Wassie M. Takele | Searcharxiv

arXiv2026

Liquid-Phase Near-Field Infrared Nanoscopy of Ion-Exchange Reactions

In situ monitoring of reactions with chemical specificity and nanometer-scale spatial resolution under aqueous conditions remains a long-standing experimental challenge. Here, we use thin SiC membrane-enabled liquid-phase infrared nanoimaging and nanospectroscopy to monitor Ca$^{2+}$/Na$^{+}$ ion exchange in a mixture of disodium terephthalate and CaCl$_2$ solution. Beneath the membrane, we follow the formation of calcium terephthalate (CaTP), a metal-organic framework (MOF)-type coordination phase in which terephthalate ligands link Ca$^{2+}$ centers. The symmetric and asymmetric carboxylate stretching modes serve as chemically specific near-field reporters of Ca$^{2+}$ coordination to the terephthalate linkers. Nano-FTIR spectra reveal pronounced peak shifts and linewidth changes in these vibrational modes, indicating modification of the carboxylate coordination environment during CaTP formation. Complementary near-field nanoimaging resolves the emergence of nanoscale precipitates beneath the SiC membrane following CaCl$_2$ injection, providing spatially resolved evidence of the formation of the CaTP coordination framework. Solid-phase nanoimaging and nanospectroscopy identify well-defined CaTP particles, providing further evidence for crystalline CaTP formation. We demonstrate that the chemistry of metal-ligand interactions can be monitored in situ via liquid-phase near-field nanoscopy. This capability could enable the study of catalytic, electrochemical, and biological processes under native aqueous conditions.

cond-mat.soft↗

arXiv2026

Distinguishing Hot-Electron and Optomechanical Pathways at Metal-Molecule Interfaces

Energy and charge transfer between molecules and metal surfaces underpin heterogeneous catalysis, surface-enhanced spectroscopies and plasmon-driven chemistry, yet the microscopic origins of vibrational excitation at metal interfaces remain unresolved. Here we use temperature-dependent surface-enhanced Raman scattering (SERS) to directly distinguish plasmon-vibration optomechanical coupling from hot-electron-driven excitation.By probing thionine adsorbed on gold nanostructures at 295 K and 3.5 K, we show that pronounced anti-Stokes scattering at cryogenic temperature arises from optical pumping of vibrational populations, whereas room-temperature spectra are governed by thermal population. Bromide co-adsorbates play a decisive role by guiding molecular alignment, inducing surface atom displacements, and enabling transient adsorption geometries that activate otherwise Raman-inactive vibrational modes. In the absence of bromide, distinct excitation pathways emerge, reflecting competition between optomechanical coupling and charge-transfer processes associated with molecular polarization along the optical field or orientation relative to the metal surface. These results establish molecular optomechanics as a sensitive probe of surface-molecule interactions and demonstrate how anion-mediated surface dynamics regulate energy flow at plasmonic interfaces.

cond-mat.mes-hall↗