Nanoscale organic electroluminescence from tunnel junctions

Abstract

Nanoscale organic electroluminescence was induced by positioning a sharp tungsten tip on the surface of a free-base porphyrin $({\mathrm{H}}_{2}\mathrm{TBPP})$ monolayer on the top of PtTBP porphyrin (PtTBPP) multilayers on a Cu(100) substrate in an ultrahigh vacuum scanning tunneling microscope (STM) system. The well-defined molecular fluorescence spectra are perfectly matched with the conventional photoluminescence spectrum from bulk ${\mathrm{H}}_{2}\mathrm{TBPP}$ molecules. The nanoscale PtTBPP multilayers do not fluoresce; rather, they act as spacers to enhance the decoupling of the electronic state of the ${\mathrm{H}}_{2}\mathrm{TBPP}$ monolayer from the Cu surface. The electronic property of molecules and the energy-level alignment of molecules with respect to the Fermi levels of electrodes are probably quite critical for observing STM-induced molecular fluorescence from molecular layers with a similar thickness. The molecule in proximity to the tip apex of a scanning tunneling microscope is locally excited by the hot electron injection mechanism, followed by radiative decay via Franck-Condon transitions.