Plasmonon-enhanced Raman spectroscopy: From plasmonic nanogap to SERS and TERS

Tip-enhanced Raman spectroscopy (TERS) witnesses its active development since more than 15 years. Based on the lightning-rod and plasmonic resonance effects, the generation of strongly confined electromagnetic field within a nanogap between a sharp tip apex and a sample surface under laser illumination even makes the visualization of single molecules possible [1–3]. Our group has actively contributed to the development of TERS in a variety of aspects [41–7], such as low temperature TERS of dye molecules at 10K [8], and TERS combined with fs- or ps-laser excitation [9, 10]. We have investigated a variety of radiation scenarios from plasmonic nanogaps. To realize a precise control over the nanogap distance down to 1 nm, we combined shear-force and tunneling current feedbacks with confocal optical microscopes. Both electrical and optical excitation can be applied to the plasmonic nanogap. This allows one to study either separately or simultaneously the plasmonic resonance effect and electric DC field on the optical radiation [9, 11]. In this talk, we will report our latest progresses of using TERS 1) to determine the nanometer-sized morphology in organic semiconductor crystalline film and 2) to identify molecular orientation changes within monolayer adsorbates. With this work, we content that TERS has past the prove-of-the-concept phase and has grown into an effective technique finding wide applications.