Grain boundaries and tilt angle-dependent transport properties of 2D

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Grain boundaries and tilt angle-dependent transport properties of 2D Mo2C superconductor Wencai Ren, Zhibo Liu, Chuan Xu, Cheng Wang, Shuang Song, Libin Wang, Yujia Wang, Ning Kang, Xiuliang Ma, and Hui-Ming Cheng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04065 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 16, 2019

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Grain Boundaries and Tilt Angle-Dependent Transport Properties of 2D Mo2C Superconductor Zhibo Liu1†, Chuan Xu1†, Cheng Wang2†, Shuang Song2, Libin Wang2, Yujia Wang1, Ning Kang2*, Xiuliang Ma1,3*, Hui-Ming Cheng1,4,5, Wencai Ren1,5* 1Shenyang

National Laboratory for Materials Science, Institute of Metal Research,

Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P. R. China. 2Key

Laboratory for the Physics and Chemistry of Nanodevices and Department of

Electronics, Peking University, 5 Yiheyuan Road, Beijing 100871, P. R. China. 3State

Key Lab of Advanced Processing and Recycling on Non-ferrous Metals,

Lanzhou University of Technology, 287 Langongping Road, Lanzhou 730050, P. R. China. 4Tsinghua-Berkeley

Shenzhen Institute (TBSI), Tsinghua University, 1001 Xueyuan

Road, Shenzhen 518055, P. R. China. 5School

of Materials Science and Engineering, University of Science and Technology

of China, 72 Wenhua Road, Shenyang 110016, P. R. China.

†These authors contributed equally to this work. *Correspondence to: [email protected] (W.C.R), [email protected] (X.L.M.), and [email protected]

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Abstract：Grain boundaries (GBs) of graphene and molybdenum disulfide have been extensively demonstrated to have strong influence on their electronic, thermal, optical and mechanical properties. 2D transition metal carbides (TMCs), known as MXenes, are a rapidly growing new family of 2D materials with many fascinating properties and promising applications. However, the GB structure of 2D TMCs and the influence of GB on their properties still remain unknown. Here, we used aberration-corrected scanning transmission electron microscopy combined with electrical measurements to study the GB characteristic of highly crystalline 2D Mo2C superconductor, a newly emerging member of 2D TMC family. 2D Mo2C superconductor shows unique tilt angle-dependent GB structure and electronic transport properties. Different from the reported 2D materials, the GB of 2D Mo2C shows peculiar dislocation configuration or sawtooth pattern depending on the tilt angle. More importantly, we found two new periodic GBs with different periodic structure and crystallographic orientations. Electrical measurements on individual GBs show that GB structure strongly affects the transport properties. In the normal state, an increasingly stronger electron localization behavior is observed at the GB region with increasing tilt angle. In the superconducting state, the magnitude of the critical current across the GBs is dramatically reduced, associated with local suppression of superconductivity at GBs. These findings provide new understandings on the GB structure of 2D TMCs and the influence of GB on 2D superconductivity, which would be helpful for tailoring the properties of 2D TMCs through GB engineering.

Keywords: 2D materials, MXene, grain boundaries, dislocations, superconductivity.


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Grain boundary (GB) is a common topological defect, which is the interface between two grains in polycrystalline materials. In two-dimensional (2D) limit, two laterally-connected grains generate one-dimensional (1D) tilt GB. In monolayer graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2), GB is a chain of polygon atomic rings such as 5/7, 6/8, 4/6, and 4/8 configurations1-6. Such 1D defects show many unique properties, such as 1D metallicity7, ferromagnetic half-metallicity and antiferromagnetic semiconductivity8, and strongly influence the mechanical, electrical, thermal, and optical properties of 2D materials1,6,9-13. 2D transition metal carbides (TMCs), known as MXenes, recently have attracted increasing interests due to their promising applications in energy storage, electromagnetic interference shielding, water purification, sensors, and catalysis14-18. Moreover, these materials have also been predicted/demonstrated to have interesting electronic, optical, magnetic, plasmonic and thermoelectric properties14,19. However, 2D TMCs synthesized by traditional chemical etching method usually have many defects and functional groups, which limit the studies on the GB structure of 2D TMCs and the intrinsic influence of GBs on their properties. Recently, several kinds of high-quality 2D TMCs free of functional groups have been synthesized by chemical vapor deposition (CVD), such as ultrathin Mo2C, WC, and TaC. As shown in Fig. S1, well-defined GBs with different structures are formed when two neighboring ultrathin Mo2C crystals joint together, which provide a good platform to understand the GB structure of 2D TMCs. In addition, CVD-grown ultrathin Mo2C crystals have been demonstrated to be a


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highly crystalline clean 2D superconductor20. The well-defined GBs also open up the possibility to reveal the GB-related 2D superconductivity phenomena, a subject of intense study from both fundamental and application points of view21,22. It is well known that various electronic states and correlation effects depend sensitively on the dimensionality of the electron system. The existence of quantum confinement and enhanced quantum fluctuations of the order parameter is expected to strongly influence the transport properties in 2D superconducting systems. Although the behavior of GBs in macroscopic superconducting crystals and films has been extensively studied, a microscopic understanding of the charge transport properties in 2D superconductors and the dependence of 2D superconductivity on the atomic structure of GB remain largely unexplored. Here, we used aberration-corrected scanning transmission electron microscopy (STEM) to study the GB atomic structure of CVD-grown 2D Mo2C superconductors, which provides some new GB structure information that were not observed in bulk TMC materials and other 2D materials. It was found that the GB structure of 2D Mo2C strongly depends on the tilt angle (θ). The low-angle GBs (LAGBs) show dislocation structure; two new high-angle GBs (HAGBs) with different periodic structures are observed, which show dislocation cores connecting end-to-end and regular sawtooth pattern, respectively; and the general HAGBs show irregular sawtooth patterns. Transport measurements on individual GBs reveal that the GB structure significantly affects 2D superconductivity. In the normal state, an increasingly stronger electron localization behavior is observed at the GB region with


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increasing θ. In the superconducting state, the presence of GBs leads to ~1 ‒ 2 orders of magnitude reduction in the critical current compared to single grains. LAGBs of 2D Mo2C crystals. As reported previously, CVD-grown Mo2C crystals have orthorhombic structure which is composed of hexagonal close-packed (HCP) molybdenum (Mo) atoms and interstitial carbon atoms orderly located in half Mo octahedra23-25. The crystal thickness is less than 20 nm. Figure S2a shows a low-magnification high-angle angular dark-field (HAADF)-STEM image of two joint 2D Mo2C crystals, which form a seamless GB. High-magnification HAADF-STEM images of two local areas in the GB indicate that these two GB segments (named GBS1 and GBS2) are composed of dislocation arrays distributed in a straight line (Fig. S2b, S2c). The θ was determined to be 6.27° by fast Fourier transformation (FFT) patterns (insets of Figs. S2b, S2c), confirming the formation of LAGB. Figure S3 shows another two LAGB structures with θ of 4.77° and 10.02°, respectively, which are also composed of dislocation arrays in a straight line. Noticeably, as shown in Figs. 1a, b and S3, the dislocation Burgers vectors (b) are perpendicular to GBs with constant dislocation distances (D) for these LAGBs, following the classic dislocation model of LAGB, D = b/2sin(θ/2). That is, the dislocation density is increased with increasing θ for the GBs with the same crystallographic orientation. Atomic-scale HAADF-STEM images show that GBS1 and GBS2 are along armchair and zigzag orientation, respectively, and consequently they have two different dislocation configurations (Fig. 1a, b). Such difference is determined by the crystal orientations of the edges of the two neighboring grains before coalescence.


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Similar to bulk ZnO26,27 and monolayer MoS24, the dislocation cores have a structure of projected 6/8 rings, which are composed of the atoms in different HCP layers. It can be seen that the dislocation cores are arranged with the same orientation in GBS1, while present 60° difference in orientation between each other in GBS2. As reported previously28, the dislocation configurations have a significant influence on the GB geometry. The orientations of the two lattices on the two sides of a GB are given by the equal and opposite rotations of the median lattice (± θ/2). The exact location of the GB plane is defined as the 0° contour, where the rotation changes from positive to negative across the boundary. Based on the elastic theory, an array of identical dislocations gives a flat boundary plane, however, an array of alternating dislocations gives a wavy boundary surface, which is independent of host material and thus a general feature of LAGB28. The geometric phase analysis (GPA) maps in Fig. 1c and 1d show the rotations of two lattices with respect to median lattice. Therefore, by mapping the local lattice rotations around dislocations via GPA29-32, we can identify the boundary plane on a near-atomic scale. It is clearly seen that GBS2 shows microscopic waviness in contrast to the straight line shape of GBS1 (Fig. 1c, d). The average D of GBS2 is smaller than that of GBS1 but the microscopic GB length of the waved GBS2 is longer at the same-sized region, suggesting that GBS2 has lower stability than GBS1. As a result, GBS1 segments are longer than GBS2 segments in both 6.27° and 11.77° GBs (Fig. S4). As we know, the intensity of HAADF-STEM image is nearly proportional to the square of atomic number (~Z2), and reflects the projected atomic density. Noticeably,


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every dislocation core we observed has the same atomic configuration but smaller atomic densities compared with those in the defect-free area (Fig. 1e). Moreover, the atoms in dislocation core have severe displacement compared with original hexagonal lattice such as the Mo atoms denoted by red arrows in Fig. 1e. In addition, the projected densities of Mo atoms are different even in the same dislocation core. For instance, the projected densities of Mo atoms denoted by pink arrows are slightly larger in comparison to those assigned by red arrows (Figs. 1e, S5). The reduction and difference of atomic density of Mo atoms in the dislocation cores in all LAGBs might be due to the interaction between surface effect and uneven spatial stress in the atomically thin sample. Integrated differential phase contrast (iDPC)33-35, a newly emerging technology, which shows a contrast that is roughly proportional to Z but can realize a high signal to noise ratio for light elements, was further used to image the carbon atoms in Mo2C lattice and GBs. It is clearly seen that the carbon atoms are located in the center of the 6 rings of Mo sublattice along the projective direction in Mo2C lattice (Fig. 1f), consistent with our previous observations by bright-field STEM25. Interestingly, we observed the interstitial carbon atoms in the 6 ring of dislocation core in LAGB (Fig. 1g). These observations are well consistent with the proposed atomic-scale structural models based on first-principle calculations (Fig. 1h, i). HAGBs of 2D Mo2C crystals. It is well known that the maximum tilt angle (θmax) of LAGBs that are composed of dislocation arrays is ~10° in bulk materials because of the presence of large lattice strain around dislocation core36. In 2D Mo2C crystals,


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the θmax of LAGBs we observed is ~11° (Fig. S4d), which inherits the nature of bulk materials. However, a straight HAGB with dislocation cores connecting end-to-end was observed (Fig. 2a, b). This HAGB has perfect symmetry and periodicity with θ = 21.63°. We define this boundary as HAGBa. When the dislocations infinitely get close to each other, asymmetric lattice strain will make GBS2 unstable (Fig. 1d). Therefore, HAGBa presents the dislocation configuration of GBS1 type. For some specific GBs, a certain fraction of atoms in one grain coincide with the positions extended from the crystal lattice of the other grain37. The coincidence sites constitute a new lattice, which is called as coincidence site lattice (CSL)37. The symbol ‘Σ’ is used to denote the degree of geometric coincidence between the two adjacent lattices. For instance, Σ7 GB refers to a GB which has CSL with Σ = 7. A coincidence site GB usually has a fine lattice matching and minimum energy along the densely packed plane of the coincidence lattice37. Interestingly, HAGBa is consistent with Σ7 GB of HCP lattice at [0001] orientation, which is similar to the GB structure of bulk ZnO bicrystal26. However, the Σ7 GB of ZnO bicrystal is composed of two structural units, namely 6/8 ring and 4/6 ring26. In sharp contrast, the HAGBa of 2D Mo2C is composed of only 6/8 rings. To understand the above difference, we constructed atomic models of two Σ7 GBs of Mo2C with 6/8 rings and 4/6 rings, respectively (Figs. 2h, S6). The calculated works of separation for the two boundary models are 4.72 J/m2 (6/8 type) and 4.55 J/m2 (4/6 type), indicating that both boundaries have strong bonding. The work of separation for the 6/8-type boundary is larger, which means that it is more stable. A


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close observation of the atomic structures of the two GB models shows that the minimal Mo-Mo bond length in the 4/6-type GB model is 2.47 or 2.51 Å, deviating much from the bulk value (2.89 ‒ 3.05 Å). In contrast, the minimal Mo-Mo bond length in the 6/8-type GB model is 2.56 or 2.60 Å, closer to the bulk value. This may be the reason why only 6/8 rings were observed in the HAGBa of 2D Mo2C lattice. Except for the CSL HAGBs, general HAGBs are popular in polycrystalline materials38, but lack of sufficient studies at the atomic level due to the complex structure and high boundary energy. The atomic thickness can largely reduce the boundary energy and simplify boundary geometry, and therefore 2D Mo2C crystals provide an ideal model to study the atomic level structure of general HAGBs. Very interestingly, we observed another new regular HAGB (HAGBb) with tilt angle of 38.18° (Fig. 2c). The equally-spaced strain dipoles suggest that this GB is also a periodic structure (Fig. 2d). Atomic-scale HAADF-STEM image shows that this periodic structure presents sawtooth pattern (Fig. 2e). Figure 2i displays the proposed atomic model of HAGBb. It can be seen that the yellow Mo atoms belonging to the two joint grains are located at “A” layer, while arrow denoted Mo atoms are sitting on the “B” layer (Fig. 2e), and the strain dipoles are nearly located on these atoms (Fig. 2f). The lattices on both sides of HAGBb are symmetric with only displacement along the GB. Therefore, HAGBb is actually a translational twin boundary39. Similar to the case of GBS1 and GBS2, as shown in Fig. 2a and 2e, HAGBa and HAGBb also have specific orientation relation, and present 90° or 30° angle between each other. However, HAGBb has larger period than HAGBa. More interestingly,


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HAGBb is not straight in a large length scale (Fig. 2c). As shown in Fig. 2g, the arrow-indicated sawtooth patterns should protrude to the right side to maintain the original periodicity of HAGBb, but protrude to the left side, leading to the deviation from the straight GB. Because the strain dipoles are centered on the GB line with small strain width, HAGBb has higher chemical-bond energy. To the best of our knowledge, this regular and periodic sawtooth GB is a new structure feature intrinsic to 2D Mo2C crystals, which has not been observed in the Mo2C bulk crystals and other 2D materials such as graphene, h-BN, and MoS2. Except for HAGBa and HAGBb, other HAGBs exhibit more complex irregular sawtooth structure. Figure 3a and 3b show the atomic-resolution HAADF-STEM images of two HAGBs with θ of 25.48° and 28.33°, respectively. Similar to the conserved atomic bonding sequences in the GBs of graphene40, numerous different structural units are connected each other in the HAGBs of 2D Mo2C crystals. As θ increases, the configurations of structural units become more regular, leading to more evident face-to-face configuration of zigzag edge and armchair edge along the GB (Fig. 3b). In addition, the centers of some local pentagons in both boundaries are inserted with additional Mo atoms (Fig. 3c, d), which can effectively reduce the boundary energy. In general, the higher angled GB has higher boundary energy41. Therefore, to improve the relative stability, 28.33° GB has larger number of inserted Mo atoms compared with 25.48° GB. Transport property across individual GBs. On the basis of understanding on the GB structure at atomic scale, we performed systematic transport measurements on a


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series of ultrathin Mo2C nanosheets to explore the GB influence on their electronic transport properties. Figure 4a shows a representative scanning electron microscopy (SEM) image of a 2D Mo2C device, consisting of two regular shaped grains that form an individual GB with θ~25°. The angle identification is shown in Fig. S7. Multi-terminal configuration was used for simultaneous transport measurements both within single grain (intragrain) and across GB (intergrain). Figure 4b displays the temperature-dependent four-terminal sheet resistance R(T) measured in a typical GB device at zero magnetic field, for the intragrain (red) and intergrain (blue) regions. Both curves exhibit a sharp drop in the resistance as the temperature is lowered below Tc~3.5 K, indicating a superconducting transition. However, the charge transport property across the GB shows two distinguishing features. First, the room-temperature resistance across the GB is significantly larger by one order of magnitude in comparison to that of single-grain region. It should be noted that the measurements on multiple devices with GBs produced similar results (Table S1), indicating that the GBs are the dominant source of electron scattering and resistance in ultrathin Mo2C crystals. The second striking feature is that the intragrain and intergrain resistance have different dependence on temperature. The resistance across the GB exhibits a nonmonotonic temperature dependence. At low temperatures, the intergrain resistance increases markedly with decreasing temperature, resulting in a resistance minimum at a characteristic temperature T* near 41 K (Fig. 4b), indicative of strong localization behavior at the GB region. In contrast, the intragrain resistance decreases monotonically with temperature, exhibiting a metallic behavior consistent with our


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previous measurements of single-grain ultrathin Mo2C crystals20. We further studied the influence of tilt angle of GB on the transport property of 2D Mo2C nanosheets. Figure 4c presents temperature-dependent intergrain resistance curves for four devices made by GB samples with different θ, normalized to the minimum value in the normal state (R(T*)) for clarity. The curves clearly exhibit a resistance minimum and an upturn toward insulator-like behavior in the normal state for T< T*, except for the sample with θ = 5.86°. As the θ is increased, the rise in R(T) at low temperatures becomes more and more evident, and the T* is shifted to a higher temperature. The observed drastic increase of resistance can be attributed to the electronic states localized at the GBs. Such localized states are a common feature in other 2D atomic crystals including polycrystalline graphene42,43 and monolayer transition metal dichalcogenides6,11. With increasing θ, the defect density in GB increases, as we found in the GB microstructures by STEM observation. Therefore, the observed tilt angle-dependent transport across ultrathin Mo2C GBs can be understood as a consequence of defect density increase that induces strong electron localization at low temperatures. In contrast to LAGBs in which disorder centers are discrete, disorder configuration is continuous for HAGBs, and 1D localized electronic states can be formed giving rise to a strong enhancement of electron scattering at the GBs. In the inset of Fig. 4b, we present a fit to the temperature-dependent conductance for the sample with θ~26.8, in terms of theoretical model of 1D weak 2 2 e 1 localization using the equation44, G (T ) ~ G0  , where L is the length of hL T 


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channel. The resistance upturn at low temperatures follows a power law with a power index of  =1/3, consistent with the localization behavior expected for the 1D character of HAGB. To find the GB effect on superconducting state properties, we have also investigated the charge transport below Tc. Figure 4d shows representative current-voltage (I-V) characteristics of the intragrain and intergrain regions for a 3.61°–GB device, measured at a temperature of 100 mK. The curves exhibit a clear dissipationless supercurrent branch with a critical current Ic, at which the sample switches from the superconducting state to the resistive state. Within single grain, the typical values of critical current are about 50 ‒ 80 A in agreement with our previous results for single-crystal ultrathin Mo2C crystals20. In contrast, the critical current across the GB is ~1-2 orders of magnitude lower than that of either of the two grains. Such large reduction in Ic suggests that the GBs strongly suppress superconductivity in ultrathin Mo2C crystals. On the one hand, the amplitude of the superconducting order parameter can be suppressed by an increasing level of disorder induced by GBs due to the destruction of Cooper pair correlations, phase fluctuations and inhomogeneities45. This is consistent with our observation of enhanced electron localization at the GB region in the normal state. On the other hand, the strain-induced structural modifications are expected to strongly affect the electron-phonon coupling, leading to the degraded superconductivity21. Further experimental and theoretical studies are needed to understand the influence of disorder and strain on the superconductivity in ultrathin Mo2C crystals.


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To summarize, we used aberration corrected electron microscopy to investigate the GB structure of highly crystalline 2D Mo2C superconductor. Some new GB structure features that are different from the bulk TMC materials and other 2D materials are observed. The GB of 2D Mo2C shows unique dislocation structure or sawtooth pattern depending on the tilt angle. Moreover, two new periodic HAGBs, a Σ7 GB with dislocation cores connecting end-to-end and a translational twin boundary with regular sawtooth pattern, are observed. The GB leads to an increasingly stronger electron localization behavior with increasing tilt angle in normal state and ~1 ‒ 2 orders of magnitude reduction in the critical current as switching from superconducting to resistive state. Note that the influence of GBs on the transport properties of 2D Mo2C superconductor observed here is much stronger than those reported in other 2D materials such as graphene and monolayer MoS210,12. These findings provide new understandings on the GB structure of 2D TMC and the influence of GB on 2D superconductivity, which would be helpful for tailoring the properties of 2D TMC through GB engineering.

Legend Figure 1. Atomic structures of a LAGB in 2D Mo2C nanosheet. (a,b) Atomic-scale HAADF-STEM images of linear (a) and waved (b) dislocation configurations of 6.27° LAGB. The bright atomic features are the signals of Mo atoms, and the carbon atoms are invisible, which are located in the lattice interstices (dark features) as shown in f. The different colored circles indicate the Mo atoms in different HCP layers, forming


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dislocation cores with projected 6/8 ring structure. The red lines around dislocation cores are Burgers circuits, and the yellow arrows represent the magnitude and direction of Burgers vectors. (c,d) Lattice rotation mappings of GBS1 (c) and GBS2 (d) analyzed by GPA. (e) High-magnification HAADF-STEM images of the dislocation cores, showing smaller and inhomogeneous atomic densities compared to the defect-free area. (f,g) Atomic-scale iDPC-STEM images of pristine Mo2C lattice (f) and the dislocation core (g) in a LAGB, clearly showing the positions of light element carbon and heavy element Mo simultaneously. (h,i) Plane-view (top) and side-view (bottom) atomic models of GBS1 (h) and GBS2 (i). The atoms at different layers are indicated by different colors. The atoms in dislocation cores are enlarged to clearly show the defect structures.

Figure 2. Atomic structures of a Σ7 HAGB and a translational twin boundary in 2D Mo2C nanosheets. (a,b) Atomic-scale HAADF-STEM image of HAGBa with dislocation cores connecting end-to-end (a) and the same image with dislocation cores superimposed by colored circles (b). (c,d) Low-magnification HAADF-STEM image of HAGBb with regular sawtooth pattern (c) and the corresponding horizontal normal strain mapping (d). (e,f) Atomic-scale HAADF-STEM image (e) obtained from the area ‘1’ in c and the corresponding horizontal normal strain mapping (f). In e, the yellow dots denote the Mo atoms belonging to the two joint grains at the same layer, and the red arrow denoted Mo atoms are at the other layer. (g) Atomic-scale HAADF-STEM image obtained from the area ‘2’ in g, showing a local curved GB.


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The yellow arrows denote the positions that lead to the change of original periodicity. (h,i) Plane-view (top) and side-view (bottom) atomic models of HAGBa (h) and HAGBb (i). The atoms at different layers are indicated by different colors. For clarity, the atoms in the GBs are enlarged.

Figure 3. General HAGBs with irregular sawtooth configurations in 2D Mo2C nanosheets. (a,b) Atomic-scale HAADF-STEM images of 25.48° (a) and 28.33° (b) HAGBs. The pink and yellow colored circles were superimposed on GB atoms to show the profile of the GB. The additional atoms that are inserted into the centers of Mo pentagons are denoted by arrows. (c,d) Local atomic structures with additional Mo atoms inserted into the pentagons, which were extracted from the boxes in 25.48° (a) and 28.33° (b) HAGBs, respectively. The red circles denote the positions of additional Mo atoms.

Figure 4. Electrical transport across individual GBs in 2D Mo2C nanosheets. (a) False-colour SEM image of ultrathin Mo2C crystal device with multiple terminals, showing device across an individual GB. (b) The sheet resistance of the intragrain (red) and intergrain (blue) regions for a typical GB device as a function of temperature. Inset: The intergrain conductance at low temperature obeys a power law dependence expected for a 1D localization behavior. (c) Temperature dependence of the normalized resistance for several GB samples with different θ. The curves are normalized to their resistance minimum value R(T*) in the normal state for clarity. (d)


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Current-voltage characteristics measured across the GB (blue) and within single grain (red), showing the critical current across the GB is strongly suppressed. Inset shows the current-voltage characteristics across the GB in an expanded current scale.

Figure 1


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Figure 4

Methods Sample preparation. The 2D Mo2C samples were fabricated by CVD as we reported previously46. A bilayer stack of a Cu foil (Alfa Aesar, 99.9% purity, 12.5 μm thickness) sitting on the top of a Mo foil (Alfa Aesar, 99.95% purity, 100μm thickness) were used as growth substrate. First, graphene film was grown on Cu/Mo substrate at a temperature below Cu melting point (1085 C). Subsequently, the substrate was heated to 1100 C for more than 20 min to grow joint 2D α-Mo2C crystals with GBs at the interface between the graphene and the liquid Cu. After growth, the samples were transferred to TEM grids or SiO2/Si substrates for further characterization and measurements.


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Structural characterization. Optical images were acquired on Nikon LV 100D. SEM images of electronic devices were obtained on Nova NanoSEM 430. HAADF-STEM images were obtained on a FEI Titan3 G2 60-300 instrument equipped with a high-brightness field-emission gun (X-FEG), double spherical aberration corrector, and a monochromator. The probe convergence angle is 21.4 mrad, and camera length is 115 mm. The iDPC-STEM images were obtained on FEI Titan Cube Themis G2 300. Theoretical calculations. All the first-principles calculations were performed using density functional theory (DFT) in the Vienna ab-initio simulation package (VASP). The Perdew-Burke-Ernzerhof form of generalized gradient approximation (GGA) was implemented as the exchange-correlation functional. Mo 4p64d55s1 and C 2s22p2 were treated as the valence states using the projector augmented wave (PAW) pseudopotentials with an energy cutoff of 500 eV. The interface models were constructed based on the experimental observations (6/8-type GB) and ZnO structure26. The Monkhorst-Pack k-point mesh was chosen as 1 × 4 × 1 for these two interface models. All these structures were optimized through the conjugate-gradient (CG) method until the Hellmann-Feynman force on each atom was less than 10 meV/Å. The works of separation for the interface models were calculated as according to the following formula: Wsep = ( Esurf × 2 − Eint ) / 2A

(1)

where Eint and Esurf are the total energies of the interface model and the slab model containing half of the atoms of the interface model, respectively, A is the interface


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area, and the factor 2 in the denominator is introduced since there are two interfaces in the models. Device fabrication and transport measurements. After transferring onto SiO2/Si substrate, the Mo2C flakes containing GBs were identified and located by optical microscope. The selected GB samples with various title angles were positioned relative to predefined markers, and the unwanted graphene layer was removed using reactive ion etching (RIE) O2 etching. To enable simultaneous transport measurements both within single grain and across a GB, Mo2C devices in a multi-terminal configuration were fabricated. Metal contacts consisting of Ti (5 nm)/Au (90 nm) were patterned using electron beam lithography followed by electron beam evaporation. Transport measurements were performed in a Physical Property Measurement System (Quantum Design DynaCool) at temperatures varying from 300 K down to 1.8 K, and a 3He/4He dilution fridge with a base temperature of 10 mK. The four-terminal resistance measurements were carried out with the standard current-biased lock-in technique and a low frequency excitation current in the range of 0.1−1 µA. I-V characteristics of the devices were measured using a four-terminal current-driven measurement scheme.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Optical images of the joint 2D Mo2C nanosheets; a 6.27°-LAGB in 2D Mo2C


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nanosheet; atomic-scale HAADF-STEM images of 4.77° and 10.02° LAGBs with different dislocation densities in ultrathin Mo2C nanosheets; LAGBs composed of GBS1 and GBS2; different projected densities of Mo atoms in dislocation core; atomic model of 4/6-type Σ7 GB; the schematic of θ determination of 2D Mo2C GB; and room temperature inergrain and intragrain resistance of several ultrathin Mo2C nanosheets with individual GBs.

ACKNOWLEDGEMENTS The authors thank Mr. Bo Wu for TEM technological support. This work was supported by National Science Foundation of China (Nos. 51325205, 51290273, 51521091, 51802315, 51802314, and 11774005), and Chinese Academy of Sciences (Nos.

KGZD-EW-303-1,

KGZD-EW-T06,

174321KYSB20160011,

and

XDB30000000), SYNL-T.S. K Research Fellowship, and the Youth Innovation Promotion Association of Chinese Academy of Sciences.

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Grain boundaries and tilt angle-dependent transport properties of 2D

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