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Figure 8.12 (a) STM image of the area around the MoS2/WS2 heterointerface (Vs = -2.5 V, It = 0.1 nA). (b) A dI/dV map acquired at a sample bias voltage of Vs = -2.19 V over the same area as in (a) (Vs = -2.5 V, It = 0.1 nA).
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Chapter 9:
Conclusions
In this work, TMDCs were mainly synthesized on exfoliated graphite with atomically-flat and clean surfaces by CVD. The substrate-dependent optical properties of TMDC monolayers demonstrate that crystals on graphite surfaces are less affected by defects and strain. Such systems assist in elucidating the electronic states of these materials based on STM/STS observations. Various TMDC heterostructures were fabricated on graphite, and the electronic states around the heterointerface were assessed. The results show that bilayer heterostructures possess unique band structure modulations along the heterostructure interface. Prior theoretical work has not identified this modulation, which might therefore be attributable to lattice strain and fixed charges. Future studies using various structures in addition to monolayers would be interesting from a band structure control perspective.
This work also developed a new method of fabricating TMDC heterostructures using organic liquid precursors, which is termed MOCVD. MOCVD allows the precursor supply to be controlled, which is challenging in conventional CVD. Using this approach, multiple heterostructures exhibiting atomically-steep, linear interfaces were successfully produced. In addition, MoS2 nanoribbons with widths of less than 20 nm were obtained using heteroepitaxy. This MOCVD method represents a novel means of fabricating a wide variety of in-plane and vertical superlattices, nanoribbons and nanowires, as a first step towards examining the unique electronic and optical properties of such materials.
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Acknowledgements
During four and a half years after I came to Department of Physics, Tokyo Metropolitan University as a graduate student, I was able to do this research with guidance and cooperation from various people. Here, I would like to express my deep appreciation.
Assoc. Prof. Miyata, thank you for taking me into your group. You taught me everything since I entered the Shinohara Laboratory at Nagoya University as an undergraduate student. You were the big motivator that I decided to enter the graduate school. The discussions with you were so exciting and fun for me. I appreciate your honesty for me during these years. Prof. Maniwa, your valuable comments and suggestions to my works have become helpful in progress and brushing my research. Also, Asst. Prof. Sakamoto and Assoc. Prof. Nakai (currently at University of Hyogo), I thankful for your kind advices for my works.
Besides, my research was able to be done with the cooperation of many cooperative researchers. They contributed greatly to part of the work done in this thesis. Prof.
Shigekawa and Asst. Prof. Yoshida (University of Tsukuba, Japan), you did excellent STM/STS measurements to observe atomic resolution structures and reveal the band structures of TMDC heterostructures. Dr. Suenaga and Dr. Liu from the national Institute of Advanced Industrial Science and Technology, thank you for performing the atomic resolution TEM observations. And, Prof. Hibino (Kwansei Gakuin University, Japan), you did special LEEM and LEED observations. Furthermore, Dr. Akinaga and Ms.
Hayama (AIST) performed the great manipulations of TMDC crystals on van der Walls substrates.
Not to forget, Assoc. Prof. Yamamoto, Dr. Takashima (Tokyo University of Science, Japan), Assoc. Prof. Konabe (Hosei University, Japan), Prof. Okada and Dr. Maruyama (University of Tsukuba, Japan), who did excellent calculations on TMDC heterostructures.
Prof. Takenobu and Asst. Prof. Pu (Nagoya University, Japan), you did unique electronic and optoelectronic measurements by using ion gels. Special thanks to Prof. Matsuda, Assoc. Prof. Miyauchi (Kyoto University, Japan), Asst. Prof. Mouri (Ritsumeikan University, Japan) for the use of lab-made optical measurement systems. The same goes to Assoc. Prof. Kitaura (Nagoya University, Japan) and Dr. Okada for very useful and interesting discussions. I would like to take this opportunity to express my gratitude to all of you.
I am grateful to all the members in the NanoScience Research Lab, who are kind and
124
helpful. Not only active discussions and experiments raise my motivations, but also there are a lot of fun. The sports event every Friday is so delightful, and the parties sometimes held in Lab room is convivial. Special mention to our devoted secretaries, Ms. Nonaka, Ms. Yamada and the staffs I the department (especially, Ms. Iwamoto), who have been helping me in many aspects.
Finally, I would like to express my gratitude to my father, mother, grandmother, brothers, and friends who supported me for my university life and encouragement every day. You always believe my decisions, and it always become my power. I’ll make you be proud of me to give back to your kindness and loves.
Yu Kobayashi August 2018
125
List of Publications
Main publications
1. "Growth and Optical Properties of High-Quality Monolayer WS2 on Graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hibino, Z. Liu, K. Watanabe, T. Taniguchi, K.
Suenaga, Y. Maniwa, Y. Miyata ACS Nano, 9 (2015) 4056-4063.
2. "Bandgap-tunable lateral and vertical heterostructures based on monolayer Mo1-xWxS2
alloys"
Y. Kobayashi, S. Mori, Y. Maniwa, Y. Miyata Nano Res., 8 (2015) 3261-3271.
3. "Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction"
Y. Kobayashi, S. Yoshida, R. Sakurada, K. Takashima, T. Yamamoto, T. Saito, S.
Konabe, T. Taniguchi, K. Watanabe, Y. Maniwa, O. Takeuchi, H. Shigekawa, Y. Miyata Sci. Rep., 6 (2016) 31223-1-8.
4. "Slidable atomic layers in van der Waals heterostructures"
Y. Kobayashi, T. Taniguchi, K. Watanabe, Y. Maniwa, Y. Miyata Appl. Phys. Express, 10 (2017) 045201-1-4.
5. "Continuous heteroepitaxy of two-dimensional heterostructures based on layered chalcogenides"
Y. Kobayashi, S. Yoshida, M. Maruyama, K. Murase, Y. Maniwa, O. Takeuchi, S. Okada, H. Shigekawa, Y. Miyata
In submitted.
126
Reference publications
1. "Microscopic basis for the band engineering of Mo1-xWxS2-based heterojunction"
S. Yoshida, Y. Kobayashi, R. Sakurada, S. Mori, Y. Miyata, H. Mogi, T. Koyama, O. Takeuchi, H. Shigekawa
Sci. Rep., 5 (2015) 14808-1-6.
2. "Local optical absorption spectra of h-BN-MoS2 van der Waals heterostructure revealed by scanning near-field optical microscopy"
J. Nozaki, Y. Kobayashi, Y. Miyata, Y. Maniwa, K. Watanabe, T. Taniguchi, K. Yanagi Jpn. J. Appl. Phys., 55 (2016) 06GB01-1-3.
3. "Growth and optical properties of Nb-doped WS2 monolayers"
S. Sasaki, Y. Kobayashi, Z. Liu, K. Suenaga, Y. Maniwa, Y. Miyauchi, Y. Miyata Appl. Phys. Express, 9 (2016) 071201-1-4.
4. "Orientation-controlled growth of hexagonal boron nitride monolayers templated from graphene edges"
E. Maeda, Y. Miyata, H. Hibin, Y. Kobayashi, R. Kitaura, H. Shinohara Appl. Phys. Express, 10 (2017) 055102-1-4.
5. "Scanning tunneling microscopy/spectroscopy on MoS2 embedded nanowire formed in CVD-grown Mo1-xWxS2 alloy"
H. Mogi, Y. Kobayashi, A. Taninaka, R. Sakurada, T. Takeuchi, S. Yoshida1, O.
Takeuchi, Y. Miyata, H. Shigekawa
Jpn. J. Appl. Phys., 56 (2017) 08LB06-1-4.
6. "Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBN"
M. Okada, A. Kutana, Y. Kureishi, Y. Kobayashi, Y. Saito, T. Saito, K. Watanabe, T. Taniguchi, S. Gupta, Y. Miyata, B. I. Yakobson, H. Shinohara, R. Kitaura
127
ACS Nano, 12 (2018) 2498-2505.
International conferences
1. "Combinatorial synthesis of single-layer Mo1-xWxS2 by sulfurization of patterned oxide thin films"
Y. Kobayashi, S. Mori, Y. Maniwa, Y. Miyata
2014 Recent Progress in Graphene Research, Taipei, Taiwan (Sep. 21-25, 2014)
2. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki1, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
The Grobal Human Resource Program Bridging across Physics and Chemistry 2015, Tokyo, Japan (Jun. 30, 2015)
3. "Growth of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
International Winterschool on Electronic Properties of Novel Materials 2015, Kirchberg in Tirol, Austria (Mar. 8-15, 2015)
4. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
Sixth Graphene and 2D Materials Satellite Symposium 2015, Nagoya, Japan (Jun. 28, 2015)
5. "Bandgap-tunable lateral and vertical heterostructures based on monolayer Mo1-xWxS2
alloys"
Y. Kobayashi, S. Mori, Y. Maniwa, Y. Miyata
128
The Sixteenth International Conference on the Science and Application on Nanotubes, Nagoya, Japan (Jun. 29- Jul. 3, 2015)
6. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
The Sixteenth International Conference on the Science and Application on Nanotubes, Nagoya, Japan (Jun. 29- Jul. 3, 2015)
7. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
JSAP-OSA Joint Symposia, Nagoya, Japan (Sep. 15, 2015)
8. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
MRS Fall Meeting, Boston, Massachusetts, USA (Nov. 29- Dec. 4, 2015)
9. "Growth and optical properties of high-quality monolayer WS2 on graphite"
Y. Kobayashi, S. Sasaki, S. Mori, H. Hiroki, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
Pacifichem 2015, Honolulu, Hawaii, USA (Dec. 15-20, 2015)
9. "Fabrication and manipulation of slidable atomic layers"
Y. Kobayashi, K. Watanabe, T. Taniguchi, Y. Maniwa, Y. Miyata
The Grobal Human Resource Program Bridging across Physics and Chemistry 2016, Tokyo, Japan (Jun. 29, 2016)
10. "Growth and characterization of MoS2/WS2 lateral heterostructures"
Y. Kobayashi, S. Yoshida, R. Sakurada, T. Saito, K. Watanabe, T. Taniguchi, Y.
129
Maniwa, H. Shigekawa, Y. Miyata
ATI NanoCarbon Zao Meeting, Yamagata, Japan (Jul. 19-20, 2016)
11. "Growth and characterization of transition metal dichalcogenides using halide-assisted MOCVD"
Y. Kobayashi, S. Yoshida, K. Murase, N. Okada, T. Irisawa, Y. Maniwa, H. Shigekawa, Y. Miyata
8th A3 Symposium of Emerging Materials: Nanomaterials for Energy and Electronics, Suzhou, China (Oct. 25-28, 2017)
12. "Growth and characterization of transition metal dichalcogenides using halide-assisted MOCVD"
Y. Kobayashi, S. Yoshida, K. Murase, N. Okada, T. Irisawa, Y. Maniwa, H. Shigekawa, Y. Miyata
2017 The Workshop on Innovative Nanoscale Devices and Systems, Kohala Coast, Hawaii, USA (Nov. 26- Dec. 1, 2017)
130
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