## 3 Results and Discussion  
To the NW's diameter optimization, the plasmon resonance in cylindrical two-dimensional (2D) NW under 785 nm laser excitation was simulated using electromagnetic PIC-code KARAT [35]. Longitudinal NW dimension was considered much larger than the transverse one. Figure 2 shows the dependence of the normalized electric field amplitude (E/E\({}_{0}\)) on single NW diameter at various distances from its surface; here, E is the amplitude of resulting electric field near the single NW, E\({}_{0}\) is the amplitude of an incident laser electric field. To describe the electrodynamic properties of NWs, the Drude model was applied taking parameters for silver with laser excitation wavelength equal to 785 nm: a plasma frequency of 8.78 eV and damping constant of 0.02 eV [36]. Simulated E/E\({}_{0}\) dependence on the Ag-NW diameter at various distances from its surface is present on Figure 2 and it is consistent with another study in which the field enhancement near the Ag-nanosphere is present [11]. It is seen that the maximum E/E\({}_{0}\) was obtained at the Ag-NWs diameter ranging from 100 to 120 nm for all distances from the Ag-NW surface. Based on the obtained results, the diameter of the Ag-NWs was chosen to be 100 nm and the substrates were synthesized. After the TM removal and following washing the substrates with the array of Ag-NWs, those substrates were left in distilled water. In this state, the Ag-NWs array remains uniformly distributed and maintains the structure of the initial TM. As distilled water was evaporated, Ag-NWs were learnt together due to the capillary force, forming a hot spot  
Figure 2: Simulated dependence of the normalized electric field amplitude (E/E\({}_{0}\)) on the Ag-NW diameter at various distances from its surface: (**1**) 2.5 nm; (**2**) 5 nm; (**3**) 10 nm. between their tips and leading to the formation of a large-area ordered array of Ag-NWs bundles along the irradiation strips of the TM. The substrate drying process and the characteristic leaned microstructure can be observed in real-time mode using an optical microscope. Figure 3 shows photos of the substrate surface with the Ag-NWs length of 12 \(\upmu\)m in a wet form (Figure 3c) and after air-drying (Figure 3d). At the moment of complete drying of the substrate, surface tension will cause the Ag-NWs to lean towards their nearest neighbors, thus creating bundles. The length of the Ag-NWs was investigated by scanning electron microscopy (SEM) (Figure 3b). It also should be noticed that the morphology of the produced SERS totally reproduces the structure of the using TM (Figure 3a). It was found that the largest length corresponds to bundles with a greater number of Ag-NWs due to their high elasticity. As a result, bundles consisting of dozens of densely leaned Ag-NWs formed without the breakaway of the Ag-NWs from the substrate. With a decrease in the length of the Ag-NWs, the number forming the bundle became smaller, and at a length in the range of 1.5-3 \(\upmu\)m it was possible to observe single leaning Ag-NWs. When the length of the Ag-NWs was less than 1 \(\upmu\)m, single standing Ag-NWs were observed on the surface without leaning. Based on the chemical compositions of studying  
Figure 3: SEM top view of the TM (**a**) and the surface of the substrate with an Ag-NWs bundles array with an Ag-NWs diameter of 100 nm and their length of 12 \(\upmu\)m. SEM top view of the Ag-NWs bundles array (**b**). Photos of the substrate are made by optical microscope (**c**) when it is in a wet state, (**d**) of the same area after drying. substrates obtained from EDX analysis we can conclude that there is no influence of the copper layer and we end up with the Ag-NWs array on Ag-substrate. To demonstrate the effect of various mechanisms on the amplification of the analyte SERS-signal, a series of samples with different Ag-NWs lengths were tested. The substrates were divided into three groups along the Ag-NWs length. Each group was split into \"dry\" and \"wet\" (Figure 4).