Self-Assembly of Chiral\nLigands on 2D Semiconductor\nNanoplatelets for High Circular Dichroism

Group II–VI semiconductor nanoplatelets (NPLs)\nwith atomically\ndefined thicknesses and extended atomically flat (001) facets are\nused for ligand binding and chiro-optical effects. In this study,\nwe demonstrate that tartrate ligands, anchored by two carboxylate\ngroups, chelate the (001) facets of NPLs at an average ratio of one\ntartrate molecule to two cadmium (Cd) surface atoms. This assembly\nof chiral molecules on inorganic nanocrystals generates a circular\ndichroism <i>g</i>-factor as high as 1.3 × 10<sup>–2</sup> at the first excitonic transition wavelength of NPLs. Tartrate ligands\ninduce an orthorhombic distortion of the initially “cubic”\ncrystal structure, classifying the NPLs within the 222-point group.\nUnlike spherical nanocrystals, where it is difficult to discern whether\nchiral ligands affect only the surface atoms or the entire crystal\nstructure, our findings unequivocally show that the crystal structure\nof NPLs is modified due to their thinness and atomically precise thickness.\nThe in-plane lattice parameters experience compressive and tensile\nstresses, significantly splitting the heavy-hole and light-hole bands.\nAdditionally, tartrate ligands adopt different conformations on the\nNPL surface over time, resulting in dynamic changes in the circular\ndichroism signal, including an inversion of its sign.