VOLUME 6, NUMBER 10, OCTOBER 2006 © Copyright 2006 by the American Chemical Society
Visualization of Localized Intense Optical Fields in Single Gold−Nanoparticle Assemblies and Ultrasensitive Raman Active Sites Kohei Imura,†,‡ Hiromi Okamoto,*,†,‡ Mohammad K. Hossain,§ and Masahiro Kitajima†,§ Institute for Molecular Science and The Graduate UniVersity for AdVanced Studies, Okazaki, Aichi 444-8585, Japan, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba, Ibaraki 305-8571, and National Institute for Materials Science, Sengen, Tsukuba, Ibaraki 305-0047, Japan Received July 18, 2006; Revised Manuscript Received September 3, 2006
ABSTRACT We demonstrate visualization of localized intense electromagnetic fields in real space in well-tailored dimeric and trimeric gold nanospheres by using near-field optical techniques. With two-photon induced luminescence and Raman measurements, we show that the electric field is confined at an interstitial site in the aggregate. We also demonstrate optical switching operations for the electric-field localized sites in the trimer structure.
Since the surface-enhanced Raman scattering (SERS) of single-molecular level sensitivity was reported,1,2 this phenomenon has attracted much attention and a number of studies have been devoted to investigate the mechanism of that.3-8 For molecules adsorbed on noble metal nanoparticles in colloidal solutions or on electrodes, very intense Raman scattering compared to ordinary Raman scattering (typical enhancement factor of the order of 106) is observed, which is known as SERS.9 For SERS from single aggregates of * Corresponding author. Tel: +81-564-55-7320. Fax: +81-564-55-4639. E-mail:
[email protected]. † Institute for Molecular Science. ‡ The Graduate University for Advanced Studies. § University of Tsukuba and National Institute for Materials Science. 10.1021/nl061650p CCC: $33.50 Published on Web 09/19/2006
© 2006 American Chemical Society
metal nanoparticles, the Raman enhancement factor in some cases reaches nearly 1014, and this huge enhancement enables the single-molecular level SERS detection. Because of the very high sensitivity, extensive applications of this phenomenon to life science, medical science, environmental science, etc. are expected.10-15 Many recent papers indeed deal with application of SERS to analytical purposes.12,15 It is of fundamental importance to reveal the origin of the huge enhancement, to develop SERS as an analytical method of very high sensitivity. Among several possible origins of the enhancement, the major factor is considered to be an electromagnetic mechanism,3,6,16 that is, electric field enhancement induced by a plasmon resonance of the metal
nanoparticle.17,18 In the case of aggregated nanoparticles in particular, it is theoretically predicted that a strong electric field is induced in interstitial gaps between the nanoparticles (“hot spot”).3,6,19,20 Raman scattering from a molecule is considered to be greatly enhanced when the molecule is adsorbed near the interstitial site. Many experimental results on SERS from molecules on aggregated nanoparticles21-26 are interpreted on the basis of the hot spot mechanism. Up to now, however, Raman enhancement in interstitial gaps between nanoparticles has not been directly observed. Rigorous examination of the role of the gap is possible only if the interstitial site is resolved from the other parts of the aggregate, which was not feasible in the previous studies because of insufficient spatial resolution.1,2,21,22,24-26 To achieve that, we need an imaging method that allows us to measure topography and electric-field distribution simultaneously with a sufficient spatial resolution and sensitivity or efficiency. It is also necessary that the apparatus is capable to observe the local Raman scattering with a minor modification of the optical arrangement. Such a method was, however, not well developed previously. We recently developed a method of near-field two-photon excitation imaging,27,28 where luminescence from gold nanoparticles is detected, and we found that this technique meets the conditions mentioned above.29 As for the samples to be measured, preparation of isolated dimeric/trimeric noblemetal nanospheres, which serve as models for hot spots, was not straightforward. Most of previous studies are on amorphous many-particles clusters. In our new method of fabricating metal nanoparticle arrays, on the one hand, many isolated dimeric/trimeric nanospheres suitable for the optical measurements are cogenerated. We are now at a position to discuss the role of the hot spots in SERS based upon observed images of optical field for aggregated nanoparticles. In this Letter, we show observations of spatial distributions of electric-field enhancement and Raman-excitation probability for dimeric/trimeric aggregates of gold nanospheres, using an apertured scanning near-field optical microscope. The results clearly demonstrate that the hot spots have major contribution to the Raman enhancement. The experimental methodology may be also served as a powerful basic tool for investigation of plasmonic interactions. Colloidal solution of spherical gold nanoparticles (diameter 100 nm) solution was purchased from BB International and used as received. Gold dimers were prepared by aggregating the gold nanospheres on a cover slip pretreated with trimethoxy[3-(methylamino)propyl]silane. The cover slip was then spin-coated by a water solution of Rhodamine 6G (R6G) molecules. The area density of R6G molecules was estimated to be less than 100 molecules per 100 nm × 100 nm, if we assume homogeneous dispersion of the molecules over the substrate. Morphology of the sample was verified by topography measurements by the scanning near-field optical microscope (SNOM) and/or by an atomic force microscope. The experimental apparatus consists of an apertured SNOM (aperture diameter ∼100 nm), an excitation light source, and a detection system. The apparatus was used under ambient 2174
Figure 1. (a) Topography of aggregated and isolated gold nanoparticles. (b, c) Near-field Raman spectra at dimers 1 and 2, respectively, taken at two different incident polarization directions (arrows). The peaks marked with # are unassigned. (d) Near-Field two-photon excitation images of dimers 1 and 2. (e) Near-field Raman images at dimer 1 and dimer 2 obtained for bands near 1600 cm-1. The images were measured at different incident polarization directions indicated. White lines in (d) and (e) indicate approximate shapes of the dimers. Polarization of the Ramanscattered signal was not selected. Image sizes are 540 nm × 540 nm.
condition. A Ti:sapphire laser (λ ) 785 nm) was used in continuous wave and pulsed operation modes, respectively, to excite the Raman scattering and two-photon-induced photoluminescence (TPI-PL). The laser power coupled to the fiber at other end of the near-field tip was less than 2 mW. Raman scattered radiation and photoluminescence (PL) was collected by an objective and detected by a polychromator-CCD and/or an avalanche photodiode. Near-field Raman spectra were typically measured at 50 nm steps across the scan area with 1 s exposure time. Figure 1a shows topography of the gold nanoparticle aggregates prepared on the cover slip. Isolated, dimeric, and trimetric nanoparticles are found. Gap distance in the dimer (denoted as 1 and 2) is estimated from the topography and scanning electron micrographs as ca. 1-10 nm. We observed very strong Raman signals at gold aggregates while there are no detectable Raman signals at isolated nanoparticles or at the substrate. Raman scattering spectra at dimer 1 and dimer 2 are shown in parts b and c of Figure 1, respectively. The low-frequency region (
