Raman scattering of 4-aminobenzenethiol sandwiched between Ag nanoparticle and macroscopically smooth Au substrate: effects of size of Ag nanoparticles and the excitation wavelength.

A nanogap formed by a metal nanoparticle and a flat metal substrate is one kind of "hot site" for surface-enhanced Raman scattering (SERS). Accordingly, although no Raman signal is observable when 4-aminobenzenethiol (4-ABT), for instance, is self-assembled on a flat Au substrate, a distinct spectrum is obtained when Ag or Au nanoparticles are adsorbed on the pendent amine groups of 4-ABT. This is definitely due to the electromagnetic coupling between the localized surface plasmon of Ag or Au nanoparticle with the surface plasmon polariton of the planar Au substrate, allowing an intense electric field to be induced in the gap even by visible light. To appreciate the Raman scattering enhancement and also to seek the optimal condition for SERS at the nanogap, we have thoroughly examined the size effect of Ag nanoparticles, along with the excitation wavelength dependence, by assembling 4-ABT between planar Au and a variable-size Ag nanoparticle (from 20- to 80-nm in diameter). Regarding the size dependence, a higher Raman signal was observed when larger Ag nanoparticles were attached onto 4-ABT, irrespective of the excitation wavelength. Regarding the excitation wavelength, the highest Raman signal was measured at 568 nm excitation, slightly larger than that at 632.8 nm excitation. The Raman signal measured at 514.5 and 488 nm excitation was an order of magnitude weaker than that at 568 nm excitation, in agreement with the finite-difference time domain simulation. It is noteworthy that placing an Au nanoparticle on 4-ABT, instead of an Ag nanoparticle, the enhancement at the 568 nm excitation was several tens of times weaker than that at the 632.8 nm excitation, suggesting the importance of the localized surface plasmon resonance of the Ag nanoparticles for an effective coupling with the surface plasmon polariton of the planar Au substrate to induce a very intense electric field at the nanogap.

Kim K ，Choi JY ，Lee HB ，Shin KS ... -  《-》

Effect of Ag and Au nanoparticles on the SERS of 4-aminobenzenethiol assembled on powdered copper.

Kim K ，Lee HS 《-》

Surface-enhanced Raman scattering characteristics of nanogaps formed by a flat Ag substrate and spherical Pt nanoparticles.

We estimated the apparent size of the 'hot site' for surface-enhanced Raman scattering (SERS) located within the gaps between Pt nanoparticles and a flat Ag substrate. Initially, no Raman peaks were detected for 4-aminobenzenethiol (4-ABT) on a flat Ag substrate. Upon attaching 68 nm-sized Pt nanoparticles onto the amine group of 4-ABT (thus denoted as Pt-4-ABT/Ag(flat)), Raman peaks were distinctly observed, not only with the excitation at 488 nm but also with the excitation at 632.8 nm. This means that electromagnetic 'hot site' had formed at the gaps between Pt nanoparticles and a flat Ag substrate. When 4-ABT molecules were adsorbed additionally onto the vacant sites of Pt nanoparticles in Pt-4-ABT/Ag(flat), the Raman signal did not increase further, suggesting that the SERS 'hot site' was very limited and located mostly at the gaps between Pt nanoparticles and a flat Ag substrate, in agreement with the finite-difference time-domain (FDTD) calculation. To a rough estimate, about 1000 molecules residing only within a ~15 nm diameter area of the center of the gap must have contributed most of the measured Raman signal of 4-ABT.

Kim K ，Lee HB ，Shin KS 《-》

Raman scattering of 4-aminobenzenethiol sandwiched between Ag/Au nanoparticle and macroscopically smooth Au substrate.

Kim K ，Yoon JK 《-》

Labeled gold nanoparticles immobilized at smooth metallic substrates: systematic investigation of surface plasmon resonance and surface-enhanced Raman scattering.

Driskell JD ，Lipert RJ ，Porter MD 《-》