Atomic Diameter Research Articles

Dual-phase glassy/nanoscale icosahedral phase materials in Cu–Zr–Ti–Pd system alloys

The present work is devoted to an investigation of the formation kinetics, stability and homogeneity area of the nanoscale icosahedral phase formed on heating in the dual-phase glassy/quasicrystalline phase Cu–Zr–Ti–Pd alloys. The data obtained indicate that the Cu–Zr–Ti–Pd icosahedral phase is not a Cu-rich part of the compositional homogeneity area of the Zr–Cu–Pd one. Moreover, Ti, as well as Pd, is found to be an important element stabilizing quasicrystalline phase in the Cu–Zr–Ti–Pd alloys. The formation criteria for Cu- and Zr/Hf-based icosahedral phases are discussed based on the quasilattice constant to average atomic diameter ratio. Deviation from a certain ratio leads to destabilization of the icosahedral phase. By using the isothermal calorimetry traces transformation kinetics above and below the glass-transition region was analyzed. Some difference in the transformation kinetics above and below the glass-transition region allows us to suggest that possible structure changes occur upon glass-transition.

Thermal stability of Ag films with various interface layers

The effects of Pd, Ni, Nb, and Ti interface layers on the thermal stability of Ag films were investigated, and those metals that are appropriate for the interface layers were summarized. The results indicated that Ti and Nb interface layers resulted in thermally stable Ag films by improving the adhesion strength of the Ag films to SiO2 substrates and by enhancing the crystal orientation of Ag(111). It was determined that appropriate interface layer metals for Ag film should have a large Gibbs free energy of formation for the oxide and an appropriate atomic diameter ratio of dmetal/dAg.

Nanoscale control designs for systems.

Nanoscale control is the science of the control of objects at dimensions with 100 nm or less and the manipulation of them at this level of precision. The desired attributes of systems under nanoscale control design are extreme high resolution, accuracy, stability, and fast response. An important perspective of investigation in nanoscale control design includes system modeling and precision control devices and materials at a nanoscale dimension, i.e., design of nanopositioners. Nanopositioners are mechatronic systems with an ultraprecise resolution down to a fraction of an atomic diameter and developed to move objects over a small range in nanoscale dimension. After reviewing a lot of existing literatures for nanoscale control designs, the way to successful nanoscale control is accurate position sensing and feedback control of the motion. An overview of nanoscale identification, linear, and nonlinear control technologies, and devices that are playing a key role in improving precision, accuracy, and response of operation of these systems are introduced in this research.

Numerical Simulation of Ejected Molten Metal Nanoparticles Liquified by Laser Irradiation: Interplay of Geometry and Dewetting

Metallic nanoparticles, liquified by fast laser irradiation, go through a rapid change of shape attempting to minimize their surface energy. The resulting nanodrops may be ejected from the substrate when the mechanisms leading to dewetting are sufficiently strong, as in the experiments involving gold nanoparticles [Habenicht etal., Science 309, 2043 (2005)]. We use a direct continuum-level approach to accurately model the process of liquid nanodrop formation and the subsequent ejection from the substrate. Our computations show a significant role of inertial effects and an elaborate interplay of initial geometry and wetting properties: e.g., we can control the direction of ejection by prescribing appropriate initial shape and/or wetting properties. The basic insight regarding ejection itself can be reached by considering a simple effective model based on an energy balance. We validate our computations by comparing directly with the experiments specified above involving the length scales measured in hundreds of nanometers and with molecular dynamics simulations on much shorter scales measured in tens of atomic diameters, as by M. Fuentes-Cabrera etal. [Phys. Rev. E 83, 041603 (2011)]. The quantitative agreement, in addition to illustrating how to control particle ejection, shows utility of continuum-based simulation in describing dynamics on nanoscale quantitatively, even in a complex setting as considered here.

Structure, equation of state, diffusion and viscosity of warm dense Fe under the conditions of a giant planet core

Fe exists abundantly in the universe. In particular, the dynamic structures and transport properties of warm dense Fe are crucial for understanding the evolution and structures of giant planets. In this paper, we present the ionic structures, equation of states, diffusion and viscosity of Fe at two typical densities of 33.385 and 45 g cm−3 in the temperature range of 1–10 eV, giving the data by the first principles calculations using quantum Langevin molecular dynamics. Furthermore, the validation of the Stokes–Einstein (SE) relation in this regime is discussed, showing the importance of choosing the effective atomic diameter. The results remind us of the careful usage of the SE relation under extreme conditions.

Fundamental quantum noise mapping with tunnelling microscopes tested at surface structures of subatomic lateral size

We present a measurement scheme that enables quantitative detection of the shot noise in a scanning tunnelling microscope while scanning the sample. As test objects we study defect structures produced on an iridium single crystal at low temperatures. The defect structures appear in the constant current images as protrusions with curvature radii well below the atomic diameter. The measured power spectral density of the noise is very near to the quantum limit with Fano factor F = 1. While the constant current images show detailed structures expected for tunnelling involving d-atomic orbitals of Ir, we find the current noise to be without pronounced spatial variation as expected for shot noise arising from statistically independent events.

Design Of Feedback Controller For Non-minimum Phase nanopositioning System

One of the most important requirement of nanotechnology is precision control and manipulation of devices and materials at nanoscale i.e. nanopositioning. Nanopositioners are precision mechatronic system designed to move objects over a small range with a resolution down to a fraction of an atomic diameter. In particular, desired specifications of any nanopositioners are fast response with no or very little overshoot, large travel range with very high resolution, extremely high precision and high bandwidth. This paper presents design and identification of nanopositioning device consisting of flexure stage, piezoelectric actuator and Linear Variable Differential Transformer (LVDT) as a sensor. Open loop behavior of the nanopositioning device on the basis of time and frequency responses is studied. To improve the system characteristics feedback controllers are used. Step response and frequency response under variety of conditions are obtained to verify the effectiveness of the proposed controllers. In this paper PI and PI2 controllers are designed and system performances are investigated for different values of feedback gain. Unfortunately nanopositioners operating in closed loop achieve high bandwidth at the cost of increased sensitivity to the measurement noise and hence reduced resolution. In this paper H infinity controller is analyzed and performance of the device is studied. Then a comparative study of traditional PI and PI2 controller with H infinity controller on the basis of time and frequency response is given to show which controller is better. Simulation results for the performance analysis are carried out in MATLAB. Copyright © 2013 VBRI press.

CARBON ATOMIC WIRES: FROM STARS TO NANOTECHNOLOGY

Carbon can produce a wide variety of systems and nanostructures with different dimensionality as exemplified by fullerenes, nanotubes and graphene, all with peculiar properties of great interest for fundamental and applied research. In this framework sp-hybridized carbon atoms can arrange to form atomic wires with size dependent functional properties. The sp form of carbon has a long story passing through astrophysics and minerals found in meteor craters and, although still considered elusive and unstable, nowadays a number of nano-systems constituted by or containing sp-carbon wires have been produced and characterized. As graphene is considered the thinnest material (one-atom thick), carbon atomic wires represent true 1-dimensional systems (one- atom diameter) with a great potential for nanoscience and nanotechnology. The status of research on carbon atomic wires is here discussed, starting from the description of ideal systems to real structures, showing that the properties can be tuned by controlling the wire length (i.e. number of carbon atoms) and termination (atom, molecular group or nanostructure). Synthesis techniques are presented as well as strategies to have stable wires. A particular attention will be given to the use of vibrational spectroscopy to provide insight on the structure and electronic properties of these systems. Perspectives for novel devices based on the exploitation of the electronic properties of these systems are also discussed.

Spin-dependent Smoluchowski effect

Electron charge near atomically sharp corrugations at the surfaces of a solid tends to spill out and smoothen the abrupt variation of the positions of the positively charged atomic nuclei. The reason is that electrons are much less localized than nuclei. This has been discussed already some 70 years ago by Smoluchowski [R. Smoluchowski, Phys. Rev. 60, 661 (1941)], and the corresponding effect of charge redistribution near surface corrugations bears his name. The Smoluchowski effect focuses on the total electron charge density. It neglects that electrons---in addition to charge---also carry a spin. We discuss spin-dependent electron spill out and demonstrate in a combined theoretical and experimental work that compelling consequences for spin-polarization and spin-dependent transport arise at the edges of magnetic nanostructures due to the spin-dependent Smoluchowski effect. We find a variation of the tunnel magnetoresistance ratio of more than 20$%$ on a length scale of a few atomic diameters.

Microscopic and mesoscopic structural features of an activated carbon sample, prepared from sorghum via activation by phosphoric acid

An acidic chemical activation procedure has been used for preparing activated carbon with a surface area exceeding 1000m2/g from sorghum. In order to reveal structural features, synchrotron X-ray diffraction measurements have been performed. The structure of the material has been characterized by the total scattering structure factor and the radial distribution function describing short-range arrangement of atoms at distances of the order of a few atomic diameters as well as correlations at a longer scale, of the order of nanometers. The atomic arrangement has been found to be consistent with that of amorphous graphite-like carbon. As far as the mesoscopic structure is concerned, the presence of a characteristic distance is suggested on the basis of the clear nanometer scale oscillations of the radial distribution function, which distance may be assigned as the mesopore size in the material. It is suggested that the approach devized here may later be applied routinely for other activated carbon samples, too, for characterizing atomic and nanoscale order simultaneously.

Band Gap Blueshift of Hollow Quantum Dots

The nanostructures with negative curvature have fascinating properties induced by a large fraction of undercoordinated atoms in inner and outer surfaces as comparable to the nanostructures with positive curvature such as nanocrystals or quantum dots. Herein, we theoretically proposed the hollow quantum dot (HQD) as a new kind of the nanostructures with the negative curvature. A sandwiched structure in shell-core-shell configuration with surface shell of an atomic diameter was considered for HQD, and an analytical model was developed to elucidate the cohesive energy and the energy band gap of HQDs. It was found that the band gap of HQD exhibits a pronounced blueshift as comparable to those of the counterpart. The lattice strain induced by surface energy and coordination number imperfection in inner and outer surfaces of HQD changes the chemical bonding among atoms, and this change leads to the band gap blueshifts. The theoretical predictions are consistent with experimental observations.

Micro-mechanism of rolling contact fatigue in Hadfield steel crossing

The formation of the fatigue cracks was due to massive vacancy clusters in the subsurface layer of Hadfield steel crossing, which are induced by the accumulated plastic deformation under the conditions of impact and contact stresses from train wheels. The high concentration layers of vacancy clusters were formed parallel to the working surface of the crossing, which caused the initial rolling contact fatigue cracks to be parallel to the working surface with a laminar distribution in the depth direction. It can be predicted that metals containing elements with larger atomic diameter should have better rolling contact fatigue and wear performances.

Toward an Atomic-Level Understanding of Size-Specific Properties of Protected and Stabilized Gold Clusters

Abstract Metal clusters consisting of fewer than 100 atoms (diameter <2 nm) are highly promising as a new class of building units for functional materials because of their novel and size-dependent properties. Nevertheless, basic and applied studies of metals clusters have been hampered by the lack of specific guidelines for design and precise synthetic methods. This account surveys recent investigations of gold clusters focusing on our effort toward an atomic-level understanding and control of their size-specific properties. We have developed a size-controlled method for synthesizing gold clusters protected by ligands, stabilized by polymers, and supported on solids. Remarkable size-effects on stabilities and various properties including catalysis were observed. Their mechanisms are discussed based on fundamental knowledge of bare gold clusters in the gas phase.

The Solute Drag Model to Calculate Grain Growth Rate at High Temperatures in Carbon Steels

Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modeled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Scaling down lateral dimensions of silicon nanopillars fabricated by reactive ion etching with Au/Cr self-assembled clusters as an etch mask

Nanodot and nanopillar structures and precisely controlled reproducible fabrication thereof are of great interest in common nanoelectronic devices, including photonic crystals and surface plasmon resonance instruments. In this work, fabrication process of the silicon nanopillar structures is described. It includes self-organization of gold and chromium clusters at thickness close to that of one atomic diameter to serve as etching masks followed by the reactive ion etching to form silicon nanopillars. Scanning electron microscopy and X-ray photoelectron spectroscopy were used to characterize self-organized gold and chromium clusters as well as the final silicon nanopillars. This method was found to produce silicon nanopillars of sub-10 nm lateral dimensions and the diameter-to-height aspect ratio of up to 1:14.

Calculation for grain growth rate of carbon steels by solute drag model considering segregation effect of each substitutional element

Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modelled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Public exhibit for demonstrating the quantum of electrical conductance

We present a robust setup that demonstrates the quantum of electrical conductance for a general audience. The setup is continuously available in a public space and allows users to manually thin a gold wire of several atoms in diameter while monitoring its conductance in real time. During the experiment, a characteristic step-like decrease in the conductance due to rearrangements of atoms in the cross-section of the wire is observed. Just before the wire breaks, a contact consisting of a single atom with a characteristic conductance close to the quantum of conductance can be maintained for up to several seconds. The setup is permanently available, needs little maintenance and can be used on different educational levels.

Effects of Bond Parameters on Hydrogen Storage Capacity of Ti-V-Cr BCC Phase Alloys

Abstract The relation between the effective hydrogen storage capacity C H of Ti-V-Cr alloys and out-layer electrons, atomic diameter difference, electronegativity difference has been studied. It is shown that their relation can be expressed as T ln C H = 0.45922 T + 5044.8 (Δ X ) 2 + 1250.6 δ 2 −285.80 n 2/3 . The factor of out-layer electrons has the largest effect on the C H of Ti-V-Cr alloys, while the factor of electronegativity difference has the smallest effect among the three bond parameters. The effective hydrogen storage capacity of Ti-V-Cr alloys is greater than 2.0wt% when n 2/3 values of Ti-V-Cr alloys are between 1.98 and 2.14, δ 2 values are between 0.255 and 0.288 and (Δ X ) 2 values are between 0.0638 and 0.0765.

SLIT-WAVE MODEL FOR BAND STRUCTURES IN SOLID STATE PHYSICS

The reason behind the entire development in silicon technology was band models in solid state physics. However, the theories postulated in order to give response to this phenomenon do not explain all kinds of materials. In a bid to overcome this limitation, we approach the problem from another point of view. In this work, the wave properties of the electrons from the external orbitals of the atoms and its diffraction patterns through the lattice structure of the material have been used to explain the band structure of metals, semiconductor and insulators. In order to probe this hypothesis, a simulation has been used and according to the relation between the lattice constant and the atomic diameter, the splitting of the bands have been observed for different kind of materials, showing a strong correlation between the simulation and the experimental results.

New insights into the pore structure of poly( d, l-lactide-co-glycolide) microspheres

The objective of this work was to develop a fast and significant method for the determination of the intraparticulate pore size distribution of microspheres. Poly(lactide-co-glycolide) (PLGA) microspheres prepared with a solvent extraction/evaporation process were studied. From the envelope and the skeletal volume of the microspheres the porosity was calculated. The skeletal volume was determined with nitrogen and helium pycnometry and mercury intrusion porosimetry. Based on single particle optical sensing (SPOS) a novel method was developed by which the envelope volume is calculated from the particle size distribution (PSD), provided that all particles have a spherical shape. The penetration capacity of the applied intrusion media is limited by their atomic or molecular diameter or by the surface tension and the pressure in case of mercury. A classification of the pore structure was obtained by comparing these different skeletal values with the values for the envelope volume. Two well separated pore fractions were found, a nanoporous fraction smaller than 0.36 nm and a macroporous fraction larger than 3.9 μm. The total porosity and the ratio between both fractions is controlled by the preparation process and was shown to depend on the solvent extraction temperature.