Kinetic coefficient of steps at the Si(111) crystal-melt interface from molecular dynamics simulations

Nonequilibrium molecular dynamics simulations are applied to the investigation of step-flow kinetics at crystal-melt interfaces of silicon, modeled with the Stillinger-Weber potential [Phys. Rev. B 31, 5262 (1985)]. Step kinetic coefficients are calculated from crystallization rates of interfaces that are vicinals of the faceted (111) orientation. These vicinal interfaces contain periodic arrays of bilayer steps, and they are observed to crystallize in a step-flow growth mode at undercoolings lower than 40 K. Kinetic coefficients for both [110] and [121] oriented steps are determined for several values of the average step separation, in the range of 7.7-62.4 A. The values of the step kinetic coefficients are shown to be highly isotropic, and are found to increase with increasing step separation until they saturate at step separations larger than approximately 50 A. The largest step kinetic coefficients are found to be in the range of 0.7-0.8 m(sK), values that are more than five times larger than the kinetic coefficient for the rough (100) crystal-melt interface in the same system. The dependence of step mobility on step separation and the relatively large value of the step kinetic coefficient are discussed in terms of available theoretical models for crystal growth kinetics from the melt.     

Kinetics of protoporphyrin IX formation in rat oral mucosa and skin after application of 5-aminolevulinic acid and its methylester

The kinetics of accumulation of protoporphyrin IX (PpIX) after topical application of 5-aminolevulinic acid (ALA) and its methylester (5-aminolevulinic acid methylester [ALA-Me]) was studied on rat oral mucosa. The accumulation of PpIX in mucosa and skin after intravenous injection of ALA and ALA-Me was also studied. The elimination rate of PpIX was dependent on drug and dose as well as on administration route. Application of ALA on rat oral mucosa and skin caused a systemic effect with PpIX building up in remote skin sites not exposed to the drugs. No such systemic effect was seen after application of ALA-Me either in mucosa or on skin. Intravenous injection of the drugs (0.2 g/kg) leads to more fluorescence in the skin than topical application of the drug (20%). For mucosa, the opposite is true. Maximal PpIX fluorescence appeared later after application of high concentrations of the drugs (around 8 h for 5% and 20% wt/wt) than after application of low concentrations (around 3-5 h for 1% and 2% wt/wt).     

Choice of optimal wavelength for PDT: the significance of oxygen depletion

We have investigated the role of tissue oxygenation on light penetration into tissue at different wavelengths. As a field of application we have chosen aminolevulinic acid-photodynamic therapy (ALA-PDT). To calculate efficiency spectra of PDT on human skin one needs to know the excitation spectrum of the photosensitizer of interest and the relative fluence rate as a function of depth in the tissue. We measured the former and computed the latter with an accurate radiative transfer algorithm. In this way we determined the efficiency spectra as functions of depth for different types of basal cell carcinomas (BCC). Our results suggest that ALA-PDT works best for nodular BCC at a wavelength of 630 nm, whereas it works best for pigmented superficial BCC at a wavelength of 390 nm. At 630 nm the light penetration into a tumor depends strongly on the oxygenation of the blood. Below a 2 mm thick, well-oxygenated, nodular BCC, we find the efficiency to be an order of magnitude larger than below a poorly oxygenated tumor. At 390 nm, the light penetration into a tumor does not depend on the oxygenation of the blood.     

Depth Profile of Protoporphyrin IX Fluorescence in an Amelanotic Mouse Melanoma Model

Protoporphyrin IX (PpIX) fluorescence was measured at different depths in a subcutaneous amelanotic melanoma model (LOX) in mice. PpIX was induced by topical application of 5‐aminolevulinic acid (ALA) and two of its derivatives, the methylester (MAL) and hexylester (HAL) onto the normal skin covering the tumor. The PpIX fluorescence intensity on the surface of the tumors was the highest for HAL, followed by ALA and MAL. Using equimolar concentrations (0.5 mmol g^?1), HAL induced nearly twice as much fluorescence as ALA did. The depth profile of PpIX fluorescence was measured at different layers of the tumor, which was carefully sliced and controlled in situ ex vivo. The PpIX fluorescence was mainly localized within the upper 2 mm of the tissue for ALA and within 1 mm for MAL and HAL. There were no significant differences in the shape of the fluorescence excitation spectra, but the long wavelength excitation peak (633 nm) was so weak that these results are unreliable for depth estimation. When considering the low fluorescence intensity (around 5% of the intensity at the tumor surface), the actual penetration depth of HAL was comparable to that of ALA. The fluorescence after topical application of ALA and HAL was significantly above the background level down to a depth of around 6 mm, and there were traces of PpIX fluorescence even at the tumor base (10 mm). The fluorescence after topical application of MAL was detectable down to 1 mm. In the depth of 2–6 mm, the fluorescence was slightly higher for HAL than for ALA. Using the estimated diffusion coefficients for topically applied ALA (0.16 ± 0.03 mm² h^?1), MAL (0.045 ± 0.005 mm² h^?1) and HAL (0.037 ± 0.003 mm² h^?1), the behavior of the drugs after different application times could be estimated in this tumor model.     

Structure and local-equilibrium thermodynamics of the c(2x2) reconstruction of rutile TiO2 (100)

We resolve the structure of a c(2x2) reconstruction of the rutile TiO2 (100) surface using a combination of transmission electron diffraction, direct methods analysis, and density functional theory. The surface structure contains an ordered array of subsurface oxygen vacancies and is in local thermodynamic equilibrium with bulk TiO2, but not the with oxygen gas-phase environment. The transition into a bulklike (1x1) reconstruction offers insights into the time-dependent local thermodynamics of TiO2 surface reconstruction under global nonequilibrium conditions.     

Large vibrational effects upon calculated phase boundaries in Al-Sc

The fcc portion of the Al-Sc phase diagram is calculated from first principles including contributions to alloy free energies associated with ionic vibrations. It is found that vibrational entropy accounts for a 27-fold increase in the calculated solubility limits for Sc in fcc Al at high temperatures, bringing calculated and measured values into very good agreement. The present work gives a clear example demonstrating a large effect of vibrational entropy upon calculated phase boundaries in substitutional alloys.     

Surface structures of SrTiO3 (001): a TiO2-rich reconstruction with a c(4 x 2) unit cell

We report the solution of the c(4 x 2) reconstruction of SrTiO(3) (001), obtained through a combination of high-resolution transmission electron microscopy, direct methods analysis, and density functional theory. The structure is characterized by a single overlayer of TiO(2) stoichiometry in which TiO(5) polyhedra are arranged into edge-shared structures, in contrast to the corner-shared TiO(6) polyhedra in bulk. This structural pattern is similar to that reported by us earlier for the (2 x 1) reconstruction of the same crystal face formed at higher temperature. We discuss probable mechanisms of surface stabilization as revealed by these two solutions which are likely to apply to other reconstructions of SrTiO(3) (001) and, possibly, other perovskites in general.