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 MS 2 (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 MoS 2 and WS 2 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 I 2 or Br 2 results in a change in the structure of restacked MoS 2, giving rise to a a × a superlattice, whereas no change is observed in the structure of restacked WS 2 . Differential Scanning Calorimetry studies show that the irreversible exothermic transition to 2H-MS 2 shifts to lower temperature with oxidation. The observed structural distortion and residual negative charge uniquely explain the thermopower measurements, which indicate that restacked MoS 2 and WS 2 are p-type metallic conductors.