Ultrafast dynamics of exciton formation in semiconductor nanowires.

The dynamics of free electron-hole pairs and excitons in GaAs-AlGaAs-GaAs core-shell-skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron-hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron-hole charge pair densities of 2-4 × 10(16) cm(-3) . The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.

Polarity driven formation of InAs/GaAs hierarchical nanowire heterostructures

The structural and morphological characteristics of InAs/GaAs radial nanowire heterostructures were investigated using transmission electron microscopy. It has been found that the radial growth of InAs was preferentially initiated on the { 112 } A sidewalls of GaAs nanowires. This preferential deposition leads to extraordinarily asymmetric InAs/GaAs radial nanowire heterostructures. Such formation of radial nanowire heterostructures provides an opportunity to engineer hierarchical nanostructures, which further widens the potential applications of semiconductor nanostructures. © 2008 American Institute of Physics.

Failure and formation of axial nanowire heterostructures in vapor-liquid-solid growth

To observe the axial growth behavior of InAs on GaAs nanowires, InAs was grown for different growth durations on GaAs nanowires using Au nanoparticles. Through transmission electron microscopy, we have observed the following evolution steps for the InAs growth. (1) In the initial stages of the InAs growth, InAs clusters into a wedge shape preferentially at an edge of the Au/GaAs interface by minimizing Au/InAs interfacial area; (2) with further growth of InAs, the Au particle moves sidewards and then downwards by preserving an interface with GaAs nanowire sidewalls. The lower interfacial energy of Au/GaAs than that of Au/In As is attributed to be the reason for such Au movement. This downward movement of the Au nanoparticle later terminates when the nanoparticle encounters InAs growing radially on the GaAs nanowire sidewalls, and with further supply of In and As vapor reactants, the Au nanoparticle assists the formation of InAs branches. These observations give some insights into vapor-liquid-solid growth and the formation of kinks in nanowire heterostructures. © 2008 Materials Research Society.

Understanding the kink formation in GaAs/InAs heterostructural nanowires

The kinks formation in heterostructural nanowires was observed to be dominant when InAs nanowires were grown on GaAs nanowires. Nanowires were grown through vapor-liquid-solid (VLS) mechanism in an MOCVD (metalorganic chemical vapor deposition) reactor. GaAs nanowires were grown in [1 1 1 ]B direction on a GaAs (1 1 1 )B substrate. When InAs nanowires grown on the GaAs nanowires, most of the InAs nanowires changed their growth directions from [1 1 1 ]B to other 〈111〉B directions. The kinks formation is ascribed to the large compressive misfit strain at the GaAs/InAs interface (7.2% lattice mismatch between GaAs and InAs) and the high mobility of indium species during MOCVD growth. The in-depth analysis of the kinks formation was done by growing InAs for short times on the GaAs nanowires and characterizing the samples. The hindrance to compressively strain InAs to form coherent layers with GaAs pushed the InAs/Au interfaces to the sides of the GaAs nanowires growth ends. New InAs/Au interfaces have generated at the sides of GaAs nanowires, due to lateral growth of InAs on GaAs nanowires. These new interfaces led the InAs nanowires growth in other 〈111〉B directions. The morphological and structural features of these heterostructural kinked nanowires were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. © 2006 IEEE.

Detailed measurements of spike formation in an axial compressor

This paper presents new experimental measurements of spike-type stall inception. The measurements were carried out in the single stage Deverson compressor at the Whittle Laboratory. The primary objective was to characterize the flow field in the tip clearance gap during stall inception using sufficient instrumentation to give high spatial and temporal resolution. Measurements were recorded using arrays of unsteady pressure transducers over the rotor tips and hot-wires in the tip gap. Pre-stall ensemble averaged velocity and pressure maps were obtained as well as pressure contours of the stall event. In order to study the transient inception process in greater detail, vector maps were built up from hundreds of stalling events using a triggering system based on the stalling event itself. The results show an embryonic disturbance starting within the blade passage and leading to the formation of a clear spike. The origins of the spike and its relation to the tip leakage vortex are discussed. It has also been shown that before stall the flow in the blade passage which is most likely to stall is generally more unsteady, from revolution to revolution, than the other passages in the annulus. Copyright © 2012 by ASME.

Influence of specific HSP70 domains on fibril formation of the yeast prion protein Ure2.

Ure2p is the protein determinant of the Saccharomyces cerevisiae prion state [URE3]. Constitutive overexpression of the HSP70 family member SSA1 cures cells of [URE3]. Here, we show that Ssa1p increases the lag time of Ure2p fibril formation in vitro in the presence or absence of nucleotide. The presence of the HSP40 co-chaperone Ydj1p has an additive effect on the inhibition of Ure2p fibril formation, whereas the Ydj1p H34Q mutant shows reduced inhibition alone and in combination with Ssa1p. In order to investigate the structural basis of these effects, we constructed and tested an Ssa1p mutant lacking the ATPase domain, as well as a series of C-terminal truncation mutants. The results indicate that Ssa1p can bind to Ure2p and delay fibril formation even in the absence of the ATPase domain, but interaction of Ure2p with the substrate-binding domain is strongly influenced by the C-terminal lid region. Dynamic light scattering, quartz crystal microbalance assays, pull-down assays and kinetic analysis indicate that Ssa1p interacts with both native Ure2p and fibril seeds, and reduces the rate of Ure2p fibril elongation in a concentration-dependent manner. These results provide new insights into the structural and mechanistic basis for inhibition of Ure2p fibril formation by Ssa1p and Ydj1p.

Modelling and design of stress-induced martensite formation in metastable β Ti alloys

The temperature dependence of the stress-induced martensite (SIM) formation in a Ti-10V-2Fe-3Al (Ti-1023) alloy under compressive loading has been studied. At low temperatures, the stress level at which martensite starts to form increases linearly with the deformation temperature, while the stress at which the deformation switches to regular plastic deformation is roughly temperature independent. A thermostatistical model for dislocation evolution is employed to describe deformation twinning in martensite. Combined effects of twinning induced plasticity and solid solution strengthening are considered in terms of temperature variations. The SIM effect disappears on deformation at temperatures beyond ~ 233 ° C, which is close to the predicted Ms temperature of 240°C. The thermostatistical model predicts a transition from twinned martensite to pure slip at 250°C. By providing a model to predict the martensite formation, and by describing deformation twinning, the present work provides a number of tools that may be employed to conceive new titanium alloys combining improved strength and ductility. © 2013 Elsevier B.V.

Detailed measurements of spike formation in an axial compressor

This paper presents new experimental measurements of spike-type stall inception. The measurements were carried out in the single stage Deverson compressor at the Whittle Laboratory. The primary objective was to characterize the flow field in the tip clearance gap during stall inception using sufficient instrumentation to give high spatial and temporal resolution. Measurements were recorded using arrays of unsteady pressure transducers over the rotor tips and hot-wires in the tip gap. Prestall ensemble averaged velocity and pressure maps were obtained as well as pressure contours of the stall event. In order to study the transient inception process in greater detail, vector maps were built up from hundreds of stalling events using a triggering system based on the stalling event itself. The results show an embryonic disturbance starting within the blade passage and leading to the formation of a clear spike. The origins of the spike and its relation to the tip leakage vortex are discussed. It has also been shown that before stall, the flow in the blade passage which is most likely to stall is generally more unsteady, from revolution to revolution, than the other passages in the annulus. © 2014 by ASME.

Towards the formation of neuro-transistor artificial chemical synapse

Highly sensitive biosensor for detection of acetylcholine (ACh) and competitive acetylcholinesterase (AChE) inhibitor, eserine, is investigated. Peculiar microelectronic configuration of an ion-sensitive field-effect transistor (ISFET) in addition to a right choice of the pH-transducing nanolayers allows recording a response of the enzyme-modified ISFET (EnFET) to a wide range of ACh concentrations. We demonstrate a remarkable improvement of at least three orders of magnitude in dose response to ACh. Described bioelectronic system reveals clear response, when the catalytic activity of the immobilized AChE is inhibited in a reversible manner by eserine, competitive inhibitor of AChE. ©2007 IEEE.