Measurement Of The Mechanical, Electrical And Tribological Properties Of Two-dimensional Materials By Scanning Probe Technique

Since Andre Geim discovered graphene in 2004,two-dimensional(2D)materials family has been greatly expanded.These atomic thin 2D materials always show different properties from bulk.Besides the first found high mobility semi-metallic graphene,there are diverse 2D materials including 2D semiconductor,for example monolayer molybdenum disulfide,the most famous two-dimensional transition metal chalcogenides,2D insulator,for example boron nitride,2D ferromagnetic,such as two-dimensional chromium triiodide and superconducting two-dimensional barium strontium calcium copper oxides.On the other hand,because layers of these 2d materials are combined by weak van der Waals forces,the stacking configurations,such as layer number and rotation angle,can be easily modulated.These eventually change their properties and even reveal new properties.Combining different twodimensional materials to form heterojunctions and regulating their stacking structures provide nearly countless ways to build heterojunctions with unimaginable properties.Some of these two-dimensional materials and their heterojunctions have excellent electrical,mechanical and optical properties under specific conditions,are superior to any other materials.Some of them have very special physical properties,which can provide a good platform for basic research.Due to atomic resolution,atom force microscope(AFM)has nature advantages to measure the properties of 2D materials.High resolution AFM not only can obtain the surface topography of 2d materials and get the information of sample thickness and roughness,but also can obtain the phase information.Z-axis pressure can be applied to the surface of 2D materials to measure their Young's modulus by using the nanoindentation method.By changing the direction of scanning,we can obtain friction information and the friction coefficient distribution of the sample surface.On the other hand,by using tips with different coating,such as conductive coating or magnetic coating,we can measure the properties of two-dimensional material transport characteristics,magnetism,piezoelectricity,surface work function/electron affinity and surface charge distribution.In addition,we can also use tip as an operating means to change the structure of two-dimensional materials.In short,atomic force microscopy is an efficient means of characterizing the properties of two-dimensional materials.In this paper,we will focus on studying the novel properties of 2D materials which includes preparation of high quality 2D materials and measuring the properties of 2D material heterojunctions by AFM.This article is summarized as follows:1.We used atomic force microscopy to study the effects of stacking-angles and pressure on the electrical properties of monolayer molybdenum disulfide/graphene heterojunctions.By using atomic force microscopy,we achieved twisting the molybdenum disulfide/graphene heterojunction and insitu measuring the electrical properties of it.We found that the Z-direction conductivity of the heterojunction is influenced by the stacking-angle.The maximum conductivities show up in 0° stacking-angle and the minimum conductivities shows up in 30°,the difference of maximum and minimum is ~ 5.We used DFT calculation to explain our observation.We found that the different conductivities at different stacking-angles results from different transmission coefficients,and the difference of transmission coefficients is due to the electron tunneling through molybdenum sulfide at different positions in K-space at different stacking-angles.Our findings may help reducing the contact resistance of molybdenum sulfide devices.We also performed an in situ measurement of the molybdenum sulfide / graphene heterojunction under pressure.We found pressure could enhance the contact quality between molybdenum sulfide and graphite.The origin of this conduct quality enhancement comes from the stretching of the molybdenum sulfide layer by tip making the molybdenum sulfide and graphene charge transfer more easily,eventually causing the reducing of Schottky barrier2.We studied the superlubricity of molybdenum sulfide/graphite heterojunctions.The friction force measurements of a monolayer of molybdenum disulfide epitaxial grown on a graphite surface using lateral force model atomic force microscope(L-AFM)show that the interface between molybdenum sulfide and graphite is super-lubricated.We also found that the friction coefficient between molybdenum disulfide and graphite is in the negative six-order magnitude of ten,which is the smallest observed friction coefficient until now.The superlubricity between molybdenum disulfide and graphite is due to the large lattice mismatch(~30%),resulting in both interfaces being incommensurate at arbitrary stacking angles.The interface energy fluctuation is extremely low,resulting in superlubricity phenomenon.We also found that because any stacking angles are incommensurate,molybdenum disulfide and graphite do not have lock-in angles like other homogenous interface,which has a very high potential for application.Due to the extremely low friction coefficient of the interface,we detected the surface energy releasing caused by the stacking angle when the molybdenum sulfide is rotated on the graphite.The extreme points of energy releasing are related to rotation direction and different in the space.Finally,we also explored the factors that affect the superlubricity properties of our heterojunctions.We found that the speed and surface contamination would not affect the superlubricity properties.However,when the steps(dislocations)appear at the interface,molybdenum sulfide and graphite will lock at the commensurate status then superlubricity disappears.Our research has great potential for large-scale applications of superlubricity.3.We have developed a non-destructive method for transferring single-layer molybdenum disulfide which epitaxial grown on wafer-scale sapphire substrate to any other substrate.Only polydimethylsiloxane(PDMS)and water were involved in the transfer.Because we do not need to corrode sapphire,the sapphire substrate can be used repeatedly.After PDMS peel the molybdenum disulfide film from sapphire substrate in water,we just directly attach it to the target substrate and then peel the PDMS,and the PDMS can be completely removed without dissolving.Compared to the wet method,our method is free of contamination.Since the molybdenum disulfide film is epitaxial grown on sapphire,it has preferred orientation.Then we can use this transfer method to explore the effects of different stacking configurations on the electrical and optical properties of the stacked samples.We found that different stacking-angle and layer numbers have great effects on peak positions of the Photo-fluorescence and Raman spectrum.The 0° stacking shows the strongest inter-layer coupling,while 30° stacking has relatively weak inter-layer coupling.In addition,we have also developed a pre-pattern transfer technology that can transfer the patterned molybdenum disulfide film on sapphire to any substrate to form a novel net-like single-double layer alternating pattern sample.