pubs.acs.org/cm Published on Web 11/17/2010 r 2010 American Chemical Society 6402 Chem. Mater. 2010, 22, 6402–6408 DOI:10.1021/cm102370a Synthesis of Magic-Sized CdSe and CdTe Nanocrystals with Diisooctylphosphinic Acid Albert D. Dukes III, † James R. McBride, † and Sandra J. Rosenthal* ,†,‡ † Department of Chemistry, Vanderbilt University, VU Station B Box 351822, Nashville, Tennessee 37235, United States, and ‡ Department of Physics and Astronomy, Department of Pharmacology, and Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States Received August 18, 2010. Revised Manuscript Received October 21, 2010 We report the observation of quantized growth in magic-sized CdSe and CdTe nanocrystals that utilize diisooctylphosphinic acid as the surface passivating ligand. The interaction between the diisooctylphosphinic acid and the coordinating solvent, hexadecylamine, results in the growth of only certain allowed sizes of nanocrystals. Magic-sized CdSe nanocrystals were synthesized with band edge absorption peaks at 414, 446, and 490 nm. Magic-sized CdTe nanocrystals were synthesized with band edge absorption peaks at 445 and 483 nm. The synthesis detailed here allows for the isolation of larger magic-sized nanocrystals than has been previously reported. The emission of diisooctylpho- sphinic acid-capped magic-sized nanocrystals is low, with a quantum yield of less than 1%. Photoluminescence excitation scans revealed a significantly reduced emission from absorption at the band edge. The diameter for the smallest CdSe magic-size was 1.7 nm, and the diameter for the smallest CdTe magic-size was 1.8 nm, as determined by transmission electron microscopy. Introduction Magic-sized nanocrystals are composed of a well- defined number of atoms and only exist at certain stable sizes. 1-3 This allows the synthesis of large quantities of nanocrystals, all with identical properties, which cannot be achieved in traditional nanocrystal synthesis. The unique stability of magic-sized nanocrystals also differ- entiates them from traditional nanocrystals. Traditional CdSe nanocrystals have either wurtzite or zinc blende structure, while the stability of magic-sized nanocrystals is thought to arise from a cluster-cage structure. 4 The cluster-cage structure has yet to be reported for bulk semiconductors. This unique structure has only been accessible by a bottom up synthetic approach. On the basis of the result of mass spectrometry experi- ments, powder X-ray diffraction measurements, and theoretical modeling, Kasuya et al. have proposed that this cluster-cage structure is composed of alternating four- and six-membered rings, which allows for only certain sizes of nanocrystals. 4 They were able to synthe- size several different sizes of the cluster-cage structure by using a reverse micelle synthesis, corresponding to (CdSe) 13 , (CdSe) 33 , and (CdSe) 34 . The (CdSe) 33 and (CdSe) 34 have band edge absorption at 414 nm. 4 No other sizes were synthesized because only these sizes result in a closed cage structure. The largest size synthesized by Kasuya et al. corresponds to an external cage of (CdSe) 28 that is stabilized by an internal structure of (CdSe) 6 . To grow the next larger allowed size, it is thought that a new layer must be grown around the existing cage. This process would be observable by the appearance of a new peak in the UV-visible absorp- tion spectrum that is red-shifted from the previous band edge absorption. Such a unique redshift has been observed. 1,5,6 It is this quantized growth of the absorption spectrum that is the hallmark of true magic-sized nanocrystals. Kudera et al. previously demonstrated quantized growth of CdSe nanocrystals. 1 Their synthesis utilized nonanoic acid to decompose CdO into a reactive pre- cursor in an amine solvent, with an injection of Se dissolved in trioctylphosphine to begin the nanocrystal synthesis. The nanocrystals in the reaction solution were allowed to grow at 80 °C for up to 2500 min. The specific sizes of nanocrystals obtained from the quantized growth were those with band edge absorption peaks at 384, 406, and 431 nm. Their magic-size CdSe nanocrystals had a Cd:Se ratio between 1.1 and 1.3:1. 1 The fluorescence spec- trum of these nanocrystals was broad and featureless, the result of trap-state emission. Kudera et al. observed band edge recombination only after passivating the surface with *To whom correspondence should be addressed. E-mail:Sandra.j.rosenthal@ vanderbilt.edu. (1) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y. Q.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. Adv. Mater. 2007, 19, 548. (2) Pedersen, J.; Bjornholm, S.; Borggreen, J.; Hansen, K.; Martin, T. P.; Rasmussen, H. D. Nature 1991, 353, 733. (3) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300. (4) Kasuya, A.; Sivamohan, R.; Barnakov, Y. A.; Dmitruk, I. M.; Nirasawa, T.; Romanyuk, V. R.; Kumar, V.; Mamykin, S. V.; Tohji, K.; Jeyadevan, B.; Shinoda, K.; Kudo, T.; Terasaki, O.; Liu, Z.; Belosludov, R. V.; Sundararajan, V.; Kawazoe, Y. Nat. Mater. 2004, 3, 99. (5) Dagtepe, P.; Chikan, V.; Jasinski, J.; Leppert, V. J. J. Phys. Chem. C 2007, 111, 14977. (6) Zanella, M.; Abbasi, A. Z.; Schaper, A. K.; Parak, W. J. J. Phys. Chem. C 2010, 114, 6205.