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Surface-enhanced Raman scattering and plasmon excitations from isolated and elongated gold nanoaggregates
Chemical Physics Letters 477 (2009) 130–134
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surface-enhanced Raman scattering and plasmon excitations from isolated and elongated gold nanoaggregates Mohammad Kamal Hossain a,1, Genin Gary Huang a, Tadaaki Kaneko b, Yukihiro Ozaki a,* a b
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan Department of Physics, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan
a r t i c l e
i n f o
Article history: Received 24 February 2009 In final form 12 June 2009 Available online 14 June 2009
a b s t r a c t Surface plasmon resonance (SPR) and subsequent surface-enhanced Raman scattering (SERS) of gold nanoaggregates were investigated with reference to hot-sites availability. Correlated SPR and SERS observation of the same sample shows that the plasmon excitations excited at the interstitials of nanoaggregates are more important rather than resonance conditions. The isolated aggregates mostly show a broadened SPR peak centered at 560 nm overlapping absorption band of analytes, whereas the elongated aggregates yield several SPR peaks in the longer wavelength region and provide higher enhancement in SERS. Finite-different time-domain analysis predicts that the localization sites get increased at least double in elongated aggregates. Ó 2009 Published by Elsevier B.V.
Localized electromagnetic (EM) field distribution in association with surface plasmon resonance (SPR) excited at the interstitials (i.e. hot-sites) of a small aggregate in nanometric space is the prime factor for giant optical field enhancement and has attracted huge interest in various fields of applications [1–4]. Aggregates of noble metal nanoparticles, particularly gold nanoparticles have shown their potential already in various sectors of nanoscience and technology including surface-enhanced Raman scattering (SERS) [4–8]. Gold nanoaggregates are capable of having localized EM field acting on them, thus creating regions of very high confined electric field named hot-spots [8–13]. Self-assembled-like nanoaggregates of gold nanoparticles deserve to be the best suited SERS-active substrate because of their active sites availability and free from uncertainty of hot-sites. As a consequence, the SERS-signal enhancement and quality using such substrates will get improved largely compared to that of irreproducible signals observed for aqueous and random nanoaggregates. Right to this point, it is a great challenge to fabricate self-assembled nanoaggregates from colloidal gold nanoparticles without using any surfactant. A unique fabrication method is adopted herewith to achieve such a SERS-active isolated and elongated gold nanoaggregates. Once the incident light becomes resonant with plasmon, the EM field localizes spatially at the junctions of noble metal nanostructure. Consequently, confined EM field polarizes the molecules adsorbed nearby by dipole–dipole coupling [14]. Thus the overall SERS enhancement appears to be approximately the product of * Corresponding author. E-mail address: [email protected] (Y. Ozaki). 1 Present address: The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Science, Victoria University of Wellington, Wellington 600, New Zealand. 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.06.043
two factors; one, due to coupling between incident photons and Loc ðxL Þ 2 Loc I localized SPR (i.e. EEI ðx , where E ðxL Þ and E ðxL Þ are the ampliÞ L
tudes of the SPR-mediated local electric field at laser angular frequency xL and incident electric field at the same angular frequency, respectively) and two, due to coupling between Raman Loc ðxR Þ 2 Loc I photons and localized SPR (i.e. EEI ðx , where E ðxR Þ and E ðxR Þ Þ R
are the amplitudes of the SPR-mediated local electric field at Raman angular frequency xR and incident electric field at the same angular frequency, respectively). If the number of EM field localization increases and spreads out, more analytes in the hot-site vicinity will be covered by SERS–EM mechanism. Any direct and correlated experiments dealing with SPR and SERS at the same spatial position would be beneficial in this respect [15,16]. Regardless of the nature of substrate, it is of fundamental interest to correlate the SPR excitations excited on nanoaggregates or at the segments of gold nanostructure to the confined optical fields, such as SERS or fluorescence images [12,13,17–19]. However, the microscopic correlation between localized EM field and confined optical field along with topographic confirmation has not yet been well studied considering whether the hot-sites are available on the substrate in advance. In this study, isolated nanoaggregates and elongated nanoaggregates of gold nanoparticles adsorbed with dye molecules are explored through SERS and dark-field microscopy done in a single platform. The correlation between SPR and SERS characteristics provides a clear evidence for the availability of hot-sites present in nanoaggregates of gold nanoparticles. Correlated SPR and SERS images as observed in this experiment reveal that the SPR mediated localized EM fields do depend on the local topography and influence the SERS enhancement. The SERS image of the same
M.K. Hossain et al. / Chemical Physics Letters 477 (2009) 130–134
sample adsorbed with dye molecules provides a supportive correlation to that of SPR excitations self-explaining the corresponding enhancements. A finite-different time domain (FDTD) analysis keeping the parameter identical to that of experimental condition (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm) was carried out. In the case of elongated aggregates, several localization sites were noted and appeared at each nanoparticle in FDTD simulation. Such multiple localizations in near-field, in fact, predict multipolar element of plasmon excitation as observed in SPR spectra too. In addition, the localized near-field distributions induced at the interstitials were found wider and close enough to merge into one. The scenario leads to having more number of analytes inside the intense EM fields and thus, enhances the Raman signal higher. And such a possible hybridization of nearby hot-sites, predicts a narrow passage for energy percolation in long-range 2D nanostructures, which are often obscured in ensemble SERS measurements, particularly using microsystem as a SERS-active substrate. Colloidal gold nanoparticles (50 nm in diameter) were used as received from BBInternational without any further modification and immobilized on a glass substrate. The glass slides were washed with ethanol in an ultrasonic bath for 15 min and then immersed into the ethanolic colloidal gold solution. Ethanolic solution served twofold purposes; one, to increase drying rate for higher convection flow of gold colloids and two, to maintain monodispersity of colloidal gold solution. To the extent, depending on the gold nanoparticles’ concentration during drying process, different kinds of nanoaggregates were observed in different places on the same glass substrate. All the nanoaggregates were confirmed by atomic force microscope (AFM). The isolated and elongated nanoaggregates of gold nanoparticles were observed to be monolayer, rather than agglomeration. A white light and dark-field condenser combination was used to observe the SPR images [20]. For SERS-activity, crystal violet (CV) was adsorbed on the nanoaggregates by dip and wash method. A Kr+ laser (for 647 nm) was used to irradiate the sample and the emission were collected through an objective lens (60, NA: 0.7) and detected by a CCD detector and a polychromator. SERS signals were collected for 10 s of exposure time with less than 4 mW of laser power at the sample surface. For nanoparticle array or few nanoparticles aggregates, the SPR peaks get shifted and several localized SPR excitations can be excited in different interstitials of the aggregates depending on the nature and shape of the aggregate [21–25]. Nanoaggregates mostly show a broad peak averaging all the excitations nearby in far-field observation [20,26,27]. Fig. 1a shows an SPR image of the isolated nanoaggregates of gold nanoparticles. The variations in color and intensity are clearly observed therein for individual nanoaggregates. For instance, the white dotted circles marked as 1, 2 and 3 represent somewhat near blue,2 greenish and reddish colors, respectively. The SERS images obtained herewith show a supportive correlation to that of aggregation-sensitive SPR excitations as shown in Fig. 1b. The nanoaggregate emitting nearly blue color (white dotted circle 1 in Fig. 1a) is found to be SERS-inactive, whereas those emitting greenish and reddish colors (white dotted circles 2 and 3 in Fig. 1a, respectively) are SERS-active, and the reddish color ones show the highest enhancement. A plausible reason will be given in the later part of the text. Fig. 1c shows an SPR spectrum obtained at the position marked as ‘X’ in Fig. 1a. A broadened SPR peak centered at 560 nm in addition to a trail in the longer wavelength region (>750 nm) is observed for the isolated nanoaggregates. Fig. 1d presents a typical SERS spectrum of CV obtained at the position marked as ‘X’ in Fig. 1b with the exposure time of 5 s. Comparing the sensitivities of SERS and bulk sample, the estimated
2 For interpretation of color in Figs. 1–3, the reader is referred to the web version of this article.
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enhancement was found to be ranging from 106 to 108 [28]. The SERS peaks of CV molecules observed herewith are consistent with the reported bands [29,30]. It is noted that the SERS spectra are comprised of background emission and mostly the background emission is termed as fluorescence emission in this respect. Details of the background emission are mentioned in the later part of the text. In the case of isolated nanoaggregates, the number of interstitials per micrometer is less and even, as usual, some nanoaggregates are not SERS-active with the laser excitation at 647 nm. On the other hand, in the case of anisotropic aggregation of gold nanoparticles, the possibility and density of localized EM fields, i.e. hot-sites, increases. Likewise the isolated nanoaggregates, color variations are observed at different segments of elongated gold nanoaggregates as shown in Fig. 2a. For instance, white dotted circles 1, 2 and 3 in Fig. 2a indicate the variations in color somewhat near blue, greenish and reddish, respectively. We recognize that the definition of the color is assumed apparently, although the variations are visible clearly as is observed in the insets of Figs. 1a and 2a. It is noteworthy that the average interparticle gaps for selfassembled gold elongated nanoaggregates was found to be 3.45 nm, which is suitable for SERS-activity (data is not shown here). The discontinuous SERS intensity shown in Fig. 2b confirms the intensity variations in relation to the SPR excitations excited along the nanoaggregates as shown in Fig. 2a. Although the nanoaggregates with nearly blue colors (e.g. white dotted circle 1 in Fig. 2a) are found to be SERS-inactive, the segments emitting greenish and reddish colors (white dotted circles 2 and 3 in Fig. 2a, respectively) are SERS-active and the segment with greenish color shows the highest enhancement unlike that of isolated nanoaggregates. The detailed morphology of self-assembled gold nanoaggregates is shown in the inset of Fig. 2b. It is noteworthy from the SPR observation that several peaks surrounding the excitation laser wavelength (647 nm) are in favor to higher SERS signal contrary to that of the isolated nanoaggregates. In the case of elongated nanoaggregates, several SPR peaks are observed centered at 510 and 610 nm in addition to a broadened peak centered at 730 nm and weak peaks in the shorter wavelength region as shown in Fig. 2c. Fig. 2d represents a typical SERS spectrum of CV obtained at the marked position ‘Y’ in Fig. 2b. The average SERS intensity is found higher compared to that of the random nanoaggregates. Once gold nanoparticles are assembled as elongated fashion, the FDTD analysis provides the evidences that the number of localization sites increases almost double and the quadrupole element of plasmon appear at each nanoparticle. An FDTD analysis keeping the parameter identical to that of experimental condition (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm) was carried out to realize the origin of higher enhancement in elongated nanoaggregates observed in this study. Spherical gold nanoparticles are used instead of hexagonal arbitrary shaped facet gold nanoparticles. A tetramer unit of the gold elongated nanoaggregates was considered as explained and shown in the inset of Fig. 2b. The arrow indicates the polarization direction of the incident excitation parallel to interparticle axis. Fig. 3a–d shows the near-field distribution of the tetramer unit with different polarization directions. It is observed that the near-field distribution is localized at the interstitials in a tiny space (0.01–0.1% of the surface area). In the case of in plane polarization parallel to the interparticle axis (Fig. 3a and b), two intense localization sites appear at each nanoparticles. It is noted that there is no possibility to be merged into one hot-site. On the contrary, if the polarization is out of plane parallel to interparticle axis (Fig. 3c), the field distribution spread out covering wider area and the number of localization sites increase. Such multiple localizations in near-field, in fact, predict multipolar element of plasmon excitation. In addition, the localized near-field distributions induced at the interstitials are
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Fig. 1. (a) A SPR image of the isolated nanoaggregates of gold nanoparticle of 50 nm diameter. Inset: a filtered image of the dashed white box shown in (a). (b) A SERS image of the same sample adsorbed with CV. (c) A SPR spectrum obtained at the marked position ‘X’ in (a). The dashed vertical down arrow in (c) indicates the laser excitation position. (d) A SERS spectrum of the CV obtained at the marked position ‘X’ in (b). The scale bar in (b) shows the size of the SPR and SERS images. The white dotted circles 1, 2 and 3 represent different types of nanoaggregates.
Fig. 2. (a) A SPR image of the elongated nanoaggregate of gold nanoparticle of 50 nm diameter. Inset: filtered images of the dashed box shown in (a). (b) A SERS image of the same sample adsorbed with CV. Inset: a AFM image of a typical elongated nanoaggregate. (c) A SPR spectrum obtained at the marked position ‘Y’ in (a). The dashed vertical down arrow in (c) indicates the laser excitation position. (d) A SERS spectrum of the CV obtained at the marked position ‘Y’ in (b). The scale bar in (b) shows the size of the SPR and SERS images. The white dotted circles 1, 2 and 3 represent isolated nanoaggregates and segments of elongated nanoaggregates.
M.K. Hossain et al. / Chemical Physics Letters 477 (2009) 130–134
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Fig. 3. (a) and (b) Near-field distribution of a tetramer unit simulated by FDTD with in plane polarization parallel to the interparticle axis (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm). (c) Near-field distribution of the same unit with the polarization out of plane parallel to the interparticle axis. A dashed two headed arrow shows the possible passage for nearby hot-sites merging. (d) Near-field distribution of the same unit with the polarization in plane and out of plane parallel to the interparticle axis (kexc = 647 nm, DAu = 50 nm, dgap = 1 nm). Arrow indicates the incident polarization direction.
close enough to merge into one. Such a possible hybridization of nearby hot-sites, indeed, predicts a narrow passage for energy percolation. The both sided dotted arrow is the guide for eyes. In reality the possibility of hybridizing adjacent hot-sites are even higher for hexagonal arbitrary shaped facet gold nanoparticles. Fig. 3d shows the near-field distribution of the same tetramer by changing interparticle gap to 1 nm only. It is noteworthy that the localization sites become more confined and the possibility of hybridization becomes less, although the intensity of each hot-site increases a little bit. The microscopic Raman measurements carried out in this study are not resonance Raman scattering, since the laser excitation wavelength is far away from the absorption band of the analyte. The laser excitation was chosen in such a way so that a possible boost-up in the excitation probability of Raman process takes place because of additional SPR peaks excited in the longer wavelength region. Indeed, strong SERS signals are observed from the gold nanoaggregates adsorbed with CV. In the case of the elongated nanoaggregate, the laser excitation matches the best with the SPR excitations excited on the aggregates. On the contrary, the SPR excitations excited in shorter wavelength region in the case of particular nanoaggregates, for instance, the white dotted circle 1 in Figs. 1a and 2a, SERS process cannot get benefit from the localized surface plasmon excitations. Hence, in this particular type of nanoaggregates, it looks like off peak position, and consequently the SERS signals are not enhanced defining the nanoaggregates as inactive sites. In the case of elongated nanoaggregates, the possibility of active hot-sites increases with wider field distribution by sacrificing the EM field intensity a bit. Even if the far-field measurement makes an average of the SERS signals from effective volume, the intensity will get enhanced; firstly, because of more active hot-sites and secondly, more analytes are polarized by wider EM field distributions. A detailed and quantitative investigation is necessary to elucidate the phenomena and the steps are being undertaken at this moment. In the case of CV, the background
emission (i.e. fluorescence) is centered much below the laser energy and only the tail appears with SERS signals [24,31,32]. In fact, several SPR peaks observed in the elongated nanoaggregates play an important role to boost-up the background emission simultaneously with SERS signals. The present investigation on the correlated SPR and SERS characteristics of isolated gold nanoaggregates and individual segments of elongated gold nanoaggregates has revealed that anisotropically self-assembled-like nanoaggregates possess higher SERS enhancement. Well-correlated SPR images and subsequent SERS images were obtained. Particular nanoaggregates (e.g. circle 3 in Fig. 1a) and segments of elongated nanoaggregate (e.g. circle 2 and 3 in Fig. 1a) with plasmons excited near the excitation wavelength show enhanced SERS signal unlike unfavourable nanoaggregates or segments of anisotropic nanoaggregates (e.g. circle 1 in Figs. 1a and 2a). The estimated enhanced was found to be reasonably high for gold nanoaggregates. An FDTD simulation with the same sample and experimental condition, provides supportive results for higher localization sites and wider EM near-field distributions. The plasmon excitations excited at the interstitials of nanoaggregates are more important in higher and ensemble SERS enhancement instead of resonance conditions and can be confirmed by a combined study of local morphology (e.g. through AFM, SEM, etc.), induced localized EM field distribution (e.g. through SPR, two-photon induced photoluminescence, etc.) and optical characteristics (e.g. through SERS, fluorescence, etc.). Such a combined observation is essential for fundamental understanding of nanostructured material and its inherent properties to realize their future potential in wide field applications. Acknowledgements The authors thank Dr. Tamitake Itoh and Dr. Vasudevan Pillai Biju (AIST, Shikoku). KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area ‘Strong Photon-Molecule Coupling
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Fields (No. 470, 20043032)’ from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) is acknowledged. References [1] K. Kneipp, M. Moskovits, H. Kneipp, Surface-enhanced Raman Scattering: Physics and Applications, Springer-Verlag, Berlin/Heidelberg, 2006. [2] G.A. Baker, D.S. Moore, Anal. Bioanal. Chem. 382 (2005) 1751. [3] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783. [4] A. Otto, J. Raman Spectrosc. 37 (2006) 937. [5] G.C. Schatz, R.P. Van Duyne, Handbook of Vibrational Spectroscopy, John Wiley and Sons Ltd., Chichester, 2002. [6] P.L. Stiles, F.A. Dieringer, N.C. Shah, R.P. Van Duyne, Annu. Rev. Anal. Chem. 1 (2008) 601. [7] A. Campion, P. Kambhampati, Chem. Soc. Rev. 27 (1998) 241. [8] K. Imura, H. Okamoto, M.K. Hossain, M. Kitajima, NanoLetters 6 (2006) 2173. [9] M.I. Stockman, V.M. Shaleav, M. Moskovits, R. Botet, T.F. George, Phys. Rev. B 46 (1992) 2821. [10] V.M. Shaleav, E.Y. Poliakov, V.K. Morkel, Phys. Rev. B 53 (1996) 2437. [11] C.E. Talley et al., NanoLetters 5 (2005) 1569. [12] K. Imura, H. Okamoto, M.K. Hossain, M. Kitajima, Chem. Lett. 35 (2005) 78. [13] M.K. Hossain, T. Shimada, M. Kitajima, K. Imura, H. Okamoto, J. Microsc. 229 (2008) 327. [14] B. Pettinger, J. Chem. Phys. 85 (1986) 7442. [15] Y. Maruyama, M. Futamata, J. Raman Spectrosc. 36 (2005) 581.
[16] M.K. Hossain, Y. Kitahama, G.G. Huang, T. Kaneko, Y. Ozaki, Appl. Phys. B 95 (2008) 165. [17] P. Zhang, S. Smith, G. Rumbles, M.E. Himmel, Langmuir 21 (2005) 520. [18] M.K. Hossain, T. Shimada, M. Kitajima, K. Imura, H. Okamoto, Langmuir 24 (2008) 9241. [19] X.X. Han, Y. Kitahama, Y. Tanaka, J. Guo, W.Q. Xu, B. Zhao, Y. Ozaki, Anal. Chem. 80 (2008) 6567. [20] T. Itoh, K. Hashimoto, Y. Ozaki, Appl. Phys. Lett. 83 (2003) 2274. [21] N. Felidj, J. Aubard, G. Levi, Phys. Rev. B 65 (2002) 075419. [22] C.L. Haynes, R.P. Van Duyne, J. Phys. Chem. B 107 (2003) 7426. [23] T. Itoh, K. Yoshida, V. Biju, Y. Kikkawa, M. Ishikawa, Y. Ozaki, Phys. Rev. B 76 (2007) 085405. [24] T. Itoh, V. Biju, M. Ishikawa, Y. Kikkawa, K. Hashimoto, A. Ikehata, Y. Ozaki, J. Chem. Phys. 124 (2006) 134708. [25] E.C. Le Ru et al., J. Phys. Chem. C 112 (2008) 8117. [26] F. Le et al., ASC Nano 4 (2008) 707. [27] L. Novotny, B. Hecht, Principles of Nano-Optics, Cambridge University Press, Cambridge, 2006. bulk ¼ IISERS NNSERS , where ISERS , Ibulk , N bulk and N SERS are SERS intensity, [28] G ¼ SSSERS bulk bulk
[29] [30] [31] [32]
intensity for bulk sample, no. of molecules for bulk sample and no. of molecules for SERS sample, respectively. E.J. Liang, X.L. Ye, W. Kiefer, J. Phys. Chem. A 101 (1997) 7330. T. Watanabe, B. Pettinger, Chem. Phys. Lett. 89 (1982) 501. T. Itoh, Y. Kikkawa, V. Biju, M. Ishikawa, A. Ikehata, Y. Ozaki, J. Phys. Chem. B 110 (2006) 21536. E.C. Le Ru, P.G. Etchegoin, J. Grand, N. Felidj, J. Aubard, G. Levi, J. Phys. Chem. C 111 (2007) 16076.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surface-enhanced Raman scattering and plasmon excitations from isolated and elongated gold nanoaggregates Mohammad Kamal Hossain a,1, Genin Gary Huang a, Tadaaki Kaneko b, Yukihiro Ozaki a,* a b
Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan Department of Physics, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda, Hyogo 669-1337, Japan
a r t i c l e
i n f o
Article history: Received 24 February 2009 In final form 12 June 2009 Available online 14 June 2009
a b s t r a c t Surface plasmon resonance (SPR) and subsequent surface-enhanced Raman scattering (SERS) of gold nanoaggregates were investigated with reference to hot-sites availability. Correlated SPR and SERS observation of the same sample shows that the plasmon excitations excited at the interstitials of nanoaggregates are more important rather than resonance conditions. The isolated aggregates mostly show a broadened SPR peak centered at 560 nm overlapping absorption band of analytes, whereas the elongated aggregates yield several SPR peaks in the longer wavelength region and provide higher enhancement in SERS. Finite-different time-domain analysis predicts that the localization sites get increased at least double in elongated aggregates. Ó 2009 Published by Elsevier B.V.
Localized electromagnetic (EM) field distribution in association with surface plasmon resonance (SPR) excited at the interstitials (i.e. hot-sites) of a small aggregate in nanometric space is the prime factor for giant optical field enhancement and has attracted huge interest in various fields of applications [1–4]. Aggregates of noble metal nanoparticles, particularly gold nanoparticles have shown their potential already in various sectors of nanoscience and technology including surface-enhanced Raman scattering (SERS) [4–8]. Gold nanoaggregates are capable of having localized EM field acting on them, thus creating regions of very high confined electric field named hot-spots [8–13]. Self-assembled-like nanoaggregates of gold nanoparticles deserve to be the best suited SERS-active substrate because of their active sites availability and free from uncertainty of hot-sites. As a consequence, the SERS-signal enhancement and quality using such substrates will get improved largely compared to that of irreproducible signals observed for aqueous and random nanoaggregates. Right to this point, it is a great challenge to fabricate self-assembled nanoaggregates from colloidal gold nanoparticles without using any surfactant. A unique fabrication method is adopted herewith to achieve such a SERS-active isolated and elongated gold nanoaggregates. Once the incident light becomes resonant with plasmon, the EM field localizes spatially at the junctions of noble metal nanostructure. Consequently, confined EM field polarizes the molecules adsorbed nearby by dipole–dipole coupling [14]. Thus the overall SERS enhancement appears to be approximately the product of * Corresponding author. E-mail address: [email protected] (Y. Ozaki). 1 Present address: The MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Science, Victoria University of Wellington, Wellington 600, New Zealand. 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.06.043
two factors; one, due to coupling between incident photons and Loc ðxL Þ 2 Loc I localized SPR (i.e. EEI ðx , where E ðxL Þ and E ðxL Þ are the ampliÞ L
tudes of the SPR-mediated local electric field at laser angular frequency xL and incident electric field at the same angular frequency, respectively) and two, due to coupling between Raman Loc ðxR Þ 2 Loc I photons and localized SPR (i.e. EEI ðx , where E ðxR Þ and E ðxR Þ Þ R
are the amplitudes of the SPR-mediated local electric field at Raman angular frequency xR and incident electric field at the same angular frequency, respectively). If the number of EM field localization increases and spreads out, more analytes in the hot-site vicinity will be covered by SERS–EM mechanism. Any direct and correlated experiments dealing with SPR and SERS at the same spatial position would be beneficial in this respect [15,16]. Regardless of the nature of substrate, it is of fundamental interest to correlate the SPR excitations excited on nanoaggregates or at the segments of gold nanostructure to the confined optical fields, such as SERS or fluorescence images [12,13,17–19]. However, the microscopic correlation between localized EM field and confined optical field along with topographic confirmation has not yet been well studied considering whether the hot-sites are available on the substrate in advance. In this study, isolated nanoaggregates and elongated nanoaggregates of gold nanoparticles adsorbed with dye molecules are explored through SERS and dark-field microscopy done in a single platform. The correlation between SPR and SERS characteristics provides a clear evidence for the availability of hot-sites present in nanoaggregates of gold nanoparticles. Correlated SPR and SERS images as observed in this experiment reveal that the SPR mediated localized EM fields do depend on the local topography and influence the SERS enhancement. The SERS image of the same
M.K. Hossain et al. / Chemical Physics Letters 477 (2009) 130–134
sample adsorbed with dye molecules provides a supportive correlation to that of SPR excitations self-explaining the corresponding enhancements. A finite-different time domain (FDTD) analysis keeping the parameter identical to that of experimental condition (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm) was carried out. In the case of elongated aggregates, several localization sites were noted and appeared at each nanoparticle in FDTD simulation. Such multiple localizations in near-field, in fact, predict multipolar element of plasmon excitation as observed in SPR spectra too. In addition, the localized near-field distributions induced at the interstitials were found wider and close enough to merge into one. The scenario leads to having more number of analytes inside the intense EM fields and thus, enhances the Raman signal higher. And such a possible hybridization of nearby hot-sites, predicts a narrow passage for energy percolation in long-range 2D nanostructures, which are often obscured in ensemble SERS measurements, particularly using microsystem as a SERS-active substrate. Colloidal gold nanoparticles (50 nm in diameter) were used as received from BBInternational without any further modification and immobilized on a glass substrate. The glass slides were washed with ethanol in an ultrasonic bath for 15 min and then immersed into the ethanolic colloidal gold solution. Ethanolic solution served twofold purposes; one, to increase drying rate for higher convection flow of gold colloids and two, to maintain monodispersity of colloidal gold solution. To the extent, depending on the gold nanoparticles’ concentration during drying process, different kinds of nanoaggregates were observed in different places on the same glass substrate. All the nanoaggregates were confirmed by atomic force microscope (AFM). The isolated and elongated nanoaggregates of gold nanoparticles were observed to be monolayer, rather than agglomeration. A white light and dark-field condenser combination was used to observe the SPR images [20]. For SERS-activity, crystal violet (CV) was adsorbed on the nanoaggregates by dip and wash method. A Kr+ laser (for 647 nm) was used to irradiate the sample and the emission were collected through an objective lens (60, NA: 0.7) and detected by a CCD detector and a polychromator. SERS signals were collected for 10 s of exposure time with less than 4 mW of laser power at the sample surface. For nanoparticle array or few nanoparticles aggregates, the SPR peaks get shifted and several localized SPR excitations can be excited in different interstitials of the aggregates depending on the nature and shape of the aggregate [21–25]. Nanoaggregates mostly show a broad peak averaging all the excitations nearby in far-field observation [20,26,27]. Fig. 1a shows an SPR image of the isolated nanoaggregates of gold nanoparticles. The variations in color and intensity are clearly observed therein for individual nanoaggregates. For instance, the white dotted circles marked as 1, 2 and 3 represent somewhat near blue,2 greenish and reddish colors, respectively. The SERS images obtained herewith show a supportive correlation to that of aggregation-sensitive SPR excitations as shown in Fig. 1b. The nanoaggregate emitting nearly blue color (white dotted circle 1 in Fig. 1a) is found to be SERS-inactive, whereas those emitting greenish and reddish colors (white dotted circles 2 and 3 in Fig. 1a, respectively) are SERS-active, and the reddish color ones show the highest enhancement. A plausible reason will be given in the later part of the text. Fig. 1c shows an SPR spectrum obtained at the position marked as ‘X’ in Fig. 1a. A broadened SPR peak centered at 560 nm in addition to a trail in the longer wavelength region (>750 nm) is observed for the isolated nanoaggregates. Fig. 1d presents a typical SERS spectrum of CV obtained at the position marked as ‘X’ in Fig. 1b with the exposure time of 5 s. Comparing the sensitivities of SERS and bulk sample, the estimated
2 For interpretation of color in Figs. 1–3, the reader is referred to the web version of this article.
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enhancement was found to be ranging from 106 to 108 [28]. The SERS peaks of CV molecules observed herewith are consistent with the reported bands [29,30]. It is noted that the SERS spectra are comprised of background emission and mostly the background emission is termed as fluorescence emission in this respect. Details of the background emission are mentioned in the later part of the text. In the case of isolated nanoaggregates, the number of interstitials per micrometer is less and even, as usual, some nanoaggregates are not SERS-active with the laser excitation at 647 nm. On the other hand, in the case of anisotropic aggregation of gold nanoparticles, the possibility and density of localized EM fields, i.e. hot-sites, increases. Likewise the isolated nanoaggregates, color variations are observed at different segments of elongated gold nanoaggregates as shown in Fig. 2a. For instance, white dotted circles 1, 2 and 3 in Fig. 2a indicate the variations in color somewhat near blue, greenish and reddish, respectively. We recognize that the definition of the color is assumed apparently, although the variations are visible clearly as is observed in the insets of Figs. 1a and 2a. It is noteworthy that the average interparticle gaps for selfassembled gold elongated nanoaggregates was found to be 3.45 nm, which is suitable for SERS-activity (data is not shown here). The discontinuous SERS intensity shown in Fig. 2b confirms the intensity variations in relation to the SPR excitations excited along the nanoaggregates as shown in Fig. 2a. Although the nanoaggregates with nearly blue colors (e.g. white dotted circle 1 in Fig. 2a) are found to be SERS-inactive, the segments emitting greenish and reddish colors (white dotted circles 2 and 3 in Fig. 2a, respectively) are SERS-active and the segment with greenish color shows the highest enhancement unlike that of isolated nanoaggregates. The detailed morphology of self-assembled gold nanoaggregates is shown in the inset of Fig. 2b. It is noteworthy from the SPR observation that several peaks surrounding the excitation laser wavelength (647 nm) are in favor to higher SERS signal contrary to that of the isolated nanoaggregates. In the case of elongated nanoaggregates, several SPR peaks are observed centered at 510 and 610 nm in addition to a broadened peak centered at 730 nm and weak peaks in the shorter wavelength region as shown in Fig. 2c. Fig. 2d represents a typical SERS spectrum of CV obtained at the marked position ‘Y’ in Fig. 2b. The average SERS intensity is found higher compared to that of the random nanoaggregates. Once gold nanoparticles are assembled as elongated fashion, the FDTD analysis provides the evidences that the number of localization sites increases almost double and the quadrupole element of plasmon appear at each nanoparticle. An FDTD analysis keeping the parameter identical to that of experimental condition (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm) was carried out to realize the origin of higher enhancement in elongated nanoaggregates observed in this study. Spherical gold nanoparticles are used instead of hexagonal arbitrary shaped facet gold nanoparticles. A tetramer unit of the gold elongated nanoaggregates was considered as explained and shown in the inset of Fig. 2b. The arrow indicates the polarization direction of the incident excitation parallel to interparticle axis. Fig. 3a–d shows the near-field distribution of the tetramer unit with different polarization directions. It is observed that the near-field distribution is localized at the interstitials in a tiny space (0.01–0.1% of the surface area). In the case of in plane polarization parallel to the interparticle axis (Fig. 3a and b), two intense localization sites appear at each nanoparticles. It is noted that there is no possibility to be merged into one hot-site. On the contrary, if the polarization is out of plane parallel to interparticle axis (Fig. 3c), the field distribution spread out covering wider area and the number of localization sites increase. Such multiple localizations in near-field, in fact, predict multipolar element of plasmon excitation. In addition, the localized near-field distributions induced at the interstitials are
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Fig. 1. (a) A SPR image of the isolated nanoaggregates of gold nanoparticle of 50 nm diameter. Inset: a filtered image of the dashed white box shown in (a). (b) A SERS image of the same sample adsorbed with CV. (c) A SPR spectrum obtained at the marked position ‘X’ in (a). The dashed vertical down arrow in (c) indicates the laser excitation position. (d) A SERS spectrum of the CV obtained at the marked position ‘X’ in (b). The scale bar in (b) shows the size of the SPR and SERS images. The white dotted circles 1, 2 and 3 represent different types of nanoaggregates.
Fig. 2. (a) A SPR image of the elongated nanoaggregate of gold nanoparticle of 50 nm diameter. Inset: filtered images of the dashed box shown in (a). (b) A SERS image of the same sample adsorbed with CV. Inset: a AFM image of a typical elongated nanoaggregate. (c) A SPR spectrum obtained at the marked position ‘Y’ in (a). The dashed vertical down arrow in (c) indicates the laser excitation position. (d) A SERS spectrum of the CV obtained at the marked position ‘Y’ in (b). The scale bar in (b) shows the size of the SPR and SERS images. The white dotted circles 1, 2 and 3 represent isolated nanoaggregates and segments of elongated nanoaggregates.
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Fig. 3. (a) and (b) Near-field distribution of a tetramer unit simulated by FDTD with in plane polarization parallel to the interparticle axis (kexc = 647 nm, DAu = 50 nm, dgap = 4 nm). (c) Near-field distribution of the same unit with the polarization out of plane parallel to the interparticle axis. A dashed two headed arrow shows the possible passage for nearby hot-sites merging. (d) Near-field distribution of the same unit with the polarization in plane and out of plane parallel to the interparticle axis (kexc = 647 nm, DAu = 50 nm, dgap = 1 nm). Arrow indicates the incident polarization direction.
close enough to merge into one. Such a possible hybridization of nearby hot-sites, indeed, predicts a narrow passage for energy percolation. The both sided dotted arrow is the guide for eyes. In reality the possibility of hybridizing adjacent hot-sites are even higher for hexagonal arbitrary shaped facet gold nanoparticles. Fig. 3d shows the near-field distribution of the same tetramer by changing interparticle gap to 1 nm only. It is noteworthy that the localization sites become more confined and the possibility of hybridization becomes less, although the intensity of each hot-site increases a little bit. The microscopic Raman measurements carried out in this study are not resonance Raman scattering, since the laser excitation wavelength is far away from the absorption band of the analyte. The laser excitation was chosen in such a way so that a possible boost-up in the excitation probability of Raman process takes place because of additional SPR peaks excited in the longer wavelength region. Indeed, strong SERS signals are observed from the gold nanoaggregates adsorbed with CV. In the case of the elongated nanoaggregate, the laser excitation matches the best with the SPR excitations excited on the aggregates. On the contrary, the SPR excitations excited in shorter wavelength region in the case of particular nanoaggregates, for instance, the white dotted circle 1 in Figs. 1a and 2a, SERS process cannot get benefit from the localized surface plasmon excitations. Hence, in this particular type of nanoaggregates, it looks like off peak position, and consequently the SERS signals are not enhanced defining the nanoaggregates as inactive sites. In the case of elongated nanoaggregates, the possibility of active hot-sites increases with wider field distribution by sacrificing the EM field intensity a bit. Even if the far-field measurement makes an average of the SERS signals from effective volume, the intensity will get enhanced; firstly, because of more active hot-sites and secondly, more analytes are polarized by wider EM field distributions. A detailed and quantitative investigation is necessary to elucidate the phenomena and the steps are being undertaken at this moment. In the case of CV, the background
emission (i.e. fluorescence) is centered much below the laser energy and only the tail appears with SERS signals [24,31,32]. In fact, several SPR peaks observed in the elongated nanoaggregates play an important role to boost-up the background emission simultaneously with SERS signals. The present investigation on the correlated SPR and SERS characteristics of isolated gold nanoaggregates and individual segments of elongated gold nanoaggregates has revealed that anisotropically self-assembled-like nanoaggregates possess higher SERS enhancement. Well-correlated SPR images and subsequent SERS images were obtained. Particular nanoaggregates (e.g. circle 3 in Fig. 1a) and segments of elongated nanoaggregate (e.g. circle 2 and 3 in Fig. 1a) with plasmons excited near the excitation wavelength show enhanced SERS signal unlike unfavourable nanoaggregates or segments of anisotropic nanoaggregates (e.g. circle 1 in Figs. 1a and 2a). The estimated enhanced was found to be reasonably high for gold nanoaggregates. An FDTD simulation with the same sample and experimental condition, provides supportive results for higher localization sites and wider EM near-field distributions. The plasmon excitations excited at the interstitials of nanoaggregates are more important in higher and ensemble SERS enhancement instead of resonance conditions and can be confirmed by a combined study of local morphology (e.g. through AFM, SEM, etc.), induced localized EM field distribution (e.g. through SPR, two-photon induced photoluminescence, etc.) and optical characteristics (e.g. through SERS, fluorescence, etc.). Such a combined observation is essential for fundamental understanding of nanostructured material and its inherent properties to realize their future potential in wide field applications. Acknowledgements The authors thank Dr. Tamitake Itoh and Dr. Vasudevan Pillai Biju (AIST, Shikoku). KAKENHI (Grant-in-Aid for Scientific Research) on Priority Area ‘Strong Photon-Molecule Coupling
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