4 p.m. - 5 p.m. Location: SLC 2.303
Dr. Mikhail Zamkov (Bowling Green State University)
We demonstrate an inverse energy-gradient nanocrystal architecture that supports the formation of two-dimensional excitons in the shell domain. The developed geometry places a wide-gap semiconductor (CdS) at the core of the composite nanoparticle in order to funnel the photoinduced energy into the low-gap CdSe surface layer. As a result, the quantum confinement is achieved in nanoparticles ranging 20-30 nm in size. The formation of excitons in the CdSe shell layer was manifested through a size-tunable emission and the characteristic step-like absorption profile. Transient absorption measurements further elucidate the dynamics of the photoinduced energy relaxation in CdS/CdSe nanoshells providing evidence that excitations of the bulk-like core domain result in a rapid, ~ 2-ps recovery of the CdS bleach attributed to electron cooling. The charge transport characteristics of nanoshell assemblies were evaluated through a side-by-side comparison with CdSe quantum dot solids. According to photocurrent measurements, nanoshell solids showed a 7-fold enhancement in the photoconductivity relative to similarly processed films of spherical CdSe nanocrystals, which was attributed to the reduced interfacial area of “large-grain” nanoshell assemblies. We expect that the developed nanoshell architecture could potentially be extended to a broader range of semiconductors (e.g. CdS/PbS, ZnS/CdS) facilitating the development of quantum dot solids offering improved charge transport characteristics.