Ordering and manipulation of MoS2 platelets on differently charged micas by atomic force microscopy

Abstract

Atomic force microscopy (AFM) has been used to investigate how dispersed MoS₂ platelets (from colloidal suspensions) deposit onto three natural (Muscovite) mica (001) surfaces. Different platelet arrangements are observed, which are attributed to defects and charging effects of the mica (as well as the concentration of starting colloid). The provenance of the mica influences the self-organisation of the platelets into long tape-like assemblies (South Dakota mica) or individual flakes (Alps mica). Atomic scale imaging of the tapes reveals a distorted octahedral (O_h)-based local structure, different from the trigonal prismatic structure found in the 2H-polytype of annealed MoS₂, in agreement with previous structural results on water-dispersed MoS₂ platelets. The buckling and susceptibility to stripping of the tapes is ascribed to the presence of a water layer between the substrate mica and MoS₂, and after stripping by the tip, the platelets ultimately form small clusters. The ordering of these clusters depends not only on the defect and charge structure of the mica, but also on complex hydration reactions between the H₂O layer associated with the MoS₂ and K ⁺ ions of the mica. Relatively symmetrical squares may be lifted out of the tapes, supporting the presence of weak bonding between tape and mica. Conversely, Alps and Bihar micas give rise to separate platelets, which are resistent to tip manipulation, which is attributed to hydrophobic interaction between MoS₂ platelets and the mica surface.