Metallic Point Contacts Research Articles

Silver electrodes for reversible oxygen sensor applications

We report a single oxygen atomic sensor based on a scanning tunneling microscope break-junction (STM-BJ) technique by using silver electrodes at room temperature. The oxygen concentration was adjusted with argon gas in a glove box. An oxygen atom was inserted in the gap of the Ag electrodes after the Ag metal point contact had been ruptured during stretching of the Ag atomic junctions by moving a piezo. We successfully measured the conductance of a single oxygen atom by forming a series with the Ag contact, Ag-O-Ag bond. We found that the probability of Ag-O-Ag junction formation increased with increasing oxygen concentration. Furthermore, we observed that the peak height in the oxygen conductance histogram was changed reversibly depending on the oxygen concentration in a glove box. It confirms that our STM-BJ can be used for atomic sensor applications in the future.

Frequency-Independent Response of Self-Complementary Checkerboard Screens.

This research resolves a long-standing problem on the electromagnetic response of self-complementary metallic screens with checkerboardlike geometry. Although Babinet's principle implies that they show a frequency-independent response, this unusual characteristic has not been observed yet due to the singularities of the metallic point contacts in the checkerboard geometry. We overcome this difficulty by replacing the point contacts with resistive sheets. The proposed structure is prepared and characterized by terahertz time-domain spectroscopy. It is experimentally confirmed that the resistive checkerboard structures exhibit a flat transmission spectrum over 0.1-1.1THz. It is also demonstrated that self-complementarity can eliminate even the frequency-dependent transmission characteristics of resonant metamaterials.

Pulling Speed Dependence of Single Molecular Plateau Length

We compared the single molecular plateau length by varying the piezo pulling speed for the Ag electrodes using a scanning tunneling microscope‐based break junction technique. We found that the molecular plateau length was longer for a high pulling speed than that for a low pulling speed. Furthermore, we observed that the gap distance between the top and bottom Ag electrodes after the metal point contact was ruptured varied with a piezo pulling speed, which is attributed to the atomic diffusion of Ag in pulling a piezo. We also found that the molecular plateau length has no pulling speed dependence for Au electrodes. Our results open a new path to characterize the electrical or mechanical properties of the organic devices based on a single molecule.

Study of the hidden-order of URu2Si2 by point contact tunnel junctions

URu2Si2 presents superconductivity at temperatures below 1.5 K and a hidden order (HO) at about 17.5 K. Both electronic phenomena are influenced by Fano and Kondo resonances. At 17.5 K the HO was related in the past to a Peierls distortion that produces an energy gap deformed by the resonances. This order has been studied for more than 20 years and still there is no clear understanding. In this work we studied the electronic characteristics of URu2Si2 in a single crystal, with tunneling and metallic point contact spectroscopies. In the superconducting state, we determined the energy gap, which shows the influence of the Fano and Kondo resonances. At temperatures where HO is observed, the tunnel junctions spectra show the influence of the two resonances. Tunnel junction characteristics show that the Fermi surface nesting depends on the crystallographic direction.

Josephson effect and Andreev reflection in Ba1–xNaxFe2As2 (x = 0.25 and 0.35) point contacts

I(V) characteristics and their first derivatives of ScS and ScN-type (S is superconductor, c is constriction, N is normal metal) point contacts (PCs) based on Ba1–xNaxFe2As2 (x = 0.25 and 0.35) were studied. ScS-type PCs with S = Nb,Ta, and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps which satisfy relation 2 eV = ℏω. The IcRN product (Ic is critical current, RN is normal state PC resistance) in these PCs is found to be up to 1.2 mV. All this data with the observed dependence of the Ic on the microwave power of ScS PCs with Pb counter electrode indicates the presence of the singlet s-wave type pairing in Ba1–xNaxFe2As2. From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap Δ ratio 2Δ/kBTc = 3.6 ± 1 for the compound with x = 0.35 was evaluated. Analysis of these dV/dI(V) at high biases V, that is well above Δ, testifies transition to the thermal regime in PCs with a voltage increase.

Lateral and temporal dependence of the transport through an atomic gold contact under light irradiation: signature of propagating surface plasmon polaritons.

Metallic point contacts (MPCs) with dimensions comparable to the Fermi wavelength of conduction electrons act as electronic waveguides and might operate as plasmon transmitters. Here we present a correlated study of optical and conductance response of MPCs under irradiation with laser light. For elucidating the role of surface plasmon polaritons (SPPs), we integrate line gratings into the leads that increase the SPP excitation efficiency. By analyzing spatial, polarization, and time dependence, we identify two SPP contributions that we attribute to transmitted and decaying SPPs, respectively. The results demonstrate the role of SPPs for optically controlling the transport in metallic nanostructures and are important for designing opto-nanoelectronic devices.

Conductance of Molecular Junctions Formed with Silver Electrodes

We compare the conductance of a series of amine-terminated oligophenyl and alkane molecular junctions formed with Ag and Au electrodes using the scanning tunneling microscope based break-junction technique. For these molecules that conduct through the highest occupied molecular orbital, junctions formed with Au electrodes are more conductive than those formed with Ag electrodes, consistent with the lower work function for Ag. The measured conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same for Ag and Au electrodes. However, the formation and evolution of molecular junctions upon elongation are very different for these two metals. Specifically, junctions formed with Ag electrodes sustain significantly longer elongation when compared with Au due to a difference in the initial gap opened up when the metal point-contact is broken. Using this observation and density functional theory calculations of junction structure and conductance we explain the trends observed in the single molecule junction conductance. Our work thus opens a new path to the conductance measurements of a single molecule junction in Ag electrodes.

Statistical evidence of strain induced breaking of metallic point contacts

A scanning tunneling microscopy in break junction regime and a mechanically controllable break junction are used to acquire thousands of conductance-elongation curves by stretching until breaking and re-connecting Au junctions. From a robust statistical analysis performed on large sets of experiments, parameters such as lifetime, elongation and occurrence probabilities are extracted. The analysis of results obtained for different stretching speeds of the electrodes indicates that the breaking mechanism of di- and mono-atomic junction is identical, and that the junctions undergo atomic rearrangement during their stretching and at the moment of breaking.

Correlating Structure, Conductance, and Mechanics of Silver Atomic-Scale Contacts

We measure simultaneously force and conductance of Ag metal point-contacts under ambient conditions at room temperature. We observe the formation of contacts with a conductance close to 1 G0, the quantum of conductance, which can be attributed to a single-atom contact, similar to those formed by Au. We also find two additional conductance features at ∼0.4 G0 and ∼1.3 G0, which have been previously ascribed to contacts with oxygen contaminations. Here, using a conductance cross-correlation technique, we distinguish three different atomic-scale structural motifs and analyze their rupture forces and stiffness. Our results allow us to assign the ∼0.4 G0 conductance feature to an Ag-O-Ag contact and the ∼1.3 G0 feature to an Ag-Ag single-atom contact with an oxygen atom in parallel. Utilizing complementary information from force and conductance, we thus demonstrate the correlation of conductance with the structural evolution at the atomic scale.

Feedback-controlled electromigration for the fabrication of point contacts

Lithographically fabricated point contacts serve as important examples of mesoscopic conductors, as electrodes for molecular electronics, and as ultra-sensitive transducers for mechanical motion. We have developed a reproducible technique for fabricating metallic point contacts through electromigration. We employ fast analog feedback in a four-wire configuration in combination with slower computer controlled feedback to avoid catastrophic instability. This hybrid system allows electromigration to proceed while dissipating approximately constant power in the wire. We are able to control the final resistance of the point contact precisely below 5 kΩ and to within a factor of three when the target resistance approaches 12 kΩ where only a single conducting channel remains.

Physics, chemistry and biology of functional nanostructures

Nanoscience emerged as a field addressing phenomena that are strongly related to and influenced by their length scale being in the nanometer range. From the very beginning its interdisciplinary character became obvious. Meanwhile, after nanoscience had left its infancy, this characteristic feature developed even further by extension of the contributing disciplines: Initially it was supported and pushed forward by physics, materials science and chemistry, with a strong interrelation with semiconductor/information technology with their road maps for the downscaling of switching and storage devices. Today, biology and thus, in a natural way, medicine are included, opening new research areas within nanoscience. As a consequence of this development towards a real transdisciplinary field of science, worldwide collaborative research networks have been installed encompassing physics, materials science, chemistry and biology as well as various areas of engineering. One such network with focus on “Functional Nanostructures” has been successfully active for almost ten years in Baden-Wurttemberg (Germany) [1]. Recent results from this network form the nucleus of the present Thematic Series “Physics, chemistry and biology of functional nanostructures” in the Beilstein Journal of Nanotechnology, complemented by contributions from international groups outside this network. Besides the interdisciplinary approaches of nanoscience, its ongoing success can be attributed to at least three more necessary conditions: Significant progress in fabricating and controlling nanometer-sized objects and functional systems. Examples are synthesis and controlled positioning of various nanoparticles and macromolecules providing, finally, specific functions if arranged on suitable platforms in an optimized way. In the context of arranging nanoobjects, the exploitation of self-organization often in combination with suitable templates plays a major role. This aspect of the “bottom-up” methods opens routes towards novel nanolithographies, and due to the practical importance of the issue, the topic will be addressed in a number of contributions to the present Thematic Series. Continuous progress related to analytical tools and facilities allowing as complete as possible characterization of nanoobjects and devices. It is of special importance to gain information on the structure as well as the chemical composition and state of nanosytems in order to relate these to the various sought-after properties. With respect to tools, immediate examples are the continuous improvements of scanning-probe measurements such as scanning tunneling or scanning force microscopy (STM, AFM) [2] and their numerous variants often combined with scanning electron microscopy (SEM) or scanning helium ion microscopy (SHIM). Remarkable also are the breakthroughs in structural insight due to the advent of Cs-corrected high-resolution transmission electron microscopy (HRTEM) [3–4] or spectroscopic imaging in scanning TEM (STEM) [5]. Related to facilities, certainly the worldwide impressive progress of synchrotron facilities has made an important contribution, now providing beams with spot sizes even below 10 nm, thus promising the application of spectroscopies such as photoelectron emission microscopy (PEEM) or X-ray magnetic circular dichroism (XMCD) on a single nanoobject [6–7]. Progress in the theoretical understanding of nanoscaled phenomena. We deem progress in theory necessary in order to gain feedback for testing new predicted phenomena or optimizing already existing nanosystems and devices. All three of these conditions together with the given examples play a major role in the present Thematic Series, which, in turn, stands alongside previous series. Of special interest appear those reports dealing with self-assembly on solid surfaces, micro- and mesoporous solids, electrical transport through nanostructures, nanooptical aspects, organic–inorganic hybrids and properties of magnetic nanoparticles. A much broader view on biomimetic approaches can be found in [8]. The choice of these recommendations becomes obvious on summarizing the main topics of the present Thematic Series: Nanolithography approaches based on self-organized colloidal systems Experimental and theoretical description of electrical transport through nanostructures. Here, focus is put on the electrochemically controlled preparation of metallic point contacts [9–10] Magnetic behavior of nanoparticles and -wires Nanophotonics Effect of nanoporosity on the catalytic properties of Au oxidizing CO Theoretical description of organic building blocks such as polythiophenes. These molecules are especially interesting for organic solar cells [11] and have been analyzed also by STS [12]. Of course, the contributions to this Thematic Series form just a snapshot of the current activities focused on functional nanostructures. We hope, however, that our choice, which was guided by the idea to present interesting examples for the three general conditions given above, will be found inspiring by the readers as well. We give thanks to all colleagues for the valuable reports on their research and the Beilstein team for the engaged editorial work. Paul Ziemann and Thomas Schimmel Ulm, Karlsruhe, November 2012

Photon generation in ferromagnetic point contacts

We show theoretically that a significant spin accumulation can occur in electric point contacts between two ferromagnetic electrodes with different magnetizations. Under appropriate conditions an inverse population of spin-split electronic levels results in stimulated emission of photons in the presence of a resonant electromagnetic field. The intensity of the emitted radiation can be several orders of magnitude higher than in typical semiconductor laser materials for two reasons. (1) The density of conduction electrons in a metal point contact is much larger than in semiconductors. (2) The strength of the coupling between the electron spins and the electromagnetic field that is responsible for the radiative spin-flip transitions is set by the magnetic exchange energy and can therefore be very large, as suggested by Kadigrobov et al. [Europhys. Lett. 67, 948 (2004)].

Revealing thermal effects in the electronic transport through irradiated atomic metal point contacts

We report on the electronic transport through nanoscopic metallic contacts under the influence of external light fields. Various processes can be of relevance here, whose underlying mechanisms can be studied by comparing different kinds of atomic contacts. For this purpose two kinds of contacts, which were established by electrochemical deposition, forming a gate-controlled quantum switch (GCQS), have been studied. We demonstrate that in these kinds of contacts thermal effects resulting from local heating due to the incident light, namely thermovoltage and the temperature dependences of the electrical resistivity and the electrochemical (Helmholtz) double layer are the most prominent effects.

Measured and Predicted Static Friction for Real Rough Surfaces in Point Contact

A numerical model to predict static friction for metallic point contacts was developed and validated by comparison to experimental measurements using a specially designed test rig. Key aspects of the numerical model were the incorporation of a digitized real rough surface profile, application of discrete convolution fast Fourier transform (DC-FFT) to predict local asperity interference, and modification of the yield strength to capture the effect of cold hardening. It was found that these model features are critically important to quantitative prediction of static friction. The model significantly underestimated the static friction coefficient if randomly generated surfaces having statistical parameters the same as the measured rough surface were used; digitized real rough surfaces enabled accurate predictions. Further, the model was able to describe the static friction of worn surfaces after cold hardening was introduced through modification of material yield strength. This work illustrates the importance of incorporating the surface features and the change of those features with wear to accurately and reliably predict static friction.

Memory effects in electrochemically gated metallic point contacts

Electrochemical gating permits the observation of few-atom processes in contact reconstruction. We monitor the junction conductance during the opening and closing of an atomic-scale metallic contact and use this as an instantaneous probe of the atomic-scale structural switching process. We observe clear correlations in the quantum conductance of a contact in subsequent switching events, demonstrating memory effects at the atomic scale. These experimental observations are supported by numerical simulations which show a conservation of the contact reconstruction process across several switching cycles. These results open a route to electrochemically control few-atom surface reconstruction events with present-day detection capabilities.

Correlation Analysis of Atomic and Single-Molecule Junction Conductance

The break-junction technique is widely used to measure electronic properties of nanoscale junctions including metal point-contacts and single-molecule junctions. In these measurements, conductance is measured as a function of electrode displacement yielding data that is analyzed by constructing conductance histograms to determine the most frequently observed conductance values in the nanoscale junctions. However much of the rich physics in these measurements is lost in this simple analysis technique. Conductance histograms cannot be used to study the statistical relation of distinct junction configurations, to distinguish structurally different configurations that have similar conductance values, or to obtain information on the relation between conductance and junction elongation. Here, we give a detailed introduction to a novel statistical analysis method based on the two-dimensional cross-correlation histogram (2DCH) analysis of conductance traces and show that this method provides new information about the relation of different junction configurations that occur during the formation and evolution of metal and single-molecule junctions. We first illustrate the different types of correlation effects by using simulated conductance traces. We then apply this analysis method to several different experimental examples. We show from break-junction measurements of different metal point-contacts that in aluminum, the first conductance histogram peak corresponds to two different junction structures. In tantalum, we identify the frequent absence of adhesive instability. We show that conductance plateaus shift in a correlated manner in iron and vanadium junctions. Finally, we highlight the applicability of the correlation analysis to single-molecule platinum-CO-platinum and gold-4,4'-bipyridine-gold junctions.

Determining transport parameters for superconductor/normal metal point contacts at fixed temperature from conductance versus magnetic field data

Transport data for superconductor/normal metal point contacts often consists of normalized conductance versus applied voltage as a function of reduced temperature, from which the barrier parameter, Z, and the broadening parameter, Γ, for the contact can be determined, in addition to other features of interest. We present a new strategy for determining these parameters, namely from normalized conductance as a function of applied magnetic field at fixed temperature, even when conductance versus variable temperature data is absent. This analysis technique will be demonstrated for several point contacts, over a wide range of parameter values.

Interplay of Conductance, Force, and Structural Change in Metallic Point Contacts

The coupling between two atomically sharp nanocontacts provides tunable access to a fundamental underlying interaction: the formation of the bond between two atoms as they are brought into contact. Here we report a detailed experimental and theoretical analysis of the relation between the chemical force and the tunneling current during bond formation in atom-scale metallic junctions and their dependence on distance, junction structure, and material. We found that the short-range force as well as the conductance in two prototypical metal junctions depend exponentially on the distance and that they have essentially the same exponents. In the transition regime between tunneling and point contact, large short-range forces generate structural relaxations which are concomitant with modifications of the surface electronic structure and the collapse of the tunneling barrier.

Electrochemically Assisted Fabrication of Metal Atomic Wires and Molecular Junctions by MCBJ and STM‐BJ Methods

Atomic wires (point contacts) and molecular junctions are two fundamental units in the fields of nanoelectronics and devices. This Minireview introduces our recent approaches aiming to develop versatile methods to fabricate and characterize these unique metallic and molecular structures reliably. Electrochemical methods are coupled with mechanically controllable break junction (EC-MCBJ) or scanning tunneling microscopy (STM) break junction (EC-STMBJ) methods to fabricate metallic point contacts and metal/molecule/metal junctions. With the designed electrodeposition method, the metal of interest (e.g. Au, Cu, Fe or Pd) is deposited in a controlled way on the original electrode pair, on a chip for MCBJ or on the STM tip, to make the metallic contact. Then, various metal atomic wires and molecular junctions can be fabricated and characterized systematically. Herein, we measured the quantized conductance through the construction of histograms of these metal atomic point contacts and of single molecules including benzene-1,4-dithiol (BDT), ferrocene-bisvinylphenylmethyl dithiol (Fc-VPM), 4,4'-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA). Finally, we briefly discussed the future of EC-MCBJ and EC-STM for nanoelectronics and devices, for example, for the formation of heterogeneous metal-based atomic point contacts and molecular junctions.

An alternative view at the zero-bias anomaly of metallic point contacts

We have studied the zero-bias anomaly of point contacts as function of contact size for a wide range of materials from simple normal metals, superconductors, and magnets to highly correlated heavy-fermion compounds. The size δR of the zero-bias anomaly varied in a universal manner proportional to the square of the contact resistance R between 1Ω and 10kΩ, that is for contact radii from about 15nm down to 0.2nm. Magnetic impurities, two-level systems, or a tunneling barrier are unlikely to be the main source of this anomaly. We suggest instead Kondo-type scattering of the conduction electrons due to a spontaneous electron spin polarization at the point contact.