In situ 1H NMR study on the trioctylphosphine oxide capping of colloidal InP nanocrystals

We used trioctylphosphine oxide (TOPO) capped colloidal InP nanocrystals (Q-InP|TOPO) to explore the potential of solution H-1 NMR spectroscopy in studying in situ the capping and copping exchange of sterically stabilized colloidal nanocrystals. The spectrum of Q-InP|TOPO shows resonances of free TOPO, superimposed on broadened spectral features. The latter were assigned to TOPO adsorbed at Q-InP by means of pulsed field gradient diffusion NMR and H-1-C-13 HSQC spectroscopy. The diffusion coefficient of Q-InP|TOPO nanocrystals was inferred from the decay of the adsorbed TOPO NMR signal. T; he corresponding hydrodynamic diameter correlates well with the diameter of Q-InP. By using the resolved methyl resonance of adsorbed TOPO, the packing density of TOPO at the InP surface can be estimated. Spectral hole burning was used to demonstrate explicitly that the adsorbed TOPO resonances are heterogeneously broadened. Exchange of the TOPO capping by pyridine was demonstrated by the disappearance of the resonances for adsorbed TOPO and the appearance of pyridine resonances in the H-1 NMR spectrum. These results show that solution NMR spectroscopy should be considered a powerful technique for the in situ study of the capping of sterically stabilized colloidal nanocrystals.