Fig. 5: Photon emission mechanism.

a dI/dV (yellow) and photon intensity (blue dots) shown as a function of sample bias. b Tentative diagram of many-body electronic states of the molecule and the tip-mediated (black arrow) and substrate-mediated (red arrow) charge-transfer events followed by trion recombination (black dashed arrow). Due to the non-negligible voltage drop across the molecule, the relative energies of the states (aligned with respect to the Ag(111) work function) are dependent on the applied bias (see Supplementary Fig. 11) with α ≈ 1/3 (gray dotted arrows). The direct contact of the molecule with the tip stabilizes the molecule in the negative doublet state \({{{\rm{D}}}}_{0}^{-}\), but when bias is applied, the molecule can be brought into the excited state \({{{\rm{D}}}}_{1}^{-}\) by first releasing an electron into the substrate (red arrow) and then capturing an electron from the tip (black arrow) as discussed in the text. c Schematic depiction of the tip-molecule-substrate geometry where voltage is applied between the two electrodes and an electron is transferred from the tip to the molecule and subsequently from the molecule into the Ag(111) substrate. Source data are provided as a Source Data file.