Stereoelectronic effects on the binding of neutral Lewis bases to CdSe nanocrystals

Using P-31 nuclear magnetic resonance (NMR) spectroscopy, we monitor the competition between tri-nbutylphosphine (Bu3P) and various amine and phosphine ligands for the surface of chloride terminated CdSe nanocrystals. Distinct P-31 NMR signals for free and bound phosphine ligands allow the surface ligand coverage to be measured in phosphine solution. Ligands with a small steric profile achieve higher surface coverages (Bu3P = 0.5 nm(-2), Me2P-n-octyl = 2.0 nm(-2), NH2Bu = >3 nm(-2)) and have greater relative binding affinity for the nanocrystal (binding affinity: Me3P > Me2P -n-octyl similar to Me2P -n-octadecyl > Et3P > Bu3P). Among phosphines, only Bu 3 P and Me2P-n-octyl support a colloidal dispersion, allowing a relative surface binding affinity (K-rel) to be estimated in that case (K-rel = 3.1). The affinity of the amine ligands is measured by the extent to which they displace Bu3P from the nanocrystals (K-rel: H2NBu similar to N-n-butylimidazole > 4-ethylpyridine > Bu3P similar to HNBu2 > Me2NBu > Bu3N). The affinity for the CdSe surface is greatest among soft, basic donors and depends on the number of each ligand that bind. Sterically unencumbered ligands such as imidazole, pyridine, and n-alkylamines can therefore outcompete stronger donors such as alkylphosphines. The influence of repulsive interactions between ligands on the binding affinity is a consequence of the high atom density of binary semiconductor surfaces. The observed behavior is distinct from the self-assembly of straight-chain surfactants on gold and silver where the ligands are commensurate with the underlying lattice and attractive interactions between aliphatic chains strengthen the binding

Inside-Out Planet Formation. V. Structure of the Inner Disk as Implied by the MRI

The large population of Earth to super-Earth sized planets found very close to their host stars has motivated consideration of $in$ $situ$ formation models. In particular, Inside-Out Planet Formation is a scenario in which planets coalesce sequentially in the disk, at the local gas pressure maximum near the inner boundary of the dead zone. The pressure maximum arises from a decline in viscosity, going from the active innermost disk (where thermal ionization of alkalis yields high viscosities via the magneto-rotational instability (MRI)) to the adjacent dead zone (where the MRI is quenched). Previous studies of the pressure maximum, based on $\alpha$-disk models, have assumed ad hoc values for the viscosity parameter $\alpha$ in the active zone, ignoring the detailed physics of the MRI. Here we explicitly couple the MRI criteria to the $\alpha$-disk equations, to find steady-state (constant accretion rate) solutions for the disk structure. We consider the effects of both Ohmic and ambipolar resistivities, and find solutions for a range of disk accretion rates ($\dot{M}$ = $10^{-10}$ - $10^{-8}$ ${\rm M}_{\odot}$/yr), stellar masses ($M_{\ast}$ = 0.1 - 1 ${\rm M}_{\odot}$), and fiducial values of the $non$-MRI $\alpha$-viscosity in the dead zone ($\alpha_{\rm {DZ}} = 10^{-5}$ - $10^{-3}$). We find that: (1) A midplane pressure maximum forms radially $outside$ the inner boundary of the dead zone; (2) Hall resistivity dominates near the midplane in the inner disk, which may explain why close-in planets do $not$ form in $\sim$50% of systems; (3) X-ray ionization can be competitive with thermal ionization in the inner disk, because of the low surface density there in steady-state; and (4) our inner disk solutions are viscously unstable to surface density perturbations.Comment: 34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical Journa

Feature-based reverse engineering of mechanical parts

Journal ArticleReverse engineering of mechanical parts requires extraction of information about an instance of a particular part sufficient to replicate the part using appropriate manufacturing techniques. This is important in a wide variety of situations, since functioning CAD models are often unavailable or unusable for parts which must be duplicated or modified. Computer vision techniques applied to 3-D data acquired using non-contact, three-dimensional position digitizers have the potential for significantly aiding the process. Serious challenges must be overcome, however, if sufficient accuracy is to be obtained and if models produced from sensed data are truly useful for manufacturing operations. This paper describes a prototype of a reverse engineering system which uses geometric representations natural to the manufacturing process. The system is interactive, which improves performance and allows for human entry of information that cannot be acquired from sensed data alone

Active inspection and reverse engineering

technical reportWe propose a new design for inspection and reverse engineering environments. In particular, we investigate the use of discrete event dynamic systems (DEDS) to guide and control the active exploration and sensing of mechanical parts for industrial inspection and reverse engineering. We introduce dynamic recursive finite state machines (DRFSM) as a new DEDS tool for utilizing the recursive nature of the mechanical parts under consideration. The proposed framework uses DRFSM DEDS for constructing an observer for exploration and inspection purposes. We construct a sensing ?? CAD interface for the automatic reconstruction of parts from visual data. We also implement a graphical interface for designing DRFSM DEDS controllers

Intermediate results in active inspection and reverse engineering

technical reportIn previous work [18], we have proposed a new design for inspection and reverse engineering environments. We have investigated the use of the dynamic recursive context of discrete event dynamic systems (DRFSM DEDS) to guide and control the active exploration and sensing of mechanical parts for industrial inspection and reverse engineering, and utilized the recursive nature of the parts under consideration. In our recent work, we construct a sensing to CAD interface for the automatic reconstruction of parts from visual data. This report includes previous results and describes this interface in greater detail, demonstrating its effectiveness with a reverse-engineered, machined part

MRI-active inner regions of protoplanetary discs. I. A detailed model of disc structure

Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magneto-rotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coupled in our model; moreover, we account for many processes omitted in previous such models, including disc heating by both accretion and stellar irradiation, vertical energy transport, realistic dust opacities, dust effects on disc ionization and non-thermal sources of ionization. For a disc around a solar-mass star with a standard gas accretion rate ($\dot{M}$$\sim$$10^{-8}$M$_\odot$yr$^{-1}$) and small dust grains, we find that the inner disc is optically thick, and the accretion heat is primarily released near the midplane. As a result, both the disc midplane temperature and the location of the pressure maximum are only marginally affected by stellar irradiation, and the inner disc is also convectively unstable. As previously suggested, the inner disc is primarily ionized through thermionic and potassium ion emission from dust grains, which, at high temperatures, counteract adsorption of free charges onto grains. Our results show that the location of the pressure maximum is determined by the threshold temperature above which thermionic and ion emission become efficient.Comment: accepted for publication in MNRA

Kinetic control over CdS nanocrystal nucleation using a library of thiocarbonates, thiocarbamates, and thioureas

We report a family of substituted thiocarbonates, thiocarbamates, and thioureas and their reaction with cadmium oleate at 180-240 degrees C to form zincblende CdS nanocrystals (d = 2.25.9 nm). To monitor the kinetics of CdS formation with UV-vis spectroscopy, the size dependence of the extinction coefficient for lambda(max)(1S(e)-1S(1/2h)) is determined. The precursor conversion reactivity spans 5 orders of magnitude depending on the precursor structure (2 degrees-thioureas > 3 degrees-thioureas >= 2 degrees-thiocarbamates > 2 degrees-thiocarbonates > 4 degrees-thioureas >= 3 degrees-thiocarbamates). The concentration of nanocrystals formed during nucleation increases when more reactive precursors are used, allowing the final size to be controlled by the precursor structure. H-1 NMR spectroscopy is used to monitor the reaction of di-p-tolyl thiocarbonate and cadmium oleate where di-p-tolyl carbonate and oleic anhydride coproducts can be identified. These coproducts further decompose into p-tolyl oleate and p-cresol. The spectral features of CdS nanocrystals produced from thiocarbonates are exceptionally narrow (95-161 meV fwhm) as compared to those made from thioureas (137-174 meV fwhm) under otherwise identical conditions, indicating that particular precursors nucleate narrower size distributions than others