## 3 Results and Discussion  
Figure 1 shows typical TEM images of bare HNTs and those with deposited Au and Ag-NPs of two different mean sizes, i.e., 5 and 10 nm. As seen from Figure 1a, bare HNTs are characterized by an outer diameter of about 50 to 80 nm and a length of about 500 to 1000 nm. Ag- and Au-NPs are randomly distributed on the external surface of HNTs and the size distributions are well-described by the log-normal function (see Figure 1b-e). An individual 10-nm Ag-NP on the HNT's surface is shown in Figure 1f. The tight binding of plasmonic NPs with HNT' surfaces was confirmed by experiments on prolonged storage of the prepared nanocomposites in water. The size distribution of Ag-NPs was found to be slightly decreased (the mean size changed from 5.4 nm to 4.8 nm) and the LSPR spectrum decreased and spectrally shifted after storage in water for one week [11]. This effect can be explained by the washing out of a part of Ag-NPs due to partial dissolution in water. Additionally, the number of large agglomerates of Ag-NPs became smaller after the storage of the samples in water [11]. Figure 2 shows simulated images of HNTs:Au-NPs and local electric field distribution under illumination with a wavelength close to the LSPR of Au-NPs. The real size distributions of NPs and their concentration are considered during the simulation. It is seen from Figure 2a that the square of the electric field, which is proportional to the local light intensity, increases by an order of magnitude in the vicinity of Au-NPs, especially nearby the NPs' aggregates. Such so-called \"hot spots\" of the local electric field are responsible for the enhanced plasmonic properties of the prepared nanocomposites. Figure 1: TEM images of (**a**) bare HNTs and (**b**,**c**) HNTs:Au-NPs, (**d**,**e**) HNTs:Ag-NPs, and (**f**) an individual Ag-NP attached on the HNT’ surface. The corresponding size distributions for Au (orange bars) and Ag (blue bars) NPs and their fits by log-normal functions (solid lines) are shown in the insets. Figure 2: (**a**) Simulation of the HNTs:Au-NPs morphology for randomly distributed Au-NPs (d\({}_{0}\) = 5 nm) and a fragment of the cross-sectional map of the electric field distribution under illumination at 514.5 nm and polarization along the HNT’ axis and (**b**) the same simulation for the light polarized in the perpendicular direction. The electric field and wavevector directions are indicated by blue and red arrows in the left-side images. The dashed lines in the right-side images indicate the HNTs’ external and internal borders, while the HNTs’ center corresponds to X = 0. The simulated images of HNTs:Ag-NPs and local electric field distributions are shown in Figure 3. The latter reveal numerous \"hot spots\" of the local electric field nearby NPs. While the strength of the electric field is higher for the field orientation along the HNT axis (Figure 3b), the spatial distribution outside the HNT excited with the perpendicular oriented field is more favorable for the SERS sensing of surrounding biomolecules and cells. Figure 4 shows simulated extinction spectra of HNTs decorated with Ag- and Au-NPs. HNTs with immobilized monodisperse Au-NPs (d = 10 nm) exhibit a sharp peak of the LSPR with a maximum at 535 nm (see Figure 4a), while the spectrum of the sample with size-distributed NPs is broadened and characterized by a lower extinction value. The larger surface density of metal NPs, the higher the amplitude of LSPR peak. Moreover, the extinction spectra of HNT with attached metal NPs of high surface density exhibit an additional long-wavelength shoulder (see Figure 4b), which can be related to the LSPR of NP's grouping ensembles. The latter is probably also responsible for the formation of \"hot spots\" shown in Figures 2 and 3. When the size distribution of NPs is considered, the maxima become weaker and broader (compare Figure 4c,d), which is obviously related to the superposition of LSPR from individual NPs and their groping ensembles and agglomerates of various sizes and topology.