Goal of this research is the study of the interaction mechanism between electonically excited molecules and semiconductor substrates. Following the isolated molecule approach, ultrathin molecular films (0.01 ML) of N,N'-dimethylperylene-3,4:9,10-bis(dicarboximide) (MePTCDI) have been deposited at helium temperature at nanometric and subnanometric distances (0-500 Angstrom) on two prototypical semiconductors, molybdenum disulphide (MoS2) and hydrogen passivated silicon (Si(111):H), making use of

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... the spacer layer technique to fix the distance and of ps-time resolved single photon counting methods to measure the fluorescence signal. By the isolated molecule approach, it is possible to stop the inter-molecular interaction (and consequently the intra-layer transport) and measure directly the effects induced by the presence of the substrate on the fluorescence properties of the molecule deposited on top. During this work, for the first time the fluorescence lifetime of isolated organic molecules has been measured as a function of the distance from semiconductor substrates. The main experimental results are summarized as follows. The electronically excited MePTCDI molecules deposited at a nanometric distance from the semiconductor surface exhibit (both on MoS2 and on Si(111):H) faster fluorescence decays than the molecule in dilute solution (prototype of intrinsic molecular fluorescence lifetime). The distance dependence of MePTCDI fluorescence lifetime as measured in the range 0-500 Angstrom for MoS2 and in 0-130 Angstrom for Si(111):H shows for the two semiconductors a quantitatively different lifetime shortening effect: on MoS2 the fluorescence lifetime is observed to be shortened more then two orders of magnitude (from 3.95 ns to 25 ps), on Si(111):H more than one (down to 145 ps). The theoretical model developed by Chance, Prock and Silbey (CPS) has been applied to describe quantitatively the observed change in fluorescence lifetime upon molecule-substrate distance variation. Within this classical model the propert [...]