2D Superconducting Mo2C

Jun 7, 2017 - The heterostructures or hybrids of graphene and superconductors offer a very interesting platform to study mesoscopic superconductivity ...
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Strongly Coupled High-Quality Graphene/2D Superconducting Mo2C Vertical Heterostructures with Aligned Orientation Chuan Xu,†,⊥ Shuang Song,‡,⊥ Zhibo Liu,† Long Chen,† Libin Wang,‡ Dingxun Fan,‡ Ning Kang,*,‡ Xiuliang Ma,† Hui-Ming Cheng,†,§,∥ and Wencai Ren*,† †

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China ‡ Beijing Key Laboratory of Quantum Devices, Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, P. R. China § Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China ∥ Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia S Supporting Information *

ABSTRACT: Vertical heterostructures of two-dimensional (2D) crystals have led to the observations of numerous exciting physical phenomena and presented the possibilities for technological applications, which strongly depend on the quality, interface, relative alignment, and interaction of the neighboring 2D crystals. The heterostructures or hybrids of graphene and superconductors offer a very interesting platform to study mesoscopic superconductivity and the interplay of the quantum Hall effect with superconductivity. However, so far the heterostructures of graphene and 2D superconductors are fabricated by stacking, and consequently suffer from random relative alignment, weak interfacial interaction, and unavoidable interface contaminants. Here we report the direct growth of high-quality graphene/2D superconductor (nonlayered ultrathin αMo2C crystal) vertical heterostructures with uniformly well-aligned lattice orientation and strong interface coupling by chemical vapor deposition. In the heterostructure, both graphene and 2D α-Mo2C crystal show no defect, and the graphene is strongly compressed. Different from the previously reported graphene/superconductor heterostructures or hybrids, the strong interface coupling leads to a phase diagram of superconducting transition with multiple voltage steps being observed in the transition regime. Furthermore, we demonstrate the realization of highly transparent Josephson junction devices based on these strongly coupled high-quality heterostructures, in which a clear magnetic-field-induced Fraunhofer pattern of the critical supercurrent is observed. KEYWORDS: graphene, 2D transition metal carbides, heterostructure, high quality, superconductivity

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disappears when the graphene is misoriented with respect to hBN.6 Such surface reconstruction leads to strong asymmetry between the sublattices in graphene, which opens a gap in the graphene electronic spectrum.7 The resultant moiré patterns lead to the reconstruction of the electronic spectrum in graphene and the formation of secondary Dirac points.6−10 The superconductor-graphene heterostructures or hybrids have offered an interesting platform to exploit various quantum phenomena and exotic devices, such as bipolar supercurrent,11

raphene and other two-dimensional (2D) crystals not only show many new physics, superior properties, and promising applications that are different from their bulk counterparts in isolated manner but also provide ideal building blocks to create 2D vertical heterostructures with unusual properties, phenomena, and the possibilities for technological use by stacking in a specific sequence.1−5 Besides the high quality, the synergistic effect between different crystals plays an important role in the properties of 2D heterostructures, which strongly depends on the interface, relative alignment, and interaction between the neighboring crystals.4−10 It has been shown that the surface reconstruction occurs for graphene on h-BN when the crystallographic orientations of the two crystals are aligned, while this effect © 2017 American Chemical Society

Received: March 8, 2017 Accepted: June 7, 2017 Published: June 7, 2017 5906

DOI: 10.1021/acsnano.7b01638 ACS Nano 2017, 11, 5906−5914

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Figure 1. Direct growth of graphene/2D α-Mo2C crystal heterostructures by CVD. (A) A Cu (top)/Mo (bottom) substrate. (B) CVD growth of graphene on top of the solid Cu/Mo substrate at 1070 °C. (C) CVD growth of 2D α-Mo2C crystals underneath graphene on the liquid Cu/ Mo substrate at 1090 °C. The optical images of the samples obtained at each step are given in the bottom.

ballistic proximity superconductivity,12,13 quantum superconductor−insulator transitions,14 and specular interband Andreev reflections.15 Recently, the experimental realization of a hybrid system with high upper critical field superconductors in contact with graphene demonstrates the coexistence of the quantum Hall state and superconductivity,16−18 making a promise for achieving Majorana zero modes in graphene systems.19,20 So far, however, these heterostructures or hybrids are fabricated by either depositing or stacking one material on the top of another,11−18,21 and therefore suffer from random relative alignment between neighboring materials, weak interfacial interaction, and unavoidable interface contaminants. Recently, chemical vapor deposition (CVD) has been extensively developed for the direct growth of various van der Waals heterostructures of 2D layered crystals such as graphene/ h-BN,22,23 h-BN/graphene,24 MoS2/graphene,25,26 MoS2/hBN,27 WS2/MoS2,28 and MoS2/WSe2/graphene.29 Compared to the stacking methods, the direct growth enables very clean interface in the heterostructures. Moreover, the van der Waals interactions during growth define the preferential growth directions so that the neighboring 2D crystals are aligned each other in some heterostructures, such as graphene/h-BN.22 However, the orientation alignment is nonuniform in most heterostructures and even absent in some heterostructures that are constructed by 2D crystals with large lattice mismatch.26,27 In addition, the interfacial interaction is weak as well in these directly grown van der Waals heterostructures.23,24 More importantly, these heterostructures usually have low quality because of the low catalytic activity of 2D crystals and the generation of many defects during the growth process as a result of the use of different reaction atmosphere at a high temperature.22,24,25 Here, we report the direct growth of high-quality graphene/ 2D superconductor (nonlayered ultrathin α-Mo2C crystal) vertical heterostructures with uniformly well-aligned lattice orientation and strong interface coupling by two-step CVD. Different from the previously reported graphene/superconductor heterostructures or hybrids, the strong interface coupling leads to a phase diagram of superconducting transition with multiple voltage steps being observed in the transition regime. Furthermore, we demonstrate the realization of highly transparent Josephson junction devices based on these strongly

coupled heterostructures, in which a clear magnetic-fieldinduced Fraunhofer pattern of the critical supercurrent is observed.

RESULTS AND DISCUSSION We fabricated the graphene/2D superconducting α-Mo2C crystal vertical heterostructures by ambient-pressure CVD, using methane as the carbon source and a bilayer of Cu/Mo foils as the substrate (see Methods Section). As shown in Figure 1A,B, we first heated the Cu/Mo substrate to 1070 °C under H2/Ar, and then introduced CH4 to initiate graphene growth on solid Cu surface, which is the key to obtain heterostructures as discussed below. After the formation of graphene, the Cu/Mo substrate was heated to 1090 °C (above the melting point of Cu), while keeping the other parameters constant, to grow 2D α-Mo2C crystals underneath the graphene on the liquid Cu/Mo substrate (Figure 1C). The direct growth of graphene/2D α-Mo2C crystal heterostructures enables a clean interface between graphene and 2D α-Mo2C crystal, similar to those reported for the CVD-grown van der Waals heterostructures.22−30 More importantly, the constant atmosphere avoids the generation of defects, enabling the formation of high-quality heterostructures. Finally, the sample was cooled down to room temperature at a cooling rate of 20−50 °C/min under the same flow rate of CH4, which is essential for obtaining intact graphene/2D α-Mo2C crystal heterostructures (Figure S1). After CVD growth, the samples were transferred from the Cu/Mo substrate to SiO2/Si substrates or transmission electron microscopy (TEM) grids by etching away the Cu layer for detailed structure characterizations and transport measurements (see Methods Section). During CVD growth, it is interesting to note that the graphene film grown on the solid Cu/Mo substrate are initially multilayers with nonuniform thickness, and it is thinned by etching once the α-Mo2C crystals are formed underneath the graphene film on the liquid substrate and eventually becomes a monolayer (Figure S2). If the substrate is directly heated over the melting point of Cu (here, 1085 °C) before introducing methane, only very thick, irregular, and sparse Mo2C crystals along with graphene are formed after growth (Figure S3). Moreover, if the graphene grown on the solid Cu/Mo substrate is discontinuous, no 2D α-Mo2C crystal is formed in the bare 5907

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Figure 2. CVD-grown graphene/2D α-Mo2C crystal heterostructures with aligned orientation. (A−D) Optical image (A) of a graphene/2D αMo2C crystal heterostructure transferred on a 290 nm-thick SiO2/Si substrate, and the corresponding Raman intensity mappings of the characteristic peak of 2D α-Mo2C crystals (B), and the G peak (C) and 2D peak (D) of graphene. (E−H) Bright-field TEM images of the heterostructures of graphene with triangular (E), hexagonal (F), octagonal (G), and nonagonal (H) 2D α-Mo2C crystals. Scale bars: 200 nm. (I−L) SAED patterns taken from the regions with 2D α-Mo2C in (E−H), showing two sets of patterns corresponding to 2D α-Mo2C (marked by red dots) and monolayer graphene (marked by green circles) with the same lattice orientation (marked by red arrows) for each case. (M) Atomic structure of the graphene/2D α-Mo2C crystal heterostructure (top view). Green balls: Mo atoms; Pink balls: carbon atoms in Mo2C; Black honeycomb lattice: graphene. (N) Atomic-resolution HAADF-STEM image of a 2D α-Mo2C crystal in a heterostructure, showing the same high quality as pure 2D α-Mo2C crystals.31

the heterostructure samples. The regions without 2D α-Mo2C crystals show typical Raman characteristics of monolayer graphene32 with very sharp and symmetrical G and 2D peaks as well as an intensity ratio of 2D peak to G peak about 2 (Figure S7), while the regions with 2D α-Mo2C show the characteristic peaks of both pure 2D α-Mo2C crystals (∼140 cm−1, Figure S8) and graphene (Figure 2B−D). Moreover, the whole film shows uniform optical contrast after etching away the 2D α-Mo2C crystals (Figure S9). These results confirm the formation of uniform monolayer graphene/2D α-Mo2C crystal heterostructures. It is worth noting that, different from the CVD-grown van der Waals heterostructures,22,24,25 the graphene in our heterostructures has a high quality without visible defect-related Raman D peak being observed (Figure S10). Moreover, these high-quality heterostructures form superconductor-normal-superconductor (SNS) junctions with graphene as a link, in which the spacing between neighboring 2D α-Mo2C crystals can be tuned by growth time and the thickness of copper foil (Figure S6). Atomic force microscope (AFM) measurements show that the thickness of the 2D αMo2C crystals in heterostructures is 5−15 nm (Figure S11).

regions on the liquid substrate (Figure S4). These results suggest that the preformation of graphene on the solid Cu/Mo substrate is the key to fabricate graphene/2D α-Mo2C heterostructures. Similar to the fabrication of pure 2D αMo2C crystals,31 the thickness of Mo2C crystals in the heterostructure samples is increased with increasing growth temperature and growth time. However, different from the fabrication of pure 2D α-Mo2C crystals,31 the density of 2D αMo2C crystals in the heterostructure samples is not sensitive to the growth temperature (Figure S5). It was found that using a thinner copper foil can decrease the nucleation density of 2D αMo2C crystals in the heterostructure samples (Figure S6). Figure 1C shows a typical optical image of graphene/2D αMo2C heterostructures grown on a Cu/Mo substrate. It is interesting to note that all the 2D α-Mo2C crystals show regular shapes and, more importantly, the same Mo-sublattice orientation, which is in sharp contrast to the random orientation of pure 2D α-Mo2C crystals grown on a Cu/Mo substrate.31 This gives strong evidence that the 2D α-Mo2C crystal growth on graphene surface follows an epitaxy mode, implying the strong interaction between graphene and 2D αMo2C crystals. We further performed Raman measurements on 5908

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Figure 3. Strong interface coupling between the graphene and 2D α-Mo2C crystal in the CVD-grown heterostructures. (A) Raman spectra taken from a pure 2D α-Mo2C crystal and the graphene (indicated by black circle) and heterostructure (indicated by red circle) in (B), showing the graphene is compressed in the heterostructure. (B−D) Optical image (B) and the corresponding G peak position (C) and 2D peak position (D) mappings of a CVD-grown graphene/2D α-Mo2C heterostructure, showing clear strain domains. Scale bars in (B−D) are 5 μm.

Figure 2I−L), indicating that 2D α-Mo2C and graphene are well aligned in the lattice orientation despite a significant lattice mismatch (∼22%) as shown in Figure 2M. Atomic-resolution high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) measurements indicate that the 2D α-Mo2C crystals in the heterostructures have the same high quality as the pure 2D α-Mo2C crystals reported in our previous work31 (Figures 2N and S18). Similar measurements and analyses of dozens of CVD-grown heterostructures of graphene and 2D α-Mo2C crystals with different shapes and thicknesses show the same structure characteristics. In contrast, the stacked graphene/2D α-Mo2C crystal heterostructures show random relative orientation (Figure S19). The uniformly aligned lattice orientation between graphene and 2D α-Mo2C crystal is a very important feature of our CVD-grown heterostructures, which confirms the epitaxy growth mode and implies strong interface coupling. In order to further understand the interfacial interactions, we analyzed the Raman spectra of the heterostructures in detail. It is important to note that both the G peak and 2D peak of the graphene across the whole heterostructure significantly shift to higher frequency (up to ∼20 and 50 cm−1 upshift, respectively) along with a greatly reduced intensity ratio of 2D peak to G peak compared to those of pure graphene, and the 2D peak exhibits a much larger upshift than the G peak for the same position in the heterostructure (Figures 3, S7, and S10). In contrast, the graphene layers in the stacked heterostructure (Figure S20) and in the reported CVD-grown van der Waals heterostructures22−24 show almost the same G and 2D peak position as pure graphene. This confirms that our CVD-grown graphene/2D α-Mo2C heterostructures have a much stronger interfacial interaction than the reported CVD-grown van der Waals heterostructures. The much larger upshift of the 2D peak than the G peak indicates that the graphene in the CVD-grown heterostructure undergoes a compression.32,35,36 The variation in the Raman positions of the G and 2D peaks across the heterostructure indicate nonuniform strain distribution in

As shown in Figures S12 and S13, 2D α-Mo2C crystals are much more stable than graphene under H2 plasma treatment. In order to identify the position of graphene in the CVD-grown heterostructures, the samples were subjected to H2 plasma treatment (Figure S14). For comparison, the stacked heterostructures of graphene (top)/2D α-Mo2C crystals (bottom) and 2D α-Mo2C crystals (top)/graphene (bottom) by transferring one 2D material onto the surface of another were also subjected to the same treatment (Figures S15 and S16). It can be seen that the graphene underneath 2D α-Mo2C crystals survives without any structure change even after a long time treatment, confirmed by the identical Raman spectra, because of the protection of stable 2D α-Mo2C crystals. In contrast, the Raman signals of graphene disappear after a short time H2 plasma treatment for both stacked heterostructures of graphene (top)/2D α-Mo2C crystals (bottom) and the CVDgrown heterostructure. This gives strong evidence that the graphene is on the top of 2D α-Mo2C crystals in the CVDgrown heterostructures. As shown in Figure S17, the growth of 2D α-Mo2C crystals is prevented by putting a piece of W foil between Cu foil and Mo foil, indicating that 2D α-Mo2C crystals are formed from the Mo atoms that diffused from the bottom Mo foil to Cu surface. This explains the stacking order of graphene and 2D α-Mo2C crystals in the CVD-grown heterostructures. We further used selected area electron diffraction (SAED) to identify the lattice alignment between the graphene and 2D αMo2C crystals in the heterostructures. Figure 2E−H and I−L show the bright-field TEM images and the corresponding SAED patterns of the CVD-grown heterostructures of graphene and triangular, hexagonal, octagonal, and nonagonal 2D αMo2C crystals, respectively. Note that all the heterostructures show two sets of SAED patterns, which correspond to monolayer graphene33 (marked by green circles) and 2D αMo2C crystals31,34 (marked by red dots). More importantly, the two sets of SAED patterns show the same orientation of 6-fold symmetry for each heterostructure (marked by red arrows, 5909

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Figure 4. Electronic transport properties of graphene/2D α-Mo2C crystal heterostructure. (A) V−I curves for a CVD-grown graphene/2D αMo2C crystal heterostructure device at different temperatures. The horizontal blue arrows point to multiple voltage steps. The lower-right inset is an expanded view of a dissipative branch, more clearly showing multiple voltage steps in the regime of superconducting transition. The upper-left inset shows a scanning electron microscopy (SEM) image of a typical four-terminal device. The graphene/2D α-Mo2C heterostructure is outlined by dashed lines. The scale bar in the image is 5 μm. (B) V−I curves for another heterostructure device at different temperatures which are expanded in the regime of superconducting transition. The inset shows V−I curve of a pure 2D α-Mo2C crystal device at 1.9 K. No step-like feature is observed. (C,D) 2D color plot of differential resistance dV/dI of CVD-grown graphene/2D α-Mo2C crystal heterostructure (C) and pure 2D α-Mo2C crystal (D) measured at 150 mK, as a function of bias current I and applied magnetic field B. The white dashed lines are a guide for the magnetic field evolution of Ic.

steps have also not been observed in the stacked graphene/2D superconductor heterostructures or hybrids reported so far.11−18,21 These indicate that the voltage step feature is intrinsic to our CVD-grown heterostructures. It has been reported that the voltage steps in the V−I characteristics have been observed in long superconducting whiskers and one-dimensional (1D) superconductors.37,38 In this case, each voltage jump corresponds to the emergence of a localized phase-slip center, which arises from the variation of the superconducting order parameter leading to a dissipative region.39 However, for our 2D heterostructure samples, the lateral size of the device is on the order of micrometer, which is much larger than the superconducting coherence length ξ. Therefore, the voltage steps observed in our heterostructures can be understood in terms of the existence of strain-induced phase-slip lines, a 2D analogue of phase-slip centers.40 As shown in Figure 3, there exist clear strain domains in our CVDgrown heterostructures, which can act as nucleation centers for localized resistive state, so that phase-slip lines form across the heterostructure. Further evidence for the influence of the strain-induced inhomogeneities on superconducting transition is indicated by the measurements under applying a magnetic field. Figure 4C shows the differential resistance dV/dI of a CVD-grown graphene/2D α-Mo2C crystal heterostructure sample as a function of both current I and magnetic field B measured at a temperature of 150 mK. Similar multiple voltage steps behavior

graphene, forming clear strain domains (Figures 3C,D and S21). Such graphene/2D superconducting α-Mo2C heterostructure provides an ideal platform for investigating the influence of interface coupling on the superconductivity and obtaining high-quality Josephson junctions. We then measured the electronic transport properties of the CVD-grown graphene/2D α-Mo2C crystal heterostructures at low temperatures. Figure 4A shows the voltage versus current (V−I) characteristics of a device fabricated from a hetrostructure (inset) at various temperatures and zero magnetic field, which show three different regions. Below a critical value of current Ic, the voltage maintains a zero value, demonstrating a dissipationless supercurrent region. Upon increasing current, the device becomes dissipative with the appearance of a finite voltage, showing nonlinear V−I curves. For larger current exceeding a certain value In, a linear V−I curve is found, and the device is driven into the normal state. Figure 4B shows the V−I characteristics of another heterostructure device at various temperatures which are expanded in the regime of superconducting transition. Interestingly, multiple voltage steps (marked by arrows) are observed in the intermediate current regime, undergoing a transition between the superconducting state and normal state. These voltage steps are producible, and more pronounced and sharper at lower temperatures. In contrast, the pure 2D α-Mo2C crystal samples exhibit a sharp superconducting transition and no step-like feature is observed at 1.9 K (inset of Figure 4B). Moreover, such multiple voltage 5910

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Figure 5. Highly transparent Josephson junction device based on CVD-grown graphene/2D α-Mo2C crystal heterostructure. (A) Schematic cross-sectional view of a Josephson junction device based on a graphene/2D superconducting α-Mo2C crystal heterostructure with graphene as a weak link. (B) False-colored SEM image of a representative device. Here, the heterostructure is shaded in dark yellow, the Ti/Au contacts blue, and the graphene junction violet. (C) V−I characteristics of the Mo2C/graphene/Mo2C junction taken at different temperatures, exhibiting typical dc Josephson response. (D) 2D color plot of the differential resistance dV/dI across the junction as a function of bias current I and magnetic field B, measured at 100 mK. The dark blue regions correspond to the zero-resistance state. The critical current Ic, as denoted by the dashed lines, exhibits a modulation as a function of magnetic field.

transition temperature of the 2D α-Mo2C crystal. In this junction device, two 2D α-Mo2C crystals have a separation of L ∼ 96 nm and lateral width W ∼ 3.1 μm. A dissipativeless supercurrent through the junction is clearly observed below the critical current Ic, a characteristic dc Josephson response due to the proximity effect. Above Ic, a finite voltage appears switching to the dissipative quasiparticle conduction. For our junction, we obtain the product IcRn ≈ 50−70 μV at low temperature, where Rn is a normal state resistance. This value is significantly smaller than the BCS superconducting gap of Mo2C crystal (Δ ∼ 1.76 kBTc ∼ 300−400 μV).31 Such suppression of the IcRn product has been commonly observed in superconductor-nanostructures weak links and has been attributed to the influence of the electromagnetic environment on Josephson junction device.41,42 We also estimated the interface transparency T of our heterostructures using the standard Blonder−Tinkham− Klapwijk (BTK) theory (Figure S22).43 The extracted value of the interface transparency is T ∼ 0.7−0.8, indicating a highly transparent high-quality graphene-2D α-Mo2C interface in comparison with previous work on graphene hybrid/heterostructure devices with either deposited or stacked superconducting contacts.12−16 After characterizing the Josephson effect in our devices, we further examined the magnetic field dependent supercurrent of the junctions. In Figure 5D, we show 2D plot of differential resistance dV/dI as a function of both current I and magnetic field B measured at a temperature of 100 mK. It can be seen that the critical current is periodically modulated by the

is observed appearing as peaks in dV/dI. The central dark area indicates transport in the supercurrent regime with zero resistance outlined with the white dash lines. The Ic and In show systematic shift toward lower currents with increasing the magnetic field. In addition, we can clearly observe a broadened superconducting transition region, which can be attributed to the existence of phase-slip lines leading to the low dissipative state above Ic. In contrast, magnetic field-current phase diagram of pure 2D α-Mo2C crystal gives a sharper phase boundary, as shown in Figure 4D. These results indicate that our graphene/ 2D superconducting α-Mo2C heterostructures are attractive for fundamental investigations of rich phase diagram of superconducting transition in the 2D limit. Finally, we used these strongly coupled graphene/2D αMo2C crystal heterostructures to create highly transparent SNS junction devices with graphene as the weak link (Figure 5A). After the as-grown graphene/2D α-Mo2C crystal heterostructures were transferred onto SiO2/Si substrates, we selected the regions where two Mo2C crystals locate closely, and deposited Ti/Au electrodes onto the top surface of the heterostructures (Figure 5B). The devices were measured in a four-probe geometry by applying bias current through junction between outer contact while measuring the voltage drop cross the inner pair of contact. The spacing between the Mo2C crystals is varied between 90 and 300 nm. Figure 5C presents the V−I characteristics of a junction device with fourterminal current-bias measurement taken at zero magnetic field and different temperatures far below the superconducting 5911

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ACS Nano magnetic field, exhibiting typical Fraunhofer-like diffraction pattern as expected for uniform Josephson current distribution over the graphene junction area. The periodicity of the pattern, ΔB = Φ0/A ∼ 1.23 mT (where Φ0 = h/2e is the flux quantum), corresponds well to an effective area (A) of junction taking into account the London penetration depth of α-Mo2C crystals. Our experiments establish graphene/2D α-Mo2C crystal heterostructures as a promising platform for realizing high-quality graphene Josephson junctions and enabling further investigations of exotic quantum transport behavior in hybrid graphene-superconductor devices.19,20

solution. After that, the separated PMMA-coated heterostructures were stamped onto a target substrate, such as SiO2/Si, or a TEM grid, and finally warm acetone (55 °C) was used to dissolve the PMMA layer to obtain clean graphene/2D α-Mo2C crystal heterostructures. Structural Characterization. The morphology of graphene/2D superconducting α-Mo2C crystal heterostructures was identified using an optical microscope (Nikon LV 100D), the crystalline quality of 2D α-Mo2C crystals and the relative lattice orientation between graphene and 2D α-Mo2C crystals by TEM (FEI Tecnai T12, 120 kV for bright field TEM imaging and SAED measurements; FEI Titan3 G2 60−300 for STEM measurements), the thickness of 2D α-Mo2C crystals by AFM (Nanoscope IIIa), and the quality of graphene and the interfacial interaction between graphene and 2D α-Mo2C crystals by Raman spectroscopy (Jobin Yvon LabRAM HR800). Note that the Raman spectra of 2D α-Mo2C crystals excited by 532 nm laser show a strong background, and the graphene shows a low Raman intensity excited by 632.8 nm laser. Therefore, the Raman spectra of 2D α-Mo2C crystals and graphene below 700 cm−1 were collected by a 632.8 nm laser, and the Raman spectra above 1,150 cm−1 were collected by 532 nm laser. All the laser powers on sample surface were below 5 mW to avoid heating effects and structure damages. Device Fabrication and Transport Measurements. We first observed graphene/2D α-Mo2C samples transferred onto 290 nm SiO2/Si substrates using an optical microscope. Then, the selected flakes were optically positioned relative to predefined alignment marks, and the unwanted graphene layer was removed using reactive ion etching (RIE) O2 etching. Finally, contact metals (90 nm-Au/5 nmTi) were patterned on the surface of the heterostructure using standard electron-beam lithography followed by electron beam evaporation. The structures of the devices were characterized by SEM (Nova NanoSEM 430, 15 kV). All electrical measurements were performed in a 3He/4He dilution fridge with a superconducting magnet and a base temperature of 10 mK. The magnetic field was applied perpendicularly to the plane of the sample. For reducing high frequency noise, all leads were filtered using a series of π-filters, copper-powder filters, and RC filters at different temperature stages.

CONCLUSIONS In summary, we have demonstrated the direct growth of graphene/2D superconducting Mo2C vertical heterostructures by CVD, in which the neighboring 2D crystals are uniformly well aligned in the lattice orientation despite a significant lattice mismatch, have very high quality thanks to the unchanged reaction atmosphere, and strongly coupled in the interface. Such strongly coupled high-quality heterostructures provide us with the possibilities for the investigation of many exciting properties and intriguing applications that are not accessible in the existing heterostructures. For example, we found a phase diagram of superconducting transition with multiple voltage steps in the transition regime and created highly transparent Josephson junction devices. In addition, such high-quality heterostructures provide a promising platform for future studies of exotic quantum transport behaviors, such as Majorna modes. It is anticipated that the CVD approach presented here can be used as a general strategy for fabricating a broad class of highquality vertical heterostructures of graphene with 2D transition metal carbides with different properties, which will greatly expand the large family of 2D heterostructures.

ASSOCIATED CONTENT

METHODS

S Supporting Information *

CVD Growth and Transfer of Graphene/2D Mo2C Vertical Heterostructures. A Cu foil (Alfa Aesar, 99.999% purity, 25 μm thick) was cut into pieces of 5 × 5 mm2 and put on the top of a Mo foil (Alfa Aesar, 99.95% in purity, 100 μm in thickness) with the same size. They were then placed in a quartz tube of outer diameter 25 mm and inner diameter 22 mm as the growth substrate of graphene/2D αMo2C crystal heterostructures. Subsequently, the Cu/Mo substrates were heated to 1070 °C in a horizontal tube furnace (Lindberg Blue M, TF55030C) under H2 (200 sccm) and Ar (500 sccm). A small amount of CH4 (1.1 sccm) was then introduced into the reaction tube at ambient pressure to initiate the growth of a graphene film. After 30 min graphene growth, the substrate was heated to 1090 °C to grow 2D α-Mo2C crystals underneath the graphene on the liquid Cu/Mo substrates. The growth lasted 5 s to 10 min. Finally, the sample was cooled down to room temperature at a cooling rate of 20−50 °C/min under the same flow rate of CH4. The transfer of graphene/2D α-Mo2C crystal heterostructures is similar to the transfer of pure 2D α-Mo2C crystals.31 A thin layer of poly(methyl methacrylate) (PMMA, weight-averaged molecular mass Mw = 600 000, 4 wt% in ethyl lactate) was first spin coated on the surface of the heterostructures at 5000 r.p.m. for 1 min and cured at 180 °C for 30 min. Subsequently, the edge of PMMA-coated sample was cut to allow the etchant to diffuse into the interface between PMMA/graphene/Mo2C stacks and copper layer. Then the PMMAcoated sample was then immersed in a 0.2 M (NH4)2S2O8 solution at 70 °C for 30 min. Because of the much higher chemical resistance of Mo and Mo2C than Cu, only the Cu layer was etched away in this process, leading to the separation of PMMA-coated graphene/Mo2C heterostructures from Mo foil. Because of the protection of top graphene layer, no etching of Mo2C crystals occurred even after the heterostructure samples stayed for a long time in (NH4)2S2O8

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01638. SEM images of graphene/2D α-Mo2C heterostructures; Raman spectra and optical images of 2D α-Mo2C crystals, graphene, and graphene/2D α-Mo2C heterostructures; a large-view atomic-level HR-STEM image of the 2D α-Mo2C crystal in a heterostructure; TEM images and SAED patterns of stacked graphene/2D α-Mo2C heterostructures; I−V characteristics of a Josephson junction device (PDF)

AUTHOR INFORMATION Corresponding Authors

*[email protected]. *[email protected]. ORCID

Wencai Ren: 0000-0003-4997-8870 Author Contributions ⊥

C.X. and S.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of China (No. 2016YFA0200101 and 2016YFA0300601), National Science Foundation of China 5912

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ACS Nano

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(Nos. 51325205, 51290273, 51521091, and 11374019), and Chinese Academy of Sciences (Nos. KGZD-EW-303-1 and KGZD-EW-T06).

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DOI: 10.1021/acsnano.7b01638 ACS Nano 2017, 11, 5906−5914

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DOI: 10.1021/acsnano.7b01638 ACS Nano 2017, 11, 5906−5914