Photon emission induced by tunneling electrons from a Cu(1 0 0) surface covered with porphyrin molecules

An ultrahigh-vacuum scanning tunneling microscope with an optically transparent tip was used to excite photon emission from a clean Cu(1 0 0) surface and a sub-monolayer of Cu-tetra-[3,5-di-t-butylphenyl]porphyrin (Cu-TBPP) molecules chemisorbed on a Cu(1 0 0) surface. Photons generated in a nanometer-scale area just under the tip were collected within the near-field region through the apex of the optical fiber tip. Two kinds of mechanisms for the photon emission from a Cu-TBPP/Cu(1 0 0) surface are proposed. The quantum efficiency for molecular fluorescence induced by inelastic tunneling was estimated to be ∼3×10−6 photons/electron.