Multifrequency Atomic Force Microscopy Research Articles

Multifrequency Atomic Force Microscopy: Compositional Imaging with Electrostatic Force Measurements

We demonstrate that single-pass Kelvin force microscopy (KFM) and dC/dz measurements in different environments expand the compositional imaging with atomic force microscopy. The KFM and dC/dz studies were performed in the intermittent contact mode with force gradient detection of tip-sample electrostatic interactions. Both factors contribute to sensitive measurements of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on a broad range of materials: metal alloys, polymers, organic layers, and liquid-like objects. For many samples the surface potential and dC/dz variations complement each other in identification of individual components of heterogeneous materials. In situ imaging in different humidity or vapors of various organic solvents further facilitate recognition of the constituents of multicomponent polymer samples due to selective swelling of components.

Simultaneous observation of surface topography and elasticity at atomic scale by multifrequency frequency modulation atomic force microscopy

The authors integrated the frequency modulation (FM) technique into multifrequency atomic force microscopy (AFM). Based on theoretical considerations, simultaneous excitation of the cantilever oscillation at the first and second flexural modes allows us to acquire the surface topography and surface elasticity simultaneously. The authors performed multifrequency FM-AFM observation using a tungsten-coated silicon cantilever on a Ge(001) surface exhibiting a dimer structure at room temperature. The topography and the elasticity of the surface were successfully obtained at the atomic scale. The authors found that the dimer atoms around a missing dimer defect have higher elasticity than the other dimer atoms. This suggests that stiffer atomic bonding of the dimer atoms occurred as a result of the additional tensile strain field from the defect. Therefore, the multifrequency FM-AFM described in the present study is expected to be useful for the investigation of the surface elasticity at the atomic scale.

Theory of Multifrequency Atomic Force Microscopy

We develop a theory that explains the origin of the high force sensitivity observed in multifrequency force microscopy experiments. The ability of the microscope to extract complementary information on the surface properties is increased by the simultaneous excitation of several flexural cantilever modes. The force sensitivity in multifrequency operation is about 0.2 pN. The analytical model identifies the virial and the energy dissipated by the tip-surface forces as the parameters responsible for the material contrast. The agreement obtained among the theory, experiments and numerical simulations validates the model.

Self-sensing, estimation and control in multifrequency Atomic Force Microscopy

Self-sensing, estimation and control in multifrequency Atomic Force Microscopy