Advanced 2D Materials E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Advance Materials Series
• Sprache: Englisch

Seitenzahl: 843

Veröffentlichungsjahr: 2016

Beschreibung

This book brings together innovative methodologies and strategies adopted in the research and developments of Advanced 2D Materials. Well-known worldwide researchers deliberate subjects on (1) Synthesis, characterizations, modeling and properties, (2) State-of-the-art design and (3) innovative uses of 2D materials including: * Two-dimensional layered gallium selenide * Synthesis of 2D boron nitride nanosheets * The effects of substrates on 2-D crystals * Electrical conductivity and reflectivity of models of some 2D materials * Graphene derivatives in semicrystalline polymer composites * Graphene oxide based multifunctional composites * Covalent and non-covalent polymer grafting of graphene oxide * Graphene-semiconductor hybrid photocatalysts for solar fuels * Graphene based sensors * Graphene composites from bench to clinic * Photocatalytic ZnO-graphene hybrids * Hydroxyapatite-graphene bioceramics in orthopaedic applications

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Part 1: Synthesis, Characterizations, Modeling and Properties

Chapter 1: Two-Dimensional Layered Gallium Selenide: Preparation, Properties, and Applications

1.1 Introduction

1.2 Preparation of 2D Layered GaSe Crystals

1.3 Structure, Characterization, and Properties

1.4 Applications

1.5 Conclusions and Perspectives

Acknowledgment

References

Chapter 2: Recent Progress on the Synthesis of 2D Boron Nitride Nanosheets

2.1 Boron Nitride and Its Nanomorphologies

2.2 Boron Nitride Nanosheets Synthesis

2.3 Conclusion

References

Chapter 3: The Effects of Substrates on 2D Crystals

3.1 Introduction

3.2 Fundamental Studies of 2D Crystals

3.3 Graphene Symmetries and Their Modification by Substrates and Functionalization

3.4 TMDs on Insulators and Metal Substrates

3.5 Conclusion

References

Chapter 4: Hubbard Model in Materials Science: Electrical Conductivity and Reflectivity of Models of Some 2D Materials

4.1 Introduction

4.2 The Hubbard Model

4.3 Calculations of Conductivity

4.4 The Hubbard Model and Optics

4.5 Conclusions

Acknowledgment

References

Part 2: State-of-the-Art Design of Functional 2D Composites

Chapter 5: Graphene Derivatives in Semicrystalline Polymer Composites

5.1 Introduction

5.2 Preparation of Polymer Nanocomposites Containing Graphene Derivatives

5.3 Properties of Graphene-based Polymer Nanocomposites

5.4 Synergic Effect of 2D/1D System

5.5 Conclusions and Future Perspectives

References

Chapter 6: Graphene Oxide: A Unique Nano-platform to Build Advanced Multifunctional Composites

6.1 Introduction to Graphene Oxide as Building Unit

6.2 Scaffolds for Tissue Engineering

6.3 Water Remediation

6.4 Multifunctional Structural Materials

6.5 Conclusions

Acknowledgments

References

Chapter 7: Synthesis of ZnO–Graphene Hybrids for Photocatalytic Degradation of Organic Contaminants

7.1 Introduction into Wastewater Treatment

7.2 Semiconductor-based Photocatalytic Degradation Mechanism

7.3 ZnO Hybridization Toward Enhanced Photocatalytic Efficiency

7.4 Synthesis Approaches for ZnO–Graphene Hybrid Photocatalysts

7.5 ZnO–Graphene Hybrid Photocatalysts

7.6 Ternary Hybrids with ZnO and rGO Materials

7.7 Conclusions

Acknowledgments

References

Chapter 8: Covalent and Non-covalent Modification of Graphene Oxide Through Polymer Grafting

8.1 Introduction

8.2 Covalent Modification of Graphene Oxide

8.3 Non-covalent Modification of Graphene Oxide

8.4 Composites and Grafts of GO with Natural Polymers

8.5 Conclusion

Acknowledgment

References

Part 3: High-Tech Applications of 2D Materials

Chapter 9: Graphene–Semiconductor Hybrid Photocatalysts and Their Application in Solar Fuel Production

9.1 Introduction

9.2 TiO2-based Photocatalyst

9.3 Non-TiO2 Semiconductors

9.4 Metal Complexes Sensitized Semiconductors

9.5 Graphene/Semicondutor/Metal Complexes-based Photocatalysts

9.6 Metal Free Dye-graphene Composite

9.7 Polymeric Semiconductors/Graphene Composites

9.8 Solar Fuel Production by Doped Graphene

9.9 Conclusion

References

Chapter 10: Graphene in Sensors Design

10.1 Introduction

10.2 Fabrication and Characterization of Graphene-based Materials

10.3 Applications

10.4 Conclusions

Acknowledgements

References

Chapter 11: Bio-applications of Graphene Composites: From Bench to Clinic

11.1 Introduction

11.2 Synthesis and Structural Features

11.3 Biomedical Applications

11.4 Conclusions (Current Limitations and Future Perspectives)

References

Chapter 12: Hydroxyapatite–Graphene Composites as Advanced Bioceramics for Orthopedic Applications

12.1 Background of Study

12.2 Literature Review

12.3 Functional Specifications

12.4 Summary and Concluding Remarks

References

Index

Advanced 2D Materials

Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106

Advanced Materials SeriesThe Advanced Materials Series provides recent advancements of the fascinating field of advanced materials science and technology, particularly in the area of structure, synthesis and processing, characterization, advanced-state properties, and applications. The volumes will cover theoretical and experimental approaches of molecular device materials, biomimetic materials, hybrid-type composite materials, functionalized polymers, supramolecular systems, information- and energy-transfer materials, biobased and biodegradable or environmental friendly materials. Each volume will be devoted to one broad subject and the multidisciplinary aspects will be drawn out in full.

Series Editor: Dr. Ashutosh TiwariBiosensors and Bioelectronics CentreLinköping UniversitySE-581 83 LinköpingSwedenE-mail: [email protected]

Managing Editors: Sachin Mishra and Sophie Thompson

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Preface

Masses of strongly bonded layers with very weak interlayer attraction exist in two-dimensional (2D) materials, which permit exfoliation into separate, atomically thick layers. Such super-thin surface percolates free electronic movements in the 2D planes and regulates motion in the third plane with a nanometer thickness. Graphene, transition metal dichalcogenides (TMDs), diatomic hexagonal boron nitride (h-BN), and emerging monoatomic buckled crystals collectively termed Xenes, which include silicene, germanene and phosphorene, are all covered in this book. The integrated unique properties of these different 2D materials therefore provide numerous possibilities to shape the future of advanced technology.

One of the possibilities is to create 2D materials by separating layered structures which are held together by weak van der Waals forces. Chapter 1 describes the various approaches used to fabricate and characterize gallium selenide and demonstrates device characteristics. The challenges associated with high growth rates to obtain atomically thin layers instead of multilayers are balanced by the possibility of controlling the shape of the gallium selenide or even exploring the stacking of 2D materials by growth on graphene. The tunable bandgap and change with number of layers create challenges in the characterization of nonlinear optical properties. The chapter ends with an overview of studies on field effect transistor and photodetector using gallium selenide. In Chapter 2, the stronger interlayer interaction in boron nitride and the difficulties in fabrication compared with graphene are described, and a range of growth approaches are also detailed. The difficulty in growth arises due to the partial ionic B-N bonds caused by the difference in electronegativity between nitrogen and boron. Ultimately this creates chemical bonds between the layers and has a strong impact on the control of the number of layers in boron nitride nanosheets.

The influence of the substrate on graphene and molybdenum disulfide is presented in Chapter 3. Defects and dangling bonds appear due to growth conditions and surface preparation. These are mostly undesired, but in some cases they may be used for internal engineering of the 2D material with the substrate. The uncertainty in the conductivity, for example, as either n- or p-type character, possibly caused by vacancies or interface properties, is an unclear issue in molybdenum disulfide. In particular, the interface has a strong impact since the 2D materials are only atomically thick. In perspective, the functionalization of the substrate with the 2D materials opens up ways to design devices once the properties are understood. Insights into the properties of 2D materials can be guided by modeling, as presented in Chapter 4 in which the Hubbard model is introduced. The results from the calculation of conductivity in one dimension are used to explore the conductivity in two dimensions. The chapter overviews previous work and in some instances extends the results of the calculations.

In Chapter 5, polymer-matrix nanocomposites for graphene derivatives are reviewed. The fabrication methods of the composites are presented. Their effect on electrical and thermal conductivity, as well as barrier properties, is shown to depend on characteristics like flake size, aspect ratio, loading, dispersion state, and alignment of nanoplatelets within the polymer matrix. As a potential field for further synergetic effects of polymers, the combination of carbon nanofillers with one or two dimensions has been raised. A nanoscale multifunctional platform using polymers and graphene oxide is given in Chapter 6. The nucleation and growth mechanisms of metallic nanoparticles depend on the degree of oxygen functionalization at the surface of graphene oxide. In addition, the graphene oxide can be given additional functionality by surface modification using a variety of polymers. The number of oxygen groups can mediate the type of binding interaction between the surfaces of graphene and graphene oxide and biomolecules to be used, for example, in tissue engineering. Graphene oxide for cleansing water or as mechanical reinforcement in structural applications is also discussed.

Chapter 7 highlights composites of graphene and zinc oxide as photocatalysts which are combined with the manufacturing capability of zinc oxide and the oxygen functionality of graphene oxide. This slows down the charge carrier recombination and improves the photooxidation since the charge transfer is more effective via the graphene. The reduced oxygen activity may also have an enhanced positive effect on the photostability given by the interaction between the graphene and zinc oxide. In addition, some ternary hybrid structures are discussed. Polymer grafting as a means of reducing the agglomeration of graphene oxide which may occur during use is presented in Chapter 8. Both covalent and noncovalent approaches are described. The range of polymers for grafting is increased by using initiators on the surface of graphene for subsequent polymer generation, or by preformed polymers which are attached to the oxygen groups on the surface of graphene oxide. The nanocomposites may also be fabricated using electrostatic interaction between modified graphene and a polymer, or by hydrogen bonding on the surface of graphene oxide.

In Chapter 9, hybrid structures of graphene and semiconductors are reviewed for use as photocatalysts. Graphene has the effect of increasing the mobility of electrons on the surface as well as enhancing the visible light absorption range of the semiconductor. These properties are reviewed with respect to water splitting and carbon dioxide conversion to liquid fuel. The bandgap of graphene oxide provides a way of making p-n junctions for separation of carriers as well as increasing the range of absorption of the solar spectrum. Further on, sensor design from graphene is described in Chapter 10. Fabrication, properties and applications of three graphene types are considered: pristine, nanocomposite and functionalized forms.

A review of graphene composites for biomedical applications is given in Chapter 11. Optical- and non-optical-based imaging is introduced and drug delivery and tissue engineering are described. Finally, bioceramic nanocomposites for orthopedic applications are presented in Chapter 12. The use of hard tissue rehabilitation materials creates a need for having a bone graft material with good mechanical and biological responses. Hydroxyapatite provides a suitable surface for bone growth and integration but has poor fracture toughness and wear resistance. Therefore, the chapter presents graphene as a secondary material in hydroxyapatite to improve the physical and biological properties.

In summation, this book brings together innovative methodologies and strategies adopted in the research and development of advanced 2D materials. Well-known worldwide researchers deliberate subjects on (1) synthesis, characterizations, modeling and properties, (2) state-of-the-art design and (3) innovative uses of 2D materials. The book is written for readers from diverse backgrounds across the fields of chemistry, physics, materials science and engineering, nanoscience and nanotechnology, biotechnology, and biomedical engineering. It offers a comprehensive overview of cutting-edge research on 2D materials and technologies.

We acknowledge the contributors and Mr. Martin Scrivener for his hard work in producing this high-quality book.

EditorsAshutosh Tiwari, PhD, DScMikael Syväjärvi, PhD22 May 2016

Part 1

SYNTHESIS, CHARACTERIZATIONS, MODELING AND PROPERTIES

Chapter 1

Two-Dimensional Layered Gallium Selenide: Preparation, Properties, and Applications

Wenjing Jie1,2 and Jianhua Hao1*

1Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

2College of Chemistry and Materials Science, Sichuan Normal University, Chengdu, Sichuan, China

*Corresponding author: [email protected]

Abstract

Gallium selenide (GaSe) is a layered III–VI semiconductor. It consists of covalently bonded stacks of four atomic layers in the sequence of Se–Ga–Ga–Se to form the tetra layers that are held together by a weak interaction of the van der Waals (vdW) force. The layered structure suggests the possibility of existence of two-dimensional (2D) GaSe like its pioneer graphene. Besides micro-mechanical exfoliation, 2D GaSe sheets can be obtained by various methods of vapor-phase mass transport, vdW epitaxy, molecular beam epitaxy, and pulse laser deposition. The fabricated 2D GaSe flakes have a tunable indirect band gap which is little lower than their direct counterpart. For monolayer, the experimental value of mobility is about 0.6 cm2 V–1 s–1 according to the transport properties of field-effect transistors (FETs). As expected, the 2D GaSe flakes exhibit layer-dependent nonlinear optical properties. The fabricated GaSe layers can enable the design of electronic and optoelectronic devices to realize functional applications of FETs and photodetectors. In this chapter, we focus on the scientific progress of 2D layered GaSe crystals to date, including various synthesis methods, characterization techniques, and electrical and optical properties as well as electronic and optoelectronic applications.

Keywords: Gallium selenide, two-dimensional materials, optoelectronic, field-effect transistors, photodetectors

1.1 Introduction

Two-dimensional (2D) layered materials have drawn extensive attention since the discovery of graphene through the method of mechanical exfoliation by Geim’s group in 2004 [1]. Graphene, with its unique 2D layered structure, exhibits outstanding and fascinating electronic, thermal, optical, and mechanical properties [2, 3]. Single-layer graphene has an ultra-high intrinsic mobility (200 000 cm2 V–1 s–1) [4] and electrical conductivity [5, 6], excellent thermal conductivity (~5000 W–1K–1) [7], transparence with very low absorption in white light spectrum (~2.3%) [8], and high Young’s modulus (~1.0 TPa) [9]. Accordingly, graphene has been explored in a wide range of applications such as optoelectronics, spintronics, sensors, supercapacitors, solar cells, and so on [10, 11]. And now, graphene is considered to be one of the most promising materials for future applications in nanoelectronics [12, 13]. The use of simple micro-mechanical cleavage technique has been expanded from graphene to other layered materials [14]. Besides graphene, a large variety of 2D materials can be exfoliated from their bulk materials with the stacked structure like graphite. A big category in 2D family is transition metal dichalcogenides (TMDCs), consisting of hexagonal layers of transition metal atoms and sandwiched between two layers of chalcogen atoms such as MoS2 and WS2 [15, 16]. The TMDCs exhibit exotic properties, especially a tunable band gap, which is absent in graphene. Among them, MoS2 is one of the most widely studied 2D materials with a tunable band gap shifting from the indirect gap of 1.29 eV to the direct gap of about 1.90 eV when decreasing the thickness from bulk to single layer [16]. MoS2 has been widely employed to integrate with many functional materials [17, 18] and 2D material of graphene [19], suggesting potential applications in future electronic and optoelectronic devices. Gallium selenide (GaSe) is a layered III–VI semiconductor, which consists of covalently bonded stacks of four atomic layers that are held together by a weak van der Waals (vdW)–type interaction. The stack is a sandwich with top and bottom layers of Se and two layers of Ga ions in the middle, i.e., in the sequence of Se–Ga–Ga–Se, with a lattice constant of 0.374 nm and a basic layer thickness of about 0.9 nm. Initially, monolayer GaSe flakes were obtained by mechanical cleavage methods [20]. The exfoliated ultrathin layers have been transferred onto SiO2/Si substrates for the fabrication of p-type field-effect transistors (FETs) and high-performance photodetectors [21, 22]. Following the roadmap of graphene, 2D GaSe crystals show potential in future applications of electronic and optoelectronic devices. In this chapter, various synthesis methods such as vapor-phase mass transport (VMT), vdW epitaxy, molecular beam epitaxy (MBE), and pulse laser deposition (PLD) are overviewed. The electrical and optical properties, especially the nonlinear optical properties of 2D layered GaSe, are summarized. The characteristics of fabricated nano- or micro-devices based on 2D GaSe flakes such as FETs and photodetectors are discussed.

1.2 Preparation of 2D Layered GaSe Crystals

1.2.1 Mechanical Exfoliation

Monolayer GaSe was firstly experimentally obtained in 2012 by Late et al. via the mechanical exfoliation method [20], similar to that employed for the production of graphene. Actually, after the discovery of graphene, the growth method has been expanded to other 2D layered materials. It is a convenient way to obtain micro-scale nanosheets with high quality from their bulk crystals in laboratory. This is also a widely used method to obtain high-quality 2D GaSe flakes. The 2D layered GaSe sheets, including mono-, bi-, and multilayer ones, are prepared by using Scotch tape from a piece of layered GaSe crystal. Then the nanosheets on the adhesive tape are transferred onto a target substrate, typically, 300-nm SiO2-coated Si substrate. Thus, the GaSe nanosheets can be prepared by using a two-step process, involving synthesis of bulk GaSe crystals and then the subsequent exfoliation of the bulk flakes onto target substrate.

1.2.1.1 Synthesis of Bulk GaSe Crystals

GaSe crystals are typically fabricated by a modified Bridgman method [23]. This process can be divided into two steps: synthesis of polycrystalline powder and single-crystal bulk GaSe. Firstly, the polycrystalline powder can be obtained by heating (typically to a temperature of above the melting point of GaSe of 960 °C for about 1 h) the mixture of gallium and selenium or Ga2Se3 and gallium at the molar ratio of 1:1, which is sealed in an evacuated quartz tube at low pressure. Then the tube is cooled to a lower temperature for a period of time followed by natural cooling to synthesize polycrystalline GaSe powder. Secondly, the synthesized GaSe powder is sealed in high-vacuum quartz ampoule, which is put in a suitable temperature gradient furnace. There are three temperature zones from top to bottom in the furnace, i.e., the high-temperature zone, the gradient zone, and the low-temperature zone. The ampoule is allowed to move from top to bottom along the axis of the gradient furnace at a very low speed and is also rotated during the downward movement to keep a uniform temperature distribution. Through the two-step process, single-crystal GaSe crystals can be well prepared for synthesis of 2D GaSe by mechanical exfoliation method.

1.2.1.2 Synthesis of 2D Nanosheets

The fabricated GaSe crystal has a layered structure with a weak interlayer coupling of vdW force, which is easy to be cleaved to synthesize 2D flakes. A small piece of GaSe crystal is put on a clean adhesive tape. Then, the tape is refolded and pressed firmly. After that, the tape is gently unfolded, leaving two mirrored areas of GaSe crystals on the tape. This process should be repeated for several times until a large dark grey portion appears. After performing these processes, some micro-scale GaSe flakes can be obtained on the adhesive tape. Then, the tape with 2D GaSe flakes is put onto a SiO2 wafer and pressed firmly, followed by gently removing the tape. Then some GaSe sheets with different layer numbers can be obtained on top surface of the SiO2 wafer.

These GaSe layers can be observed by using an optical microscope. A 300-nm-thick SiO2 is an ideal substrate because it allows GaSe nanosheets to be visible under white light, and more importantly, contrast between these nanosheets with different thicknesses is relatively high. The GaSe nanosheets prepared by the mechanical exfoliation method are in-plane micro-scale and out-of-plane atomically thin, and typically single-crystal with high quality.

1.2.2 Vapor-Phase Mass Transport

After the exfoliation of monolayer GaSe, many methods have been employed to grow these ultrathin crystals. The VMT method was firstly reported to prepare large-area atomically thin GaSe layers on insulating substrates in 2013 by Lei et al. [24]. In this method, grounded GaSe powder were used as source and small GaSe flakes as seeds for 2D crystal growth. The GaSe source and seeds were prepared with high-purity Ga2Se3 and gallium at a molar ratio of 1:1. The mixed powder was sealed in an evacuated quartz tube with argon of 10–3 Torr as protecting gas. The mixture was heated to a high temperature of 950 °C for 2 h and was maintained at this temperature for a period of time, and then the mixture was cooled to fabricate GaSe crystals with layered structure. The seeds for VMT growth were prepared by sonicating a small amount of GaSe crystals in isopropanol. Then the seeds were transferred onto a wafer-scale SiO2 substrate. Another part of GaSe crystals was ground into powder to serve as evaporation source. Then, the wafer with seeds on top and GaSe powder as source were sealed in a quartz tube as illustrated in Figure 1.1a. In a vacuum environment, the mean free path of the precursor was large enough for mass transfer. The source powder and substrate were heated, followed by a rapid cooling to room temperature. After that, the 2D GaSe flakes could be grown on the substrate.

Figure 1.1 (a) Schematic of the synthesis of GaSe flakes by the VMT method. (b) RHEED oscillations for synthesis eight-layer GaSe flakes, suggesting the layer-by-layer growth. The inset is a RHEED streaky pattern of the GaSe atomically thin film, indicating a well-ordered and atomically flat surface. (c) Schematic of experimental setup for the PLD method.

Through the VMT method, thin GaSe layers can be obtained in triangle, truncated triangle, and hexagonal shapes on the SiO2 substrate. The shape of the fabricated 2D flakes can be tuned by the distance from nucleation sites to the source. However, it is difficult to control the growth process because it is very fast. The as-grown 2D crystal becomes multilayer quickly after the nucleation. The shape of GaSe flakes could also be controlled by the concentration of the source. Similar to the growth of MoS2 atomic layers, the shape of the MoS2 flakes could be tuned to transform from triangles to hexagons by decreasing the concentration of sulfur [25, 26]. In the VMT process, two decomposed species of Se2 and Ga2Se from the source GaSe crystals could individually diffuse to the target substrate with different velocities and mean free paths. Then, the two species recombined to form GaSe nanosheets at the nucleation sites on the target substrate.

However, only few-layer GaSe sheets with thickness down to 2 nm could be achieved by this method. The controllable method to synthesize monolayer GaSe crystals is still needed.

1.2.3 Van der Waals’ Epitaxy

The GaSe epilayers grown layer by layer had identical orientations and terrace structures on pseudohexagonal mica substrates with a lattice mismatch of about 35% between them. Initially, nucleation of 2D GaSe nanosheets took place on the mica surface, followed by lateral covalent bonding of incoming atoms at the edge of the nucleation site. In vertical orientation, the top surface remained passivated by chemically saturated Se atoms. As a consequence, the crystal grew very fast in the lateral dimension compared to that in vertical dimension. Thus, it is feasible to grow single-layer GaSe, which is not achieved by the VMT method. Besides, the 2D GaSe flakes grown on flexible transparent insulating mica substrates can serve as a good platform for the measurements of their optical and optoelectronic properties.

On the other hand, GaSe epilayers on graphene started to nucleate and grow from random wrinkles or grain boundaries of the underlying graphene. The GaSe epilayers with irregular shapes had different lattice orientation from the graphene layers, which could be determined by the competition between the growth rate and the diffusion rate of the grown GaSe layers on the graphene. Generally, the formed rotation angle of 10.5 ± 0.3 was observed in the prepared GaSe epilayers. Besides, the as-prepared GaSe monolayers possessed sharp edges. Each edge was well aligned with 60° angles of neighboring contiguous islands. The vdW epitaxial single-layer GaSe flakes on graphene were large (tens of microns) irregularly shaped with single-crystalline domains and preferential nucleation on random wrinkles or grain boundaries of graphene.

1.2.4 Molecular Beam Epitaxy

Besides the aforementioned chemical deposition methods, some physical methods have been employed to grow 2D layered GaSe, such as MBE and PLD methods. MBE is well established to prepare high-quality and uniform epitaxial layers. This method has also been used for the preparation of 2D materials. Combined with in situ reflection high-energy electron diffraction (RHEED), the thickness of the prepared epitaxial layer can be precisely controlled. Very recently, Yuan et al. reported layer-by-layer growth of 2D GaSe on n-type Si substrates by the MBE method [29]. High-purity Ga and Se were evaporated and deposited on well-cleaned Si substrate with the Se/Ga flux ratio of 10:1 and the growth temperature of 580 °C. The as-prepared GaSe layers grew along [0001] direction. Periodic RHEED oscillations suggested layer-by-layer growth mode with a growth rate of about 2.8 min/layer, as shown in Figure 1.1b. The streaky RHEED pattern also indicated a well-ordered and atomically flat surface. Besides, there was a clear interfacial phase between epitaxial GaSe layers and Si substrates. GaSe layers with thickness from three layer to multilayer can be well prepared on Si substrates owning to the vdW interaction between layers. This not only provides approach to grow wafer scale 2D GaSe layers but also provides opportunity to form vdW heterostructure of p-type GaSe and n-type Si for fabrication of optoelectronic device. Furthermore, epitaxial GaSe layers can be well grown on freshly sliced mica substrates by the MBE method, suggesting an approach of substrate-independent epitaxial growth of GaSe.

1.2.5 Pulse Laser Deposition

Besides the aforementioned physical deposition method of MBE, PLD technique was also been employed to grow 2D GaSe nanosheets [30]. The GaSe target was prepared by highly pure Ga and Se via high-temperature process in a high-vacuum system. The target was irradiated by excimer laser with a wavelength of 248 nm and a pulse duration of 20 ns, as schematically shown in Figure 1.1c. The optimal growth temperature for GaSe nanosheets was 600 °C on the SiO2/Si substrate. If the deposition temperature was below 500 °C, the prepared GaSe thin films would be amorphous GaSe, and no deposition could be observed if the deposition temperature was higher than 750 °C. Both out-of-plane and in-plane growths of GaSe nanosheets were controllably synthesized by adjusting the deposition rate (via laser repetition rate) and surface diffusion rate (via substrate temperature). Besides, the stoichiometric growth of 2D GaSe nanosheets required high Ar background with a background gas pressure of about 1 Torr. Individual 1–3 layer GaSe triangular nanosheets with a domain size of about 200 nm were formed within 30 laser pulses. When the pulse number was in excess of 100, individual nanosheets could grow to form nanosheet networks. The thickness of the deposited networks increased linearly with pulse number. This growth behavior at the beginning can be described by the classic Volmer–Weber mode. A large number of small nuclei grow, forming planar, triangular, crystalline GaSe nanosheets, which then coalesce into a thin film. Similar to the aforementioned MBE method, layer-by-layer-like growth within each GaSe nanosheets is due to the weak vdW force between each unit layer of Se–Ga–Ga–Se. Through controlling the deposition temperature and rate, GaSe single layer and nanosheets network could be prepared.

1.3 Structure, Characterization, and Properties

1.3.1 Crystal Structure

Figure 1.2 (a) Crystal structure. (b) Optical image of mechanically exfoliated GaSe flakes on SiO2 substrate. (c) AFM image of monolayer triangular GaSe flake. Inset is the line profile in the direction of the dashed arrows. (d) SEM image of monolayer triangular GaSe flakes.

1.3.2 Characterization

Various techniques are employed to characterize 2D materials such as optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. These techniques have also been used to analyze 2D GaSe crystals. Among them, optical microscopy is a good method for identifying the atomically thin films by using the interference effect. Similar to graphene, this requires such 2D layers located on silicon oxide substrates for good contrast. Figure 1.2b shows optical image of exfoliated GaSe falkes [22]. A single-layer sheet located at the center part exhibits clear optical contrast and can be visible. Besides, optical microscopy is convenient, non-destructive, and also the cheapest in laboratories. However, it could not independently determine the layer number of 2D GaSe. A combination of optical microscope and other techniques could provide conclusive evidence of the crystal thickness.

AFM is an imaging technique that can completely identify the layer number of the atomically thin GaSe. By using tapping mode, the height profile can clearly determine whether a given nanosheet is single, double, or multilayered. Figure 1.2c shows the AFM image of a triangular GaSe flake [32]. The height profile clearly indicated that the GaSe triangle is single layer. Besides, AFM is also non-destructive for the 2D layers. On the other hand, SEM also could provide top morphology image of 2D GaSe, especially fabricated layers by chemical and physical deposition methods [30, 32]. Figure 1.2d shows the fabricated monolayer GaSe flakes by a controlled vapor deposition method [32]. Compared to AFM, SEM can provide relatively large-scale image of such 2D layers.

Besides optical microscopy, AFM, and SEM, HRTEM and Raman spectroscopy are frequently used to characterize 2D GaSe layers.

1.3.2.1 Transmission Electron Microscopy

In general, TEM is widely used to characterize nanoscale materials with high resolution. The operation of TEM relies on a transmitted electron beam passing through ultrathin samples and reaching to the imaging lenses and detector. After the discovery of graphene, TEM has been frequently employed to detect 2D materials. Xiao’s group has been conducting a series of research about 2D GaSe flakes by using TEM to study their grain structures and stacking orders [32, 33]. In their studies, triangular GaSe flakes were prepared by a controlled vapor deposition method, which is similar to the aforementioned vdW epitaxial growth technique. The 2D samples for TEM characterizations were grown directly on 5-nm-thick amorphous silicon films supported by a TEM grid. The grain structures and stacking orders of the 2D GaSe flakes were characterized using both bright-field imaging and electron diffraction in TEM and Z-contrast imaging in aberration-corrected scanning transmission electron microscopy (STEM), respectively. The bright-field TEM (BF-TEM) image of a single-monolayer GaSe crystal is shown in Figure 1.3a. Inset is the selected-area electron diffraction (SAED) pattern of the flake, exhibiting a single set of spots in a hexagonal pattern. The BF-TEM image shown in Figure 1.3b shows two-monolayer triangular flakes stacking together. The SAED pattern from the area in the white circle shows two sets of spots in a hexagonal pattern, indicated by red and green dashed lines, respectively. The two sets of spots are orientated with about 30° apart. All these results indicate that the fabricated GaSe triangles are single crystalline with hexagonal structure. Aberration-corrected dark-field STEM (ADF-STEM) was used to study the crystal structures of the fabricated 2D flakes. Figure 1.3c shows high-resolution ADF-STEM image of monolayer GaSe. The lattice is composed of hexagonal rings of gallium and selenium atoms. Top and side views of monolayer GaSe schematically overlaid on top of the ADF-STEM image. Inset is the corresponding FFT image. Figure 1.3d shows high-resolution ADF-STEM image of multi-layer GaSe. Top view of the multi-layer GaSe overlying on the image clearly indicates that the stacking of the multi-layer crystal can be ε- or γ-GaSe, but not β-GaSe.

Figure 1.3 Grain and crystal structures of 2D GaSe flakes. (a) BF-TEM image of a single-monolayer GaSe crystal. Inset is the SAED pattern of the flake. (b) BF-TEM image of two-monolayer triangular flakes stacking together. Inset is the SAED pattern from the area indicated by solid circles. (c) High-resolution ADF-STEM image of monolayer GaSe. (d) High-resolution ADF-STEM image of multi-layer GaSe.

Besides single- and multi-layer GaSe, it is interesting to study the crystal structures and stacking modes of bilayer GaSe flakes by TEM [33]. Figure 1.3 shows atomic resolution structures of triangle-on-triangle bilayer GaSe flakes with different stacking orders. According to the layer size and orientation, the as-prepared bilayer GaSe can be divided into three types. The top layers of the bilayer flakes have the same size and orientation as the bottom layer, which is named type I stacking, as shown in Figure 1.4a. Type II stacking is the one where top layer has same orientation but small size, while type III shows different orientations and size between top and bottom layer, as shown in the BF-TEM images in Figure 1.4b and c, respectively. Insets are SAED patterns from the selected regions as shown by white circles in Figure 1.4a–c. All the SAED patterns from both the monolayer and bilayer flakes show only one set of six-fold symmetry diffraction spots, suggesting that the as-synthesized bilayers are single crystals with hexagonal crystal structures. Z-contrast STEM imaging technique was used to directly image the stacking modes of the bilayer flakes with atomic resolution, as shown in Figure 1.4d–f. Type I and II stacked bilayer GaSe crystals show close-packed atomic structures with atoms in AB stacking order, belonging to ε-polytype. Type III GaSe flakes with a hexagonal ring structure correspond to AA’ stacking mode, belonging to β-polytype. In Z-contrast STEM image of monolayer GaSe, as shown in Figure 1.4g, the Se atoms are slightly brighter than Ga atoms due to the larger mass of Se. In bilayer AB-stacked GaSe crystals, Ga and Se atoms that are stacked in vertical alignment between top and bottom layers correspond to brighter spots, while Ga or Se atoms that are not aligned vertically correspond to less intense spots. In AA’-stacked bilayer crystals, Ga and Se atoms have similar brightness since all the Ga or Se atoms in the top layer are directly above Se or Ga in the bottom one. Besides, Z-contrast STEM imaging technique can clearly show the edge structures. As shown in Figure 1.3h and i, both AB- and AA’-stacked bilayer crystals have Ga-terminated zigzag structures.

Figure 1.4 Atomic resolution structures of 2D GaSe crystals with different stacking orders. BF-TEM images of bilayer GaSe crystal with stacking mode of (a) I, (b) II, and (c) III. Insets are SAED patterns from the area indicated by solid circles. Z-contrast STEM images of bilayer GaSe flakes with stacking mode of (d) I, (e) II, and (f) III. (g) Z-contrast STEM image of monolayer GaSe crystal. Z-contrast STEM image showing the edge structures for (h) AB- and (i) AA’-stacked bilayer crystals.

1.3.2.2 Raman Spectroscopy

Raman spectroscopy is a powerful method to characterize 2D materials, from graphene and the subsequent 2D TMDCs [34, 35]. For graphene, Raman spectroscopy can not only figure out single-layer one but also detect the defect information according to the obtained Raman spectrum. Raman spectroscopy is also a useful technique to analyze the layer number and investigate the structure and quality of 2D GaSe. For bulk GaSe, there are 12 vibrational modes, including 8 in-plane modes of E’ and E” and 4 out-of-plane modes of A1‘ and A2“ [27, 32, 36]. Among them, only A2“ mode is not Raman active. All Raman-active modes are schematically shown in Figure 1.5a, corresponding to different Raman feature peaks. Figure 1.5b shows Raman spectra of GaSe flakes with different thickness prepared by VMT method. In the Raman spectra, four feature peaks can be observed. E11g at 59 cm–1 and E2g at 212 cm–1 are in-plane vibrational modes. Other two strong feature peaks located at 132 and 305 cm–1 correspond to vibrational modes of A11g and A21g, respectively. As the number of layers decreases, the relative intensity of all Raman feature peaks decreases. The intensity decreases dramatically and even tends to vanish when the thickness approaches 1–2 layers. This is different from graphene, which shows strong 2D Raman feature peak when thickness decreases to monolayer. The A1g2 mode, which has overlaps with the Raman peak of the substrate at about 307 cm–1, show a slight red shift when thickness deceases to atomically thin, as shown in Figure 1.4c. This is due to the reduction of the force constant in the atomically thin layers. These results are consistent with those of mechanically exfoliated GaSe samples [20]. Besides, the Raman scattering intensity of out-of-plane modes is stronger than that of the in-plane modes, which also become less effective as layer number decreased.

Figure 1.5 Raman spectra of layered GaSe with various thickness. (a) Schematics of the Raman modes of GaSe. The colored arrows designate the vibrational directions of selenium atoms (orange) and gallium (green) atoms. (b) Raman spectra of 1–2 layers (blue), few layers (red), and thick flakes (black) of GaSe. (c) The A1g peak at 305.2 cm–1 shifts to 303.4 cm–1 in 1–2 layers and a few layers of GaSe.

1.3.3 Properties

1.3.3.1 Electronic Properties

For bulk GaSe, it has an indirect band gap of about 2.0 eV and a direct band gap of only about 25 meV higher according to the previous experimental results. However, the indirect band gap is almost equal to the direct one. The electron can easily transfer between the two energy levels with a small amount of thermal energy, which hampers the precise measurement of its value. In experiments, the band gap at room temperature of β- and ε-GaSe was reported to be about 2.046 and 1.996 eV, respectively [37].

In comparison with their bulk counterparts, 2D GaSe layers are expected to have a tunable band gap, similar to MoS2. In theory, the band structure of GaSe crystal has been studied by several calculation methods. However, the band gap value depended strongly on the calculation modes [38–41]. In 2011, Rybkovskiy et al. investigated the electronic band structure of the 2D and bulk GaSe crystals by using the first-principles density functional theory (DFT) calculations [42]. The local density approximation (LDA) was employed to describe the exchange and correlation. The calculations of 2D GaSe suggested a layer-dependent band gap. The calculated direct band gap value of monolayer GaSe is 2.60 eV. And, the bulk is 1.21 eV, suggesting a large disparity between the theoretical and experimental values. It is well known that DFT underestimates the conduction band energies. Therefore, the GW approximation, the expansion of the self-energy in terms of the single particle Green function G and the screened Coulomb interaction W, was used for the correction of the band-gap values. The band gap of 3.89 eV was obtained for monolayer GaSe and 2.34 eV for bulk one.

In 2014, Li et al. studied the electronic band structures of GaSe by using a first-principles quantum mechanical calculation code [32]. Similar to the aforementioned calculation method, 2D GaSe showed a tunable band gap dependent on the layer number. Although the calculated band gap of monolayer GaSe is only 2.26 eV (much smaller than the aforementioned 3.89 eV), the valence band maximum (VBM) splits in a symmetric way along the Γ point, indicating that the 2D crystals have an indirect band gap. This is significantly different from the previously reported MoS2 [16, 43], which has an indirect-to-direct band-gap transition through decreasing thickness to monolayer. Moreover, the energy difference between the direct gap and indirect gap for monolayer GaSe is so small, similar to its bulk counterpart. Therefore, electrons can easily move between the two energy levels with a small amount of thermal energy.

In experiments, Lei et al. reported an indirect band gap of 3.3 eV according to the photocurrent measurements for 2D GaSe layers [24]. While, Hu et al. obtained the bad gap of about only 20 meV higher than the bulk one based on the photoluminescence (PL) measurements for bilayer GaSe cryatsls [21]. Very recently, Jung et al. reported a band gap of 3.3 eV according to the cathodoluminescence (CL) measurements for monolayer GaSe [44]. Besides, they also investigated the electronic band structures of 2D GaSe layers by using first-principles calculations. Figure 1.6a and b, respectively, shows the band structures of mono- and bilayer GaSe performed by the DFT calculations using the HSE06 functional. The GaSe layers exhibit indirect electronic transitions, as exhibited by the red arrows. The direct transitions are indicated by the blue arrows. The detailed bans gap values of GaSe layers with different thickness and stacking mode are listed in Table 1.1. Although the calculated band structures are different by different calculation methods, all the calculation results suggest a tunable indirect band gap for 2D GaSe and the band gap increases as the thickness decreases to monolayer.

Figure 1.6 Electronic band structures of (a) mono- and (b) bilayer GaSe calculated using HSE06 functional.

Table 1.1 The direct and indirect band gaps obtained by different calculations.

Besides, GaSe is generally p-type. In bulk GaSe crystal, the mobility was reported to be about 215 cm2 V–1 s–1. However, in its monolayer form, the mobility decreased about three orders of magnitude. Compared to graphene and single-layer MoS2, 2D GaSe flakes prepared by mechanically exfoliated method have relatively low mobility, about 0.6 cm2 V–1 s–1 for single-layer GaSe [22] and 0.005 cm2 V–1 s–1 for few-layer one [45]. Few-layer GaSe nanosheets grown by the PLD method was reported to possess a mobility of about 0.1 cm2 V–1 s–1 [30].

1.3.3.2 Optical Properties

GaSe crystals possess wide transparency range from 0.65 to 18 µm, relatively high birefringence and high threshold damage value for different laser lines [46]. In comparison with their bulk counterparts, 2D GaSe layers are expected to have high photoresponsivity and high sensitivity due to the large surface-to-volume ratio and the distinct quantum confinement on their optical and electronic properties.

Two-dimensional GaSe flakes have a thickness-dependent band gap, suggesting a thickness-dependent PL. However, the emission intensity in atomically thin layers of GaSe decreases as thickness down to monolayer due to the indirect band gap. Figure 1.7a shows PL spectra of bulk and 2D GaSe flakes with different thickness [21]. In comparison to bulk GaSe, 2D GaSe layers exhibit a slight blue shift of PL peak position with a decrease in the layer number, which is in agreement with the aforementioned tunable band gap. From bulk to bilayer GaSe, there is a blue shift of 20 meV, suggesting that the peak position shifted from 620 nm (bulk) to 613 nm (two layers). This blue shift is possibly due to the modification of bandgap structure caused by the decrease in thickness of layered GaSe, differing from monolayer semiconductor whose optical properties are strongly influenced by a quantum confinement effect. For example, the optical band gap of an ultrathin MoS2 flake was transformed from indirect band gap to direct one with decreasing thickness. As a result, the MoS2 monolayer emits light more strongly than its bulk material.

Figure 1.7 PL and CL of GaSe crystals. (a) PL spectra of bulk and 2D GaSe flakes with different thickness. (b) CL spectra of the monolayer and multilayer GaSe crystals. The inset shows the photographs of blue-color emission of monolayer GaSe. The left figure is the SEM image of GaSe flakes.

Besides PL, CL of 2D GaSe layers has also been measured [44]. As shown in Figure 1.7b, there is a broad emission band (from 300 to 500 nm) in the CL spectrum for monolayer GaSe with the peak position at about 375 nm (3.3 eV). For multilayer GaSe crystal, the emission shows a red shift of about 0.1 eV. Besides, CL emission of multilayer GaSe enhanced compared to its monolayer counterpart. The CL emission of the monolayer GaSe crystals is about three times weaker than that of the multilayer ones. Similarly, the intensity of PL emission was also thickness dependent [21]. The PL intensity decreases as the layer number decreases.

1.3.3.3 Nonlinear Optical Properties

Nonlinear optics has been widely used in many important applications such as integrated optics, optical information, optical communications, and imaging techniques [47, 48]. For GaSe crystals, there are several different modifications that differ in the stacking sequence [31]. Among them, β-GaSe has two basic layers per unit cell and belongs to space group of D46h. It is centrosymmetric, which has the same crystal symmetry with MoS2 and WS2. On the other hand, other modifications are non-centrosymmetric. It should be noted that one of the most important properties of bulk non-centrosymmetric GaSe is the nonlinear optical property due to the absence of the inversion symmetric center [49, 50]. GaSe is a well-known nonlinear optical crystal. Recently, the nonlinear optical properties in 2D GaSe crystals were reported. Actually, 2D TMDCs, such as MoS2, WS2, and WSe2 thin flakes, were reported to show second-order optical nonlinearity due to the absence of inversion symmetry when decreasing the thickness to monolayer [51–55]. The intense optical second harmonic generation (SHG) was observed in odd-layered TMDCs, while vanished or degraded in even layer numbers owing to the restoration of inversion symmetry. Similar behaviors have been observed in 2D h-BN layers [55, 56]. SHG can also be observed in artificially stacked bilayer TMDCs with an arbitrary stacking angle and bilayer h-BN with broken inversion symmetry [52, 56].

After many reports about nonlinear optical properties of monolayer 2D TMDCs and the electronic and optoelectronic properties of GaSe, researchers focused on the nonlinear optical properties of 2D GaSe. Recently, Karvonen et al. reported nonlinear optical properties of SHG and third harmonic generation (THG) in multilayer GaSe crystals with thickness more than seven layers (7L) [57]. The optical image of GaSe flakes is shown in Figure 1.8a. The SHG and THG are generated by using excited femtosecond (fs) laser with a wavelength of 1560 nm. The SHG and THG images of the GaSe flakes were recorded simultaneously by employing multi-photon microscopy, as shown in Figure 1.8b and c, respectively. The SHG signal is much stronger than the THG one. To visualize the GaSe flakes, the image contrast was scaled in both SHG and THG images, where the areas with different number of layers show different signal intensity. The SHG and THG images also suggest potential application in rapidly estimating the layer number and accurately determining the shape and location of 2D GaSe flakes in the future. It was recently reported that by using edge effects of SHG in CVD-grown monolayer polycrystalline MoS2, the grain boundary can be clearly visible [58, 59]. It is common through using SHG signals to achieve high-resolution imaging for biological tissues and organic nanocrystals [60–62]. Besides, the SHG intensity is observed to decrease near the edges, while the THG signal is constant over the area with the same number of layers. The SHG photon energy is below the band gap of GaSe. Therefore, the SHG signal decreases significantly near the edges of the flake. Figure 1.7d shows the SHG (peaking at 780 nm) and THG (peaking at 520 nm) spectra from the selected area of GaSe flakes with thickness of 9L, 13L, and 40L as indicated by blue, red, and black curves, respectively. As the thickness increases, both harmonic generations are enhanced. It should be noted the peak at 390 nm, corresponding to the fourth harmonic generation (FHG), can be clearly observed from 40L GaSe flake. This peak is exactly at the fourth harmonic wavelength, but it could also be the sum frequency generation from THG and pump light. Further study is needed to confirm this.

Figure 1.8 Nonlinear optical properties of 2D GaSe flakes, (a) Optical image of a mechanically exfoliated GaSe flake. Areas marked with Roman numbers from I to VII correspond to different numbers of layers of 7, 9, 11, 12, 13, 14, and 40, respectively. (b) SHG and (c) THG image of the GaSe flake shown in (a). (d) SHG and THG spectra from three different positions with different thicknesses. (e) Thickness-dependent SHG. The inset is the SHG spectrum of GaSe flake with a thickness of 20 layers and the excitation wavelength of 800 nm. The TPEF spectra of (f) bulk and (g) 2D GaSe crystals. (h) SHG spectra from monolayer GaSe and MoS2 under different excitation wavelengths. (i) Optical image of a monolayer triangular GaSe flake on SiO2 substrate. Azimuthal angle θ is defined as the polarization direction relative to one triangular lateral side direction, as indicated by the arrow in the figure. (j) The polarization angle θ-dependent SHG intensity. (k) The crystalline lattice model of monolayer GaSe derived from the polarization-dependent pattern in (j).

Our research group also studied the nonlinear optical properties of atomically thin GaSe flakes with thickness from bilayer to multilayer by using fs laser with a wavelength of about 800 nm [63]. Figure 1.8e shows layer-dependent emission with the half wavelength and double frequency. It should be mentioned that we can get the emission from bilayer flakes. This suggest that SHG can be produced in both odd and even number layers, different from the SHG in 2D TMDCs and BN which is dependent on the parity of the layer number. As expected, we obtain quadratic dependent SHG on the layer number when the layer number is beyond 5 [64, 65]. However, when thickness is thinner than 5 layer, we get a nearly cubic dependent SHG on the layer number, suggesting the additional decrease in SHG intensity in atomically thin layers. Furthermore, Figure 1.8f and g shows two-photon excited fluorescence (TPEF) spectra for bulk and 2D GaSe, respectively. TPEF is a third order of nonlinear optical process involving absorption of the two photons and then emission according to the band gap of the semiconductor [66–68], whereas SHG is a light-scattering process involving two photons interact simultaneously with the nonlinear material without absorption. So, nonlinear material is essential for SHG, whereas TPEF does not require that the sample is nonlinear material. By considering the band gap of GaSe is about 2.0 eV as well as the incident laser is of high intensity, it is possible that the incident laser can induce TPEF in atomically thin GaSe flakes. Both emission spectra show a superposition of two peaks due to the coexisting of β- (blue curve) and ε-modifications (green curve). Firstly, from the peak position, there is a blue shift from bulk to 2D layer for both modifications, owing to the thickness reduction. Then from the peak intensity, comparable intensity for β- and ε-modifications can be achieved in atomically thin layers, while PL peak of ε-modification is much stronger than that of β-modification, suggesting that the major modification is ε-modification in bulk. Free energy calculations were performed by using first-principles method. It shows both modifications are stable in bulk, while β-GaSe is more stable compared to ε-modification in atomically thin layers. So, some external perturbations such as thermal or photo excitation may modify the stacking order in atomically thin layers. This can result in the more decrease in SHG when thickness is thinner than five layers.

Very recently, Zhou et al. reported strong SHG in CVD-grown monolayer GaSe under nonresonant excitation and emission condition [69]. Figure 1.8h shows SHG intensity from monolayer GaSe and MoS2 under different excitation wavelengths. Under the same illumination condition, the SHG intensity of GaSe is about 1–2 orders of magnitude higher than that from MoS2. The nonresonant SHG was reported to be the strongest among all the monolayer 2D crystals. Besides, the excited laser corresponding to second harmonic emission energy closer to the band gap of the monolayer generates stronger SHG. The structural symmetry properties of monolayer GaSe have also been studied by polarization-dependent SHG intensity, as shown in Figure 1.8i. Monolayer GaSe exhibits a six-fold rotational symmetry with SHG intensity varying with azimuthal angle θ. The SHG intensity I can be described as I0sin2(3θ), where I0 is the maximum of SHG intensity. The polarization angle θ-dependent SHG intensity shows a six-petal pattern, suggesting that monolayer GaSe shows three-fold lattice symmetry. According to the polarization angle-dependent SHG pattern, the intrinsic lattice model of the CVD-grown monolayer GaSe can be deduced. The maximum petal direction is parallel to the in-plane Ga–Se or Se–Ga direction. The triangular clusters exhibit a Se- or Ga-terminated zigzag edge. A possible structural model is shown in Figure 1.8k. Besides, further SHG mappings show that monolayer triangular GaSe flake has a one-to-one correspondence to the lattice symmetry, implying that the SHG spectroscopy could be utilized to investigate the in-plane grain boundary in monolayer.

1.4 Applications

1.4.1 Field-Effect Transistors

Since the discovery of graphene in 2004, it has been used as a channel material in the FETs [1]. Generally speaking, a 2D material-based FET consists of a channel of 2D layer connecting source and drain electrodes, a gate electrode and a dielectric barrier separating the gate electrode from the channel. The operation of FET relies on controlling the channel current by the gate voltage to achieve an on or off state of the channel current [70]. According to the gate dielectric position, there are three types of FETs, top-, bottom- and dual-gated, i.e. both top- and bottom-gated FETs [71]. However, as we all know, the band gap of graphene is zero. As a consequence, the graphene-based FETs cannot be really switched off, which will hamper the real applications of graphene-based FETs. Following graphene, other 2D materials have been integrated into FETs to serve as the channel materials. For example, monolayer MoS2 has been reported to have a high mobility of 200 cm2 V–1 s–1 and a high ON/OFF ratio of 108 [17]. With recent developments in large-scale production techniques such as CVD-like growth [25, 26], 2D MoS2 layers have shown some inspiring characteristics for applications in integrated devices because of the presence of band gap.

Following the roadmap of graphene and MoS2, 2D GaSe flakes have also been used in transistors as the channel material. GaSe has a suitable band gap of about 2 eV for application in FETs. In 2012, Dravid’s research group firstly reported mechanically exfoliated monolayer GaSe-based FETs by using 500 nm SiO2 as gate dielectric, as schematically shown in Figure 1.9a [22]. This is a typical bottom-gated 2D material-based FET device with Ti/Au as source and drain electrodes while Si as gate electrode. Figure 1.9b shows the room temperature output characteristics of monolayer GaSe-based FET. The curves of drain current (Ids) as function of the drain-source voltage (Vds) are gate voltage (Vg) dependent. Figure 1.9c shows transport properties of this bottom-gated FET, which is p-type with an on/off ratio of 105. The mobility of monolayer GaSe can be calculated to be about 0.6 cm2 V–1 s–1. This value is comparable with previous reported bottom-gated MoS2-based FETs [17]. However, it is much lower than the improved mobility of MoS2 and graphene as well as bulk GaSe. The low mobility can be attributed to the trap/impurity states existing at the SiO2 surface in the bottom-gated FETs. These charged impurities can degrade the performance of the fabricated FET devices, resulting in the degradation in mobility of GaSe. Reduction of the charged impurities is expected to improve the mobility of GaSe-based bottom-gated FETs. Besides, it has been reported that 2D GaSe flakes are not stable at ambient conditions and possibly creating high-density charged traps [72]. Besides, the mobility GaSe devices could be improved by using the high-K dielectric material as the top gate and with appropriate substrate engineering.

Figure 1.9 (a) Schematic of GaSe-based bottom-gated FET, (b) room temperature output characteristics, and (c) transport properties of p-type GaSe-based FET.

1.4.2 Photodetectors

A photodetector is a sensor of light that can transform incident radiation energy into an electrical signal. Generally, there are two main types of photodetectors, namely photodiodes and photoconductors [73]. The former relies on the junction built by two media with significant difference in their work functions. Under light, the electrons and holes generated by the incident photons move to an opposite direction, yielding a photocurrent corresponding to the light level. In photoconductors, the difference in conductance between in dark and light is examined to determine the light intensity. The first graphene-based photodetector was discovered by detecting the photocurrent arising from the band bending at the graphene/metal (Au or Ti) interface while locally gated by 300-nm SiO2 from a back-gated graphene transistor grown on SiO2/Si substrate [74]. Particularly, a strong photoresponse can be generated at the interfaces including single/bilayer graphene interfaces and the graphene p–n junctions [75, 76]. The first monolayer MoS2-based phototransistor exhibits a photoresponsivity of 7.5 mA/W, which is comparable to graphene-based devices [77]. Then, such monolayer MoS2-based phototransistors have been optimized to obtain a photoresponsivity of 880 A/W at a wavelength of 561 nm due to its improved mobility, as well as the contact quality and positioning technique [78].

For 2D layered GaSe flakes, both FET-based phototransistors and p–n junction photodetectors were reported recently.

1.4.2.1 Phototransistors

Based on the aforementioned FET device, mechanically cleaved GaSe flakes were transferred onto a silicon substrate with an oxidized layer of 300 nm to fabricate a phototransistor [21]. As schematically shown in Figure 1.10a, a monochromatic light is vertically incident on a GaSe device. Actually, in the experiment, few-layer GaSe flake was used to construct the phototransistor. Figure 1.10b