Symmetry superposition studied by surface second-harmonic generation

Anisotropic second-harmonic generation from monocrystalline gold flakes

Noble metals with well-defined crystallographic orientation constitute an appealing class of materials for controlling light-matter interactions on the nanoscale. Nonlinear optical processes, being particularly sensitive to anisotropy, are a natural and versatile probe of crystallinity in nano-optical devices. Here we study the nonlinear optical response of monocrystalline gold flakes, revealing a polarization dependence in second-harmonic generation from the {111} surface that is markedly absent in polycrystalline films. Our findings confirm that second-harmonic microscopy is a robust and non-destructive method for probing the crystallographic orientation of gold, and can serve as a guideline for enhancing nonlinear response in plasmonic systems.

Mapping Electrochemical Heterogeneity at Gold Surfaces: A Second Harmonic Imaging Study

Designing efficient catalysts requires correlating surface structure and local chemical composition with reactivity on length scales from nanometers to tens of microns. While much work has been done on this structure/function correlation on single crystals, comparatively little has been done for catalysts of relevance in applications. Such materials are typically highly heterogeneous and thus require methods that allow mapping of the structure/function relationship during electrochemical conversion. Here, we use optical second harmonic imaging combined with cyclic voltammetry to map the surface of gold nanocrystalline and polycrystalline electrodes during electrooxidation and to quantify the spatial extent of surface reconstruction during potential cycling. The wide-field configuration of our microscope allows for real-time imaging of an area ∼100 μm in diameter with submicron resolution. By analyzing the voltage dependence of each pixel, we uncover the heterogeneity of the second harmonic signal and quantify the fraction of domains where it follows a positive quadratic dependence with increasing bias. There, the second harmonic intensity is mainly ascribed to electronic polarization contributions at the metal/electrolyte interface. Additionally, we locate areas where the second harmonic signal follows a negative quadratic dependence with increasing bias, which also show the largest changes during successive cyclic voltammetry sweeps as determined by an additional correlation coefficient analysis. We assign these areas to domains of higher roughness that are prone to potential-induced surface restructuring and where anion adsorption occurs at lower potentials than expected based on the cyclic voltammetry.

Effect of local environment on nanoscale dynamics at electrochemical interfaces: anisotropic growth and dissolution in the presence of a step providing evidence for a schwoebel-ehrlich barrier at solid/liquid interfaces

The dynamics of nanoscale islands in the vicinity of a monoatomic step on a metal electrode surface under potential control have been investigated by potential pulse perturbation time-resolved scanning tunneling microscopy (P3 TR-STM). As the metastable islands decay, a clear anisotropy develops in the spatial distribution of islands, relative to the monoatomic step. Islands appear to be stabilized above the step, while a denuded region develops below the step over a distance of about 100 nm. Vacancy islands are observed to be more stable than adatom islands. These results provide evidence for an electrochemical Schwoebel-Ehrlich barrier to diffusion of atoms across the step.