Thermal Contrast in Nanoscale Infrared Spectroscopy (AFM-IR): Low Frequency Limit

TL;DR

This paper analyzes the contrast formation mechanism in nanoscale infrared spectroscopy (AFM-IR) at low frequencies, deriving analytical expressions for response amplitude, spatial resolution, and thermo-elastic step based on thermal and elastic parameters.

Contribution

It provides a theoretical framework for understanding contrast formation in AFM-IR at low frequencies, including analytical formulas for response amplitude and spatial resolution.

Findings

Response amplitude is linearly proportional to light intensity and thermal expansion coefficient.

Spatial resolution depends linearly on inverse light absorption coefficients and thermal transfer length.

Thermo-elastic step height relates to absorption coefficient differences and heat transfer coefficient.

Abstract

The contrast formation mechanism in Nanoscale Infrared Spectroscopy (Nano-IR or AFM-IR) is analyzed for the boundary between two layers with different light absorption, thermal and elastic parameters. Analytical results derived in the decoupling approximation for low frequency limit show that the response amplitude is linearly proportional to the intensity of the illuminating light and thermal expansion coefficient. The spatial resolution between two dissimilar materials is linearly proportional to the sum of inverse light adsorption coefficients and to the effective thermal transfer length. The difference of displacements height across the T-shape boundary ("thermo-elastic step") is proportional to the difference of the adsorption coefficients and inversely proportional to the heat transfer coefficient. The step height becomes thickness-independent for thick films and proportional to h^2 in a very thin film