Colloidal semiconductor nanoplatelets (NPLs) are highly promising luminescent materials owing to their exceptionally narrow emission spectra. While high-efficiency NPLs in non-polar organic media can be obtained readily, NPLs in aqueous media suffer from extremely low quantum yields (QYs), which completely undermines their potential, especially in biological applications. Here, we show high-efficiency watersoluble CdSe/CdS@Cd 1−x Zn x S core/crown@shell NPLs formed by layer-by-layer grown and compositiontuned gradient Cd 1−x Zn x S shells on CdSe/CdS core/crown seeds. Such control of shell composition with monolayer precision and effective peripheral crown passivation, together with the compact capping density of short 3-mercaptopropionic acid ligands, allow for QYs reaching 90% in water, accompanied by a significantly increased photoluminescence lifetime (∼35 ns), indicating the suppression of nonradiative channels in these NPLs. We also demonstrate the controlled attachment of these NPLs without stacking at the nanoscale by taking advantage of their 2D geometry and hydrophilicity. This is a significant step in achieving controlled assemblies and overcoming the stacking process, which otherwise undermines their film formation and performance in optoelectronic applications. Moreover, we show that the parallel orientation of such NPLs achieved by the controlled attachment enables directed emission perpendicular to the surface of the NPL films, which is highly advantageous for light extraction in light-emitting platforms. † Electronic supplementary information (ESI) available: Details of synthesis procedures, experimental set-up, theoretical modelling and additional figures. See

We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via Forster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs (i.e., donor-selective) or to only the acceptor QDs (i.e., acceptor-selective). Using layer-by-layer assembled colloidal QD nanocrystal solids with metal nanoparticles integrated at carefully designed spacing, we demonstrate the ability to enable/disable the coupled plasmon-exciton (plexciton) formation distinctly at the donor (exciton departing) site or at the acceptor (exciton feeding) site of our choice, while not hindering the donor exciton-acceptor exciton interaction but refraining from simultaneous coupling to both sites of the donor and the acceptor in the FRET process. In the case of donor-selective plexciton, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors, both of which shorten the donor lifetime. This consequently enhanced the acceptor emission by a factor of 1.93. On the other hand, in the complementary case of acceptor-selective plexciton we observed a 2.70-fold emission enhancement in the acceptor QDs, larger than the acceptor emission enhancement of the donor-selective plexciton, as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Here we present the comparative results of theoretical modeling of the donor-and acceptorselective plexcitons of nonradiative energy transfer developed here for the first time, which are in excellent agreement with the systematic experimental characterization. Such an ability to modify and control energy transfer through mastering plexcitons is of fundamental importance, opening up new applications for quantum dot embedded plexciton devices along with the development of new techniques in FRET-based fluorescence microscopy. KEYWORDS: Localized plasmons, nonradiative energy transfer, excitons, metal nanoparticles, semiconductor quantum dots, plexcitons, layer-by-layer assembly T he Forster-type resonance energy transfer (FRET), an important proximity effect that can strongly modify the emission kinetics of fluorophores serving as donors and acceptors, is widely used in biotechnology especially as

In this work, we report the manifestations of carrier-dopant exchange interactions in colloidal Mn(2+)-doped CdSe/CdS core/multishell quantum wells. The carrier-magnetic ion exchange interaction effects are tunable through wave function engineering. In our quantum well heterostructures, manganese was incorporated by growing a Cd0.985Mn0.015S monolayer shell on undoped CdSe nanoplatelets using the colloidal atomic layer deposition technique. Unlike previously synthesized Mn(2+)-doped colloidal nanostructures, the location of the Mn ions was controlled with atomic layer precision in our heterostructures. This is realized by controlling the spatial overlap between the carrier wave functions with the manganese ions by adjusting the location, composition, and number of the CdSe, Cd1-xMnxS, and CdS layers. The photoluminescence quantum yield of our magnetic heterostructures was found to be as high as 20% at room temperature with a narrow photoluminescence bandwidth of ∼22 nm. Our colloidal quantum wells, which exhibit magneto-optical properties analogous to those of epitaxially grown quantum wells, offer new opportunities for solution-processed spin-based semiconductor devices.

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand-alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.

1wileyonlinelibrary.com understanding fundamental excitonic processes but also because of their potential important applications in light-emitting diodes (LEDs), [ 1 ] lasers, [ 2 ] photovoltaics, [ 3 ] biological imaging, [ 4 ] and spintronics. [ 5 ] The rapidly developing fi eld of colloidal synthesis now makes custom designs of nanocrystals with a precise control over the size, shape, and composition possible. To date, semiconductor nanocrystals of various shapes including spherical dots, [ 6 ] nanorods, [ 7 ] tetrapods, [ 8 ] nanowires, [ 9 ] nanoribbons, [ 10 ] and most recently nanoplatelets (NPLs) [ 11 ] have been successfully synthesized. In these solution-processed quantum structures, an additional epitaxial growth of semiconductor shell around the starting semiconductor core leads to various architectures of nanocrystal heterostructures. By doing so, physical properties can be elegantly modifi ed with precisely controlling distribution of the composition across the heterostructure. These colloidal heteronanocrystals are possibly the best candidates for excitonic engineering and present attractive opportunities for enhanced platforms of colloidal photonics. [ 12 ] Previously, various solution-processed core/ shell quantum dots and rods have been studied for excitonically engineered properties. For example, Type-I CdSe/ZnSe core/ shell nanocrystals [ 13 ] and ZnSe/CdSe core/shell nanocrystals tunable between inverted Type-I and Type-II [ 14 ] were reported. With precise control of the shell, optical properties of core-only nanocrystals including quantum yield, [ 15 ] photostability, [ 16 ] and reduction of fl uorescence emission blinking [ 17,18 ] can be greatly enhanced, which thus make them highly attractive for colloidal LEDs, [ 19 ] colloidal lasers, [ 20 ] and biological imaging. [ 17 ] Recently, novel inverted Type-I nanocrystal heterostructures have drawn considerable interest thanks to their high charge injection effi ciency and enhanced absorption range which can be exploited in optoelectronic devices especially for applications in photovoltaics and photodetection. [ 21,22 ] CdS/HgS [ 23 ] and CdS/ CdSe [ 21,24 ] spherical core/shell architectures have been the most studied inverted Type-I colloidal nanoparticles, in each of which a thin layer of narrower bandgap material was grown onto a wider bandgap colloidal quantum dot forming a quantum dot-quantum shell heterostructure. In these architectures, quantum confi nement in the quantum shell arises from only one direction, radial, with the condition that the perimeter of the shell has to be considerably larger than the radius of the Continuously Tunable Emission in Inverted Type-I CdS/ CdSe Core/Crown Semiconductor NanoplateletsSavas Delikanli , Burak Guzelturk , Pedro L. Hernández-Martínez , Talha Erdem , Yusuf Kelestemur , Murat Olutas , Mehmet Zafer Akgul , and Hilmi V. Demir * The synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdS...

Fӧrster-type nonradiative energy transfer (NRET) is widely used, especially utilizing nanostructures in different combinations and configurations. However, the existing well-accepted Fӧrster theory is only for the case of a single particle serving as a donor together with another particle serving as an acceptor. There are also other special cases previously studied; however, there is no complete picture and unified understanding. Therefore, there is a strong need for a complete theory that models Fӧrster-type NRET for the cases of mixed dimensionality including all combinations and configurations. We report a generalized theory for the Fӧrster-type NRET, which includes the derivation of the effective dielectric function due to the donor in different confinement geometries and the derivation of transfer rates distance dependencies due to the acceptor in different confinement geometries, resulting in a complete picture and understanding of the mixed dimensionality.

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Cataloged from PDF version of article.In the past two decades, semiconductor quantum dots and wires have developed into new, promising classes of materials for next-generation lighting and display systems due to their superior optical properties. In particular, exciton-exciton interactions through nonradiative energy transfer in hybrid systems of these quantum-confined structures have enabled exciting possibilities in light generation. This review focuses on the excitonics of such quantum dot and wire emitters, particularly transfer of the excitons in the complex media of the quantum dots and wires. Mastering excitonic interactions in low-dimensional systems is essential for the development of better light sources, e.g., high-efficiency, high-quality white-light generation; wide-range color tuning; and high-purity color generation. In addition, introducing plasmon coupling provides the ability to amplify emission in specially designed exciton-plasmon nanostructures and also to exceed the Forster limit in excitonic interactions. In this respect, new routes to control excitonic pathways are reviewed in this paper. The review further discusses research opportunities and challenges in the quantum dot and wire excitonics with a future outlook