Understanding the AFM-IR signal

Abstract

Photothermal nanoscale spectroscopy using an atomic force microscope for transducing local thermal heating induced by optical absorption enable optical and chemical imaging at a spatial resolution of few nanometers. This technique - often called atomic force microscopy induced resonance (AFM-IR) – has found wide use across a range of fields: sub-cellular imaging in biology, chemical characterization of polymer materials, detection of degradation products in restoration science, and many others. However, questions remain about the way in which sample geometry and sample properties affect the AFM- IR signal. The signal transduction chain consists of optical contributions as well as thermal and mechanical steps, which affect both the signal amplitude and its spatial distribution. Our approach to understanding the AFM-IR signal consists of an analytical model describing the thermomechanical behaviour of a vertically and laterally inhomogeneous sample excited via pulsed laser heating. The results of this model can be linked to experimental data using finite element model able to account of hard to control sample imperfections. Our model shows that absorbers buried deeper inside the sample will have broader and less intense AFM-IR signal. Furthermore, we see strong dependence of the spatial resolution on pump laser repetition rate, which opens an avenue for improving spatial resolution and performing chemical imaging with vertical resolution. Further applications include signal deconvolution for computationally improving the spatial resolution of the technique.