Alexander V. Shokurov

Peculiarities of the reflection-absorption and transmission spectra of ultrathin films under normal incidence of light

This paper analyzes the prospects of studying optically anisotropic monolayers on a liquid substrate using linearly polarized light. For quantification of the electronic transmission spectra of the studied monolayers on the transparent subphase and Langmuir-Blodgett films transferred onto transparent plates as well, we obtained formulas for optical densities of transmission spectra in a triple system: surrounding medium-ultrathin film-substrate (liquid or solid). The expressions derived in this work are compared with previously obtained formulas for optical densities of spectra measured in the reflection mode for an analogous system. It is shown that, with all other conditions being equal, employment of this measurement technique allows one to obtain spectra the peak intensities of which (measured in absorbance units) are almost an order of magnitude higher than the corresponding values for transmission spectra. The regularities revealed in this work are confirmed by experimental measurements carried out for monolayers of certain conjugated organic compounds of various structures: amphiphilic chromoionofores, photosensitive oligomeric organic conductors, and electrochemically multistable rigid macrocyclic compounds. A good quantitative agreement between theory and experiment was obtained.

Orientation-induced redox transformations in Langmuir monolayers of double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] and multistability of its Langmuir-Blodgett films

The spectral, electrochemical, and optical properties of Langmuir-Blodgett films (LBFs) and cast films from a solution of new double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] (Ce(R4Pc)2) are studied. Based on analysis of compression isotherms and quantum-chemical calculations, schemes of the organization of Ce(R4Pc)2 molecules at different states of its monolayers are proposed. Correlation dependences are determined in order to relate the optical and electrochemical characteristics of monolayers and LBFs of sandwich-type lanthanide phthalocyaninates to the ionic radii of their metal centers. The valent state of Ce ions in a monolayer-forming complex is determined, and a sequence of redox transformations occurring in LBF uppon appliance of a potential is proposed, one of the transformations being associated with the Ce3+/Ce4+ redox transition. Orientation-induced intramolecular electron transfer is revealed in the planar supramolecular system. It is shown that, during the formation of a monolayer from a Ce(R4Pc)2 solution, a tetravalent metal center passes to a trivalent state. Monolayer compression to a high surface pressure reverts the complex to the electronic state typical of the solution. The reversible transformations observed upon the monolayer compression result from intramolecular electron transfer from the 4f-orbital of Ce to the phthalocyanine ring and backwards. The high operation rate and the reversibility of switching between the stable states, which are determined by means of the surface plasmon resonance technique, upon a stepwise change in the electrode potential within the range of 200–850 mV may underlie the development of optoelectronic systems. With a large number of molecules in a stacking aggregate, changes in the distance between the decks of the complex that occur with changes in the oxidation level of the metal center can substantially modulate the sizes of molecular ensembles. A supramolecular device capable of performing mechanical work can be developed based on this effect.

Decaying of Artemia salina in clay colloids: 14-month experimental formation of subfossils

Abstract. The mechanism that guides the formation of exceptionally preserved fossils with soft tissues variously displayed is a paramount challenge to paleontology. The key question for exceptional preservation is the nature of the slowdown of decay and acceleration of soft tissue mineralization. Here we report the experimental formation of subfossils of the brine shrimp Artemia salina (Crustacea, Branchiopoda), which were produced during 14 months of aging in a kaolinite clay sediment. EDS/SEM elemental analyses showed that the subfossils were preserved as thin clay-organic replicas that displayed fine anatomical details. Decomposition in the clay-colloidal solution established highly heterogeneous acidic conditions, with the lowest pH typically found in the vicinity of the buried organisms, and visually manifested in patchy coloration of the sediment. Elevated acidity is likely what ultimately slowed the decay. An acidic environment increases the rate of clay destruction and, consequently, the diffusion rate decline. As a result, the acidic products quickly accumulate around a buried body; this in turn inhibits bacterial proliferation, accelerates the acidic hydrolysis of clay and, accordingly, the release of tanning and mineralizing agents. The subfossils remained stable under experimental high pressure and temperature. These model subfossils exhibit features that are typical of some Lagerstätten fossils preserved in fine-grained sediments.

Electrochemically controlled multistability of ultrathin films of double-decker cerium phthalocyaninates

The spectral and electrochemical properties of Langmuir-Blodgett (LB) films of homoleptic double-decker cerium bis-[tetra-15-crown-5]-phthalocyaninate Ce[R4Pc]2 and its heteroleptic analog [Pc]Ce[R4Pc] were studied. It was shown that during formation of Langmuir monolayers upon contact of both complexes solutions with water subphase the metal center with valent state IV in the solution transforms into one with the valent state III in the monolayer. Upon cyclic compression-expansion of these monolayers, orientation-induced reversible intramolecular transfers of electron from 4f orbital of cerium ion to the phthalocyanine macrocycle (compression) and in reverse direction (expansion) occur in a planar supramolecular system. Scheme of reversible electrochemical transformations occurring in ultrathin films of double-decker cerium crown-phthalocyaninates, one of which is associated with the Ce+3/Ce+4 redox-transition of metal center of the complex, was proposed and substantiated using the results of spectro-electrochemical investigations. It was shown that one of these transformations is associated with the Ce+3/Ce+4 redox transition of metal center of the complex and upon change of metal center oxidation state its size changes, and, consequently, the distance between the decks of the complex also changes. This change in the distance can lead to a substantial (13–15%) modulation of the linear size of molecular assemblies with a large number of molecules in stack. On the basis of this effect the supramolecular device that can perform mechanical work can be developed. Using surface plasmon resonance technique we have demonstrated that a step change in electrode potential in region of 200–1000 mV induces a corresponding optical response reflected in a step change of the resonance angle. High operation speed and reversibility of switching between stable states can serve as a basis for development of optoelectronic switching systems.

Physicochemical properties of solutions and ultrathin films of triple-decker gadolinium tetra-15-crown-5-phthalocyaninate

The ability of the triple-decker gadolinium complex with tetra-15-crown-5-phthalocyanine Gd2(R4Pc)3, (R = 15-crown-5) (1) to form monolayers and Langmuir-Blodgett films (LBFs) was studied for the first time. The charge characteristics of molecules of a triple-decker phthalocyaninate in monolayer, as well as their orientation and adhesion to the water subphase, were controlled by changing the surface pressure, pH, and subphase composition (aqueous solutions containing triethylenetetramine (TETA) and metal cations). It was shown that the presence of Na+ and protonated TETA aminogroups in resulted in an increasing limiting monolayer area and significant decreasing of the monolayer liquid state region. It is proven that the observed effects are caused by the conformational and charge transitions of peripheral crown ethers induced by their interaction with cations. A comparison of the differential reflectance spectra of the complex monolayer on the deionized water surface with UV-Vis absorption spectra of three-layer Gd2(R4Pc)3 LBF and complex solution in chloroform shows that partial complex oxidation and intensive stacking formation occurs already at the stage of monolayer formation on the subphase surface. Electrochemical studies of three-layer LBFs performed at the indium-tin-oxide electrodes (ITO-electrodes) using cyclic voltammetry (CV) showed three reversible redox waves in the potential range −200 to +1100 mV (vs. Ag+/AgCl). All registered peaks remain the same position and intensity upon multiple cycling. plasmon resonance (SPR) measurements allow to register three stable redox states of studied LBF upon applying the external potential. Such behavior shows the possibility to use multistep redox transformations of studied complex LBF for developing of stable and reproducible switchable optoelectronic systems.

On the existence of provably secure cloud computing systems

We study a formal model of cloud computing systems with auxiliary cryptoservers. Assuming an existence of a secure threshold somewhat homomorphic public key cryptosystem we show how to build a cloud computing system secure in this model.

Software for improving the energy efficiency of a computer cluster

Methods for reducing the energy consumption of a uniform computer cluster due to flexible control strategies of the node states (waking them up or shutting down) and of the execution order of the awaiting tasks are considered. A software system developed in the Institute for System Programming of the Russian Academy of Sciences (ISP RAN) for the dynamic control of the nodes in order to reduce the energy consumption is described. Several strategies for controlling the stats of the nodes are proposed and investigated. Simulation showed that when the average density of tasks1 is 0.5, the energy saving is about 10%. When the density of the flow of tasks decreases, the effect of using the proposed system drastically increases: when the average density is 0.3, the saving is 30%; when the average density is 0.2, the saving is 50%; and when the average density is 0.1, the saving is 70%.

Conjugated Compounds in Supramolecular Informational Systems: A Review

1. Features of physico chemical properties of conju gated compounds from the standpoint of application in informational systems 2. Inter and intramolecular interactions that deter mine the chemical structure of materials based on con jugated compounds (donor–acceptor interactions, hydrogen bonds, redox isomerism) 3. Absorbance and transformation of light by solu tions of conjugated compounds upon supramolecular organization 3.1. Spectral Manifestations of Coordination Interac tions 3.2. Spectral Manifestations of Aggregation 3.3. The Influence of Aggregation on Energy Transfer in Solutions of Conjugated Compounds 4. Langmuir monolayers and langmuir—blodgett films (LBF)—basic ideas and definitions, principles of self organization of conjugated molecules 5. Features of physico chemical properties of monolayers of conjugated compounds. functional materials on their basis 5.1. Absorbance and Emission of Light in 2D Systems, the Role of Aggregation 5.2. Light Collecting Systems, Irradiation Energy Transfer 5.3. Electric Conductivity and Magnetic Properties of LBF 5.4. LBF As Semiconductor Elements for Molecular Electronics

Secure cloud computing based on threshold homomorphic encryption

We analyze the possibility of implementation of cloud computing over private data that is based on previously proposed threshold fully homomorphic cryptosystems. The main result is a protocol for securer cloud computing over private data in a model with supplementary cryptoservers. As a result, one gets a system that does not require a public key and replaces a sophisticated and costly bootstrapping by a more efficient reencryption protocol run by the cryptoservers.

On the deductive security of queries to confidential databases in cloud computing systems

We study certain simple models of confidential databases in cloud computing systems. In the framework of these models we introduce a concept of deductive security for queries to such databases, find necessary and sufficient conditions of deductive security, and describe some classes of queries which satisfy these requirements.