52 research outputs found
Exciton self-trapping causes picoseconds recombination in metal-organic chalcogenides hybrid quantum wells
Metal-organic species can be designed to self-assemble in large-scale,
atomically defined, supramolecular architectures. Hybrid quantum wells, where
inorganic two-dimensional (2D) planes are separated by organic ligands, are a
particular example. The ligands effectively provide an intralayer confinement
for charge carriers resulting in a 2D electronic structure, even in
multilayered assemblies. Air-stable metal organic chalcogenides hybrid quantum
wells have recently been found to host tightly bound 2D excitons with strong
optical anisotropy in a bulk matrix. Here, we investigate the excited carrier
dynamics in the prototypical metal organic chalcogenide [AgSePh], disentangling
three excitonic resonances by low temperature transient absorption
spectroscopy. Our analysis suggests a complex relaxation cascade comprising
ultrafast screening and renormalization, inter-exciton relaxation, and
self-trapping of excitons within few picoseconds. The ps-decay provided by the
self-trapping mechanism may be leveraged to unlock the material's potential for
ultrafast optoelectronic applications
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
Characterizing Transition-Metal Dichalcogenide Thin-Films using Hyperspectral Imaging and Machine Learning
Atomically thin polycrystalline transition-metal dichalcogenides (TMDs) are
relevant to both fundamental science investigation and applications. TMD
thin-films present uniquely difficult challenges to effective nanoscale
crystalline characterization. Here we present a method to quickly characterize
the nanocrystalline grain structure and texture of monolayer WS2 films using
scanning nanobeam electron diffraction coupled with multivariate statistical
analysis of the resulting data. Our analysis pipeline is highly generalizable
and is a useful alternative to the time consuming, complex, and
system-dependent methodology traditionally used to analyze spatially resolved
electron diffraction measurements
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
Lithographically defined synthesis of transition metal dichalcogenides
Transition metal dichalcogenides (TMDs) promise to revolutionize optoelectronic applications. While monolayer exfoliation and vapor phase growth produce extremely high quality 2D materials, direct fabrication at wafer scale remains a significant challenge. Here, we present a method that we call ‘lateral conversion’, which enables the synthesis of patterned TMD structures, with control over the thickness down to a few layers, at lithographically predefined locations. In this method, chemical conversion of a metal-oxide film to TMD layers proceeds by diffusion of precursor propagating laterally between silica layers, resulting in structures where delicate chalcogenide films are protected from contamination or oxidation. Lithographically patterned WS2 structures were synthesized by lateral conversion and analyzed in detail by hyperspectral Raman imaging, scanning electron microscopy and transmission electron microscopy. The rate of conversion was investigated as a function of time, temperature, and thickness of the converted film. In addition, the process was extended to grow patterned MoS2, WSe2, MoSe2 structures, and to demonstrate unique WS2/SiO2 multilayer structures. We believe this method will be applicable to a variety of additional chalcogenide materials, and enable their incorporation into novel architectures and devices
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