Fabrication of Insulated Tips for Scanning Tunneling Microscopy in Electrochemical Environments

Abstract

Electrochemical Scanning Tunneling Microscopy (STM) is able to image the atomic structure of electrodes and monitor how that structure changes over time. We want to use electrochemical STM to design a better fuel cell catalyst, but we must first find a cost effective way to manufacture the tips that get consumed after every scan. Production of tips for STM in an electrochemical environment is a two phase process. First platinum-iridium wire must be sharpened to a radius of curvature on the order of nanometers. We accomplished this by immersing the platinum-iridium wire in a supersaturated calcium chloride solution and running alternating current through the cell At the air-liquid interface the platinum is etched away and the meniscus formed by the surface tension of he water ensures that the platinum-iridium wire is etched on a slope that leads to an extremely point. The second phase involves preparing a tip for use in an electrochemical environment. In order to use tips in an electrochemical environment they must be insulated so no current leaks out the sides, but current can still flow through the tip. We attempted to hold the leaking current to less than 100 picoamps by coating the sides of the tip in a nonconductive material. We used an electrically deposited paint, also known as cataphoretic paint, to insulate the tip and inverted the needle while it dried so the paint would slide down and expose the tip of the needle. Our best coated tips had a leaking currents seven hundred picoamps so we were unable to test them in electrochemical environments, however we were able to use them in an air environment proving our method leaves the tip of the needle free.