Gate-controlled atomic quantum switch

Signal, detection and estimation using a hybrid quantum circuit

We investigate a hybrid device allowing a photon-phonon coupling of a transmission line radiation (TLR) and a nanoeletromechanical system (NEMS), mediated by a superconducting qubit population imbalance. We demonstrate the derivation of an effective Hamiltonian for the strongly dispersive regime for this system. The qubit works as a quantum switch, allowing a conditioned transfer of excitations between the TLR and NEMS. We show that this regime allows the system to be employed for signal processing and force estimation. Additionally, we explore the ability of the quantum switch to generate non-classical states.

Versatile Nanoscale Three-Terminal Memristive Switch Enabled by Gating

A three-terminal memristor with an ultrasmall footprint of only 0.07 μm2 and critical dimensions of 70 nm × 10 nm × 6 nm is introduced. The device's feature is the presence of a gate contact, which enables two operation modes: either tuning the set voltage or directly inducing a resistance change. In I-V mode, we demonstrate that by changing the gate voltages between ±1 V one can shift the set voltage by 69%. In pulsing mode, we show that resistance change can be triggered by a gate pulse. Furthermore, we tested the device endurance under a 1 kHz operation. In an experiment with 2.6 million voltage pulses, we found two distinct resistance states. The device response to a pseudorandom bit sequence displays an open eye diagram and a success ratio of 97%. Our results suggest that this device concept is a promising candidate for a variety of applications ranging from Internet-of-Things to neuromorphic computing.

Thermal Management in Neuromorphic Materials, Devices, and Networks

Machine learning has experienced unprecedented growth in recent years, often referred to as an "artificial intelligence revolution." Biological systems inspire the fundamental approach for this new computing paradigm: using neural networks to classify large amounts of data into sorting categories. Current machine-learning schemes implement simulated neurons and synapses on standard computers based on a von Neumann architecture. This approach is inefficient in energy consumption, and thermal management, motivating the search for hardware-based systems that imitate the brain. Here, the present state of thermal management of neuromorphic computing technology and the challenges and opportunities of the energy-efficient implementation of neuromorphic devices are considered. The main features of brain-inspired computing and quantum materials for implementing neuromorphic devices are briefly described, the brain criticality and resistive switching-based neuromorphic devices are discussed, the energy and electrical considerations for spiking-based computation are presented, the fundamental features of the brain's thermal regulation are addressed, the physical mechanisms for thermal management and thermoelectric control of materials and neuromorphic devices are analyzed, and challenges and new avenues for implementing energy-efficient computing are described.

Nanoionics from Biological to Artificial Systems: An Alternative Beyond Nanoelectronics

Ion transport under nanoconfined spaces is a ubiquitous phenomenon in nature and plays an important role in the energy conversion and signal transduction processes of both biological and artificial systems. Unlike the free diffusion in continuum media, anomalous behaviors of ions are often observed in nanostructured systems, which is governed by the complex interplay between various interfacial interactions. Conventionally, nanoionics mainly refers to the study of ion transport in solid-state nanosystems. In this review, to extent this concept is proposed and a new framework to understand the phenomena, mechanism, methodology, and application associated with ion transport at the nanoscale is put forward. Specifically, here nanoionics is summarized into three categories, i.e., biological, artificial, and hybrid, and discussed the characteristics of each system. Compared with nanoelectronics, nanoionics is an emerging research field with many theoretical and practical challenges. With this forward-looking perspective, it is hoped that nanoionics can attract increasing attention and find wide range of applications as nanoelectronics.

A first principle study of water adsorbed on flat and stepped silver surfaces

The structural, electronic and vibrational properties of a water layer on Ag(100) and Ag(511) have been studied by first principles calculations and ab initio molecular dynamics simulations. The most stable...

Controllable Spin Switching in a Single-Molecule Magnetic Tunneling Junction

A new type of spin-current filter is proposed that consists of a single-molecule magnet (SMM) coupled to two normal metal electrodes. It is shown that this tunneling junction can generate a highly spin-polarized current, whose spin polarization can be switched by means of magnetic fields and gate voltages applied to the SMM. This spin switching in the SMM tunnel junction arises from spin-selective single-electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. The electron current spectrum is still spin polarized in the absence of an external magnetic field, which can help to judge whether the molecule's spin state has reached the ground-state doublet [Formula: see text]. This device can be realized with current technologies and may have practical use in spintronics and quantum information.

Quasi-Solid-State Single-Atom Transistors

The single-atom transistor represents a quantum electronic device at room temperature, allowing the switching of an electric current by the controlled and reversible relocation of one single atom within a metallic quantum point contact. So far, the device operates by applying a small voltage to a control electrode or "gate" within the aqueous electrolyte. Here, the operation of the atomic device in the quasi-solid state is demonstrated. Gelation of pyrogenic silica transforms the electrolyte into the quasi-solid state, exhibiting the cohesive properties of a solid and the diffusive properties of a liquid, preventing the leakage problem and avoiding the handling of a liquid system. The electrolyte is characterized by cyclic voltammetry, conductivity measurements, and rotation viscometry. Thus, a first demonstration of the single-atom transistor operating in the quasi-solid-state is given. The silver single-atom and atomic-scale transistors in the quasi-solid-state allow bistable switching between zero and quantized conductance levels, which are integer multiples of the conductance quantum G₀  = 2e² /h. Source-drain currents ranging from 1 to 8 µA are applied in these experiments. Any obvious influence of the gelation of the aqueous electrolyte on the electron transport within the quantum point contact is not observed.

Atomic Scale Photodetection Enabled by a Memristive Junction

The optical control of atomic relocations in a metallic quantum point contact is of great interest because it addresses the fundamental limit of "CMOS scaling". Here, by developing a platform for combined electronics and photonics on the atomic scale, we demonstrate an optically controlled electronic switch based on the relocation of atoms. It is shown through experiments and simulations how the interplay between electrical, optical, and light-induced thermal forces can reversibly relocate a few atoms and enable atomic photodetection with a digital electronic response, a high resistance extinction ratio (70 dB), and a low OFF-state current (10 pA) at room temperature. Additionally, the device introduced here displays an optically induced pinched hysteretic current (optical memristor). The photodetector has been tested in an experiment with real optical data at 0.5 Gbit/s, from which an eye diagram visualizing millions of detection cycles could be produced. This demonstrates the durability of the realized atomic scale devices and establishes them as alternatives to traditional photodetectors.

Admittance of Atomic and Molecular Junctions and Their Signal Transmission

Atom-sized contacts of metals are usually characterized by their direct current (DC) conductance. However, when atom-sized contacts are used as device interconnects and transmit high frequency signals or fast pulses, the most critical parameter is not their DC conductance but their admittance Y(ω), in particular its imaginary part ImY(ω). In this article, I will present a brief survey of theoretical and experimental results on the magnitude of Y(ω) for atom-sized contacts of metals. Theoretical contact models are first described and followed by numerical evaluation of ImY(ω) based on these models. As for experiments on Y(ω), previous experiments conducted under time-varying biases are surveyed, and then the results of direct signal transmission through atom-sized contacts are discussed. Both theoretical and experimental results indicate that ImY(ω) is negligibly small for typical atom-sized contacts for signal frequencies up to 1 GHz.

Nanoarchitectonics for Controlling the Number of Dopant Atoms in Solid Electrolyte Nanodots

Controlling movements of electrons and holes is the key task in developing today's highly sophisticated information society. As transistors reach their physical limits, the semiconductor industry is seeking the next alternative to sustain its economy and to unfold a new era of human civilization. In this context, a completely new information token, i.e., ions instead of electrons, is promising. The current trend in solid-state nanoionics for applications in energy storage, sensing, and brain-type information processing, requires the ability to control the properties of matter at the ultimate atomic scale. Here, a conceptually novel nanoarchitectonic strategy is proposed for controlling the number of dopant atoms in a solid electrolyte to obtain discrete electrical properties. Using α-Ag_2+δ S nanodots with a finite number of nonstoichiometry excess dopants as a model system, a theory matched with experiments is presented that reveals the role of physical parameters, namely, the separation between electrochemical energy levels and the cohesive energy, underlying atomic-scale manipulation of dopants in nanodots. This strategy can be applied to different nanoscale materials as their properties strongly depend on the number of doping atoms/ions, and has the potential to create a new paradigm based on controlled single atom/ion transfer.

Macro-mechanics controls quantum mechanics: mechanically controllable quantum conductance switching of an electrochemically fabricated atomic-scale point contact

Here, we present a silver atomic-scale device fabricated and operated by a combined technique of electrochemical control (EC) and mechanically controllable break junction (MCBJ). With this EC-MCBJ technique, we can perform mechanically controllable bistable quantum conductance switching of a silver quantum point contact (QPC) in an electrochemical environment at room temperature. Furthermore, the silver QPC of the device can be controlled both mechanically and electrochemically, and the operating mode can be changed from 'electrochemical' to 'mechanical', which expands the operating mode for controlling QPCs. These experimental results offer the perspective that a silver QPC may be used as a contact for a nanoelectromechanical relay.

On-Demand Reconfiguration of Nanomaterials: When Electronics Meets Ionics

Rapid advances in the semiconductor industry, driven largely by device scaling, are now approaching fundamental physical limits and face severe power, performance, and cost constraints. Multifunctional materials and devices may lead to a paradigm shift toward new, intelligent, and efficient computing systems, and are being extensively studied. Herein examines how, by controlling the internal ion distribution in a solid-state film, a material's chemical composition and physical properties can be reversibly reconfigured using an applied electric field, at room temperature and after device fabrication. Reconfigurability is observed in a wide range of materials, including commonly used dielectric films, and has led to the development of new device concepts such as resistive random-access memory. Physical reconfigurability further allows memory and logic operations to be merged in the same device for efficient in-memory computing and neuromorphic computing systems. By directly changing the chemical composition of the material, coupled electrical, optical, and magnetic effects can also be obtained. A survey of recent fundamental material and device studies that reveal the dynamic ionic processes is included, along with discussions on systematic modeling efforts, device and material challenges, and future research directions.

Resistive Switching Performance Improvement via Modulating Nanoscale Conductive Filament, Involving the Application of Two-Dimensional Layered Materials

Reversible chemical and structural changes induced by ionic motion and reaction in response to electrical stimuli leads to resistive switching effects in metal-insulator-metal structures. Filamentary switching based on the formation and rupture of nanoscale conductive filament has been applied in non-volatile memory and volatile selector devices with low power consumption and fast switching speeds. Before the mass production of resistive switching devices, great efforts are still required to enable stable and reliable switching performances. The conductive filament, a bridge of microscopic metal-insulator-metal structure and macroscopic resistance states, plays an irreplaceable part in resistive switching behavior, as unreliable performance often originates from unstable filament behavior. In this Review, departing from the filamentary switching mechanism and the existing issues, recent advances of the switching performance improvement through the conductive filament modulation are discussed, in the sequence of material modulation, device structure design and switching operation scheme optimization. In particular, two-dimensional (2D) nanomaterials with excellent properties including and beyond graphene, are discussed with emphasis on performance improvement by their active roles as the switching layer, insertion layer, thin electrode, patterned electrode, and edge electrode, etc.

Stochastic resonance in bistable atomic switches

We have investigated the conductance of bistable gold atomic switches as a function of periodic input voltages mixed with a random noise. With increasing noise amplitude, the atomic switches biased below the threshold voltage for conductance switching start exhibiting switching in conductance between two stable states. Clear synchronization between the input and output signals is observed when an optimized noise amplitude is mixed with the periodic input voltage, even when the atomic switches are driven by an input voltage as low as approximately 10% of the threshold voltage. The observed behavior can be explained in terms of the stochastic resonance. The results presented here indicate that utilization of noise can dramatically reduce the operation voltage of metal atomic switches.

Copper atomic-scale transistors

We investigated copper as a working material for metallic atomic-scale transistors and confirmed that copper atomic-scale transistors can be fabricated and operated electrochemically in a copper electrolyte (CuSO4 + H2SO4) in bi-distilled water under ambient conditions with three microelectrodes (source, drain and gate). The electrochemical switching-on potential of the atomic-scale transistor is below 350 mV, and the switching-off potential is between 0 and -170 mV. The switching-on current is above 1 μA, which is compatible with semiconductor transistor devices. Both sign and amplitude of the voltage applied across the source and drain electrodes (Ubias) influence the switching rate of the transistor and the copper deposition on the electrodes, and correspondingly shift the electrochemical operation potential. The copper atomic-scale transistors can be switched using a function generator without a computer-controlled feedback switching mechanism. The copper atomic-scale transistors, with only one or two atoms at the narrowest constriction, were realized to switch between 0 and 1G0 (G0 = 2e2/h; with e being the electron charge, and h being Planck's constant) or 2G0 by the function generator. The switching rate can reach up to 10 Hz. The copper atomic-scale transistor demonstrates volatile/non-volatile dual functionalities. Such an optimal merging of the logic with memory may open a perspective for processor-in-memory and logic-in-memory architectures, using copper as an alternative working material besides silver for fully metallic atomic-scale transistors.

First-principles study of the structure of water layers on flat and stepped Pb electrodes

On the basis of perodic density functional theory (DFT) calculations, we have addressed the geometric structures and electronic properties of water layers on flat and stepped Pb surfaces. In contrast to late d-band metals, on Pb(111) the energy minimum structure does not correspond to an ice-like hexagonal arrangement at a coverage of 2/3, but rather to a distorted structure at a coverage of 1 due to the larger lattice constant of Pb. At stepped Pb surfaces, the water layers are pinned at the step edge and form a complex network consisting of rectangles, pentagons and hexagons. The thermal stability of the water layers has been studied by using ab initio molecular dynamics simulations (AIMD) at a temperature of 140 K. Whereas the water layer on Pb(111) is already unstable at this temperature, the water layers on Pb(100), Pb(311), Pb(511) and Pb(711) exhibit a higher stability because of stronger water-water interactions. The vibrational spectra of the water layers at the stepped surfaces show a characteristic splitting into three modes in the O-H stretch region.

Atomic Scale Plasmonic Switch

The atom sets an ultimate scaling limit to Moore's law in the electronics industry. While electronics research already explores atomic scales devices, photonics research still deals with devices at the micrometer scale. Here we demonstrate that photonic scaling, similar to electronics, is only limited by the atom. More precisely, we introduce an electrically controlled plasmonic switch operating at the atomic scale. The switch allows for fast and reproducible switching by means of the relocation of an individual or, at most, a few atoms in a plasmonic cavity. Depending on the location of the atom either of two distinct plasmonic cavity resonance states are supported. Experimental results show reversible digital optical switching with an extinction ratio of 9.2 dB and operation at room temperature up to MHz with femtojoule (fJ) power consumption for a single switch operation. This demonstration of an integrated quantum device allowing to control photons at the atomic level opens intriguing perspectives for a fully integrated and highly scalable chip platform, a platform where optics, electronics, and memory may be controlled at the single-atom level.

Conductance Quantization in Resistive Random Access Memory

The intrinsic scaling-down ability, simple metal-insulator-metal (MIM) sandwich structure, excellent performances, and complementary metal-oxide-semiconductor (CMOS) technology-compatible fabrication processes make resistive random access memory (RRAM) one of the most promising candidates for the next-generation memory. The RRAM device also exhibits rich electrical, thermal, magnetic, and optical effects, in close correlation with the abundant resistive switching (RS) materials, metal-oxide interface, and multiple RS mechanisms including the formation/rupture of nanoscale to atomic-sized conductive filament (CF) incorporated in RS layer. Conductance quantization effect has been observed in the atomic-sized CF in RRAM, which provides a good opportunity to deeply investigate the RS mechanism in mesoscopic dimension. In this review paper, the operating principles of RRAM are introduced first, followed by the summarization of the basic conductance quantization phenomenon in RRAM and the related RS mechanisms, device structures, and material system. Then, we discuss the theory and modeling of quantum transport in RRAM. Finally, we present the opportunities and challenges in quantized RRAM devices and our views on the future prospects.

Remote control of magnetostriction-based nanocontacts at room temperature

The remote control of the electrical conductance through nanosized junctions at room temperature will play an important role in future nano-electromechanical systems and electronic devices. This can be achieved by exploiting the magnetostriction effects of ferromagnetic materials. Here we report on the electrical conductance of magnetic nanocontacts obtained from wires of the giant magnetostrictive compound Tb_0.3Dy_0.7Fe_1.95 as an active element in a mechanically controlled break-junction device. The nanocontacts are reproducibly switched at room temperature between “open” (zero conductance) and “closed” (nonzero conductance) states by variation of a magnetic field applied perpendicularly to the long wire axis. Conductance measurements in a magnetic field oriented parallel to the long wire axis exhibit a different behaviour where the conductance switches between both states only in a limited field range close to the coercive field. Investigating the conductance in the regime of electron tunneling by mechanical or magnetostrictive control of the electrode separation enables an estimation of the magnetostriction. The present results pave the way to utilize the material in devices based on nano-electromechanical systems operating at room temperature.

Quantum dot made in metal oxide silicon-nanowire field effect transistor working at room temperature

We report the observation of an atomic like behavior from T = 4.2 K up to room temperature in n- and p-type Ω-gate silicon nanowire (NW) transistors. For that purpose, we modified the design of a NW transistor and introduced long spacers between the source/drain and the channel in order to separate the channel from the electrodes. The channel was made extremely small (3.4 nm in diameter with 10 nm gate length) with a thick gate oxide (7 nm) in order to enhance the Coulomb repulsion between carriers, which can be as large as 200 meV when surface roughness promotes charge confinement. Parasitic stochastic Coulomb blockade effect can be eliminated in our devices by choosing proper control voltages. Moreover, the quantum dot can be tuned so that the resonant current at T = 4.2 K exceeds that at room temperature.

Reversible mechano-electrochemical writing of metallic nanostructures with the tip of an atomic force microscope

We recently introduced a method that allows the controlled deposition of nanoscale metallic patterns at defined locations using the tip of an atomic force microscope (AFM) as a "mechano-electrochemical pen", locally activating a passivated substrate surface for site-selective electrochemical deposition. Here, we demonstrate the reversibility of this process and study the long-term stability of the resulting metallic structures. The remarkable stability for more than 1.5 years under ambient air without any observable changes can be attributed to self-passivation. After AFM-activated electrochemical deposition of copper nanostructures on a polycrystalline gold film and subsequent AFM imaging, the copper nanostructures could be dissolved by reversing the electrochemical potential. Subsequent AFM-tip-activated deposition of different copper nanostructures at the same location where the previous structures were deleted, shows that there is no observable memory effect, i.e., no effect of the previous writing process on the subsequent writing process. Thus, the four processes required for reversible information storage, "write", "read", "delete" and "re-write", were successfully demonstrated on the nanometer scale.

Atomic switch: atom/ion movement controlled devices for beyond von-neumann computers

An atomic switch is a nanoionic device that controls the diffusion of metal ions/atoms and their reduction/oxidation processes in the switching operation to form/annihilate a conductive path. Since metal atoms can provide a highly conductive channel even if their cluster size is in the nanometer scale, atomic switches may enable downscaling to smaller than the 11 nm technology node, which is a great challenge for semiconductor devices. Atomic switches also possess novel characteristics, such as high on/off ratios, very low power consumption and non-volatility. The unique operating mechanisms of these devices have enabled the development of various types of atomic switch, such as gap-type and gapless-type two-terminal atomic switches and three-terminal atomic switches. Novel functions, such as selective volatile/nonvolatile, synaptic, memristive, and photo-assisted operations have been demonstrated. Such atomic switch characteristics can not only improve the performance of present-day electronic systems, but also enable development of new types of electronic systems, such as beyond von- Neumann computers.

Lifetime analysis of individual-atom contacts and crossover to geometric-shell structures in unstrained silver nanowires

We study the crossover of quantum point contacts from (i) individual-atom contacts to (ii) electronic-shell effects and finally to (iii) geometric-shell effects in electrochemically deposited silver contacts. The method allows the fabrication of mechanically unstrained structures, which is a requirement for determining the individual atomic configuration by means of a detailed lifetime analysis of their conductance. Within the geometric-shell model, the sequence of conductance maxima is explained quantitatively based on the crystal structure data of silver, and the growth mechanism of the nanowires is discussed.

The atomic force microscope as a mechano-electrochemical pen

We demonstrate a method that allows the controlled writing of metallic patterns on the nanometer scale using the tip of an atomic force microscope (AFM) as a "mechano-electrochemical pen". In contrast to previous experiments, no voltage is applied between the AFM tip and the sample surface. Instead, a passivated sample surface is activated locally due to lateral forces between the AFM tip and the sample surface. In this way, the area of tip-sample interaction is narrowly limited by the mechanical contact between tip and sample, and well-defined metallic patterns can be written reproducibly. Nanoscale structures and lines of copper were deposited, and the line widths ranged between 5 nm and 80 nm, depending on the deposition parameters. A procedure for the sequential writing of metallic nanostructures is introduced, based on the understanding of the passivation process. The mechanism of this mechano-electrochemical writing technique is investigated, and the processes of site-selective surface depassivation, deposition, dissolution and repassivation of electrochemically deposited nanoscale metallic islands are studied in detail.

Conductance histogram evolution of an EC-MCBJ fabricated Au atomic point contact

This work presents a study of Au conductance quantization based on a combined electrochemical deposition and mechanically controllable break junction (MCBJ) method. We describe the microfabrication process and discuss improved features of our microchip structure compared to the previous one. The improved structure prolongs the available life of the microchip and also increases the success rate of the MCBJ experiment. Stepwise changes in the current were observed at the last stage of atomic point contact breakdown and conductance histograms were constructed. The evolution of 1G0 peak height in conductance histograms was used to investigate the probability of formation of an atomic point contact. It has been shown that the success rate in forming an atomic point contact can be improved by decreasing the stretching speed and the degree that the two electrodes are brought into contact. The repeated breakdown and formation over thousands of cycles led to a distinctive increase of 1G0 peak height in the conductance histograms, and this increased probability of forming a single atomic point contact is discussed.

Heteronanojunctions with atomic size control using a lab-on-chip electrochemical approach with integrated microfluidics

A versatile tool for electrochemical fabrication of heteronanojunctions with nanocontacts made of a few atoms and nanogaps of molecular spacing is presented. By integrating microfluidic circuitry in a lab-on-chip approach, we keep control of the electrochemical environment in the vicinity of the nanojunction and add new versatility for exchanging and controlling the junction's medium. Nanocontacts made of various materials by successive local controlled depositions are demonstrated, with electrical properties revealing sizes reaching a few atoms only. Investigations on benchmark molecular electronics material, trapped between electrodes, reveal the possibility to create nanogaps of size matching those of molecules. We illustrate the interest of a microfluidic approach by showing that exposure of a fabricated molecular junction to controlled high solvent flows can be used as a reliability criterion for the presence of molecular entities in a gap.

Atomic switches: atomic-movement-controlled nanodevices for new types of computing

Atomic switches are nanoionic devices that control the diffusion of metal cations and their reduction/oxidation processes in the switching operation to form/annihilate a metal atomic bridge, which is a conductive path between two electrodes in the on-state. In contrast to conventional semiconductor devices, atomic switches can provide a highly conductive channel even if their size is of nanometer order. In addition to their small size and low on-resistance, their nonvolatility has enabled the development of new types of programmable devices, which may achieve all the required functions on a single chip. Three-terminal atomic switches have also been developed, in which the formation and annihilation of a metal atomic bridge between a source electrode and a drain electrode are controlled by a third (gate) electrode. Three-terminal atomic switches are expected to enhance the development of new types of logic circuits, such as nonvolatile logic. The recent development of atomic switches that use a metal oxide as the ionic conductive material has enabled the integration of atomic switches with complementary metal-oxide-semiconductor (CMOS) devices, which will facilitate the commercialization of atomic switches. The novel characteristics of atomic switches, such as their learning and photosensing abilities, are also introduced in the latter part of this review.

Electrochemical control of stability and restructuring dynamics in Au-Ag-Au and Au-Cu-Au bimetallic atom-scale junctions

Metallic atom-scale junctions (ASJs) are interesting fundamentally because they support ballistic transport, characterized by conduction quantized in units of G(0) = 2e(2)/h. They are also of potential practical interest since ASJ conductance is extraordinarily sensitive to molecular adsorption. Monometallic Au ASJs were previously fabricated electrochemically using an I(-)/I(3)(-) medium and a unique open working electrode configuration to produce slow electrodeposition or electrodissolution, resulting in reproducible ASJs with limiting conductance <5 G(0). Here, bimetallic Au-Cu-Au and Au-Ag-Au ASJ structures are obtained by electrochemical deposition/dissolution of Cu and Ag in K(2)SO(4) supporting electrolyte. The ASJs are fabricated in Si(3)N(4)-protected Au nanogaps obtained by focused ion beam milling, a protocol which yields repeatable and reproducible Au-Cu-Au or Au-Ag-Au ASJs without damaging the Au nanogap substrates. While Au-Ag-Au ASJs are relatively stable (hours) at open circuit potential in the supporting electrolyte, Au-Cu-Au ASJs exhibit spontaneous restructuring dynamics, characterized by monotonic, stepwise decreases in conductance under the same conditions. However, the Au-Cu-Au ASJs can be stabilized by applying sufficiently negative potentials. Hydrogen adsorption and shifts in the Fermi level are possible reasons for the enhanced stability of Au-Cu-Au structures at large negative overpotentials. In light of these observations, it is possible to integrate ASJs in microfluidic devices as renewable, nanostructured sensing elements for chemical detection.

A nanoelectromechanical single-atom switch

We have exploited the electromechanical properties of gated mechanical break junctions to form single-atom relays. The gate voltage can be used to reversibly switch between a monatomic contact with a conductance around 2e(2)/h and the tunneling regime. In tunneling, the source-drain conductance varies smoothly with gate voltage. The characteristics of the devices can be understood within a simple continuum model. It indicates that the elastic properties of the substrate facilitate the electromechanical tuning and that the details of the switching depend sensitively on the nanoscale geometry of the electrode tips.

Mechanically controlled binary conductance switching of a single-molecule junction

Molecular-scale components are expected to be central to the realization of nanoscale electronic devices. Although molecular-scale switching has been reported in atomic quantum point contacts, single-molecule junctions provide the additional flexibility of tuning the on/off conductance states through molecular design. To date, switching in single-molecule junctions has been attributed to changes in the conformation or charge state of the molecule. Here, we demonstrate reversible binary switching in a single-molecule junction by mechanical control of the metal-molecule contact geometry. We show that 4,4'-bipyridine-gold single-molecule junctions can be reversibly switched between two conductance states through repeated junction elongation and compression. Using first-principles calculations, we attribute the different measured conductance states to distinct contact geometries at the flexible but stable nitrogen-gold bond: conductance is low when the N-Au bond is perpendicular to the conducting pi-system, and high otherwise. This switching mechanism, inherent to the pyridine-gold link, could form the basis of a new class of mechanically activated single-molecule switches.

Switchable wiring for high-resolution electronic measurements at very low temperatures

Low-temperature transport measurements with high energy resolution require effective filtering of high-frequency input. The high dc resistance of standard RC filters results in considerable heat input and hampers measurements with high currents or voltages. We developed a wiring scheme that incorporates a commercial latching relay at very low temperature between two sets of wires. In our application one set of wires comprises a voltage divider and a high-Ohmic reference resistance at low temperature as well. The other set has low dc resistance and no voltage divider. Both sets are high frequency filtered with very robust and compact filters, though, for insuring effective damping at gigahertz frequencies. We demonstrate that with the first set, we obtain a voltage resolution of 6 microV and a current resolution of 100 pA, which is sufficient for the recording and analysis of multiparticle transport in superconducting point contacts. The second set is used for electromigration experiments on superconducting point contacts and allows application of currents up to 1 mA and voltages up to 20 V, while the sample is at 1 K. More versatile applications of the scheme are possible.

Independently switchable atomic quantum transistors by reversible contact reconstruction

The controlled fabrication of actively switchable atomic-scale devices, in particular transistors, has remained elusive to date. Here, we explain the operation of an atomic-scale three-terminal device by a novel switching mechanism of bistable, self-stabilizing reconstruction of the electrode contacts at the atomic level: While the device is manufactured by electrochemical deposition, it operates entirely on the basis of mechanical effects of the solid-liquid interface. We analyze mechanically and thermally stable metallic junctions with a predefined quantized conductance of 1-5 G0 in experiment and atomistic simulation. Atomistic modeling of structural and conductance properties elucidates bistable electrode reconstruction as the underlying mechanism of the device. Independent room temperature operation of two transistors at low voltage demonstrates intriguing perspectives for quantum electronics and logics on the atomic scale.

Magnetoresistance of nanoscale molecular devices based on Aharonov-Bohm interferometry

Control of conductance in molecular junctions is of key importance in the growing field of molecular electronics. The current in these junctions is often controlled by an electric gate designed to shift conductance peaks into the low bias regime. Magnetic fields, on the other hand, have rarely been used due to the small magnetic flux captured by molecular conductors (an exception is the Kondo effect in single-molecule transistors). This is in contrast to a related field, electronic transport through mesoscopic devices, where considerable activity with magnetic fields has led to a rich description of transport. The scarcity of experimental activity is due to the belief that significant magnetic response is obtained only when the magnetic flux is of the order of the quantum flux, while attaining such a flux for molecular and nanoscale devices requires unrealistic magnetic fields. Here we review recent theoretical work regarding the essential physical requirements necessary for the construction of nanometer-scale magnetoresistance devices based on an Aharonov-Bohm molecular interferometer. We show that control of the conductance properties using small fractions of a magnetic flux can be achieved by carefully adjusting the lifetime of the conducting electrons through a pre-selected single state that is well separated from other states due to quantum confinement effects. Using a simple analytical model and more elaborate atomistic calculations we demonstrate that magnetic fields which give rise to a magnetic flux comparable to 10(-3) of the quantum flux can be used to switch a class of different molecular and nanometer rings, ranging from quantum corrals, carbon nanotubes and even a molecular ring composed of polyconjugated aromatic materials. The unique characteristics of the magnetic field as a gate is further discussed and demonstrated in two different directions. First, a three-terminal molecular router devices that can function as a parallel logic gate, processing two logic operations simultaneously, is presented. Second, the role of inelastic effects arising from electron-phonon couplings on the magnetoresistance properties is analyzed. We show that a remarkable difference between electric and magnetic gating is also revealed when inelastic effects become significant. The inelastic broadening of response curves to electric gates is replaced by a narrowing of magnetoconductance peaks, thereby enhancing the sensitivity of the device.

Nanoionics-based resistive switching memories

Many metal-insulator-metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

An atomic seesaw switch formed by tilted asymmetric Sn-Ge dimers on a Ge (001) surface

When tin (Sn) atoms are deposited on a clean germanium (Ge) (001) surface at room temperature, buckled dimers originating from the Sn atoms are formed at the Ge-dimer position. We identified the dimer as a heterogeneous Sn-Ge dimer by reversing its buckling orientation with a scanning tunneling microscope (STM) at 80 kelvin. An atomic seesaw switch was formed for one-dimensional electronic conduction in the Ge dimer-row direction by using the STM to reversibly flip the buckling orientation of the Sn-Ge dimer and to set up standing-wave states.

Magnetoresistance devices based on single-walled carbon nanotubes

We demonstrate the physical principles for the construction of a nanometer-sized magnetoresistance device based on the Aharonov-Bohm effect [Phys. Rev. 115, 485 (1959)]. The proposed device is made of a short single-walled carbon nanotube (SWCNT) placed on a substrate and coupled to a tip/contacts. We consider conductance due to the motion of electrons along the circumference of the tube (as opposed to the motion parallel to its axis). We find that the circumference conductance is sensitive to magnetic fields threading the SWCNT due to the Aharonov-Bohm effect, and show that by retracting the tip/contacts, so that the coupling to the SWCNT is reduced, very high sensitivity to the threading magnetic field develops. This is due to the formation of a narrow resonance through which the tunneling current flows. Using a bias potential the resonance can be shifted to low magnetic fields, allowing the control of conductance with magnetic fields of the order of 1 T.