Investigations Of Structural Damage And Optical Properties Of MoS₂ Irradiated By Swift Heavy Ions

Swift heavy ion irradiation can be used to modify the two-dimensional materials at the atomic scale as a powerful tool for defect engineering.It can be used for the preparation of nanofiltration membrane,nanopore for single biomolecule detection and nanopore ion pumps.Meanwhile,it has important applications in new nano-electronic devices,catalysis,optics and other fields.On the other hand,with the development of semiconductor industry,the device size has gradually approached the physical limitation.Therefore,it is urgent to find the new materials to replace the traditional silicon-based semiconductor materials.The transition metal sulfides represented by molybdenum disulfide（MoS₂）have two-dimensional layered structure which is different from the three-dimensional structure in the traditional silicon materials.They are easily exfoliated to several or one atomic layer due to the weak Van der Waals force between layers.They have quite different electrical,mechanical,optical and even thermal properties when they are continuously thinned.So they are considered to be the new candidate materials which can satisfied the requirements of miniaturization,lightweight and high efficient in the future semiconductor industry device.Therefore,it has a directive importance to study the irradiation effect in two-dimensional layered materials induced by swift heavy ion,which can help people to design and development of the new generation semiconductor devices serviced in the space irradiation environment.We studied the fine structures of latent tracks which irradiated by swift heavy ions in MoS₂,and proposed a new formation mechanism of latent tracks based on the thermal peak model.At the same time,we also investigated the Raman and photoluminescence spectra of MoS₂ and graphene/MoS₂ heterostructure before and after irradiation.The relationship between the internal microstructure and the optical properties changes before and after irradiation was also revealed.In these experiments,the MoS₂ powder was prepared by ultrasonic dispersion method.The single and few layer MoS₂ were synthesized by lithium intercalation method.The single layer MoS₂ and graphene/MoS₂ heterostructure were synthesized by Chemical vapor deposition.All the samples were irradiated by swift heavy ions(⁸⁶Kr、¹²⁹Xe、¹⁸¹Ta、²⁰⁹Bi)with the Heavy Ion Research Facility in Lanzhou（HIRFL）.The samples before and after irradiation were characterized by for atomic force microscopy（AFM）,Raman and photoluminescence（PL）spectrum characterization.The main results are as follows:Many circular tracks were observed after Xe,Ta and Bi ions irradiation and the number of the tracks increases with increasing ion fluence,they showed the obvious Fresnel contrast.However,no tracks were observed at the electronic energy loss threshold（dE/dx）_e=7.7,8.2 and 9.1 keV/nm after Kr ions irradiation.Therefore,（dE/dx）_e threshold for the formation of latent tracks in MoS₂ induced by～15 MeV/u heavy ions is 9.7 keV/nm,which was deduced from theoretical calculation and experimental evidences.The diameter of latent tracks increases from 1.9 to 4.5 nm with increasing（dE/dx）_e.The irradiation experiments with 30°incidence were carried out and the TEM results were revealed the various fine morphologies of latent tracks（from cylinders to sandglass and q-tips shape）.The formation mechanism of latent tracks was proposed based on the cooperative process between outflow of melting phase and recrystallization.The specific lattice structure of MoS₂ was considered as the important factor for recrystallization ability.The study of Raman spectroscopy with bulk MoS₂ before and after irradiation found the Raman peak had a red shifted with increasing the ion fluence because the appearance of tensile stress in irradiated MoS₂.The study of resonance Raman spectroscopy with bulk MoS₂ before and after irradiation found the obvious amorphous drum appears in the Raman spectrum of MoS₂ with increasing the ion fluence,which indicated the amorphization structure increases in irradiated MoS₂.The new Raman peak appeared at 190 and 230 cm^-11 which indicated the increasing of damage structures in samples.And the degree of the ratio in Raman peak intensitieswith changing（dE/dx）_e was obviously more than that of ion fluence.The study of Raman spectroscopy with single layer MoS₂ before and after irradiation found the sample amorphization increases with increasing the ion fluence.The blue shift of the Raman peaks at and indicated the introduction of compressive stress into MoS₂ due to irradiation.The study of Raman spectroscopy with Graphene/MoS₂heterostructure before and after irradiation found with increasing the ion fluence,the Raman peaks had a blue shift.The Raman peak D of graphene obviously observes when the ion fluence at 1.6×10¹² ions/cm²,which indicated the internal structure of graphene were damaged.The study of PL spectroscopy with single layer MoS₂ before and after irradiation found the intensity of PL peak decreased with increasing the ion fluence,and PL peak disappears at the maximum ion fluence.The defect energy level is introduced between the conduction band and the valence band which lead to the red shift of PL peak.In this paper,the internal structure damage and formation mechanism of MoS₂induced by swift heavy ions irradiation was studied in detail.The results provide a reference for the fine and controllable preparation of nanostructures in two-dimensional layered materials with swift heavy ions irradiation methods in the future,and create new ideas for the irradiation damage mechanism and application of two-dimensional materials.