Quantization effects in the conductance of metallic contacts at room temperature

Nanoionics enabled atomic point contact construction and quantum conductance effects

The miniaturization of electronic devices is important for the development of high-density and function-integrated information devices. Atomic-point-contact (APC) structures refer to narrow contact areas formed by one or more atoms between two conductive electrodes that produce quantum conductance effects when the electrons pass through the APC channel, providing a new development path for the miniaturization of information devices. Recently, nanoionics has enabled the electric field reconfiguration of APC structures in solid-state electrolytes, offering new approaches to controlling the quantum conductance states, which may lead to the development of emerging information technologies with low power consumption, high speed, and high density. This review provides an overview of APC structures with a focus on the fabrication methods enabled by nanoionics technology. In particular, the advantages of electric field-driven nanoionics in the construction of APC structures are summarized, and the influence of external fields on quantum conductance effects is discussed. Recent studies on electric field regulation of APC structures to achieve precise control of quantum conductance states are also reviewed. The potential applications of quantum conductance effects in memory, computing, and encryption-related information technologies are further explored. Finally, the challenges and future prospects of quantum conductance effects in APC structures are discussed.

Statistical signature of electrobreakdown in graphene nanojunctions

Controlled electrobreakdown of graphene is important for the fabrication of stable nanometer-size tunnel gaps, large-scale graphene quantum dots, and nanoscale resistive switches, etc. However, owing to the complex thermal, electronic, and electrochemical processes at the nanoscale that dictate the rupture of graphene, it is difficult to generate conclusions from individual devices. We describe here a way to explore the statistical signature of the graphene electrobreakdown process. Such analysis tells us that feedback-controlled electrobreakdown of graphene in the air first shows signs of joule heating-induced cleaning followed by rupturing of the graphene lattice that is manifested by the lowering of its conductance. We show that when the conductance of the graphene becomes smaller than around 0.1 G0, the effective graphene notch width starts to decrease exponentially slower with time. Further, we show how this signature gets modified as we change the environment and or the substrate. Using statistical analysis, we show that the electrobreakdown under a high vacuum could lead to substrate modification and resistive-switching behavior, without the application of any electroforming voltage. This is attributed to the formation of a semiconducting filament that makes a Schottky barrier with the graphene. We also provide here the statistically extracted Schottky barrier threshold voltages for various substrate studies. Such analysis not only gives a better understanding of the electrobreakdown of graphene but also can serve as a tool in the future for single-molecule diagnostics.

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.

Heat transport through atomic contacts

Heat transport and dissipation at the nanoscale severely limit the scaling of high-performance electronic devices and circuits. Metallic atomic junctions serve as model systems to probe electrical and thermal transport down to the atomic level as well as quantum effects that occur in one-dimensional (1D) systems. Whereas charge transport in atomic junctions has been studied intensively in the past two decades, heat transport remains poorly characterized because it requires the combination of a high sensitivity to small heat fluxes and the formation of stable atomic contacts. Here we report heat-transfer measurements through atomic junctions and analyse the thermal conductance of single-atom gold contacts at room temperature. Simultaneous measurements of charge and heat transport reveal the proportionality of electrical and thermal conductance, quantized with the respective conductance quanta. This constitutes a verification of the Wiedemann-Franz law at the atomic scale.

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.

Shot noise variation within ensembles of gold atomic break junctions at room temperature

Atomic-scale junctions are a powerful tool to study quantum transport, and are frequently examined through the mechanically controllable break junction technique. The junction-to-junction variation of atomic configurations often leads to a statistical approach, with ensemble-averaged properties providing access to the relevant physics. However, the full ensemble contains considerable additional information. We report a new analysis of shot noise over entire ensembles of junction configurations using scanning tunneling microscope-style gold break junctions at room temperature in ambient conditions, and compare these data with simulations based on molecular dynamics, a sophisticated tight-binding model, and nonequilibrium Green's functions. The experimental data show a suppression in the variation of the noise near conductances dominated by fully transmitting channels, and a surprising participation of multiple channels in the nominal tunneling regime. Comparison with the simulations, which agree well with published work at low temperatures and ultrahigh vacuum conditions, suggests that these effects likely result from surface contamination and disorder in the electrodes. We propose additional experiments that can distinguish the relative contributions of these factors.

Real-space finite-difference calculation method of generalized Bloch wave functions and complex band structures with reduced computational cost

Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

Study of ballistic gold conductor using ultra-high-vacuum transmission electron microscopy

Metal contacts are regarded as key elements of nanometer-scale electronics. Since gold contacts show quantized conductance even at room temperature, much effort has been devoted to understand their conductance behavior on the nanoscale. However, gold contacts do not always show quantized conductance steps during their thinning process, the reason for which has been an open question. Thus, it is necessary to investigate the relationship between the atomic structure and conductance of gold contacts. We developed a custom-made scanning tunneling microscope combined with an ultra-high vacuum transmission electron microscope to clarify the structural dependence of conductance quantization in gold contacts. We found that [111] and [001] gold contacts with a bottleneck shape showed a gradual decrease in conductance with elastic elongation and successive conductance jumps with periodic plastic deformation. In contrast, [110] gold contacts had a hexagonal prism shape (termed gold [110] nanowires). In the conductance histogram, peaks appeared nearly in steps of the quantum unit. We found that the prominent peaks corresponded to stable gold nanowires with a regular hexagonal cross-section. Following first-principles calculations, we confirmed that very thin gold [110] nanowires were ballistic conductors. The conductance behavior differed depending on the contact shape.

Field deposition from metallic tips onto insulating substrates

The deposition of gold ions from atomic force microscope cantilever tips onto bulk insulating substrates with nearby surface electrodes is discussed. Numerical models of the potential distribution are used to estimate potential barriers for the desorption process. These models indicate deposition height thresholds of 7-10 nm with the tip 20-25 nm from the metallic electrode edge over a KBr surface but greater than 20 nm high for InP/GaAs/InP substrates with a two-dimensional electron gas (2DEG) as the back electrode. Experimental results for the deposition of gold clusters over KBr surfaces near metal electrodes in ultra-high vacuum (UHV) are presented and show promising agreement with calculations of the deposition threshold heights. Deposition of clusters over InP is discussed for comparison and indicates similar trends.

Controlled electroplating and electromigration in nickel electrodes for nanogap formation

We report the fabrication of nickel nanospaced electrodes by electroplating and electromigration for nanoelectronic devices. Using a conventional electrochemical cell, nanogaps can be obtained by controlling the plating time alone and after a careful optimization of electrodeposition parameters such as electrolyte bath, applied potential, cleaning, etc. During the process, the gap width decreases exponentially with time until the electrode gaps are completely bridged. Once the bridge is formed, the ex situ electromigration technique can reopen the nanogap. When the gap is ∼ 1 nm, tunneling current-voltage characterization shows asymmetry which can be corrected by an external magnetic field. This suggests that charge transfer in the nickel electrodes depends on the orientation of magnetic moments.

Diffusion-limited and advection-driven electrodeposition in a microfluidic channel

Self-terminating electrochemical fabrication was used within a microfluidic channel to create a junction between two Au electrodes separated by a gap of 75 microm . During the electrochemical process of etching from the anode to deposition at the cathode, flow could be applied in the anode-to-cathode direction. Without applied flow, dendritic growth and dense branching morphologies were typically observed at the cathode. The addition of applied flow resulted in a densely packed gold structure that filled the channel. A computer simulation was developed to explore regimes where the diffusion, flow, and electric field between the electrodes individually dominated growth. The model provided good qualitative agreement relating flow to the experimental results. The model was also used to contrast the effects of open and closed boundaries and electric field strength, as factors related to tapering.

Screening in nanowires and nanocontacts: field emission, adhesion force, and contact resistance

The explanations of several nanoscale phenomena such as the field enhancement factor in field emission, the large decay length of the adhesion force between a metallic tip and a surface, and the contact resistance in a nanowire break junction have been elusive. Here we develop an analytical theory of Thomas-Fermi screening in nanoscale structures. We demonstrate that nanoscale dimensions give rise to an effective screening length that depends on the geometry and physical boundary conditions. The above phenomena are shown to be manifestations of the effective screening length.

A Review on the Electrochemical Sensors and Biosensors Composed of Nanowires as Sensing Material

The development and application of nanowires for electrochemical sensors and biosensors are reviewed in this article. Next generation sensor platforms will require significant improvements in sensitivity, specificity and parallelism in order to meet the future needs in variety of fields. Sensors made of nanowires exploit some fundamental nanoscopic effect in order to meet these requirements. Nanowires are new materials, which have the characteristic of low weight with extraordinary mechanical, electrical, thermal and multifunctional properties. The advantages such as size scale, aspect ratio and other properties of nanowires are especially apparent in the use of electrical sensors such as electrochemical sensors and in the use of field-effect transistors. The preparation methods of nanowires and their properties are discussed along with their advantages towards electrochemical sensors and biosensors. Some key results from each article are summarized, relating the concept and mechanism behind each sensor, with experimental conditions as well as their behavior at different conditions.

Molecular Engineering and Measurements To Test Hypothesized Mechanisms in Single Molecule Conductance Switching

Six customized phenylene-ethynylene-based oligomers have been studied for their electronic properties using scanning tunneling microscopy to test hypothesized mechanisms of stochastic conductance switching. Previously suggested mechanisms include functional group reduction, functional group rotation, backbone ring rotation, neighboring molecule interactions, bond fluctuations, and hybridization changes. Here, we test these hypotheses experimentally by varying the molecular designs of the switches; the ability of the molecules to switch via each hypothetical mechanism is selectively engineered into or out of each molecule. We conclude that hybridization changes at the molecule-surface interface are responsible for the switching we observe.

Development of a Nanowire-Based Test Bed Device for Molecular Electronics Applications

In this paper, we present a novel test bed system which we believe addresses several key challenges in molecular electronics, i.e., the need to fabricate metal-molecule-metal junctions that have the potential to facilitate single-molecule measurements, are easily characterized, and are reproducible. The system is based upon template-electrodeposited metal nanowires incorporating a self-assembled monolayer spacer that are fabricated into electrical devices using direct-write photolithography. Removal of the spacer leaves a nanometer-sized, characterizable gap to which nanoparticles or a test molecule of interest can be attached postfabrication. Here we report the fabrication procedure together with results showing the application of these devices to the study of the i/V characteristics of Au nanoparticles at cryogenic temperatures. These data demonstrate that the performance of these easily produced, inexpensive, novel devices compares favorably to that of devices made using preexisting methods.

Metal nanowire arrays by electrodeposition

We describe two related methods for preparing arrays of nanowires composed of molybdenum, copper, nickel, gold, and palladium. Nanowires were obtained by selectively electrodepositing either a metal oxide or a metal at the step edges present on the basal plane of highly oriented pyrolytic graphite (HOPG) electrodes. If a metal oxide was electrodeposited, then nanowires of the parent metal were obtained by reduction at elevated temperature in hydrogen. The resulting nanowires were organized in parallel arrays of 100-1000 wires. These nanowires were long (some > 500 microns), polycrystalline, and approximately hemicylindrical in cross-section. The nanowire arrays prepared by electrodeposition were also "portable": After embedding the nanowires in a polymer or cyanoacrylate film, arrays of nanowires could be lifted off the graphite surface thereby facilitating the incorporation of metal nanowire arrays into devices such as sensors.

Signature of atomic structure in the quantum conductance of gold nanowires

We have used high resolution transmission electron microscopy to determine the structure of gold nanowires generated by mechanical stretching. Just before rupture, the contacts adopt only three possible atomic configurations, whose occurrence probabilities and quantized conductance were subsequently estimated. These predictions have shown a remarkable agreement with conductance measurements from a break junction operating in ultrahigh vacuum, corroborating the derived correlation between nanowire atomic structure and conductance behavior.