Size-dependent melting of silica-encapsulated gold nanoparticles

Melting entropy of crystals determined by electron-beam-induced configurational disordering

Upon melting, the molecules in a crystal explore numerous configurations, reflecting an increase in disorder. The molar entropy of disorder can be defined by Boltzmann's formula ΔSd = Rln(Wd), where Wd is the increase in the number of microscopic states, so far inaccessible experimentally. We found that the Arrhenius frequency factor A of the electron diffraction signal decay provides Wd through an experimental equation A = AINTWd, where AINT is an inelastic scattering cross section. The method connects Clausius and Boltzmann experimentally and supplements the Clausius approach, being applicable to a femtogram quantity of thermally unstable and biomolecular crystals. The data also showed that crystal disordering and crystallization of melt are reciprocal, both governed by the entropy change but manifesting in opposite directions.

Self-Confined Dewetting Mechanism in Wafer-Scale Patterning of Gold Nanoparticle Arrays with Strong Surface Lattice Resonance for Plasmonic Sensing

A self-confined solid-state dewetting mechanism is reported that can fundamentally reduce the use of sophisticated nanofabrication techniques, enabling efficient wafer-scale patterning of non-closely packed (ncp) gold nanoparticle arrays. When combined with a soft lithography process, this approach can address the reproducibility challenges associated with colloidal crystal self-assembly, allowing for the batch fabrication of ncp gold arrays with consistent ordering and even optical properties. The resulting dewetted ncp gold nanoparticle arrays exhibit strong surface lattice resonance properties when excited in inhomogeneous environments under normal white-light incidence. With these SLR properties, the sensitive plasmonic sensing of molecular interactions is achieved using a simple transmission setup. This study will advance the development of miniaturized and portable devices.

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.

Electrochemical Analysis of the Thermal Stability of 0.9-4.1 nm Diameter Gold Nanoclusters

Here we report the thermal properties of weakly stabilized 0.9, 1.6, and 4.1 nm Au nanoparticles (NPs)/nanoclusters (NCs) attached to indium-tin-oxide- or fluorine-doped-tin-oxide-coated glass electrodes (glass/ITO or glass/FTO). The peak oxidation potential (E_p) for Au measured by anodic stripping voltammetry (ASV) is indicative of the NP/NC size. Heating leads to a positive shift in E_p due to an increase in NP/NC size from thermal ripening. The size transition temperature (T_t) decreases with decreasing NP/NC size following the order of 4.1 nm (509 °C) > 1.6 nm (132 °C) > 0.9 nm (90 °C/109 °C, two transitions) as compared to the bulk melting point (T_m,b) for Au of 1064 °C. The T_t generally agrees with models describing the size-dependent melting point of Au NPs (T_m,NP) for 4.1 and 1.6 nm diameter Au NPs but is higher than the models for 0.9 nm Au NCs. Scanning electron microscopy (SEM) and UV-vis size analysis confirm the electrochemical results. The thermal stability of electrode-supported metal NPs/NCs is important for their effective use in catalysis, sensing, nanoelectronics, photovoltaics, and other applications.

Surface Coating of Titanium Dioxide Nanoparticles with a Polymerizable Chelating Agent and Its Physicochemical Property

Surface modification of inorganic nanoparticles is critical for the quality and performance of pigments, cosmetics, and composite materials. We covered the titanium dioxide nanoparticles' surface with 2-(acetoacetoxy) ethyl methacrylate, a polymerizable chelating agent. Through the in situ polymerization procedure, this molecule's β-ketoester moiety quickly coordinated with the metal atoms on titanium dioxide nanoparticles, and its methacrylate group formed homogeneous coating layers. This coating layer significantly reduced the photocatalytic activity of titanium dioxide nanoparticles and prevented their aggregation. This nanoparticle dispersion showed low viscosity up to the solid content of 60% (w/w) in the liquid dispersant. As a result, it increased the UV screening performance and dispersion stability. Additionally, this coating layer widened the absorption spectrum of titanium dioxide and could change the color of nanoparticles from pale yellow to brown. It can also be helpful for cosmetic applications.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Aerosol-based functional nanocomposite coating process for large surface areas

The incorporation of nanometric-sized objects in conventional coatings can improve the properties of the matrix alone or give rise to new functionalities brought by the nanostructures. Current processes call on various shaping technologies that depend on the nature of the nano-inclusions and the matrix considered. Here, we present an integrated process based on the incorporation of nanoparticles using the aerosol route. It combines divergent nanoparticle jets with a uniform spatial profile and Physical Vapor Deposition (PVD). The chemical nature of the nanoparticles is then independent of that used for the matrix. First samples show that the morphology of nanocomposites is strongly dependent on the particle density in the films. Moreover, using several aerodynamic lens arrays combined with smart masking demonstrate the ability for coating on large surface area (40 cm²) substrates. These extended possibilities for developing new types of nanocomposites on any type of substrate and on large surface areas at low temperatures proves to be of strategic interest for various applications.

Battery-Type-Behavior-Retention Ni(OH)2-rGO Composite for an Ultrahigh-Specific-Capacity Asymmetric Electrochemical Capacitor Electrode

Nanosized battery-type materials applied in electrochemical capacitors can effectively reduce a series of problems caused by low conductivity and large volume changes. However, this approach will lead to the charging and discharging process being dominated by capacitive behavior, resulting in a serious decline in the specific capacity of the material. By controlling the material particles to an appropriate size and a suitable number of nanosheet layers, the battery-type behavior can be retained to maintain a large capacity. Here, Ni(OH)₂, which is a typical battery-type material, is grown on the surface of reduced graphene oxide to prepare a composite electrode. By controlling the dosage of the nickel source, the composite material with an appropriate Ni(OH)₂ nanosheet size and a suitable number of layers was prepared. The high-capacity electrode material was obtained by retaining the battery-type behavior. The prepared electrode had a specific capacity of 397.22 mA h g^-1 at 2 A g^-1. After the current density was increased to 20 A g^-1, the retention rate was as high as 84%. The prepared asymmetric electrochemical capacitor had an energy density of 30.91 W h kg^-1 at a power density of 1319.86 W kg^-1 and the retention rate could reach 79% after 20,000 cycles. We advocate an optimization strategy that retains the battery-type behavior of electrode materials by increasing the size of nanosheets and the number of layers, which can significantly improve the energy density while combining the advantage of the high rate capability of the electrochemical capacitor.

Photocatalytic formation of a gas permeable layer selectively deposited on supported metal nanoparticles for sintering-resistant thermal catalysis

Nanoparticle aggregation of supported metal catalysts at high temperatures is a serious problem that causes a drop in catalytic performance. This study investigates the protection of metal nanoparticles from sintering by selectively forming nanoscale SiO₂ shells on Pd supported on TiO₂ by ultraviolet (UV) light irradiation. The proton-coupled reduction reaction increases the local pH around Pd nanoparticles, resulting in hydrolysis of tetraethoxyorthosilicate (TEOS) in only the vicinity of the metal. An apparent quantum efficiency of only 0.6% is obtained for the Pd/TiO₂ catalyst in H₂ evolution from ethanol-containing water under 370 nm excitation light. Therefore, the pH of raw slurry solution should be precisely controlled to that slightly below the threshold value for the TEOS hydrolysis reaction before the photodeposition. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) clearly show that the particle size of the Pd nanoparticles (∼40 nm) with the SiO₂ shell (∼20 nm) was almost unchanged by the high-temperature treatment at 900 °C in air, suggesting that the SiO₂ shell prevented thermal aggregation of Pd nanoparticles. The Pd/TiO₂ without SiO₂ shell decoration exhibited a drop in the number of active sites, which was likely due to aggregation of the Pd catalysts. However, the number of active sites on the Pd@SiO₂/TiO₂ catalyst was maintained even after the catalyst was calcined at 900 °C. Consequently, the Pd@SiO₂/TiO₂ catalyst maintained its catalytic performance for simulated exhaust gas purification even after treatment at 900 °C. This study presents a methodology to produce sintering-tolerant supported metal nanoparticles using the photocatalytic gas permeable layer fabrication method.

Cancer Treatment Using Different Shapes of Gold-Based Nanomaterials in Combination with Conventional Physical Techniques

The conventional methods of cancer treatment and diagnosis, such as radiotherapy, chemotherapy, and computed tomography, have developed a great deal. However, the effectiveness of such methods is limited to the possible failure or collateral effects on the patients. In recent years, nanoscale materials have been studied in the field of medical physics to develop increasingly efficient methods to treat diseases. Gold nanoparticles (AuNPs), thanks to their unique physicochemical and optical properties, were introduced to medicine to promote highly effective treatments. Several studies have confirmed the advantages of AuNPs such as their biocompatibility and the possibility to tune their shapes and sizes or modify their surfaces using different chemical compounds. In this review, the main properties of AuNPs are analyzed, with particular focus on star-shaped AuNPs. In addition, the main methods of tumor treatment and diagnosis involving AuNPs are reviewed.

Multilayer gold nanoparticles as non-viral vectors for targeting MCF-7 cancer cells

Cargocomplexes play a vital role in non-viral delivery methods due to their capacity to target certain cells (or cells through the cell-division cycle) and inject their (macro)molecular "cargo" into them. The development of gene carriers that can efficiently transport and deliver genetic material into human-targeted cells with minimal toxicity is an important challenge in the field. The present study reports the straightforward preparation and testing of a modular non-viral gene carrier based on AuNPs. The design, synthesis, and in vitro evaluation of multilayer gold nanoparticles (AuNPs) as non-viral gene carriers with high transfection efficiency, reduced cytotoxicity for targeted therapeutic delivery of nucleic acids to MCF-7 cancer cells are presented. The developed non-viral vector is based on supramolecular "host-guest" inclusion complexes of β-cyclodextrin, positioned on the AuNPs surface over a layer of polyethyleneimine, and adamantyl moiety from polyethylene glycol conjugated decapeptide (WXEAAYQRFL). First, the β-CD functionalized PEI was utilized as the template for the synthesis of AuNPs of controlled sizes. The reaction produced small AuNPs with a cationic layer which is known for efficient condensation of genetic material and β-CD suitable for the decoration of the carrier with targeting moieties using "host-guest" inclusion complexation. Subsequently, adamantine-polyethylene glycol conjugated decapeptide was attached to the AuNPs. The in vitro results have validated the ability of the proposed systems to selectively target tumor cells with high efficacy and low toxicity due to the unique affinity of the aptamer-functionalized nanoparticles toward breast cancer cells. The findings of this work demonstrated that the proposed modular system may represent a very promising platform for the AuNP-based non-viral vectors mainly due to the versatility of the system, which allows for the facile exchange of several types of ligands for improving the targeting properties and transfection efficiency, or for providing better protection from the endocytotic systems.

Engineering of Oxide Protected Gold Nanoparticles

Gold nanoparticles that are partially or fully covered by metal oxide shells provide superior functionality and stability for catalytic and plasmonic applications. Yet, facile methods for controlled fabrication of thin oxide layers on metal nanoparticles are lacking. Here, we report an easy method to reliably engineer thin Ga₂O₃ shells on Au nanoparticles, based on liquid-phase chemical oxidation of Au-Ga alloy nanoparticles. We demonstrate that, with this technique, laminar and ultrathin Ga₂O₃ shells can be grown with ranging thickness from sub- to several monolayers. We show how the localized surface plasmon resonance can be used to understand the reaction process and quantitatively monitor the Ga₂O₃ shell growth. Finally, we demonstrate that the Ga₂O₃ coating prevents sintering of the Au nanoparticles, providing thermal stability to at least 250 °C. This approach, building on dealloying of bimetallic nanoparticles by the solution-phase oxidation, promises a general technique for achieving controlled metal/oxide core/shell nanoparticles.

When mechanisms of coalescence and sintering at the nanoscale fundamentally differ: Molecular dynamics study

Employing classical isothermal molecular dynamics, we simulated coalescence of mesoscopic Au nanodroplets, containing from several thousands to several hundred thousands of atoms, and sintering of mesoscopic solid Au nanoparticles. For our atomistic simulations, we used the embedded atom method. The employed open access program large-scale atomic/molecular massively parallel simulator makes it possible to realize parallel graphical processing unit calculations. We have made a conclusion that the regularities and mechanisms of the nanodroplet coalescence (temperature is higher than the nanoparticle melting temperature) and of the solid nanoparticle sintering differ from each other. We have also concluded that the nanodroplet coalescence may be interpreted as a hydrodynamic phenomenon at the nanoscale whereas sintering of solid nanoparticles is a much more complex phenomenon related to different mechanisms, including collective rearrangements of atoms, the surface diffusion, and other types of diffusion. At the same time, collective rearrangements of atoms relate not only to the solid nanoparticle sintering but also to the nanodroplet coalescence. In general, our molecular dynamics results on sintering of Au nanoparticles consisting of 10 000–30 000 atoms agree with the Ferrando–Minnai kinetic trapping concept that was earlier confirmed in molecular dynamics experiments on Au nanoclusters consisting of about 100 atoms.

Interfacial melting mechanism of nanocrystals determined by interfacial energy and interfacial stress

To clarify the interface melting mechanism, a unified analytical expression was developed to describe the depression and superheating of T_m(D) functions for metallic nanoparticles, nanostructures, and nanoparticles embedded in a coherent or incoherent interface. T_m(D) functions are determined by the sign of γ_ss, f_ss (or γ_sl and f_sl), and D₀ as caused by the change of interface environments. We found that there is TCIm(D) > TNSm(D) > TIIm(D) > TNPsm(D) for Ag nanocrystals within different interfaces. Moreover, for a given size, T_m(D)/T_m(∞) decreases with the reduction of γ_ss/f_ss for nanoparticles, nanostructures and nanoparticles embedded in incoherent interfaces, while an opposite trend occurs for the coherent interfaces. In addition, we also found that there is TNPsm(D)/T_m(∞) < TIIm(D)/T_m(∞) < TNSm(D)/T_m(∞), which is in agreement with the relation of γNPssl/fNPssl < γIIss/fIIss < γNSss/fNSss. By analyzing the γ_ss(D) (or γ_sl(D)), f_ss(D) (or f_sl(D)) and γ_ss(D)/f_ss(D) (or γ_sl(D)/f_sl(D)) functions of Ag nanocrystals and comparing with their T_m(D) functions, it is found that there is a high consistency between the variation of γ_ss(D)/f_ss(D) (γ_sl(D)/f_sl(D)) and T_m(D)/T_m, which reveals that the size dependence of T_m(D)/T_m is determined by γ_ss(D)/f_ss(D) (or γ_sl(D)/f_sl(D)). Our predictions show a good agreement with the available theoretical and experimental results.

Atomistic observation on diffusion-mediated friction between single-asperity contacts

The field of nanotribology has long suffered from the inability to directly observe what takes place at a sliding interface. Although techniques based on atomic force microscopy have identified many friction phenomena at the nanoscale, many interpretative pitfalls still result from the indirect or ex situ characterization of contacting surfaces. Here we combined in situ high-resolution transmission electron microscopy and atomic force microscopy measurements to provide direct real-time observations of atomic-scale interfacial structure during frictional processes and discovered the formation of a loosely packed interfacial layer between two metallic asperities that enabled a low friction under tensile stress. This finding is corroborated by molecular dynamic simulations. The loosely packed interfacial layer became an ordered layer at equilibrium distances under compressive stress, which led to a transition from a low-friction to a dissipative high-friction motion. This work directly unveils a unique role of atomic diffusion in the friction of metallic contacts.

Metallic core-shell nanoparticles for conductive coatings and printing

The review is focused on bimetallic nanoparticles composed of a core formed by low-cost metal having high electrical conductivity, such as Cu and Ni, and a protective shell composed of stable to oxidation noble metal such as Ag or Au. We present the chemical and physical approaches for synthesis of such particles, as well as the combination of the two, the stability to oxidation of core-shell nanoparticles at various conditions, and the formulation of conductive compositions and their application in conductive coatings and printed electronics.

C2 weakens the turnover frequency during the melting of FexCy: insights from reactive MD simulations

The carbon accumulation in the form of C2 on the surface at high temperatures blocks the surface catalytic active sites, reducing the activity of melted FexCy nanoparticles.

In Situ Transmission Electron Microscopy Investigation of Melting/Evaporation Kinetics in Anisotropic Gold Nanoparticles

We report on thermal stability and phase transition behaviors of triangular Au nanoprisms through in situ heating transmission electron microscopy. With rising temperature, Au nanoprisms exhibit fluctuating surface reconstructions at the corner regions. When a quasi-melting state is reached at the temperature below bulk melting points, the evaporation is initiated commonly at a corner with low curvature and containing sharp intersection points. The subsequent annealing process leads to the gradual evaporation, which, in the absence of thick carbon coverages, is accompanied by marked shape reconstructions. The thermal stability and evaporation behaviors are not evidently regulated by nanoprism aggregations.

Thermal Sensitivity on Eccentric Gold Hollow Nanoparticles: A Perspective from Atomistic Simulations

Eccentricity is a common feature consequence of several synthesis protocols of hollow nanoshells. Despite the crescent interest in these nanoparticles, it is still unclear how an irregular layer on the nanoparticle impacts the macroscopic properties. Here, we study the thermal stability of eccentric hollow nanoparticles (hNPs) for different sizes and eccentricity values by means of classical molecular dynamics simulations. Our results reveal that eccentricity displays a significant role in the thermal stability of hNPs. We attribute this behavior to the irregular shell contour, which collapses due to the thermal-activated diffusive process from the nanoparticle shell's most thin region. The mechanism is driven at low temperature by the nucleation of stacking faults until the amorphization for larger temperature values. Besides, for some particular eccentric hNPs, the shell suffers a surface reconstruction process, transforming the eccentric hNP into a concentric hNP. We believe that our study on thermal effects in eccentric hNPs has relevance because of their outstanding applications for plasmonic and sensing.

The Size and Shape Effects on the Melting Point of Nanoparticles Based on the Lennard-Jones Potential Function

A model is proposed to calculate the melting points of nanoparticles based on the Lennard-Jones (L-J) potential function. The effects of the size, the shape, and the atomic volume and surface packing of the nanoparticles are considered in the model. The model, based on the L-J potential function for spherical nanoparticles, agrees with the experimental values of gold (Au) and lead (Pb) nanoparticles. The model, based on the L-J potential function, is consistent with Qi and Wang’s model that predicts the Gibbs-Thompson relation. Moreover, the model based on the non-integer L-J potential function can be used to predict the melting points Tm of nanoparticles.

Plasmonic Au-Metal Oxide Nanocomposites for High-Temperature and Harsh Environment Sensing Applications

Noble metal nanoparticles like Au have long been admired for their brilliant colour, significantly influenced by plasmon resonance. When embedded in metal oxides, they exhibit unique properties which make them an excellent choice for sensing in high-temperature and harsh environment atmospheres. In this review, the various morphologies of Au nanoparticles (AuNPs) used in combination with metal oxides for sensing gases at temperatures greater than 300 °C are discussed. Theoretical discussions on the plasmon resonance properties of AuNPs as well as computational techniques like finite difference time domain (FDTD), are often used for understanding and correlating their extinction spectra and are briefed initially. The sensing properties of AuNPs embedded on a metal oxide matrix (such as TiO₂ , SiO₂ , NiO etc) for quantifying multiple analytes are then elucidated. The effect of high temperature as well as gas environments including corrosive atmospheres on such nanocomposites, and the different approaches to comprehend them are presented. Finally, techniques and methods to improve on the challenges associated with the realization and integration such Au-metal oxide plasmonic nanostructures for applications such as combustion monitoring, fuel cells, and other applications are discussed.

Atomistic processes of surface-diffusion-induced abnormal softening in nanoscale metallic crystals

Ultrahigh surface-to-volume ratio in nanoscale materials, could dramatically facilitate mass transport, leading to surface-mediated diffusion similar to Coble-type creep in polycrystalline materials. Unfortunately, the Coble creep is just a conceptual model, and the associated physical mechanisms of mass transport have never been revealed at atomic scale. Akin to the ambiguities in Coble creep, atomic surface diffusion in nanoscale crystals remains largely unclear, especially when mediating yielding and plastic flow. Here, by using in situ nanomechanical testing under high-resolution transmission electron microscope, we find that the diffusion-assisted dislocation nucleation induces the transition from a normal to an inverse Hall-Petch-like relation of the strength-size dependence and the surface-creep leads to the abnormal softening in flow stress with the reduction in size of nanoscale silver, contrary to the classical “alternating dislocation starvation” behavior in nanoscale platinum. This work provides insights into the atomic-scale mechanisms of diffusion-mediated deformation in nanoscale materials, and impact on the design for ultrasmall-sized nanomechanical devices.

As the sample size goes down to the nanoscale, the surface-related mechanism plays an important role in the deformation of nanoscale crystals. Here, the authors report breakdown of the traditional Hall-Petch-like relation in nanoscale Ag attributed to diffusion-involved nucleation behaviors.

Sub-3 nm Aluminum Nanocrystals Exhibiting Cluster-Like Optical Properties

Metal nanoclusters with distinct photophysical and photochemical properties have drawn intense research interests for their applications in optoelectronics, catalysis, and biomedicine. Herein, strong evidence is provided that light metal is capable of generating comparable optical responses of noble metal nanoclusters, but at much shorter wavelength. Air-stable, size-uniform, sub-3 nm aluminum nanocrystals are prepared with simple solution based synthetic procedures, with photoluminescence located in the ultraviolet range and short exciton lifetime. Partial modulation of the photoluminescence is achieved, indicating the key role of surface oxides. This work is envisioned to inspire new frontiers of nanocluster research with light metals.

Flexible welding of SiOx nanowire to macroporous carbon film and underlying new insights

With the continuous decreasing in sizes of functional materials and devices, people are being asked to perform a flexible, accurate, in-situ and non-thermal welding of nanowires at the nanoscale. In this work, a well deliberated procedure including three typical stages: sharpening, hooking and welding, was carried out in sequence by in-situ TEM to realize the high demand welding of SiOx nanowire to macroporous carbon film. It was found that the brittle SiOx nanowire was non-thermally softened under energetic e-beam irradiation, and the flexibility and accuracy of welding could be achieved by adjusting the beam spot size, irradiation location and irradiation time. It was demonstrated that the nanocurvature effect of SiOx nanowire and the ultra-fast energy deposition effect induced by energetic e-beam irradiation dominated the diffusion, evaporation and plastic flow of atoms and the resulting nanowire re-shaping and nanowelding processes. In contrast, the traditional knock-on mechanism and e-beam heating effect are inadequate to explain these phenomena. Therefore, such a study is crucial not only to the flexible technical controlling but also to the profound fundamental understanding of energetic e-beam-induced nanowire re-shaping and nanowelding.

Reversible disorder-order transitions in atomic crystal nucleation

Nucleation in atomic crystallization remains poorly understood, despite advances in classical nucleation theory. The nucleation process has been described to involve a nonclassical mechanism that includes a spontaneous transition from disordered to crystalline states, but a detailed understanding of dynamics requires further investigation. In situ electron microscopy of heterogeneous nucleation of individual gold nanocrystals with millisecond temporal resolution shows that the early stage of atomic crystallization proceeds through dynamic structural fluctuations between disordered and crystalline states, rather than through a single irreversible transition. Our experimental and theoretical analyses support the idea that structural fluctuations originate from size-dependent thermodynamic stability of the two states in atomic clusters. These findings, based on dynamics in a real atomic system, reshape and improve our understanding of nucleation mechanisms in atomic crystallization.

Predicting the influence of particle size on the glass transition temperature and viscosity of secondary organic material

Atmospheric aerosols can assume liquid, amorphous semi-solid or glassy, and crystalline phase states. Particle phase state plays a critical role in understanding and predicting aerosol impacts on human health, visibility, cloud formation, and climate. Melting point depression increases with decreasing particle diameter and is predicted by the Gibbs-Thompson relationship. This work reviews existing data on the melting point depression to constrain a simple parameterization of the process. The parameter [Formula: see text] describes the degree to which particle size lowers the melting point and is found to vary between 300 and 1800 K nm for a wide range of particle compositions. The parameterization is used together with existing frameworks for modeling the temperature and RH dependence of viscosity to predict the influence of particle size on the glass transition temperature and viscosity of secondary organic aerosol formed from the oxidation of [Formula: see text]-pinene. Literature data are broadly consistent with the predictions. The model predicts a sharp decrease in viscosity for particles less than 100 nm in diameter. It is computationally efficient and suitable for inclusion in models to evaluate the potential influence of the phase change on atmospheric processes. New experimental data of the size-dependence of particle viscosity for atmospheric aerosol mimics are needed to thoroughly validate the predictions.

Size-dependent melting point depression of nickel nanoparticles

We investigate the phase-transition behaviour of nickel nanoparticles (3-6 nm) via dynamic TEM. The nanoparticles were synthesized within a reverse microemulsion and then monitored via dynamic TEM simultaneously while undergoing controlled heating. The size-dependent melting point depression experimentally observed is compared with, and is in good agreement with existing thermodynamic and molecular dynamic predictions.

Determination of the melting temperature of spherical nanoparticles in dilute solution as a function of their radius by exclusively using the small-angle X-ray scattering technique

In this investigation the dependence on radius of the melting temperature of dilute sets of spherical nanocrystals with wide radius distributions was determined by a novel procedure exclusively using the results of small-angle X-ray scattering (SAXS) measurements. This procedure is based on the sensitivity of the SAXS function to small and rather sharp variations in the size and electron density of nanocrystals at their melting temperature. The input for this procedure is a set of experimental SAXS intensity functions at selected q values for varying sample temperatures. In practice, the sample is heated from a minimum temperature, lower than the melting temperature of the smallest nanocrystals, up to a temperature higher than the melting temperature of the largest nanocrystals. The SAXS intensity is recorded in situ at different temperatures during the heating process. This novel procedure was applied to three samples composed of dilute sets of spherical Bi nanocrystals with wide radius distributions embedded in a sodium borate glass. The function relating the melting temperature of Bi nanocrystals with their radius – determined by using the procedure proposed here – agrees very well with the results reported in previous experimental studies using different methods. The results reported here also evidence the predicted size-dependent contraction of Bi nanocrystals induced by the large surface-to-volume ratio of small nanocrystals and an additional size-independent compressive stress caused by the solid glass matrix in which liquid Bi nanodroplets are initially formed. This last effect is a consequence of the increase in the volume of Bi nanoparticles upon crystallization and also of differences in the thermal expansion coefficients of the crystalline phase of Bi and the glass matrix. This additional stress leads to a depression of about 10 K in the melting temperature of the Bi nanocrystals confined in the glass. The procedure described here also allowed the determination of the specific masses and thermal expansion coefficients of Bi nanoparticles in both liquid and crystalline phases.

Electrical/thermal behaviors of bimetallic (Ag-Cu, Ag-Sn) nanoparticles for printed electronics

In this work, Ag-Cu and Ag-Sn nanoparticles (NPs) were synthesized by a physical vapor condensation method, i.e. DC arc-discharge plasma. The as-prepared bimetallic NPs consist of metallic cores of Ag-Cu or Ag-Sn and ultrathin oxide shells of CuO or a hybrid of SnO and SnO₂. Ag-Sn NPs exhibit a room-temperature resistivity of 4.24 × 10^-5 Ω · cm, a little lower than 7.10 × 10^-5 Ω · cm of Ag-Cu NPs. Both bimetallic NPs demonstrate typical metallic conduction behavior with a positive temperature coefficient of resistance over 25-300 K. Ag-Sn NPs exhibit thermally competitive stability up to 230 °C and a lower resistivity of 3.18 × 10^-5 Ω · cm after sintering at 200 °C, giving it potential for application in flexible printed electronics.

Pulsed photoacoustic and photothermal response of gold nanoparticles

Gold nanospheres and nanorods are promising agents for photoacoustic imaging and photothermal therapy. In this work, seed-mediated methods were optimized for the synthesis of gold nanosphere and nanorod collides of different sizes. Nanosecond pulse photoacoustic and photothermal analysis of these nanoparticles was carried out and compared with finite element simulation. The simulations were performed to quantify the size dependent photoacoustic signal enhancement for nanospheres and nanorods. The non-sphericity contribution of nanospheres was found to enhance the photoacoustic signal. Nanosecond pulse photoacoustic studies of nanorods of different aspect ratio were carried out. Nanorods of aspect ratio ∼4.8 were found to be the most efficient photoacoustic signal generators. Photoacoustic studies of nanorods at varying laser fluence were performed and threshold fluence of 5 mJ cm^-2 was observed. Nanorods exposed to nanosecond laser pulses underwent size and shape variations which were confirmed by optical absorbance and transmission electron microscopy measurements. Simulations of nanorods of different aspect ratio and diameter were performed to investigate the photoacoustic signal enhancement and photothermal stability. The miniature size nanorod with a diameter of 10 nm and aspect ratio of 5 was found to be most appropriate in terms of photoacoustic signal generation and photothermal stability.

A closer look at the defects and luminescence of nanocrystalline fluorides synthesized via ionic liquids: the case of Ce3+-doped BaF2

Phase pure BaF₂ doped with Ce³⁺ (0.1%) nanocrystals are synthesized using an ionic liquid (IL) ([C₄mim][BF₄]) assisted solvothermal method where the IL is not only used as a reaction medium and a capping agent, but also as a reaction partner.

Ultrastable Au nanoparticles on titania through an encapsulation strategy under oxidative atmosphere

Supported gold catalysts play a crucial role in the chemical industry; however, their poor on-stream stability because of the sintering of the gold nanoparticles restricts their practical application. The strong metal-support interaction (SMSI), an important concept in heterogeneous catalysis, may be applied to construct the structure of catalysts and, hence, improve their reactivity and stability. Here we report an ultrastable Au nanocatalyst after calcination at 800 °C, in which Au nanoparticles are encapsulated by a permeable TiO_x thin layer induced by melamine under oxidative atmosphere. Owning to the formed TiO_x overlayer, the resulting Au catalyst is resistant to sintering and exhibits excellent activity and stability for catalytic CO oxidation. Furthermore, this special strategy can be extended to colloidal Au nanoparticles supported on TiO₂ and commercial gold catalyst denoted as RR2Ti, providing a universal way to engineer and develop highly stable supported Au catalysts with tunable activity.

Sintering-resistant Au nanoparticles are highly desirable due to their low Tammann temperature. Here, the authors report an ultrastable titania-supported Au nanocatalyst through an encapsulation strategy under oxidative atmosphere.

Structure and Catalytic Behavior of Alumina Supported Bimetallic Au-Rh Nanoparticles in the Reduction of NO by CO

Alumina-supported bimetallic AuRh catalysts, as well as monometallic reference catalysts, were examined with regard to their structural and catalytic properties in the reduction of NO by CO. Depending on the molar ratio of Au:Rh, the nanoparticles prepared by borohydride co-reduction of corresponding metal salt solutions had a size of 3.5–6.7 nm. The particles consisted of well-dispersed noble metal atoms with some enrichment of Rh in their surface region. NO conversion of AuRh/Al2O3 shifted to lower temperatures with increasing Rh content, reaching highest activity and highest N2 selectivity for the monometallic Rh/Al2O3 catalyst. This behavior is attributed to the enhanced adsorption of CO on the bimetallic catalyst resulting in unfavorable cationic Rh clusters Rh+-(CO)2. Doping with ceria of AuRh/Al2O3 and Rh/Al2O3 catalysts increased the surface population of metallic Rh sites, which are considered most active for the reduction of NO by CO and enhancement of the formation of intermediate isocyanate (-NCO) surface species and their reaction with NO to form N2 and CO2.

g-C3N4-Mediated Synthesis of Cu2O To Obtain Porous Composites with Improved Visible Light Photocatalytic Degradation of Organic Dyes

A highly porous architecture of graphitic carbon nitride g-C3N4/Cu2O nanocomposite in the form of cubes with a side length of ≈ 1 μm, large pores of 1.5 nm, and a high surface area of 9.12 m2/g was realized by an optimized in situ synthesis protocol. The synthesis protocol involves dispersing a suitable "Cu" precursor into a highly exfoliated g-C3N4 suspension and initiating the reaction for the formation of Cu2O. Systematic optimization of the conditions and compositions resulted in a highly crystalline g-C3N4/Cu2O composite. In the absence of g-C3N4, the Cu2O particles assemble into cubes with a size of around 300 nm and are devoid of pores. Detailed structural and morphological evaluations by powder X-ray diffraction and field emission scanning electron microscopy revealed the presence of highly exfoliated g-C3N4, which is responsible for the formation of the porous architecture in the cube like assembly of the composite. The micrographs clearly reveal the porous structure of the composite that retains the cubic shape of Cu2O, and the energy-dispersive spectroscopy supports the presence of g-C3N4 within the cubic morphology. Among the different g-C3N4/Cu2O compositions, CN/Cu-5 with 10% of g-C3N4, which is also the optimum composition resulting in a porous cubic morphology, shows the best visible light photocatalytic performance. This has been supported by the ultraviolet diffuse reflectance spectroscopy (UV-DRS) studies of the composite which shows a band gap of around 2.05 eV. The improved photocatalytic performance of the composite could be attributed to the highly porous morphology along with the suitable optical band gap in the visible region of the solar spectrum. The optimized composite, CN/Cu-5, demonstrates a visible light degradation of 81% for Methylene Blue (MB) and 85.3% for Rhodamine-B (RhB) in 120 min. The decrease in the catalyst performance even after three repeated cycles is less than 5% for both MB and RhB dyes. The rate constant for MB and RhB degradation is six and eight times higher with CN/Cu-5 when compared with the pure Cu2O catalyst. To validate our claim that the dye degradation is not merely decolorization, liquid chromatography-mass spectroscopy studies were carried out, and the end products of the degraded dyes were identified.

Hydrogel-Encapsulated Mesoporous Silica-Coated Gold Nanoshells for Smart Drug Delivery

A "smart" core@shell composite nanoparticle (NP) having dual-response mechanisms (i.e., temperature and light) was synthesized, and its efficacy in the loading and release of small molecules was explored. These core@shell NPs are composed of an optically active gold nanoshell (GNS) core and a mesoporous (m-) silica layer (m-SiO2). The GNS@m-SiO2 nanoparticles are further encapsulated within a thermo-responsive poly(N-isopropylacrylamide-co-acrylic acid) hydrogel (PNIPAM-co-AA). The multi-responsive composite NPs were designed to create thermally and optically modulated drug-delivery vehicles with a m-SiO2 layer providing additional non-collapsible space for drug storage. The influence of the m-SiO2 layer on the efficacy of loading and release of methylene blue, which serves as a model for a small-molecule therapeutic drug, was evaluated. The "smart" core@shell composite NPs having a m-SiO2 layer demonstrated an improved capacity to load and release small molecules compared to the corresponding NPs with no m-SiO2 shell. Additionally, an efficient response by the composite NPs was successfully induced by the thermal energy generated from the gold nanoshell core upon exposure to near infrared (NIR) stimulation.

Physical mechanisms involved in the formation and operation of memory devices based on a monolayer of gold nanoparticle-polythiophene hybrid materials

Understanding the physical and chemical mechanisms occurring during the forming process and operation of an organic resistive memory device is a requisite for better performance. Various mechanisms were suggested in vertically stacked memory structures, but the analysis remains indirect and needs destructive characterization (e.g. analysis of the cross-section to access the organic layers sandwiched between electrodes). Here, we report a study on a planar, monolayer thick, hybrid nanoparticle/molecule device (10 nm gold nanoparticles embedded in an electro-generated poly(2-thienyl-3,4-(ethylenedioxy)thiophene) layer), combining in situ physical (scanning electron microscopy, physicochemical thermogravimetry and mass spectroscopy, and Raman spectroscopy) and electrical (temperature dependent current-voltage) characterization on the same device. We demonstrate that the forming process causes an increase in the gold particle size, almost 4 times larger than the starting nanoparticles, and that the organic layer undergoes a significant chemical rearrangement from an sp³ to sp² amorphous carbon material. Temperature dependent electrical characterization of this nonvolatile memory confirms that the charge transport mechanism in the device is consistent with a trap-filled space charge limited current in the off state, with the sp² amorphous carbon material containing many electrically active defects.

Solution processable liquid metal nanodroplets by surface-initiated atom transfer radical polymerization

Eutectic gallium indium (EGaIn) is a liquid metal alloy at room temperature. EGaIn microdroplets can be incorporated into elastomers to fabricate highly stretchable, mechanically robust, soft multifunctional composites with high thermal stability¹ and electrical conductivity^2-4 that are suitable for applications in soft robotics and self-healing electronics^5-7. However, the current methods of preparation rely on mechanical mixing, which may lead to irregularly shaped micrometre-sized droplets and an anisotropic distribution of properties⁸. Therefore, procedures for the stabilization of sub-micrometre-sized droplets of EGaIn and compatibilization in polymer matrices and solvents have attracted significant attention^9-12. Here we report the synthesis of EGaIn nanodroplets stabilized by polymeric ligand encapsulation. We use a surface-initiated atom transfer radical polymerization initiator to covalently functionalize the oxide layer on the surface of the EGaIn nanodroplets¹³ with poly(methyl methacrylate) (PMMA), poly(n-butyl acrylate) (PBMA), poly(2-dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(n-butyl acrylate-block-methyl methacrylate) (PBA-b-PMMA). These nanodroplets are stable in organic solvents, in water or in polymer matrices up to 50 wt% concentration, enabling direct solution-casting into flexible hybrid materials. The liquid metal can be recovered from dispersion by acid treatment. The nanodroplets show good mechanical, thermal and optical properties, with a remarkable suppression of crystallization and melting temperatures (down to -80 °C from 15 °C).

Size effect on the spontaneous coalescence of nanowires

This paper investigates the size effect on the coalescence process of contacting nanoparticles. It is revealed by molecular dynamics that the nanometer-sized surface curvature coupled with the effective melting temperature exhibits a strong influence on the atom diffusion at the interface, and is therefore critical to the coalescence time. This effect is particularly pronouncing for surface curvatures below 20 nm. A phenomenological model is derived from the melting point reduction approach to describe the kinetic process of nanowire coalescence and is validated against a variety of simulation datasets. The quantitative correlation between the sample size, the sintering temperature and the contact morphology evolution is demonstrated.

Application of metallic inks based on nickel-silver core-shell nanoparticles for fabrication of conductive films

Conductive inks based on nickel nanoparticles (NPs) have attracted much attention as a low-cost replacement for the currently used silver and gold inks, for fabrication of printed electronic circuits and devices. Nickel NPs as a component of conductive inks should be stable against oxidation process at all stages of preparation of conductive patterns: ink formulation and storage, printing, and post-printing treatment. In the present study, the oxidation resistance of the Ag layer and the conductive properties of the Ni core allowed the use of nickel-silver core-shell (Ni@Ag) NPs as the component of conductive ink. Thick films composed of Ni-Ag core-shell NPs were deposited on a glass substrate and then sintered at temperatures ranging from 250 °C-370 °C. The conductivity of Ni@Ag coatings after sintering at 350 °C reached 11% of that for a bulk nickel.

Cooperative Bilayer of Lattice-Disordered Nanoparticles as Room-Temperature Sinterable Nanoarchitecture for Device Integrations

Decreasing the interconnecting temperature is essential for 3D and heterogeneous device integrations, which play indispensable roles in the coming era of "more than Moore". Although nanomaterials exhibit a decreased onset temperature for interconnecting, such an effect is always deeply impaired because of organic additives in practical integrations. Meanwhile, current organic-free integration strategies suffer from roughness and contaminants at the bonding interface. Herein, a novel bilayer nanoarchitecture simultaneously overcomes the drawbacks of organics and is highly tolerant to interfacial morphology, which exhibits universal applicability for device-level integrations at even room temperature, with the overall performance outperforming most counterparts reported. This nanoarchitecture features a loose nanoparticle layer with unprecedented deformability for interfacial gap-filling, and a compact one providing firm bonding with the component surface. The two distinct nanoparticle layers cooperatively enhance the interconnecting performance by 73-357%. Apart from the absence of organics, the internal abundant lattice disorders profoundly accelerate the interconnecting process, which is supported by experiments and molecular dynamics simulation. This nanoarchitecture is successfully demonstrated in diversified applications including paper-based light-emitting diodes, Cu-Cu micro-bonding, and SiC power modules. The strategy proposed here can open a new paradigm for device integrations and provide a fresh understanding on interconnecting mechanisms.

Conductive nanomaterials for 2D and 3D printed flexible electronics

This review describes recent developments in the field of conductive nanomaterials and their application in 2D and 3D printed flexible electronics, with particular emphasis on inks based on metal nanoparticles and nanowires, carbon nanotubes, and graphene sheets. We present the basic properties of these nanomaterials, their stabilization in dispersions, formulation of conductive inks and formation of conductive patterns on flexible substrates (polymers, paper, textile) by using various printing technologies and post-printing processes. Applications of conductive nanomaterials for fabrication of various 2D and 3D electronic devices are also briefly discussed.

High Density Static Charges Governed Surface Activation for Long-Range Motion and Subsequent Growth of Au Nanocrystals

How a heavily charged metal nanocrystal, and further a dual-nanocrystals system behavior with continuous electron charging? This refers to the electric dynamics in charged particles as well as the crystal growth for real metal particles, but it is still opening in experimental observations and interpretations. To this end, we performed an in-situ electron-beam irradiation study using transmission electron microscopy (TEM) on the Au nanocrystals that freely stand on the nitride boron nanotube (BNNT). Au nanocrystalline particles with sizes of 2–4 nm were prepared by a well-controlled sputtering method to stand on the BNNT surface without chemical bonding interactions. Au nanoparticles presented the surface atomic disorder, diffusion phenomena with continuous electron-beam irradiation, and further, the long-range motion that contains mainly the three stages: charging, activation, and adjacence, which are followed by final crystal growth. Firstly, the growth process undergoes the lattice diffusion and subsequently the surface-dominated diffusion mechanism. These abnormal phenomena and observations, which are fundamentally distinct from classic cases and previous reports, are mainly due to the overcharging of Au nanoparticle that produces a surface activation state in terms of high-energy plasma. This work therefore brings about new observations for both a single and dual-nanocrystals system, as well as new insights in understanding the resulting dynamics behaviors.

The optimization of methods of synthesis of nickel-silver core-shell nanoparticles for conductive materials

Nickel-silver core-shell (Ni@Ag) nanoparticles (NPs) were formed in a two-step process: (1) the formation of a dispersion of Ni NPs; and (2) the transmetalation (galvanic displacement) reaction, where the surface of the Ni NPs acted as the reducing agent of Ag ions. Ni NPs were synthesized by the 'wet' chemical method, i.e., by the reduction of metal ions by using NaBH₄ as the reducing agent. The influence of the concentration of polymeric stabilizer, reducing agent and Ag precursor on the properties of synthesized NPs was evaluated. In the optimal condition of synthesis, Ni@Ag NPs with about 50 and 210 nm-diameter Ni core coated with a thin (∼10-20 nm) Ag shell, were obtained. Finally, the stability of the synthesized spherical-shaped Ni@Ag NPs was tested and the results indicate long-term stability against aggregation and Ni oxidation. Thus, the resulting NPs are promising candidates for application in electronic devices, e.g., as components of conductive inks or pastes.

High-temperature stability of nanozirconate-toughed IMF material lanthanum synthesized by anin situreaction

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ (volume ratio = 1 : 1) powders were characterized by XRD, Raman spectroscopy, SEM, and TEM. XRD and Raman results showed that the samples maintained a tetragonal ZrO₂ and a pyrochlore LZ phase from 900 to 1100 °C. The addition of LZ could be helpful in the stabilization of t-ZrO₂ and decreasing the grain size of ZrO₂. The SEM results revealed that the LZ and ZrO₂ phases were homogeneously distributed in the sintered bulk. The HRTEM results suggested that the crystal orientations of the nano-LZ and nano-ZrO₂ phases were accordant; this was in agreement with the characteristics of the coherent boundaries. The fracture toughness of LZ–ZrO₂ was markedly improved by the transformation toughening of the ZrO₂ phase, and a value that was 2.2-fold that of the LZ prepared by a similar technique was achieved.

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ powders were characterized by XRD, Raman, SEM, and TEM.