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Vittorianna Tasco,a Adriana Passaseoa and Massimo Cuscunà*,a a. CNR-NANOTEC Institute of Nanotechnology, Via Monteroni, Lecce 73100, Italy b...
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Tailoring Electromagnetic Hot Spots towards Visible Frequencies in Ultra-Narrow Gap Al/Al2O3 Bowtie Nanoantennas Daniela Simeone, Marco Esposito, Mario Scuderi, Giuseppe Calafiore, Giovanna Palermo, Antonio De Luca, Francesco Todisco, Daniele Sanvitto, Giuseppe Nicotra, Stefano Cabrini, Vittorianna Tasco, Adriana Passaseo, and Massimo Cuscunà ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00665 • Publication Date (Web): 22 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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Tailoring Electromagnetic Hot Spots towards Visible Frequencies in UltraNarrow Gap Al/Al2O3 Bowtie Nanoantennas Daniela Simeone,a,‡ Marco Esposito,a,c,‡ Mario Scuderi,*,d Giuseppe Calafiore,f Giovanna Palermo,e,b Antonio De Luca,e,b Francesco Todisco,a Daniele Sanvitto,a Giuseppe Nicotra,d Stefano Cabrini,f Vittorianna Tasco,a Adriana Passaseoa and Massimo Cuscunà*,a a. CNR-NANOTEC Institute of Nanotechnology, Via Monteroni, Lecce 73100, Italy b. CNR-NANOTEC Institute of Nanotechnology, Via P. Bucci, Rende 87036, Italy c. University of Salento, Department of Mathematics and Physics Ennio De Giorgi, Via Arnesano, Lecce 73100 Italy d. CNR-IMM Institute for Microelectronics and Microsystems, Strada VIII, Catania 95121, Italy e. University of Calabria, Department of Physics, Via P. Bucci, Rende 87036, Italy f. Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley 94720, CA, United States

ABSTRACT Plasmonic bowtie nanoantennas are intriguing nanostructures, capable to achieve very high local electromagnetic (EM) field confinement and enhancement in the hot spots. This effect is strongly dependent on the gap size, which in turn is related to technological limitations. Ultra-narrow gap bowtie nanoantennas, operating at visible frequencies, can be of great impact in biosensing applications and in the study of strong light-matter interactions with organic molecules. Here, we present a comprehensive study on the structural and optical properties of aluminum bowties, realized with ultra-narrow gap by He+-ion milling lithography, and operating from the near infrared to the red part of the visible range. Most importantly, this analysis demonstrates that large EM near-field enhancement and different hot spot spatial positions, as a function of nanometer-sized gaps, are constrained by the native aluminum oxide, thus working as hot spot ruler.

KEY WORDS:

plasmon, hot spot ruler, ultra-narrow gap, native aluminum oxide, visible

frequencies Plasmonic antennas have the remarkable ability to localize light below the diffraction limit,1,2 due to the collective oscillation of the conduction electrons, known as localized surface plasmon resonances (LSPRs). In this frame, metallic nanoparticles (NPs) have already shown interesting capabilities to guide and concentrate light at the sub-wavelength scale3 and provide extremely large localized enhancement of EM fields.4 These unique properties are beneficial for applications in several fields such as plasmonic waveguiding,5 disease diagnosis,6 chemical and biological 1 ACS Paragon Plus Environment

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sensing7,8 and surface-enhanced spectroscopies.9–11 The spectral responses and spatial field distributions of LSPRs are determined by the particle material, size, shape, surface morphology and dielectric environment.12,13 In the case of sensing and spectroscopy applications, the geometry of close gap nanoparticle dimers has attracted much attention due to the dramatic electromagnetic field confinement and enhancement in the so-called ”hot spot”14,15 created into dielectric gaps between two close metallic NPs.16 When two individual metallic NPs are brought into close proximity with each other, their single surface plasmons couple electromagnetically in the non-radiative near-field region. Besides the generation of hot spots, this leads to many interesting modifications of their plasmonic behaviors, such as the evolution of hybridized plasmon modes and the shift of the LSPRs.15,17 The local EM field enhancement in gap structures is strongly correlated to the gap size narrowing starting from an enhancement factor |Emax|/|E0|of 15 with 20 nm gap18 which increases to 150 for gap reduction down to 1 nm for nanosphere dimers.19 Further downscaling to the subnanometer scale introduces detrimental electron tunneling across the metallic dimer junction or charge transfer plasmons,20–24 and non-local responses should be taken into account.25 Therefore, in order to achieve extremely high local field enhancement factors, reproducible and precise nanofabrication techniques capable to control nanometer-sized gaps are indispensable. Typically, standard fabrication processes like colloidal lithography,26 focused ion beam (FIB) milling27,28 or resist-based electron beam lithography (EBL)29,30 have made great progress toward this goal, easily achieving dimer nanogap sizes between 10 and 20 nm. Colloidal lithography can produce large-area bowtie nanoantenna arrays but with poor resolution in terms of radius of curvature (ROC) of corners.26 Concerning FIB, the most common Ga+ milling is limited in resolution to typically more than 10 nm and results in undesired Ga+-ion implantation at the milling interface.27,31 This affects both the structural quality and the dielectric function of the fabricated plasmonic structures.32 Conversely, EBL technique, could be used to fabricate single dimer antennas or antenna arrays with nanogap sizes well below 10 nm. However, such narrow nanogaps have been fabricated only with special substrates such as thin silicon nitride membranes,33 capable to reduce electron backscattering effect but difficult to handle. Most recently, milling-based He+-ion lithography (HIL) has been introduced as a resist free milling technique for the fabrication of pristine nanostructures, potentially with sub nanometer precision.34–37 Thanks to the chemical propensity of noble gases for diffusion even out of solid matter, HIL reduces permanent contamination caused by inadvertently implanted ions. An obvious disadvantage of HIL is the reduced milling speed due to the lighter ions as compared to Ga+-ion milling. In principle, a combination of a first, coarse writing step by EBL or Ga+-ion milling with a second, HIL-based fine milling step may largely overcome this limitation. Recently, HIL was used for the fabrication of gold dimer nanoantennas with gap size less than 6 nm,38 leading to a high local field enhancement in the IR spectral range. Because of the enormous impact of nanoantennas in biosensing applications,39 in advanced miniaturized photonics such as plasmonic nanolasers,40,41 or in quantum electrodynamics for strong light-matter interactions with emitting molecules,42 it is necessary to investigate and design plasmonic dimers capable to work into the UV/VIS spectral range. Ag, which supports resonances down to 350 nm, could be a good alternative to gold, but it suffers from rapid tarnishing that degrades plasmonic properties.43 2 ACS Paragon Plus Environment

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In recent years, aluminum has started driving a wide interest as an outstanding material for nanoplasmonic-based devices operating in the UV/VIS spectral region.44–46 Furthermore, as opposed to silver, Al is cheaper, more abundant and forms a few nm-thick stable oxide layer almost immediately on exposure to air,44 that preserves the nanoparticles from further degradation, ensuring long-term durability. Therefore, several practical applications for aluminumbased dimer nanoantennas with extremely close gaps and working at visible frequencies could be envisioned, even if their fabrication is still largely unexplored. Here, we demonstrate a controllable and reliable fabrication process of Al bowtie dimers with challenging nanogaps down to 3 nm, by combining EBL and HIL technologies. Most importantly, we analyze the role of the native Al oxide as gap ruler, that defines the ultimate nanofabrication limit with respect to the EM field enhancement factor and the hot spot spatial position as a function of the nanometer-sized gaps. Optical investigation of the proposed ultra-narrow gap bowtie nanoantennas highlights the possibility to observe enhanced light scattering from the near infrared to the red part of the visible range, of particular interest for disruptive biosensing and nanophotonics applications.

RESULTS AND DISCUSSION Firstly, our focus was to fabricate Al bowtie nanoantennas with ultra-narrow gaps, as sketched in Figure 1. We started by patterning (Figure 1a) a gold layer-coated glass substrate (glass prevents absorption losses) forming squared holes. After, we pursued two distinct strategies for dimer manufacturing. In the first approach, we created nanostructure arrays consisting of two connected Al equilateral triangles (30 nm thick) by EBL at the center of the holes (Figure 1b) The nanogaps of the plasmonic antennas were produced by direct milling with focused He+-ions (Figure 1c) as reported in the experimental section.

Figure 1. Sketch of the two pursued approaches to fabricate close gap Al bowtie nanoantennas. (a) Firstly, a gold layer was patterned forming squared holes sized 2x2 µm2 on glass substrate. Subsequently, first approach: (b) an array consisting of two connected Al equilateral triangles was patterned by EBL in the middle of squared holes, (c) the bowtie dimers were fabricated by separating the connected triangles with He+-ion milling. Second approach: (d) an array of Al nanorectangles centered in the squared holes of panel a) was patterned by EBL, (e) each

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nanorectangle was then milled in a “direct writing mode” using HIL to pattern nanostructures consisting of two connected triangular nanoprisms. Finally, bowtie dimers was fabricated by separating the connected triangles with + He -ion milling obtaining the nanostructures reported in the panel c).

The second approach, even though much more time consuming, provided narrower gaps than the previous strategy. We first fabricated, by using EBL technique, an array of Al nanorectangles centered in the squared holes (Figure 1d) formerly defined. The nanorectangles were then milled using He+-ion to pattern nanostructures consisting of two connected triangular nanoprisms (Figure 1e). After, the HIL-based procedure used in the first approach was exploited for separating the connected triangular nanoprisms and form the nanogaps (Figure 1c). The final result of our first pursued fabrication approach can be seen in Figure 2a, which shows helium ion microscope (HIM) images of two connected equilateral triangles produced by EBL technique (top panel), and subsequently separated with HIL by cutting the Al conductive bridge connecting them (bottom panel). The He-microscope images shown in Figure 2 are based on secondary electrons (SEs) produced by He+-ions. Despite the use of a single pixel line milling mode with low ion dose, the sputtering process induced by He+-ions47 leads to the complete Al conductive bridge removal, creating minimum gaps of about 10 nm (Figure 2a, bottom panel). Here, it is worth noting that the Al bridge is originated from Al thermal evaporation into the extremely narrow resist mask patterned by EBL on the bowtie gap region. Therefore, the bridge is very thin as confirmed by the lack of Al grains on top of it, whereas they are evident on the surface of the two triangular nanoprisms. Actually, another critical issue of the first approach is the considerable nanostructure surface roughness, typical of thermally evaporated metals (Figure 2a). Its decrease is auspicable for surface scattering loss reduction.48,49 The second adopted approach is shown in Figure 2b. The 30 nm thick Al layer of the starting nanorectangle (Figure 2b, top panel) was first He+-ion milled with nanometer accuracy to a connected bowtie dimer, with a significant sharpening of edges and corners (Figure 2b, middle panel). The second He+-ion milling stage enabled the opening of dimer gap as narrow as 3 nm, thus remarkably reduced as compared to previous approach (Figure 2b, bottom panel). A ROC value in the range 3-4 nm was estimated for the tips in proximity to the nanogap, along with a significant reduction of Al dimer outer surface roughness. The effect of roughness reduction can be ascribed to a localized heating induced by the He+-ion beam. A number of studies reported a local temperature increase during Ga+-FIB analysis and nanofabrication.50 Theoretically, the maximum temperature rise caused by local heating from an ion beam can be estimated by the following equation: T=P/(πak) where “P” is the power of the ion beam, “a” is the radius of the circular ion beam profile on the sample surface and “k” is the thermal conductivity of the substrate. In this equation, thermal conductivity plays a very important role in local heating; because of the low thermal conductivity (and diffusivity of heat) of glass substrate, used in our fabrication process, the Tmax value is larger by a factor of ∼100 than one for the most common Si substrate. During the He+-ion milling definition of the two connected triangles with large current value of about 13 pA (Figure 2b, middle panel), a morphology change was clearly noted thanks to the capability of HIM to simultaneously mill and image the nanostructures with extremely high resolution. In particular, we 4 ACS Paragon Plus Environment

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observed a surface roughness reduction, likely correlated to the formation of larger Al grains at the expenses of smaller grains51 deriving from direct thermal evaporation (Figure 2b, top panel). The reduction of grain boundaries displayed in Figure 2b (middle panel) with respect to the top panel demonstrates the grain enlargement after the He+-milling step. Hence, the last fabrication approach not only accurately provided Al dimer nanoantennas with extremely close gaps (3 nm) but also improved material quality by means of grain boundary reduction, beneficial against damping phenomena.48,52

Figure 2. Helium ion microscopy images of Al bowtie nanoantennas fabricated with two approaches: (a) two connected equilateral triangular nanoprisms produced by EBL technique (top panel) subsequently separated with HIL by cutting the conductive bridge connecting them (bottom panel), (b) two-stage He+-ion milling protocol, a nanorectangle was created by using EBL technique (top panel). It was then milled in a “direct writing mode” using HIL to pattern nanostructures consisting of two connected triangular nanoprisms (middle panel). Finally, a bowtie + dimer was fabricated by separating the connected triangles with He -ion milling (bottom panel). All scale bars are 100 nm long.

Once identified the optimal He+-fabrication procedure for the extremely narrow gap dimers and unveiled the minimum gap size achievable (3 nm), we focused on the side effect of naturally occurring oxidation process. When Al nanostructures are exposed to the ambient air, a thin native Al oxide forms on the surface,44,53 which acts as a passive and protective layer against further environmental degradation.44,54 The thickness of that layer is of a few nm, thus comparable to the narrow bowtie nanogap explored in the present work. To study the oxidation process in this system, the sample containing Al dimer antennas was exposed to indoor ambient air conditions (air-conditioned lab, 45% relative humidity at 21°C) for 40 days, a time enabling the oxidation process saturation,44 and scanning electron microscopy (SEM) analysis was then conducted. 5 ACS Paragon Plus Environment

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High-magnification SEM image of the oxidized bowtie dimer (Figure 3b) highlights a clear Al oxide growth with respect to the freshly fabricated dimer (Figure 3a). Indeed, the tips of the dimer in proximity to the nanogap appear much closer than those of the as-fabricated dimer, suggesting Al oxide growth on the pure Al outer surface. Moreover, the ROC of the tips in the nanogap region is about 5 nm, a value greater than one reported above for freshly fabricated dimers. A higher zoom could help in evaluating the gap size between the two triangular nanoprisms, but it would damage the antenna tips because of local heating induced by electron primary beam.

Figure 3. (a) HIM Image of a freshly fabricated bowtie dimer. (b) SEM image of the same bowtie dimer with geometrical parameters, defined in panel c), s = h = 140 nm after being exposed 40 days to indoor air at ambient conditions. (c) Sketch of the Al/oxide core-shell bowtie nanoantenna including the geometrical parameters of the single triangular nanoprism: base (s), thickness (t) and height (h). All scale bars are 100 nm long.

For a proper quantitative analysis of the Al oxide growth, we performed cross-sectional transmission electron microscopy (TEM) analysis combined with electron energy loss spectroscopy (EELS) of a reference 30 nm Al film deposited on silicon substrate in the same metal evaporation process of the bowtie dimer antennas. TEM micrograph of Figure S1a shows a bright-field crosssectional view of the Al film aged up to 40 days reporting a 2 nm thick native oxide-coated silicon substrate55 and an Al film covered by a clearly visible 2.5 nm thick oxide layer. Figure S1a,b highlights also the conformal oxide growth unlike what occurs for silver, the most common plasmonic metal working in the UV/VIS spectral range.43 Also, high-resolution TEM analysis didn’t detect crystal grains inside the oxide layer (Figure S1b). We also performed a chemical study of Al thin film by combining TEM with EELS, an approach enabling the analysis of the entire film volume. Figure S2 reports two main peaks ascribed to the two native oxides, one formed at the outer surface of the silicon substrate, and one formed on the evaporated Al film, respectively. The rest of the film exhibits an almost constant and negligible bulk oxygen fraction, incorporated during the Al thermal evaporation. Furthermore, in order to shed light on the stoichiometry of the native Al oxide layer discriminating between Al2O346 and Al(OH)3,56 we studied the energy-loss near edge structure (ELNES) of Al films after being exposed 40 days to indoor ambient air conditions. ELNES spectra (Figure S3) demonstrated the formation of Al2O3 instead of Al(OH)3 layer and its relative amorphous structure confirming the result obtained by HRTEM investigation of Figure S1b. Such TEM analysis (Figure S1, S2 and S3) reasonably suggests the formation of a uniform, amorphous and thin (2.5 nm) layer of Al2O3 on the outer surface of the Al bowtie dimer 6 ACS Paragon Plus Environment

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nanostructures, thus exhibiting an Al/Al2O3 core-shell structure as reported in the sketch of Figure 3c. Thereby, the oxidation process occurring on both the triangular nanoprism tips in proximity of the smallest fabricated nanogap (about 3 nm wide), would provide a large contact area of the Al2O3 shells. Actually, a touching point is barely noticeable in the SEM image of Figure 3b. Indeed, we need to take into account that in an idealized and somewhat simplified picture, the Al oxidation process is characterized by a shift of the Al/Al2O3 interface towards the Al itself, at the cost of the metal core shrinking. Considering the densities and molecular weights of Al and Al2O3 bulk,57 we can infer the consumed Al film for growing a desired oxide thickness: Thickness of Al ~ 0.39 x (Thickness of Al2O3). Therefore, a 2.5 nm thick oxide shell (α) growth consumes about 1 nm thick Al layer. This analysis poses a technological limit to manufacturing of such nanostructures: as-fabricated Al dimers with a clearly open 3 nm nanogap (Figure 3a) evolve, after air exposure, into two Al core/Al2O3 shell triangular nanoprisms with the shells mutually touching (effective null gap) (Figure 3b), and the cores few nanometers far from each other. The detailed understanding of Al oxidation process previously reported, permitted us to define appropriate input parameters in finite element method (FEM) simulations, taking into account the proper core/shell structure of fabricated samples. Figure 4 shows the near-field enhancement factor distributions calculated in Al/Al2O3 core-shell dimers (s = h = 140 nm) as a function of external surface-to-surface gaps (d = 0.5, 2 and 5 nm), at the wavelengths of the maximum field enhancement, λmax, for x-polarization (see Cartesian coordinate system in Figure 3c and Figure 4a top and middle panels) of the incident light.

Figure 4. Bowtie dimers (s = h = 140 nm) with different gap sizes: (a) 0.5 nm, (b) 2 nm and (c) 5 nm: Calculated twodimensional near-field enhancement factor |E|/|E0| (E0 = 1 V/m) distributions, at corresponding maximum wavelengths, in the Al/Al2O3 core–shell bowtie gaps (x-y view, top panels, extracted at half of the antenna height

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and x-z view, middle panels, extracted along the dimer long axis at y = 0 nm; scale bars 5nm); extinction cross section (σext) and |E|/|E0|extracted at half of the antenna height (bottom panels) for x-polarization of the incident light.

As it can be seen in Figure 4a-c top and middle panels, the hot spots arise in the gaps along the dimer x-axis and the EM field enhancement factor rapidly decreases with increasing d. Indeed, the electric field is highly confined in the gap because the refractive index of Al2O3 is higher than the Al one, thus the discontinuity of the electric field at Al2O3-air interfaces becomes larger as compared to that one at Al-Al2O3 interfaces. In a simplified picture, we can model the oxidized dimer junction region as a parallel plate capacitor at the nanometer scale (neglecting the inductance contribution),33 with sidewall area (A) of approximately 2x(ROC) x 31,5 nm2 and a dielectric gap g composed by air and Al2O3 as illustrated in Figure S4. The electric field strength in the air gap between the plates is given by E0 = Q/AƐ0, where Q is the charge accumulated on the metal surfaces and Ɛ0 is the dielectric permittivity of the air, whereas the electric field in the Al2O3 region is ED = E0/ƐD, where ƐD is the Al2O3 dielectric permittivity. Since ƐD is about 3 at optical frequencies,58 we can expect a proportionally scaled electric field decrease in the Al2O3 region as compared to E0 values. Hence, the air gap is a particularly attractive nanocavity for localizing electromagnetic fields with well-defined polarization properties to nanometric volumes (Figure 4ac top and middle panels) and to explore their enhanced coupling to nanoemitters. For each spacing d, the extinction cross sections and EM field enhancement factors calculated at half of the antenna height, in the middle of the air gap, are shown (Figure 4a-c, bottom panels). By increasing the gap, the spectral peak positions of both far field and near-field significantly blueshift because of the reduction of coupling strength.15,59 In addition, the near-field maximum is found at a slightly lower energy as compared to the far field one.60 Moreover, the expected EM field enhancement factor |E|/|E0| rapidly increases with gap narrowing (Figure 4b-c, bottom panels), up to a value of about 300 at 1150 nm for d = 0.5 nm gap (Figure 4a, bottom panel). Such value is about 20 times larger than one estimated in a recent theoretical work18 for Al nanorectangular dimers with 20 nm gap working at optical frequencies. Therefore, an extremely reduced interparticle distance in this sub-5nm range for dimer nanostructures plays a crucial role to get large EM field enhancement and localization in a very small volume. It is worth to underline that such extremely high enhancement factors are calculated at half of the antenna height, while, conversely, above the gap region, along the z-axis, the EM field enhancement factor rapidly reduces with increasing distance from the Al2O3 outer surface (Figure 4a-c, middle panels), with an estimated reduction by more than 95% at a few nm from Al2O3-air interface. Within the Al2O3 region, such factor results lower than one observed into the air gap (Figure 4a-c top and middle panels) in good agreement with the above mentioned parallel plate capacitor model. Special attention is required for the lower limit of the proposed nanofabrication approach, i.e., when successfully fabricated metal nanostructures with 3 nm nanogap undergo gap closure because of the unavoidable Al oxidation process (d = 0 nm). When two core-shell bowtie dimers touch each other (Figure 5), two hot spots symmetrically occur in the gaps near the touching point of Al2O3 shells along the perpendicular direction (y) (Figure 5a, top panel). Since these gaps deviate from the junction point, the surface charge accumulation is lower than one in the junction of core–shell dimers with d = 0.5 nm, thus the maximum near-field enhancement is reduced. More in detail, the maximum EM field enhancement factor, extracted along y direction crossing the two symmetrical hot spots (Figure 5a, top panel), approaches 170 at 1170 nm (Figure 5a, bottom 8 ACS Paragon Plus Environment

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panel). From results of Figures 4a and 5a, we can assert that a gap between freshly Al triangular nanoprisms slightly larger than 3 nm would provide the maximization of the EM field enhancement factor. In the proposed core-shell bowtie systems, despite the ultra-narrow gap, electron tunneling effect across the dimer junction or charge transfer plasmon,20 occurring in similar noble metal systems, can be avoided because of the presence of intrinsic Al2O3 layer. Figure 5b shows the same calculations related to a smaller bowtie nanoantenna, featured by s = h = 90 nm with d = 0 nm, fabricated by means of the same procedure previously mentioned. Here, we estimate the occurrence of two symmetrical hot spots in the gap (Figure 5b, top panel) as in the case of the larger bowtie dimer, but the size downscaling has blue-shifted the nanostructure optical response in the red part of the visible spectral range where the extinction cross section and the |E|/|E0| factor are now centered. In particular, a maximum EM field enhancement factor of about 70 is estimated at a wavelength of 675 nm.

Figure 5. (a) Large dimer (s = h = 140 nm, d = 0 nm), (b) Small dimer (s = h = 90 nm, d = 0 nm): Calculated twodimensional near-field enhancement factor |E|/|E0| (E0 = 1 V/m) distributions, at corresponding maximum wavelengths, in the Al/Al2O3 core–shell bowtie gaps (x-y view, top panels, extracted at half of the antenna height, and x-z view, middle panels, extracted along the dimer long axis at y = 0 nm; scale bars 5 nm); extinction cross section (σext) and |E|/|E0|extracted at half of the antenna height (bottom panels) for x-polarization of the incident light.

Experimental evidence of effective operation in the VIS is provided by directly inspecting the bowtie EM near-field by using scanning near-field optical microscopy (SNOM) analysis. Details on 9 ACS Paragon Plus Environment

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experimental conditions for such a characterization are discussed in the experimental section. The sample was oriented with the dimer long axis (x-axis) parallel to the exciting laser polarization direction, while excitation wavelength was tuned in the range 550-730 nm. The corresponding near-field scattering maps are shown in Figure 6 (top panel), as obtained by integrating the transmitted spectra around the indicated central wavelengths in a 5 nm width spectral window. As the wavelength increases, the maps show hot-spots of enhanced light scattering surrounded by absorption donuts,61 contrasting with the glass substrate background signal. The maximum contrast between scattered near-field and background signal is found at 685 nm. Figure 6 (bottom panel) shows the normalized light intensity variation ΔI/I0 (ΔI = Ihot spot – I0 where Ihot spot is the maximum light intensity on the hot spot while I0 is an average background signal measured into a squared hole sized 2x2 µm2 (Figure 1), without the bowtie dimer at the center) measured at the same wavelengths of the near-field scattering maps; as it can be noted such light intensity variation is in good agreement with the trend of the EM field enhancement factor derived from FEM simulations shown in Figure 5b (bottom panel). Given the smaller nanostructure sizes with respect to SNOM tip features, the reported normalized scattered intensity was not collected at half of the nanoantenna height but at a few nm above the gap region, evidencing a maximum value of about 3.5 at 685 nm (Figure 6, top panel) that can be assumed in first approximation to be comparable to squared |E|/|E0| calculated in Figure 5b (middle panel) at the same spatial position. In Figure S5 the polarization dependence of experimental optical response is also shown. Such optical responses demonstrate the possibility to obtain enhanced light scattering in the visible spectral range by scaling down the size (s and h) of bowtie nanoprisms, thus fabricating nanostructures suitable for disruptive biosensing and nanophotonics applications.62,63

Figure 6. SEM image and normalized near-field scattering maps (top panel), obtained by SNOM at different excitation wavelengths, of null gap Al/Al2O3 core–shell bowtie small dimer (s = h = 90 nm) with long axis parallel to the exciting laser polarization direction (x-polarization). The scattered light was normalized to an average 2 background signal (I0) measured into a squared hole sized 2x2 µm (Figure 1), without the bowtie dimer at the center.

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The graph (bottom panel) reports the normalized light intensity variation ΔI/I0 measured at the same wavelengths of the near-field scattering maps (ΔI = Ihot spot – I0 where Ihot spot is the maximum light intensity on the hot spot).

CONCLUSIONS The combination of EBL and He+-ion milling provided a few-nanometer milling precision for the fabrication of extremely narrow gap Al bowtie nanoantennas generating EM hot spots at VIS frequencies. Nanoscale structural investigation based on SEM/TEM techniques emphasized that because of the Al2O3 native oxide formation, a 3 nm gap is found as the intrinsic lower limit of the proposed nanofabrication approach. Finite element method calculations determined that large EM near-field enhancement and different hot spot spatial positions, as a function of nanometer-sized gaps, are strongly constrained by the native aluminum oxide. Because of that, we can assert that a gap between freshly Al triangular nanoprisms slightly larger than 3 nm would provide the maximization of the EM field enhancement factor. Furthermore, FEM analysis and SNOM measurements demonstrated the presence of enhanced near-field in the red side of the visible spectral range by proper downscaling the bowtie nanoprism size. These results, combining the He+-ion nanofabrication capabilities, in terms of high resolution, accuracy and high nanostructure quality, with the choice of Al as plasmonic metal can be employed to develop advanced plasmonic biosensors, novel miniaturized nanophotonic systems as well as platforms to investigate strong light-matter interactions at visible frequencies.

EXPERIMENTAL SECTION Fabrication of Extremely Close Gap Al Bowtie Dimers. We started by patterning a gold layer-coated glass substrate forming squared holes sized 2x2 µm2. This solution represents an excellent compromise for both, charging reduction during the HIL step (gold layer close to the dimers is needed) and high optical power through the holes for transmission measurements by means of a scanning near-field optical microscope. After glass substrate cleaning in acetone and isopropanol (IPA) in ultrasonic bath, a 300 nm-thick negative tone resist (ma-N 2403 from Micro Resist Technology GmbH) was spin-cast (2700 rpm, 45 s) and soft-baked at 90 °C for 60 s. Then, a conductive polymer (aquaSAVE Electronic Conductor, Mitsubishi Rayon America Inc.) was spun on to prevent charge effects during electron beam exposure. The pattern was defined by a Vistec EBL system (VB300) at 100 KeV with a beam current of 1 nA. After electron exposure, the conductive polymer was removed by a water rinse. The exposed resist was developed in MF-26A for 60 s and immediately rinsed in deionized water for 2 min. A 80 nm thick gold layer was thermally evaporated, subsequently lift-off process was performed in a remover solution and rinsed in deionized water. Following this squared hole patterning, we pursued two distinct strategies for dimer manufacturing. In the first approach, we created nanostructure arrays consisting of two connected equilateral triangles by EBL at the center of the holes through a simple alignment procedure. More in details, a 150 nm thick poly-(methylmethacrylate) (PMMA) was spin-cast (4000 rpm, 45 s) and soft-baked at 180 °C for 3 minutes. The arrays were patterned at 100 KeV with a beam current of 100 pA. The exposed resist was developed in methyl isobutyl ketone (MIBK):IPA 1:3 solution for 2 minutes and immediately rinsed in IPA for 30 s. A 30 nm-thick 99.99% pure aluminum layer was thermally evaporated -6 from tungsten boats at base pressure of 1x10 mbar and at a deposition rate of about 3 A/s. Subsequently, lift-off process was performed in a remover solution and rinsed in IPA. Such an approach provided nanostructures composed by two connected equilateral triangles with side down to 80 nm. The nanogaps of the plasmonic antennas were + produced by direct milling with focused He -ions using a Carl Zeiss Orion Nanofab HIM. For milling, we used a beam energy of 30 keV, a beam current of 2 pA, a step size of 1 nm, a single pixel line exposure with a length of 30 nm and a 2 + line dose of about 30 nC/cm . Further beam current lowering, needed for He -ion beam size reduction, was not

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possible because of the poor image contrast of aluminum. The second approach, provided narrower gaps than the previous strategy. We first fabricated an array of Al nanorectangles centered in the squared holes formerly defined. Such nanostructures were created by using EBL technique with PMMA as resist, exploiting the above mentioned procedure. The nanorectangles were then milled in a “direct writing mode” using He+-ion to pattern nanostructures consisting of two connected triangular nanoprisms. A beam energy of 30 keV and a beam current of 13 pA provided connected triangular base nanoprisms, with similar h and s size, ranging from 140 nm down to 90 nm. Finally, the HILbased procedure used in the first approach was exploited for separating the connected triangles and form the nanogaps. Electron Microscopy Characterization. A Zeiss Merlin SEM operating at 5 kV and a Carl Zeiss Orion Nanofab HIM at a beam energy of 30KeV (beam current of 2 pA) were used to image the nanostructures fabricated on glass substrates. TEM analysis was performed in a (Cs)-probe-corrected JEOL ARM200CF at a primary beam energy of 200 keV, operating in both conventional TEM and scanning-TEM mode (STEM). The microscope is equipped with a fully loaded post column GATAN GIF Quantum ER system as electron energy loss (EEL) spectrometer. The inner and outer collection angles of the annular dark-field detector are 68 and 280 mrad for high angle annular dark field (HAADF) imaging. ELNES spectra were acquired with an energy resolution of 0.8 eV and a dispersion of 0.1 eV/pixel. The convergence and collection semi-angle was 10 and 7 mrad, respectively. The spectra were acquired by using EELS simultaneous acquisition of both: the zero loss peak in the low-loss region and the interested edges in the high-loss region of the spectra (dual EELS capability). On the spectra, the background signal and plural scattering contribution was removed by using the Fourier-ratio technique. EELS analysis was performed by means of Al film cross-sectional view. To make the specimen suitable for TEM observation, the sample with a 30 nm thick Al layer, thermally evaporated on a silicon substrate, was glued together with a silicon substrate using a GATAN G-1 two components epoxy glue. Subsequently, the sample was accurately thinned by mechanical grinding, followed by dimpling and lowenergy Ar-ion milling (1 keV) in a Gatan Precision Ion Polishing System (PIPSII). Finite Element Method Simulations. A dielectric function () =  () − ∙ () was used to represent the 64 65 metallic system of interest. Here we use McPeak et al. for Al data and Boidin et al. for Al2O3. Air was chosen as the background medium with a refractive index of 1.0, and the bowtie was placed on a silica glass substrate with a refractive index of 1.446. FEM simulations were performed in terms of the scattered field in order to calculate the extinction cross section of single bowtie dimers produced in far field. A plane wave travelling in z direction (Figure 3c) was used to excite the system with impinging polarization parallel to dimer bowtie long axis (x-polarization) as a function of wavelength in the VIS/NIR range. The structure of bowtie dimers was designed directly by means of the 3D CAD module of COMSOL Multyphisics, a commercial software implementing the FEM in frequency domain. Optical Characterization. Squared holes patterned into the gold layer result in an unavoidable intense background scattering, which makes classical dark field measurements extremely noisy for our system. For this reason, the optical behavior of the individual bowtie dimers was optically characterized on a SNOM setup. The sample was mounted on an inverted microscope equipped with atomic force microscopy (AFM) piezo scanners for xy-plane (Figure 3c) raster scan. Light from an adjustable supercontinuum source (NKT photonics) was focused on the sample surface by means of an objective lens (Nikon, 20x NA 0.8), resulting in a white light spot size of around 5 µm, spanning the visible range between 550 nm and 730 nm. Scattered light from the sample was collected in the near-field through the aperture of a cantilever-based aluminum-coated hollow tip (NT-MDT, 100±20 nm aperture diameter). Broadband light from the hollow tip was finally collected by a coaxial long working distance objective lens mounted into the AFM unit (Mitutoyo, 100x NA 0.6) and analyzed, through a confocal setup, on a monochromator coupled with a CCD camera, to simultaneously allow spatial and spectral resolved measurements. In order to avoid any possible polarization effects related to setup anisotropies, measurements at different polarization angles were performed by rotating the sample in the xy-plane.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Structral and chemical characterization of Al thin film after air exposure; theoretical model and optical characterization.

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Corresponding Author *E-mail: [email protected]; [email protected]. ORCID: http://orcid.org/0000-0002-4934-3376; http://orcid.org/ 0000-0001-9026-5317 Author Contributions ‡ D.S. and M.E. contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was partially founded by the Short Term Mobility Program of CNR and the national project “Molecular Nanotechnologies for Health and Environment” (MAAT, PON02_00563_3316357 and CUPB31C12001230005). We would like to thank Antonio I. Fernández-Domínguez for scientific discussion, and Gianmichele Epifani and Iolena Tarantini for technical support. Furthermore, we would like to thank Scott Dhuey for the EBL exposures. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract no. DE-AC02-05CH11231. We would like to acknowledge Frances I Allen and Paul Lum at Biomolecular Nanotechnology Center/QB3, Stanley Hall, University of California, Berkeley, CA for the use of the Carl Zeiss Orion Nanofab.

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For Table of Contents Use Only Tailoring Electromagnetic Hot Spots towards Visible Frequencies in UltraNarrow Gap Al/Al2O3 Bowtie Nanoantennas Daniela Simeone, Marco Esposito, Mario Scuderi, Giuseppe Calafiore, Giovanna Palermo, Antonio De Luca, Francesco Todisco, Daniele Sanvitto, Giuseppe Nicotra, Stefano Cabrini, Vittorianna Tasco, Adriana Passaseo and Massimo Cuscunà

Challenging close gap Al core/Al2O3 shell bowtie nanoantennas with intrinsic photonic potentialities in the visible range.

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