One-step fabrication of sharp platinum/iridium tips via amplitude-modulated alternating-current electropolishing

The platinum/iridium (Pt/Ir) alloy tip for scanning probe microscopy was fabricated by amplitude-modulated alternating-current electropolishing. The clean tips with a radius of curvature less than 100 nm were reproducibly obtained by applying the sinusoidal voltage in the frequency (f0) of 900 Hz≤f0≤1500 Hz with amplitude modulation by the sinusoidal wave in the modulation frequency (fs) of fs=0.1f0 in CaCl2/H2O/acetone solution. The analyses by scanning electron microscopy with an energy-dispersive x-ray analyzer and atom probe tomography showed that a uniform Pt/Ir alloy was exposed on the tip surface as a clean surface without O or Cl contamination. The scanning tunneling microscopy (STM) imaging using the fabricated tip showed that it is more suitable for investigating rough surfaces than conventional as-cut tips and applicable for atomic-resolution imaging. Furthermore, we applied the fabricated tip to qPlus atomic force microscopy (AFM) analysis in liquid and showed that it has atomic resolution in both the horizontal and vertical directions. Therefore, it is concluded that the amplitude-modulated AC etching method reproducibly provides sharp STM/AFM tips capable of both atomic resolution and large-area analyses without complex etching setups.