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Stable 1T Tungsten Disulfide Monolayer and Its Junctions: Growth and Atomic Structures Yung-Chang Lin,† Chao-Hui Yeh,§ Ho-Chun Lin,∥ Ming-Deng Siao,§ Zheng Liu,⊥ Hideaki Nakajima,‡ Toshiya Okazaki,‡ Mei-Yin Chou,∥ Kazu Suenaga,†,# and Po-Wen Chiu*,§,∥
ACS Nano 2018.12:12080-12088. Downloaded from pubs.acs.org by EASTERN KENTUCKY UNIV on 01/11/19. For personal use only.
†
Nanomaterials Research Institute and ‡CNT-Application Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan § Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan ∥ Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan ⊥ Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan # Department of Mechanical Engineering, The University of Tokyo, Tokyo 113-8656, Japan S Supporting Information *
ABSTRACT: Transition-metal dichalcogenides in the 1T phase have been a subject of increasing interest, which is partly due to their fascinating physical properties and partly to their potential applications in the next generation of electronic devices, including supercapacitors, electrocatalytic hydrogen evolution, and phase-transition memories. The primary method for obtaining 1T WS2 or MoS2 has been using ion intercalation in combination with solution-based exfoliation. The resulting flakes are small in size and tend to aggregate upon deposition, forming an intercalant−TMD complex with small 1T and 1T′ patches embedded in the 2H matrix. Existing growth methods have, however, produced WS2 or MoS2 solely in the 2H phase. Here, we have refined the growth approach to obtain monolayer 1T WS2 up to 80 μm in size based on chemical vapor deposition. With the aid of synergistic catalysts (iron oxide and sodium chloride), 1T WS2 can nucleate in the infant stage of the growth, forming special butterfly-like single crystals with the 1T phase in one wing and the 2H phase in the other. Distinctive types of phase boundaries are discovered at the 1T−2H interface. The 1T structure thus grown is thermodynamically stable over time and even persists at a high temperature above 800 °C, allowing for a stepwise edge epitaxy of lateral 1T heterostructures. Atomic images show that the 1T WS2−MoS2 heterojunction features a coherent and defectless interface with a sharp atomic transition. The stable 1T phase represents a missing piece of the puzzle in the research of atomic thin van der Waals crystals, and our growth approach provides an accessible way of filling this gap. KEYWORDS: chemical vapor deposition, transition-metal dichalcogenides, phase transition, heterojunctions, 1T WS2 distortion.1,2 Driving the phase transition from the 2H to 1T phase, however, requires a considerable energy.3 This energy constraint holds the monolayer WS2 and MoS2 grown by chemical vapor deposition (CVD) solely in the 2H phase.1,4 The overwhelming majority of prior reports on 1T WS2 or MoS2 have used chemically exfoliated films grown by chemical vapor transport.5,6 Other attempts to engineer the phase include substitutional rhenium doping,7 electron-beam irradiation,8 plasmonic hot electron transfer,9 alloying,10 and
T
ungsten disulfide (WS2) and molybdenum disulfide (MoS2) are the two most intensively studied layered materials in the transition metal dichalcogenide (TMD) family. They usually appear in the 2H phase, where metal atoms are characterized by a trigonal prismatic geometry and sandwiched between two sublayers of S atoms in an eclipsing configuration (Figure 1a). A shift of the top or bottom S sublayer gives rise to another polymorph known as the 1T phase, which exhibits a tetragonal symmetry and corresponds to an octahedral coordination of the metal atoms (Figure 1a). Density functional theory calculations on the freestanding monolayer indicates that the 1T structure is unstable in the absence of external stabilizing influences, transforming into the 1T′ phase through a dimerization © 2018 American Chemical Society
Received: July 1, 2018 Accepted: December 7, 2018 Published: December 7, 2018 12080
DOI: 10.1021/acsnano.8b04979 ACS Nano 2018, 12, 12080−12088
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Figure 1. (a) Atomic model of 2H and 1T WS2. (b) Calculated formation energy of 2H and 1T phase with (green curve) and without (purple curve) the adsorption of alkali metal atoms. (c) Schematics of the experimental setup of halide-assisted chemical vapor deposition. (d) SEM image of monolayer WS2 grown on sapphire. Butterfly like grains are highlighted by yellow-dotted circles. (e) SEM images of butterfly-like grains in different joint angles. Scale bars are 10 μm. (f) XPS survey spectrum of WS2 nuclei formed in the early stage of nucleation in the halide-assisted CVD. The inset highlights the signal of Na on the WS2 nuclei. (g) XPS spectra of the W4+4f(5/2,7/2) peaks of the 1T and 2H WS2.
electrostatic gating.2,11 The phase transition mediated by charge doping is enabled through an appreciable reduction of the energy barrier, as exemplified in Figure 1b for the adsorption of Na atoms on WS2. It involves a collective motion of a group of S atoms and a displacive transformation. The 1T structure thus formed is found to be unstable and small in size (a few to tens of nanometers). Although it even persists after removal of doping, it often turns into a distorted 1T′ state upon aging or heating, forming a mixture of 2H, 1T, and 1T′ domains.6,12,13 Here, we report an energetically accessible pathway for the direct growth of 1T WS2 monolayers and lateral junctions. To this end, a proper metal oxide, along with an alkali metal halide, is introduced as a synergistic catalyst in the lowpressure CVD process, rendering the nucleation of 1T seeds in the early stage of the growth feasible. Extended edge epitaxy following the same phase structure continues until the tungsten precursor becomes deactivated or exhausted. Pure 1T WS2 with a large domain size of ∼50−80 μm can be thus formed on sapphire. Figure 1c depicts the schematic setup of the CVD system, in which a catalytic metal oxide in 3 wt % is uniformly mixed with solid WO3 precursor and placed in the close vicinity of the growth substrate. Use of different catalytic metal oxide results in a distinctive shape of WS2 grains under the same growth conditions. For example, nickel oxide or chromium oxide produce only hexagonal or truncate triangular grains (Figure S1), whereas iron oxide or indium oxide promotes the formation of butterfly like grains in addition to the typical triangular ones (Figure 1d,e). Interestingly, the WS2 butterflies consist of two symmetrical wings with one side in the 1T phase and the other in the 2H phase, separated by a sharp phase boundary. The crystalline orientation of the two
phases is independent of each other, yielding a random angle between the two wings. All butterflies are almost-uniform monolayers except for small bilayer or multilayer patches at the core of the butterflies (Figure S2).
RESULTS AND DISCUSSION To obtain 1T WS2 using the CVD method, both the alkali metal halide and the catalytic metal oxides are critical. Through the mixing of the solid WO3 precursor with Fe3O4 or In2O3 powder, the growth was found to undergo dramatic suppression. Controlling alkali metal halide in the reaction, which also acts as an effective transport agent, can promote the growth due to the formation of volatile metal-oxyhalide species and alkali metal-containing intermediate ternary.14−16 Here, we use sodium chloride as the growth promoter. Other alkali metal halides such as KBr and KI can act similarly but produce less 1T/2H butterflies, even at an optimal growth temperature. It should be noted that, unlike typical halide-assisted CVD growth of WS2.14−17 our NaCl grains are placed in a separate crucible, making independent control over vapor pressure feasible. This setup also reduces water vapor residue in the reaction chamber, facilitating the lateral growth of nuclei while suppressing nucleation.17 For chemical-state analysis, micro-XPS with a spot size of ∼20 μm was performed. Figure 1f depicts the survey spectrum of nuclei formed in the early stage of nucleation, while Figure 1g shows the W 4f core level spectra on each individual wings of a butterfly. For one wing, two prominent peaks appear at 33.1 and 35.2 eV, which are assigned to the W4+4f7/2 and W4+4f5/2 components, respectively. The peak positions correspond to a trigonal prismatic coordination of the W atoms and are nearly identical with those of 2H WS2 grown by 12081
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Figure 2. (a) Low-magnification STEM image of a single-layer WS2 butterfly. The red dotted line indicates the boundary between the left and right domains. The nucleation core is marked by a red circle. (b) An ADF image of the WS2 butterfly at the boundary taken from the yellow rectangular in panel a. The left domain shows the 2H phase, while the right domain shows the 1T phase. The phase boundary is indicated by a red dotted line. (c) The ADF intensity profile along the green dotted line in panel b. The two overlapped S atoms in the 2H phase shows ca. 2.2 times higher intensity than the two misaligned S atoms in the 1T phase. (d) A high-magnification ADF image of the 2H phase. The overlapped S atoms around a W atom are highlighted by a red triangle. (e) A high-magnification ADF image of the 1T phase. A total of six S atoms around a W atom are highlighted by a yellow hexagon. Scale bars are 0.25 nm. (f, g) The simulated STEM images of 2H and 1T WS2 corresponding to panels d and e, respectively. (h) The ADF image of the overgrown trilayer WS2 near the nucleation core, sharing the phase boundary with the monolayer region (red dotted line). (i) Local area (surrounded by cyan dotted lines) phase transformation from the 1T to the 2H phase after 3 min of e-beam scanning.
conventional CVD.12 For the other wing, this doublet peak is shifted to 32.2 and 34.4 eV. The large down-shift of the binding energies by ∼0.9 eV indicates the different coordination geometry, similar to the values observed in chemically exfoliated 1T WS2.18,19 Atomic force microscopy (AFM) and Raman spectroscopy characterizations were also applied to the WS2 butterflies and provided in Figures S3 and S4. No apparent morphology difference exists between the two wings of the butterfly in the topological image. However, the 2H and 1T phases show nearly identical Raman features, while the 1T′ phase appears to have additional Raman features at low energy (30 nm) and straight domain boundary with a tilt angle of ∼19.8° in reference to the zigzag edge, as shown in Figures 3g and S11b. In our experiment, we found that the original 1T/2H phase boundary is quite stable and shows no migration after the phase transformation at high temperature. The CVD grown 1T/2H phase boundary often contains various kinds of defects (like octagonal defects shown in Figure 3). These irregular defect structures can be the obstacles to limit the boundary migration. Lateral epitaxial TMD heterojunctions have been recently demonstrated and found to exhibit extraordinary optical and electronic properties.29−31 However, the lateral growth of 1T heterojunctions remains a challenge due to the poor stability of the 1T structure. Here, we explore the epitaxial growth of MoS2 along the edge of 1T and 2H phases of WS2 butterflies using a two-pot CVD growth, as conceptually illustrated by an atomic model in Figure 4a. The experimental results not only shed light on the thermal stability of the CVD-grown 1T structure but also convey rich information on the 1T−1T heterojunctions. Figure 4b shows the temperature−time diagram of the two-step growth, in which WS2 butterflies are first grown at 850 °C in one CVD system on sapphire, followed by a cooling and transfer process to the second system for extended growth of MoS2 at 800 °C. In the second
CONCLUSIONS The growth strategy used here follows an energetically accessible pathway to obtain 1T monolayer of W- and Mobased TMDs. Unlike the 1T structure reported previously, the CVD-grown 1T structure can be large and pure without inclusion of any other phases. Our experimental findings provide an uncommon insight into 1T WS2, which subverts our understanding of its structural stability. This result demonstrates a route through which to derive the 1T structure of other TMD materials by means of edge epitaxy. However, other intriguing properties of the 1T phase, which makes this structure of particular interest in realizing the promise of atomically thin electronic and hydrogen evolution devices, are out of the scope of current work and need to be further explored and revisited. Following the successful growth of 1Tphase WS2 and MoS2 by means of CVD method, a large variety 12085
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multiplying CCD detector (Andor) through a grating with 1800 grooves per millimiter. The CCD integration time was 10 s for all spectra. STEM Characterization and Simulation. STEM images were acquired by using JEOL ARM-200F-based ultrahigh-vacuum microscope equipped with a dodecaple Δ corrector and a cold-field emission gun operating at 60 kV. The probe current is about 28−32 pA. The convergence semi-angle is 37 mrad. and the inner acquisition semi-angle is 53 mrad. Image acquisition time is 38.5 μs for 1024 pixels × 1024 pixels imaging. The STEM simulation is performed by using MacTempas software. Computational Method. We have performed first-principles calculations using the Vienna ab initio simulation package (VASP)33−35 with the projector augmented wave method.36,37 A plane-wave energy cutoff of 400 eV and a 24 × 24 × 1 k-mesh were employed. We used the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof functional38 for the structural optimization of freestanding monolayer WS2 with a 17-Å vacuum in the supercell method. The resulting lattice constants are 3.18 Å for both 2H and 1T (unrelaxed) structures. The energy barrier between the 2H and 1T structures for pure WS2 was determined by utilizing the image nudged elastic band (NEB) method.39 Because the 1T structure is unstable in the calculation, we fixed tungsten and relaxed sulfur in the NEB study. For WS2 with adsorbed Na atoms, we used the image coordination of pure WS2 and put sodium atoms on top of the hexagonal centers in WS2. The positions of sodium atoms are optimized in the energy calculation for each image.
of ongoing experiments have been carried out to verify the intrinsic properties of the CVD-grown 1T phase as compared with those of the chemically derived 1T phase, which are embedded in a mixture of a 1T’- and 2H-phase matrix. Some interesting follow-up experiments include: (1) study to directly correlate the electronic and optical property of the 1T phase to the atomic structure by STEM experiments; (2) an elaborate impurity analysis and application of this growth mechanism to search for the 1T phase of other TMD materials; (3) study to understand how the substrate (Al- and O-termination) affects the stability of the 1T phase; (4) study to investigate the transport property of 1T heterojunction that has not been reported but may possess prolific applications in the future; and (5) study to implant various kinds of dopants into the 1T/ 2H or 1T/1T heterostructures to modulate their properties for device applications.
METHODS WS2 Growth. 1T/2H WS2 butterflies are grown inside a singlezone, 12 in. horizontal clam-shell tube furnace equipped with a 1 in. diameter quartz tube. The precursors are tungsten(VI) oxide (WO3, from Sigma-Aldrich) and elemental sulfur powder (from SigmaAldrich) with purity of 99.8% and 99.98%, respectively. A total of 1 mg of NaCl with purity of 99.5% is used as a transport agent and a catalyst in the growth. The solid precursor WO3 (300 mg) is mixed uniformly with Fe2O3 powder (10 mg) and placed on a quartz plate close to the center of the furnace. The choice of metal oxides such as Fe2O3 or In2O3 is inspired by the finding that Fe or In atoms can cause an appreciable doping effect to the WS2 layers in the transport measurements in combination with STEM inspections. We use polished sapphires as the growth substrates, which were sequentially cleaned by sonication in acetone, isopropyl alcohol, and deionized water. The cleaned sapphire substrate are placed next to the WO3/ Fe2O3 mixture. NaCl grains and sulfur powder are placed at the upstream side of the furnace at carefully adjusted locations to set the temperature. Prior to growth, the tube is purged with argon gas and then pumped to a base pressure of 0.1 Torr. Subsequently, the center of the furnace is raised to reaction temperature of 850 °C in 40 min, and the sulfur precursor and NaCl grains are kept at ∼170 °C and ∼600 °C, respectively. The growth time lasts 20 min, during which 200 sccm argon and 15 sccm hydrogen gases are constantly flowing through the tube. Cooling of the CVD furnace proceeds in two steps: initially the furnace temperature is ramped down from the maximum temperature to 600 °C at a rate of 0.3 °C/s with a constant argon flow of 100 sccm. The second step ramps the furnace down from 600 °C to room temperature at a rate of 3 °C/s. In both cooling steps, the temperature at the furnace center is controlled by the gap size of the opened clam shell. In the second cooling step, the process tube is even cooled with the aid of a fan. WS2/MoS2 Heterostructures. 1T−1T and 2H−2H WS2/MoS2 heterojunctions are grown in a two-step CVD process. We first grow the 1T/2H WS2 butterflies using the above-mentioned process, followed by a similar process for MoS2 at a reduced growth temperature. The precursors are molybdenum(VI) oxide (MO3, from Sigma-Aldrich) and elemental sulfur powder (from Sigma-Aldrich), with purity of 99.5% and 99.98%, respectively. NaCl is used to assist the growth, but no Fe3O4 is mixed with the solid MoO3 precursor. The growth is set at 800 °C for 10 min. Cooling of the CVD furnace shares the same cooling method and rates with the case of WS2. AFM and Raman Characterization. The AFM analysis was performed by using a high-vacuum scanning probe microscope (SEIKO/SPA-300HV). Micro-Raman spectra were taken on the 1T and 2H wings of the CVD-grown WS2 butterflies using a confocal microscope (Renishaw inVia) Raman system. The measurements were performed at room temperature with laser excitation wavelength of 532 nm. A 50× objective lens with NA of 0.75 was used and a low power level (∼0.5 mW) was taken to avoid any heating effect. The Raman signals from the sample were introduced to an electron
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04979. Details of specimen characterization and atomic structure identification by using SEM, STEM, AFM, and Raman spectroscopy analyses; discussion of the influence of aberration and specimen inclination to the ADF image based on STEM simulation; STEM analyses of the temperature-dependent phase transformation, the 2H-1T phase boundary, and the lateral synthesized 1T/ 1T WS2/MoS2 heterojunction; EELS analysis of the impurities existing in the WS2 butterfly (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Yung-Chang Lin: 0000-0002-3968-7239 Chao-Hui Yeh: 0000-0002-9437-055X Zheng Liu: 0000-0001-9095-7647 Po-Wen Chiu: 0000-0003-4909-0310 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS P.-W.C. appreciates the project support of Taiwan Ministry of Science and Technology: grant nos. MOST 106-2628-M-007003-MY3 and MOST 106-2119-M-007-026-FS. K.S. and Y.C.L. acknowledge the support from JSPS-KAKENHI (JP16H06333) and (18K14119). T.O. and H.N. acknowledge the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). H.-C.L. and M.-Y.C. acknowledge the Thematic Project (AS-TP-106-M07) at Academia Sinica. 12086
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