The band structures and photoemission of transition metal dichalcogenides

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Abstract
The yields and energy distributions of electrons photoemitted from MoS2, α-MoTe2, β-MoTe2, NbSe2 and ZrS2 have been investigated and the photo-thresholds and work functions established. The results confirm the basic correctness of the Wilson-Yoffe model for the electronic structure of these solids but lead to much wider band gaps for the trigonal prismatic group VIA dichalcogenides. Structure in the photoelectric yield spectra as well as the values of thresholds obtained are consistent with an energy gap separating the d-and s-like conduction bands for the trigonal prismatic compounds, but not for the octahedral materials, in agreement with theoretical predictions. The difference in electronic structure of these two groups is illustrated theoretically by extending the tight binding calculation of Bromley and his collaborators to include hypothetical octahedral non-distorted MoTe2, and the effect of lattice distortion is discussed in the light of experimental results for β-MoTe2. It is concluded that accurate theoretical models for the electronic structure of the transition metal dichalcogenides should be possible with only moderate extensions of existing models.