Energy Transfer in Nanocrystal/Semiconductor Hybrids
Energy transfer (ET)-based hybrid nanostructures is a promising class of materials offering a versatile platform for a multitude of applications.One component is characterized by its strong light-matter interaction and the other by its high charge-carrier mobilities. In such hybrid systems, the excitonic energyis transferred via non-radiative (NRET) and radiative (RET) energy transfer across the interface with the subsequent separation and transport of charge carriers entirely within the highly conductive substrate component.
We have recently established this new and rapidly growing research direction of directed energy transfer from colloidal nanocrystal quantum dots (NQDs) into functionalized Silicon substrates for light harvesting applications. Near-field interaction of the transient NQDexcitonic dipoles with electronic levels in Si leads to non-radiative (Forster type) energy transfer (NRET) at NQD-to-Si separation distances to ~ 6-7 nm (in the visible part of the spectrum). Additionally, radiative emission coupling (RET) couples NQD PL emission into the waveguiding modes propagating within high-index Si substrate. Transient photoluminescence (PL) technique has been employed to monitor dynamics of the donor nanocrystal species. Excitonic PL lifetimes of the NQD donors in the presence of the Si acceptor are shortened and rates of energy transfer are deduced by comparing to PL lifetimes on reference glass substrates.
We have demonstrated that efficient NRET and RET take place from NQD monolayers specifically grafted into Si substrates of various geometries. NRET, which is characterized by donor/acceptor dipole-dipole interaction, is shown to be an efficient mechanism for energy coupling (ENRET~65%) in the visible portion of the spectrum1, while RET waveguide couplinginto Si substrates2 plays the major role in the near-infrared,3 with both mechanisms providing overall transfer efficiency ~90% across the visible. We are utilizing ET concepts to create size-gradient multilayer NQD films with directed energy flow towards Si substrate and 3D NQD layers grafted on Si nanopillars4 to implement efficient light harvesting architectures.
 H. M. Nguyen, A. V. Malko et al., Appl. Phys. Lett.98, 161904 (2011)
 H. M. Nguyen, A. V. Malko et al., ACS Nano, 6, 5574-5582 (2012)