Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy and density functional theory

Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary up to a factor three for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single atom tips, while the intensity drops sharply for multi-atom tips. Adsorbing the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that it is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multi-atom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111).