Author

Xiu Zhu Xu

Issuing Body

Mississippi State University

Advisor

Zhang, Dongmao

Committee Member

Mlsna, Todd E.

Committee Member

Wipf, David O.

Committee Member

Fitzkee, Nicholas C.

Committee Member

Emerson, Joseph P.

Date of Degree

5-1-2020

Document Type

Dissertation - Open Access

Major

Chemistry

Degree Name

Doctor of Philosophy

College

College of Arts and Sciences

Department

Department of Chemistry

Abstract

Optical spectroscopy is an essential tool for characterization of materials’ optical properties. Quantitatively understanding material absorption, scattering and emission properties is challenging with existing spectrometric and fluorometric techniques. The direct Polarized Resonance Synchronous Spectroscopy (PRS2) is a breakthrough to the existing spectroscopic techniques for differentiation and quantification of material absorption, scattering and emission properties including their light depolarizations and optical cross-sections. However, there are some limitations and unchecked assumptions in the direct PRS2 technique that need to be addressed during my PhD study. We first examined the effect of light scattering and absorption on their innerilter-effect (IFE) and validated that sample UV-vis extinction can be approximated as absorption extinction for IFE correction in the PRS2 data processing due to the high sensitivity of PRS2 to light scattering. In the case where such approximation may produce large error, iteration PRS2 can be included to decompose the UV-vis extinction into absorption and scattering components. Compared to the direct PRS2 that must rely on several assumptions, the recent developed Bandwidth Varied PRS2 (BVPRS2) and Polarized Anti-Stokes’, On-resonance, Stokes’-shifted (PAOS) spectroscopic techniques are self-contained methods universally amenable to all kinds of fluorescent materials. BVPRS2 or PAOS is necessary for fluorescent materials that possess non-zero scattering depolarization and wavelength dependent fluorescence depolarization. Furthermore, BVPRS2 and PAOS prove that not only off-resonance fluorescence also contributes to the fluorescence signal detected in PRS2 measurement. BVPRS2 offers indirect observation that fluorescence intensity increases quadratically, while scattering signal increases linearly as wavelength bandwidth expands. More importantly, PAOS allows direct visualization of the contribution from anti-Stokes’-shifted fluorescence, ORF and Stokes’-shifted fluorescence in the detected signals, revealing the origins of the off-resonance contribution. Finally, it is illustrated that different PRS2 methods should be applied to different types of optical materials to ensure accuracy and efficiency. Direct PRS2 was readily used to study gold nanoparticles, which are light absorbers and scatterers. In contrast, PAOS-assisted PRS2 was performed on fluorescent quantum dots, which are simultaneously light absorbers, scatterers and emitters. The presented PRS2 methodology and the new insights acquired from the materials investigated should be of great significance to material design and characterization.

URI

https://hdl.handle.net/11668/16669

Comments

Spectroscopy||Optical property||Scattering||Absorption||Fluorescence||nanoparticle||quantum dot

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