Solid-State Protein Junctions:Cross-Laboratory Study Shows Preservation of Mechanism at Varying Electronic Coupling

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. Thus the question arises how far results from different laboratories and platforms are comparable and how to distinguish genuine protein electronic transport properties from platform-induced ones. We report a systematic comparison of electronic transport measurements between different laboratories, using all commonly used large-area schemes to contact a set of three proteins of largely different types. Altogether we study eight different combinations of molecular junction configurations, designed so that Ageo of junctions varies from 105 to 10−3 μm2. Although for the same protein, measured with similar device geometry, results compare reasonably well, there are significant differences in current densities (an intensive variable) between different device geometries. Likely, these originate in the critical contact-protein coupling (∼contact resistance), in addition to the actual number of proteins involved, because the effective junction contact area depends on the nanometric roughness of the electrodes and at times, even the proteins may increase this roughness. On the positive side, our results show that understanding what controls the coupling can make the coupling a design knob. In terms of extensive variables, such as temperature, our comparison unanimously shows the transport to be independent of temperature for all studied configurations and proteins. Our study places coupling and lack of temperature activation as key aspects to be considered in both modeling and practice of protein electronic transport experiments

Internal photoemission in molecular junctions: Parameters for interfacial barrier determinations

The photocurrent spectra for large-area molecular junctions are reported, where partially transparent copper top contacts permit illumination by UV-vis light. The effect of variation of the molecular structure and thickness are discussed. Internal photoemission (IPE), a process involving optical excitation of hot carriers in the contacts followed by transport across internal system barriers, is dominant when the molecular component does not absorb light. The IPE spectrum contains information regarding energy level alignment within a complete, working molecular junction, with the photocurrent sign indicating transport through either the occupied or unoccupied molecular orbitals. At photon energies where the molecular layer absorbs, a secondary phenomenon is operative in addition to IPE. In order to distinguish IPE from this secondary mechanism, we show the effect of the source intensity as well as the thickness of the molecular layer on the observed photocurrent. Our results clearly show that the IPE mechanism can be differentiated from the secondary mechanism by the effects of variation of experimental parameters. We conclude that IPE can provide valuable information regarding interfacial energetics in intact, working molecular junctions, including clear discrimination of charge transport mediated by electrons through unoccupied system orbitals from that mediated by hole transport through occupied system orbitals.Peer reviewed: YesNRC publication: Ye

Current state and perspectives of nanoscale molecular rectifiers

The concept of utilizing a molecule bridged between two electrodes as a stable rectifying device with the possibility of commercialization is a "holy grail" of molecular electronics. Molecular rectifiers do not only exploit the electronic function of the molecules but also offer the possibility of their direct integration into specific nano-electronic circuits. However, even after nearly three decades of extensive experimental and theoretical work, the concept of molecular rectifiers still has many unresolved aspects concerning both the fundamental understanding of the underlying phenomena and the practical realization. At the same time, recent advancements in molecular systems with rectification ratios exceeding 105 are highly promising and competitive to the existing silicon-based devices. Here, we provide an overview and critical analysis of the current state and recent progress in molecular rectification relying on the different design concepts and material platforms such as single molecules, self-assembled monolayers, molecular multilayers, heterostructures, and metal-organic frameworks and coordination polymers. The involvement of crucial parameters such as the energy of molecular orbitals, electrode-molecule coupling, and asymmetric shifting of the energy levels will be discussed. Finally, we conclude by critically addressing the challenges and prospects for progress in the field and perspectives for the commercialization of molecular rectifiers.Comment: 32 pages, 7 figure

Direct Optical Determination of Interfacial Transport Barriers in Molecular Tunnel Junctions

Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction