Supplementary Figure 1. Sample preparation schematic. First (Stage I), square islands of MoO 3 are prepared by either photolithography followed by

Transcription

1 Supplementary Figure 1. Sample preparation schematic. First (Stage I), square islands of MoO 3 are prepared by either photolithography followed by thermal evaporation and liftoff or by a process where an aqueous solution of ammonium heptamolybdate (AHM) is spun onto a pattern prepared by electron beam lithography, followed by liftoff. In either case, samples are annealed at o C in the CVD furnace in a flow of 500 sccm N 2. Next (Stage II) the sample is treated with an oxygen plasma, and then spin-coated with a growth promoter solution consisting of 0.1% sodium cholate in deionized water. MoS 2 is then grown using the process described in the Methods section of the main text.

2 Supplementary Figure 2. AHM deposition and AFM images showing the result of the AHM aggregation process. Panels a-d are optical micrographs. (a) Resist pattern before AHM deposition. (b) Resist pattern after AHM deposition. (c) AHM islands on SiO 2 after the liftoff step. (d) AHM islands after the aggregation process. Lower left - Atomic Force Micrograph showing that the AHM deposition process results in a set of patterned growth seeds of rectangular shape, approximately 50 nm thick in the center and slightly thicker at the edge. Lower right AFM micrograph showing that after the aggregation process discussed in Supplementary Fig. 3, the growth seed takes on a rounded shape, which enables more uniform and reproducible growth of MoS 2.

3 Supplementary Figure 3. Aggregation process for MoO 3 growth seed Panels (a) (e) show the aggregation process used for a sample with MoO 3 growth seeds created by thermal evaporation of MoO 3 along with an explanatory sketch. The same process was also effective for samples with growth seeds consisting of AHM (data not shown). (a) As-prepared sample. (b) The growth seeds show a colour change due to water accumulation as the sample temperature is lowered using a Peltier cooler. (c) When the temperature approached the dew point, water droplets were observed to accumulate on the MoO 3 pattern. (d) MoO 3 was dissolved into the droplet. (e) When the Peltier cooler was turned off, water evaporated and small MoO 3 beads were formed.

4 a b c Supplementary Figure 4. Growth of fully monolayer molybdenum disulfide flakes at controlled locations When the dimensions of the growth seed are optimized, the resulting MoS2 flakes are free of multi-layer regions. In this case, the MoO3 growth seeds were 3-μm square and 75 nm thick. (a) 7 x 7 array of flakes of monolayer MoS 2 grown by chemical vapour deposition. Scale bar is 100 μm. (b) Higher magnification image of the region shown by the dashed line in panel (a). Scale bar is 40 μm. (c) Higher magnification image of the region shown by the dashed line in panel (b). Scale bar is 20 μm.

5 Supplementary Figure 5. Correlation of the size of the MoO 3 growth seed and the number of crystalline domains of MoS 2 produced. (a) Optical micrograph of a sample with growth seeds of lateral size (5 μm) 2, (7 μm) 2, and (10 μm) 2. After growth of MoS 2, the number of crystalline domains for the monolayer regions (as indicated by the number of triangular vertices) was found to increase approximately with the perimeter of the growth seed, consistent with the growth model presented in the main text. (b) The (5 μm) 2 growth seed leads to a MoS2 region with 5 vertices, indicating the presence of 5 crystalline domains. (c-d) As the size of the growth seed is increased to (7 μm) 2 and (10 μm) 2, the number of domains increases. Scale bars in (b), (c) and (d) are 20 μm.

6 Supplementary Figure 6. Additional AFM line scans for inset of fig 3b in the main text To support the AFM data in fig 3b in the manuscript, 5 line scans are extracted from the image. The line scans show a consistent step height of approximately 0.7 nm, consistent with the expectation that the sample is monolayer MoS 2. (a) AFM image with profiles. White particles observed at the edge of the flake in the AFM image are assumed to be small aggregates of sodium cholate promoter material. The height of the particles is ~ 8 nm as seen in panel (c). (b-f) Line scans of the AFM image, as indicated in panel (a).

7 Supplementary Figure 7. AFM data to confirm synthesis of monolayer MoS2 flakes. (a-c) AFM images of other polycrystalline monolayer flakes are presented to show the lack of multilayer domains with size larger than the scan resolution (approx. 5 nm). Regions identified as monolayer are approximately 0.75 nm in thickness, and they appear flat within the noise resolution of the AFM scan. (d) Representative line scan along the line indicated in panel b showing a flake height of ~ 0.75 nm.

8 Supplementary Figure 8. 2D Raman (peak position) mapping for E 2g and A 1g mode (a-b) Spatial maps of the peak positions of the E2g (panel a) and A1g (panel b) modes in the Raman spectra. Comparison with the AFM data in Fig 3d of the main text shows that the peak positions are uniform over the monolayer regions (labelled I ) and shift to different values in multilayer regions (labelled ii ). Scale bars for panels a-b are 2 μm. (c) Representative Raman spectra from the monolayer region (i) and the multilayer region (ii). Taken together with the data in the main text, these establish that the MoS 2 flakes grown by this method consist of large, uniform monolayer regions and small multilayer regions associated with residual seed material.

9 Supplementary Figure 9. Sample preparation for transmission electron microscope (TEM) measurements. After growth of MoS 2 in a pattern similar to Fig. 2a in the main text, a gold mesh pattern was fabricated on the sample using electron beam lithography followed by lift off. This was used as a frame to avoid tearing during the transfer process onto a holey carbon TEM grid. Scale bars in figure and inset are 35 μm and 7 μm, respectively.

10 a b Supplementary Figure 10. Raw TEM image and atomic resolution image for a typical MoS 2 flake. (a) Raw TEM image used to create in Figs. 4(a-b) in the main text. The faceted monolayer MoS 2 regions are visible in this image; dashed lines around the perimeter of the faceted regions were added to yield Fig. 4b. (b) Lattice resolution TEM image for a MoS 2 flake synthesized using this method. (Cs-corrected Titan, Accelerating voltage of 80 kv). Scale bar in (a) and (b) are 2 μm and 2 nm, respectively.

11 Supplementary Figure 11. Verification of monolayer MoS 2 by analysis of the intensities of first order diffraction spots in selected area electron diffraction. Line scan showing the intensity of two neighbouring first order diffraction spots (along the blue line in the SAED pattern shown as an inset). The ratio of the intensities of the diffraction spots of 1.2 is consistent with theoretical prediction and earlier reports for monolayer MoS 2. 1 Supplementary References

12 1. Brivio, J., Alexander, D.T.L. & Kis, A. Ripples and layers in ultrathin MoS2 membranes. Nano Lett.11, (2011).