Cover: Two-Dimensional Semiconductors, First by Jingbo Li, Zhongming Wei, Jun Kang

Two-Dimensional Semiconductors

Synthesis, Physical Properties and Applications

 

 

Jingbo Li

Zhongming Wei

Jun Kang

 

 

 

 

 

 

 

 

Wiley Logo

Prof. Jingbo Li

South China Normal University

Institute of Semiconductors

No.55, West of Zhongshan Avenue

510631 Guangzhou

China

Prof. Zhongming Wei

Institute of Semiconductors, CAS

State Key Laboratory of Superlattices and Microstructures

No.A35, QingHua East Road

Haidian District

100083 Beijing

China

Prof. Jun Kang

Beijing Computational Science Research Center

Materials and Energy Division

Building 9, No.10 Xibeiwang East Road

Haidian District

100193 Beijing

China

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This book is dedicated to Prof. Jian-Bai Xia for his 80th birthday.

Preface

Ultrathin two-dimensional (2D) materials, such as graphene and MoS2, have attracted broad interest because of their exotic condensed-matter phenomena that are absent in bulk counterparts. Graphene, which is composed of a single layer of carbon atoms arranged in honeycomb lattice, has a linear dispersion near the K point, and charge carriers can be described as massless Dirac fermions, providing abundant physical picture. In contrast, 2D transition metal dichalcogenides (TMDs), transition metal oxides, black phosphorus, and boron nitride (BN) exhibit versatile optical, electronic, catalytic, and mechanical properties. It was reported that the 2D materials, especially 2D semiconductors with the intrinsic nanometer-scale size, can help to extend Moore's law, which face the challenge of further scaling down the transistor channel.

In this book, we discuss the theoretical study, synthetic method, the unique properties, the potential application, the challenges, and opportunities of the 2D semiconductors. Firstly, a general introduction of 2D materials was given. Then, the theoretical study including electronic structures and predications, the reparation, properties and applications in (opto) electronics or other devices of 2D materials/semiconductors and their alloys, and heterostructures were discussed in detail. At last, a perspective and outlook of this fast developing field is summarized.

I became a PhD student under the supervision of Prof. Jian-Bai Xia from 1998 to 2001. He gave plenty of guidelines to my study, research, and my life. We acknowledge him by dedicating this book on his 80th birthday this year. In addition, we convey our best wishes to him and his family and also to his fruitful research and healthy and happy life.

We sincerely hope this book can help researchers to understand 2D materials.

Jingbo Li

Beijing

10 December 2019

About the Authors

Photograph of Jingbo Li who worked as a professor at the Institute of Semiconductors, Chinese Academy of Sciences.

Jingbo Li received his PhD degree from the Institute of Semiconductors, Chinese Academy of Sciences, in 2001 under the supervision of Prof. Jian-Bai Xia. Then, he spent six years at the Lawrence Berkeley National Laboratory and National Renewable Energy Laboratory in USA. From 2007 to 2019, he worked as a professor at the Institute of Semiconductors, Chinese Academy of Sciences. Since 2019, he became a full-time professor and the dean of Institute of Semiconductors, South China Normal University. His research interests include the design, fabrication, and application of novel nanostructured semiconductors. He has published more than 290 scientific publications with more than 15000 citations.

Photograph of Zhongming Wei who worked as a postdoctoral fellow and then as an Assistant Professor in Prof. Thomas Bjørnholm's group at the University of Copenhagen, Denmark.

Zhongming Wei received his BS degree from Wuhan University (China) in 2005 and PhD degree from the Institute of Chemistry, Chinese Academy of Sciences, in 2010 under the supervision of Prof. Daoben Zhu and Prof. Wei Xu. From August 2010 to January 2015, he worked as a postdoctoral fellow and then as an Assistant Professor in Prof. Thomas Bjørnholm's group at the University of Copenhagen, Denmark. Currently, he is working as a professor at the Institute of Semiconductors, Chinese Academy of Sciences. His research interests include low-dimensional semiconductors and their (opto)electronic devices.

Photograph of Jun Kang who performed his postdoctoral research at the University of Antwerp in Belgium and Lawrence Berkeley National Laboratory in USA.

Jun Kang received his PhD degree from the Institute of Semiconductors, Chinese Academy of Sciences, in 2014. After that, he performed his postdoctoral research at the University of Antwerp in Belgium and Lawrence Berkeley National Laboratory in USA. In 2019, he joined Beijing Computational Science Research Center as an assistant professor. His research field is first-principles calculations on novel electronic properties of low-dimensional semiconductors. He has published over 60 peer-reviewed articles with more than 4000 citations.

Acknowledgments

Finally, I would like to thank all my group members who spent a lot time for the writing and revising of this book: Dr. Bo Li, Dr. Mianzeng Zhong, Dr. Yan Li, Dr. Le Huang, Dr. Nengjie Huo, Dr. Xiaoting Wang, Ziqi Zhou, Jingzhi Fang, Kai Zhao, Yu Cui, and Longfei Pan. Without their hard work and contribution, we would not be able to finish this book on time. Thanks to Project Editor Ms. Shirly Samuel at Wiley-VCH for all her help in the publication of this book.

1
Introduction

1.1 Background

In 2004, Ander Geim and Konstantin Novoselov from the University of Manchester, UK, first obtained graphene sheets by mechanical exfoliation method, successfully fabricated the first graphene field effect transistor (FET), and investigated its unique physical properties [1]. Before the discovery of graphene, according to the thermodynamic fluctuation law, the two-dimensional (2D) atomic thick layer under nonabsolute zero degrees is unlikely to exist stably [2]. Why is graphene stable at temperatures above absolute zero? Further theoretical studies have shown that this is because large-scale graphene is not distributed in a perfect 2D plane but in a wave-like shape. The experimental results support this view [3, 4]. Therefore, the discovery of graphene shocked the condensed matter physics community and also quickly ignited the enthusiasm of scientists to study 2D materials (a crystalline material composed of a single atomic layer or few atomic layers), indicating the arrival of the “two-dimensional material era.”

In 2010, Ander Geim and Konstantin Novoselov were awarded the Nobel Prize in Physics for their outstanding contribution to graphene (Figure 1.1) [1]. Graphene is a 2D material composed of carbon atoms and having a hexagonal lattice structure. Graphene has good toughness and its Young's modulus can theoretically reach as 1 TPa [5]. Therefore, graphene can form different structures through different curved stacks, such as zero-dimensional fullerenes, one-dimensional carbon nanotubes, and three-dimensional stacked graphite [6].

Graphene has shown many excellent physical properties resulting from the unique structure, and the disappearance of interlayer coupling makes the two carbon atoms in the cell completely equivalent, thus making the effective mass of electrons on the Fermi surface zero [7–11]. Because graphene has a unique Dirac band structure, carriers can completely tunnel in graphene, and electrons and holes in graphene have a very long free path. Therefore, the electronic transport of graphene is hardly affected by phonon collisions and temperature [8]. The mobility of electrons in monolayer graphene is much larger than that in its parent graphite (Figure 1.2c) [16]. In addition, graphene has shown good thermal conductivity (Figure 1.2d) [17], room temperature quantum Hall effect (Figure 1.2a) [12, 14], single-molecule detection (Figure 1.2b), and high light transmission [18]. Graphene is a semimetal material without band gap, it cannot form a good switching ratio in terms of regulation, thus greatly limiting the application of graphene in electronic devices. Although on the bilayer and multilayer graphene, the graphene can obtain a certain band gap by applying an electric field and stress [19]. However, this band gap is not only small but also has a low electrical on/off ratio and is difficult to apply to a controllable device. With the extensive research on two-dimensional materials, it is found that the disadvantages of graphene are compensated for in other families of 2D materials [20–26].

Graphene films. (a) Photograph of a relatively large multilayer graphene flake with thickness ~3 nm on top of an oxidized Si wafer. (b) Atomic force microscope image of 2 µm by 2 µm area of this flake near its edge. (c) AFM image of single-layer graphene. (d) SEM micrograph of an experimental device prepared from few-layer graphene, and (e) its schematic view.

Figure 1.1 Graphene films. (a) Photograph (in normal white light) of a relatively large multilayer graphene flake with thickness ∼3 nm on top of an oxidized Si wafer. (b) Atomic force microscope (AFM) image of 2 μm by 2 μm area of this flake near its edge. Colors: dark brown, SiO2 surface; orange, 3 nm height above the SiO2 surface. (c) AFM image of single-layer graphene. Colors: dark brown, SiO2 surface; brown-red (central area), 0.8 nm height; yellow-brown (bottom left), 1.2 nm; orange (top left), 2.5 nm. Notice the folded part of the film near the bottom, which exhibits a differential height of ∼0.4 nm. (d) SEM micrograph of an experimental device prepared from few-layer graphene, and (e) its schematic view.

Source: Reproduced with permission from Novoselov et al. [1]. Copyright 2004, The American Association for the Advancement of Science.

(a) Room temperature quantum Hall effect in graphene as a function of gate voltages (Vg) in a magnetic field of 29 T. (b) Single-molecule detection in graphene. (c) Mobility of graphene. The histogram depicts the number P of devices exhibiting ρmax within 10% intervals. (d) Schematic of the experiment depicting the excitation laser light focused on a graphene layer suspended across a trench.

Figure 1.2 (a) Room temperature quantum Hall effect in graphene. σxy (red) and ρxx (blue) as a function of gate voltages (Vg) in a magnetic field of 29 T. The need for high B is attributed to broadened Landau levels caused by disorder, which reduces the activation energy.

Source: Reproduced with permission from Novoselov et al. [12]. Copyright 2007, The American Association for the Advancement of Science.

(b) Single-molecule detection in graphene. Examples of changes in Hall resistivity observed near the neutrality point (|n| < 1011 cm−2) during adsorption of strongly diluted NO2 (blue curve) and its desorption in vacuum at 50 °C (red curve). The green curve is a reference – the same device thoroughly annealed and then exposed to pure He. The curves are for a three-layer device in B = 10 T. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically.

Source: Reproduced with permission from Schedin et al. [13]. Copyright 2007, Nature Publishing Group.

(c) Mobility of graphene. Maximum values of resistivity ρ = 1/σ (circles) exhibited by devices with different mobilities μ (left y axis). The histogram (orange background) shows the number P of devices exhibiting ρmax within 10% intervals around the average value of ∼h/4e2. Several of the devices shown were made from two or three layers of graphene, indicating that the quantized minimum conductivity is a robust effect and does not require “ideal” graphene.

Source: Reproduced with permission from Novoselov et al. [14]. Copyright 2005, Nature Publishing Group.

(d) Schematic of the experiment showing the excitation laser light focused on a graphene layer suspended across a trench. The focused laser light creates a local hot spot and generates a heat wave inside single-layer graphene propagating toward heat sinks.

Source: Reproduced with permission from Balandin et al. [15]. Copyright 2008, American Chemical Society.

1.2 Types of 2D Materials

The rapid pace of progress in graphene and the methodology developed in synthesizing ultrathin layers have led to exploration of other 2D materials, such as monolayer of group IVA elements (silicon, germanium, and tin) [27, 28] and their adjacent group elements (such as boron and phosphorus) monolayers; 2D layered metal oxides or metal hydroxides (octahedral or orthogonal tetrahedral structure in the layer) [29]; transition metal dichalcogenides (TMDCs) [21]; and graphene analogs such as boron nitride (BN) [30]. These 2D materials ranging from insulators (e.g. BN), semiconductors (e.g. TMDCs, tellurene, PtSe2, and BP) to semimetals (e.g. MoTe2), topological insulators (e.g. Bi2Se3), superconductors (NbSe2), and metals (1T-VS2) exhibit diverse property.

There are many 2D materials, and some literature studies have classified the 2D materials based on their positions in periodic table of elements (Figure 1.3a) [22], stoichiometric ratios [31], space groups, and structural similarities [32]. The advantage of classifying 2D materials by periodic table of elements is that 2D materials with the same group of elements often have similar properties, which has a good guiding significance for finding novel 2D materials. In 2017, Michael Ashton et al. have found that 826 2D materials can be grouped according to their stoichiometric ratios and 50% of the layered materials are represented by just five stoichiometries (Figure 1.3b) [31]. The advantage of classifying two-dimensional materials by stoichiometric ratios is to distinguish 2D materials with different stoichiometric ratios but the same elements. At the same time, when synthesizing 2D materials, the vapor pressure of growth can be adjusted according to the stoichiometric ratios, which is conductive to synthetic materials. Recently, Nicolas Mounet et al. developed a system based on high-throughput computational exfoliation of 2D materials (Figure 1.3c) [32]. They searched for materials with layered structure from more than 100 000 kinds of three-dimensional compounds in the existing database, and the 1036 kinds of easily exfoliable cases provide novel structural prototypes and simple ternary compounds by high-throughput calculations. They classify the 2D materials of the easily exfoliated group into different prototypes, according to their space groups and their structural similarities. The structure of 2D materials can be useful to search for more suitable substrates. 2D materials with a similar structure can often form stable 2D alloys. In this book, we will focus on the electronic structure, synthesis, and applications of 2D materials. We will classify 2D materials into three types based on the synthesis, structure, and application: 2D single, doped components, and van der Waals heterostructures.

2D single materials, such as graphene and MoS2, generally refer to materials that can be exfoliated from corresponding van der Waal layered three-dimensional materials. 2D doped materials include adsorption, intercalation, substitution doping, and so on. In this book, we focus on the substitution doping: transition metal element or chalcogen element is substituted by other element, such as MoS2(1−x)Se2x [33] and Fe-doped SnS2 [34]. 2D heterostructures contain vertical and lateral types (Figure 1.4).

Classification of 2D materials. (a) About 40 different layered TMDCs compounds exist. The transition metals and the three chalcogen elements that predominantly crystallize in those layered structure are highlighted in the periodic table. (b) Distribution of stoichiometries of the 826 layered compounds. (c) Polar histogram depicting the number of structures belonging to the 10 most common 2D structural prototypes in the set of 1036 easily exfoliable 2D materials. A graphical representation of each prototype is shown, together with the structure-type formula and the space group of the 2D systems.

Figure 1.3 Classification of 2D materials. (a) About 40 different layered TMDCs compounds exist. The transition metals and the three chalcogen elements that predominantly crystallize in those layered structure are highlighted in the periodic table.

Source: Reproduced with permission from Chhowalla et al. [22]. Copyright 2013, Nature Publishing Group.

(b) Distribution of stoichiometries of the 826 layered compounds.

Source: Reproduced with permission from Ashton et al. [31]. Copyright 2017, American Physical Society.

(c) Polar histogram showing the number of structures belonging to the 10 most common 2D structural prototypes in the set of 1036 easily exfoliable 2D materials. A graphical representation of each prototype is shown, together with the structure-type formula and the space group of the 2D systems. The room temperature values of the thermal conductivity in the range ∼(4.84 ± 0.44) × 103 to (5.30 ± 0.48) × 103 W/mK were extracted for a single-layer graphene from the dependence of the Raman G peak frequency on the excitation laser power and independently measured G peak temperature coefficient.

Source: Reproduced with permission from Mounet et al. [32]. Copyright 2018, Springer Nature.

Atomic structure of (a) two-dimensional (2D) single materials, (b) 2D doped materials, and (c) 2D heterostructures. Dark shaded balls represent transition metal element (M), light shaded balls represent chalcogen element (X).
Figure 1.4 Atomic structure of (a) 2D single materials, (b) 2D doped materials, and (c) 2D heterostructures. Red and blue balls stand for transition metal element (M), yellow and green balls represent chalcogen element (X).

1.3 Perspective of 2D Materials

2D materials have been attracting wide interest because of their peculiar structural properties and fascinating applications in the areas of electronics, optics, magnetism, biology, and catalysis. Overall, the current research on 2D materials is mainly in two aspects: (i) Wafer-scale growth of 2D materials and their industrial applications. (ii) Synthesis of novel 2D materials and study their physicochemical properties.

The ability to grow large, high-quality single crystals for 2D components is essential for the industrial application of 2D devices. Until now, some 2D materials, such as MoS2 (Figure 1.5a), WS2 (Figure 1.5b), InSe (Figure 1.5c), and BN (Figure 1.5d), have been synthesized as wafer scale by vapor-phase deposition or pulsed laser deposition method. Thus, developing a simple and low-cost method to synthesize wafer-scale 2D materials is a current research focus. On the other hand, taking advantage of unique characteristics of 2D materials, direct integration based on 2D heterostructures is an ingenious method (Figure 1.5e) [39].

Although some 2D materials have been synthesized and investigated now, there are more than 1000 2D materials in theory and many of them still have a lot to discover, which are suggested to have peculiar property and need further study. The efforts on exploiting the application of 2D materials in optoelectronic and electronic area, such as FET and photodetector, have been intensified in recent years. Multifunctional thermoelectric, superconducting, and magnetic devices need further investigation. For example, thermoelectric applications of 2D p–n junctions have not been thoroughly investigated yet. Giant magnetoresistance effect has been realized in CrI3, while spin–orbit torque switching, spin Hall effect in antiferromagnets, and memory transistor based on 2D materials are rarely reported.

Wafer-scale growth and integrated circuit of 2D materials. (a) Three wafer-scale MoS2 films transferred and stacked on a 4 inch SiO2/Si wafer. (b) Raman spectra of WS2 at different positions marked in the wafer-scale monolayer image. (c) Photograph of 1 × 1 cm SiO2/Si covered with InSe film. (d) Schematic diagrams highlighting the unidirectional growth of h-BN domains and the anisotropic growth speed on a Cu surface with steps. (e) Illustration of a chemically synthesized inverter based on MoTe2.

Figure 1.5 Wafer-scale growth and integrated circuit of 2D materials. (a) Three wafer-scale MoS2 films transferred and stacked on a 4 in. SiO2/Si wafer.

Source: Reproduced with permission from Yu et al. [35]. Copyright 2017, American Chemical Society.

(b) Raman spectra of WS2 at different positions marked in the wafer-scale monolayer image.

Source: Reproduced with permission from Chen et al. [36]. Copyright 2019, American Chemical Society.

(c) Photograph of 1 × 1 cm SiO2/Si covered with InSe film.

Source: Reproduced with permission from Yang et al. [37]. Copyright 2017, American Chemical Society.

(d) Schematic diagrams highlighting the unidirectional growth of h-BN domains and the anisotropic growth speed on a Cu surface with steps. This method obtained 100-cm2 single-crystal hexagonal boron nitride monolayer on copper.

Source: Reproduced with permission from Wang et al. [38]. Copyright 2019, Springer Nature.

(e) Illustration of a chemically synthesized inverter based on MoTe2.

Source: Reproduced with permission from Zhang et al. [39]. Copyright 2019, Springer Nature.

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