STM-Excited Luminescence on Organic Materials

A scanning tunneling microscope 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 Alq(3). 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 Alq(3) we find E (LUMO) = 1.57 +/- 0.15 eV above the Fermi level.