Atomistics of Asymmetric Lateral Growth of Colloidal Zincblende CdSe Nanoplatelets

Shape anisotropy in colloidal zincblende CdSe nanoplatelets (NPLs) has a direct bearing on their optical and electronic properties. The lateral aspect ratio of NPLs provides an additional knob with which one can control their optical characteristics even at a fixed thickness. For example, one can regulate the polarized emission and assembly behaviors of NPLs by varying the lateral aspect ratio while keeping the optical transition at the energy fixed by the NPL thickness. Given the isotropic nature of the zincblende crystal structure, such control over shape anisotropy is intriguing yet not fully understood. In this study, based on combined experimental and simulation works, we systematically investigate the asymmetric lateral growth behavior of colloidal 4.5 monolayer thick (5 Cd and 4 Se layers) CdSe NPLs. Experimentally, it is found that NPLs with rectangular and square shapes have different growth directions, <100> and <110>, respectively. The cadmium acetate (Cd(ac)2)-to-Se precursor ratio turns out to have an important role in altering the lateral growth kinetics of NPLs, and the deficiency of Se under high Cd-to-Se precursor ratio conditions slows down the growth and changes the growth direction from <100> to <110>. First-principles calculations reveal that the observed growth behaviors can be explained in light of the irregular wedge-shaped conformations at the NPL side surfaces that expose {110} and {111} faces. Specifically, it is identified that the adsorption of Cd(ac)2 on {110} ({111}) surfaces becomes an energetically unfavorable process in the low (high) Se coverage regime, explaining the fast <100>-direction (slow <110>-direction) growth into rectangular (square) NPLs. The presented mechanism for the lateral growth of zincblende NPLs provides an atomic-level understanding of the transition processes of NPL morphology and growth direction, with the control of unique anisotropic properties of NPLs on the line.