Langmuir 2000, 16, 2207-2212
2207
Formation of Close-Packed Silver Nanoparticle Multilayers from Electrostatically Grown Octadecylamine/Colloid Nanocomposite Precursors Vijaya Patil and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received February 2, 1999. In Final Form: August 9, 1999 The formation of close-packed silver nanoparticle thin films via a two-stage self-assembly approach is described. In the first step, surface-modified silver colloidal particles are extracted from aqueous solution via electrostatic interactions into thermally evaporated fatty amine films. Thereafter, the excess fatty amine molecules in the organic matrix are removed by dissolution in a range of organic solvents of varying dielectric properties. Thermogravimetric and quartz crystal microgravimetric studies indicate that, irrespective of whether the dissolution medium is polar or nonpolar, except for a monolayer of amine molecules in direct contact with the colloidal particle surface, almost complete fatty amine dissolution occurs leading to a considerable increase in the packing density of the silver colloidal particles. While UV-vis spectroscopy measurements of the films after amine removal suggest subtle differences in the final structure of the films prepared from the different solvents, atomic force microscopy studies show fairly aggregated colloidal particle structures in all cases.
Introduction Electrostatic interactions have long been known to play a crucial role in many biological and chemical processes.1 Recently, electrostatic interactions have been used with success in the synthesis of advanced materials through the manipulation and organization of objects on the microand nanoscale.2-5 In this laboratory, we have developed two independent routes for the electrostatic self-assembly of surface-modified colloidal particles. The methods are based on (1) electrostatic immobilization of colloidal particles at the air-water interface with suitable ionized Langmuir monolayers (and thereafter, formation of multilayer Langmuir-Blodgett films)6 and, (2) formation of nanocomposite films by electrostatically controlled diffusion of colloidal particles into suitable organic matrixes.7 In the latter route, it has been observed that under the most favorable conditions of maximum electrostatic interaction between the charged colloidal particles and the organic matrix, the cluster volume fraction in the films * To whom all correspondence is to be addressed. Tel.: +91-205893044. Fax: +91-20-5893044/5893952. E-mail: sastry@ ems.ncl.res.in. (1) Honig, B.; Nicholls, A. Science 1995, 268, 1144. (2) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349. (3) Clemente-Leon, M.; Mingotaud, C.; Angicole, B.; Gomez-Garcia, C. J.; Coronada, E.; Delhaes, P. Angew. Chem., Int. Ed. Engl. 1997, 36, 1114. (4) Taguchi, Y.; Kimura, R.; Azumi, R.; Tachibana, H.; Koshizaki, N.; Shimomura, M.; Momozawa, N.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 1998, 14, 6550. (5) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (6) (a) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575; (b) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B. 1997, 101, 4954; (c) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281; (d) Mayya, K. S.; Sastry, M. J. Phys. Chem. B. 1997, 101, 9790; (e) Mayya, K. S.; Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 3377; (f) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 74; (g) Mayya, K. S.; Patil, V.; Kumar, M.; Sastry, M. Thin Solid Films 1998, 312, 308; (h) Sastry, M.; Mayya, K. S.; Patil, V. Langmuir 1998, 14, 5921. (7) (a) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 3, 4490; (b) Patil, V.; Sastry, M. Langmuir 13 1997 5511; (c) Patil, V.; Sastry, M. J. Chem. Soc. Faraday Trans. 1997, 93, 4347; (d) Patil, V.; Sastry, M. Langmuir 1998, 14, 2707; (e) Sastry, M.; Patil, M.; Sainkar, S. R. J. Phys. Chem. B. 1998, 102, 1404.
Scheme 1 . Diagram Illustrating the Various Stages of Formation of Close-packed Silver Particle Films for Organic Nanocompositesa
a Step 1: Deposition of ODA films on suitable substrates; Step 2: formation of ODA-silver particle nanocomposite by immersion in surface-modified silver hydrosol; Step 3: dissolution of the ODA matrix by immersion in various organic solvents to yield close-packed silver particle films.
was at best 20%.7a Many applications based on the collective properties of the organizates8 require the ability to tailor the separation between the nanoparticles. We describe herein a three-step process for the generation of close-packed silver colloidal particle multilayer films, an illustration of which is given in Scheme 1. The first step consists of deposition of thin films of octadecylamine (ODA) by thermal evaporation onto suitable substrates. Formation of the silver colloidal particle composite with thermally evaporated fatty amine films by immersion of the ODA films in the colloidal solution constitutes the second step. The nanocomposite is formed by electrostatically controlled diffusion of the negatively charged silver colloidal particles into the protonated ODA films as described in detail in our earlier work.7 Fatty amine molecules are soluble in a range of polar and nonpolar organic solvents. It should be possible to dissolve the excess ODA molecules in the nanocomposite films in (8) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.
10.1021/la990103z CCC: $19.00 © 2000 American Chemical Society Published on Web 01/14/2000
2208
Langmuir, Vol. 16, No. 5, 2000
Patil and Sastry
such organic solvents and thus increase the effective packing density of the silver nanoparticle films. The dissolution of the excess ODA molecules to increase the packing density of the silver colloidal particles then forms the final step of the protocol as shown in Scheme 1. The dissolution of the ODA matrix in different organic solvents and the formation of the silver nanoparticle film has been studied using quartz crystal microgravimetry (QCM), thermogravimetric analysis (TGA), optical absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). Presented below are details of the investigation. Experimental Details The synthesis of the silver colloidal particles in aqueous medium as well as capping of the clusters with 4-carboxythiophenol (4-CTP) has been described elsewhere.7a This procedure results in the formation of stable, carboxylic acid derivatized silver colloidal particles of 70 ( 13 Å diameter. The carboxylic acid derivatized silver particles were thereafter incorporated into 500 Å thick thermally evaporated octadecylamine (ODA) films by simple immersion of the fatty amine films in the colloidal solution as also described in detail in previous reports (Scheme 1, step 2).7a,e The ODA films were deposited on gold-coated 6 MHz AT-cut quartz crystals, quartz substrates (area ) 4 cm2 ), and Si (111) wafers for QCM, UV-vis spectroscopy, FTIR spectroscopy and AFM measurements, respectively. The mass changes on the AT-cut quartz crystal were monitored using an Edwards FTM5 frequency counter with a frequency stability and resolution of ( 1 Hz. This translates to a mass resolution of 12 ng/cm2 for the 6 MHz crystal used. The frequency changes were transformed into mass variations using the standard Sauerbrey formula.9 UV-vis spectroscopy measurements on the cluster incorporated ODA films were performed on a HewlettPackard 8452A diode array spectrophotometer at a resolution of 2 nm. FTIR spectroscopy measurements of the nanocomposite films before and after immersion of the film in benzene for 12 h were performed on a Perkin-Elmer 16PC spectrophotometer in the transmission mode at a resolution of 2 cm-1. After immersion of the nanocomposite film in benzene, only 8 wt % of the ODA matrix is left in the film. To obtain sufficient signalto-noise ratio (SNR) for the ODA matrix dissolved film, 1024 scans were recorded for the FTIR measurements. Composite films of ODA complexed with the silver colloidal particles were grown at a pH of 8.5 since it has been observed that at this pH, maximum complexation of the silver particles occurs.7a,e After the cluster density in the films had equilibrated, which typically required immersion of the ODA films in the silver sol for ca. 150 h,7e the composite films were immersed in 10 mL of two polar (ethanol and acetone) and two nonpolar (benzene and carbon tetrachloride) organic solvents. All chemicals in this study were obtained from Aldrich and used as received. QCM, UV-vis spectroscopy and FTIR spectroscopy measurements followed the changes occurring in the films as a function of time of immersion in the different solvents after drying of the films in flowing N2. We would like to point out here that immersion of the nanocomposite films in the solvents and their subsequent removal was required to be performed with great care and with minimum agitation. Vigorous agitation lead to dissolution of not only the ODA matrix in the solvent but some of the silver clusters as well, an effect that could be clearly observed by the naked eye. It was possible, with the precautions listed above, to ensure minimal loss of clusters from the film and this was checked by carrying out UV-vis spectroscopy and inductively coupled plasma spectroscopy (ICP) studies of the solvents after completion of the immersion experiments. TGA for the ODA/silver cluster nanocomposite films before and after immersion in benzene for 12 h was performed on a Seiko Instruments model TG/DTA 32 thermal analysis system. Accurately weighed powders prepared by scraping off the films formed on quartz substrates were extensively dried in a vacuum at room temperature and were analyzed in a N2 atmosphere in standard aluminum pans. The TGA runs were recorded from 28 to 408 °C at a heating rate of 10 °C/min. (9) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206.
Figure 1. A. UV-vis absorption spectra recorded from a 500 Å thick ODA/silver colloidal particle composite film as a function of time of immersion in benzene. The time of immersion is indicated next to the respective curve. The dashed curve corresponds to a spectrum recorded from a separate measurement on another ODA/silver colloidal particle film under similar immersion conditions (time ) 720 min). B. UV-vis absorption spectra recorded from a 500 Å thick ODA/silver colloidal particle composite film as a function of time of immersion in carbon tetrachloride. The time of immersion is indicated next to the respective curve. The dashed curve corresponds to a spectrum recorded from a separate measurement on another ODA/silver colloidal particle film under similar immersion conditions (time ) 720 min). AFM measurements were carried out on the ODA/silver nanocomposite films deposited on Si (111) wafers before and after removal of the ODA matrix by immersion in both benzene and acetone. The measurements were carried out on a Digital Instruments microscope operated in the tapping mode. Both phase and depth profile images were obtained.
Results and Discussion The kinetics of carboxylic acid derivatized silver colloidal particle incorporation in ODA films studied by QCM and UV-vis spectroscopy has been dealt with in detail elsewhere (step 2, Scheme 1).7e The focus of this investigation will be primarily on the changes occurring in the ODA/silver nanocomposite films during immersion in the different solvents mentioned in the Experimental Section (step 3, Scheme 1). Figure 1A and B shows the UV-vis spectra recorded from 500 Å thick ODA/silver cluster composite films after various times of immersion in benzene and CCl4 (nonpolar solvents). The immersion times (in minutes) are indicated next to the corresponding curves. The spectra were recorded for the films in air after thorough drying of the films, as mentioned earlier. The time of immersion is indicated next to the respective spectra. The strong resonance at ca. 475 nm is characteristic of the silver colloidal particles in the ODA matrix7a,e and arises due to excitation of surface plasmon vibrations in the particles.10 Figure 2A and B show similar timedependent UV-vis spectra recorded from the ODA/silver cluster films after immersion in ethanol and acetone (polar solvents). The spectra shown as dashed lines in the above figures have been obtained from other ODA/silver particle composite films immersed in the respective solvents in a separate run. In Figure 1A and B, the spectra shown in dashed lines were obtained from films immersed in the respective solvents for 720 min while the time of immersion is 60 min for the dashed line spectra in Figure 2A and B. (10) (a) Henglein, A. J. Phys. Chem. 1993, 97, 5457; (b) Mulvaney, P. Langmuir 1996, 12, 788.
Nanoparticle ML from Octadecylamine Precursors
Figure 2. A. UV-vis absorption spectra recorded from a 500 Å thick ODA/silver colloidal particle composite film as a function of time of immersion in ethanol. The time of immersion is indicated next to the respective curve. The dashed curve corresponds to a spectrum recorded from a separate measurement on another ODA/silver colloidal particle film under similar immersion conditions (time ) 60 min). B. UV-vis absorption spectra recorded from a 500 Å thick ODA/silver colloidal particle composite film as a function of time of immersion in acetone. The time of immersion is indicated next to the respective curve. The dashed curve corresponds to a spectrum recorded from a separate measurement on another ODA/silver colloidal particle film under similar immersion conditions (time ) 60 min).
This was done to check the reproducibility of the procedure and unambiguously ascertain the role played by the solvent in the silver particle structure as seen by UV-vis spectroscopy. It is observed that there is fairly good agreement with spectra recorded from the silver particle films during separate runs in the different solvents, ruling out the possibility of experimental error. A glance at Figures 1 and 2 leads to the following observations. During the very initial stages of immersion of the films in both polar and nonpolar solvents (t < 30 min), the intensity of the surface plasmon resonance decreases without a detectable shift in the wavelength of the absorption maximum. Thereafter, beyond ca. 30 min of immersion, there is a pronounced broadening and further damping of the resonance, with a shift to longer wavelengths (red-shift) for the films immersed in nonpolar solvents (Figure 1A and B). In the case of films immersed in polar solvents (Figure 2A and B), prolonged immersion of the nanocomposites leads to a significantly red-shifted surface plasmon resonance component with simultaneous growth of a short wavelength component. The time taken for the clusters in the films to reach an equilibrium packing density is also observed to be different for the films immersed in the nonpolar solvents (Figure 1, 720 min) when compared with the films immersed in polar solvents (Figure 2, 120 min). We remark here that the color of the composite films (as observed by the unaided eye) changed from a dark brown before immersion in the solvents to a silvery mirrorlike color after prolonged immersion. The initial sharp fall in the surface plasmon resonance intensity observed in Figures 1 and 2 may be due to many factors, the most obvious being loss of silver clusters from the film in the different solvents. To test this hypothesis, the four solvents were subjected to UV-vis measurement before and after immersion of the nanocomposite films for 12 h in the different solvents and the spectra recorded are shown in Figure 3. (A spectrum recorded from a 1.6 × 10-5 M concentrated silver colloidal particle solution is also shown in the figure (solid line) and will be discussed
Langmuir, Vol. 16, No. 5, 2000 2209
Figure 3. UV-vis absorption spectra recorded from the different solvents before (curves shown in the lower part of figure) and after (curves in the upper half of the figure) immersion of ODA/silver colloidal particle composite films for 12 h. Triangles: ethanol; diamonds: carbon tetrachloride; circles: benzene; and squares: acetone. The spectra measured after immersion of the composite films have been vertically displaced for clarity. The solid curve corresponds to the UVvis spectrum recorded from a 1.6 × 10-5 M concentrated silver colloidal particle solution (see text for details).
subsequently). Note that the spectra recorded after the immersion cycle have been displaced for clarity and are shown in the upper part of the figure. The lack of a welldefined surface plasmon resonance feature in the spectra recorded after immersion of the nanocomposite films clearly indicates that, within the detection limits of the spectrophotometer used, there is little loss of silver colloidal particles from the film and therefore, any changes in the optical properties observed in Figures 1 and 2 must be attributed to other processes. To independently check possible loss of clusters from the film, the solutions after immersion of the silver colloidal particle composite films were subjected to ICP analysis. In all cases, the silver content in the solutions was below the detection limit of the ICP instrument, which was typically 10 parts per billion (ppb). The question of silver particle loss from the film is readdressed after presentation of the QCM results below. We point out once again that the process of immersion and removal of the nanocomposite film after dissolution of the ODA matrix required great care. Agitation of the solution leads to dissolution of a fraction of the ODA-capped clusters in the organic solvents and this aspect of the work is currently under investigation. The changes occurring in the nanocomposite films during immersion in the different solvents was also followed using QCM. The frequency changes were measured ex situ after careful washing and drying of the crystals. Figure 4 shows the mass changes recorded from two 500 Å thick nanocomposite films as a function of time of immersion in ethanol (filled circles) and benzene (filled squares). These curves are representative of the behavior observed from the other polar (acetone) and nonpolar (carbon tetrachloride) solvents. The left axis in the figure shows the overall percentage weight loss from the film while the right axis corresponds to percentage weight loss of the ODA component in the film assuming that the weight loss is due to dissolution of only the ODA molecules in the organic solvent. It is seen that there is a fairly significant mass loss with time and also that the final mass loss for both films is roughly the same. Nearly 90% of the overall ODA mass loss occurs within the first 10
2210
Langmuir, Vol. 16, No. 5, 2000
Figure 4. QCM mass loss kinetics curves for a 500 Å thick ODA-silver cluster film immersed in ethanol (circles) and benzene (squares) as a function of time of immersion. The left axis in the figure shows the overall percentage weight loss from the film while the right axis corresponds to percentage weight loss of the ODA component in the film.
Figure 5. TGA heating curve for the powder prepared by scraping the ODA/silver cluster film from the substrate (curve A) and the derivative of curve A showing three distinct mass losses (curve B). The percentage mass loss and the corresponding temperatures are given in the figure.
min of immersion after which a further 90 min is required for completion of the mass loss. Figure 5 shows the TGA heating curve A, and its derivative B, recorded from the powder prepared by scraping the ODA/silver cluster film from the substrates. These powders were collected before ODA removal by dissolution in an organic solvent. The derivative curve clearly shows that there are three distinct mass losses exhibited by the compound at 66.3, 112, and 194.3 °C. No residue was left behind on heating to ca. 400 °C. The mass losses were 4.6%, 11.5%, and 60.9% of the starting mass respectively for the above temperatures and correspond to loss of free ODA molecules (ODA molecules not coordinated with the 4-CTP on clusters), the ODA molecules directly coordinated with the 4-CTP molecules via attractive electrostatic interactions and evaporation of the clusters, respectively. The ODA weight losses determined from the TGA measurements agree quantitatively with the earlier QCM measurements on a 500 Å thick ODA film. However, the weight loss of ODA on dissolution in organic solvents (Figure 4, ca. 90% of the ODA matrix) is more than that expected by the constraint that ODA molecules required for charge neutralization
Patil and Sastry
Figure 6. FTIR spectra recorded from a 500 Å thick ODA/ silver cluster film before (curve A) and after (curve B) immersion in benzene for 12 h. Curve B has been multiplied by a factor of 3.
remain in the films as can be shown by a simple calculation.11 This indicates that ODA molecules directly coordinated with the cluster surface are also removed during immersion in the different organic solvents leaving behind roughly one ODA molecule per six 4-CTP molecules. There was no distinct weight loss in the TGA curve of the film after ODA dissolution in benzene for 12 h indicating that the film contains a negligible percentage of ODA. The total weight loss in the films during immersion in ethanol and benzene is ca. 4.4 × 10-6 gm/cm2 (90% of the ODA component in the film, ref 11). If one assumes that this weight loss is entirely due to dissolution of silver particles in the organic solvent (roughly 20% of the cluster density in the film), a simple calculation shows that for the 4 cm2 area film used, the concentration of silver atoms in solution should be nearly 1.6 × 10-5 M. A silver colloidal solution was prepared using this concentration of Ag ions and the UV-vis spectrum from this solution is shown in Figure 3 for comparison (solid line). It is clearly seen that if this degree of colloidal silver loss (20% of the silver cluster population) from the film had indeed occurred, its presence should have been detected by UV-vis measurements and this is clearly not the case (Figure 3). This concentration of silver in benzene works out to be nearly 1.5 parts per million (ppm), well above the detection sensitivity of the ICP instrument of 10 ppb. These simple calculations and measurements indicate that the weight loss observed is mainly due to dissolution of the ODA matrix in the organic solvents and that the contribution of silver particles to this loss is below the detection limits of both the UV-vis and ICP techniques used. Figure 6 shows plots of the FTIR spectra recorded in the range of 2800 to 3000 cm-1 from 500 Å thick ODA(11) For a 500 Å thick ODA film, the weight of ODA is 4.9 × 10-6 g/cm2 and the overall mass increase due to silver cluster incorporation was measured from QCM to be 23 × 10-6 g/cm2 (refs 7a, e). Thus, the weight of thermally evaporated ODA molecules in the film is 17.2% of the total weight of the film after complete cluster incorporation. For silver clusters of 70 Å diameter, the mass per cluster is ca. 2 × 10-18 g thereby leading to 1.15 × 1013 clusters/cm2 of ODA film. Assuming an area of 25 Å2 per 4-CTP molecule (Sastry, M.; Mayya, K. S.; Patil, V. J. Phys. Chem. B. 1997, 101, 1167), 616 4-CTP of molecules are on the surface of each cluster thus leading to 7.1 × 1015 4-CTP molecules in the film. This corresponds to 3.15 × 10-6 g of ODA molecules that are required to coordinate with all the 4-CTP molecules on the cluster surface for charge neutralization. This suggests that the weight of ODA coordinated with the clusters is 11.3% of the total weight of the film and that 6.4 wt % of the ODA molecules remain uncoordinated even after complete cluster incorporation.
Nanoparticle ML from Octadecylamine Precursors
Langmuir, Vol. 16, No. 5, 2000 2211
Figure 7. A. AFM tapping mode phase image of a 500 Å thick silver/ODA composite film after dissolution of the excess octadecylamine molecules in benzene for 12 h. (B) AFM tapping mode depth profile image of a 500 Å thick silver/ODA composite film after dissolution of the excess octadecylamine molecules in benzene for 12 h. The scan region is identical to that shown in Figure 7A.
silver cluster films before (curve A) and after immersion of the film in benzene for 12 h (curve B). The latter spectrum has been multiplied by a factor of 3 as indicated in the figure. The resonances observed at 2920 and 2850 cm-1 correspond to the methylene antisymmetric and symmetric vibrational modes, respectively.12 It is seen that there is a significant loss in intensity of the methylene vibrational resonances on immersion of the nanocomposite films in benzene. Since the bifunctional molecule, 4-CTP, used in this study to cap the silver cluster surface does not contain a hydrocarbon component, the above observation indicates a loss of ODA molecules from the matrix. A similar loss in intensity of the methylene vibrational modes was observed for the ODA-silver cluster composite
films after immersion in the other solvents as well. It is also to be noted that the methylene antisymmetric and symmetric vibrations occur at the same wavelengths for the ODA films before and after immersion in benzene. We could not say with certainty whether there was a corresponding loss of the silver surface-bound 4-CTP molecules electrostatically coordinated to the ODA molecules during immersion in the different organic solvents. The carbonyl stretch resonance which occurs at ca. 1678 cm-1 in benzoic acid13 could not be identified above the noise in the signal and may be a consequence of the small concentration of the carboxylic acid groups in the film. Figure 7A shows an AFM phase image of a silver/ODA composite film after dissolution of the excess ODA
(12) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604.
(13) Parikh, V. M. In Absorption Spectra of Organic Molecules; Addison-Wesley: Reading, 1974; p 77.
2212
Langmuir, Vol. 16, No. 5, 2000
molecules in benzene. Similar images were obtained for nanocomposite films immersed in the other solvents as well and are not shown for brevity. It can be seen that the particles have aggregated and are in fairly close contact. Figure 7B shows a depth profile image bringing out the aggregated nature of the colloidal particle film following ODA dissolution. It can be seen that the aggregation of the silver colloidal particles has occurred in random fashion and there does not appear to be evidence for crystalline packing of the particles in the film. This is likely to be a consequence of random piling of the silver particles as the ODA matrix is removed. Thus, multilayers of colloidal silver particles are formed, with the clusters fairly close packed. The ODA/silver particle composite film could not be imaged. The film surface was observed to be extremely sticky due to the ODA hydrocarbon chains and imaging in the tapping mode could not be done. As can be clearly seen from Figure 4, the final mass loss from the ODA/silver colloid composite films after prolonged immersion in ethanol and benzene solvents is nearly the same. Since the UV-vis spectra recorded from the different solvents at the end of the immersion cycle of the ODA/silver cluster composite films did not show the presence of silver particles in the solvents (Figure 3), the mass loss must be attributed to dissolution of the ODA molecules in the different solvents. This conclusion is supported by the FTIR results shown in Figure 6 where a significant loss in the methylene vibrational modes was observed after prolonged immersion of an ODA/silver particle film in benzene. As shown in the calculation,11 the loss in ODA corresponds to a loss of uncoordinated amine molecules in the film as well as a fraction of the molecules coordinated directly to the colloidal particle surface. To evaluate changes in the silver colloidal particle density in the films before and after dissolution of the ODA matrix in benzene, optical interferometry measurements were performed on the films. After incorporation of the silver particles in the film (step 2, Scheme 1), the film thickness increased to 1000 Å7a and this leads to a volume fraction of 20.5% for the colloidal particles in the film prior to immersion in benzene. After immersion in benzene and loss of the ODA matrix (step 3, Scheme 1), optical interferometry measurements indicated that the film thickness was reduced to 400 Å, giving a cluster volume fraction in the film of 52%. Thus, a large enhancement in the packing density of the colloidal particles occurs on dissolution of the ODA matrix. As briefly mentioned earlier, the optical absorption spectra of the ODA/silver cluster composite films do not change very much during the initial stages of ODA removal
Patil and Sastry
lasting ca. 10 min (Figures 1 and 2). A small reduction in the surface plasmon resonance intensity is observed with no appreciable broadening of the resonance. This region marks the rapid loss by dissolution of uncoordinated ODA molecules from the matrix (QCM results, Figure 4). At this preliminary stage, there is no appreciable change in the dielectric properties of the matrix surrounding the silver colloidal particles and as a consequence, there is no shift in the surface plasmon resonance.10b After ca. 30 min of immersion, however, more prominent changes are observed, as the separation between the clusters becomes smaller. We would like to point out that within 30 min of immersion, nearly all of the ODA mass loss had already occurred (Figure 4). Therefore, the time period beyond 30 min of immersion in the solvents marks the region where there is considerable reorganization in the packing of the colloidal particles in the film, an aspect to which QCM is insensitive but can clearly be observed by UV-vis spectroscopy. There are clearly some differences in the spectra recorded from the films after immersion for long time intervals in nonpolar and polar solvents as mentioned earlier (Figures 1 and 2). This may be a consequence of screening of the electrostatic interactions by the dielectric medium but more detailed measurements are required to clarify this aspect of the work. In conclusion, it has been shown that considerable enhancement in the colloidal particle density in electrostatically formed nanocomposites can be achieved by dissolution of the organic matrix in a range of organic solvents. Optical absorption studies of the aggregated silver particle films prepared by dissolution of the ODA matrix in different organic solvents show subtle differences in the spectra which appear to be related to the dielectric properties of the solvents. This approach shows promise for development in obtaining nanoparticle films of controllable architecture as well as heterocolloidal particle superlattice assemblies for a variety of applications. Acknowledgment. One of us, V.P., thanks the Department of Science and Technology (DST), Government of India, for a research fellowship. This work was funded by a grant to M.S. from the Council of Scientific and Industrial Research (CSIR), Government of India. The authors acknowledge assistance from Dr. S. D. Pradhan, NCL Pune, for the TGA measurements and Mr. Chandan Ganpule, University of Maryland, for AFM measurements.
Received February 2, 1999. In Final Form: November 8, 1999. LA990103Z
