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• Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are layered materials with fascinating properties, like the quantum spin Hall effect [1], valley polarization [2] as well as a wide range of potential applications [3], like field-effect transistors [4, 5], photodetectors [6] and functional devices [7, 8]. Among them, the growth of semiconducting VIB group TMDCs such as MoS₂ and WSe₂ has been well realized and their applications in various fields have been systematically studied [9-13]. In contrast, the growths of some metallic or narrow band-gap 2D materials with promising properties are still in their infancy. Titanium disulfide (TiS₂) and niobium disulfide (NbS₂), for example, have attracted increasing interests for their devisable electronic properties [14-18]and show application potentials as promising localized surface plasmon resonance materials [19] and superconductors. Another example, zirconium disulfide (ZrS₂) with a smaller band gap than MoS₂ displays high carrier mobility and unusual electronic properties [20], which brings the potential applications in nanoelectronics [21] and photodetectors [22-24]. However, many issues are remained to be tackled in the controllable synthesis of these ultrathin TMDCs nanosheets.

  Several top-down methods, such as mechanical exfoliation [25], electrochemical lithium intercalation exfoliation and liquid sonication exfoliation [26, 27], illustrate considerable handleability in the preparation of typical TMDs, black phosphorus (BP). However, it is usually difficult to control the shape, lateral size and thickness of the obtained nanosheets [28, 29]. Recently, bulk layered compounds [30] or large-size [31] TiS₂ flakes can be synthesized by chemical vapor transport (CVT) technique or a mixed-method of physical vapor deposition (PVD) and electron beam evaporation. Meanwhile, it is reported some substrates, such as h-BN and transferred graphene, are applied to synthesize ultrathin 2D nanocrystal by CVD [32, 33]. Nevertheless, the uniformity of the as-grown samples and the repeatability of these approaches are still not very satisfactory.

  Herein, we report a highly reproducible synthesis of MS₂ (M = Ti, Nb, Zr) nanosheets using H₂S-assisted CVD. By adjusting the growth parameters, like the growth temperature and the H₂S flow rate, the size and thickness of the flakes can be well controlled. Various characterization techniques confirm the high quality of the as-synthesized MS₂ nanosheets and FET devices based on TiS₂ nanosheets show narrow band-gap semiconductor behavior and a thickness-dependent resistance.

  The layers of MS₂ (M = Ti, Nb, Zr) are stacked via relatively weak van der Waals interactions. In each layer, M (M = Ti, Nb, Zr) atoms are sandwiched by six sulfur atoms. Fig. 1a shows the schematics of the growth of MS₂ (M = Ti, Nb, Zr) films on SiO₂/Si substrates via a CVD system together with the typical atomic structure of obtained MS₂ (M = Ti, Nb, Zr) from the side-view. More details about the growth process and experimental setup are provided in the Experimental Section and Fig. S1 (Supporting information). Briefly, sulfur powder and MO_x/NaCl mixture were used as the reactants, where NaCl powder can help to decrease the energy barrier for the reaction and thus increase the growth rate [34]. Ar is used as carrier gas and H₂S is introduced only at the high temperature around 800 ℃ for about 3 min to get ultrathin nanosheets. The typical optical microscopy (OM) images of as-grown MS₂ (M = Ti, Nb, Zr) nanosheets with H₂S-assistance are displayed in Figs. 1b-d, where the as-synthesized samples display obvious contrast with the SiO₂/Si substrates. The height profiles in Figs. 1b-d show a smooth surface with the thickness of ~2.4 nm for TiS₂, ~1.3 nm for ZrS₂ and ~1.8 nm for NbS₂, respectively. And their corresponding AFM height images of the representative MS₂ (M = Ti, Nb, Zr) nanosheets are listed as Fig. S2 (Supporting information). Furthermore, the effect of H₂S on the nucleation density and lateral size of these three kinds of samples are shown in Figs. 1e-g. As shown, the presence of H₂S can significantly reduce the nucleation density of TiS₂, ZrS₂ and NbS₂, thus, leading to much increased lateral size of the products.

  Figure 1

  

  Figure 1.  Synthesis of MS₂ (M = Ti, Nb, Zr) nanosheets. (a) Schematic illustration of MS₂ synthesis. (b-d) Typical OM images of MS₂ nanosheets grown on SiO₂/Si substrates with the corresponding height line profiles. (e-g) Sample statistical graphs of the lateral size and nucleation density of products grown with or without the assistance of H₂S.

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  To find the optimal conditions for the growth of TiS₂ nanosheets, synthesis temperature and H₂S flow are investigated in detail (Fig. 2). Keeping the other growth parameters (growth time, carrier gas, precursor quantity, etc.) constant, H₂S as auxiliary gas can obviously reduce the nucleation density and increase the lateral size of the samples compared to the growth without using H₂S. Figs. 2a-d show the typical OM images of TiS₂ grown from 750℃ to 900℃ without the help of H₂S, where the thickness increases and the lateral size changes little with increased growth temperature. Figs. 2e-h display the revolution of typical OM images of TiS₂ grown with 2 sccm H₂S from 750℃ to 900℃, in which the lateral size increases first and then the thickness becomes thicker. The corresponding statistical histogram of the number of layers increasing with temperature can be seen in Fig. S3 (Supporting information). The statistical histograms of the lateral size and nucleation density in Figs. 2i and j show the effect of H₂S clearly. The lateral size of TiS₂ nanosheets increases from ~5 µm to ~50 µm when introducing H₂S while the nucleation density decreases from ~3k mm⁻² to ~0.5k mm⁻². In addition, the change of lateral size with different temperature is observed, which may be caused by the kinetic and thermodynamic process including the source sublimation rate, the nucleation rate, the growth rate and the stability of the products [35, 36]. At relatively low temperature, the growth process is largely kinetically controlled [37]. With higher concentration of the reactants vapor at a little bit higher temperature, the product will prefer to deposit on the edge with high activity, which will lead to lateral growth. However, the growth is more thermodynamically controlled with further increased temperature, which will lead to thicker samples. Also, the supersaturated vapor source tends to increase the nucleation rate. As a result, the nanosheets tend to be thicker and smaller. However, the introduction of H₂S could promote the growth of TiS₂ in the lateral dimension from both thermodynamics and kinetics, which will be discussed in detail in the following part. The above experimental results confirm that ideal ultrathin TiS₂ nanosheets could be gained at 800℃ with the assistance of H₂S. The same research methods are used to the synthesis of ZrS₂ and NbS₂ and the characterization of the as-grown nanosheets can be seen in Supporting information (Figs. S4 and S5), showing similar effects of H₂S on the morphology control.

  Figure 2

  

  Figure 2.  Typical OM images of TiS₂ flakes grown under different conditions. OM images of TiS₂ nanosheets grown without H₂S at (a) 750 ℃, (b) 800 ℃, (c) 850 ℃, (d) 900 ℃, respectively. OM images of TiS₂ nanosheets grown with the assistance of H₂S at (e) 750 ℃, (f) 800 ℃, (g) 850 ℃, (h) 900 ℃, respectively. (i) The lateral size as a function of the growth temperature with the corresponding OM images. (j) Nucleation density of TiS₂ flakes as a function of the growth temperature. Scale bars in (a-i): 10 μm.

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  To deeply understand the mechanism of how H₂S critically affects the synthesis of TiS₂ nanosheets, the effect of H₂S flow rate on the reaction is studied in detail. Figs. 3a-f illustrate the OM images of TiS₂ grown at 800℃ with different H₂S/Ar flow rate. Obviously, the nucleation density and domain size are very sensitive to H₂S flow rate. With the increase of H₂S flow, the lateral size firstly increases and then decreases while its nucleation density firstly decreases and then keeps constant. Thus, 3 sccm H₂S flow rate is the best condition that can get the maximum size of TiS₂ nanosheets (about 60 µm). The experimental phenomenon shows that a small H₂S flow can significantly decrease the nucleation density (Fig. 3g), which is beneficial for achieving a large domain size at the proper growth time and growth temperature. A simple growth mechanism model and typical OM images of TiS₂ nanosheets are speculated in Figs. 4a-c. Firstly, compared to S, H₂S could help to produce more active Ti intermediate products with higher Gibbs free energy, which could further lower the ΔG of the reaction. This will further minimize the thermodynamic stability difference of the lateral growth product and vertical growth product. In this case, the dynamics will play a more dominant role and the source atoms will quickly add to the edge sites to realize the lateral growth [37]. Secondly, the high activity of H₂S can react with the TiO₂/NaCl powder before its evaporation, this could decrease the vapor pressure of Ti species to obtain a low nucleation density. Thus, further increasing the flow rate of H₂S will inhibit the growth of TiS₂ on the substrate.

  Figure 3

  

  Figure 3.  The effect of the flow rate of H₂S on growing TiS₂ nanosheets at 800 ℃. (a-f) OM images of TiS₂ nanosheets grown with H₂S flow range from 0 to 5 sccm. All scale bars are 10 μm. (g) TiS₂ lateral size and nucleation density as a function of the H₂S flow rate.

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  Figure 4

  

  Figure 4.  Phase characterizations of the as-grown TiS₂. (a, b) Possible growth models of TiS₂ nanosheets on SiO2/Si substrates. (c) Typical OM image of TiS₂ films grown with 3 sccm H₂S. (d, e) XPS characterization of Ti 2p and S 2p peaks of the as-grown TiS₂ sample. (f) The XRD pattern of the TiS₂ films flakes on SiO2/Si from a thick sample. (g) Raman spectrum of TiS₂ nanosheets with different layers using a 532 nm excitation laser (including a representation of A_1g and E_g vibrational modes). The dashed line in E_g highlights a minor change of the peak position. (h) Peak intensity change of the A_1g and E_g vibrational mode as a function of layer numbers.

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  X-ray photoelectron spectroscopy (XPS) was further conducted to study the chemical states and elemental composition of the as-grown samples. As Figs. 4d and e shown, the chemical states of Ti⁴⁺ 2p_3/2 and 2p_1/2 can be identified from the peaks at ~456 and 462 eV (Fig. 4d) and the peaks at ~160 and ~162 eV (Fig. 4e) correspond to the 2p_3/2 and 2p_1/2 states of S, respectively. The elemental composition of Ti: S perfectly matches the stoichiometric ratio of 1:2.

  X­ray diffraction (XRD) characterization was performed on a thick sample to further identify the crystal structure of the as-grown samples. As shown in Fig. 4f, the peak at 2θ = 15.56° of the CVD-grown sample matches well with the (001) crystal planes of TiS₂ reported in the literature [29, 38]. The peak at 2θ = 69° can be indexed to the Si substrates. Raman characterization with 532 nm laser is further applied to characterize the grown samples, as shown in Fig. 4g. It shows a peak at ~230 cm⁻¹ ascribed to the in-plane E_g, a peak at ~330 cm⁻¹ corresponding to out-of-plane A_1g modes, and a shoulder mode at about 375 cm⁻¹ assigned to A_2u, which is consistent with reported literature of TiS₂ [29]. When the thickness increases, the E_g peak exhibits a blueshift from 228 cm⁻¹ (monolayer) to 236 cm⁻¹ (bulk) while the A_1g peak at ~330 cm⁻¹ shows a negligible shift. Fig. 4h further summarizes the evolution of Raman intensities with thickness, showing decreased intensity of both E_g and A_1g peaks with increased thickness.

  Further, transmission electron microscope (TEM) was performed on the transferred samples to investigate the detailed atomic structures of TiS₂ samples. The low-magnification TEM image (Fig. 5a) shows a typical TiS₂ flake on the lacey carbon films supported by Cu grids. Fig. 5b shows a high-resolution TEM (HRTEM) image of the sample, displaying a good atomic arrangement. The lattice spacing of ~1.70 and ~2.93 Å matches the predicted spacing for the (110) and (100) planes of 1T-TiS₂. The corresponding fast Fourier transformation (FFT) image (Fig. 5c) presents only one set of hexagonal diffraction spots, which confirms the single-crystal nature and high crystal quality. Energy dispersive spectroscopy (EDS) employed along with TEM on the transferred samples identifies the chemical composition of ~1:2 for Ti: S in Fig. 5d. The EDS maps (Figs. 5e and f) clearly exhibit the spatial distribution of Ti and S atoms and their uniform color distribution demonstrates the compositional uniformity of the synthesized TiS₂ nanosheet. Together, these TEM results confirm that the H₂S-assisted CVD method can obtain ultrathin single-crystal TiS₂ nanosheets with high quality.

  Figure 5

  

  Figure 5.  TEM characterization of the as-grown TiS₂. (a) Low-magnification TEM image of TiS₂ nanosheets. (b) The high-resolution TEM image. (c) The corresponding fast Fourier transform (FFT) image. (d) Representative EDS spectrum for the as-grown TiS₂ and the corresponding elemental mapping images of Ti (e) and S (f) elements.

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  In addition, electrical measurements were performed on a series of FETs based on the as-grown TiS₂ samples with different thickness. Fig. 6a exhibits the OM image of a typical fabricated device with a channel width (W) of 8 µm and channel length (L) of 7 µm. The Ti/Au electrodes were made directly on the samples using photolithography and e-beam metal deposition (more details are provided in the Experimental Section in Supporting information). Fig. 6b shows the thickness-dependent resistance of TiS₂ devices, exhibiting that the resistance decreases with the increase of thickness. The corresponding linear current-voltage (I_ds-V_ds) curve collected at room temperature also demonstrates an Ohmic contact at the interface. The transfer characteristic curves of the devices under different V_ds in Fig. 6c demonstrate the narrow band-gap semiconductor behavior of the TiS₂ channel [39, 40].

  Figure 6

  

  Figure 6.  Electrical measurement of the TiS₂ nanosheets. (a) The OM image of a fabricated device based on TiS₂ grown on SiO₂/Si substrate. (b) I_ds-V_ds curve of the TiS₂ device collected at room temperature. (c) The transfer characteristic curves of the devices.

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  In summary, we have introduced H₂S to assist the synthesis of several 2D transition-metal dichalcogenide materials, such as TiS₂, NbS₂ and ZrS₂, in an ambient pressure CVD system. Further XRD, TEM and Raman characterizations reveal that the nanosheets are single crystals and possess high crystallinity. We take TiS₂ as an example to study the role of the H₂S in the growth, where H₂S can effectively inhibit the growth in the vertical direction and reduce the nucleation density due to its high reactivity. The method provides direct access to highly crystalline TiS₂, ZrS₂ and NbS₂ nanosheets, with superior controllability over the growth morphologies, such as the domain size and thickness. Our study hence suggests a promising CVD paradigm to synthesize high quality various 2D atomic crystals. It also provides a possible inspiration for the preparation of other 2D TMDCs and opens up opportunities for exploring a variety of intriguing physics, corresponding properties and applications of 2D TMDCs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Key R & D Program of China (No. 2018YFA0306900) and the National Natural Science Foundation of China (No. 51872012).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.07.036.

  
  
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  Figure 1  Synthesis of MS₂ (M = Ti, Nb, Zr) nanosheets. (a) Schematic illustration of MS₂ synthesis. (b-d) Typical OM images of MS₂ nanosheets grown on SiO₂/Si substrates with the corresponding height line profiles. (e-g) Sample statistical graphs of the lateral size and nucleation density of products grown with or without the assistance of H₂S.

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  Figure 2  Typical OM images of TiS₂ flakes grown under different conditions. OM images of TiS₂ nanosheets grown without H₂S at (a) 750 ℃, (b) 800 ℃, (c) 850 ℃, (d) 900 ℃, respectively. OM images of TiS₂ nanosheets grown with the assistance of H₂S at (e) 750 ℃, (f) 800 ℃, (g) 850 ℃, (h) 900 ℃, respectively. (i) The lateral size as a function of the growth temperature with the corresponding OM images. (j) Nucleation density of TiS₂ flakes as a function of the growth temperature. Scale bars in (a-i): 10 μm.

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  Figure 3  The effect of the flow rate of H₂S on growing TiS₂ nanosheets at 800 ℃. (a-f) OM images of TiS₂ nanosheets grown with H₂S flow range from 0 to 5 sccm. All scale bars are 10 μm. (g) TiS₂ lateral size and nucleation density as a function of the H₂S flow rate.

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  Figure 4  Phase characterizations of the as-grown TiS₂. (a, b) Possible growth models of TiS₂ nanosheets on SiO2/Si substrates. (c) Typical OM image of TiS₂ films grown with 3 sccm H₂S. (d, e) XPS characterization of Ti 2p and S 2p peaks of the as-grown TiS₂ sample. (f) The XRD pattern of the TiS₂ films flakes on SiO2/Si from a thick sample. (g) Raman spectrum of TiS₂ nanosheets with different layers using a 532 nm excitation laser (including a representation of A_1g and E_g vibrational modes). The dashed line in E_g highlights a minor change of the peak position. (h) Peak intensity change of the A_1g and E_g vibrational mode as a function of layer numbers.

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  Figure 5  TEM characterization of the as-grown TiS₂. (a) Low-magnification TEM image of TiS₂ nanosheets. (b) The high-resolution TEM image. (c) The corresponding fast Fourier transform (FFT) image. (d) Representative EDS spectrum for the as-grown TiS₂ and the corresponding elemental mapping images of Ti (e) and S (f) elements.

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  Figure 6  Electrical measurement of the TiS₂ nanosheets. (a) The OM image of a fabricated device based on TiS₂ grown on SiO₂/Si substrate. (b) I_ds-V_ds curve of the TiS₂ device collected at room temperature. (c) The transfer characteristic curves of the devices.

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