Structural Square Research Articles

Adlayer Structure of Shape-Persistent Macrocycle Molecules: Fabrication and Tuning Investigated with Scanning Tunneling Microscopy

The assembling structure of square and triangular macrocycle molecules constructed with diethynylcarbazole units was investigated by scanning tunneling microscopy (STM) on a graphite surface. STM observation revealed that the square macrocycle molecule (M1) forms a multilayer on the graphite surface. In the first layer, M1 assembles into medium-sized domains with a few defects and dislocations, whereas, for the second layer, most of M1 are dispersed on the first layer separately. A tentative stacking mode of the bilayer structure is provided in this paper on the basis of information given by STM experiments. Considering the interlayer distance given by the crystal data on the similar molecules and the length of the M1 alkyl chains, we think that it is possible that part of the top M1 side chains adsorbs on the cavity area of the bottom M1 and probably plays a dominant role in stabilizing the second layer. This postulation is verified by a control experiment in which coronene is filled in the cavity of M1 and no bilayer structure of M1 is found. The triangular molecule (M2) organizes into a single layer with larger and less defect domains. Two M2 are paired together in parallel, but opposite-oriented style, and are responsible for the serrate edge of the molecular row. The alkyl chains of M2 adopt rather diverse arrangements without disturbing the assembly of M2 core parts. When the solution contains both coronene and M2, no M2 coronene complex is observed and the adlayer characteristics of M2 are essentially the same as those of only M2 in solution. The results may help us to learn the stacking behavior of macrocycle molecules with different shapes, understand surface self-assembling principles, and develop high-performance devices based on related materials.

Silicon-based filters, resonators and acoustic channels with phononic crystal structures

This paper discusses the phenomenon of phononic crystal silicon-based filters, resonators and acoustic channels structured in geometrical periodic arrays created by a single silicon material. Component structured geometrical periodic array refers to a structure of square stubbed rods arranged in repeated arrays on a silicon plate. The study discovered that the band gap of the phononic crystal structure can be modulated under different heights and rotational angles of periodically arrayed square stubbed rods. In addition to band gap modulation, we used the finite element method (FEM) and supercell techniques to analyse the resonance characteristics of defect-containing phononic crystal structures with a larger band gap size design. In addition, the paper also investigated the effects on acoustic channels. Previous studies have already analysed defect-containing resonator and channel phenomenon by the plane-wave expansion method with supercell techniques. However, the FEM can solve numerical issues of extreme difficulty to reach convergence. The results of this study elaborated on the manufacturing feasibility of silicon-based acoustic resonance and filter devices under a complementary metal–oxide–semiconductor synchronization process.

Poster - Thur Eve - 74: A Fan-Beam Collimator Design for Small Animal Scintigraphy

The objective of the study was to design and conduct a performance study of a fan‐beam collimator for a multi‐modality small‐animal imaging device, which was primarily responsible yielding high‐resolution images with high sensitivity ‐ a necessity for rapid dynamic studies. The device had of a high‐resolution gamma camera that included a 2×2 array of position sensitive photomultiplier tubes and a pixilated NaI(Tl) scintillator array. The collimator's holes were square‐shaped to match the layout of the scintillation detector, with a septal thickness equals t the separation between two consecutive pixel elements. The collimator septal length (l), focal length (f), and septal thickness (t) were calculated. A Monte Carlo model was used to calculate the scattering and noise photons for a homogeneous 3D attenuating medium. The detector pixels were defined as repeated structures of square shaped NaI(Tl) crystal blocks (0.6 cm thick). The outer layer of the detector blocks was surrounded by vacuum to stop external scattering. The collimator was defined as intersecting tungsten plates of thickness 0.02cm each. The angular dependency of the fan‐beam collimator was studied and a signal to noise index was calculated within the central field of view. The detection efficiency of the fan‐beam collimator increased significantly (about 10 2 times more) in comparison to the parallel hole collimator when a point source was placed close to the focal point. The angular effect on the fan‐beam performance showed a minor effect in comparison to that of attenuation effect. The use of the fan‐beam collimator significantly increased the detection efficiency for the gamma camera.

Magnetic superlattice with two-dimensional periodicity as a waveguide for spin waves

We describe a simple method of including dissipation in the spin-wave band structure of a periodic ferromagnetic composite, by solving the Landau-Lifshitz equation for the magnetization with the Gilbert damping term. We use this approach to calculate the band structure of square and triangular arrays of Ni nanocylinders embedded in an Fe host. The results show that there are certain bands and special directions in the Brillouin zone where the spin-wave lifetime is increased by more than an order of magnitude above its average value. Thus, it may be possible to generate spin waves in such composites which decay especially slowly, and propagate especially large distances, for certain frequencies and directions in $\mathbf{k}$ space.

Propagation of designer surface plasmons in structured conductor surfaces with parabolic gradient index

In this work, we investigate the propagation of designer surface plasmons in planar perfect electric conductor structures that are subject to a parabolic graded-index distribution. A three-dimensional, fully vectorial finite-difference time-domain method was used to engineer a structure with a parabolic effective group index by modulating the dielectric constant of the structure's square holes. Using this structure in our simulations, the lateral confinement of propagating designer surface plasmons is demonstrated. Focusing, collimation and waveguiding of designer plasmons in the lateral direction is realized by changing the width of the source beam. Our findings contribute to applications of designer surface plasmons that require energy concentration, diffusion, guiding, and beam aperture modification within planar perfect electric conductors. .

Structure of liquid–vapor interface of square well fluid confined in a cylindrical pore

By means of the canonical Monte Carlo simulations, the vapor–liquid (VL) equilibrium and structure of square well (SW) fluids confined in a single cylindrical pore with repulsive surface, have been studied. Coexistence curves of the confined VL interface are determined for a wide range of temperatures and pore diameters. It is demonstrated that the confinement not only reduces the VL coexistence region but also induces strong inhomogeneities of the VL interface: coexistence liquid densities are different at the pore center and at the wall surface. It may be considered as a preliminary step for an isolated droplet formation inside the pore, as well as a tentative reason of the two VL phase transitions of simple fluids adsorbed into disordered porous media.

Computer Aided Assessment of Image Quality for Mammography Using a Contrast Detail Phantom

The aim of this study is to develop an objective method for image quality assessment for mammography using a computer code to analyse images of a contrast detail (CD) phantom. The results from the computer analysis are compared to those obtained by human observers. The CD phantom is subdivided into a matrix structure of squares. In each square two identical cylinders are milled. The diameters and depths of the cylinders vary along the axes of the CD phantom. Human observers indicate for each square if both objects are visible. To obtain an observer independent scoring, the radiographs are digitised to allow computer analysis. The computer code employs knowledge on each square, such as diameter and potential cylinder positions. The ratios in optical density are determined between the potential test spots and their background. These ratios are ranked to determine the most probable test spot locations. Various parameters are employed to assess the reliability.

Novel magnet structures for free-electron lasers

The high-energy rare-earth permanent magnets afford iron-free wiggler and twister field sources with greater field strengths in less massive structures than are possible when iron pole pieces and flux conductors are employed. However, the iron-free configurations have been notoriously difficult to adjust, as operational efficiency is very sensitive to the influence of small defects incurred in magnet manufacture and assembly. A general method for the compensation for such defects with small strategically placed magnetic dipoles is discussed and applied to structures of square, triangular, and circular cross section. The various configurations are compared with respect to design and convenience of defect compensation. The compensation of permanent magnet structures with mechanically variable field strength is also described together with a procedure for reversibly changing the function of such structures from wiggler to twister.