Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced

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Nanogapped Au Antennas for Ultrasensitive SurfaceEnhanced Infrared Absorption Spectroscopy Liangliang Dong, Xiao Yang, Chao Zhang, Benjamin Cerjan, Linan Zhou, Ming Lun Tseng, Yu Zhang, Alessandro Alabastri, Peter Nordlander, and Naomi J. Halas Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02736 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 8, 2017

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Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced Infrared Absorption Spectroscopy Liangliang Dong* §, Xiao Yang¶ §, Chao Zhang♯ §, Benjamin Cerjan¶ §, Linan Zhou* §, Ming Lun Tseng♯ §, Yu Zhang¶ §, Alessandro Alabastri♯ §, Peter Nordlander¶ ♯ § and Naomi J. Halas* ¶ ♯ § † *

Department of Chemistry, ¶Department of Physics and Astronomy, ♯Department of Electrical

and Computer Engineering, §Laboratory for Nanophotonics and the Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005

†Corresponding

Author: Naomi J. Halas

E-mail: [email protected] ; Phone: (713)348-5611 KEYWORDS: SEIRA, nanogap, bowtie antenna, mixed self-assembled monolayers, FTIR

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Abstract Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This 3D geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 107, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ~500 molecules of 4-NTP and ~600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules, and lends insight into the ultimate limit of singlemolecule SEIRA detection. Introduction Vibrational spectroscopies are powerful and universal techniques for probing the molecular structure and dynamics of molecules. Infrared and Raman spectroscopies have complementary selection rules, a property that makes them, in combination, highly valuable spectroscopies for the identification of chemical unknowns. The discovery of surface-enhanced Raman spectroscopy, where the Raman cross section of molecules is greatly enhanced in the direct vicinity of metallic 2 ACS Paragon Plus Environment

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structures, resulted in a tremendous surge in scientific effort to understand the properties of the metal structures that provide large SERS enhancements. The observation that aggregates of noble/coinage metal nanoparticles could enhance Raman signal detection down to the single molecule level1 resulted in the discovery that closely coupled pairs of metallic nanoparticles, when excited along their interparticle axis, could provide extremely large enhancements of the incident electromagnetic field.2 These enhancements, known as “hot spots”, with nanoscale optical mode volumes far smaller than what can be achieved using conventional diffraction-limited optical focusing, are one of the most important discoveries in the field of nanoscience, fueling the field of plasmonics and nanoscale chemical sensing. In contrast, SEIRA has not advanced as rapidly as SERS for several reasons. SEIRA is a linear spectroscopy, where electromagnetic enhancement provided by plasmonic focusing scales 2

as|𝐸(𝜔)|2 of the incident field, as compared to |𝐸(𝜔𝑝𝑢𝑚𝑝 )| |𝐸(𝜔𝑆𝑡𝑜𝑘𝑒𝑠 )|2 for the nonlinear Raman process. As a result, the theoretical maximum field enhancement for SEIRA is far smaller than for SERS. However, as a linear spectroscopy, the cross sections of dipole-allowed, IR-active molecular vibrational modes (e.g., ~10-19 cm2 molecule-1 for a C-H stretching mode)3, 4 are already orders of magnitude larger than the cross sections of Raman modes (e.g., ~10-28 cm2 molecule-1 for the symmetric stretch modes of benzene).5, 6 Pioneering work in the development of SEIRA was focused on the design of mid-IR antennas, which were typically excited by synchrotron IR sources. Incorporation of electromagnetic focusing of mid-IR wavelengths using IR-resonant nanoparticle aggregates,7 arrays,8, 9 and resonant antennas10, 11 designed for this spectral range have shown increasing SEIRA enhancements. While these developments have accelerated the development of SEIRA, the goal of bringing this spectroscopy into the regime of few- or singlemolecule sensitivity, analogous to SERS, is still an unmet challenge. In the area of IR antenna 3 ACS Paragon Plus Environment

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development, one current limitation is dictated by e-beam lithography, which under optimal conditions limits the size of the gap between patterned metallic nanostructures to ~5 nm. The substantially longer wavelengths required for SEIRA (3-8 microns) relative to SERS (0.5-0.8 microns) also constitute a design challenge in achieving a goal of enhanced detection sensitivity through strong electromagnetic enhancements. Efficient antenna designs with the largest possible cross sections12 are likely to be most beneficial. Many SEIRA substrate structures investigated thus far focused on rod or rod dimer-based antennas, 11, 13-20

whose aspect ratio is tailored so that their resonance matches the specific vibrational

frequency of interest of the analyte molecules. Other designs include split rings,10,

21

Fan

structures,22 asymmetric metamaterials,23 and log-periodic antennas.24 In general, these examples exploit capacitive coupling between adjacent structures to maximize field confinement and enhancement at the hot spot. The hot spot can exhibit even higher near-field enhancement by coupling plasmonic structures in narrower gaps.16, 25, 26 Nano-sized gaps have been fabricated using electron-beam lithography (EBL),27 focused ion-beam (FIB),28 electromigration,29 nanosphere lithography,30 photochemical metal deposition,16 and template stripping.25 Nevertheless, reproducible fabrication of an arbitrary plasmonic structure with nanoscale gaps remains a substantial challenge at present. In this work, we investigate the SEIRA responsivity of a bowtie31 plasmonic antenna design incorporating a sub-3 nm gap that is positioned above a Au film with a SiO2 spacer layer. We adapted a self-aligned technique32 to reproducibly fabricate antennas with ultrasmall nanometersized gaps not typically achievable using standard e-beam lithographic approaches. The extremely large field enhancement achievable with this antenna design allows us to detect very small numbers (~500) of aromatic-derivatized thiol molecules on the antenna using a standard 4 ACS Paragon Plus Environment

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commercial FTIR spectrometer. Compared to previous structures,22,

25

our optimized antenna

exhibits two orders of magnitude higher spectroscopic sensitivity. We also demonstrate controllable tunability to vibrational frequencies of several important molecules of relevance in infrared spectroscopy. Results and discussion The SEIRA samples were fabricated in patterns of four antennas (Figure 1a), separated by 16 𝜇m in the longitudinal direction and 20 𝜇 m in the transverse direction to avoid coupling. Each individual antenna (Figure 1b) consisted of two quarter circles with a radius R (2 𝜇m) and a trapezoidal portion with a shorter base i (45 nm), a longer base j (145 nm), and a height h (100 nm). The two wings of the bowtie were separated by a gap g (approximately 3 nm), whose size chiefly relies on the thickness of a sacrificial Cr layer (see Supporting Information methods). The benefit of the additional trapezoidal portion is to increase corner sharpness while maintaining the reduced polarization sensitivity of the bowtie geometry (Figure S1).

Figure 1. SEIRA antenna on a reflective substrate. (a-d) SEM images of a bowtie antenna array (a), a single antenna (b) and the nanogap at the center (c, d). The geometrical parameters are (R, i, 5 ACS Paragon Plus Environment

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j, h, g) = (2 𝜇m, 45 nm, 145 nm, 100 nm, 3 nm). (e) Illustration of the antenna-substrate geometry in (a-d). (f, g) Top-view diagram showing the geometric parameters of the bowtie antenna (f) and a zoomed-in diagram for the gap area (g). The geometric parameters, particularly the radius of the antenna (R), can be adjusted to optimize the SEIRA field enhancement (|𝐸/𝐸0 |2 ) for molecular sensing at different frequencies. For example, using a fixed gap size g = 3 nm, we adjusted R to achieve a maximum SEIRA enhancement at 2920 cm-1 (R = 650 nm), 2240 cm-1 (R = 1 𝜇m) and 1354 cm-1 (R = 3 𝜇m) (Figure S2), which correspond to the strong vibrational modes of alkane chains, nitrile groups and phosphonate compounds, respectively. The resonance wavelength is not sensitive to the other parameters, i, j, h (see Figure. 1f), and thus they are kept constant. By combining antennas with different radii, we could build up a broadband plasmonic sensor that covers a wide mid-IR spectral region. To overcome issues related to molecular absorption on gold many techniques have been previously exploited in the SERS literature. These include thiol or amine functionalization of target molecules and surface modification of the antenna with probe molecules33 or an ultrathin dielectric coating34. Such techniques could be readily applied to this device to allow for detection of a wide range of molecular species.


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Figure 2. Optical properties of SEIRA antennas on a reflective substrate. (a) FTIR reflectance spectra of antenna arrays with different R under oblique unpolarized illumination. (b) FDTD scattering (solid line) and field enhancement spectra (dash dot line) for a single antenna calculated with different R under normal unpolarized illumination. (c, d) Two-dimensional map of peak field enhancement (at 1536 cm-1) for a single antenna with R = 2 𝜇m (c) and in the junction region (d) on a log scale. (e) Illustration of antenna surface area in region 1 (blue), 2 (green), 3 (yellow) and 4 (red), corresponding to calculated SEIRA enhancements provided in Table I. Experimental reflectance spectra and theoretical scattering spectra for three radii are shown in Figure 2(a, b), respectively. The reflectance dip results from increased scattering of the antenna at its plasmon resonance frequency. The additional dip on the blue shoulder of the dipole resonance 7 ACS Paragon Plus Environment

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appears due to the oblique incidence angle used in the experiment (Figure S3). As R is increased, the plasmon resonance and the enhancement peak position redshift, and the peak field enhancement increases. The small discrepancy between the simulated and experimental antennas in the resonance frequency is likely due to the slight roughness of the experimental junction surface. Besides, the field enhancement can be improved by a factor of 2 if a surface roughness of ±1 nm is considered in the FDTD simulation (Figure S4). Spatial maps of the near-field enhancement for the longitudinal dipole mode of the full structure and the junction area are shown in Figures 2c and 2d, respectively. These maps clearly show that the strongest enhancement is in the gap region, and is most intense near the metal surface. To quantitatively investigate the spatial origin of the vibrational signals, we compared the integration of field enhancement over the surface area for four different regions (Figure 2e). The integration term considers the contribution from both the number of molecules attached to the surface (proportional to the surface area) and the field enhancement experienced by each molecule. The FDTD calculation results of this integration are tabulated in Table 1. The contribution from the junction strongly dominates; it is responsible for 93% of the total signal. Table 1. Average field enhancement (|𝐸/𝐸0 |2at 1540 cm-1), surface area (S), integration of field enhancement over the corresponding surface area and percentage of total signal from regions 1, 2, 3 and 4 shown in Figure 2(e). Region Averaged

1 (blue)

2 (green)

3 (yellow)

4 (red)

7.41E2

1.29E3

2.15E5

1.75E7

1.1E5

1.4E5

8.0E3

1.6E3

2

| 𝐸 ⁄𝐸0 |

Surface area 2

(nm )


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2

∫ | 𝐸 ⁄𝐸0 | 𝑑𝑆

8.2E7

1.8E8

1.7E9

2.8E10

6.0E-3

6.0E-2

0.93

2

(nm ) % of total signal 2.7E-3

The benefit of using a reflective substrate is clearly demonstrated in Figure S5, where SEIRA enhancement spectra are compared for an antenna with and without a reflective gold mirror. By introducing a reflective mirror, the field enhancement is increased by a factor of 12. The additional enhancement is due to the constructive interference between the incident and reflected light.35, 36 Additionally, the gold mirror supports image dipoles that couple with the LSPR of the antenna, which reduce the radiative losses and narrow the dipolar resonance linewidth.37 The thickness of the spacer layer needs to be appropriately adjusted to prevent destructive interference at the resonance frequency (Figure S6).


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Figure 3. Enhanced detection of 4-NTP and 4-MTP. (a, b) Baseline-corrected reflectance spectra of 4-NTP SAM (a) and 4-MTP SAM (b) on a single antenna (R = 2 𝜇m). Reference spectra of solid state 4-NTP and 4-MTP are shown in black, with prominent vibrations indicated by dashed lines. (c) Comparison of reflectance spectra from a single antenna after functionalization in mixed solutions (2 mM) of 4-NTP and 4-MTP with varied molar percentages. Prominent vibrations indicated in blue and red dashed lines are associated with 4-NTP and 4-MTP molecules, respectively. The nanogapped antenna was then tested for SEIRA enhancement by obtaining spectral features of 4-nitrothiophenol (4-NTP) and 4-methoxythiophenol (4-MTP) molecules. Molecular signals were extracted from the reflectance spectra by applying a baseline-corrected method adapted from asymmetric least-squares smoothing (AsLSS),38 to subtract the antenna spectrum as a baseline from the original data. The resulting baseline-corrected spectrum for a single antenna functionalized with a 4-NTP self-assembled monolayer (SAM) is shown in Figure 3a. The 10 ACS Paragon Plus Environment

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reference spectrum was acquired from a condensed slurry of 4-NTP dispersed onto a KBr disk. The blue dashed lines indicate positions of the three most prominent molecular vibrations: the aromatic ring (in-the-plane) stretch (1576 cm-1), the asymmetric NO2 stretch (1515 cm-1), and the symmetric NO2 stretch (1335 cm-1), similar to those observed in a previous study.39 Another single antenna with the same dimensions was used to analyze the vibrations of a 4-MTP SAM (Figure 3b). The red dashed lines indicate the spectral positions of the two most prominent vibrations: the aromatic ring (in-the-plane) stretches (1594 cm-1 and 1485 cm-1).40 The other two vibrational signals in the low-frequency region (i.e. C-O-C stretches at 1287 cm-1 and 1244 cm-1)40 are not observed in the antenna spectrum because of the very low SEIRA enhancement provided by the antenna at those off-resonant wavelengths. The Signal-to-noise ratio (SNR) of each vibrational spectral feature was then calculated for quantitative assessment of the signal strength. The signal intensity was found as the averaged dip depth from five individual samples in the vibrational resonance range; the noise level was found as the standard deviation in the same range measured on a single bare antenna (Figure S7). The SNR calculated for a 4-NTP SAM and for a 4-MTP SAM is tabulated in Table 2. All analyses were performed under identical conditions. Table 2. SNR calculated for a single antenna functionalized with 4-NTP SAM or 4-MTP SAM. Analytes

4-NTP SAM

4-MTP SAM

Molecular vibrations (cm-1)

1515

1335

1594

1485

Assignment

NO2 asym str.

NO2 sym str.

ring str. ip

ring str. ip

SNR (std*)

16.3 (±1.6)

16.8 (±2.4)

13.3 (±2.3)

34.2 (±6.1)

*std: Standard deviation from five individual antennas on the same substrate.


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To confirm that the dominant signal contribution is from the gap region, we compared the baselinecorrected spectra of gapped and connected antennas functionalized with a 4-MTP SAM (Figure S8). The connected antenna consists of the same two quarter circles that are connected at the tip and therefore the plasmon resonance under longitudinal polarization dramatically changes due to the bridge between two conductive regions. Nevertheless, the overall plasmon spectrum under unpolarized illumination looks very similar to that of a gapped antenna. More importantly, the near-field enhancement is negligible at the primary absorption peak of 4-MTP at 1490 cm-1. As a result, no molecular signal was observed in the spectrum of the connected antenna array after functionalization with the 4-MTP SAM. The FTIR reflectance spectra of individual antennas with varying radii (R = 2.0, 2.1, 2.2 and 2.4 𝜇m), functionalized with both 4-NTP and 4-MTP molecules, are shown in Figure S9. Two prominent vibrations were observed near 1334 cm-1 (4-NTP) and 1491 cm-1 (4-MTP). The vibration features appear as Fano resonances caused by the interference between the broad plasmon mode of the antenna and the discrete absorption peak of the molecule.41 The asymmetric lineshape is due to a phase shift in the decay of the dipole response42 and is associated with the relative peak positions of the interfering modes.43 The sensitivity of a single antenna for infrared detection was evaluated using a mixed SAM of 4NTP and 4-MTP. The mixed SAM was prepared using an ethanolic solution of 4-NTP and 4-MTP with four different ratios by molarity (with a total concentration of 2 mM). Four modes were observed (Figure 3c): the asymmetric and symmetric NO2 stretch (at 1513 cm-1 and 1332 cm-1, respectively) from 4-NTP molecules and the ring stretch (at 1599 cm-1 and 1490 cm-1, respectively) from 4-MTP molecules. As the portion of 4-NTP in the solution was decreased, its signal intensity 12 ACS Paragon Plus Environment

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became weaker, accompanied by a stronger signal intensity from the 4-MTP resonances. A closer look at Figure 3c shows random variations in the relative intensities of the asymmetric and symmetric NO2 stretching modes from the 4-NTP molecules. Such variations could be caused by differences in the specific orientations of 4-NTP molecules with respect to the enhanced near field. The vibrational mode that has its dipole moment parallel to the electric field is expected to have stronger Fano coupling. Similar effects of orientation dependence have been investigated in previous studies.44-46

Figure 4. Quantitative analysis of SEIRA signals. (a) Number of 4-NTP (blue) and 4-MTP (red) molecules on Au surface in the gap after functionalization in mixed solutions of varied molar percentages. Error bars represent standard deviations of the number of molecules obtained from five individual antennas on the same substrate. (b) Surface composition of 4-NTP in mixed SAM on a single antenna after functionalization in mixed solutions of varied molar percentages. Error bars represent standard deviations of surface coverage ratio obtained from five individual antennas on the same substrate. The dashed line is a guide to the eye for indicating nonpreferential absorption. The number of analyte molecules in the nanogap was determined using the integrated area under the absorption band and the surface packing density of pure SAMs. We first extrapolated the integrated area according to Gaussian fits as the signal intensity. The signal intensities of the strongest two vibrations for each analyte (see Figure 3c) were combined because of the random variations in their relative intensities. The combined signal intensity for 4-NTP (blue) and 4-MTP 13 ACS Paragon Plus Environment

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(red) in the mixed and their individual SAMs is shown in Figure S10. Assuming the absorption coefficients of each type of molecule in the mixed SAM are the same as their individual SAMs,47 the signal intensity is expected to be directly proportional to the number of molecules. We calculated the number of 4-NTP and 4-MTP molecules in the gap for each mixed SAM based on their signal intensities and the respective packing densities on a Au surface (4-NTP 2.07 nm-2,48 and 4-MTP 3.07 nm-2,49). Five different antennas with the same SAM were compared showing good reproducibility (Figure S11 and Figure S12). Based on this analysis, our nanogapped antenna can detect ~500 4-NTP molecules or ~600 4-MTP molecules using a conventional FTIR spectrometer with a standard broadband light source. A comparison of the surface fraction of 4-NTP versus its molar fraction in solution (Figure 4b) revealed that 4-NTP was slightly preferentially absorbed onto the nanogapped antenna. This behavior could be explained by the stronger attraction between the permanent dipole moment of 4-NTP and its image charge distribution at the substrate surface. Because of the electron acceptor properties of the nitro group and the electron donor properties of the methoxy group, 4-NTP has a larger permanent dipole moment than 4-MTP. A larger permanent dipole moment induces more image charges on the metal surface, thus forming a more stable electronic structure in the resulting SAM.50 As the concentration of 4-NTP in ethanol increases, its preferential adsorption decreases. This observation could be caused by the net intermolecular repulsion of the parallel dipoles.50 Further investigation would be necessary to definitively resolve the cause of this behavior. Nevertheless, this observation of preferential absorption is direct evidence that the single antenna is probing only a relatively small quantity of molecules in the junction. Conclusions


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In conclusion, we have demonstrated a nanogapped antenna on a reflective substrate with a field enhancement up to 107, which is two orders of magnitude higher than previous SEIRA designs.22, 25

This extremely high field enhancement enables us to detect as few as 500 molecules of 4-NTP

(and 600 molecules of 4-MTP) using a conventional FTIR spectrometer with a standard broadband light source. The device is primarily limited by photon flux – to improve SEIRA sensitivity a more powerful light source is the next step towards single-molecule measurements. Our investigations also showed that the antenna can be easily tuned to different wavelengths, allowing for broadband sensing throughout the mid-infrared range. These results represent an important step for optimizing antennas towards single-molecule mid-infrared detection. Supporting Information Supporting Information Available: [Antenna fabrication; functionalization with SAMs; FTIR spectroscopy; FDTD calculations; AsLSS; scattering and field enhancement spectra under different conditions, including polarized incidence, different radii, oblique incidence, rough gap and with no gold mirror; optimization of silica spacer thickness; baseline-corrected spectra of bare antenna; optical properties and baseline-corrected spectra of closed gap; scattering spectra of functionalized antenna with different radii; single intensity for different mixed SAMs; reproducibility for pure SAMs and mixed SAMs.].

Acknowledgments This work was supported by the Robert A. Welch Foundation under grant C-1220 (NJH) and C1222 (PN), the Army Research Office under grant W911NF-12-1-0407, the National Science Foundation (NSF) grant ECCS-1610229, the Air Force Office of Scientific Research 15 ACS Paragon Plus Environment

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Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI FA9550-15-1-0022), and the Defense Threat Reduction Agency (DTRA) under grant HDTRA116-1-0042. The authors declare no competing financial interest. References 1. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. Rev. Lett. 1997, 78, (9), 1667-1670. 2. Michaels, A. M.; Jiang, J.; Brus, L. J. Phys. Chem. B 2000, 104, (50), 11965-11971. 3. Brauer, C. S.; Blake, T. A.; Guenther, A. B.; Sharpe, S. W.; Sams, R. L.; Johnson, T. J. Atmos. Meas. Tech. 2014, 7, 3839-3847. 4. Harrison, J. J.; Allen, N. D. C.; Bernath, P. F. J. Quant. Spectrosc. Radiat. Transf. 2012, 113, 2189-2196. 5. Schomacker, K. T.; Delaney, J. K.; Champion, P. M. J. Chem. Phys. 1998, 85, 4240. 6. Trulson, M. O.; Mathies, R. A. J. Chem. Phys. 1986, 84, 2068. 7. Kundu, J.; Le, F.; Nordlander, P.; Halas, N. J. Chem. Phys. Lett. 2008, 452, (1), 115-119. 8. Pucci, A.; Neubrech, F.; Weber, D.; Hong, S.; Toury, T.; de la Chapelle, M. L. Phys. Status Solidi (b) 2010, 247, (8), 2071-2074. 9. Abb, M.; Wang, Y.; Papasimakis, N.; de Groot, C. H.; Muskens, O. L. Nano Lett. 2014, 14, (1), 346-352. 10. Cataldo, S.; Zhao, J.; Neubrech, F.; Frank, B.; Zhang, C.; Braun, P. V.; Giessen, H. ACS Nano 2012, 6, (1), 979-985. 11. Brown, L. V.; Zhao, K.; King, N.; Sobhani, H.; Nordlander, P.; Halas, N. J. J. Am. Chem. Soc. 2013, 135, (9), 3688-3695. 12. Grober, R. D.; Schoelkopf, R. J.; Prober, D. E. Appl. Phys. Lett. 1997, 70, (11), 1354-1356. 13. Adato, R.; Altug, H. Nat. Commun. 2013, 4, 2154. 14. Aksu, S.; Cetin, A. E.; Adato, R.; Altug, H. Adv. Opt. Mater. 2013, 1, (11), 798-803. 15. Bochterle, J.; Neubrech, F.; Nagao, T.; Pucci, A. ACS Nano 2012, 6, (12), 10917-10923. 16. Huck, C.; Neubrech, F.; Vogt, J.; Toma, A.; Gerbert, D.; Katzmann, J.; Härtling, T.; Pucci, A. ACS Nano 2014, 8, (5), 4908-4914. 17. Liberman, V.; Adato, R.; Mertiri, A.; Yanik, A. A.; Chen, K.; Jeys, T. H.; Erramilli, S.; Altug, H. Opt. Express 2011, 19, (12), 11202-11212. 18. Neubrech, F.; Beck, S.; Glaser, T.; Hentschel, M.; Giessen, H.; Pucci, A. ACS Nano 2014, 8, (6), 6250-6258. 19. Kusa, F.; Ashihara, S. J. Appl. Phys. 2014, 116, (15), 153103. 20. Cerjan, B.; Yang, X.; Nordlander, P.; Halas, N. J. ACS Photonics 2016, 3, (3), 354-360. 21. Cubukcu, E.; Zhang, S.; Park, Y.-S.; Bartal, G.; Zhang, X. Appl. Phys. Lett. 2009, 95, (4), 043113. 22. Brown, L. V.; Yang, X.; Zhao, K.; Zheng, B. Y.; Nordlander, P.; Halas, N. J. Nano Lett. 2015, 15, (2), 1272-1280. 23. Wu, C.; Khanikaev, A. B.; Adato, R.; Arju, N.; Yanik, A. A.; Altug, H.; Shvets, G. Nat Mater. 2012, 11, (1), 69-75. 16 ACS Paragon Plus Environment

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24. Aouani, H.; Šípová, H.; Rahmani, M.; Navarro-Cia, M.; Hegnerová, K.; Homola, J.; Hong, M.; Maier, S. A. ACS Nano 2013, 7, (1), 669-675. 25. Chen, X.; Ciracì, C.; Smith, D. R.; Oh, S.-H. Nano Lett. 2015, 15, (1), 107-113. 26. Li, Y.; Simeral, M. L.; Natelson, D. J. Phys. Chem. C 2016, 120, (39), 22558-22564. 27. Zhu, W.; Crozier, K. B. Nat. Commun. 2014, 5, 5228. 28. Melli, M.; Polyakov, A.; Gargas, D.; Huynh, C.; Scipioni, L.; Bao, W.; Ogletree, D. F.; Schuck, P. J.; Cabrini, S.; Weber-Bargioni, A. Nano Lett. 2013, 13, (6), 2687-2691. 29. Ward, D. R.; Corley, D. A.; Tour, J. M.; Natelson, D. Nat Nano. 2011, 6, (1), 33-38. 30. Hoffmann, J. M.; Janssen, H.; Chigrin, D. N.; Taubner, T. Opt. Express 2014, 22, (12), 1442514432. 31. Muhlschlegel, P.; Eisler, H. J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W. Science 2005, 308, (5728), 1607-1609. 32. Fursina, A.; Lee, S.; Sofin, R. G. S.; Shvets, I. V.; Datelson, D. Appl. Phys. Lett. 2008, 92, (11), 113102. 33. Cao, Y. C.; Jin, R.; Mirkin, C. A. Science 2002, 297, (5586), 1536. 34. Li, J. F.; Huang, Y. F.; Ding, Y.; Yang, Z. L.; Li, S. B.; Zhou, X. S.; Fan, F. R.; Zhang, W.; Zhou, Z. Y.; WuDe, Y.; et al. Nature 2010, 464, (7287), 392-395. 35. Ahmed, A.; Gordon, R. Nano Lett. 2012, 12, (5), 2625-2630. 36. Wang, D.; Zhu, W.; Best, M. D.; Camden, J. P.; Crozier, K. B. Nano Lett. 2013, 13, (5), 21942198. 37. Sobhani, A.; Manjavacas, A.; Cao, Y.; McClain, M. J.; García de Abajo, F. J.; Nordlander, P.; Halas, N. J. Nano Lett. 2015, 15, (10), 6946-6951. 38. Eilers, P. H. C. Anal. Chem. 2003, 75, (14), 3631-3636. 39. Zhang, Z.; Imae, T. J. Colloid Interface Sci. 2001, 233, (1), 99-106. 40. Colthup, N. B.; Daly, L. H.; Wiberley, S. E., CHAPTER 8 - AROMATIC AND HETEROAROMATIC RINGS. In Introduction to Infrared and Raman Spectroscopy (Third Edition), Academic Press: San Diego, 1990; pp 261-288. 41. Neubrech, F.; Pucci, A. IEEE J. Sel. Top. Quantum Electron. 2013, 19, (3), 4600809-4600809. 42. Ott, C.; Kaldun, A.; Raith, P.; Meyer, K.; Laux, M.; Evers, J.; Keitel, C. H.; Greene, C. H.; Pfeifer, T. Science 2013, 340, (6133), 716. 43. Giannini, V.; Francescato, Y.; Amrania, H.; Phillips, C. C.; Maier, S. A. Nano Lett. 2011, 11, (7), 2835-2840. 44. Gutiérrez-Sanz, O.; Marques, M.; Pereira, I. A. C.; De Lacey, A. L.; Lubitz, W.; Rüdiger, O. J. Phys. Chem. Lett. 2013, 4, (17), 2794-2798. 45. Zhang, Y.; Zhen, Y.-R.; Neumann, O.; Day, J. K.; Nordlander, P.; Halas, N. J. Nat. Commun. 2014, 5, 4424. 46. Nagasawa, F.; Takase, M.; Nabika, H.; Murakoshi, K. Chem. Commun. 2011, 47, (15), 45144516. 47. Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, (1), 100-110. 48. Nielsen, J. U.; Esplandiu, M. J.; Kolb, D. M. Langmuir 2001, 17, (11), 3454-3459. 49. Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.-h. Langmuir 1997, 13, (15), 4018-4023. 50. Kang, J. F.; Liao, S.; Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120, (37), 9662-9667. 17 ACS Paragon Plus Environment

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