Kenji Hata

Publications (59) View all

• Article: Diameter and Density Control of Single-Walled Carbon Nanotube Forests by Modulating Ostwald Ripening through Decoupling the Catalyst Formation and Growth Processes.

  Shunsuke Sakurai, Masayasu Inaguma, Don N Futaba, Motoo Yumura, Kenji Hata
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  ABSTRACT: A continuous and wide range control of the diameter (1.9-3.2 nm) and density (0.03-0.11 g cm(-3) ) of single-walled carbon nanotube (SWNT) forests is demonstrated by decoupling the catalyst formation and SWNT growth processes. Specifically, by managing the catalyst formation temperature and H2 exposure, the redistribution of the Fe catalyst thin film into nanoparticles is controlled while a fixed growth condition preserved the growth yield. The diameter and density are inversely correlated, where low/high density forests would consist of large/small diameter SWNTs, which is proposed as a general rule for the structural control of SWNT forests. The catalyst formation process is modeled by considering the competing processes, Ostwald ripening, and subsurface diffusion, where the dominant mechanism is found to be Ostwald ripening. Specifically, H2 exposure increases catalyst surface energy and decreases diameter, while increased temperature leads to increased diffusion on the surface and an increase in diameter.
  Small 04/2013; · 8.35 Impact Factor
• Article: A Torsion Sensing Material from Aligned Carbon Nanotubes Wound onto a Rod Demonstrating Wide Dynamic Range.

  Takeo Yamada, Yuki Yamamoto, Yuhei Hayamizu, Atsuko Sekiguchi, Hiroyuki Tanaka, Kazufumi Kobashi, Don N Futaba, Kenji Hata
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  ABSTRACT: A rational torsion sensing material was fabricated by wrapping aligned single walled carbon nanotube (SWCNT) thin films onto the surface of a rod with a predetermined and fixed wrapping angle without destroying the internal network of the SWCNTs within film. When applied as a torsion sensor, torsion could be measured up to 400 rad/meter, that is more than four-times higher than conventional optical fiber torsion sensors by monitoring increases in resistance due to fracturing of the aligned SWCNT thin films.
  ACS Nano 03/2013; · 10.77 Impact Factor
• Article: Unexpectedly High Yield Carbon Nanotube Synthesis from Low Activity Carbon Feedstocks at High Concentrations.

  Hiroe Kimura, Jundai Goto, Satoshi Yasuda, Shunsuke Sakurai, Motoo Yumura, Don N Futaba, Kenji Hata
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  ABSTRACT: We report a new direction for highly efficient carbon nanotube (CNT) synthesis where in place of conventional highly reactive carbon feedstocks at low concentrations, highly stable carbon feedstocks at high concentrations were shown to produce superior yields. We found that a saturated hydrocarbon that is considered to possess a low reactivity is delivered at high concentrations could achieve an extremely high growth yield, (2.5 times that when using ethylene). This result stems from the unique behavior where the CNT yield linearly increased with carbon concentration, in contrast to more reactive carbon feedstocks where the yield peaks. We propose that the mechanisms for the growth kinetics for high and low reactivity carbon feedstocks are fundamentally different where the latter benefits from a longer catalyst lifetime because of a relatively low production rate of carbon impurities.
  ACS Nano 03/2013; · 10.77 Impact Factor
• Article: Hierarchical Three-Dimensional Layer-by-Layer Assembly of Carbon Nanotube Wafers for Integrated Nanoelectronic Devices.

  Takeo Yamada, Natsumi Makiomoto, Atsuko Sekiguchi, Yuki Yamamoto, Kazufumi Kobashi, Yuhei Hayamizu, Yoshiki Yomogida, Hiroyuki Tanaka, Hisashi Shima, Hiroyuki Akinaga, Don N Futaba, Kenji Hata
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  ABSTRACT: We report a general approach to overcome the enormous obstacle of the integration of CNTs into devices by bonding single-walled carbon nanotubes (SWNTs) films to arbitrary substrates and transferring them into densified and lithographically processable "CNT wafers". Our approach allows hierarchical layer-by-layer assembly of SWNTs into organized three-dimensional structures, for example, bidirectional islands, crossbar arrays with and without contacts on Si, and flexible substrates. These organized SWNT structures can be integrated with low-power resistive random-access memory.
  Nano Letters 08/2012; 12(9):4540-5. · 13.20 Impact Factor
• Article: Alignment control of carbon nanotube forest from random to nearly perfectly aligned by utilizing the crowding effect.

  Ming Xu, Don N Futaba, Motoo Yumura, Kenji Hata
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  ABSTRACT: Alignment represents an important structural parameter of carbon nanotubes (CNTs) owing to their exceptionally high aspect ratio, one-dimensional property. In this paper, we demonstrate a general approach to control the alignment of few-walled CNT forests from nearly random to nearly ideally aligned by tailoring the density of active catalysts at the catalyst formation stage, which can be experimentally achieved by controlling the CNT forest mass density. Experimentally, we found that the catalyst density and the degree of alignment were inseparably linked because of a crowding effect from neighboring CNTs, that is, the increasing confinement of CNTs with increased density. Therefore, the CNT density governed the degree of alignment, which increased monotonically with the density. This relationship, in turn, allowed the precise control of the alignment through control of the mass density. To understand this behavior further, we developed a simple, first-order model based on the flexural modulus of the CNTs that could quantitatively describe the relationship between the degree of alignment (HOF) and carbon nanotube spacing (crowding effect) of any type of CNTs.
  ACS Nano 06/2012; 6(7):5837-44. · 10.77 Impact Factor