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We investigated the effect of cation exchange on the anionic framework of lightly doped CdSe:Te/CdS nanorods. In contrast with previously studied core/shell systems, the Te dopant, located in the center of the CdSe core, provides an extremely sensitive indicator for any structural changes of the anionic framework that may occur as a result of the cation exchange process. We first optimized the cation exchange procedure in order to retain the fluorescence properties of the CdSe:Te/CdS nanorods after exchange of Cd2+ for Cu+ and back to Cd2+. Next, using multiexciton spectroscopy, we were able to probe the magnitude of the exciton-exciton repulsion interaction and use that to assess the degree of crystal structure conservation. Our findings provide a much stronger proof that the anion framework is indeed rigid, showing no evidence of significant migration of the anionic dopant. [ABSTRACT FROM AUTHOR]

Wepropose to use the self-assembly ability of a block copolymer combinedwith compression–expansion cycles to obtain CdSe quantum dots(QDs) structures of different morphology. The methodology proposedconsists in transferring onto mica mixed Langmuir monolayers of QDsand the polymer poly(styrene-co-maleic anhydride)partial 2-butoxyethyl ester cumene terminated, PS-MA-BEE, previouslysheared by 50 compression–expansion cycles. Results indicatethat the shear stress takes out nanoparticles at the air–waterinterface from metastable states and promotes a new equilibrium stateof the Langmuir monolayer. This new state was transferred onto micaby the Langmuir–Blodgett (LB) methodology, and the morphologyof the LB films was analyzed by atomic force microscopy and transmissionelectron microscopy measurements. Our results show that when the amplitudestrain increases, the QDs domain size decreases and the QDs LB filmarrangement becomes more ordered. The dynamics of the monolayer relaxationafter cycling involves at least three time scales which are relatedto the damping of surface fluctuation, raft rearrangement, and componentmovements inside each raft. Brewster angle microscopy allowed visualizing in situthe raft rearrangement at the air–water interface. [ABSTRACT FROM AUTHOR]

We developed a three-step colloidal synthesis of near-infrared (NIR) active Cu-In-Se (CISe)-based nanocrystals (NCs) via a sequential partial cation exchange realized in one pot. In the first step, binary highly copper deficient Cu2-xSe NCs were synthesized, followed by a partial cation exchange of copper to indium ions, yielding CISe NCs. This reaction allows for a precise control of the composition of the resulting NCs through a simple variation of the ratio between guest-cation precursors and parent NCs. To enhance the stability and the photoluminescence (PL) properties of the NCs, a subsequent ZnS shell was grown in the third step, resulting in CISeS/ZnS core/shell particles. These core/shell hetero-NCs exhibited a dramatic increase in size and a restructuring to trigonal pyramidal shape. The shell growth performed at a relatively high temperature (250 °C) also led to anion exchange, in which sulfur replaced part of selenium atoms close to the surface of the NCs, forming alloyed CISeS core structure. This efficient anion exchange is rarely reported for I-III-VI-based nanomaterials. Furthermore, we demonstrated that at higher reaction temperature, it is possible to obtain In-rich CISeS/ZnS NCs whose emission was shifted to the visible region. Therefore, a careful tuning of the reaction parameters, such as the Cu:Se and Cu:In ratios, temperature, and time, enables a distinct control over the size and composition of the NCs while maintaining their crystal structure. By varying the size of the CISeS/ZnS NCs from 9 to 18 nm, the PL spectra could be tuned, covering a wide NIR range with maxima from 990 to 1210 nm. Thus, in these experiments, we demonstrated a clear dependence of the optical properties of these materials on their size and extended the PL range of CISe-based NCs further to the infrared part of the spectrum. The results obtained might be important not only to elucidation of fundamentals of ion exchange reactions but also may provide a general preparative approach to a wide variety of copper chalcogenide-based NCs with well-controlled size, shape, composition, and even crystal structure. [ABSTRACT FROM AUTHOR]

In the past few years, several protocols have been reported on the synthesis of CdSe nanoplatelets with narrow photoluminescence (PL) spectrum, high PL quantum efficiency, and short exciton lifetime. The corresponding core/shell nanoplatelets are however still mostly based on CdSe/CdS, which possess an extended lifetime and a strong red shift of the band-edge absorption and emission, in accordance with a quasi-type-II band alignment. Here we report on a robust synthesis procedure to grow a ZnS shell around CdSe nanoplatelets at moderate temperatures of 100-150 °C, to improve the optical properties of CdSe nanoplatelets via a type-I core/shell heterostructure. The shell growth is performed under ambient atmosphere, in either toluene or 1,2-dichlorobenzene. The variation of the shell thickness induces a continuous red shift of the PL peak, eventually reaching 611 nm. The PL quantum efficiency is increased compared to the original CdSe cores, with values up to 60% depending on the shell thickness. High-resolution transmission electron microscopy reveals a bending of the nanoplatelets caused by strain due to 12% lattice mismatch between CdSe and ZnS. The present procedure can easily be translated to other core/shell nanocrystals, such as CdSe/CdS and CdSe/CdZnS nanoplatelets. [ABSTRACT FROM AUTHOR]

Two dimensional (2D) colloidal PbS nanoplatelets (NPLs) with a thickness of 1.8-2.8 nm have been synthesized using a single-molecule precursor approach with lead octadecylxanthate. The lateral dimensions were tuned by varying the reaction temperature, growth time, and capping ligands. Transmission electron microscopy and X-ray diffraction reveal that the NPLs have an orthorhombic crystal structure rather than the rocksalt phase usually reported for bulk and nanostructured PbS. The 1.8 nm thickness, in combination with the tunable lateral dimensions, results in a blue-shifted absorption peak at 715-730 nm and a 48-68 nm narrow emission spectrum with a surprisingly small, 18 nm Stokes shift at room temperature. The fluorescence lifetime of these PbS NPLs is 2 orders of magnitude shorter than the typical lifetime in 0D colloidal PbS quantum dots, highlighting the advantageous properties of colloidal 2D nanostructures that combine strong transversal with weak lateral confinement. [ABSTRACT FROM AUTHOR]