Random anion distribution in MS_xSe_2−x (M = Mo, W) crystals and nanosheets

Abstract

The group VIb dichalcogenides (MX 2 , M ¼ Mo, W; X¼ S, Se) have a layered molybdenite structure in which M atoms are coordinated by a trigonal prism of X atoms. Ternary solid solutions of MS x Se 2Àx were synthesized, microcrystals were grown by chemical vapor transport, and their morphologies and structures were characterized by using synchrotron X-ray diffraction, Rietveld refinement, DIFFaX simulation of structural disorder, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Double aberration corrected scanning transmission electron microscopy was used to determine the anion distributions in single-layer nanosheets exfoliated from the microcrystals. These experiments indicate that the size difference between S and Se atoms does not result in phase separation, consistent with earlier studies of MX 2 monolayer sheets grown by chemical vapor deposition. However, stacking faults occur in microcrystals along the layering axis, particularly in sulfur-rich compositions of MS x Se 2Àx solid solutions.

Evolution of spectroscopy features in layered MoSxSe(2-x) solid solutions

“…In figure 7(a), in the range 1.8 > x > 0.4, the intensity of the MoS 2 -A 1g like phonon is enhanced, in particular close to x = 2. The enhancement can be attributed to a resonance with the A exciton, which gets closer to the laser energy at x = 1.8 [6,40]. The same reasoning can be applied to the other lasers, when we consider the resonance with the C exciton.…”

“…The MoS x Se 2−x crystals were grown by chemical vapor transport (CVT) in a two-step process previously described by Nguyen et al [6] The various percentage series was synthesized for 0 x 2 in x = 0.2 steps with an initial corresponding stoichiometric mixture of molybdenum powder (Acros Organics 99.9%), sublimated sulfur powder (J.T. Baker), and selenium powder (Acros Organics 99.5+%) placed in a vacuum sealed ampoule and heated in a tube furnace at 1000 °C for 3 days.…”

“…The layered crystals were then washed using hexane and dried under vacuum to remove solvent and residual iodine. CVT growth yields larger crystals with high quality suitable for mechanical cleavage and exfoliation for AFM characterization, while preserving the stoichiometry of the first growth step [6]. While CVT presents equilibrium conditions for solid state reactions.…”

“…The layers composing a bulk TMD crystal, can also be isolated, and a single layered TMD, such as MoS 2 , becomes a direct band gap semiconductor, in contrast with bulk crystals, which have an indirect band gap [3,4]. Additionally, their band gap can also be tuned upon changing their chemical compositions [5,6]. TMDs have been the subjects of an important research field as they offer the possibility of combining different atomically thin layers and build artificial van der Waals solids [7].…”