AFM-IR : a tool to explore the matter at the nanoscale

Infrared (IR) spectromicroscopy is an effective technique for chemically identifying compounds in a sample (by their infrared spectra) and locating them spatially. However, the main limitation of this "classic" infrared technique is its spatial resolution: this is imposed by the optical part and is of the order of a few micrometers.

It was against this backdrop that the IR-AFM technique was developed, enabling IR studies (absorption spectra and mapping) to be carried out on a scale of just a few nanometers. This fast-growing technology is based on the coupling of an atomic force microscope (AFM) and a pulsed tunable IR laser. The main idea is to do away with optical detection to study the interaction of infrared photons with matter, so as not to be limited by light diffraction. To achieve this, we follow the photothermal effect associated with the absorption of infrared light.

For this type of measurement, the sample is imaged by AFM (surface topography) and then illuminated with the IR laser source. The tip of the atomic force microscope, which is in contact with the sample, is then used to detect the photothermal effect induced in the sample by the absorption of IR radiation: if the laser wavelength corresponds to an absorption band in the sample, then part of the light is absorbed and converted into heat. This creates a local temperature rise. This temperature rise causes the sample to expand, pushing back the AFM tip. This push is extremely rapid and is felt as a shock in the tip, causing the AFM lever to oscillate. The advantage of this approach is that the amplitude of oscillation is proportional to absorbance, enabling resolution of a few nanometers (thanks to the AFM tip).

Since 2012, this technology has been routinely used worldwide for nanoscale infrared analysis by academics and industry alike, in fields as diverse as polymer science, microbiology, pharmaceuticals, the study of heritage materials and astrochemistry.

This article describes the basic principle of this technology and explains how it works, which remains relatively simple and intuitive. It is undoubtedly for the latter reasons that this technology has won over scientists from many different backgrounds and is used in a wide range of fields. The article will then illustrate the impact of this technology by describing some recent applications in polymer science and microbiology, and will conclude with a presentation of the prospects and potential developments of the technique.