Thermal Stability of Gold/Palladium Octopods Studied in Situ in 3D: Understanding Design Rules for Thermally Stable Metal Nanoparticles

In Situ Solid-State Dewetting of Ag-Au-Pd Alloy: From Macro- to Nanoscale

Metal alloy nanostructures represent a promising platform for next-generation nanophotonic devices, surpassing the limitations of pure metals by offering additional "buttons" for tailoring their optical properties by compositional variations. While alloyed nanoparticles hold great potential, their scalability and underexplored optical behavior still limit their application. Here, we establish a systematic approach to quantifying the unique optical behavior of the AgAuPd ternary system while providing a direct comparison with its pure constituent metals. Computationally, we analyze their electronic structure and uncover the transition of Pd d states to Pd/Ag hybridized s states in the bulk form, explaining the similar optical properties observed between Pd and AgAuPd. Experimentally, we fabricate pure metal and fully alloyed nanoparticles through solid-state dewetting, a scalable method. During the process, we trace the optical transition in the systems from the initial thin film stage to the final nanoparticle stage with in situ ellipsometry. We reveal the interplay between optical properties influenced by chemical interdiffusion and localized surface plasmon resonance arising from morphological changes with ex situ surface characterizations. Additionally, we analytically implement a metallic layer derived from the ternary system in a trilayer device, resulting in a single-time and irreversible color filter, to demonstrate an application encompassing a lithography-free and cost-effective route for nanophotonic devices.

Programming Intracellular Clustering of Spiky Nanoparticles via Liposome Encapsulation

The intracellular clustering of anisotropic nanoparticles is crucial to the improvement of the localized surface plasmon resonance (LSPR) for phototherapy applications. Herein, we programmed the intracellular clustering process of spiky nanoparticles (SNPs) by encapsulating them into an anionic liposome via a frame-guided self-assembly approach. The liposome-encapsulated SNPs (lipo-SNPs) exhibited distinct and enhanced lysosome-triggered aggregation behavior while maintaining excellent monodispersity, even in acidic or protein-rich environments. We explored the enhancement of the photothermal therapy performance for SNPs as a proof of concept. The photothermal conversion efficiency of lipo-SNPs clusters significantly increased 15 times compared to that of single lipo-SNPs. Upon accumulation in lysosomes with a 2.4-fold increase in clustering, lipo-SNPs resulted in an increase in cell-killing efficiency to 45% from 12% at 24 μg/mL. These findings indicated that liposome encapsulation provides a promising approach to programing nanoparticle clustering at the target site, which facilitates advances in the development of smart nanomedicine with programmable enhancement in LSPR.

In Situ Single Particle Reconstruction Reveals 3D Evolution of PtNi Nanocatalysts During Heating

Tailoring nanoparticles' composition and morphology is of particular interest for improving their performance for catalysis. A challenge of this approach is that the nanoparticles' optimized initial structure often changes during use. Visualizing the three dimensional (3D) structural transformation in situ is therefore critical, but often prohibitively difficult experimentally. Although electron tomography provides opportunities for 3D imaging, restrictions in the tilt range of in situ holders together with electron dose considerations limit the possibilities for in situ electron tomography studies. Here, an in situ 3D imaging methodology is presented using single particle reconstruction (SPR) that allows 3D reconstruction of nanoparticles with controlled electron dose and without tilting the microscope stage. This in situ SPR methodology is employed to investigate the restructuring and elemental redistribution within a population of PtNi nanoparticles at elevated temperatures. The atomic structure of PtNi is further examined and a heat-induced transition is found from a disordered to an ordered phase. Changes in structure and elemental distribution are linked to a loss of catalytic activity in the oxygen reduction reaction. The in situ SPR methodology employed here can be extended to a wide range of in situ studies employing not only heating, but gaseous, aqueous, or electrochemical environments to reveal in-operando nanoparticle evolution in 3D.

Unexpectedly high thermal stability of Au nanotriangle@mSiO2 yolk-shell nanoparticles

The shape of Au nanoparticles (NPs) plays a crucial role for applications in, amongst others, catalysis, electronic devices, biomedicine, and sensing. Typically, the deformation of the morphology of Au NPs is the most significant cause of loss of functionality. Here, we systematically investigate the thermal stability of Au nanotriangles (NTs) coated with (mesoporous) silica shells with different morphologies (core-shell (CS): Au NT@mSiO2/yolk-shell (YS): Au NT@mSiO2) and compare these to 'bare' nanoparticles (Au NTs), by a combination of in situ and/or ex situ TEM techniques and spectroscopy methods. Au NTs with a mesoporous silica (mSiO2) coating were found to show much higher thermal stability than those without a mSiO2 coating, as the mSiO2 shell restricts the (self-)diffusion of surface atoms. For the Au NT@mSiO2 CS and YS NPs, a thicker mSiO2 shell provides better protection than uncoated Au NTs. Surprisingly, the Au NT@mSiO2 YS NPs were found to be as stable as Au NT@mSiO2 CS NPs with a core-shell morphology. We hypothesize that the only explanation for this unexpected finding was the thicker and higher density SiO2 shell of YS NPs that prevents diffusion of Au surface atoms to more thermodynamically favorable positions.

Practical Considerations for Simulating the Plasmonic Properties of Metal Nanoparticles

Simulating the plasmonic properties of colloidally derived metal nanoparticles with accuracy to their experimentally observed measurements is challenging due to the many structural and compositional parameters that influence their scattering and absorption properties. Correlation between single nanoparticle scattering measurements and simulated spectra emphasize these strong structural and compositional relationships, providing insight into the design of plasmonic nanoparticles. This Perspective builds from this history to highlight how the structural features of models used in simulation methods such as those based on the Finite-Difference Time-Domain (FDTD) method and Discrete Dipole Approximation (DDA) are of critical consideration for correlation with experiment and ultimately prediction of new nanoparticle properties. High-level characterizations such as electron tomography are discussed as ways to advance the accuracy of models used in such simulations, allowing the plasmonic properties of structurally complex nanoparticles to be better understood. However, we also note that the field is far from bringing experiment and simulation into agreement for plasmonic nanoparticles with complex compositions, reflecting analytical challenges that inhibit accurate model generation. Potential directions for addressing these challenges are also presented.

Method for 3D atomic structure determination of multi-element nanoparticles with graphene liquid-cell TEM

Determining the 3D atomic structures of multi-element nanoparticles in their native liquid environment is crucial to understanding their physicochemical properties. Graphene liquid cell (GLC) TEM offers a platform to directly investigate nanoparticles in their solution phase. Moreover, exploiting high-resolution TEM images of single rotating nanoparticles in GLCs, 3D atomic structures of nanoparticles are reconstructed by a method called "Brownian one-particle reconstruction". We here introduce a 3D atomic structure determination method for multi-element nanoparticle systems. The method, which is based on low-pass filtration and initial 3D model generation customized for different types of multi-element systems, enables reconstruction of high-resolution 3D Coulomb density maps for ordered and disordered multi-element systems and classification of the heteroatom type. Using high-resolution image datasets obtained from TEM simulations of PbSe, CdSe, and FePt nanoparticles that are structurally relaxed with first-principles calculations in the graphene liquid cell, we show that the types and positions of the constituent atoms are precisely determined with root mean square displacement values less than 24 pm. Our study suggests that it is possible to investigate the 3D atomic structures of synthesized multi-element nanoparticles in liquid phase.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Asymmetric seed passivation for regioselective overgrowth and formation of plasmonic nanobowls

Plasmonic nanoparticles (NPs) have garnered excitement over the past several decades stemming from their unique optoelectronic properties, leading to their use in various sensing applications and theranostics. Symmetry dictates the properties of many nanomaterials, and nanostructures with low, but still defined symmetries, often display markedly different properties compared to their higher symmetry counterparts. While numerous methods are available to manipulate symmetry, surface protecting groups such as polymers are finding use due to their ability to achieve regioselective modification of NP seeds, which can be removed after overgrowth as shown here. Specifically, poly(styrene-b-polyacrylic acid) (PSPAA) is used to asymmetrically passivate cubic Au seeds through competition with hexadecyltrimethylammonium bromide (CTAB) ligands. The asymmetric passivation via collapsed PSPAA causes only select vertices and faces of the Au cubes to be available for deposition of new material (i.e., Au, Au-Ag alloy, and Au-Pd alloy) during seeded overgrowth. At low metal precursor concentrations, deposition follows observations from unpassivated seeds but with new material growing from only the exposed seed portions. At high metal precursor concentrations, nanobowl-like structures form from interaction between the depositing phase and the passivating PSPAA. Through experiment and simulation, the optoelectronic properties of these nanobowls were probed, finding that the interiors and exteriors of the nanobowls can be functionalized selectively as revealed by surface enhanced Raman spectroscopy (SERS).

High-Resolution Electron Tomography of Ultrathin Boerdijk-Coxeter-Bernal Nanowire Enabled by Superthin Metal Surface Coating

The rapid advancement of transmission electron microscopy has resulted in revolutions in a variety of fields, including physics, chemistry, and materials science. With single-atom resolution, 3D information of each atom in nanoparticles is revealed, while 4D electron tomography is shown to capture the atomic structural kinetics in metal nanoparticles after phase transformation. Quantitative measurements of physical and chemical properties such as chemical coordination, defects, dislocation, and local strain have been made. However, due to the incompatibility of high dose rate with other ultrathin morphologies, such as nanowires, atomic electron tomography has been primarily limited to quasi-spherical nanoparticles. Herein, the 3D atomic structure of a complex core-shell nanowire composed of an ultrathin Boerdijk-Coxeter-Bernal (BCB) core nanowire and a noble metal thin layer shell deposited on the BCB nanowire surface is discovered. Furthermore, it is demonstrated that a new superthin noble metal layer deposition on an ultrathin BCB nanowire could mitigate electron beam damage using an in situ transmission electron microscope and atomic resolution electron tomography. The colloidal coating method developed for electron tomography can be broadly applied to protect the ultrathin nanomaterials from electron beam damage, benefiting both the advanced material characterizations and enabling fundamental in situ mechanistic studies.

Multimode Electron Tomography Sheds Light on Synthesis, Structure, and Properties of Complex Metal-Based Nanoparticles

Electron tomography has become a cornerstone technique for the visualization of nanoparticle morphology in three dimensions. However, to obtain in-depth information about a nanoparticle beyond surface faceting and morphology, different electron microscopy signals must be combined. The most notable examples of these combined signals include annular dark-field scanning transmission electron microscopy (ADF-STEM) with different collection angles and the combination of ADF-STEM with energy-dispersive X-ray or electron energy loss spectroscopies. Here, the experimental and computational development of various multimode tomography techniques in connection to the fundamental materials science challenges that multimode tomography has been instrumental to overcoming are summarized. Although the techniques can be applied to a wide variety of compositions, the study is restricted to metal and metal oxide nanoparticles for the sake of simplicity. Current challenges and future directions of multimode tomography are additionally discussed.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

Thermal Activation of Gold Atom Diffusion in Au@Pt Nanorods

Understanding the thermal stability of bimetallic nanoparticles is of vital importance to preserve their functionalities during their use in a variety of applications. In contrast to well-studied bimetallic systems such as Au@Ag, heat-induced morphological and compositional changes in Au@Pt nanoparticles are insufficiently understood, even though Au@Pt is an important material for catalysis. To investigate the thermal instability of Au@Pt nanorods at temperatures below their bulk melting point, we combined in situ heating with two- and three-dimensional electron microscopy techniques, including three-dimensional energy-dispersive X-ray spectroscopy. The experimental results were used as input for molecular dynamics simulations, to unravel the mechanisms behind the morphological transformation of Au@Pt core-shell nanorods. We conclude that thermal stability is influenced not only by the degree of coverage of Pt on Au but also by structural details of the Pt shell.

Investigating Reaction Intermediates during the Seedless Growth of Gold Nanostars Using Electron Tomography

Good's buffers can act both as nucleating and shape-directing agents during the synthesis of anisotropic gold nanostars (AuNS). Although different Good's buffers can produce AuNS shapes with branches that are oriented along specific crystallographic directions, the mechanism is not fully understood. This paper reports how an analysis of the intermediate structures during AuNS synthesis from HEPES, EPPS, and MOPS Good's buffers can provide insight into the formation of seedless AuNS. Electron tomography of AuNS structures quenched at early times (minutes) was used to characterize the morphology of the incipient seeds, and later times were used to construct the growth maps. Through this approach, we identified how the crystallinity and shape of the first structures synthesized with different Good's buffers determine the final AuNS morphologies.

Five-second STEM dislocation tomography for 300 nm thick specimen assisted by deep-learning-based noise filtering

Scanning transmission electron microscopy (STEM) is suitable for visualizing the inside of a relatively thick specimen than the conventional transmission electron microscopy, whose resolution is limited by the chromatic aberration of image forming lenses, and thus, the STEM mode has been employed frequently for computed electron tomography based three-dimensional (3D) structural characterization and combined with analytical methods such as annular dark field imaging or spectroscopies. However, the image quality of STEM is severely suffered by noise or artifacts especially when rapid imaging, in the order of millisecond per frame or faster, is pursued. Here we demonstrate a deep-learning-assisted rapid STEM tomography, which visualizes 3D dislocation arrangement only within five-second acquisition of all the tilt-series images even in a 300 nm thick steel specimen. The developed method offers a new platform for various in situ or operando 3D microanalyses in which dealing with relatively thick specimens or covering media like liquid cells are required.

Robust Colloidal Synthesis of Palladium-Gold Alloy Nanoparticles for Hydrogen Sensing

Metal nanoparticles are currently used in a variety of applications, ranging from life sciences to nanoelectronic devices to gas sensors. In particular, the use of palladium nanoparticles is gaining increasing attention due to their ability to catalyze the rapid dissociation of hydrogen, which leads to an excellent response in hydrogen-sensing applications. However, current palladium-nanoparticle-based sensors are hindered by the presence of hysteresis upon hydride formation and decomposition, as this hysteresis limits sensor accuracy. Here, we present a robust colloidal synthesis for palladium-gold alloy nanoparticles and demonstrate their hysteresis-free response when used for hydrogen detection. The obtained colloidal particles, synthesized in an aqueous, room-temperature environment, can be tailored to a variety of applications through changing the size, ratio of metals, and surface stabilization. In particular, the variation of the viscosity of the mixture during synthesis resulted in a highly tunable size distribution and contributed to a significant improvement in size dispersity compared to the state-of-the-art methods.

The Influence of Size, Shape, and Twin Boundaries on Heat-Induced Alloying in Individual Au@Ag Core-Shell Nanoparticles

Environmental conditions during real-world application of bimetallic core-shell nanoparticles (NPs) often include the use of elevated temperatures, which are known to cause elemental redistribution, in turn significantly altering the properties of these nanomaterials. Therefore, a thorough understanding of such processes is of great importance. The recently developed combination of fast electron tomography with in situ heating holders is a powerful approach to investigate heat-induced processes at the single NP level, with high spatial resolution in 3D. In combination with 3D finite-difference diffusion simulations, this method can be used to disclose the influence of various NP parameters on the diffusion dynamics in Au@Ag core-shell systems. A detailed study of the influence of heating on atomic diffusion and alloying for Au@Ag NPs with varying core morphology and crystallographic details is carried out. Whereas the core shape and aspect ratio of the NPs play a minor role, twin boundaries are found to have a strong influence on the elemental diffusion.

A Perspective on Modelling Metallic Magnetic Nanoparticles in Biomedicine: From Monometals to Nanoalloys and Ligand-Protected Particles

The focus of this review is on the physical and magnetic properties that are related to the efficiency of monometallic magnetic nanoparticles used in biomedical applications, such as magnetic resonance imaging (MRI) or magnetic nanoparticle hyperthermia, and how to model these by theoretical methods, where the discussion is based on the example of cobalt nanoparticles. Different simulation systems (cluster, extended slab, and nanoparticle models) are critically appraised for their efficacy in the determination of reactivity, magnetic behaviour, and ligand-induced modifications of relevant properties. Simulations of the effects of nanoscale alloying with other metallic phases are also briefly reviewed.

Understanding and Controlling the Crystallization Process in Reconfigurable Plasmonic Superlattices

The crystallization of nanomaterials is a primary source of solid-state, photonic structures. Thus, a detailed understanding of this process is of paramount importance for the successful application of photonic nanomaterials in emerging optoelectronic technologies. While colloidal crystallization has been thoroughly studied, for example, with advanced in situ electron microscopy methods, the noncolloidal crystallization (freezing) of nanoparticles (NPs) remains so far unexplored. To fill this gap, in this work, we present proof-of-principle experiments decoding a crystallization of reconfigurable assemblies of NPs at a solid state. The chosen material corresponds to an excellent testing bed, as it enables both in situ and ex situ investigation using X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atomic force microscopy (AFM), and optical spectroscopy in visible and ultraviolet range (UV-vis) techniques. In particular, ensemble measurements with small-angle XRD highlighted the dependence of the correlation length in the NPs assemblies on the number of heating/cooling cycles and the rate of cooling. Ex situ TEM imaging further supported these results by revealing a dependence of domain size and structure on the sample preparation route and by showing we can control the domain size over 2 orders of magnitude. The application of HAADF-STEM tomography, combined with in situ thermal control, provided three-dimensional single-particle level information on the positional order evolution within assemblies. This combination of real and reciprocal space provides insightful information on the anisotropic, reversibly reconfigurable assemblies of NPs. TEM measurements also highlighted the importance of interfaces in the polydomain structure of nanoparticle solids, allowing us to understand experimentally observed differences in UV-vis extinction spectra of the differently prepared crystallites. Overall, the obtained results show that the combination of in situ heating HAADF-STEM tomography with XRD and ex situ TEM techniques is a powerful approach to study nanoparticle freezing processes and to reveal the crucial impact of disorder in the solid-state aggregates of NPs on their plasmonic properties.

Controllable Synthesis of Bimetallic Nanostructures Using Biogenic Reagents: A Green Perspective

Bimetallic nanostructures are emerging as a significant class of metal nanomaterials due to their exceptional properties that are useful in various areas of science and technology. When used for catalysis and sensing applications, bimetallic nanostructures have been noted to exhibit better performance relative to their monometallic counterparts owing to synergistic effects. Furthermore, their dual metal composition and configuration can be modulated to achieve optimal activity for the desired functions. However, as with other nanostructured metals, bimetallic nanostructures are usually prepared through wet chemical routes that involve the use of harsh reducing agents and hazardous stabilizing agents. In response to intensifying concerns over the toxicity of chemicals used in nanomaterial synthesis, the scientific community has increasingly turned its attention toward environmentally and biologically compatible reagents that can enable green and sustainable nanofabrication processes. This article aims to provide an evaluation of the green synthetic methods of constructing bimetallic nanostructures, with emphasis on the use of biogenic resources (e.g., plant extracts, DNA, proteins) as safe and practical reagents. Special attention is devoted to biogenic synthetic protocols that demonstrate controllable nanoscale features, such as size, composition, morphology, and configuration. The potential use of these biogenically prepared bimetallic nanostructures as catalysts and sensors is also discussed. It is hoped that this article will serve as a valuable reference on bimetallic nanostructures and will help fuel new ideas for the development of more eco-friendly strategies for the controllable synthesis of various types of nanostructured bimetallic systems.

Three-Dimensional Nanoparticle Transformations Captured by an Electron Microscope

ConspectusThree-dimensional (3D) morphology and composition govern the properties of nanoparticles (NPs). However, due to their high surface-to-volume ratio, the morphology and composition of nanomaterials are not as static as those for their bulk counterparts. One major influence is the increase in relative contribution of surface diffusion, which underlines rapid reshaping of NPs in response to changes in their environment. If not accounted for, these effects might affect the robustness of prospective NPs in practically relevant conditions, such as elevated temperatures, intense light illumination, or changing chemical environments. In situ techniques are promising tools to study NP transformations under relevant conditions. Among those tools, in situ transmission electron microscopy (TEM) provides an elegant platform to directly visualize NP changes down to the atomic scale. By the use of specialized holders or microscopes, external stimuli, such as heat, or environments, such as gas and liquids, can be controllably introduced inside the TEM. In addition, TEM is also a valuable tool to determine NP transformations upon ex situ stimuli such as laser excitation. However, standard TEM yields two-dimensional (2D) projection images of 3D objects. With the growing complexity of NP shapes and compositions, the information that is obtained in this manner is often insufficient to understand intricate diffusion dynamics.In this Account, we describe recent progress on measuring NP transformations in 3D inside the electron microscope. First, we discuss existing possibilities to obtain 3D information using either tomographic methods or the so-called atom counting technique, which utilizes single projection images. Next, we show how these techniques can be combined with in situ holders to quantify diffusion processes on a single nanoparticle level. Specifically, we focus on anisotropic metal NPs at elevated temperatures and in varying gas environments. Anisotropic metal NPs are important for plasmonic applications, because sharp tips and edges result in strong electromagnetic field enhancements. By electron tomography, surface diffusion as well as elemental diffusion can be tracked in monometallic and bimetallic NPs, which can then be directly related to changes in plasmonic properties of these systems. By atom counting, it has furthermore become possible to monitor the evolution of crystalline facets of metal NPs under gas and heat treatments, a change that influences catalytic properties. Next to in situ processes, we also demonstrate the value of electron tomography to assess external laser-induced NP transformations, making it viable to detect structural changes with atomic resolution. The application of the proposed methodologies is by far not limited to metal nanoparticles. In the final section, we therefore outline future material research that can benefit from tracking NP transformations from 3D techniques.

Effectiveness of reducing the influence of CTAB at the surface of metal nanoparticles during in situ heating studies by TEM

In situ TEM is a valuable technique to offer novel insights in the behavior of nanomaterials under various conditions. However, interpretation of in situ experiments is not straightforward since the electron beam can impact the outcome of such measurements. For example, ligands surrounding metal nanoparticles transform into a protective carbon layer upon electron beam irradiation and may impact the apparent thermal stability during in situ heating experiments. In this work, we explore the effect of different treatments typically proposed to remove such ligands. We found that plasma treatment prior to heating experiments for Au nanorods and nanostars increased the apparent thermal stability of the nanoparticles, while an activated carbon treatment resulted in a decrease of the observed thermal stability. Treatment with HCl barely changed the experimental outcome. These results demonstrate the importance of carefully selecting pre-treatments procedures during in situ heating experiments.

Effect of Pd Ions on the Generation of Ag and Au Heterogeneous Nanoparticles Using Laser Ablation in Liquid

Heterogeneous Ag/Au nanoparticles combined with Pd ions were generated by irradiating Ag/Au metal targets in a Pd solution with nanosecond and femtosecond lasers. AgPd and AuPd nanoparticles were generated by laser fragmentation and bonded. We numerically analyzed the hot spots with electromagnetic field enhancement of nanoparticles of different sizes separated by various distances. AgPd and AuPd nanoparticles differing in diameter were generated and showed different characteristics compared to typical core-shell heterogeneous nanoparticles. Pd ions played an important role in the generation of nanoparticles in liquid via laser ablation. The femtosecond laser produced both pure and heterogeneous nanoparticles of uniform size. The nanosecond laser produced pure nanoparticles with a relatively non-uniform size, which developed into spherical heterogeneous nanoparticles with a uniform (small) size in the presence of Pd ions. These nanoparticles could optimize applications such as photothermal therapy and catalysis.

Galvanic replacement of intermetallic nanocrystals as a route toward complex heterostructures

Galvanic replacement reactions are a reliable method for transforming monometallic nanotemplates into bimetallic products with complex nanoscale architectures. When replacing bimetallic nanotemplates, even more complex multimetallic products can be made, with final nanocrystal shapes and architectures depending on multiple processes, including Ostwald ripening and the Kirkendall effect. Galvanic replacement, therefore, is a promising tool in increasing the architectural complexity of multimetallic templates, especially if we can identify and control the relevant processes in a given system and apply them more broadly. Here, we study the transformation of intermetallic PdCu nanoparticles in the presence of HAuCl₄ and H₂PtCl₆, both of which are capable of oxidizing both Pd and Cu. Replacement products consistently lost Cu more quickly than Pd, preserved the crystal structure of the original intermetallic template, and grew a new phase on the sacrificial template. In this way, atomic and nanometer-scale architectures are integrated within individual nanocrystals. Product morphologies included faceting of the original spherical particles as well as formation of core@shell and Janus-style particles. These variations are rationalized in terms of differing diffusion behaviors. Overall, galvanic replacement of multimetallic templates is shown to be a route toward increasingly exotic particle architectures with control exerted on both Angstrom and nanometer-scale features, while inviting further consideration of template and oxidant choices.

Fast versus conventional HAADF-STEM tomography of nanoparticles: advantages and challenges

HAADF-STEM tomography is a widely used experimental technique for analyzing nanometer-scale structures of a large variety of materials in three dimensions. It is especially useful for studying crystalline nanoparticles, where conventional TEM tomography suffers from diffraction-related artefacts. Unfortunately, the acquisition of a HAADF-STEM tilt series can easily take up one hour or more, depending on the complexity of the experiment. It is therefore challenging to investigate samples that do not withstand long electron beam illumination or to acquire a large number of tilt series during a single TEM experiment. The latter would facilitate obtaining more statistically representative 3D data, and enable performing dynamic in situ 3D characterizations with a finer time resolution. Various HAADF-STEM acquisition strategies have been proposed to accelerate the tomographic acquisition and reduce the required electron dose. These methods include tilting the holder continuously while acquiring a projection "movie" and a hybrid, incremental, methodology which combines the benefits of the conventional and continuous technique. However, until now an experimental evaluation of these techniques has been lacking. In this paper, the different acquisition strategies will be experimentally compared in terms of speed, resolution and electron dose. This evaluation will be performed based on experimental tilt series, acquired for various metallic nanoparticles with different shapes and sizes. We discuss the necessary data processing and provide a general guideline that can be used to determine the most optimal acquisition strategy for specific electron tomography experiments.

Kinetically Controlled Sequential Seeded Growth: A General Route to Crystals with Different Hierarchies

The organization of natural materials into hierarchical structures accounts for the amazing properties of many biological systems; however, translating the structural motifs present in such natural materials to synthetic systems remains difficult. Inspired by how nature creates materials, this work demonstrates that kinetically controlled sequential seeded growth is a general bottom-up strategy to prepare hierarchical inorganic crystals with distinct compositions and nanostructured forms. Specifically, 85 distinct hierarchical crystals with different shape-controlled features, compositions, and overall symmetries were readily achieved by altering the kinetics of metal deposition in sequential rounds of seeded growth. These modifications in the deposition kinetics were achieved through simple changes to the reaction conditions (e.g., pH or halide concentration) and dictate whether concave or convex features are produced at specific seed locations, much in the manner that the changing atmospheric conditions account for the hierarchical and symmetrical structures of snow crystals. As such, this work provides a general paradigm for the bottom-up synthesis of hierarchical crystals regardless of inorganic material class.

Formation of Hollow Gold Nanocrystals by Nanosecond Laser Irradiation

The irradiation of spherical gold nanoparticles (AuNPs) with nanosecond laser pulses induces shape transformations yielding nanocrystals with an inner cavity. The concentration of the stabilizing surfactant, the use of moderate pulse fluences, and the size of the irradiated AuNPs determine the efficiency of the process and the nature of the void. Hollow nanocrystals are obtained when molecules from the surrounding medium (e.g., water and organic matter derived from the surfactant) are trapped during laser pulse irradiation. These experimental observations suggest the existence of a subtle balance between the heating and cooling processes experienced by the nanocrystals, which induce their expansion and subsequent recrystallization keeping exogenous matter inside. The described approach provides valuable insight into the mechanism of interaction of a pulsed nanosecond laser with AuNPs, along with interesting prospects for the development of hollow plasmonic nanoparticles with potential applications related to gas and liquid storage at the nanoscale.

Heat generation by branched Au/Pd nanocrystals: influence of morphology and composition

Bimetallic gold-palladium particles were originally proposed as catalysts with tunable reaction rates. Following the development of synthesis routes that offer better control on the morphology and composition of the particles, novel optical sensing functionalities were more recently proposed. Since temperature is a fundamental parameter that interplays with every other proposed application, we studied the light-to-heat conversion ability of Au/Pd bimetallic nanoparticles with a regular octapodal shape. Both compositional (Au-to-Pd ratio) and structural (diagonal tip-to-tip distance and tip width) characteristics were screened and found to be essential control parameters to promote light absorption and efficient conversion into heat. Electromagnetic simulations reveal that the Pd content, and specifically its distribution inside the branched particle geometry, has a profound impact on the optical properties and is an essential criterion for efficient heating. Notably, the optical and photothermal responses are shown to remain stable throughout extended illumination, with no noticeable structural changes to the branched nanocrystals due to heat generation.