Facile Film-Nanoctahedron Assembly Route to Plasmonic

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Functional Nanostructured Materials (including low-D carbon)

A Facile Film-Nanoctahedron Assembly Route to Plasmonic Metamaterial Absorbers at Visible Frequencies Haibin Zhang, Chunlin Guan, Jun Luo, Yongtao Yuan, Ning Song, Yuanyuan Zhang, Jingzhong Fang, and Hong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01088 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Materials & Interfaces

A Facile Film-Nanoctahedron Assembly Route to Plasmonic Metamaterial Absorbers at Visible Frequencies Haibin Zhang,*,†,‡ Chunlin Guan,‡ Jun Luo,† Yongtao Yuan,§ Ning Song,§ Yuanyuan Zhang,§ Jingzhong Fang,† and Hong Liu*,†,‡ †

State Key Laboratory of Optical Technologies on Nano-Fabrication and Micro-Engineering, Chinese Academy of Sciences, Chengdu, 610209, China

‡

§

University of Chinese Academy of Sciences, Beijing, 100049, China Lightweight Optics and Advanced Materials Center, Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, 610209, China

*

Address correspondence to E-mail: [email protected], [email protected]

Abstract Plasmonic metamaterial absorbers (MAs) with broadband and near-perfect absorption properties in the visible region were successfully fabricated via a facile film-colloidal nanoparticle (NP) assembly method. In this approach, colloidal octahedral Au NPs were employed as the surface meta-atoms of MAs, while the nanoscale-thick SiO2 and Al films were used as the dielectric spacer and reflector respectively. It is worth noting that the Au nanoctahedron were randomly assembled onto the Al-SiO2 films, and no effort was made to precisely control their spatial arrangements. The optical characterization showed that the as-prepared MAs exhibited broadband high absorption (average absorptivity above 85%) within the whole visible spectrum for a broad range of incident angles (0°–60°). In particular, two polarization-independent near-perfect absorption peaks (absorptance above 99%) were recorded near 540 nm and 727 nm respectively. Moreover, the absorption properties of the MAs can be

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effectively controlled and tailored by varying the geometry (the thickness of the dielectric

spacer

and

the

surface

coverages

of

the

Au

nanoctahedron).

Electromagnetic simulations further demonstrated that enhanced Mie resonances and strong plasmonic coupling effects were critical for the designed MAs. This work here may provide an efficient and alternative route for the design of scalable visible light absorbers for applications such as solar cells, photothermalvoltaics, and biochemical sensors. Keywords:

metamaterial

absorbers,

self-assembly,

aperiodic

nanostructures,

broadband and near-perfect absorption, optical impedance matching

Introduction The efficient and tunable absorption of light through the design of light absorbers has attracted widespread interest in the recent study of functional optical materials.1-6 Among these materials, plasmonic metamaterial absorbers (MAs)7-10 based on artificial metal and dielectric elements are outstanding candidates for absorbing media since a high absorptivity (near-perfect absorption), thin thickness, light weight and tunable frequency can be easily achieved by structured design. Therefore, a large range of practical applications based on MAs, including solar-energy harvesting systems,11,12 thermophotovoltaic devices,13 photonic detectors14 and topological darkness15 have received increasing attention and have been identified. To date, MAs working in different spectral regions have been proposed and constructed using conventional top-down lithographic processes,16-20 which require precise integration of dissimilar metals and dielectrics. In these structures, spatial order or periodicity of the surface meta-atoms generally exists, leading to electromagnetic resonances and interference effects; then, a high light absorptivity can be achieved. Nonetheless, top-down methods are costly, and it is difficult to achieve mass production; moreover, the absorbance of MAs is usually limited to a narrow frequency band, reducing the practical applicability of MAs to some extent. In


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turn, there is a need for alternative, cost-effective methods to fabricate scalable MAs with excellent absorption properties for practical applications, especially for the recently focused MAs at visible frequencies.21 It is known that in order to achieve broadband high absorption within a visible MA, the scattering, transmittance and reflectance of the material system must be minimized. In this case, the scattering can be ignored once the surface roughness of the MAs is controlled to be far less than the optical wavelength.3 Meanwhile, the transmittance of MAs can also be essentially eliminated when metallic nanostructures with appropriate thickness are used. However, suppressing the reflectance is more difficult, since the reflection performance of MAs is determined by their inherent electric permittivity, ε, and magnetic permeability, μ.8 For general MAs, the reflection coefficients (considering the s polarization) have the form

𝑟s =

𝑧1cos (θi) ― 𝑧2(cos θt) 𝑧1(cos θi) + 𝑧2(cos θt)

(1)

where 𝑧1 = 𝜇1 𝜀1 is the effective impedance of the MA and 𝑧2 = 𝜇2 𝜀2 is the impedance of the media. θi and θt are the incident and transmitted angles of the light, respectively. When 𝑧1 = 𝑧2 (for normal incidence θi=θt=0°), the MAs are with impedance-matched structures,22,23 where their reflectances can indeed be eliminated. This is the common rule for designing a perfect MA in the visible region. From the above theory, many researchers have focused on combining multiple plasmonic resonances24 from specific metallic nanoparticles (NPs), nanogratings and films to enhance the total visible light absorption. M. K. Hedayati el al.25 developed a plasmonic MA with more than 90% absorption of light within the visible spectrum via a composited sputtering technique. In this manner, the Au/SiO2 nanocomposites were deposited on the nanoscale-thick SiO2 spacer and Au mirror, favoring high broadband absorption of light. Y. Zhang and coworkers26 designed a metamaterial perfect absorber (MPA) with randomly formed semispherical Au NP layer constructed on a ZnO/Ag di-layer structure through a film annealing method. Their systems exhibited



near-perfect absorption near λ=677 nm. More recently, A. Moreau et al.8 and J. Geldmeier et al.27 reported a peculiar class of plasmonic MAs with Au films or grating-coupled Ag nanocubes. In this case, Ag meta-atoms were assembled onto Au layers by creating polyelectrolyte spacer layers with a controllable thickness. Approximately 80% broadband absorption of visible light was observed in their systems. In addition, core-shell Au@SnSx and Au@ZnO NPs were also assembled to realize ultrathin light absorbers with approximately 85% broadband absorption in the range of 550-650 nm.28 From these studies, it can be found that the self-assembled MAs showed relatively preferable multiple plasmonic resonances since different shaped Au or Ag NPs can be employed to design meta-atoms of MAs. Meanwhile, various metal layers can couple with the meta-atoms using flexible dielectric spacers; thus, the enhanced broadband absorption can be finally designed and tailored. However, to date, the absorption efficiency of self-assembled MAs is widely low, and their working wavebands are limited. Therefore, the realization of visible MAs with excellent broadband high or even perfect absorption using current potential self-assembly routes remains a challenge. Herein, for the first time, we demonstrate a simple and effective bottom-up approach to fabricate a plasmonic MPA that operates in wide visible range through the assembly of synthesized octahedral Au NPs onto a nanoscale-thick spacer and reflector film. The employed octahedral Au NPs are randomly arranged with an approximately equal spacing, making no effort to precisely control their spatial orientations and arrays. It shows that our designed MAs with Au nanoctahedron layer/SiO2 spacer/Al reflector configuration provide particularly broadband and near-perfect absorption, which is mainly due to the enhanced Mie resonances of the neighboring octahedral Au NPs and the strong plasmonic coupling between the Au patterns and the Al film. Furthermore, the optical absorption properties of our MAs can be effectively tailored by varying the geometrical parameters (the thickness of the SiO2 layer, the surface density of the Au NP patterns and the other dielectrics with different refractive indices). In brief, this study presents a facile self-assembly route to design scalable plasmonic MAs with excellent absorption properties at visible


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frequencies; thus, the approach may facilitate practical applications such as solar cells, thermal emitters, photothermalvoltaics, and biochemical sensors.

Results and Discussions Structural and optical characterizations. Figure 1a shows a schematic illustration of the typical fabricated plasmonic MPA in this work. Chemically synthesized colloidal Au nanoctahedron were assembled on an Al film spaced by a SiO2 dielectric layer. Figure 1b is a cross-sectional view of the real construction. Clearly, it can be seen that the sputtering-deposited Al-SiO2 films are both dense and continuous, with thicknesses of approximately 80 nm and 20 nm respectively. The Al film is opaque and can effectively eliminate the transmittance and reflect most of the incident visible-light. The top layer consists of numerous self-assembled octahedral Au NPs with a nearly identical height (~ 75 nm); in this case, the measured average root mean square (RMS) surface roughness of our MPA is ~ 0.4 nm (as shown in the inset of Figure 2), which is far less than the optical wavelength. That is, the scattering of the material system can be ignored. Plan view images of the top Au nanoctahedron layer (see Figure 1c and the inset) further illustrate that the self-assembled Au patterns with an average size of 82 nm are well dispersed and form a large-scale dense packing structure. Additionally, two neighboring Au nanoctahedron are approximately equally spaced (the average interparticle spacing is approximately 73 nm), whereas their spatial orientations and arrays are somewhat unorganized (see Figure 1d). In fact, the colloidal Au nanoctahedron arrange on the Al-SiO2 films with a coverage density of approximately 48 particles per µm2, which equates to a filling fraction of ~ 25%; thus, the MPA presents a characteristic brownish black color (as shown in the inset of Figure 1d). For the self-assembly mechanism of our MPA, we think that large amounts of synthesized Au nanoctahedron randomly assembled onto the Al-SiO2 nanofilms because of electrostatic absorption driving. It is known that Si-OH groups with negative charge are widely distributed on the surface of a SiO2 layer as the plasma sputtering process of SiO2 is conducted. On the other hand, the synthesized



Au nanoctahedron were coated by a layer of poly(diallyldimethylammonium)chloride (PDDA) molecular linkers, resulting in the positively charged surface of the Au nanoctahedron. Once the substrates with Al-SiO2 films dipped in the ethanol mixed water solution dispersed with the synthesized Au nanoctahedron, the designed self-assembly began, and the desired MPA could be controlled by adjusting the parameters, such as the duration time of the self-assembly and the concentration of the colloidal Au nanoctahedron. Figure 2 displays the experimental and simulated absorption/reflection spectra of our MPA. The average absorptivity of the material system is approximately 90% for the whole visible region at a normal incidence of 8°. Meanwhile, the corresponding average reflectivity (including the specular reflection, diffuse reflection and scattering) of the structure is nearly 10%. Interestingly, two significant near-perfect absorption peaks (the absorptance reached above 99%) were recorded near 540 nm and 727 nm respectively. In this case, the first plasmon resonance peak presents prominent broadening in contrast to the second peak. Two near-zero reflection peaks are consistent with the trend of the absorption, which is attributed to the effective impedance matching of the MA with the ambient media (i.e., air). Indeed, the enhanced localized plasmon resonances (Mie resonances)29,30 and the strong plasmonic coupling31 are critical for the broadband high absorption, which will be discussed later. Whether using an individual Au nanoctahedron layer or Al-SiO2 films, it cannot produce the particular plasmon resonances and coupling (see Figure S1, Supporting Information). In addition, the simulated absorption and reflection spectra exhibit good agreement with the experimental results, although a small redshift exists from the amorphous distribution of the separation between the nearest-neighbor Au nanoctahedron. However Figure S3 presents a distinct simulated result as spherical Au NPs with similar sizes (radius of 58 nm) and the same spatial arrangements substituting the Au nanoctahedron, owing to the different plasmonic resonance absorption behaviors determined by the construction of the meta-atoms. In our typical system, the impedance of air is 𝑧2 = 𝜇0 𝜀0 = 1, the effective impedance


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of the MPA (according to a transmission line model32) can be expressed as

𝑧1 = 𝑖𝑧Au

𝑧SiO2tan (𝑘SiO2𝑑) + 𝑧Autan (𝑘Auℎ) 𝑧Au ― 𝑧SiO2tan (𝑘SiO2𝑑)tan (𝑘Auℎ)

(2)

where 𝑧Au = 𝜇Au 𝜀Au and 𝑧SiO2 = 𝜇SiO2 𝜀SiO2 are the characteristic impedances of the SiO2 spacer and the layer of Au NPs, respectively. 𝑘SiO2 and 𝑘Au are the wave vectors of the two layers, respective, and d and h are their mean heights. At near-perfect absorption wavelengths of 540 nm and 727 nm, the individual effective impedances of our MPA calculated by Eq. (2) are 𝑧1’ = 0.997 + 0.029𝑖 and 𝑧1’’ = 0.992 ― 0.054𝑖, respectively (see Figure S4), indicating that the conditions of impedance matching are properly satisfied. To further investigate the plasmonic modes of our MPA, a simulation of the E-field densities around the Au nanoctahedron at the plasmon resonance wavelengths was performed. Figures 3a and b first show the simulated E-field strength at the resonance peak positions (λ=540 nm and λ=727 nm, respectively) in the X-Z plane. It is clear that an obvious electric field enhancement is observed between the adjacent Au nanoctahedron for the 540 nm resonance wavelength (Figure 3a) when the interparticle spacing is 73 nm. The hot spots mainly occur in the gap regions. That is, Mie resonances of neighboring Au patterns with amorphous orientations are enhanced due to the plasmonic coupling of the closely spaced Au nanoctahedron and the interferometric effect of the SiO2 layer,26,33 resulting in broadband near-perfect absorption near 540 nm. In addition, relatively larger enhancement areas were produced for the 727 nm resonance wavelength, since the hot spots even occurred across the SiO2 layer beside the gap regions (Figure 3b). In this regard, the plasmonic coupling between the Au nanoctahedron and the Al film played a major role in addition to the gap plasmon resonance. The small and appropriate spacing between the Au patterns and metal Al promoted strong plasmonic coupling;34 thus, the longwave plasmon resonance of the MAs was further enhanced, and another near-perfect absorption was finally obtained. The E-field distributions in the X-Y



plane (Figures 3c and d) show excellent agreement with those in the X-Z plane. Therefore, it can be deduced that the broadband and near-perfect absorption of our MPA is mainly from the synergistic effect of enhanced Mie resonances of neighboring Au nanoctahedron with unorganized spatial arrangements and the further enhanced plasmonic coupling between the Au patterns and the metal Al.

Polarization dependent light absorption. In this section, we investigate the influence of different polarizations on the absorption properties of our MPA. Figure 4a reveals the transverse electric (TE) polarization absorption spectra at angles ranging from 0° to 60°. One can find that the light absorption of the MPA does not change much as the incident angle increases. The two characteristic plasmon resonances (the absorption peak positions and shapes) are the same as in Figure 2, except for a slowly decreased absorption intensity. Interestingly, all of the TE polarization absorption spectra achieved a high average absorptivity above 85%. This result confirms that the broadband high absorption of our MPA is rather insensitive to the TE polarization with different angles of incidence due to the octahedral symmetry of the Au meta-atoms. Here, we believe that similar spatial arrangements of the Au meta-atoms were presented under different incident angles, although the employed Au nanoctahedron were randomly assembled onto the Al-SiO2 films. In contrast to the TE polarization results, the transverse magnetic (TM) polarization absorption shows better stability (Figure 4b) with increasing incident angles. The plasmon resonance peaks similarly appear near 540 nm and 727 nm. The corresponding absorption spectral patterns are consistent with the result under normal incidence. Notably, a smaller decreasing trend of the absorption coefficients are observed from 400 nm to 657 nm, and a slow increase in the absorption (absorptivity from 98.2% to 99.8%) was recorded near the 727 nm resonance peak. In this case, the E-field component perpendicular to the plane of incidence became much larger than the parallel component at longwave plasmon resonance when high incidence angles were employed, which enhanced the light absorption of the TM polarization. Consequently, an average absorption intensity above 87% was realized, suggesting that our designed


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MPA is also insensitive to the TM polarization. Variation in the thickness of the SiO2 spacer. In Figure 5, we show how the thickness of the SiO2 spacer affects the position and intensity of the resonance absorption peaks of the designed plasmonic MAs. When the thickness increased from 10 nm to 90 nm, the first resonance absorption peaks exhibited a subtle change. All of the peak positions stablized near 540 nm, whereas the absorption intensities increased first and then decreased, reaching a maximum when the thickness reached approximately 20 nm. In addition, there is a consistent trend in the peak widths as the thickness increases. In such case, the shortwave Mie resonances cannot be enhanced well for extremely thin or larger SiO2, because a relatively poor interference effect results from the inappropriate thickness of the spacer layer. A distinctly different change occurs for the second absorption peaks. Clearly, obvious blueshift of the peak positions from 737 nm to 646 nm was produced, and similar trends in the variations in the peak intensities and widths (as shown with the 540 nm resonance peaks) were obtained again. Indeed, 20 nm of the SiO2 spacer is critical, at which the absorption intensity and width are maximized. In addition, the absorption reaches over 99% as the spacer thickness is changed from 20 nm to 40 nm, demonstrating that the longwave perfect absorption does not change much with a variation in the spacer thickness. However the changes in the peak positions, intensities and widths are mainly due to the specific plasmonic coupling between the Au nanoctahedron and the metal Al, which depends on the interlayer thickness. Based on these results, we speculate that the small thickness of the SiO2 spacer in the range of 20-40 nm plays a critical role during the fabrication of these outstanding plasmonic MPAs. A SiO2 spacer thickness that is too small or too large is undesirable for the enhancements of the Mie resonances and plasmonic coupling, which is indispensable for the broadband and near-perfect absorption of the MAs. Impact of the Au nanoctahedron surface coverage. To study the resonance absorption behaviors influenced by the top Au nanoctahedron layer, the Au



nanoctahedron surface coverage was varied. For this purpose, self-assembly experiments were conducted by elaborately adjusting the concentration of the octahedral Au colloids in an ethanol mixed water solution and the time that the Al-SiO2 surface was exposed to the solution. Figure S5 shows surface coverages of 10%, 17%, 22%, 25%, 30% and 36% as we dipped the Al-SiO2 sample wafers in a 10 mM Au nanoctahedron solution for more than 18 h. The average interparticle distances of the Au nanoctahedron are determined to be 185 nm, 110 nm, 86 nm, 73 nm, 62 nm and 50 nm, respectively (obtained using Image Metrology SPIP software). Figure 6 shows the corresponding absorption spectra. It can be observed that by changing the surface coverage of the Au nanoctahedron from its optimum value (25%), the two characteristic resonance peaks are shifted with decreasing absorption intensities and widths. Specifically, high surface densities (more than 25%) shift the first resonance peak to the short-wavelength side, while low surface densities ( less than 25%) result in a redshift of the resonance. Here, the samples with surface coverages of 22% and 30% both display near-perfect absorption, yet their working bandwidth are narrow. On the other hand, all of the second resonance peaks show a significant blueshift compared with the optimum value. Near-perfect absorption behavior can also be observed with 22% to 30% surface coverage of Au nanoctahedron. Other surface coverages produce a damping absorptivity in agreement with the first plasmon resonance. In short, the absorption properties of our designed MAs are sensitive to the surface coverage of the colloidal Au nanoctahedron. From 22% to 30%, the surface densities of the Au nanoctahedron represent an alternative way in which the desired near-perfect absorption behaviors can be controlled and tailored in this range. This is attributed to the strong plasmonic coupling between the adjacent Au nanoctahedron with an appropriate interparticle spacing. A lower surface coverage means relatively poor plasmonic coupling, resulting in weak resonance absorption (reduced absorption intensity/width and a shifted resonance peak). A higher surface coverage causes intense reflectivity of the Au nanoctahedron layer, which is accompanied by a decrease in the absorption. Figure S6 reveals the simulated spectrum of a fully covered Au layer on an Al-SiO2 film. The enhanced reflectivity


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indeed leads to a remarkable reduction in the light absorption. Furthermore, we conducted simulation experiments varying the SiO2 spacer thickness and the Au nanoctahedron surface coverage to demonstrate an optimized design of the MPA (see Figure S7). The results show that there are no near-perfect absorption bands in the visible region with a 10 nm thickness of SiO2 (Figure S7a), although the average absorption efficiency can reach approximately 92% when the MAs are designed with a surface coverage from 25% to 36% in the short-wavelength range (from 400 to 600 nm). This can be attributed to the relatively poor interference effect and the impedance mismatch mainly caused by a thin dielectric layer. Interestingly, another two near-perfect absorption bands appear near 500 nm and 700 nm for a 40 nm thickness of SiO2 with a 22% to 30% surface coverage of Au nanoctahedron (Figure S7b), whereas the average absorption efficiency is approximately 10% less than the previous results (as shown in Figure 6) due to the relatively narrow absorption bands from the limited impedance matching and the unfavorable interference effect. However for the larger SiO2 spacer (more than 40 nm in thickness), the obtained MAs show poor absorption behavior as we vary the surface coverage of the Au nanoctahedron (shown in Figures S7c and d). Whether the thickness of the SiO2 spacer is 60 nm or 90 nm, there is no broadband and near-perfect absorption recorded in the visible spectrum. Here, we can infer that the relatively poor interference effect and the impedance mismatch again result from the inappropriate larger thicknesses of the spacer layer. According to the above results, the optimized impedance (see Figure S8) and interference effect occur for our optimized MPA with a SiO2 layer thickness of 20 nm, where the Mie resonances and plasmonic coupling effects are optimized in contrast to other SiO2 thicknesses. Effects of other dielectrics. In this part, the effects of different dielectrics on the resulting absorption are more closely explored using MgF2 and ZnO while keeping the other parameters constant. As shown in Figure 7, when using MgF2 (refractive index of =1.38) instead of SiO2 (refractive index of =1.5), the resonance absorption of the system is drastically blue-shifted with decreasing intensity. This can be attributed



to the reduced optical path difference and the disturbed impedance matching condition from the material system with a lower refractive index dielectric.25,35 In addition, one can see that the absorption shows a characteristic change when high-refractive-index ZnO (refractive index=2) is employed. With the damped intensity, the dominated absorption band is narrow, which we attribute to the increased optical path difference and the similar impedance mismatch within a higher dielectric constant environment.36 In addition, further experiments (on the impedance and the corresponding absorption behaviors influenced by the different thicknesses of SiO2, MgF2 and ZnO dielectric spacers) are performed to better demonstrate the optimized impedance matching condition and the optimized broadband and near-perfect absorption of our MAs. Indeed, Figure S8 illustrates that the effective waveband and values of the impedance (the real part of the impedance is close to 1, the imaginary part of the impedance is close to 0) are optimized for a SiO2 layer thickness of 20 nm. The impedance of other SiO2 thicknesses are partly satisfied or unsatisfied. Moreover, the results (Figures S9a and c) show that the impedances are both mismatched in the whole visible region when different thicknesses of the MgF2 or ZnO layer are used for designing the MAs. No broadband or near-perfect absorption is recorded in the visible region (Figures S9b and d). Although dual- or multiband absorption was presented with the variation in the dielectric thickness, all of the average absorption efficiencies were below 75%. Here we may infer that the MgF2 and ZnO dielectrics with different refractive indices (determined by ε and µ) produced changes in the optical path differences and the impedance conditions. Thus, a typical MPA with optimized impedance matching cannot be achieved by the other dielectrics. That is, dielectric SiO2 is absolutely necessary to fabricate the desired MPA with broadband and near-perfect absorption. Other dielectrics with a lower or higher refractive index will change the optimum condition of the Au nanoctahedron/dielectric/Al stacked structures for light absorption.


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Conclusion In summary, this study highlights the ability of a new and facile film-nanoctahedron assembly route for the design of plasmonic MAs with broadband high absorption in the visible region. This fabrication technique has the advantages of a cost-effective manufacturing process, strong operability and simple realization of large-scale production. The experimental results showed that an average absorptivity of visible light above 85% was obtained in our Au nanoctahedron/SiO2/Al stacked MPAs. In particular, there were two significant near-perfect absorption peaks near 540 nm and 727 nm, which are caused by the enhanced Mie resonances of the adjacent Au nanoctahedron and the strong plasmonic coupling between the Au patterns and the metal Al reflector. In addition, it was demonstrated that the characteristic resonance absorption of our MPA was insensitive to the polarization and the angle of incidence. Different Au nanoctahedron surface coverages and dielectric spacer layers with controllable thicknesses can effectively tailor and change the resonance absorption behaviors. Indeed, slight changes in the ideal conditions for creating the desired MPA will cause the shift of absorption peaks and a variation in the absorption intensities. We therefore believe that the work presented here may provide an effective and alternative strategy for the fabrication of visible MAs for applications such as solar cells, thermal emitters, and biochemical sensors.

Methods Typical fabrication of the plasmonic MPA. First, Al-SiO2 films were deposited in turn using vacuum magnetron sputtering (NSC-4000) onto Si or glass substrates with thicknesses of 80 nm and 20 nm, respectively. The vacuum pressure of the sputtering chamber was below 2.0×10-4 Pa, a DC magnetron sputter source was used for Al and an RF source was used for SiO2 during the ﬁlm deposition. Second, the substrates with Al-SiO2 film coatings were quickly dipped in an ethanol mixed water solution (volume ratio of 1:7) dispersed with synthesized octahedral Au NPs (concentration of 10 mM) to fabricate the top Au NP layer; in this case, large amounts



of Au nanoctahedron would randomly adhere onto the SiO2 nanofilm under electrostatic self-assembly driving. As a result, the plasmonic MPA with unorganized Au nanoctahedron arrays/SiO2 spacer/aluminum reflector was finally obtained. The coverage density of the Au nanoctahedron was controlled by varying the time (from 18 h to 72 h) over which the Al-SiO2 surface was exposed to the Au colloidal solution. Different thicknesses of the SiO2 spacer (from 10 nm to 90 nm) and other dielectrics (MgF2 and ZnO) with different refractive indices were also used to explore the MA structures and their optical absorption. Au nanoctahedron synthesis. Au nanoctahedron were synthesized similarly to our previous report.37 Briefly, 5 mL of 20 mM HAuCl4 solution in ethylene glycol was injected into a mixture solution containing 4 mL of PDDA and 80 mL of 1,5-pentanediol under constant magnetic stirring. Then, the resulting precursor was sealed and refluxed at 240 °C for 2 h in an oil bath. A series of color changes were observed before a reddish brown solution was produced. The final Au nanoctahedron were collected by repeated washing (using absolute ethanol and deionized water) and centrifugation (10000 rpm, 30 min). Afterwards, they were redispersed in an ethanol mixed with deionized water solution for the self-assembly process. Characterization. The thickness of the SiO2 and Al films and the morphology and the surface density of the Au nanoctahedron were investigated using scanning electron microscopy (SEM, ZEISS ULTRA PLUS-43-12 microscope, operated at an acceleration voltage of 15 kV). UV-Vis-NIR absorption/reflection spectra measurements were conducted using a spectrophotometer (Lambda 1050, Perkin Elmer) with a 150 mm integrating sphere. The polarization dependent absorption properties were measured with an ellipsometer (SENTECH-SE850). The surface roughness of the MAs were evaluated by a scanning white light interferometer (Bruker, NP FIEX). Simulations. The simulations of the structure and the electric field density of the MAs were performed using a commercial finite-difference time-domain software package (CST MicroWave Studio, 2006). The dielectric functions of Al, SiO2 and Au were taken from the literature.38,39 A simplified heptamer model was employed as the


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repeating unit, and periodic boundaries of the x-, y- and z-directions were used for the repeating unit. The simulated extra electromagnetic field was at normal incidence with an 8° orientation angle. The gap distance of the neighboring Au nanoctahedron was set to 73 nm, and the grid size in the simulation was set to 7.5 nm.

Supporting Information Measured absorption spectra of the self-assembled Au nanoctahedron layer and the individual sputtering-deposited Al-SiO2 films, simplified heptamer repeating unit model, simulated absorption and reflection results of the spherical Au NPs assembled on the Al-SiO2 films, the fully covered Au nanoctahedron layer on the Al-SiO2 films and the designed MAs when the SiO2 spacer thickness varied with the changes in the surface coverage, calculated impedance data of the typical MPA and the contrast of MAs with different thicknesses of MgF2/ZnO dielectrics, and SEM images of different surface coverages of Au nanoctahedron. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author: E-mail：[email protected] E-mail：[email protected]

Conflict of Interest The authors declare no conflict of interest.

Acknowledgements The authors would like to thank Prof. Xiangang Luo and Prof. Mingbo Pu (University of Chinese Academy of Sciences) for helpful discussion and technical supporting during this work. This research was supported by the National Natural Science Foundation of China (grant no. 61805250) and the Youth Innovation Promotion Association of CAS.



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Figure Captions Figure 1. (a) Schematic geometry of a typical fabricated plasmonic MPA with a Au nanoctahedron/SiO2/Al stacked configuration. The average edge length of the Au nanoctahedron is 82 nm; the thicknesses of the SiO2 spacer and Al reflector are 20 nm and 80 nm, respectively. (b) Cross-sectional SEM view of Au nanoctahedron assembled on Al-SiO2 films. (c,d) Plan view SEM images of randomly assembled Au nanoctahedron surfaces. The inset of (c) is the size distribution of the synthesized colloidal Au nanoctahedron. The inset of (d) is a photograph of the typical MPA. Figure 2. Experimental absorption (red solid line) and reflection (black solid line) spectra of the Au nanoctahedron with a surface coverage of 25% assembled on an 80 nm Al film and a 20 nm SiO2 spacer. The measured spectra are for normal incidence with an 8° orientation angle. The simulated absorption (blue dashed line) and reflection (green dashed line) spectra for the typical plasmonic MPA are recorded. The interparticle spacing of the neighboring Au nanoctahedron was set to 73 nm, and a simplified heptamer repeating unit model was constructed according to Figure 1d (see Figure S2). The inset shows the measured average surface roughness of our MPA. Figure 3. Cross-sectional view (a,b) and top view (c,d) of the calculated E-field densities at the resonance peak positions shown in Figure 2 at λ=540 nm and λ=727 nm, respectively. The gap distance for both is 73 nm. Figure 4. Absorption spectra of the typical fabricated plasmonic MPA with TE polarization (a) and TM polarization (b) at different angles of incidence.



Figure 5. Absorption spectra of the designed plasmonic MAs consisting of Au nanoctahedron with a surface coverage of 25% assembled on an 80 nm Al film with different thicknesses of the SiO2 spacer. The spectra are for normal incidence with an 8° orientation angle. Figure 6. Absorption spectra of the colloidal Au nanoctahedron with different surface coverages arranged on an 80 nm Al film with a 20 nm SiO2 spacer layer. The measured spectra are at an 8° angle of normal incidence. Figure 7. Absorption spectra of the plasmonic MAs consisting of Au nanoctahedron with a surface coverage of 25% arranged on an 80 nm Al film spaced by a 20 nm dielectric layer of SiO2 (red solid line), MgF2 (black solid line) and ZnO (light gray solid line) at an 8° angle of normal incidence.

Figure 1.


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Figure 2.

Figure 3.



Figure 4.


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Figure 5.

Figure 6.



Figure 7.


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TOC


Facile Film-Nanoctahedron Assembly Route to Plasmonic

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