Conductance steps and quantization in atomic-size contacts

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.

Unraveling the Interplay between Quantum Transport and Geometrical Conformations in Monocyclic Hydrocarbons' Molecular Junctions

In the field of molecular electronics, especially in quantum transport experiments, determining the geometrical configurations of a single molecule trapped between two electrodes can be challenging. To address this challenge, we employed a combination of molecular dynamics (MD) simulations and electronic transport calculations based on density functional theory to determine the molecular orientation in our break-junction experiments under ambient conditions. The molecules used in this study are common solvents used in molecular electronics, such as benzene, toluene (aromatic), and cyclohexane (aliphatic). Furthermore, we introduced a novel criterion based on the normal vector of the surface formed by the cavity of these ring-shaped monocyclic hydrocarbon molecules to clearly define the orientation of the molecules with respect to the electrodes. By comparing the results obtained through MD simulations and density functional theory with experimental data, we observed that both are in good agreement. This agreement helps us to uncover the different geometrical configurations that these molecules adopt in break-junction experiments. This approach can significantly improve our understanding of molecular electronics, especially when using more complex cyclic hydrocarbons.

Quantum Conductance in Memristive Devices: Fundamentals, Developments, and Applications

Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next-generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic-sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

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.

Millikelvin scanning tunneling microscope at 20/22 T with a graphite enabled stick-slip approach and an energy resolution below 8 μeV: Application to conductance quantization at 20 T in single atom point contacts of Al and Au and to the charge density wave of 2H-NbSe2

We describe a scanning tunneling microscope (STM) that operates at magnetic fields up to 22 T and temperatures down to 80 mK. We discuss the design of the STM head, with an improved coarse approach, the vibration isolation system, and efforts to improve the energy resolution using compact filters for multiple lines. We measure the superconducting gap and Josephson effect in aluminum and show that we can resolve features in the density of states as small as 8 μeV. We measure the quantization of conductance in atomic size contacts and make atomic resolution and density of states images in the layered material 2H-NbSe₂. The latter experiments are performed by continuously operating the STM at magnetic fields of 20 T in periods of several days without interruption.

Atomic scale mechanics explored by in situ transmission electron microscopy with a quartz length-extension resonator as a force sensor

An in situ transmission electron microscopy (TEM) holder equipped with a quartz length-extension resonator (LER) as a force sensor was developed to examine the elastic properties of atomic-scale materials. This holder is a useful means of studying the effects of size and crystal orientation on the properties of nanomaterials via measurements of mechanical responses while simultaneously observing atomic structures. The spring constants of nanocontacts (NCs) were determined based on shifts in the resonance frequency of the LER during TEM observations. The LER spring constant and sensitivity (the ratio of the LER induced charge to its oscillation amplitude), both of which are crucial to mechanical evaluation of NCs, were precisely calibrated from an analysis of TEM images along with the output of the electronics attached to the holder. The mechanical stability of the newly developed TEM holder was sufficient to allow chains of Pt atoms in the NC to be maintained for at least several seconds. The minimum measurable NC spring constant was on the order of 1 N m^-1, comparable to that associated with a single atomic bond. The spring constant of a NC composed of a single-bonded chain of two Pt atoms was found to be 13.2 N m^-1. This holder therefore has significant potential with regard to the characterization of nanoscale mechanical properties.

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 2G ₀ and 15G ₀ 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.

Tuning the contact conductance of anchoring groups in single molecule junctions by molecular design

A tetraphenylmethane tripod functionalized with three thiol moieties in the para position can serve as a supporting platform for functional molecular electronic elements. A combined experimental scanning tunneling microscopy break junction technique with theoretical approaches based on density functional theory and non-equilibrium Green's function formalism was used for detailed charge transport analysis to find configurations, geometries and charge transport pathways in the molecular junctions of single molecule oligo-1,4-phenylene conductors containing this tripodal anchoring group. The effect of molecular length (n = 1 to 4 repeating phenylene units) on the charge transport properties and junction configurations is addressed. The number of covalent attachments between the electrode and the tripodal platform changes with n affecting the contact conductance of the junction. The longest homologue n = 4 adopts an upright configuration with all three para thiolate moieties of the tripod attached to the gold electrode. The contact conductance of the tetraphenylmethane tripod substituted by thiols in the para position is higher than that substituted in the meta position. Such molecular arrangement is highly conducting and allows well-defined directional positioning of a variety of functional groups.

Stable, one-dimensional suspended and supported monatomic chains of pnictogens: a metal-insulator framework

Group-VA elements P, As, Sb, and Bi can construct free-standing, stable zigzag monatomic chain structures, which show unusual properties. They are normally semimetals with bands crossing at the Fermi level, but a very narrow gap opens due to spin-orbit coupling. They attain one quantum of conductance under a small bias potential; Bi, being an exception, attains two quanta of conductance. Finite size chains are magnetic semiconductors; their magnetic moments and the order of spin states show an even-odd disparity depending on the number of chain atoms. Variations of the HOMO-LUMO band gaps depending on the spin polarization and the size of the finite chains offer critical tunability. In the periodic, zigzag compound chains, a small band gap opens at the Fermi level. The mysterious zigzag geometry, cohesion, stability and band order of all these chains are well-explained by a simple bond model. When placed on the parent or other monolayers like graphene, h-BN and GaSe, these chains become weakly bound and construct a 1D metallic channel. The artificial grids or networks of these metallic chains on the insulating substrates can constitute metal-insulator frameworks of desired geometry. The zigzag phosphorene chain, having the highest stability, remains stable even at full coverage of adsorbates like H and OH, whereas other chains dissociate. While P-chains can be synthesized on GaSe and graphene substrates, phosphorene nanoribbons can transform into suspended chains under excessive tensile strain. Additionally, we showed that As, Sb, and Bi zigzag chains are weakly bound to their parent monolayers and remain stable. Nitrogen monatomic chains, on the other hand, are prone to instability. The diverse properties unveiled in this study based on the density functional method offer tunability through electric fields and strain.

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.

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.

Metallic, magnetic and molecular nanocontacts

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained.

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.

Thermopower measurements in molecular junctions

The measurement of thermopower in molecular junctions offers complementary information to conductance measurements and is becoming essential for the understanding of transport processes at the nanoscale.

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.

Single-molecule conductance in atomically precise germanium wires

While the electrical conductivity of bulk-scale group 14 materials such as diamond carbon, silicon, and germanium is well understood, there is a gap in knowledge regarding the conductivity of these materials at the nano and molecular scales. Filling this gap is important because integrated circuits have shrunk so far that their active regions, which rely so heavily on silicon and germanium, begin to resemble ornate molecules rather than extended solids. Here we unveil a new approach for synthesizing atomically discrete wires of germanium and present the first conductance measurements of molecular germanium using a scanning tunneling microscope-based break-junction (STM-BJ) technique. Our findings show that germanium and silicon wires are nearly identical in conductivity at the molecular scale, and that both are much more conductive than aliphatic carbon. We demonstrate that the strong donor ability of C-Ge σ-bonds can be used to raise the energy of the anchor lone pair and increase conductance. Furthermore, the oligogermane wires behave as conductance switches that function through stereoelectronic logic. These devices can be trained to operate with a higher switching factor by repeatedly compressing and elongating the molecular junction.

Free-Space Nanometer Wiring via Nanotip Manipulation

Relentless efforts in semiconductor technology have driven nanometer-scale miniaturization of transistors, diodes, and interconnections in electronic chips. Free-space writing enables interconnections of stacked modules separated by an arbitrary distance, leading to ultimate integration of electronics. We have developed a free-space method for nanometer-scale wiring on the basis of manipulating a metallic nanotip while applying a bias voltage without radiative heating, lithography, etching, or electrodeposition. The method is capable of fabricating wires with widths as low as 1-6 nm and lengths exceeding 200 nm with a breakdown current density of 8 TA/m(2). Structural evolution and conduction during wire formation were analyzed by direct atomistic visualization using in situ high-resolution transmission electron microscopy.

Stereoelectronic switching in single-molecule junctions

A new intersection between reaction chemistry and electronic circuitry is emerging from the ultraminiaturization of electronic devices. Over decades chemists have developed a nuanced understanding of stereoelectronics to establish how the electronic properties of molecules relate to their conformation; the recent advent of single-molecule break-junction techniques provides the means to alter this conformation with a level of control previously unimagined. Here we unite these ideas by demonstrating the first single-molecule switch that operates through a stereoelectronic effect. We demonstrate this behaviour in permethyloligosilanes with methylthiomethyl electrode linkers. The strong σ conjugation in the oligosilane backbone couples the stereoelectronic properties of the sulfur-methylene σ bonds that terminate the molecule. Theoretical calculations support the existence of three distinct dihedral conformations that differ drastically in their electronic character. We can shift between these three species by simply lengthening or compressing the molecular junction, and, in doing so, we can switch conductance digitally between two states.

Quantum thermopower of metallic atomic-size contacts at room temperature

We report conductance and thermopower measurements of metallic atomic-size contacts, namely gold and platinum, using a scanning tunneling microscope (STM) at room temperature. We find that few-atom gold contacts have an average negative thermopower, whereas platinum contacts present a positive thermopower, showing that for both metals, the sign of the thermopower in the nanoscale differs from that of bulk wires. We also find that the magnitude of the thermopower exhibits minima at the maxima of the conductance histogram in the case of gold nanocontacts while for platinum it presents large fluctuations. Tight-binding calculations and Green's function techniques, together with molecular dynamics simulations, show that these observations can be understood in the context of the Landauer-Büttiker picture of coherent transport in atomic-scale wires. In particular, we show that the differences in the thermopower between these two metals are due to the fact that the elastic transport is dominated by the 6s orbitals in the case of gold and by the 5d orbitals in the case of platinum.

Impact of Joule heating, roughness, and contaminants on the relative hardness of polycrystalline gold

Asperities play a central role in the mechanical and electrical properties of contacting surfaces. Changes in trends of uniaxial compression of an asperity tip in contact with a polycrystalline substrate as a function of substrate geometry, compressive stress and applied voltage are investigated here by implementation of a coupled continuum and atomistic approach. Surprisingly, an unmodified Au polycrystalline substrate is found to be softer than one containing a void for conditions of high stress and an applied voltage of 0.2 V. This is explained in terms of the temperature distribution and weakening of Au as a function of temperature. The findings in this communication are important to the design of materials for electrical contacts because applied conditions may play a role in reversing relative hardness of the materials for conditions experienced during operation.

Force-conductance correlation in individual molecular junctions

Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual molecular junctions. Here we report on measurements of gold-gold and gold-octanedithiol-gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force-conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule.

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.

Mechanical annealing of metallic electrodes at the atomic scale

The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately 5G0 in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which show how, after repeated indentations (mechanical annealing), the two metallic electrodes are shaped into tips of reproducible structure. These results provide a crucial insight into fundamental aspects relevant to nanotribology or scanning probe microscopies.

An approach to measure electromechanical properties of atomic and molecular junctions

We describe a new setup for simultaneous measurements of force and current in conductive nanocontacts in a liquid environment with a high sampling rate and resolution. A lab-built current-to-voltage converter allows measurements of the current over seven orders of magnitude. As examples, we studied conductances and mechanical forces upon formation and breaking of gold atomic contacts and of two molecular junctions containing 1,2-di(4-pyridyl)ethyne (M1) and 1,4-di(4-pyridyl)buta-1,3-diyne (M2). We found that the forces required to deform or break gold atomic contacts depend critically on the surrounding medium. Further, they show non-linear behaviour in dependence of the number N of gold atoms detached. The electromechanical properties of the two types of molecular junctions upon stretching were analysed by correlating breaking forces with simultaneously measured junction conductances. A rather complex behaviour in a wide range of forces was discovered. Comparison of the current-probe atomic force microscopy experiments on the rupture of molecular junctions with STM-based break junction experiments enables the assignment of breaking forces of molecular junctions to the corresponding junction conductances.

STRUCTURAL AND ELECTRONIC PROPERTIES OF AL NANOWIRES: AN AB INITIO PSEUDOPOTENTIAL STUDY

The stability and electronic structure of a single monatomic Al wire has been studied using the ab initio pseudopotential method. The Al wire undergoes two structural rearrangements under compression, i.e., zigzag configurations at angles of 140° and 60°. The evolution of electronic structures of the Al chain as a function of structural phase transition has been investigated. The relationship between electronic structure and geometric stability is also discussed. The 2p bands in the Al nanowire are shown to play a critical role in its stability. The effects of density functionals (GGA and LDA) on cohesive energy and bond length of Al nanostructures (dimer, chains, and monolayers) are also examined. The link between low dimensional 0D structure (dimer) to high dimensional 3D bulk Al is estimated. An example of optimized tip-suspended finite atomic chain is presented to bridge the gap between hypothetical infinite chains and experimental finite chains.

Real-space calculations for electron transport properties of nanostructures

Recent developments in the fabrication and investigation of conductors of atomic dimensions have stimulated a large number of experimental and theoretical studies on these nanoscale devices. In this paper, we introduce examples presenting the efficiencies and advantages of a first-principles transport calculation scheme based on the real-space finite-difference (RSFD) formalism and the overbridging boundary-matching (OBM) method. The RSFD method does not suffer from the artificial periodicity problems that arise in methods using plane-wave basis sets or the linear dependence problems that occur in methods using atomic basis sets. Moreover, the algorithm of the RSFD method is suitable for massively parallel computers and, thus, the combination of the RSFD and OBM methods enables us to execute first-principles transport calculations using large models. To demonstrate the advantages of this method, several applications of the transport calculations in various systems ranging from jellium nanowires to the tip and surface system of scanning tunneling microscopy are presented.

Two-level conductance fluctuations of a single-molecule junction

The conductance of a single-molecule junction in a low-temperature scanning tunneling microscope has been measured at nanosecond time resolution. In a transition region between tunneling and contact the conductance exhibits rapid two-level fluctuations which are attributed to different geometries of the junction. The voltage dependence of the fluctuations indicates that electrons injected into the lowest unoccupied molecular orbital may efficiently couple to molecular vibrations.

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.

Breakdown of atomic-sized metallic contacts measured on nanosecond scale

We report on a study of atomic-sized metallic contacts on a time scale of nanoseconds using a combined DC and AC circuit. The approach leads to a time resolution 3-4 orders of magnitude faster than the measurements carried out to date, making it possible to observe fast transient conductance-switching events associated with the breakdown, re-formation, and atomic scale structural rearrangements of the contact. The study bridges the wide gap in the time scales between the molecular dynamic simulations and real world experiments, and the method may be applied to study nano- and subnanosecond processes in other nanoscale devices, such as molecular junctions.

Verification of unzipping models of electromigration in gold nanocontacts by in situ high-resolution transmission electron microscopy

We observed in situ the electromigration process of gold (Au) nanocontacts (NCs) by high-resolution transmission electron microscopy. The structural dynamics of the interior and surfaces of the NCs were investigated at the atomic level. In particular, we directly verified the evidence of the unzipping model of electromigration with the in situ observation of surface-edge movement. The fundamental parameters of NCs, i.e., conductance and tensile force, were also measured during in situ lattice imaging of electromigration. Atoms migrating from the negative electrode accumulated at the most constricted regions of the NCs, leading to expansion. As a result, the NCs were compressed by the two electrodes. We demonstrated the magnitude of the force acting on the NCs during electromigration. The critical voltage of electromigration was approximately 80 mV, and the current density at the critical voltage was 60 TA m(-2). We found that Au nanogaps could be fabricated by applying this bias voltage to Au NCs.

Quantum point contact switches

Switching behavior between electron tunneling and ballistic transport states was induced by repeatedly bringing a sharpened nickel wire into contact with a gold surface. The high-conductivity ballistic state had a quantized conductance of 0.977 +/- 0.015 (2e(2)/h). Switching was accomplished by moving the electrodes with a piezoelectric actuator over a distance of 2 angstroms. The two electrodes and the actuator form a three-terminal device that is demonstrated to be a reliable digital and analog switch; it shows good discrimination between high and low states and possesses the important property of power gain. The conductance channel is most likely only one atom wide and possibly consists of a single atom.

Carbon nanotube-clamped metal atomic chain

Metal atomic chain (MAC) is an ultimate one-dimensional structure with unique physical properties, such as quantized conductance, colossal magnetic anisotropy, and quantized magnetoresistance. Therefore, MACs show great potential as possible components of nanoscale electronic and spintronic devices. However, MACs are usually suspended between two macroscale metallic electrodes; hence obvious technical barriers exist in the interconnection and integration of MACs. Here we report a carbon nanotube (CNT)-clamped MAC, where CNTs play the roles of both nanoconnector and electrodes. This nanostructure is prepared by in situ machining a metal-filled CNT, including peeling off carbon shells by spatially and elementally selective electron beam irradiation and further elongating the exposed metal nanorod. The microstructure and formation process of this CNT-clamped MAC are explored by both transmission electron microscopy observations and theoretical simulations. First-principles calculations indicate that strong covalent bonds are formed between the CNT and MAC. The electrical transport property of the CNT-clamped MAC was experimentally measured, and quantized conductance was observed.

Nanocontact resistance and structural disorder induced resistivity variation in metallic metal-oxide nanowires

Several systems of metallic metal-oxide nanowires (NWs), including pure RuO2 and as-implanted and annealed Ru(0.98)Cu(0.02)O2 and Ru(0.93)Cu(0.07)O2 NWs, have been employed in two-probe electrical characterizations by using a transmission electron microscope-scanning tunneling microscope technique with a gold tip. Thermal, mechanical, and electron beam exposing treatments are consecutively applied to reduce the electrical contact resistance, generated from the interface between the NW and the gold tip, so as to evaluate the intrinsic NW resistance. It is found that the residual contact resistance cannot be entirely removed. For each system of metallic metal-oxide NWs, several tens of NWs are applied to electrical characterizations and the total resistances unveil a linear dependence on the ratio of the length to the area of the NWs. As a result, the average resistivity and the contact resistance of the metallic metal-oxide NWs could be evaluated at room temperatures. The average resistivities of pure RuO2 NWs agree well with the results obtained from standard two- and four-probe electrical-transport measurements. In addition, the as-implanted Cu-RuO2 NWs reveal disordered crystalline structures in high-resolution TEM images and give higher resistivities in comparison with that of pure RuO2 NWs. The residual contact resistances of all kinds of metallic metal-oxide NWs unveil, more surprisingly, an approximation value of several kilohms, even though the average resistivities of these NWs change by more than one order of magnitude. It is argued that the ductile gold tip makes one or more soft contacts on the stiff metal-oxide NWs with nanometer roughness and the nanocontacts on the NWs contribute to the electrical contact resistance.

Theoretical investigation on the influence of temperature and crystallographic orientation on the breaking behavior of copper nanowire

In this paper, molecular dynamics simulations have been conducted to study the mechanical stretching of copper nanowires which will finally lead to the formation of suspended liner atomic chains. A total of 2700 samples have been investigated to achieve a comprehensive understanding of the influence of temperature and orientation on the formation of linear atomic chains. Our results prove that linear atomic chains do exist for [100], [111] and [110] crystallographic directions. Stretching along the [111] direction exhibits a higher probability in forming the two-atom contact than that along the [110] and [100] directions. However, for longer linear atomic chains, there emerges a reversed trend. In addition, increasing temperature may decrease the formation probability for stretching along [111] and [110] directions, but this influence is less obvious for that along the [100] direction.

The size effects of electrodes in molecular devices: an ab initio study on the transport properties of C(60)

The role of electrodes in the transport properties of molecular devices is investigated by taking C(60) as an example and using gold nanowire and a gold atomic chain as the electrodes. The calculations are done by an ab initio method combined with the non-equilibrium Green function technique. We find that devices in which a single C(60) molecule is connected with different electrodes show completely different transport behavior. In the case of nanowire/C(60)/nanowire the device shows a metallic behavior with a big equilibrium conductance (about 2.18G(0)) and the current increases rapidly and almost linearly starting from zero. The transmission function shows wide peaks and platforms around the Fermi level. While in the atomic-chain/C(60)/atomic-chain case, the device shows resonant tunneling behavior and the Fermi level lies between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) transmission peaks. This results in a current that is one order of magnitude smaller than that in the nanowire/C(60)/nanowire system and the current increases very slowly until the bias is big enough to include the LUMO peak in the bias window. The big difference in the conductance and the current arises from the different coupling between the electrodes and the C(60) and the different number of channels in the electrodes.

Electron transport in single molecules: from benzene to graphene

Electron movement within and between molecules--that is, electron transfer--is important in many chemical, electrochemical, and biological processes. Recent advances, particularly in scanning electrochemical microscopy (SECM), scanning-tunneling microscopy (STM), and atomic force microscopy (AFM), permit the study of electron movement within single molecules. In this Account, we describe electron transport at the single-molecule level. We begin by examining the distinction between electron transport (from semiconductor physics) and electron transfer (a more general term referring to electron movement between donor and acceptor). The relation between these phenomena allows us to apply our understanding of single-molecule electron transport between electrodes to a broad range of other electron transfer processes. Electron transport is most efficient when the electron transmission probability via a molecule reaches 100%; the corresponding conductance is then 2e(2)/h (e is the charge of the electron and h is the Planck constant). This ideal conduction has been observed in a single metal atom and a string of metal atoms connected between two electrodes. However, the conductance of a molecule connected to two electrodes is often orders of magnitude less than the ideal and strongly depends on both the intrinsic properties of the molecule and its local environment. Molecular length, means of coupling to the electrodes, the presence of conjugated double bonds, and the inclusion of possible redox centers (for example, ferrocene) within the molecular wire have a pronounced effect on the conductance. This complex behavior is responsible for diverse chemical and biological phenomena and is potentially useful for device applications. Polycyclic aromatic hydrocarbons (PAHs) afford unique insight into electron transport in single molecules. The simplest one, benzene, has a conductance much less than 2e(2)/h due to its large LUMO-HOMO gap. At the other end of the spectrum, graphene sheets and carbon nanotubes--consisting of infinite numbers of aromatic rings--have small or even zero energy gaps between the conduction and valence bands. Between these two limits are intermediate molecules with rich properties, such as perylene derivatives made of seven aromatic rings; the properties of these types of molecules have yet to be fully explored. Studying PAHs is important not only in answering fundamental questions about electron transport but also in the ongoing quest for molecular-scale electronic devices. This line of research also helps bridge the gap between electron transfer phenomena in small redox molecules and electron transport properties in nanostructures.

Surface breakdown dynamics of carbon nanocapsules

The structural variation in a single-molecule junction consisting of a hollow multi-walled carbon nanocapsule sandwiched between two gold electrodes during the breakdown of the outermost wall layer was observed in situ by high-resolution transmission electron microscopy. Simultaneously, the current-voltage curve of the junction was measured. The differential conductance of the junction decreased discontinuously from 1G(0) (where G(0) = 2e(2)/h is the conductance quantum, e is the electron charge and h is Planck's constant) to 0.5 G(0) and then to 0.2G(0) when the total number of wall layers decreased from four to three owing to the breakdown of the outermost layer. The maximum current decreased by 19 microA when the differential conductance decreased discontinuously. Our direct observation of the correlation between the atomistic structure and the conductance provides evidence for electron transport through several layers of the multi-walled nanocapsule, similar to that through carbon nanotubes with their surface in contact with electrodes.

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.

Helical [110] gold nanowires make longer linear atomic chains

Quantum mechanical molecular dynamics shows that gold nanowires formed along the [110] direction reconstruct upon stress to form helical nanowires. The mechanism for this formation is discussed. These helical nanowires evolve on stretching to form linear atomic chains. Because helical nanowires do not form symmetrical tips, a requirement to stop the growth of atomic chains, these nanowires produce longer atomic chains than other nanowires. These results are obtained resorting to the use of tight-binding molecular dynamics and ab initio electronic structure calculations.