Solution-processed quantum wells, also known as colloidal nanoplatelets (NPLs), are emerging as promising materials for colloidal optoelectronics. In this work, we report the synthesis and characterization of CdSe/CdTe core/crown NPLs exhibiting a Type-II electronic structure and Type-II specific optical properties. Here, based on a core-seeded approach, the CdSe/CdTe core/crown NPLs were synthesized with well-controlled CdTe crown coatings. Uniform and epitaxial growth of CdTe crown region was verified by using structural characterization techniques including transmission electron microscopy (TEM) with quantitative EDX analysis and X-ray diffraction (XRD). Also the optical properties were systematically studied in these Type-II NPLs that reveal strongly red-shifted photoluminescence (up to ∼150 nm) along with 2 orders of magnitude longer fluorescence lifetimes (up to 190 ns) compared to the Type-I NPLs owing to spatially indirect excitons at the Type-II interface between the CdSe core and the CdTe crown regions. Photoluminescence excitation spectroscopy confirms that this strongly red-shifted emission actually arises from the CdSe/CdTe NPLs. In addition, temperature-dependent time-resolved fluorescence spectroscopy was performed to reveal the temperature-dependent fluorescence decay kinetics of the Type-II NPLs exhibiting interesting behavior. Also, water-soluble Type-II NPLs were achieved via ligand exchange of the CdSe/CdTe core/crown NPLs by using 3-mercaptopropionic acid (MPA), which allows for enhanced charge extraction efficiency owing to their shorter chain length and enables high quality film formation by layer-by-layer (LBL) assembly. With all of these appealing properties, the CdSe/CdTe core/crown heterostructures having Type-II electronic structure presented here are highly promising for light-harvesting applications.
An all-solution processed and all-colloidal laser is demonstrated using tailored CdSe/CdS core/shell quantum dots, which exhibit highly stable and low-threshold optical gain owing to substantially suppressed non-radiative Auger recombination.
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.
Developing low-cost photovoltaic absorbers that can harvest the short-wave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Si-based and perovskite photovoltaic technologies, is a prerequisite for making high-efficiency, low-cost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganic-organic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 µm are reported, leading to a short-circuit current density of 34 mA cm and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.
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...
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