Functionality in single-molecule devices: Model calculations and applications of the inelastic electron tunneling signal in molecular junctions

We analyze how functionality could be obtained within single-molecule devices by using a combination of non-equilibrium Green's functions and ab-initio calculations to study the inelastic transport properties of single-molecule junctions. First we apply a full non-equilibrium Green's function technique to a model system with electron-vibration coupling. We show that the features in the inelastic electron tunneling spectra (IETS) of the molecular junctions are virtually independent of the nature of the molecule-lead contacts. Since the contacts are not easily reproducible from one device to another, this is a very useful property. The IETS signal is much more robust versus modifications at the contacts and hence can be used to build functional nanodevices. Second, we consider a realistic model of a organic conjugated molecule. We use ab-initio calculations to study how the vibronic properties of the molecule can be controlled by an external electric field which acts as a gate voltage. The control, through the gate voltage, of the vibron frequencies and (more importantly) of the electron-vibron coupling enables the construction of functionality: non-linear amplification and/or switching is obtained from the IETS signal within a single-molecule device.Comment: Accepted for publication in Journal of Chemical Physic

Non-equilibrium inelastic electronic transport: Polarization effects and vertex corrections to the self-consistent Born approximation

We study the effect of electron-vibron interactions on the inelastic transport properties of single-molecule nanojunctions. We use the non-equilibrium Green's functions technique and a model Hamiltonian to calculate the effects of second-order diagrams (double-exchange DX and dressed-phonon DPH diagrams) on the electron-vibration interaction and consider their effects across the full range of parameter space. The DX diagram, corresponding to a vertex correction, introduces an effective dynamical renormalization of the electron-vibron coupling in both the purely inelastic and the inelastic-resonant features of the IETS. The purely inelastic features correspond to an applied bias around the energy of a vibron, while the inelastic-resonant features correspond to peaks (resonance) in the conductance. The DPH diagram affects only the inelastic resonant features. We also discuss the circumstances in which the second-order diagrams may be approximated in the study of more complex model systems.Comment: To be published in PR

Non-equilibrium quantum transport in fully interacting single-molecule nanojunctions

Using non-equilibrium Green's functions, we derive a formula for the electron current through a lead-molecule-lead nanojunction where the interactions are not restricted to the central region, but are spread throughout the system, including the leads and the lead-molecule interfaces. The current expression consists of two sets of terms. The first set corresponds to a generalised Meir and Wingreen expression where the leads' self-energies are renormalised by the interactions crossing at the molecule-lead contacts. The second set corresponds to inelastic scattering events in the leads arising from any arbitrary interaction, including electron-electron and electron-phonon coupling, treated beyond mean-field approximations. Using different levels of approximation, we are able to recover well-known expressions for the current. We also analyse how practical calculations can be performed with our formalism by using the new concept of generalised embedding potentials.Comment: Shorter version originally submitted to PRL (29 Sep 2011). This version accepted for publication in PR

Antioxidant and antimicrobial activities of Heracleum nepalense D Don root

Purpose: The aim of the present study was to investigate antioxidant and antimicrobial effects of the methanol extract of Heracleum nepalense D.Don roots. Method: The antimicrobial effect was determined by agar dilution and disc diffusion method. The free radical scavenging potential was studied by using different antioxidants models of screening using vitamin E (5mM) as standard. Results: The crude methanol extract of H.nepalense root was found to be active against both Gram-positive and Gram-negative organisms. The ethyl acetate soluble fraction of the extract showed similar activity against these organisms. Similarly, the methanol extract at 1000 mg. ml-1 and the ethyl acetate fraction at 50 mg. ml-1 exhibited significant antioxidant activity in ferrous sulphate induced lipid peroxidation, 1,1- diphenyl- 2-picryl hydrazyl (DPPH), Hydroxyl radical and Superoxide scavenging models. Conclusions: The study confirms the possible antioxidant and antimicrobial potentiality of the plant extract. Presence of flavonoid alone or in combination with its other components could be responsible for the activity. Keywords: Heracleum nepalense, Lipid peroxidation, Superoxide scavenging, DPPH assay, Antimicrobial effect, Flavonoid> Tropical Journal of Pharmaceutical Research Vol. 4 (1) 2005: pp. 341-34

Evidence of 1D behaviour of He4^4 confined within carbon-nanotube bundles

We present the first low-temperature thermodynamic investigation of the controlled physisorption of He$^{4}$ gas in carbon single-wall nanotube (SWNT) samples. The vibrational specific heat measured between 100 mK and 6 K demonstrates an extreme sensitivity to outgassing conditions. For bundles with a few number of NTs the extra contribution to the specific heat, C$_{ads}$, originating from adsorbed He$^{4}$ at very low density displays 1D behavior, typical for He atoms localized within linear channels as grooves and interstitials, for the first time evidenced. For larger bundles, C$_{ads}$ recovers the 2D behaviour akin to the case of He$^{4}$ films on planar substrates (grafoil).Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let