Tunneling Probability Increases with Distance in Junctions Comprising Self-assembled Monolayers of Oligothiophenes

Molecular tunneling junctions should enable the tailoring of charge-transport at the quantum level through synthetic chemistry, but are hindered by the dominance of the electrodes. We show that the frontier orbitals of molecules can be decoupled from the electrodes, preserving their relative energies in self-assembled monolayers even when a top-contact is applied. This decoupling leads to the remarkable observation of tunneling probabilities that increase with distance in a series of oligothiophenes, which we explain using a two-barrier tunneling model. This model is generalizable to any conjugated oligomers for which the frontier orbital gap can be determined and predicts that the molecular orbitals that dominate tunneling charge-transport can be positioned via molecular design rather than being dominated by Fermi-level pinning arising from strong hybridization. The ability to preserve the electronic structure of molecules in tunneling junctions facilitates the application of well-established synthetic design rules to tailor the properties of molecular-electronic devices

Mechanically and Electrically Robust Self-Assembled Monolayers for Large-Area Tunneling Junctions

This paper examines the relationship between mechanical deformation and the electronic properties of self assembled monolayers (SAMs) of the oligothiophene 4-([2,2':5',2":5",2"'-quaterthiophen]-5-y1)butane-1-thiol (T4C4) in tunneling junctions using conductive probe atomic force microscopy (CP-AFM) and eutectic Ga-In (EGaIn). We compared shifts in conductivity, transition voltages, of T4C4 with increasing AFM tip loading force to alkanethiolates. While these shifts result from an increasing tilt angle from penetration of the SAM by the AFM tip for the latter, we ascribe them to distortions of the pi system present in T4C4, which is more mechanically robust than alkanethiolates of comparable length; SAMs comprising T4C4 shows about five times higher Young's modulus than alkanethiolates. Density functional theory calculations confirm that mechanical deformations shift the barrier height due to changes in the frontier orbitals caused by small rearrangements to the conformation of the quaterthiophene moiety. The mechanical robustness of T4C4 manifests as an increased tolerance to high bias in large-area EGaIn junctions suggesting that electrostatic pressure plays a significant role in the shorting of molecular junctions at high bias

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(CH2)11S–BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode. The stability of the junctions is determined by the choice of the bottom electrode, which, in turn, determines the maximum applied bias window, and the mechanism of rectification is dominated by the energy levels centered on the BTTF units. The energy level alignments of the three junctions are similar because of Fermi level pinning induced by charge transfer at the metal–thiolate interface and by a varying degree of additional charge transfer between BTTF and the metal. Density functional theory calculations show that the amount of electron transfer from M to the lowest unoccupied molecular orbital (LUMO) of BTTF follows the order Ag > Au > Pt. Junctions with Ag electrodes are the least stable and can only withstand an applied bias of ±1.0 V. As a result, no molecular orbitals can fall in the applied bias window, and the junctions do not rectify. The junction stability increases for M = Au, and the highest occupied molecular orbital (HOMO) dominates charge transport at a positive bias resulting in a positive rectification ratio of 83 at ±1.5 V. The junctions are very stable for M = Pt, but now the LUMO dominates charge transport at a negative bias resulting in a negative rectification ratio of 912 at ±2.5 V. Thus, the limitations of Fermi level pinning can be bypassed by a judicious choice of the bottom electrode material, making it possible to access selectively HOMO- or LUMO-based charge transport and, as shown here, associated reversal of rectification.The authors express thanks to the Ministry of Education (MOE) for supporting this research under award nos. MOE2018-T2-1-088 and R-143-000-B30-112. We also acknowledge the Prime Minister’s Office, Singapore, under its Medium Sized Centre program for supporting this research. This work was also funded by ITN iSwitch 642196, the DGI (Spain), projects FANCY (CTQ2016-80030-RA), GENESIS (PID2019-111682RB-I00) and MOTHER (MAT2016-80826- R), the Generalitat de Catalunya (2017-SGR-918), the Instituto de Salud Carlos III, through “Acciones CIBER”, and the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” program for Centers of Excellence in R&D (FUNFUTURE; CEX2019-000917-S). The work in Mons was financially supported by the EC through the Marie Curie project ITN iSwitch (GA no. 642196). Computational resources were provided by the Consortium des É quipements de Calcul Intensif (CÉ CI) funded by the Belgian National Fund for Scientific Research (F.R.S.-FNRS) under grant 2.5020.11. J.C. is an FNRS research director.Peer reviewe

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

Theoretical Analysis of the Conduction Properties of Self Assembled Molecular Tunnel Junctions

As the size scale of electrical devices approach the atomic scale. Moore\u27s law is predicted to be over for semiconductor devices. Studies into the replacement of semiconductor technology with organic devices was first predicted by Avriam and Ratner[1] in 1974. Since then significant research into molecular based organic devices has been conducted. The work presented in this dissertation explores the theoretical frameworks used to model transport through molecular junctions. We present studies which seek to garner a better understanding of the charge transport through molecular junctions and how the conduction properties can be optimized. We show that a single atom can change a molecule from an insulator to a conductor. We also study the effects of sigma and pi bridges on molecular rectification. We will then show molecular devices that act as viable electrical static and dynamic switches. The studies presented here help to demonstrate the viability of organic devices in the forms of rectifiers and switches with applications ranging from the replacement of traditional semiconductor devices to neuromorphic computing

Interference effects in molecular nanojunctions

Nicolò Ferri: Interference effects in molecular nanojunctions From the earliest theoretical investigation of the possibility of single molecule conductance, to today where single molecule conductance techniques are becoming routine operations, the field of molecular electronics has grown exponentially, becoming an important research topic with possible future applications. Understanding electron transport in nanojunctions has lately been a hot topic and represents a fundamental step for the advancement of molecular electronics. This thesis aims at unravelling some of the mysteries and mechanisms that revolve around the behaviour of some molecular wires. This thesis focusses its attention towards interference effects in organic molecular wires: in this thesis two main types of interferences are studied: one located on the contacts of a series of molecular wires and one regarding the structure-property relationships within molecular wires. The novelty of this work lies in the synthesis of new molecular wires and their conductance studies with STM-based techniques. The first project demonstrated the existence of strong interactions between thiol contacts and gold electrodes (so-called ‘gateway states’) that were interfering with the conductance decay of a series of double barrier tunnelling molecular wires of increasing length by suppressing the attenuation of their conductance over their length. A simple experiment supported by theoretical calculations featuring the synthesis of molecular wires with thiols replaced by thioether contacts demonstrated the existence of these interactions by cancelling their effects. The second project was focussed on the study of interference effects in thiophene-based molecular wires. Several series of molecular wires were synthesized in multiple subprojects to gain a better understanding of these effects by finely tuning the molecular wires structures such as the introduction of heteroatom bridges and the insertion of carbonyl moieties as cross conjugation points within the backbone of a molecular wire. On this project some unusual switching properties of some of the wires were discovered and investigated. Finally, this study managed to shed some light on the behaviour of all the bithiophene-based molecular wires analysed and the information discovered will be useful in the tool-box for finely tuning properties of molecular wires

Surface and Electrical Characterization of Conjugated Molecular Wires

University of Minnesota Ph.D. dissertation. SEptember 2016. Major: Material Science and Engineering. Advisor: C.Daniel Frisbie. 1 computer file (PDF); xvii, 252 pages.This thesis describes the surface and electrical characterization of ultrathin organic films and interfaces. These films were synthesized on the surface of gold by utilizing layer by layer synthesis via imine condensation. Film growth by imine click (condensation) chemistry is particularly useful for molecular electronics experiments because it provides a convenient means to obtain and extend π-conjugation in the growth direction. However, in the context of film growth from a solid substrate, the reaction yield per step has not been characterized previously, though it is critically important. To address these issues, my research focused on a comprehensive characterization of oligophenyleneimine (OPI) wires via Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), and cyclic voltammetry (CV). In addition, we had the unique opportunity of developing the first of its kind implementation of nuclear reaction analysis (NRA) to probe the intensity of carbon atoms after each addition step. Overall the combination of various techniques indicated that film growth proceeds in a quantitative manner. Furthermore, the NRA experiment was optimized to measure the carbon content in self-assembled monolayers of alkyl thiols. The results indicated well-resolved coverage values for ultrathin films with consecutive steps of 2 carbon atoms per molecule. Another fundamental problem in molecular electronics is the vast discrepancy in the values of measured resistance per molecule between small and large area molecular junctions. In collaboration with researchers at the National University of Singapore, we addressed these issues by comparing the electrical properties of OPI wires with the eutectic gallium indium alloy (EGaIn) junction (1000 µm2), and conducting probe atomic force microscopy (CP-AFM) junction (50 nm2). Our results showed that intensive (i.e., area independent) observables such as crossover length, activation energy, and decay constants agreed very well across the two junction platforms. On the other hand, the extensive (area dependent) resistance per molecule values was 100 times higher for EGaIn junction verses CP-AFM after normalizing to contact area. This was most likely due to differences in metal-molecule contact resistances. My contribution to this collaborative work is in synthesis and timely delivery of OPI wires. The structure-property relationships of OPI wires with 5 terminal F atoms were studied extensively by XPS. The results show similar crossover behavior obtained by molecular junction experiments. Saturated spacers (conjugation disruption units) were introduced into the molecular backbone, and their effects on the intensity of F 1s counts were measured. Overall, there was good correlation between the position and number of saturated units verses F 1s peak area. Even though core hole spectroscopy and time dependent density functional theory (TDDFT) calculations are required to fully understand the charge transport dynamics, the preliminary results point to a new ultrahigh vacuum method of measuring charge transfer rates. Overall, these experiments open significant opportunities to synthesize ultra-thin films and characterize a variety of donor-block-acceptor and metal complex systems in molecular electronics

Diseño de interfases funcionales: Estudio de procesos de transferencia electrónica a través de monocapas moleculares autoensambladas sobre oro

El proyecto aborda el estudio de la transferencia electrónica sobre superficies de oro modificadas con monocapas autoensambladas (SAMs) puras o mixtas, es decir, constituidas por uno o varios componentes moleculares. Este estudio contribuye al desarrollo de conceptos fundamentales en áreas de investigación relacionados con la Electroquímica Interfacial, Bioelectroquímica, Electrocatálisis, Ciencia de Superficies y Nanociencia y Nanotecnología.1-7 El interés fundamental de estos sistemas radica en las expectativas que suscita el diseño de interfaces donde la composición y distribución superficial de sus componentes puede ser controlada a escala nanométrica a partir del concepto de ensamblaje molecular. Además, la organización estructural de las SAMs también determina las interacciones con otros materiales, y la forma en que se inmovilizan. La influencia de todos estos aspectos en la transferencia electrónica de proteínas redox como la Myoglobina es básica para explicar tanto su función como la dinámica de diferentes procesos biológicos, así como para el diseño racional de sensores y biosensores electroquímicos.3-7. Con este trabajo se logra el control a nivel molecular de la organización de SAMs puras y mixtas a nivel molecular, en particular en la funcionalización superficial de sustratos de Au con alcanotioles ω-sustituidos (ej. grupos terminales –COOH, -CH3, etc.). Se estudia detalladamente con técnicas de caracterización electroquímicas (VC, EIS, etc.) y espectroscópicas (RR, IRRAS) las interacciones electrostáticas e hidrofóbicas de la Myoglobina con las interfaces funcionalizadas diseñadas, y en consecuencia su efecto sobre la transferencia electrónica y capacidad electrocatalítica del grupo redox

Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function

We report the results of an extensive investigation of metal–molecule–metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M₁–S–(C₆H₄)<i>_n</i>–S–M₂, with 1 ≤ <i>n</i> ≤ 4 and M_1,2 = Ag, Au, Pt) to examine the impact of molecular length (<i>n</i>) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = Φ_SAM – Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current–voltage (<i>I</i>–<i>V</i>) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full <i>I</i>–<i>V</i> curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment ε_h = <i>E</i>_F – <i>E</i>_HOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy <i>E</i>_F of the junction. Where photoelectron spectroscopy data are available, the ε_h values agree very well with those determined by TVS. Using a single-level model, which we justify <i>via ab initio</i> quantum chemical calculations at post-density functional theory level and additional UV–visible absorption measurements, we are able to quantitatively reproduce the <i>I</i>–<i>V</i> measurements in the whole bias range investigated (∼1.0–1.5 V) and to understand the behavior of ε_h and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal–S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in ε_h exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (<i>R</i>). Correlation among Φ, ΔΦ, <i>R</i>, transition voltage (<i>V</i>_t), and ε_h and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions