Langmuir 2000, 16, 3553-3556
Formation of Patterned, Heterocolloidal Nanoparticle Thin Films Murali Sastry,* Anand Gole, and S. R. Sainkar Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India Received July 16, 1999. In Final Form: December 17, 1999
Introduction There is much current interest in the area of nanoparticles, motivated to a large extent by their interesting optoelectronic and chemical properties.1-3 Many applications require the assembly of nanoparticles in thin film form and the ability to tailor the interaction between the particles.4 The colloidal route, with its high degree of control over particle size, monodispersity, and chemical composition,5 has emerged as a versatile means of synthesizing the nanoscale building blocks and forms the first step in the “bottom-up” approach to nanoscale assembly. Thin films of the colloidal nanoparticles are formed thereafter using techniques of self-assembly based on different types of interactions.6-8 In this laboratory, we have used electrostatic interactions for organizing charged colloidal particles at the airwater interface using oppositely charged Langmuir monolayers9 and by diffusion into thermally evaporated ionizable fatty lipid films.10 The methods mentioned above have concentrated primarily on the organization of singlecomponent nanoparticle arrays. In this Note, we advance our investigations into synthesis of colloidal nanocomposite films by electrostatic self-assembly10 one step further and demonstrate the formation of thin, patterned, heterocolloidal nanoparticle assemblies of gold, silver, and * To whom all correspondence should be addressed. Ph: +9120-5893044. Fax: +91-20-5893952/5893044. E-mail: sastry@ ems.ncl.res.in. (1) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L Nature 1997, 389, 699. (2) Jarrold, M. F. Science 1991, 252, 1085. (3) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaff, T. G.; Khoury, J.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (4) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (5) Schmid, G., Ed. Clusters and Colloids; VCH: Weinheim, 1994. (6) (a) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (b) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (c) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (d) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (e) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (7) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607 and references therein. (8) Li, M.; Wong, K. W.; Mann, S. Chem. Mater. 1999, 11, 23. (9) (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, 5198. (10) (a) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490. (b) Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 4347. (c) Patil, V.; Sastry, M. Langmuir 1997, 13, 5511. (d) Sastry, M.; Patil, V.; Sainkar, S. R. J. Phys. Chem. B 1998, 102, 1404. (e) Patil, V.; Sastry, M. Langmuir 1998, 14, 2707. (f) Patil, V.; Malvankar, R. B.; Sastry, M. Langmuir, in press.
3553
Q-state CdS. This simple technique is illustrated in Scheme 1. By judicious masking of the thermally evaporated octadecylamine (ODA) surface and sequential immersion in different carboxylic acid derivatized colloidal nanoparticle solutions (steps 1-3, Scheme 1), an in-plane spatially separated assembly of heterocolloidal particles may be obtained. The formation of the heterocolloidal nanoparticle assembly was characterized by quartz crystal microgravimetry (QCM), UV-vis spectroscopy, and energy dispersive analysis of X-ray (EDAX) measurements. We believe this technique has exciting potential for application in micropatterned chemical and biological sensors11 as well as in the growth of advanced materials with novel optical and electronic properties. Presented below are details of the investigation. Experimental Details Gold (130 ( 30 Å), silver (70 ( 13 Å), and Q-state CdS (45 ( 10 Å) colloidal particles were synthesized in an aqueous medium and capped with 4-carboxythiophenol (4-CTP) as described in refs 9a, 9b, and 10b, respectively. The chemisorption of 4-CTP onto the colloidal particle surface via thiolate bond formation leads to carboxylic acid derivatization of the surface and has been used to modify the charge on the colloidal particle surface through variation of the solution pH.9,10 Thin films of octadecylamine (ODA, 500 Å thickness) were thermally vacuum deposited in an Edwards E306 vacuum coating unit operated at a pressure of better than 1 × 10-7 Torr. The film thickness and deposition rate were monitored in situ using an Edwards QCM. ODA films 500 Å thick were deposited onto gold-coated AT-cut quartz crystals for QCM measurements, quartz substrates for UV-vis spectroscopy studies, and Si (111) wafers for EDAX studies. QCM measurements were carried out on a 6 MHz ATcut gold-coated quartz crystal. The frequency changes of the resonator were measured using an Edwards FTM5 frequency counter with a resolution and stability of 1 Hz. For the quartz crystal used, this translates into a mass resolution of 12 ng/cm2. The frequency changes were converted to mass loading using the Sauerbrey equation.12 UV-vis spectroscopy measurements were performed on a Hewlett-Packard 9452A diode array spectrophotometer operated at a resolution of 2 nm. Before immersion of the ODA films in the different colloidal particle solutions, masking of the fatty lipid film surface was effected with a Teflon mask as shown in Scheme 1. The dimensions of the film and mask are indicated in the scheme. The Teflon sheet was held in contact with the film surface by binding with Teflon tape. It was observed that use of hydrophobic masks as opposed to hydrophilic masks (such as glass) improved the colloidal particle diffusion profile into the ODA film along the mask edge. In the first stage of the patterned nanoparticle film formation, ca. 66% of the ODA film surface was masked and the film immersed in 4-CTP capped silver hydrosol until complete silver particle incorporation had occurred (step 1, Scheme 1). The kinetics and extent of colloidal particle incorporation were monitored using QCM. In the next stage, a further 33% of the ODA film surface was exposed and the film was immersed in 4-CTP derivatized colloidal gold solution until completion of colloidal gold incorporation (step 2, Scheme 1). It is to be noted that during this cycle of immersion, the film region containing colloidal silver nanoparticles is exposed to the flux of gold colloidal particles as well. During the final cycle of immersion, the mask is completely removed and the ODA film is immersed in 4-CTP capped Q-state CdS colloidal solution (step 3, Scheme 1). Note that in this last stage, the regions of the film containing colloidal silver and gold are exposed to the CdS nanoparticle flux. The pH of all the colloidal solutions was adjusted to 9 prior to immersion of the ODA films. At this pH, we have observed maximum cluster (11) Semancik, S.; Cavicchi, R. Acc. Chem. Res. 1998, 31, 279. (12) Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206.
10.1021/la990948a CCC: $19.00 © 2000 American Chemical Society Published on Web 02/23/2000
3554
Langmuir, Vol. 16, No. 7, 2000
Notes
Scheme 1. Diagram Showing the Masking Process Used for the Generation of Spatially Separate, Patterned Colloidal Particle Films by an Electrostatically Controlled Diffusion Mechanism into ODA Filmsa
a
The dimensions of the film and the mask used are indicated in the diagram.
Figure 1. QCM mass uptake measured ex situ as a function of time of immersion in the different colloidal particle solution. The different regions of immersion are indicated in the figure. incorporation in the ODA films due to complete ionization of the carboxylic acid groups on the colloidal particle surface as well as the amine groups in the ODA matrix (pKB of ODA ) 10.8)9b thus leading to maximum attractive electrostatic interaction.9,10 After formation of the patterned colloidal nanoparticle film, the edges between adjoining nanoparticle regions were analyzed by spot-profile EDAX chemical analysis carried out on a Leica Stereoscan-440 scanning electron microscope (SEM) equipped with a Phoenix EDAX attachment.
Results and Discussion Figure 1 shows the QCM mass uptake measured as a function of time of immersion of the 500 Å thick ODA film deposited on an AT-cut quartz crystal in the different
colloidal solutions. In the first stage, 66% of the ODA film surface was masked (i.e., 33% of the surface exposed; step 1, Scheme 1) and the films were immersed in the 4-CTP capped silver colloidal solution as mentioned earlier. The change in resonance frequency of the quartz crystal was measured ex situ after different times of immersion. Prior to measurement of the resonance frequency change, the quartz crystals were washed thoroughly with deionized water and dried in flowing N2. Complete silver cluster incorporation occurs within 5 h of immersion (Figure 1) and leads to a brown color in that region of the film (Supporting Information, Figure 1). It is important to achieve complete cluster incorporation in the ODA matrix since in subsequent immersion stages, the already exposed regions are left uncovered and would thus be exposed to the other colloidal solutions (viz. Au and CdS). In one experiment, we observed that incomplete incorporation of silver in the first immersion cycle resulted in diffusion of 4-CTP capped gold into the silver regions during immersion in the gold hydrosol. While the exposed regions can be masked during subsequent immersions to obviate the above-mentioned problem, we preferred this approach since truly seamless, patterned films can be achieved only in this fashion. We would like to remark here that no attempt has been made to probe the depth profile of the colloidal particles in the ODA matrix after formation of the composite film. Issues related to modulation of the electrostatic interaction between the negatively charged colloidal particles and the positively charged ODA molecules as well as detailed analysis of the incorporation of the colloidal particles in the fatty lipid matrix in terms of a one-dimensional diffusion model for colloidal silver,10d
Notes
Figure 2. UV-vis spectra recorded from the different regions of the ODA film after immersion in the different colloidal particle solutions and complete incorporation of the colloidal particle density. The spectra obtained from the different regions of the film are indicated next to the respective curves.
gold,10f and CdS10b have appeared in our earlier reports and will not be readdressed here. The main emphasis of this Note is to demonstrate the possibility of using the electrostatically controlled diffusion in forming patterned heterocolloidal nanoparticle thin films. After complete incorporation of silver particles, the Teflon mask was shifted so as to expose an additional 33% of the ODA surface contiguous to the silver colloidal particle region and the film was immersed in 4-CTP capped gold solution (step 2, Scheme 1). The QCM mass uptake recorded during this stage of gold colloidal particle diffusion was monitored ex situ and is shown in Figure 1. In this case as well, maximum cluster incorporation occurred within 5 h of immersion and led to a dark red color in the gold region of the ODA film (Supporting Information, Figure 1). In the last cycle, the mask was removed and the quartz crystal was immersed in the Q-state CdS colloidal solution (step 3, Scheme 1). The QCM mass uptake during diffusion of the CdS particles is also shown in Figure 1. This stage of cluster incorporation resulted in a light yellow color in the ODA film exposed to the CdS hydrosol (Supporting Information, Figure 1). In the procedure adopted, the silver colloidal particle region was exposed to both the gold and CdS colloidal solutions whereas the gold colloidal particle region was exposed to the CdS colloidal solution. The thickness increase in the films due to incorporation of the different colloidal particles was determined from optical interferometry measurements to be ca. 300 Å (i.e., the final thickness of the film was ∼800 Å). From the equilibrium QCM mass uptakes recorded for different colloidal particles in the ODA matrix (Figure 1), the volume fraction of silver (mass/particle ) 2 × 10-9 ng), gold (mass/particle ) 23 × 10-9 ng), and CdS (mass/particle ) 7.2 × 10-10 ng) nanoparticles in the ODA film is calculated to be 13, 19.4, and 12.8%, respectively. In a parallel experiment, the heterocolloidal particle diffusion was accomplished in ODA-coated quartz substrates by a similar masking/sequential immersion procedure and the UV-vis spectra were recorded at the end of the experiment (i.e., after complete cluster incorporation in the three different regions, the time of immersion taken from the QCM mass uptake equilibration times). Figure 2 shows the UV-vis spectra recorded from the Ag, Au,
Langmuir, Vol. 16, No. 7, 2000 3555
and CdS patterned colloidal particle regions taking care to obtain the spectra well within the individual regions (i.e., at points away from the edges). The surface plasmon resonance from the silver colloidal particles at 430 nm and the resonance from gold at 538 nm can clearly be seen in their respective diffusion regions. The “band edge” for the CdS Q-state colloidal particles at ca. 470 nm can also be observed in Figure 2 from the CdS region of the film. It is to be noted that there is no evidence (within the detection limits of the UV-vis spectrophotometer) of the presence of either gold or CdS colloidal particles in the silver particle region or CdS colloidal particles in the gold particle region of the ODA film. We recollect that the procedure adopted in this study leads to exposure of the silver colloidal particle film region to colloidal gold and CdS solutions while the gold film region is exposed to colloidal CdS solution. To check unambiguously whether there was any intermixing of the different colloidal particles in the various regions of the film, EDAX measurements were carried out in the different colloidal particle regions of the film. EDAX measurements on the silver portion of the nanoparticle film yielded 9.87 wt % silver and 90.13 wt % Si (from the silicon substrate). The gold region of the film yielded 4.04 wt % gold and 95.96% Si while the CdS region of the film yielded corresponding weight percentages of 11.41 for Cd, 4.08 for S, and 84.51 for Si. Thus, it can be seen from the EDAX measurements that there is no evidence for the presence of colloidal particles other than those incorporated by design into the respective regions of the film. This, together with the UV-vis data presented above, is an important result and indicates that once electrostatically incorporated into the ODA matrix, the colloidal particles do not “exchange” with other colloidal particles during successive immersion cycles in different hydrosols. In this sense, colloidal nanoparticle composites with fatty lipid films behave differently from salts of fatty acids where, for example, Cd2+ ions in arachidic acid salt films could be readily replaced with Pb2+ ions by immersion in PbCl2 solution.13a Optical microscopy of the interfacial regions in the heterocolloidal particle film indicated that the edges were sharp and that the electrostatically controlled diffusion of the colloidal particles into the ODA matrix was faithful to the mask morphology. However, a more exacting chemical analysis of the edge separating different colloidal particle regions would provide more information on the extent of intermixing of colloidal particles. A spot-profile EDAX measurement was carried out at the Ag-Au nanoparticle film edge as well as the Au-CdS film edge (Scheme 1 and Supporting Information Figure 1). This essentially consisted of mapping the weight percent concentration ratio of, for example, Au and Cd as a function of distance from the Au-CdS “edge”. The “edge” was chosen to be the visual edge as seen by SEM imaging of the film surface and was the origin for the distance measurement. Figure 3 shows a plot of the Cd/Au weight percent ratio (squares, left axis) measured at different distances from the edge, both within the CdS and Au colloidal particle regions. The Au/CdS weight percent ratio is also plotted (circles, right axis) in Figure 3. The individual colloidal particle regions are indicated in the figure. It is observed that the individual ratios reach saturation values well within the respective regions as is to be expected. What is surprising is the extent of the (13) (a) Ganguly, P.; Paranjape, D. V.; Pal, S.; Sastry, M. Langmuir 1994, 10, 1670. (b) Ganguly, P.; Pal, S.; Sastry, M.; Shashikala, M. N. Langmuir 1995, 11, 1078 (see these references for characterization of ion exchanged fatty lipid films).
3556
Langmuir, Vol. 16, No. 7, 2000
Figure 3. EDAX spot-profile elemental concentration ratios measured as a function of distance from the “visual” edge (see text for details). Squares refer to Cd/Au weight percent ratios (left axis) while the circles pertain to the Au/Cd weight percent ratios (right axis). The individual colloidal particle regions are indicated in the figure.
interfacial region where the density of both colloidal CdS and gold particles is very small. This interfacial region is ca. 200 µm wide as can be seen from Figure 3. A similar interfacial region was observed at the Au-Ag colloidal particle film edge, and this aspect is not understood at this moment. One contributing factor could be the roughness of the mask used where no attempt was made to control the sharpness of the mask edge. The results presented above expose the limitations of conventional microscopy and highlight the importance of a chemical profile analysis of the edge. However, intermixing of the colloidal particles within the interfacial region is not indicated and thus shows that the time-averaged trajectory of the colloidal particles as they diffuse into the ODA matrix is essentially normal to the film surface. This result supports the use of a 1-D diffusion model which we had used to understand the cluster incorporation process in such films10d and indicates that this technique can be further fine-tuned to obtain finer patterns with better colloidal particle integrity within the respective patterned regions. We proceeded further and investigated whether this process is general and can be extended to the formation of patterned fatty lipid films such as salts of fatty acids. In a simple experiment, a 500 Å thick arachidic acid film was thermally evaporated onto silicon substrates, and after masking 50% of the surface with a Teflon mask, the film was immersed in 10-5 M PbCl2 solution at pH ) 6.
Notes
After complete incorporation of Pb2+ ions (which was monitored using infrared spectroscopy),13 the film was masked on the side exposed to PbCl2 solution and immersed in 10-5 M CdCl2 solution for 1 h. The film was then analyzed using EDAX, and it was observed that there was intermixing of Pb2+ and Cd2+ ions in regions of the film well away from the interface. This clearly indicates that the diffusion of Pb2+ and Cd2+ ions into the fatty acid films occurs even within the plane of the film, and therefore, this technique cannot be used for making patterned films of metal salts of fatty lipids. A similar interdiffusion within the plane of the organic film had been observed in our earlier study of spontaneous reorganization of fatty acid films via ion exchange.13b In conclusion, it has been shown that spatially defined, patterned multicomponent colloidal particle films can be grown by a simple procedure based on blocking electrostatically driven diffusion pathways of charged colloidal particles into ionizable lipid films. The diffusion into the films appears to be normal to the film surface (i.e., little lateral diffusion within the plane of the film), and once complete cluster incorporation has been achieved, exchange of clusters with other clusters during immersion in different colloidal solutions does not occur. This result is important and indicates that seamless, multicolloidal particle films can be deposited in this fashion and can be used in the growth of multicomponent polyelectrolyte films as well. While the masks used in this preliminary study were of millimeter dimensions (Scheme 1), the technique can be advanced to grow more elaborate assemblies using complex photolithographic masking procedures. Interesting optical, electronic and chemical properties of the heteroassemblies may emerge. Attempts are underway to extend this approach to the formation of patterned films of ionizable biomacromolecules such as proteins and enzymes due to their importance in biosensor and biocatalysis applications. Acknowledgment. The authors thank Ms. Vijaya Patil for assistance with the experiments. A.G. is grateful to the Council for Scientific and Industrial Research (CSIR), Government of India, for a research fellowship. This work was supported by a special grant to M.S. from the CSIR. Supporting Information Available: Photograph of a heterocolloidal particle assembly of silver, gold, and Q-state CdS colloidal particles in an octadecylamine film by an electrostatically driven diffusion process and suitable masking. This material is available free of charge via the Internet at http://pubs.acs.org. LA990948A
