Resonant Raman spectroscopy study of swift heavy ion irradiated MoS2

Molybdenum disulphide (MoS2) crystal samples were irradiated by swift heavy ions (209Bi and 56Fe). Hillock-like latent tracks were observed on the surface of irradiated MoS2 by atomic force microscopy. The modifications of properties of irradiated MoS2 were investigated by resonant Raman spectroscopy and ultraviolet–visible spectroscopy (UV–Vis). A new peak (E1u2, ∼385.7 cm−1) occurs near the in-plane E2g1 peak (∼383.7 cm−1) after irradiation. The two peaks shift towards lower frequency and broaden due to structural defects and stress with increasing fluence. When irradiated with high fluence, two other new peaks appear at ∼ 190 and ∼ 230 cm−1. The peak at ∼230 cm−1 is disorder-induced LA(M) mode. The presence of this mode indicates defects induced by irradiation. The feature at ∼460 cm−1 is composed of 2LA(M) (∼458 cm−1) and A2uA2u (∼466 cm−1) mode. With increasing fluence, the integrated intensity ratio between 2LA(M  ) and A2uA2u increases. The relative enhancement of 2LA(M) mode is in agreement with the appearance of LA(M) mode, which both demonstrate structural disorder in irradiated MoS2. The ∼423-cm−1 peak shifts toward lower frequency due to the decrease in exciton energy of MoS2, and this was demonstrated by the results of UV–Vis spectra. The decrease in exciton energy could be due to introduction of defect levels into band gap.