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Comparing the distribution of the electronic gap of an organic molecule with its photoluminescence spectrum

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.