Direct Visualization of the Chemical Mechanism in SERRS of 4‐Aminothiophenol/Metal Complexes and Metal/4‐Aminothiophenol/Metal Junctions

Abstract

Direct visualization of photoinduced tunneling charge transfer (TCT) in an Au(5)/para-aminothiophenol (PATP)/Ag(6) junction in which Au and Ag clusters form the first and second layer, respectively, is provided by the charge difference density (see picture; green and red stand for holes and electrons, respectively). We theoretically investigate the mechanism of chemical enhancement of surface-enhanced resonance Raman scattering (SERRS) of para-aminothiophenol (PATP)/metal complexes and metal/PATP/metal junctions. The method of charge difference density is used to visualize intracluster excitation and charge transfer (CT) between PATP and metal during the process of resonant electronic transitions. It is found that the selective enhancement of the b(2) mode in SERRS spectra result not only from Albrecht's A term (the Frank-Condon term), but also from the Herzberg-Teller term (Albrecht's B mechanism) via resonant CT. For the metal/PATP/metal junctions, the calculated results reveal that the Raman spectrum is of SERRS nature and the nontotally symmetric b(2) mode is strongly enhanced at the incident wavelength of 1064 nm when Au and Ag nanoparticles are the first and second layer, respectively, and the dominant enhancement mechanism is the Herzberg-Teller term in chemical enhancement via tunneling charge transfer (intervalence electron transfer from the Ag cluster to the Au cluster). When the first and second layers were inverted (i.e. the Ag and Au nanoparticles are the first and second layers, respectively), the Raman spectrum at an incident wavelength of 1064 nm is due to normal Raman scattering, and the nontotally symmetric b(2) mode is not strongly enhanced. Our theoretical results not only support the experimental findings, but also provide a clear physical interpretation.

Surface enhanced Raman scattering of 4-aminothiophenol sandwiched between Ag nanocubes and smooth Pt substrate: The effect of the thickness of Pt film

“…However, the charge transfer enhancement has to be included for actual situations, especially for the SERS of p-ATP. [37][38][39] The involvement of the charge transfer enhancement mechanism is confirmed from the relative peak intensity of the b 2 mode to the a 1 mode. Figure 8 shows the relative peak intensity of b 2 (3 and 19b) to a 1 (7a) bands in Ag-NCs @ p-ATP/Pt structure.…”

Adjustment and control of SERS activity of metal substrates by pressure

“…With charge difference density, [45] the phenomenon of TCT has been clearly visualized in metal-molecule-metal junctions when the gap is below 1 nm. [42] When the applied pressure is large enough, the particles will be flattened and connected to each other (the distance of gaps is 0 nm), and the number of effective gaps will decrease, which will also decrease the lightning rod effect and decrease the intensity of SERS spectroscopy.…”

Direct visual evidence for chemical mechanisms of SERRS via charge transfer in Au20–pyrazine–Au20 junction

“…We also investigated the chemical enhancement mechanism (CT) on the SERS in the metal-para-aminothiophenol (PATP)-metal junction with the quantum chemical method, in which the tunneling CT at incident light of 1064 nm has been visualized with charge difference density. [12] In this paper, the direct theoretical evidence for the chemical mechanism of SERRS via CT between the molecule and the metal cluster is provided in visual way using charge difference density. The relationship between the selective chemical enhancements of the SERRS spectrum and the resonant CT transition is revealed visually, as well as the intracluster excitation, which is evidence for the local EM enhancement.…”