Interfacial Chemistry of Metal Pnictide Magic-Sized Clusters: Connecting Structure and Function through Ligand Coordination

Abstract

Synthesis development of colloidal quantum dots (QDs) has allowed for control over their distinct and unique optoelectronic properties. The continuous tunability in absorbance and emission profiles allowed by their quantum confinement when partnered with the robustness of a colloidal crystalline lattice has led to myriad applications in solid-state lighting, biosensing, photosensitive absorbing devices, and more recently quantum information science. These applications require meticulous and reproducible syntheses to generate materials with highly consistent properties and optoelectronic behaviors. During synthetic investigation toward these ends, researchers have discovered the existence of magic-sized clusters – molecular QDs that form at the early stages of nanocrystal nucleation and growth. Not only do these atomically precise materials serve as important reaction intermediates, but their precision has been heralded as a route toward eliminating heterogeneity from QD syntheses. Despite their presence being sufficiently documented optically, the mechanisms of conversion, true structural identities, and emissive behaviors remain understudied and ambiguous. This thesis seeks to develop a stronger understanding of magic-sized cluster structure and conversion (Chapters 2 and 3) as well as investigate new avenues for enhancing their emissive properties without disturbing their metastability (Chapters 4 and 5). After an introduction on the nucleation and growth of magic-sized clusters and QDs (Chapter 1), Chapter 2 focuses on the InP material system. The structural influence of ligand steric pressure on the In37P20(O2CR)51 cluster is investigated using a substituted phenylacetate ligand framework. It is shown that pressure at different angles in the ligand sphere induces structural perturbations in the internal lattice of the cluster. The influence of these structural changes is further investigated through reactions with P(SiMe3)3 in which para-substituents hinder ingress of reactive species thereby slowing cluster conversion whereas meta-substituents increase surface indium-indium separation distances and enhance cluster conversion rates. Leveraging this knowledge of controlled diffusion through ligand profile allowed for the isolation and complete structural characterization of a new magic-sized cluster intermediate, In26P13(O2CR)39. The relations of the structural motifs present in this new cluster are discussed. In Chapter 3, these structural conclusions are extended toward the InAs system. The synthesis of InAs QDs has been documented to proceed through a ubiquitous cluster intermediate with a distinct absorbance profile showing features at 425 and 460 nm. Synthetic modifications to reduce conformational flexibility of the surface ligands allowed for isolation and full structural characterization of this predominate magic-sized cluster intermediate thereby identifying it as In26As18(O2CR)24(PR’3)3. The crystal structure of this cluster shares important motifs with In26P13(O2CR)39 and In37P20(O2CR)51 in the form of an In14E13 (E = P, As) cage yet the overall structure of In26As18(O2CR)24(PR’3)3 is more anisotropic. Full characterization also allows for refinement of the surface suggesting a ligand sparsity in InAs could lead to higher reactivity. Beyond the structural investigations of magic-sized clusters, Chapter 4 presents a route toward leveraging their homogeneity to achieve narrow emission linewidths. Previous reports have shown that oleate-ligated magic-sized clusters of Cd3P2 (Cd3P2-450) and Cd3As2 (Cd3As2-525) have high emissive color purity with <100 meV linewidths but their PLQYs of 7% and 0.5%, respectively, are too low to be considered applicable emitters. In this study, a ligand exchange on these clusters for stronger binding phosphinates was developed. The bidentate coordination motif of the oleate is mimicked by the phosphinate allowing for retention of the internal structure and preservation of stability. The increased binding affinity after exchange increased the PLQY of the Cd3P2-450 to 26% and the Cd3As2-525 to 9% while maintaining the narrow linewidths. Through time-resolved spectroscopy, the mechanism toward photoluminescent enhancement was determined to be phosphinates shutting down nonradiative recombination pathways. Following up on the emission augmentation of the Cd3P2-450 and Cd3As2-525 magic-sized clusters, Chapter 5 describes a synthetic route to introducing continuous tunability in these systems that rely on discrete sizes. The anion sublattice can be alloyed using different ratios of P(SiMe3)3 and As(SiMe3)3 to generate Cd3P2-xAsx clusters while maintaining the internal structure. This alloying allows for continuous tunability between the independent emission profiles of Cd3P2-450 and Cd3As2-525 with similarly narrow linewidths across the compositional gradient. Further treatment of these materials using the previously developed phosphinate exchange leads to highly emissive, continuously tunable, magic-sized materials with PLQYs reaching as high as 33%. Following the structural investigation, using the ligand-derived diffusion management that previously resulted in the isolation of In26P13, is applied to Cd3P2-450 similarly allowing for the isolation of another smaller cluster, Cd3P2-390. The structure and conversion of this new magic-sized cluster are investigated showing its direct relation to Cd3P2-450. Overall, this thesis takes a two-pronged approach to investigating the role and behavior of magic-sized clusters in many different materials systems. In the first, structural studies of III-V clusters unveil the identity of multiple previously known and unknown intermediates in the synthesis of QDs. This allows for a rich comparison of structural motifs and identifies the M14E13 icosahedral cage as a ubiquitous element in III-V and II-VI nanomaterial growth. In the second, further development of II-V cluster systems increases applicable benchmarks through improving brightness and establishing a synthetic route towards continuous tunability in magic-sized materials. It is demonstrated how the judicious choice in cluster modification can navigate the metastability and simultaneously open new avenues to improved emissive properties.