CdSe Nanoplatelets: Living Polymers

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Probing permanent dipoles in CdSe nanoplatelets with transient electric birefringence

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Curvature and self-assembly of semi-conducting nanoplatelets

International audienceSemi-conducting nanoplatelets are two-dimensional nanoparticles whose thickness is in the nanometer range and controlled at the atomic level. They have come up as a new category of nanomaterial with promising optical properties due to the efficient confinement of the exciton in the thickness direction. In this perspective, we first describe the various conformations of these 2D nanoparticles which display a variety of bent and curved geometries and present experimental evidences linking their curvature to the ligand-induced surface stress. We then focus on the assembly of nanoplatelets into superlattices to harness the particularly efficient energy transfer between them, and discuss different approaches that allow for directional control and positioning in large scale assemblies. We emphasize on the fundamental aspects of the assembly at the colloidal scale in which ligand-induced forces and kinetic effects play a dominant role. Finally, we highlight the collective properties that can be studied when a fine control over the assembly of nanoplatelets is achieved

Influence of Monomer Feeding on a Fast Gold Nanoparticles Synthesis: Time-Resolved XANES and SAXS Experiments

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Fourier-imaging of single self-assembled CdSe nanoplatelet chains and clusters reveals out-of-plane dipole contribution

International audienceFluorescent semiconductor nanoplatelets (epitaxial quantum wells) can be synthesized with excellent monodispersity and self-assembled in highly-ordered structures. Modifications of their electronic and luminescence properties when stacked, due to strong mechanical, electronic or optical interactions between them, have been the topic of intense recent discussions. In this paper, we use Fourier imaging to measure the different dipole components of various nanoplatelet assemblies. By comparing different measurement conditions and corroborating them with polarimetric analysis, we confirm an excellent precision on the dipole components. For single nanoplatelets, only in-plane dipoles (parallel to the platelet plane) are evidenced. For clusters of 2-10 platelets and chains of 30-300 platelets, on the other hand, a clear out-of-plane dipole component is demonstrated. Its contribution becomes more significant as the number of platelets is increased. We review possible explanations and suggest that the added out-of-plane dipole can be induced by strain-induced nanoplatelet deformations

Strain-controlled fluorescence polarization in a CdSe nanoplatelet–block copolymer composite

International audienceWe dispersed semi-conducting CdSe nanoplatelets within a styrene-butadiene-styrene block copolymer matrix. We could control the orientation of the nanoplatelets by stretching the resulting material, which provides a simple and reversible way of inducing fluorescence anisotropy. Such adjustable polarization effects are useful for modulating the optical response in composite materials. Hybrid materials made of anisotropic metallic or semi-conducting nanoparticles (NPs) dispersed in a polymer matrix have become an exciting class of nanocomposites with promising applications in electronics and optics. For example, the alignment of such materials can improve current transport in electronic devices or modulate the response in optical devices. 1, 2 However, two key points have to be addressed for this purpose. Firstly, a homogeneous dispersion of inorganic nanoparticles in polymeric templates is rather hard to achieve and most often requires surface modification. 3-7 Secondly, the macroscopic orientation of the material, which is required to exploit the anisotropic properties of the individual particles, must be controlled. To date, the orientation of the NPs in polymer matrices is mostly restricted to thin films. For example, nanoparticle alignment could be obtained by stretching a thermoplastic polymer at a temperature close to T g or by applying an electric field during solvent casting. 2, 8-10 In all cases, the orientation is irreversible, which precludes applications in opto-mechanical devices, for instance. We describe here how we achieved both a homogeneous dispersion of NPs in a thick film and the reversibility of the orientation in a composite material comprised of recently discovered CdSe nanoplatelets (NPLs) dispersed within a classical thermoplastic elastomer matrix. Indeed, CdSe NPLs have attracted much attention due to their outstanding spectroscopic properties. 11-16 These flat and square 2-dimensional photoluminescent nanoparticles display a sharp emission peak that can be precisely tuned by adjusting their thickness at the atomic level. Moreover, they have a very high quantum yield and fast recombination times of the carriers that are confined in one dimension. CdSe NPLs are generally synthesized in solution but they can also be deposited flat on a substrate 17 or aggregated edge-on in micronic needles. 18 To obtain a homogeneous composite material, we carefully adapted the BCP to the oleic acid ligand grafted on the NPLs. The polymer matrix that we have chosen is a commercial thermoplastic elastomer made of Styrene-Butadiene-Styrene (SBS) whose central block is of chemical nature and polarity close to those of oleic acid (Figure 1). A crucial advantage of such a matrix is the possibility to stretch it up to high strain levels in a reversible way at room temperature. We prepared homogeneous CdSe/SBS composite films with a CdSe volume fraction of 10% where both the SBS lamellar microstructure and the CdSe NPL spectroscopic properties are preserved. Figure 1. Components of the hybrid film. Left: schematic representation of a SBS copolymer chain. Center: schematic representation of a CdSe nanoplatelet covered with ligands. Right: photography of the hybrid film (Scale bar is 2 mm). The structural properties of these hybrid films were first studied by Transmission Electron Microscopy (TEM) (Figure 2a). The TEM images, which display light and dark grey bands corresponding to PS and PB domains respectively, clearly confirm the lamellar morphology of the SBS microstructure. Despite the high load in inorganic nanoparticles, the SBS microstructure in the hybrid films is therefore not much altered compared to that of the pure block copolymer. CdSe platelets have a strong electronic density but are very thin, which explains why the only particles clearly observed on the TEM images are those seen edge-on. Moreover, close inspection of the TEM images reveals that the CdSe platelets are mostly located in the PB domains, which was expected because of the chemical similarity of OA with PB. Very occasionally, a few platelets can be seen in the PS domains, possibly because of some ligand loss durin

Gold Nanoparticle Superlattices: Conditions for Long-Range Order, Moiré Patterns, and Binary Phase from a Single Population

We report the two-dimensional self-assembly at the liquid–air interface of spherical gold nanoparticles (NPs) with diameters between 2 and 14 nm. By exploring the self-assembly conditions, such as the dispersing solvent and the coating ligand (thiols with different lengths, oleylamine, polystyrene), we identify suitable conditions for long-range close-packed monolayers obtention. We show that though NPs with diameters below 3 nm yield glassy films or fuse during self-assembly depending on the ligand length, NPs with larger sizes dispersed in toluene yield well-ordered monolayers over distances that can span tens of micrometers. Adding a free ligand in solution before the self-assembly triggers long-range ordering into close-packed structures of otherwise amorphous films. The equilibrium distance between the NPs within the monolayers is compared to predictions by packing models, and the OTM displays qualitative agreement. We also observed a CaCu5 phase in few-layers assemblies which results from the size segregation of a monomodal population of NPs into two populations of different mean sizes occupying the two different sites of this complex lattice. In some instances, Moiré patterns consisting of two close-packed hexagonal monolayers superimposed with a twist angle are evidenced. By comparing the experimental structures with numerically simulated patterns, we show that a twist angle of 30° yields a quasicrystalline order with 12-fold rotational symmetry. Our work provides insights into fundamental processes behind the self-assembly of colloidal nanocrystals into ordered mono- and few-layers as well as more complex assemblies such as quasicrystalline or Frank–Kasper phases. These structures are of great significance for bottom-up fabrication of functional devices that take advantage of (collective) plasmonic properties or surface enhanced Raman scattering

Synthesis of CdSe Nanoplatelets without Short-Chain Ligands: Implication for Their Growth Mechanisms

International audienceWe present a novel route for the synthesis of zinc blende CdSe nanoplatelets (NPLs) that exclude the use of short-chain alkyl carboxylates. CdSe NPLs obtained without acetates are shown to be extremely asymmetric and rectangular. The effects of several experimental parameters such as the nature of cadmium carboxylates, selenium precursors, and precursor concentration ratios are studied. Our experiments, together with complementary small-/ wide-angle X-ray scattering results, show that the formation of NPLs is not related to soft templating. We discuss our findings in regard to several other formation mechanisms of NPLs, which have appeared recently in the literature, and propose that the steric hindrance caused by ligand packing exerts an influence on the growth and geometry of two-dimensional NPLs

Self-Assembly of CdSe Nanoplatelets into Giant Micrometer-Scale Needles Emitting Polarized Light

We report on the self-assembly of colloidal CdSe nanoplatelets into micrometers long anisotropic needle-like superparticles (SPs), which are formed in solution upon addition of an antisolvent to a stable colloidal dispersion. Optical fluorescence microscopy, transmission electron microscopy, and small-angle X-ray scattering provide detailed structural characterization and show that each particle is composed of 10⁶ nanoplatelets organized in highly aligned columns. Within the SPs, the nanoplatelets are stacked on each other to maximize the contact surface between the ligands. When deposited on a substrate, the planes of the platelets are oriented perpendicularly to its surface and the SPs exhibit polarized emission properties

General Expression for the Size-Dependent Optical Properties of Quantum Dots

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