47 research outputs found
Mono- and Bilayer WS2 Light-Emitting Transistors
We have realized ambipolar ionic liquid gated field-effect transistors based
on WS2 mono- and bilayers, and investigated their opto-electronic response. A
thorough characterization of the transport properties demonstrates the high
quality of these devices for both electron and hole accumulation, which enables
the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for
monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the
operation of the transistors in the ambipolar injection regime with electrons
and holes injected simultaneously at the two opposite contacts of the devices
in which we observe light emission from the FET channel. A quantitative
analysis of the spectral properties of the emitted light, together with a
comparison with the band gap values obtained from transport, show the internal
consistency of our results and allow a quantitative estimate of the excitonic
binding energies to be made. Our results demonstrate the power of ionic liquid
gating in combination with nanoelectronic systems, as well as the compatibility
of this technique with optical measurements on semiconducting transition metal
dichalcogenides. These findings further open the way to the investigation of
the optical properties of these systems in a carrier density range much broader
than that explored until now.Comment: 22 pages, 6 figures, Nano Letters (2014
Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors
We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field
effect transistors exhibiting high-quality ambipolar transport. By properly
biasing the gate electrode we can invert the sign of the photocurrent showing
that the minority photocarriers are either electrons or holes. Both in the
electron- and the hole-doping regimes the photocurrent decays exponentially as
a function of the distance between the illumination spot and the nearest
contact, in agreement with a two-terminal Schottky-barrier device model. This
allows us to compare the value and the doping dependence of the diffusion
length of the minority electrons and holes on a same sample. Interestingly, the
diffusion length of the minority carriers is several times larger in the hole
accumulation regime than in the electron accumulation regime, pointing out an
electron-hole asymmetry in WS2
RecurSeed and EdgePredictMix: Single-stage Learning is Sufficient for Weakly-Supervised Semantic Segmentation
Although weakly-supervised semantic segmentation using only image-level
labels (WSSS-IL) is potentially useful, its low performance and implementation
complexity still limit its application. The main causes are (a) non-detection
and (b) false-detection phenomena: (a) The class activation maps refined from
existing WSSS-IL methods still only represent partial regions for large-scale
objects, and (b) for small-scale objects, over-activation causes them to
deviate from the object edges. We propose RecurSeed which alternately reduces
non and false-detections through recursive iterations, thereby implicitly
finding an optimal junction that minimizes both errors. We also propose a novel
data augmentation (DA) approach called EdgePredictMix, which further expresses
an object's edge by utilizing the probability difference information between
adjacent pixels in combining the segmentation results, thereby compensating for
the shortcomings when applying the existing DA methods to WSSS. We achieved new
state-of-the-art performances on both the PASCAL VOC 2012 and MS COCO 2014
benchmarks (VOC val 74.4%, COCO val 46.4%). The code is available at
https://github.com/OFRIN/RecurSeed_and_EdgePredictMix
Observation of chiral quantum-Hall edge states in graphene
In this study, we determined the chiral direction of the quantum-Hall (QH)
edge states in graphene by adopting simple two-terminal conductance
measurements while grounding different edge positions of the sample. The edge
state with a smaller filling factor is found to more strongly interact with the
electric contacts. This simple method can be conveniently used to investigate
the chirality of the QH edge state with zero filling factor in graphene, which
is important to understand the symmetry breaking sequence in high magnetic
fields (25 T).Comment: 3 pages, 3 figures. Appeared in AP
Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength Dependent Mapping
The band structure of many semiconducting monolayer transition metal
dichalcogenides (TMDs) possesses two degenerate valleys, with equal and
opposite Berry curvature. It has been predicted that, when illuminated with
circularly polarized light, interband transitions generate an unbalanced
non-equilibrium population of electrons and holes in these valleys, resulting
in a finite Hall voltage at zero magnetic field when a current flows through
the system. This is the so-called valley Hall effect that has recently been
observed experimentally. Here, we show that this effect is mediated by
photo-generated neutral excitons and charged trions, and not by inter-band
transitions generating independent electrons and holes. We further demonstrate
an experimental strategy, based on wavelength dependent spatial mapping of the
Hall voltage, which allows the exciton and trion contributions to the valley
Hall effect to be discriminated in the measurement. These results represent a
significant step forward in our understanding of the microscopic origin of
photo-induced valley Hall effect in semiconducting transition metal
dichalcogenides, and demonstrate experimentally that composite quasi-particles,
such as trions, can also possess a finite Berry curvature.Comment: accepted for publication in Nano Letter
How Team-Level and Individual-Level Conflict Influences Team Commitment: A Multilevel Investigation
We investigate how two different types of conflict (task conflict and relationship conflict) at two different levels (individual-level and team-level) influence individual team commitment. The analysis was conducted using data we collected from 193 employees in 31 branch offices of a Korean commercial bank. The relationships at multiple levels were tested using hierarchical linear modeling (HLM). The results showed that individual-level relationship conflict was negatively related to team commitment while individual-level task conflict was not. In addition, both team-level task and relationship conflict were negatively associated with team commitment. Finally, only team-level relationship conflict significantly moderated the relationship between individual-level relationship conflict and team commitment. We further derive theoretical implications of these findings
Observation of Supercurrent in PbIn-Graphene-PbIn Josephson Junction
Superconductor-graphene-superconductor (SGS) junction provides a unique
platform to study relativistic electrodynamics of Dirac fermions combined with
proximity-induced superconductivity. We report observation of the Josephson
effect in proximity-coupled superconducting junctions of graphene in contact
with Pb1-xInx (x=0.07) electrodes for temperatures as high as T = 4.8K, with a
large IcRn (~ 255 microV). This demonstrates that Pb1-xInx SGS junction would
facilitate the development of the superconducting quantum information devices
and superconductor-enhanced phase-coherent transport of graphene.Comment: 8 pages, 7 figures, accepted in PR
Beating of Aharonov-Bohm oscillations in a closed-loop interferometer
One of the points at issue with closed-loop-type interferometers is beating
in the Aharonov-Bohm (AB) oscillations. Recent observations suggest the
possibility that the beating results from the Berry-phase pickup by the
conducting electrons in materials with the strong spin-orbit interaction (SOI).
In this study, we also observed beats in the AB oscillations in a gate-defined
closed-loop interferometer fabricated on a GaAs/AlGaAs two-dimensional
electron-gas heterostructure. Since this heterostructure has very small SOI,
the picture of the Berry-phase pickup is ruled out. The observation of beats in
this study, with the controllability of forming a single transverse subband
mode in both arms of our gate-defined interferometer, also rules out the
often-claimed multiple transverse subband effect. It is observed that nodes of
the beats with an h/2e period exhibit a parabolic distribution for varying the
side gate. These results are shown to be well interpreted, without resorting to
the SOI effect, by the existence of two-dimensional multiple longitudinal modes
in a single transverse subband. The Fourier spectrum of measured conductance,
despite showing multiple h/e peaks with the magnetic-field dependence that are
very similar to that from strong-SOI materials, can also be interpreted as the
two-dimensional multiple-longitudinal-modes effect