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In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

2018-02-16T08:31:03+00:00 February 16th, 2018|Categories: Publications|Tags: , |

Abstract Molybdenum disulfide (MoS2) and bismuth telluride (Bi2Te3) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high-reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS2 and Bi2Te3 is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS2 and Bi2Te3 crystals with unprecedented resolution. Real-time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single-crystalline free-standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future. Controlled electron beam irradiation can be utilized as a tool to repair the nanopores in MoS2 and Bi2Te3 and lead to high-quality crystals with a low number of defects. The dynamic repair processes yield an in-depth understanding of the repair mechanism in 2D chalcogenides: the sites with more

Published in: "Advanced Materials".

Spatially dispersive circular photogalvanic effect in a Weyl semimetal. (arXiv:1802.04387v1 [cond-mat.mtrl-sci])

2018-02-14T19:58:49+00:00 February 14th, 2018|Categories: Publications|Tags: |

Weyl semimetals are gapless topological states of matter with broken inversion and/or time reversal symmetry, which can support unconventional responses to externally applied electrical, optical and magnetic fields. Here we report a new photogalvanic effect in type-II WSMs, MoTe2 and Mo0.9W0.1Te2, which are observed to support a circulating photocurrent when illuminated by circularly polarized light at normal incidence. This effect occurs exclusively in the inversion broken phase, where crucially we find that it is associated with a spatially varying beam profile via a new dispersive contribution to the circular photogalvanic effect (s-CPGE). The response functions derived for s-CPGE reveal the microscopic mechanism of this photocurrent, which are controlled by terms that are allowed in the absence of inversion symmetry, along with asymmetric carrier excitation and relaxation. By evaluating this response for a minimal model of a Weyl semimetal, we obtain the frequency dependent scaling behavior of this form of photocurrent. These results demonstrate opportunities for controlling photoresponse by patterning optical fields to store, manipulate and transmit information over a wide spectral range.

Published : "arXiv Mesoscale and Nanoscale Physics".

Reversible and Precisely Controllable p/n-Type Doping of MoTe2 Transistors through Electrothermal Doping

2018-02-12T08:31:18+00:00 February 12th, 2018|Categories: Publications|Tags: , |

Abstract Precisely controllable and reversible p/n-type electronic doping of molybdenum ditelluride (MoTe2) transistors is achieved by electrothermal doping (E-doping) processes. E-doping includes electrothermal annealing induced by an electric field in a vacuum chamber, which results in electron (n-type) doping and exposure to air, which induces hole (p-type) doping. The doping arises from the interaction between oxygen molecules or water vapor and defects of tellurium at the MoTe2 surface, and allows the accurate manipulation of p/n-type electrical doping of MoTe2 transistors. Because no dopant or special gas is used in the E-doping processes of MoTe2, E-doping is a simple and efficient method. Moreover, through exact manipulation of p/n-type doping of MoTe2 transistors, quasi-complementary metal oxide semiconductor adaptive logic circuits, such as an inverter, not or gate, and not and gate, are successfully fabricated. The simple method, E-doping, adopted in obtaining p/n-type doping of MoTe2 transistors undoubtedly has provided an approach to create the electronic devices with desired performance. Precisely controllable and reversible doping of molybdenum ditelluride (MoTe2) transistors is achieved by electrothermal doping (E-doping) processes. E-doping includes electrothermal annealing induced by an electric field in vacuum, which results in electron (n-type) doping, and exposure to air, which induces hole (p-type) doping. No dopant or gas is used in the E-doping processes, E-doping is a simple and efficient method.

Published in: "Advanced Materials".

Atomic structure of intrinsic and electron-irradiation-induced defects in MoTe2. (arXiv:1802.01552v1 [cond-mat.mtrl-sci])

2018-02-06T19:59:15+00:00 February 6th, 2018|Categories: Publications|Tags: , |

Studying the atomic structure of intrinsic defects in two-dimensional transition metal dichalcogenides is difficult since they damage quickly under the intense electron irradiation in transmission electron microscopy (TEM). However, this can also lead to insights into the creation of defects and their atom-scale dynamics. We first show that MoTe 2 monolayers without protection indeed quickly degrade during scanning TEM (STEM) imaging, and discuss the observed atomic-level dynamics, including a transformation from the 1H phase into 1T’, three-fold rotationally symmetric defects, and the migration of line defects between two 1H grains with a 60{deg} misorientation. We then analyze the atomic structure of MoTe2 encapsulated between two graphene sheets to mitigate damage, finding the as-prepared material to contain an unexpectedly large concentration of defects. These include similar point defects (or quantum dots, QDs) as those created in the non-encapsulated material, and two different types of line defects (or quantum wires, QWs) that can be transformed from one to the other under electron irradiation. Our density functional theory simulations indicate that the QDs and QWs embedded in MoTe2 introduce new midgap states into the semiconducting material, and may thus be used to control its electronic and optical properties. Finally, the edge of the encapsulated material appears amorphous, possibly due to the pressure caused by the encapsulation.

Published in: "arXiv Material Science".

Mechanical responses of two-dimensional MoTe2; pristine 2H, 1T and 1T’ and 1T’/2H heterostructure. (arXiv:1802.00598v1 [physics.comp-ph])

2018-02-05T19:58:47+00:00 February 5th, 2018|Categories: Publications|Tags: , |

Transition metal dichalcogenides (TMD) are currently among the most interesting two-dimensional (2D) materials due to their outstanding properties. MoTe2 involves attractive polymorphic TMD crystals which can exist in three different 2D atomic lattices of 2H, 1T and 1T’, with diverse properties, like semiconducting and metallic electronic characters. Using the polymorphic heteroepitaxy, most recently coplanar semiconductor/metal (2H/1T’) few-layer MoTe2 heterostructures were experimentally synthesized, highly promising to build circuit components for next generation nanoelectronics. Motivated by the recent experimental advances, we conducted first-principles calculations to explore the mechanical properties of single-layer MoTe2 structures. We first studied the mechanical responses of pristine and single-layer 2H-, 1T- and 1T’-MoTe2. In these cases we particularly analyzed the possibility of engineering of the electronic properties of these attractive 2D structures using the biaxial or uniaxial tensile loadings. Finally, the mechanical-failure responses of 1T’/2H-MoTe2 heterostructure were explored, which confirms the remarkable strength of this novel 2D system.

Published : "arXiv Mesoscale and Nanoscale Physics".

High-Performance Photovoltaic Detector Based on MoTe2/MoS2 Van der Waals Heterostructure

2018-01-22T08:30:29+00:00 January 22nd, 2018|Categories: Publications|Tags: , , |

Abstract Van der Waals heterostructures based on 2D layered materials have received wide attention for their multiple applications in optoelectronic devices, such as solar cells, light-emitting devices, and photodiodes. In this work, high-performance photovoltaic photodetectors based on MoTe2/MoS2 vertical heterojunctions are demonstrated by exfoliating-restacking approach. The fundamental electric properties and band structures of the junction are revealed and analyzed. It is shown that this kind of photodetectors can operate under zero bias with high on/off ratio (>105) and ultralow dark current (≈3 pA). Moreover, a fast response time of 60 µs and high photoresponsivity of 46 mA W−1 are also attained at room temperature. The junctions based on 2D materials are expected to constitute the ultimate functional elements of nanoscale electronic and optoelectronic applications. High-performance photovoltaic photodetectors based on MoTe2/MoS2 heterojunction are demonstrated. The photoresponse under different biases is measured and the corresponding light-induced charge transport is discussed. As a self-powered photodetector, the vertical 2D p–n junctions achieve fast response and broad detection wavelength range. These outstanding properties indicate that 2D van der Waals junctions possess promising applications in photodetection, on-chip logic circuits, and related applications.

Published in: "Small".

Semiconductor-metal structural phase transformation in MoTe2 monolayers by electronic excitation

2018-01-15T14:24:17+00:00 January 15th, 2018|Categories: Publications|Tags: , |

Nanoscale, 2018, Advance ArticleDOI: 10.1039/C7NR07890K, PaperAravind Krishnamoorthy, Lindsay Bassman, Rajiv K. Kalia, Aiichiro Nakano, Fuyuki Shimojo, Priya VashishtaElectronic excitation leads to soft vibration modes and reduced energy barriers for structural phase transformation in TMDCs.To cite this article before page numbers are assigned, use

Published in: "RSC Nanoscale".

Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides. (arXiv:1801.02220v1 [cond-mat.mtrl-sci])

2018-01-09T19:59:32+00:00 January 9th, 2018|Categories: Publications|Tags: , |

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

Published in: "arXiv Material Science".

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