Ligand-Mediated Phase Control in Colloidal AgInSe₂ Nanocrystals

Synthetic studies of colloidal nanoparticles that crystallize in metastable structures represent an emerging area of interest in the development of novel functional materials, as metastable nanomaterials may exhibit unique properties when compared to their counterparts that crystallize in thermodynamically preferred structures. Herein, we demonstrate how phase control of colloidal AgInSe₂ nanocrystals can be achieved by performing reactions in the presence or absence of coordinating ligands that can act as cation exchange mediators (i.e., oleylamine or 1-dodecanethiol). These coordinating ligands play a crucial role in the formation of metastable AgInSe₂ nanocrystals, as they mediate in situ topotactic conversion of an orthorhombic Ag₂Se intermediate to a metastable orthorhombic phase of AgInSe₂. We provide a detailed mechanistic crystal chemistry description of this process to structurally elucidate how the orthorhombic phase of AgInSe₂ forms. Density functional theory calculations suggest that the metastable orthorhombic phase of AgInSe₂ is metastable by a small margin, at 10 meV/atom above the thermodynamic ground state. In the absence of oleylamine or 1-dodecanethiol, Ag₂Se nanocrystal intermediates convert through kinetically slow, nontopotactic conversion processes to yield the thermodynamically preferred chalcopyrite structure of AgInSe₂. On the basis of these discoveries, we offer new insights into the prediction of novel metastable multinary nanocrystal phases that do not exist on bulk phase diagrams.