Hot Electron Emission Can Lead to Damping of Optomechanical Modes in Core–Shell Ag@TiO₂ Nanocubes

Abstract

Interactions between light and metal nanostructures are mediated by collective excitations of free electrons called surface plasmons, which depend primarily on geometry and dielectric environment. Excitation with ultrafast pulses can excite optomechanical modes that modulate the volume and shape of nanostructures at gigahertz frequencies. Plasmons serve as an optical handle to study the ultrafast electronic dynamics of nanoscale systems. We describe a method to synthesize core–shell Ag@TiO2 nanocubeswhile successfully maintaining the size and shape of the nanocube. Transient absorbance spectroscopy (TAS) is used to track photophysical processes on multiple time scales: from the ultrafast creation of hot carriers to their decay into phonons and the formation of optomechanical modes. Surprisingly, the TiO2 shell surrounding the Ag nanocubes caused no appreciable change in the frequency of the optomechanical mode, indicating that mechanical coupling between the core and shell is weak. However, the optomechanical mode was strongly attenuated by the TiO2 shell and TAS decay at ultrafast time scales (0–5 ps) was much faster. This observation suggests that up to ∼36% of the energy coupled into the plasmon resonance is being lost to the TiO2 as hot carriers instead of coupling to the optomechanical mode. Analysis of both ultrafast decay and characterization of optomechanical modes provides a dual accounting method to track energy dissipation in hybrid metal–semiconductor nanosystems for plasmon-enhanced solar energy conversion and chemical fuel generation.

In Situ Raman Probing of Hot‐Electron Transfer at Gold–Graphene Interfaces with Atomic Layer Accuracy

“…One promising way to improve the utilization efficiency of hot electrons is the development of novel nanocomposites by incorporating plasmonic metals with other functional materials, such as semiconductors, catalytic active metals, and two‐dimensional (2D) materials [29–31] . Among them, combing plasmonic metals with 2D materials has caused lots of concern, because of their unique optical and electronic properties, originating from the atomically thin thickness and 2D morphological features of 2D materials [32, 33] .…”

“…One promising way to improve the utilization efficiencyof hot electrons is the development of novel nanocomposites by incorporating plasmonic metals with other functional materials,s uch as semiconductors,c atalytic active metals,a nd twodimensional (2D) materials. [29][30][31] Among them, combing plasmonic metals with 2D materials has caused lots of concern, because of their unique optical and electronic properties,o riginating from the atomically thin thickness and 2D morphological features of 2D materials. [32,33] For example,the plasmonic metal-graphene hybrid nanostructure exhibits unique optical characteristics,r esulting from the coupling effect between plasmonic metal and graphene,which significantly increases the intensity of the interfacial electromagnetic field and changes its distribution around plasmonic metals.…”

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

“…125 As they point out, there is still no consensus regarding which mechanism is at play for the reported systems, and novel probing techniques are constantly recruited for this task. [324][325][326][327] One of the consequences of the plasmonic response is local heat generation through electron-phonon scattering, 318 which has made a distinct contribution to successful photocatalytic CO 2 methanation of Rh/TiO 2 , for example. 318 It has been suggested recently that the catalytic activity of the generated 'hotelectrons', i.e., non-thermal charge carriers in the metallic nanoparticles (which are responsible for the photocatalytic enhancement reported for Cu-Ru NPs 328 and others 312,315,317 ) disregards the local heating effect of the plasmonic structures, and that the measured enhanced photocatalytic effects should be ascribed, in lieu, to thermal effects (well-described using a simple Arrhenius model).…”

Effect of Ag Nanocube Optomechanical Modes on Plasmonic Surface Lattice Resonances

“…Transient absorption spectroscopy (TAS) has enabled researchers to better understand the dynamics of individual or isolated metal nanoparticles following the absorption of photons. − The ensuing cascade of photophysical processes underpins many of the applications of plasmonic nanostructures, but very little is known about the dynamics of highly ordered plasmonic arrays and how the ultrafast optical response of its nanoparticle constituents affect the hybrid long-range plasmonic–photonic modes of the array. At the individual nanoparticle level, the absorbed energy is transferred from the plasmon to electronic transitions and then to phonons; these phonons raise the lattice temperature and expand the lattice coordinate.…”

“…Figure illustrates the length scales of the LSPR and SLR modes ( x -axis), in addition to the time scales ( y -axis) of their decay processes, following the absorption of a photon and generation of optomechanical modes. The optomechanical modes of individual Ag nanocubes have been studied in detail. ,,, In brief, after a nanocube absorbs a photon, it expands and then evolves into a sum of its breathing eigenmodes. Ag nanocubes have two dominant breathing modes associated with its tips and faces. , The tip and face modes have different frequencies and modulate the LSPR over time .…”