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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,∥ Toai Le Quang,∥ Jean-Yves Veuillen,∥ Loïc Huder,⊥ Vincent T. Renard,⊥ Claude Chapelier,⊥ Giovanni Zamborlini,# Matteo Jugovac,# Vitaliy Feyer,# Yannick J. Dappe,¶ Pascal Pochet,§ and Matthieu Jamet*,† †
Université Grenoble Alpes, CEA, CNRS, Grenoble INP, INAC-SPINTEC, 38000 Grenoble, France Université Grenoble Alpes, CEA, LETI, Minatec Campus, F-38054 Grenoble, France ¶ SPEC, CEA, CNRS, Université Paris Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France § Université Grenoble Alpes, CEA, INAC, MEM, 38000 Grenoble, France ∥ Université Grenoble Alpes, CNRS, Institut Néel, F-38000 Grenoble, France ⊥ Université Grenoble Alpes, CEA, INAC, PHELIQS, 38000 Grenoble, France # Peter Grünberg Institute (PGI-6), Forschungszentrum Jülich GmbH, D-52425, Jülich, Germany ‡
S Supporting Information *
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 twodimensional 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 band-gap opening in the few-layer graphene. The band gap is as large as 250 meV, and we ascribed it to an interface charge transfer that 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 band-gap 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 band gap in few-layer graphene by interface charge transfer. KEYWORDS: van der Waals interaction, band-gap opening, heterojunction, few-layer graphene, MoSe2, commensurability, charge transfer
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approach that offers a fertile platform to study fascinating vdW force-driven properties: commensurate lattice coincidence, electronic structure and band structure alignment, and spin− orbit coupling or exchange coupling induced by proximity effect and inversion symmetry breaking. For application perspectives,
raphene 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 functionalities of 2D materials, three-dimensional entities, conceptualized from 2D Lego 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 © 2018 American Chemical Society
Received: October 20, 2017 Accepted: January 31, 2018 Published: January 31, 2018 2319
DOI: 10.1021/acsnano.7b07446 ACS Nano 2018, 12, 2319−2331
Article
Cite This: ACS Nano 2018, 12, 2319−2331
Article
ACS Nano
Figure 1. STM characterization of monolayer MoSe2 (1 ML-MoSe2) flakes grown on a graphitized SiC(0001) surface. (a) 3D rendered view of a constant current STM image of several 1 ML-MoSe2 flakes on bilayer graphene (2 L-Gr) terraces. Image size: 200 × 200 nm2. Sample bias: +2.2 V, tunneling current: 50 pA. (b) Similar STM image of a 1 ML-MoSe2 flake on a monolayer graphene (1 L-Gr) terrace. Image size: 60 × 60 nm2. Sample bias and tunneling current: same as in (a). A small patch of two-layer (2 ML) MoSe2 shows up in the bottom part of the 1 ML flake. (c) Profile z(x) measured along the line drawn in (b), which crosses both the 1 and 2 ML-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 letter D indicates 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 1 ML-MoSe2 flake lying on a 1 L-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 +1.3 V, tunneling current 0.4 nA.
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, a moiré pattern 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 MoSe221 on a graphene/SiC substrate. We found two important characteristics of the heterojunction: a commensurable growth of MoSe2/few-layer graphene and a band-gap opening in the fewlayer 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
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 composed of graphene and other 2D materials such as boron nitride and 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 an in situ fabrication process of both graphene and TMDs using high-vacuum chemical vapor deposition (CVD) and ultra-highvacuum molecular beam epitaxy (UHV-MBE) has also been reported.5,16 Such a fabrication of heterojunctions ensures 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, and the interplay between their structure and electronic bands. For instance, it has been found experimentally that there 2320
DOI: 10.1021/acsnano.7b07446 ACS Nano 2018, 12, 2319−2331
Article
ACS Nano
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, an 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 band gap 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 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 set points) 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 rises above the maximum noise level within the gap. Consistent values were obtained for spectra with different set points. We therefore deduce the value of the electronic band gap: 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 band gap. 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 remain almost unchanged, while the band gap becomes smaller when approaching the edges. This feature is presumably due to the change of the electronic states induced by a dangling bond in the MoSe2 layer at the edges. To briefly summarize, the
in the MoSe2 layer are imaged with STM. STS measurements show a band gap 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. This finding suggests one configuration of epitaxial registry between MBEgrown MoSe2 and graphene. We observed, however, a broadening of all in-plane MoSe2 peaks as measured by rocking scan well fitted by Gaussians with a 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 highsymmetry 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 either in STM images or in the X-ray radial scans. Thus, we cannot state that a moiré pattern is at 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 band-gap 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- and two-monolayer-thick-graphene employed for this study was elaborated by thermal decomposition in ultrahigh vacuum.22 A low MoSe2 nominal coverage (
