Swift-heavy ion irradiation-induced latent tracks in few- and mono-layer MoS2

The latent tracks in mono-and few-layer molybdenum disulfide (MoS2) induced by Bi-209 ions with energies of 0.45-1.23 GeV were characterized by atomic force microscopy (AFM). The hillock-like latent tracks were observed on the surface of irradiated monolayer MoS2. The diameter of the hillock after deconvolution procedure is 15.8 +/- 1.7 nm and the height is 1.0 +/- 0.3 nm. Hillock-like tracks are induced by energy transfer from energetic Bi-209 ions to electron system of MoS2, resulting in the ionization and excitation and then the displacement of target atoms. Since Raman spectroscopy is sensitive to damages induced by swift-heavy ion irradiation, the in-plane E-2g(1) 1 mode (similar to 385 cm(-1)) and the out-of-plane A(1g) mode (similar to 408 cm(-1)) of MoS2 were investigated. With increasing ion fluence, the A(1g) peak shifts to higher frequencies, and the intensity ratio between A(1g) and E-2g(1) pcak increases. Besides, the A(1g) peak narrows. The evolution of the structural and vibrational properties of MoS2 with fluence is discussed. It can be concluded that the blue shift and narrowing of A(1g) peak in irradiated MoS2 is due to the adsorption of oxygen molecules at latent tracks. With decreasing thickness of MoS2, the irradiation resistance decreases.