Direct Band Gap Gallium Antimony Phosphide (GaSbxP(1-x)) Alloys.

Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1-2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP(1-x) alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP(1-x). Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP(1-x) nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields.

ZnO gap engineering by doping with III-V compounds.

Gap engineering of ZnO by codoping it with III-V materials is investigated using model and ab initio calculation. Our results show that the codoped materials (ZnO)1-x (III-V)x , where (III-V) stands for GaN, AlN, AlP, BN, BP exhibit energy band gaps that get smaller as the dopant concentrations x is increased. Even at a very small dopant concentration the obtained band gaps are found to be much smaller than that of ZnO making the studied (ZnO)1-x(III-V)x materials promising candidates for photoelectrochemical water splitting.

Vortex formation and recirculation zones in left anterior descending artery stenoses: computational fluid dynamics analysis.

Flow patterns may affect the potential of thrombus formation following plaque rupture. Computational fluid dynamics (CFD) were employed to assess hemodynamic conditions, and particularly flow recirculation and vortex formation in reconstructed arterial models associated with ST-elevation myocardial infraction (STEMI) or stable coronary stenosis (SCS) in the left anterior descending coronary artery (LAD). Results indicate that in the arterial models associated with STEMI, a 50% diameter stenosis immediately before or after a bifurcation creates a recirculation zone and vortex formation at the orifice of the bifurcation branch, for most of the cardiac cycle, thus allowing the creation of stagnating flow. These flow patterns are not seen in the SCS model with an identical stenosis. Post-stenotic recirculation in the presence of a 90% stenosis was evident at both the STEMI and SCS models. The presence of 90% diameter stenosis resulted in flow reduction in the LAD of 51.5% and 35.9% in the STEMI models and 37.6% in the SCS model, for a 10 mmHg pressure drop. CFD simulations in a reconstructed model of stenotic LAD segments indicate that specific anatomic characteristics create zones of vortices and flow recirculation that promote thrombus formation and potentially myocardial infarction.

Simulation of cardiac motion on non-Newtonian, pulsating flow development in the human left anterior descending coronary artery.

This study aimed at investigating the effect of myocardial motion on pulsating blood flow distribution of the left anterior descending coronary artery in the presence of atheromatous stenosis. The moving 3D arterial tree geometry has been obtained from conventional x-ray angiograms obtained during the heart cycle and includes a number of major branches. The geometry reconstruction model has been validated against projection data from a virtual phantom arterial tree as well as with CT-based reconstruction data for the same patient investigated. Reconstructions have been obtained for a number of temporal points while linear interpolation has been used for all intermediate instances. Blood has been considered as a non-Newtonian fluid. Results have been obtained using the same pulse for the inlet blood flow rate but with fixed arterial tree geometry as well as under steady-state conditions corresponding to the mean flow rate. Predictions indicate that myocardial motion has only a minor effect on flow distribution within the arterial tree relative to the effect of the blood pressure pulse.

Rectification properties of carbon nanotube "Y-junctions".

Quantum conductivity of single-wall carbon nanotube Y-junctions is calculated. The current versus voltage characteristics of these junctions show asymmetry and rectification, in agreement with recent experimental results. Furthermore, rectification is found to be independent of the angle between the branches of these junctions, indicating this to be an intrinsic property of symmetric Y-junctions. The implications for the Y-junction to function as a nanoscale molecular electronic switch are investigated.

Catalytic action of Ni atoms in the formation of carbon nanotubes: a molecular dynamics study.

Catalytic action of Ni atoms in the growth of single-wall carbon nanotubes is investigated using tight-binding molecular dynamics and ab initio methods. Our results demonstrate this to be a two step process in which the Ni atom first creates and stabilizes defects in nanotubes. The subsequent incorporation of incoming carbon atoms anneals the Ni-stabilized defects freeing the Ni atom to repeat the catalytic process.