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Microstructural changes of stearic acid films by immersion in salt solution
Thin Solid Films 444 (2003) 174–178
Microstructural changes of stearic acid films by immersion in salt solution X.-H. Lia, M. Lia,*, L. Huangb, Z.-H. Maia a
Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-59, Beijing 100080, PR China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
b
Received 17 March 2003; received in revised form 4 June 2003; accepted 1 August 2003
Abstract X-Ray reflectivity has been used to investigate the microstructural changes of solution-cast stearic acid films before and after immersion in CoCl2 solutions. Before immersion, the films possess a well-defined layered structure with an interlayer spacing of 4.01"0.05 nm. After the films were immersed in the CoCl2 solutions, a new set of equidistant diffraction peaks emerge, the corresponding interlayer spacing of which is 5.13"0.05 nm. The X-ray photoelectron spectra of the films indicate the existence of cobalt ions inside the films after immersion. It is concluded that the permeation of the cobalt ions into the hydrophilic interlayer causes the stearic acid molecules to reorient perpendicular to the films, resulting in the increase of interlayer spacing and the roughening of the interfaces. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: X-Ray reflectivity; Microstructures; Stearic acid; Immersion
1. Introduction Over the past two decades the method of insertion of metal ions into the hydrophilic interlayers of organic films to prepare organic–inorganic hybrid multilayer films has been employed extensively. For example, in the fabrication of multilayered nanostructure of alternating polymers polystyrene-block-poly(4-vinylpyridine) (PS-b-P4vP) and gold nanoparticles w1x, the thin polymer films were immersed into the ethanol solutions of HAuCl4 in order that gold precursors can be coordinated to the pyridine units of the P4VP block and then HAuCl4 was reduced to gold nanoparticles within the P4VP layers by aqueous NaBH4 solutions. In a stepwise growth of size-confined CdS nanostructures inside the two-dimensional hydrophilic interlayers of the LB films w2x, the cadmium stearate LB films need to be repeatedly immersed in aqueous CdCl2 solutions and subsequently be exposed to H2S gas. Similar to the above, fatty acid Langmuir–Blodgett (LB) films were immersed in CuCl2 aqueous solution, and then reacted with H2S gas *Corresponding author. Tel.: q86-010-82649058; fax: q86-01082640224. E-mail address: [email protected] (M. Li).
to form fatty acidycopper sulfide hybrid multilayer films w3,4x. Sulfur species in the products were studied by the X-ray photoelectron spectra (XPS). Also, the electrical properties of copper sulfide inserted in behenic acid LB films had been investigated w5x. It was found that the most current resistivity values of the samples are approximately 10 V m and in some cases uncontrolled doping can drop the resistivity down below 0.01 V m. By immersing organic films in metal salt solutions, many organic–inorganic hybrid multilayer films have been prepared and their physical and chemical properties have been studied primarily. However, little has been done to investigate the microstructural changes of the films due to metallic ions transporting into the hydrophilic layers. In order to fabricate well-ordered and excellent-performance of organic–inorganic hybrid multilayers, it is necessary and instructive to study the microstructural changes of the films caused by metallic ions. Among the relatively few techniques, which are available for quantitative microstructural characterization of thin films, X-ray reflectivity (XRR) provides a powerful and non-destructive tool to investigate the microstructures of the films. Its application to LB films can be
0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 3 . 0 1 1 2 6 - X
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
dated back to as early as the 1930s, when Holly et al. performed the first X-ray analysis of samples prepared by Blodgett w6x. Although XRR has been applied to the studies of a variety of LB films w7–12x and selfassembled structures w1,13,14x, there are relatively few reports on the study of the microstructural changes of the films before and after immersion in salt solutions. In this work, we investigate the effect of CoCl2 solution on the microstructures of the solution-cast stearic acid films. Solution-cast films are prepared by simply depositing, by means of a microdispenser, microliter amounts of an organic solution on a desired substrate, permitting the solution to spread spontaneously and the solvent to evaporate w15x. A major advantage of this technique is the speed and ease of the process itself. Successful deposition does, however, depend on identifying a suitable combination of surfactant, spreading solvent and substrate surface. Up to now, it has been employed to produce well-structured phospholipid multilayers w16,17x. 2. Experimental details The stearic acid films were prepared on hydrophilic Si(111) substrates. Polished silicon wafers were cleaned in a mixture of H2O:H2O2:H2SO4 s5:1:1 (by volume) at 90 8C for 20 min, then rinsed with super pure water for several times; after that they were cleaned again for approximately 20 min in a mixture of H2O:H2O2:NH4OHs5:1:1 (by volume) at 90 8C, then thoroughly rinsed with super pure water until pH 7. The clean Si substrates were kept immersed in super pure water until they were used and then dried in a stream of nitrogen. Stearic acid, CoCl2 6H2O and isopropanol were all analytical reagent grade and used as supplied. A Milli-Q water purification system (Millipore Corp.) was used to produce water with a resistivity of 1.8=105 V m for all the experiments. The stearic acid was dissolved in isopropanol (2 mM) and pipetted onto the silicon substrates (10=10 mm2), permitting the solution to spread spontaneously and the solvent to evaporate. The evaporation rate of the solvent was controlled in a cascade of chambers over a period of 12 h before keeping the samples in a desiccator for another 24 h. Four films were prepared, three of which were immersed into 1 wt.% CoCl2 aqueous solutions at 23 8C for 6 h, 24 h and 56 h, respectively. Also, the last one was used as an as-deposited sample. In order to ensure that the stearic acid molecules do not desorb off the films during immersion, the solutions were presaturated with stearic acid for at least 24 h. After immersion, the films were removed from the solutions, thoroughly rinsed with super pure water for several times and gently dried with nitrogen gas. According to the volume of the deposited solution, the spreading area
175
Fig. 1. XRR spectra of the stearic acid films: Curve (a) as-deposited; curve (b) immersed in the CoCl2 solution for 6 h; curve (c) for 24 h; and curve (d) for 56 h.
of the solution on substrate and the area per molecule, it is estimated that the films contain approximately 20 bilayers. The XRR measurements were performed on a Bruker D8 Advance diffractometer at room temperature. CuKa radiation was used. The beam size was 0.2=1 mm2. XPS of the films were recorded on a ESCALAB5 instrument (VG Scientific Ltd, England) using an Mg-Ka source (1253.6 eV) with a hemispheric electron analyzer working in a constant analyzer energy mode. The electron pass energy in the analyzer was set at 50 eV. The residual spectrometer pressure was approximately 1=10y7 Pa during data collection. The surfaces of the samples were bombarded with argon ions (2000 V) for 5 min (the sputter-etching rate was approx. 0.4 nmymin) before the XPS measurements. The binding energies (Eb) were calibrated with reference to the C1s line of the aliphatic carbon of stearate molecules (284.6 eV). Quantitative analysis of atomic ratios was accomplished by determining the elemental peak areas w18x. The atomic force microscopy (AFM) measurements were performed on a NanoScope IIIa scanning probe microscope (DI Company, USA) in contact mode. Images were obtained in air at 19 8C and relative humidity of 45%. 3. Results and discussion The XRR profile of the as-deposited film is shown in Fig. 1 (curve a). The clear Bragg peaks indicate that the film exhibits a well-defined layered structure. The interlayer spacing can be calculated from the position of the Bragg peak (un) using the Bragg’s law, 2d sinunsnl where n is the order of Bragg peaks and l the wavelength of X-ray radiation. This calculation yields an average interlayer spacing of ds4.01"0.05 nm. The chain length of a stearic acid molecule is approximately 2.5 nm, and the repeating unit in the film
176
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
Fig. 2. AFM contact mode topographic images of the films. (a) top view image of the as-deposited film, with scan size 5=5 mm; (b) top view image of the film immersed in the CoCl2 solution for 56 h, with scan size 2=2 mm.
is thus a bilayer. The second-order weakening of the peak intensity is due to the electron-deficient layers at the ends of two touching hydrophobic chains, which is caused by the well-ordered assembly of the hydrocarbon chains w19x. It is known that the X-ray diffraction measurements can be applied not only to study the ordered assembly structure and the long spacing of the stearic acid films, but also to evaluate the orientation of the hydrocarbon chain axis w20x. The angle between the chain axis and the normal to the film is estimated to be 36.68. The curves (b), (c) and (d) in Fig. 1 exhibit the XRR profiles of the films immersed in the CoCl2 solution with progressively increasing immersion time at room temperature (23 8C). In comparison with the as-deposited film, there is a new set of equidistant diffraction peaks (designated as 19, 29 and 39) for the films immersed for 6 h and 24 h, respectively, besides the peaks already appeared in the as-deposited film. The corresponding interlayer spacing of these new peaks is 5.13"0.05 nm. The intensities of the original peaks (1, 2 and 3) decrease, while those of the new peaks (19, 29 and 39) increase with the increasing immersion time.
After 56 h immersion, the original peaks disappear and only the new ones remain. The reflectivity curves almost do not change when the immersion time increases beyond 90 h. Moreover, the positions of the new set of peaks do not change with the immersion time. Fig. 2 shows the AFM topographic images of the asdeposited and 56 h immersed films. One can see that the as-deposited film exhibits macroscopically flat structure. The root mean square (rms) roughness of the surface is approximately 0.98 nm. When the film was immersed in the CoCl2 solution for 56 h, the surface roughness of the sample becomes 1.6 nm and some islands emerge on the surface. The XPS spectra (shown in Fig. 3) for the films, which were immersed in the CoCl2 solution, indicate the existence of cobalt ions in the films. One sees no evidence for the existence of Cly. It indicates that when the samples are immersed in the CoCl2 solution, only cobalt ions enter into the hydrophilic area of the films, replacing the Hq. Quantitative analysis reveals that the ratio of the amount of stearic acid molecules to that of cobalt is approximately 6.0:1 and 4.4:1 for the 6 h and 24 h immersed samples, respectively. The ratio approaches 2:1 after 56 h immersion. It is well known that XRR is sensitive to the electron density of each sub-layer in the film and the surface and interfacial roughness as well. Structural information can be obtained by theoretical simulation of the reflectivity curves according to the method reported in Refs. w21,22x. The XRR curves of the as-deposited film and the one after 56 h immersion were fitted, as shown in Fig. 4. The fitting shows that the electron density in the hydrophilic area of the films after immersion is higher than that of the as-deposited film. The increase in electron density is due to the permeation of the cobalt ions into the hydrophilic part, replacing the hydrogen ions. This is in agreement with the XPS results. The reflectivity spectrum of the sample after 56 h immersion is similar to that of cobalt stearate LB films
Fig. 3. X-Ray photoelectron spectra of the films immersed in the CoCl2 solution for (a) 6 h; (b) 24 h; and (c) 56 h.
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
177
surface of the films. However, the chains remain inclined in the non-permeated regions. Near the boundary between the two kinds of regions, the stearic acid molecules are squeezed and stress is produced so that the ordered arrange of the neighbor is destroyed. The stress at the boundaries increases with the increasing of the permeation areas. It will not release until it meets the defeats in the films. Moreover, in the course of microstructural changes, the surface of the film is inclined to gather defects and stress so that the surface roughness becomes rougher. It may be noted that the second-order weakening phenomenon of the peak intensity disappears after 56 h immersion. It is attributed to the fact that the increase of the interfacial roughness disarrays the end of hydrocarbon chains. Therefore, the electron-deficient layers at the two hydrophobic ends of the chains disappear. 4. Conclusion
Fig. 4. XRR spectra of the films: (a) as-deposited; and (b) immersed in the solution for 56 h. Symbols are the experimental data. Solid line represents the simulation. Part of the electron density profiles are shown in the insets.
in which the hydrocarbon chains are oriented perpendicular to the interfaces w20,23x. It is reasonable to assume that the interlayer spacing increase of 1.12 nm for the immersed films is due to the cobalt ions and the change in orientation of the stearic acid molecules. When cobalt ions transport into the hydrophilic layers, the cobalt ions combine together with stearic acid molecules via electrostatic interaction. This leads to the reorientation of the hydrocarbon chains. The surface roughness obtained from simulation for the as-deposited film is 1.0"0.1 nm and that for the film immersed in the CoCl2 solution for 56 h is 1.7"0.2 nm. These values are in agreement with the AFM observations. The simulations indicate that the absolute values of the electron density decrease as a function of displacement from the substrate. This implies increased roughness and formation of defects due to incomplete bilayer coverage, towards the top of the films. The interfacial roughness for the as-deposited film increases from 0.5"0.07 to 1.0"0.1 nm as the position of the interface goes from the bottom to the top. After the film is immersed in the CoCl2 solution for 56 h, the interfacial roughness increases from 0.7"0.07 to 1.7"0.2 nm as the position of the interface goes from the bottom to the top. It indicates the interfaces become rougher after immersion in the salt solutions. This may be explained as follows. During the immersion of the films, the cobalt ions in the permeation regions force the hydrocarbon chains to reorient perpendicular to the
The effect of CoCl2 solution on the microstructure of the solution-cast stearic acid films is investigated by XRR. Before immersion, the film possesses a welldefined layered structure. When the films are immersed in the CoCl2 solutions, a new set of equidistant diffraction peaks emerge and the peaks corresponding to the structure of the as-deposited film disappear gradually. Only the new set of peaks exists after 56 h immersion. The interlayer spacing increases by 1.12 nm after immersion and the rms interfacial roughness increases as well. It is concluded that the increase of interlayer spacing and the roughening is due to the permeation of the cobalt ions into the hydrophilic layers and the reorientation of the stearic acid molecules. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 10274096 and 50173001) and by the Chinese Academy of Sciences. References w1x B.H. Sohn, B.H. Seo, Chem. Mater. 13 (2001) 1752. w2x I. Moriguichi, K. Hosoi, H. Nagaoka, I. Tanaka, Y. Teraoka, S. Kagawa, J. Chem. Soc. Faraday Trans. 90 (1994) 349. w3x H.J. Chen, X.D. Chai, Q. wei, Y.S. Jiang, T.J. Li, Thin Solid Films 178 (1989) 535. w4x J. Leloup, A. Ruaudel-Teixier, A. Barraud, H. Roulet, G. Dufour, Appl. Surf. Sci. 68 (1993) 231. w5x J. Leloup, A. Ruaudel-Teixier, A. Barraud, Thin Solid Films 210 (1992) 407. w6x C. Holly, S. Bernstein, Phys. Rev. 49 (1936) 403. w7x W. Lesslauer, Acta Cryst. B30 (1974) 1927. w8x W. Lesslauer, Acta Cryst. B30 (1974) 1932. w9x P. Ganguly, M. Sastry, S. Choudhury, D.V. Paranjape, Langmuir 13 (1997) 6582.
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w10x K. Kago, M. Furst, H. Matsuoka, H. Yamaoka, T. Seki, Langmuir 15 (1999) 2237. w11x C. Mingotaud, C. Lafuente, J. Amiell, P. Delhaes, Langmuir 15 (1999) 289. w12x J. Bolze, M. Takahasi, J. Mizuki, T. Baumgart, W. Knoll, J. Am. Chem. Soc. 124 (2002) 9412. w13x J. Schmitt, G. Decher, W.J. Dressick, S.L. Brandow, R.E. Geer, R. Shashidhar, J.M. Calvert, Adv. Mater. 9 (1997) 61. w14x G. Evmenenko, M.E. Van der Boom, J. Kmetko, S.W. Dugan, T.J. Marks, P. Dutta, J. Chem. Phys. 115 (2001) 6722. w15x M. Seul, M.J. Sammon, Thin Solid Films 185 (1990) 287. w16x M. Vogel, C. Munster, W. Fenzl, T. Salditt, Phys. Rev. Lett. 84 (2000) 390.
w17x G. Pabst, J. Katsaras, V.A. Raghunathan, Phys. Rev. Lett. 88 (2002) 128 101. w18x D. Briggs, M.P. Seah, Practical Surface Analysis, Auger and X-Ray Photoelectron Spectroscopy, 1,, Wiley, New York, 1990. w19x A. Matsuda, M. Sugi, T. Fukui, S. Lizima, M. Miyahara, Y. Otsubo, J. Appl. Phys. 48 (1977) 771. w20x X. Luo, Z. Zhang, Y. Liang, Langmuir 10 (1994) 3213. w21x V. Nitz, M. Tolan, J.P. Schlomka, O.H. Seeck, J. Stettner, W. Press, Phys. Rev. B 54 (1996) 5038. w22x M. Tolan, in: G. Hohler (Eds.), X-Ray Scattering from SoftMatter Thin Films, Springer Tracts in Modern Physics, 148, Springer, New York, 1999, p. 33. w23x E. Hatta, T. Maekawa, K. Mukasa, Phys. Rev. B 60 (1999) 14 561.
Microstructural changes of stearic acid films by immersion in salt solution X.-H. Lia, M. Lia,*, L. Huangb, Z.-H. Maia a
Institute of Physics, Chinese Academy of Sciences, P.O. Box 603-59, Beijing 100080, PR China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China
b
Received 17 March 2003; received in revised form 4 June 2003; accepted 1 August 2003
Abstract X-Ray reflectivity has been used to investigate the microstructural changes of solution-cast stearic acid films before and after immersion in CoCl2 solutions. Before immersion, the films possess a well-defined layered structure with an interlayer spacing of 4.01"0.05 nm. After the films were immersed in the CoCl2 solutions, a new set of equidistant diffraction peaks emerge, the corresponding interlayer spacing of which is 5.13"0.05 nm. The X-ray photoelectron spectra of the films indicate the existence of cobalt ions inside the films after immersion. It is concluded that the permeation of the cobalt ions into the hydrophilic interlayer causes the stearic acid molecules to reorient perpendicular to the films, resulting in the increase of interlayer spacing and the roughening of the interfaces. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: X-Ray reflectivity; Microstructures; Stearic acid; Immersion
1. Introduction Over the past two decades the method of insertion of metal ions into the hydrophilic interlayers of organic films to prepare organic–inorganic hybrid multilayer films has been employed extensively. For example, in the fabrication of multilayered nanostructure of alternating polymers polystyrene-block-poly(4-vinylpyridine) (PS-b-P4vP) and gold nanoparticles w1x, the thin polymer films were immersed into the ethanol solutions of HAuCl4 in order that gold precursors can be coordinated to the pyridine units of the P4VP block and then HAuCl4 was reduced to gold nanoparticles within the P4VP layers by aqueous NaBH4 solutions. In a stepwise growth of size-confined CdS nanostructures inside the two-dimensional hydrophilic interlayers of the LB films w2x, the cadmium stearate LB films need to be repeatedly immersed in aqueous CdCl2 solutions and subsequently be exposed to H2S gas. Similar to the above, fatty acid Langmuir–Blodgett (LB) films were immersed in CuCl2 aqueous solution, and then reacted with H2S gas *Corresponding author. Tel.: q86-010-82649058; fax: q86-01082640224. E-mail address: [email protected] (M. Li).
to form fatty acidycopper sulfide hybrid multilayer films w3,4x. Sulfur species in the products were studied by the X-ray photoelectron spectra (XPS). Also, the electrical properties of copper sulfide inserted in behenic acid LB films had been investigated w5x. It was found that the most current resistivity values of the samples are approximately 10 V m and in some cases uncontrolled doping can drop the resistivity down below 0.01 V m. By immersing organic films in metal salt solutions, many organic–inorganic hybrid multilayer films have been prepared and their physical and chemical properties have been studied primarily. However, little has been done to investigate the microstructural changes of the films due to metallic ions transporting into the hydrophilic layers. In order to fabricate well-ordered and excellent-performance of organic–inorganic hybrid multilayers, it is necessary and instructive to study the microstructural changes of the films caused by metallic ions. Among the relatively few techniques, which are available for quantitative microstructural characterization of thin films, X-ray reflectivity (XRR) provides a powerful and non-destructive tool to investigate the microstructures of the films. Its application to LB films can be
0040-6090/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 3 . 0 1 1 2 6 - X
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
dated back to as early as the 1930s, when Holly et al. performed the first X-ray analysis of samples prepared by Blodgett w6x. Although XRR has been applied to the studies of a variety of LB films w7–12x and selfassembled structures w1,13,14x, there are relatively few reports on the study of the microstructural changes of the films before and after immersion in salt solutions. In this work, we investigate the effect of CoCl2 solution on the microstructures of the solution-cast stearic acid films. Solution-cast films are prepared by simply depositing, by means of a microdispenser, microliter amounts of an organic solution on a desired substrate, permitting the solution to spread spontaneously and the solvent to evaporate w15x. A major advantage of this technique is the speed and ease of the process itself. Successful deposition does, however, depend on identifying a suitable combination of surfactant, spreading solvent and substrate surface. Up to now, it has been employed to produce well-structured phospholipid multilayers w16,17x. 2. Experimental details The stearic acid films were prepared on hydrophilic Si(111) substrates. Polished silicon wafers were cleaned in a mixture of H2O:H2O2:H2SO4 s5:1:1 (by volume) at 90 8C for 20 min, then rinsed with super pure water for several times; after that they were cleaned again for approximately 20 min in a mixture of H2O:H2O2:NH4OHs5:1:1 (by volume) at 90 8C, then thoroughly rinsed with super pure water until pH 7. The clean Si substrates were kept immersed in super pure water until they were used and then dried in a stream of nitrogen. Stearic acid, CoCl2 6H2O and isopropanol were all analytical reagent grade and used as supplied. A Milli-Q water purification system (Millipore Corp.) was used to produce water with a resistivity of 1.8=105 V m for all the experiments. The stearic acid was dissolved in isopropanol (2 mM) and pipetted onto the silicon substrates (10=10 mm2), permitting the solution to spread spontaneously and the solvent to evaporate. The evaporation rate of the solvent was controlled in a cascade of chambers over a period of 12 h before keeping the samples in a desiccator for another 24 h. Four films were prepared, three of which were immersed into 1 wt.% CoCl2 aqueous solutions at 23 8C for 6 h, 24 h and 56 h, respectively. Also, the last one was used as an as-deposited sample. In order to ensure that the stearic acid molecules do not desorb off the films during immersion, the solutions were presaturated with stearic acid for at least 24 h. After immersion, the films were removed from the solutions, thoroughly rinsed with super pure water for several times and gently dried with nitrogen gas. According to the volume of the deposited solution, the spreading area
175
Fig. 1. XRR spectra of the stearic acid films: Curve (a) as-deposited; curve (b) immersed in the CoCl2 solution for 6 h; curve (c) for 24 h; and curve (d) for 56 h.
of the solution on substrate and the area per molecule, it is estimated that the films contain approximately 20 bilayers. The XRR measurements were performed on a Bruker D8 Advance diffractometer at room temperature. CuKa radiation was used. The beam size was 0.2=1 mm2. XPS of the films were recorded on a ESCALAB5 instrument (VG Scientific Ltd, England) using an Mg-Ka source (1253.6 eV) with a hemispheric electron analyzer working in a constant analyzer energy mode. The electron pass energy in the analyzer was set at 50 eV. The residual spectrometer pressure was approximately 1=10y7 Pa during data collection. The surfaces of the samples were bombarded with argon ions (2000 V) for 5 min (the sputter-etching rate was approx. 0.4 nmymin) before the XPS measurements. The binding energies (Eb) were calibrated with reference to the C1s line of the aliphatic carbon of stearate molecules (284.6 eV). Quantitative analysis of atomic ratios was accomplished by determining the elemental peak areas w18x. The atomic force microscopy (AFM) measurements were performed on a NanoScope IIIa scanning probe microscope (DI Company, USA) in contact mode. Images were obtained in air at 19 8C and relative humidity of 45%. 3. Results and discussion The XRR profile of the as-deposited film is shown in Fig. 1 (curve a). The clear Bragg peaks indicate that the film exhibits a well-defined layered structure. The interlayer spacing can be calculated from the position of the Bragg peak (un) using the Bragg’s law, 2d sinunsnl where n is the order of Bragg peaks and l the wavelength of X-ray radiation. This calculation yields an average interlayer spacing of ds4.01"0.05 nm. The chain length of a stearic acid molecule is approximately 2.5 nm, and the repeating unit in the film
176
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
Fig. 2. AFM contact mode topographic images of the films. (a) top view image of the as-deposited film, with scan size 5=5 mm; (b) top view image of the film immersed in the CoCl2 solution for 56 h, with scan size 2=2 mm.
is thus a bilayer. The second-order weakening of the peak intensity is due to the electron-deficient layers at the ends of two touching hydrophobic chains, which is caused by the well-ordered assembly of the hydrocarbon chains w19x. It is known that the X-ray diffraction measurements can be applied not only to study the ordered assembly structure and the long spacing of the stearic acid films, but also to evaluate the orientation of the hydrocarbon chain axis w20x. The angle between the chain axis and the normal to the film is estimated to be 36.68. The curves (b), (c) and (d) in Fig. 1 exhibit the XRR profiles of the films immersed in the CoCl2 solution with progressively increasing immersion time at room temperature (23 8C). In comparison with the as-deposited film, there is a new set of equidistant diffraction peaks (designated as 19, 29 and 39) for the films immersed for 6 h and 24 h, respectively, besides the peaks already appeared in the as-deposited film. The corresponding interlayer spacing of these new peaks is 5.13"0.05 nm. The intensities of the original peaks (1, 2 and 3) decrease, while those of the new peaks (19, 29 and 39) increase with the increasing immersion time.
After 56 h immersion, the original peaks disappear and only the new ones remain. The reflectivity curves almost do not change when the immersion time increases beyond 90 h. Moreover, the positions of the new set of peaks do not change with the immersion time. Fig. 2 shows the AFM topographic images of the asdeposited and 56 h immersed films. One can see that the as-deposited film exhibits macroscopically flat structure. The root mean square (rms) roughness of the surface is approximately 0.98 nm. When the film was immersed in the CoCl2 solution for 56 h, the surface roughness of the sample becomes 1.6 nm and some islands emerge on the surface. The XPS spectra (shown in Fig. 3) for the films, which were immersed in the CoCl2 solution, indicate the existence of cobalt ions in the films. One sees no evidence for the existence of Cly. It indicates that when the samples are immersed in the CoCl2 solution, only cobalt ions enter into the hydrophilic area of the films, replacing the Hq. Quantitative analysis reveals that the ratio of the amount of stearic acid molecules to that of cobalt is approximately 6.0:1 and 4.4:1 for the 6 h and 24 h immersed samples, respectively. The ratio approaches 2:1 after 56 h immersion. It is well known that XRR is sensitive to the electron density of each sub-layer in the film and the surface and interfacial roughness as well. Structural information can be obtained by theoretical simulation of the reflectivity curves according to the method reported in Refs. w21,22x. The XRR curves of the as-deposited film and the one after 56 h immersion were fitted, as shown in Fig. 4. The fitting shows that the electron density in the hydrophilic area of the films after immersion is higher than that of the as-deposited film. The increase in electron density is due to the permeation of the cobalt ions into the hydrophilic part, replacing the hydrogen ions. This is in agreement with the XPS results. The reflectivity spectrum of the sample after 56 h immersion is similar to that of cobalt stearate LB films
Fig. 3. X-Ray photoelectron spectra of the films immersed in the CoCl2 solution for (a) 6 h; (b) 24 h; and (c) 56 h.
X.-H.-H. Li et al. / Thin Solid Films 444 (2003) 174–178
177
surface of the films. However, the chains remain inclined in the non-permeated regions. Near the boundary between the two kinds of regions, the stearic acid molecules are squeezed and stress is produced so that the ordered arrange of the neighbor is destroyed. The stress at the boundaries increases with the increasing of the permeation areas. It will not release until it meets the defeats in the films. Moreover, in the course of microstructural changes, the surface of the film is inclined to gather defects and stress so that the surface roughness becomes rougher. It may be noted that the second-order weakening phenomenon of the peak intensity disappears after 56 h immersion. It is attributed to the fact that the increase of the interfacial roughness disarrays the end of hydrocarbon chains. Therefore, the electron-deficient layers at the two hydrophobic ends of the chains disappear. 4. Conclusion
Fig. 4. XRR spectra of the films: (a) as-deposited; and (b) immersed in the solution for 56 h. Symbols are the experimental data. Solid line represents the simulation. Part of the electron density profiles are shown in the insets.
in which the hydrocarbon chains are oriented perpendicular to the interfaces w20,23x. It is reasonable to assume that the interlayer spacing increase of 1.12 nm for the immersed films is due to the cobalt ions and the change in orientation of the stearic acid molecules. When cobalt ions transport into the hydrophilic layers, the cobalt ions combine together with stearic acid molecules via electrostatic interaction. This leads to the reorientation of the hydrocarbon chains. The surface roughness obtained from simulation for the as-deposited film is 1.0"0.1 nm and that for the film immersed in the CoCl2 solution for 56 h is 1.7"0.2 nm. These values are in agreement with the AFM observations. The simulations indicate that the absolute values of the electron density decrease as a function of displacement from the substrate. This implies increased roughness and formation of defects due to incomplete bilayer coverage, towards the top of the films. The interfacial roughness for the as-deposited film increases from 0.5"0.07 to 1.0"0.1 nm as the position of the interface goes from the bottom to the top. After the film is immersed in the CoCl2 solution for 56 h, the interfacial roughness increases from 0.7"0.07 to 1.7"0.2 nm as the position of the interface goes from the bottom to the top. It indicates the interfaces become rougher after immersion in the salt solutions. This may be explained as follows. During the immersion of the films, the cobalt ions in the permeation regions force the hydrocarbon chains to reorient perpendicular to the
The effect of CoCl2 solution on the microstructure of the solution-cast stearic acid films is investigated by XRR. Before immersion, the film possesses a welldefined layered structure. When the films are immersed in the CoCl2 solutions, a new set of equidistant diffraction peaks emerge and the peaks corresponding to the structure of the as-deposited film disappear gradually. Only the new set of peaks exists after 56 h immersion. The interlayer spacing increases by 1.12 nm after immersion and the rms interfacial roughness increases as well. It is concluded that the increase of interlayer spacing and the roughening is due to the permeation of the cobalt ions into the hydrophilic layers and the reorientation of the stearic acid molecules. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 10274096 and 50173001) and by the Chinese Academy of Sciences. References w1x B.H. Sohn, B.H. Seo, Chem. Mater. 13 (2001) 1752. w2x I. Moriguichi, K. Hosoi, H. Nagaoka, I. Tanaka, Y. Teraoka, S. Kagawa, J. Chem. Soc. Faraday Trans. 90 (1994) 349. w3x H.J. Chen, X.D. Chai, Q. wei, Y.S. Jiang, T.J. Li, Thin Solid Films 178 (1989) 535. w4x J. Leloup, A. Ruaudel-Teixier, A. Barraud, H. Roulet, G. Dufour, Appl. Surf. Sci. 68 (1993) 231. w5x J. Leloup, A. Ruaudel-Teixier, A. Barraud, Thin Solid Films 210 (1992) 407. w6x C. Holly, S. Bernstein, Phys. Rev. 49 (1936) 403. w7x W. Lesslauer, Acta Cryst. B30 (1974) 1927. w8x W. Lesslauer, Acta Cryst. B30 (1974) 1932. w9x P. Ganguly, M. Sastry, S. Choudhury, D.V. Paranjape, Langmuir 13 (1997) 6582.
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