Assembly and local structure of MoS2 colloidal platelets on quartz and muscovite Mica surfaces

Abstract

Aqueous suspensions of MoS₂ colloidal platelets and their assembly after casting on quartz and cleaved muscovite mica (001) surfaces have been investigated by optical spectroscopy and atomic force microscopy (AFM). Optical spectra are consistent with the presence of non-metallic distorted pseudo-octahedral (MoS₂)_n-based moieties. Features observed between 800–1300 nm in single layers, at low platelet stacking order and when traces of Li⁺ remain (but not in either 2H-MoS₂ or 1T-MoS₂) are assigned to transitions from a split d_x²,yz,xzO_h MoS₂ band to other d levels in a complex (rather than band) formulation. Differences in optical spectra of platelets on (001) mica and on quartz, both as-cast and after annealing at > 120 °C (when the films undergo a phase transition) are ascribed to differences in platelet assembly involving water bilayers (H₂O vibrational combination bands are observed in mica but not in quartz). AFM micro-imaging shows MoS₂ colloidal platelets assemble randomly, including as almost vertical packets (as on γ-alumina) on quartz over a wide concentration range. Instead, on mica (001) surfaces the cast assembly orders as a network of laterally developed tapelike structures (albeit with stacking faults). Single tapes are 200–300 nm wide, ca. 4 nm high and > 3 µm long and probably arise via side-on platelet fixture involving a complex platelet parking mechanism. It is suggested that horizontal dispersion and re-stacking of the platelets is directed by the charged mica surface. After annealing at 200 °C, MoS₂ platelets on mica give single layers of minimum thickness 6.2 ± 0.2 Å, having a₀= 3.2 Å(by AFM) as expected for a 2H-MoS2 structure. [MoS₂ films prepared by reaction of MoO₃ or MoCl₅+ H₂S via hard methods (CVD, sputtering, etc.) give vertically-oriented MoS₂ platelets.] Colloidal MoS₂ cast on GaAs(111) and Si(111) surfaces gives rise to a micro-crystalline environment via platelet scission and surface reactions. At the limit, annealing MoS₂ plates on both surfaces at 200 °C leads to complete disappearance of MoS₂ islands due to reaction of S^2– with the surface.