Synthesis of an Insulator-Coated Metal Tip with a 50-nm-Diameter Conductive Region at the Apex

We present a method for the fabrication of a passivated probe with a nanoscale conductive region. For an electrochemically sharpened W tip wholly coated with inner PtIr and outer SiO2 thin layers by pulsed laser deposition, the local removal of the SiO2 layer from the tip apex was achieved by electron-beam irradiation in a transmission electron microscope. The bared area of the tip apex was less than 50 nm in diameter. Scanning tunneling microscope imaging using the tip revealed that the active area at the apex was conductive. Moreover, the passivated tip showed stability to a biological culture solution.