Swift Heavy Ion Irradiation Effect On Mono-And Few-layer MoS₂

In recent years, mono- and few-layer MoS₂ has attracted great attention for its promising application prospect in nanoelectronic and optoelectronic devices. Mircomechanical exfoliation method was employed to prepare mono- and few-layer MoS₂. The identification of layer thickness was done by optical microscope and Raman measurement. Irradiation experiments were performed on Heavy Ion Research Facility in Lanzhou（HIRFL） in vacuum at room temperature under normal incidence.Samples were irradiated by 209 Bi and 86 Kr ions with initial kinetic energy of 9.5 and 19.5 Me V/u, respectively, and 56 Fe ions with initial kinetic energy of 5.66 Me V/u. The morphology of irradiation-induced latent tracks was characterized by atomic force microscopy（AFM） and transmission electron microscopy（TEM）. Modification of the properties of irradiated MoS₂ was studied by Raman and resonant Raman spectroscopy, Ultraviolet-Visible（UV-Vis） spectroscopy and X-ray photoelectron spectroscopy（XPS）. The main results are as follows.After irradiation, hillock-like latent tracks were observed by AFM on the surfaces of trilayer MoS₂, monolayer MoS₂ supported bySi O₂/Si substrate and bulk MoS₂. The track-formation-probability of hillocks for MoS₂ with different thicknesses is almost the same（ 0.7）. The diameters of hillocks on bulk, trilayer and monolayer MoS₂ are tested to be 24.6±1.5, 22.2±0.3 and 26.2±1.1 nm, respectively. After deconvolution, the diameters are 16.5±1.8, 12.0±1.6 and 15.8±2.1 nm, respectively. The heights of hillocks on trilayer and monolayer MoS₂ are 1.0±0.2 and 1.0±0.3 nm, which are higher than the value for bulk MoS₂, which is 0.6±0.1 nm. This may be due to that recrystallization is more prominent in bulk than in atomically thin material. With increasing fluence, the root-mean-square surface roughness of MoS₂ increases.According to the high-resolution TEM images, the plane vertical to the ion-beam is the basal plane of MoS₂. The low-resolution bright-field images show that black dots which are homogeneously distributed and of consistent size are irradiation-induced latent tracks. Irradiated by 1.27 Ge V Bi ions, the track-formation-probability was counted to be 0.95. The diameter of tracks is around 7.1±0.7 nm. Lattice structure inside tracks are partly disordered. According to the brightness of the dots and the halo around them in electron diffraction pattern, the dimmer the dots is and much more halo around them indicate more severely disordered lattice structure.The XPS spectra of MoS₂ irradiated with 0.97 Ge V Bi ions show that Mo6+ state appears and the intensity of O 1s peak increases with increasing fluence, which indicate the formation of Mo O3 or Mo-O bonds. There is no peak that belongs to Mo O3 was observed in Raman spectrum. Thus it can be concluded that Mo-O bonds was formed after irradiation. The formation of Mo-O bonds can be attributed to the adsorption of oxygen molecules at latent tracks. Oxygen molecules can drain out electrons from MoS₂, causing the decrease of electron density, i.e., p-type doping to MoS₂, which performs as the blue shift of A_1g peak. The blue shift of A_1g peak is more obvious for mono-layers than few-layers and bulk ones, indicating a higher amount of defects and weaker irradiation resistance in thinner samples. With increasing ion fluence, the intensity ratio between A_1g and E_2g¹ peak increases because of lattice defects and decrease in electron density. Besides, the A_1g peak narrows due to decrease in electron density.The modifications of properties of MoS₂ crystal samples irradiated by Bi and Fe ions were investigated by resonant Raman spectroscopy and UV-Vis spectroscopy. A new peak(E_1u², ³85.7 cm^-1) occurs near the in-plane E_2g¹ 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 ¹90 cm^-1 peak is attributed to be a difference combination process involving ⁴23-cm^-1 peak and LA（M） mode. The feature at ⁴60 cm^-1 is composed of 2LA（M）(⁴58 cm^-1) and 2 uA(⁴66 cm^-1) mode. With increasing fluence, the integrated intensity ratio between 2LA（M） and 2 uA(R_{2LA（M）/A2n}) increases. As in case of the fluence is two orders lower for Bi than Fe ions,R_{2LA（M）/A2n} is almost the same because Bi ions are of much higher electron energy loss（（d E/dx）e） than Fe ions, indicating the key role of（d E/dx）e during irradiation. 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.MoS₂ was irradiated by 0.97 Ge V Bi ions with the fluence of 1×1010- 1×1012 ions/cm2. The defects induced by irradiation decrease the thermal conductivity of MoS₂, so local temperature increase dramatically when Raman spectrum is excited by high laser power, which cause the downshift of peaks position and broadening of E_2g¹ and A_1g peak. Furthermore, according to Raman spectra measured at different laser power, thermal conductivity of MoS₂ before and after irradiation was calculated, which show that the thermal conductivity of MoS₂ decreases with increasing fluence, from 563 to 132 W/m K for pristine and 1×1012 ions/cm2 irradiated MoS₂, respectively.