Hiroki Ago

Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube

Conventional electronic devices generally utilize only the charge of conduction electrons; however, interest is growing in ‘spin-electronic’ devices, whose operation depends additionally on the electronic spin. Spin-polarized electrons (which occur naturally in ferromagnetic materials) can be injected from a ferromagnet into non-ferromagnetic materials, or through oxide tunnel barriers. The electron-scattering rate at any subsequent ferromagnetic/non-ferromagnetic interface depends on the spin polarity, a property that is exploited in spin-electronic devices. The unusual conducting properties of carbon nanotubes offer intriguing possibilities for such devices; their elastic- and phase-scattering lengths are extremely long, and carbon nanotubes can behave as one-dimensional conductors. Here we report the injection of spin-polarized electrons from ferromagnetic contacts into multi-walled carbon nanotubes, finding direct evidence for coherent transport of electron spins. We observe a hysteretic magnetoresistance in several nanotubes with a maximum resistance change of 9%, from which we estimate the spin-flip scattering length to be at least 130 nm—an encouraging result for the development of practical nanotube spin-electronic devices.

Epitaxial Chemical Vapor Deposition Growth of Single-Layer Graphene over Cobalt Film Crystallized on Sapphire

Epitaxial chemical vapor deposition (CVD) growth of uniform single-layer graphene is demonstrated over Co film crystallized on c-plane sapphire. The single crystalline Co film is realized on the sapphire substrate by optimized high-temperature sputtering and successive H(2) annealing. This crystalline Co film enables the formation of uniform single-layer graphene, while a polycrystalline Co film deposited on a SiO(2)/Si substrate gives a number of graphene flakes with various thicknesses. Moreover, an epitaxial relationship between the as-grown graphene and Co lattice is observed when synthesis occurs at 1000 °C; the direction of the hexagonal lattice of the single-layer graphene completely matches with that of the underneath Co/sapphire substrate. The orientation of graphene depends on the growth temperature and, at 900 °C, the graphene lattice is rotated at 22 ± 8° with respect to the Co lattice direction. Our work expands a possibility of synthesizing single-layer graphene over various metal catalysts. Moreover, our CVD growth gives a graphene film with predefined orientation, and thus can be applied to graphene engineering, such as cutting along a specific crystallographic direction, for future electronics applications.

Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire.

On single-crystal substrates, such as sapphire (alpha-Al 2O 3) and quartz (SiO 2), single-walled carbon nanotubes (SWNTs) align along specific crystallographic axes of the crystal, indicating that the SWNT growth is influenced by the crystal surface. Here, we show that not only the orientation, but also the diameter and chirality of SWNTs are affected by the crystal plane of the sapphire substrate. The aligned SWNTs grown on the A- and R-planes of sapphire have narrower diameter distributions than randomly oriented tubes produced on the C-plane sapphire and amorphous SiO 2. Photoluminescence measurements reveal a striking difference between the aligned SWNTs: near-zigzag tubes are observed on the A-plane and near-armchair tubes on the R-plane. This study shows the route for the diameter and chirality control of SWNTs by surface atomic arrangements of a single-crystal substrate.

Dispersion of metal nanoparticles for aligned carbon nanotube arrays

We report that Co metal nanoparticles (an average diameter of 4 nm) chemically synthesized by a reverse micelle method catalyzes the growth of multiwall carbon nanotubes (MWNTs) aligned perpendicular to a substrate. The surface of the nanoparticles is covered with surfactants so that the nanoparticles can be dispersed in organic solvent. The dispersion of the nanoparticles was cast directly onto a plane Si substrate for thermal pyrolysis of acetylene. We have found that the pretreatment of the metal nanoparticles with hydrogen sulfide before the pyrolysis straightens the MWNTs, suggesting sulfurization of the nanoparticle catalyst plays an important role in regular growth of the MWNTs. The dispersion of the nanoparticles offers a conventional and processible approach to synthesize large area aligned MWNT arrays.

Catalytic Growth of Graphene: Toward Large-Area Single-Crystalline Graphene.

For electronic applications, synthesis of large-area, single-layer graphene with high crystallinity is required. One of the most promising and widely employed methods is chemical vapor deposition (CVD) using Cu foil/film as the catalyst. However, the CVD graphene is generally polycrystalline and contains a significant amount of domain boundaries that limit intrinsic physical properties of graphene. In this Perspective, we discuss the growth mechanism of graphene on a Cu catalyst and review recent development in the observation and control of the domain structure of graphene. We emphasize the importance of the growth condition and crystallinity of the Cu catalyst for the realization of large-area, single-crystalline graphene.

Thermal and electrical conductivity of a suspended platinum nanofilm

This letter reports on the measurements of the in-plane thermal conductivity and the electrical conductivity of a microfabricated, suspended, nanosized platinum thin film with the width of 260nm, the thickness of 28nm, and the length of 5.3μm. The experimental results show that the electrical conductivity, the resistance-temperature coefficient and the in-plane thermal conductivity of the nanofilm are greatly lower than the corresponding bulk values from 77to330K. The comparison results indicate that the relation between the thermal conductivity and the electrical conductivity of this nanofilm might not follow the Wiedemann–Franz law that describes the relation between the thermal conductivity and the electrical conductivity of a bulk metallic material.

Ink-jet printing of nanoparticle catalyst for site-selective carbon nanotube growth

We report on site-selective growth of multiwalled carbon nanotubes (MWNTs) from a Co nanoparticle catalyst patterned by an ink-jet printing (IJP) technique. The dispersion of the Co nanoparticles was employed as “catalyst ink” for the IJP, and the catalyst pattern was subjected to chemical vapor deposition of acetylene gas. The patterned array of MWNTs was obtained with a dot size around 5–30 μm and showed field emission of electrons corresponding to the printed pattern. The present method offers a simple and powerful means to pattern carbon nanotubes at desired positions with any patterns.

Enhanced Chemical Reactivity of Graphene Induced by Mechanical Strain

Control over chemical reactivity is essential in the field of nanotechnology. Graphene is a two-dimensional atomic sheet of sp(2) hybridized carbon with exceptional properties that can be altered by chemical functionalization. Here, we transferred single-layer graphene onto a flexible substrate and investigated the functionalization using different aryl diazonium molecules while applying mechanical strain. We found that mechanical strain can alter the structure of graphene, and dramatically increase the reaction rate, by a factor of up to 10, as well as increase the final degree of functionalization. Furthermore, we demonstrate that mechanical strain enables functionalization of graphene for both p- and n-type dopants, where unstrained graphene showed negligible reactivity. Theoretical calculations were also performed to support the experimental findings. Our findings offer a simple approach to control the chemical reactivity of graphene through the application of mechanical strain, allowing for a tuning of the properties of graphene.

Strain engineering the properties of graphene and other two-dimensional crystals

Graphene has been widely studied for its many extraordinary properties, and other two-dimensional layered materials are now gaining increased interest. These excellent properties make thin layer materials very attractive for integration into a wide variety of technologies, particularly in flexible optoelectronic devices. Therefore, gaining control over these properties will allow for a more focused design and optimisation of these possible technologies. Through the application of mechanical strain it is possible to alter the electronic structures of two-dimensional crystals, such as graphene and transition metal dichalcogenides (e.g. MoS2), and these changes in electronic structure can alter their behaviour. In this perspective we discuss recent advances in the strain engineering of thin layer materials, with a focus on using Raman spectroscopy and electrical transport to investigate the effect of strain as well as the effect of strain on the chemical functionalisation of graphene.

Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene

Multilayered heterostructures of two-dimensional materials have recently attracted increased interest because of their unique electronic and optical properties. Here, we present chemical vapor deposition (CVD) growth of triangular crystals of monolayer MoS2 on single-crystalline hexagonal graphene domains which are also grown by CVD. We found that MoS2 grows selectively on the graphene domains rather than on the bare supporting SiO2 surface. Reflecting the heteroepitaxy of the growth process, the MoS2 domains grown on graphene present two preferred equivalent orientations. The interaction between the MoS2 and the graphene induced an upshift of the Raman G and 2D bands of the graphene, while significant photoluminescence quenching was observed for the monolayer MoS2. Furthermore, photoinduced current modulation along with an optical memory effect was demonstrated for the MoS2-graphene heterostructure. Our work highlights that heterostructures synthesized by CVD offer an effective interlayer van der Waals interaction which can be developed for large-area multilayer electronic and photonic devices.