NANO LETTERS
Colloidal CdSe Quantum Wires by Oriented Attachment
2006 Vol. 6, No. 4 720-724
Narayan Pradhan,*,† Huifang Xu,‡ and Xiaogang Peng*,† Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701, and Department of Geology and Geophysics, and Materials Science Program, UniVersity of Wisconsin, Madison, Wisconsin 53706 Received December 19, 2005; Revised Manuscript Received February 14, 2006
ABSTRACT We report here a relatively low temperature (100−180 °C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as “pearl-necklace” or “string-of-pearls” in the literature, which are loosely associated and chemically fused sections.
Synthesis of colloidal semiconductor nanocrystals has been a major topic in the field of material chemistry in the recent years. Among them, CdSe nanocrystals have been acting as the model system1-3 in terms of synthesis. Despite the success of synthesizing high-quality CdSe quantum dots,2,3 quantum rods,4-6 and quantum wells,7 colloidal CdSe nanowires in a quantum confinement size regime (quantum wires) have proven to be a challenge. The solution-liquidsolid method8-10 and nanoporous template method11,12 have generated CdSe quantum wires, but the diameter of the resulting nanowires has been quite large, barely in the quantum confinement size regime. This report intends to demonstrate a possible means to solve this challenge. The current synthetic methods for high-quality CdSe quantum rods were derived mostly from synthesis of CdSe quantum dots,3,4 which seem to not be a choice for the growth of quantum wires. Detailed kinetics study illustrated that, as the length of the quantum rods reached about 50 nm, uniformity of the nanorods along the long axis became a problem and branching was often evidenced.4,9,13,14 It is worth mentioning that the smallest diameter of the quantum rods reported so far, about 2 nm, was synthesized under relatively low temperatures(∼100 °C).13 For other nanorods, such as ZnS15 and ZnSe,16 very small nanorods with diameters in the range of 1 to 2 nm were also grown at similar temperatures although the mechanism of such growth is not clear yet. This existing literature motivated us to explore the growth of CdSe nanowires using relatively low reaction temperatures, between 100 and 180 °C. However, it is * Corresponding authors. E-mail:
[email protected];
[email protected]. † University of Arkansas. ‡ University of Wisconsin. 10.1021/nl052497m CCC: $33.50 Published on Web 03/07/2006
© 2006 American Chemical Society
necessary to keep in mind that the low-temperature approaches mentioned above, in contrast to the typical high temperature (250-360 °C) routes for growing CdSe quantum rods and dots, could only generate nanorods with very small diameters,4,5,17 typically below 2 nm. In addition to typical template methods, there is an interesting growth approach for nanowires, oriented attachment. Oriented attachment refers to the phenomenon that generates nanowires by attaching existing dot-shaped nanocrystals along a given crystal orientation. Since it was first reported a few years ago,18 this mechanism has been observed for several systems, such as CdTe19 and PbSe nanowires20 and ZnO21 and ZnSe nanorods.16,17 A recent report by Murray’s group documented impressive evidence of the importance of the dipole moment of PbSe nanodots for the oriented attachment.20 As for CdSe nanowire formation, Kotov’s group mentioned it briefly in their influential publication in 2002 although the main content was related to CdTe nanowires.19 Formation of CdSe quantum wires was observed by reacting cadmium acetate (CdAc2) and selenourea in amines in the temperature range between 100 and 180 °C (Figure 1). Details of the experiments are provided in the Experimental Section and Supporting Information. Amines were found to be necessary to activate the cadmium precursors, which in turn decreased the reaction temperature. Cadmium fatty acid salts with a long hydrocarbon chain, such as cadmium stearate, were not sufficiently reactive even with the presence of the amines. Consequently, stable magic-sized CdSe clusters were the only products. Magic-sized clusters22 are those that have bulk crystalline atomic packing and a fully closed outer shell. The special configuration allows a
Figure 1. Formation of CdSe quantum wires. A photoluminescence (PL) image of the prewire sample is shown as an inset for the middle panel. TEM images after sonication, direct annealed, and washed and annealed are shown in left-bottom, right-top and right-bottom panels, respectively. The scale bar represents 50 nm.
Figure 2. Left: formation of the quantum wires, cluster, and prewire stages (top) and solid wire stage (bottom). Right: sonicaiton experiments for prewire aggregates (top) and solid wires (bottom).
magic-sized cluster be at a local free energy minimum in comparison to other nanoclusters/nanocrystals, and thus they are exceptionally stable. This stability is observed commonly during the kinetic studies of growth of semiconductor nanocrystals, indicated by the persistent and sharp absorption peak at a given wavelength and sudden jump of the this persistent spectrum for magic-sized clusters to a continuous shifting spectral region.22 Three different growth stages for CdSe quantum wires were identified. The system generated magic-sized CdSe clusters in the beginning (Figure 2, top left), which was readily observed by UV-vis absorption (Figure 2, top left, solid curve) and photoluminescence (PL) spectra although these clusters were too small to be observed under TEM. This stage typically lasted for about tens of seconds. As the reaction proceeded, a spectral tail at the low-energy side of the magic-sized absorption features (see dotted line in Figure 2, top left) appeared, which indicates that the reaction went into the second stage, formation of prewire aggregates. The prewire aggregates, or pearl-necklace-shaped nanowires observed by others,15,16,19,23 were formed by the aggregation of the magic-sized clusters (Figure 1, middle panel). Further Nano Lett., Vol. 6, No. 4, 2006
heating of these pearl-necklace-shaped nanowires yielded solid quantum wires in the third stage (Figure 1,right panel). The representative UV-vis spectra of the related wires are in Figure 2, bottom left. Although spherical clusters and short wires were not obvious under TEM by mixing in the nanowire bundles, one can conclude that these species might often be present along with the solid wires by comparing the UV-vis spectra before and after purification via sizeselective precipitation (Figure 2, bottom left). The UV-vis spectrum of a solid wire sample shifted red considerably from that of a dot sample with the same size as the diameter of the nanowires. For example, the first excitation peak in the UV-vis absorption spectrum for the solid nanowires after selective precipitation (Figure 2 bottom left) was centered at ∼511 nm and the diameter of the nanowires was determined to be 2.1 ( 0.3 nm. It is known that CdSe dots with this diameter should have their first exciton absorption peak at ∼475 nm.24 The appearance of pearl-necklace-shaped wires,19-21 such as the ones in Figure 1 (middle panel) has been considered as key evidence of the oriented attachment mechanism, provided the preexistence of nanodots with sizes similar to the diameter of the pearls in the solution. Figure 2 (left) indicates that the concentration of the magic-sized clusters decreased as the reaction proceeded and new absorption features appeared on the low-energy side of the absorption spectra, further implying the aggregation of the magic-sized nanodots in this specific case. Because the nanodot precursors were the extremely small magic-sized clusters, interactions between them for formation of pearl-necklace-shaped nanowires should be reasonably small no matter if the interactions were dipole-moment-originated or other types of molecular interactions. This motivated us to see if these pearl-necklaceshaped nanowires could be broken down by sonication before they were converted into solid wires. Figure 2 (top right) shows the UV-vis spectra of the pearlnecklace-shaped nanowires before and after sonication. Upon sonication, although there was no apparent change of the solid wires (Figure 2, bottom right), the peak intensity of 721
the magic-sized clusters increased dramatically for the prewire sample. This is consistent with the fact that each prewire aggregate should consist of many magic-sized clusters. As expected, the increase of the intensity of the absorption due to the magic-sized clusters was accompanied by weakening of the absorption feature at the low-energy side. Under TEM, the pearl-necklace-shaped nanowires almost disappeared completely. However, smaller sized fragments and short wires and rods were observed after the sonication (Figure 1, bottom left). Again, magic-sized clusters in the prewire sample after sonication were difficult to observe under TEM although their high population was evidenced by UV-vis measurements (Figure 2, top right). The small-sized fragments all seemed to be solid rods (Figure 1, bottom left), indicating that some parts of the prewire aggregates were already chemically fused together. Additional experiments described below were carried out to further confirm this hypothesis. If the prewire aggregates contained two different sections, chemically fused fragments and loosely associated nanocluster sections, then one could destroy the loosely associated sections by repeated precipitation, dissolution, and washing with a strong solvent of the surface amine ligand, such as methanol. This treatment should eventually get rid of all loosely associated sections and leave behind the chemically fused fragments. Such purified fragments were put back into hot toluene or 1-octadecene. As expected, only quantum rods with various lengths were observed (Figure 1, bottom right). Unfortunately, the magic-sized clusters were too small to identify their atomic packing and relative orientation using HRTEM. It was thus not possible to judge whether oriented attachment occurred in the loosely associated sections of the prewire aggregates. As mentioned above, typical low-temperature approaches only yielded nanorods with extremely small diameters,13,15,16 between 1 and 2 nm. The diameter of the quantum wires, however, was found to be variable between about 1.5 and 6 nm, the typical strong quantum confinement size regime of CdSe nanocrystals. This was achieved by changing the chain length of the amines (Figures 1 (right panel) and 3 (top panel)) in a particular reaction condition. The length of the wires was as long as a few micrometers (Figure 3, middle panel). The smallest nanowires, with diameters similar to the size of the magic-sized clusters, could be made with a very high yield (Figure 1, right). These ultrathin nanowires were often observed as random bundles, and no specific selfassembly was noticed. Formation of ultrathin nanorods was also observed, which is similar to ZnS15 and ZnSe cases.16,17 The diameter of the quantum wires generally increased as the chain length of the amines increased at a particular synthetic temperature. Under the synthetic conditions tested, no formation of nanowires was observed in a single type of fatty amine with its chain length longer than 14 carbons. Interestingly, it was possible to form wires by mixing a longchain amine with a relatively short-chain one (Figure 3, bottom) in our reaction condition. When the short chain amine had a dominating concentration over the long chain one, the quantum wires typically had small diameters (Figure 722
Figure 3. Top: quantum wires with different diameters as labeled and synthesized from the mixture of amines. Middle: a single quantum wire. Bottom: quantum wires formed with different dodecylamine (DDA) and octadecylamine (ODA) ratios. The scale bars are 10 nm (top), 0.2 µm (middle), and 50 nm (bottom).
3, bottom left). Significantly thicker quantum wires were dominating the sample if more long-chain amines presented in the solution (Figure 3, bottom right). An equal mixture of the long-chain and short-chain amines yielded a more or less equal amount of quantum wires with two diameters (Figure 3, bottom middle). However, we could not quantitatively estimate the exact percentage of nanowires with different diameters because the lengths of the nanowires varied substantially in a given sample. When exclusively octylamine (or decylamine) is taken, only thin nanowires (either bundled or scattered) were observed, but in the presence of octadecylamine with octylamine, ∼5 nm diameter nanowires are obtained along with thin nanowires. A table summarizing the synthetic conditions and resulting nanowires is provided as Supporting Information. The chain length of the amines also affects the selfassembly of the nanowires (Figure 3). As mentioned above, very small nanowires, such as the ones in Figure 1 (right panel, about 1.5 nm in diameter), did not form regular selfassembly patterns. Wires larger than 2 nm in diameter and/ or amines with greater than 12-carbon chain lengths can form regular patterns on TEM substrates (Figure 3, top). However, because of their substantial length, such assembly only occurred locally. The distance between two adjacent wires when they were closely packed seemed to be related to the diameter of the wires. The longer the hydrocarbon chain of the amines was, the wider they were separated (Figure 4, top, and Figure 3, top). Again, this self-assembly was observed as local events, and the long and bended nature of the nanowires made separation distances between wires varied in places (Supporting Information, Figure S2). All solid nanowires were found to be single-crystalline with wurtzite crystal structure (Figure 4) although they were grown in the typical zinc blende preferred temperatures, between 120 and 180 °C. The long axis of these quantum wires were found to be the c axis of the wurtzite structure, which is the same as the quantum rods formed under high Nano Lett., Vol. 6, No. 4, 2006
Figure 4. High-resolution TEM images of CdSe quantum wires. Bottom inserts (A-D) are FFT patterns of corresponding places marked on the nanowire. These nanowires are formed in a mixture of DDA and HDA.
temperatures.4,5,17 A similar orientation was also observed for the catalyst-promoted growth of CdSe wires in both solution9 and the gas phase.25 In literature, all magic-sized clusters for CdSe22 and other II-VI compounds26 with determined structures were zinc blende packed. If this was the case, then the oriented attachment observed here should have gone through a phase transition in terms of atomic packing in the third stage, from prewire aggregates (presumably zinc blende packing) to solid-state wires (determined to be wurtzite). Such a phase transition might have also occurred for the formation of CdSe nanorods under high temperatures5 and also the formation of CdTe nanowires through oriented attachment.19 Magicsized clusters were found as the common initial products for all reactions studied no matter what diameter the final wires had. However, relatively largely sized dot-shaped nanocrystals were also observed along with the solid wires with similar diameters. The experimental results obtained so far cannot clearly identify the role of these nanocrystals. In summary, we presented a low-temperature synthetic route to CdSe quantum wires with tunable wire diameters in the strong quantum confinement size regime, between 1.5 and 6 nm. Convincing evidence was observed for formation of these quantum wires through oriented attachment. Different from existing examples for oriented attachment, the relatively weak interaction between the magic-sized cluster precursors allowed us to reveal further insight of oriented attachment. Reversible association of magic-sized clusters in prewire aggregates was observed, and coexistence of Nano Lett., Vol. 6, No. 4, 2006
loosely associated nanoclusters and chemically fused fragments of nanowires were evidenced. Experimental Section. Synthesis of Pearl-NecklaceShaped Nanowires. Stable and well designed pearl-necklaceshaped nanowires were synthesized using octylamine or oleylamine (70%) as the solvent. In a typical reaction, 2.5 g oleylamine (or 2.0 g octylamine) and 0.02 g cadmium acetate are loaded in a vial with septum (or three-necked flask) and degassed by purging Argon. No rigorous inert atmosphere is necessary in this case. A separate stock solution of 0.076 g selenourea and 1.4 g oleylamine (or 1.0 g octylamine) is prepared in another vial (details are provided in the Supporting Information) and 0.2 g is injected to previously prepared cadmium acetate and amine solution at 100 °C. The reaction turned yellow and then slowly orange. The reaction is monitored using UV-vis spectra as reported in Figure 2. After 2 h of the reaction, most of the dots were aligned but this stage is stable until 8-10 h at 120 °C. Finally it is precipitated with methanol and centrifuged. These are dissolved several times and precipitated with hexane and methanol to remove excess ligands and finally dissolved in toluene for TEM analysis. The solution sometimes needs sonication to give a clear solution. Synthesis of Multiwidth Nanowires. For multiwidth nanowires, two amines are taken in the reaction system. In a typical reaction a mixure of 1.0 g dodecylamine, 1.0 g octadecylamine, and 0.02 g cadmium acetate are loaded in a vial (or three-necked flask), degassed, and heated to 120 °C to make a clear solution or heated through a heat gun. A stock solution of 0.076 g selenourea is prepared in 3 g dodecylamine, and 0.8 g from this stock solution is injected to cadmium solution at 120 °C. The reaction turned yellow to red very fast and maintained at 120 °C for hours with slow stirring. After 6 h of reaction, two separate width nanowires are formed along with some free spherical nanodots. The reaction is increased to 140-150 °C for 2030 min for annealing and precipitated with methanol. Details and additional specific information are provided in Supporting Information. Acknowledgment. N.P. acknowledges Professor Shlomo Efrima (1949-2005) for the initial discussion of CdSe nanocrystals using such a method. Financial support from the National Science Foundation and Wisconsin Alumni Research Foundation are acknowledged. Supporting Information Available: Details of the synthetic procedure and additional supporting results. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Brennan, J. G.; Siegrist, T.; Carroll, P. J.; Stuczynski, S. M.; Brus, L. E.; Steigerwald, M. L. J. Am. Chem. Soc. 1989, 111, 4141-3. (2) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-15. (3) Peng, X.; Manna, U.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Allvisatos, A. P. Nature (London) 2000, 404, 59-61. (4) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700-12706. 723
(5) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389-1395. (6) Bunge, S. D.; Krueger, K. M.; Boyle, T. J.; Rodriguez, M. A.; Headley, T. J.; Colvin, V. L. J. Mater. Chem. 2003, 13, 1705-1709. (7) Battaglia, D.; Li, J. J.; Wang, Y.; Peng, X. Angew. Chem., Int. Ed. 2003, 42, 5035-5039. (8) Tang, K.-b.; Qian, Y.-t.; Zeng, J.-h.; Yang, X.-g. AdV. Mater. (Weinheim, Ger.) 2003, 15, 448-450. (9) Grebinski, J. W.; Hull, K. L.; Zhang, J.; Kosel, T. H.; Kuno, M. Chem. Mater. 2004, 16, 5260-5272. (10) Yu, H.; Li, J.; Loomis, R. A.; Gibbons, P. C.; Wang, L.-W.; Buhro, W. E. J. Am. Chem. Soc. 2003, 125, 16168-16169. (11) Pena, D. J.; Mbindyo, J. K. N.; Carado, A. J.; Mallouk, T. E.; Keating, C. D.; Razavi, B.; Mayer, T. S. J. Phys. Chem. B 2002, 106, 74587462. (12) Xu, D.; Chen, D.; Xu, Y.; Shi, X.; Guo, G.; Gui, L.; Tang, Y. Pure Appl. Chem. 2000, 72, 127-135. (13) Thoma, S. G.; Sanchez, A.; Provencio, P. P.; Abrams, B. L.; Wilcoxon, J. P. J. Am. Chem. Soc. 2005, 127, 7611-7614. (14) Artemyev, M.; Moeller, B.; Woggon, U. Nano Lett. 2003, 3, 509512. (15) Pradhan, N.; Efrima, S. J. Phys. Chem. B 2004, 108, 11964-11970.
724
(16) Panda, A. B.; Acharya, S.; Efrima, S. AdV. Mater. (Weinheim, Ger.) 2005, 17, 2471-2474. (17) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296-1306. (18) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969-971. (19) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237-240. (20) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140-7147. (21) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188-1191. (22) Peng, Z. A.; Peng, X. J. Am. Chem. So. 2002, 124, 3343-3353. (23) Osterloh, F. E.; Martino, J. S.; Hiramatsu, H.; Hewitt, D. P. Nano Lett. 2003, 3, 125-129. (24) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 28542860. (25) Ma, C.; Wang, Z. L. AdV. Mater. (Weinheim, Ger.) 2005, 17, 26352639. (26) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S.-I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662-5670.
NL052497M
Nano Lett., Vol. 6, No. 4, 2006
