Plasmonic Nanocone Scanning Antenna: Fabrication and Optical Properties

Optical antennas are nanostructures that introduce unprecedented possibilities for light–matter interaction at the nanoscale. An appropriately tailored plasmonic antenna can enhance the total radiative decay rate and modify the angular radiation pattern of a single‐quantum emitter through controlled near‐field coupling. Despite their ability to surpass the fundamental diffraction limit and confine the electromagnetic field to a tiny mode volume, fabricating 3D sharp scanning nanoscale plasmonic structures with desired aspect ratio is yet an ambitious goal. The fabrication of nanocones by gold evaporation on commercial atomic force microscopy probes followed by a focused ion beam milling technique is presented. The method is versatile and allows the fabrication of nanocones with desired dimensions around 100 nm along with an aspect ratio of ≈1. Their optical properties are studied and it is shown how the variation in the refractive index of the dielectric substrate affects the localized surface plasmon resonance of the nanocones, the decay rate enhancement, and the quantum yield of an emitter relevant for fluorescence/Raman scanning experiments. Theoretical studies using finite‐difference time‐domain calculations have guided the fabrication process and are consistent with experimental results.


Introduction
For the past decades, research in nano-optics has been growing at a mind-blowing speed.The implementation of strongly enhanced, tightly confined electromagnetic radiation is of great interest for a wide range of applications, including manipulating the photophysics of quantum emitters, [1][2][3] sensing, [4] information processing, [5,6] scanning near-field microscopy, [7] etc.The ideal candidates employed for these applications are often plasmonic nanostructures acting as optical antennas. [8,9]Here, the localized surface plasmon resonance (LSPR) of the nanostructures can be tuned by modifying the aspect ratio and it can achieve further field enhancement by introducing sharp features to the nanostructures.Hence, the electromagnetic radiation from a distant source is concentrated near the sharp edges of the antenna through the so-called lightning rod effect, beating the fundamental diffraction limit.The primary proof of this concept was done by Synge et al. in 1928, [10] which was later demonstrated in the optical spectral range by Pohl et al. [11] in 1984, and finally translated from the context of near-field optics to optical antennas later in 2005. [12]Optical antennas are also able to enhance the spontaneous emission rate of quantum emitters via efficient near-field coupling, but one must find a balance between the Purcell enhancement and the antenna efficiency.The Purcell enhancement [13] is calculated as the enhancement of the radiative decay rate, and the antenna efficiency represents the ratio of far-field power to the total emitted power. [9]Previous studies have shown that nanocones deliver strong confinement of electromagnetic fields compared to nanorods and nanospheroids; [14] introducing a sharp tip on one end promotes field enhancement and increasing the other end (the base of the cone) help to increase the volume and thus the antenna efficiency. [15]ltrasharp nanoantennas fabricated on scanning near-field probes have shown controlled manipulation of the photophysics of quantum emitters. [16]The most commonly used scanning probes nanostructures are uncoated or metalized atomic force microscopy (AFM) probes.Hecht et al. [17] showed photoluminescence enhancement over the nonradiative losses of a single quantum dot coupled with a bowtie antenna.Here the antenna fabricated at the apex of a pyramidal AFM probe by focused ion beam (FIB) milling enabled efficient light-matter interaction of the hybrid system.Similarly, the influences of different scanning probes on the excitation intensity, excited state lifetime, and the angular emission of single molecules are also reported. [18]OI: 10.1002/adpr.202300058Optical antennas are nanostructures that introduce unprecedented possibilities for light-matter interaction at the nanoscale.An appropriately tailored plasmonic antenna can enhance the total radiative decay rate and modify the angular radiation pattern of a single-quantum emitter through controlled near-field coupling.Despite their ability to surpass the fundamental diffraction limit and confine the electromagnetic field to a tiny mode volume, fabricating 3D sharp scanning nanoscale plasmonic structures with desired aspect ratio is yet an ambitious goal.The fabrication of nanocones by gold evaporation on commercial atomic force microscopy probes followed by a focused ion beam milling technique is presented.The method is versatile and allows the fabrication of nanocones with desired dimensions around 100 nm along with an aspect ratio of ≈1.Their optical properties are studied and it is shown how the variation in the refractive index of the dielectric substrate affects the localized surface plasmon resonance of the nanocones, the decay rate enhancement, and the quantum yield of an emitter relevant for fluorescence/Raman scanning experiments.Theoretical studies using finite-difference time-domain calculations have guided the fabrication process and are consistent with experimental results.
van Hulst et al. demonstrated the possibility of nanometer resolution optical microscopy by exploiting a resonant optical nanoantenna positioned at the end of a metal-coated glass fiber near-field probe. [19]De Angelis et al. introduced conical structures based on the principle of nanofocusing for surfaceenhanced Raman scattering. [20]A highly reproducible near-field optical imaging with a sub-20 nm resolution based on templatestripped gold pyramids was established by Lukas Novotny et al. in 2012. [21]owever, the bottom line for an efficient near-field coupling was the nanofabrication of sharp plasmonic nanostructures.In the last decade, scientists have developed several fabrication techniques to create sharp plasmonic structures for various applications in information processing, [17,18] sensing, [22] spectroscopy, [21] microscopy, [19] etc.The underlying physics of these nanostructures relies on the light-matter interaction at a single emitter level.Based on the fabrication technique, the control of the ideal/critical dimension of the nanostructures varies accordingly.Thus, achieving suitable structures without sophisticated or expensive fabrication methods is still under investigation.Nanofabrication techniques, in general, can be categorized mainly into: the 'top-down' approach and the 'bottom-up' approach. [23]As the name suggests, the top-down approach creates nanostructures by deconstructing bulk materials via lithographic tools or by dry etching.Such methods allow the devising of rigorously controlled complex 2.5D geometries with high precision.However, creating high-quality 3D structures is still challenging.On the other hand, the bottom-up approach creates nanostructures from the self-organization of basic atomic/ molecular levels amid great control over the fabrication of 2.5D and 3D molecular structures.However, the technique needs compatible surfaces and selectively adding atoms to create the desired nanostructures is not trivial.The widely experimented fabrication techniques to create 3D nanocones comprise etching processes like wet chemical etching or dry reactive ion etching (RIE) and various lithography processes such as FIB lithography and electron beam lithography, natural lithography, nanoimprint lithography (NIL), and colloidal lithography.Among these largely practiced top-down approaches are FIB etching and electron beam lithography, while colloidal lithography and RIE processes are the broadly used bottom-up approaches. [24,25]his article shows a versatile, simple, and relatively fast fabrication approach of plasmonic gold nanocones by combining gold sputtering followed by FIB milling on a commercially available AFM cantilever.These nanostructures fabricated on the tip apex of sharp AFM probes can be exploited for a plethora of near-field optical phenomena.For instance, plasmonic nanostructures fabricated on the tip of AFM probes can be exploited for controlled near-field coupling.For instance, the coupling strength can be manipulated by precisely controlling the distance between the emitter and the plasmonic structure. [26]Particularly, if the emitter is embedded in a relatively thin medium, as in the case of color centers in thin diamond membranes, AFM probe-based plasmonic nanostructures allow efficient near-field coupling scheme.Employing a dark-field spectroscopy setup, we have optically characterized the nanocones and ensured that the LSPR is in near-infrared region (NIR) at the desired wavelength range for future experiments.
The theoretical studies have already verified that nanocones of dimensions around 100 nm with an aspect ratio of ≈1 can enhance the spontaneous emission rate of quantum emitters without compromising the quantum efficiency (QE). [15,27]We also address the role of the substrate and of the intrinsic quantum yield in determining the decay rates of a quantum emitter coupled to a nanocone.Starting from quantitative expressions that offer more insight into the process, we present finitedifference time-domain (FDTD) simulations under different conditions to guide future experimental work on fluorescence enhancement.

Results and Discussions
For tailoring the accurate cone geometry to enhance the nearfield coupling with a quantum emitter, we have performed FDTD calculations. [28]In this section, we show the theoretical findings on the spectral dependencies of the decay rate, and the QE on the refractive index of the dielectric substrate (n d ).The decay rate can also be written as the sum of the radiative decay rate, the intrinsic nonradiative decay rate, and the additional nonradiative decay rate due to absorption in the antenna system.See the discussion in the supplementary material.

Simulation and Theory
The spontaneous emission is not an intrinsic property of the quantum emitter, and it is also dependent on the electromagnetic environment; hence, it is important to consider the modification in the spontaneous emission by the interaction of plasmonic gold nanostructures in the presence of a dielectric substrate. [9]We have calculated this quantity by FDTD (Lumerical FDTD Solutions, the mesh size is 0.1 nm near the dipole-nano-cone gap and 1 nm elsewhere) calculations of a classical dipole placed in proximity of a plasmonic gold nanocone, in which the n d varies.
The normalized decay rate (Γ tot ) is calculated as the ratio of the power radiated by the classical dipole in the presence of the nanocone, hence considering possible metal losses, to the power radiated by the dipole in free space.The QE is represented by the symbol ('η') and it is defined as the ratio between the power radiated to the total power transferred from the emitter to the antenna.The intrinsic QE (η 0 ) is defined as the ratio of the radiative decay rate to the total decay rate in free space.In the ideal case, we assume that η 0 is 100% to calculate both η and Γ tot .In reality, η 0 of SiV color centers in thin polycrystalline diamond (PCD) membranes is presumably not more than 10%. [29]hus, in the case of an emitter-antenna interaction, both QE and total radiative decay rate must take η 0 into account.The expressions are given as follows, where the meaning of the primed quantities Γ 0 tot and η 0 is explained in the next paragraph.
Γ rad is the normalized radiative decay rate in the coupled system, that is, without the additional nonradiative decay rate due to absorption by the antenna.The derivation of the equations is shown in the supplementary material.
We remark that Γ 0 tot , for the case of an emitter-antenna system (see Figure 1a), is normalized with respect to the normalized decay rate (Γ diel ) in the dielectric substrate, to consider the actual experimental conditions.Likewise, η 0 represents the QE normalized with respect to that in the dielectric substrate.Here the modification factor also includes the fact that the spontaneous emission rate in a bulk medium is equal to the value in free space multiplied by the refractive index, indicated here as n d (because all the calculated values are normalized with respect to free space by the FDTD software).Notice that for η 0 equal to 1, the two equations reduce to Γ tot /Γ diel and to η, respectively.
Previously, we have delineated the design considerations and reasons for choosing nanoantennas with plasmon resonance in the NIR. [30]Any plasmonic nanostructure can manipulate the photonic environment of a quantum emitter, thanks to their LSPR, leading to a confined electric field in a tiny mode volume.However, to have an efficient near-field coupling between the antenna and the quantum emitter, we should optimize the design parameters of the antenna so that we can achieve spectral matching between the antenna and the emitter emission.That results in the enhancement of the total radiative decay rate of the quantum emitter with a more significant antenna efficiency of 80%.However, the interaction of the nanocone plasmonic antenna with a dielectric substrate can lead to the LSPR shift of the nanocones due to the change in the dielectric environment near the tip.Indeed, the main factor responsible for this is the radiation coupling due to the dielectric substrate.We have investigated the LSPR shift in correlation with the change in the dielectric environment of gold nanocones by FDTD calculations (previously, we have performed simulations on the optimal aspect ratio for the nanocones, which is not shown in this article). [30]o set the ground, we have simulated the near-field interaction of a nanocone of an aspect ratio of ≈1 with the presence of dielectric substrates, such as a (semi-infinite) glass coverslip (n = 1.5) and 130 nm thick PCD membrane (n = 2.4), respectively (where n is the refractive index of the dielectric substrate).The substrates will not give rise to relevant waveguiding or Fabry-Perot effects at wavelengths around 700 nm.The nanocone dimension used for the FDTD simulation is based on a real fabricated structure (Probe 1, i.e., base, b = 85 nm, height, h = 103 nm, radius of curvature, r = 25 nm).The simulated structure is a gold nanocone created on a quartz AFM probe situated on a quartz substrate of a few micrometer thickness along with a gold layer thickness of t = 30 nm.The schematics depicted in Figure 1a is a singlequantum emitter embedded Q = 5 nm beneath a dielectric substrate placed closer to a plasmonic nanocone structure [Figure 1a].The electric dipole source oriented along the symmetry axis of the gold nanocone placed at a variable distance P from the nanocone tip, which efficaciously excited the longitudinal plasmon mode of the nanocone.
Figure 1b represents the spectral dependencies of Γ 0 tot and η 0 of an emitter embedded (Q = 5 nm) in a dielectric substrate coupled with the plasmonic gold nanocone, where η 0 is assumed to be 100%.The dielectric-antenna distance P is kept as 1 nm for all the calculations in Figure 1b, but similar findings occur for larger distances, provided that near-field coupling with the substrate occurs.It is evident that a small change in the refractive index of the photonic environment shifts the LSPR resonance by around 50-80 nm due to the modification of the local environment caused by the boundary conditions at the interface.Apart from that, the decay rate and the QE of the quantum emitter become evidently dependent on the substrate (see Figure 1b). [27]Here, Γ (n = 1.5) and Γ (n = 2.4) correspond to the spectral dependencies of Γ 0 tot , whereas η (n = 1.5) and η (n = 2.4) correspond to the shift in QE with respect to a glass coverslip and diamond membrane respectively.The LSPR linewidth gets broader with a higher refractive index due to the radiative broadening, [9] as observed in Figure 1b, the blue curve corresponds to Γ (n = 2.4) .Moreover, the overall enhancement gets larger, despite the linewidth broadening.This apparently contradictory trend can be explained by the fact that Γ 0 tot is normalized with respect to the decay rate in the substrate.We have found that the latter environment can significantly change the local density of states (LDOS).
For example, an ideal emitter in a PCD membrane exhibits a radiative decay rate that is more than an order of magnitude smaller than in free space, depending on the dipole position and orientation.On the other hand, a dipole in a semi-infinite glass substrate exhibits a moderate change in the spontaneous emission rate, hence the smaller enhancement with respect to air.Thus, the very same nanostructure can lead to significantly different enhancements depending on the substrate parameters.Another interesting effect of a dielectric substrate is the increase in the quantum yield, which results from the radiative broadening of the LSPR and to a weaker excitation of nonradiative modes. [15,27]The emitter's total radiative decay rate, Γ 0 tot , enhanced by one order of magnitude, and the QE, η 0 , enhanced up to 70% as the dielectric substrate's refractive index increased for the similar emitter-antenna separation (P = 1 nm).As the distance P increases, Γ 0 tot rapidly decreases; hence, it is desirable to optimize P for controlled, efficient near-field coupling (as demonstrated in Figure 1c).However, the change in η 0 is less than 10% even if the distance P is 25 nm.At minimal distances, it is known that the nonradiative channels of the plasmonic antenna result in significant fluorescence quenching.Hence, it is ultimately essential to devise a structure that could provide more significant enhancement in Γ 0 tot while keeping a higher value for η 0 as shown here.Interestingly, for the same distance, a nanocone near a PCD membrane provides a considerably larger enhancement, although it should be kept in mind that the absolute rate might not be higher than in the case of a nanocone in free space, given the different values of Γ diel in the denominator.Nonetheless, a stronger relative change of the rates may be advantageous for investigating the photophysics of the emitter under a wider parameter space.
In Figure 1d, we present Γ 0 tot (solid curves with filled symbols) and η 0 (solid curves with hollow symbols) for different values of n d , (n = 1.5 and n = 2.4) in correlation with the change in distance P in nm for η 0 = 10%.There is an evident change with respect to the previous situation.In the case of Γ (n = 2.4) , it decreased compared to Γ (n = 1.5) due to the smaller value of Γ diel in the PCD membrane.The modification of η 0 with a change in n d reduces when Γ tot is larger than about 1/η 0 as it can be inferred from Equation (2).However, for n = 2.4 this occurs only for distances smaller than 5 nm.Furthermore, the variation of η 0 with distance encodes the relationship between Γ tot and η 0 .One can thus gain more insight into the modification of η 0 in complex environments, such as polycrystalline versus nanocrystalline host matrices or bulk versus surface boundaries, for the same emitter.

Tuning LSPR of Gold Nanocones
Usually, the nanoantennas are grown on a substrate, but here they are fabricated on a quartz AFM probe via gold evaporation followed by FIB milling, which creates a gold nanocone structure at the tip apex.Previous studies have shown that the changes in the substrate on which the nanostructures are grown can shift the LSPR.Mohammadi et al. showed that the nanocone grown on a glass substrate can shift its LSPR by more than 50 nm. [15]evertheless, the LSPR shows extensive radiative broadening, enhancing the antenna efficiency with a trade-off of reduction in the Purcell factor and the field enhancement. [15]Thus, it's essential to know how much the LSPR shift is attributed to an ideal nanocone aspect ratio when changing the substrate or its near-field environment from air to dielectric. [27]The nanostructures are engineered to have resonances in the NIR as the absorption in gold is smaller in this spectral region, also resulting in a smaller relative intrinsic linewidth of the LSPR.In addition, the emission of some of photostable quantum emitters, for example, silicon, nickel, and chromium-related color centers in diamond, are in the NIR region.Furthermore, it is of interest for biomedical applications as tissue has transparent window in the NIR. [31,32]To attain LSPR at the desired spectral wavelength, we have fabricated them with an approximate aspect ratio of 1-1.5 (height in the range of 60-150 nm and base in the range of 70-120 nm) along with a gold layer thickness of ≈30 nm on a commercial AFM probe.
Using a home-built dark-field microscopy setup, we have characterized the gold nanocone and its LSPR. Figure 2a shows the LSPR of individual nanocones for various aspect ratios.The nanocone samples are named as follows: probe 1 (b = 85 nm, h = 103 nm), probe 2 (b = 111 nm, h = 150 nm), and probe 3 (b = 92 nm, h = 107 nm).The dotted green curve represents the laser spectrum taken on a clean glass coverslip, where the laser shows oscillation and a peak at 806 nm related to the Raman line.Usually, the LSPR shifts toward the red with an increase in aspect ratio; however in here, the dimension of the plasmonic gold nanocone, including the height, base, thickness of the gold layer deposited, and the separation of the nanocone from the truncated cone, is not similar.Thus, the LSPR redshift in correlation with an increase in aspect ratio is not clearly visible.However, the fabricated structures show LSPR in the NIR region as desired.The difference in the LSPR linewidth is attributed to the variations in shape, size, radius of curvature, and sample quality related to surface smoothness.Compared to the LSPR spectral linewidths of several plasmonic nanoantennas, these gold nanocones show a narrow linewidth of 30-60 nm in air. [16,33,34]The highest Q-factor we obtained was around 22, other geometries report Q factors as high as 30. [35]he linewidth of the LSPR spectra is broadened when the plasmonic structure is near a dielectric substrate.This is due to the collective electron oscillation contributed by both radiative and dissipative mechanisms. [36]Comparing the LSPR linewidths of Probe 3 (b = 92 nm, h = 107 nm), it is noticeable that the linewidth upsurges from 60 to 80 nm as the plasmonic antenna is brought closer to dielectric (see Figure 2a,c).As the aspect ratio increases, the linewidth of the LSPR spectra becomes smaller, while for larger sizes, radiation damping is more significant, which in turn determines the linewidth (see Figure 2a). [36,37]igure 2b,c demonstrates the LSPR shift corresponding to the nanocone with b = 92 and h = 107 nm corresponding to two scenarios: nanocone near glass coverslip and PCD membrane.Figure 2b represents the LSPR shift as the nanocone is closer to a glass coverslip.The red curve indicates the LSPR in the air without the presence of a dielectric substrate.The peak around 805 nm is due to laser scattering (see the laser spectrum Figure 2a, green curve).Figure 2a shows more prominent noise compared to Figure 2b,c due to the choice of different integration times during the experimental sessions.The integration time used for Figure 2a was 30 s, while for Figure 2b,c was 60 s, respectively.
Initially, the nanocone approached the glass coverslip using the AFM piezostage.A setpoint force (few nN) is chosen so that the probe will be at proximity, around 8 AE 1 nm from the substrate.We define 0 when the probe and the surface are in contact; at this point, further approach will not change the spectral features.Our starting point is X = 8 nm from the substrate.From X, the nanocone was brought at different distances from the glass coverslip by moving the AFM cantilever using a piezostage in nm precision.The LSPR was monitored during various positions, as depicted in Figure 2b.When the distance between the nanocone and the glass coverslip reduces from 8 to 0 nm, the LSPR redshifts from 725 to 775 nm.Furthermore, as shown in Figure 2c, the LSPR becomes significantly broader and shifts more toward a higher NIR region when the index of refraction of the substrate increases to n = 2.4, PCD membrane.Once the nanocone encounters the membrane, the LSPR shifts to ≈820 nm, as shown in Figure 2c.The ripples shown in the LSPR spectra correspond to the supercontinuum laser itself.

Conclusion
In conclusion, we have shown the fabrication technique and optical characterization of sharp plasmonic gold nanocones fabricated through gold sputtering on commercial AFM probes followed by the FIB milling technique.This method shows an alternative approach for the fabrication of plasmonic nanostructures of dimensions around 100 nm, with an aspect ratio of about 1, for quantum nano-optics applications.By changing the dimension of the structures, we tuned the aspect ratio of nanocones and hence the LSPR into the NIR regime.The optical properties of the nanocones are studied using an inverted dark-field spectroscopy system combined with AFM.Since the gap between the nanocone and the truncated AFM tip varies for different nanostructures, the extent of the LSPR shift is different.However, the LSPR linewidth is relatively narrow (around 30-50 nm) and spans the required NIR spectral region, enabling them to implement for near-field coupling with NIR quantum emitters.We have also demonstrated the dielectric effect on the LSPR of the nanocones as the LDOS with the nanocone changes upon substrate variations.Nanocone with approximately an aspect ratio of 1 shows 100 nm redshift as the refractive index of the dielectric changes from n = 1 to n = 2.4.Also, we have revealed that the experimental results are in correlation with the FDTD simulations of the spontaneous emission rate and quantum yield.Our analysis unveils the relationship between the modification of the decay rate for an emitter in the dielectric substrate with and without nanocone, considering the intrinsic quantum yield.The FDTD calculations provide spectral features in good agreement with the experimental findings.However, our fabrication technique does not promise extremely sharp features (down to <10 nm radius of curvature) and reproducible nanocone dimension.As the thickness of evaporated gold layer increases, the radius of curvature of the nanocones tends to increase even though the initial AFM probe has relatively sharp (about 10 nm) tips.By optimizing the FIB milling parameters, the aspect ratio of the antenna as well as the surface plasmon coupling from the truncated gold part to the nanocones can be amended in the future.Alternatively, the nanocones can be fabricated on a commercial AFM probe by adapting the electron beam-induced metal deposition (EBID) fabrication technique, [23,37] which allows even better control over the nanostructure; however, the method is time-consuming.The goal of this work is to show the potential of scanning plasmonic gold nanocones with NIR resonances in a plethora of nanooptics applications, for example, to modify the spontaneous emission rate and the quantum yield of quantum emitters to study their photophysics in different dielectric environments.

Experimental Section
Fabrication: Here, we summarize a few of the fabrication techniques in chronological order, which have marked their success in the history of plasmonic nanostructure fabrication, especially the fabrication of nanocones.Ching Mei Hsu et al. [38] developed a technique to fabricate closely packed sharp silicon nanotips by combining Langmuir-Blodgett assembly and RIE.Monodispersed SiO 2 nanoparticles assembled into a closely packed monolayer on a silicon wafer via Langmuir-Blodgett mechanism.Utilizing the RIE process, they tuned the nanoparticles' diameter and spacing.Even though this approach has unprecedented control over the dimension of the nanoparticle, the fabrication of nanocones at the desired location using this method is wholly arduous.Monika.Fleischer et al. [25] created nanocones by merging thin-film deposition, electron beam lithography, and FIB techniques.A thin film of titanium was deposited by electron beam evaporation followed by thermal evaporation of the gold layer.Then, the etch mask was patterned onto the gold film by electron beam lithography.Finally, using a FIB milling system, the unprotected metal and the etching mask were dry etched to create nanostructures.The conical shapes were created because of the ion bombardment of gold, where the cone axis pointed along the direction of ion incidence.However, the ultimate shape, cone angle, and size of the nanocone depended entirely on lots of fabrication parameters such as lateral erosion, sputtering yield, etching conditions, etc. Wei Wu et al. [24] used the NIL technique to fabricate nanocones.Initially, a master mold of silicon nanocones created by the Bosch etching process was pressed into a polymer layer to make a daughter mold.Then polymer cones were imprinted from the daughter mold, followed by gold coating to create 3D nanocone arrays.NIL is cost-effective and deterministic; however, the quality of the imprinting structure is highly dependent on proper NIL resist.Moreover, the mechanical stiffness of the polymer is one of the prerequisites for nanocone fabrication; the softer the mold, the higher the chances of getting deformed nanostructures.The vital drawback of this fabrication method is the formation of nanocones with multiple tips.The lack of circular tips eventually prevents them from potential applications such as near-field coupling with quantum emitters.Kontio et al. [39] introduced a nanocone fabrication technique by combining UV-NIL (UV-NIL) and electron beam evaporation on flat surfaces.First, cylindrical holes were patterned onto a thick resist layer using UV-NIL and RIE, followed by metal layer deposition onto the holes by e-beam evaporation until the holes were filled completely.Finally, a lift-off process using ultrasonic agitation was performed to create an array of scalable metallic nanocones on the substrate.The metal evaporation makes the top of the hole shrink, leaving a conical shape for the nanostructure.The technique is relatively simple, low cost, and versatile, but controlling the radius of curvature of the nanocone is challenging.Moreover, the structure's sharpness and final height solely depend on the evaporation material.Mana Toma et al. [40] used a combination of colloidal lithography and oxygen plasma to fabricate nanocone arrays.This process is also known as nanosphere lithography (NSL).The 2D periodic nanostructures were created by single-step simultaneous distinctive oxygen plasma etching of polystyrene bead monolayers on Teflon films.Similarly, Zhang et al. [41] also used a similar technique, where NSL was combined with RIE to fabricate ordered arrays of nanocones.The major drawback of this fabrication technique was poorly sharpened tips of gold nanocones over simultaneous etching, which hinders their applications from near-field coupling with quantum emitters.
Moreover, obtaining good quality gold nanocones with the desired size, lattice periods, and sharp tips depends on various parameters such as the nanosphere size, the etching power, and the etching time.Björn Hoffmann et al. [33] reported the fabrication of nanocones using FIB milling of sputtered nanocrystalline gold layers.This process enables precision control; however, it is a multistep FIB etching process where the removal speed of each processing stage and patterning resolution needs different ion beam properties to create desired nanocone structures.Thus, this topdown process is also not suitable for large-scale fabrication of nanocones, as it is time-consuming and limited in the total obtainable structured area.Previously the crystallization of excimer laser and the formation of surface texturing were already reported.However, the formation of a well-defined array of nanocones at desired location with controlled dimensions has not yet been shown.
Recently, controlled fabrication of sharp plasmonic gold nanocones on a 100 nm-thick silicon nitride (Si3N4) transparent substrate via EBID followed by a gold deposition technique was reported by Flatae et al. [37] This technique enables fabrication control over the size, shape, and radius of curvature of the nanocone.Magdi et al. [42] demonstrated the fabrication of an array of random silicon nanocones via a lithography-free one-step process by KrF excimer laser.Here, the silicon nanocones were not formed by etching; instead, the silicon mass was redistributed to form nanocones.It is a relatively fast process that does not require sophisticated fabrication environments.However, according to the change in the maximum laser energy used, arrays of nanocones with random sizes were formed.
Unlike conventional fabrication methods, our process enables the fabrication of high-quality, sharp (tip radius of curvature approx.20 nm) metallic gold nanocones on a commercially available AFM probe (qp-fast, NanoAndMore GmbH) apt for several applications such as singlemolecule fluorescence microscopy, tip-enhanced Raman spectroscopy, etc.The antennas were fabricated at the apex of the conical-shaped quartz (SiO 2 ) AFM probe using gold deposition followed by FIB milling, as shown in Figure 3a.Initially, 30 nm of gold was deposited on the AFM probe via the sputtering technique, using a Kenosistec KS500 system at a power of 18 W, which resulted in a deposition rate of 0.47 Å s À1 .Since the metal film was conductive, the precise focusing of the ion beam at a distinct position for the ideal nanocone fabrication was possible.FEI Helios Nanolab 650 Dual Beam system (combined with a scanning electron microscope (SEM)) and FIB was used for both sample inspection and FIB milling.The milling was performed by bombardment using high-energy gallium ions at 30 kV and 1.1 pA, removing the deposited gold in a controlled way to define the nanostructure shape and dimension.The ion beam waist was around 40-60 nm.The AFM tip was placed in a rotating stage and the longitudinal axis was at an angle of 90°with the beam axis.After one of the sides were exposed, the stage was rotated to expose the other end (see supplementary material).
The probes characterization to optimize the desired geometry of the tip (base, height, and radius of curvature) before and after FIB milling was possible using the integrated SEM.Moreover, in order to try to improve as much as possible the nanocone quality by enlarging the grain sizes and increasing the surface smoothness, [43] we tested two different annealing processes: 2 h at 100 °C and 1 h at 200 °C, on a hot plate in a nitrogen atmosphere.The nanostructures did not show any change in shape or roughness, while the metal on the rest of the tip appeared ruined after the annealing.We think that these different effects can be due to the discontinuity in the thermally conductive metal film, which prevents heat from reaching the nanostructure.Hence, in the final samples, we did not perform annealing.All the fabrication steps were performed at the Clean Room Facility of the Istituto Italiano di Tecnologia (IIT) in Genova.
A schematic representation of the fabrication process is depicted in Figure 3a. Figure 3b,c shows the SEM images of the FIB nanocone structures.The fabricated nano-cones have different bases, heights, curvature radius, and gold thicknesses.Hence the LSPR varies according to the overall shape of the nanostructure on each AFM chip.According to our previous theoretical studies, the ideal aspect ratio required for strong modification in the spontaneous emission using the nanocone should be nearly 1, where the base and height of the nanocones were around 100 nm. [15,30]We successfully fabricated nanocones with base diameters in the range of 70-120 nm and height in the range of 60-150 nm, respectively.The aspect ratio of the nanostructures was between 1 and 1.5, and the radius of curvature was down to 20 nm.
Different from Maouli, et al. [44] we were able to produce controlled structures with significantly smaller dimensions and with a stronger electromagnetic decoupling between the nanocone and the gold-coated AFM probe and also verified by the field plots in the Supporting Information (see Figure S4).Vasconcelos et al. [45] followed a different approach than ours.They milled a gap in the AFM probe and then evaporated a metal from the front side.Overall, the structures have dimensions comparable to ours, but the rugosity of the deposited metal film was substantial, and part of the metal evaporation could fill the gap between the apex and the AFM probe.
Spectroscopy of Nanocones: We investigated the optical properties of gold nanocones fabricated on a commercially available quartz AFM probe (qp-fast, NanoAndMore GmbH).We primarily addressed the longitudinal plasmon resonance of the nanocone to match the emission of a quantum emitter.For this purpose, we optically characterized the nanocones with an inverted microscope in a dark-field microscopy configuration. [37]This enabled us to filter the weak scattering signal of the nanostructures effectively.The mode of illumination to excite both longitudinal and transversal modes is demonstrated in Figure 4a.A linearly polarized collimated supercontinuum laser (SuperK COMPACT, NKT Photonics) illuminated the nanostructures using a high-numerical aperture (NA) microscope objective (ZEISS Epiplan-NEOFLUAR 100X/0.75NA-BDDIC).The central part of the excitation laser was blocked by a central beam blocker, creating a hollow ring beam.The polarization orientation of the ring beam was adjusted via a half-wave plate.To manipulate the polarization of the hollow ring beam, part of the beam was blocked using a beam blocker (BB) before entering the objective.Thus, the partially blocked ring beam can generate both S-and P-polarization once it is focused on the dielectric (namely, we obtained S-polarization along the substrate transverse axis and P-polarization along the nanocone longitudinal axis).Apart from that, to excite the plasmon modes of the nanocone, it is significant to generate P-polarization along the longitudinal axis..In general, the spot size was around 1.3 μm, but the use of confocal pinhole defines the detection volume and makes the lateral and axial resolution of detection around 529 nm and 1.4 μm, respectively.We scanned through the focal plane to increase the signal-to-noise ratio of the nanoparticles at the tip of the AFM probe.
The partially blocked hollow ring beam entered the objective lens via a reflecting mirror (RM) (M).The mirror sent the beam toward the beam splitter (BS).The BS sent the broken ring beam to excite the nanocones through the high-NA objective and blocked the reflected laser part which came toward the collection beam path.The collection optics were maintained so that only the scattered light from the nanoparticles will enter the objective for spectroscopic measurements.As shown in Figure 4a, the illumination supercontinuum laser beam (red line) along the collection path is blocked by an iris; thus, only the desired scattering signal (signal coming from the nanocone) was filtered later by the confocal system (confocal system consisting of two planoconvex lenses (L) and a pinhole (P) at the focus).Then the collected signal was reflected from a RM and sent to an electron multiplying charge-coupled device (EM-CCD) camera (Princeton Instruments, ProEMHS: 512 BX3, back-illuminated EM-CCD, more than 90% QE in the NIR region) for dark-field imaging by a flippable mirror (FM).FM can be switched to access the spectrometer (Andor, Newton DU970P-BVF) to measure the LSPR or toward the EM-CCD camera for dark-field imaging.
Additionally, an AFM scan head (Nanowizard4, JPK Instruments AG) was placed on top of the microscope, allowing the AFM probe to be positioned at the center of the optical axis.The nanocones fabricated on the tip apex of the AFM cantilevers were mounted on the AFM for controlled positioning of the nanocone in the vicinity of a dielectric substrate (see Figure 4b).Figure 4c depicts the wide-field image of an AFM cantilever closer to a PCD membrane with the plasmonic gold nanocone (a bright spot at the center of the cantilever represents the gold nano-cone).PCD membrane is a good platform for photostable quantum emitters; for example, SiV color centers can be easily implanted in the first few nanometer layers for near-field plasmon coupling. [30,46]he AFM head facilitated three directional degrees of freedom to control the probe.Besides, the sample was mounted on an XY piezo stage, and thus in total, we had five directional degrees of freedom.In this fashion, our setup can move both the emitter and the nanocone with nanometer precision.Considering we have used a coherent broadband white light source to illuminate the sample, the scattering signal (I s ) encompasses contributions from the white light illumination (I w ).Hence, to get I s , we must normalize the measured signal I m with I w .We should also consider the background signals incorporated into I m , such as weak scattering from the substrate (in this case, it would be the scattering from the cantilever) and dark counts of the detector.Since the background signal from the cantilever and the dark counts from the detector is fairly negligible, we can eliminate them.
The LSPR of the nanoparticles depends on their dielectric environment.Hence, the change in the refractive index of the nearby medium changes its resonance position and the LDOS at the emitter's position.The LSPR shift with a dielectric substrate was verified experimentally as follows: initially, we looked at the LSPR of one nanocone (with an aspect ratio of about 1) in the air as a reference point without the presence of any dielectric.Then we investigated the shift in LSPR under two cases: the same nanocone was brought closer to a dielectric, and the LSPR shift was monitored in terms of the change in distance between the nanoantenna and the dielectric.1) First, the scattering measurements were done for the nanocone closer to a commercial glass coverslip.2) Second, the measurements were repeated with a thin PCD membrane. [46]

Figure 1 .
Figure 1.a) Schematics of the simulated structure showing metal-dielectric antenna system.The electric dipole is embedded Q = 5 nm beneath the dielectric substrate, and the minimum distance between the nanocone and the dielectric is P = 1 nm; it can be varied.The thickness of the gold layer is t = 30 nm.The sketch is not to scale.Here in the FDTD calculations, the mesh in the gap is 0.1 and 1 nm elsewhere.b) The shift in the normalized decay rate (Γ 0 tot ) correlates with the change in n d , where P = 1 nm and Q = 5 nm.Here n d varies from n = 1 (air: dashed blue curve), 1.5 (glass slide: dotted red curve), and 2.4 (diamond membrane (130 nm): solid black curve), respectively.The shift in QE ('η 0 ') in correlation with n d is represented by η (n = 1) , η (n = 1.5) , and η (n = 2.4) , indicated by the blue curve (plus symbol), red curve (line symbol), and black curve (square symbol), where the intrinsic QE (η 0 ) is 100%.c) Modification of Γ 0 tot (solid curves with solid symbols) and η 0 (solid curves with hollow symbols) of an electric dipole embedded in glass (n = 1.5, red curve) and diamond (n = 2.4, black curve) concerning distance P, where the intrinsic QE (η 0 ) is 100%.d) Modification in Γ 0 tot (solid curves with filled symbols) and 'η 0 ' (solid curves with hollow symbols) of an electric dipole embedded in glass (n = 1.5, red curve) and diamond (n = 2.4, black curve) for distance P, where the intrinsic QE (η 0 ) is 10%.

Figure 2 .
Figure 2. a) LSPR spectra of gold nanocones in the air for different dimensions.The dotted green curve represents the supercontinuum laser spectrum.b) LSPR shift of a gold nanocone (dimension b = 92 nm, h = 107 nm) when approaching a glass cover slide.The red curve shows the LSPR in air without the presence of the dielectric.The LSPR shift is ≈50 nm when the nanocone and glass coverslip are in contact.c) LSPR shift of a gold nanocone (b = 92 nm, h = 107 nm) when approaching a PCD membrane.The LSPR shift is ≈100 nm when the nanocone and membrane are in contact.

Figure 3 .
Figure 3. a) Schematic representation of the FIB milling of deposited gold from the AFM probe to create plasmonic gold nanocone at the tip apex.(The sketch is not to scale.)b) The SEM image of the fabricated gold nanocone on the AFM probe.c) The SEM images of the nanocone structures.Each AFM chip contains three cantilevers, and the fabricated nanocones have different aspect ratios.The values of the base (b) and height (h) of the nanostructures are also indicated in (nm).

Figure 4 .
Figure 4. a) Schematics of the experimental setup for dark-field microscopy of nanostructures.A BB is used to create a longitudinal polarization in the focal spot of the microscope objective.b) Schematic representation of a hybrid quantum system.Plasmonic gold nanocone fabricated on AFM cantilever is brought closer to a quantum emitter embedded in the dielectric.c) A wide-field image of plasmonic gold nanocone closer to a PCD membrane.