Photothermal AFM-IR Spectroscopy and Imaging: Status, Challenges, and Trends

This review appeared in the January 2022 edition of the nanoIR Journal Club — a monthly email brief highlighting leading-edge research and the latest discoveries supported by Bruker nanoIR technology.

AFM-IR combines the high spatial resolution of atomic force microscopy (AFM) with the chemical identification capability of infrared (IR) spectroscopy to achieve nanoscale physico-chemical analyses. Since the invention of this technique, technological improvements have dramatically advanced its capabilities in terms of spatial and spectral resolution, measurement time, sensitivity, and potential applications. 

Current Technology Review

This review article discusses all major advancements in AFM-IR technology and operational modes, as well as their current technical limitations. The developments discussed include those responsible for expanding the variety of samples that can be studied by AFM-IR, achieving monolayer sensitivity and sub-10 nm spatial resolution, and shortening spectral acquisition time to several seconds.

The authors also discuss how the AFM-IR signal is impacted by photoacoustic effect, thermal expansion and heat diffusion, phase-locked loop, polarization of the IR beam, and enhancement of electric field by the cantilever metal coating.

New Directions in Photothermal AFM-IR

The authors propose three new promising application fields that they expect will advance the AFM-IR technique in the next decade. Those fields are:

1. AFM-IR in a liquid environment, in which strong absorption of liquid water in the mid IR range can dominate the signal: Current data suggests that Photothermal Tapping AFM-IR works better than resonance enhanced mode for samples in a fluid environment. The result can open up new exciting areas of development for fluid-based IR applications including life science and corrosion.
2. Surface Sensitive AFM-IR, which is aimed to measure only the top layer by limiting the contribution of sub-surface material: This method provides great benefits for directly measuring the surface contaminants without further processing or the top layer of a thick sample without the requirement of microtoming. Through non-linear interaction between tip modulation and sample thermal expansion, this new mode can achieve a surface sensitivity much better than either resonance-enhanced mode or Tapping AFM-IR mode. This publication highlights that, so far, a probing depth of about 20–30 nm has been demonstrated for a typical polymer sample and more effort is underway to better understand and further improve the surface sensitivity.
3. Analyses of minerals, meteorites, and other inorganic species with thermal expansion coefficients at least one to two orders of magnitude below those of polymers: Sample properties, such as crystallinity and orientation can make the data analysis even more complex.
   
   

      _{KEY TERMS:}

• Photothermal AFM-IR; AFM-IR Modes; Infrared Nanospectroscopy; Surface Analysis Techniques; Physico-Chemical Analyses