A novel self-assembling monolayer film containing a sulfone

Sep 1, 1990 - Bin Zhao, David Mulkey, William J. Brittain, Zhihao Chen, and Mark D. Foster ... Krishna M. R. Kallury , John D. Brennan , and Ulrich J...
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Langmuir 1990, 6, 1512-1518

1512

A Novel Self -Assembled Monolayer Film Containing a Sulfone-Substituted Aromatic Group Nolan Tillman, Abraham Ulman,* and James %. Elman Corporate Research Laboratories, Eastman Kodak Co., Rochester, New York 14650-2109 Received July 31, 1989. In Final Form: March 2, 1990 The preparation of a novel self-assembling monolayer film containing an aromatic ring substituted with

a sulfonyl group is described. The quality of the monolayer film is evaluated by spectroscopic methods,

including FTIR and XPS,and by physical methods, including ellipsometry and contact angle measurements.

A comparison with an analogous aromatic ester-containing monolayer film is presented. The results indicate that a donor-acceptor substituted phenyl ring containing the sulfonyl group can be incorporated into selfassembled monolayers. The ester-containing material formed monolayers with anomalously low contact angles for nonpolar liquids such as hexadecane, indicating greater disorder in the monolayer structure.

Introduction Recently, we demonstrated that monolayers containing simple chromophores and aromatic rings can be fabricated with good ordering and orientation of monolayer structural features,'$ and it has been shown that multilayer structures of up to 25 layers in thickness can be ~repared.~ However, films w i t h more p o l a r c h r o m o p h o r e s h a v e n o t b e e n reported so far. It is by no means clear what the precise effects of incorporating polar structures will be on the quality of the monolayer and multilayer films and on t h e process of self-assembly. The effects of such parameters as the size of the chromophore, its shape, the magnitude of its dipole m o m e n t , and t h e effect of dipole-dipole interactions are i m p o r t a n t questions which need t o be addressed. We report here that we have obtained good, oleophobic monolayer films from dodecyl 4- [ (11-(trichlorosily1)undecy1)oxylphenyl sulfone (1) (Figure 1). W e have evaluated this film b y a n u m b e r of spectroscopic a n d physical methods. We present these results a n d compare them with results obtained with analogous monolayer f i i s from the analogous, ester-containing material, nonyl

4-[ (11-(trichlorosilyl)undecyl)oxy]benzoate (2).

Experimental Section Experimental methods for ellipsometric measurement of film thicknesses, FTIR-ATR spectroscopy of monolayers, contact angle measurements, and determination of critical surface tensions have been adequately described previously.1 FTIR spectra were run on an IBM IR44 spectrometer, and peak frequencies are accurate to f2 cm-l. Substrates. Substrates employed were cut, cleaned silicon p-doped, test grade silicon wafers (Wacker Chemitronic GMBH) or ATR crystals (Harrick), cleaned with detergent by using a soft, camel-hair brush followed by radio frequency plasma cleaning in 0.1-Torr argon at 30 W power for -30 min. In place of the radio frequency plasma treatment, brief (15-30 s) immersion of silicon wafers in CrO,/sulfuric acid, followed by rinsing with water and distilled water and drying with nitrogen gave results which were indistinguishable. Aluminized silicon wafers were prepared by vapor deposition (at 104 Torr) loo0 A of aluminum onto cleaned silicon wafers (75 X 25 mm). The thickness was monitored with a quartz crystal thickness monitor. The chamber was back-filled with nitrogen after the deposition. (1)Tillman, N.; Ulman, A,; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. SOC. 1988,110,6136. (2) Tlllman, N.; Ulman, A.; Elman, J. F. Langmuir 1989,5, 1020. (3) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989,5, 101.

0743-7463/90/2406-1512$02.50/0

1

2

Figure 1. Structures of sulfone 1 and ester 2. Monolayer Preparation. Preparation of self-assembled monolayers of I and 2 follows previously published methods.' Monolayers were prepared by immersing the cleaned silicon wafers or ATR crystals in freshly prepared, filtered silanizing solutions which were nominal 0.1% (-2 mM) in concentration. The silane was weighed directly into a solvent mixture consisting of 411 hexadecane/CC&. Silane solutions of 1 were heated with a heat gun to facilitate dissolving of the trichlorosilane.The hexadecane/CC& solvent mixture had been allowed to stand overnight over a few drops of water and was decanted prior to use. Silane solutions of 1 were particularily prone to cloud upon standing, and consequently they were gravity filtered through Whatman no. 2 qualitative filter paper prior to use; this removed the cloudiness successfully for a brief period of time, and the silanizing solutions were used rapidly after filtration. The substrates were immersed for 3-10 min, removed, and rinsed with heptane, methanol, and water, in that order. Then the monolayer-coated substrates were cleaned with detergent/ water, rinsed with hot tap water followed by distilled water, and dried in a stream of nitrogen. Ellipsometry and contact angle measurements indicate that this treatment removes loosely adsorbed material and does not disrupt the monolayer per se or result in adsorption of impurities onto the monolayer surface. This process was repeated until stable films with maximum hexade0 1990 American Chemical Society

Substituted Self-Assembling Monolayer Film

Langmuir, Vol. 6, No. 9, 1990 1513

lb

I

3

4

I

1

Figure 2. Conditions: (a) Br(CH2)&Hs, EtONa/EtOH; (b) H202/HOAc; (c)H&=CH(CH&,OH, NaH/DMSO; (d) HSiCla, cat. HpPtCl~/MeO(CH2)20Me. cane contact angles and unchanging film thicknesses (from ellipsometry measurements) were obtained. Monolayers of 1 on aluminized silicon wafers were prepared by immersing the clean substrate into a ca. 5 X 10-3 M silanizing solution, with use of only short immersion times (C20 s).l The monolayers were rinsed only with CCld and dried with a steam of nitrogen. X-ray Photoelectron Spectroscopy (XPS). Routine XPS spectra, both survey and high-resolutionscans, were obtained on a Surface Science Instruments SSX-100 photoelectron spectrometer with a monochromatic A1 K a X-ray source (1486 eV) with an electron acceptance half-angle cone of 12O. Angleresolved XPS spectra were obtained on a Hewlett Packard 5950A photoelectron spectrometer equipped with an A1 K a X-ray source and a Surface Science Laboratory Model 259 angular-rotation probe. Previously published procedures were followed.2 The magnification of the four-element electron lens was -2.3. The electron acceptance half-angle cone was 2.8O. All samples were analyzed at ambient temperature and at a typical pressure of 2 X 1O-BTorr. No evidence was obtained indicating radiation damage for any samples. All spectra were referenced to the C 1s peak of neutral carbon, which was assigned a value of 284.6 eV. The estimated error in the ESCA experiments is *5%.

Results and Discussion The preparation of molecule 1was carried out according to the procedures described in Figure 2. Hydrosilation4 of sulfone 3 proceeded without difficulty in 1,2-dimethoxyethane solvent and gave 1 in 71% yield. Thus, the sulfonyl group does not appear to inhibit CPA-catalyzed hydrosilation (CPA = chloroplatinic acid hexahydrate) and should be easily fabricated into a variety of materials for self-assembled monolayer preparation. The solvent was removed by distillation and vacuum pumping, leaving an off-white solid residue which was >85% pure by lH NMR and was used as is. We note that high molecular weight alkyltrichlorosilane derivatives of aromatic compounds are very difficult (if not impossible) to purify. These moisture-sensitive materials cannot be chromatographed, distilled, or recrystallized. We, therefore, recommend that the final hydrosilation reaction will be carried out with highly pure starting materials and solvents and with enough excess of trichlorosilane to drive the reaction to a quantitative yield. (4) Lukevics, E. Usp. Khim. 1977,46,507.

Efforts at crystallization of 1 from nonpolar, anhydrous solvents such as heptane proved unsuccessful. Estertrichlorosilane 2 was prepared by hydrosilation of the appropriate ester-alkene using the same procedure as for 1 and could be distilled through a short Vigreux column at high vacuum, without, however, much improvement in purity by 'H NMR. The nature of the impurities appeared to be the same as for 1, Le., some residue from the dimethoxyethane. The yield was 64%, and the purity was 85 5% by 'H NMR. The preparation of oleophobic monolayers from 1 on cleaned silicon wafers (we employ the notation 1/Si for this monolayer) and on ATR crystals was straightforward and followed published procedures.' Silicon wafers coated with monolayers of 1 could be retracted completely dry from the filtered silanizing solution (-2 mM concentration of 1 in 4/ 1hexadecane/CCh) after 1-3 min of immersion. Such short immersion times are usually experienced for good monolayer-formingmaterials, where the competition with the bulk film formation is negligible.' However, to ensure reproducibility, monolayers were immersed, removed, rinsed and cleaned, and reimmersed (using 3-min immersion times) until stable film thicknesses (measured by ellipsometry) and contact angles (for water and n-hexadecane) were obtained. In most cases, maximal thickness and highest contact angles were achieved immediately after the first immersion, although in other instances further increases in these parameters (indicating improvement in monolayer quality and close-packing) were obtained with further immersion. The procedure for preparing 2/23 monolayers followed that for l/Si; however, monolayers of 2/Si could not be obtained which were completely oleophobic to a silanizing solution of 2 (ca. 5 mM) in hexadecane/CC&. Contact angles parallel this observation (see below), with hexadecane contact angles found for monolayers of 2 being considerably lower than those found for 1. The f i thickness of monolayers was determined within 30 min of fabrication by ellip~ometry.~?~ The calculation of film thickness values employed an estimated film refractive index of nf= 1.55 for both 1and 2.' The results were a thickness of 30 f 3 A for 1 and 31 f 3 8, for 2. This compares with estimated monolayer thicknesses of 37 A for 1 and 35 A for 2, if it is assumed that the monolayer is formed with all-trans alkyl chains with the chain axes perpendicular to the substrate (although we believe that a certain concentration of gauche bonds should exist at room temperature). According to this model, each C-C bond and the aromatic ring C1-C4 axis would be tilted by 35' from the alkyl chain axis and the axis normal to the substrate surface (consider Figure 1). The observed film thicknesses are consistent with monolayers which have alkyl chains tilted at an angle of approximately 30'. It is important to emphasize that this is not a registered tilt, and thus no conclusion about the degree of order in these molecular assemblies can be drawn from the ellipsometric results. However, ellipsometry, contact angle measurements, XPS, and IR spectroscopy (see below) are inconsistent with a film structure of molecules which lay flat on the substrate surface, and thus they support the suggested monomolecular layer structure. The wettability of a monolayer surface provides a characteristic property of monolayers which reflects the composition of the monolayer-air interface. Contact angles

-

(5) Allara, D. L.; Nuzzo, R. G. Longmuir 198S,l, 45. (6) Bashara, N. M.; Hall, A. C.; Buckman, A. G. Ellipsometry. In Techniques of Chemistry; Weissberger, A., Rossiter, B., Ede; Wiley: New York, 1972; Vol. 1, Part IIIC, p 453.

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Tillman et al.

Table I. Wettability Data for USi, 2/Si, and OTS/Si Monolayers on Silicon Wafers contact angle

B,O

deg

monolayer

HzO

CHzIz

n-C16H~

l/Sic 2/SP OTS/Si OTS/Si

104

71 62 73

42 22 45 46

100 111 112

ref 20.0 23.4 20.2

this work this work 1 8

Contact angles are advancing angles. Critical surface tension (mN m-l); for details, see text. Estimated error in contact angle measurements is on the order of f 2 O .

appear to be sensitive only to surface-localized functional groups which are in direct contact with the test liquid.7 Monolayers of 1, if formed with an approximately closepacked, perpendicular arrangement of the alkyl chains, should form a monolayer surface composed entirely of close-packed methyl groups. Therefore, the wettability properties should be comparable to OTS/Si (OTS = octadecyltrichlorosilane)and other monolayers with such methyl surface^.^**^^ Any disorder of the C12 alkyl chain due to, for example, a considerable number of gauche bonds should be manifested by a decrease in contact angles (especially that of hexadecane).' We have measured contact angles for water and other test liquids and determined critical surface tensions (yc) by the method of Zismanlousing a series of n-alkane liquids. The results for 1/Si and 2/Si are presented in Table I. Indeed, the value of yc is, within experimental error ( f l mN m-l), equal to that found for OTS/Si,l as expected for an ordered methyl surface, indicating that the alkyl chain did not fold and that concentration of gauche bonds at the surface should not be high. The water contact angle is lowered from the OTS value by 7-8O. Similar behavior, Le., reduction in water contact angle, accompanied by a relatively small reduction in HD contact angle was observed by Bain and Whitesides in their work on depth sensitivity of wetting." Note also that in the case of 2/Si the advancing water contact angle is 100" (less than on polyethylene), while that of HD is 22O. Thus, water and HD contact angles do not always go "hand-in-hand", and we believe that these data should not be used to imply a degree of disorder in the monolayer. It may well be that details such as the exact orientation of the methyl and methylene groups at the surface are important, and contact angles are really not well understood at this molecular level. This pattern observed for the various test liquids on 1/Si is comparable to data published by the Sagiv group for partial monolayers of OTS/Si a t ca. 65-70% surface coverage.8 We also calculate similar coverage (-70% f l o % ,relative to OTS/Si) for the 1/Si monolayers from the FTIR data obtained on silicon ATR crystals (see below). It is important to note that the cross sectional area of a tilted phenyl ring (- 30°)is 25 A2 and that of an alkyl chain is -20 A2;thus, a coverage of 80% for 1/Si and 2/Si (based on OTS)is in fact 100% coverage based on the phenyl cross sectional area. However, this coverage does not by any means imply that the assembly should be disordered. In fact, work in our laboratory on alkanethiol molecules similar to l, even with a smaller total number of methylene groups, indicates the facile formation of self-assembled monolayers on gold surfaces, with wetting

-

(7) Holmes-Farley, S. R.; Whitesides, G. M. Langmrrir 1987,3,62. (8)Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (9) Pomerantz, M.; Segmuller, A.; Netzer, L.; Sagiv, J. Thin Solid Films 1985.132. - - , - - - , 153. ~ - (10) Zisman, W. A. Adu. Chem. 1963,43, 1. (11) Bain,C. D.; Whitesides,G. M. J . Am. Chem. SOC.1988,110,5897.

properties similar to that of 1/Si.12 Furthermore, results of 200-ps molecular dynamics (MD) simulation on molecules similar to 1 indicate that while the alkyl chain facing the substrate is somewhat disordered, but tilted, the chain facing the air-monolayer interface is ordered, mostly all-trans (-2% gauche bonds near the surface), and tilted (the molecular tilt is -3Oo).I3 Thus, the MD simulation is in agreement with our wetting results, i.e., the fact that we basically observe a methyl surface. On the other hand, monolayers of 2/Si showed not only diminished water contact angles relative to OTS/Si but also very low n-hexadecanecontact angles (120O lower than expected) and a critical surface tension over 3 mN mhl greater than that characteristic of OTS/Si. This is unlikely to be due exclusively to the shorter length of the C9 alkyl chain than the C12 chain on 1, since previous results from this lab for a monolayer containing a midchain phenyl ether with a Cg chain exposed to the interface gave oleophobic contact angles and a yc value equal to those of OTS/ Si.'J4 Nor is there any evidence from ellipsometry or IR data (see below) for 2/Si or other ester-containing m o n ~ l a y e r s , l ,which ~ J ~ indicates that extensive hydrolysis of the ester group occurs either during the monolayer formation process or as a result of our sample handling. Therefore, we believe that these data indicate that a significantly greater amount of alkyl chain disorder occurs in the 2/Si samples than in the 1/Si or OTS/Si monolayers. Molecular dynamics results in our lab indicate that the sulfone group plays a vital role in the packing of molecules similar to 1, through electrostatic interactions,13 which may provide some explanation to our observations in the present work. Grazing angle FTIR spectroscopy represents an extremely useful method for estimating alkyl chain orientations in monolayers adsorbed on reflecting metallic surfaces due to the strong polarization normal to the metal surface which is e ~ t a b l i s h e d . ~ J ~In- ' an ~ oriented monolayer, transition dipoles show varying tilt angles with respect to this polarization vector, and the orientation of various molecular features can be estimated from the relative intensities of the bands observed. Previous investigators have used this technique to estimate the orientation of the chains in long-alkyl-chainthiols and disulfides on g0ld,~9,2~ of octadecyltrichlorosilane (OTS) on gold,21and of OTS and phenoxy-containing alkyltrichlorosilane monolayers on aluminized surfaces.l A grazing angle IR spectrum of 1/Al is presented in Figure 3 (monolayers of 2 could not be obtained on aluminized surfaces). The stretching modes between 2700 and 3000 cm-' are well characterized CH2 and CH3 vibrations, and the intensity ratios of u,(CH2)/uS(CH3) (2852/2878 cm-', 1.40) and of ua(CH2)/v,(CH3) (2921/ (12) Evans, S. D.; Ulman, A,; Shenidman, Y.; Eilers, J. E.; Ferris, N. Submitted to J . Am. Chem. SOC. (13) Shnidman, Y.; Ulman,A.; Eilera, J. A.; Evans,S. D., in preparation. (14) Furthermore, results with compounds similar to 2, but with a CIS chain replacing the CSchain or with the aromatic ester-ether inverted and a CISchain facing the surface, also gave deficient n-hexadecanecontact angles (536') (Tillman, N.; Ulman, A., unpublished results). (15) Maoz, R.; Netzer, L.; Gun, J.; Sagiv, J . J. Chem. Phys. (Les Ulis, Fr.) 1989,85, 1059. (16) Greenler, R. G. J . Chem. Phys. 1966,44,310. (17) Rabolt, J. F.; Jurich, M.; Swalen, J. D. Appl. Spectrosc. 1985,39, 269. (18) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J.Chem. Phys. 1983, 78, 946. (19) Nuzzo, R. G.; Fusca, F. A,; Allara, D. L. J . Am. Chem. SOC.1987, ~~

109. 23.58. --,

(20) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C.F. D. J. Am. Chem. SOC.1987, 109,3559. (21) Finklea, H . 0.; Avery, S.;Lynch, M.; Furtach, T. Langmuir 1987, 3, 409.

Langmuir, Vol. 6, No. 9, 1990 1515

Substituted Self-Assembling Monolayer Film

Table 11. IR Spectral Data for Monolayers of 1/Si and 2/Si Compared with OTS/Si intensity, 4 band, assign- abs units (calcd), run' cm-1 mentb X 102c Rd de@ 2959 dCH.4 wf 2921 2.08 0.98 63 1.25 0.98 63 2851 0.59 37 1598 0.34 0.73 458 1580 0.158 0.73 0.15 45 1500 OTS/Si 2960 Wf 2919 1.07 3.53 80 2.23 80 2851 1.07 wf 1/Si 2959 2.11 2921 1.07 80 1.25 2852 1.10 90 0.35 1598 0.45 31 0.158 1580 0.63 39 0.20 0.57 1499 36 wf OTS/Si 2960 3.54 1.08 85 2919 2.23 1.11 90 2851 Wf 1/Si 2959 2921 2.47 1.01 69 2852 1.48 1.00 67 0.46 30 1598 0.37 1580 0.178 0.718 438 0.60 38 0.15 1500 2/Si 2960 Wf 2923 0.95 60 1.84 0.94 1.13 2853 58 1713 0.43 0.48 0.46 33 1609 0.75 0.13 47 1513 wf OTS/Si 2960 1.05 75 2918 3.56 1.08 86 2.25 2851 a Each run was performed on different silicon ATR crystals. Assignments for aromatic ring stretching modes @a, Bb, 19a) are based on comparisons with closely related compounds as compiled in ref 36. e Maximum band intensity for p-polarized light (Zlill). d R = Zl(Z~ll. e Calculated from R as described in ref 14. f Weak intensity; shoulder on wa(CH2) band. 8 Uncertain because band is incompletely resolved from vibration 8a. Uncertain due to base-line drift during experiment. monolayer 1/Si

1 " ' l " " l " " l " " l " ' ~ I 3400 3000 2500 2000 1500 Wavenumbers

1000

Figure 3. Grazing angle FTIR spectra of l/Al.

2965 cm-l, 3.55) indicate that the alkyl chains are not perpendicular to the surface. It is important to note that such resolved narrow C-H vibration peaks, as those in Figure 3,are typical of a monolayer assembly; however, this spectrum does not preclude the existence of gauche bonds, especially in the lower part of the molecule, as was indicated above, and in low concentration. Therefore, the IR results can give only an estimate of the tilt angles. In the case of a bulk film of molecules that lay flat on the surface, however, the CH2 vibrations appear as broad peaks, and the CH3 vibrations are not resolved. The peaks a t 1500 and 1599 cm-l are in-plane C=C aromatic stretching modes and are of symmetry class CzU and species al, in the direction along the principal axis (between the C1 and Cq phenyl c a r b o n ~ ) . ~ ~ 1 ~ 3 The sulfone (S02) symmetric stretch (us) is assigned to the strong band at 1152 cm-l and the asymmetric stretch (Va) to the weaker band at 1300 cm-l. These intensities suggest that there is more tilt of the aromatic moiety in the plane of the ring (which affects us) and much less in the direction perpendicular to the plane of the phenyl ring (which affects v,). The ether (=COC) vibrations are assigned to the bands at 1090 and 1179 cm-l for vg and v,, respectively.22~~3 Monolayers of 1/Si and 2/Si for FTIR-ATR were prepared on silicon with 45O incidence angle internal reflection elements in the same fashion as that followed for monolayer preparation on silicon wafers. The FTIRATR t e c h n i q ~ e ~provides l ~ ~ f ~ a~ method with sufficient sensitivity to detect monolayer films. The method provides information by which relative surface coverage can be estimated, relative to a standard monolayer film, as well as information on peak frequencies and line widths, which, in the case of C-H stretching modes, has been qualitatively associated with relative 'liquidity" or "solidity" of the films.1J9*2s-27T h e use of polarized light allows the measurement of ATR dichroic ratios, which are known to vary with the tilt of IR transition dipoles relative to the axis perpendicular to the substrate surface.s*2s The preparation of monolayers of 1 and 2 on silicon ATR crystals was similar to that on silicon wafers and gave contact angles and film thicknesses equal to the values found on silicon wafers. ~

~~

~~

(22) Nakanishi, K. Infrared Absorption Spectroscopy (Practical);

Holden-Day: San Francisco, 1966. (23)Bellamy, L. J. The Infrared Spectra of Complex Molecules, Part One, 3rd ed.; Chapman and Hall: London, 1975;Part Two, 2nd ed., Chap man and Hall: London, 1980. (24) Harrick,N. J. Internul Relection Spectroscopy; Harrick Scientific: Ossining, New York, 1979. (25)Mirabella, F. M., Jr. Appl. Spectrosc. Reu. 1985,2I, 45. (26) Zbinden,R. Infrared Spectroscopy of High Polymers; Academic: New York, 1964. (27) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.

*

Some FTIR spectral features of 1/Si and 2/Si monolayers are collected in Table 11, along with comparable results found for OTS/Si monolayers on the same ATR crystals. We present here frequencies, band assignments, intensities, and dichroic ratios, R. We define R = IL/Ili, where II and 11, refer to the intensities of light polarized parallel (p-polarized) and perpendicular (s-polarized) to the incident plane, respectively. Also included are calculated transition dipole orientations, 4 (see below). The assignments for the aromatic ring stretching bands (sa, ab, and 19a) of Table I1 are made on the basis of assignments for closely analogous structures compiled by Varsanyi.28 Peak frequencies for the methylene stretching bands may be compared for the monolayer samples with solution values of the monomeric compounds.mfl These are known to shift to lower frequencies (by ca. 10 cm-1 for the va. (CH2) band) for long, paraffinic, alkyl chains in the solid phase relative to the liquid state. The v,(CH2) bands occur in CCl, solution at 2928-2930 cm-l for OTS, 1, and 2. These bands are shifted in the monolayers from liquid phase values by Av = -10 cm-l for OTS/Si, -7 cm-' for l/Si, and -8 cm-1 for 2/Si, where Av = v m - us (the subscripts m and (28) (a) Varsanyi, G. Vibrational Spectra of Benzene Deriuatiues; Academic: New York, 1969; pp 142-178. (b) Varsanyi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Wiley: New York, 1974; Vol 1.

1516 Langmuir, Vol. 6, No. 9, 1990 s refer to the monolayer and solution samples, respectively).

The width of C-H stretching peaks is also known to decrease for solid-phase samples, relative to liquid or solution samples.n We have measured peak widths at halfheight (w)for the u,(CH2) mode and found w m = 15-17 cm-' for OTS/Si, 26-28 cm-1 for l/Si, and 29-30 cm-l for 2/SiB These compare with solution values of w0 = 25 cm-I for OTS, 30 cm-' for 1, and 31 cm-l for 2. Thus, if we define Aw = wm - wO,then we find Aw = -9 cm-l for OTS, -3 cm-' for 1, and 0 cm-' for 2. These data qualitatively indicate that the chains of the 1/Si and 2/Si monolayers are less tightly packed and more liquid in nature than OTS/Si monolayers, which is in agreement with the slight decrease in contact angles (Table I). From the dichroic ratios in the FTIR-ATR spectra (Table II), we can estimate the orientation of molecular features in monolayer assemblies, provided that the orientation of IR transition dipoles can be assigned, relative to molecular coordinates. Our method of calculating the orientation of molecular features in monolayer assemblies by ATR has been published elsewhere.' We calculate transition dipole orientations 4, where 4 is the angle between the transition dipole direction and the axis normal to the substrate surface (the "z-direction"), from the measured dichroic ratios Ra30 The v,(CH2) and Va(CH2) vibrations are known to have transition dipoles which lie perpendicular to the chain axes for all-trans, fully extended alkyl chains and parallel to and perpendicular to the planes bisecting the CH2 units, respectively.31 We assign the aromatic ring stretching vibrations (8a, 8b, and 19a) and the orientation of their transition dipole moments (relative to molecular coordinates) by comparison with similar assignments collected in Varsanyi as follows: 8a, parallel to the principal ring axis (Cl--c4);8b, perpendicular to the principal axis and in the ring plane; 19a, parallel to the principal ring axis.@ These aromatic ring transition dipole moment orientations are expected to deviate from the assigned directions due to deviations from perfect Cpu symmetry of the aromatic ring.28 The three independent samples for the 1/Si monolayers afforded average dichroic ratios of 1.02 f 0.05 for the v,(CH2) vibration and 1.03 f 0.06 for the u,(CH2) vibration, from which we estimate e& = 31' f or about 16' more than what is observed for OTS/Si.33 Note that IR dichroic ratios do not allow one to separate the contributions of the C n and Cll chains to the measured chain tilt or to unequivocally distinguish true, uniform alkyl chain tilt from such disorder or "randomization" of the monolayer structure (e.g., through trans-gauche isomerism or chain entanglement). However, it seems that for the first approximation the IR results are in agreement with our MD simulations, Le., assigning an average tilt of >30° to the alkyl chains. Taking the same degree of caution, the 2/Si monolayer example presented in Table I1 indicates that the alkyl chains in this monolayer are still more tilted ( 8 =~42') and/or disordered. Therefore, from these data (29) Bandwidths are reported for p-polarization (I,,)but were usually within 1 cm-l of these values for s-polarization (It). (30) We assume uniaxial distribution of transition dipoles about the z-axis and no preferred dipole orientation in the x or y directions, where the x y plane is the plane containing the substrate surface. (31) Allara, D.L.;Swalen, J. D. J.Phys. Chem. 1982,86, 2700. (32) We note here that the total chain axis tilt (0,) contains both a component due to an in-plane tilt and a component due to an out-ofplane tilt and therefore is greater than 90- 4. See the following reference. (33) We calculate for this example ii: 31' by simple vector analysis for a unit vector, initially pointed in the z-direction (perpendicular to the substrate surface),which is then tilted by 90- 69 = 21O in the plane which contains the C-C bonds and bisects the H-C-H bond angles (the "inplane" tilt) and then tilted by 90 - 67 = 23' in the direction orthogonal to this plane (the 'out-of-plane" tilt).

Tillman et al.

the measured values of R and the calculated values of Balk could be due either to uniform, registered chain tilting or to folding or trans-gauche conformational isomerism of the alkyl chains. A combination of these factors is also possible. We conclude that, qualitatively, the IR data for peak frequencies and especially bandwidths (see above) suggest that the alkyl chains in the 1/Si and 2/Si monolayers are not as uniformly oriented as in the OTS/Si monolayers. However,wetting results indicate that folding of the alkyl chains is probably not occurring; hence, it can be suggested that the monolayer 1/Si is mainly oriented (although somewhat disordered). Although similar arguments apply for aromatic ring dichroic ratios, the rigidity of the system and its fewer degrees of freedom make their analysis more reliable. The dichroic ratios for the aromatic ring stretching vibrations present in both the 1/Si and 2/Si monolayers give, in a similar fashion, estimates of the orientation of the aromatic rings. For the 1/Si monolayers, the results from the 8a and 19a bands yield an average tilt angle 4 = 36' f 6'. The quoted error (and all errors quoted from the IR data) indicates the precision of the measurement and comes from the standard deviation in R; in addition, deviations of the transition dipole orientation from the assigned directions will add an additional, unknown, systematic error. Since the transition dipoles for these vibrations are approximately parallel to the aromatic ring Cov axis (along the Cl(0)C4(SO2) direction), 4 = Oaro, where Oar0 is the orientation angle of the ring axis. A tilting of 35' for the aromatic ring axis is predicted by using standard bond angles if the molecules simply orient with alkyl chains perpendicular to the substrate surface in a fully extended, all-trans conformation (see Figure 1). The 8b vibration is interesting because its direction is orthogonal to vibrations 8a and 19a and in the plane of the ring. If the C1-C4 axis is aligned along the 36" direction, then this band should have a tilt angle which might vary from 90° to 90 - 36 = 54', depending upon the rotation of the aromatic ring. For the three examples in Table 11, we in fact calculate 4eb = 42' f 3', which is, however, less than the minimum of 54' that one should observe. This could be due to deviations from the assigned transition dipole orientations or to a measurement inaccuracy resulting, perhaps, from the fact that the 8b band is a shoulder on the 8a band, preventing accurate determination of R8b. In any case, the observed value of R deviates least from the expected value if it is assumed that the phenyl ring is rotated so as to lie in the plane of the paper in Figure 1. For the 2/Si monolayer sample, the tilt angles calculated from the 8a and 19a vibrations differ by 14', with an average of 40'. The observed difference of 4' from what is observed for 1/Si is probably not significant. There is no observable 8b band for this monolayer. The model presented so far for 1/Si is consistent with a model of a monolayer structure with a sulfone group sandwiched between two alkyl chains. Results from XPS studies confirm this interpretation (Figure 4). A survey scan showed peaks attributable to carbon, sulfur, silicon, and oxygen (Figure 4a). No traces of chlorine (which may be present as SiC1, from incompletely hydrolyzed 2 in the monolayer or from chlorination of the carbon atom a to the sulfone during synthesis) or other elements could be detected. The sulfur 2p peak is seen to show two maxima in a high-resolution scan (Figure 4b) at binding energies of 167.67 and 168.97 eV, with relative integrated areas of 71% and 29% area, respectively. These are assigned to the S2p3p and S2p1/2 orbitals, respectively. This peak position reflects the oxidation of the sulfur, since a similar

Langmuir, Vol. 6, No. 9,1990 1517

Substituted Self-Assembling Monolayer Film

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Figure 4. XPS results for l/Si monolayers. Presented is (a,top) a survey scan from 0 to 600 eV; the following binding energies and atomic percents (in parentheses) were found: 0 2s,24.8 eV; Si 2p, 98.4; Si 2s, 150.0 (12.34 atom %, as 5.0% oxide and 7.3% metal, from the ratio of the Si2p peak intensities);S 2p, 167.4; S 28, 231.7 (1.77); C ls, 284.6 (72.30); 0 Is, 531.7 (13.58). (b, Bottom) High-resolutionscan of the S 2p peak. The electron takeoff angle was 25' for both spectra.

Figure 5. Atom percentages determined by XPS vs electron takeoff angle for (a, top) carbon and (b, bottom) silicon (metal),

silicon (oxide),and sulfur. and an aromatic sulfone group which is buried below the surface but not adsorbed on the silicon oxide layer of the substrate.

Conclusions This paper presents the first experimental results comparing the detailed molecular orientation of selfhigh-resolution scan of an aromatic sulfide-containing assembled monolayers containing polar, donor-acceptor monolayer showed S2p3p and S2p1p peaks at 163.28 and substituted aromatic groups in a midchain position and 164.44 eV, respectively.2 The observed sulfone peak indicates that the sulfonyl group is a suitable moiety for position of -168 eV also agrees with typical literature incorporation into self-assembled monolayers. values for the S2p binding energy of the sulfonyl g r o ~ p . 3 ~ IR data for l/Si monolayers suggest a density of surface In Figure 5 are plotted the results of variable-angle XPS coverage which is ca. 70% f 10% of that found for the spectroscopy. High-resolution scans were used, and OTS/Si model system. This is reasonable, considering the intensities were measured for the C Is, Si 2p, and S 2p larger cross sectional area of the ArSO2 group, relative to peaks. The electron takeoff angle is defined here as the the alkyl chains. FTIR results also show that the alkyl angle between the emitted electron and the surface, and chains of the 1/Si monolayers are tilted by 31' f lo', or it is expected that elements which are present at higher -16-20' more than is observed for OTS/Si (6, I 10concentration at the surface should show increasing atomic 15'). The measurements of film thickness by ellipsompercent with decreasing takeoff angle, while the reverse etry are qualitatively consistent with this observation, shce should be the case for elements which are concentrated the 30 f 3 A observed thickness is just within experimental more deeply (-50 A) in the monolayer/substrate.3s We error of the 33 A estimated for 30' tilting of the alkyl observe these expected trends for carbon (present at the chains. Such tilting is expected to occur as a result of the highest atomic percent at the surface) and silicon metal chain-chain separation imposed by the ArSOz group.mThe (present at the highest concentration at the limit of depth O-Ar-SO2 group c1-c4 axis is tilted, and the observed sensitivity of the XPS technique). Sulfur and silicon (as value of -36' is remarkably close to the value of 35O SO,) both show an initial increasing intensity with estimated by using the simplifying assumptions that the increasing takeoff angle, followed by a drop in intensity, alkyl chains stand perpendicular to the substrate surface with the maximum for SiO, occurring at a higher takeoff (see Figure 1). Variable-angle XPS studies qualitatively angle than that observed for sulfur. This is consistent with support an ordered monolayer structure, with the sulfothe proposed structure of a monolayer of tilted alkyl chains nyl groups sandwiched between the two alkyl chains. Contact angles and the critical surface tension for the 1/Si system indicate that the monolayer surface is constituted (34) Handbook of X-ray Photoelectron Spectroscopy; Muilenberg, G . E., Ed; Perkin-ElmerCorp., ElectronicsDivision: Eden Prairie, primarily of methyl groups, and the contact angles agree - Physical MN, 1979; p 56. with published results for partial OTS monolayers in which (35) (a) Andrade, J. D. In Surface and ZnterfaciolAspects ofBiomedicaZ the surface coverage has been deliberately reduced by 30Polymers; Andrade, J. D., Ed;Plenum: New York, 1987; Vol. 1, p 175. (b) Paynter, R. W.; Ratner, B. D. Ibid. Vol. 2, pp 198-9. 40 % from full close-packing.

1518 Langmuir, Vol. 6, No. 9,1990 FTIR and ellipsometry experiments for 2/Si give results which are very similar to the results for 1/Si. However, the contact angles for 2/Si are consistentlyand significantly lower, especially the n-hexadecane contact angle, which is 120° lower than the contact angles observed for 2/Si and OTS/Si, while the critical surface tension yc is -3 mN m-1 higher. For polyethylene, a surface of which consists solely of methylene groups, yc = 31 mN m-l, compared with yc = 20 mN m-1 for OTS/Si.15 Therefore, it appears that the surface of 2/Si is not one of perfectly close-packed methyl groups and that the Cg chains entangle, fold, or are defect ridden enough to allow

Tillman et al.

exposure of the test liquid to a significant quantity of methylene groups.

Supplementary Material Available: Text showing preparation of compounds 1-7 (5 pages). Ordering information is given on any current masthead page. Registry No. 1, 128843-72-1;2, 128843-73-2; 3, 128843-674; 4, 128843-68-5;5, 128843-69-6;6, 128843-70-9; 7, 128843-710; Si, 7440-21-3;CH&, 75-11-6; 4-vluorothiophenol,371-42-6;p-hydroxybenzoicacid, 99-96-7; 1l-bromo-1-undecene, 7766-50-9; hexadecane, 544-76-3.