Fabrication and study of 2D materials – transition metal dichalcogenides
The transition metal dichalcogenides are a class of materials with the formula MX2, where M is a transition metal element from group IV (Ti, Zr, Hf, and so on), group V (for instance V, Nb, or Ta) or group VI (Mo, W, and so on), and X is a chalcogen (S, Se, or Te). These materials have crystal structures consisting of weakly coupled sandwich layers X–M–X, where a M-atom layer is enclosed within two X layers and the atoms in layers are hexagonally packed (Fig. 1). Adjacent layers are weakly held together by van der Walls interaction to form the bulk crystal in a variety of polytypes, which vary in stacking orders and metal atom coordination. The overall symmetry of transition metal dichalcogenides is hexagonal or rhombohedral, and the metal atoms have octahedral or trigonal prismatic coordination.
Fig. 1 Three-dimensional schematic representation of a typical MoS/Se2 structure, with the chalcogen atoms (X) in yellow and the metal atoms (M) in blue.
Belonging to the family of layered transition metal dichalcogenides, molybdenum disulfide (MoS2) has been widely used in numerous areas, such as hydrodesulfurization catalyst, photovoltaic cell, photocatalyst, nanotribology, lithium battery, and dry lubrication, due to their distinctive electronic, optical, and catalytic properties. Bulk MoS2 is a semiconductor with an indirect bandgap of 1.2 eV. The monolayer MoS2 has recently attracted great interest because of its potential applications in two-dimensional nanodevices, although it had been obtained and studied in the past several decades. The monolayer MoS2 is a direct gap semiconductor with a bandgap of 1.8 eV, and can be easily synthesized by using scotch tape or lithium-based intercalation. The mobility of the monolayer MoS2 can be at least 200 cm2V-1s-1 at room temperature using hafnium oxide as a gate dielectric, and the monolayer transistor has the room temperature current on/off ratios of 1 x 108 and ultralow standby power dissipation. The experimental achievements triggered the theoretical interests on the physical and chemical properties of two-dimensional MoS2 nanostructures to reveal the origins of the observed electrical, optical, mechanical, and magnetic properties, and guide the design of novel MoS2-based devices.
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