General Strategy for Two-Dimensional Transition Metal

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General Strategy for Two-dimensional Transition Metal Dichalcogenides by Ion Exchange Huihui Chen, Zhuo Chen, Binghui Ge, Zhen Chi, Hailong Chen, Hanchun Wu, Chuanbao Cao, and Xiangfeng Duan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b03523 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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Chemistry of Materials

General Strategy for Two-dimensional Transition Metal Dichalcogenides by Ion Exchange †

†

‡

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Huihui Chen , Zhuo Chen* , Binghui Ge* , Zhen Chi§, Hailong Chen§, Hanchun Wu , Chuanbao †

Cao and Xiangfeng Duan †

#

Department of Materials Physics and Chemistry, Beijing Key Laboratory of Construction

Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology Institution, Beijing 100081, P. R. China ‡

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, P. R. China §

Institute of Physics, Chinese Academy of Science, Beijing 100190, PR China

‖

Department of Physics, Beijing Institute of Technology, Beijing 100081, P. R. China

#

Department of Chemistry and Biochemistry, University of California, Los Angeles, California

90095, United States KEYWORDS: Ion exchange, Two-dimensional materials, Transition metal dichalcogenides, Layered semiconductor, Nanosheet

ACS Paragon Plus Environment

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ABSTRACT: The ability to control and vary the atomic compositions of two-dimensional (2D) layered semiconductors is of considerable importance for tailoring their electronic and optoelectronic properties. The current methods to tailor chemical composition of 2D layered semiconductors is largely limited to vapor phase chemistry, and solution phase chemistry is insufficiently explored to date. Here, we report a general approach to prepare 2D layered transition metal dichalcogenides (TMDs) by ion exchange reactions in solution phase. By choosing four typical layered metal dichalcogenides including MoS2, MoSe2, WS2 and SnS2 as representative cases, the feasibility and versatility of both cation and anion exchange reactions in layered metal dichalcogenides are confirmed. Transient absorption results indicate that exciton lifetime of these samples as excitation energy is increased. The optoelectronic properties of these TMD nanosheets after the ion exchange exhibit potential for future devices. Our strategy can be employed not only to modulate the atomic composition and electronic structures of layered TMDs, but also open up a new pathway for the fabrication of various hybrid heterostructures.

During the past decade, two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received intense attention since they not only have unique physical and chemical properties, but also have great potential for diverse applications in optoelectronic and valleytronic devices, as well as energy conversion and storage.1-7 Exploring effective and controllable approaches to tailor the atomic and electronic structures of the 2D layered semiconductors via chemical composition is crucial for facilitating their application in optoelectronic devices and the field of energy, and presents a significant challenge in the subject of 2D layered materials. Up to now, four approaches have been developed to prepare 2D TMDs, including mechanical exfoliation,2,8 liquid exfoliation,9 chemical vapor deposition10-12 and wet chemical synthesis.13-20 Among various methods mechanical exfoliation can yield some of the highest-quality mono- or few-


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layered TMDs, however, this method is not suitable for practical technologies due to its limited scalability for production. Liquid exfoliation method can permit a large-scale preparation, but its relatively poor quality control restricts its useful applications. Chemical vapor deposition, based on solid precursors, has become a popular method for the growth of 2D TMDs, alloys and heterostructures. For chemical vapor deposition, the quality of product strongly depends on the super-saturation and vapor transport of the growth species. Unfortunately, for solid sources it is usually difficult to overcome the fluctuations of vapor partial pressure and non-uniformity in the spatial distribution.21 As a result, it remains an open challenge to grow continuous high quality 2D TMD thin films in a precisely controllable way. Although the solution method as a low-cost bottom-up approach has been extensively used for various colloidal nanocrystals, its controllability for 2D TMDs is far less successful than when applied for the growth 0D nanocrystal, with respect to size, shape and composition.

In general, the TMDs is defined by strong intra-layer covalent bonds and weak inter-layer van der Waals forces, leading to a large difference in surface energies along the plane and c-axis, respectively.13 According to the Wulff construction for the equilibrium shape of crystals, the anisotropic surface energy of layered TMDs implies the crystal should grow much faster along the plane than along c-axis, to minimize the total surface energy. Theoretically, this is favorable for achieving anisotropic 2D growth, but practical growth processes usually deviate largely from the ideal situation, especially for solution methods. Recently, many efforts have been devoted to exploit various solution-based approaches for the synthesis of 2D metal dichalcogenides. For example, the Cheon group demonstrated colloidal synthesis of uniform, disc-shaped ZrS2 nanosheets by injecting CS2 into a mixture of ZrCl4 and oleylamine at 300 °C,22 whereafter this hot injection approach was also extended to other group IV and V TMDs.23 An alternative one-


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pot colloidal synthesis method is also used for a wider range of IV-VI semiconductors. The Schaak group reported a thickness-controlled synthesis of single crystal SnSe nanosheets with uniform boundary via oriented attachment growth.19 More recently, by adjusting the binding energies of surface capping ligands on the TMD edge facets, anisotropic lateral growth of WSe2 and MoSe2 nanosheets with tunable thickness was achieved by Cheon and co-workers.14 Although great progress has been made in controlling the physical dimensions and aspect ratios of TMD nanosheets such as the edge dimensions, lateral uniformity, and thickness, the reliability and precision of synthesis method based on the solution methods have plenty of room for improvement. Moreover, in addition to their physical dimension, the properties of TMDs are strongly dependent to their composition. Thus a successful recipe or experimental conditions may only be suitable for a certain material when using a solution approach. Therefore, it is extremely worthwhile to explore a generalized composition-controlled solution strategy to 2D TMDs. Here, we demonstrate a general approach to prepare 2D layered TMDs by ion exchange reactions. Four typical layered metal dichalcogenides including MoS2, MoSe2, WS2 and SnS2 were selected to test the ion exchange strategy. Structural and chemical composition characterizations were used to determine the success of anion or cation exchange reactions in 2D TMDs. Optoelectronic property investigations indicate that the 2D TMD nanosheets obtained by the ion exchange strategy have potential for various optoelectronic devices. Although ion exchange has been extensively used in II−VI and III−V colloid nanocrystals,24-28 there is no report on the ion exchange method for 2D layered TMDs to date. This strategy can not only be used to synthesize a wide range of 2D TMD nanosheets, but also be extended to fabricate various hybrid heterostructures, such as metal-TMDs and van der Waals heterojunctions.


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Figure 1. Schematic of the general strategy for TMD producted by ion exchange. A schematic of the general strategy to prepare 2D TMDs proposed in this work is shown in Figure 1. Following this strategy, 2D layered compounds, such as MoS2 and SnS2, are first prepared by liquid exfoliation.9,29 Second, the chemical compositions of the obtained TMDs can be changed by ion exchange, both cations and anions. Theoretically, an ion exchange reaction strictly depends on thermodynamic and kinetic factors.30 Among these factors, the crystal structures of the reactant and product and the relative thermodynamic stabilities are key in determining the feasibility of an ion exchange process. Here, several typical layered TMD crystals were selected to demonstrate the ion exchange strategy, chosen because they are of the same phase structure and similar lattice constants, as shown in Table 1. To the best of our knowledge, this is the first report on the solution synthesis of layered TMD crystals through ion exchange method. The strategy not only provides a general protocol for the synthesis of 2D TMD nanosheets, but also can be adopted for the growth of other alloy TMDs with tunable band gaps.


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Table 1. Lattice parameters and crystal structures of layered compounds TMD

a (Å)

c (Å)

MoS2

3.15

12.30

MoSe2

3.29

12.90

WS2

3.15

12.36

SnS2

3.64

5.89

top view

side view

Figure 2. (a) HRTEM image of a SnS2 nanosheet before cation exchange. (b) HRTEM and element mapping images of a MoS2 after cation exchange. (c) STEM and element mapping images of a MoSe2 after anion exchange. (d) STEM and element mapping images of a WS2 after cation exchange. (e) XRD spectra of hexagonal phases of SnS2, MoSe2, MoS2 and WS2


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nanosheets (JCPDS No. 89-3198, 77-1715, 77-1716 and 87-2417). (f) XPS spectra of WS2, MoSe2 and MoS2 nanosheets. To verify the feasibility and universality of ion exchange reaction in 2D layered TMDs, three reactions were carried out as below: 1) from SnS2 to MoS2, 2) from MoS2 to MoSe2, 3) from MoS2 to WS2. Figure 2a shows a high-resolution transmission electron microscopy (HRTEM) image of an SnS2 nanosheet before cation exchange (reaction 1). According to the lattice fringes of the SnS2 crystal, the lattice spacing between two planes is ∼0.31Å, corresponding to the distance of two {100} planes. Figure 2b displays a HRTEM image after fast Fourier transform (FFT) filtering of the product after cation exchange (reaction 1), in which the atomic arrangements of Mo and S are marked by the overlaid cyan and yellow symbols, respectively. It reveals a lattice spacing of 2.7 Å, which is consistent with the distance of two {100} planes of MoS2. From energy dispersive spectroscopy (EDS) analyses, it was determined that the nanosheet is composed of Mo and S atoms and elemental mapping images show a relatively uniform distribution of Mo and S. Moreover, both annular dark-field (ADF) scanning TEM (STEM) and EDS elemental mapping images of MoSe2 and WS2 indicate that the MoS2 nanosheets can be further converted to MoSe2 and WS2 by anionic or cationic substitution reactions, as shown in Figure 2c and 2d. It means our strategy based on ion exchange reactions successfully modulate the atomic structure of 2D TMDs via chemical composition. Figure 2e shows the X-ray diffraction (XRD) patterns of the MoSe2, MoS2 and WS2 nanosheets, which indicates the crystalline characteristics and phase of these samples annealed at 500 °C. All the peaks can be indexed to the hexagonal phases of MoSe2, MoS2 and WS2 (JCPDS No. 77-1715, 77-1716 and 87-2417), respectively. X-ray photoelectron spectroscopy (XPS) was used to confirm the chemical composition of the products, as shown in Figure 2f. The peaks at


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163.8, 162.6, 35.1 and 33 eV correspond to the S2- 2p1/2, S2- 2p3/2, W4+ 4f5/2 and W4+ 4f7/2, respectively. The peaks at 231.7 and 228.5 eV represent the Mo 3d3/2 and Mo 3d5/2 binding energies for Mo4+, and the Se 2p1/2 and Se 2p3/2 orbital of divalent selenide ions (Se2-) are identified at 56 and 54.2 eV. The peaks at 232.5, 229.3, 163.4 and 162.2 eV can be assigned to the Mo4+ 3d3/2, Mo4+ 3d5/2, S2- 2p1/2 and S2- 2p3/2, respectively. These XPS datum are in agreement with previous reports,31-33 further confirming that the three ion exchange processes are successfully completed. To further acquire the detailed knowledge of the ion exchange reaction, we performed structural and chemical composition characterizations of the products before annealing and the intermediate products with ion exchange reaction time, as shown in Figure S1 to S12 and Table S1 to S4. For the anion exchange reaction from MoS2 to MoSe2, most of the intermediate products at different reaction time display a polycrystal state. Basically the morphologies of the intermediate products are nanosheets, except a few nanoparticle-like products as shown in Figure S5(c). For the cation exchange reaction from MoS2 to WS2, the morphologies of the intermediate products are no obvious change with prolonged reaction time, and most of these products exhibit a single crystal state. For the cation exchange reaction from SnS2 to MoS2, the intermediate products look like nanosheets, but their surfaces obviously change from smooth to rough as reaction time increases, and all of them are almost in amorphous state. The conversion ratios of three ion exchange reactions gradually increase as reaction time increases. Based on the experimental observations, in morphology retention and crystal quality aspects the exchange reaction from MoS2 to WS2 is the best, followed by that from MoS2 to MoSe2, and that from SnS2 to MoS2 is the worst.


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Furthermore, the reverse exchange reaction from MoS2 to SnS2 was carried out, as shown in Figure S13 and S14. TEM results show that the intermediate and final products obtained at 3h and 5h, respectively, are almost in amorphous state, which should be attributed to a relatively large lattice constant difference between MoS2 and SnS2. In morphology aspect it basically keeps nanosheet after the ion exchange reaction. The exchange reaction from SnS2 to SnSe2 also was performed, as shown in Figure S15 and S16. TEM results indicate that the intermediate and final products respectively obtained at 3h and 5h almost display a single crystal state. EDS elemental mapping images show a relatively uniform distribution of Mo, Sn and S for the intermediate, Sn and S for the final ones, Sn, Se and S for the intermediate and Sn and Se for the final ones, respectively. The XRD results in Figure S17 and S18 further confirm that the MoS2 and SnS2 nanosheets can be converted to SnS2 and SnSe2 by our ion exchange strategy. These results mean that the reverse ion exchange reaction is feasible, extending the generality of the ion exchange strategy. Usually, ion exchange reactions occur at relatively low temperature and short time. But the mild reaction conditions are only suitable for ionic crystals with nanometer in size. For nanomaterials with covalent lattices, the high binding energy and low diffusivity of ions make it difficult to undergo ion exchange process in such gentle conditions. A high temperature is necessary to complete the conversion for III−V nanocrystals, such as GaAs, InAs, GaP, and InP, as reported by Alivisatos28. In addition, the reaction conditions of ion exchange are also strongly dependent on the size, except for the binding energy. The size has great influences on the thermodynamics and kinetics of reactions. Alivisatos has already investigated the size effect in ion exchange24. For only a few nanometers in size, cation exchange reactions can occur completely and reversibly at room temperature with unusually fast reaction rates, but for


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extended solids, reactions are in general very slow because of high activation energies for the diffusion of atoms and ions in the solid, thus requiring high temperatures and long times24,26,28,30. To further understand the ion exchange reaction, we calculated the lattice energies of TMDs by the Kapustinskii equation34

U POT =

121.4 za zbv 0.0345 [1 − ] (ra + rb ) (ra + rb ) ,

where za and zb are the moduli of the charges on the v ions in the lattice and ra and rb (in nm) are the thermochemical radii. The ra for metal ions is taken to be the Goldschmidt radius.35 Since the change in T∆S is typically one or two orders of magnitude smaller than that from the enthalpy change (∆H),36 the thermodynamic driving force should be dominated by the lattice energy. Compared the non-layered metal sulfides as shown in Table S5, the relatively high lattice energy means higher thermodynamic energy barrier, thereby requiring longer reaction time and higher reaction temperature. On the other hand, for layered TMDs, weak van der Waals forces and relatively large distance between layers could provide an ion diffusion channel to facilitate the ion exchange reaction. Thus, the host ions replaced by the guest ions could take place in crystal interior, distinct from the non-layered metal sulfide, in which ion substitution reactions occur by ion diffusion from outer to inner crystal. In addition, for non-layered metal sulfides (CdS, PbS and Ag2S etc.), the anion exchange reaction often results in poor morphology retention due to lower diffusivity of lattice anions and different phase structures before and after anion exchange.30 In our case, the morphology of the products basically keep in similar with that of the reactants except that a few samples change their morphologies. We think that there are three possible reasons as below. Firstly, relatively


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large distance between layers greatly facilitate ion diffusion, and all of the chalcogen atoms in unique sandwich structure are exposed on surface or interface, thereby benefiting the anion exchange reaction. Secondly, anion in the host lattice should be gradually replaced with the reaction time. Thirdly, in our work the crystal structures of TMDs before and after exchange reaction are hexagonal phase, leading to smaller lattice distortion. Thus, the morphology can be well maintained before and after anion exchange. Actually, the anion exchange with excellent morphology retention under chalcogen vapor has been reported by David B. Geohegan37, Ludwig Bartels,38 Anlian Pan,39 and Lain-Jong Li40, respectively. Although our as-synthesized TMDs by ion exchange exhibit a relatively poor crystallinity, it can be improved by annealing at relatively high temperature.

Figure 3. (a) Raman and (b) UV−vis absorption spectra of MoSe2, WS2 and MoS2 nanosheets. Raman spectroscopy is a powerful technique to investigate the formation of various 2D TMD crystals, as shown in Figure 3a. The two distinct peaks in the Raman spectra correspond to the in-plane E2g at 382 cm−1 and the out-of plane A1g at 408 cm−1 for MoS2, E2g at 353 cm−1 and A1g at 422 cm−1 for WS2, respectively, and one peak is identified as A1g at 243 cm−1 for MoSe2, which are consistent with previous reports.41,42 No peak shift was observed due to the multilayer


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nature of the products. UV-vis absorption properties are strictly related to the electronic structure of a material. In the UV-vis absorption spectra there exist two absorption peaks in the spectral range of 500~800 nm, corresponding to A and B excitons as labeled in Figure 3b. The A and B excitons result from direct transitions due to a valence band splitting associated with the combined effects of interlayer interaction and spin-orbit coupling.43 Compared with the corresponding bulk materials, A and B exciton energy levels exhibit a blue-shift to a certain extent, mainly arising from quantum confinement effects as the number of layers of the 2D TMDs decreases.44,45 This result means the electronic structures of 2D layered TMDs can be modulated by the ion exchange method. Femtosecond transient absorption (TA) spectroscopy has been utilized as a useful tool to characterize the ultrafast dynamics of photoexcited carriers in 2D TMDs. TA spectra of various 2D TMD nanosheets in ethanol following photoexcitation at 400 nm are shown in Figure 4a-4c. The highly structured features with series of alternating narrow positive and negative bands were observed in all TA spectra, which is in accord with previous reports.46,47 The positive bands with peaks located at ~675 and ~625 nm for MoS2, ~805 and ~695 nm for MoSe2, and ~638 nm for WS2 are assigned to photobleaching of excitonic A and B transitions.48,49 The negative bands are attributed to the transient absorption of photoexcited carriers.50 Figure 4e-4g present A exciton dynamics for all three samples in ethanol with different pump fluence and detecting at exciton resonance peaks. All dynamic curves exhibit multi-exponential decay features and can be well fitted using a Gaussian response function convoluted with multi-exponential decay functions. The fitting parameters are displayed in Table S6. All signals present a dominated decay time in order of ~2 ps. Commonly, the primary decay channels in 2D TMDs are nonradiative recombination via defects and Auger-type exciton-exciton annihilation51,52. For the latter


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mechanism, Auger-type decay constant usually highly depends on the pump density, i.e., increase of recombination rate with increasing pump fluence. However, our experimental results seem to be opposite as shown in Figure 4e-4g. Therefore, the observed initial fast decay can be attributed to the trap of excitons by surfaces and interfacial defects.48 Under the same pump condition, the decay constant of photoexcited excitons strongly relies on the amount of defects in the sample. Generally, the increase of defect density will result in the decrease of decay constant. The observed similar decay constants for all three samples indicate they have roughly the same amount of defects, mainly because of the same synthesis method and layers. It further demonstrates the morphologies of 2D TMD nanosheets as well as inside dynamics of photoexcited carriers keep almost unchanged after ion exchange reactions from MoS2 to MoSe2 and WS2. The pump power dependent exciton dynamic curves also exhibit another interesting phenomenon: the slow component of dynamics becomes much slower and more significant with increasing excitation fluence. It has been demonstrated that a main source for pump-induced absorption changes of 2D TMDs is the transfer of the excitation to phonon energies, which followed by cooling of the material through heat transfers53. Therefore, the slow component can be attributed to the thermal effect, which turns out to be more significant with increasing excitation fluence because of more redundant excitation energies. Both the hot electron relaxation inside the crystal and electron-phonon scattering on surfaces could contribute the rapid increase of local temperature in these nanosheets, which will be further confirmed in our next work.


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Figure 4. TA spectra recorded following photoexcitation at 400 nm with energy of 200 nJ/ pulse: (a) MoS2, (b) MoSe2, and (c) WS2. Normalized energy-dependent A exciton dynamics excitation with 400 nm, and probe wavelengths and pulse energy as marked: (e) MoS2, (f) MoSe2, and (g) WS2. Circles are data, and the solid lines are fits of exponential decay function.


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Figure 5. (a) The dark I-V curves and (b) photo responses of MoSe2, WS2 and MoS2 nanosheets under 500 nm light illumination. To date, 2D TMD nanosheets have been extensively employed in many hot fields, such as optoelectronic devices, energy conversion and storage and catalysis. As a representative characteristic of 2D atomic crystals, we chose the optoelectronic properties to evaluate the performances of our 2D TMD nanosheets produced by the ion exchange strategy. The dark and photo illuminated I−V behaviors of the MoSe2, MoS2 and WS2 nanosheets on SiO2(200nm)/Si substrates were measured at room temperature and in air, respectively. Ti (∼5nm)/Au (∼100nm) electrodes deposited by thermal evaporation, were used as metal contacts. The dark I-V curves in Figure 5a show a good ohmic contact between the TMD nanosheets and electrodes. Figure 5b shows the photo responses of the MoS2, MoSe2 and WS2 devices under 500 nm light illumination. The photocurrents are about 0.23 nA for MoS2, 59 nA for MoSe2 and 99 nA for WS2, respectively. The photo responsivity can be calculated from the photocurrent Iph divided by the incident power: Rph = Iph/Pin. The power density of the light is 100 mW cm–2. The corresponding responsivities are obtained: 2.87 mA W-1 for MoS2, 0.74 A W-1 for MoSe2 and 1.24 A W-1 for WS2, which are comparable to previous reports.46 The results indicate the 2D TMD nanosheets obtained by the ion exchange strategy have potential for various optoelectronic devices. In summary, we successfully developed an effective ion exchange route for 2D TMD nanosheets. Structural and chemical composition characterizations confirm the feasibility and versatility of the ion exchange reaction in 2D layered TMDs. Moreover, the 2D TMD nanosheets obtained by the ion exchange strategy were evaluated by the optoelectronic properties and exhibit potential for various optoelectronic devices. Our strategy could be further extended to


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prepare a wide range of 2D TMD nanosheets and even heterostructures, such as Metal-TMDs and van der Waals heterojunctions etc.

EXPERIMENTAL SECTION Liquid exfoliation of MoS2 and SnS2 Liquid exfoliation was carried out by ultrasonicating MoS2 or SnS2 in ethanol for 10 hours. The MoS2 or SnS2 nanosheets were collected by centrifugation. Cation exchange reaction from SnS2 to MoS2 All ion exchange reactions were carried out under N2 using standard Schlenk technique. A few layered SnS2 nanosheets (0.1mmol), oleylamine (OLA, 10 mmol), and 5 mL of 1-octadecene (ODE) were loaded into a 50 mL three-necked flask. The mixture (A) was degassed at 120 °C under a nitrogen atmosphere. A mixture of MoCl5 (2mmol), oleic acid (OA, 3ml) and TOP (3ml) was injected into the precursor mixture A. The reaction was kept at 320 °C and the reaction time was kept for 5 hours. After the mixture was cooled down to room temperature, the products were precipitated by adding ethanol and collected by centrifugation. Cation exchange reaction from MoS2 to WS2 MoS2 nanosheets (0.1mmol), oleylamine (OLA, 10 mmol), and 5 mL of 1-octadecene (ODE) were loaded into a 50 mL three-necked flask. The mixture (A) was degassed at 120 °C under a nitrogen atmosphere. A mixture of WCl6 (2mmol), oleic acid (OA, 3ml) and TOP (3ml) was injected into the precursor mixture A. The reaction was kept at 320 °C and the reaction time was


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kept for 5 hours. After the mixture was cooled down to room temperature, the products were precipitated by adding ethanol and collected by centrifugation. Anion exchange reaction from MoS2 to MoSe2 A few layered MoS2 nanosheets (0.1mmol), oleylamine (OLA, 10 mmol), and 5 mL of 1octadecene (ODE) were loaded into a 50 mL three-necked flask. The mixture (A) was degassed at 120 °C under a nitrogen atmosphere. Then Se powder (2 mmol) dissolved in 5 mL of TOP was quickly injected into the above mixture. The reaction was heated to 300 °C and kept for 5 hours and then stopped by removing the heater. After the mixture was cooled down to room temperature, the products were precipitated by adding ethanol and collected by centrifugation. High temperature annealing of the products All the obtained 2D TMD nanosheets were annealed at high temperature to improve crystallinity for further characterization. Firstly, the nanosheets were kept at 350 oC for 1h to remove the residual solvent and capping ligands. Second, the temperature was increased to 500 o

C for 3h. During the annealing, Ar mixed with 7% H2 as flowing gas was used to protect the

nanosheets against oxidation. Device fabrication Fabrication of MoS2 MoSe2 or WS2 nanosheet devices: An ethanol solution of suspended TMD nanosheets was drop-cast onto the Si/SiO2 (200 nm) substrate, followed by annealing at 300 oC for 1 h under Ar (7% H2) flow. Then Ti (5 nm)/Au (100 nm) electrodes were fabricated by e-beam lithography (EBL) and thermal evaporation. The channel area was about 80 µm2. The current-voltage (I-V) curve was measured using Keithley 4200 and probe station. A Xe solar


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simulator with a band-pass filter (500 nm) was used as the light source and light intensity was 100 mW/cm2. Characterization Methods Transmission electron microscopy (FEI Tecnai G2 F20, 200 kV) was used to observe the morphology of the samples. The high-resolution HAADF images and energy dispersive spectroscopy (EDS) spectra were taken by 200 keV JEOL ARM 200 equipped with a probe corrector and a cold FEG with spatial resolution of 0.08 nm, and a energy-dispersive X-ray detector. Data analysis was performed using the INCA software. X-ray diffraction (XRD) spectrometer (Philips X’Pert Pro MPD with a standard Cu-Kα radiation source, λ = 0.15418 nm) was used to study the phase and crystal structure of MoS2, MoSe2 and WS2. It operated at 40 kV and 40 mA at room temperature and scanned in the 2θ range from 10° to 80°. X-ray photoelectron spectroscopy (XPS): The chemical states of atoms in MoS2, MoSe2 and WS2 were measured by PHI Quantera II with an Al K =280.00 eV excitation source. Raman spectra were collected by a HORIBA HR800 equipped with a 532 nm laser. ICP-OES (inductively coupled plasma optical emission spectrometry) was performed on a Vista-MPX (Varian) instrument. Absorption spectra were measured on a Hitachi U4100 spectrometer at room temperature. The nanosheets were dispersed into ethanol for absorption measurements. Ultrafast spectroscopy measurements were performed using the output pulses from a regenerative Ti:sapphire amplifier (Spectra-Physics) with time duration of 150 fs, 800 nm center wavelength, and repetition rate of 1 kHz. The output was split, and a fraction of the beam was used to pump the BBO nonlinear crystal to produce its second harmonic at 400 nm pulse served as excitation pulses. Another fraction of the beam was focused into a sapphire plate to generate the broadband white light continuum pulses that were used as probe pulses. Excitation pulses were sent through a delay


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stage and then noncollinearly focused on the sample where they overlapped spatially and temporally with the probe pulses. The spectrum of the differential transmission change of the probe pulses after photoexcitation was recorded by a phase-locked amplifier after frequency resolved by a spectrograph.

Supporting Information: Structural and chemical composition characterizations of the intermediates and the products; The crystal structures, lattice energies and ion radii of layered and non-layered metal chalcogenides; The fitting parameters of the decay curves of TMDs.

ACKNOWLEDGEMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 51472031 and 51102017).

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General Strategy for Two-Dimensional Transition Metal

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