Plasmonic Heating Effects in Tip-Enhanced Raman Spectroscopy (TERS)

Abstract

Tip-enhanced Raman spectroscopy (TERS) is a spectroscopy technique that possesses single-molecule sensitivity and subnanometer spatial resolution. These unique properties are achieved thanks to the extremely high electromagnetic field confinement at the apex of the scanning probe. However, such strong field confinement can lead to photodecomposition and thermal decomposition of the analytes. Here, we demonstrate that the use of an aqueous solvent as tip−sample junction mediator drastically reduces possible molecule degradation. Using a combination of electrodynamic and heat transport simulations, we provide some theoretical insight into the plasmonic heating of the TERS system. The simulations of a realistic model system show that upon illumination of the tip−sample junction the temperature at the tip apex can increase by ∼180 K when TERS is performed in air with optical powers of ∼100 μW. On the other hand, in aqueous media, the temperature increase of the tip remains significantly lower (a few kelvins) thanks to the higher thermal conductivity of water.

Metal–Adsorbate Interactions Modulate Plasmonic Reactivity of Chemisorbed Nitrophenyl Derivatives

“…As shown in detail by our previous work, the overall reaction kinetics were essentially modulated by the photoactivated O 2 – radicals at its steady state concentrations, which exhibited a linear dependence on P ex . The linear P ex -dependence of k obs also indicated that under this reaction condition, the pNTP coupling reaction was driven by plasmonic hot electrons without any significant influence by photothermal heating. ,, Because of the small excitation volume of the confocal illumination and high thermal conductivity of water, the heat produced through photothermal transduction was rapidly dissipated to the aqueous reaction environment, resulting in a rather insignificant elevation of the local temperatures on the photocatalyst surfaces. , In alkaline environments, 785 nm excitations gave rise to sublinear P ex -dependence of k obs for both pNTP and pNPA coupling reactions (lower panel of Figure D). Such sublinearity in power dependence is a common feature of semiconductor-driven photocatalytic reactions when the bulk electron–hole pair recombination becomes prevalent but has been rarely observed in plasmon-driven photocatalysis on metallic nanostructures .…”

Playing catch and release with single molecules: mechanistic insights into plasmon-controlled nanogaps

“…21,27 Since the rate constant (K) equals the reciprocal of τ, 56 we can deduce the SM dissociation and association kinetics to the nanogap (K DIS ¼ 1=τ CAP and K ASS ¼ 1=τ REL , respectively 47,49,56 ) at the temperature at which the experiment was performed (RT). Despite the use of low and medium power (far-field) intensities, the photothermal effects (optical heating) [57][58][59] of exposure of the BDT molecule to laser illumination and the possible repercussions over its capture/release kinetics should be discussed here. As a very recent study revealed, 59 the expected local temperature increase is estimated to be below 3 K for the nearfield in a nanogap of Au-tip|Au-surface configurations in aqueous media using a tip radius of 25 nm, and under the same laser power densities here employed, even considering a field magnification of the order of 50, almost twice that of ours.…”

“…Despite the use of low and medium power (far-field) intensities, the photothermal effects (optical heating) [57][58][59] of exposure of the BDT molecule to laser illumination and the possible repercussions over its capture/release kinetics should be discussed here. As a very recent study revealed, 59 the expected local temperature increase is estimated to be below 3 K for the nearfield in a nanogap of Au-tip|Au-surface configurations in aqueous media using a tip radius of 25 nm, and under the same laser power densities here employed, even considering a field magnification of the order of 50, almost twice that of ours. This temperature increase is insufficient to promote Au-S desorption, 60 Au-Au fracture, 61 or thiol-Au bond instabilities that have been estimated to be significant from a temperature increase of 30 K. 62 Accordingly, the expected small (local) thermal increase will not affect the stability of the junction.…”

Nanophotonics for Chemical Imaging and Spectroscopy

“…Indeed, engineered hybrid molecular-plasmonic nanostructures have enabled single molecule detection and identification via surface-enhanced Raman scattering (SERS). , The same effect is at the core of visualizing single molecules with subnanometer spatial resolution via tip-enhanced Raman scattering (TERS). − More generally, modern applications of plasmon-enhanced spectroscopy and microscopy fueled advances in fields as diverse as quantum and semiconductor materials, heterogeneous catalysis, and the health sciences, to name a few. , All of these exciting developments and many more are addressed in this virtual special issue (VSI) on nanophotonics. From ultraprecise chemical imaging measurements performed in a scanning tunneling microscope chamber − to single molecule spectroscopic measurements performed at solid–air and solid–liquid interfaces, , this VSI includes contributions from active practitioners that are all striving to better understand increasingly complex interfaces encountered in the biological, materials, and chemical sciences (see Figure ).…”

“…6,7 All of these exciting developments and many more are addressed in this virtual special issue (VSI) on nanophotonics. From ultraprecise chemical imaging measurements performed in a scanning tunneling microscope chamber 3−5 to single molecule spectroscopic measurements performed at solid−air and solid−liquid interfaces, 8,9 this VSI includes contributions from active practitioners that are all striving to better understand increasingly complex interfaces encountered in the biological, materials, and chemical sciences (see Figure 1).…”