Letter Cite This: Nano Lett. XXXX, XXX, XXX−XXX
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Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe2 Single Crystals and Their Thickness-Dependent Electronic Properties Ma Huifang,†,& Peng Chen,†,& Bo Li,‡ Jia Li,† Ruoqi Ai,† Zhengwei Zhang,† Guangzhuang Sun,† Kangkang Yao,† Zhaoyang Lin,§ Bei Zhao,† Ruixia Wu,† Xuwan Tang,† Xidong Duan,*,† and Xiangfeng Duan§ †
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China ‡ Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China § Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe2) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe2 nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 μm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-raydiffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies show characteristics Eg and A1g vibration modes at ∼109 and ∼155 cm−1, with a systematic red shift with increasing nanosheet thickness. Electrical transport studies show the 2D PtTe2 nanosheets display an excellent conductivity up to 2.5 × 106 S m−1 and show strong thickness-tunable electrical properties, with both the conductivity and its temperature dependence varying considerably with the thickness. Moreover, 2D PtTe2 nanosheets show an extraordinary breakdown current density up to 5.7 × 107 A/cm2, the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides to date. KEYWORDS: Chemical vapor deposition, PtTe2 nanosheets, 2D materials, electrical conductivity, breakdown current density
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hanks to inspiration from the discovery of graphene,1−8 two-dimensional (2D) layered atomic crystals9−15 (e.g., MoS2,16,17 MoSe2,18−22 WS2,23−26 WSe2,27 CdI2,28 and BiI3,29 SnS2,30,31 GaSe,32 hexagonal BN,33 and black phosphorus34) have drawn rapidly growing scientific interest for their rich physical and chemical properties and exciting potential for applications in future electronic and optoelectronic devices, catalysis, energy technologies, biosensors, and magnetic devices.31 Beyond the widely explored graphene and semiconducting transition-metal dichalcogenides (TMDs), metallic TMDs (MTMDs) (e.g., TaS2, TaSe2, NbSe2, Td-MoTe2, and VS2) have recently attracted considerable interests for 2D superconductors and charge-density waves as well as novel electronic applications. TaS2 and NbSe2 have been proven to be ideal systems for exploring 2D collective electronic states down to the limit of single-layer thickness.35−37 The thicknessdependent charge-density-wave phase transitions of TaS2 offers an interesting system for exploring novel many-body physics.35 The NbSe2 are shown to exhibit a thickness-dependent © XXXX American Chemical Society
superconducting properties with a transition temperature increasing from 1.0 K in monolayers to 4.56 K in 10 layers.38 TaSe2 flakes have been shown to exhibit a best breakdown current density of up to 3.7 × 107 A/cm2,39 with exciting potential for spin-torque devices and 2D interconnects. The TdMoTe2 exhibits a unique set of properties, including a topological superconducting phase.40,41 The VS2 nanosheets show spontaneous superlattice periodicities and excellent electrical conductivities (3 × 105 S m−1).42 Together, MTMDs exhibit rich physical properties and are attracting rapidly growing interest as a new branch of 2D materials research. However, most studies to date are limited to exfoliated flakes, with limited controllability, yield, and reproducibility, which are Received: February 9, 2018 Revised: April 18, 2018
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DOI: 10.1021/acs.nanolett.8b00583 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters clearly not scalable for practical applications. Here, we report the synthesis of few-layer PtTe2 nanosheets with controllable thickness. PtTe2 has been reported as a type-II Dirac semimetal, offering a rich playground for investigating novel quantum phenomena (e.g., novel transport related to chiral anomaly and topological phase transition).43 Similarly, Dirac and Weyl semimetals (3D Cd3As2,44,45 YPd2Sn,46 PdTe2,47 3D Na3Bi,48 and 3D WTe2)49 have attracted intense attention among scientific community for their potential applications in nextgeneration spintronics and quantum computing. There is considerable interest in exploring Dirac and Weyl semimetals at the limit of single or few-atom thickness. However, the controlled synthesis of 2D Dirac and Weyl semimetals, particularly at single-atom or few-atom thickness, remains a significant challenge to date. Here, we report, for the first time, a one-step chemical vapor deposition (CVD) approach for the direct synthesis of fewlayer PtTe2 monocrystalline nanosheets on SiO2/Si substrates. Optical microscopy (OM) and scanning electron microscopy (SEM) studies show the resulting PtTe2 nanosheets typically exhibit a hexagonal or triangular shape with the lateral domain size varying from 2 to ∼80 μm. Atomic force microscopy (AFM) studies reveal the thickness of the nanosheets ranges from 1.8 nm to ∼20 nm. X-ray diffraction (XRD) studies indicate that the nanosheets are the hexagonal-phase PtTe2 structure with (001) plane oriented parallel to the substrate. The high-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) characterizations further confirm the PtTe2 nanosheets are high-quality single crystals. Raman spectroscopy studies show the characteristic in-plane Eg vibration mode at ∼109 cm−1 and the out-of-plane A1g vibration mode at 155 cm−1, respectively, with the resonant frequency showing a systematic red shift with increasing thickness. Electrical transport studies show 2D PtTe2 has a high conductivity, the highest breakdown current density reported for 2D-MTMDs to date, and strong thickness-tunable electronic properties, suggesting that the 2D PtTe2 is an exciting platform for investigating the novel quantum phenomenons. Results and Discussion. The PtTe2 nanosheets are synthesized using a home-built CVD system in a 1 in. tube furnace at ambient pressure, with the platinum powder and tellurium power as the source materials and SiO2/Si substrate as the growth substrate (Figure S1). Briefly, the precursor powder Te (∼600 mg) and Pt (∼40 mg) were placed in the furnace at temperatures of ∼420 and 1100 °C, respectively. The center of the furnace was heated to 1100 °C. Pieces of SiO2/Si (285 nm SiO2) substrates were placed at certain positions in the downstream end of the furnace. Before heating, the system was purged with 1000 standard cubic centimeters per minute (sccm) ultrahigh-purity argon (Ar) gas for 2 min, ramped to the desired source temperature (420 °C for Te and 1100 °C for Pt) at a rate of 22 °C/min, and kept at desired temperature for 30 min under a continuous Ar flow of 120 sccm. The growth was terminated by shutting off the power of the furnace, and the system was naturally cooled down to room temperature. Figure 1a,b show the stick-and-ball crystal structure model of PtTe2, which has a layered structure with a hexagonal unit cell and crystallizes in P 3m ̅ 1 space group. In the crystal of PtTe2, each Pt atom is surrounded by six Te atoms (Figure 1a). The resulting nanosheets show highly distinct optical contrast under a bright-field optical microscope (Figure 1c), which can be attributed to different optical interference resulting from the
Figure 1. (a) Side view of the PtTe2 layered structure. (b) Top view of the PtTe2 layered structure. Pt atoms are green, and Te atoms are red. (c) Optical microscopy (OM) image of PtTe2 nanosheets with different thickness grown on SiO2/Si. The scale bar is 10 μm. (d) SEM image of hexagonal PtTe2 nanosheets grown on the SiO2/Si substrate. The scale bar is 10 μm. (e) OM and the (f) corresponding atomic force microscopy images (1.8 nm) of PtTe2 nanosheets with the thickness of 1.8 nm grown on SiO2/Si. Scale bars in panels (e) and (f) are 2 μm.
variable thickness. The OM (Figures 1c,e and S3) and SEM (Figure 1d) images reveal PtTe2 nanosheets are mostly of hexagonal or triangular shapes with the lateral size varying from a few microns up to ∼80 μm. The thickness of the resulting nanosheets can be tailored down to 1.8 nm (Figure 1f) and varied up to ∼20 nm (Figure S2), as confirmed by AFM studies. To tailor the thickness of the resulting nanosheets, we have conducted systematic studies to investigate the effect of substrate temperature and flow rate of carrier gas (Figure 2). With the source temperature kept constant at 1100 °C and Ar flow rate constant at 120 sccm, the nanosheets produced at different deposition temperature zones show a clear thickness and morphology evolution. At higher substrate temperature of ∼690 °C, relatively thick PtTe2 nanosheets (∼11−23 nm) are produced with most domains adopting hexagonal shape (Figure 2a). With the decrease of the substrate temperature to ∼645 °C, the hexagonal shaped PtTe2 domains remain with the thicknesses of ∼5−10 nm (Figure 2b). When the growth temperature is decreased to ∼585 °C, the nanosheets largely remain hexagonal with further reduced thickness to 3.7−5 nm (Figure 2c). As the growth temperature is further decreased to ∼450 °C, the thickness of the resulting nanosheets is further reduced to ∼3.0 nm, with the domains typically adopting a triangular shape (Figure 2d). Overall, we observed two main trends as the growth temperature decreases. First, the layer numbers of PtTe2 decrease with reducing substrate temperature. Second, the PtTe2 nanosheets evolves from mostly hexagonal toward triangular morphology with decreasing growth temperatures. These results resemble MoS2 and WSe2 crystals grown on SiO2/Si.50,51 These studies clearly show that the growth temperature is critical in affecting PtTe2 growth kinetics. In general, the higher substrate temperature enhances surface diffusion of the chemical precursors and promotes the nucleation of the extra atomic layers to produce multilayers27 (Figure 2i), while at lower growth temperatures, the active reactants are less mobile and quickly add to the fastest growth front dominated by edge energetics under specific conditions, leading to thinner nanosheets with a more-dominant triangular shape. B
DOI: 10.1021/acs.nanolett.8b00583 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 2. (a−d) The source temperature (1100 °C) and Ar flow rate (120 sccm) and acquisition of nanosheets with various average thicknesses in different deposition-temperature zones. The deposition temperature of panels a−d are ∼690, ∼645, ∼585, and ∼450 °C, respectively, and the corresponding thicknesses of nanosheets are ∼11−23, ∼5−10, ∼3.7−5, and ∼2.9−3.7 nm, respectively; all of the scale bars are 10 μm. (e−h) The source temperature of 1148 °C, the deposition temperature zone of ∼570 °C, and different Ar flow rates can result in nanosheets with various average thicknesses. The flow of Ar for panels e−h are 60, 120, 200, and 400 sccm, respectively, and the corresponding thickness of PtTe2 nanosheets are ∼11−23, ∼4−7, ∼3−4, and ∼1.8−3 nm, respectively; all of the scale bars are 10 μm. (i) Average thickness of PtTe2 nanosheets plotted as a function of different deposition-temperature zones. Insets are optical images of PtTe2 nanosheets on 285 nm SiO2/Si, and all of the scale bars are 5 μm, showing contrast evolutions from white to purple when their thicknesses decrease. (j) Average thickness of PtTe2 nanosheets plotted as a function of Ar flow rate. Insets are optical images of PtTe2 nanosheets on 285 nm SiO2/Si, and all of the scale bars are 5 μm, showing contrast evolutions from white to purple when their thicknesses decrease.
We have also conducted systematic studies to investigate the effect of flow rate of carrier gas under a constant source temperature (1148 °C for Pt) (Figure 2e−h). In the deposition temperature zone ∼570 °C, a clear trend is observed in the nanosheet thickness with Ar flow rate. Relatively thick PtTe2 nanosheets (∼11−23 nm) are obtained at a low Ar flow rate (60 sccm). With increasing Ar flow rate, the nanosheets get thinner and thinner (∼4−7 nm at 120 sccm; 3−4 nm at 200 sccm; and ∼1.8−3 nm at 400 sccm) (Figure 2f−h). A similar evolution from mostly hexagonal thicker domains to triangular thinner domains is also observed with increasing flow rate (Figure 2j), suggesting the growth at high flow rate is more kinetically dominated. This growth trend is similar to the synthesis of vanadium diselenide by chemical vapor deposition.52 The XRD studies indicate that the diffraction pattern can be indexed to space group P 3̅m1 (164) with lattice parameters of a = b = 4.026 Å, c = 5.221 Å (JCPDS no. 18-0977; Figure 3a). The three primary diffraction peaks can all be indexed to the (001), (002), and (003) family planes of hexagonal-phase PtTe2, respectively, suggesting that the nanosheets are all wellaligned with the [001] direction normal to the growth substrates. Raman spectroscopic studies show two prominent resonance modes at the ∼109 and ∼155 cm−1 range, corresponding to the in-plane Eg vibration mode and out-ofplane A1g vibration mode (Figure 3b), respectively,43 which are characteristic for 1T-PtTe2 structure.53 A close inspection of the
Figure 3. (a) XRD pattern of PtTe2 nanosheets grown on the SiO2/Si substrate. (b) Raman spectra of PtTe2 nanosheets with different thickness. (c) Optical microscopy of and the corresponding (d) Raman intensity map of the synthesized PtTe2 nanosheet with a Raman peak located at 155 cm−1. Raman experiments were performed in a confocal spectrograph using a 633 nm excitation laser. Scale bars in panels (c) and (d) are 5 μm.
Raman shift versus the layer thickness reveals a systematic shift. The Eg mode shifts from 109.05 to 111.72, 114.76, and 121.79 C
DOI: 10.1021/acs.nanolett.8b00583 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters cm−1 when the thickness is reduced from 10.8 to 4.2, 3.7, and 2.9 nm, respectively; while at the same time, the A1g mode shifts from 154.74 to 155.91, 156.13, and 157.19 cm−1, respectively. Thus, both Eg and A1g modes show a red shift with increasing layer number. The trend is similar to the red shift of E12g peak of multilayer MoS2 with increasing layer number.54 In general, the interlayer van der Waals force suppresses atomic vibration with the increasing layer number, which is expected to lead to higher force constants and blue shift of E12g and A1g modes in thicker materials.55 However, the stacking-induced structure changes or long-range Coulombic interlayer interactions56,57 could soften the vibration and lead to a lower force constant and a red shift in the corresponding Raman modes in thicker materials, as observed in multilayer MoS2. The observed red shift for Eg and A1g modes in thicker PtTe2 suggest that van der Waals interlayer interactions do not play a dominant role here. In addition, the OM image (Figure 3c) of a typical PtTe2 hexagonal domain and the corresponding spatially resolved mapping of the Raman signals (155 cm−1) show uniform contrast (Figure 3d), indicating a highly uniform crystal structure across the entire domain. We have further analyzed the structural and chemical composition of the synthesized PtTe2 nanosheets using TEM, energy-dispersive spectroscopy (EDS), EDS elemental mapping, and HRTEM. Figure 4a shows high-angle annular dark-
corresponding to the (110) and (100) planes of the PtTe2 hexagonal structure (Figure 4e). The SAED is further used to characterize the crystal structure of the nanosheet. The single set of diffraction spots with 6-fold symmetry (Figure 4f) demonstrates that the triangular PtTe2 nanosheet is a single crystal with hexagonal structure. To study the electrical transport properties of the synthesized PtTe2 nanosheets, we fabricated PtTe2 devices on a SiO2/Si substrate, in which the silicon substrate may be used as a gate to tune the electrostatic potential of the PtTe2 device, like a fieldeffect transistor (FET). The output characteristics (Ids versus Vds) of a 2.6 nm thick PtTe2 nanosheet devices show a linear and symmetric relationship (Figure 5a), indicating that good ohmic contacts are formed. Different from 2D PtS2 and PtSe2 at same thickness, the gate voltage shows no obvious influence on the source-drain current, suggesting a metallic behavior of 2D PtTe2. The flat transfer characteristics (Ids versus Vg) further verified the metallic behavior (Figure S4). The conductivity of PtTe2 nanosheets range from 1 × 105 S to 2.5 × 106 S m−1 at room temperature, which is considerably larger than that of PtS2 and PtSe2, suggesting an enhanced metallic nature in PtTe2.58,59 Our experimental observation is consistent with the theory predictions.60 The conductivity of PtTe2 strongly depends on the thickness. Upon an increase in the thickness of PtTe2 nanosheets from 2.6 to 14.5 nm, the conductivity increased about 1 order (Figure 5b), which is due to the increased metallic nature of PtX2 as the thickness increased.59,60 The temperature-dependent conductivity of the PtTe2 nanosheets with different thickness further verified the thicknessdependent electronic properties (Figures 5c and S5). The conductivity of the PtTe2 nanosheet with thickness above ∼5 nm increases as the temperature decreases, while the conductivity of the PtTe2 nanosheets with thickness ∼4 nm or less decreases slightly as the reducing temperature(Figure S5), suggesting the reduced metallic nature of PtTe2 nanosheets in the ultrathin regime. Hall measurements reveal interesting temperature induced carrier type switching characteristics (Figure S6 and Table S5), which has been reported in Dirac semimetals due to their small conduction and valenceband overlapping.61−64 Additionally, the Hall measurements also indicate that the carrier concentration at room temperature in thinner nanosheets (∼3 nm) is about 1 order of magnitude lower than that in thicker nanosheets (∼5 nm), which might be attributed to reduce band overlapping between the conduction band and valence band in thinner nanosheets.58,59 We have evaluated the breakdown current density of the PtTe2 nanosheets. Measurement of the breakdown current density is performed by continuously increasing the bias voltage on the device until a sudden decrease of the current to zero. The breakdown current density is taken as the current density right before the sharp decrease in current. Figure 5d shows the current density Jds as a function of Vds. The maximum breakdown current density observed in the PtTe2 FET is up to 5.7 × 107 A/cm2, which is the best value achieved on 2D MTMDs and comparable with graphene nanoribbons65 and carbon nanotubes.66 This large breakdown current density will be valuable for their applications in on-chip electrical interconnects or spin torque devices. Conclusions. In summary, we have successfully synthesized highly crystalline PtTe2 nanosheets for the first time. The obtained PtTe2 nanosheets exhibit hexagonal and truncated triangle and triangle shapes, with a lateral size of up to ∼80 μm and a thickness as small as 1.8 nm. HRTEM and SAED studies
Figure 4. (a) High-angle annular dark-field scanning transmission electron microscopy image of a PtTe2 nanosheet achieved on STEM grids. (b, c) EDS mapping images of Pt and Te for a triangular PtTe2 nanosheet achieved on a STEM grid, respectively. (d) EDS elemental analysis of the transferred sample achieved on STEM grids. (e) HRTEM image and (f) SAED image of the transferred PtTe2 nanosheets on a copper mesh with carbon film, respectively. Scale bars in panels a−c are 200 nm; in panels e and f, scale bars are 1 and 2 1/nm, respectively.
field scanning transmission electron microscopy (HAADFSTEM) image of a PtTe2 nanosheet, confirming the wellfaceted triangular geometry. The corresponding EDS elemental mapping images (Figure 4b,c) clearly show the spatial distribution of the Pt and Te elements in the PtTe2 nanosheet. The uniform color contrasts demonstrate the compositional uniformity of the PtTe2 nanosheet. The quantification of EDS spectrum shows that the atomic ratio between Pt and Te is approximately 1:2 within the range of experimental error (Figure 4d), consistent with the expected stoichiometry ratio. The HRTEM image reveals a clearly resolved hexagonal lattice arrangement with the lattice spacings of 0.201 and 0.345 nm D
DOI: 10.1021/acs.nanolett.8b00583 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 5. Electrical properties of PtTe2 devices. (a) Output characteristics of a 2.6 nm thick PtTe2 device (Vg: −60 to 60 V). The inset shows the OM image of a typical PtTe2 FET device; the scale bar is 10 μm. (b) Conductivity of PtTe2 nanosheets as a function of thickness. (c) Temperaturedependent conductivity of PtTe2 with different thicknesses. (d) Breakdown current-density measurement.
solution to etch the underlying SiO2 layer and release the PMMA/PtTe2 from the substrate. The released PMMA/PtTe2 membrane was then transferred onto a copper grid with carbon film, and the PMMA was dissolved with acetone. Sample Characterizations. The morphology of the synthesized PtTe2 nanosheets is characterized by optical microscope (DP27, OLYMPUS). The thickness of PtTe2 nanosheets is determined by an atomic force microscope (Bioscope system, BRUCKER). The morphology of the obtained PtTe2 nanosheets on the SiO2/Si substrate is also directly characterized by SEM (Zeiss, Germany). TEM characterization was performed using a JEM-2100F, JEOL, operating at 200 kV and equipped with an EDS system. Raman spectra were collected from PtTe2 nanosheets using a confocal microscope (invia-reflex, Renishaw) with a 633 nm laser as the excitation source. Device Fabrication and Characterization. The FET and Hall devices were fabricated by e-beam lithography followed by the deposition of Cr/Au for electrical contact (Cr: 10 nm; Au: 40 nm). The conductivity is obtained by the four-terminal measurement. The Hall measurement is performed with the sixterminal Hall bar structure in a physical property measurement system (Quantum design) using a lock-in amplifier (Stanford SR830).
demonstrate that the resulted PtTe2 nanosheets are single crystals. Raman spectroscopic studies show a red shift of the inplane Eg vibration mode and out-of-plane A1g vibration mode with increasing layer number. Electrical transport studies of the PtTe2 nanosheets show excellent electrical conductivity and the highest breakdown current density reported for 2D-MTMDs to date. Our results show CVD produced PtTe2 nanosheets an excellent candidate for the design of van der Waals heterostructure devices and may open a new material platform for investigating topological semimetals at the atomic scale. Methods. Preparation of PtTe2 Nanosheets. 2D PtTe2 nanosheets were synthesized on SiO2/Si substrates using a home-built CVD system (Figure S1). Pt powder (40 mg) (99.9%, metal basis, particle size of ≤1 μm, Aladdin) in a ceramic boat was placed in the center of the furnace, and Te powder (600 mg) (99.9%, Alfa) in a ceramic boat was placed at the upstream end of the quartz tube furnace, where the temperature would roughly reach 420 °C when the center hot zone of the furnace is set at 1100 °C. A SiO2/Si substrate is placed at the downstream end of the tube furnace as the growth substrate. The quartz tube was purged with ultrahigh-purity argon (Ar) gas (99.999%) and then ramped up to 1100 °C in 50 min and held at 1100 °C for 30 min under a constant argon flow of 120 sccm under atmospheric pressure. The furnace was then turned off, and the sample was naturally cooled to ambient temperature under a 120 sccm Ar flow. Finally, the different thicknesses PtTe2 nanosheets can be grown on different positions of the SiO2/Si substrate. We adjusted different the source temperature (1000−1200 °C) and Ar flow rate (50−400 sccm) to get nanosheets of various thicknesses. Transfer of PtTe2 Nanosheets onto Copper Grids. A thin layer of PMMA was spin-coated (low speed of 700 rpm for 10 s and high speed of 2000 rpm for 60 s) on top of the PtTe2/ SiO2/Si surface, and then the sample was baked at 100 °C for 2 min. The PtTe2 sample was then put into sodium hydroxide
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.8b00583. Additional details on Hall measurements of ultrathin PtTe2 and the experimental setup. Figures showing a schematic of the CVD system, optical microscopy and atomic force microscopy images, transfer characteristics, and temperature-dependent conductivity. A table showE
DOI: 10.1021/acs.nanolett.8b00583 Nano Lett. XXXX, XXX, XXX−XXX
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ing a comparison of the electrical conductivity, breakdown current density, temperature-dependent synthesis, and Ar flow-rate-dependent synthesis of PtTe2; and carrier concentration and mobility derived from the Hall measurement. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiangfeng Duan: 0000-0002-4321-6288 Author Contributions &
H. Ma and P. Chen contributed equally. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support from the National Natural Science Foundation of China (no. 61528403) and the Fundamental Research Funds of the Central Universities (no. 531107051078, 531107051055).
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ABBREVIATIONS MTMDs, metallic transition-metal dichalcogenides; 2D, twodimensional; OM, optical microscopy; AFM, atomic force microscopy; XRD, X-ray diffraction; SEM, scanning electron microscopy; TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; SAED, selected-area electron diffraction; HAADF-STEM, high-angle annular dark-field scanning transmission electron microscopy; R−T, resistivity−temperature
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