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

2018-09-21T14:33:18+00:00 September 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".

Quantum Nonlinear Optics in Atomically Thin Materials

2018-09-21T12:34:31+00:00 September 21st, 2018|Categories: Publications|

Author(s): Dominik S. Wild, Ephraim Shahmoon, Susanne F. Yelin, and Mikhail D. LukinWe show that a nonlinear optical response associated with a resonant, atomically thin material can be dramatically enhanced by placing it in front of a partially reflecting mirror, rendering otherwise weakly nonlinear systems suitable for experiments and applications involving quantum nonlinear opti…[Phys. Rev. Lett. 121, 123606] Published Fri Sep 21, 2018

Published in: "Physical Review Letters".

Tuning Anti-Klein to Klein Tunneling in Bilayer Graphene

2018-09-21T12:34:22+00:00 September 21st, 2018|Categories: Publications|Tags: |

Author(s): Renjun Du (杜人君), Ming-Hao Liu (劉明豪), Jens Mohrmann, Fan Wu (吴凡), Ralph Krupke, Hilbert von Löhneysen, Klaus Richter, and Romain DanneauWe show that in gapped bilayer graphene, quasiparticle tunneling and the corresponding Berry phase can be controlled such that they exhibit features of single-layer graphene such as Klein tunneling. The Berry phase is detected by a high-quality Fabry-Pérot interferometer based on bilayer graphene. B…[Phys. Rev. Lett. 121, 127706] Published Fri Sep 21, 2018

Published in: "Physical Review Letters".

Strain-tunable molecular doping in germanane: a first-principles study

2018-09-21T12:33:59+00:00 September 21st, 2018|Categories: Publications|

Germanane, fully hydrogenated germanene, has recently attracted great interest, both theoretical and experimental. In this paper we thoroughly study strain-tunable n/p-type doping in germanane by adsorption of tetrathiafulvalene (TTF)/tetracyanoquinodimethane (TCNQ) molecules through first-principles calculations. The results show that both TTF and TCNQ molecules can non-covalently functionalize the electronic properties of germanane. Not surprisingly, TTF molecular adsorption induces n-type doping in germanane because the TTF molecule is a typical electron donor. Moreover, a linearly tunable band gap of germanane and differing n-type doping strengths can be realized by a biaxial strain ranging from −3% to 3%. Analysis indicates that tensile strain would promote the doping effect whereas compressive strain would inhibit it. Comparatively, TCNQ molecular adsorption induces a germanane/TCNQ system which exhibits metallic characteristics. Surprisingly, however, under a tensile stra…

Published in: "Nanotechnology".

Resonant edge-state switching in polariton topological insulators. (arXiv:1809.07565v1 [physics.optics])

2018-09-21T04:30:24+00:00 September 21st, 2018|Categories: Publications|Tags: |

Topological insulators are unique devices supporting unidirectional edge states at their interfaces. Due to topological protection, such edge states persist in the presence of disorder and do not experience backscattering upon interaction with defects. We show that despite the topological protection and the fact that such states at the opposite edges of a ribbon carry opposite currents, there exists a physical mechanism allowing to resonantly couple topological excitations propagating at opposite edges. Such mechanism uses weak periodic temporal modulations of the system parameters and does not affect the internal symmetry and topology of the system. We study this mechanism in truncated honeycomb arrays of microcavity pillars, where topological insulation is possible for polaritons under the combined action of spin-orbit coupling and Zeeman splitting in the external magnetic field. The temporal modulation of the potential leads to a periodic switching between topological states with the same Bloch momentum, but located at the opposite edges. The switching rate is found to increase for narrower ribbon structures and for larger modulation depth, though it is changing nonmonotonically with the Bloch momentum of the input edge state. Our results provide a promising realization of a coupling device based on topologically protected states. The proposed scheme can be used in other topological photonic and condensed matter systems, including Floquet insulators and graphene.

Published : "arXiv Mesoscale and Nanoscale Physics".

Collisionless Transport Close to a Fermionic Quantum Critical Point in Dirac Materials. (arXiv:1801.03495v2 [cond-mat.mes-hall] UPDATED)

2018-09-21T04:30:22+00:00 September 21st, 2018|Categories: Publications|

Quantum transport close to a critical point is a fundamental, but enigmatic problem due to fluctuations, persisting at all length scales. We report the scaling of optical conductivity (OC) in the emph{collisionless} regime ($hbar omega gg k_B T$) in the vicinity of a relativistic quantum critical point, separating two-dimensional ($d=2$) massless Dirac fermions from a fully gapped insulator or superconductor. Close to such critical point gapless fermionic and bosonic excitations are strongly coupled, leading to a emph{universal} suppression of the inter-band OC as well as of the Drude peak (while maintaining its delta function profile) inside the critical regime, which we compute to the leading order in $1/N_f$- and $epsilon$-expansions, where $N_f$ counts fermion flavor number and $epsilon=3-d$. Correction to the OC at such a non-Gaussian critical point due to the long-range Coulomb interaction and generalizations of these scenarios to a strongly interacting three-dimensional Dirac or Weyl liquid are also presented, which can be tested numerically and possibly from non-pertubative gauge-gravity duality, for example.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electrical Parameters for Planar Transport in Graphene and 2-D Materials. (arXiv:1809.07319v1 [cond-mat.mes-hall])

2018-09-21T04:30:18+00:00 September 21st, 2018|Categories: Publications|Tags: |

Classical electrodynamics has been revisited with a view to recast the electrical parameters for planar transport in 2-dimensional (2-D) materials like graphene. In this attempt a new line integral, named transverse line integral, with extensive applications in 2-D, is defined. Since the existing divergence theorem in not applicable in 2-D, we introduced a new divergence theorem. A new definition for the in-plane flux of any 2-D vector is introduced. A new vector named electric vector potential is defined and Gauss law is modified in terms of the 2-D flux of the new vector. The new Gauss law in presence of dielectric is obtained and a new electric displacement vector is defined for the 2-D materials. The conduction and displacement current densities in 2-D are defined. Resistance and resistivity in 2-D materials are discussed. The continuity equation for planar transport is derived.

Published : "arXiv Mesoscale and Nanoscale Physics".

Quantum Hall Effect in Electron-Doped Black Phosphorus Field-Effect Transistors. (arXiv:1809.07536v1 [cond-mat.mtrl-sci])

2018-09-21T02:29:26+00:00 September 21st, 2018|Categories: Publications|Tags: |

The advent of black phosphorus field-effect transistors (FETs) has brought new possibilities in the study of two-dimensional (2D) electron systems. In a black phosphorus FET, the gate induces highly anisotropic 2D electron and hole gases. Although the 2D hole gas in black phosphorus has reached high carrier mobilities that led to the observation of the integer quantum Hall effect, the improvement in the sample quality of the 2D electron gas (2DEG) has however been only moderate; quantum Hall effect remained elusive. Here, we obtain high quality black phosphorus 2DEG by defining the 2DEG region with a prepatterned graphite local gate. The graphite local gate screens the impurity potential in the 2DEG. More importantly, it electrostatically defines the edge of the 2DEG, which facilitates the formation of well-defined edge channels in the quantum Hall regime. The improvements enable us to observe precisely quantized Hall plateaus in electron-doped black phosphorus FET. Magneto-transport measurements under high magnetic fields further revealed a large effective mass and an enhanced Land’e g-factor, which points to strong electron-electron interaction in black phosphorus 2DEG. Such strong interaction may lead to exotic many-body quantum states in the fractional quantum Hall regime.

Published in: "arXiv Material Science".

Fabrication of photonic nanostructures from hexagonal boron nitride. (arXiv:1809.07561v1 [physics.app-ph])

2018-09-21T02:29:24+00:00 September 21st, 2018|Categories: Publications|Tags: |

Growing interest in devices based on layered van der Waals (vdW) materials is motivating the development of new nanofabrication methods. Hexagonal boron nitride (hBN) is one of the most promising materials for studies of quantum photonics and polaritonics. Here, we report in detail on a promising nanofabrication processes used to fabricate several hBN photonic devices using a hybrid electron beam induced etching (EBIE) and reactive ion etching (RIE) technique. We highlight the shortcomings and benefits of RIE and EBIE and demonstrate the utility of the hybrid approach for the fabrication of suspended and supported device structures with nanoscale features and highly vertical sidewalls. Functionality of the fabricated devices is proven by measurements of high quality cavity optical modes (Q~1500). Our nanofabrication approach constitutes an advance towards an integrated, monolithic quantum photonics platform based on hBN and other layered vdW materials.

Published in: "arXiv Material Science".

Controlled Inertial Cavitation as a Route to High Yield Liquid Phase Exfoliation of Graphene. (arXiv:1809.07630v1 [cond-mat.mtrl-sci])

2018-09-21T02:29:22+00:00 September 21st, 2018|Categories: Publications|Tags: |

Ultrasonication is widely used to exfoliate two dimensional (2D) van der Waals layered materials such as graphene. Its fundamental mechanism, inertial cavitation, is poorly understood and often ignored in ultrasonication strategies resulting in low exfoliation rates, low material yields and wide flake size distributions, making the graphene dispersions produced by ultrasonication less economically viable. Here we report that few-layer graphene yields of up to 18% in three hours without introduction of basal plane defects can be achieved by optimising inertial cavitation during ultrasonication. We demonstrate that the yield and the graphene flake dimensions exhibit a power law relationship with inertial cavitation dose. Furthermore, inertial cavitation is shown to preferentially exfoliate larger graphene flakes which causes the exfoliation rate to decrease as a function of sonication time. This study demonstrates that measurement and control of inertial cavitation is critical in optimising the high yield sonication-assisted aqueous liquid phase exfoliation of size-selected nanomaterials. Future development of this method should lead to the development of high volume flow cell production of 2D van der Waals layered nanomaterials.

Published in: "arXiv Material Science".

Atomically Thin CBRAM Enabled by 2-D Materials: Scaling Behaviors and Performance Limits

2018-09-21T00:34:31+00:00 September 21st, 2018|Categories: Publications|Tags: |

Reducing the energy and power dissipation of conductive bridge random access memory (CBRAM) cells is of critical importance for their applications in future Internet of Things (IoT) device and neuromorphic computing platforms. Atomically thin CBRAMs enabled by 2-D materials are studied theoretically by using 3-D kinetic Monte Carlo simulations together with experimental characterization. The results indicate the performance potential of attoJoule energy dissipation for intrinsic filament formation and a filament size of a single atomistic chain in such a CBRAM cell. The atomically thin CBRAM cells also show qualitatively different features from conventional CBRAM cells, including complete rupture of the filament in the reset stage and comparable forming and set voltages. The scaling and variability of the CBRAM cells down to sub-nanometer size of the switching layer as realized in the experiment are systematically studied, which indicates performance improvement and increased relative variability as the switching layer scales down. The results establish the ultimate limits of the size and energy scaling for CBRAM cells and illustrate the unique application of 2-D materials in ultralow power memory devices.

Published in: "IEEE Transactions on Electron Devices".

Modeling of Electron Devices Based on 2-D Materials

2018-09-21T00:34:29+00:00 September 21st, 2018|Categories: Publications|Tags: , |

The advent of graphene and related 2-D materials has attracted the interest of the electron device research community in the past 14 years. The possibility to boost the transistor performance and the prospects to build novel device concepts with 2-D materials and their heterostructures has awakened a strong experimental interest that requires continuous support from modeling. In this paper, we review the state of the art in the simulation of electron devices based on 2-D materials. We outline the main methods to model the electronic bandstructure and to study electron transport, classifying them in terms of accuracy and computational cost. We briefly discuss, how they can be combined in a multiscale approach to provide a quantitative understanding of the mechanisms determining the operation of electron devices, and we examine the application of these methods to different families of 2-D materials. Finally, we shortly analyze the main open challenges of modeling 2-D-based electron devices.

Published in: "IEEE Transactions on Electron Devices".

<italic>Ab Initio</italic> Simulation of Band-to-Band Tunneling FETs With Single- and Few-Layer 2-D Materials as Channels

2018-09-21T00:34:28+00:00 September 21st, 2018|Categories: Publications|

Full-band atomistic quantum transport simulations based on first principles are employed to assess the potential of band-to-band tunneling FETs (TFETs) with a 2-D channel material as future electronic circuit components. We demonstrate that single-layer (SL) transition metal dichalcogenides are not well suited for TFET applications. There might, however, exist a great variety of 2-D semiconductors that have not even been exfoliated yet; this paper pinpoints some of the most promising candidates among them to realize highly efficient TFETs. SL SnTe, As, TiNBr, and Bi are all found to ideally deliver ON-currents larger than 100 $mu text{A}/mu text{m}$ at 0.5-V supply voltage and 0.1 nA/$mu text{m}$ OFF-current value. We show that going from single to multiple layers can boost the TFET performance as long as the gain from a narrowing bandgap exceeds the loss from the deteriorating gate control. Finally, a 2-D van der Waals heterojunction TFET is revealed to perform almost as well as the best SL homojunction, paving the way for research in optimal 2-D material combinations.

Published in: "IEEE Transactions on Electron Devices".

Channel Thickness Optimization for Ultrathin and 2-D Chemically Doped TFETs

2018-09-21T00:34:26+00:00 September 21st, 2018|Categories: Publications|Tags: , |

The 2-D material-based TFETs are among the most promising candidates for low-power electronics applications since they offer ultimate gate control and high-current drives that are achievable through small tunneling distances ($Lambda $ ) during the device operation. The ideal device is characterized by a minimized $Lambda $ . However, devices with the thinnest possible body do not necessarily provide the best performance. For example, reducing the channel thickness (${T}_{text {ch}}$ ) increases the depletion width in the source, which can be a significant part of the total $Lambda $ . Hence, it is important to determine the optimum ${T}_{text {ch}}$ for each channel material individually. In this paper, we study the optimum ${T}_{text {ch}}$ for three channel materials: WSe2, black phosphorus, and InAs using full-band self-consistent quantum transport simulations. To identify the ideal ${T}_{text {ch}}$ for each material at a specific doping density, a new analytic model is proposed and benchmarked against the numerical simulations.

Published in: "IEEE Transactions on Electron Devices".

The Understanding of SiNR and GNR TFETs for Analog and RF Application With Variation of Drain-Doping Molar Fraction

2018-09-21T00:34:24+00:00 September 21st, 2018|Categories: Publications|Tags: , , |

The 1-D nanoribbon (NR) of monolayer materials has gained immense interest due to their unique properties qualitatively distinct from their bulk properties and the demand for nanoscale applications. In this paper, the quantum transport properties of two most prominent 2-D materials, i.e., silicene NR (SiNR) and a graphene NR (GNR) tunnel field-effect transistor (TFET) with the effect of different dopant molar fractions in the drain region are studied numerically using nonequilibrium Green’s function formalism. In SiNR TFET, higher on-state current (${I}_{ mathrm{scriptscriptstyle ON}}$ ) is observed due to wider tunneling energy window and high transmission probability of carriers. In order to observe the effect of variation of doping density in the drain region, we have studied analog figures of merit such as the transconductance (${g}_{m}$ ), output resistance (${r}_{o}$ ), transconductance generation factor (${g}_{m}/{I}_{D}$ ), and the intrinsic gain (${g}_{m}{r}_{o}$ ) for different molar fractions. Similarly, we have evaluated the RF performance of the SiNR and GNR TFETs as a function of cutoff frequency (${f}_{T}$ ), gate capacitance (${C}_{G}$ ), and transport delay ($tau$ ).

Published in: "IEEE Transactions on Electron Devices".

An Intuitive Equivalent Circuit Model for Multilayer Van Der Waals Heterostructures

2018-09-21T00:34:22+00:00 September 21st, 2018|Categories: Publications|Tags: , , |

Stacks of 2-D materials, known as van der Waals (vdW) heterostructures, have gained vast attention due to their interesting electrical and optoelectronic properties. This paper presents an intuitive circuit model for the out-of-plane electrostatics of a vdW heterostructure composed of an arbitrary number of layers of 2-D semiconductors, graphene, or metal. We explain the mapping between the out-of-plane energy band diagram and the elements of the equivalent circuit. Although the direct solution of the circuit model for an n-layer structure is not possible due to the variable, nonlinear quantum capacitance of each layer, this paper uses the intuition gained from the energy band picture to provide an efficient solution in terms of a single variable. Our method employs Fermi–Dirac statistics while properly modeling the finite density of states (DOS) of 2-D semiconductors and graphene. The approach circumvents difficulties that arise in commercial TCAD tools, such as the proper handling of the 2-D DOS and the simulation boundary conditions when the structure terminates with a nonmetallic material. Based on this methodology, we have developed 2dmatstacks, an open-source tool freely available on nanohub.org. Overall, this paper equips researchers to analyze, understand, and predict experimental results in complicated vdW stacks.

Published in: "IEEE Transactions on Electron Devices".

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