Strong Effect of Nonpolar Solvent Molecular Structure on CdSe Nanoplatelet Stacking

Abstract

We report a drastic difference in stacking behavior of oleic acid-stabilized 4-monolayer (4ML) CdSe nanoplatelets (NPLs) in toluene and methylcyclohexane (MCH), two nonpolar solvents that differ in conformational flexibility of the molecules. Using liquid cell transmission electron microscopy (TEM) and small angle scattering (SAXS) techniques, we show that NPLs form microns long ribbons consisting of 4 mL CdSe NPLs in toluene, the solvent widely used to form stable colloidal solutions of a broad range of quasi-spherical nanoparticles. In contrast, 4 ML CdSe NPLs are well-dispersed in MCH. Molecular dynamics simulations reveal that toluene better solvates the oleate ligands, resulting in extended ligand shells and increased NPL twisting, features that may promote stacking. In contrast, MCH induces a bimodal oleate span distribution, resulting in increased solubility of CdSe NPLs. The difference in stacking behavior of NPLs in toluene and MCH suggests that the conformational flexibility of the solvent, such as the ability to adopt multiple chair conformations, modulates nanoplatelet interactions by increasing the range of accessible molecular arrangements. We show that destabilization of toluene solution containing the ribbons of 4 ML CdSe NPLs without CdS shell results in the formation of NPL assemblies with amplified spontaneous emission (ASE) with a low threshold of 14 µJ/cm² that is comparable with that of CdSe/CdS core/shell NPLs. Our results emphasize that the solvent itself plays a major role in mediating interactions between NPLs and hence their processability for fabrication of functional structures.