Publication on 2D research

Stabilizing the metastable superhard material wurtzite boron nitride by three-dimensional networks of planar defects [Engineering]

2019-05-18T02:31:46+00:00May 18th, 2019|Categories: Publications|Tags: |

Wurtzite boron nitride (w-BN) is a metastable superhard material that is a high-pressure polymorph of BN. Clarifying how the metastable high-pressure material can be stabilized at atmospheric pressure is a challenging issue of fundamental scientific importance and promising technological value. Here, we fabricate millimeter-size w-BN bulk crystals via the hexagonal-to-wurtzite…

Published in: "PNAS (Ahead)".

Engineering single-atom dynamics with electron irradiation

2019-05-18T00:36:41+00:00May 18th, 2019|Categories: Publications|Tags: |

Atomic engineering is envisioned to involve selectively inducing the desired dynamics of single atoms and combining these steps for larger-scale assemblies. Here, we focus on the first part by surveying the single-step dynamics of graphene dopants, primarily phosphorus, caused by electron irradiation both in experiment and simulation, and develop a

Published in: "Science Advances".

Selective trapping of hexagonally warped topological surface states in a triangular quantum corral

2019-05-18T00:36:30+00:00May 18th, 2019|Categories: Publications|Tags: |

The surface of a three-dimensional topological insulator (TI) hosts two-dimensional massless Dirac fermions (DFs), the gapless and spin-helical nature of which leads to their high transmission through surface defects or potential barriers. Here, we report the behaviors of topological surface states (TSS) in a triangular quantum corral (TQC) which, unlike

Published in: "Science Advances".

Aromatic molecular junctions between graphene sheets: a molecular dynamics screening for enhanced thermal conductance

2019-05-17T12:33:19+00:00May 17th, 2019|Categories: Publications|Tags: |

RSC Adv., 2019, 9,15573-15581DOI: 10.1039/C9RA00894B, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Alessandro Di Pierro, Maria Mar Bernal, Diego Martinez, Bohayra Mortazavi, Guido Saracco, Alberto FinaThe tuning of covalently bound molecular junctions could increase heat

Published in: "RSC Advances".

Highly effective photocatalytic performance of {001}-TiO2/MoS2/RGO hybrid heterostructures for the reduction of Rh B

2019-05-17T12:33:16+00:00May 17th, 2019|Categories: Publications|Tags: , |

RSC Adv., 2019, 9,15033-15041DOI: 10.1039/C9RA02634G, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Ya Gao, Yongjie Zheng, Jixing Chai, Jingzhi Tian, Tao Jing, Deqing Zhang, Junye Cheng, Huiqing Peng, Bin Liu, Guangping ZhengEffective separation and rapid

Published in: "RSC Advances".

Surfactant-dependant thermally induced nonlinear optical properties of L-ascorbic acid-stabilized colloidal GNPs and GNP–PVP thin films

2019-05-17T12:33:13+00:00May 17th, 2019|Categories: Publications|

RSC Adv., 2019, 9,15502-15512DOI: 10.1039/C9RA01598A, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.A. L. Sunatkari, S. S. Talwatkar, Y. S. Tamgadge, G. G. MuleyGold nanoparticle (GNP) colloids stabilized with various concentrations of L-ascorbic acid were

Published in: "RSC Advances".

Black phosphorus electronic and optoelectronic devices

2019-05-17T10:34:41+00:00May 17th, 2019|Categories: Publications|Tags: |

This paper reviews the recent progress on electronic and optoelectronic devices based on 2D black phosphorus (BP). First, the crystal structure, band structure, and optical properties of BP, as well as some currently-known passivation methods used for making BP stable in ambient conditions are briefly summarized. Device architectures and operating principles of the state-of-the-art few-layer BP based electronic and optoelectronic devices will then be discussed in detail, with a focus on field-effect transistors, heterojunction diodes, and photodetectors. Next, solution-based exfoliation methods aimed for addressing the scalability challenge faced by BP are briefly discussed, followed by their potential applications in gas sensors and biomedicine. By reviewing recent process and discussing remaining challenges faced by BP, this paper aims to provide perspectives on opportunities and future research directions for utilizing BP and moving towards practical applications.

Published in: "2DMaterials".

Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics

2019-05-17T10:34:39+00:00May 17th, 2019|Categories: Publications|Tags: , |

We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect for the nonlinear optics of graphene are then discussed. Our analysis shows that the conventional perturbative treatment of the nonlinear optics, i.e. expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and larg…

Published in: "2DMaterials".

Contactless millimeter wave method for quality assessment of large area graphene

2019-05-17T10:34:37+00:00May 17th, 2019|Categories: Publications|Tags: |

We demonstrate that microwave absorption experiments offer a route for efficient measurements of transport properties for fast and accurate quality control of graphene. This conctactless characterization method can be used to quickly evaluate transport properties over large areas without recourse to complex lithographic methods making it suitable as a probe of quality during wafer scale fabrication. In particular, we demonstrate that absorption measurement of transport properties is sensitive to inhomogeneities in sample transport properties. This is in contrast to traditional methods using electrical contacts which tend to overestimate transport properties due to the formation of preferential conducting channels between the electrodes. Here we compare Shubnikov–de Haas oscillations simultaneously detected by microwave absorption and by conventional contact Hall bar measurements in fields up to 15 T on quasi-free standing, large area ( ##IMG## […]

Published in: "2DMaterials".

Large-area synthesis of 2D MoO 3− x for enhanced optoelectronic applications

2019-05-17T10:34:35+00:00May 17th, 2019|Categories: Publications|

Two-dimensional (2D) molybdenum trioxide has been attracting research interest due to its bandgap tunability and a wide variety of desirable electronic/optoelectronic properties. However, the lack of a reproducible synthesis process for obtaining large coverage 2D MoO 3 has limited the use of this material. Here we report the synthesis of large area 2D MoO 3− x via physical vapor deposition, using MoO 3 powder as the precursor. The as-grown layers are directly deposited on SiO 2 /Si, eliminating the necessity for any transfer process. These as-grown MoO 3− x layers allow for the large-scale fabrication of planar device arrays. The applicability of 2D MoO 3− x in optoelectronics is established via the demonstration of low-power ultraviolet (UV) sensor arrays, with rapid response times (200 µ s) and responsivity up to 54.4 A · W −1 . At a bias voltage of 0.1 V, they are at least 400 time…

Published in: "2DMaterials".

Preparation of graphene oxide/poly(vinyl alcohol) composite membrane and pervaporation performance for ethanol dehydration

2019-05-17T10:33:29+00:00May 17th, 2019|Categories: Publications|Tags: , |

RSC Adv., 2019, 9,15457-15465DOI: 10.1039/C9RA01379B, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Xue Cheng, Weibin Cai, Xiaohan Chen, Zhen Shi, Jiding LiThe separation factor of the composite GOP-2.0 membranes could reach 3 059, which is

Published in: "RSC Advances".

Phase space analysis of quantum transport in graphene. (arXiv:1905.06637v1 [cond-mat.mes-hall])

2019-05-17T04:30:31+00:00May 17th, 2019|Categories: Publications|Tags: |

Prominent among the many fascinating properties of graphene are its surprising electronic transport characteristics which are commonly studied theoretically and numerically within the Landauer-B”uttiker formalism. Here a device is characterized by its scattering properties to and from reservoirs connected by perfect semi-infinite leads, and transport quantities are derived from the scattering matrix. In many respects, however, the device becomes a `black box’ as one only analyses what goes in and out. Here we use the Husimi function as a complementary tool for understanding transport in graphene nanodevices. It is a phase space representation of the scattering wavefunctions that allows to link the scattering matrix to a more semiclassical and intuitive description and gain additional insight in to the transport process. In this article we demonstrate the benefits of the Husimi approach by analysing emph{Klein tunneling} and emph{intervalley scattering} in two simple graphene nanostructures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Large enhancement of the effective second-order nonlinearity in graphene metasurfaces. (arXiv:1905.06497v1 [physics.optics])

2019-05-17T04:30:27+00:00May 17th, 2019|Categories: Publications|Tags: |

Using a powerful homogenization technique, one- and two-dimensional graphene metasurfaces are homogenized both at the fundamental frequency (FF) and second harmonic (SH). In both cases, there is excellent agreement between the predictions of the homogenization method and those based on rigorous numerical solutions of Maxwell equations. The homogenization technique is then employed to demonstrate that, owing to a double-resonant plasmon excitation mechanism that leads to strong, simultaneous field enhancement at the FF and SH, the effective second-order susceptibility of graphene metasurfaces can be enhanced by more than three orders of magnitude as compared to the intrinsic second-order susceptibility of a graphene sheet placed on the same substrate. In addition, we explore the implications of our results on the development of new active nanodevices that incorporate nanopatterned graphene structures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Tunable valley Hall effect in gate-defined graphene superlattices. (arXiv:1905.06373v1 [cond-mat.mes-hall])

2019-05-17T04:30:25+00:00May 17th, 2019|Categories: Publications|Tags: |

We theoretically investigate gate-defined graphene superlattices with broken inversion symmetry as a platform for realizing tunable valley dependent transport. Our analysis is motivated by recent experiments [C. Forsythe et al., Nat. Nanotechnol. 13, 566571 (2018)] wherein gate-tunable superlattice potentials have been induced on graphene by nanostructuring a dielectric in the graphene/patterneddielectric/gate structure. We demonstrate how the electronic tight-binding structure of the superlattice system resembles a gapped Dirac model with associated valley dependent transport using an unfolding procedure. In this manner we obtain the valley Hall conductivities from the Berry curvature distribution in the superlattice Brillouin zone, and demonstrate the tunability of this conductivity by the superlattice potential. Finally, we calculate the valley Hall angle relating the transverse valley current and longitudinal charge current and demonstrate the robustness of the valley currents against irregularities in the patterned dielectric.

Published : "arXiv Mesoscale and Nanoscale Physics".

Ultra-compact photodetection in atomically thin MoSe$_2$. (arXiv:1905.06794v1 [])

2019-05-17T04:30:20+00:00May 17th, 2019|Categories: Publications|Tags: |

Excitons in atomically-thin semiconductors interact very strongly with electromagnetic radiation and are necessarily close to a surface. Here, we exploit the deep-subwavelength confinement of surface plasmon polaritons (SPPs) at the edge of a metal-insulator-metal plasmonic waveguide and their proximity of 2D excitons in an adjacent atomically thin semiconductor to build an ultra-compact photodetector. When subject to far-field excitation we show that excitons are created throughout the dielectric gap region of our waveguide and converted to free carriers primarily at the anode of our device. In the near-field regime, strongly confined SPPs are launched, routed and detected in a 20nm narrow region at the interface between the waveguide and the monolayer semiconductor. This leads to an ultra-compact active detector region of only ~0.03$mu m ^2$ that absorbs 86% of the propagating energy in the SPP. Due to the electromagnetic character of the SPPs, the spectral response is essentially identical to the far-field regime, exhibiting strong resonances close to the exciton energies. While most of our experiments are performed on monolayer thick MoSe$_2$, the photocurrent-per-layer increases super linearly in multilayer devices due to the suppression of radiative exciton recombination. These results demonstrate an integrated device for nanoscale routing and detection of light with the potential for on-chip integration at technologically relevant, few-nanometer length scales.

Published : "arXiv Mesoscale and Nanoscale Physics".

Outstanding strength, optical characteristics and thermal conductivity of graphene-like BC$_3$ and BC$_6$N semiconductors. (arXiv:1905.06819v1 [cond-mat.mtrl-sci])

2019-05-17T02:29:51+00:00May 17th, 2019|Categories: Publications|Tags: |

Carbon based two-dimensional (2D) materials with honeycomb lattices, like graphene, polyaniline carbon-nitride (C$_3$N) and boron-carbide (BC$_3$) exhibit exceptional physical properties. On this basis, we propose two novel graphene-like materials with BC$_6$N stoichiometry. We conducted first-principles calculations to explore the stability, mechanical response, electronic, optical and thermal transport characteristics of graphene-like BC$_3$ and BC$_6$N monolayers. The absence of imaginary frequencies in the phonon dispersions confirm dynamical stability of BC$_3$ and BC$_6$N monolayers. Our first principles results reveal that BC$_3$ and BC$_6$N present high elastic moduli of 256 and 305 N/m, and tensile strengths of 29.0 and 33.4 N/m, with room temperature lattice thermal conductivities of 410 and 1710 W/m.K, respectively. Notably, the thermal conductivity of BC$_6$N is one of the highest among all 2D materials. According to electronic structure calculations, monolayers of BC$_3$ and BC$_6$N are indirect and direct bandgap semiconductors, respectively. The optical analysis illustrate that the first absorption peaks along the in-plane polarization for single-layer BC$_3$ and BC$_6$N occur in the visible range of the electromagnetic spectrum. Our results reveal outstandingly high mechanical properties and thermal conductivity along with attractive electronic and optical features of BC$_3$ and BC$_6$N nanosheets and present them as promising candidates to design novel nanodevices.

Published in: "arXiv Material Science".

Dimensional crossover and topological nature of the thin films of a three-dimensional topological insulator by band gap engineering. (arXiv:1905.06776v1 [cond-mat.mtrl-sci])

2019-05-17T02:29:49+00:00May 17th, 2019|Categories: Publications|Tags: |

Identification and control of topological phases in topological thin films offer great opportunity for fundamental research and the fabrication of topology-based devices. Here, combining molecular beam epitaxy, angle-resolved photoemission spectroscopy and ab-initio calculations, we investigate the electronic structure evolution in (Bi1-xInx)2Se3 films with thickness from 2 to 13 quintuple layers. We identify several phases with their characteristic topological nature and evolution between them, i.e., dimensional crossover from a three-dimensional topological insulator with gapless surface state to its two-dimensional counterpart with gapped surface state, and topological phase transition from topological insulator to a normal semiconductor with increasing In concentration x. Furthermore, by introducing In alloying as an external knob of band gap engineering, we experimentally demonstrated the trivial topological nature of Bi2Se3 thin films (below 6 quintuple layers) as two-dimensional gapped systems, in consistent with our theoretical calculations. Our results provide not only a comprehensive phase diagram of (Bi1-xInx)2Se3 and a route to control its phase evolution, but also a practical way to experimentally determine the topological properties of a gapped compound by topological phase transition and band gap engineering.

Published in: "arXiv Material Science".

Competition between canted antiferromagnetic and spin-polarized quantum Hall states at $nu$ = 0 in graphene on a ferrimagnetic insulator. (arXiv:1905.06866v1 [cond-mat.mes-hall])

2019-05-17T02:29:46+00:00May 17th, 2019|Categories: Publications|Tags: , |

The $nu$ = 0 quantum Hall state in graphene has attracted experimental and theoretical interest. Graphene supports four zero-energy Landau levels which are described by spin and valley degeneracies. These lead to a number of approximately degenerate symmetry-broken states. Electron-electron and electron-phonon interactions break valley-symmetry and determine the ground state of the $nu$ = 0 state. The consensus emerging from theory and experiment is that these interactions favour an antiferromagnetic insulating state which supports long-range spin-polarized edge transport. Here we report a competition between canted antiferromagnetic and ferromagnetic quantum Hall states in graphene placed on a ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ (YIG), which induces a uniform magnetic exchange field in graphene of the order 60 T. The magnetic order and energy gap of the edge modes in graphene are tunable with an 8 T out-of-plane magnetic field at 2.7 K.

Published in: "arXiv Material Science".

Dramatic effects of vacancies on phonon lifetime and thermal conductivity in graphene. (arXiv:1905.06672v1 [cond-mat.mtrl-sci])

2019-05-17T02:29:43+00:00May 17th, 2019|Categories: Publications|Tags: , |

Understanding thermal transport in 2D materials and especially in graphene is a key challenge for the design of heat management and energy conversion devices. The high sensitivity of measured transport properties to structural defects, ripples and vacancies is of crucial importance in these materials. Using a first principle based approach combined with an exact treatment of the disorder, we address the impact of vacancies on phonon lifetimes and thermal transport in graphene. We find that perturbation theory fails completely and overestimates phonon lifetimes by almost two orders of magnitude. Whilst, in defected graphene, LA and TA modes remain well defined, the ZA modes become marginal. In the long wavelength limit, the ZA dispersion changes from quadratic to linear and the scattering rate is found proportional to the phonon energy, in contrast to the quadratic scaling often assumed. The impact on thermal transport, calculated beyond the relaxation time approximation and including first principle phonon-phonon scattering rates as reported recently for pristine graphene, reveals spectacular effects even for extremely low vacancy concentrations.

Published in: "arXiv Material Science".

Tuning the work function of graphene toward application as anode and cathode. (arXiv:1905.06594v1 [])

2019-05-17T02:29:41+00:00May 17th, 2019|Categories: Publications|Tags: , |

The rapid technological progress in the 21st century demands new multi-functional materials applicable to a wide variety of industries. Two-dimensional (2D) materials are predicted to have a revolutionary impact on the cost, size, weight, and functions of future electronic and optoelectronic devices. Graphene, which shows potential as an alternative to conventional conductive transparent metal oxides, may play a central role. Since its work function (WF) is tunable, graphene exhibits the interesting ability to serve two different roles in electronic and optoelectronic devices, both as an anode and a cathode. After introducing some basic concepts, this work reviews the most important advances in controlling the tuned WF of graphene, highlighting special features of graphene electronic band structure and recognizing different methods for measuring WF. The impact of thickness, type of contact, chemical doping, UV and plasma treatments, defects, and functional groups of graphene oxide are considered and related with the applications of the modulated material. The results of the review, organized in lookup tables, have been used to identify the advantages and main challenges of the tuning methods.

Published in: "arXiv Material Science".

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