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Thickness-Dependent Binding Energy Shift in FewLayer MoS Grown by Chemical Vapor Deposition 2

Yu-Kai Lin, Ruei-San Chen, Tsu-Chin Chou, Yi-Hsin Lee, Yang-Fang Chen, Kuei-Hsien Chen, and Li-Chyong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06615 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

Thickness-Dependent Binding Energy Shift in FewLayer MoS2 Grown by Chemical Vapor Deposition Yu-Kai Lin1,2,3, Ruei-San Chen4, Tsu-Chin Chou3, Yi-Hsin Lee5, Yang-Fang Chen1,3*, Kuei-Hsien Chen3,6, and Li-Chyong Chen3* 1

Department of Physics, National Taiwan University, Taipei, 10617, Taiwan

2

Nanoscience and Technology Program, Taiwan International Graduate Program, Academia

Sinica and National Taiwan University 3

Center for Condensed Matter Sciences, National Taiwan University, Taipei, 10617, Taiwan

4

Graduate Institute of Applied Science and Technology, National Taiwan University of Science

and Technology, Taipei 10607, Taiwan 5

Department of Materials Science and Engineering, National Taiwan University of Science and

Technology, Taipei 10607, Taiwan 6

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan

KEYWORDS MoS2, thickness, Fermi level, band bending, x-ray photoelectron spectroscopy, chemical vapor deposition, hydrogen evolution reaction. Corresponding Author E-mail: [email protected] & [email protected]

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ABSTRACT

The thickness-dependent surface states of MoS2 thin films grown by the chemical vapor deposition process on the SiO2-Si substrates are investigated by x-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS2 films to be ranging from 3 to 10 layers. Both the core levels and valance band edges of MoS2 shift downward ~0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valance band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS2 surface. Bulk defects in thick MoS2 may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS2 thin films, the Fermi level downshifts and the binding energy reduces due to the holedoping characteristics of Au and easy charge transfer from the surface defect sites of MoS2. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS2 film thickness. The tunable Fermi level and the high chemical stability make our MoS2 a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts.


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INTRODUCTION Since the discovery of graphene, two-dimensional layered materials, especially the transition metal dichalcogenides (TMDs), have drawn lots of attentions for the study of new fundamental sciences and technological applications.1-3 Unlike the semi-metal graphene, molybdenum disulfide (MoS2) is one of the TMD family that has a semiconductor behavior with a sizable band gap between 1~2 eV. The unique indirect to direct band gap transition at monolayer MoS2 further broadens its applications in catalysis, energy storage, sensing and electronic devices such as field-effect transistors and logic circuits.2-5 Since the high surface to volume ratio in the atomically thick MoS2, the surface properties play a crucial role to dominate the carrier transport on its applications. For examples, the surface structure of MoS2 can be engineered as edgepreferred to promote the electrocatalytic hydrogen evolution.6,

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The work function of MoS2

interfacial layers can be efficiently manipulated by p- or n-doping treatments to enhance the performance in polymer solar cells.8 The type-II p-n junctions formed at the van der Waals or at the lateral heterostructures between MoS2 and the other TMD materials, such as WSe2, also showed the potentials on optoelectronics and energy conversion by manipulating the interlayer tunneling recombination across the interface.9 Although MoS2 has versatile applications, it has been demonstrated that we cannot predict the contact properties of metal-MoS2 by merely considering the work functions and the Schottky theory.10-12 Au/MoS2 interface is still in debate on the Schottky or ohmic behavior, and the formation mechanism of the contact is vague so far.4, 12-15 Thus, the elusive surface properties of MoS2 may result in the limited device performance and impede the development to compete with Si-based devices. This situation will become more complicated when the layer number of MoS2 reduces because the band structure changes with the thickness.16-18 The optical properties and the


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transport properties through the interface will then be affected. In our study, we reported the thickness-dependent variations in surface states of CVD-grown MoS2 thin films on the SiO2-Si substrate by the x-ray photoelectron spectroscopy (XPS). The unique binding energy shift with film thickness was observed, and the reasons for this phenomenon were systematically discussed. The potential application of our MoS2 films was also demonstrated. We believe our findings could help clarify the surface/interface properties of MoS2 and facilitate the development of MoS2-related applications.

RESULTS AND DUSCUSSION Figure 1(a) shows Raman spectra of the MoS2 thin films prepared by sulfurization of MoO3 with various thicknesses at 1000oC for 30 min. Two characteristic peaks are observed at around 382 cm-1 and 407 cm-1, corresponding to the E2g1 and A1g vibrational modes, respectively. The out-of-plane A1g mode illustrates the symmetric vibration of S atoms along the c-axis, and the inplane E2g1 mode illustrates the opposite vibration of two S atoms with respect to the Mo atom.20 After normalization to the intensity of Si signal, the peak intensities of A1g and E2g1 increase from MS3 to MS20, merely reflecting the increase in the thickness of MoS2. The Raman frequencies of A1g and E2g1 peaks have been demonstrated to vary with the layer number of MoS2 in the opposite way mainly due to the changes in the interlayer coupling.21,

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The

frequency differences between A1g and E2g1 from MS3 to MS10 are 23.5, 24.8, and 25.8 cm-1, respectively, which correspond to around 3, 5, and 10 layers of MoS2.21, 23 On the other hand, MS20 does not show obvious peak shift compared to MS10, which indicates that MS10 has already possessed bulk properties of MoS2. Thicker MoS2 might show similar behaviors with MS10. AFM images with height profiles were analyzed to confirm the film thickness before and


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after the MoS2 formation, as illustrated in Figs. S3 and S4. The oxide thickness is as our expectation ranging from 3 to 20 nm and is homogeneous on the SiO2-Si substrate. After sulfurization, the thickness of formed MoS2 reduces by around 30% compared to its oxide precursor; specifically, the resultant film thickness is 2.3, 3.5, 6.9, and 14.1 nm for MS3, MS5, MS10, and MS20, respectively, corresponding to around 3 to 20 layers of MoS2. In order to estimate the band gaps of prepared MoS2 thin films, the absorption spectra were measured, as shown in Fig. 1(b). The prominent A/B peaks (1.7 to 2.0 eV) are the spin-orbit-split pair arising from the band edge exitonic transitions at the Brillouin zone K point. The broad C/D peaks are interband transitions from the occupied dz2 orbital to unoccupied dxz, dyz, dxy, and dx2-y2 orbitals.24, 25 As illustrated in Fig. 1(b), the A/B peaks show a slight blue shift from MS10 to MS3, which can be attributed to the quantum confinement effect.16 We estimated the optical band gap by the tauc plot with the indirect allowed transition for the few-layered MoS2, as shown in the inset of Fig. 1(b). The increase of the optical band gaps from 1.57eV to 1.66eV is apparent with the reduction of the film thickness from MS10 to MS3. To determine the layer number precisely, we performed the HRTEM and collected the crosssectional images of our MoS2 films. Figure 2(a) illustrates the photographs of the MoO3 thin films with various thicknesses and the corresponding MoS2 films after sulfurization. Our twostep process has successfully achieved large-area and homogeneous growth of MoS2 film with good thickness control. Figure 2(b), 2(c), and 2(d) illustrates the cross-sectional TEM images of FIB-cutting MS10, MS5 and MS3 specimens, respectively, where it elucidates that our MoS2 films are continuous and flat rather than discrete random-orientated particles. The corresponding HRTEM images of higher magnification, Figs. 2(e), 2(f), and 2(g), clearly show that the layer


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number of MoS2 for MS10, MS5, and MS3 is around 10, 5, and 3, respectively, which is consistent with the AFM and Raman results. The surface chemical states of prepared MoS2 thin films analyzed by XPS are shown in Fig. 3. The detailed results are listed in the Table 1. The MoS2 exhibits two characteristic peaks at around 229.6 and 232.7 eV, corresponding to the doublet Mo 3d5/2 and 3d3/2 for Mo4+. There are one peak at around 226.7 eV and two peaks at around 162.4 and 163.6 eV assigned as S-2s, and the doublet S 2p3/2 and 2p1/2, respectively. After adjusting the energy by the C-1s position, both Mo-3d and S-2p core levels are found to shift ~0.2 eV toward high binding energy (BE) as the film thickness decreases. Meanwhile Si-2p and O-1s core levels from the substrate remain in the same positions and their peak intensities increase with reducing film thickness. On the other hand, the binding energy does not show obvious shift for thicker MoS2 film (MS20) in either Mo-3d or S-2p compared to MS10, as shown in Fig. S6. This indicates that the BE shift may relate to the surface effect on MoS2 rather than the bulk. To clarify the correlations for the unique BE shift with the film thickness, we discuss as follows. First, the BE shift may result from the variations in the chemical environment of the surface Mo and S atoms due to the improper control during the synthesis. This indicates the oxidation states of Mo and S may change with the film thickness. Originally they should be Mo4+ and S2- in an ideal MoS2. Since we prepared the MoS2 from MoO3, where it is Mo6+, if part of the oxides is not fully sulfurized, especially in the thicker film, mixed valances of Mo will be observed in the Mo-3d core-level spectrum. However, TEM-EDX mapping (Fig. S7) and XRD profiles (Fig. S8) indicates that there is no detectable oxide impurity in our MoS2 thin films. If the sulfur deficient sample is obtained, the zero valance Mo may appear (MoSx MoS2 + Mo), which will also contribute to the Mo-3d spectrum.26, 27 During the sulfurization, sulfur-rich phase


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MoS3 could form as an intermediate state, which can be considered as MoIV(S2-)(S22-) and reflects on the changes in S-2p core level.28 However, we do not observe the peak broadening, shape change, or shoulders in the spectra because of the average oxidation state of Mo or S increases or decreases. Therefore, we rule out the possibility that BE changes with valence states. Second, the surface of MoS2 could be charged positively when the photoelectrons leave the sample surface. The accumulated positive charges at the sample surface will trap the following photoelectrons leading to the increase in the measured BE. Although the electron flood gun was employed to neutralize the surface charges, we still observe the BE shift in Mo-3d and S-2p core-level spectra. As illustrated in Fig. 1(b), the band gap energy of MoS2 increases when the film thickness reduces, which intuitively implies the conductance of MoS2 thin film decreases and results in surface charging. Nevertheless, it was reported that the conductance of MoS2 has little dependence with the thickness.29 On the contrary, the conductivity of MoS2 increases as the film thickness decreases, which indicates that the surface of MoS2 is more conductive than the bulk. In our work, the thicknesses of three MoS2 thin films are all less than 10 nm, and get thinner from MS10 to MS3, which means the surface properties are dominant. Figure S9 depicts the electrical measurements of our MoS2 thin films with van der Pauw configuration, where it shows all the MoS2 films have the conductance in the same order of magnitude regardless of the film thickness. However, the conductivity of MS3 is more than 20 times higher than that of MS10, indicating that the surface is more conductive than the bulk. Thus, surface charging shall not account for the BE shift, especially for MS3 which exhibits the highest binding energy of Mo-3d and S-2p among these three samples. To further confirm the surface charging effect, we also prepared reference samples, which are MoS2 grown on the conducting substrate (graphene paper) via the same processes as those grown


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on the SiO2-Si substrates. As shown in Fig. S10, the Raman results demonstrate that MoS2 thin films were successfully grown on the conducting carbon material with similar qualities to MoS2 grown on the SiO2-Si. The XPS core-level spectra of MoS2 grown on various substrates are illustrated in Fig. S11. Comparatively, it is quite clear that the substrate will not influence the Mo-3d and S-2p core levels significantly, whereas the change of film thickness induces the BE shift. This result also suggests that there is no or negligible surface charging effect on our MoS2 thin film. Third, the composition deviation from the stoichiometry at the MoS2 surface may lead to the BE shift. It was demonstrated that the binding energy of Mo-3d5/2 decreased progressively as the x in MoSx decreased, while the binding energy of S was independent of the MoSx stoichiometry.27 Recent reports also elucidated that the variations in the S/Mo ratio would reveal the n-type or p-type behavior of MoS2.12 Sulfur-deficient MoS2 (S/Mo < 2) generally has n-type feature because the local formed sulfur vacancies act as electron donors, shifting the Fermi level toward the conduction band minimum (CBM). On the contrary, sulfur-rich or Mo-deficient (S/Mo > 2) will lead to the p-type feature, causing the Fermi level to move away from the CBM. Intentional surface treatment to turn the S/Mo ratio was also discovered to change the threshold voltage of MoS2 field-effect transistors.30 Since the BE detects the energy differences from core levels referenced to the Fermi level of the system, the variations in the Fermi level will influence the measured BE. However, it is noted that it seems no strong correlation between the film thickness and MoS2 stoichiometry. In our work, both Mo-3d and S-2p core levels vary monotonically with film thickness, and the calculated S/Mo ratios are all closed to 2, as listed in Table 1. Thus, the deviations in composition shall not be blamed for the BE shift in our case.


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Except for the chemical composition, the extrinsic doping may be another way to vary the Fermi level in two-dimensional materials. For example, impurities, like Na, at the MoS2/SiO2 interface will generate a defect-induced shallow donor level right below the Fermi level, resulting in n-type doping to the MoS2 conduction band.30 It has also been proposed that impurities in natural MoS2, whose concentration is less than the detection limit of XPS, can have significant impacts on its electronic properties.32 However, we prepared each MoS2 film via the same CVD process, which is much different from the natural environment. The concentration and composition of impurities shall be similar in our MoS2 films, and their effects on the material properties could be independent with the film thickness. When we changed the substrate from SiO2-Si to graphene paper (GP), we still observed BE shifts with MoS2 thickness (Fig. S11). Thus, the impurity effects on the BE may not be the suitable answer in our case. Moreover, recent study illustrated that the physically adsorbed O2/H2O at the MoS2 surface reduces the Fermi level due to its hole-doping characteristic.31 The competing screening effect between the surface- and the substrate-O2/H2O molecules was claimed for the work function variations with the MoS2 thickness. However, it is not the scenario for the BE shift in our case. The characteristic peak of O-1s in Fig. 2(e) is located at around 533.1 eV, corresponding to the SiO2 from the substrate. Its intensity decreases with increasing the MoS2 thickness, and becomes invisible for the MS10, which implies the oxygen signal in our case mainly originates from the substrate rather than the adsorbed O2/H2O. The other possible reason for the BE shift is the variations in the measured valance band position and/or the core levels at the MoS2 surface corresponding to the surface band bending (SBB). As shown in Table 1, both Mo-3d and S-2p core levels downshift as the film thickness increases, and the energy differences between Mo-3d and S-2p (ECLMo-ECLS) are comparable,


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which imply that SBB would occur in our MoS2 thin films. Addou et al. has proposed the concept of SBB on natural MoS2, which was attributed to the influences of surface defects.32 Mahatha et al. was also reported that the metallic edge steps and negative-charged dislocations would generate the electric field gradient on the MoS2 surface, leading to the inhomogeneous SBB so as localized Fermi level pinning (FLP) which caused the BE shift in XPS.34 Figures 4(a) to 4(c) are AFM images, where they exhibit the surface topographies of MoS2 thin films. Within the 2 x 2 µm, the MoS2 thin films prepared from the thermal-evaporated MoO3 on the SiO2-Si substrate show polycrystalline features with the grain size of ~100 nm. The high density of edges and grain boundaries (GBs) also reflects on the Raman results shown in Fig. 1(a), where it demonstrates that the intensity of E2g1 is lower than that of A1g. However, the edge densities among these three samples are so similar that the impacts on the BE shift may be ignored. To further clarify the effect of SBB, the valance band spectra of MoS2 thin films were collected by fine scanning near the Fermi level of the system, where the binding energy is 0. Figure 4(d) illustrates the valance band spectra of MoS2 thin films and the details are listed in Table 2. The valance band maximum (VBM) of MoS2 can be considered as the density-of-state (DOS) of Mo-4d, which locates at 1.55 eV relative to the Fermi level for MS3 and decreases the relative energy to 1.31 eV for MS10. According to the optical band gap estimated in Fig. 1(b), it illustrates that our MoS2 is n-type. It has been reported that the VBM of MoS2 at the Γ point downshifts with respect to the local maximum at the K point as the layer number decreases.17 However, based on the direct measurement of VBM by angle-resolved photoemission spectroscopy (ARPES), we can regard it as almost no variations in VBM when the MoS2 is more than 3 layers.35 The energy differences among the three samples at valence band edges are almost the same to those at core levels, indicating that the SBB does occur in our MoS2. It is


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worth to note that there is a small band tail in front of the valance band edge, which could be regarded as a defect-induced gap states. Zhou et al.’s calculation proposed that the sulfur vacancies would create a shallow occupied defect level right adjacent to the VBM, and two deep unoccupied defect levels below the CBM in n-type MoS2.36 Thus, the small defect band that we observed in Fig. 4(d) may be originated from the local sulfur vacancies due to the high edge densities and GBs in our MoS2 thin films. Since the defect bands are attributed to the sulfur vacancies, these positively charged defects may form donor-like (or metallic-like) surface states that contribute additional free electrons to the MoS2 surface and generate an electric field pointing from the surface to the bulk.12 The electrical potential formed near the surface will then bend the bands downward. Figure 4(e) illustrates the band offsets of MoS2 films with various thicknesses, which clearly shows the consequence of band bending when the film thickness reduces. Although the BE refers to the energy difference between core levels and Fermi level at the MoS2 surface, it is an average outcome of the detected volume in the real case rather than a single measured plane. Therefore, we obtain larger BE in thinner MoS2 than the thicker one due to the bending down feature. The SBB we proposed also implies that thinner MoS2 possesses higher conductivity because higher electron density at the surface leads to CBM close to Fermi level, which is consistent with our electrical measurement results (Fig. S9). Other two-dimensional materials were also found to have such thickness-dependent variations in conductivity and might be explained by the SBB effect. Besides, MoS2 has been demonstrated as a good piezoelectric material.37-39 The electric field applied from the S site to the Mo site will stretch the Mo-2S dipole, creating tensile stress along the field direction and compressive stress at the orthogonal direction. Therefore, the builtin surface potential in our MoS2 thin film could induce the piezoelectric effect to elongate its


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lattice constant in c-axis and reduce in a/b, corresponding to increasing E2g1 and decreasing A1g in Raman. Similar phenomenon has been discovered on GaN nanorods.40 This effect is proportional to the field strength; i.e., the thinner MoS2, the lattice alters more. The frequency evolutions of E2g1 and A1g with MoS2 thickness in Fig. 1(a) also reflect on the changes of lattice constants due to the piezoelectric effect in a certain extent. Although SBB seems to match our observation, the variations of defect densities in our MoS2 films may not be ignored, which would shift Fermi level as well. As shown in Fig. 4(d), the DOS of defect bands slightly increases from MS3 to MS10, which implies that defects in the bulk may also contribute to the defect band near the VBM. The thicker MoS2 films probably have higher defect number in between MoS2 layers, such as the point defect of interstitial or the line defect of dislocation. Those bulk defects would form localized trap states in the band gap of MoS2, and part of them locate near the VBM.41, 42 Thus, the higher DOS of defect bands in MS10 might result from the formation of additional acceptor-like defect states in the bulk, as shown in the Fig. 4(f). The bulk defects would capture free electrons leading to lower Fermi level. However, sulfur vacancy is the main defect in MoS2 due to its low formation energy so that it is expected to have the greatest impacts on the Fermi level compared to other structural defects. 36, 41

Addition to the defects, strain in our MoS2 films, originated either from the substrate or the

thermal treatment in CVD process, is also possible to alter the band structure leading to the BE shift. The largest direct band gap shift can be estimated to 300 meV per 1% biaxial compressive strain in a tri-layer MoS2.43 However, it is difficult to make such high strain difference among our MoS2 films without bending the substrate. Part of the strain could also be relaxed by the GBs in our polycrystalline MoS2 films. Thus, the effect of strain-induced band structure variations shall be limited in our case. Based on previous discussion, though slight quality variation can be


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observed among our MoS2 films, which would affect the Fermi level so as the BE, surface states is still the dominant effect on the BE shift. In addition, the proposition of the donor-like surface states in the CVD-grown MoS2 films could also explain the downward SBB observed in Fig. 4(e). A thin layer of Au (~1 nm) was deposited by utilizing the e-beam evaporator under high vacuum to see how the BE varied at the MoS2 surface with a metal contact. Figure 5(a) to 5(c) illustrates the core-level spectra of Au-deposited MoS2, where Mo-3d and S-2p core levels down shift after the Au deposition, while Au-4f shifts oppositely toward higher energy, implying the charge transfer between Au and MoS2. The valance band spectrum of Au/MoS2 shows three distinct edges, as labeled in a, b, and c in Fig. 5(d). Compared to bare MoS2 and pure Au, the energy positions of a and c are right consistent with the band edges of Au, while b is the contribution of Mo-4d with the energy position shifting ~0.3 eV toward the Fermi level. This indicates that the high work function metal (φAu = 5.1 eV) serves as a p-type material and attracts electrons from the n-type MoS2 surface when they contact each other.44 After reaching the thermal equilibrium, the accumulated electrons at the MoS2 surface will be reduced so as the Fermi level position, leading to BE downshift. It is worth to note that Mcdonnell et al. have shown if there is no chemical reaction occurring at the metal/MoS2 interface, the BE will not shift.12 In Fig. 5(a) and 5(b), the core-level spectra exhibit no chemical reaction or surface damage at the Au/MoS2 interface, indicating the BE shift after Au deposition in our case shall result from other factors. The metallic behavior of MoS2 has been discovered at the Mo-terminated edges (sulfur-deficient sites) or two different types of 60o GBs consisting of distinct 4-fold ring chains.36, 45 These defects have been shown to transfer charges easily, leading to good catalytic performance.6, 36 The Schottky barrier at MoS2 and Au interface is also discovered lower than expectation because the metallic-like surface defects on


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MoS2.12 Since our MoS2 thin films exhibit polycrystalline characteristics with many edges and GBs, they may provide free charges at the interface to realign the Fermi level after contact with metal and shift the BE. Furthermore, it has been reported that p-doped MoS2 gives rise to the increase of relative intensity and frequency difference between A1g and E2g1 modes.44 The Raman spectra of MoS2 before and after Au deposited were compared in Fig. 5(e). After normalized the peak intensity of E2g1, the A1g mode of Au-deposited MoS2 shows higher intensity and blue-shift to higher frequency than bare MoS2, which also illustrates the hole-doping characteristics of Au on our MoS2 films. Since our previous results show that Fermi level in our MoS2 monotonically changes with film thickness and the polycrystalline feature may facilitate the charge transfer at the surface, we try to demonstrate its catalytic behavior by hydrogen evolution reaction (HER). Figure 6(a) shows the linear sweep voltammetry (LSV) curves of our MoS2 grown on the GP. It is obvious that the onset potential dramatically downshifts when there is MoS2 on the GP surface, and further decreases when the MoS2 thickness reduces. The corresponding potential at -10 mA/cm2 decreases 140 mV between GP and MS10, and 70 mV between MS10 and MS3, respectively. The onset potential here represents the potential that is required to drive HER, which is related to the Fermi level of our MoS2 film and the chemical potential of HER. As a result of the identical electrolyte we used for each specimen, the lower onset potential indicates lower reaction barrier required to be overcome for HER, as shown in Fig. 6(b). The evolution of the onset potential with the MoS2 film thickness in Fig. 6(a) implies that MS3 has the highest ground state in the potential energy surface for HER, i.e. the highest Fermi level near the CBM, which is consistent with the trends we observed in the XPS.


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To further illustrate the charge transfer between our MoS2 films and the reactants (hydrogen ions) in the electrolyte, the impedance measurements of the electrochemical system were carried out. Figure 6(c) shows the equivalent circuit in our electrochemical system, where RS is the resistance including the electrode resistance and the electrolyte resistance, Cdl is the double layer capacitance near the MoS2 surface, RCT is the charge transfer resistance between our MoS2 and reactants in the electrolyte, and ZW is the Warburg impedance related to the mass transfer of the hydrogen ions from the electrolyte to the MoS2 surface. RCT can be extracted from the Bode plot because it is independent of applied frequencies. Figure 6(d) shows the Bode plot for bare GP and MoS2/GP specimens at -10 mA/cm2. After subtracting the effect from RS, RCT decreases monotonically from MS10 to MS3, indicating the charge transfer from MoS2 surface to the hydrogen ions becomes faster and more efficient when the MoS2 thickness reduces. This result is consistent with our electrical measurement, which exhibits higher conductivity for thinner MoS2. Moreover, comparing the Raman spectra of MoS2 films before and after HER (Fig. S13), it is interesting to note that the profiles clearly overlap for either thin (MS3) or thick (MS10) MoS2 indicating the high chemical stability of our MoS2 under a catalytic reaction.

CONCLUSION We prepared MoS2 thin films on SiO2-Si with thickness ranging from 3 to 10 layers via CVD process, and observed the BE of Mo-3d and S-2p monotonically downshift ~0.2 eV with an increase in film thickness by XPS analyses. The probable reasons for the BE shift were discussed systematically. Defect-induced Fermi level variation is suggested to explain the thicknessdependent behaviors. Polycrystalline features shown in the AFM images and the defect bands appeared adjacent to the VBM of MoS2 which indicates the sulfur vacancies exist in our MoS2


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thin films. A downward band bending is proposed based on the defect-induced surface states to explain the BE shift with MoS2 thickness. Meanwhile, the bulk defects appearing in the thicker MoS2 may form acceptor-like trap states in the band gaps, giving rise to lower Fermi level. The thickness-dependent Fermi level variation can also be applied on other two-dimensional materials where the conductivity increases with a decrease in film thickness. The electrical field generated at the MoS2 surface might trigger the piezoelectric effect to alter the lattice constants, which is consistent to our frequency evolutions of E2g1 and A1g in Raman. Depositing Au on the MoS2 is found to reduce the BE ~0.3 eV because of its hole-doping characteristics, which implies the Fermi level can be tuned by the surface treatment. The Fermi level in the MoS2 films affects the interfacial transport properties in MoS2-related applications. We have also demonstrated here that the onset potential for HER evolves with MoS2 film thickness, which is related to the Fermi level variation in our MoS2 films. The lower charge transfer resistance in thinner MoS2 is also an indirect support for the thickness-dependent conductivity and SBB effect. The tunable Fermi level and the high chemical stability make MoS2 a potential material for catalytic reactions. We believe our discoveries will also stimulate novel concepts in designing new catalyst systems and flexible electronic devices based on the two-dimensional materials.

EXPERIMENTAL SECTION MoS2 thin films were prepared by a simple two-step process which is similar to that reported by Y. C. Lin et al.19 First, a thin MoO3 layer was thermal evaporated onto the SiO2-Si wafer. The thickness of the oxide layer was controlled as 3, 5, and 10 nm. Then all the specimens were sulfurized in 10% H2S gas flow (mixed with H2 and Ar) at various temperatures and time. The sulfurized MoS2 samples were named as MS3, MS5, and MS10 corresponding to the original


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oxide layer of 3, 5, and 10 nm, respectively. The optimized conditions for preparing the MoS2 thin film were found by analyzing the Raman spectra, as shown in Fig. S1. Then the MoS2 thin films of different thicknesses were prepared and analyzed by Raman, photoluminescence (PL), and absorption spectra to understand how the material properties varied with thickness. The Raman and PL spectra were recorded using a Jobin-Yvon LabRAM H800 with a 532 nm Nd:YAG laser as the excitation source and the spot size was ~1 µm. Spectra at several positions inside the continuous films were taken to check the uniformity of the films. UV−visible absorption spectrometry was adopted by using a JASCO 670 spectrometer. Atomic force microscopy (AFM) images with height profiles were carried out by a Veeco Dimension-Icon system with the scan rate of 1 Hz and 1024 scanning lines to examine the thickness of MoO3 precursors and the corresponding MoS2 films. High resolution cross-sectional image of grown MoS2 thin film was performed by the field emission transmission electron microscope (FETEM, JEOL JEM-2100) to confirm the layer number of MoS2. The specimens for TEM were prepared by focused-ion-beam (FIB) cutting with 50 nm BCP as a protection layer. TEM-EDX mapping with Quantmap adjustment and x-ray diffraction (XRD, Bruker D8) patterns were analyzed to identify the composition and structure of MoS2 films. The surface properties were characterized right after the CVD process with high-resolution Xray photoelectron spectroscopy (XPS, Thermo Scientific Theta Probe) using Al Kα radiation (1486.6 eV). The spot size we used to probe the sample was fixed at ~400 µm. All XPS peaks were corrected for inelastic scattering by initially subtracting the Shirley background, followed by fitting the peaks with Voigt shape peaks. The peak positions were adjusted carefully with respect to C-1s and Au-4f core levels. The atomic sensitivity factors used in this work were 3.321 for Mo and 0.666 for S, respectively. Each specimen was measured five different points to


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confirm the spectra coherency. The deviation of measured binding energy (BE) between each point after calibration was around 0.02~0.04 eV. The spectra of valence band edges were recorded by fine scanning the energy range from -10 to 10 eV. To observe the surface topographies of MoS2 thin films, the tapping-mode atomic force microscopy (AFM) images were collected by a JPK NanoWizard II. The conductivities of MoS2 films were calculated by van der Pauw method where Keithley 4200 was used to perform the I-V curves. The MoS2 for electrochemical studies was grown on the 1 x 1.5 cm graphene paper (GP, AzTrong) with thickness control. Hydrogen evolution reaction (HER) was carried out in the 0.1 M HClO4 solution by a three-electrode system with Ag/AgCl as the reference electrode and a graphite rod as the counter electrode. Linear sweep voltammetry with scan rate of 2 mV/s, and AC impedance spectroscopy with 0.1 to 100k Hz and amplitude of 10 mV were recorded by a Zahner Zennium workstation.


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FIGURES

Figure 1 (a) Raman spectra of MoS2 with various film thicknesses. The inset figure illustrates the frequency evolutions of A1g and E2g1 modes with deposited MoO3 thickness; (b) Absorption spectra of MoS2 with various film thicknesses. The inset figure illustrates the transformed tauc plot.


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Figure 2 (a) Photographs of the as-deposited MoO3 films on 1 x 1.5 cm SiO2-Si substrates, and the corresponding MoS2 films after the oxides were sulfurized. Cross-sectional HRTEM images of MoS2/SiO2-Si with various MoS2 thicknesses: (b), (e) MS10; (c), (f) MS5; and (d), (g) MS3.


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Figure 3 XPS core-level spectra of (a) Mo-3d, (b) S-2p, (c) C-1s, (d) Si-2p, and (e) O-1s, collected from MoS2 with various film thicknesses.


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Figure 4 AFM topographies of (a) MS3, (b) MS5, and (c) MS10; (d) Valance band spectra of various MoS2 film thicknesses; (e) Calculated band offsets in our MoS2 films from (d) and Fig. 1(b); (f) Schematic diagrams for the band structures of MS3 and MS10.


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Figure 5 XPS core-level spectra of (a) Mo-3d, (b) S-2p, and (c) Au-4f of MS5 before and after Au deposition; (d) Valance band spectra of bare MS5, Au-deposited MS5, and pure Au; (e) Raman spectra of MS5 before and after Au deposition.


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Figure 6 (a) LSV curves for the GP substrate and MoS2/GP with various MoS2 thicknesses (MS3, MS5, and MS10); (b) Schematic diagram of potential energy surface on reaction coordinate for HER; (c) Schematic diagram of the equivalent circuit in our electrochemical system for HER (d) Corresponding Bode plot for HER at -10 mA/cm2 in (a), obtained by the impedance measurement from 0.1 to 100k Hz with amplitude of 10 mV.


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TABLES Table 1 The energy values of Mo-3d and S-2p core levels obtained from Figure 2. All the values in the table are in the unit of eV. MoS2

MS10

Mo

∆SO

3d3/2

3d5/2

232.62

229.49

3.13

ECLMo

∆ECLMo

231.06

S 2p1/2

2p3/2

163.50

162.32

∆SO

ECLS

1.18

162.91

0.11 MS5

232.74

229.59

3.15

231.17

232.83

229.68

3.15

163.58

231.26

ECLMo-ECLS

S/Mo

68.15

2.01

68.18

2.02

68.19

1.98

0.08 162.40

1.18

162.99

0.09 MS3

∆ECLS

0.08 163.66

162.48

1.18

163.07

SO refers to the spin orbit splitting. ECLX refers to the core-level energy for element X (X = Mo or S)

Table 2 The valance band edges of various MoS2 thicknesses obtained from Figure 3(b). All the values in the table are in the unit of eV. MoS2

MS10 MS5 MS3

EV1

EV2

∆E12

defect

Mo-4d

EV2-EV1

0.65

1.31

0.66

0.77 0.89

1.44 1.55

∆EV1

∆EV2

0.12

0.13

0.12

0.11

0.67 0.68

ECLMo-EV2

ECLS-EV2

EG*

229.75

161.60

1.57

229.73

161.55

1.62

229.71

161.52

1.66

* The band gap energy EG is obtained from Figure 1(b).


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional illustration of growth procedures of few-layer MoS2 thin films, Raman characterization, photoluminescence (PL) spectra, atomic force microscopy (AFM) images with height profiles, high resolution transmission electron microscopy (HRTEM) images with EDXmapping, x-ray diffraction (XRD) patterns, electrical measurements, scanning electron microscopy (SEM) images, as well as x-ray photoelectron spectroscopy (XPS) analyses.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] E-mail: [email protected] Author Contributions Y.K. conceived the idea, designed the experiments, and carried out most of the measurements. R.S. and Y.F. built the physical concept for the observed phenomenon. T.C. and Y.H. demonstrated the application. Y.K., K.H., and L.C. analyzed the data and wrote the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the Taiwan International Graduate Program and grants from the Institute of Atomic and Molecular Sciences, Academia Sinica, Center for Condensed Matter Sciences, National Taiwan University, and Ministry of Science and Technology (project numbers: MOST 103-2745-M-002-006-ASP & MOST 104-2745-M-002-004-ASP).

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Table of Contents Graphic


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