Giant negative electrostriction and dielectric tunability in a van der Waals layered ferroelectric. (arXiv:1803.08142v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:25+00:00 March 23rd, 2018|Categories: Publications|Tags: |

The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we reveal giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), with an electrostrictive coefficient Q33 as high as -3.2 m$^{4}$/C$^{2}$ and the resulting bulk piezoelectric coefficient d$_{33}$ of -85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few $mu$C/cm$^{2}$. Moreover, in the paraelectric state, readily accessible owing to low transition temperature, CIPS exhibits large dielectric tenability, similar to widely-used barium strontium titanate, and as a result both giant and continuously tunable electromechanical response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials and might facilitate the quest towards novel ultrathin functional devices incorporating electromechanical response.

Published in: "arXiv Material Science".

Goos-H”anchen shifts due to 2D materials with complex conductivity. (arXiv:1803.08223v1 [physics.optics])

2018-03-23T19:59:24+00:00 March 23rd, 2018|Categories: Publications|Tags: |

We investigate theoretically the Goos-H”anchen (GH) shift of a p-polarized terahertz beam incident on a 2D material surface with complex conductivity. Taking monolayer graphene to be the model material, we determine the dependence of GH shifts on the Fermi level and incident frequency. Both spatial and angular GH shifts are present. For both GH shifts in general, we find that increasing the Fermi level shifts the incident angle at which the maximum GH shifts arise. Moreover, we see that at higher frequencies, the amount of beam shift decreases with the Fermi level when the incident frequency is changed. At lower frequencies, however, the shift becomes proportional with the Fermi level. Upon obtaining the measurable shifts, the angular GH shift dominates the spatial GH shift given appropriate experimental parameters. Our results may pave the way for these material’s use in optoelectronics devices, and fundamentally, to determine properties of 2D materials with complex conductivity.

Published in: "arXiv Material Science".

Single-layer graphdiyne on Pt(111): Improved catalysis confined under two-dimensional overlayer. (arXiv:1803.08231v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:23+00:00 March 23rd, 2018|Categories: Publications|Tags: , |

In recent years, two-dimensional confined catalysis, i.e. the enhanced catalytic reactions in confined spaces between metal surface and two-dimensional overlayer, makes a hit and opens up a new way to enhance the performance of catalysts. In this work, graphdiyne overlayer was proposed as a more excellent material than graphene or hexagonal boron nitride for two-dimensional confined catalysis. Density functional theory calculations revealed the superiority of graphdiyne overlayer originated from the steric hindrance effect which increases the catalytic ability and lowers the reaction barriers. Moreover, with the big triangle holes as natural gas tunnels, graphdiyne possesses higher efficiency for the transit of gaseous reactants and products than graphene or hexagonal boron nitride. The results in this work would benefit future development two-dimensional confined catalysis.

Published in: "arXiv Material Science".

A Primary Exploration to Quasi-Two-Dimensional Rare-Earth Ferromagnetic Particles: Holmium-Doped MoS2 Sheet as Room-Temperature Magnetic Semiconductor. (arXiv:1803.08232v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:22+00:00 March 23rd, 2018|Categories: Publications|Tags: |

Recently, two-dimensional materials and nanoparticles with robust ferromagnetism are even of great interest to explore basic physics in nanoscale spintronics. More importantly, room-temperature magnetic semiconducting materials with high Curie temperature is essential for developing next-generation spintronic and quantum computing devices. Here, we develop a theoretical model on the basis of density functional theory calculations and the Ruderman-Kittel-Kasuya-Yoshida theory to predict the thermal stability of two-dimensional magnetic materials. Compared with other rare-earth (dysprosium (Dy) and erbium (Er)) and 3d (copper (Cu)) impurities, holmium-doped (Ho-doped) single-layer 1H-MoS2 is proposed as promising semiconductor with robust magnetism. The calculations at the level of hybrid HSE06 functional predict a Curie temperature much higher than room temperature. Ho-doped MoS2 sheet possesses fully spin-polarized valence and conduction bands, which is a prerequisite for flexible spintronic applications.

Published in: "arXiv Material Science".

Phonon-assisted magnetic Mott-insulating state in the charge density wave phase of single-layer 1TNbSe2. (arXiv:1803.08361v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:20+00:00 March 23rd, 2018|Categories: Publications|Tags: |

We study the structural, electronic and vibrational properties of single-layer 1TNbSe$_2$ from first principles. Within the generalized gradient approximation, the 1T polytype is highly unstable with respect to the 2H. The DFT+U method improves the stability of the 1T phase, explaining its detection in experiments. A charge density wave occurs with a $sqrt{13}timessqrt{13}~R30^{circ}$ periodicity, in agreement with STM data. At $U=0$, the David-star reconstruction displays a flat band below the Fermi level with a marked d$_{z^2-r^2}$ orbital character of the central Nb. The Hubbard interaction induces a magnetic Mott insulating state. Magnetism distorts the lattice around the central Nb atom in the star, reduces the hybridization between the central Nb d$_{z^2-r^2}$ orbital and the neighbouring Se p-states and lifts in energy the flat band becoming non-bonding. This cooperative lattice and magnetic effect amplifies the Mott gap. Single-layer 1TNbSe$_2$ is then a phonon-assisted spin-$1/2$ Magnetic Mott insulator.

Published in: "arXiv Material Science".

Origin of the monolayer Raman signature in hexagonal boron nitride: a first-principles analysis. (arXiv:1803.08454v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:19+00:00 March 23rd, 2018|Categories: Publications|Tags: |

Monolayers of hexagonal boron nitride (h-BN) can in principle be identified by a Raman signature, consisting of an upshift in the frequency of the E2g vibrational mode with respect to the bulk value, but the origin of this shift (intrinsic or support-induced) is still debated. Herein we use density functional theory calculations to investigate whether there is an intrinsic Raman shift in the h-BN monolayer in comparison with the bulk. There is universal agreement among all tested functionals in predicting the magnitude of the frequency shift upon a variation in the in-plane cell parameter. It is clear that a small in-plane contraction can explain the Raman peak upshift from bulk to monolayer. However, we show that the larger in-plane parameter in the bulk (compared to the monolayer) results from non-local correlation effects, which cannot be accounted for by local functionals or those with empirical dispersion corrections. Using a non-local-correlation functional, we then investigate the effect of finite temperatures on the Raman signature. We demonstrate that bulk h-BN thermally expands in the direction perpendicular to the layers, while the intralayer distances slightly contract, in agreement with observed experimental behavior. Interestingly, the difference in in-plane cell parameter between bulk and monolayer decreases with temperature, and becomes very small at room temperature. We conclude that the different thermal expansion of bulk and monolayer partially “erases” the intrinsic Raman signature, accounting for its small magnitude in recent experiments on suspended samples.

Published in: "arXiv Material Science".

Symmetry regimes for circular photocurrents in monolayer MoSe2. (arXiv:1803.08289v1 [cond-mat.mtrl-sci])

2018-03-23T19:59:18+00:00 March 23rd, 2018|Categories: Publications|Tags: |

In monolayer transition metal dichalcogenides helicity-dependent charge and spin photocurrents can emerge, even without applying any electrical bias, due to circular photogalvanic and photon drag effects. Exploiting such circular photocurrents (CPC) in devices, however, requires better understanding of their behavior and physical origin. Here, we present symmetry, spectral, and electrical characteristics of CPC from excitonic interband transitions in a MoSe2 monolayer. The dependence on bias and gate voltages reveals two different CPC contributions, dominant at different voltages and with different dependence on illumination wavelength and incidence angles. We theoretically analyze symmetry requirements for effects that can yield CPC and compare these with the observed angular dependence and symmetries that occur for our device geometry. This reveals that the observed CPC effects require a reduced device symmetry, and that effects due to Berry curvature of the electronic states do not give a significant contribution.

Published in: "arXiv Material Science".

High density carriers at a strongly coupled graphene-topological insulator interface. (arXiv:1803.08260v1 [cond-mat.mes-hall])

2018-03-23T19:58:51+00:00 March 23rd, 2018|Categories: Publications|Tags: |

We report on a strongly coupled bilayer graphene (BLG) – bise device with a junction resistance of less than 1.5 k$Omegamu$m$^2$. This device exhibits unique behavior at the interface, which cannot be attributed to either material in absence of the other. We observe quantum oscillations in the magnetoresistance of the junction, indicating the presence of well-resolved Landau levels due to hole carriers of unknown origin with a very large Fermi surface. These carriers, found only at the interface, could conceivably arise due to significant hole doping of the bilayer graphene with charge transfer on the order of 2$times$10$^{13}$ cm$^{-2}$, or due to twist angle dependent mini-band transport.

Published : "arXiv Mesoscale and Nanoscale Physics".

Evidence for a topological “exciton Fermi sea” in bilayer graphene. (arXiv:1803.08077v1 [cond-mat.str-el])

2018-03-23T19:58:50+00:00 March 23rd, 2018|Categories: Publications|Tags: , |

The quantum Hall physics of bilayer graphene is extremely rich due to the interplay between a layer degree of freedom and delicate fractional states. Recent experiments show that when an electric field perpendicular to the bilayer causes Landau levels of opposing layers to cross in energy, a even-denominator Hall plateau can coexist with a finite density of inter-layer excitons. We present theoretical and numerical evidence that this observation is due to a new phase of matter — a Fermi sea of topological excitons.

Published : "arXiv Mesoscale and Nanoscale Physics".

Nonlocal hydrodynamic phonon transport in two-dimensional materials. (arXiv:1803.08372v1 [cond-mat.mes-hall])

2018-03-23T19:58:50+00:00 March 23rd, 2018|Categories: Publications|

We study hydrodynamic phonon heat transport in two-dimensional (2D) materials. Starting from the Peierls-Boltzmann equation within the Callaway model, we derive a 2D Guyer-Krumhansl-like equation describing non-local hydrodynamic phonon transport, taking into account the quadratic dispersion of flexural phonons. In additional to Poiseuille flow, second sound propagation, the equation predicts heat current vortices and nonlocal thermal conductance in 2D materials. Our results illustrate the universal transport behavior of hydrodynamics, independent on the type of quasi-particles and their microscopic interactions.

Published : "arXiv Mesoscale and Nanoscale Physics".

Topological exciton Fermi surfaces in two-component fractional quantized Hall insulators. (arXiv:1611.01171v2 [cond-mat.str-el] UPDATED)

2018-03-23T19:58:49+00:00 March 23rd, 2018|Categories: Publications|Tags: , |

A wide variety of two-dimensional electron systems (2DES) allow for independent control of the total and relative charge density of two-component fractional quantum Hall (FQH) states. In particular, a recent experiment on bilayer graphene (BLG) observed a continuous transition between a compressible and incompressible phase at total filling $nu_T = frac{1}{2}$ as charge is transferred between the layers, with the remarkable property that the incompressible phase has a finite interlayer polarizability. We argue that this occurs because the topological order of $nu_T = frac{1}{2}$ systems supports a novel type of interlayer exciton that carries Fermi statistics. If the fermionic excitons are lower in energy than the conventional bosonic excitons (i.e., electron-hole pairs), they can form an emergent neutral Fermi surface, providing a possible explanation of an incompressible yet polarizable state at $nu_T = frac{1}{2}$. We perform exact diagonalization studies which demonstrate that fermionic excitons are indeed lower in energy than bosonic excitons. This suggests that a “topological exciton metal” hidden inside a FQH insulator may have been realized experimentally in BLG. We discuss several detection schemes by which the topological exciton metal can be experimentally probed.

Published : "arXiv Mesoscale and Nanoscale Physics".


2018-03-23T19:56:57+00:00 March 23rd, 2018|Categories: Patents|Tags: |

The present invention discloses a method and an apparatus for converting incident electromagnetic radiation into acoustic and/or ultrasonic waves, comprising: a) a 3D graphene structure comprising a number of graphene sheets for receiving the incident electromagnetic radiation and responding to the incident electromagnetic radiation by an acoustic and/or ultrasonic signal; b) an electro-acoustic transducer being coupled to the graphene sponge and receiving the acoustic and/or ultrasonic signal and outputting in response to the acoustic and/or ultrasonic signal an electric signal; and c) an evaluation unit being enable to detect the course of the intensity of the electric signal. It further discloses a method and an apparatus for converting incident acoustic and/or ultrasonic waves into electromagnetic radiation, comprising: a) a 3D graphene structure comprising a number of graphene sheets for receiving the incident acoustic and/or ultrasonic waves and responding to the incident acoustic and/or ultrasonic waves by electromagnetic radiation; and b) an electro-acoustic transducer being coupled to the 3D graphene structure outputting the acoustic and/or ultrasonic waves. The present invention allows simple, efficient and prompt conversion of high frequency electromagnetic radiation, in the range between MHz and 100 THz, into acoustic waves at frequencies between few Hz to more than 100 kHz and possibly vice versa.

Published in: "Patentscope".


2018-03-23T19:56:48+00:00 March 23rd, 2018|Categories: Patents|Tags: , |

In order to reduce the risk of infection in medical establishments, the surface (10, 15) of a medical device (6, 7) having a surface (10, 15) that runs the risk of being contaminated is provided with a self-disinfecting property. For this purpose, the surface (10, 15) is provided with a coating (11, 16) that substantially consists of graphene oxide or a graphene oxide-containing composite material, or a component of the medical device (7) comprising the surface (10) is manufactured from a graphene oxide-containing composite material.

Published in: "Patentscope".

Low-field magnetotransport in graphene cavity devices

2018-03-23T12:31:10+00:00 March 23rd, 2018|Categories: Publications|Tags: |

Confinement and edge structures are known to play significant roles in the electronic and transport properties of two-dimensional materials. Here, we report on low-temperature magnetotransport measurements of lithographically patterned graphene cavity nanodevices. It is found that the evolution of the low-field magnetoconductance characteristics with varying carrier density exhibits different behaviors in graphene cavity and bulk graphene devices. In the graphene cavity devices, we observed that intravalley scattering becomes dominant as the Fermi level gets close to the Dirac point. We associate this enhanced intravalley scattering to the effect of charge inhomogeneities and edge disorder in the confined graphene nanostructures. We also observed that the dephasing rate of carriers in the cavity devices follows a parabolic temperature dependence, indicating that the direct Coulomb interaction scattering mechanism governs the dephasing at low temperatures. Our results demonstrate…

Published in: "Nanotechnology".

Temperature-dependent Raman spectroscopy studies of the interface coupling effect of monolayer ReSe 2 single crystals on Au foils

2018-03-23T12:31:09+00:00 March 23rd, 2018|Categories: Publications|Tags: |

Rhenium diselenide (ReSe 2 ), which bears in-plane anisotropic optical and electrical properties, is of considerable interest for its excellent applications in novel devices, such as polarization-sensitive photodetectors and integrated polarization-controllers. However, great challenges to date in the controllable synthesis of high-quality ReSe 2 have hindered its in-depth investigations and practical applications. Herein, we report a feasible synthesis of monolayer single-crystal ReSe 2 flakes on the Au foil substrate by using a chemical vapor deposition route. Particularly, we focus on the temperature-dependent Raman spectroscopy investigations of monolayer ReSe 2 grown on Au foils, which present concurrent red shifts of E g -like and A g -like modes with increasing measurement temperature from 77–290 K. Linear temperature dependences of both modes are revealed and explained from the anharmonic vibration of the ReSe …

Published in: "Nanotechnology".

Real-time and stepwise deoxidization processes to tune the photoluminescence properties of graphene oxide using EC-SPR spectroscopy

2018-03-23T12:29:08+00:00 March 23rd, 2018|Categories: Publications|Tags: , |

RSC Adv., 2018, 8,11557-11565DOI: 10.1039/C7RA13594G, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Nan-Fu Chiu, Cheng-Du YangThe development of a stepwise deoxidized process and real-time monitoring of the large-scale mass production of electrochemically reduced graphene oxide

Published in: "RSC Advances".

Semimetallic Vanadium Molybdenum Sulfide For High-Performance Battery Electrodes

2018-03-23T03:18:00+00:00 March 23rd, 2018|Categories: Publications|Tags: , |

J. Mater. Chem. A, 2018, Accepted ManuscriptDOI: 10.1039/C8TA00995C, PaperQingfeng Zhang, Longlu Wang, Jue Wang, Xinzhi Yu, Junmin ge, Hang Zhang, Bingan LuThe ultrathin thickness and lateral morphology of the two dimensional (2D) MoS2 nanosheet contribute to its high surface-to-volume ratio and short diffusion

Published in: "Journal of Materials Chemistry A".

Complementary Resistive Switching Observed in Graphene Oxide-Based Memory Device

2018-03-22T23:20:43+00:00 March 22nd, 2018|Categories: Publications|Tags: , |

In this letter, complementary resistive switching (CRS) was demonstrated in a single-stack graphene oxide (GO) memory cell for the first time, where the high resistance state can be distinguished into ”0” and ”1” states by different bias polarities. The high switching uniformity ensures reliable reading/writing operations. By changing the compliance currents in the forming and switching processes, the quantity of oxygen defects required for the conducting-filament (CF) formation and supplied by the GO layer ( ${Q} _{R}$ and ${Q} _{S}$ ) was adjusted to determine their influence on the CRS. It was found that the CRS only occurred at the condition of ${Q} _{R} > {Q}_{S}$ and its mechanism is due to the inversion of CF geometry.

Published in: "IEEE Electron Device Letters".

Some say, that 2D Research is the best website in the world.