© Copyright 1999 by the American Chemical Society
VOLUME 103, NUMBER 12, MARCH 25, 1999
LETTERS Electrostatic Self-Assembly of Highly-Uniform Micrometer-Thick Fullerene Films Yanjing Liu,†,‡ Youxiong Wang,† Hongxia Lu,† and Richard O. Claus*,†,§ Fiber & Electrooptics Research Center, The Bradley Department of Electrical and Computer Engineering, Department of Material Science and Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061-0356, and Nanosonic Incorporated, 509 Rose AVenue, Blacksburg, Virginia 24060 ReceiVed: October 23, 1998; In Final Form: February 2, 1999
The layer-by-layer electrostatic self-assembly monolayer (ESAM) method has been successfully utilized to fabricate ultrathin multilayer films of fullerenes. Highly uniform, transparent, micrometer-thick fullerene films have been fabricated and characterized by UV-vis spectroscopy, ellipsometry, and atomic force microscopy.
Since the first demonstration of fullerenes in 1985,1 there has been an explosion of research activity into the unique physical and chemical properties of this new material form. Extensive studies have been carried out to obtain thin fullerene films with two- and three-dimensionally ordered structures. These studies indicate epitaxial films formed under vapor sublimation,2 Langmuir-Blodgett (LB) films,3 chemically self-assembled films,4 and sol-gel films.5 Much of this activity is motivated by the important role that high-quality films can play in furthering a general understanding of fullerenes and leading to potential applications.6 The sublimation films of fullerenes have been reported to have close-packing structures and relatively low roughness.6,7 Poor-quality LB films, however, have been reported by many groups.8 The reported chemically selfassembled films consist of only a few layers due to the requirement of high-yield chemical reactions for the film fabrication process. In this paper, we report that fullerenes can be electrostatically self-assembled9 into multilayer films in a layer-by-layer fashion at room temperature. Specifically, this study shows that it is possible to fabricate more than 200 bilayers (>1 µm in total thickness) of fullerene films with excellent homogeneity. † The Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University. ‡ Nanosonic Inc. § Department of Material Science and Engineering, Virginia Polytechnic Institute and State University.
The fullerenes, a C60 and C70 mixture (fullerite, 9:1), and poly(diallyldimethylammonium, chloride) (PDDA) were obtained from Aldrich. The hydroxylated fullerenes, fullerols, were synthesized as previously reported.10 The fabrication of fullerene films was carried out as follows. The substrates used were single-crystal silicon (p-100), glass, and quartz slides. In each case, the slide was first immersed in a 1% (w/w) aqueous solution of cationic PDDA (pH 9) for 3 min, then rinsed extensively with water (pH 9). Subsequently, the slide was dipped into an aqueous solution of anionic fullerol (1.0 × 10-4 mol/L, pH 9) for 5 min, followed by thorough washing with water (pH 9). By repeating the above procedure in an alternating fashion, multilayer assemblies of fullerol/PDDA thin films were fabricated on the substrates. Optical UV-vis spectroscopy (Hitachi U-2010) was used to characterize the growth of the multilayer structures and the relative amounts of material deposited per layer. Figure 1 shows the absorption spectra of the multilayer (from 10 to 80 bilayers) fullerol films. As shown in this figure, the consecutive adsorption of layers is stepwise and the deposition process is linear and consistent from layer to layer. The 200 bilayer fullerol films are extremely uniform and display a homogeneous brown color across the entire substrate area, from the edges to the center. Direct measurements of the increase in the thickness of the grown fullerenol films by multiwavelength ellipsometry (Auto EL II-3W, Rudolph Technologies, Inc.) are shown in Figure 2.
10.1021/jp9841539 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/04/1999
2036 J. Phys. Chem. B, Vol. 103, No. 12, 1999
Letters
Figure 3. Taping-mode AFM images of two bilayers of ESAM fullerol film deposited on a single-crystal silicon substrate.
Figure 1. Optical absorption spectra of multilayer ESAM fullerenol films deposited on quartz substrate.
respectively. As the film thickness increases, it appears that the fullerol clusters form larger domed elements with increasing dimension. In summary, we have demonstrated that highly homogeneous and micrometer-thick fullerene films can be prepared using the ESAM method. This is the first example of the preparation of uniform fullerene films through solution chemistry, a “bottomup” method. The optical limiting properties of such films are now under detailed investigation. An analysis of the electrical, optical, and electrooptical properties of metal nanocluster/ fullerene ESAM films is also in progress. Acknowledgment. This work was supported in part by ARO Grant No. DAA G55-97-1-0101 and Air Force Contract No. F33615-98-C5421. References and Notes
Figure 2. Dependence of ellipsometrically determined thickness of ESAM fullerol films deposited on a single-crystal silicon substrate: odd numbers correspond to PDDA and even numbers to fullerenol.
Each data point is the average of at least 10 measurements taken at a minimum of 10 locations on the film. A linear increase in the thickness with the number of layers of the ESAM fullerenol films is observed, which indicates a narrow size distribution of the assembled fullerene clusters. The average thicknesses added per layer of PDDA and of fullerol are 6.1 and 54 Å, respectively. The layer-by-layer growth of the ESAM fullerenol films was also investigated using atomic force microscopy (AFM Nanoscopy III, Digital Instrument). An AFM image of two bilayers of a fullerol film is shown in Figure 3. Locally domed surface perturbations with an average diameter of about 20 nm are typical for such films. The average surface roughness values for one, two, and three bilayers is 8.5, 13.7, and 18.5 Å,
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