Publication date: 1st July 2014
In colloidal nanoparticles (NPs) devices, trap state densities at their surface exert a profound impact on the rate of charge carrier recombination and, consequently, on the deterioration of the device performance. Here, we report on the successful application of an atomic-ligand strategy to effectively passivate the surface of cuprite (Cu2O) NPs. Cu2O NPs were prepared by means of a novel synthetic route based on flame spray pyrolysis. FTIR, XRD, XPS and HRTEM measurements corroborate the formation of cubic cuprite Cu2O nanocrystals, excluding the possible presence of undesired CuO or Cu phases. Most importantly, steady-state emission and transient absorption assays document that surface passivation results in substantial changes in the intensity of emissive excitonic states – centered at copper and oxygen vacancies – and in the lifetime of free excitons near the band edge. To shed light onto ultrafast processes in Cu2O nanocrystals additional pump probe experiments on the femtosecond and nanosecond timescales were carried out. Two discernible species were observed. On one hand, an ultra-fast component (~ps) that relates to the free excitons; on the other hand, a long-lived component (~µs) that originates from the defects / trap states. Such knowledge about structure property relationship is decisive for the performance optimization of all oxide colloidal NPs devices.
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