23 research outputs found
Broadband Near-Infrared to Visible Upconversion in Quantum Dot–Quantum Well Heterostructures
Upconversion is a nonlinear process
in which two, or more, long
wavelength photons are converted to a shorter wavelength photon. It
holds great promise for bioimaging, enabling spatially resolved imaging
in a scattering specimen and for photovoltaic devices as a means to
surpass the Shockley–Queisser efficiency limit. Here, we present
dual near-infrared and visible emitting PbSe/CdSe/CdS nanocrystals
able to upconvert a broad range of NIR wavelengths to visible emission
at room temperature. The synthesis is a three-step process, which
enables versatility and tunability of both the visible emission color
and the NIR absorption edge. Using this method, one can achieve a
range of desired upconverted emission peak positions with a suitable
NIR band gap
Super-resolved CARS by coherent image scanning
We present super-resolved coherent anti-Stokes Raman scattering (CARS) microscopy by implementing phase-resolved image scanning microscopy (ISM), achieving up to two-fold resolution increase as compared with a conventional CARS microscope. Phase-sensitivity is required for the standard pixel-reassignment procedure since the scattered field is coherent, thus the point-spread function (PSF) is well-defined only for the field amplitude. We resolve the complex field by a simple add-on to the CARS setup enabling inline interferometry. Phase-sensitivity offers additional contrast which informs the spatial distribution of both resonant and nonresonant scatterers. As compared with alternative super-resolution schemes in coherent nonlinear microscopy, the proposed method is simple, requires only low-intensity excitation, and is compatible with any conventional forward-detected CARS imaging setup
Long-Lived Population Inversion in Isovalently Doped Quantum Dots
Optical gain from colloidal quantum dots has been desired for several decades since their discovery. While gain from multiexcitations is by now well-established, nonradiative Auger recombination limits the lifetime of such population inversion in quantum dots. CdSe cores isovalently doped by one to few Te atoms capped with rod-shaped CdS are examined as a candidate system for enhanced stimulated emission properties. Emission depletion spectroscopy shows a behavior characteristic of 3-level gain systems in these quantum dots. This implies complete removal of the 2-fold degeneracy of the lowest energy electronic excitation due to the large repulsive exciton–exciton interaction in the doubly excited state. Using emission depletion measurements of the trap-associated emission from poorly passivated CdS quantum dots, we show that 3-level characteristics are typical of emission resulting from a band edge to trap state transition, but reveal subtle differences between the two systems. These results allow for unprecedented observation of long-lived population inversion from singly excited quantum dots
Quantum Dot Antennas for Photoelectrochemical Solar Cells
The use of Förster resonant energy transfer (FRET) has recently shown promise for significant improvement in various aspects of photoelectrochemical cells. Considering the particular case of semiconductor quantum dot donors, we show that they can enable broadening of the spectral response and increased optical density of the cell, thus increasing the current while potentially decreasing the electrode thickness. Moreover, the use of FRET and the separation of optical and electrical function within the cell provide flexibility in the choice of materials for both the antenna and sensitizer, opening new paths for performance optimization and achievement of long-term cell stability
ExampleData2.zip
<div>Yonatan Israel, Ron Tenne, Dan Oron, and Yaron Silberberg,<br></div><div>Quantum correlation enhanced super-resolution localization using a fiber bundle camera</div><div>2017, Nature Communications</div><div><br></div><div>Description:Â ExampleData2 - Folder containing exmaple data used to generate Fig. 3 in the manuscript.</div
Type-II Quantum-Dot-Sensitized Solar Cell Spanning the Visible and Near-Infrared Spectrum
Type-II heterostructure CdTe/CdSe core/shell nanocrystals (quantum
dots, QDs) are explored as sensitizers in a QD-sensitized photoelectrochemical
solar cell. These QDs comprise a hole-localizing core and an electron-localizing
shell. Among their advantages is the significant red shift of the
absorption edge of the heterostructured QD relative to its two constituents
due to spatially indirect absorption leading to improved absorption
characteristics, intraparticle exciton dissociation upon photoexcitation,
and a relatively small content of the less abundant tellurium element.
Upon incorporation in a sensitized solar cell utilizing a porous TiO<sub>2</sub> and a polysulfide electrolyte, these QDs exhibited efficient
charge separation and high internal quantum efficiency despite hole
localization in the CdTe core. Monochromatic incident photon-to-current
conversion efficiency (IPCE) measurement shows a spectrally broad
photoresponse spanning the whole visible spectrum and reaching up
to ∼900 nm
Enhanced Third-Harmonic Generation from a Metal/Semiconductor Core/Shell Hybrid Nanostructure
Nonlinear optical processes can be dramatically enhanced <i>via</i> the use of localized surface plasmon modes in metal nanoparticles. Here we show how more elaborate structures, based on shape-controlled Au/Cu<sub>2</sub>O core/shell nanostructures, enable further enhancement of the nanoparticle third-harmonic scattering cross-section. The semiconducting component takes a twofold role in this structure, both providing a knob to tune the resonant frequency of the gold plasmon and providing resonant enhancement by virtue of its excitonic states. The advantages and deficiencies of using such core/shell metal/semiconductor structures are discussed
Nucleation, Growth, and Structural Transformations of Perovskite Nanocrystals
Despite
the recent surge of interest in lead halide perovskite
nanocrystals, there are still significant gaps in the understanding
of nucleation and growth processes involved in their formation. Using
CsPbX<sub>3</sub> as a model system, we systematically study the formation
mechanism of cubic CsPbX<sub>3</sub> nanocrystals, their growth via
oriented attachment into larger nanostructures, and the associated
phase transformations. We found evidence to support that the formation
of CsPbX<sub>3</sub> NCs occurs through the seed-mediated nucleation
method, where Pb° NPs formed during the course of reaction act
as seeds. Further growth occurs through self-assembly and oriented
attachment. The polar environment is a major factor in determining
the structure and shape of the resulting nanoparticles, as confirmed
by experiments with aged seed reaction mixtures, and by addition of
polar additives. These results provide a fundamental understanding
of the influence of the environment polarity on self-assembly, self-healing,
and the ability to control the morphology and structure over the perovskite
structures. As a result of this understanding, we were able to extend
the synthesis to produce other materials such as CsPbBr<sub>3</sub> nanowires and orthorhombic CsPbI<sub>3</sub> nanowires with tunable
length ranging from 200 nm to several microns
Study of Quantum Dot/Inorganic Layer/Dye Molecule Sandwich Structure for Electrochemical Solar Cells
A highly efficient quantum dot (QD)/inorganic layer/dye
molecule sandwich structure was designed and applied in electrochemical
QD-sensitized solar cells. The key component TiO<sub>2</sub>/CdS/ZnS/N719
hybrid photoanode with ZnS insertion between the two types of sensitizers
was demonstrated not only to efficiently extend the light absorption
but also to suppress the charge recombination from either TiO<sub>2</sub> or CdS QDs to electrolyte redox species, yielding a photocurrent
density of 11.04 mA cm<sup>–2</sup>, an open-circuit voltage
of 713 mV, a fill factor of 0.559, and an impressive overall energy
conversion efficiency of 4.4%. More importantly, the cell exhibited
enhanced photostability with the help of the synergistic stabilizing
effect of both the organic and the inorganic passivation layers in
the presence of a corrosive electrolyte