Phase-engineered transition-metal dichalcogenides for energy and electronics

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Phases in transition-metal dichalcogenides The three phases of semiconducting transition-metal dichalcogenides (TMDs) are shown in Figure 1.1–6 The thermodynamically stable 2H phase in TMDs is semiconducting and is the trigonal prismatic structure shown in Figure 1a. It is referred to as the 2H phase because the unit cell extends into two basal planes. This convention is also used for monolayered TMDs. The octahedral metal-ion coordinated 1T phase is metallic and is not found in naturally formed minerals because it is recognized as being unstable (Figure 1b). The 1T′ phase is a distorted version of the 1T phase (Figure 1c). An additional 3R phase is found only in bulk compounds and relaxes to the 2H phase upon mild heating.7 Theoretical calculations have indicated that the 1T phase is inherently unstable and cannot be realized, whereas the 1T′ and 2H phases are stable.8,9 Experimentally, however, it has been found that the 1T′ phase is the least stable and relaxes to the 1T phase that, in turn, relaxes to the most stable 2H configuration (see atomic resolution transmission electron microscopy images in Figure 1e–g).4 The 1T phase was realized several decades ago in alkali-metal (Li and K) intercalated TMDs.1,2,10,11 Intercalation of TMDs leads to large expansion of the interlayer spacing such

that solvation and reduction of intercalants lead to exfoliation into individual layers.2,3,10,12–14 These monolayered nanosheets are composed of a large fraction of the 1T phase.5,15 The conversion of the 2H phase to the 1T phase during intercalation is attributed to charge transfer from the alkali atoms to the nanosheets. This additional charge results in density of states at the Fermi level, rendering the material metallic. More specifically, crystal mean field theory shows that the 2H phase of TMDs such as MoS2 is semiconducting because of symmetry-induced splitting of the Mo 4d orbitals into three groups: the completely occupied Mo 4dz2 orbital; Mo 4dxy and Mo 4dx2– y2; and Mo 4dxz and Mo 4dyz that are unoccupied. The S 3p states do not influence the electronic structure of the material, as they are located approximately 3 eV away from the Fermi level. In the case of the 1T phase, the Mo 4d orbitals split into two groups: three degenerate Mo 4dxy,yz,xz orbitals occupied by two electrons; and the unoccupied Mo 4dz2 and Mo 4dx2– y2. The incomplete occupation of the Mo 4dxy,yz,xz orbitals renders the 1T phase metallic but also makes it unstable. Completing the occupation of the Mo 4dxy,yz,xz orbitals via additional electrons from dopants stabilizes the 1T phase but destabilizes the 2H phase, thus allowing phase conversion to occur.9,16,17

Manish Chhowalla, Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, USA; [email protected] Damien Voiry, Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, USA; [email protected] Jieun Yang, Department of Materials Science and Engineering, Rutgers, The State University of New Jersey, USA; juliay