Ligand-Mediated Phase Control in Colloidal AgInSe₂ Nanocrystals

Abstract

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 1-dodecanethiol. The thiol plays a crucial role in formation of metastable AgInSe2 nanocrystals, as it mediates an in-situ topotactic cation exchange from an orthorhombic Ag2Se intermediate to a metastable orthorhombic phase of AgInSe2. We provide a detailed mechanistic description of this cation exchange 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 1-dodecanethiol, a mixture of Ag2Se nanocrystal intermediates form that convert through kinetically slow, non-topotactic exchange processes to yield the thermodynamically preferred chalcopyrite structure of AgInSe2. Finally, we offer new insight into the prediction of novel metastable multinary nanocrystal phases that do not exist on bulk phase diagrams.

Polymorphic Metastability in Colloidal Semiconductor Nanocrystals

“…Our proposed mechanism for this transformation is akin to the mechanism we proposed to explain the formation of metastable wurtzite‐like CuInSe 2 from a copper selenide intermediate [65] . To account for the tetragonal Ag 2 Se intermediate in this mechanism, we hypothesize that tetragonal Ag 2 Se converts first to orthorhombic Ag 2 Se, which can then undergo cation exchange to orthorhombic AgInSe 2 [67] . Indeed, it is known that, when ligated with oleylamine, tetragonal Ag 2 Se undergoes an irreversible phase transition to orthorhombic Ag 2 Se at 60–67 °C [13] …”

“…A highly related system, AgInS 2 , also crystallizes in the chalcopyrite structure at low temperatures, but adopts an orthorhombic wurtzite‐like structure (space group , Figure 9b) at T >913 K, as shown in Figure 9d; [103–105] this phase has also been isolated for AgInS 2 nanocrystals [106] . Interestingly, AgInSe 2 nanocrystals also can crystallize in an analogous orthorhombic structure, despite this phase not being present on the bulk phase diagram for AgInSe 2 [18,67,99,107–110] …”

“…In the last ten years, dichalcogenide precursors have been employed in the syntheses of ternary I‐III‐VI 2 semiconductor nanocrystals [20,57,66,67] . Ternary I‐III‐VI 2 semiconductor nanocrystals with an A + B 3+ E 2− 2 composition are of interest as relatively non‐toxic alternatives to cadmium and lead‐containing semiconductors, with applications in thin film solar cells, light emitting diodes (LEDs), photocatalysis, and bioimaging [68] .…”

“…Recently, we showed that the judicious selection of coordinating ligands enables polymorphic control in the syntheses of AgInSe 2 nanocrystals [67] . In a general synthesis, the Ag + and In 3+ precursors were dissolved together in a flask in either excess oleylamine or excess oleic acid.…”

“…While insightful, it is difficult to extend this principle to the nanoscale due to the aforementioned complexity of parameters that influence thermodynamic stabilities of nanomaterials, including surface energy, morphology, defects, surface ligand interactions, size effects, composition, etc. Recently, we proposed a conceptual framework that utilizes thermodynamic bulk phase diagrams to target material systems that may yield novel metastable polymorphs on the nanoscale [67] . This framework was developed after noting emerging patterns in our studies of the mechanisms of formation for metastable wurtzite‐like CuInSe 2 and AgInSe 2 nanocrystals; in both cases, the metastable ternary wurtzite‐like phases form as In 3+ reacts with binary selenide intermediates (Cu 3 Se 2 or orthorhombic Ag 2 Se, respectively) that possess pseudo‐hexagonal Se 2− sub‐lattices and are metastable at the reaction temperatures (vide supra) [65,67] .…”