Abstract by: Alberto Morgante

Ultrafast Charge Injection at Complex Interfaces

Interface processes strongly affect the performances and efficiency of organic based devices. The integration of 2D materials like graphene in organic devices (for example as electrodes) is expected to improve the overall device performances. There is a need therefore for a deeper understanding and control of processes at interfaces between organic films, graphene and metals and charge transfer (CT) is one of the most critical. Charge injection across molecular junctions can occur at the femtosecond time scale or even shorter. In most cases this time frame is still out of reach of the pump probe spectroscopies. Here we use X-ray spectroscopies to investigate charge injection in complex hetero-structures that include organic molecules, graphene and metallic substrates. We show that the Core hole clock implementation of the Resonant Photoemission spectroscopy (RESPES) allows us to determine charge dynamics in both directions (to/from the molecule) at these interfaces and can give clues on the interface parameters that can increase/decrease the charge transfer efficiency. Examples of model systems will be discussed.

It will be shown how RPES allows us to elucidate the role of inter-molecular interaction on through-space charge transfer characteristics in π-stacked molecular systems [1], the electronic coupling, morphology and charge transfer rates at the donor-acceptor (D/A) interfaces between C60 and either flat- or contorted hexabenzocorones (HBC) [2] and the relation with improved internal (IQE) and external (EQE) quantum efficiency of devices based on these shape-matched molecular systems. The case of ammine and pyridine terminated organic overlayers will be discussed in connection with recent results of break junction experiments [3]. In this case we show how core-hole clock spectroscopy can be used to measure charge transfer through noncovalent interactions and map charge delocalization times from carbon and nitrogen sites on the molecules. Comparison of charge transfer rates between different substrates (metal, graphene and graphene nanoribbons) will be discussed elucidating the role of the local density of states of these materials and the level alignment in the charge transfer process [4].

References

[1] A. Batra, G. Kladnik, H. Vazquez, et al. Nature Communications. 3, 1086 (2012). http://dx.doi.org/10.1038/ncomms2083.

[2] T. Schiro, et al., Advanced Energy Materials. 3, 894 (2013). http://dx.doi.org/10.1002/aenm.201201125.

[3] G. Kladnik, et al, J. Phys. Chem. C 117, 16477 (2013). http://dx.doi.org/10.1021/jp405229b.

[4] O. Adak, Nanoletters. 15, 8316 (2015). http://dx.doi.org/10.1021/acs.nanolett.5b03962