Synthesis, structural characterization, optical and electrical properties of mos2

VIETNAM NATIONAL UNIVERSITY – HO CHI MINH HO CHI MINH UNIVERSITY OF TECHNOLOGY LE NGOC LONG SYNTHESIS, STRUCTURAL CHARACTERIZATION, OPTICAL AND ELECTRICAL PROPERTIES OF MoS2/GRAPHENE NANOCOMPOSITE A DISSERTATION SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY HO CHI MINH CITY, 2022 VIETNAM NATIONAL UNIVERSITY – HO CHI MINH HO CHI MINH UNIVERSITY OF TECHNOLOGY LE NGOC LONG SYNTHESIS, STRUCTURAL CHARACTERIZATION, OPTICAL AND ELECTRICAL PROPERTIES OF MoS2/GRAPHENE NANOCOMPOSITE A DISSERTATION SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Major: Materials Engineering Major code: 62520309 Independent reviewer: Prof Nguyen Cuu Khoa Independent reviewer: Assoc Prof Ha Thuc Chi Nhan Reviewer: Assoc Prof Tran Ngoc Quyen Reviewer: Assoc Prof Ho Thi Thanh Van Reviewer: Dr Le Thi Duy Hanh SUPERVISORS: Assoc Prof Tran Van Khai Assoc Prof Pham Trung Kien i This dissertation has been conducted and completed at Ho Chi Minh University of Technology (HCMUT), Vietnam National University Ho Chi Minh City (VNU-HCM) and, The Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology (KYUTECH), Japan under the Cooperative Education between HCMUT and KYUTECH ii STATEMENT OF ORIGINALITY I, Long, am the sole author of this work, and all text contained herein is of my own invention unless otherwise indicated Any text, figures, theories, results or designs that are not of my own devising are appropriately referenced in order to give credit to the original author(s) All sources of assistance have been assigned due acknowledgment Author of Dissertation, Le Ngoc Long iii ABSTRACT This dissertation is focused on the hydrothermal synthesis of MoS2/graphene nanocomposite (MoS2/C NC) and the study of the impact of key experimental parameters on microstructures, morphology that define their electrical, electrochemical and optical properties Thereby determining the appropriate technological parameters to fabricate MoS2/C NC systems with high conductivity and broad band luminescence for energy storage device or optoelectronic applications The results of this work show that MoS2/C NC systems have been successfully synthesized at 230 °C, in ~2 h, from graphene oxide (GO) dispersion (1.0 mg L−1, ~84.73% C) and Mo4+ and S2− sources The molar ratio (Mo4+ : C) and pH are controlled at ~(1.46 : 1) and ~7.2–8.8, respectively Two-dimensional (2D) MoS2 petal-liked crystals, thickness of ~0.63–3.69 nm was in-situ grown on graphene sheets forming “sandwich”, “layer-by-layer”, “vertical-stacked” and “anchored” nanocomposite structures The MoS2/C NC systems dominating with ultrathin (~1–6 monolayers) metallic 1T-MoS2 phase on graphene, have high electrical conductivity (G ~ 0.180 μS) and large specific capacitance (Csp ~ 122.20 F g−1) were successfully synthesized at temperature and (Mo4+ : C) molar ratio below ~230 °C and ~(1.5 : 1), respectively Meanwhile, nanocomposite systems with semiconductive 2H-MoS2 phase were obtained at temperatures above 230 °C exhibit strong optical absorption (~82 %) and wide luminescence with large band gap of ~1.31– 2.34 eV The underlying mechanism of improving the conductivity of MoS2/C NC originates from the metallic boundary structure forming at the contact layers between the dispersed phase 1T-MoS2 and the graphene matrix that facilitate its sp2 hybridization restructured, and increased the charge carriers’ mobility While, the donor-acceptor electronic structure at the ultrathin 2D semiconductive 2H-MoS2 phase and graphene interface breaks the band symmetry of 2D MoS2 This structure forms mid-gap bands and allows the intralayer photoexcited electron-hole generation, excitons transition or electron tunneling mechanism which results exceptional photoresponse in various types of MoS2/C NC Such structure broadens the optical band gap, enhances the nonlinear optical property, strong optical absorption and broad band photoluminescence of dispersed MoS2 phase in MoS2/C NC The results of this thesis also suggest that the iv metallic (1T) to semiconductive (2H) phase transition and large band gap with mid-gap bands structure of 2D MoS2 can be engineered by controlling reaction temperature and pH value through the synthesis of MoS2/C NC The ability to tune the band gap and nonlinear optical property as well as the memristive property of the 2H-MoS2 semiconductive phase which originates from the double-layer electrochemical storage mechanism of the Mo atomic layers and the MoS2–graphene contact layer, that make them promise for applications in broad band electromagnetic wave absorbers, optoelectronics and memristors The proposed synthetic approach in this dissertation can also be applied as a reliable and scalable strategy to fabricate MoS2/graphene nanocomposite and various functional materials with tunable structures, electrical and optical properties v TÓM TẮT LUẬN ÁN Luận án tập trung nghiên cứu quy trình tổng hợp vật liệu nanocompozit MoS2/graphene (MoS2/C NC) phương pháp thủy nhiệt, đồng thời khảo sát ảnh hưởng thông số thực nghiệm đến cấu trúc, tính chất điện quang học vật liệu tổng hợp; từ xác định chế độ cơng nghệ thích hợp để chế tạo hệ MoS2/C NC có độ dẫn điện cao, phát quang rộng nhằm ứng dụng thiết bị lưu trữ lượng hay quang điện tử Kết Luận án cho thấy hệ MoS2/C NC tổng hợp thành công nhiệt độ 230 °C, ~2 h, từ dung dịch phân tán graphene oxide (GO) (1,0 mg L−1, ~84,73% C) muối chứa Mo4+ S2− Tỷ lệ mol (Mo4+ : C) pH, theo thứ tự, kiểm soát khoảng ~(1,46 : 1) ~7,2–8,8 Các tinh thể hai chiều (2D) MoS2 dạng “cánh hoa”, dày ~0,63–3,69 nm hình thành graphene tạo thành cấu trúc nanocompozit dạng “sandwich”, “xếp theo lớp”, “xếp chồng lên nhau” “neo” Các hệ MoS2/C NC với chủ yếu pha kim loại 1T-MoS2 mỏng ~1–6 đơn lớp Mo–S–Mo phân tán graphene, có độ dẫn điện cao (G ~ 0.180 μS) điện dung riêng lớn (Csp ~ 122.20 F g−1) tổng hợp thành công điều kiện nhiệt độ ~230 °C tỉ lệ mol (Mo4+ : C) ~(1.5 : 1) Trong đó, hệ nanocomposite với pha phân tán bán dẫn 2H-MoS2 tổng hợp nhiệt độ 230 °C thể tính chất hấp thụ quang học mạnh (~82 %) phát quang rộng với vùng cấm ~1,31–2,34 eV Cơ chế cải thiện độ dẫn điện MoS2/C NC bắt nguồn từ cấu trúc biên giới kim loại lớp tiếp xúc pha phân tán 1T-MoS2 graphene tạo điều kiện phục hồi liên kết (sp2) nó, tăng khả truyền dẫn hạt tải điện Trong khi, đặc điểm cấu trúc điện tử kiểu cho-nhận biên giới pha bán dẫn 2H-MoS2 graphene phá vỡ tính đối xứng, hình thành khe trung gian (mid-gap band) mở rộng vùng cấm 2D MoS2; cho phép kích thích điện tử-lỗ trống vận chuyển excitons điện tử xun hầm qua lớp tiếp xúc hình thành tính chất quang phi tuyến hấp thụ quang học mạnh MoS2 vật liệu nanocompozit Kết luận án gợi ý rằng, trình chuyển pha kim loại (1T) – bán dẫn (2H) cấu trúc dải vùng cấm rộng với khe trung gian vật liệu 2D MoS2 điều khiển cách kiểm sốt nhiệt độ phản ứng giá trị pH thông qua tổng hợp MoS2/C NC Khả điều chỉnh vùng cấm, tính chất quang phi tuyến tính chất ghi nhớ pha bán dẫn 2H-MoS2 xuất phát vi từ chế lưu trữ điện hóa lớp kép lớp nguyên tử Mo lớp tiếp xúc MoS2– graphene, khiến chúng trở nên đầy hứa hẹn cho ứng dụng linh kiện hấp thụ sóng điện từ băng rộng, tách sóng quang nhớ (memristors) quang điện tử Quy trình tổng hợp đề xuất luận án xem chiến lược tin cậy, dễ mở rộng để chế tạo vật liệu nanocompozit MoS2/graphene hệ vật liệu tiên tiến khác có cấu trúc tính chất điện, quang học mong muốn vii ACKNOWLEDGMENTS This dissertation has been conducted and completed at Ho Chi Minh University of Technology (HCMUT), Vietnam National University Ho Chi Minh City (VNU-HCM), Vietnam and The Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology (KYUTECH), Japan, under the Cooperative Education between HCMUT and KYUTECH The first acknowledgement in any thesis should, in general, be reserved for one’s supervisor This dissertation is of no exception The tireless hard work and consistently good ideas of Assoc Prof Tran Van Khai and Assoc Prof Pham Trung Kien have been the driving force behind all the material presented in this dissertation Moreover, I would like to thank all the people I have had the pleasure of working with during the course of my Ph.D A special thanks go to Professor Hirofumi TANAKA at KYUTECH, JAPAN who has worked with me on most of the measurement the electrical properties include nano devices fabrication, I – V measurement presented here Deep in my heart, I would like to thank my loving family for their love, support in all respects, and continuous encouragement, which is meaningful not only to my work but also to my life Author of Dissertation, Le Ngoc Long viii TABLE OF CONTENTS STATEMENT OF ORIGINALITY iii ABSTRACT iv TÓM TẮT LUẬN ÁN vi ACKNOWLEDGMENTS viii LIST OF FIGURES xii LIST OF TABLES xiv LIST OF ABBREVIATIONS .xv INTRODUCTION Motivation Objectives and Scope Novelty .4 Major contributions Research contents .5 Organization of this Dissertation .6 THEORETICAL BACKGROUND .7 Structure, properties and applications of MoS2/C NC .7 2.1.1 Carbon 2.1.2 Graphene .8 2.1.3 Graphene oxide 10 2.1.4 Molydenum disulfide (MoS2) 11 2.1.5 Optical property of 2D MoS2 and graphene 13 2.1.6 Optical property of MoS2/C NC 14 Synthetic approaches for MoS2/C NC .16 2.2.1 Synthesis of graphene and 2D MoS2 nanocrystals 16 2.2.2 Synthesis of MoS2/C NC 16 Process engineering for synthesis of MoS2/C NC 19 2.3.1 Hydrothermal method .19 2.3.2 Properties of supercritical water and reactor design 19 2.3.3 Nucleation and growth in hydrothermal condition 20 ix ... dependent band gap of 2D MoS2 Electronic band structures of MoS2 calculated by DFT method [26] 2.1.5 Optical property of 2D MoS2 and graphene Among many interesting properties of monolayer MoS2, graphene... UNIVERSITY OF TECHNOLOGY LE NGOC LONG SYNTHESIS, STRUCTURAL CHARACTERIZATION, OPTICAL AND ELECTRICAL PROPERTIES OF MoS2/ GRAPHENE NANOCOMPOSITE A DISSERTATION SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY... property of MoS2/ C NC 40 Measuring optical absorption property MoS2/ C NC 40 Measuring photoluminescence property of MoS2/ C NC 40 SYNTHESIS, CHARACTERIZATION, ELECTRICAL AND OPTICAL