Tip Engineering for STM-TERS and Other Surface Chemistry Applications

Science Center, Bent Corridor

Tip-Enhanced Raman Spectroscopy (TERS) is a technique used to obtain spectra which act as vibrational fingerprints for molecules. This technique depends upon a conductive metal needle with apex tens of nanometers wide (tip). Typical Raman Scattering is performed to analyze ensembles, allowing the scattering of enough photons to produce obvious spectra. TERS is used to analyze systems of few molecules, prompting need for enhancement of observed signals. Scanning Tunneling Microscopy (STM) is an imaging technique based on electron tunneling that also involves the use of a tip. The ideal tip is atomically sharp and tungsten is easily made this sharp. The enhancement in TERS is two-fold: part comes from image dipole interactions between surface and tip and part from plasmonic enhancements, known to be affected by the metal’s dielectric constant (Ag is optimal) and tip shape. The latter dependence is not understood as thoroughly. This uncertainty means the qualities defining a “good” TERS tip are not well known. Our lab utilizes a three step etching process: the first two work electrochemically, and the last functions using an Ar+ ion beam. The primary focus of this research lies in the third etching step: through systematic control of the Ar+ etcher, I will determine how to control the shape of tips. Because the best tip shape for TERS is unknown, this systematic creation of differently shaped tips will pave the way to test many tip styles, with the ultimate goal being to solve the mystery of what the ideal TERS tip is.