Preparation Of Carbon Fiber Tip Of AFM And Its Application In The Structural Characterization

Atomic force microscope(AFM)is an analysis instrument which is used to te st the properties of sample on the nanometer scale.At present,the most commonly used AFM is based on the micro cantilever force sensor,and used to detect the vertical information of materials.However,there are many defects of the cantilever,such as large light detection system,low quality factor(Q),low sensitivity,and limited test conditions.In this dissertation,AFM based on carbon fiber(CF)tip and quartz tuning fork(QTF)force sensor was prepared,and a method to realize 3D in situ characterization on the nanometer scale was firstly proposed.It is well known that CFs are high-modulus,high-strength,and low-density structural materials that are widely used in many fields,such as aerospace,aircraft industry,vehicles,electronics,and sports equipments,etc.They are an indispensable basic material.CF tip is applied in AFM,which can significantly improve the overall performance of the system.Electrochemical etching method was used to prepare CF tip.A feasible method,named as radius profiling,was presented to study the structure–mechanical heterogeneity of CFs T700 and T300.CF monofilaments were peeled layer by layer by plasma etching,and the crystallinity and modulus distribution along the fiber radius direction were measured experimentally.The results are demonstrated that the electrical and mechanical properties of the core of polyacrylonitrile(PAN)-based CFs is poor because of their heterogeneous skin–core structure,causing poor performances of prepared PAN-based CF tip.Therefore,asphaltic base CFs was used to prepare CF tips according to its homogeneous properties along radial direction.CF tip was prepared by electrochemical etching using self-developed tip etching control circuit.And the influences of the preparation conditi ons on the CF tip were investigated.The results show that the CF probe with smooth surface and exponential shape can be easily obtained under the optimal conditions(high-modulus CF M40,5 V onset etching voltage,1.6 V reference voltage,and 4 mol·L-1 Na OH electrolyte solution).The curvature radius of the CF tip is less than 100 nm,and the success rate is more than 50%.The self-developed AFM with CF probe is mainly composed of hardware,control,and software system.The lateral and vertical resolution of self-made AFM system can be up to 0.22 nm and 0.024 nm in atmospheric environment,respectively.The design of scanning probe for compact rigid structure ensures the mechanical stability of the system and reduces the influence of external environme nt on the system.The design of slip-stick style piezoelectric motor enables the system to reach 100 nm step precision in x,y,and z axis.Meanwhile a preamplifier design was presented to digitally and accurately compensate the stray capacitance of QTF force sensors.Three composites,gold nanoparticles/thermoplastic acrylic resin(Au NPs/TPAR),styrene and methyl methacrylate block copolymer(PMMA-b-PS)and polystyrene/low density polyethylene(PS/LDPE),were used to verify the self-made AFM system with CF tip and QTF force sensors.First of all,it can be used to characterize the morphology and roughness of the surface.Then the electrical property of the materials can be tested by utilizing the good conductivity of the CF tip.The test result of contact potential difference of the PMMA-b-PS LB film is consistent with that reported in literature.In addion,combining CF tip with Lift–off mode(LOM)AFM,the mechanical properties of the heterogeneous materials can be rapidly determined at the nanoscale.Bimodal,2D and 3D AFM based on CF tip and QTF force sensor were designed and developed.A high efficient welding technology based on low temperature glass solder was used to prepare a new-type QTF force sensor.It is composed of two QTFs in different ways.The force sensor is simultaneously excited by the first two flexural modes.The surface morphology of materials is measued by detecting the information of the first frequency,and the structure and properties of materials are measued by detecting the information of the second frequency.The combination QTF force sensors have high and stable Q,ensuring the high scanning accuracy of the AFM system.Bimodal AFM based on the combination QTF force sensor with CF tip has high spatial resolution and sensitivity.With two-dimensional driven AFM force sensor,it can be easily and efficiently realized in situ measurement of the 2D properties of materials in nanoscale,which is mean that it can be simultaneously obtained the forces on the x and y coordinates paralleling to the sample surface,such as the friction force and shear modulus etc.Combined with the above two-dimensional driven AFM force sensor and QTF sample stage,an in situ 3D exciting and detecting AFM system in nanoscale was firstly prepared.This AFM technology can be further used in analysis and characterization of the anisotropy of various physical properties of materials in nanoscale.The development of self-made AFM system with CF tip and QTF force sensor has significantly opened up new areas of application of the traditional AFM.The development of 3D AFM will help enhance the understanding of the microstructure of materials,and shows great potentials in the field of nanotechnology and plays a very important role in biological,material,physical,and chemical research fields.