2D Inorganic Materials beyond Graphene

The following sections are included:

• Preface
• About the Authors
• Contents

The importance of two-dimensional (2D) materials needs to be recognised in a big way due to the discovery of graphene. It is noteworthy that research on 2D inorganic layered materials in the last few years has shown that these materials are equally fascinating, exhibiting several novel properties and phenomena. Especially, noteworthy is the variety of fascinating properties of single and few layers of MoS₂ and related dichalcogenides. WTe₂ exhibits the largest magnetoresistance recorded in materials. Then there are topological insulators such as Bi2Se3. Besides chalcogenides, there are materials such as boron nitrides and Borocarbonitrides with several interesting features. This chapter presents a brief review of the various 2D materials of interest today.

In this chapter, we review the topological aspect of valleytronics in the two- and three-dimensions. We first take an example of the honeycomb lattice, where K and K′ points act as the valley degree of freedom. The valley-selective optical absorption known as the circular dichroism occurs when we apply a circularly polarised light into the honeycomb system. We show that the K and K′ points have the opposite Chern numbers. The K and K′ points shift and annihilate for highly anisotropic honeycomb lattices. We also review the basic of the Weyl semimetal, where the Weyl and anti-Weyl points act as the valley-degrees of freedom. The Weyl and anti-Weyl points have the opposite monopole charges in the momentum space. Accordingly, the Weyl and anti-Weyl points will never be gapped out without a pair annihilation of them.

This chapter describes the recent developments in the area of two-dimensional, layered compounds for oxygen reduction reaction (ORR). After a brief introduction to the ORR, the thermodynamics involved and the mechanistic details of the reaction are given, followed by the use of layered materials as catalysts. The catalytic activity of graphene and its analogues such as silicene, chalcogenides, carbides, borides, borocarbonitrides and layered double hydroxides are discussed. Towards the end, a summary is given along with possible way forward.

Among the two-dimensional (2D) materials, phosphorene is very attractive as an alternative to graphene and transition metal dichalcogenides for use in future electronic devices, due to its finite band gap and high carrier mobility. A remarkable anisotropy in the crystal and electronic structure of phosphorene is associated with charge redistribution in the intra- and inter-layer regions. A suitable in-plane strain can make phosphorene a potential photocatalyst, and also tune the anisotropy in electrical conductance. The mutually orthogonal directions of prominent electrical and thermal transport make phosphorene a promising material for application in thermoelectric energy conversion. The normal insulating state transforms to topological insulating state through band-inversion upon application of external electric field.

The isolation of graphene and subsequently other two-dimensional (2D) materials like transition metal dichalcogenide’s (TMDCs) and topological insulators have led to the discovery of a plethora of new fundamental phenomena and concepts in atomically thin materials. While single layer of graphene has no bandgap and can host high mobility carriers, TMDCs show a thickness dependent bandgap, but respond sensitively to optical illumination. It is however, often desirable, to combine these contrasting individual material properties for enhanced functionality. An emerging method to achieve this is called the van der Waals epitaxy, where atomic and/or molecular layers of different 2D materials are physically stacked on top of each other in appropriate sequences. In this chapter, we have reviewed various aspects pertinent to 2D van der Waals hybrids. In Sec. 5.2, we introduce some of the methods of fabricating such heterostructures, and in Sec. 5.3 we discuss about characterising them. In Secs. 5.4–5.6, we discuss several electrical, optical and thermal phenomena related to these hybrids. In Sec. 5.7, we review some of the emerging phenomena in these systems. The van der Waals hybrids seem to hold great promise in the context of both unanticipated fundamental discoveries and device applications, and we are still only at the incipient stages.

Nearly, ∼65% utilised energy being lost as waste heat. Thermoelectric materials can directly and reversibly convert heat to electrical energy, thereby thermoelectrics has an important role to play in the future energy management. Recently, layered metal chalcogenides has gained a momentous attention in the field of thermoelectrics as some of them exhibit record high values of thermoelectric figure of merit due to their intrinsically low thermal conductivity. For example, layered Bi₂Te₃ and its alloys are known for their high thermoelectric performance near room temperature. Mention must be made that remarkably high thermoelectric figure of merit of ∼2.6 has been recently achieved in layered SnSe due to its ultra-low thermal conductivity. Herein, we discuss about the synthesis, structure and thermoelectric properties of important layered chalcogenides.

The growth of 2D materials using plasma chemical and vapour deposition techniques is reviewed in this chapter. We first explain the growth mechanisms of single- and few-layer graphene, and graphene nanopetals in plasma chemical vapour deposition systems. Thereafter, physical vapour depositions of h-BN by pulsed laser deposition and MoS₂ using magnetron sputtering are described. These plasma systems provide many benefits including low growth time, low growth temperatures, high growth rate, and continuous morphology of the deposited films. Moreover, an overview of in situ diagnostic methods in plasma systems is covered, including optical emission spectroscopy, plasma imaging, laser absorption spectroscopy, and coherent anti-Stokes Raman spectroscopy. These diagnostics methods are applied to understand, monitor and control the growth of 2D materials in a plasma environment. Overall, plasma systems are projected to be efficient and effective in mass production of 2D materials for existing and future applications.

The performance of transistors based on 2D transition metal dichalcogenides such as MoS₂ is significantly limited by large contact resistance at the metal-MoS₂ interface. In this chapter, we present a comprehensive review of metal-MoS₂ contact interfaces and recent progress made towards realising low contact resistance on MoS₂. Review of metal-semiconductor contact and Fermi-level pinning theory is followed by an analysis of contact resistance and Schottky barrier height data, along with methods used for their extraction, for different work function metals on MoS₂ as collated from several experimental and simulation studies. Reduction in contact resistance towards its quantum limit using conventional (doping and interfacial layers) and unconventional (different contact geometries) methods, and different Fermi-level pinning mechanisms responsible for n-type conduction in MoS₂ have also been discussed and compared in detail.

One of the fascinating properties of the new families of two-dimensional (2D) crystals is their high stretchability, which gives rise to possibility of using external strain to manipulate their optical and electronic properties in a controlled manner. The objective of this chapter is to give an overview of the recent progresses made in controlling the electronic, optical and vibrational properties of 2D transition metal dichalcogenides (MX₂, where M = Mo, W and X = S, Se, Te), by means of strain engineering. We will discuss the electronic origin of strain induced semiconductor-metal transition in these 2D materials. We will present the role played by Raman spectroscopy in mapping out the changes in vibrational properties of these 2D materials under applied strain. The effect of strain induced changes in electronic structure on controlling the transport properties of these materials will also be covered. Finally, we will review the recent experimental results, which verifies some of the theoretical predictions and show the immense potential of strain engineering in the range of applications.

In contrast to graphene, two-dimensional (2D) materials such as h-BN and TMX₂ exhibit a non-zero band-gap which makes them suitable for applications in FETs. It is important to understand the nature of defect in these materials, and how they influence their properties. Here, we present analysis of point defects, interfaces and planar defects in these 2D materials and highlight the impact of these defects on electronic and vibrational properties. We discuss carbon doping in h-BN, which is interesting for engineering its electronic structure and band-gap through both compositional and configurational modification. We show that the planar structure h-BN with grain boundaries (GBs or interfaces) exhibits instability with respect to buckling (out-of-plane deformation), resulting in the formation of a wrinkle at the GB and long lengthscale rippling of the structure. We present the electronic and vibrational signatures of these interfaces and STM images of the most stable interface, which should facilitate their experimental characterisation. Planar defects (faults) essentially give different polymorphs of materials such as TMX₂ compounds (M = (Mo,W); X = (S, Se)), whose monolayer consists of 2 or 3 atomic layers. Introduction of these planar faults leads to transition from insulating 1H structure to metallic c1T structure. We present the energy landscape associated with the fault or 1H to 1T phase transition, leading to prediction of novel 1T structures and intermediate structures associated with structural instabilities of the 1T structure. The metallic centrosymmetric 1T (c1T ) structure of these compounds is shown to be unstable with respect to dimerisation or trimerisation of transition metal atoms, leading to a competing metallic $\sqrt{3}\times 1$ 1T form and ferroelectric semiconducting $\sqrt{3}\times \sqrt{3}$ 1T form respectively. While the former is a more stable 1T form of MoSe₂, WS₂ and WSe₂, the latter is a more stable 1T form of MoS₂ exhibiting rich ferroelectric dipolar domain structure. Due to vicinity to such metal-semiconductor transitions, their semiconducting forms are shown to exhibit anomalous response to electric field. To facilitate experimental verification of these subtle features of 1T forms of MX₂ monolayers, we present comparative analysis of their vibrational properties, and identify their Raman and Infra-Red spectroscopic signatures.

The following section is included:

• Index