Three-dimensional dielectric nanostructures have been analyzed using field ion microscopy (FIM) to study the electric dc field penetration inside these structures. The field is proved to be screened ...within a few nanometers as theoretically calculated taking into account the high-field impact ionization process. Moreover, the strong dc field of the order of 0.1 V/Å at the surface inside a dielectric nanostructure modifies its band structure leading to a strong band gap shrinkage and thus to a strong metal-like optical absorption near the surface. This metal-like behavior was theoretically predicted using first-principle calculations and experimentally proved using laser-assisted atom probe tomography (APT). This work opens up interesting perspectives for the study of the performance of all field-effect nanodevices, such as nanotransistor or super capacitor, and for the understanding of the physical mechanisms of field evaporation of dielectric nanotips in APT.
To understand atom probe results on the field evaporation of oxides we use density functional theory on MgO clusters to follow the structural changes during field evaporation and toobtain potential ...energy curves, partial charges and desorption pathways. It is straightforward to understand that Mg evaporates doubly charged. We also show that MgO+, MgO2+, MgO2+ and O+ ions leave the surface. Two questions are however new for oxides. (1) Where do the electrons go? When the oxides are deposited on a metal tip it can be assumed that the electrons are used to complete the electrical circuit. However this leaves the second question unanswered, namely (2) what happens to the oxygen? We will argue that there are two channels for the oxygen, namely (a) To travel down the (metallic) surface of the tip and eventually to desorb either as atoms or molecules. (b) The oxygen can recombine within the oxide layer itself and desorbs as a neutral molecule accelerated in the inhomogeneous field due to its induced dipole.
► Density functional theory is used on MgO clusters to follow the changes during field evaporation. ► We also show that Mg2+ , MgO+, MgO2+ and O+ ions leave the surface. ► The calculations agree with the experimental results.
In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a ...mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface.
We look at the new challenges associated with Atom Probe Tomography of insulators and semiconductors with regard to local fields inside and on the surface of such materials. The theoretical discovery ...that in high fields the band gap in these materials is drastically reduced to the point where at the evaporation field strength it vanishes will be crucial in our discussion. To understand Atom Probe results on the field evaporation of insulators and semiconductors we use density functional theory on ZnO clusters to follow the structural and electronic changes during field evaporation and to obtain potential energy curves, HOMO–LUMO gaps, field distributions, desorption pathways and fragments, dielectric constants, and polarizabilities. We also examine the effects of electric field reversal on the evaporation of ZnO and compare the results with Si.
•The effects of high electric fields on ZnO are analyzed by ab-initio calculations.•High electric fields induce an insulator to metal transition.•Closure of the HOMO–LUMO gap occurs during field evaporation.•A small cluster can reproduce bulk dielectric values reasonably well.
Three different tungsten alloys (pure W, a lanthanum-oxide dispersion strengthened W-alloy – WL10 and a potassium doped tungsten alloy – WVM) in different fabrication conditions (sintered and rolled ...rods) were tested to characterize their fracture behaviour at different temperatures (from −196 to 800
°C). Additionally severe plastic deformed tungsten alloys were produced. It is shown that an ultra fine grained microstructure with a diameter of about 300
nm is able to decrease the brittleness at room temperature significantly. Such W-alloys are very attractive candidate materials for fusion applications.
Digital in-line holography with numerical reconstruction has been developed into a new tool, specifically for biological applications, that routinely achieves both lateral and depth resolution, at ...least at the micron level, in three-dimensional imaging. The experimental and numerical procedures have been incorporated into a program package with a very fast reconstruction algorithm that is now capable of real-time reconstruction. This capability is demonstrated for diverse objects, such as suspension of microspheres and biological samples (diatom, the head of Drosophila melanogaster), and the advantages are discussed by comparing holographic reconstructions with images taken by using conventional compound light microscopy.
Iron−silicon steels have excellent magnetic and electrical properties, especially, with increased silicon and aluminum contents. However, increasing alloying with Si and Al reduces the workability of ...the material, and spontaneous cleavage fracture may occur during production. The main influence factors on the occurrence of cleavage fracture of Fe−Si−Al alloys are investigated in this paper. Fracture mechanics tests are performed at various temperatures and loading rates. The effects of specimen orientation and strain aging are studied. The tests result surprisingly high fracture toughness values. However, cleavage fracture regions are found in the pre-fatigue regions of the specimens, i.e. at much lower loads. In order to resolve this effect, the strain rates during the various fatigue- and fracture experiments are estimated by an analytical model. Additional fatigue experiments are performed at various frequencies where the stress intensity range is increased until cleavage fracture is detected. Similar experiments are conducted at increasing temperatures. The analyses reveal a significant transition behavior of the stress intensity at the onset of cleavage fracture, where the effects of decreasing temperatures and increasing strain rates are equivalent. Measures for avoiding cleavage fracture can be deduced from the results of this investigation.
Digital in-line holographic microscopy is a promising new tool for high resolution imaging. We demonstrate, by using latex beads, that a considerable increase in numerical aperture, and, therefore, ...resolution can be achieved if the space between a source and a CCD camera chip is filled with a high refractive index medium. The high refractive index medium implies a shorter effective wavelength so that submicrometer resolution can be obtained with laser light in the visible range.
High electrostatic fields cause major changes in polymers, structural (e.g. electrostriction) and electronic (e.g. reduction of the “band gap” with final metallization). Using density functional ...theory we have studied field effects on amino-alkane-thiols and perfluoro-alkane-thiols adsorbed on a metal substrate. Our results agree well with the APT fragmentation spectra obtained by Stoffers, Oberdorfer and Schmitz and shed light on disintegration pathways. We demonstrate that in SAMs the HOMO/LUMO gap is again reduced as a function of the field strength and vanishes at evaporation. We also follow the field dependence of the dielectric constant and polarizability.
•Simple model of thiolate polymers is used to understand trends leading to field evaporation.•Potential energy curves followed by dipole moment, electric polarizability and dielectric constant are examined.•Features including coil, tilt and electrostriction are featured alongside evaporated species and HOMO/LUMO gap.
PDF
