Synthesis, Characterization, and Photophysical Investigations of Doped Semiconductor Nanocrystals

Doped semiconductor nanocrystals (NCs) are an emerging class of new materials, as incorporation of guest ions allows for refinement of their electronic, optical, and magnetic properties beyond that afforded by quantum confinement effects. However, doping semiconductors by impurity atoms has proven elusive for strongly confined colloidal semiconductor nanocrystals due to the synthetic challenges, as well as the lack of fundamental understanding of the electronic structure of doped NCs. Therefore, it is the aim of this dissertation to gain deeper insight into the underlying mechanism of the synthesis of doped materials and the impact of impurities on the electronic structure of the host matrix. Our research has resulted in a microscopic description of dopant electronic structure in the ground and excited states, including the discovery that charge carrier localization on a dopant may result in a modulation of the guest ion’s bonding to the underlying semiconductor matrix. These findings provide a sound scientific for the future developments of more efficient electronic devices

Anomalous Perturbation of the O2 Sensitivity of Poly(aromatic) Hydrocarbons by Magnetic Quantum Dots

Molecular oxygen is known to be an efficient quencher of the excited states of organic molecules. This fact has been exploited to develop fluorescent O2 sensors, which is topical for cancer screening and many other applications. In this regard, our group and others have reported the development of robust, ratiometrically reporting chemosensors by conjugating O2-sensitive organic chromophores to inorganic quantum dots. Recently, an attempt was made to prepare a multifunctional sensor system by attaching the emissive poly(aromatic) hydrocarbons pyrene and perylene to magnetic nanomaterials, specifically CdSe/CdMnZnS and ZnSe/ZnMnS/ZnS quantum dots. However, the fluorescence of both dyes became invariant to oxygen levels, even if the solutions were fully saturated. We have ruled out any material dependency beyond the presence of Mn2+ ions by studying control samples, while molecular dynamics simulations negated any possibility of spatial sequestration of O2 by the magnetic fields. In the case of pyrene, the proximity of the magnetic dots induces significant singlet-triplet mixing; however, this does not explain this curious observation. As such, we believe that the exchange interaction between O2 and the dyes is perturbed by both the microenvironment of the system and the inhomogeneous magnetic fields such that quenching is not observed

In Vitro Detection of Hypoxia Using a Ratiometric Quantum Dot-Based Oxygen Sensor

A quantum-dot based ratiometric fluorescent oxygen probe for the detection of hypoxia in live cells is reported. The system is composed of a water-soluble near-infrared emissive quantum dot conjugated to perylene dye. The response to the oxygen concentration is investigated using enzymatic oxygen scavenging in water, while in vitro studies were performed with HeLa cells incubated under varying O₂ levels. In both cases a significant enhancement in dye/QD emission intensity ratio was observed in the deoxygenated environment, demonstrating the possible use of this probe for cancer research

Charge Carriers Modulate the Bonding of Semiconductor Nanoparticle Dopants As Revealed by Time-Resolved X‑ray Spectroscopy

Understanding the electronic structure of doped semiconductors is essential to realize advancements in electronics and in the rational design of nanoscale devices. Reported here are the results of time-resolved X-ray absorption studies on copper-doped cadmium sulfide nanoparticles that provide an explicit description of the electronic dynamics of the dopants. The interaction of a dopant ion and an excess charge carrier is unambiguously observed <i>via</i> monitoring the oxidation state. The experimental data combined with DFT calculations demonstrate that dopant bonding to the host matrix is modulated by its interaction with charge carriers. Furthermore, the transient photoluminescence and the kinetics of dopant oxidation reveal the presence of two types of surface-bound ions that create midgap states