276 research outputs found
Diffuse neutron reflectivity and AFM study of interface morphology of an electro-deposited Ni/Cu film
We present a detailed study of the interface morphology of an
electro-deposited (ED) Ni/Cu bilayer film by using off-specular (diffuse)
neutron reflectivity technique and Atomic Force Microscopy (AFM). The Ni/Cu
bilayer has been electro-deposited on seed layers of Ti/Cu. These two seed
layers were deposited by magnetron sputtering. The depth profile of density in
the sample has been obtained from specular neutron reflectivity data. AFM image
of the air-film interface shows that the surface is covered by globular islands
of different sizes. The AFM height distribution of the surface clearly shows
two peaks [Fig. 3] and the relief structure (islands) on the surface in the
film can be treated as a quasi-two-level random rough surface structure. We
have demonstrated that the detailed morphology of air-film interfaces, the
quasi-two level surface structure as well as morphology of the buried
interfaces can be obtained from off-specular neutron reflectivity data. We have
shown from AFM and off-specular neutron reflectivity data that the morphologies
of electro-deposited surface is distinctly different from that of
sputter-deposited interface in this sample. To the best of our knowledge this
is the first attempt to microscopically quantify the differences in
morphologies of metallic interfaces deposited by two different techniques viz.
electro-deposition and sputtering
Electronic and magnetic properties of the (111) surfaces of NiMnSb
Using an ab-initio electronic structure method, I study the (111) surfaces of
the half-metallic NiMnSb alloy. In all cases there is a very pronounced surface
state within the minority gap which destroys the half-metallicity This state
survives for several atomic layers below the surface contrary to the (001)
surfaces where surface states were located only at the surface layer. The lower
dimensionality of the surface leads in general to large enhancements of the
surface spin moments
Optical and Structural Properties of Ultra-thin Gold Films
Realizing laterally continuous ultra-thin gold films on transparent
substrates is a challenge of significant technological importance. In the
present work, formation of ultra-thin gold films on fused silica is studied,
demonstrating how suppression of island formation and reduction of plasmonic
absorption can be achieved by treating substrates with (3-mercaptopropyl)
trimethoxysilane prior to deposition. Void-free fi lms with deposition
thickness as low as 5.4 nm are realized and remain structurally stable at room
temperature. Based on detailed structural analysis of the fi lms by specular
and diffuse X-ray reflectivity measurements, it is shown that optical
transmission properties of continuous ultra-thin films can be accounted for
using the bulk dielectric function of gold. However, it is important to take
into account the non-abrupt transition zone between the metal and the
surrounding dielectrics, which extends through several lattice constants for
the laterally continuous ultra-thin films (film thickness below 10 nm). This
results in a significant reduction of optical transmission, as compared to the
case of abrupt interfaces. These findings imply that the atomic-scale interface
structure plays an important role when continuous ultra-thin films are
considered, e.g., as semi-transparent electrical contacts, since optical
transmission deviates significantly from the theoretical predictions for ideal
films.Comment: appears in Advanced Optical Materials 201
Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys
We study the origin of the gap and the role of chemical composition in the
half-ferromagnetic Heusler alloys using the full-potential screened KKR method.
In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap.
Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are
semiconductors, but when Z_t > 18 antibonding states are also populated, thus
the paramagnetic phase becomes unstable and the half-ferromagnetic one is
stabilized. The minority occupied bands accommodate a total of nine electrons
and the total magnetic moment per unit cell in mu_B is just the difference
between Z_t and . While the substitution of the transition metal
atoms may preserve the half-ferromagnetic character, substituting the atom
results in a practically rigid shift of the bands and the loss of
half-metallicity. Finally we show that expanding or contracting the lattice
parameter by 2% preserves the minority-spin gap.Comment: 11 pages, 7 figures New figures, revised tex
Theoretical investigation of the count rate capabilities of inâ pixel amplifiers for photon counting arrays based on polycrystalline silicon TFTs
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146384/1/mp13128_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146384/2/mp13128.pd
Europium-155 as a source for dual energy cone beam computed tomography in adaptive proton therapy: A simulation study
To investigate the feasibility of a 3D imaging system utilizing a 155 Eu source and pixelated cadmium-zinc-telluride (CZT) detector for applications in adaptive radiotherapy. Specifically, to compare the reconstructed stopping power ratio (SPR) values of a head phantom obtained with the proposed imaging technique with theoretical SPR values.A Geant4 Monte Carlo simulation was performed with the novel imaging system. The simulation was repeated with a typical 120 kV X-ray tube spectrum while maintaining all other parameters. Dual energy 155 Eu source cone beam computed tomography (CBCT) images were reconstructed with an iterative projection algorithm known as total variation superiorization with diagonally relaxed orthogonal projections (TVS-DROP). Single energy 120 kV source CBCT images were also reconstructed with TVS-DROP. Reconstructed images were converted to SPR with stoichiometric calibration techniques based on ICRU 44 tissues. Quantitative accuracy of reconstructed attenuation coefficient images as well as SPR images were compared.Images generated by gamma emissions of 155 Eu showed superior contrast resolution to those generated by the 120 kV spectrum. Quantitatively, all reconstructed images correlated with reference attenuation coefficients of the head phantom within 1 standard deviation. Images generated with the 155 Eu source showed a smaller standard deviation of pixel values. Use of a dual energy conversion into SPR resulted in superior SPR accuracy with the 155 Eu source.155 Eu was found to display desirable qualities when used as a source for dual energy CBCT. Further work is required to demonstrate whether the simulation results presented here can be translated into an experimental prototype.Jiahua Zhu, Scott N. Penfol
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