Tunneling spectroscopy study of 3,4,9,10-perylenetetracarboxylic dianhydride on Au(1 0 0)

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 pronounced peak at +0.8 to 0.9 V sample bias. No peak in the normalized differential conductivity was observed for uncovered gold regions. The STS results indicate tunneling via the lowest unoccupied molecular orbital at adequate bias voltages.