5,856 research outputs found
Efficiency of Energy Conversion in Thermoelectric Nanojunctions
Using first-principles approaches, this study investigated the efficiency of
energy conversion in nanojunctions, described by the thermoelectric figure of
merit . We obtained the qualitative and quantitative descriptions for the
dependence of on temperatures and lengths. A characteristic temperature:
was observed. When , . When , tends to a saturation value. The dependence of
on the wire length for the metallic atomic chains is opposite to that for
the insulating molecules: for aluminum atomic (conducting) wires, the
saturation value of increases as the length increases; while for
alkanethiol (insulating) chains, the saturation value of decreases as the
length increases. can also be enhanced by choosing low-elasticity bridging
materials or creating poor thermal contacts in nanojunctions. The results of
this study may be of interest to research attempting to increase the efficiency
of energy conversion in nano thermoelectric devices.Comment: 2 figure
Correlation of interfacial bonding mechanism and equilibrium conductance of molecular junctions
We report theoretical investigations on the role of interfacial bonding
mechanism and its resulting structures to quantum transport in molecular wires.
Two bonding mechanisms for the Au-S bond in an
Au(111)/1,4-benzenedithiol(BDT)/Au(111) junction were identified by ab initio
calculation, confirmed by a recent experiment, which, we showed, critically
control charge conduction. It was found, for Au/ BDT/Au junctions, the hydrogen
atom, bound by a dative bond to the Sulfur, is energetically non-dissociative
after the interface formation. The calculated conductance and junction
breakdown forces of H-non-dissociative Au/BDT/Au devices are consistent with
the experimental values, while the H-dissociated devices, with the interface
governed by typical covalent bonding, give conductance more than an order of
magnitude larger. By examining the scattering states that traverse the
junctions, we have revealed that mechanical and electric properties of a
junction have strong correlation with the bonding configuration. This work
clearly demonstrates that the interfacial details, rather than previously
believed many-body effects, is of vital importance for correctly predicting
equilibrium conductance of molecular junctions; and manifests that the
interfacial contact must be carefully understood for investigating quantum
transport properties of molecular nanoelectronics.Comment: 18 pages, 6 figures, 2 tables, to be appeared in Frontiers of Physics
9(6), 780 (2014
Density functional calculations of nanoscale conductance
Density functional calculations for the electronic conductance of single
molecules are now common. We examine the methodology from a rigorous point of
view, discussing where it can be expected to work, and where it should fail.
When molecules are weakly coupled to leads, local and gradient-corrected
approximations fail, as the Kohn-Sham levels are misaligned. In the weak bias
regime, XC corrections to the current are missed by the standard methodology.
For finite bias, a new methodology for performing calculations can be
rigorously derived using an extension of time-dependent current density
functional theory from the Schroedinger equation to a Master equation.Comment: topical review, 28 pages, updated version with some revision
Effect of Thermoelectric Cooling in Nanoscale Junctions
We propose a thermoelectric cooling device based on an atomic-sized junction.
Using first-principles approaches, we investigate the working conditions and
the coefficient of performance (COP) of an atomic-scale electronic refrigerator
where the effects of phonon's thermal current and local heating are included.
It is observed that the functioning of the thermoelectric nano-refrigerator is
restricted to a narrow range of driving voltages. Compared with the bulk
thermoelectric system with the overwhelmingly irreversible Joule heating, the
4-Al atomic refrigerator has a higher efficiency than a bulk thermoelectric
refrigerator with the same due to suppressed local heating via the
quasi-ballistic electron transport and small driving voltages. Quantum nature
due to the size minimization offered by atomic-level control of properties
facilitates electron cooling beyond the expectation of the conventional
thermoelectric device theory.Comment: 8 figure
A fluidic device for the controlled formation and real-time monitoring of soft membranes self-assembled at liquid interfaces
The work was supported by the European Research Council Starting Grant (STROFUNSCAFF) and the
Marie Curie Career Integration Grant (BIOMORPH). L.B. acknowledges fnancial support from the European
Community through grant no. 618335 ‘FlowMat: Flow and Capillarity in Materials Science’ and ERC Starting
Grant FLEXNANOFLOW no. 715475. Te authors thank Karla E. Inostroza-Brito for the constructive support
in this work
Physicochemical Characterization of Passive Films and Corrosion Layers by Differential Admittance and Photocurrent Spectroscopy
Two different electrochemical techniques, differential admittance and photocurrent spectroscopy, for the characterization of electronic and solid state properties of passive films and corrosion layers are described and critically evaluated. In order to get information on the electronic properties of passive film and corrosion layers as well as the necessary information to locate the characteristic energy levels of the passive film/electrolyte junction like: flat band potential (Ufb), conduction band edge (EC) or valence band edge (EV), a wide use of Mott-Schottky plots is usually reported in corrosion science and passivity studies. It has been shown, in several papers, that the use of simple M-S theory to get information on the electronic properties and energy levels location at the film/electrolyte interface can be seriously misleading and/or conflicting with the physical basis underlying the M-S theory. A critical appraisal of this approach to the study of very thin and thick anodic passive film grown on base-metals (Cr, Ni, Fe, SS etc..) or on valve metals (Ta, Nb, W etc..) is reported in this work, together with possible alternative approach to overcome some of the mentioned inconsistencies. At this aim the theory of amorphous semiconductor Schottky barrier, introduced several years ago in the study of passive film/electrolyte junction, is reviewed by taking into account some of the more recent results obtained by the present authors. Future developments of the theory appears necessary to get more exact quantitative information on the electronic properties of passive films, specially in the case of very thin film like those formed on base metals and their alloys.
The second technique described in this chapter, devoted to the physico-chemical characterization of passive film and corrosion layers, is a more recent technique based on the analysis of the photo-electrochemical answer of passive film/electrolyte junction under illumination with photons having suitable energy. Such a technique usually referred to as Photocurrent Spectroscopy (PCS) has been developed on the basis of the large research effort carried out by several groups in the 1970’s and aimed to investigate the possible conversion of solar energy by means of electrochemical cells. In this work the fundamentals of semiconductor/electrolyte junctions under illumination will be highlighted both for crystalline and amorphous materials. The role of amorphous nature and film thickness on the photo-electrochemical answer of passive film/solution interface is reviewed as well the use of PCS for quantitative analysis of the film composition based on a semi-empirical correlation between optical band gap and difference of electronegativity of film constituents previously suggested by the present authors. In this frame the results of PCS studies on valve metal oxides and valve metal mixed oxides will be discussed in order to show the validity of the proposed method. The results of PCS studies aimed to get information on passive film composition and carried out by different authors on base metals (Fe, Cr, Ni) and their alloys, including stainless steel, will be also compared with compositional analysis carried out by well-established surface analysis techniques
Hamilton-Jacobi Tunneling Method for Dynamical Horizons in Different Coordinate Gauges
Previous work on dynamical black hole instability is further elucidated
within the Hamilton-Jacobi method for horizon tunneling and the reconstruction
of the classical action by means of the null-expansion method. Everything is
based on two natural requirements, namely that the tunneling rate is an
observable and therefore it must be based on invariantly defined quantities,
and that coordinate systems which do not cover the horizon should not be
admitted. These simple observations can help to clarify some ambiguities, like
the doubling of the temperature occurring in the static case when using
singular coordinates, and the role, if any, of the temporal contribution of the
action to the emission rate. The formalism is also applied to FRW cosmological
models, where it is observed that it predicts the positivity of the temperature
naturally, without further assumptions on the sign of the energy.Comment: Standard Latex document, typos corrected, refined discussion of
tunneling picture, subsection 5.1 remove
The radial arrangement of the human chromosome 7 in the lymphocyte cell nucleus is associated with chromosomal band gene density
This is the author's accepted manuscript. The final published article is available from the link below. Copyright @ Springer-Verlag 2008.In the nuclei of human lymphocytes, chromosome territories are distributed according to the average gene density of each chromosome. However, chromosomes are very heterogeneous in size and base composition, and can contain both very gene-dense and very gene-poor regions. Thus, a precise analysis of chromosome organisation in the nuclei should consider also the distribution of DNA belonging to the chromosomal bands in each chromosome. To improve our understanding of the chromatin organisation, we localised chromosome 7 DNA regions, endowed with different gene densities, in the nuclei of human lymphocytes. Our results showed that this chromosome in cell nuclei is arranged radially with the gene-dense/GC-richest regions exposed towards the nuclear interior and the gene-poorest/GC-poorest ones located at the nuclear periphery. Moreover, we found that chromatin fibres from the 7p22.3 and the 7q22.1 bands are not confined to the territory of the bulk of this chromosome, protruding towards the inner part of the nucleus. Overall, our work demonstrates the radial arrangement of the territory of chromosome 7 in the lymphocyte nucleus and confirms that human genes occupy specific radial positions, presumably to enhance intra- and inter-chromosomal interaction among loci displaying a similar expression pattern, and/or similar replication timing
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