**2. Basic concept about 2D materials**

2D materials, also known as 2D topological materials or single‐layer materials, can be defined as crystalline materials with a single layer of atoms or chemical compounds. These materials are substances with a thickness of a few nanometres or less (one or two atoms thick). The extraordinary properties of these 2D materials have opened new opportunities for scientists and engineers to increase the research activities in this novel engineering materials field [4]. Electronic transport is realized in the 2D plane of the material, where electrons move freely [2]. In addition, electrons have restricted motion in the third direction, due to which this phenom‐ enon is governed by quantum mechanics. They can be constituted by 2D allotropes of diverse chemical elements or compounds. Actually, graphene is the most researched 2D material till now. In addition, they can be classified as elemental, compounds, or van der Waals hetero‐ structures. The elemental 2D materials use the ‐*ene* suffix in their names, for example, graphene, germanene, borophene, silicene, stanene, and phosphorene. Compounds consist of two or more covalently bonded elements and carry the ‐*ane* or ‐*ide* suffixes in their names, for example, graphane, hexagonal boron nitride (*h*BN), germanane, chalcogenide, Molybdenum disulfide (MoS2), and tungsten diselenide (WSe2). Van der Waals heterostructures are constituted of layered combinations of different 2D materials and as examples they are presented as MXenes (few atoms thick layers of transition metal carbide or carbonitrides) or organic compounds, such as Ni3(HITP)2, where HITP is 2, 3, 6, 7, 10, 11‐hexaaminotriphenylene.

combinations. Until now, many papers presenting reviews related to 2D materials have been presented [1–5]; however, a direct comparison of these materials for electronic applications is necessary. This study will allow us to know the advantages and disadvantages of 2D materi‐ als for electronic applications. A clear trend related to the choice of these materials for deter‐ mined applications must be established. In this context, a comparison of the physical properties of these materials is used to exploit them from several technical points of view. Moreover, the possible synergy between 2D materials is presented as a strategic way to exploit these materi‐ als completely in more complex applications such as the development of hybrid or multifunc‐

104 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

Several 2D materials, such as carbon‐based 2D materials, silicate clays, transition metal dichalcogenides (TMDs), and transition metal oxides (TMOs), have been used in electronic devices. Particularly, materials such as graphene, molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum trioxide (MoO3), and silicon carbide (SiC) provide enhanced physical and chemical functionality making use of uniform shapes, high surface‐to‐volume ratios, and surface charge [1–5]. While dichalcogenides and buckled nanomaterials have sizeable band gaps, graphene has zero band gap and they also become semiconducting or metallic materials. These materials are very sensitive to the number of layers, ranging from indirect band‐gap semiconductor in the bulk phase to direct band‐gap semiconductor in monolayers. 2D materials are leading to ubiquitous flexible and transparent electronic systems for applications in integrated circuits, solar cells, and storage energy [2, 4]. Comparison of the

performance in electrical and optical properties of 2D materials is presented here.

presented. Finally, conclusions about the work are given in Section 7.

**2. Basic concept about 2D materials**

A few decades ago, the potential of the electronics industry depended entirely on silicon. New materials such as carbon allotropes of the groups III, IV and V are being introduced to increase efficiency, specific capacity, and speed of information processing. Actually, in electronics, 2D materials are used in the manufacture of supercapacitors, batteries, field‐effect transistors (FETs), solar cells, light‐emitting diodes, transparent electrodes, coatings for electrostatic dissipation, and/or electromagnetic interference shielding, etc. The potential of the 2D materials has not been fully discovered yet; however, new potential applications are being invented and others are emerging from the laboratory, which are beneficial for the develop‐ ment of materials science and engineering. The use of 2D materials in the electronic industry will be extended in the design of new electronic devices being applied either individually or as a component within a composite, hybrid, or functional material. This chapter has been divided as follows: Section 2 introduces basic concepts about 2D materials. Graphene and its derivatives are studied in Section 3. Section 4 analyses different allotropes based on chemical elements with the exception of carbon. 2D materials such as hexagonal boron nitride (*h*BN), TMDs, and MXenes are discussed in Section 5. In Section 6, optoelectronic applications are

2D materials, also known as 2D topological materials or single‐layer materials, can be defined as crystalline materials with a single layer of atoms or chemical compounds. These materials

tional materials.

The first 2D material discovered was graphene in 2004, which was isolated from graphite. Since 2010, graphene is not alone. Since this success a large amount of research has been realized to isolate other 2D materials due to their remarkable chemical, electronic, optical, and mechanical properties. 2D materials possess exceptional properties, since they are strong, lightweight, flexible, and good conductors of heat and electricity. 2D materials exhibit diverse electronic properties, ranging from insulating, semiconducting to semimetallic properties [3]. Some 2D materials are nearly optically transparent. Such properties proceed from the variations in structure (tunable architectures) as a consequence of changing metal centres [6]. Despite great advances achieved until now, none of these materials has been used for large‐scale commercial applications with the possible exception of graphene. Their main areas of applications are electronics and optoelectronics, sensors, biological engineering, filtration (water purification), lightweight/strong composite materials, photovoltaic systems (electrodes), medicine, quan‐ tum dots, thermal management, ethanol distillation, and energy storage [3, 7].

Optical and electronic properties of the 2D materials are completely different from those of the bulk materials due to confinement of electrons and absence of interlayer interactions, which play an important role in the band structure presented in these materials [6]. On the other hand, mechanical and chemical properties are obtained thanks to geometric effects (chirality, dilution, defects, etc.) and to the high surface‐volume ratio (it trends to infinity in the thinnest materials) [8]. Graphene and TMDs are building blocks for optoelectronic applications in areas such as ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges [3, 7].

Three different strategies have been developed to achieve a finite band gap in 2D materials: (1) chemical modification, (2) deposition on substrates, and (3) application of 2D chemical compounds [1–5]. Each of the above strategies has a direct influence on the properties and applications of each of the different types of 2D materials that have been discovered. Section 3 studies graphene‐based materials; the 2D allotropes of groups III, IV, and V, with the exception of the carbon (C), are analyzed in Section 4; the *h*BN, 2D TMDs, and MXenes are discussed in Section 5, and finally, the optoelectronic applications are analyzed in Section 6.
