Two-dimensional layered transition-metal dichalcogenides for versatile properties and applications
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Introduction
The bonds
There are a wide variety of materials that exhibit layered structures. These materials (e.g., graphite, boron nitride, and molybdenum disulfide) lend themselves very well to being mechanically cleaved along the layers, which led to the utilization of these materials as dry lubricants.1 The last 10 years have shown us that these materials are also incredibly interesting for electronic and optoelectronic applications.2,3 By continuous cleaving of these layered materials, one can readily thin them down to a single layer,4,5 which results in the realization of new properties different from the bulk. These single layers, commonly known as two-dimensional (2D) materials, are receiving significant consideration due to their unique and tunable material properties:2 (1) They have no surface dangling bonds. (2) They are an ideal quantum well with wide-ranging direct and indirect energy bandgaps. (3) They can be superconducting or magnetic. (4) They can have enhanced spin–orbit coupling for applications in spin-based or valley-based electronics (i.e., use of the spin or momentum [valley] of an electron to encode the digital logic state). The unique properties of transition-metal dichalcogenides (TMDCs) are now being engineered to solve fundamental scientific and technological challenges.
TMDCs are a subset of the layered materials family, where the intra-layer bonding is covalent and the inter-layer bonding consists of van der Waals (vdW) forces. This type of bond structure leads to highly anisotropic properties, where in-plane mechanical, thermal, and electrical properties significantly outperform the out-of-plane properties.6 Within a single plane, there are two types of crystal structures that dictate the electronic properties of the layers—trigonal prismatic (2H) and octahedral (1T). The trigonal prismatic structure is thermodynamically favorable for the majority of Mo and W-based TMDCs and leads to semiconducting behavior in the layers, while the octahedral phase is more stable for Hf and Zr-based TMDCs.7 While Hf and Zr-based TMDCs are semiconducting, many octahedrally bound TMDC layers (for example, 1T-TaS2) exhibit metallic or semi-metallic behavior, and each phase (trigonal or octahedral) can be transformed to the other via atomic gliding of the layers.
A brief history Naturally occurring TMDCs (specifically MoS2) have been around for billions of years,8 but scientific publications on
Eric M. Vogel, Georgia Institute of Technology, USA; [email protected] Joshua A. Robinson, Materials Science and Engineering Department, The Pennsylvania State University, USA; [email protected] DOI: 10.1557/mrs.2015.120
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MRS BULLETIN • VOLUME 40 • JULY 2015 • www.mrs.org/bulletin
© 2015 Materials Research Society
TWO-DIMENSIONAL LAYERED TRANSITION-METAL DICHALCOGENIDES FOR VERSATILE PROPERTIES AND APPLICATIONS
synthetic TMDCs did not begin to appear until the 1950s.9 The first reports of MoS2 synthesis were from the reduction of molybdenum trisulfide (MoS3)9,10 and high temperature solution-ba
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