Exfoliated and restacked MoS₂ and WS₂: Ionic or neutral species? Encapsulation and ordering of hard electropositive cations

The relationship between charge and structure in restacked MS₂ (M = Mo, W) has been probed by cation encapsulation and chemical oxidation and characterized by elemental analysis, electron diffraction, X-ray diffraction, and Differential Scanning Calorimetry. Alkali cations have been encapsulated in MoS₂ and WS₂ without the presence of a co-intercalated counterion, suggesting a negative charge in the 0.15-0.25 electrons per M atom range. Electron diffraction studies show ordering of these cations between the layers. Chemical oxidation with I₂ or Br₂ results in a change in the structure of restacked MoS₂, giving rise to a √3a x √3a superlattice, whereas no change is observed in the structure of restacked WS₂. Differential Scanning Calorimetry studies show that the irreversible exothermic transition to 2H-MS₂ shifts to lower temperature with oxidation. The observed structural distortion and residual negative charge uniquely explain the thermopower measurements, which indicate that restacked MoS₂ and WS₂ are p-type metallic conductors.