Role of ligands in an hybrid CdSe Nanocrystal-Organic interface

Functional 'hybrid' materials based on combinations of organic molecules and inorganic nanoparticles open the possibility to modify the optical and charge transport properties of the inorganic component by chemical bridging with the organic functional moiety. Moreover, such hybrid systems allow the assembly of the two components into an ordered supramolecular structure with a simple process. Among the several techniques known for the formation of the hybrid NC-organic molecule material, the layer-by-layer (LBL) self-assembling technique offers several advantages over other solution-based techniques for the achievement of efficient charge transport in thin films. This method leads to densely packed layers with good surface regularity, also featuring really small interparticle distances. In this work, we report on femtosecond to millisecond pump-probe experiments performed on a multilayered structure composed by the alternation of CdSe nanocrystals (NCs) and poly(p-styrenesulphonic acid)(PSSH) obtained by LBL (see Figure 1). We found that, after a few picoseconds of pumping at high intensity, an intense photoinduced absorption band and a strong derivative feature emerge in the differential absorption spectrum, both overlapping with the positive bleaching bands of the filled exciton states. We attribute the first to the population of charged trap states, located close to the NC surface, and the second to an induced Stark effect due to the strong electric field that those charged trap states generate inside the NC [1]. A combined approach based on ultrafast photophysics characterization and ab initio simulation also indicates that these states are localized close to the nanoparticle surface, and that electrons and holes are separated across the hybrid interface. By using density functional calculations we identify shallow hole traps only for the deprotonated SSH attached to the CdSe surface and not for more common amine-terminated NC surfaces pointing out the effects of surfactant capping molecules on the optoelectronic properties of the interface [2].