Characteristics of Nanogaps Formed by Planar Au and Pt Nanoparticles Revealed by Raman Spectroscopy

Raman peaks are barely detectable for molecules present on macroscopically smooth metal substrates. Raman peaks can be observed, however, by allowing Ag or Au nanoparticles to overlie the adsorbate molecules. For instance, no Raman peaks are detected for 4-aminobenzenethiol (4-ABT) on a flat Au substrate. Upon attaching Ag nanoparticles onto 4-ABT, however, Raman peaks are distinctly observed, not only with excitation at 632.8 nm but also with excitation at 514.5 nm, though the peaks at 632.8 nm are several tens of times more intense than those at 514.5 nm. When Au nanoparticles are attached, Raman peaks are negligibly observable with the excitation at 514.5 nm, though distinctly observed with the excitation at 632.8 nm. Interestingly, Raman peaks are observable even at 514.5 nm by attaching Pt nanoparticles onto 4-ABT on a flat Ag substrate to form Pt@4-ABT/Ag(flat). The larger the Pt nanoparticle is, the greater the electromagnetic field induced in the nanogap. Much the same observation is made for the Pt@4-ABT/Au(flat) system. However, the excitation wavelength dependence is opposite such that the higher intensity is observed in the order of excitations 632.8 > 568 > 514.5 > 488 nm for the Pt@4-ABT/Au(flat) system, while in the order of excitations 488 > 514.5 > 568 > 632.8 nm for the Pt@4-ABT/Ag(flat) system. The maximum enhancement factor (EF) at 632.8 nm excitation in the Pt@4-ABT/Au(flat) system is nonetheless only about one-half of the EF value observable at 632.8 nm excitation in the Pt@4-ABT/Ag(flat) system, suggesting that the planar Ag substrate should be far more effective than the planar Au substrate for the induction of SERS by virtue of Pt nanoparticles overlaid thereon.