Effect of layer structures of gold nanoparticle films on surface enhanced Raman scattering

Analytica Chimica Acta 649 (2009) 111–116

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Effect of layer structures of gold nanoparticle films on surface enhanced Raman scattering Min Kyung Oh a , Sukang Yun a , Seong Kyu Kim a,∗ , Sungho Park a,b,c,∗ a b c

Department of Chemistry, Sungkyunkwan University, Suwon 440-746, Republic of Korea Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea

a r t i c l e

i n f o

Article history: Received 20 May 2009 Received in revised form 8 July 2009 Accepted 9 July 2009 Available online 15 July 2009 Keywords: Nanoparticle Gold film Surface enhanced Raman scattering Plasmon Extinction spectroscopy

a b s t r a c t Using a method of collecting nanoparticles at a water/hexane interface in a close-packed monolayer film and transferring such films onto a solid substrate, three-dimensional multilayer films of nanoparticles were formed. The packed nanoparticles were gold nanospheres (NS) with a 26 nm diameter or gold nanorods (NR) with a 31 nm diameter and 74 nm length. We investigated variations in the surface enhanced Raman scattering (SERS) intensities from such nanoparticle films as the layer compositions were changed. The films stacked with NR layers generated much higher SERS intensity than those of NS layers. The SERS intensities from both kinds of films increased as the number of layers were increased. However, when the NR layer and NS layer were stacked alternately, SERS intensity varied in a zigzag fashion. It was found that the structure of top layer plays a distinguishable role in generating strong SERS enhancement while the lower layers contribute to SERS with less dependency on structures. Interlayer coupling as well as intralayer coupling was considered in order to explain the observations. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Surface enhanced Raman scattering (SERS) has been considered a useful method in elucidating characteristics of adsorbate–metal interactions [1]. It utilizes the electric field enhancement induced by localized surface Plasmon resonance in noble metal nanostructures and the adsorbate–metal charge transfer resonance. To increase its usefulness as an analytical tool, numerous methods have been attempted to develop effective SERS substrates [2–8]. An effective substrate is likely to carry a high density of ‘hot spots’ [9–19] for the local electric field induced by an excitation laser. When the high density of hot spots is produced from a collection of closely spaced nanoparticles, a long range interparticle Plasmon coupling or higher order Plasmon resonance is considered a major mechanism for high local electric field generation. The interparticle Plasmon coupling and its relation to SERS have been elucidated for several well-defined model nanostructures, nanoparticle dimers [20,21] and lithographically patterned arrays [22–28]. Most model nanostructures have been arranged two-dimensionally on a flat surface, to which an excitation light is incident normal; thereupon, the Plasmon coupling is made laterally on the two-dimensional surface.

∗ Corresponding authors: Tel.: +82 31 290 7069; fax: +82 31 290 7075. E-mail addresses: [email protected] (S.K. Kim), [email protected] (S. Park). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.07.025

While the SERS effects from three-dimensional architectures are seldom studied, a relatively clean and stable three-dimensional SERS substrate system based on gold nanoparticles (Au NPs) has been developed recently [7,8]. It was assembled by successive transfer of a close-packed Au NP monolayer film onto a solid substrate [29,30]. In this method, Au NPs synthesized in an aqueous medium are collected at the water/non-polar liquid interface by surface destabilization of the Au NPs. Pressure induced by a Langmuir–Blodgett (LB) trough produces a closepacked Au NP monolayer film with a mirror-like reflection, which enables the transfer onto a solid substrate while maintaining the close-packing structure. Analysis with a field emission scanning electron microscope (FESEM) demonstrated that a layer-by-layer transfer as many times as desired is possible to a satisfactory level. The ␨-potential measurement showed that capping materials that stabilized Au NPs in water were largely removed [8], so that their vibrations did not appear in the SERS measurements [7]. The SERS intensity of the Au NP film as a substrate, measured for benzenethiol as an analyte, increased almost linearly up to eight monolayers [8]. Therefore, a three-dimensional architecture is proven to be a more efficient SERS substrate than a two-dimensional. The effectiveness of the three-dimensional architecture as a SERS substrate has opened new questions regarding its mechanism, especially the coupling along the normal direction to the substrate plane. In the work of this paper, in order to elucidate the SERS mechanism in the three-dimensional architecture, we have var-

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ied the layer compositions with different shapes of nanoparticles: nanosphere (NS) and nanorod (NR).

3. Results 3.1. FESEM results

2. Experimental 2.1. Preparation of gold NS and NR solutions An aqueous solution of 30 nm diameter gold NS was prepared by growing 13 nm diameter Au NPs. First, the 13 nm diameter Au NPs were synthesized by reducing 100 mL of 1.0 mM aqueous HAuCl4 ·3H2 O with 10 mL of aqueous 38.8 mM trisodium citrate. Then, for the additional growth, 15 mL of the prepared 13 nm gold solution was added to 155 mL of triply deionized water (Millipore) with 4.0 mL of 20 mM HAuCl4 and 400 ␮L of 10 mM AgNO3 . Subsequently, 30 mL of 0.1 M ascorbic acid was added at a rate of 0.5 mL min−1 . The synthesis of an aqueous solution of gold NR followed a method reported elsewhere [31]. First, a seed solution of gold was prepared by reducing 2.5 × 10−6 mol of HAuCl4 in 10 mL of a 0.1 M cetyltrimethylammonium bromide (CTAB) solution with 600 ␮L of 6.08 mM NaBH4 . Second, a growth solution of gold was prepared by slowly adding 0.1 M ascorbic acid into 100 mL of a 0.1 M CTAB solution containing 0.05 mmol HAuCl4 and 6.0 mmol AgNO3 . Then, the orange color of the gold salt solution disappeared. Finally, 120 ␮L of the seed solution was added to the growth solution while stirring mildly. After 12 h without agitation, the final solution was centrifuged twice to remove the CTAB. 2.2. Layer-by-layer assembly of Au NPs film The prepared Au NP aqueous solution (30 mL) was transferred into a LB Teflon cell (inner dimension: 7.5 × 4.0 × 1.5 cm3 ), and 10 mL of hexane was added to form an immiscible water/hexane interface. Then, ethanol was added at a rate of 0.6 mL min−1 , using a mechanical syringe pump. The Au NPs destabilized by the ethanol addition gathered at the water/hexane interface, displaying a shiny gold color. Addition of the ethanol continued until the mirror reflection was apparent. After forming the shiny interface, the hexane layer was evaporated spontaneously. The shiny interface indicates that a majority of Au NPs was packed at this stage. Further packing to remove visible void areas was achieved by compressing the film with two LB barriers from the opposite sides. Finally, in order to transfer the Au NP monolayer film onto a solid substrate, a 1 × 1 cm2 segment of either a silicon wafer (for SERS measurement) or a glass substrate (for extinction spectroscopy) was dipped into the LB solution at a 45◦ incident angle and pulled out vertically. All the substrates were modified with poly(4-vinylpyridine) (PVP) to maintain good adhesion to the Au NPs. 2.3. Characterization FESEM images were obtained to analyze nanoparticle size and shape using JEOL 7000F. The extinction spectra of the Au NP films on glass were obtained using a UV-vis spectrophotometer (Scinco, S3100). Each Au NP film prepared on a silicon wafer was immersed into 0.1 M benzenethiol in ethanol for an hour and used for the SERS measurement. A micro-Raman spectrometer (Renishaw, InVia) was used to record the SERS. The excitation was given by a He–Ne laser at 632.8 nm in wavelength. The intensity of a 1572 cm−1 peak, a C–C stretching vibration of the benzenethiol, was recorded for each sample. The excitation beam was delivered through an objective lens (NA = 0.75) and was incident normally onto the sample surface. The scattering at the 0◦ angle was collected with the same objective lens before further optical filtering and dispersion.

Fig. 1 shows examples of FESEM images of the Au NP films. Fig. 1(A) shows the image of a monolayer (ML) film of Au NS. From the FESEM image, the average diameter was determined to be 26.4 (±2.5) nm. The NSs were well packed in most of the area. Excluding rather large void spots, the packing density was 1408 particles ␮m−2 , which corresponds to a 77.1% packing efficiency, smaller than that of 2-dimensional hexagonal closest packing of spheres, 90.7%. Fig. 1(B) is the FESEM image of a 2 ML Au NP film. The bottom layer was packed with NSs and the top layer with NRs. From the top layer image, it can be confirmed that most of the NRs were lying flat on the film plane. The average diameter and the length of the NRs were estimated to be 31.7 (±4.4) nm and 74.8 (±6.7) nm, respectively. The packing density was 358 particles ␮m−2 , giving 77.2% packing efficiency, the same as the packing efficiency of the NS layer. Fig. 1(C) and (D) are the FESEM images of 3 ML and 4 ML Au NP films, respectively, where each layer was packed with NS or NR alternately. On the whole, Fig. 1 shows that the layer-bylayer assembly was achieved as controlled by the LB-to-substrate transfer process. 3.2. Extinction spectroscopy Extinction spectra of the gold NS and gold NR dispersed in the aqueous media before they were assembled onto the films were taken (shown in the supplementary materials). The spectrum of the NS solution exhibited a band at 520 nm, while the spectrum of the NR solution exhibited a large band at 640 nm, in addition to a small band at 520 nm. These two wavelengths can be assigned to the transverse (for 520 nm) and longitudinal (for 640 nm) excitations of localized surface Plasmon. Fig. 2 shows the extinction spectra of Au NP films formed on glass substrates. In the spectra, the ordinate is the optical density or the extinction, defined as −log(transmittance). Fig. 2(a) is for the NS films, where a Plasmonic band appeared at ca. 560 nm, shifting slightly to the blue as the number of layers increased. Fig. 2(b) is for the NR films, where a broad Plasmonic band appeared around 900 nm, also shifting to the blue as the number of layers increased. The NR spectra also show an additional tiny Plasmonic band at ca. 560 nm. These two Plasmonic wavelengths are shorter than those in this laboratory’s previous reports [7,8]. As a matter of fact, the extinction spectra change upon nanoparticle shapes and other experimental conditions such as pressure of the LB trough. Therefore, instead of attempting to give details of physical origin for these two bands, the ‘NS band’ label is attributed for the shorter wavelength and the ‘NR band’ for the longer wavelength. In Fig. 2(c), the extinction spectra for the NS–NR composite films are shown. The 1 ML film was made of the NS layer. The 2 ML film was made by stacking a NR layer onto the 1 ML film; then, the NR band grew while the NS band of the 1 ML film hardly changed. The 3 ML film was made by stacking a NS layer onto the 2 ML film; then, the NS band grew while the NR band of the 2 ML film did not change. This trend continued for the films with a higher number of layers stacked alternately with the NS and NR layers. The spectra in Fig. 2(c) can be described by linear combinations of the NS spectrum and the NR spectrum with the corresponding layer numbers, which indicates that a layer-by-layer assembly was achieved. The two Plasmonic bands around 560 nm and 900 nm were sitting on a broad background band, which must consist of scattering and absorption due to an interband transition [32–34] of gold. In general, the interband transition band of gold nanoparticle samples rises rapidly as the wavelength drops below 500 nm and forms a long tail into the long wavelength region of the extinction spec-

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Fig. 1. FESEM images of Au NP films: (A) a single layer of NS; (B) a double layer of NR (top) and NS (bottom); (C) a triple layer of NS (top), NR (middle), and NS (bottom); (D) a quadruple layer of NR (top), NS, NR, NS (bottom). Inset images show an edge structure of each film.

trum. The excitation of the interband transition turns into heat and does not induce Plasmons. In each spectral window, the excitation wavelength of the SERS experiments, 632.8 nm, is shown with a dotted line, and the optical density at this wavelength as a function of the number of layers is shown in the inset. The excitation wavelength is at the red edge

of the NS band and at the blue edge of the NR band. The optical density at this wavelength has the combined contribution from non-Plasmonic and Plasmonic bands. For every film type, the optical density increases linearly with film thickness, implying that the flux of incident light decays exponentially along the film stacking direction. This Beer-Lambert type behavior appears to apply to all

Fig. 2. Extinction spectra of Au NP films formed on glass substrates: (a) the NS films from 1 ML to 4 ML; (b) the NR films from 1 ML to 3 ML; (c) the NS–NR composite films in alternating layers. In (c), the 1 ML film was made of the NS layer; the 2 ML film was made of NS (bottom) and NR (top); the 3 ML film was made of NS (bottom), NR (middle), and NS (top); the 4 ML film was made of NS (bottom), NR, NS, and NR (top); the 5 ML film was made of NS (bottom), NR, NS, NR, and NS (top). In each spectral window, a dotted line indicates the wavelength of the SERS experiment, 632.8 nm. In addition, the optical density at this wavelength as a function of the number of layers is shown in the inset of each spectral window.

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Fig. 3. (Left) The SERS intensity of the Au NP film as a function of number of layers: : for the NS films; 䊉 for the NR films; : for the NS–NR composite films. (Right) The corresponding SERS spectra for benzenethiol adsorbed on the Au NPs films in the region of 1570 cm−1 .

bands, since the shapes of the spectra in Fig. 2(a) or (b) do not change upon a change in film thickness. The Plasmonic excitation profile in the SERS experiments is likely to decay exponentially along the propagation direction of the incident light, which will eventually lead to more dominant SERS enhancement from the first NP layer than from the lower layers. 3.3. SERS Fig. 3 shows the variations in SERS intensity from the three kinds of Au NP film when the number of layers was changed. The SERS intensity of the NR film increased with the number of layers up to 7 ML, while that of the NS film increased up to 5 ML before saturation. The SERS intensity of the NR film was several times higher than that of the NS film of the same thickness. An interesting phenomenon was found in the behavior of the NS–NR composite film, where the NS and NR layers were stacked alternately. The 1 ML film was made of the NS layer. The 2 ML film was made of the NR layer on top and the NS layer on the bottom. The 3 ML film was made of the NS layer stacked on the 2 ML film, and so on. The SERS intensity of this series of the composite NS–NR film varied in a zigzag fashion. The SERS intensities of the even numbered layers followed the upper trend in the zigzag fashion, and those of the odd numbered layers followed the lower trend. The upper trend is close to the SERS variation of the pure NR layer films and the lower trend is close to that of the pure NS layer films. To investigate further the effect of layer structure on the SERS intensity, a series of host–guest NS–NR films was synthesized, where a guest layer, either NS or NR, was inserted into host multilayers, either NR or NS. Fig. 4 shows the variation in the SERS intensity as a function of the location of the guest layer. Here, all the films were 5 ML. In the film series with a NR host (denoted with 䊉), the film with the guest NS layer on the top showed the weakest SERS intensity, but the SERS intensity doubled when the guest NS layer went to the second layer from the top. From the second layer to the third layer for the location of the guest NS layer, the SERS intensity increased only by 10%. The SERS intensities for the third, fourth, and fifth layers for the guest NS location were similar to one another and to the pure NR film (denoted with ). The trend in the series of NS host (denoted with ) is completely opposite; the presence

of the guest NR layer on the top gave the maximum SERS intensity, the presence of the guest NR in the second layer gave a reduced SERS intensity, while that in the lower layers was indistinguishable from the pure NS film (denoted with ). Fig. 4 suggests that in contributing to the SERS intensity, the structure of the top layer plays a distinguishable role, while that of the second layer plays a much reduced role, and that of the lower layers a minor role at best. This idea is also consistent with the trend of the NS–NR composite films in Fig. 3; the films with even numbered layers had the NR layer on the top, and their SERS intensity variation was close to the trend of

Fig. 4. The SERS intensity variation of the host–guest Au NP film: : NS guest layer inside NR host layer; 䊉 NR guest layer inside NS host layer;  pure NR layer, : pure NS layer. All films were 5 ML. The x-axis represents the layer number (from the top) where the guest layer was inserted.

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the pure NR films, and the films with odd numbered layers had the NS layer on the top, and their SERS intensity variation was close to the trend of the pure NS films.

4. Discussions Results of the SERS study shown in Figs. 3 and 4 can be summarized as follows. (1) Within the size regime of Au NP in this study, the NR films generate stronger SERS signal than the NS films. (2) The SERS intensity increases almost linearly as the thickness of pure Au NP film increases until a saturation thickness is reached. (3) In the NS–NR composite films and the host–guest films, the structure of the top layer is much more sensitive in contributing to the SERS intensity than that of lower layers. First, the stronger SERS signal in the NR film than that in the NS film can be understood in the following way. In a close-packed assembly of nanostructures, the hot spots are likely to be formed at the crevice areas between two Au NPs since the local field increases exponentially as the interparticle distance is reduced down to a subnanometer range [35]. The NR assembly has a larger crevice area than the NS assembly since most NR–NR contact is made sideby-side, while most NS–NS contact is made point-by-point. This difference in the structures is likely to produce a more efficient interparticle Plasmon coupling in the NR layer than in the NS layer. Second, the increase in SERS intensity with the number of layers is deliberated. As mentioned in the Results section, the incident light flux must be distributed exponentially along the film stacking direction or the light propagation direction. From the slopes of optical density versus number of layers in Fig. 2, the penetration depths for the 632.8 nm light are calculated: 1.07 ML for the NS films and for the NS–NR composite films, 0.96 ML for the NR films. The penetration depth of the 1 ML (or ca. 30 nm) implies that 63% of the light flux inside the film is resident at the top layer, 23% at the second layer, 9% at the third layer, and so on. Unless there is no interlayer coupling, this ratio corresponds to the distribution of Plasmons over the film layers. In this case, the relative contribution of each layer to the SERS intensity must roughly be the square of the above number since the Raman scattering from each layer must escape from the film (toward the top) with the same ratio of probability. For instance, in the case of the 5 ML film, the SERS contribution from each layer is estimated as follows: 86% from the first layer, 12% from the second layer, 2% from the third layer, and almost 0% from the fourth and fifth layers. This simple calculation contradicts the observations in the SERS intensity variations over film thickness, where contributions from all layers appear to be similar in a pure Au NP film series. This suggests that an interlayer coupling must be considered in addition to the intralayer Plasmon coupling. An interparticle Plasmon coupling is most effective when the coupling direction is in the plane of the electric field of the incident light [18,35]. Such a radiation induced coupling must have produced high local field, polarized in plane, at crevices among Au NPs in each layer. In the three-dimensional nanostructures of this study, the direct radiation induced coupling must be weak along the film stacking direction. However, an electron induced coupling between two layers is expected. In this mechanism, an electron oscillation in one layer may induce an oscillation of image charges in the other layer; so that a longitudinally polarized oscillating dipole is produced at the interlayer. This mechanism is thought to be responsible for the SERS phenomena found in noble metal nanoparticles sitting on top of flat noble metal films [36,37]. There are several theoretical works related to this mechanism [38–40]. We suppose that in our samples the electron induced oscillation is produced in a cascade way and propagates slowly along the film stacking direction. Thus, as virtually all conduction electrons in the film can be excited to

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a certain depth (around 5 ML for NS films and 7 ML for NR films in this study), the SERS intensity contribution of each layer can be equalized. Third, the distinguishable role of the top layer is also explained with the electron induced coupling between layers in addition to the intralayer radiation induced coupling. Since the intralayer coupling is more efficient in the NR layer and is dominant in the top layer (and in the second layer to a reduced extent), the presence of the NR layer on the top initially produces a larger amplitude of Plasmon oscillations. The interlayer coupling produces the electron oscillations whose amplitudes follow the initial Plasmon excitation. The interlayer coupling is more dependent on the density of the conduction electrons than the intralayer structure or intralayer coupling. Thus, the position of the guest layer in other than the top layer is not important to SERS intensity. Finally, we emphasize the close-packing of nanoparticles in our film fabrication methodology to achieve the three-dimensional coupling. The increases of SERS intensity with the number of layers in other laboratory’s reports [41,42] are not as dramatic as our case. This is probably because in other laboratory’s layer-by-layer film there were intervening layers which weaken the interlayer coupling. 5. Conclusion Through a series of the SERS experiments with varied layer compositions of Au NP film, we demonstrated that the threedimensional nanoscale architecture can be a more effective SERS substrate than two-dimensional, even though the light-driven Plasmon excitation is localized to the top few layers. The SERS intensity is roughly determined by the contribution from the structure of the top layer multiplied by the number of layers. This phenomenon is explained in terms of interlayer electron induced coupling in addition to intralayer radiation induced coupling. As most SERS or surface Plasmon research has focused on two-dimensional nanostructures, the study on the interlayer coupling has been limited to cases of a nanoparticle on a flat film. While more attention to the three-dimensional system is called upon, we are currently studying this phenomenon with a computational electrodynamic method. Acknowledgements This work was supported by the Korea Research Foundation grant funded by the Korean Government (MEST, KRF-2008-005J00702) and the Korea Science and Engineering Foundation (Nano R&D program: 2008-04285, 2009-0060482). The support of the SRC program (Center for Nanotubes and Nanostructured Composites) is also acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.aca.2009.07.025. References [1] [2] [3] [4] [5]

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