REALIZATION OF PLASMONIC 2D AND 3D NANOSTRUCTURES FOR SURFACE ENHANCED RAMAN SCATTERING DETECTION Yang Jing (B. Sci., Peking University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINNERING NATIONAL UNIVERSITY OF SINGAPORE 2015 Acknowledgements I would like to express my heart felt appreciation and gratitude to my supervisors, Prof. Hong Minghui and Dr. Teng Jinghua for their invaluable guidance and great support throughout my PhD course. I am grateful to Prof. Hong Minghui for his high standard on me. Without his dedicate care, my research would be slowed down. His strong motivation and passion in research inspire me to work hard. It is my pleasure to express my appreciation to all my lab members, Dr. Luo Fangfang, Dr. Du Zheren, Dr. Qin Fei, Ms. Wu Mengxue, Mr. Dacheng, Mr. Ma Zhijie, Mr. Chen Yiguo, Mr. Xu Kaichen, Mr. Gu Guoqiang, Mr. Liu Hongliang, Mr. Xu Jianfeng, Dr. Tang min, Dr. Chen Zaichun, Dr. Xu Le, Dr. Thanh Nguyen, Dr. Ng Binghao, Ms. Li Jiabao as well as my friends in IMRE, Mr. Huang Jian, Mr. Yang Chenyuan, Dr. Liu Hong, Ms. Chew Ah Bian, Dr. Liu Yanjun and Dr. Zhang Nan. I deeply appreciate the time shared with you and I wish you the best luck in your careers. I also thank Prof. Gong Qihuang, Prof. Gu Min, Prof. Qiu Chenwei and Prof. Chen Xudong for the valuable discussions and comments. I would like to express my gratitude for the financial support from the Singapore Peking Oxford Research Enterprise (SPORE). I also thank my roommate Dr. Chen Chao, Mr. Liu Xiao, Mr. Wang Guanfeng, Ms. Bian Jinwen, Ms. Shen Yanyan and Mr. Chai Bo for you great care in my daily life. Last but the most importantly, I would like to give my great thanks to my mother Madame Huang Zongqun, my brother Mr. Yang Lei and my lovely wife Ms. Yang Jie. Thank you for your love all the while which gives me the strength to carry on. II Table of Contents DECLARATION I Acknowledgements . II Table of Contents . III Summary .VI List of Tables VIII List of Figures .IX List of Publications . XIII Chapter Introduction 1.1 Background: Raman Scattering and SERS . 1.2 Various types of SERS detection systems . 1.3 Challenges for current SERS research . 11 1.4 Significance and scope 12 1.5 Organization of thesis 13 Chapter Electromagnetic Enhancement Theory for SERS Detection . 16 2.1 Localized Surface Plasmon Resonance . 16 2.1.1 Physics on Plasmonics 17 2.1.2 Excitation of LSPR 19 2.2 Electromagnetic enhancement of SERS . 21 2.2.1 Hot spots’ generation for SERS 23 2.3 Enhancement factor . 26 Chapter Methodology . 28 3.1 2D micro/nano-structuring of SERS substrates 28 3.1.1 Laser ablation . 29 3.1.2 Electron beam evaporation . 31 3.2 Fabrication of 3D SiNW SERS substrates . 33 3.2.1 Sample cleaning 34 III 3.2.2 Photoresist coating . 35 3.2.3 Laser interference lithography . 35 3.2.4 Reactive-ion etching . 38 3.2.5 Metal assisted chemical etching 39 3.2.6 Thermal annealing . 41 3.2.7 Redox reaction for nanoparticles’ decoration . 41 3.2.8 Probing molecules’ absorption 42 3.3 Characterization methods . 43 3.3.1. Scanning electron microscope and transmission electron microscope . 43 3.3.2 UV-Visible spectroscope and Raman spectroscope 46 3.4 FDTD simulation 49 Chapter 2D Laser Hybrid Micro/nano-structuring of Si Surfaces in Air and its Applications for SERS Detection . 51 4.1 Introduction 51 4.2 Experimental . 53 4.3 Results and discussion 54 4.3.1 Synthesis of nanoparticles during laser ablation in air 54 4.3.2 Single line laser ablation of Si for SERS detection . 57 4.3.3 Large area laser hybrid micro/nano-structures for SERS detection . 59 4.4 Summary . 70 Chapter Fabrication of 3D SiNW Array with Ag Film Coating/Ag Nanoparticles’ Decoration for SERS Detection . 72 5.1 Introduction 72 5.2 Experimental . 74 5.3 Results and discussion 75 5.3.1 SERS performance of Ag film coated SiNWs at different heights . 75 5.3.2 Comparison of Ag NPs’ decorated and Ag film coated SiNWs 81 5.4 Summary . 84 Chapter Aspect Ratio Effect in Ag Nanoparticles’ Decorated 3D SiNW for SERS Detection 85 6.1 Introduction 85 6.2. Experimental 87 IV 6.3 Results and discussion 88 6.3.1 Aspect ratio effect of Ag decorated SiNW array for SERS 88 6.3.2 A comparison of well-ordered and randomly arranged SiNWs 94 6.4 Summary . 97 Chapter Conclusions and Future Work 99 7.1 Conclusions . 99 7.2 Future Work 101 References . 104 V Summary Raman scattering is an important method to obtain the vibrational spectroscopic information of molecules by their inelastic scattering with photons. Surface enhanced Raman scattering (SERS) greatly enhances the Raman scattered light and makes it possible for applications in material analyses and low-concentration bio-chemical molecules’ detection. It is necessary to develop SERS substrates with high enhancement factors (EFs) and good signal homogeneities. Laser processing is a versatile tool to create micro/nano-structures for SERS detection. It provides high potential to rapidly fabricate SERS substrates in large area. The studies in this thesis design and fabricate 2-dimensional (2D) and 3-dimensional (3D) plasmonic nanostructures as SERS substrates by laser means. The SERS substrates can exhibit good signal enhancement and uniformity over the sample surface. A rapid two-step approach to fabricate 2D SERS substrates with high controllability in ambient air is developed. Dynamic laser ablation directly creates microgrooves on the Si substrate. During laser ablation, nanoparticles (NPs) are synthesized via the nucleation of laser induced plasma species and the air molecules. With Ag film coating, these NPs can function as hot spots for SERS. Microsquare arrays are fabricated on the Si surface as large-area SERS substrates by the laser ablation in horizontal and vertical directions. In each microsquare, it exhibits quasi-3D structures with randomly arranged and different shaped NPs aggregated in more than one layer. Uniform SERS signals are obtained by detecting the probing molecules adsorbed on the substrates. With the optimal laser fluence, the SERS signals show a uniform enhancement factor up to 5.5 x 106. VI To further develop the surface area of nanostructures to improve the SERS detection sensitivity, well-ordered Si nanowires (SiNWs) are applied as 3D SERS substrates. Laser interference lithography (LIL) and metal-assisted chemical etching are used to fabricate various aspect ratios of 3D SiNWs. Ag thin films are deposited on the SiNWs for SERS detection. By gradually increasing the height of SiNWs, a better SERS performance is observed. Compared to Ag thin film deposition, Ag NPs’ decoration via different fabrication techniques are carried out for SERS detection. AgNPs’ decoration by redox reaction exhibits the best SERS performance due to the generation of high-density hot spots. The aspect ratio effect of SiNWs with AgNPs’ decoration on SERS has been further investigated in detail. As the height of the SiNWs increases, the light scattering inside the structures is enhanced. The number of the probing molecules within the detection volume is increased as well. These factors contribute to higher SERS signal intensity. However, the light trapping effect for higher SiNWs may prevent the collection of SERS signals. An optimized aspect ratio ~ 5:1 (1 µm height and 200 nm width) for the SiNW array is found for SERS detection. The well-ordered SiNWs demonstrate much better SERS signal intensity and uniformity than the randomly arranged SiNWs with ultra-high aspect ratio. VII List of Tables Table 4.1 Composition determined using EDX of flat Si surfaces and the substrates fabricated at different laser fluences.························································56 VIII List of Figures Figure 1.1 The energy-level diagram in Rayleigh scattering and Raman scattering. ························································································2 Figure 1.2 Illustration of localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS). ··············································5 Figure 2.1 Illustration of the localized surface plasmon.···························25 Figure 2.2 The local field enhancement of Ag sphere monomer (radius: 100 nm) and Ag dimer (radius: 100 nm; inter-particle gap: 50 nm) at their LSPRs.··········· 29 Figure 2.3 The maximum localized E-fields observed in the hotpots of Ag dimers with different inter-particle gaps at 532 nm. ······································30 Figure 3.1 The fabrication process of 2D micro/nano-structured SERS substrates. ·······················································································35 Figure 3.2 Schematic of laser ablation system for 2D hybrid micro/nanostructures fabrication. ···························································37 Figure 3.3 The schematic of the electron beam evaporator. ·······················39 Figure 3.4 The fabrication process for 3D periodical SiNWs. ····················40 Figure 3.5 Different approaches to make the SiNW array SERS active. ········41 Figure 3.6 a) The Lloyd mirror configuration of a LIL setup and b) the light intensity distribution of the interference pattern. ··········································43 Figure 3.7 The LIL setup used for exposure. ········································44 Figure 3.8 SEM images of a) positive photoresist (S1805) and b) negative photoresist (N1407) patterns after the double LIL exposure and developing, respectively. ····················································································45 Figure 3.9 The skeletal formulas of 2-naphthylamine and 4-methylbenzenethiol molecules. ······················································································49 IX 2. Design of 3D structures with more spatial diversities: The surface area of the nanostructures plays an important role in the SERS performance of the substrates. The 3D structures investigated in this study are nanowires with different heights. Such structures exhibit large surface areas by greatly increasing the vertical dimension. To design 3D nanostructures not only with high vertical dimension but also with more structure diversities in lateral direction will further increase the surface area of the nanostructures, which contributes to the sensitivity of the SERS detection. The diversity in the lateral direction can induce the coupling of the metallic nanostructures and thus enhance the LSPR for SERS. However, the fabrication of complex 3D nanostructure will be very challenging. Advanced fabrication techniques, such as two photon polymerization, might be considered. 3. SERS substrates with hybrid metal-dielectric structures: As mentioned above, the fabrication of complex 3D structures remains challenging as well, especially in a large area assay. A compromising design of the 3D SERS substrate is the multilayer metal-dielectric hybrid nanostructures. Metal-dielectricmetal (MDM) oligomers are investigated in my previous study [122]. It is found that in the MDM oligomers, not only the E-component of incident field drives plasmon oscillations, but also the H-component plays an important role to excite magnetic plasmons. These magnetic plasmons give rise to a magnetic resonance in addition to classical Fano Resonance (FR). One key feature of the FR is the strong localization of the light among the nanostructures, which can be used for SERS detection, especially at single molecule level [123]. Besides the investigations only on the SERS substrates, the integration of those substrates into a portable bio-chemical SERS sensing device arouses more industrial 102 interests. Conventional SERS detection system includes the laser excitation, sensing platform (SERS substrates) and Raman spectroscope. Each component is an expensive individual setup, making the whole SERS detection system large size and complex. It prevents the practical field applications. Portable SERS detectors or even on-chip SERS sensor can be realized by integrating small size optical components, such as laser diode, waveguides, SERS substrates and micro-photomultiplier-tubes. The Si based SERS substrates studied in this work can play an important role for the device integration because the sensitivity greatly is improved by the surface structures. Meanwhile, the Si substrate is compatible with current semi-conductor fabrication process. 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Nano letters, 12(3), 1660-1667. 117 [...]... from 2D to 3D, the aspect ratio and surface area of the nanostructures play a vital role to improve the EF of the SERS substrates To fully develop the advantages of the 3D nanostructures and to optimize their vertical dimension for SERS remain as an intriguing topic for the research Thirdly, the reproducibility and signal uniformity of 2D and 3D SERS substrates need to be improved [44, 45, 49] for industrial... molecules, which can be used for material analyses and bio-chemical sensing [1] However, the Raman scattering cross section is intrinsically small in a normal Raman process, which prevents its practical applications In order to improve the Raman signal intensity, surface enhanced Raman scattering (SERS) has been widely investigated Using plasmonic nanostructures as the sensing platform, SERS techniques can... the influence of the aspect ratio is seldom investigated In this thesis, the aspect ratio effect is studied in detail, keeping the periodicity of the nanostructures constant The vertical dimensions of 3D SERS substrates will be optimized to enhance the SERS signal intensity, uniformity and reproducibility The results of the present study have high impact on the realization of 2D and 3D plasmonic SERS... sensitivity of the SERS systems [4042] The development in advanced lithography methods provide the possibility to create more versatile surface structures in terms of size and shape for SERS applications The 3D configuration of substrates can promote stronger coupling of the plasmonic nanostructures and greatly improve the electromagnetic enhancement One good attempt to further explore the capability of 3D. .. conventional 2D SERS substrates These substrates showed excellent enhancement in the detection of the analyte molecules at low concentrations However, in these studies, the surface structures were not in well-ordered arrangement, which might influence the uniformity of the SERS signals over the substrates Besides the studies to improve the SERS performance of the substrates by surface patterning, enhanced Raman. .. material science However, Raman scattering is extremely weak when compared to Rayleigh scattering because only a small fraction of the photons are inelastically scattered [10] Meanwhile, Raman scattering is also very weak when compared to fluorescence A typical non-resonant Raman scattering cross-section (~10-30 cm2) of a dye molecule is about 15 orders of magnitude lower than that of fluorescence (~10-15... fabricate 2D and 3D plasmonic nanostructures on Si substrates via laser processing for SERS detection applications The study will first explore a cost-effective and high speed method to fabricate 2D plasmonic SERS substrates Then the 3D SERS substrates will be investigated to further increase the SERS sensitivity In previous researches, the influence on SERS brought by laterally directional features of nanostructures, ... of Raman signal arises from the electromagnetic field enhancement at rough metal surfaces It is realized by the amplification of light due to the excitation of localized surface plasmon resonances (LSPRs) LSPRs are supported by the collective oscillation of charges in plasmonic nanostructures with the incident light at the resonant frequency [14, 15] The light concentration occurs at the surfaces of. .. well-ordered 3D SiNWs as the base for SERS substrates would benefit the understanding on the influence of SERS brought by the vertical dimension of the nanostructures It promotes the future design of nanowirebased or black Si based SERS substrates The introduced fabrication approaches can 12 be used for new types of plasmonic sensors with good periodicity and large surface area as well 1.5 Organization of thesis... fabrication and characterization of 2D and 3D plasmonic nanostructures as SERS substrates Chapter 2 is a brief description of the fundamental physics of SERS The origin of LSPR based on the metallic NPs is presented Their function as hot spots for the SERS electromagnetic enhancement is discussed The corresponding numerical simulations are also provided for a better understanding of the LSPR and hot spots’ . REALIZATION OF PLASMONIC 2D AND 3D NANOSTRUCTURES FOR SURFACE ENHANCED RAMAN SCATTERING DETECTION Yang Jing A THESIS SUBMITTED FOR. Background: Raman Scattering and SERS 1.2 Various types of SERS detection systems 1.3 Challenges for current SERS research 1.4 Significance and scope 1.5 Organization of thesis . Enhancement Theory for SERS Detection 2.1 Localized Surface Plasmon Resonance 2.1.1 Physics on Plasmonics 2.1.2 Excitation of LSPR 2.2 Electromagnetic enhancement of SERS 2.2.1