Ligand control of growth, morphology, and capping structure of colloidal CdSe nanorods

The organic capping ligands used in the colloidal synthesis of CdSe nanorods have a profound impact on nanorod shape, dimensions, and capping structure as is shown here by varying the length of the alkylphosphonic acid ligands used in synthesis. The shorter the ligand, the more elongated and branched are the resulting nanorods; when mixtures of alkylphosphonic acids are used, the higher the molar fraction of the shorter ligand, the more elongated and branched are the nanorods. P-31 and H-1 nuclear magnetic resonance (NMR) studies indicate that shorter ligands preferentially remain on the nanorod surfaces when mixtures of phosphonic acids are used during synthesis. Optical transmission spectroscopy shows decreased solubility of CdSe nanorods capped by relatively long ligands because of nanorod aggregation. The results suggest that substantial control of nanorod dimensions and properties can be achieved by selecting the appropriate ligands in the synthesis process.