Codrin Cionca

Experimental System for One-Dimensional Rotational Brownian Motion

We present here an experimental, strictly one-dimensional rotational system, made by using single magnetic Janus particles in a static magnetic field. These particles were half-coated with a thin metallic film, and by turning on a properly oriented external static magnetic field, we monitor the rotational brownian motion of single particles, in solution, around the desired axis. Bright-field microscopy imaging provides information on the particle orientation as a function of time. Rotational diffusion coefficients are derived for one-dimensional rotational diffusion, both for a single rotating particle and for a cluster of four such particles. Over the studied time domain, up to 10 s, the variation of the angle of rotation is strictly brownian; its probability distribution function is gaussian, and the mean squared angular displacement is linear in time, as expected for free diffusion. Values for the rotational diffusion coefficients were also determined. Monte Carlo and hydrodynamic simulations agree well with the experimental results.

A quantitative investigation of additive noise reduction for active matrix flat-panel imagers using compensation lines

A quantitative investigation of a technique for reducing correlated noise in indirect detection active matrix flat-panel imagers has been reported. Correlated noise in such systems arises from the coupling of electronic noise, originating from fluctuations in external sources such as power supplies and ambient electromagnetic sources, to the imaging array via its address lines. The noise reduction technique involves the use of signals from columns of compensation line pixels located in relatively close proximity to the columns of normal imaging pixels on the array. Compensation line pixels are designed to be as sensitive to externally-coupled noise as columns of normal imaging pixels but are insensitive to incident radiation. For each imaging pixel, correlated noise is removed by subtracting from the imaging pixel signal a signal derived from compensation line pixels located on the same row. The effectiveness of various implementations of this correction has been examined through measurements of signal and noise from individual pixels as well as of noise power spectra. These measurements were performed both in the absence of radiation as well as with x rays. The effectiveness of the correction was also demonstrated qualitatively by means of an image of a hand phantom. It was found that the use of a single compensation line dramatically reduces external noise through removal of the correlated noise component. While this form of the correction increases non-radiation-related uncorrelated noise, the effect can be largely reduced through the introduction of multiple compensation lines. Finally, a position-dependent correction based on compensation lines on both sides of the array was found to be effective when the magnitude of the correlated noise varied linearly across the array.

Correlated structural and magnetization reversal studies on epitaxial Ni films grown with molecular beam epitaxy and with sputtering

We have studied the correlation between film structure and the azimuthal dependence of the magnetization reversal in (001) and (111) epitaxial Ni films grown on MgO substrates using two different deposition techniques: molecular beam epitaxy (MBE) and dc magnetron sputtering. The films were grown and in situ annealed under identical conditions. The magnetization reversal was investigated using MOKE. The coercive field in the sputtered (001) Ni films exhibits fourfold azimuthal symmetry as expected for crystalline films of good epitaxial quality, while MBE (001) Ni grown films exhibit an additional uniaxial symmetry superimposed to the fourfold symmetry. We performed high-resolution XRD studies as well as cross sectional TEM studies in order to establish similarities and differences in the structure of the films. Both types of films exhibit epitaxial growth and very good crystalline quality with no indication of strain. The main difference between the films is the different magnetic anisotropy. We postulat...

Resonant coherent Bragg rod analysis of strained epitaxial heterostructures

The resonant response of the complex x-ray scattering factor has been used in conjunction with the coherent Bragg rod analysis phase-retrieval algorithm to determine the composition and strain profiles of ultrathin layers of GaAs grown on InGaAs buffers. The buffer layers are nominally latticed matched with the InP substrate and the subsequent GaAs growth is compared at two different temperatures: 480 and 520°C. We show that electron density maps extracted from Bragg rod scans measured close to the Ga and As K-edges can be used to deconvolute roughness and intermixing. It is found that indium incorporation and roughening lead to a significant reduction of the strain in this system.

MEMS Gate Structures for Electric Propulsion Applications

Abstract : A MEMS gate prototype is under development to extract and accelerate charged particles for use with field emission cathodes and the nanoparticle field extraction thruster at the University of Michigan. Preliminary simulations suggest the desirability of a unity aspect ratio in the emission channel design to achieve electric field uniformity. Low emission threshold cubic boron nitride films have been grown, and gated testing with these films along with carbon nanotubes is in progress.

Magnetic confinement of Brownian rotation to a single axis and application to Janus and cluster microparticles

We present an experimental, one-dimensional, Brownian rotation system in which the free rotation is confined to a single axis. Control of the rotational diffusion of a single microparticle, or particle aggregate, around a chosen axis, was performed by using a static 1.0 mT external magnetic field. The confined object rotated freely around the chosen axis, and that axis was confined to within 3.9°. This method presents several advantages and may have wide applicability in biological and physical systems of interest.

Strain and composition mapping of epitaxial nanostructures

We have used surface x-ray diffraction and a direct method of phase reconstruction to obtain subangstrom resolution maps of an ion-beam milled In0.27Ga0.73As∕GaAs(001) thin film exhibiting three-dimensional (3D) epitaxial nanostructures. The 3D electron density was calculated based on the diffraction pattern along the Bragg rods measured with synchrotron radiation, from which the chemical composition, strain profile, and average nanostructure shape were extracted. The film maintained a wetting layer exhibiting a sharp strain gradient, which extended into the substrate. Atop the wetting layer, the ion-beam milled islands possessed an apical shape and were depleted in indium.

Theoretical investigation of advanced design active matrix flat-panel imagers for mammography

A theoretical investigation of the system performance of active matrix, flat-panel imagers (AMFPIs) under mammographic imaging conditions is reported. These imagers employ either indirect or direct detection of the incident radiation. The x- ray converter materials assumed in these studies were Gd2O2S:Tb and CsI:Tl scintillators for indirect detection and a-Se and PbI2 photoconductors for direct detection. A model based on cascaded systems formalism was used to predict the detective quantum efficiency (DQE) of various AMFPI designs incorporating these converters. The functional dependence of DQE performance on pixel-to-pixel pitch, collection fill factor and additive electronics noise was investigated under mammographic conditions. Incorporation of a continuous photodiode structure in indirect detection AMFPI arrays is necessary in order to achieve reasonable DQE performance for devices with pixel pitches significantly below 100 micrometer. In the case of CsI:Tl, a-Se, and PbI2, through optimization of the converter thickness, the DQE performance of these advanced AMFPI designs is predicted to exceed that of an AMFPI system incorporating a conventional Gd2O2S:Tb mammographic screen. Finally, the model predicts that the incorporation of a high gain converter such as PbI2 leads to a high value of DQE which is independent of exposure over the mammographic exposure range at all spatial frequencies, even at 50 micrometer pitch.

Real time structural modification of epitaxial FePt thin films under x-ray rapid thermal annealing using undulator radiation

Modification of chemical order in epitaxial FePt binary alloy thin films deposited on MgO (100) substrates was induced and investigated in real time using x-ray rapid thermal annealing (XRTA). This is possible because synchrotron undulator radiation has sufficient power density to induce significant structural modifications in thin films and its energy can be tuned to optimize absorption in the sample. A monochromatic portion of the pink beam diffracted from the epitaxial FePt sample was used to probe microstructure evolution in real time and significant changes in chemical order were observed. In particular, the relative amount of L10 phase remained practically unchanged whereas the amount of L12 phase was significantly decreased in the FePt thin film sample during XRTA.

Surface morphology and magnetization reversal

We have studied the microstructure and surface morphology in annealed and nonannealed (001) epitaxial Ni films deposited on MgO single crystalline substrates. The correlation between microstructure and the magnetic anisotropy properties of the films such as the magnetization reversal has been determined. Scanning tunneling microscopy images of the surface of the annealed films indicate self-assembled periodic stripe nano-patterning, while images of the nonannealed films show typical mounded surfaces. The azimuthal dependence of the coercive field in the nonannealed films exhibits fourfold symmetry as expected from the symmetry of the crystal. The annealed films exhibit an additional uniaxial symmetry superimposed to the fourfold symmetry. Additional structural studies to understand the origin of the surface nano-patterning observed on (001) Ni films will also be presented.