Dependences on Excitation Wavelength and Hot-Spot Gap - American

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Study of Signal-to-Background Ratio of Surface-Enhanced Raman Scattering: Dependences on Excitation Wavelength and Hot-Spot Gap Mykhaylo M. Dvoynenko, Huai-Hsien Wang, Hui-Hsin Hsiao, Yuh-Lin Wang, and Juen-Kai Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08362 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Study of Signal-to-Background Ratio of SurfaceEnhanced Raman Scattering: Dependences on Excitation Wavelength and Hot-Spot Gap Mykhaylo M. Dvoynenko,1,4 Huai-Hsien Wang,1 Hui-Hsin Hsiao3 Yuh-Lin Wang1,3 and Juen-Kai Wang1,2* 1

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan

2

Center for Condensed Matter Sciences, National Taiwan University, Taipei, 10699, Taiwan

3

Department of Physics, National Taiwan University, Taipei, 10699, Taiwan

4

V.E. Lashkarov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,

Kyiv, Ukraine

ABSTRACT. Signal-to-background ratio of surface-enhanced Raman scattering (SERS) plays an important role in the analytic applications of SERS. Its dependences on excitation wavelength and gap between metal particles were studied theoretically and experimentally. We show that this ratio is higher at smaller gaps for the same excitation wavelength and is higher for the wavelengths having higher values of the absolute values of the dielectric function of the metal. This study thus results in design guidelines in the fabrication of SERS-active substrates with high signal-tobackground ratio.

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I.

INTRODUCTION

Surface-enhanced Raman scattering (SERS) has attracted much attention,1-5 because of enhanced local field induced by collective oscillation of free electrons—plasmon—in metallic nanostructures,6,7 allowing for detection of trace chemicals. One key issue in realizing repeatable SERS signal is the creation of stable local enhanced field in the gaps of metallic nanostructures— “hot spots”,6 as the SERS signal of the molecules residing in the gaps dominates the ultimate SERS contribution. The extra SERS enhancement has been attributed to plasmonic coupling between the metallic nanostructures surrounding the gap. Because of many successful demonstrations of these deliberately fabricated metallic nanostructures for SERS, the developers have turned their attention to another critical issue: measured SERS signal is always accompanied with a continuum background.

The continuum background in SERS spectra would compromise the dynamical range of detection. In other words, the lower limit in SERS sensitivity is often restricted by the existence of the SERS background: Its fluctuation would lead to uncertainty in quantification with SERS. The nature of this background was suggested previously.8-27 An interesting hypothesis about the source of the SERS background was made by Baumberg’s team:26,27 Based on the facts that the SERS signal always accompanies the background and higher enhancement leads to higher background, the authors supposed that this background is caused by the dipole of the adsorbed molecule by photoexcitation and its induced mirror image in an adjacent metallic object. Specifically, the fast relaxation of the image dipole, owing to prompt and vast electron-electron collisions, engenders broadening of all Raman lines. However, our analysis28 showed that the spectral broadenings of both fluorescence and Raman radiation from an adsorbed molecule on metal surface are much smaller than the experimentally observed one. Alternatively, it was supposed that the background is caused by the interband sp-d transition,8-10 intraband transitions11-13 in metal, and inelastic


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scattering of light on electron plasma.14-19 All these processes can be efficiently enhanced by electromagnetic resonance of the metallic nanostructures.20-25

Here, we are considering that the background is produced by the metal nanoparticles which serve the basic structural constituents in SERS-active substrates—i.e., by the penetrated electric field inside the nanoparticles—without the revelation of the nature of the SERS background (photoluminescence and/or electronic Raman scattering of the metal nanoparticles). The simplest version of SERS enhancers is two adjacent nanometer-sized metallic spheres whereas near-field interaction between them takes place. In this study, theoretical analysis was first performed to reveal how the ratio between the signal and the background of SERS, ⁄ , depends on the material and structural aspects of this simple SERS enhancer—i.e., material of the two spheres and gap between them. The theoretically predicted ⁄ values were compared with the SERS spectra of adenine that were obtained with use of Ag-nanoparticle array. The agreement between them confirms the theoretical derivation of ⁄ and thus provides additional optimization consideration in the fabrication of SERS enhancers besides enhancing SERS signal on which SERS studies have often been focused. II.

THEORETICAL PREDICTION

If the SERS background is contributed by the photoinduced radiation by the metal spheres, ⁄ is determined by the ratio between the field amplitude just outside the sphere and that inside : is responsible for the SERS signal, while determines the SERS background. The SERS signal, , of a specific Raman peak at frequency Ω can be expressed29,30 as , Ω = , Ω , , !" , , #

(1)

where is optical instrument function, is the Raman cross-section of molecules adsorbed on metal spheres that accounts for the chemical enhancement factor, is the number density of molecules uniformly adsorbed on the surface of metal spheres, , = $ , ⁄ % is the local field enhancement factor—equal to the ratio between the amplitudes of local field just 3 Environment ACS Paragon Plus


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outside the metal spheres, $ , and incident field, % , at excitation wavelength , , ! is the corresponding local field enhancement factor at the Stokes radiation wavelength &' of Raman scattering &' , = − Ω⁄) !, " is excitation power, and c is light velocity. The

surface integral is performed over the sphere surface to reflect the understanding that the SERS signal originates from adsorbed molecules. The SERS signal depends on the field just outside the spheres, because only the first-layer molecules adsorbed on the surface the nanoparticles make the contribution.31 On the other hand, the SERS background, * , depends on in a complicated manner, because is inhomogeneous as anticipated and the whole region inside the spheres excited by this field radiates. As a consequence, * ,* ! can be expressed as follows: * , ,* ! = + * , ,* , ! - ,, ,* , !" , , , #

(2)

where + is electron density of metal spheres, * , ,* , ! is cross-section of photoexcited luminescence and/or electronic Raman scattering of electrons inside metal spheres at the wavelength ,* near the Stokes radiation wavelength of Raman scattering , —namely, &' &' ,* ≈ , . Furthermore, * , ,* , ! is generally a function of to reflect the

dependence of the background on the adsorbed molecules—The SERS background often exhibits positive correlation with the SERS signal,32 although this behavior is not a subject of this study here. For the sake of easy discussion, , is set to equate ,*—namely, , = ,* ≡ . , , = , ⁄ % .

(3)

is the enhancement factor at excitation wavelength and , , is the corresponding enhancement factor at emission frequency . Both , and , are functions of position inside the spheres. The volume integral is performed over the sphere volume 0 . is the same optical instrument function as in eq. (1). Although it is generally difficult to obtain analytical relations between , , and , as well as between , , and , , they can be estimated qualitatively with the following consideration. A comparison of field outside 4 Environment ACS Paragon Plus

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and inside the spheres can be started from the fact that hot spots provide the main contribution to SERS signal,6 because the electromagnetic enhancement factor of two coupled metal spheres is much higher than that of two well-separated spheres. The maximal value of the field enhancement appears in the gap between the spheres, as the incident field direction is aligned along the line connected with the centers of the two spheres. The local field on the line is perpendicular to the surface of the spheres. Applying the boundary condition for normal component of the field, one can obtain $ = |2| $ , where $ and $ are the field amplitudes just outside and inside the surface of the spheres, and ε is the dielectric function of the metal spheres. It means that the ratio $ ⁄ $ = |2| depends on the wavelength as 2 = 2 .

(4)

For example, since |2| of Ag in the visible region

increases with the wavelength,33 $⁄ $ increases with the wavelength.

This wavelength

dependence of 2 is generally true according to Drude model.34 Consequently, ⁄ is higher at a longer wavelength, because the signal and background bear positive correlation with $ and $, respectively, according to eqs. (1) and (2). This is the first inference resulted from this simple consideration. Furthermore, the penetrated field inside the spheres in the case of two coupled metal spheres can be revealed by a simple argument based on electrostatic approximation—i.e., the diameter of the two spheres and the gap between them are much smaller than the excitation wavelength. It is clearly seen in Figure 1(a) that the distance between the positive and negative charges on the surfaces in the gap determines the penetration of .

Since is primarily

determined by the opposite surface charges in the gap, the fields thus produced by the opposite charges inside the sphere ( 3 and & ) but away from the gap nearly cancel each other—namely, they are almost equal in amplitude but out of phase. On the contrary, there is no such compensation at a location near the gap, because the distances to the two opposite charges differ significantly. Thus, the smaller the gap is the less the field penetrates inside the particle. One can state that the penetration depth of the light field into the metal sphere is roughly equal to the gap value, if the gap is smaller than the skin depth. Therefore, a smaller gap between the metal spheres leads to a 5 Environment ACS Paragon Plus


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smaller penetration depth of the field into the spheres, a smaller SERS background level, and thus a higher ⁄ value, yielding the second inference. These two inferences are valid for two coupled cylindrical particles too.

The qualitative consideration above can be confirmed by

numerical calculation of field distribution inside and outside metal spheres. The calculation method of the two coupled metal spheres is based on the expansion of the scattering field inside and outside the spheres by vector spherical harmonics.35-38 Let us consider the centers of the two Ag spheres to be at the coordinates 0,0,0 and 0,0, + 6, where d is the diameter of the spheres and 6 is the gap between them. The wavevector of the incidence plane wave is parallel to the x axis and the polarization is directed along the y axis. The field amplitude distribution in the yz plane at the incident wavelength of 532 nm is presented in Figure 1(b), as an example. The maximal value of the local field amplitude outside the spheres is about 18.4 of the incident field amplitude—i.e., 7 532 nm, ⁄ % 532 nm>? = 18.4 —and resides in the gap between the two spheres just near its surfaces on the line between the two centers, as shown previously.39

On

the

other

hand,

the

other

calculation

alike

gives

7 632.8 nm, ⁄ % 632.8 nm>? = 13.5. The field amplitude inside the spheres is much smaller than that in the gap: Since 2EF532 nm = −11.24 + G0.445 and 2EF632.8 nm = −17.7 + G0.616,33 the field inside the sphere for both excitation wavelengths of 532 and 632.8 nm is at least one order of magnitude less than that outside according to eq. (4). The field distribution inside the spheres can be glimpsed from the field distribution along the z axis. Figure 2(a) shows 0,0, I⁄ % = , I of the two identical Ag spheres with a diameter of 50 nm and a gap of 5 nm excited at 532 and 632.8 nm. It is seen that the field amplitude inside the spheres relative to the incident field at 532 nm is consistently larger than that at 632.8 nm. For example, one can find J, 532 K⁄, 632.8 KLMNO % = 1.58

that

is

equal

to

P2EF 632.8 nmPQP2EF532 nmP according to boundary conditions. Since the relative value of the field amplitude inside the sphere for 632.8 nm is lower than the one for 532 nm, * excited at 632.8


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nm is smaller than that excited at 532 nm and, subsequently, ⁄ for 632.8 nm would be higher than that for 532 nm. The similar comparison is made for the two different gaps of 5 nm and 10 nm, shown in Figure 2(b), for the excitation wavelength of 532 nm. , I in both cases is normalized with respect to their corresponding values at z = 25 nm to illustrate the difference in the penetration of inside the nanoparticle for the two gap values. It is seen that , I of the 5-nm gap decays faster into the sphere than that of the 10-nm gap. The quantitative comparison of ⁄ for different excitation wavelengths and intersphere gaps can be roughly done on the basis of the calculation of the ratio of the enhancement factors and cross-sections. The cross-sections can be roughly estimated by molecule and metal absorption at these different conditions, because both photoluminescence and Raman scattering from excited electrons inside metal spheres are determined by absorption of the field inside the spheres. According to eqs. (1) and (2), the ratio between the SERS signal and background is roughly proportional to the following form

⁄ ~

ST SU

WXYZX [\] ,^ _ [\] ,[\` ,ST

×W

×

T ab7c\] [\] ,d>e #

h afJcg,\] [\] ,dL #

e

.

(5)

This rough estimation neglects the differences between and and between , and , as usually made in the previous SERS studies. As a note, the unit of j j ⁄+ is cm, while ⁄* is unitless, while i kQi- ,, k bear a unit of cm-1. As #

#

a consequence, ⁄ is unitless. The calculated ⁄ of the two Ag spheres based on eq. (5) is then compared with the experimental SERS study with use of Ag-nanoparticle array as a SERS-active substrate. III.

EXPERIMENT AND DISCUSSION

SERS-active substrates used in the experiments were made of an array of Ag nanospheres imbedded in anodic aluminum oxide—dubbed as AgNP/AAO. The fabrication procedure was



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described previously.40 Briefly, an aluminum-coated glass slide prepared by sputtering was anodized and then chemically etched under different conditions to create two-dimensional hexagonally packed arrays of nanochannels with an average nanochannel diameter of 50 nm as well as average gaps between nanochannels of 5 and 10 nm. Silver electrochemical plating was then used to grow Ag nanoparticles in the nanochannels. Subsequent chemical etching of aluminum oxide resulted in the half-sphere round top exposed above alumina, yielding two-dimensional hexagonally-packed Ag-nanosphere array with average sphere diameter of 50 nm and mean gaps of 5 and 10 nm. Individual SERS-active substrates were then cleaned by rinsing with deionized water, followed by being vacuum-sealed in plastic bags for storage. To minimize surface contamination, each substrate was freshly used in the SERS measurement. The substrates were characterized with scanning electron microscopy and scattering spectroscopy to reveal their structural and optical propensities.41 The scanning electron microscope (SEM) images of the SERS substrates with the mean 5- and 10-nm gaps are presented in Figure 3, while the dark-field scattering spectra for these substrates are presented in Figure 4. SERS measurements were performed with a home-built Raman system. A continuous-wave narrow-linewidth frequency-doubled Nd:YAG laser emitting at 532 nm and a HeNe laser emitting at 632.8 nm served as two excitation sources. Appropriate laser-line filters for them were used to remove residual emission outside their respective passbands (2 nm for the 532-nm filter and 2.4 nm for the 632.8 nm). The laser beam was focused by a 10× micro-objective lens (NA = 0.25) at the surface of the SERS-active substrates. The scattered radiation was collected backward with the same objective lens and sent to a 14-cm spectrometer plus a thermoelectric-cooled charge-coupled device (CCD) for spectral recording. The spectrometer with such small focal length was used to cover the whole spectrum of interest (from 100 to 2000 cm-1) with one spectral recording. The spectral calibration was done with a Hg-Ne lamp. The spectral resolution was 18 cm-1 and the spectral error was about 1 cm-1. Typical acquisition time ranged from 30 to 60 sec. The laser


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irradiation power was set at 0.25 mW to avoid laser-induced damage and photodegradation of SERS signal. Adenine dissolved in water (2×10-5 M) were used in the SERS measurements. All the SERS measurements for comparison were performed in one substrate to avoid the signal variation due to the disparities in SERS sensitivity between different substrates. The SERS signal was varied within 10% over 1 cm × 2 cm region of the substrate. 2 µL of the solution sample was placed on the substrate, forming a droplet with a diameter of ~2 mm. A big water drop (~0.5 mL) was placed beside the substrate. The substrate, containing sample droplets, and this water reservoir were then enclosed by a rectangular stainless-steel frame and covered with a low-fluorescence glass plate. In this way, the water vapor pressure inside the chamber was quickly equilibrated by the water reservoir and, therefore, the concentration of these sample droplets was invariant during the SERS measurements. The invariability of the droplet concentration was verified by the continual SERS measurements (>1 hr.). The ultimate SERS spectrum for subsequent analysis was obtained by averaging five spectra measured at five randomly selected locations within the droplet. The drop could be kept during 30 minutes without changing in its size. It means that the number of adsorbed molecules of the adenine on the SERS-active substrate remained the same during this period. The number of the adsorbed molecules depends on the molecules affinity and the solution concentration. As a final note, the SERS measurement performed in this way bears one additional advantage: the adenine molecules would be uniformly adsorbed on the surface of the SERS substrate within the droplets so that well-known coffee-ring effect42 would happen to engender variant dispersal of deposited adenine molecules. The acquired SERS spectra of adenine on the AgNP/AAO substrates of 5- and 10-nm gaps with two excitation wavelengths of 532 and 632.8 nm are presented in Figure 5. The well-known breathing mode adenine at about 740 cm-1 and the broadened feature in the range from 1250 to 1750 cm-1 are readily recognized.43 The background around the 740-cm-1 peak in the SERS spectra of



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adenine was determined by averaging the data outside the spectral range of the 740-cm-1 peak (from 720 to 760 cm-1) but within the range from 700 to 800 cm-1, while the signal of the 740-cm-1 peak was by the subtraction of the data at 740-cm-1 peak from the background above. The resultant error depended on the signal level and change from few % for large SERS signal (Figure 5(b)) to 20 % for small signal (Figure 5(c)). We have added errors in the experimental data of Table 1. Two inferences can be obtained from these data. Firstly, ⁄ of the 740-cm-1 peak obtained with the excitation wavelength of 632.8 nm is consistently larger than that obtained with the excitation wavelength of 532 nm for the two gaps. Secondly, the ⁄ obtained on the substrate of the 5-nm gap is consistently larger than that on the substrate of the 10-nm gap for the two excitation wavelengths. Two questions then emerge: (1) Does this dependence of ⁄ on excitation wavelength show good correspondence to theoretical estimation and (2) how does ⁄ depend on the gap between Ag nanospheres? Even though the theoretical estimation of Eq. (5) is done with two coupled Ag spheres, the comparison with the experimental results would shed light on the answers to these two questions. Before proceeding to the comparison results, the different contributions of ⁄ are discussed. According to Eq. (5), ⁄ is a product of three terms: ⁄+ , ⁄* , and j j i kQi- ,, k. #

#

The first team reflects the quantity ratio between the adsorbed

molecules on the surface of the metal spheres and the electrons inside them, the second one represents the cross-section ratio between molecular Raman scattering and electronic Raman scattering/luminescence processes, while the third one accounts for the electrodynamic propensity of the metal nanostructures. In this study, since the concentration of adenine used in the SERS experiments was fixed at 2×10-5 M and the diameter of the Ag nanoparticles was fixed at 50 nm, ⁄+ is constant in the comparison study. Secondly, both the excitation light waves at 532 and 632.8 nm are away from resonance excitation condition of Raman scattering, only the wavelength dependence of pre-resonance Raman scattering of adenine is considered:44


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'

'

j

∝ m pq pq r × i k , [ &[ [ no

\]

(6)

\`

where ?s is the lowest-energy absorption wavelength of adenine at ~260 nm. &j accounts for the wavelength dependence in the emission process.

Here, adenine is assumed to be physically

adsorbed on the surface of silver nanoparticles, as supported by many previous studies:45-47 Only weak charge transfer takes place between adenine and silver. The frequencies of the spectral signatures in the SERS spectrum of adenine are similar to that is its normal Raman spectrum, except that their signal strengths differ probably owing to adenine-silver interaction. Accordingly, the excitation-wavelength dependence of of adenine is regarded as that of normal Raman cross section of adenine. As to * , it originates from two sources: (1) radiative transition from electrons in sp-band to holes in d-band of silver;48 (2) electronic Raman scattering. Since the d-band of silver is located at ~4 eV below its Fermi energy in the sp-band, the photoluminescence of silver nanoparticles ranges from 325 to 400 nm.48 Consequently, this photoluminescence would not be produced by the excitation of the 532- and 632.8-nm photons, leaving non-resonant intraband electronic Raman scattering49 to be the sole source of continuum background in this study. In fact, a

recent experimental

study

by

Baumberg’s

group19

posited

that

the

“hot-electron

photoluminescence” emitted by silver nanorods observed by Lin et al.12 ought to originate from electronic Raman scattering. The fundamental difference between photoluminescence and Raman scattering lies on the fact that photoluminescence is outcome of spontaneous emission from an excited state to a lower state while Raman scattering is a coherent inelastic scattering event. They become less contractive for metals in both the portrayed steady-state spectroscopic and timeresolved results. Electronic Raman scattering of metals has a long history of study.49 According to its theory, the cross section of electronic Raman scattering follows the form below: '

j

∝ tuv , Ω × i[ k , \`


(7)


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where tuv contains non-resonant, mixed and resonant contributions of electronic Raman j j kQi- ,, k, depends on scattering, respectively.50 Thirdly, the third term in eq. (5), i #

#

the excitation wavelength as well as the inter-sphere gap as reflected in the electromagnetic resonance propensity of coupled Ag spheres. Theoretical estimations and experimental data are summarized in Table 1. Without getting into details of these three terms of electronic Raman scattering of silver, we will discuss two extreme cases: only non-resonant term and only resonant term. The non-resonant term, tuv,wv , does not depend on , while the resonant term bears the similar behavior as molecular resonance: &

& i.e., tuv,v ∝ & a→by − !

, where a→by the wavelength corresponding to the excitation

photon energy of the → z transition of silver. The lower theoretical values in the ratio of ⁄ at 632.8 nm and that at 532 nm take into account only non-resonant contribution tuv,wv while the higher one only the resonant term tuv,v . The resonant absorption wavelength for silver at 320 nm was used in the resonant term. An absorption peak at 320 nm can be confirmed from the optical absorbance in the form 1 − || , where is the Fresnel reflection coefficient at the normal incidence. Note that the theoretical values of the two extreme cases are varied by less than 50% and are approximately within the range of the corresponding experimental values. Three inferences thus can be deduced from this comparison. Firstly, the ⁄ values deduced from the 740-cm-1 peak in the experimental SERS spectra of adenine correlate qualitatively well with the theoretical prediction. Secondly, at the same excitation wavelength, ⁄ is consistently higher for the substrate with the 5-nm gap.

Thirdly, for the substrates with the same gap,

⁄ is higher for the long excitation wavelength of 632.8 nm. It should be noted that we have estimated ⁄ for the simple model of the two coupled spheres while on the experiment there are six inter-sphere gaps surrounding individual Ag nanospheres in the two-dimensional hexagonally-packed nanosphere array while only one hot spot in the case of the two Ag nanospheres.

Based on our previous high-precision numerical calculation on the local field


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distribution of the array,51 more hot spots are resulted from the hexagonally packed configuration. This is the first factor influenced on ⁄ . The second possible reason can be a result of the fluctuation of the gap values. Our previous study showed that the variation in gap decreases with the increase in averaged gap.40 The following two nonlinear behaviors would engender the smaller gaps to be dominant in the experimental SERS signal as well as background: Equation (1) j proclaims that ∝ ; depends nonlinearly on the gap, as they approaches infinity #

at zero gap with local response of metal.52 That is, the average “effective” gap value is less than the geometrical one. Note also that the wavelength behavior of the background41 for a flat substrate ratio as well as resonance Raman scattering for adsorbed molecules influents on the studied. We also note that some contribution to the background can be caused by the photoluminescence of impurities and defects in the anodic aluminum oxide matrix, yielding some additional fluorescence and thus affecting the ultimate ⁄ value.

For the gaps larger than 10 nm the light

penetration is determined by the skin effect not by the gap by which the resultant S/B would be relatively insensitive to the gap. On the other hand, nonlocal dielectric response of metal would become important when the gap is less than 2 nm and quantum tunneling effect would dominate for the gap less than 1 nm. Accordingly, our model would be modified for such small gaps. It is expected that these two effects would confer modified wavelength dependences. Furthermore, the use of shorter excitation wavelengths might produce additional SERS background owing to fluorescence from molecules of interest which was considered in the derivation of our theoretical model and from the impurities and defects in the anodic aluminum oxide (AAO) matrix. If the excitation wavelength reaches absorption of the AAO matrix, the local field surrounding the Ag particles would be greatly changed. IV CONCLUSIONS In sum, the signal-to-background ratio of surface-enhanced Raman scattering is investigated in the case of two-dimensional hexagonally-packed Ag nanoparticles. Theoretical estimation of the signal-to-background ratio in the case of two Ag spheres is compared with the experimental results 13 Environment ACS Paragon Plus


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in the consideration that the SERS background is attributed to the field inside Ag spheres. Although the two cases differ, their results show good agreement in the dependence on excitation wavelength and inter-sphere gap. Two conclusions are resulted from this study: the smaller the gap is, the larger the SERS signal-to-background ratio is; the smaller absolute value of dielectric function of Ag begotten at a longer excitation wavelength results in smaller SERS background and thus higher signal-to-background ratio. A smaller gap gives higher local field in the gap region, leading to the first conclusion. The second conclusion originates from the fact that the field just outside the metallic sphere surface in the gap region is equal to that just inside the sphere surface times the absolute value of the dielectric function of the metal. This study has proposed two guidelines in the design and fabrication of SERS-active substrates with high signal-to-background ratio. The first one is that the nanostructural substrate with high volume ratio between hot spot and metal would confer high SERS signal-to-background ratio. The second guideline is that the metal used in the fabrication of SERS-active substrates is the one with a high real part of dielectric function and a small imaginary part at excitation wavelength of interest, because the real part of dielectric function of metal reflects the field outside the metal with respect to the field inside and the imaginary part contributes to SERS background. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) ACKNOWLEDGMENT The authors thank the financial support from the National Science Council and Academia Sinica in Taiwan.

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Surface-Enhanced Raman Scattering: Physics and Applications. Kneip, K.; Moskovits, M.; Kneip, H. eds.; Springer: Berlin, 2006.

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Table 1. Comparison of SERS signal-to-background ratio, ⁄ , gap , obtained by theoretical estimation (Theory) on two Ag nanospheres (diameter = 50 nm, gap = 5 and 10 nm) and that of adenine obtained by SERS experiment (Exp.) on AgNP/AAO substrates (average diameter = 50 nm, average gap = 5 and 10 nm) with two excitation wavelengths (λex = 532 and 632.8 nm). ⁄ 632.8 nm ⁄ 532 nm

⁄ 5 nm ⁄ 10 nm

gap = 5 nm

gap = 10 nm

= 632.8 nm

= 532 nm

Theory

4.2-5.7

4.4-5.9

4.3

4.5

Exp.

5±1

4±1.5

5.5±1

4.5±1.5



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Figure 1. (a) Schematic layout of two coupled spheres aligned along z axis induced by applied electric field Ein and induced charge distribution on them; (b) field distribution on yz plane of two coupled Ag spheres with a diameter of 50 nm and a gap of 5 nm excited at 532 nm. Dotted circles represent the boundary of the spheres.


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Figure 2. (a) Field enhancement factors, , = 0,0, I⁄ % , along z axis of two identical Ag spheres with a gap of 5 nm and a diameter of 50 nm, depicted in Fig. 1(a), inside the left sphere centered at 0,0,0 excited with incident light field along z axis at 532 nm (solid) and 632.8 nm ∗ (dashed); (b) normalized field enhancement factors, , = , I⁄, I = 25 nm , of two

identical Ag spheres with gaps of 10 nm (solid) and 5 nm (dotted) excited with incident light field at 532 nm and for the gaps of 10 nm (solid) and 5 nm (dotted).



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Figure 3. SEM images of AgNP/AAO substrates with particle diameter of 50 nm and gaps of (a) 5 nm and (b)10 nm.


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Figure 4. Dark-field scattering spectra of AgNP/AAO substrates with particle diameter of 50 nm and gaps of (a) 5 nm and (b)10 nm.



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Figure 5. SERS spectra of adenine dissolved in water (2×10-5 M) on AgNP/AAO substrates with two average gaps (6) and two excitation wavelengths ( ): (a) 6, = 5 nm, 532 nm , (b) 6, = 5 nm, 632.8 nm

,

(c)

6, = 10 nm, 532 nm

,

and

6, = 10 nm, 632.8 nm. Star marks indicate the prominent 740-cm-1 peak of adenine.


(d)

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TOC Graphic


Dependences on Excitation Wavelength and Hot-Spot Gap - American

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