Tip and Substrate Preparation for Tip-Enhanced Raman Spectroscopy
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Tip-enhanced Raman spectroscopy (TERS) combines scanning tunneling microscopy with Raman spectroscopy to obtain both the morphology and the chemical composition of molecules on a surface. A sharp noble metal tip acts as a Raman active sensor for Raman scattering while the material and morphology of the substrate affects the Raman signal enhancement. This research approaches the application of TERS from both the tip and the substrate aspects. Electrochemical fabrication procedures are investigated to etch sharp gold and silver tips. Molybdenum disulfide (MoS2) substrates are prepared using molecular exfoliation and studied using TERS for the employment of semiconductor Raman active substrates. Gold tips were successfully etched at voltages ranging from 1.5 to 4.0 V. After analysis with a scanning electron microscope, tips etched at 3.5 V show the best combination of shape and sharpness. A cutoff circuit was integrated into the etching setup in an attempt to increase tip sharpness by reducing cutoff time. Analysis of silver tips etched with the cutoff circuit showed blunter tips than those etched without the circuit. MoS2 multilayer samples visualized with an optical microscope showed flakes of varying thicknesses. Atomic Force Microscopy revealed that the flakes are composed of bulk MoS2 as opposed to monolayers. Raman mapping, used to measure signal intensity of materials, showed greater signal intensity for regions of thinner MoS2.