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Search Results: 1 - 10 of 841820 matches for " M.C.;García-Suárez "
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Some intrinsic and extrinsic factors of acetylated starches: morphological, physicochemical and structural characteristics
Nú?ez-Santiago, M.C.;García-Suárez, F.J.;Gutierrez-Meraz, F.;Sánchez-Rivera, M.M.;Bello-Pérez, L.A.;
Revista mexicana de ingeniería química , 2011,
Abstract: degree substitution (ds), morphological characteristics, x-ray diffraction (xrd), pasting properties, thermal properties and amylopectin chain length distribution were used to study the effect of botanical source (potato starch, ps; and maize starch, ms) and reagent type (acetic anhydride and vinyl acetate) on starch acetyration. the reagent type produced a different effect depending on the botanical source: for ms, the reaction with acetic anhydride produced higher ds than ps, whereas that in the reaction with vinyl acetate resulted in an inverse pattern. in general, modified starches had a maltase cross, except for ps when acetylated with acetic anhydride. native ps had the highest crystallinity level and trie acetylated starches had a decreased amount of crystallinity (between 10% and 12 %) compared to the native starches. acetylation produced a decrease in pasting temperature, and changes in the pasting characteristics were more evident wish higher ds. structural changes in starch components due to the acetylation reaction produced a decrease in the temperature and enthalpy of gelatinization. in general, acetylated starches had minor retrogradation. regardless of reagent used, acetylated ms had higher retrogradation than acetylated ps. higher ds values resulted in greater amounts of short chains in both starches.
Tailoring the Fermi level of the leads in molecular-electronic devices
V. M. García-Suárez,C. J. Lambert
Physics , 2008, DOI: 10.1103/PhysRevB.78.235412
Abstract: The dependence of the transport properties on the specific location of the Fermi level in molecular electronics devices is studied by using electrodes of different materials. The zero-bias transport properties are shown to depend dramatically on the elemental composition of the electrodes, even though the shape of the transmission coefficients is very similar. By using alkaline materials it is possible to move the Fermi level from the HOMO-LUMO gap to the LUMO resonance and change dramatically the length dependence of the conductance of molecular wires, which opens the possibility of using molecules with different lengths and very similar conductances in nanoscale circuits. This method shows how to dramatically increase the conductance of molecular devices and alter qualitatively and quantitatively their electronic and transport properties.
Non-trivial length dependence of the conductance and negative differential resistance in atomic molecular wires
V. M. García-Suárez,C. J. Lambert
Physics , 2008, DOI: 10.1088/0957-4484/19/45/455203
Abstract: We study the electronic and transport properties of two novel molecular wires made of atomic chains of carbon atoms (polyynes) capped with either, benzene-thiols or pyridines. While both molecules are structurally similar, the electrical conductance of benzene-thiol-capped chains attached to gold electrodes is found to be much higher than that of pyridine-capped chains. We predict that the conductance is almost independent of molecular length, which suggests that these molecules could be ideal molecular wires for sub-10 nm circuitry. Both systems exhibit negative differential resistance (NDR) but its origin and characteristics depend on the type of molecule. We find a novel type of NDR mechanism produced by the movement of the LUMO resonance with bias. We also show that by gating the pyridine-capped molecules it is possible to make the NDR disappear and dramatically modify the $I$-$V$ characteristics and the length dependence.
Giant thermopower and figure of merit in single-molecule devices
C. M. Finch,V. M. García-Suárez,C. J. Lambert
Physics , 2008, DOI: 10.1103/PhysRevB.79.033405
Abstract: We present a study of the thermopower $S$ and the dimensionless figure of merit $ZT$ in molecules sandwiched between gold electrodes. We show that for molecules with side groups, the shape of the transmission coefficient can be dramatically modified by Fano resonances near the Fermi energy, which can be tuned to produce huge increases in $S$ and $ZT$. This shows that molecules exhibiting Fano resonances have a high efficiency of thermoelectric cooling which is not present for conventional un-gated molecules with only delocalized states along their backbone.
Electronic Properties of Alkali- and Alkaline-Earth-Intercalated Silicon Nanowires
S. Sirichantaropass,V. M. García-Suárez,C. J. Lambert
Physics , 2006, DOI: 10.1103/PhysRevB.75.075328
Abstract: We present a first-principles study of the electronic properties of silicon clathrate nanowires intercalated with various types of alkali or alkaline-earth atoms. We find that the band structure of the nanowires can be tuned by varying the impurity atom within the nanowire. The electronic character of the resulting systems can vary from metallic to semiconducting with direct band gaps. These properties make the nanowires specially suitable for electrical and optoelectronic applications.
Strongly correlated electron physics in nanotube-encapsulated metallocene chains
V. M. García-Suárez,J. Ferrer,C. J. Lambert
Physics , 2006, DOI: 10.1103/PhysRevB.74.205421
Abstract: The structural, electronic and transport properties of metallocene molecules (MCp$_2$) and isolated or nanotube-encapsulated chains are studied by using a combination of density functional theory and non-equilibrium Green's functions. The analysis first discusses the whole series of isolated MCp$_2$ molecules, where M = V, Cr, Mn, Fe, Co, Ni, Ru, and Os. The series presents a rich range of electronic and magnetic behaviors due to the interplay between the crystal field interaction and Hund's rules, as the occupation of the $d$ shell increases. The article then shows how many of these interesting properties can also be seen when MCp$_2$ molecules are linked together to form periodic chains. It is argued that encapsulation of the chains inside carbon nanotubes, that is exothermic for radii larger than 4.5 \AA, provides the missing mechanical stability and electrical isolation. The structural stability, charge transfer, magnetic and electronic behavior of the ensuing chains, as well as the modification of the electrostatic potential in the nanotube wall produced by the metallocenes as thoroughly discussed. We argue that the full devices can be characterized by two doped, strongly correlated Hubbard models whose mutual hybridization is almost negligible. The charge transferred from the chains produces a strong modification of the electrostatic potential in the nanotube walls, which is amplified in case of semiconducting and endothermic nanotubes. The transport properties of isolated metallocenes between semi-infinite nanotubes are also analyzed and shown to lead to important changes in the transmission coefficients of clean nanotubes for high energies.
Quantum Interference in Single Molecule Electronic Systems
R. E. Sparks,V. M. García-Suárez,D. Zs. Manrique,C. J. Lambert
Physics , 2011, DOI: 10.1103/PhysRevB.83.075437
Abstract: We present a general analytical formula and an ab initio study of quantum interference in multi-branch molecules. Ab initio calculations are used to investigate quantum interference in a benzene-1,2-dithiolate (BDT) molecule sandwiched between gold electrodes and through oligoynes of various lengths. We show that when a point charge is located in the plane of a BDT molecule and its position varied, the electrical conductance exhibits a clear interference effect, whereas when the charge approaches a BDT molecule along a line normal to the plane of the molecule and passing through the centre of the phenyl ring, interference effects are negligible. In the case of olygoynes, quantum interference leads to the appearance of a critical energy $E_c$, at which the electron transmission coefficient $T(E)$ of chains with even or odd numbers of atoms is independent of length. To illustrate the underlying physics, we derive a general analytical formula for electron transport through multi-branch structures and demonstrate the versatility of the formula by comparing it with the above ab-initio simulations. We also employ the analytical formula to investigate the current inside the molecule and demonstrate that large counter currents can occur within a ring-like molecule such as BDT, when the point charge is located in the plane of the molecule. The formula can be used to describe quantum interference and Fano resonances in structures with branches containing arbitrary elastic scattering regions connected to nodal sites.
The Smeagol method for spin- and molecular-electronics
A. R. Rocha,V. M. García-Suárez,S. Bailey,C. Lambert,J. Ferrer,S. Sanvito
Physics , 2005,
Abstract: {\it Ab initio} computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating $I$-$V$ characteristics of atomic junctions, which integrates the non-equilibrium Green's function method with density functional theory. This is currently implemented in the package {\it Smeagol}. The heart of {\it Smeagol} is our novel scheme for constructing the surface Green's functions describing the current/voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian, and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of {\it Smeagol} together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular-electronics.
Conformation dependence of molecular conductance: chemistry versus geometry
C. M. Finch,S. Sirichantaropass,S. W. Bailey,I. M. Grace,V. M. García-Suárez,C. J. Lambert
Physics , 2007, DOI: 10.1088/0953-8984/20/02/022203
Abstract: Recent experiments by Venkatamaran {\em et al.} [Nature (London) {\bf 442}, 904 (2006)] on a series of molecular wires with varying chemical compositions, revealed a linear dependence of the conductance on $\mathrm{cos}^2\theta$, where $\theta$ is the angle of twist between neighboring aromatic rings. To investigate whether or not this dependence has a more general applicability, we present a first principles theoretical study of the transport properties of this family of molecules as a function of the chemical composition, conformation and the contact atom and geometry. If the Fermi energy $E_\mathrm{F}$ lies within the HOMO-LUMO gap, then we reproduce the above experimental results. More generally, however, if $E_\mathrm{F}$ is located within either the LUMO or HOMO states, the presence of resonances destroys the linear dependence of the conductance on $\mathrm{cos}^2\theta$ and gives rise to non-monotonic behaviour associated with the level structure of the different molecules. Our results suggest that the above experiments provide a novel method for extracting spectroscopic information about molecules contacted to electrodes.
Non-universal behavior of the parity effect in monovalent atomic wires
Péter Major,V. M. García-Suárez,S. Sirichantaropass,József Cserti,C. J. Lambert,J. Ferrer,Géza Tichy
Physics , 2005, DOI: 10.1103/PhysRevB.73.045421
Abstract: We propose a mixed analytical-ab-initio method for the accurate calculation of the conductance in monovalent atomic wires. The method relies on the most general formula for ballistic transport through a monovalent wire, whose parameters can be determined from first-principles calculations. Our central result is the demonstration of the highly non-universal behavior of the conductance, which depends on the fine details of the contacts to the leads. We are therefore able to reconcile a large number of the apparently contradictory results that have recently appeared in the literature.
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