Field-induced self-assembly: does size matter?

ABSTRACT

Simulated external electric fields are applied to polarisable species containing either a monodisperse of bidisperse distribution of polarisabilities. The magnitude of the external field and the polarisabilities are systematically varied. The application of an external field (of sufficient magnitude) is found to induce chain formation (as expected). The monodisperse systems are found to ‘self-assemble’ with larger induced dipole moments effectively clustering in chains as a result of significant dipole-induced dipole effects. The distribution of the chain lengths is characterised as a function of the applied field and the atom polarisability. For the bidisperse systems, the external field induces chain formation and a partial segregation, in which the more polarisable species preferentially form chains. The chain lengths are again determined as a function of field strength and the atom polarisabilities. Scaling behaviour is analysed.     

Theoretical insights into the formation of thiolate-protected nanoparticles from gold(Ⅲ) chloride

Reaction pathways for the formation of thiolate-gold nanoparticles are investigated by density functional theory(DFT) and a new mechanism upon solvent polarity and tetraalkylammonium is obtained. In solvents with high polarities, [Au(I)SR]n polymers can be formed as the precursor of metal ions prior to the addition of a reducing agent; while a product of [Cl···AuCl(HSR)] is identified as the precursor in solvents with low polarities, such as toluene and chloroform.In addition, tetraalkylammonium also has an obvious effect on the reactions when it is used as a phase transfer agent in the two-phase synthesis. These findings offer a systematic analysis on the pathways to thiolate-stabilized nanoparticles and give a favorable explanation by comparison with those in an experimental system.     

Structural transition dynamics of the formation of warm dense gold: From an atomic scale view

With the highly optimized embedded-atom-method(EAM)potential and electron-phonon coupling factor obtained from experimental data,the dynamics of the formation of warm dense gold and the nuclear response of gold foils upon intense laser excitation were investigated using two-temperature molecular dynamics simulations.Considering laser energy densities ranging from 0.18 to 1.17 MJ/kg,we provide a microscopic picture of the formation of warm dense gold.A threshold(0.19 MJ/kg)for the laser energy density was determined,identifying two different melting mechanisms.For an energy density below 0.19 MJ/kg,the melting of the foil is controlled by the propagation of melt fronts from external surfaces,which results in heterogeneous melting on the time scale of hundreds of picoseconds.For an energy density above 0.19 MJ/kg,homogeneous nucleation and growth of liquid regions inside the foil play the leading role,and homogeneous melting occurs with several picoseconds.Compared with previous simulations and experimental measurements,the evaluated different threshold value indicates that the improvement in the electron heat capacity for the two-temperature model by including the kinetic information of electrons may predict better laser-matter interactions under such extreme non-equilibrium conditions.     

Nanotechnology,voluntary oversight,and corporate social performance: does company size matter?

In this article, we examine voluntary oversight programs for nanotechnology in the context of corporate social performance (CSP) in order to better understand the drivers, barriers, and forms of company participation in such programs. At the theoretical level, we use the management framework of CSP to understand the voluntary behavior of companies. At the empirical level, we investigate nanotech industry participation in the Environmental Protection Agency’s Nanoscale Materials Stewardship Program (NMSP) as an example of CSP, in order to examine the effects of company characteristics on CSP outcomes. The analysis demonstrates that, on the average, older and larger companies for which nanotech is one of the many business activities demonstrate greater CSP as judged by company actions, declarations, and self-evaluations. Such companies tended to submit more of the requested information to the NMSP, including specific information about health and safety, and to claim fewer of the submitted items as confidential business information. They were also more likely to have on-line statements of generic and nano-specific corporate social responsibility principles, policies, and achievements. The article suggests a need to encourage smaller and younger companies to participate in voluntary oversight programs for nanotechnology and presents options for better design of these programs.     

Nonlinear conductance of long quantum wires at a conductance plateau transition: where does the voltage drop?

We calculate the linear and nonlinear conductance of spinless fermions in clean, long quantum wires, where short-ranged interactions lead locally to equilibration. Close to the quantum phase transition, where the conductance jumps from zero to one conductance quantum, the conductance obtains a universal form governed by the ratios of temperature, bias voltage, and gate voltage. Asymptotic analytic results are compared to solutions of a Boltzmann equation which includes the effects of three-particle scattering. Surprisingly, we find that for long wires the voltage predominantly drops close to one end of the quantum wire due to a thermoelectric effect.     

Fifty years of M?ssbauer spectroscopy: from alloys and oxides to glasses and nanoparticles

The M?ssbauer Effect was discovered in 1957. In 1960 M?ssbauer spectroscopy was born when two important papers appeared on (i) the magnetic hyperfine interaction and (ii) the electric monopole (isomer shift) and quadrupole interactions. These transformed an interesting phenomenon into a method for probing solids. Applications to magnetism, metals and alloys, chemical compounds, biological molecules, geology, archaeology and other sciences followed and are still of current interest. Two areas of research where M?ssbauer spectroscopy is making unique contributions are in determining oxidation states in (i) glasses and (ii) nanoparticles. Some recent measurements are described.     

Toxicity and cellular uptake of gold nanoparticles: what we have learned so far?

Gold nanoparticles have attracted enormous scientific and technological interest due to their ease of synthesis, chemical stability, and unique optical properties. Proof-of-concept studies demonstrate their biomedical applications in chemical sensing, biological imaging, drug delivery, and cancer treatment. Knowledge about their potential toxicity and health impact is essential before these nanomaterials can be used in real clinical settings. Furthermore, the underlying interactions of these nanomaterials with physiological fluids is a key feature of understanding their biological impact, and these interactions can perhaps be exploited to mitigate unwanted toxic effects. In this Perspective we discuss recent results that address the toxicity of gold nanoparticles both in vitro and in vivo, and we provide some experimental recommendations for future research at the interface of nanotechnology and biological systems.     

Melting of sodium clusters: where do the magic numbers come from?

Melting temperatures of Na clusters show size-dependent fluctuations that have resisted interpretation so far. Here we discuss that these temperatures, in fact, cannot be expected to exhibit an easily understandable behavior. The energy and entropy differences between the liquid and the solid clusters turn out to be much more relevant parameters. They exhibit pronounced maxima that correlate well with geometrical shell closings, demonstrating the importance of geometric structure for the melting process. Icosahedral symmetry dominates, a conclusion corroborated by new photoelectron spectra measured on cold cluster anions. In the vicinity of the geometrical shell closings the measured entropy change upon melting is in good agreement with a simple combinatorial model.     

Particle production at a finite potential step: transition from Euler–Heisenberg to Klein paradox

The European Physical Journal A -     

Insoluble organic matter in chondrites: Archetypal melanin-like PAH-based multifunctionality at the origin of life?

An interdisciplinary review of the chemical literature that points to a unifying scenario for the origin of life, referred to as the Primordial Multifunctional organic Entity (PriME) scenario, is provided herein. In the PriME scenario it is suggested that the Insoluble Organic Matter (IOM) in carbonaceous chondrites, as well as interplanetary dust particles from meteorites and comets may have played an important role in the three most critical processes involved in the origin of life, namely 1) metabolism, via a) the provision and accumulation of molecules that are the building blocks of life, b) catalysis (e.g., by templation), and c) protection of developing life molecules against radiation by excited state deactivation; 2) compartmentalization, via adsorption of compounds on the exposed organic surfaces in fractured meteorites, and 3) replication, via deaggregation, desorption and related physical phenomena. This scenario is based on the hitherto overlooked structural and physicochemical similarities between the IOM and the dark, insoluble, multifunctional melanin polymers found in bacteria and fungi and associated with the ability of these microorganisms to survive extreme conditions, including ionizing radiation. The underlying conceptual link between these two materials is strengthened by the fact that primary precursors of bacterial and fungal melanins (collectively referred to herein as allomelanins) are hydroxylated aromatic compounds like homogentisic acid and 1,8-dihydroxynaphthalene, and that similar hydroxylated aromatic compounds, including hydroxynaphthalenes, figure prominently among possible components of the organic materials on dust grains and ices in the interstellar matter, and may be involved in the formation of IOM in meteorites. Inspired by this rationale, a vis-à-vis review of the properties of IOM from various chondrites and non-nitrogenous allomelanin pigments from bacteria and fungi is provided herein. The unrecognized similarities between these materials may pave the way for a novel scenario at the origin of life, in which IOM-related complex organic polymers delivered to the early Earth are proposed to serve as PriME and were preserved and transformed in those primitive forms of life that shared the ability to synthesize melanin polymers playing an important role in the critical processes underlying the establishment of terrestrial eukaryotes.     

A theoretical study of N–H ··· π H-bond interaction of pyrrole: from clusters to the liquid

The N–H?···?π H-bond interactions of clusters and liquid formed by pyrrole molecules were investigated by Quantum Mechanics (QM) and classical Molecular Dynamics (MD), respectively. Based on the optimized geometry at the B97-D/aug-cc-pVTZ level of theory including dispersion correction, the nature and the origin of N–H?···?π H-bond interactions were unveiled by atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). Among them, the AIM analysis gives evidence to the presence of N–H?···?π H-bond interactions, the NBO examination reveals that π?→?σ* donor-acceptor orbital interaction is of great importance. EDA study indicates that N–H?···?π interactions are governed by the electrostatic and dispersion term. Meanwhile, MD simulation with OPLS-AA (optimized potentials for liquid simulations all-atom) was applied to study the pure liquid pyrrole at different temperature. The results confirm the existence of the N–H?···?π H-bond in the pure liquid pyrrole, and further characterized the structures of this H-bond which is somewhat different to the clusters.     

Growth of ZnO nanorod arrays by sol–gel method: transition from two-dimensional film to one-dimensional nanostructure

ZnO nanorod arrays were prepared by a sol–gel method in the present work. The effects of doping concentration and annealing time on the morphologies of ZnO:Al (ZAO) layers were investigated to clearly explore the growth process of ZnO nanorods by designing gradient structures and adjusting the annealing time. The results show that the doping level in the films is a key factor for the formation of nanorods and they cannot form at a low doping level. Out-of-plane anisotropic grain growth instead of the traditional in-plane coarsening process is observed with increasing annealing time. The growth model of nanorods is proposed in terms of the surface diffusion and Ehrlich–Schwoebel barrier (ES barrier) theory.     

Graphene edge from armchair to zigzag: the origins of nanotube chirality?

The energy of an arbitrary graphene edge, from armchair (A) to zigzag (Z) orientation, is derived in analytical form. It contains a "chemical phase shift" determined by the chemical conditions at the edge. Direct atomistic computations support the universal nature of the relationship, definitive for graphene formation, and shapes of the voids or ribbons. It has further profound implications for nanotube chirality selection and possibly control by chemical means, at the nucleation stage.     

The effect of ionisation of silica nanoparticles on their binding to nonionic surfactants in oil–water system: an atomistic molecular dynamic study

ABSTRACT

In this study, the adsorption of nonionic surfactant, triethylene glycol monododecyl ether (C₁₂E₃), on a surface of silica nanoparticle (NP) has been studied with variation in the degree of ionisation (DI) of silica NP using all-atom molecular dynamic simulations in hexadecane–water system. Hydrogen bonding is found to be responsible for the adsorption of C₁₂E₃ on NP, particularly at low DI. We observe that with increasing DI of NP, the amount of adsorption of C₁₂E₃ on NP reduces, which is negligible beyond DI ～ 0.5. The decrease in the adsorption with increasing DI is due to the decrease in the number of hydrogen bonds formed by the silica NP with surfactant molecules. Potential of mean force (PMF) profiles indicate attractive interactions between NP and C₁₂E₃ for DI < 0.5, and for larger DI depletion effect is observed. This work explains the unusual effect of nonionic surfactant on interfacial tension in the presence of silica particles as observed in recent experiments.     

The effect of climate on acoustic signals: does atmospheric sound absorption matter for bird song and bat echolocation?

The divergence of signals along ecological gradients may lead to speciation. The current research tests the hypothesis that variation in sound absorption selects for divergence in acoustic signals along climatic gradients, which has implications for understanding not only diversification, but also how organisms may respond to climate change. Because sound absorption varies with temperature, humidity, and the frequency of sound, individuals or species may vary signal structure with changes in climate over space or time. In particular, signals of lower frequency, narrower bandwidth, and longer duration should be more detectable in environments with high sound absorption. Using both North American wood warblers (Parulidae) and bats of the American Southwest, this work found evidence of associations between signal structure and sound absorption. Warbler species with higher mean absorption across their range were more likely to have narrow bandwidth songs. Bat species found in higher absorption habitats were more likely to have lower frequency echolocation calls. In addition, bat species changed echolocation call structure across seasons, using longer duration, lower frequency calls in the higher absorption rainy season. These results suggest that signals may diverge along climatic gradients due to variation in sound absorption, although the effects of absorption are modest.     

Diffuse T1 reduction in gray matter of sickle cell disease patients: evidence of selective vulnerability to damage?

The objective of our study was to test the hypothesis that subtle brain abnormality can be present in pediatric sickle cell disease (SCD) patients normal by conventional MR imaging (cMRI). We examined 50 SCD patients to identify those patients who were normal by cMRI. Quantitative MR imaging (qMRI) was then used to map spin-lattice relaxation time (T1) in a single slice in brain tissue of all 50 patients and in 52 healthy age-similar controls. We also used a radiofrequency (RF) pulse to saturate blood spins flowing into the T1 map slice, to characterize the effect of blood flow on brain T1. Abnormalities were noted by cMRI in 42% (21/50) of patients, with lacunae in 32%, and encephalo malacia in 20%. Brain T1 in patients normal by cMRI was significantly lower than controls, in caudate, thalamus, and cortex (p < or =0.007), and regression showed that gray matter T1 abnormality was present in caudate and cortex by age 4 (p < or =0.002). In patients abnormal by cMRI, T1 reductions in gray matter were larger and more significant. White matter T1 was not significantly increased except in patients abnormal by cMRI. RF saturation in a slab below the T1 map produced no significant change in T1, compared to RF saturation in a slab above the T1 map, suggesting that inflow of untipped spins in blood does not cause an artifactual shortening of T1. Gray matter T1 abnormality was present in patients normal by cMRI, while white matter T1 abnormality was present only in patients also abnormal by cMRI. These findings suggest that gray matter is selectively vulnerable to damage in pediatric SCD patients and that white matter damage occurs later in the disease process. Our inability to find an effect from saturation of inflowing blood implies that rapid perfusion cannot account for T1 reduction in gray matter.