Single Crystals with Highly Uniform Thickness Distributions

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Synthetic Control of Two-Dimensional NiTe2 Single Crystals with Highly Uniform Thickness Distributions Bei Zhao,‡,† Weiqi Dang,‡,† Yuan Liu,†,§ Bo Li,†,§ Jia Li,† Jun Luo,# Zhengwei Zhang,† Ruixia Wu,† Huifang Ma,† Guangzhuang Sun,† Yu Huang,¶ Xidong Duan,*,† and Xiangfeng Duan*,∥

J. Am. Chem. Soc. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/22/18. For personal use only.

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Hunan Key Laboratory of Two-Dimensional Materials, State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China § 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 Materials Science Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States ∥ Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States S Supporting Information *

ABSTRACT: Two-dimensional (2D) layered materials have stimulated extensive research interest for their unique thickness-dependent electronic and optical properties. However, the layer-number-dependent studies on 2D materials to date are largely limited to exfoliated flakes with relatively small lateral size and poor yield. The direct synthesis of 2D materials with a precise control of the number of atomic layers remains a substantial synthetic challenge. Here we report a systematic study of chemical vapor deposition synthesis of large-area atomically thin 2D nickel telluride (NiTe2) single crystals and investigate the thickness dependent electronic properties. By controlling the growth temperature, we show that the highly uniform NiTe2 single crystals can be synthesized with precisely tunable thickness varying from 1, 2, 3, . . . to multilayers with a standard deviation (∼0.3 nm) of less than the thickness of a monolayer layer NiTe2. Our studies further reveal a systematic evolution of single crystal domain size and nucleation density with the largest lateral domain size up to ∼440 μm. X-ray diffraction, transmission electron microscopy, and high resolution scanning transmission electron microscope studies demonstrate that the resulting 2D crystals are high quality single crystals and adopt hexagonal 1T phase. Electrical transport studies reveal that the 2D NiTe2 single crystals show a strong thickness-tunable electrical properties, with an excellent conductivity up to 7.8 × 105 S m−1 and extraordinary breakdown current density up to 4.7 × 107 A/cm2. The systematic study and robust synthesis of NiTe2 nanosheets defines a reliable chemical route to 2D single crystals with precisely tailored thickness and could enable the design of new device architectures based on thickness-tunable electrical properties.

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thickness decreases from bulk to monolayer.15−19 NbSe2 exhibits a thickness-dependent superconducting properties with a transition temperature increasing from 1.0 K in monolayer to 4.56 K in 10-layer.20 CrI3 exhibits layerdependent magnetic phase transitions, displaying ferromagnetism in the monolayer, antiferromagnetism in bilayer, and back to ferromagnetism in the trilayer and bulk.21 These initial studies demonstrate the critical role of the layer number in determining the fundamental physical properties. However, the studies to date are largely limited to exfoliated flakes with relatively small lateral size and poor yield, which is intrinsically not scalable in both the quantity of sheets produced and the maximum size of each sheet.22,23 It remains a significant

INTRODUCTION Two-dimensional layered atomic crystals (2DLACs) such as graphene,1,2 black phosphorus (BP),3 metal halides,4,5 and transition metal dichalcogenides (TMDCs)6,7 have drawn intense scientific and engineering interest for their unique layer-number dependent properties and potential applications in electronics, optoelectronics, valleytronics, spintronics and catalysis, etc.8−11 With a strong covalent bond in each atomic layer and weak van der Waals (vdW) interaction between the layers, 2DLACs can be readily isolated or synthesized as singleor few-atom layers. In general, 2DLACs exhibit fascinating properties that are tunable by the number of atomic layers.12,13 For example, PtSe2 shows a transition from metal to semiconductor while the thickness decreases from ∼13 to 2.5 nm.14 Semiconducting MoS2, MoSe2, WS2, and WSe2 show a transition from indirect to direct band gap when their © XXXX American Chemical Society

Received: July 31, 2018

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DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 1. Thickness tunable growth of NiTe2 2D crystals with varying substrate temperature (TSub) (and NiCl2 source temperature). (a−f) OM images of ultrathin NiTe2 single crystals grown at different substrate temperature. The Te powders evaporation temperature was set at ∼570 °C, the carry gas flow rate was Ar/H2 = 60/5 sccm, and the growth time was 15 min. The substrate temperatures of panels a, b, c, d, e, and f are ∼530, ∼550, ∼570, ∼590, ∼620, and ∼650 °C, respectively. Scale bars: 20 μm. The distinct and uniform contrast in each image suggesting highly uniform thickness distribution within each sample and highly tunable thickness distribution by controlling the growth conditions.

μm. By controlling the growth temperatures, the layer number of NiTe2 nanosheets can be precisely controlled from multilayer down to monolayer. The high-crystalline quality of the 1T-NiTe2 nanosheets were confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and high angle annular dark field scanning transmission electron microscope (HAADF-STEM) studies. Electronic transport studies show that NiTe2 single crystals exhibit metallic behavior and strong thickness-tunable electrical properties. These studies defines a reliable way to 2D NiTe2 single crystals with precisely controlled layer number, which is essential for fundamental investigation of the thickness-dependent properties of 2D NiTe2, such as electronic and optoelectronic properties, superconductivity, charge−density−wave order, and their potential applications.

challenge to produce high quality single crystals of 2DLACs with precisely controlled layer numbers. The chemical vapor deposition (CVD) approach provides a more scalable and controllable way to high quality and large area 2D materials.24 Considerable efforts have been devoted to the growth of single crystal domains of 2DLACs with controlled thickness, domain shape and size. For example, large-area CVD MoS2 films with the number of layers from one to four have been achieved using oxygen-plasma treated substrate.25 MoS2 crystals with the number of layers tunable from one to six have been reported by using different 2D materials, such as SnS2, TaS2, and graphene, as substrates in a typical low-pressure CVD system.26 MoSe2 crystals with the number of layers from one to four have been synthesized by increasing the growth temperatures from 750 to 900 °C in a one-step CVD process.27 NiTe2 is a layered compound sharing the same structure with other MX2 compounds (where M = transition metal and X = chalcogen), with each layer of the crystal composed of 2D hexagonally arranged Ni atoms sandwiched between Te atoms in a 1T structure. It is predicted that NiTe2 single crystals adopt CdI2-type structure with a complicated Fermi surface and a large density of states at the Fermi level through linear muffin-tin orbital method with the atomic sphere approximation (LMTO-ASA).28,29 Very recently, various methods have been employed to synthesize NiTe2 crystals. For example, Chia et al. produced NiTe2 bulks crystals by chemical vapor transport (CVT).30 Wang et al. synthesized NiTe2 microplates on Ti mesh substrate via anion exchange reaction under hydrothermal conditions, with thicknesses ranging from ∼3 μm to ∼4 μm.31 Bhat et al. obtained ∼55 nm-thick NiTe2 nanoplates by further optimizing the growth conditions.32 However, the layer-controlled synthesis of high quality ultrathin NiTe2 single crystal, particularly in monolayer form, is scarce to date. Herein, we report a one-step CVD approach to ultrathin NiTe2 nanosheets with precisely controlled layer numbers. Typical optical microscopy (OM) images show the resulting NiTe2 nanosheets mostly exhibit a hexagonal or triangular shape with the lateral domain size varying from ∼5 to ∼440

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RESULTS AND DISCUSSION The NiTe2 nanosheets were synthesized using an atmospheric pressure CVD (APCVD) process with a two-zone tube furnace. Tellurium powders and nickel dichloride (NiCl2) powders were chosen as the chemical precursors. The tellurium powders were placed in the upstream zone (heated to 550−680 °C). The NiCl2 powers with a piece of tilted SiO2/Si substrate were placed in the center of the downstream zone (heated to 530−700 °C). A continuous Ar flow of 60− 300 sccm is employed during the entire process, and a flow 5− 25 sccm H2 (one twelfth of Ar flow) was introduced during the growth stage to ensure the uniform nucleation and growth. More details about the sample synthesis are described in the Experimental Section (Figure S1). Typical OM images (Figure 1a−f, Figure 2a−l) show the resulted NiTe2 nanosheets on SiO2/Si mostly exhibit hexagonal or triangular shape. The OM images of the samples produced at each given temperature show relative uniform contrast among different nanosheets, and the samples produced at different temperature display a systematic evolution of optical contrast with the increasing growth temperature. In particular, the AFM study for the sample grown at 530 °C demonstrates highly uniform thickness around 0.9 nm (Figure S2), suggesting the formation of a B

DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. OM (a−l) and AFM (a1−l1) images of NiTe2 flaks obtained at different substrate temperature. The Te powders evaporation temperature was set at ∼570 °C, the carry gas flow rate was Ar/H2 = 60/5 sccm, and the growth time was 15 min. (a, b, c, d, e, f, g, h, i, j, k, and l) Individual NiTe2 flakes with different thicknesses (0.9, 1.6, 2.3, 2.9, 3.5, 4.1, 4.8, 6.2, 7.6, 9, 10, and 11 nm, respectively). Scale bars: 5 μm. (m, n, o, p, q, r) The corresponding thickness histogram distributions of NiTe2 single crystals synthesized with growth temperature set at ∼530, ∼550, ∼570, ∼590, 620, and ∼650 °C, respectively. The white spots observed in some NiTe2 nanosheets are resulted from degradation after long-term exposure to air before AFM characterizations.

monolayer NiTe2.33,34 AFM studies of the sample grown at different temperature (Figure 2a1−l1) further confirm highly uniform thickness distribution within each sample and a systematic thickness evolution from monolayer, bi-, tri-, four-, five-, six-, seven-layer NiTe2 nanosheets and so on, up to ∼30 nm thick nanoplates. The lateral domain size of CVD-grown NiTe2 flakes can reach up to ∼440 μm (Figure S3). Furthermore, large-area fully continuous NiTe2 nanosheets can also be deposited on SiO2/Si substrates by adjusting the CVD parameters such as the heating temperature of Te, the carry gas flow rate and the growth time (Figure S4). By systematically controlling the growth temperatures, highly uniform NiTe2 single crystals with nearly monodispersed layer number can be readily obtained, as distinguished by the thickness-dependent optical contrast (Figure 1a−f, Figure 2a−l) and representative AFM images (Figure 2a1−l1). It can be seen that the growth temperature has a significant effect on the growth behavior of NiTe2 nanosheets, including

the layer number, lateral dimension, nucleation density and shape. At lower substrate temperature (and NiCl2 source temperature) of ∼530 °C, 1−2 layer NiTe2 nanosheets (with average edge size of ∼4−5 μm) were obtained with a nucleation density of ∼7000/mm2 and most domains adopting truncated triangular shapes (Figure 1a and Figure 2a1,b1). Increasing the substrate temperature (and NiCl2 source temperature) to ∼550 °C (Figure 1b), ∼570 °C (Figure 1c), and ∼590 °C (Figure 1d), the layer number of NiTe2 nanosheets (typically in triangular shape with an average lateral size of ∼10−13 μm) increased to 3−4 (Figure 2c1,d1), 5−6 (Figure 2e1,f1), and 7−8 (Figure 2g1) with a reducing nucleation density to be ∼4950, ∼3900, and ∼2000/mm2, respectively. When the substrate temperature (and NiCl2 source temperature) is increased to 620 °C (Figure 1e), truncated triangular NiTe2 (∼9−11 layer (Figure 2h1,i1) with average edge size of 15 μm) started to appear with the domain density reduced to ∼1800/mm2. As the substrate temperature C

DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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In addition, we have also explored the influence of other parameters such as the temperature of Te powders (TTe) and the flow rate on growth behavior under specific experimental conditions. For example, with increasing TTe, a systematic evolution of the thickness, morphology, and nucleation density is observed. With increasing the TTe from ∼570 to ∼660 °C, the NiTe2 thickness increases from 4 to 35 nm, the lateral dimension increases from 12 to 38 μm, the nucleation density decreases from ∼3900 to 1200/mm2 and the shape changes from triangles to hexagons (Figure S6a−d). Relatively higher TTe will induce the higher Te partial pressure to produce thicker and larger hexagonal NiTe2 nanosheets. Under the same source temperature and growth temperature, the change of carrier gas (Ar/H2) flow rate can also result in a systematic evolution in thickness, lateral size, and nucleation density, while the morphology is largely unchanged triangular shape. With increasing the flow rate of Ar/H2, both the thickness and the size of the NiTe2 nanosheets are increased, while the nucleation density of NiTe2 nanosheets is reduced (Figure S6e−h). For example, as the Ar/H2 flow rate is changed from 60/5 to 120/10 and 300/25 sccm, the thickness, lateral size, and nucleation density evolve from ∼2.3 nm, 11 μm and 4590/ mm2, to ∼4 nm, 19 μm and 1031/mm2, and then ∼5 nm, 31 μm and 440/mm2. In general, under a give source temperature (sublimation rate) and substrate temperature, a lower flow rate retains relatively high partial pressure of both Te and NiCl2 vapor near growth substrate, leading to more rapid nucleation and rapid addition to the energetic growth front, produce a higher nucleation density, smaller and thinner domains. While a high flow rate effectively dilute the chemical vapor source, reduces the supersaturation, moving the growth to slightly more thermodynamically controlled regime to resulting in larger and thicker nanosheets with a lower nucleation density. To examine the crystallinity and the quality of the as-grown 1-T NiTe2 nanosheets, XRD, TEM and HAADF-STEM analysis were also performed. The XRD pattern (Figure 4a) can be indexed to space group P3(−)m1-164 with lattice parameters of a = b = 3.843 Å, c = 5.265 Å (JCPDS no. 080004). The two main diffraction peaks can all be indexed to the (001) and (002) planes of the hexagonal NiTe2 crystals, suggesting that the as-grown nanosheets are all well-aligned with the [001] direction normal to the growth substrate. The elemental analysis of the transferred NiTe2 nanosheets through energy dispersive spectrometry (EDS) demonstrates that the NiTe2 nanosheets consist only of Ni and Te elements, with the atomic ratio of Ni and Te is approximately 1:2 (Figure 4b), consistent with the expected stoichiometric ratio. The HRTEM image of NiTe2 (Figure 4c) reveals a nearly perfect periodic atom arrangement with the clearly resolved lattices spacing of 0.33 and 0.19 nm corresponding to the (100) and (110) planes of the NiTe2 hexagonal structure. The corresponding SAED patterns in the insets of Figure 4c show a single set of hexagonally arranged diffraction spots, strongly suggesting that NiTe2 nanosheet is a single crystal with hexagonal structure. The corresponding EDS elemental mapping images (Figure 4d,e) clearly show the spatial distribution of the element Ni and Te, with the uniform color contrasts further confirming the compositional homogeneity. The HAADF-STEM image of NiTe2 further shows the inplane crystal structure of the hexagonal 1T-phase with each hexagonally arranged Ni atom surrounded by six Te atoms (Figure 4f). For typical 1T phase structure, each hexagonally

(and NiCl2 source temperature) was further increased to 650 °C (Figure 1f), hexagonal NiTe2 (13−17 layer (Figure 2j1−l1), with average edge size of 17 μm) were produced with even lower nucleation density of ∼650/mm2. As the substrate temperature continued to increase, the NiTe2 domains remained hexagonal shape, and the average edge size and the thickness of NiTe2 nanosheets increased continuously along with a systematic reduction in the nucleation density (Figure S5). In general, by decreasing the substrate temperature (and NiCl2 source temperature) from 650 to 620, 590, 570, 550 and 530 °C, the NiTe2 ultrathin single crystals with ∼13−17, ∼9− 11, ∼7−8, ∼5−6, ∼3−4, and ∼1−2 layers could be obtained. These systematic studies clearly demonstrated that the layercontrolled CVD synthesis of ultrathin NiTe2 single crystal is relative precise (Figure 2m−r). Notably, the single crystal nanosheets obtained in each synthetic conditions show highly uniform thickness distribution with the standard deviation (∼0.3 nm) less than the thickness of one monolayer materials (∼0.6 nm). A summary of the evolution of nucleation density, lateral dimension, thickness and shapes as a function of growth temperature (∼530, ∼550, ∼570, ∼590, 620, and ∼650 °C) is shown in Figure 3.

Figure 3. Evolution of nucleation density, lateral dimension, the representative morphologies and thickness (inset) of NiTe2 single crystals grown at ∼530, ∼550, ∼570, ∼590, ∼620, ∼650, and ∼700 °C. Scale bars: 5 μm.

Overall, a higher growth temperature yields thicker NiTe2 domains with a larger lateral dimension and a lower nucleation density, and the NiTe2 nanosheets evolves from mostly triangular shape to hexagonal shape with increasing growth temperature. These trends are consistent with the growth of MoS2,35 MoSe2,27 and WSe236,37 reported previously. During NiTe2 and other TMDC growth, the growth temperature affects the kinetic/thermodynamic process such as source sublimation rate, surface atom diffusion rate and the nucleation/growth rate. On the basis of the nucleation model of the vapor phase deposition developed by W. K. Burton and N. Cabrera, the nucleation probability is inversely proportional to the growth temperature.38 At lower substrate temperature, the growth is largely kinetically controlled. The nucleation rate is higher, and the source atoms quickly add to the fastest growth front dominated by edge energetics, leading to thinner and smaller nanosheets with dominant triangular shape. At higher substrate temperature, the growth is more thermodynamically controlled, leading to a lower nucleation rate to produce thicker nanosheets with mostly hexagonal shape. D

DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 4. (a) XRD pattern of synthetic NiTe2 nanosheets. (b) Elemental analysis of the NiTe2 nanosheets by EDS achieved on STEM grids. (c) HRTEM image of the NiTe2 nanosheet transferred onto a copper mesh; inset shows the corresponding selected area electron diffraction (SAED) pattern. (d, e) EDS mapping images of Te and Ni for a hexagonal NiTe2 nanosheet on STEM grid, respectively. (f) Atomic-resolution HAADFSTEM image (filtered) of a NiTe2 nanosheet. (g) Zoomed-in image of the region highlighted by the blue dashed rectangle in panel f. (h) Intensity line profile along the white dashed line in panel g. (i) Top view of NiTe2 structure. Red and green spheres correspond to tellurium and nickel, respectively.

Figure 5 shows the electrical measurement results of the vdW contacted NiTe2 devices. The Ids−Vds output character-

arranged Ni atom is surrounded by six Te atoms. Ni atoms and Te atoms in the 1T-NiTe2 single layer are light gray and white spots owing to their small and large atomic numbers (Z).39 In the zoomed-in image (Figure 4g) (highlighted by the blue dashed rectangle in Figure 4f), the Ni and Te atoms can be more clearly differentiated as green and red balls, respectively. The corresponding contrast intensity line profile (Figure 4h) shows the in-plane lattice constant (a) of NiTe2 nanosheets to be ≈0.386 nm, which is in good agreement with literature values. Figure 4i and Figure S7 show the structural model of hexagonal NiTe2 with top views, 3D schematic and side view. Apparently, hexagonal NiTe2 is a typically layered material, with each NiTe2 layer consisting of one central Ni atom and six Te atoms located at octahedral points (CdI2-type structure). To further investigate the electrical properties of synthesized NiTe2, we have fabricated two terminal NiTe2 devices by simply laminating Pt metal electrode pairs onto the NiTe2 nanosheets as vdW contact electrodes.40 The detailed fabrication approaches are described in the Experimental Section. The typical lithography approach leave polymeric residue on the 2D nanosheets, and typical “high-energy” metal deposition processes usually involve atom or cluster bombardment and strong local heating to the contact region, which could damage the crystal lattice at or near the interface, leading to nonideal transport in 2D materials, particularly for the ultrathin flakes that could be more easily damaged by these processes.41,42 We note the vdW metal contact here could minimize such nonideal factors and is essential for probing the intrinsic electrical properties of 2D NiTe2 by greatly minimizing lithography and deposition induced defects and interface disorder, which would otherwise reduce the device current level by more than 1 order of magnitude (as experimentally confirmed in Figures S8 and S9).

Figure 5. Electrical properties of NiTe2 devices with transferred Pt contacts. (a) Ids−Vds output characteristics of NiTe2 device (Vg: −60 to +60 V). The inset shows the OM image of a typical NiTe2 device (scale bar is 10 μm). (b) Transfer characteristics of NiTe2 device. (c) Electrical conductivity of NiTe2 as a function of flake thickness. (d) Breakdown current-density measurement of NiTe2 device. (e) Measured breakdown current density of NiTe2 as a function of flake thickness. (f) Measured breakdown current density as a function of flake resistivity.

istics demonstrate a linear and symmetric relationship, indicating the formation of Ohmic contact between vdW metal electrodes and the 2D NiTe2 nanosheets (Figure 5a). Furthermore, the device current Ids is insensitive to the back gate voltage Vgs (Figure 5b, from −60 to +60 V), suggesting the metallic behavior, which is consistent with previous reports.43,44 We have further extracted electrical conductivity E

DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society of the NiTe2 nanosheets at various flake thickness (Figure 5c). In general, the flake conductivity increases linearly with flake thickness, with a value of 2 × 104 S m−1 for 1.6 nm thick samples, and increasing to 7.8 × 105 S m−1 in 35 nm thick nanosheet. To the best of our knowledge, the conductivity of 7.8 × 105 S m−1 represents an extraordinary value for 2D materials, as shown in the comparison Table S1 in the Supporting Information. Furthermore, we have also measured the breakdown current density of NiTe2 devices (Figure 5d), by continuously increasing the Vds until a sudden decrease of the current to zero. As shown in Figure 5e, the breakdown current density also scales linearly with the material thickness, similar to previous studies on graphene nanoribbons.45 With the decrease of layer number of NiTe2, the flake resistivity increases, which results in a degradation of breakdown current density (Figure 5f). Importantly, the highest breakdown value of 4.7 × 107 A/cm2 can be achieved in 35 nm thick sample, which is comparable with state-of-the-art nanomaterials such as carbon nanotubes46 or other 2D materials47 (as compared in Table S2). The ultrahigh conductivity and breakdown current density of NiTe2 could be important for its practical application as vdW metal electrode, on-chip electrical interconnects or spin torque devices.

JEOL, 343) operating at 200 kV and equipped with an EDS system and STEM (Titan Cubed Themis G2300). Device Fabrication and Characterization. 50 nm thick Pt (or Au) electrode pairs are first fabricated on atomically flat substrate (Si/ SiO2) using photolithography and high-vacuum electron-beam evaporation. Next, hexamethyldisilazane (HMDS) was used to functionalize the whole wafer, and followed by spin-coating ∼1 μm thick poly(methyl methacrylate) (PMMA) to fully wrap the metal electrodes. With the prefunctionalization by HMDS, the PMMA layer has weak adhesion to the Si/SiO2 substrate and can be mechanically peeled through adhesive tape and physically laminated onto the surface of NiTe2. Finally, the PMMA on top of the contact pads is removed using standard electron-beam lithography and development processes, leaving the exposed metal pads for electrical probing and measurements. Electrical measurement of the fabricated FETs was conducted in a Lake Shore TTPX Probe Station (in vacuum at room temperature) and Agilent 1500A semiconductor parameter analyzer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b08124. Experimental setup, atomic force microscopy AFM image of NiTe2 monolayer, optical microscopy (OM) images of large size NiTe2 nanoplates and large-area NiTe2 few-layers, OM images of ultrathin NiTe2 single crystals grown at 700 °C, growth of NiTe2 with varying temperature of Te powders and flow rate of Ar/H2, 3D schematic and side view of NiTe2 structure, electrical properties of NiTe2 devices with transferred Au contact and deposition Au contact, table of comparison of the electrical conductivity of NiTe2 and other 2D materials, table of comparison of the breakdown current density of NiTe2 and other materials (PDF)

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CONCLUSION In summary, we have systematically investigated the layernumber controlled synthesis of ultrathin NiTe2 single crystals with systematically variable thickness down to monolayer regime and lateral dimension up to ∼440 μm. By controlling the growth temperatures, the layer number of NiTe 2 nanosheets can be precisely controlled from 1, 2, 3 to multilayers. The resulted NiTe2 nanosheets are characterized by XRD, TEM and STEM, and display high-quality singlecrystalline 1T phase structures. Electrical transport studies of the NiTe2 nanosheets show excellent electrical conductivity and the extraordinary breakdown current density. Our study defines a reliable route to 2D NiTe2 nanosheets with precisely controlled thickness and could enable the design of new device architectures based on thickness-tunable electrical properties.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Bo Li: 0000-0002-5802-7519 Jun Luo: 0000-0001-5084-2087 Yu Huang: 0000-0003-1793-0741 Xiangfeng Duan: 0000-0002-4321-6288

EXPERIMENTAL SECTION

Preparation of NiTe2 Nanosheets. Two-dimensional NiTe2 nanosheets were grown on SiO2/Si substrates using a two-zone tubular furnace under atmospheric pressure (Figure S1). Tellurium power (0.1 g) (99.99%, Alfa) in a ceramic boat was placed in the upstream zone. Nickel dichloride power (0.1 g) (>98%, Energy Chemical) in a ceramic boat was placed in the middle of the second temperature zone with a piece of SiO2/Si substrate tilted above nickel dichloride powder. Before the heating process, the system was purged with ultrahigh purity argon (Ar) gas (Rizhen, ∼99.999%) to completely remove oxygen and moisture from the quartz tube. Next, the first zone (T1) and second zone (T2) were heated up to 550−680 °C and 530−700 °C under a constant argon flow of 60−300 sccm. The temperatures then were both held for 15 min in the flow mixed gases of 5−25 sccm H2 and 60−300 sccm Ar. After a 15 min growth period, the heating process was terminated and the furnace was naturally cooled down to room temperature without changing the carrier gases. Materials Characterization. The morphology of the assynthesized NiTe2 nanosheets was characterized by optical microscope (DP27, OLYMPUS). The thickness of NiTe2 nanosheets was determined by atomic force microscope (Bioscope system, BRUCKER). The crystal structures and phase purities of NiTe2 were analyzed by XRD (XRD, D8-Advance, Bruker at room temperature in the 2λ range of 10−60°), TEM (a JEM-2100F,

Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the 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).

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REFERENCES

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DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.8b08124 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX