Controlling and Understanding Atomic-Scale Dynamics within Plasmonic Nanogaps
Plasmonic nanogaps, such as the ∼1 nm gaps between two gold surfaces in a nanoparticle-on-mirror (NPoM) structure, are able to confine light to nanoscale volumes and enhance optical fields by orders of magnitude. This enhancement allows a few hundred molecules placed in this gap, such as biphenyl-4-thiol, to be probed by techniques like surface-enhanced Raman spectroscopy (SERS). In addition, the enhanced fields induce a range of atomic-scale dynamics in the gap, resulting in blinking emission phenomena such as picocavities and plasmonic flares. Picocavities are fleeting narrow-band spectral lines attributed to transiently forming metallic adatoms that confine light to a <1 nm3 volume, inducing stronger SERS from a nearby single molecule. Flares are flashes of broadband emission explained by patches of the metal surface peeling off and enhancing metal emission. In this thesis, optical control over formation and stability of picocavity adatoms is first explored. Such control is achieved by pulses of visible light on an NPoM, where they rapidly write an adatom in the nanogap. The adatoms are shown to be storable in ambient conditions for up to a week in the dark and are read using low-intensity SERS. Writing at higher optical intensities stabilises the atomic protrusion through light-induced local restructuring. Fluctuations in the picocavity SERS spectra show that light also accelerates the energy landscape exploration by the adatom. Optical control over single metal atom dynamics opens promising avenues for next-generation microelectronics and catalysis. Next, the nanogap blinking is studied at 10 μs time resolution to expand the mechanistic understanding of the associated atomic-scale dynamics. To maximise the detection of these fast fluctuations, a custom avalanche photodiode (APD) device is built, capable of 10 μs resolution with negligible read noise contribution. Furthermore, superefficient plasmonic nanoarchitectures for Raman kinetics (SPARKs) are implemented and optimised to improve the blinking event intensities by orders of magnitude compared to NPoMs. Besides picocavities, SPARKs exhibit unique broadband blinking emission distinct from flares, which could result from cascaded formation of a "sea" of many adatoms. The fast blinking APD data for both picocavities and broadband events reveals complex state switching dynamics at all timescales. A custom analysis pipeline is implemented to extract discrete emissive states, featuring machine-learning-assisted denoising, edge detection, and custom background detection. Both metastable state and blinking event "on" durations are found to follow scale-free power laws, suggesting that the associated atomic-scale systems undergo many possible dynamics. The adatom "sea" attributed to broadband blinking seems to explore a landscape with exponentially distributed energy barriers, while for the picocavity adatoms, the barriers are distributed uniformly. Picocavity data also shows evidence of the adatom cycling through a loop of favourable states. The energy landscape insights gained in this work could serve as a base for atomistic modelling of the associated dynamics to elucidate concrete mechanisms. This mechanistic understanding can then be used to devise strategies to favour and control these atomic-scale dynamics for applications like microelectronics and catalysis.
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