354 research outputs found
Electrically driven photon emission from individual atomic defects in monolayer WS2.
Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources
Resonant and bound states of charged defects in two-dimensional semiconductors
A detailed understanding of charged defects in two-dimensional semiconductors is needed for the development of ultrathin electronic devices. Here, we study negatively charged acceptor impurities in monolayer WS2 using a combination of scanning tunneling spectroscopy and large-scale atomistic electronic structure calculations. We observe several localized defect states of hydrogenic wave function character in the vicinity of the valence band edge. Some of these defect states are bound, while others are resonant. The resonant states result from the multivalley valence band structure of WS2, whereby localized states originating from the secondary valence band maximum at Γ hybridize with continuum states from the primary valence band maximum at K/K′. Resonant states have important consequences for electron transport as they can trap mobile carriers for several tens of picoseconds
Bioimpedance cardiography in pregnancy: A longitudinal cohort study on hemodynamic pattern and outcome
Background: Pregnancy associated cardiovascular pathologies have a significant impact on outcome for mother
and child. Bioimpedance cardiography may provide additional outcome-relevant information early in pregnancy
and may also be used as a predictive instrument for pregnancy-associated diseases.
Methods: We performed a prospective longitudinal cohort trial in an outpatient setting and included 242 pregnant
women. Cardiac output and concomitant hemodynamic data were recorded from 11th-13th week of gestation
every 5th week as well as at two occasions post partum employing bioimpedance cardiography.
Results: Cardiac output increased during pregnancy and peaked early in the third trimester. A higher heart rate
and a decreased systemic vascular resistance were accountable for the observed changes. Women who had a
pregnancy-associated disease during a previous pregnancy or developed hypertension or preeclampsia had a
significantly increased cardiac output early in pregnancy. Furthermore, an effect of cardiac output on birthweight
was found in healthy pregnancies and could be confirmed with multiple linear regression analysis.
Conclusions: Cardiovascular adaptation during pregnancy is characterized by distinct pattern described herein.
These may be altered in women at risk for preeclampsia or reduced birthweigth. The assessment of cardiac
parameters by bioimpedance cardiography could be performed at low costs without additional risks
Enlarged magnetic focusing radius of photoinduced ballistic currents
We exploit GaAs-based quantum point contacts as mesoscopic detectors to
analyze the ballistic flow of photogenerated electrons in a two-dimensional
electron gas at a perpendicular magnetic field. Whereas charge transport
experiments always measure the classical cyclotron radius, we show that this
changes dramatically when detecting the photoinduced non-equilibrium current in
magnetic fields. The experimentally determined radius of the trajectories
surprisingly exceeds the classical cyclotron value by far. Monte Carlo
simulations suggest electron-electron scattering as the underlying reason.Comment: pdf-file includes both main article and supplementary informatio
How Substitutional Point Defects in Two-Dimensional WS Induce Charge Localization, Spin-Orbit Splitting, and Strain
Control of impurity concentrations in semiconducting materials is essential
to device technology. Because of their intrinsic confinement, the properties of
two-dimensional semiconductors such as transition metal dichalcogenides (TMDs)
are more sensitive to defects than traditional bulk materials. The
technological adoption of TMDs is dependent on the mitigation of deleterious
defects and guided incorporation of functional foreign atoms. The first step
towards impurity control is the identification of defects and assessment of
their electronic properties. Here, we present a comprehensive study of point
defects in monolayer tungsten disulfide (WS) grown by chemical vapor
deposition (CVD) using scanning tunneling microscopy/spectroscopy, CO-tip
noncontact atomic force microscopy, Kelvin probe force spectroscopy, density
functional theory, and tight-binding calculations. We observe four different
substitutional defects: chromium (Cr) and molybdenum
(Mo) at a tungsten site, oxygen at sulfur sites in both bottom and
top layers (O top/bottom), as well as two negatively charged
defects (CDs). Their electronic fingerprints unambiguously corroborate the
defect assignment and reveal the presence or absence of in-gap defect states.
The important role of charge localization, spin-orbit coupling, and strain for
the formation of deep defect states observed at substitutional defects in
WS as reported here will guide future efforts of targeted defect
engineering and doping of TMDs
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