José Antonio Gil

Large negative differential conductance in single-molecule break junctions

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Greens functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

Stretching-Induced Conductance Increase in a Spin-Crossover Molecule

We investigate transport through mechanically triggered single-molecule switches that are based on the coordination sphere-dependent spin state of Fe(II)-species. In these molecules, in certain junction configurations the relative arrangement of two terpyridine ligands within homoleptic Fe(II)-complexes can be mechanically controlled. Mechanical pulling may thus distort the Fe(II) coordination sphere and eventually modify their spin state. Using the movable nanoelectrodes in a mechanically controlled break-junction at low temperature, current-voltage measurements at cryogenic temperatures support the hypothesized switching mechanism based on the spin-crossover behavior. A large fraction of molecular junctions formed with the spin-crossover-active Fe(II)-complex displays a conductance increase for increasing electrode separation and this increase can reach 1-2 orders of magnitude. Theoretical calculations predict a stretching-induced spin transition in the Fe(II)-complex and a larger transmission for the high-spin configuration.

Flat sheet or hollow fibre — comparison of full-scale membrane bio-reactor configurations

Abstract Membrane bioreactors (MBRs) are widely used for wastewater treatment and reuse applications. Selection of a membrane configuration is a crucial step in the design process and has a high impact on further plant operations. Despite increasing experience with full-scale applications, practical knowledge concerning the impact of different membrane configurations on process performance and operational costs is still lacking. This paper provides full scale MBR performance data comparing the use of flat sheet and hollow fibre membranes and analyses the consequences on operation, performance and treatment efficiency. Hollow fibre configurations, comparing to the flat sheet, are designed for higher fluxes, operated at lower concentrations, cleaned more often and protected by stricter pre-treatment. Filterability of activated sludge from municipal MBRs is better than from industrial MBRs and does not depend on membrane configuration. The energy consumption depends more on the influent type than on the memb...

Filtration Characterization Method as Tool to Assess Membrane Bioreactor Sludge Filterability—The Delft Experience

Prevention and removal of fouling is often the most energy intensive process in Membrane Bioreactors (MBRs), responsible for 40% to 50% of the total specific energy consumed in submerged MBRs. In the past decade, methods were developed to quantify and qualify fouling, aiming to support optimization in MBR operation. Therefore, there is a need for an evaluation of the lessons learned and how to proceed. In this article, five different methods for measuring MBR activated sludge filterability and critical flux are described, commented and evaluated. Both parameters characterize the fouling potential in full-scale MBRs. The article focuses on the Delft Filtration Characterization method (DFCm) as a convenient tool to characterize sludge properties, namely on data processing, accuracy, reproducibility, reliability, and applicability, defining the boundaries of the DFCm. Significant progress was made concerning fouling measurements in particular by using straight forward approaches focusing on the applicability of the obtained results. Nevertheless, a fouling measurement method is still to be defined which is capable of being unequivocal, concerning the fouling parameters definitions; practical and simple, in terms of set-up and operation; broad and useful, in terms of obtained results. A step forward would be the standardization of the aforementioned method to assess the sludge filtration quality.

MBR performance: Operational problems in industry

A major study undertaken by a number of institutions and a leading engineering consultancy has investigated factors affecting the performance in operation of membrane bioreactors in a variety of different industrial applications. Jose Antonio Gil summarises the findings.

Humidity-controlled rectification switching in ruthenium-complex molecular junctions

Although molecular rectifiers were proposed over four decades ago1,2, until recently reported rectification ratios (RR) were rather moderate2–11 (RR ~ 101). This ceiling was convincingly broken using a eutectic GaIn top contact12 to probe molecular monolayers of coupled ferrocene groups (RR ~ 105), as well as using scanning tunnelling microscopy-break junctions13–16 and mechanically controlled break junctions17 to probe single molecules (RR ~ 102–103). Here, we demonstrate a device based on a molecular monolayer in which the RR can be switched by more than three orders of magnitude (between RR ~ 100 and RR ≥ 103) in response to humidity. As the relative humidity is toggled between 5% and 60%, the current–voltage (I–V) characteristics of a monolayer of di-nuclear Ru-complex molecules reversibly change from symmetric to strongly asymmetric (diode-like). Key to this behaviour is the presence of two localized molecular orbitals in series, which are nearly degenerate in dry circumstances but become misaligned under high humidity conditions, due to the displacement of counter ions (PF6–). This asymmetric gating of the two relevant localized molecular orbital levels results in humidity-controlled diode-like behaviour.The rectification ratio of a molecular junction made of a self-assembled monolayer of di-nuclear ruthenium-complex molecules can be varied by more than three orders of magnitude by controlling relative humidity.