Synthesis of enantiomerically enriched amino sulfide building blocks from acyclic chiral amino allylsilanes

An efficient synthesis of various protected syn-β-sulfenyl amides is described. These are prepared from the corresponding enantiopure amino allylsilanes which are in turn obtained from naturally occurring amino acids. The key step for introduction of the sulfur substituent is a diastereoselective electrophilic sulfodesilylation which is carried out with phthalimidesulfenyl chloride. The resulting homochiral β-phthalimidesulfenyl amines with an allylic sulforated stereogenic center are useful building blocks, as they represent a starting point for subsequent functional manipulations.     

Tuning the Dimensions of ZnO Nanorod Arrays for Application in Hybrid Photovoltaics

ZnO nanorod arrays are a very eligible option as electron acceptor material in hybrid solar cells, owing to their favorable electrical properties and abundance of available, easy, and low‐cost synthesis methods. To become truly effective in this field, a major prerequisite is the ability to tune the nanorod dimensions towards optimal compatibility with electron‐donating absorber materials. In this work, a water‐based seeding and growth procedure is used to synthesize ZnO nanorods. The nanorod diameter is tuned either by modifying the zinc concentration of the seeding solution or by changing the concentration of the hydrothermal growth solution. The consequences of this morphological tailoring in the performance of hybrid solar cells are investigated, which leads to a new record efficiency of 0.82 % for hydrothermally grown ZnO nanorods of size 300 nm in combination with poly(3‐hexylthiophene‐2,5‐diyl) (P3HT). This improvement is attributed to a combined effect of nanorod diameter and orientation, and possibly to a better alignment of the P3HT backbone resulting in improved charge transport.     

Study of interface properties in CuPc based hybrid inorganic-organic solar cells

Metal-substituted phthalocyanine thin films such as copper-phthalocyanine (CuPc) are often used as photo-active and hole transporting layers (HTLs) in fully organic photovoltaic devices. In this work, CuPc is vacuum sublimated on an electron acceptor layer of mesoporous titania (TiO(2)) for the formation of hybrid TiO(2):CuPc solar cell devices. The performance of these hybrid solar cell devices was demonstrated without and with dye sensitization at the TiO(2):CuPc interface. The charge separation and photocurrent contribution at the interfaces in these multilayer hybrid devices was studied by using a variety of optoelectrical and photophysical characterization techniques. It is important to understand the fundamental interface properties of these multilayer hybrid solar cell devices for optimized performance.     

Numerical Solution of non-Homogeneous Semi-Markov Processes in Transient Case*

In this article a numerical solution for the evolution equation of a continuous time non-homogeneous semi-Markov process (NHSMP) is obtained using a quadrature method. The paper, after a short introduction to continuous time NHSMP, presents the numerical solution of the process evolution equation with a general quadrature method. Furthermore, the paper gives results that justify this approach, proving that the numerical solution tends to the evolution equation of the continuous time NHSMP. Moreover, the formulae related to some specific quadrature methods are given and a method for obtaining the discrete time NHSMP by applying a very particular quadrature formula for the discretization is shown. In this way the relation between the continuous and discrete time NHSMP is proved. Then, the problem of obtaining the continuous time NHSMP from the discrete one is considered. This problem is solved showing that the discrete process converges in law to the continuous one if the discretized time interval tends to zero. In addition, the discrete time NHSMP in matrix form is presented, and the fact that the solution to this process always exists is proved. Finally, an algorithm for solving the discrete time NHSMP is given. To illustrate the use of this algorithm for a discrete NHSMP, an example in the area of finance is presented.     

Superconductivity and magnetic properties of YBa2Cu3O7-δ annealing and quenching effects

Summary Crystal structure, superconductivity and magnetic properties as functions of annealing and quenching temperatures for high-T _c YBa₂Cu₃O_7-δ superconductor were investigated. It was found that the ?orthorhombic character? of the superconducting phase is affected by the quenching process through a mechanism in which both density and order of oxygen vacancies in the Cu (X, Y)-plane of the unit cell may be involved. The quenching-induced effects manifest themselves onT _c and on the behaviour of normal-state resistivity and magnetic susceptibility, whose values are analysed and discussed.

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Riassunto Sono state studiate le proprietà strutturali, superconduttive e magnetiche del composto YBa₂Cu₃O_7-δ in funzione delle condizioni di annealing e tempra. Si è trovato che il ?carattere ortorombico? della fase superconduttrice è modificato dai processi di tempra attraverso un meccanismo nel quale possono essere coinvolti sia la concentrazione delle vacanze di ossigeno che la loro distribuzione nel pianoX-Y della cella unitaria. Gli effetti indotti dalla tempra si riflettono sul valore diT _c e sull'andamento della resistività e della suscettività magnetica nello stato normale, i cui valori sono analizzati e discussi.

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Резюме Исследуются кристаллическая структура, сверхпроводящие и магнитные свойства, как функция температур отжига и закалки для высокостемпературногоT _c сверхпроводника YBa₂Cu₃O_7-δ. Получено, что ?орторомбический характер? сверхповодящей фазы изменяется в процессе закалки через механизм, в котором учитываются и плотность и порядок кислородных вакансиь в CuX-Y плоскости элементарной ячейки. Эффекты закалки в явном виде влияют наT _c и на поведение сопротивления нормальной фазы и магнитной восприимчивости, значеня которых анализируются и обсуждаются.

     

Nanostructured organic pn junctions towards 3D photovoltaics

The working principle of so-called organic bulk heterojunction solar cells prepared with blends of poly(2-methoxy-5-(3,7-dimethyl-octyloxy))-p-phenylene vinylene (MDMO-PPV), acting as an electron donor, and (6,6)-phenyl-C₆₁-butyric-acid methyl ester (PCBM) (a soluble C60 derivative), acting as electron acceptor, is based on the presence of three-dimensional nanostructured pn junctions and percolation paths for charge transport. At high PCBM contents, spontaneous phase separation occurs giving rise to PCBM-rich spherical/ellipsoidal regions (electron transport) embedded in a MDMO-PPV-rich matrix (hole transport). With transmission electron microscopy and scanning probe microscopy techniques it has been demonstrated that the size of the PCBM-rich region depends strongly on the preparation conditions such as solvents and drying conditions. The morphology of the active films in high-performance bulk heterojunction solar cells is characterized by a significantly higher number and a smaller size (nanoscale) of the PCBM-rich regions than for the low-performance cells. This morphology yields both an increase of the useful photoactive volume and an increase of the percolation paths for charge transport. Towards mature and high-performance organic-based three-dimensional photovoltaics, it is clear that besides mastering the electro-optical properties of the constituting materials it also of key importance to control the nanomorphology of the solid-state blends in order to obtain efficient interpenetrating pn networks. PACS 68.37.Lp; 72.80.Le; 73.50.Pz     

Downward migration credit risk problem: a non-homogeneous backward semi-Markov reliability approach

International organizations evaluate credit risk and rank firms according to risk by assigning them a ‘rating’. The time evolution of a rating can be studied by means of Markov models. Some papers have outlined the problem pertaining to the unsuitable fitting of Markov processes in a credit risk environment. This paper presents a model that overcomes the problems given by the Markov rating models. It includes non-homogeneity, the downward problem and the randomness of time in the transitions of states, thus making it possible to consider the duration inside a state in a complete way. In this paper, both, the transient and asymptotic analyses are presented. The asymptotic analysis is performed by using a mono-unireducible topological structure. Moreover, a real data application is conducted using the historical database of Standard & Poor’s as the source.     

Statistical mechanics of holonomic systems as a Brownian motion on smooth manifolds

The statistical mechanics of arbitrary holonomic scleronomous systems subjected to arbitrary external forces is described by specializing the Lagrange and Hamilton equations of motion to those of the Brownian motion on a manifold. In this context, the Klein‐Kramers and Smoluchowski equations are derived in covariant form, and it is demonstrated that these equations have equilibrium solutions corresponding to the Gibbs distribution, in agreement with standard thermodynamics. At last, the Langevin dynamics corresponding to the Smoluchowski limit is found to exactly correspond to the Brownian motion on a smooth manifold. These results find significant applications in the study of several statistical properties of constrained molecular assemblies (e.g. polymers) of interest in chemistry, physics and biology.