Mechanical Characterization Based On Higher Harmonic Atomic Force Microscopy

After half a century of rapid development,nanoscience and nanotechnology has become an emerging research field with frontier,interdisciplinary and multidisciplinary characteristics.In the fields including composite materials,biological cells and micro-nano devices,there is an urgent need for characterization methods that can realize nondestructive,unlabeled,rapid and high-resolution imaging of sample surface,interface and subsurface structure,as well as provide information about material properties.As a key instrument for the development of nanoscience and nanotechnology in the past 30 years,atomic force microscopy(AFM)has shown great potential for meeting the above requirements.Among its many modes,higher harmonic AFM based on multi-frequency mode has an important prospect in certain fields due to its advantages such as nanometer spatial resolution and ultra-high force detection sensitivity.In tapping mode AFM,the microprobe is excited to vibrate at its resonant frequency.When the gap between the tip and sample surface is moderate,it will make intermittent contact with the sample in each cycle.The vibration of cantilever beam is highly nonlinear because of the interaction force.This nonlinearity contains a large number of higher harmonics responses related to sample properties.Due to simplicity,high force sensitivity and high imaging resolution,it has been widely concerned.In this dissertation,the problems of higher harmonic AFM in nanomechanical characterization and imaging of subsurface structures are discussed.Based on probe's vibration mechanics,the dynamical analysis of micro cantilever beam,applications development and signal enhancement of higher harmonics as well as evaluation of subsurface imaging capabilities are mainly carried out.In terms of theoretical analysis,starting from the basic principle of amplitude modulated atomic force microscopy,and taking the mechanical model of probe vibration and the model of the interaction between tip and sample as the theoretical foundations,we analyzed the dynamic characteristics of the cantilever beam under the interaction field,and explored the generation mechanism and imaging characteristics of the high-order harmonics.In terms of parameter optimization of harmonic imaging,we studied the influence of the main influencing factors such as excitation frequency,modulation amplitude and laser spot location on the mechanical characterization via higher harmonic.The results show that the modulation amplitude significantly affects the harmonic signal.The spot located at the free end of the cantilever can provide the highest harmonic sensitivity.The proper choice of excitation frequency can avoid the contrast inversion of harmonic imaging.Through the comprehensive analysis of the influencing factors,the optimization of higher harmonic imaging contrast can be realized,which is applied to the discrimination of heterogeneous materials with difference in mechanical properties,and further applied to the quantitative measurement of the mixing ratio of nanoparticles in composite materials.In terms of harmonic signal enhancement,we adjust and control the cantilever's frequency characteristics by selective material removal of the beam,In such a way,the higher-order eigenmode frequency falls at the location of the higher harmonic and enhance the corresponding harmonic response signal strength is enhanced.By using theoretical analysis and finite element simulation,we optimized the design of parameters such as the position and size of material removal,and used focused ion beam(FIB)etching on conventional AFM cantilevers.Imaging results on LDPE/PS hybrid material samples show that the optimized higher harmonic signal can be enhanced by 6 times.Finally,higher harmonic AFM is applied to image subsurface structures owing to its ability to detect mechanical properties.In air,PDMS materials embedded with hard spherical particles were used as reference samples to carry out quantitative evaluation of subsurface detection ability of higher harmonics,and to compare with contact resonance and bimodal AFM.The results show that the higher harmonic has higher sensitivity of mechanical properties,and the subsurface detection depth can exceed 100 nm.Secondly,the subsurface imaging capability of higher harmonic in liquid was studied on the reference sample with cavity structure and compared with that in air.The results demonstrate that the transient excitation of the high-order eigenmode of the cantilever in high-damping environment can enhance the signal strength of the harmonics,and the sensitivity of the mechanical properties of the higher harmonics in liquid is an order of magnitude higher than that in air.Harmonic AFM in liquid can detect the cavities under HOPG plate with thickness over 200 nm.