Steric effects of carboxylic capping ligands on the growth of the CdSe quantum dots

Abstract

Cadmium selenide quantum dots were grown using different carboxylic capping ligands in order to study the steric effects of the ligand chain length and chain branching. Straight chain and branched saturated carboxylic acids (fatty acids) were used as capping ligands. Hot injection synthesis was performed in 1-octadecene via the reaction between Cd carboxylates and trioctylphosphine selenide. At specific times, samples were withdrawn from reaction mixtures. For straight chain carboxylic acids, namely stearic, hexadecanoic, dodecanoic, decanoic, and octanoic, the decrease of the chain length leads to the increase of the CdSe particle sizes due to the faster transport of the monomers through the capping layer. For decanoic and octanoic acids, fast diffusion through the capping layer leads to the reaction-controlled kinetics, and the particles grow as tetrapods instead of dots. α-Branched double chain acids, 2-butyloctanoic and especially 2-hexyldecanoic, create more steric hindrance compared to the straight chain acids of comparable length and produce smaller quantum dots. α-Branched triple chain acids, neodecanoic and 1-adamantanecarboxylic, were expected to show superior steric hindrance during the synthesis. Neodecanoic acid, compared to 2-butyloctanoic acid, the closest to it in length, created less hindrance for the monomer diffusion, probably because two of its three branches are very short. 1-Adamantanecarboxylic acid performed comparable to 2-butyloctanoic acid, despite a much shorter length. This is attributed to the bulky rigid adamantane cages forming a dense capping layer. Straight chain and branched chain ligands lead to different photoluminescence patterns. When capped with straight chain carboxylates, CdSe quantum dots exhibit only sharp bandgap fluorescence, consistent with a high degree of surface site coverage. All of the α-branched ligands enabled intense surface trap emission, likely because of incomplete passivation of the surface atoms due to the nearest neighbor steric repulsion.