Photochemical Sciences Ph.D. Dissertations

Computational Raman Spectroscopy of Heterogeneous Organic-Inorganic Interfaces

Date of Award

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Alexey Zayak (Advisor)

Second Advisor

Peter Lu (Committee Member)

Third Advisor

John Cable (Committee Member)

Fourth Advisor

Hee Soon Lee (Other)

Abstract

Nanoscale molecular structures are central to many current and emerging technologies. Integrating different materials into hybrid structures allows for combining the individual materials' strengths while compensating for deficits. However, an in-depth characterization of such heterogeneous structures is a significant challenge. Their physical and chemical properties vary on the scale of a few chemical bonds, which is extremely difficult to access by most existing spectroscopic techniques. "Raman scattering" is an optical characterization tool that gives precise chemically specific information about molecules' vibrational energies and provides unique spectral signatures for any chemical species. The development of Surface Enhanced Raman Spectroscopy (SERS)-like techniques drastically increased the scattering Raman cross-section, which made this approach applicable at the nanoscale. A new aspect that was not present in the conventional Raman is that SERS reports not only about the molecule identity, but also about its local chemical environment, encoded in the Chemical Enhancement (CE). This new information remains a complex theoretical problem. However, with good computational insight, this phenomenon may become a unique tool for studying the chemistry of interfaces. The first part of this work presents an example of a theoretical-computational study that contributes to a real-life problem that demands a precise understanding of the atomic-scale chemical interface. The project utilizes the power of the Density Functional Theory (DFT) to represent a complex chemical system with great detail. The computational data employs to explain new experimental data based on the well - understandable parameters of the interfacial electronic structure. The second part of this dissertation focuses on the Chemical Enhancement mechanism in SERS by using a two-state model. The results of this work show a stronger electronic coupling between an organic molecule and graphene quenches the Raman activity of the organic molecule, an opposite effect with previously observed on metals and semiconductors. The quenching attributes to the coexistence of two damping mechanisms. One is related to the orbital relaxation or the environment's response to the electron perturbed by the external electric field (electron damping). Another mechanism is the damping of molecular vibrations caused by the dynamic interfacial charge transfer driven by the vibrations (phonon damping).

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