NANO LETTERS
Connecting Nanowires Consisting of Au55 with Model Electrodes
2002 Vol. 2, No. 10 1097-1099
Nan Lu,† Jiwen Zheng, Michael Gleiche, Harald Fuchs, and Lifeng Chi* Physikalisches Institut, Westfa¨ lische Wilhelms-UniVersita¨ t Mu¨ nster, Wilhelm-Klemm-Strasse 10, 48149 Mu¨ nster, Germany
Olivia Vidoni, Torsten Reuter, and Gu1 nter Schmid Institut fu¨ r Anorganische Chemie, UniVersita¨ t Essen, UniVersita¨ tsstrasse 5-7, 45117 Essen, Germany Received July 18, 2002; Revised Manuscript Received July 26, 2002
ABSTRACT We demonstrate an approach to connect nanowires consisting of ligand-stabilized Au55 clusters with metal arrays prepared by using metal evaporation through a mask of monodispersed latex beads. The metal arrays serve as a model for micro- or nanoelectrodes, depending on the size of the beads. The silicon surfaces bearing such model electrodes were used as substrates for transferring nanowires consisting of Au55 prepared with the Langmuir−Blodgett technique on a poly(vinylpyrrolidone) (PVP) subphase. Single connection and multi-connection are obtained by controlling the structure density on surface.
Metallic and semiconductive nanoclusters have been studied extensively because of their special optical1,2 and electronic3,4 properties, which are caused by quantum effects resulting from their restricted size. Nanoclusters are expected to play an important role in future nanoelectronics. Their two- (2D) and one-dimensional (1D) arrangements have thus received significant attention over a number of years.5-10 Among these nanoparticles, ligand-stablized gold nanoclusters, Au55, which consist of a 1.4 nm gold core enveloped by an organic shell from 0.35 to 2.8 nm have been found to act as single-electron transistors, even at room temperature.11 In the past years the electrical properties and 2D arrangements of nanoclusters have been studied extensively.12 However, the 1D arrangement of clusters and the interconnection of 1D structures with external electrodes in simple and reliable ways is still a great challenge. In our previous work10 we presented the results of generating 2D networks of Au55 nanoclusters based on chainlike structures prepared with the Langmuir-Blodgett technique, and the successful transfer of the structures on various planar solid substrates. Here we report the connection of such structures with model electrodes. To prepare chainlike and network structures of Au55, we spread a solution of 3.8 × 10-6 mol/L Au55(PPh3)12Cl613 clusters, which are dissolved in dichloromethane (Fluka), onto the surface of an aqueous solution containing 1 mg/L * Corresponding author. E-mail:
[email protected]. Tel: +49 (0)251 83-33651. Fax: +49 (0)251 83-33602. † Permanent address: Chemistry College, Jilin University, 130023, Changchun, P.R. China. 10.1021/nl025710c CCC: $22.00 Published on Web 09/12/2002
© 2002 American Chemical Society
Figure 1. SFM topographical image (3D presentation) of chainlike and network structures of Au55 on Si/SiO2 surface (1.5 µm × 1.5 µm).
poly(vinylyirrolidone) (PVP) (-K30). Silicon is used as the substrate and cleaned with the following procedure. First, silicon wafers (WaferNet Co., type N, 0.5 mm thick, orientation 〈100〉 , resistivity 2-5 Ωcm) were ultrasonicated in acetone, chloroform, ethanol, and deionized water (Millipore, resistance 18.2 MΩcm) successively for 10 min, then treated with oxygen plasma (Templa System 100-E plasma system) for 2 min at a power of 300 W and rinsed with water. This treatment made the silicon surfaces hydrophilic. The experiments were carried out with a standard Langmuir-
Figure 2. SFM topographical images of (a) silicon surface patterned with latex spheres (6.4 µm × 6.4 µm); (b) metal arrays (as model electrodes) fabricated by evaporating Cr and Au with the latex spheres as the mask (1.5 µm × 1.5 µm).
Blodgett device (Nima 611M). After evaporation of the solvent, the surface was slowly compressed to a certain target surface pressure. The assembled structures on the surface of the aqueous subphase can be transferred onto various solid substrates such as mica, silicon wafer, and glass, as shown in Figure 1. Numerous LB transfer experiments indicated the reproducibility of the procedure. The cluster concentration and spreading volume of the cluster solution, target pressure, compressing speed, and transfer speed turned out to be critical parameters for obtaining cluster chains and networks. The results were summarized in our previous work.10 As the second step, we have transferred these structures (2D networks and quasi 1D chains of Au55 clusters) onto silicon surfaces with prefabricated microelectrodes. Due to the experimental limitation of fabricating real electrodes in our own laboratory and the demand for a large amount of samples to optimize the experimental conditions, we used the so-called natural nanolithography14,15 to fabricate model electrodes. We prepared monolayers of monodisperse latex spheres with diameters of 439 nm on silicon surfaces and used them as mask to evaporate metal arrays. Regular patterned latex spheres on silicon surfaces can be created (Figure 2a) after dropping diluted latex solution and evaporating the solvent. Model electrodes were prepared by thermal 1098
Figure 3. SFM topographical image of (a) network (800 nm × 800 nm) and (b) nanowires of Au55 connected with model electrodes (350 nm × 350 nm).
evaporation (Edwards E306) of Cr and Au in high vacuum through the latex mask sequentially. After removing the latex beads, only the triangle electrodes were left on the silicon surface (Figure 2b). The thickness of the metal islands can be adjusted. The minimum distance between the metal islands is about 20 nm. The LB transfer on metal patterned silicon surfaces is different from that on unpatterned surfaces. The surface cleanliness and the height of the metal patterns turned out to be critical to achieve a successful transfer. By using the clean silicon substrate with model electrodes with heights of 12.5 nm, we transferred structures, as shown in Figure 3. Both networks (Figure 3a) and single chains (Figure 3b) were observed. The target pressure was 3.0 mN/m. On unpatterned Si/SiO2 surfaces with this pressure, we obtained only networks. To obtain noninterlinked chainlike structures, lower pressures (ca. 1.0 mN/m) have to be used if the transfer is carried out on planar Si/SiO2 surfaces. It seems that the local pressure at the contact line between the monolayer covered aqueous surface and the metal array patterned Si/ SiO2 surface is lower than the read-out pressure measured by surface pressure sensor. This implies that the probability Nano Lett., Vol. 2, No. 10, 2002
of obtaining single-chain or network structures on a patterned surface is not very sensitive to the target pressure. In fact, even at a target pressure of 5.0 mN/m, in addition to the pure networks that were found on planar Si/SiO2 surfaces as reported in our previous paper,10 we also find single-chain structures between the model electrodes in the present system. The effect might be induced by the local pressure variation due to the presence of metal patterns through (a) geometry factor and (b) different surface properties of Au and Si/SiO2 (the Au surface is hydrophobic after storing freshly evaporated Au in air for some hours, whereas the Si/SiO2 surface remains hydrophilic). To get only single connections, a target pressure lower than 3.0 mN/m should be used. As observed in Figure 3, the chainlike structures can connect two model electrodes separated by a distance of several tens of nanometers, while network structures link more metal islands. The latter shows its similarity to neuron synapses growing on electronics.16 Although we cannot prove right now the connection properties at the interface between the model metal electrodes and the created structures consisted of Au55 and PVP, an electrical contact is plausible through the water bridge: the polymer PVP is water soluble, and a certain amount of water may still exist after the structure is deposited.17 To study the electron-transfer properties, real electrodes have to be used. The experiences and experimental conditions we gained from this work can be transferred to the further studies. We demonstrate here that networks and chainlike structures consisting of Au55(PPh3)12Cl6 clusters, generated by LB preparation with the polymer PVP in the subphase, can be transferred onto a Si/SiO2 surface with prefabricated model
Nano Lett., Vol. 2, No. 10, 2002
Au electrodes, with a high probability that the model electrodes are connected by the structures. It provides an alternative approach and a promising route for connecting nanomaterials with addressable micro/nanoelectrodes. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft (DFG) and the state of North-Rhine Westphalia (NRW) for the financial support. References (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (2) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (3) Scho¨nenberger, C.; van Houten, H.; Donkersloot, H. C. Europhys. Lett. 1992, 20, 249. (4) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, M. S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323. (5) Liu, Y.; Schmann, M.; Raschke, T.; Radehaus, C.; Schmid, G. Nano Lett. 2001, 1, 405. (6) Wyrwa, D.; Beyer, N.; Schmid, G. Nano Lett. 2002, 2, 419. (7) Vidoni, O.; Reuter, T.; Torma, V.; Meyer-Zaika, W.; Schmid, G. J. Mater. Chem. 2001, 11, 3188. (8) Hoeppener, S.; Chi, L. F.; Fuchs, H. Nano Lett. 2002, 2, 459. (9) Okawa, Y.; Aono, M. Nature 2001, 409, 683. (10) Reuter, T.; Vidoni, O.; Torma, V.; Schmid, G.; Lu, N.; Gleiche, M.; Chi, L. F. Nano Lett. 2002, 2, 709. (11) Chi, L. F.; Hartig, M.; Drechsler, T.; Schwaack, Th.; Seidel, C.; Fuchs, H.; Schmid, G. Appl. Phys. A 1998, A66, 187. (12) Schmid, G.; Chi, L. F. AdV. Mater. 1998, 10, 515. (13) Schmid, G. Inorg. Synth. 1990, 835. (14) Fischer, U. C.; Zingsheim, H. P. J. Vac. Sci. Technol. 1981, 19, 1881. (15) Winzer, M.; Kleiber, M.; Dix, N.; Wiesendanger, R. Appl. Phys. A 1996, 63, 617. (16) Fromherz, P.; Offenha¨usser, A.; Vetter, T.; Weis, J. Science 1991, 252, 1290. (17) Johnston, R. R. Ph.D. Thesis, 1992, Mainz University.
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