Switching magnetization with a Weyl semimetal

2019-08-20T12:39:51+00:00August 20th, 2019|Categories: Publications|Tags: |

Nature Nanotechnology, Published online: 19 August 2019; doi:10.1038/s41565-019-0541-8Energy-efficient magnetization manipulation is a prerequisite for competitive spintronic devices. The Weyl semimetal WTe2 can act as a spin current source that enables magnetization switching of an adjacent ferromagnet at low power consumption and additionally induces chiral magnetism.

Published in: "Nature Nanotechnology".

All-electric magnetization switching and Dzyaloshinskii–Moriya interaction in WTe<sub>2</sub>/ferromagnet heterostructures

2019-08-20T12:39:46+00:00August 20th, 2019|Categories: Publications|Tags: , |

Nature Nanotechnology, Published online: 19 August 2019; doi:10.1038/s41565-019-0525-8The Weyl semimetal WTe2 possesses strong spin–orbit coupling and time-reversal-protected spin polarization in surface and bulk states. In a WTe2/permalloy heterostructure, WTe2 can act as a spin current source that enables magnetization switching at low current densities.

Published in: "Nature Nanotechnology".

Strain engineering on the electronic states of two-dimensional GaN/graphene heterostructure

2019-08-20T12:37:21+00:00August 20th, 2019|Categories: Publications|Tags: , |

RSC Adv., 2019, 9,26024-26029DOI: 10.1039/C9RA03175H, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Zhongxun Deng, Xianhui WangCombining two different layered structures to form a van der Waals (vdW) heterostructure has recently emerged as an intriguing way

Published in: "RSC Advances".

Anomalous temperature dependent thermal conductivity of two-dimensional silicon carbide

2019-08-20T10:39:10+00:00August 20th, 2019|Categories: Publications|Tags: |

Recently, two-dimensional silicon carbide (2D-SiC) has attracted considerable interest due to its exotic electronic and optical properties. Here, we explore the thermal properties of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. At room temperature, a thermal conductivity of ∼313 W mK −1 is obtained for 2D-SiC which is one order higher than that of silicene. Above room temperature, the thermal conductivity deviates the normal 1/ T law and shows an anomalous slowly decreasing behavior. To elucidate the variation of thermal conductivity, the phonon modes at different length and temperature are quantified using Fourier transform of the velocity auto-correlation of atoms. The calculated phonon density of states at high temperature shows a shrinking and softening of the peaks, which induces the anomaly in the thermal conductivity. On the other hand, quantum corrections are applied to avoid the freezing effects of phonon modes on the thermal condu…

Published in: "Nanotechnology".

Two-dimensional MoSe 2 nanosheets via liquid-phase exfoliation for high-performance room temperature NO 2 gas sensors

2019-08-20T10:39:07+00:00August 20th, 2019|Categories: Publications|Tags: |

Molybdenum selenide (MoSe 2 ) has drawn significant interest due to its typical semiconductor properties. MoSe 2 is a relatively novel material in the field of gas sensors especially at room temperature. Herein, we utilize a facile and efficient synthetic method of liquid-phase exfoliation to exfoliate bulk MoSe 2 into nanosheets. Anhydrous ethanol is used as dispersant, so the low boiling point makes it easy to be removed from MoSe 2 nanosheets, which does not affect the subsequent sensing performance. The exfoliated few-layered MoSe 2 nanosheets shows significantly enhanced NO 2 gas response (1500% to 10 ppm NO 2 which is 18 times greater than pristine bulk MoSe 2 ), a low detection concentration (50 ppb), an outstanding repeatability, a remarkable selectivity, and a reliable long-term device durability (more than 60 d) at room temperature (25 °C). The reason of the significant improvement in gas sen…

Published in: "Nanotechnology".

Exfoliation and characterization of a two-dimensional serpentine-based material

2019-08-20T10:39:05+00:00August 20th, 2019|Categories: Publications|

We report on an experimental investigation of serpentine, an abundant phyllosilicate, as an alternative source of two-dimensional (2D) nanomaterials. We show, through scanning probe microscopy (SPM) measurements, that natural serpentine mineral can be mechanically exfoliated down to few-layer flakes, where monolayers can be easily resolved. The parent serpentine bulk material was initially characterized via conventional techniques like XRD, XPS, FTIR and Raman spectroscopies and the results show that it is predominantly constituted by the antigorite mineral. From ab initio calculations using density functional theory, we also determine the geometry and electronic structure of antigorite, the observed structural form of serpentine. Additionally, we further characterized electrical and mechanical properties of the obtained 2D material flakes using SPM and broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the serpentine vibrational resonances, assigned to…

Published in: "Nanotechnology".

Light-weight and low-cost electromagnetic wave absorbers with high performances based on biomass-derived reduced graphene oxides

2019-08-20T10:39:02+00:00August 20th, 2019|Categories: Publications|Tags: |

Rational structure design of microwave absorption material is extremely significant from the perspectives of enhancing the electromagnetic microwave absorption (EMA) performance and adapting to cost-effective and sustainable industrial applications. Here, reduced graphene oxides (rGOs) with curl structures derived from corn stover are applied for the absorption of electromagnetic waves. The results suggest that biomass-rGO show the maximum reflection loss of −51.7 dB and an effective absorption bandwidth 13.5 GHz (4.5–18 GHz) at a thickness of 3.25 mm, implying the unique critical role of the microstructure in adjusting the EMA performance. Moreover, the successful conversion of waste biomass into widely used electromagnetic wave absorbing materials could solve the problems of environmental pollution caused by straw burning.

Published in: "Nanotechnology".

Direction and strain controlled anisotropic transport behaviors of 2D GeSe-phosphorene vdW heterojunctions

2019-08-20T10:38:59+00:00August 20th, 2019|Categories: Publications|Tags: , , |

Vertical van der Waals (vdW) heterostructures made up of two or more 2D monolayer materials provide new opportunities for 2D devices. Herein, we study the electronic transport properties of vertical integration of 2D GeSe-phosphorene(GeSe–BP) heterostructure, using the nonequilibrium Green’s function formalism combined with the density-functional theory. The results reveal that the directional dependency and strain tunable transport anisotropic behavior appears in GeSe/BP-stacking vdW heterostructures. The current–voltage ( I – V ) characteristics indicate that the electric current propagates more easily through the perpendicular buckled direction ( Y ) than the linear atomic chain direction ( X ) in the low bias regime regardless of the GeSe–BP stacking, which is supported by the underlying electronic structures along Γ– Y and Γ– X lines. The anisotropic transmission spectra indicate an over 10 5 on/off ratio between the I …

Published in: "Nanotechnology".

Electrical-field tuned thermoelectric performance of graphene nanoribbon with sawtooth edges

2019-08-20T10:38:57+00:00August 20th, 2019|Categories: Publications|Tags: |

By using first-principle calculations combined with the non-equilibrium Green’s function approach, we report that a vertical electrical field can modulate the thermoelectric performance of a graphene nanoribbon with sawtooth edges. The results show that the sawtooth graphene nanoribbon exhibits the spin-dependent Seebeck effect under the temperature gradients, and is strengthened by increasing the width of the sawtooth graphene nanoribbon. When the vertical electrical field is applied to the device, the spin-dependent Seebeck effect can also be enhanced. The vertical electrical field can modulate the device transferring from hole-conducting to electron-conducting. This opens up the possibility of tuning the thermal transport properties of the device by the electrical field.

Published in: "Nanotechnology".

Size effects in the resistivity of graphene nanoribbons

2019-08-20T10:38:53+00:00August 20th, 2019|Categories: Publications|Tags: |

The electrical resistivity of single-layer graphene nanoribbons has been studied experimentally for ribbon widths from 16 to 320 nm and is shown to validate the expected quantum scattering model for conduction through confined graphene structures. The experimental findings are that the resistivity follows a more dramatic trend than that seen for metallic nanowires of similar dimensions, due to a combination of the nature of the charge carriers in this 2D material, surface scattering from the edges, bandgap related effects and shifts in the Fermi level due to edge effects. We show that the charge neutrality point switches polarity below a ribbon width of around 50 nm, and that at this point, the thermal coefficient of resistance is a maximum. The majority doping type therefore can be controlled by altering ribbon width below 100 nm. We also demonstrate that an alumina passivation layer has a significant effect on the mean free path of the charge carriers within the graphene, whic…

Published in: "Nanotechnology".

Core–shell structured graphene sphere-silver nanowire hybrid filler embedded polydimethylsiloxane nanocomposites for stretchable conductor

2019-08-20T10:38:50+00:00August 20th, 2019|Categories: Publications|Tags: |

Three-dimensional (3D) core–shell structured graphene-silver nanowire (AgNW) hybrid fillers are prepared through facile spray drying and an optical welding process. The spray drying process enables formation of a core–shell structure with AgNWs attached onto the spherical graphene surface by van der Waals force and surface tension during evaporation. AgNW shell is optically welded for enhanced mechanical stability and interfacial resistance reduction. 3D core–shell structured graphene-AgNW hybrid fillers are partially embedded into polydimethylsiloxane (PDMS) to fabricate highly stretchable and conductive nanocomposites. The electrical conductivity of nanocomposites largely increases up to ∼116 S cm −1 and the electrical properties are well maintained under high stretchability of ∼140% strain with 100 stretching cycles despite small amount of AgNW. These enhancements are attributed to the formation of electrically conducting network by excellent dispersion property of …

Published in: "Nanotechnology".

Isotropic charge screening of the anisotropic black phosphorus revealed by potassium adatoms. (arXiv:1908.06554v1 [cond-mat.mes-hall])

2019-08-20T04:30:50+00:00August 20th, 2019|Categories: Publications|Tags: , |

Black phosphorus has attracted great research interest due to its numerous applications in electronic devices, optoelectronic devices, energy storages and so on. Compared with the majority of two-dimensional materials, black phosphorus possesses a unique property, i.e. the strong in-plane anisotropy. All the properties reported so far, including its effective mass, electron mobility, light absorption, thermal conductivity and so on, have shown great anisotropy in the basal plane. This property renders black phosphorus unique applications not achievable with other two-dimensional materials. In this work, however, we discover a remarkable isotropic behavior in the strongly anisotropic black phosphorus, i.e. its electrostatic screening of point charges. We use the tip-induced band bending of a scanning tunneling microscope to map out the Coulomb field of ionized potassium adatoms on black phosphorus, and reveal its isotropic charge screening. This discovery is important for understanding electron scattering and transport in black phosphorus.

Published : "arXiv Mesoscale and Nanoscale Physics".

Relaxation of electronically confined states in Pb/Si(111) thin films from Master Equation with first-principles-derived rates. (arXiv:1908.06119v1 [cond-mat.mes-hall])

2019-08-20T04:30:47+00:00August 20th, 2019|Categories: Publications|Tags: |

Atomically thin films of Pb on Si(111) provide an experimentally tunable system comprising a highly structured electronic density of states. The lifetime of excited electrons in these states is limited by both electron-electron (e-e) and electron-phonon (e-ph) scattering. We employ the description by a Master equation for the electronic occupation numbers to analyze the relative importance of both scattering mechanisms. The electronic and phononic band structures, as well as the matrix elements for electron-phonon coupling within deformation potential theory were obtained from density functional calculations, thus taking into account quantum confinement effects. For the relaxation dynamics, the contribution of impact ionization processes to the lifetime is estimated from the imaginary part of the electronic self-energy calculated in the GW approximation. By numerically solving rate equations for the occupations of the Pb-derived electronic states coupled to a phononic heat bath, we are able to follow the distribution of the electronic excitation energy to the various modes of Pb lattice vibrations. While e-e scattering is the dominant relaxation mechanism, we demonstrate that the e-ph scattering is highly phonon-mode-specific, with a large contribution from surface phonons. At electron energies of about 0.3eV above the Fermi surface, a ‘phonon bottleneck’ characteristic of relaxation in nanostructures with well-separated electronic states is observed. The time scales extracted from the simulations are compared to data from pump-probe experiments using time-resolved two-photon photoemission.

Published : "arXiv Mesoscale and Nanoscale Physics".

Scanning tunneling microscope characterizations of a circular graphene resonator realized with p-p junctions. (arXiv:1908.06582v1 [cond-mat.mes-hall])

2019-08-20T02:30:20+00:00August 20th, 2019|Categories: Publications|Tags: , |

Using low-temperature high-magnetic-field scanning tunneling microscopy and spectroscopy (STM/STS), we systematically study a graphene quantum dot (GQD) defined by a circular graphene p-p junction. Inside the GQD, we observe a series of quasi-bound states arising from whispering-gallery-mode (WGM) confinement of the circular junction and directly visualize these quasi-bound states down to atomic dimensions. By applying a strong magnetic field, a large jump in energy of the quasi-bound states, which is about one-half the energy spacing between the quasi-bound states, is observed. Such a behavior results from turning on a {pi} Berry phase of massless Dirac fermions in graphene by a magnetic field. Moreover, our experiment demonstrates that a quasi-bound state splits into two peaks with an energy separation of about 26 meV when the Fermi level crosses the quasi-bound state, indicating that there are strong electron-electron interactions in the GQD.

Published in: "arXiv Material Science".

Relativistic Artificial Molecules Realized by Two Coupled Graphene Quantum Dots. (arXiv:1908.06580v1 [cond-mat.mes-hall])

2019-08-20T02:30:15+00:00August 20th, 2019|Categories: Publications|Tags: |

Coupled quantum dots (QDs), usually referred to as artificial molecules, are important not only in exploring fundamental physics of coupled quantum objects, but also in realizing advanced QD devices. However, previous studies have been limited to artificial molecules with nonrelativistic fermions. Here, we show that relativistic artificial molecules can be realized when two circular graphene QDs are coupled to each other. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe the formation of bonding and antibonding states of the relativistic artificial molecule and directly visualize these states of the two coupled graphene QDs. The formation of the relativistic molecular states strongly alters distributions of massless Dirac fermions confined in the graphene QDs. Because of the relativistic nature of the molecular states, our experiment demonstrates that the degeneracy of different angular-momentum states in the relativistic artificial molecule can be further lifted by external magnetic fields. Then, both the bonding and antibonding states are split into two peaks.

Published in: "arXiv Material Science".

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