STM-excited luminescence on organic materials

A scanning tunneling microscope (STM) is used to generate electroluminescence from thin films of tris(8-hydroxyquinolato) aluminum (Alq3) deposited on Au(111) substrates. The emission spectra are highly dependent on the local structural features of the thin film. Furthermore, the intensity distribution is modulated by the collective charge-carrier excitations of the tip and the substrate. This is manifested as a red-shifted emission spectrum characterized by relatively narrow emission lines from the organic material. The luminescence signal appears to be a linear superposition of two different kinds of Alq3. We show also that the energy of the lowest unoccupied molecular orbital (LUMO) can be determined with the aid of measurements of the threshold for tunneling-excited electroluminescence. For Alq3 we find ELUMO = 1.57 ± 0.15 eV above the Fermi level.