Effect of the bulkiness of alkyl ligands on the excited-state dynamics of ZnO nanocrystals

Abstract

Organic ligands on the surface of nanocrystals (NCs) are extremely important in influencing various physical properties, such as dispersibility, electrical properties, and optical properties. Recent studies have revealed that a slight difference in the molecular structure of aliphatic organic ligands significantly affects the dispersibility of the NCs. On the other hand, the effects of the difference in the molecular structure of ligands on the excited-state dynamics of NCs remain elusive. In this study, we synthesized a series of colloidal ZnO NCs capped with different alkyl phosphonic acids and investigated their photophysical properties using emission decay measurements and transient absorption spectroscopy. The spectral shape and lifetime of the emission originating from the surface oxygen defects of ZnO NCs are almost the same irrespective of the alkyl phosphonic ligands used, indicating that the electronic states of the surface oxygen defects are not affected by the bulkiness of the ligand. On the other hand, the emission quantum yield correlates with the rate of carrier trapping by oxygen defects, suggesting that the rate of carrier trapping reflects the number of oxygen defects. Revealing the detailed relationship between molecular structures of organic ligands and the optical properties of NCs is important for advanced photofunctional superstructures using semiconductor NCs.