Kinetic Studies of Cation Exchange Reactions in II-VI Quantum Dots

Abstract

The quest to synthesize excellent materials with qualities ranging from size-tunability, photoluminescence quantum yield and storability has led to the study of cation exchange reaction as one of the many methods explored. Cation exchange reactions have been heavily studied with CdSe as starting materials to synthesize Ag2Se quantum dots (QDs) and other QDs. In this work, ZnSe QDs has been studied and successfully used as a starting material for cation exchange for Ag2Se QDs. The reaction was carried out at room temperature with cheap and readily available materials. Kinetics of reaction was studied to understand the rate of cation exchange reaction using ZnSe compared to known and heavily studied CdSe QDs. The fitting of the kinetic curves was done with multiple exponential function with fastest rate constant (k) value 1.9 × 102 s-1 obtained for ZnSe and a later slower reaction rate, which is the reverse for CdSe. It is understood that the cation exchange at the surface process for ZnSe system was faster, but the interior exchange process inside the crystal was slower. The observed rates are due to shorter bond length and stronger bonds in ZnSe QDs. How ligands on the surface of CdSe QDs affect the rate of exchange with Ag+ ion was investigated. CdSe QDs were treated with hexadecylamine (HDA), trioctylamine (TOA), dodecylamine (DDA), oleylamine (OlAm), stearate, and oleate as capping ligands. The amines show difference in their rate constant (k) values with DDA being the fastest, followed by OlAm, HDA, and TOA being the slowest. This trend is attributed to the basis of steric effect of these ligands preventing diffusion of ions in and out of QD in the solution. Oleate and stearate have similar chain-length (C18), but show different exchange rate constant k1, with oleate (1.8 × 103 s-1) being faster than 3.8 × 102 s-1 observed with stearate. These rates are attributed to steric hindrance caused by these ligands on the surface of the QDs, which is consistent with the trend observed with the amines.