1 Optical Characterization of Gold Nanoblock Dimers: From Capacitive

Subscriber access provided by University of South Dakota

C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Optical Characterization of Gold Nanoblock Dimers: From Capacitive Coupling to Charge Transfer Plasmons and Rod Modes Man-Nung Su, Quan Sun, Kosei Ueno, Wei-Shun Chang, Hiroaki Misawa, and Stephan Link J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05755 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

The Journal of Physical Chemistry

Optical Characterization of Gold Nanoblock Dimers: From Capacitive Coupling to Charge Transfer Plasmons and Rod Modes

Man-Nung Su,1 Quan Sun,2 Kosei Ueno,2 Wei-Shun Chang,1Hiroaki Misawa,2,3* and Stephan Link,1,4* 1

Department of Chemistry, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States

2

3

4

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan

Department of Applied Chemistry, National Chiao Tung University, Hsinchu 30010, Taiwan.

Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States

*Corresponding authors’ e-mail: [email protected], [email protected]

1 ACS Paragon Plus Environment

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

Abstract The optical properties of plasmonic dimers consisting of two adjacent metal nanoparticles can be tuned over a broad spectral range by changing only slightly the dimer geometry. Most drastic are the changes for the smallest interparticles distances, and often only optical spectroscopy together with electromagnetic simulations yield insights into the geometry of the junction. Here we study the coupling of gold nanoblock dimers, two square nanoantennas with different nanogaps between their closest corners. We identify three different coupling regimes – capacitively coupled, conductively bridged, and fused dimers – and optically characterize the transitions between them. By combining sample array fabrication, single-particle hyperspectral measurements, and electromagnetic simulations, we were able to examine in detail the effects of junction geometry on the resonance energy and intensity of the plasmon modes supported by gold nanoblock dimers.


Page 2 of 23

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


Introduction Plasmonic nanostructures have been widely studied in the past decades because of their potential applications in photocatalysis,1-3 solar cells,4 biomedical treatments,5 biosensing,6, 7 or optomechanical devices8, 9 to name a few examples. They possess unique optical properties due to the localized surface plasmon resonance (LSPR), which is the collective oscillation of conduction band electrons in response to an external electromagnetic field.10 LSPRs enhance light-matter interactions and result in larger optical cross sections than the physical cross sections of the nanostructures. Furthermore, the high sensitivities of LSPRs to the sizes, shapes, materials, and local environments of the nanostructures make it possible to specifically tailor their optical properties for desired applications.11, 12 For example, closely arranged metallic nanoparticle pairs, also known as plasmonic dimers, offer broader tunabilities, additional plasmon modes,13-18 and stronger localized electric field enhancements19-23 compared to the individual plasmonic monomers. When two nanostructures are in close proximity, their LSPRs couple with each other. This coupling alters the overall optical properties in a manner that is highly sensitive to interparticle geometries including distance and relative orientation.14,

15, 17, 24-28

This sensitive

geometry dependence not only can be utilized as a molecular ruler at the nanometer scale29, 30 but also provides an important parameter to manipulate the plasmon line shape over a wider spectral range involving multiple new plasmon modes.24 Moreover, strong localized electric field enhancements are generated at the junctions in between the nanoparticles, especially for dimers arranged in a tip-to-tip geometry that creates a small mode volume. The enhanced localized electric fields make such dimers great candidates for photocatalysis,31, spectroscopies,20, 33-35 and nonlinear optics.23, 36


32

surface-enhanced


Page 4 of 23

Because of these enhanced optical properties, significant research efforts have been devoted to study plasmonic dimers, including chemically prepared15-17,

27, 28, 37-40

and

lithographically fabricated13, 14, 30, 41-43 dimers investigated both at the ensemble14, 17, 30, 37, 41, 43 and single particle13, 15, 16, 27, 28, 38-40, 42 levels. While preparing dimers chemically offers larger sample throughput, fabricating dimers lithographically makes it easier to manipulate their sizes, shapes, and relative positions. However, even a dimer presents already a complex plasmonic system with several geometric degrees of freedom, and as each one is varied structural heterogeneity increases together with pronounced effects on the overall optical response, often beyond the resolution of ensemble measurements. Therefore, single-particle spectroscopy carried out on lithographically fabricated nanostructures becomes a powerful approach to quantitatively characterize plasmonic dimers.10 A general view of the effect of interparticle gap on the optical properties of plasmonic dimers has been established, and the electrical conductivity between the two nanoparticles plays a key role.18 When two nanoparticles are separated by a few nanometers, no electrons can pass through the gap. The surface plasmon dipole modes of the individual nanoparticles are then capacitively coupled and form a bonding dipolar plasmon (BDP).14,

15, 43

The BDP redshifts

monotonously as the separation decreases, which is well-explained by plasmon hybridization theory.44 Smaller gaps lead to stronger interactions between the two dipole modes and therefore lower the energy of the hybridized mode. When a conducting bridge is created between two nanoparticles in a dimer, electrons can transfer through the junction, and charge transfer plasmons (CTPs) are observed.13, 14, 17, 18 This conducting bridge can take on many geometries including sub-nanometer gaps where quantum tunneling effects enable weak conductivity in between the nanoparticles. The energy of CTPs is lower compared to BDPs and is very sensitive 4 ACS Paragon Plus Environment

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


to the conductivity of the junction, making CTPs highly tunable and for gold often lie in the near IR region.13, 14, 17 Meanwhile, BDPs are replaced by screened bonding dipolar plasmons (SDPs) in the presence of a conducting medium.45 Compared to BDPs, SDPs show lower intensities at higher resonance energies as the screened charge distribution reduces the strength of plasmon coupling. Finally, when the two nanoparticles are highly fused together into one single nanostructure, the overall geometry can be considered a type of nanorod.13 Although this general picture helps to understand most of the optical properties of plasmonic dimers, details still differ among various plasmonic dimers, especially when dimers consist of two nanoparticles with anisotropic shapes. Furthermore, while the properties of BDPs and CTPs have been studied separately, there exists an important lack of knowledge regarding the spectral evolution from capacitively to conductively bridged and then fused together. In this work, we studied the spectral evolution of gold nanoblock dimers arranged in an corner-to-corner configuration23, 32, 35 instead of face-to-face in order to minimize contact area and mode volume. A wide range of interparticle distances spanning electrically isolated, conductively bridged, and fused dimers was fabricated by electron-beam lithography (EBL) and optically characterized using single-particle dark-field scattering spectroscopy. With the aid of electromagnetic simulations we were able to assign the different plasmon modes and clearly identified the transitions between three coupling regimes: capacitively coupled, conductively bridged, and fused together.



Experimental Methods Nanoblock dimer fabrication. Planar gold nanoblock dimers were fabricated on glass substrates using EBL. The glass substrates were sequentially cleaned with acetone, methanol, and ultra-pure water in an ultrasonic bath. A conventional copolymer resist (ZEP520, Zeon Chemicals) diluted with a ZEP thinner (1:1) was then spin-coated onto the substrates at 1000 rpm for 10 s and at 4000 rpm for 90 s. The substrates were then prebaked on a hot plate for 2 min at 150 °C. In this study, a high-resolution EBL system (ELS-7000HM, Elionix) operated at 100 kV was used for sample fabrication. The EBL was conducted at a dose rate of 128 µC/cm2. After the development, a 2 nm-thick titanium adhesion layer was first deposited via sputtering (MPS4000, ULVAC), followed by deposition of a 30 nm-thick gold film. Lift-off was performed by successively immersing the sample in anisole, acetone, methanol, and ultra-pure water in an ultrasonic bath. All dimers with nanoblock edge lengths of 80 and 100 nm were fabricated on the same substrate. Results for the 100 nm edge length nanoblock dimers are only shown in the supporting information (SI). The quoted dimensions are in all cases the design parameters when writing the EBL pattern. Single-particle scattering spectroscopy. A home-built hyperspectral microscope was used to take single-particle scattering spectra.46 An oil immersion dark-field condenser (Zeiss, numerical aperture (NA) = 1.4) mounted on an inverted dark-field microscope (Axio Observer m1, Zeiss) created a total-internal-reflection excitation geometry with an incident angle of 67o with respect to the normal of the substrate. The light source was a halogen lamp. The scattered light from the sample was collected by an air-spaced 50× objective (Zeiss, NA = 0.8), passing through a polarizer, and was sent to an imaging spectrograph (SpectraPro 2150i, Acton Research Corporation) and CCD camera (PIXIS 400BR, Princeton Instruments). Raw spectra were 6 ACS Paragon Plus Environment

Page 6 of 23

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


corrected subtracting the background and normalizing by the lamp spectrum. Although the intensities are reported in arbitrary units, spectral intensities can be directly compared considering our hyperspectral acquisition scheme. Scanning electron microscopy (SEM). The morphology of the nanostructures was examined by the SEM observation function of a high-resolution EBL system (ELS-F125-U, Elionix) operating at 125 kV. The spatial resolution of the SEM images was estimated to be 1.8 nm with a beam current of 100 pA. Prior to SEM measurements, an approximately 2 nm thin layer of Pd/Pt was deposited on the surface of the sample to reduce the charge accumulation on the nonconductive glass substrates. The spatial resolution and metal deposition made it hard to accurately characterize sub-nanometer gaps. Main conclusions are therefore derived from experimental spectra in comparison to simulations. Finite-difference time-domain (FDTD) simulations. Numerical simulations of the optical properties of the gold nanoblock dimers were performed using the FDTD software package Lumerical. The glass substrate was assumed to have a constant refractive index of n = 1.5 within the wavelength range of 500 – 1150 nm. The optical properties of gold were obtained using the data from Johnson and Christy.47 A total field and scattering field (TFSF) source was used for calculating the scattering spectra. The TFSF source was incident on the dimers at an incidence angle of 50°. A nonuniform mesh grid was used for discretization, but a 0.5- to 2-nmresolution overlaid mesh grid was used around the dimer gaps. For the charge distributions calculated at plasmon resonances, the uniform mesh was used over the whole monitor region. Perfectly-matched layer boundary conditions were imposed at all boundaries. Rounded corners and edges of the nanoblocks were chosen to have a 16 nm radius of curvatures. In general,



geometrical parameters and especially the gap size were considered as adjustable parameters to best reproduce experimental trends.

Results and Discussion

Figure 1. Sample design combined with hyperspectral measurements enable an accurate systematic study of nanoblock dimers with varying gap sizes. (a) Designed geometry of nanoblock dimers. The edge lengths (l) of the nanoblock were either 80 or 100 nm. The estimated gaps (g) were 16, 11, 6, 0, -5, -10, -15, -21, -26, and -31 nm. Longitudinal (L) and transverse (T) modes were measured with detection polarization parallel and perpendicular to the dimer axis, respectively. (b) SEM image of a nanoblock dimer with a designed length of 80 nm and estimated gap of 11 nm. Scale bar: 100 nm. (c) Scattering image of a nanoblock dimer sample with a designed edge length of 80 nm. The gap size was varied in the vertical directions, showing here 5 of the 10 gap sizes. In the horizontal direction, nanoblock dimers with the same dimensions and gap size were reproduced ten times. The distance between all neighboring dimers was 4 µm. The bright spot in the middle was a gold residue left from the fabrication


Page 8 of 23

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


process. (d) Single-particle dark-field scattering spectra of the row of 10 nanoblock dimers highlighted in (c), acquired with longitudinal (left) and transverse (right) detection polarizations.

In order to directly compare the optical properties of nanoblock dimers with varying separations, we performed single-particle measurements under exactly the same conditions on a sample array that contained ten-fold copies of all different dimer geometries. Figure 1 illustrates our experimental approach. The dimers consisted of two nanoblocks with a designed edge length of either 80 or 100 nm and a thickness of 30 nm. The nanoblocks were positioned with their vertical edges aligned instead of face-to-face (Figure 1a and 1b) in order to minimize the potential contact area and mode volume (for separated nanostructures). The designed gaps varied from 12 nm (positive) to -35 nm (negative). Negative gaps indicate that the two nanoblocks overlapped with one another. Rounded corners instead of sharp corners were fabricated due to the nature of EBL and therefore the physical gaps of the dimers are larger than the designed ones (Figure S1). We estimated a difference of 4 nm between the designed and physical gap sizes (Table S1) via a comparison with simulated spectra. To simplify the discussion, estimated gap sizes are always quoted. The gap size was varied within a column of an array, while each row consisted of 10 dimers with the same designed dimensions. Figure 1c shows a dark-field scattering image of half the array comprising 5 of the 10 gap sizes. Column and row markers enabled the straightforward one-to-one correlation between optical images and SEM of individual dimers. Hyperspectral dark-field spectroscopy (see Experimental Methods) allowed us to measure the scattering spectra of all 50 dimers shown in Figure 1c under the same condition and then to shift the sample to collect 10 spectra each for the other 5 gap sizes. This approach of sample array fabrication and hyperspectral microscopy facilitated the optical characterization of



nanoblock dimers without introducing errors caused by switching between samples and instrument realignment. The interparticle separation including electrical contact between the nanoblocks in the dimer is probed by the longitudinally polarized scattering spectra. Depending on the direction of the electric field oscillation of the excitation light, two modes are observed for a plasmonic dimer: longitudinal (L) and transverse (T) polarized modes, in which the LSPR oscillates parallel and perpendicular to the dimer axis, respectively. To isolate these two modes, we collected polarized scattering spectra by placing a linear polarizer in the detection path, which is equivalent to modulating the excitation polarization. Figure 1d plots the 10 polarized scattering spectra for dimers with a designed gap of 11 nm and an edge length of 80 nm. These spectra show very good particle to particle reproducibility and furthermore demonstrate the shift of the L-mode to lower energies by 100 meV compared to T-mode due to capacitive plasmon coupling.44 All 10 spectra for each gap size and both polarizations (200 total) are shown in the SI (Figures S2 and S3). Figures S4 and S5 furthermore show the results for nanoblock dimers with an edge length of 100 nm. While the T-mode spectra, which are mainly determined by the diagonal length of the individual nanoblock, show little dependence on the gap sizes (Figures S3 and S5),41 the L-mode spectra depend strongly on the gap sizes (Figures S2 and S4). In order to simplify the mode assignment by avoiding contributions from the individual nanoblock, we will exclusively discuss the L-modes in the following text.


Page 10 of 23

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


Figure 2. Capacitive coupling in electrically isolated nanoblock dimers. (a) Single-particle scattering spectra and (b) SEM images of nanoblock dimers with a designed edge length of 80 nm and estimated gaps of 16 (blue) and 6 (red) nm. The scale bars are 100 nm. (c) Simulated scattering spectra and (d) charge distributions at the resonance peaks for nanoblock dimers with an edge length of 80 nm and gaps of 16 (cyan) and 6 (magenta) nm. The size of the charge plots is 160×240 nm2.

For dimers with an obvious gap (>1 nm), the scattering spectra are dominated by the BDP mode that redshifts as the gap size decreases (Figure 2). Dimers with a designed edge length of 80 nm and gaps between 16 and 6 nm show one coupled plasmon peak, assigned to the BDP (Figures 2a and S2). SEM images verify the existence of clear gaps between nanoblocks (Figure 2b). The small spectral variations among the different dimers for each gap size demonstrate our good control over nanostructure geometry (Figure S2). The coupled plasmon mode redshifts as the gap size decreases, which is illustrated by comparing typical spectra of



dimers with gaps between 16 and 6 nm (Figure S2 and S6). This experimental trend is reproduced in FDTD simulations (Figure 2c) when the gold nanoblock dimer geometry, glass substrate, and excitation geometry are all considered. Charge plots calculated at the respective resonance energies support the assignment to the BDP mode (Figure 2d). A redshift was also observed experimentally in nanoblock dimers with a designed edge length of 100 nm and decreasing gap sizes between 16 and 6 nm (Figure S4). In this capacitive coupling region, the spectral shift of the BDP is strongly correlated with the gap size, consistent with numerous previous reports.14, 15, 43, 44

Figure 3. The variety of scattering spectra measured for the same gap size of 0 nm illustrates the transition from electrically isolated to conductively bridged dimers. (a) Correlated SEM images and (b) single-particle scattering spectra of dimers with a gap of 0 nm and edge length of 80 nm. BDP: bonding dipolar plasmon. SDP: screened bonding dipolar plasmon. MP: multiple peaks. The scale bars are 100 nm. (c) Simulated scattering spectra of nanoblock dimers with a length of 80 nm and gaps of 4 nm (top), -1 nm (middle), and -2 nm (bottom). An offset is applied for visualization. (d) Charge plots corresponding to the dimers in (c) calculated at the respective resonance energies. The size of the charge plots is 160×240 nm2. 12 ACS Paragon Plus Environment

Page 12 of 23

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


However, the optical properties change dramatically when two nanoblocks are almost or slightly touching (Figure 3). This sensitivity is revealed by the large heterogeneity observed in the 10 single-particle spectra of dimers with a gap size of 0 nm and an edge length of 80 nm (Figure S2). Note that only a gap size of 0 nm produced large spectral variations, indicative of both structural heterogeneity and a strong dependence of the optical properties on nanostructure geometry. The heterogeneous distribution of spectra can be classified into three groups with representative spectra from each of the groups given in Figure 3b: (i) one peak at lower energy with higher intensity (blue), (ii) one peak at higher energy with lower intensity (cyan), and (iii) multiple peaks (red). A similar grouping can also be applied to the spectra of dimers with a designed edge length of 100 nm and gap of 0 nm (Figures S4 and S7). The spectral features of these groups can be used to infer the detailed relative geometry between the two nanoblocks. For dimers Group (i), the peak around 1.8 eV follows the evolution of the BDP discussed in Figure 2 with similar intensities but lower energies, suggesting the existence of a small gap which is still too large for electrons to tunnel through. For dimers in Group (ii), the spectra show a single peak with lower intensities at higher energies, which can be identified as a SDP mode that occurs when a CTP is formed.13, 14 The corresponding CTP is outside our spectral detection window.14 The existence of a SDP mode implies that an electrical junction between the two nanoblocks is formed. Finally for dimers in Group (iii), multiple peaks emerge that might result from couplings of higher-order modes through a very small gap48 or quantum effects.22, 38, 49 SEM images cannot provide further structural information of the junctions (Figure 3a) because of limited resolution (see Experimental Methods for more details).



Instead of SEM imaging, we therefore relied on FDTD simulations to support our conclusions (Figure 3c). Nanoblock dimers with a designed edge length of 80 nm and gap size around 0 nm were considered in the simulations. The exact 0-nm case was avoided because it is physically ill-defined.50 When the gap size is larger than 0 nm, the BDP redshifts as the gap size decreases (Figure 3c, magenta line) compared to Figure 2c. This trend is the same as described for dimers in Group (i) (also see Figure S6). When the gap size is smaller than 0 nm (Figure 3c, green and black lines), CTP and SDP modes emerge as for the dimers in Group (ii). The CTP mode is still at energies lower than 1.24 eV, but for the -2 nm gap dimer (black in Figure 3c) the tail of the CTP becomes obvious. Figure S8 plots the entire CTP peaks observed in the near-IR spectral range on a scale wider than the experimental spectral window. The SDP mode blueshifts with increasing overlap (green vs. black lines in Figure 3c). Dimers in Group (iii) are not observed in the simulations as we performed classical simulations with limited mesh sizes. The charge plots of the three spectra in Figure 3c at their respective resonance energies again show a dipolar mode (Figure 3d), which is expected for both BDP and SDP modes.13 These results demonstrate that optical spectroscopy is extremely sensitive to the gap geometry and therefore serves as a convenient tool to probe the relative geometry between two nanoblocks even for subnanometer separations and in particular the transition from capacitive to conductive coupling. It should be noted that the resistance of junction deviates from the Drude value when the width of the junction is smaller than the electron mean free path of gold (~ 40 nm). The resistance of the junction then has a strong influence on the CTP mode, as has been shown previously.13 Considering the resolution of our electron microscopy images and the spectral window of our single-particle spectra, we can, however, not comment further on this aspect here.


Page 14 of 23

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


Figure 4. Transition from conductively bridged to fused dimers. (a) SEM images (scale bar: 100 nm) and (b) correlated scattering spectra of nanoblock dimers with an edge length of 80 nm and gap sizes of -5, -10, -21, and -31 nm (top to bottom). CTP: charge transfer plasmon. DP: dipolar plasmon. QP: quadruple plasmon. (c) Simulated scattering spectra of nanoblock dimers with an edge length of 80 nm and gap sizes of -5, -10, -20, and -32 nm (top to bottom). An offset is applied for visualization. (d) Charge plots at the indicated energies corresponding to the dimers in (c). The charge plot sizes in the top two rows are 160×240 nm2. The bottom row is a zoom-in at the dimer gap for the high energy peak with an image size of 65×55 nm2. (e) Cartoons illustrating the difference in charge distributions expected for pure SDP and QP modes.



A dimer transitions from a conductively bridged to a fused nanostructure occurs when electrons can freely oscillate in the whole nanostructures with low resistance.13 By decreasing the gap further from 0 nm (Figure 4a), we were able to follow this transition through the evolution of the single-particle dark-field scattering spectra. For the -5 nm gap, the edge of the lowest energy peak started to emerge in our spectral detection (Figure 4b). Overall, the spectra contain two peaks for these well-overlapped dimers and are reproduced by simulations (Figures 4b and 4c). Unlike the transition from electrically isolated to conductively bridged dimers where sharp differences in the spectra can be observed, the transition between conductively bridged and fused dimers is less clear at first, because the spectra for all dimers with gaps of -5 nm and smaller have two resonance peaks. The low energy peaks are assigned to CTPs and dipolar plasmons (DPs) for conductively bridged and fused dimers, respectively. The spectral shifts of the low energy peaks cannot help in differentiating CTPs and DPs as both of them blueshift as the gap size decreases due to higher conductivities in the junction11, 13, 14, 33 and smaller aspect ratios of a rod-like nanostructure,51 respectively. This trend is observed in both experiments and simulations. The simulated charge distributions of the low energy peaks are also similar (top row of Figure 4d). Opposite charges are always observed on the two nanoblocks. Although the low energy peaks relate to how much the two nanoblocks overlap through their spectral shifts, it is not possible to establish a transition from conducting to fused dimers. The high energy peaks in connected dimers, on the other hand, provide insight into the transition between conductively bridged and fused nanostructures through their charge plots. The high energy peaks are assigned to SDPs and quadruple plasmons (QPs) for conductively bridged and fused dimers, respectively. Because SDPs and QPs share similar peak energies, it is hard to differentiate these two modes in the scattering spectra. Calculated charge plots, however, allow 16 ACS Paragon Plus Environment

Page 16 of 23

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


us to make a distinction and to identify the transition to a fused nanostructure. As Figure 4e illustrates, the charge distribution of SDPs possesses opposite charges at the two sides of the junction and the two ends of the nanostructure.13 In contrast, the same charge is observed at the junction and the ends in the charge distribution of QPs. All the charge distributions of the high energy peaks appear like QPs at first glance (middle row of Figure 4d). However, careful inspection (bottom row of Figure 4d) reveals opposite charges at the junction for the dimers with the gaps of -5 and -10 nm, allowing us to assign the corresponding resonance to SDP modes of conductively bridged dimers. For dimers with even larger overlap, pure QPs of fused dimers were observed. The charge plots of the higher energy peaks therefore allow us to identify a transition from conductively coupled dimers to a fused rod-like nanostructure. While the experimental and simulated results in Figure 4b and 4c show overall good agreement, a minor discrepancy is present for the high energy peaks in the highly overlapping dimers (bottom two spectra in Figure 4b and 4c). The simulated spectra display a low intensity for the QPs because it is a dark mode consistent with the charge plot in Figure 4e.52 However, a significant intensity was observed in our measurements for the QP of the fused dimer. One difference between the experiments and simulations was that the former utilized a dark-field illumination that matched the critical angle for total-internal-reflection (TIR) excitation. It has been demonstrated that the evanescent wave in such an illumination geometry produces more efficient far-field scattering from dark plasmon modes,15 consistent with our experimental results and explaining the difference seen compared to the non-TIR dark-field scattering simulations.



Conclusions In conclusion, we examined the plasmonic evolution of gold nanoblock dimers as a function of gap sizes, covering a wide range of interparticle gaps from capacitively coupled to fused nanostructures. Metal contact allowing for electrical conductivity between the dimer constituents caused the most drastic spectral shifts as new plasmon modes emerge. In particular, the single-particle scattering spectra revealed a change from a single plasmon mode (BDP) to two plasmon modes (CTP and SDP). As the two nanoblocks fused further together, the transition from conductively bridged dimers to rod-like nanostructures was furthermore assigned with the help of simulated charge plots. Our findings provide especially new insight into the transition from touching to fused plasmonic nanoparticle dimers.


Page 18 of 23

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


Supporting information Definition of designed vs. estimated gap sizes, individual scattering spectra recorded with both longitudinal and transverse polarizations for all nanoblock dimers studied (nanoblock edge lengths of 80 nm and 100 nm with gaps of 16, 11, 6, 0, -5, -10, -15, -21, -26, and -31 nm), scattering peak energy as a function of gap size, and simulated spectra showing the CTP mode plotted on a wider spectral window. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements SL thanks the Robert A. Welch Foundation (C-1664), the National Science Foundation (ECCS1608917 and supplemental funding of CHE- 0955286 for an international collaboration), and the Air Force (MURI FA9550-15-1-0022) for financial support. This work was partially supported by JSPS KAKENHI (Grant Nos. JP18H05205, JP17H01041, JP17H05245, and JP17H05459), the Nanotechnology Platform (Hokkaido University), and the Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials (Five-Star Alliance) of MEXT. The authors acknowledge Ms. Yuko Mori and Ms. Wakako Nakano for the fabrication of Au nanoblock dimer. The authors are grateful to Dr. Olivier Lecarme for the fabrication of samples at the initial stage of this collaborative work. The authors thank Dr. Kyle W. Smith for help with editing the manuscript.



References 1. Wu, K.; Chen, J.; McBride, J. R.; Lian, T., Efficient hot-electron transfer by a plasmoninduced interfacial charge-transfer transition. Science 2015, 349, 632-635. 2. Xuming, Z.; Yu Lim, C.; Ru-Shi, L.; Din Ping, T., Plasmonic photocatalysis. Rep. Prog. Phys. 2013, 76, 046401. 3. Kamat, P. V., Photophysical, photochemical and photocatalytic aspects of metal nanoparticles. J. Phys. Chem. B 2002, 106, 7729-7744. 4. Atwater, H. A.; Polman, A., Plasmonics for improved photovoltaic devices. Nat. Mater. 2010, 9, 205. 5. Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y., Gold nanostructures: engineering their plasmonic properties for biomedical applications. Chem. Soc. Rev. 2006, 35, 1084-1094. 6. Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P., Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7, 442. 7. Stewart, M. E.; Anderton, C. R.; Thompson, L. B.; Maria, J.; Gray, S. K.; Rogers, J. A.; Nuzzo, R. G., Nanostructured plasmonic sensors. Chem. Rev. 2008, 108, 494-521. 8. Kippenberg, T. J.; Vahala, K. J., Cavity optomechanics: back-action at the mesoscale. Science 2008, 321, 1172-1176. 9. Chang, W.-S.; Wen, F.; Chakraborty, D.; Su, M.-N.; Zhang, Y.; Shuang, B.; Nordlander, P.; Sader, J. E.; Halas, N. J.; Link, S., Tuning the acoustic frequency of a gold nanodisk through its adhesion layer. Nat. Commun. 2015, 6, 7022. 10. Olson, J.; Dominguez-Medina, S.; Hoggard, A.; Wang, L.-Y.; Chang, W.-S.; Link, S., Optical characterization of single plasmonic nanoparticles. Chem. Soc. Rev. 2015, 44, 40-57. 11. Bryant, G. W.; García de Abajo, F. J.; Aizpurua, J., Mapping the plasmon resonances of metallic nanoantennas. Nano Lett. 2008, 8, 631-636. 12. Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C., The optical properties of metal nanoparticles: the influence of size, shape, and dielectric environment. J. Phys. Chem. B 2003, 107, 668-677. 13. Wen, F.; Zhang, Y.; Gottheim, S.; King, N. S.; Zhang, Y.; Nordlander, P.; Halas, N. J., Charge transfer plasmons: optical frequency conductances and tunable infrared resonances. ACS Nano 2015, 9, 6428-6435. 14. Atay, T.; Song, J.-H.; Nurmikko, A. V., Strongly interacting plasmon nanoparticle pairs: from dipole−dipole interaction to conductively coupled regime. Nano Lett. 2004, 4, 1627-1631. 15. Yang, S.-C.; Kobori, H.; He, C.-L.; Lin, M.-H.; Chen, H.-Y.; Li, C.; Kanehara, M.; Teranishi, T.; Gwo, S., Plasmon hybridization in individual gold nanocrystal dimers: direct observation of bright and dark modes. Nano Lett. 2010, 10, 632-637. 16. Wang, L.-Y.; Smith, K. W.; Dominguez-Medina, S.; Moody, N.; Olson, J. M.; Zhang, H.; Chang, W.-S.; Kotov, N.; Link, S., Circular differential scattering of single chiral self-assembled gold nanorod dimers. ACS Photonics 2015, 2, 1602-1610. 17. Fang, L.; Wang, Y.; Liu, M.; Gong, M.; Xu, A.; Deng, Z., Dry sintering meets wet silverion “soldering”: charge-transfer plasmon engineering of solution-assembled gold nanodimers from visible to near-infrared I and II regions. Angew. Chem. Int. Ed. 2016, 55, 14296-14300. 18. Schnell, M.; García-Etxarri, A.; Huber, A. J.; Crozier, K.; Aizpurua, J.; Hillenbrand, R., Controlling the near-field oscillations of loaded plasmonic nanoantennas. Nat. Photonics 2009, 3, 287. 20 ACS Paragon Plus Environment

Page 20 of 23

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


19. Mühlschlegel, P.; Eisler, H.-J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W., Resonant optical antennas. Science 2005, 308, 1607-1609. 20. 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. 21. Chu, M.-W.; Myroshnychenko, V.; Chen, C. H.; Deng, J.-P.; Mou, C.-Y.; García de Abajo, F. J., Probing bright and dark surface-plasmon modes in individual and coupled noble metal nanoparticles using an electron beam. Nano Lett. 2009, 9, 399-404. 22. Zuloaga, J.; Prodan, E.; Nordlander, P., Quantum description of the plasmon resonances of a nanoparticle dimer. Nano Lett. 2009, 9, 887-891. 23. Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H., Clusters of closely spaced gold nanoparticles as a source of two-photon photoluminescence at visible wavelengths. Adv. Mater. 2008, 20, 26-30. 24. Halas, N. J.; Lal, S.; Chang, W.-S.; Link, S.; Nordlander, P., Plasmons in strongly coupled metallic nanostructures. Chem. Rev. 2011, 111, 3913-3961. 25. Slaughter, L. S.; Wu, Y.; Willingham, B. A.; Nordlander, P.; Link, S., Effects of symmetry breaking and conductive contact on the plasmon coupling in gold nanorod dimers. ACS Nano 2010, 4, 4657-4666. 26. Funston, A. M.; Novo, C.; Davis, T. J.; Mulvaney, P., Plasmon coupling of gold nanorods at short distances and in different geometries. Nano Lett. 2009, 9, 1651-1658. 27. Kumar, J.; Wei, X.; Barrow, S.; Funston, A. M.; Thomas, K. G.; Mulvaney, P., Surface plasmon coupling in end-to-end linked gold nanorod dimers and trimers. PCCP 2013, 15, 42584264. 28. Sheikholeslami, S.; Jun, Y.-w.; Jain, P. K.; Alivisatos, A. P., Coupling of optical resonances in a compositionally asymmetric plasmonic nanoparticle dimer. Nano Lett. 2010, 10, 2655-2660. 29. Sönnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P., A molecular ruler based on plasmon coupling of single gold and silver nanoparticles. Nat. Biotechnol. 2005, 23, 741. 30. Jain, P. K.; Huang, W.; El-Sayed, M. A., On the universal scaling behavior of the distance decay of plasmon coupling in metal nanoparticle pairs: a plasmon ruler equation. Nano Lett. 2007, 7, 2080-2088. 31. Kulkarni, V.; Manjavacas, A., Quantum effects in charge transfer plasmons. ACS Photonics 2015, 2, 987-992. 32. Wu, B.; Ueno, K.; Yokota, Y.; Sun, K.; Zeng, H.; Misawa, H., Enhancement of a twophoton-induced reaction in solution using light-harvesting gold nanodimer structures. J. Phys. Chem. Lett. 2012, 3, 1443-1447. 33. Nie, S.; Emory, S. R., Probing single molecules and single nanoparticles by surfaceenhanced raman scattering. Science 1997, 275, 1102-1106. 34. Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E., Large single-molecule fluorescence enhancements produced by a bowtie nanoantenna. Nat. Photonics 2009, 3, 654. 35. Ito, F.; Ohta, R.; Yokota, Y.; Ueno, K.; Misawa, H.; Nagamura, T., Near infrared fluorescence enhancement by local surface plasmon resonance from arrayed gold nanoblocks. Optics and Photonics Journal 2013, 3, 27.



36. Schuck, P. J.; Fromm, D. P.; Sundaramurthy, A.; Kino, G. S.; Moerner, W. E., Improving the mismatch between light and nanoscale objects with gold bowtie nanoantennas. Phys. Rev. Lett. 2005, 94, 017402. 37. Fontana, J.; Charipar, N.; Flom, S. R.; Naciri, J.; Piqué, A.; Ratna, B. R., Rise of the charge transfer plasmon: programmable concatenation of conductively linked gold nanorod dimers. ACS Photonics 2016, 3, 904-911. 38. Scholl, J. A.; García-Etxarri, A.; Koh, A. L.; Dionne, J. A., Observation of quantum tunneling between two plasmonic nanoparticles. Nano Lett. 2013, 13, 564-569. 39. Byers, C. P.; Hoener, B. S.; Chang, W.-S.; Link, S.; Landes, C. F., Single-particle plasmon voltammetry (spPV) for detecting anion adsorption. Nano Lett. 2016, 16, 2314-2321. 40. Lerch, S.; Reinhard, B. M., Effect of interstitial palladium on plasmon-driven charge transfer in nanoparticle dimers. Nat. Commun. 2018, 9, 1608. 41. Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht, B.; Aussenegg, F. R., Optical properties of two interacting gold nanoparticles. Optics Communications 2003, 220, 137141. 42. Wu, L.; Tan, S. F.; Bosman, M.; Yang, J. K. W.; Nijhuis, C. A.; Bai, P., Charge transfer plasmon resonances across silver-molecule-silver junctions: estimating the terahertz conductance of molecules at near-infrared frequencies. RSC Advances 2016, 6, 70884-70894. 43. Ueno, K.; Misawa, H., Spectral properties and electromagnetic field enhancement effects on nano-engineered metallic nanoparticles. PCCP 2013, 15, 4093-4099. 44. Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P., A hybridization model for the plasmon response of complex nanostructures. Science 2003, 302, 419-422. 45. Pérez-González, O.; Zabala, N.; Borisov, A. G.; Halas, N. J.; Nordlander, P.; Aizpurua, J., Optical spectroscopy of conductive junctions in plasmonic cavities. Nano Lett. 2010, 10, 30903095. 46. Hartsfield, T.; Chang, W.-S.; Yang, S.-C.; Ma, T.; Shi, J.; Sun, L.; Shvets, G.; Link, S.; Li, X., Single quantum dot controls a plasmonic cavity’s scattering and anisotropy. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 12288-12292. 47. Johnson, P. B.; Christy, R. W., Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370-4379. 48. Lassiter, J. B.; Aizpurua, J.; Hernandez, L. I.; Brandl, D. W.; Romero, I.; Lal, S.; Hafner, J. H.; Nordlander, P.; Halas, N. J., Close encounters between two nanoshells. Nano Lett. 2008, 8, 1212-1218. 49. Tame, M. S.; McEnery, K. R.; Ozdemir, S. K.; Lee, J.; Maier, S. A.; Kim, M. S., Quantum plasmonics. Nat. Phys. 2013, 9, 329-340. 50. Romero, I.; Aizpurua, J.; Bryant, G. W.; Abajo, F. J. G. d., Plasmons in nearly touching metallic nanoparticles: singular response in the limit of touching dimers. Opt. Express 2006, 14, 9988-9999. 51. Encina, E. R.; Coronado, E. A., Resonance conditions for multipole plasmon excitations in noble metal nanorods. J. Phys. Chem. C 2007, 111, 16796-16801. 52. Yang, Z.-J.; Zhang, Z.-S.; Zhang, L.-H.; Li, Q.-Q.; Hao, Z.-H.; Wang, Q.-Q., Fano resonances in dipole-quadrupole plasmon coupling nanorod dimers. Opt. Lett. 2011, 36, 15421544.


Page 22 of 23

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


TOC Graphic


1 Optical Characterization of Gold Nanoblock Dimers: From Capacitive

Recommend Documents