Redox-Activated Covalent Functionalization of Semiconductor Surfaces

Citation

Morla, Maureen Baradi (2023) Redox-Activated Covalent Functionalization of Semiconductor Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1exn-0t72. https://resolver.caltech.edu/CaltechTHESIS:04252023-051729803

Abstract

Semiconducting materials are those with a band gap across which electrons can be excited when the material absorbs photons with sufficient energy. Surface functionalization of semiconductors involves manipulation of the properties of the material by attaching organic small molecules through a surficial covalent bond. By controlling the surface properties of the material, functionalization has enabled the application of semiconductors in a myriad of fields, prompting a highly active field of research. To aid in this effort, we explore a new reaction methodology based on redox-mediated surface functionalization, where an outer-sphere, one-electron metallocene reductant or oxidant is added to the solution medium containing the semiconductor and the small molecule to be added to its surface. Using density functional theory, we elucidated the thermodynamic and kinetic factors that limit the experimentally observed upper coverage bound of reductant-activated methylation of 1T′-molybdenum disulfide by determining two governing factors: 1) sulfur sites with longer Mo–S bonds are more thermodynamically favorable for methyl addition, and 2) sulfur sites with fewer adjacent methylated sulfur sites are preferentially functionalized due to steric hindrance. We then expanded this reductant-activated reaction methodology to silicon(111) surfaces and demonstrated that the reductant solution potential must lie near or above the silicon(111) conduction band edge to observe reactivity. By extending this study to silicon nanocrystals of different sized diameters and different conduction band edges, we found that the extent of surface reactivity relied heavily on reductant strength, but the energy difference between the conduction band edges was too small to observe a distinct dependency on nanocrystal size. The work encompassed in this thesis expanded our understanding of redox-mediated reactions on semiconductor surfaces, providing a new avenue for attaining atomic-level control of the surficial properties of the material using mild reaction conditions and no specialized equipment. Furthermore, redox-activated addition enables the use of new functional groups that would otherwise be reactive in other functionalization methods, promoting an abundance of opportunities to explore new applications of semiconductor materials.

Thesis Files