The role of ad-atoms in the coalescence of alkanethiol-passivated gold nanoparticles

Electron irradiation effects in transmission electron microscopy: Random displacements and collective migrations

Electron beam damage in transmission electron microscopy (TEM) is complicated because the damage phenomena can be the result of random atomic displacements or collective migrations. The former is categorized as the primary beam effects and the latter is the secondary beam effects. The mechanisms for these two distinguishing atomic processes of damage are different. The primary beam effects can be caused by the mechanisms of knock-on and/or radiolysis, while the secondary effects must be driven by a field that is induced by electron irradiation. One such field has been identified to be the electric field produced by the accumulated charges due to the ejection of secondary and Auger electrons from the irradiated region. One convincing example is the electron irradiation-induced domain switch in ferroelectric materials, in which the collective cation displacements are driven by the induced electric field. A detailed interpretation is given in this review. The sintering of metal NPs under electron irradiation is a secondary beam effect and is most likely also caused by the induced electric fields. The interactions between the charged NP and substrate, and between charged NPs, result in NP motion. Interchanging atoms between NPs during the sintering may also be driven by the electric fields. Although many beam-damage phenomena in C nanotubes and layered materials, such as graphene, BN, and transition metal dichalcogenides, are caused by the primary beam effects and have been well studied experimentally and theoretically in the literature, some phenomena from the secondary beam effects have also been identified in this review. These phenomena are sensitive to electron current density, the shape and orientation of the specimen, and even the illumination mode (i.e., TEM or STEM). Unfortunately, the mechanisms responsible for these phenomena still need to be clarified.

Nanowelding in Whole-Lifetime Bottom-Up Manufacturing: From Assembly to Service

The continuous miniaturization of microelectronics is pushing the transformation of nanomanufacturing modes from top-down to bottom-up. Bottom-up manufacturing is essentially the way of assembling nanostructures from atoms, clusters, quantum dots, etc. The assembly process relies on nanowelding which also existed in the synthesis process of nanostructures, construction and repair of nanonetworks, interconnects, integrated circuits, and nanodevices. First, many kinds of novel nanomaterials and nanostructures from 0D to 1D, and even 2D are synthesized by nanowelding. Second, the connection of nanostructures and interfaces between metal/semiconductor-metal/semiconductor is realized through low-temperature heat-assisted nanowelding, mechanical-assisted nanowelding, or cold welding. Finally, 2D and 3D interconnects, flexible transparent electrodes, integrated circuits, and nanodevices are constructed, functioned, or self-healed by nanowelding. All of the three nanomanufacturing stages follow the rule of "oriented attachment" mechanisms. Thus, the whole-lifetime bottom-up manufacturing process from the synthesis and connection of nanostructures to the construction and service of nanodevices can be organically integrated by nanowelding. The authors hope this review can bring some new perspective in future semiconductor industrialization development in the expansion of multi-material systems, technology pathway for the refined design, controlled synthesis and in situ characterization of complex nanostructures, and the strategies to develop and repair novel nanodevices in service.

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.

Successful Synthesis of Gold Nanoparticles through Ultrasonic Spray Pyrolysis from a Gold(III) Nitrate Precursor and Their Interaction with a High Electron Beam

Herein, we report for the first time the successful preparation of a gold(III) nitrate [Au(NO3)3] water-based precursor for use in a bottom-up ultrasonic spray pyrolysis (USP) process. Due to its limited solubility in water, the precursor was prepared under reflux conditions with nitric acid (HNO3) as the solvent and ammonium hydroxide (NH4OH) as a neutralizer. This precursor enabled the USP synthesis of gold nanoparticles (AuNPs) and the in situ formation of low concentrations of NO2- and NO3- ions, which were caught directly in deionized water in a collection system. These ions were proven to act as stabilizers for the AuNPs. Investigations showed that the AuNPs were monodispersed and spherically shaped with a size distribution over three groups: the first contained 5.3 % AuNPs with diameters (2 r) <15 nm, the second contained 82.5 % AuNPs with 2 r between 15 and 200 nm, and the third contained 12.2 % AuNPs with 2 r>200 nm. UV/Vis spectroscopy revealed the maximum absorbance band of the AuNPs at λ=528 nm. Additionally, scanning transmission electron microscopy (STEM) observations of the smallest AuNPs (2 r<5 nm) revealed atomically resolved coalescence phenomena induced by interaction with the electron beam. Four stages of the particle-growth process were distinguished: 1) movement and rotation of the AuNPs; 2) necking mechanism; 3) orientated attachment at matching facets; 4) reshaping of the AuNPs by surface diffusion. This provided important insight into the formation/synthesis process of the AuNPs.

Structural order in ultrathin films of the monolayer protected clusters based upon 4 nm gold nanocrystals: an experimental and theoretical study

The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a number of application areas but can be difficult to demonstrate by conventional methods, particularly when the metallic core dimension, d, is in the intermediate size-range, 1.5 < d < 5.0 nm. Here, improved techniques for the synthesis of monodisperse thiolate-protected gold nanoparticles have made possible the production of dodecane-thiolate saturated ∼4 ± 0.5 nm Au clusters with single-crystal core structure and morphology. An ultrathin ordered film or superlattice of these nanocrystal-core MPCs is prepared and investigated using aberration corrected scanning/transmission electron microscopy (STEM) which allowed imaging of long-range hexagonally ordered superlattices of the nanocrystals, separated by the thiolate groups. The lattice constants determined by direct imaging are in good agreement with those determined by small-angle electron diffraction. The STEM image revealed the characteristic grain boundary (GB) with sigma (Σ) 13 in the interface between two crystals. The formation and structures found are interpreted on the basis of theoretical calculations employing molecular dynamics (MD) simulations and coarse-grained (CG) approach.