Local atomic order and infrared spectra of biogenic calcite.
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Calcification, Physiologic↗
Biomedical subjects
Leeor Kronik
Publications and source records attributed to Leeor Kronik.
Radiation damage to alkyl chain monolayers on semiconductor substrates investigated by electron spectroscopy.
Monolayers of alkyl chains, attached through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO2, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation- or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) pi resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.
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Photoelectron spectroscopy as a structural probe of intermediate size clusters.
We examine the utility of photoelectron spectroscopy (PES) as a structural probe of Si(n) (-) in the n=20-26 size range by determining isomers and associated photoelectron spectra from first principles calculations. Across the entire size range, we consistently obtain a good agreement between the theory and experiment [Hoffmann et al., Eur. Phys. J. D 16, 9 (2001)]. We find that PES can almost invariably distinguish between structurally distinct isomers at a given cluster size, but that structurally similar isomers usually cannot be reliably distinguished by PES. For many, but not all, sizes the isomer giving the best match to experiment is the lowest-energy one found theoretically. Thus, combining theory with PES experiments emerges as a useful source of structural information even for intermediate size clusters.
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Size-dependent spintronic properties of dilute magnetic semiconductor nanocrystals.
The electronic structure and magnetic properties of Mn-doped Ge, GaAs, and ZnSe nanocrystals are investigated using real space ab initio pseudopotentials constructed within the local spin-density approximation. The ferromagnetic and half-metallicity trends found in the bulk are preserved in the nanocrystals. However, the Mn-related impurity states become much deeper in energy with decreasing nanocrystalline size, causing the ferromagnetic stabilization to be dominated by double exchange via localized holes rather than by a Zener-like mechanism.
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Electrical response of molecular chains from density functional theory.
The electrical response of molecular chains is dramatically overestimated by local and semilocal density functionals. We show that Kohn-Sham density-functional theory yields accurate linear and nonlinear polarizabilities when the exact exchange energy is employed together with the corresponding exact Kohn-Sham potential upsilonx(r). We further show that approximations to upsilonx(r) that are very accurate for the ground-state energy can nevertheless fail badly for the response because of potential barriers that have little effect on the ground-state energy but strongly affect the electron mobility.
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Hybridization and bond-orbital components in site-specific X-ray photoelectron spectra of rutile TiO2.
We have determined the Ti and O components of the rutile TiO2 valence band using the method of site-specific x-ray photoelectron spectroscopy. Comparisons with calculations based on pseudopotentials within the local density approximation reveal the hybridization of the Ti 3d, 4s, and 4p states, and the O 2s and 2p states on each site. These chemical effects are observed due to the large differences between the angular-momentum dependent matrix elements of the photoelectron process.
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Highest electron affinity as a predictor of cluster anion structures.
Small clusters have a range of unique physical and chemical phenomena that are strongly size dependent. However, analysis of these phenomena often assumes that thermodynamic equilibrium conditions prevail. We compare experimentally measured and ab initio computed photoelectron spectra of bare and deuterated silicon cluster anions produced in a plasma environment. We find that the isomers detected experimentally are usually not the ground-state isomers, but metastable ones, which indicates that cluster relaxation is strongly limited kinetically by a dwell time that is much shorter than the relaxation time. We show that, under these conditions, the highest electron affinity replaces the traditional lowest total energy as the appropriate criterion for predicting isomer structures. These findings demonstrate that a stringent examination of non-equilibrium effects can be crucial for a correct analysis of cluster properties.
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