We report on the effects of Lewis bases and other ligands on radiative recombination in CdSe quantum dots (QDs) in several solvents. Long-chain primary amines are found to be the most efficacious capping agents for CdSe QDs in nonpolar solvents. Primary alkylamines are superior to secondary and tertiary alkylamines. The kinetics of chemisorption and desorption in less polar solvents, such as hexane or chloroform, are temperature controlled and obey a Langmuir isotherm. Mercaptan adsorption also obeys a Langmuir isotherm, and alkylmercaptans rapidly displace amines, leading to luminescence quenching. In more polar solvents, such as toluene, ligands desorb, leading to luminescence quenching. It is proposed that surface Cd vacancies function as nonradiative recombination centers. The adsorption of a Lewis base to the QD raises the surface vacancy energy close to, or above, the conduction band edge and eliminates electron capture by the surface vacancies. Solvent polarity has a strong effect on luminescence since the solvent determines the extent of ligand adsorption to the QD surface.

The growth kinetics of CdSe nanocrystals nucleated from TOPSe and cadmium oleate were investigated in octadecene, a non-coordinating solvent. The effects of temperature and the oleic acid concentration on the kinetics of both nucleation and particle growth were investigated. It was found that increasing oleic acid concentrations led to smaller numbers of nuclei, smaller initial nuclei size, and larger final particle sizes. The rate constant for steady-state CdSe deposition was found to be 2.2 × 10 -6 cm s -1 at ≈ 265 °C, far slower than the diffusion limit. The number of growing particles remained constant following the initial nucleation step. The radius of the primary CdSe nuclei varied from 1.0 ± 0.1 nm down to 0.8 ± 0.2 nm at lower oleic acid concentrations. Between 2 and 8% of the available Cd was consumed during nucleation. From the residual TOPSe and cadmium oleate concentrations at the onset of Ostwald ripening, the solubility of 2.2 nm CdSe in octadecene is measured to be 6.4 × 10 -5 M 2 at 265 °C. The surface free energy of CdSe in octadecene was found to be 0.17 J/cm 2 , which leads to an estimate of a 9% size distribution in the nanocrystals at the moment of nucleation.

High quality CdSe nanocrystals have been prepared using elemental selenium as the chalcogenide precursor dispersed in 1-octadecene (ODE). The conditions used to prepare the Se precursor were found to be critical for successful nanocrystal synthesis. Systematic titration of the Se precursor solution with tri-n-octylphosphine (TOP) allowed the Se reactivity to be tuned and the final particle size to be controlled. Band-edge and surface related emission were observed for samples prepared in the presence and absence of added TOP. In the absence of a selenium passivant, the crystal structure of CdSe nanocrystals could be altered from zinc blende to wurtzite by the addition of bis(2,2,4-trimethylpentyl)phosphinic acid (TMPPA).