First-principles prediction of doped graphane as a high-temperature electron-phonon superconductor

Engineering the electronic structure of graphene

Graphene exhibits many unique electronic properties owing to its linear dispersive electronic band structure around the Dirac point, making it one of the most studied materials in the last 5-6 years. However, for many applications of graphene, further tuning its electronic band structure is necessary and has been extensively studied ever since graphene was first isolated experimentally. Here we review the major progresses made in electronic structure engineering of graphene, namely by electric and magnetic fields, chemical intercalation and adsorption, stacking geometry, edge-chirality, defects, as well as strain.

Evidence for Dirac fermions in a honeycomb lattice based on silicon

Silicene, a sheet of silicon atoms in a honeycomb lattice, was proposed to be a new Dirac-type electron system similar to graphene. We performed scanning tunneling microscopy and spectroscopy studies on the atomic and electronic properties of silicene on Ag(111). An unexpected √3 × √3 reconstruction was found, which is explained by an extra-buckling model. Pronounced quasiparticle interferences (QPI) patterns, originating from both the intervalley and intravalley scatter, were observed. From the QPI patterns we derived a linear energy-momentum dispersion and a large Fermi velocity, which prove the existence of Dirac fermions in silicene.

A first-principles prediction of two-dimensional superconductivity in pristine B₂C single layers

Based on first-principles lattice dynamics and electron-phonon coupling calculations, B(2)C sheets are predicted to be a two-dimensional (2D) phonon-mediated superconductors with a relatively high transition temperature (T(c)). The electron-phonon coupling parameter was calculated to be 0.92 and it is mainly contributed by low frequency out-of-plane phonon modes and electronic states with a π character. When the Coulomb pseudopotential, μ*, is set to 0.10, the estimated temperature, T(c), is 19.2 K. To the best of our knowledge, B(2)C is the first pristine 2D superconductor with a T(c) higher than the boiling point of liquid helium.

Large conductance modulation of gold thin films by huge charge injection via electrochemical gating

By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n(2D) as high as 3.5×10(15)  e/cm(2) in gold films and to observe large relative variations in the film resistance, ΔR/R', up to 10% at low temperature. ΔR/R' is a linear function of n(2D)-as expected within a free-electron model-if the film is thick enough (≥25  nm); otherwise, a tendency to saturation due to size effects is observed. The application of this technique to 2D materials might allow extending the field-effect experiments to a range of charge doping where large conductance modulations and, in some cases, even the occurrence of superconductivity are expected.

Graphane sheets and crystals under pressure

Eight isomeric two-dimensional graphane sheets are found in a theoretical study. Four of these nets—two built on chair cyclohexanes, two on boat—are more stable thermodynamically than the isomeric benzene, or polyacetylene. Three-dimensional crystals are built up from the two-dimensional sheets, and their hypothetical behavior under pressure (up to 300 GPa) is explored. While the three-dimensional graphanes remain, as expected, insulating or semiconducting in this pressure range, there is a remarkable inversion in stability of the five crystals studied. Two stacking polytypes that are not the most stable at ambient pressure (one based on an unusual chair cyclohexane net, the other on a boat) are significantly stabilized with increasing pressure relative to stackings of simple chair sheets. The explanation may lie in the balance on intra and intersheet contacts in the extended arrays.

Controlled hydrogenation of graphene sheets and nanoribbons

The electronic properties of graphene sheets and nanoribbons with different degrees of hydrogenation have been investigated using a combination of charge transport and Raman spectroscopy experiments. The field-effect transistor mobility of graphene is shown to be highly sensitive to the treatment time during atomic hydrogen dose and follows an exponential decrease with time. Raman spectroscopy demonstrates linearly increasing defect-band intensity, and when considered together with transport data, the relationship between graphene mobility and the crystalline size of intact sp(2) carbon regions can be derived. Further, the increase in width of the voltage plateau for monolayer and bilayer graphene points to the formation of midgap states. For partially hydrogenated graphene, the temperature-dependent transport in these states shows a weak insulating behavior. A comparison of Raman spectrum and conductivity data of partially hydrogenated monolayer and bilayer graphene suggests that the latter is also quite susceptible to adsorption of hydrogen atoms, despite a stiffer lattice structure.

Electron-phonon renormalization of the direct band gap of diamond

We calculate from first principles the temperature-dependent renormalization of the direct band gap of diamond arising from electron-phonon interactions. The calculated temperature dependence is in good agreement with spectroscopic ellipsometry measurements, and the zero-point renormalization of the band gap is found to be as large as 0.6 eV. We also calculate the temperature-dependent broadening of the direct absorption edge and find good agreement with experiment. Our work calls for a critical revision of the band structures of other carbon-based materials calculated by neglecting electron-phonon interactions.