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Nilusha Perera Thesis

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Study of Far-Side Geometrical Enhancement in Surface-Enhanced Raman Scattering A thesis submitted for the degree of Doctor of Philosophy by Nilusha Perera Centre for Quantum and Optical Science Faculty of Science, Engineering and Technology Swinburne University of Technology Melbourne, Australia 2017 “One of the basic rules of the universe is that nothing is perfect. Perfection simply doesn't exist... Without imperfection, neither you nor I would exist.” Stephen Hawking i Dedication This thesis is dedicated to my parents (Thaththa Edward and Amma Desy) who have provided me endless love and courage to follow my dream over the years and to my loving husband Rajith who has always been with me throughout this wonderful journey of PhD. iii Abstract Surface-enhanced Raman scattering (SERS) is a promising method for detection of biomolecules and trace chemicals. The well-established concepts of electromagnetic and chemical enhancements provide the theoretical background of SERS. Over the years since the discovery of SERS, a few research groups have observed an additional enhancement in the SERS signal when exciting the SERS substrate through an underlying transparent dielectric substrate (referred to here as ‘far-side geometrical enhancement’). The experimental evidence was not only limited to planar SERS substrates, but also observed for some SERS-active angled fibre tips. The first significant attempts to explain the origin of this geometrically-enhanced SERS signal appeared in 2013. The supporting information in those works was limited to a single excitation wavelength and provided only qualitative agreement between the proposed theories and the experimental results. Consequently, there remains a need to more carefully study this extra enhancement by addressing some of the gaps in the proposed theories, and comparing the theoretical predictions against more detailed experiments. This information will be useful for a range of applications where it is necessary to understand the effective optical behaviour of thin plasmonic films, including optical windows and in designing miniaturised optical fibre sensors for remote sensing applications. Therefore in this thesis, the far-side geometrically-enhanced SERS signal was further explored in a more rigorous theoretical and experimental basis. Nanostructured silver films fabricated on transparent substrates, using oblique angle deposition (OAD) at an angle of 86° with a nominal thickness of 100 nm, were used as the SERS substrate. The effective optical constants of transparent substrates and the nanostructured Ag layer were carefully determined by spectroscopic (standard) ellipsometry and generalised Mueller matrix ellipsometry, respectively. The generalised Mueller matrix ellipsometry data analysis revealed a uniaxial optical response in the Ag OAD86° 100 nm film, with the optical axis inclined towards the source of the deposition vapour flux. The wavelength-dependent effective optical constants as determined by the uniaxial model were represented by complex numbers with real and imaginary parts representing refractive index and extinction coefficient, respectively. The effective optical constants differed substantially v from those of the bulk silver, with a significant localised surface plasmon resonance (LSPR) peak in the extinction coefficient. As has been identified previously, this geometrical enhancement appears to follow the E 4 approximation in SERS. Therefore, the wavelength-dependent complex effective optical constants were incorporated in the Fresnel equations to calculate the corresponding electric fields at the metal-dielectric boundary for near-side and far-side excitation of the SERS substrate. The theoretical model derived with the complex reflection and transmission coefficients in the Fresnel equations consistently showed a geometrically-enhanced SERS signal for the far-side excitation compared to the near-side. Ellipsometrically-determined optical constants of Ag OAD86° 100 nm films deposited on glass and sapphire substrates were used to model the substrate material dependence. Further, the extended Fresnel model predicted that the highest geometrical enhancement factors should arise close to the LSPR peak of the sample, with a significant red-shift of the peak for increasing refractive index of the underlying substrate. Silver OAD86° 100 nm substrates functionalised with thiophenol were used to study the dependence of the geometrically-enhanced SERS signal on the excitation wavelength, supporting substrate material and excitation angle of incidence. The experimental SERS results confirmed that the highest geometrical enhancement factors up to 3.2-3.5 are seen closer to the LSPR of the metallic nanostructures, with a slight red-shift from the theoretical predictions. Both the theoretical predictions and experimental results generally agree with each other for the dependence on the wavelength and substrate material, although precise quantitative comparison was not achievable due to the relatively large variability (± 10-15%) in the SERS measurements. However, increasing angle of incidence showed a decrease in the experimental geometrical enhancement factor, which contradicts the gradual increase predicted by the extended Fresnel model. Further validation of the extended Fresnel model will require substrates with reduced variability and avoiding systematic errors when measuring the dependence on angle of incidence. In view of the results of this study, it appears likely that the far-side SERS signal is amplified by the net enhancement in the electric field at the metal-dielectric substrate interface compared to the near-side excitation. Given this improved understanding of the far-side geometrical SERS enhancement, the potential for further signal amplification and optimisation for practical sensing applications can now be considered. This study also provides a pathway to use ellipsometry to characterise different SERS substrates when investigating the far-side geometrically-enhanced SERS signal. vi Acknowledgements This document is not only a thesis but also represents the most challenging, yet inspiring and exciting chapter of my life. I have been blessed enough to be surrounded by lovely people and the work of this thesis would not have been possible without the support of most of them. A big thank you to my principal coordinating supervisor Prof. Paul Stoddart who has not just been my supervisor but also my mentor. I am forever grateful to have met a person like you who gave me the space and time to mature during tough times of the PhD when I was planning to quit. Thank you Paul for guiding me through the challenges and problems of my project with your expertise knowledge and experience. No meeting was ever boring and you always had the power to lift my spirits when the days were dark and gloomy. I could not have hoped for a better supervisor for my PhD project. It has been a privilege to work with a person like you, and your inspirational thoughts will forever be a treasure of my life. I would like to thank Prof. Russell McLean, Prof. Margaret Reid and Dr. Qiongyi He from Centre for Quantum and Optical Science (CQOS) for introducing me to the Applied Optics Group and giving me the opportunity to work with them. A special thank you to Prof. Saulius Juodkazis for the advices and support given during my PhD candidature as the coordinating supervisor of the project. It has been a great experience to share the work with the Applied Plasmonic Group at Centre for Micro- Photonics (CMP). I kindly thank Mr. Keith Gibbs for all the discussions we had, for the time you especially dedicated to read my writing and your efforts to provide me constructive feedback. Some days were hard and it was not easy to convince you with my vocabulary. It has been a pleasure to have met an enthusiastic physicist like you to discuss my work. Thanks again for your insights and suggestions. I want to say thank you to Prof. Sally McArthur, Prof. Peter Cadusch, A/Prof. Scott Wade, Prof. Simon Moulton and Dr. Michelle Dunn for their constructive feedback during my presentations at group meetings. vii I also owe a special thanks to my panel members of the PhD progress reviews including A/Prof. Scott Wade, Prof. Simon Moulton, A/Prof. James Chon and Prof. Damien Hicks for their feedback and suggestions based on diverse experience and immense knowledge on different research fields. I am grateful to my Swinburne colleagues Ms. Jennifer Hartley, Dr. Ricardas Buividas, Dr. Gediminas Seniutinas, Dr. Gediminas Gervinskas, Dr. Chiara Paviolo, Dr. Nirali Shah, Dr. Smitha Jose, Dr. Myintzu Hlaing, Dr. Muhammad Awais, Dr. Prince Surendran, Ms. Emma Carland, Dr. Hitesh Pingle, Mr. Nainesh Godhani, Mr. Armandas Balcytis, Mr. Xuewen Wang, Mr. Philip Kinsella, Dr. Sepideh Mina, Dr. Martina Abrigo, Dr. Hannah Askew, Dr. Priyamvada Venugopalan, Mr. Racim Radjef, Ms. Litty Thekkekara, Ms. Elena Goi, Mr. Md Mamun and Mr. Ahmed Osman. Thank you for providing such a friendly and cheerful environment in the research labs and around the Swinburne premises. I would like to acknowledge the huge support given to me by Dr. Sylvia Mackie, Dr. Brenton Hall, Ms. Tatiana Tchernova, Ms. Johanna Cooper and Ms. Susan Lloyd-Angol for different tasks throughout this entire period at Swinburne. This PhD journey would not have been possible without the financial support received through the Vice Chancellors Research Scholarship from Swinburne University of Technology. Thanks again Swinburne, for giving me the opportunity to fulfil
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