Nanotribological Properties Of Two-dimensional Transition Metal Dichalcogenides Based On Atomic Force Microscopy

Molybdenum disulfide?MoS2?and selenide molybdenum?MoSe2?belong to van der waals layered dielectric materials,and are characterized by weak van der Waals force between layers.The weak van der Waals force results unique mechanical properties of low slip strength between layers,which make it widely used in nanotribology and other fields.The research on micro-nano tribology is still in the primary stage,there are a few reports about the affectting friction properties of MoS2 and MoSe2 factors.In this paper,we investigated on the tribological properties of MoS2nanosheets prepared by different methods and the tribological properties of different grain boundary distribution regions on polycrystalline MoS₂ and MoSe₂ nanosheet and the effect of illumination on the friction properties of MoS2 nanosheet.It is of great significance to study the affect factors of tribological properties for promoting the development of nano-tribology and solving the problems in practical applications.The research aspects of thesis are as follows.?1?The chemical vapor deposition?CVD?parameters were adjusted to prepare high quality MoS2 and MoSe2 nanosheets and the thin layer MoS2 nanosheet was prepared by micromechanical exfoliation.?2?The external light source was introduced by using optical fiber and was focussed on the experimental sample by used the focus lens.The tribological performance test device was successfully built under illumination and the device not affects the results tested by AFM.?3?The friction force of the sample was characterized by using the friction force microscope module?FFM?.The contact potential difference?CPD?between the probe and the sample was characterized by using the kelvin probe module?KPFM?.And the adhesion of the sample was characterized by using the adhesion module.?4?For MoS2 nanosheets with the same number of layers,the friction coefficient of MoS2nano-sheet prepared by mechanical stripping method are obviously larger than that prepared by CVD method.Friction coefficients in different grain boundary distribution regions of single polycrystalline MoS₂ and MoSe₂ canbe arranged in order of size:the region with obvious grain boundary,the region no obvious grain boundary,the region no grain boundary.When the probe slides at the obvious grain boundary of the sample,it may be affected by the double effects of ratchet effect and piezoelectric effect,resulting in larger deformation of the probe and more energy dissipation.?5?With the increase of the layer number of MoS₂ nanosheet,the shielding effect to van der Waals force between the tip and the substrate becomes more and more obvious,and the adhesive force decreases gradually.And it is found that the CPD decreases with the increase of the number of MoS₂ nanosheet.Under the illumination condiation,the friction coefficient of odd layer MoS₂nanosheet decreases and the maximum reduction is 41%.The friction coefficient of even layer MoS₂ nanosheet increases and the maximum increment is 55%.