Residual strain measurements in InGaAs metamorphic buffer layers on GaAs

This work deals with the strain relaxation mechanism in InGaAs metamorphic buffers (MBs) grown on GaAs substrates and overgrown by InAs quantum dots (QD). The residual strain is measured by using Raman scattering and X-ray diffraction, both in Reciprocal Space Map and in single ω-2θ scan modes (ω and θ being the incidence angles on the sample surface and on the scattering planes, respectively). By relating the GaAs-like longitudinal optical phonon frequency ω^LO of InGaAs MBs to the in-plane residual strain ε measured by means of photoreflectance (PR), the linear ε-vs.-ω^LO working curve is obtained. The results of Raman and XRD measurements, as well as those obtained by PR, are in a very satisfactory agreement. The respective advantages of the techniques are discussed. The measurements confirm that strain relaxation depends on the thickness t of the buffer layer following a ~t^-1/2 power law, that can be explained by an energy-balance model.     

Interaction of radiatively cooled plasma jets with neutral gases for laboratory astrophysics studies

A supersonic (Mach～2–3), radiatively cooled plasma jet is produced by the ablation of aluminium plasma from a radial foil, a disc subjected to a ～1.4 MA, 250 ns current from the MAGPIE pulsed-power generator. The ablated plasma converges on axis, producing a steady and collimated jet with axial velocities reaching ～100 km/s. The study of jet-ambient interactions is achieved by introducing a neutral gas above the foil using a fast valve with a supersonic gas nozzle. The system has flexibility to study different interaction geometries in order to vary critical dimensionless parameters for astrophysical studies. In particular the effects of radiative cooling on the working surface of the jet are strongly affected by varying the gas composition. Experimental results are compared to numerical simulations using the 3-D MHD code GORGON.     

Impedance measurement technique for high-sensitivity cell detection in microstructures with non-uniform conductivity distribution

Particle detection in microstructures is a key procedure required by modern lab-on-a-chip devices. Unfortunately, state of the art approaches to impedance measuring as applied to cell detection do not perform well in regions characterized by non-homogeneous physical parameters due, for example, to the presence of air-liquid interfaces or when the particle-electrode distance is relatively high. This paper presents a robust impedance measurement technique and a circuit for detecting cells flowing in microstructures such as microchannels and microwells. Our solution makes use of an innovative three-electrode measurement scheme with asymmetric polarization in order to increase cell detection ability in microstructures featuring large electrode distances of up to 100 μm as well as to limit signal loss due to cell position relative to the electrodes. Compared to standard techniques, numerical simulations show that, with the proposed approach, the cell detection sensitivity is increased by more than 40%. In addition, we propose a custom circuit based on division instead of difference between signals, as in standard differential circuits, so as to reduce the baseline signal drift induced by non-homogeneous conductivity. A simplified analytical model shows an increase in the signal-to-noise-ratio comprised in the range 3.9-5.9. Experimental results, carried out using an open-microwell device made with flexible printed circuit board technology, are in agreement with simulations, suggesting a six-fold increase of the signal-to-noise ratio compared to the differential measurement technique. We were thus able to successfully monitor the process of isolating K562 leukemia cells inside open-microwells determining all single-cell events with no false positive detection.     

Experimental study of shock waves from the interaction of a supersonic plasma jet with an ambient gas

Preliminary results of the interaction of a supersonic, radiatively cooled plasma jet with an ambient gas are presented. The experimental setup consists of a radial foil, a mum-thick aluminium disc held between two concentric electrodes and subjected to a 1.4 MA, 250-ns current pulse from the MAGPIE generator. The plasma flow, with typical velocities of ~70?C90?km/s, is produced by the J?× B force acting on the plasma ablated from the foil. A jet is formed from the convergence of this ablated plasma on the axis of the system. A new setup allows the jet to interact with an argon ambient (particle density N ~10^16-17 cm^?3) from a supersonic gas nozzle (Mach ~9). First results are characterised by the presence of several (previously unseen) shock structures, which are formed from the interaction of the jet with the argon ambient.     

Deposition of polyaniline on RVC electrodes: effect of substrate thickness

Polyaniline films, obtained by either chemical or electrochemical deposition on reticulated vitreous carbon (RVC), were investigated as a function of the substrate thickness. The electrochemical properties of these RVC/Pani electrodes were assessed by cyclic voltammetry and electrochemical impedance spectroscopy (EIS), whereas the morphology of the Pani films on RVC was analyzed by scanning electron microscopy (SEM). The cyclic voltammetric results revealed that the oxidation/reduction charges for electrodeposited polyaniline decrease as the RVC thickness is increased. Conversely, the charge densities for the chemically deposited films do not present a significant dependence on the substrate thickness. Two time constants, appearing in all the EIS spectra, indicate that an ohmic drop effect within the RVC substrate affects the polymer electrodeposition and the electrochemical behavior of the obtained electrodes. Therefore, an electric equivalent circuit considering the different electrochemical environments at the outer and inner RVC surfaces was proposed to analyze the EIS data.     

Inverted open microwells for cell trapping, cell aggregate formation and parallel recovery of live cells

The inverted open microwell is a novel microstructure supporting isolation and trapping of cells, analysis of cell-cell and cell-molecule interactions and functional cell sorting. This work introduces the inverted open microwell concept, demonstrating successful isolation of K562 cells in 75 μm microwells fabricated on a flexible printed circuit board substrate, and recovery of viable cells onto standard microtiter plates after analysis and manipulation. Dielectrophoresis (DEP) was used during the delivery phase to control cell access to the microwell and force the formation of cell aggregates so as to ensure cell-cell contact and interaction. Cells were trapped at the air-fluid interface at the bottom edge of the open microwell. Once trapped, cells were retained on the meniscus even after DEP de-activation and fluid was exchanged to enable perfusion of nutrients and delivery of molecules to the microwell, as demonstrated by a calcein-staining protocol performed in the microsystem. Finally, cell viability was assessed on trapped cells by a calcein release assay and cell proliferation was demonstrated after multiple cells had been recovered in parallel onto standard microtiter plates.     

Direct conversion of electrodeposited nanocrystalline ε-MnO2 into LiMn2O4 by microwave calcination

Journal of Solid State Electrochemistry - Firstly, ε-MnO2 films with good adhesion and high surface area were electrolytically deposited on platinum plates. After being soaked for...     

X-Ray double crystal rocking curves of Ga1−x Al x As/GaAs laser structures

Summary The double crystal X-ray rocking curves of Ga_1−x Al _x As/GaAs laser structures, with both a single and double confinement, have been calculated on the basis of the Takagi-Taupin dynamica theory. It has been demonstrated that very small changes in the thickness and composition of the active and the internal confining layers give rise to dramatic modifications of the rocking curves; this offers in principle a very powerful tool for measuring very precisely thickness and composition of these layers. However, the shape of the Bragg peak of the external confining layers exhibits a nearly period behaviour as a function of the thickness of the active or the internal confining layer; a simple relation between the thickness period and the composition difference of the considered layers has been obtained for the first time. Finally, the effect of the interchange of the confining layers on the rocking curves has been discussed.     

Numerical simulations of Z-pinch experiments to create supersonic differentially-rotating plasma flows

The aim of this work is to produce and study a high energy density laboratory plasma relevant to astrophysical accretion disks. To this end, an experimental setup based on a modified cylindrical wire array was devised, which employs a cusp magnetic field to introduce angular momentum into the system. The setup was studied numerically with the three-dimensional, resistive magneto-hydrodynamic code GORGON. Simulations show that a differentially-rotating flow is formed, with typical rotation velocity and Mach number values of 60 km/s and M_φ ～ 5 respectively. The plasma is radiatively cooled and presents a Reynolds number higher than 10⁷. In addition, the magnetic Reynolds number and the plasma β are >1. Such a plasma is of interest for the study of hydrodynamic and magneto-hydrodynamic instabilities, and turbulence generation in differentially-rotating plasma flows.     

Electro-optical properties of InGaAs layers grown by hydride vapour phase epitaxy

A series of epitaxial layers of the InGaAs alloy were deposited on (001) oriented InP substrates by using hydride VPE technique. The layers were characterized by Double Crystal Diffractometry (DCD), Photoluminescence (PL), Hall effect and Capacitance-Voltage (C-V) measurements. The growth parameters and the quality of the grown layers are discussed on the basis of electrical and structural data analysis.