Low-energy density has long been the major limitation to the application of supercapacitors. Introducing topological defects and dopants in carbon-based electrodes in a supercapacitor improves the performance by maximizing the gravimetric capacitance per mass of the electrode. However, the main mechanisms governing this capacitance improvement are still unclear. We fabricated planar electrodes from CVD-derived single-layer graphene with deliberately introduced topological defects and nitrogen dopants in controlled concentrations and of known configurations, to estimate the influence of these defects on the electrical double-layer (EDL) capacitance. Our experimental study and theoretical calculations show that the increase in EDL capacitance due to either the topological defects or the nitrogen dopants has the same origin, yet these two factors improve the EDL capacitance in different ways. Our work provides a better understanding of the correlation between the atomic-scale structure and the EDL capacitance and presents a new strategy for the development of experimental and theoretical models for understanding the EDL capacitance of carbon electrodes.

The two inequivalent valleys in graphene are protected against long range scattering potentials due to their large separation in momentum space. In tailored √ 3N × √ 3N or 3N × 3N graphene superlattices, these two valleys are folded into Γ and coupled by Bragg scattering from periodic adsorption. We find that, for top-site adsorption, strong inter-valley coupling closes the bulk gap from inversion symmetry breaking and leads to a single-valley metallic phase with quadratic band crossover. The degeneracy at the crossing point is protected by C3v symmetry. In addition, the emergence of pseudo-Zeeman field and valley-orbit coupling are also proposed, which provide the possibility of tuning valley-polarization coherently in analogy to real spin for spintronics. Such valley manipulation mechanisms can also find applications in honeycomb photonic crystals. We also study the strong geometry-dependent influence of hollow-and bridge-site adatoms in the inter-valley coupling.

We theoretically report that, with in-plane magnetization, the quantum anomalous Hall effect (QAHE) can be realized in two-dimensional atomic crystal layers with preserved inversion symmetry but broken out-of-plane mirror reflection symmetry. We take the honeycomb lattice as an example, where we find that the low-buckled structure, which makes the system satisfy the symmetric criteria, is crucial to induce QAHE. The topologically nontrivial bulk gap carrying a Chern number of C = ±1 opens in the vicinity of the saddle points M , where the band dispersion exhibits strong anisotropy. We further show that the QAHE with electrically tunable Chern number can be achieved in Bernalstacked multilayer systems, and the applied interlayer potential differences can dramatically decrease the critical magnetization to make the QAHE experimentally feasible.