Nanoscale Probing of Dynamics in Local Molecular Environments

Abstract

Vibrational spectroscopy can provide information about structure, coupling, and dynamics underlying the properties of complex molecular systems. While measurements of spectral line broadening can probe local chemical environments, the spatial averaging in conventional spectroscopies limits insight into underlying heterogeneity, in particular in disordered molecular solids. Here, using femtosecond infrared scattering scanning near-field optical microscopy (IR s-SNOM), we resolve in vibrational free-induction decay (FID) measurements a high degree of spatial heterogeneity in polytetrafluoroethylene (PTFE) as a dense molecular model system. In nanoscopic probe volumes as small as 10(3) vibrational oscillators, we approach the homogeneous response limit, with extended vibrational dephasing times of several picoseconds, that is, up to 10 times the inhomogeneous lifetime, and spatial average converging to the bulk ensemble response. We simulate the dynamics of relaxation with a finite set of local vibrational transitions subject to random modulations in frequency. The combined results suggest that the observed heterogeneity arises due to static and dynamic variations in the local molecular environment. This approach thus provides real-space and real-time visualization of the subensemble dynamics that define the properties of many functional materials.

Plasmonic nanofocusing – grey holes for light

“…Often, they are performed on diluted samples in order to artificially increase the distance between adjacent emitters and to facilitate their spatial isolation. As such, the local environment of the emitter is necessarily different from the native environment and this may significantly affect the ultrafast dynamics of its optical excitations [18][19][20].…”

Heterogeneous Cation–Lattice Interaction and Dynamics in Triple-Cation Perovskites Revealed by Infrared Vibrational Nanoscopy

“…The spatio-spectral IR s -SNOM imaging of the FA cation’s CN antisymmetric stretch mode provides for a spatially resolved mapping of composition within a spin-coated FAMACs film. Furthermore, with subwavenumber (<0.1 meV) precision resolved variations in vibrational peak position and line width, we are able to probe disorder in the cation–lattice interactions and their coupled vibrational dynamics. − To study the evolution of heterogeneity induced by an external perturbation, we expose the perovskite film to high humidity that results in the formation of larger vapor-annealed grains as previously observed in different perovskites, yet surprisingly with no correlated change in chemical heterogeneity. We observe that stronger cation–lattice interactions facilitate vibrational damping, suggesting a varying degree of lattice contraction potentially due to a nonuniform distribution of cesium cations.…”

Far Infrared Synchrotron Near-Field Nanoimaging and Nanospectroscopy

“…Near-field nanoscopy attracts increasing scientific attention, specifically in the implementation of infrared scattering type scanning near-field optical microscopy (IR s -SNOM). − It provides for nanoimaging and nanospectroscopy down to a few nanometer length scales, gaining insight into molecular orientation and coupling, catalytic activity, heterogeneity in electron and lattice dynamics, , and plasmonic and polaritonic effects in quantum matter − with recent extension to the low temperature , and ultrafast regimes. − Despite these significant developments, s -SNOM has largely been limited to a narrow range of the electromagnetic spectrum of the near- to mid-IR at high frequencies, and the RF and low THz regime at low frequencies.…”