Large scale plasmonic cell fabrication literature review

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This page is the literature review of large scale plasmonic cell fabrication and severs as part of my PhD project at Michigan Tech under supervision of Dr. Pearce.

Controlled distance nanosphere lithography literature reviews

Nanosphere lithography fabrications

Nanostructure Array Fabrication with a Size-Controllable Natural Lithography[1]

Abstract: A simple technique for size-controllable nanostructure array formation has been developed, using self-assembled polystyrene beads whose diameters can be arbitrarily reduced by reactive ion etching. We have produced a hole array of 83 and 157 nm diameter with 200 nm pitch on Si substrate. This technique can find potential applications in many areas of science and technology.

• X-ray, e-beam and ion beam have many advantages but they are large, expensive and not easy to handle; natural lithography which relies on small structure self-organization is a relative simple and inexpensive way for large scale manufactory.
• polystyrene beads size is controlled by subsequent RIE, the array pitch is determined by the size of the sphere thus also subjected to the etch of the beads. The experiment is conducted on Si(100) with polystyrene beads with initial size of 200nm
• obtain the polystyrene beads array: Si(100) wafer rinsed in acetone then ultrasonic for 1h, the cleaned wafer was preserved in pure water to keep a hydrophilic surface. To form the array, wafer was taken out from pure water and tilted and suspension of beads diluted pure water was dropped onto it, the substrate was kept still until all the water was evaporated
• styrene beads were attenuated by oxygen RIE, the silicon wafer was not affected because the etch rate for silicon in oxygen is much smaller than the rate of the beads.
• Masking(this process can also be used for some plasmonic metal masking): The Pt-Pd mask was deposited using sputtering for about 5nm in thickness, the polystyrene beads with Pt-Pd on top were removed by rubbing the surface with acetone-soaked cotton bud.
• Lithography: The sample was etched by RIE using a mixture of oxygen and carbon fluoride(CF4:O2=9:1), since Pt-Pd mask has a much higher resistance than Si does, the Si wafer is etched and an array of Si holes formed.
• Structure nonuniformity was found in the hole size, position and shape, ascribed to the initial nonuniform bead size, rate of RIE and the non uniformity of the self-organization, however, the overall array, seeing from the SEM images attached, is quite uniform and conformal.
• By changing the RIE time for the beads, different hole diameters were obtained, by changing the time of Si etching, the pitch depth can be adjusted.
• the author summarized an empirical equation to show the diameter of the hole is in relation with the etching time of the beads, d = d0cos[arcsin(kt/2d0)], d is the diameter of the hole(equivalent to the diameter of the beads after etch), d0 is the initial diameter of the beads, k is a constant, depending on etching conditions and t is the bead etching time.
• the authors said in summary that the hole diameter being tested falls in the range of 87nm to 157nm with a pitch depth of 200nm

Fabrication of Nanopillars by Nanosphere Lithography[2]

Abstract: A low cost nanosphere lithography method for patterning and generation of semiconductor nanostructures provides a potential alternative to the conventional top-down fabrication techniques. Forests of silicon pillars of sub-500 nm diameter and with an aspect ratio up to 10 were fabricated using a combination of the nanosphere lithography and deep reactive ion etching techniques. The nanosphere etch mask coated silicon substrates were etched using oxygen plasma and a time-multiplexed ‘Bosch’ process to produce nanopillars of different length, diameter and separation. Scanning electron microscopy data indicate that the silicon etch rates with the nanoscale etch masks decrease linearly with increasing aspect ratio of the resulting etch structures.

• The general purpose of the paper is to use the nanosphere lithography to produce nano pillars and they demonstrate that not only the size but also the packing density resist the etching so the time of etching needed to be precisely controlled to yield distance-specified separation.
• experiment briefs: 2 by 2 cm Si wafer goes through RCA clean at 70 Celsius for 30min followed by DI cleaning and N2 flow drying. 350ul of PS beads with diameter 500nm was diluted with a mixed solution(50ul Triton X-100 in methanol by 1:400), the solution was SPIN-COATED on the samples with a spin coater, the spin program consists three parts:1) 400rpm 10s to spread the solution evenly; 2) 800rpm for 2min to spin away the excess bead solution; 3) 1400rpm 10 s to spin off the excess materials from the edges. the samples were then tailored by a parallel plate RIE etcher with 200sccm of oxygen and 8.4 sccm of tetrafluoromethane at a pressure of 200mTorr and RF power of 100W
• Nano pillar fab: alternative cycles of RIE in a flow of SF6 (12 sccm 12 sec) and passivation in a flow of C4F8( 85 sccm 9 sec) were used to etch the unprotected areas and to deposite fluorinated polymer to protect the side walls of the resulting etched structures. RF power = 600W, pressure = 4.5 mTorr, temperature = 25 C.
• the spin-coating was a more controllable process than the traditional dip-drying process in spreading the beads on the sample surface, especially for patterning a monolayer of beads.
• During the fab of nano pillars from the bead mask, the reactive ions preferentially attack the bottom of the pitch. The side walls were etched only minimally due to the parallel directionality of the ions to the side walls and the protection of deposited fluorinated polymers.
• This is likely to say: without the alternative etching cycles, the side walls may be attenuated from top to bottom, resulting in a spike-like nano pillar. For pillars with high aspect ratio this etching process is crucial but for pillars that is very shallow the prectetion of the polymer is not further demonstrated to be necessary
• the difference between this paper and the previous one is that in the first paper it is the areas that covered by beads being 'etched', this one etched the unprotected areas which are the areas without beads on them!
• silicon etch rate drops as the aspect ratio goes higher, beads mask was degraded in prelonged silicon etch period

Surface patterning by nanosphere lithography for layer growth with ordered pores[3]

Abstract: Porous Ta layers were grown by glancing angle deposition (GLAD) onto two types of regular surface patterns: honeycomb nanodot arrays and pyramidal hole arrays. The patterning technique employs colloidal self-assembly of 260- to 700-nm-diameter SiO2 nanospheres that form hexagonal close packed monolayers on Si(001) surfaces. Directional evaporation through the holes between the nanospheres yields honeycomb nanodot patterns, while sputter deposition through the nanospheres leads to a thin film mesh that acts as a mask during subsequent anisotropic etching, resulting in an array of inverted pyramid holes. GLAD on nanodot patterns results in honeycomb nanopillar arrays containing a regular array of 450-nm-wide pores that are each surrounded by six 286-nm-wide pillars. GLAD on inverted pyramid hole patterns leads to porous layers with arrays of 280-nm-wide vertical nanochannels separated by 60–130 nm wide Ta nanorod walls. These results demonstrate that substrate patterning by nanosphere lithography is effective in controlling both the size and the arrangement of pores during GLAD.

• This paper shows a particular application of NSL(nanosphere lithography): To pattern the surface for growth of GLAD porous materials. The paper combines GLAD and NSL to produce honeycomb shaped nano pillar and vertical nano channel array.
• two PVD methods being used are 1) e-beam evaporation and 2) magnetron sputtering, the e-beam of Cr comes perpendicular to the surface of the sample through the pores of the closed packed nanosphere array, resulting in small pyramids of Cr clusters forming a honeycomb array, the sputtering is not direction selective so that it comes at all direction, giving a broader yet thinner layer that almost cover everywhere on the sample surface other than those pre-occupied by the big silica sphere, in other word, it forms a Cr thin interconnected two dimensional mesh.
• nano spheres were removed chemically, using 0.5% HF solution. for the Cr mesh formed in sputtering process, the author performed a second etch using 25% sodium hydroxide at 70 Celsius for 30 sec, gives an inverted pyramid hole array(the masked area is protected and remains intact during etching).
• The author then did the so-called GLAD in a UHV charmber using magnetron sputter deposition system. Ta target is used with a angle of 84 degree. In the e-beam one the Ta is grown on the Cr pyramids to form an array of pillars, six such pillars encircles a pore; for the sputtered one Ta grows on the thin mesh layer(with inverted pyramidal holes). Deposition i nthe holes is suppressed by atomic shadowing, leading to a squared shaped nano channels that elongate along the growth direction perpendicular to the surface.

Ordered nanostructures array fabricated by nanosphere lithography[4]

Abstract: Ordered PS array was deposited on the silicon substrate and the rigid monolayer formed due to the interconnection by the sodium dodecyl sulfate. Metal film with nanoholes was fabricated by depositing the film onto the PS array. Co nanorings were fabricated in the following Ar ion etching process masked by PS particles. Silicon-based nanoholes were also fabricated by RIE process after the removal of the PS particles by chemical solution. The diameter of PS particle was thinned by the O2 etching process, and thus it is possible to control the diameters of the nanorings and the nanoholes.

• This paper, differed from other NSL methods, used a easy self-assembly method to form a close packed polystyrene nanosphere array that is much more uniform than tradition spin-coating or Langmuir-Blodgett methods(from those attached SEM images), the following RIE and sputtering have no fancy stuff, just to accomplished a complete NSL course..
• Experimental: start from commercial 200nm monodisperse polystyrene particles. Wafer was then immersed into water and polystyrene particles solution to form a normal Langmuir-Blodgett monolayer, the layer at this moment is, however, less close packed and less uniform. The nanosphere floats randomly on the surface for the sake of surface strain, the pattern showed no long-range order. fancy stuff begins here, they dipped sodium dodecyl sulfate solution into the wafer surface and this drove the monolayer into highly ordered, dense packed hexagonal pattern over 1cm2,
• The author doesn't mention what tricky chemical reaction happen there but just said the dodecyl sulfate solution acts as interconnector between spheres. I guess the dodecyl sulfate anion behaves as a surfactant with hydrophilic end(negatively charged) closed to the (static electronically positive) organic sphere while hydrophobic chain stretching out like a long tail, because the hydrophobic nature they tend to crosslink each other, hence the loose and less close packed sphere array became denser.
• So the key to this is to make sure the surfactant falls onto it the monolayer properly so that there won't be any three dimensional crosslink, and the structure of the monolayer is kept intact but denser. Thus why the author doing the lift off very carefully.
• Let's review the whole process: first, diluted polystyrene solution applied to a modified substrate and spread all over the substrate;second, a monolayer of spheres formed on the surface of water; third, carefully slide the wafer into the glass vessel filled with water and polystyrene particles on surface; dip sodium dodecyl sulfate solution onto the water surface; lift off the silicon wafer carefully.take a look at ref [19] for more information!!!
• similiar empirical oxygen RIE equation for bead etching was obtained, D = D0cos(arcsin(kt)/2D0), in this experiment the author took k=4.0 as an empirical value. data agrees well with the equation, according to the author, thus controlling of the size of the beads become feasible.
• either fancy hexagonal pyramids by e-beam eva or Au spread films with holes by sputtering is applicable as important applications of NSL, many of those applications find their further development in optical, especially in plasmonic related areas.
• tetrahydrofuran can etch away the polystyrene beads, do it in ultrasonic bath!!!

Application of two-dimensional polystyrene arrays in the fabrication of ordered silicon pillars[5]

Abstract: 2D ordered Si-based pillars were fabricated by nanosphere lithography. Polystyrene nanosphere monolayer was deposited onto the silicon substrate by the self-organization technique, which was used as mask in the following RIE process. The polystyrene nanospheres were thinned by oxygen plasma, and thus the radius of silicon pillars can be controlled.

• This paper is a proceeding work from the previous one, the author is the same author, method is the same method, the only difference is that the author find a new application to make nano pillar using the polystyrene beads NSL, the resulting nano pillars are spike-like shaped, not column which has uniform diameter from top to bottom, this is probably because the author didn't use any protection methods to protect the sidewalls of newly grown nano pillars, the protection approach introduced in the 2nd paper would be useful for this kind of application.
• The author wrote a more detailed lab instruction this time, but I'm still confused with some of the description. Posted the full context here for further analysis. Experimental: polystyrene particles purchased were in 10wt% aqueous solution, which was then diluted by equal amount of ethanol. Si(100) wafer boiled in solution of NH4OH, H2O2 and H2O(volume 1:2:6) for 5min. Then the wafer were washed thoroughtly and were kept in 10% sodium dodecyl sulfate solution for 24h to yield a hydrophilic surface. (they dried the wafer or not?)7uL diluted polystyrene solution mentioned above was applied onto the modified substrate(make sure it spread all over the substrate!). The wafer was then slowly immersed into the glass vessel filled with water and polystyrene particles started to form an unordered monolayer on the water surface(I don't what solution is filled into the glass vessel, nor do the author specifies it!Is that the )Some sodium dodecyl sulfate solution was dipped onto the water surface and a large monolayer with highly ordered areas was obtained. Such monolayers were then lifted off using previous mentioned silicon wafers.
• So, the monolayer is formed on the water surface, then my spread 7uL diluted PS solution onto the modified substrate? And what is actually filled in the glass vessel before immersing the wafer?
• All the RIE stuff is no fancy at all, it is a copy of those mentioned before.

Nanosphere Lithography:  A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics(Review paper)[6]

Abstract: Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 20?1000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed in detail. More specifically, the effects of nanoparticle size, shape, interparticle spacing, nanoparticle-substrate interaction, solvent, dielectric overlayers, and molecular adsorbates on the LSPR spectrum of Ag nanoparticles are presented. This systematic study of the fundamentals of nanoparticle optics promises to find application in the field of chemical and biological nanosensors; herein, the initial data demonstrate that LSPR spectroscopy of Ag nanoparticles can be used to sense specifically bound analytes with zeptomole per nanoparticle detection limits and no detectable nonspecific binding.

• LSPR: localized surface plasmon resonance, occurs when the correct wavelength of light strikes a metallic nanoparticle, causing the plasma of conduction eletrons to oscillate collectively. The term LSPR is used because this collective oscillation is localized within the near surface region of the nanoparticle. The two consequences of exciting the LSPR are 1) selective photon absorption and 2) generation of locally enhanced or amplified electromagnetic fields at the nanoparticle surface. LSPR for noble metal nanoparticles in the 20 to a few hundred nanometer size regime occurs in the visible and IR regions of the spectrum and can be measured by UV-VIS-IR extinction spectroscopy.
• To spread the nanospheres across the substrate to form a uniform monolayer, chemical modification of the nanosphere surface is often done to have cnegatively charged functional group such as carboxylate or sulfate coated surface that is electronstatically repelled by the negatively charged surface of a substrate such as mica or glass. Only hexagonally closed-packed nanospheres is self-assembled onto the substrate.
• double layer periodic particle arrays can form if one increases the concentration of nanospheres in the solution to be self-assembled onto the substrate, a significant portion of the colloidal crystal will consist of double layers of hexagonally close=packed nanospheres. The resulting double layer has two possible configurations with the third layer to be assembled onto it, 1)ABAB, 2)ABCABC, the latter offers no space for metal to be deposited onto the substrate. Double layer has promising application in increasing the store capacity in magnetic storage material.double layer array offers a much smaller space for deposition, therefore the resulting metal array will be consisted of dots rather than triangular shaped domains. Diameter of those dots can be adjusted down to 30nm according to the author
• This may be useful in plasmonic implementation, tracking!!! Nanoring: An array of nanorings within a single layer NS array formed by e-beam deposition of high melting point transition metals, Hypothesis: low kinetic energy atoms that travel along a direct line of sight from the depositon target to the substrate and stick where they strike the substrate to give a triangular array; high kinetic energy atoms(1-10eV) that travel along off-normal trajectories are able to continue their journey after the first strike. After several times bounce between the substrate and the NS mask they lose all their KE and finally settled close to the foot of NS to form a nanoring.
• Angle-resolved NSL: Control of the deposition angle to produce new deposition motifs.(not quite useful for our purpose)
• LSPR with Ag particles.Figure 11 is very important here! It shows the maximum absorption wavelength varies with shape, height and length of Ag array. YOU MUST CHECK THE FIGURE!!

Wafer-Scale Periodic Nanohole Arrays Templated from Two-Dimensional Nonclose-Packed Colloidal Crystals[7]

Abstract: This communication reports a simple yet versatile nonlithographic approach for fabricating wafer-scale periodic nanohole arrays from a large variety of functional materials, including metals, semiconductors, and dielectrics. Spin-coated two-dimensional (2D) nonclose-packed colloidal crystals are used as first-generation shadow masks during physical vapor deposition to produce isolated nanohole arrays. These regular nanoholes can then be used as second-generation etching masks to create submicrometer void arrays in the substrates underneath. Complex patterns with micrometer-scale resolution can be made by standard microfabrication techniques for potential device applications. These 2D-ordered nanohole arrays may find important technological applications ranging from subwavelength optics to interferometric biosensors.

• In this paper the author demonstrate a 2D nonclosed-packed colloidal crystal array(nanosphere array) using spin-coating. The principle is that the normal pressures produced by spin-coating and monomer photopolymerization squeeze particles against the substrate to form nonclose-packed 2D array. In their SEM images the beads are separated from each other at a distant of about 1.4D, where D is the diameter of the beads..
• The spheres function as shadow masks to obstruct material deposition underneath them. As the colloids are not in contact in the original templates, the deposited materials fill interstitials to form continuous films rather than distinct islands as in nanosphere lithography

Sub-100 nm Triangular Nanopores Fabricated with the Reactive Ion Etching Variant of Nanosphere Lithography and Angle-Resolved Nanosphere Lithography[8]

Abstract: Nanosphere lithography (NSL) is combined with reactive ion etching (RIE) to fabricate ordered arrays of in-plane, triangular cross-section nanopores. Nanopores with in-plane widths ranging from 44 to 404 nm and depths ranging from 25 to 250 nm are demonstrated. The combination of angle-resolved nanosphere lithography (AR NSL) and RIE yields an additional three-fold reduction in nanopore size.

• The author demonstrate that by using angle resolved nanosphere lithography(AR NSL), the criterion for nanopores with the following properties can be met:1) extremely uniform nanopores(in-plane widths = 44nm-404nm); 2)controlled nanopore depth(25nm-250nm); 3) controlled, uniform nanopore shapes; and 4) high areal density(~1010pores cm-2)..
• The nanopores formed by simply employing a single layer PS beads array followed by reactive ion etch looks uniform in wafer scale with slight amount of slip dislocations( dislocation free region ~= 10-100um2, given the etching condition, by changing the etching time(2-10min), nanopores depth ranges from ~25nm - 250nm.
• cross-sectional SEM and AFM are used in analysis the nanopores, however, AFM tip is sometimes too big for a small pore thus tip convolution is inevitable.
• Theoretically predicted in-plane with goes in agreement with experiment results! slight deviation can be attributed to AFM tip convolution, anisotropic RIE process.
• The RIE variant of AR NSL yields a further reduction in nanopore in-plane widths by approximately a factor of 3. In addition, a variety of nanopore aperture shapes are possible with AR NSL. However, tilting of the substrate will affect the uniformity of the etching parameters. resulting in further deviation of the experimental nanopore geometry from the predicted nanopore geometry. The tilted sample yields generally more stretched nanopores in terms of the tilt angle.
• personally, this paper may not be a Nano Letter level paper!!!

Optical properties

Nanosphere Lithography:  A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics(Review paper)[6]

Abstract: Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 20?1000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed in detail. More specifically, the effects of nanoparticle size, shape, interparticle spacing, nanoparticle-substrate interaction, solvent, dielectric overlayers, and molecular adsorbates on the LSPR spectrum of Ag nanoparticles are presented. This systematic study of the fundamentals of nanoparticle optics promises to find application in the field of chemical and biological nanosensors; herein, the initial data demonstrate that LSPR spectroscopy of Ag nanoparticles can be used to sense specifically bound analytes with zeptomole per nanoparticle detection limits and no detectable nonspecific binding.

• LSPR: localized surface plasmon resonance, occurs when the correct wavelength of light strikes a metallic nanoparticle, causing the plasma of conduction eletrons to oscillate collectively. The term LSPR is used because this collective oscillation is localized within the near surface region of the nanoparticle. The two consequences of exciting the LSPR are 1) selective photon absorption and 2) generation of locally enhanced or amplified electromagnetic fields at the nanoparticle surface. LSPR for noble metal nanoparticles in the 20 to a few hundred nanometer size regime occurs in the visible and IR regions of the spectrum and can be measured by UV-VIS-IR extinction spectroscopy.
• To spread the nanospheres across the substrate to form a uniform monolayer, chemical modification of the nanosphere surface is often done to have cnegatively charged functional group such as carboxylate or sulfate coated surface that is electronstatically repelled by the negatively charged surface of a substrate such as mica or glass. Only hexagonally closed-packed nanospheres is self-assembled onto the substrate.
• double layer periodic particle arrays can form if one increases the concentration of nanospheres in the solution to be self-assembled onto the substrate, a significant portion of the colloidal crystal will consist of double layers of hexagonally close=packed nanospheres. The resulting double layer has two possible configurations with the third layer to be assembled onto it, 1)ABAB, 2)ABCABC, the latter offers no space for metal to be deposited onto the substrate. Double layer has promising application in increasing the store capacity in magnetic storage material.double layer array offers a much smaller space for deposition, therefore the resulting metal array will be consisted of dots rather than triangular shaped domains. Diameter of those dots can be adjusted down to 30nm according to the author
• This may be useful in plasmonic implementation, tracking!!! Nanoring: An array of nanorings within a single layer NS array formed by e-beam deposition of high melting point transition metals, Hypothesis: low kinetic energy atoms that travel along a direct line of sight from the depositon target to the substrate and stick where they strike the substrate to give a triangular array; high kinetic energy atoms(1-10eV) that travel along off-normal trajectories are able to continue their journey after the first strike. After several times bounce between the substrate and the NS mask they lose all their KE and finally settled close to the foot of NS to form a nanoring.
• Angle-resolved NSL: Control of the deposition angle to produce new deposition motifs.(not quite useful for our purpose)
• LSPR with Ag particles.Figure 11 is very important here! It shows the maximum absorption wavelength varies with shape, height and length of Ag array. YOU MUST CHECK THE FIGURE!!

Parallel Fabrication of Plasmonic Nanocone Sensing Arrays[9]

Abstract: A fully parallel approach for the fabrication of arrays of metallic nanocones and triangular nanopyramids is presented. Different processes utilizing nanosphere lithography for the creation of etch masks are developed. Monolayers of spheres are reduced in size and directly used as masks, or mono- and double layers are employed as templates for the deposition of aluminum oxide masks. The masks are transferred into an underlying gold or silver layer by argon ion milling, which leads to nanocones or nanopyramids with very sharp tips. Near the tips the enhancement of an external electromagnetic field is particularly strong. This fact is confirmed by numerical simulations and by luminescence imaging in a confocal microscope. Such localized strong fields can amongst others be utilized for high-resolution, high-sensitivity spectroscopy and sensing of molecules near the tip. Arrays of such plasmonic nanostructures thus constitute controllable platforms for surface-enhanced Raman spectroscopy. A thin film of pentacene molecules is evaporated onto both nanocone and nanopyramid substrates, and the observed Raman enhancement is evaluated.

• Sharp cone tips lead to well-defined highly localized area of excitation and high field intensity.
• The author use single and double layer nanosphere mask on top of Au/Ti film combining with RIE and Argon ion milling to create pyramids and cones
• The tip of nanotriangles can be tailored from the triangular to an approximately circular shape by annealing, which means if we do it more gently we may end up with more round-like tips rather than turn the whole triangle to sphere. and the shape of the tip matters a lot in current simulation. An individual paper discuss the effect of tip sharpness could be possible
• Beads are removed by ultrasonication in toluene
• The author showed the patterned sphere monolayer can be obtained over several cm^2
• Field enhancement simulation shows localized surface plasmon is the major cause for extinction, the simulation is done in COMSOL Multiphysics and near field figures for |E|, |Ex,y,z| are plotted and related to tip excitation. Very importantly, in their simulation they argue that the plasmonic cone should be kept separated from the surface of silicon substrate with tiny air gap for numerical reasons, this should be applied in our simulation

Mapping of plasmonic resonances in nanotriangles(this is another optical overkill paper)[10]

Abstract: Plasmonic resonances in metallic nano-triangles have been investigated by irradiating these structures with short laser pulses and imaging the resulting ablation and melting patterns. The triangular gold structures were prepared on Si substrates and had a thickness of 40 nm and a side length of ca. 500 nm. Irradiation was carried out with single femtosecond and picosecond laser pulses at a wavelength of 800 nm, which excited higher order plasmon modes in these triangles. The ablation distribution as well as the local melting of small parts of the nanostructures reflect the regions of large near-field enhancement. The observed patterns are reproduced in great detail by FDTD simulations with a 3-dimensional model, provided that the calculations are not based on idealized, but on realistic structures. In this realistic model, details like the exact shape of the triangle edges and the dielectric environment of the structures are taken into account. The experimental numbers found for the field enhancement are typically somewhat smaller than the calculated ones. The results demonstrate the caveats for FDTD simulations and the potential and the limitations of “near field photography” by local ablation and melting for the mapping of complex plasmon fields and their applications.

• System: Au triangle on silicon substrate with thickness of 40nm and side length of 500nm
• Uses (black technology) femtosecond and picosecond laser at 800nm to excite higher order plasmon modes in triangle array
• The substrates carrying the nanostructures are irradiated with pulsed laser light. The ablation experiments are realized by using irradiation with femtosecond laser pulses, i.e., pulses shorter than the internal heat diffusion times. When using pico- and nanosecond pulses instead, the heat diffusion results in controlled melting of the nanostructures
• The author provide detailed analysis on shape of nanotriangles by AFM and SEM imaging, they argued that nanosphere lithography has the advantages in making small feature than conventional tools, the tips of nanotriangle can be made as small as 5nm, the down side is the inclined walls (shown in AFM image) and the triangle seem to be surrounded by ensemble of small solid droplets as a result of evaporation This could be another explanation to the discrepancies between simulation and experiments The author uses AFM images as a guide for their simulation
• Field enhancement should be averaged over the height of the triangle
• There are Intensity enhancement, Absorption Enhancement and Field Distribution pictures. I assume the intensity enhancement if emw.normE and absorption enhancement is emw.Qh, but I should verify this with Dr Guney By the way, in our simulation I show the 1-R-T is roughly equals to emw.Qh, which means dissipation is almost the entire reason for extinction, this result can go to the optical paper
• The field enhancement (ewm.normE) doubled for straight-edged triangle compared to concave triangle, so our simulation should definitely go consistently with our AFM images, but sometimes it's difficult to do so due to the perturbation of evaporation, which results in unpredictable structures

Exploiting the localized surface plasmon modes in gold triangular nanoparticles for sensing applications[11]

Abstract: In this study we investigate and exploit, for optical sensing, the surface plasmon excitation in gold triangular nanoparticles with high aspect ratios (i.e., the ratio of the edge length of the triangles with the height) prepared by nanosphere lithography. As shown previously, the shape and size of these nanoparticles were used to tune their optical properties, monitored by far field extinction spectroscopy. Interestingly, several localized surface plasmon resonances were detected in the visible and near infrared regions and were attributed to dipole and quadrupole modes. These modes, identified from numerical simulations, “red-shift” as the aspect ratio of the particles increases. The plasmon modes observed for larger triangles exhibit unexpected sensitivity with a change in the refractive index. From experiments and numerical simulations, this higher sensitivity has been attributed to an increase of the local field enhancement for sharper tips. This new effect can provide important information for the design of particles as building blocks for sensing applications.

• I read this paper three times, it's an optical overkill again
• The author made Au triangular array from nanosphere lithography with beads of 4 difference diameters and the following deposition yields 40nm and 80nm in thickness. The main point of this paper is to show how aspect ratio(AR) of Au particles affects the extinction spectrum, this paper should be considered as an extension study of ref6
• The Author first investigate the plasmon modes of Au nano triangle array by experiment and simulation: They found 1) increasing AR leads to a red-shift as well as an increase in intensity of resonance peaks'
• COMSOL simu is done to confirm and analysis the experiment result, the made the exactly same model as I did - using 1/4 of the unit cell with normal incidence from port - and the calculation is exactly the same too - doing 1 - T to numerically calculate extinction spectrum where T is the normalized transmitted power through a plane underneath the particles. The difference would be: 1) they use n = 1.5426 for the substrate, 2) they do not derive T from port under substrate, the obtain T from particle/substrate interface, 3) they don't use PML, but I'm not sure, they didn't mention that, but not using PML is likely to yield erroneous result in this type of study
• There's a sentence in the article: Please note that care has to be given to the geometric parameters used to model the experiments, in order to fall well within the range of the experimentally measure values. Which means they tried desperately on tuning the geometry of the triangle array in order to match their experiment result, geometry of the triangle matters a lot and worth further investigation
• Dipole and Quadpole modes are identified by experiment and simulation. In our result, the two modes shown in experiment but in simulation the quadpole is not well recognized, we should try add PORT right underneath the particle (not the port underneath the substrate) and try again
• Another thing to notice is that their simu peaks are sharp and smooth, our simulated peaks are zigzagged, not just the simu data, the experiment transmission is also zigzagged, I guess it's possibly because of the use of extinction spectrometer and PORT as mentioned earlier
• The author gives near field |E| distribution vs. AR, which is sort of the standard procedure for this type of paper, both dipole and quadpole peaks red-shift as AR goes up.
• In the end the paper did simulation with medium of different n to test the sensitivity of the Au triangle array as a potential candidates for LSPR or SERS.

Localized Surface Plasmon Resonance Spectroscopy of Single Silver Triangular Nanoprisms[12]

Abstract: The plasmonic properties of single silver triangular nanoprisms are investigated using dark-field optical microscopy and spectroscopy. Two distinct localized surface plasmon resonances (LSPR) are observed. These are assigned as in-plane dipolar and quadrupolar plasmon excitations using electrodynamic modeling based on the discrete dipole approximation (DDA). The dipole resonance is found to be very intense, and its peak wavelength is extremely sensitive to the height, edge length, and tip sharpness of the triangular nanoprism. In contrast, the intensity of the quadrupole resonance is much weaker relative to the dipole resonance in the single particle spectra than in the ensemble averaged spectrum. Several parameters relevant to the chemical sensing properties of these nanoprisms have been measured. The dependence of the dipole plasmon resonance on the refractive index of the external medium is found to be as high as 205 nm RIU-1 and the plasmon line width as narrow as ?0.17 eV. These data lead to a sensing figure of merit (FOM), the slope of refractive index sensitivity in eV RIU-1/line width (eV), as high as 3.3. In addition, the LSPR shift response to alkanethiol chain length was found to be linear with a slope of 4.4 nm per CH2 unit. This is the highest short-range refractive index sensitivity yet measured for a nanoparticle.

• This is all about round tip and height effect
• This is done in solution environment, which is not the same case, the author investigate single particle LSPR, which means diffraction is not covered by the paper
• A good way to analysis scattering is to use dark field spectroscopy, which means if you put Ag or Au triangle array on dark field imaging tool, and if all the theory goes correct, you gonna see colorful scattered light from the objective lens, and more important, if grating happens, you gonna see colorful strips changing with respect to incident angle. Find out on campus who has the dark field spectroscopy will help in analysis scattering and diffraction
• The sharper the tip, the broader the peak line width, the broadest resonance comes from perfect sharp tips.

Tuning nanopatterns on fused silica substrates: a theoretical and experimental approach[11]

Abstract: In this study we develop a novel approach to tune nanopatterns on fused silica substrates exploiting the polarization dependence of the strongly localized near field of highly ordered triangular nanoparticle arrays. For this purpose such arrays were prepared by nanosphere lithography on fused silica substrates and subsequently irradiated with single 35 fs long laser pulses. The irradiation leads to the excitation of localized surface plasmon polariton resonances, followed by ablation of the nanoparticles and partially of the substrate. By this means, nanostructures were generated on the substrate surface, reflecting the local fields in the vicinity of the triangular nanoparticles. Depending on the applied fluence, small holes as well as extended nanostructures with dimensions well below the diffraction limit have been created. Furthermore, by rotating the linear polarization of the laser light by 90° with respect to the orientation of triangular nanoparticles, different plasmon modes have been excited, which in turn, alter the local field distribution. As a result, either nanochannels or bone like shaped nanogrooves in a chequered structure were generated on the fused silica substrates. Finite-difference time-domain simulations demonstrate, that the results can, in fact, be explained by the enhanced near field distribution, which is dominated by the excitation of localized surface plasmon polariton resonances in the triangular nanoparticles. It is shown, that the fluence and the polarization of the laser light are the key parameters in nanogroove and nanochannel formation.

• The author generate nanopatterns on fused silica substrate using polarization dependence of strong localized near field of highly ordered triangular nanoparticle array
• By rotating the linear polarization of the laser light by 90 deg with respect to the orientation of triangular nanoparticles, different plasmon modes have been excited
• Field distribution is analyzed using FDTD

Facile multifunctional plasmonic sunlight harvesting with tapered triangle nanopatterning of thin films[13]

Abstract: Plasmonic absorbers have recently become important for a broad spectrum of sunlight-harvesting applications exploiting either heat generation, such as in thermal photovoltaics and solar thermoelectrics, or hot-electron generation, such as in photochemical and solid state devices. So far, despite impressive progress, combining the needed high performance with fabrication simplicity and scalability remains a serious challenge. Here, we report on a novel solar absorber concept, where we demonstrate and exploit simultaneously a host of absorption phenomena in tapered triangle arrays integrated in a metal–insulator–metal configuration to achieve ultrabroadband (88% average absorption in the range of 380–980 nm), wide-angle and polarization-insensitive absorption. Furthermore, this absorber is subwavelength in thickness (260 nm) and its fabrication is based on a facile, low-cost and potentially scalable method. In addition, the geometry of our design makes it compatible for both heat and hot electron generation.

• The importance of this one is that they are related to PV, however, they didn't do cell simulation
• The paper takes advantages of plasmonic multiple resonances in a metal-insulator-metal(MIM) structure, MIM is the stack of silver particles - dielectric - silver film. (In the paper they use gold instead of silver)
• The author define two variable for triangle array tip tapering. Rb = diameter of the beads, R = the curvature of the patter, R = Rb + deltaR, where deltaR >= 0, then deltaR > 0, the curvature of the particle is larger than the beads, the result is larger and more closed triangle, we know that closer tips yield higher enhancements, that's why the author want to change R, R is tuned by gentle RIE
• The use home-built invert microscope with the functionality to excite and collect plane wave to and from the sample, they claim their system is capable of collecting both reflection and scattered light. A = (I-R)/I is used to generate absorption spectrum, where I is measured by reflecting from an broad flat silver mirror
• they use PEC/PMC for normal incidence and PEC or PMC + Floquet for oblique incidence in their simulation, the unit cell size is 1/4 of the actually unit cell, this is the as we did in our lab
• both simulation and measured data shows that the closer the two tips, the more broadband absorption happens, however, the tip should still be kept separated as connected tip has way less enhancement. They claim absorption stays >85% with spectral range of 380-850nm and slowly drop to 70% at 980nm
• Angular dependency: normal incidence is broadband absorption, absorption at long wavelength quickly decreases as incident angle goes more oblique. at 40 deg oblique, the absorption above 850nm is about 50% of that of normal incidence.
• the optimal insulator thickness for their system is 60nm. optimal beads radius is about 200nm, they made a pretty nice contour graph showing how maximum absorption is obtained by varying beads size and insulator thickness.
• The author attribute the enhancement in absorption to 1) grating-mediated coupling of EM field to the surface plasmon polariton(SPP) exicted at the interface between the dielectric and back reflector, 2)dipole-dipole interaction between the triangles and the back reflector, 3)localized surface plasmon resonance of the individual tapered triangles, and 4) energy dissipation at tips.
• Total absorption is (optimized) 85% using Rb = 200nm and h(insulator) = 45nm.
• They didn't consider Fabry-Perot effect as a major enhancement factor

Near-field effects and energy transfer in hybrid metal-oxide nanostructures[14]

Abstract: One of the big challenges of the 21st century is the utilization of nanotechnology for energy technology. Nanoscale structures may provide novel functionality, which has been demonstrated most convincingly by successful applications such as dye-sensitized solar cells introduced by M. Grätzel. Applications in energy technology are based on the transfer and conversion of energy. Following the example of photosynthesis, this requires a combination of light harvesting, transfer of energy to a reaction center, and conversion to other forms of energy by charge separation and transfer. This may be achieved by utilizing hybrid nanostructures, which combine metallic and nonmetallic components. Metallic nanostructures can interact strongly with light. Plasmonic excitations of such structures can cause local enhancement of the electrical field, which has been utilized in spectroscopy for many years. On the other hand, the excited states in metallic structures decay over very short lifetimes. Longer lifetimes of excited states occur in nonmetallic nanostructures, which makes them attractive for further energy transfer before recombination or relaxation sets in. Therefore, the combination of metallic nanostructures with nonmetallic materials is of great interest. We report investigations of hybrid nanostructured model systems that consist of a combination of metallic nanoantennas (fabricated by nanosphere lithography, NSL) and oxide nanoparticles. The oxide particles were doped with rare-earth (RE) ions, which show a large shift between absorption and emission wavelengths, allowing us to investigate the energy-transfer processes in detail. The main focus is on TiO2 nanoparticles doped with Eu3+, since the material is interesting for applications such as the generation of hydrogen by photocatalytic splitting of water molecules. We use high-resolution techniques such as confocal fluorescence microscopy for the investigation of energy-transfer processes. The experiments are supported by simulations of the electromagnetic field enhancement in the vicinity of well-defined nanoantennas. The results show that the presence of the nanoparticle layer can modify the field enhancement significantly. In addition, we find that the fluorescent intensities observed in the experiments are affected by agglomeration of the nanoparticles. In order to further elucidate the possible influence of agglomeration and quenching effects in the vicinity of the nanoantennas, we have used a commercial organic pigment containing Eu, which exhibits an extremely narrow particle size distribution and no significant agglomeration. We demonstrate that quenching of the Eu fluorescence can be suppressed by covering the nanoantennas with a 10 nm thick SiOx layer.

• Please simply jump to Discussion B. Systems with Ag nanoantennas from NSL and C. Numerical simulation of the eletrical field distribution
• Dark technology - confocal microscopy used to focus on a single triangle to perform optical study
• The author is being able to somehow separate the R, T, fluorescence and scattered light, the result is discovery of the dark area around nano triangles, indicating strong scattering happens
• COMSOL simu tells the normE/E0 for triangle array, the most enhanced area is the gap between the bowtie triangle pair with polarization direction aligned with the tie bisector, other polarization gives much much much weaker enhancement
• The difference between n(air) and n(sub) is also important, as the difference increases, the enhancement peak (=extinction peak) strongly blue-shifted. In general, the peak position is affected by triangle size, shape, tip sharpness, spacing and periodicty and surrounding environment

Nonlinear optical enhancement caused by a higher order multipole mode of metallic triangles[15]

Abstract: We describe a nonlinear optical study of gold triangles that exploits a higher order plasmonic resonance. A comprehensive nonlinear optical characterisation was performed both by second harmonic generation (SHG) and two photon fluorescence spectroscopy (2PF). We demonstrate and explain the enhancement of the coherent and incoherent nonlinear optical emission by a higher order multipolar mode of the plasmonic structure. The peculiarities of the mode shape and its influence on intensity and polarisation of the nonlinear signal are experimentally and numerically confirmed.

• The paper emphases on the non-linear optical behavior caused by higher order multipole mode of triangles
• They use much larger beads, the periodicity = 900nm and plasmonic height = 40nm, the high AR gives a much red-shifted transmission graph with dipole ~= 2500nm and quadpole ~= 1500nm (the graph is quite ugly)
• The rest of the paper talking about the dark arts (advanced equipment) they used to describe the high order modes, less relevant

Light scattering and surface plasmons on small spherical particles[16]

Abstract: Light scattering by small particles has a long and interesting history in physics. Nonetheless, it continues to surprise with new insights and applications. This includes new discoveries, such as novel plasmonic effects, as well as exciting theoretical and experimental developments such as optical trapping, anomalous light scattering, optical tweezers, nanospasers, and novel aspects and realizations of Fano resonances. These have led to important new applications, including several ones in the biomedical area and in sensing techniques at the single-molecule level. There are additionally many potential future applications in optical devices and solar energy technologies. Here we review the fundamental aspects of light scattering by small spherical particles, emphasizing the phenomenological treatments and new developments in this field.

• This is a review article focuses on light scattering on sub-wavelength sphere particles, going from theory to experiments and simulation
• Physical understanding of scattering: constant EM field phase on a small region induce polarization which in turn results in light scattering, this is the simple dipole mode, the dipole moment (polarization) is determined by material's dielectric function. For non-conducting materials the dielectric function can be expressed by Lorentz model, for conducting material, free electron contribution need to be added, yields a model known as the Lorentz-Drude model.
• For simulation concern: Qsc and Qabs are refering to a single particle, if you use PBC or PEC/PMC as boundary condition in 2nd step, the scattering field you derived is the result of interference of scattering field in an array of particles, it can be erroneous.
• The scattering and absorption cross section formula inplies that as the particle size is decreased, the efficiency of absorption will dominate over the scattering efficiency, and the dipole moment will dominate other types of resonance. The dipole resonance is where \epsilon(particle)\epsilon(medium) = -2 happens, the induced surface plasmon resonance frequency is \omega(particle)/sqrt(3)
• The author derived the dipole scatter cross section for sphere particles (both conductive and non-conductive particles)
• For small size dielectric particles, the scattering cross-section will increases with increasing refractive index. But the near field electric field intensity from dipolar mode will not become stronger as the size of the dielectric particles decreases, instead, the enhanced field change its distribution as the particle size changes. Note that this statement hold valid only for dipolar mode
• For metal sphere the high order modes have resonant frequencies freq(l)^2 = freq(p)^2 * l/(2l+1), freq(p) is the particle's nature frequency, clearly, higher modes resonant peaks go blue. The author then derived the scattering cross-section for higher order resonance modes. The scattering cross-section increases as modes goes higher
• Fano resonance: arises from the constructive and destructive interference between a narrow resonant mode and a broad background spectral line, Fano spectra exhibits an asymmetric shape, namely, forward scattering, and backward scattering, For PV applications, we definitely want to maximize the forward scattering while minimize the backward scattering.
• My idea: One way to engineering the Fano scattering to make it in favor of PV application is to have two different size/shape/material particles to give a mixture array, the scattering field from one particle interference with the field come from another particle to produce more forward scattering while minimizing the backward scattering. The good and bad thing is, Fano spectra characteristics cannot be found in single particle cross-section spectra, which mean they are hard to be monitored by simulation, the good side is you can direction monitor the cell performance and attribute the enhancement to Fano
• Usually the Fano comes from the interference of dipole and quadpole plasmon modes from the same particle, the asymmetry occurs mainly at quadpole peak with a sharp line width, the dipole peak remains symmetric for backward and forward scattering. one thing to notice is that to use Fano, the dissipative losses of plasmonic materials much be weak for the Fano resonance to appear, since the higher-order modes are rapidly suppressed when dissipative losses increases.(the particle eat the light it just emitted)
• The major dissipative losses arises from the inter-band transitions of electrons from lower-lying d-bands to the sp-hybridized conduction bands
• Au and Ag: dielectric function of Au has flat real part at <500nm, and flat imaginary part at around the same range, epsilon_Au_real ~= -2, epsilon_Au_imaginary ~= 6, which is high, this is why Au is good for bio-, organic- molecules detection: large dissipation losses result in greater absorption cross-section and hence more sensitive to the change of refractive index. Ag has relatively small k at >320nm, which is less dissipative, resulting in way more enhanced near field, Ag and Au particles of the same size at the resonance peak, (E(Ag)/E0)^2 = 457 and (E(Au)/E0)^2 = 11. However the real part of epsilon_Ag decreases faster at >300nm, which gives a quick shift of the peak to red region.
• The rest of the paper analyze the surface plasmon in core-shell structure which is not of interested

Plasmonic solar cells

Design of Plasmonic Thin-Film Solar Cells with Broadband Absorption Enhancements[17]

Abstract: Noble metal nanostructures can enhance absorption in thin-film solar cells by simultaneously taking advantage of i) high near-fields surrounding the nanostructures close to their surface plasmon resonance frequency and ii) coupling to waveguide modes. We develop basic design rules for the realization of broadband absorption enhancements for such structures.

• Texturing surfaces of thin film cells is not ideal as it leads to enhanced surface recombination. .
• The author used a simple and physically intuitive structure to demonstrate his concept. Ag strip on top of silica-Si-silica structure.

Ag-strip: two parameters(width and length) easy for optimization, studied optimum would be 50-100nm. SiO2:highly transparency and can provide for excellent eletrical surface passivation of the Si. Si: active material responsible for light absorption. The thickness of Si(precisely controlled using thermal oxidation) is very critical simply because it determines the number of allowed waveguide modes and their dispersion. The optimum coupling results when the reciprocal lattice vector of the Ag strip-array is matched to the k-vector of a waveguide mode supported by the Si-slab

Harnessing plasmonics for solar cells[18]

Abstract: Plasmons are free-electron oscillations in a conductor that allow light to be manipulated at the nanoscale. The ability of plasmons to guide and confine light on subwavelength scales is opening up new design possibilities for solar cells.

• This is an introduction paper.
• The core of light trapping is to enhance the light pathlength, which can be done through, scattering, diffraction/grating, Febry-Perot resonance, wave-guide modes and many others. The limit of pathlength enhancement can be as large as 4n^2, where n is the refractive index of the cell material, for silicon, it can be amazing 50-fold enhancement (This limited is not valid for diffraction using photonic structures, where the new limit can be much larger than 4n^2)
• scattering or diffraction trap the light by utilizing the materials escape cone, which goes sharper and pointer as the refractive index goes up, light not fall within the cone will be reflected internally
• Recent work suggests that aluminium particles may be more effective than noble metals for reducing reflection. This advantage is attributed to the higher resonant frequency of aluminium, as particles absorb light at wavelengths below their resonance frequency.
• The advantage of using hemisphere or triangle particles is that they can utilize the contact surface to excite surface plasmon polaritons(SPPs), which makes them superior than sphere-like particles.
• Evanescent waves can extend further into the dielectric by the inverse ratio of respective dielectric constants, which is roughly (km/ns)^2. larger metal k and smaller dielectric n results longer penetration depth for scattered field or SPP.
• Metal absorption is an issue, studies show that small value of n(metal)/k(metal)^3 maximize desirable absorption in the semiconductor.

Facile multifunctional plasmonic sunlight harvesting with tapered triangle nanopatterning of thin films[13]

Abstract: Plasmonic absorbers have recently become important for a broad spectrum of sunlight-harvesting applications exploiting either heat generation, such as in thermal photovoltaics and solar thermoelectrics, or hot-electron generation, such as in photochemical and solid state devices. So far, despite impressive progress, combining the needed high performance with fabrication simplicity and scalability remains a serious challenge. Here, we report on a novel solar absorber concept, where we demonstrate and exploit simultaneously a host of absorption phenomena in tapered triangle arrays integrated in a metal–insulator–metal configuration to achieve ultrabroadband (88% average absorption in the range of 380–980 nm), wide-angle and polarization-insensitive absorption. Furthermore, this absorber is subwavelength in thickness (260 nm) and its fabrication is based on a facile, low-cost and potentially scalable method. In addition, the geometry of our design makes it compatible for both heat and hot electron generation.

• The importance of this one is that they are related to PV, however, they didn't do cell simulation
• The paper takes advantages of plasmonic multiple resonances in a metal-insulator-metal(MIM) structure, MIM is the stack of silver particles - dielectric - silver film. (In the paper they use gold instead of silver)
• The author define two variable for triangle array tip tapering. Rb = diameter of the beads, R = the curvature of the patter, R = Rb + deltaR, where deltaR >= 0, then deltaR > 0, the curvature of the particle is larger than the beads, the result is larger and more closed triangle, we know that closer tips yield higher enhancements, that's why the author want to change R, R is tuned by gentle RIE
• The use home-built invert microscope with the functionality to excite and collect plane wave to and from the sample, they claim their system is capable of collecting both reflection and scattered light. A = (I-R)/I is used to generate absorption spectrum, where I is measured by reflecting from an broad flat silver mirror
• they use PEC/PMC for normal incidence and PEC or PMC + Floquet for oblique incidence in their simulation, the unit cell size is 1/4 of the actually unit cell, this is the as we did in our lab
• both simulation and measured data shows that the closer the two tips, the more broadband absorption happens, however, the tip should still be kept separated as connected tip has way less enhancement. They claim absorption stays >85% with spectral range of 380-850nm and slowly drop to 70% at 980nm
• Angular dependency: normal incidence is broadband absorption, absorption at long wavelength quickly decreases as incident angle goes more oblique. at 40 deg oblique, the absorption above 850nm is about 50% of that of normal incidence.
• the optimal insulator thickness for their system is 60nm. optimal beads radius is about 200nm, they made a pretty nice contour graph showing how maximum absorption is obtained by varying beads size and insulator thickness.
• The author attribute the enhancement in absorption to 1) grating-mediated coupling of EM field to the surface plasmon polariton(SPP) exicted at the interface between the dielectric and back reflector, 2)dipole-dipole interaction between the triangles and the back reflector, 3)localized surface plasmon resonance of the individual tapered triangles, and 4) energy dissipation at tips.
• Total absorption is (optimized) 85% using Rb = 200nm and h(insulator) = 45nm.
• They didn't consider Fabry-Perot effect as a major enhancement factor

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