges. To shortly summarize, the important results we could extract from atomic-scale characterizations are: the presence of point defects and twin boundaries in low density and the n-type doping of MoSe2 . To get more insight into the microscopic properties of the heterojuction, we performed GIXRD and k -PEEM. GIXRD enables us to probe the microscopic structure of the whole stack of the heterojunction, thus, its epitaxial registry. For this purpose, we grew a set of three samples having nominal thicknesses of 0.7 , 1.0 and 3.5 monolayers MoSe2 on multilayergraphene/SiC substrates, which were prepared by graphitization in an Ar/H2 mixture gas flow. Note that the number of graphene sheets in this case (estimated from GIXRD data) is larger than in the STM/STS study. However, the growth mechanism, which is mostly related

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to the last free-standing-graphene layers, should to be identical. 31 Figure 2 shows an in-plane reciprocal space map (rsm) measured on the 3.5 ML sample, covering one-sixth of angular range of the rsm. The h, k and ℓ indices are reciprocal space units using the 6H-SiC reciprocal lattice. SiC has the Wurtzite structure (space group P63mc), and unit cell parameters a = b = 3.73 ˚ A; c = 10.053 ˚ A and α = β = 90◦ ; γ = 120◦ . The maps measured on the 0.75 ML and 1 ML samples are very similar, showing equal graphene and SiC Bragg intensities, and weaker intensities of MoSe2 reflections. In addition to the rsm, very precise radial scans were ¯ (Figure 3a) and (hh 2h0) ¯ (Figure 3b), performed along the high symmetry directions (h0h0) as well as precise rocking scans (Figure 3c) and crystal-truncation-rod (CTR) scans along (10¯1ℓ) and (11¯2ℓ) (Figure 3d). ℓ was varied by increasing the exit angle with respect to the surface while keeping the incident angle fixed. The diffraction peaks from three different hexagonal lattices are clearly visible in the in-plane rsm: the visible peaks of the SiC(0001) substrate; the peaks from the few-layer graphene on top of SiC, and those of the MoSe2 thin layer, in the form of broad, in-plane textured, rings of scattering. No other feature is visible. The position of these peaks (together with out-of-plane ones, not shown), yields the following epitaxial relationships: SiC[10¯10](0001)//Gr[11¯20](0001)//MoSe2 [11¯20](0001). This finding indicates that the in-plane lattices of graphene and MoSe2 are aligned to each other, whereas the SiC substrate lattice is rotated with an angle of 30◦ with respect to the two adjacent overlayers. The relative lattice orientation between graphene and SiC substrate corresponding to a six-fold symmetry is commonly observed for graphitization process. However, no other rotational variants are seen for MoSe2 with respect to graphene. Only the lattice registry following the six-fold symmetry is featured and that is found to be different than the CVD growth. In fact, in the CVD growth without any nucleation control, in addition to the 60◦ variant, MoS2 growth on graphene also shows a significant signature of a 30◦ variant. 5 Thus, it turns out that vdW interaction between MoSe2 and graphene layers are robust enough and could induce a driving force for the in-plane epitaxial alignment of the MoSe2 lattice with the graphene. As for the 0.7 ML and 1.0 ML MoSe2 samples, tuning

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MoSe2 thickness does not affect systematically the relative orientation of MoSe2 and graphene lattices nor the mosaic spread of the MoSe2 layer (see Table S1, Supporting Information), suggesting that the first layer of TMD can serve as a flexible template for further growth of 2D layers onto graphene-based heterojunction. All the parameters extracted from the GIXRD measurements are summarized in Table S1 (Supporting Information). The lattice parameters of the few-layer graphene and MoSe2 were extracted by fitting the positions of the corresponding Bragg peaks along radial scans. The in-plane lattice constant of MoSe2 with different thicknesses remains almost unchanged and identical to that of the bulk, indicating a strong proof of a vdW epitaxy of the heterojunction. Moreover, following Renaud et al. 32 and from the Bragg peaks width, we can estimate the in-plane domain size of MoSe2 according to: (∆Q)2 = (2π/D)2 + Q2 (∆a/a)2 , where ∆Q is the peak full width at half maximum (FWHM) in nm−1 , D is the domain size (nm), Q the reciprocal space position (or momentum transfer in nm−1 ) and ∆a/a the FWHM of a possible inhomogeneous distribution of the in-plane lattice parameter. The lowest bound of the domain sizes are 25 ± 10 nm, 18 ± 10 nm, 30 ± 10 nm for 0.7 ML, 1.0 ML and 3.5 ML-thick MoSe2 , respectively. A linear evolution of the square of the MoSe2 peak width with Q2 was indeed found, revealing a distribution of the in-plane lattice parameter, i.e. some inhomogeneous strain (see Figure S2, Supporting Information). The in-plane mosaic spreads were deduced from rocking scans across the few-layer graphene and MoSe2 peaks. No evolution was found with varying momentum transfer Q, thus showing that the peak widths are completely dominated by the in-plane mosaic spread. The mosaic spread of the MoSe2 layer (8◦ ) is much larger than the one of the graphene layer underneath but still smaller than that of epitaxial TMD grown on AlN/Si substrates by MBE. 33 The resulting inhomogeneous strain and in-plane mosaic spread would imply a certain degree of lattice distortion or shearing of the MoSe2 layer, which could be induced by the defects, mirror twin boundaries in the MoSe2 layer. CTR measurements were performed in order to obtain the out-of-plane parameters of the stack. We extracted the few-layer graphene thickness,

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which corresponds to about 7 graphene sheets, by fitting the graphene rods perpendicular to the surface, using the ROD software. 34 Quantitative MoSe2 rod measurements were also performed, and the intensities fitted using the ROD program. Four rods (10¯1ℓ), (11¯2ℓ), (20¯2ℓ) and (30¯3ℓ) were fitted simultaneously, with the following free parameters: an overall scale factor, the distance between each MoSe2 layer and their occupancies. The thickness deduced from the fits of out-of-plane rods is almost nominal (see Table 1, Supporting Information). The thicker sample (3.5 ML) yields quite different results by this fitting process, presumably because of additional roughness and/or contamination due to air transfer whose simulation would need too many additional parameters. Note that the other samples were subjected to being transferred under UHV condition to the European Synchrotron Radiation Facility (ESRF) using a vacuum suitcase. Interestingly, we find ∼ 1.2% contraction of the MoSe2 inter-plane distance perpendicular to the surface compared to the theoretical value (3.38 ˚ A) of free-standing MoSe2 , which was obtained by DFT calculations. This vertical contraction relative to the Se and Mo planes, which has never been reported before in an as-grown TMD, may result from the interaction between MoSe2 and graphene. To shortly summarize the GIXRD data, the important results of the microstructure analysis are: the alignment of the MoSe2 and few-layer graphene lattices and the vertical contraction of the atomic planes (Mo, Se) of MoSe2 Finally, the electronic interaction between MoSe2 and graphene at the microscopic scale was investigated using k -PEEM. The multilayer-graphene/SiC substrates are the same as in GIXRD measurements, i.e. about 7 graphene sheets. Figure 4a presents a cut along the ′

high symmetry K -Γ-K direction of one-layer-thick MoSe2 /few-layer graphene. This cut was extracted from a stack made of a series of 2D momentum maps measured at different binding energies. Three of these maps, taken at binding energies of 0.0, 1.3 and 1.4 eV, are shown in Figure 4b. The band structure of MoSe2 and few-layer graphene, on the other hand, can be very distinctively identified with characteristic features: first, the intense density at the Γ point (originating from the delocalized out-of-plane Mo 4dz 2 orbitals of MoSe2 and

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′

Se 3pz orbitals); second, the two sets of six-fold symmetry bands at K-K : one at higher binding energy (cuts 2 and 3) mostly due to the localized in-plane Mo 4dx2 −y2 /dxy orbitals of MoSe2 and the other one at much lower binding energy (cut 1) due to graphene with its distinct Dirac cones 35,36 (see Figure 4b). In terms of geometry, further structural information regarding the MoSe2 layer and graphene is also given by the angular alignment of the K points of graphene and MoSe2 relative to the center of the Brillouin zone: ortho-radial intensity plots at the Kgraphene and KM oSe2 diameters show a negligible angular deviation of 0.3◦ , showing the perfect structural alignment of MoSe2 on graphene. Together with the reciprocal map acquired by GIXRD, Figure 4b confirms a commensurate lattice matching of the MoSe2 and graphene layers. Thereby, the measured ratio Γ-K (MoSe2 )/Γ-K (graphene), i.e. commensurate ratio, is equal to 0.77, which is in reasonable agreement with the value determined by synchrotron diffraction (0.74). A close inspection to the in-plane lattice constant of the MoSe2 layer by measuring the Γ-K (MoSe2 ) distance gives 1.305 ± 0.091 ˚ A−1 yielding an in-plane lattice parameter of 3.21 ± 0.22 ˚ A, which is again in reasonable agreement within error bars with the bulk and X-ray diffraction values. This figure is indicative of the lattice relaxation of MoSe2 , as expected in vdW growth. Regarding the energy band structure of MoSe2 , the upper VBM characteristic of a single-layer MoSe2 is clearly seen, with the VBM binding energy at the K point at about 1.3 eV being closer to the Fermi level than the VBM at the Γ point, thus reflecting a direct bandgap character expected in the monolayer limit. 37 We measure EV BM (K )-EV BM (Γ) = 0.21 eV using the second derivative of intensity plots, which is in a good agreement with measurements performed with similar energy resolution. 38 This energy difference is however smaller than the value of 0.38 eV derived from angle-resolved photoemission spectroscopy (ARPES) experiments conducted at a much higher energy resolution of 0.025 eV, which is able to resolve the 0.180 eV spin-splitting of the VBM at the K point. 27,29 The much lower value found here is, therefore, directly related to the broadening of the band at the K point resulting from the experimental conditions, leading to a large uncertainty in the determination of the VBM. Another parameter one can

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derive from the band structure of MoSe2 is the hole effective mass using a parabolic fit of the bands at the Γ and K points within the free electron approximation for the final state. We find hole effective masses of ∼ 2.73 m0 and 1.82 m0 with m0 being the free electron mass, respectively at the Γ and K points. These effective masses are larger than the calculated values for one monolayer MoSe2 . 39,40 This might be due to the dependence on the high-symmetry directions along which the effective mass is calculated. 41 Further experiments such as magneto-transport in high magnetic fields characterized by Shubnikov-de-Haas oscillations 42 are needed to confirm these values. In Figure 4a, the band dispersion of few-layer graphene is recognized as distinct Dirac cones. The linear dispersion of few-layer graphene shows a gap in the vicinity of the Fermi level. This feature is further investigated by looking at the slices of the few-layer graphene band structure perpendicular to the Γ-K direction and about the K point, in order to visualize the graphene dispersion of π and π ∗ bands (Figure 5a). A bare few-layer graphene/SiC substrate was also measured for comparison and the result is shown in Figure 5b. In the Figure 5a, there is a clear decrease of the photoemission signal in the vicinity of the region at 0.3 eV below the Fermi level, i.e. a bandgap opening. Moreover, a broadening of the linear dispersion branches as compared to bare graphene can be seen as MoSe2 is deposited on graphene. The Dirac cones give Fermi velocities of 1.02 x 106 m/s and 1.31 x 106 m/s for bare graphene and MoSe2 /graphene, respectively. Thus, the evolution of the π bands of graphene including the bandgap opening and the broadening of the Dirac linear dispersion implies a radical change of the electronic states, which is a result of interlayer interaction between the 2D layers. This feature is further highlighted in the plot of the integrated intensity as a function of the binding energy in the area around kk = 0 of the heterojunction and bare graphene (Figure 5c). A distinct dip at 0.3 eV below the Fermi level for the heterojunction is observed. That is unambiguously ascribed to the existence of a bandgap in few-layer graphene. Details about the interpretation of Figure 5c will be given thanks to theoretical computation. The bandgap opening in few-layer graphene is approximately 250 meV which is two or-

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ders of magnitude larger than the minigaps found in the graphene-based heterojunction. 43 It is thus interesting to investigate further the origin of this bandgap by taking into account all possible interactions between MoSe2 and few-layer graphene beyond a simple vdW interaction. First, a strong chemical interaction may damage the chemical configuration of graphene even switching from sp2 to sp3 orbitals, thus, leading to a destruction of the linear dispersion cones. This is not the scenario in the present heterojunction. Second, interactions in a 2D heterojunction may result in a relative orientation of lattices, which induces a superlattice potential associated with a long-range Moiré pattern or a breaking of inversion symmetry in the graphene sublattices. 44,45 In fact, Moiré patterns stem from a local change in atomic density, thus charge density, at the corners of Moiré supercells, giving rise to a regular potential repeating over the surface. In addition to weak vdW coupling, this periodic potential has a real impact on the band structure of the heterojunction, especially on the π bands of graphene. The linear dispersion of the π bands built up from delocalized π bonds of out-of-plane p orbitals is very fragile due to the side-on-side nature of orbital overlapping. Therefore, when the π bands are subjected to this potential, it may result in a very small gap opening of a few meV, called mini-gaps, located at energy levels far away from the Fermi level. This has been observed in several 2D heterojunctions like hBN/graphene 45 and MoS2 /graphene 15 obtained with mechanical exfoliation and transfer process. By taking the lattice mismatch of MoSe2 and graphene m = 36% and a relative rotation of lattices of 0◦ , we find the Moiré period: λ = (1/m + 1)a ∼ 1 nm. 44 We should see the signature of the Moiré pattern if this short period exists. Indeed, the GIXRD is a highly sensitive technique to atomic displacement, therefore, it was capable of resolving such short-period Moiré superlattices. If the Moiré effect is merely electronic phenomenon, i.e. electronic Moiré pattern, STM measurements could unveil its existence in our scanning conditions. However, our experimental data from GIXRD and STM did not reveal any Moiré signature in the heterojunction. We finally believe that the interlayer interaction involving a charge transfer is probably

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at the origin of the band structure evolution and the gap opening in few-layer graphene. The charge transfer results from the proximity effect inside the vdW gap through orbitals overlapping. This process gives rise to a shift of the Fermi level, thus, a hole or electron doping in few-layer graphene. According to STS measurements, the MoSe2 is highly n-type doped, meaning that there is a significant electron transfer from graphene to MoSe2 . This scenario is also confirmed by a shift to higher binding energies of the graphene C 1s peak (that can be distinguished from the C 1s peak in the SiC substrate) for MoSe2 /graphene compared to the C 1s of bare graphene (see Figure S1, Supporting Information). Consequently, the doping effect results in an enhancement of electrostatic potential in few-layer graphene, i.e. a reinforcement of dipole fields, that lifts the subbands of graphene sheets and gives rise to a large bandgap in the vicinity of the Fermi level. 46 To support this experimental observation, we performed first-principles calculations taking into account vdW interaction between different layers. The computational details are given in the Methods section. We considered two different systems: bilayer (BL) graphene/SiC and MoSe2 /BL graphene/SiC. The corresponding unit cells used for ab initio calculations are shown in Figure 6a and 6b. The carbon buffer layer between SiC and graphene as formed during the silicon sublimation process is also considered for the calculations. 47 First, from a structural point of view, we find a vertical contraction (6.5%) of the MoSe2 layer, from 3.38 ˚ A for a free standing layer down to 3.16 ˚ A for MoSe2 /BL graphene/SiC. This contraction is connected to the electron transfer between the adjacent graphene and MoSe2 . However, the layer thickness extracted from x-ray diffraction is 3.34 ˚ A which is 5.7% larger than the calculated value. The LDA-DFT technique is known to underestimate lattice parameters by approximately 5%, which explains this difference well. We also quantitatively estimated the charge distribution in both systems. They are indicated as numbers in units of elemental charge e− per unit cell for each layer in Figures 6a and 6b. Positive blue (resp. negative red) numbers are for an excess (resp. a deficit) of electrons. In MoSe2 , we find an electronic polarity in agreement with the layer contraction found by the calculations. This result is also

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in line with STS measurements presented before where n-type doping in MoSe2 is observed. Note that the electronic doping is one of the consequences of the charge transfer, which is in turn the result of the interaction between MoSe2 and graphene. The charge transfer might in fact also induce delocalized states in the gap of the semiconductor, as earlier described by Tejedor and Flores. 48 This feature could result in the background of the photoemission signal around the Γ and K points of MoSe2 and the non-zero tunneling current in the gap of MoSe2 in the STM/STS measurements. By increasing the number of graphene layer from 2 to 4, the charge distribution between MoSe2 and graphene layers was found to be almost unchanged, indicating that the number of graphene layers seems to have a minor impact on the charge transfer process. This theoretical finding strongly supports the experimental observation of a significant charge transfer in the heterojunction MoSe2 /few-layer graphene, though the graphene/SiC substrates used are made of 7 graphene sheets as extracted from the GIXRD data. In Figures 6c and 6d, we show the calculated electronic band structure of BL graphene/SiC and MoSe2 /BL graphene/SiC. The band structure of bilayer graphene is highlighted in red and blue showing two distinct cones for the conduction and valence bands. It shows that adjacent graphene layers are electronically decoupled leading to two nearly independent linearly dispersing bands at the K point of graphene. 49 Moreover, in both systems, we clearly observe a bandgap between the maximum of the valence band and the minimum of the conduction band: 158 meV for BL graphene/SiC and 256 meV for MoSe2 /BL graphene/SiC. The bandgap opens because of the breaking of sublattice symmetry within graphene sheets owing to the electric field produced by the charge transfers between the different layers of the stack 50 . In BL graphene/SiC, the charge transfer between graphene and the SiC substrate leads to the bandgap opening. However, as shown in the experimental data of Figure 5b, we do not observe any bandgap in the few-layer graphene. Instead, at the K point, we observe a single cone with very broad branches. This is still a consequence of the number of graphene sheets in the sample. Furthermore, the vanishing dipole electric field within the 7 graphene

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layers leads to a vanishing bandgap that cannot be resolved in k -PEEM measurements. On the other hand, in the energy cut at the K point shown in Figure 5c, we observe an intense peak in the density of states at the Fermi level which can be attributed to localized states at the interface between SiC and the carbon buffer layer. 51 Note that the dotted line presented in Figure 5c is a result of a subtraction of these localized states, allowing for a straightforward comparison with the heterojunction. For MoSe2 /BL graphene/SiC, due to the strong charge transfer between MoSe2 and the top graphene layer, the resulting electric field gives rise to a bandgap in graphene which is larger than in BL graphene/SiC. Hence, we can distinctly observe a bandgap of 250 ± 100 meV as a dip in k -PEEM data, as shown in Figure 5c. Though the density of states in the gap does not drop down to zero due to the experimental energy resolution and a probable distribution of bandgap widths, we succeeded in demonstrating the existence of a bandgap in few-layer graphene as a consequence of charge transfer with MoSe2 .

Conclusion In summary, we have investigated the structural and electronic properties of the vdW heterojunction MoSe2 /few-layer graphene which has been fabricated by MBE with a high quality of vdW interface. The properties of the heterojunction were investigated in detail from atomic resolution to microscopic range thanks to a substrate-scale layer and its homogeneity. We found typical point defects and twin boundaries in the top-layer MoSe2 with a semiconducting gap locally determined by STS. The crystallographic directions of the MoSe2 lattice were found to align along those of graphene by GIXRD. The resulting lattice orientation was further confirmed by constant energy curves k -PEEM measurements showing a spatial coincidence of the first two Brillouin zones of the 2D layers. Another striking feature resulting from the interlayer interaction was also reported, that is the presence of unoccupied states, i.e. a bandgap, around the Fermi level of few-layer graphene. The origin of the bandgap

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opening was interpreted as a result of a significant electron transfer from graphene sheets to MoSe2 . Here, we pointed out that few-layer graphene was subjected to charge transfer processes from both the substrate and the TMD layer, leading to an enhancement of the bandgap-induced electric field in the graphene sheets. Together with the commensurable growth, we have shown the features in this heterojunction which are beyond the characteristics of the vdW interaction. Finally, the vdW heterojunction will permit the making of gapped few-layer graphene. The empty states are easy to reach since they are located at energies close to the Fermi level. This will offer perspectives of graphene-based heterojunctions not only for band engineering but also for exploring physics at the frontier of 2D and 3D worlds.

Methods Synthesis of MoSe2 by MBE All samples were grown in a MBE chamber with a base pressure of about 5x10−10 mbar which increased up to about 2x10−8 mbar during the co-deposition of Mo and Se. The deposition of large-scale MoSe2 layers on graphene was carried out using MBE technique. The Mo and Se were respectively evaporated by an e-gun evaporator with a home built evaporation cell. The Se pressure measured at the sample position with a retractable Bayard-Alpert gauge is about 2x10−6 mbar. The Mo deposition rate measured with a quartz balance monitor was 1.5 ˚ A.min−1 . The post-annealing was performed at 720 ◦ C for 15 minutes under Se flux.

Scanning tunneling microscopy and spectroscopy STM and STS measurements were performed under UHV conditions at a temperature of 8.4K. The data were recorded using a Nanonis electronics and dedicated software. For the spectroscopic measurements, the dI/dV(V) spectra were recorded with an open feedback loop using the lock-in technique, with a 10 mV (peak-to-peak) amplitude bias modulation 17


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at frequency 477 Hz. All data was analyzed using the WSXM software. 52

Grazing incidence x-ray diffraction using synchrotron radiation The Grazing Incidence X-ray Diffraction (GIXRD) measurements were performed using the UHV-MBE-CVD diffractometer 53 of the BM32 beamline (www.esrf.eu/UsersAndScience/ Experiments/CRG/BM32) at the European Synchrotron Radiation Facility ESRF (www.esrf.fr) in Grenoble using an X-ray energy of 12.5 keV (1.008 ˚ A), below the K-edge of Se to avoid background fluorescence. The incident beam, of energy resolution 10−4 eV, had a very small (5x10−5 rad) divergence parallel to the surface. The incident angle was set at 0.14◦, which is slightly below the critical angle for total external reflection perpendicular to the surface, in order to enhance the surface signal while minimizing the background from the SiC substrate. The incident beam size on the sample was 20 µm thick and 300 µm wide. The diffracted/scattered beam was measured using a MaxiPix 5x1 2D hybrid pixel detector having 1280 x 256 pixel of 55 x 55 µm2 , located 0.77 m away from the sample. Its long (70 mm) direction was oriented perpendicular to the surface, with a 1 mm guard slit parallel to the sample, 370 mm away from it. Three samples have been characterized with MoSe2 thicknesses of ∼ 0.7 ML, 1 ML and 3.5 ML deposited on graphene/SiC(0001).

Photoemission electron microscopy k -PEEM k -PEEM measurements were performed at the NanoESCA beamline of the ELETTRA synchrotron radiation storage ring in Trieste, Italy, using an electrostatic photoelectron emission microscope (PEEM). 54 The instrument, in the reciprocal space operation mode, can acquire 2 dimensional momentum maps in the k //[-2.0, 2.0] range within a single data acquisition. The photon energy was set to 70 eV, providing a linearly out-of-plane polarized light illuminating the sample with a 25◦ angle relative to the surface. Prior to the acquisition, the MoSe2 /graphene heterojunction sample was heated at 250 ◦ C for 2h under UHV conditions. k -PEEM was performed at 130 K, at an overall energy resolution of 85 meV and a momentum 18


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resolution of 0.03 ˚ A−1

Theoretical calculation Our calculations have been performed using the very efficient Density functional Theory localized orbital Fireball code. 55 Since the main features have been widely described elsewhere, 56 we just mention here that this code uses a self-consistent version of the Harris-Foulke LDA functional 55,57 and that the self-consistency is achieved over the occupation numbers. Optimized numerical basis sets have been used for molybdenum (Mo), Selenium (Se) and carbon (C) with respective cutoff radii in atomic units of s = 4.5, p = 4.5 for C, 58 s = 4.3, p = 4.8, d = 5.3 for Se and s = 5.4, p = 4.7, d = 4.8 for Mo. 59 The Fireball code has been used to optimize the different structures considered in this work, namely : a 3×3 unit cell of graphene / MoSe2 interface, a 3×3 bilayer graphene on a graphene buffer layer above a 5 layers SiC slab, and finally a 3×3 MoSe2 on the previous bilayer graphene / graphene buffer layer / SiC unit cell. Those systems have been optimized until the forces were below 0.1 eV/˚ A. Since vdW interaction is not properly described in DFT, we have considered an extra perturbation theory method, the LCAO-S2 + vdW approach 60 to accurately determine the interlayer equilibrium configuration. This approach is based on the dipolar approximation for vdW interaction and has already been used successfully in graphitic and 2D materials, in good agreement with experimental determinations. 43,61 Once the equilibrium configuration is found, the electronic structure of each system has been analyzed by means of Density of States (DOS), band structure and charge transfer calculations. A set of specific k-points along the Γ-K-M path has been used for band structure calculations.

Acknowledgement The authors thank Vincent Mareau, Laurent Gonon and Joao Paulo Cosas Fernandes for their assistance with Raman spectroscopy. The authors acknowledge Lucien Notin for tech19


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nical assistance with MBE, V. Cherkez and J.F. Motte for their participation to the development of the cryogenic STM. This work is supported by the CEA-project 2D FACTORY, Agence Nationale de la Recherche within the ANR MoS2ValleyControl, 2D Transformers contracts. The French state funds LANEF (ANR-10-LABX-51-01), Equipex (ANR-11-EQPX0010) and ANR-J2D are also acknowledged for their support with mutualized infrastructure. Part of this work was funded by the EC Graphene Flagship project (no. 604391). M.T. Dau acknowledges the support from the CEA-Enhanced Eurotalents program.

Supporting Information Available Supplementary information is available in the online version of the paper. It contains the XPS spectra, the linear evolution of the MoSe2 peak width with the momentum transfer Q2 , and the table of structural parameters extracted from the GIXRD data. This material is available free of charge via the Internet at http://pubs.acs.org/.

References 1. Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; Liang, L.; Louie, S. G.; Ringe, E.; Zhou, W.; Kim, S. S.; Naik, R. R.; Sumpter, B. G.; Terrones, H.; Xia, F.; Wang, Y. et al. Recent Advances in Two-Dimensional Materials beyond Graphene. ACS Nano 2015, 9, 11509–11539. 2. Duan, X.; Wang, C.; Pan, A.; Yu, R.; Duan, X. Two-Dimensional Transition Metal Dichalcogenides as Atomically Thin Semiconductors: Opportunities and Challenges. Chem. Soc. Rev. 2015, 44, 8859–8876. 3. Li, M.-Y.; Chen, C.-H.; Shi, Y.; Li, L.-J. Heterostructures Based on Two-Dimensional Layered Materials and Their Potential Applications. Mater. Today 2016, 19, 322 – 335. 4. Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huan, Y.; Duan, X. Van der Waals Heterostructures and Devices. Nat. Rev. Mater. 2016, 1, 16024. 20


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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5. Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2016, 10, 1067–1075. 6. Avsar, A.; Tan, J. Y.; Taychatanapat, T.; Balakrishnan, J.; Koon, G.; Yeo, Y.; Lahiri, J.; Carvalho, A.; Rodin, A. S.; Farrell, E. O.; Eda, G.; Neto, A. H. C.; Ozyilmaz, B. Spinorbit Proximity Effect in Graphene. Nat. Commun. 2014, 5, 4875. 7. Georgiou, T.; Jalil, R.; Belle, B. D.; Britnell, L.; Gorbachev, R. V.; Morozov, S. V.; Kim, Y.-J.; Gholinia, A.; Haigh, S. J.; Makarovsky, O.; Eaves, L.; Ponomarenko, L. A.; Geim, A. K.; Novoselov, K. S.; Mishchenko, A. Vertical Field-effect Transistor Based on Graphene-WS2 Heterostructures for Flexible and Transparent Electronics. Nat. Nanotech. 2013, 8, 100. 8. Ross, J. S.; Rivera, P.; Schaibley, J.; Lee-Wong, E.; Yu, H.; Taniguchi, T.; Watanabe, K.; Yan, J.; Mandrus, D.; Cobden, D.; Yao, W.; Xu, X. Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction. Nano Lett. 2017, 17, 638–643. 9. Shanmugam, M.; Jacobs-Gedrim, R.; Song, E. S.; Yu, B. Two-dimensional Layered Semiconductor/Graphene Heterostructures for Solar Photovoltaic Applications. Nanoscale 2014, 6, 12682–12689. 10. Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S.-H. An Efficient Molybdenum Disulfide/Cobalt Diselenide Hybrid Catalyst for Electrochemical Hydrogen Generation. Nat. Commun. 2015, 6, 5982. 11. Loan, P. T. K.; Zhang, W.; Lin, C.-T.; Wei, K.-H.; Li, L.-J.; Chen, C.-H. Graphene/MoS2 Heterostructures for Ultrasensitive Detection of DNA Hybridisation. Adv. Mater. 2014, 26, 4838–4844. 12. Li, Z.; Wu, S.; lu, W.; Shao, J.-J.; Kang, F.; Yang, Q.-H. Graphene Emerges as a Versatile Template for Materials Preparation. Small 2016, 12, 2674. 21


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 38

13. Woods, C. R.; Britnell, L.; Eckmann, A.; Ma, R. S.; Lu, J. C.; Guo, H. M.; Lin, X.; Yu, G. L.; Cao, Y.; Gorbachev, R. V.; Kretinin, A. V.; Park, J.; Ponomarenko, L. A.; Katsnelson, M. I.; Gornostyrev, Y. N.; Watanabe, K.; Taniguchi, T.; Casiraghi, C.; Gao, H.-J.; Geim, A. K. et al. Commensurate-Incommensurate Transition in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2014, 10, 451. 14. Coy Diaz, H.; Avila, J.; Chen, C.; Addou, R.; Asensio, M. C.; Batzill, M. Direct Observation of Interlayer Hybridization and Dirac Relativistic Carriers in Graphene/MoS2 Van der Waals Heterostructures. Nano Lett. 2015, 15, 1135–1140. 15. Pierucci, D.; Henck, H.; Avila, J.; Balan, A.; Naylor, C. H.; Patriarche, G.; Dappe, Y. J.; Silly, M. G.; Sirotti, F.; Johnson, A. T. C.; Asensio, M. C.; Ouerghi, A. Band Alignment and Minigaps in Monolayer MoS2 -Graphene Van der Waals Heterostructures. Nano Lett. 2016, 16, 4054–4061. 16. Miwa, J. A.; Dendzik, M.; Gronborg, S. S.; Bianchi, M.; Lauritsen, J. V.; Hofmann, P.; Ulstrup, S. Van der Waals Epitaxy of Two-Dimensional MoS2 -Graphene Heterostructures in Ultrahigh Vacuum. ACS Nano 2015, 9, 6502–6510. 17. Dean, C. R.; Wang, L.; Maher, P.; Forsythe, C.; Ghahari, F.; Gao, Y.; Katoch, J.; Ishigami, M.; Moon, P.; Koshino, M.; Taniguchi, T.; Watanabe, K.; Shepard, K. L.; Hone, J.; Kim, P. Hofstadters Butterfly and The Fractal Quantum Hall Effect in Moiré Superlattices. Nature 2013, 497, 598. 18. Mucha-Kruczy´ nski, M.; Wallbank, J. R.; Fal’ko, V. I. Moiré Miniband Features in the Angle-resolved Photoemission Spectra of Graphene/hBN Heterostructures. Phys. Rev. B 2016, 93, 085409. 19. Moon, P.; Koshino, M. Electronic Properties of Graphene/Hexagonal-Boron-Nitride Moiré Superlattice. Phys. Rev. B 2014, 90, 155406.

22


Page 23 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20. Pochet, P.; McGuigan, B. C.; Coraux, J.; Johnson, H. T. Toward Moiré Engineering in 2D Materials, Using Dislocation Theory. Appl. Mater. Today 2017, 9, 240–250. 21. Dau, M. T.; Vergnaud, C.; Marty, A.; Rortais, F.; Beigné, C.; Boukari, H.; BelletAmalric, E.; Guigoz, V.; Renault, O.; Alvarez, C.; Okuno, H.; Pochet, P.; Jamet, M. Millimeter-Scale Layered MoSe2 Grown on Sapphire and Evidence for Negative Magnetoresistance. Appl. Phys. Lett. 2017, 110, 011909. 22. Mallet, P.; Varchon, F.; Naud, C.; Magaud, L.; Berger, C.; Veuillen, J.-Y. Electron States of Mono- and Bilayer Graphene on SiC Probed by Scanning Tunneling Microscopy. Phys. Rev. B 2007, 76, 041403. 23. James, P. B.; Lavik, M. T. The Crystal Structure of MoSe2 . Acta Crystallogr. 1963, 16, 1183. 24. Liu, H.; Jiao, L.; Yang, F.; Cai, Y.; Wu, X.; Ho, W.; Gao, C.; Jia, J.; Wang, N.; Fan, H.; Yao, W.; Xie, M. Dense Network of One-Dimensional Midgap Metallic Modes in Monolayer MoSe2 and Their Spatial Undulations. Phys. Rev. Lett. 2014, 113, 066105. 25. Barja, S.; Wickenburg, S.; Liu, Z.-F.; Zhang, Y.; Ryu, H.; Ugeda, M. M.; Hussain, Z.; Shen, Z.-X.; Mo, S.-K.; Wong, E.; Salmeron, M. B.; Wang, F.; Crommie, M. F.; Ogletree, D. F.; Neaton, J. B.; Weber-Bargioni, A. Charge Density Wave Order in 1D Mirror Twin Boundaries of Single-Layer MoSe2 . Nat. Phys. 2016, 12, 751. 26. Peng, J.-P.; Guan, J.-Q.; Zhang, H.-M.; Song, C.-L.; Wang, L.; He, K.; Xue, Q.-K.; Ma, X.-C. Molecular Beam Epitaxy Growth and Scanning Tunneling Microscopy Study of TiSe2 Ultrathin Films. Phys. Rev. B 2015, 91, 121113. 27. Ugeda, M. M.; Bradley, A. J.; Shi, S.-F.; da Jornada, F. H.; Zhang, Y.; Qiu, D. Y.; Ruan, W.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X.; Wang, F.; Louie, S. G.; Crommie, M. F. Giant Bandgap Renormalization and Excitonic Effects in a Monolayer Transition Metal Dichalcogenide Semiconductor. Nat. Mater. 2014, 13, 1091. 23


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

28. Bradley, A. J.; M. Ugeda, M.; da Jornada, F. H.; Qiu, D. Y.; Ruan, W.; Zhang, Y.; Wickenburg, S.; Riss, A.; Lu, J.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X.; Louie, S. G.; Crommie, M. F. Probing the Role of Interlayer Coupling and Coulomb Interactions on Electronic Structure in Few-Layer MoSe2 Nanostructures. Nano Lett. 2015, 15, 2594– 2599. 29. Zhang, Y.; Chang, T.-R.; Zhou, B.; Cui, Y.-T.; Yan, H.; Liu, Z.; Schmitt, F.; Lee, J.; Moore, R.; andHsin Lin, Y. C.; Jeng, H.-T.; Mo, S.-K.; Hussain, Z.; Bansil, A.; Shen, Z.X. Direct Observation of The Transition From Indirect to Direct Bandgap in Atomically Thin Epitaxial MoSe2 . Nat. Nanotech. 2014, 9, 111–115. 30. Zhang, C.; Chen, Y.; Johnson, A.; Li, M.-Y.; Li, L.-J.; Mende, P. C.; Feenstra, R. M.; Shih, C.-K. Probing Critical Point Energies of Transition Metal Dichalcogenides: Surprising Indirect Gap of Single Layer WSe2 . Nano Lett. 2015, 15, 6494–6500. 31. Vishwanath, S.; Liu, X.; Rouvimov, S.; Mende, P. C.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Feenstra, R. M.; Furdyna, J. K.; Jena, D.; Xing, H. G. Comprehensive Structural and Optical Characterization of MBE Grown MoSe2 on Graphite, CaF and Graphene. 2D Mater. 2015, 2, 024007. 32. G. Renaud, A. B.; Robach, O. Growth, Structure and Morphology of the Pd/MgO(001) Interface: Epitaxial Site and Interfacial Distance. Phys. Rev. B 1999, 60, 5872. 33. Aminalragia-Giamini, S.; Marquez-Velasco, J.; Tsipas, P.; Tsoutsou, D.; Renaud, G.; Dimoulas, A. Molecular Beam Epitaxy of Thin HfTe2 Semimetal Films. 2D Mater. 2017, 4, 015001. 34. Vlieg, E. ROD: a Program for Surface X-ray Crystallography. J. Appl. Crystallogr. 2000, 33, 401–405. 35. Coehoorn, R.; Haas, C.; Dijkstra, J.; Flipse, C. J. F.; de Groot, R. A.; Wold, A. Electronic

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Structure of MoSe2 , MoS2 , and WSe2 . I. Band-structure Calculations and Photoelectron Spectroscopy. Phys. Rev. B 1987, 35, 6195–6202. 36. Jin, W.; Yeh, P.-C.; Zaki, N.; Zhang, D.; Sadowski, J. T.; Al-Mahboob, A.; van der Zande, A. M.; Chenet, D. A.; Dadap, J. I.; Herman, I. P.; Sutter, P.; Hone, J.; Osgood, R. M. Direct Measurement of the Thickness-Dependent Electronic Band Structure of MoS2 Using Angle-Resolved Photoemission Spectroscopy. Phys. Rev. Lett. 2013, 111, 106801. 37. Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2 . Nano Lett. 2010, 10, 1271–1275. 38. Xenogiannopoulou, E.; Tsipas, P.; Aretouli, K. E.; Tsoutsou, D.; Giamini, S. A.; Bazioti, C.; Dimitrakopulos, G. P.; Komninou, P.; Brems, S.; Huyghebaert, C.; Radu, I. P.; Dimoulas, A. High-Quality, Large-Area MoSe2 and MoSe2 /Bi2 Se3 Heterostructures on AlN(0001)/Si(111) Substrates by Molecular Beam Epitaxy. Nanoscale 2015, 7, 7896– 7905. 39. Chang, J.; Register, L. F.; Banerjee, S. K. Ballistic Performance Comparison of Monolayer Transition Metal Dichalcogenide MX2 (M = Mo, W; X = S, Se, Te) Metal-OxideSemiconductor Field Effect Transistors. J. Appl. Phys. 2014, 115, 084506. 40. Zibouche, N.; Philipsen, P.; Heine, T.; Kuc, A. Electron Transport in MoWSeS Monolayers in The Presence of an External Electric Field. Phys. Chem. Chem. Phys. 2014, 16, 11251–11255. 41. Peelaers, H.; Van de Walle, C. G. Effects of Strain on Band Structure and Effective Masses in MoS2 . Phys. Rev. B 2012, 86, 241401. 42. Fallahazad, B.; Movva, H. C. P.; Kim, K.; Larentis, S.; Taniguchi, T.; Watanabe, K.; Banerjee, S. K.; Tutuc, E. Shubnikov de Haas Oscillations of High-Mobility Holes in

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Monolayer and Bilayer WSe2 : Landau Level Degeneracy, Effective Mass, and Negative Compressibility. Phys. Rev. Lett. 2016, 116, 086601. 43. Pierucci, D.; Henck, H.; Naylor, C. H.; Sediri, H.; Lhuillier, E.; Balan, A.; Rault, J. E.; Dappe, Y. J.; Bertran, F.; Lefevre, P.; Johnson, A. T. C.; Ouerghi, A. Large Area Molybdenum Disulphide-Epitaxial Graphene Vertical Van der Waals Heterostructures. Sci. Rep. 2016, 6 . 44. Yankowitz, M.; Xue, J.; Cormode, D.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Jacquod, P.; LeRoy, B. J. Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2012, 8, 382. 45. Hunt, B.; Sanchez-Yamagishi, J. D.; Young, A. F.; Yankowitz, M.; LeRoy, B. J.; Watanabe, K.; Taniguchi, T.; Moon, P.; Koshino, M.; Jarillo-Herrero, P.; Ashoori, R. C. Massive Dirac Fermions and Hofstadter Butterfly in a Van der Waals Heterostructure. Science 2013, 340, 1427–1430. 46. Ohta, T.; Bostwick, A.; Seyller, T.; Horn, K.; Rotenberg, E. Controlling the Electronic Structure of Bilayer Graphene. Science 2006, 313, 951–954. 47. Pierucci, D.; Sediri, H.; Hajlaoui, M.; Velez-Fort, E.; Dappe, Y. J.; Silly, M. G.; Belkhou, R.; Shukla, A.; Sirotti, F.; Gogneau, N.; Ouerghi, A. Self-Organized MetalSemiconductor Epitaxial Graphene Layer on Off-Axis 4H-SiC(0001). Nano Res. 2015, 8, 1026–1037. 48. Flores, F.; Tejedor, C. Energy Barriers and Interface States at Heterojunctions. J. Phys. C 1979, 12, 731. 49. Sprinkle, M.; Siegel, D.; Hu, Y.; Hicks, J.; Tejeda, A.; Taleb-Ibrahimi, A.; Le Fèvre, P.; Bertran, F.; Vizzini, S.; Enriquez, H.; Chiang, S.; Soukiassian, P.; Berger, C.; de Heer, W. A.; Lanzara, A.; Conrad, E. H. First Direct Observation of a Nearly Ideal Graphene Band Structure. Phys. Rev. Lett. 2009, 103, 226803. 26


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50. Zhou, S. Y.; Gweon, G.-H.; Fedorov, A. V.; First, P. N.; de Heer, W. A.; Lee, D.H.; Guinea, F.; Neto, A. H. C.; Lanzara, A. Substrate-Induced Bandgap Opening in Epitaxial Graphene. Nat. Mater. 2007, 6, 770. 51. Chen, M.; Sun, W.; Guo, G.-C.; He, L. Substrate Induced Bandgap in Multilayer Epitaxial Graphene on the 4H-SiC (000¯1) Surface. Phys. Status Solidi B 2011, 248, 1690–1695. 52. Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. 53. Cantelli, V.; Geaymond, O.; Ulrich, O.; Zhou, T.; Blanc, N.; Renaud, G. The in situ Growth of Nanostructures on Surfaces (INS) Endstation of the ESRF BM32 Beamline: a Combined UHV-CVD and MBE Reactor for in situ X-Ray Scattering Investigations of Growing Nanoparticles and Semiconductor Nanowires. J. Synchrotron Radiat. 2015, 22, 688. 54. Schneider, C.; Wiemann, C.; Patt, M.; Feyer, V.; Plucinski, L.; Krug, I.; Escher, M.; Weber, N.; Merkel, M.; Renault, O.; Barrett, N. Expanding the View into Complex Material Systems: From Micro-ARPES to Nanoscale HAXPES. J. Electron Spectros. Relat. Phenomena 2012, 185, 330 – 339. 55. Harris, J. Simplified Method for Calculating the Energy of Weakly Interacting Fragments. Phys. Rev. B 1985, 31, 1770–1779. 56. Jel´ınek, P.; Wang, H.; Lewis, J. P.; Sankey, O. F.; Ortega, J. Multicenter Approach to the Exchange-Correlation Interactions in ab initio Tight-Binding Methods. Phys. Rev. B 2005, 71, 235101. 57. Foulkes, W. M. C.; Haydock, R. Tight-Binding Models and Density-Functional Theory. Phys. Rev. B 1989, 39, 12520–12536.

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58. Basanta, M.; Dappe, Y. J.; Jelinek, P.; Ortega, J. Optimized Atomic-Like Orbitals for First-Principles Tight-Binding Molecular Dynamics. Comput. Mater. Sci. 2007, 39, 759 – 766. 59. Di Felice, D.; Abad, E.; Gonzalez, C.; Smogunov, A.; Dappe, Y. J. Angle Dependence of the Local Electronic Properties of the Graphene/MoS2 Interface Determined by ab initio Calculations. J. Phys. D 2017, 50, 17LT02. 60. Dappe, Y. J.; Ortega, J.; Flores, F. Intermolecular Interaction in Density Functional Theory: Application to Carbon Nanotubes and Fullerenes. Phys. Rev. B 2009, 79, 165409. 61. Savini, G.; Dappe, Y. J.; Oberg, S.; Charlier, J.-C.; Katsnelson, M.; Fasolino, A. Bending Modes, Elastic Constants and Mechanical Stability of Graphitic Systems. Carbon 2011, 49, 62–69.

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Figure captions Figure 1. STM characterization of monolayer MoSe2 (1ML-MoSe2 ) flakes grown on a graphitized SiC(0001) surface. (a) 3D rendered view of a constant current STM image of several 1ML-MoSe2 flakes on bilayer graphene (2L-Gr) terraces. Image size: 200 x 200 nm2 . Sample bias: +2.2 V, tunneling current: 50 pA. (b) Similar STM image of a 1ML-MoSe2 flake on a monolayer graphene (1L-Gr) terrace. Image size: 60 x 60 nm2 . Sample bias and tunneling current: same as in (a). A small patch of two layers (2ML) MoSe2 shows up in the bottom part of the 1ML flake. (c) Profile z(x) measured along the line drawn in (b), which crosses both the 1ML and 2ML-MoSe2 regions. The sample bias is +2.2 V. The different measured step heights are sketched on the profile. (d) Upper part: Zoom-in on the rectangular box plotted in (b). The image is differentiated to enhance the different defects present within the TMD flake. The label TB indicates a twin boundary, the letters D indicate three atomic point-like defects (Se vacancies). Bottom part: constant current STM image with atomic resolution, corresponding to the dashed rectangle plotted in the upper image. (e) STS performed far from defect on another 1ML-MoSe2 flake lying on 1L-Gr terrace. The dI/dV(V) spectrum is displayed using a logarithmic vertical scale (see text). The energy positions of the valence band maximum (VBM) and of the conduction band minimum (CBM) are indicated by the two plain vertical lines. The Fermi energy position EF is also given (dashed vertical line). Stabilization parameters: sample bias +1.3V, tunneling current 0.4 nA. Figure 2. In-plane reciprocal space map (rsm) of the 3.5ML-thick MoSe2 sample measured by rocking the sample over 80◦ at increasing values of the in-plane SiC(0001) reciprocal lattice units h and k with increments of 0.01. The SiC unit cell, is hexagonal with 3.079 ˚ A and 10.053 ˚ A lengths, respectively in-plane and out-of-plane lattice parameters. The outof-plane ℓ value is close to zero; the intensity being integrated over △ℓ = 0.1. Note that a 3D measurement is actually performed thanks to the 5◦ long detector perpendicular to 29


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the surface, covering an ℓ-range between 0 and 0.75. The color scale is logarithmic, the highest (red) intensity being 103 photons/s and the background 1 photons/s (blue and white colors of respectively 1 and 0.1 photon/s). The SiC and graphene Bragg peaks are circled in red and green, respectively; the other, very wide peaks arising from MoSe2 . The SiC and graphene unit cells are highlighted by red and green lozenges, respectively, showing that the graphene lattice is rotated by 30◦ with respect to the SiC one. The hexagonal four index notation (hki ℓ) with i =-(h+k) is used. (b) Schematic of the reciprocal space map where contours of recorded spots are reported with different colors, depending on their origin (SiC (red), MoSe2 (black) and Graphene (green)), together with the corresponding reciprocal unit cells and primitive vectors. The SiC and graphene peaks are schematized by circles, while the MoSe2 peaks are schematically represented by ellipses with increasing widths in both radial and rocking directions as a function of distance, respectively due to inhomogeneous strain and dominating in-plane mosaic spread (corresponding to a constant angular width, as indicated by the two dashed line). The two radial directions of scans reported in Fig. 3a and 3b are indicated in black. ¯ direction (SiC) for the 1ML- (red) and Figure 3. (a) Radial scan along the in-plane (h0h0) 3.5 ML- (blue, multiplied by 10) MoSe2 -thick samples, crossing the following Bragg peaks, in order of increasing h: MoSe2 (11¯20), Gr(11¯20), MoSe2 (22¯40), Gr(22¯40). The small peak ¯ left on the SiC(10¯10) is not assigned. (b) Radial scan along the in-plane (hh 2h0) direction (SiC) for the 1ML- (red) and 3.5ML- (blue, multiplied by 10) MoSe2 -thick samples, crossing ¯ ¯ with the following Bragg peaks, in order of increasing h = k : MoSe2 (0H H0), Gr(0H H0) H = 1, 2 and 3. (c) Azimuthal rocking scans across the MoSe2 (01¯10) reflection, for 3.5 ML (red) and 1 ML (black). (d) Measured intensity along the (10¯1ℓ) (red) and (11¯2ℓ) (blue) rods of MoSe2 for the 1ML-thick sample together with simulated rods (green and black lines, respectively) for a perfectly 1ML-thick MoSe2 layer of H-type structure. The hexagonal four index notation (hki ℓ) with i =-(h+k) is used.

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Figure 4. (a) Band structure of MoSe2 /graphene heterojunction: cut along the K -Γ-K

′

direction of MoSe2 . Sets of energy constant cuts in (b): at E = EF , E = 1.3 eV (valence-band maximum at K point) and E = 1.4 eV, the onset of MoSe2 valence band maximum at Γ point. The arrows indicate the K -Γ direction of graphene (white) and MoSe2 (blue). (c) is ′

a cut along the K -Γ-K direction of bare graphene/SiC. Figure 5. Dispersion of graphene π bands as a function of the kx direction in the case of the MoSe2 /graphene heterojunction (a) and of the graphene/SiC substrate (b).(c) shows integrated intensity curves over binding energy from EF to EF + 1.4 eV, extracted from the area around kk = 0 (white rectangles in (a) and (b)). The dip corresponding to the bandgap is highlighted by arrows. Figure 6. Theoretical calculation results obtained from heterojunction and MoSe2 -free substrate with BL graphene: charge distribution in different layers in the heterojunction (a) and MoSe2 -free substrate (b). The amounts of transferred charge calculated for each atomic layer are also displayed (positive or negative signs indicating an excess or a deficit of electrons, respectively). Band structures of heterojunction and MoSe2 -free substrate are shown in (c) and (d), respectively, where the valence band (VB) and conduction band (CB) of graphene are highlighted in red and blue colors. Other bands are generated from the SiC substrate and MoSe2 .

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Figure 2

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Figure 4

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Figure 5

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Beyond van der Waals Interaction: The Case of MoSe2 Epitaxially

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