TcSUH
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Special Seminar
III-Nitrides Materials Grown by Plasma-Assisted Molecular Beam Epitaxy
by: Prof. Li-Wei Tu
Date: Monday February 05, 2007
Time: 1:00 pm – 1:45 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
GaN, InN, AlN, and their alloys have tremendous usages in many different areas of electronics and optoelectronics. Light-emitting diodes for lighting, decoration, displays, traffic signals, and laser diodes for pointers, DVD players and recorders, sensors in detectors, solar cells for energy conversion, field-emission transistors in power devices, etc. are but some examples. In this talk, I would like to discuss some contemporary issues faced by the community and then present some interesting topics in which our laboratory has engaged. After an introductive overview of the III-nitride family, I will discuss an unusual controversy on the fundamental band gap of InN. The debates, which started in 2002 and continued unabated even up to this day, are mainly on whether the material’s energy band gap is ~1.9 eV or ~0.7 eV. I will present the current status also on the non-polar nitrides’ growth and their advantages over the traditional polar ones, which is a relevant issue challenging the viability of the III-nitride technology. I will conclude with the nanostructures of III-nitrides and their potential applications in modern technology, which include the use of some basic forms such as nanorods and nanotips.
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Special Seminar
In-Situ Formation of Nanoscaled Al2O3 in Al-ZnO System by Friction Stir Technique
by: Prof. New-Jin Ho
Date: Monday February 05, 2007
Time: 12:00 pm – 12:45 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
In past years, we have successfully blended nano-Al2O3 into aluminum alloys with excellent dispersion by friction stir processing. The strength and ductility of this composite is superior to those made by sintering and casting. In addition to successful blending of nano particles in the stir zone, we also found ultra-fine grains ranging from several microns to one hundred nanometers. Nevertheless, it was difficult to make clustering and grain sizes smaller even with adding a large amount of nano-particles unless an in-situ formation of nano oxides occurs.
It was well understood that aluminum can redox most of metal oxides to produce Al2O3 during sintering process at high temperature. An Al-ZnO system was first chosen because zinc is soluble in aluminum at high temperature to form stable alloys at room temperature. The preliminary results indicate that Al2O3 as small as 2 nm~10 nm without nano-clustering can be made in the Al-Zn matrix. According to Orowon strengthening mechanism, the shear strength can be as high as G/20, or 1.5GPa.
Further investigations will be carried out to (1) understand how a large amount of ultra-fine nano Al2O3 were formed at temperatures lower than 600°C, and within a short period (<1 sec) of the friction stir process, (2) understand the crystallographic change in 2nm~10nm Al2O3 from amorphous state to gamma- and to alpha-phase, and at what size is the change irreversible comparing to constraint-free nano-Al2O3, and (3) understand the effect of such fine nano-Al2O3 in excellent dispersion condition on the mechanical properties—such as strength, ductility, low cycle fatigue, fatigue crack propagation, and fracture toughness.
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Special Seminar
Structural Kinetics, Electronic and Optical Properties of Organic Molecules Adsorbed on Solid Surfaces
by: Prof. Vladimir I. Gavrilenko
Date: Friday January 12, 2007
Time: 2:00 pm – 3:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
Structural kinetics of molecular aggregates and adsorption of organic molecules on metallic and semiconductor surfaces are studied by first principle methods within Density Functional Theory (DFT). Equilibrium geometries of molecular aggregates and molecules adsorbed on solid surfaces are determined through the total energy minimization method. Electron energy structure and optical functions of solid surfaces and effects of the adsorption of organic molecules are studied by first principle evaluation of eigen-value and eigen-vector problems using ab initio pseudopotentials (PP). Intermolecular interaction is shown to be responsible for substantial modifications of optical spectra of molecular aggregates. It is demonstrated that adsorption of water on transition metal surfaces (silver, gold) could be monitored through differential linear optics (e.g. differential reflectance anisotropy). Electronic structure and optical properties of complex organic dye molecules (Rhodamine 6G) strongly depends on the molecular aggregation. Predicted electron energy structure modifications of silicon surfaces due to the adsorption of ethanol explain observed enhancement of sum frequency generation intensity from nanostructured silicon. Theoretical results are discussed in comparison with optical spectroscopy data (luminescence, optical absorption, reflectance, etc).
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Special Seminar
Hybrid Physical-Chemical Vapor Deposition for MgB2 Coated-Conductors
by: Prof. Xiaoxing Xi
Date: Tuesday December 12, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
MgB2 thin films and coated-conductor fibers, grown by Hybrid Physical-Chemical Vapor Deposition (HPCVD), show extraordinarily high values of upper critical field Hc2 (over 60 T) and irreversibility field Hirr (over 40 T), which are much higher than those of Nb-based superconductors. This is the result of two-band superconductivity, a unique feature of MgB2, and the modification of the inter-band and intra-band scattering of the two bands. The carbon alloying also dramatically increases the critical current density in magnetic field. The carbon content-dependence of the lattice constants in the HPCVD films are found to be different from those in bulk carbon-doped MgB2, which may explain their superior high field properties. Coated conductor wires and tapes are a potentially viable approach to turn such properties into practical high-field conductors for applications such as magnetic resonance imaging.
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Special Seminar
Half-Metallic Oxides for Spintronics
by: Prof. Arunava Gupta
Date: Friday December 08, 2006
Time: 12:00 pm – 1:00 pm
Location: Houston Science Center – Building 593 — Room 102
Overview
The emerging field of spintronics aims to exploit the electron spin, in addition to its charge, to create a new class of devices that scale down to much smaller dimensions with possibly added functionalities. Of particular relevance are magnetic tunnel junctions (MTJs), consisting of two ferromagnetic thin film electrodes separated by an insulating barrier, that exhibit large tunneling magnetoresistance (TMR) at relatively low fields. The MTJs are promising for a host of applications including magnetic memory (MRAM), sensors, and storage devices. Most of the studies on MTJs have thus far focused on using transition metal ferromagnets (Fe, Ni, Co) and their alloysâ€â€typically with spin polarization values less than 50%â€â€where the maximum observed TMR is limited to about 40–50% at room temperature using an amorphous aluminum oxide barrier. There is obvious interest in further enhancing the TMR by using materials with a higher degree of spin polarization. Half-metallic systems, which contain a gap in one spin band at the Fermi level and no gap in the other spin band, are expected to have a spin polarization value approaching 100%. We have fabricated MTJ devices using half-metallic oxides, such as the mixed-valence manganites (La1-xAxMnO3, A=Ba, Sr, or Ca) and chromium dioxide (CrO2), that exhibit reproducible tunneling characteristics with high TMR values. However, the TMR enhancement has thus far been limited to low temperature. I will present an overview of MTJs, particularly related to the MRAM application, and then focus on the fabrication and properties of tunnel junctions using the half-metallic oxides and the challenges.
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