Modifying Plasmonic-Field Enhancement and Resonance Characteristics of Spherical Nanoparticles on Metallic Film: Effects of Faceting Spherical Nanoparticle Morphology

A three-dimensional finite-difference time-domain study of the plasmonic structure of nanoparticles on metallic film (NPOM) is presented in this work. An introduction to nanoparticle (NP) faceting in the NPOM structure produced a variety of complex transverse cavity modes, which were labeled S11 to S13. We observed that the dominant S11 mode resonance could be tuned to the desired wavelength within a broadband range of ~800 nm, with a maximum resonance up to ~1.42 µm, as a function of NP facet width. Despite being tuned at the broad spectral range, the S11 mode demonstrated minimal decrease in its near field enhancement characteristics, which can be advantageous for surface-enhanced spectroscopy applications and device fabrication perspectives. The identification of mode order was interpreted using cross-sectional electric field profiles and three-dimensional surface charge mapping. We realized larger local field enhancement in the order of ~109, even for smaller NP diameters of 50 nm, as function of the NP faceting effect. The number of radial modes were dependent upon the combination of NP diameter and faceting length. We hope that, by exploring the sub-wavelength complex optical properties of the plasmonic structures of NPOM, a variety of exciting applications will be revealed in the fields of sensors, non-linear optics, device engineering/processing, broadband tunable plasmonic devices, near-infrared plasmonics, and surface-enhanced spectroscopy.


Introduction
Local-field or near field enhancement is an attractive property in the field of metallic nanostructures, yielding a variety of potential applications in the area of plasmonics [1][2][3][4][5]. In metallic nanostructures, optical responses are primarily influenced by surface plasmon resonance (SPR) [4][5][6]. This SPR property arises from the collective oscillations of conduction electrons that can be excited at the metal surface as light interacts with the metallic nanostructures. This SPR-based collective oscillation of charge carriers can be primarily categorized into two types based on their optical nature: surface plasmon polariton (SPP), arising from the propagation of charge carriers along a planar surface, and localized surface plasmon resonance (LSPR), observed from electromagnetic confinement in

Materials and Methods
The schematic of the NPOM design is shown in Figure 1. An NP diameter "D" of 100 nm and a dielectric layer thickness of 2 nm were fixed throughout our simulations. The NP diameter was given by "D" and the faceting parameter "f " (in a circular shape) was introduced at the NP-dielectric interface (Figure 1a,b). The "f " parameter was related to the sphere "D" percentage ( Figure 1c,d). A 100% sphere shape was formed when f = 0 nm. When "f " increased, NP faceting was introduced at the NP-dielectric interface. The hemisphere was structurally formed when "f " = 100 nm. Three-dimensional (3D) finite-difference time-domain (FDTD) was employed to analyze the optical characteristics of the NPOM plasmonic nanostructure (Lumerical Solutions Inc., Vancouver, Canada). The NPOM structure was excited with incident plane wave light E 0 in "Z" direction in relation to the structure. The NPOM plasmonic nanostructure was surrounded by perfectly matched layers (PML). The high-resolution mesh size of 0.5 nm was employed to extract the proper solutions. A box field monitor was used to extract the broadband near field enhancement |E/E 0 | 4 spectra. For this purpose, average near field enhancement spectroscopy was used to obtain the average integral volume of |E/E 0 | 4 [6,12,14,17,36,37]: Coatings 2019, 9, x FOR PEER REVIEW 3 of 11 Figure 1. (a) Schematic illustration of NP on a metallic mirror (NPOM) structure. Gold was employed as metal, and the dielectric layer was inserted between NP and the metallic film. Faceting parameter influence on NP morphology is shown in (b), and an example cross-section view is shown in (c). NP diameter changes (%) from a perfect sphere to a hemisphere as a function of the facet "f" parameter is shown in (d).

Materials and Methods
The schematic of the NPOM design is shown in Figure 1. An NP diameter "D" of 100 nm and a dielectric layer thickness of 2 nm were fixed throughout our simulations. The NP diameter was given by "D" and the faceting parameter "f" (in a circular shape) was introduced at the NP-dielectric interface (Figure 1a,b). The "f" parameter was related to the sphere "D" percentage ( Figure 1c,d). A 100% sphere shape was formed when f = 0 nm. When "f" increased, NP faceting was introduced at the NP-dielectric interface. The hemisphere was structurally formed when "f" = 100 nm. Threedimensional (3D) finite-difference time-domain (FDTD) was employed to analyze the optical characteristics of the NPOM plasmonic nanostructure (Lumerical Solutions Inc., Vancouver, Canada). The NPOM structure was excited with incident plane wave light E0 in "Z" direction in relation to the structure. The NPOM plasmonic nanostructure was surrounded by perfectly matched layers (PML). The high-resolution mesh size of 0.5 nm was employed to extract the proper solutions. A box field monitor was used to extract the broadband near field enhancement |E/E0| 4 spectra. For this purpose, average near field enhancement spectroscopy was used to obtain the average integral volume of |E/E0| 4 [6,12,14,17,36,37]: Figure 1. (a) Schematic illustration of NP on a metallic mirror (NPOM) structure. Gold was employed as metal, and the dielectric layer was inserted between NP and the metallic film. Faceting parameter influence on NP morphology is shown in (b), and an example cross-section view is shown in (c). NP diameter changes (%) from a perfect sphere to a hemisphere as a function of the facet "f " parameter is shown in (d).
Here, E (E x , E y , E z ) is the local electric field, E 0 is the amplitude of the incident electric field and V is the volume at a certain distance limit (2 nm) within the metal-NP surface. In our NPOM simulation model, we used gold as a material for the NPs and metallic mirror. The refractive index of n = 1.5 was used for the dielectric layer. In the case of gold, the Johnson and Christy database was used and modeled by a Lorentz-Drude dispersion model fitting [42,43]. The parameters used in Equation (2) are explained as follows: Drude model fitting is shown as the first term in the equation where "w p " is the plasma frequency with "f 0 " oscillator strength and damping constant "Γ 0 ". The last term shows the Lorentz modification, where "m" represents a number of oscillations with frequency "w j ", damping constant "Γ j " and strength "f j ".

Results and Discussion
Three-dimensional FDTD simulation analysis of the 100% perfect sphere was carried out and structural modifications of NP faceting "f " were introduced by changing the sphere's diameter "D" percentage ( Figure 2). For the 100% sphere diameter, or perfect sphere, the resonance peak was found at 606 nm with a maximum near field intensity of |E/E 0 | 4 of~1 × 10 9 . As the sphere diameter D percentage decreased from 100%, a stronger red-shift was observed, and additional weaker resonance at shorter wavelengths arose. The dashed lines in Figure 2a indicate the resonance modes and are termed S mn (m = 1, n = 1, 2, 2 , 3). Their naming is further explained later in the Results section. One of the key results was the dominant S 11 mode red-shifting its resonance to near-infrared at~1.42 µm (NP at hemisphere shape) wavelength from a visible region (NP at perfect sphere shape) as sphere D% decreased (or as NP facet width "f " increased). Please note that despite the S 11 mode resonance red-shift, the |E/E 0 | 4 intensity decreased more slowly (from~1 × 10 9 to~3.2 × 10 8 ), which is helpful for SERS-based applications and device processing. This optical property showed that sphere diameter D% modification helped tune the S 11 mode to the desired wavelength within a broad range (a span of 800 nm till 50% sphere D% modification) with little compromise in terms of near field enhancement (Figure 2b,c). damping constant "Γ0". The last term shows the Lorentz modification, where "m" represents a number of oscillations with frequency "wj", damping constant "Γj" and strength "fj".

Results and Discussion
Three-dimensional FDTD simulation analysis of the 100% perfect sphere was carried out and structural modifications of NP faceting "f" were introduced by changing the sphere's diameter "D" percentage ( Figure 2). For the 100% sphere diameter, or perfect sphere, the resonance peak was found at 606 nm with a maximum near field intensity of |E/E0| 4 of ~1 × 10 9 . As the sphere diameter D percentage decreased from 100%, a stronger red-shift was observed, and additional weaker resonance at shorter wavelengths arose. The dashed lines in Figure 2a indicate the resonance modes and are termed Smn (m = 1, n = 1, 2, 2′, 3). Their naming is further explained later in the Results section. One of the key results was the dominant S11 mode red-shifting its resonance to near-infrared at ~1.42 µm (NP at hemisphere shape) wavelength from a visible region (NP at perfect sphere shape) as sphere D% decreased (or as NP facet width "f" increased). Please note that despite the S11 mode resonance redshift, the |E/E0| 4 intensity decreased more slowly (from ~1 × 10 9 to ~3.2 × 10 8 ), which is helpful for SERS-based applications and device processing. This optical property showed that sphere diameter D% modification helped tune the S11 mode to the desired wavelength within a broad range (a span of ~800 nm till 50% sphere D% modification) with little compromise in terms of near field enhancement (Figure 2b,c). (a) Broadband |E/E0| 4 results for sphere diameter D% modification from 100% to 50% for NP D = 100 nm in NPOM plasmonic nanostructure. Resonance wavelengths (b) and near field intensities (c) for S11, S12, S12′ and S13 modes extracted from Figure 2a. (d) Schematic illustration of NP facet contact shape with a dielectric layer concerning sphere diameter % explaining the reason for the resonance red-shift. The inset figure displays the similarity of the metal-insulator-metal structure.
At this point, we tried to understand the optical phenomenon behind the larger red-shift for the dominant S11 mode upon modification of the sphere diameter "D" percentage (or NP facet "f"). As shown in Figure 2d's inset, the NPs on the film structure were approximated to a metal-insulator- At this point, we tried to understand the optical phenomenon behind the larger red-shift for the dominant S 11 mode upon modification of the sphere diameter "D" percentage (or NP Coatings 2019, 9, 387 5 of 11 facet "f "). As shown in Figure 2d's inset, the NPs on the film structure were approximated to a metal-insulator-metal (MIM) resonator [44][45][46]. For this kind of structure gap plasmons (cavity mode) exist, which is a type of electromagnetic wave strongly localized to the gap between metallic films that propagates along the plane of the film (for example, propagation along the x-axis and confinement along the y-axis) [46][47][48][49][50][51][52][53]. Upon light excitation, counter propagating gap plasmons formed standing waves, which was like the Fabry-Perot condition. Using the Fabry-Perot as the interpretation of resonances and an effective refractive index of the MIM cavity (n eff ), the resonance wavelength λ mn of our current structure as a function of resonator width "f " was satisfied by the following equation [41,42,[44][45][46]: Here β is an appropriate reflection phase, a mn is the nth root of the mth order Bessel function J m . In terms of mode labeling in the case of S mn , m denotes the number of angular modes and n represents the number of radial modes. From Equation (3), it is clearly visible that λ mn is linearly proportional to resonator width f, which is an NP facet in our NPOM plasmonic structure. As schematically illustrated in Figure 2d, the circular NP facet area, as a function of percent of sphere diameter "D", increased until it became a hemisphere. This explained the red-shift of the dominant S 11 mode as well as other weaker resonance modes (S 12 , S 12 and S 13 ) at shorter wavelengths.
To understand the mode properties, the electric field amplitude profiles of NPOM (sphere diameter D% = 80%) are shown in the XZ cross-section of Figure 3a-d. The XZ cross-sectional electric field profiles were extracted from S 11 (λ = 1171 nm), S 12 (λ = 739 nm), S 12 (λ = 649 nm) and S 13 (λ = 589 nm) modes, as shown in Figure 2b. The maximum |E/E 0 | 4 values obtained were in order of~6.5 × 10 8 , 1.9 × 10 7 , 5.7 × 10 5 , 2.7 × 10 5 for S 11 to S 13 modes respectively. Please note that in all S modes, the near field enhancement was dominantly observed from the cavity located at the NP-mirror region. As observed in the cross-sectional XZ electric field amplitude profiles, the number of nodes increased with the order of mode number. In order to deeply understand the complicated and hybridized plasmonic modes, it is good to utilize three-dimensional mapping of the surface charge distributions. This surface charge density (ρ) was calculated by considering the skin effect and by integrating Gauss's law [38,41,42]: Here, the outward normal vector of the spherical NP surface is given by n = (n x , n y , n z ), E = (E x , E y , E z ) is the local electric field, the permittivity of the vacuum is termed ε 0 , and δ is the skin depth [54,55]. Figure 3e-h shows the 3D mapping of surface charge distributions for NPs with facet modification for S 11 , S 12 , S 12 and S 13 modes respectively. The 3D surface charge distributions clearly show the transverse dipole mode for the entire NP resonance in Figure 3e-h. Clear differences are seen in the NPOM hot spot or cavity area, where different orders of Fabry-Perot-like resonances were characterized. Please note that the difference between the S 12 and S 12 modes was a charge switching of a similar number of nodes in the cavity of the NPOM structure. Here, the nodes were either confined inside the cavity area (S 12 ) or populated around the edges of the facet (S 12 ), which is seen in Figure 3e-h. From the obtained electric amplitude profiles and 3D surface charge mapping, it is clearly understood that the cavity mode of NPOM plasmonic structures provides maximum contribution to the near field enhancement. The significant observation shows that the cavity modes are directly dependent on the NP faceting parameter.
In addition, gold NPs with diameters ranging from 50 to 90 nm were simulated to reveal the size-related effects. Faceting morphology changes based on a sphere diameter of 100%, 80% and 50% were studied for above mentioned NP sizes. Figure 4a-c shows the broadband near field spectrum for NP diameters of 50 to 90 nm as a function of sphere percentage, 100%, 80%, and 50%, respectively. In the case of a perfect sphere (Figure 4a), we saw the near field enhancement for S 11 mode increase as NP diameter got bigger (~1.9 × 10 6 for D = 50 nm,~5.4 × 10 8 for D = 90 nm). When the NP faceting effect was introduced, larger near field enhancements were observed for the dominant S 11 mode wavelength while its resonance wavelength red-shifted, and additional weaker resonance modes (S 12 , S 12 , S 13 ) at shorter wavelengths were noted (Figure 4b,c). The most attractive feature to be pointed out was the role of the NP faceting effect in Figure 4d: for NPs with D = 50 nm, near field enhancement values of~9.4 × 10 7 and~1.5 × 10 8 were seen for 80% and 50% sphere diameters; more importantly their near field enhancements were very close to that of a bigger non-faceted NPs (D = 90 nm,~5.4 × 10 8 ). Thus, upon introducing a faceting morphology to smaller NPs (example D = 50 nm), it would be possible to extract larger near field enhancement close to that of larger NPs. Please note that similar optical properties can be observed with other commonly used plasmonic NP materials such as Ag, Al, etc. The only difference was the resonance position. The dielectric layer thickness parameter played a critical role in near field enhancement properties [36], but, most importantly, higher near field enhancement would be possible with 50% faceted NP D when compared with non-faceted NPs at similar dielectric spacer thickness conditions. Coatings 2019, 9, x FOR PEER REVIEW 5 of 11 metal (MIM) resonator [44][45][46]. For this kind of structure gap plasmons (cavity mode) exist, which is a type of electromagnetic wave strongly localized to the gap between metallic films that propagates along the plane of the film (for example, propagation along the x-axis and confinement along the yaxis) [46][47][48][49][50][51][52][53]. Upon light excitation, counter propagating gap plasmons formed standing waves, which was like the Fabry-Perot condition. Using the Fabry-Perot as the interpretation of resonances and an effective refractive index of the MIM cavity (neff), the resonance wavelength λmn of our current structure as a function of resonator width "f" was satisfied by the following equation [41,42,[44][45][46]: Here β is an appropriate reflection phase, amn is the nth root of the mth order Bessel function Jm. In terms of mode labeling in the case of Smn, m denotes the number of angular modes and n represents the number of radial modes. From Equation (3), it is clearly visible that λmn is linearly proportional to resonator width f, which is an NP facet in our NPOM plasmonic structure. As schematically illustrated in Figure 2d, the circular NP facet area, as a function of percent of sphere diameter "D", increased until it became a hemisphere. This explained the red-shift of the dominant S11 mode as well as other weaker resonance modes (S12, S12′ and S13) at shorter wavelengths. Electric field amplitude and 3D surface charge distribution profiles taken from NPOM plasmonic structure with sphere "D" 80% for S11 (λ = 1171 nm), S12 (λ = 739 nm), S12′ (λ = 649 nm) and S13 (λ = 589 nm) modes. Cross-sectional XZ (a-d) electric field amplitude profiles and related 3D surface charge distributions from NP standalone view (e-h).
To understand the mode properties, the electric field amplitude profiles of NPOM (sphere diameter D% = 80%) are shown in the XZ cross-section of Figure 3a-d. The XZ cross-sectional electric field profiles were extracted from S11 (λ = 1171 nm), S12 (λ = 739 nm), S12′ (λ = 649 nm) and S13 (λ = 589 nm) modes, as shown in Figure 2b. The maximum |E/E0| 4 values obtained were in order of ~6.5 × 10 8 , 1.9 × 10 7 , 5.7 × 10 5 , 2.7 × 10 5 for S11 to S13 modes respectively. Please note that in all S modes, the near field enhancement was dominantly observed from the cavity located at the NP-mirror region. As observed in the cross-sectional XZ electric field amplitude profiles, the number of nodes increased with the order of mode number. In order to deeply understand the complicated and hybridized plasmonic modes, it is good to utilize three-dimensional mapping of the surface charge distributions. This surface charge density (ρ) was calculated by considering the skin effect and by integrating Gauss's law [38,41,42]: ρ is approximated as = · + · + · (4) Figure 3. Electric field amplitude and 3D surface charge distribution profiles taken from NPOM plasmonic structure with sphere "D" 80% for S 11 (λ = 1171 nm), S 12 (λ = 739 nm), S 12 (λ = 649 nm) and S 13 (λ = 589 nm) modes. Cross-sectional XZ (a-d) electric field amplitude profiles and related 3D surface charge distributions from NP standalone view (e-h). Figure 5 shows the data for S 11 , S 12 and S 13 radial mode resonance wavelength positions as a function of sphere diameter percentage (100%, 80%, 50%) for different NP Ds (50 to 90 nm). The dominant S 11 radial mode was observed for all NP Ds and facet parameters. For non-faceted NPs, except in the presence of S 11 radial mode, no other modes were present. As the faceting effect (e.g., 80% or 50%) was introduced to NP, additional radial modes (S 12 , S 13 ) were seen. On the other hand, observation of additional radial modes was reduced even in the presence of faceting, based on the NP D. Figure 5c shows one such NP size related effect for smaller NP sizes (e.g., D = 50 and 60 nm, 50% sphere diameter) where S 13 radial mode was absent. With our obtained data, it was possible to conclude that the number of radial modes was dependent on NP diameter size along with faceting "f " length. We hope that the possibility for high near field enhancement for smaller sized NPs upon faceting will boost several applications in the fields of plasmonics, photonics, and SERS-based sensors. Understanding the origin of radial mode numbers as a function of NP size and faceting effect will be helpful for optimizing practical near field enhancement-based applications.
characterized. Please note that the difference between the S12 and S12′ modes was a charge switching of a similar number of nodes in the cavity of the NPOM structure. Here, the nodes were either confined inside the cavity area (S12′) or populated around the edges of the facet (S12), which is seen in Figures 3e-h. From the obtained electric amplitude profiles and 3D surface charge mapping, it is clearly understood that the cavity mode of NPOM plasmonic structures provides maximum contribution to the near field enhancement. The significant observation shows that the cavity modes are directly dependent on the NP faceting parameter. Extracted maximum near field enhancement for S11 mode for different sphere D% from figures (a-c). 3D surface charge distributions for S11, S12, and S12′ modes taken from hemispherical NPs with D = 50 nm (e-g).
In addition, gold NPs with diameters ranging from 50 to 90 nm were simulated to reveal the size-related effects. Faceting morphology changes based on a sphere diameter of 100%, 80% and 50% were studied for above mentioned NP sizes. Figure 4a-c shows the broadband near field spectrum for NP diameters of 50 to 90 nm as a function of sphere percentage, 100%, 80%, and 50%, respectively. In the case of a perfect sphere (Figure 4a), we saw the near field enhancement for S11 mode increase as NP diameter got bigger (~1.9 × 10 6 for D = 50 nm, ~5.4 × 10 8 for D = 90 nm). When the NP faceting effect was introduced, larger near field enhancements were observed for the dominant S11 mode wavelength while its resonance wavelength red-shifted, and additional weaker resonance modes (S12, S12′, S13) at shorter wavelengths were noted (Figure 4b,c). The most attractive feature to be pointed out was the role of the NP faceting effect in Figure 4d: for NPs with D = 50 nm, near field enhancement values of ~9.4 × 10 7 and ~1.5 × 10 8 were seen for 80% and 50% sphere diameters; more importantly their near field enhancements were very close to that of a bigger non-faceted NPs (D = 90 nm, ~5.4 × 10 8 ). Thus, upon introducing a faceting morphology to smaller NPs (example D = 50 nm), it would be possible to extract larger near field enhancement close to that of larger NPs. Please note that similar optical properties can be observed with other commonly used plasmonic NP materials such as Ag, Al, etc. The only difference was the resonance position. The dielectric layer thickness parameter played a critical role in near field enhancement properties [36], but, most importantly, higher near field enhancement would be possible with 50% faceted NP D when compared with non-faceted NPs at similar dielectric spacer thickness conditions. Figure 5. Extracted resonance wavelengths for S11 mode (a), S12 mode (b), and S13 mode (c) obtained from broadband |E/E0| 4 spectrum of Figure 4a-c. Figure 5 shows the data for S11, S12 and S13 radial mode resonance wavelength positions as a function of sphere diameter percentage (100%, 80%, 50%) for different NP Ds (50 to 90 nm). The dominant S11 radial mode was observed for all NP Ds and facet parameters. For non-faceted NPs, except in the presence of S11 radial mode, no other modes were present. As the faceting effect (e.g. 80% or 50%) was introduced to NP, additional radial modes (S12, S13) were seen. On the other hand, observation of additional radial modes was reduced even in the presence of faceting, based on the NP D. Figure 5c shows one such NP size related effect for smaller NP sizes (e.g. D = 50 and 60 nm, 50% sphere diameter) where S13 radial mode was absent. With our obtained data, it was possible to conclude that the number of radial modes was dependent on NP diameter size along with faceting "f" length. We hope that the possibility for high near field enhancement for smaller sized NPs upon faceting will boost several applications in the fields of plasmonics, photonics, and SERS-based sensors. Understanding the origin of radial mode numbers as a function of NP size and faceting effect will be helpful for optimizing practical near field enhancement-based applications.
Please note that previously reported tunable plasmonic nanostructures, for example, the most commonly used dimer designs involving bow-tie antenna, disks, spheres, or rods, showed limitations with large near field enhancement of surface modifications. Moreover, it is difficult to consistently fabricate or reproduce a gap as small as ~2 nm in plasmonic dimer nanoparticles. Whereas, in case of the NPOM design, with the help of recent advances in deposition techniques such as e-beam deposition, atomic layer deposition, and highly ordered self-assembly bio-fabrication methods, it is possible to deposit and reproduce a thinner dielectric layer [56][57][58]. Thus, practical fabrication of extremely small sized gaps of the NPOM plasmonic nanostructure is possible, and the process methodology is less complex when compared with other geometrical designs. Combined with the introduction of geometrical errors, such as NP faceting, retaining identical or little reduced near field enhancement characteristics will enhance flexibility of fabrication as well as optical characterization. By using a biopolymer layer (for example, genetically engineered M13 bacteriophage) as a dielectric spacer it is possible to realize a highly efficient plasmonic sensing device with high selectivity and Figure 5. Extracted resonance wavelengths for S11 mode (a), S12 mode (b), and S13 mode (c) obtained from broadband |E/E 0 | 4 spectrum of Figure 4a-c.
Please note that previously reported tunable plasmonic nanostructures, for example, the most commonly used dimer designs involving bow-tie antenna, disks, spheres, or rods, showed limitations with large near field enhancement of surface modifications. Moreover, it is difficult to consistently fabricate or reproduce a gap as small as~2 nm in plasmonic dimer nanoparticles. Whereas, in case of the NPOM design, with the help of recent advances in deposition techniques such as e-beam deposition, atomic layer deposition, and highly ordered self-assembly bio-fabrication methods, it is possible to deposit and reproduce a thinner dielectric layer [56][57][58]. Thus, practical fabrication of extremely small sized gaps of the NPOM plasmonic nanostructure is possible, and the process methodology is less complex when compared with other geometrical designs. Combined with the introduction of geometrical errors, such as NP faceting, retaining identical or little reduced near field enhancement characteristics will enhance flexibility of fabrication as well as optical characterization. By using a biopolymer layer (for example, genetically engineered M13 bacteriophage) as a dielectric spacer it is possible to realize a highly efficient plasmonic sensing device with high selectivity and sensitivity, which is critical for biosensor applications [58][59][60]. With the positives mentioned above, the NPOM plasmonic nanostructure introduces new possibilities for the high-precision analysis of optical properties and optoelectronic/photochemical processes, and interpretation of morphological changes at the sub-nanometer scale.

Conclusions
We simulated and numerically characterized effects of NP faceting, which structurally introduced modifications in the cavity of the NPOM plasmonic nanostructure. The variations in NP faceting introduced the following significant results: (i) the dominant S 11 mode resonance wavelength was tuned in a span of~800 nm from the visible to the near-infrared region where the sphere diameter "D" percentage was reduced from 100% to 50%; (ii) minimal reduction of S 11 mode's near field enhancement was noted even when NP facet structural modifications were introduced; (iii) the reason for S 11 mode resonance tuning was found to be dependent on the NP facet "f " parameter; (iv) three-dimensional surface charge distributions revealed unchanged transverse dipolar mode characteristics of NPs and, at same time, revealed changes in the number of modes with respect to the S 11 , S 12 , S 12 and S 13 order at the NPOM cavity; (v) NP faceting played a significant role practical device fabrication as the near field enhancement was negligibly affected even in the presence of sphere diameter variations from 100% to 50%. This means that, in-spite of the NP faceting error, a highly efficient plasmonic SERS device can still be realized. More importantly, it would be possible to demonstrate higher near field enhancement, in the order of~10 9 , from smaller sized NPs if a faceting morphological change were introduced (D = 50 nm, 50% sphere D). We hope that a better understanding of this sub-nanometer optical phenomenon in plasmonic nanostructures will be helpful for device fabrication and open up new opportunities in the fields of tunable (and high) near field enhancement SERS applications, biosensors, and so on.