Evidence for directed self-assembly of quantum dots in a nematic liquid crystal

Abstract

Assembling quantum dots (QDs) into nanoscale configurations over macroscopic dimensions is an important goal to realizing their electro-optical potential. In this Rapid Communication, we present a detailed study of a pentylcyanobiphenyl liquid crystal (LC) and a CdS QD colloidal dispersion by probing the dielectric property epsilon and relaxation as a function of an applied ac electric field Eac. In principle, dispersing QDs in a nematic LC medium can direct the dots to align in nearly one-dimensional chainlike structures along the nematic director and these assemblies of QDs can be directed by external electric fields. In a uniform planar aligned cell, the Fréedericksz switching of the LC+QDs appears as a two-step process with the same initial switching field as the bulk but with the final epsilon value larger than that for an aligned bulk LC. The relaxation of epsilon immediately following the removal of Eac follows a single-exponential decay to its original value that is slower than the bulk but becomes progressively faster with increasing Eac, eventually saturating. These results suggest that the arrangement of the QDs is mediated by the LC.