A fundamental theoretical understanding of SERS, and SERS hotspots, leads to new design principles for SERS substrates and new applications in nanomaterials and chemical analysis.

The solid-electrolyte interphase (SEI) plays crucial roles for the reversible operation of lithium metal batteries. However, fundamental understanding of the mechanisms of SEI formation and evolution is still limited. Herein, we develop a depth-sensitive plasmon-enhanced Raman spectroscopy (DS-PERS) method to enable in-situ and nondestructive characterization of the nanostructure and chemistry of SEI, based on synergistic enhancements of localized surface plasmons from nanostructured Cu, shell-isolated Au nanoparticles and Li deposits at different depths. We monitor the sequential formation of SEI in both ether-based and carbonate-based dual-salt electrolytes on a Cu current collector and then on freshly deposited Li, with dramatic chemical reconstruction. The molecular-level insights from the DS-PERS study unravel the profound influences of Li in modifying SEI formation and in turn the roles of SEI in regulating the Li-ion desolvation and the subsequent Li deposition at SEI-coupled interfaces. Last, we develop a cycling protocol that promotes a favorable direct SEI formation route, which significantly enhances the performance of anode-free Li metal batteries.

Sum-frequency generation (SFG) spectroscopy is a highly versatile tool for surface analysis. Improving the SFG intensity per molecule is important for observing low concentrations of surface species and intermediates in dynamic systems. Herein, Shell-Isolated-Nanoparticle-Enhanced SFG (SHINE-SFG) was used to probe a model substrate. The model substrate, $p$-mercaptobenzonitrile adsorbed on a Au film with SHINs deposited on top, provided an enhancement factor of up to ${10}^5$. Through wavelength- and polarization-dependent SHINE-SFG spectroscopy, the majority of the signal enhancement was found to come from both plasmon enhanced emission and chemical enhancement mechanisms. A new enhancement regime, i.e., the nonlinear coupling of SHINE-SFG with difference frequency generation, was also identified. This novel mechanism provides insight into the enhancement of nonlinear coherent spectroscopies and a possible strategy for the rational design of enhancing substrates utilizing coupling processes.

Interparticle spacing was controlled by evaporating water on 2D Au nanoparticles arrays.

After surface-enhanced Raman spectroscopy (SERS) was initiated over four decades ago, its practical application seems to be far behind the fundamental research that has made tremendous progress. SERS as a highly sensitive technique has not been widely adopted by the materials science and surface science communities or in the market of analytical instruments. In this discussion, we first classify the previous approaches along this direction over the past four decades and divide them into three strategies. Based on our recent theoretical and experimental approaches, we discuss in more detail the third strategy related to shell-isolated nanostructures. It can significantly expand the SERS study on nontraditional SERS-active (i.e. weakly SERS-active) materials (e.g. Pt, Ni, Fe, etc.) and even SERS-inactive materials (e.g. Si and Al₂O₃). We then focus on a single shell-isolated nanoparticle and how to controllably locate the strong electromagnetic field just at the probe surface of various materials. The use of side illumination at a high incident angle and/or nanocubes can further enhance the Raman signal by one to two orders of magnitude, which could be helpful for quantitative studies for surface science, heterogeneous catalysis, and soft matter science.