Raman Scattering of 4-Aminobenzenethiol Sandwiched between Au Nanoparticles and a Macroscopically Smooth Au Substrate: Effect of Size of Au Nanoparticles

Abstract

In order to obtain information on the nature of the "hot" sites in surface-enhanced Raman scattering, we have fabricated a model nanostructure composed of 4-aminobenzenethiol (4-ABT) that has further been sandwiched between Au nanoparticles and a macroscopically smooth Au substrate. In the absence of Au nanoparticles, that is, when 4-ABT is adsorbed solely onto a planar Au substrate, no Raman peak is identified at all using 632.8 nm radiation as the excitation source. When Au nanoparticles are attached onto the amine groups of 4-ABT on a planar Au substrate, Raman peaks are readily observed, however. What is intriguing is that a more intense Raman signal is obtained with increasing size of the Au nanoparticles at least with dimensions of under ∼100 nm. Much the same trend is confirmed from the finite-difference time-domain calculation. This is thought to suggest that the electromagnetic coupling of the localized surface plasmon of Au nanoparticles with the surface plasmon polariton of the underneath smooth Au substrate occurs more favorably with largersized nanoparticles leading to the molecules trapped in the gap exhibiting strong Raman peaks.

Surface-Enhanced Raman Scattering: A Powerful Tool for Chemical Identification

“…[32][33][34] The gap formed by a metal nanoparticle and a flat metal substrate is functioning as a "hot spot" for SERS. [18][19][20][21] The EFs derived from the attachment of a single, 55-nm-sized nanoparticle onto 4-ABT on Au were estimated to be as large as 8.3 × 10 5 or 5.0 × 10 5 , for Ag or Au nanoparticles, respectively (for the ring 3 band (b2) near 1390 cm -1 ), in which a factor of ~10 2 was presumed to be due to a chemical effect, with the remaining EF contributed by the electromagnetic effect. 18 What is intriguing is that a more intense Raman signal is obtained with increasing size of the Au nanoparticles, at least with dimensions of under ~100 nm.…”

“…Much the same trend has been confirmed from a finite-difference time-domain (FDTD) calculation. 19 This is thought to suggest that the electromagnetic coupling of the localized surface plasmon of Au nanoparticles with the surface plasmon polariton of the underlying smooth Au substrate occurs more favorably with larger-sized nanoparticles, leading to the molecules trapped in the gap exhibiting strong Raman peaks. Orendorff et al have also investigated the SERS spectra of 4-mercaptobenzoic acid molecules sandwiched between Au nanoparticles of various shapes and a smooth Au substrate.…”

“…As mentioned above, very intense SERS spectra could be obtained for 4-ABT assembled on a macroscopically flat Ag or Au substrate simply by attaching Ag or Au nanoparticles to them. 18,19 Based on these observations, we investigated whether unaggregated Ag nanoparticles can also induce SERS for 4-ABT assembled on a macroscopically smooth Pt substrate (Fig. 1).…”

“…4,[14][15][16][17] We have recently confirmed that dramatic increases in Raman scattering can occur even for molecules sandwiched between Au (Ag) nanoparticles and macroscopically smooth Au (Pt) substrates. [18][19][20][21] The gap formed by a metal nanoparticle and a flat metal substrate, or the gap between two metal nanoparticles is indeed the SERS-active site. Regarding the "hot spot", it has been well established theoretically that as two particles approach each other, their transition dipoles couple in such a way that the enhanced EM fields around each particle create a pattern of coherent interference.…”

“…By using 4-aminobenzenthiol (4-ABT) or 1,4-phenylenediisocyanide (1,4-PDI) molecules we can characterize the gap formed by a planar Au (Pt) and an Au (Ag) nanoparticle, or the gap between two spherical Au nanoparticles. [18][19][20][21]27,28 These nanogaps are indeed the SERS-active sites, and the apparent size of the "hot spot" is found to be very limited. Silver thin film can be reproducibly fabricated simply by soaking glass substrates in ethanolic solutions of AgNO3 and butylamine.…”