Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C

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Epitaxial synthesis of molybdenum carbide and formation of a Mo2C/ MoS2 hybrid structure via chemical conversion of molybdenum disulfide Jaeho Jeon, Yereum Park, Seunghyuk Choi, Jinhee Lee, Sung Soo Lim, Byoung Hun Lee, Young Jae Song, Jeong Ho Cho, Yun Hee Jang, and Sungjoo Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06417 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C/MoS2 Hybrid Structure via Chemical Conversion of Molybdenum Disulfide Jaeho Jeon†, Yereum Park‡, Seunghyuk Choi†, Jinhee Lee‡, Sung Soo Lim‡, Byoung Hun Lee§, Young Jae Song†, Jeong Ho Cho†, Yun Hee Jang‡,*, and Sungjoo Lee†,* † SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Korea ‡ Department of Energy Systems Engineering, DGIST (Daegu Gyeongbuk Institute of Science & Technology), Daegu 42988, Korea § School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Korea Abstract The epitaxial synthesis of molybdenum carbide (Mo2C, a 2D MXene material) via chemical conversion of molybdenum disulfide (MoS2) with thermal annealing under CH4 and H2 is reported. The experimental results show that adjusting the thermal annealing period provides a fully converted metallic Mo2C from MoS2 and an atomically sharp metallic/semiconducting hybrid structure via partial conversion of the semiconducting 2D material. Mo2C/MoS2 hybrid junctions display a low contact resistance (1.2 kΩ·µm) and low Schottky barrier height (26 meV), indicating the material’s potential utility as a critical hybrid structural building block in future device applications. Density functional theory calculations are used to model the mechanisms by which Mo2C grows and forms a Mo2C/MoS2 hybrid structure. The results show that Mo2C conversion is initiated at the MoS2 edge and undergoes sequential hydrodesulfurization and carbide conversion steps, and an atomically sharp interface with MoS2 forms through epitaxial growth of Mo2C. This work provides the areacontrollable synthesis of a manufacturable MXene from a transition metal dichalcogenide (TMD) material and the formation of a metal/semiconductor junction structure. The present results will be of critical importance for future 2D heterojunction structures and functional device applications. Keywords : transition metal carbide, transition metal dichalcogenides, heterostructure, lateral contact, metal semiconductor junction, chemical conversion MXenes, which form a family of two-dimensional materials, have attracted extensive interest in recent years as part of efforts to explore two-dimensional (2D) materials beyond graphene. Layered ternary carbides and nitrides were initially studied in the 1960s,1 and the resurgence of interest in MXenes has mainly arisen from the attractive properties of MXene, which overcome certain

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fundamental limits of other 2D materials. Recent experimental studies have demonstrated excellent properties of MXenes, including a capacity to shield electromagnetic interference (Ti3C2Tx),2 to provide ultra-high electrical conductivity (Mo2C),3 to provide excellent thermal conductivity (Ti2CO2),4 to provide good mechanical properties with flexibility (c11 > 500 GPa),5 and to provide optoelectronic activities (Ti2CTx).6,7 Despite the demonstrated outstanding properties of MXenes, applications of MXenes are hindered by a lack of scalable preparation methods that yield stable MXene films. Significant efforts have advanced the scalable synthesis of 2D materials such as graphene and transition metal dichalcogenides (TMD), whereas few studies have examined MXene synthesis. Unlike other 2D materials, most MXenes are prepared by etching the corresponding MAX (M: early transition metal, A: group IIIA or IVA element, X: carbon and/or nitrogen) phases, which include strong covalent, metallic, and ionic bonds of M-X unlike weak metallic bond of M-A. Selective etching of the A components from MAX were performed using a variety of chemical etchants, followed by mechanical exfoliation and/or thermal annealing to prepare the 2D structured MXenes. Although this synthetic method is appropriate for studies of the fundamental properties of MXenes, implementation of these materials in device applications would require scalable synthesis methods. Furthermore, the intrinsic physical properties of MXenes depend significantly on their surface groups,4,8-10 which are unavoidably introduced during the MXene preparation process from MAX. Therefore, the development of a scalable MXene synthesis method that could minimize surface group generation in a controllable way through an understood growth mechanism is critical to extending fundamental studies of MXene properties toward real practical device application areas. Large-scale (~ 4 cm) Ti2C films for use as transparent conductive electrodes (TCEs) have been prepared using a combination of plasma etching and solution-based nanolamination processes.7 Despite the attainment of promising TCE properties, the conductivity and transparency of this material tends to degrade during typical delamination processes. Molybdenum Carbides have been synthesized by several methods such as the carburization of MoS211-16, use of MoCl517,18 precursors and the delamination with Ga intercalated MAX phase (Mo2Ga2C)19,20 to provide an effective electrocatalyst for the hydro evolution reaction (HER)12,13,15-18,21 or an anode material of Li-ion battery13,20 with the nanoparticle, nanorods, fullerene-like structures. The applicability in lowdimensional structures with a controllable synthesis method for electronic device applications has not yet been demonstrated. Recently, Ren et al., reported the chemical vapor deposition (CVD) synthesis of α-Mo2C nanosheets on the micrometer scale at the interface between Mo and Cu foils under CH4 and H2,22 and they studied the superconducting properties of the synthesized Mo2C nanosheets. High-quality thin α-Mo2C films were synthesized using this CVD method and demonstrated superconductivity under 2.85 K. This approach, however, requires high-temperature processes over a limited temperature range (1085–1096°C), and the growth mechanism was not explored.


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In this work, we demonstrated the epitaxial synthesis of 2D metallic Mo2C layers via the chemical conversion of 2D MoS2 crystals via the carburization of exfoliated MoS2 flakes during 820°C thermal annealing under CH4 and H2, using Cu foil as a catalyst. The converted Mo2C area was experimentally shown to depend on the thermal annealing period. Theoretical calculations revealed that this conversion occurred from the edge of the MoS2 films. The Mo2C synthesis mechanism was identified as a sequential hydrodesulfurization and carbide conversion process using density functional theory (DFT) calculations, and the epitaxially formed interface between Mo2C and MoS2 was characterized using a well-designed crystal structure model that corresponded to the TEM and STEM results. The electronic properties of the synthesized Mo2C were investigated using 4-probe and hall measurements. Fully converted Mo2C nanosheets demonstrated an excellent sheet resistance (123.6 Ω sq–1) and carrier concentration (5.84 × 1013 cm–2). Partial conversion of MoS2 by adjusting the annealing period resulted in the formation of a metallic/semiconducting (Mo2C/MoS2) junction with an atomically sharp interface and a low contact resistance (1.2 kΩ·µm) and SBH (26 meV).

Results and Discussion Epitaxial Synthesis of 2D Mo2C from MoS2 Figure 1a illustrates the process by which Mo2C nanosheets were synthesized from MoS2 crystals under CH4 and H2 under 820°C thermal heating. The experimental conditions and processes are described in detail in the experimental section. As illustrated in Figure 1a and supported by DFT calculations (see below), the chemical conversion of Mo2C began at the edge of MoS2, where hydrodesulfurization and carbide conversion occurred. S atoms at the edge were detached by a reaction with H· radicals, and CH3· radicals combined with the Mo-terminated edge (red dotted box in Figure 1a) after decomposition of CH4 in the presence of the Cu catalyst. The reduction of S atoms occurred proximally to the CH3-terminated Mo, followed by the formation and lateral extension of Mo-C-Mo bonds, yielding H2S as a by-product. A detailed exploration of the Mo2C growth mechanism using DFT calculations is provided in the next section. The Mo2C converted area was found to increase as the thermal annealing time increased, and complete conversion from MoS2 to Mo2C was achieved after a 4 hour annealing process. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical configurations of the converted Mo2C and the pristine and unconverted MoS2 over various thermal annealing time conditions. Mo was present in the Mo2+ and Mo4+ oxidation states, corresponding to Mo2C and MoS2, respectively.23-25 The Mo 3d spectra of Mo2C and MoS2 consisted of 2 doublet peaks (Figure S1). As shown in Figure S1a, a doublet corresponding to Mo4+ 3d5/2 and 3d3/2 was observed at 228.7 eV and 232.0 eV for the pristine MoS2 prior to thermal annealing. After thermal annealing under H2 + CH4, one more doublet appeared at 227.5 eV and 231.1 eV, with peaks corresponding to Mo2+ 3d5/2 and


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Mo2+ 3d3/2, respectively.23,24 Doublets corresponding to Mo2+ and Mo4+ were present in the spectra collected at the 1 h, 2 h, and 3 h thermal annealing time conditions (Figure S1b, c, and d). Figure 1b shows the areal intensity percentages of the Mo 3d peaks corresponding to each oxidation state. The Mo2+ oxidation state contributed 0% to the intensity prior to annealing, and the portion of Mo2+ increased to 18.27%, 37.96%, and 72.03% at 1 h, 2 h, and 3 h annealing times. The entire MoS2 film was converted to Mo2C after a 4 h thermal annealing, displaying a doublet peak corresponding to Mo2+ 3d, as shown in Figure S1e. This trend in the Mo 3d spectra supported the substitutional conversion from MoS2 to Mo2C. Figure S2 shows the C1s spectra obtained from different time conditions. Each annealing time condition displayed a carbidic carbon binding peak at 282.8 eV, with a binding energy that corresponded to the Mo-to-C bond in Mo2C.26 No carbidic peaks were observed prior to annealing (Figure S2a). Figure 1c shows an OM image of a few-layer exfoliated MoS2 flake prior to annealing, and Figure 1d displays the corresponding OM image with deep color contrast after 3 h annealing. Here, the MoS2 crystal was fully converted to Mo2C in the half-down side of the flake (red dotted area of Figure 1d). We measured the Raman spectra at point 1 in Figure 1c (before annealing) and at two points in Figure 1d (after annealing, point 2: converted Mo2C, point 3: unconverted MoS2). Figure 1e shows the Raman spectra measured at 3 points. The out-of-plane vibration mode of the sulfur atoms (A1g) at 406 cm–1 and the in-plane vibrations of molybdenum and sulfur atoms (E12g) at 382 cm–1 were observed at points 1 and 3, indicating that the unconverted area retained two distinct vibration modes characteristic of the few-layer MoS2.27 At point 2, which converted to Mo2C after 3 h annealing, two distinct Raman peaks were observed at 140 cm–1 and 240 cm–1. These peaks agreed well with the 2Eg (in-plane) and A1g (out-of-plane) Raman vibration modes of Mo2C.28 At point 2 the A1g and E12g modes of MoS2 were clearly removed from the converted area due to complete conversion to Mo2C. No G (~1350 cm-1) or D (~1600 cm-1) modes were detected with wide Raman spectra (0–3500 cm-1) from the synthesized Mo2C and the remaining MoS2 regions (Figure S4). The Mo2C conversion over time is supported in Figures 1f, 1g, and 1h, which show the Raman mapping images of the A1g modes of MoS2 (blue area) and Mo2C (red area) at different annealing times. An exfoliated MoS2 flake (before annealing) is shown in blue in Figure 1f. After 1 h annealing, a partially converted Mo2C region was observed, as shown in red in Figure 1g. This conversion initiated at the edge of MoS2, and the unconverted area assumed a clear blue color. Figure 1h shows that the whole bottom side of the MoS2 flake was converted to Mo2C, and the partial edge of the upper MoS2 was converted after 3 h annealing. The Raman mapping results demonstrated that the carbide conversion of MoS2 increased with the annealing time in the lateral direction. A detailed statistical analysis of the Mo2C conversion with the annealing time is provided in Figure S3. The Mo2C conversion was found to be accelerated by thermal annealing at higher temperatures. Figures S5 and S6 exhibit the annealing time-dependent Mo2C conversion from the MoS2 edge at 850°C and 950°C. The work function is a key parameter of an electrode material and plays a crucial role in


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determining the device performance, such as the barrier height in contact with the channel material. Kelvin probe force microscopy (KPFM) measurements were used to measure the work functions of the synthesized Mo2C and MoS2 materials. The contact potential difference (△VCPD) was calculated based on work function measurements collected between the KPFM tip and the sample according to the equation e·△VCPD = Φtip–Φsample, where Φtip is the work function of the tip, Φsample is the work function of the sample, and e is the electric charge. We calibrated the Φtip at 4.75 eV using the KPFM measurements collected from highly ordered pyrolytic graphite. Φsample mapping images were obtained from the △VCPD values measured from the pristine MoS2 prior to annealing (Figure 1i), after 1 h annealing (Figure 1j), and after 3 h annealing (Figure 1k). The dark areas in Figure 1i display the uniform work function distributions obtained from the exfoliated MoS2 flake. The value of ΦMoS2 was found to be 4.3 eV, similar to the theoretical work function of MoS2.29 Figures 1j and 1k were obtained under the conditions used to obtain Figures 1g and 1h and show the work function difference between the synthesized Mo2C region (bright area) and the remnant MoS2 region (dark area). The synthesized Mo2C exhibited a higher work function (ΦMo2C = 4.45 eV). We utilized high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS) to characterize the crystal structure, junction interface, and chemical composition of the synthesized Mo2C and remnant MoS2 (Figure 2). First, we focused on an investigation of the MoS2–Mo2C interface formed over a thermal annealing time of than 3 h. Figure 2a shows the HRTEM top-view image of the MoS2–Mo2C interface and its fast Fourier transform (FFT) patterns (inset). The remnant MoS2 area observed in the left half diagonal area displayed a hexagonal structure corresponding to MoS2, with a 3.2 Å Mo-toMo atomic distance; however, right half diagonal region in Figure 2a shows a different crystalline structure and exhibits a 1.8 Å d-spacing corresponding to the d-spacing value reported for Mo2C (102) .30 Figure 2b shows the cross-sectional HRTEM images of the Mo2C-MoS2 junction. In the MoS2 region, the layer-by-layer distance was estimated to be 6.2 Å, whereas the region in Mo2C displayed different crystalline structures with a 1.8 Å d-spacing value along the Mo2C (102) direction. The red dotted lines in Figures 2a and 2b indicate the interface between MoS2 and Mo2C. A 131° crystal direction difference between the MoS2 (100) plane and Mo2C (102) plane was observed. The formation of heterostructures through the epitaxial growth of 2D materials on other 2D materials has been reported recently, for example, MoS2/WS2,31 MoSe2/Wse2,32 and Gr/h-BN.33 Our results indicate that an atomically sharp metallic/semiconducting junction can be formed through the partial chemical conversion of a semiconducting 2D material. Investigations into the chemical components of a junction interface were performed using EELS chemical component mapping images. Figure 2c displays low-resolution STEM images of the Mo2C/MoS2 junction, in which the EELS was measured. Figure 2d shows a sulfur L-edge mapping image taken from the red dotted rectangular area in Figure


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2c. The left light blue area corresponded to remnant MoS2. Figure 2e exhibits the carbon atom distribution in the same area. The area almost exclusively included carbon, except for an area in which the sulfur was preserved. On the other hand, Figures 2f–j show that MoS2 fully converted to Mo2C after 4 h thermal annealing. Figure 2f shows HRTEM images and the corresponding electron diffraction pattern (inset) along the [0001] direction (top-view) of the fully converted Mo2C. A 5.8 Å Mo-to-Mo atomic distance was observed, in good agreement with previous experimental measurements34 and theoretical calculations of the Mo2C lattice constant in the b direction.35 Figures 2g and 2h show HRTEM and STEM images with the corresponding side-view FFT pattern (inset in Figure 2g), revealing the layered structure of the synthesized Mo2C. A layer-by-layer Mo2C structure with a 4.8 Å atomic spacing was observed, corresponding to the lattice parameter reported for Mo2C in the c-direction.23,34,36 Figures 2i and 2j, respectively, show the sulfur L-edge and carbon K-edge mappings of the fully converted Mo2C layers. The mapping area is displayed as a dotted red rectangle in the low-resolution STEM image (Figure 2h). These results indicated that the S components could be ignored in the fully converted Mo2C layers; however, the entire area contained carbon atoms due to the substitutional conversion process. The Mo2C thickness was also found to be tunable through the selection of the MoS2 thickness. Figure S7 shows that the thickness of the converted Mo2C depended linearly on the MoS2 thickness with a ratio of 0.83 (Mo2C/MoS2).

Carbide Growth Mechanism: DFT Calculations There have been several reports on MoS2-Mo2C composites,13-16 and their focus was mostly on synthesizing high-performance catalysts (for hydrogen evolution, in particular) rather than determining the atomic-level structure of the MoS2-Mo2C interface or understanding the mechanism of the MoS2-to-Mo2C conversion, which are the purpose of this work. A DFT study has been carried out on MoS2-Mo2C hybrid nanosheets,12 but again it focused on calculating their free energy of hydrogen adsorption as a descriptor of their catalytic activity towards hydrogen evolution rather than on understanding the formation mechanism of the MoS2-Mo2C hybrid nanosheets by determining the energy change along the structural evolution during the MoS2-to-Mo2C conversion. The HRTEM image in Figure 2a shows a distinct interface along the edge of the basal plane of MoS2. The reaction most likely begins at the edge rather than at the basal plane.37 The edge states of MoS2 layers are known to be Mo edges covered 100% or 50% by S (Figure 3, i-ii) .38,39 We also notice that only a small area of the MoS2 edge was converted to carbide without H2. The indispensability of H2, in addition to CH4, for the carbide conversion indicates that hydrodesulfurization (HDS; reduction of S on the edge to gas-phase H2S) should initiate the reaction. However, the HDS by H2 is endothermic (∆E > 1.4 eV),40,41 explaining why the


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carbide conversion did not occur at temperature lower than 800°C even in the presence of CH4 and H2. Most likely, H⋅ and CH3⋅ radicals produced after decomposition of H2 and CH4 in the presence of Cu catalysts are prerequisite of the reaction. The HDS by H⋅ is indeed exothermic [∆E = E(Mo edge) + E(H2S) – E(S-covered Mo edge) – 2E(H⋅) < –2.6 eV per H2S; Figure 3, a] as is the adsorption of CH3⋅ and H⋅ radicals to the under-coordinated S-free Mo edge to form Mo-S-Mo(H)(CH3) on the edge (∆E < –4.5 eV; Figure 3, b). Dehydrogenation from the adsorbed CH3⋅ and H transfer would form Mo-S(H)-Mo(H)(CH2) (Figure 3, c). Further dehydrogenation from the adsorbed CH2⋅ would complete HDS and remove S as H2S from the edge, and the resultant CH⋅ would move into the Mo--Mo vacancy to form Mo-CH-Mo(H) (Figure 3, d-e). The final dehydrogenations from Mo-CH-Mo(H) would remove neighboring S as H2S by HDS and again produce a Mo--Mo vacancy to receive another CH produced from CH3 adsorbed nearby. Such a chain reaction would complete the formation of the first row of Mo2C on the MoS2 edge (Figure 3, f) and further grow multiple rows of Mo2C phase (Figure 3, g). The overall reaction from the post-HDS S-free MoS2 edge (Figure 3, a) to the Mo2C/MoS2 hybrid edge (Figure 3, g) would be endothermic and unfavorable (∆E ≈ 2.2 eV) according to our single-layer model calculations (most likely due to the endothermic nature of a series of dehydrogenation steps). However, the stacking of a multiple of such MoS2/Mo2C hybrid layers (Figure 3, h) is exothermic and favorable (-3.3 eV; Figure 3, h) and would compensate the endothermic nature of the single-layer MoS2-to-Mo2C conversion and drive it forward. In other words, the conversion to Mo2C would occur only when it starts from thick (or bulk) multilayers of MoS2 (as in Figure 3, h) rather than from a single layer (as in Figure 3, g). It explains why the conversion to Mo2C has not been achieved with single- or few-layered MoS2. The square planar termination and the step-like layered structure of the converted Mo2C layers optimized from DFT calculations resembles the (102) planes of the β-Mo2C crystal, as confirmed from the d-spacing (1.8 Å) in the HRTEM images of the MoS2-Mo2C junction (Figure 2a). While such static calculations provide clues to the reaction mechanism by identifying possible intermediates, the information on the reaction rate or the activation barrier is missing and thus, we cannot guarantee that each step of the reaction would occur in a given time. Moreover, there can be unexpected processes and intermediates that we missed during those calculations. We find that AIMD (ab initio molecular dynamics) simulations are useful to confirm whether the dynamic structure evolution (i.e., reaction) proposed from the static calculations really occur as a function of time at the reaction temperature. As the first examples, the exothermic (i to a, Figure 3) and endothermic (c to d, Figure 3) hydrodesulfurization steps are confirmed to occur within 100 fs of our AIMD simulation42,43 performed at 800°C starting from the geometries optimized by the static DFT calculations. In fact, the AIMD simulation exposes an


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interesting feature of the latter (c-to-d) H2S removal step which we had expected but could not see from the static calculations. This step was originally suggested as a two-step process (endothermic c-to-d H2S removal and exothermic d-to-e CH migration, Figure 3) but it turns out to be a concerted one, in which the CH2 group migrates to fill up the vacancy Mo--Mo created by H2S desorption). It would be interesting to see the effect of the presence of an extra amount of CH3⋅/H⋅ radicals. The complete AIMD study will be carried out and reported separately in near future.

Electrical Characteristics of the Synthesized Mo2C When Used as a 2D Electrode The carrier transport properties of converted Mo2C films were investigated using 4-probe and Hall measurements, as shown in Figure 4. The Mo2C device structure and fabrication processes are described in the experimental section. Figure 4a shows the I-V characteristics obtained from the 4probe measurements. The voltage difference between electrodes 2 and 3 was measured while the current was swept between electrodes 1 and 4 (red curve). The sheet resistance (Rsh) of Mo2C was 123.6 Ω sq–1, extracted from the expression Rsh = (V23/I14) × (W/L). This value was comparable to that of other 2D electrodes, such as graphene (102 ~103 Ωsq–1 ),44,45 and Ti2CTx (75 ~ 300 Ωsq–1).7,46,47 The contact resistance between Mo2C and Ti was extracted from the slope of the black curve shown in Figure 4a and was found to be 0.2148 Ωmm, lower than the values reported for other metallic 2D material–3D metal junctions, such as graphene/Cr,48,49 graphene/Ti,49 and Ti2C(OH)xFy/Cr.6 The gate bias-independent I-V characteristics of Mo2C (Figure 4b) indicated its metallic properties. The carrier concentration of the converted Mo2C film was extracted from the Hall measurements and found to be 5.84 × 1013 cm–2, based on the slope of the VH (Hall voltage)–B (magnetic field) curve (Figure 4c). Figure 4d shows an OM image of an FET device fabricated using a lateral Mo2C/MoS2 junction fabricated under the partial conversion conditions. Figure 4e compares the ID–VG transfer curves of a MoS2 FET (L = 2.5 µm, W = 10.2 µm). The red curves represent electrons injected from the Mo2C electrode into the MoS2 channel, measured under positive VDS (case A), whereas black curves were measured under a negative VDS, which resulted from electron injection from the Ti electrode to the MoS2. The Mo2C/MoS2 heterostructure FET device showed similar n-type transport characteristics with positive and negative VDS. The on-current corresponding to electron injection from the lateral Mo2C electrode was four times the value obtained from the vertical Ti electrode. The Schottky barrier height (SBH) in the lateral Mo2C/MoS2 junction was estimated to be 26 meV (Figure 4f), lower than the values obtained from the vertical Ti/MoS2 (66 meV) and Pd/MoS2 (130 meV). Similar low SBHs were reported for the 2D lateral MoS2 junctions, such as 1T-MoS2,50 and a graphene electrode.51 SBH calculations are discussed in Figure S13 and in the supporting information. We calculated the contact


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resistance (Rc) in the lateral MoS2-Mo2C junction using gate bias-induced 4-probe measurements. The calculation methods are described in detail in the supporting information (Figure S14). Figure 4g plots the Rc values calculated with various channel sheet resistance (Rchannel) values obtained through gate modulation. Extremely low Rc values (20 kΩ·µm to 1.2 kΩ·µm with Rchannel from 106 to 105 Ωsq-1) were obtained from the lateral Mo2C/MoS2 junctions, about two orders of magnitude lower than the values obtained from the vertical Ti/MoS2 junction and comparable to the values reported for the 1TMoS2/2H-MoS2 junction.50 This result was attributed to the lower SBH and the atomically sharp clean Mo2C/MoS2 junction. The trends in Rc α √(ρc·Rchannel) were retained in both junctions, according to the transmission line model,52,53 indicating that the lateral contact geometry of the Mo2C/MoS2 junction overcame the structural limits of the vertical 2D junction at which the increase in Rc occurred for a contact length shorter than the transfer length.52

Conclusions In this work, we report the synthesis of metallic 2D Mo2C electrode materials through the chemical conversion of 2D semiconducting MoS2 films. Our experimental results showed that thermal annealing under CH4 and H2 fully converted the Mo2C nanosheets with controllable thickness (3–100 nm) and lateral size (100 µm). These materials demonstrated an excellent sheet resistance (123.6 Ω sq–1) and carrier concentration (5.84 × 1013 cm–2). Partial conversion of MoS2 by adjusting the annealing period yielded a metallic/semiconducting (Mo2C/MoS2) junction that featured an atomically sharp interface and low contact resistance (1.2 kΩ·µm), potentially a critical hybrid structural building block for future device applications. The mechanisms by which Mo2C was grown and the Mo2C/MoS2 hybrid structure was formed was elucidated in detail using DFT calculations, which revealed that the Mo2C conversion proceeded at the MoS2 edge through sequential hydrodesulfurization and carbide conversion steps, and that an atomically sharp interface was formed by the epitaxial growth of Mo2C.

Experimental Section [Mo2C conversion from MoS2] 1.1 cm x 1.1 cm SiO2 (285 nm)/p+doped Si substrates were cleaned by acetone and IPA for 15 min with ultrasonication and dried with N2. MoS2 flakes were obtained via mechanical exfoliation onto a substrate from bulk MoS2 using the tape (224SPV, Nitto) exfoliation method. The tape residue was removed by placing the samples in an acetone bath at 140°C for 30 min. The MoS2 samples were then converted to Mo2C in a low-pressure CVD system. The MoS2 samples were positioned below a square folded Cu foil (99.9% metal basis, Alfa Aesar) and placed at the center of the heating zone of a 4 inch quartz tube. The furnace was heated to 820°C over 1 hour at a


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heating rate of 13.75°C/min under 100 sccm Ar. The MoS2 was fully converted to Mo2C at 820°C over 4 h under 100 sccm CH4, 5 sccm H2, and 100 sccm Ar. After the conversion process, the furnace was slowly cooled to room temperature at a cooling rate of 4.75°C/min. [Mo2C characterization] After the conversion process, the thickness and morphology of the synthesized Mo2C were measured using AFM (Park systems Corp.). Raman spectra and Raman mapping images were obtained using a Raman microscopy system (Kaiser Optical Systems Model RXN) equipped with a 532 nm laser. The work function was measured using the KPFM method in an AFM–Raman measurement system equipped with a Kelvin probe (NTEGRA Spectra, NT-MDT 830). The chemical configurations were examined using XPS (ESCA2000, VG Microtech Inc.) and a Mg Kα X-ray source. TEM measurements were obtained by transferring fully converted Mo2C or partially converted Mo2C flakes onto copper without carbon-coated TEM grids using a wet transfer method comprising coated polymethyl methacrylate (PMMA) and a buffer oxide etcher (BOE) solution. Cross-sections of the samples were prepared using the focused ion beam method (SMI3050TB). TEM images were captured by HRTEM (JEOL, JEM ARM 200F) with an incident electron beam energy of 80 keV. [DFT calculation] Spin-polarized DFT calculations implemented in the VASP 5.3.5 code54 were carried out to optimize the geometry of the 100% and 50% S-covered Mo edges of a single-layer MoS2, and the intermediate states during their conversion to Mo2C caused by H⋅ and CH3⋅ radicals. Those edges were modeled by infinitely long (i.e., periodic) strips of a single-layer MoS2 (a = 6.32, b = 12.3, c = 35.0; in Å; Figure 3). These were built by inserting a vacuum layer of 15 Å along the c axis into a (2×2) supercell of the single-layer MoS2 placed in the ac plane. This model has been widely used to represent the MoS2 edges involved in various catalytic reactions12. Moreover, larger edge models such as a wider (in the c direction) strip built with a (2×3) supercell [instead of the (2×2) supercell] or a thicker (in the b direction) strip built with bilayer MoS2 or bulk MoS2 have resulted in negligible change in the edge formation energy and dehydrosulfurization energy (within 5%; 4.9–5.1 eV and –2.8 to –2.7 eV, respectively), which can serve as a measure of edge reactivity towards its conversion to Mo2C, and thus, justifies the size of our edge model. During the geometry optimization, the lattice parameters were fixed at the bulk values (6.32, 12.3, 35.0; in Å), and the bottom two S-MoS rows were fixed at their bulk positions. A 4×1×1 Monkhorst-Pack mesh55 was used for the k-point sampling. The Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional56 was used with the dispersion correction scheme D3 of Grimme.57 The core potential was replaced by a pseudopotential augmented wave and the valence electrons were represented by plane-wave basis sets with a kinetic energy cutoff of 600 eV. The energy convergence criterion for the geometry optimization was 0.02 eV/Å, and the dipole interaction in the z-direction (perpendicular to the edge) was taken into account with a total energy correction factor. The same level of DFT (PBE-D3) was also used to carry out the


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AIMD simulations42,43 of a canonical (constant-NVT) ensemble at 1100 K which is kept constant with the Nose-Hoover thermostat.58 Starting from each of the DFT-optimized reactants and the intermediates along the reaction coordinate (represented by even smaller models which have one less MoS2 strip than in the static calculations), the energy and its gradients (i.e., the force) calculated with DFT at each time step are used to build the interatomic potentials required to calculate the geometry for the next time step (0.5 fs apart) by integrating Newton’s equations of motion with the Verlet algorithm.59 This is repeated for 103–106 time steps to build a trajectory for a given period of time. [Mo2C/MoS2 device fabrication with top-contact structure] The synthesized Mo2C samples on SiO2/Si substrate were coated with A6 PMMA and the SiO2 layer was etched by a BOE solution. The PMMA coated Mo2C samples were rinsed with DI water several times to remove the remaining BOE residue and transferred onto another cleaned 285 nm SiO2/ p+doped Si substrate, and samples were annealed on hot plate 110 °C for 15 min. The PMMA coated layer was removed by an aceton bath for 30 min. The Mo2C/MoS2 vertical heterostructure device was fabricated by using the dry stacking method. The source/drian electrodes were patterned by using E-beam lithography, and followed by metal deposition with Ti (10 nm), and Au (50 nm) by an electron-beam evaporation. After metal deposition, the metal lift-off process was followed. [Mo2C/MoS2 device fabrication with lateral-contact structure and characterization] Mo2C samples were prepared using the wet transfer method using a coated top surface A6 PMMA and a BOE solution-etched SiO2 bottom layer substrate. The PMMA-coated Mo2C samples were rinsed with DI water several times to remove residual BOE and were transferred onto another cleaned 285 nm SiO2/ p+doped Si substrate. Samples were annealed on a hot plate at 110°C for 15 min. The PMMA coated layer was removed in an acetone bath for 30 min. The Mo2C–MoS2 lateral heterostructure device was fabricated through 820°C thermal annealing over 50 min under H2 and CH4. The source– drain electrodes were patterned using e-beam lithography, followed by Ti (10 nm)/Au (50 nm) or Pd (50 nm) deposition via e-beam evaporation. After metal deposition, a metal lift-off process was applied. Low-temperature measurements and the electrical device properties were measured in a vacuum probe station chamber (under 10–4 Torr) using a Keithley 4200 parameter analyzer.


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Figure 1: (a) Schematic illustration of the Mo2C synthesis via thermal annealing under CH4 and H2. (b) Statistical analysis of the Mo2C/MoS2 XPS data to obtain the Mo4+ 3d and Mo2+ 3d XPS peak integrated intensity percentages of the Mo 3d total intensity. (c) OM image of an exfoliated MoS2 flake, (d) and OM image of a flake after partial conversion to Mo2C over 3 h thermal annealing (d). (e) Raman spectra obtained at the points indicated in (c) and (d). Raman mapping image of the A1g modes of MoS2 and Mo2C before annealing (f), after 1 h annealing (g), and after 3 h annealing (h). Work function mapping images extracted from the KPFM measurements obtained before annealing (i), after 1 h annealing (j), and after 3 h annealing (k).


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Figure 2: (a) Top view HRTEM image of the Mo2C – MoS2 junction and the corresponding FFT images (inset). (b) Cross-sectional view HRTEM image of the Mo2C – MoS2 junction. (c) low magnification STEM images of the Mo2C – MoS2 junction (upper panel) and high magnification STEM images taken from the blue rectangle in the upper image (lower panel), EELS element mapping image of the sulfur L-edge (d) and carbon K-edge (e), over the area indicated by the red rectangle in (c). (f) Top view HRTEM image of the fully converted Mo2C, and the FFT images (inset). (g) Cross-sectional view HRTEM image of the fully converted Mo2C. (h) STEM images of the fully converted Mo2C and EELS element mapping image of the sulfur L-edge (i) and carbon K-edge (j), in the area indicated by the red rectangle in (h).


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Figure 3. (A) DFT reaction energy profiles for the carbide conversion at the edge of a MoS2 monolayer: (i and ii; a) H radicals desulfurize 100% and 50% S-covered edges; (b) methyl radicals are attached to the Mo edge; the S edge is (c-d) reduced to H2S and (e-g) replaced by carbon through consecutive dehydrogenation of the methyl groups; (h) interface model of bulk MoS2 and Mo2C converted from MoS2. (B) AIMD snapshots and Mo-S(C) distance profiles for the (i)-to-(a) and (c)-to(d) DFT steps. Color code: Mo (purple), S (yellow), C (brown), and H (white).


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Figure 4: (a) Voltage–current curves obtained from the four-terminal Mo2C device, and schematic diagram of the device structure used in each measurement (inset). (b) Current–voltage characteristics obtained from a two-terminal Mo2C device under various back gate bias conditions, and schematic diagram of the device structure (inset). (c) Hall voltage measurements under various magnetic fields, and OM images of the fully converted Mo2C device with 4-terminal electrodes (inset). (d) OM images of MoS2 FET devices with top-contact Ti/MoS2 and lateral-contact Mo2C/MoS2 structures. (e) IDS=VG curves obtained from electrons injected from Mo2C into MoS2 (Case A) and electrons injected from Ti into MoS2. (f) Electron barrier heights extracted from ln(IDS2/T) – q/kBT for a lateral Mo2C/MoS2 contact, and (g) Rc for the channel sheet resistance (Rchannel) obtained using the 4-probe measurement method for vertical Ti/MoS2 contact (black rectangles) and lateral Mo2C/MoS2 contact (red circles), and OM image of the devices (insets).


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:... Detail XPS spectra, analysis of conversion area with the annealing periods or temperatures, Raman spectra with wide range, AFM & KPFM measurements, line profile result with EELS measurement and electrical characteristics are included as Figures S1−S14.

Acknowledgment This research was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) of the National Research Foundation of Korea and the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2014M3C1A3053029), and the Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government

(MSIP)

(grant

no.:

2015R1D1A1A09057297,

2017R1A4A1015400).


2015M3A7B7045496,

and

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Epitaxial Synthesis of Molybdenum Carbide and Formation of a Mo2C

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