Exfoliated and Restacked MoS₂ and WS₂: Ionic or Neutral Species? Encapsulation and Ordering of Hard Electropositive Cations

Abstract

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 a × a 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.