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Plasmonic Nanocrystal Arrays on Photonic Crystals with Tailored Optical Resonances

Hierarchical plasmonic–photonic microspheres (PPMs) with high controllability in their structures and optical properties have been explored toward surface-enhanced Raman spectroscopy. The PPMs consist of gold nanocrystal (AuNC) arrays (3rd-tier) anchored on a hexagonal nanopattern (2nd-tier) assembled from silica nanoparticles (SiO2NPs) where the uniform microsphere backbone is termed the 1st-tier. The PPMs sustain both photonic stop band (PSB) properties, resulting from periodic SiO2NP arrangements of the 2nd-tier, and a surface plasmon resonance (SPR), resulting from AuNC arrays of the 3rd-tier. Thanks to the synergistic effects of the photonic crystal (PC) structure and the AuNC array, the electromagnetic (EM) field in such a multiscale composite structure can tremendously be enhanced at certain wavelengths. These effects are demonstrated by experimentally evaluating the Raman enhancement of benzenethiol (BT) as a probe molecule and are confirmed via numerical simulations. We achieve a maximum SERS enhancement factor of up to ∼108 when the resonances are tailored to coincide with the excitation wavelength by suitable structural modifications.


INTRODUCTION
Surface-enhanced Raman spectroscopy (SERS) has attracted increasing attention in both scientific research and practical applications due to its high sensitivity and specificity. It relies on enhancing the Raman signal from molecules close to a supporting SERS-active substrate, mostly made from noble metals. This method allows nondestructive and label-free detection of a wide range of molecules with near single molecule detection limit. 1−3 Therefore, it has been applied in various fields such as chemistry, biology, medicine, and environmental science. 4,5 The SERS phenomenon is mainly attributed to a combination of an electromagnetic (EM) enhancement (due to the excitation of surface plasmon polaritons, i.e., an excitation where the free electrons in the metal couple resonantly to a driving EM field) with a chemical enhancement (charge transfer (CT) between SERS-active substrates and analyte molecules). 3,6,7 Generally, the contribution of the EM enhancement is much stronger than that of the CT enhancement. 8 It has been reported that the EM enhancement can be manipulated by tailoring the characteristic dimensions and optical properties of the SERS-active substrates. 9 A variety of SERS-active substrates/structures have been fabricated using top−down methods 9 (e.g., electron-beam lithography and colloidal lithography), bottom−up methods 10,11 (e.g., colloid assembly), or hybrid methods (top−down combined with bottom−up approach) 11 to provide and control the localized surface plasmon resonance (LSPR) by varying the geometry of nanostructures (e.g., nanogap size and shape and periodicity), plasmonic resonance frequencies and modes, 12 thereby resulting in an enhanced local EM field on demand. Nevertheless, the development of cost-effective, reproducible approaches to fabricate SERS-active substrates with high sensitivity and high enhancement factors (EFs) are still at stake and become an increasingly significant topic with the ultimate goal to eventually achieve near single molecule level detection. 13 Noble metals have been mostly used as the base materials for SERS-active substrates (e.g., Au, Ag, and Cu) as they possess unique surface plasmon resonance (SPR) properties upon illumination with incident light. 14 Noble metal nanostructures can confine light into small volumes to enhance the local EM field close to their surface. Such greatly enhanced local EM field areas are defined as "hotspots" in SERS-active substrates, as in turn the weak Raman scattering signal is particularly enhanced if the molecules are located in these spatial locations. It has to be reminded that the EM SERS enhancement factor is approximately proportional to the fourth power of the electric field amplitude. 15 Many metal SERS-active substrates, such as colloidal metal nanoparticles (e.g., dimers and aggregates), metal nanopatterns, or three-dimensional (3D) plasmonic structures have been reported. 16−18 However, either expensive equipment or complex chemical synthesis is required to fabricate these metal nanostructures.
It is worth noting that photonic crystal (PC) structures made from dielectrics have also long been considered for SERS application and not just metals. They are either used as SERSactive substrate themselves 19 or serve as a substrate to provide structural support for metal films or metal nanoparticles. 20 PC structures consist of a periodic arrangement of materials with alternative permittivities. PC structures can tailor the propagation of light and can improve the light−matter interaction thanks to multiple scattering effects. Particularly, PC structures offer photonic stop bands (PSBs) in certain wavelength regions where the propagation of light along a specific direction is forbidden. Also, close to the edge of the PSB, the group velocity of light can be slowed down. 21,22 For example, Qi et al. 19 have reported a plasmon-free TiO 2 photonic microarray as a SERS-active substrate. They explained the enhanced light−matter interaction by the repeated and multiple light scattering in their PC structures. It has been demonstrated that the SERS enhancement strongly depends on spectral alignment between the PSB and the laser wavelength used in the SERS measurements. 19 PC structures with a PSB center near the laser excitation wavelength show lower SERS signal owing to the reduced local density of states. In contrast, PC structures with PSB edges in the proximity of the laser excitation wavelength show higher SERS signal that is explained by the increased local density of states that accompanies the slow light effect. 19 In addition, our group has previously reported the fabrication of PC microspheres with metal-covered nanoarrays causing LSRP, which showed a high SERS EF (>10 7 ), which we attributed to the high density "hotspots" confined in a single microsphere. 11 However, herein, we mainly focus on the enhancement of the EM field by a composite structure, possessing both PSB and LSPR properties, to investigate the Raman enhancement performance. Till now, only a few studies 23−25 have shown the fabrication of SERS-active substrates with both LSPR (resulting from the noble metal nanoparticles/2D nanoarrays/3D nanostructures) and PSB properties (resulting from the PC structures) to improve the enhanced EM field synergistically, thereby leading to a further enhanced SERS intensity. EM modeling of a bioinspired PC structure incorporated with Au nanocrystals for enhancing the localized electric field was explored by Zheng et al. 26 They showed that the Auincorporated system can produce a stronger EM field due to the presence of the PC structure. Franzl et al. 25 have reported an SERS-active substrate by covering a one-dimensional PC with ordered metal nanoparticle arrays by electrochemical etching and the nanosphere lithography method, exhibiting a significant interaction of the plasmonic resonance with the PSB. Such an SERS-active substrate leads to a highly confined EM field at the interface between both structures and thus showing a significant enhancement in Raman signal with EFs up to 10 5 . Lee et al. 27 have also demonstrated that the SERS behavior of amorphous TiO 2 −Ag nanoarchitectures depends on the plasmonic− photonic interference coupling where SERS-active units were fabricated by a two-step anodization on a titanium substrate and thermal annealing to obtain silver nanoparticles on top of the titanium dioxide nanotube (TNT) structures. However, there are still ample opportunities to improve the plasmonic− photonic structures for SERS enhancement in a facile and robust manner to render structures with more designability and controllability with respect to the hierarchical structures and the resulting optical properties of both LSPR and PSB.
In this work, a robust and cost-effective approach has been developed to fabricate hierarchical plasmonic−photonic microspheres (PPMs) by combining droplet microfluidics with metal film deposition and thermal annealing. The fabricated PPMs possess both PSB properties, resulting from periodic silica nanoparticle (SiO 2 NP) arrangements of the 2nd-tier, and SPR properties, resulting from the AuNC arrays of the 3rd-tier. The uniform microsphere backbone is regarded as the 1st-tier. We investigated the SERS sensitivity dependent on the PSB edge of the PC structures by varying the size of the SiO 2 NPs. We observe a complicated impact of the size of the SiO 2 NP on the achievable SERS enhancement because (i) the spectral position of the PSB is modified but also (ii) because the size of the SiO 2 NP affects the geometrical properties of the deposited metallic structures and with that the spectral position of the SPR. The complicated interplay can, nevertheless, be reproduced in dedicated full-wave optical simulations that we perform to provide further insights to the experimental results. We demonstrate the synergistic effect of the multiple scattering of light in the PC structure and the LSPR caused by AuNC arrays as the presence of "hotspots" by tailoring the nanogaps between adjacent AuNCs of the 3rd-tier of the PPMs. An SERS enhancement up to a factor of 10 8 is experimentally achieved. Additionally, this multilevel hybrid structure has potential in photocatalysis and sensing according to its dual tunability in Bragg mode and plasmonic mode. 28,29 2. RESULTS AND DISCUSSION 2.1. Fabrication of Hierarchical Plasmonic−Photonic Microspheres. Figure 1 shows the process for fabricating PPMs 30 serving as SERS-active substrates. In this process, we combine droplet microfluidics with metal film deposition and thermal annealing. Note that the annealing process in this work is different from our previous work 28 where an inert gas of nitrogen (N 2 ) was applied for formation of the AuNC arrays, avoiding surface cross contamination and ensuring the chemical properties. It is of great significance for subsequent molecule immobilization with respect to bioassays. Briefly, photonic crystal microsphere (PMs) were obtained using a microfluidic droplet generator (Figure 1a,b). These PMs possess PSB properties, resulting from the periodic SiO 2 NP arrangements that offer the required periodically modulated permittivity. 31 By tailoring the SiO 2 NP diameter (d SiO2 ), the PSB properties are precisely controlled. Depositing a thin layer of a Au film onto the as-prepared PMs ( Figure 1c) and annealing them in N 2 at 800°C for 1 h results in PPMs consisting of both the PC structures and well-spaced AuNC arrays (Figure 1d). The surface morphology of the AuNC arrays and the AuNC diameters strongly depend on the thickness of the deposited Au films. The formation of AuNCs is attributed to the dewetting of the Au film deposited onto the surface of SiO 2 NP arrangements. 32 Both the AuNC diameter (d Au ) and their edge-to-edge distance (p Au ) increase with increasing d SiO2 ( Figure S1) where PPMs feature one large AuNC on top and multiple small AuNCs along the rim of each SiO 2 NP. Subsequent deposition of a second continuous Au film on these as-prepared PPMs leads to a narrowing of the p Au (Figure 1e). This can result in a plasmon coupling effect with near-field EM enhancement, thereby creating high density hotspots. 33 Figure 2d). In this manner, the nanogaps among the adjacent AuNCs can be precisely tailored to achieve an EM field enhancement.
2.2. Optical Properties of Hierarchical Plasmonic− Photonic Microspheres. Figure 3 shows optical microscopy (OM) images in bright-field (BF) and dark-field (DF) illumination modes, and their corresponding reflection spectra of the fabricated PMs and PPMs. The detailed optical reflection spectra measurements are described in the Methods section. The PMs demonstrate PSB properties (solid curves in Figure  3a, Figure 3a) with increasing d SiO2 . 34 The PSB is clearly visible in these spectra as a local peak that shifts toward longer wavelengths with increasing d SiO2 . Side lobes that would appear in a perfect PC structure with a planar interface are not visible because of the spherical shape of the PC structure and possibly also due to presence of defects and disclinations in the fabricated samples. The PPMs possess both PSB properties (resulting from the PC structures) and SPR properties (resulting from the AuNC arrays). Their reflection spectra (dotted curves in Figure 3a,b) show strong dips at approximately 517 nm wavelength, resulting from the AuNC array absorption. 35 Figure 3b) is observed for which the Bragg diffraction peak (in the blue spectral region/high energy region) is separated from the Au scattering peak (in the red spectral region/low energy region). The scattering peaks from the AuNC arrays on the PPMs consisting of SiO 2 NPs of 250 and 280 nm in diameter show a red shift and a larger magnitude with increasing the d SiO2 (blue dotted and red dotted curves in Figure 3b). This is attributed to the overlap of the Bragg diffraction from the PC structures and AuNC array scattering. The different behavior with wavelength of the scattering of the AuNC arrays in DF compared to that in BF is noteworthy and not yet completely understood. We attribute the difference to the complex 3D structure of the AuNC arrays favoring specular reflection when illuminated from above compared to the more sideway scattering required for the DF signal. Figure 3c,d shows the BF and DF images and their corresponding reflection spectra of these PPMs measured in hexadecane with a refractive index (n) of 1.4 instead of in ambient air (n = 1.0), respectively. We found that these PPMs show similar dips as in air but shift to approximately 530 nm wavelength ( Figure 3c Figure 3d). The red shift of the scattering peaks is attributed to an increase in d Au .
However, when the Bragg diffraction is in the blue spectral region/high energy region (black dotted curve in Figure 3d), Au scattering shows a broad peak with a low intensity in the red spectral region/low energy region, which is consistent with the spectra measured in ambient air. Hereby, it shows high potential to confine light of a specific wavelength to induce a locally amplified EM field incorporated with AuNC arrays for the SERS application, and this hypothesis will be investigated below.

Hierarchical PPMs as SERS-Active Substrates.
Since the fabricated PPMs possess not only multilevel structures ranging from nanometers to micrometers but also integrated optical properties of both PSB and SPR, their SERS performance was explored using a laser emitting at 632.8 nm for the excitation. Figure 4 shows the Raman spectra of various fabricated SERS-active substrates, including 100 nm Au-coated AuNCs randomly distributed on a silicon (Si) wafer (t Au  Table S2) with the dominant Raman active vibrational modes of benzenethiol (BT) molecules. 36 Note that the slight shifts of vibration frequencies to higher wavenumbers in comparison with the isolated bulk molecules ( Figure S2) can be explained by the chemical interaction between the adsorbed molecules and the underneath supporting SERS-active sub-strate. 37 The calculation of the EF A is shown in the Methods section and the Supporting Information where the subscript A indicates that the area occupied by immobilized molecules was equated to the microscope objective spot area. To be more precise, the enhancement factor was also estimated by calculating the BT-immobilized Au surface area, and denoted as EF Au . The detailed calculation of EF Au is shown in the Supporting Information. It is worth noting that the absence of the 909 cm −1 vibration mode in these SERS-active substrates, compared to the normal Raman spectrum of neat BT ( Figure  S2), indicates the formation of a monolayer of BT molecules on these SERS-active substrates. 15 Furthermore, a three-dimensional (3D) spatial Raman mapping of t Au 100 nm @Au 15 s 200 W d SiO2 200 nm @ PPM results in a signal distribution along the z axis by manual focusing with a step of 1 μm ( Figure S3), suggesting that only the area in focus shows an enhanced signal. Figure 4 shows that hierarchical microspheres improve the Raman signal, compared to the sample with randomly distributed Au-coated AuNCs on a Si substrate (t Au @PMs) is caused by the LSPR from high density hotspots and has already been reported in our previous work. 11 The 100 nm Au-coated PMs do not offer optimized conditions for producing the amplified EM field caused in these hotspots. The Raman signal of these Au-coated PMs (t Au 100 nm @ d SiO2 200 nm @PMs) can be maximized by adjusting the nanogap between adjacent metal-coated nanoparticles ( Figure S4). When this distance approaches 2 nm, a much stronger EM field can be obtained. 15 However, in this work, we focus on the 200W d SiO2 200nm @ PPMs), indicating the possibility of interplay between the PC structure and the AuNC arrays, which will be further investigated in the following sections. In addition, we should consider the signal contribution from the total Au surface area since Au-coated PPMs have a larger surface area than PPMs without Au film coating. To quantify this contribution, the Au surface areas of Au 15  @PPMs with EF Au = 1.20 × 10 7 , which agrees with the variation of EF A between these two structures.
2.4. Effect of the PSBs of PPMs on SERS Signals. Here, we investigate the interaction of the PSBs of PPMs with the laser excitation wavelength and its effect on the SERS performance. Figure 5a shows the Raman spectra of the Au 15 s 200 W d SiO2 x @PPMs and their corresponding surface morphologies (close-up HR-SEM images). The Raman signal increases with decreasing d SiO2 .
One possible reason is that the PC structures play a critical role in the Raman enhancement. 19 PC structures have PSB properties characterized by a Bragg diffraction peak (indicating a spectral region in which the light propagation is forbidden in certain direction) and a slow light effect at the edge of the Bragg diffraction peak with a reduced group velocity. 38 The sinusoidal standing wave at the blue edge of the Bragg diffraction peak is mainly localized in the low refractive index medium of the PC structures, i.e., in the air voids, whereas at the red edge of the Bragg diffraction peak, the highest amplitude of the standing wave is localized in the high refractive index medium of the PC structures, i.e., in the PC matrix or the matter adsorbed on the PC surface. This implies that adsorbates with a high refractive index confined on the PC structure strongly interact with the light at the red edge region of the Bragg diffraction peak. As discussed above (Figure 3), the Bragg diffraction peaks of the d SiO2 250 nm @PMs and the d SiO2 280 nm @PMs are located near the laser excitation wavelength, which would suppress the light propagation in such structures. 19 In contrast, the Bragg diffraction peak of the d SiO2 @PPMs. It should be noted however that the increase in p Au caused by the increasing d SiO2 (dashed curves in Figure 5d and Figure S1) in this case could possibly lead to a decrease in the Raman signal.
To further study the effect of the PC structures on the Raman signals, PPMs featuring a high density of hotspots with a relatively equivalent d Au and p Au are investigated (Figure 5b and solid curves in Figure 5d). These PPMs are fabricated by  (Figure 5b), which is consistent with the Raman intensity change of the previously used PPMs (Figure 5a,c). The enhancement factors (EF A ) of these structures are a function of d SiO2 , as shown in Figure 5c, whereby the tendency agrees well with EF Au presented in Table S3. We attribute the fact that the EF Au of Au 15 s 200 W d SiO2 @PPM is larger than the corresponding EF A to the fact that the estimated Au surface area is smaller than the used microscope objective spot (calculation details are shown in the Supporting Information). In summary, we found indications that when the PSBs of PPMs are spectrally located near the laser excitation wavelength used in the SERS experiments, the light− matter interaction is suppressed and the Raman signal is decreased. This agrees with a previous study 19 and is confirmed by the experimental results.
Additionally, Au-coated monolayers of SiO 2 NPs with different diameters were investigated to further confirm the PSB effect of PPMs on the Raman enhancement ( Figure S6). It was found that the Raman signal is not substantially affected by the size of the SiO 2 NPs. This can be clearly seen in Figure S6 where the Raman intensity at 1067 cm −1 as a function of SiO 2 NP diameter is shown. We would like to note that the monolayer of SiO 2 NPs serves as a platform to provide geometrical support for the metallic structures on top but that the exact geometrical dimension in the present regime does not noticeably affect the specific properties of the samples. Furthermore, compared to the PPMs, the Raman signals of the monolayers of close-packed SiO 2 NPs of different diameters were all lower than those of Au 15  @PPM. This suggests that the PSBs resulting from the photonic crystals play a role in the localized amplified EM field.
2.5. Synergistic Effect of the Tunable LSPR and PSB on the SERS Signals. The Raman performance of PPMs coated with a layer of the Au film is investigated for different Au thicknesses (t Au ) ( Figure 6). The nanogaps between the adjacent nanoparticles can be adjusted by controlling the t Au , which can further enhance the localized EM field by the formation of hotspots. We found a significant change in the obtained Raman signals with an increase in t Au . As t Au increased from 0 to 300 nm, a change of surface morphology results in the increase of d Au and decrease of p Au at the same time (close-up @PPM) reaching the highest signal. We attribute this to the synergistic effect of the PSB properties with the nearfield plasmon coupling at the hotspots. We hypothesize that the Raman intensity of t Au 150 nm @Au 15 s 200 W d SiO2 200nm @PPMs is decreased with respect to the PPMs with 100 nm Au coating because the increased Au film started to bury the initial SiO 2 NP lattices, leaving only the AuNC arrays. The light interaction in the PC structures then weakened while another type of PC structure featuring AuNC arrays is formed where the thick Au film behaves like a mirror with high reflection. Figure S7 shows the corresponding optical properties of Au-coated PPMs under the BF and DF illumination modes, respectively. It is obvious to see that the samples show a dip approximately at 480 nm caused by a layer of Au film with increasing t Au , especially when t Au ≥ 200 nm ( Figure S7b) @PPMs, and then again decreases with further increasing the t Au . For t Au ≥ 200 nm, the hexagonal SiO 2 NP nanopatterns are completely covered by the corresponding AuNC arrays so that the Raman enhancement only resulted from the LSPR of the hotspots. The distance of the nanogaps remains relatively stable (HR-SEM images in Figure  6c−f and h), but the density of nanogaps decreases as the nanogaps partly joined together (e.g., t Au = 300 nm), decreasing the surface roughness and resulting in a reduced number of hotspots with decreased Raman signal. Summarizing, we found that the Au-coated PPM with 100 nm Au film thickness (t Au 100 nm @Au 15 s 200 W d SiO2 200 nm @PPM) shows the highest Raman intensity. We attribute this to the synergistic effect of the PC structures with the LSPR by hotspot formation. The LSPRinduced amplified EM then dominates the PC structure with increasing t Au of the Au-coated PPMs.
To more precisely determine the maximal synergistic effect of the PC structure and the LSPR of Au-coated PPMs, the Raman performance of PPMs with a continuous Au film in the range of 0−100 nm is studied. It was found that the EF A reaches a maximum of 10 8 when t Au = 60 nm and then decreases again with increasing t Au (Figure 7a,b). This confirms the hypothesis formulated in the previous section that the amplified EM field raised by the nanogaps between the adjacent AuNCs does not play the dominant role in the explanation of the Raman enhancement. Rather, it is the interplay of field enhancement caused by the PC structures and LSPR. We also found that the EF A of Au-coated PPMs is approximately three times larger than the EF Au (listed in Table S4), which we attribute to the Au surface area being larger than the planar surface area on the same projected area (the area of the used microscope objective). However, both the EF A and EF Au follow the same tendency when varying the Au thickness t Au . Figure 7c shows  @PPM. Figure 7d shows the d Au and p Au varying with t Au .
In addition, the uniformity and reproducibility have also been investigated for the hybrid microspheres t A u 60 nm @ Au 15 Figure S8.  Figure S8b) and found a relative standard deviation (RSD) of 12%, demonstrating that the hybrid PPMs can be fabricated with high reproducibility and relative uniformity. Additionally, these hybrid PPMs show potential application in the sensing field, as demonstrated by the small Raman cross-sectional molecule detection using L-methionine ( Figure S9).
It remains to be mentioned that an unambiguous assignment of which of the effects causes the observed SERS enhancement is not that obvious. To clear the misunderstanding, we want to indicate these possible effects in a list: • The spectral alignment of the PSB relative to the excitation wavelength is important due to the aforementioned slow light effect and the localization of the EM field in either the high or the low refractive index region. • The spectral alignment of the PSB relative to the excitation wavelength also affects the amount of backreflected light and its phase. Considering that the molecules are deposited in a tiny layer above the structure, a fraction of the light that experiences multiple scatterings inside the PC structures is finally back-reflected where it interferes with the illumination. This interference can be either constructive or destructive. This leads to a lowering or enhancement of the local EM field in specific spatial regions. Depending on its alignment with the molecular layer, the SERS signal is enhanced differently. • Finally, any modification of the PC structure that serves as the backbone for the plasmonic structure changes the geometry of the metallic constituents and with that the exact spectral position of the plasmonic resonances. If the spectral position of the LSPR is in agreement with the wavelength that matters in the SERS process, the signal gets tremendously enhanced.
Even though the disentanglement of the impact of all parameters is too complex and beyond the scope of the present work, we perform full-wave optical simulations of selected structures to discuss the importance of different effects and to verify the experimental findings.
2.6. Full-Wave Numerical Simulations. Figure 8 shows the numerical simulation results of the local electric field of SERS-active substrates with different relevant geometries. First, we consider a pure photonic microstructure that consists of a hexagonal lattice of SiO 2 NPs (d SiO2 200 nm @PM, Figure 8a). For numerical simulation convenience, the periodicity is chosen to be slightly larger than the SiO 2 NP diameter (210 nm), which should not affect the conclusion to be drawn. The PC structure is finite and made of 20 layers of SiO 2 NPs. It is illuminated at normal incidence with linearly polarized plane wave oscillating at a wavelength of 632.8 nm according to the experimental condition. The amplitude of the electric field is shown in some selected central cross section, and we concentrate on the spatial region directly above the PC structure. We furthermore consider a PPM structure (Au 15 s We found that the maximum enhancement of the local electric field is obtained with the Au-coated PPM (Figure 8c), which is in line with our experimental results (Figure 7). This suggests that the presence of nanogaps between the AuNCs and the Au shell and in between the shell-coated SiO 2 NPs causes LSPR at the excitation wavelength as used in the SERS experiments, which is mainly responsible for the Raman enhancement. When considering only an isolated AuNC on top of the PC structure, the enhancement is much weaker (Figure 8b). We do see some field enhancement (amplitude enhanced by a factor of 4). Also, the PC structure alone offers a local enhancement in some spatial regions that is a factor of 2. This corresponds in good approximation to the value that one would expect at most for a perfect reflector. The illumination is back-reflected and interferes with the incident field in some spatial region, so the total amplitudes can double or can be suppressed. Also, we are operating the PC structure at a wavelength longer than the wavelength of its PSB. The period of the structure is smaller than the wavelength, and the PC structure offers only a zerothdiffraction order. The amplitude of this zeroth-diffraction order gets strongly affected by multiple scattering effects, but it cannot be tremendously enhanced. However, this back-reflected zerothdiffraction order excites the plasmonic structure as well where it gets further enhanced. It is that combined action that explains the excellent performance of the suggested structure. So, we exploit not only the plasmonic field due to the excitation with the actual illumination but also the back-reflected field from the PSB structure, adding to the excitation of the plasmonic field and further enhancing the local electric field. As the SERS signal is proportional to the fourth power of the electric field amplitude, this additional field translates to a much higher SERS signal.
In addition, we investigate the effect of the PC structure of Aucoated PPMs on the electric field with varying d SiO2 ( Figure  S10a−c). The simulated results agree with our experimentally determined Raman enhancement ( Figure S10d), further confirming that the PC structure can improve the Raman signal according to their enhanced light−matter interaction due to the multiple scattering in combination with the plasmonic effect these samples sustain. Herein, the simulated EFs are determined as |E| 4 /|E 0 | 4 at the spatial location where the electric field is the highest. 39 Experimental EFs are obtained by the corresponding sample measurements with the maximum Raman signal enhancement. The deviation of the EFs between the simulated and experimentally determined ones is mainly attributed to the imperfection of the PC structures, which can cause differences between the simulated electric field and the real produced electric field. Also, of course, molecules are not just situated in the hotspot but also in other spatial regions. There they contribute as well to the Raman signal. However, it should be noted that the estimated EF according to the simulations is not the spatial average. This could be another point to cause the estimated EF larger than the experimental EF. Nevertheless, the theoretically predicted functional dependency and particularly the conclusion on the most optimal sample agree with the experimental findings.

CONCLUSIONS
Hierarchical plasmonic−photonic microspheres consisting of the PC structure of close-packed colloids and well-spaced AuNC arrays have been successfully fabricated by using a robust and cost-effective approach with high throughput, designability, and reproducibility. The fabricated PPMs possess both PSB properties resulting from the periodic SiO 2 NP arrangements and LSPR resulting from the Au-coated AuNC arrays. The multilevel structures and optical properties of these fabricated PPMs can be adjusted by tailoring the size of the building blocks and the nanogaps among adjacent AuNCs and thus achieving a synergistic interaction of PSB and LSPR. Moreover, the Raman performance of these PPMs has been evaluated by detection of the chemisorbed monolayer of BT molecules on these fabricated SERS-active units. The obtained enhancement factor is on the order of magnitude of 10 8 for the 1067 cm −1 of the thiol−Au bond vibration. Among these PPMs, the t Au 60 nm @ Au 15 (Figure 1a). 40 The SiO 2 NPs with different diameters (200 ± 10, 250 ± 10, and 280 ± 10 nm) were purchased from the Nanjing Rainbow company (Nanjing, China). These PMs (Figure 1b) were then used as templates for manufacturing PPMs via the Au thin film deposition ( Figure 1c) and thermal annealing (Figure 1d). The sputtering of Au films was conducted in an ion-beam sputtering system (home-built T'COathy system, MESA+, the Netherlands) at 200 W and 6.6 × 10 −3 mbar. The thermal annealing process was conducted in a tube furnace (Nabertherm, Nabertherm GmbH, Germany) in a N 2 environment at atmospheric pressure. Sputtering the second Au film onto the as-prepared PMs and PPMs can further modify their structures and optical properties (Figure 1e).

Reflection Measurements.
A monolayer of fabricated microspheres was patterned on a piece of clean silicon wafer. The reflection spectra and corresponding OM images were measured on the pre-patterned samples in the BF and DF illumination modes, using a microscope with a 20×/0.4 NA objective integrated with a white light source (100 W tungsten xenon lamp) with a light spot size around 1.1 mm in diameter and an Ocean Optics HR4000 visible fiber optic spectrometer. The reflected light was collected by the same objective and passed through a multimode fiber (QP450−1-XSR, Ocean Optics) to the spectrometer with an integrated detector (HR4000, Ocean Optics). A clean silicon wafer without any patterns was used as 100% reflection in BF mode. The observed intensities of samples from the microscope are normalized to the intensity as observed with a clean piece of silicon wafer without any patterns. The used microscope uses a higher illumination intensity in the DF mode than that in the BF mode. The fabricated samples were measured both in ambient air and hexadecane.

SEM Characterization.
A HR-SEM (GeminiSEM 500, Carl Zeiss Microscopy GmbH) was used to characterize the surface topology of the fabricated hierarchical microspheres at acceleration voltages ranging from 0.7 to 2 kV with different detectors: secondary electron detector (SE2) or in-lens detector.

SERS Measurements.
Prior to the measurements of Raman spectra, all prepared substrates were immersed into the BT solution (10 mM) in ethanol overnight followed by gently rinsing with dehydrated ethanol and passively drying at room temperature. A confocal Raman microscope (Alpha300R, Witech GmbH) consisted of a TE-cooled charge-coupled device (CCD) camera (DU970P-BV, Andor Technology, Belfast, Northern Ireland) at −60°C and a He−Ne laser (P max = 24 mW and λ ex = 632.8 nm), and a UHTS300 spectrometer (f/4300 mm FL, and grating of 600 lines mm −1 ) was used to collect the Raman spectra. All measurements were performed by using a 100×/0.9 NA microscope objective and a He−Ne excitation laser of 632.8 nm wavelength. Raman spectra of chemisorbed BT on the fabricated SERSactive substrates were measured in ambient air with an excitation laser power of 500 μW (measured at the entrance of the microscope objective), an integration time (t int ) of 1 s, and 10 times accumulation. Whereas, the Raman mapping measurements were conducted with an excitation laser power of 500 μW, an integration time of 1 s, and onetime accumulation. The collected Raman signal was presented in CCD counts. Spatial imaging was performed over 10 × 10 μm 2 area with 40 lines and 50 measurements per line, yielding a total of 2000 measurements per scanned image. Particularly, as the SERS-active unit was in 3D, the spatial imaging along the z axis was collected in every step of 1 μm by manual focusing.
4.5. Calculation of EFs. EFs were calculated by using the equation EF = (I SERS /I Bulk ) × (N Bulk /N SERS , where I SERS and I Bulk are the intensities of the same band of the Raman signal from the analytical molecules on a SERS-active unit and on the pure bulk analyte, respectively, and N SERS and N Bulk are the number of the analytical molecules probed on the SERS-active unit and on the pure bulk analyte, respectively. The number of chemisorbed BT molecules probed on the SERS-active unit is estimated by N SERS = A s D BT . D BT is the surface density of BT molecules chemisorbed on an Au(111) surface 41 and equals 3.3 × 10 18 molecules·m −2 . A s is the surface area of molecules immobilized. Here, either the area of the used microscope objective spot (1 μm in diameter) or the Au surface area of each structure was used to estimate A s and thus EF A and EF Au can be obtained. More details on N SERS is described in the Supporting Information. The number of bulk BT molecules in the confocal volume (V CV ) of the conventional Raman measurements is estimated by N Bulk = N A M BT −1 ρ BT V cv ≈ 3.56 × 10 10 molecules, where N A = 6.02 × 10 23 ACS Applied Materials & Interfaces www.acsami.org Research Article molecules mol −1 , M BT = 110.18 g mol −1 is the BT molecular weight, ρ BT is the molecule density, and V CV ≈ 6.03 × 10 −18 m 3 is the experimentally determined confocal volume of the measurement system. The collection volume is dependent on the excitation diameter and the effective probe depth of the laser excitation. The effective probe depth (h obj ) was obtained by adjusting the substrate stage out of the laser focus plane till the disappearance of the peak intensity with a step of 1 μm and recording the characteristic silicon peak value at 934 cm −1 by using a laser excitation wavelength of 632.8 nm and a 100×, 0.9 NA microscope objective. Then, an h obj value of 7.68 μm was obtained in this experiment.
In this work, the Raman band at 1067 cm −1 was chosen to estimate the EFs. The integrated intensity of the conventional Raman spectrum from a neat BT solution in a seal glass vial was approximately 0.55 counts (λ ex = 632.8 nm, P ex = 500 μW, 10 times accumulation, and t int = 1 s). Details of the estimation of the Raman intensity of the neat BT solution are given in the Supporting Information. The average integrated intensity of the SERS measurement was 4142 counts from the SERS-active unit of t Au  Figure 8a). Then, a hexagonally well-spaced AuNC pattern anchoring on the top of the pure photonic structure was used to represent PPM (the structure used in Figure 8b). Also, the Au-coated PPM is modeled by introducing the other layer of the thin Au film before anchoring a hexagonal AuNC pattern on the pure photonic structure, as shown in Figure 8c. The maximum thickness of the Au shell is d SiO2NP /8. All of the structures used for investigation of the electric field were illuminated by a normal incident plane wave, and the resulting electric field distribution were collected near the interface between the plasmonic nanoparticle arrays and the PC structures, where the largest values of the electric field are expected. All absolute values of the collected electric field were normalized with the ones of a vacuum-filed simulation, i.e., the magnitude of the incident plane wave. The simulated EF can be approximately expressed by (|E|/|E 0 |) 4 where |E| is the localized electric field and |E 0 | is an incident plane wave magnitude used for normalization. In this work, the maximum electric field value |E max | is used for |E|, and thus the resulting EF is the maximum evaluation for the respective simulation. We also simulated the electric fields of Au-coated PPMs with varying d SiO2 to investigate the effect of the PC structure on the Raman performance, as presented in Figure S9.
■ ASSOCIATED CONTENT * sı Supporting Information