/Tag: WS2

Nanodots of transition metal (Mo, W) disulfides grown on NiNi prussian blue analogue nanoplates for efficient hydrogen production

2018-09-07T02:33:11+00:00September 7th, 2018|Categories: Publications|Tags: , |

Chem. Commun., 2018, Accepted ManuscriptDOI: 10.1039/C8CC04844D, CommunicationGuo Chen, Wenfei Dong, Yang Hui Deng, Bang Lin Li, Xiao Lin Li, Hong Qun Luo, Nianbing LiLowering the dimension of transition metal dichalcogenides is an efficient approach to expose more S-edge-sites. Here, zero-dimension MoS2 and WS2 nanodots are successfully prepared with the assistance of template NiNi…The content of this RSS Feed (c) The Royal Society of Chemistry

Published in: "Chemical Communications".

Optical harmonic generation in monolayer group-VI transition metal dichalcogenides. (arXiv:1808.09494v1 [physics.optics])

2018-08-30T04:30:21+00:00August 30th, 2018|Categories: Publications|Tags: , , , |

Monolayer transition metal dichalcogenides (TMDs) exhibit high nonlinear optical (NLO) susceptibilities. Experiments on MoS$_2$ have indeed discovered very large second-order ($chi^{(2)}$) and third-order ($chi^{(3)}$) optical susceptibilities. However, third harmonic generation of other layered TMDs has not been reported. Further, the reported $chi^{(2)}$ and $chi^{(3)}$ of MoS$_2$ vary by several orders of magnitude, and a reliable quantitative comparison of optical nonlinearities across different TMDs has remained elusive. Here, we demonstrate third harmonic generation in WSe$_2$, MoSe$_2$ and WS$_2$, and three-photon photoluminescence in TMDs for the first time. We also report the first experimental study of $chi^{(2)}$ and $chi^{(3)}$ of four common TMD materials (MoS2, MoSe2, WS2 and WSe2) by placing different TMD flakes in close proximity to each other on a common substrate, allowing their NLO properties to be accurately obtained from a single measurement. $chi^{(2)}$ and $chi^{(3)}$ of the four monolayer TMDs have been compared, indicating that they exhibit distinct NLO responses. We further present theoretical simulations of these susceptibilities in qualitative agreement with the measurements. Our results of comparatively studying the NLO responses of different two-dimensional layered materials allow us to select the best candidates for atomic-scale nonlinear photonic applications, such as frequency conversion and all-optical signal processing.

Published : "arXiv Mesoscale and Nanoscale Physics".

Screened Strong Coupling of Excitons in Multilayer WS2 with Collective Plasmonic Resonances. (arXiv:1808.08388v1 [cond-mat.mtrl-sci])

2018-08-28T02:29:31+00:00August 28th, 2018|Categories: Publications|Tags: , |

We demonstrate the strong coupling of direct transition excitons in tungsten disulfide (WS2) with collective plasmonic resonances at room temperature. We use open plasmonic cavities formed by periodic arrays of metallic nanoparticles. We show clear anti-crossings with monolayer, bilayer and thicker multilayer WS2 on top of the nanoparticle array. The Rabi energy of such hybrid system varies from 50 to 110 meV from monolayer to sixteen layers, while it does not scale with the square root of the number of layers as expected for collective strong coupling. We prove that out-of-plane coupling components can be disregarded since the normal field is screened due to the high refractive index contrast of the dielectric layers. Even though the in-plane dipole moments of the excitons decrease beyond monolayers, the strong in-plane field distributed in the flake can still enhance the coupling strength with multilayers. However, the screened out-of-plane field leads to the saturation of the Rabi energy. The achieved coherent coupling of TMD multilayers with open cavities could be exploited for manipulating the dynamics and transport of excitons in 2D semiconductors and developing ultrafast valley/spintronic devices.

Published in: "arXiv Material Science".

Spatial extent of the excited exciton states in WS$_2$ monolayers from diamagnetic shifts. (arXiv:1808.07786v1 [cond-mat.mes-hall])

2018-08-24T04:30:17+00:00August 24th, 2018|Categories: Publications|Tags: , , |

We experimentally study the radii of excitons in hBN-encapsulated WS2 monolayers by means of magneto-optical reflectance spectroscopy at cryogenic temperatures in magnetic fields up to 29 T. We observe field-induced energy shifts of the exciton ground and excited states due to valley Zeeman and diamagnetic effects. We find the g factor of the first excited state of $-4.2(+/-0.1) to be essentially equal to that of the ground state of -4.35(+/-0.1). From diamagnetic shifts we determine the root mean square radii of the excitons. The radius of the first excited state is found to be 5-8 nm and that of the ground state around 2 nm. Our results further confirm the Wannier-Mott nature of the exciton quasiparticles in monolayer semiconductors and the assignment of the optical resonances in absorption-type measurements. They also provide additional support for the applicability of the effective mass hydrogenlike models in these systems.

Published : "arXiv Mesoscale and Nanoscale Physics".

Observation and active control of a collective polariton mode and polaritonic band gap in few-layer WS2 strongly coupled with plasmonic lattices. (arXiv:1808.06554v1 [physics.optics])

2018-08-21T02:29:20+00:00August 21st, 2018|Categories: Publications|Tags: , |

Two-dimensional semiconductors host excitons with very large oscillator strengths and binding energies due to significantly reduced carrier screening. Two-dimensional semiconductors integrated with optical cavities are emerging as a promising platform for studying strong light-matter interactions as a route to explore a variety of exotic many-body effects. Here, in few-layered WS2 coupled with plasmonic nanoparticle lattices, we observe the formation of a collective polaritonic mode near the exciton energy and the formation of a complete polariton band gap with energy scale comparable to the exciton-plasmon coupling strength. A coupled oscillator model reveals that the collective mode arises from the cooperative coupling of the excitons to the plasmonic lattice diffraction orders via exciton-exciton interactions. The emergence of the collective mode is accompanied by a superlinear increase of the polariton mode splitting as a function of the square root of the exciton oscillator strength. The presence of these many body effects, which are enhanced in systems which lack bulk polarization, not only allows the formation of a collective mode with periodically varying field profiles, but also further enhances the exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice device with a field-effect transistor, we demonstrate active tuning of the collective mode and the polariton band gap. These systems provide new opportunities for obtaining a deeper and systematic understanding of many body cooperative phenomena in periodic photonic systems and for designing more complex and actively controllable polaritonic devices including switchable polariton lasers, waveguides, and optical logical elements.

Published in: "arXiv Material Science".

Femtosecond valley polarization and topological resonances in transition metal dichalcogenides

2018-08-15T16:33:21+00:00August 15th, 2018|Categories: Publications|Tags: , |

Author(s): S. Azar Oliaei Motlagh, Jhih-Sheng Wu, Vadym Apalkov, and Mark I. StockmanWe theoretically introduce the fundamentally fastest induction of a significant population and valley polarization in a monolayer of a transition metal dichalcogenide (i.e., MoS2 and WS2). This may be extended to other two-dimensional materials with the same symmetry. This valley polarization can be…[Phys. Rev. B 98, 081406(R)] Published Wed Aug 15, 2018

Published in: "Physical Review B".

Zeeman Splitting and Inverted Polarization of Biexciton Emission in Monolayer ${mathrm{WS}}_{2}$

2018-08-02T14:33:48+00:00August 2nd, 2018|Categories: Publications|Tags: |

Author(s): Philipp Nagler, Mariana V. Ballottin, Anatolie A. Mitioglu, Mikhail V. Durnev, Takashi Taniguchi, Kenji Watanabe, Alexey Chernikov, Christian Schüller, Mikhail M. Glazov, Peter C. M. Christianen, and Tobias KornAtomically thin semiconductors provide an ideal testbed to investigate the physics of Coulomb-bound many-body states. We shed light on the intricate structure of such complexes by studying the magnetic-field-induced splitting of biexcitons in monolayer WS2 using polarization-resolved photoluminescen…[Phys. Rev. Lett. 121, 057402] Published Thu Aug 02, 2018

Published in: "Physical Review Letters".

kinetic Monte Carlo perspectives on substrate-induced anisotropic growth of monolayer tungsten disulfide. (arXiv:1807.09323v1 [cond-mat.mtrl-sci])

2018-07-26T02:29:12+00:00July 26th, 2018|Categories: Publications|Tags: |

Controlled anisotropic growth of two-dimensional (2D) materials provides a distinct approach for the synthesis of large single crystals and nanoribbons, which will be very promising due to its potential applications in low-dimensional semiconductors and next-generation optoelectronic devices. In particular, the anisotropic growth of transition metal dichalcogenides (TMDs) induced by the substrate is of great interest due to its operability. For the first time, the proof-of-concept experiment presents the extremely anisotropic growth of monolayer tungsten disulfide (WS2) on the ST-X quartz substrate by chemical vapor deposition (CVD), and the mechanism of the substrate-induced anisotropic growth of monolayer WS2 is carefully examined by virtue of the kinetic Monte Carlo (kMC) method. In addition, we propose a quantitative method for classifying and measuring the morphology of 2D flakes and derive the formula of anisotropic growth ratio (AGR) with regard to the extent of the growth anisotropy. The results of kMC simulations show that, besides the variation of substrate adsorption, the chalcogen to metal (C/M) ratio is a major contribution to the surge of growth anisotropy and the polarization of undergrowth and overgrowth; either perfect isotropy or high anisotropy can be expected when the C/M ratio equals to 2 by properly controlling the linear relationship between gas flux and temperature.

Published in: "arXiv Material Science".

Ni-P coatings electroplating – A review, Part II: Ni-P composites. (arXiv:1807.04692v1 [])

2018-07-13T02:29:25+00:00July 13th, 2018|Categories: Publications|Tags: , , |

Ni-P coatings produced by electrodeposition are characterized by good mechanical, tribological and electrochemical properties, they exhibit catalytic activity and beneficial magnetic behaviour. With subsequent thermal treatment hardness of this binary metal-metalloid system can approach or be even higher than that of hard Cr coatings. Electrochemical co-deposition of homogeneously dispersed second phase particles within the Ni-P metal matrix can result in the enhancement of alloy’s favourable properties and in the possibility of completely new engineering applications. The purpose of this paper is to provide a general overview of the research work regarding the electrodeposition of Ni-P matrix based composite coatings. Advances in the research of Ni-P composites reinforced by: SiC, B4C, WC, Al2O3, SiO2, TiO2, CeO2, MWCNT, MoS2, WS2, TiN, hBN, PTFE and their combinations are covered, with the consideration of the main models proposed for the co-deposition of particles, influence of the system hydrodynamics and other process factors.

Published in: "arXiv Material Science".

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