Super-radiant Plasmon Mode is more Efficient for SERS than the Sub-radiant Mode in Highly Packed 2D Gold Nanocubes Arrays

Transcription

1 Super-radiant Plasmon Mode is more Efficient for SERS than the Sub-radiant Mode in Highly Packed 2D Gold Nanocubes Arrays Mahmoud A. Mahmoud * Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia * mmahmoud@gatech.edu Abstract The field coupling in highly packed plasmonic nanoparticle arrays is not localized due to the energy transport via the sub-radiant plasmon modes, which is formed in addition to the regular super-radiant plasmon mode. Unlike the sub-radiant mode, the plasmon field of the super-radiant mode cannot extend over long distances since it decays radiatively with a shorter life time. The coupling of the plasmon fields of gold nanocubes (AuNCs) when organized into highly packed 2D arrays was examined experimentally. Multiple plasmon resonance optical peaks are observed for the AuNC arrays and are compared to those calculated using the discrete dipole approximation (DDA). The calculated electromagnetic plasmon fields of the arrays displayed high field intensity for the nanocubes located in the center of the arrays for the lower energy super-radiant mode, while the higher energy sub-radiant plasmon mode displayed high field intensity at the edges of the arrays. The Raman signal enhancement by the super-radiant plasmon mode was found to be one hundred fold greater than that by sub-radiant plasmon mode because the super-radiant mode has higher scattering and stronger plasmon field intensity relative to the sub-radiant mode. Keywords: 2D plasmon field coupling, Super-radiant, sub-radiant, Langmuir-Blodgett, SERS 1

2 Introduction Plasmonic nanoparticles have shown to possess exciting optical and opto-electronic properties. 1, 2 Plasmon fields are generated due to the collective oscillation of the conduction band free electrons of the plasmonic nanoparticles, resulting in strong absorption and scattering spectra. 3 The plasmon fields are capable of enhancing both radiative electronic transitions such as the fluorescence of fluorophores 4, 5 or the Raman scattering of photons 6, 7. Non-radiative electronic transitions such as the rate of non-radiative electronic relaxation in semiconductors and exciton exciton annihilation 8 can also be enhanced by plasmon fields. The efficiency of plasmonic nanoparticles in most of their applications relies on the intensity of their electromagnetic plasmon fields Efforts are still ongoing to improve the strength of these plasmon electromagnetic fields, with some efforts centering on the control of the shape and structure of the nanoparticles while others are focusing on the assembly of the plasmonic nanoparticles in order to create strong coupling interactions between the plasmon fields of neighboring nanoparticles. 3 Tailoring the shape of plasmonic nanoparticles to have sharp corners and edges such as in the cases of nanocubes 16, 17, octahedra 18, prisms 19-21, rods 22-24, stars 25, 26, and bars 27 can improve the plasmon field intensity. However, the plasmon field intensity is typically only highest on the sharp corners and edges for these types of structures. 3 In order to increase the surface area of high plasmon field intensity locations, hollow plasmonic nanoparticles such as hollow cubes 16, hollow prisms 28, cubical frames 10, 29, octahedral frames 30, hollow rice 9, and hollow spheres 31 have been synthesized. Higher plasmon field strengths of hollow nanostructures are present due to the existence of plasmon fields on both the interior and exterior surfaces; these two fields can 2

3 couple to one another and generate high field enhancements on both the inner and outer walls of the hollow nanoparticles. 32 Different approaches such as nanosphere lithography techniques 33, electron beam lithography 34, self-assembly 35, Langmuir-Blodgett techniques 36-40, magnetic field assembly of plasmonic nanoparticles functionalized with magnetic nanoparticles 41, template assembly 42, and DNA functionalization 43 have been used to engineer the assembly behavior of nanoparticles in order to improve their plasmon field enhancements and resulting optical properties. The surface-enhanced Raman spectroscopy (SERS) by the plasmonic nanoparticles involves the enhancement of the incident laser photons by the plasmon field before interacting with the analyte. 44 However, the Raman signal of the analyte generates as a result of the scattering of the enhanced incident laser photons by the analyte. 44 The Raman signal of the analyte get enhanced by the plasmon field of the nanoparticles, as suggested by the electromagnetic mechanism of SERS. 6 For the analytes, which are capable of binding to the surface of the nanoparticles, further SERS enhancement is possible through the chemical enhancement mechanism. 45 In order to improve the SERS efficiency, the following requirements are necessary: 1) the laser photon has to be in resonance with the localized surface plasmon resonance (LSPR) spectrum of the plasmonic nanoparticles, 2) as the Raman photon (stokes) has energy lower than the incident photons, slightly broad LSPR spectrum is strongly recommended to overlap with both the incident laser photons and the scattered Raman photons 44, 3) since the SERS is a scattering process, the ratio between the intensity of scattering spectrum to that of the absorption spectrum has to be large, 4) the plasmon field of the nanoparticles should be strong enough, 5) the analyte has to be close to the surface of the nanoparticle where the plasmon field intensity is maximum and the chemical enhancement is allowed. Briefly, the SERS enhancement 3

4 factor depends greatly on the plasmon field strength and the scattering efficiency of the plasmonic nanoparticles. This study examines the optical properties and the nature of the plasmonic coupling of the well-organized two-dimensional arrays of gold nanocubes which are fabricated by the Langmuir-Blodgett (LB) technique. The optical measurements are conducted by the technique of hyperspectral microscopy on these AuNC arrays. These measurements displayed multiple localized surface plasmon resonance (LSPR) scattering peaks. The experimental results were compared to the results of the theoretical calculations carried out by the discrete dipole approximation (DDA) simulations. The study is also extended to cover 2D AuNC arrays having different separation distances between the individual gold nanocubes within the arrays. The separation distance between the nanocubes is finely controlled by changing the chain length of the polymer functionalized with the surface of the nanocubes. The LSPRs measured and simulated for the arrays suggest that the plasmon fields of the individual nanocubes inside the AuNC 2D array experience strong coupling interactions with those of nearby nanocubes. Due to the highly packed structure of the AuNCs forming the arrays, the plasmon mode splits into two modes sub-radiant and super-radiant. However, the sub-radiant mode transports the plasmon energy to the end of the arrays, while the super-radiant is localized and decays radiatively. The presence of the two modes is confirmed by the DDA calculation for the plasmon field distribution which show high field intensity located around nanocubes in the center of the array for the lower energy super-radiant plasmon mode. Whereas, the higher energy sub-radiant plasmon mode showed strong field intensity between nanocubes located at the edges of the arrays. Plasmon field coupling interactions are found to depend both on the number of the nanocubes forming the 2D array as well as the interparticle separation distance between 4

5 individual AuNCs. The SERS enhancement factor by the sub-radiant and super-radiant plasmon modes of AuNCs 2D arrays is determined. The SERS measurements conducted for 4- nitrothiophenol while adsorbed on the surface of the AuNCs arrays after exciting the sub-radiant plasmon mode using 532 nm and after the super-radiant plasmon mode excitation by the 785 nm laser. Experimental Gold nanocubes of 40 nm edge length, capped with cetyltrimethylammonium bromide (CTAB), were prepared by the seed-mediated approach as reported earlier 17 (see supporting info). The CTAB bound to the surface of AuNCs was exchanged with polyethylene glycol-thiol of molecular weight of 6,000 (6K PEG) and 2,000 (2K PEG), before transferring the AuNCs into a chloroform solvent for LB film preparation (supporting info). The AuNCs dispersed in chloroform were sprayed over the water sub-layer of a Nima 611D LB trough. The surface area occupied by the AuNCs was controlled by the mechanical barrier of the LB trough. The AuNC monolayer was then transferred to the surface of a silicon wafer substrate by the vertical dipping technique (supporting info). A Zeiss Ultra60 scanning electron microscope (SEM) was used to image the nanoparticles. Hyperspectral microscopy images were collected using a CytoViva Hyperspectral imaging system utilizing a diffraction grating spectrophotometer with a spectral range of nm and a spectral resolution of 2.8 nm. Both 50X and 100X dark field objectives (NA: 0.80, 0.90) in reflectance mode were used to scan the surface with a 10 nm step size scan resolution. Hyperspectral maps of the samples were normalized by a Lambertian reflectance standard (Labsphere, SRS ). Approximately 1,000 spectra were collected per scan to obtain individual sample averages. The surface Raman measurements were conducted using a Renishaw In via Raman microscope of 50x lens. The AuNCs arrays assembled on the 5

6 surface of silicon substrate was immersed in 0.1 mm aqueous solution of 4-nitrothiophenol (4NTP) and left for 2 hours. The arrays were washed five times by deionized water (immersion and gently shaking). The Raman spectra were collected by using 532 nm and 785 nm laser excitations. The regular Raman measurement carried out for thin film of 4NTP made by drop casting 4NTP paste on the surface of aluminum foil. This paste was made by mixing 0.2 g 4NTP with 0.1 ml of deionized water. Results and Discussion Assembly of gold nanocubes into well-organized, highly packed 2D arrays with different separation distances Nanoparticles prepared by the colloidal chemical technique are usually functionalized with capping materials which control their shape and size during their synthesis and also prevent aggregation. 46 Recently, gold nanocubes were successfully assembled into highly packed 2D arrays of different structures using the LB technique. 39 The osmotic pressure of free CTAB-PEG nanomicelles proved to be responsible for the assembly of AuNCs into highly packed structures according to experimental and theoretical calculations. 39 Changing the separation distance between the individual nanocubes inside of the 2D arrays without disrupting their organization is not an easy task. In the previous study on the assembly of AuNCs, a 6k PEG polymer was used to change the separation distance between the AuNCs inside the arrays compared with that when using 2K PEG, but the organization of the AuNCs inside the arrays was poor. 39 In fact, the former study accorded well with what has been observed by Tao s group 47 in the chain assembly of silver nanocubes; the short chain polyvinyl pyrrolidone polymer bound to the surface of the silver nanocubes drives them to assemble face-to-face, while the long chain polymers supported edge-to-edge assembly (less organization). In this former study, the concentration of nanomicelles was intentionally lowered to decrease the depletion force that drives the AuNCs to 6

7 assemble, facilitating the structure change of the 2D arrays upon changing the LB surface pressure. The goal here is to fabricate 2D arrays of well-organized AuNCs with controllable separation distances. The organization of the AuNCs is increased by increasing the amounts of nanomicelles present while the separation distance between the nanocubes inside the 2D arrays is controlled by changing the chain length of the polymer. Gold nanocubes functionalized with PEG of different chain lengths 2k PEG and 6k PEG were assembled into monolayers using LB. The AuNC monolayer was transferred to the surface of substrate at two different surface pressures using the vertical dipping technique. However, AuNCs functionalized with 2k PEG were transferred to the surface of silicon substrate at surface pressures of 0 and 7 mn/m while 0.3 and 9 mn/m surface pressures were used for the transfer of AuNCs functionalized with 6K PEG. AuNCs are arranged into highly packed 2D arrays with circular voids and a large numbers of cracks are observed, especially when the 2k PEG is used (see the SEM images in Figure S1 48 ). The percent of coverage from the surface of the silicon substrate by the AuNCs 2D arrays was estimated from the low magnification SEM images shown in Figure S1 48 using ImageJ program. For the AuNC arrays of 2K PEG, the coverage density was found to be 20 % and 59 %, which resulted from an increase in the fabrication surface pressure from 0 to 7 mn/m, respectively. The 6k PEG AuNCs arrays fabricated at surface pressure of 0.3 and 9 mn/m produced a percent of surface coverage of 15 % and 46 %, respectively. Figure 1A and 1B show the SEM images of AuNC monolayer arrays when functionalized with 2k PEG and deposited on the surface of silicon substrates at surface pressures of 0 and 7 mn/m, respectively. SEM images of the 2D arrays of AuNCs functionalized with 6KPEG and assembled at surface pressures of 0.3 and 9 mn/m are shown in Figure 1C and 1D, respectively. It is clear that the width of the 2D Au NC arrays functionalized with 2k PEG 7

8 increases as the surface pressure is increased but the number of nanocubes per array is not clearly affected, due to the presence of cracks separating the arrays into groups. The width of the 2D arrays of AuNCs functionalized with 6k PEG was also found to increase upon increasing the LB surface pressure. Interestingly, the cracks are rare and the AuNCs are well-organized inside the arrays. The estimated length of the stretched 2k and 6k PEG functionalized with the surface of the AuNCs is 20 and 60 nm, respectively. The separation distance between the individual AuNCs inside the 2D arrays is also affected and changed from 4.1 ± 1.3 nm when using 2k PEG to 6.4 ± 2.5 nm when using 6k PEG. In principle, the value of the distance between the AuNCs inside the arrays should not be that small. A possible reason for the observed organization of the AuNCs is that the free nanomicelles are present in high concentrations and can apply strong osmotic pressure on the nanocube arrays, forcing them to organize in 2D arrays. This high osmotic pressure overcomes the strong steric force between the long chains of the 6k PEG arranged in brush-like structures on the surface of the AuNCs. Such organization was not observed for low micelle concentrations. In addition to the depletion force of the free nanomicelles, the van der Waals attraction forces between the nanocubes and the polymer chains drive the polymer chains to pack and crystalize together. 39 The potential energy of the AuNC 2D arrays depends on the interparticles separation distance. 39 The optimum separation distance between the AuNCs forming the arrays that minimizes the value of the potential energy is obtained. 39 Due to the cubical shape of the nanoparticles forming the arrays, the voids are expected to be cubical as common in the fractal phenomena. As a result of the presence of cracks in the 2k PEG AuNCs arrays and the small tilting of the AuNCs in the arrays containing 6k PEG, the circular and semi-hexagon structure was obtained instead of the cubical ones. 8

9 A B C D Figure 1 SEM images of 2D arrays of gold nanocubes functionalized with short chain 2k PEG polymers and assembled on the surface of silicon substrates at surface pressures of: A) 0 mn/m, B) 7 mn/m. The gold nanocubes are arranged into highly organized 2D arrays of different widths. Cracks that partition the arrays into smaller groups and a fixed separation distance between individual nanoparticles (4.2 ± 1.5 nm) are observed in the two samples. SEM image of gold nanocube 2D arrays functionalized with long chain 6k PEG deposited on a silicon substrate at surface pressures of: C) 0.3 mn/m and D) 9 mn/m. The nanocubes are well-organized inside the arrays with a 6.4 ± 2.5 nm separation distance between the individual particles; the cracks are rarer compared with the short chain polymer assemblies. The inset magnified images of figure 1B and C are for part of the arrays 2k PEG and 6k PEG AuNC arrays. Optical properties of the highly packed 2D arrays of gold nanocubes The plasmon field of a nanoparticle can couple with that of other fields of nanoparticles in close proximity, generating amplified plasmon field enhancements known as hot spots. For dimers, this magnified field is concentrated in the gap between the nanoparticles, and the strength of the plasmon field is increased as the separation gap is decreased. 3 As shown in the 9

10 SEM images in Figure 1, AuNCs were assembled into highly packed 2D arrays, the advantage of these organized structures being that the separation distance between the many individual nanocubes in the arrays is very small (4.1 ± 1.3 nm for 2K chains and 6.4 ± 2.5 nm for 6k PEG chains). The 2D arrays presented here therefore display exciting optical phenomena compared to the simpler case of nanoparticle dimers. It is well-known that the LSPR spectrum of a nanoparticle dimer shifts to a lower energy compared to the LSPR spectrum of individual nanoparticles, 49 but the LSPR spectrum shifts, both shape and peak position, are less wellknown for these highly packed 2D arrays. The plasmon field distribution intensity for these arrays is more complex than that of a dimer, since each nanocube is surrounded by four other nanocubes. Due to the dislocation of some rows of nanocubes compared to the others, even for very small dislocation, some of the AuNCs inside the 2D arrays are surrounded with other six particles. The presence of such particles surrounding with different numbers of particles is expected to add to the heterogonous broadening of the LSPR spectrum. Figure 2A shows the LSPR scattering spectra of AuNC 2D arrays functionalized with 6k PEG and deposited on a silicon substrate at LB surface pressures of 0.3 and 9 mn/m. Although multiple LSPR scattering peaks are observed in the two samples, the peak positions are different and the peak of lower energy becomes clearer in the arrays when prepared at 9 mn/m. A sharp LSPR scattering peak is located at 675 nm in addition to four shoulders at 546, 584, 632, and 775 nm for the arrays fabricated at 0.3 mn/m. For the arrays prepared at a surface pressure of 9 mn/m, the sharp scattering peak was observed at 721 nm, while the shoulders were at 546, 608, 649, and 836 nm. Figure 2B shows the LSPR scattering spectra of 2D AuNC arrays functionalized with the 2k PEG polymer and deposited at LB surface pressure of 0 and 7 mn/m on the surface of a silicon substrate. These arrays displayed four shoulders and a sharp LSPR 10

11 scattering peak; three shoulders are located at 526, 725, and 811 nm in the two samples while a small shift is observed in both the central sharp LSPR peak and the third shoulder as well. Although the LSPR scattering spectra for the two surface pressures do not display strong shifts, the ratios between the peak intensities are different. The LSPR peak shoulders are located at 632 and 642 nm while the sharp peaks are centered at 682 and 692 nm for the 2D arrays assembled at respective surface pressures of 0 and 7 mn/m. Scattering Intensity A 546 nm 675 nm 632 nm 650 nm 721 nm 775 nm 6k PEG-SH 836 nm Scattering Intensity B 632 nm 682 nm 2k PEG-SH 725 nm 811 nm 526 nm 725 nm 642 nm 811 nm 692 nm nm 0.3 mn/m 9 mn/m Wavelength (nm) Wavelength (nm) Figure 2 Localized surface plasmon resonance scattering spectra of gold nanocubes assembled into 2D arrays. A) Gold nanocubes are functionalized with the long chain 6k PEG polymer generating 6.4 ± 2.5 nm separation distances between the individual cubes. Multiple plasmon peaks are observed in the two samples, one of them being located at 546 nm for the two arrays fabricated at surface pressures of 0.3 and 9 mn/m. The peak positions of the other plasmon peaks vary depending on the LB surface pressure. B) The gold nanocubes in the arrays are functionalized with the short chain 2k PEG polymer, leading to 4.1 ± 1.3 nm separation distances; multiple shoulders are obtained in addition to the main sharp central plasmon peak which is not affected much by a change in surface pressure from 0 to 7 mn/m nm 0 mn/m 7 mn/m DDA Simulation of the LSPR spectra for highly packed 2D gold nanocube arrays A single gold nanocube (isotropic shape) and a nanocube dimer are characterized by the presence of a single LSPR extinction plasmon peak. It was observed from the above 11

12 experimental measurements that AuNC 2D arrays possess multiple plasmon peaks. In order to assign these peaks to plasmon modes and to determine the number of nanocubes that couple together within the 2D array to produce the observed LSPR scattering spectra, discrete dipole approximation (DDA) calculations were carried out. Figure 3A shows the LSPR scattering spectrum, calculated by the DDA technique, for 25 AuNCs arranged into 2D arrays (5 x 5 nanocubes) with a 6 nm separation distance between the individual nanocubes, the dielectric of the surrounding medium was taken to be a mixture of PEG and air (1 to 1 ratio). The simulation carried out for the array placed on the surface of silicon oxide substrate of 1 nm separation distance and dipole density of 1 dipole for each 1 nm. The experimental LSPR scattering spectrum of the AuNCs functionalized with the long chain 6k PEG polymer and assembled on a silicon substrate at an LB surface pressure of 0.3 mn/m is also included in Figure 3A (red color). As was experimentally observed, the theoretical spectrum has five plasmon peaks at 536 nm, 585 nm, 643 nm, 675 nm (sharpest peak), and 756 nm. The experimental and theoretical spectra in Figure 3A appear to be comparable, although the calculated LSPR scattering spectrum of the nanocubes arrays composed of 25 particles would poorly fit the experimental LSPR spectra for arrays assembled at a surface pressure of 9 mn/m. The calculation was therefore extended to include 2D arrays composed of 49 AuNCs arranged in a square 7 x 7 nanocube formation. Figure 3B shows the LSPR scattering spectrum for 49 AuNCs with a 6 nm separation distance calculated by the DDA technique and the experimental LSPR scattering spectrum measured for an AuNC 2D array with the 6k PEG polymer assembled at a surface pressure of 9 mn/m. The theoretical spectrum exhibited four plasmon peaks at 536, 604, 660, 714, and 830 nm and closely matches the experimental spectra that were obtained for the 2D arrays fabricated at an LB surface pressure of 9 mn/m. 12

13 In order to assign plasmon modes to the experimental LSPR scattering spectra of the AuNC arrays functionalized with the short chain 2k PEG polymer at different LB surface pressures, DDA calculations were carried out for 25 and 49 nanocube 2D square arrays with a 4 nm separation distance between the nanocubes. Figure 3C shows the simulated LSPR scattering spectrum for a 25 AuNC 2D array (5 x 5 particles) with 4 nm separation distances and the experimental LSPR spectrum of AuNCs functionalized with the short chain 2k PEG polymer and assembled on a silicon substrate at surface pressures of 0 and 7 mn/m. The simulated spectrum correlates well with the experimental results. This suggests that the plasmon field coupling interactions occurring in the AuNC 2D arrays functionalized with the short chain polymers have significant long-range coupling effects on the order of ~ 25 nanocubes. The obtained results were confirmed by running the calculation for 49 particles assembled with 4 nm separation gaps. The calculated LSPR scattering spectrum showed LSPR scattering peaks at 529, 615, 693, 749, and 905 nm which did not correlate with any experimental results (see Figure 3D). DDA calculation for small arrays such as 9 nanocubes was examined. However, a single peak LSPR spectrum was obtained when the separation distance between the individual nanocubes in the 3 x 3 arrays was taken to be 4 and 6 nm. It is clear from the DDA simulation, that the LSPR spectrum of the AuNC 2D arrays red shifts as the number of nanocubes forming the arrays is increased. The arrays of large numbers of AuNC such as 49 particles separated with 4 nm showed LSPR spectrum of lower energy. Based on the results obtained from the simulation, the LSPR spectrum of AuNC arrays composed of more than 49 nm particles is expected to locate at wavelength longer than that of the LSPR spectrum measured for all the studied arrays. 13

14 It is useful to mention that, both the simulated and measured LSPR spectrum of the AuNC 2D arrays are broad, centered at the same wavelength position, and have multiple peaks. The calculated LSPR spectrum is much sharper and more resolved compared with the experimental spectrum. The broader and less resolved peaks measured LSPR spectrum (heterogeneous broadening) of the arrays are attributed to the different size distribution of the AuNCs and the imperfect organization of the AuNCs inside the 2D arrays. It is unlikely to achieve identical match between the experiments the theoretical calculations, but to obtain the highest possible accuracy in the calculation. A mn/m 1.0 B 9 mn/m Scattering Intensity Scattering Intensity DDA 25 particles Exp. PEG 6k 25 particles coupling 6 nm separation Wavelength (nm) DDA 49 particles Exp. PEG 6k 49 particles coupling 6 nm separation Wavelength (nm) 1.0 C 0 mn/m 7 mn/m 1.0 D Scattering Intensity Scattering Intensity DDA 25 particles Exp. PEG 2k 25 particles coupling 4 nm separation Wavelength (nm) Wavelength (nm) Figure 3 Surface plasmon resonance scattering spectra of gold nanocube 2D arrays calculated using the discrete dipole approximation technique (black spectrum) and experimentally measured DDA 49 particles 49 particles coupling 4 nm separation 14

15 (red and blue spectra): A) The 2D array in the simulation consists of 25 nanocubes placed at 6 nm separation distances and the experimentally measured spectrum is for AuNCs functionalized with the long chain 6k PEG polymer and fabricated at a surface pressure of 0.3 mn/m. B) Simulation consists of 49 nanocubes in a square 2D array with 6 nm separation distances, and the experimental sample is for AuNCs functionalized with the long chain 6K polymer and fabricated at a 9 mn/m surface pressure. C) The 2D array used in the simulation was composed of 25 nanocubes with 4 nm separation distances while the experimental spectrum is for AuNCs functionalized with the short chain 2k PEG polymer and assembled at surface pressures of 0 mn/m (red) and 7 mn/m (blue). D) The theoretical calculation is for a 49 nanocube 2D array with 4 nm separation distances. The effect of number of AuNCs forming the 2D arrays on their optical properties was examined by studying the optical properties of 2D arrays of different dimensions and shapes. Figure 4A shows the dark field image of the 2D arrays of AuNCs of different dimensions remains after cleaning part of the connected monolayer of AuNCs functionalized with 2k PEG and fabricated at LB surface pressure of 7 mn/m. The SEM image of the arrays of the connected monolayer before is shown in Figure 4B. The corresponding LSPR scattering spectrum of the arrays of different dimensions as well as that of the connected 2D arrays are shown in Figure 4C. The LSPR scattering spectrum of the single AuNC measured on the surface of silicon substrate in a highly separated AuNCs 2D assembly is also shown in Figure 4C. Figure S2A shows the SEM image of the highly separated AuNCs monolayer assembly. 48 The single AuNCs have sharp LSPR scattering peak of higher energy compared with that of the arrays. The measured LSPR spectrum of the single AuNC was confirmed by DDA calculation in Figure S2B. 48 Wellagreement between the theoretical and experimental spectrum of the single AuNC was observed. As in case of the connected layer 2D arrays of AuNCs, the LSPR spectrum of the arrays of different dimensions is broad and has multiple peaks. However, the center of the spectrum blue shifted as the surface area of the arrays was decreased (see Figure 3C), which confirms that the LSPR spectrum of the AuNC arrays depends on the number of AuNCs forming the arrays. 15

16 Moreover, arrays highlighted by the cyan color showed LSPR spectrum matches the measured LSPR spectrum for the connected arrays. It was found that the cyan color arrays composed of ~ 50 AuNCs. This suggests that the maximum number of AuNCs that their plasmon field can be coupled together in the prepared arrays does not exceed 50 particles. Ultimately, the dark field of the connected arrays in Figure S3A shows that the scattering intensity on the cracks and voids is much stronger than that on the body of the arrays. 48 This supports the idea that the plasmon field coupling between the AuNCs inside the arrays is not extended over the whole arrays. For more clarification of the 2D plasmon field coupling in the AuNC arrays, the LSPR scattering spectrum of 2k PEG AuNCs 2D arrays fabricated at 7 mn/m was collected from the center of the arrays and from the circular void edges (see Figure S3C). 48 The LSPR spectrum measured from the center of the voids is broad and showed three peaks at 546, 651, and 775 nm. Interestingly, the LSPR peak appeared at 811 nm when the spectrum was collected from the center of the arrays disappeared when the spectrum collected from the center of the void. A 10 µm C Single Particle connected layer Green array Cyan array B Scattering Intensity Wavelength (nm) Figure 4 A dark field image of 2D arrays of AuNCs functionalized with 2k PEG of different dimensions. The arrays are generated from cleaning up different parts from the connected 16 Yellow array Blue array Magenta array

17 monolayer of AuNCs assembled on the surface of silicon substrate at LB surface pressure of 7 mn/m. B) SEM image of the AuNCs monolayer before partially cleaned. C) The LSPR scattering spectrum of single AuNC measured on the surface of silicon substrate, the LSPR scattering spectrum of 2D arrays of AuNCs of different dimensions, and the LSPR spectrum of the connected 2D AuNCs arrays. The single AuNCs have a sharp plasmon peak while the 2D arrays of the different dimensions and the connected layer have similar LSPR scattering spectra (multiple peaks and broad spectrum). As the surface area of the 2D arrays is decreased, their LSPR spectrum is blue shifted, and the intensity of the LSPR scattering peak at the shorter wavelength becomes more intense, while the intensity of the peaks at longer wavelengths decreased. The LSPR spectrum of the cyan arrays showed a great match with the LSPR spectrum of the connected arrays. This confirms that ~ 50 AuNCs couple together inside the connected arrays. Plasmon field coupling and distribution in the highly pack 2D nanocubes arrays The LSPR scattering spectra calculated for 25 and 49 gold nanocubes arranged into 2D square arrays with separation distances of 4 and 6 nm allow the interpretation of the measured LSPR scattering spectra of the AuNC 2D arrays. The plasmon field calculated by the DDA technique for the top and lower plane of a single AuNC placed on the surface of silicon substrate and excited at 544 nm showed strong field intensity on the corners of the AuNC (see Figure S2C and D). 48 The plasmon field intensity close to the surface of the substrate is much stronger than that far from the surface of the substrate. For any pair of plasmonic nanoparticles that are close together, the coupled plasmon field intensity will be highest in the gap between them. However, the plasmon field distribution for these types of arrays with small nanoparticle gaps in each direction is less intuitive. DDA field enhancement calculations were therefore performed for 25 and 49 nanocube arrays with separation distances of 4 and 6 nm and with the incident light polarized along the array parallel to the substrate, the arrays are 1 nm far from the surface of the silicon coated with oxide layer substrate. The simulations were carried out for both the top plane of the arrays (far from the surface of the substrate) and the bottom plane (close to the surface of the substrate). The plasmon field on the top plane of the arrays is shown in Figure 5, while the 17

18 plasmon field distribution on the bottom plane is shown in Figure S4. 48 Figure 5A plots the plasmon field intensity distribution of 2D nanocube array composed of 25 particles with 6 nm separation distances for the 675 nm plasmon mode. The plasmon field intensity was found to be highest around the nanocubes located at the edges of the arrays compared to ones located in the center. In contrast to the 675 nm plasmon mode, the plasmon field intensity for the 756 nm plasmon mode displayed the highest field intensities in the center of the AuNC 2D arrays and lower field intensities between the cubes at the edges (see Figure 5B). When the separation distance in the 25 nanocube arrays is reduced to 4 nm, the behavior of the plasmon field intensity distribution for the 714 nm mode shown in Figure 5C is similar to that observed at 675 nm plasmon mode for the separation distance of 6 nm. The plasmon field intensity distribution of the 25 nanocube 2D arrays with 4 nm separation distances displayed the highest plasmon field intensities around the central nanocubes and lower field intensities between those at the edges when the 830 nm plasmon mode was excited. The field intensity of the 25 nanocube arrays with 4 nm separation distances exhibited higher field intensities for both the 714 nm and 830 nm modes when compared with the field intensities of the 675 nm and 756 nm plasmon modes with 6 nm separation distances. DDA calculations were also carried out for the 49 nanocube arrays with 6 nm separation distances for both the 719 nm and 828 nm plasmon modes. In the case of the plasmon mode at 719 nm, the plasmon field intensity around the nanocubes located in the upper and lower edge rows is high while the field intensity in the remainder of the array is relatively low, as shown in Figure 5E. Figure 5F shows the calculated plasmon field intensity distribution for the 49 nanocube 2D arrays with 6 nm separation distances for the 828 nm plasmon mode. The field distribution is highest in between the nine central nanocubes while the field distribution around the nanocubes at the edges is much lower. Similar field distributions are 18

19 obtained for the 693 nm and 905 nm plasmon modes for the 49 nanocube 2D arrays when the separation distance is reduced to 4 nm (see Figure S5). 48 Figure S6 shows the plasmon field distribution for a 49 AuNC array with 6 nm separation distances calculated using the DDA technique for the high energy plasmon mode of 536 nm. 48 It is clear that the plasmon field distribution for this mode is similar to that calculated at 719 nm but the field intensity is low on the AuNCs located at the corner of the arrays. The plasmon field distribution for nanoparticles that were organized into chains was proven to be strong in the gaps between the chain due to the linear plasmon field coupling. 50 The plasmon energy transport in metal nanoparticle chains model (PETM) succeeded to describe the plasmon field coupling in the 1- or 2- D arrays PETM suggests that, for the close-packed multiple plasmonic nanoparticles, due to the particle-particle interaction, the plasmon mode splits into two modes. 51 The first is sub-radiant plasmon mode, that propagates along the arrays 53, 55 with minimum radiative losses and so having a longer life time. 56 The second mode is the regular super-radiant plasmon mode, which decays radiatively at a shorter life time due to the coupling of the near field of the individual nanoparticles inside the arrays and the far-field of the neighboring nanoparticles The sub-radiant plasmon mode appears usually at wavelengths shorter than that of the super-radiant plasmon mode. 51 Interestingly, the PETM presented a wellinterpretation for the plasmon coupling highly-packed 2D arrays of AuNCs. However, the plasmon field distribution calculated at shorter wavelengths (sub-radiant plasmon modes) by the DDA technique showed high field intensity on the AuNCs located at the ends of the arrays. This is because of the plasmon energy transportation along the arrays when the sub-radiant mode is excited (see Figure 5A, C, and E). 53, 55 Conversely, the plasmon field calculated at the superradiant plasmon mode of lower energy showed high field intensity on the AuNCs located on the 19

20 center of the arrays. This is because of the radiative plasmon losing and the destructive plasmon field coupling between the far-field and near field, the field cannot transfer to the end of the arrays. Based on PETM51, the multiple LSPR peaks of the AuNCs arrays that experimentally observed and obtained from the DDA calculation can be assigned as follow: the high energy plasmon modes are for the sub-radiant plasmon mode (main and multipolar), while the low energy plasmon mode is for the super-radiant plasmon mode. This assignment was supported by the results of the LSPR spectrum collected from the center of the voids of the 2k PEG AuNC arrays fabricated at 7 mn/m (see Figure S3C).48 Comparing the LSPR spectrum collected from the center of the arrays with that collected from the center of the voids, the LSPR peak at 811 nm did not appear when the measurement taken from the voids area. This suggests that the peak at 811 nm corresponding to the plasmon field concentrated on the center of the arrays is obtained from the DDA simulation. 790 nm top_6 nm 712 nm top_6 nm A B nm Mode E k 723 nm top_4 nm C nm Mode nm top _4 nm D nm Mode nm Mode

21 852 nm top _ 6 nm 735 nm top _ 6 nm E F nm Mode nm Mode Figure 5 Plasmon field intensity distributions for the 2D gold nanocube arrays placed on the surface of silicon oxide substrate calculated on the top plane of the arrays by the discrete dipole approximation technique. A) A 25 nanocube array with 6 nm separation distances calculated for the 675 nm plasmon mode. The plasmon field shows high intensities in between nanocubes located at the edges. B) A 25 nanocube array with 6 nm separation distances calculated for the 756 nm plasmon mode. The plasmon field intensity is highest in between the central nanocubes. C) A 25 nanocube array calculated for the 714 nm plasmon mode when the distance between the individual nanocubes inside the array was reduced to 4 nm. The intensity of the plasmon field is stronger than that calculated for the 25 particle array when the separation distance was 6 nm. The field intensity for this mode is higher in between the central nanocubes compared with those located at the edges. D) A 25 nanocube array with 4 nm separation distances. The field intensity is highest in between the central nanocubes for the 830 nm plasmon mode. E) A 49 nanocube array with 6 nm separation distances calculated for the 719 nm plasmon mode. The plasmon field is concentrated in between the nanocubes located at the edges. F) A 49 nanocube array with 6 nm separation distances calculated for the 828 plasmon mode. The field distribution is localized in the center of the 2D array. SERS by super and sub-radiant plasmon modes of highly packed 2D arrays of AuNCs In order to examine the SERS enhancement by the super and sub-radiant plasmon modes, Raman measurement was conducted for 4NTP adsorbed on the surface of the AuNCs arrays using 532 nm laser (excites the sub-radiant plasmon mode) and 785 nm laser that excites the super-radiant plasmon mode. Figure 6A shows the SERS spectrum of 4NTP adsorbed on the surface of AuNCs-6k PEG arrays fabricated at surface pressure of 0.3 and 9 mn/m and on the surface of AuNCs-2k PEG arrays fabricated at 0 and 7 mn/m LB surface pressures collected upon 532 nm laser excitation. Sharp SERS bands at 1573, 1340 and 1084 cm-1 correspond to parallel C-C stretching mode, symmetric nitro stretching mode vs(no2), and ν(c S) stretching mode of 4NTP, respectively. Interestingly, no SERS bands corresponding to the presence of PEG 21

22 or CTAB molecules were observed. Complete removal of CTAB used during the synthesis of the AuNCs is impossible, but the concentration was minimized by the multiple washing of the nanocubes before LB assembly. Although both PEG and CTAB exist with the AuNC 39, they have a low Raman cross-section compared to 4NTP. Therefore, SERS bands of 4NTP were only detected. The regular Raman spectrum measured for a thin film of 4NTP-water paste is shown in Figure S7. 48 The SERS enhancement factor (EF) was calculated using the Raman band intensities of 4NTP (located at 1573, 1340 and 1084 cm -1 ) in both the SERS spectrum collected for the arrays and the regular Raman spectrum measured for thin film. The EF values calculated for the three SERS bands at 1573, 1331 and 1084 cm -1 was found to be comparable. When the 532 nm laser that excites the sub-radiant plasmon mode of the AuNCs arrays was used, the average value of the EF calculated using the three SERS bands was found to be ~ 1.8 x10 9 and 3.2x10 9 for the AuNCs functionalized with 6k PEG and fabricated at 0.3 and 9 mn/m. The average value of EF of arrays containing 6k PEG did not much change from that calculated for the AuNCs-2k PEG arrays, which showed average EF of 2.1x10 9 and 4.6x10 9 when the arrays fabricated at 0 and 7 mn/m, respectively. The chain length of the PEG functionalized with the surface of AuNCs forming the arrays did not show observable effect on the SERS values. When the 785 nm laser was used in the SERS measurement, the average value of EF calculated for 4NTP was found to be 4.8x10 10 and 1.6x10 11 for the AuNCs-6k PEG arrays fabricated at 0.3 and 9 mn/m, respectively (see Figure 6 B). The respective average value of EF by the AuNCs-2k PEG arrays fabricated at 0 and 7 mn/m was 1.1x10 11 and 2.1x The 785 nm laser is able to excite the super-radiant plasmon mode of the AuNCs 2D array (see Figure 2). The efficiency of SERS by the super-radiant plasmon mode in the highly packed AuNCs 2D 22

23 arrays is ~100 times greater than that by the sub-radiant plasmon mode. The SERS enhancement factor increases by increasing the strength of the plasmon field and the scattering efficiency of the plasmonic nanoparticles. The super radiant plasmon is characterized by its strong plasmon field and the strong radiative scattering efficiency which is necessary to improve the SERS EF Raman Unit A 6K PEG 0.3 mn/m 6K PEG 9 mn/m 2K PEG 0 mn/m 2K PEG 7 mn/m 532 nm Laser excitation Raman Unit B 6K PEG 0.3 mn/m 6K PEG 9 mn/m 2K PEG 0 mn/m 2K PEG 7 mn/m 785 nm Laser excitation Wavenumber (cm -1 ) Wavenumber (cm -1 ) Figure 6 SERS of 4-nitrothiophenol measured while adsorbed on the surface of AuNCs functionalized 6k PEG organized into arrays, when fabricated at LB surface pressures of 0 mn/m (black spectrum) and 9 mn/m (red spectrum). The SERS spectrum of 4-nitrothiophenol adsorbed on the surface of the arrays of AuNCs functionalized with 2k PEG and assembled at LB surface pressure of 0 and 7 mn/m are in blue and olive color, respectively: A) 532 nm Laser was used for Raman excitation, B) The Raman measurement was conducted at 785 nm laser excitation. The 532 nm laser excites the sub-radiant plasmon mode which less efficient in SERS than the super-radiant plasmon mode excited by the 785 nm laser. Conclusions In order to study the two-dimensional plasmon field coupling and the Raman enhancement factor of highly packed plasmonic nanoparticles, 2D arrays of gold nanocubes were fabricated on silicon substrates using the Langmuir-Blodgett technique via vertical dipping. The nanocubes were prepared by the colloidal chemical approach using CTAB as a capping agent. After the nanocubes were synthesized, CTAB was exchanged with thiolated PEG of two different chain lengths. Two-dimensional arrays of gold nanocubes with separation distance between the 23

24 individual nanocubes of 4.1 ± 1.3 and 6.4 ± 2.5 nm were fabricated by changing the chain length of PEG functionalized on the surface of the AuNCs from 2K to 6K, respectively. In order to increase the number of AuNCs per array, the surface pressure of the Langmuir-Blodgett trough was increased. Increasing the surface pressure succeeded for AuNCs functionalized with 6k PEG, but did not work with the short chain 2k PEG due to the presence of cracks in the array that split the arrays into smaller group of particles. Optical measurements of the 2D gold arrays showed multiple LSPR scattering spectral peaks compared to the single peak that is well-known and understood for individual gold nanocubes. Discreet dipole approximation (DDA) simulations for the LSPR scattering spectrum suggested that the plasmon fields of the nanocubes organized into the 2D arrays couple together to generate multiple plasmon peaks. As the AuNCs 2D arrays are highly packed, the plasmon energy transport in metal nanoparticle chains model was applied and succeeded to describe the plasmon field coupling in the arrays. The plasmon modes of the 2D arrays was assigned to be due to sub-radiant modes of energy higher, which can be transported to the end of the arrays and a super-radiant plasmon mode of lower energy. The plasmon field distribution calculated for the super-radiant plasmon mode showed high field intensities in between nanocubes located in the center of the 2D arrays. While the sub-radiant plasmon modes generated high field intensities in between nanocubes located at the edges of the arrays. Because of the radiative nature of the super-radiant mode and the strong plasmon field, the SERS by the super-radiant mode was found to be more efficient than that by the sub-radiant mode. Finally, the prepared arrays are useful SERS substrate for detection of large molecules. For example, proteins that can be accommodated inside the large surface area voids between the arrays where the plasmon field is very strong. Selective detection by the prepared arrays is possible; however, for samples that have two sizes analyte one is large and the other is small. 24

25 The small one can locate in between the nanocubes inside the arrays and the large one locates inside the voids. SERS excited by a short wavelength laser leads to the detection of the large analyte in the voids, while the longer wavelength laser make the detection of the small analyte in between the nanocubes is possible. Acknowledgement This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award # DE-FG02-09ER I would like to thank Mr Jeffrey Geldmeier from the school of materials science and engineering at Georgia Tech for helping in the optical measurements. References 1. U. Kreibig and M. Vollmer, Optical Properties of Metal Clusters. (Springer Berlin 1995). 2. D. L. F. Fedlheim, C. A., Metal Nanoparticles: Synthesis, Characterization, and Applications. (Marcel-Dekker,, New York, 2002). 3. K. L. Kelly, E. Coronado, L. L. Zhao and G. C. Schatz, The Journal of Physical Chemistry B 107, 668 (2003). 4. J. R. Lakowicz, C. D. Geddes, I. Gryczynski, J. Malicka, Z. Gryczynski, K. Aslan, J. Lukomska, E. Matveeva, J. A. Zhang, R. Badugu and J. Huang, Journal of Fluorescence 14, 425 (2004). 5. M. A. Mahmoud, A. J. Poncheri and M. A. El-Sayed, The Journal of Physical Chemistry C 116, (2012). 6. D. L. Jeanmaire and R. P. Van duyne, Journal of Electroanalytical Chemistry 84, 1 (1977). 25

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31 A B C D

32 Scattering Intensity A 546 nm 675 nm 632 nm 650 nm 721 nm 775 nm 6k PEG-SH 836 nm Scattering Intensity B 526 nm 632 nm 682 nm 2k PEG-SH 725 nm 811 nm 725 nm 642 nm 811 nm 692 nm nm 0.3 mn/m 9 mn/m Wavelength (nm) nm 0 mn/m 7 mn/m Wavelength (nm)

33 A mn/m 1.0 B 9 mn/m Scattering Intensity Scattering Intensity DDA 25 particles Exp. PEG 6k 25 particles coupling 6 nm separation Wavelength (nm) DDA 49 particles Exp. PEG 6k 49 particles coupling 6 nm separation Wavelength (nm) 1.0 C 0 mn/m 7 mn/m 1.0 D Scattering Intensity Scattering Intensity DDA 25 particles Exp. PEG 2k 25 particles coupling 4 nm separation Wavelength (nm) DDA 49 particles 49 particles coupling 4 nm separation Wavelength (nm)

34 A A B B µm µm Scattering Intensity Scattering Intensity C C Single Single Particle Particle connected connected layer layer Green Green array array Cyan Cyan array array Yellow Yellow array array Blue Blue array array Magenta Magenta array array Wavelength (nm) (nm)

35 790 nm top_6 nm 712 nm top_6 nm A B nm Mode E k 723 nm top_4 nm C 714 nm Mode 735 nm top _ 6 nm E 719 nm Mode nm Mode nm top _4 nm D nm Mode 852 nm top _ 6 nm F nm Mode