950 publications from this institution
We report on transport and capacitance measurements of graphene devices in magnetic fields up to 30 T. In both techniques, we observe the full splitting of Landau levels and we employ tilted field experiments to address the origin of the observed broken symmetry states. In the lowest energy level, the spin degeneracy is removed at filling factors $\nu=\pm1$ and we observe an enhanced energy gap. In the higher levels, the valley degeneracy is removed at odd filling factors while spin polarized states are formed at even $\nu$. Although the observation of odd filling factors in the higher levels points towards the spontaneous origin of the splitting, we find that the main contribution to the gap at $\nu= -4,-8$, and $-12$ is due to the Zeeman energy.
Electrocatalysts are core materials in metal–air batteries and fuel cells. How to prepare catalysts with special morphologies and design efficient active sites is always a challenging task. Fluorine has the highest electronegativity. In addition, F atoms have three lone pairs of electrons, which allow F to efficiently influence the charge distribution on the surface of the material. Here, some F, Fe/Zn codoped graphene-like nanoribbons are prepared in a controlled manner. Due to this structure, the active sites could be fully exposed, enhancing the catalytic efficiency in the oxygen reduction reaction (ORR). The results show that the ORR activity of F-doped graphene-like nanoribbon catalysts is 30 mV higher than that of commercial Pt/C catalysts. Density functional theory (DFT) calculations were performed to understand the mechanisms that underpin these results. DFT studies showed that (1) in the ORR process, the F-doped graphene nanoribbons are beneficial to oxygen molecules using the end-on adsorption model (Pauling model) and reduce the rate-determining step (RDS) barrier. (2) F-doping can weaken the bonding energy of the FeN4 site 3d orbital, which reduces the RDS barrier and is conducive to the formation and desorption of *OH.
