Solid state molybdenum carbide nanomotors driven via high temperature carbon-decomposition catalytic reactions.

The motion of solid state nanomotors, i.e., molybdenum carbide nanoparticles, which were driven via carbon-decomposition catalytic reactions at ∼2900 K, was directly observed by in situ transmission electron microscopy. The nanomotors exhibited unidirectional linear motions inside the hollow space of multiwall carbon nanotubes, reciprocating motions around the nanotube endcaps, and rotational motions in the hollow spaces of carbon nanocapsules. The inner atomic wall-layers of carbon nanotubes and nanocapsules were consumed during the nanomotor motions.

High-power laser irradiation for high-temperature in situ transmission electron microscopy.

We developed high temperature in situ transmission electron microscopy using a high-density laser irradiation device (nominal maximum laser density ~9.4 GW/m2) and a corresponding heat shielding sample mount device. The spatial line resolution of the microscope was maintained to be 0.14 nm at ambient temperatures after the installation of the laser irradiation device. The system was applied to the investigation of high temperature structural variation in tungsten plates. When the laser power was increased up to irradiation densities of approximately 61-280 MW/m2 (laser source output: 130-590 mW) to degrade tungsten plates, the microscope was undamaged. The surface dynamics was observed in situ by lattice imaging at irradiation densities of approximately 61-75 MW/m2 (laser source output: 130-160 mW); the spatial line resolution of the microscope was maintained to be 0.23 nm at high temperatures. It was expected that high temperature observation is realized using this heating system, which can be applied to the investigation of various advanced heat-resistant materials. We found using this heating system that degradation in tungsten plates started from surfaces and progressed through the preferential generation of characteristic defects, such as atomistic and nanometer holes and rods, and their subsequent evolution in thinner regions during the heating. It was demonstrated that the holes and rod were truncated with {110} sidewalls, i.e., these surfaces were stable in tungsten at high temperatures.

Crossing interfacial conduction in nanometer-sized graphitic carbon layers.

Graphitic carbon layers (GCLs), exemplified by graphene, have been proposed for potential application in high-performance functional devices due to superior electrical properties, e.g., high electron mobility. In state-of-the-art electronics, it is required that GCLs are miniaturized to nanometer scales and incorporated into the integrated circuits to exhibit novel functions at nanometer scales. However, the implementation of nanometer-scale GCLs is suspended; the function in devices is deteriorated by increasing contact resistance in miniaturized GCL/electrode interfaces. In this study, nanometer-sized GCL/gold (Au) interfaces were fabricated via atomistic visualization of nanomanipulation, and simultaneously their contact resistance was measured. We showed that the contact resistivity of the interfaces was decreased to the order of 10-10Ω cm2, which was 104 times smaller than that of micrometer-sized or larger GCL/metal interfaces. In addition, it was revealed that peculiar electrical conduction at the nanometer-sized GCL/Au interfaces emerged; current flows throughout the entire area of the interfaces unlike micrometer-sized or larger GCL/metal interfaces. These results directly contribute to actual application of GCLs in advanced nanodevices.

Element Mapping in Single-Atom-Width Platinum-Iridium Wires.

Single-atom-width wires (SAWWs) of platinum-iridium (PtIr) alloy were produced by mechanical breaking inside a transmission electron microscope. The formation dynamics, the atomic configuration, and the conductance were observed in situ. From the observed lattice images of the SAWWs and image simulation, the structure models, i.e., the configurations of atom position and element allocation, were constructed. Using the experiment-based structural models, the first-principle calculation of the conductance was performed. The atomic configuration and element allocation of the observed SAWWs were identified via the combination of the lattice imaging and calculation. The conductance of PtIr SAWWs changed in complexity for different element allocation in addition to the wire length and the configuration of the constituent atoms, which was difficult to presage from the conductance features of pure metal SAWWs. The present study revealed that the conductance of alloy SAWWs can be controlled by element allocation.

Surface reconstruction in gold nanowires.

Surface reconstructions are caused by structural stabilization resulting from the modulation of surface atomic positions. Studies on surface reconstruction have been conducted for substantially large surfaces, rather than at the size of reconstructed surface unit cells. Hence, well-known surface reconstruction manners may not be applicable for the surfaces of nanometer-sized isolated crystals, such as nanoclusters, nanowires and nanotubes. This is because they have high surface area-to-interior volume ratios exceeding several tens of percent, and their surface structures significantly affect the stabilization of their entire structures. In this study, we demonstrate the inherent surface reconstruction of gold nanowires via nanosecond-pulsed electromigration with the application of tensile stresses. The results lead to evolutions in basic studies relating to surface reconstruction and nanostructures and in applications of nanowires, for which stabilization is essential when they are used in extremely miniaturized integrated circuits for next-generation electronics.

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.

Growth Texture and Mechanism of Zinc Nanowires Produced by Mechanical Elongation of Nanocontacts.

Two zinc nanotips were brought into contact and elongated inside a transmission electron microscope, thereby growing single-crystal nanowires. The growth dynamics was observed in situ via a lattice imaging method. The preferential crystal growth directions were identified as [101-0], [112-0], [101-2-], and [0001]. Of these, the nanowires grown along the [101-0] and [112-0] directions accounted for 75% of the total and were surrounded by low-energy side surfaces, i.e., {0001}, {101-1}, and {101-0}. On the basis of these features, models of the nanowire morphology were proposed. In either growth direction, the tensile force aligned parallel to the direction along which slip events corresponding to the predominant slip system were unlikely to occur. This led to a high tensile stress for extracting atoms from the growth region, i.e., the promotion of nanowire growth.

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.

Young's Modulus of Fullerene C60-C70 Alloy Crystalline Nanowhiskers.

Crystalline nanowhiskers (NWs) composed of fullerene C60 and C70 molecules, i.e., alloy NWs, were synthesized by a liquid-liquid interfacial precipitation method. The nominal composition of C70 ranged from 0 to 40 mass%. The bending tests of the alloy NWs were performed inside a high-resolution transmission electron microscope, and the deformation behavior was observed in situ. The bending force acting on the NWs were measured simultaneously by an optical deflection method, and the Young's modulus was estimated from the resulting force-flexure curves. The average Young's modulus was found to increase to approximately 30 GPa as the C70 composition was increased to the solubility limit. In contrast, the Young's modulus decreased with increasing NW diameter caused by the addition of C70.

Surface Morphology Analysis of Zirconium Dioxide Nanoparticles at 1200 K by Transmission Electron Microscopy.

Zirconium dioxide (ZrO2) nanoparticles were heated at 1200 K in a transmission electron microscope, and their surfaces were observed in situ via lattice imaging. The nanoparticles exhibited well-defined crystal habits while fluctuating the surface terrace structures. The stable surfaces and ridges were inferred from the fluctuation of the terraces, leading to the construction of the three-dimensional surface structures at 1200 K. By contrast, after the specimen cooled to room temperature, the surface fluctuation of the nanoparticles stopped and the crystal habit disappeared, implying that the crystal habit was maintained because of atomic diffusion during the surface fluctuation.

Transformation from slip to plastic flow deformation mechanism during tensile deformation of zirconium nanocontacts.

Various types of nanometer-sized structures have been applied to advanced functional and structural devices. Inherent structures, thermal stability, and properties of such nanostructures are emphasized when their size is decreased to several nanometers, especially, to several atoms. In this study, we observed the atomistic tensile deformation process of zirconium nanocontacts, which are typical nanostructures used in connection of nanometer-sized wires, transistors, and diodes, memory devices, and sensors, by in situ transmission electron microscopy. It was found that the contact was deformed via a plastic flow mechanism, which differs from the slip on lattice planes frequently observed in metals, and that the crystallinity became disordered. The various irregular relaxed structures formed during the deformation process affected the conductance.

Development of 2000 K Class High Temperature In Situ Transmission Electron Microscopy of Nanostructured Materials via Resistive Heating.

We report the development of a new type of a 2000 K class high temperature stage for transmission electron microscopy (TEM) of various shaped nanostructured materials, i.e., nanometer-sized isolated nanostructures, such as particles, fibers, and thin films, and nanocrystalline bulk materials. The maximum temperature of the heating stage was 300­700 K higher than that of prevailing heating stages. In addition, we found that since the structure of the developed heating stage is simple, the stage can be applied to most of transmission electron microscopes without any additional modification. The stability of the heating stage was confirmed by In Situ high-resolution observation of the lattice fringes of the zirconium dioxide nanoparticles heated at 1183 K. We observed In Situ the nanoscale structural variation of titanium plate surfaces around 900 K and the melting of nanocrystalline iron thin films around 1808 K by this method. It was demonstrated that the heating stage is useful for the analysis of high temperature structural dynamics of various shaped nanostructured materials.

Structures and electrical properties of single nanoparticle junctions assembled using LaC2-encapsulating carbon nanocapsules.

As the miniaturization of integrated circuits advances, electronics using single molecules and nanosize particles are being studied increasingly. Single nanoparticle junctions (SNPJs) consist of two electrodes sandwiching a single nanoparticle. Nanocarbons with nanospaces in their center, such as fullerenes, carbon nanotubes, and carbon nanocapsules (CNCs), are expected to be elements of advanced SNPJs. In this study, SNPJs were assembled using lanthanum dicarbide (LaC2)-encapsulating CNCs and two gold (Au) electrodes by a nanotip operation inside a high-resolution transmission electron microscope. The atomic configuration and electrical resistance of the SNPJs were investigated in situ. The results implied that the electrical resistance of the SNPJ depended on the interface structures of the contacts between the CNC and Au electrodes, i.e., the contact electrical resistance, and the greatest portion of the current through the SNPJ flowed along the outermost carbon layer of the CNC. Thus, the resistance of the SNPJs using the CNCs was demonstrated and the electrical conduction mechanism of one of the CNC was discussed in this study.

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 In Situ 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.

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.

The structure of graphene/nickel interfaces within nickel-encapsulating carbon nanocapsules studied by high-resolution transmission electron microscopy.

Nickel (Ni)-encapsulating carbon nanocapsules (CNCs) were synthesized by vacuum deposition. The graphene/Ni interfaces within the CNCs were observed by high-resolution transmission electron microscopy. From lattice imaging, the orientational relationship and atomic configuration at the interfaces were investigated. It was found that the interlayer spacing at the (0001)graphene/(001)Ni interfaces was 0.21 +/- 0.05 nm. The interfaces within Ni3C-encapsulating CNCs, synthesized by the same method, were also examined.

Interface structure of niobium carbide-encapsulating carbon nanocapsules studied by high-resolution transmission electron microscopy.

Carbon nanocapsules (CNCs) encapsulating niobium carbide (NbC) crystals with a sodium chloride structure were synthesized via a gas-evaporation method by arc-discharge heating. CNCs were observed by high-resolution transmission electron microscopy, and the atomic configurations at the graphene/NbC interfaces were investigated. The NbC crystals within the nanospaces of CNCs were truncated by the {100}, {110}, and (111} facets and were coated with several graphene layers. It was found that the interlayer spacings for the graphene{0001}/NbC{100} and graphene{0001}/NbC{110} interfaces were 0.33 +/- 0.05 nm, whereas those for the graphene{0001}/{111}NbC interfaces were 0.28 +/- 0.05 nm.

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.

Electrical conductivity of single molecular junctions assembled from Co- and Co3C-encapsulating carbon nanocapsules.

Single molecular junctions (SMJs) were assembled from cobalt (Co)- and Co carbide (Co3C)-encapsulating carbon nanocapsules (CNCs) and two gold electrodes inside a high-resolution transmission electron microscope equipped with a specimen-piezomanipulation system. The structure and electrical transport properties of the SMJs were investigated in situ. The current density depended on the perimeter of the contact area between CNCs and the electrodes, showing that the current flowed not through the encapsulated region but rather along the graphene layers of CNCs. It was demonstrated that the properties of graphene can be applied to nanodevices using CNCs irrespective of the encapsulating materials.