Multilayers of Carbodithioate and Sulfide-Linked CdSe Nanocrystals: Progressive Increasing of Exciton Delocalization

Multilayers of sulphur-based anchoring group linkers and CdSe nanocrystals (NCs) were realized on ITO substrate via layer-by-layer alternation. The materials were investigated by UV-vis and FTIR spectroscopy, photoluminescence and photoconductivity. We found that the change of NCs packing during the multilayers build-up produces a progressive increase of exciton delocalization. In fact during multilayer deposition the NCs first excitonic peak shifts progressively towards longer wavelengths. We found a linear relation between the optical bang gap shift ("?" E_gn) and the inverse of number of CdSe-NCs layers ( n ) for n > 2, in particular the decrease of its value for the infinite multilayer from the one of the solution (2.10 eV) goes from 18 meV for the 1,2-ethylene-bis(dithiocarbamate) (EDTC) linker to 66 meV for sulphide dianions, corresponding to an apparent increase of the exciton radius of 0.20 nm and 0.85nm respectively.