Leaving Förster Resonance Energy Transfer Behind: Nanometal Surface Energy Transfer Predicts the Size-Enhanced Energy Coupling between a Metal Nanoparticle and an Emitting Dipole

The interaction of a fluorescent molecule with a gold nanoparticle is complex and can lead to excited-state enhancement or quenching. Many attempts have been made to explain the observed interaction when in close proximity to the metal surface; yet no single model has been capable of explaining the observations. In this work, we show that by accurately describing the interaction in terms of an induced image dipole modified within the gold nanoparticle by the size-dependent changes in absorptivity and dielectric constant, the oscillator interaction can be fully described in terms of a surface-moderated interaction. Comparison of experimental and theoretical data confirms the validity of the model for a selected range of separation distances, nanoparticle radii, and fluorescent molecule selection. The results of the study illustrate the importance of nonradiative pathways for modifying the decay of a fluorescent molecule by coupling to the image dipole, thus providing a firm understanding of the reported variance in behavior for an emitting species in close proximity to nanometal surfaces. A more significant impact of the results is the ability to apply nanometal surface energy transfer methods as a molecular ruler to probe physical questions at much greater distances (>400 Å) than previously achievable

Ostwald’s Rule of Stages and Its Role in CdSe Quantum Dot Crystallization

A century ago Ostwald described the “Rule of Stages” after deducing that crystal formation must occur through a series of intermediate crystallographic phases prior to formation of the final thermodynamically stable structure. Direct evidence of the Rule of Stages is lacking, and the theory has not been implemented to allow isolation of a selected structural phase. Here we report the role of Ostwald’s Rule of Stages in the growth of CdSe quantum dots (QDs) from molecular precursors in the presence of hexadecylamine. It is observed that, by controlling the rate of growth through the reaction stoichiometry and therefore the probability of ion-packing errors in the growing QD, the initially formed zinc blende (ZB) critical nuclei representing the kinetic phase can be maintained at sizes >14 nm in diameter without phase transformation to the thermodynamic wurtzite (WZ) structure. An intermediate pseudo-ZB structure is observed to appear at intermediate reaction conditions, as predicted by Ostwald. The ZB and pseudo-ZB structures convert to the WZ lattice above a critical melting temperature. This study validates Ostwald’s Rule of Stages and provides a phase diagram for growth of CdSe QDs exhibiting a specific crystallographic motif