Supplementary Figure S1 Photograph of MoS 2 and WS 2 flakes exfoliated by different metal naphthalenide (metal = Na, K, Li), and dispersed in water.

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1 Supplementary Figure S1 Photograph of MoS 2 and WS 2 flakes exfoliated by different metal naphthalenide (metal = Na, K, Li), and dispersed in water.

2 Supplementary Figure S2 AFM measurement of typical LTMDs nanosheets, deposited on Si/SiO 2 substrate, corresponding EDS and photograph of dispersion in water. (a) TiSe 2, (b) NbSe 2, and (c) MoSe 2 give average thickness of ~2 nm, confirming that few layers (2-4 layers) were successfully produced by our method.

3 Supplementary Figure S3 XRD pattern of Li, Na, K exfoliated high quality MoS 2 and amorphous MoS 2 after annealing at 150 C.

4 Supplementary Figure S4 TEM image of sub-micron sized MoS 2 sheet exfoliated by n-butyl lithium. The same hydrazine treatment is applied to create pre-expanded MoS 2, which is then used as starting material for exfoliation. Compared to our exfoliation methods which give large-sized flakes, the flakes exfoliated by n-butyl lithium are typically sub-micron sized and are full of cracks.

5 Supplementary Figure S5 SEM image of sub-micron sized MoS 2 sheet exfoliated by n-butyl lithium.

6 Supplementary Figure S6 AFM image of sub-micron sized MoS 2 sheet exfoliated by n-butyl lithium.

7 Supplementary Figure S7 Histogram showing the size distribution of the exfoliated flakes. (a) Na-exfoliated MoS 2, (b) nbuli-exfoliated MoS 2. Histogram shows the thickness (number of layers) distribution of the exfoliated flakes; (c) Na-exfoliated MoS 2, (d) n-buli exfoliated. All MoS 2 sheets were exfoliated from the same starting material. Our naphtalenide intercalation method is conducted at low temperatures and full exfoliation can be achieved within 8 hours. In contrast, the n-butyl lithium intercalation method requires elevated reaction temperatures (e.g. 100 o C) and takes three days. As observed in the TEM and AFM images, the exfoliated sub-micron sized sheets are full of cracks and covered by impurities such as Mo nanoparticles. Our method can exfoliate monolayer MoS 2 sheets with high yield and integral large size.

8 Supplementary Figure S8 Edge structures of MoS 2 and WS 2. (a) High resolution TEM (HRTEM) image of edge area of MoS 2 in Fig 3g, (b) TEM image of few layer MoS 2, (c) HRTEM image of edge area of few layer MoS 2 in b show several clear fringes, (d) TEM of WS 2 sheets.

9 Supplementary Figure S9 Histogram showing the thickness (number of layers) distribution of the exfoliated flakes collected from the supernatant of centrifuged Na-exfoliated MoS 2. AFM was used to analyse the layer thickness.

10 Supplementary Figure S10 Structure of 2H- and 1T-MoS 2.

11 Supplementary Figure S11 XPS spectra of MoS 2. (a) Mo 3d and (b) S 2p XPS spectra of Na-exfoliated MoS 2, before and after annealing, after Shirley background subtraction, the Mo 3d and S 2p peaks are deconvoluted to show the components assignable to the 2H and 1T phases, represented by red and green plots, respectively. Before annealing the atomic ratio of Mo:S = 1:1.99. After annealing the atomic ratio of Mo : S=1:1.97. The flake is stoichiometric with MoS 2.

12 Supplementary Figure S12 Raman spectra illustrating that the exfoliated MoS 2 sheets contain a mixture of 1T and 2H phase. After annealing the sample shows the characteristic 2H phase.

13 Supplementary Figure S13 Photoluminescence spectrum of Na-exfoliated single-layer MoS 2 sheet deposited on Si/SiO 2 substrate before and after annealing.

14 Supplementary Figure S14 EDS spectra of MoS 2 and WS 2. (a) EDS spectra of N 2 H 4 -exfoliated MoS 2. No N 1s peak is detected, and the Mo: S ratio is nearly 1:2. (b) EDS spectra of Na-exfoliated MoS 2 and (c) EDS spectra of Na-exfoliated WS 2.

15 Supplementary Figure S15 FET device characterization of MoS 2. (a) Output characteristic of monolayer MoS 2 flake (b) transfer characteristics of the monolayer MoS 2 flake, (c) Output characteristic of three layer MoS2 flake (d) transfer characteristics of the three layer MoS 2 flake.

16 Supplementary Figure S16 Enlarged SEM image of MoS 2 thin film-coated optical fiber pigtail in Fig 5d. The pigtail is coated with uniform MoS 2 film, which attests to the good printability of MoS 2 flakes prepared by our methods.

17 Supplementary Figure S17 (a) photo image of a printed MoS 2 dot array. (b) photo image of a printed MoS 2 line arrays, and the similar SEM image for different printed lines. (c) SEM image of MoS 2 flakes in printed MoS 2 line. (d) SEM image of printed MoS 2 drop point. (e) photo image of a printed MoS 2 thin film. (f) SEM image of printed MoS 2 thin film.

18 Supplementary Note 1 Phase transformation of MoS 2 As-prepared Na-exfoliated MoS 2 sheets are dominated by the 1T phase (metallic). Annealing at 200 o C causes the phase transformation to the 2H phase (semiconductor). This phase transformation has been characterized by XPS, Raman, PL, UV and PSD spectra. The XPS Mo 3d, S 2s, and S 2p regions for exfoliated MoS 2 before and after annealing are shown in Figure S12 The Mo 3d core levels consist of the spin orbit doublet Mo 3d 3/2 and Mo3d 5/2. For 2H phase, these doublet peaks appear at 232 and 229 ev, respectly. In the 1T phase, these peaks are shifted to lower binding energies by 0.9 ev. A similar shift is observed in the S 2p emission. The spin orbit S peaks of 2H-MoS 2, S 2p 1/2, and S 2p 3/2, appear at and 162 ev, respectively. In the 1T phase these peaks become shifted towards the lower binding energies. The phase transition from 1T to 2H occurs after annealing at 200 o C. Based on the deconvoluted peak area, the relative proportion of the 2H and1t phases can be determined. As-prepared MoS 2 sheets are composed of about 60 % 1T phase whereas after annealing at 200 o C, 90% of the material is converted to 2H phase. Figure S12 shows a typical Raman spectrum obtained from exfoliated MoS 2 sheet. Before annealing there are peaks at 154, 204, 310, 356, and 407 cm -1. Only a very weak peak is present near 385 cm -1. The 385 cm -1 peak is usually strong in 2H-MoS 2. The peaks at 154 and 356 cm -1 which appear in the spectra of sample before annealing are not normally seen in the Raman spectrum of 2H-MoS 2 crystals. The presence of these peaks at at 154, 204, 310, 356, and 407 cm -1, in conjunction with the weak peak at 385 cm -1, are related to the octahedral coordination which gives rise to the metallic 1T phase, consistent with the report published in Physical Review B by Yang in 1991 [43, 14, page 12053]. After annealing at 200 o C the peaks due to the metallic 1T phase become attenuated while the peaks associated with 2H phase appear and exhibit relatively strong intensity. No PL can be observed on the as-exfoliated samples whereas PL can be observed after a brief bake at 200 ºC. The PL spectrum of a Na-exfoliated single layer MoS 2 exhibits a peak centered at 668 nm (1.86 ev) with a shoulder at 623 nm (1.99 ev), which agrees with excitonic peaks arising from the K point of the Brillouin zone.

19 Supplementary Note 2 FET device of MoS 2 To evaluate the electrical performance of the MoS 2 sheets, we fabricated FETs with ~500 nm to 1.5 μm channel lengths on monolayer and few layer (2-4 layers) MoS 2 flakes using 5 nm Cr/50 nm Au as source and drain electrodes, 300-nm-thick SiO 2 as dielectrics and p++ Si as the back gate by electron beam lithography and electron beam deposition. To avoid the doping of oxygen, all devices were tested in argon at room temperature. Fig. S15 shows the typical electrical performance of MoS 2 FETs. Both of them show n-type behavior, consistent with previous reports. Electron mobility can be extracted from the linear regime of the transfer characteristics, using μ=[(δi DS /V DS )(L/W)]/Cox ΔV G where L and W are channel length and width respectively, Cox is silicon oxide gate capacitance, I DS, V DS, and V G are drain-source current, drain-source voltage and gate voltage, respectively. Twenty FET were measured, the mobility of monolayer MoS 2 devices were in the range of 1~8 cm 2 /(V s)while few layer were in 20~80 cm 2 /(V s), which were comparable with those of backgated FETs made with mechanically exfoliated MoS 2 flakes (10~20 cm 2 /(V s)and 100~200 cm 2 /(V s) respectively ) 37,39.

20 Supplementary Note 3 Ink-jet printing of MoS 2 The ink is made from 0.02 mg/ml MoS 2 fully dispersed in ethanol/ water (2:1 volume) solution (viscosity 2.64 cp and surface tension 34.3 mn/m). To print high resolution patterns and uniform films, 10 μm diameter printer nozzle is selected, and the wafers are heated to 60 o C before printing. Unlike nanoparticles, monolayer MoS 2 is soft and flexible, as a result even large sized flakes (~ 20 μm) can easily pass through a 10 μm diameter glass printer nozzle. The large sized flake will fold in the ink drop and unfold when ink drop spread out on wafer. As a result, we can print uninterrupted for hours without clogging the print head. Fig. S17 a and b show high throughput arrays of dots and lines we can print using the MoS 2 ink. The printing is highly stable and repeatable as can be judged from SEM imaging of ribbon lines in Fig. S17c, there are many flakes that have sizes > 10 µm. As shown in Fig. S17c, the SEM images of different regions from different ribbons show high consistency in terms of the flakes size. Fig. 17d shows the SEM image of a printed array dot. It is clear that large sized flakes of about 20 µm are present. Fig. S17e shows a printed continuous and uniform thin film which nearly covers an entire 4 inch wafer. Fig. S17f shows the SEM image of the continuous printed film.

21 Supplementary Reference 39. S. H. Wan et al. Comparative study of chemically synthesized and exfoliated multilayer MoS2 field-effect transistors. Appl. Phys. Lett. 102, (2013).