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Nanoporous Gold Nanoparticles and Au/Al2O3 Hybrid Nanoparticles with Large Tunability of Plasmonic Properties Wenye Rao, Dong Wang, Thomas Kups, Eszter Baradács, Bence Parditka, Zoltán Erdélyi, and Peter Schaaf ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13602 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 2, 2017

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ACS Applied Materials & Interfaces

Nanoporous Gold Nanoparticles and Au/Al2O3 Hybrid Nanoparticles with Large Tunability of Plasmonic Properties Wenye Rao†, Dong Wang†*, Thomas Kups†, Eszter Baradács‡, Bence Parditka‡, Zoltán Erdélyi‡, and Peter Schaaf† †

Group Materials for Electronics, Institute of Materials Engineering and Institute of Micro- and

Nanotechnologies MacroNano®, Technische Universität Ilmenau, Gustav-Kirchhoff-Str. 5, 98693 Ilmenau, Germany ‡

Department of Solid State Physics, University of Debrecen, P.O. Box 2, H-4010 Debrecen,

Hungary

* Corresponding author: [email protected]

ABSTRACT Nanoporous gold nanoparticles (NPG-NPs) with controlled particle size and pore size are fabricated via a combination of solid-state dewetting and a subsequent dealloying process. Due to the combined effects of size and porosity, the NPG-NPs exhibit greater plasmonic tunability and significantly higher local field enhancement as compared to solid NPs. The effects of the nanoscale porosity and pore size on the optical extinction are investigated for the NPG-NPs with different particle sizes experimentally and theoretically. The influences of both porosity and pore size on the plasmonic properties are very complicated and clearly different for small particles with dominated dipole mode and large particles with dominated quadrupole mode. Au/Al2O3 hybrid porous NPs with controlled porosity and composition ratio are fabricated through plasma enhanced atomic layer deposition of Al2O3 into the porous structure. In the Au/Al2O3 hybrid porous NPs, both Au and Al2O3 components are bi-continuously percolated over the entire structure. A further red-shift of the plasmon peak is observed in the hybrid NPs due to the change of the environmental refractive index. The high tunability of the plasmonic resonances in the NPG-NPs and the hybrid porous NPs can be very useful for many applications in sensing biological and organic molecules.

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Keywords: nanoporous gold, nanoparticle, plasmon resonance, hybrid, nanophotonics 1. Introduction Small metal particles exhibit complex optical and physical properties. The most striking phenomenon encountered in metal nanoparticles (NPs) is electromagnetic resonance. Noble metal NPs have attracted an increased attention due to their effect of the localized surface plasmon resonances (LSPRs) which originate from collective oscillations of the conduction band electrons. LSPRs induce a strong interaction with light, and the wavelength at which this resonance occurs depends on local dielectric environment, size, shape, and composition of the NPs.1-4 In another side, due to the extraordinary properties, such as low density, large surface area, excellent electric and thermal conductivities, and high surface-to-volume ratios,5-7 nanoporous gold (NPG) as a nanostructured semi-infinite thin film material has attracted intense research interest for the application in catalysis, energy storage, sensing, actuation, and biotechnology.8-19 Furthermore, the optical properties of NPG are quite interesting, and NPG exhibits both propagating surface plasmon resonance (PSPR) along the metal film surface and LSPR within its interconnected nanoporous framework.20 Currently, nanoporous metals can be fabricated by using various methods including wet-chemistry,21 dealloying,22-24 galvanic replacement,25-28 and dual-templating.29-31 Using these techniques, many different shapes of NPG have been fabricated and investigated, including nanoporous gold disks,32-35 nanoporous gold nanowires,36-40 and nanoporous gold nanoparticles (NPG-NPs).21, 41-47 Three-dimensional (3D) NPG-NPs have triggered tremendous research interest because their optical properties can be influenced by both porous structure and the confinement of the limited particle size. Recently, Zhang and coworkers have studied NPG-NPs which combine highly tunable plasmon resonances and intense local electric field enhancements and thus are exploitable for single-particle surfaceenhanced Raman spectroscopy (SERS).21 The plasmonic properties of the individual NPG-NPs have been studied by single particle spectroscopy, and a large red-shift of the LSPR even in the near infrared (NIR) region and a strong polarization dependence of the scattering spectra have been observed.47 All this means that an additional influence factor, porosity or porous configuration, has been introduced for the LSPRs of the NPs. The controllable and tunable fabrication of NPG-NPs is desired for these applications, which benefit from large specific surface area, catalytic function and plasmonic properties. Therefore, quantitative information on


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the influences of the porous structure of NPG-NPs becomes essential for further optimizing their optical properties for the specific applications. In addition, other materials can be filled inside of the NPG to obtain the hybrid nanomaterials with modified properties. For instances, Al2O3 and TiO2 thin layers have been deposited into the NPG via atomic layer deposition (ALD), and the thermal stability and mechanical properties of the NPG have been dramatically improved.48, 49 Porous Au-Ag nanospheres were prepared for SERS analysis with high density and highly accessible hotspots.50 Porous core-shell nanostructures consisting of gold skeletons and silver shells were fabricated by controllable electroless plating of Ag into the NPG films, showing a considerable improvement in SERS.51 Holey Au-Ag alloy nanoplates possess enormous internal hotspots for high sensitivity in the SERS analysis.52 Ag-Au hybrid nanoporous NPs with distinct plasmonic properties were obtained by cyclic electroless deposition of Ag into the NPG-NPs.53 In this work, NPG-NPs with tunable particle size and pore size are fabricated, and the influences of the particle size, porosity and pore size on the plasmon resonances are studied experimentally and theoretically by finite-different time domain (FDTD) method. The NPG-NPs exhibit very large tunability of plasmonic properties over a much broader spectral range with significantly intensified near-field enhancements in comparison to the solid NPs. It is found that nanoscale porosity and pore size of the NPG-NPs have profound influences on both the far- and near-field optical properties of the particles. When nanoscale porosity was introduced into the gold NPs, the higher-order mode (quadrupole) was significantly dampened, whereas the dipole plasmon mode remained robust. A clear red-shift of plasmon resonances with increasing porosity is observed. However, the influences of the porosity are clearly different for the particles with different particle sizes. Furthermore, there is a slight blue-shift of plasmon resonances with increasing pore size, and a correlation between pore size effect and particle size effect is also noticed. Furthermore, Au/Al2O3 hybrid porous NPs are fabricated and there is a clear red-shift of the main plasmon resonance due to presence of the high refractive index Al2O3. The intensity of the 2nd peak of plasmon resonance located in small wavelength is clearly improved by the Al2O3. Moreover, it is usually observed that these hybrid nanostructures even possess novel properties, which are distinct from that of the single-component NPs. Therefore, hybrid nanomaterials with tailored structures are expected further to improve the tunability and versatility of the plasmonic properties.



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2. Experimental Section 2.1 Preparation of the nanoporous gold nanoparticles (NPG-NPs) NPG-NPs were fabricated via a combination of solid state dewetting of Ag/Au bi-layers and a subsequent dealloying. The Au/Ag bi-layers were deposited on fused silica by using electron beam evaporation, and then annealed at 900 °C in Ar for 15 min to induce the dewetting. Ag-Au alloy NPs were formed after dewetting. The samples were then submerged in HNO3 aqueous solutions for dealloying. By dealloying, the Ag was removed from the Ag-Au alloy NPs and NPG-NPs were formed. For the investigation of the correlation between the particles size effect and porosity effect on the plasmonic properties, 4 types of Ag/Au bi-layers with same thickness ratio or atomic ratio but different total thicknesses (4 nm Ag/2 nm Au, 8 nm Ag/4nm Au, 12 nm Ag/6 nm Au and 20 nm Ag/10 nm Au) were deposited for preparing the NPG-NPs. The dealloying process was performed with a 65 wt% HNO3 aqueous solution at 21 °C for 5 min. For the investigation of the pore size effect on the plasmonic properties, the bi-layers of 20 nm Ag/8 nm Au were used. The Ag-Au alloy NPs were obtained after the dewetting induced at 900 °C in Ar for 15 min. Then, NPG-NPs with different pore size were obtained by dealloying on following different conditions, respectively: (a) in a diluted HNO3 aqueous solution (volume ratio between 65 wt% HNO3 and H2O is 1:2) at 21 °C for 5 min, (b) in a 65 wt% HNO3 aqueous solution at 21 °C for 5 min, and (c) in a diluted HNO3 aqueous solution (volume ratio between 65 wt% HNO3 and H2O is 1:2) at 60 °C for 5 min. NPG-NPs prepared with 20 nm Au/8 nm Ag bi-layers were used as templates, then 5 nm and 10 nm thick Al2O3 layers were deposited with plasma- enhanced atomic layer deposition (ALD, Beneq TFS 200). The Al2O3 layers were well homogeneously deposited inside of the porous structure, and the Au/Al2O3 Hybrid Nanoparticles were obtained. Trimethyl aluminium (TMA) and oxygen plasma were used as the precursors, and the deposition was performed at 150 °C. The ALD growth cycle consisted of the following steps: 450 ms TMA dose at 1 mbar, 3 s of nitrogen purge, 6 s of oxygen plasma at 50 W, 3 s of nitrogen purge. The growth rate of the layers was 1.43 Å/cycle. 2.2 Experimental characterization


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The NPs were investigated by using scanning electron microscope (SEM, Hitachi S-4800). The particle size (diameter, D) was determined by thresholding the image contrast in the SEM images and performing a pixel counting. The mean ligament size (diameter), , and pore size (pore channel diameter), , of every type of NPG-NPs were determined by measuring a minimum of 20 ligaments and pore channels in the SEM images and averaging. Transmission electron microscopy (TEM, FEI Tecnai 20) was used to investigate the interior cross sectional structure. Selected area electron diffraction (SAED) has been recorded and the energy-dispersive X-ray spectroscopy (EDS, Thermo Scientific) was used to investigate the chemical composition. The TEM sample was prepared by milling with focused ion beam (FIB, Carl Zeiss Auriga 60 crossbeam). The optical extinction of the NPG-NPs on fused silica (1.0 × 1.0 cm2) was measured by a transmission accessory of an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrometer (Cary 5000 UV-Vis-NIR).

2.3 Numerical modeling Finite-difference time-domain (FDTD) simulations were performed to calculate the extinction spectra and the near-field enhancement by using the commercial software FDTD Solutions (version 8.12.501, Lumerical Solutions Inc.). Due to the complexity of the porous structure, it is very difficult to build a model with the exact same percolated porous structure. Probably 3D reconstruction with FIB milling could be useful but it is a time-consuming and challenging task. Here a simplified model is used. The model of the NPG-NPs was constructed with a large gold sphere etched by N randomly distributed spherical air pores. The pore size of the air pores corresponded to the pore size in the NPG-NPs. A total volume overlap of the small air pores was assumed as about 20%. This allows for the formation of the bi-continuous network throughout the nanoparticles. The volume porosity of the NPG-NPs was varied by changing the number of air pores. The permittivity of gold from Johnson and Christy54 was used and the surrounding medium of the model structure was set to be air. The Total Field/Scattered Field (TFSF) source was applied.

3. Results and Discussion



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3.1 Investigation of the influences of the porosity on the plasmonic properties NPG-NPs were fabricated via a combination of solid-state dewetting of Ag/Au bi-layers and a subsequent dealloying.41-43 Ag-Au alloy NPs were obtained by the solid-state dewetting of the Ag/Au bi-layers, and then Ag were removed and NPG-NPs were formed during the dealloying process. 4 types of Ag/Au bi-layers with same thickness ratio (or atomic ratio) but different total thicknesses of bi-layers (4 nm Ag/2 nm Au, 8 nm Ag/4nm Au, 12 nm Ag/6 nm Au and 20 nm Ag/10 nm Au) were deposited for preparing the NPG-NPs. The same thickness ratio was kept to assure that the similar ligament and pore sizes (: 12.2 ~ 18.3 nm; : 7.9 ~ 12.4 nm) and porosity (η = 66%) can be obtained under the same dealloying conditions. The particle size is increased with increasing total thickness. Figure 1 shows the representative scanning electron microscopy (SEM) images of the formed NPG-NPs from the 4 types of Ag/Au bi-layers. The measured particle size distributions and the characteristic particle spacing for these samples are shown in Figure S1 (Supporting Information). The mean particle diameters (D) of the formed NPG-NPs are 54, 130, 233, and 393 nm, respectively. The contours of the alloy particles before dealloying can be also identified in the SEM image, indicating that there was a slight shrinkage of particle size during dealloying. The measured ligament size and pore size distributions are shown in Figure S2 (Supporting Information), and the mean pore size is generally little smaller than the mean ligament size for the same type of NPG-NPs. Figure 2a shows the cross-sectional TEM image of a NPG-NP, and it can be clearly observed that the porous structure is over the entire particle. In addition, Figure 2b shows the selected area electron diffraction (SAED) pattern which reveals the whole NPG-NP is single crystalline. The obtained polycrystalline rings in the SAED pattern correspond to the FIB deposited Pt cap. The high resolution TEM (HRTEM) image in Figure 2c supports the single crystallinity interpretation of the SAED pattern. The Inset shows the FFT of the HRTEM image. It was also reported that micrometer-sized singlecrystalline porous Au particles can be prepared from the Au-Ge eutectic system.46 Most of studies in literature dealing with the formation of NPG out of bulk or thin film alloys have reported the formation of polycrystalline structure.16,17 Furthermore, a little amount of Ag (7at%) has been found in the as-prepared NPG-NPs by using EDS, as shown in Figure S3 (Supporting Information). The influence of the residual Ag in the NPG-NPs on the optical properties is neglected in the discussion and in the simulation work.


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Figure 1. SEM images of the NPG-NPs with different mean particle sizes of (a) 54 nm, (b) 130 nm, (c) 233 nm, and (d) 393 nm formed from the 4 nm Ag/2 nm Au bi-layers, 8 nm Ag/4nm Au bi-layers, 12 nm Ag/6 nm Au bi-layers and 20 nm Ag/10 nm Au bi-layers, respectively.

Figure 2. (a) Cross-sectional TEM images of a NPG-NP formed from the 20 nm Ag/10 nm Au bi-layer, (b) the corresponding selected area electron diffraction (SAED) image. The diffraction rings fit to the polycrystalline FIB deposited Pt. (c) High resolution TEM image and FFT (inset) to clarify the single crystallinity of the NPG-NP.

The Ag-Au alloy NPs and the corresponding NPG-NPs exhibit size-dependent LSPRs, and the peak position is highly tunable across the visible and near-infrared (NIR) spectral regions. Figure 3 demonstrates the UV-Vis-NIR extinction spectra of 4 types of solid Ag-Au alloy NPs and the corresponding NPG-NPs on fused silica (1.0 × 1.0 cm2). For both, the solid Ag-Au alloy NPs



and the NPG-NPs, the plasmon peak is progressively red shifted and became increasingly broadened upon an increase of the particle size. There is also a big red-shift due to the porous structure. For example, comparing to the solid Ag-Au alloy NPs (D = 54 nm) formed from 4 nm Ag/2 nm Au bi-layer with a plasmon peak centered at 428 nm (the peak position located between the 350 nm (for pure Ag NPs) and 520 nm (pure Au NPs) due to the composition effect55), the corresponding NPG-NPs (with the similar particle size) possess a plasmon peak at 685 nm (even clearly larger than the value of 520 nm for pure Au NPs). Noteworthy is that there was a small size reduction of the NPG-NPs after dealloying, which can induce a shift toward lower wavelengths. But the shift in peak position is mainly governed by the presence of nanoporosity. When the particle size is beyond the quasi-static limit (beyond ~ 60 nm), the dipole plasmon mode of the solid NPs red shifted and broadened. The larger solid Ag-Au alloy NPs with diameter of 233 nm (formed from the 12 nm Ag/6 nm Au bi-layer) and 393 nm (formed from the 20 nm Ag/10 nm Au bi-layer) show a broad peak in large wavelength and a narrow peak in low wavelength, respectively. Remarkably, when the nanoscale porosity was introduced into the large NPs, the dipole plasmon modes red shifted and narrowed. The narrowed resonance peak suggests that the losses are not very pronounced.47

0.5 Extinction (arb. unit)

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Ag-Au alloy NPs (D=54 nm) Ag-Au alloy NPs (D=130 nm) Ag-Au alloy NPs (D=233 nm) Ag-Au alloy NPs (D=393 nm)

NPG-NPs (D=54 nm) NPG-NPs (D=130 nm) NPG-NPs (D=233 nm) NPG-NPs (D=393 nm)

0.4 0.3 0.2 0.1 0.0 400

600 800 1000 1200 Wavelength (nm)


1400

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Figure 3. UV-vis-NIR extinction spectra of the Ag-Au alloy NPs (dashed lines) and the corresponding NPG-NPs (solid lines) with different mean particle sizes.

Figure 4. Calculated extinction spectra of (a) NPG-NPs and (b) solid gold NPs with various particle sizes. The volume porosity of the NPG-NPs is 66% and the pore size d = 20 nm. The diameter of NPs is 54, 130, 233 and 393 nm, respectively. (c) Cross-sectional views of the calculated near-field enhancements of the NPG-NPs (D = 393 nm) and solid gold NPs (D = 393 nm) at extinction of different wavelengths. To investigate the structure-property relationship of the NPG-NPs, FDTD simulations were performed to calculate their extinction spectra. The models for the NPG-NPs were constructed with a large Au sphere etched by N randomly distributed spherical air pores, and details can be seen in the Experimental Section. Figure 4a shows the normalized extinction spectra of NPGNPs with different particle size. The volume porosity of NPG-NPs was assumed as 66% and the pore size was fixed at 20 nm. The plasmon resonance progressively red-shifted and became increasingly broadened as the overall particle size increased, and this agrees well with the experimental data. For NPG-NPs with D = 54 nm show a peak centered at ~545 nm, this is a dipole plasmon peak. For NPG-NPs with D = 130 nm there are two clearly distinct peaks. The peak around 645 nm is attributed to dipole plasmon peak, the other peak at shorter wavelength around 420 nm corresponds to quadrupole plasmon peak. The NPG-NPs with D = 393 nm show three distinct peaks. The peak around 1175 nm is attributed to a dipole plasmon peak, the peak at 780 nm corresponds to quadrupole plasmon peak, and the peak at 430 nm could be an octupole plasmon peak. Quadrupole and octupole plasmon modes are not visible in experimental measurements, although FDTD calculation shows the peaks. The discrepancies of the LSPR wavelength between experiment and FDTD calculation are attributed to two reasons: (1) non-



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uniform particle size and shape, and (2) interactions between substrate and NPG-NPs which are absent in the FDTD simulation.56, 57 For comparison, the calculated extinction spectra of solid gold NPs with different sizes are depicted in Figure 4b. It is apparent that the plasmon resonance depends much more sensitively on the overall size when the particles become porous. In addition, when nanoscale porosity was introduced into the gold NPs, the higher-order mode (quadrupole) was significantly dampened, whereas the dipole plasmon mode remained robust. Size dependent plasmonic shifts in dipole mode resonance are shown in the Figure S4 (Supporting Information). Figure 4c demonstrates the cross-sectional views of the calculated near-field enhancements of the NPG-NPs and solid gold NPs at extinction of different wavelengths. Figure S5 (Supporting Information) shows the polarization vectors of solid gold NPs (D = 393 nm) at different plasmon mode resonances. It is clear that more hotspots are excited in NPG-NPs. In addition, it is obvious that the higher-order multipoles emerging in solid gold NPs are much easier than the same size of the NPG-NPs. It can be seen from Figure 3 and Figure 4 that the influence of the porosity on the plasmonic properties is very complicated, especially when the joint influence of the particle size is also present. To explore the relationship between the extinction spectra and the volume porosities, FDTD simulation has been performed to obtain the normalized extinction spectra of NPG-NPs with different porosities for different size. The pore size was fixed at 20 nm. Figure 5a shows the extinction spectra of the NPG-NPs with a small size of 130 nm and different volume porosities from 0% to 66%. Figure 5b and Figure 5c show the wavelength of the plasmon peaks and the intensity ratio of the dipole mode and quadrupole mode plotted as a function of the volume porosities, respectively. Regardless of various volume porosities, two characteristic resonant peaks can be observed in all the NPG-NPs. The peak position at a low wavelength of 420 nm (quadrupole) does not change when the porosity increases from 0% to 66%, whereas the peak at a high wavelength (dipole plasmon peak) gradually shifts from 540 nm to 645 nm (Figure 5b). The intensity ratio of dipole mode and quadrupole mode increases with an increase of volume porosity, but generally is more than 1, meaning that dipole resonance dominates in the extinction spectrum and the contributions of higher order multipoles are less pronounced (Figure 5c). Figure 5d shows the normalized extinction spectra of the large NPG-NPs with diameter of 393 nm and different volume porosities. Both the dipole and quadrupole plasmon peaks progressively


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red shift, when the porosity increases from 0% to 66%. Figure 5e and Figure 5f show the wavelength of the plasmon peaks and the intensity ratio between the dipole mode and quadrupole mode as a function of the volume porosities, respectively. The peak of dipole mode at a high wavelength increases from 1100 to 1175 nm and the peak of quadrupole mode at low wavelength shifts from 620 to 780 nm (Figure 5e) with increasing porosity. The relative intensity of the quadrupole mode gradually decreases while the intensity of the dipole plasmon mode increases upon an increase of the porosity (Figure 5d). This means the dipole mode becomes stronger and the quadrupole mode becomes weak with increasing porosity, suggesting the higher-order mode (quadrupole) was significantly dampened and a transition of the domination from quadrupole to dipole mode. In addition, when the porosity decreases to 30%, the octupole mode appears, the peak of octupole mode shifts to low wavelengths and the intensity gradually increases upon a decrease of the porosity. Furthermore, the extinction spectra of the NPG-NPs with a medium size of 233 nm and different porosity are also calculated and shown in Figure S6 (Supporting Information). It is clear that the influence of the porosity is different for small particles and large particles. For small particles in which dipole mode is dominated, a red-shift of the dipole mode is presented due to the increasing porosity. For large particles in which quadrupole or high order modes become more important, both the dipole mode and the quadrupole mode show a red-shift, and even there is a transition from quadrupole mode dominated to dipole mode dominated when the porosity increases.



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Figure 5. Calculated extinction spectra of spherical NPG-NPs: (a) D = 130 nm and (d) D = 393 nm of various volume porosities. (b and e) The relationship between the wavelength of plasmon mode peak and the volume porosity of the NPG-NPs. (c and f) The intensity ratio of dipole mode and quadrupole mode plotted as a function of the volume porosity. The pore size is fixed at 20 nm. The volume porosities are 0%, 10%, 20%, 30%, 40%, 50%, 60% and 66%, respectively.

3.2 Influence of the pore size on the plasmonic properties 20 nm Ag/8 nm Au bi-layers were used to prepare the NPG-NPs with different mean pore size, d. The pore size is dependent on the alloy concentration and the dealloying conditions, and here was tuned by changing the dealloying conditions such as temperature and concentration of the HNO3 aqueous solution, as described in Experimental Section. NPG-NPs with mean ligament sizes of 11.6, 22.8 and 50.4 nm were obtained, as shown in Figures 6a-c. The measured ligament size and pore size distributions are shown in Figure S7 (Supporting Information), and the mean pore size is generally little smaller than the mean ligament size for the same type of NPG-NPs. However, the particle size is also clearly influenced by the dealloying conditions, and strong size shrinkage has been observed at higher temperature or in the HNO3 aqueous solution with higher concentration. Therefore, the NPG-NPs with a smaller mean pore size have also a smaller mean


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particle size. Figure 6d shows the measured extinction spectra of the NPG-NPs with different pore size. There is a clear blue-shift of the plasmon resonance peak from the NPG-NPs with small pore size to the NPG-NPs with large pore size. Blue shift can be also resulted from the decrease in particle size. Thus it is not clear whether this observed blue-shift is partially resulted from the increasing pore size or just only from the decreasing particle size.

Figure 6. SEM images of the formed NPG-NPs from 20 nm Ag/8 nm Au bi-layers on different conditions: (a) in a diluted HNO3 aqueous solution (volume ratio between 65 wt% HNO3 and H2O is 1:2) at 21 °C for 5 min, (b) in a 65 wt% HNO3 aqueous solution at 21 °C for 5 min, and (c) in a diluted HNO3 aqueous solution (volume ratio between 65 wt% HNO3 and H2O is 1:2) at 60 °C for 5 min, respectively. (d) UV-Vis-NIR extinction spectra of the NPG-NPs with different pore size of 11.6, 22.8, 50.4 nm, respectively. To explore the relationship between the extinction efficiency and pore size, FDTD simulation has been performed for both small (D = 130 nm) and large (D = 393 nm) NPG-NPs with different pore size. The volume porosity is fixed at 66%. The normalized extinction spectra of the NPG-NPs with different pore sizes, the relationship between the wavelength of plasmon peaks and the pore size and the intensity ratio of dipole mode and quadrupole mode plotted as a function of the pore size are plotted in Figures 7a-f. For small NPG-NPs, as can be seen in evidently in Figure 7a-c, regardless of pore sizes, two characteristic resonant peaks are observed in all the NPG-NPs. The quadrupole peak at a low wavelength of 415 nm does not change when pore sizes reduce from 50 to 10 nm, whereas the dipole peak at a high wavelength shifts from 590 to 650 nm. It is apparent that the ratio of dipole mode and quadrupole mode deceases with



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increase of the pore size. For large NPG-NPs (Figures 7d-f), with the pore size increase from 20 nm to 100 nm, the peak of dipole mode gradually shifts from 1175 nm to 1115 nm, while the peak of quadrupole mode at low wavelength shows a minor fluctuation. Remarkably, the ratio of dipole mode and quadrupole mode deceases with an increase of the pore size. In addition, FDTD simulation has also been performed for the NPG-NPs with middle size of D = 233 nm and with different pore sizes, as shown in Figures S8 (Supporting Information). It is clearly seen that there is a slight blue-shift of plasmon resonance with increasing pore size, and the effect of pore size is also different for small particles and large particles. Lang and coworkers have studied that smaller pore size of NPG yield stronger SERS enhancements due to the enhanced localized electromagnetic fields by shorter inter-ligaments distances.58 When the gap between ligaments becomes smaller, plasmon coupling oscillation strength produces a strong confinement of the local electric field and leads to an intense enhancement of the spectroscopic signals.59

Figure 7. Calculated extinction spectra of spherical NPG-NPs: (a) D = 130 nm and (d) D = 393 nm of various pore sizes. (b and e) The relationship between the wavelength of plasmon mode peak and the pore size of the NPG-NPs. (c and f) The intensity ratio of dipole mode and quadrupole mode plotted as a function of the pore size. The volume porosity of the NPG-NPs is fixed at 66%.


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3.3 Au/Al2O3 hybrid porous NPs and the influence of the Al2O3 on the plasmonic properties 20 nm Ag/8 nm Au bi-layers were used to prepare the NPG-NPs, and then Au/5nm-Al2O3 and Au/10nm-Al2O3 hybrid porous NPs were obtained by deposition of Al2O3 layers into the NPGNPs with plasma enhanced ALD at 150 °C. Both dosing and purging time was tripled as standard process time during the ALD process, so that the precursor molecules can diffuse into the nanoporous structure. Figure 8a shows the representative SEM image of the formed NPG-NPs. The mean particle diameter (D) of the formed NPG-NPs is 272 nm. Figure 8b and c show the representative SEM image of Au/Al2O3 hybrid porous NPs with 5 nm and 10 nm Al2O3 layers, respectively. After Al2O3 deposition, the pore size decreases. Figure 9a shows the cross-sectional TEM image of an Au/5nm-Al2O3 hybrid porous NP. It can be clearly observed that the 5 nm thick Al2O3 layer was homogenously deposited inside of the NPG-NP. This can be also confirmed by the EDS mapping (Figure S9, Supporting Information). Figure 9b shows the SAED pattern which reveals that the Au framework in the hybrid porous NP is single crystalline. The obtained polycrystalline rings in the SAED pattern correspond to the FIB deposited Pt cap.

Figure 8. SEM images of (a) nanoporous gold nanoparticles, (b) Au/5 nm-Al2O3 hybrid porous nanoparticles and (c) Au/10 nm-Al2O3 hybrid porous nanoparticles.



Figure 9. (a) Cross-sectional TEM image of an Au/5nm-Al2O3 hybrid porous NP. (b) the corresponding selected area electron diffraction (SAED) image. The diffraction rings fit to the polycrystalline FIB deposited Pt. (c) High resolution TEM image and FFT (inset) to clarify the single crystallinity of the NPG-NP.

0.3

Extinction (arb. unit)

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0.2

NPG-NPs Au/5nm-Al2O3 hybrid NPs

0.1

0.0 400

Au/10nm-Al2O3 hybrid NPs

600

800

1000

1200

Wavelength (nm)

1400

1600

Figure 10. (g) UV-vis-NIR extinction spectra of the NPG-NPs, Au/5 nm-Al2O3 hybrid NPs and Au/10 nm-Al2O3 hybrid NPs. Figure 10 demonstrates the UV-Vis-NIR extinction spectra of NPG-NPs, Au/5nm-Al2O3 hybrid NPs and Au/10nm-Al2O3 hybrid NPs. The main plasmon peak in NIR range is progressively red


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shifted and becomes increasingly broadened when Al2O3 was deposited into the porous structure. The refractive index of Al2O3 is clearly larger than that of air, and therefore, after the deposition of the Al2O3 layers, the effective environment refractive index nm is increased, leading to the redshift.17 The broadening of the LSPR can be resulted from radiative damping or/and electron scattering. In addition, the 2nd peak centered at 520-530 nm was detected. As the amount of the Al2O3 was increased, this peak became more pronounced.

4. Conclusion In conclusion, the structure-property relationship of the NPG-NPs has been investigated. NPGNPs possess a very large tunability of the plasmonic property contributed from the combined influences of particle size, volume porosity, pore size and even environmental medium. For instance, for the NPG-NPs with particle sizes from 54 to 393 nm, the plasmon peak can be shifted in a wide spectral range from 685 to 1070 nm. The influences of both porosity and pore size are very complicated especially when the joint effect of particle size is present. For small particles with dominated dipole mode, the dipole plasmon peak progressively red shifts and the quadrupole plasmon peak remains in the same position upon an increase of the porosity. For large particles with dominated quadrupole mode, both peaks of dipole and quadrupole plasmon modes red shift and there is also a transition from dominated quadrupole mode to dominated dipole mode with increasing porosity. There is a slight blue-shift of the plasmon peaks with increasing pore size, but the influence of pore size is as well clearly different for small particles compared to large particles. It is believed that the high tunability of the plasmonic resonances in the NPG-NPs and the hybrid porous NPs can be very interesting for many potential applications.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Additional information on the particle size distribution, polarization vectors of the solid gold NPs for the plasmon resonances, calculated extinction spectra, and EDS mapping.



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Author information Corresponding author: Email: [email protected]; tel.: +49-3677-69-3170; fax: +49-3677-69-3171 Notes: The authors declare no competing financial interest.

Acknowledgments The authors are grateful to Ms. Diana Rossberg for the TEM sample preparation with FIB, and Mrs. Birgitt Hartmann, Mrs. Birgit Kolodziejczyk, Mrs. Ilona Marquardt, Mrs. Gabriele Harnisch, and Mr. Joachim Döll from TU Ilmenau for their help with sample preparation. This work is funded by the Deutsche Forschungsgemeinschaft (DFG, Grant SCHA 632/24), by the state of Thuringia (TMWAT, BioMacroNano2020, 2010-2012; Grant B715-10009) and by the OTKA Board of Hungary (No. NF101329) and the GINOP-2.3.2-15-2016-00041.

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