Generalized Voigt broadening due to thermal fluctuations of electromechanical nanosensors and molecular electronic junctions. (arXiv:1811.05486v1 [cond-mat.mes-hall])

2018-11-15T04:30:23+00:00November 15th, 2018|Categories: Publications|Tags: |

Graphene and other 2D materials give a platform for electromechanical sensing of biomolecules in aqueous, room temperature environments. The electronic current changes in response to mechanical deflection, indicating the presence of forces due to interactions with, e.g., molecular species. We develop illustrative models of these sensors in order to give explicit, compact expressions for the current and signal-to-noise ratio. Electromechanical structures have an electron transmission function that follows a generalized Voigt profile, with thermal fluctuations giving a Gaussian smearing analogous to thermal Doppler broadening in solution/gas-phase spectroscopic applications. The Lorentzian component of the profile comes from the contact to the electrodes. After providing an accurate approximate form of this profile, we calculate the mechanical susceptibility for a representative two-level bridge and the current fluctuations for electromechanical detection. These results give the underlying mechanics of electromechanical sensing in more complex scenarios, such as graphene deflectometry.

Published : "arXiv Mesoscale and Nanoscale Physics".

Visualizing Encapsulated Graphene, its Defects and its Charge Environment by Sub-Micrometer Resolution Electrical Imaging. (arXiv:1811.05912v1 [cond-mat.mes-hall])

2018-11-15T04:30:20+00:00November 15th, 2018|Categories: Publications|Tags: , , |

Devices made from two-dimensional (2D) materials such as graphene or transition metal dichalcogenides possess interesting electronic properties that can become accessible to experimental probes when the samples are protected from deleterious environmental effects by encapsulating them between hexagonal boron nitride (hBN) layers. While the encapsulated flakes can be detected through post-processing of optical images or confocal Raman mapping, these techniques lack the sub-micrometer scale resolution to identify tears, structural defects or impurities, which is crucial for the fabrication of high-quality devices. Here we demonstrate a simple method to visualize such buried flakes with sub-micrometer resolution, by combining Kelvin force probe microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which measures surface potential fluctuations, is extremely effective in spotting charged contaminants within and on top of the heterostructure, making it possible to distinguish contaminated regions in the buried flake. When applying a tip bias larger than the surface potential fluctuations, EFM becomes extremely efficient in highlighting encapsulated flakes and their sub-micron structural defects. We show that these imaging modes, which are standard extensions of atomic force microscopy (AFM), are perfectly suited for locating encapsulated conductors, for visualizing nanometer scale defects and bubbles, and for characterizing their local charge environment.

Published : "arXiv Mesoscale and Nanoscale Physics".

Anderson-Bogoliubov and Carlson-Goldman modes in counterflow superconductors. The case study of a double monolayer graphene. (arXiv:1811.05899v1 [cond-mat.mes-hall])

2018-11-15T04:30:16+00:00November 15th, 2018|Categories: Publications|Tags: |

The impact of electron-hole pairing on the spectrum of plasma excitations of double layer systems is investigated. The approach accounts coupling of the scalar potential oscillations with fluctuations of the order parameter $Delta$. The theory is developed with reference to a double monolayer graphene. We find that the spectrum of antisymmetric (acoustic) plasma excitations contains a weakly damped mode below the gap $2Delta$ and a strongly damped mode above the gap. The lower mode can be interpreted as an analog of the Carlson-Goldman mode. This mode has an acoustic dispersion relation at small wave vectors and it saturates at the level $2Delta$ at large wave vectors. Its velocity is larger than the velocity of the Anderson-Bogoliubov mode $v_{AB}=v_F$/$sqrt{2}$, and it can be smaller as well as larger than the Fermi velocity $v_F$. The damping rate of this mode strongly increases under increase in temperature. Out-of-phase oscillations of the order parameters of two spin components are also analyzed. This part of the spectrum contains two modes one of which is recognized as the Anderson-Bogoliubov (phase) mode and the other, as the Schmid (amplitude) mode. With minor modifications the theory describes collective modes in the double bilayer graphene system as well.

Published : "arXiv Mesoscale and Nanoscale Physics".

Characterization of Lattice Thermal Transport in Two-Dimensional BAs, BP, and BSb: A First-Principles Study. (arXiv:1811.05597v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:22+00:00November 15th, 2018|Categories: Publications|Tags: |

Ever since the high thermal conductivity in cubic boron arsenide (c-BAs) was predicted theoretically by Lindsay et. al in 2013, countless studies have zeroed in on this particular material. Most recently, c-BAs has been confirmed experimentally to have a thermal conductivity of around 1,100 W/m-K. In this study, we investigate the seldom studied two dimensional hexagonal form of boron arsenide (h-BAs) using a first-principles approach and by solving the Boltzmann Transport Equation for phonons. Traditionally, a good indicator of a high thermal conductivity material is its high Debye temperature and high phonon group velocity. However, we determine h-BAs to have a much lower Debye temperature and average phonon group velocity compared to the other monolayer boron-V compounds of boron nitride (h-BN) and boron phosphide (h-BP), yet curiously it possesses a higher thermal conductivity. Further investigation reveals that this is due to the phonon frequency gap caused by large mass imbalances, which results in a restricted Umklapp phonon-phonon scattering channel and consequently a higher thermal conductivity. We determine the intrinsic lattice thermal conductivity of monolayer h-BAs to be 362.62 W/m-K at room temperature, which is considerably higher compared to the other monolayer boron-V compounds of h-BN (231.96 W/m-K), h-BP (187.11 W/m-K), and h-BSb (87.15 W/m-K). This study opens the door for investigation into a new class of monolayer structures and the properties they possess.

Published in: "arXiv Material Science".

Unique gap structure and symmetry of the charge density wave in single-layer VSe$_2$. (arXiv:1811.05690v1 [cond-mat.mes-hall])

2018-11-15T02:29:20+00:00November 15th, 2018|Categories: Publications|Tags: |

Single layers of transition metal dichalcogenides (TMDCs) are excellent candidates for electronic applications beyond the graphene platform; many of them exhibit novel properties including charge density waves (CDWs) and magnetic ordering. CDWs in these single layers are generally a planar projection of the corresponding bulk CDWs because of the quasi-two-dimensional nature of TMDCs; a different CDW symmetry is unexpected. We report herein the successful creation of pristine single-layer VSe$_2$, which shows a ($sqrt7 times sqrt3$) CDW in contrast to the (4 $times$ 4) CDW for the layers in bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single layer shows a sizable ($sqrt7 times sqrt3$) CDW gap of $sim$100 meV at the zone boundary, a 220 K CDW transition temperature twice the bulk value, and no ferromagnetic exchange splitting as predicted by theory. This robust CDW with an exotic broken symmetry as the ground state is explained via a first-principles analysis. The results illustrate a unique CDW phenomenon in the two-dimensional limit.

Published in: "arXiv Material Science".

Quasi-free-standing single-layer WS2 achieved by intercalation. (arXiv:1811.05748v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:18+00:00November 15th, 2018|Categories: Publications|Tags: |

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single-layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single-layer’s native properties. This dilemma can potentially be solved by decoupling the single-layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer WS2-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer WS2.

Published in: "arXiv Material Science".

Composition and Stacking Dependent Topology in Bilayers from the Graphene Family. (arXiv:1811.05525v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:17+00:00November 15th, 2018|Categories: Publications|Tags: , , |

We present a compositional and structural investigation of silicene, germanene, and stanene bilayers from first-principles. Due to the staggering of the individual layers, several stacking patterns are possible, most of which are not available to the bilayer graphene. This structural variety, in conjunction with the presence of the spin-orbit coupling, unveil a diversity of the electronic properties, with the appearance of distinct band features, including orbital hybridization and band inversion. We show that for particular cases, the intrinsic spin Hall response exhibits signatures of non-trivial electronic band topology, making these structures promising candidates to probe Dirac-like physics.

Published in: "arXiv Material Science".

Toward the Identification of Atomic Defects in Hexagonal Boron Nitride: X-Ray Photoelectron Spectroscopy and First-Principles Calculations. (arXiv:1811.05924v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:15+00:00November 15th, 2018|Categories: Publications|Tags: , |

Defects in hexagonal boron nitride (hBN) exhibit single-photon emission (SPE) and are thus attracting broad interest as platforms for quantum information and spintronic applications. However, the atomic structure and the specific impact of the local environment on the defect physical properties remain elusive. Here we articulate X-ray photoelectron spectroscopy (XPS) and first-principles calculations to discern the experimentally-observed point defects responsible for the quantum emission observed in hBN. XPS measurements show a broad band, which was deconvolved and then assigned to $N_{B}V_{N}$, $V_{N}$, $C_{B}$, $C_{B}V_{N}$, and $O_{2B}V_{N}$ defect structures using Density Functional Theory (DFT) core-level binding energy (BE) calculations.

Published in: "arXiv Material Science".

Narrow photoluminescence and Raman peaks of epitaxial MoS 2 on graphene/Ir(1 1 1)

2018-11-14T18:33:53+00:00November 14th, 2018|Categories: Publications|Tags: |

We report on the observation of photoluminescence (PL) with a narrow 18 meV peak width from molecular beam epitaxy grown ##IMG## [http://ej.iop.org/images/2053-1583/6/1/011006/tdmaaebd3ieqn001.gif] on graphene/Ir(1 1 1). This observation is explained in terms of a weak graphene-MoS 2 interaction that prevents PL quenching expected for a metallic substrate. The weak interaction of MoS 2 with the graphene is highlighted by angle-resolved photoemission spectroscopy and temperature dependent Raman spectroscopy. These methods reveal that there is no hybridization between electronic states of graphene and MoS 2 as well as a different thermal expansion of both materials. Molecular beam epitaxy grown MoS 2 on graphene is therefore an important platform for optoelectronics which allows for large area growth with controlled properties.

Published in: "2DMaterials".

Polytypism in ultrathin tellurium

2018-11-14T18:33:51+00:00November 14th, 2018|Categories: Publications|

We report the synthesis of ultrathin tellurium films, including atomically thin tellurium tri-layers, by physical vapor deposition (PVD) as well as larger area films by pulsed laser deposition (PLD). PVD leads to sub-nanometer, tri-layer tellurene flakes with distinct boundaries, whereas PLD yields uniform and contiguous sub-7 nm films over a centimeter square. The PLD films exhibit the characteristic hexagonal crystal structure of semiconducting tellurium, but high resolution transmission electron microscopy (HRTEM) reveals a unique stacking polytype in the thinner PVD-grown material. Density Functional Theory calculations predict the possible existence of three polytypes of ultrathin Te, including the α -type experimentally observed here. The two complementary growth methods afford a route to controllably synthesize ultrathin Te with thicknesses ranging from three atomic layers up to 6 nm with unique polytypism. Lastly, temperature dependent Raman studies suggest the pos…

Published in: "2DMaterials".

Screening of long-range Coulomb interaction in graphene nanoribbons: Armchair versus zigzag edges

2018-11-14T16:33:30+00:00November 14th, 2018|Categories: Publications|Tags: |

Author(s): H. Hadipour, E. Şaşıoğlu, F. Bagherpour, C. Friedrich, S. Blügel, and I. MertigWe study the electronic screening of the long-range Coulomb interaction in graphene nanoribbons (GNRs) with armchair and zigzag edges as a function of the ribbon width by employing ab initio calculations in conjunction with the random-phase approximation. We find that in GNRs with armchair edges qua…[Phys. Rev. B 98, 205123] Published Wed Nov 14, 2018

Published in: "Physical Review B".

Preparation, characterization and reusability efficacy of amine-functionalized graphene oxide-polyphenol oxidase complex for removal of phenol from aqueous phase

2018-11-14T14:32:32+00:00November 14th, 2018|Categories: Publications|Tags: , |

RSC Adv., 2018, 8,38416-38424DOI: 10.1039/C8RA06364H, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Pravin M. D., Chris Felshia S., A. GnanamaniSchematic representation of the present study describing amine functionalization followed by enzyme immobilization and degradation of

Published in: "RSC Advances".

Interface conjugation enhances the interface adhesion performance of carbon fiber-reinforced phthalonitrile composites by π–π stacking

2018-11-14T12:33:15+00:00November 14th, 2018|Categories: Publications|Tags: |

RSC Adv., 2018, 8,38210-38218DOI: 10.1039/C8RA07680D, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Changping Yin, Liping Sheng, Yudong Yang, Gengyuan Liang, Suli Xing, Jingcheng Zeng, Jiayu XiaoCross-linking products and graphene can interact with each other through

Published in: "RSC Advances".

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