Effects of Structural Asymmetries in Branched Nanocrystals: Implications for Electron Collection Efficiency in Single-Nanotetrapod Devices

Geometrically anisotropic branched nanocrystals can occur either accidentally or deliberately during the growth process. Although the additional degree of freedom provided by such anisotropies has the potential to further broaden the functionality and applicability of these versatile nanostructures, this aspect of shape engineering has received little attention in the literature. We investigate theoretically the effects of the anisotropic growth in the width or length of a single arm on the electronic and optical properties of short- and long-armed tetrapods with type I and type II band alignment, identifying signatures of their presence and possible applications. Although there are some common features in both types, we find that many of the properties vary between branched structures with type I and type II band structures, and, for tetrapods of the same material, with the aspect ratio. Our work also addresses the nontrivial question, relevant to theorists and experimentalists alike, of whether the properties of large, long-armed branched structures with tens of thousands of atoms can be predicted based on those observed in (or theoretically calculated for) small, short-armed tetrapods with a few thousands of atoms, providing an easy-to-use catalogue of features which can and cannot. Most of them can be traced back to the characteristics of the charge density distribution in these systems and lead to different applications. In particular, the degree of wave function localization in a single arm found in some of the anisotropic structures considered here could be exploited to enhance the electron collection efficiency in single-tetrapod devices, where type I tetrapods would provide fast exciton recombination for efficient light emission, whereas type II structures would ensure long electron lifetimes for applications in electronics, photovoltaics, and photocatalysis.