Geometry and electronic structure of iridium adsorbed on graphene

2019-02-22T18:37:04+00:00February 22nd, 2019|Categories: Publications|Tags: |

Author(s): Bradford A. Barker, Aaron J. Bradley, Miguel M. Ugeda, Sinisa Coh, Alex Zettl, Michael F. Crommie, Steven G. Louie, and Marvin L. CohenWe report investigation of the geometry and electronic structure of iridium atoms adsorbed onto graphene through a combined experimental and theoretical study. Ir atoms were deposited onto a flake of graphene on a Pt(111) surface and found to form clusters even at low temperatures. The areal density…[Phys. Rev. B 99, 075431] Published Fri Feb 22, 2019

Published in: "Physical Review B".

Enhanced oxygen evolution reaction on amine functionalized graphene oxide in alkaline medium

2019-02-22T14:32:24+00:00February 22nd, 2019|Categories: Publications|Tags: , |

RSC Adv., 2019, 9,6444-6451DOI: 10.1039/C8RA10286D, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Vijay S. Sapner, Balaji B. Mulik, Renuka V. Digraskar, Shankar S. Narwade, Bhaskar R. SatheMetal free tyramine functionalized graphene oxide (T-GO) is a promising

Published in: "RSC Advances".

Sulfur-doped graphene for efficient electrocatalytic N2-to-NH3 fixation

2019-02-22T12:43:10+00:00February 22nd, 2019|Categories: Publications|Tags: , |

Chem. Commun., 2019, Accepted ManuscriptDOI: 10.1039/C9CC00602H, CommunicationLi Xia, Jia-Jia Yang, Huanbo Wang, Runbo Zhao, hongyu chen, Wei-hai Fang, Abdullah M. Asiri, Fengyu Xie, Ganglong Cui, Xuping SunIndustrial NH3 synthesis mainly relies on the carbon-emitting Haber–Bosch process operating under severe conditions. Electrocatalytic N2-to-NH3 fixation at ambient conditions is an attractive approach to reduce energy consumption and avoid…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

Toward High Power‐High Energy Sodium Cathodes: A Case Study of Bicontinuous Ordered Network of 3D Porous Na3(VO)2(PO4)2F/rGO with Pseudocapacitance Effect

2019-02-22T10:45:17+00:00February 22nd, 2019|Categories: Publications|Tags: , , |

A bicontinuous ordered network of 3D porous Na3(VO)2(PO4)2F/reduced graphene oxide (NVOPF/rGO) is prepared by a two‐step strategy (solvothermal plus electrostatic spray deposition method). Such electrodes exhibit excellent rate capability and cycling stability for both half cell and full cell, as well as a significant contribution of pseudocapacitance, resulting in real high power‐high energy sodium batteries. Abstract Developing high power‐high energy electrochemical energy storage systems is an ultimate goal in the energy storage field, which is even more difficult but significant for low‐cost sodium ion batteries. Here, fluoride is successfully prepared by the electrostatic spray deposition (ESD) technique, which greatly expands the application scope of ESD. A two‐step strategy (solvothermal plus ESD method) is proposed to construct a bicontinuous ordered network of 3D porous Na3(VO)2(PO4)2F/reduced graphene oxide (NVOPF/rGO). This two‐step strategy makes sure that NVOPF can be prepared by ESD, since it avoids the loss of F element during synthesis. The obtained NVOPF particles are as small as 15 nm, and the carbon content is only 3.5% in the final nanocomposite. Such a bicontinuous ordered network and small size of electroactive particles lead to the significant contribution of the pseudocapacitance effect to sodium storage, resulting in real high power‐high energy sodium cathodes. The cathode exhibits excellent rate capability and cycling stability, whose rate performance is one of the best ever reported in both half cells and full cells. Moreover, this work provides a general and promising strategy for developing high power‐high energy electrode materials for various electrochemical energy storage systems.

Published in: "Small".

Negative Friction Coefficients in Superlubric Graphite-Hexagonal Boron Nitride Heterojunctions. (arXiv:1902.08067v1 [cond-mat.mes-hall])

2019-02-22T08:31:02+00:00February 22nd, 2019|Categories: Publications|Tags: |

Negative friction coefficients, where friction is reduced upon increasing normal load, are predicted for superlubric graphite-hexagonal boron nitride heterojunctions. The origin of this counterintuitive behavior lies in the load-induced suppression of the moir’e superstructure out-of-plane distortions leading to a less dissipative interfacial dynamics. Thermally induced enhancement of the out-of-plane fluctuations leads to an unusual increase of friction with temperature. The highlighted frictional mechanism is of a general nature and is expected to appear in many layered material heterojunctions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Valley isospin of interface states in a graphene $pn$ junction in the quantum Hall regime. (arXiv:1902.07975v1 [cond-mat.mes-hall])

2019-02-22T08:30:58+00:00February 22nd, 2019|Categories: Publications|Tags: |

In the presence of crossed electric and magnetic fields, a graphene ribbon has chiral states running along sample edges and along boundaries between $p$-doped and $n$-doped regions. We here consider the scattering of edge states into interface states, which takes place whereever the $pn$ interface crosses the sample boundary, as well as the reverse process. For a graphene ribbon with armchair boundaries, the evolution of edge states into interface states and {em vice versa} is governed by the conservation of valley isospin. Although valley isospin is not conserved in simplified models of a ribbon with zigzag boundaries, we find that arguments based on isospin conservation can be applied to a more realistic modeling of the graphene ribbon, which takes account of the lifting of electron-hole degeneracy. The valley isospin of interface states is an important factor determining the conductance of a graphene $pn$ junction in a quantizing magnetic field.

Published : "arXiv Mesoscale and Nanoscale Physics".

Spin-orbit coupling and spin relaxation in phosphorene: Intrinsic versus extrinsic effects. (arXiv:1902.07957v1 [cond-mat.mes-hall])

2019-02-22T08:30:51+00:00February 22nd, 2019|Categories: Publications|Tags: |

First-principles calculations of the essential spin-orbit and spin relaxation properties of phosphorene are performed. Intrinsic spin-orbit coupling induces spin mixing with the probability of $b^2 approx 10^{-4}$, exhibiting a large anisotropy, following the anisotropic crystalline structure of phosphorene. For realistic values of the momentum relaxation times, the intrinsic (Elliott–Yafet) spin relaxation times are hundreds of picoseconds to nanoseconds. Applying a transverse electric field (simulating gating and substrates) generates extrinsic $C_{2v}$ symmetric spin-orbit fields in phosphorene, which activate the D’yakonov–Perel’ mechanism for spin relaxation. It is shown that this extrinsic spin relaxation also has a strong anisotropy, and can dominate over the Elliott-Yafet one for strong enough electric fields. Phosphorene on substrates can thus exhibit an interesting interplay of both spin relaxation mechanisms, whose individual roles could be deciphered using our results.

Published : "arXiv Mesoscale and Nanoscale Physics".

Efficient and Robust Carbon Dioxide Electroreduction Enabled by Atomically Dispersed Snδ+ Sites

2019-02-22T06:37:14+00:00February 22nd, 2019|Categories: Publications|Tags: , |

A positively charged single‐atom metal electrocatalyst to largely reduce the overpotentials for carbon dioxide electroreduction is fabricated. Synchrotron‐radiation X‐ray absorption fine structure and high‐angle annular dark‐field scanning transmission electron microscopy demonstrate that atomically dispersed tin atoms are positively charged, which enables carbon dioxide activation and protonation proceed spontaneously, affirmed by in situ Fourier transform infrared spectra and Gibbs free energy calculations. Abstract Electrocatalytic CO2 reduction at considerably low overpotentials still remains a great challenge. Here, a positively charged single‐atom metal electrocatalyst to largely reduce the overpotentials is designed and hence CO2 electroreduction performance is accelerated. Taking the metal Sn as an example, kilogram‐scale single‐atom Sn δ + on N‐doped graphene is first fabricated by a quick freeze–vacuum drying–calcination method. Synchrotron‐radiation X‐ray absorption fine structure and high‐angle annular dark‐field scanning transmission electron microscopy demonstrate the atomically dispersed Sn atoms are positively charged, which enables CO2 activation and protonation to proceed spontaneously through stabilizing CO2•−* and HCOO−*, affirmed by in situ Fourier transform infrared spectra and Gibbs free energy calculations. Furthermore, N‐doping facilitates the rate‐limiting formate desorption step, verified by the decreased desorption energy from 2.16 to 1.01 eV and the elongated SnHCOO− bond length. As an result, single‐atom Sn δ + on N‐doped graphene exhibits a very low onset overpotential down to 60 mV for formate production and shows a very large turnover frequency up to 11930 h−1, while its electroreduction activity proceeds without deactivation even after 200 h. This work offers a new pathway for manipulating electrocatalytic performance.

Published in: "Advanced Materials".

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

2019-02-22T04:52:45+00:00February 22nd, 2019|Categories: Publications|Tags: |

Here, a sizable valley polarization (P c) is obtained by in‐situ electrostatic and optical doping approaches. More than threefold enhancement of P c in monolayer WS2 at 80 K and off‐resonance excitation is obtained, which is competitive with previously reported values at cryogenic temperature and high magnetic field. The enhancement is attributed to the carrier screening effect which suppresses intervalley scattering process. Abstract The emerging field of valleytronics has boosted intensive interests in investigating and controlling valley polarized light emission of monolayer transition metal dichalcogenides (1L TMDs). However, so far, the effective control of valley polarization degree in monolayer TMDs semiconductors is mostly achieved at liquid helium cryogenic temperature (4.2 K), with the requirements of high magnetic field and on‐resonance laser, which are of high cost and unwelcome for applications. To overcome this obstacle, it is depicted that by electrostatic and optical doping, even at temperatures far above liquid helium cryogenic temperature (80 K) and under off‐resonance laser excitation, a competitive valley polarization degree of monolayer WS2 can be achieved (more than threefold enhancement). The enhanced polarization is understood by a general doping dependent valley relaxation mechanism, which agrees well with the unified theory of carrier screening effects on intervalley scattering process. These results demonstrate that the tunability corresponds to an effective magnet field of ≈10 T at 4.2 K. This work not only serves as a reference to future valleytronic studies based on monolayer TMDs with various external or native carrier densities, but also provides an alternative approach toward

Published in: "Small".

Monolithic Integration of All‐in‐One Supercapacitor for 3D Electronics

2019-02-22T04:38:17+00:00February 22nd, 2019|Categories: Publications|Tags: , , |

An all‐in‐one asymmetrical supercapacitor is prepared by integrating a 2D MnO2 cathode, a holey reduced graphene oxide/carbon nanotube anode, Ni/Au current collector, and a separator into one monolithic glass fiber (GF) membrane. The all‐in‐one supercapacitror is embedded into GF/polydimethylsiloxane substrate to realize the integration with the 3D electronic system. Abstract A supercapacitor is usually stacked in the configuration of a layered sandwiched architecture, and has been adopted as discrete energy storage device or circuit component. However, this stacked structure decreases mechanical integrity, leads to low specific capacity, and prevents high‐density monolithic integration. Here all‐in‐one supercapacitors are fabricated by integrating cathode, anode, current collector, and separator into one monolithic glass fiber (GF) substrate together with other circuit components through matured and scalable fabrication techniques, the all‐in‐one supercapacitor is embedded as a component for 3D electronics. This all‐in‐one architecture demonstrates its effectiveness in the prevention of the delamination of the sandwiched supercapacitor and the minimization of the proportion of inactive materials. The supercapacitor delivers high power density (320 mW cm−3) and energy density (2.12 mWh cm−3), and exhibits a capacitance retention of 100% even after a continuous cycling of 431 h. Furthermore, a 3D polydimethylsiloxane/GF architecture is constructed for driving a flash light emitting diode system, where the all‐in‐one supercapacitor is monolithically integrated in the 3D system, and each layer is connected via vertical through‐holes. This all‐in‐one device can be constructed with a macroscopically available membrane and readily integrated into 3D systems without secondary packaging, providing the potential for high‐density heterogeneous 3D

Published in: "Advanced Energy Materials".

High‐Performance Solid Polymer Electrolytes Filled with Vertically Aligned 2D Materials

2019-02-22T04:33:44+00:00February 22nd, 2019|Categories: Publications|Tags: |

Vertically aligned 2D materials filler for solid polymer electrolytes is demonstrated. The aligned, continuous, run‐through polymer‐filler interfaces enhance the ionic conductivity, Li+ transference number, mechanical modulus, and electrochemical stability of solid polymer electrolytes. LiFePO4 in lithium metal batteries with the electrolyte could deliver a specific capacity of 167 mAh g−1 at 0.1 C at 35 °C. Abstract Solid state lithium metal batteries are the most promising next‐generation power sources owing to their high energy density and safety. Solid polymer electrolytes (SPE) have gained wide attention due to the excellent flexibility, manufacturability, lightweight, and low‐cost processing. However, fatal drawbacks of the SPE such as the insufficient ionic conductivity and Li+ transference number at room temperature restrict their practical application. Here vertically aligned 2D sheets are demonstrated as an advanced filler for SPE with enhanced ionic conductivity, Li+ transference number, mechanical modulus, and electrochemical stability, using vermiculite nanosheets as an example. The vertically aligned vermiculite sheets (VAVS), prepared by the temperature gradient freezing, provide aligned, continuous, run‐through polymer‐filler interfaces after infiltrating with polyethylene oxide (PEO)‐based SPE. As a result, ionic conductivity as high as 1.89 × 10−4 S cm−1 at 25 °C is achieved with Li+ transference number close to 0.5. Along with their enhanced mechanical strength, Li|Li symmetric cells using VAVS–CSPE are stable over 1300 h with a low overpotential. LiFePO4 in all‐solid‐state lithium metal batteries with VAVS–CSPE could deliver a specific capacity of 167 mAh g−1 at 0.1 C at 35 °C and 82% capacity retention after 200 cycles at

Published in: "Advanced Functional Materials".

Selective Defect Formation in Hexagonal Boron Nitride. (arXiv:1902.07932v1 [])

2019-02-22T02:29:27+00:00February 22nd, 2019|Categories: Publications|Tags: |

Luminescent defect-centers in hexagonal boron nitride (hBN) have emerged as a promising 2D-source of single photon emitters (SPEs) due to their high brightness and robust operation at room temperature. The ability to create such emitters with well-defined optical properties is a cornerstone towards their integration into on-chip photonic architectures. Here, we report an effective approach to fabricate hBN single photon emitters (SPEs) with desired emission properties in two isolated spectral regions via the manipulation of boron diffusion through copper during atmospheric pressure chemical vapor deposition (APCVD)–a process we term gettering. Using the gettering technique we deterministically place the resulting zero-phonon line (ZPL) between the regions 550-600 nm or from 600-650 nm, paving the way for hBN SPEs with tailored emission properties across a broad spectral range. Our ability to control defect formation during hBN growth provides a simple and cost-effective means to improve the crystallinity of CVD hBN films, and lower defect density making it applicable to hBN growth for a wide range of applications. Our results are important to understand defect formation of quantum emitters in hBN and deploy them for scalable photonic technologies.

Published in: "arXiv Material Science".

Ab-initio calculations of carbon and boron nitride allotropes and their structural phase transitions using periodic coupled cluster theory. (arXiv:1902.08100v1 [cond-mat.mtrl-sci])

2019-02-22T02:29:25+00:00February 22nd, 2019|Categories: Publications|Tags: , |

We present an ab-initio study of boron nitride as well as carbon allotropes. Their relative thermodynamic stabilities and structural phase transitions from low- to high-density phases are investigated. Pressure-temperature phase diagrams are calculated and compared to experimental findings. The calculations are performed using quantum chemical wavefunction based as well as density functional theories. Our findings reveal that predicted energy differences often depend significantly on the choice of the employed method. Comparison between calculated and experimental results allows for benchmarking the accuracy of various levels of theory. The produced results show that quantum chemical wavefunction based theories allow for achieving systematically improvable estimates. We find that on the level of coupled cluster theories the low- and high-density phases of boron nitride become thermodynamically degenerate at 0 K. This is in agreement with recent experimental findings, indicating that cubic boron nitride is not the thermodynamically stable allotrope at ambient conditions. Furthermore we employ the calculated results to assess transition probabilities from graphitic low-density to diamond-like high-density phases in an approximate manner. We conclude that the stacking order of the parent graphitic material is crucial for the possible formation of meta-stable wurtzite boron nitride and hexagonal carbon diamond also known as lonsdaleite.

Published in: "arXiv Material Science".

The $8Pmmn$ borophene sheet: A solid-state platform for space-time engineering. (arXiv:1902.07767v1 [cond-mat.mes-hall])

2019-02-22T02:29:24+00:00February 22nd, 2019|Categories: Publications|

We construct the most generic Hamiltonian of the $8Pmmn$ structure of borophene sheet in presence of spin-orbit, as well as background electric and magnetic fields. In addition to spin and valley Hall effects, this structure offers a framework to conveniently manipulate the resulting “tilt” of the Dirac equation by applying appropriate electric fields. Therefore, the tilt can be made space-, as well as time-dependent. The border separating the low-field region with under-tilted Dirac fermions from the high-field region with over-tilted Dirac fermions will correspond to a black-hole horizon. In this way, space-time dependent electric fields can be used to design the metric of the resulting space-time felt by electrons and holes satisfying the tilted Dirac equation. Our platform offers a way to generate analogues of gravitational waves by electric fields (instead of mass sources) which can be detected in solid state spectroscopies as waves of enhanced superconducting correlations.

Published in: "arXiv Material Science".

Visualization of Local Conductance in MoS2/WSe2 Heterostructure Transistors. (arXiv:1902.08147v1 [])

2019-02-22T02:29:21+00:00February 22nd, 2019|Categories: Publications|Tags: , , , |

The vertical stacking of van der Waals (vdW) materials introduces a new degree of freedom to the research of two-dimensional (2D) systems. The interlayer coupling strongly influences the band structure of the heterostructures, resulting in novel properties that can be utilized for electronic and optoelectronic applications. Based on microwave microscopy studies, we report quantitative electrical imaging on gated molybdenum disulfide (MoS2)/tungsten diselenide (WSe2) heterostructure devices, which exhibit an intriguing anti-ambipolar effect in the transfer characteristics. Interestingly, in the region with significant source-drain current, electrons in the n-type MoS2 and holes in the p-type WSe2 segments are nearly balanced, whereas the heterostructure area is depleted of mobile charges. The configuration is analogous to the p-i-n diode, where the injected carriers dominate in the recombination current. The spatial evolution of local conductance can be ascribed to the lateral band bending and formation of depletion regions along the line of MoS2-heterostructure-WSe2. Our work vividly demonstrates the microscopic origin of novel transport behaviors, which is important for the vibrant field of vdW heterojunction research.

Published in: "arXiv Material Science".

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