Effects Of Ionizing Radiation On Opto Mechanical Electro Magnetic Coupling Properties Of 2D WSe₂

The two-dimensional materials have layered structure with atomic thickness,and can be compatible with the existing device manufacturing process.Reducing the size of individual transistors can make the integrated circuits more powerful and flexible.This is highly attractive for space electronics given the cost of launching objects into space.Semiconducting two-dimensional materials have a layered structure with a thickness of individual atoms at the monolayer limit,and are compatible with existing device fabrication processes.The layered materials with atomic layer thickness are self-assembled into van der Waals structures by weak van der Waals forces without considering lattice matching.However,the space environment imposes an additional constraint on the device performance of radiation hardness.As such,the stability of WSe₂ against various forms of ionizing radiation should be critically examined to determine if it possesses sufficient radiation hardness for space applications.In this artical,we explored that theγ-ray treatment was a feasible method to control the defects of WSe₂ single-layer and multi-layer crystals.Transmission electron microscopy,Raman spectroscopy,photoluminescence（PL）spectroscopy and Kelvin probe force microscopy were used to study the optical,nano-frictional and electrical properties of WSe₂ under different radiation doses.Furthermore,the DFT simulation was carried out,and the experimental results were compared with the theoretical calculation.The main research contents are as follows,（1）The WSe₂ films were grown on a sapphire substrate by using chemical vapor deposition（CVD）.Then,WSe₂ flakes were irradiated withγ-ray from a ⁶⁰Co source in oxygen.Irradiation was performed on a ⁶⁰Co irradiator（22.57 GBq in total activity）with a photon energy of 1.3 Me V.The irradiation doses were 0,5,10,15 and 20 k Gy,respectively.Another group of flakes were irradiated in Ar.The irradiation doses were0,20,40,60,80 and 100 k Gy.Morphologies of synthesized samples were characterized by atomic force microscopy（AFM）,scanning electron microscope（SEM）,transmission electron microscopy（TEM）and selected area electron diffraction（SAED）.Most samples clearly exhibit the quasi-equilateral triangular shapes,which are the signature of the WSe₂ crystal,and this is further confirmed in the magnified SEM image.Moreover,both AFM and SEM images indicate rather high yields contributed by the CVD method.The TEM and SAED images demonstrate the well-defined chemical and structural of TMDs monolayer WSe₂crystal with atomically sharp interface.（2）The effects of ionizing radiation on the microstructure and optical properties of WSe₂ were researched by TEM,Raman and PL spectra.During theγ-ray irradiation in the air,high-energy photons facilitate the exoenergetic process,which generates selenium vacancies that can be subsequently occupied by oxygen atoms.This passivation reaction can be summarized as:2WSe₂+7O₂→2WO₃+4Se O₂.With respect to the as-grown monolayer,the creation of W⁶⁺and consequently the formation of WO₃ may introduce lattice distortion and alter the restoring force in it,which will be reflected in Raman spectra.Change in the lattice vibrational modes induced by passivation of oxygen is captured by Raman spectroscopy.We attribute this Raman peak shift to the change in bond restoring force as a response to the oxygen substitution.The linear relationship between the changes in Raman modes and the oxidation content allows us to engineer the monolayer properties at a quantitative level,which is highly desirable in device design and fabrication.The first-principles calculation suggests that an increase in the anion vacancy population is generally accompanied by a transition from a direct gap material to an indirect one.（3）The effects of ionizing radiation on the surface contact potential distribution（CPD）of WSe₂ were investigated by using Kelvin Probe Force Microscopy（KPFM）.The obvious dependency of CPD values on the radiation dose implies the significant role of irradiation-introduced oxygen atoms,which shares consistency with the CPD values calculated using DFT.The First-principle calculations are also performed to determine the band structure due to the passivation of oxygen at different defect concentrations.Afterγ-ray irradiation treatment,a monolayer tungsten diselenide could be transitioned into an n-doped semiconductor due to the anion vacancies created by the radiation.（4）The effects of the ionizing radiation and the number of layers on the optical,frictional and electrical properties of WSe₂ nanostructures are systematically investigated by Raman,PL spetra,friction force microscope（FFM）and KPFM.The optical property of WSe₂ nanoflakes was investigated by using Raman spectroscopy,photoluminescence spectroscopy of WSe₂ nanoflakes with different layers.The surface potential of WSe₂ nanostructures increase with the number of WSe₂ layers,suggesting screening effect due to the charge exchange at the interface between WSe₂ and substrates.Gamma-ray radiation is a practical way to engineer the frictional and electrostatic properties of WSe₂ nanostructures.Our work provides an important indication to modify the optical,frictional and electrical properties of WSe₂-based devices by choosing a suitable thickness,properly irradiation and thermal treatment.（5）The effects of ionizing radiation on the magnetic property of WSe₂ were investigated by magnetic force microscope（MFM）.The results show that there is almost no magnetic domain reversal in the samples without irradiation.With the increase of ionizing radiation,the magnetic domain reversal occurs at the edge of the samples.