/Tag: WSe2

Low temperature photoconductivity of few layer p -type tungsten diselenide (WSe 2 ) field-effect transistors (FETs)

2018-10-01T12:33:45+00:00October 1st, 2018|Categories: Publications|Tags: |

We report on the low-temperature photoconductive properties of few layer p -type tungsten diselenide (WSe 2 ) field-effect transistors (FETs) synthesized using the chemical vapor transport method. Photoconductivity measurements show that these FETs display room temperature photo-responsivities of ∼7 mAW −1 when illuminated with a laser of wavelength λ = 658 nm with a power of 38 nW. The photo-responsivities of these FETs showed orders of magnitude improvement (up to ∼1.1 AW −1 with external quantum efficiencies reaching as high as ∼188%) upon application of a gate voltage ( V G  = −60 V). A temperature dependent (100K < T < 300 K) photoconductivity study reveals a weak temperature dependence of responsivity for these WSe 2 phototransistors. We demonstrate that it is possible to obtain stable photo-responsivities of ∼0.76 ± 0.2 AW −1 (with applied V G = −60 V), at low tem…

Published in: "Nanotechnology".

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".

Valley polarization of exciton-polaritons in monolayer WSe2 in a tunable microcavity. (arXiv:1809.09571v1 [cond-mat.mes-hall])

2018-09-26T04:30:17+00:00September 26th, 2018|Categories: Publications|Tags: , |

Monolayer transition metal dichalcogenides, known for exhibiting strong excitonic resonances, constitute a very interesting and versatile platform for investigation of light-matter interactions. In this work we report on a strong coupling regime between excitons in monolayer WSe2 and photons confined in an open, voltage-tunable dielectric microcavity. The tunability of our system allows us to extend the exciton-polariton state over a wide energy range and, in particular, to bring the excitonic component of the lower polariton mode into resonance with other excitonic transitions in monolayer WSe2. With selective excitation of spin-polarized exciton-polaritons we demonstrate the valley polarization when the polaritons from the lower branch come into resonance with a bright trion state in monolayer WSe2 and valley depolarization when they are in resonance with a dark trion state.

Published : "arXiv Mesoscale and Nanoscale Physics".

Channel Thickness Optimization for Ultrathin and 2-D Chemically Doped TFETs

2018-09-21T00:34:26+00:00September 21st, 2018|Categories: Publications|Tags: , |

The 2-D material-based TFETs are among the most promising candidates for low-power electronics applications since they offer ultimate gate control and high-current drives that are achievable through small tunneling distances ($Lambda $ ) during the device operation. The ideal device is characterized by a minimized $Lambda $ . However, devices with the thinnest possible body do not necessarily provide the best performance. For example, reducing the channel thickness (${T}_{text {ch}}$ ) increases the depletion width in the source, which can be a significant part of the total $Lambda $ . Hence, it is important to determine the optimum ${T}_{text {ch}}$ for each channel material individually. In this paper, we study the optimum ${T}_{text {ch}}$ for three channel materials: WSe2, black phosphorus, and InAs using full-band self-consistent quantum transport simulations. To identify the ideal ${T}_{text {ch}}$ for each material at a specific doping density, a new analytic model is proposed and benchmarked against the numerical simulations.

Published in: "IEEE Transactions on Electron Devices".

Multiphysics Modeling and Simulation of Carrier Dynamics and Thermal Transport in Monolayer MoS<sub>2</sub>/WSe<sub>2</sub> Heterojunction

2018-09-21T00:34:00+00:00September 21st, 2018|Categories: Publications|Tags: , |

Computational study of atomically thin monolayer MoS2/WSe2 heterojunction with focus on carrier dynamics and thermal transport is performed by using finite-difference method to solve carrier transport equations and heat conduction equation. Carrier transport in the horizontal direction is in the micrometer scale, while in the vertical direction it is on the subnanometer scale. Carrier transport is modeled as diffusive transport in the horizontal direction with tunneling-assisted carrier recombination between two monolayers in the vertical direction as in the previous study. Band profile, quasi-Fermi level splitting, carrier distribution, and heat generation are investigated in detail. The heterojunction with high-doping concentration has larger current compared to that with low-doping concentration. Heat generation has two components which are due to the Joule heating and inelastic carrier recombination, respectively. The former is provided by the applied voltage, while the latter is from surrounding environment which acts as thermal reservoir. Since the heterojunction works very differently from the conventional p-n diode, it is compared to typical field-effect transistors due to similar device structures. The heat generation of the heterojunction is eight orders lower than that of typical nanoscale field-effect transistors because of its low-current density. Temperature increase in the heterojunction is also several orders of magnitude lower compared to typical nanoscale field-effect transistors.

Published in: "IEEE Transactions on Electron Devices".

Synthesizing Coulombic superconductivity in van der Waals bilayers

2018-09-19T16:33:07+00:00September 19th, 2018|Categories: Publications|Tags: , , |

Author(s): Valla Fatemi and Jonathan RuhmanThe emerging field of van der Waals heterostructures promises the engineering of quantum ground states by combining materials with different properties. A special point of interest is the synthesis of new superconductors. Here, the authors explore the idea of combining very fast electrons with very slow ones, proposing a bilayer heterostructure consisting of monolayer graphene (fast) on top of a monolayer semiconducting transition metal dichalcogenide WSe2 (slow). This interesting two liquid model generates a slow plasmonic mode that acts somewhat like phonons in a crystal, which can mediate an attractive interaction to make graphene superconducting. This mechanism for superconductivity is found to be uniquely optimized in van der Waals heterostructures, and the transition temperature is estimated to be in the range of 0.1 Kelvin. If such a superconductor were realized in an experiment, it would be a special example in which attraction between electrons is clearly mediated by electrons.[Phys. Rev. B 98, 094517] Published Wed Sep 19, 2018

Published in: "Physical Review B".

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".

Spin-Conserving Resonant Tunneling in Twist-Controlled WSe2-hBN-WSe2 Heterostructures. (arXiv:1809.02639v1 [cond-mat.mes-hall])

2018-09-11T02:29:15+00:00September 11th, 2018|Categories: Publications|Tags: , , , |

We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe2) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe2 monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe2 monolayers allows us to probe the conservation of spin-valley degree of freedom in tunneling. In heterostructures where the two WSe2 monolayers have a 180{deg} relative twist, such that the Brillouin zone of one layer is aligned with the time-reversed Brillouin zone of the opposite layer, the resonant tunneling between the layers is suppressed. These findings provide evidence that in addition to momentum, the spin-valley degree of freedom is also conserved in vertical transport.

Published in: "arXiv Material Science".

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".

Environmental effects on the electrical characteristics of back-gated WSe2 field effect transistors. (arXiv:1808.08372v1 [cond-mat.mes-hall])

2018-08-28T04:30:24+00:00August 28th, 2018|Categories: Publications|Tags: |

We study the effect of polymer coating, pressure and temperature on the electrical characteristics of monolayer WSe2 back-gated transistors with quasi-ohmic Ni/Au contacts. We find that the removal of a layer of poly(methyl methacrylate) or decreasing the pressure change the device conductivity from p to n-type. We study the current-voltage characteristics as a function of the temperature and measure a gate-tunable Schottky barrier at the contacts with a height of 60 meV in flat-band condition. We report and discuss a change in the mobility and the subthreshold slope observed with increasing temperature. Finally, we estimate the trap density at the WSe2/SiO2 interface and study the spectral photoresponse of the device, achieving a responsivity of 0.5 AW^-1 at 700 nm wavelength and 0.37 mWcm^-2 optical power.

Published : "arXiv Mesoscale and Nanoscale Physics".

A WSe2 vertical field emission transistor. (arXiv:1808.02127v1 [cond-mat.mes-hall])

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

We report the first observation of gate-controlled field emission current from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour deposition on SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated transistors, under high vacuum, exhibit n-type conduction and drain-bias dependent transfer characteristics, which are attributed to oxygen/water desorption and drain induced Schottky barrier lowering, respectively. The gate-tuned n-type conduction enables field emission, i.e. the extraction of electrons by quantum tunnelling, even from the flat part of the WSe2 monolayers. Electron emission occurs under an electric field ~100 V {mu}m^(-1) and exhibit good time stability. Remarkably, the field emission current can be modulated by the back-gate voltage. The first field-emission vertical transistor based on WSe2 monolayer is thus demonstrated and can pave the way to further optimize new WSe2 based devices for use in vacuum electronics.

Published : "arXiv Mesoscale and Nanoscale Physics".

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