Interfacial interaction and enhanced image contrasts in higher mode and bimodal mode atomic force microscopy

Abstract

Higher mode and bimodal atomic force microscopy (AFM) are two recently developed imaging modes of dynamic AFM for improving resolution. In higher mode, the higher flexural mode of the cantilever instead of the traditional fundamental eigenmode is excited. In bimodal mode, two flexural modes of the cantilever are simultaneously excited for obtaining more information about the properties of the material. The first three flexural modes for higher mode and superposition of two excitation signals for bimodal mode are explored and compared by imaging a polymer blend of polystyrene (PS) and low density polyethylene (LDPE). The effects of different operating conditions of the two imaging modes are researched to improve image contrast and material discrimination. Dissipated power and virial are employed to explain the origin of contrast for the complex and highly nonlinear dynamical tip-sample interfacial system. Amplitude and phase contrasts of each single mode and bimodal mode are calculated by Ashman's D statistical equation. It is found that higher single modes with small free amplitudes show enhanced phase contrast. The bimodal of the first and the third modes gains a clear advantage over the bimodal of the first and second modes for phase and amplitude image contrasts. In addition, the best contrast of bimodal imaging occurs when it is a combination of a large free amplitude for the first mode and a small free amplitude for the third mode.