Clusters Of Different Sizes Research Articles

A spectroscopic study of the cis/trans-isomers of penta-2,4-dienoic acid attached to gold nanoclusters.

In this theoretical work, we present a spectroscopic analysis of the cis/trans-isomers of a molecular switch, penta-2,4-dienoic acid, attached to gold clusters of different size (1, 2 and 20 gold atoms). We have simulated 4 different spectroscopic techniques: Infrared spectroscopy, normal Raman scattering, absorption spectra and resonance Raman scattering. We discuss how the position and the conformation of the molecule determine the electronic structure and hence, the spectra. The calculations have been performed using density functional theory for the properties of the ground state and time-dependent density functional theory for the excited-state properties. Special emphasis is put on the resonance Raman spectra for the study of the isomers. In the present case, resonance Raman scattering is best suited to discriminate between the isomers on the gold clusters.

Permeabilization of yeast Saccharomyces cerevisiae cell walls using nanosecond high power electrical pulses

The electrical field-induced changes of the yeast Saccharomyces cerevisiae cells permeabilization to tetraphenylphosphonium (TPP+) ions were studied using square-shaped, nanosecond duration high power electrical pulses. It was obtained that pulses having durations ranging from 10 ns to 60 ns, and generating electric field strengths up to 190 kV/cm significantly (up to 65 times) increase the absorption rate of TPP+ ions without any detectible influence on the yeast cell viability. The modelling of the TPP+ absorption process using a second order rate equation demonstrates that depending on the duration of the pulses, yeast cell clusters of different sizes are homogeniously permeabilized. It was concluded, that nanosecond pulse-induced permeabilization can be applied to increase the operational speed of whole cell biosensors.

An Exploration of Transport Within Microdroplet and Nanodroplet Clusters During Dropwise Condensation of Water on Nanostructured Surfaces

Experimental studies of dropwise condensation have generally indicated that higher heat transfer coefficients correspond to smaller mean sizes for droplets growing through condensation on the surface. Recent investigations of dropwise condensation on nanostructured surfaces suggest that optimizing the design of such surfaces can push mean droplet sizes down to smaller values and significantly enhance heat transfer. This paper summarizes a theoretical exploration of the limits of heat transfer enhancement that can be achieved by pushing mean droplet size to progressively smaller sizes. A model analysis is developed that predicts transport near clusters of water droplets undergoing dropwise condensation. The model accounts for interfacial tension effects on thermodynamic equilibrium and noncontinuum transport effects, which become increasingly important as droplet size becomes progressively smaller. In this investigation, the variation of condensing heat transfer coefficient for droplet clusters of different sizes was explored for droplet diameters ranging from hundreds of microns to tens of nanometers. The model predictions indicate that the larger droplet transport trend of increasing heat transfer coefficient with decreasing mean droplet size breaks down as droplet size becomes smaller. The model further predicts that as drop size becomes smaller, a peak heat transfer coefficient is reached, beyond which the coefficient drops as the size continues to diminish. This maximum heat transfer coefficient results from the increasing importance of surface tension effects and noncontinuum effects as droplet size becomes smaller. The impact of these predictions on the interpretation of dropwise condensation heat transfer data, and the implications for design of nanostructured surfaces to enhance dropwise condensation are discussed in detail.

Understanding resonant tunnel transport in non-identical and non-aligned clusters as applied to disordered carbon systems

We study the conductance spectra and the corresponding current-voltage characteristics of a set of three impurity clusters of different sizes arranged in the form of a scalene triangle and compare with the transport of their horizontal and vertical configurations. The tuning capability of resonant tunnelling features in a quantum dot device made of these non-aligned impurity clusters is demonstrated by re-distributing their diameters and inter-cluster distances in a systematic manner. By manipulating the inter-cluster coupling for a scalene triangular configuration, the transition of current-voltage curves from a step-like feature to a negative differential resistance can be produced. This process also yields conductance features for triangular configurations, which can be compared to the quantum dot structures perfectly aligned to the direction of the propagating wavevector. The strength of inter-cluster coupling or order parameter for these configurations is analysed from the relative variation of the width and the energy difference of the sharp and broad peaks observed in the density of states spectra. Based on the relative change of the inter-cluster coupling with the cluster configurations, a transport model applicable to structurally inhomogeneous systems is proposed in order to explain the experimentally observed variation of the energy band gap with the disorder parameters.

Empirical investigation of SEA-based dependence cluster properties

Dependence clusters are (maximal) groups of source code entities that each depend on the other according to some dependence relation. Such clusters are generally seen as detrimental to many software engineering activities, but their formation and overall structure are not well understood yet. In a set of subject programs from moderate to large sizes, we observed frequent occurrence of dependence clusters using Static Execute After (SEA) dependences (SEA is a conservative yet efficiently computable dependence relation on program procedures). We identified potential linchpins; these are procedures that can primarily be made responsible for keeping the cluster together. Furthermore, we found that as the size of the system increases, it is more likely that multiple procedures are jointly responsible as sets of linchpins. We also give a heuristic method based on structural metrics for locating possible linchpins as their exact identification is unfeasible in practice, and presently there are no better ways than the brute-force method. We defined novel metrics to be able to uncover clusters of different sizes in programs, and also to relate programs in terms of their degree of clusterization. Finally, we present a possible application of SEA-based dependences in change impact analysis, and investigate the effect of dependence clusters on the successfulness of this activity.

Spatial Averaging: Sampling Enhancement for Exploring Configurational Space of Atomic Clusters and Biomolecules.

Spatial averaging Monte Carlo (SA-MC) is an efficient algorithm dedicated to the study of rare-event problems. At the heart of this method is the realization that from the equilibrium density a related, modified probability density can be constructed through a suitable transformation. This new density is more highly connected than the original density, which increases the probability for transitions between neighboring states, which in turn speeds up the sampling. The first successful investigations included the diffusion of small molecules in condensed phase environments and characterization of the metastable states of the migration of the CO ligand in myoglobin. In the present work, a general and robust implementation including rotational and torsional moves in the CHARMM molecular modeling software is introduced. Also, a procedure to estimate unbiased properties is proposed in order to compute thermodynamic observables. These procedures are suitable to study a range of topical systems including Lennard-Jones clusters of different sizes and the blocked alanine dipeptide (Ala)2 in implicit and explicit solvent. In all cases, SA-MC is found to outperform standard Metropolis simulations in sampling configurational space at little extra computational expense. The results for (Ala)2 in explicit solvent are in good agreement with previous umbrella sampling simulations.

Atomic domain magnetic nanoalloys: interplay between molecular structure and temperature dependent magnetic and dielectric properties in manganese doped tin clusters.

We present extensive temperature dependent (16-70 K) magnetic and electric molecular beam deflection measurements on neutral manganese doped tin clusters Mn/SnN (N = 9-18). Cluster geometries are identified by comparison of electric deflection profiles and quantum chemical data obtained from DFT calculations. Most clusters adopt endohedral cage structures and all clusters exhibit non-vanishing magnetic dipole moments. In the high temperature regime all species show exclusively high field seeking magnetic response and the magnetic dipole moments are extracted from the shift of the molecular beam. At low nozzle temperatures, some of the clusters show considerably broadened beam profiles due to non-uniform deflection in the magnetic field. The results reflect the influence of the chemical environment on the magnetic properties of the transition metal in atomic domain magnetic nanoalloys. Different ground state spin multiplicities and coupling of rotational and vibrational degrees of freedom with the spin angular momentum of isolated clusters of different size apparently cause these variations of spin orientation. This is discussed by taking electronic and molecular structure data into account.

A NEW METHOD TO SEARCH FOR HIGH-REDSHIFT CLUSTERS USING PHOTOMETRIC REDSHIFTS

We describe a new method (Poisson Probability Method, PPM) to search for high redshift galaxy clusters and groups by using photometric redshift information and galaxy number counts. The method relies on Poisson statistics and is primarily introduced to search for Mpc-scale environments around a specific beacon. The PPM is tailored to both the properties of the FR I radio galaxies in the Chiaberge et al. (2009) sample, that are selected within the COSMOS survey, and on the specific dataset used. We test the efficiency of our method of searching for cluster candidates against simulations. Two different approaches are adopted. i) We use two z~1 X-ray detected cluster candidates found in the COSMOS survey and we shift them to higher redshift up to z=2. We find that the PPM detects the cluster candidates up to z=1.5, and it correctly estimates both the redshift and size of the two clusters. ii) We simulate spherically symmetric clusters of different size and richness, and we locate them at different redshifts (i.e. z = 1.0,1.5, and 2.0) in the COSMOS field. We find that the PPM detects the simulated clusters within the considered redshift range with a statistical 1-sigma redshift accuracy of ~0.05. The PPM is an efficient alternative method for high-redshift cluster searches that may also be applied to both present and future wide field surveys such as SDSS Stripe 82, LSST, and Euclid. Accurate photometric redshifts and a survey depth similar or better than that of COSMOS (e.g. I<25) are required.

Propene epoxidation with O2 or H2-O2 mixtures over silver catalysts: theoretical insights into the role of the particle size.

The adsorption and dissociation of molecular O2 over silver extended surfaces, nanoparticles and clusters of different size, as well as the reaction between adsorbed O2 and H2 to form less nucleophilic hydroperoxide groups have been investigated by means of periodic DFT calculations. Hydroperoxide formation from O2 and H2 is in all systems kinetically favoured over O2 dissociation, which becomes energetically forbidden on small planar clusters. The nature and reactivity towards propene of all oxygen species present on silver, including adsorbed molecular O2, atomic O, and hydroperoxide groups, have been theoretically explored. Formation of the oxametallacycle intermediate involved in propene epoxidation is energetically accessible on subnanometric three-dimensional silver nanoparticles, but competitive pathways leading to hydrogen abstraction and allyl formation always involve lower activation barriers. Theoretical findings have been experimentally confirmed by Raman spectroscopy of O2 adsorption and catalytic testing of planar and three dimensional silver clusters.

Probing the structure and dynamics of molecular clusters using rotational wave packets.

Rotational wave packets of the weakly bound C(2)H(2)-He complex have been created using impulsive alignment. The coherent rotational dynamics were monitored for 600 ps enabling extraction of a frequency spectrum showing multiple rotational energy levels up to J = 4. spectrum has been combined with ab initio calculations to show that the complex has a highly delocalized structure and is bound only by ca. 7 cm(-1). The experiments demonstrate how highly featured rotational spectra can be obtained from an extremely cold environment where only the lowest rotational energy states are initially populated.

Device design and materials optimization of conformal coating for islets of Langerhans

Encapsulation of islets of Langerhans may represent a way to transplant islets in the absence of immunosuppression. Traditional methods for encapsulation lead to diffusional limitations imposed by the size of the capsules (600-1,000 μm in diameter), which results in core hypoxia and delayed insulin secretion in response to glucose. Moreover, the large volume of encapsulated cells does not allow implantation in sites that might be more favorable to islet cell engraftment. To address these issues, we have developed an encapsulation method that allows conformal coating of islets through microfluidics and minimizes capsule size and graft volume. In this method, capsule thickness, rather than capsule diameter, is constant and tightly defined by the microdevice geometry and the rheological properties of the immiscible fluids used for encapsulation within the microfluidic system. We have optimized the method both computationally and experimentally, and found that conformal coating allows for complete encapsulation of islets with a thin (a few tens of micrometers) continuous layer of hydrogel. Both in vitro and in vivo in syngeneic murine models of islet transplantation, the function of conformally coated islets was not compromised by encapsulation and was comparable to that of unencapsulated islets. We have further demonstrated that the structural support conferred by the coating materials protected islets from the loss of function experienced by uncoated islets during ex vivo culture.

Effect of magnetic field and iron content on NMR proton relaxation of liver, spleen and brain tissues.

Iron accumulation is observed in liver and spleen during hemochromatosis and important neurodegenerative diseases involve iron overload in brain. Storage of iron is ensured by ferritin, which contains a magnetic core. It causes a darkening on T2 -weighted MR images. This work aims at improving the understanding of the NMR relaxation of iron-loaded human tissues, which is necessary to develop protocols of iron content measurements by MRI. Relaxation times measurements on brain, liver and spleen samples were realized at different magnetic fields. Iron content was determined by atomic emission spectroscopy. For all samples, the longitudinal relaxation rate (1/T1 ) of tissue protons decreases with the magnetic field up to 1 T, independently of iron content, while their transverse relaxation rate (1/T2 ) strongly increases with the field, either linearly or quadratically, or a combination thereof. The extent of the inter-echo time dependence of 1/T2 also varies according to the sample. A combination of theoretical models is necessary to describe the relaxation of iron-containing tissues. This can be due to the presence, inside tissues, of ferritin clusters of different sizes and densities. When considering all samples, a correlation (r(2) = 0.6) between 1/T1 and iron concentration is observed at 7.0 T. In contrast the correlation between 1/T2 and iron content is poor, even at high field (r(2) = 0.14 at 7.0 T). Our results show that MRI methods based on T1 or T2 measurements will easily detect an iron overloading at high magnetic field, but will not provide an accurate quantification of tissue iron content at low iron concentrations.

Determination of size of molecular clusters of ethanol by means of NMR diffusometry and hydrodynamic calculations.

The microscopic structure of ethanol in the liquid state is characterized as a dynamic equilibrium of hydrogen-bonded clusters of different sizes and topologies. We have developed a novel method for determination of the average size of the clusters that combines the measurement of diffusion coefficient by means of NMR diffusometry technique and hydrodynamic simulations. The approach includes the use of HydroNMR [J. Garcı̀a de la Torre, M. L. Huertas, and B. Carrasco, J. Magn. Reson. 147, 2000, 138] for small molecules, which is attained here by the calibration procedure using a dilute solution of tetramethylsilane. It is thus possible to correlate the experimentally determined diffusion coefficient of ethanol with calculated diffusion coefficients of the modeled clusters of different sizes. We found that average size of the clusters in 0.16 M solution of ethanol in n-hexane corresponds to the monomer above 300 K and to the pentamer/hexamer below 240 K. The clusters in the case of 0.44 M solution are generally slightly larger, from the average size corresponding to the dimer at 320 K and the hexamer at 210 K.

Multiphoton ionization of large water clusters.

Water clusters are multimers of water molecules held together by hydrogen bonds. In the present work, multiphoton ionization in the UV range coupled with time of flight mass spectrometry has been applied to water clusters with up to 160 molecules in order to obtain information on the electronic states of clusters of different sizes up to dimensions that can approximate the bulk phase. The dependence of ion intensities of water clusters and their metastable fragments produced by laser ionization at 355 nm on laser power density indicates a (3+1)-photon resonance-enhanced multiphoton ionization process. It also explains the large increase of ionization efficiency at 355 nm compared to that at 266 nm. Indeed, it was found, by applying both nanosecond and picosecond laser ionization with the two different UV wavelengths, that no water cluster sequences after n = 9 could be observed at 266 nm, whereas water clusters up to m/z 2000 Th in reflectron mode and m/z 3000 Th in linear mode were detected at 355 nm. The agreement between our findings on clusters of water, especially true in the range with n > 10, and reported data for liquid water supports the hypothesis that clusters above a critical dimension can approximate the liquid phase. It should thus be possible to study clusters just above 10 water molecules, for getting information on the bulk phase structure.

Effective field renormalization group approach to classical antiferromagnets on d-dimensional corner-sharing hyper-tetrahedral lattices

A real-space renormalization group method for classical interacting spins on a d-dimensional lattice of corner-sharing hyper-tetrahedra is presented. These include the classical Heisenberg model on the kagome and pyrochlore lattices as particular cases. The partition function of clusters of different sizes and the corresponding order parameters are calculated to leading order in the symmetry breaking fields created by the spins outside the cluster and the corresponding scaling relations and critical behavior of these lattices are analyzed. To illustrate the simplicity of the method, a reduced subset of the possible ordered states of these lattices for the isotropic nearest-neighbour interaction problem is studied in detail. It is found that the method correctly predicts the absence of the ordered state on the antiferromagnetic sector of the model no matter the dimensionality of the lattice or the number of spin components. Possible ways to consider more complex ordered states and higher-order effects —such as order by disorder— in the present framework are also discussed.

Level-spacing statistics and spectral correlations in diffuse van der Waals clusters

We present a statistical analysis of eigenenergies and discuss several measures of spectral fluctuations and spectral correlations for the van der Waals clusters of different sizes. We show that the clusters become more and more complex with increase in cluster size. We study nearest-neighbour level spacing distribution $P(s)$, the level number variance $\Sigma^2(L)$, and the Dyson-Mehta $\Delta_3-$statistics for various cluster sizes. For large clusters we find that although the Bohigas-Giannoni-Schmit (BGS) conjecture seems to be valid, it does not exhibit true signatures of quantum chaos. However contrasting conjecture of Berry and Tabor is observed with smaller cluster size. For small number of bosons, we observe the existence of large number of quasi-degenerate states in low-lying excitation which exhibits the Shnirelman peak in $P(s)$ distribution. We also find a narrow region of intermediate spectrum which can be described by semi-Poisson statistics whereas the higher levels are regular and exhibit Poisson statistics. These observations are further supported by the analysis of the distribution of the ratio of consecutive level spacings $P(r)$ which is independent of unfolding procedure and thereby provides a tool for more transparent comparison with experimental findings than $P(s)$. Thus our detail numerical study clearly shows that the van der Waals clusters become more correlated with the increase in cluster size.

DFT Computational and Experimental Study of Electroless Nickel Deposition

The kinetic mechanism of the electroless deposition of NiP from hypophosphite solutions was investigated from first principles with the intent of determining a set of elementary reactions that could effectively identify the most relevant steps of this complex process. The outcome of the computational approach is discussed in terms of experimental results obtained by electrochemical methods and surface analysis techniques [1,2]. The computational study was performed without considering explicitly the effect that complexing agents introduced in solution may have on the deposition kinetics. Simulations were performed with Density Functional Theory DFT using the B3LYP exchange and correlation functionals [3], the 6-311+G(d,p) basis set for P,C,H and O atoms [4], and the Stuttgart effective core potential basis sets for Ni [5]). The interaction of Ni++ and H2PO2- with the Ni surface was studied using Ni clusters of different sizes (from 4 to 12 atoms). Solvation in water was modeled using the implicit polarizable continuum model [6]; calculations were performed using the G09 computational suite [7]. Simulations were initially devoted to the identification of the first reaction steps that lead to the adsorption of Ni++ on the Ni surface and to its successive reduction by means of the hypophosphite anion. Though several theories were proposed to describe this key step in the literature, our search was guided by the original insight published several years ago [8,9], according to which the reduction of Ni++ requires the formation of an intermediate species containing the NiOH group. Such species would then take active part in the transfer of the OH group to H2PO2-, thus favoring its oxidation to H2PO3-. In addition, it was imposed that the searched elementary mechanism satisfies the experimental observation [10], obtained with marked deuterium species, that each molecule of hydrogen that is evolved contextually to the deposition of NiP contains two H atoms coming from two H2PO2- anions. The possibility that NiOH+ is formed in solution prior to deposition was investigated determining computationally the pKa of Ni++. Results revealed that the formation of NiOH+ is not favored in the pH ranges in which NiP is usually deposited. A similar result was obtained when considering the possibility that NiOH+, after being formed in solution, is stabilized through the formation of a complex with H2PO2-. A thermodynamically favored set of reactions was instead found when it was investigated the possibility that NiOH is formed at the surface contextually with a Ni-H2PO2 complex. It was in fact found that the following set of reactions has a globally slightly negative (≈ -5 ± 5 kcal/mol) free energy change: R1) Ni ++ + surf --> Niads ++ R2) Niads ++ + 2 H2O + H2PO2 - --> NiOH(H2PO2)ads + H3O+ The two reactions set R1-R2 provides a mechanism for the initial step of reduction of Ni++, thermodynamically viable and consistent with the hypotheses formulated above. The results of this analysis suggest therefore that the initial step of Ni++ reduction during NiP deposition involves a concerted mechanism in which adsorbed Ni++ contextually reacts with H2PO2- and a water molecule. This mechanism can be also supported by electrochemical measurements, indicating the need of OH funtionalization of the catalytic surface in order to have the reaction of hyposphite. References P.L. Cavallotti, L. Magagnin, Influence of added elements on ACD electroless NiP, ECS Transactions, 50 (53) 1-8 (2013).P.L. Cavallotti, L. Magagnin, C. Cavallotti, Influence of added elements on autocatalytic chemical deposition electroless NiP, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.09.083.A. D. Becke, J. Chem. Phys. 98, 1372. (1993), C.T. Lee, W.T. Yang, R. G. Parr, Phys. Rev. B, 37, 785 (1988).K. Raghavachari, J. S. Binkley, R. Seeger, and J. A. Pople, J. Chem. Phys. 72, 650 (1980).P. Fuentealba, H. Preuss, H. Stoll, L.v. Szentpaly, Chem. Phys. Lett., 89, 418 (1982).B. Mennucci, E. Cances, J. Tomasi, J. Phys. Chem. B 101, 10506 (1997).M. J. Frisch et al., Gaussian 09, Revision A.1; Gaussian, Inc.: Wallingford CT, 2009.P. Cavallotti, G. Salvago, Electrochimica Metallorum III, 239 (1968).G. Salvago, P.L. Cavallotti, Plating, 59(7), 665 (1972).A.A. Sutyagina, K.M. Gorbunova, P.M. Glazunov, Dokl. Ak. NAUK SSSR, 147, 1133 (1962).

NUCLEATION AND STABILIZATION OF CARBON-RICH STRUCTURES IN INTERSTELLAR MEDIA

We study conditions under which carbon clusters of different sizes form and stabilize. {We describe an approach to equilibrium by simulating tenuous carbon gas dynamics to long times.} First, we use reactive molecular dynamics simulations to describe the nucleation of long chains, large clusters, and complex cage structures in carbon and hydrogen rich interstellar gas phases. We study how temperature, particle density, presence of hydrogen, and carbon inflow affect the nucleation of molecular moieties with different characteristics, in accordance with astrophysical conditions. We extend the simulations to densities which are orders of magnitude lower than current laboratory densities, to temperatures relevant to circumstellar environments of planetary nebulae, and to longtime (microsecond) formation timescales. We correlate cluster size distributions from dynamical simulations with thermodynamic equilibrium intuitions, where at low temperatures and gas densities, entropy plays a significant role.

Real-space renormalization group method for quantum 1/2 spins on the pyrochlore lattice

A simple phenomenological real-space renormalization group method for quantum Heisenberg spins with nearest and next nearest neighbour interactions on a pyrochlore lattice is presented. Assuming a scaling law for the order parameter of two clusters of different sizes, a set of coupled equations that gives the fixed points of the renormalization group transformation and, thus, the critical temperatures and ordered phases of the system is found. The particular case of spins 1/2 is studied in detail. Furthermore, to simplify the mathematical details, from all the possible phases arising from the renormalization group transformation, only those phases in which the magnetic lattice is commensurate with a subdivision of the crystal lattice into four interlocked face-centred cubic sublattices are considered. These correspond to a quantum spin liquid, ferromagnetic order, or non-collinear order in which the total magnetic moment of a tetrahedral unit is zero. The corresponding phase diagram is constructed and the differences with respect to the classical model are analysed. It is found that this method reproduces fairly well the phase diagram of the pyrochlore lattice under the aforementioned constraints.

Natural Rubber Latex: Study of a Novel Carrier for Casearia sylvestris Swartz Delivery

Natural rubber latex (NRL) from Hevea brasiliensis has showed interesting biomedical properties as improving wound healing, cell adherence, tissue formation, and angiogenesis. It is used for biosynthesis of nanoparticles, sensors and prosthesis and for drug delivery systems (for drugs, plant extracts, and nanoparticles). To enhance its wound healing properties was incorporated Casearia sylvestris Swartz extract, whose pharmacological activity includes anti-inflammatory, analgesic, antiseptic, antiulcer, and antitumor due to its casearins and phenols. Results showed the prolonged release of its compounds (35 days) and the mechanism of release is super case II (n&gt;1) by Korsmeyer-Peppas model. Although SEM shows different sizes of clusters at the surface, the release is homogeneous through the biomembrane. FTIR shows no interaction between the matrix and the extract, with computation of the presence of some casearins.