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At least 1,279 records · Page 71

Ultra-violet Sensing Characteristic and Field Emission Properties of Vertically Aligned Aluminum Doped Zinc Oxide Nanorod Arrays

Ultra-violet (UV) sensing behavior and field emission characteristic have been investigated on vertically aligned aluminum (Al) doped zinc oxide (ZnO) nanorod arrays prepared using sol-gel immersion method. Uniform and high coverage density of ZnO nanorod arrays have been successfully deposited on seeded-catalyst coated substrates. The synthesized nanorods have diameter sizes between 50 nm to 150 nm. The XRD spectra show Al doped ZnO nanorod array has high crystallinity properties with the dominancy of crystal growth along (002) plane or c-axis. UV photoresponse measurement indicates that Al doped ZnO nanorod array sensitively detects UV light as shown by conductance increment after UV illumination exposure. The nanorod array shows good field emission properties with low turn on field and threshold field at 2.1 V/{mu}m and 5.6 V/{mu}m, respectively. The result suggested that Al doped ZnO nanorod arrays prepared by low-cost sol-gel immersion method show promising result towards fabrication of multi applications especially in UV photoconductive sensor and field emission displays.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Band alignment at the Cu{sub 2}ZnSn(S{sub x}Se{sub 1-x}){sub 4}/CdS interface

Energy band alignments between CdS and Cu{sub 2}ZnSn(S{sub x}Se{sub 1-x}){sub 4} (CZTSSe) grown via solution-based and vacuum-based deposition routes were studied as a function of the [S]/[S+Se] ratio with femtosecond laser ultraviolet photoelectron spectroscopy, photoluminescence, medium energy ion scattering, and secondary ion mass spectrometry. Band bending in the underlying CZTSSe layer was measured via pump/probe photovoltage shifts of the photoelectron spectra and offsets were determined with photoemission under flat band conditions. Increasing the S content of the CZTSSe films produces a valence edge shift to higher binding energy and increases the CZTSSe band gap. In all cases, the CdS conduction band offsets were spikes.

36 MATERIALS SCIENCE↗

Tokamak equilibria with non field-aligned axisymmetric divergence-free rotational flows

Rotational ideal divergence-free magnetohydrodynamic (MHD) equations are expressed in terms of transformed variables w-vector{sub *}=({mu}{rho}){sup 1/2}v-vector and {mu}p{sub *} = ({mu}p + w{sub *}{sup 2}/2), where v-vector, p, and {rho} are plasma velocity, pressure, and mass density, respectively. With divergence-free flows, {nabla}{center_dot}v-vector=0, the plasma density {rho} does not appear in the MHD equations written in terms of w-vector{sub *} and {mu}p{sub *}. The non field-aligned rotational Grad-Shafranov equation is represented in spherical coordinates. Tokamak-like axisymmetric equilibria with v-vector perpendicular {nabla}{rho} are obtained analytically by solving for torus solutions under only three source functions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Studies of Valence Band Alignment Between Nitrided GaPN/GaP (111) Interface Using X-ray Photoelectron Spectroscopy

The analysis of core levels positions of Ga{sub 3}d, N{sub 1s} and P{sub 2}p at different etching depth from the plasma nitrided GaP (111) surface shows that the nitrogen ions interact with both Gallium and Phosphorous ions with nearly equal probability. The analysis of valence band spectra shows the type-II band alignment between GaPN{sub 0.22}/GaP and the valence band offset is {approx}2.2{+-}0.1 eV.

36 MATERIALS SCIENCE↗

Aligned and exchange-coupled FePt-based films

Two-phase nanostructures of hard L1{sub 0}-ordered FePt and soft iron-rich fcc Fe-Pt are investigated experimentally and by model calculations. The Fe-Pt thin films were produced by epitaxial co-sputtering onto MgO and have a thickness of about 10 nm. They form two-phase dots that cover a large fraction of the surface but are separated from each other. X-ray diffraction and TEM show that the c-axis of the phase FePt is aligned in the direction normal to the film plane. The experimental and theoretical hysteresis loops indicate archetypical exchange coupling, and excellent magnetic properties are obtained. The largest values of coercivity, saturation magnetization, and nominal energy product obtained in the samples studied are 51 kOe, 1287 emu/cc, and 54 MGOe, respectively.

36 MATERIALS SCIENCE↗

Orbital alignment in photodissociation probed using strong field ionization

The photodissociation of molecules often produces atomic fragments with polarized electronic angular momentum, and the atomic alignment, for example, can provide valuable information on the dynamical pathways of chemical reactions unavailable by other means. In this work, we demonstrate for the first time that orbital polarization in chemical reactions can be measured with great sensitivity using strong field ionization by exploiting its extreme nonlinearity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases {approx}7 times while the QD height decreases {approx}50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

36 MATERIALS SCIENCE↗

Aligned and exchange-coupled L1{sub 0} (Fe,Co)Pt-based magnetic films

Films of aligned L1{sub 0}-structure (Fe,Co)Pt with fcc Fe(Co,Pt) are synthesized by co-sputtering Fe, Co, and Pt on an (001) MgO substrate with in situ heating at 830 deg. C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID). The compositions of the samples (Fe,Co){sub x}Pt{sub 1-x} are designed to maintain an atomic Fe: Co ratio of 65: 35 while increasing the Fe,Co content in each successive sample. In samples with low Fe and Co concentration, the XRD patterns exhibit three strong peaks, namely L1{sub 0} (Fe,Co)Pt (001), L1{sub 0} (Fe,Co)Pt (002), and MgO (002). A fourth peak is observed in samples with high Fe and Co concentration and identified as fcc (002). The XRD patterns confirm the formation of L1{sub 0}-ordered (Fe,Co)Pt and its epitaxial growth on MgO. TEM shows that the (Fe,Co)Pt films form isolated magnetic grains of about 100 nm in diameter. Hysteresis-loop measurements show that the increase of the Fe,Co concentration from 57.3 to 68.3 at % enhances the saturation magnetization M{sub s} from 1245 emu/cm{sup 3} to 1416 emu/cm{sup 3}, and the coercivity decreases from 32 kOe to 8.9 kOe. The nominal maximum energy product per grain is 64 MGOe.

36 MATERIALS SCIENCE↗

Modeling the propagation of whistler-mode waves in the presence of field-aligned density irregularities

We present a numerical study of propagation of VLF whistler-mode waves in a laboratory plasma. Our goal is to understand whistler propagation in magnetic field-aligned irregularities (also called channels or ducts). Two cases are examined, that of a high-frequency ({omega}>{Omega}{sub ce}/2) whistler in a density depletion duct and that of a low-frequency ({omega}<{Omega}{sub ce}/2) whistler in a density enhancement. Results from a numerical simulation of whistler wave propagation are compared to data from the UCLA Los Angeles Physics Teachers Alliance Group plasma device and whistler propagation in pre-existing density depletion and density enhancement ducts is demonstrated.

58 GEOSCIENCES↗

Field-aligned accelerations by plasma shocks propagating through interstellar magnetic fields

A kinetic model of particle acceleration by plasma shocks is analyzed theoretically and with numerical calculations. The shocks are propagating through weakly magnetized background plasmas, namely interstellar magnetic fields (IMFs). Particles located at the shock front are accelerated parallel to the magnetic field of the shock; this is defined as the field-aligned acceleration (FAA). The cross angle between IMF and the magnetic field of the shock plays an important role in creating the magnetic neutral sheet at the shock front. A test particle trapped by the neutral sheet obtains enormous energy due to the FAA. A reasonable formula for the highest energy gain is derived from theoretical analysis of the relativistic equations of motion. A possible configuration of the electric and magnetic fields in supernova remnants is also proposed by way of example.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Band alignment of InGaZnO{sub 4}/Si interface by hard x-ray photoelectron spectroscopy

Although amorphous InGaZnO{sub 4} has intensively been studied for a semiconductor channel material of thin-film transistors in next-generation flat-panel displays, its electronic structure parameters have not been reported. In this work, the electron affinities ({chi}) and the ionization potentials (I{sub p}) of crystalline and amorphous InGaZnO{sub 4} (c-IGZO and a-IGZO) were measured using bulk-sensitive hard x-ray photoelectron spectroscopy. First, the {chi} and I{sub p} values of c-IGZO and a-IGZO thin films were estimated by aligning the Zn 2p{sub 3/2} core level energies to a literature value for ZnO, which provided {chi} = 3.90 eV and I{sub p} = 7.58 eV for c-IGZO and 4.31 eV and 7.41 eV for a-IGZO. It was also confirmed that the escape depth of the photoelectrons excited by the photon energy of 5950.2 eV is 3.3 nm for a-IGZO and large enough for directly measuring the interface electronic structure using a-IGZO/c-Si heterojunctions. It provided the valence band offset of {approx}2.3 eV, which agrees well with the above data. The present results substantiate that the a-IGZO/c-Si interface follows well the Schottky-Mott rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Assembly of uniaxially aligned rare-earth-free nanomagnets

We report HfCo7 nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K-1 approximate to 10 Mergs/cm(3), coercivity H-c approximate to 4.4 kOe, and magnetic polarization J(s) approximate to 10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having J(s) as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo7 as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4753950]

Balamurugan, B↗

Asymmetric chiral alignment in magnetized plasma turbulence

Multi species turbulence in inhomogeneous magnetised plasmas is found to exhibit symmetry breaking in the dynamical alignment of a third species with the fluctuating electron density and vorticity with respect to the magnetic field direction and the species' relative background gradients. The possibility of truly chiral aggregation of charged molecules in magnetized space plasma turbulence is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Photoelectron spectroscopic study of band alignment of polymer/ZnO photovoltaic device structure

Using x-ray photoelectron spectroscopy, we investigated the band alignment of a Ag/poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO photovoltaic structure. At the P3HT/ZnO interface, a band bending of P3HT and a short surface depletion layer of ZnO were observed. The offset between the highest occupied molecular orbital of P3HT and the conduction band minimum of ZnO at the interface contributed to the open circuit voltage (Voc) was estimated to be approximately 1.5 {+-} 0.1 eV, which was bigger than that of the electrically measured effective Voc of P3HT/ZnO photovoltaic devices, meaning that the P3HT/ZnO photovoltaic structure has the potential to provide improved photovoltaic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Enrichment of magnetic alignment stimulated by {gamma}-radiation in core-shell type nanoparticle Mn-Zn ferrite

Core shell type nanoparticle Mn{sub x}Zn{sub 1-x}Fe{sub 2}O{sub 4} systems with x=0.55, 0.65 and 0.75 were prepared using autocombustion method. The systems were characterized using tools like XRD and IR for structure confirmation. Magnetic parameter measurements like Saturation magnetization and coercivity were obtained from hysteresis loop which exhibited a symmetry shift due to core shell nature of the nanoparticles. Nanoparticles of particle size between 21.2nm to 25.7nm were found to show 20 percent shrinkage after being radiated by the {gamma}-radiation. This is due to variation in the cation distribution which also affects the cell volume of the cubic cell. Lattice constant reduction observed is reflected in the magnetic properties of the samples. A considerable hike in the saturation magnetization of the samples was observed due to enrichment of magnetic alignment in the magnetic core of the particles. Samples under investigation were irradiated with gamma radiation from Co{sup 60} source for different time intervals.

36 MATERIALS SCIENCE↗

Magnetotransport properties of quasi-one-dimensionally channeled vertically aligned heteroepitaxial nanomazes

A unique quasi-one-dimensionally channeled nanomaze structure has been self-assembled in the (La{sub 0.7}Sr{sub 0.3}MnO{sub 3}){sub 1−x}:(ZnO){sub x} vertically aligned nanocomposites (VANs). Significantly enhanced magnetotransport properties have been achieved by tuning the ZnO composition x. The heteroepitaxial VAN thin films, free of large angle grain boundaries, exhibit a maximum low-field magnetoresistance (LFMR) of 75% (20 K and 1 T). The enhanced LFMR close to the percolation threshold is attributed to the spin-polarized tunneling through the ferromagnetic/insulating/ferromagnetic vertical sandwiches in the nanomazes. This study suggests that the phase boundary in the nanomaze structure is an alternative approach to produce decoupled ferromagnetic domains and thus to achieve enhanced magnetoresistance.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Energy band alignment of atomic layer deposited HfO{sub 2} oxide film on epitaxial (100)Ge, (110)Ge, and (111)Ge layers

Crystallographically oriented epitaxial Ge layers were grown on (100), (110), and (111)A GaAs substrates by in situ growth process using two separate molecular beam epitaxy chambers. The band alignment properties of atomic layer hafnium oxide (HfO{sub 2}) film deposited on crystallographically oriented epitaxial Ge were investigated using x-ray photoelectron spectroscopy (XPS). Valence band offset, {Delta}E{sub v} values of HfO{sub 2} relative to (100)Ge, (110)Ge, and (111)Ge orientations were 2.8 eV, 2.28 eV, and 2.5 eV, respectively. Using XPS data, variation in valence band offset, {Delta}E{sub V}(100)Ge>{Delta}E{sub V}(111)Ge>{Delta}E{sub V}(110)Ge, was obtained related to Ge orientation. Also, the conduction band offset, {Delta}E{sub c} relation, {Delta}E{sub c}(110)Ge>{Delta}E{sub c}(111)Ge>{Delta}E{sub c}(100)Ge related to Ge orientations was obtained using the measured bandgap of HfO{sub 2} on each orientation and with the Ge bandgap of 0.67 eV. These band offset parameters for carrier confinement would offer an important guidance to design Ge-based p- and n-channel metal-oxide field-effect transistor for low-power application.

36 MATERIALS SCIENCE↗

Interfacial chemical bonding state and band alignment of CaF{sub 2}/hydrogen-terminated diamond heterojunction

CaF{sub 2} films are deposited on hydrogen-terminated diamond (H-diamond) by a radio-frequency sputter-deposition technique at room temperature. Interfacial chemical bonding state and band alignment of CaF{sub 2}/H-diamond heterojunction are investigated by X-ray photoelectron spectroscopy. It is confirmed that there are only C-Ca bonds at the CaF{sub 2}/H-diamond heterointerface. Valence and conductance band offsets of the CaF{sub 2}/H-diamond heterojunciton are determined to be 3.7 {+-} 0.2 and 0.3 {+-} 0.2 eV, respectively. It shows a type I straddling band configuration. The large valence band offset suggests advantage of the CaF{sub 2}/H-diamond heterojunciton for the development of high power and high frequency field effect transistors.

36 MATERIALS SCIENCE↗