Effect of metal ion solubility on the oxidative assembly of metal sulfide quantum dots

The versatility of the oxidative assembly method for the creation of 2D and 3D quantum dot (QD) architectures represents both an opportunity and a challenge as a method enabling controlled placement of chemically distinct QDs in multicomponent systems. The opportunity lies in the ability to independently tune the kinetics of the different components so that they are similar (leading to well-mixed systems) or different (enabling gradient or phase-segregated composites) using a wide range of variables; the challenge lies in understanding those variables and how their interplay affects the overall kinetics. Here, we show that the identity of the cation in the sulfide matrix (M = Cd²⁺ vs Zn²⁺) plays a large role in the kinetics of assembly of mass spectrometry QDs, attributed to differences in solubility. Time resolved dynamic light scattering is used to monitor the hydrodynamic radius, $R¯h$⁠. ZnS shows an exponential growth associated with reaction-limited cluster aggregation (RLCA), whereas CdS demonstrates a significant induction period (10–75 min) followed by a growth step that cannot be distinguished between RLCA and diffusion limited cluster aggregation. These data correlate with relative solubilities of the nanoparticles, as probed by free-cation concentration. Data also confirm prior studies showing that cubic-closest-packed (ccp) lattices are kinetically slow relative to hexagonally closest-packed (hcp); using the slope of the ln $R¯h$ vs time plot for the rate constant, the values of 0.510 s⁻¹ and 3.92 s⁻¹ are obtained for ccp ZnS and hcp ZnS, respectively. Thus, both the structure and the solubility are effective levers for adjusting the relative reactivity of QDs toward oxidative assembly.