Surface functionalization of quasi-two-dimensional MoS2 with nitrogen and oxygen plasma

The paper deals with the experimental and theoretical study of the interaction of quasi-two-dimensional MoS2 films with nitrogen and oxygen plasmas in order to reveal the main mechanisms of their surface functionalization and to analyze its effects on the structure and properties of the films. Diagnostics of samples before and after treatment with radicals and ions was performed ex situ by Raman spectroscopy, spectroscopic ellipsometry, atomic force microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The results of the experimental study showed that nitrogen and oxygen plasma treatment of ultrathin MoS2 films leads to the modification of the near-surface layers of the samples due to the removal of sulfur and the incorporation of incident atoms into forming vacancies. Nitrogen and oxygen ions with even low (less than 30 eV) energy causes partial removal of upper layers of the films leading to their partial destruction, while remote plasma (mainly by thermal radicals) guarantees more ‘soft’ treatment of films. The obtained spectroscopic data also indicated different effects of radicals (thermal O and N atoms) on the MoS2 surface. To analyze these effects, the computer modelling with density functional theory method was carried out. Its results enables to reveal the main mechanisms of the surface functionalization of MoS2 monolayers and to explain the experimental data.