Piotr Mazurek

Xanthophyll pigments lutein and zeaxanthin in lipid multibilayers formed with dimyristoylphosphatidylcholine

This paper reports the research on the effect of two main carotenoid pigments present in the membranes of macula lutea of the vision apparatus of primates, including humans, lutein and zeaxanthin, on the structure of model membranes formed with dimyristoylphosphatidylcholine (DMPC). The effects observed in DMPC are compared to the effects observed in the membranes formed with other phosphatidylcholines (PC): egg yolk PC (EYPC), and dipalmitoyl-PC (DPPC). The analysis has been focused, in particular, on the following aspects of the organization of lipid-carotenoid membranes: aggregation state of pigments, an effect on a thickness of the bilayer and pigment orientation within the membranes. These problems have been addressed with the application of UV-Vis absorption spectroscopy, linear dichroism measurements and the diffractometric technique. (1) Both lutein and zeaxanthin appear in a partially aggregated form in the oriented DMPC multibilayers, even at molar fractions as low as 2 mol.% with respect to lipid. (2) Orientation of the transition dipole of both xanthophylls with respect to the axis normal to the plane of DMPC membrane is different in the case of a monomeric form (34+/-3 degrees in the case of lutein and 26+/-3 degrees in the case of zeaxanthin) but essentially the same in the case of aggregated forms of both pigments (42+/-3 degrees in the case of lutein and 40+/-5 degrees in the case of zeaxanthin). It was found that only lutein has an effect on the increase in the thickness of the DMPC membranes (by about 3 A at 25 degrees C). A similar effect was observed also in the case of DPPC at the same temperatures despite the differences in the physical state of both membrane systems. The differences between the effects of lutein and zeaxanthin observed are interpreted in terms of differences of stereochemical structure of both xanthophylls leading to the different localization in the lipid phase. The results demonstrate significant differences in the behavior of lutein and zeaxanthin in model membranes, which may contribute to their different physiological functions and different efficacy as membrane antioxidants.

RHEED intensity oscillations observed during growth of Ge on Si(111) substrates

Reflection high-energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy growth of Ge on a Si(111) surface. At 250°C the oscillations continue up to 6 ML. When Ge is grown at room temperature on the GeSi(111) surface the oscillations continue up to 14 ML. During the growth of Ge thin films on a clean Si(111)-7 × 7 surface at room temperature the oscillations continue up to 10 ML. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A simple birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250°C.

RHEED intensity oscillations observed during the growth of YSi2 − x on Si(111) substrates

Abstract For the first time reflection high-energy electron diffraction intensity oscillations were observed during reactive deposition epitaxy (RDE) growth of YSi2 − x (x ≅0.3) on the Si(111) surface. The YSi2 − x crystallographic structure consists of Y planes alternating with Si planes parallel to the Si(111) substrate planes. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A birth-death model was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the growth of YSi2 − x on the Si(111) surface.

Direct observation of rare-earth silicide epilayer formation by RHEED technique

Abstract The solid phase reaction of yttrium thin film and silicon substrate was investigated in situ with help of RHEED technique. For the first time, we have measured directly yttrium silicide formation temperature which can be as low as 120 °C for a thin (60 A) metal layer deposited on a Si(111) substrate. For this purpose, we developed a new experimental technique. A study of the growth of thin (10–150 A) silicon overlayers on yttrium and dysprosium silicide films epitaxially grown on Si(111) was also made. In this paper, an in situ reflection high energy electron diffraction (RHEED) pattern and also azimuthal plot investigation of Si/RE-silicide/Si double heterostructures grown by solid phase epitaxy and reactive deposition method are presented.

Epitaxial silicides: new results from RHEED analysis

Abstract The growth kinetics and surface structure of Y, Dy and Co suicides are examined using a new reflection high-energy electron diffraction (RHEED) method. We prepared epitaxial silicide films on (111) Si by (1) deposition of metal and contact reaction (solid phase epitaxy), and (2) deposition of metal on hot substrates (reactive deposition). Subsequent annealing at temperatures up to 600 °C yielded monocrystalline, continuous layers, whose properties were examined by means of RHEED (in situ) and X-ray diffraction and scanning electron microscopy (ex situ). Method 2 was shown to give better results. We have measured directly the yttrium silicide formation temperature which can be as low as 120 °C for thin (60 A) metal layer deposited on (111)Si substrate. RHEED azimuthal plots of the Si layer which was grown on silicides depends strongly on the crystalline quality of the epitaxial layers. A study of the growth of thin (10–150 A) silicon overlayers on yttrium, dysprosium and cobalt suicide films epitaxially grown on (111)Si was also made.

Monte-Carlo simulation of Ge on Si(111) MBE growth: analysis of percolative structure

Abstract Reflection high energy electron diffraction (RHEED) intensity oscillations were observed during molecular beam epitaxy (MBE) growth of Ge on Si(111) surface. At 250 °C the oscillations continue up to 6 monolayers. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. Monte-Carlo simulation was used for the growth simulation in order to investigate fundamental behaviours of reflectivity change during the Stranski-Krastanov growth of Ge on the Si(111) surface at 250 °C.

Distributed growth model used for the interpretation of RHEED intensity oscillations observed during the growth of Pb on Si(111) substrates

Abstract Analyses of reflection high-energy electron diffraction (RHEED) intensity changes observed during initial stages of heteroepitaxial growth of Pb on Si(111) substrates were presented. The intensity of the reflected beam is calculated by solving the one-dimensional Schrodinger equation. A simple birth-death model was used for the growth simulation in order to investigate the fundamental behaviours of reflectivity change during the growth of Pb on the Si(111) surface.