111 research outputs found
Cathodoluminescence-based nanoscopic thermometry in a lanthanide-doped phosphor
Crucial to analyze phenomena as varied as plasmonic hot spots and the spread
of cancer in living tissue, nanoscale thermometry is challenging: probes are
usually larger than the sample under study, and contact techniques may alter
the sample temperature itself. Many photostable nanomaterials whose
luminescence is temperature-dependent, such as lanthanide-doped phosphors, have
been shown to be good non-contact thermometric sensors when optically excited.
Using such nanomaterials, in this work we accomplished the key milestone of
enabling far-field thermometry with a spatial resolution that is not
diffraction-limited at readout.
We explore thermal effects on the cathodoluminescence of lanthanide-doped
NaYF nanoparticles. Whereas cathodoluminescence from such lanthanide-doped
nanomaterials has been previously observed, here we use quantitative features
of such emission for the first time towards an application beyond localization.
We demonstrate a thermometry scheme that is based on cathodoluminescence
lifetime changes as a function of temperature that achieves 30 mK
sensitivity in sub-m nanoparticle patches. The scheme is robust against
spurious effects related to electron beam radiation damage and optical
alignment fluctuations.
We foresee the potential of single nanoparticles, of sheets of nanoparticles,
and also of thin films of lanthanide-doped NaYF to yield temperature
information via cathodoluminescence changes when in the vicinity of a sample of
interest; the phosphor may even protect the sample from direct contact to
damaging electron beam radiation. Cathodoluminescence-based thermometry is thus
a valuable novel tool towards temperature monitoring at the nanoscale, with
broad applications including heat dissipation in miniaturized electronics and
biological diagnostics.Comment: Main text: 30 pages + 4 figures; supplementary information: 22 pages
+ 8 figure
Blue-Light-Emitting Color Centers in High-Quality Hexagonal Boron Nitride
Light emitters in wide band gap semiconductors are of great fundamental
interest and have potential as optically addressable qubits. Here we describe
the discovery of a new color center in high-quality hexagonal boron nitride
(h-BN) with a sharp emission line at 435 nm. The emitters are activated and
deactivated by electron beam irradiation and have spectral and temporal
characteristics consistent with atomic color centers weakly coupled to lattice
vibrations. The emitters are conspicuously absent from commercially available
h-BN and are only present in ultra-high-quality h-BN grown using a
high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these
emitters originate from impurities or related defects specific to this unique
synthetic route. Our results imply that the light emission is activated and
deactivated by electron beam manipulation of the charge state of an
impurity-defect complex
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
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Electronic interactions between gold nanoclusters in constrainedgeometries
Near-monochromatic tuneable cryogenic niobium electron field emitter
Creating, manipulating, and detecting coherent electrons is at the heart of
future quantum microscopy and spectroscopy technologies. Leveraging and
specifically altering the quantum features of an electron beam source at low
temperatures can enhance its emission properties. Here, we describe electron
field emission from a monocrystalline, superconducting niobium nanotip at a
temperature of 5.9 K. The emitted electron energy spectrum reveals an
ultra-narrow distribution down to 16 meV due to tunable resonant tunneling
field emission via localized band states at a nano-protrusion's apex and a
cut-off at the sharp low-temperature Fermi-edge. This is an order of magnitude
lower than for conventional field emission electron sources. The self-focusing
geometry of the tip leads to emission in an angle of 3.7 deg, a reduced
brightness of 3.8 x 10exp8 A/(m2 sr V), and a stability of hours at 4.1 nA beam
current and 69 meV energy width. This source will decrease the impact of lens
aberration and enable new modes in low-energy electron microscopy, electron
energy loss spectroscopy, and high-resolution vibrational spectroscopy.Comment: to be published in Phys. Rev. Lett. (2022
Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory
Chalcogen vacancies are considered to be the most abundant point defects in
two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and
predicted to result in deep in-gap states (IGS). As a result, important
features in the optical response of 2D-TMDs have typically been attributed to
chalcogen vacancies, with indirect support from Transmission Electron
Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM
imaging measurements do not provide direct access to the electronic structure
of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a
direct probe of local electronic structure, the interpretation of the chemical
nature of atomically-resolved STM images of point defects in 2D-TMDs can be
ambiguous. As a result, the assignment of point defects as vacancies or
substitutional atoms of different kinds in 2D-TMDs, and their influence on
their electronic properties, has been inconsistent and lacks consensus. Here,
we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS,
and state-of-the-art ab initio density functional theory (DFT) and GW
calculations to determine both the structure and electronic properties of the
most abundant individual chalcogen-site defects common to 2D-TMDs.
Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our
results and analysis strongly suggest that the common chalcogen defects in our
2D-TMDs, prepared and measured in standard environments, are substitutional
oxygen rather than vacancies
Autonomous Investigations over WS and Au{111} with Scanning Probe Microscopy
Individual atomic defects in 2D materials impact their macroscopic
functionality. Correlating the interplay is challenging, however, intelligent
hyperspectral scanning tunneling spectroscopy (STS) mapping provides a feasible
solution to this technically difficult and time consuming problem. Here, dense
spectroscopic volume is collected autonomously via Gaussian process regression,
where convolutional neural networks are used in tandem for spectral
identification. Acquired data enable defect segmentation, and a workflow is
provided for machine-driven decision making during experimentation with
capability for user customization. We provide a means towards autonomous
experimentation for the benefit of both enhanced reproducibility and
user-accessibility. Hyperspectral investigations on WS sulfur vacancy sites
are explored, which is combined with local density of states confirmation on
the Au{111} herringbone reconstruction. Chalcogen vacancies, pristine WS,
Au face-centered cubic, and Au hexagonal close packed regions are examined and
detected by machine learning methods to demonstrate the potential of artificial
intelligence for hyperspectral STS mapping.Comment: Updates from final journal publicatio
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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