Nanoscale chemical imaging with novel fiber tip-enhanced Raman spectroscopy and microscale study of surface reactions on monolayer MoS2.
Study of molecule-surface interaction dynamics requires nanoscale chemical and topological information. Full dynamic understanding cannot be provided by microscale optical spectroscopy and has been aided by nanoscale imaging techniques, such as scanning probe microscopy (SPM). Recently, Tip-Enhanced Raman spectroscopy (TERS) has demonstrated single molecule chemical imaging with <1 nm resolution. However, nanoscale experiments require restrictive sample conditions and TERS microscope configurations prohibit easy operation in real-world environments, such as liquid. In this work, I helped develop and demonstrate a novel fiber optic-based technique, fiber-based TERS (FTERS), which overcomes the limitations of traditional SPMs, microspectroscopies, and current TERS techniques. The FTERS probe utilizes plasmonic coupling between an optical fiber and metal coating to excite and collect nanoscale chemical scattering and emission. FTERS probes were fabricated and plasmonic nanofocusing of FTERS was demonstrated experimentally. Delivery of optical excitation to a diffraction limited volume was confirmed by polarization dependent optical imaging of FTERS probe emission. The back-collection plasmonic coupling of FTERS was demonstrated by Photoluminescence (PL) emission of quantum dots (QDs) placed on the tip. Fiber-in excitation fiber-out collection of PL from QDs on a Au surface was demonstrated in Scanning Tunneling Microscopy (STM) configuration. Single-nanometer dependence of the tip-sample distance confirms the plasmonic excitation/collection mechanism. Finally, FTERS probes were demonstrated to operate in liquid for Raman collection and STM surface imaging. Two studies of the industrial material monolayer (ML) MoS2 were conducted with scanning Raman microspectroscopy in environment-controlled cells. ML MoS2 has a direct bandgap that is dependent on its local surface structure and is modulated by a small number of adsorbed molecules. The first study imaged localized photoreactions with ambient atmospheric components O2, N2, and H2O at flake edges. Laser irradiation in ambient conditions causes increasing PL at edge sites of ML MoS2, but has little effect on the basal plane PL. A second study showed a dramatic PL modulation by reactant phase: gaseous pyridine and thiophene nearly completely quenched PL but liquid exposure reduced emission by only half. These studies demonstrate molecule interaction dynamics are strongly dependent on localized surface features and full understanding requires accessible nanoscale chemical imaging techniques.