4:00pm - 4:30pmInvited
Sequential Tunneling in Strongly Coupled Carbon-based Molecular Junctions
1University of Alberta, Canada; 2National Institute for Nanotechnology, Canada
Carbon-based molecular junctions involve aromatic molecules covalently bonded to graphitic carbon contacts, and electronic coupling between the electrodes and molecules is generally strong. Transport for four different aromatic oligomers was strongly dependent on structure once the molecular layer thickness exceeded 5 nm, and was weakly temperature dependent, with no linear regions in an Arrhenius plot for the range 200-440 K. For anthraquinone and fluorene, lnJ vs thickness was linear from 2-10 nm, with a slope of 2.7 nm-1. UV-Vis absorption spectroscopy revealed that the optical gaps differed for the four molecules, in the range of 3.5 - 4.7 eV. Although the HOMO and LUMO energies of the free molecules showed no correlation with current density (R2 <0.25), the optical absorption maximum strongly correlated with current density (R2 >0.96) for 8 nm thick devices and V= 0.5 V. The results are consistent with transport controlled by the energy levels of the molecular layer interior rather than the offset between the HOMO or LUMO and the electrode Fermi level. We propose a multistep tunneling mechanism in which the important barrier is the HOMO-LUMO gap. Provided the electronic coupling between the electrodes and the molecules is stronger than that between oligomeric subunits, interfacial electron transport is not the rate limiting step.
4:30pm - 4:45pmOral
Coulomb Blockade in Protein Monolayers
1Weizmann Institute of Science, Israel; 2Universidad Autonoma de Madrid, Spain
Coulomb Blockade, CB, is a phenomenon that is generally observed if a sample is small enough so that the electrons inside the device will create sufficiently strong Coulomb repulsion to prevent an additional electron to enter the sample, so that Ohm’s law is no longer obeyed. We measured electron transport across a redox active protein in a solid state junction using a Au (substrate)/protein//linker/ Au (top) tunnel junction configuration, with the Cu(II) redox centre of the electron transfer protein Azurin (Az) shielded from direct interaction with the Au (top) electrode, by a monolayer of (< 1 nm) linker molecules, bound covalently to the Au (top) electrode. In contrast to what is the case without such linker layer, the differential conductance of the current-voltage (I-V) characteristics observed at low temperatures (~6 K), shows clear steps and its first derivative a peak-like structure, rather than the earlier observed inelastic electron tunneling spectrum. Given the nature of the junction, the most likely cause for this result is the redox active protein, specifically its redox-active moiety, the Cu ion. The non- linearities in the conductance can then be explained by CB, due to charging of the Cu redox centre. Increasing the temperature yields results that are consistent with tunnelling through discrete molecular levels. This implies that transport is coherent throughout the tunnelling event. If the Cu ion is removed from the protein, yielding apo-Az, the CB effect disappears. While CB is well known in single molecule systems, our experiments are done on monolayers; the results thus imply either single protein events across the monolayer or coordinated charge transport across several proteins.
4:45pm - 5:00pmOral
Gating Electron Transfer in Peptides Towards Molecular Switches
The University of Adelaide, Australia
Electron transfer in proteins plays a crucial role in energy conversion and storage processes in all living organisms, thus providing an opportunity to mimic nature for applications in bio-inspired molecular electronics. However, the vast complexities of such systems is somewhat limiting to progress, with model synthetic peptides presenting as ideal alternatives in this context. Here, electrochemical studies are reported on a series of peptides to determine the influence of different side-chains and backbone rigidity on electron transfer, to progress the field of molecular electronics. Specifically, these peptides share either a common helical or β-strand conformation to cover a range of secondary structures, to fully investigate the influence of backbone rigidity. Two types of side-chain tethers, either triazole-containing or alkene-containing, are also compared to investigate these effects on electron transfer. Our results showed that the observed formal potentials (Eo) and apparent electron transfer rate constants (ket) fall into two distinct groups. The peptides constrained via a side-chain tether exhibited high formal potentials and low electron transfer rate constants, whereas the linear peptides displayed low formal potentials and high electron transfer rate constants. This was found to occur irrespective of the backbone conformation, or the nature of the side-chain constraint. The vast formal potential shifts (as much as 482 mV) and the large disparity in the electron transfer rate constants (as much as 97%) between the constrained and linear peptides, provides two distinct states (i.e. on/off) with a sizeable differential, which is ideal for the design of molecular switches.
5:00pm - 5:30pmInvited
Charge Transport and Molecular Conductance of Nucleic Acids and Peptides on Electrodes
University of Pittsburgh, United States
We present new results on charge transport in biomolecular systems, nucleic acids and chiral peptides. Molecular conductance and charge transfer are different manifestations of a molecule’s ability to transmit electric charge. We report on recent experiments and theoretical modelling that examines how the electron transport depends on temperature, length, sequence, and chirality. In addition, we will present recent findings on the chiral induced spin-selectivity effect and its importance for understanding the fundamental nature of charge transfer.
5:30pm - 5:45pmOral
Statistical Tools to Reveal Effect of Dipole Moments in Charge Transport
1Iowa State University, United States; 2DOE Ames National Lab, Ames, United States; 3Micro-electronics Research Center, Iowa State University, United States; 4University of California San Diego, United States
Delineating role of dipoles in large area junctions based on self-assembled monolayers (SAMs) is challenging due to molecular tilt, surface defects, inter-chain coupling among other features. This is even more challenging in polyfunction molecules when the polar moieties are electronically isolated. First we deployed SAMs that orient along the surface normal to delineate the role of magnitude and direction of dipole moment. Then using simple statistical tools, we demonstrate that when a molecule contains multiple isolated polar moieties, the effect of the coupled tensors (dipole moments) can be revealed. Using a case where no effect had been reported, we demonstrate that evaluating the nature of the distributions shows a general trend that relates to the magnitude and orientation of the dipole moments across two electronically isolated polar functional groups.
5:45pm - 6:00pmOral
Spin-current Induced Conductance Switching in EGaIn-Based bio-macular Magnetic Tunneling Junctions
1Department of Chemistry, National University of Singapore, Singapore; 2Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore; 3School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore; 4Analysis and Characterization, Institute of Materials Research & Engineering (IMRE), Singapore; 5Department of Physics, National University of Singapore, Singapore
Development and room temperature operation of magnetic molecular materials are currently receiving extensive attention for applications in high density data storage, spintronics, and quantum information processing.1 Magnetic tunnel junction based molecular devices (MTJ) are highly promising candidates for flexible devices and have the potential to reduce fabrication costs. Self-assembly of molecular nanomagnets on a ferromagnetic (FM) surface with controlled magnetic interactions with the surface and between the deposited molecular nanomagnet is most challenging task. Ferritin, an iron storage protein, has significant potential in this context since the iron content can be controlled. We developed a method to form highly dense monolayers of ferritin, without inter-particle and surface magnetic interaction on a FM surface. To form junctions, we used a non-invasive top-electrode of GaOx/EGaIn stabilised in a through-hole device.2,3 We adsorbed ferritin on ultra-smooth template stripped nickel (NiTS) surface4. We observed temperature independent electron transport through ferritin, which seems to suggest that coherent tunnelling is the dominant mechanism of charge transport. The magnetic property of ferritin immobilized NiTS surface has been investigated by X-ray magnetic circular dichroism spectroscopy. The data reveal that in the iron oxide core of ferritin does not have a magnetic coupling with the nickel surface. The field-dependent magnetization suggests that the spin ground state in the monolayer is perpendicular to substrate at room temperature. These findings suggest that significant changes of the electronics structure, molecular geometry and magnetic properties take place upon surface adsorption. This study helps to improve our ability to design in biomolecular based magnetic tunnelling junction.