Surface Derivatization of Nanocrystalline CdSe Semiconductors
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(a)< 5%.
Surface Exchanged QDs To modify the surface of size selected CdSe QDs with different ligands we employed the following procedure. For a surface modification with pyridine (or 4-picoline) size selected QDs passivated with TOPO/TOPSe were recovered as powder and redispersed in a small volume (typically 5-10 mL) of neat pyridine (or 4-picoline). After overnight stirring the QDs were 323
Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
precipitated from solution by adding an excess of n-hexane. The resulting suspension was then centrifuged and the recovered precipitate dispersed in neat pyridine (or 4-picoline). The solution was left to stir for 5 hours whereupon the dots were precipitated, redispersed in pyridine (or 4picoline) as above and left to stir for an additional 5 hours. The QDs were later precipitated from solution and dried under N2 . To passivate the surface of QDs with thiophenol, size selected QDs were recovered as a powder by drying the QD precipitate under a continuous flow of N 2 . The isolated nanocrystallites were then brought into a glove box where they were dissolved in about 10 mL of thiophenol. The mixture was left to stir for a day inside the glove box. The QDs were recovered as precipitate by adding an excess of n-hexane to the solution. Dilute solutions of 4(trifluoromethyl)thiophenol in CH 3C1 were used to exchange the surface with 4(trifluoromethyl)thiophenol. In each case the isolated QDs were dissolved in 3 mL of CH3 C1 and an estimated 20 fold excess of 4-(trifluoromethyl)thiophenol was added to the CH 3CI solution. The mixture was left to stir for 4 days in a glove box. At the end of the exchange reaction, the QDs were recovered by precipitation with methanol. Control Experiments Various control experiments were conducted to monitor the extent of ligand exchange in our reactions. QDs were prepared in several different ways from the standard preparation described above. This was done to evaluate the extent of surface passivation by TOPO and TOPSe. Clusters were prepared with an excess of TOPO over TOP (trioctylphosphine). This involved excluding TOP in our injection solution. QDs were also prepared exclusively in TOPO by replacing our TOPSe precursor with bis(trimethylsilyl)selenium, (TMS) 2 Se, and excluding TOP in the injection solution. RESULTS 'H NMR Measurements of Surface Ligands In our ligand exchange experiments we found ourselves unable to completely change the surface of the QDs. This was confirmed by 'H NMR experiments. The NMR spectra of QDs passivated with thiophenol, 4-(trifluoromethyl)thiophenol, pyridine and 4-picoline all show residual alkyl protons upfield from the aromatic resonances characteristic of these ligands. Based on the integrated intensities of the proton peaks we find that 85-90% of the ligands passivating the surface of the QDs are the newly introduced capping species. The remaining 10-15% are native capping ligands. Similar results have been seen by Katari et al. in CdSe nanocrystallites exchanged with pyridine. 5 To overco
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