On Low-Resistance Contacts to 2-D MoTe<sub>2</sub> by Crystalline Phase Junctions

2018-04-10T02:29:36+00:00 April 10th, 2018|Categories: Publications|Tags: |

Low contact resistance to 2-D semiconductor materials plays a critical role on their device applications. It has been experimentally demonstrated recently that the crystalline phase homojunctions between the 1T’ metallic and 2H semiconducting phases of 2-D transition metal dichacolgenide (TMDC) materials can be formed by a variety of fabrication techniques. A multiscale simulation approach that integrates atomistic ab initio simulations with quantum transport calculations based on the nonequilibrium Green’s function formalism is used to examine the contact properties of the crystalline phase junctions of monolayer MoTe2. It is shown that the following mechanisms can contribute to the low contact resistance of crystalline phase metal-semiconductor junctions of 2-D materials. First, the electric field is significantly enhanced at the 2-D phase junction interface due to the extremely thin body, which results in a thin Schottky barrier. Second, the coupling of electron wave functions cross the 1T’-2H junction interface is strong. Third, different from 3-D bulk metal-semiconductor junctions, metal-induced band gap states (MIGS) do not pin the Fermi level in the 2-D material junctions due to low dimensionality of the MIGS charge. The results provide insights into the possibility and limits of achieving low-contact-resistance contacts to 2-D TMDC semiconductors by using crystalline phase metal-semiconductor junctions.

Published in: "IEEE Transactions on Electron Devices".

Vacancy-driven phase transition in MoX2 (X: S, Se and Te) nanoscrolls

2018-03-29T02:24:19+00:00 March 29th, 2018|Categories: Publications|Tags: , , |

Nanoscale, 2018, Accepted ManuscriptDOI: 10.1039/C7NR08634B, CommunicationDa Young Hwang, Kyoung Hwan Choi, Dong Hack SuhAtomically thin MoX2 (MoS2, MoSe2 and MoTe2) exhibits semiconducting, metallic, and semi-metallic properties associated with different polymorphic phases such as 2H, 1T and distorted 1T (1T’), respectively. The phase transitions…The

Published in: "RSC Nanoscale".

Calculations of point defects in the layered MX2 (M=Mo, W; X=S, Te): Substitution by the groups III, V and VII elements. (arXiv:1803.04334v1 [cond-mat.mtrl-sci])

2018-03-13T20:00:32+00:00 March 13th, 2018|Categories: Publications|Tags: , , , , |

Dopability in semiconductors plays a crucial role in device performance. Using the first-principles density-functional theory calculations, we investigate systematically the doping properties of layered MX2 (M= Mo, W; X=S, Te) by replacing M or X with the groups III, V and VII elements. It is found that the defect BM is hard to form in MX2 due to the large formation energy originating from the crystal distortion, while AlM is easy to realize compared to the former. In MoS2, WS2 and MoTe2, Al is the most desirable p-type dopant under anion-rich conditions among the group III components, since AlM has relatively low transition and formation energies. With respect to the doping of the group V elements, it is found that the substitutions on the cation sites have deeper defect levels than those on the anion sites due to the strong electronegativity. AsTe and SbTe in MoTe2 and WTe2 are trend to form shallow acceptors under cation-rich conditions, indicating high hole-concentrations for p-type doping, whereas SbS in MoS2 and PTe in WTe2 are shown to be good p-type candidates under cation-rich conditions. In despite of that the substitutions of group VII on X site have low formation energies, the transition energies are too high to achieve n-type MoS2 and WS2. Nevertheless, for MoTe2, the substitutions with the group VII elements on the anion sites are suitable for n-type doping on account of the shallow donor levels and low formation energies under Mo-rich condition. As to WTe2, F is the only potential

Published in: "arXiv Material Science".

Sub 20 meV Schottky barriers in metal/MoTe2 junctions. (arXiv:1803.04164v1 [cond-mat.mes-hall])

2018-03-13T20:00:28+00:00 March 13th, 2018|Categories: Publications|Tags: |

The newly emerging class of atomically-thin materials has shown a high potential for the realisation of novel electronic and optoelectronic components. Amongst this family, semiconducting transition metal dichalcogenides (TMDCs) are of particular interest. While their band gaps are compatible with those of conventional solid state devices, they present a wide range of exciting new properties that is bound to become a crucial ingredient in the future of electronics. To utilise these properties for the prospect of electronics in general, and long-wavelength-based photodetectors in particular, the Schottky barriers formed upon contact with a metal and the contact resistance that arises at these interfaces have to be measured and controlled. We present experimental evidence for the formation of Schottky barriers as low as 10 meV between MoTe2 and metal electrodes. By varying the electrode work functions, we demonstrate that Fermi level pinning due to metal induced gap states at the interfaces occurs at 0.14 eV above the valence band maximum. In this configuration, thermionic emission is observed for the first time at temperatures between 40 K and 75 K. Finally, we discuss the ability to tune the barrier height using a gate electrode.

Published : "arXiv Mesoscale and Nanoscale Physics".

Highly Active, Durable Ultrathin MoTe2 Layers for the Electroreduction of CO2 to CH4

2018-03-08T14:32:39+00:00 March 8th, 2018|Categories: Publications|Tags: |

Abstract The electroreduction of CO2 to CH4 is a highly desirable, challenging research topic. In this study, an electrocatalytic system comprising ultrathin MoTe2 layers and an ionic liquid electrolyte for the reduction of CO2 to methane is reported, efficiently affording methane with a faradaic efficiency of 83 ± 3% (similar to the best Cu-based catalysts reported thus far) and a durable activity of greater than 45 h at a relatively high current density of 25.6 mA cm−2 (−1.0 VRHE). The results obtained can facilitate research on the design of other transition-metal dichalcogenide electrocatalysts for the reduction of CO2 to valuable fuels. An ultrathin MoTe2 electrode/ionic liquid electrolyte system is put forward to achieve efficient and robust CO2 reduction through offering a high faradaic efficiency (similar to the best Cu-based catalyst reported thus far) and superior structural stability (up to 45 h).

Published in: "Small".

Room-temperature nanoseconds spin relaxation in WTe2 and MoTe2 thin films. (arXiv:1803.00305v1 [cond-mat.mtrl-sci])

2018-03-02T19:59:20+00:00 March 2nd, 2018|Categories: Publications|Tags: , |

The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically-protected spin-polarized states and can carry a very large current density. In addition, the intrinsic noncentrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry-controlled spin-orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, we report a room-temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time-resolved Kerr rotation (TRKR). Based on ab initio calculation, we identify a mechanism of long-lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron-hole recombination rate and suppression of backscattering required by time-reversal and lattice symmetry operation. In addition, we find the spin polarization is firmly pinned along the strong internal out-of-plane magnetic field induced by large spin splitting. Our work provides an insight into the physical origin of long-lived spin polarization in Weyl semimetals which could be useful to manipulate spins for a long time at room temperature.

Published in: "arXiv Material Science".

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