Acoustic vibrations in plasmonic nanoparticles, monitored by an all-optical means, have attracted significant increasing interest because they provide unique insight into the mechanical properties of these metallic nanostructures. Al nanostructures are a recently emerging alternative to noble metal nanoparticles, because their broad wavelength tunability and high natural abundance make them ideal for many potential applications. Here, we investigate the acoustic vibrations of individual Al nanocrystals using a combination of electron microscopy and single-particle transient extinction spectroscopy, made possible with a low-pulse energy, high sensitivity, and probe-wavelength-tunable, single-particle transient extinction microscope. For chemically synthesized, faceted Al nanocrystals, the observed vibration frequency scales with the inverse particle diameter. In contrast, triangularly shaped Al nanocrystals support two distinct frequencies, corresponding to their inand out-of-plane breathing modes. Unlike ensemble measurements, which measure average properties, measuring the damping time of the acoustic vibrations for individual particles enables us to investigate variations of the quality factor on the particle-to-particle level. Surprisingly, we find a large variation in quality factors even for nanocrystals of similar size and shape. This observed heterogeneity appears to result from substantially varying degrees of nanoparticle crystallinity even for chemically synthesized nanocrystals.

Finite-field EOM-CCSDT: a highly accurate method for the theoretical prediction of excitation energies and electronic spectra in strong magnetic fields.

Acoustic vibrations in plasmonic nanostructures provide deep insight into mechanical properties at the nanoscale for potential applications including optomechanical devices. Lithographic fabrication of plasmonic nanostructures allows precise control over shape and size as well as position. Here, we present a summary of our recent ultrafast studies of lithographically fabricated Au and Al nanostructures using single-particle transient extinction spectroscopy to measure the size- and shape-dependent acoustic frequencies and homogeneous damping times. Electron-beam lithography coupling with single-particle measurements necessitate the presence of a substrate, which we found to cause a blue shift in the acoustic vibration frequencies. This frequency shift enables the determination of the binding strength between Au nanostructures and the substrate. The substrate furthermore facilitates vibrational coupling between adjacent Au nanostructures. Electron-beam lithography also makes it possible to explore other plasmonic metals such as Al, which as the Earth’s most abundant metal creates a sustainable pathway toward applications. We compared the ultrafast relaxation dynamics and acoustic properties of Al nanodisks to similar Au nanostructures. For both Au and Al nanostructures, we found an acoustic vibration quality factor which we ascribed to internal defects in the polycrystalline nanostructures that dominate the energy dissipation pathway. These findings provide significant insight into the optomechanical properties of nanostructures fabricated by electron-beam lithography.

Lithographically prepared plasmonic nanoparticles are ideal mechanical probes, as their vibrational behavior can be precisely tuned through particle size and shape. But these particles exhibit strong intrinsic and extrinsic damping that results in small vibrational quality (Q) factors. Here, we perform single-particle transient transmission microscopy to investigate the effect of substrate-particle binding strength on the vibrational Q-factor of lithographically prepared gold nanodisks on glass. Weak and strong binding is realized through titanium adhesion layers of variable thickness. We find that strong binding leads to the generation of several new acoustic modes with varying Q-factors that depend on the particle aspect ratio and substrate material. Our work proposes an approach to tune enhanced acoustic Q-factors of lithographically prepared nanoparticles and offers a comprehensive description of their damping mechanism.

Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions.