Coexistence of metallic and insulating-like states in graphene

New Ferroelectric Phase in Atomic-Thick Phosphorene Nanoribbons: Existence of in-Plane Electric Polarization

Ferroelectrics have many significant applications in electric devices, such as capacitor or random-access memory, tuning the efficiency of solar cell. Although atomic-thick ferroelectrics are the necessary components for high-density electric devices or nanoscale devices, the development of such materials still faces a big challenge because of the limitation of intrinsic mechanism. Here, we reported that in-plane atomic-thick ferroelectricity can be induced by vertical electric field in phosphorene nanoribbons (PNRs). Through symmetry arguments, we predicted that ferroelectric direction is perpendicular to the direction of external electric field and lies in the plane. Further confirmed by the comprehensive first-principles calculations, we showed that such ferroelectricity is induced by the electron-polarization, which is different from the structural distortion in traditional ferroelectrics and the recent experimental discovery of in-plane atomic-thick ferroelectrics (Science 2016, 353, 274). Moreover, we found that the value of electronic polarization in bilayer is much larger than that in monolayer. Our results show that electron-polarization ferroelectricity maybe the most promising candidate for atomic-thick ferroelectrics.

Dirac cone move and bandgap on/off switching of graphene superlattice

Using the density functional theory with generalized gradient approximation, we have studied in detail the cooperative effects of degenerate perturbation and uniaxial strain on bandgap opening in graphene. The uniaxial strain could split π bands into π_a and π_z bands with an energy interval E_s to move the Dirac cone. The inversion symmetry preserved antidot would then further split the π_a (π_z) bands into π_a1 (π_z1) and π_a2 (π_z2) bands with an energy interval E_d, which accounts for the bandgap opening in a kind of superlattices with Dirac cone being folded to Γ point. However, such antidot would not affect the semimetal nature of the other superlattices, showing a novel mechanism for bandstructure engineering as compared to the sublattice-equivalence breaking. For a superlattice with bandgap of ~E_d opened at Γ point, the E_s could be increased by strengthening strain to close the bandgap, suggesting a reversible switch between the high velocity properties of massless Fermions attributed to the linear dispersion relation around Dirac cone and the high on/off ratio properties associated with the sizable bandgap. Moreover, the gap width actually could be continuously tuned by controlling the strain, showing attractive application potentials.

A promising way to open an energy gap in bilayer graphene

There has been huge research interest in the energy gap problem of monolayer and bilayer graphene due to their great potential in practical applications. Herein, based on first-principles calculations, we report a promising way to open a large band gap in bilayer graphene (BLG) by sandwiching it between two substrates, although this is not usually expected to occur due to the weak interlayer interactions dominated by van der Waals forces. Taking surface-functionalized boron-nitrides as substrates, we predict from first-principles calculations that BLG can have energy gaps ranging from 0.35 eV to 0.55 eV, depending on the substrates and stacking order. Compared to other methods of band-gap manipulation in BLG, the structural integrity of BLG is well-preserved in our study, and the predicted energy gap is suitable for electric devices. Since the proposed method is easily realized in experiments, our results will hopefully accelerate the application of graphene in semiconductor devices and promote the development of graphene technology.