5,811 research outputs found
Theoretical study of the charge transport through C60-based single-molecule junctions
We present a theoretical study of the conductance and thermopower of
single-molecule junctions based on C60 and C60-terminated molecules. We first
analyze the transport properties of gold-C60-gold junctions and show that these
junctions can be highly conductive (with conductances above 0.1G0, where G0 is
the quantum of conductance). Moreover, we find that the thermopower in these
junctions is negative due to the fact that the LUMO dominates the charge
transport, and its magnitude can reach several tens of micro-V/K, depending on
the contact geometry. On the other hand, we study the suitability of C60 as an
anchoring group in single-molecule junctions. For this purpose, we analyze the
transport through several dumbbell derivatives using C60 as anchors, and we
compare the results with those obtained with thiol and amine groups. Our
results show that the conductance of C60-terminated molecules is rather
sensitive to the binding geometry. Moreover, the conductance of the molecules
is typically reduced by the presence of the C60 anchors, which in turn makes
the junctions more sensitive to the functionalization of the molecular core
with appropriate side groups.Comment: 9 pages, 7 figure
Ab-initio study of the thermopower of biphenyl-based single-molecule junctions
Employing ab-initio electronic structure calculations combined with the
non-equilibrium Green's function technique, we study the dependence of the
thermopower Q on the conformation in biphenyl-based single-molecule junctions.
For the series of experimentally available biphenyl molecules, alkyl side
chains allow us to gradually adjust the torsion angle \phi\ between the two
phenyl rings from 0 to 90{\deg} and to control in this way the degree of
\pi-electron conjugation. Studying different anchoring groups and binding
positions, our theory predicts that the absolute values of the thermopower
decrease slightly towards larger torsion angles, following an a+b*cos^{2}\phi\
dependence. The anchoring group determines the sign of Q and a,b,
simultaneously. Sulfur and amine groups give rise to Q,a,b>0, while for cyano
Q,a,b<0. The different binding positions can lead to substantial variations of
the thermopower mostly due to changes in the alignment of the frontier
molecular orbital levels and the Fermi energy. We explain our ab-initio results
in terms of a \pi-orbital tight-binding model and a minimal two-level model,
which describes the pair of hybridizing frontier orbital states on the two
phenyl rings. The variations of the thermopower with \phi\ seem to be within
experimental resolution.Comment: 8 pages, 4 figues, 3 table
Two-phase quasi-equilibrium in β-type Ti-based bulk metallic glass composites
The microstructural evolution of cast Ti/Zr-based bulk metallic glass composites (BMGCs) containing β-Ti still remains ambiguous. This is why to date the strategies and alloys suitable for producing such BMGCs with precisely controllable volume fractions and crystallite sizes are still rather limited. In this work, a Ti-based BMGC containing β-Ti was developed in the Ti-Zr-Cu-Co-Be system. The glassy matrix of this BMGC possesses an exceptional glass-forming ability and as a consequence, the volume fractions as well as the composition of the β-Ti dendrites remain constant over a wide range of cooling rates. This finding can be explained in terms of a two-phase quasi-equilibrium between the supercooled liquid and β-Ti, which the system attains on cooling. The two-phase quasi-equilibrium allows predicting the crystalline and glassy volume fractions by means of the lever rule and we succeeded in reproducing these values by slight variations in the alloy composition at a fixed cooling rate. The two-phase quasi-equilibrium could be of critical importance for understanding and designing the microstructures of BMGCs containing the β-phase. Its implications on the nucleation and growth of the crystalline phase are elaborated
Understanding the Electronic Transport Through Single Noble Gas Atoms
We present a theoretical study of the conductance of atomic junctions
comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold
electrodes. The aim is to elucidate how the presence of noble gas atoms affects
the electronic transport through metallic atomic-size contacts. Our analysis,
based on density functional theory and including van der Waals interactions,
shows that for the lightest elements (He and Ne) no significant current flows
through the noble gas atoms and their effect is to reduce the conductance of
the junctions by screening the interaction between the gold electrodes. This
explains the observations reported in metallic atomic-size contacts with
adsorbed He atoms. Conversely, the heaviest atoms (Kr and Xe) increase the
conductance due to the additional current path provided by their valence p
states.Comment: 3 figure
Binge Alcohol Exposure Transiently Changes the Endocannabinoid System: A Potential Target to Prevent Alcohol-Induced Neurodegeneration
Excessive alcohol consumption leads to neurodegeneration, which contributes to cognitive decline that is associated with alcohol use disorders (AUDs). The endocannabinoid system has been implicated in the development of AUDs, but little is known about how the neurotoxic effects of alcohol impact the endocannabinoid system. Therefore, the current study investigated the effects of neurotoxic, binge-like alcohol exposure on components of the endocannabinoid system and related N-acylethanolamines (NAEs), and then evaluated the efficacy of fatty acid amide hydrolase (FAAH) inhibition on attenuating alcohol-induced neurodegeneration. Male rats were administered alcohol according to a binge model, which resulted in a transient decrease in [3H]-CP-55,940 binding in the entorhinal cortex and hippocampus following two days, but not four days, of treatment. Furthermore, binge alcohol treatment did not change the tissue content of the three NAEs quantified, including the endocannabinoid and anandamide. In a separate study, the FAAH inhibitor, URB597 was administered to rats during alcohol treatment and neuroprotection was assessed by FluoroJade B (FJB) staining. The administration of URB597 during binge treatment did not significantly reduce FJB+ cells in the entorhinal cortex or hippocampus, however, a follow up “target engagement” study found that NAE augmentation by URB597 was impaired in alcohol intoxicated rats. Thus, potential alcohol induced alterations in URB597 pharmacodynamics may have contributed to the lack of neuroprotection by FAAH inhibition
Growth and mortality estimates of Sardinella brasiliensis in the southeastern Brazilian Bight
Filling some black holes: modeling the connection between urbanization, infrastructure, and global service intensity
This empirical article combines insights from previous research on the level of knowledge-intensive service in metropolitan areas with the aim to develop an understanding of the spatial structure of the global service economy. We use a stepwise regression model with the Globalization and World Cities research network's measure of globalized service provisioning as the dependent variable and a range of variables focusing on population, infrastructure, urban primacy, and national regulation as independent variables. The discussion of the results focuses on model parameters as well as the meaning of outliers and is used to explore some avenues for future research
CO2 and temperature effects on the asphaltene phase envelope as determined by a quartz crystal resonator
Knowledge of the asphaltene phase envelope (APE) is crucial for oil companies, especially when enhanced oil recovery is applied. An innovative quartz crystal resonator (QCR) technique was employed to assess the phase behavior of asphaltene under reservoir conditions. The effect of CO2 injection coupled to temperature changes on the APE of a recombined oil with a very low asphaltene content (0.235% w/w of C7 asphaltene in dead oil) are reported. It has been shown that QCR is an appropriate and highly sensitive nondestructive experimental technique for detecting asphaltene flocculation. Pressure onsets were found to be dependent on the depressurization rate.Knowledge of the asphaltene phase envelope (APE) is crucial for oil companies, especially when enhanced oil recovery is applied. An innovative quartz crystal resonator (QCR) technique was employed to assess the phase behavior of asphaltene under reservoir281167806787sem informaçãosem informação(2012), http://www.iea.org/publications/freepublications/publication/key_world_energy_stats-1.pdf, International Energy Agency. 2011 Key World Energy Statistics. (accessed Aug 27)Speight, J.G., (1999) The Chemistry and Technology of Petroleum, pp. 215-241. , 3 rd ed. CRC Press: New YorkFan, T., Buckley, J.S., Rapid and Accurate SARA Analysis of Medium Gravity Crude Oils (2002) Energy Fuels, 16, pp. 1571-1575Andersen, S.I., Speight, J.G., Petroleum resins separation, character, and role in petroleum (2001) Pet. Sci. 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J.Daridon, J.L., Cassiěde, M., Nasri, D., Pauly, J., Carrier, H., Probing Asphaltene Flocculation by a Quartz Crystal Resonator (2013) Energy Fuels, 27, pp. 4639-4647Abudu, A., Goual, L., Adsorption of Crude Oil on Surfaces Using Quartz Crystal Microbalance with Dissipation (QCM-D) under Flow Conditions (2009) Energy Fuels, 23, pp. 1237-1248Farooq, U., Sjöblom, J., Øye, G., Desorption of Asphaltenes from Silica-Coated Quartz Crystal Surfaces in Low Saline Aqueous Solutions (2011) J. Dispersion Sci. Technol., 32, pp. 1388-1395Tavakkoli, M., Panuganti, S.R., Vargas, F.M., Taghikhani, V., Pishvaie, M.R., Chapman, W.G., Asphaltene Deposition in Different Depositing Environments: Part 1. Model Oil (2014) Energy Fuels, 28, pp. 1617-1628Daridon, J.L., Cassiěde, M., Paillol, J.H., Pauly, J., Viscosity Measurements of Liquids under Pressure by Using the Quartz Crystal Resonators (2011) Rev. Sci. 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Cluster-based density-functional approach to quantum transport through molecular and atomic contacts
We present a cluster-based density-functional approach to model charge
transport through molecular and atomic contacts. The electronic structure of
the contacts is determined in the framework of density functional theory, and
the parameters needed to describe transport are extracted from finite clusters.
A similar procedure, restricted to nearest-neighbor interactions in the
electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)].
Here, we show how to systematically improve the description of the electrodes
by extracting bulk parameters from sufficiently large metal clusters. In this
way we avoid problems arising from the use of nonorthogonal basis functions.
For demonstration we apply our method to electron transport through Au contacts
with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure
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