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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Using a Fabry-Perot Cavity to Augment the Enhancement Factor for SERS and TERS Yinsheng Guo, Song Jiang, Xu Chen, Michael Mattei, Jon A. Dieringer, John P. Ciraldo, and Richard P. Van Duyne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05253 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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The Journal of Physical Chemistry

Using a Fabry-Perot Cavity to Augment the Enhancement Factor for SERS and TERS Yinsheng Guo,† Song Jiang,† Xu Chen,‡ Michael Mattei,† Jon. A. Dieringer,† John P. Ciraldo,¶ and Richard P. Van Duyne∗,†,‡

† Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡ Applied Physics Graduate Program, Northwestern University, Evanston, Illinois 60208, United States ¶ Northwestern University Micro/Nano Fabrication Facility (NUFAB), Evanston, Illinois 60208, United States

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ABSTRACT

We propose the use of Fabry-Perot cavities as a means to augment the enhancement factor in SERS and TERS experiments by optical interference. Using both clean-room and bench-top fabrication approaches, we demonstrate such a design can be readily realized and provides an additional 12× SERS enhancement and 5× TERS enhancement, in good agreement with expectations from electromagnetic modeling . The mechanism of optical interference enhancement is of far-field nature, and is independent of the enhancement mechanisms relying on plasmonic and molecular resonances. Therefore the Fabry-Perot cavity substrate can be applied generally without material and molecular limitations. The Fabry-Perot cavity structure provides enhancement at large incidence angles away from surface normal, particularly suitable for low-light Raman measurements with side illumination.

Introduction As surface-enhanced Raman spectroscopy (SERS) has evolved over the years, it has not only become a major analytical technique reaching single-molecule sensitivity, but has also been combined with ultrafast spectroscopies1,2 and scanning probe microscopies3,4 to yield chemical information at molecular time and length scales.5–7 In pursuit of the ultimate sensitivity and space-/time- resolution, signal photons become increasingly scarce, motivating the need to maximize all possible enhancement mechanisms. The keys to previous successes of singlemolecule surface-enhanced Raman spectroscopy (SMSERS)8 and tip-enhanced Raman spectroscopy (TERS)9–11 investigations are plasmonic and molecular resonance enhancement.12,13 These enhancement mechanisms entail the use of a plasmonic substrate and focus on molecules with large systems of conjugated π-bonds. Substrate and molecule generality


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is crucial for developing TERS into a wide spread tool of nanoscale chemical imaging.14,15 In studies of heterogeneous catalysis and electrocatalysis,14,16,17 for example, many chemically significant systems involve non-plasmonic metals and non-resonant small molecules.18 To realize signal enhancement of the scarce Raman photons in a materials-general way, we demonstrate the use of Fabry-Perot cavities as the means to augment the enhancement factor in SERS and TERS experiments by optical interference. Methods SERS substrate fabrication To fabricate the reflector-spacer stack for SERS measurements, a 10 nm Ti layer, 100 nm Ag layer and 60 nm silicon oxide layer were sequentially deposited on top of 1 mm thick glass substrates. The Au nanoparticles used in the SERS measurements were provided by Cabot Security Materials and used as received. Briefly, trans-1, 2-bis(4pyridyl)ethylene (BPE) molecules were absorbed onto the surface of Au nanoparticles. The subsequently induced particle aggregation was arrested by the formation of silica shells. The few-particle aggregates enclosed by silica shells were separated and sorted by ultracentrifugation. Colloidal solutions with enriched dimer and/or trimer particles were used for the SERS measurements. Typical dimensions of the samples are about 60 nm thickness for the silica shell, and about 100 nm diameter for the Au nanoparticle.43 The SERS measurements were performed with 785 nm laser excitation at normal incidence. Fabry-Perot cavity and control substrate fabrication For Fabry-Perot cavities with PMMA spacers, glass substrates were first cleaned with piranha solution, rinsed with pure water (MilliQ), and blown dry with nitrogen. The bottom mirror was made by depositing 200 nm Au film


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onto the glass substrate using e-beam evaporation. The PMMA film was spin coated onto the Au surface. The top mirror was then made by depositing 20 nm Au film using e-beam evaporation. For Fabry-Perot cavities with a SiO2 spacer, a 200 nm Au film was deposited onto freshly cleaved mica using e-beam evaporation. A SiO2 film of specified thickness was then prepared using plasma-enhanced chemical vapor deposition (PECVD). Finally a 20 nm Au film was deposited using e-beam evaporation. Control samples were made for each Fabry-Perot cavity fabrication process using identical procedures, with only the deposition of cavity spacer excluded. Far field Raman spectroscopy The Fabry-Perot cavity and control samples were incubated in 10 µM Nile Blue ethanol solution for 15 min, and then rinsed with ample water. Raman spectroscopy was measured with 633 nm laser excitation. Laser power was kept at 1 mW for all far field normal Raman measurements. The laser is coupled into a home-built microscope and focused by an objective with 0.5 NA onto sample surface at 55 degrees from surface normal. The scattered light was collected by the same objective, filtered and coupled into a multi-mode optical fiber. The fiber transmitted signal was then dispersed by a spectrograph and imaged onto a CCD. All experimental parameters were kept the same for both Fabry-Perot cavity and control samples. Tip-enhanced Raman spectroscopy The FP cavity and Au film control samples were immersed in about 8 mM biphenyl-4-thiol (BPT) benzene solution overnight. The samples were rinsed with benzene and methanol, then blown dry. STM-based side-illuminated TERS measurements were done under ambient conditions using the home-built microscope as described in the above section. An electrochemically etched Au tip was used as the STM probe. All TERS spectra were


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collected under 0.1V bias and 1nA current. The laser power was kept at 0.8 mW for all TERS measurements. Micro-reflectance spectroscopy The cavity length was obtained by measuring the interference fringes produced by the Fabry-Perot cavity in reflectance, and fitting to the transfer matrix model. Briefly, a quartz-tungsten-halogen fiber lamp was used as the broad band white light source. A 50 µm pinhole was mounted after the lamp output to serve as a point source. The white light emitted through the pinhole was collimated by an achromat lens, coupled into an inverted microscope, and imaged onto the Fabry-Perot cavity substrate at the focal plane of the objective. The reflected light was collected by the same objective, transmitted through a broad band beam splitter, and sent into a spectrometer equipped with a liquid-nitrogen cooled CCD. Results and Discussion Optical interference from multiple reflections in a thin film stack is a well-understood phenomenon. Tuning such interference effects to achieve desired reflection and transmission is part of the core techniques of optical engineering.19 Harnessing the optical interference effect was a key for the discovery of graphene,20 and was later used to understand the enhancement of Raman scattering seen in 2D materials.21,22 Optical interference within a thin film stack can be calculated by tracing individual reflections in simple cases,21,22 and can also be modelled in a general manner using the transfer matrix method.19,23 Figure 1 demonstrates that optical interference within a simple thin film stack provides an additional 12× enhancement factor for SERS. Aggregates of about 100 nm diameter Au nanoparticle24–26 were functionalized with trans-1, 2-bis(4-pyridyl)ethylene (BPE) molecules and encapsulated in 60 nm silica shell. Such a configuration has already proved to yield a SERS


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enhancement factor of a few times 108.25 The simplest configuration for multi-beam optical interference is perhaps a spacer-reflector stack. Raman enhancement from optical interference has been reported for 2D materials and molecular intercalates inside 2D materials.21,22 Utilization of similar interference enhancement has also been reported for SERS.27 When the Au nanoparticle aggregates were placed on top of the spacer-reflector stack (60 nm SiO2 – 100 nm Ag), the SERS signals were robustly enhanced by an extra factor of about 12 with 785nm Raman excitation. This additional factor reliably brings the achievable enhancement factor into the regime of a few times 109.24,28 We modeled the optical interference enhancement with transfer matrix methods. This signal enhancement from optical interference agrees well with modeling within the uncertainty of the exact positions of hot spots. The simple spacer-reflector stack is of value for normal Raman and SERS at normal incidence. Challenges arise for TERS applications. STM-based TERS measurements entail the use of conductive substrates. Many TERS measurements are also done with a side illumination/collection geometry. Under these considerations, we show below that a Fabry-Perot cavity is an ideal design which is both simple and efficient for typical TERS experiments. Figure 2 shows the proposed Fabry-Perot cavity substrate structure. It consists of two planar mirrors aligned in parallel to face each other. A light beam travelling inside of the FabryPerot cavity reflects off the end mirrors multiple times. All orders of reflections are superimposed to render multi-beam interference. The Fabry-Perot cavity substrate is made of a thin film stack of two metallic reflective layers sandwiching a dielectric spacer layer. The optical path length within the dielectric layer is determined by its thickness and refractive index. The exact thickness of the dielectric spacer can be optimized for specific experimental conditions such as laser wavelength and incidence angle. The top mirror is thin and semi-transparent to


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allow coupling between optical fields inside and outside of the Fabry-Perot cavity. This stack of thin films are deposited on to a solid support. Sample molecules are deposited on the outer surface of the top mirror. In the experiments to be reported below, side illumination configuration with a single microscope objective was used; both excitation and scattered light were defined by the same objective at the same angle away from surface normal. We note the model and design demonstrated here can accommodate arbitrary experimental configurations, including different incidence and scattering angles, and various excitation and signal wavelengths. Figure 3 shows the signal enhancement factor as a function of Fabry-Perot cavity parameters, calculated using the transfer matrix model. At a finite incidence angle away from surface normal, the Fabry-Perot cavity Raman substrate provides a far-field enhancement that depends on the polarization state of light. For TERS in particular, the alignment of tip-samplesubstrate is along the surface normal. This configuration favors p polarized excitation.29 Figure 3a and 3b map the enhancement as a function of cavity length and incidence angle. As incidence angle increases, the effective enhancement factor increases. For common incidence angles in a side illumination configuration, an enhancement factor on the order of a few tens can be expected. In the experimental demonstration reported in Figure 4, we used 633 nm excitation incident at 55 degrees. Figure 3c and 3d plots the enhancement profile of the optimized cavity structure under these conditions. The thickness of the top mirror determines the coupling of light waves in and out of the cavity, and thus the eventual enhancement factor. A top mirror that is too thin suffers from reduced reflectivity and a weakened interference effect, and results in limited enhancement. A top mirror that is too thick suffers from minimal transmissivity and the FabryPerot cavity eventually reduces to bulk metal substrate. Figure 3e shows this trade-off of top mirror thickness on interference enhancement.


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Next we experimentally verify the performance of signal enhancement of the structural design. We constructed a Fabry-Perot cavity using parameters determined in Figure 3. Au cavity end mirrors were fabricated using e-beam evaporation. The thickness of the bottom and top mirrors are 200 nm and 20 nm respectively. A layer of SiO2 cavity spacer with a precise thickness of 192 nm was deposited using plasma-enhanced chemical vapor deposition (PECVD). A non-intrusive measurement of local thickness is done by measuring the optical reflectance using a white light spot with a radius of a few microns. The reflectance spectrum thus measured carries interference fringes with which one is able to uniquely determine the local cavity length. (Supporting Information figure S2) Figure 4a shows the interference enhancement of far field resonance Raman scattering on a Fabry-Perot cavity. Physisorbed Nile Blue (NB) was used as a test molecule. Raw Raman spectra showed sharp molecular Raman modes riding on top of a prominent luminescence background, subsequently removed by baseline fitting (Supporting Information figure S3). Comparing with the Au film, Raman scattering on the Fabry-Perot cavity substrate is enhanced across the entire recorded spectral range. Three relatively strong modes of NB are selected to provide a quantitative measure of enhancement factor. The inset of Figure 4a shows the comparison of NB modes at 590 cm−1, 1180 cm−1, and 1640 cm−1 on the Fabry-Perot cavity and the Au film substrates. The Raman scattering intensities for all three modes are enhanced by about a factor of 20. By averaging over the micron-sized illumination area, far field Raman scattering provides a robust assessment of the overall enhancement capability. Figure 4b shows the interference enhancement of a near field TERS measurement on the Fabry-Perot cavity. Chemisorbed biphenyl-4-thiol (BPT) was used as the test molecule. Owing to the lack of electronic resonance at the laser excitation wavelength, the Raman cross-section of


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BPT is ~ 10-29 cm2sr-1 and can be observed in our experiment only with the near field enhancement from the tip plasmons. Similar to the far-field case, the Raman scattering intensity is enhanced across the spectral range, as shown by the comparison of BPT modes at 1079 cm−1, 1282 cm−1, and 1584 cm−1. For TERS spectra shown in figure 4b, the Fabry-Perot substrate provided an additional interference enhancement factor of about 5×. We note that the TERS results exhibit more variability, and consequently a broader range of enhancement factors, than that of the far field measurements. (Supporting Information figure S4). This is likely due to inadvertent change of tip/sample morphology during measurements, and/or spatially varying tipsample interactions. The Fabry-Perot cavity substrates can be readily produced with a variety of techniques without using lithographical methods. In the demonstration of Figure 4, both end mirrors and the dielectric spacer were fabricated with vapor deposition techniques. Physical and chemical vapor deposition (PVD and CVD), as well as other thin film technologies are already widely used in optical and photonic engineering. Besides PVD/CVD based methods, a variety of other options are available for the fabrication of Fabry-Perot cavities. We also fabricated Fabry-Perot cavities using spin-coated poly(methyl methacrylate) (PMMA) as the dielectric spacer layer. Similar enhancement from optical interference is obtained. (Supporting Information figure S5 and S6) Compared to PVD, spin coating polymers is a more facile, accessible, and low-cost fabrication route, with trade-offs of having less accurate thickness control and larger spatial variance. For fabrication of cavity end mirrors, template stripping can be used to provide atomically flat metal films.30–32 Colloidal synthesis provides a chemical route to produce large (tens of µm in xy length) and thin (tens of nm in z height) single crystal Au platelets.33–35 The above mentioned techniques provide complementary alternatives to PVD based fabrications.


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Utilizing the interference enhancement of a Fabry-Perot cavity design presents several advantages. The signal enhancement is produced by modulating the far-field propagating light through optical interference within the stack structure. Such an enhancement mechanism is independent of the enhancement from plasmonic and molecular resonances. Thus the enhancement from optical interference is generally applicable to wide classes of materials and molecules. The enhancement from a Fabry-Perot cavity can be obtained in conjunction with established near-field enhancement mechanisms with little complication, in a multiplicative manner. The top mirror layer can be fabricated with noble metals, as shown here, to support gap mode plasmonic resonances in the same way as previous SERS and TERS studies.9,11,36 Although the enhancement from optical interference might seem modest compared with plasmonic and resonant mechanisms, the one to two orders of magnitude stronger signal directly translates into shorter acquisition time, contributing to higher imaging speed and faster kinetics measurement. Such improvements are particularly valuable in low-light situations such as SMSERS and TERS. Apart from the spectroscopic advantages, the design of a Fabry-Perot cavity inherently involves metal surfaces for reflection of electromagnetic fields. This accessibility of top metal surface makes Fabry-Perot cavity compatible with electrochemical measurements and scanning tunneling microscopy. Compared with the homologous reflector-spacer substrate, the Fabry-Perot cavity features a reflector-spacer-reflector geometry. Such a simple difference has important consequences. At large incidence angles on the reflector-spacer substrate, the angular tolerance range of the enhancement becomes increasingly narrow, with a constant maximal enhancement factor. In contrast, the Fabry-Perot cavity performs well at large incidence angles; the angular tolerance range of the enhancement broadens at large incidence angles. The enhancement factor


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also increases as incidence angle increases. Both behaviors render the Fabry-Perot cavity a uniquely suitable substrate design for measurements with side illumination.37 In addition, a unique feature of the Fabry-Perot cavity substrate is the large difference in the polarization response to p- and s- polarized light at intermediate incidence angles. This is produced by the phase jump at reflection as determined by Fresnel’s law.19 Compared with the reflector-spacer structure (Supporting Information figure S1), the Fabry-Perot cavity has more orders of reflected light significantly participating in modulating the final output. Thus the differential polarization response becomes pronounced and visible. This feature could provide signal enhancement for polarization based optical measurements. The Fabry-Perot cavity is a simple and elegant structure that has been used in many fields. For instance, it has been used to couple optical field with atoms,38 molecules and elementary excitations in condensed matter systems39. Similarly such a cavity is used to encapsulate photovoltaic materials to optimize their power conversion performance.40 For these examples the absorber or emitter is placed inside the cavity to modulate the available local photonic density of states.40 In other works, the Fabry-Perot cavity is used to boost sensitivity to changes in the extended optical path length. Such changes come from variation of refractive indices41 or even the space itself42. Different from these examples above, here we have designed and utilized the Fabry-Perot cavity as a photonic platform that couples to far-field propagating light fields with tunable coupling strength and adjustable angular distribution. This is an open platform with emitters situated outside the cavity, accessible to external chemistry, and compatible with near field probes. The signal enhancement applies not only to spontaneous Raman processes, but also to nonlinear optical spectroscopies (e.g. stimulate Raman scattering,


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coherent anti-Stokes Raman scattering, etc) and photoluminescence, as well as other photophysical measurements such as photocurrent mapping and photoconductivity spectroscopy. Conclusion In summary, we have proposed and demonstrated the use of Fabry-Perot cavities as Raman substrate to obtain about an order of magnitude signal enhancement from optical interference. In this design, signal enhancement comes from constructive interference in the propagating light waves. This type of far-field optical enhancement does not depend on, and applies in parallel to the enhancements from plasmonic and molecular resonances. Fabry-Perot cavities excel at signal enhancement with angled excitation and collection, and provide accessible metal surface to enable simultaneous STM and/or electrochemical measurements. These features are particularly beneficial to broadening the scope of SM-SERS and developing TERS for more general applications including catalysis and electrochemistry studies.


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FIGURES

Figure 1: Interference enhancement of SERS signal on spacer-reflector substrate. a) Schematics of SERS measurements of the silica encapsulated Au nanoparticle aggregates dispersed on a spacer-reflector substrate. b) Enhancement factor from optical interference calculated as a function of SiO2 spacer thickness. The exact distribution of hot spot locations are not known. The gray area between dashed lines denote the probable range. The lower limit is set by the spacer thickness, and the upper limit is set by the radius of the Au nanoparticle. c) Experimental observation of the enhanced Raman scattering of BPE. The enhancement factor on the reflector was typically ~12 times larger than on glass, bringing the overall enhancement factor into the 109 regime.


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Figure 2: Schematics of the Fabry-Perot cavity substrate. The Fabry-Perot cavity is fabricated by depositing stacks of thin films on a flat solid support. The bottom mirror is fully reflective, with its thickness larger than the optical skin depth. The cavity spacer is made of transparent dielectric material of predetermined thickness. The top mirror is semi-transparent for coupling the cavity mode to the incidence and scattered light. Sample molecules are deposited onto the top mirror just outside of the Fabry-Perot cavity. Raman excitation laser is focused onto the sample surface at an angle θ. Collection of Raman scattered light is done using the same focusing optics at the identical angle θ.


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Figure 3: Modeling interference enhancement of Raman scattering on top of the Fabry-Perot cavity substrate. Enhancement factor (EF) is calculated as a function of incidence angle and cavity length, for (a) p-polarized incident light and p-polarized component of scattered light. (b) p-polarized incident light and s-polarized component of scattered light. Refractive index of SiO2 is used for the cavity spacer. Excitation wavelength is chosen to be 633 nm. Using experimental parameters, line sections of the enhancement profile are shown in (c) for EF as a function of cavity length at an incidence angle of 55 degree, and in (d) for EF as a function of incidence angle at a cavity length of 192 nm. In (e) EF is plotted as a function of top mirror thickness.


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Figure 4: Comparison of measured Raman scattering intensity on Fabry-Perot cavity and on Au film substrates. The FP cavity was formed by depositing 192 nm of SiO2 using PECVD method. Raman scattering was measured using 633 nm laser excitation at 55 degree incidence angle, parameters identified in Figure 3. a) Far field normal Raman scattering of physisorbed Nile Blue (NB) molecules on FP cavity (red) and on Au film (black and blue). Inset shows Raman scattering intensity comparison of three strong modes across the spectral range. Blue traces are Raman spectra of NB on Au film scaled by a factor of 20. Spectra are vertically offset for the clarity of display b) Tip-enhanced Raman scattering of chemisorbed biphenyl-4-thiol (BPT) molecules on FP cavity (red) and on Au film (blue). Black trace shows no BPT modes were observable in the far-field Raman scattering when tip was withdrawn. For TERS spectra shown in b), the BPT Raman intensity on Fabry-Perot cavity is about a factor of 5 stronger than on Au film.


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ASSOCIATED CONTENT Description of transfer matrix modelling, modelling of the spacer-reflector configuration, optical reflectance measurement of cavity spacer thickness, background continuum on Fabry-Perot cavity substrate, Variability of TERS intensity of BPT, Fabry-Perot cavity with spin-coated PMMA spacer. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID? Yinsheng Guo 0000-0002-0571-8447 Song Jiang 0000-0002-2223-8517 Xu Chen 0000-0002-2603-4837 Michael Mattei 0000-0002-8276-5562 Richard P. Van Duyne 0000-0001-8861-2228 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We thank Steven J. Byrnes for helpful discussions. Y.G. and R.P.V.D acknowledge support from the Department of Defense Vannevar Bush Faculty Fellowship program (N00014-17-1-3024) and the National Science Foundation Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CaSTL) Grant CHE-1414466. S.J., X.C., and M.M. acknowledge support from the Air Force Office of Scientific Research MURI (FA9550-14-1-0003). This work utilized Northwestern University Micro/Nano Fabrication Facility (NUFAB), which is partially supported by Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS1542205), the Materials Research Science and Engineering Center (NSF DMR-1121262), the State of Illinois, and Northwestern University.

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