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Langmuir 1993,9,3446-3451
Binary Self-Assembled Monolayers As Prepared by Successive Adsorption of Alkyltrichlorosilanes Klemens Mathauer and Curtis W. Frank* Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025 Received May 5,1993. In Final Form: September 20, 199P Binary monolayers of octadecyltrichlorosilane (OTS) and ll-(2-naphthyl)undecyltrichlorosilane(2Np) were prepared by backfilling of partial monolayers of one component as a result of exposure to solutions of the other component. The comparison with coadsorbed monolayers showsthat both procedures lead to the same kind of binary monolayers. The ratio of the UV-absorption bands at 224 nm (Bb) and 280 nm (La) of the naphthalene chromophore in UV spectra of binary monolayers is independent of composition, indicating constant orientation of the chromophore. Thus, the UV absorbance of the naphthalene tag in backfilled monolayers could be used to determine the surface composition and thus the original coverage of the partial monolayers. The transient growth process monitored in this fashion does not follow irreversible first-order kinetics. This is proposed to be the result of the rinsing process that is carried out after removing the substrates from the adsorption solution. Part of the monolayers are removed by this process and the portion of removable material depends on surface coverage. Partial monolayers exhibit a liquid character as shown by orientational disorder (UV), reduced lateral interaction of alkyl chains (FTIR), and the occurrence of the full-sandwich excimer in partial monolayers of 2-Np. Backfilling solidifies the monolayers. In such systems the naphthalene chromophores become oriented, the alkyl chains show higher lateral interaction and the full-sandwich excimer disappears in favor of the partial-overlap excimer. Backfilled monolayers show less defects with lower surface density. This is proposed to be the result of the second immersion in a fresh alkyltrichlorosilane solution that contains more monomeric silanetriols to fill vacancies in the monolayer.
Introduction The reaction of alkyltrichlorosilaneswith hydroxylated surfaces to form physically robust monolayers is used for the modification of silica and related surfaces.lI2 With n-alkyl derivatives (n > 10) very uniform, so-called selfassembled monolayers are formed on flat surfaces.3-5The study of the kinetics of the formation of these monolayers and the investigation of the structure of partial monolayers can provide information about the growth mechanism. Of special interest is the question of the relative magnitudes of the contributions of head group/substrate and intermolecular interactions. Weak head group/substrate interactions (like the physisorption of fatty acids on high energy surfaces) can allow for lateral diffusion a t the surface. An island structure of partial monolayers was observed for the adsorption of fatty acids on glass surfaces? For the adsorption of fatty acids onto aluminum surfaces, the contribution of the head group/substrate interaction has been shown to be about equal to the van der Waals interaction of Cl~-chains.~ For the adsorption of octadecyltrichlorosilane on siliconailicon dioxide surfaces, partial monolayers are uniform with a decreased thickness compared to the full monolayer as shown by ellipsometry and by X-ray refle~tivity.~ It is reasonable to assume that in this case the monolayer grows randomly because the reaction of alkyltrichlorosilanes with surface hydroxyl groups should inhibit surface diffusion. However, an island growth process was also observed recently for monolayers ~~
@Abstractpublished in Advance ACS Abstracts, November 1, 1993. (1) Leyden, D. E., Ed. Silylated Surfaces; Gordon & Breach New York, 1980. (2) Grushka, E., Ed. Bonded Stationary Phoses in Chromatography; Ann Arbor Science Publication: Ann Arbor, MI 1974. (3) Sagiv, J. J. Am. Chem. SOC. 1980,102,92. (4) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Axe, J. D. J. Am. Chem. SOC.1989, 111, 5852. (5) Wasserman, S.R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (6) Brockway, L. 0.;Jones, R. L. Adu. Chem. Ser. 1964, No. 43,275. (7) Chen, S. H.; Frank, C. W. Langmuir 1989,5, 978.
of octadecyltrichlorosilane (OTS) on mica that had been treated with steam.8 In situ vibrational spectroscopy methods show that the adsorption of OTS followsa Langmuirian irreversiblefirstorder kinetic p r o c e ~ s . ~In J ~earlier experiments on the kinetics of self-assembly the substrate was removed from the adsorption solution after specified times and then c h a r a c t e r i ~ e d .This ~ ~ ~ procedure ~ is obviously a strong disturbance to the monolayers because the substrates have to be rinsed after taking them out of the solution. On the other hand, this interrupted procedure allows more independent methods to be used to characterize the partial monolayers. Moreover, the rinsing process after removing the substrates can distinguish between chemisorbed and physisorbed material in the monolayer. It is therefore interesting to compare both approaches to the determination of the kinetics of formation of these monolayers. The present paper describes the use of a naphthalene tagged undecyltrichlorosilane (11-(2-naphthyl)undecyltrichlorosilane, 2-Np) to characterize the growth mechanism of monolayers of alkyltrichlorosilaneson fused silica. In a previous study we investigated the coadsorption of 2-Np and OTS on quartz by UV, FTIR, and fluorescence spectroscopy.12 By using excimer fluorescence as a molecular probe for the analysis of the morphology of twocomponent monolayers, we showed that OTS and 2-Np form monolayers with a homogeneous distribution of the chromophores. The average orientation of the naphthalene with respect to the substrate surface was found to be independent of the composition of the monolayers, as indicated by a constant ratio of the Bb to the La transition in the UV spectra. (8)Schwartz, D. K.; Steinberg, S.;Israelachvili, J.; Zasadzinski,J. A.
N.Phys. Rev. Lett.
1992,69,3354. (9) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.: Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (10) Cheng, S. S.; Scherson, D. A.: Sukenik. C. N. J. Am. Chem. SOC. 1992, 114, 5436. (11) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (12) Mathauer, K.; Frank, C. W. Submitted to Langmuir.
0743-7463/93/2409-3446$04.00/0 0 1993 American Chemical Society
Binary Monolayers of OTS and 2-Np
A general problem that arises when determining surface coverage of partial monolayers from measurement of absorbance involves the unknown changes in orientation of the adsorbed molecules that occur with increasing surface coverage. To circumvent this problem, we have converted each partial monolayer to a binary monolayer by backfilling with the second component. UV, FTIR, and fluorescence spectroscopy are used to investigate the influence of backfilling on the orientation and local environment of the naphthalene chromophores in partial 2-Np monolayers. The binary monolayers prepared in this manner are then compared to monolayers prepared by coadsorption of the same components.12 Under the assumption that this procedure leads to full binary monolayers (comparable to monolayers prepared by coadsorption) the UV-absorbance of naphthalene in these full monolayers can then be used to determine the monolayer composition and, therefore, the original partial surface coverage. The objective of this paper is not to specificallyidentify or characterize the kinetics of adsorption of alkyltrichlorosilane self-assembled monolayers. Rather, we want to explore in some detail the experimental consequences of the substrate removal procedure that is commonly used. However, the kinetics determined by our sequential adsorption procedure will be discussed and compared to that obtained by in situ techniques. Our more general molecular-level objective is to explore the nature of the monolayer formation of alkyltrichlorosilanes in terms of island vs random growth, The comparison of the excimerto-monomer ratio in fluorescence spectra of backfilled (partial 2-Np monolayers subsequently filled with OTS and partial OTS monolayers subsequently filled with 2-Np) and coadsorbed monolayers gives information about the growth mechanism of these monolayers. Any inhomogeneity in the partial monolayers as a consequence of island growth should increase the excimer-to-monomer ratio in the fluorescence spectra compared to the coadsorbed case.
Experimental Section
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Time of Immersion [min] Figure 1. Absorbance of the L, band of naphthalene at 280 nm (squares) and the Bb band at 224 nm (circles) as a function of immersion time for the adsorption of 2-Np on fused silica (6 X 106 M, hexadecane). The empty symbols represent data for partial monolayers of 2-Np, whereas the solid symbols represent data for the monolayers backfilled with OTS. for material deposited as a consequence of the wiping procedure. The backfilling of partial monolayers was performed with a 2 mM solution of the second component for 24 h, approximately 8 h after the preparation of the partial monolayers. Before reimmersing the substrates in the trichlorosilane solutions, they were rinsed with methylene chloride. FTIR transmission spectra of the monolayers on the quartz slides were recorded on a BIORAD Digilab FTS-GOA single beam spectrometer equipped with a He-Ne laser and a TGS detector. Spectra were recorded at 4-cm-1 resolution and 1024 scans were coadded. A single beam reference spectrum of a freshly cleaned quartz slide was recorded before each series of measurements (usually five spectra) and used as a background spectrum in the recording of the monolayer spectra. UV spectra on 2-Np were obtained with a Varian CARY 3 UV-vis spectrometer with a clean slide as reference. Steady-state fluorescence spectra were recorded on a SPEX Fluorolog 212 spectrometer equipped with a DM3000F data system. Corrected emission spectra were recorded in the front-face mode in air with an excitation wavelength of 285 nm and slits of 4 mm.
The synthesis of ll-(2-naphthyl)undecyltrichiorosilane(2-Np) is described elsewhere.12 Octadecyltrichlorosilanewas purchased from Hlils and used as received. Hexadecane (Aldrich) used for the adsorption solutions was passed through neutral alumina (Brockman Activity I) prior to use. Fused silica slides used as substrates were first cleaned with 2-propanol and wiped with Kimwipes, rinsed with chloroform and then treated with a mixture of concentrated HzSO, and HzO2 (7/3 (v/v))lS for 30 min at 90 OC. After cooling to room temperature, the slides were rinsed with deionized water and dried with a stream of nitrogen. To prepare the partial monolayers the freshly cleaned slides were immersed in a 6 X 1W solution of alkyltrichlorosilane in hexadecane in 20-mL scintillation vials that had been cleaned by storing for a minimum of 24 h in a 20% KOH solution in 2-propanol and thoroughly rinsed with deionized water and dried at 150 OC in an oven. All solutions were prepared and handled in a nitrogen atmosphere (AldrichAtmosbag). After different times the adsorption was stopped by transferring the substrates to scintillation vials containing methylene chloride. In order to remove any solutiondeposited material, such as siloxane polymers resulting from solution condensation, the substrates were wiped off with 100% cotton tissues that had been dipped in 2-propanol. The substrates were then rinsed with chloroform and ethanol and dried with a stream of nitrogen. The fused silica slides used as reference samples for spectroscopywere treated in the same manner except that pure hexadecane was used instead of a trichlorosilane solution. UV, FTIR, and fluorescencespectra showedno evidence
Results UV Spectroscopy. Figure 1shows the absorbance of the La and Bb bands of naphthalene as a function of immersion time in 6 X 106 M hexadecane solution for the partial monolayers of 2-Np before and after backfilling with OTS. A striking feature is the strong decrease in absorbance for the Bb band at 224 nm after backfilling, whereas the absorbance of the La band at 280 nm remains the same. The drop in absorbance for the Bb band can be explained by a reorientation of the naphthalene with the long axis (which is the direction of the transition dipole moment for this band14)becoming more perpendicular to the substrate due to the backfilling. As Figure 2 demonstrates, this orientation occurs also at higher surface coverage of a partial monolayer of 2-Np. Going from low to high surface coverage,the average orientation reflected in the BdLa ratio changes gradually from a value close to that found in a solution spectrum (&/La = 19.3) to one that is observed in coadsorbed monolayers of 2-Np and OTS for all compositions (&,/La = 5 ) . This clearly demonstrates that it is very important to consider orientation effects when interpreting transmission absorbance values in terms of a particular model for adsorption kinetics. As we can see from Figure 1only the Bb band at 224 nm and, thus, the orientation of the long axis transition of
(13)Pintchovski, F.; Price, J. B.; Tobin, P. J.; Peavey, J.; Kobold, K. J . Electrochem. Soe. 1979,26, 1428.
(14) (a) Klevens, H. B.; Platt, J. J. Chem. Phys. 1949, 17, 470. (b) Platt, J. J . Chem. Phys. 1949, 17, 484.
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2-Np Surface Coverage in Backfilled Monolayers Figure 2. BdL. ratio as a function of surface coverage of 2-Np (determined from the absorbance of the Bb band of backfilled monolayers (Abs(100T) = 0.065)). The empty circles represent the ratio for partial monolayers of 2-Np and the solid circles are for partial monolayers of 2-Np backfilled with OTS. The solid triangles represent data of partial monolayers of OTS that were backfilled with 2-Np. The surface coverage is also that of 2-Np as determinedby the UV-absorbanceof the backfied monolayers.
naphthalene with respect to the surface changes as a function of surface coverage. We can therefore estimate the average tilt angle a of the long axis of the naphthalene with respect to the surface normal using the isotropic solution case as reference (sin2a = (5n9.2) ( sin2a)).This leads to an average tilt angle of 25O in complete monolayers, a value which is in the range of the tilt in close-packed naphthalene planes of crystals (33O).'5 For the backfilled monolayers of OTS and 2-Np the &/La ratio is independent of the immersion time and is in close agreement with the value for the coadsorbed monolayers. Therefore, it is possible to use the absorbance of the backfilled samples to determine the surface coverage. It can also be seen in Figure 2 that backfilling of partial OTS monolayers with 2-Np leads to the same value of &/La for all different surface concentrations. From the UV results we conclude that the orientation of 2-Np molecules in partial monolayers is considerably different than the situation in a complete monolayer. A t low surface coverage (
