We correlate spatially resolved fluorescence (-lifetime) measurements with X-ray nanodiffraction to reveal surface defects in supercrystals of self-assembled cesium lead halide perovskite nanocrystals and study their effect on the fluorescence properties. Upon comparison with density functional modeling, we show that a loss in structural coherence, an increasing atomic misalignment between adjacent nanocrystals, and growing compressive strain near the surface of the supercrystal are responsible for the observed fluorescence blueshift and decreased fluorescence lifetimes. Such surface defect-related optical properties extend the frequently assumed analogy between atoms and nanocrystals as so-called quasi-atoms. Our results emphasize the importance of minimizing strain during the self-assembly of perovskite nanocrystals into supercrystals for lighting application such as superfluorescent emitters.
We report an optically gated transistor composed of CdSe
nanocrystals
(NCs), sensitized with the dye zinc β-tetraaminophthalocyanine
for operation in the first telecom window. This device shows a high
ON/OFF ratio of 6 orders of magnitude in the red spectral region and
an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption
spectroscopy, we reveal that this unexpected infrared sensitivity
is due to electron transfer from the dye to the CdSe NCs within 5
ps. We show by time-resolved photocurrent measurements that this enables
fast rise times during near-infrared optical gating of 47 ± 11
ns. Electronic coupling and accelerated nonradiative recombination
of charge carriers at the interface between the dye and the CdSe NCs
are further corroborated by steady-state and time-resolved photoluminescence
measurements. Field-effect transistor measurements indicate that the
increase in photocurrent upon laser illumination is mainly due to
the increase in the carrier concentration while the mobility remains
unchanged. Our results illustrate that organic dyes as ligands for
NCs invoke new optoelectronic functionalities, such as fast optical
gating at sub-bandgap optical excitation energies.
The electronic structure of mono and bilayers of colloidal 2H‐MoS2 nanosheets synthesized by wet‐chemistry using potential‐modulated absorption spectroscopy (EMAS), differential pulse voltammetry, and electrochemical gating measurements is investigated. The energetic positions of the conduction and valence band edges of the direct and indirect bandgap are reported and observe strong bandgap renormalization effects, charge screening of the exciton, as well as intrinsic n‐doping of the as‐synthesized material. Two distinct transitions in the spectral regime associated with the C exciton are found, which overlap into a broad signal upon filling the conduction band. In contrast to oxidation, the reduction of the nanosheets is largely reversible, enabling potential applications for reductive electrocatalysis. This work demonstrates that EMAS is a highly sensitive tool for determining the electronic structure of thin films with a few nanometer thicknesses and that colloidal chemistry affords high‐quality transition metal dichalcogenide nanosheets with an electronic structure comparable to that of exfoliated samples.
CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid material under optical excitation. This work highlights that combining semiconductor nanocrystals with molecular aggregates, which exhibit aggregation induced emission, can result in unprecedented emerging optical properties.
CdSe quantum dots are functionalized with the organic dye iron β-tetraaminophthalocyanine to reward a solution-processable hybrid material with two individually addressable optical resonances. We exploit this dual functionality during optical write/optical read patterning experiments and show that it is possible to simultaneously write complex optical patterns with positive and negative fluorescence contrast. This is enabled by a fluorescence enhancement under near-resonant excitation of the quantum dots in combination with a fluorescence bleaching during excitation of the singlet transition of the phthalocyanine. The presence of the organic dye not only enables negative optical patterning but also enhances the contrast during positive patterning. Furthermore, the patterning result is strongly dependent on the excitation wavelength during readout. Our results highlight the new possibilities that arise from combining inorganic quantum dots and organic π-systems into hybrid nanocomposites.
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