Multi-eigenmode control for high material contrast in bimodal and higher harmonic atomic force microscopy

Abstract

High speed imaging and mapping of nanomechanical properties in atomic force microscopy (AFM) allows the observation and characterization of dynamic sample processes. Recent developments involve several cantilever frequencies in a multifrequency approach. One method actuates the first eigenmode for topography imaging and records the excited higher harmonics to map nanomechanical properties of the sample. To enhance the higher frequencies' response two or more eigenmodes are actuated simultaneously, where the higher eigenmode(s) are used to quantify the nanomechanics. In this paper, we combine each imaging methodology with a novel control approach. It modifies the Q factor and resonance frequency of each eigenmode independently to enhance the force sensitivity and imaging bandwidth. It allows us to satisfy the different requirements for the first and higher eigenmode. The presented compensator is compatible with existing AFMs and can be simply attached with minimal modifications. Different samples are used to demonstrate the improvement in nanomechanical contrast mapping and imaging speed of tapping mode AFM in air. The experiments indicate most enhanced nanomechanical contrast with low Q factors of the first and high Q factors of the higher eigenmode. In this scenario, the cantilever topography imaging rate can also be easily improved by a factor of 10.

Quantitative Visualization of the Nanomechanical Young’s Modulus of Soft Materials by Atomic Force Microscopy

“…In addition, the Fast Force Mapping mode of AFM (Asylum Co., Santa Barbara, CA, USA) was introduced for overcoming time consumption, in which the tip–sample distance is modulated in a sinusoidal motion at up to 300 Hz. The Multifrequency AFM utilizes multiple eigenmodes of the cantilever’s resonance frequency to detect surface morphology as well as nanomechanical properties of the samples [ 14 , 15 , 16 ].…”

Review: Advanced Atomic Force Microscopy Modes for Biomedical Research

“…Since a lock-in amplifier can extract oscillation information at a specific frequency, several cantilever resonance frequencies can be excited simultaneously during imaging to create multiple material property maps with high contrast. Cantilever geometry optimization to control eigenmode frequency difference and specialized control algorithms have been developed to improve this technique [ 74 , 75 , 76 ]. The remaining challenges and potential of multifrequency AFM for property mapping have attracted researchers to develop new machine learning and control algorithms.…”

Multifunctional cantilevers for simultaneous enhancement of contact resonance and harmonic atomic force microscopy

“…In harmonic AFM, the probe works in tapping mode and the harmonic responses are detected [10]. Both CR-AFM and harmonic AFM have been frequently employed to characterize the mechanical properties of samples [11][12][13][14][15] including their Young's modulus [11,12]. Furthermore, subsurface nanostructures, for example, inclusions in heterogeneous materials [16][17][18] or embedded defects in micro-and nanodevices [19][20][21], can also be sensed by mechanical imaging.…”