/Tag: hBN

Giant Effective charges and Piezoelectricity in Gapped Graphene. (arXiv:1903.09407v1 [cond-mat.mtrl-sci])

2019-03-25T02:30:09+00:00March 25th, 2019|Categories: Publications|Tags: , |

Since the first realization of reversible charge doping in graphene via field-effect devices, it has become evident how the induction a gap could further enhance its potential for technological applications. Here we show that the gap opening due to a sublattice symmetry breaking has also a profound impact on the polar response of graphene. By combining ab-initio calculations and analytical modelling we show that for realistic band-gap values ($Deltalesssim 0.5$ eV) the piezoelectric coefficient and the Born effective charge of graphene attain a giant value, independent on the gap. In particular the piezoelectric coefficient per layer of gapped mono- and bilayer graphene is three times larger than that of a large-gap full polar insulator as hexagonal Boron Nitride (h-BN) monolayer, and 30% larger than that of a polar semiconductor as MoS$_2$. This surprising result indicates that piezoelectric acoustic-phonons scattering can be relevant to model charge transport and charge-carrier relaxation in gated bilayer graphene. The independence of the piezoelectric coefficient and of the Born effective charge on the gap value follows from the connection between the polar response and the valley Chern number of gapped Dirac electrons, made possible by the effective gauge-field description of the electron-lattice/strain coupling in these systems. In the small gap limit, where the adiabatic ab-initio approximation fails, we implement analytically the calculation of the dynamical effective charge, and we establish a universal relation between the complex effective charge and the so-called Fano profile of the phonon optical peak. Our results provide a general theoretical framework to

Published in: "arXiv Material Science".

High thermal conductivity of high-quality monolayer boron nitride and its thermal expansion. (arXiv:1903.08862v1 [cond-mat.mtrl-sci])

2019-03-22T02:29:18+00:00March 22nd, 2019|Categories: Publications|Tags: , |

Heat management becomes more and more critical, especially in miniaturized modern devices, so the exploration of highly thermally conductive materials with electrical insulation and favorable mechanical properties is of great importance. Here, we report that high-quality monolayer boron nitride (BN) has a thermal conductivity (k{appa}) of 751 W/mK at room temperature. Though smaller than that of graphene, this value is larger than that of cubic boron nitride (cBN) and only second to those of diamond and lately discovered cubic boron arsenide (BAs). Monolayer BN has the second largest k{appa} per unit weight among all semiconductors and insulators, just behind diamond, if density is considered. The k{appa} of atomically thin BN decreases with increased thickness. Our large-scale molecular dynamic simulations using Green-Kubo formalism accurately reproduce this trend, and the density functional theory (DFT) calculations reveal the main scattering mechanism. The thermal expansion coefficients (TECs) of monolayer to trilayer BN at 300-400 K are also experimentally measured, and the results are comparable to atomistic ab initio DFT calculations in a wider range of temperatures. Thanks to its wide bandgap, high thermal conductivity, outstanding strength, good flexibility, and excellent thermal and chemical stability, atomically thin BN is a strong candidate for heat dissipation applications, especially in the next generation of flexible electronic devices.

Published in: "arXiv Material Science".

Twists in ferromagnetic monolayers with trigonal prismatic symmetry

2019-03-20T14:35:34+00:00March 20th, 2019|Categories: Publications|Tags: , |

Author(s): Kjetil M. D. Hals and Karin Everschor-SitteTwo-dimensional materials such as graphene or hexagonal boron nitride are indispensable in industry. The recently discovered 2D ferromagnetic materials also promise to be vital for applications. In this work, we develop a phenomenological description of noncentrosymmetric 2D ferromagnets with trigon…[Phys. Rev. B 99, 104422] Published Wed Mar 20, 2019

Published in: "Physical Review B".

Characterization of Hydrogen Plasma Defined Graphene Edges. (arXiv:1903.07002v1 [cond-mat.mes-hall])

2019-03-19T04:30:33+00:00March 19th, 2019|Categories: Publications|Tags: , |

We investigate the quality of hydrogen plasma defined graphene edges by Raman spectroscopy, atomic resolution AFM and low temperature electronic transport measurements. The exposure of graphite samples to a remote hydrogen plasma leads to the formation of hexagonal shaped etch pits, reflecting the anisotropy of the etch. Atomic resolution AFM reveals that the sides of these hexagons are oriented along the zigzag direction of the graphite crystal lattice and the absence of the D-peak in the Raman spectrum indicates that the edges are high quality zigzag edges. In a second step of the experiment, we investigate hexagon edges created in single layer graphene on hexagonal boron nitride and find a substantial D-peak intensity. Polarization dependent Raman measurements reveal that hydrogen plasma defined edges consist of a mixture of zigzag and armchair segments. Furthermore, electronic transport measurements were performed on hydrogen plasma defined graphene nanoribbons which indicate a high quality of the bulk but a relatively low edge quality, in agreement with the Raman data. These findings are supported by tight-binding transport simulations. Hence, further optimization of the hydrogen plasma etching technique is required to obtain pure crystalline graphene edges.

Published : "arXiv Mesoscale and Nanoscale Physics".

Luminescent emission of excited Rydberg excitons from monolayer WSe2. (arXiv:1903.07073v1 [cond-mat.mes-hall])

2019-03-19T04:30:23+00:00March 19th, 2019|Categories: Publications|Tags: , , |

We report the experimental observation of radiative recombination from Rydberg excitons in a two-dimensional semiconductor, monolayer WSe2, encapsulated in hexagonal boron nitride. Excitonic emission up to the 4s excited state is directly observed in photoluminescence spectroscopy in an out-of-plane magnetic field up to 31 Tesla. We confirm the progressively larger exciton size for higher energy excited states through diamagnetic shift measurements. This also enables us to estimate the 1s exciton binding energy to be about 170 meV, which is significantly smaller than most previous reports. The Zeeman shift of the 1s to 3s states, from both luminescence and absorption measurements, exhibits a monotonic increase of g-factor, reflecting nontrivial magnetic-dipole-moment differences between ground and excited exciton states. This systematic evolution of magnetic dipole moments is theoretically explained from the spreading of the Rydberg states in momentum space.

Published : "arXiv Mesoscale and Nanoscale Physics".

Morphology, ordering, stability, and electronic structure of carbon-doped hexagonal boron nitride. (arXiv:1903.07357v1 [cond-mat.mtrl-sci])

2019-03-19T02:29:29+00:00March 19th, 2019|Categories: Publications|Tags: , , |

We present theoretical studies of morphology, stability, and electronic structure of monolayer hexagonal CBN alloys with rich content of h-BN and carbon concentration not exceeding 50 %. Our studies are based on the bond order type of the valence force field to account for the interactions between atomic constituents and Monte Carlo method with Metropolis algorithm to establish equilibrium distribution of atoms over the lattice. We find out that the phase separation into graphene and h-BN domains occurs in the majority of growth conditions. Only in N-rich growth conditions, it is possible to obtain quasi uniform distribution of carbon atoms over boron sublattice. We predict also that the energy gap in stoichiometric C$_x$(BN)$_{1-x}$ alloys exhibits extremely strong bowing.

Published in: "arXiv Material Science".

Thermal boundary conductance and phonon transmission in hexagonal boron nitride/graphene heterostructures. (arXiv:1903.06385v1 [cond-mat.mes-hall])

2019-03-18T04:30:31+00:00March 18th, 2019|Categories: Publications|Tags: , , |

Increased power density in modern microelectronics has led to thermal management challenges which can cause degradation in performance and reliability. In many high-power electronic devices, the power consumption and heat removal are limited by the thermal boundary conductance (TBC) at the interfaces of dissimilar materials. Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) have attracted interest as a conductor/insulator pair in next-generation devices because of their unique physical properties; however, the thermal transport at the interfaces must be understood to accurately predict the performance of heterostructures composed of these materials. We use time-domain thermoreflectance (TDTR) to estimate the TBC at the interface of h-BN and graphene to be 35.1 MW/m2-K. We compare the phonon transmission and TBC at the h-BN/graphene interface predicted by two different formulations of the diffuse mismatch model (DMM) for anisotropic materials. The piecewise anisotropic DMM model, which uses two different phonon velocities near the center and at edge of the first Brillouin zone, results in better prediction of phonon transmission rates. The phonon transmission and temperature dependence of TBC confirms the flexural branch in ab-plane and c-plane longitudinal acoustic branch of graphene and h-BN are the dominant contributor when implementing both the A-DMM and PWA-DMM models. The methodology used here can be employed to heterostructures of other 2D materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Indirect bandgap of hBN-encapsulated monolayer MoS2. (arXiv:1903.06427v1 [cond-mat.mes-hall])

2019-03-18T04:30:29+00:00March 18th, 2019|Categories: Publications|Tags: , |

We present measurements of temperature dependence of photoluminescence intensity from monolayer MoS2 encapsulated by hexagonal boron nitride (hBN) flakes. The obtained temperature dependence shows an opposite trend to that of previously observed in a monolayer MoS2 on a SiO2 substrate. Ab-initio bandstructure calculations have revealed that monolayer MoS2 encapsulated by hBN flakes have no longer a direct-gap semiconductor but an indirect-gap semiconductor. This is caused by orbital hybridization between MoS2 and hBN, which leads to upward shift of gamma-valley of MoS2. This work shows an important implication that the hBN-encapsulated structures used to address intrinsic properties of two-dimensional crystals can alter basic properties encapsulated materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Resonant optical second harmonic generation in graphene-based heterostructures. (arXiv:1903.06641v1 [cond-mat.mes-hall])

2019-03-18T04:30:24+00:00March 18th, 2019|Categories: Publications|Tags: , , , |

An optical Second-Harmonic Generation (SHG) allows to probe various structural and symmetry-related properties of materials, since it is sensitive to the inversion symmetry breaking in the system. Here, we investigate the SHG response from a single layer of graphene disposed on an insulating hexagonal Boron Nitride (hBN) and Silicon Carbide (SiC) substrates. The considered systems are described by a non-interacting tight-binding model with a mass term, which describes a non-equivalence of two sublattices of graphene when the latter is placed on a substrate. The resulting SHG signal linearly depends on the degree of the inversion symmetry breaking (value of the mass term) and reveals several resonances associated with the band gap, van Hove singularity, and band width. The difficulty in distinguishing between SHG signals coming from the considered heterostrusture and environment (insulating substrate) can be avoided applying a homogeneous magnetic field. The latter creates Landau levels in the energy spectrum and leads to multiple resonances in the SHG spectrum. Position of these resonances explicitly depends on the value of the mass term. We show that at energies below the band-gap of the substrate the SHG signal from the massive graphene becomes resonant at physically relevant values of the applied magnetic field, while the SHG response from the environment stays off-resonant.

Published : "arXiv Mesoscale and Nanoscale Physics".

Propane oxidative dehydrogenation over highly selective hexagonal boron nitride catalysts: The role of oxidative coupling of methyl

2019-03-16T02:37:08+00:00March 16th, 2019|Categories: Publications|Tags: |

Hexagonal boron nitride (h-BN) catalyst has recently been reported to be highly selective in oxidative dehydrogenation of propane (ODHP) for olefin production. In addition to propene, ethylene also forms with much higher overall selectivities to C2-products than to C1-products. In this work, we report that the reaction pathways over the

Published in: "Science Advances".

Enhanced toughness and thermal conductivity for epoxy resin with a core–shell structured polyacrylic modifier and modified boron nitride

2019-03-15T14:32:49+00:00March 15th, 2019|Categories: Publications|Tags: |

RSC Adv., 2019, 9,8654-8663DOI: 10.1039/C8RA10645B, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Chen Xu, Taoguang Qu, Xiaojie Zhang, Xiongwei Qu, Nongyue Wang, Qingxin Zhang, Beckry Abdel-Magid, Guohua LiA ternary composite is fabricated with exfoliated boron

Published in: "RSC Advances".

The electronic properties and band-gap discontinuities at the cubic boron nitride/diamond hetero-interface

2019-03-14T16:32:18+00:00March 14th, 2019|Categories: Publications|Tags: |

RSC Adv., 2019, 9,8435-8443DOI: 10.1039/C9RA00784A, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Dehe Zhao, Wei Gao, Yujing Li, Yuyuan Zhang, Hong YinClarifying the electronic states and structures of the c-BN/diamond interface is of extreme importance

Published in: "RSC Advances".

Weak localization in boron nitride encapsulated bilayer ${mathrm{MoS}}_{2}$

2019-03-14T14:35:13+00:00March 14th, 2019|Categories: Publications|Tags: , |

Author(s): Nikos Papadopoulos, Kenji Watanabe, Takashi Taniguchi, Herre S. J. van der Zant, and Gary A. SteeleWe present measurements of weak localization on hexagonal boron nitride encapsulated bilayer MoS2. From the analysis we obtain information regarding the phase coherence and the spin diffusion of the electrons. We find that the encapsulation with boron nitride provides higher mobilities in the sample…[Phys. Rev. B 99, 115414] Published Mon Mar 11, 2019

Published in: "Physical Review B".

Hybrid superlattices of graphene and hexagonal boron nitride: A ferromagnetic semiconductor at room temperature

2019-03-14T14:34:31+00:00March 14th, 2019|Categories: Publications|Tags: , |

Author(s): Rita Maji and Joydeep BhattacharjeeCarbon (C) -doped hexagonal boron nitride (hBN) has been experimentally reported to be ferromagnetic at room temperature. Substitution by C in hBN has also been reported to form islands of graphene. In this work, we derive a mechanistic understanding of ferromagnetism with graphene islands in hBN fr…[Phys. Rev. B 99, 125409] Published Fri Mar 08, 2019

Published in: "Physical Review B".

Quantitative analysis of the electronic decoupling of an organic semiconductor molecule at a metal interface by a monolayer of hexagonal boron nitride

2019-03-14T14:34:21+00:00March 14th, 2019|Categories: Publications|Tags: , |

Author(s): Christine Brülke, Timo Heepenstrick, Ina Krieger, Beatrice Wolff, Xiaosheng Yang, Ali Shamsaddinlou, Simon Weiß, François C. Bocquet, F. Stefan Tautz, Serguei Soubatch, and Moritz SokolowskiThe adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a…[Phys. Rev. B 99, 121404(R)] Published Mon Mar 11, 2019

Published in: "Physical Review B".

Conformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipation

2019-03-13T10:37:02+00:00March 13th, 2019|Categories: Publications|Tags: , |

Conformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipationConformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipation, Published online: 13 March 2019; doi:10.1038/s41467-019-09016-0Plasma-enhanced chemical vapour deposition (PECVD) is an industrially compatible microelectronics technology. Here, the authors use PECVD to obtain low-temperature, catalyst-free growth of poly-crystalline two-dimensional hexagonal-boron nitride, thus enabling superior thermal dissipation in WSe2 field-effect transistors with mobility up to 121 cm2 V−1 s−1.

Published in: "Nature Communications".

Hexagonal Boron Nitride Growth on Cu‐Si Alloy: Morphologies and Large Domains

2019-03-12T10:34:49+00:00March 12th, 2019|Categories: Publications|Tags: , , |

Large domains of hexagonal boron nitride (h‐BN) are deposited on silicon‐doped Cu substrate. A rich variety of h‐BN domain morphologies can be delicately controlled through the growth temperature, from fractal tree patterns to dendritic triangles. Abstract Controllable synthesis of high‐quality hexagonal boron nitride (h‐BN) is desired toward the industrial application of 2D devices based on van der Waals heterostructures. Substantial efforts are devoted to synthesize h‐BN on copper through chemical vapor deposition, which has been successfully applied to grow graphene. However, the progress in synthesizing h‐BN has been significantly retarded, and it is still challenging to realize millimeter‐scale domains and control their morphologies reliably. Here, the nucleation density of h‐BN on Cu is successfully reduced by over two orders of magnitude by simply introducing a small amount of silicon, giving rise to large triangular domains with maximum 0.25 mm lateral size. Moreover, the domain morphologies can be modified from needles, tree patterns, and leaf darts to triangles through controlling the growth temperature. The presence of silicon alters the growth mechanism from attachment‐limited mode to diffusion‐limited mode, leading to dendrite domains that are rarely observed on pure Cu. A phase‐field model is utilized to reveal the growing dynamics regarding B‐N diffusion, desorption, flux, and reactivity variables, and explain the morphology evolution. The work sheds lights on the h‐BN growth toward large single crystals and morphology probabilities.

Published in: "Small".

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