Phase Transition and Electronic Structure Investigation of MoS$_2$-rGO Nanocomposite Decorated with AuNPs.

In this work a simple approach to transform MoS$_2$ from its metallic (1T' to semiconductor 2H) character via gold nanoparticle surface decoration of a MoS$_2$ graphene oxide (rGO) nanocomposite is proposed. The possible mechanism to this phase transformation was investigated using different spectroscopy techniques, and supported by density functional theory theoretical calculations. A mixture of the 1T'- and 2H-MoS2 phases was observed from the Raman and Mo 3d High Resolution X-ray photoelectron (HRXPS) spectra analysis in the MoS$_2$-rGO nanocomposite. After surface decoration with gold nanoparticles the concentration of the 1T' phase decreases making evident a phase transformation. According to Raman and valence band spectra analyses, the AuNPs induces a p-type doping in MoS$_2$ -rGO nanocomposite. We proposed as a main mechanism to the MoS$_2$ phase transformation the electron transfer from Mo 4d-xy,xz,yz in 1T' phase to AuNPs conduction band. At the same time, the unoccupied electronic structure was investigated from S K-edge Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy. Finally, the electronic coupling between unoccupied electronic states was investigated by the core hole clock approach using Resonant Auger spectroscopy (RAS), showing that AuNPs affect mainly the MoS2 electronic states close to Fermi level.