Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2

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Te-Assisted Low-Temperature Synthesis of MoS and WS Monolayers Yongji Gong, Zhong Lin, Gonglan Ye, Gang Shi, Simin Feng, Yu Lei, Ana Laura Elías, Nestor PereaLopez, Robert Vajtai, Humberto Terrones, Zheng Liu, Mauricio Terrones, and Pulickel M Ajayan ACS Nano, Just Accepted Manuscript • Publication Date (Web): 26 Oct 2015 Downloaded from http://pubs.acs.org on October 27, 2015

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Te-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers Yongji Gong1,2 †, Zhong Lin3 †, Gonglan Ye2, Gang Shi2, Simin Feng3, Yu Lei4, Ana Laura Elías3, Nestor PereaLopez3, Robert Vajtai2, Humberto Terrones5, Zheng Liu6, Mauricio Terrones3,4,7*, Pulickel M. Ajayan1,2 * 1 Department of Chemistry, Rice University, Houston, TX 77005, USA 2 Department of Materials Science & NanoEngineering, Rice University, Houston, TX 77005, USA 3 Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA 4 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA 5 Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Johnson-Rowland Science Center, 110 Eighth Street, Troy, NY 12180, USA. 6 School of Materials Science and Engineering, School of Electrical and Electronic Engineering, Nanyang Technological University, 639798, Singapore 7 Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA † Y.G. and Z.L. contributed equally to this work * Corresponding authors: [email protected]; [email protected]

ABSTRACT Chemical vapor deposition (CVD) is a scalable method able to synthesize MoS2 and WS2 monolayers. In this work, we reduced the synthesis temperature by 200 oC only by introducing tellurium (Te) into the CVD process. The as-synthesized MoS2 and WS2 monolayers show high phase purity and crystallinity. The optical and electrical performance of these materials is comparable to those synthesized at higher temperatures. We believe this work will accelerate the industrial synthesis of these semiconducting monolayers.

KEYWORDS: two-dimensional materials; transition metal dichalcogenides; molybdenum disulfide; tungsten disulfide; chemical vapor deposition.

The research on graphene over the past decade has triggered explosive interest in two-dimensional and van der Waals solids such as hexagonal boron nitride and transition metal dichalcogenides (TMDs).1-4 TMDs exhibit a variety of electronic properties, ranging from semiconducting to superconducting depending on the elemental composition and the crystal structure. Among all the few-layered TMDs, the semiconducting members such as MoS2, and WS2 have so far received more attention due to their novel properties associated with reduced dimensionality such as enhanced photoluminenscence,5-7 valley polarization8 and second harmonic generation.9-12 Unlike graphene, which shows limitations due to its zero band gap and inert chemical properties, semiconducting TMDs (sTMDs) possess a sizeable gap and versatile chemical properties, thus opening opportunities for numerous applications in electronics and optoelectronics.13-18 Recently, the band gap

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tunability has been tested via defect engineering,19-20 alloying21-26 and layer-by-layer stacking.27-31 Considerable efforts have been devoted to the synthesis of sTMD thin layers by both top-down and bottom-up approaches.1,

7, 32-50

Due to the weak interlayer van der Waals interactions, mono- and few-layers can be

obtained by mechanical exfoliation,1, 32 and liquid-based exfoliation.33-34 Chemical vapor deposition (CVD) has also been developed to achieve scalable synthesis of MoS2 and WS2 atomic layers with controlled thickness.7,

35-45

Thus far, deposition of micron-sized disulfide monolayers generally requires a high

temperature at around 700-800 oC.7, 38 This high temperature poses strong limitations for applying CVD grown sTMDs. For example, similar to graphene, sTMD monolayers are flexible, and could be used in flexible electronics platforms if they can directly be grown on suitable substrates.51 However, flexible substrates such as polymers normally cannot withstand high temperatures. Another potential application of sTMDs monolayers is to complement/substitute silicon is electronics.52 Therefore, the temperature to grow sTMD layers should also be lower and compatible with established silicon-based fabrication technologies. Industrial applications require reducing the synthesis temperature without sacrificing the material crystallinity. This challenge is nontrivial since a lower growth temperature generally results in smaller grain size or lower degree of crystallinity. In order to address this issue, a deeper understanding about CVD growth and new strategies are needed. Three steps are generally involved in the CVD synthesis of sTMD monolayers. First, the transport of transition metal precursors; second, the sulfurization of transition metal precursors by either S vapor, or H2S; and lastly the crystal nucleation followed by lateral expansion of crystalline domains. Some attention has been paid to facilitating the crystal nucleation by seeding or patterning,37, 45 but little progress has been achieved regarding the metal transport and sulfurization processes. For example, the synthesis of polycrystalline MoS2 layers can be carried out by sulfurizing Mo films thermally deposited on an insulating substrate.36 However, to the best of our knowledge, bulk W powder as a metal precursor has not been directly used to produce WS2 monolayers due to its higher melting point (3422 oC), and difficulty to vaporize W powders inside the CVD tube. Therefore, a new strategy is necessary to reduce the metal transport temperature so that new options will be available for transition metal precursors. The sulfurization process constitutes another key issue that requires understanding of the underlying chemistry and still needs further optimization. Sulfurization of MoO3, for example, requires a temperature as high as 700oC.38 Thermal energy from an external source is necessary 2


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for this reduction reaction because sulfur exhibits a lower electron affinity than oxygen. The discrepancy in electron affinity between oxygen and chalcogen precursors leads to an energy barrier that needs to be overcome during the chemical reaction. The even weaker electron affinity of selenium makes direct selenization of MoO3 challenging unless H2 is introduced to form an intermediate MoO3-xHy phase.50 In this work, we demonstrate that when the metal transport and sulfurization processes are carefully addressed, new precursors can be employed and the synthesis conditions can be further relaxed. In particular, after introducing Te powders into the reaction chamber, the synthesis of MoS2 and WS2 can be carried out at 500 oC, which constitutes a temperature reduction of at least 200 oC. Furthermore, with the assistance of Te powders, bulk W metal powders can be directly used as a solid precursor. The resulting WS2 and MoS2 monolayers also exhibit high optical and electrical performance. RESULTS AND DISCUSSION Using Te to control the growth morphology of disulfides was introduced in our previous work on the synthesis of MoS2/WS2 heterostructures,31 in which Te was added for the purpose of slowing down the deposition of WS2, so that phase segregated MoS2/WS2 heterostructures are obtained, instead of MoxW1-xS2 alloys. In the previous work, we demonstrated that WS2 monolayers can be grown either via lateral covalent epitaxy from fresh edges of MoS2, or via vertical van der Waals (vdW) epitaxy on top of freshly deposited MoS2.31 However, it remains unclear if using mixed Te/W precursors could result in high crystalline quality WS2 monolayers directly grown on amorphous substrates (such as SiOx), without using MoS2 seeding. Along this line, Figure 1(a) shows a schematic representation of the Te-assisted synthesis approach of WS2 crystalline monolayers. In particular, W powders with Te powders were physically ground with a mortar. We observed that monocrystalline WS2 monolayers with lateral sizes up to 30 µm are obtained (see Figure 2(a)) at 500 oC, a temperature much lower when compared to that of CVD syntheses using WO3 precursors (800 oC).7 We note that adding Te is necessary to grow WS2 monolayers at such a low temperature. In a control experiment, we sulfurized micron-sized W powders at 500 oC without Te. In this case, micron-sized WS2 powders instead of WS2 monolayers were obtained (see supplementary Figure S1). This result is a consequence of the high melting point of W. Although in situ monitoring of a deposition process remains extremely challenging, the control experiment together with the Te-W binary phase diagram suggests the following growth mechanism: 3


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When Te powders are mixed with W powders, Te melts at a relatively low temperature (~450 oC) and a small fraction of W powders dissolve into Te (see shaded region in Figure 1(b) and supplementary Figure S2).53 In a sulfur-rich reaction tube, WS2 crystals precipitate from Te agglomerates and grow on SiO2 substrates. Note that W reacts in molten solution rather than in a vapor form, so the term CVD may not strictly apply to this case. Interestingly, in the final reaction product Te phases are not detected by X-ray photoelectron spectroscopy (XPS) (see Figure 1(c)). This observation is consistent with previous results reported in the literature in which MoTexS2-x and WTexS2-x ternary alloys are thermally unstable.54 The W:S atomic ratio determined by XPS is 1:2.05, which is fairly close to its nominal stoichiometry. By introducing Te in the reaction chamber it is also possible to relax the synthesis conditions for the metal oxide CVD. For example, as shown in Figure 1(d), MoO3 bulk powder is loaded into the center of a quartz tube and Te powders are distributed on a SiO2 substrate next to the MoO3 particles. The synthesis is carried out at 500 oC, and MoS2 crystalline monolayers are formed on the substrate (see Figure 2(b)). According to the XPS data (see Figure 1(f)) the addition of Te powders does not leave detectable residues in MoS2 monolayers. The atomic ratio of Mo:S is 1:1.97. It has been shown in the literature that a temperature higher than 700 oC is needed to fully sulfurize MoO3 in the absence of Te.38 It seems that in our synthesis, Te is involved in a metastable reaction intermediate and then leaves the final product. In order to verify this conjecture, we carried out a control experiment: A MoO3/Te mixture supported by a SiO2 substrate is heated up to 500 oC without adding any S powders, and then quenched to room temperature. We examined the composition of the residue deposits on SiO2 by XPS (see supplementary Figure S3), and noted that the residue has a composition of TeMo5.3O15.6, which is close to the TeMo5O16 reported in the literature.55 The residue deposits were then loaded back to the CVD chamber for sulfurization at 500 oC. Upon sulfurization, MoS2 triangular monolayers were then obtained (see supplementary Figure S3). This control experiment indicates the importance of Te in the low temperature synthesis by the formation of an intermediate Te containing phase. Since MoSxTe2-x alloy is unstable in a sulfur rich environment (see Figure 1(e)),54 Te leaves the reaction product and is carried away by Ar flow to the downstream. The WS2 and MoS2 monolayers mainly grow as triangular islands with domain sizes up to tens of micrometers (see Figure 2(a) and 2(b)). Secondary layers can also be observed in the center of some triangles, thus 4


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indicating the growth of islands is induced by nucleation. The monolayer nature of these islands is directly confirmed by atomic force microscopy (see Figure 2(c) and 2(d)). The thickness of as-grown WS2 monolayers is 0.8 nm, which is consistent with both WS2 monolayers obtained by exfoliation and higher temperature CVD using WO3 precursors.7 In addition, the sample surface is smooth (surface roughness ~0.3 nm), indicating that the introduction of Te powders does not leave residues between WS2 monolayers and the substrates. The crystallinity, optical and electronic properties of the as-synthesized MoS2 and WS2 monolayers are further examined. As-grown WS2 and MoS2 monolayers can then be transferred onto Transmission Electron Microscope (TEM) grids using wet chemical methods. Figures 2(e) and 2(f) show high resolution TEM images of MoS2 and WS2 monolayers exhibiting the hexagonal lattice. The monocrystalline nature of the monolayers is confirmed by Fast Fourier Transformations of TEM images. Therefore, it is clear that the introduction of Te into the reaction does not degrade/affect the crystallinity of the as-synthesized MoS2 and WS2 monolayers. We then examined the optical performance of the synthesized materials. Figure 3(a) shows Raman spectra of WS2 monolayers acquired with two different laser excitations. When exciting with the 488 nm laser line, the in-plane vibrational mode (E’) and the out-of-plane mode (A’1) are the most prominent. Considering that WS2 monolayers do not have inversion symmetry (point group of D3h), the Raman notations of monolayers should be carefully distinguished from their bulk counterpart (point group of D6h), and flakes with even number of layers (point group of D3d).56 The second order longitudinal acoustic mode 2LA(M) is weak with the 488 nm laser illumination, but it is resonant when a 514.5 nm laser is used; its intensity unambiguously indicates the monolayer nature of WS2 triangles.57 Several higher order Raman modes and combinational modes are also identified and indexed by employing multi-peak Lorentzian fitting. The main features shown in the Raman spectrum of WS2 monolayers synthesized in this work are essentially identical to those of monolayers synthesized in previous works at higher temperatures and without Te.57 We neither find new modes associated with WTe2 nor notice apparent shifts in the frequencies of WS2 related Raman modes. In addition, the LA(M) mode is a defect-activated Raman mode since the M point lies on the edge of the Brillouin zone (see the inset of Figure 3(a)).58 The intensity of this mode indicates average density of structural defects.59 Compared to previous results,57 the addition of Te does not enhance the LA(M) mode. A typical photoluminescence (PL) spectrum of WS2 monolayer is shown in Figure 3(b). A single and intense 5


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PL peak located at 1.98 eV associated with A1 exciton is observed, consistent with the direct band gap of WS2 monolayers.7 Notably, the full width at half maximum (FWHM) of the PL peak is ~43 meV, comparable to the peak widths (~42-68 meV) reported by earlier works using other synthesis methods and onto comparable substrates.7 The PL studies presented here clearly confirm that as-grown WS2 monolayers starting from W/Te precursors exhibit an optical quality comparable to higher temperature CVD grown samples, and do not suffer any drawback for using Te. The optical uniformity of WS2 triangular islands is further examined. The triangle shown in Figure 3(c) was mapped, and the corresponding Raman and PL intensity maps are depicted in Figure 3(e) and 3(f), respectively. For clarity, Raman and PL intensity profiles along the shown arrow in Figure 3(c) were also acquired (see insets of Figure 3(e) and 3(f)). Although the Raman intensity of the E’ mode is uniform across the entire triangular domain, the PL intensity along the edges of triangles is stronger than in the center, possibly due to local chemistry at the edge.7 However, in a typical PL mapping, a laser beam with a spot size of ~1 µm is scanned over the sample surface. The step size during mapping is typically a few hundred nanometers, which limits the spatial resolution of PL intensity maps. To better visualize the edge effect, fluorescence imaging is also performed. The fundamental principles of fluorescence microscopy are similar to PL microscopy, but fluorescence microscopy allows the acquisition of ‘snapshots’ with a higher spatial resolution. As shown in Figure 3(d), the fluorescence image of WS2 monolayers clearly shows the edge effect, with a fluorescence enhancement localized at the triangle’s edge. The addition of Te also enables the growth of extra-large MoS2 triangles with multiple ad-layers. The synthesized MoS2 triangles were observed to have a broad size distribution, ranging from a few to hundreds of micrometers. Small monocrystalline triangles show higher uniformity in terms of thickness, as confirmed by PL mappings (see Figure 4(a)). When increasing the size of triangles, thicker layers appear inside the triangular domains as a common by-product of the nucleation induced growth. Figure 4(b) shows an extreme case in which many ad-layers embed into a large (~100 µm) MoS2 triangular monolayer. The monolayered region shows characteristic Raman modes E’ and A’1 with a frequency difference of 21 cm-1 (see Figure 4(d)), consistent with previous results.23, 36 In the few-layered region, the Raman intensity of MoS2 modes increases with respect to the Si peak due to a screening effect, and the separation between these two modes changes to 6


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23 cm-1 in response to an increase in vdW interactions and reduced Coulomb interactions.60 When compared with monolayers, the few-layered regions show suppressed fluorescence (see Figure 4(c)) and very low PL intensity (see Figure 4(e)), as a result of transitions from direct to indirect band gap semiconductors as the samples thicken.5 Since ad-layers grow with various orientations within a single monolayer matrix, this work provides a new platform to study few-layered TMDs. In particular, the relatively large lateral size of the adlayers (~3 µm) and a clean interface between the matrix and ad-layers make this material especially suitable for studies of stacking order in few-layered TMDs.61-62 The large size of the resulting monolayer triangles further allows the fabrication of diverse device structures to probe the electronic properties of these semiconducting TMDs. Figure 5(a) shows an optical micrograph of MoS2 monolayers consisting of a Hall bar structure with a channel width and length of ~5 µm and 30 µm, respectively. The inset shows a schematic side view of the field effect transistor showing the different layers of the device. Figure 5(b) depicts the drain current as a function of the gate voltage. The inset also displays the linear IV plots of the drain and reveals the change of the channel conductivity as the gate voltage increases. The on-off ratio reaches 105. Considering the dimensions of the device and the dielectric SiO2 layer (285 nm), the mobility in this device was 4.5 cm2V-1S-1, similar to values (6.0 cm2V-1S-1) reported for MoS2 grown at 850 o

C.63 An additional advantage of the low temperature growth of TMDs is related to the preservation of the SiO2

layer integrity. In conventional TMD synthesis at 700-850 ᵒC, it has been frequently observed that “pinholes” appear in the SiO2 layer, thus destroying its dielectric capability. The dielectric nature of the SiO2 was preserved in all the tested devices, when using the Te-assisted growth approach. CONCLUSION To summarize, we have developed a Te-based strategy to synthesize crystalline WS2 and MoS2 monolayers. Introducing Te into the synthesis relaxed the reaction conditions without leaving unwanted residues. The asgrown monolayers show good optical characteristics and electrical performance comparable to samples grown by CVD at higher temperatures and onto similar substrates. In 2010, it was reported that CVD synthesis of graphene could be carried out at a temperature as low as 325 oC.64 The low-temperature synthesis of graphene constitutes an essential step towards its commercialization. In this work we show that sTMDs can be synthesized at a reduced temperature of ~500 oC. Our work may, therefore, promote the industrial interest of 7


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these emerging 2D semiconductors. METHODS Material synthesis. A tellurium-assisted chemical vapor deposition (CVD) method has been developed to synthesize MoS2 or WS2 atomic layers at a lower temperature under atmospheric pressure in a quartz reaction tube (inner diameter ~ 5 cm). For WS2 synthesis, mixed W and Te powders were scattered on a SiO2/Si substrate. While for MoS2 growth, MoO3 powders were loaded into the center of the tube. SiO2/Si substrate with Te powder was placed slightly downstream of the MoO3 powder. The quartz tube was heated up to 500 oC at a rate of 50 oC/min, and was then kept at 500 oC for 15 min before cooling. The sulfur powder was loaded

at the upstream zone inside the same quartz tube with a temperature of about 200 °C. 100 sccm argon is used as protection from oxidation and carrier gas during growth. Sample transfer and TEM characterization. The transfer process was performed by a poly (methyl

methacrylate) (PMMA) assisted method. 2M KOH was used to etch the SiO2 layer after PMMA was spincoated on the sample. The lifted off PMMA/sample layer was then transferred to the TEM grid. Lastly, the PMMA was washed away by acetone and isopropanol. JEOL-2100 was used to characterize the sample with a voltage of 200 KV. AFM measurements. The AFM measurements were performed using the tapping mode, with an Asylum Research MFP-3D AFM. Raman and PL measurement. Raman and PL spectra acquisition was performed with an inVia confocal Renishaw Raman spectrometer. Samples were mounted under a 100× objective and excited by either 488 nm or 514.5 nm laser light. Fluorescence imaging. A Carl Zeiss Axio Imager microscope with a 50× objective was used for fluorescence imaging. The as grown MoS2 and WS2 triangles were optically excited by green incident light and emitted red fluorescent light. Device fabrication and electrical measurements. Conventional e-beam lithography was used to etch a hall

bar shape from a monolayered MoS2 triangle and also to selectively deposit the metal contacts, consisting of 30 nm gold on 3 nm titanium by electron beam evaporation. Before electrical measurements, the FET

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device was annealed in vacuum for 2 h at 120 oC.

Conflict of interest: The authors declare no competing financial interest. Acknowledgement. PMA and MT acknowledge the financial support from the U.S. Army Research Office under MURI ALNOS project No. W911NF-11-1-0362. MT also thanks Penn State Center for Nanoscale Science for the seed grant on 2D Layered Materials (DMR-0820404), and Center for 2-Dimensional and Layered Materials at The Pennsylvania State University. ALE and HT acknowledge support from the

National Science Foundation EFRI 2-DARE grant No. 1433311. We thank Ethan Kahn for his valuable comments on the manuscript. Supporting information available. Raman, SEM and XPS data related to the growth mechanism of sTMD monolayers. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. (a) Schematics of Te-assisted CVD synthesis of WS2 monolayers; (b) binary Te-W phase diagram.53 The reaction conditions are indicated by a shaded area in the phase diagram. Te exhibits a melting point around 451 oC. At around this temperature, W metal can slightly dissolve into Te. L stands for liquid; (c) XPS of WS2 monolayers; (d) Schematics of Te-assisted CVD synthesis of MoS2 monolayers; (e) binary Te-S phase diagram.53 Within the region marked in the phase diagram, Te and S could form a liquid solution. L stands for liquid; (f) XPS of MoS2 monolayers.

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Figure 2. (a) and (b) Optical images of WS2 and MoS2 monolayers grown on SiO2 substrates; (c) AFM topography of a WS2 monoalyer; (d) shows the corresponding height profile along the red line; (e) and (f) High-resolution TEM images of WS2 and MoS2 monolayers. The Fast Fourier Transformations (FFT) are shown as insets.

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Figure 3. Optical characterization of WS2 monolayers. (a) Typical Raman spectra of WS2 monolayers measured with 488 nm and 514.5 nm laser excitations. Active Raman modes are assigned based on multi-peak Lorentzian fittings; (b) Typical PL spectrum of WS2 monolayers measured with the 488 nm laser. The PL intensity is normalized with respect to the A’1 Raman mode; (c-f) Optical image, fluorescence image, Raman intensity mapping and PL intensity mapping of a WS2 triangular monolayer. Raman and PL line scanning is performed along the arrow indicated in (c), and the corresponding intensity profiles are shown as insets in (ef). The 488 nm laser is used for acquiring the mapping and the line scan. The scale bar is 2 µm.

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Figure 4. Optical characterization of MoS2 monolayers. (a) PL peak intensity mapping of a relatively small (~5 µm) MoS2 triangular monolayer; (b) Optical and (c) fluorescence images of a large (~100 µm) MoS2 triangle. The darker regions inside the large triangle correspond to few-layers; (d) Raman spectra of a large MoS2 triangle acquired from a monolayer and a few-layer regions. The Raman intensity is normalized with respect to Si peak at 520.5 cm-1, and (e) PL spectra of the large MoS2 triangle recorded from a monolayer and a few-layer regions. The PL intensity is normalized with respect to the A’1 Raman mode. The 488 nm excitation is used for Raman and PL characterizations.

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Figure 5. (a) Optical micrograph of the MoS2 device constructed on SiO2/Si used to probe the electrical properties of these TMDs synthesized at a low temperature. The inset shows a schematic of the different layers and terminals of the device; and (b) Transport characteristic curve of the back gated MoS2 FET when the drainsource voltage (VDS) was fixed at 0.5 V. The inset displays the linear drain IV plots for small VDS values and shows the strong effect of the gate voltage on the channel conductivity.

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Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2

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