The carboxylate-ligated In37P20 is an intriguing magic-sized cluster (MSC) whose high stability (i.e., magic size) stems from a delicate balance between the energy cost and gain associated with its partially disordered, In-rich core and its passivation by the bidentate ligands. In order to use such MSCs as intermediates for non-classical nucleation and growth of quantum dots, it is essential to control the reactivity (or stability) of MSCs by disrupting the energetic balance. Here, using ab initio molecular dynamics simulations, we reveal the destabilization process of the InP MSC induced by a modification of the surface ligand network beyond a critical limit. When three In(O2CR)3 subunits are released from the cluster at high temperatures, the remaining In34P20 core suddenly loses its stability and undergoes a structural transformation through In–P bond breaking and rearrangement. The net effect of the isomerization is an In–P bond exchange between a pair of In atoms, thereby leading to a rupture on the cluster surface. We elucidate the mechanism for the MSC instability by studying the intricate interactions between the surface ligand network and the inorganic core. Finally, we discuss the similarity and fundamental differences in the cluster isomerization of group III–V InP and group II–VI CdS MSCs.

The recent discovery of chemically reversible isomerization of CdS clusters (Williamson et al. Science2019, 363, 731) shows that the structural transformation of such inorganic clusters has essential characteristics of both small-molecule isomerization and solid−solid transformation. Despite its importance in synthesizing colloidal quantum dots from cluster intermediates (so-called "magic-sized clusters" or MSCs), the underlying mechanism for such inorganic isomerization is not yet understood. Here, using ab initio simulated spectroscopy, we propose a microscopic mechanism for the multiscale isomerization of CdS MSC. When triggered by hydroxyl adsorption, a carboxylate-ligated CdS cluster undergoes a structural transformation through Cd−S bond exchanges at the bond-length scale (molecular isomerization), which induces the change in the stacking sequence of the partially ordered CdS lattice (solid−solid transformation). The creation of the bond-exchange defects in the CdS core and "self-healing" by ligand rearrangements on the surface play a central role in the isomerization. MSCs can be thus made susceptible to forming a particular type of point-like defect (e.g., bond-exchange defect), which provides useful insights into understanding the stability and structural activation of MSCs.