Nanocontacts Research Articles

Stepwise conductance control of niobium nanocontacts by partial phase transformation

The structural dynamics of pure niobium (Nb) contacts with a width of several nanometers (nanocontacts (NCs)) during the energization using nanosecond pulse-width voltages was observed in situ by transmission electron microscopy. The conductance of Nb NCs was simultaneously measured. As pulse waves with 4 ns width and 0.88 V in average height were impressed to crystal-state Nb NCs, phase transformation into amorphous states was observed. Subsequently, average pulse-width voltages lower than that of the amorphization voltage, i.e., 0.32 V, were impressed to amorphous state NCs, resulting in the inverse phase transformation, i.e., crystallization. In particular, the impression of the lower pulse-width voltages caused stepwise crystallization in the amorphous state NCs, corresponding to multiple stepwise conductance variations. The step height in the stepwise conductance variation depended on the external shape of NCs, the injected energy, and the stage of the stepwise crystallization. This stepwise conductance variation leads to the applications of Nb NCs to optimized ultimate fine multi-level nonvolatile memory devices.

Critical shear stress of gold nanocontacts estimated by in situ transmission electron microscopy equipped with a quartz length-extension resonator

The critical shear stress (CSS) of gold nanocontacts (NCs) was estimated using a home-made in situ transmission electron microscopy holder equipped with a quartz length-extension resonator (LER) as a force sensor. The LER was being oscillated at its resonance frequency utilizing frequency modulation method, enabled us to measure the dissipative energy in the NCs. From the LER amplitude dependence of dissipative energy, we determined the value of CSS to be 0.94 GPa. The dissipation was reproduced by applying this CSS value to a simple slip model with a hysteretic force–displacement curve, suggesting the possibility of elucidating the plastic deformation mechanism.

Thermal origin of light emission in nonresonant and resonant nanojunctions

Electron tunneling is associated with light emission. In order to elucidate its generating mechanism, we provide a novel experimental ansatz that employs fixed-distance epitaxial graphene as metallic electrodes. In contrast to previous experiments, this permits an unobscured light spread from the tunnel junction, enabling both a reliable calibration of the visible to infrared emission spectrum and a detailed analysis of the dependence of the parameters involved. In an open, non-resonant geometry, the emitted light is perfectly characterized by a Planck spectrum. In an electromagnetically resonant environment, resonant radiation is added to the thermal spectrum, both being strictly proportional in intensity. In full agreement with a simple heat conduction model, we provide evidence that in both cases the light emission stems from a hot electronic subsystem in interaction with its linear electromagnetic environment. These very clear results should resolve any ambiguity about the mechanism of light emission in nano contacts.

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.

Mechanical analysis of gold nanocontacts during stretching using an in-situ transmission electron microscope equipped with a force sensor

Gold (Au) nanocontacts (NCs) being stretched in the [110] direction were analyzed using a transmission electron microscopy (TEM) holder equipped with a force sensor to measure spring constants. The Au NCs showed similar change in spring constant with electrical conductance, regardless of the initial shape of NC formed during stretching of contacted two Au wires. The Young’s modulus of the Au NC was estimated to be close to the bulk value. This study demonstrated the performance of TEM-combined method to characterize the mechanical properties of nanomaterials via knowing their shape and size.

Amorphization of pure hafnium nanocontacts and continuous conductance control via phase transition treatment using nanosecond pulse voltage energization

The amorphization of a pure metal nanocontacts (NCs) via pulse voltage energization was verified in pure hafnium (Hf) NCs with a hexagonal close-packed structure. The structural dynamics during amorphization was examined via in situ lattice imaging of transmission electron microscopy with simultaneous conductance measurements. The similarities and difference in the phase transition dynamics and conductance between Hf NCs and pure metal NCs having body-centered cubic structures were demonstrated. In Hf NCs, step-by-step crystallization from amorphous phases could be coordinated by the adjustment of the energization manner of nanosecond pulse voltages, i.e., the pulse height and the energization number. This led to gradual conductance control of the NCs; the conductance could be decreased down to 0.72 of the conductance of the crystalline phase.

Structural transformation between crystal and amorphous states and relating conductance variation in pure molybdenum nanocontacts

The amorphization and crystallization of the molybdenum (Mo) nanocontacts (NCs) via pulsed voltage application were observed in situ at the atomic resolution by transmission electron microscopy. Simultaneously, the variation in conductance of NCs during structural transformation was measured. The structural transformation could be controlled by the height of applied pulse waves. The amorphization was accelerated when NCs have longer constriction regions, which has not been found in NCs comprised of other metallic elements. The amorphized states of NCs were maintained during the observation at room temperature. The conductance ratio of amorphous Mo NCs to crystal Mo NCs was 0.62, which is lower than the ratio of liquid to solid Mo and is the lowest among those of other metallic NCs, leading to applications to advanced switching and memory functions.

Structure control of tungsten nanocontacts through pulsed-voltage application

The structural variation in tungsten nanocontacts (NCs) during a pulsed-voltage application was observed in situ by high-resolution transmission electron microscopy. The direction of electromigration in the NCs changed from the well-known direction to the opposite direction at a critical voltage of 0.9 V. Upon applying a higher pulsed voltage of 2.5 V, the NC structure changed to amorphous, with an average conductance density decreased to 82% of that of the crystalline NCs. We demonstrated that the external shape and texture of tungsten NCs can be controlled with an atomic precision through electromigration and amorphization by a pulsed-voltage application.

Deformation Behavior and Critical Shear Stress of Copper Nanocontacts Studied by In Situ Transmission Electron Microscopy.

Tensile deformation of dislocation-free copper (Cu) nanocontacts (NCs) having widths of less than 6 nm was performed in a transmission electron microscope. The deformation behavior was observed In Situ via lattice imaging and the force acting on the NCs was simultaneously measured. The critical shear stress (CSS) for slip occurred in the NCs was estimated using the observed atomic configuration and the values of the forces. The relation of CSS with the minimum cross-sectional area of the NCs was investigated. The value of CSS was found to be at least two orders of magnitude higher than that observed for the well-known mechanism of dislocation-mediated slip in coarsegrained Cu. Furthermore, it was inferred that when the minimum cross-sectional width was larger than the critical value, i.e., 2.3 nm, deformation occurred via the introduction of dislocations into dislocation-free NCs and subsequent dislocation-mediated slip. When the minimum cross-sectional width was less than the critical value, the deformation mechanism transformed into simultaneous slip, i.e., no dislocation was mediated.

Atomistic Structural Variation via Electromigration in Molten-State Gold Nanocontacts.

The atomistic structural variation in gold nanocontacts (NCs) due to applied voltage pulses was examined inside a transmission electron microscope. This examination was accompanied by simultaneous conductance measurements. The NCs melted during the passage of 4 ns long voltage pulses, with voltages in the 0.75-1.30 V range. Electromigration occurred in the molten state, and was directed from the positively-biased electrode to the negatively-biased electrode, which opposes the observed direction convention of most solid state metals. Voltage pulses higher than 1.30 V resulted in the formation of nanoscaled gaps. This study demonstrates the ability to control the external shape of NCs and to induce nanoscaled gap formation by utilizing the molten state electromigration phenomenon using voltage pulsing.

Influences of magnetic field on the fractal morphology in copper electrodeposition

Copper magneto-electrodeposition (MED) is used decrease roughening in the copper electrodeposition process. This technology plays a vital role in electrodeposition process to synthesize metal alloy, thin film, multilayer, nanowires, multilayer nanowires, dot array and nano contacts. The effects of magnetic fields on copper electrodeposition are investigated in terms of variations in the magnetic field strength and the electrolyte concentration. Based on the experimental results, the mere presence of magnetic field would result in a compact deposit. As the magnetic field strength is increased, the deposit grows denser. The increment in concentration also leads to the increase the deposited size. The SEM image analysis showed that the magnetic field has a significant effect on the surface morphology of electrodeposits.

Atomic Configuration and Conductance of Tantalum Single-Atom Contacts and Single-Atom Wires

The tensile deformation and successive fracture process of tantalum (Ta) nanocontacts (NCs) while applying various bias voltages was observed in situ by high-resolution transmission electron micros...

Atomistic Structural Dynamics and Current Density Variations during the Transformation from Crystalline to Amorphous States in Tantalum Nanocontacts

Pulsed voltages were applied to crystalline tantalum (Ta) nanocontacts (NCs) having a width of approximately 1 nm in a transmission electron microscope and the structural dynamics during their transformation to amorphous states was observed in situ at atomic resolution with simultaneous conductance measurements. Amorphization was caused when a Ta NC was the grain boundary, i.e., the orientations of the two regions adjacent to the contact boundary were different. The amorphous states transformed into crystalline states when the two regions were oriented along the same direction upon the application of the same pulsed voltage. Thus, it was found that the structural transformation of Ta NCs by pulsed voltages depends on the orientational relationship at the contact boundaries. The current density at the minimum cross-sectional area of the amorphized states decreased to 0.71–0.91 relative to that of the crystalline states.

Plastic Flowlike Deformation and Its Relation to Aperiodic Peaks in Conductance Histograms of Molybdenum Nanocontacts

We observed the tensile deformation of molybdenum (Mo) nanocontacts (NCs) and simultaneously measured their conductance by in situ transmission electron microscopy. During deformation, the contact width decreased from several nanometers to a single-atom size. Mo NCs were thinned via a plastic flowlike deformation process. The process differs from the slip on lattice planes, which is frequently observed in NCs made of noble metals. We plotted histograms of the time–conductance traces measured during the tensile deformation of Mo NCs. In the conductance histograms, we observed peaks at 1.8G0 (G0 = 2e2/h, where e is the electron charge and h is Planck’s constant), 3.6G0, and 4.4G0. When the minimum conductance (1.8G0) was measured, the minimum cross-sectional widths of the NCs were 3–7 atoms. These NCs exhibited relaxed structures that formed irregularly after the plastic flowlike deformation occurred in the final stage of the tensile process. We inferred that the aperiodic peaks observed in the conductance ...

Critical Shear Stress of Rhodium Nanocontacts Studied by In Situ High-Resolution Transmission Electron Microscopy.

The structural dynamics during tensile deformation of rhodium (Rh) nanocontacts (NCs) at room temperature was observed by in situ high-resolution transmission electron microscopy. The critical shear stress for a {111}-(110) slip system was estimated from simultaneous measurement of the force acting on the NCs. It was found that the critical shear stress increased to ~5 GPa, which was comparable with that for a homogeneous slip, as the minimum cross-sectional width of the NCs decreased to less than 1.6 nm. This result implies that the slip mechanism in Rh transformed from dislocation-mediated slip to homogeneous slip when the width decreased to less than the critical size.

Structure and Conductance of Aluminum Nanocontacts Studied by <I>In Situ</I> High-Resolution Transmission Electron Microscopy

The structural dynamics and conductance of aluminum nanocontacts (NCs) during mechanical breaking was investigated in situ by high-resolution transmission electron microscopy. When the minimum cross-sectional width of the NCs was found to decrease to less than 1.3 nm at a bias voltage of 12.9 mV, a large strain was introduced in the minimum cross section region. The critical width of straining increased with bias voltage. Below the critical width, the current density started to decrease.

Distance control of electromigration-induced silver nanogaps.

Electromigration (EM) in Ag nanocontacts (NCs) was observed in situ on an atomic scale using simultaneous measurements of electrical conductance and mechanical stress. The in situ observations showed that the critical bias voltage of EM was 45 mV. As the bias voltage was increased to 100-200 mV, the NCs broke and gaps with distances of 1.3 +/- 0.8 nm were obtained for NCs having widths smaller than 6 nm. When the bias voltage was further increased to 200-300 mV, the gaps expanded to more than 3 nm, regardless of the NC width. It was found that the nanogap distance could be controlled to fit specific molecular sizes by appropriately selecting the bias voltage and NC width.

Formation and anisotropic magnetoresistance of Co/Pt nano-contacts through aluminum oxide barrier

We report on the observation of anisotropic magnetoresistance (AMR) in vertical asymmetric nano-contacts (NCs) made through AlOx nano-oxide layer (NOL) formed by ion-assisted oxidation method in the film stack of Co/AlOx-NOL/Pt. Analysis of NC formation was based on in situ conductive atomic force microscopy and transmission electron microscopy. Depending on the purity of NCs from Al contamination, we observed up to 29% AMR ratio at room temperature.

Propagation of magnetic vortices using nanocontacts as tunable attractors

Magnetic vortices in thin films are in-plane spiral spin configurations with a core in which the magnetization twists out of the film plane. Vortices result from the competition between atomic-scale exchange forces and long-range dipolar interactions. They are often the ground state of magnetic dots, and have applications in medicine, microwave generation and information storage. The compact nature of the vortex core, which is 10-20 nm wide, makes it a suitable probe of magnetism at the nanoscale. However, thus far the positioning of a vortex has been possible only in confined structures, which prevents its transport over large distances. Here we show that vortices can be propagated in an unconstrained system that comprises electrical nanocontacts (NCs). The NCs are used as tunable vortex attractors in a manner that resembles the propelling of space craft with gravitational slingshots. By passing current from the NCs to a ferromagnetic film, circulating magnetic fields are generated, which nucleate the vortex and create a potential well for it. The current becomes spin polarized in the film, and thereby drives the vortex into gyration through spin-transfer torques. The vortex can be guided from one NC to another by tuning attractive strengths of the NCs. We anticipate that NC networks may be used as multiterminal sources of vortices and spin waves (as well as heat, spin and charge flows) to sense the fundamental interactions between physical objects and fluxes of the next-generation spintronic devices.

Current–Voltage Characteristics of Silver Nanocontacts Studied byIn situTransmission Electron Microscopy

Silver (Ag) nanocontacts (NCs) were produced by the contact of two Ag nanotips at room temperature inside a transmission electron microscope equipped with two piezodriven goniometer stages. The NCs were thinned during tensile deformation using piezoelements while applying alternating bias voltage. The variations in the bias voltage, current, conductance of the NCs, force, and stress acting on the NCs were measured during dynamic image observation. The atomic configurations of the NCs with various contact widths were analyzed. The current–voltage characteristics were investigated for each configuration. As a result, the present study demonstrated the relationship between the contact structure and the conduction of the NCs; the nonlinearity of the current–voltage curves increased as the NCs were thinned.