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

Abstract

Chloride-terminated, tri- n -butylphosphine (Bu 3 P) bound CdSe nanocrystals were prepared by cleaving carboxylate ligands from CdSe nanocrystals (2.5 carboxylate/nm 2 ) with chlorotrimethylsilane in Bu 3 P solution. 1 H and 31 P{ 1 H} nuclear magnetic resonance (NMR) spectra of the isolated nanocrystals allowed assignment of distinct signals from several free and bound species, including surface-bound Bu 3 P ( δ = −13 ppm, fwhm = 908 Hz) and [Bu 3 P–H] + [Cl] − ligands as well as a Bu 3 P complex of cadmium chloride. NMR spectroscopy supports complete cleavage (>99%) of the X-type carboxylate ligands. Primary n- alkylamines rapidly displace the bound Bu 3 P on mixing, leading to amine-bound nanocrystals with higher dative ligand coverages (1.8 RNH 2 /nm 2 vs 0.5 Bu 3 P/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 Cd 2+ 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.