Revealing the Presence of Mobile Molecules on the Surface

Mobile molecules crossing freely underneath the scanning tip of a scanning tunneling microscope create a uniform diffusive noise, making the identification of single molecules on the surface a challenge. We demonstrate the possibility of detecting mobile molecules on a surface by scanning tunneling microscopy and reveal how the diffusive noise is created. Additionally, we show that a molecule caught in the tip–sample junction allows us to explore the potential energy surface of the system. Finally, voltage pulses disturb the mobile molecules, causing the loss of that ability. They also allow the creation of islands on the surface. Most of the investigations were done for Co- and Cu-phthalocyanine (Pc) on Ag(100). However, the concept is limited to neither Pc molecules nor Ag(100), as shown for a different organic molecule, astraphloxin, on Cu(111).