Plasmonic Nanolenses: Electrostatic Self-Assembly of Hierarchical

Plasmonic Nanolenses: Electrostatic SelfAssembly of Hierarchical Nanoparticle Trimers and Their Response to Optical and Electron Beam Stimuli Julian A. Lloyd,†,‡ Soon Hock Ng,†,‡ Amelia C. Y. Liu,§,∥ Ye Zhu,† Wei Chao,† Toon Coenen,⊥ Joanne Etheridge,†,§ Daniel E. Gómez,‡,#,∇ and Udo Bach*,†,‡,# †

Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia Melbourne Centre for Nanofabrication, Wellington Road 151, Clayton, Victoria 3168, Australia § Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia ∥ School of Physics, Monash University, Clayton, Victoria 3800, Australia ⊥ DELMIC BV, Thijsseweg 11, 2629 JA, Delft, The Netherlands # Commonwealth Scientific and Industrial Research Organisation, Manufacturing, Research Way, Clayton, Victoria 3168, Australia ∇ School of Applied Science, RMIT University, Melbourne, Victoria 3000, Australia ‡

S Supporting Information *

ABSTRACT: Asymmetric nanoparticle trimers composed of particles with increasing diameter act as “plasmonic lenses” and have been predicted to exhibit ultrahigh confinement of electromagnetic energy in the space between the two smallest particles. Here we present an electrostatic self-assembly approach for creating gold nanoparticle trimers with an assembly yield of over 60%. We demonstrate that the trimer assembly leads to characteristic red-shifts and show the localization of the relevant plasmon modes by means of cathodoluminescence and electron energy loss spectroscopy. The results are analyzed in terms of surface plasmon hybridization. KEYWORDS: gold nanoparticle assembly, trimer, nanolens, cathodoluminescence, EELS very strong field localization in the gap between the two smallest particles (see Figure 1).26 This plasmonic lensing effect cannot be achieved with chains of equally sized particles. To date, the number of experimental demonstrations of such structures is very limited as their fabrication has been challenging, requiring a higher degree of control over the assembly technique than, for example, particle chains of samesized particles. Recent studies have shown progress and promising properties for hierarchical trimers (i.e., structures comprising three nanoparticles as suggested by Li et al.; see Figure 1). However, those studies utilized lithographic methods for the fabrication or surface pattering,27,28 which limited the interparticle gaps to

A

fter significant research activity in the past decades, plasmonic nanostructures are now employed in a wide range of fields including energy harvesting,1−6 biomedical applications,7,8 sensing,9−12 imaging devices13−15 and light guiding.16−19 One key aspect of plasmonic nanostructures is their ability to confine or focus electromagnetic energy in their near-fields, an effect that is greatly amplified in nanostructures that have nanometer-scale gaps due to the excitation of so-called plasmonic hotspots.20−24 Plasmonic hotspots have been studied intensively for nanoparticle dimers and it has been found that besides the shape of the plasmonic nanoparticles, the size of the gap between two individual particles is of major importance for the localization and intensity of the electric field in these hotspots.25 However, Li et al.26 predicted that much greater field localization could be achieved with self-similar arrangements of at least three nanoparticles, where there would be plasmon modes with a © 2017 American Chemical Society

Received: October 31, 2016 Accepted: February 6, 2017 Published: February 6, 2017 1604

DOI: 10.1021/acsnano.6b07336 ACS Nano 2017, 11, 1604−1612

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>10 nm and accordingly the magnitude of field confinement. These gaps are much larger than can be achieved with selfassembly techniques.29,30 Smaller interparticle gaps lead to stronger coupling of interparticle plasmon modes, resulting in stronger field localization (with gap sizes >0.5 nm).25,31,32 On the other hand, not all self-assembly techniques are applicable for the reproducible production of larger-scale arrays of trimers needed for many applications (e.g., DNA-directed selfassembly).30,33 In this study we investigate the plasmonic properties and the lensing effect of hierarchical gold nanoparticle (AuNP) trimers (nanolenses) as shown in Figure 1. An electrostatic selfassembly method is chosen to produce the trimer structures due to its simplicity and intrinsic independence of the particle material, while offering the required degree of control. The ability of the self-assembled trimers to focus light is ascertained experimentally via optical, cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) and theoretically via an analysis of the optical properties of the nanoparticle trimers in terms of surface plasmon eigenmodes.

Figure 1. Schematic (top inset): Geometry of the gold nanoparticle trimers investigated in this study with different particle radii (r1 = 10 nm, r2 = 15 nm, r3 = 25 nm), center−center distances (s12, s23) and interparticle gaps (d1, d2). 3D-plot: Electrical field enhancement plot calculated by a fully retarded boundary element method for a trimer upon incident plane-wave illumination at 610 nm perpendicular to the plane (without substrate). The field is calculated for a plane containing the centers of all the particles. Particle cross sections are indicated in yellow.

Figure 2. Overview of the electrostatic assembly process: (a) Total interaction energy map for a 20 nm AuNP approaching a 30 nm core particle on the substrate in step 2. Repulsive forces are indicated in red, attractive in blue. The dashed lines indicate where Etot = 1.5 kBT. (b) Similar map as in (a) for step 3 with a 20 and a 30 nm particle already on the substrate and a 50 nm satellite approaching. (c) Schematic of the proposed assembly process from negatively charged core particles on a positively charged substrate in step 1 to trimers by subsequently adding positively charged particles of different sizes in steps 2 and 3. Surface confined charges are indicated in green (positive) and red (negative). (d,e,f) High resolution SEM micrographs showing the results of each assembly step (scale bar: 250 nm). 1605


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RESULTS AND DISCUSSION Fabrication. The method for the hierarchical assembly of arrays of trimers is based on a protocol used previously by Zheng et al. for dimer assembly.29 The 3-step-assembly process fully relies on electrostatic interactions between a charged substrate and particles with alternating positive or negative surface charges as illustrated in Figure 2. To confine charges on the AuNP surface, thiol chemistry was used to bind thiolated single stranded DNA (Fidelity Systems Inc.) to the 30 nm particles (for negative charge) and N,N,N-trimethyl-(11mercaptoundecyl)ammonium chloride (TMAC, ProChimia) for the positively charged AuNPs. SiO2-coated Si and glass substrates were functionalized with (3-aminopropyl)triethyoxysilane (APTES, Sigma-Aldrich) to confer a positive charge. Therefore, the assembly is in principle independent of the NP or substrate material as long as a suitable surface chemistry to confine charged molecules on the surfaces is available (Supporting Information Figures S2 and S3). To advance the original dimer assembly protocol to trimers, DLVO theory (Derjaguin, Landau, Verwey and Overbeek)29,34−36 was employed to understand and visualize the potential gradients driving the dimer (Figure 2a) and trimer (Figure 2b) assembly. Within this theory, we take into account the attractive electrostatic interactions between positively and negatively charged particles, repulsive electrostatic interactions between positively charged particles, the substrate and other positively charged particles, as well as van der Waals interactions (see theory section in Supporting Information). In the first step (Figure 2c, step 1), an APTES-modified substrate with a positive surface charge is immersed into a colloidal solution of negatively charged AuNPs (cores). Due to the opposite charges, the particles assemble readily on the substrate with a uniform nearest neighbor distance (Figure 2d, Supporting Information Figure S1). These negatively charged 30 nm core particles (r2 = 15 nm) provide adsorption points for positive particles (satellites) in the subsequent steps. In the second step, the so-produced substrates are then immersed in a colloidal solution of positively charged 20 nm satellites (r1 = 10 nm). Using DLVO theory, we calculate the total interaction energy Etot for such a 20 nm satellite at varying positions (d, z) approaching a core particle on the substrate. The resulting interaction energy map (Figure 2a) shows a funnel-shaped gradient as a result of the interplay of the attractive interaction between the positive satellite and negative core and the repulsive interaction between satellite and substrate. This funnel guides the positive satellites to the negatively charged cores and leads to dimer formation (Figure 2e). It is also evident that the particles (with a mean thermal energy of 1.5 kBT, dashed lines in Figure 2a) have to overcome a potential barrier (red indicates repulsive interaction) during the adsorption process. Once a satellite attaches to a core, it greatly reduces the chances of a second satellite attaching to the same core due to its positive charge (i.e., the process is mostly self-limiting after dimer formation). This only changes after drying said substrates, which allows the satellites to drop onto the surface next to the core thereby “opening” the funnel again for the next assembly step. In the last step, trimer formation is completed (Figure 2f) by immersing the substrates with the AuNP dimers in a colloidal solution of positively charged 50 nm satellites (r3 = 25 nm). The calculated map (Figure 2b) shows again a funnel similar to the previous step. The repulsive barrier for the trimer-forming

step is now higher due to the previously attached 20 nm satellite and the increased size of the approaching satellite. By varying the ionic strength of the assembly solution, this barrier height can be adjusted (as shown before by Zheng et al.29) to allow maximum adsorption yield, while still retaining the selflimiting effect (i.e., prevent uncontrolled multimer formation). A concentration of 25 μM NaCl resulted in the highest trimer yield for an assembly temperature of 25 °C (Supporting Information Figure S4). Upon drying, the 50 nm satellite gets dragged down on the surface, as previously shown for dimers.29 Due to the asymmetry of the assembly and therefore the funnel (see Supporting Information Figure S5) the 50 nm satellite settles preferentially opposite to the 20 nm particle resulting in a rather linear arrangement of the trimer. There is however a finite distribution of nonlinear trimers, which does not significantly impact the plasmonic properties (as shown further down). Structural Characterization. In Figure 3a, a larger scale SEM micrograph of the final assembly with trimers highlighted

Figure 3. (a) SEM overview image of particle assemblies on a SiO2coated Si substrate with trimeric structures colored in green (scale bar: 500 nm). Inset: Statistical evaluation of the assemblies based on SEM data comprising 621 individual assemblies (assembly patterns indicated in white, 6% unassigned; Supporting Information Figure S6). (b) Top: UV−vis absorption spectra for core (orange), dimer (blue) and trimer (green) assemblies on a glass substrate. Bottom: Retarded boundary element method calculations for absorption spectra for dimer and trimer (combining different arrangements according to their statistical occurrence) assemblies. Dashed lines indicate the spectra for a single linear trimer excited along the longitudinal (dark gray) or transversal (light gray) direction. 1606


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Figure 4. Calculation results for electric fields and surface charges of an AuNP trimer: (a) Local electric field enhancement map at the center plane of the trimer for an optical excitation of the longitudinal resonance (614 nm excitation wavelength, calculated by retarded BEM). Disks indicate the particle positions. (b) Surface charge distribution of the trimer for the same excitation as in (a). (c) Contributions of the first 100 eigenmodes to the longitudinal resonance feature of the trimer. Heights indicate the relative amplitudes of the different modes with the sum of all modes being normalized to 1. Surface charge distributions are depicted for the 5 most intense modes.

Note that the refractive index surrounding the core particles is inhomogeneous due to the presence of the substrate, resulting in a spectral redshift as documented in the literature.39 When the dimers are formed, an additional resonance at around 575 nm arises, as evidenced by the shoulder in the spectrum shown in Figure 3b (blue). This resonance occurs at a wavelength that is different from that of the localized plasmon resonance of individual core or satellite particles (see Supporting Information Figure S8, S9 and S10). The new absorption band occurs due to the near-field interaction (coupling) of the particles which leads to a longitudinal coupled-resonance of the dimer.29,40,41 Figure 3b shows the spectrum of the final trimer assembly, which exhibits a further red-shifted shoulder at 610 nm. This new absorption band occurs due to the interparticle interactions of the 3 particles in the trimer assemblies. They result in a localized plasmon resonance which we now proceed to discuss in detail. We describe the interaction of light with the self-assembled structures using a (retarded) boundary element method (BEM) to solve the Maxwell equations in the systems (as implemented for Matlab in the MNPBEM toolbox,42,43 see Supporting Information Figure S11). The starting point in these numerical simulations is the geometry of the trimer (or dimer) and the dielectric properties of the trimer and its surroundings. Transmission electron microscopy (TEM) was used to confirm the sizes of the nanoparticles. For the calculations, we assumed perfect spherical particles with diameters of 20, 30, and 50 nm, thereby neglecting the distribution of particle sizes and shapes in the experiment. Different particle center heights on a substrate were taken into account as shown in Supporting Information Figure S12. Dimer assemblies were used to estimate the average interparticle gaps for 20−30 nm dimers (d1 = 1.1 nm) and for 30−50 nm dimers (d2 = 1.7 nm) via a pseudo fitting procedure described by Zheng et al.29,41,44 For the dielectric constants of gold, we interpolated the tabulated values by Johnson and Christy.45 The refractive index of the surrounding medium was fixed to 1.3 as it was found that this value closely represents the dielectric environment of metal particles located at an air/glass interface.29,39,46 As the analysis of SEM images revealed, not only linear but also angled trimers are present (as shown in Figure 3a). We

in green shows the distribution and variety of the assemblies. The results of a statistical analysis of the assemblies (of several similar SEM images) are presented in the inset. 63% of all observed structures are trimer structures, of these, 59% are rather linear, as expected from the DLVO calculations (Supporting Information Figure S5 and Figure S6). Twenty % of the structures are dimers (20−30 nm) which did not form trimers in the third step. However, further lowering of the adsorption barrier to facilitate a higher yield would lead to increased nonspecific adsorption of 50 nm satellites on the substrate. One can also see the limitations of the dimer forming step (step 2), as 12% of the 30 nm cores have two 20 nm satellites attached. A less pronounced selflimiting effect (i.e., exactly one satellite attaching to the core) for the dimer forming step for smaller satellites compared to bigger satellites has been observed before.29 It can be explained by comparing the potential maps for the 20 nm satellite and the 50 nm satellite approach (see also Supporting Information, Figure S7): For the 20 nm particle, the funnel is wider and the repulsive potential lower in a broader area above the core particle (similar to Figure 2b and c). As such, it is more likely for two 20 nm satellites to attach to the same core than for two 50 nm AuNPs (in the respective assembly steps). Such assemblies with two 20 nm satellites result in tetramers when adding the 50 nm satellite. Optical Properties. In order to investigate the plasmonic properties of the assembled nanolenses, the optical absorbance spectra (measured by UV−vis spectroscopy of assemblies on glass substrates) for the core, dimer and trimer assemblies are shown in Figure 3b. Starting from the core particles, the spectrum exhibits a single absorption band, characteristic of a localized surface plasmon resonance of individual particles (Figure 3b, orange). Within the electrostatic limit, applicable in this instance due to the small particle size (λ/size ≫1), the position in wavelength of this plasmon band is expected to occur at ∼526 nm according to the Froehlich resonance condition:37,38 Reε(ωp , m) = −2εb

(1)

with ε(ω) the metal permittivity and εb the permittivity of the surrounding medium. 1607


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simulate the adsorption spectra for the 4 major geometric arrangements present (accounting for 80% of all found structures, see Figure 3a): the three differently aligned trimer species and a 20−30 nm dimer. The calculated spectra of these structures were then combined to produce the final spectrum using their statistical occurrence as weight (weighted sum according to Figure 3a; see also Supporting Information Figure S13). The results agree well with experimental data in terms of relative peak heights and positions. We observe a slightly blueshifted higher energy peak for the calculations, which is expected as we did not include the substrate explicitly in the calculations. The measured spectra also have less pronounced peaks than the calculated ones, which can be attributed to the finite particle and gap size distribution of the AuNP colloids. When separating the spectrum of a linear trimer into a transversal (perpendicular to trimer axis) and a longitudinal plasmon resonance, we can see that the lower energy resonance is purely longitudinal (Figure 3b). At this wavelength, the surface charge distribution of the free electrons is predicted (using a retarded BEM calculation) to exhibit a strong localization in the region of space between the two smaller particles (see Figure 4a). This leads to a strong localization of electric field in what has previously been called a plasmon lensing ef fect.26 The lensing effect is evident on a calculated map of the electric fields around the trimers (for a 614 nm excitation wavelength) as shown in Figure 4a, which shows a strong |E/ E0|2 ∼ 105 localization of the electric field in the gap between the two smallest particles (Supporting Information Figure S14). Analysis of the Surface Plasmon Eigenmodes. In order to get a more physical description of the interparticle interactions responsible for this effect, we now proceed to describe the trimer in terms of superpositions of eigenmodes, an approach that is strictly applicable only to assemblies with subwavelength dimensions (electrostatic limit).37,47,48 In general, the plasmon resonance peak positions derived from this electrostatic eigenmode method (EEM) can differ slightly from those obtained by the retarded BEM calculations;42 however, this method offers the opportunity to provide a more physically tangible interpretation of the numerical BEM results. In the EEM description, illumination of a trimer results in the excitation of a surface charge distribution σ, which is a superposition of plasmon eigenmodes σm of the system: σ ( r ⃗) =

∑ am̃ (ω)σm( r ⃗) m

(3)

where C12 = C21 is the coupling coefficient describing the evanescent interaction between the 20 and 30 nm particle and C23 = C32 the coupling between 30 and 50 nm particle in dipole approximation.54 Given the geometry of the assembled trimers it is C12 > C23 (see Supporting Information for further details). The surface plasmon eigenmodes of the trimer are the eigenvectors of  . The eigenvector corresponding to the lowest energy bright mode (m = 1) is given by (see Supporting Information EEM section, not normalized): ⎞ C12/C23 ⎛ a1 ⎞ ⎛ ⎟ ⎜a ⎟ ⎜ 2 2 0.5 ⎜⎜ 2 ⎟⎟ = ⎜(C12 + C23) /C23 ⎟ ⎟ ⎝ a3 ⎠ ⎜⎝ ⎠ 1

(4)

where each entry represents the contribution of a dipolar localized surface plasmon from each particle to the plasmon eigenmode of the trimer. The plasmonic lensing effect of the trimer is described by the fact that most of the excitation is localized between the two smallest particles (given that C12 > C23, i.e., stronger coupling between 20 and 30 nm particle than between 30 and 50 nm particle, a3 < a1 and a2). The lensing effect within this level of approximation originates from asymmetric coupling among the nanoparticle trimer, which consequently leads to a dominant eigenmode characterized by a strong localization of the electric field in the gap between the two smallest particles. As some trimers are not linear but rather in an angled arrangement, the impact of this bending on the plasmon modes was investigated. It was found that the contributions of the relevant eigenmodes do not change significantly (when staying below 60° bending angle) and the field enhancement map (lensing) is maintained (see Supporting Information Figure S15). This is consistent with previous observations.17 Cathodoluminescence and Electron Energy Loss Measurements. The measurements presented so far have addressed the response of an ensemble of trimers to optical excitation, whereas the theoretical considerations were focused on individual trimers. It is therefore essential to perform measurements on individual trimers, to better correlate the experiment with the theory and get experimental information about the spatial distribution of the different plasmon modes within one trimer.25 This information provides insight in the predicted lensing capabilities of the trimer. Due to the high density of trimers implicit with our self-assembly method, darkfield optical spectroscopy is not possible as it would require beam focusing below the diffraction limit. We therefore utilized cathodoluminescence (CL) and electron energy loss spectroscopy (EELS). Both techniques use an electron beam as excitation source, which provides the required spatial accuracy in positioning for probing the plasmonic lensing effect. The electron beam acts like a localized broad-band point source and excites resonant modes in the nanostructure with an efficiency that depends on the electron trajectory with respect to the symmetry of the object.55−58 In the case of CL, the excited modes are detected via the radiative decay of the collective surface plasmon resonance.59 Whereas in EELS, the excited modes perpendic-

(2)

with ãm the expansion coefficients. We can now expand the surface charge distribution obtained from the retarded BEM calculations (which are in good agreement with the experiments) in terms of these eigenmodes as they provide an orthogonal basis.37 In Figure 4c these (normalized) amplitudes of such expansion are plotted for an excitation at the 614 nm resonance of the trimer assembly. It can be seen that only a few eigenmodes have a significant contribution to the resonance, the main one corresponds to the lowest-energy eigenmode (Figure 4c, highest bar): this mode is characterized by having a strong dipolar character, with a dipole moment aligned to the axis connecting the particles.49,50 Within the EEM, the origin of this eigenmode (m = 1) can be approximately described by considering only nearestneighbor interactions between nanoparticles. Accordingly, the interaction matrix which describes the localized plasmon resonances of the trimer is of the form37,51−53 1608


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Figure 5. Electron beam excitation of plasmon modes of a trimer: (a) Calculated CL spectra for different electron beam positions. Beam positions are indicated in the inset on the left. (b) Measured CL spectrum with the electron beam positioned on the 20 nm AuNP (see corresponding SEM image on the right). Green dots indicate the measured data while the solid green line represents the fitted spectrum consisting of two Lorentzians (dashed and dotted gray lines). (c) Measured CL spectrum for an electron beam centered on the 50 nm AuNP (see corresponding SEM image on the right). (d) Calculated electron energy loss probabilities for different beam positions of a trimer structure as depicted in the annular dark-field scanning TEM (STEM) image on the right with a schematic indicating the beam positions (left). (e) Principal component analysis components of the EELS data corresponding to the lower energy resonance of the trimer. Right: experimental and calculated loss maps for the lower energy resonance. (f) Similar to (e) but for the higher energy resonance. (Scale bars b and c: 100 nm; d: 25 nm).

ular to the beam direction are detected by measuring the energy losses experienced by the electron beam. These two techniques are complementary, as depending upon the instrumentation used, CL typically offers superior energy resolution but inferior spatial resolution relative to EELS.60−62 As for the UV−vis measurements, retarded BEM calculations for CL (Figure 5a) and EELS (Figure 5d) spectra with the electron beam impinging at different positions on a single trimer were used to provide insight into the localization of the different modes and help to interpret the experimental results. A Jacobian transformation (hc/E2) was applied to the CL data to convert to intensity per unit energy.63 The results are summarized in Figure 5. CL Spectra. Figure 5b and c show CL spectra with beam positions on the 20 and the 50 nm particle, respectively. The beam positions are indicated in the SEM images on the right. When the beam is centered on the 20 nm particle (Figure 5b) two main features are visible in the spectrum that were fitted with Lorentzians: A dominant resonance at around 1.77 eV and a smaller resonance at 2.06 eV. When the beam is positioned on the 50 nm particle (Figure 5c), the lower energy feature is suppressed below the detection limit and a single resonance at 2.15 eV remains. In the following, the measured CL spectra are compared to those obtained by the BEM calculations (Figure 5a) to assign the observed peaks to longitudinal and transversal plasmon modes. For the beam position on the 20 nm particle, the calculated CL signal shows only one clearly visible resonance at ∼2.02 eV (614 nm). The additional higher energy resonance visible in the CL experiments is attributed to the wide beam of the SEM. This beam size leads to the excitation of additional modes and was not taken into account in these simulations. There is also an obvious redshift in the experimental data. Such energy differences between theory and experiment have

previously been attributed to a thin dielectric shell (ligands and carbon coating in our case) and substrate effects.62,64,65 When the beam is positioned at the opposite side of the trimer (on the 50 nm AuNP), the low-energy resonance is still visible but with a greatly reduced intensity in the calculations. Due to the low signal-to-noise ratio this reduced lower energy feature cannot be identified in the experiment. The calculated frequency of ∼2.31 eV (537 nm) of this higher energy mode matches the resonance frequency of a single 50 nm AuNP and also the transversal mode observed with the optical excitation. The transversal character can be further supported by the increase of the calculated signal when moving the excitation source from the center of the 50 nm AuNP to its off-axis side (Figure 5a, blue). A transversal mode can be excited more efficiently at this position than at the particle center.50,58,66 Similarly, the longitudinal character of the low energy mode is supported by its visibility for both on-axis excitations in the calculations (Figure 5a, green and orange). By comparing the peak positions of the higher energy resonances in Figure 5b and c (orange), one can observe that the peak in Figure 5b is red-shifted. This can be explained by a slight accumulation of carbonaceous material deposited around the assembly while performing the line scan (as the scan was performed starting at the 50 nm particle).50 In summary, the CL measurements and calculations show results matching with UV−vis data and the developed theory. It can be seen that when placing the beam on the 20 nm AuNP, the lower energy resonance, which is responsible for the desired lensing effect, is excited almost exclusively, which further supports the lensing idea (as will be discussed below). EEL Spectra. To map the different plasmon modes with a nm resolution, we proceeded with EELS measurements on a trimer and show the results in Figure 5d−f. Similar to UV−vis and CL measurements, we can identify two main peaks: one above and one below 2 eV. Due to the 1609


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then concentrated to 0.1 mL. Then 25 μL (2.4 μL) of 24 mM (12 mM) TMAC and 0.1 mL of 6 mM Cetyltrimethylammonium bromide (CTAB, GFS Chemicals) are added consecutively under continuous vortexing. The solution is then incubated at 20 °C overnight. After washing by centrifugation 5 times, the colloid was redispersed to optical density 2.5 with Milli-Q water. Substrate Surface Functionalization with APTES. To modify the SiO2-coated Si or glass substrates (4 × 6 mm) with positively charges molecules, they were cleaned in piranha solution before functionalization. After drying, the substrates were immersed in a solution of 9.5 mL 99.9% Ethanol, 0.2 mL APTES and 0.3 mL Milli-Q water mix for 1 h. After 1 h, the substrates were washed 3 times in ethanol, dried under a nitrogen stream and baked for 10 min at 110 °C. Assembly Process. An APTES functionalized substrate is immersed in 0.2 mL of DNA modified AuNPs (optical density: 0.1) at 65 °C for 2 h. The substrate is then washed 3 times in Milli-Q water and dried under a stream of nitrogen. The substrate is then incubated in 0.2 mL of TMAC modified 20 nm AuNPs (optical density: 1) at 25 °C for 2.5 h. After washing 5 times with Milli-Q water, it is naturally dried in air. In a final step the substrate is then immersed in 0.2 mL of TMAC modified 50 nm AuNPs (optical density: 2.5) with 2.5 μL 2 mM NaCl for 3 h. After that it is again washed 5 times in Milli-Q water and dried in air. UV−Vis Measurements. An Agilent Technologies Cary 60 UV− vis spectrometer was used for the absorbance spectra measurements. Solution measurements were done using a Starna 50 μL quartz cuvette with a path length of 10 mm. Spectra of the assemblies were taken by positioning the glass substrates with the assemblies in the beam path. A baseline subtraction using either Milli-Q water or a functionalized substrate (without particles) was performed for all measurements. CL Measurements. CL measurements were conducted with a Delmic SPARC Cathodoluminescence System mounted on an FEI Nova NanoSEM 450. An Andor Shamrock 303i spectrometer with a 150 l/mm grating and Andor iVac spectral camera (2000 × 256 pixels) were employed to collect spectra with 1 s exposures. An accelerating voltage of 18 kV was used with an approximate current of 1.2 nA to achieve a good signal-to-noise ratio (∼10 nm beam size). As immersion mode imaging was used on the SEM, a voltage higher than 18 kV (which would lead to a higher plasmon excitation efficiency) was not possible. These measurements were performed on assemblies on TEM grids with SiO/Formvar support films to reduce the background signal from the substrate. A thin 3 nm carbon coating had to be applied to the final assemblies to avoid charging of the grids. Background subtraction was performed with background spectra taken in the vicinity of the measured trimer immediately after each measurement. EELS Measurements. STEM imaging and EELS mapping were carried out on a double-Cs corrected FEI Titan3 80−300 S/TEM equipped with a Gatan Image Filter (GIF Tridiem 863P), operating at 80 kV and with a standard Schottky field emission gun (FEG) without a monochromator. The electron gun lens setting was adjusted to reduce the beam current, so that the EELS energy resolution could be improved to ∼0.5 eV (from 0.8 to 0.9 eV for a Schottky FEG) as measured from the fwhm of the zero-loss peak. A ∼17 mrad convergence angle was used to yield a ∼2 Å diameter electron beam, and a ∼16 mrad collection angle was used for EELS mapping. The EELS map in Figure 5e and 5f was taken with 35 ms acquisition time for each pixel and 1.8 nm step size. Principal component analysis and non-negative matrix factorization68 were employed separately to disentangle the signal (different plasmon modes and zero-loss peak). BEM Modeling. MNPBEM toolbox42,43 in combination with Matlab (R2014a) was used for the BEM calculations. Retarded and quasistatic eigenmode BEM simulations were used with logarithmic distributed grid (see Supporting Information Figure S11), providing a high mesh density at interparticle gaps. An electron beam size of 5 nm was chosen for the CL and EELS calculations to get sufficient spatial distribution of the different modes.

high spatial resolution, we can now plot the intensity of the lower energy component (which was attributed earlier to the relevant longitudinal resonance) for the different beam positions. This results in the map shown in Figure 5e (right) which is in excellent agreement with the calculated EELS map. The map shows that the low energy resonance can most efficiently be excited at the smallest particle of the trimer, more specifically at the tip of the trimer structure. This is in good agreement with the CL measurements and the theoretical considerations presented above (eq 2). In contrast to the CL measurements, the expected (weak) excitation of the low energy mode at the edge of the biggest particle can also be observed providing further evidence for its longitudinal character. While a direct imaging of the plasmonic hotspot associated with the lensing effect is not possible with neither CL nor EELS, the strong agreement between the presented data and its theoretical description provides strong evidence for the nanolensing capabilities of the nanolensing capabilities of the self-assembled trimers. It is important to emphasize that neither CL nor EELS produce direct maps of plasmonic hot-spots. It has been noted in the literature that when exciting with an electron beam, one may “be blind to hotspots” that occur in the space between adjacent nanoparticles.55,67 On the contrary, it is possible to excite the resonances responsible for the plasmonic lensing, when the electron beam is positioned away from the associated hotspot.67 The data of Figure 5 shows that this position is located at the tip of the trimer, while the plasmonic lensing hotspot is located in the gap between the two smallest particles.

CONCLUSION In conclusion, we demonstrate the strong plasmonic lensing effect of hierarchical gold nanoparticle trimers that has been at the focal point of theoretical studies for more than ten years. A wet-chemical, electrostatically driven self-assembly process allowed the fabrication of the nanolenses with sub 2 nm interparticle gaps at high density at a macroscopic scale. We achieved high trimer yields of over 60%. We present a characterization of the plasmonic properties by means of UV−vis, CL and EEL spectroscopy and match it with numerical BEM calculations. This enables us to identify the lower energy resonance as a coupled longitudinal dipole mode, which is responsible for the strong lensing effect toward the gap of the smallest particles. More specifically, we demonstrate that this resonance can be efficiently excited when focusing an electron beam on the small particle supporting the idea of a strong localization of the electric field. METHODS Particle Surface Functionalization with DNA. Citrate stabilized gold nanoparticle were purchased from TedPella. Thirty nm particles were modified with thiolated single stranded DNA (Fidelity Systems Inc., [HS]-C6-T15-TAA TCA GGG TCA TAA) to convey negative charge on them. A initial volume of one milliliter of 30 nm AuNPs was concentrated to 0.1 mL and then mixed with 5 μL of a 2% TWEEN 20 (Sigma-Aldrich) solution, 30 μL of 0.1 M phosphate buffer (pH 7), 50 μL of 2 M NaCl solution, 20 μL of 100 mM thiolated DNA solution and 5 μL 100 mM Bis(p-sulfonatophenyl)phenylphosphine dihydrate dipotassium (BSPP, Sigma-Aldrich). Following an overnight incubation at 20 °C, the mix was washed by centrifugation (5 times) and redispersed to 1 mL with Milli-Q water. Particle Surface Functionalization with TMAC. One mL of 20 nm (50 nm) AuNP colloid was washed once by centrifugation and 1610


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07336. Further details on the modeling of the electrostatic assembly; Large scale SEM micrograph of a core assembly; Additional details about the BEM and EEM calculations and further calculation results (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Ye Zhu: 0000-0002-5217-493X Udo Bach: 0000-0003-2922-4959 Notes

The authors declare the following competing financial interest(s): T. C. is employee of Delmic BV, a company that develops and sells the SPARC cathodoluminescence system that was used in this work.

ACKNOWLEDGMENTS This study has been supported by CSIRO through the OCE Science Leader program and the Australian Research Council through an Australian Research Fellow grant (DP110105312) to UB, a Future Fellowship to DEG (FT140100514) and a Discovery Project grant to JE (DP150104483). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors acknowledge use of facilities within the Monash Centre for Electron Microscopy, including instrumentation funded by ARC grants LE0454166 and LE140100104. REFERENCES (1) Chen, X.; Jia, B.; Saha, J. K.; Cai, B.; Stokes, N.; Qiao, Q.; Wang, Y.; Shi, Z.; Gu, M. Broadband Enhancement in Thin-Film Amorphous Silicon Solar Cells Enabled by Nucleated Silver Nanoparticles. Nano Lett. 2012, 12, 2187−2192. (2) Reineck, P.; Lee, G. P.; Brick, D.; Karg, M.; Mulvaney, P.; Bach, U. A Solid-State Plasmonic Solar Cell via Metal Nanoparticle SelfAssembly. Adv. Mater. 2012, 24, 4750−4755. (3) Linic, S.; Christopher, P.; Ingram, D. B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911−921. (4) Shen, S.; Mao, S. S. Nanostructure Designs for Effective Solar-toHydrogen Conversion. Nanophotonics 2012, 1, 31−50. (5) Ng, C.; Cadusch, J. J.; Dligatch, S.; Roberts, A.; Davis, T. J.; Mulvaney, P.; Gómez, D. E. Hot Carrier Extraction with Plasmonic Broadband Absorbers. ACS Nano 2016, 10, 4704−4711. (6) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 865−865. (7) Rosi, N. L.; Mirkin, C. a. Nanostructures in Biodiagnostics. Chem. Rev. 2005, 105, 1547−1562. (8) Heuer-Jungemann, A.; Harimech, P. K.; Brown, T.; Kanaras, A. G. Gold Nanoparticles and Fluorescently-Labelled DNA as a Platform for Biological Sensing. Nanoscale 2013, 5, 9503−9510. (9) Yang, A.; Huntington, M. D.; Cardinal, M. F.; Masango, S. S.; Van Duyne, R. P.; Odom, T. W. Hetero-Oligomer Nanoparticle Arrays for Plasmon-Enhanced Hydrogen Sensing. ACS Nano 2014, 8, 7639− 7647. (10) Guo, L.; Chen, L.; Hong, S.; Kim, D.-H. Single Plasmonic Nanoparticles for Ultrasensitive DNA Sensing: From Invisible to Visible. Biosens. Bioelectron. 2016, 79, 266−272. 1611


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Plasmonic Nanolenses: Electrostatic Self-Assembly of Hierarchical

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