Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy Kunttal Keyshar,† Morgann Berg,‡ Xiang Zhang,† Robert Vajtai,† Gautam Gupta,§ Calvin K. Chan,‡ Thomas E. Beechem,‡ Pulickel M. Ajayan,† Aditya D. Mohite,*,§ and Taisuke Ohta*,‡ †
Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77005, United States Sandia National Laboratories, Albuquerque, New Mexico 87185, United States § Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ‡
S Supporting Information *
ABSTRACT: The values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron−hole separation in photovoltaics. KEYWORDS: transition metal dichalcogenide, molybdenum disulfide, tungsten disulfide, molybdenum selenide, band alignment, ionization energy, photoemission electron microscopy
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Owing to this general importance, ionization energies of atomically thin TMDs have been systematically investigated using density functional theory (DFT).9−14 A fundamental question is how the absolute band edge positions and the ionization energies in TMDs vary for the isoelectronic metal cations (e.g., Mo, W) and chalcogen anions (S, Se, Te). These DFT studies have suggested that the ionization energies of TMDs decrease for the constituent elements with smaller electronegativities. Thus, a simple argument based on the electronegativities could explain the general trend of ionization energies in TMDs. This hypothesis, however, has not yet been experimentally verified. In particular, it remains unclear whether isolated TMD monolayers would display their intrinsic (or native) behaviors while not impacted by the underlying substrate or surface dipole. The scarcity of experimental reports15−17 stems from the difficulties in characterizing TMDs using photoemission spectroscopy, which is typically used to determine ionization
emiconductor homo- and heterojunctions have been key elements in high-performance electronic and optoelectronic devices. In addition to a tunable band gap and carrier control via valley degrees of freedom (termed valleytronics), atomic layers of transition metal dichalcogenides (TMDs) with various chemical compositions have been regarded as alternatives to build semiconductor heterojunctions. Due to the lack of chemical bonding between the layers, heterostructures using TMDs obviate the mismatch of lattice parameters and chemical compatibilities, commonly restricting the heteroepitaxy of conventional semiconductors.1−4 Knowledge of the positions of the valence band maximum and conduction band minimum energies with respect to the vacuum level is essential to designing semiconductor heterojunctions. These absolute band edge positions dictate how the band gaps of semiconductors are aligned and determine charge transfer when one semiconductor is brought into contact with another semiconductor or metal.5 The ionization energy, defined as the energy separation between the vacuum level and the highest occupied state, is of particular importance for predicting the type of band alignment (i.e., straddling gap, staggered gap, or broken gap) of semiconductor heterojunctions involving TMDs.6−8 © 2017 American Chemical Society
Received: May 10, 2017 Accepted: July 14, 2017 Published: July 19, 2017 8223
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Figure 1. PEEM images of MoSe2, WS2, and MoS2 acquired at 0.1 eV below Evac of the 1 ML area. The photon energy used is 6.53 eV (λ = 190 nm). Higher photoemission intensity is shown as darker gray.
Figure 2. False-color images of photoemission spectra of MoSe2, WS2, and MoS2 acquired using selected photon wavelengths along the dashed lines in Figure 1. The black/white grayscale is linear to the photoemission intensity, with higher intensity shown as darker gray. Blue and red dots illustrate the fitted positions of the vacuum level and the highest occupied states near Γ-point (BZ center). The photon wavelength and the TMD thicknesses are labeled in each image. The origin of the energy scale is referenced to the vacuum level of one monolayer TMDs.
energies and band offsets.18−20 Specimens of TMDs are typically micron-size flakes supported on SiO2 films (typically ∼100 or ∼300 nm thick) on silicon wafers. This sample configuration allows the flakes to be located and identified under optical microscopes21 but requires a microspectroscopic approach. Additionally, this arrangement is subject to sample charging during photoemission measurements because of the insulating nature of SiO2. Other methods based on scanning probe microscopy also suffer from the need of conduction paths to samples and from environmental effects, such as water absorption and aging, when the measurement is conducted in air.22,23 Consequently, DFT-calculated absolute band edge positions and ionization energies have yet to be compared to experimentally determined values. We present a systematic determination of the ionization energies of few-layer MoSe2, WS2, and MoS2 using the imaging spectroscopy capabilities of photoemission electron microscopy
(PEEM) with deep ultraviolet (DUV) illumination. Using this method, we found that we can spatially map the ionization energy determined from measurements of the energy positions of the vacuum level cutoff and the highest occupied states near the Brillouin zone center. This measurement is enabled by the absence of gas absorption in ultrahigh vacuum and minimal electronic interaction of the TMD layers with the underlying SiO2 support. The band alignments presented here represent the electronic properties of as-grown atomically thin TMDs. The measured ionization energies decrease from MoS2, to WS2, to MoSe2 in close agreement with the DFT calculations based on the GW approximation. Given this quantitative agreement, the extended list of ionization energies for TMDs and 2D crystals would facilitate designing and fabricating atomically thin semiconductor heterostructures with desired properties. The paper is organized as follows. The next section describes the measurements of the ionization energies of three TMDs 8224
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Figure 3. Absolute band edge diagrams of (a) MoSe2, (b) WS2, and (c) MoS2 as a function of their thicknesses, as constructed based on fits to the photoemission spectra shown in Figure 2. The hashed boxes represent the conduction bands, and the solid boxes represent the valence bands. The blue dash-dot line in (c) indicates the Fermi level of MoS2 reported in ref 37. The highest occupied state at K-point for trilayer WS2 is not specified due to the lack of a reported value. The origin of the energy scale is referenced to the vacuum level of one monolayer TMDs.
in common is the higher intensities of the bare SiO2 area relative to the TMD flakes at longer photon wavelengths (i.e., lower photon energy). The relative intensity of the bare SiO2 maximizes at 260−280 nm (hν = 4.43−4.77 eV, data not shown) consistent with previous reports on the low photon energy photoemission of glassy SiO2 and quartz implanted with metals ions.28,29 Fitting the leading (lower relative electron kinetic energy side) and trailing (higher relative electron kinetic energy side) edges of the photoemission spectra yields variations of the vacuum level (Evac)30 and the highest occupied states near Γpoint (near the BZ center because of the low photon energy used for the measurements).31 The energies of the fitted edges are indicated in Figure 2 by red dots for Evac and by blue dots for the highest occupied states. The photoemission data acquired using photons with λ = 185 and 190 nm show the clearest contrast of Evac and the highest occupied states. Most likely, the suppressed photoemission intensity from the underlying SiO2 at lower photon wavelength helps us to extract the Evac and the highest occupied states in the fitting process. For MoS2, both the energy positions of Evac (red dots) and the highest occupied states (blue dots) shift toward higher electron kinetic energy as a function of the layer thickness. A similar trend is seen for MoSe2, except that Evac and the highest occupied states of its bilayers (2 ML) are slightly lower than those of 1 ML. For WS2, the highest occupied states near Γpoint (blue dots) move closer to Evac with additional layers, similar to the cases of MoS2 and MoSe2. In contrast, Evac (red dots) lowers in energy for thicker regions. We presume that the surface dipole originated from the interface dipoles formed beneath flakes, and this is one main cause of the Evac variations for different thicknesses of TMDs. Surface vacancies and oxygen absorption,32 especially at the outer layer of TMDs, could present additional surface dipoles. We note that TMD flakes with thicknesses that exhibit a lower Evac tend to be located at the edges of flakes. For example, 2 and 3 ML WS2 display Evac lower than that of 1 ML and have a tendency to cover the edges of flakes preferentially, as shown
(MoSe2, WS2, and MoS2) and their comparisons to DFT predictions. Subsequently, we present the spatial variations of electronic properties at the edges of MoSe2 and WS2 flakes that presumably evolve with aging.
RESULTS AND DISCUSSION Measurement of Ionization Energies. We measured the ionization energy of three TMDs: MoSe2, WS2, and MoS2, grown on SiO2-covered Si wafers using chemical vapor deposition (CVD) method (see the Methods section for detail). Typical PEEM images of the TMD flakes are shown in Figure 1. The images were acquired at an electron kinetic energy 0.1 eV below the vacuum level (Evac) of the 1 ML area. Each flake consists of regions with varying thicknesses, which appear as different image intensities. Higher photoemission intensity is shown as darker gray. The layer thicknesses were verified using Raman spectroscopy, photoluminescence mapping, or optical contrast and are labeled in each image. The crystallographic alignment between layers can be inferred from the relative orientations of the edges of triangular flakes.24 Most thin flakes display 2H or 3R stacking with random stacking orientation for thicker parts in the middle of the flakes (see, for example, a small flake in the left side of Figure 1a).25 The PEEM intensity does not change monotonically as a function of local thickness. The 1 ML regions display the strongest photoemission intensity among different thicknesses for MoSe2 and MoS2. Meanwhile, 1 ML shows the least photoemission intensity for WS2. In general, photoemission intensity increases with the quantity of the probed material, that is, thickness in this case. We postulate that these nonmonotonic variations in photoemission intensity arise due to abrupt alterations of the photoemission cross section with thickness that occur at the DUV wavelength of the experiment.26,27 Figure 2 shows representations of the series of photoemission spectra taken along the dashed lines in Figure 1 acquired using photons with various wavelengths. For all three TMDs, photoemission intensity varies significantly as a function of photon energy. One feature that Figure 2a−c has 8225
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ACS Nano in Figure 1b. In contrast, most edges of the MoS2 flakes are terminated by 1 ML. Although it is tempting to draw a simple conclusion, numerous studies suggest that the growth of TMDs via CVD or vapor transport processes is highly anisotropic and far from thermodynamic equilibrium. It is well-known that the relative positions of precursor materials and the growth substrate in the furnace could play important roles in determining the resulting growth morphologies (see, for example, Figure S2 for multilayer morphology of WS2 different from that shown in Figure 1b but found on the same sample).33 Establishing an explicit correlation between the growth morphology and the work function variation requires further understanding of the kinetic processes during the growth. We constructed the absolute band diagrams of MoSe2, WS2, and MoS2 based on the average of the energy positions of Evac and the highest occupied states near the Γ-point using the photoemission spectra of λ = 185 and 190 nm (from Figure 2). First, as shown in Figure S1, the highest occupied states (Γpoint) are shifted by the excitation photon energy to represent their initial states with respect to Evac. Based on these diagrams, the energy positions of Evac and the highest occupied states (Γpoint) were extracted, as illustrated in Figure 3 and listed in Table 1. The energy scale is referenced to Evac of 1 ML TMDs.
apparent in Figure 3 that the valence band maximum and the conduction band minimum shift toward the Evac from MoS2, to WS2, to MoSe2. In other words, ionization energy (i.e., the energy separations between the vacuum level and the valence band maximum) decrease monotonically from MoS2, to WS2, to MoSe2, as anticipated from the electronegativities of the constituent atoms. To make this point clearer, the experimentally determined absolute band edge energies of 1 ML MoSe2, WS2, and MoS2 are compared to those predicted using the DFT calculations based on the GW approximation in Figure 4 and Table 1.12 The experimental results are in
Table 1. Ionization Energies of MoSe2, WS2, and MoS2 in Electronvolts, Determined from the Photoemission Spectra (Figure 2) and Based on Reported Values from Density Functional Theory Calculations12 1 ML MoS2 WS2 MoSe2
2 ML
3 ML
bulk
PEEM
theory
PEEM
PEEM
theory
5.77 5.74 5.34
5.86 5.50 5.23
5.66 5.42 5.29
5.61 5.34 5.30
5.44 5.26 5.08
Figure 4. Absolute band diagrams of 1 ML MoSe2, WS2, and MoS2. The experimentally determined diagrams (blue boxes) are compared to predicted values from density functional theory calculations (black boxes).12 As in Figure 3, the hashed boxes represent the conduction bands, and the solid boxes represent the valence bands. The red dash-dot and dashed lines are the redox potentials for the reduction (H+/H2) and oxidation (H2O/O2) of water, respectively.
The details of the approach used to analyze the data shown in Figure S1 are described in ref 31. As in ref 31, the predicted energy difference of the valence band edges at the Γ-point versus the K-point (ΔEΓ‑K) and the reported optical band gap were used to evaluate the positions of the highest occupied states at the K-point and the conduction band minimum, respectively.34−36 Additionally for MoS2, we assumed the work function for 1 ML to be 4.52 eV, which was determined from photoemission spectroscopy on large-area 1 ML MoS2 films assembled onto silicon from chemically exfoliated flakes.37,38 The expected position of the Fermi level is shown as a blue dash-dot line in Figure 3c, indicating that the MoS2 flakes are ntype.31 Figure 3 shows the general trend that both the valence band maximum and the conduction band minimum shift away from the vacuum level (Evac) as the thickness of the TMDs increases. This trend is consistent with what is predicted for 1 ML versus bulk absolute band positions of TMDs.9−13 Also, the types of the band alignment at the lateral (in-plane) junctions of the same TMD with different thicknesses can be evaluated from the absolute band diagrams. WS2 and MoS2 display type-I junctions between 1, 2, and 3 ML, as shown in Figure 3b,c. In contrast, homojunctions made of MoSe2 are likely to exhibit type-II staggered band alignment. Experimentally Determined Ionization Energies in Comparison to Theoretical Predictions. We first compare the influence of isoelectronic metals (Mo and W) and the chalcogen family (S and Se) on their band positions. It is
excellent agreement with the DFT-predicted values. The ionization energies of 2 and 3 ML TMDs determined experimentally also follow the expected decrease from MoS2, to WS2, to MoSe2 as tabulated in Table 1. This general trend of ionization energies to decrease from MoS2, to WS2, to MoSe2 is further confirmed by measuring the photoemission threshold, where the photoemission intensity is recorded as a function of the photon wavelength. The bidirectional red arrows in Figure S1 (determined along the dashed lines in Figure 1) highlight the photoemission thresholds. This method of measuring the photoemission threshold represents an alternative way to determine the ionization energy39 and is shown to work well for multilayer MoS2.31 Overall, the experimentally determined ionization energies of TMDs follow the systematic trend predicted by DFT calculations reported in refs 11−13. How the valence band maximum and conduction band minimum align between different TMDs is one of the most important criteria for designing TMD-based heterostructures. Here, we comment on the vertical heterojunction (or heterobilayer) comprising any of the three TMD 1 MLs based on information from Figure 4 and Table 1. Simplifying the discussion by assuming that the interlayer coupling due to the van der Waals interaction is not significant, Figure 4 suggests that vertical heterojunctions comprising any of the 1 8226
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oxidized TMDs, which are likely the product of aging). Tungsten oxidizes into semiconducting WO3 with a large ionization energy of 9.8 eV.48 Molybdenum forms stable oxides, which have the stoichiometry of MoO2 and MoO3, also with large work function and the ionization energy of 6.0 and 9.4 eV, respectively.49 When WO3 (or MoO2/MoO3) comes into contact with WS2 (or MoSe2) with an ionization energy of 5.74 eV (or 5.34 eV for MoSe2; see Table 1), we anticipate that electrons would transfer from TMDs to oxides, forming an electron-depleted region. Such a depletion region is expected to develop an upward band bending at the edges of TMDs that is contrary to the downward bending shown in Figure 2a,b. We note that the ionization energy of the interior of the MoSe2 flakes exposed to air for a few weeks show an inhomogeneous increase to 5.6−5.9 eV, consistent with the partial oxidation of the flakes, validating the notion of edge band bending due to aging. Photoluminescence (PL) of WS2 further supports the hypothesis of aging initiated from the edges for the sample used in this work. Figure S2 shows the PL mapping image (a) and corresponding spectra (b) of a WS2 flake, which was found on the same sample as shown in Figure 1b. The flake exhibits nearly 10 times enhancement at the edges relative to that of the flake’s interior. Similar enhancement has been observed by multiple groups50−54 and attributed to oxygen-chemisorbed sulfur vacancies within the flake.50 The sulfur vacancies would lead to the formation of localized states within the band gap that act as nonradiative recombination centers that reduce PL intensity. With chemisorption of oxygen at the vacancy sites,43,44 however, states within the band gap are removed, allowing for a “recovery” of the PL efficiency.51,55 We speculate that oxygen chemisorption occurs predominantly near the edges, resulting in the observed localized PL enhancement. Assuming that passivation of sulfur vacancies by the oxygen leads to a net change toward n-type behavior,50,52 >1 μm long extension of the aged area, as shown in Figure S2, is consistent with the spatial extent of downward band bending observed by PEEM, as shown in Figure 2b. The apparent lack of edge band bending in freshly grown MoS2 at the limit of our lateral resolution indicates a higher chemical stability of MoS2 compared to that of the other two TMDs. The emergence of metallic edge states and the associated edge band bending are observed for the MoS2 flakes using scanning tunneling microscopy.56,57 The reported band bending induced by metallic edge states appears at a length scale of a few nanometers, suggesting a different physical origin from the ones we observed in MoSe2 and WS2 flakes. We can also exclude the possible influence of stray built-in fields affecting the PEEM images, which would skew the trajectory of photoemitted electrons30 because the band bending was absent at the edges of MoS2 flakes. In the end, we surmise that the electrical transport of air-exposed TMDs could be significantly influenced by conductance through the edges and the overall aging.
ML MoSe2, WS2, and MoS2 are likely to form a staggered (type-II) band alignment. Type-II band alignment is advantageous for hosting interlayer or indirect excitons expected to display a long lifetime and for efficient charge separation in photovoltaic applications. Next, we consider the influence of the interlayer interaction on the band structure of vertical heterojunctions. In bilayer TMDs, the interlayer coupling splits the highest occupied state at the Γ-point as much as ∼0.8 eV.17,35 As a result, the splitting makes this state the valence band maximum by pushing it closer to the conduction band than the band edge at the K-point. If we assume that the interlayer coupling shifts the occupied band edge at the Γ-point by 0.4 eV (half the interlayer splitting, ∼0.8 eV), a heterobilayer with MoS2 or WS2 would have the highest occupied state located at the Γ-point because ΔEΓ‑K for MoS2 and WS2 are smaller than 0.4 eV (0.05 eV for MoS2 and 0.22 eV for WS2).34 Consequently, the heterobilayers that include MoS2 or WS2 would have an indirect band gap with almost no valence band offset, thus deviating from a type-II heterojunction. On the other hand, MoSe 2 incorporated in heterobilayers is likely to keep the valence band maximum at the K-point because of the sizable ΔEΓ‑K. Additionally, type-II band alignment is likely to be maintained due to MoSe2’s smaller ionization energy (Table 1). We expect that the details of the resulting band alignment also depend on the charge transfer from the supporting substrate, the spin−orbit effect, the atomic arrangement of the interfaces (azimuthal misalignment, or twist, between TMD layers), and defects or impurities, influencing the interlayer distance and the surface dipoles. Systematic verification of the ionization energies also allows us to assess the electrochemical activities of TMDs.10,11,13,40−42 The red dash-dot and dashed lines in Figure 4 indicate the redox potentials of the reduction (H+/H2) and oxidation (H2O/O2) of water, respectively, for photocatalytic water splitting reaction. The data shown in Figure 4 suggest that the 1 ML MoS2 can, in principle, be used as a cathode as well as an anode as its conduction band minimum is above the H+/H2 line, and its valence band minimum is below the H2O/O2 line. On the contrary, WS2 and MoSe2 can be used only as cathode materials to produce hydrogen. It has been reported that the calculated conduction band minimum of MoS2 based on density functional theory differs significantly between the local density approximation (LDA) and GW approximation.13 As a result, LDA calculation suggests that 1 ML MoS2 is not suitable as a cathode, yet the GW approximation predicts the contrary. Our experimental result favors the prediction based on GW approximation. Spatial Dependence of TMD Electronic Properties. A closer inspection of the PEEM micrographs and the photoemission spectra allows us to examine the spatial variation of the electronic properties in TMDs. In Figure 2a,b, we found a sizable shift of the electronic bands at the edges of MoSe2 and WS2 flakes, extending a few microns toward the flake interior as highlighted by the ovals with black dotted lines. The shift is as large as 0.5 eV, as shown for the Evac and highest occupied states displayed by the red and blue dots in Figure 2a,b. We presume that this edge band bending results from the aging of MoSe2 and WS2 flakes. Recently, several groups have reported that aging is initiated from the edges of TMDs.43,44 In this context, the downward band bending in TMDs can be explained by Fermi level pining by defects45 or oxygenchemisorbed sulfur vacancies near edges46,47 but not by charge transfer between TMDs and transition metal oxides (the
CONCLUSIONS We report the ionization energies of three prototypical TMDsmonolayers and few-layer MoSe2, WS2, and MoS2 on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. Based on this imaging spectroscopy, we extracted the ionization energy from absolute band positions. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with density functional 8227
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ACKNOWLEDGMENTS We thank A. McDonald for his help on the photoluminescence mapping. We also thank R.G. Copeland for his help in constructing the tunable DUV light source, and N. Bartelt for fruitful discussions. The PEEM work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science (DE-AC04-94AL85000). K.K. was supported by the Army Research Office MURI Grant W911NF-11-1-0362. The work performed by M.B. and C.C. was supported by a U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy SunShot Initiative award for Bridging Research Interactions through Collaborative Development Grants in Energy (BRIDGE, DE-FOA-0000654 CPS25859). A.D.M. is supported by the LANL LDRD program. T.O. was supported by the CINT user program and Sandia LDRD. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
theory calculations. Combined with the measured energy position of the valence band edge at the Brillouin zone center, we deduce that a heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment in the absence of the interlayer coupling. This study reveals a way to systematically determine ionization energies of as-grown 2D crystals thanks to the lack of gas absorption in ultrahigh vacuum, and the SiO2 supports minimally interacting with the overlying TMDs. Building an extended knowledge of the ionization energies of 2D crystals would enable informed design decisions to fabricate atomically thin heterojunctions.
METHODS TMD flakes were synthesized using a CVD technique based on previously reported procedures.24,58−60 The 285 nm thick SiO2, thermally grown on a highly doped silicon wafer (p-type, 12.0−16.0 Ω· cm), was chosen as a substrate to produce optical contrast for asgrown flakes. The layer number of each TMD flake was verified using Raman spectroscopy (MoS2), photoluminescence mapping (WS2), or optical contrast (MoSe2). Prior to PEEM measurements, samples were annealed for 4−12 h at ∼300 °C in ultrahigh vacuum with a typical final pressure below 1 × 10−10 Torr to remove adsorbed water and hydrocarbons from their surfaces. To minimize the influence of aging, we introduced the sample to the ultrahigh vacuum condition of PEEM within a few days of the sample growth. PEEM measurements were conducted in a LEEM-III system (Elmitec Elektronenmikroskopie GmbH) coupled to a tunable DUV light source. The spectral width of the DUV light was set to 50−100 meV throughout the wavelength range used for the measurement. Photoemission spectra were acquired by sweeping through the kinetic energies of the photoemitted electrons with an electron energy filter. No aperture restricted the emission angle of the photoelectrons. Thus, the data presented here correspond to emission-angle-integrated spectra. The energy resolution of the electron energy filter was set to 0.5 eV. The leading (lower relative electron kinetic energy side) and trailing (higher relative electron kinetic energy side) edges of photoemission spectra yield information about the energies of the vacuum level and the highest occupied states near the Γ-point (the center of the BZ), respectively.61 The vacuum level and the highest occupied states were obtained by fitting the photoemission spectra at each pixel and by correcting for the dispersion of the electron energy filter using the SiO2 background. Details of the measurement setup and the fitting procedure are reported in ref 31. Throughout this paper, the origin of the energy scale is referenced to the vacuum level of one monolayer (1 ML) TMDs.
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ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03242. Visualization of the vacuum level and the highest occupied states in the photoemission spectra, and the photoluminescence study of WS2 (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Aditya D. Mohite: 0000-0001-8865-409X Taisuke Ohta: 0000-0002-0827-5960 Notes
The authors declare no competing financial interest. 8228
DOI: 10.1021/acsnano.7b03242 ACS Nano 2017, 11, 8223−8230
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DOI: 10.1021/acsnano.7b03242 ACS Nano 2017, 11, 8223−8230
