invited article plasmonic growth of patterned metamaterials with fractal geometry

Invited Article: Plasmonic growth of patterned metamaterials with fractal geometry Nobuyuki Takeyasu, Natsuo Taguchi, Naoki Nishimura, Bo Han Cheng, and Satoshi Kawata Citation: APL Photonics 1, 050801 (2016); doi: 10.1063/1.4952997 View online: http://dx.doi.org/10.1063/1.4952997 View Table of Contents: http://aip.scitation.org/toc/app/1/5 Published by the American Institute of Physics Articles you may be interested in Invited Article: Broadband highly efficient dielectric metadevices for polarization control APL Photonics 1, 030801030801 (2016); 10.1063/1.4949007 Optofluidics of plants APL Photonics 1, 020901020901 (2016); 10.1063/1.4947228 Anti-reflective surfaces: Cascading nano/microstructuring APL Photonics 1, 076104076104 (2016); 10.1063/1.4964851 Ultrafast, broadband, and configurable midinfrared all-optical switching in nonlinear graphene plasmonic waveguides APL Photonics 1, 046101046101 (2016); 10.1063/1.4948417 APL PHOTONICS 1, 050801 (2016) Invited Article: Plasmonic growth of patterned metamaterials with fractal geometry Nobuyuki Takeyasu,a Natsuo Taguchi, Naoki Nishimura, Bo Han Cheng,b and Satoshi Kawatac Department of Applied Physics, Osaka University, Suita 565-0871, Japan (Received March 2016; accepted 17 May 2016; published online 23 June 2016) Large-scale metallic three-dimensional (3D) structures composed of sub-wavelength fine details, called metamaterials, have attracted optical scientists and materials scientists because of their unconventional and extraordinary optical properties that are not seen in nature However, existing nano-fabrication technologies including two-photon fabrication, e-beam, focused ion-beam, and probe microscopy are not necessarily suitable for fabricating such large-scale 3D metallic nanostructures In this article, we propose a different method of fabricating metamaterials, which is based on a bottom-up approach We mimicked the generation of wood forest under the sunlight and rain in nature In our method, a silver nano-forest is grown from the silver seeds (nanoparticles) placed on the glass substrate in silver-ion solution The metallic nano-forest is formed only in the area where ultraviolet light is illuminated The local temperature increases at nano-seeds and tips of nano-trees and their branches due to the plasmonic heating as a result of UV light excitation of localized mode of surface plasmon polaritons We have made experiments of growth of metallic nano-forest patterned by the light distribution The experimental results show a beautiful nano-forest made of silver with self-similarity Fractal dimension and spectral response of the grown structure are discussed The structures exhibit a broad spectral response from ultraviolet to infrared, which was used for surface-enhanced Raman detection of molecules C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4952997] I INTRODUCTION Nanostructured metals have a great potential as functional nanomaterials with its unique electrical, mechanical, chemical, and optical (plasmonic) properties The interaction of nano-metallic structures with photons exhibits extraordinary phenomena such as field enhancement and nanoscale photon-confinement due to the localization of surface plasmon polaritons (SPPs) at the metal structures.1 A variety of metallic nanostructured particles, such as nano-spheres, nano-rods, and nano-stars, have been fabricated with the use of chemical synthesis methods.2,3 However, the size of these nanoparticles is practically limited in small scale If the total size of metallic nanostructures can be as large as bulky materials, and if a production method is available, nanostructures become more attractive for optical scientists and materials scientists Such bulky metallic materials known as metamaterials that are composed of subwavelength units of plasmonic metallic nanostructures have been one of the hottest topics in photonics due to their exotic optical properties such as negative refraction,4–6 cloaking,7 and optical coordinate a Present address: Graduate School of Natural Science & Technology, Okayama University, Okayama, Okayama, 700-8530, Japan b Present address: National Taipei University of Technology, 1, Sec 3, Zhongxiao E Rd Taipei 10608, Taiwan c Author to whom correspondence should be addressed Electronic mail: kawata@skawata.com 2378-0967/2016/1(5)/050801/8 1, 050801-1 © Author(s) 2016 050801-2 Takeyasu et al APL Photonics 1, 050801 (2016) transforms.8,9 Meta-surface or a two-dimensional (2D) version of metamaterials is also promising for modifying the optical wave-front at the surface with nanostructures.10 However, the fabrication of complex metallic nanostructures in large scale in three-dimensional (3D) is still under the development Most of exciting demonstration of exotic optical properties by metamaterials has been shown with thin or 2D structures, rather than thick bulky metamaterials, except a few examples such as 3D array of sprit ring resonators in microwave region by Smith and Burckel.4,11 To fabricate metamaterials in optical frequency region, the available nanotechnology is mostly based on silicon fabrication technology or top-down approach, which is a 2D technology to fabricate or modify the surface with deposition, oxidization, photo patterning, and etching of surface with light, e-beam, or focus ion beam For extending the dimensions into 3D as metamaterials, 2D-layer stacking method has been used such as a multi-layered resonator array by Giessen12 and a layered fishnet by Zhang.13 Two-photon laser drawing is another method for realizing real 3D nano-fabrication.14,15 We have fabricated 3D spring array by electroless metal plating on two-photon polymerized microsprings.16 An array of square-pyramid frames without polymer has been also drawn with two-photon photo-reduction.17 Two-photon drawing is useful for fabricating arbitrary three-dimensional plasmonic structures in three dimensions while it takes an extremely long time for fabricating a large object in 3D In this article, we discuss about a bottom-up approach for producing 3D metamaterials in a large scale Bottom-up approach is more advantageous for building up or self-growing large-scale complex nanostructures in 3D Self-assembling of metallic nanoparticles has been studied by other groups where the nanoparticles are used as building-blocks.18–20 The metamaterials we grow can be as large as possible but with fine details It is not a periodic structure like a photonic crystal, which is not necessarily essential as metamaterials II FABRICATION PROTOCOL AND EXPERIMANTAL RESULTS We mimicked the process of generation of wood forest in nature Trees are grown from seeds on the ground through exposure to rain and sunshine Finally, a wood forest is patterned according to the casting of sunlight The forest has hierarchal levels of trees, branches, sub-branches, and leaves with self-similarity In our proposed method, we plant silver nano-seeds on the glass substrate, pour silver-ion solution on them, and illuminate the area with UV patterned light As a result, a silver nano-forest is formed according to light illumination pattern Figure shows the recipe for the experiment in detail First, we coat a glass substrate with amino-propyl trimethoxysilane (APTMS) monolayer Silver nano-seeds of a few to several nm in diameter, stabilized by citric acid, are fixed on the APTMS-coated glass substrate Then, we remove the APTMS, which is hydrophilic by UV ozone cleaning (upper row of Fig 1) APTMS is replaced by dimethyldichlorosilane, which exhibits hydrophobicity (second row of Fig 1) Then the silver ion solution is dropped onto it (left bottom row of Fig 1) The solution is composed of acetone-water solution of silver nitrate and L-ascorbic acid (see Fig S1 of the supplementary material).33 We purposely not add any surfactant, which may act as a template21 for not determining the unit structure Hence, the geometry of the structure is not given by the surfactant, but by temperature, surface tension, and viscosity of solvent, as shown in Fig S1.33 Those experimental parameters are optimized to produce fractal geometries of structures Next, the substrate with the silver solution droplet is illuminated with a UV laser The wavelength of laser is 355 nm, which is close enough to the plasmonic resonant peak of the silver nano-seeds The illumination optics contains a photomask to produce the pattern of forest The discussion on the minimum exposure time and laser power to grow nano-trees appears in supplementary material (Figs S2 and S3).33 Growth mechanism of silver nano-trees contains two processes: needle growing and branching Silver nano-seeds are pre-placed on a glass substrate in the acetone-water solution consisting of silver nitrate and L-ascorbic acid (Fig 2(a)) Seeds are plasmonically heated up due to the excitation of localized surface plasmon resonance of silver nano-particle with UV light The solution is kept cooled and concentration is low enough to avoid unnecessary thermal reduction The temperature increase of the nano-seed induces a dramatic increase of the chemical reaction rate Rrea in the volume near the nano-seed Since Rrea is higher than the diffusion rate Rdiff in the same area, silver 050801-3 Takeyasu et al APL Photonics 1, 050801 (2016) FIG Growth of silver nano-trees under UV light illumination Substrate preparation and optical system for photofabrication growth AgNPs adsorb on a chemically modified glass surface (inset SEM image), which performs as seeds to mediate the growth of the nano-trees The growth area of the forest of silver nano-trees is controlled by a photomask that transfers the desired pattern of UV light illumination on to the substrate The scale bar in the image of the photomask is 10 mm ions are locally reduced and contribute to crystal growth of silver needle (Fig 2(b)) This process results in the formation of a concentration gradient near the seed for both silver ions and ascorbate ions The ions diffuse towards the silver crystal to proceed a needle growing.22 The fluctuation in concentration is amplified to produce protrusions on the sides of the needle to initiate side-branching (Fig 2(c)).23 The side-branches grow at 60◦ relative to the parent branch due to its lowest potential for silver crystallization The branching repeats at different scales to produce fractal geometry As a result, hierarchical self-similar structures (fractal structures) are massively grown (Fig 2(d)) Figure shows SEM images of resultant silver nano-forests A silver forest is grown as large as 1.2 × 1.2 mm2 It looks as a Chinese character of “tree” according to the mask pattern for illumination as shown in the left part of Fig The middle part of Fig is an enlarged view of the central area of the “tree” with a magnification of 800×, showing massive nano-trees similar to a forest The right image shows further enlargement of the image in the middle with a magnification of 8000×, showing tree-like structures made of silver, typically with a length of a few tens of µm, with branches of a few µm and leaves of a few tens of nm III SPECTRAL RESPONSE AND FRACTAL DIMENSION We have investigated the optical properties of the grown silver nano-trees Figure 4(a) shows the measured extinction spectrum of nano-trees (as a red curve) It is broadly spanning the visible and near infrared region with a resonant peak at 396 nm This broadband feature indicates the characteristics of fractal geometry, i.e., complexity In contrast, the green curve in Fig 4(a) shows the measured extinction spectrum of nano-seeds, which has a resonant peak at 385 nm and has 050801-4 Takeyasu et al APL Photonics 1, 050801 (2016) FIG Schematics of the growth mechanism of the silver nano-trees under UV laser illumination Silver ion (+) concentration is initially homogeneous in the solution (a) When the solution is illuminated by UV laser, silver ions around the silver (Ag) nano-seed are locally reduced by L-ascorbic acid (−) due to the plasmonic heating, resulting to an Ag sprout (b) Since the reduction rate around the silver structures is much faster than the diffusion rate of silver ions, this leads to a gradient of concentration and results to the growth of a tree-like structure with several branches (c) The branches with faster growing rate become large branches where the next generation of branches (side-branches) develop This growth process continues to form the silver nano-tree (d) a much narrower band than the nano-tree spectrum To understand the broadband character of nano-trees, we decompose the spectrum of nano-trees into the spectra of several nano-rods because nano-trees are composed of nano-rods with different sizes Four dotted curves depicted in blue in Fig 4(a) are obtained through 3D-Finite-Difference Time-Domain (FDTD) calculation for silver nano-rods with different lengths and a sphere More than four components were necessary to well fit to the nano-tree spectrum As a result of fitting, we found three rods with the lengths of L = 82 nm, 114 nm, and 150 nm with a sphere of 20 nm in radius gave the best fit, and the peaks are seen at 416 nm, 555 nm, 686 nm, and 830 nm, respectively The radius of sphere (20 nm) and that of rod (25 nm) is different in simulation This is due to the difference in the experimental results in SEM images Four dotted curves represent the absolute values of extinction function of single nano-sphere and single nano-rods, but multiplied individually by 7.2, 1.6, 1.0, and 2.2, respectively, to have a best fit This number set indicates the ratio of numbers between the sphere and the three different length rods in trees FIG SEM images of silver nano-trees grown on an area patterned after the Chinese character of “tree” (left) The area size is 1.2 × 1.2 mm2 The scale bar is 500 µm An enlarged view of the central area shows a forest of massive nano-trees (middle) The scale bar is 20 µm Further enlargement of the view area shows fine silver tree-like structures exhibiting self-similarity referred to as fractals (right) The scale bar is µm 050801-5 Takeyasu et al APL Photonics 1, 050801 (2016) FIG Extinction spectra and fractal dimensions of silver nano-trees (a) Extinction spectra of silver nano-trees (red solid) and silver nano-seeds (green solid) were measured with an absorption spectrometer The dotted curves in blue are the calculation results for a silver nano-sphere and silver nano-rods with lengths of 82 nm, 114 nm, and 150 nm The radius of the sphere and three nano-rods are 20 nm and 25 nm, respectively (b) SEM image of a silver nano-tree and its diffractogram obtained through Fourier transformation (c) Fractal dimensions D f of silver nano-trees analyzed by means of a box counting method The inset images show two grown nano-trees with D f of 1.717 (red) and 1.864 (green) The fractal dimensions of three different tree patterns whose ratios R of the length of the branch to the distance between neighboring branches are 0.5 (blue), (black), and (magenta) were calculated obtaining a D f of 1.84, 1.66, and 1.58, respectively Although the light used in the experiment was not polarized, in the simulation, the incident light was assumed linearly polarized with respect to the long-axis of nano-rods The polarization along to the minor axis may also resonate and contribute to the spectrum of the sphere In the simulation, we did not either deal the interaction among nano-rods while the resonance of connected multiple nano-rods would be seen far beyond the spectral range of interest These results indicate that the tree spectrum is composed of multiple plasmon modes corresponding to the individual branches of nano-rods with different lengths In addition, we obtained a SEM image of one of silver nano-trees and its diffractogram to confirm the crystal structure of 050801-6 Takeyasu et al APL Photonics 1, 050801 (2016) the grown metallic nanostructures Figure 4(b) shows the SEM image (top) and its diffractogram (2D Fourier transform, bottom) The diffractogram shows a six-fold rotational symmetry, which confirms the crystallinity of the nano-trees and highlights the face centered cubic (fcc) lattices of silver crystals Next, we examined the fractal nature of the metal nano-trees A dendritic structure is characterized by a fractal dimension D f ,24 a parameter representing the filling factor of the structure in space, and is given by the following equation: Df = log N , log( r1 ) (1) where N is the number of self-similar sub-pieces (branches from the nano-tree stem) required for filling the space of the object (a nano-tree), and r is the scale ratio, respectively D f can be measured from the SEM image through a calculation method called box counting,25 where n is the number of unit square cells whose size is l × l (l is the unit length), occupying the object in the SEM image Hence, Eq (1) can be expressed as Df = log n log( 1l ) (2) or more precisely D f = −liml→ ∞ log n log l (3) The fractal dimension of nano-trees clearly depends on the geometry of the structure, in particular, the density of the branches Figure 4(c) shows plots of log (n) to log (1/l) for different nano-trees The fractal dimension D f is given by the slope The resulting red plot for the SEM image shown in Fig 3(b) shows a fractal dimension D f of 1.717 This value is quite comparable to those for metallic metamaterials that were lithographically fabricated as “H”-shaped fractal structures of self-similarity; D f of Zhou’s structure was ∼1.6 (Ref 26) Fractal dimension of metal aggregation in diffusion has been also known through simulations by Erzan et al.,28 and it was ∼1.7, again comparable to our result Fractal dimension tells us the complexity of the structures, and the higher value indicates the higher complexity Since metamaterials are basically the complex metallic structures made of subwavelength units, fractal dimension is a good measure of the performance of metamaterials As a reference, fractal dimension of natural coastal line is typically ∼1.25 (the west coast of Britain).24 Figure 4(c) includes another plot (in green), which shows D f ∼ 1.864 This plot was derived from another nanostructure fabricated with a different experimental parameter (the temperature during the fabrication was ∼290 K for the plot in green while it was 310 K for the plot in red) As a conclusion, the fractal dimension is given as a function of experimental parameters The dependence is shown in supplementary material (Fig S1).33 The higher temperature leads to smaller D f and the larger viscosity leads to larger D f As a reference, Fig 4(c) includes three plots in blue, black, and in magenta, each corresponds to the calculation result of a simple nano-tree model with different density to indicate the relationship between fractal dimension and complexity of the structures IV DISCUSSIONS The fractal geometry analyzed above exhibits another exotic optical property Zhou26 and Miyamaru27 have reported fractal metamaterials with the “H” shaped geometry Fractal geometry possesses self-similarity, giving multiple frequency operation Self-similarity in the structural geometry can result in a mode consisting of multiple hot spots, which can be localized or spread over the entire system However, the fractals discussed in the above works are limited to in 2D because it is difficult to make 3D fractal metamaterials The advantage of fractal geometry of metamaterials is their complexity, density, and broad Fourier spectral response Such a character can be 050801-7 Takeyasu et al APL Photonics 1, 050801 (2016) FIG Enhanced Raman spectroscopy of Rhodamine 6G (R6G) molecules using silver nano-trees (a) SEM image of the grown silver nano-trees The scale bar is 500 µm (b) Fluorescence microscope image of (a) covered with R6G (10 µM) aqueous solution The fluorescence from R6G is quenched by the silver nano-trees, resulting in a dark star pattern (c) SERS spectrum of R6G on the silver nano-trees obtained in the red circle area in (a) and (b) Strong Raman peaks assigned to vibrational modes of R6G are clearly observed, which can be attributed to the large enhancement effects provided by the silver nano-trees (d) Same as (c) but taken on a different area without the silver nano-trees denoted by the green circle area in (a) and (b) For both measurements, the exposure time was sec, and the laser power was 1.0 and 0.13 mW, respectively utilized as an ideal substrate for surface-enhanced Raman spectroscopy We have made an experiment of Raman scattering detection of fluorescent molecules Figure 5(a) shows an SEM image of a silver nano-forest grown on a glass substrate A star-like metallic nano-forest is seen as in the center of Fig 5(a) Aqueous solution of Rhodamine 6G (R6G) molecules (10 µM) was dropped both on and out of the forest, and Raman scattering and fluorescence were measured Figure 5(b) shows a fluorescence microscope image of the sample Figure 5(c) shows a Raman shift spectrum detected at the position of a red circle in Figs 5(a) and 5(b) Vibration modes of R6G molecule on the silver nano-forest were successfully enhanced and detected as peaks at 610, 772, 1190, 1360, 1509, 1572, and 1649 cm−1, which are attributed to R6G in Raman shift spectrum Figure 5(d) shows the spectrum taken at the position marked with a green circle that is out of silver forest Contribution to the spectral intensity was mostly fluorescence while Raman peak was buried in fluorescence intensity On the contrary, inside the forest, the fluorescence has been fully quenched by metallic structures as shown in Fig 5(c) We confirmed the suppression of the fluorescence on the silver nano-trees in another experiment, which is shown in Fig S4 of the supplementary material Another experiment was made with the same sample, with and without the silver metamaterials on the glass substrate, to estimate the factor of surface enhancement by the metamaterials The excitation laser was at 785 nm in near infrared to avoid fluorescence and compared the Raman scattering intensity at 1509 cm−1 (aromatic stretching mode); the power and the measurement time were 1.0 mW and s, respectively The concentration of R6G was 10 µM on the metamaterials substrate, whereas it was 10 mM for the experiment without metamaterials Through this experimental the Surface-Enhanced Raman Scattering (SERS) enhancement factor due to the metallic metamaterials was found as ∼5 × 106 This result shows that our metallic nano-forest produces Raman enhancement and fluorescence quenching, demonstrating its potential for molecular analytical microscopy.29 050801-8 Takeyasu et al APL Photonics 1, 050801 (2016) Although this article features a fabrication method of metamaterials rather than its applications, the study on optical properties and their applications is another interesting and important topic There have been intensive studies on the negative permeability or negative refraction of fishnet structures of layered dendrites by Zhao.30–32 The demonstration of such a property of our non-layered bulky dendrite structure is left to us for the future study In summary, we have developed a self-growing method of metallic metamaterials that exhibit fractal geometry with self-similarity Plasmonic heating with UV light illumination triggers the growth at seeds and needle tips Silver nano-trees are grown in silver ion solution due to the diffusion of ions and reduction of silver crystals No use of surfactant helps to form trees with branches and sub-branches We described the protocol of the self-growth fabrication and analyzed the fractal dimensions of the structures An application of the developed structures was shown through experiments of surface enhanced Raman sensing of molecules with large 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