Welcome to the 2...Welcome to the 2nd International Workshop on 2D Materials This event is a part of activities of A3 Foresight Program “Joint Research on Novel Physical Properties

Welcome to the 2nd International Workshop on 2D Materials

This event is a part of activities of A3 Foresight Program “Joint Research on Novel Physical Properties and Functionalities of Emerging 2D Materials

and van der Waals Heterostructures”

which will last for five years from August 2018 to July 2023.

The Asia-Pacific region, including A3 countries (Japan, China, and Korea), has come to occupy an

increasingly important position in the 2D materials researches. The purpose of this Project is to

promote the exchange of young researchers and international collaborations, and to make the A3 countries a world-class center of excellence in the 2D materials science. The 1st International

Workshop on 2D Materials has been successfully held in Tokyo organized by Prof. Yoshihiro Iwasa

from the University of Tokyo in Nov. 2018. This is the second event of the A3 foresight Program and will be organized by Nanjing university in Nanjing, China from Feb. 20 to Feb. 23 in 2019.

The main activities in this A3 program are

-Triannual seminars (one per each country per year) including one or two school-format. -Support of young researchers’ exchange and international collaborations.

The purpose of this workshop is that all the members will be acquainted with each other, stimulate

discussions on the forefront of research and find some seeds for future collaboration researches. We sincerely hope that the scope of the workshop will serve the interest of the A3 foresight program

and the scientific community. Wish you a stimulating workshop and interactions in Nanjing.

Program Leaders

Prof. YUAN Hongtao (Nanjing University) Prof. IWASA Yoshihiro (University of Tokyo)

Prof. CHEONG Hyeonsik (Sogang University)

A3 Project Supported by：

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https://www.jsps.go.jp/english/index.html

https://www.jsps.go.jp/english/e-foresight/index.html

Two-dimensional (2D) layered materials have recently attracted much interest due to their

unique physical and chemical properties and potential applications in electronics,

optoelectronics, spintronics and valleytronics. A wealth of unusual physical phenomena

occurs when charges are confined into a plane lattice and dominated by their two-

dimensional structural units. Research on 2D materials has thus become a major branch in

the field of materials sciences. Since a large amount of research budget has been invested

in 2D materials researches in Asian countries, Asia has nowadays become a leading area

of the world for 2D material community. According to the statistics by Prof. Castro Neto

in National University of Singapore, China, Korea and Japan were ranked as 1, 4, 3,

respectively, in terms of academic publications, and 1, 3, 4 in terms of patents. In order to

take a leading role, Asian countries are organizing two major international conferences

every year inviting topnotch scientists from all over the world and try to boost the related

research for applications. However, Asian countries still lag behind the United States and

Europe in fundamental physics of 2D materials. Since industrial application or

commercialization cannot be successful without a firm foundation of basic research, it is

important for the Asian countries to strengthen basic research in 2D materials. Since there

are already several groups in these countries carrying out frontier research, forming an

alliance of leading scientists from the three countries would be a great step forward and be

beneficial to the scientific advancement in all three countries.

The purpose of this A3 Foresight Program is to establish a world-leading COE of 2D

materials and to cultivate the next generation researchers. To realize this goal, (1) we will

support further individual collaborations among A3 countries, (2) we will hold three

seminars per year (one per country) including a summer or winter school for students and

young researchers, and we will also organize international conferences, and (3) we will

support the mutual exchanges of young researchers including PhD students. Our main

focus is on such non-graphene 2D materials and their van der Waals heterostructures, which

are fabricated by stacking various monolayer materials. With such novel 2D materials and

artificial heterostructures, we are anticipating observing novel lattice structures and

electronic properties which cannot be achieved in conventional bulk materials. Also, with

such unique structures, we will realize unique optical and electronic devices. Among a wide

variety of research directions of 2D materials, we will focus on physical aspects and related

device functions. In particular, the emphasis will be given on the following four subjects:

(1) Growth/characterization of 2D materials and van der Waals heterostructures

(2) Spin structures and quantum transport

(3) Exciton physics and non-linear photonics

(4) Functionalized devices

The A3 consortium was made so as to maximize the potential of the A3 countries. The three

countries’ activities are highly complementary: in short, Japan, Korea, and China are strong

in basic physics, device physics, and materials synthesis and characterization, respectively.

The researchers from the 3 countries will contribute to one or more of the four main topics,

and they can be assigned as follows, as shown in below figure.

The distribution of the researchers clearly demonstrates the complementary nature of the

three countries and the comprehensiveness of the expertise. A number of achievements

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have been published from the international collaborations between the three countries, and

this program will not only support existing collaborations but also stimulate new

collaborations. According to the statistics explained above, the alliance of China, Korea,

and Japan through the A3 project will make this geographically close countries the word-

top and largest community in the 2D materials researches.

The research topics on 2D materials of the A3 project.

In order to cultivate the next generation of researchers, we will organize a summer or winter

school every year. These schools will be open to those who are not members of the

consortium so that a wider audience could benefit from them. This will particularly benefit

the students in the country where the school is held. For this reason, we will hold three

schools during the first two years, one at each country. The lecturers will be recruited

among the participating scientist, but some external experts will be invited, if necessary. In

addition, we will include a tutorial session at regular seminar meetings so that the students

can learn some latest developments in the field.

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DATE

Feb. 21 - Feb. 23

REGISTERATION

300 RMB, paid on-site in cash at registration desks either at hotels or in the meeting hall.

VENUE

Lecture Hall of Yifu science and technology building

Nanjing University (Gulou campus) 南京大学鼓楼校区逸夫科技馆会议中心

Venue

Lecture Hall of Yifu science and technology building

Venue

ACCOMMODATION

Nanjing Greenland InterContinental Hotel

No. 1 Zhongyang Road, Gulou District, Nanjing (南京鼓楼区中央路 1号，近南京大学)

Jingli Hotel (晶丽宾馆)

No. 7 Beijing west street, Gulou District, Nanjing (南京鼓楼区北京西路 7号，近南京大学)

TRANSPORTATION

1）By Taxi:

Lukou International Airport to Nanjing University (Gulou campus) or the hotels nearby will take

about 1 hour and costs around 160-190 Chinese Yuan.

2）By Subway:

Lukou International Airport to Zhujianglu subway station near Nanjing University Gulou campus

will take about 50 minutes and costs 10 Chinese Yuan. In this case, the passage needs to take

Airport Subway Line to Nanjing South Railway station and then take Line 1 to Zhujianglu

Substation to reach Nanjing University.

Venue

Thursday, Feb. 21

Feb. 21, Morning Session, Chair: Yoshihiro Iwasa

8:55-9:00 Opening remark 5 min

9:00-10:00 Lecture

J1 Graphene optoelectronics and plasmonics for terahertz device applications

Taiichi Otsuji

Tohoku University

10:00-11:00 Lecture

K1

Solid-state electronic devices using 2D

materials

Hyun-Jong

Chung

Konkuk

University

11:00-11:20 20-min Break

11:20-12:20 Lecture

C1

Emerging two-dimensional materials

beyond graphene

Yuanbo

Zhang

Fudan

University

12:20-13:30 Lunch

Feb. 21, Afternoon Session, Chair: Hyeonsik Cheong

13:30-14:30 Lecture

J2 Superconductivity in 2D Materials

Yoshihiro

Iwasa

University of

Tokyo

14:30-15:30 Lecture

K2

Electrical contacts to two-dimensional

semiconductors Chul-Ho Lee

Korea

University

15:30-15:50 20-min Break

15:50-16:50 Lecture

C2

Quantum anomalous Hall effect in

magnetic topological insulators Ke He

Tsinghua

University

16:50-18:00

Poster Session

18:30-20:30 Welcome Reception

Friday, Feb. 22

Feb. 22, Morning Session, Chair: Yuanbo Zhang

9:00-9:30 Invited

J1

Synthesis of boron nitride single

crystals under high pressure and their

properties

Takashi

Taniguchi NIMS

9:30-10:00 Invited

K1

Engineering ferromagnetic lines in

graphene by local functionalization

using AFM lithography

Bae-Ho Park Konkuk

University

10:00-10:30 Invited

C1

Quantum phase transition in 2D

superconductors Jian Wang

Peking

University

Tim

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10:30-10:50 20-min Break

10:50-11:20 Invited

J2

Electronic structures of topological materials clarified by spin- and angle-

resolved photoemission spectroscopy

Masato Sakano University of

Tokyo

11:20-11:50 Invited

K2

Novel states in stacked two-

dimensional crystals Young-Woo Son

Korea

Institute for

Advanced

Study

11:50-12:20 Invited

C2

Topological axion states in magnetic

insulator MnBi2Te4 Haijun Zhang

Nanjing

University

12:20-13:30 Lunch

Feb. 22, Afternoon Session, Chair: Jian Wang

13:30-13:45 Contributed

J1

Layer rotation angle dependent ultrafast

charge transfer dynamics

of interlayer excitons in vdW

heterostructure

Pranjal Kumar

Gogoi AIST


J2

Room-Temperature Valley-Polarized

Light-Emitting Devices via strained

monolayer semiconductors

Jiang Pu Nagoya

University


K1

Suppression of magnetic ordering in

XXZ-type antiferromagnetic NiPS3 in

atomically thin limit

Kang-Won

Kim

Sogang

University


K2

Topological and ferromagnetic properties

of iron-based van der Waals metals JunHo Seo POSTECH


C1

Saddle points induced by band inversion

in topological materials

Huaiqiang

Wang

Nanjing

University


C2

Electronic structure of exfoliated 2H-

MoTe2 atomically thin flakes by using

NanoARPES

Hongyun

Zhang

Tsinghua

University

15:00-16:00 Lab tour

16:00-18:00 Confucius Temple (科举博物馆)

18:30-20:30 Banquet, Qinhuai River

Saturday, Feb. 23

Feb. 23, Morning Session, Chair: Hongtao Yuan

9:00-9:30 Invited

J3

Autonomous robotic searching and

assembly of two dimensional crystals to build van der Waals superlattices

Satoru Masubuchi

Univ. of Tokyo

9:30-10:00 Invited

K3

Magnetic van der Waals: hopes and

challenges Je-Geun Park

Seoul

National

University

Tim

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edule

10:00-10:30 Invited

C3

Photoemission Study of Topological

Semimetals Zhongkai Liu

ShanghaiTech

University

10:30-11:00 Invited

J4

High-resolution ARPES studies of

atomic-layer group-V transition metal

dichalcogenides

Katsuaki

Sugawara

Tohoku

university

11:00-11:10 Conclusion remark

Tim

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Nanjing A3 Meeting Poster Session: 16:50-18:00, Thursday Feb 21

Number Title Name Affiliation

Poster-1

Terahertz light emitting and lasing operation

in graphene-based heterostructure 2D material systems

Takayuki Watanabe Tohoku

University

Poster-2 Gate-Tunable Valley Polarization in WSe2

Field-Effect Transistor Keisuke Shinokita Kyoto University

Poster-3 Nonreciprocal transport in

noncentrosymmetric van der Waals crystals Toshiya Ideue

University of

Tokyo

Poster-4 Coexistence of charge ordering and 2D

superconductivity in IrTe2 Sun-Gyu Park POSTECH

Poster-5 Quantum transport in graphene/hexagonal

boron nitride heterostructures Takuya Iwasaki NIMS

Poster-6

Observation of selective charge-density-wave

pinning depending on the atomic structure of

native defects in 2H-NbSe2

Eunseok Oh POSTECH

Poster-7

Influence of Chemical Etching Treatment on

Copper Foils for Single-Layer Graphene CVD Growth

Naoki Yoshihara Fukuoka

University

Poster-8

Synaptic devices implemented with two-

dimensional layered single crystal chromium

thiophosphate (CrPS4)

Mi-Jung Lee Konkuk

University

Poster-9 Gate-tunable room-temperature

ferromagnetism in two-dimensional Fe3GeTe2 Yujun Deng Fudan University

Poster-10

Spin-orbit-interaction - driven magnetic

anisotropy in dimerized honeycomb-lattice

ruthenate Li2RuO3

Seokhwan Yun Seoul National

University

Poster-11 Large-scale fabrication of highly-transparent

solar cell with 2D material Xing He

Tohoku

University

Poster-12 Fuel cell based on AA′-stacked trilayer

hexagonal boron nitride Seong-In Yoon UNIST

Poster-13

Light Soaking Phenomena in Organic-

inorganic Mixed Halide Perovskite Single Crystals

Hye-Ryung Byun Sungkyunkwan

University

Poster-14 Visualizing the electronic structure of

monolayer Bi2Sr2CaCu2O8+δ Liguo Ma Fudan University

Poste

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Poster-15

Temperature-dependent phonon vibration and

excitonic transition in a van der Waals

heterostructure

Hamin Park

School of

Electrical

Engineering,

KAIST

Poster-16 Spin-Valley dependent optical response at monolayer transition metal

dichalcogenide/ferrimagnet interface

Takatoshi Akamatsu

The University of Tokyo

Poster-17 Thermo-driven Crystalline Phase Transition of

Epitaxial Metastable 1T’-WSe2 Thin Film Wang Chen

Nanjing

University

Poster-18 Spatially Resolved Electronic Structure of Bi2Sr2-xLaxCuO6+δ in the Two-dimensional

Limit

Hengsheng Luo Fudan University

Poster-19

Electronic structure of Weak topological

insulator candidate CaSn studied by Angle

Resolved Photoemission Spectroscopy

Lixuan Xu ShanghaiTech

University

Poster-20 Vertical 1T-TaS2 synthesis on nanoporous gold for high-performance electrocatalytic

applications

Yahuan Huan Peking University

Poster-21 The properties of MoS2 FET with functional

TMPS gate insulators Minjung Shin Konkuk university

Poster-22 Wafer-scale epitaxial growth of 2D semiconductor single-crystal film

Congwei Tan Peking University

Poster-23

Fabrication of Size-Controlled Graphene

Quantum Dots Embedded in Hexagonal Boron

Nitride monolayer

Gwangwoo Kim UNIST

Poster-24 Raman study of polytypism in 2-dimensional Gallium Selenide

Soo-Yeon Lim Sogang University

Poster-25

WS2(1-x)Te2x Alloy Monolayer grown by

ChemicalVapor Deposition with Tunable Band

Structures

Haolin Wang Nanjing

University

Poster-26 Van der Waals pn Junction Field Effect Transitors using Transition Metal

Dichalcogenides

June-Yeong Lim Yonsei University

Poster-27 Band Engineering in Epitaxial TMDC Alloy

MoxW(1-x)Se2 thin films Xuedong Xie

Nanjing

University

Poster-28

Fowler-Nordheim Tunneling current

modulation by controlling a barrier height in

graphene/hexagonal boron nitride heterostructure

Junho Lee Konkuk

University

Poster-29 Integrated circuits based on chemical

patterning of 2D Bi2O2Se Tianran Li Peking University

Poste

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Poster-30 Experimental setup for second harmonic

generation spectroscopy Jungcheol Kim Sogang University

Poster-31 Thermal stability study on 2D Bi2O2Se Teng Tu Peking University

Poster-32 Gap-mode Plasmon-Induced Photoresponse in

Vertical Structure of Multilayer graphene Khang-June Lee KAIST

Poster-33

Generation of MoOx on the surface of MoTe2

by O2 plasma and its effect on MoTe2 based

transistors.

Yongjae Cho Yonsei University

Poster-34 Ultralow-threshold whispering-gallery-mode

lasing in CdS nanoplatelets Liyun Zhao Peking University

Poster-35 Towards Neutral Interlayer Exciton Transport

in Transition Metal Dichalcogenide

Takamoto

Yokosawa

The University of

Tokyo

Poster-36

Electronic Structure of K-doped IrTe2 Single

Crystal Resolved by Angle-Resolved

Photoemission Spectroscopy

Haoxiong Zhang Tsinghua

University

Poster-37

Polycrystalline Bi2O2Se thin film prepared

with spin-coating method for flexible thin film

transistors

Congcong Zhang Peking University

Poster-38 Direct EL Imaging of Transition Metal

Dichalcogenide Light-emitting Devices Hirofumi Matsuoka

Nagoya

University

Poster-39 Lasing in mechanically exfoliated single phase

2D perovskite crystals Yin Liang Peking University

Poster-40

Electric-field-induced Metal-Insulator

Transition and Quantum Transport in Large-

Area Polycrystalline MoS2 Monolayers

Tomoyuki Yamada Nagoya

University

Poster-41 Local chemical modification of MoS2 layer

using AFM lithography Dayea Oh

Konkuk

University

Poster-42

Synaptic plasticity selectively activated by

polarization-dependent energy-efficient ion

migration in an ultrathin ferroelectric tunnel

junction

Chansoo Yoon Konkuk

University

Poster-43 Na-assisted fast growth of large single-crystal MoS2 on sapphire

Yuping Shi Peking University

Poster-44 Electrical properties of individual CaVO

(CalciumVanadate) nanowire HyunJeong Jeong

Ewha Womans

Univ.

Poste

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ch

ed

ule

Poster-45 2D hard ferromagnet Co2FeGeTe2 Inho Hwang Seoul National

University

Poster-46

Structurally engineered carrier dynamics in

hybrid quasi-two-dimensional perovskite thin

films

Qiuyu Shang Peking University

Poster-47 Monolithic 2D Oxide/Semiconductor

Superlattice for Efficient Light Emitters Yoonseok Kim Korea University

Poste

r S

ch

ed

ule

Lecture-J1

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2nd International Workshop on 2D Materials

Graphene optoelectronics & plasmonics for terahertz device applications

First Name: Taiichi

Last Name: Otsuji

Affiliation: Research Institute of Electrical Communication, Tohoku

University, Sendai, Japan

Email: [email protected]

Short Biography:

Taiichi Otsuji received the Ph.D in Electron. Eng. from Tokyo Inst. Tech., Japan in 1994. After working for NTT Lab. (1984~1999) and Kyushu Inst. Tech. (1999-2005), Japan, he has been working at RIEC, Tohoku University as a full professor. He has been served an IEEE Electron Device Society Distinguished Lecturer since 2013. He is a Fellow of the IEEE, OSA, and JSAP, and a senior member of the IEICE, and a member of the MRS and SPIE.

Abstract: Despite its tremendous application potential, the terahertz (THz) frequency range of the electromagnetic spectrum is not widely populated yet because of the lack of commercially available microelectronic or nanoelectronic devices that can generate, detect, or manipulate the THz waves efficiently at room temperature. In such a situation graphene has attracted considerable attention due to its extraordinary optoelectronic and plasmonic properties so that it has been expected to be a promising material that can bridge over the technological ‘THz gap’ [1]. Mass-less Dirac fermions of electrons and holes in gapless and linear symmetric band structures in graphene enable a relatively weak gain in a wide THz frequency range under optical or electrical pumping [1]. We’ve recently demonstrated 1-8-THz broadband amplified spontaneous THz emission as well as single-mode THz lasing at 5.2 THz both at 100K in graphene-channel laser transistor under current-injection pumping [2]. The excitation of surface plasmon polaritons in population-inverted graphene can dramatically enhance the THz gain (Fig. 1) [3]. Introduction of a gated double-graphene-layered (G-DGL) van der Waals heterostructure in which gate-bias tuned THz radiation emission is obtained via plasmon- and/or photon-assisted quantum-mechanical tunneling is a promising rout to further increase operation temperature as well as output intensity [4]. We experimentally demonstrated the proof of concept of such an operation mechanism [5]. The important physics behind is the acoustic plasmon modes in the DGL that can enormously enhance the quantum efficiency by orders for dc electric power to THz photo radiation power conversion in comparison with that for a simple graphene-channel transistor laser structure [6]. We have proposed a cascading of the G-DGL unit element working as a new type of THz quantum-cascade lasers [7]. This work was supported by JSPS KAKENHI #16H06361 and #18H05331, Japan.

[1]A. Tredicucci, IEEE JSTQE 20, 8500109 (2014). [2]D. Yadav et al., Nanophoton. 7, 741-752 (2018). [3] T. Watanabe et al., New J. Phys. 15, 075003 (2013). [4] V. Ryzhii et al., Appl. Phys. Lett. 103, 163507 (2013). [5]D. Yadav et al., 2D Mater. 3, 045009 (2016). [6]D. Svintsov et al., Phys. Rev. B 94, 115301 (2016).

Fig. 1. THz light amplification by stimulated emission of radiation via surface plasmon polaritons in optically pumped graphene.



Lecture-K1


Solid-state electronic devices using 2D materials

First Name: Hyun-Jong

Last Name: CHUNG

Affiliation: Department of Physics, Konkuk University, Seoul, Korea


Short Biography:

In 2005, he received Ph. D degree in Seoul National University, Korea. Then, he moved to Korea research institute of standard science as a postdoctoral researcher. Since he joined Samsung Advanced Institute of Technology in 2006, where he initiated graphene research, he has conducted research in graphene electronics. Based on the research, he co-authored a review article in Nature with the president of Samsung Advanced Institute of Technology. To solve the well-known switching problem of graphene transistor, he invented a new electronic device, modulating the graphene-Si Schottky-barrier height, published in Science. He named the device “barristor,” meaning barrier transistor, since the current can be modulated by modulating the Schottky-barrier. Then he moved to Konkuk University in 2013 and conducted research to discover the barristor's potential and limitations. To discover the potential, he is now conducting a research on the graphene electronic device as a sensor framework.

Abstract:

Graphene has been attracting many intentions for a post-Si material due to its high mobility [1]. Since its gapless band structure keeps from turning-off the devices in traditional way, applications for the graphene transistors are limited to analog amplifiers which does not have to be turned off during the operation [2]. Recently, new device structures have been proposed to solve the issue: graphene barristor [3]. It is a diode-based switching device with hetero junction between graphene and semiconductor, unlike the traditional transistor-based switching devices [4].

In this tutorial, the switching mechanism of the traditional solid-state devices, especially field effect transistor(FET), will be summarized, followed by the review of the research on graphene electronics. Then, the potential applications of graphene electronic devices will be discussed.

[1] K. Kim, et al., Nature, 479, 338 (2011)

[2] I. Meric, et al., Nature Nanotechnol., 3, 654 (2008)

[3] H. Yang, et al., Science, 336, 1140 (2012)

[4] S. Jhang, et al., New Physics: Sae Mulli, 66, 1201 (2016)

Fig. 1. Surface charge vs. Fermi level for Si and graphene FET



Lecture-C1


Emerging two-dimensional materials beyond graphene

First Name: Yuanbo

Last Name: Zhang

Affiliation: Department of Physics, Fudan University, China


Short Biography:

Yuanbo Zhang received his B.S. degree from Peking University in 2000 and his Ph.D. in Physics from Columbia University in 2006. He was a Miller Research Fellow at the University of California at Berkeley from Sept. 2006 to Jun. 2009, a postdoc research associate at IBM Almaden Research Center from Mar. 2010 to Sept. 2010, and a professor of Fudan University since 2011. His main research interests are: Electronic transport in low-dimensional systems including graphene; Scanning probe techniques and their application in studying low-dimensional nanostructures. Major honors include: IUPAP Young Scientist Prize, International Union of Pure and Applied Physics (2010); Nishina Asia Award, Nishina Memorial Foundation, Japan (2014); Changjiang Scholarship, Ministry of Education, China (2017)

Abstract:

Two-dimensional (2D) atomic crystals, best exemplified by graphene, have emerged as a new class of material that may impact future science and technology. From a material physicist’s point of view, 2D materials provides vast opportunities on two fronts: first, the reduced dimensionality in these 2D crystals often leads to novel material properties that are different from those in the bulk; second, the entire 2D crystal is a surface, so it is possible to have better control of their material properties with external perturbations. In this talk I will first illustrate these two points with two examples: black phosphorus and 1T-TaS2. These two layered materials have vastly different properties. Black phosphorus is a 2D semiconductor, and its superior material quality has recently enabled us to observe the quantum Hall effect. 1T-TaS2, on the other hand, is a metal with a rich set of charge density wave phases; we explore their electronic properties while the doping and dimensionality of the 2D systems are modulated. I will then discuss a few other emerging 2D materials that include 2D ferromagnet metal Fe3GeTe2, and 2D high Tc superconductor Bi2Sr2CaCu2O8+.

Lecture-J2


Superconductivity in 2D Materials

First Name: Iwasa

Last Name: Yoshihiro

Affiliation: Department of Applied Physics, University of Tokyo

RIKEN Center for Emergent matter Science, Japan


Short Biography:

Team Leader, RIKEN, 2010-present Professor, Quantum Phase Electronics Center, University of Tokyo, 2010 - present Professor, Institute for Materials Research, Tohoku University, 2001 - 2009 Associate Professor, Japan Advanced Institute of Science and Technology, 1994 - 2001 Visiting Researcher, AT&T Bell Laboratories, Murray Hill NJ, 1993 - 1994 Lecturer, Department of Applied Physics, University of Tokyo, 1991 - 1994 Research Associate, Department of Applied Physics, University of Tokyo, 1986 – 1991

Ph.D., Department of Applied Physics, University of Tokyo, 1986 M.E., Department of Applied Physics, University of Tokyo, 1983 B. E., Department of Applied Physics, University of Tokyo, 1981

Abstract: In the past decade, technological advances of materials fabrication have led to discoveries of a variety of highly crystalline two-dimensional (2D) superconductors at heterogeneous interfaces and in ultrathin films [1]. These systems are offering opportunities of searching for superconductivity at higher temperatures as well as investigating the intrinsic nature of 2D superconductors. This is because all the recently found 2D superconductors are highly crystalline, showing marked contrast with the conventional 2D superconductors with the amorphous or granular structure, which were extensively investigated in the last century. Thus the new 2D superconductors could be a new plat form of physics of 2D superconductivity. In this presentation, we report recent progress on gate-induced superconductivity of exfoliated layered materials, such as ZrNCl and MoS2, which has become an archetypal 2D superconductor with high crystallinity [2]. Discussion is given on the quantum phase transitions at low temperatures [3] and the peculiar transport reflecting the broken inversion symmetry of the crystals, such as enhanced upper critical fields [4] and nonreciprocal supercurrent [5]. References 1. Y. Saito et al., Nat. Rev. Mater. 2, 16094 (2016). 2. J. T. Ye et al., Science 338, 1193 (2012). 3. Y. Saito et al., Science 350, 409 (2015). 4. Y. Saito et al., Nat. Phys. 12, 144 (2016). 5. R. Wakatsuki et al., Sci. Adv. 3, e1602390 (2017).

Lecture-K2


Electrical contacts to two-dimensional semiconductors

First Name: Chul-Ho

Last Name: Lee

Affiliation: KU-KIST Graduate School of Converging Science and

Technology, Korea University, Seoul, Korea


Short Biography:

Chul-Ho Lee received his B.S. (2005) and Ph.D. (2011) from the Department of Materials Science and Engineering of Pohang University of Science and Technology (POSTECH), Korea. After his Ph. D course, he worked in the Department of Physics (with Prof. Philip Kim) at Columbia University, United States, as a postdoctoral fellow. In 2014, then, he joined the faculty of the KU-KIST Graduate School of Converging Science and Technology at Korea University. His current researches focus on MOCVD growth of 2D semiconductors and physics of devices based on 2D materials.

Abstract:

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) have attracted tremendous attention as an active material for next-generation electronic and optoelectronic devices. The performances of these devices can be significantly affected by the electrical contact between metals and 2D semiconductors. In this tutorial talk, I will present fundamental device physics on metal-semiconductor contacts and specific issues in the case of atomically thin 2D semiconductors. Furthermore, recent progresses and challenges in making good electrical contacts to 2D semiconductors will be discussed.

[1]A. Allain et al., Nat. Mat. 14, 1195 (2015). [2] D. S. Schulman et al., Chem. Soc. Rev. 47, 3037 (2018).

Lecture-C2


Quantum anomalous Hall effect in magnetic topological insulators

First Name: Ke

Last Name: HE

Affiliation: Department of Physics, Tsinghua University, Beijing, China


Short Biography:

Ke He graduated from Department of Physics, Shandong University in 2000, and received his PhD in 2006 from Institute of Physics, Chinese Academy of Sciences (IOP, CAS). After that, he worked in the University of Tokyo as a postdoctoral researcher for three years. From 2009 to 2013, he worked in the IOP CAS as an associate professor. He joined Department of Physics, Tsinghua University in October, 2013, and became a full professor from July, 2016. The main research interests of Ke He in recent years are molecular beam epitaxy growth of topological quantum materials and investigations on their novel quantum effects. He and his collaborators for the first time realized the quantum anomalous Hall effect in experiment [Science 340, 167 (2013)]. Up to now,Ke He have published over 60 peer-reviewed papers which have been cited above 4000 times in total.

Abstract:

The quantum anomalous Hall (QAH) effect is a quantum Hall effect induced by spontaneous magnetization instead of an external magnetic field. The effect occurs in two-dimensional (2D) insulators with topologically nontrivial electronic band structure characterized by a non-zero Chern number. The experimental observation of the QAH effect in thin films of magnetically doped (Bi,Sb)2Te3 topological insulators (TIs) paves the way for practical applications of dissipationless quantum Hall edge states, but an ultralow temperature of 30 mK is required to reach a perfect quantization [1]. I will introduce the experimental studies on the QAH effect in magnetic TI films in the past years [2-5]. These results clarify the relationship between the QAH effect and the energy band structure, electronic localization and magnetism of magnetically doped TI films and indicate the ways to increase the temperature of the QAH effect and to manipulate the chiral QAH edge states for various purposes.

[1] C. -Z. Chang et al., Science 340, 167 (2013).

[2] X. Feng et al., Adv. Mater. 28, 6386 (2016).

[3] Y. Ou et al., Adv. Mater. 30, 1703062 (2018).

[4] G. Jiang et al., Chin. Phys. Lett. 35, 076802 (2018).

[5] Y. Gong et al., arXiv:1809.07926 (2018).



Invited-J1


Synthesis of boron nitride single crystals under high pressure and their properties

First Name: Takashi

Last Name: Taniguchi

Affiliation: National Institute for Materials Science

1-1 Namiki Tsukuba 305-0044 Japan


Short Biography:

Dr Takashi Taniguchi is Fellow of National Institute for Materials Science, Tsukuba Japan. He received his PhD from Tokyo Institute of technology. His current research interests concern materials synthesis under high pressure and high temperature by using belt-type high pressure apparatus. Especially studies for boron nitride crystals as superhard and wideband gap materials are major topics in his research activity. He was a former president of the Japan society of High Pressure Science and Technology.

Abstract: Hexagonal boron nitride BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. Among those BN crystals, some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a growth solvent material at high pressure (HP) of 5.5GPa[1]. Band-edge natures (cBN Eg=6.2eV and hBN Eg=6.4eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the HP growth circumstances. Then an attractive potential of hBN as a deep ultraviolet (DUV) light emitter [2] and also superior properties as substrate of graphene devices [3] were realized. While the current subject is to realize how the major impurities such as carbon and oxygen affect the properties of hBN and cBN, some progerss for the realization for the application of 2D’s substrates and photonic materials have been achieved. Also, controlling of boron and nitrogen isotope ratio (10B,11B and 15N) in hBN and cBN crystals can be now carried out by metatheses reaction under HPHT.

In this paper, our recent studies on hBN and cBN single crystals growth under HP with respect to impurity / isotope controls and their functionalizations will be reported.

[References] [1] T.Taniguchi, K.Watanabe, J.Cryst.Growth , 303,525 (2007). [2] K.Watanabe,T.Taniguchi,A.Niiyama,K.Miya, M.Taniguchi, Nature Photonics 3, 591(2009). [3] C.R. Dean, A.F. Young, I. Meric, C. Lee, W. Lei, S. Sorgenfrei, K Watanabe, T. Taniguchi, P. Kim,

K.L.Shepard, J. Hone, Nature Nanotechnology, 5,722 (2010).

Invited-K1


Engineering ferromagnetic lines in graphene by local functionalization using AFM lithography

First Name: Bae, H.

Last Name: Park

Affiliation: Department of Physics, Konkuk University, Seoul 143-701,

Korea


Short Biography:

Prof. Bae Ho Park received his B.S. (1993), M.S. (1995), and Ph.D. (1999) degrees in physics from Seoul National University.

Before coming to the Konkuk University in 2001, he was a Director-funded Post-doctoral Fellow at Los Alamos National

Laboratory. He has been trying to find origins of suppressed electrical performances of emerging nano-materials, such as

graphene, transition metal dichalcogenides, and oxide nano-structures, applicable to next-generation electronic devices

and modify materials or device structures for improvement of electrical properties. He published more than 195 journal

articles (including 1 Science, 1 Nature, 3 ACS Nano, 5 Advanced Materials, 6 Nano Letters, 6 ACS Applied Materials &

Interfaces, 3 NPG Asia Materials, and 42 Applied Physics Letters) and got more than 10,740 citations (h-index of 45). He

received the POSCO TJ Park Science Prize from POSCO TJ Park Foundation in 2015, C. N. Yang Award from Association of

Asia Pacific Physical Societies in 2013, and the Best Academic Achievement Award from Korea Physical Society in 2009.

He was nominated as the Scientist of the Month from Korea Ministry of Education, Science, and Technology and the

President Trusted Professor from Konkuk University in 2012. He is currently serving as an editorial board member of

Scientific Reports (Nature Publishing Group), Journal of the Korean Physical Society, and Nano Convergence.

Abstract:

Monolayer graphene with sp2-carbon-atom network is a promising platform for next-generation spintronic devices due

to its high carrier mobility and long spin relaxation length. For implementation of practical and high-density graphene-

based spintronic devices, we need to define nanoscale areas with ferromagnetic properties on graphene. Up to now,

conventional ferromagnetic metal electrodes accompanied by barrier insulators have been used for injection and

detection of polarized spins in graphene-based spintronic devices. If graphene-based materials show ferromagnetic

behaviors, they will become ideal candidates for spin injectors and detectors, because they structurally, chemically, and

electrically match well with pristine graphene.

In this presentation, I will report on local magnetic characteristics of nanoscale graphene oxidized and hydrogenated by

atomic force microscope (AFM) lithography without conventional sources of surface contamination and chemical agents.

By using AFM lithography, we can selectively control functional

groups and their coverages on the nanoscale at the surface of

graphene. By performing magnetic force microscope (MFM)

measurement, we can clearly distinguish local magnetic signal of

selectively oxidized or hydrogenated graphene from that of

surrounding pristine graphene which does not produce

ferromagnetic signal. The nanoscale oxidized and hydrogenated

graphene show experimental evidences for room-temperature

ferromagnetism. From x-ray magnetic circular dichroism

photoemission electron microscope (XMCD-PEEM)

measurement, we also identified remarkable asymmetry in

carbon K edge XMCD spectra, which strongly indicates that the

observed ferromagnetic order in functionalized graphene layers

is intrinsic.

Fig. 1. XMCD spectra measured on the locally oxidized graphene layers fabricated

with three different bias voltages.



Invited-C1


Quantum phase transition in 2D superconductors

First Name: Jian (健)

Last Name: Wang (王)

Affiliation: International Center for Quantum Materials, School of Physics, Peking University, Beijing, China


Short Biography:

Jian Wang, Changjiang Distinguished Professor of China's Ministry of Education, received his bachelor’s degree in Physics from Shandong University in 2001, and PhD degree in condensed matter physics from Institute of Physics, Chinese Academy of Sciences in 2007. From 2006 to 2011, he worked as a Postdoc and Research Associate at Penn State University, USA. He became a Professor at Peking University in 2017. He was selected to Changjiang Distinguished Professor of China's Ministry of Education in 2016 and Chief Scientist for National Key R&D Program of China in 2018. He won Sir Martin Wood China Prize in 2015. His current research interests are qauntum transport properties of low dimensional superconductors and topological materials. Jian Wang with collaborators discovered quantum Griffiths singularity in 2D superconductors, firstly demonstrated high Tc in one unit cell thick FeSe films by direct transport and Meissner evidences, for the first time found interface-modulated Ising superconductivity, developed a new method to detect topological superconductivity (tip-induced unconventional superconductivity in topological materials), was first to reveal eletron-electron interaction in topological materials, and studied the heterostructures of topological/ferromagnetic materials and superconductors etc as one of the pioneers. In recent years, he has authored more than 70 papers including Science, Science Advances, Nature Materials, Nature Physics, Nature Nanotechnology, Nature Communications, Physical Review X, Physical Review Letters, Nano Letters, JACS, Advanced Materials etc. Jian Wang’s lab at Peking University possesses low temperature scanning tunneling microcopy/spectroscopy-molecular beam epitaxy combined ultrahigh vacuum system, ultralow temperature-high magnetic field measurement system etc.

Abstract: Quantum phase transition is one of most important topics in condensed matter physics. When we study the superconductor-metal transion in ultrathin crystalline Ga films grown on GaN substrate [1], for the first time quantum Griffiths singularity in two dimensional (2D) systems [2] is discovered as a new quantum phase transition in 2D superconductors. The coexistence of quantum Griffiths singularity and Ising (Zeeman-protected) superconductivity is further revealed in monolayer NbSe2 films[3]. We also demonstrate that by interface engineering Zeeman-protected superconductivity can be artificially induced in crystalline 2D superconducting heterostructures [4], which also show anomalous quantum Griffith singularity. [1] Physical Review Letters 114, 107003 (2015) (Editors’ Suggestion) [2] Science 350, 542 (2015) (with a perspective article: Science 350, 509(2015)) [3] Nano Letters 17, 6802 (2017) [4] Phys. Rev. X 8, 021002 (2018)

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03026

e in 32 mm height x 24 mm width

Invited-J2


Electronic structures of topological materials clarified by spin- and angle-resolved photoemission spectroscopy

First Name: Masato

Last Name: Sakano

Affiliation: Quantum-Phase Electronics Center, The University of Tokyo, Tokyo, Japan


Short Biography:

Masato Sakano holds a doctoral degree of engineering from the University of Tokyo. He finished his PhD at the Department of Applied Physics, the University of Tokyo in 2016. Thereafter, he worked as a postdoctral researcher at the institute of Solid State Physics, the University of Tokyo (2016-2017). Since April 2017, he is a research associate at Quantum-Phase Electronics Center, the University of Tokyo. His present research is on studying the electronic structures in strongly spin-orbit coupled materials and topological materials by means of photoemission spectroscopy, and developing the laser angle-resolved photoemission spectroscopy system.

Abstract:

Topological materials of which electronic structures are characterized by the non-zero topological invariants attract much attentions because of the peculiar transport phenomena and magneto-electric effects. On the 3-dimensional (3D) topological material, the existence of 2-dimensional (2D) topological surface state corresponding to the topological invariant is guaranteed by the bulk-edge correspondence. In nearly a decade, by utilizing the advantage of the surface-sensitivity of the angle-resolved photoemission spectroscopy (ARPES), the characteristic 2D surface and 3D bulk electronic structures of the topological materials have been directly observed and widely investigated. In this talk, I present the investigations on the electronic structures of the topological superconductor candidate material -PdBi2 [1] and the Weyl semimetal candidate material -MoTe2 [2], by using the ARPES and spin-resolved ARPES. Layered bismuth compound -PdBi2 (Tc = 5.3 K) is a centrosymmetric superconductor. Beside the spin-degenerate bulk bands, several topologically protected spin-polarized surface bands, some of which crossing the Fermi level, are observed. It indicates that this material may act as a topological superconductor without any carrier doping or applying pressure. Transition metal dichalcogenides MoTe2 is the topological Weyl semimetal candidate realized through the nonpolar-polar structural transition. Two kinds of the bulk polar domain with different surface band dispersions are observed by the laser ARPES. For both domains, some segment-like band features (Fig. 1a,b) resembling the 2D Fermi arc are clearly observed. Our result strongly suggests that the Fermi arc connects the identical pair of Weyl nodes on one side of the polar crystal surface (Fig. 1b), whereas it connects between the different pairs of Weyl nodes on the other side (Fig. 1a).

[1]M. Sakano, et al., Nat. Commun. 6, 8595 (2015). [2]M. Sakano et al., Phys. Rev. B 95, 121101 (2017).

Fig. 1. 2D Fermi arc-like band features on

the surface A (a) and B (b) of -MoTe2.

surface A surface B

mm height x

24 mm width

Invited-K2


Novel states in stacked two-dimensional crystals

First Name: Young-Woo

Last Name: Son

Affiliation: Korea Institute for Advanced Study, Seoul, Korea


Short Biography:

Dr. Young-Woo Son received his Ph. D. from Department of Physics, Seoul National University in 2004. After then, he moved to UC Berkeley as a post doctor and came back to Korea in 2007. From Aug. 2008, he has been a professor of School of computational sciences, Korea Institute for Advanced Study (KIAS), Seoul, Korea. He has studied various interesting electronic, magnetic, structural and electron-phonon-photon interactions in low dimensional materials using several theoretical and computational methods.

Abstract:

Recent advances in fabricating stacked two-dimensional crystals realize interesting electronic structures in low dimensions. For example, the bilayer graphene system with a twist angle of 30 degrees [1] offers a new type of quasicrystals uniting the 12 fold quasicrystalline order and relativistic Dirac fermions. On the other hand, atomically thin magnetic materials and their stacked forms also offer fruitful playgrounds in studying many-body magnetism in low dimension. In this talk, first, I will introduce a new Hamiltonian formalism for graphene quasicrystal. With the model, the spatially localized 12-fold resonant states are shown to exist together with fractal scaling in their wavefunction [2]. Second, I will also discuss a possible modification of magnetism through interlayer interactions in magnetic van der Waals materials [3].

[1] S. J. Ahn et al., Science 361, 782 (2018).

[2] P. Moon, M. Koshino, Y.-W. Son, submitted [arXiv:1901.04701] (2019).

[3]. S. Lee and Y.-W. Son, in preparation.

Invited-C2


Topological axion states in magnetic insulator MnBi2Te4

First Name: Haijun

Last Name: ZHANG

Affiliation: School of Physics, Nanjing University, Nanjing, China


Short Biography:

2000-2004 University of Science of Technology of China, Hefei, China. Bachelor

2004-2010 Institute of Physics, Chinese Academy of Sciences, Beijing, China. Ph.D

2010-2015 Stanford University, USA. Postdoctoral scholar

2015- Nanjing University, Nanjing, China. Professor

My interest is to discover and understand all kinds of novel electronic structure in condensed matter physics, including superconductors, topological materials, oxide com-pounds and diluted magnetic semiconductors, based on first-principles calculations, tight-binding models and the kp method.

Abstract: Topological states of quantum matter have attracted great attention in condensed matter physics and materials science. The study of time-reversal-invariant (TRI) topological states in quantum materials has made tremendous progress in both theories and experiments. As a great success, thousands of TRI topological materials are predicted through sweeping search. Richer exotic phenomena are expected to appear in magnetic topological materials because of varied magnetic configurations, but this study falls much behind due to the complex magnetic structures and transitions. Here, we predict the tetradymite type compound MnBi2Te4 and its related materials host interesting magnetic topological states. The magnetic ground state of MnBi2Te4 is an antiferromagnetic phase which leads to an antiferromagetic topological insulator state with a large topologically non-trivial energy gap (~0.2 eV). It is the parent state for the axion state, which has gapped bulk and surface states, and quantized topological magnetoelectric effect. The ferromagnetic phase of MnBi2Te4 leads to an ideal minimal type-II Weyl semimetal with two Weyl points accompanied by one hole-type and one electron-type Fermi pocket at the Fermi level, which has never been discovered elsewhere.

[1] arXiv: 1808.0801

Fig. 1. Topological Surface states. (a) The surface states on (111) surface are fully gapped due to the S symmetry broken. (b) On the (011) surface, the topological insulator surface states are gapless with the preserved S symmetry.

Invited-J3


Autonomous robotic searching and assembly of two-dimensional crystals to build van der Waals superlattices

First Name: Satoru

Last Name: Masubuchi

Affiliation: Institute of Industrial Science, Univ. of Tokyo, Tokyo, Japan


Short Biography:

Satoru Masubuchi is a lecturer at Institute of Industrial Science, University of Tokyo. He completed his Ph.D. and MS in applied physics, both at the University of Tokyo. His research interests are quantum transport phenomena in van der Waals (vdW) heterostructures, and the application of computer vision and robotic technologies to build automated system for assembling vdW heterostructures.

Abstract:

Van der Waals heterostructures are comprised of stacked atomically thin two-dimensional crystals and serve as novel materials providing unprecedented properties. However, the random natures in positions and shapes of exfoliated two-dimensional crystals have required the repetitive manual tasks of optical microscopy-based searching and mechanical transferring, thereby severely limiting the complexity of heterostructures.

To solve the problem, we develop a robotic system that automatically searches exfoliated 2D crystals and assembles them into vdW superlattices inside the glovebox [1]. The system can autonomously detect 400 monolayer graphene flakes per hour and stack four cycles of the designated two-dimensional crystals per hour with few minutes of human intervention for each stack cycle. The system enabled fabrication of the vdW superlattice structures consisting of 29 alternating layers of the graphene and the hexagonal boron nitride flakes. Fabricated graphene devices exhibited unprecedented charge carrier mobilities (>1,000,000 cm2/Vs), demonstrating the suitability of the method for prototyping variety of high quality vdW superlattices. The fabrication efficiency can be further enhanced by developing the machine-learning algorithm for automatically identifying graphene flakes form the optical microscopy images [2], which eliminates the parameter tuning process to detect graphene flakes.

[1] S. Masubuchi et al., Nature Communications 9, 1413 (2018).

[2] S. Masubuchi et al., npj 2D Materials and Applications 3, 4 (2019).

Fig. (top) The schematics of automated assembly system. (middle) Fabrication process. (bottom) vdW heterostructures. Scale bar corresponds to 5 µm.

Invited-K3


Magnetic van der Waals: hopes and challenges

First Name: Je-Geun

Last Name: Park

Affiliation: IBS-CCES, Dept. Physics & Astronomy, Seoul National University, Seoul, KOREA


Short Biography:

Je-Geun Park is professor of Department of Physics and Astronomy, Seoul National University, and associate director of Center for Correlated Electron Systems, Institute for Basic Science, Korea. He received his Ph.D. from Imperial College London, and his research covers wide areas of strongly correlated electron systems.

Abstract:

There has been a huge increase of interests in two-dimensional van der Waals materials over the past ten years or so. Despite the impressive list of new materials and the novel physics it has come to offer, there is the conspicuous absence of one particular class of materials: magnetic van der Waals systems. It is certainly a sorry status of materials science given the huge impact the magnetic materials have had on both the fundamental understanding and the diverse applications. In this talk, I will identify and illustrate how we might be able to benefit from exploring these so-far neglected materials.

[1] Je-Geun Park, J. Phys. Condens. Matter 28, 301001 (2016). [2] Jae-Ung Lee, et al., Nano Lett. 16, 7433 (2016). [3] Cheng-Tai Kuo, et al., Scientific Reports 6, 20904 (2016). [4] So Yeun Kim, et al., Phys. Rev. Lett. 120, 136402 (2018). [5] Kenneth S. Burch, David Mandrus, and Je-Geun Park, Nature 563, 47 (2018). [6] Kangwon Kim, et al., Nature Communications 10, 345 (2019).



Invited-C3


Photoemission Study of Topological Semimetals

First Name: Zhongkai

Last Name: Liu

Affiliation: School of Physical Science and Technology, ShanghaiTech University, Shanghai, China


Short Biography:

Zhongkai Liu received B.S. in physics from Tsinghua University and Ph. D in Stanford University under the supervision of Prof. Zhi-Xun Shen. After one year experience working as a postdoc in I05, DLS, he started as an assistant professor at ShanghaiTech University. His research interests lie in the photoemission study of topological quantum materials, low dimensional functional materials and development of synchrotron based spatial resolved ARPES.

Abstract: Topological quantum matter represents one of the most intensively investigated fields in condensed

matter physics and material science in the past few years. After the successful theoretical prediction

and experimental discovery for topological insulators, the concept of topological classification has

been generalized to metals where the topological Dirac and Weyl semimetals have been proposed and

studied. Dirac semimetals host bulk Dirac points and 3D Dirac fermions; by breaking the time reversal

symmetry or inversion symmetry, both the Dirac point and Dirac fermion could split into a pair of

Weyl points and Weyl fermions, leading to the formation of Weyl semimetals.

To explore the complex electronic structures in topological semimetals (e.g., bulk Dirac/Weyl fermions

and surface Fermi arcs), synchrotron based ARPES serves as a powerful tool in the investigation. Here

I would like to describe our efforts in the discovery and studies on several typical topological

semimetal systems, including the Dirac semimetal Na3Bi (1) and Cd3As2 (2), inversion symmetry

breaking semimetal TaAs (3,4), time reversal symmetry breaking semimetal Co3Sn2S2 and type-II

Dirac/Weyl semimetals (5,6).

[1] Z. K. Liu, et al., Science, 343, 864 (2014).

[2] Z. K. Liu, et al., Nature Materials, 13, 677–681 (2014).

[3] L. X. Yang, et al., Nature Physics, 11, 728–732 (2015).

[4] Z. K. Liu, et al., Nature Materials, 15, 27–31 (2016).

[5] J. Jiang, et al., Nature Communications, 8, 13973 (2017).

[6] Yiwei Li, et al., Physical Review Materials, 1, 074202, (2017).



Invited-J4


High-resolution ARPES studies of atomic-layer group-V transition metal dichalcogenides

First Name: Katsuaki

Last Name: Sugawara

Affiliation: Department of Physics, Tohoku university, Sendai, Japan


Short Biography:

Ph. D of Physics, Tohoku University, Japan (2009). Recent research interests are the electronic structure of graphene and related atomic-layer materials (graphene superconductor, TMDs).

Abstract: The two-dimensional (2D) atomic-layer materials have been a target of intensive studies by the discovery of Dirac fermion in thinnest limit of graphite and its related anomalous physical properties such as integer quantum Hall effect and valley polarization [1]. Amongst 2D layer materials, group-V transition-metal dichalcogenides (TMDs) MX2 has been also studied more than 40 years ago since it shows various strong-correlated electron phenomena such as Mott insulator, charge-density-waves, and superconductivity [2]. On the other hands, the electronic structures in atomic-layer group-V TMDs have not been investigated. In this talk, we will introduce the recent progress of high-resolution angle-resolved photoemission spectroscopy studies on monolayer group-V TMDs films [3-5] as follows. (1) We find the insulating band dispersions in monolayer NbSe2 different from band structures of bulk 2H-NbSe2, which indicate Mott-insulator by strong coulomb repulsion. [3]. (2) We find the insulating ground state at low temperature in monolayer VSe2 which different from the metallic of bulk 1T-VSe2. In addition, we also find the Fermi arc with pseudo gap at room temperature, which indicate the strong-correlated electron systems in the analogous to high-Tc cuprate superconductivity [4]. [1] A. H. Catro Neto, et al, Rev Mod. Phys., 81, 109 (2009). [2] M. Chhowalla, et al, Nature Chem., 5, 263-275 (2013). [3] Y. Nakata, et al, NPG Asia Mater., 8, e321-1-5 (2016). [4] Y. Umemoto, et al, Nano Res., 12, 165-169 (2018).

Contributed-J1


Layer rotation angle dependent ultrafast charge transfer dynamics of interlayer excitons in vdW heterostructure

First Name: Pranjal Kumar

Last Name: Gogoi

Affiliation: Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan


Short Biography: Pranjal Kumar Gogoi received his Ph.D. degree from the physics department of National University of Singapore (NUS) in 2013. His PhD work was focused on optical spectroscopy of graphene. Currently he is a JSPS postdoctoral fellow in the Electron Microscopy Group at National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan. His research interests include STEM, EELS in the core-loss and valence-loss energy range, and optical spectroscopy of low-dimensional materials, especially 2D transition metal dichalcogenides.

Abstract:

Fig: (a) Schematic of the STEM-EELS experimental set-up. (b) Comparison of low-loss EEL spectra for monolayer WSe2, anti-aligned MoS2/WSe2 heterostructure (MW 60˚), and mis-aligned MoS2/WSe2 heterostructure (MW 21˚).

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide monolayers are a fascinating class of 2D materials with unique properties. Presence of interlayer exciton, where the electron and the hole remain spatially separated in the two layers due to the ultrafast charge transfer, is an intriguing feature of these heterostructures. However, the role of the relative rotation angle of the constituent layers on this charge transfer dynamics is not known yet. Investigating MoS2/WSe2 vdW heterostructures with aberration-corrected monochromated low-loss electron energy loss (EEL) spectroscopy combined with scanning transmission electron microscopy, we report that momentum conservation is a critical factor in the charge transfer dynamics of vdW heterostructures. The rotation angle dependent EEL spectra reveal that - in the aligned (or anti-aligned) case, the charge transfer rate can be about one order-of-magnitude faster than in the misaligned cases. Our results provide a deeper insight into the role of the fundamental principle of momentum conservation in the 2D vdW heterostructure charge transfer dynamics. This work is (partially) supported by JSPS KAKENHI (16H06333, 17H04797, 18K14119, and P18350).



Contributed-J2


Room-Temperature Valley-Polarized Light-Emitting Devices via strained monolayer semiconductors

First Name: Jiang

Last Name: Pu

Affiliation: Dept. of Applied Physics, Nagoya University, Nagoya, Japan


Short Biography:

Dr. Jiang Pu is an assistant professor in Department of Applied Physics at Nagoya University, Japan. He received his B.E and M.E degrees in Applied Physics from Waseda University, Japan. He completed his Ph.D. in the Leading Graduate Program in Science and Engineering at Waseda University in 2017, supported by Ministry of Education, Culture, Sports, Science and Technology (MEXT). During his Ph.D. program, he also was selected as the Research Fellowship for Young Scientists from Japan Society of Science (JSPS).

Abstract:

The valley contrasted electronic structure in monolayer transition metal dichalcogenides (TMDCs) offers unique optical functionalities, such us circularly polarized light emission [1]. In particular, the electrical control of circularly polarized light emission is one of the most desired device applications. However, the reported TMDC chiral light-emitting devices have mostly realized at low temperatures and/or required high magnetic fields [2,3]. Therefore, the light-emitting device that can control valley-polarized electroluminescence (EL) at room temperature is a significant challenge [4]. Interestingly, we recently found out that the local strains implanted inside CVD-grown TMDCs might serve as a key role to generate circularly polarized EL nearly room temperature. On the basis of this observation, here, we try to achieve the room-temperature valley-polarized light-emitting devices by combining the electrolyte-based structures with the strained monolayers.

The monolayer WS2 flakes were grown on sapphires by CVD method, followed by transferring them onto PEN substrates. After that, two Au electrodes were deposited on monolayer flakes, and then, the ion-gel films were spin-coated to construct two-terminal light-emitting structure (Fig. 1a) [5]. The devices were set on the home-built bending stage with the polarization-resolved optical set-up. Figs. 1b and 1c show the PL mapping of channel regions with flat (Fig. 1b) and strained conditions (Fig. 1c). We can notice obvious PL red-shifts in the strained devices, corresponding to 1 % strain induced in monolayers. In the strained conditions, we applied AC voltage to the devices to integrate EL intensity, and the chirality of EL (σ+ and σ-) was selectively detected by controlling quarter-wave plate. We observed EL polarizations ([σ+-σ-]/[σ++σ-]) up to 20 % at room temperature. Importantly, we can also switch the EL polarizations by inversing the current direction (Fig. 1d). These results provide the electrical generation and control of valley-polarized EL at room temperature via strains.

[1] X. Xu, et al. Nat. Phys. 10, 343 (2014) [2] Y. J. Zhang, et al. Science 344, 725 (2014) [3] Y. Ye, et al. Nat. Nanotechnol. 11, 598 (2016) [4] J. Pu and T. Takenobu Adv. Mater. 30, 1707627 (2018) [5] J. Pu, et al. Adv. Mater. 29, 1606918 (2017)

Contributed-K1


Suppression of magnetic ordering in XXZ-type antiferromagnetic NiPS3 in atomically thin limit

First Name: Kangwon

Last Name: Kim

Affiliation: Department of Physics, Sogang University, Seoul, Korea


Short Biography:

Kangwon Kim is a Ph.D. candidate in the Department of Physics at Sogang University, where he received his MS. and B.S. degrees in 2014 and 2016, respectively.

He study physical properties of 2-dimenional materials by using optical spectroscopy.

Abstract:

Magnetism in low dimension is important for both in fundamental science and application. Transition metal phosphorus trisulfides (TMPS3) are a new class of magnetic van der Waals materials and show different magnetic ground states depending on the TM element: FePS3[1], NiPS3, and MnPS3 have Ising-, XXZ-, and Heisenberg-type antiferromagnetic ordering, respectively. We investigated XXZ-type antiferromagnetic NiPS3 in the 2D limit by Raman spectroscopy. Raman spectroscopy is a powerful technique for studying 2D magnetic materials, as it can investigate phonon scattering as well as magnetic scattering. Below the Néel temperature, several Raman signatures due to antiferromagnetic phase transition are observed in the Raman spectrum of bulk NiPS3: 2-magnon scattering, Fano resonance[2], suppression of the quasi-elastic scattering, and splitting of a phonon mode. The Néel temperature (TN) of NiPS3 can be estimated by analyzing the temperature dependence of these signatures. We synthesized bulk single-crystal NiPS3 by the vapour transport method and prepared atomically thin samples down to the monolayer by using mechanical exfoliation. We measured temperature dependent Raman spectra of few-layer NiPS3 samples and found that the Néel temperature shows slightly difference from bulk for the thickness down to bilayer, but seems to be suppressed significantly for monolayer[3]. We note that all these experimental observations are in good agreement with the theoretical predictions of the XY model.

[1] J.-U. Lee et al., Nano Lett. 16, 7433 (2016).

[2] S. Rosenblum et al., Phys. Rev. B 49, 4352 (1994).

[3] K. Kim et al., Nat. Commun. 10(1), 345 (2019).

Fig. 1. (a) Raman spectra of bulk NiPS3 at low and room temperatures. (b) Estimated TN for various thicknesses by using phonon splitting.



Contributed-K2


Topological and ferromagnetic properties of iron-based van der Waals metals

First Name: Junho

Last Name: Seo

Affiliation: 1Center for Articial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), Pohang 37673, Korea, 2Department of Physics, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea


Short Biography:

Junho Seo is PhD student of Prof. Jun Sung Kim’s lab at POSTECH, Pohang, Korea. He mainly do bulk single crystal growth, especially van der Waals ferromagnetic materials, and measuring magnetotransport properties of them.

Abstract:

Topological semimetals, new states of matters whose low energy electronic structure possesses several band contact points or lines, are generally expected to exhibit intriguing topological responses. Up to now, most of the studies on topological semimetals are limited to non-magnetic materials with time-reversal symmetry. However, magnetic materials can also be endowed with topological band structures in which the interplay of magnetism and band topology can generate novel correlated topological phenomena. In this talk, I will introduce iron-based van der Waals (vdW) materials, where combination of magnetism, spin-orbit interaction, and topological band structures gives rise to unusual physical properties and magnetic tunability. This demonstrates that topological and ferromagnetic vdW materials have great potential for various spin-dependent electronic functionalities.



Contributed-C1


Saddle points induced by band inversion in topological materials

First Name: Huaiqiang

Last Name: Wang



Short Biography:

My interest mainly focuses on novel electronic structures and corresponding transport phenomena in topological quantum systems, including topological insulators, topological superconductors, and various kinds of topological semimetals. Specifically, transition metal dichalcogenides and non-Hermitian topological phases have been my recent research interest.

2008-2012 Nanjing University, Nanjing, China. Bachelor

2012-2017 Nanjing University, Nanjing, China. Ph. D

2017- now Nanjing University, Nanjing, China. Postdoctoral scholar

Abstract:

Saddle points in electronic band structures have always been interesting mostly because of their divergent density of states (Van Hove singularities). Here, we propose a general mechanism that saddle points can be induced by band inversion in both two- and three- dimensional topological materials. The emergence of saddle points requires large band hybridization and small band inversion. As concrete examples, through both first-principles calculations and 𝑘 ⋅ 𝑝 analysis, we demonstrate the existence of saddle points in monolayer 1T’-WS2, topological insulator Bi2Te3, and topological semimetal Na3Bi. When tuning the Fermi level to the saddle point, the log-divergent density of states may lead to instabilities in the presence of electron-electron interaction.

Figure: Schematic illustration of the saddle points induced by band inversion.

[1] Jiawei Ruan, Huaiqiang Wang, and Haijun Zhang, in preparation.



Contributed-C2


Electronic structure of exfoliated 2H-MoTe2 atomically thin flakes by using NanoARPES

First Name: Hongyun

Last Name: Zhang

Affiliation: Department of Physics, Tsinghua University, Beijing, China


Short Biography:

In 2015, she received B.S. degree in Tianjin University, China. Then, she joined Prof. Shuyun Zhou’s Group as a Ph.D. student in Tsinghua University. She is now focusing on the electronic structure of 2D materials and heterostructures by using ARPES and Nano-ARPES.

Abstract:

Transition-metal dichalcogenides exhibit strong quantum confinement effects, and their electronic structure is strongly dependent on the number of layers. Resolving the thickness-dependent electronic structure is important. While the electronic structure of atomically thin 2H-MoSe2 or 2H-MoS2 have been explored, information on the experimental electronic structure of 2H-MoTe2 is still missing. Here, by using nanospot angle-resolved photoemission spectroscopy (nanoARPES), we reveal the first experimental electronic structure of exfoliated 2H-MoTe2 thin flakes with different thickness (three, five, and seven monolayers). [1] Well-separated quantum-well states are clearly observed in thin 2H-MoTe2 flakes at deep valence bands at energies between −3 to −5 eV, while those at the top of the valence band between −1 and −2 eV are much more closely spaced compared with those from 2H-MoSe2 and 2H-MoS2. First-principles calculation shows that the main difference is attributed to the weaker hybridization and smaller energy difference between Mo 4dz

2 and Te 5pz orbitals as compared with Se 4pz and S 3pz orbitals.

[1] Hongyun Zhang, et al., Nano Lett, 18, 4664-4668, (2018)

Fig. 1. Electronic structure of exfoliated 2H-MoTe2 flakes (3, 5 and 7ML) revealed by NanoARPES.



Poster-1


Terahertz light emitting and lasing operation in graphene-based heterostructure 2D material systems

First Name: Takayuki

Last Name: Watanabe

Affiliation: Research Institute of Electrical Communication Tohoku University, Sendai, Japan


Short Biography:

Dr. Takayuki Watanabe received the Dr. Eng. degree in Electrical and Communication Engineering from the Graduate School of Engineering, Tohoku University in March 2014. He is currently working at the Ultra-Broadband Signal Processing Laboratory, Research Institute of Electrical Communication, Tohoku University as an associate professor.

Abstract: Carrier-injection pumping of graphene enables negative-dynamic conductivity in the terahertz (THz) range, which may lead to new types of THz lasers [1]. We designed/fabricated the distributed feedback (DFB) dual-gate graphene-channel FET (DG-GFET) (Fig. 1) [2]. The GFET channel consists of a few layer (non-Bernal) epitaxial graphene [3], providing an intrinsic field-effect mobility exceeding 100,000 cm2/Vs [4]. The teeth-brash-shaped DG forms the DFB cavity having the fundamental mode at 4.96 THz (designed). The modal gain and the Q factor at 4.96 THz were simulated to be ~5 cm-1 and ~240, respectively [2]. Broadband rather intense (up to ~80 μW) amplified spontaneous emission from 1 to 7.6 THz and weak (up to ~0.1 μW) single-mode lasing at 5.2 THz [2] were observed at 100K in different samples (Fig. 2).

When the substrate-thickness dependent THz photon field distribution could not meet the maximal available gain-overlapping condition, the DFB cavity cannot work properly, resulting in broadband LED-like incoherent emission. To increase the operating temperature and lasing radiation intensity, further enhancement of the THz gain by increasing the THz photon-field confinement and the cavity Q factor are mandatory. Plasmonic metasurface structures promoting the superradiance and/or instabilities [5] are promising for giant THz gain enhancement.

[1] V. Ryzhii et al., J. Appl. Phys. 101, 083114 (2007); V. Ryzhii et al., J. Appl. Phys. 110, 094503 (2011). [2] G. Tamamushi, et al., 74th Dev. Res. Conf. Dig., 1, 225-226 (2016). [3] H. Fukidome et al., Appl. Phys. Lett. 101, 041605 (2012). [4] A. Satou et al., IEEE Trans. Electron Dev., 63, 3300-3306 (2016). [5] V.V. Popov et al., Phys. Rev. B 86, 195437 (2012); Y. Koseki et al., Phys. Rev. B 93, 245408 (2016).

Fig. 2. Broadband and single-mode emissions from fabricated devices.

Fig. 1. Schematics of fabricated DG-GFET.

Poster-2


Gate-Tunable Valley Polarization in WSe2 Field-Effect Transistor

First Name: Keisuke (啓介)

Last Name: Shinokita (篠北)

Affiliation: Institute of Advanced Energy, Kyoto University, Kyoto, Japan


Short Biography:

Dr. Keisuke Shinokita obtained his B.Sc. (2008), M.Sc. (2010), and D.Sc. (2013) degrees from Kyoto University. He was a postdoctoral fellow at University of Groningen (2013-2015) and a guest researcher at Max Born Institute (2015-2017). Since 2017, he has been working at institute of advanced energy in Kyoto University as a program-specific assistant professor. His main topic is to study novel light-matter interaction in two dimensional materials.

Abstract:

Valley pseudospin that gives new degrees of freedom in momentum space has attracted tremendous attention for both research and application as a new binary index of electron systems [1]. Currently, the most important issues in valley physics are maintaining and further controlling the valley-polarized states by suppressing valley relaxation [2].

Herein, we present an alternative way to suppress and control the valley relaxation of electron–hole pairs (neutral excitons) in monolayer WSe2 [3]. By using carrier doping by applying a bias gate voltage with a field-effect-transistor (FET), we demonstrate the suppression of valley relaxation between the K and –K valleys resulting in the enhancement of valley polarization of the neutral exciton. The valley control under carrier doping was achieved by modulation of long-ranged electron-hole exchange interaction and it does not require high external field to break the time-reversal and spatial-inversion symmetries. This approach to manipulate valley polarization will allow the potential of valleytronics and quantum information-processing applications using valley index of exciton as an information carrier in these exotic two-dimensional materials to be developed further.

[1] X. Xu, W, Yao, D. Xiao, T. F. Heinz, Nat. Phys. 10. 343 (2014). [2] Y. Miyauhi, K. Matsuda et al., Nat. Commun 9. 2598 (2018). [3] K. Shinokita, X. Wang, Y. Miyauchi, K. Matsuda, submitted for publication.

Fig. 1. Gate-dependent polarization-resolved PL spectra under σ+ excitation.

Poster-3


Nonreciprocal transport in noncentrosymmetric van der Waals crystals

First Name: Toshiya

Last Name: Ideue

Affiliation: Quantum-Phase Electronics Center (QPEC) and Department of Applied Physics, the University of Tokyo, Tokyo, Japan.


Short Biography:

Research Associate, Department of Applied Physics, University of Tokyo, 2015–

Ph.D., Department of Applied Physics, University of Tokyo, 2015

Fujifilm Corporation 2011-2012

M.E., Department of Applied Physics, University of Tokyo, 2011 B. E., Department of Applied Physics, University of Tokyo, 2009

Abstract: Noncentrosymmetric crystals are attracting a growing interest as an ideal platform for novel physical properties and functionalities. One of the manifestations of the lattice symmetry breaking in electric transport is the nonreciprocal magneto resistance, in which forward and backward current becomes inequivalent under magnetic field. In this poster presentation, I will review our recent experimental results on the nonreciprocal magneto transport measurements in van der Waals crystals [1-3]. Rectification effect originating from crystal symmetry breaking has been observed in several noncentrosymmetric van der Waals crystals. In all the materials, nonreciprocal transport satisfies the characteristic selection rule reflecting the crystal symmetry. Microscopic mechanism of the nonreciprocal transport in each material will be also discussed.

Fig. 1. Nonreciprocal l transport in several noncentrosymmetric van der Waals crystals

[1]T. Ideue et al. Nature Physics, 13, 578 (2017) [2]F. Qin et al. Nature Communications, 8, 14465 (2017) [3]R. Wakatsuki et al. Science Advances, 3, e1602390 (2017)

Poster-4


Coexistence of charge ordering and 2D superconductivity in IrTe2 nanosheets

First Name: Sun-gyu

Last Name: Park

Affiliation: CALDES, IBS, Pohang, Republic of Korea


Short Biography:

Post-doctor, Center for Artificial Low D Electronic Systems, Institute for Basic Science (Oct. 2017 - )

Post-doctor, Department of Physics, KAIST (Mar. 2014 – Sept. 2017)

Ph. D. in Physics, Department of Physics, KAIST (Mar. 2007 – Feb. 2014)

Research interests: Phase transition of 2D superconducting film tuned by magnetic field and pressure

Abstract:

IrTe2 holds a unique position among transition metal dichalcogenides (TMDCs) because of its intriguing many-body states such as various types of charge orders [1] and metastable superconductivity [2,3]. In bulk IrTe2 crystals with chemical doping or thermal quenching, the stripe charge order competes with the superconducting order, often leading to macroscopic phase separation. In this work, we report that mechanically-exfoliated IrTe2 crystals host both stripe charge order and superconductivity together without phase separation. Using transport measurements and scanning tunnelling microscopy experiments, the clear signature of first-order stripe phase transition is observed at high temperatures, while two-dimensional superconductivity occurs at Tc ~ 2 K with strong anisotropy in the upper critical fields. Our findings reveal the coexistence of stripe charge order and superconducting order in thinned IrTe2 crystals, demonstrating effective thickness tuning of competing phases in correlated TMDCs.

This work was supported by the Institute for Basic Science (IBS) through the Center for Artificial Low Dimensional Electronic Systems (no. IBS-R014-D1), by POSCO through the Green Science programme, and also by the National Research Foundation (NRF) of Korea through the SRC (no. 2011-0030785)

[1] M. J. Eom et al., PRL 113, 266406 (2014). [2] M. Yohida et al., Nano Lett. 18, 3113 (2016). [3] H. Oike et al., Sci. Adv. 4, (2018).



Poster-5


Quantum transport in graphene/hexagonal boron nitride heterostructures

First Name: Takuya

Last Name: Iwasaki

Affiliation: International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, Japan


Short Biography:

Takuya Iwasaki received B.Sc. from Tokyo Denki University in 2012, and M.Sc. from Japan Advanced Institute of Science and Technology (JAIST) in 2014. He received Ph.D. from JAIST and completed The Collaborative Education and Research Co-supervision Programme undertaken in collaboration with Univ. of Southampton in 2017. He was Japan Society for the Promotion of Science (JSPS) Fellow (PD) in JAIST (2017–2018). Currently, he is International Center for Young Scientists (ICYS) Research Fellow at National Institute for Materials Science (NIMS). His research interest is transport property in nanostructures based on two-dimensional materials, such as graphene.

Abstract:

Graphene has attracted attention because of its unique electronic properties that relativity and chirality. Since the development of the fabrication technology for two-dimensional (2D) material heterostructures, high quality graphene devices are available by encapsulating graphene by hexagonal boron nitride (hBN) layers, atomically flat 2D insulator. Both the technique and high quality hBN have allowed to study the carrier transport property reflecting the essential material property, which could not be accessed in graphene on the conventional SiO2/Si substrate due to its surface roughness. Furthermore, the stacking of graphene and hBN with certain crystallographic angle alignment gives rise to superlattice potential, leading to breaking inversion symmetry in graphene [1]. Accordingly, fascinating phenomena in graphene/hBN heterostructure has been studied extensively.

Here we report the transport properties of graphene/hBN-based quantum point contact (QPC), graphene/hBN superlattice, and bilayer graphene(BLG)/hBN superlattice devices. In the QPC, the dependence of the open/close state transition on the gap between the split gates and Fabry-Perot interference are investigated [2]. In the graphene/hBN superlattice [3] and BLG/hBN superlattice devices, the fractal spectrum in the local resistance mapping plot of gate voltage and perpendicular magnetic field is observed. By using the H-shaped geometry we measured non-local resistance as shown in Fig. 1, which is caused by the valley Hall and its inverse effect.

[1] L. A. Ponomarenko et al., Nature 497, 594-597 (2014). [2] N. F. Ahmed et al., Appl. Phys. Lett. 114, 023101 (2019); N. F. Ahmed et al., arXiv: 1901.06143. [3] K. Komatsu et al., Sci. Adv. 4, eaaq0194 (2018).

Fig. 1. (a) Schematic of the graphene/hBN device with non-local measurement configuration. (b) Non-local resistance in bilayer graphene/hBN device at 6 K.



Poster-6


Observation of selective charge-density-wave pinning depending on the atomic structure of native defects in 2H-NbSe2

First Name: Eunseok

Last Name: Oh

Affiliation: 1Center for Artificial Low dimensional Electronic Systems, Institute for Basic Science (IBS), 2Department of Physics, Pohang University of Science and Technology(POSTECH) /Pohang 37673/Republic of Korea


Short Biography:

2017.9 –current : Research assistant in CALDES, IBS

2017.9 –current : Ph.d Student in Physics department, POSTECH

2017.8: B.S., in Physics, POSTECH, Pohang, Korea

Research Topics: Charge-density wave, Superconductivity, Transition metal dechalcogenide, STM

Abstract:

Charge-density-wave (CDW) in transition-metal dichalcogenide (TMDC) 2H-NbSe2 is widely known for several decades [1], while microscopic mechanism of CDW formation are not fully understood. Native defects have been known to have a self-doping effects on the material, affect the surrounding electronic structure, and be also related to the CDW [2-3]. We investigate the native defects in 2H-NbSe2 and classify three major atomic defects by means of scanning tunneling microscope (STM). By density functional theory (DFT) calculations, the atomic structures of these major atomic defects are analyzed and compared. In the STM measurement, specific CDW phase are selectively pinned around native defects depending on their atomic structure above the CDW transition temperature. These results give the basis for the further defect studies in 2H-NbSe2 and other 2D TMDC materials, and show microscopically relation between CDW and defects.

Figure. 1

[1]D. E. Moncton .et al., Phys. Rev. B 16, 801 (1977). [2] B. Hildebrand et al., Phys. Rev. Lett. 112, 197001 (2014). [3] C. J. Arguello., Phys. Rev. B 89, 235115 (2014).



Poster-7


Influence of Chemical Etching Treatment on Copper Foils for Single-Layer Graphene CVD Growth

First Name: Naoki

Last Name: Yoshihara

Affiliation: Department of Chemical Engineering, Fukuoka University, Fukuoka, 8-19-1 Nanakuma, Jonan-ku, Japan


Short Biography:

Naoki Yoshihara received his PhD from Kyushu University in 2009 for experimental research on carbon nanotube synthesis. After spending four years at Mitsui Chemicals Inc., he was appointed as assistant professor of Fukuoka University. His current research focuses on the controlled synthesis of graphene and related crystal materials as well as experimental investigation of their electrochemical properties.

Abstract:

Single-layer graphene growth on commercially available copper foils by chemical vapor deposition (CVD) is paid the most attention as a mass production technique for practical applications in numerous fields. Chemical etching treatment of the copper foils is essential as a pre-treatment for the graphene CVD growth. This chemical etching plays the role of between the surface flatness and the removal of impurities and the naturally copper oxide layer. However, the graphene CVD growth on etched copper foils is still unclear. Here, we investigated the influence of the chemical etching treatment on copper foils for single-layer graphene CVD growth.

Figure 1 showed the optical micrographs of graphene domains grown on the copper foil (thickness: 25 m) treated with 0.2 mol/L iron trichloride (FeCl3) aqueous solution for the different etching time. We synthesized graphene on the etched copper foil under a mixed flow of CH4, H2, and Ar atmosphere at 1000 °C for 1 hr. In all etching conditions, the hexagonal graphene domains were observed on etched copper foils. Interestingly, the graphene domain size increased with extending the etching time and the graphene domain on copper foils etched for 120 sec is around 4 orders of magnitude larger than that for 15 sec.

It is likely that the observed effect of chemical etching treatment on the copper surface reflects the graphene nucleation density. Recently, it has been reported that the presence of oxygen in copper foils is a key factor for decreasing the graphene nucleation density and accelerating graphene domain growth [1-2]. However, our results cannot account for only this explanation because a large amount of copper oxide layer on subsurface of copper foils is eliminated by the longer etching treatment. This might be related to surface morphology of the copper foil after the chemical etching treatment.

[1]Y. Hao et al., Science 342, 720 (2013). [2] D. Ding et al., Appl. Surf. Sci. 408, 142 (2017).

Fig. 1. The optical micrographs of graphene domains on the etched copper foil treated with FeCl3 aqueous solution for a) 15, b) 30, c) 60, and d) 120 sec.

All scale bars are 100 m.

a) b) c) d)

Poster-8


Synaptic devices implemented with two-dimensional layered single crystal chromium thiophosphate (CrPS4)

First Name: Mi Jung

Last Name: Lee

Affiliation: Department of Physics, Konkuk University, Seoul 143-701, Korea


Short Biography:

Mi Jung Lee received his B.S. (2010), M.S. (2012), Ph.D. (2018) degrees in physics from Konkuk University, respectively. She is currently a post-doctoral researcher at the Konkuk University in Korea. Her research interests focus on conductive bridging memory based memristive devices and synaptic devices using two-dimensional materials.

Abstract:

Two-dimensional (2D) van der Waals (vdW) materials have recently attracted considerable attention due to their excellent electrical and mechanical properties. TmPSx (where Tm = a transition metal), which is a new class of 2D vdW materials, is expected to show various physical phenomena depending on the Tm used.

Here, we demenstrated the unprecedented synaptic behavior of a vertical Ag/CrPS4/Au capacitor structure. Multi-stable resistive states were obtained using an external voltage of less than 0.3 V. Both short-term plasticity and long-term potentiation were observed by controlling the interval of the external voltage pulse. Simple mechanical exfoliation was used to develop a synaptic device based on a very thin CrPS4 layer with a thickness of ~17 nm. The devices’ behavior is attributed to the electrochemically induced migration of silver ions across the CrPS4 layer, leading to the formation and rupture of a conducting filament between the electrodes. Therefore, it was demonstrated that vertical Ag/CrPS4/Au structure is promising for computing technologies that imitate neural pathways in the nervous system.

Fig. 1. The current-voltage (I-V) curves show the bias-polarity-dependent bipolar resistive switching (RS) behaviors of the Ag/CrPS4/Au device.



Poster-9


Gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2

First Name: Yujun

Last Name: Deng

Affiliation: State Key Laboratory of Surface Physics and Department of Physics, Fudan University, Shanghai, China


Short Biography:

Yujun Deng is currently a PhD student in Prof. Yuanbo Zhang’s group in Fudan University, Shanghai, China. Her current research interest is focused on new fabrication methods and transport properties of novel 2D materials with an emphasis on magnetic 2D materials. Her representative work is tunable magnetic properties of 2D Fe3GeTe2.

Abstract:

The advent of two-dimensional van der Waals crystals creates new possibilities in developing novel spintronic devices. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 and CrI3 at low temperatures. Here, we developed a new device fabrication technique, and successfully isolated monolayers from layered metallic magnet Fe3GeTe2. We found that the itinerant ferromagnetism persists in Fe3GeTe2 down to monolayer. The ferromagnetic transition temperature, Tc, is suppressed in pristine Fe3GeTe2 thin flakes. An ionic gate, however, dramatically raises the Tc up to room temperature. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics.

Fig. 1. Hall resistance Rxy of a four-layer Fe3GeTe2 flake

under a gate voltage of Vg = 2.1 V.

Poster-10


Spin-orbit-interaction - driven magnetic anisotropy in dimerized honeycomb-lattice ruthenate Li2RuO3

First Name: Seokhwan

Last Name: Yun

Affiliation: Department of Physics and Astronomy, Seoul National University, Seoul, Republic of Korea


Short Biography:

Seokhwan Yun is currently a PhD student at the Seoul National University. He researches involves studies of spin-orbital entangled and electron correlated system, and employs several synchrotron radiation techniques including resonant elastic x-ray scattering (REXS) to study such systems.

Abstract:

Li2RuO3 has a layered honeycomb structure like α-Li2IrO3, but it has the unique lattice symmetry (P21/m). It is also known to be non-magnetic down to very low temperatures. Instead, it has a transition to a valence bond solid (VBS) phase at a fairly high temperature of ~540 K [1]. At the transition, the otherwise uniform honeycomb lattice of Ru gets distorted and forms a herring-bone structure. This transition was interpreted to arise from dimerized Ru 4d band through strong lattice effects, involving off-centering of Li atoms at the center of the Ru honeycomb lattice [2].

Despite several studies on the origin of the VBS transition [1-5], however much of the detailed mechanism still remains unresolved. In order to get better understanding of the high-temperature transition, we studied the anisotropy of physical properties such as resistivity and magnetic susceptibility using single crystal Li2RuO3. According to our results, there are clear anisotropy in both measured properties. Using various both experimental and theoretical considerations, we conclude that our results can only be understood to be due to the spin-orbit coupling. We will present the orbital patterns that can be consistent with our results.

[1] Y. Miura et al, J. Phys. Soc. Jpn. 76, 033705 (2007)

[2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009)

[3] S. A. J. Kimber et al, Phys. Rev. B 89, 081408(R) (2014)

[4] S. V. Streltsov and D. I. Khomskii, Phys. Rev. B 89, 161112(R) (2014)

[5] Junghwan Park, et al., Scientific Reports 6, 25238 (2016)

Poster-11


Large-scale fabrication of highly-transparent solar cell with 2D material

First Name: Xing

Last Name: He

Affiliation: Department of Electronic Engineering, Tohoku University, Sendai, Japan


Short Biography:

Xing He is a PhD student under the supervisor of Prof. Kaneko and Dr. Kato at Tohoku University. Her current research interests focus on TMD growth and transparent solar cell fabrication. She has ever published two papers on SCI as first author and four papers as contributed author. She received funding from the China Scholarship Council (CSC).

Abstract:

Layered transition metal dichalcogenide (TMD) is known as a true 2D material with excellent semiconducting properties. TMD is one of the most attractive materials for future transparent and flexible optoelectrical devices due to their atomically thin structure, band gap in visible light range, and high optical transparency [1-3].

Recently, we have developed a new fabrication process of TMD-based solar cell [4]. In our process, Schottky type device configuration is utilized, which can be simply formed by asymmetrically contacting electrodes and TMD. The power conversion efficiency clearly depended on the work function difference between two electrodes (ΔWF), and a higher efficiency could be obtained with higher ΔWF (Pd-Ni), which is consistent with our concept, where Ni and Pd can form large and small Schottky barriers to operate as power-generation and carrier-collect regions, respectively. Based on the optimizations of electrodes and distance, the power conversion efficiency can be reached up to 0.7 %, which is the highest value for solar cell with similar TMD thickness [4] (Fig. 1).

Also, in order to improve the transparency of whole device, we use indium tin oxide (ITO) as electrodes. The directly grown large area WS2 film are also used to overcome the limited device size. After controlling the ΔWF of ITO electrodes and optimizing the synthesis conditions of WS2, clear power generation can be observed with ITO/WS2 based transparent solar cell in large scale. Since our simple fabrication process includes high potential for large scale fabrication, this achievement is very important for realizing the industrial application of TMD as a transparent and flexible solar cell.

[1] T. Kato and T. Kaneko, ACS Nano 8, 12777 (2014). [2] T. Kato and T. Kaneko, ACS Nano 10, 9687 (2016). [3] C. Li, Y. Yamaguchi, T. Kaneko, and T. Kato, Appl. Phys. Express 10, 075201 (2017). [4] T. Akama, W. Okita, R. Nagai, C. Li, T. Kaneko, and T. Kato, Sci. Rep. 7, 11967 (2017).

Fig. 1. Typical optical microscope image

of transparent solar cell made by few-

layered TMD.

Poster-12


Fuel cell based on AA′-stacked trilayer hexagonal boron nitride

First Name: Seong In

Last Name: Yoon

Affiliation: Energy engineering, UNIST, Ulsan, Korea


Short Biography:

Seong In Yoon received his B.S. degree in energy engineering in Ulsan National Institute of Science and Technology (UNIST) in 2013, and he is now a PhD candidate in the energy engineering of UNIST, South Korea. He is currently studying on membrane application of two-dimensional (2D) materials such as 2D materials for proton exchange membrane and graphene sandwich as nanoreactor.

Abstract: Hexagonal boron nitride (h-BN) and graphene have emerged as promising materials for proton exchange membranes because of their high proton conductivity and chemical stability.[1] However, the defects and grain boundaries generated during the growth and transfer of two-dimensional materials limit their practical applicability. Here, we report the fabrication of membrane electrode assemblies using large-area single-oriented AA′-stacked trilayer h-BN (3L-BN), which exhibits very few defects during the growth and transfer, as a proton exchange membrane for use in fuel cell systems. The fuel cell based on AA′-stacked 3L-BN showed a H2 permeation current density as low as 2.69 mA cm−2 and an open circuit voltage (OCV) as high as 0.958 V; this performance is much superior to those for cells based on Nafion (3.7 mA cm−2 and 0.942 V, respectively) and single-layer h-BN (10.08 mA cm−2 and 0.894 V, respectively). Furthermore, the fuel cell with the AA′-stacked 3L-BN membrane almost maintained its original performance (OCV, maximum power density, and H2 permeation current density) even after 100 h of an accelerated stress test at 30% RH and 90 oC, while the fuel cells with the Nafion and single-layer BN membranes exhibited severely deteriorated performances. The stability of the cell based on the AA′-stacked 3L-BN membrane was better because the membrane prevented gas crossover and suppressed the generation of reactive radicals during cell operation.

[1]S. Hu et el., Nature, 2014, 516, 227.

Fig. 1. Schematic illustration of fuel cell based on AA′-stacked 3L-BN


32 mm height x 2 4 mm width

Poster-13


Light Soaking Phenomena in Organic-inorganic Mixed Halide Perovskite Single Crystals

First Name: Hye Ryung

Last Name: Byun

Affiliation: Department of Energy Science, Sungkyunkwan University, Suwon, 16419, Republic of Korea


Short Biography:

Mar, 2015 – present

Ph.D course, Department of Energy Science, Sungkyunkwan University, Suwon, South Korea

Mar, 2013 – Feb, 2015

Master course, Department of Physics, Kangwon National University, South Korea

Mar, 2008 – Feb, 2013

Bachelor of Science, Kangwon National University, South Korea

Abstract:

Recently, organic-inorganic mixed halide perovskite (MAPbX3; MA = CH3NH3+, X = Cl-, Br-, or I-) single

crystals with low defect densities have been highlighted as candidate materials for high efficiency photovoltaics and optoelectronics. Here we report the optical and structural investigations of mixed halide perovskite (MAPbBr3-xIx) single crystals. Mixed halide perovskite single crystals showed strong light soaking phenomena with light illumination conditions which were correlated to the trapping and detrapping events from defect sites. By systematic investigation with optical analysis, we found that the pseudocubic phase of mixed halide perovskites generates light soaking phenomena. These results indicate that photo-induced changes are related to the existence of multiple phases or halide migrations.

Fig. 1. Snap shot of Mixed halide perovskite single crystal growth at different mixing ratios: Br/I 3:0, Br/I 5:1, Br/I 2:1, and Br/I 0:3 (top). Contour plots of PL spectra of Br/I :1 and Br/I 2:1 single crystals as a function of laser irradiation time. (bottom)

Poster-14


Visualizing the electronic structure of monolayer Bi2Sr2CaCu2O8+δ

First Name: Liguo

Last Name: Ma

Affiliation: Department of Physics, Fudan University, Shanghai, China


Short Biography:

Ma is a PhD. student of Department of Physics, Fudan University, China. He received B.S. degree in physics from Shandong University. He joined Yuanbo Zhang’s group of two dimensional electron system in 2013. His research interests are focused on studying layered correlated materials by scanning probe method.

Abstract:

Although high-temperature superconductors are a complex and diverse family of materials, they all adopt a layered crystal structure, in which two-dimensional lattices stack together to form the three-dimensional bulk. This curious fact begs the question: does high-temperature superconductivity (HTS) exist in an isolated monolayer, and if so, is the two-dimensional HTS—and various other correlated phenomena related to HTS—different from its three-dimensional counterpart? The answer to these questions may provide important insights on the role of dimensionality in HTS]. Here, we fabricate atomically thin cuprate Bi2Sr2CaCu2O8+δ that retains HTS down to monolayer (i.e. half unit cell) limit. The electronic structure of monolayer Bi2Sr2CaCu2O8+δ is probed with scanning tunneling microscopy and spectroscopy. Survey of the electronic phases – superconductivity, pseudogap, charge order and Mott insulating state – reveals that they are indistinguishable from those in the bulk. Our results, therefore, indicates that essential physics of HTS is contained in a monolayer Bi2Sr2CaCu2O8+δ.

Fig. 1. (a) Atomic structure model of Bi2Sr2CaCu2O8+δ. (b) Optical image of a mechanically cleaved monolayer (1L) Bi2Sr2CaCu2O8+δ accessed by STM tip (tip is sketched).

Poster-15


Temperature-dependent phonon vibration and excitonic transition in a van der Waals heterostructure

First Name: Hamin

Last Name: Park

Affiliation: School of Electrical Engineering, KAIST, Daejeon, Korea


Short Biography:

Mr. Hamin Park is a Ph.D. candidate of the electrical engineering in Korea Advanced Institute of Science and Technology (KAIST), member of Molecular and Nano Device Lab, and member of Center for Advanced Materials Discovery towards 3D Display. He got his B.S. and M.S. in electrical engineering at KAIST. He is interested in the research topic of physical characterization and application into device based on two-dimensional materials under the supervision of Prof. Sung-Yool Choi.

Abstract:

Two dimensional (2D) van der Waals (vdW) heterostructures have attracted explosive interest because of their potential ability to exhibit novel quantum phenomena based on condensed matter physics.[1] The 2D vdW heterostructures are formed by stacking different types of 2D materials, including graphene, transition metal dichalcogenide (TMD) and hexagonal boron nitride (h-BN).[2,3] Each layer is dangling-bond-free and weakly bound to neighboring layers by vdW interactions. The 2D vdW heterostructures have exhibited novel quantum phenomena emerging from layer-layer interaction, such as electron-electron interaction and electron-phonon interaction. As a template for an application to the electronic device, the layer-layer interaction between TMD and h-BN layer needs to be significantly investigated, because the TMD and h-BN exhibit a semiconducting and insulating characteristic, respectively. In this study, we investigate the interlayer interaction between TMD and h-BN by observing spectroscopic characteristics from the 2D vdW heterostructures. We fabricated the heterostructures by transferring a top-layer flake onto bottom-layer flake using a pick-up transfer technique based on polydimethylsiloxane (PDMS)/ polypropylene carbonate (PPC) stamp. The investigation of Raman scattering and photoluminescence (PL) reveals the nature of phonon vibration and excitonic transition of the heterostructures. We expect that the fundamental understanding of the layer-layer interaction in the 2D vdW heterostructures plays a key role for future applications to the electronic devices based on 2D materials.

[1] A. Geim et al., Nature 499, 419 (2013). [2] K. S. Novoselov et al., Science 353, aac9439 (2016). [3] F. Pizzocchero et al., Nat. Commun. 7, 11894 (2016).

Fig. 1. Optical image of the van der Waals heterostructure.

Poster-16


Spin-Valley dependent optical response at monolayer transition metal dichalcogenide/ferrimagnet interface

First Name: Takatoshi

Last Name: Akamatsu

Affiliation: Department of Applied Physics, the University of Tokyo, Tokyo, Japan.


Short Biography:

B. E., Department of Applied Physics, University of Tokyo, 2018

Abstract: Monolayer transition metal dichalcogenides (TMDs) is one of the promising candidates for valley-based electronics, since valley degree of freedom in TMDs couples with orbital and spin degree of freedoms, so that it can be manipulated by optical or magnetic method. So far, valley -optical responses related with orbital degree of freedom have been elucidated in monolayer TMDs. Recently, TMDs/magnet interfaces are widely studied as an ideal platform for studying the spin-valley related phenomena[1][2]. In this poster presentation, I will report the characteristic optical responses at new interface of TMDs and ferrimagnetic Fe3O4. Valley-Zeeman effect and the potential signature of the spin-valley relaxation, which has been studied in previous works of TMDs/ferromagnet interfaces, have been also observed at this new interface. I will also discuss the microscopic origin of the observed spin-valley relaxation.

Fig. 1. a : Crystal structure and Brillouin zone of monolayer TMDs. b : Schematic of TMDs/ Fe3O4 interface.

[1] D. Zhong. et al., Science Advances 3, e1603113 (2017). [2] Y. Ye. et al., Nature Nanotech. 11, 598 (2016).

Poster-17


Thermo-driven Crystalline Phase Transition of Epitaxial Metastable 1T’-WSe2 Thin Film

First Name: Wang

Last Name: Chen

Affiliation: National Laboratory of Solid State Microstructure, School of Physics, Nanjing, 210093, China


Short Biography:

Wang Chen receive his bachelor's degree in physics at CSU(Central South University) in 2015.After that he is studying for PhD in Prof. Yi Zhang’s group, Nanjing University. His research interests is using the Molecular beam epitaxial(MBE), Scanning tunnel microscopy (STM) and Angle resolved photoemission spectroscopy(ARPES) to explore the structure and electrical property include topological materials, metal dichalcogenides MX2, 2D heterojunction, etc.

Abstract:

Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices [1, 2]. The 1T’ crystalline phase transition can drive the monolayer MX2 to be a 2D topological insulator [3]. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T’ and 2H phase monolayer WSe2 on bilayer graphene (BLG) substrate [4]. The crystalline structures of these two phases were characterized using scanning tunneling microscopy. The monolayer 1T’-WSe2 was found to be metastable, and can transform into 2H phase under post-annealing procedure. The phase transition temperature of 1T’-WSe2 grown on BLG is lower than that of 1T’ phase grown on 2H-WSe2 layers. This thermo-driven crystalline phase transition makes the monolayer WSe2 to be an ideal platform for the controlling of topological phase transitions in 2D materials family.

[1] Y. Zhang, et al., Nature Nanotechnology, 9, 111-115 (2014)

[2] Y. Zhang, et al., Nano Letters, 16, 2485-2491 (2016)

[3] X Qian, et al.., Science, 346, 1344-1347 (2014)

[4] W Chen, et al., arXiv:1810.05981

Fig 1. STM image of 1T’-WSe2

monolayer

Poster-18


Spatially Resolved Electronic Structure of Bi2Sr2-xLaxCuO6+δ in the Two-dimensional Limit

First Name: Hengsheng

Last Name: Luo

Affiliation: Department of Physics, Fudan University, Shanghai, China


Short Biography: He joined Yuanbo Zhang’s group as an undergraduate in 2016 where he worked on atomically thin BSCCO thin film. In order to investigate the dimensional effect on this traditional high Tc superconducting material, he uses scanning tunneling microscope and transport measurement to characterize the monolayer Bi2Sr2-xLaxCuO6+δ and finds the monolayer cuprate, which contains only one Cu-O plane, is still superconducting. He received bachelor degree of physics at Fudan University in 2018 and became a graduate student at Fudan University. Now he focused on tuning the doping

level of the monolayer superconductor.

Abstract: Dimensionality plays a fundamental role in high-temperature superconductivity; all high-temperature superconductors adopt a layered crystal structure, and much of high-temperature superconductor theory is based on purely two-dimensional (2D) models. Here we exfoliate Bi2Sr2-

xLaxCuO6+δ (referred to as La-Bi2201) single crystals down to monolayers (half unit cell) at liquid nitrogen temperature under ultra-high vacuum. A monolayer La-Bi2201 contains only a single layer of CuO2 plane, and therefore represents a cuprate superconductor in the ultimate 2D limit. We study the electronic structure of monolayer La-Bi2201 at atomic scale using scanning tunneling microscopy and spectroscopy.

Poster-19


Electronic structure of Weak topological insulator candidate CaSn studied by Angle Resolved Photoemission Spectroscopy

First Name: Lixuan

Last Name: Xu

Affiliation: School of Physical Science and Technology, ShanghaiTech University, Shanghai, China


Short Biography:

In 2005, she graduated from Nanjing University of Science and Technology after completing the Bachelor’s Program in Materials Science and Engineering. Now, she is a Ph.D candidate in Dr.Liu's Research Group in ShanghaiTech University, Shanghai, China. They focus on investigating the electronic structure of unique quantum materials by the combination of Angle Resolved Photoemission Spectroscopy and Scanning Tunnelling Microscope experiments.

Abstract:

Topological materials have attracted much attention because of their unique characters including non-trivial Dirac surface state, transport Properties and so on [1]. This field has evolved rapidly in recent years thanks to the well agreement and effective mutual promotion between experimental observation and theoretical calculation. Materials of

Cmcm space group involving ZrTe5[2]、HfTe5 [3] have been investigated the electronic structure to address their topological nature by Angle Resolved Photoemission Spectroscopy (ARPES) and Scanning Tunnelling Microscope (STM) experiments. Simple compounds CaSn has the the same space group and symmetry properties of ZrTe5, and has been classified

as Weak Topological Insulator（WTI）based on the first principle calculation recently. In this work, We systematically investigated the electronic structure of bulk CaSn to explore its topological nature using Angle Resolved Photoemission Spectroscopy (ARPES) and the first principle calculation.

[1] Y.L.Chen, et al., Science,325,178 (2009)

[2]X.B.Li, et al., Physical Review Letters,116, 176803 (2016) [3] S. Liu, et al., APL Material,6,12111 (2018)

Fig. 1. 3D illustration of the in-plane

electronic structure on the (010) surface of CaSn

E-E

F(eV)

G

Z

Z

A

X

AA

X



Poster-20


Vertical 1T-TaS2 synthesis on nanoporous gold for high-performance electrocatalytic applications

First Name: Yahuan

Last Name: HUAN

Affiliation: Department of Materials Science and Engineering, Peking University, Beijing, China


Short Biography:

She received her B.S. degree in Nanjing University of Science and Technology. She is a Ph.D. candidate under the tutelage of Professor Xiaoqin Yan at University of Science & Technology Beijing. She is now a visiting Ph.D. student at Peking University in Prof. Yanfeng Zhang 's group. Her research interests focus on controllable synthesis and applications of 2D materials.

Abstract:

2D metallic TaS2 is acting as an ideal platform for exploring fundamental physical issues (superconductivity, charge-density wave, etc.) and for engineering novel applications in energy-related fields. The batch synthesis of high-quality TaS2 nanosheets with a specific phase is crucial for such issues. Herein, the successful synthesis of vertically oriented 1T-TaS2 nanosheets on nanoporous gold substrates is reported, via a facile chemical vapor deposition route. By virtue of the abundant edge sites and excellent electrical transport property, such vertical 1T-TaS2 is employed as high-efficiency electrocatalysts in hydrogen evolution reaction, featured with Tafel slopes ≈67–82 mV dec−1 and exchange current density ≈67.61 μA cm−2. The influence of phase states of 1T- and 2H-TaS2 on the catalytic activity is discussed with the combination of density functional theory calculations. This work hereby provides fundamental insights into the controllable syntheses and electrocatalytic applications of vertical 1T-TaS2 nanosheets achieved through the substrate engineering.

Fig. 1. APCVD synthesis of vertically oriented 1T-TaS2 nanosheets on NPG substrates

Poster-21


The properties of MoS2 FET with functional TMPS gate insulators

First Name: Minjung

Last Name: Shin

Affiliation: Division of Quantum Phases and Devices, Department of Physics, Konkuk university, Seoul, Korea


Short Biography:

Master's degree in Physics, in the doctor's course in Physics Research interests : 2D material, Semiconductor, TMDC, TMPS, Field-effect transistor

Abstract:

Intriguing characteristics of 2D materials including MoS2 and other materials is attracting attention. Because 2D material is sensitive to its environment due to the nature of atomic scale material itself, controlling environments such as substrate, contact with electrode, and surface or junction with other materials is important for studying and application of it. By making contact of 2D material with diverse materials, we can improve the properties of device or figure out the effects of environments. We are interested in transition metal phosphide (TMPS) which is one of the emerging materials and a new class of 2D vdW materials. TMPS has a various physical properties depending on its transition metal such as magnetism and ferroelectricity. Therefore, TMPS can be considered as promising candidates to study synergistic effects when integrated with other 2D materials. Because TMPS is layered material, we can get flat and clean surface which is required for improving the properties of 2D heterostructures. In this research, we used CrPS4 as top gate insulator in MoS2 FET device on SiO2/Si substrates. We fabricated the FET device using e-beam lithography and evaporator and performed AFM to confirm the surface and thickness of MoS2 and CrPS4. We used 2-layer MoS2 and CrPS4 with 66 nm thickness. In our experiment, SiO2/Si substrate is used for back gating and Ti/Au (5 nm/ 80 nm) metal is used as gate, source, and drain electrodes of FET device. I-V characteristics shows the electrical properties of the device with and without junction of CrPS4. The mobility with CrPS4 is 6 times higher value than without it for back-gated FET and on/off ratio is about 106 for both of them. We also measured top-gated I-V curve using CrPS4 as a gate insulator and leakage current level is 10-

11 A, on/off ratio is 2.1 x 105 and the subthreshold swing (SS) is 1.06 V/dec which is lower than in back-gated FET. In dual-gated FET, we sweep top gate voltage with controlling doping level of MoS2 channel by back gating. Our results confirm that CrPS4 can be used as gate insulator and junction material to improve the performance of FET device based on 2D material. Furthermore, we can study more interesting physical phenomena in TMPS on MoS2 FET device by precisely controlling temperature or external field. This study will help us to check properties of 2D materials, effects of its environments and possibility for its potential applications.

Fig. 1. Dual-gated I-V curve of junction FET device with MoS2 and CrPS4

-15 -10 -5 0 5 101E-13

1E-11

1E-09

1E-07

1E-05 Vds

= 0.3 V

Vbg

I ds (

A)

Vtg

(V)

10 V

0 V

-10 V

-20 V

Poster-22


Wafer-scale epitaxial growth of 2D semiconductor single-crystal film

First Name: Congwei

Last Name: Tan Affiliation: College of Chemistry and Molecular Engineering, Peking University, Beijing, China


Short Biography:

In 2015, he received B.S. degree of Physics in Shaanxi Normal University, Xian, China. Then, he moved on to pursue his Ph.D of Physical Chemistry in PKU, working on the controllable synthesis of high-quality two dimen-sional semiconductor film and the characterization of physical properties at the nanoscale.

Abstract:

High-mobility two-dimensional (2D) semiconductors hold great promise for applications in next-generation electronics and photoelectronics. The availability of single-crystalline film without defected grain boundaries on a wafer-scale dimensions is fundamental to achieve those high-mobility 2D semiconductors-based potential applications. So far, although the wafer-scale growth of some 2D semiconductors such as polycrystalline MoS2 thin film had been achieved [1], the uniform growth of single-crystal semiconducting thin film of 2D semiconductors with wafer-scale homogeneity remains a challenge. Here, we introduce an epitaxial growth to obtain the single-crystal film of high-mobility 2D Bi2O2Se semiconductors [2] on commercial-available wafers of SrTiO3 (001), LaAlO3 (001), (La, Sr)(Al, Ta)O3 (001). The fourfold symmetry, negligible lattice mismatch and strong interaction between the epi-layer of non-neutral layered Bi2O2Se [2] and the related perovskite-type substrates enable the unidirectional alignment of multiple seed, which would merge into single-crystal continuous film without twin grain boundaries. The single-crystal Bi2O2Se thin film show excellent spatial homogeneity over the entire wafer, and allow for the batch fabrication of high-performance field-effect devices with high mobility of ~150 cm2 V-1 s-1 at room temperature, excellent switching behavior with large on/off ratio of >105, and high drive current of ~45 μA μm-1 at a channel length of ~5 μm.

[1] K. Kang, et al., Nature 2001, 409, 66-69. [2] J. X. Wu, et al., Nature nanotechnol. 2017, 12, 530.

Fig. 1. Optical graph of as-synthesized 2D Bi2O2Se single-crystal film on a wafer-scale.

Poster-23


Fabrication of Size-Controlled Graphene Quantum Dots Embedded in Hexagonal Boron Nitride monolayer

First Name: Gwangwoo.

Last Name: Kim

Affiliation: Energy Engineering, UNIST, Ulsan, South Korea


Short Biography:

Gwangwoo Kim received his BS degree in chemical engineering in Ulsan National Institute of Science and Technology (UNIST) in 2013, and he is now a Ph.D. candidate in energy engineering, UNIST, South Korea. His current research is focused on the growth of 2D materials by using chemical vapor deposition method.

Abstract:

Graphene quantum dots (GQDs) have received tremendous attention because their band gap can be controlled. Although many approaches have been developed to fabricate GQDs, they are time-consuming and costly, and furthermore, precise control over the morphology and size distribution of GQDs remains challenging. Here, we demonstrate spatially controlled conversion of hexagonal boron nitride (h-BN) to graphene on an array of Pt nanoparticles (NPs) to realize an array of uniform GQDs embedded in an h-BN sheet.[1] A uniform Pt NP array was formed on a SiO2/Si substrate with the aid of self-patterning diblock copolymer micelles,[2] and the h-BN sheet was transferred on the Pt NPs array, followed by the conversion of h-BN on Pt to GQDs.[3] The size of the obtained GQDs corresponded with the sizes of the Pt NPs due to the selective conversion of h-BN on top of Pt NPs. Uniform and precisely controlled size of the GQDs ranging from 7 to 13 nm was achieved. Finally, we demonstrate electron transport by the size-controlled GQDs isolated by insulating h-BN like a Coulomb blockade, indicating that the splitting energy of the GQD is 80–160 meV, compatible with its dimension.

[1] G. Kim et al., Nat. Commun. 10, 230-238 (2019). [2] S. -S. Kim et al., Chem. Mater. 27, 7003-7010 (2015). [3] G. Kim et al., Nano Lett. 15, 4769-4775 (2015).

Fig. 1. The fabrication steps of GQD-hBN in-plane heterostructure

Poster-24


Raman study of polytypism in 2-dimensional Gallium Selenide

First Name: Soo Yeon

Last Name: Lim

Affiliation: Department of Physics, Sogang University, Seoul, South Korea


Short Biography:

Soo Yeon Lim is a Ph. D. candidate in department of physics at Sogang University, where she received her M.S. and B.S. degrees in 2017 and 2015, respectively. She is a recipient of a fellowship from Hyundai Chung Mong-Koo Foundation. She studies physical properties of 2-dimenional materials by using Raman spectroscopy.

Abstract:

Gallium selenide (GaSe) is a layered semiconductor composed of post-transition metal Ga and chalcogen Se. It has a direct bandgap of ~ 2.0 eV in bulk phase unlike other typical transition metal dichalcogenides (TMDs). It can be used in a photodevice such as a photodetector thanks to high photoresponsivity and external quantum efficiency [1]. Four different polytypes in bulk GaSe are designated as β-, ε-, γ- and δ-type. Since different polytypes result in different optical and electrical properties even for the same thickness, identifying the polytype is highly important.

We investigated few-layer GaSe by using polarized Raman spectroscopy. We determined the thickness of GaSe by using AFM and from the positions of the A1g

1 and the shear mode. We found that the low-frequency shear modes due to in-plane interlayer vibrations show different spectra even in the same-thickness flakes. In particular, we investigated trilayer GaSe and found four-types of low-frequency Raman spectra corresponding to different polytypes [2]. The results of Raman measurements are consistent with theoretical calculations and HR-STEM measurements.

[1]P. Hu et al., ACS Nano 6, 5988-5994 (2012). [2]J.-U. Lee et al., ACS Nano 10, 1948-1953 (2016).

Fig. 1. Four different low-frequency Raman spectra in trilayer GaSe.



Poster-25


WS2(1-x)Te2x Alloy Monolayer grown by Chemical Vapor Deposition with Tunable Band Structures

First Name: Haolin

Last Name: WANG



Short Biography:

Dr. Wang is currently a postdoc in Nanjing University. He received his B.S. degree in material chemistry from China University of Geosciences Sciences Beijing (CUGB) in 2011 and completed Ph.D. degree in material physics and chemistry from Institute of Semiconductors, Chinese Academy of Sciences (CAS) in 2016. Dr Wang's research interests lie in the area of scalable synthesis of two-dimensional materials, in particular CVD and PVD deposition of h-BN, TMDCs, and Xene, and their applications in electronic and optoelectronic devices.

Abstract: Two-dimensional (2D) transition-metal tellurides have recently emerged as a nontrivial material due to their unique structures and fascinating properties. Alloying Te with other 2D sulphides or selenides will give rise to new interesting phenomena and find broad opportunities in device areas. However, the studies to date are largely limited to the growth of such alloys due to the huge affinity difference. Here, for the first time, we have refined chemical vapour deposition (CVD) strategy to grow monolayer WS2(1-x)Te2x alloys with a continuously varying composition, from semiconducting 2H to semimetallic 1T’ phase. With the increase of Te components, the optical band gap of 2H phase alloys have redshifted. And the phase transition appeared in high ternary ratio, which was identified by the evolution of Raman spectra and chemical valence states. Microscopically, Te dopants were observed as a random distribution within the highly regular 2H structure, while S atoms displayed the anisotropic ordering in 1T’ phase. Our work offers a facile method to control the composition and phase in alloy engineering, which expands the library of 2D materials and inspires the fundamental studies in nanoelectronics and nanophotonics.

Poster-26


Van der Waals pn Junction Field Effect Transitors using Transition Metal Dichalcogenides

First Name: June Yeong

Last Name: Lim

Affiliation: Department of Physics, Yonsei University, Seoul, Korea


Short Biography:

JUNE YEONG LIM received the B.S. degree in physics from Yonsei University, Korea, in 2015, where he is currently pursuing the Ph.D. degree with the Institute of Physics and Applied Physics. His research topic is to fabrication of 2-D devices based on TMD nanosheets.

Abstract:

Two dimensional (2D) transition metal dichalcogenides (TMDs)-based van der Waals (vdW) PN junctions have been used for heterojunction diodes which basically utilize out-of-plane current across the junction interface.[1], [2] Van der Waals heterostructures of atomically thin semiconductors can be junction field effect transistors (JFETs). JFETs was fabricated using a heterojunction of semiconducting p-MoTe2 (or p-WSe2) and n-MoS2. The p-type TMDs works as a gate for the n-type TMDC channel, whereas the n-type TMDs operates as a gate for the p-type TMDs channel. Owing to the low density of traps at the van der Waals interface, the devices exhibit low hysteresis of 0.05–0.1 V and achieve a subthreshold swing of ~100 mV/decade. The highest mobility reaches 500 cm2/V·s for the n-channel JFET with MoS2, whereas the p-channel JFET with MoTe2 is characterized by a much lower mobility of ~13 cm2/V·s. The highest ON/OFF current ratio was observed to be >104.

[1]Zhang, K. et al., ACS Nano 10, 3852-3858 (2016). [2] Wang, B. et al., Nanoscale 9, 10733-10740 (2017).

Fig. 1. (a) Optical microscope image of van der Waals JFET. (b) Schematic views of JFET (c) Output Characteristics of n- and p-channel JFET.

Poster-27


Band Engineering in Epitaxial TMDC Alloy MoxW(1-x)Se2 thin films

First Name: Xuedong

Last Name: Xie



Short Biography:

2015-present Ph.D Candidate in Prof. Yi Zhang’s group

2011-2015 Bachelor in Wuhan University of Science and

Technology

Abstract:

Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices [1, 2]. The bandgap of monolayer MX2 can be engineered in MoxW(1-x)Se2 alloy [3]. Here we realized the MBE growth of monolayer MoxW(1-x)Se2 thin films on BLG substrate. The quality and morphology of the grown films was characterized using in-situ RHEED and STM. Combining the in-situ ARPES and XPS measurements, we further studied the evolution of the band structures and spin-splitting of valence band with the stoichiometric x in MoxW(1-x)Se2, which can be subtly controlled by the flux ratio between Mo and W.

[1] Y. Zhang, et al., Nature Nanotechnology, 9, 111-115 (2014)

[2] Y. Zhang, et al., Nano Letters, 16, 2485-2491 (2016)

[3] Y. Chen, et al., ACS Nano, 7, 4610-4616 (2013)

Poster-28


Fowler-Nordheim Tunneling current modulation by controlling a barrier height in graphene/hexagonal boron nitride heterostructure

First Name: Junho

Last Name: Lee

Affiliation: Department of Physics, Konkuk University, Seoul


Short Biography:

I am a PhD student within department of physics at konkuk university in south korea. My thesis is concerned with modulating a tunneling barrier height in graphene/hBN/metal heterostructure and its application to photodetector. In addition, I have researched a transfer method for 2D materials to make 2D heterostructure that has a clean interface.

Abstract:

A tunneling barristor, switching Fowler-Nordheim tunneling current through vertically-staked graphene/hBN/metal, will be presented in this poster. Graphene is a 2-dimensional plane of carbon with one-atom thick of graphite. Because of its low density of states near the Dirac point, its Fermi level can be modulated with the accumulated charge by 0.5~0.6 eV from charge neutral point [1]. Hexagonal boron nitride, the tunneling barrier, is composed of boron and nitrogen, and has the same structure with graphene. However, it is an insulator, whose breakdown voltage and dielectric constant are similar to those of SiO2, which is ~0.8 V/nm and 4 respectively [2]. The current is modulated by tuning the barrier height between h-BN and Graphene like the barristor [3]. Interestingly, depending on gate voltage. While the former depends on the density of states, and modulates Ion/Ioff ~ 100, the latter depends on the barrier height, and modulates Ion/Ioff ~ 106. Therefore, our device can be switched by the barrier-height modulation. In this work, we achieved 106 of Ion/Ioff ratio with 50 nm thick hBN at 300 K and 103 with 32 nm thick hBN at 1.78 K.

[1] I. Gierz et al., Nano Lett. 8, 12 (2008). [2] G. H. Lee et al., Appl. Phys. Lett. 99, 243114 (2011). [3] H. J. Yang et al., Science 336, 1140 (2012).

Fig. 1. FNT current modulation at 1.78 K



Poster-29


Integrated circuits based on chemical patterning of 2D Bi2O2Se

First Name: Tianran

Last Name: Li

Affiliation: College of Chemistry and Molecular Engineering, Peking University, Beijing, China


Short Biography:

In 2016, he received Bachelor degree of Chemistry in Peking University, China. He moved on to be a PhD student of Physical Chemistry in PKU ever since. In 2017, he initiated research on fabricating FET and logic circuits on 2D Bi2O2Se.

Abstract:

2D semiconductor has been viewed as potential competitor of silicon due to its atomic thickness which is naturally resistant to short channel effect. However, lack of consistency has post a serious limitation on future application of 2D semiconductors. Up to now, most researchers focus on fabricating isolated transistors and evaluate their device performance.

In 2017, Wu et al. synthesized a ternary 2D semi-conductor[1] Bi2O2Se which has shown high electron mobility(~450 cm2V-1s-1) and excellent stability against oxygen and moisture. Meanwhile, the authors demonstrated highly efficient chemical patterning of 2D Bi2O2Se and successfully fabricated photodetector arrays[2]. Based on these researches, we are now able to fabricate logic circuits on wet-etched CVD Bi2O2Se samples. Figure.1 shows Vin-Vout curve on thus fabricated inverter. These devices could work in 2V and gain is over 30. More complex circuits are also available.

[1] Wu, Jinxiong, et al. Nature nanotechnology 12.6 (2017): 530.

[2] Wu, Jinxiong, et al. Advanced Materials 29.44 (2017): 1704060.

Figure.1 Performance of inverter circuit built on wet-etched 2D-Bi2O2Se.



Poster-30


Experimental setup for second harmonic generation spectroscopy

First Name: Jungcheol

Last Name: Kim

Affiliation: Department of Physics, Sogang University, Seoul, Korea


Short Biography:

Jungcheol is a Ph. D. candidate in department of physics at Sogang University, where he received his M.S. and B.S. degrees in 2016 and 2014, respectively. He studies about physical properties of 2-dimenional materials using optical measurements such as second harmonic generation, photoluminescence and Raman spectroscopy.

Abstract:

Second harmonic generation (SHG) spectroscopy is a fascinating experimental method to investigate exciton state in 2-dimensional materials. Fixed wavelength SHG is commonly used to determine the crystallographic orientation in transition metal dichalcogenides. Additionally, the electronic states can be investigated by measuring the SHG signal with varying the wavelength of the excitation source. Using photoluminescence excitation (PLE) which is useful method to investigate the energy states, only s-exciton states can be measured due to the selection rule. In contrast, both s- and p- exciton state resonances are observed in the SHG spectroscopy, even though signal only occurs in structures with broken inversion symmetry [1].

For measuring the SHG spectroscopy on 2-dimensional materials, tunable-wavelength ultrafast pulsed laser is needed. We combined a supercontinuum laser excitation source with a microscopic measurements system. Using the system, micron-scale 2-dimesional samples can be measured with various excitation wavelengths from 420 to 1700 nm.

[1] G. Wang et al., Phys. Rev. Lett. 114, 097403 (2015).

Fig. 1. Experimental scheme of SHG spectroscopy

Poster-31


Thermal stability study on 2D Bi2O2Se

First Name: Teng

Last Name: Tu

Affiliation: Center for Nanochemistry, College of Chemistry and Molecular Engineering, Peking University, Chengfu Road No.202, Beijing 100871, China


Short Biography:

In 2016, she received her Bachelor of Science degree in Tongji University, China. Then, She joined Center for Nanochemistry to pursue her PhD at Peking University ever since. Since 2017, she starts research on thermal stability and chemical modification on 2D Bi2O2Se.

Abstract:

High mobility, a suitable bandgap and good environmental stability are three essential requirements for 2D semiconductor’s future development. Nevertheless, intensively investigated materials such as MoS2 and black phosphorous can not meet these requirements at the same time. In 2017, an air-stable 2D Bi2O2Se with high mobility and large band gap appeared [1], which could be a highly competitive emerging 2D semiconductor material. However, its thermal stability that could restrict the processing technology and application field has not been deeply studied.

We discovered that 2D Bi2O2Se could keep stable under the heating temperature of 290 ℃ in atmospheric environment. Water vapor did little effect to accelerate its oxidation, which means Bi2O2Se is insensitive to moisture. When the temperature became higher such as 340 ℃ , 2D Bi2O2Se was oxidized, but still remained a smooth surface with a mild increase of thickness. Good thermal stability of 2D Bi2O2Se is beneficial to expanding its future applications.

[1] J. Wu, et al. Nature nanotechnol., 12, 530 (2017).

Fig. 1. The optical and AFM images of 2D Bi2O2Se under different heating temperatures for 10 min.

mailto:[email protected]



Poster Poster-32


Gap-mode Plasmon-Induced Photoresponse in Vertical Structure of Multilayer graphene

First Name: Khang June

Last Name: Lee

Affiliation: School of Electrical Engineering, KAIST, Daejeon, Korea


Short Biography:

Mr. Khang June Lee received his B.S. and M.S. degrees from the School of Electrical Engineering, KAIST, in 2014, 2016, respectively. Now he is in Ph.D. candidates with the same department in KAIST. During the M.S. study, he worked on the field of surface enhanced Raman spectroscopy (SERS) which is closely related with plasmonics. Currently, he is focusing on research of optoelectronic devices based on two-dimensional materials using plasmonics under the supervision of Prof. Sung-Yool Choi.

Abstract:

Despite a broadband absorption and high quantum efficiency of graphene, its applications to photovoltaic devices are known to be severely hindered by a low optical absorption and short carrier lifetime in the graphene. Here, we present a gap-mode plasmon-induced asymmetric junction device to investigate photoresponse in the multilayer graphene. Gold nanoparticles are introduced below the multilayer graphene so that they can induce the gap-mode plasmonic field enhancement by coupling with metal electrode[1]. Multilayer graphene acts as a spacer for the plasmonic field enhancement and simultaneously generate the photo-excited carriers upon illumination. In this way, the photoresponsivity of up to 2.9 mA/W and external quantum efficiency of up to 0.57 % have been achieved and the photoresponse time is believed to be extremely short because of the atomically short channel length by the vertical structure[2]. Such structural advantages are considered to be able to serve as a platform for building more efficient devices that overcome the trade-off relationship of photoresponsivity and photoresponse time.

Figure 1. (a) Schematic illustration of vertical homojunction device. (b) Laser on-off photocurrent modulation with and without nanoparticles cases.

[1] Khang June Lee et al., Adv. Funct. Mater. 26, 5093 (2016)

[2] Jing-Jing Chen et al., ACS Nano 9, 8851 (2015)



Poster-33


Generation of MoOx on the surface of MoTe2 by O2 plasma and its effect on MoTe2 based transistors.

First Name: Yongjae

Last Name: Cho

Affiliation: Department of Physics, Yonsei University, Seoul, South Korea


Short Biography:

Yongjae CHO received the B.S. degree in physics from Yonsei University, Korea, in 2017, where he is currently pursuing the Ph.D. degree with the Institute of Physics and Applied Physics. The research area is doping of TMD based transistors.

Abstract:

We have fabricated transparent p-MoTe2 2D transistors. To improve the performance of the transistor, we applied O2 plasma treatment and deposited an ultra-thin Pt layer between the p-MoTe2 surface and ITO S/D electrodes. Consequently, almost transparent 2D FETs are obtained with a decent mobility of ~5 cm2/V s, a high ON/OFF current ratio of ~105, and 70% transmittance. O2 plasma treatment on the S/D area of p-MoTe2 flake generates formation of MoOx on the surface of p-MoTe2. The induced MoOx layer greatly improves a hole injection characteristics at S/D contacts of the transistor. Using one identical MoTe2 flake, we fabricated two transistors with and without O2 plasma treatment to separately investigate the effect of an O2-plasma-induced MoOx layer on a p-MoTe2 transistor.

[1] Chuang, S et al., Nano Lett., 14 (3), pp 1337–1342 (2014)

[2] Pezeshki, A et al., ACS Nano 10, 1118–1125 (2016).

Fig 1. Transparent p-MoTe2 channel p-FET with MoOx/ultrathin Pt/ ITO for S/D (top left). Linear

mobility plot and inset output characteristic curve (top right). Snapshot photo (bottom left).

Transmittance characteristics (bottom right)



Poster-34


Ultralow-threshold whispering-gallery-mode lasing in CdS nanoplatelets

First Name: Liyun

Last Name: ZHAO

Affiliation: College of Engineering, Peking University, Beijing, China


Short Biography:

In 2016, she began joint cultivation in Prof. Qing Zhang’s group in Peking University (PKU, China), and then further studied as a Ph.D. student in Peking University under the supervision of Prof. Qing Zhang in 2018. Her research focuses on light-matter interaction of low dimensional semiconductor and optical microcavity laser spectroscopy.

Abstract:

Low threshold micro/nanolasers have attracted considerable attention for applications in high-throughput sensing, high-density storage and optical communications. However, constrained by quantum efficiency and crystalline quality, conventional small-size semiconductor lasers are still subjected to high lasing threshold. In this talk, we demonstrate a low-threshold planar laser based on high-quality single-crystalline hexagonal CdS nanoplatelets using self-limited epitaxial growth method. The as-grown hexagonal CdS nanoplatelets show multiple whispering-gallery-mode (WGM) lasing at room temperature with an ultralow threshold of 0.6 µJ/cm2, which is several times lower than those values in previous reported CdS based lasers. Our experimental results advocate the promise of non-layered semiconductor materials for the development of novel optoelectronic devices.

Fig. 1. (a) Emission spectra of a typical CdS nanoplatelet at pump fluences from 0.3 to 1 µJ/cm2. Inset: photoluminescence emission image above lasing threshold (0.6 µJ/cm2). (b) Integrated intensity and line width of the dominant emission peak as a function of pump fluence.

[1] Y. Mi, Q. Zhang, et al., Unpublished.

500 510 520 530

Wavelength (nm)

PL I

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nsity (

a.u

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Poster-35


Towards Neutral Interlayer Exciton Transport in Transition Metal Dichalcogenide

First Name: Takamoto

Last Name: Yokosawa

Affiliation: Department of Applied Physics, The University of Tokyo, Tokyo, Japan.


Short Biography:

Mr. Takamoto Yokosawa received his B.E. from The University of Tokyo in 2017. He is now a master course student of The University of Tokyo. His research interest includes interlayer exciton in TMDC van der Waals heterostructures.

Abstract:

Valley degree of freedom has recently attracted much attention as a new information carrier. One of the most promising platforms for valley manipulation is a layered honeycomb lattice material, such as graphene and transition metal dichalcogenides (TMDCs). TMDCs have moderate band gaps enabling optical access while graphene has no or only small gap. However, crystal quality of TMDCs is lower than graphene so that valley relaxation time is shorter.

In van der Waals heterostructures of TMDCs, such as MoSe2 and WSe2, optically excited electrons and holes immediately separate into different layers and form spatially separated indirect excitons, i.e., interlayer excitons (ILEs), which have longer valley life time. Furthermore, finite out-of-plane electric dipoles of ILEs may allow us electrical manipulation of them and generation of exciton flow. This can expand the valleytronics field.

Recently, electrical modulation of ILE diffusion by vertical electric fields was reported (Fig. 2) [1]. But the scheme will need more complicated device structures for arbitral operation in future. There is another report that interlayer trions are transported by an in-plane electric field in one of the layers [2]. It is unknown if neutral ILEs can also be transported by such an in-plane electric field.

In this poster, I will present our research towards electrical control of ILEs in TMDC driven by an in-plane electric field in a layer (Fig.1). Especially, I will focus on fabrication process of samples and their difficulties.

[1] D. Unuchek et al., Nature, 560, 340 (2018). [2] L. A. Jauregui et al., arXiv:1812.08691 (2018).

Fig. 1. A schematic to drive and detect ILE flow



Poster-36


Electronic Structure of K-doped IrTe2 Single Crystal Resolved by Angle-Resolved Photoemission Spectroscopy

First Name: Haoxiong

Last Name: Zhang

Affiliation: Department of Physics, Tsinghua University, Ph. D. student, Beijing, China


Short Biography:

After receiving B.S degree of Physics from department of physics in Nankai University in 2016, I began my Ph. D. journey in Prof. Shuyun Zhou's group in Tsinghua University. Now, I am working on the single crystal synthesis and electronic structure of quasi two dimensional materials by using ARPES.

Abstract:

Topological semimetal as host materials of Dirac and Weyl fermions attract extensive research interests recently. Lorentz-violating Dirac fermions which are classified as type-II Dirac fermions are first experimentally found in transition metal dichalcogenide PtTe2

[1]. In the PtTe2 class transition metal dichalcogenides, PtSe2, PdTe2 and IrTe2 are also predicted to exist type-II Dirac fermions[2]. We have synthesized high quality IrTe2 single crystal to find type-II Dirac fermions. But the type-II Dirac point of IrTe2 lies at about 0.18 eV above Fermi energy along ΓA direction[1]. Conventional Angle-Resolved Photoemission Spectroscopy (ARPES) is unsuitable for the measurement. By Pt doping, IrTe2 could reveal novel topological properties. Ir1-xPtxTe2 has a type-II Dirac point lies at Fermi energy along ΓA direction [1]. Besides, doping K, as a common method to induce electrons into host materials, should tune the type-II Dirac point to the Fermi energy too. Electron doping could shift bands to lower energy. We try to tune the type-II Dirac point to the Fermi energy by doping K. To resolve the band structure changes, ARPES is used to directly detect materials’ electronic structures by measuring the energy-momentum dispersion. We have doped K on its fresh cleaved surface. But the type-II Dirac point hasn’t been tuned to Fermi energy within the doping time which sample still have clear dispersion. That could result from the lack of electron transfer for per unit cell. There also could have some kind of chemical reaction between IrTe2 and K due to band structure changes with K doping. [1] Yan, Mingzhe, et al. " Nature communications 8.1

(2017): 257. [2] Huang, Huaqing, et al. Physical Review B 94.12

(2016): 121117.

Fig. 1. A-H direction dispersion of K-doped IrTe2 as doping time increases.



Poster-37


Polycrystalline Bi2O2Se thin film prepared with spin-coating method for flexible thin film transistors

First Name: Congcong

Last Name: Zhang

Affiliation: College of Chemistry and Molecular Engineering, Peking University, Beijing, China


Short Biography:

In 2017, she received Bachelor degree of Chemistry in Shandong University, China. Then, she joined Center for Nanochemistry to pursue her PhD at Peking University ever since. Since 2018, she starts research on preparation of polycrystalline Bi2O2Se thin film and its application in TFTs. Abstract:

Two-dimensional semiconductors (2DSCs) have attracted considerable attention as atomically thin channel materials with great electrostatic control for field effect transistors. 2DSCs with high mobility and natural flexibility exactly meet the requirements of flexible thin film transistors (TFTs). 2D Bi2O2Se synthesized with chemical vapor deposition method have shown great performance with high electron mobility (~450 cm2 V-1 s-1), high on/off ratio (>106) and ideal device subthreshold swing (~65 mV dec-1)[1]. Preparation of Bi2O2Se thin film with low cost is important to its application in flexible TFTs.

We prepared Bi2O2Se thin film with spin-coating and solution-assisted method. The Bi2O2Se thin film is polycrystalline and the c-axis is vertical to the thin film, which indicates high mobility because electrons flow in the Bi-O layers. We patterned the Bi2O2Se thin film with wet chemical etching[2] and then fabricated TFTs. The Bi2O2Se TFTs showed uniform performance with high mobility (~60 cm2 V-1 s-1) and high on/off ratio (~105).

[1] J. Wu, et al. Nature nanotechnol., 12, 530 (2017)

[2] J. Wu, et al. Adv.Mater., 29, 1704060 (2017)

Figure.1 Performance of Bi2O2Se TFTs

n 32 mm height x 24 mm width

Poster-38


Direct EL Imaging of Transition Metal Dichalcogenide Light-emitting Devices

First Name: Hirofumi

Last Name: Matsuoka

Affiliation: Department of Applied Physics, Nagoya Univ., Nagoya, Japan


Short Biography:

Hirofumi Matsuoka received his engineering bachelor’s degree from Nagoya University in 2017. In his graduate research, He worked on the research of chemical carrier doping into Transition Metal Dichalcogenide (TMDC) monolayers [1]. From 2017, he has gone to the graduate school of Nagoya University. He has been researching the EL characteristics of TMDC monolayers light-emitting devices using the electrolyte, and the flexible TMDCs light-emitting device generating circular polarized light emissions in room temperature.

Abstract:

Transition Metal Dichalcogenide (TMDC) monolayers have attracted much interest as active materials for light-emitting devices [2]. Most relevant TMDC light-emitting devices are made using tiny samples [3,4] and it is highly required to fabricate devices using large-area TMDC monolayers for practical applications. Hence, we recently proposed a novel device structure, in which two electrodes were deposited on a large-area TMDC film and TMDC film was covered with an electrolyte [5].

The centimeter-scale polycrystalline WS2, WSe2, and MoSe2 monolayer films were synthesized on sapphire substrates by CVD method. The devices only need two electrodes (Au/Ni) deposited on the surfaces of TMDCs, followed by spin-coating ion gels (Figure 1). Figure 2 shows the EL image for WSe2 device. We clearly observed light emission between cathode and anode in several TMDC (WSe2, WS2, and MoSe2) devices due to electrolyte-induced p-i-n junctions. Interestingly, we observed the variation of light-emitting positions depending on both materials and applied voltages. To understand the material dependences and light-emitting device mechanism, we evaluate the relationships between electrical transports and light-emitting positions. As a result, we found out that the EL positions in our TMDC light-emitting devices are universally dominated by the ratio of electron and hole mobilities of materials.

[1] H. Matsuoka et al., Jpn. J. Appl. Phys. 57, 02CB15 (2018). [2] F. Xia et al., Nat. Photonics 8, 899 (2014) [3] Y. J. Zhang et al., Science 344, 725 (2014) [4] J. S. Ross et al., Nat. Nanotechnol. 9, 268 (2014) [5] J. Pu et al., Adv. Mater., 29, 1606918 (2017).

Fig. 1. The schematic of TMDC

light-emitting device.

Fig. 2. The EL image of

the polycrystalline WSe2 monolayer.

Au

Sapphire substrate

Ion gel

Cation Anion

TMDC monolayer



Poster-39


Lasing in mechanically exfoliated single phase 2D perovskite crystals

First Name: Yin

Last Name: Liang

Affiliation: College of Engineering, Peking University, Beijing, China


Short Biography:

As an undergraduate, he has just started his 2D Ruddlesden-Popper perovskite research. To gain an insight into the potentials and limitations of this emerging material, he is now working on nano-laser device using 2D perovskite as gain media. In 2019, he will receive bachelor degree in Peking University, Beijing.

Abstract:

2D Ruddlesden-Popper perovskite (RPP) has attracted great attention in optical applications due to its high mobility and fast excitonic recombination rate. However, multi-phase components in solution-possessed RPP crystals hinder the exploration of its intrinsic properties. To solve this problem, mechanical exfoliation is a feasible and convenient way to get high quality pure phase RPP thin films.

In this poster, basic crystal structures of (C4H9NH3, BA)2(CH3NH3, MA)n-1PbnI3n+1 (n = 1-5) will be introduced, followed by a summarization of carrier behaviors in multi-phase and pure phase RPP crystals. Then, n-dependent low-threshold lasing in mechanically exfoliated single phase 2D RPP crystals will be demonstrated. And the underlying mechanism, including Auger recombination and exciton-phonon coupling will be discussed.

[1] Y. Liang, Q. Zhang, et al., Unpublished.

Fig. 1. Lasing spectra of exfoliated (BA)2(MA)n-1PbnI3n+1 (n = 2-5)



Poster-40


Electric-field-induced Metal-Insulator Transition and Quantum Transport in Large-Area Polycrystalline MoS2 Monolayers

First Name: Tomoyuki

Last Name: Yamada

Affiliation: Dept. of Applied Physics, Nagoya University, Nagoya, Japan


Short Biography:

Tomoyuki Yamada is a first year master student in graduate school of engineering at Nagoya University, Japan. He received his B.E. in applied physics from Nagoya University in 2018. He is currently focusing on electronic and optoelectronic devices using electrolyte-gating methods.

Abstract:

Molybdenum disulfide (MoS2) is attracting increasing interests because the exfoliated single crystalline flakes have realized high mobility, metal-insulator transition, and superconductivity [1]. In addition, the quantum transports arising from strong spin-orbit interaction have also been observed; thus, these superior transport properties offer potentials for exploring quantum devices [2]. Although it is necessary to apply large-area films for scalable device applications, the chemically synthesized polycrystalline films commonly showed poor transports, such as low mobility, due to the effects of grain boundaries and/or impurities. To overcome this issue, we combined electric double layer transistors (EDLTs) with polycrystalline MoS2 monolayers, and achieved high mobility of 30 cm2/Vs at room temperature. Here, we apply EDLTs to investigate the temperature- and carrier-density- dependence of resistance and magnetoresistance, resulting in the observation of metallic conduction and quantum transport in large-area polycrystalline MoS2 monolayers.

The cm-scale polycrystalline MoS2 monolayers were grown by chemical vapor deposition, and then, the ion-gel films were spin-coated on MoS2 and on deposited electrode surfaces (Fig. 1). Figure 2 shows the temperature dependence of the sheet resistance (RS) with different gate voltage (Vg). At low Vg, the RS increases as decreasing temperature (T), meaning the insulating behavior. In contrast, the RS shows metallic conduction down to T of 1.9 K at high Vg, which clearly indicates the metal-insulator transition. The Hall measurement also revealed the carrier density and mobility up to 1.0 × 1014 /cm2 and 100 cm2/Vs, repectively, at low temperature. Moreover, we observed unique magnetoresistance characteristics; the crossover from weak localization to weak anti-localization as shown in Fig. 3. These results indicate that high carrier density eliminates the adverse effects of grain boundaries, consequently realizing quantum transports in polycrystalline MoS2 monolayers. We will also discuss the possibility of superconductivity by evaluating the obtained phase diagram.

[1] J. T. Ye, et al. Science 338, 1193 (2012), [2] H. Schmidt, et al. Phys. Rev. Lett. 116, 046803 (2016)

Poster-41


Local chemical modification of MoS2 layer using AFM lithography

First Name: Dayea

Last Name: Oh



Short Biography:

DaYea Oh received his B.S. (2013) degrees in physics from Konkuk University. She is currently pursuing a Ph.D. degree at the Konkuk University in Korea. She has been focused on chemical modification of 2D materials using AFM lithography and spintronic devices based on ferromagnetic materials or 2D materials.

Abstract:

As the demand for nano scaled devices is increasing, Two dimensional (2D) materials have been theoretically and experimentally investigated in the last few decades. Among 2D materials, TMD(Transition Metal Dichalcogenide) materials which have layered structure shows extensively magnetic, electrical, and mechanical properties [1]. Especially, hydrogenation of MoS2 by high temperature and MoS2 irradiated by proton shows unexpected ferromagnetic behavior which would lead to new spintronics devices [2].

In this works, we fabricate locally hydrogenated or oxidized MoS2 using AFM lithography and confirm specific magnetic properties. Through Raman measurement, we identify that the pure MoS2 surface modify hydrogenated or oxidized one under different lithographic condition. Also, Magnetic Force Microscopy (MFM) measurement support that hydrogenated or oxidized MoS2 using AFM lithography shows novel magnetic properties comparing with pristine MoS2. This result may attribute to the H or O atoms deposited on MoS2 defect by AFM lithography.

[1] S. Ahmed. et al. Journal of Alloys and compounds 746 (2018) 399-404

[2] S.W. Han. et al. Physical Review Letters 110 (2013) 247201-5

Fig 1. (a) Schematic image of AFM lithography method and (b) AFM image of hydrogenated or oxidized MoS2 using AFM lithography



Poster-42


Synaptic plasticity selectively activated by polarization-dependent energy-efficient ion migration in an ultrathin ferroelectric tunnel junction

First Name: Chansoo

Last Name: Yoon



Short Biography:

Chansoo Yoon received his B.S. (2011), M.S. (2015) degrees in physics from Kyungsung University and Konkuk University, respectively. He is currently pursuing a Ph.D. degree at the Konkuk University in Korea. His research interests focus on nanomaterials and memristive devices based on nanoionics for memory and neuromorphic computing applications.

Abstract: Selectively activated inorganic synaptic devices, showing a high on/off ratio, ultrasmall dimensions, low power consumption, and short programming time, are required to emulate the functions of high-capacity and energy-efficient reconfigurable human neural systems combining information storage and processing.

Here, we demonstrated that such a synaptic device is realized using a Ag/PbZr0.52Ti0.48O3(PZT)/La0.8Sr0.2MnO3 (LSMO) ferroelectric tunnel junction (FTJ) with ultrathin PZT (thickness of ~4nm). Ag ion migration through the very thin FTJ enables a large on/off ratio (107) and low energy consumption (potentiation energy consumption= ~22 aJ and depression energy consumption = ~2.5pJ). In addition, the simple alignment of the downward polarization in PZT selectively activates the synaptic plasticity of the FTJ and the transition from short-term plasticity to long-term potentiation.

Fig. 1 Potentiation and depression of an Ag/PZT/LSMO FTJ.



Poster-43


Na-assisted fast growth of large single-crystal MoS2 on sapphire

First Name: Yuping

Last Name: SHI

Affiliation: Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, China


Short Biography:

Yuping Shi received her B.S. degree in Central South University in 2017. She is currently a Ph.D. candidate in Department of Materials Science and Engineering, Center for Nanochemistry, Peking University, and her research interests center on controllable syntheses of MX2 and MX2 heterostructures.

Abstract:

Monolayer molybdenum sulfide (MoS2), a typical semiconducting transition metal dichalcogenide, has emerged as a perfect platform for next-generation electronics and optoelectronics due to its sizeable band gap and strong light–matter interactions. Nevertheless, the controlled growth of a monolayer MoS2 single-crystal with a large-domain size and high crystal quality still faces great challenges. Herein, we demonstrate the fast growth of a large-domain monolayer MoS2 on the c-plane sapphire substrate with the assistance of sodium chloride (NaCl) crystals as the intermediate promoter. Particularly, the volatilization temperature of the NaCl crystal and the growth temperature of MoS2 are established to be the key parameters that influence the growth efficiency of MoS2 at an optimized growth condition. Monolayer triangular MoS2 domain with an edge length ~300 μm is obtained within 1 min, featured with a growth rate ~5 μm/s. The Na element from the NaCl crystal is found to be able to facilitate the two dimensional growth of monolayer MoS2. This work thus offers novel insights into the high-efficiency production of large-domain monolayer MoS2 on insulating growth substrates.

Poster-44


Electrical properties of individual CaVO (CalciumVanadate) nanowire

First Name: HyunJeong

Last Name: Jeong

Affiliation: Department of Physics, Ewha Womans Univ., Seoul, Korea


Short Biography:

HyunJeong Jeong received the B.S. degree from the School of Physics, Ewha Womans University, Seoul, Korea, in 2017. She is currently working at the Department of Physics, Ewha Womans University, Seoul, Korea, as a research intern.

Abstract: Low-dimensional nanostructures have gained wide attention recently. V2O5 (vanadium pentoxide) nanowire is one of such low-dimensional material. Its electrical properties are widely studied from various researches. Especially, it is well known that V2O5 shows outstanding conductivities ranging from 10-6 S/cm to almost 1 S/cm [1,2]. We wonder what will happen if we add Calcium to V2O5. In this study, we measured the electrical properties of CaVO and investigated how its electric properties change by doping(adding) Ca. Our sample consists of three mixed materials: V2O5, Ca0.17V2O5, CaV2O6. CaVO nanowires were synthesized by sol-gel method by adding acidic vanadium solution and calcium hydroxide solution. Electrical device of CaVO nanowire was fabricated by conventional nano-lithography processes. The basic current-voltage and transfer characteristics were measured and its semiconducting properties were studied. Details of synthesis, fabrication, and electrical properties of our nanowire and devices will be explained in this presentation.

Acknowledgements

This research was supported by the National Research Foundation of Korea (NRF).

Figure1. Scanning Electron microscope image of CaVO nanowires

Reference [1]J. Bullot et al., Appl.Phys. Lett. 36, 986 (1980). [2] A. L. Pergament et al., Appl. Phys. 35, 2187 (2002).



Poster-45


2D hard ferromagnet Co2FeGeTe2

First Name: Inho

Last Name: Hwang

Affiliation: Seoul National University, Seoul, Korea


Short Biography:

Inho Hwang is a graduate student in physics at Seoul National University (SNU). Under professor Je-Geun Park, he is studying magnetic van der Waals layered material. He graduated from SNU with Bachelor’s degrees in physics in 2016.

Abstract:

With metallic property, combined with van der Waals (vdW) layered ferromagnetism, Fe3GeTe2 (FGT) has been widely studied. Though its high Curie temperature and gate-tunable magnetism [1] allows high perspective in spintronic applications, magnetic property in FGT changes quite drastically as iron deficiency happens. To study defect dependent property in FGT, here we introduce the derivative of FGT, Co2FeGeTe2. By substituting iron atoms into cobalt atoms, coercivity was enhanced. Here includes the transport, magnetic property data of the bulk Co2FeGeTe2, compared to pristine Fe3GeTe2 single crystal. By investigating the doped sample, we expect to study how the magnetism is related to the exotic properties reported in FGT.

[1]Y. Deng et al., Nature. 563, 94-99 (2018).



Poster-46


Structurally engineered carrier dynamics in hybrid quasi-two-dimensional perovskite thin films

First Name: Qiuyu

Last Name: SHANG

Affiliation: Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, China


Short Biography:

In 2017, he received Bachelor Degree in University of Science & Technology Beijing, China. Then, he was admitted into College of Engineering, Peking University as a graduate student majored in materials science and engineering. He is now conducting researches focus on light-matter interaction in low- dimensional perovskite nanomaterials.

Abstract:

Metal halide perovskites have recently attracted great attentions for their breakthrough in many photoelectric devices, such as solar cell, LED, photodetectors, etc. Especially, (quasi) two-dimensional (2D) perovskite with multiple quantum well structure exhibits large exciton binding energy, better environment stability, flexible composition and structure homogeneity, ect, which have led to successful applications in emitting diode and light-harvesting devices. However, insightful understanding of photophysics and carrier dynamics in 2D perovskite are still inadequate.

In this talk, we introduced transient absorption spectroscopy and photoluminescence spectroscopy to explore structure engineered carrier dynamics in quasi two-dimensional perovskite (PEA)2(MA)n-

1PbnI3n+1. It found that the multiple phases of perovskites are arranged perpendicularly to substrate from small to large n, and also co-exist randomly in the same horizontal planes. A carrier self-separation dynamics was proven that electrons transfer from small-n to large-n perovskite phases and holes transfer reversely. The potential applications of two-dimensional layered perovskite films in solar energy conversion and photoelectric detection devices will be further discussed.

[1] Q. Shang, et al., J. Phys. Chem. Lett., 8, 4431 (2017)

[2] M. Yuan, et al., Nature Nanotechnol., 11, 872 (2016)

[3] N. Wang, et al., Nat. Photonics, 10, 699 (2016)

[4] J. Liu, et al., J. Am. Chem. Soc., 139, 1432 (2017)

Fig. 1. Schematic diagram of carrier dynamics in hybrid quasi-two-dimensional perovskite thin films.

Poster-47


Monolithic 2D Oxide/Semiconductor Superlattice for Efficient Light Emitters

First Name: Yoonseok

Last Name: Kim

Affiliation: KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, Korea


Short Biography:

Academic degree: B.S Degree in material science and engineering from Yonsei University

Research interests: Luminescence, Material science

Academic honours/awards : The best poster awards (The 5th graphene symposium)

Abstract: Semiconducting transition metal dichalcogenides (TMDCs) have been attracted enormous attention

because of exceptional optical properties such as large exciton binding energy and direct bandgap

transition at the monolayer limit. Such remarkable properties make them promising for high-

performance light-emitting devices such as LEDs, LASERs, and single-photon quantum emitters.

However, highly efficient luminescence in the two-dimensional (2D) semiconductor heterostructures

is inherently limited due to the indirect band-gap transition at the multilayer regime as well as low

quantum yield of constituent monolayers.

Here, we propose a new approach to fabricate a high-efficiency luminescent 2D superlattice by

monolithic phase engineering and van der Waals stacking. To achieve that, first, the bilayer WSe2 was

converted to the WOx/WSe2 heterolayer by the layer-by-layer oxidation. Then, by multiply stacking

the monolithically-phase-engineered WOx/WSe2 building blocks, we successfully achieve the 2D

oxide/selenide superlattice structure. Unlike the case of stacking monolayers only, the

photoluminescence (PL) characteristic was not quenched in this stacked heterostructure. As the number

of the stacked WOx/WSe2 structures increases, PL intensity increases several times more than the

simple sum of the single WOx/WSe2. This is presumably because the WOx layer acts as a decoupling

layer between two adjacent monolayers, allowing to preserve the direct bandgap nature of monolayers

even in the staked heterostructure. Our work suggests a new approach to fabricate 2D-based quantum

heterostructures for high-performance light emitters.

Fig 1. Schematic of Monolithically 2D Oxide/Semiconductor superlattice structure and band alignment

Note

Documents

Welcome to the 2...Welcome to the 2nd International Workshop on 2D Materials This event is a part of activities of A3 Foresight Program “Joint Research on Novel Physical Properties