Nanoscale organic electroluminescence from tunnel junctions


X.-L. Guo,1,* Z.-C. Dong,1 A. S. Trifonov,1 K. Miki,1 Y. Wakayama,1 D. Fujita,1 K. Kirnura,1 S. Yokoyama,2 and S. Mashiko2

1National institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
2Communication Research Laboratory, Kobe, Hyogo 651-2401, japan
(Received 7 April 2004; revised manuscript received 1 1 August 2004; published 27 December 2004)

　Nanoscale organic electroluminescence was induced by positioning a sharp tungsten tip on the surface of a
free-base porphyrin (H2TBPP) moonolayer on the top of PtTBP porphyrin (PtTBPP) multilayers on a Cu(lOO)
substrate in an ultrahigh vacuum scanning tunneling microscope (STM) system. The well-defined molecular
fluorescence spectra are perfectly matched with the conventional photoluminescence spectrum from bulk
H2TBPP molecules. The nanoscale PtTBPP multilayers do not fluoresce; rather, they act as spacers to enhance
Ihe decoupling of the electronic stale of the H2TBPP monolayer from the Cu surface. The electronic property
of molecules and the energy-level alignment of molecules with respect to the Fermi levels of electrodes are
probably quite critical for observing STM-induced molecular fluorescence from molecular layers with a similar
thickness. The molecule in proximity to the tip apex of a scanning tunneling microscope is locally excited by
the hot electron injection mechanism, followed by radiative decay via Franck-Condon transitions.
D01: 10.1103/PhysRevB.70.233204
PACS number(s): 68.37.Ef, 73.20.- r, 33.80.-b