Correlating structural distortions and optical shifts in carboxylate-exchanged CdSe nanoplatelets

Quasi-2D nanomaterials such as semiconducting nanoplatelets (NPLs) have drawn considerable interest due to their tunable optical properties and large surface to volume ratios. Cadmium selenide (CdSe) NPLs are of particular fundamental interest since their thicknesses can be controlled with atomic precision using well-established solution-phase synthetic techniques. Additionally, their large surface area makes them especially susceptible to changes in the identity of the capping ligands and, therefore, good model systems for understanding surface chemistry. In the current work, we explore the role of these ligands in altering the lattice parameters and optical properties of CdSe NPLs. We build on prior research that has employed varying binding groups, including thiols, phosphonic acids, and halides, to demonstrate ligand-dependent optical bandgap changes and concomitant lattice distortions as determined by powder x-ray diffraction (PXRD). Our work investigates the correlations between ligand-induced optical and structural changes with a series of ligands that maintain a consistent carboxylic acid binding group, thus allowing us to probe secondary ligand effects. We perform ligand exchanges on oleic acid-capped CdSe NPLs with benzoic acids, cinnamic acids, and cyclohexanecarboxylic acid. In all cases, the optical bandgap decreases upon ligand exchange, and a correlated expansion in the thickness of the NPLs is observed via PXRD. We also observe that the benzoic acids produce larger optical and structural distortions than the cinnamic acids. We show that the optical and structural correlation is nearly quantitatively described by quantum confinement effects, with the thicker quantum wells exhibiting smaller energy gaps.