Structure and material engineering to investigate enhancement mechanism of surface-enhanced raman scattering (SERS)

Abstract

Raman spectroscopy provides many advantages, such as high accuracy, high speed, and non-destructiveness for the characterization of a wide range of materials. However, despite these outstanding characteristics, Raman spectroscopy fails to offer sufficient detection sensitivity, because of its weak signal intensity. Surface-enhanced Raman scattering (SERS) is considered the most successful strategy for significantly enhancing the scattering cross-section and, consequently, the signal intensity. The proposed theory about origin of SERS effect can be divided into two mechanisms. One is the electro-magnetic enhancement which is the enhancement caused by the surface plasmon resonance generated on the novel metal surface. The other is the chemical enhancement by the interaction between molecular electronic state and metal surface. If an effective SERS substrate is fabricated through the understanding of the SERS effect, it is possible to maximize the advantages of SERS to realize label free detection, safe and fast detection using laser, and single molecule detection ability using high sensitivity. However, in order to fabricate an effective SERS substrate, it is necessary to develop and apply a technique of fabricating a large-area pattern with sub-10 nm in order to maximize the SERS effect. Also, better understanding of SERS mechanism is necessary. In the chapter 2, we report a bottom-up pathway to fabricate dual-length-scale Au nanostructures that are constructed with concentric Au nanorings with sub-10 nm plasmonic nanogaps embedded in large-area Au nanomeshes. This hierarchically constructed nanostructure is achieved by the synergic combination of sub-micron-scale polystyrene (PS) particle self-assembly and sub-10 nm BCP self-assembly without using high-cost top-down lithography tools such as optical scanner or electron-beam exposure system. Self-organized PS spheres are used to generate signal-enhancing plasmonic nanohole arrays, which, in turn, serve as circular topographic templates for the for-mation of BCP patterns and sub-10 nm plasmonic nanogaps after pattern transfer. In this manner, the SERS effect from the rings-in-mesh structure benefits from both the plasmonic nanomesh structures and circular nanogap structures at the sub-10 nm regime. Moreover, the isotropic features of the nanoscale motifs (holes and rings) used to construct the hierarchical nanostructure provide additional advantages of uniform SERS signal intensity regardless of incident directions. In the chapter 3, we successfully developed the fabrication method that satisfies synergistic condition for highly reproducible and extremely sensitive SERS substrate by matching the plasmonic resonance peak with excitation laser wavelength and generating the ultra-high density of “hot spot” on measured area. By adapting the electrochemical reduced anodic (RA) method on flat gold films, the nanoporous gold structures with ultra-high density sub-5 nm gaps were successfully fabricated. Also, by controlling the reduction current on oxidized gold films, the resultant nanoporous gold structure exhibit the tunable plasmonic properties, which further increase the SERS activity at measured laser wavelength. As a result, a remarkable enhancement factor with a $10^5$ on entire measured area is achieved where the near-electric field is maximized as the nanogap size is decreases. Moreover, the resulting nanostructure based SERS substrate also acted as Hg2+ ion sensor by aptamer functionalization. By fully utilizing the surface area of nanoporous gold structures, the Hg2+ ion selective aptamer was successfully attached on the sub-5 nm gap nanoporous gold structures. This aptamer based SERS sensor shows excellent selectivity and sensitivity even on the six different ion mixture condition and also can be used as a sensor in conventional beverage. In the chapter 4, we report the SERS effect of MoTe2, MoS2 and WS2 which have various band-gap from 1.1 eV to 2.1 eV at their monolayer states. Also, Copper Phthalocynine (CuPC) and these layered materials can mutually interact as a face-on configuration, enabling the uniform signal enhancement over the entire samples. The different SERS effect for these layered materials suggests that energy level similarity of two-dimensional semiconductor materials is one of the dominant factors in the degree of signal enhancement. Moreover, it was confirmed that additional SERS enhancement effect could be obtained by combining the metal nanostructure and two-dimensional semiconductor materials. The systemic observation of CM effect could be helpful in designing of more effective SERS substrates for practical sensing applications. This chap-ter provides not only the clear explanation of SERS enhancement mechanisms but also realization of hybrid SERS substrate. In the chapter 5, we introduce novel self-electrical modulated SERS based substrate. This substrate is real-ized by combining the SERS active structures with electron transfer and energy conversion system which is triboelectric device. The SERS signal can be enhanced up to 8 times by simply physical contact the triboelec-tric device with a finger, and detection is carried out in the air condition. By selection of electrically different materials, the degree of transferred charge density is can be enhanced. Also, by changing the contact area between SERS active substrate and counter material, the degree of enhancement is further enhanced. The key enhancement mechanism is the electron transfer to SERS active substrate which can enhance the electric field in SERS substrate. The enhanced charge density is naturally released in 30 minutes after triboelectrification. Also, we demonstrate theuniversality of this system with explosives, biomolecules and organic dyes, at trace levels.Our substrates are also easy to fabricate, reusable at various condition. We believe that this study will help better understand the SERS enhancement and fabricate an efficient SERS substrate. Through the extension of this study, it is expected that the application field can be expanded to practical fields such as detection of trace amounts and detection of disease factors.