Direct Experimental Evidence of Hot Carrier-Driven Chemical Processes in Tip-Enhanced Raman Spectroscopy (TERS)

Abstract

Nanoscale localization of electromagnetic fields using metallic nanostructures can catalyze chemical reactions in their immediate vicinity. Local optical field confinement and enhancement is also exploited to attain single-molecule detection sensitivity in surface-and tip-enhanced Raman (TER) spectroscopy. In this work, we observe and investigate the sporadic formation of 4-nitrobenzenethiolate upon TER imaging of a 4-nitrobenzenethiol (4NBT) monolayer on Au(111). Density functional theory (DFT), finite-difference timedomain (FDTD), and finite element method (FEM) calculations together confirm that this chemical reaction does not occur as a result of thermal desorption of the molecule, which requires temperatures in excess of 2100 K at the tip−sample junction. Our combined experimental and theoretical analyses strongly suggest that the chemical transformations observed throughout the course of TERS mapping is not driven by plasmonic photothermal heating, but rather by plasmon-induced hot carriers.

Nanoscale structural characterization of plasmon-driven reactions

“…Wang and Kurouski recently reported a new class of plasmon-driven scissoring reactions. The researchers found that 4-NBT could be converted into 4-nitrobenzothiolate (4-NBT -) on the surface of AuNPs [26]. This conclusion was made by an observation of the shift of the 1335 cm −1 vibration of 4-NBT to the 1305 cm −1 vibration of 4-NBT - (Figure 10).…”

“…There are several plasmon-driven scissoring reactions including cleavage of carbon-carbon (C-C) [46], nitrogennitrogen (N=N) [47] and gold-sulfur (Au-S) [26] bonds, as well as dissociation of H 2 and O 2 that are caused by scissoring of H-H [48] and O-O [49] bonds respectively. Sun and coworkers showed that under acidic conditions, DMAB could be cleaved by hot carriers producing two molecules of 4-ATP, Figure 4.…”

“…We also demonstrated that the observed spectroscopic changes could not be caused by a change in molecular orientation of the analyte relative to the metal surface. It should be noted that thermal desorption of 4-NBT would require 2100 K in the tip-sample junction, which cannot be achieved in TERS [26].…”

“…Although the yield of 4-NBT ionization on AuNPs is relatively low (∼1.5%) [26], the El-Khoury's group showed that AgNPs exhibited nearly 100% conversion of 4-NBT to 4-NBT - [98]. Thus, the yield of the Au-S scissoring reaction is determined by the choice of the plasmonic metal.…”

“…This experimental evidence demonstrated that plasmon-driven chemistry observed on SERS platforms (noble metal nanostructures and SERS substrates) is not thermally driven. Large group used finite density time domain (FDTD) calculations to predict changes in temperature in tip-sample junction upon its illumination with red (λ = 671 nm) laser light [26]. The researchers found that a temperature change in the tipsample junction directly depends on the distance between the tip and the sample.…”