Organic Surfactant-Controlled Composition of the Surfaces of CdSe Quantum Dots

The ratio of Cd to Se (Cd/Se) within colloidal CdSe quantum dots (QDs) synthesized with 90% trioctylphosphine oxide (TOPO) as the coordinating solvent increases from 1.2:1 for QDs with radius R >= 3.3 nm to 6.5:1 for R = 1.9 nm, as measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The highest value of Cd/Se reported previously for CdSe QDs was 1.8:1. The dependence of Cd/Se on R fits a geometric model that describes the QDs as CdSe cores with Cd/Se approximate to 1:1 encapsulated by a shell of Cd-organic complexes. Use of 99% TOPO as the coordinating solvent produces QDs with Cd/Se 1:1 for all values of R, and use of 99% TOPO doped with n-octylphosphonic acid (OPA), an impurity in 90% TOPO, produces QDs with values of Cd/Se up to 1.5:1. These results imply that Cd enrichment of the QDs is driven by tight-binding Cd(2+)-alkylphosphonate complexes that stabilize the interface between the polar CdSe core and the organic medium.