Dissertation Defense: Joanna Xu

February 27, 2020

3:00 pm

Department of Chemistry
Mississippi State University

Thursday, February 27, 2020
3:00 PM
Mitchell Library Auditorium

Polarized Resonance Synchronous Spectroscopy for Characterization of Material Optical Properties

Optical spectroscopy is an essential tool for characterization of material 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 inner-filter-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, which fail to fulfil the assumptions required in the direct PRS2 method. Furthermore, BVPRS2 and PAOS advance the direct PRS2 method by providing evidence that not only on-resonance fluorescence (ORF), but off-resonance fluorescence also contributes to the fluorescence signal detected with resonance excitation and detection conditions. 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 (AuNPs), which are light absorbers and scatterers. In contrast, PAOS-assisted PRS2 was performed on fluorescent quantum dots (QDs), 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.

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