Experimental
Single and few-layer MoS2 were prepared by mechanically exfoliating 2H-MoS2 crystals (SPI, molybdenite) onto a Si wafer substrate covered with a 285 nm thick amorphous SiO2 layer.11 Thin MoS2 sheets of several μm across were identified under an optical microscope for faster screening and were probed with Raman spectroscopy for their thickness. 11 Raman and PL spectra were obtained with a homebuilt micro-Raman spectrometer setup that has been described in detail elsewhere.27-29 Briefly, an Ar ion laser beam (514.5 nm, 0.17 mW) was focused onto a sample (spot size < 1 μm) using a microscope objective (40X, numerical aperture = 0.60). Back-scattered Raman or PL signal was collected with the same objective and guided to a spectrometer combined with a liquid nitrogen-cooled CCD detector. The overall spectral accuracy was better than 1 cm−1 and 1.5 meV for Raman and PL spectra, respectively. Atomic force microscopy (AFM) was also employed to reveal the nanoscopic morphology of the samples. To modify the PL characteristics, some samples were annealed in a vacuum (< 3 mtorr) or H2 gas (99.999%, flow rate = 30 mL/min) using a tube furnace. The annealing time was one hour unless stated otherwise.
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