Bias dependence of tunneling-electron-induced molecular fluorescence from porphyrin films on noble-metal substrates

We investigated scanning tunneling microscope (STM)-excited luminescence from porphyrin (PhTPP and H₂TPP) thin films on metal substrate (Au and Ag) under ambient conditions. Molecular fluorescence similar to the corresponding photoluminescence was observed from PhTPP/Au and H₂TPP∕Ag at both STM bias polarities. We found that at the same experimental condition and parameters, the STM-induced luminescence intensities of maxima peak are similar for PhTPP and H₂TPP but weaker by a factor of about 4 than that for H₂TBPP which we studied previously, probably due to fluorescence quenching by aggregation. The polarity dependence and the bias voltage dependence of the fluorescence intensity for the PhTPP/Au were similar to that for the respective pristine metal. These facts support our proposal that intense molecular fluorescence from porphyrin film on the noble metals is a result of the enhancement of molecular excitation by substrate surface plasmons.