Transmission electron microscopy investigation of segregation and critical floating-layer content of indium for island formation in InGaAs

We have investigated InGaAs layers grown by molecular-beam epitaxy on GaAs(001) by transmission electron microscopy (TEM) and photoluminescence spectroscopy. InGaAs layers with In-concentrations of 16, 25 and 28 % and respective thicknesses of 20, 22 and 23 monolayers were deposited at 535 C. The parameters were chosen to grow layers slightly above and below the transition between the two- and three-dimensional growth mode. In-concentration profiles were obtained from high-resolution TEM images by composition evaluation by lattice fringe analysis. The measured profiles can be well described applying the segregation model of Muraki et al. [Appl. Phys. Lett. 61 (1992) 557]. Calculated photoluminescence peak positions on the basis of the measured concentration profiles are in good agreement with the experimental ones. Evaluating experimental In-concentration profiles it is found that the transition from the two-dimensional to the three-dimensional growth mode occurs if the indium content in the In-floating layer exceeds 1.1+/-0.2 monolayers. The measured exponential decrease of the In-concentration within the cap layer on top of the islands reveals that the In-floating layer is not consumed during island formation. The segregation efficiency above the islands is increased compared to the quantum wells which is explained tentatively by strain-dependent lattice-site selection of In. In addition, In0.25Ga0.75As quantum wells were grown at different temperatures between 500 oC and 550 oC. The evaluation of concentration profiles shows that the segregation efficiency increases from R=0.65 to R=0.83.Comment: 16 pages, 6 figures, 1 table, sbmitted in Phys. Rev.

n-Однородные C*-алгебры

The classical results by J. Fell, J. Tomiyama, M. Takesaki describe n-homogeneous С*-algebras as algebras of all continuous sections for an appropriate algebraic bundle. By using this realization, several authors described the set of n-homogeneous С*-algebras with different spaces of primitive ideals. In 1974 F. Krauss and T. Lawson described the set of all n-homogeneous С*-algebras whose space Prim of primitive ideals is homeomorphic to the sphere S2. Suppose the space PrimA of primitive ideals is homeomorphic to the sphere S3 for some n-homogeneous С*-algebra A. In this case, these authors showed that the algebra A is isomorphic to the algebra C(S3,Cn×n). If n ≥ 2 then there are countably many pairwise non-isomorphic n-homogeneous С*-algebras A such that PrimA ≅ S 4. Further, let n ≥ 3. There is only one n-homogeneous С*-algebra A such that PrimA ≅ S 5. There are two non-isomorphic 2-homogeneous С*-algebras A and B with space PrimA ≅ S 5. On the other hand, algebraic bundles over the torus T 2 are described by a residue class [p] in Z/nZ = π1(PUn). Two such bundles with classes [pi] produce isomorphic С*-algebras if and only if [p1] = ±[p2]. An algebraic bundle over the torus T 3 is determined by three residue classes in Z/nZ. Anatolii Antonevich and Nahum Krupnik introduced some structures on the set of algebraic bundles over the sphere S2. Algebraic bundles over the compact connected two-dimensional oriented manifolds were considered by the author. In this case, the set of non-equivalent algebraic bundles over such space is like the set of algebraic bundles over the torus T2. Further advances could be in describing the set of algebraic bundles over the 3-dimensional manifolds.Рассматриваются результаты, касающиеся n-однородных С*-алгебр. Приводятся классические результаты Ж. Фелла, Ж. Томияма, М. Такесаки, описывающие n-однородную С*-алгебру как алгебру всех непрерывных сечений соответствующего алгебраического расслоения. Посредством этой геометрической интерпретации, различные авторы описывали классы n-однородных С*-алгебр с пространством примитивных идеалов, гомеоморфным двумерной сфере S2, трехмерной сфере S3, двумерному тору T2, трехмерному тору T3, произвольному связному ориентируемому и неориентируемому компактному двумерному многообразию. Также А. Антоневич и Н. Крупник задавали различные структуры на множестве классов эквивалентности алгебраических расслоений на сферах. Дальнейшая работа в этом направлении может состоять в описании классов эквивалентности алгебраических расслоений над трехмерными, четырехмерными многообразиями и т. д

Binding mechanism of the model charged dye carboxyfluorescein to hyaluronan/polylysine multilayers

Biopolymer-based multilayers become more and more attractive due to the vast span of biological application they can be used for, e.g., implant coatings, cell culture supports, scaffolds. Multilayers have demonstrated superior capability to store enormous amounts of small charged molecules, such as drugs, and release them in a controlled manner; however, the binding mechanism for drug loading into the multilayers is still poorly understood. Here we focus on this mechanism using model hyaluronan/polylysine (HA/PLL) multilayers and a model charged dye, carboxyfluorescein (CF). We found that CF reaches a concentration of 13 mM in the multilayers that by far exceeds its solubility in water. The high loading is not related to the aggregation of CF in the multilayers. In the multilayers, CF molecules bind to free amino groups of PLL; however, intermolecular CF–CF interactions also play a role and (i) endow the binding with a cooperative nature and (ii) result in polyadsorption of CF molecules, as proven by fitting of the adsorption isotherm using the BET model. Analysis of CF mobility in the multilayers by fluorescence recovery after photobleaching has revealed that CF diffusion in the multilayers is likely a result of both jumping of CF molecules from one amino group to another and movement, together with a PLL chain being bound to it. We believe that this study may help in the design of tailor-made multilayers that act as advanced drug delivery platfor

Formation and stability of self-assembled coherent islands in highly mismatched heteroepitaxy

We study the energetics of island formation in Stranski-Krastanow growth within a parameter-free approach. It is shown that an optimum island size exists for a given coverage and island density if changes in the wetting layer morphology after the 3D transition are properly taken into account. Our approach reproduces well the experimental island size dependence on coverage, and indicates that the critical layer thickness depends on growth conditions. The present study provides a new explanation for the (frequently found) rather narrow size distribution of self-assembled coherent islands.Comment: 4 pages, 5 figures, In print, Phys. Rev. Lett. Other related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

The influence of surface stress on the equilibrium shape of strained quantum dots

The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained islands with sizes >= 10,000 atoms) grown on a GaAs (001) substrate are studied using a hybrid approach which combines density functional theory (DFT) calculations of microscopic parameters, surface energies, and surface stresses with elasticity theory for the long-range strain fields and strain relaxations. In particular we report DFT calculations of the surface stresses and analyze the influence of the strain on the surface energies of the various facets of the quantum dot. The surface stresses have been neglected in previous studies. Furthermore, the influence of edge energies on the island shapes is briefly discussed. From the knowledge of the equilibrium shape of these islands, we address the question whether experimentally observed quantum dots correspond to thermal equilibrium structures or if they are a result of the growth kinetics.Comment: 7 pages, 8 figures, submitted to Phys. Rev. B (February 2, 1998). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

On the Stabilizing Effect of Surfactants Adsorption Layers

Cohesion forces in contacts of discrete spherical particles (having either hydrophilic or hydrophobic surface) were measured directly in aqueous or heptane solutions. It is shown that the stabilizing action of tensides in the hydrophobic particle system was brought about by a two-dimensional pressure of the adsorbed tenside which is easily removed · from the contact zone of the particle. The stabilizing action of tensides in the hydrophilic particle systems is a consequence of the formation of a mechanical barrier which has a high apparent strength due to the adsorbed layer on the solid surface

High-Efficiency Impact Electromechanical Converter

An algorithm for choosing the parameters of the impact electromechanical converter based on the genetic algorithms and the flexible polyhedron method joint use is developed. The spatial temporal distribu tion of integral and local electrodynamics forces that act on the accelerated anchor is found

Development of a Comprehensive Approach to Assessing the Content of Elemental Contaminants in Native Herbal Medicinal Products, Using Pumpkin Seeds as a Case Study

Native herbal medicinal products (NHMPs) are medicinal products derived from plant raw materials. NHMPs are characterised by a complete transfer of contaminants from raw materials to finished products and comparability of therapeutic doses to daily consumption of many foods. The aim of the study was to develop an approach to assessing the content of elemental contaminants in NHMPs, based on public health risk assessment methods, and exemplify its use by applying it to pumpkin seeds. Materials and methods: the study analysed national and international regulatory documents governing the control of elemental contaminants in medicinal products and food, as well as literature on the ability of pumpkin seeds to accumulate such contaminants. In order to determine permitted concentrations, hazard quotients and cancer slope factors, the authors used internationally accepted risk assessment criteria. Results: the study identified the elemental contaminants to be controlled during risk assessment of NHMPs (i.e., As, Cd, Co, Cr, Pb, Hg, Ni, and V). The authors calculated permitted concentrations of the elements for a therapeutic dose of pumpkin seeds; hazard quotients for the essential (Co, Cr, Cu, Fe, Mn, Mo, Se, Zn), probably essential (As, Ni, V) and toxic (Al, Cd, Hg, Pb, Sr, Tl) elements consumed with the dose; and individual cancer risks associated with As, Cd, Cr, Pb. Conclusions: the total health impact of the studied elements in a therapeutic dose of pumpkin seeds is permissible, and there is no risk of non-carcinogenic effects. The content of each carcinogenic element in pumpkin seeds corresponds to the acceptable level of maximum individual cancer risk. These risk levels are to be controlled at all times. The results of this study may support the development of a compendial monograph on pumpkin seeds