One-step Fabrication of Sharp Platinum/Iridium Tips via Amplitude-Modulated Alternating-Current Electropolishing

Abstract

The platinum/iridium (Pt/Ir) alloy tip for scanning probe microscopy (SPM) was fabricated by amplitude-modulated alternating-current (AC) electropolishing. The clean tips with a radius of curvature less than 100 nm were reproducibly obtained by applying the sinusoidal voltage in the frequency ($f_0$) of $900\ \mathrm{Hz} \leq f_0 \leq 1500\ \mathrm{Hz}$ with amplitude modulation by the sinusoidal wave in the modulation frequency ($f_s$) of $f_s=0.1f_0$ in $\mathrm{CaCl}_2$/$\mathrm{H_2O}$/acetone solution. The analyses by scanning electron microscopy with an energy-dispersive X-ray analyzer (SEM-EDX) and atom probe tomography (APT) showed that a uniform Pt/Ir alloy was exposed on the tip surface as a clean surface without O or Cl contamination. The 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 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.