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C: Plasmonics, Optical Materials, and Hard Matter

Interaction of 1-Dimensional Photonic Crystals and Metal Nanoparticle Arrays and Its Application for Surface-Enhanced Raman Spectroscopy Martin Fränzl, Stefan Moras, Ovidiu D. Gordan, and Dietrich R. T. Zahn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02241 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Interaction of 1-Dimensional Photonic Crystals and Metal Nanoparticle Arrays and Its Application for Surface-Enhanced Raman Spectroscopy Martin Fränzl†, Stefan Moras, Ovidiu D. Gordan, and Dietrich R. T. Zahn* Semiconductor Physics, Technische Universität Chemnitz, 09107 Chemnitz, Germany Abstract We introduce a new concept to localize and strongly enhance electromagnetic fields by covering 1-dimensional photonic crystals with ordered metal nanoparticles arrays. When designed properly, the combined photonic-plasmonic composite shows a significant interaction of the plasmonic resonance and the photonic band gap. For this purpose we fabricated 1-dimensional photonic crystals based on porous silicon by electrochemical etching of silicon in hydrofluoric acid and deposited a silver nanoparticle array on top by nanosphere lithography. The composite structure was designed in such a way that the plasmonic resonance coincides with the photonic band gap leading to highly confined electromagnetic fields at the interface between both structures. The samples were characterized using spectroscopic ellipsometry and reflectance measurements and were modeled using effective medium theories and finite-element methods. Surface-enhanced Raman spectroscopy measurements of this unique photonic-plasmonic hybrid system show extraordinary enhancement factors that can only be explained by an interaction

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mechanism. The optical properties of the composite structure are very versatile, providing a promising platform for improved sensing applications and superior substrates for surface enhanced Raman spectroscopy. Introduction Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive spectroscopic technique that allows the detection of molecules at extremely low concentrations.1-5 The effect primarily relies on the presence of intense electromagnetic fields on the surface of SERS substrates.6 Even though this technique is already widely used in commercial systems, substrates with localized and strongly enhanced electromagnetic fields are heavily required. In recent years the development of cost-effective, reproducible fabrication techniques of substrates with high enhancement factors (EFs) has become a very important topic.7 Here, we introduce a new approach combining 1-dimensional photonic crystals with plasmonic nanostructures in a composite structure by placing metal nanoparticle arrays on top of periodic dielectric layer stacks (Figure 1c). When designed properly, the combination of these distinct systems shows a significant interaction of the photonic band gap and the plasmonic resonance, revealing a new possible way to localize and strongly enhance electromagnetic fields. This leads to further improved performance of such composite structures as SERS substrates. 1-dimensional photonic crystals (1DPCs) are multilayer stacks with alternating refractive indices that can have a broad spectral range of very high reflectivity (Figure 1a), referred to as photonic band gap (PBG).8,9 The PBG is a result of the fact that electromagnetic waves are partially reflected at each layer interface and, for a specific range of wavelengths, the multiple reflections interfere constructively. 1DPCs, also commonly known as distributed Bragg


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reflectors (DBRs), are already the basis of many devices including dielectric reflectors, filters, and distributed feedback lasers.8-11 On the other side, metal nanoparticles (NPs) exhibit a plasmonic resonance (PR) due to an oscillation of the free electrons in the metal NPs leading to an increased extinction at the resonance frequency (Figure 1b).12,13 These nanostructures can serve as nanoantennas that localize electromagnetic fields with numerous applications in optical sensing and surfaceenhanced spectroscopy.12-16

Figure 1. (a) Sketch of a 1DPC and its reflectance spectrum at normal incidence. (b) Sketch of the extinction spectrum of the metal NP array. (c) Illustration of the composite structure consisting of an 1DPC covered by an array of metal NPs. The aim of this work is to study the interplay of the PR with the PBG from the viewpoint of fundamental physics and applications for SERS substrates. For this purpose, we fabricated 1DPCs based on porous silicon (PS) by electrochemical etching of silicon in hydrofluoric acid (HF) and deposited a metal NP array on top by nanosphere lithography (NSL).17,18 The structures were designed in such a way that the PR coincides with the PBG leading to highly confined


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electromagnetic fields at the interface between both structures. These SERS substrates show EFs for the Raman signals of thin organic layers up to 105 and 1000 in comparison to bare silicon and NP arrays on bare silicon, respectively. Experimental Section PS is formed by electrochemical etching of p-doped crystalline silicon in HF.19 The morphology depends on the etching current, the silicon wafer doping, and the HF concentration, as well as other factors.20 A detailed discussion of the formation mechanism can be found in the literature.21,22 By varying the etching current, while keeping the other etching parameters constant, it is possible to change the porosity of the etched layers in depth. The higher the etching current, the higher the porosity and the lower the effective refractive index of the layer. The thickness of the layer is determined by the etching time. In particular, the etching process is self-limited and takes place only at the pore tips. Thus, the formation of a new layer beneath an existing layer does not affect the properties of the previously etched layers. For the preparation of the 1DPC samples an electrochemical etching cell (Supporting Information, Figure S2) was built.22 In this work, we used (100) oriented p-type silicon wafers with a resistivity of about 0.01 Ω cm as substrates. The etchant was composed of 50% HF and ethanol with a HF concentration of 15%. The doping of the silicon wafer and HF concentration was chosen to access a large range of porosities. The addition of ethanol ensures the complete infiltration of the HF into the pores and improves the uniformity of the porosity. Before etching, the samples were subjected to standard cleaning procedures.22 To get an ohmic back-contact, the native oxide was removed by rinsing the sample in 5% HF. Subsequently, the sample was mounted in the etching cell where an area of 1.5 cm2 was exposed to the etchant. During etching, the current was controlled by a programmable power supply (KORAD KA3005P). After etching, the sample was


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rinsed with ethanol and dried with a nitrogen gas flow. We first prepared single layers of PS with varying currents and etching times to find a relation between the current density, porosity, and etch rate. The samples were characterized by spectroscopic ellipsometry (J.A. Woollam, VASE). The data were modeled using effective medium theories and the WVASE32 software.23-25 The thickness and the effective refractive index of the PS layer were then extracted from the model fit. The modeling and the calibration are described in detail in the Supporting Information. After calibration, the etch rate and porosity of the etched layers can be tuned precisely by the etching current. By applying a periodic etching current, a periodic variation of the porosity and with that a periodic variation of the effective refractive index can be accomplished (Figure 2a,b). The current-time-profile is mapped to a porosity-depth-profile. The etching stops in the current profile (Figure 2a) ensure a compensation of the HF concentration at the pore tips during the etching process.

Figure 2. (a) Plot of an etching current profile. (b) Cross section SEM image of a PS 1DPC with 10 layer pairs. (c) Top view SEM image of a silver NP array fabricated on a PS substrate for nanospheres with 450 nm diameter.


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The NSL process uses the self-assembly of monodisperse nanospheres into a hexagonal closed-packed monolayer on a substrate which is then used as a mask for metal evaporation. We achieved the self-assembly at the air-water interface by a procedure adapted from Weekes and Vogel et al.26,27 using a suspension of monodisperse, carboxyl-functionalized polystyrene spheres with 450 nm diameter (Postnova Analytics). Before usage, the aqueous suspension was diluted 1:1 with ethanol, which acts as a spreading agent.26 The solution was then attached to the water surface using a glass ramp and a micropipette. Upon contact with the air-water interface, the spheres immediately start assembling and form a monolayer, which can be identified by optical interference effects. The monolayer was then lifted with the substrate and the sample was left for drying. After drying, 99.99% pure silver was deposited in a vacuum chamber by thermal evaporation. Finally, the polystyrene spheres were removed with sticky tape revealing an array of silver, triangular shaped NPs adhering on the substrate. A typical SEM image of the resulting nanostructures can be seen in Figure 2c. We demonstrated that it is possible to use porous silicon as a substrate for this NSL process. This is not evident since porous silicon is very hydrophobic. To attain a maximum reflectance the 1DPCs were fabricated as quarter-wave stacks, where the optical thickness of the layers is ݊ଵ ݀ଵ = ݊ଶ ݀ଶ = ߣ଴ /4 and the PBG is centered at the wavelength ߣ଴ or the photon energy ‫ܧ‬଴ for normal incidence. Consequently, the spectral position of the PBG center can be tuned by the thicknesses of the layer pairs. The width of the PBG is determined by the contrast of the refractive indices. Further details on 1DPCs are provided in the Supporting Information. The samples were characterized by reflectance measurements at 15° incidence (J.A. Woollam, VASE). Figure 3a,b shows the reflectance spectrum of a 10 period 1DPC designed with a PBG at ‫ܧ‬଴ = 2.0 eV corresponding to Figure 2b in comparison to the silicon substrate before etching. The modeled spectrum is shown in Figure 3c. The shift of the


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PBG center to higher energies can be explained by the non-normal incidence, while the decreasing reflectance for energies above 2.5 eV is caused by an increasing optical absorption of silicon and the roughness of the interfaces (Supporting Information, Figure S5).

Figure 3. (a) Measured reflectance spectra of the silicon substrate and (b) the PS 1DPC shown in Figure 2b. (c) Modeled reflectance spectrum of the PS 1DPC. (d) Geometry used in the COMSOL® simulation. The sharpness of the tips can be adjusted by the angle θ. (e) Measured transmittance (green) and reflectance spectra (blue) of the silver NP array on a glass substrate. (f) Simulated electric field at the PR. (g) Simulated absorbance (red), transmittance (green) and reflectance spectra (blue) of the silver NP array on glass. In order to study the optical properties of the silver NP array, we first prepared samples with 450 nm diameter nanospheres on glass substrates. The desired silver film was 75 nm and monitored by a quartz crystal microbalance. After the nanosphere mask was removed with sticky


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tape silver triangles with an edge length of about 100 nm remained on the substrate. The samples were then characterized by transmittance and reflectance measurements (J.A. Woollam, VASE) at normal and 15° incidence, respectively (Figure 3e). The peak in the reflectance and the attenuated transmittance in Figure 3e indicate an increased extinction and correspond to the PR of the silver nanostructure. The spectral position of the PR depends on the metal, shape, and sizes of the NPs which were investigated elsewhere.28-30 Here, the parameters of the metal NP array were fixed to the values stated above. To get a better insight into the optical properties, we simulated the silver NP array with COMSOL® using finite-element methods. Figure 3d depicts the schematic of the geometry used in the simulation. The structure was modeled using periodic boundary conditions on the sides of a hexagonal domain and port boundary conditions at the top and bottom. For the refractive index of silver we used the data of Johnson and Christy.31 For the substrate ݊ = 1.5 was employed. The other parameters were adjusted to match the silver nanostructure shown in Figure 2c. The polarization was chosen to be along the bisector of the triangles. Figure 3g shows the simulated absorbance, transmittance, and reflectance spectra. The experimentally measured shapes of the PR (Figure 3e) are less distinct and broader than the ones determined from the simulation, which can be attributed to deviations from perfect periodicity and NP geometry in the samples. The spectra in Figure 3g are remarkable, since only about 10% of the glass surface is covered by silver leading to an almost zero transmission at the PR. This can be explained by a collective interaction of the NPs and the excitation of a so-called lattice plasmon resonance. The arrangement of these nanoparticles in periodic arrays under conditions of coupling leads to an amplification of the PR compared to the single nanoparticle case due to a collective interaction.32,33


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Results and Discussion We fabricated composite structures (Figure 1a) using the PS based 1DPC as substrate for the NSL process. Again, the samples were characterized by reflectance measurements. The resulting spectra are shown in Figure 4a-c. The position of the PBG of the 1DPC was tuned to meet the PR of the silver NP array. In Figure 4a the reflectance spectrum of the 1DPC is nearly unchanged by the PR. If the PBG is shifted to higher energies, the reflectance in the PBG suddenly drops (Figure 4b,c), indicating a significant interaction of the structures. However, the coupled resonance seems to shift to higher energies with respect to the PR on glass. Because of its high porosity, the effective refractive index of the top layer is smaller than the refractive index of glass (Supporting Information, Figure S4), leading to a shift of the PR to higher energies.29 As a result, the resonance of the composite structure emerges at higher energies. Since the 1DPC is highly reflective in the PBG, the reflectance dip can only be attributed to an enhanced optical extinction. A possible explanation is that the silver NP array in combination with the 1DPC acts as a nanocavity and leads to a confinement of the electromagnetic field at the surface of the 1DPC and to an enhanced optical extinction in the silver NPs. This mechanism is comparable to the excitation of Tamm plasmons reported previously where the interface of a 1DPC and a thin metal layer was considered.34 There, the electromagnetic field is vertically confined at the interface due to the reflection of the metal on one side and the PBG on the other (Supporting Information, Figure S6). Here, instead of the metal layer, the partial reflection of the silver NP array at the PR (Figure 3e,g) leads to a localization of the electromagnetic field at the surface if the PR coincides with the PBG. The key advantage is that the highly confined and localized field can interact with materials deposited on the surface.


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Figure 4. (a-c) Plots of the reflectance of the composite structure (blue) for different spectral positions of the PBG center (solid gray) along with the transmittance of the corresponding silver NP array on a glass substrate (dashed grey). (d) Raman spectra of about 2 nm CoPc on top of silicon (black), PS 1DPC (green), silver NP array on silicon (red), and the resonant composite structure (b) (blue). The samples were excited with a photon energy of 1.92 eV (647.1 nm) close to the resonant interaction of the composite structure (b). To demonstrate the localized field enhancement and its application for SERS, we performed Raman spectroscopy after depositing an ultra-thin film of about 2 nm of the organic molecule cobalt phthalocyanine (CoPc) on top of the relevant structures (Figure 4d). The spectra were measured using a DILOR XY 800 (Jobin Yvon) spectrometer in a backscattering macroconfiguration with a spot size of about 300 µm. All samples were excited with a Kr+ laser (647.1 nm, 1.92 eV) to achieve resonant Raman scattering and to be close to the resonant interaction of the composite structure in Figure 4b. A power of about 10 mW, resulting in a power density of about 35 mW/cm2, was used. The spectra in Figure 4d were obtained after subtraction of the background. For the determination of the EF the Raman peak at 1541 cm-1 was


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used and the EF was calculated by EF = ‫ܫ‬ୗ୉ୖୗ /‫ܫ‬଴ where ‫ܫ‬ୗ୉ୖୗ are the SERS intensity and ‫ܫ‬଴ the Raman intensity on the silicon substrate. For clarity the EF was simply calculated by the ratio of the peak intensity. The peak maxima were obtained by a fit. One can see that the intensity of the Raman signal of 2 nm CoPc on silicon, despite the resonance Raman condition, is very weak. For the PS 1DPC, the signal is significantly enhanced by a factor of about 10. Since the reflectance is only increased by a factor of two (Figure 3a,b), the enhancement effect is likely to be related to an interference enhanced Raman scattering effect.35,36 The roughness of the surface also contributes to the EF. The silver NP array on silicon, which is commonly employed as a SERS substrate, shows a large EF of about 100 due to the excitation of surface plasmons.8,37 The substrates were stored in air between the preparation and the CoPc deposition. Therefore, the silver is covered by an oxide layer which prevents the direct contact of the CoPc molecules and the silver reducing contributions of chemical enhancement.38 Compared to a thick layer of CoPc no shifts in the Raman spectrum are present confirming a weak influence of chemical enhancement effects. Finally, for the combined structure we observe an extraordinary enhancement of several orders of magnitude larger than any of the individual structures. The measurement was performed for the composite structure shown in Figure 3b. In reference to bare silicon as substrate the signal is enhanced by a factor of about 105. We fabricated two different batches of samples with a sample-to-sample variation of the EFs of less than 10%. For each sample the SERS spectrum was measured for 10 different spots on an area of 10 mm2 with a spot-to-spot variation of about 10%. The EF of the combined structures in comparison to the silver NP array on silicon cannot be explained by the increased reflectivity alone, but needs an interaction mechanism. The already strongly enhanced signal of the silver NP array is further enhanced by a factor of 1000. The off-resonance structure (Figure


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4a) shows an EF of about 100, close to the EF of the silver triangles alone, providing further evidence for a coupling. Previous reports on SERS utilizing PS were either based on disordered silver NP on a single PS layer or silver-coated silicon nanopores.39,40 Here, we go well beyond these techniques by combining periodic PS layers and ordered silver NP arrays. Conclusion In conclusion, we developed a unique photonic crystal plasmonic hybrid system that might provide a new way of controlling and enhancing the optical response of plasmonic resonances. We provide an efficient SERS substrate with EFs up to 105 where we gained a factor of 1000 by placing a well-known SERS structure on top of a porous silicon distributed Bragg reflector. The etching of PS outperforms other fabrication techniques due to the simplicity of fabrication and the compatibility with silicon technology. In order to get the desired optical properties, the porosity and thickness of the layers can be easily tuned by the etching current, even making it possible to change the effective refractive index continuously.41 Furthermore, the porosity of the structure yields a very large surface area and allows the infiltration of fluids into the structure, opening further perspectives for refractive index based sensing. NSL is a cost-effective and relatively easy-to-implement method leading to highly reproducible plasmonic structures. The optical properties of the composite structures are very versatile, offering a promising platform for improved sensing applications and SERS substrates. Supporting Information The Supporting Information is available on the ACS Publications Website at DOI: Details on 1D Photonic Crystals, Sketch of the Etching Cell, Optical Model for Porous Silicon, Calibration Measurements, Reflectance Modeling, Numerical Simulations of a Tamm Plasmon


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Corresponding Author *[email protected] Present Addresses †Molecular Nanophotonics, Universität Leipzig, 04317 Leipzig, Germany Conflict of Interest The authors declare no competing financial interest. Acknowledgments This research received no specific grant from any funding agency. The authors acknowledge Axel Fechner for his technical support and helpful discussions. References (1)

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