title: Low-temperature catalyst activator: mechanism of dense carbon nanotube forest growth studied using synchrotron radiation.
abstract: The mechanism of the one-order-of-magnitude increase in the density of vertically aligned carbon nanotubes (CNTs) achieved by a recently developed thermal chemical vapor deposition process was studied using synchrotron radiation spectroscopic techniques. In the developed process, a Ti film is used as the underlayer for an Fe catalyst film. A characteristic point of this process is that C2H2 feeding for the catalyst starts at a low temperature of 450°C, whereas conventional feeding temperatures are ∼800°C. Photoemission spectroscopy using soft and hard X-rays revealed that the Ti underlayer reduced the initially oxidized Fe layer at 450°C. A photoemission intensity analysis also suggested that the oxidized Ti layer at 450°C behaved as a support for nanoparticle formation of the reduced Fe, which is required for dense CNT growth. In fact, a CNT growth experiment, where the catalyst chemical state was monitored in situ by X-ray absorption spectroscopy, showed that the reduced Fe yielded a CNT forest at 450°C. Contrarily, an Fe layer without the Ti underlayer did not yield such a CNT forest at 450°C. Photoemission electron microscopy showed that catalyst annealing at the conventional feeding temperature of 800°C caused excess catalyst agglomeration, which should lead to sparse CNTs. In conclusion, in the developed growth process, the low-temperature catalyst activation by the Ti underlayer before the excess Fe agglomeration realised the CNT densification.
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Akito
authors: A
authors: Takashima
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Yudai
authors: Y
authors: Izumi
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Eiji
authors: E
authors: Ikenaga
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Takuo
authors: T
authors: Ohkochi
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Masato
authors: M
authors: Kotsugi
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Tomohiro
authors: T
authors: Matsushita
authors: Japan Synchrotron Radiation Research Institute (JASRI) , 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
authors: Takayuki
authors: T
authors: Muro
authors: Collaborative Research Team Green Nanoelectronics Center (GNC), National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
authors: Akio
authors: A
authors: Kawabata
authors: Collaborative Research Team Green Nanoelectronics Center (GNC), National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
authors: Tomo
authors: T
authors: Murakami
authors: Collaborative Research Team Green Nanoelectronics Center (GNC), National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
authors: Mizuhisa
authors: M
authors: Nihei
authors: Collaborative Research Team Green Nanoelectronics Center (GNC), National Institute of Advanced Industrial Science and Technology (AIST) , 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan.
authors: Naoki
authors: N
authors: Yokoyama
keywords: X-ray absorption spectroscopy (XAS); dense vertically aligned carbon nanotubes; growth mechanism; hard X-ray photoemission spectroscopy (HAXPES); photoemission electron microscope (PEEM); soft X-ray photoemission spectroscopy (SXPES); synchrotron radiation; thermal chemical vapor deposition