Carbon-based Molecular Junctions Research Articles

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.

Electronic transport in molecular junctions: The generalized Kadanoff-Baym ansatz with initial contact and correlations.

The generalized Kadanoff-Baym ansatz (GKBA) offers a computationally inexpensive approach to simulate out-of-equilibrium quantum systems within the framework of nonequilibrium Green's functions. For finite systems, the limitation of neglecting initial correlations in the conventional GKBA approach has recently been overcome [Karlsson et al., Phys. Rev. B 98, 115148 (2018)]. However, in the context of quantum transport, the contacted nature of the initial state, i.e., a junction connected to bulk leads, requires a further extension of the GKBA approach. In this work, we lay down a GKBA scheme that includes initial correlations in a partition-free setting. In practice, this means that the equilibration of the initially correlated and contacted molecular junction can be separated from the real-time evolution. The information about the contacted initial state is included in the out-of-equilibrium calculation via explicit evaluation of the memory integral for the embedding self-energy, which can be performed without affecting the computational scaling with the simulation time and system size. We demonstrate the developed method in carbon-based molecular junctions, where we study the role of electron correlations in transient current signatures.

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 (β < 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.

Molecular spin–photovoltaic device based on a graphene nanoflake

We generate a spin-polarized current in a hexagonal zigzag graphene nanoflake (hZGNF) by integrating the photovoltaic and spin-dependent transport effects. We consider three different hZGNF configurations and simulate their spin–photovoltaic properties using two probe models in the presence of ferromagnetic contacts as well as the magnetization of zigzag edges. Our results reveal acceptable spin-dependent quantum efficiency, full optical spin polarization, and good optically induced magnetoresistance up to 900%, which can be modified by adjusting the photon energy, by varying the configuration, and also by introducing monovacancy. Interestingly, switching the magnetization of ferromagnetic contacts can approximately invert the spin characteristic of the photocurrent, and so the sign of optical spin polarization. Our findings may provide an efficient way to enhance radiation-induced magnetoresistance in carbon-based molecular junctions.

Unipolar Injection and Bipolar Transport in Electroluminescent Ru-Centered Molecular Electronic Junctions

Bias-induced light emission and light-induced photocurrents were used as independent probes of charge transport in carbon-based molecular junctions containing Ru(bpy)3. The thickness, bias, and tem...

(Invited) Spectroscopy and Photocurrents in All-Carbon Molecular Electronic Devices

One or both contacts in all-carbon molecular junctions can be sufficiently transparent to permit optical spectroscopy as a probe of device structure, and photocurrent generation to investigate photon-induced electron transport. Current-voltage (JV) curves for aromatic molecules depends strongly on molecular structure when molecular layers are more than 5 nm thick, 1 and photocurrents were used to determine structural factors determining JV behavior. 2 As shown in the left figure below, the photocurrent spectrum tracks the molecular absorption spectrum determined directly in the completed molecular junction. A molecular bilayer consisting of an electron donor and acceptor layers yields significantly higher photocurrents, which depend on the order of the layer deposition (right image below). 3 The use of photocurrents and Raman spectroscopy for characterization of molecular junction structure and operation will be discussed. 4 (1) Morteza Najarian, A.; McCreery, R. L.; Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions; ACS Nano 2017, 11, 3542. (2) 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. (3) Smith, S. R.; McCreery, R.; Photocurrent, Photovoltage and Rectification in Large-Area Bilayer Molecular Electronic Junctions submitted 2018. (4) 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. Figure 1

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

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.

Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions

Carbon-based molecular junctions consisting of aromatic oligomers between conducting sp2 hybridized carbon electrodes exhibit structure-dependent current densities (J) when the molecular layer thickness (d) exceeds ∼5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically by a factor of 2 over the range of 200-440 K. Fluorene and anthraquinone show linear plots of ln J vs d with nearly identical J values for the range d = 3-10 nm, despite significant differences in their free-molecule orbital energy levels. The observed current densities for anthraquinone, fluorene, nitroazobenzene, and bis-thienyl benzene for d = 7-10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV-vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV-vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO-LUMO energy gap. We conclude that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO-LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial "injection" at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the "bulk" properties of the molecular layer interior.

Effects of electronic coupling and electrostatic potential on charge transport in carbon-based molecular electronic junctions.

Molecular junctions consisting of 2–20 nm thick layers of organic oligomers oriented between a conducting carbon substrate and a carbon/gold top contact have proven to be reproducible and reliable, and will soon enter commercial production in audio processing circuits. The covalent, conjugated bond between one or both sp2-hybridized carbon contacts and an aromatic molecular layer is distinct from the more common metal/molecule or silicon/molecule structures in many reported molecular junctions. Theoretical observations based on density functional theory are presented here, which model carbon-based molecular junctions as single molecules and oligomers between fragments of graphene. Electronic coupling between the molecules and the contacts is demonstrated by the formation of hybrid orbitals in the model structure, which have significant electron density on both the graphene and the molecule. The energies of such hybrid orbitals correlate with tunneling barriers determined experimentally, and electronic coupling between the two graphene fragments in the model correlates with experimentally observed attenuation of transport with molecular layer thickness. Electronic coupling is affected significantly by the dihedral angle between the planes of the graphene and the molecular π-systems, but is absent only when the two planes are orthogonal. Coupling also results in partial charge transfer between the graphene contacts and the molecular layer, which results in a shift in electrostatic potential which affects the observed tunneling barrier. Although the degree of partial charge transfer is difficult to calculate accurately, it does provide a basis for the “vacuum level shift” observed in many experiments, including transport and ultraviolet photoelectron spectroscopy of molecular layers on conductors.

Theoretical Modeling of Tunneling Barriers in Carbon-Based Molecular Electronic Junctions

Density functional theory (DFT) is applied to three models for carbon-based molecular junctions containing fragments of graphene with covalent edge-bonding to aromatic and aliphatic molecules, with the graphene representing a sp2 hybridized carbon electrode and the molecule representing a molecular layer between two electrodes. The DFT results agree well with experimental work functions and transport barriers, including the electronic coupling between molecular layers and graphitic contacts, and predict the compression of tunnel barriers observed for both ultraviolet photoelectron spectroscopy (UPS) and experimental tunneling currents. The results reveal the strong effect of the dihedral angle between the planes of the graphene electrode and the aromatic molecule and imply that the molecules with the smallest dihedral angle are responsible for the largest local current densities. In addition, the results are consistent with the proposal that the orbitals which mediate tunneling are those with significant electron density in the molecular layer. These conclusions should prove valuable for understanding the relationships between molecular structure and electronic transport as an important step toward rational design of carbon-based molecular electronic devices.

Conductivity of carbon-based molecular junctions from ab-initio methods

Carbon nanomaterials (CNMs) are prompting candidates for next generational electronics. In this review we provide a mini overview of recent results on the conductivity of carbon-based molecular junctions obtained from ab-initio methods. CNMs used as nanoelectrodes and molecular materials in molecular junctions are discussed. The functionalities that include the nanomechanically controlled molecular conductance switches, negative differential resistance devices, and electronic rectifiers realized by using CNMs have been demonstrated.

Molecular rectification and conductance switching in carbon-based molecular junctions by structural rearrangement accompanying electron injection.

Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm(2), or about 10(5) e(-)/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e(-)/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude.

Electronic Conductance Behavior of Carbon-Based Molecular Junctions with Conjugated Structures

A novel molecular junction based on a monolayer between carbon and mercury “contacts” was investigated by examining current/voltage behavior as a function of temperature and monolayer thickness. Monolayers of phenyl, biphenyl, and terphenyl were covalently bonded to flat, graphitic carbon, then a top contact was formed with a suspended mercury drop. Similar molecular junctions were formed from multilayer nitroazobenzene (NAB) films of 30 Å and 47 Å thickness, and junctions were examined over the temperature range of +80 °C to −50 °C. Junction resistances were a strong function of molecular length and structure, with mean resistances for 0.78 mm2 junctions of 34.4 Ω, 13.8 KΩ, and 41.0 KΩ for phenyl, biphenyl, and terphenyl junctions. The i/V characteristics of biphenyl and phenyl junctions were nearly independent of temperature, while those of terphenyl and NAB junctions were temperature independent below 0 °C but thermally activated above 10 °C. The results are consistent with a tunneling process at low temperature, where the molecular conformations are apparently fixed. For the thicker terphenyl and NAB junctions, the tunneling rate is sufficiently slow to observe a thermally activated conduction process at higher temperatures. The observed activation barriers of 0.3 to 0.8 eV are in the range expected for phenyl ring rotation, implying that the coplanar conformer of terphenyl has a significantly higher conductivity. Below 0 °C, the junction is presumably “frozen”, with only a small fraction of terphenyl molecules in the conductive conformation. Calculated HOMO-LUMO gaps for the planar and twisted conformations of terphenyl predict that the planar geometry is five times more conductive than the twisted conformation. In addition to presenting a new type of molecular electronic junction, the results bear on the widespread topic of electronic conductivity of organic molecules.