The organic molecule 3,4,9,10-perylenetetracarboxylic-3,4,9,10-dianhydride
(PTCDA) deposited on Au(1 0 0) has been investigated by scanning tunneling
spectroscopy (STS). For this purpose, highly ordered ultrathin PTCDA films
have been prepared by organic molecular beam epitaxy. Single point current-
voltage spectroscopy at fixed tip-sample separations has been performed in
UHV on closed PTCDA monolayers as well as on PTCDA islands. The normalized
differential conductivity curves calculated from these I-V data reveal a pr
onounced peak at +0.8 to 0.9 V sample bias. No peak in the normalized diffe
rential conductivity was observed for uncovered gold regions. The STS resul
ts indicate tunneling via the lowest unoccupied molecular orbital at adequa
te bias voltages. (C) 2001 Elsevier Science B.V. All rights reserved.