Site control of InAs quantum dot nucleation by ex situ electron-beam lithographic patterning of GaAs substrates

Conventional electron-beam lithographic patterning of GaAs substrates followed by reactive-ion etching of small holes has been successfully used to control the nucleation of InAs dots. We have observed >50% single dot occupancy for holes wide and deep and show that the dot occupancy and dot size can be varied by changing the size of the holes. Luminescence from an array of these site-controlled dots has been demonstrated. Thus this use of substrate patterning is a viable technique to controllably place single dots at pre-determined positions in devices.     

Intermediate temperature creep mechanisms in Ni 3 Al

The intermediate-temperature creep response of single-crystal Ni 3 Al(Ta) has been investigated along both [ ] and [001] axial orientations. The effect of the existing deformation structure (i.e. pre-straining) on the [ ] creep response was reported. The creep responses of virgin specimens and specimens prestrained at room temperature (RT) and 520°C are compared. In order to compare the dislocation structures prior to creep, the microstructure of specimens which had been deformed at a constant strain rate at RT and 520°C, but not subjected to creep, was also examined. Creep curves show that the temperature of pre-strain influences the subsequent creep properties. The primary creep response, like the yielding response, appears to be controlled by the kink size distribution, while the secondary creep response is thought to be controlled by the kink separation (or the length of the Kear-Wilsdorf locks). Specimens crept along [ ] display steady state creep properties and rectangularly oriented [ ](010) dislocations, while a virgin specimen crept along [001] displays an increasing secondary creep rate (inverse creep) and d110 ¢{100}-type dislocations. Inverse creep along [001] is thought to be the result of an increasing density of edge kink octahedral sources where there is little resolved shear stress on the cube planes.     

Radiation energetics of ICF-relevant wire-array Z pinches

Short-implosion-time 20-mm diameter, 300-wire tungsten arrays maintain high peak x-ray powers despite a reduction in peak current from 19 to 13 MA. The main radiation pulse on tests with a 1-mm on-axis rod may be explained by the observable j x B work done during the implosion, but bare-axis tests require sub-mm convergence of the magnetic field not seen except perhaps in >1 keV emission. The data include the first measurement of the imploding mass density profile of a wire-array Z pinch that further constrains simulation models.     

Measurement of temperature, density, and particle transport with localized dopants in wire-array Z pinches

Axially localized NaF dopants are coated onto Al cylindrical wire arrays in order to act as spectroscopic tracers in the stagnated z-pinch plasma. Non-local-thermodynamic-equilibrium kinetic models fit to Na K-shell lines provide an independent measurement of the density and temperature that is consistent with spectroscopic analysis of K-shell emissions from Al and an alloyed Mg dopant. Axial transport of the Na dopant is observed, enabling quantitative study of instabilities in dense z-pinch plasmas.     

Binary alloys of Ge and Te: order, voids, and the eutectic composition

The liquid and amorphous structures of Ge0.15Te0.85 and GeTe alloys are characterized using combined density functional/molecular dynamics simulations. Te is threefold coordinated, in contrast with predictions of the "8-N rule," and Ge atoms (fourfold coordinated) show octahedral and tetrahedral bonding angles. Cubic local environment occurs in both materials, and GeTe shows a pronounced alternation of atomic types. Tetrahedral Ge coordination is more common in the eutectic Ge0.15Te0.85, which comprises corner- and edge-sharing GeTe4 units surrounded by Te. There is no Te segregation, and the material resembles neither GeTe nor Te. The ubiquitous cavities (voids) have been overlooked in Ge0.15Te0.85, where they comprise over 25% of the volume.     

Solid Cylinder Torsion for Large Shear Deformation and Failure of Engineering Materials

Experimental Mechanics - Using a thin-walled tube torsion test to characterize a material’s shear response is a well-known technique; however, the thin walled specimen tends to buckle before...     

Composite wraps for aging infra-structure

The post war years, with Australia's rapid economic growth, was the period which began the construction of much of the current infra-structure. However, there are now doubts on its durability and structural integrity. Composite repairs and external composite reinforcement, may well be one method for addressing the problems of aging infra-structure. To this end the present paper evaluates the ability of externally bonded composite wraps to increase the load carrying capacity of concrete columns and beams. In this study the failure loads were found to occur when the composite wraps reached a critical strain level.     

Medium effects on fluorescence quantum yields and lifetimes for coumarin laser dyes

Fluorescence quantum yields and lifetimes of coumarin dyes are sharply reduced in polar solvents if amine substituent groups are free to rotate. The polar solvent effect is interpreted in terms of relaxation of excited dye from an initial planar conformation to a twisted zwitterionic state.     

Reconstruction at a metallic interface studied by field ion and field emission microscopy

Reconstruction of a tungsten surface by adsorbed layers of gold, silver and copper has been studied by field emission and field ion microscopy. Gold reconstructs the surface in three ways, termed the α, β and γ rearrangements. The α rearrangement, which results in a smoothing of the tungsten surface, takes place at around 400° K with gold coverages of 5 monolayers (5θ), and is thought to be an increase in structural perfection of the tungsten surface by gold-assisted surface diffusion of tungsten atoms, β-reconstruction takes place in the temperature range 480–950°K at coverages ? 1.7θ, producing a faceted surface which comprises {211} and {110} facets, and is thought to result from the need to minimise the free energy at the gold/tungsten interface. The γ structure, which appears above 1400°K, is believed to represent a change in the shape of the tip by transport of tungsten to the (110) locality. Adsorbed silver produces neither β nor γ structures, and the degree of α rearrangement is very small, being confined to the {230} regions of the substrate. Copper lies between silver and gold in its ability to rearrange the tungsten surface, some degree of α rearrangement is detectable, and the β structure is very poorly developed unlike the γ structure which is clearly formed. The binding strength of copper to tungsten is greater than that of silver, but less than that of gold; the capacity of an adsorbate, to reconstruct the tungsten substrate is therefore thought to be related to the strength of the adsorbate-substrate bond.