Langmuir 1992,8, 947-954
947
Interaction of Silicon Surfaces Silanized with Octadecylchlorosilanes with Octadecanoic Acid and Octadecanamine Studied by Ellipsometry, X-ray Photoelectron Spectroscopy, and Reflectance Fourier Transform Infrared Spectroscopy Krishna M. R. Kallury and Michael Thompson* Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A1, Canada
Carl P. Tripp and Michael L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada Received November 12, 1991
Treatment of silicon substrates with octadecyltrichlorosilane(OTS)or chlorodimethyloctadecylsilane
(CDOS)in toluene or hexadecane results in different surfaceconcentrationsof the silanes depending upon
the solvent and the structure of the silanizing agent. With hexadecane, voids are created in the surface structures upon removal of the intercalated hexadecane with chloroform into which octadecanoic acid and octadecanamine fit in with head groups-up orientation. Additional acid or amine also gets deposited through hydrogen-bonding interactions. Excellent correlation could be obtained between the ellipsometric, X-ray photoelectron spectroscopic, and reflectance Fourier transform infrared probe techniques used to follow these interactions.
Introduction Silanization of materials possessing surface hydroxylic functionalities with alkoxy- or chlorosilanes is a reaction which finds extensive applicability in divergent areas. In the domain of analytical chemistry, this surface modification technique is widely utilized in high-performance liquid chromatography to reduce the polarity of stationary phase materials such as silica to prevent the interaction between polar analytes (organic or bioorganic molecules) and the surface hydroxyls of the adsorbent.lI2 Among the chlorosilanes used for this purpose, octadecyltrichlorosilane and chlorodimethyloctadecylsilane are two important reagents and treatment of silica with these silanes furnishes octadecylsilylated surfaces which are extremely hydrophobic in Such C-18 surfaces have also been utilized in environmental chemical analyses for the preconcentration of toxic chemicals such as herbicides, pesticides, and fungicides in water sample^.^ These (2-18 surfaces are also used in fundamental biochemical studies such as protein adsorption on material surfaces,6 in electrochemistryfor the generation of blocked interfaces,' in ion-exchange chromatography for the separation of anions or cations,8 and in friction/lubrication ~ t u d i e s . ~ Sagiv'O investigated the formation of oleophobic monolayers on glass substrates utilizing mixtures of octade(1) Unger, K. K. Porous Silica; Elsevier: Amsterdam, 1989. (2) Nawrocki, J.; Buszewski, J. Chromatogr. Reu. 1988, 449, 1. (3) Dorsey, J. G.; Dill, K. A. Chem. Reu. 1989, 89, 331. (4) Chemically modified surfaces; Leyden, E., Collins, W. T., Eds.; Gordon and Breach New York, 1986 (Vol. 11, 1988 (Vol. 2), and 1990 (Vol. 3). (5) MacCarthy, P.; Klusman, R. W.; Cowling, S. W.; Rice, J. A. Anal. Chem. 1991,63, 301R-342R. (6) Zimmermann,R.M.;Schmidt, C.F.;Gaub,H. E. J.Colloidlnterface Sci. 1990, 139, 268. (7) (a) Clichet, P.; Jaffrezic-Renault, Ado. Mater. 1990, 2, 293. (b) Rusling, J. F.; Zhang, H.; Willis, W. S. Anal. Chim. Acta 1990,235,307. (8) (a) Liu, H.; Cantwell, F. F. Anal. Chem. 1991,63,993. (b) Ito, K.; Ariyoshi, Y.; Tanabiki, F.; Sunahara, H. Anal. Chem. 1991, 63, 273. (9) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868. (10) Sagiv, J. J. Am. Chem. SOC.1980, 102, 92.
0743-7463/92/2408-0947$03.00/0
cyltrichlorosilane (OTS) and eicosanoic acid (a CZOfatty acid) at different concentrations of the two components in a mixed solvent system consisting of chloroform/carbon tetrachloride/hexadecane(81280). When equimolar concentrations (5 X M)of both components in hexadecane were reacted with glass substrates for 2 min, the surface film consisted of a monolayer of OTS only. When excess of the fatty acid was employed (molar ratio 30:l for Czo/OTS),a monolayer of the fatty acid was formed in 30 s; however, for adsorption times longer than 1 min, the quality of the monolayer became poor as was evidenced by the receding solvent leaving long wet tails on the emerging glass slides. For times longer than 22 min, monolayers of essentially all OTS were again obtained, and this was attributed to replacement of the fatty acid from the surface by the chlorosilane. In a subsequent study, Gun and Sagivll examined the effect of solvent variation during the formation of monolayers from either OTS or eicosanoic acid on ZnSe or silicon and found that solvent incorporation into the silane film is facilitated by geometrical matching between the solvent and the nonpolar portion of the amphiphile. Thus, hexadecane was retained in the monolayers to a considerable extent while bicyclohexyl was incorporated to only a very small extent, if at all. The authors concluded that in order to prepare high-quality, solvent-free monolayers by adsorption from organic solutions, geometrical matching between solvent and amphiphile should be avoided and that amphiphiles capable of irreversible binding to the surface should be employed. Attempts have been made by several research groups to develop molecular recognitive systems based on the above observations of Sagiv and co-workers. Transduction techniques such as cyclic voltammetry, ellipsometry, surface-enhanced resonance Raman spectroscopy and fluorescencehave all been employed and it has been shown that biologicallysignificant molecules such as vitamin K1, vitamin E, and cholesterol as well as fluorescent dyes such (11) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986, 112,457.
0 1992 American Chemical Society
Kallury et al.
948 Langmuir, Vol. 8, No. 3, 1992 as tetraphenylporphyrin could be incorporated into the films and quantified.12-16 The protocol consisted of initially forming mixed monolayers of OTS and the template guest molecule and then extracting out the template guest with a solvent. Removal of the guest leaves cavities in the residual silane layer which correspond to the dimensions of the guest. Treatment of the “activefilm with the guest molecule will enable reconstitution of the same molecule into the cavities while analytes of different dimensions are rejected by the template. Andersson et al.15 utilized the solvent hesadecane itself as the template-formingentity during silanization with OTS and showed that molecules with long alkyl chains can fit into the cavities vacated by the solvent. Yamamura et al.16 reported that by using N-(3,6,9-trioxaheptacosy1)ethylenediamine as the template guest, basic surfaces could be created which interacted with negatively charged liposomes carrying fluorescent probes and the reaction was followed by fluorescence measurements. The results of these workers point to the fact that simple long chain molecules with a terminal functionality (in this case an amine) orient with this functional group facingoutward from the OTSmonolayer when they occupy the cavity left behind by hexadecane. Similar results were documented by Fox and co-workers,17who used octadecanethiol-treated gold surfaces together with guest molecules such as N-methyl-N’-octadecyl 4,4’-dipyridinium dibromide (viologen) or dodecyltrimethylammoniumbromide. When added to the electrolyte solution, these compounds produced a further decrease in the residual current under cyclic voltammetric conditions for the electron transfer to Ru(NH3),j3+. Contact angle studies indicated that some of the viologen intercalates with the alkanethiol monolayer and is more difficult to remove by solvent treatment. It was postulated that the viologen head group is situated closer to the air-monolayer interface rather than to the gold surface. Collard and Foxla demonstrated in a subsequent publication that the w-mercaptoundecyloxy or hexadecyloxy esters of ferrocenecarboxylic acid could be incorporated along with hesadecyl mercaptan as mixed monolayers onto gold surfaces and using cyclic voltammetry showed that the guest molecules could be exchanged with alkanethiols. The present work was undertaken with the view of evaluating whether molecular templates could be generated from the monofunctional silane, chlorodimethyloctadecylsilane (CDOS),in hexadecane solution using silicon wafers containing naturally grown oxide layers as substrates. For comparison, the template-forming reactions were also carried out using octadecyltrichlorosilane (OTS) as the silanizing agent. Octadecanoic acid and octadecylamine were used as representative template guests. The formation of the templates and their interactions with the above analytes were followed by ellipsometry,contact angle measurements, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The relative extents of silanization of the silicon substrates with the above mono- and trichlorosilanes, when hesadecane and (12) Tabushi, I.; Kurihara, K.; Naka, K.; Yamamura, K.; Hatakeyama, H. Tetrahedron Lett. 1987,28, 4299. (13) Yamamura, K.; Hatakeyama, H.; Naka, K.; Tabushi,I.; Kurihara, K.J. Chem. Soc., Chem. Commun. 1988, 79. (14) Kim, J.-H.; Cotton, T. M.; Uphaus, R. A. J.Phys. Chem. 1988,92, 5575. (15) Andersson, L. I.; Mandenius, C. F.;Mosbach, K. Tetrahedron Lett. 1988, 29, 5437. (16) Yamamura, K.; Hatakeyama, H.; Tabushi, I. Chem. Lett. 1988, 99. (17) Creager, S. E.; Collard, D. M.; Fox,M. A. Langmuir 1990,6,1617. (18) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192.
Table I. Ellipsometric Data on the Surfaces Obtained by the Treatment of Silicon Substrates with Octadecyltrichlo&ilsine (OTS)and Chlorodimethyloctadecyldlane (CDOS)in Toluene and Hexadecane and on the Templates 7-11 time, temp, thickness, solventa min O C A Silanization Reactions octadecyltrichloroeilane(OTS) T 15 20 11h2 T 30 20 15h1 T 60 20 18 2 T 90 20 25 1 T 120 20 35 1 HD 15 20 l8* 1 HD 30 20 30 2 HD 60 48 5 20 HD 120 20 55 h 4 chlorodimethyloctadecylsilane T 30 20 7*1 20 (CDOS) T 90 9*l 20 T 120 12* 1 T b 20* 1 20 T 120 C 19* 1 HD 120 20 7*2 HD b 20 35 2 Template Reactions silicon/OTS/hexadecane(2 h) CHCl, 30 20 23 2 DAd 15 20 30* 1 ODAd 15 32 2 20 silicon/CDOS/hexadecane CHCl, 30 26* 1 20 (24 h) SAd 15 34* 1 20 ODAd 15 36h 1 20 ~~~
*
* *
Key: T = toluene; HD = hexadecane. * Overnight. Reflux. Key: SA = stearic acid in methanol (lo-, M) [the common name stearic acid is used throughout for octadecanoic acid]; ODA = octadecanamine in methanol (10-3M).
toluene were used as solvents, were ascertained by reflectance FTIR and XPS techniques. The time required to form monolayer level films by the above two chlorosilanes on silicon substrates was also computed through ellipsometric measurements.
Experimental Section Materials. The silicon wafer substrates utilized in the present work were obtained from the International Wafer Service, Menlo supplied by Aldrich, Park, CA. Octadecyltrichlorosilane(OW), and chlorodimethylodadecylsilane(CDOS),purchasedfrom Hule America,Bristol, PA, were used as obtained. Toluene and hexadecane were dried over molecular seives and distilled prior to use. Octadecanoicacid and octadecanaminewere Aldrich samples used as such. Methanol was distilled over potassium hydroxide prior to use. Instrumentation. Ellipsometric measurementa were made on an Auto EL-I1ellipsometer,Rudolph Research, Flanders, NJ. The laser sourcewas a 1-mWcontinuous-wave h e l i d n e o n laser, with a wavelengthof 6328A. The angle of incidencewas 70° and the spot size 2-3 mm. A refractive index of 1.5 was utilized for a l l the silane layers. The data were analyzed on a Hewlett-Packard 85 computer using film 85 software package, version 30, program 13,and the film thickness waa calculated using the McCrackin program. Contact angles were measured on a Rame-Hart Model 100 goniometer at room temperature and 100% relative humidity with water as the test liquid. X-ray photoelectron spectra reported were recorded on a Leybold MAX-200 ESCA spectrometer using an unmonochromated Mg K a source run at 12 kV and 25 mA. The shape of the spectra indicated that no compensation for differential surface charging was needed. The binding enesgy scale was calibrated to 285 eV for the main C(1s) (C-C) feature. Spectra were run in both lowresolution (passenergy = 192 eV) and high-resolution (passenergy = 48 eV) modes for the C(ls), N(ls), and Si(2p) regions. Each sample was analyzed at a 90° angle relative to the electron detector. An analysis area of 4 X 7 mm was used for rapid data collection (typically5min for the C(ls) high-resolutionspectrum).
Langmuir, Vol. 8,No. 3,1992 949
Silicon Surfaces 0
I
CI, SI-C&,,
CH,
(CDOS. 2)
-OH
Toluene
CH,
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CH,
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(01s. 1) Toluene
0
4 (No reaction with octadecanoic acid
,
or octadecanamine)
hexodecane (5)
j
3
1 , hexodecone
(No reaction wth octadecanoic acid
or octodecanamine)
14-1 '0
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CH, CH,
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CH,
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CH, CH,
CH,
0
I
*
I
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CH,
CH,
CH,
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X
X-R
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":> '0
I
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Ila
(x
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Figure 1. Silanization of silicon substrates with octadecyltrichlorosilane(OTS) and chlorodimethyloctadecylsilane (CDOS) and reactivity of the silanized surfaces with octadecanoic acid and octadecanamine (it is noted that the exact details of the interaction between OTS and CDOS and the silicon surface are not defined). Elemental compositions were calculated from the satellitesubtracted low-resolution spectra normalized for constant transmission using the software supplied by the manufacturer. The sensitivity factors employed in these computations were C(1s) = 0.34, N(1s) = 0.54, Si(2p) = 0.4, and O(1s) = 0.78, empirically derived for the MAX-200 spectrometer by Leybold. Infrared spectra were recorded on a modified Bomem Michelson 110-E FTIR at 4-cm-I resolution. Typically, 100 signal averaged scans requiring approximately 2 min to record were co-added for each spectrum. The details of the FTIR and the experimental procedures for recording spectra are described in detail elsewhere.19 In brief, spectra were recorded in reflectance operating at approximately 70° angle of incidence. The relative quantity of material adsorbed on the surface was determined by measuring the change in the peak intensity for the CH2 stretching mode located near 2920 cm-'. Reflectance infrared was used rather than transmission because the Si wafers used were polished on one side only and this rendered them opaque for transmission infrared studies. In several test samples using Si wafers polished on both sides we have found a good correlation between the intensity of the bands measured in reflectance and in transmission. Absolute coverages were determined from a Beer's law relationship using calibrated solutions of the adsorbate in CCL. Surface Modification Procedure. For determining the conditions required for monolayer-leveldepositions, the silicon wafers were initially cleaned according to the followingprocedure. The Si wafers were dipped for 10 min in warm concentrated HCl, washed thoroughly with distilled water, and then dipped for 10 min in warm 5% HzOz. Prior to use, the wafers were then washed (19) Tripp, C. P.; Hair, M. L. Appl. Spectrosc. 1992, 46, 100.
with distilled water and then dipped in chloroformfor 1h. The wafers were silanized by treatment with a 1mM solution of the appropriate chlorosilane in toluene for varying periods of time (seeTable I) at room temperature under nitrogen. The modified wafers were washed thoroughly with chloroformand acetone and dried under vacuum and the surface silane thickness waa measuredby ellipsometry. For the templating reactions,reactions times of 2 h for OTS and 24 h for CDOS were utilized employing 1 mM solutions of either silane in hexadecane-carbon tetrachloride (91). After the reaction, the wafers were washed with acetone, dried under nitrogen, and subjected to ellipsometric measurements. The protocols are summarized in Figure 1. The substrates were then soaked in chloroform for 30 min and dried with nitrogen for 30 min. The resulting surfaces were treated with 1mM solutions of octadecanoic acid or octadecanamine in methanol for 15 min, washed with chloroform, and dried before optical or spectroscopic measurements.
Results and Discussion Ellipsometry. The ellipsometric data on siliconwafers treated with OTS and CDOS for various lengths of time in toluene or hexadecane, are included in Table I. If we use the value of 26.2 A which was computed for the monolayer thickness of OTS on a silicon surface by Whitesides and co-workers20(with the octadecyl chain in an all-trans conformation),the data in Table I for OTS deposited from (20) Wasserman, S.R.; Tao, Y.; Whitesides, G . M. Langmuir 1989,5, 1074.
950 Langmuir, Vol. 8,No. 3, 1992
Kallury et al.
Table 11. X-ray Photoelectron Spectroscopic Data on Silicon Substrates with Octadecyltrichlorosilane(OTS) and Chlorodimethyloctadecylsilane (CDOS) in Toluene and Hexadecane and of Surfaces Templated with Stearic Acid and Octadecanamine high-resolution data elemental composition surfaceo
%N
C(lS) binding energy,eV % 285.0 100
%C 63.4
%Si 20.6
%0
Si/OTS/toluene Si/OTS/hexadecane
53.3
26.7
20.0
285.0
100
SI/OTS/hexadecane, then CHC13, then SA
68.3
18.1
13.6
92
Si/OTS/hexadecane, then CHC13, then ODA
64.2
19.6
15.0
100
16.0
Si/CDOS/toluene
42.4
31.0
26.6
285.0 288.6 285.0 286.1 285.0
Si/CDOS/hexadecane
32.4
43.9
23.7
285.0
100
Si/CDOS/hexadecane, then CHCl3, then SA
44.3
30.4
25.3
Si/CDOS/hexadecane, then CHC13, then ODA
49.3
29.7
19.1
285.0 288.8 285.0 286.2
91 9 91 9
4
1.2
1.8
8 90
10
Si(2p) binding energy,eV 99.6 103.0 99.6 103.0 99.6 103.0 99.5 102.8 99.5 102.9 99.7 102.8 99.6 102.9 99.6 102.8
%
N(ls) binding energy,eV
%
399.1 402.0
58 42
399.4 402.3
59 41
75 25 71 29 72 28
72 28
80 20 80 20 73 27 75 25
For details of abbreviations used, see Table I.
toluene indicates that it takes about 80-90 min to form a complete monolayer. Whitesides et al. used bicyclohexyl as the solvent and reported that under dry atmospheric conditions, formation of a complete monolayer required about 5 h, whereas experiments conducted under air at 30% relative humidity provided monolayers in about 1h. Our depositions (from toluene) were carried out under humidity conditions close to the latter and it therefore appears that the behavior of the solvent toluene in the current study is analogous to that of bicyclohexylused by Whitesides et al. On the other hand, when hexadecane was used as the solvent, the thickness of the film was larger than that expected for a monolayer level coating of the silane after only 30 min of reaction time and the final value leveled off after 2 h at about 55 A. This suggests a bilayer-level surface film formation and it seems likely that the hexadecane forms an additional layer over the surface-linked silane layer generated by OTS. With CDOS, monolayer level depositions could be obtained only after an overnight treatment in toluene at room temperature or upon refluxing for 2 h in the same solvent. An overnight treatment with CDOS in hexadecane resulted in the same bilayer level thickness reading as was recorded for OTS reaction after only 30 min. It is interesting to note that Rangnekar and Oldham21investigated the interaction of CDOS with quartz surfaces under a variety of conditions using FTIR spectroscopy and observed that either a room temperature reaction in water or ethanol or refluxing in toluene furnished only physisorbed silane layers which could be completely removed by washing with dichloromethane. These authors found that CDOS forms surface siloxane bonds with quartz upon refluxing with the pure silane or by initially heating the quartz to 1000 "C and then reacting the cooled surface with CDOS in carbon tetrachloride. On the other hand, Angst and Simmons22have reported that silicon wafers could be silanized with CDOS in a mixture of CC4 and Isopar G (a branched hydrocarbon solvent) and obtained thickness values of 8 and 5 A for the hydrated and dry oxide surfaces, respectively, after 30 min of reaction times. Our current results are in agreement with the ellipsometric and infrared data of Angst and Simmons. (21) Rangnekar, V. M.; Oldham, P.B. Spectrosc. Lett. 1989,22,993. (22) Angst, D. L.; Simmons, G. W. Langmoir 1991, 7, 2236.
Table 111. Infrared Data on Silicon Substrates Reacted with OTS and CDOS in Toluene and Hexadecane and of Surfaces Templated with Octadecanoic Acid and Octadecanamine surface vc~,(2920cm-~), ccA Si/OTS/toluene 520 380 Si/OTS/hexadecane Si/OTS/toluene + stearic acid 540 Si/OTS/hexadecane + 660 stearic acid Si/OTS/toluene + 540 octadecanamine Si/OTS/hexadecane + 660 octadecanamine Si/CDOS/toluene 250 Si/CDOS/ hexadecane 135 Si/CDOS/toluene + stearic acid 250 Si/CDOS/hexadecane + 250 stearic acid Si/CDOS/toluene + 250 octadecanamine Si/CDOS/hexadecane + 290 octadecanamine The absorbance figures are reproducible within &lo PA.
Washing the OTS or CDOS in hexadecane-treated silicon surfaces with chloroform results in a reduction of about 10A in the thickness of both of the surfaces. When these chloroform-washed surfaces were treated with solutions containing octadecanoic acid or octadecanamine, an increase in layer thickness of about 6-8 A was obtained for both surfaces. These values indicate that these acid or amine probes do not deposit as an additional layer over the respective silane layers, but rather intercalate between the silane layers. Further, advancing contact angle measurements, using water as the test liquid, show that these acid- or amine-treated surfaces have values of about 90-92". This is to be compared with values of 110" recorded for the originalsilanized surfaces. This behavior is similar to that recorded by Fox and co-workers17 for gold surfaces treated with octadecanethiol. Those workers report that the contact angle of the surface was lowered from 108"to95"upon reaction withviologen and explained this as being due to the upward orientation of the polar head groups of the viologen in the intercalated structures. It is important to note in our work on the silane structures that neither the originalfilm nor its intercalated
Silicon Surfaces
Langmuir, Vol. 8, No. 3, 1992 951
I
I
Figure 2. X-ray photoelectron spectra of (A) Si/OTS/toluene, (B)Si/OTS/hexadecane (CHCla washed), (0Si/CDOS/toluene, (D) Si/CDOS/hexadecane (CHC13washed), (E)C(ls) binding energy region of A (1 eV correction), and (F)C(1s) binding energy region of C (5 eV correction).
molecules could be washed out by either acetone or water. This result was confirmed by XPS data. X-ray Photoelectron and IR Spectroscopic Measurements. The XPS and IR data collected on the silicon wafer surfaces silanized with OTS or CDOS in toluene or hexadecane and washed thoroughly with chloroform are presented in Table I1 and Table 111, respectively. The results obtained from the infrared are in accord with the data obtained from XPS measurements. The infrared measurements were easy to perform and much faster to obtain than the XPS results. This proved to be very useful in many facets of the experiments such as in the monitoring of the quality of the chloroform rinse. Also, as noted later, the IR method is potentially more accurate than XPS in quantitative comparisons. The X P spectra for the surfaces 3,4,7, and 10 are shown in Figure 2. For both the silanes, high-resolution XPS measurements in the C(1s)binding energyregion indicated a single peak around 285 eV, representing the carbons on the octadecyl chain (see inserts in Figure 2). The two methyl groups on the silicon atom of CDOS appear to merge with this hydrocarbon C(1s) binding energy peak. In the Si(2p) binding energy region, two peaks were recorded around 99.6 f 0.1 eV and 103 f 0.2 eV, corre-
sponding to the substrate silicon and the silane silicon atoms, respectively. A relative assessment of the surface coverages of the two silanes obtained with either of the solvents toluene and hexadecane could be made by comparing the C(ls) (si1ane):Si (2p) (substrate) binding energy peak ratios. Thus, whereas this ratio was found to be 4:l for OTS in toluene, a value of 2.8:l was computed for the same silane in hexadecane. Similarly, with CDOS, the reaction in toluene yields a value of 1.7 for this ratio, whereas a value of only 0.92 is obtained for the reaction in hexadecane. A similar trend is obtained from the infrared data which is shown in Table 111. The intensity of the infrared band at 2920 em-] obtained for OTS for the surface layer after reaction in toluene was 520 PA and this can be compared with the intensity of 380 PA when the reaction was carried out in hexadecane (see Figure 3). For CDOS the values were 250 and 135 PA, respectively. The adsorption of 520 PA measured for OTS can be converted to a surface coverage of approximately 2 mg/m2,which is in agreement with the monolayer coverage reported by other^.^^*^^ Two distinct trends can be drawn from these ratios. First, the amount of the silane deposited on the (23)Maoz, R.;Sagiv, J. J. Colloid Interface Sci. 1984, 100, 466.
952 Langmuir, Vol. 8, No. 3,1992
Kallury et al.
to the amino moiety in the amine-treated surfaces. In contrast, no acid or amine carbon C(ls) peaks in the XPS spectra could be detected for the substrates silanized with OTS or CDOS in toluene and subsequently treated with 500 uA either octadecanoic acid or octadecanamine. Nor was there an increase in intensity of the infrared band at 2920 cm-' or an enhancement in the C(1s) binding energy peak at 285 eV. The C(1s) (silane):Si(2p) (substrate) binding energy peak ratios of the surfaces 8 and 11 are twice those obtained for the templated surfaces 7and 10,respectively. This enhancementis expected in view of the incorporation of an additional 18-carbon chains (devoid of any silicon) to structures 7 and 10. However, it is considerably more than the amount of the intercalated hexadecane on the silane surfaces 6 and 9, indicating that more octadecanoic acid or octadecanamine have been introduced onto the templates 7 and 10,especially the latter. It is apparent 7 that apart from the intercalated stearic acid or octadecan2850 2900 2950 amine occupying the places vacated by hexadecane, an cm- 1 additional 20-30% of these molecules go onto the surface. Figure 3. ReflectanceFTIR of (a) Si/OTS/toluene,(b)Si/OTS/ For this additional acid or amine to be incorporated onto hexadecane (CHC13 washed), (c) Si/OTS/hexadecane (CHC13 the silanized silicon surfaces and remain stable to washing washed) treated with octadecanoic acid, and (d) Si/OTS/hexadecane (CHC13 washed) treated with octadecanamine. with chloroform, strong hydrogen-bonding interactions are to be expected. Such polar interactionscan occur either silicon surface employing toluene as the solvent is about with the residual hydroxyls on the silicon surface or with 30 5% more than that with hexadecane in the case of OTS the functionalities on the intercalated octadecanoic acid and about 75-80 % more with CDOS. Secondly, with either or octadecanamine molecules. In view of the fact that solvent, the quantity of OTS going onto the surface is neither the acid nor the amine interacts with the surfaces more than twice that of CDOS. In the first case the 3 and 4 (obtained with OTS and CDOS, respectively, in difference represents the amount of hexadecane that has toluene), the involvement of the residual silanols in such intercalated between the octadecylsilane chains during polar interactions appears to be minimal. If the interthe film formation. That this is removed by chloroform calated acid or amine are the species interacting with the treatment is easily shown by the IR analysis of the eluent. corresponding functionalities on the additional molecules The reduced reaction of CDOS relative to OTS suggests deposited, the polar groups on the former should be that the two methyl groups on the silicon of CDOS not oriented away from the silicon surface and toward the only present considerable steric hindrance for the surface surface film-air interface. This hypothesis is supported reaction but also prevent close packing of the silane by the observed depression of about 20° recorded for the molecules. It is interesting to note that Fox and cocontact angles on the surfaces 8 and 11. Further support workers18 have computed that the amount of 16-mercapis derived from the fact that the high-resolution N(1s) tohexadecyl ester of ferrocenecarboxylic acid which is binding energy region scans of the surfaces 8b and l l b in deposited onto a hexadecanethiol-treated gold surface can the XPS spectra exhibit nonbonded and bonded amino be attributed to a 33% displacement of the hexadecane peaks around 399 and 402 eV in a ratio of 3:2 for both from the surface layer. The extent of intercalated hexasurfaces (see Figure 4). Assuming that each of the amino decane which is found in the current work with OTS corgroups from the additional material interacts with only responds roughly to the above exchange which was one intercalated amino group on the surface, the perattributed to the replacement of the thiol at the defect centage of amino groups not involved in such bonding sites by Fox et al.l* as well as by Chidsey et al.,= suggesting amounts to 60% of the total amino groups. Of the the probability of a similar process being associated with remaining 40 % involved in hydrogen bonding, half must hexadecane. come from the intercalated amine. Thus, the additional The templates 7 and 10 were generated by the chloro20% must represent the amount of octadecanaminegoing form wash of the surfaces 6 and 9, respectively (see Figure on top of the intercalated structures 8b and llb,respec1for structures). Further treatment with octadecanoic tively. Unfortunately, a similar XPS analysis could not acid or octadecanaminesolutions in methanol resulted in be carried out on the C(ls) binding energy peak of the the intercalation of the acid or amine onto the silane films carbonyl carbon of the carboxyl with the octadecanoic (structures 8 or 11). The XPS spectra of these surfaces acid-treated surfaces, since both bonded and nonbonded (see Table I1 for details and Figure 3 for spectra) exhibit acid C(1s) peaks appear in the same region and could not strong C(1s) binding energy peaks comparable in intenbe distinguished. sities with the corresponding peaks in the spectra of the It is perhaps worth commentingbriefly on the combined silanized surfaces which had been obtained when toluene use of the XPS and IR techniques in this work. The data was used as the solvent (structures 3 and 4). However, in in Table IVshow that, in most of the cases, the quantitative addition to the strong hydrocarbon peak at 285 eV the data obtained from the two techniques are not in exact high-resolution C(ls) binding energy region scans also agreement. The ratios from the XPS measurements are indicate broad peaks around 288.5 and 286 eV for uniformly higher in comparison with the FTIR results. It structures 8a and8b, respectively. These can be attributed has been shown by Whitesides and co-workers26that to carboxylic carbons in the former and to the carbon CY considerable changes in the attenuation length (or escape depth) of photoelectrons representing gold substrate peaks (24) Tillman, N.; Ulman,A.; Schildkraut, J. S.; Penner, T. L. J. Am. can occur depending upon the chain length of the alkaneChem. SOC.1988,110, 6136.
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(25) Chideey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J . Am. Chem. SOC.1990,112,4301.
(26) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989,93,1670.
Silicon Surfaces
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is not needed in the calculation). The resulta obtained by this method are more likely to be the quantitatively correct ones. On the other hand, the IR technique is insufficiently sensitiveto detect the presence of the carboxylic and amine functions which are revealed by the XPS. The combination is therefore extremely useful.
Conclusion In conclusion we have confirmed that silicon substrates can be treated with CDOS or OTS to give silanizedsurfaces. The surface concentration depends upon the structure of
Kallury et al. the silanizing agent and the solvent. When hexadecane is used as a solvent, it is incorporated into the surface and ita removal with chloroformgives rise to an "active" surface containing voids into which octadecanoic acid and octadecanamine will fit with head-groups-up orientation. Additional acid or amine is also deposited through hydrogen bonding interactions. Ellipsometric, XPS and reflectance FTIR techniques are in agreement, with each contributing its own specific detail. Registry No. SA,57-11-4; ODA,60085-15-6;Si,7440-21-3; CHCl3, 67-66-3; toluene, 108-88-3;hexadecane, 544-76-3.
