Cadmium-Free Colloidal Branched Nanocrystals with Optical Anisotropy Induced by Symmetry Breaking

Abstract

Geometric anisotropy in colloidal semiconductor nanocrystals (NCs) induces polarized light absorption and emission that play an important role in optoelectronic applications. So far, studies of shape and optical anisotropy have been mostly limited to Cd-or Pb-containing NCs because of the technical difficulties involved in the synthesis of anisotropic NCs using, for example, III-V crystals. Here, we report the linearly polarized light absorption and emission of the Cd-free branched NCs (BNCs), wherein wurtzite ZnS arms grow off the zincblende seeds (InAs, InP, or ZnSe NCs) in the presence of oleylamine at a high temperature (300 degrees C). By the proper control of the size and composition of the seed NCs, the linearly polarized light emission of Cd-free BNCs can be tuned from ultraviolet to near-infrared. From small-angle X-ray scattering analysis, we find that the linear polarization of InP/ZnS BNCs originates mainly from the broken symmetry in shape after the depletion of the precursor to minimize the surface energy. In addition, we demonstrate uniaxial alignment of BNCs under the external electric field, which implies the formation of the permanent dipole moment in anisotropic BNCs. This work sheds light on the utilization of optically anisotropic NCs in practical applications that essentially demand the nontoxicity of materials.