Light emission from PTCDA bilayer on Au(111) induced in scanning tunneling microscopy

► STM tunneling light emission was detected from 2-ML PTCDA on Au(111). ► Two light emission peaks at 1.71 eV and 2.35 eV of photon energy were detected. ► The peak at 2.35 eV is special; out of Au local surface plasmon excitation range. ► Very weak interaction of PTCDA with the substrate allows this transition.