From prism to flat silicon: ​ Novel approach for AFM-IR measurements in liquids​

Abstract

AFM-IR is a hybrid technique combining atomic force microscopy and mid-infrared spectroscopy. Pulses of a tuned EC-QCL source cause local short-lived photo thermal expansion, which excites the resonant oscillation of the AFM cantilever in contact with the sample. The oscillation amplitude is directly proportional to the absorption and an absorption spectrum is generated [1]. It is possible to perform experiments in liquid. However, this requires a bottom illuminated setup. IR Measurements in liquids (e.g., water) suffer from high absorption, thus conventional measurements require an IR transparent prism (e.g. ZnSe) on which the sample sits. Here, the ingoing undergoes a total internal reflection and the developed evanescent field on the crystal face interacts with the sample. Nevertheless, using a ZnSe prism comes with drawbacks, like not being suitable for every type of sample, difficulty of handling and sample preparation, functionalization of ZnSe is not trivial and there is little literature to this, also the cost of an ATR prism is not insignificant. We introduce a novel sample carrier for liquid AFM measurements on a flat, silicon ATR crystal. This opens up new ways for measuring AFM-IR in liquids. The flat shape allows easy handling and spin coating of samples on the carrier. Because it’s made of silicon, surface functionalization is very easy, and one can refer to a lot of literature. We have shown that our new sample carrier allows cost-effective, functionalizeable measurements on the nanoscale in liquids.