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Two-dimensional Transition Metal Dichalcogenides: A General Overview

Chi Sin Tang1,2 and Xinmao Yin2

1National University of Singapore, Singapore Synchrotron Light Source (SSLS), 5 Research Link, Singapore, 117603, Singapore

2Shanghai University, Shanghai Key Laboratory of High Temperature Superconductors, Shanghai Frontiers Science Center of Quantum and Superconducting Matter States, Physics Department, 99 Shangda Road, Shanghai, 200444, China

1.1 Introduction to 2D-TMDs

Since graphene was first exfoliated from graphite using the mechanical cleavage method [1], 2D materials have garnered widespread interest. Atomically thin 2D-TMDs, with a formula of MX2 (M: transition metal atom; X: chalcogen atom), form a diverse class of 2D materials with about 60 members. While one can trace back the extensive studies on bulk and multilayer TMD materials to more than half a century ago [2], it was only the groundbreaking emergence of graphene of single-atom thickness [1, 3] that led to the tremendous progress of monolayer van der Waals systems within the last two decades. With unique optoelectronic properties and robust mechanical features, 2D-TMD is a class of low-dimensional materials ideal for multiple applications in areas such as electronics, optoelectronics, and valleytronics [4-6]. They surpass graphene in terms of their functionality due to the combination of their non-zero bandgap electronic structures and their pristine yet robust layered surface properties. 2D-TMD is also a favorable class of materials in practical applications related to field-effect-transistor (FET) based systems. Hence, extensive research studies have taken center stage over the past decade to uncover both the fundamental physical properties and to unleash new frontiers for possible 2D-TMD-based device applications.

At the molecular level, diverse variations to the chemical bonding and crystal configurations of the transition metal atom component in 2D-TMDs have led to multiple structural phases that possess unique electronic properties. The semiconducting 1H phase is a quintessential example. Structural changes to one of the chalcogen planes will result in the metallic 1T phase. In addition, a unique quasi-metallic 1T'phase arises due to its distorted sandwich structure, where an array of one-dimensional zigzag transition metal chains are formed [7, 8].

1.2 Crystal Structures of 2D-TMDs in Different Phases

TMDs present various structural polymorphs which have attracted huge interest in the last decade, both as an ideal platform for the fundamental study of layered quantum systems and their potential for multiple applications. Structurally, a unit layer is made up of a transition metal layer sandwiched between two chalcogen layers. Interestingly, TMDs, whether in the mono or multi-layer form, manifest themselves in different structural phases arising due to different configurations of the transition metal atom component. The common polymorphs of 2D-TMDs are the trigonal prismatic 1H phase and the octahedral 1T phase. In the case of the octahedral 1T phase structure, it has been experimentally and theoretically shown that it is dynamically unstable under free-standing conditions [7-9]. Consequently, similar to the Peierls distortion, the 1T phase will relax and buckle spontaneously into a thermodynamically more stable distorted structure known as the 1T´ phase [7-9]. Hence, 1T phase 2D-TMDs can further stabilize under favorable chemical, thermal and mechanical conditions [10], particularly into the 1T´phase.

To better understand the diverse structural properties of 2D-TMDs, the respective structural phases can be visualized by the stacking configurations of the three atomic planes (i.e. the X-M-X structure). The 1H phase corresponds to an ABA stacking configuration where the chalcogen atoms at the top and bottom atomic planes are in the same vertical position and are located on top of each other in a direction perpendicular to the layer (Figure 1.1a). In contrast, the 1T structural phase has an ABC stacking configuration displayed in Figure 1.1b. Since the 1T phase structure is unstable under freestanding conditions, it will buckle and distort into the 1T´ structure where the transition metal atoms sandwiched between the upper and lower atomic layers distort. Consequently, it forms a period doubling 2?×?1 structure. As viewed from the top, this structural phase consists of an array of 1D zigzag transition metal chains (Figure 1.1c). Indeed, recent investigations related to symmetry-reducing CDW properties [12] in metallic 1T phase 2D-TMDs have created new opportunities for integrated low-dimensional material-based applications, including transistor systems, nanoscale charge channels and gate switching devices [13-15]. Besides, 1T´ phase 2D-TMDs are known to possess anisotropic electronic and optical features. An in-depth understanding of its unique structure can bring new insights to its characteristics, which can then be exploited for directionally regulated charge or photon channel applications in optoelectronics and electronics.

As discussed thereafter, electronic structure calculations have indicated that while the 1T phase 2D-TMD is metallic [10, 16], the 1T´ phase counterpart possesses a unique quasi-metallic electronic structure that will be discussed later. To clearly distinguish between these two structural phases, note that while low-temperature charge density wave (CDW) phases are typically observed in 1T phase TMD systems (e.g. TCDW ~120?K for TaSe2 and TCDW~35?K for NbSe2 [17, 18]), a CDW-like lattice distortion in the form of periodic 1D zigzag chain structure unique to the 1T´phase 2D-TMD can be observed even at room temperature [7, 8]. Morphologically, while the low-temperature commensurates CDW in 1T phase, TMDs can be distinguished by a unique star-of-David superlattice [19-21] typically characterized using scanning tunneling microscopy. Conversely, the 1D zigzag chains in the 1T´ phase are clearly distinguished via high-resolution transmission electron microscopy.

Figure 1.1 Lattice structures of 2D-TMDs in the (a) trigonal prismatic (1H), (b) octahedral (1T), and (c) distorted (1T´) phases. Stacking configuration of the atomic planes have been indicated. (d) The stacking orders that distinguish between the 1T´ and the 1Td phases. The red dashed boxes serve as visual guides. Source: Tang et al. [11]/With permission of AIP Publishing.

1.2.1 Other Structural Phases

Apart from the three common structural phases, TMDs are also present in other structural phases each with their unique optical and electronic properties. For example, MoTe2 and WTe2 would undergo a first-order phase transition from the monoclinic 1T´ phase to form the orthogonal 1Td phase as temperature decreases. While the structures of both the 1T´ and 1Td phase are rather similar, their key differences lie with the dislocations between the stacking layers depicted in the layer distortion in Figure 1.1d. These seemingly trivial dislocations can lead to a significant symmetry change between the two structural phases [22, 23]. Nevertheless, with a similar structure present in the respective layer, 1Td phase TMDs still possess similar quasi-metallic electronic properties as that of its 1T´ phase counterparts [9, 22, 23].

Figure 1.2 Atomic structures of the 1T?-phase 2D-TMD with the atomic planes indicated. Source: Reproduced with permission from Ma et al. [24]. Copyright 2016, The Royal Society of Chemistry.

In the monolayer regime, apart from the formation of the 1T´ structural phase due to distortion from its 1T phase counterpart, other stable polymorphs have also been reported. This includes another octahedrally coordinated 1T? phase (Figure 1.2) [25]. While there is an association between the transition metal atoms in the M-M configuration in the 1T´ and 1T? distorted phases, the transition metal atoms are dimerized in the 1T´ phase while trimerization takes place in the 1T? phase [26]. Besides, while the 1T´ phase possesses quasi-metallic electronic properties, computational studies have suggested that the 1T? phase is a wide bandgap semiconductor where 1T? phase monolayer-MoS2 possesses an indirect bandgap of ~0.27?eV [24]. Nevertheless, no consensus has been reached in terms of the relative stability between the octahedral phases, particularly for monolayer-MoS2, where different studies have reported different stability levels between the structural phases [24, 27, 28].

1.2.2 Phase Stability

Theoretical studies have shown that the 1H-1T´ energy differences vary between different Mo- and W-based 2D-TMD...