Probing Surface Electrostatics on Metal Chalcogenide Nanocrystals Using Surface-Bound Metal Carbonyl Fragments

This dissertation describes efforts to develop spectroscopic reporters to understand better the surface redox chemistry of colloidal semiconductor metal chalcogenide nanocrystals. Through the use of surface-bound metal carbonyl organometallic fragments, changes to the surface electrostatics were observed through IR spectroscopy. Chemical reduction and oxidation, photoinduced charge trapping, and modifications to the metal-to-chalcogenide surface stoichiometry caused observable shifts to the C–O stretching frequencies of the functionalized metal carbonyls.

Chapter 2 describes the synthetic pathways for functionalized CdE-CdM(CO)₄ NCs (E = Se, S; M = Fe, Co) through X-type ligand exchange. Through the addition of increasing equivalents per nanocrystal of chemical reductant and oxidant, the relationship between the change in C–O stretching frequencies was related to the number of electrons added or removed from the surface. The differences in the reducing potential of the [Fe(CO)₄]^2- precursor were explored through the initial energies of the C–O stretching band and the change in C–O stretching frequencies from the addition of chemical reductant and oxidant was compared between the samples. The C–O stretching force constants and group dipole moment derivatives for each species were also calculated and showed upon the addition of reductant, reduced π contribution from the surface Cd to the metal carbonyl center. Altogether, these results showed that CdSe-CdFe(CO)₄ NCs were sensitive to changes to surface reduction and oxidation and that the metal carbonyl bonding interactions could be tuned by altering the NC surface.

Chapter 3 describes a new synthetic pathway to metal carbonyl functionalized metal chalcogenide nanocrystals through Z-type addition of [CdFe(CO)₄]. The Z-type exchange mechanism for functionalization was explored. The surface metal-to-chalcogenide ratio was changed through L-type assisted Z-type displacement, and the changes to the surface electrostatics were observed through IR spectroscopy. The displacement isotherms of CdX₂ from the surface were also calculated and showed heterogeneity in the effect each binding site had on the surface electrostatics. These results were used to demonstrate the use of CdSe-CdFe(CO)₄ NCs as a probe sensitive to changes to the surface electrostatics induced by modifying the surface metal-to-chalcogenide ratio.

Chapter 4 describes the instability of cobaltocene under certain photochemical conditions. The released cyclopentadiene radical was trapped and observed through EPR spectroscopy. The wavelength dependency of the radical release was detailed. A reactive [CoCp^I] species was proposed to be generated and was shown to facilitate the catalytic trimerization of alkynes.

Appendix A describes efforts to probe the surface of metal chalcogenide nanocrystals during photoexcitation and photocatalytic conditions. Appendix B contains additional characterization of precursors and photocatalysis products.