Langmuir 2008, 24, 9043-9049
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Synthesis of High Aspect Ratio Quantum-Size CdS Nanorods and Their Surface-Dependent Photoluminescence Aaron E. Saunders,† Ali Ghezelbash, Preeti Sood, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 ReceiVed March 27, 2008. ReVised Manuscript ReceiVed May 21, 2008 Colloidal CdS nanorods with diameters near 4 nm and narrow size distributions (∼(10%) were synthesized up to 300 nm long by a sequential reactant injection technique that utilizes phosophonic acids as capping ligands. The phosphonic acid strongly passivates the nonpolar CdS surfaces and sequential reactant injection provides controlled CdS formation kinetics to enable heterogeneous and facet-selective CdS deposition on the more reactive {002} surfaces. With this process, the nanorod length can be systematically increased by increasing reactant addition to extend nanorod growth. The phosphonic acid concentration, however, is quite important, as “low” concentrations allow radial deposition and branching to occur. These high aspect ratio (>100) CdS nanorods luminesce with relatively high efficiencies of 10.8% quantum yield at room temperature. The luminescence, however, mostly arises from trap-related recombination, and the emission is significantly red-shifted from the absorption edge. Various surface passivation treatments were explored to eliminate trap emission and increase the luminescence quantum yield. Thiol and amine passivation both significantly reduced trap emission and enhanced band-edge emission, but the total luminescence quantum yields dropped significantly, with a maximum measured value of 1.5% for the amine-passivated CdS nanorods.
Introduction Semiconductor nanorods and nanowires with sufficiently narrow diameters to induce radial quantum confinement are an interesting class of materials. Compared to spherical nanocrystals, nanorods can have higher photon absorption cross sections1,2 and optical gain lifetimes (important for lasing),2 relatively strong permanent electric dipoles,3–5 unique electrorheological properties,6 enhanced photovoltaic efficiencies,7 and size-dependent8 and polarized light absorption,1 scattering,9 and emission.10,11 High aspect ratio rods are particularly interesting because the electron and hole are delocalized along the length of the nanowire, but confined in the radial direction, enabling the role of dimensionality and its influence on electrical and optical properties to be examined.10,12 Colloidal synthetic routes provide one effective way to obtain large amounts of crystalline semiconductor nanorods with high aspect ratios and diameters sufficiently narrow to observe quantum confinement. Three general approaches to semiconductor nanorod growth in solution have emerged: (1) metal particle-seeded growth,10,12–14 (2) oriented attachment,15–18 * Corresponding author: e-mail,
[email protected]; telephone, (512) 471-5633; fax, (512) 471-7060. † Present address: Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309-0424.
(1) Protasenko, V. V.; Hull, K. L.; Kuno, M. AdV. Mater. 2005, 17, 2942. (2) Htoon, H.; Hollingsworth, J. A.; Malko, A. V.; Dickerson, R.; Klimov, V. I. Appl. Phys. Lett. 2003, 82, 4776–4778. (3) Li, L.-S.; Alivisatos, A. P. Phys. ReV. Lett. 2003, 90, 097402. (4) Nann, T.; Schneider, J. Chem. Phys. Lett. 2004, 384, 150–152. (5) Ghezelbash, A.; Koo, B.; Korgel, B. A Nano Lett 2006, 6, 1832–1836. (6) Lozano, K.; Hernandez, C.; Petty, T.; Sigman, M. B.; Korgel, B. A. J. Colloid Interface Sci. 2006, 297, 618–624. (7) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425– 2427. (8) Buhro, W. E.; Colvin, V. L. Nat. Mater. 2003, 2, 138–139. (9) Sigman, M. B.; Korgel, B. A. J. Am. Chem. Soc. 2005, 127, 10089–10095. (10) Kan, S.; Mokari, T.; Rothenberg, E.; Banin, U. Nat. Mater. 2003, 2, 155–159. (11) Hu, J.; Li, S.-S.; Yang, W.; Manna, L.; Wang, L.-W.; Alivisatos, A. P. Science 2001, 292, 2060–2063. (12) Wang, F.; Buhro, W. E. J. Am. Chem. Soc. 2007, 129, 14381–14387. (13) Holmes, J. D.; Johnston, K. P.; Doty, R. C.; Korgel, B. A. Science 2000, 287, 1471–1473.
and (3) ligand-assisted kinetically controlled growth.19–26 Although significant chemical understanding has accumulated about these processes, there is still much to learn, particularly with respect to how to systematically control nanorod length while maintaining a narrow diameter and diameter distribution. The Cd chalcogenidessCdS, CdSe, and CdTesare particularly prone to anisotropic crystallization and nanorod growth by arrested precipitation due to their hexagonal crystal structure and significant difference in reactivity between their polar and nonpolar surface facets. CdS27 and CdSe22,23 were in fact the first materials systems observed to generate a high yield of nanorods by ligand-assisted kinetically controlled growth. Since these initial studies, the role of surfactant and reactant composition and how the reactants are added and the reaction temperature tuned have been intensively studied in order to understand how to control nanorod formation and growth and achieve high yields of nanorods with narrow diameter and length distributions. It is now relatively well understood that capping ligands are crucial to nanorod formation and must inhibit growth from particular (14) Wang, F. D.; Dong, A. G.; Sun, J. W.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511–7521. (15) Tang, Z. Y.; Kotov, N. A.; Giergsig, M. Science 2002, 297, 237–240. (16) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140–7147. (17) Barnard, A. S.; Xu, H. J. Phys. Chem. C 2007, 111, 18112–18117. (18) Barnard, A. S.; Xu, H.; Li, X.; Pradhan, N.; Peng, X. Nanotechnology 2006, 17, 5707–5714. (19) Shieh, F.; Saunders, A. E.; Korgel, B. A. J. Phys. Chem. B 2005, 109, 8538–8542. (20) Joo, J.; Son, J. S.; Kwon, S. G.; Yu, J. H.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 5632–5633. (21) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183–184. (22) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700–12706. (23) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59–61. (24) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296–1306. (25) Jun, Y.-W.; Lee, S.-M.; Kang, N.-J.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150–5151. (26) Park, J.; Koo, B.; Yoon, K. Y.; Hwang, Y.; Kang, M.; Park, J.-G.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 8433–8440. (27) Yang, J.; Zeng, J.-H.; Yu, S.-H.; Yang, L.; Zhou, G.-E.; Qian, Y.-T. Chem. Mater. 2000, 12, 3259–3263.
10.1021/la800964s CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
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crystal facets. In the case of CdS, CdSe, and CdTe, long-chain phosphonic acids are well-known to have this effect.28,29 The second aspect of nanorod growth is to control reactant decomposition kinetics so that anisotropic crystallization without homogeneous particle nucleation can occur in solution. This aspect has been perhaps the most elusive to control and understand, and there have been many different recipes showing how to inject reactants and ramp temperature; however, there is still only an intuitive understanding of how to do this effectively.11,20–23,25,27,29–35 One emerging approach is sequential controlled reactant injection, in which one of the reactants, for example the Cd reactant, is first added to the reaction mixture and heated and then the second reactant, the chalcogen for example, is injected slowly into the reactant mixture over the course of tens of minutes.19,24,26,36 This approach enables a burst of particle nucleation, followed by controlled facet-selective epitaxial deposition to promote nanorod growth. Here, it is demonstrated that CdS nanorods with narrow diameters (
