OVERALL CONCLUSIONS
6.1 Overall Conclusion
This thesis has been devoted to fundamental study on designed synthesis of single crystalline metal oxide nanowires by a vapor-liquid-solid (VLS) process. The literate survey in chapter II has revealed the interests and the weakness of single crystalline metal oxide nanowires in VLS process.
To obtain the wide range applications of the nanowires, the fundamental study on designed synthesis is strongly required. Thus, this study aims to 1) fabricate novel metal oxide nanowires via VLS process by clarifying the fundamentals of VLS growth mechanism of metal oxide nanowires, 2) propose a rational concept to reduce the growth temperature of VLS growth method, 3) make a strategy to realize wide range control of carrier density of semiconducting metal oxide nanowires. The major implications in this thesis are summarized below.
Chapter III demonstrated a strategy to fabricate a novel metal oxide nanowires via VLS route by clarifying the fundamental growth mechanism of metal oxide nanowires in VLS process. In this section, we demonstrated a rational strategy to firstly fabricate single crystalline NiO nanowires via vapor-liquid-solid (VLS) route, which essentially allows us to tailor the diameter, the spatial position and the heterostructures. Our strategy is based on the suppression of the vapor-solid (VS) nucleation at the substrate surface, which promotes the crystal growth of nanowires only at liquid-solid (LS) interface. Manipulating the supplied material fluxes (oxygen and metal) and the growth temperature enables to enhance the nucleation only at LS interface. Furthermore, this strategy allows us to reduce the growth temperature of single crystalline NiO nanowires via VLS route down to 550 °C, which is the lowest growth temperature so far reported. In addition, the fabricated single crystalline NiO nanowires exhibited the superior memristive properties.
Chapter IV proposed a rational concept to reduce the growth temperature of the VLS nanowire growth of various metal oxides. MD simulation theoretically predicted the possibility to reduce
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the growth temperature of VLS nanowire growth by precisely controlling the vapor flux. This concept is based on the temperature dependent “material flux window” that the appropriate vapor flux for VLS nanowire growth decreases with decreasing the growth temperature. Experimentally, we found the applicability of this concept for reducing the growth temperature of VLS processes for various metal oxides including MgO, SnO2 and ZnO. In addition, we showed the successful applications of this concept to ITO glass and PI substrates.
Chapter V demonstrates 6 orders of magnitude control of electrical conductivity of single crystalline Sb-doped SnO2 nanowires grown via VLS process with the spatial homogeneity of dopants. This electrical conductivity range via an intentional impurity doping is the widest of the ones in metal oxide semiconducting nanowires reported so far. Strictly tailoring two crystal growth interfaces (LS and VS) via a flux control is required to suppress an unintentional carrier doping during a crystal growth in the presence of impurity dopants. This is because an unintentional carrier doping occurs at VS interface, controlling a crystal growth only at LS interface is essential to promote an intentional impurity doping at LS interface. In addition, the presence of Sb dopants strongly enhances a SnO2 crystal growth at VS interface, which requires further strict flux controls to suppress a detrimental unintentional carrier doping, as theoretically revealed by molecular dynamics simulations. This wide range electrical controllability of single crystalline metal oxide nanowires will be a foundation to design various electronic and energy device applications utilizing their semiconducting properties.
The conclusions show that understanding VLS growth mechanism of metal oxide nanowires enables us to exploit novel metal oxide nanowires fabrication, and tailor the nanowires, such as controlling the diameter and spatial position. Furthermore, further understanding of growth mechanism enables us to reduce the growth temperature and control the electrical conductivity of
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semiconductor nanowires. These achievements have high potential to promote the developments of metal oxide nanowires in novel oxide nanodevice application in the future.
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List of publications
Scientific Journals in This Thesis
1. “Tailoring Nucleation at Two Interfaces Enables Single Crystalline NiO Nanowires via Vapor-Liquid-Solid Route”
K. Nagashima, H. Yoshida, A. Klamchuen, M. Kanai, G. Meng, F. W. Zhuge, Y. He, H. Anzai, Z. Zhu, M. Suzuki, M. Boudot, S. Takeda and T. Yanagida
ACS Applied Materials & Interfaces 8, 27892-27899 (2016)
2. “Rational Concept for Reducing Growth Temperature in Vapor-Liquid-Solid Process of Metal Oxide Nanowires”
Z. Zhu, M. Suzuki, K. Nagashima, H. Yoshida, M. Kanai, G. Meng, H. Anzai, F. W. Zhuge, Y.
He, M. Boudot, S. Takeda and T. Yanagida Nano Letters 16, 7495-7502 (2016)
(日刊工業新聞 (21 面) "低温合成条件を発見 金属酸化物ナノワイヤ 阪大・九大、
VSL法で" 2017年10月31日)
3. “6 Orders of Magnitude Control of Electrical Conductivity of Sb-Doped SnO2 Nanowires via True Vapor-Liquid-Solid Process”
Z. Zhu, K. Nagashima, T. Takahashi, M. Suzuki, M. Kanai, G. Zhang, T. Hosomi, H. Saito, S.
Hata and T. Yanagida In submission
Collaborated Scientific Journals
4. “True Vapor-Liquid-Solid Process Suppresses Unintentional Carrier Doping of Single Crystalline Metal Oxide Nanowires”
H. Anzai, M. Suzuki, K. Nagashima, M. Kanai, Z. Zhu, Y. He, M. Boudot, G. Zhang, T.
Takahashi, K. Kanemoto, T. Seki, N. Shibata and T. Yanagida Nano Letters 17, 4698-4705 (2017)
5. “Engineering Nanowire-Mediated Cell Lysis for Microbial Cell Identification”
T. Yasui, T. Yanagida, K. Otsuka, M. Takeuchi, K. Nagashima, N. Kaji, S. Rahong, T. Naito, H. Yasaki, A. Yonese, R. Magofuku, Z. Zhu, M. Kanai, T. Kawai, and Y. Baba
141 In submission
6. “Ultra-Uniform Sized Zinc Oxide Nanowires from Random Sized Seed Crystals”
X. Zhao, K. Nagashima, G. Zhang, Z. Zhu, T. Takahashi, T. Hosomi, M. Kanai,H. Yoshida, S.
Takeda and T. Yanagida In submission
Proceedings
1. “Rational Concept for Reducing Growth Temperature of Vapor-Liquid-Solid Metal Oxide Nanowire Growth”
Z. Zhu, T. Takahashi, M. Suzuki, K. Nagashima, H. Yoshida, M. Kanai, G. Meng, H. Anzai, F. W. Zhuge, Y. He,M. Boudot, S. Takeda and T. Yanagida
CSS-EEST, Kyushu University, Fukuoka, Japan, 2017.11.29 pp170
2. “Dual Impact of Impurity on Crystal Growth Interface and Electrical Homogeneity of Metal Oxide Nanowires in Vapor-Liquid-Solid Process”
Z. Zhu, T. Takahashi, K. Nagashima, M. Suzuki, M. Kanai, G. Zhang, H. Anzai and T.
Yanagida
2018 International Conference on Solid State Device and Materials, Tokyo, 2018, pp729-730
International Conferences (First author)
1. “Rational Concept for Reducing Growth Temperature of Vapor-Liquid-Solid Metal Oxide Nanowire Growth”
Z. Zhu, M. Suzuki, K. Nagashima, G. Meng, M. Kanai, H. Anzai, F. W. Zhuge, Y. He and T.
Yanagida
International Symposium on Materials for Chemistry and Engineering (IMCE 2017), Fukuoka, Japan, 2017.2.3
2. “Rational Concept for Reducing Growth Temperature of Vapor-Liquid-Solid Metal Oxide Nanowire Growth”
Z. Zhu, T. Takahashi, M. Suzuki, K. Nagashima, H. Yoshida, M. Kanai, G. Meng, H. Anzai, F. W. Zhuge, Y. He,M. Boudot, S. Takeda and T. Yanagida
CSS-EEST, Kyushu University, Fukuoka, Japan, 2017.11.29
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3. “Dopant incorporation at two crystal growth interfaces in vapor-liquid-solid process of metal oxide nanowires”
Z. Zhu, K. Nagashima, T. Takahashi, M. Kanai, H. Anzai, G. Zhang and T. Yanagida 2018 IMCE International Symposium, Fukuoka, Japan, 2018.3.16.
4. “Dual Impact of Impurity on Crystal Growth Interface and Electrical Homogeneity of Metal Oxide Nanowires in Vapor-Liquid-Solid Process”
Z. Zhu, T. Takahashi, K. Nagashima, M. Suzuki, M. Kanai, G. Zhang, H. Anzai and T.
Yanagida
2018 International Conference on Solid State Device and Materials (SSDM 2018), University of Tokyo, Tokyo, Japan, 2018.9.11-13.
Domestic Conferences (First author)
1. “単結晶酸化物ナノワイヤのVLS結晶成長における低温化の理論設計指針”
Zetao Zhu, 長島一樹, 鈴木 将, 金井真樹, 柳田 剛
第77回応用物理学会秋季学術講演会, 朱鷺メッセ, 新潟, 2016年9月13日～16日 2. “Rational Concept for Reducing Growth Temperature in Vapor-Liquid-Solid Process of Metal
Oxide Nanowires”
Zetao Zhu, 鈴木 将, 長島一樹, 金井真樹, Gang Meng, 安西宇宙, Fuwei Zhuge, Yong He, Mickaël Boudot, 柳田 剛
第7回分子アーキテクト二クス研究会, 九州大学, 福岡, 2016年10月20日～21日 3. “Investigation on Physical Properties of Low Temperature Grown VLS Oxide Nanowires”
Zetao Zhu, 長島 一樹, 吉田 秀人, 安西 宇宙, 酒井 大樹, 井上 暉英, 中村 千枝, 金井 真 樹, Yong He, 竹田 精治, 柳田 剛
第64回応用物理学会春季学術講演会, パシフィコ横浜, 神奈川, 2017年3月14日～
17日
4. “Optical and Electrical Properties of Low Temperature Grown VLS Oxide Nanowires via Material Flux Window Concept”
Zetao Zhu, 長島 一樹, 吉田秀人, 安西宇宙, 酒井大樹, 井上暉英, 中村 千枝, 金井真 樹, Yong He, 竹田精治, 柳田剛
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第64回応用物理学会春季学術講演会, パシフィコ横浜, 神奈川, 2017年3月14日～
17日
5. “Impact of Crystal Growth Interfaces on the Electrical Properties of SnO2 Nanowire Field-Effect Transistors”
Zetao Zhu, Tsunaki Takahashi, Hiroshi Anzai, Masaki Kanai, Guozhu Zhang, Yong He, Kazuki Nagashima and Takeshi Yanagida
第78回応用物理学会秋季学術講演会, 福岡国際会議場, 福岡, 2017年9月5日～8日 6. “Dual Impact of Impurity on Crystal Growth Interface and Electrical Homogeneity of Metal
Oxide Nanowires in Vapor-Liquid-Solid Process”
Z. Zhu, K. Nagashima, T. Takahashi, M. Suzuki, M. Kanai, G. Zhang, T. Hosomi, H. Anzai and T. Yanagida
2018 (平成30年度) 応用物理学会九州支部学術講演会, 福岡大学, 福岡, 2018年12月8 日～9日
Collaborated International and Domestic Conferences
1. “Universal Design Rule for Single Crystalline Oxide Nanowires Growth”
K. Nagashima, H. Anzai, D. Sakai, Z. Zhu, A. Inoue, C. Nakamura, A. Klamchuen, M. Suzuki, H. Yoshida, M. Kanai, G. Meng, F. W. Zhuge, Y. He, S. Takeda, T. Kawai and T. Yanagida 29th International Microprocesses and Nanotechnology Conference, Kyoto, Japan, 2016.11.8-11
2. “Rational Concept for Reducing Growth Temperature of Vapor-Liquid-Solid Metal Oxide Nanowire Growth”
K. Nagashima, Z. Zhu, M. Suzuki, G. Meng, M. Kanai, H. Anzai, F. W. Zhuge, Y. He and T.
Yanagida
Material Research Society Fall Meeting 2016, Boston, USA, 2016.11.27-12.2 3. “Universal Design Concept for Single Crystalline Oxide Nanowires Growth”
K. Nagashima, Z. Zhu, D. Sakai, A. Inoue, C. Nakamura, A. Klamchuen, M. Suzuki, H. Anzai, H. Yoshida, M. Kanai, Y. He, S. Takeda and T. Yanagida
International Symposium on Materials for Chemistry and Engineering (IMCE 2017), Fukuoka, Japan, 2017.2.3
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4. “True Vapor-Liquid-Solid Process Prevents Unintentional Doping of Single Crystalline SnO2
Nanowires”
H. Anzai, K. Nagashima, M. Suzuki, G. Meng, Z. Zhu, F. W. Zhuge, K. Kanemoto, T. Seki, N.
Shibata and T. Yanagida
International Symposium on Materials for Chemistry and Engineering (IMCE 2017), Fukuoka, Japan, 2017.2.3
5. “Lowering the Growth Temperature of Vapor-liquid-solid Oxide Nanowires”
長島一樹, Zetao Zhu, Gang Meng, 金井真樹, 安西宇宙, Fuwei Zhuge, Yong He, 柳田 剛 第 63 回応用物理学会春季学術講演会, 東京工業大学大岡山キャンパス, 東京, 2016 年3月19日～22日
6. “単結晶酸化物ナノワイヤにおける 2つの結晶成長界面が有する相反する電子輸送特 性”
安西宇宙, 長島一樹, Zetao Zhu, Gang Meng, 金井真樹, Fuwei Zhuge, Yong He, 関 岳人, 柴田直哉, 柳田剛
九州表面・真空研究会2016, 九州大学筑紫キャンパス, 福岡, 2016年6月11日 7. “固液界面アニール現象を利用した単結晶酸化物ナノワイヤのUnintentional Dopingの
劇的抑制と超高純度化の実現”
安西宇宙, 長島一樹, 鈴木将, Zetao Zhu, 金井真樹, Yong He, Guozhu Zhang, 鐘本勝一, 関岳人, 柴田直哉, 柳田剛
第64回応用物理学会春季学術講演会, パシフィコ横浜, 神奈川, 2017年3月14日～
17日
Scientific Awards
1. Best Poster Award Zetao Zhu
“Rational Concept for Reducing Growth Temperature in Vapor-Liquid-Solid Process of Metal Oxide Nanowires”
第5回分子アーキテクト二クス若手研究会, 2016年10月19日 2. Poster Award