2, University of California, Riverside, Riverside, California, United States
3, University of California, Riverside, Riverside, California, United States
Exploring both topographical and chemical information down to nanoscale has intrigued broad research attention. Tip-enhanced Raman spectroscopy (TERS) imaging, which combines both the high spatial resolution of Scanning Probe Microscopy (SPM) and the enhanced signal sensitivity of surface-enhanced Raman Spectroscopy, has been demonstrated as a promising technique for the near-field Raman imaging research. The quality of TERS heavily relies on the Raman signals Enhancement Factor (EF) using the metallic tip. Here, we have developed a reliable and low-cost fabrication method to prepare STM-TERS tip. Using a Sharp-Tip Silver Nanowire (ST-AgNW) introduced in our previous work, both high spatial resolution and high EF can be achieved. By putting the ST-AgNW on the platinum-iridium wire with a micromanipulator, the protruding length of ST-AgNW can be precisely controlled and reduced to less than 5 µm, which results in the minimized thermal vibration. Utilizing conical shape at the ST-AgNW apex, atomic scale resolution (~0.1 nm) STM image of Highly Ordered Pyrolytic Graphite (HOPG) has been achieved and multi-channel tunneling current can be avoided. Moreover, TERS performances of the ST-AgNW have been evaluated by approaching the ST-AgNW to monolayer graphene on the 30nm gold substrate. Owing to the small tip radius of our ST-AgNW (~5 nm), a gap mode with small mode volume between the ST-AgNW and the gold substrate can be supported when the incident laser was tightly focused into the tip-substrate gap. Comparing with the regular AgNW tips in other TERS tip preparation methods, a much higher enhancement factor (EF) from our STAgNW tip has been demonstrated by both Raman spectroscopy measurement and finite element analysis simulation.