Advanced 2D Materials

Ashutosh Tiwari, Mikael Syväjärvi, Ashutosh Tiwari, Mikael Syväjärvi

• Verlag: Wiley-Scrivener
• Jahr: 2016
• ISBN:
  9781119242802
• Auflage: 1
• Thema: Technik & Maschinenbau
• Thema: Werkstoffwissenschaft

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

Part 1

SYNTHESIS, CHARACTERIZATIONS, MODELING AND PROPERTIES

Chapter 1

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

Wenjing Jie^1,2 and Jianhua Hao^1*

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 ...

Inhaltsverzeichnis

• Cover
• Half Title page
• Title page
• Copyright page
• Preface
• Part 1: Synthesis, Characterizations, Modeling and Properties
• Part 2: State-of-the-Art Design of Functional 2D Composites
• Part 3: High-Tech Applications of 2D Materials
• Index