Direct observation of ordered configurations of hydrogen adatoms on graphene.

Ordered configurations of hydrogen adatoms on graphene have long been proposed, calculated, and searched for. Here, we report direct observation of several ordered configurations of H adatoms on graphene by scanning tunneling microscopy. On the top side of the graphene plane, H atoms in the configurations appear to stick to carbon atoms in the same sublattice. Scanning tunneling spectroscopy measurements revealed a substantial gap in the local density of states in H-contained regions as well as in-gap states below the conduction band due to the incompleteness of H ordering. These findings can be well explained by density functional theory calculations based on double-sided H configurations. In addition, factors that may influence H ordering are discussed.

Temperature dependent Raman and photoluminescence of vertical WS2/MoS2 monolayer heterostructures.

Heterostructures from two-dimensional transition-metal dichalcogenides MX2 have emerged as a hot topic in recent years due to their various fascinating properties. Here, we investigated the temperature dependent Raman and photoluminescence (PL) spectra in vertical stacked WS2/MoS2 monolayer heterostructures. Our result shows that both E12g and A1g modes of WS2 and MoS2 vary linearly with temperature increasing from 300 to 642K. The PL measurement also reveals strong temperature dependencies of the PL intensity and peak position. The activation energy of the thermal quenching of the PL emission has been found to be equal to 69.6meV. The temperature dependence of the peak energy well follows the band-gap shrinkage of bulk semiconductor.

Active Light Control of the MoS2 Monolayer Exciton Binding Energy.

Plasmonic excitation of Au nanoparticles deposited on a MoS2 monolayer changes the absorption and photoluminescence characteristics of the material. Hot electrons generated from the Au nanoparticles are transferred into the MoS2 monolayers, resulting in n-doping. The doping effect of plasmonic hot electrons modulates the dielectric permittivity of materials, resulting in a red shift of both the absorption and the photoluminescence spectrum. This spectroscopic tuning was further investigated experimentally by using different Au nanoparticle concentrations, excitation laser wavelengths, and intensities. An analytical model for the photoinduced modulation of the MoS2 dielectric function and its exciton binding energy change is developed and used to estimate the doping density of plasmonic hot electrons. Our approach is important for the development of photonic devices for active control of light by light.