Research On The Controllable Preparation, Mechanical And Electric Properties And The Nitridation Of MoO₂ Nanosheets

Monoclinic molybdenum dioxide（MoO₂）nanosheet is a metallic metal oxide with excellent electronic conductivity and ionic conductivity,which has wide application prospects in the fields of catalysis,sensing and energy storage.Chemical vapor deposition（CVD）method is a common method of preparation of nanosheet materials.The horizontal growth of nanosheet（parallel to the substrate）and vertical growth（perpendicular to the substrate）dominate their performance and application.Horizontal growing nanosheets have important application prospects in fields such as logical electronic devices and optoelectronic devices.Vertical nanosheets have great potential in high density applications such as high density transistors and high performance catalysis with their unique vertical architecture and super large exposed surface area.However,the controlled growth of vertical nanosheets is still a challenging subject,and the mechanism of vertical growth of nanosheets needs to be further revealed.At the same time,the process of nanosheets from preparation,transfer to application will inevitably involve the force loading.Therefore,it is particularly important for the application of nanosheets to study the mechanical properties of nanosheets and the influence of force on the electrical properties.In this paper,horizontal and vertical MoO₂ nanosheets were prepared by CVD method,and the vertical growth mechanism,electrical performance and force regulation of MoO₂ nanosheets were systematically studied.Further,molybdenum nitride nanosheets were prepared by nitriding MoO₂ nanosheets,and molybdenum nitride nanosheets have been found to have a good surface enhanced Raman scattering（SERS）effect.The specific research content is as follows:The horizontal（parallel to the substrate）and vertical（vertical to the substrate）growth of two-dimensional（2D）materials dominates their propreties and applications.Horizontal growth of 2D materials is the most common state,with important application prospects in the fields of logic electronic devices and optoelectronic devices,and easy to integrate with traditional bulk semiconductor devices to expand the functions of traditional semiconductor devices.Corresponding to it,vertical 2D materials have great potential in applications like high density transistors and high performance catalysis because of their unique vertical architecture and huge exposed surface area.However,the controlled growth of vertical 2D materials is still a challenging subject,and the vertical growth mechanism of different 2D materials has not been well revealed.At the same time,the process of 2D materials from preparation,transfer to application will inevitably involve the force loading.Therefore,it is particularly important for the application of 2D materials to study the mechanical properties of 2D materials and the influence of force on the electrical properties.In this paper,horizontal and vertical MoO₂nanosheets were prepared by CVD method,and the vertical growth mechanism,electrical performance and force regulation of MoO₂ nanosheets were systematically studied.Further,molybdenum nitride nanosheets were prepared by nitriding MoO₂ nanosheets,and molybdenum nitride nanosheets have been found to have a good surface enhanced Raman scattering（SERS）effect.The specific research content is as follows:（1）Controllable preparation of MoO₂ nanosheets:With molybdenum trioxide as the precursor,MoO₂ nanosheets were prepared by atmospheric CVD,and the morphology of MoO₂nanosheets was regulated by changing the substrate species,growth temperature and growth time.Meanwhile,a transition from horizontal MoO₂ nanosheets to vertical MoO₂ nanosheets was observed on different regions of the c-Al₂O₃ substrate.On the confined contact region between the substrate and the quartz boat sides,the MoO₂ nanosheets grew vertically,and the vertical MoO₂ nanosheets were mainly oriented in three directions with the interval age of 120°.According to our specially designed confined space experiments and the available nucleation theory revealed the horizontal and vertical growth mechanism of MoO₂ nanosheets:the vertical growth of MoO₂ nanosheets were enabled by the synergistic effect of a low precursor concentration and a low carrier gas flow rate.The mechanism of vertical MoO₂ directional growth was revealed by the binding energies between MoO₂ and c-Al₂O₃ calculeted by density functional theory,thus realized the controllable preparation of MoO₂ nanosheets.（2）Mechanical and electric properties of MoO₂ nanosheets:The Young’s modulus of the vertical MoO₂ nanosheets grown on c-Al₂O₃ substrate was tested using atomic force microscopy（AFM）.The bending behavior of the MoO₂ nanosheets was observed in situ under an optical microscope using displacement platform.The conductivity of the MoO₂ nanosheets was tested using a two-electrode method.And the effect of mechanical loading on the conductivity of MoO₂ nanosheets in the suspended state and on the flexible substrate was also explored.The experimental results show that the Young’s modulus of vertical MoO₂ nanosheets is up to 324±10 GPa,the maximum strain can withstand up to 7.6%,the conductivity can reach1.9×10⁵ S/m,and the conductivity is insensitive to strain.（3）Preparation of molybdenum nitride nanosheets and their SERS effect:Horizontal MoO₂ nanosheets grown on SiO₂/Si were nitrated for various times.The thickness of MoO₂nanosheets before and after nitride was characterized by AFM,and the thinnest molybdenum nitride nanosheets was up to 3.9 nm.The SERS effect of molybdenum nitride nanosheets was explored using rhodamine as a probe molecule.The results show that the detection limit of R6G by molybdenum nitride SERS substrate can reach 10^-8 M,and the enhancement factor can reach3×10⁵,which is much higher than other 2D materials SERS substrate.