key: cord-0046654-gt294acg
authors: Yang, Eui-Hyeok; Datta, Dibakar; Hader, Grzegorz (Greg); Ding, Junjun
title: Overview
date: 2020-06-26
journal: Synthesis, Modelling and Characterization of 2D Materials and their Heterostructures
DOI: 10.1016/b978-0-12-818475-2.00001-5
sha: 17c15e15255ddebfd1a0997121d0200c867759a5
doc_id: 46654
cord_uid: gt294acg

nan

In December of 1959, the physicist, Richard P. Feynman, gave a lecture titled, "There's Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics." This lecture would become the advent to the scientific field of nanotechnology. Feynman's lecture on the manipulation of atoms would eventually become reality when researchers demonstrated the precise placement of individual atoms by synthesizing graphene nanoribbons into specific patterns [1] . His radical idea to make machines at a small scale would eventually culminate in the development of microelectromechanical systems (MEMS) starting in the 1980s. Fabrication processes of microscaled electromechanical devices were based on techniques adapted from the integrated circuit (IC) industry. It was this synergy between the IC industry and the need for MEMS that would bring consumers a wealth of technology advancements in cell phones, automobiles, gaming, robotics, fitness/health trackers, airplanes, many military applications, and last but not least, drones, which would not have been possible without MEMS technology. This synergistic relationship is now being explored between twodimensional (2D) materials and the silicon-based semiconductor industry [2] . Due to the extraordinary properties of atomically thin 2D materials, they have now made their way to the forefront of several research areas, including electronics, photonics, electrophotonics, catalysis, and energy. There have been extensive research efforts on the mechanical, thermal, optical, and electrical properties, including modeling, synthesis, and their applications. As the need for new high-performance materials continues to push toward the mantra of lighter, stronger, and faster, bringing credence to the lecture by Feynman, that there is still "Plenty of Room at the Bottom," leaves one to image what new technology lies beyond the horizon.

Over a decade has passed since the seminal work in isolating graphene by Sir Andre Geim and Sir Konstantin Novoselov, which started a revolution in the research of a new family of materials with atomic thickness and planar dimensionality. Graphene is a monolayer of carbon atoms arranged in a hexagonal lattice. Its high degree of crystallinity and outstanding electronic, mechanical, thermal, and optical properties leads to the term the new wonder material [3] and makes graphene an ideal candidate for novel high-speed (GHzÀTHz) optoelectronic devices [4, 5] . Graphene is a gapless semimetal with a linear dispersion relation in the low bias transport regime. The research on graphene has opened the floodgates to a vast library of other 2D-layered materials [6] , including the fabrication of heterostructures, all at atomic thicknesses. Although the micromechanical exfoliation technique has been adopted for rapid material characterization and demonstration of innovative device ideas based on these 2D systems, significant advances have recently been made in largescale homogeneous and heterogeneous growth of these materials. The emergence of these new 2D materials dramatically broadens the spectrum of properties. Unlike the zerobandgap graphene, hexagonal-boron nitride (h-BN) is an insulator with a similar atomic structure to graphene, while monolayer transition metal dichalcogenides such as molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ), and tungsten diselenide (WSe 2 ) are direct bandgap semiconductors. The diverse properties of these 2D material systems make it flexible for the use of various applications. Mechanical, thermal, optical, and electrical properties of 2D materials will be further discussed in Chapters 2, 3, 4, and 5, respectively.

With the constant discovery of new 2D materials and 2D heterostructures, the development of 2D materials opens up a completely new territory for both experimental studies and computational studies. Recent advances in the modeling of phenomena during the nanofabrication and mechanics of controllable synthesis of 2D materials have paved the way for various applications. With the continuous increase in computing power and significant advancements of theoretical methods and algorithms, the modeling for physical properties of 2D materials and 2D heterostructures has shown comparable accuracy to experiments, while keeping the cost down. The advantages of computational materials databases are not limited to speed and cost as compared to experimental efforts. The computational work makes it possible for sharing and comparison of research data with reduced duplication of research efforts. The increasing volume of databases enables the application of machinelearning techniques for the discovery of new 2D materials and designing materials with tailored properties. Modeling topics on atomistic modeling, molecular dynamics simulation, Monte Carlo methods, and continuum modeling are covered in Chapters 6, 7, 8, and 9, respectively.

To characterize the layer-dependent properties of 2D properties, it is essential to synthesize 2D materials in a controllable manner. Other than the micromechanical exfoliation technique, many strategies have been reported to synthesize monolayer or few-layer 2D materials, such as chemical vapor deposition (CVD) method, chemical exfoliation, and hydrothermal method. These methods show their advantages and disadvantages in terms of quality, production volume, and layer control, which determines the applications of these synthesized 2D materials. The synthesis of 2D heterostructures often requires a more complicated process, which integrates two or more synthesis methods for each layer of 2D materials. The synthesis methods of graphene, h-BN, TMD, and 2D heterostructures are introduced in depth in Chapters 10, 11, 12, and 13, respectively. Chapter 14 discusses the characterization techniques utilized for the confirmation and analysis of natural and synthesized 2D materials. This chapter outlines transmission electron microscopy (TEM), Raman Spectroscopy, atomic force microscopy (AFM), including other surface and atomic characterization tools to gain insight into the 2D materials physics, chemistry, and material science.

The understanding of the physical properties of 2D materials and the synthesis of 2D materials and their heterostructures make it possible to design electrical and optoelectronic devices with superb performance. The photodetector, which converts photons into electrical signals, can be redesigned with 2D materials other than the conventional semiconductors, such as silicon and indium gallium arsenide. The 2D materials and 2D heterostructures enable new photoresponse effects at much greater sensitivities and provide photodetection covering UV, visible, IR, and THz ranges. The unique mechanical properties also ensure the fascinating processing of photodetection in flexible electronics as well as bioelectronics. Detailed discussion on 2D material-based photodetectors is shown in Chapter 15, 2D Materials and Hybrid Systems for Photodetection. In addition to optoelectronics, 2D materials have found a wide range of applications in electronic devices such as conductors, thinfilm transistors, sensors, and energy storage devices. Solution-processed 2D materials bear high potential due to the advantages of low cost and high-volume production, which is critical for the fast-growing demands of printed electronics and other electronics applications. The exfoliated 2D materials are solution-processable so that the 2D materials can be easily assembled into designed layered structures on arbitrary substrates, which is important for flexible electronics. Most 2D materials can be chemically exfoliated, while more researchers are trending toward synthesis by other methods such as CVD. Therefore potential applications with 2D heterostructures can be realized by solution-processed 2D materials using methods such as layer-by-layer assembly, LangmuirÀBlodgett assembly, spin coating, electrophoretic deposition, inkjet printing, and vacuum filtration. The details on solutionprocessed 2D materials for electronic applications are discussed in Chapter 16, Electronic Devices Based on Solution-Processed 2D Materials.

Due to the extraordinary electrical properties, 2D materials have been extensively explored as additives in composites for electrodes in energy storage devices in order to increase electronic conductivity and mechanical stability and provide additional Li storage sites for lithium-based batteries. The 2D materials and 2D heterostructures are excellent candidates as anodes and help provide high porosity, good electron mobility, lightweight, high charge capacity, high rate capability, and increased operational voltage. Many researchers have reported improvements in the performance of anodes in lithium-ion batteries and offer 2D materials as an alternative option to anodes fabricated with Li metal, which is prone to deadly dendrite formation [7] . While monolayers of most 2D materials are not ideal candidates for battery electrodes, van der Waals layered heterostructures offer possibilities to Chapter 1 • Overview 5 design battery electrodes for fast diffusion kinetics, high structural integrity, and excellent electron conductivity. However, there are many questions to be answered to fully understand the mechanics of 2D electrodes and how to optimize the performance for energy storage devices. Chapter 17, 2D Materials and Its Heterostructures for Energy Storage, provides a systematic review on the state-of-the-art of 2D materials and their heterostructures for energy storage applications. The World Health Organization (WHO) announced a global pandemic on March 11, 2020, due to the uncontrolled outbreak of the novel coronavirus (COVID-19) . The study of low dimensional materials in virology and living organisms is discussed in Chapter 18 and provides insight into how these materials are prime candidates for the capture, detection, and analysis of biological systems.

Despite extensive research efforts of 2D materials in the last two decades, the 2D materials and their heterostructures have greatly expanded their territory for more opportunities to explore. With the development of computational power and algorithms, computational modeling has grown into an important tool for the discovery of new 2D materials and prediction of their physical properties [8] . The increasingly large database of 2D materials and their 2D heterostructures offers endless possibilities in designing electronic, photonic, optoelectronic, and energy storage devices [6, 8] . Chapter 19, Machine Learning in Materials Modeling-Fundamentals and the Opportunities in 2D Materials, discusses the emerging field of machine learning for 2D materials research. This book provides an overview of the synthesis, modeling, and characterization of 2D materials and their heterostructures. Applications are provided to the reader throughout the text as well as current technological breakthroughs, outlining recent scientific progress in the fast-paced research environment of 2D materials.

Atomically precise bottom-up fabrication of graphene nanoribbons

Graphene and two-dimensional materials for silicon technology

Graphene: status and prospects

Gate-variable optical transitions in graphene

Graphene photonics and optoelectronics

Data mining for new two-and one-dimensional weakly bonded solids and latticecommensurate heterostructures

Self-heatingÀinduced healing of lithium dendrites

Two-dimensional materials from high-throughput computational exfoliation of experimentally known compounds