Soluble functionalized carbon nanohorns

Carbon nanahorns (CNH) were functionalized following the methodology of 1,3-dipolar cycloaddition of azomethine ylides and found to form stable solutions in either organic solvents or water. The number of added functional units, in the form of pyrrolidine moieties, was calculated when a pyrene chromophore was utilized in the modification scheme. Moreover, complementary theoretical calculations revealed that reactivity enhancement is expected at locations near the conical-shaped tip of CNH, where the highest curvature and strain exist. Finally, additional organic transformation of already modified CNH was exploited by covalently linked ferrocene units.     

Propagation of shear waves generated by a modulated finite amplitude radiation force in a viscoelastic medium

An effective way to generate localized narrow-band low-frequency shear waves within tissue noninvasively, is by the modulated radiation force, resulting from the interference of two confocal quasi-CW ultrasound beams of slightly different frequencies. By using approximate viscoelastic Green's functions, investigations of the properties of the propagated shear-field component at the fundamental modulation frequency were previously reported by our group. However, high-amplitude source excitations may be needed to increase the signal-to-noise-ratio for shear-wave detection in tissue. This paper reports a study of the generation and propagation of dynamic radiation force components at harmonics of the modulation frequency for conditions that generally correspond to diagnostic safety standards. We describe the propagation characteristics of the resulting harmonic shear waves and discuss how they depend on the parameters of nonlinearity, focusing gain, and absorption. For conditions of high viscosity (believed to be characteristic of soft tissue) and higher modulation frequencies, the approximate shear wave Green's function is inappropriate. A more exact viscoelastic Green's function is derived in k-space, and using this, it is shown that the lowpass and dispersive effects, associated with a Voigt model of tissue, are more accurately represented. Finally, it is shown how the viscoelastic properties of the propagating medium can be estimated, based on several spectral components of the shear wave spectrum.     

Narrowband shear wave generation by a Finite-Amplitude radiation force: The fundamental component

A highly localized source of low-frequency shear waves can be created by the modulated radiation force resulting from two intersecting quasi-continuous-wave ultrasound beams of slightly different frequencies. In contrast to most other radiation force-based methods, these shear waves can be narrowband. Consequently, different frequency-dependent effects will not significantly affect their spectrum as they propagate within a viscoelastic medium, thereby enabling the viscoelastic shear properties of the medium to be determined at any given modulation frequency. This can be achieved by tracking the shear wave phase delay and change in amplitude over a specific distance. In this paper we explore the properties of short duration (dynamic) low-frequency shear wave propagation and study how the shear displacement field depends on the excitation conditions. Our investigations make use of the approximate Green's functions for viscoelastic media, and the evolution of such waves is studied in the spatiotemporal domain from a theoretical perspective. Although nonlinearities are included in our confocal source model, just the properties of the fundamental shear component are examined in this paper. We examine how the shear wave propagation is affected by the shear viscosity, the coupling wave, the spatial distribution of the force, the shear speed, and the duration of the modulated wave. A method is proposed for estimating the shear viscosity of a viscoelastic medium. In addition, it is shown how the Voigt model paremeters can be extracted from the frequency-dependent speed and attenuation.     

Watermark detection for noisy interpolated images

In this brief, the case where the watermark is detected in a noisy interpolated version of the originally watermarked image is investigated. Polyphase decomposition is utilized at the detection side in order to enable the flexible formation of a fused image, which is appropriate for watermark detection. The optimal fused correlator, obtained by combining information from different image components, is derived through a statistical analysis of the correlation detector properties, followed by Lagrange optimization. It is shown that it is preferable to perform detection in a fused image rather than the original image.     

Theoretical study of benzene,toluene, and dibromobenzene at a Si(111)7×7 surface

Theoretical Austin model 1 (AM1) calculations on the adsorption of benzene and toluene on Si(111)7×7 are presented. Both physisorbed and chemi-sorbed states have been calculated for up to three adsorbed molecules per half unit cell of the Si(111)7×7 surface. Secondly, theoretical calculations on the induced attachment of benzene as well as rationalization of the dynamics of the halogenation reaction of 1,2- and 1,4-dibromobenzene on Si(111)7×7 are reviewed. The main incentive for this study was the interpretation of recent experimental scanning tunneling microscopy (STM) results from the Toronto laboratory on a new electron-induced or photo-induced attachment process for benzene on Si(111), and, particularly, experimental results related to the thermal dissociative reactions of 1,2- and 1,4-dibromobenzene on a Si(111)7×7 surface. The central objective is to relate the reagent geometry in 1,2-dibromobenzene and 1,4-dibromobenzene to the Br-Br pair distance of dibrominated Si(111)7×7. For benzene, we propose a possible path for the conversion from the normal strained di-sigma-bound state (S) at Si(111) to a more strongly bound state (B) consisting of a phenyl plus an H-atom adsorbed species. For 1,2- and 1,4-dibromobenzene dibromination of silicon, evidence has been found for two mechanisms of reaction. One reaction pathway involves intermediate binding of the organic molecule on the Si surface through C-Si bonds, analogous to the benzene S structure. The second dynamical pathway involves intermediate binding through weak Br. Si attachment followed by formation of pairs of covalently-bound Br-Si. The outcomes from the two dynamical pathways are consistent with the observed STM patterns for pairs of Br-Si at Si(111) 7×7 due to the reaction of 1,2- and 1,4-dibromobenzene.     

Theoretical study of halogen-substituted benzene at a Si(111)7×7 surface

The adsorption of halobenzene (for halogens F, Cl, Br, and I) on Si(111)7×7 was investigated using AM1 quantum mechanical calculations. First the 1,4-cyclohexadiene type of chemisorbed structures with two C—Si bonds at C atoms 1 and 4 have been calculated. Generally, the calculated binding energy increases with the size of the halogen atom, in the series F, Cl, Br, and I. The sp² carbon positions are the most favorable for the halogen atom, and for bromobenzene and iodobenzene there is significant additional stabilization for the geometry that allows interaction of the halogen atom with a nearby Si adatom. This stabilization hinders the transfer of the heavier halogen atoms, Br and I, to the surface. Other chemisorbed structures, involving formally divalent halogen atoms in a C-X-Si type of binding, have also been found to correspond to energy minima in the AM1 calculations. Furthermore, it was possible to calculate physisorbed structures for chlorobenzene and bromobenzene once the Si rest atoms were capped with H atoms. Two types of binding structures are suggested for chemisorption in excess of three molecules per half-unit cell: one is a radical structure binding by a single C-Si adatom bond, and the second one is binding by a single C-X-Si adatom interaction. Both of these singly-bonded structures have been calculated to correspond to energy minima with binding energies smaller than the 1,4-cyclohexadiene type of chemisorbed structure.