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