On the nanoparticle to molecular size transition: Fluorescence quenching studies

Semiconductor nanoparticles, NPs, exhibit fluorescence properties that are closely related to the nature of their surface. CdSe NPs that range in size from 2 to 15 nm in diameter fluoresce with both near-band-edge emission an d deep-trap emission, depending on the quality of the surface. When butylam ine is added to colloidal solutions of NPs in this size range, the amine is thought to bind to the NP surface and eliminate radiant recombination path ways that lead to luminescence. Since the amine binds to the NP surface, th e decrease in fluorescence intensity does not follow standard molecular mod els of collisional fluorescence quenching. NPs that are smaller than simila r to2 nm are composed entirely of a discontinuous arrangement of atoms that are all in constant chemical contact with capping material, solvent, and c ontaminants. When butylamine is added to solutions of these smaller NPs, th e fluorescence quenching follows more standard collisional quenching models . Thus, by monitoring the interaction between NPs and a hole acceptor such as butylamine, one can observe the transition from NP, with atoms both in t he core and on the surface, to molecular cluster, with no core atoms to dom inate the electron density, but with only surface atoms that can participat e in molecular processes such as electron transfer.