Unconventional pairing symmetry of interacting Dirac fermions on a $π$-flux lattice

2018-04-20T14:30:22+00:00 April 20th, 2018|Categories: Publications|

Author(s): Huaiming Guo, Ehsan Khatami, Yao Wang, Thomas P. Devereaux, Rajiv R. P. Singh, and Richard T. ScalettarThe pairing symmetry of interacting Dirac fermions on the π-flux lattice is studied with the determinant quantum Monte Carlo and numerical linked-cluster expansion methods. The s*- (i.e., extended s-) and d-wave pairing symmetries, which are distinct in the conventional square lattice, are degenerat…[Phys. Rev. B 97, 155146] Published Fri Apr 20, 2018

Published in: "Physical Review B".

Topological phase transitions and quantum Hall effect in the graphene family

2018-04-20T14:30:15+00:00 April 20th, 2018|Categories: Publications|Tags: |

Author(s): P. Ledwith, W. J. M. Kort-Kamp, and D. A. R. DalvitMonolayer staggered materials of the graphene family present intrinsic spin-orbit coupling and can be driven through several topological phase transitions using external circularly polarized lasers and static electric or magnetic fields. We show how topological features arising from photoinduced pha…[Phys. Rev. B 97, 165426] Published Fri Apr 20, 2018

Published in: "Physical Review B".

Efficient charge pump by pure mechanical resonators in graphene

2018-04-20T12:29:41+00:00 April 20th, 2018|Categories: Publications|Tags: |

Graphene is an ideal two-dimensional nanoelectromechanical material due to its outstanding elastic properties and superior electro-mechanical coupling. We study a graphene-based charge pump by two mechanical resonators out of phase. It is found that in the adiabatic limit, the pumped charge per mode is quantized in a pumping cycle and the electro-mechanical conversion efficiency is maximally saturated, as long as the mechanical lattice deformations produce a transport gap for massless Dirac electrons. The efficient charge pump originates from the definite chirality of Dirac electrons as well as the possible topological interface state forming in the evanescent modes. Our findings might shed light on enhancing the electro-mechanical conversion efficiency of graphene-based devices.

Published in: "EPL".

A Facile Strategy for Fabricating the Hierarchical Nanocomposites of V2O5 Nanowire Arrays on Three-Dimension N-doped Graphene aerogel with Synergistic Effect for Supercapacitor

2018-04-20T11:18:02+00:00 April 20th, 2018|Categories: Publications|Tags: , |

J. Mater. Chem. A, 2018, Accepted ManuscriptDOI: 10.1039/C8TA01448E, PaperWei Sun, Guohua Gao, Yuchuan Du, Kun Zhang, Guangming WuAs one of the most promising candidates for supercapcitor electrodes, vanadium pentoxide (V2O5) usually suffers from the poor electronic conductivity and small ionic diffusivity. The key

Published in: "Journal of Materials Chemistry A".

Biological recognition of graphene nanoflakes

2018-04-20T10:31:19+00:00 April 20th, 2018|Categories: Publications|Tags: |

Biological recognition of graphene nanoflakesBiological recognition of graphene nanoflakes, Published online: 20 April 2018; doi:10.1038/s41467-018-04009-xThe production of graphene nanoflakes for biological studies is usually done in a controlled chemical environment. Here, the authors develop a method to produce graphene nanoflakes in biofluids and find that their protein coats are different from that of other materials.

Published in: "Nature Communications".

Homogeneous growth of TiO 2 -based nanotubes on nitrogen-doped reduced graphene oxide and its enhanced performance as a Li-ion battery anode

2018-04-20T10:30:00+00:00 April 20th, 2018|Categories: Publications|Tags: , , |

The pursuit of a promising replacement candidate for graphite as a Li-ion battery anode, which can satisfy both engineering criteria and market needs has been the target of researchers for more than two decades. In this work, we have investigated the synergistic effect of nitrogen-doped reduced graphene oxide (NrGO) and nanotubular TiO 2 to achieve high rate capabilities with high discharge capacities through a simple, one-step and scalable method. First, nanotubes of hydrogen titanate were hydrothermally grown on the surface of NrGO sheets, and then converted to a mixed phase of TiO 2 -B and anatase (TB) by thermal annealing. Specific surface area, thermal gravimetric, structural and morphological characterizations were performed on the synthesized product. Electrochemical properties were investigated by cyclic voltammetry and cyclic charge/discharge tests. The prepared anode showed high discharge capacity of 150 mAh g −1 at 1 C current rate after 5…

Published in: "Nanotechnology".

Epitaxial and atomically thin graphene-metal hybrid catalyst films: the dual role of graphene as the support and the chemically-transparent protective cap

2018-04-20T10:28:17+00:00 April 20th, 2018|Categories: Uncategorized|Tags: , |

Energy Environ. Sci., 2018, Advance ArticleDOI: 10.1039/C8EE00539G, PaperAli Abdelhafiz, Adam Vitale, Parker Buntin, Ben deGlee, Corey Joiner, Alex Robertson, Eric M. Vogel, Jamie Warner, Faisal M. AlamgirRevolutionary catalyst protection by single layer graphene capping, tremendous catalyst lifetime longevity and activity enhancement towards oxygen reduction reaction.To cite this article before page numbers are assigned, use the DOI form of citation above.The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Energy & Environmental Science".

Correction: Interfacial growth of a metal-organic framework (UiO-66) on functionalized graphene oxide (GO) as a suitable seawater adsorbent for extraction of uranium(VI)

2018-04-20T09:17:59+00:00 April 20th, 2018|Categories: Publications|Tags: , |

J. Mater. Chem. A, 2018, Advance ArticleDOI: 10.1039/C8TA90090F, Correction Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Peipei Yang, Qi Liu, Jingyuan Liu, Hongsen Zhang, Zhanshuang Li, Rumin Li, Lianhe Liu, Jun WangTo cite this article before

Published in: "Journal of Materials Chemistry A".

Highly Interconnected Hollow Graphene Nanospheres as an Advanced High Energy – High power Cathode for Sodium Metal Batteries

2018-04-20T03:17:58+00:00 April 20th, 2018|Categories: Publications|Tags: , |

J. Mater. Chem. A, 2018, Accepted ManuscriptDOI: 10.1039/C8TA00153G, CommunicationRanjith Thangavel, Aravindaraj G Kannan, Rubha Ponraj, Xueliang Sun, Dong-Won Kim, Yun-Sung LeeDeveloping sodium based energy storage systems that retain high energy density at high power along with stable cycling is of paramount importance to

Published in: "Journal of Materials Chemistry A".

Probing the ultimate plasmon confinement limits with a van der Waals heterostructure

2018-04-20T02:27:39+00:00 April 20th, 2018|Categories: Publications|Tags: , , |

The ability to confine light into tiny spatial dimensions is important for applications such as microscopy, sensing, and nanoscale lasers. Although plasmons offer an appealing avenue to confine light, Landau damping in metals imposes a trade-off between optical field confinement and losses. We show that a graphene-insulator-metal heterostructure can overcome that trade-off, and demonstrate plasmon confinement down to the ultimate limit of the length scale of one atom. This is achieved through far-field excitation of plasmon modes squeezed into an atomically thin hexagonal boron nitride dielectric spacer between graphene and metal rods. A theoretical model that takes into account the nonlocal optical response of both graphene and metal is used to describe the results. These ultraconfined plasmonic modes, addressed with far-field light excitation, enable a route to new regimes of ultrastrong light-matter interactions.

Published in: "Science".

Origin of Nonideal Graphene-Silicon Schottky Junction

2018-04-20T00:29:29+00:00 April 20th, 2018|Categories: Publications|Tags: |

In this paper, we investigate the origin of the nonideal turn- ON characteristics of the graphene-silicon Schottky junction. Native oxide (SiO2) is proved to play a critical role in determining the behavior of a graphene-Si junction. Within the metal–oxide–semiconductor structure, the effective voltage drop across the junction degrades due to the capacitor network, which contributes to an increased ideality factor. Residual metal catalysts are detected, which act as recombination centers in the silicon and further degrade the ideality factor by enhancing the recombination current. The recombination current is found to be the dominant factor in making the junction nonideal. Forming gas annealing and the insertion of an interfacial dielectric restore the ideality of a graphene-Si junction by reducing the interface states and bulk recombination centers.

Published in: "IEEE Transactions on Electron Devices".

Highly Sensitive and Optically Transparent Resistive Pressure Sensors Based on a Graphene/Polyaniline-Embedded PVB Film

2018-04-20T00:29:27+00:00 April 20th, 2018|Categories: Publications|Tags: |

The development of a facile, low-temperature, and low-cost method to fabricate highly sensitive and optically transparent resistive pressure sensors remains a challenge. In this paper, a graphene/polyaniline-embedded polyvinyl butyral (GPANI-PVB) composite film is employed as the active layer in a resistive pressure sensor; this film is ultrathin, highly sensitive, optically transparent, anisotropically conductive, highly durable, highly flexible, and bending insensitive. A flexible touch panel based on the GPANI-PVB composite film is fabricated using a facile process, which supports multitouch and multilevel pressure detection. The GPANI-PVB composite film offers great potential for applications in flexible and transparent interactive electronic devices.

Published in: "IEEE Transactions on Electron Devices".

Spectrally narrow exciton luminescence from monolayer MoS2 exfoliated onto epitaxially grown hexagonal BN. (arXiv:1804.06623v1 [cond-mat.mtrl-sci])

2018-04-19T19:59:14+00:00 April 19th, 2018|Categories: Publications|Tags: , , |

The strong light-matter interaction in transition Metal dichalcogenides (TMDs) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation, which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS$_2$ and MoSe$_2$ MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then we cover them with exfoliated hBN to finally obtain an encapsulated sample : exfoliated hBN/TMD ML/MBE hBN. We observe an improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO$_2$ substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge free substrate for fabricating TMD-based heterostructures on a larger scale.

Published in: "arXiv Material Science".

Probing Nonlocal Effects in Metals with Graphene Plasmons. (arXiv:1804.06478v1 [cond-mat.mes-hall])

2018-04-19T19:59:11+00:00 April 19th, 2018|Categories: Publications|Tags: , |

In this paper we analyze the effects of nonlocality on the optical properties of a system consisting of a thin metallic film separated from a graphene sheet by a hexagonal boron nitride (hBN) layer. We show that nonlocal effects in the metal have a strong impact on the spectrum of the surface plasmon-polaritons on graphene. If the graphene sheet is shaped into a grating, we show that the extinction curves can be used to shed light on the importance of nonlocal effects in metals. Therefore, graphene surface plasmons emerge as a tool for probing nonlocal effects in metallic nanostructures, including thin metallic films. As a byproduct of our study, we show that nonlocal effects lead to smaller losses for the graphene plasmons than what is predicted by a local calculation. We show that these effects can be very well mimicked using a local theory with an effective spacer thickness larger than its actual value.

Published : "arXiv Mesoscale and Nanoscale Physics".

Deterministic coupling of site-controlled quantum emitters in monolayer semiconductors to plasmonic nanocavities. (arXiv:1804.06541v1 [cond-mat.mes-hall])

2018-04-19T19:59:10+00:00 April 19th, 2018|Categories: Publications|Tags: , |

Solid-state single-quantum emitters are a crucial resource for on-chip photonic quantum technologies and require efficient cavity-emitter coupling to realize quantum networks beyond the single-node level. Previous approaches to enhance light-matter interactions rely on forming nanocavities around randomly located quantum dots or color centers but lack spatial control of the quantum emitter itself that is required for scaling. Here we demonstrate a deterministic approach to achieve Purcell-enhancement at lithographically defined locations using the sharp corner of a metal nanocube for both electric field enhancement and to deform a two-dimensional material. For a 3 by 4 array of strain-induced exciton quantum emitters formed into monolayer WSe2 we show spontaneous emission rate enhancement with Purcell-factors (FP) up to FP=1050 (average FP=272), single-photon purification, and cavity-enhanced quantum yields increasing from initially 1 % to 15 %. The utility of our nanoplasmonic platform is applicable to other 2D material, including boron nitride, opening new inroads in quantum photonics.

Published : "arXiv Mesoscale and Nanoscale Physics".

Room Temperature Valley Polarization and Coherence in Transition Metal Dichalcogenide-Graphene van der Waals Heterostructures. (arXiv:1804.06725v1 [cond-mat.mes-hall])

2018-04-19T19:59:08+00:00 April 19th, 2018|Categories: Publications|Tags: , , , |

Van der Waals heterostructures made of graphene and transition metal dichalcogenides (TMD) are an emerging platform for opto-electronic, -spintronic and -valleytronic devices that could benefit from (i) strong light-matter interactions and spin-valley locking in TMDs and (ii) exceptional electron and spin transport in graphene. The operation of such devices requires significant valley polarization and valley coherence, ideally up to room temperature. Here, using a comprehensive Mueller polarimetry analysis, we report textit{artifact-free} room temperature degrees of valley polarization up to $40~%$ and, remarkably, of valley coherence up to $20~%$ in monolayer tungsten disulfide (WS$_2$)/graphene heterostructures. Valley contrasts have been particularly elusive in molybdenum diselenide (MoSe$_2$), even at cryogenic temperatures. Upon interfacing monolayer MoSe$_2$ with graphene, the room temperature degrees of valley polarization and coherence are as high as $14~%$ and $20~%$, respectively. Our results are discussed in light of recent reports of highly efficient interlayer coupling and exciton transfer in TMD/graphene heterostructures and hold promise for room temperature chiral light-matter interactions and coherent opto-valleytronic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Tunable Berry curvature, valley and spin Hall effect in Bilayer MoS$_2$. (arXiv:1804.06830v1 [cond-mat.mes-hall])

2018-04-19T19:59:07+00:00 April 19th, 2018|Categories: Publications|Tags: |

The chirality of electronic Bloch bands is responsible for many intriguing properties of layered two-dimensional materials. We show that in bilayers of transition metal dichalcogenides (TMDCs), unlike in few-layer graphene and monolayer TMDCs, both intra-layer and inter-layer couplings give important contributions to the Berry-curvature in the $K$ and $-K$ valleys of the Brillouin zone. The inter-layer contribution leads to the stacking dependence of the Berry curvature and we point out the differences between the commonly available 3R type and 2H type bilayers. Due to the inter-layer contribution the Berry curvature becomes highly tunable in double gated devices. We study the dependence of the valley Hall and spin Hall effects on the stacking type and external electric field. Although the valley and spin Hall conductivities are not quantized, in MoS$_2$ 2H bilayers they may change sign as a function of the external electric field which is reminiscent of the behaviour of lattice Chern insulators.

Published : "arXiv Mesoscale and Nanoscale Physics".

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