Chemical, Mechanical and Optical Property Mapping at the Nanoscale

K. Kjoller, E. Dillon, Q. Hu, A. Roy, H. Yang, C. Prater
Ansys Instruments,
United States

Keywords: atomic force microscopy, FTIR, AFM-IR, s-SNOM, nanoIR

Summary:

The recent rapid growth of nanoscale chemical characterization coupled to Atomic Force Microscopy (AFM) has added a new layer of analysis allowing researchers to gain a much greater understanding of the complex, heterogeneous materials common in industry and academia. AFM has for many years provided nanoscale characterization of the topography of a sample as well as a wide range of material properties, including nanomechanical information. Frequently the mechanical or other material properties of the sample measurable using AFM were used as a proxy for the chemical variation in the sample because of the challenges in obtaining nanoscale chemical information. These challenges included obtaining the chemical information at relevant length scales in a fast, robust, repeatable fashion that did not require significant expertise in the analytical technique. Conventional infrared spectroscopy is one of the most widely used tools for chemical analysis, but optical diffraction limits its spatial resolution to the scale of many microns. This presentation will discuss the advances in two infrared techniques (1) AFM-based infrared spectroscopy (AFM-IR)1 and (2) scattering scanning near field optical microscopy (s-SNOM)2. Both of these techniques overcome the diffraction limit, providing the ability to measure and map chemical and optical properties with nanoscale spatial resolution. AFM-IR is a photothermal technique which generates chemical information analogous to FTIR. s-SNOM is a near field optical technique which generates nanoscale measurements of the real and imaginary index of refraction, similar to spectroscopic ellipsometry. Recent speed improvements in the collection of IR spectra and images using these techniques has made it feasible to perform hyperspectral imaging of samples allowing a more complete picture of the chemistry and/or optical properties. Also improved sensitivity in AFM-IR has allowed measurements of monolayer samples such as functionalized graphene and nanoparticles in addition to materials which traditionally have been challenging to measure in the AFM-IR mode. Tapping AFM-IR was recently developed using principles from heterodyne AFM3 in which multiple flexural eigenmodes of the AFM cantilever are excited or measured to enhance the sensitivity of the probe sample interaction. This has improved the spatial resolution in chemical imaging to