Plasmon Coupling in Self-Assembled Gold Nanoparticle-Based

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Plasmon Coupling in Self-Assembled Gold Nanoparticle Based Honeycomb Islands Sebastian P Scheeler, Stefan Mühlig, Carsten Rockstuhl, Shakeeb-bin Hasan, Simon Ullrich, Frank Neubrech, Stefan Kudera, and Claudia Pacholski J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405560t • Publication Date (Web): 13 Aug 2013 Downloaded from http://pubs.acs.org on August 16, 2013

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The Journal of Physical Chemistry

Plasmon Coupling in Self-Assembled Gold Nanoparticle Based Honeycomb Islands Sebastian P. Scheeler†,‡, Stefan Mühlig§, Carsten Rockstuhl§, Shakeeb Bin Hasan§, Simon Ullrich†,‡, Frank Neubrech#, Stefan Kudera†,‡, and Claudia Pacholski† ,* †

Max-Planck Institute for Intelligent Systems, Dept. New Materials and Biosystems, Heisenbergstrasse 3, 70569 Stuttgart, Germany

‡

Department of Biophysical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

§

Institute of Condensed Matter Theory and Solid State Optics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, 07743 Jena, Germany

#

University of Stuttgart, 4th Physics Institute, Pfaffenwaldring 57, 70569 Stuttgart, Germany

E-mail: [email protected] Phone: +49 711 6893620, FAX: Phone: +49 711 6893612

Corresponding Author * [email protected]

ACS Paragon Plus Environment

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Abstract Metallic nanostructures that sustain plasmonic resonances are indispensable ingredients for many functional devices. Whereas structures fabricated with top-down methods entail the advantage of a nearly unlimited control over all plasmonic properties, they are in most cases unsuitable for a low cost fabrication on large surfaces; and eventually a truly nanometric size domain is difficult to reach due to limitations in the fabrication resolution. Although ordinary bottom-up techniques based on colloidal nanolithography promise to lift these limitations, they often suffer from their incapability to self-assemble nanoparticles at large surfaces and at a density necessary to observe effects that strongly deviate from those of isolated nanoparticles. Here, we rely on the application of sequential bottom-up fabrication steps to realize honeycomb structures from gold nanoparticles that show strong extinction bands in the near infrared. The extraordinary properties are only facilitated by densely packing the nanoparticles into clusters with a finite size; causing the clusters to act as plasmonic macromolecules. These strongly interacting bottom-up materials with a deterministic geometry but fabricated by self-assembly might be of use in future sensing applications and in material platforms to mediate strong light-matter-interactions. Keywords Plasmon coupling, gold nanoparticles, self-assembly, honeycomb structure, bottom-up, degree of order


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Introduction In 2001 Maier et al. popularized the term Plasmonics for the vast expanding research on the interaction of light with metallic nanostructures which, in essence, allows to tame light at the nanoscale.1 This is only possible by coupling electromagnetic fields with the conduction band electrons in the metallic nanostructure that leads to the formation of localized surface plasmon polaritons (LSPPs). Their properties can be controlled by defining the size, shape, and environment of individual metallic nanostructures. Furthermore, the resonance strength, width and position of the LSPPs can be controlled by the assembly of isolated metal nano-objects into larger 2D/3D structures.2 Below a critical gap size, the LSPP as sustained by the individual metallic building blocks strongly couple. This enables many applications for which surfaceenhanced spectroscopy applications are potentially a referential example.3 Such techniques profit from tunable resonance wavelengths and intense localized electromagnetic fields, so called ‘hot spots’, whose minute details highly depend on the separation distance between the involved metallic nanostructures.4 To date, mainly top-down methods such as e-beam lithography and nanoimprint lithography have been used for the fabrication of plasmonic nanostructures. This allows for the precise definition of the geometrical dimensions of the nanostructure including structure size, morphology, and separation distance. Especially appealing is the step-wise assembly of well-defined gold nanostructures which permit optical studies of single metallic building blocks and their repetitive extensions into larger arrays. 5-8 However, the top-down approach is challenged by the required gap sizes, the demand for large-scale structures, and the amount of grain boundaries resulting from the metal deposition by thermal evaporation. The latter usually has a negative impact on the optical response of the metallic nanostructure since it increases radiative and non-radiative losses. An elegant way to circumvent at least some of these


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problems is to combine top-down and bottom-up techniques. 9, 10 Template assisted selfassembly of gold nanoparticles on lithographically structured substrates has extensively been studied for this purpose. 11, 12 The inevitable low yield is not critical for fundamental optical studies but should be improved. Moreover, a densification of the nanoparticle arrangement is often necessary to achieve structures with an optical response that deviates from the response of its isolated constituents. Metallic nanostructures fabricated by using solely bottom-up methods require cumbersome correlation of single particle optical measurements and electron microscopy images. 13 In these bottom-up methods metallic nanostructures are usually self-assembled from metallic nanoparticles with the aid of specially designed molecular linkers such as DNA, peptides strands, or selective binding ligands. 14-16 In contrast, geometrical undefined networks of linear and branched gold nanoparticle chains can be obtained by ‘controlled’ aggregation employing light triggered molecules or partial ligand exchange. 17-20 Extended layers of gold nanoparticles comprising no order have been extensively investigated and plasmonic coupling between adjacent particles leads to a red-shift and a broadened LSPP band. 21-25 An extension of these layers to the third dimension by using spherical instead of planar substrates enabled the fabrication of novel nanophotonic structures by bottom-up techniques. For example, magnetic dipole resonances in the visible 26 as well as spherical clusters that highly improve the cross section for surface enhanced Raman scattering 27 were demonstrated. Ordered metallic nanostructures have been fabricated using self-assembly methods such as block copolymer nanolithography (BCML) or polymer-coated nanoparticles but did not show plasmon coupling due to the obtained interparticle distances. 28-30 In a nutshell, until now either random metallic nanostructures with sufficient interparticle distances for plasmon coupling or ordered nanoparticle arrays whose plasmons do not strongly interact were fabricated using bottom-up


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strategies. In addition, self-assembled gold nanoparticle arrays on large surfaces are mainly arranged in a hexagonal pattern limiting the control of their optical response. In this paper we present a novel bottom-up approach for the fabrication of honeycomb island structures composed of gold nanoparticles. The developed method allows for the positioning of gold nanoparticles in a more advanced geometry leading to a considerable decrease in the interparticle distance. Thereby strong plasmon coupling is enabled resulting in extinction resonances in the near infrared. The fabrication process started with the equipment of wetchemically synthesized gold nanoparticles with a polymer shell and their deposition onto glass substrates by spin-coating. By alternating plasma treatment and spin-coating cycles honeycomb island structures composed of gold nanoparticles are formed which show strong plasmonic coupling effects in the near infrared wavelength range. The experimental extinction spectra were verified by numerical simulations, emphasizing the necessity of honeycomb islands for the observed plasmon resonances in the near infrared spectral domain. Experimental Methods Materials & Methods. Gold(III) chloride trihydrate (HAuCl4 * 3 H2O) (Aldrich), sodium citrate dihydrate (Sigma), cetyltrimethylammonium bromide (CTAB) (≥99.0%, Aldrich), oleylamine (C18 content 80-90%, Acros), toluene (for analysis, Merck), thiol terminated polystyrene (P4434-SSH, Mn = 50000 gmol-1, Mn/Mw = 1.06) (Polymersource), hydrogen peroxid (Merck), concentrated sulfuric acid (Carl Roth), and methanol (VLSI grade, J. T. Baker) were commercially obtained and used as received. Water was deionized to a resistance of at least 18.2 MΩ (Ultra pure water system (TKA, Germany)) and then filtered through a 0.2 µm filter. Glass substrates ( 2 x 2 cm cover glass) were obtained from Carl Roth (Germany).


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Acrodisc® 25 mm Syringe Filters (0.8 µm Versapore® Membrane) were received from Pall Life Sciences. Spin coating was carried out with a WS-400B-6NPP/LITE spin-coater from Laurell Technologies Corporation. Plasma treatment was performed using a PVA TePla 100 Plasma System. Transmission spectra under normal incidence of light were recorded with a Cary 5000 UV-VIS-NIR spectrometer (Varian, USA) whereas angle-dependent spectra were taken with an IR microscope (Bruker IRscope II) attached to an FTIR spectrometer (Bruker Vertex 80). Samples were imaged with a Zeiss Ultra 55 "Gemini" scanning electron microscopy (SEM) after coating with carbon.

Gold Nanoparticle Synthesis Gold nanoparticles covered with thiol terminated polystyrene ligands were prepared according to reference. 31 Briefly, gold nanoparticels with diameters of approximately 15 nm were wetchemically synthesized and grown to a diameter of approximately 50 nm. The resulting particles were transferred from the aqueous solution to an organic phase (toluene) by ligand exchange using oleylamine. Finally, the oleylamine coated gold nanoparticles in toluene were treated with an excess of thiol terminated polystyrene. Two fractions of nanoparticles with different diameters (47 ± 5 nm and 57 ± 7 nm) were obtained by size selective precipitation. Assembly of Honeycomb Structures Glass cover slips were cleaned in piranha solution (3:1, V:V, conc. H2SO4:H2O2) overnight and washed 3 times with deionized water in an ultrasonic bath for 15 minutes. Then, the glass cover slips were dried in a stream of nitrogen and mounted in the spin-coater. 15 µL of appropriately concentrated polymer coated gold nanoparticles in toluene were applied to the cleaned glass substrate at 2000 rpm. The spin-coating process was stopped after 30 s. The resulting gold


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nanoparticle monolayers were treated with W10 (10% hydrogen, 90% argon) plasma at 0.4 mbar and 150 watt for 45 minutes in order to remove the polymer and fix the particles on the substrates. Subsequently, a second spin coating cycle was performed to assemble a second gold nanoparticle monolayer onto the first layer. The polymer of the second layer was removed by plasma treatment (O2 plasma at 0.4 mbar and 150 watt for 45 minutes), while the second layer of particles sank into the plain of the first layer. Finally, the samples were coated with a polystyrene film by spin-coating (2000 rpm, 30 seconds) using 15 µL of a thiol terminated polystyrene solution in toluene (5mg / mL).

Results & Discussion The fabrication process for honeycomb structures based on gold nanoparticles is depicted in scheme 1. Starting point is the deposition of quasi-hexagonally ordered gold nanoparticle monolayers on glass substrates by spin-coating according to a recently published method. 31


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Scheme 1. Fabrication process of plasmonic honeycomb structures. A quasi-hexagonally ordered array of polystyrene-coated gold nanoparticles was formed on glass by spin-coating (a). The polymer shell was removed and the gold nanoparticles fixed to the surface by plasma treatment (b). A second layer of polystyrene-coated gold nanoparticles was deposited on top of the first layer (c). Subsequent plasma treatment led to the formation of a 2D honeycomb structure (d). Finally, the gold nanostructure was embedded in polystyrene (e).


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Briefly, gold nanoparticles with diameters of approximately 50 nm were wet-chemically synthesized using a seeded growth approach and equipped with a polystyrene shell by ligand exchange. The concentration of the polystyrene coated gold nanoparticles in toluene was adjusted in order to receive the desired gold nanoparticle arrays (a). In order to suppress the disintegration of the array upon subsequent nanoparticle deposition, the substrates were plasma treated (b). Thereby the polymer shell was removed and the gold nanoparticles were fixed to the glass surface. Similar observations were already described for gold nanoparticle arrays prepared using block copolymer micelle nanolithography (BCML) 28or ligand-coated gold nanoparticles. 29

A second layer of polystyrene-coated gold nanoparticles was deposited on top of the first layer

composed of an ordered gold nanoparticle array using spin-coating (c). Here, the first layer acted as a template and the gold nanoparticles in the second layer settled into the voids formed by three gold nanoparticles in the first layer. The occupation of the three neighboring voids was successfully prevented by the size of the polystyrene-coated gold nanoparticles. Related results have been obtained for the formation of binary colloidal arrays. 32-35 However, in these experiments the first colloidal layer was always hexagonally closed packed and only in the case that differently sized colloids were deposited in the second layer, kagome lattices or honeycomb structures were formed - depending on the diameter of the employed colloids. In the experiments described in the present report, the resulting 3D structure consisted of two triangularly arranged gold nanoparticle arrays and was transferred into a 2D honeycomb structure by subsequent plasma treatment (d). Finally the whole nanostructure was embedded in polystyrene in order to stabilize it and to provide comparable extinction spectra of all fabricated gold nanoparticle based structures by using the same surrounding material with the same permittivity (ε).


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SEM images of fabricated structures based on gold nanoparticles are shown in figure 1. Two types of polystyrene coated gold nanoparticles were used which differ in their diameter. Figures 1 a)-c) display SEM images of gold nanoparticle arrays prepared using nanoparticles which are 47 ± 5 nm in diameter whereas structures shown in figures 1 d)-f) are composed of larger gold nanoparticles with a diameter of 57 ± 7 nm. In both cases, a single spin-coating step of the polystyrene-coated gold nanoparticles led to the formation of quasi-hexagonally ordered gold nanoparticle arrays [figures 1a) & d)]. The removal of the polymer matrix did not change the geometrical dimensions of the array such as gold nanoparticle distance and gold nanoparticle diameter. Subsequent deposition of a second layer of gold nanoparticles resulted in the assembly of honeycomb-like structure composed of hexagons of gold nanoparticles. Here, the hexagons are arranged in a ‘double layer’ of gold nanoparticles and therefore resemble a 3D structure (figure 1 b) & e)). After plasma treatment, the particles sank into the plane, leading to the formation of a 2D structure and reducing the interparticle distance (surface to surface) to approximately 1 nm. However, this value for the interparticle distance is only a rough estimation as the resolution of the SEM does not allow for a proper interparticle distance determination. SEM images of the fabricated honeycomb structures are displayed in figure 1 c) and 1 f) as well as in figure S1. Depending on the diameter of the employed gold nanoparticles, they either fit the average gap size between three particles of the first gold nanoparticle layer (57 nm) or are slightly smaller (47 nm). The formation of straight lines of gold nanoparticles was also observed which is based on a mismatch between the gap size and the diameter of the newly incorporated gold nanoparticles caused by the size distribution of the employed gold nanoparticles. Hence, the glass substrate surfaces is not homogenously covered with a gold nanoparticle based honeycomb


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structure but composed of differently sized islands of honeycomb structures surrounded by short gold nanoparticle chains and randomly distributed gold nanoparticles.

Figure 1. SEM images of fabricated plasmonic nanostructures using either gold nanoparticles with diameters of 47 ± 5 nm (a-c) or gold nanoparticles with diameters of 57 ± 7 nm (d-f). A quasi-hexagonally ordered monolayer of gold nanoparticles coated with polystyrene (a, d) was deposited onto glass substrates. After removal of the polymer by plasma treatment a second layer of polystyrene-coated gold nanoparticles was spin-coated on top of the first layer (b, e). Subsequent plasma treatment led to the formation of 2D gold nanoparticle honeycomb structures (c, f). Scale bar is 200 nm. The SEM images were analyzed with image analysis software (ImageJ) to determine the nanoparticle radius, area, and x,y coordinates. They are required for the determination of the interparticle distances by calculating the corresponding radial distribution function given by: g(r)=N(r)/(2πr∆rρ),

(1)


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where N(r) is the number of particles in an annular disc of radii r and r + ∆r drawn with a particle at the centre and ρ is the number of particles per cm2. Details of the calculation method have been reported elsewhere. 36-39 In figure 2 a) the radial distribution functions of the prepared quasi-hexagonally ordered gold nanoparticle arrays (shown in figure 1) and the theoretical radial distribution function of a perfect hexagonal lattice are displayed. The peak positions of the array coincide with the predicted peak positions for a perfect hexagonal lattice. The interparticle distance in the fabricated nanoparticle arrays can be extracted from the position of the first maximum and is 56 ± 1 nm. The different gold nanoparticle diameters did not have a significant influence on the particle spacing. The slow decay of the radial distribution function amplitude indicates the appearance of regular domains in the size range of 4-5 lattice constants. Figure 2 b) shows the radial distribution functions for the fabricated honeycomb structures in comparison to a perfect hexagonal lattice and a perfect honeycomb structure, which are normalized to the particle density of the gold nanoparticle monolayer. The radial distribution functions of the fabricated gold nanoparticle based honeycomb structures are reflected in the radial distribution of a perfect honeycomb structure. The peak positions as well as the amplitude of the radial distribution function of the prepared honeycomb structures coincide with the predicted values. The three perfect lattices employed for simulation purposes are shown in figure S2. The interparticle distance of the gold nanoparticles could not be extracted from the first maximum due to the resolution of the SEM images. However, the position of the second peak of the radial distribution function coincides with the nearest particle distance in the gold nanoparticle monolayer implying that the first gold nanoparticle layer is fixed on the glass substrate and not affected by the deposition of additional particles. Moreover, we used the coordinates of the center of mass of the particles determined by imageJ's Particle Analyze Plugin for analyzing the


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distribution of islands of honeycomb structures on the surface. An algorithm was employed to find points which were arranged in honeycomb structures. Only points with distances less than 80 nm to the next point in the arrangements were considered. The angles between 3 adjacent points in a honeycomb were limited to 90 - 150°. In figure 2 c) a representative SEM image and the corresponding analysis result are shown. The position of the gold nanoparticles is depicted as black squares whereas the honeycomb structures are displayed as red dots. The presence of islands of honeycomb structures can easily be noticed. Further SEM images and image analysis results can be found in figure S3.

Figure 2. Radial distribution functions of fabricated gold nanoparticle monolayers (a) and honeycomb structures (b) in comparison to the radial distribution functions of ‘perfect’ hexagonal as well as honeycomb lattices. (c) Representative SEM image of a honeycomb structure and corresponding image analysis showing the distribution of honeycomb islands (black squares: center of mass of gold nanoparticles, red dots: center of hexagons). VIS/NIR spectra of the fabricated gold nanoparticle structures shown in figure 1 d)-f) are displayed in figure 3 a). Here, only extinction spectra of structures prepared from gold nanoparticles with a diameter of 57 ± 7 nm are presented for the sake of clarity but comparable


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results are also obtained for the smaller nanoparticles and are discussed. Due to the refractive index of 1.58 of polystyrene, the extinction of the structures covered with polymer would be increased compared to the particles surrounded by air. Therefore, we have embedded the honeycomb structures in polystyrene in order to receive consistent data. The extinction spectrum of the gold nanoparticle monolayer shows the expected plasmon resonance peak at 551 nm. After the deposition of a second layer of polystyrene coated particles (diameter 57 nm) the plasmon peak showed a red shift of 72 nm to a wavelength of 623 nm (585 nm for 47 ± 5 nm particles) and a broadening of the peak, indicating plasmonic coupling of adjacent particles. This sample is denoted as 3D honeycomb. The spectra of the structures after removal of the polymer by plasma treatment and back-filled with polystyrene showed three extinction peaks at 591 nm, 823 nm and 1593 nm (587 nm, 796 nm, and 1172 nm for 47 ± 5 nm particles), which indicates strong plasmonic coupling of nearly touching particles. This sample is denoted as 2D honeycomb. In contrast to previously published spectra on nearly touching particles, these structures showed additionally a surprising broad extinction in the near infrared.


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Figure 3. (a) Experimental extinction spectra of plasmonic nanostructures consisting of gold nanoparticles with a diameter of 57 ± 7 nm. (b) Simulated extinction spectra of gold nanoparticle based structures solely considering the near field interaction between neighboring nanoparticles (dimer approximation). In order to unravel the origin of all the resonances observed in the measured extinction spectra, strongly coupled gold nanoparticles were simulated. The starting point is the simulation of the extinction spectrum of a perfect hexagonally ordered gold nanoparticle monolayer with identical geometrical parameters to that observed experimentally. The gold nanoparticles are assumed as perfect spheres in the simulations (57 nm diameter) and the material parameters were taken from


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literature. 40 The incident field is a linearly polarized plane wave under normal incidence. The well-known Korringa-Kohn-Rostocker method 41 was applied to simulate the single hexagonal layer and the results are shown results in figure 3 b) (monolayer). The resonance wavelength of the hexagonal lattice perfectly coincides with the experimental spectrum as well as with the extinction spectrum from an isolated sphere obtained from Mie theory (not shown here). Therefore, it can be safely assumed that the coupling between adjacent gold nanoparticles can be neglected in the extinction spectrum for the monolayer. Deposition of a second layer of gold nanoparticles leads to the formation of a 3D honeycomb structure which is characterized in the experimental extinction spectrum by a redshift and a broadening of the plasmon resonance. Since the optical response of the monolayer of gold nanoparticles was dominated by the single sphere resonance we assume the extinction spectra of the 3D honeycomb structure to be dominated by the coupling of gold spheres from adjacent nanoparticle layers. A so called dimer approximation is applied and only two spheres are considered in simulations with an interparticle distance of 11 nm (along the connection line of the dimer). Please note that the two spheres are also separated about 10 nm normal to the substrate. The simulations were performed by an extension of the well-known Mie-theory that can handle arbitrary arrangements of spheres. 42, 43 The incident field is polarized at 45° to the connection line of the dimer and the surrounding is set to the permittivity of polystyrene. The results presented in figure 3b) (3D honeycomb) resemble the experimental spectra, i.e. the red-shift and the broadening of the plasmon resonance compared to the monolayer are observed in simulations. After plasma treatment the 3D honeycomb structure is transferred into a 2D structure and the interparticle distance is considerably decreased [c.f. figures 1 d) and e)]. The measured extinction spectra shows two peaks in the visible for the 2D honeycomb structure at 591 nm and 823 nm. Again, the dimer approximation is applied to


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simulate the extinction curves but now both spheres are lying on the substrate and the interparticle distance is drastically decreased, as it was observed in experiments. Since the exact distance between neighboring spheres in experiments cannot be safely extracted we vary the distance in simulations from 0.5 - 1 nm in 0.1 nm steps and superimpose the resulting spectra afterwards. Again, the two measured extinction resonances in the visible can be nicely explained by considering solely the coupling between two gold nanoparticles [c.f. figure 3)]. The two resonance positions at 591 nm and 823 nm nicely coincide in experiments and simulations. However, the plasmon resonances in the near infrared observed experimentally are not reproduced in the simulated extinction spectrum. Therefore we hypothesized that these resonances should be sustained by the coupling between more than two gold nanoparticles. This is investigated in detail in the following. At first, infinite honeycomb lattices of gold nanoparticles with a diameter of 57 nm were calculated with the commercial FEM based Maxwell solver COMSOL for different filling fractions. The simulated extinction spectra did not show a resonance in the near infrared (figure S4), as expected for infinite lattices. Therefore, one possible explanation is that the IR resonance is caused from clusters with a finite size as they occur in the investigated samples. The structures would correspond to something what we wish to call plasmonic macromolecules. Examples would be polycyclic aromatic hydrocarbons like coronene 44 where carbon atoms are replaced by metallic nanoparticles. The previously discussed figure 2 already suggests that there exists some near field order in the fabricated honeycomb structure, since the radial distribution function offers multiple peaks that are all separated by the same lattice constant. However, only four to five peaks could be identified. In other words, the fabricated samples offer a near field order in the range from one to approximately five lattice constants in the honeycomb lattice. We


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systematical study the effect of this near field order on the optical response of the fabricated gold nanoparticle based nanostructures. To this end, we calculated the extinction spectra of gold nanostructures while gradually increasing the number of considered gold nanoparticles that contribute to the formation of a finite honeycomb structure. Starting from a single gold nanosphere and incorporating additional gold nanoparticles (dimer, quadrumer, honeycomb, etc.) leads to increased regions where short range order between adjacent nanospheres exists. This is depicted by the sketches of the structures under investigation in figure 4. Drastic changes in the simulated extinction spectrum are observed when increasing the number of gold nanoparticles that are ordered towards a honeycomb structure. The extinction spectrum of a single gold nanoparticle exhibits the expected LSPP resonance at around 550 nm. Also in the case of a gold nanoparticle dimer the simulated extinction spectrum consists of the well-known resonances at approximately 550 nm (transversal) and 700 nm (longitudinal), as already observed in figure 3. A 2D quadrumer of gold nanoparticles shows plasmon resonances at approximately 550 nm and at approximately 800 nm. Hence, the low-energy plasmon resonance is red-shifted and in addition was considerable broadened in comparison to the longitudinal resonance in the gold nanoparticle dimer. The broadening of the plasmon resonance peak located at approximately 800 nm is increasing if six gold nanoparticles were assembled into a hexamer (benzol). Further extension of the geometrical dimensions by using hexamers as building blocks results in the formation of honeycomb structures which show broadband extinction resonance in the infrared, as it was also observed in angle- and polarization dependent absorption measurement of the 2D honeycomb structure. These measurements further prove the local character of the plasmonic excitation (see Figure S5). The physical origin of the low energy modes in such finite honeycomb lattices can be understood by considering at first as a simplifying model a linear


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chain of nanoparticles. In this case the electric dipoles of all nanoparticles can oscillate in phase along the connection line of the nanoparticles leading to a resonance which is shifted to lower energies with increasing chain length. 13 Even though the finite honeycomb lattices show more complex spectra the mode at the lowest energy remains to be a mode where nearly all particles are involved and where they oscillate all in phase. Hence, an increase in the size of the finite clusters results in a shift of the mode to lower energies (figure 4).

Figure 4. Simulated extinction spectra of gold nanoparticle assemblies whose lateral dimensions are gradually increased. Only spherical gold nanoparticles with a diameter of 57 nm and a distance of 0.7 nm are considered. A direct comparison of experimental extinction spectra of the fabricated gold nanoparticle based honeycomb structures to the simulated spectra of gold nanoparticle dimers and honeycomb


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structures is displayed in figure 5. The position of the plasmon resonances in the experimental and calculated extinction spectra are in good agreement indicating the importance of honeycomb islands for the observed plasmon resonances in the near infrared. Neither for gold nanoparticle chains nor random nanoparticle networks (cluster) similar optical properties have been described. In fact, aperiodic plasmonic structures show a broadening in their spectra if multiple spatial frequencies occur 45 but in contrast to our honeycomb lattices the resonances of aperiodic structures are evoked in the entire (infinite) structure. The resonances of the honeycomb islands are based on the finite nanoparticle clusters consisting of several, but of finite number, plasmonic nanoparticles. The plasmon resonance of gold nanoparticle chains are characterized by a red shift with increasing chain length but approaches towards a finite value for chains consisting of more than six particles. 13 The extinction spectra of more complex structures such as gold nanoparticle clusters have been successfully simulated by assuming that these structures are composed of onedimensional chains which dictate their plasmon resonances. 46, 47 The appearance of plasmon resonances in the infrared could also be traced back to charge-transfer plasmons which have been theoretical predicted for almost touching gold nanoparticle dimers. 48-50 However, in our experiments the formation of a two-dimensional honeycomb island structure was crucial for the observed plasmon resonances in the infrared. In the case of gold nanoparticle double layers in which the gold nanoparticles have the same distance of less than 1 nm to their next neighbor plasmon resonances were not obtained in the infrared region of the extinction spectrum (figure S6). Hence, the plasmon resonance in the presented gold nanoparticle based honeycomb island is distinct from plasmon coupling in chains and highly depends on the complex honeycomb. Stochastic imperfections in the honeycomb structure do not circumvent a well-defined optical


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response as the structures are characterized by nominal geometry that can be very well controlled, although the exact appearance of a cluster of a certain size cannot be adjusted.

Figure 5. Experimental (a) and simulated (b) extinction spectra of 2D honeycomb structures composed of gold nanoparticles with a diameters of 47 or 57 nm. Conclusions To summarize, gold nanoparticles have been self-assembled into 2D honeycomb structures using alternating spin-coating and plasma treatment cycles. The fabricated gold nanostructures show strong plasmon resonances in the visible and near infrared wavelength range which could be traced back to two distinct effects. Whereas, the entire optical response in the visible could be


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fully explained by just considering the coupling of nearest neighbors in the fabricated samples that have to be densely packed (dimer approximation), the resonances in the IR could only be explained by short range order up to five lattice constants for a honeycomb structure. The presented results demonstrate that self-assembled metallic nanostructures with their inherent short range order provide valuable optical properties which are unobtainable in ‘perfectly’ ordered systems. In other words, the imperfections in the fabrication process (only short range order and no perfect infinite honeycomb lattice) has to be seen as an advantage regarding the optical properties since it allows for novel resonances in the system. Acknowledgment The authors acknowledge support from the German Federal Ministry of Education and Research (BMBF, project PhoNa, contract no. 03IS2101E) and the Max Planck Society. Supporting Information Detailed experimental methods, overview SEM image, perfect lattices used for calculation purposes, honeycomb island distribution on surfaces, calculated extinction spectra, and measured angle-dependent extinction spectra. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1)

Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Meltzer, S.; Requicha, A. A. G.; Atwater, H. A. Plasmonics - A Route to Nanoscale Optical Devices. Adv. Mater. 2001, 13, 15011505.


22

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2)

Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913-3961.

(3)

Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P. Controlled Plasmonic Nanostructures for Surface-Enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41, 1653-1661.

(4)

Cialla, D.; Maerz, A.; Boehme, R.; Theil, F.; Weber, K.; Schmitt, M.; Popp, J. Surfaceenhanced Raman Spectroscopy (SERS): Progress and Trends. Anal. Bioanal. Chem. 2012, 403, 27-54.

(5)

Hentschel, M.; Dregely, D.; Vogelgesang, R.; Giessen, H.; Liu, N. Plasmonic Oligomers: The Role of Individual Particles in Collective Behavior. ACS Nano 2011, 5, 2042- 2050.

(6)

Liu, N.; Mukherjee, S.; Bao, K.; Brown, L. V.; Dorfmueller, J.; Nordlander, P.; Halas, N. J. Magnetic Plasmon Formation and Propagation in Artificial Aromatic Molecules. Nano Lett. 2012, 12, 364-369.

(7)

Liu, N.; Mukherjee, S.; Bao, K.; Li, Y.; Brown, L. V.; Nordlander, P.; Halas, N. J. Manipulating Magnetic Plasmon Propagation in Metallic Nanocluster Networks. ACS Nano 2012, 6, 5482-5488.

(8)

Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.; Halas, N. J.; Aizpurua, J.; Nordlander, P. Metallic Nanoparticle Arrays: A Common Substrate for Both SurfaceEnhanced Raman Scattering and Surface-enhanced Infrared Absorption. ACS Nano 2008, 2, 707-718.

(9)

Zhao, J.; Frank, B.; Burger, S.; Giessen, H. Large-Area High-Quality Plasmonic Oligomers Fabricated by Angle-Controlled Colloidal Nanolithography. ACS Nano 2011 5, 9009-9016.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(10)

Page 24 of 29

Adelung, R.; Aktas, O. C.; Franc, J.; Biswas, A.; Kunz, R.; Elbahri, M.; Kanzow, J.; Schurmann, U.; Faupel, F. Strain-Controlled Growth of Nanowires Within Thin-film Cracks. Nat. Mater. 2004, 3, 375-379.

(11)

Slaughter, L. S.; Willingham, B. A.; Chang, W.-S.; Chester, M. H.; Ogden, N.; Link, S. Toward Plasmonic Polymers. Nano Lett. 2012, 12, 3967-3972.

(12)

Fan, J. A.; Bao, K.; Sun, L.; Bao, J.; Manoharan, V. N.; Nordlander, P.; Capasso, F. Plasmonic Mode Engineering with Templated Self-Assembled Nanoclusters. Nano Lett. 2012, 12 , 5318-5324.

(13)

Barrow, S. J.; Funston, A. M.; Gomez, D. E.; Davis, T. J.; Mulvaney, P. Surface Plasmon Resonances in Strongly Coupled Gold Nanosphere Chains from Monomer to Hexamer. Nano Lett. 2011, 11, 4180-4187.

(14)

Srivastava, S.; Kotov, N. A. Nanoparticle Assembly for 1D and 2D Ordered Structures. Soft Matter 2009, 5, 1146-1156.

(15)

Auyeung, E.; Cutler, J. I.; Macfarlane, R. J.; Jones, M. R.; Wu, J.; Liu, G.; Zhang, K.; Osberg, K. D.; Mirkin, C. A. Synthetically Programmable Nanoparticle Superlattices Using a Hollow Three-Dimensional Spacer Approach. Nat. Nanotechnol. 2012, 7, 24-28.

(16)

Zeng, X. B.; Liu, F.; Fowler, A. G.; Ungar, G.; Cseh, L.; Mehl, G. H.; Macdonald, J. E. 3D Ordered Gold Strings by Coating Nanoparticles with Mesogens. Adv. Mater. 2009, 21, 1746-1750.

(17)

Yang, Y.; Matsubara, S.; Nogami, M.; Shi, J. L.; Huang, W. M. One-Dimensional SelfAssembly of Gold Nanoparticles for Tunable Surface Plasmon Resonance Properties. Nanotechnology 2006, 17, 2821-2827.


24

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(18)

Lai, J.; Xu, Y.; Mu, X.; Wu, X.; Li, C.; Zheng, J.; Wu, C.; Chen, J.; Zhao, Y. LightTriggered Covalent Assembly of Gold Nanoparticles in Aqueous Solution. Chem. Commun. 2011, 47, 3822-3824.

(19)

Li, M.; Johnson, S.; Guo, H.; Dujardin, E.; Mann, S. A Generalized Mechanism for Ligand-Induced Dipolar Assembly of Plasmonic Gold Nanoparticle Chain Networks. Adv. Funct. Mater. 2011, 21, 851-859.

(20)

Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self-Assembly of Nanoparticles. ACS Nano 2010, 4, 3591-3605.

(21)

Andres, C. M.; Kotov, N. A. Inkjet Deposition of Layer-by-Layer Assembled Films. J. Am. Chem. Soc. 2010, 132, 14496-14502.

(22)

Cunningham, A.; Muehlig, S.; Rockstuhl, C.; Buergi, T. Coupling of Plasmon Resonances in Tunable Layered Arrays of Gold Nanoparticles. J. Phys. Chem. C 2011, 115, 8955-8960.

(23)

Malassis, L.; Masse, P.; Treguer-Delapierre, M.; Mornet, S.; Weisbecker, P.; Kravets, V.; Grigorenko, A.; Barois, P. Bottom-up Fabrication and Optical Characterization of Dense Films of Meta-Atoms Made of Core-Shell Plasmonic Nanoparticles. Langmuir 2013, 29, 1551-61.

(24)

Jiang, C. Y.; Markutsya, S.; Tsukruk, V. V. Collective and Individual Plasmon Resonances in Nanoparticle Films Obtained by Spin-Assisted Layer-by-Layer Assembly. Langmuir 2004, 20 , 882-890.

(25)

Lu, C.; Moehwald, H.; Fery, A. Plasmon Resonance Tunable by Deaggregation of Gold Nanoparticles in Multilayers. J. Phys. Chem. C 2007, 111, 10082- 10087.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26)

Page 26 of 29

Muehlig, S.; Cunningham, A.; Scheeler, S.; Pacholski, C.; Buergi, T.; Rockstuhl, C.; Lederer, F. Self-Assembled Plasmonic Core-Shell Clusters with an Isotropic Magnetic Dipole Response in the Visible Range. ACS Nano 2011, 5, 6586-6592.

(27)

Gellner, M.; Steinigeweg, D.; Ichilmann, S.; Salehi, M.; Schuetz, M.; Koempe, K.; Haase, M.; Schluecker, S. 3D Self-Assembled Plasmonic Superstructures of Gold Nanospheres: Synthesis and Characterization at the Single-Particle Level. Small 2011, 7, 3445-3451.

(28)

Glass, R.; Moller, M.; Spatz, J. P. Block Copolymer Micelle Nanolithography. Nanotechnology 2003, 14, 1153-1160.

(29)

Sivaraman, S. K.; Santhanam, V. Realization of Thermally Durable Close-Packed 2D Gold Nanoparticle Arrays Using Self-assembly and Plasma Etching. Nanotechnology 2012, 23, 255603-255603.

(30)

Kiely, C. J.; Fink, J.; Zheng, J. G.; Brust, M.; Bethell, D.; Schiffrin, D. J. Ordered Colloidal Nanoalloys. Adv. Mater. 2000, 12, 640-643.

(31)

Ullrich, S.; Scheeler, S. P.; Pacholski, C.; Spatz, J. P.; Kudera, S. Formation of Large 2D Arrays of Shape-Controlled Colloidal Nanoparticles at Variable Interparticle Distances. Part. Part. Syst. Charact. 2012, 30, 102-108.

(32)

Velikov, K. P.; Christova, C. G.; Dullens, R. P. A.; van Blaaderen, A. Layer-by-Layer Growth of Binary Colloidal Crystals. Science 2002, 296, 106-109.

(33)

Schaak, R. E.; Cable, R. E.; Leonard, B. M.; Norris, B. C. Colloidal Crystal Microarrays and Two-Dimensional Superstructures: A Versatile Approach for Patterned Surface Assembly. Langmuir 2004, 20, 7293-7297.


26

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34)

Huang, W.-H.; Sun, C.-H.; Min, W.-L.; Jiang, P.; Jiang, B. Templated Fabrication of Periodic Binary Nanostructures. J. Phys. Chem. C 2008, 112, 17586-17591.

(35)

Zhou, Z.; Yan, Q.; Li, Q.; Zhao, X. S. Fabrication of Binary Colloidal Crystals and NonClose-Packed Structures by a Sequential Self-Assembly Method. Langmuir 2007, 23, 1473-1477.

(36)

Becherer, G.; Herms, G.; Ahrenholz, P. Die Ermittlung von Radialen Verteilungsfunktionen aus dem Beugungsbild Zweidimensionaler Amorpher Strukturen. Ann. Phys. 1966, 17, 166-176.

(37)

Kaatz, F. H. Measuring the Order in Ordered Porous Arrays: Can Bees Outperform Humans? Naturwissenschaften 2006, 93, 374-378.

(38)

Kaatz, F. H.; Bultheel, A.; Egami, T. Order Parameters from Image Analysis: A Honeycomb Example. Naturwissenschaften 2008, 95, 1033-1040.

(39)

Kaatz, F. H.; Bultheel, A.; Egami, T. Real and Reciprocal Space Order Parameters for Porous Arrays From Image Analysis. J. Mater. Sci. 2009, 44, 40-46.

(40)

Johnson, P. B.; Christy, R. W. Optical Constants of Noble Metals. Phys. Rev. B 1972, 6, 4370-4379.

(41)

Stefanou, N.; Yannopapas, V.; Modinos, A. MULTEM 2: A New Version of the Program for Transmission and Band-Structure Calculations of Photonic Crystals. Comput. Phys. Commun. 2000, 132, 189-196.

(42)

Xu, Y. L. Electromagnetic Scattering by an Aggregate of Spheres. Appl. Opt. 1995, 34, 4573-4588.

(43)

Muehlig, S.; Rockstuhl, C.; Pniewski, J.; Simovski, C. R.; Tretyakov, S. A.; Lederer, F. Three-Dimensional Metamaterial Nanotips. Phys. Rev. B 2010, 81, 075317.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(44)

Page 28 of 29

Wan, X.; Chen, K.; Liu, D.; Chen, J.; Miao, Q.; Xu, J. High-Quality Large-Area Graphene from Dehydrogenated Polycyclic Aromatic Hydrocarbons. Chem. Mater. 2012, 24, 3906-3915.

(45)

Dal Negro, L.; Boriskina, S. V. Deterministic Aperiodic Nanostructures for Photonics and Plasmonics Applications. Laser & Photon. Rev. 2012, 6, 178-218.

(46)

Esteban, R.; Taylor, R. W.; Baumberg, J. J.; Aizpurua, J. How Chain Plasmons Govern the Optical Response in Strongly Interacting Self-Assembled Metallic Clusters of Nanoparticles. Langmuir 2012, 28, 8881-8890.

(47)

Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. One-Dimensional Plasmon Coupling by Facile Self-Assembly of Gold Nanoparticles into Branched Chain Networks. Adv. Mater. 2005, 17, 2553-2559.

(48)

Romero, I.; Aizpurua, J.; Bryant, G. W.; Garcia de Abajo, F. J. Plasmons in Nearly Touching Metallic Nanoparticles: Singular Response in the Limit of Touching Dimers. Opt. Express 2006, 14, 9988-9999.

(49)

Perez-Gonzalez, O.; Zabala, N.; Aizpurua, J. Optical Characterization of Charge Transfer and Bonding Dimer Plasmons in Linked Interparticle Gaps. New J. Phys. 2011, 13, 083013.

(50)

Esteban, R.; Borisov, A. G.; Nordlander, P.; Aizpurua, J. Bridging Quantum and Classical Plasmonics with a Quantum-Corrected Model. Nat. Commun. 2012, 3, 825.


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Plasmon Coupling in Self-Assembled Gold Nanoparticle-Based

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