Near-field Imaging of Surface-enhanced Raman Active Sites in Aggregated Gold Nanoparticles

Abstract

We observed near-field images of aggregates of gold nanospheres, by detecting TPI-PL from gold and Raman scattering from Rhodamine 6G adsorbed on the nanoparticles. The results were analyzed in relation to the localized Raman active site (hot spot) of surface-enhanced Raman scattering.Although chemical specificity of Raman spectra is expected to be useful in various fields of science, small cross section of Raman scattering sometimes causes difficulties in practical applications. To extend the applicability of Raman spectroscopy to wider purposes, it is essential to enhance the scattering cross section. In surface-enhanced Raman scattering (SERS), 1 the Raman signal is so drastically enhanced that it would give an effective solution to the difficulties. Even single-molecule level Raman spectroscopy has been reported recently by using the SERS scheme.2 In the mechanism of SERS, it is considered that enhancement of the electric field by plasmon in noble metal nanoparticles plays a crucial role. This mechanism of SERS is called electromagnetic (EM) enhancement.3 In addition to the EM mechanism, chemical effects are also considered to have some contribution to the SERS enhancement. To understand and control the SERS process, knowledge about spatial structures and other characteristics of plasmons is indispensable.In single-molecule level SERS, spatially localized plasmon resonance, called ''hot spot,'' is believed to be essential for the Raman enhancement.3 Understanding of the physical properties of hot spots and imaging of them are hence important. From electromagnetic calculations on dimers of noble metal spherical nanoparticles, it has recently been revealed that the radiation field is strongly enhanced in the interstitial space between the particles (sometimes called as ''junction'').3 Such a structure may be regarded as a model system for the hot spot, but up to now direct microscopic observation of the dimeric structures and radiation field distribution has not been reported.The present authors recently reported that the plasmonic modes are visualized using near-field two-photon induced photoluminescence (TPI-PL). 4 Applying this method to the aggregates of metal nanoparticles, distribution of the electric field can be imaged. By monitoring Raman signals, direct visualization of hot spot distribution is also possible. In this letter, we report imaging of hot spots on gold nanosphere aggregates by near-field TPI-PL and Raman imaging. We consider that such direct optical imaging with high spatial resolution is essential to understand structures of hot spots and the electromagnetic effects of junctions in SERS process.Gold nanosphere (diameter 100 nm) solution was purchased from BB International. Gold dimers were prepared by aggregating the gold nanospheres on a cover-slip treated with trimethoxy[3-(methylamino)propyl]silane. The cover-slip was then spin-coated by a methanol/water solution of Rhodamine 6G (R6G) molecules. The area density of R6G molecules was estimated to be less than 15 molecules per 100 Â 100...

Surface‐Enhanced Raman Scattering Imaging: Application and Experimental Approach by Far‐Field with Conventional Setup

“…This technique has been widely used for monitoring by correlated two-photon-induced photoluminescence and Raman scattering imaging. It has been detected that the photoluminescence and SERS are emitted from the gap of a noble metal nanodimer by combination with topographic measurements [17,[22][23][24]. This is consistent with the calculated results by a finite-difference time-domain (FDTD) calculation [8,9,18,[25][26][27][28].…”

Quantitative evaluation of electromagnetic enhancement in surface-enhanced resonance Raman scattering from plasmonic properties and morphologies of individual Ag nanostructures

“…[5][6][7][8][9] Because plasma resonance in metal nanostructures is highly sensitive to the subtle difference in their shape and size, their morphologies affect strongly SERRS enhancement under the SERRS-EM model. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] Ensemble measurement averages over spectra of plasma resonance and SERRS of individual Ag nanostructures. Indeed, the ensemble measurement obscures the plasma resonance inducing SERRS and the morphology of Ag nanostructures that determines the plasma resonance.…”

Nanooptical Studies on Physical and Chemical Characteristics of Noble Metal Nanostructures

“…We applied nearfield imaging to assembled gold nanoparticles and succeeded in experimental visualization of localized enhanced optical fields for the first time. 25,26 I felt that this finding might also stimulate a novel research area, as in the standing wave observation of a single nanoparticle mentioned above, and made some efforts to develop the study. As a result we have derived some guidelines for the structures of nanoparticle assemblies to get efficient field enhancement.…”

“…Figure 9 shows near-field two-photon excitation images for gold nanosphere dimers using a femtosecond Ti:sapphire laser (wavelength 780 nm) as an excitation source. 25,26 In this measurement the system is illuminated by the near-field radia- AWARD ACCOUNTS Bull. Chem.…”