Exploiting the Surface-Enhanced IR Absorption Effect in the Photothermally Induced Resonance AFM-IR Technique toward Nanoscale Chemical Analysis.

Abstract

The photothermally induced resonance AFM-IR technique (denoted as PTIR) is a promising and still developing analytical method that can provide nanoscale chemical and topographical information. Herein, by taking advantage of a customized PTIR system with either top-down or bottom-up incidence mode for a quantum cascade laser (QCL), we explore how the surface-enhanced IR absorption (SEIRA) effect due to the Au-coated AFM tip and/or substrate may affect the PTIR signals from 25 to 580 nm thick p-nitrobenzoic acid (PNBA) samples, as a function of sample thickness, incidence mode, laser polarization, and Au film morphology. By analysis of the νas(NO2) band intensity, it is revealed that the SEIRA effect may increase the PTIR signals by 1.5-8.3 times, with that from the Au-coated substrate being greater than that from the Au-coated tip. Nevertheless, the overall PTIR signal goes up monotonically over the entire thickness range for the top-down incidence mode, while it increases and then decreases with the sample thickness for the bottom-up incidence mode. The p-polarized laser enhances the PTIR signal more than does the s-polarized laser, especially on the Au-coated substrate. The significant loss of the PTIR signal of a PNBA sample corroborates the substantial loss of the SEIRA effect of an annealed Au film. The present work may promote the application of the SEIRA effect to the PTIR technique and provides hints for developing the PTIR technique into a more versatile analytical tool.