Raman and hyper-Raman spectroscopy with nanomaterials

Abstract

Raman spectroscopy is a powerful analytical tool widely used for material identification and characterization. This thesis focuses on experimental techniques for investigating the linear and nonlinear Raman scattering characteristics of molecules, with a particular emphasis on Surface-Enhanced Raman Scattering (SERS) materials and chiral optical (chiroptical) effects. The aim of this thesis is to investigate the chiroptical properties of chiral metasurfaces using surface-enhanced Raman scattering (SERS) with a focus on both linear and nonlinear Raman scattering phenomena, achieved using circularly polarized light. The main objective is to convince the reader that the ultimate results demonstrate the observation of a chiroptical effect in hyper-Raman scattering (a three-photon nonlinear scattering process) which was first predicted in 1979. In general, this thesis provides valuable insights for various applications in fields such as materials science, chemistry, biology, and environmental analysis.

The author's original intention was to create a thesis by portfolio of publishable papers. However, the thesis presented here is a hybrid format. Chapters 1 to 4 follow the format of a traditional thesis while Chapters5 to 7 follow the format of a portfolio thesis. The reader could choose to begin reading this thesis from Chapter 5 and refer back to previous chapters for detailed information about the background and basic techniques.

The thesis begins with a comprehensive overview of Raman spectroscopy in Chapter 2, laying the foundation for the subsequent chapters. It covers the principles of spontaneous Raman scattering, nonlinear susceptibility, surface plasmons, and surface-enhanced Raman scattering. These fundamental concepts highlight the potential of SERS materials for enhancing Raman signals and demonstrate the wide applicability of Raman spectroscopy in various fields.

Chapter 3 focuses on the experimental considerations and analysis techniques necessary for accurate Raman spectroscopy measurements. It provides a detailed description of the instrumentation, SERS-specific considerations, and the processing and analysis of Raman spectra. The Renishaw-inVia Raman microscope is discussed as a fundamental tool for conducting advanced experiments presented in the following chapters.

Chapter 4 investigates linear Raman spectra of bulk materials to showcase the versatility of Raman spectroscopy for material characterization. Crystalline and polymer samples are analysed, providing insights into the measurement capabilities of the inVia Raman microscope. This chapter demonstrates the power of Raman spectroscopy in analysing a wide range of materials and its potential for applications such as quality control, forensic analysis, and materials research.

Chapter 5 explores the application of nanomaterials for Raman sensing, specifically focusing on SERS substrates. Dense arrays of Ag and Au-based nanohelices, G-shaped Au nanostructured substrates, and Au Conglomerate Nanoparticles (Au-CNPs) SERS substrates are fabricated and characterized. The results highlight the significant enhancements in Raman signals achieved with these nanomaterials, enabling ultrasensitive detection and analysis. The chapter illustrates the potential of SERS substrates for applications such as chemical sensing, environmental monitoring, and biomedical diagnostics. Crucially, this chapter introduces Raman measurement techniques with circularly polarised light, laying the foundations for the observation of a chiroptical effect in hyper-Raman scattering in Chapter 7.

In Chapter 6, the thesis extends the investigation to hyper-Raman spectroscopy. The enhancement in hyper-Raman scattering with dense arrays of Ag and Au-based nanohelices are explored with two different species of molecules (crystal violet and rhodamine 6G). The results demonstrate the ability to obtain complementary spectroscopic information about the symmetry of vibrational modes. This chapter also shows that surface chemical interactions can be a significant factor in determined SERS performance in hyper-Raman spectroscopy. Crucially, this chapter builds on from Chapter 5 with the nonlinear spectroscopic capabilities that were essential to the observation of a chiroptical effect in hyper-Raman scattering in Chapter 7.

Chapter 7 presents an observation of chiroptical effect in hyper-Raman scattering. As with the previous two chapters, this effect was achieved by coating achiral molecules (crystal violet) to act as reporter molecules on chiral metasurfaces which provide a SERS effect. The chiroptical effect is demonstrated by measuring the strength of the hyper-Raman signal from achiral molecules for left- and right-handed circularly polarised light on the chiral metasurface. The experiment heavily builds-on from those presented in Chapters 5 and 6 with the inclusion of experimental automation.

The thesis concludes with Chapter 8, summarizing the research findings, major contributions, and future directions. It emphasizes the significance of the experimental techniques developed in this research, highlighting their potential for chiroptical Raman spectroscopy. Future research directions include further optimization of SERS materials and instrument development with real-world applications.

Date of Award	4 Dec 2023
Original language	English
Awarding Institution	University of Bath
Sponsors	Renishaw plc
Supervisor	Daniel Wolverson (Supervisor) & Ventsislav Valev (Supervisor)