Friction as Contrast Mechanism in Heterodyne Force Microscopy

TL;DR

This paper reveals that friction at material boundaries causes contrast in heterodyne force microscopy, enabling quantitative subsurface imaging by ruling out other mechanisms through analysis and simulations.

Contribution

It provides the first quantitative analysis identifying boundary friction as the contrast mechanism in heterodyne force microscopy.

Findings

Friction at material boundaries causes contrast in heterodyne force microscopy.

Other mechanisms like Rayleigh scattering and elastic damping are ruled out.

The analytical model enables quantitative subsurface imaging.

Abstract

The nondestructive imaging of subsurface structures on the nanometer scale has been a long-standing desire in both science and industry. A few impressive images were published so far that demonstrate the general feasibility by combining ultrasound with an Atomic Force Microscope. From different excitation schemes, Heterodyne Force Microscopy seems to be the most promising candidate delivering the highest contrast and resolution. However, the physical contrast mechanism is unknown, thereby preventing any quantitative analysis of samples. Here we show that friction at material boundaries within the sample is responsible for the contrast formation. This result is obtained by performing a full quantitative analysis, in which we compare our experimentally observed contrasts with simulations and calculations. Surprisingly, we can rule out all other generally believed responsible mechanisms, like Rayleigh scattering, sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound attenuation. Our analytical description paves the way for quantitative SubSurface-AFM imaging.