Injection of charge carriers into the LUMO and HOMO levels of copper phthal
ocyanine (CuPc) is investigated by scanning tunneling microscopy (STM) and
spectroscopy. The threshold for injection of charge carriers emitted by the
tip of the STM allows us to determine the so-called single-particle band g
ap (E-gsp) Of CuPc polymorphs as well as the band edge alignment with respe
ct to the Fermi level of the Au(1 1 1) substrate. We find E-gsp values sign
ificantly lower than the optical band gap. This indicates the presence of i
ntermolecular interactions, giving rise to in-gap states that can play a pr
edominant role in controlling injection as well as transport properties. Th
e lowest value of E-gsp is found for the crystalline phases of CuPc obtaine
d by thermal annealing, whereas the largest band gap and inferior charge tr
ansport properties are found for samples grown on substrates at room temper
ature. (C) 2001 Elsevier Science B.V. All rights reserved.