Soluble, Chloride-Terminated CdSe Nanocrystals: Ligand Exchange Monitored by 1H and 31P NMR Spectroscopy

Chloride-terminated, tri-n-butylphosphine (Bu3P) bound CdSe nanocrystals were prepared by cleaving carboxylate ligands from CdSe nanocrystals (2.5 carboxylate/nm(2)) with chlorotrimethylsilane in Bu3P solution. H-1 and P-31{H-1} nuclear magnetic resonance (NMR) spectra of the isolated nanocrystals allowed assignment of distinct signals from several free and bound species, including surface-bound Bu3P (delta = -13 ppm, fwhm = 908 Hz) and [Bu3P-H](+)[Cl](-) ligands as well as a Bu3P complex of cadmium chloride. NMR spectroscopy supports complete cleavage (>99%) of the X-type carboxylate ligands. Primary n-alkylamines rapidly displace the bound Bu3P on mixing, leading to amine-bound nanocrystals with higher dative ligand coverages (1.8 RNH2/nm(2) vs 0.5 Bu3P/nm(2)) and greatly increased photoluminescence quantum yields (33 +/- 3% vs <1%). Combined with measurements of the Se:Cd:Cl ratio (1:1.16:0.28) using Rutherford backscattering spectrometry, these studies support a structural model of nanocrystals where chloride ligands terminate the crystal lattice by balancing the charges of excess Cd2+ ions. The adsorption of dative amine and phosphine ligands leads to nanocrystals whose solubility is afforded by reversibly bound and readily exchanged L-type ligands, for example, primary amines and phosphines. The importance of ligand coverage to both the UV-visible absorption and photoluminescence spectra are discussed.