Plasmon-polariton fractal spectra in quasiperiodic photonic crystals with graphene

2019-12-09T20:34:40+00:00December 9th, 2019|Categories: Publications|Tags: |

In this work we study the plasmon-polariton spectra in one-dimensional photonic crystals based on the Fibonacci, Thue-Morse and double-period sequences, where we have graphene at the interface of one of the constituent building blocks ( A = SiO 2 /graphene, B = SiO 2 ). The dispersion relations for each quasicrystal are numerically calculated by using a transfer-matrix approach. The spectra of these structures are shown to be fractals (for the bulk bands distribution), obeying a power law in a particular frequency region and these are induced specifically by the quasiperiodic ordering in the unit cells of each system studied. We also report a strong dependence of the width of the bulk bands on the wave vector for this particular frequency region, whereas for large wave vectors the fractal properties in these spectra are absent.

Published in: "EPL".

First-principles insights of electronic and optical properties of F-doped hexagonal boron nitride nanosheets for photo-catalytic water splitting

2019-11-08T16:32:32+00:00November 8th, 2019|Categories: Publications|Tags: |

Two-dimensional hexagonal boron nitride nanosheets ( h -BNNSs) have a wide band-gap of over 6.0 eV, which makes them pervious to visible light. In the current report, based on first-principles calculations, we demonstrate that h -BNNS bi-layers can be converted to visible light absorption materials through band-gap tuning carried out by fluorine (F) doping. Bi-layer structures with different F doping ratios can be formed via van der Waals interaction, whose band-gaps are greatly reduced comparing with those of pure h -BNNS bi-layers and isolated h -BN mono-layers. Furthermore, the optical absorption of F-doped h -BNNS bi-layers is observably enhanced in the ultraviolet-visible light range, which makes F-doped h -BNNS bi-layers potential metal-free visible-light–driven photo-catalyst for water splitting.

Published in: "EPL".

Interlayer friction properties of oxygen-doped hexagonal boron nitride bilayers

2019-08-14T14:43:02+00:00August 14th, 2019|Categories: Publications|Tags: , |

Understanding the interfacial tribological behavior of two-dimensional materials is essential for their actual application. In this letter, the tribological properties between two oxygen-doped hexagonal boron nitride nanosheets ( h -BNNSs) are investigated using first-principles calculation. Oxygen atoms can significantly affect the interlayer interaction energy by influencing the geometry and electron distribution of h -BNNS bilayers, thus changing the interlayer friction performance. The interlayer friction depends on the competition between like-magnetostriction and anchoring effects, and high anchoring effect will cause the increase of the interface friction. This result provides theoretical bases for further investigation of the interlayer friction properties of other oxygen-doped two-dimensional materials.

Published in: "EPL".

Manipulating the anomalous Josephson effect by interface valley-polarized mixing

2019-07-19T13:12:58+00:00July 19th, 2019|Categories: Publications|Tags: |

We theoretically investigate the supercurrent through a Josephson junction with at least one superconducting electrode directly coupled to a valley-polarized graphene sheet. The anomalous Josephson effect is shown and remarkably manipulated by interface valley-polarized mixing together with combination of the static staggered potentials and off-resonant circularly polarized light field inducing the valley polarization. The unconventional Josephson effect results from the breaking of underlying chiral (polarized-valley rotational) and time-reversal symmetries, which in turn shows the equivalence of the valley and spin freedoms. The valley-polarized interface-tunable phase offset will be of great interest in the designing and fabrication of such novel devices based on valleytronics as superconducting flux- and phase-based quantum bits, phase batteries and rectifiers.

Published in: "EPL".

Substrate-induced half-metallic property in epitaxial silicene

2019-07-08T14:49:23+00:00July 8th, 2019|Categories: Publications|Tags: , |

For most practical applications in electronic devices, two-dimensional materials should be transferred onto semiconducting or insulating substrates, since they are usually generated on metallic substrates. However, the transfer often leads to wrinkles, damages, contaminations and so on which would destroy the intrinsic properties of samples. Thus, directly generating two-dimensional materials on nonmetallic substrates has been a desirable goal for a long time. Here, via a swarm structure search method and density functional theory, we employed an insulating N-terminated cubic boron nitride(111) surface as a substrate for the generation of silicene. The result tells that the silicene exhibits a ferromagnetic half-metal with a half-metallic band gap of ##IMG## [http://ej.iop.org/images/0295-5075/126/5/57006/epl19703ieqn1.gif] {${sim}0.11 text{eV}$} . This feature is driven by the strong interaction between silicon and surface nitrogen atoms. The magnetic m…

Published in: "EPL".

Excitation of SPPs in graphene by a waveguide mode

2019-05-20T16:33:07+00:00May 20th, 2019|Categories: Publications|Tags: , |

We present a semi-analytical model that predicts the excitation of surface-plasmon polaritons (SPPs) on a graphene sheet located in front of a sub-wavelength slit drilled in a thick metal screen. We identify the signature of the SPP in the transmission, reflection, and absorption curves. Following the previous literature on noble-metal plasmonics, we characterize the efficiency of excitation of SPPs in graphene computing a spatial probability density. This quantity shows the presence of plasmonics resonances dispersing with the Fermi energy, E F , as ##IMG## [http://ej.iop.org/images/0295-5075/126/2/27001/epl19656ieqn1.gif] {$sqrt{E_F}$} an unambiguous signature of graphene plasmons.

Published in: "EPL".

Tunable transmission due to defects in zigzag phosphorene nanoribbons

2019-04-26T16:32:45+00:00April 26th, 2019|Categories: Publications|Tags: , |

Transport of the edge-state electrons along zigzag phosphorene nanoribbons in the presence of two impurities/vacancies is analytically investigated. Considering the places of the defects, a number of different situations are examined. When both defects are placed on the edge zigzag chain, as is expected, by changing the energy of the traveling electrons the electrical conductance exhibits a resonance behavior. In this case, for two vacancies the observed resonant peaks become extremely sharp. An amazing behavior is seen when the second vacancy is located along an armchair chain while the first is placed at the intersection of the edge zigzag and this armchair chain. In this case, in a considerable range of energy, the conductance is strongly strengthened. In fact the presence of the second vacancy creates a shielded region around the first vacancy, consequently, the traveling wave bypasses this region and enhances the conductance. The analytical results are compared with numeric…

Published in: "EPL".

Thermal transport in graphene/h-BN lateral heterostructures with interface compositional diffusion

2019-03-28T20:36:06+00:00March 28th, 2019|Categories: Publications|Tags: , |

The synthesis of two-dimensional lateral structures is often associated with compositional transition, causing graded interfaces. Our molecular-dynamics simulations show that the graded interface can significantly increase the thermal resistance of graphene/h-BN lateral heterostructures. The thermal conductivity is therefore lowered depending on the length of the interface diffusion. The phonon participation ratio calculations indicate the occurrence of phonon localization in the composition-graded heterostructures. Meanwhile, thermal rectification is observed in the graphene/h-BN lateral heterostructures, and the heat transport from h-BN to graphene is favored, particularly in the system with abrupt interface. These results provide critical information for the novel design of two-dimensional lateral heterostructures in terms of thermoelectrics and thermal diodes.

Published in: "EPL".

Andreev reflection and 0- π transition in graphene-based antiferromagnetic superconducting junctions

2019-02-26T14:33:51+00:00February 26th, 2019|Categories: Publications|Tags: |

We investigate the Andreev reflection and 0- π transition in the graphene-based antiferromagnetic superconducting junctions on the SiC substrate, respectively. The differential conductance of Andreev reflection is reduced in the presence of a gap induced by the antiferromagnet or the substrate. Interestingly, although the gap induced by the antiferromagnet is the same to the one induced by the substrate, their differential conductances of the Andreev reflection are absolutely different, which can be used to detect antiferromagnetism in experiment. Although the interaction between the antiferromagnet and the substrate cannot show special differential conductance, their interaction can bring the 0- π transition. This breaks up the conventional wisdom that the antiferromagnetism-induced 0- π transition shows a rigorous atomic-scale dependence on the interlayer thickness. Furthermore, compared with the conventional atomic-scale dependent 0- π transition, our …

Published in: "EPL".

Nonlinear pull-in instability of suspended graphene-based sensors

2019-02-26T12:35:21+00:00February 26th, 2019|Categories: Publications|Tags: |

Applying an external electric field on a graphene surface is an important way to improve the molecular adsorption capability of graphene, thus pull-in instability of suspended graphene sensors becomes a critical issue. Incorporating residual built-in strains, fringing fields and intermolecular forces, an electromechanical model is developed to characterize the nonlinear pull-in behaviors of suspended graphene-based sensors. The obtained results of pull-in voltages agree well with the reported experimental data. Moreover, the fracture failure of graphene sensors is initially compared to the pull-in failure. To avoid pull-in instability and fracture failure, critical formulas of axial pre-stress for zigzag-oriented and armchair-oriented graphene sensors are derived. It is demonstrated that axial pre-stress is an effective and controllable way to improve the pull-in stability of graphene sensors.

Published in: "EPL".

Unconventional charge and spin-dependent transport properties of a graphene nanoribbon with line-disorder

2018-12-27T20:33:27+00:00December 27th, 2018|Categories: Publications|Tags: |

Electronic transport with a line (or a few lines) of Anderson-type disorder in a zigzag graphene nanoribbon is investigated in the presence of Rashba spin-orbit interaction. Such line-disorders give rise to a peculiar behavior in both charge as well as spin-polarized transmission in the following sense. In the weak-disorder regime, the charge transport data show Anderson localization up to a certain disorder strength, beyond which the extended states emerge and start dominating over the localized states. These results are the hallmark signature of a selectively disordered (as opposed to bulk disorder) graphene nanoribbon. However, the spin-polarized transport shows a completely contradicting behavior. Further, the structural symmetries are shown to have an important role in the spintronic properties of the nanoribbons. Moreover, the edge-disorder scenario (disorder selectively placed at the edges) seems to hold promise for the spin-filter and switching device applications.

Published in: "EPL".

Surface-induced transition of nematic liquid crystals on graphene/SiC substrate

2018-12-11T16:40:52+00:00December 11th, 2018|Categories: Publications|Tags: |

A nematic liquid-crystal director aligns along the armchair direction of graphene grown on a SiC substrate. The temperature-dependent textural change in the nematic phase is different from the usual texture on the alignment layer. The isotropic-nematic phase transition temperature decreases over time after cell fabrication. Tiny domains occur with the phase transition and appear to be merged from a critical temperature with decreasing temperature in the nematic phase. This is thought to be due to the transition occurring at the interface. On the other hand, the nematic liquid crystal in graphene grown on a Cu foil does not show textural changes, and the phase transition temperature does not decrease even after a long time has elapsed. X-ray fluorescence measurements indicate that silicon atoms exist in the liquid crystal possibly extracted from the SiC substrate. A model of first-order phase transition on the graphene surface has been proposed. This transition is accompanied by …

Published in: "EPL".

Controlled engineering of spin-polarized transport properties in a zigzag graphene nanojunction

2018-11-13T12:33:23+00:00November 13th, 2018|Categories: Publications|Tags: |

We investigate a novel way to manipulate the spin-polarized transmission in a two-terminal zigzag graphene nanoribbon in the presence of the Rashba spin-orbit (SO) interaction with a circular-shaped cavity engraved into it. A usual technique to control the spin-polarized transport behaviour of a nanoribbon can be achieved by tuning the strength of the SO coupling, while we show that an efficient engineering of the spin-polarized transport properties can also be done via cavities of different radii engraved in the nanoribbon. Simplicity of the technique in creating such cavities in the experiments renders an additional handle to explore transport properties as a function of the location of the cavity in the nanoribbon. Further, a systematic assessment of the interplay of the Rashba interaction and the dimensions of the nanoribbon is presented. These results should provide useful input to the spintronic behaviour of such devices. In addition to the spin polarization, we have also …

Published in: "EPL".

On the influence of dilute charged impurity and perpendicular electric field on the electronic phase of phosphorene: Band gap engineering

2018-11-13T12:33:20+00:00November 13th, 2018|Categories: Publications|Tags: |

Tuning the band gap plays an important role for applicability of 2D materials in the semiconductor industry. The present paper is a theoretical study on the band gap engineering using the electronic density of states (DOS) of phosphorene in the presence of dilute charged impurity and of a perpendicular electric field. The electronic DOS is numerically calculated using a combination of the continuum model Hamiltonian and the Green’s function approach. Our findings show that the band gap of phosphorene in the absence and presence of the perpendicular electric field decreases with increasing impurity concentration and/or impurity scattering potential. Further, we found that in the presence of opposite perpendicular electric fields, the electronic DOS of disordered phosphorene shows different changing behaviors stemming from the Stark effect: in the positive case the band gap increases with increasing electric-field strength; whereas in the negative case the band gap disappears. The…

Published in: "EPL".

Andreev reflection across a Kane-Mele normal-superconductor nano-junction

2018-11-08T16:34:09+00:00November 8th, 2018|Categories: Publications|Tags: |

We have investigated the transport properties of a Kane-Mele normal-superconductor (NS) nano-junction using the familiar Blonder-Tinkham-Klapwijk (BTK) theory. The effects of the Rashba and the intrinsic spin-orbit coupling are mimicked by the inclusion of different transition metal adatoms adsorbed in a graphene nanoribbon. Specifically, we have focussed on the Andreev reflection phenomena for a range of Rashba and intrinsic coupling strengths. We have computed the spin resolved tunneling conductance where we found that the conductance characteristics are very sensitive to the strengths of the spin-orbit couplings. Further an interesting interplay between the Rashba and the intrinsic spin-orbit couplings is observed and its effects on the tunneling conductance are explored in detail. The possibility of tuning the spin-orbit couplings via different metal adatoms provides an experimental handle for achieving tunable conductance properties of this Kane-Mele nano-junction.

Published in: "EPL".

Enhancing thermal rectification in graphene-carbon nanotube junctions by tuning the chirality of pillar

2018-09-21T14:33:18+00:00September 21st, 2018|Categories: Publications|Tags: |

This letter investigates thermal rectification (TR) in graphene-carbon nanotube (GN-CNT) junctions formed by SWCNT(12, 12) connected with a single-layer graphene nanosheet (GN-SWCNT(12, 12)). It is found that the TR ratio of GN-SWCNT junction can be enhanced dramatically by tuning the chirality of the pillar. TR ratio of the GN-SWCNT(12, 12) junction can respectively reach up to 1487% and 2586.4% at temperatures of 300 K and 200 K ##IMG## [http://ej.iop.org/images/0295-5075/123/4/44004/epl19293ieqn1.gif] {$(vert Delta vert =0.5)$} , much higher than those previously reported for the pillared graphene and GN-CNT junctions. The influences of the geometric parameters on the thermal rectification are discussed. The results could offer useful guidelines to the design and performance improvement of the GN-CNT–based thermal rectifier.

Published in: "EPL".

Wannier-Koopmans method calculations of organic molecule crystal band gaps

2018-09-04T14:33:11+00:00September 4th, 2018|Categories: Publications|

It is important to accurately predict the band gaps of crystals, including organic crystals, with low computational cost. Despite the significant underestimation of the crystal band gap by the density functional theory (DFT), a recently proposed Wannier-Koopmans method (WKM) based on DFT calculations seems to yield accurate band gaps for a wide class of materials including common semiconductors, alkali halides and 2D materials. It is nevertheless important to test the limit of WKM, in particular in systems with unique characteristics. In this work, we apply the WKM to 10 organic small molecule crystals and find that the WKM calculated band gaps agree well with GW results. We also introduce a new way to calculate the Wannier functions in the WKM calculations.

Published in: "EPL".

Edge-mode–based graphene nanomechanical resonators for high-sensitivity mass sensor

2018-09-04T14:33:09+00:00September 4th, 2018|Categories: Publications|Tags: |

We perform both molecular dynamics simulations and theoretical analysis to study the sensitivity of the mass sensor based on graphene nanomechanical resonators, which are actuated following the global extended mode or the localized edge mode. We find that the mass detection sensitivity corresponding to the edge mode is about three times higher than that corresponding to the extended mode. Our analytic derivations reveal that the enhancement of the sensitivity originates from the reduction of the effective mass for the edge mode due to its localizing feature. Our findings shed light on improving the sensitivity of the graphene-based mass spectrometry by utilizing the localized edge mode.

Published in: "EPL".

Electron scattering in gapped graphene quantum dots

2018-08-20T14:33:36+00:00August 20th, 2018|Categories: Publications|Tags: , |

Due to Klein tunneling in graphene only quasi-bound states are realized in graphene quantum dots by electrostatic gating. Particles in the quasi-bound states are trapped inside the dot for a finite time and they keep bouncing back and forth till they find their way out. Here we study the effect of an induced gap on the scattering problem of Dirac electrons on a circular electrostatically confined quantum dot. Introducing an energy gap inside the quantum dot enables us to distinguish three scattering regimes instead of two in the case of gapless graphene quantum dot. We will focus on these regimes and analyze the scattering efficiency as a function of the electron energy, the dot radius and the energy gap. Moreover, we will discuss how the system parameters can affect the scattering resonances inside the dot.

Published in: "EPL".

A continuum model of lithium ion transport inside graphene

2018-08-13T18:33:27+00:00August 13th, 2018|Categories: Publications|Tags: , |

In this letter, we demonstrate the usage of a continuum equation in conjunction with Poisson equation and mean-field theory to investigate the ion transport and storage pattern of lithium ions between double-layer graphene. The majority of recent research on the ion transport for such batteries merely focuses on the effect of an external electric field acting on ions. Here, we emphasize the nanoscale mechanics of the graphene anode so that the forces between ions and the host material, and steric effects between lithium ions are incorporated. Under certain electric fields, multi-layers are formed between graphene sheets, and some related storage phenomenon is also investigated for potential applications in lithium ion battery and providing further understanding of ion transport inside biological channels.

Published in: "EPL".

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