Surface Ligand Binding Motif Chemistry on Semiconductor Nanoclusters and Quantum Dots

Abstract

Semiconductor nanocrystals (NCs) offer utility across a range of optoelectronic devicesand applications such as diodes, superlattice solar cells, and lasing. This versatility is due to their synthetically based size tunability, narrow linewidths, and strong ab- sorptive cross sections. It is essential to understand the photophysical properties of semiconductor NCs such that they can be utilized to their fullest extent. Much of the photophysics of nanocrystals results from their core structure, but the surface de- fects and unbalanced charges of the core can be catastrophically destructive to these properties. Organic ligands with carboxylic acid head groups can be used to stabilize these defects, and the interconversion of head group binding motifs in the ligand shell is dynamic with increased free ligand concentration, elevated temperature, and changes in solvent identity. Carboxylic acid head groups bind to the NC core surface with a variety of different binding motifs with indicative frequencies that can be detected with infrared spectroscopy. However, these frequencies are impacted by both ligand and core identity. Fitting the infrared signature of the carboxylic acid binding motifs with gaussians allows for characterization of surface ligand populations. This thesis documents the surface ligand binding populations and solution interaction of long chain carboxylic acid ligands on InP magic size clusters and CdS Quantum Dots (QDs). Chapter 1 describes an overview and perspective of carboxylic acid binding motifs and experimental methods used to characterize them. Emphasis is put on using infrared spectroscopy to identify binding motif and applied Bayesian inference to deconvolute complicated infrared spectra. Chapter 2 documents the relative binding energies of carboxylic acid binding mo- tifs on the surface of nanocrystals found through temperature dependent FTIR fit with a Markov-Chain Monte Carlo algorithm. The relative binding motif energies found through globally fitting a temperature dependent FTIR spectra of InP magic sized clusters (MSCs) with a Markov-Chain Monte Carlo global fitting algorithm are suggest previously unquantified binding motif lability. The chelating and syn-syn binding conformations are 0.7±0.3 kcal/mol and 1.1±0.5 kcal/mol more stable than the monodentate motif. Additionally, we find that the free ligand has nearly the same relative energy as the monodentate stretching frequency, with a relative energy of 4.52±0.05 kcal/mol, or 1582±19 cm−1, which may suggest that ligand vibrational play a key role in ligand dissociation. Chapter 3 documents how surface ligand populations change with the formation of macroscopic gels of CdS QDs. Through FTIR and 1HNMR spectroscopy, we de- termine that the defused Z-type ligand cadmium oleate is essential for the formation of gels. We also determine that washing CdS QDs with ethanol successfully prevents gelation due to removal of weakly bound Z-type ligands from CdS surface. Finally, by comparing the IR spectra across temperature of these ethanol and acetonitrile washed QDs, we determine that there are two mechanisms for gelation – physical gelation and chemical gelation – that are spectroscopically distinct. This research gives insight into the interface of semiconductor QD surface and their ligands as well as how the surface ligands interact with solvent. This knowledge can be utilized for synthetic control of colloidal quantum dots to better optimize their physical properties. Likewise, QD ligand solvent interactions allow for optimization of QD nanostructures. Furthermore, characterization of the ground state allows us to begin quantification of ligand binding moiety populations and dynamics in the excited state.