Langmuir 1996,11,1304-1312
1304
Structure of Alkylsiloxane Monolayers on Silicon Surfaces Investigated by External Reflection Infrared Spectroscopy Helmuth Hoffmann,* Ulrich Mayer,* and Anton Krischanitz Department of Inorganic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Wien, Austria Received October 20, 1994. In Final Form: January 19, 1995@ Self-assembled monolayers o f homologous alkyltrichlorosilanes(CH3(CHdnSiC13,n = 10, 13-17) on bulk silicon substrates have been prepared by adsorption from dilute solutions under dry inert gas conditions and the hydrocarbon chain orientation in the monolayer films has been investigated by external reflection infrared spectroscopy. A quantitative analysis of the monolayer reflection spectra based on a spectral simulation and fit procedure reveals that the hydrocarbon chains in these monolayers are tilted by an average of 8"toward the surface normal. Alinear relationship was found between the hydrocarbon chain length of the adsorbate molecules and the v(CH2)absorptionintensities in the monolayer reflection spectra, which further supports an essential conservation of an ordered film structure with a constant chain tilt angle within this series of adsorbate compounds. A slight increase of structural disorder with decreasing chain length was indicated,however, by peak shifts and band broadenings ofthe CH stretching absorptions and has been ascribedtoan increased amount of conformationallydisordered hydrocarbon chains. Significant changes in the film structure were observed for monolayers of short-chain compounds (n = 10, n = 13) prepared under ambient atmospheric conditions. "he films consist of isotropically disordered,liquid-like regions and ordered, crystalline domains. The long-chain compounds, on the other hand, yielded identical film structures under both atmospheric and dry inert gas conditions.
Introduction Self-assembled monolayers (SAMs)have recently received growing attention both as model systems for highly organized biological structures such as biomembranes, vesicles, and micelles and as candidates for a variety of technological applications.' The key to their successful exploitation is a detailed knowledge of their molecular composition and structure as a function of the various parameters governing the film formation process, aiming a t the fabrication of synthetic supramolecular structures with tailored physical, chemical, or biological properties. Today the most extensively investigated systems consist of aliphatic organosulfur compounds or fatty acids adsorbed on metal surfaces such as Au, Ag, Cu, and Al,which are based on a fairly simple chemical adsorption process and are accessible to a variety of optical, electrochemical, and surface science characterization techniques.1,2 An equally interesting, although less thoroughly studied, group of SAMs are organosilane compounds adsorbed on oxidic surfaces like glass, mica, Al203, SiOz, or Sn02.3-14 Abstract published in Advance ACS Abstracts, April 1, 1995. (1)Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly;Academic Press: San Diego, 1991. (2)(a)Nuzzo,R.G.;Allara,D.L.J.Am.Chem.Soc. 1983,105,44814483. (b) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1, 52-65. (c) Whitesides, G. M.; Laibnis, P. E. Langmuir 1990,6,87-96.(d) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992,43,437-463. (e) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K ; Majdy, M.; Melroy, 0.;Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J.Phys. Chem. 1993,97,7147-7173. (0 Tao, Y.-T. J . Am. Chem. SOC.1993,115,4350-4358.(g) Chailapakul, 0.;Sun, L.; Xu,C.; Crooks, R. M. J . Am. Chem. SOC.1993,115,1245912467. (h) Offord, D. A.; Griffin, J. H. Langmuir 1993,9,3015-3025, and references cited therein. (3)Sagiv, J. J. Am. Chem. SOC.1980,102,92-98. (4)Maoz, R.; Sagiv, J. J.Colloid Interface Sci. 1984,100,465-496. ( 5 ) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J . Am. Chem. SOC.1988,110,6136-6144. (6) Ulman, A. Adu. Mater. 1990,2,573-582. (7)Wasserman, S. R.;Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5,1074-1087. (8)Angst, D. L.;Simmons, G. W. Langmuir 1991,7,2236-2242. (9)Kallury, K. M. R.; Thompson, M.; Tripp, C. P.; Hair, M. L. Langmuir 1992,8,947-954. (10)Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J . J. Langmuir 1991,7,1647-1651. @
The silanization of surfaces with various alkoxy- or chlorosilaneshas found extensive use in ~hromatography'~ and biochemistry16 and has recently shown promising applications in the field oftribology" as well as for chemical sensors18and microelectronic device fabri~ati0n.l~ Comparatively little is known, however, about the structure of organosilane monolayers and the underlying adsorption mechanism. It is generally believed today that the adsorption of organosilane compounds having hydrolyzable bonds such as Si-C1 or Si-OR proceeds on hydroxylated surfaces via the formation of silanols as intermediates, which are then covalently bonded to the surface through a condensation reaction with the surface OH An additional (11)Kessel, C. R.; Granick, S. Langmuir 1991, 7,532-538. (12)Nakagawa, T.; Ogawa, K.; Kurumizawa, T. Langmuir 1994,10, 525-529. (13)Yamamura, K.; Hatakeyama, H.; Naka, IC;Tabushi, I.; Kurihara, K. J. J . Chem. SOC.,Chem. Commun. 1988,79-81. (14)Kallury, K. M.R.; Cheung, M.; Ghaemmaghami, V.; Krull, U. J.;Thompson, M. Colloids Surf. 1992,63,1-9. (15)(a) Plueddemann, E. P. Silane Coupling Agents; Plenum: New York, 1991. (b) Nawrocki, J.; Buszewski, J. J . Chromatogr. 1988,449, 1-24. (c) Liu, H.; Cantwell, F. F.Ana1. Chem. 1991,63,993-1000.(d) Jones, K. J . J. Chromatogr. 1987,392,1-10 and 11-16. (e) Ito, K.; Ariyoshi,Y.; Tnabiki, F.; Sunahara, H.Anal. Chem. 1991,63,273-276. (0 Wirth, M. J.; Fatunmbi, H. 0. Anal. Chem. 1992,64,2783-2786. (16)(a)Zimmermann, R. M.; Schmidt, C. F.; Gaub, H. E. J . Colloid InterfaceSci. 1990,139,268-280.(b)Markovich,R.J.; Qiu,X.;Nichols, D. E.; Pidgeon, C. E.; Invergo, B.; Alvarez, F. M. Anal. Chem. 1991,63, 1851-1860. (c) Muramatsu, H.; Dicks, J . M.; Tamiya, E.; Karube, I. Anal. Chem. 1987,59,2760-2763. (17)(a)DePalma, V.; Tillman, N. Langmuir 1989,5,868-872. (b) Ando, E.;Goto, Y.: Morimoto, K.; Ariga, K.; Okahata, Y. Thin Solid Films 1989,180,287-291.(c)Ruhe, J.;Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993,9,2383-2388. (18)(a) Lee, Y. W.; Reed-Mundell, J.; Sukenik, C. N.; Zull, J. E. Langmuir 1993,9,3009-3014. (b) Gebbert, A.; Alverez-Icaza, M.; Stochlein, W.; Schmid, R. D. Anal. Chem. 1992,64,997.(c) Heckl, W. M.; Marassi, F. M.; Kallury, K M. R.; Stone, D. C.; Thompson, M. Anal. Chem. 1990,62,32-37.(d) Kurth, D. G.; Bein, T. Langmuir 1993,9, 2965-2973. (e)Jin, Z.H.; Vezenov, D. V.; Lee, Y. W.; Zull, J. E.; Sukenik, C. N.; Savinell, R. F. Langmuir 1994,10,2662-2671. (19)(a) Dulcey, C. S.; Georger, J. H.; Krauthamer, V.; Fare, T. L.; Stenger, D. A.; Calvert, J. M. Science 1991,252,551-554.(b) Calvert, J. M. In Organic Thin Films and Surfaces; Ulman, A., Ed.; Academic Press: San Diego, CA, 1993. (c) Fontaine, P.; Goguenheim, D.; Deresmes, D.; Vuillaume, D.; Garet, M.; Rondolez, F. Appl. Phys. Lett. 1993,62,2256-2258.
0743-746319512411-1304$09.00/0 0 1995 American Chemical Society
Structure of Alkylsiloxane Monolayers
Langmuir, Vol. 11, No. 4, 1995 1305
cross linking takes place for film molecules such as external reflection technique. First, they are weak alkyltrichlorosilanes (RSiC13) having more than one reflectors in the infrared region (typically less than 20% of the incident radiation is reflected depending on incidence hydrolyzable group at the silicon atom: which results in angle and polarization) and require a carefully optimized the formation of mechanically, thermally and chemically instrumentation in order to achieve monolayer sensitivextremely stable alkylsiloxane surface layers.7J2~21-24 Film ity.37,38 Second, their optical properties can result in thickness measurements by ellipsometry and X-ray repronounced optical effects on peak positions, shapes, and flectivity combined with wettability studies have indicated a high degree of structural order in these films,4,5,7,8J0,25-27 intensities leading to complex absorption profiles in the adsorbate reflection ~ p e c t r a . , ~ -In ~ ~turn, this strong but there is yet little direct spectroscopic information on dependence of external reflection infrared spectra on the the adsorbate structure. Shen's group28and Watanabe substrate's optical properties and on the incidence angle et aLZ9have used vibrational sum frequency generation and polarization of the infrared radiation makes this (SFG) spectroscopy to determine the hydrocarbon chain technique in combination with spectral simulations a very orientations in monolayers of alkyltrichlorosilanes on silica powerful tool for the surface chemistry of nonmetallic substrates. On the basis of the selection rules of this substrate^.^^-^ method the absence of v(CH2)absorptions ofthe methylene In a brief report we have recently compared monolayer groups in the monolayer spectra provided evidence of a reflection spectra of octadecyltrichlorosilane (OTS) adpredominantly all-trans geometry of the hydrocarbon sorbed on silicon and of dioctadecyl disulfide (ODS) chains.29 From the v(CH3) absorptions the tilt angle of adsorbed on gold30and have shown that similar sensitivity the terminal methyl groups against the surface normal and accurate structural information can be obtained on was determined as 45 f 20" 29 and 45 f 5°,28from which both surfaces. On the basis of a spectral simulation and an overall chain tilt angle a of less than 15"was derived.28 fit procedure, a tilt angle of -8" f 3"was determined for Similar results (10" a 18")were obtained with internal the hydrocarbon chains in OTS monolayers. In this paper reflection infrared spectroscopy (ATR) for octadecylwe will give a more detailed account on the quantitative trichlorosilane monolayers on silicon surface^,^ although structural analysis of alkylsiloxane monolayers from their a large experimental uncertainty (f18")was inherent to external reflection infrared spectra and will extend our these results. previous findings to homologous compounds with different This lack of accurate structural data for alkylsiloxane hydrocarbon chain lengths. We will also present some monolayers has prompted us to investigate the application preliminary results which show that this technique has a unique sensitivity for differentiating between liquidof external reflection infrared spectroscopy, which unlike, isotropic regions and oriented crystalline domains in doubtedly has contributed significantly to the detailed partially disordered monolayers. knowledge on the structure of S A M films on metal surfaces. Previous approaches in this direction were based on the Experimental Methods use of "artificial" silicon substrates prepared by either implanting a metal layer by ion bombardment into a silicon Synthesisof Alkyltrichlorosilanes. Alkyltrichlorosilanes (CH3(CH2),SiC13) with hydrocarbon chain lengths between n = substrate31 or depositing a thin layer of silicon or silicon 10 and n = 16 were synthesized in 50%to 70%yield by radical oxide onto a metal Thereby a top surface addition of trichlorosilane (HSiC13) to the corresponding l-allayer similar to native silicon wafers was obtained without kenes CHs(CH2),-2CH=CH2 according to the following reaction sacrificing the advantages of a metal substrate in the scheme:& external reflection infrared measurements (high reflectivity and easily interpretable spectra, whose band AIBN, hv HSiC1, CH,(CH,),-,CH=CH, CH,(CH,),SiCl, intensities are governed by the metal surface selection rule). Very small v(CH2) absorption intensities were observed in alkylsiloxane monolayer spectra on these In a typical procedure, 75 mmol of the 1-alkene was mixed in a s u b ~ t r a t e s lin ~ ~qualitative ,~~ agreement with close-todry nitrogen atmosphere with an equimolar amount of HSiC13 (Merck, 99% purity), a small amount (1-3 mmol) of a,a'vertically oriented hydrocarbon chains, but no quantitative azoisobutyronitrile(AIBN,Aldrich, 98%purity) was added, and structural data were given. Two major problems are the reaction mixture was stirred for 9 h under U V light inherent to the use of bulk silicon substrates for the
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(20)Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120-1126. (21) Balachander, N.; Sukenik, C. N.Langmuir 1990,6,1621-1627. (22) Maoz, R.; Sagiv, J. Langmuir 1987,3, 1045-1051. (23) Ulman, A. Adu. Mater. 1991, 3, 298-303. (24) Bourdieu, L.; Maaloum,M.; Silberzan, P.;Ausserre, D.; Coulon, G.; Chatenay, D. Ann. Chim. Fr. 1992,17, 229-239. (25) Wasserman, S. R.; Whitesides, G. M.; Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.;Axe, J. D. J.Am. Chem. SOC.1989,111,5852-5861. (26) Brzoska, J. B.;Shahidzadeh, N.;Rondolez, F. Nature 1992,360, 719-721. (27) Parikh, A. N.; Allara, D.L.;Azouz, I. B.; Rondolez, F. J . Phys. Chem. 1994,98, 7577-7590. (28) Guyot-Sionnest,P.;Superfine, R.; Hunt, J. H.;Shen,Y. R. Chem. Phys. Lett. 1988, 144, 1-5. (29)Watanabe, N.;Yamamoto, H.; Wada, A,; Domen, K.; Hirose, C. Spectrochim. Acta 1994,50A, 1529-1537. (30) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vib. Spectrosc. 1995, 8, 151. (31)Ehrley, W.; Butz, R.; Mantl, S. S u f . Sci. 1991,248, 193-200. (32) Bermudez, V. M.; Prokes, S.M. S u f . Sci. 1991,248,201-206. (33) McGonigal, M.; Bermudez, V. M.; Butler, J. E. J. Electron Spectrosc. Relat. Phenom. 1990, 54/55, 1033-1044. (34) McGonigal, M.; Bermudez, V. M. Su$. Sci. 1991,241,357-368. (35) Finke, S. J.; Schrader, G. L.Spectrochim. Acta 1990,46A, 9196. (36) Ohtake, T.;Mino, N.;Ogawa, K. Langmuir 1992,8,2081-2083.
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irradiation. The products were isolated by vacuum distillation as colorless liquids and their purity was checked by 1H-NMR spectroscopy(Bruker, 250 MHz, CDCl3 solutions) and transmission infrared spectroscopy (undecyltrichlorosilane(UTS),bp 82 "C/0.04 Torr, lH-NMR 6 0.88 (t, CH3), 1.7-1.2 (m, (CH2)lo); tetradecyltrichlorosilane(TTS) bp 114-118 "C/0.2Torr, 'H-NMR (37)Chesters, M. A.; Horn, A. B.; Kellar, E. J. C.; Parker, S. F.; Raval, R. Nato ASZSer., Ser. B 1989,198, 103-109. (38) Because of the partial transparency of silicon, adsorbate spectra are commonly measuredby internal reflection (ATR)spectroscopy,which has a substantial sensitivity advantage due to the multiple reflections within an ATR prism. The strong absorptions of silicon at wavenumbers < 1500 cm-l, however, block the low-frequency range in ATR spectra. Additionally, ATR prisms are expensive and must be reused, which in this particular system (alkylsiloxane monolayers) requires harsh cleaning procedures with unknown effects on the morphology and composition of the prism surface. (39) Wong, J. S.;Yen, Y. S. Appl. Spectrosc. 1988, 42, 598-604. (40) Ishino, Y.; Ishida, H. Langmuir 1988,4, 1341-1346. (41)Yen, Y. S.; Wong, J. S. J. Phys. Chem. 1989, 93, 7208-7216. (42) Parikh, A. N.; Allara, D. L. J . Chem. Phys. 1992,96,927-945. (43) Mielczarski, J. A. J . Phys. Chem. 1993, 97, 2649-2663. (44) Mielczarski, J. A.; Yoon, R. H. J . Phys. Chem. 1989,93,20342038. (45) Eaborn, C.; Harrison, M. R.;Walton, M. R. J.Organomet.Chem. 1971,31,43-46.
Hoffmann et al.
1306 Langmuir, Vol. 11,No. 4, 1995 6 0.88 (t, CH3), 1.7-1.2 (m, (CH2)13);pentadecyltrichlorosilane (PTS), bp 128-130 "C/0.06 TOIT,'H-NMR 6 0.88 (t, CH3), 1.71.2 (m, (CH2)14);hexadecyltrichlorosilane (HXTS), bp 131-138 W0.2-0.3 Torr, 'H-NMR 6 0.88 (t, CHd, 1.7-1.2 (m, (CH2)15); heptadecyltrichlorosilane (HPTS),bp 149-151 "C/0.03Torr, 'HNMR 6 0.88 (t, CH3), 1.7-1.2 (m, (CH2)16)). Octadecyltrichlorosilane (OTS)was obtained from Aldrich (95% purity) and was used without further purification. Substrates. p-Doped,(100)oriented and single-sidedpolished silicon wafers (test grade, 14-30 Q cm resistivity, 100 mm diameter, 0.5 mm thickness) were obtained from Wacker Chemitronic and cut into 25 x 20 mm2 pieces with a diamondtipped stylus. They were cleaned immediately before adsorption by sonication in HzSOflz02 solution (4:1, v/v) for a few minutes followed by extensive rinsing with doubly distilled water (specific conductivity < 10-6 W 1cm-l) and acetone (Aldrich, 99%purity), after which they were dried in an oven at 40 "C. This treatment yields substrates with an approximately 20A thick surface layer of silicon dioxide (determined ellipsometrically)and an OH-group surface concentration in the order of 5 x 1014 cm-2.6 Monolayer Adsorption. Unless otherwise noted, all operations were carried out in a glovebox filled with dry nitrogen. Solutions of the adsorbate compounds in absolute toluene (Aldrich, 99% purity, distilled over Na, water content < 1mmol L-1) were prepared volumetrically a t concentrations of 1mmol L-l. The cleaned substrates were placed in an adsorption vessel similar to a TLC chamber, which was filled with the adsorbate solution so that the lower half of the substrate area (25 x 10 "2) was covered, allowing the upper, clean halfof the substrate to be used as "internal" reference for the infrared measurements (see below). A potential limitation of this technique is a contamination of the reference surface via gas phase adsorption or creeping of the adsorbate solution during the adsorption process. Therefore the cleanliness of the reference was periodically checkedby ratioing its IR spectrum against a freshly cleaned, separate substrate and was always found to be satisfactory within the detection limits of the IR method. The substrates were left in the adsorbate solutions overnight, after which they became autophopic; i.e., they emerged completely dry from the solution. This was taken as a qualitative criterion for complete monolayer f ~ r m a t i o n .Longer ~ ) ~ adsorption times (up to 3 days) showed no noticable effect on the wetting properties and the infrared spectra of the films. The final step ofthe monolayer preparation involved rinsing and sonication of the substrates with absolute toluene and drying in air. External Reflection Infrared Measurements. Infrared spectra were measured with a custom-made external reflection optical system connected to a Mattson RS1 FT-IR spectrometer, which is described in detail elsewhere.30 p-Polarized light at an incidence angle of 80 & 3" was used. A total of 1024 scans a t 4 cm-1 resolution were averaged resulting in spectral peak-topeak noise levels of 3 x 10-5 absorbance unit at 3000 cm-l. The substrates were mounted on an optically flat sample holder connected to a remote-controlledmicrometer stage, which allowed for a rapid interchange between sample and reference position without opening the purged optical system. Baseline artifacts such as interference fringes, slopes, or miscancelled background absorptions, which are caused by varying purge conditions or long term drifts of the spectrometer stability and are usually far more critical for the overall spectral quality than the random noise, could be largely avoided in this manner. SpectralSimulations. On the basis of a previously described semiempirical matrix method42a computer program has been developed, which allows a simulation of external reflection infrared spectra as a function of the optical parameters of substrate and adsorbate, film thickness, incidence angle, and the molecular orientation of the adsorbate molecules on the surface. The mathematical algorithm of this program follows closely the original work42and will therefore not be described here. Several simplifications can be made, however, for the particular systems in this study (alkylsiloxane monolayers on silicon substrates): A three-layer model with parallel phase boundaries consisting of an isotropic, semiinfinite incidence medium, an anisotropic adsorbate film of thickness d, and an isotropic, semiinfinite substrate is used.46 Both the incidence phase (air)and the substrate (silicon)are treated as nonabsorbing in the infrared (absorption coefficient k = 0) and are assigned
frequency-independent refractive indices (n, = 1,ns,= 3.4240*41). A uniaxial symmetry around the surface normal is assumed for the adsorbate layer, which reduces the general (4 x 4) matrix method used in ref 42 to a (2 x 2) formalism and allows separate treatment of p-polarized and s-polarized radiation. The optical function (complex refractive index li = n ik) of the adsorbate can then be described as a diagonalized, second rank tensor with the diagonal components lix2, fiy2, fizz (fix = liJ, where z is the symmetry axis (surface normal). This tensor is constructed as described in ref 42 from the isotropic absorption coefficients k,(v)of a suitable reference compound and the internalcoordinates of the vibrations, from which the anisotropic vector components {kf,kp,k:} for a certain orientation ofthe molecule on the surface are obtained by transformation of the internal coordinates into surface coordinates. The corresponding real parts {nf, np, n:} of the tensor elements are calculated from {kf, kp, kf} by Kramers-Kronig t r a n s f o r m a t i ~ nassuming ~~ a frequency-independent background component n, = 1.50.5 A least-squares fit procedure finally allows the determination of an optimum set of parameters for which the best agreement with the experimental spectrum is obtained.
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Results and Discussion Molecular Orientation Analysis from IR Reflection Spectra. When the surface ofbulk silicon as a typical dielectric substrate in the infrared region (refractiveindex n = 3.42, absorption coefficient k = 039241)is probed with p-polarized light, the resulting electric field has sizable components both parallel and perpendicular to the surface, whose relative amplitudes depend strongly on the light incidence angle.40 Therefore both the parallel and perpendicular vibrational components of an adsorbate film absorb radiation but cause reflectivity changes of the adsorbatehubstrate system in opposite directions. Thus, the resultant intensity and the sign of an absorption band in the reflection spectrum depend on the dipole moment orientation of the particular vibration on the surface and on the incidence angle.39-" This is illustrated in Figure 1, where the calculated intensities of an absorption band with an isotropic absorption index k,,, = 0.10 a t 3000 cm-l in a hypothetical monolayer film ( n = 1.50,d = 25 A) on silicon are shown as a function of the light incidence angle. The particular values for the optical constants and film thickness were chosen to simulate a CH-stretching absorption in a monolayer film of a hydrocarbon compound. The approximate equations given by Mielczarsky et a1.44 were used for these calculation^.^^ Ax and A, in Figure 1 represent the absorption intensities for parallel and perpendicular orientations of the vibrational dipole moments on the surface. The corresponding anisotropic absorption indices k, and k, were calculated from k,,, as k, = 3k, and k, = 3k,,J2, assuming a uniaxially symmetric distribution of the dipole moments around the surface Some important conclusions for the design and interpretation of external reflection infrared measurements on silicon substrates can be drawn from Figure 1: A, and A, are always opposite in sign. Their absolute intensities increase exponentially toward the substrate's Brewster angle OB (OB(&) = 74"))where the signs change (46) More precisely, a five-layer model (air-adsorbate-substrateadsorbate-air) would be required to account also for multiple reflections within the substrate and absorption from the sample adsorbed on the backside of the substrate. Simulations based on this extended model yield spectra which are completely dominated by large interference fringes with an approximate spacing of 3 cm-' for 80"incidence and 0.5 mm thickness of the silicon substrate. Since we did not observe such interferences in the experimental spectra, we conclude that most of the light is diffusely scattered on the roughened backside of the substrates used in the present study and contributions from multiple reflections to the specularily reflected radiation on the front surface are negligible. (47) The equations for A, and Az derived in ref 44 are valid for films whose thickness d is small with respect to the infrared wavelengths and whose absorption intensities therefore scale linearily with d (see ref 41).
Structure of Alkylsiloxane Monolayers I I
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Langmuir, Vol.11,No. 4,1995 1307 I
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-6 -9
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Incidence angle 8 Figure 1. Calculated IR intensities of an absorption (YO = 3000 cm-', kbo = 0.10) in a hypothetical organic monolayer (d = 25 A, n = 1.50)on silicon for parallel (A,) and perpendicular (A,) orientation ofthe vibrational dipole moments on the surface as a function of the light incidence angle for p-polarized radiation. Equations from ref44 were used for the calculations. and a discontinuity appears. A, is negative and A, is positive for incidence angles 8 < 8B and the reverse applies for 8 > OB. An incidence angle slightly above or slightly below 8B is therefore expected to be the optimum choice in terms of ~ e n s i t i v i t y .For ~ ~80" incidence, the calculated intensities for the example shown in Figure 1 are A, = f0.001 for parallel dipole moment orientation and Az = -0.0047 for perpendicular orientation, respectively. Thus, for a certain tilt angle 4 of the dipole moment toward the surface normal the absorption is expected to vanish due to mutual cancellation of the parallel and perpendicular components. This is illustrated in Figure 2, where the calculated intensities are shown as a function ofthe dipole moment tilt angle 4 for three different absorptions with kiso values of 0.05, 0.15, and 0.3, spanning the range of typical absorption intensities expected from an organic compound. The curves in Figure 2 represent the sum of the parallel and perpendicular components and were calculated as A(#) = A&(@)) A,(kZ(4))with k, = 3klS0 sin2412 and k, = 3klS0cos2442using the previous equations for A, and A,.44 With 4 decreasing from parallel (4 = 90") to perpendicular (4 = 0") orientation, the absorptions decrease in intensity, pass through zero for 4 67", and grow in the negative direction afterward. A tilt angle of 4 = 54" represents the pseudoisotropic case, i.e., an orientation with equal x - and z-components, for which negative absorptions are predicted in Figure 2. Isotropic films on a silicon substrate, in which each vibration has equal averaged parallel and perpendicular components because of the random molecule orientations, are therefore
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(48) Concurrentwith the exponential increase of the band intensities toward the Brewster angle the absolute reflectivity of the substrate approaches zero and the spectral noise increases correspondingly, resulting in t w o counteracting effects on the signal to noise ( S N ) ratio. A more detailed analysis shows that the optimum angle of incidence is not where the relative band intensity A = -log(R/Ro) is highest but where the absolute reflectivity change IR -Rol has its maximum (Rand Ro being the reflectivities ofthe adsorbate-coveredand the bare substrate surface, respectively). An optimum angle of 85" is thereby calculated for p-polarized light on silicon substrates. See refs 37 and 49. (49) Udagawa,A.; Matsui, T.;Tanaka, S. Appl. Spectrosc. 1986,40, 794-797.
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k=0.30 0
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60
30 Tilt angle
90
4
Figure 2. Calculated IR intensities of three absorptions with different isotropic absorption coefficients k in a hypothetical organic monolayer (d = 25 A, n = 1.50)on silicon as a function of the tilt angle between their vibrational dipole moments and
the surface normal (p-polarizedradiation, 80" incidence).
expected to yield inverted absorbance spectra under these experimental conditions (80" incidence, p-polarized radiation). Anisotropic films, on the other hand, in which a certain uniform molecule orientation on the surface imposes different tilt angles for each molecular vibration, can give very complex reflection spectra consisting of superimposed positive and negative absorption bands. This is exemplified in Figure 3, where the simulated CH stretching absorptions of an octadecylcompound adsorbed on silicon are shown for different tilt angles a of the hydrocarbon chains toward the surface normal. A simple geometrical model of an all-trans alkyl group with tetrahedral bonding angles (110") and a film thickness of 25 A (the average thickness of a monolayer of octadecyltrichlorosilane') were assumed for these simulations. The isotropic spectral parameters were derived from a literature spectrum of the model compound dioctadecyl disulfide50 and are listed in Table 1 together with the corresponding transition dipole moment directions. Fairly complex absorption profiles result in the simulated reflection spectra due to overlapping absorptions with different dipole moment orientations. Each band intensity, however, follows the simple angle dependence shown in Figure 2. The vS(CH2)absorption at 2851 cm-l in Figure 3, for example, shows its maximum positive intensity for a vertically oriented chain (a= 0'1, where its transition dipole moment lies parallel to the surface (4 = 90"). Upon tilting the chain in either the +a or -a direction, the absorption decreases concurrent with its tilt angle 4 toward the surface normal (4 = 90 - lal)and inverts from a positive to a negative band between a = 20"and a = 30". A small shift toward higher wavenumbers occurs upon inversion resulting from a refractive index dispersion effect, which has a much stronger influence on the perpendicular vibrational components than on the parallel vibration^.^^ It is also seen from Figure 3 that the vs(CH2) intensities at 2851 cm-l are equal for positive and negative a and change very little in the range -10" < a < lo", indicated by the flat slope of the intensity versus angle (50) Laibnis,P.E.;Whitesides,G.M.;Allara,D.L.;Tao,Y.-T.;Parikh,
A. N.; Nuzzo, R. G.J . Am. Chem. SOC.1991,113, 7152-7167.
1308 Langmuir, Vol. 11, No. 4, 1995
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Table 1. Spectral Parameters and Transition Dipole Moment Orientations for the CH Stretching Absorptions of an Octadecyl ( C l a 7 ) Group, Derived from an Absorption Coefficient (k-)Spectrum of Dioctadecyl Disulfideso Peak band width frequency absorption (hhmP vibration" (cm-l) coefficient (cm-l)
transition dipole moment unit vector {z,y,z ) ~
2962 0.030 12 {sin 55", 0,cos 55") vaS(C&)ip v,(CH3),, 2950 0.023 10 {O,l,OI 0.008 12 {-cos 55",0,sin 55") Y.(CH~)FR 2937 vas(a-CH2Y 2927 0.070 10 {0,1,01 v ~ C H Z ) 2919 0.305 12 {O,l,OI Y ~ ( C H ~ )2906 ~ 0.028 11 {1,0,0) vs(CH2)m 2892 0.030 17 {1,0,01 2878 0.018 10 {-cos 55",0,sin 55") vACHS) vs(a-CH2Y 2860 0.015 16 {1,0,0) VS(P-CHZY 2853 0.021 10 {LO,O} vS(CHz) 2851 0.174 12 U,O,O) a fwhm, full width at half maximum; ip, in plane;op, out of plane; FR, Fermi resonance. Unit vector coordinates in a molecular coordinate system defined by the hfdrocarbon chain axis (z-axis) and the C-atom backbone plane (zj-plane). Absorptions of the CH2 groups in a-and ,&position to the disulfide group.
*
0,
0 C
0
e 0
v)
n 4
2800
'
2900
'
3000
wavenumbers (cm -9 Figure 4. CH-stretching absorptions in external reflection infrared spectra of monolayers of homologous alkyltrichlorosilanes CH3(CHz),SiCls (n= 10-17) adsorbed on silicon surfaces measured with p-polarized light at 80" f 3" incidence.
Figure 3. Simulated monolayer reflection spectra (p-polarized radiation, 80" incidence) of an octadecyl compound on silicon for different tilt angles of the hydrocarbon chain axis toward
The intensity ofthe v,(CHd vibration at 2919 cm-', whose dipolemoment is perpendicular to the C-atom plane (Table 1)and remains parallel to the substrate surface for all tilt angles a, does not change with a in this simplified geometrical model (see below). The calculated absolute band intensities change by as much as a factor of 4 within an incidence angle range 77" < 6 < 83" (see Figure 1)-a parameter which is hardly determinable with an accuracy better than a few degrees and represents just a mean value for the focused infrared beam in the experiment. Additionally,the film absorbance scales linearily with other parameters such as the surface coverage, the film thickness, and the absorption coefficients, all of which are not directly determinable and depend on certain models and assumptions. A quantitative orientation analysis with this method should therefore rely primarily on the relative band intensities in the experimental reflection spectra.51
curves (Figure 2) in the region close to $I = 90".In contrast, the methyl group absorptions vas(CH3Iipat 2962 cm-' and v,(CH3) at 2878 cm-l, whose dipole moment tilts are 55" and 35" for vertical chain orientation, show markedly different intensities for positive and negative a along with significant intensity changes in the region around a = 0".
(51)In alkanethiol monolayerspectra on metal surfaces,for example, the hydrocarbon chain tilt angle a can be determined only from the absolute intensities of the v(CH2) absorptions (see ref 50). Any error in the assumed film thickness, incidence angle, surface coverage, or absorption coefficientsresulta in a wrong value for a. This is probably the main reason for the significant variations of the literature values for this parameter (see ref 2d). On silicon substrates, where also the parallel vibrational components contribute to the overall intensities, a can be determined from the relative v(CH2)intensities (see Figure 3).
2800
2900
3000
Wavenumbers (cm-1]
the surface normal. See text for details.
Structure of Alkylsiloxane Monolayers
Langmuir, Vol. 11, No. 4, 1995 1309
Table 2. Vibrational Assignments, Peak Positions, and Half Widths of the CH Stretching Absorptions in Monolayer Infrared Spectra of AlkyltrichlorosilanesCHq(CHa),SiCLq Adsorbed on Silicon Substrates ~~~~
peak position
UTS (n = 10) vibration
vmax
2968 vS(CH3) 2880 vas(CH2) 2924 vs(CH2) 2855 vs(CHz)mc 2890(sh) vas(CH3)ipa
ip, in plane. Not
AVUZ 8 b 22 18 b
Vmax
TI'S (n = 13)
PTS (n = 14)
vmax
Avl/z
Vmax
Av1/2
vmax
2968 2879 2920 2851 2890(sh)
9 b 17 12 b
2968 2879 2919 2851 2890(sh)
9 b 17 12 b
2968 2879 2919 2851 2890(sh)
/
1
CH,(CH,),SiCI,
160-
0
C
9
10
11
~
~~
~~~~
HXTS (n = 15) AVUZ 9 b 16 12 b
HPTS (n = 16)
OTS ( n = 17)
Vmax
AV1/2
Vmax
Av1/2
2968 2879 2918 2851 2890(sh)
10 b 16 12 b
2968 2879 2918 2851 2890(sh)
10 b 16 12 b
determinable with sufficientaccuracy. FR, Fermi resonance band sh, shoulder.
200 r v)
~
and half width Avllz (cm-')
12
13
14
15
16
17
18
n
Figure 5. v(CH2) absorption'intensitiesin monolayer spectra of homologous alkyltrichlorosilanes adsorbed on silicon sub-
strates (Figure4) as a function ofthe hydrocarbonchain length.
Hydrocarbon Chain Orientation in Alkylsiloxane Monolayers. In Figure 4 external reflection infrared spectra in the CH stretching region are shown for a series of homologous alkylsiloxane monolayers prepared by adsorption of alkyltrichlorosilanes (CH3(CH2),SiC13,n = 10-17) on silicon surfaces. Peak positions and half-widths of the major absorption bands are listed in Table 2 together with their vibrational assignments. The data in Table 2 represent the average between three and six different samples per compound and were reproducible to within 1 cm-'. The peak frequencies and line widths are largely independent of the chain length of the adsorbate molecules except for the shortest-chain compound UTS ( n = lo), for which a shift of the CH2-absorptions v,(CHz) and ~as(CH2) to higher wavenumbers together with a distinct band broadening is observed. An approximately linear relationship is found between the intensities (peak heights) of the v,(CH2) and vas(CH2)absorptions and the chain length of the adsorbate molecules (Figure 5). Because of the complex overlapping absorptions no attempt was made to evaluate the integrated peak areas. The v(CH3) absorptions at 2968 and 2879 cm-' appear as negative (downward looking) features in the monolayer spectra corresponding to an increase in reflectivity of the monolayerhubstrate system compared to the bare substrate surface. Their intensities are independent of the chain length within the accuracy of these measurements. The only exception is again the UTS monolayer spectrum, which shows a significantly weaker vas(CH3)ipabsorption. A quantitative analysis of the OTS monolayer spectra based on the simulation and fit procedure described before yields an average hydrocarbon chain tilt angle of -8" f 3" toward the surface normal.30 The accuracy of this determination, which is based on the best least-squares fits for five different OTS samples, is illustrated in Figure 6, which shows the experimentalOTS monolayer spectrum together with simulated spectra for tilt angles a of +8",
-8", -3", and -13".52 The same parameters as in Figure 3 were used for the simulated spectra53 except for the
incidence angle 8, which was used as a scaling factor to match the absolute intensities between the experimental and calculated spectra (see previous section). The best agreement was obtained for 8 = 83", which is still within the estimated error range (80"f 3")of this parameter in our optical system. Apart from small band shifts of the vs(CH3)and vas(CH3)absorptions between the calculated and the experimental spectra in Figure 6, which might be caused by the unique environment of the terminal methyl groups at the monolayerlair interface, the experimental spectrumis accurately reproduced for a = -8". Significant differences in the v(CH3) intensities are observed in all other cases, while the v(CH2) intensities are fairly insensitive within this range of tilt angles (see previous section). Extendingthis analysis to the homologous shorter-chain compounds in this study, the linear decrease of the v(CH2) absorption intensities (Figure 5 ) and the constant peak positions and line shapes (Table 2) in the experimental reflection spectra are strong indications for a conservation of a crystalline, densely packed film structure with a constant chain tilt angle. Similar conclusions have been derived from infrared reflection spectra of other SAM films on metal s ~ b s t r a t e sand ~ ~are ~ ~further ~ , ~ ~supported here by the constant contact angles and the linear change of the film thickness with the hydrocarbon chain length observed previously for alkylsiloxane monolayer^.^ Only the UTS monolayer spectrum in Figure 4 indicates a beginning transition toward a disordered film structure judged from the shift of the v(CH2) absorptions to higher wavenumbers and the increase in bandwidths. This will be discussed in more detail in the following section. A constant tilt angle imposes different surface orientations of the terminal methyl groups for chains with odd and even numbers of CH2 groups. Consequently, the IR intensities of the v,(CH3) and vas(CH3)ip modes are expected to alternate between odd- and even-numbered chains, which has been experimentally observed in numerous previous studies of S A M films on metal surfaces.2f~50,55-57 For a -8" chain tilt the angles between the v,(CH3) and v,,(CH3)ip dipole moments and the surface normal change from 43" and 47" for odd-numbered chains to 27" and 63" (52)The sign ofthe tilt angle a is herein defined according to previous conventionss0as having the Si-C(l) bond tilted toward the surface for positive a and away from the surface for negative a. (53)Strictly speaking, the experimental substrates are Si/SiOz structures and would require a two-phase model with separate optical constants for Si and SiOz. Ellipsometric measurements have shown, however, that a thin oxide layer (d 20 A) has a negligible effect on the optical constants of a silicon substrate, which can therefore be approximated a8 a pseudo-one-phase structure (see ref 27). (54)Porter, M.D.;Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1989,109,3559-3568. (55)Tao,Y. T.; Lee, M. T.; Chang, S. C. J.Am. Chem. SOC.1993,115, 9.547-9.5.5.5. - - - . .- - -. (56)Walczak, M.M.;Chung, C.; Stole, S.M.; Widrig, C. A,; Porter, M. D.J.Am. Chem. SOC.1991,113,2370-2378. (57)Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993,97,8032-8038.
-
Langmuir, Vol. 11, No. 4, 1995
HofFnann et al.
-s
: -
2800
2900
3000
2800
2900
3000
Wavenu mbers [cm-1] Figure 6. Comparison of the experimental monolayer reflection spectraof octadecyltrichlorosilane (OTS) adsorbed on silicon (solid lines) with simulated spectra for different hydrocarbon chain orientations on the surface (broken lines). a denotes the tilt angle between the chain axis and the surface normal (see Figure 3).
for even-numbered chains, which should result in an alternation of their absorption intensities equivalent to flipping the chain tilt angle between -8" and $8". The effect predicted by the simulations (see Figure 6) is well above the reproducibility of the experimental band intensities, which show no detectable differences for odd and even chains. There are several possible interpretations for the absence of this odd-even effect. The first and intuitively most likely explanation would be a partly disordered film surface due to structural defects (gauche-trans isomers, freely rotating CH3 groups) at the chain termina, which could diminish the intensity alternations between odd and even chains below the experimental detectability. For the limiting case of a completely randomized film surface the dipole moments of the v(CH3) vibrations would assume an average 54" tilt toward the surface normal (isotropic angle)independent ofthe chain length, which is reasonably close to the angles of43" and 47"resulting for v,(CHs) and Y~~(CH in~a)perfectly ,~ ordered OTS monolayer for a -8" chain tilt. Direct experimental evidence for some (not complete) structural disorder at the surface of alkanethiol monolayers on gold has previously been provided by helium diffraction and temperature-dependent IR studies.58s59A comparison of the v(CH2) frequency shifis with temperature between alkanethiol and alkylsiloxanefilms27 indicates, however, a more ordered film structure in the latter systems. This was ascribed to differences in the free volume of the monolayers, which is created by a thermally induced transition from the tilted to an upright orientation of the film molecules and is proportional to the chain tilt angle in the ordered monolayer structure.27 The almost vertical chain Orientation in alkylsiloxane monolayers leaves less additional space for such conformational disordering than alkanethiol monolayers, in which the hydrocarbon chains are tilted by an average of 30".27Another explanation for invariant dCH3) intensities (58) Camillone,N.;Chidsey,C. E. D.;Liu, G.;F'utvinski, T. M.; Scoles, G . J . Chem. Phys. 1991,94,8493-8502. (59)Dubois, L.H.; Zegarski,B.R.; Nuzzo, R. G.J.Electron Spectrosc.
Relat. Phenom. 1990,54/55,1143-1152.
in accordance with a highly ordered film surface would be an alternating chain tilt angle (-8" for odd and +Bo for even chains), which conserves the surface orientation of the terminal methyl groups, but implies a corresponding alternation of the bonding angles a t the substrate surface. In other words, a surface energy minimum for a certain methyl group orientation might dictate the bonding geometry at the substrate. Such a case was previously reported for alkanethiol monolayers on Ag5O and was explained by the largely ionic character of the Ag-S surface bond. At first sight an analogous interpretation for alkylsiloxane monolayers seems hardly justified due to the rigid, covalently bonded polysiloxane network to which the hydrocarbon chains are attached. It appears more plausible, however, in light of very recent studies on the film formation mechanism,27in which a highly mobile, Langmuir-Blodgett-type film of hydrolyzed silanol molecules floating on a thin water layer a t the substrate surface was proposed as the intermediate stage in the film formation process. Since the film is completely decoupled from the substrate a t this transient stage, the packing and orientation of the film molecules are determined solely by the interchain and film interface energetics. A conservation of this structure in the final crosslinking and surface-coupling step would be explainable by the fairly flexible geometry of the Si-0-Si bond (bond angles as low as 94" (0-Si-0) and 86" (Si-0-Si) have been found in amorphous Si02 compared to the corresponding values of 110" and 152" in an idealized tetrahedral structure60) and/or a slight distortion of the alltrans chain geometry within a short distance from the substrate/film interface. Experimental evidence for the existence of such strained regions has recently been provided for fatty acid monolayers on copper and aluminum surfaces.2f Finally, a more sophisticated geometrical model accounting also for a rotation of the alkyl groups around the chain axis (twist angle p ) might be necessary to describe the structure of these films. An increase in @ has only a minor effect here on the v(CH2) intensities (60)Chiang, C.-M.; Zegarski, B. R.; Dubois, L. H. J . Phys. Chem. 1995,97,6948-6950.
Langmuir, Vol. 11, No. 4, 1995 1311
Structure of Alkylsiloxane Monolayers because of their essentially parallel dipole moment orientations on the surface, but diminishes the odd-even effect on the v(CH3) absorptions, which vanishes completely for p = 90". Unfortunately, an accurate determination ofp is prohibited here by the insensitive response of the absorption intensities as a consequence of the almost vertical chain orientation and the uniaxial film symmetry (note that for a vertically oriented chain (a= 0') the chain axis becomes the symmetry axis in a uniaxially symmetric film structure and the twist angle /3 shows a random distribution). Further studies on the surface structure of the terminal methyl groups have to be awaited for a more detailed interpretation of the infrared spectra. Additional information can also be expected from external reflection spectra using different incidence angles and polarizations. Such experiments are currently in progress in our laboratory. Structural Order-Disorder Transitions. Previous experimental protocols used for the preparation of alkylsiloxane monolayers vary largely with respect to the pretreatment of the substrates, the concentration of the adsorbate solutions, the adsorption time, or the water content of the adsorbate solutions,' ranging, for example, from a few minutes adsorption in water-saturated solutions5J7" to overnight adsorption under rigorous exclusion of humidity.zh,61For the most part, only qualitative relationships between these parameters and their influence on the film structure have been reported. In a very recent detailed study on OTS monolayersz7 the influence of the film preparation temperature has been investigated. Monolayers prepared below a certain threshold temperature T,exhibited a densely packed, highly ordered film structure with very few gauche defects (
