The optical properties of the as-grown MoS2 flakes were studied by Raman and PL spectroscopy, as shown in Figure 3j−k. A strong PL signal is located at 1.83 eV, and the frequency difference
between the Raman modes E2g (382 cm−1) and A1g (402 cm−1) is 20 cm1, indicating a high crystallinity of the as-grown MoS2 monolayer.