The study of acoustic vibrations in nanoparticles provides unique and unparalleled insight into their mechanical properties. Electron-beam lithography of nanostructures allows precise manipulation of their acoustic vibration frequencies through control of nanoscale morphology. However, the dissipation of acoustic vibrations in this important class of nanostructures has not yet been examined. Here we report, using single-particle ultrafast transient extinction spectroscopy, the intrinsic damping dynamics in lithographically fabricated plasmonic nanostructures. We find that in stark contrast to chemically synthesized, monocrystalline nanoparticles, acoustic energy dissipation in lithographically fabricated nanostructures is solely dominated by intrinsic damping. A quality factor of Q = 11.3 ± 2.5 is observed for all 147 nanostructures, regardless of size, geometry, frequency, surface adhesion, and mode. This result indicates that the complex Young's modulus of this material is independent of frequency with its imaginary component being approximately 11 times smaller than its real part. Substrate-mediated acoustic vibration damping is strongly suppressed, despite strong binding between the glass substrate and Au nanostructures. We anticipate that these results, characterizing the optomechanical properties of lithographically fabricated metal nanostructures, will help inform their design for applications such as photoacoustic imaging agents, high-frequency resonators, and ultrafast optical switches.

Controlling the morphology of noble-metal nanoparticles is mandatory to tune specific properties such as catalytic and optical behavior. Heterodimers consisting of two noble metals have been synthesized, so far mostly in aqueous media using selective surfactants or chemical etching strategies. We report a facile synthesis for Au@Pd and Pd@Au heterodimer nanoparticles (NPs) with morphologies ranging from segregated domains (heteroparticles) to core–shell structures by applying a seed-mediated growth process with Au and Pd seed nanoparticles in 1-octadecene (ODE), which is a high-boiling organic solvent. The as-synthesized oleylamine (OAm) functionalized Au NPs led to the formation of OAm-Au@Pd heteroparticles with a “windmill” morphology, having an Au core and Pd “blades”. The multiply twinned structure of the Au seed particles (⌀ ≈ 9–11 nm) is associated with a reduced barrier for heterogeneous nucleation. This leads to island growth of bimetallic Au@Pd heteroparticles with less-regular morphologies. The reaction process can be controlled by tuning the surface chemistry with organic ligands. Functionalization of Au NPs (Ø ≈ 9–11 nm) with 1-octadecanethiol (ODT) led to the formation of ODT-Au@Pd NPs with a closed Pd shell through a strong ligand–metal binding, which is accompanied by a redistribution of the electron density. Experiments with varied Pd content revealed surface epitaxial growth of Pd on Au. For OAm-Pd and ODT-Pd seed particles, faceted, Au-rich domain NPs and impeded core–shell NPs were obtained, respectively. This is related to the high surface energy of the small Pd seed particles (⌀ ≈ 5–7 nm). The metal distribution of all bimetallic NPs was analyzed by extended (aberration-corrected) transmission electron microscopy (HR-TEM, HAADF-STEM, EDX mapping, ED). The Au and Pd NPs, as well as the ODT-Au@Pd and OAm-Pd@Au heteroparticles, catalyze the reduction of 4-nitrophenol to 4-aminophenol with borohydride. The catalytic activity is dictated by the particle structure. OAm-Au@Pd heteroparticles with faceted Au domains had the highest activity because of a mixed Au–Pd surface structure, while ODT-Au@Pd NPs, where the active Au core is covered by a Pd shell, had the lowest activity.

Biological systems exhibit strikingly sophisticated properties, including adaptability, directed motion, regulation, and self-organization. Such systems are often described as being “nonequilibrium” or “out-of-equilibrium”, and it can be instructive to think of them as adopting thermodynamic states that require a constant supply of energy to maintain. Despite their ubiquity, systems that demonstrate these abilities require a remarkably stringent set of chemical requirements to exist. Broadly speaking, they must be (a) capable of consuming some external source of energy that (b) acts as a fuel to do some form of work, (c) all while maintaining highly organized structural features at the nanometer length scale that persist in space and over time. It remains a grand challenge in the field of chemistry to synthesize artificial systems capable of similarly complex nonequilibrium behavior both as a means for greater fundamental understanding and as a way to imbue non-natural structures with dynamic behavior for various applications. Yet an oft-overlooked challenge in this field involves not just the synthesis of nonequilibrium materials but also their characterization. The requirements for measuring nanometer-scale systems of nonequilibrium building blocks with the appropriate temporal and spatial resolution are demanding and have heretofore been largely unavailable to researchers. In this Perspective, we highlight challenges and recent advances in the measurement of dynamic nanoscale systems. We argue that progress in this area is crucial and must occur in parallel to synthetic goals if any meaningful understanding is to occur.

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