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2008, 112, 4033-4035 Published on Web 02/28/2008
Near-Field Study on Correlation of Localized Electric Field and Nanostructures in Monolayer Assembly of Gold Nanoparticles Toru Shimada,† Kohei Imura,‡,§ Mohammad Kamal Hossain,†,|,⊥ Hiromi Okamoto,‡,§ and Masahiro Kitajima*,†,‡,| National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan, Institute for Molecular Science (IMS), Okazaki, Aichi 444-8585, Japan, The Graduate UniVersity for AdVanced Studies, Okazaki, Aichi 444-8585, Japan, and Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-0005, Japan ReceiVed: January 17, 2008
We clarified the relationship between near-field optical properties and the local structure in monolayer assembly of gold nanoparticles by using scanning near-field optical microscopy (SNOM) and scanning electron microscopy (SEM). The success of superposition of the SNOM image with the SEM image allows us to find that the intense electromagnetic field is localized at the rim part, especially the interstitial site of the dimerlike structure.
Surface plasmon (SP) resonance of noble metal nanoparticle has attracted much interest, because it exhibits strong electromagnetic (EM) field enhancement.1 This enhanced EM field has been recognized as the predominant source of surfaceenhanced Raman scattering (SERS).2 Raman enhancement factor as high as 1014-1015 has been reported.3 Because of the very high sensitivity, extensive applications of this phenomenon to life, medical, environmental, and other fields of science are expected.4 To utilize SERS as an analytical method of very high sensitivity, it is of fundamental importance to reveal the correlation between nanostructures and their optical properties. Recently, we succeeded in visualization of the highly localized EM field at interstitial sites in gold-nanoparticle dimers by using scanning near-field optical microscope (SNOM).5,6 For larger nanostructures, near-field SERS mapping was attempted on selfaffine silver films, and irregular distributions of SERS intensity over the surface of the films were reported.7,8 Their topographic images, however, are not clear enough to discuss the correlation between the structure and the SERS activity. Although SNOM is a prominent tool for imaging optical-field distributions with subwavelength resolution, the topography of nanostructures is broadened due to finite radius of curvature of the near-field probe tip. To characterize the detailed structure, the lateral spatial resolution of topographic measurements by SNOM is not sufficient. Scanning electron microscopy (SEM) is one of the most suitable tools for this purpose. In this communication, we present the correlation between the near-field optical properties and the particle configuration in monolayer assemblies of gold * Corresponding author. Tel: +81-29-859-2836. Fax: +81-29-859-2801. E-mail:
[email protected]. † National Institute for Materials Science (NIMS). ‡ Institute for Molecular Science (IMS). § The Graduate University for Advanced Studies. | University of Tsukuba. ⊥ Present address: School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan.
10.1021/jp8004508 CCC: $40.75
nanoparticles by using SNOM and SEM. The superimposition of the SNOM image with the SEM image clarifies the relation between the local structure and the optical properties, revealing peculiar localization of EM field in the assemblies. Colloidal solution of spherical gold nanoparticle (diameter 100 nm) was purchased from BBInternational and used as received. Monolayer assembly of gold nanoparticles was prepared on a glass cover slip. Morphology of the sample was verified by topography measurements by SNOM and SEM. An apertured-type SNOM (aperture diameter 50-100 nm) was used under ambient condition. A Ti:sapphire laser (λ ) 785 nm) was used for transmission and two-photon-induced photoluminescence (TPI-PL) excitation measurements.9 The near-field transmission and the TPI-PL excitation images were obtained simultaneously. To clarify the relation between the local structure and the optical properties precisely, the SEM image of the assembly must be well registered with the SNOM counterparts. Projective transformation of the SEM image was employed to compensate the distortion due to the subtle difference in holding angle of the sample between the SEM and the SNOM setups. The SEM image was spatially transformed to the SNOM topography by using a simple point-to-point matching correlation procedure.10,11 The identical transformed SEM image was used to superimpose on the optical images by the SNOM. Figure 1 shows near-field transmission images superimposed on the transformed SEM image. The SEM image clearly resolves each gold nanoparticle in the assembly. The interparticle distances are found to be less than 10 nm for the close-packed portions. Such detailed structures cannot be identified by the topography observed by the SNOM because of the lower lateral spatial resolution. When the near-field transmission image is taken with the polarization parallel to the incident one (//-polarization), the transmission is reduced almost homogeneously over the as© 2008 American Chemical Society
4034 J. Phys. Chem. C, Vol. 112, No. 11, 2008
Figure 1. (a,b) Transmission SNOM images (green) of monolayer assembly of gold nanoparticles measured around 785 nm. Images are superimposed on the transformed SEM image (black/white). Arrows indicate the polarization direction of the incident and the detected light, respectively. Transmission images are taken with a polarization (a) parallel (//-polarization) and (b) perpendicular (⊥-polarization) to the incident one, respectively.
Figure 2. (a,b) Near-field TPI-PL excitation images of monolayer assembly of gold nanoparticles superimposed on the transformed SEM image. The cross-section profile of TPI-PL intensity along the dashed line is given on the left for panel a. Arrows indicate the polarization direction of the incident light and the detected PL, respectively. These TPI-PL images and the transmission images in Figure 1 were obtained simultaneously. The color scale for (b) is identical to that for (a).
sembly (Figure 1a). The reduction in transmission indicates that plasmon excitation takes place in the assembly. The enhancement at the isolated nanoparticles results from the conversion of evanescent wave (antenna effect).12 When the image is taken with the polarization perpendicular to the incident one (⊥polarization), that is, crossed Nicol condition, the transmission is observed at the rim of the assembly (Figure 1b). This means that depolarization occurs at the rim. The distribution of EM field was imaged by the near-field TPI-PL measurements.9,13 The EM field is found to be enhanced over the whole area of assembly (Figure 2a cross section), which is consistent with the uniform extinction in Figure 1a. The signal is especially intense at the rim of the assembly in both of the detected polarizations, as shown in Figure 2. This shows that the depolarization is related to the intense localized EM field. The difference between the inner part and the rim may be related to delocalization of SP excitation in two-dimensional nanostructures. In the inner part, the nanoparticles are surrounded by other particles from all the directions and the SP excitation may be propagated in all the directions through the
Letters interactions with neighboring particles. In contrast, the nanoparticles at the rim are not surrounded from all the directions by other particles. This condition restricts the propagation of the SP excitation and would make the EM-field confinement remain strong. From another point of view, it might be related to the localized EM field in the boundaries of photonic crystals.14 The SEM image allows us to determine the precise structure of the sites where the intense EM fields are localized. When the image is taken with the //-polarization, the strongest EM fields are observed at the dimerlike structures aligned nearly parallel to the incident light polarization (Figure 2a, A and B). The perpendicular component of EM field is not negligible in site B, however, which will be discussed later. The measured signal intensity at the interstitial site of A is estimated to be about twenty times higher than that at the inner part of the assembly. It is noted that not all of the dimerlike units in the assembly exhibit the intense EM field. When the image is taken with the ⊥-polarization, the strongest EM fields are observed at the sites marked as C and D in Figure 2b. Because the nanostructure of the assembly in C and D are rather anisotropic, the transition moment of the PL must be not parallel to the incident light polarization,15 and the PL contains a perpendicular polarization component. At the dimerlike site B, weakly enhanced EM fields are observed (Figure 2b, B) as mentioned above, which is different from the isolated dimer case. (For an isolated dimer, only a very weak signal is detected under the crossed Nicol condition when excited along the dimer axis.6) In the case of site B, other nanoparticles are in close proximity, which may interact with the dimer and cause depolarization of the PL. The SEM image reveals the existence of many defects in the inner part of the assembly. The nanoparticles at the defects are not completely surrounded by other particles as is similar to the situation at the rim. A slight intense localized EM field is also found at the defect, but the intensity is in general not very strong. The optical near-field in the neighbor of the particle is spread over approximately the particle size,16 and therefore SPs on two nanoparticles can interact to each other through space if the interparticle distance is not very far compared with the particle diameter. This may explain the observation mentioned above. Some authors have reported that separated metal nanoparticles can interact with each other through the plasmon couplings,17,18 which is consistent with our results. In summary, we have accomplished the image matching between the SEM and the SNOM counterparts. The SEM image allows us to determine the precise structure that gives the intense EM field. The near-field TPI-PL images reveal that EM field driven by SP excitation preferentially localized at the rim of the assembly. The nonuniform nature of the enhanced EM field in the assembly found here gives a new guideline for designing highly sensitive SERS substrate. Acknowledgment. T.S. would like to thank Professor U. C. Fischer for stimulating discussion and Dr. K. Ushida for fruitful suggestions for the registration of images. This study was supported by Grants-in-Aid for Scientific Research (Grants 18205004, 18685003, and 19049015) from JSPS and Nanotechnology Support Projects by MEXT. References and Notes (1) (a) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (b) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212-4217. (2) (a) Xu, H. X.; Aizpurua, J.; Ka¨ll, M.; Apell, P. Phys. ReV. E 2000, 62, 4318-4324. (b) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.;
Letters Hollars, C. W.; Lane, S. M.; Huser, T. R.; Noldlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569-1574. (c) Aravind, P. K.; Nitzan, A.; Metiu, H. Surf. Sci. 1981, 110, 189-204. (3) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (4) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629-1632. (b) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. ReV. 1999, 99, 2957-2975. (c) Yonzon, C. R.; Haynes, C. L.; Zhang, X. Y.; Walsh, J. T.; Van Duyne, R. P. Anal. Chem. 2004, 76, 78-85. (d) Haes, A. J.; Zou, S. L.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 2004, 108, 109116. (e) Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17930-17935. (f) Zhang, Y.; Gu, C.; Schwartzberg, A. M.; Zhang, J. Z. Appl. Phys. Lett. 2005, 87, 123105. (5) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Chem. Lett. 2006, 35, 78-79. (6) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173-2176. (7) Zhang, P.; Smith, S.; Rumbles, G.; Himmel, M. E. Langmuir 2005, 21, 520-523.
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4035 (8) Moskovits, M. Top. Appl. Phys. 2006, 103, 1-17. (9) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B 2005, 109, 13214-13220. (10) Image registrations were performed with Matlab (Mathworks, Inc., Natick, MA) and its Image Processing Toolbox. (11) Jin, R. C.; Jureller, J. E.; Scherer, N. F. Appl. Phys. Lett. 2006, 88, 263111. (12) Imura, K.; Nagahara, T.; Okamoto, H. Chem. Phys. Lett. 2004, 400, 500-505. (13) Imura, K.; Nagahara, T.; Okamoto, H. J. Am. Chem. Soc. 2004, 126, 12730-12731. (14) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, 1995. (15) Xu, H. X.; Ka¨ll, M. Chem. Phys. Chem. 2003, 4, 1001-1005. (16) Novotny, L.; Hecht, B. Principles of Nano-Optics; Cambridge University Press: Cambridge, 2006. (17) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2000, 104, 10549-10556. (18) Maier, S. A.; Brongersma, M. L.; Kik, P. G.; Atwater, H. A. Phys. ReV. B 2002, 65, 193408.
