The deposition of these three layers was achieved without breaking the vacuum. 30 nm Ni

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1 Transfer-free Growth of Atomically Thin Transition Metal Disulfides using a Solution Precursor by a Laser Irradiation Process and their Application in Low-power Photodetectors Chi-Chih Huang 1, Henry Medina 1, Yu-Ze Chen 1, Teng-Yu Su 1, Jian-Guang Li 1, Chia-Wei Chen 1, Yu-Ting Yen 1, Zhiming M. Wang 2 and Yu-Lun Chueh 1 * 1 Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, R.O.C. 2 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, People s Republic of China. These authors contributed equally to this work ylchueh@mx.nthu.edu.tw Methods Deposition of Ni, TiO 2 and WO 3 The deposition of these three layers was achieved without breaking the vacuum. 30 nm Ni (99.99%) was first deposited by e-beam evaporation with a deposition rate of 0.3 nm s 1 in a high vacuum (10 6 torr). In a similar way, TiO 2 and WO 3 films were deposited with thickness of 10 nm and 7 nm on the Ni layer, respectively. Sulfur solution preparation 0.64 g of 99.98% sulfur mixing with 20 ml oleylamine (technical grade, 70%) was heated at 120 C and stirred for an hour. MS 2 synthesis Figure 1 shows steps to grow MS 2 (M=W, Mo). First, nickel (laser absorption layer), TiO 2 (barrier layer) and MO 3 (M=W, Mo) thin films are deposited (Step 1). Then, sulfur in oleylamine was spin-coated (Step 2) on the layered structure and then irradiated by a 808 nm CW laser (Step 3) at room temperature. A low level of vacuum is still needed to pump out the evaporated oleylamine solvent (bp. ~365 C) from the rapid heating induced by the laser.

2 Device Fabrication Processes Ni was used as a back electrode while WS 2 and the WO 3 capping layers serve as a light absorbing layer. Photo lithography and standard lift-off process were used to define the top electrodes. Finally, silver was deposited as top electrode. All measurements were carried out under ambient conditions in a two probe configuration. Characterization Raman spectroscopy was recorded using 514 nm laser excitation. Information of the microstructures was from TEM operated at 200 and 350 kv with resolution of 0.17 nm. The chemical bonding between tungsten and oxygen on the surface and the interior of the film were certified by X-ray photoelectron spectroscopy PHI 5000 VersaProbe II. The photoelectric device was characterized with a Keithley 4200-SCS semiconductor parameter analyzer under different kinds of light (410nm, 514nm, and 633nm).

3 Figure S1. XPS after laser irradiation without barrier layer. (a)an OM image of the laser irradiated sample without the TiO 2 layer. XPS signals of Ni and S from (b) WO 3 and (c) Ni layer of irradiated sample without the TiO 2 layer Figure S2. Raman spectra of WS 2 after laser irradiation.

4 Figure S3. Sulfur diffusion into the WO 3 layer. (a) A schematic illustration for the layer structure of the sample after the low power laser irradiation. XPS spectra from the surface of low power (4.02 W for 90 s) laser irradiated sample for (b) W 3d and (c) S 2p. The spectra from the interior of WO 3 layer for (d) W 3d and (e) S 2p. Figure S4 Raman spectra of WS 2 after the laser irradiation with a high power.

5 Figure S5. Cross sectional TEM (a) low and (b) high magnification images of the sample exposed to high power laser operated at 8.25 W for 90 s.

6 Normalized current (a.u.) Figure S6. Comparison of signal to noise of normalized current vs. time plot measured under red (633 nm) light with 0.1 V and 1 V biases. Vbias = 0,1 V Vbias = 1 V Time (s) Figure S7. Band alignment of the WS 2 based photodetector.

7 Figure S8. Normalized responsivity of the WS 2 photodetector kept on atmospheric conditions and measured up to 80 days after the synthesis. Figure S9. (a) I-t curve measured under red (633 nm) light with a 0.1 V bias in a device without the laser irradiation. (b) I-V curves measured with and without light illumination.

8 Figure S10. (a) I-t curve measured under blue (410 nm) light with a 0.1 V bias. (b) Normalized I-t curves measured under red (633 nm), green (532 nm) and blue (410 nm) lasers. Figure S11. Fitting curves of the photoresponse and time constant estimation for different laser wavelengths at (a) 633 nm, (b) 532 nm and (c) 410 nm, respectively.

9 Table S1. Rise and fall time under different excitation wavelengths.