Doping the dots: doped quantum dots for luminescent solar concentrators

Abstract

In this thesis, synthesis methods for luminescent organically capped colloidal ZnSe QDs of different sizes, ranging from 4.0 to 7.5 nm are reported. These QDs are analyzed using TEM, absorption spectroscopy, photoluminescence measurements and temperature dependent photoluminescence decay measurements. A similar trend is observed for the band-edge photoluminescence decay of all investigated sizes: the decay time is short (~5 ns) above 20 K and increases sharply below 20 K, eventually reaching a constant value (270 - 400 ns) at sufficiently low temperatures (< 4 K). The temperature regime in which the decrease of lifetime occurs depends on the QD size and is lower for larger QDs. This behavior can be modeled by a Boltzmann distribution between a lower long-lived and a higher short-lived exciton states, with an energy separation ranging from from 3.3±0.2 to 1.5±0.1 meV in the 4.0±0.3 nm to 7.5±0.5 nm size range. We show that this energy separation is consistent with coupling of the lowest exciton state to a confined acoustic phonon. We report successful doping of these ZnSe QDs with Mn2+ via a cation exchange method. The ZnSe QDs are prepared and purified, they are subsequently doped with Mn2+ via cation exchange. The doped NCs show dual emission: an excitonic emission band around ~ 425 nm and a second emission band centered at 580 nm with a radiative decay time of 242 µs, characteristic of the Mn2+ 4T1à6A1 emission transition in a ZnSe host. Furthermore, we report on the successful synthesis of ZnTe magic size nanocrystals (MSNCs) doped with Mn2+. Colloidal ZnTe MSNCs are prepared via a hot-injection method and doped with Mn2+ via cation exchange. The doped MSNCs show an emission band centered at 620 nm with a radiative decay time of 45 µs, characteristic of Mn2+ in ZnTe. The excitation spectrum of the Mn2+ emission shows narrow absorption bands corresponding to different sizes of ZnTe MSNCs providing further evidence that the 620 nm emission originates from Mn2+ incorporated in the ZnTe host, rather than Mn2+ bound to the surface. The Mn2+-doped ZnTe clusters may serve as nuclei for the growth of larger ZnTe quantum dots doped with a single Mn2+ ion. Finally, we present a successful approach to incorporate of Yb3+ ions into CdSe QDs. CdSe QDs are grown using a hot injection method, Yb3+ ions are attached to the surface and subsequently overgrown with more CdSe. Our spectroscopic data shows strong coupling of the Yb3+ to the QD resulting in energy transfer from the excited QD to the Yb3+ ion and subsequent narrow line emission from the Yb3+ ions, successfully combining the strong size-tunable absorption features of the QD with the efficient narrow line emission from the incorporated lanthanide ion. Overgrowth with Se greatly improves the Yb3+ luminescence intensity which is also reflected by a longer emission lifetime. The luminescence intensity and lifetime are further improved by growing up to two monolayers of CdSe using the successive ionic layer adsorption and reaction (SILAR) method.