It is conventionally assumed that the growth of monodisperse colloidal nanocrystals requires a temporally discrete nucleation followed by monomer attachment onto the existing nuclei. However, recent studies have reported violations of this classical growth model, and have suggested that inter-particle interactions are also involved during the growth. Mechanisms of nanocrystal growth still remain controversial. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution onto the existing particles or by coalescence between the particles. Surprisingly, an initially broad size distribution of the nanocrystals can spontaneously narrow. We suggest that nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies. These observations are expected to be highly relevant for other nanocrystal systems.

Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures.

Plasmonic nanomaterials coupled with catalytically active surfaces can provide unique opportunities for various catalysis applications, where surface plasmons produced upon proper light excitation can be adopted to drive and/or facilitate various chemical reactions. A brief introduction to the localized surface plasmon resonance and recent design and fabrication of highly efficient plasmonic nanostructures, including plasmonic metal nanostructures and metal/semiconductor heterostructures is given. Taking advantage of these plasmonic nanostructures, the following highlights summarize recent advances in plasmon-driven photochemical reactions (coupling reactions, O2 dissociation and oxidation reactions, H2 dissociation and hydrogenation reactions, N2 fixation and NH3 decomposition, and CO2 reduction) and plasmon-enhanced electrocatalytic reactions (hydrogen evolution reaction, oxygen reduction reaction, oxygen evolution reaction, alcohol oxidation reaction, and CO2 reduction). Theoretical and experimental approaches for understanding the underlying mechanism of surface plasmon are discussed. A proper discussion and perspective of the remaining challenges and future opportunities for plasmonic nanomaterials and plasmon-related chemistry in the field of energy conversion and storage is given in conclusion.

Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis.

Noble metal nanoparticles due to their unique optical properties arising from their interactions with an incident light have been intensively employed in a broad range of applications. This review comprehensively describes fundamentals behind plasmonics, used to develop applications in the fields of biomedical, energy, and information technologies. Basic concepts (electromagnetic interaction and permittivity of metals) are discussed through Mie theory presented as the main model for interpreting phenomena of optical absorption and scattering. The effects of near-field enhancement, shape, composition, and surrounding medium of nanoparticles on optical properties are described in detail. The review explores and identifies the potential of plasmonic nanoparticles based on their optical properties (e.g., light absorption, scattering, and field enhancement) for developing different applications (biomedical, energy and information technologies). Due to a significant impact of plasmonic nanoparticles on medicine and healthcare products and technologies, the review initially focuses on biomedical applications extensively benefited from optical features of these nanoparticles. Advantages of the optical properties outstandingly implemented are also briefly discussed in other applications, including energy and information technologies. This review concisely summarizes the explored areas based on plasmonic properties, compares advantages of plasmonic nanoparticles over other types of nanomaterials and highlights challenges.

Optical Properties and Applications of Plasmonic-Metal Nanoparticles

Local hot charge density regulation: Vibration-free pyroelectric nanogenerator for effectively enhancing catalysis and in-situ surface enhanced Raman scattering monitoring

Surface-enhanced Raman spectroscopy (SERS) is advantageous over fluorescence for bioimaging due to ultra-narrow linewidth of the fingerprint spectrum and weak photo-bleaching effect. However, the existing SERS imaging speed lags far behind practical needs, mainly limited by Raman signals of SERS nanoprobes. In this work, we report ultrabright gap-enhanced Raman tags (GERTs) with strong electromagnetic hot spots from interior sub-nanometer gaps and external petal-like shell structures, larger immobilization surface area, and Raman cross section of reporter molecules. These GERTs reach a Raman enhancement factor beyond 5 × 109 and a detection sensitivity down to a single-nanoparticle level. We use a 370 μW laser to realize high-resolution cell imaging within 6 s and high-contrast (a signal-to-background ratio of 80) wide-area (3.2 × 2.8 cm2) sentinel lymph node imaging within 52 s. These nanoprobes offer a potential solution to overcome the current bottleneck in the field of SERS-based bioimaging. The speed of surface-enhanced Raman spectroscopy (SERS) imaging is generally limited due to low Raman signals. Here, the authors develop bright gap-enhanced Raman tags with external hot spots and demonstrate their use in fast near-infrared bioimaging.

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Ultrabright gap-enhanced Raman tags for high-speed bioimaging.

Collective oscillation of quasi-free electrons on the surface of metallic plasmonic nanoparticles (NPs) in the ultraviolet to near-infrared (NIR) region induces a strong electromagnetic enhancement around the NPs, which leads to numerous important applications. These interesting far- and near-field optical characteristics of the plasmonic NPs can be typically obtained from numerical simulations for theoretical guidance of NP design. However, traditional numerical simulations encounter irreconcilable conflicts between the accuracy and speed due to the high demand of computing power. In this work, we utilized the machine learning method, specifically the deep neural network (DNN), to establish mapping between the far-field spectra/near-field distribution and dimensional parameters of three types of plasmonic NPs including nanospheres, nanorods, and dimers. After the training process, both the forward prediction of far-field optical properties and the inverse prediction of on-demand dimensional parameters of NPs can be accomplished accurately and efficiently with the DNN. More importantly, we have achieved for the first time ultrafast and accurate prediction of two-dimensional on-resonance electromagnetic enhancement distributions around NPs by greatly reducing the amount of electromagnetic data via screening and resampling methods. These near-field predictions can be realized typically in less than 10-2 seconds on a laptop, which is 6 orders faster than typical numerical simulations implemented on a server. Therefore, we demonstrate that the DNN is an ultrafast, highly efficient, and computing resource-saving tool to investigate the far- and near-field optical properties of plasmonic NPs, especially for a number of important nano-optical applications such as surface-enhanced Raman spectroscopy, photocatalysis, solar cells, and metamaterials.

https://pubs.rsc.org/en/content/articlepdf/2019/NR/C9NR03450A

Plasmonic nanoparticle simulations and inverse design using machine learning

Oxygen reduction reaction (ORR) is of paramount importance in polymer electrolyte membrane fuel cells due to its sluggish kinetics. In this work, a plasmon-induced hot electrons enhancement method is introduced to enhance ORR property of the silver (Ag)-based electrocatalysts. Three types of Ag nanostructures with differently localized surface plasmon resonances have been used as electrocatalysts. The thermal effect of plasmonic-enhanced ORR can be minimized in our work by using graphene as the support of Ag nanoparticles. By tuning the resonance positions and laser power, the enhancement of ORR properties of Ag catalysts has been optimized. Among these catalysts, Ag nanotriangles after excitation show the highest mass activity and reach 0.086 mA/μgAg at 0.8 V, which is almost 17 times that of a commercial Pt/C catalyst after the price is accounted. Our results demonstrate that the hot electrons generated from surface plasmon resonance can be utilized for electrochemical reaction, and tuning the resonance positions by light is a promising and viable approach to boost electrochemical reactions.

Jing He

Papers

Ultrabright gap-enhanced Raman tags for high-speed bioimaging.

Plasmonic nanoparticle simulations and inverse design using machine learning

Plasmonic-Enhanced Oxygen Reduction Reaction of Silver/Graphene Electrocatalysts

Surface‐Enhanced Raman Nanoprobes with Embedded Standards for Quantitative Cholesterol Detection

Shape Transformation Mechanism of Gallium–Indium Alloyed Liquid Metal Nanoparticles