631 research outputs found
Morphology of passivating organic ligands around a nanocrystal
Semiconductor nanocrystals are a promising class of materials for a variety
of novel optoelectronic devices, since many of their properties, such as the
electronic gap and conductivity, can be controlled. Much of this control is
achieved via the organic ligand shell, through control of the size of the
nanocrystal and the distance to other objects. We here simulate ligand-coated
CdSe nanocrystals using atomistic molecular dynamics, allowing for the
resolution of novel structural details about the ligand shell. We show that the
ligands on the surface can lie flat to form a highly anisotropic 'wet hair'
layer as opposed to the 'spiky ball' appearance typically considered. We
discuss how this can give rise to a dot-to-dot packing distance of one ligand
length since the thickness of the ligand shell is reduced to approximately
one-half of the ligand length for the system sizes considered here; these
distances imply that energy and charge transfer rates between dots and nearby
objects will be enhanced due to the thinner than expected ligand shell. Our
model predicts a non-linear scaling of ligand shell thickness as the ligands
transition from 'spiky' to 'wet hair'. We verify this scaling using TEM on a
PbS nanoarray, confirming that this theory gives a qualitatively correct
picture of the ligand shell thickness of colloidal quantum dots.Comment: 17 Pages, 9 Figure
Measuring ligand-cell surface receptor affinities with axial line-scanning fluorescence correlation spectroscopy
Development and homeostasis of multicellular organisms is largely controlled by complex cell-cell signaling networks that rely on specific binding of secreted ligands to cell surface receptors. The Wnt signaling network, as an example, involves multiple ligands and receptors to elicit specific cellular responses. To understand the mechanisms of such a network, ligand-receptor interactions should be characterized quantitatively, ideally in live cells or tissues. Such measurements are possible using fluorescence microscopy yet challenging due to sample movement, low signal-to-background ratio and photobleaching. Here, we present a robust approach based on fluorescence correlation spectroscopy with ultra-high speed axial line scanning, yielding precise equilibrium dissociation coefficients of interactions in the Wnt signaling pathway. Using CRISPR/Cas9 editing to endogenously tag receptors with fluorescent proteins, we demonstrate that the method delivers precise results even with low, near-native amounts of receptors
Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?
During the exothermic adsorption of molecules at solid surfaces dissipation
of the released energy occurs via the excitation of electronic and phononic
degrees of freedom. For metallic substrates the role of the nonadiabatic
electronic excitation channel has been controversially discussed, as the
absence of a band gap could favour an easy coupling to a manifold of
electronhole pairs of arbitrarily low energies. We analyse this situation for
the highly exothermic showcase system of molecular oxygen dissociating at
Pd(100), using time-dependent perturbation theory applied to first-principles
electronic-structure calculations. For a range of different trajectories of
impinging O2 molecules we compute largely varying electron-hole pair spectra,
which underlines the necessity to consider the high-dimensionality of the
surface dynamical process when assessing the total energy loss into this
dissipation channel. Despite the high Pd density of states at the Fermi level,
the concomitant non-adiabatic energy losses nevertheless never exceed about 5%
of the available chemisorption energy. While this supports an electronically
adiabatic description of the predominant heat dissipation into the phononic
system, we critically discuss the non-adiabatic excitations in the context of
the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed
V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin
asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)
Prevalence and patterns of self-reported animal-related injury among veterinarians in metropolitan Kampala
To establish the prevalence, patterns and risk factors of animal-related injuries among veterinarians, self-administered questionnaires were given to 60 veterinarians practicing in metropolitan Kampala. The prevalence of animal-related injuries in metropolitan Kampala was 72% (95%CI, 57~84). Some veterinarians (34%) suffered multiple injuries with a mean and median of 2.1 and 2.0 injuries per veterinarian, respectively. Of a total of 70 self-reported animal related injuries, cattle accounted for 72%, cats for 25%, dogs for 23%, self inoculation for 15% and birds for 13%. Injuries associated with poultry did not require hospital treatment. The upper limb was the most the frequently (68%) injured anatomical body part of veterinarians, and vaccination of animals (25%) was the major activity associated with injury. Animal-related injuries are common among practicing veterinarians in metropolitan Kampala; however, they did not differ significantly based on the veterinarian's gender, experience or risk awareness
Symmetric Versus Nonsymmetric Structure of the Phosphorus Vacancy on InP(110)
The atomic and electronic structure of positively charged P vacancies on
InP(110) surfaces is determined by combining scanning tunneling microscopy,
photoelectron spectroscopy, and density-functional theory calculations. The
vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge
transfer level 0.75+-0.1 eV above the valence band maximum. The scanning
tunneling microscopy (STM) images show only a time average of two degenerate
geometries, due to a thermal flip motion between the mirror configurations.
This leads to an apparently symmetric STM image, although the ground state
atomic structure is nonsymmetric.Comment: 5 pages including 3 figures. related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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