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Electrochemical Wet Etching of Silver STM Tips Presentation By: Anthony R. Davis Senior Electrical Engineering Major Hampton University Mentor: Dr. Wilson Ho Graduate Student: Xiuwen Tu Outline • • • • • Background (Tips, STM) Research Inspiration (Why Silver Tips) Process/Results/Findings Future Applications Acknowledgements What are “tips” • Used in STMs • Atomic-sized radius of curvature • Etched using electrochemistry • Usually made of tungsten metal Tips usage in STMs Atomic Mapping: The Art of Drawing Atoms Outline • • • • • Background (Tips, STM) Research Inspiration (Why Silver Tips) Process/Results/Findings Future Applications Acknowledgements Ho Lab Group Goals & Research Inspiration • Nanoscale Chemical & Physical Phenomena • Detailed Descriptions of Individual Molecules and Atoms What’s Special About Silver Tips? • Research suggests that Ag tips have higher photon-coupling efficiency than tungsten tips • Enhanced Photo-Induced Tunneling yields more comprehensive data from specimens scanned Why is Etching Silver Tips so Challenging Compared to Etching other Metal Tips? • Malleable/Soft composure of Ag • Quick evaporation of electrolyte Solution • Appearance of AgOH reside • Silver tip etching is relatively new; etching methods are primitive • Well known methods & commerical kits are available for etching other metals tips (W, Au, Pt-Ir); but not Ag Outline • • • • • Background (Tips, STM) Research (Why Silver Tips) Process/Results/Findings Future Applications Acknowledgements Primary Objective of Research The challenge presented is optimizing the procedure and parameters for etching silver tips to ensure efficiency and reproducibility. 2-Step Electrochemical Etching 1st Etch 2nd Etch Setup of Primary Etch System (Electropolishing) Graphite Tip Electrolyte Objectives for Optimizing 1st Etch • Prevent Solution concentration for changing – Retard electrolyte evaporation • Increase Speed of First Etch – Obtain & interpret voltage vs. time data • Automate tip etching to increase efficiency – Create circuitry to monitor current flow through tip over time Stabilize that Solution • Began using a new recipe NH4:C2H6:H2O2 to prevent evaporation • This solution works just as well as NH4 • Needs to be replenished after 3 hours • Liquid Levels are consistent over time DC Voltage Slows and Speeds Etching Effect of Voltage on Etching Time 30 Time (minutes) 25 20 Time Trial 1 15 Time Trial 2 Time Trial 3 10 5 0 0 10 20 DC Voltage (Volts) 30 40 A Need For Automation • The electrochemical reaction is lengthy & subtle • Visual Inspection Required • Cannot Perform both 1st and 2nd Etches at the same Time What’s the “Current” Trend? Current Flow Through Silver Tips While Etching Current (mA) 6 5 4 V=24V 3 V=30V 2 V=21V 1 0 0 10 20 Time (minutes) 30 How Can we Measure It? Measure voltage Across a Resistor using a Voltmeter Current I R Use LabVIEW software to analyze signal Voltmeter Resistor The New System: From Idea to Reality Triggered Alarms I = 0 mA I < 2 mA Variation in Apex Shape After First Etch Secondary Etch Setup Diagram DC Voltage 5-12V Tip NH4 Second Etch: Useful Findings Outline • • • • • Background (Tips, STM) Research (Why Silver Tips) Process/Results/Findings Future Applications Acknowledgements Innovation Future Possibilities/Applications: Chemical/Optical Sensors High Density Data Storage Nanoelectronic Circuit Components Bio-toxin & Pollution Removal Quantum Computing Acknowledgements University of California, Irvine National Science Foundation (NSF) The UROP Team Department of Engineering Department of Physics & Astronomy Mentor: Dr. Wilson Ho Graduate Student: Xiuwen Tu