Angstroms In Size Research Articles

Tuning Pore Size in Graphene in the Angstrom Regime for Highly Selective Ion-Ion Separation.

Zero-dimensional pores spanning only a few angstroms in size in two-dimensional materials such as graphene are some of the most promising systems for designing ion-ion selective membranes. However, the key challenge in the field is that so far a crack-free macroscopic graphene membrane for ion-ion separation has not been realized. Further, methods to tune the pores in the Å-regime to achieve a large ion-ion selectivity from the graphene pore have not been realized. Herein, we report an Å-scale pore size tuning tool for single layer graphene, which incorporates a high density of ion-ion selective pores between 3.5 and 8.5 Å while minimizing the nonselective pores above 10 Å. These pores impose a strong confinement for ions, which results in extremely high selectivity from centimeter-scale porous graphene between monovalent and bivalent ions and near complete blockage of ions with the hydration diameter, DH, greater than 9.0 Å. The ion diffusion study reveals the presence of an energy barrier corresponding to partial dehydration of ions with the barrier increasing with DH. We observe a reversal of K+/Li+ selectivity at elevated temperature and attribute this to the relative size of the dehydrated ions. These results underscore the promise of porous two-dimensional materials for solute-solute separation when Å-scale pores can be incorporated in a precise manner.

Anomalously enhanced ion transport and uptake in functionalized angstrom-scale two-dimensional channels.

Emulating angstrom-scale dynamics of the highly selective biological ion channels is a challenging task. Recent work on angstrom-scale artificial channels has expanded our understanding of ion transport and uptake mechanisms under confinement. However, the role of chemical environment in such channels is still not well understood. Here, we report the anomalously enhanced transport and uptake of ions under confined MoS2-based channels that are ~five angstroms in size. The ion uptake preference in the MoS2-based channels can be changed by the selection of surface functional groups and ion uptake sequence due to the interplay between kinetic and thermodynamic factors that depend on whether the ions are mixed or not prior to uptake. Our work offers a holistic picture of ion transport in 2D confinement and highlights ion interplay in this regime.

Rigid gigaporous chromatographic media and their potential impact on downstream processing

More and more biomolecules are being produced by the biotechnology industry for applications ranging from medicine and food to engineering materials. Liquid chromatography plays a center-stage role in a typical downstream process producing biomolecules such as recombinant proteins. Rigid gigaporous media are porous particles possessing large transecting through-pores with a pore-to-particle diameter ratio of d pore/ d particle> 0.01. They allow convective flow in the large through-pores, while the smaller diffusion-pores (typically several hundred angstroms in size) supply the needed surface areas. Because of the transecting gigapores, a portion of the mobile phase flows through the pores in addition to fluid flow in the interstitial spaces between the particles in a packed-bed column. This considerably lowers the operating column pressure drop. This lower pressure drop makes axial-direction scale-up of chromatographic columns possible to avoid pancake columns that invariably degrade separation resolution. The large gigapores also make the binding sites on the diffusion pore surfaces more accessible, thus increasing the loading capacity of large protein molecules that can be hindered sterically if only diffusion pores are present. This work discusses the development of rigid gigaporous media and their potential impact on the design of multi-stage downstream process from the angle of multi-scale analysis.

Liquid–liquid phase separation in alkali-borosilicate glass.: An impedance spectroscopy study

We followed the first stages of liquid–liquid phase separation in an alkali-borosilicate glass by complex impedance spectroscopy at constant temperatures below and above the glass transition temperature, T g. We found a new feature of the electrical conductivity behavior: at temperatures below T g, the electrical conductivity, σ, diminished with time, while above T g, the electrical conductivity increased. The activation energy, E a, for electrical conductivity was largest for samples heat-treated below T g. Other techniques, such as simultaneous small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and transmission electron microscopy (TEM) were also used. After annealing the glass inside the immiscibility gap, TEM shows a structure with a few Angstroms in size. Above and below the glass transition temperature, in situ SAXS isothermal experiments show an increase of the scattered intensity with time, while WAXS confirms that no crystallization occurred. These facts suggest that glass heat treated below T g develops a SiO 2 rich matrix with lower electrical conductivity and isolated sodium- rich regions. Annealing above T g (of the quenched glass) seems to produce an interconnected Na-rich phase, which dominates the electrical conductivity.

The reduction of aqueous Au3+by sulfide minerals and green rust phases

The reactions of Au3+ with green rust, stibnite, pyrite, and chalcopyrite were investigated in the laboratory and the size of the gold clusters formed was measured using extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The individual clusters produced were between approximately 29 Å and 77 Å in diameter and may occur individually or as composite clusters from hundreds to thousands of angstroms in size. This work shows that it is possible to form “invisible” gold through reduction of Au3+ by green rust phases and the surfaces of sulfide minerals.

Phase transitions in silicon single crystals resulting from directional plastic deformation

The formation of polytypic modifications is observed in dislocation-free silicon single crystals under directional plastic deformation. It is shown that the deformation-stimulated phase appears on the surface of the sample in the form of small grains ranging from several hundred to several thousand angstroms in size. A twin structure in the individual grains is observed.

Nucleation and growth kinetics in diblock styrene-butadiene

Abstract Time-resolved small-angle x-ray scattering has been used to study the phase transformation kinetics of a thermally quenched, asymmetric, diblock copolymer. The roles played by homogeneous and heterogeneous nucleation in the microdomain ordering process are considered through comparison of results from pure samples and samples doped with impurities. This crystallization process is unique in that it involves constituent particles (microdomains) that are hundreds of angstroms in size ordering onto a lattice with spacings on a similar length scale. Models derived from classical nucleation and growth theories are used to analyze results from pure and doped samples. Clear experimental evidence of a heterogeneous nucleation process occurring in the doped samples suggests that impurity effects cannot be ignored in these materials. In addition, it is proposed that the late-time coexistence of crystalline and amorphous microdomain structures is due to strain energy effects induced by the volume constraints...

Diffusion-Induced Precipitation in Arsenosilicate Glass (AsSG)

ABSTRACTDiffusion-induced precipitation in arsenosilicate glass (AsSG) under high temperature anneal is studied by high resolution Transmission Electron Microscopy (TEM), and other techniques. Polycrystalline precipitates of a few hundred angstroms in size have been observed near the interface of AsSG and the silicon substrate, covering the entire interface area after high temperature anneal. It is proposed that high concentrations of arsenic (above the solid solubility limit) precipitate initially at nucleation sites near the AsSG/silicon interface. Further anneal-induced diffusion of arsenic to the interface promotes growth of the precipitates. As a result, the precipitates cover the entire interface area, and impede further As diffusion into the Si substrate. Techniques to avoid the precipitation process without compromising the thermal budget or reduced arsenic diffusion have been explored and will be discussed.

Artificial Atoms

The wizardry of modern semiconductor technology makes it possible to fabricate particles of metal or “pools” of electrons in a semiconductor that are only a few hundred angstroms in size. Electrons in these structures can display astounding behavior. Such structures, coupled to electrical leads through tunnel junctions, have been given various names: single-electron transistors, quantum dots, zero-dimensional electron gases and Coulomb islands. In my own mind, however, I regard all of these as artificial atoms—atoms whose effective nuclear charge is controlled by metallic electrodes. Like natural atoms, these small electronic sytems contain a discrete number of electrons and have a discrete spectrum of energy levels. Artificial atoms, however, have a unique and spectacular property: The current through such an atom or the capacitance between its leads can vary by many orders of magnitude when its charge is changed by a single electron. Why this is so, and how we can use this property to measure the level spectrum of an artificial atom, is the subject of this article.

A Laser-Based Technique for Particle Sizing to Study Two-Phase Expansion in Turbines

Efficiency of binary power cycles can be improved by expanding the hydrocarbon working fluids through two-phase region in a turbine and exiting at saturated or superheated condition. This improvement can be achieved if there is no condensation during the expansion or if there is condensation, the droplet size is extremely small. In order to verify this, a particle sizing technique for extremely small particles in flow is needed. In this study, a laser-based technique is developed by which it is possible to detect particles as small at ten angstroms in size. The basis of the technique is that particles of size less than one third of the wavelength of the incident radiation will scatter according to Rayleigh scattering theory. According to this theory, the intensity of the scattered light will be the same in the forward as well as in the backward directions. Therefore, measurement of the scattered intensity at two or three different angles will confirm the presence of Rayleigh scattering. The size of the particles can, then, be calculated from the measured scattered intensity according to the Rayleigh scattering equation.

Superfluid Transition in Porous Media Showa Puzzling Features

Liquid helium in the superfluid phase behaves as if a fraction of the liquid has zero viscosity—the liquid flows through narrow tubes even when there is no pressure gradient across the tubes' ends. This “superfluid fraction” is zero above the critical temperature for the onset of superfluidity, and it increases to unity when the temperature is decreased to absolute zero in the superfluid phase. When pure helium is three dimensional—for example, when it fills a beaker—the critical temperature for superfluidity is 2.17 K. When only a thin film of helium covers a plane substrate, however, the critical temperature depends on the film thickness and the substrate. Since the late 1970s, when our understanding of the superfluid transition in two and three dimensions was essentially completed, experimenters have been studying the superfluid transition when helium fills pores, on the order of a few tens of angstroms in size, in highly connected porous structures. These experiments have shown features in the past few years that both theorists and experimenters say they do not understand.

Ammonia synthesis over a supported iron catalyst prepared from an amorphous iron-zirconium precursor: II. Surface morphological changes during the genesis of the catalyst

Amorphous Fe 91Zr 9, when exposed to ammonia synthesis conditions, undergoes drastic changes in the bulk and surface structure which finally lead to an active ammonia synthesis catalyst. Investigation of the surface morphology of the precursor using both scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) revealed that the surface of the amorphous alloy is not flat, but consists of disk-shaped metal particles several hundred angstroms in size with clear valleys separating them. Upon activation, crystallization and reconstruction of the precursor surface occurs and results finally in the active catalyst consisting of iron particles stabilized by poorly crystalline zirconia. SEM showed that the active iron particles exist predominantly in the form of stacks of disks and well-developed crystals. Both morphologies are uncommon in conventional ammonia synthesis catalysts. The genesis of these morphologies is discussed based on the results of SEM and STM investigations.

Topographie de surfaces Si(111) après bombardement d'ions Ar+ et recuit thermique

Flat Si(111) surfaces were submitted to a 10 keV argon bombardment and then thermally annealed, the surface being simultaneously investigated by R.H.E.E.D. and Reflection Electron Microscopy. It was found that the annealing temperature needed for recovering a trace of crystallinity and the surface roughness were strongly dependent on the implanted dose. Damage and range calculations were performed in order to estimate Ar concentrations and damage distributions in the implanted layer and close to the surface. For a dose of 6×10 13 ions/cm 2 , which corresponds to the extinction of R.H.E.E.D. spots and the disappearance of atomic steps in the REM image, 30 min-575°C anneal restores the initial topography. Far from the amorphous state, the surface layer thus desorbs the argon atoms embedded in the lattice. For a dose of 4×10 15 ions/cm 2 which corresponds to the formation of an amorphous layer extending from the surface to a depth of 220 A, a 20 h-750°C anneal is needed for a partial epitaxial regrowth of the surface layer (microcrystals of a few thousand angstroms in size). For an intermediate dose of 6×10 14 ions/cm 2 where defect rich and amorphous regions may be present at the surface and in the implanted layer, a 90 min-700°C anneal is needed to recover a monocrystalline but very rough surface. These experiments suggest that a local epitaxial regrowth is obtained only if the argon concentration is lower than some upper limit in the interfacial Etude de la reorganisation, par recuit thermique, de surface Si(111) prealablement bombardees par des ions argon d'energie incidente E 0 =10 keV, en utilisant les techniques de diffraction electronique RHEED et d'imagerie REM. L'influence de la dose implantee sur la temperature et la duree du recuit necessaire, ainsi que la topographie de la surface obtenue, a ete mise en evidence. La theorie LSS et un programme de calcul d'energie deposee permettent d'interpreter l'ensemble des resultats

Local-field enhancement on rough surfaces of metals, semimetals, and semiconductors with the use of optical second-harmonic generation

Optical second-harmonic generation was used to study the local-field enhancement due to surface roughness on various materials ranging from the alkalis to a semiconductor. The roughness morphology was standardized by evaporating each material onto the same chemically etched glass slide, having microstructures hundreds to thousands of angstroms in size. With the laser excitation at 1.06 \ensuremath{\mu}m, the observed second-harmonic enhancements for different materials varied from 27 to 1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ times that of silver. They were in fair agreement with a simple model calculation assuming that the rough surface is composed of a distribution of noninteracting hemispheroids on a plane. The results are used to predict some rather substantial enhancements for surface Raman scattering for a number of substrate materials.

Measurements of temporal and spatial sequences of events in periodic precipitation processes

A series of new experiments on Liesegang ring (or band) formation is presented which is concerned with the temporal and spatial evolution of the process of structure formation. We have chosen NH4OH and MgSO4 to form rings of Mg(OH)2 precipitate in a gelatin gel, as well as KI and Pb(NO3)2 for periodic precipitation of PbI2 in an agar gel. A temporal sequence of events during the entire period from the start of a Liesegang experiment in a test tube to the completion of the final ring pattern has been determined at many locations in the tube by visual observations and by measurements of transmitted light, of scattered light, of deflection of the transmitted light beam, and of gravity effects. After diffusion of one electrolyte into the gel medium containing the second electrolyte results in an ion product larger than three times the solubility product, at any and all points in space, we observe the onset of homogeneous nucleation of colloidal particles by a steplike increase of the index of refraction. The colloid concentration and the particle number density at the nucleation site are estimated to be 10−2 mol/l and 1015 to 1016 cm−3, respectively. Nucleation is followed by the growth of colloidal particles which gives rise to distinct light scattering (turbidity). Both nucleation and colloid formation take place in space continuously; the fronts of these phenomena move through the system and obey a simple diffusion law. A substantial time interval after their passage, there arises a localized gradient of the index of refraction at the prospective ring positions which indicates onset of structure formation by means of a focusing mechanism. While the localized gradient becomes more pronounced and narrower in space, the turbidity in the regions on either side of the ring location decreases, which indicates a depletion in colloidal material in the neighboring zones. Eventually, a sharp band of visible precipitate appears, which is clearly separated from the preceding ring. We conclude that the ring formation is a postnucleation phenomenon in that structure arises from a spatially continuous region of colloid a long time after nucleation has occurred, and propose that it is associated with the autocatalytic growth of colloidal particles. The location of rings is not determined by the spatial pattern of nucleation and colloid deposition as predicted by the Ostwald–Wagner–Prager theory. Our conclusions are supported by experiments on the influence of gravity on the ring locations, which provide evidence for the existence of colloidal particles of several hundred angstroms in size for a substantial fraction of the time required for the formation of a visible structure.

Studies of zinc-chromium hydroxy salts. I. Thermal decomposition of [Zn 2Cr(OH) 6] X · nH 2O, where [formula omitted

We have characterized the layered anion exchange compounds [Zn 2Cr(OH) 6] X · nH 2O, where X −= F −, CI −, Br −, I −, 1 2 CO 2− 3, and NO − 3 , and studied their thermal decomposition. The precipitated particles were less than a few hundred angstroms in size, and could not be easily aged. Upon heating, a considerable amount of surface water, up to n = 1 in the formula, was initially lost, followed by the ill-defined stages represented by loss of water of hydration ( n ≤ 2), first stage of dehydroxylation, loss of X (over temperature ranges between 200 and 500°C) and final dehydroxylation (up to 700°C). Interest in such possible mixed O 2−/OH −/ X − decomposition intermediates as ionic conductors is discussed.

On the mechanism of Weigert effect in silver halide materials

The mechanism of Weigert effect (WE) was determined. Photolitic silver particles was shown to be anisotropic and a few dozen angstroms in size. The particles were pointed out to be mainly oblate. Selective influence of monochromatic polarized light to the particles of the certain form and orientation might lead to their more faster destruction (WE under bleaching), suppression of growth (WE under colouring) and changing of shape (reversion of dichroism). To verify the mechanism advanced a number of experiments were cited.

Preparation of Tin Oxide Films from Ultrafine Particles

A highly reproducible procedure for preparing films from ultrafine particles of metal oxides has been developed. The ultrafine particles of several tens of angstroms in size are grown by the evaporation of the metal in low oxgen gas pressure, and subsequently, the film is constructed from the ultrafine particles aligned in a specific direction on the substrate. The particles' size and film structure were studied under various deposition conditions, i.e., oxygen gas pressure, rf activation power, evaporation temperature, and others. The ultrafine particle films prepared with this procedure may serve as a highly sensitive gas sensor.

Preparation and properties of alumina supermicrogrid

It is often required in high resolution electron microscopy to use very thin supporting films for the reduction of random noise. When the specimen is crystalline film, it is also desirable that the thickness is controlled to be less than a hundred angstroms so that the specimen can be dealt with as a thin phase object. In both cases, a suitable supporting technique is necessary to secure the stability and mechanical strength of these very thin films and perhaps very small fragments of a few hundred angstroms in size. The present work was undertaken to prepare rigid supporting films by means of chemical thinning of alumina films produced by the anodic oxidation of aluminium plate. The overall procedure is outlined in Fig. 1.The aluminium to be oxidized is cut in a form of a 1cm × 2cm plate from extra pure foil whose thickness ranges between 15 to 30μm.

Electron microscopy and small angle x-ray studies of amorphous polymers

Abstract The small-angle X-ray scattering (SAXS) from polycarbonate, polymethyl methacrylate, and polyethylene terephthalate seem inconsistent, both in the magnitude of the SAXS intensity and in its variation with scattering angle, with nodular features as representative of the bulk polymers. Rather than nodules, the SAXS data indicate the presence of thermal density fluctuations frozen-in at the glass transitions, together with a small concentration of large heterogeneities (hundreds to thousands of angstroms in size). Electron microscopy (EM) of amorphous polycarbonate provides further support for the suggestion that nodular features are not representative of the bulk material. The only structure seen in bright field is the “salt and pepper” background common to all materials. In dark field, very small structures (<30 Å in size) occupying small volume fractions were observed. These results are at variance with previously reported EM observations of nodular features in polycarbonate. It is suggested that the structure of these polymers be regarded as that of a random amorphous solid, and that the nodular structures may reflect surface features of the materials.