Article pubs.acs.org/JPCC
Size Effect on SERS of Gold Nanorods Demonstrated via Single Nanoparticle Spectroscopy Kai-Qiang Lin,†,‡ Jun Yi,‡ Shu Hu,‡ Bi-Ju Liu,‡ Jun-Yang Liu,‡ Xiang Wang,‡ and Bin Ren*,†,‡,§ †
Collaborative Innovation Center of Chemistry for Energy Materials, ‡State Key Laboratory of Physical Chemistry of Solid Surfaces, and §The MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *
ABSTRACT: Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive analytical method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in experiment, as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) molecule as the probe molecule. The dark field scattering intensity was found to increase with the increase of the size of nanoparticles, whereas the SERS intensity increases with the decrease of the size as a result of the stronger lightning rod effect and weaker radiation damping. We further explored the size-dependent effect for the coupled nanorod dimer system. The SERS activity was also found to increase with a decrease of the particle size when the excitation is close to the LSPR wavelength. Understanding of the size effect on the local field enhancement may help to design and fabricate SERS substrate and TERS tips with high SERS activity.
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INTRODUCTION Surface-enhanced Raman scattering (SERS) has found important applications in chemistry, physics, material and biomedical sciences, and so on,1−6 since its first observation around 40 years ago.7−9 It can provide rich fingerprint information on molecules via their vibrational spectra, with a high sensitivity down to a single molecule level.10−12 The SERS enhancement relies on the excitation of the localized surface plasmon resonance (LSPR) of certain metal nanostructures, which provides a strong local electric field enhancement to amplify the Raman signal of nearby target molecules.13 Consequently, the magnitude of SERS enhancement is crucially dependent on the plasmonic properties of metal nanostructures, which is determined not only by the metal material, but also by the size, shape, and interparticle coupling of nanoparticles.14−18 Although single molecule sensitivity has already been demonstrated for SERS, there is still an increasing interest in optimizing the SERS enhancement for detection of molecules with a weak Raman signal or low affinity to SERS substrates.14−17 Despite that various types of SERS substrates have been fabricated, the in-depth understanding of the factors influencing the SERS enhancement factor has not yet been well established, even for the size effect, which hampers to some extend the rational optimization of SERS substrates. The size © 2016 American Chemical Society
effect on SERS has been theoretically explored since three decades ago and it was found that smaller nanoparticles tend to produce higher local field enhancement due to the simultaneous operation of the lightning rod effect and radiation damping.13,19−24 The booming of nanoscience and nanotechnology since 1990s allows the synthesis of monodisperse nanoparticle with well controlled size and shape.25−27 The progress makes the verification the size effect in experiment possible. Ever since, there have been extensive reports on the size effect of nanoparticles on the SERS enhancement, especially for nanospheres and triangular nanostructures.28−35 However, different experimental results concluded with very different optimal size, ranging from 60 to 170 nm.29−34,36−38 Such a discrepancy is mainly due to the difference in the experimental conditions, including the existing state of nanoparticles and the laser wavelength used. It is known that nanospheres may have different LSPR for different particle sizes and when they present in solution as uncontrollable nanoaggregates39 and on substrate.40 It has been found that the relative position of excitation wavelength to the LSPR Special Issue: Richard P. Van Duyne Festschrift Received: February 29, 2016 Revised: April 2, 2016 Published: April 5, 2016 20806
DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
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After cooling to 30 °C, 1.8 mL of 4 mM silver nitrate (AgNO3) solution and 25 mL of 1 mM HAuCl4 solution were then added. A total of 150 μL of hydrochloric acid (37 wt % in water) was added to adjust the pH. Right after, 125 μL of ascorbic acid (AA) was added, followed by the addition of 200 μL of the above seed solution. The solution was kept undisturbed at 30 °C overnight after being gently shaken. The amounts of CTAB, NaOL, AgNO3, HCl, and seed solution were adjusted to control the average localized surface plasmon resonance (LSPR) peak of nanorods. The length and the diameter of three batches of gold nanorods used were characterized through SEM to be 85.6 ± 5.5 nm/22.4 ± 2.1 nm, 116.3 ± 6.5 nm/34.6 ± 1.7 nm, and 129.9 ± 5.4 nm/45.8 ± 2.3 nm. Sample Preparation. As shown in Scheme 1, 1.5 mL of gold nanorod solution was centrifuged first to remove the majority of CTAB and NaOL. The nanorods precipitates were incubated with the 1.5 mL 10−4 M modified malachite green isothiocyanate (MGITC) molecule solution for more than 2 h. The mixture was further centrifuged to remove the excess MGITC molecules and dispersed with 1.5 mL of Milli-Q water again. To make sure nanorods can be separately dispersed on the substrate, the obtained nanorod solution was further diluted with water for more than 200 times, which varies depending on the concentration of the nanorod solution. The three batch of solution containing nanorods with three different sizes were dropped at different position of a same quartz substrate with colocalization markers. The sample was put in a vacuum desiccator and dried under vacuum condition using a water circulation vacuum pump. The sample was further washed with Milli-Q water followed by blow-drying with nitrogen gas. Fabrication of Markers on a Quartz Substrate and SEM Characterization. The colocalization markers on the quartz plate were fabricated by microfabrication techniques. The quartz plate was cleaned in a piranha solution and then rinsed in the chromic acid to remove surface impurities and
wavelength will result in different SERS enhancement.13,41−43 Therefore, it is highly important to strictly control the experimental condition to allow reliable investigation of the size effect. Fortunately, recently there are some reports on the development of correlated LSPR-scanning electron microscope (SEM)/transmission electron microscopy (TEM)-SERS techniques to achieve reliable quantitative structure-LSPR-SERS relationships for single nanoparticles.44−46 Thus, if the SERS can be obtained from single nanoparticles, it may be possible to obtain a clear correlation between size and SERS enhancement factor experimentally. Herein, we systematically investigate the size effect on SERS enhancement through single nanoparticle spectroscopy. We synthesize gold nanorods with different size but supporting a same LSPR wavelength. By using a confocal Raman microscope with the dark field module, we perform a detailed single nanoparticle SERS study on each gold nanorod. Tunable LSPR of gold nanorods enables us to explore the relation between the size effect and LSPR wavelength. Theoretical simulations based on the measured SEM image of each nanorod were further performed to understand the correlation. Moreover, we explore the coupling system by comparing the gold nanorod dimers with different sizes but a same LSPR and gap distance to understand the generality of the size effect on SERS.
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EXPRIMENTAL AND THEORETICAL METHODS Gold Nanorods Synthesis. Gold nanorods were synthesized following a reported method.47 Briefly, 5 mL of 0.5 mM HAuCl4 was mixed with 5 mL of 0.2 M hexadecyltrimethylammonium bromide (CTAB) solution in a 25 mL roundbottom flask first. Then, 0.6 mL of fresh 0.01 M sodium borohydride (NaBH4) was diluted to 1 mL with water, which was then injected to the HAuCl4−CTAB solution under vigorous stirring to obtain the seed solution. To synthesize gold nanorods, 0.90 g CTAB and 0.15 g sodium oleate (NaOL) were first dissolved in 25 mL of 50 °C water in a 50 mL cuvette. 20807
DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
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Figure 1. (a) Single nanoparticle dark field scattering spectra of the three gold nanorods showing different scattering intensity. (b) Normalized dark field scattering spectra. (c) Single nanoparticle SERS spectra of the three gold nanorods modified with MGITC. (d) Background-subtracted SERS spectra of the three gold nanorods. Both the white light and laser polarization were kept parallel to nanorods. (e) SEM images of three gold nanorods with different size but similar LSPR wavelength.
morphology change during measurements. Upon laser illumination, the intensity of the SERS spectra first decreased and became stable after about 30 s. The SERS spectra were acquired afterward for 10 s with 3 accumulations, without observable difference between each accumulation. The laser polarization was always kept parallel to the longitudinal axis of the nanorods controlled by a half-wave plate. The white light polarization was also kept parallel to the longitudinal axis of the nanorods for every scattering spectrum using a high transmission polarizer. To make sure no morphology change during the laser illumination, the scattering spectra were measured before and after SERS spectra acquisitions of every nanorod. The scattering spectra were given by
residues. Photoresist was then coated on the quartz substrates. Using conventional photolithography, the patterns of a selfdesigned photolithographic mask were transferred to the photoresist surface. After aluminum evaporation and the liftoff process, the photoresist was removed and only the aluminum patterns were left on the substrate. As shown in Figure S1, one pattern consists of 5 × 5 units and each unit consists of 22 × 11 squares of 60 μm2. To identify every single square, each row and column was labeled with a Chinese coordinates. We can thus locate the nanoparticles inside one square through these labels quickly. A photo of one inch quartz substrate with markers and a dark-field image of part of one unit are shown in Figure S1. Correlation between single particle spectra and morphology of the nanorods was accomplished using this substrate for identification in the dark-field microscope and the scanning electron microscope (SEM; Hitachi S-4800). Since quartz substrate is not conductive, 3 nm platinum was deposited after the SERS measurement and before SEM imaging. Single Nanoparticle Spectroscopy. Single nanoparticle spectroscopic measurements were carried out on Renishaw inVia Raman spectrometer with a home-built dark-field microscope (inverted, Leica). A motorized stage (Prior ProScan II) was used to trace the nanoparticle position. A schematic diagram of the experimental setup is shown in Scheme 1. A same objective lens (NA = 0.64, 50×, Leica) was used to collect the elastic scattering and SERS spectra. The transmission darkfield scattering experiment was conducted with a 100 W halogen lamp illumination through an air condenser (NA = 0.80−0.95, Leica). The dark-field image was recorded by an EXi Aqua camera from Q-Imaging. A 30 μm slit was set in the detection path and a 150 grooves/mm grating was used to disperse the scattering and SERS spectra. To measure the single nanoparticle SERS spectra, the power of 633 nm laser was set to a very low value of 3.9 μW to avoid the laser-induced
Iscattering =
Iparticle − Isubstrate Isubstrate
where Iscattering is the real scattering spectrum from a single nanoparticle, Iparticle is the original scattering spectrum collected by CCD, and Isubstrate is the background spectrum collected in the dark region nearby without nanoparticles. Theoretical Simulations. A commercial finite-element method simulation software (COMSOL multiphysics package 4.4) was used to model the nanorods. We created a rectangular domain around a single nanorod and perfectly matched layers (PMLs) were employed to simulate an open boundary. The permittivity values of the silver and gold nanorods were taken from Johnson and Christy53 and the quartz substrate beneath the nanorod was assumed to be semi-infinite with a refractive index n = 1.4585. The medium over this substrate is air. The rod geometry was modeled as a cylinder with a semisphere attached at the two ends. The diameter and length of gold nanorods were set based on the SEM images of the gold nanorods. 20808
DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
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RESULTS AND DISCUSSION The advantage of gold nanorods for the size-dependent SERS study is that their LSPR peak can be kept at the same wavelength by properly choosing the size and aspect ratio of the nanorods. We would expect large nanorods have a much stronger scattering as a result of the increasing cross section and a broader LSPR peak as a result of the stronger radiation damping.13,19 This can indeed be seen from Figure 1a. This set of three gold nanorods with different scattering intensity but supporting similar LSPR wavelength around 660 nm were first chosen through dark field microscope for investigation. The scattering intensity of the Large-1 gold nanorod (green line) is 1.5× larger than that of the Medium-1 gold nanorod (red ine), and 6× larger than that of the Small-1 gold nanorod (black line). After normalizing the scattering spectra (Figure 1b), we found that the LSPR line width Γ of Larger-1 gold nanorod (188.6 meV) is broader than that of Medium-1 (171.6 meV) and Small-1 (131.7 meV). We can thus calculate the surface plasmon dephasing time of the longitude LSPR mode via τ = 2ℏ/Γ of the three nanorods,48 which are approximately 7, 8, and 10 fs for the large, medium, and small nanorods, respectively. This is in good agreement with a faster plasmon dephasing time in large nanoparticles as a result of a stronger radiation damping compared with small nanoparticles.13,19 To understand the size effect on SERS, we further measured the single nanoparticle SERS of the three nanorods. Surprisingly, as shown in Figure 1c, the Large-1 gold nanorod with a stronger scattering shows an inversely lowest single nanoparticle SERS signal. We notice that there are strong continuum backgrounds under the Raman peaks,49 the origin of this background and the correlation of it with SERS signal will be detailed elsewhere. In this work, we tentatively subtract the backgrounds and obtain clean SERS spectra (Figure 1d). We can see that the Raman peaks located at the peak (660 nm) of the LSPR band shows highest SERS enhancement, which agrees well with the predictions of the electromagnetic enhancement mechanism and wavelength-scanned SERS experiment of Van Duyne group.41,42 The SERS spectra in Figure 1d clearly shows that gold nanorods with a stronger scattering intensity have a weaker SERS intensity. Finally, to verify the size of the nanorods, we characterize their morphology through SEM after all the optical measurements. The SEM images in Figure 1e clearly demonstrate that the three nanorods have different size, which is in good agreement with the predicted size sequence. We thus plot the scattering intensity, full width at halfmaximum (fwhm) of the scattering peak, and absolute SERS intensity at 790 cm−1 of the three nanorods as a function of size of nanorods in Figure 2. We can see that the scattering intensity and fwhm of LSPR increase while absolute SERS intensity
decreases as the size of nanorod becomes larger. The SERS intensity of the large nanorod is about 4 times weaker than that of the small nanorod. If we further consider that the large nanorod has a larger surface area which can adsorb more MGITC molecules, the difference in the absolute SERS enhancement will be even large. We then plot the surface area of each nanorod also in Figure 2. Notably, the surface area of larger nanorods is 2 times larger than that of small nanorods. Therefore, the absolute SERS enhancement factor of large nanorod is about 8× weaker than that of small nanorod. It is generally considered that the SERS enhancement factor is roughly proportional to the fourth power of the local electric field intensity.13 In the present study, the local field and far field scattering was found to be inversely correlated as the size of nanorods changed, which seems to be counterintuitive because both SERS intensity and far field scattering generally increase as the size of the nanoparticles becomes larger in the case of nanoaggregates on substrate.30,31,35,37,50 In fact, as the size of nanoparticles becomes larger in nanoaggregate systems, the LSPR changes and the number of molecules and hot spots also changes. Therefore, it is still challenging to unambiguously analyze the size effect. To further investigate whether such a size effect on SERS is LSPR dependent, we measured three more sets of gold nanorods, and the results are shown in Figure 3. The SEM images of these nanorods are shown as insets in Figure 3g−i. Each set includes three nanorods with different sizes but supporting very similar LSPR wavelength, and three different sets correspond to three different LSPR wavelengths (680, 690, and 700 nm). The dark field scattering spectra of the three sets of gold nanorods in Figure 3a−c consistently show that larger nanorods have much stronger scattering intensity. After normalization, we can also find that the band widths of larger nanorods are obviously broader for all three sets of nanorods, which confirms that larger gold nanorods have shorter plasmon dephasing time again. The original single nanoparticle SERS spectra in Figure 3g−i clearly show that smaller nanorods have much stronger SERS intensity, which agrees well with the conclusion obtained from Figure 1. Since the main Raman peaks of MGITC are located at a frequency lower than 1650 cm−1 (707 nm for 633 nm excitation), nanorods with LSPR at longer wavelengths (>700 nm) do not show a clear difference in the relative intensity of Raman peaks. To further understand the size effect, we simulate the gold nanorods in Figure 1 based on their SEM images. As shown in Figure 4b, the simulated scattering spectra both in the peak position and intensity agree well with that in Figure 1a, showing that larger gold nanorods have stronger scattering intensity. The bandwidth of the scattering spectra can be easily identified from normalized spectra (inset of Figure 4b), which also matches with the experimental result (Figure 1b). We further simulate the local fields of the three nanorods. As shown in Figure 4a, the smallest gold nanorod shows the strongest local electric field, and the field strength decreases with the increase of size of gold nanorods. We then obtained the SERS enhancement factor EF = |E633/E0|2 × |Eem/E0|2 for the three gold nanorods, where E0, E633, and Eem are the incident electric field, and the local electric field at 633 nm and at the emission wavelength (where Raman scattering signal emits), respectively. After integrating over the surface and dividing by the surface area, we obtained the average (absolute) SERS enhancement factors of the three gold nanorods as a function of emission wavelength in Figure 4c. It can be seen that the Small-1 gold
Figure 2. Scattering intensity, fwhm, surface area, and SERS intensity (790 cm−1) as a function of the size. 20809
DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
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Figure 3. Scattering and SERS spectra of three sets of gold nanorods. Gold nanorods in each set have different sizes, but similar LSPR wavelength. Three different sets correspond to three different LSPR wavelengths. (a−c) Scattering spectra of three sets of gold nanorods. (d−f) Normalized scattering spectra of (a)−(c). (g−i) SERS spectra of three set of gold nanorods. Insets in (g)−(i) show SEM images of corresponding gold nanorods.
Figure 4. Theoretical simulated local electric field (a), scattering cross section (b), normalized scattering spectra (inset in b), and average SERS enhancement factor (c) of the three gold nanorods shown in Figure 1e. (d) Comparison between experimental SERS intensity after normalized with the surface area and simulated average SERS enhancement factor of the three nanorods.
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DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
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Figure 5. Theoretical simulations of gold nanorod dimers with different size but a same gap distance of 2 nm. Simulated local electric field at 750 nm (a), scattering cross section (b), normalized scattering spectra (c), average electric field enhancement factor (d), SERS enhancement factor for 630 nm excitation (e), SERS enhancement factor for 670 nm excitation (f), and SERS enhancement factor for 710 nm excitation (g) of the three gold nanorods. Large dimer, blue lines; Medium dimer, red lines; Small dimer, green lines.
nanorod has five times stronger EF than the Large-1. Interestingly, if we compare the experimental SERS intensity after taking into account of the surface area of the three nanorods with the theoretically obtained average SERS EF in Figure 4d, we obtained a very good agreement between experiment and theory. This result strongly supports the experimental observation of the size effect on SERS, that is, the smaller gold nanorod gives stronger SERS signal. It should be especially pointed out that although the scattering intensity of a single nanoparticle can be used to predict the local field enhancement at different wavelengths, the scattering intensity cannot be used to compare the local field enhancement of different nanoparticles. With the understanding of the size effect on SERS in a single nanoparticle system, we further explored how the size of nanoparticles will affect the SERS enhancement in a coupling system. In a typical coupling system of nanospheres, a change in the size would generally lead to the change of the LSPR position. Therefore, to obtain a same LSPR position for nanosphere dimers with different sizes, the gap distance of nanosphere dimers should be modified, which on the other hand leads to a significant change of the electric field and makes the investigation of the size effect alone impossible. To avoid the simultaneous change of the LSPR position and gap distance with the change of the size, we simulated three dimers made up of gold nanorods with different sizes. The gap distance was kept 2 nm. As shown in Figure 5b, although the LSPR of the three
nanorod dimers are at the same wavelength, the larger gold nanorod dimer shows much stronger scattering cross section. After normalizing the scattering spectra in Figure 5c, we can clearly find that larger gold nanorod shows much broader plasmon bandwidth, which indicates faster dephasing time in larger gold nanorod dimers. Actually, both the stronger scattering and faster dephasing time can be attributed to the stronger radiation damping for larger nanorods, which agrees well with the previous single nanorod results. We further simulated the local electric field of the three dimers. As shown in Figure 5a, contrary to the scattering signal, the local electric field of smaller gold nanorod dimer is obviously stronger at the LSPR wavelength (750 nm). Moreover, we further plotted the average electric field enhancement of the three dimers as a function of the incident wavelength in Figure 5d. It is apparently that the average electric field enhancement of the smaller nanorod dimer is much stronger than that of the larger nanorod dimer, especially at the LSPR peak wavelength. Only when the wavelength is shorter than 670 nm, we observed a stronger average electric field enhancement for the larger nanorod dimers (as shown in the inset of Figure 5d). This is due to the much narrower LSPR bandwidth of the smaller nanorod dimer. When the excitation wavelength is far from the LSPR wavelength, the excitation enhancement of larger nanorod dimers can even be stronger than the smaller dimers. Since the emission enhancement of larger nanorod dimers is weaker than the smaller nanorod dimer, there would be a best 20811
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suppress the local field, further experimentally investigation to understand the competition between the lighting rod effect and nonlocal effect would be highly valuable.
size of highest SERS enhancement factor for each excitation wavelength combining excitation enhancement factor and emission enhancement factor. Then, we calculated the average SERS enhancement factors of the three nanorod dimers at three excitation wavelengths over a large emission range (where Raman scattering signal emits). As shown in Figure 5e, contrary to the average electric field enhancement, when the excitation wavelength is at 630 nm, the larger nanorod dimer having broader LSPR bandwidth has higher average SERS enhancement factor over a broad emission wavelength range. Only for the Raman bands appearing at the peak position show a weaker signal than the smaller size. The trend changes when the excitation wavelength red shifts toward the LSPR wavelength. For example, with the 670 nm excitation, the smaller nanorod dimer starts to show higher average SERS enhancement factor for Raman bands appearing around the LSPR wavelength range (Figure 5f). However, the Raman bands at different Stokes shifts show different dependence on the size for the dimer systems, as shown in Figure 5f. A clear size dependence can be found when the excitation wavelength becomes very close to the LSPR wavelength, for example 710 nm. As shown in Figure 5g, the SERS enhancement is higher for smaller nanorods over a large wavelength range. The main reason is that the smaller nanorods produce stronger lighting rod effect and weaker radiation damping.13,19,22 Therefore, to achieve a higher SERS enhancement, it is better to use a smaller nanorod and an excitation close to the LSPR wavelength. However, the LSPR band of small nanostructures tends to be narrow, which may result in a sensitive response of the SERS enhancement to the excitation wavelength. Quality factor (Q factor), which is proportional to the reciprocal of the LSPR bandwidth, has been well correlated with the SERS intensity in nanoaggregates system.51,52 In agreement with that observed in the nanoaggregates system, we demonstrated at the single nanoparticle level that a higher Q factor leads to a stronger SERS intensity.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02098. Experimental and theoretical details and additional data (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86-592-2186532. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge support from MOST (2011YQ03012400 and 2013CB933703) and NSFC (21227004, 21321062, and J1310024) and MOE (IRT13036).
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REFERENCES
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CONCLUSIONS In summary, we investigated the size effect of gold nanorods on their SERS enhancement through correlated single nanoparticle LSPR-SEM-SERS technique. Gold nanorods with different sizes but supporting a same LSPR wavelength were synthesized so that size dependence can be studied while the LSPR of nanorod can still remain the same. We found both experimentally and theoretically that large nanorods show stronger scattering intensity but much weaker SERS intensity than small nanorods. The stronger SERS signal at a smaller size is a synergistic effect of the stronger lighting rod effect and weaker radiation damping resulting in a stronger local field enhancement. Such a size effect has been generalized in the coupling system of gold nanorod dimers with a same LSPR wavelength but different sizes. The conclusion achieved in this study may help to design and fabricate SERS substrates and TERS tips with high SERS activity. The propagation electromagnetic (EM) waves coupled with dipolar plasmonic resonance are localized around nanorods and efficiently scattered out as propagation EM waves, which enables us to observe the size effect on the optical local field by the far field SERS measurement. Such a size effect on the optical local field can also be generalized to other local field enhanced spectroscopies. Our work has practical important for those who design the plasmonic nanostructures for the applications of strong photon−particle interactions. Since smaller size may evoke the nonlocal effect which will 20812
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Article
The Journal of Physical Chemistry C
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DOI: 10.1021/acs.jpcc.6b02098 J. Phys. Chem. C 2016, 120, 20806−20813
