Single-atom Contacts Research Articles

Electrically Controlled Bimetallic Junctions for Atomic-Scale Electronics.

Forming atomic-scale contacts with attractive geometries and material compositions is a long-term goal of nanotechnology. Here, we show that a rich family of bimetallic atomic-contacts can be fabricated in break-junction setups. The structure and material composition of these contacts can be controlled by atomically precise electromigration, where the metal types of the electron-injecting and sink electrodes determine the type of atoms added to, or subtracted from, the contact structure. The formed bimetallic structures include, for example, platinum and aluminum electrodes bridged by an atomic chain composed of platinum and aluminum atoms as well as iron-nickel single-atom contacts that act as a spin-valve break junction without the need for sophisticated spin-valve geometries. The versatile nature of atomic contacts in bimetallic junctions and the ability to control their structure by electromigration can be used to expand the structural variety of atomic and molecular junctions and their span of properties.

Inelastic Light Scattering in the Vicinity of a Single-Atom Quantum Point Contact in a Plasmonic Picocavity.

Electromagnetic fields can be confined in the presence of metal nanostructures. Recently, subnanometer scale confinement has been demonstrated to occur at atomic protrusions on plasmonic nanostructures. Such an extreme field may dominate atomic-scale light-matter interactions in "picocavities". However, it remains to be elucidated how atomic-level structures and electron transport affect plasmonic properties of a picocavity. Here, using low-temperature optical scanning tunneling microscopy (STM), we investigate inelastic light scattering (ILS) in the vicinity of a single-atom quantum point contact (QPC). A vibration mode localized at the single Ag adatom on the Ag(111) surface is resolved in the ILS spectrum, resulting from tip-enhanced Raman scattering (TERS) by the atomically confined plasmonic field in the STM junction. Furthermore, we trace how TERS from the single adatom evolves as a function of the gap distance. The exceptional stability of the low-temperature STM allows to examine distinctly different electron transport regimes of the picocavity, namely, in the tunneling and QPC regimes. This measurement shows that the vibration mode localized at the adatom and its TERS intensity exhibits a sharp change upon the QPC formation, indicating that the atomic-level structure has a crucial impact on the plasmonic properties. To gain microscopic insights into picocavity optomechanics, we scrutinize the structure and plasmonic field in the STM junction using time-dependent density functional theory. The simulations reveal that atomic-scale structural relaxation at the single-atom QPC results in a discrete change of the plasmonic field strength, volume, and distribution as well as the vibration mode localized at the single atom. These findings give a qualitative explanation for the experimental observations. Furthermore, we demonstrate that strong ILS is a characteristic feature of QPC by continuously forming, breaking, and reforming the atomic contact and how the plasmonic resonance evolves throughout the nontunneling, tunneling, and QPC regimes.

Antibody specificity against highly conserved membrane protein Claudin 6 driven by single atomic contact point.

The tight junction protein claudin 6 (CLDN6) is differentially expressed on cancer cells with almost no expression in healthy tissue. However, achieving therapeutic MAb specificity for this 4 transmembrane protein is challenging because it is nearly identical to the widely expressed CLDN9, with only 3 extracellular amino acids different. Most other CLDN6 MAbs, including those in clinical development are cross-reactive with CLDN9, and several trials have now been stopped. Here we isolated rare MAbs that bind CLDN6 with up to picomolar affinity and display minimal cross-reactivity with CLDN9, 22 other CLDN family members, or across the human membrane proteome. Amino acid-level epitope mapping distinguished the binding sites of our MAbs from existing clinical-stage MAbs. Atomic-level epitope mapping identified the structural mechanism by which our MAbs differentiate CLDN6 and CLDN9 through steric hindrance at a single molecular contact point, the γ carbon on CLDN6 residue Q156.

Structural Asymmetry of Metallic Single-Atom Contacts Detected by Current-Voltage Characteristics.

The complex behavior of the simplest atomic-scale conductors indicates that the electrode structure itself is significant in the design of future nanoscale devices. In this study, the structural asymmetry of metallic atomic contacts formed between two macroscopic Au electrodes at room temperature was investigated. Characteristic signatures of the structural asymmetries were detected by fast current-voltage (I-V) measurements with a time resolution of approximately 100 μs. Statistical analysis of more than 300,000 I-V curves obtained from more than 1000 contact-stretching processes demonstrates that the current rectification properties are correlated with the conductance of the nanocontacts. A substantial suppression of the variation in current rectification was observed for the atomic contacts with integer multiples of the conductance quantum. Statistical analysis of the time-resolved I-V curves revealed that the current rectification variations increased significantly from 500 μs onward before the breakage of the atomic contacts. Ab initio atomistic simulations of the stretching processes and corresponding I-V characteristics confirmed the magnitude of the rectification and related it to the structural asymmetries in the breakdown process of the junctions. Overall, we provide a better understanding of the interplay between geometric and electronic structures at atomically defined metal-metal interfaces by probing charge transport properties in extremely sensitive nanocontacts.

Revealing channel polarization of atomic contacts of ferromagnets and strong paramagnets by shot-noise measurements

We report measurements of the shot noise of atomic contacts of the ferromagnets cobalt and gadolinium and the strong paramagnets palladium, platinum, and iridium. The contacts are fabricated using the mechanically controlled break-junction technique at 4 K. Single-atom contacts of Co and Gd with conductance smaller than the conductance quantum show reduced noise compared to the expectation for the spin-degenerate single-channel case. We discuss by how far this observation gives evidence for spin-polarized transport. Pd, Pt, and Ir reveal noise levels which are above, but close to the threshold to the spin-degenerate single-channel situation. Since for such multivalent metals more than one channel is expected to contribute to the transport, our findings do not exclude spin-polarized transport for these elements. We observe an anticorrelation between the minimum noise and the bulk Stoner parameter of these elements, supporting that spin polarization might be reflected in the shot-noise signal in strong paramagnets.

Single‐Atom Quantum‐Point Contact Switch Using Atomically Thin Hexagonal Boron Nitride

The first report of a quantized conductance atomic threshold switch (QCATS) using an atomically-thin hexagonal boron nitride (hBN) layer is provided. This QCATS has applications in memory and logic devices. The QCATS device shows a stable and reproducible conductance quantization state at 1·G0 by forming single-atom point contact through a monoatomic boron defect in an hBN layer. An atomistic switching mechanism in hBN-QCATS is confirmed by in situ visualization of mono-atomic conductive filaments. Atomic defects in hBN are the key factor that affects the switching characteristic. The hBN-QCATS has excellent switching characteristics such as low operation voltage of 0.3V, low "off" current of 1 pA, fast switching of 50 ns, and high endurance> 107 cycles. The variability of switching characteristics, which are the major problems of switching device, can be solved by reducing the area and thickness of the switching region to form single-atom point contact. The switching layer thickness is scaled down to the single-atom (≈0.33nm) h-BN layer, and the switching area is limited to single-atom defects. By implementing excellent switching characteristics using single-layer hBN, the possibility of implementing stable and uniform atomic-switching devices for future memory and logic applications is confirmed.

Mechanisms of jump to contact and conductance plateau formation in copper atomic junctions in vacuum and aqueous environments

The interplay between groups of water molecules and single-atom contacts, as reflected in the electrical conductances and mechanical forces of copper atomic junctions, is explored by means of first-principles theory and semi-empirical calculations. We study the influence of the atomic geometries of copper electrodes with pyramidal and non-crystalline structures in the presence and absence of water on the conductance profiles as the electrodes approach each other. It is shown that the atomic arrangements of nano-contacts have crucial effects on the formation of plateaus and the conductance values. Groups of hydrogen bonded water molecules bridge the junction electrodes before a direct Cu-Cu contact between the electrodes is made. However, the bridging of the two copper electrodes by a single H$_{2}$O molecule only occurs in the junctions with pyramidal electrodes. Our findings reveal that the presence of H$_{2}$O molecules modifies strongly the conductance profile of these junctions. In the absence of water molecules, the pyramidal junctions exhibit continuous transitions between integer conductance plateaus, while in the presence of H$_{2}$O molecules, these junctions show abrupt jump to contact behavior and no well-defined conductance plateaus. By contrast, in the absence of H$_{2}$O molecules, the non-crystalline junctions display jump to contact behavior and no well-defined plateaus, while in the presence of H$_{2}$O molecules they exhibit a jump to contact and abrupt transitions between fractional and integer plateaus.

Electromigration-induced directional steps towards the formation of single atomic Ag contacts.

Even though there have been many experimental attempts and theoretical approaches to understand the process of electromigration (EM), it has not been quantitatively understood for ultrathin structures and at grain boundaries. Nevertheless, we showed recently that it can be used reliably for the formation of single atomic point contacts after careful pre-structuring of the initial Ag nanostructures. The process of formation of nanocontacts by EM down to a single-atom point contact was investigated for ultrathin (5 nm) Ag structures at 100 K by measuring the conductance as a function of the time during EM. In this paper, we compare the process of thinning by EM of structures with constrictions below the average grain size of Ag layers (15 nm) with that of structures with much larger initial constrictions of around 150 nm having multiple grains at the centre constriction prior to the formation of a point contact. Even though clear morphological differences exist between both types of structures, quantized conductance plateaus showing the formation of single point contacts have been observed for both. Here we put emphasis on the thinning process by EM, just before a point contact is formed. To understand this thinning process, the semi-classical regime before the contact reaches the quantum regime was analyzed in detail. For this purpose, we used experimental conductance histograms in the range between 2G0 and 15G0 and their corresponding Fourier transforms (FTs). The FT analysis of the conductance histograms exhibits a clear preference for thinning along the [100] direction. Using well-established models, both atom-by-atom steps and ranges of stability, presumably caused by electronic shell effects, can be discriminated. Although the directional motion of atoms during EM leads to specific properties such as the instabilities mentioned, similarities to mechanically opened contacts with respect to cross-sectional stability were found.

Directional bonding explains the high conductance of atomic contacts in bcc metals

Atomic-sized junctions of iron, created by controlled rupture, present unusually high values of conductance compared to other metals. This result is counter-intuitive since, at the nanoscale, body-centered cubic metals are expected to exhibit lower coordination than face-centered cubic metals. In this work, classical molecular dynamics simulations of contact rupture, using an interatomic potential that accounts for directional bonding, yield highly-coordinated stable structures before rupture, unlike an isotropic bonding potential, which results in the expected stable single-atom contacts. Density functional theory electronic transport calculations show that conductance values of these highly coordinated and highly stable structures, can explain the experimentally measured values for conductance of body-centered cubic atomic contacts, thus revealing the important role of directional bonding in these metals.

Tunnel magnetoresistance in theB24N24 cage by the considering contacts type

In this research, spin-transport properties of the B24N24 molecule, when it is connected through a single and multiple atom contacts to two ferromagnetic electrodes have been investigated using the Landauer formula and the non-equilibrium Green’s function. The results show that the current has a stepwise behavior against the bias voltage; the off state of the B24N24 molecule occurs at low voltages, independent of the type of contact. The characteristic of the current in terms of the gate voltage is dependent on the contact type in usage, and generally less for single contact configurations as compared to those seen in multiple contact configurations. In the case of contacting one atom of the B24N24 molecule to electrodes, the B24N24 has conduction property, while in multiple contacts it shows property of a semiconductor. The Tunneling Magnetic Resistance (TMR) contained three peaks no matter of the contact type, and whose maxima for single and 4-atom (about 42%) and for 8-atom (about 37%) all occurred at zero bias voltage. The maxima of TMR for 6-atom arrangement went up at the bias voltages − 1.2 V and + 1.2 V, about 53%. And, last but not the least, the TMR maxima change locations as the voltage increases.

Heat dissipation in quasi-ballistic single-atom contacts at room temperature

We report on evaluations of local heating in Au single-atom chains at room temperature. We performed onsite thermometry of atomic-scale Au junctions under applied sinusoidal voltage of variable amplitudes. The AC approach enabled to preclude electromigration effects for characterizing the influence of energy dissipations on the lifetime. We elucidated nonlinear increase in the effective temperature of the current-carrying single-atom chains with the voltage amplitudes, which was attributed to subtle interplay between electron-phonon scattering and electron-mediated thermal transport in the quasi-ballistic conductor. We also found that only 0.2% of the electric power contributed to local heating while the majority was consumed at the diffusive bank. The present findings can be used for thermal management of future integrated nanoelectronics.

Statistical analysis of electronic and phononic transport simulations of metallic atomic contacts

We adapt existing phonon heat transport methods to compute the phononic thermal conductance of metallic atomic contacts during a stretching process. Nonequilibrium molecular dynamics simulations are used to generate atomic configurations and to simultaneously determine the phononic thermal conductance. Combining the approach with established electronic structure calculations based on a tight-binding parametrization allows us to calculate in addition charge transport properties of each contact geometry within the Landauer-B\"uttiker formalism. The method is computationally fast enough to perform a statistical analysis of many stretching events, and we apply it here to atomic junctions formed from three different metals, namely gold (Au), platinum (Pt), and aluminum (Al). The description of both phononic and electronic contributions to heat transport allows us to examine the validity of the Wiedemann-Franz law at the atomic scale. We find that it is well obeyed in the contact regime at room temperature for Au and Al as far as only electronic contributions are concerned, but deviations of up to 10% arise for Pt. If the total thermal conductance is studied, deviations of typically less than 10% arise for Au and Al, which can be traced back mainly to phononic contributions to the thermal conductance, while electronic and phononic contributions can add up to some 20% for single-atom contacts of Pt.

Switching Dynamics and Conductance Quantization of $Aloe$ Polysaccharides-Based Device

The switching behaviors of polysaccharides-based resistive random access memories change substantially depending on the electrical inputs. Here, the switching dynamics of the device are presented by varying the applied current compliance (ICC) and voltage sweeping rate (v). The results show that the device resistance in the low-resistance state (RLRS) can be modulated over five orders of magnitude by varying ICC and v in the typical current- voltage measurements. The RLRS modulation is attributed to the variable tunneling gap between the filament tip and the top electrode (TE). Conductance quantization is observed once a single-atomic contact with resistance ≤12.9 kΩ is formed. Depending on the TEs, both integer and half-integer multiples of quantization levels are being observed, demonstrating its potential for multilevel data storage. In addition, the results unveil the stochastic strengthening and rupturing of the filament as modulated by periodic voltage pulses, thus enabling the device to operate in both volatile and nonvolatile modes. The device offers excellent switching dynamics for preliminary emulation of synapselike learning and forgetting behaviors in neural junctions for next-generation neuromorphic computing systems.

Quantized thermal conductance in metallic heterojunctions

To develop next-generation electronics and high efficiency energy-harvesting devices, it is crucial to understand how charge and heat are transported at the nanoscale. Metallic atomic-size contacts are ideal systems to probe the quantum limits of transport. The thermal conductance and electrical conductance of gold atomic contacts have been recently proven to be quantized at room temperature. However, a big experimental challenge in such measurements is represented by the fast breaking dynamics of metallic junctions at room temperature, which can exceed the typical response time of the thermal measurement. Here, we use a break-junction setup that combines Scanning Tunneling Microscopy with suspended microelectro-mechanical systems with a gold-covered membrane and an integrated heater acting also as a thermometer. By using other metals as tip materials, namely, Pt, PtIr, and W, we show heat transport measurements through single gold atomic contacts. The dependence of the thermal conductance is analysed as a function of contact size and materials used. We find that by using Pt and Pt-Ir tips, we can maximize the mechanical stability and probability of forming single Au atomic contacts. We then show the quantization of the electrical and thermal conductance with the verification of the Wiedemann-Franz law at the atomic scale. We expect these findings to increase the flexibility of experimental techniques probing heat transport in metallic quantum point contacts and to enable the investigation of thermal properties of molecular junctions.

Nanotribological properties of bulk metallic glasses

Bulk metallic glasses (BMGs) exhibit unique mechanical properties; however, a detailed understanding of nanotribological properties of BMGs is still lacking. In this article, alloy composition, surface disorder, normal load, and velocity dependence of friction between a single asperity contact and the BMG surface is explored with two-dimensional Prandtl-Tomlinson (PT) model. Our numerical results reveal distinct regimes for load and velocity dependence of friction at small length scales. The friction is inert to alloy composition and surface roughness for small loads in wearless regime. Surface corrugation increases with the number of alloy elements and dominates the friction for normal loads that may lead wear. Calculations for the effect of sliding velocity on friction between a single-atom contact and different BMGs show that velocity dependence of friction is inert to alloy composition and surface structure for sliding velocities smaller than 1.3 μm/s. Besides, the friction evolves non-monotonically with alloy composition and surface disorder for larger sliding velocities. Our numerical results show that tracking the minimum energy path and slip-stick motion determine the nanotribological properties of BMGs. The single asperity can trace the minimum energy path at small loads and sliding velocities while increasing load and sliding velocities impede following the minimum energy path with inflation in slip-stick motion. Our analyses disclose that BMGs unveil two unique frictional properties at small length scales: (I) friction is inert to alloy composition and surface disorder in wearless friction regime, (II) velocity dependence of friction does not depend on composition and disorder for small sliding speeds. These results promote BMGs as ideal materials for applications that require high durability and low friction in quasi-dynamic operating conditions.

Thermal conductance of metallic atomic-size contacts: Phonon transport and Wiedemann-Franz law

Motivated by recent experiments [Science 355, 1192 (2017); Nat. Nanotechnol. 12, 430 (2017)], we present here an extensive theoretical analysis of the thermal conductance of atomic-size contacts made of three different metals, namely gold (Au), platinum (Pt), and aluminum (Al). The main goal of this work is to elucidate the role of phonons in the thermal transport through these atomic contacts as well as to study the validity of the Wiedemann-Franz law, which relates the electrical and the thermal conductance. For this purpose, we have employed two different custom-developed theoretical approaches. The first one is a transport method based on density functional theory (DFT) that allows one to accurately compute the contributions of both electrons and phonons to the thermal transport in few-atom-thick contacts. The second technique is based on a combination of classical molecular dynamics (MD) simulations and a tight-binding model that enables the efficient calculation of the electronic contribution to the thermal conductance of atomic contacts of larger size. Our DFT-based calculations show that the thermal conductance of few-atom contacts of Au and Pt is dominated by electrons, with phonons giving a contribution typically below 10% of the total thermal conductance, depending on the contact geometry. For these two metals we find that the small deviations from the Wiedemann-Franz law, reported experimentally, largely stem from phonons. In the case of Al contacts we predict that the phononic contribution can be considerably larger with up to 40% of the total thermal conductance. We show that these differences in the phononic contribution across metals originate mainly from their distinct Debye energies. On the other hand, our MD-based calculations demonstrate that the electronic contribution to the thermal conductance follows very closely the Wiedemann-Franz law, irrespective of the material and the contact size. Finally, the ensemble of our results consistently shows that the reported observation of quantized thermal transport at room temperature is restricted to few-atom contacts of Au, a monovalent metal in which the transport is dominated by the $s$ valence orbitals. In the case of multivalent metals like Pt and Al this quantization is statistically absent due to the fact that additional orbitals contribute to the transport with conduction channels that have intermediate transmissions between 0 and 1, even in the case of single-atom contacts.

Two-probe STM experiments at the atomic level

Direct characterization of planar atomic or molecular scale devices and circuits on a supporting surface by multi-probe measurements requires unprecedented stability of single atom contacts and manipulation of scanning probes over large, nanometer scale area with atomic precision. In this work, we describe the full methodology behind atomically defined two-probe scanning tunneling microscopy (STM) experiments performed on a model system: dangling bond dimer wire supported on a hydrogenated germanium (0 0 1) surface. We show that 70 nm long atomic wire can be simultaneously approached by two independent STM scanners with exact probe to probe distance reaching down to 30 nm. This allows direct wire characterization by two-probe I–V characteristics at distances below 50 nm. Our technical results presented in this work open a new area for multi-probe research, which can be now performed with precision so far accessible only by single-probe scanning probe microscopy (SPM) experiments.

Break voltage of Au single-atom contacts formed by junction closure

We produced Au single-atom contacts (SACs) by contact closing and found that they show higher break voltages than SACs formed by the conventional break junction method. Our finding clearly indicates that the tensile forces inside SACs are important to obtain their high-bias stability and the SAC break voltage can be increased by reducing the tensile force. The break voltages of Pb SACs formed by scanning tunneling microscopy tip-sample touching and the break junction method have also been compared.

Multiplicity of atomic reconfigurations in an electrochemical Pb single-atom transistor

One focus of nanoelectronics research is to exploit the physical limits in size and energy efficiency. Here, we demonstrate a device in the form of a fully metallic atomic-scale transistor based on a lead (Pb) single-atom quantum point contact. The atomic configuration of the point contact determines the conductance of the Pb atomic-scale transistor. The conductance multiplicity of the Pb single-atom transistor has been confirmed by performing switching between an electrically nonconducting ``off-state'' and conducting ``on-states'' at $1{\mathrm{G}}_{0}({\mathrm{G}}_{0}=2{e}^{2}/h$, where $e$ is the electron charge, and $h$ Planck's constant), $2.0{\mathrm{G}}_{0}$, $3.0{\mathrm{G}}_{0}$, $1.5{\mathrm{G}}_{0}$, $2.4{\mathrm{G}}_{0}$, $2.7{\mathrm{G}}_{0}$, $2.8{\mathrm{G}}_{0}$, and $5.4{\mathrm{G}}_{0}$, respectively. Our density-functional calculations for various ideal Pb single-atom contacts explain the atomic-configuration-related conductance multiplicity of the Pb single-atom transistor. The performance of the Pb single-atom transistors indicates that both the signatures of atomic valence and conductance quantization play roles in electron transport and bistable reconfiguration. The bistable reconfiguration of the electrode tips is an underlying mechanism in the switching of the Pb atomic-scale transistors. The absolute value of the electrochemical potential applied to the gate electrode is less than 30 mV. This merit suggests Pb [besides silver (Ag)] atomic-scale transistors as potential candidates for the development of electronic circuits with low power consumption. The dimension of the switching unit in the Pb single-atom transistor is in the range of 1 nm, which is much smaller than the projected scaling limit of the gate lengths in silicon transistors (5 nm). Therefore, the metallic single-atom transistors may provide perspectives for electronic applications beyond silicon.

Quantized conductance operation near a single-atom point contact in a polymer-based atomic switch

Highly-controlled conductance quantization is achieved near a single-atom point contact in a redox-based atomic switch device, in which a poly(ethylene oxide) (PEO) film is sandwiched between Ag and Pt electrodes. Current–voltage measurements revealed reproducible quantized conductance of ∼1G0 for more than 102 continuous voltage sweep cycles under a specific condition, indicating the formation of a well-defined single-atom point contact of Ag in the PEO matrix. The device exhibited a conductance state distribution centered at 1G0, with distinct half-integer multiples of G0 and small fractional variations. First-principles density functional theory simulations showed that the experimental observations could be explained by the existence of a tunneling gap and the structural rearrangement of an atomic point contact.