Thickness-dependent in-plane polarization and structural phase transition in van der Waals Ferroelectric CuInP2S6. (arXiv:1911.08639v1 [cond-mat.mtrl-sci])

2019-11-21T02:29:34+00:00November 21st, 2019|Categories: Publications|Tags: |

Van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intra-layer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high-density storage devices. As the thickness going thinner, the competition between the surface energy, depolarization field and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, we report the existence of the intrinsic in-plane polarization in vdW ferroelectrics CuInP2S6 (CIPS) single crystals, whereas below a critical thickness between 90-100 nm, the in-plane polarization disappears. The Young’s modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, we ascribe these behaviors to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. Taken together, these findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.

Published in: "arXiv Material Science".

Active spatial control of terahertz graphene plasmons by tailoring carrier density profile. (arXiv:1911.08672v1 [cond-mat.mtrl-sci])

2019-11-21T02:29:32+00:00November 21st, 2019|Categories: Publications|Tags: |

Graphene offers a possibility for actively controlling plasmon confinement and propagation by tailoring its spatial conductivity pattern. However, implementation of this concept has been hampered because uncontrollable plasmon reflection is easily induced by inhomogeneous dielectric environment. In this work, we demonstrate full electrical control of plasmon reflection/transmission at electronic boundaries induced by a zinc-oxide-based dual gate, which is designed to minimize the dielectric modulation. Using Fourier-transform infrared spectroscopy, we show that the plasmon reflection can be varied continuously with the carrier density difference between the adjacent regions. By utilizing this functionality, we show the ability to control size, position, and frequency of plasmon cavities. Our approach can be applied to various types of plasmonic devices, paving the way for implementing a programmable plasmonic circuit.

Published in: "arXiv Material Science".

Systematization of MAX phases formability and exploration of their new phases for future MXenes. (arXiv:1911.08735v1 [cond-mat.mtrl-sci])

2019-11-21T02:29:31+00:00November 21st, 2019|Categories: Publications|Tags: |

As a fast-growing family of 2D materials, MXenes are rising into the limelight due to their attractive properties that leading to applications in energy storage, catalysis, microwave attenuation, etc. As their precursors, MAX phases are a family of layered, hexagonal-structure ternary carbides and nitrides. The exploration of novel MAX phases would largely expand the candidates for future MXenes with various component elements and microstructures. Meanwhile, apart from their indispensable role in Top-down synthesis of MXenes, MAX phase materials show remarkable combination of characteristics of metals and ceramics, endowing them great interest as both structural and functional materials. Up to now, there are more than 100 MAX compositions synthesized to date; and discoveries of new MAX phases (Ti3AuC2, Ti3IrC2, Ti2ZnC, Nb2CuC et al.) are continuously being reported by experimental scientists. To increase the efficiency in the successful search for novel MAX phases, a general guideline should be set up to direct experiment through an information-prediction system incorporating all MAX phases’ databases and high-quality regularities. In this work, we introduce the structure mapping methodology, which has shown its viability of being an effective guideline in design of MAX phases, and also provide simple principles for experimental scientists in the search for novel MAX phases. In fact, above-mentioned new MAX phases are falling into structure-stability regime that predicted by this structure mapping methodology. These new MAX phases, whatever experimentally reported or theoretically predicted, will largely enrich the structure-modification and property-tuning of derived MXenes.

Published in: "arXiv Material Science".

Excitonic Transport and Intervalley Scattering in Exfoliated MoSe2 Monolayer Revealed by Four-Wave-Mixing Transient Grating Spectroscopy. (arXiv:1911.08939v1 [cond-mat.mtrl-sci])

2019-11-21T02:29:29+00:00November 21st, 2019|Categories: Publications|Tags: |

Exciton intervalley scattering, annihilation and relaxation dynamics, and diffusive transport in a monolayer transition metal dichalcogenides are central to the functionality of devices based on them. This motivated us to investigate these properties in exfoliated high-quality monolayer MoSe2 using heterodyned nonlinear four-wave-mixing transient grating spectroscopy. While free exciton excitations are found to be long-lived (~230 ps), an extremely fast intervalley scattering ( less than 120 fs) is observed leading to a negligible valley polarization, consistent with steady state photoluminescence measurements. The exciton population decay shows an appreciable contribution from exciton-exciton annihilation reactions with an annihilation constant of ~ 0.01 cm2s-1, which in addition leads to an extra contribution to the transient grating response. The underlying excitonic dynamics were numerically modeled by including exciton-exciton annihilation, also in the diffusion equation, which allows extraction of the diffusion constant, D ~1.4 cm2s-1. Our results provide a method that allows for the disentanglement of the intricate dynamics involving many-body annihilation processes and a detailed characterization of the excitonic properties of monolayer MoSe2

Published in: "arXiv Material Science".

Supersonic Cold Spraying for Energy and Environmental Applications: One‐Step Scalable Coating Technology for Advanced Micro‐ and Nanotextured Materials

2019-11-21T00:33:27+00:00November 20th, 2019|Categories: Publications|Tags: , , |

Supersonic cold spraying, which can deposit various nanoscale materials onto nearly any substrate rapidly and economically, is emerging as a new coating technique for energy and environmental products. Fundamentals of the supersonic cold spraying technique and recent progress in its development are briefly discussed and reviewed, illustrating its advantages and utility for energy and environmental applications. Abstract Supersonic cold spraying is an emerging technique for rapid deposition of films of materials including micrometer‐size and sub‐micrometer metal particles, nanoscale ceramic particles, clays, polymers, hybrid materials composed of polymers and particulates, reduced graphene oxide (rGO), and metal–organic frameworks. In this method, particles are accelerated to a high velocity and then impact a substrate at near ambient temperature, where dissipation of their kinetic energy produces strong adhesion. Here, recent progress in fundamentals and applications of cold spraying is reviewed. High‐velocity impact with the substrate results in significant deformation, which not only produces adhesion, but can change the particles’ internal structure. Cold‐sprayed coatings can also exhibit micro‐ and nanotextured morphologies not achievable by other means. Suspending micro‐ or nanoparticles in a liquid and cold‐spraying the suspension produces fine atomization and even deposition of materials that could not otherwise be processed. The scalability and low cost of this method and its compatibility with roll‐to‐roll processing make it promising for many applications, including ultrathin flexible materials, solar cells, touch‐screen panels, nanotextured surfaces for enhanced heat transfer, thermal and electrical insulation films, transparent conductive films, materials for energy storage (e.g., Li‐ion battery electrodes), heaters, sensors, photoelectrodes for

Published in: "Advanced Materials".

Directional Janus Metasurface

2019-11-21T00:33:25+00:00November 20th, 2019|Categories: Publications|

Direction‐encoded wave manipulations are experimentally achieved through the use of Janus metasurfaces composed of cascaded metasheets. By introducing a rotational twist in metasurface geometry, asymmetric electromagnetic wavefront manipulation can be realized. This is demonstrated both theoretically and experimentally by a series of passive metadevices, which enable functionalities including one‐way anomalous refraction, one‐way focusing, asymmetric focusing, and direction‐controlled holograms. Abstract Janus monolayers, a class of two‐faced 2D materials, have received significant attention in electronics, due to their unusual conduction properties stemming from their inher