Carbon-Based Molecular Junctions for Practical Molecular Electronics.

The field of molecular electronics has grown rapidly since its experimental realization in the late 1990s, with thousands of publications on how molecules can act as circuit components and the possibility of extending microelectronic miniaturization. Our research group developed molecular junctions (MJs) using conducting carbon electrodes and covalent bonding, which provide excellent temperature tolerance and operational lifetimes. A carbon-based MJ based on quantum mechanical tunneling for electronic music represents the world's first commercial application of molecular electronics, with >3000 units currently in consumer hands. The all-carbon MJ consisting of aromatic molecules and oligomers between vapor-deposited carbon electrodes exploits covalent, C-C bonding which avoids the electromigration problem of metal contacts. The high bias and temperature stability as well as partial transparency of the all-carbon MJ permit a wide range of experiments to determine charge transport mechanisms and observe photoeffects to both characterize and stimulate operating MJs. As shown in the Conspectus figure, our group has reported a variety of electronic functions, many of which do not have analogs in conventional semiconductors. Much of the described research is oriented toward the rational design of electronic functions, in which electronic characteristics are determined by molecular structure.In addition to the fabrication of molecular electronic devices with sufficient stability and operating life for practical applications, our approach was directed at two principal questions: how do electrons move through molecules that are components of an electronic circuit, and what can we do with molecules that we cannot do with existing semiconductor technology? The central component is the molecular junction consisting of a 1-20+ nm layer of covalently bonded oligomers between two electrodes of conducting, mainly sp2-hybridized carbon. In addition to describing the unique junction structure and fabrication methods, this Account summarizes the valuable insights available from photons used both as probes of device structure and dynamics and as prods to stimulate resonant transport through molecular orbitals.Short-range (<5 nm) transport by tunneling and its properties are discussed separately from the longer-range transport (5-60 nm) which bridges the gap between tunneling and transport in widely studied organic semiconductors. Most molecular electronic studies deal with the <5 nm thickness range, where coherent tunneling is generally accepted as the dominant transport mechanism. However, the rational design of devices in this range by changing molecular structure is frustrated by electronic interactions with the conducting contacts, resulting in weak structural effects on electronic behavior. When the molecular layer thickness exceeds 5 nm, transport characteristics change completely since molecular orbitals become the conduits for transport. Incident photons can stimulate transport, with the observed photocurrent tracking the absorption spectrum of the molecular layer. Low-temperature, activationless transport of photogenerated carriers is possible for up to at least 60 nm, with characteristics completely distinct from coherent tunneling and from the hopping mechanisms proposed for organic semiconductors. The Account closes with examples of phenomena and applications enabled by molecular electronics which may augment conventional microelectronics with chemical functions such as redox charge storage, orbital transport, and energy-selective photodetection.

Evaluation of Carbon Based Molecular Junctions as Practical Photosensors.

Molecular junctions with partially transparent top contacts permit monitoring photocurrents as probes of transport mechanism and potentially could act as photosensors with characteristics determined by the molecular layer inside the device. Previously reported molecular junctions containing nitroazobenzene (NAB) oligomers and oligomers of two different aromatic molecules in bilayers were evaluated for sensitivity, dark signal, responsivity, and limits of detection, in order to determine the device parameters which have the largest effects on photodetection performance. The long-range transport of photogenerated charge carriers permits the use of molecular layers thick enough to absorb a large fraction of the light incident on the layer. Thick layers also reduce the dark current and its associated noise, thus improving the limit of detection to a few nanowatts on a detector area of 0.00125 cm2. Since the photocurrents have much lower activation energy than dark currents do, lowering the detector temperature significantly improves the limit of detection, although the present experiments were limited by environmental and instrumentation noise rather than detector noise. The highest specific detectivity (D*) for the current molecular devices was 3 × 107 cm s1/2 /W (∼109, if only shot noise is considered) at 407 nm in a carbon/NAB/carbon junction with a molecular layer thickness of 28 nm. Although this is in the low end of the 106-1012 range for commonly used photodetectors, improvements in device design based on the current results should increase D* by 3-4 orders of magnitude, while preserving the wavelength selectivity and tunability associated with molecular absorbers. In addition, operation outside the 300-1000 nm range of silicon detectors and very low dark currents may be possible with molecular junctions.

Comment on "Extent of conjugation in diazonium-derived layers in molecular junction devices determined by experiment and modelling" by C. Van Dyck, A. J. Bergren, V. Mukundan, J. A. Fereiro and G. A. DiLabio, Phys. Chem. Chem. Phys., 2019, 21, 16762.

Misinterpretation of scanning tunnelling microscopy results yielded incorrect conclusions about the flatness of a carbon electrode substrate used for molecular electronic devices. Furthermore, the results are not supported statistically and likely not representative of materials used in numerous publications.

Photostimulated Near-Resonant Charge Transport over 60 nm in Carbon-Based Molecular Junctions.

The bias and temperature dependence of both dark and photoinduced currents in carbon-based molecular junctions were examined over a wide range of oligomeric layer thickness (d) values from 4 to 60 nm. The dark current density versus bias (JV) response of nitroazobenzene molecular junctions exhibits the exponential thickness dependence consistent with coherent tunneling when d < 5 nm, but becomes weakly dependent on d and temperature (T) for d = 15-60 nm. The photocurrent (PC) response is orders of magnitude higher than the dark current for the same d and bias, with very different curve shape and much earlier onset with bias. Although the dark and PC differed greatly in magnitude for d > 14 nm, they both exhibit near zero attenuation coefficients (ß < 0.05 nm-1) and are activationless (Eact < 5 meV) below ∼200 K. For d > 14 nm, both dark and PC become electric field (E) dependent and exhibit approximate overlap of J versus E response for d = 14-60 nm. The value of ln J versus E1/2 is linear for both PC and dark current, with very different magnitudes and slopes. We propose an orbital mediated transport for PC, which involves sequential tunneling of photogenerated electrons and holes between frontier orbitals of adjacent, weakly interacting oligomeric subunits. Such transport is "bulk-limited", E dependent, and nearly activationless due to small tunneling barriers and short distances between adjacent molecular orbitals. In contrast, the dark current is activated and injection limited due to an interfacial energy barrier much larger than that for bulk transport in the junction interior. Rapid, low-barrier transport between orbitals in adjacent molecules should significantly extend the "range" of molecular electronics to >50 nm and avoid the usually strong temperature dependence observed in thicker organic films.

Ion-Assisted Resonant Injection and Charge Storage in Carbon-Based Molecular Junctions.

Resonant injection and resulting charge storage were examined in a large-area carbon/tetraphenylporphyrin(TPP)/LiF/carbon junction, where the LiF layer provides mobile ions in acetonitrile (ACN) vapor. Resonant electron transfer into TPP molecules occurs at <+1 V in the presence of mobile ions, enabled by ionic screening of the carbon electrode. Injection of holes, i.e. formation of the TPP radical cation, inside the junction was monitored by in situ photocurrent measurements. Following the injection, despite the lack of a redox counter-reaction or conventional electrolyte, persistent faradaic current peaks dominate the IV cycle of the junction (±2 V) in ACN vapor, enhancing the reversible charge storage by a factor of 78 compared to that in vacuum.

Evaluation of the electroanalytical performance of carbon-on-gold films prepared by electron-beam evaporation.

Carbon film electrodes can often be used without pretreatment, and their fabrication allows for flexibility in size and shape and for mass production. In this work, we are exploring layered structures comprised of thin films of carbon on gold (eC/Au) prepared by electron-beam evaporation. These extremely flat films are not pyrolyzed and are comprised of mainly amorphous carbon but still exhibit reasonable conductivity due to the underlying gold layer. eC/Au electrodes, without any pretreatment, yield similar heterogeneous electron-transfer rates for benchmark redox systems and significantly lower background current in comparison with polished glassy carbon. Interestingly, they show insignificant adsorption of quinones, which is uncommon for most carbon electrode materials. However, eC/Au is still prone to adsorption of airborne hydrocarbons when exposed to ambient air like most graphitic materials. With reproducibly fast electron transfer kinetics, low background current, negligible adsorption, and ultraflat surface, eC/Au films are a promising candidate for electrochemical and sensor applications.

Introducing mesoscopic charge transfer rates into molecular electronics.

It has been demonstrated that mesoscopic rates operate in nanoscale electrochemical systems and, from a fundamental point of view, are able to establish a bridge between electrochemical and molecular electronic concepts. In the present work we offer additional experimental evidence in support of this statement.

Redox Flow Batteries: How to Determine Electrochemical Kinetic Parameters.

Redox flow batteries (RFBs) are promising energy storage candidates for grid deployment of intermittent renewable energy sources such as wind power and solar energy. Various new redox-active materials have been introduced to develop cost-effective and high-power-density next-generation RFBs. Electrochemical kinetics play critical roles in influencing RFB performance, notably the overpotential and cell power density. Thus, determining the kinetic parameters for the employed redox-active species is essential. In this Perspective, we provide the background, guidelines, and limitations for a proposed electrochemical protocol to define the kinetics of redox-active species in RFBs.

Large Capacity Enhancement of Carbon Electrodes by Solution Processing for High Density Energy Storage.

An inexpensive, solution phase modification of flat carbon electrodes by electrochemical reactions of a 1,8-diaminonaphthalene derivative results in a 120- to 700-fold increase in capacity by formation of a 15-22 nm thick organic film. Modification of high surface area carbon electrodes with the same protocol resulted in a 12- to 82-fold increase in capacity. The modification layer contains 9-15% nitrogen present as -NH- redox centers that result in a large Faradaic component involving one H+ ion for each electron. The electrodes showed no capacity loss after prolonged cycling in 0.1 M H2SO4 and exhibited significantly higher charge density than similar reported electrodes based on graphene and polyaniline. Investigation of the deposition conditions revealed that N-doped oligomeric ribbons are formed both by diazonium ion reduction and diaminonaphthalene oxidation, and the 1,8 isomer is essential for the large capacity increases. The capacity increase has at least three contributions: increased microscopic surface area from ribbon formation, Faradaic reactions of nitrogen-containing redox centers, and changes in ribbon conductivity resulting from polaron formation. An aqueous fabrication process was developed which both increased capacity and improved stability and was amenable to industrial production. The high charge density, low-cost fabrication, and <25 nm thickness of the diaminonaphthalene-derived films should prove attractive toward practical application on both flat surfaces and in high surface area carbon electrodes.

Hubbard Nonequilibrium Green's Function Analysis of Photocurrent in Nitroazobenzene Molecular Junction.

We present a combined experimental and theoretical study of photoinduced current in molecular junctions consisting of monolayers of nitroazobenzene oligomers chemisorbed on carbon surfaces and illuminated by ultraviolet-visible light through a transparent electrode. Experimentally observed dependence of the photocurrent on light frequency, temperature, and monolayer thickness is analyzed within first-principles simulations employing the Hubbard nonequilibrium Green's function diagrammatic technique. We reproduce qualitatively correct behavior and discuss mechanisms leading to the characteristic behavior of dark and photoinduced currents in response to changes in bias, frequency of radiation, temperature, and thickness of molecular layer.

Long-Range Activationless Photostimulated Charge Transport in Symmetric Molecular Junctions.

Molecular electronic junctions consisting of nitroazobenzene oligomers covalently bonded to a conducting carbon surface using an established "all-carbon" device design were illuminated with UV-vis light through a partially transparent top electrode. Monitoring junction conductance with a DC bias imposed permitted observation of photocurrents while varying the incident wavelength, light intensity, molecular layer thickness, and temperature. The photocurrent spectrum tracked the in situ absorption spectrum of nitroazobenzene, increased linearly with light intensity, and depended exponentially on applied bias. The electronic characteristics of the photocurrent differed dramatically from those of the same device in the dark, with orders of magnitude higher conductance and very weak attenuation with molecular layer thickness (ß = 0.14 nm-1 for thickness above 5 nm). The temperature dependence of the photocurrent was opposite that of the dark current, with a 35% decrease in conductance between 80 and 450 K, while the dark current increased by a factor of 4.5 over the same range. The photocurrent was similar to the dark current for thin molecular layers but greatly exceeded the dark current for low bias and thick molecular layers. We conclude that the light and dark mechanisms are additive, with photoexcited carriers transported without thermal activation for a thickness range of 5-10 nm. The inverse temperature dependence is likely due to scattering or recombination events, both of which increase with temperature and in turn decrease the photocurrent. Photostimulated resonant transport potentially widens the breadth of conceivable molecular electronic devices and may have immediate value for wavelength-specific photodetection.

Self-Inhibitory Electron Transfer of the Co(III)/Co(II)-Complex Redox Couple at Pristine Carbon Electrode.

Applications of conducting carbon materials for highly efficient electrochemical energy devices require a greater fundamental understanding of heterogeneous electron-transfer (ET) mechanisms. This task, however, is highly challenging experimentally, because an adsorbing carbon surface may easily conceal its intrinsic reactivity through adventitious contamination. Herein, we employ nanoscale scanning electrochemical microscopy (SECM) and cyclic voltammetry to gain new insights into the interplay between heterogeneous ET and adsorption of a Co(III)/Co(II)-complex redox couple at the contamination-free surface of electron-beam-deposited carbon (eC). Specifically, we investigate the redox couple of tris(1,10-phenanthroline)cobalt(II), Co(phen)32+, as a promising mediator for dye-sensitized solar cells and redox flow batteries. A pristine eC surface overlaid with KCl is prepared in vacuum, protected from contamination in air, and exposed to an ultrapure aqueous solution of Co(phen)32+ by the dissolution of the protective KCl layer. We employ SECM-based nanogap voltammetry to quantitatively demonstrate that Co(phen)32+ is adsorbed on the pristine eC surface to electrostatically self-inhibit outer-sphere ET of nonadsorbed Co(phen)33+ and Co(phen)32+. Strong electrostatic repulsion among Co(phen)32+ adsorbates is also demonstrated by SECM-based nanogap voltammetry and cyclic voltammetry. Quantitatively, self-inhibitory ET is characterized by a linear decrease in the standard rate constant of Co(phen)32+ oxidation with a higher surface concentration of Co(phen)32+ at the formal potential. This unique relationship is consistent not with the Frumkin model of double layer effects, but with the Amatore model of partially blocked electrodes as extended for self-inhibitory ET. Significantly, the complicated coupling of electron transfer and surface adsorption is resolved by combining nanoscale and macroscale voltammetric methods.

Internal Electric Field Modulation in Molecular Electronic Devices by Atmosphere and Mobile Ions.

The internal potential profile and electric field are major factors controlling the electronic behavior of molecular electronic junctions consisting of ∼1-10 nm thick layers of molecules oriented in parallel between conducting contacts. The potential profile is assumed linear in the simplest cases, but can be affected by internal dipoles, charge polarization, and electronic coupling between the contacts and the molecular layer. Electrochemical processes in solutions or the solid state are entirely dependent on modification of the electric field by electrolyte ions, which screen the electrodes and form the ionic double layers that are fundamental to electrode kinetics and widespread applications. The current report investigates the effects of mobile ions on nominally solid-state molecular junctions containing aromatic molecules covalently bonded between flat, conducting carbon surfaces, focusing on changes in device conductance when ions are introduced into an otherwise conventional junction design. Small changes in conductance were observed when a polar molecule, acetonitrile, was present in the junction, and a large decrease of conductance was observed when both acetonitrile (ACN) and lithium ions (Li+) were present. Transient experiments revealed that conductance changes occur on a microsecond-millisecond time scale, and are accompanied by significant alteration of device impedance and temperature dependence. A single molecular junction containing lithium benzoate could be reversibly transformed from symmetric current-voltage behavior to a rectifier by repetitive bias scans. The results are consistent with field-induced reorientation of acetonitrile molecules and Li+ ion motion, which screen the electrodes and modify the internal potential profile and provide a potentially useful means to dynamically alter junction electronic behavior.

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.

Orbital Control of Photocurrents in Large Area All-Carbon Molecular Junctions.

Photocurrents generated by illumination of carbon-based molecular junctions were investigated as diagnostics of how molecular structure and orbital energies control electronic behavior. Oligomers of eight aromatic molecules covalently bonded to an electron-beam deposited carbon surface were formed by electrochemical reduction of diazonium reagents, with layer thicknesses in the range of 5-12 nm. Illumination through either the top or bottom partially transparent electrodes produced both an open circuit potential (OCP) and a photocurrent (PC), and the polarity and spectrum of the photocurrent depended directly on the relative positions of the frontier orbitals and the electrode Fermi level (EF). Electron donors with relatively high HOMO energies yielded positive OCP and PC, and electron acceptors with LUMO energies closer to EF than the HOMO energy produced negative OCP and PC. In all cases, the PC spectrum and the absorption spectrum of the oligomer in the molecular junction had very similar shapes and wavelength maxima. Asymmetry of electronic coupling at the top and bottom electrodes due to differences in bonding and contact area cause an internal potential gradient which controls PC and OCP polarities. The results provide a direct indication of which orbital energies are closest to EF and also indicate that transport in molecular junctions thicker than 5 nm is controlled by the difference in energy of the HOMO and LUMO orbitals.

Nanometric building blocks for robust multifunctional molecular junctions.

Much of the motivation for developing molecular electronic devices is the prospect of achieving novel electronic functions by varying molecular structure. We describe a "building block" approach for molecular junctions resulting in one, two or three nanometer-thick molecular layers in a commercially proven junction design. A single layer of anthraquinone between carbon electrodes provides a tunnel device with applications in electronic music, and a second layer of a thiophene derivative yields a molecular rectifier with quite different audio characteristics. A third layer of lithium benzoate produces a redox-active device with possible applications in non-volatile memory devices or on-chip energy storage. The building block approach forms a basis for "rational design" of electronic functions, in which layers of varying structure produce distinct and desirable electronic behaviours.

Ultraflat, Pristine, and Robust Carbon Electrode for Fast Electron-Transfer Kinetics.

Electron-beam (e-beam) deposition of carbon on a gold substrate yields a very flat (0.43 nm of root-mean-square roughness), amorphous carbon film consisting of a mixture of sp2- and sp3-hybridized carbon with sufficient conductivity to avoid ohmic potential error. E-beam carbon (eC) has attractive properties for conventional electrochemistry, including low background current and sufficient transparency for optical spectroscopy. A layer of KCl deposited by e-beam to the eC surface without breaking vacuum protects the surface from the environment after fabrication until dissolved by an ultrapure electrolyte solution. Nanogap voltammetry using scanning electrochemical microscopy (SECM) permits measurement of heterogeneous standard electron-transfer rate constants (k°) in a clean environment without exposure of the electrode surface to ambient air. The ultraflat eC surface permitted nanogap voltammetry with very thin electrode-to-substrate gaps, thus increasing the diffusion limit for k° measurement to >14 cm/s for a gap of 44 nm. Ferrocene trimethylammonium as the redox mediator exhibited a diffusion-limited k° for the previously KCl-protected eC surface, while k° was 1.45 cm/s for unprotected eC. The k° for Ru(NH3)63+/2+ increased from 1.7 cm/s for unprotected eC to above the measurable limit of 6.9 cm/s for a KCl-protected eC electrode.

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature.

Thin layers of oligomers with thickness between 7 and 9 nm were deposited on flat gold electrode surfaces by electrochemical reduction of diazonium reagents, then a Ti(2 nm)/Au top contact was applied to complete a solid-state molecular junction. The molecular layers investigated included donor molecules with relatively high energy HOMO, molecules with high HOMO-LUMO gaps and acceptor molecules with low energy LUMO and terminal alkyl chain. Using an oligo(bisthienylbenzene) based layer, a molecule whose HOMO energy level in a vacuum is close to the Fermi level of the gold bottom electrode, the devices exhibit robust and highly reproducible rectification ratios above 1000 at low voltage (2.7 V). Higher current is observed when the bottom gold electrode is biased positively. When the molecular layer is based on a molecule with a high HOMO-LUMO gap, i.e., tetrafluorobenzene, no rectification is observed, while the direction of rectification is reversed if the molecular layer consists of naphtalene diimides having low LUMO energy level. Rectification persisted at low temperature (7 K), and was activationless between 7 and 100 K. The results show that rectification is induced by the asymmetric contact but is also directly affected by orbital energies of the molecular layer. A "molecular signature" on transport through layers with thicknesses above those used when direct tunneling dominates is thus clearly observed, and the rectification mechanism is discussed in terms of Fermi level pinning and electronic coupling between molecules and contacts.

Robust Bipolar Light Emission and Charge Transport in Symmetric Molecular Junctions.

Molecular junctions consisting of a Ru(bpy)3 oligomer between conducting carbon contacts exhibit an exponential dependence of junction current on molecular layer thickness (d) similar to that observed for other aromatic devices when d < 4 nm. However, when d > 4 nm, a change in transport mechanism occurs which coincides with light emission in the range of 600-900 nm. Unlike light emission from electrochemical cells or solid-state films containing Ru(bpy)3, emission is bipolar, occurs in vacuum, has rapid rise time (<5 ms), and persists for >10 h. Light emission directly indicates simultaneous hole and electron injection and transport, possibly resonant due to the high electric field present (>3 MV/cm). Transport differs fundamentally from previous tunneling and hopping mechanisms and is a clear "molecular signature" relating molecular structure to electronic behavior.

Characterization of Growth Patterns of Nanoscale Organic Films on Carbon Electrodes by Surface Enhanced Raman Spectroscopy.

Electrochemical deposition of aromatic organic molecules by reduction of diazonium reagents enables formation of molecular layers with sufficient integrity for use in molecular electronic junctions of interest to microelectronics. Characterization of organic films with thicknesses in the 1-10 nm range is difficult with Raman spectroscopy, since most molecular structures of electronic interest have Raman cross sections which are too small to observe as either thin films on solid electrodes or within intact molecular junctions. Layer formation on a 10 nm thick Ag island film on a flat carbon surface (eC/Ag) permitted acquisition of structural information using surface enhanced Raman spectroscopy (SERS), in many cases for molecules with weak Raman scattering. Raman spectra obtained on eC/Ag surfaces were indistinguishable from those on carbon without Ag present, and the spectra of oligomeric molecular layers were completely consistent with those of the monomers. Layer growth was predominantly linear for cases where such growth was sterically allowed, and linear growth correlated strongly with the line width and splitting of the C═C phenyl ring stretches. Molecular bilayers made by successive reduction of different diazonium reagents were also observable and will be valuable for applications of 1-20 nm organic films in molecular electronics.