Large-area Molecular Junctions Research Articles

Understanding Keesom Interactions in Monolayer-Based Large-Area Tunneling Junctions.

Charge transport across self-assembled monolayers (SAMs) has been widely studied. Discrepancies of charge tunneling data that arise from various studies, however, call for efforts to develop new statistical analytical approaches to understand charge tunneling across SAMs. Structure-property studies on charge tunneling across SAM-based junctions have largely been through comparison of average tunneling rates and associated variance. These early moments (especially the average) are dominated by barrier width-a static property of the junction. In this work, we show that analysis of higher statistical moments (skewness and kurtosis) reveals the dynamic nature of the tunnel junction. Intramolecular Keesom (dipole-dipole) interactions dynamically fluctuate with bias as dictated by stereoelectronic limitations. Analyzing variance in the distribution of tunneling data instead of the first statistical moment (average), for a series of n-alkanethiols containing internal amide and aromatic terminal groups, we observe that the direction of dipole moments affects molecule-electrode coupling. An applied bias induces changes in the tunneling probability, affecting the distribution of tunneling paths in large-area molecular junctions.

(Invited) Fabrication and Characterization of Carbon-Based Nanoscale Devices: Insights and Applications

Nanoscale devices made from carbon-based materials are investigated for a variety of unique properties and features, including their promise to improve performance, decrease production costs, or provide totally new functionality relative to extant electronic devices. Because these devices have active areas composed of materials that are much thinner than those used in conventional devices, the details of the interfaces can often act to control the physics that lead to the operational device characteristics in unexpected ways. This is particularly true in molecular electronics, where design rules based on chemical intuition often fail to account for key physical aspects that dominate device behaviour. Thus, the ability to modify interfaces in a way that leads to control of device properties is a key to enabling next-generation devices based on nanoscale phenomenon. In addition, it is important to achieve an understanding of the processes that occur during carrier transport in nanoscale devices in order to obtain desirable functionality. This presentation will describe several aspects of carbon-based nanoscale devices, including fabrication and operation of molecular electronics and graphene field effect transistors (GFETs). After a discussion of some general physical principles that operate within the interfacial energy level alignment regime in molecular electoronics, a discussion of how light emission from nanoscale devices can be used to characterize them will be provided. In particular, we have used light emission from both large area molecular junctions and GFETs to understand important distance scales (elastic limits) and processes (transport and emission mechanisms). The data indicate that processes that involve hot carrier interactions with plasmonic structures in the devices lead to light emission, and that this phenomenon can be used to measure energy losses of carriers as they traverse a molecular layer. Results indicate that carriers can travel approximately 7 nm through a molecular junction before energy losses become significant, indicating that elastic transport is achieved for thin layers, but a transition in mechanism occurs for thicker films. The characteristics of the energy losses are reported for several different structures, providing insights into the nature of the transport in molecular electronics. Light emission from GFETs, on the other hand, appears to follow a novel mechanism that involves the excitation of plasmons in the graphene by hot carriers followed by decay through photon emission. By controlling the location of defects in the graphene lattice and the nanostructures around these scattering sites, emission could be localized to regions where this coupling is more optimized. In this second case, the devices may be able to be engineered through nanostructuring in order to gain control over the character of light emission. Finally, a few novel and emerging applications of nanoscale devices will be discussed, including using molecular devices in audio circuits, as well as for high frequency harmonic generation.

(Invited) Towards a Perfect Junction: Effect of Smoothening Electrodes on Tunneling Behaviors across Large Area Molecular Junctions

Large area self-assembled monolayers (SAMs) based tunneling junctions have recently received heightened attentions, in part, due to high yield of working junctions, ease of fabrication, and data reproducibility. Eutectic gallium indium (EGaIn) and ultra-flat metal substrate are used to form these molecular junctions. Despite the advances, effect of sub-nanometer roughness inhibits delineation of second-order effects on tunneling characteristics. These effects, like intra-molecular keesom interactions, can lead to new approaches to tuning charge transport behavior – a fete that is yet to be harnessed. To address this challenge, we deployed a three-pronged approach entailing; i) polishing of the top-electrode, ii) tuning roughness of the bottom electrode and the angstrom scale, and iii) deploying statistical analysis to uncover these effects. In this talk, effect of these modifications on SAM and associated charge transport behavior will be discussed.

Charge Transport and Practical Applications of All-Carbon Molecular Electronic Devices

Molecular electronic junctions can be made by covalently bonding aromatic molecules and graphene ribbons to a flat, sp2 hybridized carbon electrode followed by a “top contact” of electron-beam deposited carbon. The entire active region of the structure consists of carbon and hydrogen, and electronic properties range from insulators to efficient conductors, with excellent lifetime and tolerance to temperature excursions. 1-3 Of particular interest is a carbon/graphene/carbon device containing 5-carbon wide graphene ribbons, which avoids torsional disorder common to aromatic molecular components (shown schematically below, left). The graphene devices have the highest molecular conductance reported to date for large-area molecular junctions, and is stable for at least five million current/voltage cycles to ±4 A/cm2. 4 An application of all-carbon molecular tunnel junctions in electronic music will be described, which resulted in the first known commercial product involving molecular electronics (below, right). 5 (1) Morteza Najarian, A.; Bayat, A.; McCreery, R. L.; Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions; Journal of the American Chemical Society 2018, 140, 1900. (2) Supur, M.; Smith, S. R.; McCreery, R. L.; Characterization of Growth Patterns of Nanoscale Organic Films on Carbon Electrodes by Surface Enhanced Raman Spectroscopy; Analytical Chemistry 2017, 89, 6463. (3) Morteza Najarian, A.; McCreery, R. L.; Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions; ACS Nano 2017, 11, 3542. (4) Supur, M.; Van Dyck, C.; Bergren, A. J.; McCreery, R. L.; Bottom-up, Robust Graphene Ribbon Electronics in All-Carbon Molecular Junctions; ACS Applied Materials & Interfaces 2018, 10, 6090. (5) Bergren, A. J.; Zeer-Wanklyn, L.; Semple, M.; Pekas, N.; Szeto, B.; McCreery, R. L.; Musical molecules: the molecular junction as an active component in audio distortion circuits; Journal of Physics: Condensed Matter 2016, 28, 094011. Figure 1

(Invited) Towards a Perfect Junction: Effect of Smoothening Electrodes on Tunneling Behaviors across Large Area Molecular Junctions

Eutectic gallium indium (EGaIn), as top electrode for self-assembled monolayers (SAMs) based tunneling junction, has recently received heightened attentions, in part, due to its ease of fabrication, high-yield working junctions, and reproducibility. Despite these advances, effect of sub-nanometer roughness inhibits delineation of second-order effects on tunneling characteristics that are inherent in the molecules or molecular assemblies. Some other effects, such as molecule/electrode interactions and field-dependent intra-molecular van der Waals interactions, can lead to new approaches of tuning charge transport properties. One challenge, however, is that junctions have inherent defects that can significantly skew interpretation of obtained results. Addition challenge is the decoupling of subtle molecular effects from vast number of measurements that usually give large distributions. To mitigate these challenges, we deployed a three-pronged approach entailing; i) tuning roughness of the bottom electrode at the angstrom scale, ii) polishing of the top-electrode, and iii) deploying statistical analysis to uncover interface effects. In this paper, the effect of these modifications on SAM and associated charge transport behavior will be discussed.

Bottom-up, Robust Graphene Ribbon Electronics in All-Carbon Molecular Junctions.

Large-area molecular electronic junctions consisting of 5-carbon wide graphene ribbons (GR) with lengths of 2-12 nm between carbon electrodes were fabricated by electrochemical reduction of diazotized 1,8-diaminonaphthalene. Their conductance greatly exceeds that observed for other molecular junctions of similar thicknesses, by a factor of >1 × 104 compared to polyphenylenes and >1 × 107 compared to alkane chains. The remarkable increase of conductance of the GR nanolayer results from (i) uninterrupted planarity of fused-arene structure affording extensive π-electron delocalization and (ii) enhanced electronic coupling of molecular layer with the carbon bottom contact by two-point covalent bonding, in agreement with DFT-based simulations.

Bottom-electrode induced defects in self-assembled monolayer (SAM)-based tunnel junctions affect only the SAM resistance, not the contact resistance or SAM capacitance.

In large area molecular junctions, defects are always present and can be caused by impurities and/or defects in the electrode materials and/or SAMs, but how they affect the electrical characteristics of junctions has rarely been studied. Usually, junctions are characterized by two-terminal current–voltage measurements where only the total current across the junction is measured, but with these methods one cannot distinguish how the individual components of the junctions are altered by the defects. Here we show that the roughness of the bottom-electrode is a crucial factor in determining the electrical properties of self-assembled monolayer (SAM)-based junctions. We used potentiodynamic impedance spectroscopy to reveal which components of the junctions are altered by defective bottom electrodes because this method allows for direct determination of all components that impede charge transport in the equivalent circuit of the junctions. We intentionally introduced defects via the roughness of the bottom electrode and found that these defects lower the SAM resistance but they do not alter the capacitance of the SAM or the contact resistance of the junction. In other words, defective junctions can be seen as “leaky capacitors” resulting in an underestimation of the SAM resistance of two orders of magnitude. These results help to improve the interpretation of data generated by SAM-based junctions and explain in part the observed large spread of reported tunneling rates for the same molecules measured across different platforms.

Electrochemistry does the impossible: Robust and reliable large area molecular junctions

This communication describes the advances made in the past five years in molecular electronic when the electroreduction of diazonium compounds is used to attach a molecular layer on a bottom electrode and to generate molecular junctions through fully CMOS compatible processes. It describes some of the many electronic functions that can be realized and gives some perspectives.

A study on electrical contact at the PEDOT:PSS electrode/molecule interface in large-area molecular junctions

We have investigated the effect of the PEDOT:PSS electrode/molecule contact on the charge transport characteristics of large-area molecular junctions. We incorporated two different benzenethiolate molecules into the molecular junctions: 4-methylbenzenethiol (MBT) and benzene- 1,4-dithiol (BDT). They have an identical backbone structure but different top end-groups. From statistical analysis, we found that the tunneling transport behavior showed a significant difference between the two prototype conjugated molecules, which was attributed to their distinct electrical contacts at the PEDOT:PSS electrode/molecule interface. We also observed an enhancement of the junction conductance at elevated temperatures, which would be caused by the increased grain size of the conducting PEDOT-rich cores and the removal of residual solvents or water in the junctions.

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $(<80\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $(0--400\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$ where infrared spectroscopy is lacking.

Chemical Modification of Semiconductor Surfaces for Molecular Electronics.

Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current-voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers. Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another one, concerning the formation of large-area molecular junctions and charge transport that is dominated solely by molecules.

Electrografted monolayer based on a naphthalene diimide-ruthenium terpyridine complex dyad: efficient creation of large-area molecular junctions with high current densities.

An electron donor-acceptor dyad has been designed for the creation of large-area molecular junctions (MJ). Diazonium cation electrografting was used to form well controlled monolayers. The robustness of the monolayer enabled the creation of MJs using direct top-coat evaporation with a high yield of operating devices.

Monitoring of Energy Conservation and Losses in Molecular Junctions through Characterization of Light Emission

Emission of visible light from large area molecular junctions provides a direct measure of the energy of carriers when they encounter a conducting contact and stimulate photon emission. For carbon/molecule/carbon molecular junctions containing aromatic molecular layers with thicknesses less than 5 nm, transport is elastic, and the maximum emitted photon energy (i.e., “cut‐off” energy, hvco) is equal to eVapp, where Vapp is the bias across the molecular junction. hvco increases monotonically with Vapp, is symmetric with polarity, but is weakly dependent on the nature of the contact material. Light emission from molecular junctions containing oligomeric films of anthraquinone, nitroazobenzene, naphthalene diimide, and bis‐thienyl benzene with thicknesses of 4.5–59 nm is observed as a function of bias. For layers thicker than 5–7 nm, hvco < eVapp, indicating loss of energy and therefore inelastic transport. The energy loss depends strongly on molecular structure and is linear with molecular layer thickness. When the molecular layer thickness exceeds 5–7 nm, the results provide strong evidence for a transition from elastic to inelastic transport and for stepwise, activationless transport up to 65 nm molecular layer thicknesses. Such information proves valuable for determining transport mechanisms and ultimately designing molecular junctions with desirable electronic properties.

Pronounced Environmental Effects on Injection Currents in EGaIn Tunneling Junctions Comprising Self-Assembled Monolayers.

Large-area tunneling junctions using eutectic Ga–In (EGaIn) as a top contact have proven to be a robust, reproducible, and technologically relevant platform for molecular electronics. Thus far, the majority of studies have focused on saturated molecules with backbones consisting mainly of alkanes in which the frontier orbitals are either highly localized or energetically inaccessible. We show that self-assembled monolayers of wire-like oligophenyleneethynylenes (OPEs), which are fully conjugated, only exhibit length-dependent tunneling behavior in a low-O2 environment. We attribute this unexpected behavior to the sensitivity of injection current on environment. We conclude that, contrary to previous reports, the self-limiting layer of Ga2O3 strongly influences transport properties and that the effect is related to the wetting behavior of the electrode. This result sheds light on the nature of the electrode–molecule interface and suggests that adhesive forces play a significant role in tunneling charge-transport in large-area molecular junctions.

Robust All-Carbon Molecular Junctions on Flexible or Semi-Transparent Substrates Using "Process-Friendly" Fabrication.

Large area molecular junctions were fabricated on electron-beam deposited carbon (eC) surfaces with molecular layers in the range of 2-5.5 nm between conducting, amorphous carbon contacts. Incorporating eC as an interconnect between Au and the molecular layer improves substrate roughness, prevents electromigration and uses well-known electrochemistry to form a covalent C-C bond to the molecular layer. Au/eC/anthraquinone/eC/Au junctions were fabricated on Si/SiOx with high yield and reproducibility and were unchanged by 10(7) current-voltage cycles and temperatures between 80 and 450 K. Au/eC/AQ/eC/Au devices fabricated on plastic films were unchanged by 10(7) current density vs bias voltage (J-V) cycles and repeated bending of the entire assembled junction. The low sheet resistance of Au/eC substrates permitted junctions with sufficiently transparent electrodes to conduct Raman or UV-vis absorption spectroscopy in either reflection or transmission geometries. Lithographic patterning of Au/eC substrates permitted wafer-scale integration yielding 500 devices on 20 chips on a 100 mm diameter wafer. Collectively, eC on Au provides a platform for fabrication and operation of chemically stable, optically and electrically functional molecules on rigid or flexible materials. The relative ease of processing and the robustness of molecular junctions incorporating eC layers should help address the challenge of economic fabrication of practical, flexible molecular junctions for a potentially wide range of applications.

Musical molecules: the molecular junction as an active component in audio distortion circuits

Molecular junctions that have a non-linear current–voltage characteristic consistent with quantum mechanical tunneling are demonstrated as analog audio clipping elements in overdrive circuits widely used in electronic music, particularly with electric guitars. The performance of large-area molecular junctions fabricated at the wafer level is compared to currently standard semiconductor diode clippers, showing a difference in the sound character. The harmonic distributions resulting from the use of traditional and molecular clipping elements are reported and discussed, and differences in performance are noted that result from the underlying physics that controls the electronic properties of each clipping component. In addition, the ability to tune the sound using the molecular junction is demonstrated. Finally, the hybrid circuit is compared to an overdriven tube amplifier, which has been the standard reference electric guitar clipped tone for over 60 years. In order to investigate the feasibility of manufacturing molecular junctions for use in commercial applications, devices are fabricated using a low-density format at the wafer level, where 38 dies per wafer, each containing two molecular junctions, are made with exceptional non-shorted yield (99.4%, representing 718 out of 722 tested devices) without requiring clean room facilities.

Stereo-Electronic Effects in Tunneling Junctions: Revisiting the Platform

Stereo-electronic effects in through-bond tunneling in large area molecular junctions have not been widely studied, largely due to a lack of a clear understanding of basic interfacial properties of self-assembled monolayers (SAMs) — the platform on which these junctions are built. Due to a large volume of work in SAMs, it has been largely assumed that their characteristics are well understood, among these, the so-called odd-even effect. This report demonstrates that there is still a lot to be done, or revisited, with regards to SAM formation, characterization, and, potential, if molecular junctions are to emerge as a useful technology. To illustrate this point, the Whitesides-Porter odd-even quandary of the early 90s (in SAM hydrophobicity) is revisited with a new set of data. An overview of recent efforts in our group to improve quality of tunneling junctions is also presented from three key perspective, viz; i) Surface: optimizing the surface roughness (Au, RMS roughness <2Å). ii) Interface: studies on the properties of the unbound SAM interface. iii) Molecule: Engineering the stereo-electronic properties to reveal the role of conformational effects on tunneling currents.

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.

Tuning the thickness of electrochemically grafted layers in large area molecular junctions

We have investigated the thickness, the surface roughness, and the transport properties of oligo(1-(2-bisthienyl)benzene) (BTB) thin films grafted on evaporated Au electrodes, thanks to a diazonium-based electro-reduction process. The thickness of the organic film is tuned by varying the number of electrochemical cycles during the growth process. Atomic force microscopy measurements reveal the evolution of the thickness in the range of 2–27 nm. Its variation displays a linear dependence with the number of cycles followed by a saturation attributed to the insulating behavior of the organic films. Both ultrathin (2 nm) and thin (12 and 27 nm) large area BTB-based junctions have then been fabricated using standard CMOS processes and finally electrically characterized. The electronic responses are fully consistent with a tunneling barrier in case of ultrathin BTB film whereas a pronounced rectifying behavior is reported for thicker molecular films.

Preparation of large-area molecular junctions with metallic conducting Langmuir–Blodgett films

Metallic conducting Langmuir–Blodgett (LB) films were used as soft electrodes to fabricate molecular junctions with self-assembled monolayers (SAMs) of alkanethiols (CH3(CH2)n−1SH) on an Au surface. Alkanethiols can form highly ordered, stable dielectric SAMs on metal surfaces over large areas. However, it is difficult to establish electrical contacts on such SAMs, which has limited their application. In this work, we used metallic conducting LB films composed of bis(ethylenedioxy)tetrathiafulvalene and stearic acid as a soft electrode onto alkanethiol SAMs (Cn-SAM, n=12, 14, 16, 18) to prepare Au/SAM/metal junctions of relatively large size (~15.6×103μm2). The current density–voltage (J–V) characteristics across the junctions exhibited rectifying behavior with a ratio R of ~5 (R=|J(V)|/|J(−V)| at ±1V). The lower transfer rate corresponding to the electron transport from Au to the LB films exhibited nonlinear J–V characteristics, while the higher transfer rate of electrons from the LB film to Au showed linear J–V characteristics. Kelvin probe force microscopy revealed that the work function of the metallic LB films was smaller than that of Au. The observed rectification behavior is probably caused by different electron transport mechanisms between the two current directions.