Unraveling the Structural and Electronic Properties at the WSe2

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Unravelling the Structural and Electronic Properties at the WSe-Graphene Interface for a Rational Design of Van der Waals Heterostructures Stefano Agnoli, Alberto Ambrosetti, Tevfik Onur Mente#, Alessandro Sala, Andrea Locatelli, Pier Luigi Silvestrelli, Mattia Cattelan, Sarah M. Eichfeld, Donna D. Deng, Joshua A. Robinson, José Avila, Chaoyu Chen, and Maria Carmen Asensio ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00315 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Nano Materials

Unravelling the Structural and Electronic Properties at the WSe2-Graphene Interface for a Rational Design of Van der Waals Heterostructures Stefano Agnoli, a* Alberto Ambrosetti,b Tevfik Onur Menteş,d Alessandro Sala, d Andrea Locatelli, d Pier Luigi Silvestrelli, b Mattia Cattelan,a,c† Sarah Eichfeld e Donna D. Deng,e Joshua. A. Robinson, e José Avila, f Chaoyu Chen, f Maria Carmen Asensio f

a

b

Department of Chemical Sciences, University of Padua Via F. Marzolo 1 35131, Padova, Italy Department of Physics and Astronomy, University of Padua, Via F. Marzolo 8, 35131, Padova, Italy c

d

School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK

Elettra - Sincrotrone Trieste, S.S. 14 km 163.5 in AREA Science Park, Basovizza, I-34149 Trieste, Italy

e

Department of Materials Science and Engineering and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, US

ACS Paragon Plus Environment

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Synchrotron SOLEIL, L’Orme des Mérisiers, Saint Aubin-BP 48, 91192 Gif sur Yvette Cedex, France

KEYWORDS Van der Waals heterostructures, chemical vapor deposition, transition metal dichalcogenides, graphene, low energy electron microscopy, angle resolved photoemission spectroscopy, DFT

ABSTRACT WSe2 thin films grown by chemical vapor deposition on graphene on SiC(0001) are investigated using photoelectron spectro-microscopy and electron diffraction. By tuning the growth conditions, micron-sized single or multilayer WSe2 crystalline islands preferentially aligned with the main crystallographic directions of the substrate are obtained. Our experiments suggest that the WSe2 islands nucleate from defective WSex seeds embedded in the support. We explore the electronic properties of prototypical Van der Waals heterostructures, by performing µ-angle resolved photoemission spectroscopy on WSe2 islands of varying thickness (mono-, and bi-layer) supported on single, bi- and tri-layer graphene. The experiments are substantiated by DFT calculations indicating that the interaction between WSe2 and graphene is weak and the electronic properties of the resulting heterostructures are unaffected by the thickness of the supporting graphene layer or by the crystallographic orientation. Yet, the WSe2-graphene distance and the WSe2/WSe2 interlayer separation strongly influence the electronic band alignment at the high symmetry points of the Brillouin zone. The values of technology relevant quantities such as splitting of spin polarized bands and effective mass of electrons at band


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valleys are extracted from experimental angle resolved spectra. These findings establish further strategies for tuning the morphology and electronic properties of artificially fabricated Van der Waals heterostructures that may be used in the fields of nanoelectronics and valleytronics.

Introduction The recent availability of thin films of transition metal dichalcogenides (TMDCs) has created an extremely rich and fertile ground for fundamental studies on 2D materials, which may lead to important technological developments.1,2 As a matter of fact, TMDCs offer a huge variety of electronic properties ranging from semiconducting to metallic behaviour, or even superconductivity, as well as intriguing optical properties such as strong photoluminescence and an intrinsically strong interaction with the electromagnetic radiation due to the presence of Van Hove singularities.1 Moreover, they exhibit dramatic changes in the electronic structure when the film thickness is reduced to the monolayer (ML).3 The availability of such a large number of different materials sharing the same 2D nature, but endowed with very diverse physicochemical properties, has quickly led to the idea of realizing artificially stacked solids, called van der Waals (vdW) heterostructures.4 These novel materials combine unique fundamental properties according to a design that aims at obtaining highly specialized multifunctional systems.5 This vision poses two great challenges, which have not been met yet. First, it demands a precise understanding of the subtle interactions that take place when different nanosheets are coupled together,1 and second, it requires the ability to synthesise and manipulate at the nanometer level different 2D materials. Solving these issues will permit to open new pathways for tuning electronic and photonic properties, thus fostering new concepts in the design of advanced multifunctional devices.4,5


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The study of the fundamental physics of vdW heterostructures is getting more and more momentum and is already providing an extremely rich phenomenology.6 It is now clear in fact, that electronic hybridization at the interface between 2D nanosystems can lead to profound modifications in the band structure,7 such as the formation of mini gaps,8 delay in the transition from direct to indirect band gap as a function of thickness,9 band bending and lift of band degeneracy,10 and profound modification of physical properties such as work functions and quasi particle band gap.11 The control of interfacial interactions therefore, offers a unique possibility to manipulate the fundamental properties of materials and to prepare nanoassemblies with novel properties. For example, it has been shown that the photoluminescence properties of assemblies of vertically staked TMDCs nanosheets can be precisely modulated by the insertion of h-BN layers, and the electronic properties of graphene are greatly improved when supported on hBN.12 Moreover, it has been reported that the electronic coupling at the interface between graphene and topological insulators or graphene and metal oxide interfaces may induce anomalous magnetotransport properties.13,14 Concerning the synthesis and manipulation of VdW heterostructures, quite interesting results have been obtained through manual stacking of different layers of micromechanically exfoliated nanosheets. On the contrary the development of bottom-up synthesis of such heterostructures is highly desirable and has emerged only recently.11,15,16,17,18,19,20,21,22,23 In this regard, chemical vapour deposition (CVD) represents a rather powerful method that, under optimized conditions, allows preparing clean and precisely oriented vdW heterostructures. Moreover, at variance with sequential nano-manipulation methods, CVD has the merit of being highly scalable, making it suitable for high yield technological applications.


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Recently, the CVD growth of TMDCs and in particular of WSe2 has received great attention.24,25 It has been demonstrated that SiC represents an optimal substrate for the growth of different types of graphene/TMDC vdW heterostructures. In the past, this material has been thoroughly studied for the growth of graphene layers, whose properties (thickness and morphology) were shown to be efficiently controlled by tuning the duration and temperature of the thermal treatment.26 In the present work, epitaxial graphene (EG) films obtained on SiC(0001) single crystals by thermal removal of Si atoms, were used as substrate for the growth of WSe2 thin films by CVD using metalorganic precursors. The so formed WSe2/EG heterostructures have been investigated by a combination of microscopic techniques enabling access to their structural and electronic properties. Low energy electron microscopy (LEEM), µ- low energy electron diffraction (µLEED) and x-ray photoemission electron microscopy (XPEEM) have been used to probe the structural and chemical properties of a series of EG/WSe2 vdW heterostructures with varying thickness of the single components as well as different crystallographic orientation between the layers. Then, the electronic band structures of selected heterostructures have been investigated in detail by spatially resolved angle resolved photoemission spectroscopy (µ-ARPES). The experiments were complemented by ab-initio calculations, determining important properties such type of band gap, band spin splitting, effective mass of charge carriers. This experimental approach based on spatially resolved electron spectroscopies is very innovative since similar investigations on single or few layer heterostructures have been generally carried out either locally by STM, without detailed information about the momentum resolved band structure,11,18 or by area averaging techniques, which may overlook the complexity connected to the local structure.17 The body of information acquired at the nanoscale has also allowed to understand the


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growth mechanism of WSe2 layer of EG providing a rationale for controlling and directing their growth, in particular we uncovered the special role of defective seeds on the heterogeneous nucleation of crystalline WSe2 islands. Moreover, we have identified the most important parameters that may be exploited to control the electronic coupling between vertically stacked heterostructures.

Results and discussion EG thin films have been grown on the (0001) plane of 6H-SiC single crystal by Si sublimation at 1725°C in ultrapure Ar flow at 200 Torr. Under this conditions, large flat terraces covered mainly by single and bilayer graphene are obtained. However, in some minority regions, step bunching can be observed with the consequent formation of multilayer graphene (up to 6 layers, see figure 1). WSe2 films have been deposited by metal organic CVD (MOCVD) using dimethylselenium ((CH3)2Se) and tungsten hexacarbonyl (W(CO)6) as precursors and Ar as gas carrier, at a temperature of 800°C. Details about the growth procedure can be found in a previous paper (see scheme 1).24


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Scheme 1. Synthesis scheme of the Van der Waal heterostructures: I) 6H-SiC(0001) substrate, II) growth of epitaxial graphene by sublimation of Si at 1750°C in inert atmosphere, III) deposition by CVD deposition of WSe2 at 800°C using W(CO)6 and dimethyl selenium. Figure 1a reports a bright field (BF) LEEM image showing the typical morphology of WSe2 thin films grown on EG. Triangular adislands can be seen on the surface. Their thickness generally corresponds to a monolayer (ML), (W1 or W2) or bilayer (BL) (W3), whereas three layer thick structures occur only occasionally (W4). The thickness has been determined by analyzing the energy-dependent intensity (I/V) profiles in low-energy electron reflectivity extracted from the BF LEEM images acquired at different electron energies (see supplementary movie movie_S1.mov reporting a full LEEM I/V data set from 0 to 35 eV). In addition, the thickness was independently determined from the XPEEM data by monitoring the attenuation of the C 1s and Si 2p core-level photoemission signal due to the presence of the WSe2 overlayer. Details of the thickness assignments are given in the supporting information (see Figure S1). The WSe2 film covers about 40% of the substrate, although the islands are distributed in a rather inhomogeneous fashion according to the thickness variations in the underlying graphene layer. Also in the case of graphene layers, the precise determination of the film thickness was obtained by inspecting the LEEM I/V profiles (see Figure S2 in the SI).27 WSe2 islands show a strong preference to nucleate on ML graphene (MLG), whereas on bilayer graphene (BLG) the island density is lower. From the statistical analysis of several LEEM images, it results that on a SiC substrate comprising about 50% of single layer graphene and 40% of bilayer graphene (the remaining surface is covered by multilayer graphene) almost 80% of the islands of WSe2 can be found on MLG. On the SiC surface some large stripes of multilayer graphene several microns


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wide can be observed (see top left region in Figure 1). Usually, on such regions, only few WeSe2 islands can be observed. The WSe2 islands exhibit very sharp edges oriented along the zig-zag direction ([-1010]), which are indicative of a well-ordered and crystalline structure. In general, their shapes are triangular, but sometimes more complicate polygonal structures are formed. As shown by darkfield (DF) LEEM images (figure S3 and the discussion in section S3 of the SI), perfectly triangular islands are single crystalline, whereas those with irregular shape, are polycrystalline, and are likely formed by the coalescence into a single unit of different growing nuclei. Interestingly, the majority of multilayer islands are polycrystalline with a central thicker core flanked by thinner grains with a different crystallographic orientation. (see W4 and W3 in figure 2a and the corresponding DF image in figure S3). We also note that most of the WSe2 islands show a small region or core at their center, which stems out in LEEM for exhibiting a different contrast, making it appear darker than its surrounding.


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The crystallinity of the films has been investigated also by µ-LEED. A sharp hexagonal pattern corresponding to the (0001) surface of 2H-WSe2 can be observed, which is overlapped

a)

b)

Figure 1. (a) Bright field LEEM image in false colors (sample bias voltage 5 eV) of the WSe2 islands on EG. The meaning of the labels is reported in the main text. (b) µ-LEED pattern on a cluster of WSe2 showing two domains rotated by about 20°. The red circles indicate the diffraction spots of graphene, whereas dark and light blue ones the two domains of WSe2. (Ekin=40 eV) with the pattern corresponding to graphene or to the (6√3×6√3)R30° surface reconstruction (see figure 1b). Within the experimental accuracy (±2%) no strain is detected. We observed that the WSe2 lattice is most often epitaxially aligned to the EG substrate. However, less frequently, other orientations were found, namely: ±5°(±2°), ±11°(±2°), ±25°(±2°). The variability in the epitaxial relationship between the substrate and the overlayer suggests the existence of a rather weak interaction between the two materials, as is expected for vdW solids and has already been reported for other TMDCs on EG.15,28 Previous cross sectional transmission electron microscopy measurements indicates that all interfaces in the WSe2/EG/SiC(0001) are defect free.29,30


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The chemical state of WSe2 has been addressed by microscopically probing the emission from the W 4f and Se 3d core levels using XPEEM. Spectra measured on the crystalline, flat part of the islands and on its nucleus are plotted in Figure 2a and 2b, respectively. On the flat part of the island, the W 4f7/2 core level emission is peaked at 32.5 eV binding energy (BE), the Se 3d5/2 at 55.5 eV. These values are perfectly compatible with stoichiometric WSe2 (see figure 2a and 2b).24,31 Two different types of samples have been analyzed: bare WeSe2 thin films transferred in air from the CVD growth chamber to the XPEEM/LEEM analysis chamber and samples that, after the deposition process, were in situ capped with a multilayer protective coating of selenium, which was removed directly in the analysis chamber by thermal annealing in UHV conditions. Both samples were left degassing at about 573 K for several hours at a pressure better than 10-8 mbar. According to photoemission spectra, no contamination by oxygen has been observed, not even on island edges, and the electronic fingerprints of W 4f and Se 3d core levels is the same for both types of samples. This indicates a very good resistance toward environmental contamination and oxidation for the systems WSe2/graphene. Laterally resolved W 4f and Se 3d spectra corresponding to the core region display a clear shift of 0.15 and 0.2 eV toward lower BE in the W 4f and Se 3d core levels, respectively. We suggest that it is the “seed” from which the island nucleated, which then becomes entrapped in the middle of the growing nucleus. A recent work using scanning transmission electron microscopy reported on fullerene-like structures that are 10-30 nm in diameter similarly located at the center of triangular islands of MoS2-MoSe2 alloys.32 These structures, formed by an oxide core wrapped by a TMDC layer, are thought to provide the anchoring point for the nucleation and growth of the TMDC islands. In the same work, the authors suggested that such a behavior must be


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common to all TMDCs prepared by CVD technique using oxide precursors (e.g. WO3, Na2WO4). Similar but larger seeds have been recently documented by XPEEM and optical microscopy also in CVD grown WS2.33 In the present case, the “seed mediated” nucleation is also observed, but the initial defect is not an oxide (no oxygen was detected on these areas by XPEEM spectra) and the W precursor is a carbonyl. The seeds were visible both on single or multiple WSe2 islands grown on single or multiple graphene layers. Notably no differences can be clearly detected on the signal of C 1s, however the spectroscopic evidence of the presence of isolated carbon defects is likely beyond the sensitivity of the technique. Local XPEEM spectra acquired on the seed region show that overall intensity and the intensity ratio between W 4f and Se 3d photoemission line is the same as in the surrounding islands, the only difference being a small shift toward lower BE, which have been previously attributed to an amorphous phase.28 The same low BE component is observed in the W4f photoemission spectra of WSe2 grown on epitaxial graphene on Ni(111) at room temperature (see figure S4 in SI), however it disappears after annealing at 500°C when the structure become fully ordered as demonstrated by the appearance of a LEED pattern. In the low temperature spectrum, the W 4f peak may be fitted by two components at 32.2 eV and 32.5 eV, whereas after annealing and consequent formation of an ordered layer, the peak becomes narrower and can be fit by a single component centered at 32.5 eV. Moreover, in the I/V profiles extracted by BF images, these morphological features remain constantly dark, indicating lack of crystallinity or a high number of defects, or presence of curvature.


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Figure 2. Spatially resolved Se 3d (a) and W 4f (c) spectra taken on a WSe2 ML islands (red) and on a “seed” (black). (b) BF LEEM image of a cluster of islands, each showing the dark central part, which might possibly be a seed to initiate the island growth.

Based on the detailed information provided by µ-LEED and LEEM about the thickness and orientation of both graphene and WSe2 we have undertaken a detailed experimental and theoretical investigation of the electronic structure of the different vdW heterostructures that can be formed. Figure 3a shows the µ-ARPES measurement on a single crystalline ML WSe2 island supported on ML EG along the Γ to K direction. Figure 3b displays the second derivative vs energy of these data for a better visualization of the bands. On this island, EG and WSe2 are perfectly aligned as demonstrated both by µ-LEED and µ-ARPES measurements, so the Γ-K direction is the same for both materials. The photoemission spectra show that the top of the valence band of WSe2 is characterized by an intense band that has a maximum at the Γ point 1.25±0.1 eV of binding energy and then on its way toward the K point separates into two sub-bands that reach a maximum in K, at a BE of 0.85±0.1 eV and 1.35±0.1 eV, respectively as determined by fit of the


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single valence band spectra (see figure 3b and 3c). Interestingly, we observe a rather large splitting of the two spin polarized bands at the K point, which results to be 0.50±0.10 eV, which is slightly larger than the value observed for the bulk material.34,35 The effective mass of electrons in the two spin polarized bands was determined to be 0.40±0.2 me and 0.70±0.2 me in the upper and lower band, respectively (see figure S5), which are suitable values for the development of spintronics devices. At higher k// values (see figure 3a) it is possible to observe the π band of graphene showing the typical linear dispersion of the Dirac cones of ML films. A small doping of 0.1 eV is observed on graphene (Figure 3d) even if it is generally reported that on SiC(0001) the Dirac energy should be found at a BE of 0.4 eV,36 on the other hand the WSe2 is slightly n-doped. These two facts seem to suggest the existence of a modest charge transfer from EG to WSe2.

Figure 3. µ-ARPES measurement on a single crystalline ML WSe2 island supported on single layer EG taken with 26 eV photons. a): energy dispersion curve along the Γ-K direction (orange line in e)) of the single layer WSe2 supported on ML EG. b), c) valence band spectra at the K and


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Γ points, respectively, reporting the multi-peak analysis for the identification of the band center;d) section of the Dirac cone taken along the direction perpendicular to Γ-K direction (i.e. white line in e)). e) section of the band structure in the k// plane at a BE of 1.7 eV.

We also measured the electronic structure of a rotated heterostructure where the WSe2 lattice is rotated by 11.5 ±0.5° with respect to EG, as determined by azimuthal alignment of the graphene and WSe2 Brillouin zones observed in the µ-ARPES spectra. This low angle rotation does not allow a strong interaction between the two materials because of the lack a simple commensurate superstructure. Nonetheless, within the experimental uncertainty, the resulting band structure of WSe2 is essentially identical to that of the aligned vdW heterostructure (see figure S6). We can observe only a small increase in the doping level of about 0.1 eV with respect to the aligned case, i.e. the bands of WSe2 are rigidly shifted downward. Similarly, ML WSe2 islands have been identified on bilayer and trilayer graphene. Also in this case the band dispersion observed is essentially the same (i.e. no experimentally appreciable changes in the E vs k dispersion) as for the ML graphene case (see figure S7 reporting ML WSe2 on BL graphene). Both these experimental findings indicate that the interaction between graphene and WSe2 is very scarce, even when the two lattices are perfectly aligned. As a matter of fact, the two materials potentially present some electronic states that overlap with each other in the energy/momentum space, but no significant effects are observed, differently from other systems such as monolayer graphene supported on MoS2 single crystals that exhibit distinctive mini gaps,8 or twisted graphene layers that on crossing points develop Van Hove singularities.37 When bilayer WSe2 is formed on EG, regardless of the number of the layers of the support, we can observe the expected change in the band structure that is typical of many TMDC.35,38 The K


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and the Γ point are almost at the same energy, that is 1.30±0.1 eV, below the Fermi energy. Moreover, at the Γ point a new band is observed, which is located at a BE of 1.95 eV (see Figure 4a-c). In this case therefore, WSe2 results to be n-doped probably because of the presence of Se vacancies, whereas graphene exhibits the typical 0. 4 eV n-doping (figure S8) that is typical of EG/SiC(0001).36 A more detailed characterization of the electronic structure of the 2 ML films can be obtained using averaged ARPES on a sample characterized by a homogeneous deposition as reported in figure 4d-f. Here the higher resolution allows refining the energy resolution of the microscopic data: the upper band at the K point is only located 0.01 eV below the upper band at the Γ point, and at K the spin orbit splitting between the two bands is 0.52 eV, (see figure 4c and 4d).


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Figure 4. µ-ARPES measurement on a single crystalline 2 ML WSe2 island supported on single layer EG taken (a): energy dispersion curve along the Γ-K direction (Ephoton=32 eV). b), c) valence band spectra at the K and Γ points, respectively, reporting the multi-peak analysis for the identification of the band center; d) area averaged ARPES data along the Γ-K direction (Ephoton=58 eV), e) second derivative vs energy of image d). f) valence band spectra at the K and Γ points, respectively, reporting the multi-peak analysis for the identification of the band center

We have also calculated the apparent electron mass for this systems that resulted to be around the K point of 0.40±0.05 me and 0.61±0.05 me, (see figure S5). It has also to be mentioned that in bulk WSe2 the maximum of the valence band has been recently located at the Γ point, which however is only 0.05 eV higher than the local maximum at the K point.3 Recently, it was suggested by spatially averaged ARPES measurements on single crystals SiC(0001) that the formation of a well aligned vdW heterostructure between WSe2 and EG is responsible for the delay of the transition between monolayer to bulk properties.9 On the other hand, MoS2 thin films grown on EG/SiC, which is a system very similar to the present one, do


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not exhibit any delay in the transition from direct to indirect band gap on passing from the ML to the BL.20 In order to shed light on this phenomenon we have carried out dispersion-corrected39 ab-initio density functional theory (DFT) calculations based on the Perdew Burke Ernzerhov functional40 (see Supporting Information for details). Single layer Graphene has been used to support single and double layer WSe2. For the feasibility of the calculations we adopted a hexagonal supercell matching four unit cells (4×4) of graphene with three (3×3) cells of WSe2, with a lattice constant of 9.90 Å, obtained upon total energy minimization. However, the above mentioned data on rotated ML WSe2, suggest that the specific epitaxial relationship between the layers does not affect appreciably the electronic properties of the vdW heterostructure. Spin-orbit coupling (SOC) effects were explicitly included, given the presence of heavy elements (W and Se). In figure 5 we report the band structure obtained for the different vdW heterostructures. Going from WSe2 monolayer to bilayer we observe the expected direct-to-indirect band-gap transition both in the presence and absence of the Graphene substrate. We may also notice that not significant renormalization of the band gap of WSe2 is observed as found on other systems.41 Notably, while DFT calculations for WSe2 bilayer performed in the absence of SOC predict the valence band maximum to be clearly situated at the unit cell Γ point42 (see Fig. S9 in Supplementary Material), SOC effects tend to make the valence band maxima at K and Γ almost isoenergetic. In fact, according to our calculations, at equilibrium geometry the two valence electronic levels differ by only ~0.03 eV. Although photoemission spectroscopy data suggest the actual valence band maximum be locate at Γ, the two levels experimentally differ by only ~0.01 eV so that the observed K-Γ alignment is well reproduced by our theoretical data. Moreover, DFT calculations provide clear suggestion that valence K-Γ alignment can vary depending on key geometrical parameters, such as interlayer distances, and in-plane lattice constant. In


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particular, an increase of ~0.15 Å in the WSe2 - WSe2 interlayer separation (6.31 Å at equilibrium) is sufficient to shift the valence band maximum to the unit cell K point. As from Fig. 5, we also observe that the graphene substrate tends to cause a slight K-Γ alignment, reducing the energy difference by ~0.008 eV. Reduction of the graphene– WSe2 interlayer distance can further enhance this effect. Finally, a ~1% increase in the in-plane supercell lattice constant also results in a 0.009 eV reduction of the K-Γ levels splitting. Only minor modifications of the band structure are found when considering graphene bilayer as a support (see Supplementary Information). By an energetic analysis, we estimate that the binding energy of single layer graphene to WSe2 bilayer amounts to 88 meV per C atom (or, equivalently, 156 meV per WSe2 unit), thus falling in the typical range for small molecule adsorption on graphene/graphite.43 A larger binding energy is predicted instead for WSe2 monolayer adhesion to graphene-supported WSe2 , namely 544 meV per (monolayer) WSe2 unit. Finally, a small charge transfer from graphene to WSe2 (0.06 and 0.09 electrons per unit cell for WSe2 monolayer and bilayer, respectively) is found by integrating the charge density difference between the interacting graphene-WSe2 complex and the two isolated layers, up to the plane located at half interlayer distance. While our calculations confirm a limited effect of graphene in altering the overall WSe2 band structure at the equilibrium geometry, they suggest viable pathways for the fine tuning of valence electronic levels, and support the experimental observation of small charge transfer between the two materials.


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Figure 5.: DFT band structure of WSe2 monolayer in the unit cell (a), in the adopted (3×3) supercell (b), and in the presence of a graphene support (c). Analogous band structures are given also for WSe2 bilayer (d-f). Green (valence band maximum), yellow (valence band local maxima) and blue (conduction band minimum) spots help identifying the band folding in the (3×3) supercell: the K point of the unit cell Brillouin zone is shifted to Γ in the (3×3) supercell.


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Red arrows indicate the nature of the HOMO-LUMO gap, dotted horizontal lines at 0 eV identify the Fermi level. SOC effects are explicitly included in our calculations (see text).

Conclusions Different vdW heterostructures between EG and WSe2 have been prepared by a CVD route obtaining a full gamut of different structural configurations. The WSe2 islands prefer to align with the main crystallographic orientation of EG, but also different arrangements can be observed, though less frequently. Most WSe2 islands grow on monolayer graphene through a mechanism possibly involving the nucleation around “seeds” that are likely connected to defects coming from the SiC substrates. This suggests that intentionally created defects can be used to pattern with high spatial precision the growth of WSe2 thin films.44 A systematic investigation by µ-ARPES of the band structure of a wide set of heterostructures indicates that the WSe2 islands present a thickness dependent electronic structure, whereas the effects induced by the type of underlying graphene support, either the orientation or the thickness are minimal (less than 0.1 eV). ML layer WSe2 exhibits the maximum of the valence band at the K point whereas for BL WSe2 the band maximum of the valence band at the Γ and K points are essentially isoenergetic. State of the art DFT calculations provide complementary insight to our experimental measurements, demonstrating that the presence of ML or BL graphene alters slightly the band dispersion of WSe2. On the other hand, the interfacial distance between the two materials and the precise value of the lattice constants of WSe2 are critical parameters for controlling the band line up at the K and Γ points. Overall, our results suggest that by means of CVD, very controlled VdW heterostructures can be prepared on a large scale, and EG in particular is an ideal substrate because of its limited interaction with WSe2.


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Moreover, the modification of parameters such as the in-plane lattice parameter or the interlayer distance by chemical functionalization, gas intercalation or mechanical strain, could be used to modify the electronic properties of the heterostructures and therefore their functional properties.45

MATERIALS AND METHODS Heterostructure synthesis Epitaxial graphene was grown on 6H-SiC (0001) single crystals by silicon sublimation and carbon segregation by keeping the substrate for 15 min at 1700°C in 1 Torr of Ar pressure. Tungsten diselenide thin films were prepared on epitaxial graphene by using tungsten hexacarbonyl (Sigma-Aldrich 99.99%) as metal precursor and dimethylselenium (SAFC (99.99%) or STREM Chemical (99%)) as chalcogenide source by using a vertical cold wall reactor with an induction heated susceptor as described in ref 24. Two different transport lines ensured the possibility to supply the two precursors independently. The carrier gas was a mixture of N2 and H2. The substrates were heated to 500°C at 80°C/min and annealed for 15 min to eliminate any trace of water, then the growth temperature of 800°C was reached at a rate of 80°C/min, and the precursors were introduced into the reaction chamber. To deposit the Se capping layer, after the growth of the WSe2 films, the tungsten hexacarbonyl supply was interrupted providing only dimethylselenium. XPEEM, LEEM and ARPES measurements The photoemission microscopy measurements were acquired with the spectroscopic photoemission and low energy electron microscope (SPELEEM) at the Nanospectroscopy beamline, in Elettra Synchrotron Light Laboratory.46 LEEM was used in both bright- and dark-


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field modes, respectively, utilizing the first or secondary order diffraction beam for imaging. The microscope lateral resolution in LEEM mode approaches a few tens of nanometers; whereas in XPEEM mode the spatial and energy are respectively, 30 nm and 0.3 eV. µ-ARPES spectra were acquired from area of about 2 µm in diameter, by inserting a field limiting aperture in the optical path of the instrument using a photon energy of a 26 or 32 eV. Area averaged ARPES spectra were acquired at the ANTARES beamline in SOLEIL synchrotron with an energy of 57 eV. The angular and energy resolution of the beamline are ~0.2 Å and ~10 meV, respectively, in the used conditions. All ARPES spectra were collected at room temperature.

DTF calculations Ab-initio DFT calculations are based on the PBE generalized-gradient approximation to the exchange-correlation functional including the DFT-D2 correction proposed by Grimme39 for dispersion forces. All calculations were carried out by Quantum Espresso code,47 based on ultrasoft pseudopotentials and periodically repeated supercells. Wave functions were expanded in plane waves imposing an energy cutoff of 35 Ry. An empty space of 15 Å along the z direction was adopted to minimize the interaction with periodic replicas. The Brillouin zone of the (3×3) WSe2 hexagonal supercell corresponding to a (4×4) graphene hexagonal supercell (containing 9 WSe2 units and 32 C atoms, respectively) was sampled through a uniform mesh of 12×12 k-points. Geometry optimization via a quasi-Newton algorithm as implemented in Quantum Espresso was performed prior to band calculations.


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ASSOCIATED CONTENT Supporting Information. Additional data regarding the electronic structure of different VdW heterostructures and I/V LEEM profiles of WSe2 and graphene thin films with different thickness, and additional DFT calculations, the fit of the E vs moment curve for the determination of the electron apparent mass and a movie reporting LEEM i/V data are provided. AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses †

School of Chemistry University of Bristol Cantock's Close Bristol BS8 1TS United Kingdom

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript ACKNOWLEDGMENT This work was partially supported by the Italian MIUR through the national grant Futuro in Ricerca 2012 RBFR128BEC “Beyond graphene: tailored C-layers for novel catalytic materials and green chemistry”. The authors wish to acknowledge the award of beamtime on the Antares beamline at Synchrotron SOLEIL under proposal number 2015023.


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J.A.R., D.D., and S.M.E. are supported by the Center for Low Energy Systems Technology

(LEAST).

LEAST

is

one

of

six

Semiconductor

Research

STARnet

centers sponsored by MARCO and DARPA.

ABBREVIATIONS EG, epitaxial graphene; VdW, Van der Waals; ML, monolayer; BL, bilayer; TL trilayer;

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Unraveling the Structural and Electronic Properties at the WSe2

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