WS2 Layered Heterostructures and

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One-Step Synthesis of MoS/WS Layered Heterostructures and Catalytic Activity of Defective Transition Metal Dichalcogenide Films John Michael Woods, Yeonwoong Jung, YuJun Xie, Wen Liu, Yanhui Liu, Hailiang Wang, and Judy J. Cha ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06126 • Publication Date (Web): 02 Feb 2016 Downloaded from http://pubs.acs.org on February 3, 2016

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One-Step Synthesis of MoS2/WS2 Layered Heterostructures and Catalytic Activity of Defective Transition Metal Dichalcogenide Films John M. Woods1,2, Yeonwoong Jung*,1,2, Yujun Xie1,2,3 , Wen Liu2,4, Yanhui Liu1,3, Hailiang Wang2,4, and Judy J. Cha1,2,3

1

Department of Mechanical Engineering and Materials Science, Yale University, New Haven,

Connecticut 06511, United States 2

Energy Sciences Institute, Yale University West Campus, West Haven, Connecticut 06477, United

States 3

Center for Research on Interface Structure and Phenomena, Yale University, New Haven, CT 06511,

USA 4

Department of Chemistry, Yale University, New Haven, CT 06511, USA

*Present address: Nanoscience Technology Center, Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32826, USA

CORRESPONDING AUTHOR FOOTNOTE: *

Corresponding author. E-mail: [email protected]

ACS Paragon Plus Environment

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Telephone number: +1 (203) 737-7293, Fax number: (203) 432-6775

KEYWORDS: Molybdenum disulfide, tungsten disulfide, Two-dimensional materials, layered materials, van der Waals heterostructures, chemical synthesis, hydrogen evolution reaction

Abstract:

Transition metal dichalcogenides (TMDCs) are a promising class of two-dimensional (2D) materials for use in applications such as 2D electronics, optoelectronics, and catalysis. Due to the van der Waals (vdW) bonding between layers, vdW heterostructures can be constructed between two different species of TMDCs. Most studies employ exfoliation or co-vapor growth schemes which are limited by the small size and uneven distribution of heterostructures on the growth substrate. In this work we demonstrate a one-step synthesis procedure for large-area vdW heterostructures between horizontal TMDCs MoS2 and WS2. The synthesis procedure is scalable and provides patterning ability, which is critical for electronic applications in integrated circuits. We demonstrate rectification characteristics of large-area MoS2/WS2 stacks.

In addition,

hydrogen evolution reaction performance was measured in these horizontal MoS2 and WS2 thin films, which indicate that, in addition to the catalytically active sulfur edge sites, defect sites may serve as catalyst sites.


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Two-dimensional (2D) materials have been studied extensively in recent years,1-3 in part due to potential to create van der Waals (vdW) heterostructures by stacking the layers of different 2D materials.4 These vdW heterostructures could realize the construction of 2D electronics that are thin, flexible, and transparent.5-11 Unlike epitaxially grown 2D films, vdW heterostructures are possible even with lattice mismatch owing to the weak vdW interactions. The strain due to the lattice mismatch may affect materials properties of the heterostructures.12-14

VdW

heterostructures also offer unique device structures in which electrical transport is through the vdW gap, providing a new paradigm for device physics.

In particular, transition metal

dichalcogenide (TMDC)15 films and vdW heterostructures are promising for thin film optoelectronics16-17 due to layer-dependent bandgaps and direct bandgap18 in the monolayer limit. The properties of TMDCs are highly dependent on the film quality, and defects, such as grain boundaries, which can affect the performance of TMDC devices.19 Large-scale, uniform growth by metal–organic chemical vapour deposition,20 atomic layer deposition,21 pulsed laser deposition,22 and chemical vapor deposition23-27 of TMDCs such as MoS2 has been demonstrated,20, 28 and these films can be constructed on a variety29 of substrates either by direct growth or film transfer. However, TMDC heterostructures have been produced mostly by mechanical exfoliation30-31 and co-vapor growth32-33 methods.

More work is needed to

demonstrate the ability to easily pattern TMDC heterostructures at large-scale, necessary for any industrial scale implementation of 2D electronics based on TMDC heterostructures. Growth by seed layers allows for large-area patterned TMDC film synthesis.34-36 Recent work has shown the location-defined growth of monolayer MoS2 from MoO3 seed layer.37 Metallic seed layers have been shown to be appropriate for synthesis of TMDCs on many substrates.38 Recently, we showed that cm-scale MoS2/WS2 heterostructures where the building block was


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placed side-by-side were possible using the metallic seed layer growth39.

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A stacked

heterostructure of MoS2 and WSe2 was also demonstrated by sequential deposition and chalcogenide reaction of metallic layers.40 For both heterostructures however, the molecular layers of TMDCs were vertically oriented. Thus, although the synthesis methods are facile and potentially scalable, the resulting heterostructures are not ideally suited for electronic applications due to large scattering across vdW gaps. We found that the seed-layer thickness controlled the transition between vertical and horizontal TMDC films.41 If the metallic seed layers are thinner than ~3nm, the synthesized films will be in the horizontal orientation, desirable for electronic applications.41 Drawing on the results of our previous study, we selected a “goldilocks” seed layer thickness of 1nm to synthesize a stacked heterostructure of horizontally oriented MoS2 and WS2. In this work we demonstrate the one-step synthesis of a cm-scale, stacked heterostructure between horizontally oriented MoS2 and WS2 from patterned metal seed layers.

Results and Discussion A stack of Mo and W leads to a stack of the corresponding TMDCs upon reaction with a chalcogenide (in our case, sulfur). Figure 1(a) shows the schematic of our growth scheme. Stacks of Mo or W elemental metals films, patterned by electron beam lithography, were deposited on a Si/SiO2 substrate by sequential magnetron sputtering of W and Mo then reacted with sulfur, as described in the methods section, to produce the stacked heterostructure of MoS2 and WS2. Both MoS2/WS2 and WS2/MoS2 are possible by changing the deposition order of Mo and W. Figure 1(b) shows an optical microscopy image of the patterned heterostructure films


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after the reaction with sulfur. After the reaction, the color changes from blue to light green, indicating sulfurization was carried out. The TMDC heterostructure films maintain the specified geometry of the lithography. The geometry of the pattern remains constant. The average width of the square pattern is 199.0±1.1 and 200.4±1.0 micrometers before and after sulfurization. We measured multiple patterns and find that the lateral size increases by ~ 0.68 % +/- 0.74 %, which is minimal. The MoS2/WS2 heterostructures were analyzed by scanning electron microscopy (SEM) in Figure 1(c-f). Figure 1(c) shows the corner of one of the patterned regions of the heterostructure. Figure 1(d-f) show SEM energy dispersive spectroscopy (EDS) maps of (d) S, (e) Mo, and (f) W, which matches the defined patterning of the film. The Mo and S peaks overlap for the EDS SEM mapping, preventing careful analysis of the nature of the heterostructure. However, the uniform distribution of the W and Mo maps suggest that Mo and W did not form segregated alloys of varying Mo/W ratios. The MoS2/WS2 heterostructure films were further characterized by Raman spectroscopy. Figure 2 shows a comparison between the Raman spectra of single-species MoS2 and WS2 grown by metal-seed layers and the Raman spectrum of our synthesized heterostructure. Present in the spectrum of our sample are peaks from both WS2 (E12g ~ 353 cm-1 and A1g ~ 419 cm-1) and MoS2 (E12g ~ 383 cm-1 and A1g ~ 408 cm-1). This indicates that we have synthesized a MoS2/WS2 heterostructure and not an alloyed MoxW1-xS2 film, because in the Raman spectrum of a MoxW1-xS2 alloy we would expect two main peaks at positions intermediary to the locations of the E12g and A1g peaks in pure MoS2 or WS2.42-44

The stacked MoS2/WS2 heterostructures were investigated by scanning transmission electron microscopy (STEM) imaging. Although Raman characterizations indicate that our


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sample is a heterostructure and not alloy, it is possible that the heterostructure in question could be MoS2 and WS2 grains in a single film instead of the stacked heterostructure as depicted in Figure 1(a). This could arise from incomplete coverage of the sputtered W film before Mo deposition.

Figure 3(a) shows a HAADF-STEM image of the heterostructure flake, and the

inset shows the corresponding diffraction pattern for the MoS2/WS2 flake. The diffraction pattern indicates poly-crystalline MoS2 and WS2. The average grain size from dark field TEM characterization (Supplementary Figure S1) is 10 +/- 3 nm. Compared to reference diffraction patterns of MoS2 and WS2, the diffraction pattern of the heterostructure shows that the lattice constant is similar to (or in between) those of MoS2 and WS2. Selected area diffraction patterns of MoS2, WS2, and the MoS2/WS2 heterostructure film show minimum lattice mismatch (Supplementary Figure S2). Figure 3(b) shows a TEM image of the heterostructure, which shows faint lattice fringes on top of an amorphous carbon film. Although weak, the hexagonal pattern of the fringes shows that the MoS2 and WS2 layers are horizontally oriented instead of vertically oriented.41 The Fourier transform of Figure 3(b) shows blurring of the 6-fold spots, indicating the polycrystalline nature of the MoS2/WS2. The height changes before and after sulfurization of Mo, W, and Mo/W metal stacks are characterized by atomic force microscopy (Supplementary Figure S3).

We observe minimal height change after sulfurization, which

further confirms that the molecular layers are oriented horizontal to the substrate, instead of vertical. Figure 3(c,d,e) show the EDX maps for Mo, W, and S respectively. The uniform spatial distribution of Mo and W in the flake indicates that MoS2 and WS2 are continuous films, instead of separate grains of MoS2 and WS2 existing side-by-side within a single layer. Higher magnification EDS maps (Supplementary Figure S4) further illustrate the uniform spatial distribution of Mo and W (and therefore MoS2 and WS2).


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To confirm the stacked nature of the heterostructure, cross-section TEM samples were prepared by focused ion beam (FIB). Figure 4(a) shows the high angle annular dark field (HAADF) STEM of the cross-sectioned WS2/MoS2 heterostructure. The (bottom) WS2 layer appears brighter than the (top) MoS2 layer due to Z-contrast resulting from tungsten’s higher atomic number. Figure 4(b) shows a composite EDS map of Mo and W that clearly shows that the synthesized heterostructures are composed of stacked layers of MoS2 and WS2. Figure 4(c-e) show the EDS maps for (c) Mo, (d) W, and (e) S. The thickness of MoS2 is 10.3nm and that of WS2 is 10.8nm. Using the layer thickness ~0.63nm, this corresponds to ~16 and 17 layers of MoS2 and WS2 for the MoS2/WS2 heterostructure. The FIB-prepared sample was too thick to obtain high-resolution TEM images.

For the cross-section STEM, we deliberately chose a

thicker film in order to increase the chance of observing the MoS2 and WS2 layers separately. For a stacked heterostructure of MoS2 and WS2 we anticipate rectification of current through the interface. We thermally evaporated Cr/Au electrodes on the stack to verify the rectification. The resulting I-V curve in Figure 5(a) shows the rectification behavior we would expect for a heterojunction, but the electronic behavior of the device is negatively affected by the many grain boundaries of the polycrystalline films. Electrical measurements of single-species of (a) MoS2 and (b) WS2 using the same contact metals (Supplementary Figure S5) show that the rectification is not primarily due to Schottky contacts. MoS2 and WS2 have also been studied as Hydrogen Evolution Reaction (HER) catalysts.45-46 Sulfur edges of MoS2 have shown to be catalytically active for HER. 47-48 Indeed, grown vertically oriented MoS2 films show HER behavior, as shown in Figure 5 (b), with a Tafel slope of 127mV/dec (Supplementary Figure S6) A question remains however whether horizontally aligned MoS2 may exhibit comparable HER activity. Recently horizontal, and


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continuous MoS2 films have shown HER activity,49 suggesting additional catalytic sites are possible. To answer this, we measured hydrogen evolution reaction (HER) of individual MoS2 and WS2 films with 1 nm seed-layer thickness. We show that even though our films are horizontally oriented, HER is observed. The Tafel slope for 1nm horizontally oriented MoS2 is 161 mV/dec. WS2 films show a similar behavior. The Tafel slope for horizontally oriented films is much higher than those from the vertically oriented MoS2 films, yet it is surprising that we see HER activity given the lack of the S edge sites in the horizontally oriented films. Because our films are poly-crystalline as shown by the ring diffraction patterns (Figure 3a inset), we suspect that the HER activity is coming from defect sites such as grain boundaries and sulfur vacancies. This suggests that catalytically active sites are not limited to the S edge sites. Conclusion In summary we have demonstrated a one-step synthesis route for stacked heterostructures of MoS2 and WS2 with horizontal layer orientation. The MoS2/WS2 heterostructure films were characterized by Raman spectroscopy, SEM, and cross-sectional STEM-EDX. The quick easy reaction route of MoS2/WS2 heterostructures outlined in this manuscript can be generally applied to other TMDC heterostructures. The major benefit of the presented synthetic route is the ability to pattern structures at large-scale. Current challenge is the polycrystalline nature of the TMDC layers, reflected in the rather poor rectification behavior. Further work is necessary to improve the crystalline quality such as increasing the grain size. Lastly, we show HER activity from these horizontally-oriented TMDC films, which point to defect sites such as grain boundaries being catalytically active in addition to the established S-edge sites.


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Methods Materials Growth and Substrates Preparation. The Mo and W metal seed layers each of nominal thickness of 1nm were deposited on SiO2/Si (300nm SiO2 thickness) or on carbon fiber paper (AvCarb P75T, Fuel Cell Store) (nominal thickness of 0.5nm, 1nm, 2nm, 4nm, and 7nm) by magnetron sputtering (AJA International) at a rate of 1.8nm/min.

This rate was

measured by a SQM-160Rate/Thickness Monitor and combined with the exposure time yields the nominal thickness of the metal seed films. The base pressure before sputtering was better than 10-5 Pa. Pattern seed layers were generated by electron beam lithography (Vistec EBPG 5000+) (~1000nm PMMA photoresist). The TMDC films/heterostructures were grown in a single-zone, horizontal tube furnace (Lindberg/Blue M). After multiple purge/flush cycles with Ar gas, the furnace was pumped down to a base pressure below 50 mTorr. The Ar flow at 50 S.C.C.M. brought the operating pressure to ~300 mTorr. The furnace was brought to 750⁰C with a 20 min ramp, held at 750⁰C for a 15 min dwell, and allowed to cool naturally. During the reaction sulfur precursor (Sigma Aldrich 99.998%) located upstream was at a temperature of around 300⁰C to 500⁰C. Morphological/Electrical Characterizations. The TMDC films/heterostructures were characterized by optical microscopy and Raman spectroscopy (Bruker Senterra Raman Microscope Spectrometer, 532nm illumination). The morphology and chemical composition were further characterized by SEM (Hitachi SU-87 & Hitachi SU-8230 w/ Bruker QUANTAX FlatQUAD EDX detector) and by TEM/STEM (FEI Technai Osiris200kV).

TEM sample

preparation was performed by etching away SiO2 layer with KOH and transferring the delaminated TMDC films by pipette to C/Cu TEM grids (Ted Pella). Cross-sectional TEM samples were prepared by Focus Ion Beam (FEI Fib200) at IBM’s T. J. Watson Research


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Facility. The pattern shown in Figure 5a inset schematic was created by depositing W selectively on a masked Mo film. This yielded a substrate with a region with only Mo film and another region with W film on top of Mo film.

This sample was sulfurized and electrodes were

deposited to yield a sample in the specified configuration.

For electrical characterization

(AgilentB1500A), Au electrodes (~200nm) were deposited by thermal evaporation (MBRAUN MB-EcoVap) with a Ti or Cr wetting layer (~10nm). Hydrogen Evolution Reaction Measurements.

HER catalytic measurements were

performed with a CHI 760D electrochemistry workstation (CH instruments, USA). A conventional three electrode cell configuration was employed. A saturated calomel electrode (SCE) was used as the reference electrode, and a graphite rod was used as the counter electrode. 0.5 M H2SO4 solution was used as electrolyte. Linear sweep voltammetry was recorded at scan rate of 5 mV s-1. All polarization curves were iR-corrected. The reference electrode was calibrated against the reversible hydrogen electrode (RHE). All the potentials reported in our work were converted according to E (vs. RHE) = E (vs. SCE) + 0.278 V. Author Info *Address correspondence to [email protected] Author Contributions: The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: SEM EDX mapping, AFM Acknowledgment.

Authors acknowledge support from NSF DMR 1402600. Facilities used

in this work were supported by the Yale Institute for Nanoscience and Quantum Engineering and


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National Science Foundation MRSEC DMR 1119826. Raman characterization was conducted at the Yale Institute for the Preservation of Cultural Heritage. Some SEM characterization was conducted with the assistance of Hitachi High Technologies America. We thank Rosa Zartman Goss from IBM T. J Watson Research Center for assisting in FIB usage.

Supporting Information Available: Higher magnification TEM EDS maps, MoS2 and WS2 single film electrical behavior, and Tafel plots for horizontal and vertical MoS2 films. This material is available free of charge via the Internet at http://pubs.acs.org.

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27. van der Zande, A. M.; Huang, P. Y.; Chenet, D. A.; Berkelbach, T. C.; You, Y.; Lee, G. H.; Heinz, T. F.; Reichman, D. R.; Muller, D. A.; Hone, J. C., Grains and Grain Boundaries in Highly Crystalline Monolayer Molybdenum Disulphide. Nat. Mater. 2013, 12, 554-561. 28. Yu, Y.; Li, C.; Liu, Y.; Su, L.; Zhang, Y.; Cao, L., Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-Layer MoS2 Films. Sci. Rep. 2013, 3, 1866. 29. Lee, Y. H.; Yu, L.; Wang, H.; Fang, W.; Ling, X.; Shi, Y.; Lin, C. T.; Huang, J. K.; Chang, M. T.; Chang, C. S.; Dresselhaus, M.; Palacios, T.; Li, L. J.; Kong, J., Synthesis and Transfer of Single-Layer Transition Metal Disulfides on Diverse Surfaces. Nano Lett. 2013, 13, 1852-1857. 30. Cheng, R.; Li, D.; Zhou, H.; Wang, C.; Yin, A.; Jiang, S.; Liu, Y.; Chen, Y.; Huang, Y.; Duan, X., Electroluminescence and Photocurrent Generation from Atomically Sharp WSe2/MoS2 Heterojunction p-n Diodes. Nano Lett. 2014, 14, 5590-5597. 31. Lee, C. H.; Lee, G. H.; van der Zande, A. M.; Chen, W.; Li, Y.; Han, M.; Cui, X.; Arefe, G.; Nuckolls, C.; Heinz, T. F.; Guo, J.; Hone, J.; Kim, P., Atomically Thin p-n Junctions with van der Waals Heterointerfaces. Nat. Nanotechnol. 2014, 9, 676-681. 32. Gong, Y.; Lin, J.; Wang, X.; Shi, G.; Lei, S.; Lin, Z.; Zou, X.; Ye, G.; Vajtai, R.; Yakobson, B. I.; Terrones, H.; Terrones, M.; Tay, B. K.; Lou, J.; Pantelides, S. T.; Liu, Z.; Zhou, W.; Ajayan, P. M., Vertical and In-Plane Heterostructures from WS2/MoS2 Monolayers. Nat. Mater. 2014, 13, 1135-1142. 33. Huang, C.; Wu, S.; Sanchez, A. M.; Peters, J. J.; Beanland, R.; Ross, J. S.; Rivera, P.; Yao, W.; Cobden, D. H.; Xu, X., Lateral Heterojunctions Within Monolayer MoSe2-WSe2 Semiconductors. Nat. Mater. 2014, 13, 1096-1101. 34. Lee, Y.; Lee, J.; Bark, H.; Oh, I. K.; Ryu, G. H.; Lee, Z.; Kim, H.; Cho, J. H.; Ahn, J. H.; Lee, C., Synthesis of Wafer-Scale Uniform Molybdenum Disulfide Films with Control Over the Layer Number Using a Gas Phase Sulfur Precursor. Nanoscale 2014, 6, 2821-2826. 35. Browning, P.; Eichfeld, S.; Zhang, K.; Hossain, L.; Lin, Y.-C.; Wang, K.; Lu, N.; Waite, A. R.; Voevodin, A. A.; Kim, M.; Robinson, J. A., Large-Area Synthesis of WSe2 from WO3 by Selenium–Oxygen Ion Exchange. 2D Materials 2015, 2, 014003. 36. Orofeo, C. M.; Suzuki, S.; Sekine, Y.; Hibino, H., Scalable Synthesis of Layer-Controlled WS2 and MoS2 Sheets by Sulfurization of Thin Metal Films. Appl. Phys. Lett. 2014, 105, 083112. 37. Han, G. H.; Kybert, N. J.; Naylor, C. H.; Lee, B. S.; Ping, J.; Park, J. H.; Kang, J.; Lee, S. Y.; Lee, Y. H.; Agarwal, R.; Johnson, A. T., Seeded Growth of Highly Crystalline Molybdenum Disulphide Monolayers at Controlled Locations. Nat. Commun. 2015, 6, 6128. 38. Wang, H.; Kong, D.; Johanes, P.; Cha, J. J.; Zheng, G.; Yan, K.; Liu, N.; Cui, Y., MoSe2 and WSe2 Nanofilms with Vertically Aligned Molecular Layers on Curved and Rough Surfaces. Nano Lett. 2013, 13, 3426-3433. 39. Jung, Y.; Shen, J.; Yong, S.; Cha, J. J., Chemically Synthesized Heterostructures of TwoDimensional Molybdenum/Tungsten-Based Dichalcogenides with Vertically Aligned Layers. ACS Nano 2014, 8, 9550-9557. 40. Yu, J. H.; Lee, H. R.; Hong, S. S.; Kong, D.; Lee, H. W.; Wang, H.; Xiong, F.; Wang, S.; Cui, Y., Vertical Heterostructure of Two-Dimensional MoS2 and WSe2 with Vertically Aligned Layers. Nano Lett. 2015, 15, 1031-1035. 41. Jung, Y.; Shen, J.; Liu, Y.; Woods, J. M.; Sun, Y.; Cha, J. J., Metal Seed Layer Thickness-Induced Transition from Vertical to Horizontal Growth of MoS2 and WS2. Nano Lett. 2014, 14, 6842-6849.


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Figure 1. Patterned growth of MoS2 and WS2 stacked heterostructures. (a) Schematic of TMDC growth from patterned metallic seed layers. (b) Optical image of patterned MoS2 and WS2 stacked heterostructure after sulfurization. (c) Top-down SEM image of the analyzed region. (d-f) Top-down SEM-EDS maps showing spatial distribution of (d) S, (e) Mo, and (f) W.


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Figure 2.

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Comparison of Raman spectra of MoS2, WS2, and our stacked MoS2/WS2

heterostructure. Four distinct peaks are observed from the MoS2/WS2 heterostructure, ruling out the possibility of alloying.


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Figure 3. Top-down HAADF STEM-EDS mapping of patterned heterostructure. (a) HAADF STEM image of the heterostructure flake and corresponding selected area diffraction (inset). (b) TEM image of the heterostructure flake showing lattice fringes associated with horizontally oriented TMDCs. The inset shows the FFT of the image. (c-e) Top-down STEM-EDS maps showing spatial distribution of (b) M, (c) W, and (d) S. From the maps, we confirm that TMDC films are continuous and no segregation of MoS2 and WS2 are observed.


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Figure 4. Cross-sectional scanning transmission electron microscopy of the WS2/MoS2 stacked heterostructure. (a) Cross-section HAADF STEM image of the heterostructure film.

(b)

Composite STEM-EDS map of the heterostructure film. (d-f) Cross-section STEM-EDS maps showing the spatial distribution of (d) Mo, (e) W, and (f) S.


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Figure 5. (a) Electrical behavior of MoS2/WS2 stacked heterostructure, verifying the expected rectification across the junction. (inset) Schematic of partial coverage of WS2 on MoS2 with Au electrodes. (b) HER data of MoS2 (red) and WS2 (green). Vertical (blue) MoS2 and (black) WS2 shown for comparison.


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90x39mm (300 x 300 DPI)


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WS2 Layered Heterostructures and

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