Fast, High Resolution and Wide Modulus Range Nanomechanical Mapping with Bimodal Tapping Mode

TL;DR

This paper introduces bimodal tapping mode AFM, called AM-FM imaging, which enables high-resolution, wide-range nanomechanical mapping of materials and biological samples with quantitative stiffness and modulus measurements.

Contribution

The paper presents a novel bimodal tapping mode technique that allows direct, quantitative nanomechanical property mapping across a broad modulus range with high spatial resolution.

Findings

Able to measure stiffness from 100 MPa to 100 GPa.

Achieves sub-nanometer resolution in stiffness mapping.

Operates effectively at line scan rates up to 40 Hz.

Abstract

Tapping mode atomic force microscopy (AFM), also known as amplitude modulated (AM) or AC mode, is a proven, reliable and gentle imaging mode with widespread applications. Over the several decades that tapping mode has been in use, quantification of tip-sample mechanical properties such as stiffness has remained elusive. Bimodal tapping mode keeps the advantages of single-frequency tapping mode while extending the technique by driving and measuring an additional resonant mode of the cantilever. The simultaneously measured observables of this additional resonance provide the additional information necessary to extract quantitative nanomechanical information about the tip-sample mechanics. Specifically, driving the higher cantilever resonance in a frequency modulated (FM) mode allows direct measurement of the tip-sample interaction stiffness and, with appropriate modeling, the setpoint-independent local elastic modulus. Here we discuss the advantages of bimodal tapping, coined AM-FM imaging, for modulus mapping. Results are presented for samples over a wide modulus range, from a compliant gel (~100 MPa) to stiff materials (~100 GPa), with the same type of cantilever. We also show high-resolution (sub-nanometer) stiffness mapping of individual molecules in semi-crystalline polymers and of DNA in fluid. Combined with the ability to remain quantitative even at line scan rates of nearly 40 Hz, the results demonstrate the versatility of AM-FM imaging for nanomechanical characterization in a wide range of applications.