/Tag: WS2

Quasi-free-standing single-layer WS2 achieved by intercalation. (arXiv:1811.05748v1 [cond-mat.mtrl-sci])

2018-11-15T02:29:18+00:00November 15th, 2018|Categories: Publications|Tags: |

Large-area and high-quality single-layer transition metal dichalcogenides can be synthesized by epitaxial growth on single-crystal substrates. An important advantage of this approach is that the interaction between the single-layer and the substrate can be strong enough to enforce a single crystalline orientation of the layer. On the other hand, the same interaction can lead to hybridization effects, resulting in the deterioration of the single-layer’s native properties. This dilemma can potentially be solved by decoupling the single-layer from the substrate surface after the growth via intercalation of atoms or molecules. Here we show that such a decoupling can indeed be achieved for single-layer WS2 epitaxially grown on Ag(111) by intercalation of Bi atoms. This process leads to a suppression of the single-layer WS2-Ag substrate interaction, yielding an electronic band structure reminiscent of free-standing single-layer WS2.

Published in: "arXiv Material Science".

Atomic process of oxidative etching in monolayer molybdenum disulfide. (arXiv:1811.04242v1 [cond-mat.mtrl-sci])

2018-11-13T02:29:19+00:00November 13th, 2018|Categories: Publications|Tags: , , , |

The microscopic process of oxidative etching of two-dimensional molybdenum disulfide (2D MoS2) at an atomic scale is investigated using a correlative TEM-etching study. MoS2 flakes on graphene TEM grids are precisely tracked and characterized by TEM before and after the oxidative etching. This allows us to determine the structural change with an atomic resolution on the edges of the domains, of well-oriented triangular pits and along the grain boundaries. We observe that the etching mostly starts from the open edges, grain boundaries and pre-existing atomic defects. A zigzag Mo edge is assigned as the dominant termination of the triangular pits, and profound terraces and grooves are observed on the etched edges. Based on the statistical TEM analysis, we reveal possible routes for the kinetics of the oxidative etching in 2D MoS2, which should also be applicable for other 2D transition metal dichalcogenide materials like MoSe2 and WS2.

Published in: "arXiv Material Science".

Super-ideal diodes at the Schottky-Mott limit in gated graphene-WSe$_2$ heterojunctions. (arXiv:1811.02660v1 [cond-mat.mes-hall])

2018-11-08T04:30:19+00:00November 8th, 2018|Categories: Publications|Tags: , , , , |

Metal-semiconductor interfaces, known as Schottky junctions, have long been hindered by defects and impurities. Such imperfections dominate the electrical characteristics of the junction by pinning the metal Fermi energy. We report measurements on a boron nitride encapsulated graphene-tungsten diselenide (WSe$_2$) Schottky junction which exhibits ideal diode characteristics and a complete lack of Fermi-level pinning. The Schottky barrier height of the device is rigidly tuned by electrostatic gating of the WSe$_2$, enabling experimental verification of the Schottky-Mott limit in a single device. Utilizing this exceptional gate control, we realize a super-ideal gated-Schottky diode which surpasses the ideal diode limit. Our results provide a pathway for defect-free electrical contact to two-dimensional semiconductors and open up possibilities for circuits with efficient switching characteristics and higher efficiency optoelectronic devices.

Published : "arXiv Mesoscale and Nanoscale Physics".

Excitonic emission in van-der-Waals nanotubes of transition metal dichalcogenides. (arXiv:1811.01195v1 [cond-mat.mtrl-sci])

2018-11-06T05:29:32+00:00November 6th, 2018|Categories: Publications|Tags: , |

Nanotubes (NTs) of transition metal dichalcogenides (TMDs), such as MoS2 and WS2, were first synthesized more than a quarter of a century ago; nevertheless, many of their properties have so far remained basically unknown. This review presents the state of the art in the knowledge of the optical properties of TMD NTs. We first evaluate general properties of multilayered TMD crystals, and analyze available data on electronic band structure and optical properties of related NTs. Then, the technology for the formation and the structural characteristics of TMD NTs are represented, focusing on the structures synthesized by chemical transport reaction. The core of this work is the presentation of the ability of TMD NTs to emit bright photoluminescence (PL), which has been discovered recently. By means of micro-PL spectroscopy of individual tubes we show that excitonic transitions relevant to both direct and indirect band gaps contribute to the emission spectra of the NTs despite the presence of dozens of monolayers in their walls. We highlight the performance of the tubes as efficient optical resonators, whose confined optical modes strongly affect the emission bands. Finally, a brief conclusion is presented, along with an outlook of the future studies of this novel member of the family of radiative NTs, which have unique potential for different nanophotonics applications.

Published in: "arXiv Material Science".

Suppressing Diffusion-Mediated Exciton Annihilation in 2D Semiconductors Using the Dielectric Environment. (arXiv:1811.01066v1 [cond-mat.mtrl-sci])

2018-11-06T04:30:26+00:00November 6th, 2018|Categories: Publications|Tags: , |

Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit nonlinear exciton-exciton annihilation at notably low excitation densities (below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at which annihilation occurs can be tuned by changing the underlying substrate. When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate constant for second-order exciton-exciton annihilation, k_XX [cm2/s], is reduced by one or two orders of magnitude, respectively. Using transient photoluminescence microscopy, we measure the effective room-temperature exciton diffusion coefficient in chemical-treated MoS2 to be D = 0.06 +/- 0.01 cm2/s, corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of {tau} = 20 ns, which is independent of the substrate. These results, together with numerical simulations, suggest that the effective exciton-exciton annihilation radius monotonically decreases with increasing refractive index of the underlying substrate. Exciton-exciton annihilation limits the overall efficiency of 2D semiconductor devices operating at high exciton densities; the ability to tune these interactions via the dielectric environment is an important step toward more efficient optoelectronic technologies featuring atomically thin materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

Facile access to shape-controlled growth of WS 2 monolayer via environment-friendly method

2018-10-30T12:39:48+00:00October 30th, 2018|Categories: Publications|Tags: |

2D materials with tailored morphologies exhibit distinctive shape-dependent properties especially in electrocatalysis and gas sensing, extending their applications in nanoelectronics. Atomic-layer tungsten disulfide (WS 2 ) stands out as a promising candidate, but the controllable synthesis of WS 2 still faces unavoidable obstacles, such as strict parameter requirements, low efficiency and serious pollution during the preparation. Here we report an elegant technique for growing homogeneous-luminescence WS 2 monolayer with desired shapes and developed electrochemical properties, through the improved chemical vapor deposition method with a semi-closed airflow environment. It is an effective, economic and especially environment-friendly approach with atmospheric pressure and hydrogen-free condition, aiming at synthesizing high-quality and large-scale monolayer WS 2 crystals with only one heating zone. Compared with the existing methods, this o…

Published in: "2DMaterials".

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals. (arXiv:1810.08927v1 [cond-mat.mes-hall])

2018-10-23T04:30:27+00:00October 23rd, 2018|Categories: Publications|Tags: , , , |

To explore new constituents in two-dimensional materials and to combine their best in van der Waals heterostructures, are always in great demand as being unique platform to discover new physical phenomena and to design novel functionalities in modern electronics. Herein, we introduce PbI2 into two-dimensional system by synthesizing PbI2 crystals down to atomic scale, and further assemble them with transition metal dichalcogenide monolayers, as a good demonstration of flexibly designing different types of band alignment and interlayer interactions in PbI2-based interfacial semiconductors. The photoluminescence of MoS2 is strongly enhanced in MoS2/PbI2 stacks, while a dramatic quenching of WS2 or WSe2 is revealed in WS2/PbI2 and WSe2/PbI2 stacks. This is attributed to the effective heterojunction formation between PbI2 and these two-dimensional materials, but type I band alignment in MoS2/PbI2 stacks where fast-transferred charge carriers accumulate in the semiconductor with a smaller band gap and type II in WS2/PbI2 and WSe2/PbI2 stacks with separated electrons and holes. Our results demonstrate that MoS2, WS2, WSe2 monolayers with very similar electronic structures themselves, show distinct optical properties when interfacing with atomically thin PbI2 crystals, providing unprecedent capabilities to engineer and optimize the device performance based on two-dimensional heterostructures.

Published : "arXiv Mesoscale and Nanoscale Physics".

Tuning Bandgap and Energy Stability of Organic-Inorganic Halide Perovskites through Surface Engineering. (arXiv:1810.07297v1 [])

2018-10-18T02:29:28+00:00October 18th, 2018|Categories: Publications|Tags: , , , , , , |

Organohalide perovskite with a variety of surface structures and morphologies have shown promising potential owing to the choice of the type of heterostructure dependent stability. We systematically investigate and discuss the impact of 2-dimensional molybdenum-disulphide (MoS2), molybdenum-diselenide (MoSe2), tungsten-disulphide (WS2), tungsten-diselenide (WSe2), boron- nitiride (BN) and graphene monolayers on band-gap and energy stability of organic-inorganic halide perovskites. We found that MAPbI3ML deposited on BN-ML shows room temperature stability (-25 meV~300K) with an optimal bandgap of ~1.6 eV. The calculated absorption coefficient also lies in the visible-light range with a maximum of 4.9 x 104 cm-1 achieved at 2.8 eV photon energy. On the basis of our calculations, we suggest that the encapsulation of an organic-inorganic halide perovskite monolayers by semiconducting monolayers potentially provides greater flexibility for tuning the energy stability and the bandgap.

Published in: "arXiv Material Science".

Spatially selective reversible charge carrier density tuning in WS 2 monolayers via photochlorination

2018-10-17T12:37:49+00:00October 17th, 2018|Categories: Publications|Tags: , |

Chlorine-doped tungsten disulfide monolayer (1L-WS 2 ) with tunable charge carrier concentration has been realized by pulsed laser irradiation of the atomically thin lattice in a precursor gas atmosphere. This process gives rise to a systematic shift of the neutral exciton peak towards lower energies, indicating reduction of the crystal’s electron density. The capability to progressively tune the carrier density upon variation of the exposure time is demonstrated; this indicates that the Fermi level shift is directly correlated to the respective electron density modulation due to the chlorine species. Notably, this electron withdrawing process enabled the determination of the trion binding energy of the intrinsic crystal, found to be as low as 20 meV, in accordance to theoretical predictions. At the same time, it is found that the effect can be reversed upon continuous wave laser scanning of the monolayer in air. Scanning auger microscopy (SAM) and x-ray photoelectron s…

Published in: "2DMaterials".

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2. (arXiv:1810.04745v1 [cond-mat.mtrl-sci])

2018-10-12T02:29:42+00:00October 12th, 2018|Categories: Publications|Tags: , , , , |

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.

Published in: "arXiv Material Science".

Large spin-orbit splitting of deep in-gap defect states of engineered sulfur vacancies in monolayer WS2. (arXiv:1810.02896v1 [cond-mat.mtrl-sci])

2018-10-09T02:29:29+00:00October 9th, 2018|Categories: Publications|Tags: |

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping in semiconductors. Specifically, deep in-gap defect states of chalcogen vacancies have been associated with intriguing phenomena in monolayer transition metal dichalcogenides (TMDs). Here, we report the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS2 by a combination of CO-tip noncontact atomic force microscopy (nc-AFM) and scanning tunneling microscopy (STM). Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states of the vacancy provide a unique fingerprint of this defect. Direct imaging of the defect orbitals by STM and state-of-the-art GW calculations reveal that the large splitting of 252 meV between these defect states is induced by spin-orbit coupling. The controllable incorporation and potential decoration of chalcogen vacancies provide a new route to tailor the optical, catalytic and magnetic properties of TMDs.

Published in: "arXiv Material Science".

Strain tuning of excitons in monolayer ${mathrm{WSe}}_{2}$

2018-09-27T16:34:10+00:00September 27th, 2018|Categories: Publications|Tags: , |

Author(s): Ozgur Burak Aslan, Minda Deng, and Tony F. HeinzThe authors investigate the influence of strain on the electronic properties of monolayer WSe2 using optical absorption and photoluminescence spectroscopy. The linewidth of the A exciton exhibits a significant and unexpected decrease, from 42 to 24 meV at room temperature. A slightly different behavior is observed for WS2; its linewidth decreases from 30 to 24 meV. They provide a model that explains both the decrease in linewidth and differences in the magnitude of the effect in these two similar material systems. Their findings reveal the utility of strain tuning for probing subtle aspects of 2D materials.[Phys. Rev. B 98, 115308] Published Thu Sep 27, 2018

Published in: "Physical Review B".

Understanding Interlayer Coupling in TMD-hBN Heterostructure by Raman Spectroscopy

2018-09-21T00:33:59+00:00September 21st, 2018|Categories: Publications|Tags: , , |

In 2-D van der Waals heterostructures, interactions between atomic layers dramatically change the vibrational properties of the hybrid system and demonstrate several interesting phenomena that are absent in individual materials. In this paper, we have investigated the vibrational properties of the heterostructure between transition metal dichalcogenide (TMD) and hexagonal boron nitride (hBN) on gold film at low- and high-frequency ranges by Raman spectroscopy. Nineteen Raman modes have been observed from the sample, including a new interlayer coupling mode at 28.8 cm−1. Compared to reported experimental results of tungsten disulfide (WS2) on SiO2/Si substrates, the Raman spectrum for WS2 on hBN/Au emerges a blue shift of about 8 cm−1. Furthermore, a remarkable enhancement of Raman intensity can be obtained when tuning hBN thickness in the heterostructure. Through systematic first-principles calculations, numerical simulations, and analytical calculations, we find that the 28.8 cm−1 mode originates from the shearing motion between monolayer TMD and hBN layers. In addition, the gold substrate and hBN layers form an optical cavity and the cavity interference effects enhance the obtained Raman intensity. This paper demonstrates the novel vibrational modes of 2-D van der Waals heterostructure as an effective tool to characterize a variety of such heterostructures and reveals a new method to enhance the Raman response of 2-D materials.

Published in: "IEEE Transactions on Electron Devices".

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