Date on Master's Thesis/Doctoral Dissertation

12-2019

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Degree Program

Chemistry, PhD

Committee Chair

Liu, Jinjun

Committee Co-Chair (if applicable)

Mendes, Sergio

Committee Member

Mendes, Sergio

Committee Member

Handa, Sachin

Committee Member

Thompson, Lee

Author's Keywords

molecular spectroscopy; laser spectroscopy; ultrafast; high-resolution; peroxy radicals; perovskite

Abstract

Studying molecular dynamics and chemical kinetics is important to understand the chemical behavior of renewable energy sources. Laser spectroscopy techniques are powerful tools for the identification and diagnosis of such processes. In this dissertation, using our laser spectroscopy techniques, biofuels and solar energy were targeted as renewable energy sources study. We have studied the ultrafast exciton dynamics for a series of methylammonium lead bromide (CH3NH3PbBr3) nanostructures, nanocrystals (NCs, 0D), nanowires (NWs, 1D), and nanoplatelets (NPs, 2D) as a promising solar energy materials.1-2 Aided by analysis of UV−visible absorption and photoluminescence spectra, features in the transient absorption (TA) spectra are assigned to different charge carrier processes, time constants of which are determined in fitting the transient kinetics. Immediately after photoexcitation, the charge carrier thermalization process occurs within the instrument response function time and results in a quasi-equilibrium distribution of charge carriers. It is followed by charge carrier cooling on a sub-picosecond time scale. The charge carrier recombination process obeys the rate law of a second-order reaction, which suggests that it occurs mainly via the bimolecular nongeminate recombination process. Dependence of the charge recombination rate constant on the initial charge carrier density has also been investigated in fluence-dependence measurements. The sensitivity of the recombination rate constant to initial charge carrier density is different for the three perovskite nanostructures, signifying the strong impact of quantum confinement on exciton dynamics. Using room-temperature cavity ring-down spectroscopy (CRDS), we obtained the spectra of the à ← X̃ electronic transition of tetrahydrofuranyl peroxy (THFOO•) and tetrahydropyranyl peroxy (THPOO•) radicals.3-4 The peroxy radicals were produced by Cl-initiated oxidation of tetrahydrofuran and tetrahydropyran. Quantum chemical calculations of the lowest-energy conformers of all regioisomers of these two peroxy radicals have been carried out to aid the spectral simulation. Conformational identification and vibrational assignment were achieved by comparing the experimentally obtained spectra to the simulated ones. The absence of α-THPOO• absorption peaks in the CRD spectrum is attributed to ring-opening due to its weak Cα′O bond.

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