Nanoscale spectroscopic studies of two different physical origins of the tip-enhanced force: dipole and thermal

TL;DR

This paper investigates the different physical origins of tip-enhanced forces in nanoscale spectroscopy, distinguishing between dipole-induced and thermal forces through theoretical and experimental methods, to improve chemical specificity in PiFM.

Contribution

It provides a detailed analysis and experimental demonstration of how thermal and dipole forces contribute to photo-induced forces in PiFM, clarifying their spectral behaviors and origins.

Findings

Thermal force follows a dissipative Lorentzian lineshape.

Dipole force exhibits a dispersive spectrum related to polarizability.

Thermal effects can be distinguished by tuning sample relaxation time.

Abstract

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can con-tribute to the measured photo-induced force simultaneously. Of particular interest are the instantaneous force be-tween the induced dipoles in the tip and in the sample and the force related to thermal heating of the junction. A key difference between these two force mechanisms is their spectral behaviors. The magnitude of the thermal response follows a dissipative Lorentzian lineshape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Be-cause the two interactions are sometimes comparable in magnitude, the origin of the nanoscale chemical selectivity in the recently developed photo-induced force microscopy (PiFM) is often unclear. Here, we demonstrate theoretically and experimentally how light absorption followed by nanoscale thermal expansion generates a photo-induced force in PiFM. Furthermore, we explain how this thermal force can be distinguished from the induced dipole force by tuning the relaxation time of samples. Our analysis presented here helps the interpretation of nanoscale chemical measure-ments of heterogeneous materials and sheds light on the nature of light-matter coupling in van der Waals materials.