Investigation of the Exciton Formation in a Single Molecule with a Scanning Tunneling Microscope*

The recombination of holes and electrons in organic molecules produce excitons and the radiative decay from the excitons is used in organic light emitting diodes (OLEDs). Controlling the exciton formation is one of the primary strategy for improving the device properties of OLEDs. Thus, it is required to investigate and describe the exciton formation process at well-defined molecular systems. In this study, we prepared single molecule system, which is ultimate well-defined system, and conducted experiments with a scanning tunneling microscope (STM). Recent progress in STM experiment enables to detect luminescence from a single molecule on ultrathin insulating film. Here, we applied this technique to 3,4,9,10-perylenetetracarboxyricdianhydride (PTCDA) adsorbed on NaCl ultrathin film grown on Ag(111). PTCDA is known as a prototypical n-type organic semiconductor with strong optical response, and we succeeded in detecting luminescence from PTCDA. The precise bias voltage dependent electroluminescence measurement and differential conductance measurement revealed the exciton formation mechanism in this system.