Low-dimensional transition-metal dichalcogenides

Agnieszka Kuc

DOI: 10.1039/9781782620112-00001

1 Introduction

Nanomaterials form a field of materials science, which is devoted to the production and properties of systems with at least one dimension at the nanometre scale. If any of the dimensions is restricted, layered 2D materials are formed; if restrictions appear in two dimensions, one obtains 1D polymer-like systems; and finally, if all three dimensions are scaled down to the range of only few nanometres, 0D clusters or nanoflakes are in subject. These considerations are particularly applicable to the case of carbon, where 3D graphite can be exfoliated down to the 2D graphene monolayers (MLs), which in turn can be rolled up to form 1D nanotubes (NTs) or 0D fullerenes (see Fig. 1). Each of these sp2 carbon allotropes exhibits very different physical properties, especially the electronic structure differs significantly between those allotropes. For example, the parabolic dispersion relation in graphite's band structure – resulting in a zero band gap – changes to linear band behaviour in graphene, where it is described by massless Dirac fermions. On the other hand, NTs can either be metallic or semiconducting, depending on the size and chirality. Fullerenes are always insulators with a large finite band gap, independent of size and shape. Among these carbon nanomaterials, graphene research has been developing extremely fast ever after the successful separation from bulk graphite, what led to the Nobel Prize for Novoselov and Geim in 2010.

Low-dimensional nanomaterials are important in many fields of research and technology. Some examples cover silicon-based semi-conductor devices, optical coatings, micro-electromechanical systems, biomedical research, lasers and electro-optics. Recently, they became extremely interesting as building blocks of next-generation devices for (opto-)electronic applications. As modern electronic devices are strongly miniaturized (nanoscale), several problems start to occur. Traditional electronics with silicon-based field-effect transistors (FETs) often suffers from heat dissipation. At this scale, also quantum effects become very important. To overcome problems of silicon-based technology at nanoscale, one could replace it with nanomaterials that perform better at atomic scale.

In the world of 2D materials, graphene has gained enormous attention, especially for its applications in nanoelectronics. High electron mobility, long-distance spin-transport, or exceptional mechanical properties of graphene are very attractive. Graphene has a potential as a spin-conserver system and it is attractive for spintronic applications. However, weak spin–orbit coupling and zero-band gap disregard graphene as switching material in chargeand spin-based transistors.

These difficulties can be overcome in the semiconducting 2D materials. After the discoveries of CNTs and graphene, other layered and corresponding tubular materials have gained considerable attention. The successful methodologies and knowledge gained in the search for graphite monolayers and CNTs have been extended to other inorganic materials. Though graphene is presently a cutting-edge system, it opens up a variety of new possibilities going beyond the limits of its own properties and applications.

Many materials exist in the layered 3D bulk forms, which can be easily confined to lower dimensions resulting in single layers or tubular structures. Among them, the most known are boron nitride, transitionmetal chalcogenides (TMCs), TX2 (T–Mo, W, Nb, Re, Ti, etc.; X–S, Se, Te), halides (Cl, Br, I), or oxides. Layered 3D TMCs of TX2 type have been extensively studied on experimental and theoretical bases for the last 50 years. There is a huge number of theoretical works on various properties of the TMC layered materials reported to date in the literature. Some of the possible elemental compositions of layered TMCs are schematically shown in Fig. 2.

Weak non-covalent interactions between the adjacent sheets and the anisotropic character of TMC structures result in easy shearing of the layers even under high pressure, leading to very good lubricant properties. Other applications, such as catalysis, optoelectronics and photovoltaics, have been proposed and investigated for this family of compounds. However, it was only in 2011, when TMCs have started their renaissance as potential materials for nanoand opto-electronics after seminal works of Nicolosi and co-workers, and Kis and coworkers. The group of Nicolosi have reported that large-area single layers of TMC can be easily produced using liquid exfoliation technique. Using such a single layer TMC, the group of Kis have produced the first FET based on MoS2-ML (see Fig. 3). Pioneering measurements of this MoS2-ML-based device have shown that at room temperature the mobility is about 200 cm2 V s-1, when exfoliated onto the HfO2 substrate, however, it decreases down to the 0.1–10 cm2 V s-1 range if deposited on SiO2. Various electronic devices have been fabricated based on the MoS2-ML, including thin film transistors, logical circuits, amplifiers and photodetectors.

On the other hand, TMC-NTs have been known for about two decades. In 1992 and 1993, Tenne and co-workers have shown that layered WS2 and MoS2 form, in analogy to carbon, inorganic nanotubes and fullerenelike nanoparticles. TMC-NTs can be produced using, for example, chemical vapour transport technique or by high-temperature annealing of the respective metal trisulphides. TMCs-NTs behave as exceptional lubricants. If the MoS2 NTs or nanoonions are added to base grease, the friction coefficient remains low, even at very high loads. Moreover, MoS2 NTs have been used for catalytic conversion of carbon monoxide and hydrogen into methane and water. These findings are quite unexpected, as the fully bonded sulphur atoms in the TMC surfaces are not expected to be chemically active. Their electronic properties are very intriguing, as depending on the chirality, they resemble monolayers or bulk forms.

The promising use of TMC low-dimensional materials as building blocks in nanoelectronic devices calls for detailed investigations of their physical properties. Therefore, in the...