Modelling electron tunnelling in the presence of adsorbed materials

University of Technology, Sydney. Institute for Nanoscale Technology.The transport characteristics of single-molecule Au(lll) junctions are investigated using density functional theory (DFT) together with the non-equilibrium Green’s functions formalism (NEGF). DFT optimisations of the adsorption of various molecules on a Au(lll) surface are used as starting points for the equilibrium junction geometries. Test calculations are performed to find a recommended set of parameters for the final DFT results. The interaction energies of several molecules with the Au(lll) surface obtained within the same level of theory are compared. Amine compounds bind preferentially in an adatom geometry and weakly in the out op site. A Z-matrix optimiser is implemented in the SIESTA code as a useful tool for future surface and molecular junction optimisations. Transport properties are calculated for molecular junctions in their equilibrium geometry. While the conductances are orders of magnitude larger than experimental data, the sizes are in line with expectation. The junction geometries are altered in various ways. Changing the binding site or altering the nature of the sulphur-gold interaction in a phenylenedimethanethiol junction, reduces the conductance by a factor of two. Orders of magnitude reduction of conductance is only observed when increasing the distance between a physisorbed molecule and the surface. Increasing this distance for a chemisorbed molecule, results in a surprising increase in conductance. This is attributed to an interplay between the coupling strength of the molecule with the surface and the location of the molecular energy levels relative to the Fermi level. When the chemical bond is broken, the system is spin-polarised and the conductances for electrons of opposite spin types are different by a factor of 250 - the junction acts as a spin-filter. When stretching a diethynylbenzene junction, the strong gold-carbon bond does not break, but rather extracts a gold atom from the surface. In this case the conductance decreases rapidly with stretching. A WKB tunnel barrier model is used as an alternate much faster method for calculating I(V) characteristics. With the surface work functions acting as barrier heights, the relative junction conductances are in good agreement with the DFT results. However, the direction of asymmetry in the I(V) characteristics predicted by the two levels of theory are opposite. More sophisticated barrier shapes may be needed to correctly predict the asymmetries. The tunnelling model is used in conjunction with the DFT results to quantify the effect a gap between an STM tip and monolayer may have on STS measurements

Effect of dipole moment on current-voltage characteristics of single molecules

We perform empirical calculations of the tunneling current through various small organic molecules sandwiched between gold electrodes by using the Wenzel-Kramers-Brillouin (WKB) approximation. The barrier to tunneling is taken to be the work function of gold and calculated from a first principles electronic structure code. The current-voltage characteristics of these molecules are compared in the context of existing first principles and experimental results. In this model the surface dipole moment, induced by the adsorbed molecule, can have a significant effect on the current and hence dipole moments may be an important property for prediction of the conductance characteristics of a molecule. © 2006 IEEE

A new class of self-assembled monolayers on gold using an alkynyl group as a linker

The geometry and energetics for adsorption of ethynylbenzene on Au(111) have been studied using Density Functional Theory. The alkynyl group, following removal of the terminal H atom, adsorbs covalently to the surface in the fcc hollow site with a bond energy of about 70 kcal.mol-1. Intermediate adsorption states are also possible via a hydrogen 1,2 shift to form a surface-bound vinylidene, or through the opening of the C-C triple bond without removing the hydrogen atom. © 2006 IEEE

Adsorption of amine compounds on the Au(111) surface: A density functional study

A Density Functional Theory study of the adsorption energetics of various amine compounds on the gold-(111) surface revealed that preferential binding occurs in under-coordinated sites. The largest binding energy is obtained when a gold adatom is placed in the fee position and the amine positioned with the nitrogen above the adatom. The results are compared with previous calculations for thiols, phosphines, and ethynylbenzene molecules to provide a meaningful comparison within a consistent computational framework. The systematic increase in binding energy with methyl group substitution previously observed for phosphine compounds is not observed for the amine analogues. The binding energy of the amines is considerably lower than that for thiols and binding is indicated for only the adatom geometry-a result consistent with experimental data. © 2007 American Chemical Society

The effect of reciprocal-space sampling and basis set quality on the calculated conductance of a molecular junction

We perform density functional theory and non-equilibrium Green's function calculations of the conductance of a gold wire and a 1,4-phenylenedimethanethiol (XYL) molecule adsorbed between Au(111) electrodes using the TranSIESTA software package. The effect of varying different computational parameters is investigated. We find that the conductance is more sensitive to the reciprocal-space sampling grid than the quality of the basis set employed. The conductance can vary up to a factor of five as a result of the choice of computational parameters. We report a set of computational parameters that yields a well-converged conductance value

Evaluation of half wave induction motor drive for use in passenger vehicles

Research performed at the University of Missouri-Columbia to devise and design a lower cost inverter induction motor drive for electrical propulsion of passenger vehicles is described. A two phase inverter motor system is recommended. The new design is predicted to provide comparable vehicle performance, improved reliability and a cost advantage for a high production vehicle, decreased total rating of the power semiconductor switches, and a somewhat simpler control hardware compared to the conventional three phase bridge inverter motor drive system. The major disadvantages of the two phase inverter motor drive are that it is larger and more expensive than a three phase machine, the design of snubbers for the power leakage inductances produce higher transient voltages, and the torque pulsations are relatively large because of the necessity to limit the inverter switching frequency to achieve high efficiency

Exploring the performance of molecular rectifiers: Limitations and factors affecting molecular rectification

There has been significant work investigating the use of molecules as nanoscale rectifiers in so-called "molecular electronics". However, less attention has been paid to optimizing the design parameters of molecular rectifiers or to their inherent limitations. Here we use a barrier tunneling model to examine the degree of rectification that can be achieved and to provide insight for the design and development of molecules with optimum rectification responses. © 2007 American Chemical Society

Mice with genetic deletion of group VIA phospholipase A2β exhibit impaired macrophage function and increased parasite load in Trypanosoma cruzi-induced myocarditis

Trypanosoma cruzi infection, which is the etiological agent of Chagas disease, is associated with intense inflammation during the acute and chronic phases. The pathological progression of Chagas disease is influenced by the infiltration and transmigration of inflammatory cells across the endothelium to infected tissues, which are carefully regulated processes involving several molecular mediators, including adhesion molecules and platelet-activating factor (PAF). We have shown that PAF production is dependent upon calcium-independent group VIA phospholipase A(2)β (iPLA(2)β) following infection of human coronary artery endothelial cells (HCAECs) with T. cruzi, suggesting that the absence of iPLA(2)β may decrease the recruitment of inflammatory cells to the heart to manage parasite accumulation. Cardiac endothelial cells isolated from iPLA(2)β-knockout (iPLA(2)β-KO) mice infected with T. cruzi demonstrated decreased PAF production compared to that by cells isolated from wild-type (WT) mice but demonstrated increases in adhesion molecule expression similar to those seen in WT mice. Myocardial inflammation in iPLA(2)β-KO mice infected with T. cruzi was similar in severity to that in WT mice, but the iPLA(2)β-KO mouse myocardium contained more parasite pseudocysts. Upon activation, macrophages from iPLA(2)β-KO mice produced significantly less nitric oxide (NO) and caused less T. cruzi inhibition than macrophages from wild-type mice. Thus, the absence of iPLA(2)β activity does not influence myocardial inflammation, but iPLA(2)β is essential for T. cruzi clearance

Immune responses to gp82 provide protection against mucosal Trypanosoma cruzi infection

The potential use of the Trypanosoma cruzi metacyclic trypomastigote (MT) stage-specific molecule glycoprotein-82 (gp82) as a vaccine target has not been fully explored. We show that the opsonization of T. cruzi MT with gp82-specific antibody prior to mucosal challenge significantly reduces parasite infectivity. In addition, we investigated the immune responses as well as the systemic and mucosal protective immunity induced by intranasal CpG-adjuvanted gp82 vaccination. Spleen cells from mice immunized with CpG-gp82 proliferated and secreted IFN-γ in a dose-dependent manner in response to in vitro stimulation with gp82 and parasite lysate. More importantly, these CpG-gp82-immunized mice were significantly protected from a biologically relevant oral parasite challenge.Saint Louis University Department of Molecular MicrobiologyUniversidade Federal de São Paulo (UNIFESP) Escola Paulista de Medicina Departamento de Microbiologia, Imunologia e ParasitologiaUNIFESP, EPM, Depto. de Microbiologia, Imunologia e ParasitologiaSciEL

PO and ID BCG vaccination in humans induce distinct mucosal and systemic immune responses and CD4(+) T cell transcriptomal molecular signatures.

Protective efficacy of Bacillus Calmette-Guérin (BCG) may be affected by the methods and routes of vaccine administration. We have studied the safety and immunogenicity of oral (PO) and/or intradermal (ID) administration of BCG in healthy human subjects. No major safety concerns were detected in the 68 healthy adults vaccinated with PO and/or ID BCG. Although both PO and ID BCG could induce systemic Th1 responses capable of IFN-γ production, ID BCG more strongly induced systemic Th1 responses. In contrast, stronger mucosal responses (TB-specific secretory IgA and bronchoalveolar lavage T cells) were induced by PO BCG vaccination. To generate preliminary data comparing the early gene signatures induced by mucosal and systemic BCG vaccination, CD4(+) memory T cells were isolated from subsets of BCG vaccinated subjects pre- (Day 0) and post-vaccination (Days 7 and 56), rested or stimulated with BCG infected dendritic cells, and then studied by Illumina BeadArray transcriptomal analysis. Notably, distinct gene expression profiles were identified both on Day 7 and Day 56 comparing the PO and ID BCG vaccinated groups by GSEA analysis. Future correlation analyses between specific gene expression patterns and distinct mucosal and systemic immune responses induced will be highly informative for TB vaccine development.Mucosal Immunology advance online publication 30 August 2017; doi:10.1038/mi.2017.67