J Fontana Et Al (Adv. Optical Mater 2013 1 100 ) Au Np

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Jake Fontana,* Jawad Naciri, Ronald Rendell, and Banahalli R. Ratna

Macroscopic Self-Assembly and Optical Characterizationof NanoparticleLigand Metamaterials

Dr. J. Fontana, Dr. J. Naciri, Dr. R. Rendell,Dr. B. R. RatnaNaval Research Laboratory4555 Overlook Ave, SW, Washington, D.C, 20375, USAE-mail: [email protected]

DOI: 10.1002/adom.201200039

1. Introduction

Metamaterials are a large class of engineered materials, rangingfrom optical devices[1] to high-strength composites.[2] Recentlyinterest has grown in optical metamaterials where the indexof refraction approaches zero.[3] In such materials the phasevelocity of the electromagnetic wave becomes large (low wavenumber), leading to a spatially uniform phase inside the mate-rial, yet remains dynamic in time allowing for energy transport,providing unique opportunities for applications.[4]

If gold nanospheres, with negative permittivity values at

prescribed visible wavelengths, are assembled into sufficientlyhigh-densities the index of refraction of the metamaterial canreach positive near-zero values. These metamaterials are lim-ited to thin materials, only a few metal layers thick, due to thesignificant intrinsic losses associated with the metal constitu-ents at optical frequencies.

Traditionally, top-down lithographic techniques[5] have beenused to produce the nanoscopic constituents used for opticalmetamaterials. These techniques are complex, time consuming,expensive, producing primarily microscopic fixed materialswith limited particle resolution. Alternatively bottom-up or self-assembly can be used, offering a powerful and easier route, forcreating optical metamaterials and is advantageous because of

the ease of synthesis, feature size, and scalability.To create such films aqueous suspensions of colloidal goldnanoparticles can be synthesized using wet chemistry methods [6]and charge-stabilized surfactants. However, surfactants permitnanoparticle aggregation since the global energy minimum

occurs when the nanoparticles are in con-tact. To circumvent this issue gold nano-particles can be functionalized with thiolor amine terminated ligands bonded tothe gold surface to stabilize against aggre-gation through entropic repulsion. How-ever, most of these ligands are not solublein aqueous solvents, thus requiring thenanoparticles to be phase transferred toorganic solvents. Though numerous tech-niques have been developed over the yearsto phase transfer nanoparticles, it hasremained challenging to efficiently func-

tionalize metallic nanoparticles without aggregation.[7]Once the nanoparticles are successfully capped with the

appropriate ligand self-assembly can be utilized to createmacroscopic, high-density, monolayer films. Bigioni et al. [8]created macroscopic (mm) monolayers of dodecanethiolcapped 6 nm gold nanospheres using excess ligand and rapidsolvent evaporation. The technique required the nanospheres tobe phase transferred prior to reaction, several hours to completethe reaction and the resultant film could not be easily trans-ported onto other substrates. Further, they only demonstrated

the technique using a specific ligand with nanospheres. Mayyaet al.[9] immobilized nanospheres at a water-toluene interfaceusing electrostatic attraction of charged aqueous nanoparticlesand transferred them onto substrates using surface tensiongradients from two immiscible fluids. Similarly Johnson etal.[10] suspended silver nanospheres in excess ligand and thentranslate the nanoparticles up the walls of the vial using surfacetension gradients from two immiscible fluids. The techniquesrequired the nanoparticles to be functionalized with appro-priate ligands prior to the reaction and the films were not dem-onstrated to be uniform, high-density monolayers over macro-scopic length scales.

Such thin, high-density films make it challenging to deter-mine the bulk properties such as the index of refraction ormore accurately for these films an effective index of refrac-tion. Ponizovskaya et al.[11] performed finite difference timedomain simulations of close-packed monolayer films of silvernanospheres and indicated the films can achieve a positive near-zero refractive index at visible wavelengths. Alaeian et al.[12]performed rigorous coupled wave analysis on gold nanospherefilms retrieving the refractive index, electric permittivity and pro-posed the magnetic permeability can reach non-unity values foreven a single monolayer film at optical frequencies. Experimen-tally, Wan et al.,[13]Kooji et al.[14]and Wang et al.[15]used spec-troscopic ellipsometry to determine the refractive index of high-density gold nanosphere monolayers films. These studies report

A self-assembled metamaterial exhibiting a positive near-zero index of refrac-

tion at visible wavelengths is demonstrated. The metamaterial consists of

thiolene-functionalized gold nanospheres self-assembled into macroscopic,

crosslinked, monolayers. By measuring the real and imaginary parts of the

phase shift of light transmitted through the self-assembled films the effective

index of refraction is determined as a function of wavelength. These findings

may pave a way to simply and efficiently self-assemble and optically charac-

terize multifunctional, multilayer nanoparticleligand metamaterials.

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large discrepancies in the refractive index that can be attributedto the sensitive nature of the nanoparticle structures,[13] dem-onstrating the need for additional techniques to assemble andcharacterize thin, high-density nanoparticle films.

In this paper we introduce a single, efficient, robust andscalable technique that combines (i) the phase transfer of nan-

oparticles using a water miscible organic solvent, (ii) the self-assembly of nanoparticles into macroscopic (>cm), high-density,monolayer films using phase separation and (iii) the transportof the nanoparticle-ligand films onto non-functionalized sub-strates using surface tension gradients. We optically charac-terized a self-assembled, crosslinked, 17 nm gold nanospheremonolayer film by measuring the real and imaginary parts ofthe phase shift of light transmitted through the films using aconoscopic Mach-Zehnder interferometer and a spectrophoto-meter and determine the effective optical refractive index. Tothe best of our knowledge these are the first spectroscopic, realand imaginary phase shift measurements based on transmit-tance reported for high-density, macroscopic, nanoparticle-ligand monolayer films.

2. Results and Discussion

The films were self-assembled using three primary constituents:citrate stabilized 17 nm diameter gold nanospheres dispersed

in water, tetrahydrofuran (THF) and thiol-ligands. The chem-ical structure of the two ligands thiol-ene (SC6V[16]) and di-thiol(SC6S) are shown in Figure1ab.

Macroscopic, self-assembled gold nanosphere monolayerfilms were produced by placing the aqueous suspended nano-particles into a glass vial and mixing with the thiol-ligands sus-

pended in THF and a small amount of photoinitiator. The sus-pension mixture was then vigorously shaken (Figure 1c).

The thiol-ene and di-thiol ligands rapidly form a covalentbond to the gold nanoparticles in suspension, causing the goldnanospheres to phase transfer, becoming hydrophobic andmore miscible in THF. The lower density of THF, relative towater, causes the THF to move towards the air-fluid interface,carrying the phase transferred nanospheres. The nanoparticlesare confined at the air-fluid interface by the reduction in the freeenergy (Figure 1d), approximately kBTper nanoparticle, wherekB is the Boltzmann constant and T is absolute temperature.

In addition to mixing, shaking the vial also leads to wettingthe sides of the vial with a thin film of solution. The THF inthe thin film evaporates faster than the water, causing a surface

tension gradient.[9]The fluid flows from the low (bulk suspen-sion) to high (thin film) surface tension area carrying the nano-particle films up the sides of the vial with velocities as high asmillimeters per second (Figure 1e). Within minutes nearlyall nanoparticles are removed from the suspension (Figure 1f,movie available in the Supporting Information). The rate the

Figure 1. Self-assembly technique. (a) SC6V; 4-(5-hexenyloxy)phenyl 4-(6-mercaptohexyloxy)-benzoate. (b) SC6S; 1,6-hexanedithiol. (c) Vial containinga suspension of metallic nanoparticles, ligands, tetrahydrofuran and water. Inserted image: reaction mixture. (d) Phase separation and metallic nano-particle film formation. Inserted image: initial gold nanosphere film formation. (e) Metallic nanoparticle film transport using surface tension gradients.Inserted image: gold films translating up the sides of the vial. (f) End of the phase transfer and transport technique. Inserted image: gold nanospherefilms attached to the sides of the vial.



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the initial monolayer, creating a bilayer film. The reproduc-ibility and mechanisms for creating the bi-layer films are notwell understood and will be studied in future experiments. Theabsorbance for a SC6V-SC6S capped 30 nm gold nanospheremono/bi-layer crosslinked film on a glass substrate is shown inFigure3a. The absorbance roughly doubles (discretely) in mag-nitude translating from the monolayer regions onto the bilayerregions. Although shifted due to particle-particle coupling, theplasmon resonances are preserved well for the crosslinked filmsgiven the ultra high-density of nanospheres. The technique canalso be used to form high-density monolayers of gold nanorods(details in Supporting Information).

The technique can also be used as a simple, fast and effi-cient phase transfer tool to functionalize nanoparticles of dif-ferent compositions, shapes and sizes with nearly 100% par-ticle transfer and recovery, in contrast to previous work.[10]Onceall the nanoparticles were functionalized and removed from theinitial reaction suspension (Figure 1f) the remaining solutioncan be decanted and the vial dried. The nanoparticles on theside of the vial can then be quickly re-suspended in organic sol-vents. Figure 3b shows the normalized absorbance spectra forgold and silver nanospheres and nanorods initially suspendedin water and after being phase transferred with dodecanethioland re-suspended in chloroform.

nanoparticles are removed from suspension also depends onthe concentration of ligand (details in Supporting Information).If gold nanospheres suspended in water and THF are mixedtogether the reaction does not occur. If small quantities of lig-ands are added, sufficiently less than the surface area of thenanoparticles, the film does not form at the air-liquid interface

or rise up the side of the vial. Once enough ligand was addedto cover approximately the entire surface area of the nanopar-ticles film formation was observed at the air-liquid interfaceand the films would translate up the side of the vial. Addingexcess ligands greatly increased the speed and final height thefilms reached on the vial surface. The excess ligand most likelylowers the surface tension in the bulk suspension, enhancingthe surface tension gradients. If the THF is replaced with asimilar surface tension, lower vapor pressure organic liquidsuch as dimethyl sulfoxide the film assembly rate and height ofthe film on the side of the vial is significantly reduced.

The nanoparticle films can be transferred onto substrates,such as glass or silicon wafers, by placing the substrate into thevial prior to shaking. After shaking, the nanoparticle films trans-

late up the substrate in the same manner as the sides of theglass vial. Once the reaction is complete the thiol-ene cappednanospheres are exposed to UV-light for several seconds to ini-tiate ligand crosslinking, via thiol-ene click chemistry[17]to createa crosslinked film of nanoparticles. A chloroform wash of thefilms did not alter the monolayer uniformity over centimeter-size domains, providing evidence they are crosslinked films. Incontrast, if the films were not exposed to UV light they wereeasily washed away with organic solvents (details in SupportingInformation).

Figure 2 shows the monolayer scale (cm) achievable withthis self-assembly technique without coffee-ring effects orregions of multiple layers on a non-functionalized substrate.

Figure 2a is an image of a self-assembled, crosslinked, 17 nmgold nanosphere monolayer film on a glass substrate partiallytransmitting light (left side), demonstrating preservation of theplasmon resonances, uniformity, optical clarity and reflectinglight (right side), demonstrating a high volume fraction of nano-particles (photograph details are in the experimental section).Figure 2b is an SEM image of self-assembled, crosslinked,SC6V-SC6S capped 17 nm gold nanosphere monolayer film onsilicon wafer substrates. In addition to the SEM images dem-onstrating the films are monolayers over centimeter lengthscales (Figure 2b) the thickness of the film, t, on the glasssubstrates was calculated from absorbance, x, measurementsto be t = x/ 16 nm, corresponding to approximately onemonolayer, where 3 1023 [NP/m3] is the number density

and 1.3 1016 [m2/NP] is the absorption cross-section.[18]The fast Fourier transform (FFT) was calculated from the imageusing ImageJ, from the Fourier transform the center-to-centerspacing was measured to be 21.5 0.82 nmwhere the uncer-tainty is the standard deviation between the measured values.This corresponds to a particle-particle spacing of d = 4.5 nm(Figure 2b inset).

Multi-functional, multi-layered films can be achieved usingthe self-assembly technique. After one monolayer film wastransferred onto a glass substrate the substrate was removedfrom the vial, held outside the vial for 30s and then placedback into the vial. Another monolayer film was translated onto

Figure 2. Macroscopic gold nanoparticle monolayers. (a) gold nano-sphere-crosslinked film on a glass substrate is partially transmitting light(left side) and reflecting light (right side). (b) False-color SEM image andFFT (inset) of the crosslinked 17 nm gold nanosphere film.



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From the experimental absorbance data the SC6V-SC6Scapped nanosphere film in air had an absorbance peak at =

693 nm Figure 4a), a considerable peak shift of = 174 nmfrom the dilute aqueous suspension. The COMSOL simulationscalculate an interparticle spacing of d = 1 nm to achieve thisshift, but a d = 4.5 1.6 nm spacing was measured using SEMand FFT. The expected interparticle spacing for the SC6V-SC6Scapped nanospheres film is approximately d = 34 nm (esti-mated using ACD/Labs). The measured value is comparable tothe expected value but varies considerably from the simulationvalue. A possible source of uncertainty is the estimated refrac-tive index of the ligand shell, though a refractive index of 2.8is required in the simulations to shift the peak to 700 nmwithan interparticle spacing of 4 nm which is physically unlikely.

Figure 4a shows the significant absorbance peak shift for aSC6V-SC6S capped 17 nm gold nanosphere crosslinked mono-

layer film relative to a dilute water suspension of 17 nm goldnanospheres. The large non-linear absorption peak shift is usu-ally attributed to the near field coupling between nanoparticles.[19]Finite-element simulations were undertaken using COMSOLMultiphysics 4.2a to develop an understanding of the absorbancepeak shift from the nanoparticle-ligand interactions. Figure 4bshows the absorbance peak position versus interparticle separa-tion. SC6V-SC6S ligand shells of various thickness, with refractiveindex ns = 1.6 (estimated using ACD/Labs), were placed aroundthe gold nanospheres and hexagonally packed in a unit cell on aglass substrate surrounded by air with periodic boundary condi-tions and probed with unpolarized light, inset Figure 4b.

Figure 3. Absorbance spectra. (a) absorbance of monolayer and bilayercrosslinked 30 nm gold nanosphere film. Inserted image: mono/bi-layerfilm on a glass substrate (b) absorbance of aqueous and organic sus-pensions of various size, shape and material nanoparticles: 20 nm silvernanospheres (blue). 10 nm gold nanospheres (green). 10 nm 45 nmgold nanorods (red).

Figure 4. Absorbance peak shift. (a) normalized experimental absorbancedata for a dilute aqueous 17 nm gold nanospheres suspension(dottedline) and a SC6V-SC6S capped 17 nm gold nanosphere crosslinkedmonolayer film (solid line). (b) COMSOL Multiphysics 4.2a simulationsfor the absorbance peak shift vs. interparticle separation. The inset is animage generated from COMSOL Multiphysics 4.2a of the nanoparticle-ligand film.



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system and a spectrophotometer (Figure 5a, see the Experi-mental Section for details).

To measure the real phase shift of the nanosphere-ligandfilm, relative to air, two lenses were placed into one arm of aMach-Zehnder interferometer to focus then collimate thebeam. The film, on a glass substrate, was positioned normal

to the beam near the common focus of the two lenses. If thefilm is translated out of the beam, probing only the glass sub-strate, a symmetric concentric ring interference pattern isformed. Translating the film halfway across the cross-sectionof the beam, such that half of the interference pattern probedthe air-glass and the other half was film-glass, causes the film-glass fringes to translate radially relative to the air-glass fringepositions, but remain concentric. The film was then translatedin the probe beam direction to image the film in the far-fieldon a screen and the interference pattern was recorded with aCCD camera as a function of wavelength. The exact center ofthe interference pattern was determined and the distances fromthe center to the first order intensity maximum, for both theair-glass and film-glass halves of the interference patterns were

measured (details in Supporting Information). The differencein these measurements determined the relative real phase shift= 2x(nfilm nair)/ between air and the film as a functionof wavelength where nfilm,air is the real index of refraction of thefilm and air, respectively, and is the free-space wavelength.To determine the imaginary phase shift the absorbance forthe film was measured and the imaginary phase is calculatedfilm = x/2 = 2xnfilm/, where nfilm is the imaginary indexof refraction of the film. The real and imaginary index of refrac-tion was calculated by inverting the phase shift and solving fornfilm; nfilm = nair + /2x, nfilm = film/2x.

Figure 5b shows the index of refraction for the SC6V-SC6S17 nm gold nanosphere monolayer film probed from = 590

680 nm. The refractive index varies from n = 0.19 + 1.45i at= 630 nm to n= 2.35 + 1.97i at = 680 nm. The error barsfor nmeasurements represent the standard deviation betweenthree separate data sets and for n are the uncertainty in themeasurement. In Figure 5b, the refractive index remainspositive, though reaching near-zero values at = 630 nm. Thefigure of merit is calculated FOM = |n/n| for the film (insetin Figure 5b) and varied from 0.1 at = 640 nm to 1.4 at= 590 nm.

3. Conclusion

We developed an efficient, robust and scalable self-assembly

technique that can be generalized using particles of differentcomposition, size and shape to create high-density macroscopicmetamaterials. Metallic nanoparticles can be quickly phasetransferred, with a variety of thiol-ligands, and re-suspendedin organic solvents with nearly 100% nanoparticle transfer andrecovery. The technique self-assembles macroscopic nanopar-ticle-ligand monolayers and transports them onto non-func-tionalized substrates using a single technique. We establisheda simple method to determine the effective optical refractiveindex and demonstrated positive near-zero index of refraction, atprescribed wavelengths, for self-assembled, crosslinked, 17 nmgold nanosphere monolayer films.

Wessels et al.[20] experimentally demonstrated that increasingthe conjugation of the thiol-ligand red-shifted the absorbance

spectra for nanosphere-ligand films. They attributed the differ-ences to the formation of a resonant state at the interface dueto an overlapping molecular orbital and metal wave function,causing an apparent increase in nanosphere diameter. Theincrease in conjugation may help explain the large red-shiftsobserved in our films.

We determined the refractive index of the self-assembled,crosslinked, 17 nm gold nanosphere monolayer film by meas-uring the real and imaginary phase shift of light transmittedthrough the film. The real and imaginary parts of the phaseshift were determined using a conoscopic Mach-Zehnder inter-ferometer probed with a spectroscopic continuous-wave laser

Figure 5. Conoscopic Mach-Zehnder interferometer. (a) Schematic forthe experimental setup to measure the real phase shift of light trans-mitted through the gold nanosphere monolayer films. (b) Refractive indexof crosslinked 17 nm gold nanosphere monolayer film: (left) real and(right) imaginary refractive index as a function of wavelength. The insetis the figure of merit.



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and a lens expands the interference pattern onto a screen. The film wasthen translated in the beam propagation direction to image the film inthe far-field on the screen and the interference pattern was recorded witha CCD camera (Thorlabs USB2.0 CMOS camera, 12 mm EFL f/1.8 lens)as a function of wavelength.

Supporting Information

Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements

This work was supported with funding provided from the Office ofNaval Research. J. Fontana is a NRC-NRL postdoctoral resident at theNaval Research Laboratory. J. Naciri and B. Ratna are affiliated with theCenter for Biomolecular Science and Engineering and R. Rendell withthe Electronics Science and Technology division at the Naval ResearchLaboratory. J. Fontana acknowledges the National Research Council fora postdoctoral associateship and also thanks P. Palffy-Muhoray for past

discussions. We thank C. Spillmann and S. Trammell for reviewing themanuscript.

Received: August 16, 2012Revised: October 23, 2012

[1] V. M. Shalaev, W. S. Cai, U. K. Chettiar, H. K. Yuan, A. K. Sarychev,V. P. Drachev, A. V. Kildishev, Optics Lett. 2005, 30, 3356.

[2] P. Podsiadlo, A. K. Kaushik, E. M. Arruda, A. M. Waas, B. S. Shim,J. D. Xu, H. Nandivada, B. G. Pumplin, J. Lahann, A. Ramamoorthy,N. A. Kotov, Science 2007, 318, 80.

[3] A. Alu, M. G. Silveirinha, A. Salandrino, N. Engheta, Phys. Rev. B2007, 75.

[4] S. Kocaman, M. S. Aras, P. Hsieh, J. F. McMillan, C. G. Biris,N. C. Panoiu, M. B. Yu, D. L. Kwong, A. Stein, C. W. Wong, Nat.Photonics 2011, 5, 499.

[5] a) S. M. Xiao, U. K. Chettiar, A. V. Kildishev, V. P. Drachev,V. M. Shalaev, Optics Lett. 2009, 34, 3478; b) A. Sundaramurthy,P. J. Schuck, N. R. Conley, D. P. Fromm, G. S. Kino,W. E. Moerner, Nano Lett. 2006, 6, 355; c) S. R. Quake,A. Scherer, Science 2000, 290, 1536; d) S. H. Ahn, L. J. Guo, Adv.Mater. 2008, 20, 2044; e) D. Chanda, K. Shigeta, S. Gupta, T. Cain,A. Carlson, A. Mihi, A. J. Baca, G. R. Bogart, P. Braun, J. A. Rogers,Nat. Nanotechnol. 2011, 6, 402.

[6] G. Frens, Nat. Phys. Sci. 1973, 241, 20.[7] R. A. Sperling, W. J. Parak, Philos. T. R. Soc. A 2010, 368, 1333.[8] T. P. Bigioni, X.-M. Lin, T. T. Nguyen, E. I. Corwin, T. A. Witten,

H. M. Jaeger, Nat. Mater. 2006, 5.[9] K. S. Mayya, M. Sastry, Langmuir 1999, 15, 1902.

[10] E. M. Spain, D. D. Johnson, B. Kang, J. L. Vigorita, A. Amram,J. Phys. Chem. A 2008, 112, 9318.

[11] E. V. Ponizovskaya, A. M. Bratkovsky,Appl. Phys. A-Mater. 2007, 87,175.

[12] H. Alaeian, J. A. Dionne, Opt. Express 2012, 20, 15781.[13] D. H. Wan, H. L. Chen, Y. S. Lin, S. Y. Chuang, J. Shieh,

S. H. Chen,ACS Nano 2009, 3, 960.[14] E. S. Kooij, H. Wormeester, E. A. M. Brouwer, E. van Vroonhoven,

A. van Silfhout, B. Poelsema, Langmuir 2002, 18, 4401.[15] D. S. Wang, C. W. Lin, Optics Lett. 2007, 32, 2128.[16] J. Lub, D. J. Broer, N. vandenBroek, Liebigs Ann. Recl. 1997,

2281.

We envision that the self-assembly technique willenable the development of multi-functional, multi-layerednanoparticle-ligand structures and provide a simple and effi-cient route to enable the large scale production of metamate-rials for applications.[21] In addition, our optical characteriza-tion technique provides a direct measurement for the real and

imaginary phase shifts from the nanoparticle-ligand film anda straightforward means to determine the effective refractiveindex as a function of wavelength.

4. Experimental Section

Self-Assembly Technique: The technique consisted of freshlysynthesized citrate stabilized 17 1.4 nm diameter gold nanospheresdispersed in one milliliter of water (SPI supplies; volume fraction 104),thiol-ligands; SC6V (4-(5-hexenyloxy)phenyl 4-(6-mercaptohexyloxy)-benzoate;[16] used at 5 mg/mL) and SC6S (Sigma 1,6-hexanedithiol;used at 5 mg/mL) with photoinitiator (Irgacure 369 used at 1% wt)dispersed in one milliliter of tetrahydrofuran (THF). The technique isnot constrained to unique ligands; dodecanethiol and undec-10-ene-1-thiol were also used with no noticeable differences in the kinetics ofthe reaction. The constituents were then placed into a 20 ml vial andvigorously shaken. After shaking, the metallic nanoparticle films translateup the glass substrate (1.25 25 1 mm). If capped with properligands the nanoparticles can be exposed to UV-light (Dymax Bluewave200, = 280450 nm, I = 40 W/cm2) for several seconds crosslinkingthe nanoparticles together. The films were washed with chloroform toremove any excess ligands and dried with N2.

Transmission/Reflection Image: The image in Figure 2a was taken witha Canon EOS Rebel T3i camera by placing the film on a glass substrateonto a backlight which was then tilted until the reflection from theoverhead room lights covered half of the film.

SEM Measurements: Gold nanoparticles were capped with ligands,self-assembled into a monolayer, and transferred onto silicon wafersubstrates (cleaned with Piranha solution; 3:1 sulfuric acid to 30%

hydrogen peroxide), using the technique described above, to examinethe nanoscopic film using a scanning electron microscope, SEM (CarlZeiss, Model 55).

Absorbance Measurements: Absorbance measurements were carriedout using an unpolarized white light (Mikropack DH-2000) passingnormally through the film or suspension, the transmitted light wascollected with a 400 m core diameter fiber optic cable connected to aphotospectrometer (OceanOptics Redtide USB650).

Real Phase Shift Measurements: The refractive index for the monolayerfilms was determined by measuring the real and imaginary phaseshift of light transmitted through the film. The real phase shift of thefilm, relative to air, was measured using a conoscopic Mach-Zehnderinterferometer (Figure 5a). A Coherent I-308 CW Argon laser (verticallinear polarized) was used to pump a Coherent 59921 Dye laser, with arange of wavelengths = 580700 nm. From the dye laser the beam issplit with a beamsplitter one path goes into a spectrometer (OceanOptics

Redtide USB650) to measure the output wavelength. The other beamenters a spatial filter consisting of a microscope lens with a 7 mm focallength focused onto a 10 m pin hole plate to expand the beam. A lensis then used to collimate the beam. The beam enters the Mach-Zehnderinterferometer and enters a 50/50 unpolarized beamspliter (BS1). Avariable attenuator was placed in the upper-right arm (BS1-M2-BS2).Two lenses (L1, L2;Newport, M-10x, 0.25NA) were placed into lower-leftarm (BS1-M1-BS2) of the Mach-Zehnder interferometer to focus thencollimate the beam. The film, on a glass substrate, was translated intothe beam near the common focus of the two lenses. The beam thenenters a planar liquid crystal cell (WP), oriented in the vertical, applyinga voltage to the liquid crystal cell aligns the E7liquid crystal in the beampropagation direction, decreasing the phase through the cell determiningthe sign of the interference fringe shift. Both arms are combined (BS2)



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[20] J. M. Wessels, H. G. Nothofer, W. E. Ford, F. von Wrochem,F. Scholz, T. Vossmeyer, A. Schroedter, H. Weller, A. Yasuda, J. Am.Chem. Soc. 2004, 126, 3349.

[21] a) C. D. Zangmeister, J. M. Beebe, J. Naciri, J. G. Kushinerick,R. D. van Zee, Small 2008, 4, 1143; b) S. M. Adams,S. Campione, J. D. Caldwell, F. J. Bezares, J. C. Culbertson,F. Capolino, R. Ragan, Small 2012, DOI: 10.1002/smll.201102708.

[17] J. P. Phillips, N. M. Mackey, B. S. Confait, D. T. Heaps, X. Deng,M. L. Todd, S. Stevenson, H. Zhou, C. E. Hoyle, Chem. Mater. 2008,20, 5240.

[18] O. L. Muskens, P. Billaud, M. Broyer, N. Fatti, F. Vallee, Phys. Rev. B

2008, 78.[19] I. Romero, J. Aizpurua, G. W. Bryant, F. J. G. de Abajo, Opt. Express

2006, 14, 9988.


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