The alloy Au–Ag system is an important noble bimetallic phase, both historically (as “Electrum”) and now especially in nanotechnology, as it is applied in catalysis and nanomedicine. To comprehend ...the structural characteristics and the thermodynamic stability of this alloy, a knowledge of its phase diagram is required that considers explicitly its size and shape (morphology) dependence. However, as the experimental determination remains quite challenging at the nanoscale, theoretical guidance can provide significant advantages. Using a regular solution model within a nanothermodynamic approach to evaluate the size effect on all the parameters (melting temperature, melting enthalpy, and interaction parameters in both phases), the nanophase diagram is predicted. Besides an overall shift downward, there is a “tilting” effect on the solidus–liquidus curves for some particular shapes exposing the (100) and (110) facets (cube, rhombic dodecahedron, and cuboctahedron). The segregation calculation reveals the preferential presence of silver at the surface for all the polyhedral shapes considered, in excellent agreement with the latest transmission electron microscopy observations and energy dispersive spectroscopy analysis. By reviewing the nature of the surface segregated element of different bimetallic nanoalloys, two surface segregation rules, based on the melting temperatures and surface energies, are deduced. Finally, the optical properties of Au–Ag nanoparticles, calculated within the discrete dipole approximation, show the control that can be achieved in the tuning of the local surface plasmon resonance, depending of the alloy content, the chemical ordering, the morphology, the size of the nanoparticle, and the nature of the surrounding environment.
Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous gold clusters protected with p-mercaptobenzoic acid ligands (pMBA) of composition Au n (pMBA)p or (n, ...p). The Au144(pMBA)60, (144, 60), or gold-144 aqueous gold cluster is considered special because of its high symmetry, abundance, and icosahedral structure as well as its many potential uses in material and biological sciences. Yet, to this date, direct confirmation of its precise composition and total structure remains elusive. Results presented here from characterization via high-resolution electrospray ionization mass spectrometry on an Orbitrap instrument confirm Au102(pMBA)44 at isotopic resolution. Further, what usually appears as a single band for (144, 60) in electrophoresis (PAGE) is shown to also contain the (130, 50), recently determined to have a truncated-decahedral structure, and a (137, 56) component in addition to the dominant (144, 60) compound of chiral-icosahedral structure. This finding is significant in that it reveals the existence of structures never before observed in all-aromatic water-soluble species while pointing out the path toward elucidation of the thermodynamic control of protected gold nanocrystal formation.
MoS2 has been the focus of extensive research due to its potential applications. More recently, the mechanical properties of MoS2 layers have raised interest due to applications in flexible ...electronics. In this article, we show in situ transmission electron microcsopy (TEM) observation of the mechanical response of a few layers of MoS2 to an external load. We used a scanning tunneling microscope (STM) tip mounted on a TEM stage to induce deformation on nanosheets of MoS2 containing few layers. The results confirm the outstanding mechanical properties on the MoS2. The layers can be bent close to 180°. However, when the tip is retrieved the initial structure is recovered. Evidence indicates that there is a significant bond reconstruction during the bending with an outstanding capability to recover the initial bond structure. The results show that flexibility of three layers of MoS2 remains the same as a single layer while increasing the bending modulus by 3 orders of magnitude. Our findings are consistent with theoretical calculations and confirm the great potential of MoS2 for applications.
Gold nanoclusters (AuNCs) with well-defined structure and arrangement possess particular physical and functional properties. AuNCs that differ only by less than 1 nm in diameter corresponding to one ...atomic layer show different structural, optical, and physicochemical properties in a size-dependent mode, making their analytical characterization a challenge. Herein we describe an integrative approach to characterization of larger aqueous AuNC (Au102-pMBA44, Au144pMBA60 and higher) selected by gel electrophoresis (PAGE). We employ UV–vis, dynamic light scattering, and zeta-potential in combination with high-performance analytical techniques such as multiwavelength analytical ultracentrifugation and electrospray ionization mass spectrometry were used to separate aqueous AuNCs and to determine their specific hydrodynamic diameter, partial abundance, molecular weight, and mass/charge ratios when present in a complex mixture of AuNCs containing Au102 (1.6 nm), Au144 (2 nm), and Au288–328 (2.5 nm). Advanced analytical electron microscopy imaging (spherical aberration corrected BF/HAADF-STEM at low voltages dose) also revealed the structures of discrete arrangements of gold nanocluster populations.
Growth of anisotropic nanostructures enables the manipulation of optical properties across the electromagnetic spectrum by fine morphological tuning of the nanoparticles. Among them, stellated ...metallic nanostructures present enhanced properties owing to their complex shape, and hence, the control over the final morphology becomes of great importance. Herein, a seed-mediated method for the high-yield production of goldrich–copper concave branched nanostructures and their structural and optical characterization is reported. The synthesis protocol enabled excellent control and tunability of the final morphology, from concave pentagonal nanoparticles to five-fold branched nanoparticles, named “nanostars”. The anisotropic shape was achieved via kinetic control over the synthesis conditions by selective passivation of facets using a capping agent and assisted by the presence of copper chloride ions, both having a crucial impact over the final structure. Optical extinction measurements of nanostars in solution indicated a broad spectral response, hiding the properties of the individual nanostars. Hence, single-particle scattering measurements of individual concave pentagonal nanoparticles and concave nanostars were performed to determine the origin of the multiple plasmon bands by correlation with their morphological features, following their growth evolution. Finite-difference time-domain calculations delivered insights into the geometry-dependent plasmonic properties of concave nanostars and their packed aggregates. Our results uncover the intrinsic scattering properties of individual nanostars and the origin of the broad spectral response, which is mostly due to z-direction packed aggregates.
Gold nanoparticles with anisotropic structures have tunable absorption properties and diverse bioapplications as image contrast agents, plasmonics, and therapeutic–diagnostic materials. Amino acids ...with electrostatically charged side chains possess inner affinity for metal ions. Lysine (Lys) efficiently controlled the growing into star-shape nanoparticles with controlled narrow sizes (30–100 nm) and produced in high yields (85–95%). Anisotropic nanostructures showed tunable absorbance from UV to NIR range, with extraordinary colloidal stability (−26 to −42 mV) and surface-enhanced Raman scattering properties. Advanced electron microscopy characterization through ultra-high-resolution SEM, STEM, and HR-TEM confirmed the size, nanostructure, crystalline structure, and chemical composition. Molecular dynamics simulations revealed that Lys interacted preferentially with Au(I) through the −COOH group instead of their positive side chains with a binding free energy (BFE) of 3.4 kcal mol–1. These highly monodisperse and colloidal stable anisotropic particles prepared with biocompatible compounds may be employed in biomedical applications.
The aim of this study was to investigate the effect of the nanostructured hydroxyapatite (NHAp) and titanium dioxide nanoparticles (NTiO2) on dispersion in an adhesive containing monomers of Dipenta ...erythritol penta-acrylate monophosphate (PENTA) and Urethane dimethacrylate (UDMA), as well as evaluating the structural, optical and mechanical behavior of the composite material for dental aesthetic application.
The NHAp powders were synthesized through the wet chemical methods of hydrothermal and ultrasound-assisted precipitation. The microstructure, morphology and composition analysis of the powder of NHAp and NTiO2 were performed by scanning and transmission electron microscopy. The optical microscopic identification of the different colors was obtained due to varying the amounts of NHAp and NTiO2 in the adhesive. On the other hand, the diffuse reflectance spectra of the coatings were evaluated every 2nm with the wavelength from 400 to 800nm for combined specular and diffuse reflectance. The nanomechanical properties of the aesthetic coating such as (H), elastic modulus (E) and nanoscratching were evaluated by nanoindentation. The roughness of the composite coatings were evaluated by AFM.
From different powders combinations, NHAP 75%Wt-NTiO2 %25Wt, at (10Wt %) into a dental adhesive, the resulting mixture manifested the optimum aesthetic white appearance. The scanning and transmission electron microscopy images confirmed that the HAp nanorods and TiO2 nanoparticles sized were 55nm and 20nm respectively prepared by the high-energy ball mixed process. The values of nanomechanical properties of the optimum aesthetic coating were hardness, H=3.2±0.3GPa, elastic modulus, E=78±3GPa, Yield point, Y=107MPa±2 and scratching, maximum wear track deformation 3.7±0.12 μm2. The percentage of reflectance to optimum aesthetic white appearance was of 46.83% at 423nm of wavelength.
The nanocomposite PENTA/UDMA with mixtures of Nanohydroxyapatite and titanium dioxide may be considerate as a mechanical toughened, also an option to modify shade qualities for dental aesthetic applications.
Given the binary nature of nanoalloy systems, their properties are dependent on their size, shape, structure, composition, and chemical ordering. When energy and entropic factors for shapes and ...structure variations are considered in nanoparticle growth, the spectra of shapes become so vast that even metastable arrangements have been reported under ambient conditions. Experimental and theoretical variations of multiply twinned particles have been observed, from the Ino and Marks decahedra to polyicosahedra and polydecahedra with comparable energetic stability among them. Herein, we report the experimental production of a stable doubly truncated double-icosahedron structure (TdIh) in Au–Ag nanoparticles, in which a twinned Ag-rich alloyed shell is reconstructed on a Au–Ag alloyed Ino-decahedral core. The structure, chemical composition, and growth pathway are proposed on the basis of high-angle annular dark-field scanning transmission electron microscopy analysis and excess energy calculations, while its structural stability is estimated by large-scale atomic molecular dynamics simulations. This novel nanostructure differs from other structures previously reported.
Abnormal accumulation of brain metals is a key feature of Alzheimer's disease (AD). Formation of amyloid-β plaque cores (APC) is related to interactions with biometals, especially Fe, Cu and Zn, but ...their particular structural associations and roles remain unclear. Using an integrative set of advanced transmission electron microscopy (TEM) techniques, including spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM), nano-beam electron diffraction, electron holography and analytical spectroscopy techniques (EDX and EELS), we demonstrate that Fe in APC is present as iron oxide (Fe3O4) magnetite nanoparticles. Here we show that Fe was accumulated primarily as nanostructured particles within APC, whereas Cu and Zn were distributed through the amyloid fibers. Remarkably, these highly organized crystalline magnetite nanostructures directly bound into fibrillar Aβ showed characteristic superparamagnetic responses with saturated magnetization with circular contours, as observed for the first time by off-axis electron holography of nanometer scale particles.
In the search for new mechanisms to improve and control the mechanical properties of nanostructures, the idea of tuning the strength through composition is appealing because of the extensive ...experimental availability of nanoparticles with segregated configurations, such as core–shell nanoparticles. However, not much is known about the deformation mechanism of these types of systems because of the lack of correlation between theoretical predictions and experimental observations. In this work, we investigate the atomistic mechanical response of Au–Pd core–shell and Pd nanocubes under indentation, using molecular dynamics simulations. These results are compared to experimental observations of in situ transmission electron microscopy (TEM) nanoindentation on similar nanoparticles. Our study resolves the nucleation of Shockley partial dislocations and their propagation in Au–Pd core–shell and single-crystalline Pd nanocubes. In the latter, Shockley partial dislocations originate at the cube corners and create stacking faults that propagate across the nanoparticle, creating the so-called V-shaped defects. In contrast, in Au–Pd core–shell nanocubes, nucleation starts at the semicoherent Au–Pd interface, where a network of misfits acts as dislocation storage, reducing the nanocube’s strength. We explore the effect of the core size and its function as a dislocation barrier for nanocubes of smaller sizes. Additionally, strain hardening was observed as the core size increased and, for the case of the largest core (Au30Pd70) at strain values above 20%, where a complex network of different types of dislocations, including sessile dislocations, is observed. Our results suggest a clear agreement between simulation and experiments, which points to a promising field in which combining two or more metals in a core–shell configuration can be used to tune and control the mechanical properties at the nanoscale.
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