pubs.acs.org/cm Article Thickness-Controlled Synthesis of CoX2 (X = S, Se, and Te) Single Crystalline 2D Layers with Linear Magnetoresistance and High Conductivity Xingguo Wang, Zhang Zhou, Peng Zhang, Shuqing Zhang, Yang Ma, Weiwei Yang, Hao Wang, Bixuan Li, Lingjia Meng, Huaning Jiang, Shiqiang Cui, Pengbo Zhai, Jing Xiao, Wei Liu, Xiaolong Zou, Lihong Bao,* and Yongji Gong* Cite This: Chem. Mater. 2020, 32, 2321−2329 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Two-dimensional (2D) materials especially transition metal dichalcogenides (TMDs) have drawn intensive interest owing to their plentiful properties. Some TMDs with magnetic elements (Fe, Co, Ni, etc.) are reported to be magnetic theoretically and experimentally, which undoubtedly provide a promising platform to design functional devices and study physical mechanisms. Nevertheless, plenty of theoretical TMDs remain unrealized experimentally. In addition, the governable synthesis of these kinds of TMDs with desired thickness and high crystallinity poses a tricky challenge. Here, we report a controlled preparation of CoX2 (X = S, Se, and Te) nanosheets through chemical vapor deposition. The thickness, lateral scale, and shape of the crystals show great dependence on temperature, and the thickness can be controlled from a monolayer to tens of nanometers. Magneto-transport characterization and density function theory simulation indicate that CoSe2 and CoTe2 are metallic. In addition, unsaturated and linear magnetoresistance have been × 6 × 6 observed even up to 9 T. The conductivity of CoSe2 and CoTe2 can reach 5 10 and 1.8 10 S/m, respectively, which is pretty high and even comparable with silver. These cobalt-based TMDs show great potential to work as 2D conductors and also provide a promising platform for investigating their magnetic properties. 1. INTRODUCTION predicted by theoretical calculation, till date have hardly been 34 The family of two-dimensional (2D) materials such as explored on the scale of atomic layers experimentally, and the 1,2 3 4,5 6 7 scalable preparation of these TMDs with controllable layer graphene, h-BN, black phosphorus, CrI3, FePS3, and Downloaded via INST OF PHYSICS on November 7, 2020 at 07:23:11 (UTC). transition metal dichalcogenides (TMDs) has shown exciting numbers and high quality still remains a serious challenge. potentials in various aspects, including both scientific research For instance, layered TMD CoX2 (X = S, Se, and Te) is (superconductivity, charge density wave, magnetism, topo- composed of hexagonal unit cells, where each Co atom is 8−14 bonded with six X (chalcogen) atoms. Because of the existence See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. logical properties, etc.) and practical applications (elec- − tronics, optoelectronics, catalysis, energy storage, etc.).15 25 of cobalt elements, CoX2 is possible to behave as a magnet Among these, TMDs have drawn intensive attention because with atomic thickness. A few studies have been conducted on of their abundant types and thickness-dependent proper- the preparation and application of CoX2. For example, a ties.26,27 For example, the superconducting transition temper- nonlayered material CoS2 is reported as ferromagnetic metal with the Curie temperature TC of 124 K, and CoSe2 with the ature of NbSe2 nanosheets is strongly dependent on their layer 28 ff − − pyrite-type structure is regarded as the controversial Pauli numbers. Three di erent charge density wave phases have 35−37 29 paramagnet or the antiferromagnet. Some initial efforts been observed in TaS2. Linear magnetoresistance (MR) and topological superconductivity have also been discovered in have also been made on CoTe2 in two dimensions recently. 30 Duan and co-workers reported the layer-controlled growth of WTe2 nanosheets. Recently, some TMDs with magnetic elements such as CrSe2, CrTe2, VTe2, VTe2, and so forth have been predicted theoretically or proved experimentally to have Received: October 28, 2019 − some magnetic properties,31 33 which can be served as a Revised: March 4, 2020 powerful platform to design functional devices and study Published: March 4, 2020 physical magnetic mechanisms. Generally, plenty of funda- mental physical phenomena and potential applications can be realized in 2D TMDs. However, some new kinds of TMDs, © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.chemmater.9b04416 2321 Chem. Mater. 2020, 32, 2321−2329 Chemistry of Materials pubs.acs.org/cm Article Figure 1. Preparation and morphology of CoX2 (X = S, Se, and Te). (a) Schematic illustration of the growth process. Mixture of Co3O4 and NaCl are used as Co precursors which can react with S or Se or Te gas at a certain temperature and atmosphere. (b−d) OM images and height profiles of ultrathin CoS2, CoSe2, and CoTe2 nanoplates on SiO2/Si. (e) OM image of the large view of CoTe2 with uniform thickness. (f) Diagrams of the smallest thickness of CoTe2 nanosheets plotted as a function of the growth temperature. (g) Evolution of the largest lateral scale of CoTe2 nanosheets with the growth temperature. CoTe2 and CoSe and studied the electric properties of the the conductivity of silver showing promising application resulting nanosheets.38,39 Wang and co-workers found tunable potential as 2D conductors. According to the magneto- ff magnetic properties of CoTe2 treated with di erent concen- transport and superconducting quantum interference device 40 trations of NaOH. However, high-quality ultrathin-layered (SQUID) test, both CoSe2 and CoTe2 seem likely to be CoX2 (X = S and Se) has not been synthesized yet. Moreover, paramagnetic in nature. Density function theory (DFT) is CoX2 nanosheets with well-controlled thickness down to the further used to simulate the band structures of CoX2, further monolayer are desirable to explore the layer-dependent confirming their metallic nature. Altogether, the studies magnetic properties of these materials. illustrate a controllable way to synthesize 2D CoX2, which Herein, we report a chemical vapor deposition (CVD) may provide a promising opportunity for studying the approach to synthesize few-layer CoX2 nanosheets with the properties of 2D CoX2 with various layers, such as electronic, controlled thickness on SiO2/Si substrates by tuning the magnetic, catalytic, and other properties. growth temperatures. By using monolayer MoSe2 as substrates (monolayer MoSe2 developed on SiO2/Si substrates in 2. RESULTS AND DISCUSSION fi advance), a single-layer CoX2 can be synthesized for the rst The CoX2 (X = S, Se, and Te) nanosheets on 285 nm SiO2/Si time. In addition, the growth temperature plays an important substrates are synthesized using a traditional CVD system, as role in the evolution of the lateral size and shape. Optical shown in Figure 1a, with the mixture of Co3O4 (0.1 g) and microscopy (OM) images illustrate the morphologies of the as- NaCl (0.03 g) as the Co source while different chalcogen grown CoX2 nanosheets, mostly displaying a hexagonal or element powders as X precursors. Although the growth time ∼ triangular shape with the lateral domain size ranging from 5 and the flow of carrier gas can also cause a certain influence on ∼ μ to 100 m. Atomic force microscopy (AFM) studies reveal the thickness, which was reported in other TMDs such as the thickness of the nanosheets varying from 1.18 to ∼20 nm. 41 ff NiTe2, the thickness of MX2 can still be tuned e ectively by X-ray photoemission spectroscopy (XPS) is carried out to controlling the growth temperature in this study. More details analyze chemical states, and Auger electron spectroscopy of the sample synthesis are described in the Methods section. − (AES) mapping indicates the compositional homogeneity of The OM images, as shown in Figure 1b d, indicate that CoX2 the as-grown CoX2 samples. The crystal structure and high (X = S, Se, and Te) nanosheets exhibit hexagonal or triangular crystalline quality are confirmed by high-resolution trans- shapes with various lateral size. The AFM results demonstrate mission electron microscopy (HRTEM) and selected-area that the typical thickness of CoS2, CoSe2, and CoTe2 is about electron diffraction (SAED) characterizations. Magneto-trans- 7.2, 5.1, and 3.9 nm, respectively. Figure 1e shows the OM port studies show that CoX2 (X = Se and Te) single crystals image of the uniform CoTe2 nanosheets in a large view. exhibit metallic behaviors. Additionally, unsaturated linear MR In order to achieve controllable synthesis of these 2D (LMR) up to 9 T occurs in both CoSe2 and CoTe2 nanosheets. materials, we take CoTe2 as an example and conduct ff Meanwhile, both CoSe2 and CoTe2 with various thicknesses systematic studies to investigate the e ect of growth temper- 6 fl show outstanding conductivity up to 10 S/m, which is close to ature on the thickness and lateral size. With the Ar/H2 ow 2322 https://dx.doi.org/10.1021/acs.chemmater.9b04416 Chem. Mater. 2020, 32, 2321−2329 Chemistry of Materials pubs.acs.org/cm Article Figure 2. Characterizations of chemical states and compositional homogeneity. (a and e) XPS spectra of Co 2p from CoSe2 and CoTe2 nanosheets. (b and f) XPS spectra of Se 3d and Te 3d. Indexed peaks are formed by CoSe2 or CoTe2, while the others come from some contaminations during the CVD process. (c,d,g, and h) AES mapping spectra of CoSe2 and CoTe2 nanosheets. ° rate constant at 50 sccm in which 10% of the gas is H2 and the to 780 C. This phenomenon can be explained by the local time of deposition process constant is 3 min, the nanosheets changes in the Co/Te ratio at different temperatures, which ff ° 45 produced at di erent temperatures from 720 to 800 C show a has been discussed in the CVD growth of MoS2. clear thickness evolution, as shown in Figure 1f. With Although the growth temperature and deposition time can ff increasing growth temperatures, thicker CoTe2 nanosheets e ectively tune the thickness of CoX2, it still remains a are grown.