Evaluation of Surface-Enhanced Raman Spectroscopy Substrates from Single-Molecule Statistics

Accurate quantification of substrate characteristics is a central pursuit within the field of surface-enhanced Raman spectroscopy (SERS). A theory based on single-molecule SERS (SM-SERS) statistics was developed for comprehensive substrate evaluation. This approach is applicable to general substrates possessing many hotspots and is capable of quantifying hotspot strength variation using a minimal set of fitting parameters. The model was validated for simulated substrates and then applied to the SM-SERS statistics of a roughened silver electrode, for which the degree of hotspot uniformity was quantified. The fitted model parameters provide important information concerning the structure–activity relationship of hotspots and can be used to directly compare SERS substrates. Overall, our results present an experimentally determinable parameter set that potentially improves upon the widely used “average enhancement factor” metric currently used for SERS substrate evaluation

Microscopic Dynamics of Li⁺ in Rutile TiO₂ Revealed by ⁸Li β‑Detected Nuclear Magnetic Resonance

We report measurements of the dynamics of isolated ⁸Li⁺ in single crystal rutile TiO₂ using β-detected nuclear magnetic resonance. From spin–lattice relaxation and motional narrowing, we find two sets of thermally activated dynamics: one below 100 K and one at higher temperatures. At low temperature, the activation barrier is 26.8(6) meV with prefactor 1.23(5) × 10¹⁰ s^–1. We suggest this is unrelated to Li⁺ motion and rather is a consequence of electron polarons in the vicinity of the implanted ⁸Li⁺ that are known to become mobile in this temperature range. Above 100 K, Li⁺ undergoes long-range diffusion as an isolated uncomplexed cation, characterized by an activation energy and prefactor of 0.32(2) eV and 1.0(5) × 10¹⁶ s^–1, respectively, in agreement with macroscopic diffusion measurements. These results in the dilute limit from a microscopic probe indicate that Li⁺ concentration does not limit the diffusivity even up to high concentrations but that some key ingredient is missing in the calculations of the migration barrier. The anomalous prefactors provide further insight into both Li⁺ and polaron motion