Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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Chemical Vapor Deposition Growth of Single Crystalline CoTe2 Nanosheets with Tunable Thickness and Electronic Properties Huifang Ma,†,⊥ Weiqi Dang,†,⊥ Xiangdong Yang,† Bo Li,‡ Zhengwei Zhang,† Peng Chen,† Yuan Liu,‡ Zhong Wan,†,∥ Qi Qian,†,∥ Jun Luo,§ Ketao Zang,§ Xiangfeng Duan,∥ and Xidong Duan*,†
Chem. Mater. Downloaded from pubs.acs.org by UNIV OF RHODE ISLAND on 12/10/18. For personal use only.
†
Hunan Key Laboratory of Two-Dimensional Materials and 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 § Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin 300384, China ∥ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States S Supporting Information *
ABSTRACT: Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDs) have recently drawn increasing interest for fundamental studies and potential applications in catalysis, charge density wave (CDW), interconnections, spintorque devices, as well superconductors. Despite some initial efforts, the thickness-tunable synthesis of atomically thin MTMDs remains a considerable challenge. Here we report controlled synthesis of 2D cobalt telluride (CoTe2) nanosheets with tunable thickness using an atmospheric pressure chemical vapor deposition (APCVD) approach and investigate their thicknessdependent electronic properties. The resulting nanosheets show a well-faceted hexagonal or triangular geometry with a lateral dimension up to ∼200 μm. Systematic studies of growth at varying growth temperatures or flow rates demonstrate that nanosheets thickness is readily tunable from over 30 nm down to 3.1 nm. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution scanning transmission electron microscope (STEM) studies reveal the obtained CoTe2 nanosheets are high-quality single crystals in the hexagonal 1T phase. Electrical transport studies show the 2D CoTe2 nanosheets display excellent electrical conductivities up to 4.0 × 105 S m−1 and very high breakdown current densities up to 2.1 × 107 A/cm2, both with strong thickness tunability. (HER) with low overpotentials and small Tafel slopes,33 and as potential catalysts for the oxygen evolution reaction (OER) as well.34−36 K. De Clippeleir et al. have reported that orthorhombic marcasite-type CoTe2 has a paramagnetic phase above 90 K.37 However, studies of ultrathin CoTe2 single crystals and their thickness-dependent properties are scarce to date. Herein, we report a chemical vapor deposition (CVD) growth of few-layer metallic CoTe2 single crystals. Morphological characterizations by optical microscopy (OM) and atomic force microscopy (AFM) show that the resulting CoTe2 nanosheets typically display a hexagonal or triangular shape with the lateral domain size up to >200 μm, and the nanosheet thicknesses are systematically tunable from 3 nm to ∼30 nm depending on the growth temperature or the carrier gas flow rate. Structural characterizations by X-ray diffraction
1. INTRODUCTION Two-dimensional (2D) layered atomic crystals have increasing interest for their unique electronic and chemical properties and exciting potentials for electronics,1−5 optoelectronics,6−10 capacitive energy storage,11 catalysis,12 and biosensors.13−17 More recently, the layered metallic transition metal dichalcogenides (MTMDs) have attracted considerable research interest for exploring a wealth of exotic properties, including superconductivity,18−20 magnetism,21−24 charge density waves (CDW),25−28 and other novel quantum phenomena at the 2D limit.29−31 Despite these various potentials, the experimental studies about MTMDs to date are limited to exfoliated flakes, and the scalable preparation of MTMDs with controllable thickness remains a significant challenge. For example, CoTe2 has a layered structure with a hexagonal unit cell and crystallizes in the P3̅m1 (No. 164) space group, with each Co atom surrounded by six Te atoms. CoTe2 has trigonal CdI2-type and the orthorhombic marcasite-type structures.32 Orthorhombic CoTe2 has been reported as an effective electrocatalyst for the hydrogen evolution reaction © XXXX American Chemical Society
Received: September 24, 2018 Revised: November 20, 2018
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DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials (XRD), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and scanning transmission electron microscope (STEM) imaging studies reveal that the resulting CoTe2 nanosheets are highquality single crystals in the hexagonal 1T phase. Electrical transport studies show 2D CoTe2 nanosheets are metallic with thickness-tunable conductivity and an excellent breakdown current density. Our work on controllable CVD synthesis of CoTe2 nanosheets may open up opportunities for studying the layer-number-dependent electrical properties, catalytic properties, and magnetism in the 2D layered CoTe2 nanosheets.
2. RESULTS AND DISCUSSION The synthesis of CoTe2 nanosheets was carried out in a homeconstructed CVD system in a 1 in. tube furnace at ambient pressure under relatively low temperatures (580−640 °C) (Figure 1a). The cobalt chloride (CoCl2) powder and
Figure 2. (a−d) Optical microscopy (OM) images of the CoTe2 nanosheets on the SiO2/Si substrate obtained at increasing temperatures of ∼580, ∼600, ∼620, and ∼640 °C, respectively, under a constant carrier gas flow rate 70 sccm (Ar/H2 mixture with 10% H2), scale bars 20 μm. (e−h) The representative atomic force microscopy images of CoTe2 nanosheets obtained at increasing temperatures of ∼580, ∼600, ∼620, and ∼640 °C, respectively. (i−l) The corresponding thickness distribution histograms of the CoTe2 nanosheets obtained at ∼580, ∼600, ∼620, and ∼640 °C, showing an average thickness of 3.8, 6.6, 8.6, and 30.1 nm, respectively.
Figure 1. (a) Schematic illustration of the home-built CVD system for the growth of CoTe2 nanosheets. (b) Optical microscopy (OM) image of CoTe2 nanosheets grown on SiO2/Si. (c) Optical microscopy (OM) image and (d) the corresponding atomic force microscopy (AFM) image of a CoTe2 nanosheet with a thickness of 3.1 nm grown on SiO2/Si.
tellurium (Te) power were used as the source materials, and the silicon chips (with 285 nm SiO2) were used as the growth substrate. Briefly, the Te powder (2.2 g) (99.9%, Alfa) was placed in a ceramic boat at the upstream end of the CVD tube as Te source, and the CoCl2 powder (1.2 g) (99.7%, Aladdin) was placed in a quartz boat at the center of the furnace as the Co source. A piece of Si growth substrate was placed at the center obliquely covering the CoCl2 quartz boat as the growth substrate. The CVD system was purged with 1000 sccm ultrahigh-purity Ar gas for 2 min before ramping up to the desired source temperature (e.g., 420 °C for Te and 615 °C for CoCl2) at a rate of 31 °C/min under a continuous Ar/H2 carrier gas (e.g., 100 sccm with 10% H2). The system was kept at the targeted growth temperature for 15 min and then was naturally cooled down to room temperature by shutting off the power of the furnace. The OM images show that the resulting CoTe2 nanosheets mostly display well-faceted triangular or hexagonal geometries (Figures 1b,c, 2, and 3) with lateral sizes up to ∼200 μm (Figure S1). To controllably synthesize the CoTe2 nanosheets with tailored thickness, we have carried out systematic studies to probe the impact of the growth temperature and the carrier gas flow rate. By varying the center temperature while keeping all other conditions (e.g., flow rate) constant, the thickness and morphology of the resulted nanosheets show a systematic
Figure 3. (a−d) Optical microscopy (OM) images of CoTe2 nanosheets on the SiO2/Si substrate obtained at increasing carrier gas flow rates of ∼50, ∼80, ∼100, and ∼120 sccm, respectively (Ar/ H2 mixture with 10% H2), under a constant temperature 615 °C, scale bars 20 μm. (e−h) The representative atomic force microscopy images of the CoTe2 nanosheets obtained at increasing carrier gas flow rates of ∼50, ∼80, ∼100, and ∼120 sccm, respectively. (i−l) The corresponding thickness distribution histograms of the CoTe2 nanosheets obtained at ∼50, ∼80, ∼100, and ∼120 sccm, respectively, showing an average thickness of 30.5, 12.2, 6.3, and 4.2 nm, respectively.
evolution with increasing temperature, as distinguished by the OM images (Figure 2a−d) and representative AFM images (Figure 2e−h). At a low growth temperature of ∼580 °C, relatively thin triangular CoTe2 nanosheets (∼3.8 nm) are obtained (Figure 2a). Upon an increase of the growth temperature to ∼600 °C, the CoTe2 domains show average thicknesses of ∼6.6 nm with triangular shapes (Figure 2b). When the center temperature is increased to ∼620 °C, the nanosheets become hexagonal with a further increased thickness to ∼8.6 nm (Figure 2c). As the center temperature B
DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials is further increased to ∼640 °C, the resulting nanosheets mostly exhibit hexagonal shapes with further increased thickness to ∼30.1 nm (Figure 2d). The corresponding histograms for thickness distribution of the nanosheets obtained at different temperature (Figure 2i−l) clearly show a rather narrow thickness distribution with a standard deviation as small as 0.5 nm, and a systematic thickness evolution under different growth temperatures. These temperature-dependent studies clearly show that the growth temperature considerably affected the shape and layer numbers of the resulting CoTe2 nanosheets. First, the CoTe2 nanosheets evolve from mostly triangular toward hexagonal morphology with increasing growth temperatures. Second, the layer number or the thickness of CoTe2 nanosheets also increases with increasing growth temperature. These results are consistent with the growth behavior of ultrathin single crystal domains of MoS2 and WSe2.38,39 These studies suggest that the growth temperature can significantly modulate the CoTe2 growth kinetics. The process is more kinetically controlled at a lower growth temperature. In this case, the addition of precursor atoms to the growing domain is dominated by edge energetics, with the atoms rapidly attaching to the fastest growth front at the edge of the 2D crystals, resulting in thinner nanosheets in triangular shapes. On the other hand, with increasing temperature, the process becomes more and more dictated by thermodynamics, producing a thicker hexagonal domain with lower surface and edge energy. We have also investigated the impact of the carrier gas flow rate on the CoTe2 growth behavior while keeping all other growth conditions the same (Figure 3a−d). In general, a low carrier flow rate (50 sccm) produces relatively thick CoTe2 nanosheets (∼30.5 nm). Increasing the flow rate leads to thinner and thinner nanosheets (∼12.2 nm at 80 sccm, 6.3 nm at 100 sccm, and ∼4.2 nm at 120 sccm), as clearly revealed in OM images and the representative AFM images (Figure 3e− h). An increasing flow rate also produces a systematic morphology evolution from mostly hexagonal thicker domains at lower flow rates to triangular thinner domains at higher flow rates. These observations suggest the process is more kinetically controlled at high flow rate, similar to results from previous studies on CVD growth of MoS2 and VSe2.38,40 The histograms of the thickness distributions of CoTe2 single crystals clearly show nearly monodispersed thickness distribution with a standard deviation of ∼0.6 nm (Figure 3i−l), and a systematic evolution of the nanosheet thickness with increasing flow rate. The X-ray diffraction (XRD) pattern of the resulting nanosheets can be indexed to space group P3̅m1 (No. 164) with lattice parameters of a = b = 3.802 Å, c = 5.411 Å (PDF no. 04-007-6577) (Figure 4a). Only the diffraction peaks corresponding to the (001) family planes of hexagonal phase CoTe2 are prominently displayed in the XRD pattern, which can be attributed the well-oriented nanosheets with the [001] direction normal to the substrates. We have further analyzed the chemical composition and the crystalline structure of the resulting CoTe2 nanosheets using energy-dispersive spectroscopy (EDS), TEM, and HRTEM. The EDX spectrum reveals that the presence of Co and Te with an atomic ratio approximately 1:2, consistent with the expected stoichiometry ratio within experimental error (Figure 4b). The lattice resolved TEM image clearly shows a hexagonal lattice with the interplane distances of 0.191 and 0.328 nm, which are consistent with the (110) and (100) planes of the CoTe2
Figure 4. (a) XRD pattern of the CoTe2 nanosheets grown on the SiO2/Si substrate. The XRD pattern shows strong (001) and (002) peaks but not other peaks (inlcuding the strongest (101) peak) due to the preferential orientation of the [001] direction perpendicular to the growth substrates. (b) The EDX analysis of the CoTe2 nanosheets grown on the SiO2/Si substrate. (c) HRTEM image and (d) SAED pattern of a transferred CoTe2 nanosheet on a copper mesh with carbon film. (e) Atomic-resolution HAADF-STEM image (filtered) of the CoTe2 single crystals. The measured lattice constant (∼3.81 Å) matches well with the theoretical value of hexagonal CoTe2 (a = 3.802 Å). (f) Zoomed-in image of the region highlighted by the white solid rectangular frame in part e, false-color-coded according to the HAADF intensity. The positions of the Te and Co atoms are colored red and blue, respectively. (g) Intensity line profile along the dashed line in image e. (h) Side view of the CoTe2 layered structure.
hexagonal structure (Figure 4c). The SAED pattern shows a single set of diffraction spots with 6-fold symmetry (Figure 4d), further confirming the single crystalline nature of the CoTe2 nanosheet. Figure 4e shows an atomically resolved HAADF-STEM image of a CoTe2 nanosheet. The Te (white spots) and Co (light gray) atoms can be readily identified by their different contrasts due to their distinct atomic numbers (Z). The observed atomic arrangement shows that each Co atom is surrounded by six Te atoms, which is consistent with the 1T phase structure of CoTe2. Figure 4f further shows a zoomed-in HAADF-STEM image of a CoTe2 nanosheet, with the Co and Te atoms more clearly differentiated as blue and red balls, respectively. An intensity line profile is also shown in Figure 4g. The periodicities in the intensity line profile demonstrate that the lattice constant (a) of the resulting nanosheets can be derived to be ∼3.81 Å (6.6/√3). Overall, the results derived from these structural characterizations are very consistent with the hexagonal structure of CoTe2 (Figure 4h). To further study the electronic properties of the resulting CoTe2 nanosheets, two-terminal CoTe2 devices were fabricated on a SiO2/Si substrate, in which the Si substrate may be used to modulate the electrostatic potential of the CoTe2 device (Figure 5a), similar to that in a field-effect transistor. The contact electrodes were fabricated by directly laminating prefabricated Au metal electrodes on the CoTe2 nanosheets as van der Waals contact electrodes (see Methods section for details).41 Compared with the typical lithography approach, the transferred metal electrodes method reduces the polymeric residue and metal-deposition induced damage to ensure an optimum charge transfer across the contact interface.41−43 Figure 5a shows the OM image of a typical CoTe2 device with transferred Au contacts. The drain-source current (Ids) vs drain-source voltage (Vds) output characteristics of a 6.5 nm thick CoTe2 device show a linear and symmetric relationship C
DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
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3. CONCLUSION Together, our systematic studies have demonstrated successful synthesis of ultrathin CoTe2 single crystal nanosheets with tunable thickness down to 3.1 nm. High-resolution STEM studies reveal that the CoTe2 nanosheets adopt a hexagonal 1T phase. Electrical transport studies of the CoTe2 nanosheets show excellent electrical conductivity and breakdown current density. The scalable synthesis of CoTe2 nanosheets with systematically tunable thickness will define a valuable 2DMTMD material system for studying the layer-numberdependent electrical properties, catalytic properties, and magnetism. 4. METHODS
Figure 5. Electrical transport properties of CoTe2 nanohseet devices. (a) Optical microscopy (OM) image of a typical CoTe2 device with transferred Au contacts, scale bar 40 μm. (b) Output characteristics of a 6.7 nm thick CoTe2 device with transferred Au contacts (Vg −60 to 60 V). The inset shows the transfer characteristics of the device. (c) Conductivity of the CoTe2 nanosheets as a function of thickness measured with transferred Au contacts. (d) Conductivity of CoTe2 nanosheets as a function of thickness measured with deposited Au contacts. The inset shows the output characteristics of a 6.3 nm thick CoTe2 device with deposited Au contacts (Vg −60 to 60 V). (e) Breakdown current density measurement of an 8.0 nm thick CoTe2 device with transferred Au contacts. (f) Breakdown current density of the CoTe2 nanosheets as a function of thickness with transferred Au contacts.
Preparation of CoTe2 Nanosheets. The CoTe2 nanosheets were grown on SiO2/Si substrates using a home-built CVD system (Figure 1a). Te powder (2.2 g) (99.9%, Alfa) was placed in a ceramic boat at the upstream end of the quartz tube furnace. CoCl2 powder (1.2 g) (99.7%, Aladdin) was placed in a quartz boat at the center of the furnace with a piece of SiO2/Si chips tilted above cobalt chloride powder as the growth substrate. The quartz tube was purged with ultra-high-purity argon (Ar) gas (99.999%) for 2 min before ramping up to 580, 600, 620, and 640 °C in about 20 min (Te powder was kept at 420 °C) under a constant carrier gas flow rate 70 sccm (Ar/H2 mixture with 10% H2) under atmospheric pressure. The temperatures were held for 15 min before shutting off to allow the sample to naturally cool to ambient temperature without changing the carrier gases. Similarly for the flow rate dependence study, we kept the center temperature at 615 °C, with Te powder at 420 °C under increasing carrier gas flow rates of ∼50, ∼80, ∼100, and ∼120 sccm, respectively (Ar/H2 mixture with 10% H2). Sample Characterizations. An optical microscope (DP27, OLYMPUS) and an atomic force microscope (Bioscope system, BRUCKER) were used to characterize the morphology and the thickness of the resulting CoTe2 nanosheets. XRD (XRD, D8Advance) was used to determine the crystal structures and phase purities of the CoTe2. EDX analysis (equipped with a Zeiss SEM) was used to determine the chemical composition of the obtained CoTe2 nanosheets; TEM characterization was performed using a JEM-2100F, JEOL, operating at 200 kV and STEM (Titan Cubed Themis G2300). Device Fabrications and Characterizations. For devices with transferred Au contacts, Au electrodes (50 nm thick) were first patterned on a silicon substrate (SiO2) by using standard photolithography or electron-beam lithography followed by an electronbeam evaporation process. Next, the substrate was functionalized with hexamethyldisilazane (HMDS) and then spin-coated with ∼1 μm thick poly(methyl methacrylate) (PMMA) to fully wrap the Au electrodes. The PMMA enwrapped Au electrodes were then mechanically released through adhesive tape and physically laminated onto CoTe2 nanosheets. The PMMA on top of the probing pads was removed using electron-beam lithography and development processes, exposing metal pads for electrical probing. The devices with deposited Au contacts were fabricated by electron-beam lithography followed by the electron-beam evaporation deposition of Au. Electrical measurement was carried out in a Lake Shore TTPX Probe Station using an Agilent 1500A semiconductor parameter analyzer. All electrical measurements were conducted in vacuum, at room temperature.
(Figure 5b), suggesting the formation of satisfactory Ohmic contacts. The gate-dependent measurement shows that the drain-source current is essentially constant under different gate voltages (Vg) (Figure 5b, inset), suggesting metallic behavior. The measured conductivity of the CoTe2 nanosheets ranges from 2.4 × 104 to 4.0 × 105 S m−1 at room temperature with transferred Au contacts (Figure 5c). These conductivity values are higher than those of hydrogen arc discharge exfoliated graphene,44 CVD growth VS2,45 and so on. With the increasing thickness from 4.3 to 17.4 nm, the conductivity of the CoTe2 nanosheets increased by about 1 order of magnitude. It is also noted that the device fabricated with deposited electrodes shows about 2 orders of magnitude smaller current and conductivity values (Figure 5d and inset), demonstrating the importance of the use of van der Waals integration to achieve the optimum contact with minimum interface damage to the atomically thin 2D CoTe2 nanosheets. We have also characterized the maximum breakdown current density of the CoTe2 nanosheets with transferred Au contacts. Upon a continuous increase of the drain-source voltage, a linear increase of current is observed until there is a sudden drop of the current to zero. The maximum current density before the sudden drop represents the breakdown current density. Figure 5e shows the current density Jds of an 8.0 nm thick CoTe2 nanosheet as a function of Vds with transferred Au contacts. The breakdown current density of CoTe2 is strongly dependent on the thickness. With the increasing nanosheet thickness from 3.4 to 12.1 nm, the breakdown current density is increased by about 1 order of magnitude (Figure 5f). The maximum breakdown current density is among the highest values achieved on 2D MTMDs, comparable to the graphene nanoribbon,46 mechanical exfoliation MoS2,47 CVD growth PtTe2,48 mechanical exfoliation TaSe2,49 and so on.
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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.chemmater.8b04069. Optical microscopy (OM) images of large-sized CoTe2 nanosheets (PDF) D
DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Bo Li: 0000-0002-5802-7519 Yuan Liu: 0000-0002-0024-9290 Jun Luo: 0000-0001-5084-2087 Xiangfeng Duan: 0000-0002-4321-6288 Xidong Duan: 0000-0002-4951-901X Author Contributions ⊥
H.M. and W.D. contributed equally. The manuscript was written through 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 the financial support from the Fundamental Research Funds of the Central Universities (no. 531107051078), the Double First-Class University Initiative of Hunan University (no. 531109100004), and the 111 Project of China (No. D17003). We also acknowledge the support from the National Natural Science Foundation of China (no. 751214296, 51802090, 61874041, 61804050), Hunan Key Laboratory of Two-Dimensional Materials (no. 801200005).
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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; STEM, scanning transmission electron microscope; HAADF-STEM, high-angle annular dark field scanning transmission electron microscopy
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DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.8b04069 Chem. Mater. XXXX, XXX, XXX−XXX
