## 3 Results and Discussion
As there is an insignificant difference in the Ag-NWs lengths along the substrate and some Ag-NWs are shorter than another, after the drying process the tip of one Ag-NW will be close to the side surface of its neighbor Ag-NW. For the \"wet\" substrates there will be a nanometer gap between the Ag-NW tip and the side surface of its neighbor, and for the \"dry\" ones there will be a point of contact between them. \"Short NWs\" are not elastic and the capillary force is not enough to lean them, and the increase of analyte SERS-signal is very slight (Figure 7c). The optimal aspect ratio is found to be 160. Figure 6: SERS-spectra of 10 \(\upmu\)g/mL 4-MPBA adsorbed on the “dry” and “wet” substrates with Ag-NWs with a diameter of 100 nm collected in 1571 cm\({}^{-1}\). To study the effect of the gap on the electric field enhancement the simulation was performed by code KARAT for the case of \"long\" and \"medium-sized\" Ag-NWs with a monochromatic incident laser wavelength of 785 nm. The laser pulse with linear (E\({}_{\text{x}}\), B\({}_{\text{y}}\)) polarization propagated from the top down along z-direction (Figure 8), where the amplitude of the incident wave was taken as a unit of arbitrary units (a.u.). The normalized electric field amplitude E/E\({}_{0}\) was obtained by the following procedure: the field amplitudes were calculated at all points of the computational area at each moment of time, then the ratio of the field amplitude at a certain point to the amplitude of the incident was found. A large E-field was found to be at the gap in the spacing between the adjacent Ag-NWs for the case of \"wet\" substrates, where Ag-NWs were not pre-leaned (Figure 8a,c). Figure 8: _Cont_. Figure 7: Feature of the leaning of the 100 nm diameter Ag-NWs, depending on their length. (**a**) shows that sufficiently long NWs form the bundle, while so-called “middle-sized” NWs form the point contact between their tips (**b**). NWs with the length of fewer than 1 \(\upmu\)m are not leaning due to its short length (c). The maximum E/E\({}_{0}\) for the \"long\" Ag-NWs in the case of \"wet\" substrates was found to be 21.1 in the gap (order of 1 nm) between neighboring Ag-NWs (Figure 8a). For the case of \"dry\" substrates (NWs that are tightly leaning to each other), large E-field was found at the Ag-NWs touchpoint with the maximum E/E\({}_{0}\) of 13.1 (Figure 8b). For \"long\" Ag-NWs (the leaning angle is of 21 degrees) it was found out that the E/E\({}_{0}\) depends on the gap size x (nm) according to the exponential scaling law, derived by fitting, and for x varying from 1 to 10 nm takes the following form:  
\[\frac{\mathrm{E}}{\mathrm{E}_{0}}=113\times\mathrm{e}^{-\mathrm{x}}+4\]  
We can conclude that E/E\({}_{0}\) decay drastically with gap size x. The case of the leaning geometry, demonstrated by the example of \"medium-sized\" NWs, is extremely promising. Depending on the angle between neighboring NWs, the E/E\({}_{0}\) can be widely varied. This geometry leads to the concentration of electromagnetic energy in a narrowing gap between NWs and to the formation of a plasmon in it with several regions of electromagnetic field amplification, with a maximum E/E\({}_{0}\) equals 74.5 for the case of \"wet\" substrate (the opening angle of the NWs in the calculated case is equal to 2 degrees). E/E\({}_{0}\) is of 20 for the \"dry\" one in case of \"medium-sized\" NWs (Figure 8c,d). For the \"wet\" substrates the enhancement is strongly dependent on the geometry of leaning Ag-NWs (angle between them, length of the gap, etc.). The E/E\({}_{0}\) obtained for \"medium-sized\" NWs can be significantly enhanced due to the incident field polarization. As it is shown in [38] the SERS signal enhancement factor (EF) is connected with the field enhancement as (E/E\({}_{0}\))4, considering its average over all illuminated molecules. Thus for the \"medium-sized\" NWs in case of the \"wet\" substrates EF is approximately equal to 3 \(\times\) 10\({}^{7}\).