Takanari Saito

In situ atomic force microscopy imaging of structural changes in metal nanowires during feedback-controlled electromigration

The authors present the real-time atomic force microscopy (AFM) imaging of structural changes in gold (Au) nanowires during the feedback-controlled electromigration (FCE) process. The resistance increases during the FCE process and is associated with drastic changes in the nanowire morphology, suggesting successful control of electromigration (EM) through the FCE scheme. Moreover, the AFM images taken after performing FCE indicate a redeposition of matter along the nanowire in the direction of the anode side. The grains show faceting structures at the anode side. Furthermore, to obtain quantitative information on the height of structures, cross-sections of the nanowire obtained from the AFM images during FCE were investigated. The height evolution of the narrowest part of the wire perpendicular to the electron flow was obtained, showing that void nucleation and void growth along the grain boundaries, which are located on the border of the nanowire, start in the vicinity of the nanowire constriction at the...

A new computing architecture using Ising spin model implemented on FPGA for solving combinatorial optimization problems

Recently, the new computing architecture using Ising spin model has been attracting considerable attention. It is well known that the Ising spin model represents the physical properties of ferromagnetic materials in terms of statistical mechanics. In this model, the spin states are varied in order to minimize the system energy automatically, by the interaction between connected adjacent spins. The new computing scheme maps combinatorial optimization problems based on Ising model and solves these problems by using ground state search operations exploiting its convergence property. In this report, a new computing architecture using Ising spin model was implemented using field-programmable gate array (FPGA), and Ising computing using FPGA was investigated to solve combinatorial optimization problems.

Prompt decision method for ground-state searches of natural computing architecture using 2D ising spin model

Recently, the ability to analyze big data has been required for the optimization of social systems and the development of artificial intelligence. Consequently, the solution of combinatorial optimization problems has become important in recent years, especially for the suitable operation of infrastructures. However, combinatorial optimization problems have unique properties that the number of candidate solutions increases explosively as the number of parameters is increased. One possible resolution of this issue involves artificial and/or simulated Ising spin system. In this work, we have implemented such a spin system using “prompt decision logic”. The convergence operation was successfully observed in prompt decision method for spin interaction. Therefore, it is indicated that Ising computing by prompt decision logic could resolve combinatorial optimization problems.

Wearable strain sensors based on thin graphite films for human activity monitoring

Wearable health-monitoring devices have attracted increasing attention in disease diagnosis and health assessment. In many cases, such devices have been prepared by complicated multistep procedures which result in the waste of materials and require expensive facilities. In this study, we focused on pyrolytic graphite sheet (PGS), which is a low-cost, simple, and flexible material, used as wearable devices for monitoring human activity. We investigated wearable devices based on PGSs for the observation of elbow and finger motions. The thin graphite films were fabricated by cutting small films from PGSs. The wearable devices were then made from the thin graphite films assembled on a commercially available rubber glove. The human motions could be observed using the wearable devices. Therefore, these results suggested that the wearable devices based on thin graphite films may broaden their application in cost-effective wearable electronics for the observation of human activity.

Tuning of channel conductance of au nanowires using ultrafast electromigration controlled by a field-programmable gate array

Feedback-controlled electromigration (FCE) has been employed to control metal nanowires with quantized conductance and to create nanogaps. However, the formation of nanostructures by conventional FCE procedure using a microprocessor-based controller with a general purpose operating system (GPOS) is considerably slow process. Therefore, we proposed an ultrafast FCE method using a field-programmable gate array (FPGA) to immediately and precisely control the channel conductance of Au nanowires. In this report, we study the tuning of quantized conductance of Au nanowires by ultrafast FCE using FPGA-based control system with a constant-voltage (CV) method. First, in the FCE procedure, preset values of quantized conductance of Au nanowires were defined as 25 G0, 15 G0, and 5 G0 (G0 = 2e2/h). The conductance of the Au nanowires during FCE procedure decreased until the conductance reached the preset values within an order of a millisecond. Furthermore, the quantized conductance plateaus of the Au nanowires were precisely controlled by the CV procedure with the preset values of 15 G0 and 5 G0. These results imply that the combination of FCE and CV procedures with FPGA-based control system can precisely and stably tune the channel conductance of Au nanowires with millisecond-scale resolution.

Investigation of strain sensors based on thin graphite wires

In this study, the electrical properties of thin graphite wires were investigated for strain sensors. The thin graphite wires were simply and easily fabricated from pyrolytic graphite sheet, which can be formed by firing a polymer film (such as a polyimide film) at high temperatures. The resistance of the thin graphite wires increased under increasing tensile bending strains and decreased under increasing compressive bending strains. Notably, the sensitivity of the sensors increased when the thickness of the thin graphite wires was reduced. This property was investigated via modeling of the strain-induced changes in the overlap area and conduction pathways of the graphite flakes. Multiple-cycle tests were carried out to evaluate the long-term stability of the thin graphite wires; specifically, the electrical response was monitored under repeated cycling, for approximately 1000 cycles. The thin graphite wires were assembled on ultrathin gloves to fabricate data gloves that could detect finger motions. The ...

In-situ temperature measurements of Joule-heated graphene using near-infrared CCD imaging system

We report temperature distribution of graphene during Joule heating process using in-situ near-infrared (NIR) charge-coupled device (CCD) imaging system. Graphene layers were prepared using mechanical exfoliation of a pyrolytic graphite sheet (PGS), which is commercially available from an industrial materials company, and were then deposited on SiO2/Si substrates with approximately 780 nm thermally grown oxide. Thickness of the graphene layers was optically determined to be 20-80 nm using Fresnel theory. In order to investigate the heating process of the graphene, the temperature of the graphene under current flow was estimated using NIR microscopy with a CCD detector. A hand-made, in-situ experimental setup consists of an IR microscope, a NIR CCD, and an image enhancer. The CCD detector is mounted on the IR microscope with objective 20×. Heating experiments were carried out in obscurity. Joule heating process controlled with applied bias voltages was performed for the graphene in vacuum/ambient air, and the temperature distribution of the graphene during NIR emission was successfully studied by in-situ NIR CCD imaging system. The temperature of Joule-heated graphene was detected to be approximately 800 K. These results imply that NIR CCD imaging system is a useful tool for the investigation of temperature distribution of graphene.

In-situ observation of temperature distribution of microheaters using near-infrared CCD imaging system

Temperature distribution of microheaters during electromigration (EM) was observed by near-infrared (NIR) charge coupled device (CCD) imaging system. The temperature of the hot spot, located at the tip of the microheater, was well controlled with varying the applied voltage. Microheaters were broken during EM processes, and breakdown region clearly corresponded to the position of the hot spot. These results imply that NIR CCD imaging system is a useful tool for the investigation of the temperature distribution of microheaters.

Nanoscale mechanical scratching of graphene using scanning probe microscopy

Nanolithography of graphene surfaces using scanning probe microscopy (SPM) scratching with a diamond-coated tip was systematically investigated. The graphene films were obtained by mechanical exfoliation of pyrolytic graphite sheet (PGS). The groove size increased linearly with the applied force. Furthermore, there were no effects of scan speed and scratch angle on the groove size. These results imply that SPM scratch nanolithography is promising for the fabrication of nanoscale graphene devices.