Understanding Surface-enhanced Raman Scattering Using a Raman Bond Model
Open Access
- Author:
- Chen, Ran
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 13, 2022
- Committee Members:
- Benjamin Lear, Major Field Member
William Noid, Major Field Member
Lasse Jensen, Chair & Dissertation Advisor
Michael Janik, Outside Unit & Field Member
Philip Bevilacqua, Program Head/Chair - Keywords:
- Surface-enhanced Raman Scattering
Plasmonics
Raman Bond Model
Electronic Structure Methods
Density Functional Theory - Abstract:
- Surface-enhanced Raman scattering (SERS) is widely applied because of its high sensitivity and chemical specificity. SERS enhancements arise from two complementary mechanisms: the electromagnetic mechanism (EM) and the chemical mechanism (CM). EM helps to explain the high sensitivity, and the chemical specificity is mainly explained by CM. In EM, SERS enhancements are explained by local field enhancements due to surface plasmons and SERS enhancements approximately scale with field enhancements to the fourth power. In CM, SERS enhancements are attributed to bonding interactions between molecules and metal, and electronic structure simulations are usually applied to evaluate chemical enhancements. CM models have been developed to interpret chemical enhancements based on electronic structure simulations, and a transition based interpretation is shared by most of the models. In the transition based interpretation, chemical enhancements are explained by molecular transitions with decreased energy gaps and new emerged charge transfer transitions. Such transition based interpretation works well when specific transitions can be identified as the main contributions to the enhancements. However, when a large number of transitions contribute, a quantitative explanation requires an analysis of many transitions while a simple explanation is made by focusing on few transitions. To build a simple and quantitative interpretation of chemical enhancements, we develop a Raman bond model. The Raman bond model adopts Hirshfeld partitioning and charge density localization to express polarizability derivatives as charge flow modulations named Raman bonds. Chemical enhancements can accordingly be explained by Raman bonds near molecule-metal interfaces, and a charge based interpretation is obtained. The Raman bond model is demonstrated to work consistently for different types of molecule-metal bonds, different resonance conditions, and model systems with or without periodic conditions. Model systems consisting of pyridine, thiol, carbene, and CO molecules with localized clusters or periodic slabs are studied at different excitation frequencies and applied electric fields. Various factors such as electric fields, chemical substitutions, molecular structures, excitation frequencies, surface coverages, and surface roughness are consistently explained by Raman bond distributions near molecule-metal interfaces. Stronger chemical enhancements can be achieved by increasing the Raman bond conjugation near molecule-metal interfaces. The Raman bond model also helps quantify different enhancement mechanisms by mapping Raman bonds in the molecule, the inter-fragment bond, and the metal to enhancement contributions of molecular, charge transfer, and plasmon resonance. The mapping interprets EM as charge flow modulations in the metal. Such interpretation of EM may provide insights for building one holistic theory of SERS which includes CM and EM in a full quantum mechanical theory. The Raman bond model also reveals a linear correlation between chemical enhancements and inter-fragment charge flows to the fourth power. Based on the correlation, we show that SERS spectra of localized and periodic systems normalized by inter-fragment charge flows can be unified.