  The electronic gap structure the organic molecule
N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, or TPD,
has been studied by means of a Scanning Tunneling Microscope (STM) operated
under ambient conditions, and by Photoluminescence (PL) analysis. Thousands of
current-voltage characteristics have been measured at different spots of the
sample showing the typical behavior of a semiconductor. The analysis of the
curves allows us to construct a gap distribution histogram which reassembles
the PL spectrum of this compound. Our analysis shows that in the TPD films most
of the observed distribution of the gap comes mainly from an uncertainty in the
position of the LUMO levels of the molecular compound as would be expected from
theory. This analysis demonstrates that STM can give relevant information, not
only related to the expected value of a semiconductor gap, but also on its
distribution which affects its physical properties such as the case of the PL
and absorption distributions as here is reported.
