Stable, Single-Layer MX₂ Transition-Metal Oxides and Dichalcogenides in a Honeycomb-Like Structure

Abstract

Recent studies have revealed that single-layer transition-metal oxides and dichalcogenides (MX 2 ) might offer properties superior to those of graphene. So far, only very few MX 2 compounds have been synthesized as suspended single layers, and some of them have been exfoliated as thin sheets. Using first-principles structure optimization and phonon calculations based on density functional theory, we predict that, out of 88 different combinations of MX 2 compounds, several of them can be stable in free-standing, single-layer honeycomb-like structures. These materials have two-dimensional hexagonal lattices and have top-view appearances as if they consisted of either honeycombs or centered honeycombs. However, their bonding is different from that of graphene; they can be viewed as a positively charged plane of transition-metal atoms sandwiched between two planes of negatively charged oxygen or chalcogen atoms. Electron correlation in transition-metal oxides was treated by including Coulomb repulsion through LDA + U calculations. Our analysis of stability was extended to include in-plane stiffness, as well as ab initio, finite-temperature molecular dynamics calculations. Some of these single-layer structures are direct-or indirect-band-gap semiconductors, only one compound is half-metal, and the rest are either ferromagnetic or nonmagnetic metals. Because of their surface polarity, band gap, high in-plane stiffness, and suitability for functionalization by adatoms or vacancies, these single-layer structures can be utilized in a wide range of technological applications, especially as nanoscale coatings for surfaces contributing crucial functionalities. In particular, the manifold WX 2 heralds exceptional properties promising future nanoscale applications.

Atomic defect states in monolayers of MoS2 and WS2

“…2. Accordingly, our method is not only able to clearly demonstrate the electronic band structure of defective MoS 2 and WS 2 monolayers, but also is very computationally affordable and can be easily generalized to study very large systems with a random distribution of single defects and other types of vacancies such as MoS double vacancies, MoS 2 triple vacancies and antisite defects 2,18 .…”

“…Since the model presents an accurate description for the band structure of LTMDs, the application of this model to defective MoS 2 and WS 2 provides a more realistic understanding of the electronic states contributing to the process of vacancy formation and the accurate location of defect states within the bandgap. Moreover, the optimized geometries of the monolayers obtained by ab initio calculations have demonstrated that atomic vacancies do not cause a considerable geometry deformation and the neighboring atoms around the vacancies do not show any visible displacement 2,22 . Therefore, the defect-induced deformation is ignored.…”

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

“…Many TMDCs have band structures that are similar in their general features, as shown by first principles and tight-binding approximations 25,[80][81][82][83][84][85] and measured using a variety of spectroscopic tools 31,32,[86][87][88][89] . In general, MoX 2 and WX 2 compounds are semiconducting whereas NbX 2 and TaX 2 are metallic 25,[80][81][82][83][84] .…”

“…In general, MoX 2 and WX 2 compounds are semiconducting whereas NbX 2 and TaX 2 are metallic 25,[80][81][82][83][84] . The band structures of bulk and monolayer MoS 2 and WS 2 calculated from first principles are shown in Fig.…”

High-energy collective electronic excitations in layered transition-metal dichalcogenides

“…This stimulated the search for other families of semiconducting layered materials. Among them, transition metal dichalcogenides (TMD) [7] are probably the materials with the most versatile electronic properties [8,9]. Their chemical formula is MX 2 , where M is a transition metal and X a chalcogen (S, Se, and Te).…”