Langmuir 1995,11, 4449-4453
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Mechanistic Studies of Film Growth of Zirconium Bis(phosphonate)Mono- and Multilayer Thin Films. These Things Grow Darned Flat! Houston Byrd,? Jonathan L. Snover, and Mark E. Thompson* Department of Chemistry, University of Southern California, Los Angeles, California 90089 Received J u n e 15, 1995. I n Final Form: July 24, 1995@ Atomic force microscopy (AFM) was used to study thin film growth of zirconium bis(phosphonate)films on silicon substrates under water. We observed a monolayer of zirconium 1,16-hexadecanediylbis(phosphonate) [ Z ~ ( O ~ P - ( C H Z ) ~ ~ -depositing P O ~ ) ] as “islands” on a zirconium-derivatized silicon wafer. ~HZ Images of the zirconated substrate obtained after short exposure to a H z O ~ P - ( C H Z ) ~ ~ - P O(C16BPA) solution correspond to an incomplete monolayer. The surface roughness for an incomplete monolayer is seven times greater than the initial zirconated surface. Upon further exposure t o the ClsBPA solution, the surface roughness decreases and is ultimately very close to that of the original zirconated substrate. 0 AF’Mimages of an The Zr*C16BPAfilm is almost completely formed after a deposition time of ~ 2 0 min. incomplete bilayer film show regions correspondingto the zirconated substrate and monolayer and bilayer coverage.
Introduction The design of new materials using self-assembled organic thin films is currently a n area of interest in materials research. While there are many different approaches in the area of self-assembled films,l one promising and successful method, developed by Mallouk and co-workers, is the layer-by-layer deposition of transition metal p h o s p h o n a t e ~ . ~In - ~this three-step procedure, multilayered films possessing various organic molecules can easily be prepared. In the first step a substrate is chemically derivatized producing a phosphonic acid terminated surface. Examples of such chemical derivatizations include the reactions of chlorosilanes with hydroxylated surfaces (such as glass or silicon) or the reaction of alkylthiols with coinage metal surfaces.l Once formed, the acid-terminated substrate is placed in a solution containing metal ions. In this step, the metal ions are coordinated to the acid surface converting it to a metal-terminated surface. The final step involves preparing a “new” phosphonic acid surface, which can again coordinate metal ions. This is accomplished by placing the metal-terminated surface in a solution containing a n a,w-bis(phosphonic acid). One end of the a,wbis(phosphonic acid) binds to the metal surface leaving the remaining end of the acid uncoordinated. This process is illustrated in Figure 1. Repeating deposition steps a and b by alternating the metal and a,w-bisphosphonic acid solutions results in multilayered films which are constructed one layer at a time. Using this type of approach, thin films have been designed with specific ~ p t i c a l , ~catalytiqs -~ electroni~,~ and ~ ~nonlinear J~ opti*Author to whom correspondence should be addressed. ‘Permanent address: Department of Chemistry, Montevallo University, Montevallo, AL 35115. Abstract published inAduunce ACSAbstructs, October 1,1995. (1) Ulman, A. A n Introduction to Ultrathin Organic Films: From Langmuir-Blodgettto Self-Assembly;Plenum Press: Boston, MA, 1991. (2) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J . A m . Chem. SOC. 1988, 110, 618-620. (3) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597-2601. (4) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420-427. (5) Katz, H. E. Chem. Mater. 1994, 6 , 2227-2232. (6) Vermeulen, L. A,; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. SOC.1993, 115, 11767-11774. @
Figure 1. Steps in the growth of multilayer metal phosphonate thin films. Multilayers are formed by repeating steps a and b.
ca15J1J2properties. While this method has been used extensively, the exact mode of deposition is not clearly understood. There have been many macroscopic and surface science techniques utilized to elucidate the structure of the metal phosphonate films. Ellipsometry, a n optical technique used in determining film thickness, has been the most practiced method in characterizing these f i l m ~ . ~ , ~ J ~ J ~ Ellipsometric measurements have shown a linear increase in total film thickness as a function of the number of layers deposited. 1nfraredl4-l7 and UV-visible6J0Js spectroscopic studies of the deposition process are consistent with the ellipsometric results. These studies have demonstrated a linear increase in the absorbance of the film as a function of the number of layers deposited, indicating that the same amount ofmaterial is placed on the substrate (7) Ungashe, S. B.; Wilson, W. L.; Katz, H. E.; Scheller, G. R.; Putivinski, T. M. J. Am. Chem. SOC.1992, 114, 8717-8719. ( 8 ) Snover, J. L.; Thompson, M. E. J. Am. Chem. SOC.1994, 116, 765-766. .. (9) Kepley, L. J.; Sackett, D. D.; Bell, C. M.; Mallouk, T. E. Thin Solid Films 1992, 208, 132-136. (10)Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Muter. 1991, 3, 699-703. (11)Katz. H. E.: Scheller. G.: Putvinski. T. M.: Schilling. M. L.: Wilson. W. L.; Chidsey, C. E. D. Science 1991,254, 1485-1487. (12) Putvinski, T. M.; Schilling, M. L.; Katz, H. E.; Chidsey, C. E. D.; Mujsce, A. M.; Emerson, A. B. Langmuir 1990, 6, 1567-1571. (13) Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J . Am. Chem. SOC.1994, 116, 9158-9165. (14) O’Brien, J. T.; Zeppenfeld, A. C.; Richmond, G. L.; Page, C. J. Langmuir 1994, 10, 4657-4663. (15) Schiling, M. L.; Katz, H. E.; Stein, S. M.; Shane, S. F.; Wilson, W. L.; Buratto, S.; Ungashe, S. B.; Taylor, G. N.; Putvinski, T. M.; Chidsey, C. E. D. Langmuir 1993, 9, 2156-2160. (16) Yang, H. C.; Aoki, K.; Hong, H.-G.; Sackett, D. D.; Arendt, M. F.; Yau, S.-L.; Bell, C. M.; Mallouk, T. E. J . Am. Chem. SOC.1993,115, 11855-11862. (17) Frey,B. L.;Hanken,D.G.;Corn,R.M.Langmuir1993,9,18151820. (18) Byrd, H.;Whipps, S.; Pike, J. K.; Ma, J.;Nagler, S. E.; Talham, D. R. J. Am. Chem. SOC.1994, 116, 295-301.
0743-746319512411-4449$09.00/00 1995 American Chemical Society
Byrd et al.
4450 Langmudr, Vol. 11, No. 11, 1995
t = 20 min, RMS = 12.8 A
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Figure 2. AFM images of various stages of the deposition of ClsBPA onto a zirconium rich surface. 300 nm x 300 nm images were recorded before treatment with CleDPA (t = 0 min) and after 20, 110, and 200 min. The asterisks indicate the lines along which the line scans of Figure 3 were measured.
after each deposition cycle. Page and co-workers,13using grazing angle X-ray diffraction, have shown uniform growth in the metal phosphonate films from one layer to the next. Variable take-off angle X-ray photoelectron spectroscopy ( X P S )studies revealed that discrete metal layers exist within the film, which suggests that the metal phosphonate films are l a ~ e r e d . l ~ > The ~ Olayered nature of the metal phosphonate films has been confirmed with the
observation of Bragg reflections from films grown on silica The coordination ofthe metal ions by the phosphonic acid groups has been examined by infrared spec(19) Umemura, Y.; Tanaka, K.; Yamagishi, A. J. Chem. SOC.,Chem. Commun. 1992, 67-68. (20) Akhter, S.; Lee, H.; Hong, H.-G.; Mallouk, T. E.; White, J. M. J. Vac. Sei. Technol., A 1989, 7 , 1608-1613. (21) Hong, H.-G.; Sackett, D. D.; Mallouk, T. E. Chem. Muter. 1991, 3, 521-527.
Growth
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Zirconium Bis(phosphonate) Films
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Figure 3. Line scans of the images shown in Figure 2. a, b, c, and d refer to t = 0, 20, 110, and 200 min, respectively. The lines used for these scans are shown by the asterisks shown in Figure 2 (connecting the asterisks).
(a)
h
Langmuir, Vol. 11, No. 11, 1995 4451 used as purchased from the Aldrich Chemical Company (Milwaukee, WI). The surface-priming agent (3-aminopropy1)dimethylethoxysilane (APS)was used as received from Gelest Inc. (Tullytown, PA). Polished silicon wafers were used as substrates. The wafers were a cleaned with 3:l HZSO&O’%H,Oz mixture. (Caution should be used; this mixture is extremely exothermic!) Deposition Methods. The cleaned silicon substrates were placed in a 3%solution (by volume)of APS in toluene overnight. The amine-primedsubstrates were then rinsed with toluene and acetonitrile. The amine surface was phosphorylated using a mixture of POC13 and 2,g-lutidine by the method described by KatzlO producing a surface covered with acidic NHP03H2 groups. The acid-terminated substrates were then placed in a 5 mM solution of Zr-acac in ethanol overnight. Zr-acac is a monomeric species in ethanol solution. It was used here to promote the attachment of uniform monolayer thick films of zirconium species, in a solvent system compatiblewith C&PA, which is soluble in ethanol but insoluble in water. The films were grown from 5 mM solutions in ethanol of C16BPA and Zr-acac. AFM. Atomic force microscopy was performed using a Nanoscope I11 scanning probe instrument from Digital Instruments (Santa Barbara, CAI. Films were imaged in water using a Digital Instruments fluid cell apparatus. The images were taken in contact mode with 200pm narrow leg sharpened silicon nitride tips. The force applied to the surface was between 0.1 and 0.2 nN. Scan rates for the experiments were between 0.3 and 0.5 Hz.
Results and Discussion
Figure 4. Different types of structures for bis(phosphonicacids)
at the surface. (a)Structure expected from formation of lamellar assemblies in solution and (b) one of the structures expected in the early stages offilm growth for single molecules interacting with the surface. t r o s ~ o p y . ~ ~ The - ’ ~ ,data ~ ~ indicate that the phosphonic acid groups fully coordinate the metal ions and that the same number of metal ions are incorporated in the film (ratio of metal to bis(phosphonic acid)) after each deposition cycle.). The metal to phosphonic acid stoichiometry determined by XPS for these films is consistent with the stoichiometry observed in the bulk metal phosphonates (ie.,ratio = 1:l).l6J*Although all of these methods have been useful in understanding the structure of the metal phosphonate films, the exact manner in which the molecules are deposited has yet to be probed. Herein we report evidence, using fluid cell atomic force microscopy (AFM), that demonstrates H203P-(CHz)16PO3Hz (C16BPA) molecules deposit initially as “islands” on the surface. Imaging the film at selected time intervals reveals a n increase in the size and number of islands until the surface is uniformly covered. A sample was prepared which had a n incomplete bilayer. AFM studies of this film show clear steps which correspond to only mono and bilayers of ClsBPA being deposited at the surface. The AFM images indicate that Zr.C16BPA forms extremely uniform films.
In the experiments reported here we used atomic force microscopy (AFM) to examine growth of zirconium bis(phosphonate) thin films on planar substrates. Recently, AFM has been used to image the growth ofboth o r g a n i ~ * ~ , ~ ~ andinorganic crystalsz4in situ. The substrate was a single crystal of silicon, which had been suitably derivatized to produce a zirconium-terminated surface (see experimental section). The silicon substrate was placed in the fluid cell of the AFM, and a 5 mM ethanolic solution of C1~BPA was injected into the fluid cell. We had hoped to be able to monitor the film growing i n situ as a function of time, but our attempts to image the surface directly in ethanol failed. When the fluid cell was charged with water, however, we obtained very good images. Therefore, for the experiments reported here, the substrate was exposed to a n ethanol solution of either C16BPA or Zr-acac in the fluid cell of the AFM, and then the cell was flushed with ethanol followed by water. Images were then obtained in pure water. After imaging, the cell was again flushed with ethanol to remove the water, and the appropriate Zr or C16BPA solution was injected into the cell. The turbulence of flowing a liquid through the cell was great enough that it was not possible to maintain contact between the tip and the surface while the liquid was flowing. For this reason the sequential images shown below are not of the same exact spot on the substrate. We overcame this problem by imaging several different areas after each deposition sequence to ensure that images were consistent over the substrate. AFM images recorded at various stages ofthe deposition of C16BPA onto a zirconium rich surface are shown in Figure 2. The first image (Figure 2a) depicts the zirconium-terminated surface before the CleBPA solution has been injected into the liquid cell. This image shows a highly uniform surface, similar to the initial silicon substrate. Parts b-d of Figure 2 show the zirconated substrate after a 20, 110, and 200 min exposure to the ~~
Experimental Section Materials. H~O~P-(CH~)I~-PO~HZ (C16BPA)was graciously provided by Professor Thomas E. Mallouk (The Pennsylvania
State University). Zirconium(1V)acetylacetonate(Zr-acac)was
(22)Hillier, A. C.; Schott, J. H.; Ward, M. D. Adu. Mater. 1995,7 ,
409-413. (23) Hillier, A. C.; Maxson, J. B.; Ward, M. D. Chem. Mater. 1994, 6,2222-2226. (24) Hillner, P. E.; Manne, S.; Hansma, P. K.; Gratz, A. J. Faraday Discuss. 1993,95,191-197.
Byrd et al.
4452 Langmuir, Vol. 11, No. 11, 1995
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Figure 5. AFM images of two layers of zr*c~&PA.The growth time for layer (60 min) was chosen sto give only partial layer formation at each step; regions of the bare substrate, as well as one layer and two layers of ZraC16BPA,are seen. The line scan is along a line running from the lower left to the upper right corner. The lines in the scan are at f25 and -25 A.
C&PA solution. After 20 min, formation of islands is clearly seen by the sharp contrast between green (film) regions and red (substrate) regions. Further exposure of the C&PA solution produces less of a contrast indicating that more film is being deposited on the surface. The
image obtained after 200 min approaches that of the original zirconated substrate. Quantification of these images were attained by investigating the average height deviation from the surface. The root mean square (rms)of the height deviation for the
Growth
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Langmuir, Vol. 11, No. 11, 1995 4453
Zirconium Bis(phosphonate) Films
A
zirconated surface is 1.8 . First exposure of this substrate to the C16BPA solution increases the rms to 12.8fi indicating the surface is approximately seven times as rough as the initial surface. U on successive expoture, the rms value decreases to 7.8 and finally to 3.6 A . It should be pointed out that the rms values reported here are for the individual images shown in Figure 2 but are very close to the average observed for several different images. We imaged at least four separate areas before and after each deposition (20,110, and 200 min), and the rms values for all of the images of a given deposition step fall within 10% of the reported values. This final rms value indicates that the C16BPA monolayer is slightly rougher, but approaching, the initial zirconated surface. After the final deposition, there is still a small area (lower left corner of Figure 2, t = 200 min) where bare silicon can be observed. Ifthe rms i? calculated excluding this region, the value drops to 2.8 A. The small difference between the monolayer and the initial substrate can possibly be explained by an uneven hydrogen-bonding network between the terminated phosphonic acid groups. Figure 3 shows the height deviation in the films via line traces of the AFM images. The lines used to generate the traces are indicated by the asterisks in Figure 2 (the line which joins the asterisks). The first exposure to the c16BPA solution (Figure 3b) clearly shows steps that are consistent with monolayer coverage. For an all-trans alkane conformation the C16BPA molecule possesses a length of ~ 2 9l 3 and if tilted a t 31" , asFeen in the bulk ~ o l i d , ' would ~ , ~ ~ possess a height of a 5 A. The line scan reveals steps that range between 25 and 30 A , These values are consistent with what is expected for a monolayer. After the second exposure, the step edges are decreased indicating that the bare zirconated substrate areas are beginning to be filled with C&PA molecules. The line scans of the zirconated surface and the image taken after 200 min (Figure 3a and 3d) are nearly identical and demonstrate very small differences in the height of the surface. The data also suggest that deposition is almost complete after ~ 2 0 min 0 which is consistent with the time scale reported by Mallouk.16 We speculate that van der Waals interactions between the hydrocarbon chains lead to lamellar assemblies of the bis(phosphonic acids) in solution. The islands are formed by association of these CleBPA aggregates with the surface (Figure 4a). This aggregate hypothesis is supported by the observation that in the early stages ofgrowth the images are composed predominantly of bare substrate and film of 25-30 fi in thickness. If a significant number of single C16BPA molecules were reaching the surface we would expect to see portions of the film formed by both acid ends of a single molecule adhering to the surface ( F i p r e 4b), which would lead to a film ranging from 5-15 A depending on the distance between the phosphonate groups.
K
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(25) Thompson, M. E. Chem. Mater. 1994,6 , 1168-1175
In order to examine multilayer film growth, two layers of the Zr.Cl&PA were deposited. In this experiment the zirconated surface was exposed to a 5 mM C16BPA solution for 60 min, and then after rinsing the cell with ethanol, the cell was filled with a 5 mM Zr-acac solution for 120 min. The Zr solution was removed, and the cell was again filled with a 5 mM solution of C16BPA for an additional 60 min. The deposition times were chosen so that an incomplete monolayer would be formed in each C&PA growth step, so that an image could be obtained simultaneously showing regions with no film, one layer, and two layers of material. Figure 5 is a surface AFM image obtained after the two C&PA treatments. The AFM image shows three distinct regions: the zirconated surface (red), monolayer coverage (yellow/green), and bilayer coverage (purple). The rainbow color scheme was chosen to illustTate that not very much of the film is between 5 and 20 A (orange) or between 35 and 50 fi (blue), which would be less than a monolayer and between a monolayer and a bilayer, respectively. These three regions are seen more clearly as a height deviation plot obtained from a line trace of the image as shown at the bottom of figure 5. The vertical distances from the highest points to the lowest points on the line trace range from 57 to 61 A, which corresponds to a bilayer of C16BPA. The vertical distances from the midpoint line to the lowest points range from 27 to 31 corresponding to monolayer coverage. There are also regions which possess heights in between these ranges and are possibly due to regions of molecules that are tilted over at the edges of plateaus or are raised due to being zirconium capped. Because the deposition times were less than the time required for complete monolayer coverage, these variations in height are expected. This data taken in conjunctionwith the previous experiment suggests that the ZreC16BPAfilms do initially deposit as fairly large islands on the surface. Upon completion of the deposition, the monolayer film is uniform and indistinguishable from the starting surface.
A
Conclusion AFM images have demonstrated that the deposition of C16BPA on a zirconated silicon substrate occurs as islands. Once formed, the bare areas around the islands are filled until monolayer coverage is complete. The roughness of the monolayer approaches that of the initial zirconated substrate and indicates that a uniform film has been prepared. Bilayer films prepared with sub-monolayer deposition times exhibit step edges that correspond to heights consistent with no coverage, monolayer coverage, and bilayer coverage. The data from both experiments suggest that the deposition of the ZrC16BPA films first occurs as islands, but uniform films are indeed formed.
Acknowledgment. The authors would like to thank The American Biomimetics Corporation and the National Science Foundation (CHE-9312856) for their financial support of this work. LA950471C
