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SWCNT-MoS2-SWCNT Vertical Point Heterostructures

December 7th, 2016|Publications|

By Jin Zhang, Yang Wei, Fengrui Yao, Dongqi Li, He Ma, Peng Lei, Hehai Fang, Xiaoyang Xiao, Zhixing Lu, Juehan Yang, Jingbo Li, Liying Jiao, Weida Hu, Kaihui Liu, Kai Liu, Peng Liu, Qunqing Li, Wei Lu, Shoushan Fan, Kaili Jiang

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A vertical point heterostructure (VPH) is constructed by sandwiching a two-dimensional (2D) MoS2 flake with two cross-stacked metallic single-walled carbon nanotubes. It can be used as a field effect transistor with high on/off ratio and a light detector with high spatial resolution. Moreover, the hybrid 1D–2D–1D VPHs open up new possibilities for nanoelectronics and nanooptoelectronics.

Published in: "Advanced Materials".

Transport properties of chemically synthesized MoS2 – Dielectric effects and defects scattering

December 7th, 2016|Publications|

By Massimo Mongillo, Daniele Chiappe, Goutham Arutchelvan, Inge Asselberghs, Marta Perucchini, Mauricio Manfrini, Dennis Lin, Cedric Huyghebaert and Iuliana Radu

We report on the electrical characterization of synthetic, large-area MoS layers obtained by the sulfurization technique. The effects of dielectric encapsulation and localized defect states on the intrinsic transport properties are explored with the aid of temperature-dependent measurements. We study the effect of dielectric environment by transferring as-grown MoS
films into different dielectrics such as SiO, AlO, HfO, and ZrO with increasing dielectric permittivity. Electrical data are collected on a statistically-relevant device ensemble and allow to assess device performances on a large scale assembly. Our devices show relative in-sensitiveness of mobility with respect to dielectric encapsulation. We conclude that the device behavior is strongly affected by several scattering mechanisms of different origin that can completely mask any effect related to dielectric mismatch. At low temperatures, conductivity of the devices is thermally activated, a clear footprint of the existence of a mobility edge separating extended states in the conduction band from impurity states in the band-gap.

Published in: "Applied Physics Letters".

Nano-gap between a gold tip and nanorod for polarization dependent surface enhanced Raman scattering

December 7th, 2016|Publications|

By Zhengmin Cao, Yingbo He, Yuqing Cheng, Jingyi Zhao, Guantao Li, Qihuang Gong and Guowei Lu

We demonstrate experimentally that a nano-gap could be constructed by using a scanning probe microscope to allow a gold tip to approach a gold
nanorod immobilized on a glass coverslip. The nano-gap can enhance Raman scattering of graphene sandwiched between the tip and the nanorod. The Raman intensity was strongly dependent on the incident light polarization. Here, linear, radial, azimuthal, and intermediate states between radial and azimuthal polarization were investigated and compared in detail. The maximum surface-enhanced Raman scattering effect of the nano-gap occurred for the intermediate states between the radial and azimuthal polarized light.

Published in: "Applied Physics Letters".

Monitoring electrostatically-induced deflection, strain and doping in suspended graphene using Raman spectroscopy

December 7th, 2016|Publications|

By Dominik Metten, Guillaume Froehlicher and Stéphane Berciaud

Electrostatic gating offers elegant ways to simultaneously strain and dope atomically thin
membranes. Here, we report on a detailed in situ Raman scattering study on graphene, suspended over
a Si/SiO 2 substrate. In such a layered structure, the intensity of the Raman G- and 2D-mode
features of graphene are strongly modulated by optical interference effects and allow an accurate
determination of the electrostatically-induced membrane deflection, up to irreversible collapse. The
membrane deflection is successfully described by an electromechanical model, which we also use to
provide useful guidelines for device engineering. In addition, electrostatically-induced tensile
strain is determined by examining the softening of the Raman features. Due to a small residual
charge inhomogeneity, we find that non-adiabatic anomalous phonon softening is negligible compared
to strain-induced phonon softening. These results open perspectives for innovative Raman
scattering-base…

Published in: "2DMaterials".

Increasing the light extraction and longevity of TMDC monolayers using liquid formed micro-lenses

December 7th, 2016|Publications|

By C S Woodhead, J Roberts, Y J Noori, Y Cao, R Bernardo-Gavito, P Tovee, A Kozikov, K Novoselov and R J Young

The recent discovery of semiconducting two-dimensional materials is predicted to lead to the
introduction of a series of revolutionary optoelectronic components that are just a few atoms thick.
Key remaining challenges for producing practical devices from these materials lie in improving the
coupling of light into and out of single atomic layers, and in making these layers robust to the
influence of their surrounding environment. We present a solution to tackle both of these problems
simultaneously, by deterministically placing an epoxy based micro-lens directly onto the materials’
surface. We show that this approach enhances the photoluminescence of tungsten diselenide (WSe 2 )
monolayers by up to 300%, and nearly doubles the imaging resolution of the system. Furthermore, this
solution fully encapsulates the monolayer, preventing it from physical damage and degradation in
air. The optical solution we have developed could become a key enabling technology for the mass
pr…

Published in: "2DMaterials".

A simple process for the fabrication of large-area CVD graphene based devices via selective in situ functionalization and patterning

December 7th, 2016|Publications|

By Arseny M Alexeev, Matthew D Barnes, V Karthik Nagareddy, Monica F Craciun and C David Wright

We report a novel approach for the fabrication of micro- and nano-scale graphene devices via the in
situ plasma functionalization and in situ lithographic patterning of large-area graphene directly on
CVD catalytic metal (Cu) substrates. This enables us to create graphene-based devices in their
entirety prior to any transfer processes, simplifying very significantly the device fabrication
process and potentially opening up the route to the use of a wider range of target substrates. We
demonstrate the capabilities of our technique via the fabrication of a flexible, transparent,
graphene/graphene oxide humidity sensor that outperforms a conventional commercial sensor.

Published in: "2DMaterials".

Optical fingerprint of dark 2p-states in transition metal dichalcogenides

December 7th, 2016|Publications|

By Gunnar Berghäuser, Andreas Knorr and Ermin Malic

Atomically thin transition metal dichalcogenides exhibit a remarkably strong Coulomb interaction.
This results in a fascinating many-particle physics including a variety of bright and dark excitonic
states that determine optical and electronic properties of these materials. So far, the impact of
dark states has remained literally in the dark to a large extent, since a measurement of these
optically forbidden states is very challenging. Here we demonstrate a strategy to measure a direct
fingerprint of dark states even in standard linear absorption spectroscopy. We present a microscopic
study on bright and dark higher excitonic states in the presence of disorder for the exemplary
material of tungsten disulfide (WS 2 ). We show that the geometric phase cancels the degeneration of
2s and 2p states and that a significant disorder-induced coupling of these bright and dark states
offers a strategy to circumvent optical selection rules. As a proof, we show a clear fingerprint
o…

Published in: "2DMaterials".

Silicon photonic crystal cavity enhanced second-harmonic generation from monolayer WSe 2

December 7th, 2016|Publications|

By Taylor K Fryett, Kyle L Seyler, Jiajiu Zheng, Chang-Hua Liu, Xiaodong Xu and Arka Majumdar

Nano-resonators integrated with two-dimensional materials (e.g. transition metal dichalcogenides)
have recently emerged as a promising nano-optoelectronic platform. Here we demonstrate
resonator-enhanced second-harmonic generation (SHG) in tungsten diselenide using a silicon photonic
crystal cavity. By pumping the device with ultrafast laser pulses near the cavity mode at the
telecommunication wavelength, we observe a near visible SHG with a narrow linewidth and near unity
linear polarization, originated from the coupling of the pump photon to the cavity mode. The
observed SHG is enhanced by factor of ∼200 compared to a bare monolayer on silicon. Our results
imply the efficacy of cavity integrated monolayer materials for nonlinear optics and the potential
of building a silicon-compatible second-order nonlinear integrated photonic platform.

Published in: "2DMaterials".