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Microtribologic Properties of a Covalently Attached Nanostructured Self-Assembly Film Fabricated from Fullerene Carboxylic Acid and Diazoresin Tingbing Cao, Fang Wei, Yanlian Yang, Lan Huang, Xinsheng Zhao, and Weixiao Cao* College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received March 4, 2002. In Final Form: April 16, 2002 A novel self-assembly ultrathin film was fabricated using a C60 carboxylic acid derivative and diazoresin through electrostatic interaction on different substrates. Following UV irradiation and the decomposition of the diazonium group, the bond nature of the film converted from ionic into covalent and the film got to be very stable. The microtribologic properties of the C60-containing film were investigated using atomic force microscopy/friction force microscopy devices, and the result showed that rigid C60 molecules in the film have load-bearing capacity. To increase the lubricative property of the film, a soft poly(acrylic acid) chain was incorporated into the film to form a ternary composite film and the polymer-bound C60 film showed not only good load-bearing but also low friction properties.
Introduction Buckminsterfullerene, C60, and its derivatives have drawn wide attention due to their unique molecular structure and unusual physical, chemical, and electronic properties.1,2 C60 molecules are near-perfect spheres, and in the solid crystal each molecule rotates at a high frequency while remaining within its lattice position.3 Much speculation has centered around this property as being conducive for good lubrication because the molecules could mimic ball bearings,4-6 but experimental observations of the lubricating properties of C60 films have been inconsistent.7 Numerous efforts have been devoted to the elaboration of well-ordered thin films containing C60, in particular using the Langmuir-Blodgett (LB) technique8-10 and self-assembly technique.11,12 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.13 The layer-by-layer self-assembly technique was rapidly developed because it is simple in procedure, easy to automate, and friendly to the environment.14,15 However, similar to the poor-quality LB films, the general self(1) Acc. Chem. Res. 1992, 25, 97 (special issue on buckminsterfullerenes). (2) Physics and Chemistry of Fullerenes; Stephens, P. W., Ed.; World Scientific: Singapore, 1993. (3) Luengo, G.; Campbell, S. E.; Srdanov, V. I.; Wudl, F.; Israelachvili, J. N. Chem. Mater. 1997, 9, 1166. (4) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354. (5) Bhushan, B.; Gupta, B. K. J. Appl. Phys. 1994, 75, 6156. (6) Schwarz, U. D.; Allers, W.; Gensterblum, G.; Wiesendanger, R. Phys. Rev. B 1995, 52, 14976. (7) Lee, S.; Shon, Y. S.; Lee, T. R.; Perry, S. S. Thin Solid Films 2000, 358, 152. (8) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. (9) Ravaine, S.; Mingotaud, C.; Delhaes, P. Thin Solid Films 1996, 284, 76. (10) Zhang, P. Y.; Lu, J. J.; Xue, Q. J.; Liu, W. M. Langmuir 2001, 17, 2143. (11) Liu, Y. J.; Wang, Y. X.; Lu, H. X.; Claus, R. O. J. Phys. Chem. B 1999, 103, 2035. (12) Zhang, W.; Shi, Y. R.; Gan, L. B.; Wu, N. Z.; Huang, C. H.; Wu, D. G. Langmuir 1999, 15, 6921. (13) Hebard, A. F.; Zhou, O.; Zhong, Q.; Flemming, R. Thin Solid Films 1995, 257, 147. (14) Decher, G. Science 1997, 277, 1232. (15) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1995, 11, 942.
assembly films are not stable toward polar solvents or wearing because of the Coulombic interaction between the layers. Recently, we revealed a kind of ultrathin film based on diazoresin (DR) using the layer-by-layer selfassembly technique; following the decomposition of the diazonium group of DR by UV irradiation, the selfassembly films were linked by covalent bonds with excellent properties against etching from polar solvents.16-19 In this article, we fabricated a self-assembly ultrathin film using a C60 carboxylic acid derivative (C63(COOH)6) and DR through electrostatic attraction in aqueous solution; following the decomposition of the diazonium group by UV irradiation, the bond nature between film layers was converted from ionic into covalent (as shown in Scheme 1). The covalently attached film is very stable toward polar solvents, and moreover it has better character to endure wear and tear. Consequently, the microtribologic properties can be measured for this kind of C60-containing self-assembly film. Experimental Section Materials. C63(COOH)6, DR, and poly(acrylic acid) (PAA) were synthesized according to refs 20-22 in our lab. Mn values of DR and PAA were determined to be about 2000 and 70 000 g/mol, respectively. Process to Fabricate Self-Assembly Film. DR was dissolved in deionized water, and C63(COOH)6 or poly(acrylic acid) was dissolved in a weak alkaline aqueous solution (pH ) 8) with the concentration of 0.5 mg/mL. To fabricate C63(COOH)6/DR film, the quartz (mica or CaF2 wafer) substrate was first immersed in DR solution for 5 min and then thoroughly rinsed with water and dried by flow air, followed by immersion in C63(COOH)6 solution for 5 min, rinsing, and drying to complete a fabrication (16) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 17, 1853. (17) Chen, J. Y.; Huang, L.; Ying, L. M.; Luo, G. B.; Zhao, X. S.; Cao, W. X. Langmuir 1999, 15, 7208. (18) Cao, T. B.; Chen, J. Y.; Yang, C. H.; Cao, W. X. Macromol. Rapid Commun. 2001, 22, 181. (19) Cao, T. B.; Chen, J. Y.; Yang, C. H.; Cao, W. X. New J. Chem. 2001, 25, 305. (20) (a) Bingel, C. Chem. Ber. 1993, 126, 1957. (b) Lamparth, I.; Hirsch, A. Chem. Commun. 1994, 1727. (21) Cao, S. G.; Zhao, C.; Cao, W. X. Polym. Int. 1998, 45, 142. (22) (a) Luo, H.; Chen, J. Y.; Luo, G. B.; Chen, Y. N.; Cao, W. X. J. Mater. Chem. 2001, 11, 419. (b) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620.
10.1021/la025691m CCC: $22.00 © 2002 American Chemical Society Published on Web 05/11/2002
Film of Fullerene Carboxylic Acid and Diazoresin
Figure 1. UV-vis absorption spectra of the C63(COOH)6/DR layer-by-layer self-assembly film. The number of bilayers is 2, 4, 6, 8, 10, 12, 14, and 16 (from bottom to top).
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Figure 3. FTIR spectroscopy of a 50 bilayer C63(COOH)6/DR film on a CaF2 wafer before and after UV irradiation. Irradiation intensity (at 360 nm): 230 µW/cm2; irradiation time: 10 min.
Figure 4. The AFM image of a 2 bilayer ionic bond attached C63(COOH)6/DR film on a mica substrate.
Figure 2. UV-vis absorption spectra of a 16 bilayer C63(COOH)6/DR film upon UV irradiation for different times. Irradiation intensity (at 360 nm): 230 µW/cm2. Scheme 1. Bond Conversion (from Ionic into Covalent) Taking Place in C63(COOH)6/DR Multilayer Film under UV Irradiation
Figure 5. Diagram of frictional signal vs applied load for a 2 bilayer UV-irradiated C63(COOH)6/DR film on a mica substrate.
cycle. Thus, a bilayer of C63(COOH)6/DR film was deposited on the substrate. To fabricate C63(COOH)6/DR/PAA ternary film, the substrate was immersed first in DR solution, then in C63(COOH)6 solution and in DR solution again, and finally in poly(acrylic acid) solution. After the cycle, the C63(COOH)6/DR/PAA ternary film was deposited on the substrate.
Characterization. UV-vis spectra were recorded on a Shimadzu 2100 spectrometer. Fourier transform infrared (FTIR) spectra were recorded on a Bruker Vector 22 FTIR spectrometer. Microtribologic Measurements. The morphology and microtribology measurements of the ultrathin films were performed by atomic force microscopy (AFM)/friction force microscopy (FFM) with a Nanoscope IIIa (Digital Instruments, Inc.) equipped with a bioscope G scanner in contact mode. A commercial Si3N4 cantilever with a spring constant of K ) 0.06 N m-1 was used to obtain the roughness and friction images in air at 25 °C and relative humidity of 50%. The typical scan rates were 2 Hz. Roughness images and RMS (roughness mean square) were obtained simultaneously when the scan angle was 0°. Friction
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Figure 6. AFM morphology of the C63(COOH)6/DR film (a) and the C63(COOH)6/DR/PAA film (b) on a mica substrate. Scheme 2. Photoreaction of the Ternary Composite Film Fabricated from C63(COOH)6, DR, and PAA
images in trace and retrace scan directions were recorded when the scan angle was 90°; the friction forces (V) were then plotted as a function of load to yield a friction-load map.
Results and Discussion Preparation of Covalently Attached Multilayer Assembly Films. As seen from the UV-vis spectra shown in Figure 1, the fabrication of the self-assembly film from C63(COOH)6 and diazoresin proceeds regularly on a quartz substrate. The characteristic band with λmax at 380 nm belongs to the diazonium group of diazoresin; the inset plot shows a good linear relationship between the absorbance at 380 nm and the bilayer number, which indicates that a smooth step-by-step fabrication has taken place on the substrate. In weak alkaline solution, some of the carboxylic acid in C63(COOH)6 will convert into COO-, and thus the C63(COOH)6 can be linked via electrostatic interaction with the cationic diazonium group (-N2+) of DR to form a layerby-layer ultrathin film. Figure 2 shows the UV-vis absorption spectra of a 16 bilayer film of C63(COOH)6/DR on a quartz substrate under different UV irradiation times. As indicated in this figure, the diazonium groups decompose gradually with the decrease of the absorbance at 380 nm and concomitant increase of the absorbance at 292 nm. An isosbestic point at 335 nm appears. Throughout the experiments, we found that the decomposition of this 16 bilayer C63(COOH)6/DR film proceeds completely within 10 min.
To obtain the evidence of the bond conversion, the FTIR spectra of a film with 50 bilayers of C63(COOH)6/DR fabricated on a CaF2 wafer were recorded before and after irradiation (as shown in Figure 3). From the figure, we can find that the peak at 2163 cm-1 (stretching vibration of the diazonium group) disappears after irradiation of UV light, indicating that the diazonium groups comprised in the C63(COOH)6/DR film decompose completely. The absorbance at 1580 cm-1 is attributed to the phenyl groups conjugated with the diazonium of DR; it disappears under UV irradiation. Meanwhile, the peak at 1740 cm-1 increases obviously, indicating the formation of the -COO- ester bond in the film. The Microtribologic Properties of C60-Containing Self-Assembly Film. The covalently attached films are rather more stable than the ionically attached selfassembly ultrathin film, which has been described in our previous work.17 Because the C63(COOH)6/DR self-assembly film is a blend of C60 and polymer and covalent bonds between film layers make the film much firmer than ordinary C60-containing LB films or common ionic bond attached self-assembly films, the microtribological properties of this kind of C60-containing ultrathin film are expected to show relative predominance. The microtribology properties of a 2 bilayer C63(COOH)6/ DR film on a mica substrate before and after UV irradiation were studied by AFM/FFM. The frictional images were obtained through the frictional channel of the AFM
Film of Fullerene Carboxylic Acid and Diazoresin
instrument with the scan angle of 90°. As the lateral force constant of the V-shaped cantilevers and the lateral sensitivity of the optical detector were not measured, we could not obtain the absolute frictional force and thus the frictional signals (in volts) were adopted to represent the relative frictional force. The imaging load force was calculated according to ref 23. Figure 4 shows the morphology of unirradiated C63(COOH)6/DR film in contact mode of AFM. The inner 2.5 × 2.5 µm part of the figure shows an obvious difference from the others; the flange is stacked by the tip of AFM in the trace-retrace process for only three cycles with very low applied load. Because of the relatively weak interaction between layers for the ionic bond linkage, the film can be easily scratched by the AFM tip of contact mode with the minor load of less than 10 nN, and the friction test cannot further proceed for the unirradiated film. In contrast with the unirradiated film, the irradiated self-assembly film shows significant stability to bear the applied load in contact mode of AFM/FFM. We show in Figure 5 the friction force signals versus applied load for the 2 bilayer UV-irradiated C63(COOH)6/DR film. The frictional signals increase linearly with the applied load, and the load added on the UV-irradiated film through the AFM tip is much higher than that of the unirradiated film, which indicates that the covalently attached film has much better load-bearing properties. Zhang et al.24 have proposed a model of plain molecular bearing to explain the tribological behavior of the C60 LB film. The key point of their model lies in two aspects: the first is the role of long-chain molecules in reducing friction and localizing the C60 molecules, and the other aspect is the load-bearing ability of C60. Though their model lacked the support of experimental evidence in microscale, we still believed that the C60 LB or self-assembly film is a considerable model system for the research of “tribology of C60” because (a) it is easy to fabricate different types of C60 ultrathin film with different soft chains and (b) it is easy to fabricate multilayer C60 films with different configurations. In the covalently attached C63(COOH)6/DR self-assembly film, though the C60 has the virtue of load-bearing ability, diazoresin, the polymer cannot act in a soft chain role to reduce friction because of the high density of phenyl groups in its molecule chain. To increase the lubricative properties of the ultrathin film, PAA, a polymer with soft chains, was chosen to sandwich into the film. PAA can be easily self-assembled with diazoresin to achieve a covalently attached ultrathin film.22 Since diazoresin has plenty of diazonium groups on its chain, when some of the diazonium groups (cationic) were linked with COO- of PAA, the remainder can also link with carboxyl groups of C63(COOH)6 through electrostatic interaction (shown in Scheme 2). When the ternary composite film undergoes the irradiation of UV light, all of the ionic bonds between bilayers will convert into covalent bonds. The morphologies of a 2 bilayer C63(COOH)6/DR film and a 1 cycle C63(COOH)6/DR/PAA film are shown in Figure 6. From Figure 6a, we can find that the film easily (23) Li, J. W.; Wang, C.; Shang, G. Y.; Xu, Q. M.; Lin, Z.; Guan, J. J.; Bai, C. L. Langmuir 1999, 15, 7662. (24) Xue, Q. J.; Zhang, J. Tribol. Int. 1995, 28, 287.
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Figure 7. Diagrams of frictional signals vs applied load for the C63(COOH)6/DR film and the C63(COOH)6/DR/PAA film.
forms an “island” structure, that is, because the diazoresin is hydrophobic due to many phenyl groups contained in the chains, which have the tendency to aggregate under UV irradiation. For the PAA-containing composite film, as shown in Figure 6b, the soft PAA layer can fill the uneven part of the ternary film through the movement of chain segments, and thus the film shows obvious flatness. Through changing the applied load on the C63(COOH)6/ DR/PAA ternary film, the frictional signals also show linear increase with the corresponding load as shown in Figure 7, and the film has no damage under rather high load. The most interesting part in Figure 7 is that under the same applied load, the ternary film shows rather lower frictional signals, which indicates that the soft chain polymer containing composite film has better lubricative properties than the C63(COOH)6/DR film. The results are coincident with their morphologies and experimentally substantiate the model of tribology of C60. Therefore, through forming a covalently attached self-assembly film, the rigid C60 molecules and soft polymer chains can be integrated into a potentially solid lubricant film to be used in microelectromechanical systems. Conclusion In conclusion, using a C60 carboxylic acid derivative and diazoresin, a novel self-assembly ultrathin film has been fabricated through electrostatic interaction on quartz, mica, and CaF2 substrates. The diazonium group between layers is photosensitive and easy to decompose when the film is irradiated by UV light, and the bond nature between layers converted from ionic into covalent following the decomposition of the diazonium cation. The covalently attached film is very stable toward polar solvents and applied load; thus, the microtribologic properties of the C60-containing film can be investigated using atomic force microscope/friction force microscope devices, and the results show that rigid C60 molecules in the film have load-bearing capacity. To increase the lubricative property of the film, a soft poly(acrylic acid) chain was incorporated into the film to form a ternary composite film and the polymer-bound C60 film showed not only good load-bearing but also low friction properties. Acknowledgment. The authors are grateful to the NSFC (Contract No. 50173001, 50173002) for financial support of this work. LA025691M
