IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA9
Can Cu-TBP Porphyrin Molecules Emit Light under an STM Excitation?

Monday, October 29, 2001, 4:40 pm, Room 120

Session: Innovations in Surface Science
Presenter: Z.-C. Dong, National Institute for Materials Science, Japan
Authors: D. Fujita, National Institute for Materials Science, Japan
H. Nejo, National Institute for Materials Science, Japan
S. Yokohama, National Institute for Materials Science, Japan
S. Mashiko, National Institute for Materials Science, Japan
Z.-C. Dong, National Institute for Materials Science, Japan
Correspondent: Click to Email
Photon emission on metal surfaces induced by tunneling electrons is thought to arise from the radiative decay of localized surface plasmons. While this theory is still not yet complete due to the negligence of non-local effects, the situation becomes more complicated and difficult to study when molecules are sandwiched between the tip and surface. It remains unclear how molecules couple with the electrodes and electromagntic field to give out light. We demonstrate here a technique to produce tunneling-electron induced photon emission from Cu-TBP porphyrins on Cu(100). The emission intensity and optical spectra show not only the enhancement effects, but also new features revealing the molecular origin of light emission. The quantum efficiency is estimated to be 10@super -4@ photons per electron. The observation of a broad peak around 630 nm associated with molecules is particularly exciting but puzzling, since Cu-centered porphyrin molecules are known not to fluoresce under photoexcitation. This broad peak appears dramatically enhanced when the bias voltage is above 3 V, in good agreement with the high B-band absorption via optical excitation. These observations suggest that while energy absorption behavior by electrons appears in parallel with that by photons, the excitation or decay mechanism might be different in the two processes, and may thus bring up new physics. Tunneling electrons can excite surface plasmons in metal substrates, which causes the enhancement of local electromagnetic field. Excitation of molecules occurs via the resonant coupling of molecular electronic states with the localized electromagnetic modes between the tip and substrate. Light is emitted when excited molecules decay to the ground state. The decoupling of luminescent porphyrin cores from the substrate, a means to suppress the fast nonradiative energy dissipation, is found to be important for efficient light emission.