A Method to Investigate the Mechanism of Charge Transport Across Bio-Molecular Junctions with Ferritin.

To investigate the mechanisms of charge transport (CT) across biomolecular tunnel junctions, it is required to make electrical contacts by a non-invasive method that leaves the biomolecules unaltered. Although different methods to form biomolecular junctions are available, here we describe the EGaIn-method because it allows us to readily form electrical contacts to monolayers of biomolecules in ordinary laboratory settings and to probe CT as a function of voltage, temperature, or magnetic field. This method relies on a non-Newtonian liquid-metal ally of Ga and In with a few nm thin layer of GaO floating on its surface giving this material non-Newtonian properties allowing it to be shaped in to cone-shaped tips or stabilized in microchannels.

Temperature-Dependent Coherent Tunneling across Graphene-Ferritin Biomolecular Junctions.

Understanding the mechanisms of charge transport (CT) across biomolecules in solid-state devices is imperative to realize biomolecular electronic devices in a predictive manner. Although it is well-accepted that biomolecule-electrode interactions play an essential role, it is often overlooked. This paper reveals the prominent role of graphene interfaces with Fe-storing proteins in the net CT across their tunnel junctions.

The Role of Structural Order in the Mechanism of Charge Transport across Tunnel Junctions with Various Iron-Storing Proteins.

In biomolecular electronics, the role of structural order in charge transport (CT) is poorly understood. It has been reported that the metal oxide cores of protein cages (e.g.

Biomolecular control over local gating in bilayer graphene induced by ferritin.

Electrical field-induced charge modulation in graphene-based devices at the nanoscale with ultrahigh density carrier accumulation is important for various practical applications. In bilayer graphene (BLG), inversion symmetry can simply be broken by an external electric field. However, control over charge carrier density at the nanometer scale is a challenging task.

Role of Order in the Mechanism of Charge Transport across Single-Stranded and Double-Stranded DNA Monolayers in Tunnel Junctions.

Deoxyribonucleic acid (DNA) has been hypothesized to act as a molecular wire due to the presence of an extended π-stack between base pairs, but the factors that are detrimental in the mechanism of charge transport (CT) across tunnel junctions with DNA are still unclear. Here we systematically investigate CT across dense DNA monolayers in large-area biomolecular tunnel junctions to determine when intrachain or interchain CT dominates and under which conditions the mechanism of CT becomes thermally activated. In our junctions, double-stranded DNA (dsDNA) is 30-fold more conductive than single-stranded DNA (ssDNA).

Room temperature conductance switching in a molecular iron(iii) spin crossover junction.

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [Fe(qsal-I)]NTf (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current-voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling.

Bias-Polarity-Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction.

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region.

The energy level alignment of the ferrocene-EGaIn interface studied with photoelectron spectroscopy.

The energy level alignment after the formation of a molecular tunnel junction is often poorly understood because spectroscopy inside junctions is not possible, which hampers the rational design of functional molecular junctions and complicates the interpretation of the data generated by molecular junctions. In molecular junction platforms where the top electrode-molecule interaction is weak; one may argue that the energy level alignment can be deduced from measurements with the molecules supported by the bottom electrode (sometimes referred to as "half junctions"). This approach, however, still relies on a series of assumptions, which are challenging to address experimentally due to difficulties in studying the molecule-top electrode interaction.

Temperature Dependence of Charge and Spin Transfer in Azurin.

The steady-state charge and spin transfer yields were measured for three different Ru-modified azurin derivatives in protein films on silver electrodes. While the charge-transfer yields exhibit weak temperature dependences, consistent with operation of a near activation-less mechanism, the spin selectivity of the electron transfer improves as temperature increases. This enhancement of spin selectivity with temperature is explained by a vibrationally induced spin exchange interaction between the Cu(II) and its chiral ligands.

Reversal of the Direction of Rectification Induced by Fermi Level Pinning at Molecule-Electrode Interfaces in Redox-Active Tunneling Junctions.

Control over the energy level alignment in molecular junctions is notoriously difficult, making it challenging to control basic electronic functions such as the direction of rectification. Therefore, alternative approaches to control electronic functions in molecular junctions are needed. This paper describes switching of the direction of rectification by changing the bottom electrode material M = Ag, Au, or Pt in M-S(CH)S-BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode.

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Successful integration of proteins in solid-state electronics requires contacting them in a non-invasive fashion, with a solid conducting surface for immobilization as one such contact. The contacts can affect and even dominate the measured electronic transport. Often substrates, substrate treatments, protein immobilization, and device geometries differ between laboratories.

Protective Layers Based on Carbon Paint To Yield High-Quality Large-Area Molecular Junctions with Low Contact Resistance.

A major obstacle for transforming large-area molecular junctions into a viable technology is the deposition of a top, metallic contact over the self-assembled monolayer (SAM) without chemically damaging the molecules and preventing an interface-limited charge transport. Often a thin conducting layer is softly deposited over the SAM to protect it during the deposition of the metal electrode which requires conditions under which organic molecules are not stable. We report a new protective layer based on carbon paint which is highly conductive and has metallic-like behavior.

A Black Phosphorus Carbide Infrared Phototransistor.

Photodetectors with broadband detection capability are desirable for sensing applications in the coming age of the internet-of-things. Although 2D layered materials (2DMs) have been actively pursued due to their unique optical properties, by far only graphene and black arsenic phosphorus have the wide absorption spectrum that covers most molecular vibrational fingerprints. However, their reported responsivity and response time are falling short of the requirements needed for enabling simultaneous weak-signal and high-speed detections.

Long-Range Tunneling Processes across Ferritin-Based Junctions.

The mechanism of long-range charge transport across tunneling junctions with monolayers of ferritin is investigated. It is shown that the mechanism can be switched between coherent tunneling, sequential tunneling, and hopping by changing the iron content inside the ferritin. This study shows that ferritins are an interesting class of biomolecules to control charge transport.

Spin selectivity in electron transfer in photosystem I.

Photosystem I (PSI) is one of the most studied electron transfer (ET) systems in nature; it is found in plants, algae, and bacteria. The effect of the system structure and its electronic properties on the electron transfer rate and yield was investigated for years in details. In this work we show that not only those system properties affect the ET efficiency, but also the electrons' spin.

A device for measuring spin selectivity in electron transfer.

A new type of device is presented that allows direct measurement of spin selectivity in charge transfer processes occurring in adsorbed molecules. The new device provides direct information about the nature of the charge being transferred (electrons or holes) and on spin selectivity, if it exists. Here the device is applied for establishing the spin-dependent electron transfer through double-stranded DNA and its variation with the length of the oligomer.

Quantitative analysis and characterization of self-assembled DNA on a silver surface.

Self-assembled monolayers of DNA on a silver surface were prepared and characterized by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), fluorescence imaging, and (32)P radioactive labeling. The buffer concentration of the DNA solution and the surface roughness of the silver substrate were found to affect the surface coverage of DNA and its hybridization. At low buffer concentrations, surface coverage and hybridization were greatly reduced.

Existence of a new emitting singlet state of proflavine: femtosecond dynamics of the excited state processes and quantum chemical studies in different solvents.

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity.