548 research outputs found
Quantifying Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) has recently been introduced as a
spectroscopic tool for molecular junctions where it offers the possibility to
probe molecular level energies at relatively low bias voltages. In this work we
perform extensive ab-initio calculations of the non-linear current voltage
relations for a broad class of single-molecule transport junctions in order to
assess the applicability and limitations of TVS. We find, that in order to
fully utilize TVS as a quantitative spectroscopic tool, it is important to
consider asymmetries in the coupling of the molecule to the two electrodes.
When this is taken properly into account, the relation between the transition
voltage and the energy of the molecular orbital closest to the Fermi level
closely follows the trend expected from a simple, analytical model.Comment: 5 pages, 4 figures. To appear in PR
Improving Transition Voltage Spectroscopy of Molecular Junctions
Transition voltage spectroscopy (TVS) is a promising spectroscopic tool for
molecular junctions. The principles in TVS is to find the minimum on a
Fowler-Nordheim plot where is plotted against and relate the
voltage at the minimum, , to the closest molecular level.
Importantly, , is approximately half the voltage required to see a
peak in the curve. Information about the molecular level position can
thus be obtained at relatively low voltages. In this work we show that the
molecular level position can be determined at even lower voltages, by finding the minimum of with .
On the basis of a simple Lorentzian transmission model we analyze theoretical
{\it ab initio} as well as experimental curves and show that the voltage
required to determine the molecular levels can be reduced by as
compared to conventional TVS. As for conventional TVS, the symmetry/asymmetry
of the molecular junction needs to be taken into account in order to gain
quantitative information. We show that the degree of asymmetry may be estimated
from a plot of vs. .Comment: 6 pages, 8 figure
Electrochemical control of quantum interference in anthraquinone-based molecular switches
Using first-principles calculations we analyze the electronic transport
properties of a recently proposed anthraquinone based electrochemical switch.
Robust conductance on/off ratios of several orders of magnitude are observed
due to destructive quantum interference present in the anthraquinone, but
absent in the hydroquinone molecular bridge. A simple explanation of the
interference effect is achieved by transforming the frontier molecular orbitals
into localized molecular orbitals thereby obtaining a minimal tight-binding
model describing the transport in the relevant energy range in terms of hopping
via the localized orbitals. The topology of the tight-binding model, which is
dictated by the symmetries of the molecular orbitals, determines the amount of
quantum interference.Comment: 6 pages, 6 figure
Multiterminal single-molecule--graphene-nanoribbon thermoelectric devices with gate-voltage tunable figure of merit ZT
We study thermoelectric devices where a single 18-annulene molecule is
connected to metallic zigzag graphene nanoribbons (ZGNR) via highly transparent
contacts that allow for injection of evanescent wave functions from ZGNRs into
the molecular ring. Their overlap generates a peak in the electronic
transmission, while ZGNRs additionally suppress hole-like contributions to the
thermopower. Thus optimized thermopower, together with suppression of phonon
transport through ZGNR-molecule-ZGNR structure, yield the thermoelectric figure
of merit ZT ~ 0.5 at room temperature and 0.5 < ZT < 2.5 below liquid nitrogen
temperature. Using the nonequilibrium Green function formalism combined with
density functional theory, recently extended to multiterminal devices, we show
how the transmission resonance can also be manipulated by the voltage applied
to a third ZGNR electrode, acting as the top gate covering molecular ring, to
tune the value of ZT.Comment: 5 pages, 4 figures, PDFLaTe
Electronic transport in Si nanowires: Role of bulk and surface disorder
We calculate the resistance and mean free path in long metallic and
semiconducting silicon nanowires (SiNWs) using two different numerical
approaches: A real space Kubo method and a recursive Green's function method.
We compare the two approaches and find that they are complementary: depending
on the situation a preferable method can be identified. Several numerical
results are presented to illustrate the relative merits of the two methods. Our
calculations of relaxed atomic structures and their conductance properties are
based on density functional theory without introducing adjustable parameters.
Two specific models of disorder are considered: Un-passivated, surface
reconstructed SiNWs are perturbed by random on-site (Anderson) disorder whereas
defects in hydrogen passivated wires are introduced by randomly removed H
atoms. The un-passivated wires are very sensitive to disorder in the surface
whereas bulk disorder has almost no influence. For the passivated wires, the
scattering by the hydrogen vacancies is strongly energy dependent and for
relatively long SiNWs (L>200 nm) the resistance changes from the Ohmic to the
localization regime within a 0.1 eV shift of the Fermi energy. This high
sensitivity might be used for sensor applications.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
Escherichia coli contamination and health aspects of soil and tomatoes (Solanum lycopersicum L.) subsurface drip irrigated with on-site treated domestic wastewater.
Faecal contamination of soil and tomatoes irrigated by sprinkler as well as surface and subsurface drip irrigation with treated domestic wastewater were compared in 2007 and 2008 at experimental sites in Crete and Italy. Wastewater was treated by Membrane Bio Reactor (MBR) technology, gravel filtration or UV-treatment before used for irrigation. Irrigation water, soil and tomato samples were collected during two cropping seasons and enumerated for the faecal indicator bacterium Escherichia coli and helminth eggs. The study found elevated levels of E. coli in irrigation water (mean: Italy 1753 cell forming unit (cfu) per 100 ml and Crete 488 cfu per 100 ml) and low concentrations of E. coli in soil (mean: Italy 95 cfu g(-1) and Crete 33 cfu g(-1)). Only two out of 84 tomato samples in Crete contained E. coli (mean: 2700 cfu g(-1)) while tomatoes from Italy were free of E. coli. No helminth eggs were found in the irrigation water or on the tomatoes from Crete. Two tomato samples out of 36 from Italy were contaminated by helminth eggs (mean: 0.18 eggs g(-1)) and had been irrigated with treated wastewater and tap water, respectively. Pulsed Field Gel Electrophoresis DNA fingerprints of E. coli collected during 2008 showed no identical pattern between water and soil isolates which indicates contribution from other environmental sources with E. coli, e.g. wildlife. A quantitative microbial risk assessment (QMRA) model with Monte Carlo simulations adopted by the World Health Organization (WHO) found the use of tap water and treated wastewater to be associated with risks that exceed permissible limits as proposed by the WHO (1.0 × 10(-3) disease risk per person per year) for the accidental ingestion of irrigated soil by farmers (Crete: 0.67 pppy and Italy: 1.0 pppy). The QMRA found that the consumption of tomatoes in Italy was deemed to be safe while permissible limits were exceeded in Crete (1.0 pppy). Overall the quality of tomatoes was safe for human consumption since the disease risk found on Crete was based on only two contaminated tomato samples. It is a fundamental limitation of the WHO QMRA model that it is not based on actual pathogen numbers, but rather on numbers of E. coli converted to estimated pathogen numbers, since it is widely accepted that there is poor correlation between E. coli and viral and parasite pathogens. Our findings also stress the importance of the external environment, typically wildlife, as sources of faecal contamination
Graphical prediction of quantum interference-induced transmission nodes in functionalized organic molecules
Quantum interference (QI) in molecular transport junctions can lead to
dramatic reductions of the electron transmission at certain energies. In a
recent work [Markussen et al., Nano Lett. 2010, 10, 4260] we showed how the
presence of such transmission nodes near the Fermi energy can be predicted
solely from the structure of a conjugated molecule when the energies of the
atomic p_z orbitals do not vary too much. Here we relax the assumption of equal
on-site energies and generalize the graphical scheme to molecules containing
different atomic species. We use this diagrammatic scheme together with
tight-binding and density functional theory calculations to investigate QI in
linear molecular chains and aromatic molecules with different side groups. For
the molecular chains we find a linear relation between the position of the
transmission nodes and the side group pi orbital energy. In contrast, the
transmission functions of functionalized aromatic molecules generally display a
rather complex nodal structure due to the interplay between molecular topology
and the energy of the side group orbital.Comment: 7 page
Comparison of Different Strategies for Selection/Adaptation of Mixed Microbial Cultures Able to Ferment Crude Glycerol Derived from Second-Generation Biodiesel
Objective of this study was the selection and adaptation of mixed microbial cultures (MMCs), able to ferment crude glycerol generated from animal fat-based biodiesel and produce building-blocks and green chemicals. Various adaptation strategies have been investigated for the enrichment of suitable and stable MMC, trying to overcome inhibition problems and enhance substrate degradation efficiency, as well as generation of soluble fermentation products. Repeated transfers in small batches and fed-batch conditions have been applied, comparing the use of different inoculum, growth media, and Kinetic Control. The adaptation of activated sludge inoculum was performed successfully and continued unhindered for several months. The best results showed a substrate degradation efficiency of almost 100% (about 10 g/L glycerol in 21 h) and different dominant metabolic products were obtained, depending on the selection strategy (mainly 1,3-propanediol, ethanol, or butyrate). On the other hand, anaerobic sludge exhibited inactivation after a few transfers. To circumvent this problem, fed-batch mode was used as an alternative adaptation strategy, which led to effective substrate degradation and high 1,3-propanediol and butyrate production. Changes in microbial composition were monitored by means of Next Generation Sequencing, revealing a dominance of glycerol consuming species, such as Clostridium, Klebsiella, and Escherichia
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