High sensitivity infrared nano-spectroscopy in ambient and water environment

Mid-infrared (mid-IR) vibrational spectroscopy is a universal label–free tool for identifying molecular compounds via their ‘fingerprint’ vibrational absorption lines. Infrared nano-spectroscopies with nanoscale spatial resolution can reveal the chemical and physical properties at a few nanometers, and several scanning probe techniques¹⁻⁹ have been developed to address this need. Among these methods, AFM-IR and scattering nearfield scanning microscopy (s-NSOM) have produced outstanding results in mid-IR and far-IR spectral range. In this thesis, a set of experiments are presented, which have substantially improved AFM-IR in terms of the sensitivity and resolution, and have implemented the system for operation under different conditions. We demonstrated high-sensitivity AFM-IR on monolayer molecules via molecular expansion force. AFM-IR microscopy and spectroscopy is also demonstrated on thin films in aqueous environment, which offers the possibility of in vivo studies of biological samples using IR nanospectroscopies¹⁰. Apart from AFM-IR, s-NSOM imaging using the self-mixing detection in mid-IR quantum cascade lasers (QCL) is also presented in this dissertation. The phase and amplitude of the scattered light field can be analyzed from the voltage signal on the QCL, without the need of external-cavity interferometric optical setup.