NANO LETTERS
Two-Dimensional Networks via Quasi One-Dimensional Arrangements of Gold Clusters
2002 Vol. 2, No. 7 709-711
Torsten Reuter, Olivia Vidoni, Viktoria Torma, and Gu1 nter Schmid* Institut fu¨ r Anorganische Chemie, UniVersita¨ t Essen, UniVersita¨ tsstrasse 5-7, 45117 Essen, Germany
Lu Nan, Michael Gleiche, Lifeng Chi,* and Harald Fuchs Physikalisches Institut, UniVersita¨ t Mu¨ nster, Wilhelm-Klemm-Strasse 9, 48149 Mu¨ nster, Germany Received May 14, 2002; Revised Manuscript Received May 22, 2002
ABSTRACT A new approach to one-dimensionally organize the quantum dot Au55(PPh3)12Cl6 is described by using poly(vinyl-pyrrolidone) (PVP) molecules as templates. AFM investigations indicate chemisorption of the nanoclusters at the surface of the polymers. Using Langmuir−Blodgett techniques, networks of cluster-loaded polymer chains on mica or silicon are generated. The pattern of the networks can be designed from widely distributed but often interconnecting filaments, up to almost densely packed structures, depending on the surface pressure. Those networks are rigid enough to be transferred from the water surface onto solid substrates without degradation. Bare PVP molecules do not give comparable networks under equivalent conditions, indicating that the nanoclusters act as junctions between the chains.
Introduction The application of nanoparticles strongly depends on their assembly in ordered one- or two-dimensional arrangements. Several reports concerning the fabrication of closed packed monolayers of nanoscaled particles have been published; however, only a few examples exist concerning their formation in linear chains.1-3 This kind of organization may play an important role in electronic devices and other applications. For the generation of such chains, the use of templates supporting the arrangement of the particles is attractive. As shown by Brust et al., an artificial and linear template can be used for the self-assembly of gold nanowires.3 After evaporation of the solvent, the gold clusters spontaneously form parallel rows of particles. Fitzmaurice et al. were able to decorate carbon nanotubes up to 10 µm with gold particles.4 Heath et al. observed the spontaneous alignment of silver particles to wires of the length of several µm and 20 to 300 nm in width at the water/air interface.5 Hoppener et al. reported the achievement of linearly arranged Au55 cluster strands by using molecular templates.6 Here we report the elaboration of a quasi one-dimensional arrangement of the gold clusters Au55(PPh3)12Cl6 [Au55]7 in the presence of the water-soluble polymer poly(vinyl* Corresponding authors. E-mail:
[email protected]; chi@ nwz.uni-muenster.de. 10.1021/nl025608f CCC: $22.00 Published on Web 06/05/2002
© 2002 American Chemical Society
pyrrolidone) (PVP) at the water/air interface. The wires (30 nm in width and up to 1 µm in length) are connected by junctions of cluster islands to a complete two-dimensional network. This structure can be moved to various substrates such as mica or silicon.
Experimental Section The synthesis of Au55(PPh3)12Cl6 is described elsewhere.7 The experiments were carried out with a standard LangmuirBlodgett device (Nima 6100). A typical experiment was as follows: a solution of 350 µL of Au55(PPh3)12Cl6, dissolved in dichloromethane (Fluka, p.a.) (3.8 × 10-6 M) was spread carefully on the water surface containing poly(vinyl-pyrrolidone) (molecular weight: 40.000)(1 mg/L). After evaporation of the solvent, the surface was slowly compressed up to a required surface pressure in the range of 2.5-10 mN m-1 for transferring the monolayer at different surface pressures. The assembled structure on the surface was transferred onto the substrate with a speed of 1 mm min-1. Silicon substrates (WaferNet Co., Type N, 0.5 mm thick, orientation 〈100〉, resistivity 2-5 Ohm cm) were ultrasonicated successively in acetone, chloroform, ethanol, and water, respectively for 10 min. Then they were rinsed with water (Millipore resistivity 18.2 MΩ cm) and blown dry with high-purity nitrogen. Finally they were treated with oxygen plasma for
Figure 1. (a) AFM image (1.6 × 1.6 µm2) of a network of Au55/ PVP transferred onto mica surface; (b) cross section.
2 min at 300 W (Templa System 100-E plasma system) and rinsed again with deionized water. Due to these treatments, the silicon surface became fully hydrophilic. Mica (PLANO) was freshly cleaved and rinsed with deionized water just before usage. Characterization was achieved by atomic force microscopy (AFM digital instruments, Nanoscope IIIa, Dimension 3000) and transmission electronic microscopy (Philips CM 2000 FEG at 200 kV).
Results and Discussion Recently, we succeeded in the generation of a close-packed 2D arrangement of gold clusters at the water/dichloromethane interface.8 The organization process was supported by an additive that assembles at the phase boundary. It stabilizes the formed monolayer of gold clusters and assists the transfer to a solid substrate. Without additives, smaller islands of well 710
Figure 2. AFM images of Au55/PVP, transferred onto mica with different surface pressures. (a) 2.5 mN m-1, (b) 4 mN m-1, (c) 10 mN m-1.
ordered Au55(PPh3)12Cl6 are formed as well at the water/air phase boundary.9 In this work, a completely networked twoNano Lett., Vol. 2, No. 7, 2002
dimensional structure out of quasi one-dimensional gold cluster wires was generated at the water/air phase boundary. It was received in the presence of the water-soluble additive poly(vinyl-pyrrolidone) using the Langmuir-Blodgett technique. The LB trough was filled with an aqueous solution of PVP and a very dilute solution of the clusters was spread on the surface. After evaporation of the solvent the substrate, generally mica or silicon, was pulled-out with a speed of 1 mm min-1. A typical assembly obtained by this procedure is displayed in Figure 1a. The network obtained consisted of wires of 30 nm in width, branched and connecting each other. The thickness of the structures was measured to be 2.5 nm (Figure 1b), which corresponds well with the size of the Au55(PPh3)12Cl6 cluster, including the ligand shell of 2.2-2.3 nm and the supporting PVP layer. The evolution of the network starts with single wires at low surface pressure and ends up in a network at higher pressures. This is shown by samples prepared at different values of surface pressure (Figure 2). At low surface pressure (Figure 2a), the wires on the surface seem to be widely distributed and not very well packed. They were transferred with similar thickness and width as at high surface-pressure transfer. An increase of the surface pressure induces only a higher density of the wires without changing of their structural aspect. The higher the pressue, the denser the coverage of a definite surface area. This leads to a connection of the wires into a network by higher pressure and higher coverage (Figure 2c). Some junctions consist of small islands of gold clusters. This network structure, as mentioned above, is rigid enough to stay intact during the transfer, so we were able to cover areas of several tens of micrometers with a homogeneous network. Furthermore, not only is the possible size of the transferred area promising, but the fact that the transfer can be performed on different substrates is also encouraging. At this moment, we successfully tried out mica and silicon. For silicon and mica we received the same network structure, as can be seen from Figure 2 and Figure 3. To determine the causing of the formation of networks described above, control experiments were made with the single components. Obtaining a network of the polymer without clusters was unsuccessful because no structure was detectable on the surface. Spreading a thin film of a Au55 solution in dichloromethane on the surface of pure water without polymer built up well-ordered closed layers but no wires.2,9 This indicates that the network may be caused by specific interactions between the polymer and the gold clusters, which need to be proved by further work. In this work we have reported the elaboration of quasi one-dimensional arrangements of Au55(PPh3)12Cl6 clusters
Nano Lett., Vol. 2, No. 7, 2002
Figure 3. AFM image of a network transferred onto silicon wafer substrate with a pressure of 5 mN m-1.
supported by a thin layer of poly(vinyl-pyrrolidone) using the Langmuir-Blodgett technique. The length of these wires is around 1 µm, but can be longer. The wires form a network of quantum dots, which is completely interconnected. We have also shown that it is possible to transfer this network onto different substrates (mica, silicon) without structure modification. Further measurements are now necessary to have a better understanding of those networks. Acknowledgment. G.S. and L.Ch. acknowledge support by the Deutsche Forschungsgemeinschaft (DFG, SPP 1072); G.S. achnowledges further help by the DFG-Graduiertenkolleg (689/1) and the Fonds der Chemischen Industrie, Frankfurt, Germany. References (1) Schmid, G.; Liu, Y.-P.; Schumann, M.; Raschke, Th.; Radehaus, Ch. Nano Lett. 2001, 1, 405. (2) Vidoni, O.; Reuter, T.; Torma, V.; Meyer-Zaika, W.; Schmid, G. J. Mater. Chem. 2001, 11, 3188. (3) Hutchinson, T.; Liu, Y.; Kiely, C.; Brust, M. AdV. Mater. 2001, 13, 1800. (4) Fullam, S.; Cottel, D.; Rensmo, H.; Fitzmaurice, D. AdV. Mater. 2000, 12, 1430. (5) Chung, S. W.; Markovich, G.; Heath, J. R. J. Phys. Chem. B 1998, 102, 6686. (6) Hoeppener, S.; Chi, L. F.; Fuchs, H. Nano Lett. 2002, 2, 459. (7) Schmid, G. Inorg. Synth. 1990, 7, 214. (8) Schmid, G.; Beyer, N. Eur. J. Inorg. Chem. 2000, 835. (9) Chi, L. F.; Rakers, S.; Hartig, M.; Oleiche, M.; Fuchs, H. Colloids Clusters 2000, 171, 241.
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