Characteristic Length and Temperature Dependence of Surface

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Characteristic Length and Temperature Dependence of Surface Enhanced Raman Scattering of Nanoporous Gold X. Y. Lang, P. F. Guan, L. Zhang, T. Fujita, and M. W. Chen* WPI AdVanced Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan ReceiVed: April 5, 2009; ReVised Manuscript ReceiVed: May 9, 2009

Surface enhanced Raman scattering (SERS) of nanoporous gold (NPG) with various nanopore sizes was investigated at temperatures ranging from 80 to 300 K using rhodamine 6G as probe molecules. It was found that the SERS intensity dramatically increases as temperature and nanopore size decrease. Theoretical analysis indicates that the improved SERS enhancements result from the confluence effects of the electron-phonon scattering, the finite ligament size effect, and the electromagnetic coupling between neighboring gold ligaments, which demonstrate the electromagnetic origin of the SERS effect of NPG. 1. Introduction

2. Experimental Methods

Nanostructured noble and transition metals have been widely investigated because of their extraordinary optical properties that arise from surface plasmon resonance (SPR).1,2 One of the most important applications of the surface-plasmon-based analytical technique is surface enhanced Raman scattering (SERS) that has attracted considerable attention because of its distinctive function in trace-level molecular detection and analysis in chemical and biological systems.3–6 Because of electromagnetic (EM) resonance and the coupling of metal nanostructures,4–8 SERS enhancements essentially depend on the nanoscaled characteristics of metallic substrates, such as size,9,10 shape,11 and assembly of nanostructures.12,13 This has led to the surge in exploring reliable SERS substrates with high enhancement factors.1–3,9 Recently, free-standing nanoporous gold (NPG) films are of special interest as promising SERS-active substrates by virtue of a unique bicontinuous nanoporosity with a high surface-tovolume ratio as well as excellent chemical stability and biocompatibility. The proper nanostructures consisting of quasiperiodic ligaments and nanopore channels14–16 enable NPG to provide a reliable, stable, and uniform SERS signal from the free-standing films.14,16–18 It has been found that the SERS enhancement is susceptible to the characteristic lengths of nanoporosity that are affected by the thickness of the films and dealloying conditions.14,16–18 Excellent SERS enhancements of NPG have recently been achieved by reducing nanopore sizes,14 roughening gold ligament surfaces,17 and introducing fracture surfaces through the so-called antenna effect.14,18 Although both chemical and EM effects have been suggested,1,3,9 the underlying mechanisms of the SERS effects of NPG remain unclear. Since the size of nanostructures and measuring temperature are two essential factors responsible for the optical properties of nanostructured metals,19–25 in this study we systemically investigate SERS effects of NPG with characteristic lengths of ∼12-45 nm at a temperature ranging from 80 to 300 K. It reveals that the SERS enhancements increase with the decrease of temperature and nanopore size as the results of the weakening of electron-phonon scattering and the strengthening of EM coupling between gold ligaments.

All chemicals were used as received without any further purification. NPG films with the thickness of ∼100 nm were fabricated by dealloying commercial Ag75Au25 (atom %) alloy leaves. The films were floated on a concentrated HNO3 solution at three scheduled temperatures (293, 273, and 253 K) that were controlled by a commercial cooling/heating setup with a temperature accuracy of (2 K.26 Intermediate nanoporous structures were quenched by pure water (18.2 MΩ · cm), and the residual acid within the nanopore channels was removed by water rinsing. The synthesized NPG films were placed on Cu grids for transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray energy dispersive spectroscopy (EDS) characterizations. The 3D morphology of the NPG films was investigated by using a computer controlled scanning TEM (Tecnai F30, FEI Co.) equipped with a high-tilt specimen holder.15 The NPG specimens with various nanopore sizes were subjected to SERS experiments with R6G as the test molecules. SERS spectra were collected with a Renishaw Raman microscope operating with a 514.5 nm Ar ion laser (beam size: ∼1 µm), a 50× working distance objective, 1 mW power, and 10 s integration times. For each sample, five spectra were acquired from different spots and were averaged as the representative spectrum used in this paper. DDSCAT 7.0 code developed by Draine and Flatau27,28 was employed to calculate the near-field distributions of NPG. The sizes of the ligament and nanopore are set to equal to each other with D ) 10, 50, and 80 nm, similarly to the intrinsic feature of NPG determined by electron tomography.15,29,30 The dielectric properties of the nanostructure were simulated by the bulk dielectric function of Au,31 which was scaled by the dielectric constant of the surrounding environment/medium (εm ) 1). The plane wave with a wavelength of 514 nm propagates along the direction normal to the top surface of the nanostructure.

* Corresponding author. E-mail address: [email protected].

3. Results and Discussion During the dealloying process, nanoporosity evolves by the migration of Au atoms via surface diffusion at solid/electrolyte interfaces with Ag atoms dissolving.32,33 By carefully controlling the dealloying time (t) and temperature (T), NPG films with different characteristic lengths (D) were achieved. Figure 1a shows the representative top-view SEM micrograph of NPG

10.1021/jp903137n CCC: $40.75  2009 American Chemical Society Published on Web 06/02/2009

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Figure 1. (a) Representative SEM image of NPG film with a characteristic length of 26 nm. (b) 3D tomographic reconstruction of NPG with a characteristic length of 18 nm. TEM micrographs of NPG films with characteristic lengths of (c) 15, (d) 26, and (e) 41 nm, respectively. (f) Representative SEM EDS of NPG films. The Cu peak comes from the Cu grid.

films with D ≈ 26 nm, illustrating the NPG microstructure with open nanoporosity. Figure 1b shows an example of the 3D structure of NPG, revealed by electron tomography, which demonstrates that the nanoporous structure is bicontinuous in three dimensions.15 Although nanopore sizes vary with dealloying conditions, it is interesting to note that in each sample with a different characteristic length, the morphologies and sizes of nanoporous channels and gold ligaments are statistically identical. This is also verified by TEM micrographs of NPG films with D ≈ 15, 26, and 41 nm, which are shown in Figure 1c-e, respectively. However, the observations are different from those of Ro¨sner et al.34 In their electron tomograph images, the NPG prepared by dealloying Ag80Au20 shows much of the open nanoporous structure, and the mean pore size (∼28 ( 8 nm) is about twice as large as the ligament size (∼16 ( 5 nm). The remarkable dissimilarity is probably caused by the different dealloying approaches. Free-chemical etching used in our study may allow the nanoporous structure to have enough time to relax by surface diffusion for a more stable nanostructure. In contrast, the applied potential (∼600 mV) for electrochemical dealloying in the study of Ro¨sner et al. may lead to the formation of gold hydroxide that inhibits the coarsening of gold ligaments and the relaxation of the porous structure.35 The characteristic length scales of NPG, defined by the equivalent diameters of nanopores or the gold ligaments, are measured from statistical analysis of digital TEM and SEM images by a rotational fast Fourier transform method.29 A representative X-ray EDS spectrum shown in Figure 1f demonstrates that only ∼1-2 atom % silver is left behind in the nanoporous structure, implying that the

Figure 2. Relationship between the characteristic length (D) of thoroughly rinsed NPG films and the dealloying time (t) at 293, 273, and 253 K, respectively.

influence of residual Ag on the SERS effect of NPG is negligible. Figure 2 shows the relationship between characteristic length D(t,T) with the dealloying time (t) and temperature (T), indicating that D increases with t and T.26 This suggests that the etching process allows us to fine-tune the characteristic length of NPG from tens to several nanometers by adjusting the dealloying conditions.26 In particular, low temperatures can effectively delay the coarsening of the nanoporous structure, resulting in very small nanopore sizes. To test the Raman-enhancing capability of NPG films, all of the NPG specimens were respectively immersed in a water solution of R6G (1 × 10-7 mol/L) and then dried in air for the

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Lang et al. TABLE 1: Necessary Parameters Used for Calculations of Eqs 2 and 3 parameters

gold21

ω0 θD VF lb T0

2.0477 × 1013 rad/s 170 K 1.4 × 106 m/s 47.5 nma 300 K

lb is calculated in terms of lbFb(T0) ) l0F0 ) 9.6 × 10-6 µΩ · cm2 and Fb(T0) ) 2.02 µΩ · cm.42 a

Figure 3. (a) Representative SERS spectra of NPG films with different characteristic lengths at the concentration of 1 × 10-7 mol/L R6G. Laser excitation: 514.5 nm. (b) Characteristic length dependence of the relative SERS intensities [mSERS(T0,D)] of the 1650 cm-1 Raman band of R6G molecules adsorbed on NPG films fabricated at 293 ([), 273 (b), and 253 K (2), respectively, and the mSERS,lig(T0,D) function (the solid line). The latter is calculated in terms of eq 3 at T0 with the necessary parameters listed in Table 1. The data (9) are taken from ref 14. (c) The relative SERS intensity induced by EM coupling mSERS,coup(T0,D) as a function of characteristic length of NPG films, wherein all of the symbols are of the same meaning as the aforementioned. The solid line is described by the exponential function mSERS,coup(T0,D) ) A exp(-D/l) with A ) 81 and l ) 6 nm.

Raman scattering measurements. The representative SERS spectra of R6G molecules physisorbed on NPG films with various characteristic lengths at room temperature (T0) are shown in Figure 3a, demonstrating that SERS enhancements are remarkably improved with decreasing D. High-intensity Raman peaks corresponding to the characteristic vibrational modes of R6G molecules can be observed, similarly to the previous observations.12,14 To further illustrate the dependence of SERS intensity on characteristic length, the relationship of the relative integrated intensities [mSERS(T0,D)] of the R6G Raman band with D is plotted in Figure 3b, where mSERS(T0,D) ) ISERS(T0,D)/ ISERS(T0) with ISERS(T0,D) denoting the characteristic length dependent SERS intensity, and ISERS(T0) is the SERS intensity

of NPG with D ) 50 nm that is approximately equal to the mean free path of bulk Au.1 In this plot, the Raman band located at 1650 cm-1, corresponding to the CdC stretch of the xanthene skeleton of R6G molecules, was used to count the Raman scattering intensity. As seen from this plot, the SERS intensity of NPG films with D ≈ 12 nm is ∼20 times stronger than that of the NPG film with D ) 50 nm. The continuous increase of the SERS enhancements with the decreasing pore size is contrary to the maximum SERS enhancement at a pore size of ∼250 nm observed by Kucheyev et al.17 The anomalous enhancement from large pore size most likely arises from the rough surfaces of gold ligaments, which produce the so-called antenna effect for high SERS enhancements.14,18 To explore the mechanism of NPG SERS enhancements, we measured the SERS spectra of R6G molecules physisorbed on NPG films with D ≈ 15, 26, and 41 nm at temperatures ranging from 80 to 300 K, which are shown in Figure 4a-c, respectively. Obviously, for the given NPG films with D, the SERS enhancements are dramatically improved with the decrease of temperature. No peak shifts can be observed when the substrate temperatures decrease from 300 to 80 K, indicating that the bond nature of R6G molecules does not change during the course of temperature variation. The temperature and characteristic length dependence of the integrated intensity [ISERS(T,D)] is plotted in Figure 4d. It can be seen that ISERS(T,D) increases as T and D decrease, suggesting that small pore sizes and low temperatures lead to high SERS enhancements. Furthermore, the degree of increment of ISERS(T,D) with decreasing T is strongly dependent on the characteristic length of NPG films. For instance, ISERS(T ) 80 K,D) is larger than ISERS(T ) 300 K,D) by factors of ∼1.9, 2.0, and 2.7 for D ≈ 15, 26, and 41 nm, respectively. Although the small pore size offers large internal surfaces for the adsorption of target molecules and thereby strong Raman signals, both quantitative electron tomograph and electrochemical measurements suggest that the internal surface areas only increase about 2-3 times with the decrease of nanopore size from ∼50 nm to 10 nm. As D decreases to ∼10 nm, the one order of magnitude discrepancy between the surface increment and the SERS improvement evidently indicates that the SERS effect on NPG mainly arises from EM field enhancement caused by plasmon excitation of NPG through incident laser light. According to the EM enhancement mechanism for the SERS effect based on the quasi-static approximation,1,3,9 ISERS(T,D) ∝ |Ein(T,D)/E0|4 ) |3εm/[ε(ω,T,D) + 2εm]|4,1,3,9,36 where Ein(T,D) and E0 denote the average electric field inside the NPG films and the externally applied electric field, respectively, εm is the dielectric constant of the surrounding medium, and ε(ω,T,D) ) ε1(ω,T,D) + iε2(ω,T,D) ) 1 - ωp2/ω[ω + iωc(T,D)] is the dielectric constant of metals according to the Drude model with ε1(ω,T,D) ) 1 - ωp2/[ω2 + ωc2(T,D)], ε2(ω,T,D) ) ωp2ωc(T,D)/ {[ω2 + ωc2(T,D)]ω}, and ωp2 ) 4e2πN/m0.22,31,37 Here ω is the angular frequency of incident light, N is the density of conductor

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J. Phys. Chem. C, Vol. 113, No. 25, 2009 10959 electrons, and e and m0 denote the charge and the effective optical mass, respectively; ωc(T,D) is the collision frequency of electrons depending on T and D. Because of ω2 . ωc2(T,D) and the negligible temperature19–25 and size38 dependence of ωp2, ε1(ω) ) 1 - ωp2/ω2 and ε2(ω,T,D) ) ωp2ωc(T,D)/ω3 for further simplicity.19–21,24 Usually, εm is taken as a real constant throughout the visible range, and it is possible to find a resonance frequency at which [ε1(ω) + 2εm]2 + [ε2(ω)]2 ) minimum.1,3,9,36 In short, under a resonance frequency with ε1(ω) ) -2εm, ISERS(T,D) ∝ {3εmω3/[ωp2ωc(T,D)]}4, or

ISERS(T, D) ∝ 1/[ωc(T, D)]4

(1)

where ωc(T,D) consists of two parts that arise from the phonon-electron scattering [ωc(T)]21–23,39 and the electron mean free path effect [ωc(D)]1,40 in the light of Matthiessen’s rule, that is, ωc(T,D) ) ωc(T) + ωc(D). On the basis of the simple Debye model for the phonon spectrum, ωc(T) is derived by Holstein in the limit of EF . pω . kBT and kBθD,21–23,39 ωc(T) ) ω0[2/5 + 4(T/ΘD)5∫0θD/Tz4dz/(ez - 1)], with EF as the Fermi energy, kB as Boltzmann’s constant, and ΘD as the Debye temperature. The 2/5 term in this equation is due to spontaneous emission of a phonon by a photon-excited electron, while the temperature-dependent term corresponds to phonon adsorption and stimulated-emission processes. ω0 is a constant that can be independently determined from the known value of the dc conductivity (σ0). Since for the dc case pω , kBT and kBθD, the Holstein expression becomes ωc(T,ωf0) ) ωp2/(4πσ0) ) ω0[4(T/ΘD)5∫0θD/Tz5 dz/[(ez - 1)(1 - e-z)]].21–23,39 On the other hand, for NPG with D smaller than the electronic mean free path lb of bulk Au, the effective mean free path leff is dominated by collisions with the surfaces of gold ligaments. As a result, ωc(D) can be given by ωc(D) ) 8VF/(3leff).1,41 In terms of eq 1, the scaling relationship of ISERS(T,D) with T for the given NPG films with D can be given by eq 2 when taking T0 as a reference point

ISERS(T, D)/ISERS(T0, D) ) [ωc(T0, D)/ωc(T, D)]4

Figure 4. SERS spectra of NPG films with a characteristic length of (a) 15, (b) 26, and (c) 41 nm at the concentration of 1 × 10-7 mol/L R6G under the temperatures of 80, 100, 150, 200, 250, and 300 K. (d) Temperature and characteristic length dependence of integrated intensities of the 1650 cm-1 Raman band of R6G molecules on NPG films over the 80-300 K temperature range according to the figure: (a) 15 (2), (b) 26 ([), and (c) 41 nm (9), respectively. The solid lines denote the predictions of ISERS(T,D) in terms of eq 2 with ISERS(T0,D) ) 8178, 5958, and 2595 for 15, 26, and 41 nm NPG films, respectively. The other necessary parameters are listed in Table 1.

(2)

Comparisons between eq 2 and experimental results are shown in Figure 4d. The good consistence interprets that the gradual increases of ISERS(T,D) with decreasing T result from the reduced effect of electron-phonon scattering. While T drops to ∼30 K, the ISERS(T,D) approaches to a constant, similarly to the inverse of temperature dependence of the electronic resistivity of bulk and nanostructured Au,41,42 which implies the underlying correlation between the SERS effect and the natural properties of the electron transport in the nanostructured metal, that is, effective electronic mean free path. In the NPG films, the effective mean free path decreases with D as a result of the inner surface scattering of NPG,30 giving rise to the decrease of ε2(ω,T,D) and thus the decrease of EM enhancements.9,10,43 This makes it reasonable that the degree of enhancements of ISERS(T ) 80 K,D) in comparison with ISERS(T ) 300 K,D) decreases with decreasing D and illustrates the contribution of ligament sizes to SERS intensity. While considering the mean free path effect of separate ligaments at T0, in terms of eq 1, the size dependent function of the relative SERS intensity induced by a individual ligament [mSERS,lig(T0,D)] can be expressed as

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mSERS,lig(T0, D) ) ISERS,lig(T0, D)/ISERS(T0) ) [ωc(T0)/ωc(T0,D)]4 (3) with ISERS,lig(T0,D) denoting the SERS intensity contributed from an individual ligament. According to eq 3, the mSERS,lig(T0,D) function shown as the solid line in Figure 3b reveals that the decreasing ligament size results in the weakening of SERS enhancements due to the dramatic increase of electronic scattering at the inner surface, the same as the observations in nanoparticles or nanowires.9,10,44,45 This decreasing trend with D is totally contrary to the experimental measurements that the SERS enhancements of NPG films increase with decreasing characteristic length, as shown in Figure 3b and Figure 4d. Consequently, in addition to the finite size effect of an individual ligament, there should be other factors responsible for the large SERS enhancements of NPG with a small nanopore size. In view of the unique nanostructure of NPG films, a smaller nanopore size represents a closer interligament distance and large local curvature. The extra enhancements most likely result from the EM coupling between neighboring ligaments and intensified localized SPR,18 which are verified by the near-field calculations with the method of the discrete dipole approximation (DDA).27,28 In view of the complicated structure of NPG, a simplified 2D nanoporous structure with identical pore and ligament sizes (Figure 5a) is introduced to qualitatively simulate the EM fields of NPG. Under the plane wave with wavelength of 514 nm propagating along the direction normal to the top surface of the nanostructure, the near-field distributions on the top surfaces are shown in Figure 5b-j for NPG films with D ) 10, 50, and 80 nm, respectively. The EM field enhancements arising from the EM coupling between neighboring ligaments (Ecoup) are illustrated in Figure 5h-j, where Ecoup ) E - E*.44 In the simulations, E is the total EM field distributions of NPG (Figure 5b-d), calculated on the basis of the nanostructure (Figure 5a) with both A and B materials set as the gold substance. E* denotes the localized electric field distribution without the EM coupling effect (Figure 5e-g), which is the linear sum of electric field distributions of the left (A ) gold and B ) air, Eleft) and right (A ) air and B ) gold, Eright) parts of the nanostructure (Figure 5a). Note that although the E* located at the boundary between the left and the right parts is higher than that in other spots in the nanostructure because of a boundary effect, it does not influence remarkably the evaluation of electric field distribution in the nanoporous channels. As shown in these plots, the large enhancements of the EM field of NPG with decreasing D arise from both the localized EM increments and the EM coupling between the face-to-face ligaments, in particular in NPG with the 10 nm pore. This result is qualitatively in accordance with the simulated results of the boundary element method46 and the plasmon hybridization approach47 for thin gold films with single nanoholes. Therefore, the measured SERS intensity of NPG films apparently consists of the contributions of ISERS,lig(T0,D) from the size effect of individual gold ligaments and the EM coupling between neighboring ligaments [ISERS,coup(T0,D)].44 As a consequence, ISERS,coup(T0,D) ) ISERS(T0,D) - ISERS,lig(T0,D). The relative SERS intensity induced by the EM coupling [mSERS,coup(T0,D) ) ISERS,coup(T0,D)/ISERS(T0)] is shown in Figure 3c. Similarly to the optical properties of NPG influenced by a evanescent field,48 mSERS,coup(T0,D) dramatically increases with decreasing D by an exponential function, which can be well-fitted by the scaling behavior of distancedecayofplasmoncouplingbetweenmetalnanoparticles,49–51 mSERS,coup(T0,D) ) A exp(-D/l), where A is a constant and l denotes the decay length. From the fitting curve in Figure 3c,

Figure 5. (a) Schematic nanostructure used for DDA simulations. Calculated total electric field (E) (b, c, d), localized electric field (E*) (e, f, g), and EM coupling electric field (Ecoup) distributions (h, i, j) on the top surface of the NPG with D ) 10, 50, and 80 nm, respectively. The calculations were produced using the method of DDA, and the incident light with a wavelength of 514 nm was polarized in the x direction.

l ) 6 nm, which approximately corresponds to the decay length of the EM coupling between Au nanoparticles.49,50,52,53 This qualitative agreement with other experimental results of assembled nanoparticles suggests that the EM coupling between neighboring ligaments gives rise to a large number of “hot spots” in the quasi-periodic nanostructure when nanopore sizes become small and thus results in the reliable and uniform SERS spectra with high enhancement from the whole specimens.14,17,18 4. Conclusions In summary, the characteristic length and temperature effects on the SERS enhancements of NPG films have been systematically investigated. The SERS intensity of NPG is dramatically enhanced with decreasing nanopore size and temperature. The improved SERS enhancements of NPG primarily originate from the EM coupling between neighboring ligaments in comparison with the confluence effects of the electron-phonon scattering and the finite ligament size effect, which is qualitatively consistent with DDA simulation. The relative SERS intensity induced by the EM coupling between neighboring ligaments follows a simple exponential function that is similar to the exponential behavior of distance decay of plasmon coupling between nanoparticles. Acknowledgment. Research was sponsored by the Global COE for Materials Science, WPI Initiative for Atoms, Molecules and Materials, MEXT, Japan, and Iketani Science and Technology Foundation. X.Y.L. thanks the support of the JSPS Postdoctoral Fellowship Program and Grants (Grant No. P07373). References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Cluster; Springer: Berlin, 1995.

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