Bridging the Nanogap with Light: Continuous Tuning of Plasmon

Heesun Jung, Hoon Cha, Daedu Lee, and Sangwoon Yoon*

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Bridging the Nanogap with Light: Continuous Tuning of Plasmon Coupling between Gold Nanoparticles Department of Chemistry, Dankook University, 152 Jukjeon-ro, Suji-gu, Yongin, Gyeonggi 448-701, Korea

ABSTRACT The control of nanogaps lies at the heart of

plasmonics for nanoassemblies. The plasmon coupling sensitively depends on the size and the shape of the nanogaps between nanoparticles, permitting fine-tuning of the resonance wavelength and near-field enhancement at the gap. Previously reported methods of molecular or lithographic control of the gap distance are limited to producing discrete values and encounter difficulty in achieving subnanometer gap distances. For these reasons, the study of the plasmon coupling for varying degrees of interaction remains a challenge. Here, we report that by using light, the interparticle distance for gold nanoparticle (AuNP) dimers can be continuously tuned from a few nanometers to negative values (i.e., merged particles). Accordingly, the plasmon coupling between the AuNPs transitions from the classical electromagnetic regime to the contact regime via the nonlocal and quantum regimes in the subnanometer gap region. We find that photooxidative desorption of alkanedithiol linkers induced by UV irradiation causes the two AuNPs in a dimer to approach each other and eventually merge. Light-driven control of the interparticle distance offers a novel means of exploring the fundamental nature of plasmon coupling as well as the possibility of fabricating nanoassemblies with any desired gap distance in a spatially controlled manner. KEYWORDS: dimer . plasmon coupling . nanogap control . nonlocal effect . quantum regime

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nderstanding the nature of interactions between plasmonic nanoparticles in close proximity is essential for a wide variety of applications of nanoparticle assemblies. Plasmon coupling gives rise to a new resonance band in a longer wavelength region compared to the surface plasmon resonance (SPR) band of the individual nanoparticles.1,2 The shift of the plasmon coupling band sensitively depends on the extent of the interaction between the nanoparticles.3,4 The best conceivable means of controlling this interaction is to modify the interparticle distance. As the interparticle distance decreases, the interaction between the nanoparticles strengthens and the plasmon coupling band undergoes a further redshift.5
7 A classical electromagnetic model explains this redshift in terms of hybridization of the dipolar plasmon modes.8 Such plasmon coupling permits fine control over the optical properties of the nanoparticles. Furthermore, the local electric fields at a nanogap are significantly JUNG ET AL.

intensified as the plasmon coupling increases, effectively focusing the light into a nanometer-scale volume.9,10 This leads to various near-field enhanced phenomena such as surface-enhanced Raman scattering (SERS) and surface-enhanced fluorescence (SEF).11,12 As the interparticle distance reaches the subnanometer range, however, deviation from the classical electromagnetic model is observed.13
18 The influence of the surrounding charge distribution becomes significant for these narrow nanogaps in describing the induced polarization in a material. These nonlocal e€ects lead to a smaller redshift and broadening of the plasmon coupling mode in the optical response of dimers compared to the classical local description.15
18 A further decrease in the interparticle distance gives rise to electron tunneling across the gaps between the nanoparticles, which drastically changes the plasmonic response of the dimers. In this quantum regime, the tunneling current neutralizes the capacitive charges at VOL. XXX

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* Address correspondence to [email protected]. Received for review September 4, 2015 and accepted October 14, 2015. Published online 10.1021/acsnano.5b05568 C XXXX American Chemical Society

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deformations of the nanoparticles, complicating the nanogap analysis.26
28 Continuous tuning of the interparticle distance would simultaneously resolve those two major difficulties.29 If one could continuously modify the interparticle distance in a dimer from a few nanometers to zero (i.e., direct contact between the two nanoparticles), then the dimer would experience subnanometer gap distances along the way and consequently reveal the evolution of plasmonic interactions between the nanoparticles as the nanogap closes. Here, we report that UV irradiation of alkanedithiol-linked gold nanoparticle (AuNP) dimers leads to a gradual shortening of the interparticle distance, eventually merging the particles. The extinction spectra of the AuNP dimers reflect those changes in the interparticle distance.

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the junction, causing the redshifting plasmon coupling mode to diminish. Instead, new resonance modes arising from the charge transfer plasmon (CTP) emerge even before the nanoparticles come in contact with each other, contrary to the prediction of the classical electromagnetic theory. The CTP mode blueshifts as the nanogap narrows further. The local electric field enhancement also decreases in the quantum regime as a result of the screening of charges at the gap by the electron transfer.14,19 Numerous theoretical studies, as discussed above, enabled us to better understand the plasmonic interactions between nanoparticles within subnanometer separations. Despite this significant progress on the theoretical side, experimentally probing such interactions remains far more difficult. It was not until 2012, for example, that the quantum effect was observed for the first time.20 Baumberg and co-workers, in their elegantly designed experiments, attached gold nanospheres (diameter ∼300 nm) to atomic force microscopy (AFM) tips. As they brought the tips closer together, they observed the evolution of dark-field scattering spectra similar to the prediction of quantum theory. Dionne and co-workers and, more recently, Nijhuis and co-workers directly observed the CTP mode in their electron energy loss spectra (EELS) for a silver nanoparticle dimer with the nanogap narrowed by electron beams or molecular linkers, revealing quantum plasmon interactions.21,22 Whereas those experiments were performed for a single dimer, our group prepared colloidal solutions of molecularly linked nanoparticle assemblies of ultrahigh purity and optically measured the quantum plasmon coupling at the ensemble level.23,24 Despite rapidly growing interest and recent advances in both theory and experiments, presented above, our fundamental understanding of plasmon coupling, particularly in the subnanometer gap region, is far from perfect. The breakthrough in this quest must rely upon our capability to precisely control the interparticle distance down to the subnanometer region and to correlate the exact interparticle distance with the optical spectra, both of which are very challenging tasks. In our previous work, we tuned the interparticle distance on the molecular scale using a series of alkanedithiol linkers, the shortest being 1,2ethanedithiol.23,24 Consequently, the interparticle distance was no shorter than ∼0.7 nm and the changes in the interparticle distance were limited to a discrete value corresponding to the length of a methylene unit (∼2 Å). Even if a subnanometer gap can somehow be achieved, measuring the exact gap distance is even more difficult. In practice, transmission electron microscopy (TEM) does not offer such high resolution, particularly for the gaps of nanosphere dimers with three-dimensionally curved boundaries.25 Furthermore, the accelerated electrons from TEM often cause

RESULTS AND DISCUSSION We prepared AuNP dimers on glass slides using the masked desilanization method.23 The detailed procedures are presented in the Methods section. In brief, AuNPs with a diameter of 26.5 ( 2.9 nm are used as the monomer unit. The citrate-capped AuNPs are adsorbed on an amine-coated glass slide via the electrostatic interaction. Hereafter, these AuNPs are referred to as the first AuNPs to distinguish them from the AuNPs added later for dimer formation. The AuNPanchored glass slide is reacted with NaOH to remove the amine layer on the glass surfaces, except in the area where the first AuNPs are adsorbed (masked desilanization). Then, the first AuNPs are functionalized with thiol. The ligand exchange of citrate with alkanedithiol, SH(CH2)nSH (abbreviated as Cn), results in the formation of self-assembled monolayers (SAMs) of Cn with a thiol terminal group exposed to the outside. Finally, the second AuNPs are adsorbed on the thiolfunctionalized first AuNPs, not on the glass slide, which now lacks an amine coating due to the desilanization, leading to the high-yield formation of AuNP dimers (Figure 1a). Scanning electron microscopy (SEM) images show that AuNP dimers are formed on the glass slides with a good yield (Figure 1b). Statistical analysis of the images of the AuNP dimers with C8 linkers reveals that 88 ( 2% of the 634 total particles are dimers (Figure 1c). Other assemblies (trimers and multimers) constitute only 5.6% of the particles on the glass slide, suggesting that their contribution to the optical response should be negligible. The UV
vis spectrum of the dimers on a glass slide exhibits two distinct extinction bands (Figure 1d): one at a wavelength similar to the SPR band of the AuNP monomers used in the assembly (λ = 528 nm) and the other at a greatly red-shifted wavelength (λ = 624 nm for C8-linked AuNP dimers). The 528 nm band is assigned to the transverse plasmon coupling mode.8,30 A weak coupling between the VOL. XXX

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plasmon dipoles of each nanoparticle perpendicular to the interparticle axis leads to nearly the same energy as the SPR of the individual AuNPs. The extinction band at 624 nm is attributed to the plasmon coupling along the interparticle axis.8,30 This longitudinal plasmon coupling mode is therefore sensitive to changes in the interparticle distance. The AuNP dimers on a glass slide were irradiated with white light from a Xe arc lamp (Figure 2a). The spectral evolution was measured with increasing irradiation time. The glass slide supporting the AuNP dimers was kept immersed in ethanol during irradiation and measurement to prevent aggregation of the nanoparticles caused by the drying effect. Figure 2b presents the UV
vis extinction spectra at selected irradiation times. Drastic changes in the spectra are observed, particularly in the longitudinal plasmon coupling mode, as marked by red triangles. The two-dimensional surface contour plot clearly shows the spectral evolution (Figure 2c). We also plotted the changes in the peak wavelength and intensity as functions of the irradiation time in Figure 2, panels d and e, respectively. During the early stage of irradiation (0
2 h), the longitudinal plasmon coupling mode (initially at 624 nm) gradually redshifts and decreases in intensity. The transverse plasmon coupling mode, as marked by blue triangles, also shifts but toward shorter wavelengths in contrast to the longitudinal mode, although the extent of the shift is much smaller (see Figure 2d). After 2 h of irradiation, the band assigned to the longitudinal plasmon coupling mode broadens significantly (Figure 2b,c). This band continues to redshift (to 683 nm) until the irradiation time reaches 5 h. At 5 h, the direction of the shift in the longitudinal coupling band reverses from a redshift to a blueshift. This JUNG ET AL.

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Figure 1. Characterization of AuNP dimers prepared on glass slides. (a) Schematic illustration of the AuNP dimers. (b) A representative SEM image of the AuNP dimers (linker = C8). (c) Population distribution of the particles on the glass slides after assembly; 88% of the 634 total particles correspond to dimers. (d) UV
vis extinction spectrum of the C8-linked AuNP dimers on a glass slide (red line) compared with that of the first AuNPs before dimer formation (blue line).

blueshift continues as the irradiation period is prolonged to 15 h. In this regime, the intensity also increases continually and the peak slightly narrows. Meanwhile, the transverse plasmon coupling band remains at the same wavelength after 5 h of irradiation with a steadily decreasing intensity. The observed spectral evolution is reminiscent of the changes in the plasmon coupling with decreasing interparticle distance. It is strikingly similar to the measured spectral changes previously reported as the interparticle gap distance is reduced until the two nanoparticles are merged.20,21 Quantum models in which electron tunneling is taken into account in the plasmon coupling also report a similar spectral pattern: the initial redshift of the plasmon coupling band with its intensity decreasing and a smooth transition to a CTP band that blueshifts with decreasing interparticle distance.13
15 Such similarity strongly suggests that irradiation in our experiments reduces the interparticle distance of the AuNP dimers linked by C8 SAMs from ∼1.3 nm (corresponding to the length of C8) to negative values (corresponding to merged particles). Accordingly, the plasmon coupling between the two AuNPs experiences a transition from the classical regime to the contact regime (vide infra). Measuring the exact gap distance and correlating it to the spectral change as it is reduced by irradiation must be ideal. However, even state-of-the-art highresolution TEM (Titan 80-300, FEI) measurements did not produce consistent and reproducible results. Obtaining clear images of nanogaps with high contrast remains challenging because of the difficulty in focusing on high-curvature boundaries of three-dimensional spheres. Additionally, high-energy electron beams often cause structural deformation of the nanoparticles, complicating measurement of the interparticle distances.26
28 These experimental difficulties led us to elicit information on the interparticle distances from finite-difference timedomain (FDTD) calculations. We simulated the extinction spectra for AuNP dimers with interparticle distances ranging from 1.4 to 0 nm (Figure S1, Supporting Information) and matched the calculated peak positions to the measured ones. The open circles in Figure 2c mark the peak wavelengths of the simulated extinction spectra at the selected interparticle distances denoted on the right axis of the graph. The FDTD simulations based on the classical electromagnetic model indicate that the plasmon coupling band continually redshifts as the interparticle distance decreases until the two nanoparticles finally touch each other (dashed line in Figure 2c). Upon contact, the resonance band appears at a wavelength of ∼680 nm, abruptly blueshifted from the trend. The continuous redshift with decreasing interparticle distance and the singularity (sudden blueshift) of the resonance band upon contact are the key features of the classical electromagnetic model.13

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ARTICLE Figure 2. Changes in the spectra of the AuNP dimers (linker = C8) upon irradiation. (a) Schematic illustration. (b) UV
vis extinction spectra of the AuNP dimers at selected irradiation times. The blue and red triangles indicate the transverse and longitudinal plasmon coupling modes, respectively. (c) Two-dimensional surface contour plot for the spectral evolution of the AuNP dimers with increasing irradiation time. The red and blue lines trace the changes in the peak positions during irradiation. The plasmon coupling smoothly transitions from the classical regime (CR) to the nonlocal regime (NLR), to the quantum regime (QR), and finally to the touching regime (TR). (d and e) Plots of the changes in the peak wavelength and peak intensity as functions of irradiation time. (f) SEM images of the AuNP dimers before (0 h) and after (15 h) irradiation. The scale bars in the magnified images represent 50 nm. The open circles mark the plasmon coupling band wavelengths calculated from finite-difference time-domain (FDTD) simulations for gap distances d corresponding to the values on the right axis. The best fit of the calculated peak positions to an exponential function is represented by the dashed line.

A comparison between the observed and calculated spectral evolutions reveals how the nature of plasmon coupling changes with decreasing interparticle distance induced by irradiation. The longitudinal plasmon coupling band gradually redshifts as the interparticle distance decreases from ∼1.3 nm (for C8 linkers) by irradiation for up to 2 h. The observed redshift agrees with the FDTD simulation in this early stage of irradiation, suggesting that the classical electromagnetic coupling dominates the interaction between the AuNPs in this regime (classical regime). The slight blueshift of the transverse plasmon coupling mode is also explained within the framework of the classical electromagnetic model. Its antibonding dipolar interaction causes the resonance to shift to a higher energy as the coupling between the two AuNPs strengthens by the reduced interparticle distance.8 As irradiation continues, the nanoparticles more closely approach each other. Then, the plasmon coupling begins to deviate from the classical electromagnetic model when the nanogap narrows below 1 nm. In this subnanometer nanogap regime, the measured extinction spectra are characterized by broadening and a smaller redshift than the FDTD calculations predict. These spectral characteristics are consistent with the key features of the nonlocal effects identified by many theoretical studies (nonlocal regime).15
18 Broadening may arise from inhomogeneous distribution of the gap distances present in an ensemble of dimers. JUNG ET AL.

However, this possibility is excluded because broadening occurs only after irradiation for a certain period of time and varies with linkers (Figure 3). Broadening systematically appears at later times as the initial interparticle distance becomes longer, indicating that it is more likely attributed to the plasmonic interactions. Further irradiation brings the two nanoparticles even closer together and the coupling enters the regime in which quantum effects predominate over classical electromagnetic coupling. The redshifting plasmon coupling band transitions to a blueshifting band after 5 h of irradiation, which corresponds to an interparticle distance of ∼0.3 nm. The optical response observed at this stage is fully consistent with the signatures of the quantum regime.13
15,20,21 Previous studies found that in the quantum regime, CTP modes that blueshift with decreasing interparticle distance emerge even before the two nanoparticles touch each other. The reported onset distance for the quantum effect is typically in the range of ∼0.3
0.5 nm, also agreeing with our observation. Although we set the sharp boundaries between the regimes in Figure 2c, we note that the division of each regime may not be as clear as presented. The dimers are likely to reach a certain regime at slightly different irradiation times because of the distribution of the gap sizes present in the ensemble. Therefore, there are uncertainties about the interparticle distances at which transition between each regime occurs. The uncertainty VOL. XXX

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ARTICLE Figure 3. Surface contour plots of the changes in the UV
vis extinction spectra of AuNP dimers linked by Cn SAMs (Cn = SH(CH2)nSH, n = 2, 3, 4, 5, 6, 8, and 10) with increasing irradiation time. The black lines trace the peak maxima.

particularly makes the observation of the quantum regime difficult because electron tunneling occurs in the small gap range (between 0 and ∼0.3
0.5 nm) and is more sensitive to the interparticle distance than other plasmonic interactions such as the Coulomb interaction in the classical regime. This is probably the reason for the less pronounced blueshift observed in our experiments compared to theory and single-particle measurements.13
15,20,21 Continuing irradiation past ∼6 h causes the AuNPs to come in contact with each other and even merge. In this touching regime, the CTP band continues to blueshift and intensify as the bridge in the gap widens under prolonged irradiation (Figure 2c).21,31
33 The SEM images of AuNP dimers irradiated for 15 h show merged particles, in comparison with the dimers prior to irradiation (Figure 2f). More SEM images depicting the structural changes induced by irradiation in AuNP dimers with other linkers are available in the Supporting Information (Figure S2). From our observations and comparisons with theory, we conclude that the irradiation of AuNP dimers linked by alkanedithiol (C8) results in a continuous decrease in the interparticle distance. Accordingly, the plasmon coupling between the two AuNPs in each dimer transitions from the classical electromagnetic regime to the nonlocal regime, to the quantum regime, and finally to the touching regime. Similar spectral evolutions upon irradiation were observed for other AuNP dimers with different alkanedithiol linkers. Figure 3 shows that AuNP dimers with gap distances ranging from C4 to C10 exhibit the common pattern of a redshift of the plasmon coupling band, followed by a sudden broadening and then a crossover to a blueshift, as they are irradiated with a Xe lamp. A subtle difference among these dimers is that increasingly less irradiation time is required for the longitudinal plasmon coupling band to switch from JUNG ET AL.

redshifting to blueshifting as the initial interparticle distance decreases. This observation again supports our conjecture that irradiation causes the interparticle distances of Cn-linked AuNP dimers to shorten. When the gap distance is as short as C3 or C2, the resonance band is broad and blueshifts immediately upon irradiation. This result indicates that the interparticle distance of the AuNPs separated by C2 or C3 is already in the subnanometer range, and thus, the plasmon coupling readily accesses the quantum regime. The results presented so far strongly suggest that continuous tuning of the interparticle distance, and thus the plasmon coupling, is feasible. Central to these new findings is the interaction between light and Cn-linked AuNP dimers. We investigated which color component of the white light from the Xe lamp was responsible for the changes in the AuNP dimers to gain insight into how the interparticle distance is reduced by such irradiation. We used optical filters to select specific ranges of wavelengths from the Xe lamp, corresponding to the UV (318 ( 87.5 nm), visible (>420 ( 6 nm), and near-IR (>830 nm) regions, as well as wavelengths of 525 ((12.5) nm and 625 ((12.5) nm. The transmittance curve of each filter is presented in Figure 4a against a backdrop of the extinction spectrum of the AuNP dimers. This figure indicates that irradiation in each wavelength region will induce photochemistry of the alkanedithiol linkers (UV), SPR excitation of the AuNP monomers (525 nm) or dimers (625 nm), plasmon enhancement or photothermal heating of the nanoparticles (visible), or direct thermal excitation (near-IR). We found that the shifting of the plasmon coupling band was caused only by UV excitation (Figure 4b). Neither excitation across the entire visible region nor excitation specifically at the SPR wavelengths caused any difference in the extinction spectra of the dimers. Therefore, the change in the dimer structure does not arise from plasmon-induced VOL. XXX

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ARTICLE Figure 5. Effect of irradiation on the dithiol linkers. (a) Irradiation of the first AuNPs with dithiol linkers (C8 SAMs) before they are combined with the second AuNPs significantly lowers the yield for dimer formation to 7%, as a result of the desorption of the C8 linkers. The corresponding SEM image shows mostly monomers. (b) In the absence of irradiation, the rate of dimer formation is as high as 88%. Figure 4. (a) Transmittance curves of the optical filters used in the experiments. The extinction spectrum of the C8-linked AuNP dimers (black curve) is also presented to illustrate the excitation region covered by each filter. (b) The shifts in the plasmon coupling band of AuNP dimers irradiated with light at the wavelengths selected by the filters represented in (a). White light refers to irradiation from the Xe lamp without any filters.

field enhancement or thermal effects, but rather, it must be associated with the photochemistry of the molecules (surface ligands) or atoms in the AuNPs that can be excited by UV light.25,34
37 UV excitation removes the alkanedithiol linkers. To determine the role of UV excitation, we inserted an irradiation step into the dimer assembly process and compared the outcomes. The sample was irradiated for 3 h between the steps in which the first AuNPs were functionalized with thiol using the C8 SAMs and in which the second AuNPs were attached to them. Figure 5 shows that the yield for dimer formation drops to 7% when the thiol-functionalized first AuNPs are irradiated before being combined with the second AuNPs, in contrast with the 88% yield achieved in the normal assembly process. This result strongly suggests that irradiation removes the C8 SAM linkers and, as a result, causes the second AuNPs to fail to adsorb on the first AuNPs. The desorption of the dithiol linkers as a result of irradiation is confirmed by the shift of the SPR band of the AuNPs. The SPR band position is sensitive to changes in the dielectric constant of the local environment.38
40 As dithiol is desorbed upon irradiation, the medium surrounding the AuNPs changes from a monolayer of dithiol (refractive index, n = 1.50) to ethanol (n = 1.36). The UV
vis extinction spectrum of JUNG ET AL.

the first AuNPs on the glass slide shows that the SPR band shifts to shorter wavelengths (Δλ =
4 nm) upon irradiation (Figure S3, Supporting Information). This result is consistent with previous studies that demonstrated that the SPR band of AuNPs blueshifts as the dielectric constant of the medium surrounding the AuNPs decreases.38
40 The bare surfaces of irradiated AuNPs can be refunctionalized with thiol. We placed the irradiated AuNP/glass substrates (from Figure 5a) in dithiol solution once again and then allowed them to react with the second AuNPs. We again observed the formation of dimers (Figure S4, Supporting Information), suggesting that the surfaces of AuNPs can be repeatedly modified such that they are activated or deactivated for the adsorption of secondary AuNPs by alternately exposing them to irradiation and dithiol. The light-driven desorption of thiol occurs via photooxidation. Numerous studies using X-ray photoelectron spectroscopy (XPS), mass spectrometry, SERS, and scanning probe microscopies have revealed that thiols in SAMs on two-dimensional Au substrates become oxidized into sulfonate species and desorbed from the Au surfaces upon UV irradiation.41
45 In our experiments, such photooxidative desorption of the alkanedithiols constituting a SAM on the AuNP surfaces leads to the less-ordered, loose SAM structure, which subsequently causes the two AuNPs separated by the SAM to move closer to each other. Figure 6 schematically illustrates the proposed mechanism. Alkanedithiols initially form a close-packed, highly ordered, stable SAM structure on the first AuNPs as well as between the first and the second AuNPs (Figure 6a). Upon VOL. XXX

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ARTICLE Figure 6. Proposed mechanism of light-induced changes in the interparticle distance of AuNP dimers. (a) The AuNPs in the dimer are connected by the highly ordered alkanedithiol SAMs formed on the first AuNPs. (b) UV irradiation causes photooxidative desorption of the dithiol linkers. (c) Further desorption of the dithiols weakens the interchain van der Waals interactions, leading to a less-ordered loose SAM structure. The collapse of the SAMs results in a decrease in the interparticle distance between the two AuNPs. (d) Complete desorption of the dithiol linkers brings the AuNPs into contact. (e) The two AuNPs merge together, creating a neck between the nanoparticles, through lattice ionization and the surface diffusion of the Au atoms.

irradiation, the alkanedithiols begin to desorb from the AuNP surfaces (Figure 6b). The dithiols that are bound only to the first AuNPs are likely to desorb first because the dithiols at the gap are held by the two AuNP surfaces. As the desorption occurs progressively, the interchain van der Waals interaction that keeps the SAM structure maintained weakens. The collapse of the semirigid SAMs into a less-ordered loose structure results in closing a gap between the two AuNPs (Figure 6c). Complete desorption of the dithiol linkers ultimately leads to contact between the AuNPs (Figure 6d). Further UV irradiation causes lattice ionization and subsequently surface diffusion and migration of the Au atoms in the contact region of the AuNPs (Figure 6e), causing them to coalesce into a peanutshell-like blob, as shown in Figure 2f. Further studies using SERS are underway in our laboratory to elucidate the detailed mechanism of the UV-induced changes in the interparticle distance of dithiol-linked AuNP dimers. High-yield preparation of AuNP dimers and continuous tuning of the plasmon coupling in these dimers together offer a novel means of studying plasmonic interactions from the classical to the quantum regime. Furthermore, light-driven control of the interparticle distance enables the preparation of nanoparticle dimers with any desired gap distance in a given space.

CONCLUSIONS

METHODS

Preparation of AuNP Dimers. The AuNPs were assembled into dimers on glass slides using the masked desilanization method described in a previous publication.23 In brief, glass slides were coated with amine via a silanization reaction with (3-aminopropyl)trimethoxysilane (APTMS) (step 1: silanization). For this purpose, the glass slides (12  25 mm2) were thoroughly cleaned and immersed in an ethanol solution of APTMS (1% v/v, 5 mL) for 30 min. After the glass slides were washed with ethanol and dried in an oven (120 °C, 3 h), they were immersed in a solution of 26.5 nm citrate-capped AuNPs (100 pM, 5 mL) for 3 h to adsorb the AuNPs (step 2: adsorption of the first AuNPs). Then, the amine coatings were peeled off the glass slides (step 3: masked desilanization). The glass slides were

Chemicals. All chemicals used for the synthesis and assembly of the nanoparticles were purchased and used without further purification. The list of these compounds and their purities is available in the Supporting Information. Preparation and Characterization of AuNPs. We employed the seeded growth method to prepare colloidal AuNPs (26.5 ( 2.9 nm).46,47 Seed nanoparticles (11.3 ( 1.4 nm) were first prepared using the Turkevich method (citrate reduction of Au3þ) and grown to the desired size by adding appropriate amounts of growth solutions (HAuCl4 and sodium citrate) and adjusting the reaction time.48 The detailed reaction conditions and characterization data are presented in the Supporting Information.

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We found that the interparticle distances of AuNP dimers prepared on glass slides can be tuned with light. The irradiation of AuNP dimers linked by alkanedithiol reduces the interparticle gap distance from the initial distance to negative values (representing merged particles). As the two AuNPs in a dimer approach each other, the plasmon coupling between the nanoparticles transitions from the classical electromagnetic regime to the nonlocal and quantum regimes and finally to the touching regime. The longitudinal plasmon coupling band in the extinction spectra gradually redshifts, suddenly broadens with a smaller redshift, and then crosses over to a blueshift as the plasmon coupling passes through each interaction regime with decreasing interparticle distance. Selection of the irradiation wavelengths and examination of the effect of light on the surface ligands reveal that the photooxidative desorption of alkanedithiol linkers from the AuNP surfaces is responsible for the change in the interparticle distance in these AuNP dimers. The capability to continuously tune the interparticle distance with light, as demonstrated here, offers new opportunities for exploring the fundamental nature of plasmon coupling as well as the possibility of fabricating nanoassemblies with any desired gap distance in a spatially controlled manner.

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Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05568. Simulated extinction spectra of AuNP dimers with various gap distances, SEM images of the AuNP dimers before and after irradiation, additional evidence for the desorption of the dithiol linkers induced by irradiation, and preparation and characterization of the AuNPs (PDF) Acknowledgment. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2008336).

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25 nm Separation Range: Experimental Indications of Distinct Plasmon Coupling Regimes. J. Phys. Chem. C 2010, 114, 4901–4908. 8. Nordlander, P.; Oubre, C.; Prodan, E.; Li, K.; Stockman, M. I. Plasmon Hybridization in Nanoparticle Dimers. Nano Lett. 2004, 4, 899–903. 9. Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Surface-Enhanced Raman Scattering from Individual Au Nanoparticles and Nanoparticle Dimer Substrates. Nano Lett. 2005, 5, 1569–1574. 10. Kawata, S.; Inouye, Y.; Verma, P. Plasmonics for near-Field Nano-Imaging and Superlensing. Nat. Photonics 2009, 3, 388–394. 11. Schlücker, S. Surface-Enhanced Raman Spectroscopy: Concepts and Chemical Applications. Angew. Chem., Int. Ed. 2014, 53, 4756–4795. 12. Zhang, J.; Fu, Y.; Chowdhury, M. H.; Lakowicz, J. R. MetalEnhanced Single-Molecule Fluorescence on Silver Particle Monomer and Dimer: Coupling Effect between Metal Particles. Nano Lett. 2007, 7, 2101–2107. 13. Esteban, R.; Borisov, A. G.; Nordlander, P.; Aizpurua, J. Bridging Quantum and Classical Plasmonics with a Quantum-Corrected Model. Nat. Commun. 2012, 3, 825. 14. Zuloaga, J.; Prodan, E.; Nordlander, P. Quantum Description of the Plasmon Resonances of a Nanoparticle Dimer. Nano Lett. 2009, 9, 887–891. 15. Esteban, R.; Zugarramurdi, A.; Zhang, P.; Nordlander, P.; Garcia-Vidal, F. J.; Borisov, A. G.; Aizpurua, J. A Classical Treatment of Optical Tunneling in Plasmonic Gaps: Extending to the Quantum Corrected Model to Practical Situations. Faraday Discuss. 2015, 178, 151–183. 16. García de Abajo, F. J. Nonlocal Effects in the Plasmons of Strongly Interacting Nanoparticles, Dimers, and Waveguides. J. Phys. Chem. C 2008, 112, 17983–17987. 17. Mortensen, N. A.; Raza, S.; Wubs, M.; Søndergaard, T.; Bozhevolnyi, S. I. A Generalized Non-local Optical Response Theory for Plasmonic Nanostructures. Nat. Commun. 2014, 5, 3809. 18. Raza, S.; Bozhevolnyi, S. I.; Wubs, M.; Mortensen, N. A. Nonlocal Optical Response in Metallic Nanostructures. J. Phys.: Condens. Matter 2015, 27, 183204. 19. Zhu, W.; Crozier, K. B. Quantum Mechanical Limit to Plasmonic Enhancement as Observed by SurfaceEnhanced Raman Scattering. Nat. Commun. 2014, 5, 5228. 20. Savage, K. J.; Hawkeye, M. M.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; Baumberg, J. J. Revealing the Quantum Regime in Tunnelling Plasmonics. Nature 2012, 491, 574–577.

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reacted with NaOH (1.5 mM, 5 mL) for 5 h to remove all amine functional groups from the glass surfaces except for those binding the first AuNPs. Linker molecules (alkanedithiol) were attached to the first AuNPs (step 4: thiol functionalization of the first AuNPs). Immersion of the first AuNPs/glass in an ethanol solution of alkanedithiol (1 mM, 5 mL, 1 h) led to the formation of SAMs with thiol terminal groups on the first AuNPs. Finally, dimers were formed by binding citrate-capped AuNPs (the second AuNPs) to the thiolfunctionalized first AuNPs (step 5: adsorption of the second AuNPs). Immersion of the thiol-functionalized first AuNPs/ glass in a citrate-capped AuNP solution (150 pM, 5 mL) for 8 h led to the adsorption of the second AuNPs on the first AuNPs, forming dimers. Note that the second AuNPs did not adsorb on the glass slides because the amine functional groups had been removed from the glass surfaces in the masked desilanization step. We displaced weakly bound citrates on the second AuNPs with carboxylates using 11-mercaptoundecanoic acid (1 mM, 5 mL, 1 h) to ensure the stability of the dimers. Irradiation. Each glass slide supporting AuNP dimers was transferred to a quartz cuvette (100-10-40, Hellma Analytics) filled with ethanol (3 mL) for irradiation of the dimers. The dimers were illuminated with light from a Xe lamp (300 W Ozone-free Xe Arc Lamp, Oriel Instruments) positioned 90 cm from the cuvette. The Xe lamp was equipped with a liquid filter to block IR. For the selection of specific ranges of wavelengths, we used the following filters: a UV filter (318 ( 87.5 nm, Schott UG5 bandpass filter), a visible filter (>420 ( 6 nm, Schott GG420 long-pass filter), a near-IR filter (>830 nm, Schott RG830 color filter), a bandpass filter (525 ( 12.5 nm, Edmund Optics), and another bandpass filter (625 ( 12.5 nm, Edmund Optics). The power density at the sample for each selected wavelength range was as follows: 282 mW 3 cm
2 for white light, 121 mW 3 cm
2 for UV light, 308 mW 3 cm
2 for visible light, 38 mW 3 cm
2 for near-IR light, 11 mW 3 cm
2 for 525 nm light, and 10 mW 3 cm
2 for 625 nm light. Measurements and Simulations. The extinction spectra of the AuNP dimers after irradiation were measured using a UV
vis spectrometer (PerkinElmer, Lambda 25). The structural and morphological changes in the nanoparticles and dimers were probed using high-resolution TEM (JEM-2100F, JEOL or Titan 80-300, FEI) and SEM (S-4300 or S-4800, Hitachi). The irradiation power was measured using a power meter (Orion TH, OPHIR) with a thermal surface absorbing head (3A-SH, OPHIR). FDTD simulations were performed using a calculation package from Lumerical Solutions, Inc. (FDTD Solutions ver. 8.5). Two spherical AuNPs with a diameter of 26.5 nm were separated by a gap distance ranging from 1.4 to 0.0 nm in increments of 0.2 nm. We used a dielectric function from Johnson and Christy for the AuNPs.49 To simulate the longitudinal plasmon coupling band, light was allowed to propagate in the direction perpendicular to the interparticle axis with a polarization parallel to the axis. The simulation region had dimensions of 1000  1000  1000 nm3, with an override region in a 32  32  60 nm3 box inclusive of the dimer, in which a mesh size of 0.2 nm was used to improve calculation accuracy. The simulation time and the refractive index of the medium were set to 1000 fs and 1.3614 (ethanol), respectively.

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