NANO LETTERS
One-Dimensional Arrangements of Metal Nanoclusters†
2002 Vol. 2, No. 4 419-421
Daniel Wyrwa, Norbert Beyer, and Gu2 nter Schmid* Institute of Inorganic Chemistry, UniVersity of Essen, UniVersita¨ tsstrasse 5-7, 45117 Essen, Germany Received December 20, 2001; Revised Manuscript Received January 16, 2002
ABSTRACT A promising route to strictly one-dimensional arrangements of Au55(PPh3)12Cl6 clusters is described. On the basis of the successful generation of ordered two-dimensional cluster architectures by self-assembly processes at the phase boundary between water and dichloromethane, the formation of single cluster rows could be initiated by dilution effects. Concentrations of 0.5 µg/L of isooctyl-substituted poly(paraphenylenethinylen) (PPE-i-octyl) in CH2Cl2 lead to thin bundles of the polymer molecules that are partially decorated with a few, but also with single, rows of the [Au55] quantum dots and that can be collected from the water surface after evaporation of the dichloromethane. TEM investigations have been applied to characterize the hybrid systems.
Low-dimensional assemblies of nanoparticles are becoming of increasing interest if they behave as quantum dots (QDs). Two- and one-dimensional QD arrangements are expected to play a dominant role in future nanoelectronics, above all in storage devices. An ideal and easily available quantum dot is Au55(PPh3)12Cl6, [Au55], a 1.4 nm metal particle, enveloped by a shell of PPh3 molecules and some chlorine atoms, enlarging it to the total size of 2.2-2.3 nm. Currentvoltage (I-U) studies of individual [Au55] clusters exhibited their capability to act as single electron transistors (SET) even at room temperature.1 Perfectly ordered two-dimensional (2D) arrangements of these QDs have become available routinely2 and first electrical studies showed them to behave as quantum wells.3 Quantum behavior was also observed for very short cluster chains (6-10 clusters) between electrodes.4 However, inherently stable, onedimensional arrays of these QDs could not be generated up to now, except for quasi one-dimensional stripes of 3-4 rows of [Au55] clusters, which could recently be fabricated by us using a modified Langmuir-Blodgett technique.5 The formation of the above-mentioned ordered monolayers of [Au55] clusters is performed at the phase boundary between water and dichloromethane, supported by a thin film of appropriate polymer molecules, as described in ref 2. As it turned out, those polymers showing weak interactions with the ligand shells, here it is PPh3, gave best results. Variously ortho-substituted poly(p-phenyleneethynylenes) (PPEs) (substituents were -CO2C8H17, PPE-i-oct, and -CO2CH2C6H5, PPE-benz)6 showed the best conditions to chemisorb the † This paper is dedicated to Prof. Joachim Stra ¨ hle on the occasion of his 65th birthday. * Corresponding author. E-mail address:
[email protected]. Phone: +49 201 183 2401. Fax: +49 201 183 4195.
10.1021/nl0157086 CCC: $22.00 Published on Web 02/06/2002
© 2002 American Chemical Society
aromatic rings on the cluster surfaces due to π-π interactions, weak enough to let the clusters order themselves. Furthermore, they tend to form organized structures of linearly oriented molecules.7,8 Our strategy to come from 2D to 1D cluster assemblies on PPE molecules is simply based on dilution effects. Following the procedure described in ref 2, a clean water surface was covered with a very thin film of dichloromethane, where the cluster and the polymer were dissolved. After slow evaporation of CH2Cl2, a TEM grid was dipped underneath the surface, carefully drawn back and dried in air for about 30 min. The formed structures were then studied by high-resolution transmission electron microscopy (TEM). Figures 1-4 show the development of 1D structures from 2D networks by reduction of the concentration of PPE and [Au55] from 1 mg/L via 1 µg/L to 0.5 µg/L. Figure 1 shows a cutout of a predominantly ordered cluster monolayer on PPE-benz of the hexagonal structure, produced from CH2Cl2 solutions of 1 mg/L PPE-benz and [Au55] each. The real extension of that monolayer was about 1 µm2. PPEMe and PPE-i-oct also support formation of monolayers, but usually those of smaller dimensions. The reason may be that the phenyl groups of the benzyl substituents in PPE-benz support the interaction with the clusters additionally. This also makes it understandable that PPE-i-oct gives better results with respect to 1D structures than PPE-benz does. A dichloromethane film of 1 µg/L [Au55] and PPE-i-oct each on the water surface resulted in structures as shown in Figure 2. Formation of bands of polymer molecules, hundreds of nanometers long and all covered with clusters, could be observed.
Figure 1. TEM of a cutout of a closed hexagonally ordered monolayer of Au55(PPh3)12Cl6 on a PPE-benz film, produced from a solution of a 1 mg/L cluster and polymer, respectively.
Figure 3. TEM image of a string of PPE-i-oct, decorated with two chains of [Au55] clusters and formed from a 0.5 µg/L solution.
Figure 2. Dilution to 1 µg/L gives broad bands of PPE-i-oct, randomly covered by [Au55] clusters.
Reducing the concentration of both components to 0.5 µg/L resulted in structures with partially perfectly onedimensionally arranged [Au55] clusters along bundles of polymers. These are still relatively thick, 10-12 nm in diameter. Despite that, many of them are decorated with only a single wire, some with up to 3 or 4 rows. Figure 3 shows a TEM image of a ca. 250 nm long bundle of PPE-i-oct polymer molecules, decorated by mainly two cluster wires. From the magnified parts it can be seen that there are almost no defects along the observed distance. Figure 4 finally shows one of the combinations with definitely a single cluster wire of about 450 nm in length. The question, if such a cluster wire on PPE can be used for electrical measurements, the final goal of these investigations, cannot yet be answered. We just began to prepare corresponding experiments in collaboration with physical laboratories. However, the case of solving the contact problems these hybrid systems might give us first insight into electron transport mechanisms in one-dimensional quantum dot arrangements at room temperature. It is especially the question if the conductivity of the PPE molecules will not suppress the superimposed electronic structure of the QD wires. 420
Figure 4. TEM image of single cluster wire on a ca. 250 mm long string of PPE-i-oct (0.5 µg/L).
Acknowledgment. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (SFB 452 and Graduiertenkolleg 689/1). We also thank the Fonds der Chemischen Industrie, Frankfurt, for the continuous and generous support. References (1) (2) (3) (4)
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(5) Vidoni, O.; Reuter, T.; Torma, V.; Meyer-Zaika, W.; Schmid, G. J. Mater. Chem. 2001, 11, 3188-3190. (6) Heitz, W. Makromol. Chem., Macromol. Symp. 1991, 48/49, 15. (7) Samori, P.; Francke, V.; Mu¨llen, K.; Rabe, J. P. Chem. Eur. J. 1999, 5, 2312.
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(8) Schnablegger, H.; Antonietti, M.; Go¨ltner, Ch.; Hartmann, J.; Co¨lfen, H.; Samori, P.; Rabe, J. P.; Ha¨ger, H.; Heitz, W. J. Colloid Interface Sci. 1999, 212, 24.
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