CdSe Clusters: At the Interface of Small Molecules and Quantum

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CdSe Clusters: At the Interface of Small Molecules and Quantum Dots Brandi M. Cossairt and Jonathan S. Owen* Department of Chemistry, Columbia University, Havemeyer Hall, MC 3121, 3000 Broadway, New York, New York 10027, United States

bS Supporting Information ABSTRACT: We have prepared and isolated a variety of cadmium selenide cluster molecules by the reaction of Cd(O2C
R)2(H2NR0 )2 (R = C4H9, 4-X-Ph; X = H,
OMe,
SMe,
Me,
F, and
CF3; R0 = dodecyl, phenethyl) with tri-n-octylphosphine selenide (TOPSe) and diphenylphosphine selenide (DPPSe). Reactions conducted with 1.0 equiv of DPPSe lead to a cluster with a lowest energy electronic transition at 418 nm (d ≈ 1.5 nm, ε(418 nm) = 189 000 ( 27 000 M
1 cm
1) and a broad (fwhm = 0.75 eV) and intense white light emission profile (ΦPL = 0.11(03)). Nuclear magnetic resonance spectroscopy, Rutherford backscattering spectrometry and solution molecular weight determinations support a molecular formula of [(CdSe)4(Cd(O2C
C6H5)2)(H2NC12H25)2]7((1). Mechanistic studies indicate that 2 equiv of DPPSe convert to [Se2PPh2]
[H3NR0 ]þ and diphenyl phosphine upon mixing with H2NR0 and prior to reaction with cadmium benzoate. Reactions with TOPSe result in a series of CdSe nanocrystals characterized by discrete steps in size with absorption features between 330 and 550 nm. The importance of precursor reactivity to the outcome of the cluster growth is discussed. KEYWORDS: CdSe, magic-size, diselenophosphinate, nanocrystal, cluster

’ INTRODUCTION Inorganic nanostructures with dimensions in the range of 1
2 nm adopt thermodynamically stable structures or “magic sizes” with uniformity in their chemical composition approaching that of small molecules.1
5 A number of groups have pursued this class of nanostructures and reported syntheses of cadmium chalcogenide (CdE) clusters,1
3,6
16 sheets,17 and ribbons.18,19 Although sheet and ribbon structures are uniform in one- and two-dimensions, there are a number of reports of single-sized clusters that, in some cases, could be structurally characterized by single-crystal diffraction.6,7,9,15 The reproducible synthesis and isolation of well-defined clusters has proven challenging. Hence most detailed studies of CdE clusters focus on the structurally characterized, chalcogenolate-ligated cluster molecules.7,15,20 Owing to their surface ligands, these clusters display weak luminescence at room temperature.21,22 Recently, Rosenthal and co-workers prepared CdSe clusters in the presence of tech-grade TOPO and phosphonic acid ligands that display broad photoluminescence and fluorescence lifetimes characteristic of surface trapped excited states.23
25 Other reports document related nanostructures with similar photophysical characteristics,2,10,26,27 though in both cases their chemical formulas and structural features remain largely unexplored. Furthermore, with the exception of the structurally characterized chalcogenolate-ligated clusters, it remains unclear whether these samples are composed of clusters with an identical composition or a distribution of formulas. If a detailed understanding of their structure and composition can be obtained, such clusters would offer a valuable model system by which to study structure
property relationships. En route to this goal, we have developed a novel synthesis of a sub-2 nm CdSe cluster and characterized its chemical formula as well as the chemistry r 2011 American Chemical Society

underlying its formation. Precise tuning of the reaction conditions further unveils an accessible quantized growth regime, giving rise to clusters ranging in size from 1 to 4 nm in diameter.

’ EXPERIMENTAL SECTION CdSe(418 nm) Synthesis and Isolation: Se (310 mg, 3.9 mmol) and diphenylphosphine (DPP, 730 mg, 3.9 mmol) are combined in 10 mL of toluene and allowed to stir for 14 h. CdO (0.5 g, 3.9 mmol), benzoic acid (1.5 g, 12.3 mmol), and dodecylamine (2.0 g, 10.8 mmol) are combined in a 25 mL, 3-neck round-bottom flask equipped with a glass thermocouple adapter, condenser, and a septum and are degassed at 110 °C for 40 min. The temperature is then elevated to 210 °C for 10 min to allow the cadmium benzoate precursor to form. After the solution has become colorless and homogeneous, the temperature is decreased to 110 °C and the mixture is degassed for an additional 40 min. Finally, the temperature is decreased to 45 °C and allowed to equilibrate under argon. Once equilibrated, the diphenylphosphine selenide (DPPSe) solution is injected and the temperature is allowed to recover to 45 °C. After 150 min, UV
vis indicates the reaction is at completion and the mixture is cannula transferred into a thick-walled glass reactor equipped with a Teflon stopper. This bright-yellow reaction mixture is brought into the glovebox (N2) where it is taken to dryness under reduced pressure. The resulting yellow residue is then slurried in 20 mL of pentane and centrifuged to remove colorless insoluble materials. The pentane solution is placed in the glovebox freezer at
35 °C for at least 5 h, during which time additional colorless material crystallizes. Filtration removes these remaining organic coproducts. The pentane filtrate is then taken to dryness and dissolved in 15 mL of toluene. Addition of acetonitrile Received: March 25, 2011 Revised: May 12, 2011 Published: May 24, 2011 3114

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Chemistry of Materials (30 mL) affords a milky yellow solution, which leaves a yellow film of particles upon centrifugation. The toluene/acetonitrile supernatant is removed and the yellow film is dissolved in toluene (5 mL) and reprecipitated with acetonitrile (10 mL). The resulting yellow powder is dissolved in pentane and dried to constant mass affording approximately 0.8
1 g of particles in a typical run (71
89% yield of clusters based on the reported theoretical mass of 10 046 g mol
1). CdMe2 can also be used in place of CdO, eliminating the need for extensive degassing. CdMe2 is added dropwise to a mixture of benzoic acid and dodecylamine at 45 °C under argon causing vigorous evolution of methane upon addition. Following equilibration of the temperature at 45 °C, DPPSe is added and the reaction followed as described above. Syntheses conducted with butyric acid were regulated at 30 °C during growth. CdSe From TOPSe Synthesis and Isolation: For synthesis and isolation of the quantized growth clusters, tri-n-octylphosphine selenide (TOPSe) replaces DPPSe in either of the above procedures with growth temperatures ranging from 50 to 115 °C. Isolation is carried out in the same manner as described for CdSe(418 nm).

’ RESULTS AND DISCUSSION Benzoate-terminated CdSe clusters were synthesized by combining a mixture of cadmium benzoate (prepared from CdMe2 or CdO) and dodecylamine with diphenylphosphine selenide (DPPSe) at 45 °C in toluene. The temporal evolution of the reaction to a single, stable size (CdSe(418 nm)) is evident from the narrow absorption maximum at 418 nm shown in Figure 1, which is similar to the absorption of clusters observed previously.2,3,23,27

Figure 1. UV
visible absorption and fluorescence spectra of CdSe(418 nm). Dashed lines are the absorbance from aliquots taken at the times indicated in the legend.

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Growth of the cluster nears completion in 150 min, after which virtually no changes are observed, indicating that ripening does not follow consumption of the cadmium precursor.28 Similar reports of stability toward ripening in this temperature regime have been documented in other syntheses.2,27 In a typical run, 0.8
1.0 g of bright yellow powder is isolated without affecting the absorption or fluorescence spectrum, provided it is protected from air (see Figure 1S and 2S in the Supporting Information).29 Although powders of CdSe(418 nm) are indefinitely stable under nitrogen at room temperature, exposure of the isolated solids to air results in a slow color change from yellow to red. Exposure of toluene solutions of CdSe(418 nm) to air, on the otherhand, immediately quenches the fluorescence; however, no red shift is observed even over several days.30,31 Interestingly, the reaction coproducts observed in this synthesis are dodecylammonium benzoate, tetraphenyl diphosphine (δ(31P)
14.5 ppm), and N-dodecyl-1,1-diphenylphosphinamine selenide (δ(31P) 57.3 ppm, δ(77Se)
275 ppm, 1J31P/77Se 771 Hz; Figures 3S and 4S in the Supporting Information), which was verified by an independent synthesis of an authentic sample.32 Small amounts (550 nm, we suggest these particles arise from assembly of cluster building blocks. We are actively pursuing the mechanism of these reactions and the structure of both types of particles. ’ ASSOCIATED CONTENT

Figure 4. Absorption and fluorence spectra of CdSe clusters grown from TOPSe at 80 °C before and after isolation.

bS

Supporting Information. Details concerning reagents, instrumentation, and characterization (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. TEM images of CdSe nanoparticles grown from TOPSe at 115 C. The scale bar for the left image is 10 nm and for the right image is 2 nm. 3117

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported as part of the Center for Re-Defining Photovoltaic Efficiency Through Molecule Scale Control, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award DE-SC0001085. B.M.C. would like to thank the NIH for postdoctoral funding under NRSA Grant 10484140. The authors would like to thank Dr. Eric Stach of Brookhaven National Laboratory for obtaining high-resolution TEM images. ’ REFERENCES (1) Kudera, S.; Zanella, M.; Giannini, C.; Rizzo, A.; Li, Y.; Gigli, G.; Cingolani, R.; Ciccarella, G.; Spahl, W.; Parak, W. J.; Manna, L. Adv. Mater. 2007, 19, 548. (2) 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. (3) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (4) Chen, H. S.; Kumar, R. V. J. Phys. Chem. C 2009, 113, 31. (5) McBride, J. R.; Dukes, A. D. I.; Schreuder, M. A.; Rosenthal, S. J. Chem. Phys. Lett. 2010, 498, 1. (6) Vossmeyer, T.; Reck, G.; Katsikas, L.; Haupt, E. T. K.; Schulz, B.; Weller, H. Science 1995, 267, 1476. (7) Eichhofer, A.; Hampe, O.; Blom, M. Eur. J. Inorg. Chem. 2003, 1307. (8) Corrigan, J. F.; Fuhr, O.; Fenske, D. Adv. Mater. 2009, 21, 1867. (9) Herron, N.; Calabrese, J. C.; Farneth, W. E.; Wang, Y. Science 1993, 259, 1426. (10) Dai, Q.; Li, D.; Chang, J.; Song, Y.; Kan, S.; Chen, H.; Zou, B.; Xu, W.; Xu, S.; Liu, B.; Zou, G. Nanotechnology 2007, 18, 405603. (11) Jiang, Z. J.; Kelley, D. F. ACS Nano 2010, 4, 1561. (12) Ouyang, J.; Zaman, B.; Yan, F. J.; Johnston, D.; Li, G.; Wu, X.; Leek, D.; Ratcliffe, C. I.; Ripmeester, J. A.; Yu, K. J. Phys. Chem. C 2008, 112, 13805. (13) Riehle, F. S.; Bienert, R.; Thomann, R.; Urban, G. A.; Kruger, M. Nano Lett. 2009, 9, 514. (14) Yu, K.; Hu, M. Z.; Wang, R.; Le Piolet, M.; Frotey, M.; Zaman, B.; Wu, X.; Leek, D. M.; Tao, Y.; Wilkinson, D.; Li, C. J. Phys. Chem. C 2010, 114, 3329. (15) Behrens, S.; Bettenhausen, M.; Eichhofer, A.; Fenske, D. Angew. Chem., Int. Ed. 1997, 36, 2797. (16) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Diesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 7665. (17) Ithurria, S.; Dubertret, B. J. Am. Chem. Soc. 2008, 130, 16504. (18) Joo, J.; Son, J. S.; Kwon, S. G.; Yu, J. H.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 5632. (19) Pradhan, N.; Xu, H.; Peng, X. Nano Lett. 2006, 6, 720. (20) Eichhofer, A. Eur. J. Inorg. Chem. 2005, 2005, 1245. (21) Aharoni, A.; Eichhofer, A.; Fenske, D.; Banin, U. Opt. Mater. 2003, 24, 43. (22) Soloviev, V. N.; Eichhofer, A.; Fenske, D.; Banin, U. J. Am. Chem. Soc. 2001, 123, 2354. (23) Bowers, M. J. I.; McBride, J. R.; Rosenthal, S. J. J. Am. Chem. Soc. 2005, 127, 15378. (24) Schreuder, M. A.; Xiao, K.; Ivanov, I. N.; Weiss, S. M.; Rosenthal, S. J. Nano Lett. 2010, 10, 573.

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