On the morphology and optical properties of molybdenum disulfide nanostructures from a monomolecular layer to a fractal-like substructure

The impact of layer thickness on the morphology and optical properties of MoS$_2$ nanostructures, including monomolecular layers, formed by the carrier-gas-assisted transport of sulfur vapor to the hot zone of a reactor containing metallic molybdenum and subsequent deposition on mica (muscovite) substrates is investigated. Molybdenum disulfide nanostructures of different thicknesses grown at different temperatures of gas-transport synthesis are studied by atomic-force microscopy, optical absorption spectroscopy, and Raman spectroscopy. It is found that synthesis at temperatures of 525–600$^\circ$C makes it possible to obtain monomolecular MoS$_2$ layers containing trigonal domains and featuring direct-gap optical transitions at 1.84 eV with the formation of excitons at room temperature. Fractal-type MoS$_2$ substructures are obtained for the first time. The frequencies of intralayer and interlayer vibrational modes $E^{1}_{2g}$ and $A_{1g}$, respectively, in their Raman spectra (377.5 and 403.8 cm$^{-1}$, respectively) differ both from the corresponding values for a monomolecular layer and the known frequencies for bulk samples. The frequency of the $E^{1}_{2g}$ intralayer mode in these samples (377.5 cm$^{-1}$) is the lowest of all previously reported.