Single Junctions Research Articles

Towards single molecule switches.

The concept of using single molecules as key building blocks for logic gates, diodes and transistors to perform basic functions of digital electronic devices at the molecular scale has been explored over the past decades. However, in addition to mimicking the basic functions of current silicon devices, molecules often possess unique properties that have no parallel in conventional materials and promise new hybrid devices with novel functions that cannot be achieved with equivalent solid-state devices. The most appealing example is the molecular switch. Over the past decade, molecular switches on surfaces have been intensely investigated. A variety of external stimuli such as light, electric field, temperature, tunneling electrons and even chemical stimulus have been used to activate these molecular switches between bistable or even multiple states by manipulating molecular conformations, dipole orientations, spin states, charge states and even chemical bond formation. The switching event can occur either on surfaces or in break junctions. The aim of this review is to highlight recent advances in molecular switches triggered by various external stimuli, as investigated by low-temperature scanning tunneling microscopy (LT-STM) and the break junction technique. We begin by presenting the molecular switches triggered by various external stimuli that do not provide single molecule selectivity, referred to as non-selective switching. Special focus is then given to selective single molecule switching realized using the LT-STM tip on surfaces. Single molecule switches operated by different mechanisms are reviewed and discussed. Finally, molecular switches embedded in self-assembled monolayers (SAMs) and single molecule junctions are addressed.

Gating of single molecule junction conductance by charge transfer complex formation.

The solid-state structures of organic charge transfer (CT) salts are critical in determining their mode of charge transport, and hence their unusual electrical properties, which range from semiconducting through metallic to superconducting. In contrast, using both theory and experiment, we show here that the conductance of metal |single molecule| metal junctions involving aromatic donor moieties (dialkylterthiophene, dialkylbenzene) increase by over an order of magnitude upon formation of charge transfer (CT) complexes with tetracyanoethylene (TCNE). This enhancement occurs because CT complex formation creates a new resonance in the transmission function, close to the metal contact Fermi energy, that is a signal of room-temperature quantum interference.

Negative Differential Resistance due to Nonlinearities in Single and Stacked Josephson Junctions

Josephson junction systems with a negative differential resistance (NDR) play an essential role for applications. As a well-known example, long Josephson junctions of the BSCCO type have been considered as a source of terahertz radiation in recent experiments. Numerical results for the dynamics of the fluxon system have demonstrated that a cavity induced NDR plays a crucial role for the emission of electromagnetic radiation. We consider the case of an NDR region in the McCumber curve itself of a single junction and found that it has an effect on the emission of electromagnetic radiation. Two different shapes of NDR region are considered, and we found that it is essential to distinguish between current bias and voltage bias.

Fabrication of single linear aromatic molecular junction with high formation probability

The fabrication of single molecular junctions (SMJs) with high formation probabilities and high conductance values is important to the progress of molecular electronics. The relationship between formation probability and the structure of a single molecular junction is not clear because of the lack of systematic studies. Here, we propose a guideline for the fabrication of SMJs with high formation probabilities, focusing on the nanogap and molecular sizes. The investigation of simple linear aromatic compounds, which consist of a small number of benzene rings (i.e., benzene, naphthalene, and anthracene), using Au, Ag, and Cu electrodes revealed that SMJs with high formation probabilities and high conductance values can be obtained when the molecular size is equivalent to or smaller than the nanogap size.

Quantum interference in off-resonant transport through single molecules

We provide a simple set of rules for predicting interference effects in off-resonant transport through single molecule junctions. These effects fall into two classes, showing, respectively, an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with $GW$ calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions interference causes no transmission nodes, but we argue that it may lead to a nonstandard gate dependence of the zero-bias Kondo resonance.

Thermal transport through ac-driven transparent Josephson weak links

We discuss how phase coherence manifests in the heat transport through superconducting single and multichannel Josephson junctions in time dependent situations. We consider the heat current driven through the junction by a temperature difference in dc voltage and ac phase biased situations. At low bias, the electromagnetic driving mainly modifies the form of the coherent resonance that transports a large part of the heat current, rather than simply dissipating energy in the junction. We find a description for the heat current in terms of quasiparticle n-photon absorption and emission rates, and discuss analytical and numerical results concerning them. In addition to the ensemble average heat transport, we describe also its fluctuations.

Cost analysis of roll-to-roll fabricated ITO free single and tandem organic solar modules based on data from manufacture

We present a cost analysis based on state of the art printing and coating processes to fully encapsulated, flexible ITO- and vacuum-free polymer solar cell modules. Manufacturing data for both single junctions and tandem junctions are presented and analyzed. Within this calculation the most expensive layers and processing steps are identified. Based on large roll-to-roll coating experiments the exact material consumptions were determined. In addition to the data for the pilot scale experiment presented here, projections to medium and large scale scenarios serve as a guide to achieve cost targets of 5 €ct per Wp in a detailed material and cost analysis. These scenarios include the replacement of cost intensive layers, as well as process optimization steps. Furthermore, the cost structures for single and tandem devices are listed in detail and discussed. In an optimized model the material costs drop below 10 € per m2 which proves that OPV is a competitive alternative to established power generation technologies.

Regulating a benzodifuran single molecule redox switch via electrochemical gating and optimization of molecule/electrode coupling.

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal-molecule-metal (m-M-m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (-CS2(-)) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8.

Interfacial electronic transport phenomena in single crystalline Fe-MgO-Fe thin barrier junctions

Spin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3–8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Δ1-like and Δ5-like symmetries.

Printing-based assembly of quadruple-junction four-terminal microscale solar cells and their use in high-efficiency modules.

Expenses associated with shipping, installation, land, regulatory compliance and on-going maintenance and operations of utility-scale photovoltaics can be significantly reduced by increasing the power conversion efficiency of solar modules through improved materials, device designs and strategies for light management. Single-junction cells have performance constraints defined by their Shockley-Queisser limits. Multi-junction cells can achieve higher efficiencies, but epitaxial and current matching requirements between the single junctions in the devices hinder progress. Mechanical stacking of independent multi-junction cells circumvents these disadvantages. Here we present a fabrication approach for the realization of mechanically assembled multi-junction cells using materials and techniques compatible with large-scale manufacturing. The strategy involves printing-based stacking of microscale solar cells, sol-gel processes for interlayers with advanced optical, electrical and thermal properties, together with unusual packaging techniques, electrical matching networks, and compact ultrahigh-concentration optics. We demonstrate quadruple-junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1,000 suns, and modules with efficiencies of 36.5%.

High-Conductance Conformers in Histograms of Single-Molecule Current–Voltage Characteristics

Understanding charge transport across single molecular junctions is essential for the rational design and optimization of molecular device components. However, the correlation between calculated and experimental transport properties of single molecules probed by current–voltage (I–V) characteristics is often uncertain. Part of the challenge is that molecular conductance is sensitive to several factors that are difficult to control, including molecular orientation, conformation, aggregation, and chemical stability. Other challenges include the limitations of computational methodologies. Here, we implement the Σ-Extended Huckel (EH) nonequilibrium Green’s function (NEGF) method to analyze the histogram of I–V curves of 4,4′-diaminostilbene probed by break-junction experiments. We elucidate the nature of the molecular conformations with a widespread distribution of I–V curves, typically probed under experimental conditions. We find maximum conductance for molecules that are not at the minimum energy configur...

Influence of Bound Metals on the Electrical Properties of Single Molecule Junction Porphyrin-Imides Linked to SWNTs

Since Aviram and Ratner proposed that a single molecule could act as a molecular rectifier, containing a donor and an acceptor separated by a sigma bond, many such molecules have been synthesized for use in devices. Among these, porphyrins are widely used as molecular electronic devices owing to their structural flexibility as well as their well-developed synthetic chemistry, which allows their physical and chemical properties to be adjusted, making them ideally suited for binding to metals. In this research, porphyrin-imide dyads with perpendicular arrangements containing different metals (zinc and rhodium) bound to porphyrin molecules have been synthesized (Figure 1) and the effect of these “center metals” on the electrical properties of single molecule junction porphyrin-imides connected to SWNTs have been investigated. DFT calculations for porphyrin-imide dyads revealed that the HOMO and LUMO energy levels of the dyads were completely separated, indicating the absence of any intramolecular interactions between the donor and acceptor. Furthermore, these calculations also revealed that the molecular orbital of rhodium porphyrin-imide for HOMO-2, HOMO-3, and LUMO-1 were entirely localized on the rhodium center, whereas no localization on the zinc center of the molecular orbital for zinc porphyrin-imide was observed. Based on these DFT results, it was predicted that employing zinc and rhodium as center metals would result in the corresponding dyads having different electrical properties. The connection of single molecule junctions for porphyrin-imides linked covalently to SWNTs was evidenced by the coordination of the rhodium porphyrin-imide dyad with pyridine-tert-dodec-AuNPs (~3 nm in size) used as a marker. AFM topography was employed in order to confirm that the AuNPs were attached to the molecule. Moreover, the topography revealed the presence of particles with a height of 2.6 nm, which subsequent TEM observations revealed to be part of a single-molecule junction (Figure 2c). The electrical properties of these dyads were measured by point-contact current imaging atomic force microscopy (PCI-AFM). The results for the rectification behavior of single molecule junction zinc porphyrin-imide revealed larger values, with an average rectification ratio of 26, compared to the rhodium porphyrin-imide that had an average rectification ratio of 6. This differential conductance indicated that the HOMO-LUMO gap can be controlled by changing the center metal.

Electron Transport through Single Alkanedicarboxylic Acids Junctions with Pd Electrode

not Available.

Observation of spin-dependent quantum well resonant tunneling in textured CoFeB layers

We report the observation of spin-dependent quantum well (QW) resonant tunneling in textured CoFeB free layers of single MgO magnetic tunnel junctions (MTJs). The inelastic electron tunneling spectroscopy spectra clearly show the presence of resonant oscillations in the parallel configuration, which are related with the appearance of majority-spin Δ1 QW states in the CoFeB free layer. To gain a quantitative understanding, we calculated QW state positions in the voltage-thickness plane using the so-called phase accumulation model (PAM) and compared the PAM solutions with the experimental resonant voltages observed for a set of MTJs with different CoFeB free layer thicknesses (tfl = 1.55, 1.65, 1.95, and 3.0 nm). An overall good agreement between experiment and theory was obtained. An enhancement of the tunnel magnetoresistance with bias is observed in a bias voltage region corresponding to the resonant oscillations.

Density functional theory based calculations of the transfer integral in a redox-active single-molecule junction

There are various quantum chemical approaches for an ab initio description of transfer integrals within the framework of Marcus theory in the context of electron transfer reactions. In our paper, we aim to calculate transfer integrals in redox-active single molecule junctions, where we focus on the coherent tunneling limit with the metal leads taking the position of donor and acceptor and the molecule acting as a transport mediating bridge. This setup allows us to derive a conductance, which can be directly compared with recent results from a nonequilibrium Green's function approach. Compared with purely molecular systems we face additional challenges due to the metallic nature of the leads, which rules out some of the common techniques, and due to their periodicity, which requires k-space integration. We present three different methods, all based on density functional theory, for calculating the transfer integral under these constraints, which we benchmark on molecular test systems from the relevant literature. We also discuss many-body effects and apply all three techniques to a junction with a Ruthenium complex in different oxidation states.

Investigation of multi-junction solar cells using electrostatic force microscopy methods

Multi-junction III-V solar cells are designed to have a much broader absorption of the solar spectrum than Si-based or single junctions, thus yield the highest conversion. The conversion efficiency can be further scaled with sun concentration. The ability of high conversion efficiencies makes multi-junction prime candidates for fine-tuning explorations aimed at getting closer to the theoretical efficiencies. In this paper, we report on electrostatic force microscopy (EFM) measurements of the built-in potential of multi-junction III-V semiconductor-based solar cells. Kelvin probe force microscopy (KPFM) was employed to qualitatively study the width and electrical properties of individual junctions, i.e., built-in potential, activity, and thickness of the p-n junctions. In addition, the voltage drops across individual solar cell p-n junctions were measured using Kelvin probe microscopy under various operation conditions: dark; illuminated; short-circuit; and biased. We present a method which enables the measurement of a working structure, while focusing on the electrical characteristics of an individual junction by virtue of selecting the spectral range of the illumination used. We show that these pragmatic studies can provide a feedback to improve photovoltaic device design, particularly of operation under a current mismatched situation. This new analysis technique offers additional insights into behavior of the multi-junction solar cell and shows promise for further progress in this field.

Electrical conductivity of single molecular junctions assembled from Co- and Co3C-encapsulating carbon nanocapsules.

Single molecular junctions (SMJs) were assembled from cobalt (Co)- and Co carbide (Co3C)-encapsulating carbon nanocapsules (CNCs) and two gold electrodes inside a high-resolution transmission electron microscope equipped with a specimen-piezomanipulation system. The structure and electrical transport properties of the SMJs were investigated in situ. The current density depended on the perimeter of the contact area between CNCs and the electrodes, showing that the current flowed not through the encapsulated region but rather along the graphene layers of CNCs. It was demonstrated that the properties of graphene can be applied to nanodevices using CNCs irrespective of the encapsulating materials.

Enhancement of tunnel magnetoresistance in magnetic tunnel junction by a superlattice barrier

Tunnel magnetoresistance of magnetic tunnel junction improved by a superlattice barrier composed of alternate layers of a nonmagnetic metal and an insulator is proposed. The forbidden band of the superlattice is used to predict the low transmission range in the superlattice barrier. By forbidding electron transport in the anti-parallel configuration, the tunnel magnetoresistance is enhanced in the superlattice junction. The results show that the tunnel magnetoresistance ratio for a superlattice magnetic tunnel junction is greater than that for traditional single or double barrier junctions.

Promising anchoring groups for single-molecule conductance measurements.

The understanding of the charge transport through single molecule junctions is a prerequisite for the design and building of electronic circuits based on single molecule junctions. However, reliable and robust formation of such junctions is a challenging task to achieve. In this topical review, we present a systematic investigation of the anchoring group effect on single molecule junction conductance by employing two complementary techniques, namely scanning tunneling microscopy break junction (STM-BJ) and mechanically controllable break junction (MCBJ) techniques, based on the studies published in the literature and important results from our own work. We compared conductance studies for conventional anchoring groups described earlier with the molecular junctions formed through π-interactions with the electrode surface (Au, Pt, Ag) and we also summarized recent developments in the formation of highly conducting covalent Au-C σ-bonds using oligophenyleneethynylene (OPE) and an alkane molecular backbone. Specifically, we focus on the electron transport properties of diaryloligoyne, oligophenyleneethynylene (OPE) and/or alkane molecular junctions composed of several traditional anchoring groups, (dihydrobenzo[b]thiophene (BT), 5-benzothienyl analogue (BTh), thiol (SH), pyridyl (PY), amine (NH2), cyano (CN), methyl sulphide (SMe), nitro (NO2)) and other anchoring groups at the solid/liquid interface. The qualitative and quantitative comparison of the results obtained with different anchoring groups reveals structural and mechanistic details of the different types of single molecular junctions. The results reported in this prospective may serve as a guideline for the design and synthesis of molecular systems to be used in molecule-based electronic devices.

Investigation on the Pyrazine Molecular Junction Studied by Conductance Measurement and Near Edge X-ray Absorption Fine Structure

A pyrazine molecular junction was investigated using mechanically controllable break junction (MCBJ) technique at 10 K. The conductance measurements revealed the single pyrazine molecular junctions showed two distinct conductance values of 0.27 ± 0.04 and 1.0 ± 0.2 G 0 (2e 2/h). The conductance value of the single pyrazine molecular junction was comparable with that of the metal atomic junction. The interface between pyrazine molecule and Pt surface was investigated by near edge X-ray absorption fine structure (NEXAFS). The broadening of the π* peak in N K-edge NEXAFS spectra suggested that the pyrazine molecule connected to Pt surface via a nitrogen atom. Based on the measurements of the conductance and NEXAFS, we could propose the structural models of two distinct conductance states for the single pyrazine molecular junction.