Graphene

/Tag: Graphene

Graphene on Group‐IV Elementary Semiconductors: The Direct Growth Approach and Its Applications

2019-02-16T22:37:40+00:00February 16th, 2019|Categories: Publications|Tags: , |

Research on the integration of graphene with group‐IV elementary semiconductors, which can complement and enhance the intrinsic properties of the semiconductor through graphene’s superior and unique properties, is actively under way. The recent progress in the direct growth of graphene on the Si and Ge surface and its applications are comprehensively summarized. Abstract Since the first development of large‐area graphene synthesis by the chemical vapor deposition (CVD) method in 2009, CVD‐graphene has been considered to be a key material in the future electronics, energy, and display industries, which require transparent, flexible, and stretchable characteristics. Although many graphene‐based prototype applications have been demonstrated, several important issues must be addressed in order for them to be compatible with current complementary metal‐oxide‐semiconductor (CMOS)‐based manufacturing processes. In particular, metal contamination and mechanical damage, caused by the metal catalyst for graphene growth, are known to cause severe and irreversible deterioration in the performance of devices. The most effective way to solve the problems is to grow the graphene directly on the semiconductor substrate. Herein, recent advances in the direct growth of graphene on group‐IV semiconductors are reviewed, focusing mainly on the growth mechanism and initial growth behavior when graphene is synthesized on Si and Ge. Furthermore, recent progress in the device applications of graphene with Si and Ge are presented. Finally, perspectives for future research in graphene with a semiconductor are discussed.

Published in: "Advanced Materials".

Improved Charge Extraction Beyond Diffusion Length by Layer‐by‐Layer Multistacking Intercalation of Graphene Layers inside Quantum Dots Films

2019-02-16T22:37:33+00:00February 16th, 2019|Categories: Publications|Tags: |

A novel architecture based on layer‐by‐layer multistacking intercalation of graphene inside quantum dot (QD) films is studied. The intercalated graphene layers ensure improved charge extraction beyond the diffusion length of the QDs, offering superior quantum efficiency over single‐bottom graphene/QD devices, and overcoming the restriction that the diffusion length imposes on film thickness. Abstract Charge collection is critical in any photodetector or photovoltaic device. Novel materials such as quantum dots (QDs) have extraordinary light absorption properties, but their poor mobility and short diffusion length limit efficient charge collection using conventional top/bottom contacts. In this work, a novel architecture based on multiple intercalated chemical vapor deposition graphene monolayers distributed in an orderly manner inside a QD film is studied. The intercalated graphene layers ensure that at any point in the absorbing material, photocarriers will be efficiently collected and transported. The devices with intercalated graphene layers have superior quantum efficiency over single‐bottom graphene/QD devices, overcoming the known restriction that the diffusion length imposes on film thickness. QD film with increased thickness shows efficient charge collection over the entire λ ≈ 500–1000 nm spectrum. This architecture could be applied to boost the performance of other low‐cost materials with poor mobility, allowing efficient collection for films thicker than their diffusion length.

Published in: "Advanced Materials".

Graphene Electrodes: Quantitative Principles for Precise Engineering of Sensitivity in Graphene Electrochemical Sensors (Adv. Mater. 6/2019)

2019-02-16T22:37:27+00:00February 16th, 2019|Categories: Publications|Tags: |

In article number 1805752, Davood Shahrjerdi and co‐workers reveal the quantitative relationships between the structural defects of graphene and the sensitivity of electrodes made of it. Using the physics of point defects in graphene, a microscopic model is proposed, which can quantitatively explain the relationship between sensitivity and average density of point defects in graphene electrodes. In addition, the model is shown to accurately guide nanoengineering of the structure of graphene, resulting in homogeneous miniaturized sensors with unprecedented sensitivity.

Published in: "Advanced Materials".

Photodetectors: Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions (Adv. Mater. 6/2019)

2019-02-16T22:37:25+00:00February 16th, 2019|Categories: Publications|Tags: , , , |

In article number 1805656, Rui Chen, Liyuan Zhang, Youpin Gong, and co‐workers develop an h‐BN/MoTe2/graphene/SnS2/h‐BN van der Waals heterostructure to realize an ultrahigh‐sensitivity broadband (405–1550 nm) photodetector, due to its unique advantages for high‐efficiency light absorption and exciton dissociation. Graphene plays a key role in enhancing the sensitivity and broadening the spectral range, providing a viable approach toward future ultrahigh sensitivity and broadband photodetectors.

Published in: "Advanced Materials".

Kerr Nonlinearity in 2D Graphdiyne for Passive Photonic Diodes

2019-02-16T22:37:23+00:00February 16th, 2019|Categories: Publications|Tags: |

2D graphdiyne is demonstrated to exhibit a strong light–matter interaction with high stability to achieve a broadband Kerr nonlinear optical response, which is useful in photonic devices. Based on the strong Kerr nonlinearity of 2D graphdiyne, a nonlinear photonic diode that breaks time‐reversal symmetry to realize unidirectional excitation of Kerr nonlinearity is demonstrated. Abstract Graphdiyne is a new carbon allotrope comprising sp‐ and sp2‐hybridized carbon atoms arranged in a 2D layered structure. In this contribution, 2D graphdiyne is demonstrated to exhibit a strong light–matter interaction with high stability to achieve a broadband Kerr nonlinear optical response, which is useful for nonreciprocal light propagation in passive photonic diodes. Furthermore, advantage of the unique Kerr nonlinearity of 2D graphdiyne is taken and a nonreciprocal light propagation device is proposed based on the novel similarity comparison method. Graphdiyne has demonstrated a large nonlinear refractive index in the order of ≈10−5 cm2 W−1, comparing favorably to that of graphene. Based on the strong Kerr nonlinearity of 2D graphdiyne, a nonlinear photonic diode that breaks time‐reversal symmetry is demonstrated to realize the unidirectional excitation of Kerr nonlinearity, which can be regarded as a significant demonstration of a graphdiyne‐based prototypical application in nonlinear photonics and might suggest an important step toward versatile graphdiyne‐based advanced passive photonics devices in the future.

Published in: "Advanced Materials".

Extraordinary tensile strength and ductility of scalable nanoporous graphene

2019-02-16T04:47:11+00:00February 16th, 2019|Categories: Publications|Tags: |

While the compressive strength-density scaling relationship of ultralight cellular graphene materials has been extensively investigated, high tensile strength and ductility have not been realized in the theoretically strongest carbon materials because of high flaw sensitivity under tension and weak van der Waals interplanar bonding between graphene sheets. In this study,

Published in: "Science Advances".

Geometry modulation of ion diffusion through layered asymmetric graphene oxide membranes

2019-02-15T20:34:50+00:00February 15th, 2019|Categories: Publications|Tags: , |

Chem. Commun., 2019, Accepted ManuscriptDOI: 10.1039/C9CC00239A, CommunicationJinlei Yang, Xiaopeng Zhang, Fengxiang Chen, Lei JiangThe asymmetric ion diffusion phenomenon through 2D nanofluidic thickness gradient membrane under concentration gradient is reported. The preferential direction for ion diffusion is from thick side toward thin side of…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

Valley-polarized tunneling currents in bilayer graphene tunneling transistors

2019-02-15T14:37:32+00:00February 15th, 2019|Categories: Publications|Tags: , |

Author(s): J. J. P. Thompson, D. J. Leech, and M. Mucha-KruczyńskiWe study theoretically the electron current across a monolayer graphene/hexagonal boron nitride/bilayer graphene tunneling junction in an external magnetic field perpendicular to the layers. We show that change in effective tunneling barrier width for electrons on different graphene layers of bilaye…[Phys. Rev. B 99, 085420] Published Fri Feb 15, 2019

Published in: "Physical Review B".

Correlated Insulating and Superconducting States in Twisted Bilayer Graphene Below the Magic Angle. (arXiv:1902.05151v1 [cond-mat.mes-hall])

2019-02-15T04:30:35+00:00February 15th, 2019|Categories: Publications|Tags: , |

The emergence of flat bands and correlated behaviors in ‘magic angle’ twisted bilayer graphene (tBLG) has sparked tremendous interest, though many aspects of the system are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of approximately 0.93, which is smaller than the magic angle by 15%. At an electron concentration of +/-5 electrons per moire unit cell, we observe a narrow resistance peak with an activation energy gap of approximately 0.1 meV, indicating the existence of an additional correlated insulating state. This is consistent with theory predicting the presence of a high-energy band with an energetically flat dispersion. At a doping of +/-12 electrons per moire unit cell we observe a resistance peak due to the presence of Dirac points in the spectrum. Our results reveal that the magic range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG.

Published : "arXiv Mesoscale and Nanoscale Physics".

Breakdown of the Hebel-Slichter effect in superconducting graphene due to the emergence of Yu-Shiba-Rusinov states at magnetic resonant scatterers. (arXiv:1902.05474v1 [cond-mat.mes-hall])

2019-02-15T04:30:29+00:00February 15th, 2019|Categories: Publications|Tags: |

We study theoretically the relaxation of electron spins in graphene in proximity to an $s$-wave superconductor in the presence of resonant magnetic and spin-orbit impurities. Off resonance, the relaxation behaves as predicted from superconducting coherence: with lower temperatures the spin relaxation increases when electrons scatter off magnetic impurities (Hebel-Slichter effect), and decreases when the scatterers induce spin-orbit coupling. This distinct temperature dependence, not available in the normal state, can uniquely discriminate between the two scattering mechanisms. But the Hebel-Slichter picture breaks down at resonances. The emergence of Yu-Shiba-Rusinov bound states within the superconducting gap shifts the spectral weight of the magnetic resonances and leads to a significant decrease of the spin relaxation rate at lower temperatures. Our findings should be valid for generic $s$-wave superconductors that host resonant magnetic impurities.

Published : "arXiv Mesoscale and Nanoscale Physics".

Graphene thermal break-down induced by anharmonic bending mode. (arXiv:1902.05111v1 [cond-mat.mes-hall])

2019-02-15T04:30:25+00:00February 15th, 2019|Categories: Publications|Tags: , |

The abrupt loss of mechanical stability of two-dimensional graphene-type crystals at a certain transition temperature is described. At this temperature, the graphene state with practically zero-speed bending sound and developed bending fluctuations becomes energetically favorable. Such phenomenon, akin to melting, is naturally caused by the anharmonicity of crystal oscillations. In order to circumvent the known difficulties associated with taking into account the anharmonic effects, we propose an original pseudo-harmonic approximation, within which we determine the free energy of the anharmonic crystal and find a numerical characteristic for the intensity of bending vibrations at transition temperature. This characteristic is similar to the empiric Lindemann criterion for the melting phenomenon. At the same time, in contrast to the conventional Lindemann criterion, the found characteristic is explicitly expressed through the graphene bending moduli of the second, third, and fourth orders.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observation of pseudospin Berry phase as a signature of nontrivial band topology in a coupled electron-hole system. (arXiv:1902.05310v1 [cond-mat.mes-hall])

2019-02-15T02:30:58+00:00February 15th, 2019|Categories: Publications|Tags: , |

Electron motion in crystals is governed by the coupling between crystal momentum and internal degrees of freedom such as spin implicit in the band structure. The description of this coupling in terms of a momentum-dependent effective field and the resultant Berry phase has renovated the understanding of diverse phenomena including various Hall effects, which underlies the discovery of new states of matter exemplified by topological insulators. While experimental studies on topological systems have focused on the gapless states that emerge at the surfaces or edges, the underlying nontrivial topology in the bulk has not been manifested. Here we report the observation of Berry’s phase in magneto-oscillations and quantum Hall effects of a coupled electron-hole system hosted in quantum wells with inverted bands. In contrast to massless Dirac fermions in graphene, for which Berry’s phase $Gamma$ is quantized at $pi$, we observe that $Gamma$ varies with the Fermi level $E_mathrm{F}$, passing through $pi$ as $E_mathrm{F}$ traverses the energy gap that opens due to electron-hole hybridization. We show that the evolution of $Gamma$ is a manifestation of the pseudospin texture that encodes the momentum-dependent electron-hole coupling and is therefore a signature of the nontrivial band topology. Our results suggest a way to engineering Berry curvature exploiting the bulk states, which may provide new functionalities in spintronics applications.

Published in: "arXiv Material Science".

Negative reflection of elastic guided waves in chaotic and random scattering media. (arXiv:1710.06150v2 [cond-mat.mtrl-sci] UPDATED)

2019-02-15T02:30:49+00:00February 15th, 2019|Categories: Publications|Tags: |

The propagation of waves in complex media can be harnessed either by taming the incident wave-field impinging on the medium or by forcing waves along desired paths through its careful design. These two alternative strategies have given rise to fascinating concepts such as time reversal or negative refraction. Here, we show how these two processes are intimately linked through the negative reflection phenomenon. A negative reflecting mirror converts a wave of positive phase velocity into its negative counterpart and vice versa. In this article, we experimentally demonstrate this phenomenon with elastic waves in a 2D billiard and in a disordered plate by means of laser interferometry. Despite the complexity of such configurations, the negatively reflected wave field focuses back towards the initial source location, thereby mimicking a phase conjugation operation while being a fully passive process. The super-focusing capability of negative reflection is also highlighted in a monochromatic regime. The negative reflection phenomenon is not restricted to guided elastic waves since it can occur in zero-gap systems such as photonic crystals, chiral metamaterials or graphene. Negative reflection can thus become a tool of choice for the control of waves in all fields of wave physics.

Published in: "arXiv Material Science".

Optical absorption in monolayer ${mathrm{SnO}}_{2}$

2019-02-14T14:41:18+00:00February 14th, 2019|Categories: Publications|Tags: |

Author(s): C. E. EkumaSince the discovery of graphene, considerable research efforts have focused on understanding the properties of other two-dimensional materials. Herein, based on ab initio many-body calculations, we report the optical properties of monolayer SnO2. First, we apply the first-principles density function…[Phys. Rev. B 99, 075421] Published Thu Feb 14, 2019

Published in: "Physical Review B".

Tuning the topological insulator states of artificial graphene

2019-02-14T14:41:16+00:00February 14th, 2019|Categories: Publications|Tags: , |

Author(s): H. D. Scammell and O. P. SushkovWe develop a robust, nonperturbative approach to study the band structure of artificial graphene. Artificial graphene, as considered here, is generated by imposing a superlattice structure on top of a two-dimensional hole gas in a semiconductor heterostructure, where the hole gas naturally possesses…[Phys. Rev. B 99, 085419] Published Thu Feb 14, 2019

Published in: "Physical Review B".

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