UNDERSTANDING STRUCTURAL INFLUENCES IN GOLD MONOLAYERED-PROTECTED CLUSTERS USING MAGNETO-OPTICAL SPECTROSCOPY AND TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPY
Open Access
- Author:
- Mitra, Utsab
- Graduate Program:
- Chemistry
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- June 13, 2019
- Committee Members:
- Kenneth Luther Knappenberger, Jr., Thesis Advisor/Co-Advisor
John B Asbury, Committee Member
Benjamin James Lear, Committee Member
Venkatraman Gopalan, Committee Member - Keywords:
- Nanoclusters
Ultrafast Spectroscopy
Two-dimensional Electronic Spectroscopy
Magneto-Optical Spectroscopy
Superatoms
Gold Clusters
Zeeman Effect - Abstract:
- Variable Temperature-Variable Field Magnetic Circular Photoluminescence (VTVH-MCPL) has been developed to characterize the unique structure-dependent optical properties of monolayer-protected gold nanoclusters. VTVH-MCPL compliments the absorption-based Magnetic Circular Dichroism (MCD) technique and is capable of analyzing the electronic Landé g-factors, zero-field energy splitting and the relative A, B and C-term contribution to the photoluminescence response of the system. Au25(SC8H9)18 MPC has been taken as a model system to provide a state-resolved characterization of the electronic structure and relaxation dynamics by the application of such a technique. VTVH-MCPL has been able to characterize at least three relaxation channels contributing to the global photoluminescence, a low energy intraband transition (1.67 eV) translating to the relaxation between superatomic-D to superatomic-P orbitals localized in the core of the MPC, another two higher energy interband transitions (1.72 eV and 1.96 eV) corresponding to relaxation between superatomic-D to lower energy states localized in the semiring of the cluster. Such a study helped in extracting information such as at lower temperature (<60K), the preferential pathway of relaxation is via core-based states, while at higher temperature (>60K) a ligand-based emission pathway is preferred. Explanation of such observation has been done in terms of relative vibrational coupling strengths of the electronic states involved. Quantification of spectroscopic term-symbols relating to the total angular momentum and the Landé g-factors of each electronic states reveals the structure-dependent electronic properties of Au MPCs. 2D Electronic Spectroscopic studies on Au25(SC8H9)19 MPC introduces a second dimensional and enables to extract further information about the system. Evidence of a potential energy cross-curve in the LUMO + n levels has been observed through the analysis of the time domain measurements, which displayed a sigmoidal feature when the amplitude of the 2D cross-peaks were plotted against excitation energies. Evidence of a fast-oscillatory mode of frequency 12 THz has also been observed using 2D Electronic Spectroscopy.