Tuning the optical band gap of quantum dot assemblies by varying network density.

The effect of bulk network density on the extent of quantum confinement (probed by optical band gap) in CdSe quantum dot gels is evaluated. The CdSe gels were produced from controlled removal of surface thiolate ligands from CdSe quantum dots by adding 3% tetranitromethane. Two main techniques were employed to systematically vary the bulk density. First, different amounts of oxidizing agent were added to change the monolith density of the wet gel, followed by supercritical CO(2) drying to yield CdSe aerogels with different bulk densities. Experimental results suggest that a gradual and almost linear band gap decrease is observed when increasing the bulk density at the aerogel level. The fact that quantum confinement effects are largely preserved in aerogel constructs is consistent with SAXS data revealing the fractal nature of the network. Second, for a constant amount of oxidant, different drying techniques were used to yield CdSe gels with a larger density variation: aerogels (supercritical CO(2) dried), ambigels (hexane dried), and xerogels (acetone dried). A nonlinear trend for band gap decrease was found when comparing CdSe aerogels, ambigels, and xerogels, and the more dense ambi- and xerogels have broader absorption edges, suggestive of resonance transfer effects due to dipole-dipole interactions in non-homogeneous interacting systems. This is attributed to increased aggregation in the denser constructs (supported by TEM and SAXS data). Together, these data suggest that highly porous architectures, such as aerogels, are best suited for maintaining localized quantum confinement effects in 3D connected nanoparticle networks.