Letter Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Frequency Modulated Raman Spectroscopy Silvio Greco,†,‡ Simone Dal Zilio,† Alpan Bek,§ Marco Lazzarino,*,† and Denys Naumenko† †
CNR-IOM, TASC Laboratory, AREA Science Park, 34143 Trieste, Italy Ph.D. School of Nanotechnology, University of Trieste, 34100 Trieste, Italy § Department of Physics, Orta Doğu Teknik Ü niversitesi, 06800 Ankara, Turkey ‡
S Supporting Information *
ABSTRACT: The coupling of plasmonic and mechanical properties at the nanoscale is of great potential for the development of next generation devices capable to detect weak forces, mass changes, minute displacements and temperatureinduced effects. Both the transduction of mechanical motion to the scattered light fields in term of polarization or intensity modulation and plasmon-driven mechanical oscillations have already been demonstrated. Quasi static tunable hot spots have recently been designed and applied to surface-enhanced Raman spectroscopy (SERS). Here we fabricated a plasmomechanical device, with a plasmonic hot spot modulated at the oscillator eigenfrequency, and demonstrated that the nonlinear modulation of polarization-dependent SERS signal from a synthetic dye can be analyzed with lock-in techniques, thus, realizing frequency modulated Raman spectroscopy. KEYWORDS: MEMS, SERS, Raman, frequency modulation
R
In a hot spot, both the intensity and the wavelength of the plasmon resonance frequency depend on the size, the shape, and the separation between nanoparticles; the latter dependence, in particular, has been used to create so-called plasmonic rulers, nanostructures in which a nanometric tuning of the interparticle distance is converted to a more directly measurable spectral shift.6 This property of the hot spot has recently been addressed in order to statically and reversibly tune the plasmonic resonance of a substrate. This tuning was obtained by several approaches including magnetic,7 thermal,8 pH9 and biochemical interaction.10 The modulation of the optical properties was exploited through the coupling of plasmonic nanostructures with micromechanical and nanomechanical structures, by which applications such as metamaterials with tunable negative refractive indices, optical cloaking, hyperlensing, and controlled luminescence have been demonstrated.11 The frequency modulation of the plasmonic properties of a gold nanostructure integrated onto a nanomechanical resonator represents a good candidate for coupling SERS and frequency modulation for lock-in detection. It was first proposed by Rutger Thijssen and co-workers, who fabricated a plasmon nanocavity in which nanomechanical frequencies and plasmonic resonances are coupled.12 The same group integrated a plasmonic dimer on two Si3N4 double clamped beam separated by a nanometric gap. When the beams oscillate at their resonant frequency, the gap in the dimer is modulated. By
aman scattering is an extremely sensitive vibrational spectroscopy that allows investigating the structural and chemical properties in fine detail for a wide range of materials and applications,1 but that suffers of a very low cross section. To overcome this limitation, surface enhanced Raman scattering (SERS), first observed at the end of the 1970s,2 takes advantage of the electric field enhancement at the surface of noble metals, thanks to the onset of plasmonic resonances in the visible spectrum. In the last 40 years, huge efforts were dedicated to improving the signal-to-noise ratio (SNR) of SERS, with essential improvement originating from field enhancement by plasmon resonances, with success. Meanwhile, a common alternative approach utilized in spectroscopy to increase the SNR, namely, lock-in amplification based frequency modulation of the signal, was neglected in Raman scattering. Our goal here is to realize an effective coupling of plasmon mediated field enhancement and frequency modulation techniques to achieve high SNR SERS. Bringing back our attention to SERS alone, the frequency, intensity and spatial extensions of the enhanced EM field depend on several parameters, such as the size and the shape of the noble metal and its dielectric surrounding.2 By using isolated gold nanoparticles (AuNP) enhancements of the order of 103 can be obtained.3 However, when more particles get in close proximity the constructive interference of the EM fields from different surface plasmons, enhances enormously the field in the gaps, resulting in an amplification of the Raman signal of up to 8 orders of magnitude.4 These nanometric regions exhibiting highly enhanced field characteristics are known as hot spots.5 © XXXX American Chemical Society
Received: September 8, 2017 Published: December 8, 2017 A
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics
Figure 1. SEM image of a fabricated resonator. (a) Top view of the full device; (b) detail of the tip-wall gap. The yellow contours represent minimum and maximum displacements of the resonating arm at first resonant mode.
Figure 2. (a, b) SSCM images of static and actuated structure obtained with 660 nm excitation wavelength. Scale bars are 2 μm. (c) Frequency sweep of the structure. The vertical dotted line indicates the resonant frequency at 90.02 kHz. (d) Voltage sweep at the resonance frequency. The vertical dotted line indicates the voltage at 5.5 V. The laser beam (∼400 nm diameter at focus) in (c) and (d) is focused in the gap at the distance of 200 nm from the steady wall to maximize the position sensitivity of the Rayleigh reflection in proximity to the contact region. The measurements were performed in air.
with IR radiation. In spite of the demonstrated potentiality in the IR range,17−19 this approach was only recently extended to visible light to visualize near field distributions on plasmonic nanostructures,20,21 but not applied to Raman spectroscopy. Tip enhanced Raman scattering (TERS) also makes use of an oscillating AFM tip, which creates a modulated enhanced field, which in turn can be used as a localized source for Raman spectroscopy. Here continuous wave (CW) Raman signal is usually collected which enables to reach nanometric resolutions when high Raman cross section materials are investigated such as carbon nanotubes22 or silicon.23 Single molecule resolution in ultrahigh vacuum scanning tunnelling microscopy (UHVSTM) configuration was demonstrated using a plasmonic cavity generated between the tip and a clean silver substrate.24 By synchronizing a multichannel detector with tapping oscillation of the metallic nanotip, and thus acquiring both near- and farfield Raman signals during the periodic oscillation of the tip, the group of Satoshi Kawata has demonstrated higher contrast in TERS imaging.25 So far, probably because of the low signal intensity, no developments in synchronized TERS has been demonstrated, and moreover, no frequency modulation Raman has been attempted neither in scanning probe systems nor using micromechanical systems. Here we propose to exploit the mechanical coupling of a microfabricated plasmonic hot spot with a micromechanical oscillator, in which the hot spot is modulated in frequency. A Raman reporter is placed on the
measuring the transmission at the dimer plasmon resonance, a plasmonic modulation is recorded.13 By raster scanning a structure through the microscope focus and acquiring the signal spectral density power at the mechanical resonance frequency the oscillating hot spot was localized. The measured motion approached the thermal limit to force sensitivity of the resonator. On the contrary, Silvan Schmid and co-workers coupled a plasmonic slit and a nanomechanical resonator via plasmon-driven photothermal effect.14 Recently, a reversible switch of a plasmonic hot spot has been demonstrated using a micromechanical approach. In particular, the enhancement of Raman signal was demonstrated15 by actuating a vertical pillar, which is separated by a submicrometer gap from a steady wall. However, in none of those examples, the resulting frequency modulated Raman signal was extracted. Taking advantage of the nonlinear modulation of the EM field enhancement that is generated in the gap between an atomic force microscope (AFM) tip and the substrate Bernhard Knoll and Fritz Keilmann were the first to develop the scattering type scanning near-field optical microscopy approach.16 In their seminal paper, the authors separate the component of light scattered by an AFM tip in contact with a dielectric substrate from the background. Because of the exponential decay of the scattering cross section with tip− sample distance, third harmonic demodulation produced the highest contrast, reaching a spatial resolution as low as 0.01λ, B
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics
reports the frequency modulated Raman maps for 532 nm excitation wavelength.
gold surface and the Raman signal is collected from the gap and analyzed with lock-in techniques, thus, realizing frequency modulated surface enhanced Raman scattering (FM-SERS).
■
RESULTS AND DISCUSSION The micromechanical resonators are designed to have a welldefined contact region in which the tunable hot spot is localized. Indeed, a certain degree of uncertainty in the hot spot location is still present due to nanofabrication resolution and metal roughness, but better than the spatial definition of the laser beam focused on the device to produce the Raman spectrum. The resonator top-view is shown in Figure 1a: the thick parts are anchored to the substrates, while the long and thin resonating arm (RA) with a diamond shape is released from the substrate and can oscillate. Figure 1b shows a tilted view of the RA from which it is possible to appreciate its aspect ratio (h, 15 μm; w, 2 μm), which implies that the first resonant mode is in the substrate plane, as indicated by the yellow contours. The resonators fabrication and functionalization with benzotriazole-azo dye as Raman reporter26 are described in Methods. Finite element analysis of the frequency response is discussed in Supporting Information, S1. To actuate the RA and, thus, create a frequency modulated hot spot between the RA and the steady wall, the resonators were mounted on a piezoelectric actuator driven by an arbitrary function generator. A 660 nm wavelength laser source was used to perform a sample scanning confocal microscopy (SSCM) and visualize both the RA and the steady wall, first in static condition (Figure 2a) and then in dynamic condition, at RA’s resonant frequency (Figure 2b). Here the RA edges look blurred due to the oscillations of the RA at frequency higher than the SSCM sampling ratio (90 kHz and 5 Hz, respectively). Focusing the laser in the gap between the RA and the steady wall (the red circle in Figure 2b), the intensity of the Rayleigh scattered light was analyzed using a lock-in amplifier while sweeping the actuation frequency (Figure 2c). The resulting curve showed a distinct resonance behavior corresponding to the first flexural mode of the cantilever oscillating in the plane parallel to the surface, in agreement with finite element analysis. To define the actuation voltage required to bring the oscillating RA in contact with the steady wall, the same measurement was repeated while sweeping the actuation voltage at resonant frequency (Figure 2d). The resulting curve exhibits a sigmoidlike behavior and can be used to determine the lowest voltage required to achieve contact between the RA and the steady wall (dashed line in Figure 2d). The obtained saturation voltage was used for actuation for the subsequent Raman mapping. Different RA batches exhibited different performances, both in resonance frequency and oscillation amplitude; therefore, every RA was characterized prior to use in Raman mapping. The Raman spectra of benzotriazole-azo (BT-Azo) were acquired using both 660 and 532 nm excitation wavelength. However, as previously demonstrated,27 at 660 nm, a preresonant SERS condition is satisfied, which maximizes the electromagnetic enhancement homogeneously for all the Raman modes in BT-Azo dye.4,28 Therefore, in the following we will discuss the results recorded with 660 nm excitation, and we will focus our attention to the mode at 1290 cm−1, corresponding to quadrant aromatic stretches with an associated azo displacement,26,27 only because it is the more intense Raman band in these conditions. A Raman spectrum of the RA tip coated with 50 nm Au and a continuous layer of BTAzo is displayed in Figure 3a. Supporting Information, S6,
Figure 3. (a) Raman spectrum of BT-Azo dye acquired at 660 nm excitation wavelength. Inset: molecular structure of BT-Azo dye. (b) Raman mapping of nonactuated structure. The normalized intensity of 1290 cm−1 peak is displayed. Scale bar is 2 μm.
The proper functionalization of the structure with BT-Azo dye was verified on the nonactuated device by mapping with 300 nm steps (slightly below the laser post size 400 nm; Figure 3b). The observed Raman signal nonhomogeneity (±20%) is due to the randomness of the Au film on which SERS properties were not optimized. Most importantly, the signal is homogeneous within ±10% at the structure edges, which is the device part of interest. When the RA is set into oscillation with an amplitude large enough to reach and impact against the opposite wall (impact oscillator), a frequency-modulated (FM) plasmonic hot-spot is generated: a similar condition was already observed for a different geometry and higher resonant frequency in our group.15 However, because of the nonfavorable ratio of the hotspot area (∼100 nm2) with the area integrated by the optics (∼105 nm2), the overall increase of the Raman spectral intensity mediated over a whole oscillation period did not exceed a factor 2 in the best conditions.15 With these limitations, it is difficult to separate the contribution coming from the micromechanical hot spot from that coming from the supporting areas and to appreciate spectral differences from the two regions, if any. However, since the Raman signal coming from the hot spot is modulated at the same frequency of the RA resonance, we can separate the hot spot contribution from the background using a lock-in amplifier. Moreover, since the plasmonic near-field and, thus, the Raman intensity depend exponentially on the tip-wall distance, while the separation between the RA and the steady wall has a sinusoidal timedependence, the signal coming from the hot spots will be composed of infinite Fourier components at integer multiples of the RA fundamental frequency. Finally, since a hot spot is C
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics formed only if the incident field polarization is aligned along the hot spot axis, we can use perpendicular and parallel polarization to further discriminate the signal origin. The maps obtained by acquiring the first, second and third harmonic both in perpendicular and parallel polarization, using 660 nm laser excitation and collecting the lock-in signal coming from BT-Azo dye (1290 cm−1 peak) are displayed in Figure 4,
period. Therefore, each signal is modulated while the RA passes under the excitation laser and then retracts. This effect is dominant for the first order, and loses intensity for higher orders. The higher the harmonic used for the demodulation, the lower the signal coming from the scattered light modulated by the mechanical motion of the RA only. More details on this mechanism are provided in Supporting Information, S5. Indeed, a bright spot appears at the very tip of the RA, generated by the strongly nonlinear plasmonic field at higher harmonics. To better determine the hot spot contribution at each harmonic, we subtracted the parallel polarization signal from the perpendicular one pixel-wise. The stochastic origin of the signal ensured that polarization does not influence its intensity distribution, so as a result, an isolated bright spot emerged. The resulting images are displayed in Figure 4a3,b3,c3, where the hot spot contribution is finally distinguished. The third harmonic difference image provided the highest contrast, thus revealing a highly confined high-level signal in the tip-wall approach region, where we expected the hot spot to emerge. The apparent size of the hot spot, barely two pixels, is indeed determined by the diffraction-limited resolution of the collection lens. The obtained results are summarized in Figure 4d, where the values of deconvoluted signal for two orthogonal polarizations in the hot spot area, difference signal, and the ratio of signals are plotted for various harmonics. Better contrasts were achieved at higher harmonics. However, the fourth harmonic required operating the lock-in amplifier at the limit of its sensitivity, hence, displayed a low SNR, concluding further analysis as unnecessary. The data for third harmonic were used to estimate the enhancement factor in the hot spot. Outside the hotspot the average Raman signal is not sensitive to the light polarization. On the contrary in the hotspot the EM field is enhanced only if it is polarized parallel to the gap axis. From Figure 4d, it can be seen that the integrated signal over the laser spot size is 2.5 fold enhanced in the presence of the hot spot formation. To calculate the effective enhancement, we take into account the fact that all molecules in the laser spot contribute to the nonenhanced signal, while only those in the hot spot provide the enhancement signal. The laser spot area is 2πR2, while the hot spot area is defined by the width of the contact area by the thickness of the molecular layer on the RA and the steady wall. The resulting expression for the enhancement is
Figure 4. Mapping of the region of interest at different harmonics and polarizations: (row a) first harmonic; (row b) second harmonic; (row c) third harmonic; (column 1) perpendicular polarization; (column 2) parallel polarization. (column 3) Difference maps obtained by subtraction of the maps with parallel polarization from perpendicular polarization. The scale bars are 2 μm. (d) The values of deconvoluted signal for two orthogonal polarizations in the area of hot spot, difference signal, and the ratio of signals (right axis) for various harmonics.
E=
⎤ 2πR2 ⎡ I⊥ ⎢ − 1⎥ 2dl ⎢⎣ I ⎦⎥
This expression results in E = 3140 for R = 200 nm, d = 1 nm, and l = 60 nm (as estimated by finite elements analysis, as discussed in Supporting Information, S1), and the integral Raman signal ratio of 2.5. Moreover, we need to consider that the hot spot is formed only during the fraction of the oscillation period in which the tip is closer than few nm to the steady wall, which is in our case approximately 15% of the period (see Supporting Information, S4). Taking this factor into account, we finally obtain a hot spot enhancement larger than 2 × 104 at the third harmonic. This still represents a lower limit since the continuous gold layer plasmonic properties are not strictly tuned for the used 660 nm excitation wavelength. Better confinement of electromagnetic field and therefore higher
panels a1, a2, b1, b2, c1, and c2, respectively. For harmonics above the third, the Raman signal was too weak to enable the map acquisition using reasonable integration times. It is worthwhile to stress here that a1, a2, b1, b2, c1, and c2 maps are acquired sequentially, and we did not observe any signal deterioration in the impact region due to RA operation. First we notice that, in all harmonics and polarization, only the signal from the moving part is detected, with no counts from the steady wall. However, the whole RA surface seems to contribute, although not on the full surface rather in striped features, whose number increases with the harmonic order. This comes from the fact that the signal is acquired pixel by pixel for an integration time much longer than an oscillation D
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics
Device Functionalization. Freshly fabricated devices were coated with a 50 nm thin layer of gold in a high vacuum e−-gun evaporator. A 1 mM BT-Azo solution was prepared by the dissolution of 2 mg of N1-(4-((1H-benzo[d][1,2,3]triazol5yl)diazenyl)naphthalen-1-yl)ethane-1,2-diamine powder in 6.04 mL of methanol (99.9%, J.T. Baker). The functionalization of the devices was performed by immersing the samples in solution for overnight and subsequent gentle rinsing with methanol. Thanks to the relatively low production cost, after measurements the devices were disposed and not reused to avoid cross-contamination. SSCM Imaging. Rayleigh and Raman scattering measurements were performed on an inverted optical microscope (Axiovert 200, Zeiss) coupled with a 750 mm long spectrometer (Shamrock SR-750, Andor Technology plc). The sample was scanned on the XY plane with a 100 × 100 μm piezo scanner (JPK−TAO system) holder in order to obtain the laser scanning confocal images. A 660 nm (Laser Quantum Torus, 140 mW, bandwidth 1 MHz) CW laser was used as the excitation light for Raman spectroscopy. A 100× air objective (NA 0.8, EC Epiplan, Zeiss) was used to focus the laser on the sample surface resulting in a spot diameter of around 0.4 μm. The Rayleigh scattered light was collected by the same objective and then directed into the spectrometer with a mounted silicon avalanche photodiode (SPCM-AQRH-15, PerkinElmer). The Raman scattered light was directed into a spectrometer through an edge filter (664 nm RazorEdge ultrasteep long-pass edge filter), dispersed by a diffractive grating of 600 lines mm−1, and finally analyzed using either TEcooled EMCCD (Newton DU971-UVB, Andor Technology plc) or above-mentioned APD and lock-in amplifier (7280 DPS, Ametek Advanced Measurement Technology, Inc.).. The lock-in integration time was selected according to the image size and the scan speed. The shown maps were acquired using 3″ of integration time. The pixel size for each image was selected according to signal intensity and acquisition time. In any case, the pixel size was significantly smaller that the laser spot (typically from 200 to 300 nm) in order not to affect the spatial resolution determined by the optical system. More details can be found in Supporting Information, S2. Deconvoluted SSCM Imaging. On the same setup described above, a piezoelectric actuator driven by a function generator (33120A 15 MHz, Agilent) is mounted on the XY piezo stage. The APD output signal is lock-in amplified using the TTL output of the function generator as a reference signal. The lock-in amplifier output is recorded pixel by pixel to generate a Raman map. More details can be found in Supporting Information, S3.
enhancement factors can be achieved using well-shaped nm size plasmonic structures, which however require costly techniques such as focused ion beam milling or similar. While the Raman spot was localized within a single pixel of the size of the diffraction limit, we can assume that the deconvoluted signal mainly constitutes of signals coming from those few molecules that are located in the gap where the hot spot is formed. This allows for a much more precise localization of the Raman signal for which, in principle, the precision depends only on the spatial resolution of the used nanofabrication technique.
■
CONCLUSIONS
In this Letter, we report a demonstration of frequency modulated Raman spectroscopy that allows to localize and separate the Raman contributions arising from a lithographically defined hot spot. The hot spot was designed to be part of a micromechanical resonator, and thus, it was possible to modulate its formation at the resonator eigenfrequency. The Raman signal was then analyzed in frequency and polarization; first, second, and third harmonic components were extracted and finally a hot spot enhancement factor larger than 2 × 104 was calculated. With this approach, two main issues were addressed: first, we were able to localize and separate the Raman signal of the hot spot area from that of the surrounding with a precision that, by construction, is better than a few nm. Second, by using a lock-in based deconvolution, we were able to separate the different harmonic contribution, thus, exploiting the nonlinear character of the plasmonic field. We think that this approach could help in investigating several spectroscopic features which have not yet been addressed. These include the effect of the electric field intensity and the mechanical pressure generated by the microelectromechanical oscillation on the target molecules, such as modification in the molecular assembly, order−disorder transitions, and pump and probe experiments. Moreover, downscaling the size of the RA would provide access to molecular conformation transitions and faster kinetics. For instance, linking a protein to the RA on one side and to the steady wall on the other side, folding−unfolding pathways could be investigated by Raman spectroscopy, while operating at nanoscale oscillation amplitudes.
■
METHODS Device Fabrication. The devices were fabricated on a Silicon-On-Insulator (SOI) substrate by electron beam lithography (EBL). However, since the proposed geometry features are larger than 1 μm, a serial optical lithography on large wafer scale is also possible. Briefly, PMMA (R-P 671.05) was spin coated on the substrate and then patterned using a scanning electron microscope (LEO 1540 XB, Zeiss) equipped with a pattern generator (Raith). After the exposure, the pattern was transferred in chromium with standard lift−off protocols. The substrate was then etched using a Bosch-like process in an inductively coupled plasma reactive ion etching (ICP-RIE) reactor down to the oxide layer. The parameters of the Bosch process were tuned in order to produce a slight underetching of the vertical walls to force the RA to impact point the top surface were the gold layer is evaporated.29 To release the RA, underlying silicon oxide was etched in HF:H2O 1:10 solution; a limited and noninfluent underetching was observed also under the steady structures.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01026. Supporting Information S1−S6 (PDF).
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alpan Bek: 0000-0002-0190-7945 Marco Lazzarino: 0000-0003-1077-6569 E
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
Letter
ACS Photonics Author Contributions
(16) Knoll, B.; Keilmann, F. Enhanced Dielectric Contrast in Scattering-Type Scanning near-Field Optical Microscopy. Opt. Commun. 2000, 182, 321−328. (17) Hillenbrand, R.; Taubner, T.; Keilmann, F. Phonon-Enhanced Light − Matter Interaction at the Nanometre Scale. Nature 2002, 418, 159−162. (18) Alonso-González, P.; Albella, P.; Schnell, M.; Chen, J.; Huth, F.; García-Etxarri, A.; Casanova, F.; Golmar, F.; Arzubiaga, L.; Hueso, L. E.; et al. Resolving the Electromagnetic Mechanism of SurfaceEnhanced Light Scattering at Single Hot Spots. Nat. Commun. 2012, 3, 684. (19) Neuman, T.; Alonso-González, P.; Garcia-Etxarri, A.; Schnell, M.; Hillenbrand, R.; Aizpurua, J. Mapping the near Fields of Plasmonic Nanoantennas by Scattering-Type Scanning near-Field Optical Microscopy. Laser Photon. Rev. 2015, 9, 637−649. (20) Zhou, Z.; Xue, J.; Zheng, Z.; Li, J.; Ke, Y.; Yu, Y.; Han, J.; Xie, W.; Deng, S.; Chen, H.; et al. A Centimeter-Scale Sub-10 Nm Gap Plasmonic Nanorod Array Film as a Versatile Platform for Enhancing Light−matter Interactions. Nanoscale 2015, 7, 15392−15403. (21) Dubrovkin, A. M.; Adamo, G.; Yin, J.; Wang, L.; Soci, C.; Wang, Q. J.; Zheludev, N. I. Visible Range Plasmonic Modes on Topological Insulator Nanostructures. Adv. Opt. Mater. 2017, 5, 1−4. (22) Cançado, L. G.; Hartschuh, A.; Novotny, L. Tip-Enhanced Raman Spectroscopy of Carbon Nanotubes. J. Raman Spectrosc. 2009, 40, 1420−1426. (23) Bek, A.; De Angelis, F.; Das, G.; Di Fabrizio, E.; Lazzarino, M. Tip Enhanced Raman Scattering with Adiabatic Plasmon Focusing Tips. Micron 2011, 42, 313−317. (24) Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chen, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y.; et al. Chemical Mapping of a Single Molecule by Plasmon-Enhanced Raman Scattering. Nature 2013, 498, 82−86. (25) Yu, J.; Saito, Y.; Ichimura, T.; Kawata, S.; Verma, P. Far-Field Free Tapping-Mode Tip-Enhanced Raman Microscopy. Appl. Phys. Lett. 2013, 102, 123110. (26) Graham, D.; Brown, R.; Ewen Smith, W. SERRS Detection of PNA and DNA Labelled with a Specifically Designed Benzotriazole Azo Dye. Chem. Commun. 2001, 1002−1003. (27) Naumenko, D.; Stolzer, L.; Quick, A. S.; Abt, D.; Wegener, M.; Barner-Kowollik, C.; Zilio, S. D.; Marmiroli, B.; Amenitsch, H.; Fruk, L.; Lazzarino, M. Design of Broadband SERS Substrates by the LaserInduced Aggregation of Gold Nanoparticles. J. Mater. Chem. C 2016, 4, 6152−6159. (28) McNay, G.; Eustace, D.; Smith, W. E.; Faulds, K.; Graham, D. Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Resonance Raman Scattering (SERRS): A Review of Applications. Appl. Spectrosc. 2011, 65, 825−837. (29) Melli, M.; Pozzato, A.; Lazzarino, M. Inverted tapered pillars for mass sensing. Microelectron. Eng. 2010, 87, 730−733.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
̇ AK-CNR Bilateral Project Grant Nos. 113F375 and TÜ BIT 115F603. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We thank Ljiliana Fruk for providing us the Raman reporter, and Remo Proietti and Alois Bonifacio for fruitful discussions.
■
REFERENCES
(1) Sharma, B.; Frontiera, R. R.; Henry, A.-I.; Ringe, E.; Van Duyne, R. P. SERS: Materials, Applications, and the Future. Mater. Today 2012, 15, 16−25. (2) Fleischmann, M.; Hendra, P. J. J.; McQuillan, A. J. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (3) Naumenko, D.; Zannier, V.; Grillo, V.; Cassese, D.; Priante, G.; dal Zilio, S.; Rubini, S.; Lazzarino, M. Enhanced Plasmonic Properties of Gold-Catalysed Semiconductor Nanowires. Nanoscale 2014, 6, 13651−13659. (4) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111, 13794−13803. (5) Stockman, M. I.; Pandey, L. N.; George, T. F. Inhomogeneous Localization of Polar Eigenmodes in Fractals. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 53, 2183−2186. (6) Sönnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. A Molecular Ruler Based on Plasmon Coupling of Single Gold and Silver Nanoparticles. Nat. Biotechnol. 2005, 23, 741−745. (7) Guo, Q.-H.; Zhang, C.-J.; Wei, C.; Xu, M.-M.; Yuan, Y.-X.; Gu, R.-A.; Yao, J.-L. Controlling Dynamic SERS Hot Spots on a Monolayer Film of Fe3O4@Au Nanoparticles by a Magnetic Field. Spectrochim. Acta, Part A 2016, 152, 336−342. (8) Gehan, H.; Fillaud, L.; Chehimi, M. M.; Aubard, J.; Hohenau, A.; Felidj, N.; Mangeney, C. Thermo-Induced Electromagnetic Coupling in Gold/Polymer Hybrid Plasmonic Structures Probed by SurfaceEnhanced Raman Scattering. ACS Nano 2010, 4, 6491−6500. (9) Taladriz-Blanco, P.; Buurma, N. J.; Rodríguez-Lorenzo, L.; PérezJuste, J.; Liz-Marzán, L. M.; Hervés, P. Reversible Assembly of Metal Nanoparticles Induced by Penicillamine. Dynamic Formation of SERS Hot Spots. J. Mater. Chem. 2011, 21, 16880. (10) Piantanida, L.; Naumenko, D.; Torelli, E.; Marini, M.; Bauer, D. M.; Fruk, L.; Firrao, G.; Lazzarino, M. Plasmon Resonance Tuning Using DNA Origami Actuation. Chem. Commun. 2015, 51, 4789− 4792. (11) Zheludev, N. I.; Plum, E. Reconfigurable Nanomechanical Photonic Metamaterials. Nat. Nanotechnol. 2016, 11, 16−22. (12) Thijssen, R.; Verhagen, E.; Kippenberg, T. J.; Polman, A. Plasmon Nanomechanical Coupling for Nanoscale Transduction. Nano Lett. 2013, 13, 3293−3297. (13) Thijssen, R.; Kippenberg, T. J.; Polman, A.; Verhagen, E. Plasmomechanical Resonators Based on Dimer Nanoantennas. Nano Lett. 2015, 15, 3971−3976. (14) Schmid, S.; Wu, K.; Larsen, P. E.; Rindzevicius, T.; Boisen, A. Low-Power Photothermal Probing of Single Plasmonic Nanostructures with Nanomechanical String Resonators. Nano Lett. 2014, 14, 2318−2321. (15) Naumenko, D.; Toffoli, V.; Greco, S.; Dal Zilio, S.; Bek, A.; Lazzarino, M. A Micromechanical Switchable Hot Spot for SERS Applications. Appl. Phys. Lett. 2016, 109, 131108. F
DOI: 10.1021/acsphotonics.7b01026 ACS Photonics XXXX, XXX, XXX−XXX
