UC Berkeley

Semiconducting quantum dots hold a great deal of promise in a variety of light-based applications like displays and bioimaging due to their bright and relatively narrow emissions. For each of these applications, an understanding of the surface of the quantum dots is important to maximize their efficiency, as the surface is home to most of the trap states in a quantum dot. One can either make use of these states as intermediates for energy transfer, or remove them to maximize the quantum dot’s native emission. This dissertation will investigate both the coupling of a luminescent species to the surface of InP quantum dots, as well as investigate ligand-based passivations of the surface of InP quantum dots.

Chapter 1 of this dissertation gives an overview of the variety of materials that have been made into quantum dots, along with some of their applications. The coupling of quantum dots to emissive dopants is discussed, along with two passivation strategies that have been studied in nanoparticles: the growth of a passivating shell material around the emissive core and the use of ligands bound to the surface of the quantum dot to passivate trap states.

Chapter 2 details the coupling of InP quantum dots to emissive Yb3+ or Nd3+ contained in a YF3 shell around the quantum dot, which are finally passivated with a LuF3 or YF3 shell to reduce non-radiative quenching. This geometry allows for the broad absorbance of the quantum dot core to be coupled to the narrow emission from the Yb 3+ f – f transition at ∼976 nm. This work utilizes a wide variety of electron microscopy and x-ray characterization methods to investigate the structure of this novel geometry. Optical characterizations were used to show that excitation of the quantum dot core led to a transfer to an intermediate trap state before transfer to the emissive rare earth ion. Though the total system quantum efficiency was only around 0.5%, this work introduces a novel method of creating extremely narrow emissions from quantum dot based systems.

Chapter 3 details initial investigations on the thermodynamics of ligand interactions with the surface of InP quantum dots. Isothermal titration calorimetry was used to investigate the heats of reactions of a variety of organic ligands with InP quantum dots, while quantitative nuclear magnetic resonance spectroscopy was used to investigate the extent of ligand binding to the surface of the quantum dots. It was found that carboxylate ligands, which are the most commonly used ligand in InP quantum dot synthesis, are not necessarily fully passivating the surface, but amine ligands strongly bind to the quantum dots and passivate more of the surface than the carboxylates do.