# Enhancing SERS Intensity by Coupling PSPR and LSPR in a Crater Structure with Ag Nanowires
## 1 Introduction  
Surface plasmon resonance (SPR) is a phenomenon in which free electrons within the metal surface oscillate collectively when light is irradiated on the metal-dielectric interface [1, 2]. SPR is classified into two types: propagating surface plasmon resonance (PSPR), which occurs in the form of surface plasmon polaritons (SPPs) along smooth metal-dielectric interfaces, and localized surface plasmon resonance (LSPR), which is generated locally between metal nanostructures [3]. There are several methods for generating SPR, such as making micro- and nano-sized holes [4, 5] or creating a rough surface using nanoparticles [6, 7], nanospheres [8, 9], and nanowires [10, 11] on metal films such as Ag and Au. The electric field is enhanced through resonance between the incident light source that has a specific wavelength, and either a sharp corner of the metal nanostructures or the narrow gap between the nanoparticles, which is called the \"hot spot\" [12]. The electromagnetic field can, therefore, be enhanced by generating PSPR and LSPR through these various methods, but the enhancement effect is limited when only one type of SPR exists. A higher electromagnetic field enhancement can be obtained by coupling PSPR with metal micro and nanostructures and LSPR by hot spots on the surface [13]. For example, [PERSON] et al. obtained ultra-high field enhancement through coupling localized and propagating surface plasmons by fabricating grating geometry structures and depositing nanoparticles [14]. The enhanced electromagnetic field causes a surface-enhanced Raman scattering (SERS) phenomenon in which a weak Raman signal due to a small scattering cross-sectional area is greatly amplified [15]. SERS is a key interest of many researchers because it can be applied to various fields, such as marine environment, medicine, food science, biology, and optics, as a sensitive technology for detecting very low concentrations of molecules [16; 17; 18; 19]. In particular, SERS is very useful for searching various poisons or toxins present in seawater or ocean creatures. [PERSON] et al. analyzed saxitoxin using cysteine-modified Au nanoparticles [20]. [PERSON] [PERSON] et al. detected poly-nuclear aromatic hydrocarbons with high carcinogenicity using Ag colloid as a SERS probe [21]. One method of creating SERS is to use Ag nanowires (AgNWs), which are one-dimensional metal nanostructures with a high aspect ratio. They are widely used as transparent electrodes due to their excellent electrical conductivity and optical transparency [22]. Additionally, they are suitable nanostructures for producing the SERS phenomenon due to their large surface area, good crystallinity, and wide plasma resonance band between the UV to the near-infrared region [23; 24]. [PERSON] et al. enhanced SERS intensity by crossing a pair of metal nanowires [25]. [PERSON] et al. fabricated a layer-by-layer assembly of AgNPs to obtain high-density hot spots [26]. [PERSON] et al. fabricated a simple SERS architecture by forming hot spots between a metal film and a nanowire [27]. In this case, the substrate for SERS requires a uniform distribution of nanostructures for the uniformity and stability of the Raman signal. To create a uniform distribution, the self-assembled monolayer, electrochemical deposition, and spray coating methods are adopted [28; 29; 30]. Within this study, the spray coating method was chosen and applied, which deposits relatively uniformly over a large area. To further enhance the SERS effect, a volcanic crater type structure capable of condensing light was fabricated, and a metal film and the AgNPs were sequentially deposited therein. The deposited AgNPsNWs form a high-density hot spot due to the gap between the AgNPsNWs and the metal film. Thus, the LSPR generated by a hot spot can be coupled to the PSPR generated by the metal film deposited inside the crater.