Generation of sub-20-fs pulses from a graphene mode-locked laser. (arXiv:1901.05069v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:55+00:00January 17th, 2019|Categories: Publications|Tags: , |

We demonstrate, what is to our knowledge, the shortest pulses directly generated to date from a solid-state laser, mode locked with a graphene saturable absorber (GSA). In the experiments, a low-threshold diode-pumped Cr3+:LiSAF laser was used near 850 nm. At a pump power of 275 mW provided by two pump diodes, the Cr3+:LiSAF laser produced nearly transform-limited, 19-fs pulses with an average output power of 8.5 mW. The repetition rate was around 107 MHz, corresponding to a pulse energy and peak power of 79 pJ and 4.2 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed started by the GSA and not by Kerr lens mode locking.

Published in: "arXiv Material Science".

An Element Replacement Approach by Reaction with Lewis acidic Molten Salts to Synthesize Nanolaminated MAX Phases and MXenes. (arXiv:1901.05120v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:53+00:00January 17th, 2019|Categories: Publications|

Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this element replacement reaction between Zn2+ cation in molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), the novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excessive ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. Moreover, the etching effect of Lewis acid in molten salts provides a clean and safe chemistry for the delamination of MAX phase to obtain MXene when compared with the commonly used HF-etching process.

Published in: "arXiv Material Science".

Discovery of Hidden Classes of Layered Electrides by Extensive High-throughput Materials Screening. (arXiv:1901.05121v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:49+00:00January 17th, 2019|Categories: Publications|Tags: |

Despite their extraordinary properties, electrides are still a relatively unexplored class of materials with only a few compounds grown experimentally. Especially for layered electrides, the current researches mainly focus on several isostructures of Ca2N with similar interlayer two-dimensional (2D) anionic electrons. An extensive screening for different layered electrides is still missing. Here, by screening materials with anionic electrons for the structures in Materials Project, we uncover 12 existing materials as new layered electrides. Remarkably, these layered electrides demonstrate completely different properties from Ca2N. For example, unusual fully spin-polarized zero-dimensional (0D) anionic electrons are shown in metal halides with MoS2-like structures; unique one-dimensional (1D) anionic electrons are confined within the tubes of the quasi-1D structures; a coexistence of magnetic and non-magnetic anionic electrons is found in ZrCl-like structures and a new ternary Ba2LiN with both 0D and 1D anionic electrons. These materials not only significantly increase the pool of experimentally synthesizable layered electrides but also are promising to be exfoliated into advanced 2D materials.

Published in: "arXiv Material Science".

Tin+1Cn MXene with fully saturated and thermally stable Cl terminations. (arXiv:1901.05212v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:45+00:00January 17th, 2019|Categories: Publications|Tags: |

MXenes are a rapidly growing family of 2D materials that exhibit a highly versatile structure and composition, allowing for significant tuning of the material properties. These properties are, however, ultimately limited by the surface terminations, which are typically a mixture of species, including F and O that are inherent to the MXene processing. Other and robust terminations are lacking. Here, we apply high-resolution scanning transmission electron microscopy (STEM), corresponding image simulations and first-principles calculations to investigate the surface terminations on MXenes synthesized from MAX phases through Lewis acidic melts. The results show that atomic Cl terminates the synthesized MXenes, with mere residual presence of other termination species. Furthermore, in situ STEM-electron energy loss spectroscopy (EELS) heating experiments show that the Cl terminations are stable up to 750 {deg}C. Thus, we present an attractive new termination that widely expands the MXenes functionalization space and enable new applications.

Published in: "arXiv Material Science".

Phonon polaritonics in two-dimensional materials. (arXiv:1901.05308v1 [cond-mat.mes-hall])

2019-01-17T02:29:43+00:00January 17th, 2019|Categories: Publications|Tags: , |

Extreme confinement of electromagnetic energy by phonon polaritons holds the promise of strong and new forms of control over the dynamics of matter. To bring such control to the atomic-scale limit, it is important to consider phonon polaritons in two-dimensional (2D) systems. Recent studies have pointed out that in 2D, splitting between longitudinal and transverse optical (LO and TO) phonons is absent at the $Gamma$ point, even for polar materials. Does this lack of LO–TO splitting imply the absence of a phonon polariton in polar monolayers? Here, we derive a first-principles expression for the conductivity of a polar monolayer specified by the wavevector-dependent LO and TO phonon dispersions. In the long-wavelength (local) limit, we find a universal form for the conductivity in terms of the LO phonon frequency at the $Gamma$ point, its lifetime, and the group velocity of the LO phonon. Our analysis reveals that the phonon polariton of 2D is simply the LO phonon of the 2D system. For the specific example of hexagonal boron nitride (hBN), we estimate the confinement and propagation losses of the LO phonons, finding that high confinement and reasonable propagation quality factors coincide in regions which may be difficult to detect with current near-field optical microscopy techniques. Finally, we study the interaction of external emitters with two-dimensional hBN nanostructures, finding extreme enhancement of spontaneous emission due to coupling with localized 2D phonon polaritons, and the possibility of multi-mode strong and ultra-strong coupling between an external emitter and hBN phonons. This may lead to

Published in: "arXiv Material Science".

First Principles Study of Intrinsic and Extrinsic Point Defects in Monolayer WSe2. (arXiv:1901.05238v1 [cond-mat.mtrl-sci])

2019-01-17T02:29:36+00:00January 17th, 2019|Categories: Publications|Tags: , |

We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the formation energies of these defects significantly. For the extrinsic point defects, we focus on O, O2, H, H2 and C interacting with perfect WSe2 and its intrinsic point defects. The preferred binding site in perfect WSe2 is the interstitial site for atomic O, H and C. These interstitial defects have no gap states. The gap states of the intrinsic defects are modified by interaction with O, O2, H, H2 and C. In particular, the gap states of Sevac and 2Sevac are completely removed by interaction with O and O2. This is consistent with the significantly larger stability of O-related defects compared to H- and C-related defects. The preferred binding site for O is Sevac, while that for H is SeW. H bonded to SeW results in spin-polarized gap states, which may be useful in defect engineering for spintronics applications. The charge transition levels and ionization energies of these defects are also computed. H in the interstitial site

Published in: "arXiv Material Science".

Two-dimensional materials in semiconductor photoelectrocatalytic systems for water splitting

2019-01-16T16:32:40+00:00January 16th, 2019|Categories: Uncategorized|Tags: |

Energy Environ. Sci., 2019, 12,59-95DOI: 10.1039/C8EE00886H, Review ArticleMonireh Faraji, Mahdieh Yousefi, Samira Yousefzadeh, Mohammad Zirak, Naimeh Naseri, Tae Hwa Jeon, Wonyong Choi, Alireza Z. MoshfeghHydrogen production via solar water splitting can be enhanced by combining semiconductors with various 2-dimensional materials.The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Energy & Environmental Science".

Multiscale Design of Graphyne‐Based Materials for High‐Performance Separation Membranes

2019-01-16T14:37:26+00:00January 16th, 2019|Categories: Publications|

The mechanical, thermal, and chemical properties of different types of graphynes are reviewed. The studies include the analysis of crack propagation, stress concentrations, atomic stress distributions under tensile loads, and Fukui function, among others. Graphyne’s superior performance for separation and desalination membranes are also reviewed. Abstract By varying the number of acetylenic linkages connecting aromatic rings, a new family of atomically thin graph‐n‐yne materials can be designed and synthesized. Generating immense scientific interest due to its structural diversity and excellent physical properties, graph‐n‐yne has opened new avenues toward numerous promising engineering applications, especially for separation membranes with precise pore sizes. Having these tunable pore sizes in combination with their excellent mechanical strength to withstand high pressures, free‐standing graph‐n‐yne is theoretically posited to be an outstanding membrane material for separating or purifying mixtures of either gases or liquids, rivaling or even dramatically exceeding the capabilities of current, state‐of‐art separation membranes. Computational modeling and simulations play an integral role in the bottom‐up design and characterization of these graph‐n‐yne materials. Thus, here, the state of the art in modeling α‐, β‐, γ‐, δ‐, and 6,6,12‐graphyne nanosheets for synthesizing graph‐2‐yne materials and 3D architectures thereof is discussed. Different synthesis methods are described and a broad overview of computational characterizations of graph‐n‐yne’s electrical, chemical, and thermal properties is provided. Furthermore, a series of in‐depth computational studies that delve into the specifics of graph‐n‐yne’s mechanical strength and porosity, which confer superior performance for separation and desalination membranes, are reviewed.

Published in: "Advanced Materials".

2D Ferroelectrics: Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure (Adv. Mater. 3/2019)

2019-01-16T14:37:23+00:00January 16th, 2019|Categories: Publications|

The underlying mechanism of the transition temperature in ferroelectric 2D SnTe films with a thickness of as little as 2 atomic layers is determined by Stuart S. P. Parkin and co‐workers, as described in article number 1804428. The observed high transition temperature, together with the strong spin‐orbit coupling and van der Waals structure, underlines the potential of atomically thin γ‐SnTe films for the development of novel spontaneous‐polarization‐based devices.

Published in: "Advanced Materials".

Heterostructured Nanocube‐Shaped Binary Sulfide (SnCo)S2 Interlaced with S‐Doped Graphene as a High‐Performance Anode for Advanced Na+ Batteries

2019-01-16T14:34:05+00:00January 16th, 2019|Categories: Publications|Tags: , , |

A heterogeneous nanocube‐shaped binary sulfide interlaced with S‐doped graphene is fabricated as an anode for sodium storage. Its unique heterointerfacial structure can increase reaction kinetic and maintain structural stability, resulting in ultrahigh rate capacity with ultralong life. Furthermore, the fundamental mechanism of synergistic effects for heterogeneous is demonstrated by in‐situ measurements, confirming that constructing a stable Sn/Na2S interface can effectively enhance the reversibility of the conversion reaction. Abstract Heterostructuring electrodes with multiple electroactive and inactive supporting components to simultaneously satisfy electrochemical and structural requirements has recently been identified as a viable pathway to achieve high‐capacity and durable sodium‐ion batteries (SIBs). Here, a new design of heterostructured SIB anode is reported consisting of double metal‐sulfide (SnCo)S2 nanocubes interlaced with 2D sulfur‐doped graphene (SG) nanosheets. The heterostructured (SnCo)S2/SG nanocubes exhibit an excellent rate capability (469 mAh g−1 at 10.0 A g−1) and durability (5000 cycles, 487 mAh g−1 at 5.0 A g−1, 92.6% capacity retention). In situ X‐ray diffraction reveals that the (SnCo)S2/SG anode undergoes a six‐stage Na+ storage mechanism of combined intercalation, conversion, and alloying reactions. The first‐principle density functional theory calculations suggest high concentration of p–n heterojunctions at SnS2/CoS2 interfaces responsible for the high rate performance, while in situ transmission electron microscopy unveils that the interlacing and elastic SG nanosheets play a key role in extending the cycle life.

Published in: "Advanced Functional Materials".

Planar and van der Waals heterostructures for vertical tunnelling single electron transistors

2019-01-16T10:39:13+00:00January 16th, 2019|Categories: Publications|Tags: , , |

Planar and van der Waals heterostructures for vertical tunnelling single electron transistorsPlanar and van der Waals heterostructures for vertical tunnelling single electron transistors, Published online: 16 January 2019; doi:10.1038/s41467-018-08227-1The possibility to combine planar and van der Waals heterostructures holds great promise for nanoscale electronic devices. Here, the authors report an innovative method to synthesise embedded graphene quantum dots within hexagonal boron nitride matrix for vertical tunnelling single electron transistor applications.

Published in: "Nature Communications".

Fine-tuning of wettability in a single metal–organic framework via postcoordination modification and its reduced graphene oxide aerogel for oil–water separation

2019-01-16T10:32:32+00:00January 16th, 2019|Categories: Publications|Tags: , |

Chem. Sci., 2019, Advance ArticleDOI: 10.1039/C8SC04581J, Edge Article Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Sunhwi Eom, Dong Won Kang, Minjung Kang, Jong Hyeak Choe, Hyojin Kim, Dae Won Kim, Chang Seop HongPostcoordination modification of Mg2(dobpdc) with monoamines of various alkyl chain lengths enables facile fine-tuning of wettability and efficient oil–water separation.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: "Chemical Science".

Quasicrystalline electronic states in 30$^circ$ rotated twisted bilayer graphene. (arXiv:1901.04701v1 [cond-mat.mes-hall])

2019-01-16T04:30:57+00:00January 16th, 2019|Categories: Publications|Tags: |

The recently realized bilayer graphene system with a twist angle of $30^circ$ offers a new type of quasicrystal which unites the dodecagonal quasicrystalline nature and graphene’s relativistic properties. Here, we introduce a concise theoretical framework that fully respects both the dodecagonal rotational symmetry and the massless Dirac nature, to describe the electronic states of the system. We find that the electronic spectrum consists of resonant states labeled by 12-fold quantized angular momentum, together with the extended relativistic states. The resulting quasi-band structure is composed of the nearly flat bands with spiky peaks in the density of states, where the wave functions exhibit characteristic patterns which fit to the fractal inflations of the quasicrystal tiling. We also demonstrate that the 12-fold resonant states appear as spatially-localized states in a finite-size geometry, which is another hallmark of quasicrystal. The theoretical method introduced here is applicable to a broad class of “extrinsic quasicrystals” composed of a pair of two-dimensional crystals overlaid on top of the other with incommensurate configurations.

Published : "arXiv Mesoscale and Nanoscale Physics".

Moir’e ordered current loops in the graphene twist bilayer. (arXiv:1901.04712v1 [cond-mat.mes-hall])

2019-01-16T04:30:53+00:00January 16th, 2019|Categories: Publications|Tags: |

While a typical material exhibits field induced currents only at the boundary, a uniform out-of-plane magnetic field applied to two mutually rotated layers of graphene is shown to result in an ordered array of permanent current loops throughout the material. Each current loop consists of an interlayer current flowing through the open AA stacked regions of the moir’e created by rotation, which then flows back through the neighboring AB regions to form a circuit, with significant current strength even at small fields. Similar moir’e ordered arrays of current loops are also shown to exist in non-equilibrium transport states, where they manifest as current back flowing against the applied bias in the device. Such current loops thus represent an intrinsic feature of the twist bilayer in conditions of broken time reversal symmetry, and exist both as a low field imprint of the moir’e lattice on Landau physics, and as measurable moir’e scale current configurations in transport states.

Published : "arXiv Mesoscale and Nanoscale Physics".

Negative flatband magnetism in a spin-orbit coupled kagome magnet. (arXiv:1901.04822v1 [cond-mat.mes-hall])

2019-01-16T04:30:46+00:00January 16th, 2019|Categories: Publications|Tags: , |

It has long been speculated that electronic flatband systems can be a fertile ground for hosting novel emergent phenomena including unconventional magnetism and superconductivity. Although flatbands are known to exist in a few systems such as heavy fermion materials and twisted bilayer graphene, their microscopic roles and underlying mechanisms in generating emergent behavior remain elusive. Here we use scanning tunnelling microscopy to elucidate the atomically resolved electronic states and their magnetic response in the kagome magnet Co3Sn2S2. We observe a pronounced peak at the Fermi level, which is identified to arise from the kinetically frustrated kagome flatband. Increasing magnetic field up to +-8T, this state exhibits an anomalous magnetization-polarized Zeeman shift, dominated by an orbital moment in opposite to the field direction. Such negative magnetism can be understood as spin-orbit coupling induced quantum phase effects tied to non-trivial flatband systems. We image the flatband peak, resolve the associated negative magnetism, and provide its connection to the Berry curvature field, showing that Co3Sn2S2 is a rare example of kagome magnet where the low energy physics can be dominated by the spin-orbit coupled flatband. Our methodology of probing band-resolved ordering phenomena such as spin-orbit magnetism can also be applied in future experiments to elucidate other exotic phenomena including flatband superconductivity and anomalous quantum transport.

Published : "arXiv Mesoscale and Nanoscale Physics".

Lateral heterostructures and one-dimensional interfaces in 2D transition metal dichalcogenides. (arXiv:1901.04931v1 [cond-mat.mes-hall])

2019-01-16T04:30:39+00:00January 16th, 2019|Categories: Publications|Tags: |

The growth and exfoliation of two-dimensional (2D) materials have led to the creation of edges and novel interfacial states at the juncture between crystals with different composition or phases. These hybrid heterostructures (HSs) can be built as vertical van der Waals emph{stacks}, resulting in a 2D interface, or as emph{stitched} adjacent monolayer crystals, resulting in one-dimensional (1D) interfaces. Although most attention has been focused on vertical HSs, increasing theoretical and experimental interest in 1D interfaces is evident. In-plane interfacial states between different 2D materials inherit properties from both crystals, giving rise to robust states with unique 1D non-parabolic dispersion and strong spin-orbit effects. With such unique characteristics, these states provide an exciting platform for realizing 1D physics. Here, we review and discuss advances in 1D heterojunctions, with emphasis on theoretical approaches for describing those between semiconducting transition metal dichalcogenides $MX_{2}$ (with $M$=Mo, W and $X$= S, Se, Te), and how the interfacial states can be characterized and utilized. We also address how the interfaces depend on edge geometries (such as zigzag and armchair) or strain, as lattice parameters differ across the interface, and how these features affect excitonic/optical response. This review is intended to serve as a resource for promoting theoretical and experimental studies in this rapidly evolving field.

Published : "arXiv Mesoscale and Nanoscale Physics".

Dark-Exciton-Mediated Fano Resonance from a Single Gold Nanostructure Deposited on Monolayer WS2 at Room Temperature. (arXiv:1812.00770v2 [physics.optics] UPDATED)

2019-01-16T04:30:29+00:00January 16th, 2019|Categories: Publications|Tags: |

Strong spatial confinement and highly reduced dielectric screening provide monolayer transition metal dichalcogenides (TMDCs) with strong many-body effects, thereby possessing optically forbidden excitonic states (i.e., dark excitons) at room temperature. Herein, we explore the interaction of surface plasmons with dark excitons in hybrid systems consisting of stacked gold nanotriangles (AuNTs) and monolayer WS2. We observe a narrow Fano resonance when the hybrid system is surrounded by water, and we attribute the narrowing of the spectral Fano linewidth to the plasmon-enhanced decay of dark K-K excitons. Our results reveal that dark excitons in monolayer WS2 can strongly modify Fano resonances in hybrid plasmon-exciton systems and can be harnessed for novel optical sensors and active nanophotonic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

4.4 V supercapacitors based on super-stable mesoporous carbon sheets made of edge-free graphene walls

2019-01-16T02:32:11+00:00January 16th, 2019|Categories: Uncategorized|Tags: , |

Energy Environ. Sci., 2019, Accepted ManuscriptDOI: 10.1039/C8EE03184C, CommunicationKeita Nomura, Hirotomo Nishihara, Toshihiro Asada, Naoya Kobayashi, Takashi KyotaniTowards the application of supercapacitors to objectives like automobiles requiring high voltage and high temperature operation, the development of new electrode materials equipped with sufficient stability under such harsh conditions…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Energy & Environmental Science".

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