Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities

Abstract

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

Keywords: SERS; atomic hopping; crystal facet; nano-optics; nanocavity; picocavity; single-molecule.

Figure 1

Atomic hopping and nanoparticle reconstruction. (a) Schematic of NPoM system illustrating atom hopping gradually growing the facet width under intense light illumination. (b) Primary crystallographic facets of globular nanoparticles. (c) TEM of 80 nm diameter NP. (d) Schematic showing NCoM sustains its shape under illumination due to large energy barriers at edges. (e, f) As for (b, c) for 75 nm NCoM. Molecular layer is biphenyl-4-thiol (BPT) that creates nanogaps of 1.3 ± 0.1 nm.

Figure 2

Nanostructure resilience to laser illumination. (a, b) Scattering spectra of (a) NPoM and (b) NCoM, measured before (orange) and after 30 s (blue) of continuous laser illumination. (c) SERS time scans for NPoM, showing unstable backgrounds and appearance of new vibrational lines. (d) Variation in SERS intensity of 1580 cm^–1 vibrational mode (orange) and background (purple dashed). (e, f) As (c, d) for NCoM, showing no new vibrational lines or changes in the background. Pump laser is 633 nm (red arrows in a,b) with intensity 200 μW/μm² in all cases.

Figure 3

Nanoparticles shape reconstruction seen via dark-field scattering and SERS. (a, c) Shift in spectral position (Δλ) of plasmon resonances recorded before (red) and after (blue) 30 s of continuous laser illumination. Outline of box indicates 85% deviation from mean value (bold horizontal line). (b, d) Variation of SERS intensity for NPoM and NCoM nanocavities, fit to Gaussian distribution. Irradiation intensity is 200 μW/μm².

Figure 4

Plasmon resonance shifts of NCoMs with laser intensity. (a) Dark-field scattering spectra with increasing laser intensity. (b, c) Plasmon resonance shifts of two NCoMs for (10) and (20) modes vs laser intensity. (d) Intensity thresholds extracted from sigmoidal fits (box outlines 85% closest to mean).

Figure 5

Facet stability effects on vibrational spectroscopy of molecular monolayers. (a) SERS time scan over 20 s for NPoM showing creation of new and previously dark vibrational modes due to multiple picocavity events. (b) SERS time scan over 600 s for NCoM revealing high stability of nanocavity modes. (c) Averaged SERS for NCoM constructs (orange) and calculated DFT (blue) spectra for MPA attached to Au surface and a monodendate O–Au bond with nanoconstruct facet.