Optical Response Properties of Stable and Controllable Au Nanorod

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Optical Response Properties of Stable and Controllable Au Nanorod Monolayer Meta-arrays Renming Liu, Li Jiang, Guanghui Liu, GengYan Chen, Junyu Li, Jin Liu, and Xue-Hua Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02700 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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The Journal of Physical Chemistry

Optical

Response

Properties

of

Stable

and

Controllable Au Nanorod Monolayer Meta-arrays

Renming Liu, † Li Jiang,§ Guanghui Liu,† Gengyan Chen,⊥ Junyu Li, † Jin Liu, † Xue-Hua Wang*,†

†State

Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and

Engineering, Sun Yat-sen University, Guangzhou 510275, China. §College

of Science, Guilin University of Technology, Guilin 541004, China.

⊥ School

of Optoelectronic Engineering, Guangdong Polytechnic Normal University, Guangzhou

510665, China. .

ABSTRACT： Ordered assemblies of individual metal nanoparticle building blocks have been important platforms in exploring plasmon-based light–matter interactions. Despite of the fact that such assemblies have been extensively constructed, the systemic investigations of their optical response properties have remained lacking. In addition, the poor structural stability of these assembled metastructures treated with organic or inorganic solvents is another open question. Here, we develop a facile and robust approach for constructing structurally stable and controllable Au nanorod monolayer meta-arrays (NRMMAs) vertically aligned on silicon substrate, via evaporation-induced self-assembly and electron beam exposure. By precisely

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controlling zeta-potential on the surface of Au nanorods (NRs) and the Debye length of the colloidal dispersion, we demonstrate a good manipulation on edge-to-edge gap distance between the NRs, which agrees well with our theoretical predictions. Based on this precise control, farand near-field optical responses of the Au NRMMAs are systematically demonstrated. This controllable and stable vertically aligned Au NRMMA with abundant, uniform and tunable “hot spots” provide a useful optical material for practical applications.

INTRODUCTION Ordered assemblies of colloidal nanoparticles have great potential applications from selfassembled electronics, nano-optics, catalysis, to biosensors.1,2 Recent advances in self-assembly technologies makes such assemblies provide a simple and robust route to optical metamaterials and superlattices.3-9 Since these assembled materials have significant differences in optical response from their individual components,10,11 the construction of ordered assemblies from the monodisperse metal nanoparticles has been a rapidly developing field of research in nanotechnology and nanophotonics.12 Especially, the ordered assemblies of the anisotropic metal nanoparticles have attracted increasing interest because of their rich assembling behaviours caused by their reduced shape symmetry, as well as their remarkable collective plasmonic properties governed by the excitation of localized surface plasmon resonances,13 making them ideal candidates for exploring light concentrators and resonators, single-molecule sensors, enhanced fluorescence and surface-enhanced Raman spectroscopy (SERS ).14-19 To obtain ordered assemblies, different approaches have been exploited, such as association in solution,20,21 drying at an interface,22-26 templating,3,27-30 and DNA-mediated nanoparticle assembly.31,32 Compared to isotropic nanostructures, however, the ordered assemblies of


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anisotropic nanostructures, such as Au NRs, are still challenging, because the anisotropic shape results in various assembling behaviours.17,33 Despite the fact that based on the coffee-ring effect, the evaporation-induced self-assembly have been developed to transfer the NRs to the droplet edge and assemble them into ordered multilayer superarrays,22,34-37 the number of the layers and the edge-to-edge gap distance (EEGD) between the nanorods (NRs) in the ordered arrays has remained random and difficult to be controlled. Recently, Peng et al. demonstrate for the first time that it is possible to directly self-assemble CTAB-coated Au NRs into highly organized vertical monolayer arrays,38 opening up possibilities for precisely manipulating the optical responses of the monolayer arrays and therefore finding their great potential applications in plasmonics. However, there are still some challenges that hinder such metastructures in practical applications. The first challenge is the poor structural stability of such oriented assemblies when treated with the organic or inorganic solvent (e.g. water or ethanol); the second one is to precisely control the EEGD between the nanoparticles and revel the underlying mechanism behind it; the last one is that the far- and near-filed optical responses of the ordered assemblies has remained confused. In this work, we develop a simple, two-step method for assembling meta-arrays of vertically aligned Au NRs in monolayer with high structural stability using the evaporation-induced selfassembly of Au NRs and electron beam (e-beam) exposure. By controlling the zeta-potential on the surface of the NRs, we demonstrate a facile and robust way for precisely scaling the EEGD between the NRs in the monolayer arrays from ~7 to 11 nm, agreeing well with our theoretical predictions. In addition, we demonstrate the control of far- and near-field optical response of the vertically aligned Au NR monolayer arrays (NRMMAs) based on the precise control of EEGD


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between the NRs, suggesting a great potential in coherent manipulation of light-matter interactions. EXPERIMENTAL SECTION Preparation of vertically aligned Au NRMAs with high stability. Materials, the preparation and purification of Au NRs can be found in Supporting Information. Evaporation-induced selfassembly and e-beam exposure of CTAB-coated Au nanorods were used to prepare of stable and controllable Au NRMMAs vertically aligned on silicon substrate. Specifically, 4 mL of the purified Au nanorods was centrifugated at the speed of 7000 rpm for 10 min, and the precipitates were redispersed averagely in CTAB solution (1 mL, 2.5 mM) containing NaCl whose concentration was ~5×10-4, 1×10-3, 3×10-3, 8×10-3, 1×10-3 and 3×10-2 M. Then, 10 μL dispersion of the CTAB-coated Au NR solution was dropped on the silicon substrate cleaned with acetone and ethanol, respectively. The samples were kept in Petri dish with cover at ~20 °C for 24 h in the atmosphere with humidity of ~80%. After dried of the droplet, the vertically aligned Au NRMMAs with different EEGDs were fabricated (Figure 1a). To improve the structural stability of the obtained Au NRMMAs, an e-beam of 20 kV from the electron gun integrated in the scanning electron microscope (SEM) machine was used to expose the surface of the samples for 10 min. After this treatment, CTAB molecules play a role similar to the negative photoresists, protecting the samples from being destroyed when washed with organic or inorganic solvents. Characterization. The extinction spectra were recorded on an ultraviolet-visible-near-infrared (UV-VIS-NIR) spectrophotometer (PerkinElmer Lambda 950). The reference we adopted was 3 mL deionized water. A Malvern Zetasizer NanoZS90 instrument was employed to record the


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particle’s zeta-potential. SEM images were performed using a Zeiss Auriga-39-34 SEM machine with a resolution of 1 nm operated at an accelerating voltage of 5.0 kV. The dark-field scattering images and spectra of the Au NRMA samples were recorded on a darkfield optical microscope (Olympus BX51, OlympusInc.) that was integrated with a quartz tungsten halogen lamp (100W), a monochromator (Acton SpectraPro 2360, Acton Inc.), and a charge-coupled device camera (Princeton Instruments Pixis 400BR_eXcelon). The camera was thermoelectrically cooled to -70 °C during the measurements. The light was launched from a dark-field objective (100×, numerical aperture 0.80), and the light scattered in the backward direction was collected by the same objective. In measurements, the spectrograph equipped with an entrance slit was set at 0.6 μm, and the scattering signals were collected from the area away from the fringes of the samples, which not only can record the collective plasmonic optical properties originated from the coupling of the NRs, but also effectively avoid the influence of strong scattering arising from the sample edges. The collected scattering signals were corrected by first subtracting the background spectra taken from the adjacent regions without Au NRMAs and then dividing them with the calibrated response curve of the entire optical system. Colour scattering images were captured using a colour digital camera (ARTCAM-300MI-C, ACH Technology Inc.) mounted on the imaging plane of the microscope. The s-SNOM nanoimagings were conducted using a scattering-type near-field optical microscope (NeaSNOM, Neaspec GmbH). To image the near-filed responses in real-space, a laser with an excitation wavelength of 633 nm was focused onto the sample with a metal-coated AFM tip (Arrow-IrPt, Nanoworld), the curvature radius r0~15 nm. The back-scattered light from the tip was demodulated and detected at a fourth harmonic of the tip vibration frequency.


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RESULTS ABD DISCUSSIONS Preparation and morphology characterization of the vertically aligned Au NRMMAs. The vertically aligned Au NRMMAs were fabricated using the evaporation-induced self-assembly of CTAB-stabilized Au NRs.38 This approach is based on near quilibrium status at the internal region of the drying droplet, which eliminates the complex ligand exchange reaction (Figure 1a). Before the Au NRMMAs were fabricated, the purified and uniform Au NRs with ~86 ± 6 nm in length and ~30 ± 4 nm in diameter were prepared using a seed-mediated method39 combined with surface area-based purification40. By using this approach, we successfully removed the spherical Au nanoparticles and the Au NRs with larger surface areas, which promises the obtained Au NRs with high purity (~99 %) and uniformity (Figure 1b and Figure S1). For fabrication of the vertically aligned Au NRMMAs, 10 µL Au NR dispersion containing CTAB (2.5 mM) and NaCl (8.0 mM) was drop-casted on a silicon substrate and kept in a Petri dish with cover at room temperature (~20 °C) and 80 % humidity. After 24 h, the aqueous NR dispersions were slowly dried in humid atmosphere and the Au NRs were spontaneously and reproducibly crystallized into island-like monolayer meta-arrays vertically aligned on silicon substrate (Figure 1c and Figure S2), and the vertical Au NRs are highly organized and arranged in hexagonal lattice with an average EEGD between the NRs of ~8.3 ± 0.6 nm (Figure 1d). Figure 1e shows the edge image of the Au NRMMA, clearly indicating that the array is in monolayer fashion. It should be mentioned that this fabrication method is general, which not only promises the formation of the Au NRMMAs, but also enables the assembly of monolayer arrays from colloidal Au triangular nanoprisms and nanospheres (Figure S3). In fabrications, NaCl was used to change the zeta-potential (ζ) of the NRs and the Debye length (κ-1) of the colloidal aqueous dispersion. We find that the two parameters, especially the ζ, play


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the key role in controlling the EEGDs between the NRs. On the other hand, CTAB molecules adsorbed on the Au NRs is another important factor that affects the ζ. However, these molecules are easily dissolved in organic and inorganic solvents, such as ethanol and water. Our experimental results show that once the formed Au NRMMAs immersed in such solvents with 1~2 min, CTAB molecules in the gaps between the NRs will be dissolved and make the capillary force balance between the NRs in the structure to be broken, leading to the decrease of the EEGDs between the NRs, as well as random or fractal crack patterns are formed (Figure 1f and Figure S4), despite the fact that this is a general mean for cleaning the samples (Figure S5). In this case, the average EEGD of the Au NRMMAs can be decreased to ~1.3 ± 0.5 nm (Figure 1g). With the prolongation of washing time, the EEGDs between the NRs can be further decreased to a subnanometer scale, which has been proved very powerful in SERS detections.26,38


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Figure 1. Morphology characterization of the vertically aligned Au NRMMAs on silica substrate. (a) Schematic showing the preparation of the vertically aligned Au NRMMAs. (b) Extinction spectrum of the purified Au NRs used to fabricate the Au NRMMAs. Inset: Scanning electron microscope (SEM) image of the purified Au NRs. (c) SEM image of a vertically aligned Au NRMMA viewed from the top. (d) Distribution of the EEGDs for the Au NRMMA shown in (c). The average EEGD is ~8.3 ± 0.6 nm. (e) SEM image of the vertically aligned Au NRMMA shows its monolayer feature. The inset is the atomic force microscope (AFM) image of the vertically aligned Au NRMMA on silica substrate. (f) SEM image of the Au NRMMA after immersed and washed in the ethanol with concentration of 50 % for 1 min. (g) Distribution of the EEGDs for the vertically aligned Au NRMMA shown in (f). Note that, here the cracks were not considered as gap. However, such a poor structural stability is adverse for practical applications of these monolayer meta-arrays. Because that a poor structural stability maybe results in poor controllability and repeatability of the optical response property of a nanodevice.41,42 Our investigations suggest that this challenge can be removed by e-beam “lithography”. In specific, an e-beam of 20 kV was used to expose the surface of the prepared Au NRMMA samples for 10 min. After this treatment, the monolayer mate-arrays become very stable without any cracks even if they were immersed and washed in water or ethanol for 20 min (Figure 2). Figures 2a and 2b give the SEM images of two vertically aligned Au NRMMA samples after exposed by e-beam (20 KV) for 10 min. Although the two samples were immersed and washed in water (Figure 2d) or ethanol with a concentration of 20 % (Figure 2e) for ~20 min, their structures kept almost unchanged compared to those shown in Figure. 2a,b. However, for the samples without the treatment of e-beam exposure, random or fractal crack patterns formed quickly after they immersed and washed in


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water (Figure 2e) or ethanol (20 % in concentration, Figure 2f) for 1 min. By comparing the structural stability of the samples immersed and washed in water or ethanol for different time, we find that there is ~20 times improvement of the structural stability for the Au NRMMAs with the exposure of e-beam exposure. It is believed that the CTAB molecules adsorbed on the surface of the Au NRs play a role similar to the negative photoresists. Once exposed with e-beam, the CTAB molecules become crosslinked or polymerized, and difficult to be dissolved in organic or inorganic solvent (Figure S6), which can effectively protect the samples from being destroyed in the washing process. This improved stability of the vertically aligned Au NRMMAs will make them more useful in future applications.

Figure 2. High structural stability of the vertical aligned Au NRMMAs. (a,b) SEM images of two vertically aligned Au NRMMA samples after exposed by e-beam (20 kV) for 10 min. (c) SEM image for the Au NRMMA sample in (a) after immersed and washed in water for 20 min. (d) SEM image for the Au NRMMA sample in (b) after immersed and washed in ethanol with the concentration of 20 % for 20 min. (e) SEM image of a vertically aligned Au NRMMA sample (without the treatment of e-beam exposure) after immersed and washed in water for 1


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min. (f) SEM images the vertically aligned Au NRMMA samples (without the treatment of ebeam exposure) after immersed and washed in ethanol with concentrations of 20 % for 1 min. The insets are SEM images with higher magnification taken from the areas marked with dashed orange square frames. Precise control of the EEGDs between the Au NRs. The precise control of EEGDs between the NRs plays a key role in manipulating the far- and near-field optical response of the Au NRMMAs. Since Au NRs synthesized using the seed-mediated method are protected by a positive-charge CTAB bilayer, electrostatic repulsion forces (Fele) between the NRs makes them stay away from each other. As solvent evaporates colloidal naoparticles collect at the contact line which is a three-phase contact line where solvent, substrate, and vapour meet via capillary flow mechanism.1,43 On the other hand, the van der Waals forces (FvdW) between the Au NRs drag them much closer.38,44 Also, the attractive depletion forces (Fdep) arise from the presence of CTAB micelles leads to a further close between the Au NRs. By tailoring the three nanoscale forces, one can control the EEGDs between the Au NRs in the arrays. By considering two parallel Au NRs (the inset of Figure 3a), the relationships of the three forces (Fele, FvdW and Fdep) and the EEGD (x) between the two NRs can be expressed as (Supporting Information).38,45,46

 .e  ( x  2t  1/2  1 

Fele  2l  r  r 0 2 .

CTAB )

Ad A1lr1/2  52 FvdW =  [ .x + . 2 CTAB x 2 ] 16 48

(1)

(2)


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Fdep  2l .[(r  tCTAB,eff 

d CTAB,eff 2

x ) 2  (r  ) 2 ]1/2 2

(3)

where, l and r are the length and the radius of the Au NR; ε and ε0 are the relative electric permittivity of the electrolyte solution and the electric permittivity of a vacuum, respectively; κ-1 is the Debye length; ζ is the zeta-potential on the surface of the Au NR; x is the separation between the surfaces of the two NRs; tCTAB is the thickness (~3.2 nm)47 of the CTAB bilayer on the surface of the Au NR; tCTAB,eff  tCTAB     -1 is the effective thickness of the CTAB bilayer in nonneutral solutions; dCTAB is the diameter (~5.8 nm)43 of the CTAB micelles; d CTAB,eff  d CTAB     -1 is the effective diameter of the CTAB micelles in nonneutral solutions, 

was set at 5 in our calculations;48  is the number of the CTAB molecule pairs on the two Au NRs (here, we see CTAB bilayer on the NR as micelles); A1 and A2 is the Hamaker coefficient;  is the osmotic pressure which can be found in ES. In eqs 2 and 3, the first minus is just

indicating the direction of force. From eq 1, one can see that zeta-potential, ζ, and Debye length, κ-1, are two key parameters that can be experimentally controlled. Therefore, the balance of the three nanoscale forces can be manipulated by controlling the two parameters, which enables the control of the EEGDs between the Au NRs. The three forces and their total interaction force ( Ftotal  Fele  FvdW  Fdep ) change along with the varying EEGDs at the given ζ (36 mV) and κ-1 (3.16 nm) can be found in Figure S7. Importantly, the two parameters can be simultaneously controlled by one-step operation of adding negatively charged ions to neutralize the positive charges arising from CTAB bilayer on the surface of the Au NRs,49 and to decrease the Debye length, κ-1 (nm) = 0.3/ (I (M))1/2 of the colloidal aqueous dispersion,44 where I is the ionic strength expressed in molar (mol/L). Figures


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3a-c show the changes of the three nanoscale forces of FvdW, Fele and Fdep between two parallel Au NRs with varying ζ and κ-1 ranging from 20 to 51 mV and 1.73 to 15 nm, respectively. Figure S7 shows that Fdep is much smaller than FvdW and Fele, making it can be neglected in EEGD controlling. It is clearly found that all the three nanoscale forces decrease as the EEGD increases (Figure. 3a-c), and Fele and FvdW changes more quickly. The total interaction force, Ftotal, first decreases to zero and then increases as the EEGD increases, but it is not monotonous (Figure 3d). When the Ftotal comes to zero, it means that three forces are at a balance state, and the EEGD between the two parallel Au NRs can be determined. In experiments, Cl- is used to adjust both the ζ and κ-1 of the Au NRs, as shown in Figure 3e. It should mentioned that here we directly demonstrate that the key factor in precisely controlling the EEDG between the NRs in the Au NRMMAs is the ζ on the NRs, to our knowledge, which has not been definitely proposed and demonstrated in previous works, depict of the fact that this key factor has been included in the previous theory38. In addition, we also give a more comprehensive consideration on the FvdW, another key factor determining the EEGD between the NRs. Therefore, a much better agreement between the measuremental and the theoretical EEDGs can be found in Figure 3f. By using this facile method, we obtained varying average EEGDs ranging from ~11.0 ± 0.8 to 6.9 ± 0.5 nm (Figure 3f and Figure S8). With ζ and κ-1 are respectively adjusted to 20 mV and 1.7 nm, the EEGD between adjacent vertical Au NRs decreases to 6.9 ± 0.5 nm, which is approximately twice the length of the CTAB bilayer. This value may be the lower limit one can achieve using this self-assembly method. To obtain much smaller EEGDs, the CTAB molecules between the gaps should be cleaned and make the EEGD between the NRs further decrease. (Figure 1e,f).


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Figure 3. Theoretical and experimental precise control on EEDGs between the Au NRs in the vertically aligned Au NRMMAs. (a-d) Interaction nanoscale forces as a function of the EEGD in the cases: van der Waals force, FvdW (a), electrostatic repulsive force, Fele (b), depletion force, Fdep (c), and total interaction force, defined by FTotal = FvdW + Fdep + Fele (d), at different zeta potential (ζ) and Debye length (κ-1). In the calculations, l= 86 nm, r= 15 nm, A1 = 2A2 = 1×10-19J 40,

η=5×103, tTDBC = 3.2 nm, dTDBC = 5.8 nm. The inset in (a) is the configuration of two adjacent

Au NRs. (e) I: The measured zeta potential (ζ) as a function of the concentration of Cl-. II: The calculated Debye length (κ-1) as a function of the concentration of Cl-. (f) Theoretical predictions and (black circles) and experimental measurements (red dots) for the average EEGDs as functions of ζ and κ-1. The experimental results are in good agreement with the theoretically predictions. Manipulating on the far-field optical response of the vertically aligned Au NRMMAs. It is well known that metal materials participating light-matter interactions are fundamentally


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dominated by their plasmonic resonances corresponding to the collective oscillation of electron gas.50,51 Therefore, the manipulation of plasmonic modes plays a central role in light interacting with materials. To demonstrate the plasmonic mode tailoring of the Au NRMMAs with different average EEGDs, dark-field (DF) scattering measurements were performed. In measurements, the entrance slit of the spectrograph was set at 0.6 μm, which can effectively avoid the influence of strong edge-scattering from the samples (Figure S9). Figure 4a gives the scattering spectra of the Au NRMMAs with different average EEGDs. As revealed by the extinction spectrum of the colloidal Au NRs, the individual Au NRs exhibits two plasmon resonance modes (dashed gray curve in Figure 4a), with a low-energy mode (longitudinal surface plasmon resonance: LSPR) oscillating along the length axis of the NR, and a high-energy mode associated with excitation perpendicular to the short axis of the NR (transverse surface plasmon resonance: TSPR). As the average EEGD changes from ~10.8 to 1.3 nm, the LSPR band of the Au NRs in the arrays undergoes redshifts from ~750 to 926 nm. In the spectral region of 400-600 nm, the TSPR band at ~520 nm of the colloidal Au NRs is replaced by two plasmonic resonance modes (at ~435 and 610 nm) of Au NRs in the vertically aligned NRMMAs. These phenomena can be interpreted: as the distance between the adjacent Au NRs decreases, the plasmon supported by an individual NR strongly coupled with those of the neighboring nanorods and form collective plasmon modes when they are assembled closely in ordered VAGNRM arrays, leading to energy level broadening in their conduction bands and further resulting in an increasing redshift of the longitudinal mode resonance and the broadening of the corresponding spectral band.52 In addition, the two wavy plasmonic resonance modes at ~435 and 610 nm are originated from the coupling of the collective plasmon resonance mode of the NRMMAs with the Fabry–Pérot modes excited in the dielectric CTAB film deposited between the silicon substrate and the


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NRMMAs.53 Figure 4b-d demonstrate the representative DF images of the Au NRMMA samples with different average EEGDs of ~10.8, 7.4 and 1.3 nm, and their corresponding SEM images can be found in Figure 4e-g. The insets show the details of the locations in the arrays that the scattering spectra were collected, demonstrating a fine scaling of the average EEGDs in arrays. These scattering spectra reveal the far-filed optical responses of the vertically aligned Au NRMMAs with different average EEGDs.

Figure 4. Far-field optical response of the vertically aligned Au NRMMAs. (a) Doted gray line: Extinction spectrum of the purified colloidal Au NRs in aqueous solution. Solid lines: Scattering spectra of the vertically aligned Au NRMMAs with different average EEGDs of ~10.8, 8.6, 7.4, 3.2 and to 1.3 nm, respectively. (b-d) Representative dark-field (DF) images of the Au NRMMAs with different average EEGDs of ~10.8, 7.4 and 1.3 nm, respectively. (e-g) Representative SEM images of the Au NRMMAs shown in (b), (c) and (d), respectively. The insets are the SEM images higher magnification taken from the areas marked with dashed red square frames on the samples in (e-g), where the DF scattering spectra were collected. The scale bar in the insets is 100 nm.


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Manipulating on the near-field optical response of the vertically aligned Au NRMMAs. To demonstrate the near-filed optical properties of the Au NRMMAs, the scattering-type scanning near-field optical microscope (s-SNOM) was used to excite and image the sample with a ppolarized 633 nm excitation. The measurements were based on an atomic force microscope (AFM), as shown in Figure 5a. The typical topography images of the vertically aligned Au NRMMAs with the average EEGDs of ~7 and 2 nm are presented in Figure 5b,c, showing welldefined hexagonal lattice patterns. In measurements, a sharp AFM probe (curvature radius r0~15 nm) was employed as a nanoantenna to focus the free-space light into the gap between the tip and sample, the strongly confined light field thereafter provided sufficient momentum to launch the plasmons. Figure 5d,e demonstrates the real-space near-field optical images of the two vertically aligned Au NRMMAs with the average EEGDs of ~7 and 2 nm, respectively. From the near-field optical images, the regions with distinct optical amplitudes can be seen in the gaps between the adjacent Au NRs. These enhanced electric fields localized within the gaps forms lots of “hot spots”54,55, providing great facilities for the surface enhanced Raman scattering (SERS) detections and enhanced spontaneous emission.38,56-59. Figure 5f,g depicts the FDTD simulated electric field distributions for the unit cells of the Au NRMMAs with the average EEGD of 7 and 2 nm, respectively. To simulate the experimental setups, the angle of incidence was set at 70° (Figure S10). From the calculated results one can clearly see that the calculated electric field intensity in the gaps between the NRs is enhanced by ~1.5 times with the EEGD decreases from ~7 to 2 nm (Figure 5f,g and Figure S10), agreeing with optical near-field results for the two cases (Figure 5d,e). In addition, the local electric field enhancement factor (|E/E0|4) within the gaps can be found in Figure 5 h. It can be seen that when the EEGD was lowered to the subnanometre scale (0.5 nm) the maximum electric field enhancement factor can be up to ~2.5×106


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at the excitation of 633 nm and under a vertical incidence, giving rise to the important applications of SERS detection based on these arrays. It should be mentioned that, limited by the resolution of our s-SNOM system at present, much stronger electric field enhancement in smaller EEGD (< 2 nm) arrays cannot be characterized and have not been displayed in Figure 5.

Figure 5. Near-field optical response of the vertically aligned Au NRMMAs. (a) Schematic showing the s-SNOM measurements. (b,c) Atomic force microscope (AFM) topography images of the vertically aligned Au NRMMA samples with the average EEGDs of ~7 (b) and 2 nm (c), respectively. (d,e) Optical near-field amplitude (4th harmonics) at the excitation of 633 nm recorded in the same area as that in (b) and (c). (f, g) FDTD calculated electric field intensity contours of the unit cell in the Au NRMMAs with the EEGDs of 7 (f) and 2 nm (g), respectively. The contours are obtained on the cross section 5 nm below the top of the nanorod. The


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NRMMAs are excited by a p-polarized light at an incidence angle of 70°. The excitation wavelength is localized at 633 nm. (h) The maximum electric field enhancement factor (|E/E0|4) calculated as a function of the EEGD at the excitation of 633 nm. The angle of incidence was set at 0° (i.e. vertical incidence, see the inset). In calculations, dielectric with the refractive index of 1.4, a common vale for molecules, was filled in the gaps of the NRMMAs to simulate the crosslinked CTAB molecules. CONCLUSIONS In summary, we have developed a facile approach to fabricate vertically aligned Au NRMMAs with largely improved structural stability via evaporation-induced self-assembly and e-beam exposure. By theoretically analyzing, we find that the zeta-potential on the surface of Au NRs and the Debye length of the colloidal dispersion are two key factors determining the balance of nanoscale forces between the CTAB-coated Au NRs, which can be synchronously controlled by adding Cl- in Au NR colloid. In this way, the EEGD between the Au NRs can be precisely controlled from ~7 to 11 nm. In addition, the far- and near-field optical responses of the Au NRMMAs with different average EEGDs were also systematically investigated. It is revealed that such stable Au NRMMAs are ideal platforms for plasmonic resonance mode control, and will find their important roles in future biosensing, light field controlling, nanolasing, as well as quantum coherent manipulating. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications web.


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Detailed theoretical considerations of Fele, FvdW and Fdep, synthesis and purification of Au NRs, vertically aligned Au NRMMAs preparation, optical measurements of the far- and near-field response for the Au NRMMAs (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Prof. H. J. Chen and Dr. Z. B. Zheng for their assistance with the measurements of dark-field scattering and near-field optics. We thank the national supercomputer center in Guangzhou for helping with numerical calculations. This work was supported by the National Key R&D Programs of China (Grant No. 2016YFA0301300), the National Natural Science Foundations of China (Grant Nos. 11874438, 91750207 and 11761141015), the Natural Science Foundation of Guangdong (Grant No. 2018A030313722), the Fundamental Research Funds for the Central Universities (17lgpy22) and the project was supported by Open Fund of IPOC (BUPT) (Grant No. IPOC2018B007).


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Morphology characterization of the vertically aligned Au NRMMAs on silica substrate 197x147mm (300 x 300 DPI)



High stability of the vertical aligned Au NRMMAs. 202x101mm (300 x 300 DPI)


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Theoretical and experimental precise control on EEDGs between the Au NRs in the vertically aligned Au NRMMAs 203x120mm (300 x 300 DPI)



Far-field optical response of the vertically aligned Au NRMMAs 159x83mm (300 x 300 DPI)


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Near-field optical response of the vertically aligned Au NRMMAs 209x150mm (300 x 300 DPI)


Optical Response Properties of Stable and Controllable Au Nanorod

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