Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation

Abstract

Conspectus Historically, molecular spectroscopists have focused their attention to the right-hand side of the Schrödinger equation. Our major goal had and still has to do with determining a (bio)­molecular system’s Hamiltonian operator. From a theoretical spectroscopist’s perspective, this entails varying the parameters of a model Hamiltonian until the predicted observables agree with their experimental analogues. In this context, less emphasis has been put on the left-hand side of the equation, where the interplay between a system and its immediate local environment is described. The latter is particularly meaningful and informative in modern applications of optical microscopy and spectroscopy that take advantage of surface plasmons to enhance molecular scattering cross-sections and to increase the attainable spatial resolution that is classically limited by diffraction. Indeed, the manipulation of light near the apex of a metallic nanotip has enabled single molecule detection, identification, and imaging. The distinct advantages of the so-called tip-enhanced optical nanospectroscopy/nanoimaging approaches are self-evident: ultrahigh spatial resolution (nanometer or better) and ultimate sensitivity (down to yoctomolar) are both attainable, all while retaining the ability to chemically fingerprint one molecule at a time (e.g., through Raman scattering). An equally interesting aspect of the same approach stems from using the properties of a single molecule to characterize the local environment in which it resides. This concept of single molecule spectroscopy on the left-hand side of the Schrödinger equation is certainly not novel and has been discussed in pioneering single molecule studies that ultimately led to a Nobel prize in chemistry. That said, local environment mapping through ultrasensitive optical spectroscopy acquires a unique flavor when executed using tip-enhanced Raman scattering (TERS). This is the subject of this Account. In a series of recent reports, our group utilized TERS to characterize different properties of nanolocalized and enhanced optical fields. The platforms that were used to this end consist of chemically functionalized plasmonic nanostructures and nanoparticles imaged using visible-light-irradiated gold- or silver-coated probes of an atomic force microscope. Through a detailed analysis of the recorded spectral nanoimages, we found that molecular Raman spectra may be used to track the magnitudes, resonances, spatiotemporal gradients, and even vector components of optical fields with nanometer spatial resolution under ambient conditions. On the other side of the equation, understanding how spatially varying optical fields modulate molecular nano-Raman spectra is of utmost importance to emerging areas of nanophotonics. For instance, tracking plasmon-enhanced chemical transformations via TERS necessitates a deeper fundamental understanding of the optical signatures of molecular reorientation and multipolar Raman scattering, both of which may be driven by local optical field gradients th...

A Closer Look at Tip-Enhanced Raman Chemical Reaction Nanoimages

“…Given the extreme confinement of the optical field in the vicinity of the silver nanocube (∼1–2 nm) in this study, , the optical response is also governed by tensorial Raman scattering. In other words, the attainable high spatial resolution translates into an ultrasensitive optical response that is nascent from a single/a few molecules in the probing volume . The disappearance of a peak, herein the vinyl stretching vibration at ∼1630 cm –1 , is not the result of chemical transformation.…”

Multimodal (Non)Linear Optical Nanoimaging and Nanospectroscopy

“…This is most evident in measurements that targeted perfect spherical gold nanoparticles and nanorods, as summarized elsewhere. 29 Some aspects of the spectral nanoimages that are shown in Figure 2D,E are worth highlighting. First, as the tip makes its way toward the edges of the cube, a seemingly (see below) gentle rise in signal is observed.…”

“…22 Model systems that are often used to access the few-nanometer length scale in optical spectroscopy include bare and chemically functionalized plasmonic silver and gold nanoparticles. 29 In the TOM geometry, spectral nanoimages of plasmonic particles trace localized and enhanced optical fields that are governed by the interaction between the plasmonic probe and the particle. For example, in the case of plasmonic silver nanocubes, a square-like scattering pattern is observed in TERS (see Figure 2).…”

“…Even though this Perspective is focused on ambient TOM, one cannot but mention the seminal UHV nano-Raman measurements that allowed the real space visualization of molecules and their vibrational modes with subnanometer spatial resolution. , Under ambient conditions, spatial resolution on the order of 1 to a few nanometers was repeatedly demonstrated. ,− Note that this is the case not only for Raman, ,, photoluminescence, , and extinction ( vide infra ) but also for nonlinear nano-optical processes, such as tip-enhanced four-wave mixing . Model systems that are often used to access the few-nanometer length scale in optical spectroscopy include bare and chemically functionalized plasmonic silver and gold nanoparticles . In the TOM geometry, spectral nanoimages of plasmonic particles trace localized and enhanced optical fields that are governed by the interaction between the plasmonic probe and the particle.…”

“…For example, in the case of plasmonic silver nanocubes, a square-like scattering pattern is observed in TERS (see Figure ). , Here, molecular reporters at nanojunctions formed between the tip (its shaft and apex) and the sides/edges of the nanocubes are optimally enhanced and dominate the recorded spectral nanoimages. , In some cases, the choice of excitation wavelength and operative resonances allow the observation of the local optical fields of the nanoparticles themselves. This is most evident in measurements that targeted perfect spherical gold nanoparticles and nanorods, as summarized elsewhere …”