Langmuir 1996,11, 3811-3814
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Self-Assembled Multilayer Formation on Predefined Templates S. D. Evans,”??T. M. Flynn,? and A. UlmanS Physics Department, University of Leeds, Leeds LS2 9JT, U.K., and Brooklyn Polytechnic University, New York, New York Received April 12, 1995. I n Final Form: June 28, 1995@ The ability to form high-definition self-assembled monolayer patterns enables one t o create templates t o control the growth of overlyingstructures. In this paper, we explorethe use of surface plasmon microscopy to study the growth of self-assembled multilayer structures on predefined templates.
Introduction The Langmuir-Blodgett (LB) technique has proved popular as a means for producing organic thin films for situations in which control of the film structure at the molecular level is desired in the z direction (Le., perpendicular to the substrate). The technique and the films formed via it, however, do have several drawbacks. Firstly, their production requires expensive equipment a n d clean room facilities. Secondly, the films formed tend to be mechanically a n d chemically unstable. Thus, there is a demand for any alternative technique which gives the same high degree of control at the molecular level but in which at least some of the limitations mentioned above may be overcome. Self-assembled multilayers have both advantages a n d disadvantages when compared with their LB counterparts. In general, only relatively thin films have been built to date, though there have been some notable exceptions to this. Sagiv et al. prompted interest in this area, showing that, in principle, multilayer systems can be prepared using trichlorosilane derivatives.’ Unfortunately, the propagation of defects led to a rapid deterioration in film quality, a n d only a three-layer sample could be prepared. By taking care with the surface treatment, Tillman et al. demonstrated that regular layer structures of u p to 30 layers could be built.2 Attempts to prevent defect propagation by using polymeric annealing layers have recently proved s u ~ c e s s f u l . ~ Attention has also been paid to alternative techniques based on selective ionic interactions a n d polyelectrolyte^,^-^ the latter having the potential to form optically clear films consisting of hundreds of layers, albeit with a less well-defined layer structure. The potential benefits ofproducing multilayer films via self-assembly include (i) the ease of fabrication, (ii) their high degree ofchemical a n d mechanical stability, and (iii) the high-order parameter achieved in the z direction, i.e., perpendicular to the plane of the substrate surface. The recent work on the patterning of self-assembled monolayers (SAMs) opens exciting new possibilities i n
* Author to whom correspondence should be addressed. +Universityof Leeds. Brooklyn Polytechnic University. Abstract published in Advance ACS Abstracts, September 1,
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1995. (1)Pomerantz, M.; Segmuller, A,; Netzer, L.; Sagiv, J. Thin Solid Films 1985, 132 (NO. 1-41, 153-162. (2) Tillman, N.; Ulman, A,; Penner, T. L. Langmuir 1989,5 (No. 11, 101-111. (3) Marks, T.J.;Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.;Allan, D. S.; Lin, W. P.; Wang, G. K. Langmuir 1993,9, 388. (4) Evans, S. D.; Ulman, A,; Goppertberarducci, K. E.; Gerenser, L. J. J . Am. Chem. SOC.1991, I13 (No.151, 5866-5868. (5) Mallouk, T. E.; Lee, H.; Kepley, L. J.; Hong, H.-G. J.Am. Chem. SOC.1988, 100, 619. (6) Decher, G.; Hong, J. D. Int. J . Phys. Chem. 1991, 95 (No. 111, 1430- 1434.
the design of monolayer/multilayer system^.^ In particular, it leads to the ability to construct multilayer systems on predefined templates, thus obviating the need for postassembly processingllithography. Such templates may have uses, for example, in the growth of optical wave guide structures a n d optical devices. In this paper, we present the results of our recent investigations into the formation of self-assembled multilayers on predefined templates a n d illustrate the use of surface plasmon resonance (SPR)techniques for monitoring the formation and quality of monolayer and multilayer films.
Experimental Section Materials. All thiol molecules were available from previous ~ t u d i e s .Gold ~ wire (99.99+%)was obtained from Advent Ltd. Chromium rods were obtained from Megatech Ltd. Ethanol (HPLC grade) was obtained from Aldrich Chemical Co. SYLGARD elastomer was obtained from Farnell Electronic Components Ltd., matching fluid ( n = 1.5151) was obtained from McCrone Scientific. Substrate Preparation. Gold films of thickness -500 A were evaporated onto glass slides primed with a thin (-30-A) chromium layer. The Cr layer promoted adhesion between the Au and the glass and was necessary to ensure the mechanical stability of the Au film. The metal films were deposited using an Edwards Auto 306 Turbo evaporator at a pressure of 1 3 x mbar; both the gold and chromium layers were deposited at a rate of 1-2 s-l. The AdCr substrates were cleaned prior to use by immersing for 2-3 min in a 7:3 mixture of HzS04: H ~ O ZThe . ~ slides were then rinsed thoroughly in Millipore water and stored under Millipore water until used. All cleaning procedures performed in the course of this work were done so with the proper safety precautions (fume cupboard and appropriate safety clothing). Formation of ElastomericStamps. The use of elastomeric stampsto produce patterned SAMs was first introduced by Kumar et al., who demonstrated that high-resolutionpatterns could be produced by a simple stamping technique without the need for expensive clean-room and lithographic fa~ilities.~ The stamps were formed using the procedure given in ref 9. A 10:1 (vol to vol) mixture of SYLGARD silicone elastomer base and SYLGARD silicone curing agent was poured over the appropriate stamp master and was allowed to cure for 24 h at room temperature. For the results presented here, a simple master was used which consisted of a brass block with grooves of dimension 1mm (wide) x 1 mm (depth) X 19 mm (length) machined into it at 1-mm intervals. The produced stamp gives a striped template which has an interval of 1mm between stripes. The stamp was rinsed thoroughly in ethanol before use.
a
(7) Kumar, A,; Biebuyck, H. A,; Whitesides, G. M. Langmuir 1994, 10 (NO.5), 1498-1511. (8) Caution must be applied when using “phirana” solution. Dobbs, D. A,; Bergman, R. G.; Theopold, K. H. Chem. Eng. News 1990,68 (No. ,v,
0
1 1 1 , Y.
(9) Kumar, A,;Whitesides, G. M.AppZ. Phys. Lett. 1993,63 (No. 141, 2002-2004.
0743-7463/95/2411-3811$09.00/00 1995 American Chemical Society
3812 Langmuir, Vol. 11, No.10, 1995
Evans et al.
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Figure 1. Formation of a self-assembled multilayer film on a preformed template. (A) The template pattern is transferred to the substrate via the stamp. (B) The remaining surface of the sample is then derivatized with the carboxylic acid terminated molecule, giving a patterned monolayer sample. (C) The sample is then immersed in an aqueous solution of copper acetate to provide a bridging moiety for the next thiol layer. After adsorption of the bridging layer, the sample is rinsed thoroughly and reimmersed into the acid-functionalized thiol solution. This process can be repeated until the desired number of layers is reached. Multilayer and Template Formation. The alkanethiol “ink” used in our studies was a 1 mM solution of octadecyl mercaptan (ODT) dissolved in an ethanoVdichloromethane mixture. Only a small amount (1-2 mL) of dichloromethane was used to enhance the solubility of the ODT. Monolayer templates were formed by placing the inked stamp in contact with a clean gold surface (a slight hand pressure was applied to ensure that the pattern was transferred to the gold surface). The sample was then rinsed thoroughly in ethanol and placed in a 1 mM ethanolic solution of COOH-(CHz)lS-SH to derivatize the remaining bare gold surface (immersion time -1 h). Figure 1 shows schematically the steps involved in the formationof a multilayer film on a S A M template. The multilayer structures were fabricated using a sequential deposition technique, Figure 2, employingselective ionic i n t e r a c t i ~ n s . ~After J~ the first layer of the multilayer had been deposited (see above), the patterned sample was ultrasonicated in clean ethanol for 10-20 s and then rinsed thoroughly in Millipore water before being dried in a stream of nitrogen. It was then placed into an aqueous solution of CuAc for 4 min: this provides a bridging moiety for the adsorption of the subsequent thiol layer. The sample was rinsed in millipore water, dried, and reimmersed into the acid thiol solution for the deposition of the next layer in the multilayer structure. Using this procedure, it is possible to build structures of 20+ layers4 Surface Plasmon Imaging. The surface plasmon images of the low-dimensional structures were taken using a system developed at the University of Leeds based on that described by Rothenhausler and Kn01l.l~A BK7 prism (n = 1.515)was used to couple 632.8-nm radiation from a HeNe laser into the gold (10)Freeman, T.L.;Evans, S.D. Thin Solid Films 1994,244,784788. (11)Rothenhausler, B.; Knoll, W.Nuture 1988,332(No.61651,615617.
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Figure 2. Schematic illustration of the selective ionic deposition technique. (A) Formation of the acid-functionalized monolayer. (B) Following the deposition of the copper acetate bridging layer, a second thiol layer is adsorbed. sample under investigation. The subsequent surface plasmon images were collected via the use of a simple planoconvex lens of focal length 40 mm and a square pixel Sony CCD camera (512 x 484 pixels). High-magnificationimages were obtained by the use of a zoom lens attached to the camera. An index matching fluid layer was used between the prism and the sample to reduce back-reflection and was prepared in a laminar flow cabinet to reduce the amount of dust caught in the layer. In the system’snormal mode of operation, it is usual to monitor the reflected light intensity as a function of the angle of incidence of the incident light. CurveA in Figure 3 showsthe characteristic reflectance vs angle of incidenceexpectedfor a region of gold film (500-Athick) coated with a ODT thiol monolayer (18 A). Similarly, curve B shows that calculated for a multilayer coating three layers thick (15 A per layer), i.e., a difference of 27 A. Clearlythe position of the resonance minimumis shifted to higher angles of incidence for the thicker film. By fixing the angle of incidencetoAmin, one can clearly see that the regions of film that are three layers thick will give a greater reflected intensity (i.e., will appear bright). The contrast (difference in reflectivity) between such regions is illustrated in curve C. Note the calculations presented here are obtained using the Fresnel equations (see, for example, ref 12) and assuming a refractive index of the films to be 1.50. (12)de Bruijn, H.E.; Kooyman, R. P. H.; Greve, J.Appl. Opt. 29 (No. 13). 1974-1977.
Langmuir, Vol. 11,No. 10, 1995 3813
SA Multilayer Formation on Predefined Templates 1.o -CIS
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Figure 3. Diagrammatic explanation of the contrast mechanism for SPR imaging. The calculated surface plasmon curves for a template region and a multilayer region are compared (NB, the multilayer film consists of three layers, 15A per layer), assuming a refractive index of 1.5. The lower diagram shows the contrast expected between these regions as a function of the angle of incidence. Positive contrast is defined a when the multilayer region appears bright and, hence, the ODT region dark and vice versa for negative contrast. While the contrast is greatest at 43.90",the results presented here were mostly obtained at A m i n and Bmin. Operating at B m i n would make the multilayer region appear dark, while the ODT region would appear bright. For obtaining maximum contrast within a film, one would clearly like to work at 43.90'; however, for thicker films, i.e., multilayers, this rapidly leads to saturation of the contrast, thus making it beneficial to work closer to Amin or Bmin. Wetting images were obtained by the use of a poly(tetraflu0roethylene) (PTFE) cell to hold Millipore water in contact with the sample. Image Analysis. The images were analyzed using ACCUWARE software in their raw format. Analysis functions in the software package were used to obtain the average pixel intensity over a user-definable area of pixels. We define contrast simply as the differencebetween the average pixel intensity in the ODT and the COOH-terminatedregions. The contrast between regions was measured for each image. EllipsometryMeasurements. The film thickness measurements were made using a BeagleholeInstruments ellipsometer. Software was written to calculate the thickness values and substrate constants using a three-layer model to describe the substrate, film, and ambient structure, all assumed to be isotropic. All samples were precharacterized by ellipsometry before use; hence, n and k were obtained for the clean gold surfaces before film deposition.
Results and Discussion A monolayer template was formed by stamping a striped pattern of ODT onto a gold surface, and the resulting sample was placed in a solution containing the acid functionalized thiol derivative. The sample was then
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Figure 5. SPR image for a three-layer film. Image a is taken a t the resonance position of the ODT film (Amin). Image b is taken a t the resonance position of the multilayer film (Bmin) [NB, reversal of contrast].
mounted vertically, as the back face of a PTFE cell, and Millipore water was added via injection ports located at the top of the cell. Figure 4 shows an SPR image of the three-phase line of water in contact with the striped monolayer sample. Clearly the acid-terminated regions are more hydrophilicthan the ODT stripes, as evidenced by the meniscus profile. Since there is only a small difference, a few angstroms, in the thickness of the monolayer-forming species, the SPR image shows little contrast in the template film. Upon adsorption of additional layers, via selectiveionic interactions, the contrast between the ODT regions and the multilayer regions increases, as the resonanceposition of the multilayer film moves to a higher internal angle (see Figure 3). Figure 5 shows images of a three-layer sample, illustrating the use of SPR as a means of following the multilayer formation. The image in Figure 5a, was taken at A m i n , i.e., ODT on resonance. Thus, the brighter regions correspond to the multilayer film. In contrast, Figure 5b was obtained with the multilayer film "at resonance", i.e., at B m i n , and shows the expected
3814 Langmuir, Vol. 11, No. 10, 1995 A
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reversal in contrast. The stamping technique employed in this work enables one to create a highly defined and uniform template for the growth of multilayer films. However, it is noticeable that defects do occur;at present, we believe that these are due to defects in the stamp itself rather than any deficiency in the technique. The image presented here was specifically chosen to highlight the defect present in the template region and demonstrates the use of surface plasmon microscopy as a powerful tool for the investigation of the uniformity of such structures. In Figure 6, the simple imaging lens has been replaced by a zoom lens and the images presented show how the system can be used to investigate film uniformity in greater detail. It is evident in the images shown, for example, in Figure 6b, that there are some small regions of higher contrast in the template (ODT) region itself. This could be due to penetration of the template layer by the acidterminated thiols, or it could be due to extraneous material on the surface of the sample. In general, we have found the uniformity of these templates to be of a high quality and suitable for controllingthe growth of multilayer films. Surface plasmon microscopy obviously has a lot to offer for determining the qualityhniformity in thin films. The thicknesses of the ODT and the acid-terminated regions were determined independently using ellipsometry. The thickness of the ODT region remained constant, indicating that there was no adsorption of material onto this region during multilayer buildup. Figure 7A shows the variation in contrast (seenin the SPR images) between the multilayer and the ODT regions as a hnction of layer number. The contrast from the SPR images was determined via the method described in the Experimental Section. If the optical constants and thickness of the gold substrate are not known, it should be possible to fit the contrast response as shown in Figure 7A to determine these parameters. The contrast obtained is very dependent on the refractive index and thickness of the gold substrate (n, k, and d), as these determine the shape of the resonance curve. If one knows the refractive index of the gold film, one can convert the changes in intensity to thicknesdcoverage information about the overlying films deposited on the gold. Figure 7B shows the variation in
Layer no.
Figure 7. Variation of SPR contrast and ellipsometricthickness as a function of the number of layers. (A) The variation of contrast between monolayer and multilayer regions as a function of layer number. (B) Variation in thickness, of the multilayer and the ODT region, as a function of layer number.
thickness (determined by ellipsometry) of the ODT template and the multilayer regions as a function of layer number. It is evident that there is negligible change in the ODT region of the low-dimensional structure. Conclusions The use of patterning to form predefined templates for self-assembled multilayer formation has potential for controlling the growth of organic thin films and reduces the degree of postsample fabrication patterning required. The multilayer formation process has been followed using SPR and ellipsometry. In particular, SPR has been used to image the multilayer structures. We are currently extending this technique to look.for inhomogeneities in the monolayer templates and multilayer films. Ellipsometric determination of the “stamped” ODT regions reveals that these films are “thinner” than usually found for ODT monolayers and indicates an average decrease in the coverage of approximately 12%. By choosing an angle so as to give maximum contrast, a greater “thickness” resolution can be obtained for such uniformity studies. At present, our lateral resolution has been restricted by the wavelength of the laser chosen; moving to a shorter wavelength will increase the possible resolution by a factor of -10, thus enabling imaging on the 1-pm scale. LA950293N
