MoSe2

/Tag: MoSe2

Two dimensional crystals in three dimensions: electronic decoupling of single-layered platelets in colloidal nanoparticles. (arXiv:1811.05238v1 [cond-mat.mtrl-sci])

2018-11-14T05:29:24+00:00November 14th, 2018|Categories: Publications|Tags: , |

Two-dimensional crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley- and spinorbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, we propose the formation of stacks of two-dimensional crystals intercalated with small surfactant molecules. We show, using first principles calculations, that already the very short surfactant methyl amine electronically decouples the layers. We demonstrate the indirect-direct band gap transition characteristic for Group 6 transition metal dichalcogenides experimentally by observing the emergence of a strong photoluminescence signal for ethoxide-intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the two-dimensional crystals with electronic properties typical for monolayers. Variation of the surfactant’s chemical potential allows fine-tuning of electronic properties and potentially elimination of trap states caused by defects.

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".

Spatial control of carrier capture in two-dimensional materials: Beyond energy selection rules

2018-11-09T20:33:11+00:00November 9th, 2018|Categories: Publications|Tags: , |

Author(s): Roberto Rosati, Frank Lengers, Doris E. Reiter, and Tilmann KuhnThe carrier capture from a two-dimensional transition metal dichalcogenide monolayer into a quasi-zero-dimensional potential is a decisive process to exploit these remarkable materials as, e.g., single-photon sources. Here, we study theoretically the phonon-induced carrier capture in a MoSe2 monolay…[Phys. Rev. B 98, 195411] Published Fri Nov 09, 2018

Published in: "Physical Review B".

2D Crystals in Three Dimensions: Electronic Decoupling of Single‐Layered Platelets in Colloidal Nanoparticles

2018-11-07T08:35:19+00:00November 7th, 2018|Categories: Publications|Tags: , |

Optoelectronic properties of 2D crystals are distinct from their bulk counterparts, and advantageous for applications. Band gaps are larger, and sometimes direct, well‐suited for optoelectronic and photocatalytic applications. However, being atomically thin, low density is a bottleneck for quantitative performance. The proposed intercalation of layered materials with short surfactants yields electronically decoupled layers with similar properties as monolayers, but higher cross‐sections. Abstract 2D crystals, single sheets of layered materials, often show distinct properties desired for optoelectronic applications, such as larger and direct band gaps, valley‐ and spin‐orbit effects. Being atomically thin, the low amount of material is a bottleneck in photophysical and photochemical applications. Here, the formation of stacks of 2D crystals intercalated with small surfactant molecules is proposed. It is shown, using first principles calculations, that the very short surfactant methyl amine electronically decouples the layers. The indirect–direct band gap transition characteristic for Group 6 transition metal dichalcogenides is demonstrated experimentally by observing the emergence of a strong photoluminescence signal for ethoxide‐intercalated WSe2 and MoSe2 multilayered nanoparticles with lateral size of about 10 nm and beyond. The proposed hybrid materials offer the highest possible density of the 2D crystals with electronic properties typical of monolayers. Variation of the surfactant’s chemical potential allows fine‐tuning of electronic properties and potentially elimination of trap states caused by defects.

Published in: "Small".

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

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".

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".

Inversion domain boundaries in MoSe2 layers

2018-09-27T12:32:52+00:00September 27th, 2018|Categories: Publications|Tags: |

RSC Adv., 2018, 8,33391-33397DOI: 10.1039/C8RA07205A, Paper Open Access &nbsp This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.Quang Duc Truong, Nguyen Tuan Hung, Yuta Nakayasu, Keiichiro Nayuki, Yoshikazu Sasaki, Devaraju Murukanahally Kempaiah, Li-Chang Yin, Takaaki Tomai, Riichiro Saito, Itaru HonmaStructural

Published in: "RSC Advances".

Electrical control of excitons in van der Waals heterostructures with type-II band alignment

2018-09-12T16:33:07+00:00September 12th, 2018|Categories: Publications|Tags: , , |

Author(s): A. Chaves, J. G. Azadani, V. Ongun Özçelik, R. Grassi, and T. LowWe investigate excitons in stacked transition-metal dichalcogenide layers under a perpendicularly applied electric field, herein MoSe2/WSe2 van der Waals heterostructures (vdWHs). Band structures are obtained with density functional theory (DFT), along with electron and hole wave functions in conduc…[Phys. Rev. B 98, 121302(R)] Published Wed Sep 12, 2018

Published in: "Physical Review B".

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".

Symmetry regimes for circular photocurrents in monolayer MoSe<sub>2</sub>

2018-08-21T10:35:47+00:00August 21st, 2018|Categories: Publications|Tags: |

Symmetry regimes for circular photocurrents in monolayer MoSe2Symmetry regimes for circular photocurrents in monolayer MoSe<sub>2</sub>, Published online: 21 August 2018; doi:10.1038/s41467-018-05734-zCircular photocurrents emerge in atomically thin transition metal dichalcogenides as a result of circular photogalvanic and photon drag effects. Here, the authors identify two different circular photocurrent contributions in monolater MoSe2, dominant at different voltages and with different dependence on illumination wavelength and incidence angles.

Published in: "Nature Communications".

Biexcitonic optical Stark effects in monolayer molybdenum diselenide

2018-07-30T16:34:21+00:00July 30th, 2018|Categories: Publications|Tags: |

Biexcitonic optical Stark effects in monolayer molybdenum diselenideBiexcitonic optical Stark effects in monolayer molybdenum diselenide, Published online: 30 July 2018; doi:10.1038/s41567-018-0216-7Light–matter interactions in monolayer MoSe2 can be dramatically modified by the interactions between the excitonic states, leading to a rich set of light-driven coherent phenomena.

Published in: "Nature Physics".

Semiconducting van der Waals Interfaces as Artificial Semiconductors. (arXiv:1807.08282v1 [cond-mat.mes-hall])

2018-07-24T04:30:20+00:00July 24th, 2018|Categories: Publications|Tags: , , , , |

Recent technical progress demonstrates the possibility of stacking together virtually any combination of atomically thin crystals of van der Waals bonded compounds to form new types of heterostructures and interfaces. As a result, there is the need to understand at a quantitative level how the interfacial properties are determined by the properties of the constituent 2D materials. We address this problem by studying the transport and optoelectronic response of two different interfaces based on transition-metal dichalcogenide monolayers, namely WSe2-MoSe2 and WSe2-MoS2. By exploiting the spectroscopic capabilities of ionic liquid gated transistors, we show how the conduction and valence bands of the individual monolayers determine the bands of the interface, and we establish quantitatively (directly from the measurements) the energetic alignment of the bands in the different materials as well as the magnitude of the interfacial band gap. Photoluminescence and photocurrent measurements allow us to conclude that the band gap of the WSe2-MoSe2 interface is direct in k space, whereas the gap of WSe2/MoS2 is indirect. For WSe2/MoSe2, we detect the light emitted from the decay of interlayer excitons and determine experimentally their binding energy using the values of the interfacial band gap extracted from transport measurements. The technique that we employed to reach this conclusion demonstrates a rather-general strategy for characterizing quantitatively the interfacial properties in terms of the properties of the constituent atomic layers. The results presented here further illustrate how van der Waals interfaces of two distinct 2D semiconducting materials are composite systems that truly behave as artificial

Published : "arXiv Mesoscale and Nanoscale Physics".

Moir’e Excitons in Van der Waals Heterostructures. (arXiv:1807.03771v1 [cond-mat.mes-hall])

2018-07-11T04:30:19+00:00July 11th, 2018|Categories: Publications|Tags: , , |

In van der Waals (vdW) heterostructures formed by stacking two monolayer semiconductors, lattice mismatch or rotational misalignment introduces an in-plane moir’e superlattice. While it is widely recognized that a moir’e superlattice can modulate the electronic band structure and lead to novel transport properties including unconventional superconductivity and insulating behavior driven by correlations, its influence on optical properties has not been investigated experimentally. We present spectroscopic evidence that interlayer excitons are confined by the moir’e potential in a high-quality MoSe2/WSe2 heterobilayer with small rotational twist. A series of interlayer exciton resonances with either positive or negative circularly polarized emission is observed in photoluminescence, consistent with multiple exciton states confined within the moir’e potential. The recombination dynamics and temperature dependence of these interlayer exciton resonances are consistent with this interpretation. These results demonstrate the feasibility of engineering artificial excitonic crystals using vdW heterostructures for nanophotonics and quantum information applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

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