Tunneling electron induced photon emission from monolayered H₂TBP porphyrin molecules on Cu( 1 0 0 )

Positioning of a clean scanning tunneling microscope tip above a monolayer of free-base porphyrin (H₂TBPP) molecules on Cu(100) is found to induce merely plasmon-mediated emission with molecular fluorescence completely quenched. The molecule acts as a spacer to increase the tip-metal substrate distance to make spectra blue-shifted. Additional broad emissions at low energies may be associated with the molecules either adsorbed onto the tip apex or on the second monolayer and might suggest the involvement of modified molecular fluorescence. In both cases, the energy transfer from molecular excited states to the metal substrate is overwhelmingly dominant, leading to enhanced plasmon-mediated emission.