Formation of Chiral Helices by Self-Assembling Molecules on Semiconductor Nanosubstrates

Among semiconductor colloidal nanocrystals, 2D nanoplatelets (NPLs) are geometrically seen as well-defined flexible substrates for the self-assembly of molecules. In the presence of mechanical stress brought by surface stabilizers, helical structures are formed according to the parameters of the initial material after internal energy relaxation.<br/>Here, we demonstrate the tuning of the NPLs helices radii through the organic ligands, described as an anchoring group and an aliphatic chain of a given length. A perfect control in surface chemistry allows the switch between different morphological features of these nanohelices. Nonetheless, their optical properties in the visible region are well-preserved upon such modifications. Numerical simulations and as well as structural X-ray diffraction studies done on these anisotropic nanoparticles, unveil a strong preferential orientation effect on their resulting scattered patterns.<br/>A mechanical model accounting for the misfit strain between the inorganic core and the organic ligands, enables to predict the nanohelices radii. The model treats these different layers of substrate, anchoring group and aliphatic chain contributions individually and demonstrates good agreement for all studied homo- and hetero-structure cadmium-based NPLs. It reveals ultimately that the self-assembly of surface molecules is equivalent to a layer, whose Young modulus in lateral compression can be estimated close to 0.9 GPa.<br/>Furthermore, the chirality of the semiconductor nanohelices shown in this work is dictated by the ligands anchoring group and can be inverted from one population to another. Chiroptical properties exhibited in circular dichroism measurements show a special interest in investigating this type of mirror-symmetry-breaking particles and could undoubtedly, lead to the emission of circularly polarized light in the visible spectrum.