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

We studied the light emission induced by tunneling electrons of scanning tunneling microscopy (STM) from well-defined 3,4,9,10-perylenetertracarboxylic dianhydride (PTCDA) bilayer, vapor deposited under ultrahigh vacuum condition on Au(111) single crystal surface. We detected visible fluorescence over the second layer of PTCDA. The light emission spectrum contained a fluorescence peak at 1.75 eV of photon energy at STM bias voltages below 2.5 V. When the bias voltage was set above 2.5 V, another fluorescent peak was observed at 2.35 eV. This peak is out of the range of Au local plasmon excitation, and is attributed to the transition from S₁ excited state to S₀ ground state of PTCDA molecule excited by direct electron injection. The fluorescence near 2.35 eV was reported in dilute solution of PTCDA by optical excitation. This intrinsic molecular fluorescence indicates that the 2nd layer PTCDA molecules were significantly isolated from environment over the presence of the 1st layer on Au(111).