Langmuir 1996,11, 578-584
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Optical Effects in Reflection-Absorption IR Spectroscopy of Thin Films of Silane Coupling Agents on Metallic Surf-aces Dirk Giinter Kurth and Thomas Bein* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received May 9, 1994. I n Final Form: November 9, 1994@ The influence of the anomalous dispersion on the appearance of reflection-absorption bands in the infrared (RAIR) is evaluated, and quantitative trends concerning optically induced peak shifts for the analysis of reflection spectra are presented. We measured RAlR spectra and optical response functions of several trialkoxysilanes. In particular, we present results for (3-aminopropy1)triethoxysilane. Calculations of RAIR spectra demonstrate the influence of the anomalous dispersion on the appearance of band contours of Si-0 modes. As a result, peak positions in reflection spectra do not always correlate with transmission reference data, because these optical effects are absent in the latter. We solve this problem by transforming transmission spectra into reflection spectra. These calculated reflection spectra serve as reference for the identification of experimental reflection spectra. Alkoxysilanes also undergo hydrolysis and condensation reactions that cause additional shifts of Si-0 band positions.
Introduction Thin organic films and interfaces play a key role in biological systems and can be important in future techno1ogies.l Several promising applications of thin films have been identified, including microsensors, patternable materials, protective layers, and optical devices.2 However, more scientific investigations are needed to realize these application^.^ It will also be necessary to further develop surface sensitive characterization tools and to improve our knowledge of surface-confined phenomena. For instance, the success of structure/function correlation depends on the accurate interpretation of complex vibrational spectra. Some important examples of thin films include polymer surfaces, self-assembled monolayers (SAMs),as well as Langmuir-Blodgett films.4 Self-assembly relies on binding of molecules to a substrate whereas in the LangmuirBlodgett technique molecules are transferred onto the substrate. Two notable classes of self-assembling molecules are based on thiols, or silanes ofthe general formula RSi(X)B, where R is a n organic group and X is a n hydrolyzable group, typically alkoxy or chloride. Monolayers of organosilanes adsorbed on siliconwere pioneered by S a g i ~ He . ~ found that several trichlorosilanes spontaneously assemble a t the liquid-solid interface. The binding mechanism of silanes to the surface consists of a complex series of hydrolysis and condensation reactions of the hydrolyzable group^.^ The product of the hydrolysis and condensation reactions is a cross-linked network of siloxanes that may exceed monolayer coverage under Abstract published inAduance ACSAbstracts, January 1,1995. (1)(a)Swalen, J. D.; Allara, D. L. ; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.;McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J . F.; Wynne, K. J.;Yu, H. Langmuir 1987,3,932.(b) Wegner G.Adu. Mater. 1991,3,8. (c)Fuchs, H.;Ohst, H.; Prass, W.Adu. Mater. 1991, 3, 10. (2)Allara, D. L. Crit. Reu. Surf Chem. 1993,2,199. (3) (a)Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L.; Ulman A. Crit. Reu. Surf. Chem. 1993, 3, 49. (b) Supramolecular Architecture; Bein, T., Ed.; ACS Symposium Series 499;American Chemical Society: Washington, DC, 1992. (4)Ulman, A. A n Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston 1991. (5)(a) Gun, J.; Sagiv J. J . Colloid Interface Sci. 1986,112,457.(b) (c) Pomerantz, M.;Segmuller, Maoz, R.; Sagiv J. ibid. 1984,100,465. A.; Netzer, L.; Sagiv J. Thin Solid Films 1985,132,153.(d) Sagiv J. J.Am. Chem. SOC.1980,102 , 92.(e) Tilman, N.; Ulman, A,; Penner T. L. Langmuir 1989,5,101. @
certain conditions. In the presence of hydrolyzable surface groups, such as metal oxide-hydroxy groups, covalent surface bonding has been p o ~ t u l a t e d .The ~ final structure of the film is a function of several parameters, including the nature of the unhydrolyzed group a t the silicon, the number of hydrolyzable groups of the silane, the solvent used during deposition, and the amount of water present in the solvent and on the surfacee8 As a result, the structure of the films varies greatly, ranging from welldefined monolayer to polymeric siloxane aggregates. The presence of several species a t the surface complicates the interpretation of surface IR spectra. Thiols on the other hand self-assemble on gold and form highly ordered films;g the bonding mechanism probably involves formation of a thiolate adsorbed on a gold(1) surface.1° Because of the simplicity of the assembly process, mercaptans adsorbed on various surfaces have been the subject of many studies as model organic surfaces.ll Surface IR spectroscopy in particular has proven very useful in these studies. Allara has pioneered computer simulations of specular reflection spectra.12The particular optical response of a n electronic conductor a t glancing incident angle results in an enhanced, anisotropic wave field on the surface that forms the basis of the socalled “surface selection rules”.13 The structural analysis of thin films relies on calculating reflection-absorption (RA) spectra as a function of the orientation of the transition dipole moments on the surface.12 Careful (6)(a)McNeil, K.J.; Dicaprio, J. A.; Walsh, D. A.; Pratt, R. F. J.Am. Chem. SOC.1980,102,1859. (b)Pohl, E.R.; Osterholtz, F. D. InSilanes, Surfaces and Interfaces; Leyden, D., Ed.; Gordon & Breach: New York, 1986.(c) Pohl, E.R.; Osterholtz, F. D. In Chemically Modified Surfuces in Science and Industry; Leyden, E.; Collins W., Eds.; Gordon & Breach: New York, 1988. (7)(a) Dubois, L. H.; Zegarski B. R. J . Am. Chem. SOC.1993,115, 1190. (b) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8 , 1120. (c) Wasserman, S. R. ; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989,5, 1074. (8)Plueddeman, E. P. In Silane Coupling Agents; Plenum 1991. For other (9)(a)Bain, C. D.; Whitesides, G. M. Science 1988,240,62. than gold surfaces, see: Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. SOC.1992,114,1990. (10)Whitesides, G. M.;Laibinis, P. E. Langmuir 1990,6 , 87. (11) Bain, C.D.; Whitesides, G. M. Angew. Chem. Int. Ed. Engl. 1989, 28,506. (12)Parikh, A. N. ; Allara, D. L. J . Chem. Phys. 1992,96,927. (b)Yen,Y.(13)(a)Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1,52. S.;Wong, J. S. J . Phys. Chem. 1989,93,7208. (c)Greenler,R. G.; Snider, (d) Umemura, D. R.; Witt, D.; Sorbello, R. S. Surf Sci. 1982,118,415. J.;Kamata, T.; Kawai, T.; Takenaka T.J . Phys. Chem. 1990,94,62.
0743-7463/95/241 1-0578$09.00/0 0 1995 American Chemical Society
RAIR Spectroscopy of Silanes analysis of the surface IR spectra has produced a detailed understanding of the structure of self-assembled monolayers on gold surfaces.14 The initial analysis of surface IR spectra relies on comparison with transmission data. It can be shown that for weak absorbers the band positions and band shapes remain fairly unaffected in the reflection-absorption experiment; the relative band intensities are, however, a function of the orientation of the transition dipole moment with respect to the surface. The underlying physical process of specular reflection spectroscopy with infrared radiation was first described by Greenler for the metalfilm-ambient system. Greenler already pointed out the effects of the anomalous dispersion on the reflectance of the sample-substrate interface; for strong absorbers we must expect to see band shifts, asymmetry, splitting, and distortions.15 In this case transmission spectra make poor candidates for peak identification, because the absorbance in the transmission spectrum is fairly independent of the anomalous dispersion.16 For complex systems, such as polyatomic molecules adsorbed on imperfect interfaces, a semiquantitative analysis is useful. The discussion is greatly simplified if we use a n experimentally determined optical response function for the adsorbate, e.g. from transmission measurements of the bulk material, with the assumption that the optical response function of the adsorbate can be approximated with the response function of the bulk material. This optical response function is in turn used to calculate a specular reflection spectrum. The overall process transforms a transmission spectrum into a specular reflection spectrum as a function ofvarious parameters, such as film thickness, angle of incidence, optical response function of the substrate, or geometric parameters of the adsorbate. In this paper we discuss the analysis of RAIR spectra of functionalized trialkoxysilanes adsorbed on metallic surfaces. We identify two important reasons that can cause peak shifts in reflection spectra of silanes: (a) hydrolysis and condensation reactions (chemical shift)and (b)the contribution from the anomalous dispersion (optical shift). Therefore, transmission spectra are not reliable references for the analysis of RAIR spectra by direct comparison. To avoid erroneous interpretation of RA spectra, we use the following approach: We transform transmission IR spectra of the organosilane into RAIR spectra; these calculated RAIR spectra serve as reference for the interpretation of the experimental surface IR spectrum of that particular silane. We note that other effects such as vibrational coupling may also play a role in some systems, but they are not considered in this article.
Experimental Section All materials were used as received. Silanes, including (3-aminopropy1)triethoxysilane(APS),(3-glycidoxypropy1)trimethoxysilane (GPS), (3-(methacryloxy)propy1)trimethoxysilane (TPM),trimethoxyvinylsilane (VS), tetraethoxysilane (TEOS), (3-bromopropy1)trimethoxysi(14)(a) Laibinis, P.E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N . ; Nuzzo, R. G. J . Am. Chem. Soc. 1991,113,7152.(b) Laibinis, P.E.; Whitesides, G. M. J.Am. Chem. Soc. 1992,114,1990. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992,43,437. (15)Greenler, R. G.; Rahn, R. R.; Schwartz J. P. J.Cutul. 1971,23, 42. (16)Transmission spectroscopy is not free of optical artifacts; other effects, such as cell effects, can cause distortions of bands. We took extreme care in transmission measurements in order to obtain quantitative results. See: (a)Hirschfeld, T. Appl. Spectrosc. 1978,32 , 508. (b) Zbid. 1976,30,549.(c) Hirschfeld, T.Anal. Chem. 1979,51, 495.(d) Brown, C.W.; Lynch, P. F.; Obremski, R. J. Appl. Spectros. 1982,36,539.
Langmuir, Vol. 11, No. 2, 1995 579
lane (BPS),and (3-mercaptopropy1)trimethoqsilane(MI'S) were purchased from Aldrich or Huls. Silanes were hydrolyzed with water and acetic acid, if necessary. Transmission spectra were recorded periodicallyin a NaBr cell or from pressed KBr pellets. The optical response functions were obtained from transmission spectra of the silane in various states. The C-N stretch at 2252 cm-l of acetonitrile was used as a calibration standard. The optical response function of acetonitrile was reported by J0nes.l' In a typical experiment, 0.321 g of APS (1.3 mmol) was mixed with 1.7 g of acetonitrile (40 mmol). A drop of that liquid was placed between two polished NaBr disks and immediately transferred into the spectrometer for analysis. We also took spectra of the silane in Nujol, Fluorolube, and tetrachlorocarbon. Typically four scans a t 4 cm-I were recorded (approximately 8 s) with a Mattson RS-1 spectrometer purged with boil-off nitrogen. Generally, the resulting path length is below 10 pm. For this reason it is necessary to account for optical cell effects. This is achieved by an iterative algorithm as described below. RAIR spectra were taken on a Mattson RS-1 FTIR spectrometer, equipped with a liquid nitrogen-cooledMCT detector. We employed f720 self-built optics to collect RAIR spectra. One hundred scans a t a resolution of 4 cm-I were averaged. The sample compartment was evacuated and the interferometer was purged with boil-off nitrogen. In a typical experiment, 200 pL of APS were dissolved in 30 mL of methanol; 1mL of this solution was diluted with 30-100 mL of methanol; 30-100 pL of this solution was applied with a GC microsyringe to the precleaned evaporated gold-on-siliconwafer (10 mm x 15mm in size) and directly transferred to the spectrometer for analysis. Films prepared in this fashion are typically several nanometers in thickness, comprising one to three layers ofthe silane. The film thickness was determined by optical ellispometry with a wavelength of 633 nm. The surface of the wafer was heated radiatively with a commercial 100 W light source placed approximately 7 mm above the wafer (the temperature is 75 f 2 "C). RAIR spectra were recorded with the light source turned off.
Results and Discussion In the analysis of IR spectra of silicon compounds we need to consider the following points. The high mass of silicon compared to carbon or oxygen provides vibrational insulation from the molecule, which gives rise to very characteristic frequencies of adjacent groups that are of great value in identification. These modes may react sensitively to the local environment of the molecule.ls The Si-0 bond is strongly polarized and has a large anomalous dispersion. Cross-linking, under formation of Si-0-Si bonds, is expected to increase the Si-0 transition dipole moment and bandwidth.lg Differences of reflection and transmission spectra of silanes can be attributed to the chemical constitution of the adsorbate, the particular orientation of the dipole moments, the physical environment of the adsorbate, and optical effects. Previously, we reported reflection-absorption IR (RAIR) spectra of various silane coupling agents (SCA)on metallic surfaces. We discussed the Si-0 modes in terms of the hydrolysis and condensation reactions of the surfaceconfined molecules. In this article, we wish t o integrate optical effects into the analysis. Our approach involves (17) Goplen, T. G.; Cameron, D. G.; Jones, R. N.Appl.Spectros. 1980, 34,657. (18)Porro, T. J.; Pattacini, S. C. Spectroscopy 1993,8 , 40. (19)Smith, A. L.Spectrochim. Acta 1960,16,87.
Kurth and Bein
580 Langmuir, Vol. 11, No. 2, 1995 the simulation of isotropic RA spectra from the optical response function determined from transmission spectra. Comparison of these calculated RA spectra with the experimental RA spectrum reveals details of the chemical and physical nature of the adsorbate, e.g. the degree of condensation or the local environment. We note that other mechanisms can also affect the band profile, including dephasing, motional narrowing, and vibrational decoupling; however, such a n analysis is beyond the scope of this article. Before we discuss our results we wish to briefly outline the calculation. We model our thin film systems with a planar, conducting surface covered with a thin, homogeneous, and isotropic film. The optical response function of the is adsorbate, given by N ( v ) = n ( v ) - ik(v) (i = chosen to be that of some reference state, for example the bulk liquid or solid. The reflectance, R , of the samplesubstrate interface is calculated from the electric fields on the surface by
20
,
I
m),
E"
=
121
IEiI and IEoI are the amplitudes of the incident and reflected electric field vectors perpendicular to the surface. The electric fields are calculated from the optical response functions, the optical constants of the substrate, the film thickness, and the angle of incidence according to the description given by Greenler.20 We chose to calculate the absorbance according to
where R is the reflectance of the sample-substrate interface and R, is the reflectance of the substrate. We obtain the optical response function for the adsorbate from bulk transmission IR spectra via a Kramers-Kronig transform, as outlined by Jones.21 This method relies on an iterative procedure that takes into consideration optical effects of the transmission cell. In addition, we use a n internal standard for calibration. Acetonitrile is an excellent standard for our purposes, because it dissolves organosilanes readily and has a n isolated, characteristic band a t 2252 cm-l (C-N stretch) of moderate strength. We obtain the optical response function from the following reference sources: APS dissolved in acetonitrile, liquid APS,partially polymerized APS (liquid), and polymerized APS (solid).22 In the overall process, the transmission spectrum is transformed into a RAIR spectrum. Generally, the final band contour is a function of several parameters, including the bandwidth (FWHH, full width a t half-height), the optical response function of the film and the substrate, the angle of incidence, and the film thickness. Some qualitative trends can be established for a general discussion of the to do so, we employ a synthetic intensity profile based on the Cauchy function (see below). In this case, the optical response function is obtained from a Kramers-Kronig transform ofthe Cauchy contour and the absorbance is calculated as outlined above. The intensity profile of a n isolated RA band is a sensitive function of k and the bandwidth, expressed as FWHH. In Figure 1we plot the peak maximum ofbands with a FWHH of 20 and 40 cm-l (located a t 1000 cm-l) as a function of (20)Greenler, R. G. J. Chem. Phys. 1966,44, 310. (21)(a) Hawranek, J. P.; Neelakantan, P.; Young, R. P.; Jones R. N. Spectrochim. Acta 1976,32A,75. (b)Ibid. 1976,32A,85. ( c ) Hawranek, J. P.; Jones, R. N. Ibid. 1976,32A, 99. (22)Kurth, Dirk G. Ph. D. Thesis, Purdue University, 1993. (23) Allara, D. L.; Baca, A,; Pryde, C. A. Mucromol. 1978,11, 1215.
k(fi1m). The substrate refractive index, the angle of incidence, and the film thickness were chosen arbitrarily (ns = 10-50, typical for metals in the IR region, BO", and 1nm, respectively, see discussion below). These are typical examples encountered for many organic molecules adsorbed on a good conductor; for instance, we find that k, is approximately 0.15 for a C-H stretch, 0.5 for C=O,and 0.9 for Si-0. The reason for the shifts shown in Figure 1 is that n and k are interdependent. Commonly the relation of n and k can be expressed by the Kramers-Kronig transform
where n, is the refractive index for the high frequency limit (P denotes the principal value of the integral).24 Inspection shows that if k or the FWHH is large, the dispersion curve, n(v), is wider in the wings. Consequently, the effect of n on the reflectance increases in the wings of the reflection-absorption band. At the high energy side of the peak maximum less radiation is reflected (small n )and a t the low energy side (large n)more radiation is reflected toward the detector. As a result the band is shifted asymmetrically to higher energy.25 The asymmetry is a result of the nonlinear dependence of the absorbance on n and k, as shown in Figure 2. So far we have considered only individual bands; now we turn our attention to overlapping bands. For simplicity, we separate the discussion ofn and k; we associate n with the reflectivity and k with absorbance.26 In a propagating wave field, that is in absorption spectroscopy, the spectral envelope is assumed to be the direct sum of individual spectral intensities of contributing absorption lines; if we express the absorption band by a Cauchy function, the spectral envelope is given by
(24)The Kramers-Kronig intergral can generally not be evaluated analytically; we chose the method proposed by Maclaurin for our calculations (Ohta, K.; Ishida, H. Appl. Spectr. 1988, 42, 952). (25)For very large k, which are not discussed here, the band can split showing two maxima (see the discussion by Greenler in ref 20). (26)It is important to recognize that the individual parameters discussed here are not separate entities but have a causality.
RAlR Spectroscopy
of
Langmuir, Vol. 11, No. 2, 1995 581
Silanes
0.001
0.0008 0.0006 0.0004
0.0002 I 3
0 ' 1
1.4
2.2
1.0
2.6
n
1150
850
950
1050
750
cm'l
0.004
Figure 3. (top) The dispersion,n, for separated and overlapping Cauchy functions (FWHH, 20 cm-l; n, = 1.4).(bottom)The k spectrum for each band (kmm = 0.5).
n-1.3
0.0°6
0.005
0 ' 0
1
0.4
0.0
1.2
1.6
1 ~
1055
I 2
k
Figure 2. The dependence of the absorbance as a function of the film refractive index, n and k, for the incidence angles of 83" (solid line) and 87" (dashed line) for a good conductor.
0 1200
where a l b 2 = k,, and 2b = FWHH.27 In the simplest form the dispersion equation for multiple absorbers with absorption frequencies vj is derived from classical mechanics and is given by the sum of the individual dispersion curves:
ejis a factor that contains the charge, the mass, the number density, and the force constants.28 Figure 3 shows the dispersion curves, n(v), for overlapping and separated bands located a t 850, 1010, and 1050 cm-' (Cauchy functions, k,, = 0.5; FWHH = 20 cm-l). According to the dispersion equation, n(v)is reduced a t the high-energy side and increased a t the low-energy side of the anomalous dispersion by contributions from vicinal bands. As a result of the standing wave field a t a surface the amount of radiation absorbed from the incident beam is governed by n(v)and K(v),that is, the spectral envelope is a combination of the reflection and absorption of radiation at the interface governed by n(v)and M y ) . The behavior of the dispersion equation and the dependence of the absorbance on n(v)and k ( v ) (Figure 2) result in a (27) A physicochemical implication for this profile was derived by Lorentz in terms of a collisional broadening of a damped harmonic oscillator (Lorentz,H. A. KoninkZ. Ned. h a d . Wetenschap. Proc. 1906, 8,591). To avoid connotations on the origin of the band profile we use empiricalCauchy functions;additional Gaussian perturbationsare not taken into considerationfor simplicity (see, for instance: Young, R. P.; Jones, R. N. Chem. Rev. 1971, 71, 219). (28) (a)Born, M.;Wolf, E. InPrincipZes ofOptics, 6th ed.;Pergamon: New York, 1993, Chapter 2. For applications to infrared spectroscopy, see: (a)Spitzer, W. G.;Kleinman, D. A. Phys. Rev. 1961,121,1324. (b) Schatz, P. N.;Maeda, S.;Hollenberg,J. L.;Dows, D. A. J. Chem. Phys. 1961, 34, 175. (c) Schatz, P. N. J. Chem. Phys. 1960, 32, 894.
_.., 1100
.._. 1000
, 900
800
700
cm-'
Figure 4. Reflection-absorption bands of separated and overlapping Cauchy functions in the case of a good conductor.
spectral envelope than may not be commensurate with corresponding absorption spectra, especially if bands overlap. Figure 4 shows the reflection-absorption spectra for the bands shown in Figure 3 (the angle of incidence, the substrate refractive index, and the film thickness are chosen arbitrarily; see below). As expected, separated bands show a reflection-absorption spectrum (solid line) similar to a n absorption spectrum, except for the exact positions and intensities. A frequency dependent term in the equations describing the reflection-absorption process increases the band intensity toward higher frequency.20 The spectral envelope of the overlapping bands, however, does not resemble a direct sum of its components. In contrast to absorption spectroscopy where the peak maxima are of equal intensity, the peak maxima of the final intensity envelope are dissimilar in the reflectionabsorption spectrum.29 The intensity of the band located a t 1050 cm-l is enhanced because, as pointed out above, the reflectivity is reduced (small n(v))by contributions from the neighboring band. The intensity of the band a t 1010 cm-l is diminished because the reflectivity is enhanced (large n(v)). The relative intensities in Figure 4 are not associated with the surface selection rules that relate the intensity to the orientation of the transition dipole moments. We note in conclusion that the intensities and peak positions in reflection-absorption spectra are affected by vicinal bands if there is significant overlap in the dispersion spectrum n(v). The RA-intensity profile is insensitive to the angle of incidence and the substrate refractive index, if the angle (29) The intensity expressed by Beer's law is a function of k, path length, and frequency.
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582 Langmuir, Vol. 11, No. 2, 1995
Table 1. Band Positions of Substituted SiloxanesSs type Si-0-Si
Si-0-Et
Si-CH2
1250-1200
Si-0-Me
1200
1100
1000
1130-1100 1070-1040 1150- 1000 1175-1135 1100-1050 990-940 1107-1100 1190 110011075
Si-0-R
1300
position (cm-') several bands depending on structure alkyl-substituted disiloxanes siloxane olymer (several maxima possibfe) Si-0-R Si-0-CHS; asym Si-0-C stretch CH3, rock Si-O-CHzCH3; a s p Si-0-C stretch (doublet)
900
cm.'
Figure 5. Normalized transmission spectra of APS: (a)APS/ acetonitrile,(b)neat APS,(c) partially polymerized APS (liquid), and (d) polymerized APS (solid).Bands with an asterisk are due to acetonitrile.
of incidence is less than 85" and the substrate is a good conductor (kS(v)exceeds 20). For glancing angles and poor conductors we must expect severe distortions; such extreme conditions are not considered here.30 For most metals the condition that k s exceeds 20 is satisfied in the IR region. In this case the intensity is fairly independent of Because there is no critical angle for conducting substrates, we can rule out band inversions that are encountered for other substrate^.^^ The band contour is also quite insensitive to the angle of incidence, although the latter affects the intensity. I t is often found that high angles of incidence give more signal, and generally a high angle of incidence is chosen for the experiment. In summary, the band contour in reflection-absorption spectroscopy is rather insensitive to experimental parameters under the conditions outlined above. However, reflection-absorption and transmission spectra of a n isotropic compound can show significant differences in the relative peak intensities, peak positions, and band contours. Transmission IR spectra of the APS Si-0 modes are shown in Figure 5. The bands a t 1040 and 918 cm-' are due to acetonitrile. The band positions and assignments of substituted siloxanes in this region are summarized in Table 1. The band contours ofthe Si-0 modes are strongly affected by the physical (mixtureheatliquid) and chemical state (condensation) of the material. If APS is dissolved in acetonitrile (40 mmol, spectrum a) we observe two well resolved bands a t 1077 and 1100 cm-l, whereas the neat liquid shows broad bands and different intensities (spectrum b). The band positions are not affected significantly. This effect is independent of solvent. Due to the permanent dipole moments in the S i 0 moiety, we anticipate permanent dipole interactions resulting in association of molecules. The random nature of such interactions is expected to increase the bandwidth through lifetime broadening, vibrational coupling, and other inhomogeneous broadening mechanisms. We expect that in dilute solution molecular attractions and associations are diminshed by the presence of the surrounding solvent. The translational and rotational motions may fluctuate more rapidly in the absence of intermolecular attractions, (30) For angles of incidence higher than 85" we observe severe distortions; such high angles are experimentally unfeasible and we will not discuss them here. (31)For angles of incidence larger than 85",ks does affect the intensity; highly reflective surfaces will produce a strong signal. However, the band contour is also more distorted at large angles. (32)(a) Porter, M.D.; Bright, T. B.; Allara D. L.; Kuwana, T. Anal. (b) Wong, J.S.;Yen, Y.-S.AppZ. Spectrosc. 1988, Chem. 1986,58,2461. 42, 598.
a
CHi'def. (wagging)
The methylene C-H modes adjacent to oxygen are weak. 1.a 1.7 1.6 1.s E
1.4 1.3 1.2 1.1 -1 1300
1200
1100
1000
900
1000
900
cm" 0.0
0.7
0.6 0.5 Y
0.4 0.3 0.2 0.1
0
1300
1200
1100
cm"
Figure 6. The optical response function of neat APS (solid line) and APS/acetonitrile (dashed line).
resulting in motional narrowing of the bands.33 We rationalize the spectra in terms of the particular local environment of the Si-0 moieties, that is, intermolecular attractions exist in liquid phase but are diminished in dilute solution. As expected, the S i 0 bands broaden and also shiR in frequencywhen APS hydrolyses and condenses (spectra c and d). Other organosilanes that we studied in our laboratory show the same trends in transmission spectra. Figure 6 shows the optical response functions of neat APS and APS dissolved in acetonitrile. Polymerized APS that is solid shows further broadening in the n and k (33)Steele, D.;Yarwood J. (Ed.) In Spectroscopy and Relaxation of Molecular Liquids; Elsevier: Amsterdam, 1991.A detailed line shape and relaxation analysis of such complex molecules is beyond the scope of this paper.
RATR Spectroscopy of Silanes
1300
1200
1100
Langmuir, Vol. 11, No. 2, 1995 583
1000
900
cm" Figure 7. Calculated RA spectra of APS on gold based on the
transmission spectra of Figure 5 (angle of incidence, 84";film thickness, 9 A). Spectra are offset for clarity. (Spectrum a is reduced in scale.)
1300
1200
1100
1000
900
cm" Figure 8. RA spectra of APS adsorbed on gold as a function of the hydrolysis and condensation reactions (1, freshly deposited APS (no heating); 2, plus 4 min at 75 "C; 3, plus 6 min; 4, plus 6 min; 5, plus 20 min; 6, after 12 h at 75 "C).
spectra (data not shown for clarity). Each case discussed so far, that is, dissolved APS, neat APS, and solid APS, has a distinct optical response function. Therefore we can expect to see distinct optical effects in RA spectra. This is shown in the corresponding RA simulations in Figure 7. The film thickness was chosen to be 9 A, approximately the length of a n extended APS molecule. The optical response function of the substrate, in this case gold, was taken from the literature, and the angle of incidence was 84". Note the frequency shifts of the band maxima and the alteration of the relative intensities in the simulations compared to the transmission spectra. The dispersion of the S i - 0 modes in silanes has a significant contribution to the reflectance in RA spectra. The important point is that these altered band contours are entirely due to the optical effects ofthe film-substrate interface. We emphasize the bands at 1100 and 1077 cm-l in spectrum a in Figure 5 and the corresponding simulation, spectrum a in Figure 7. The relative intensities of these two bands are inverted as a result of the anomalous dispersion. The particular relative intensities in this example are not a consequence of the surface selection rules but merely a result of optically induced shifts. Recall that the calculated reflection spectrum was obtained from a n isotropic response function. As the material hydrolyzes and cross-links, we observe a continuous shift of the Si-0 modes towards higher frequency. Figure 8 shows the RAIR spectra of APS adsorbed on polycrystalline gold as a function of hydrolysis and condensation reactions. To record early stages of hydrolysis and condensation reactions, APS was applied directly from solution to the wafer.34 This method produces film thicknesses from one to three molecular layers as determined with optical ellipsometry. The APSgold sample was heated in the spectrometer, and RAIR spectra were recorded periodicallyuntil no further changes in the M R spectrum were observed. From the previous discussion we may expect to see environmentally sensitive relaxation phenomena in RAIR spectra ofAPS in addition to optical effects. We may analyze these different effects with the help of previously calculated RAIR spectra. Freshly deposited APS (spectrum 1)shows Si-0 modes with maxima a t 1126 and 1091 cm-l. Compared with the RA simulation, we observe that the bands a t 1163 and 963 cm-l are of reduced intensity, indicating some loss of ethoxy groups (compare Table 1). This APS film shows a Si-0 band contour that is similar to the RA simulation
based on APS dissolved in acetonitrile (see Figure 7, spectrum a). The experimentally observed peak positions are in excellent agreement with the simulated RA spectrum (1126, 1091 versus 1125, 1088 cm-l). On the basis of the similarity of the APS RAIR spectrum with the simulation based on APS dissolved in acetonitrile, we propose that (a) the degree of cross-linking in the film is small and (b)the molecular environment in both systems is similar, that is, molecular associations in APS films are largely absent. The film most likely consists of loosely connected APS oligomers of low molecular weight. One would expect molecular interactions to be diminished in films where the silane groups are attached to the surface and the aminopropyl groups are oriented away from the surface. As the hydrolysis and condensation reactions proceed, we observe a continuous shift of the Si-0 modes to higher frequencies as predicted by the RA simulations (see Figure 7). The last RAIR spectrum in this series (spectrum 6) corresponds to a fully polymerized APS film; we did not observe any further changes in the RAIR spectrum upon additional heating and exposure to water. The C-H stretching bands of the ethoxy groups are greatly diminished a t this point, indicating that the film is cross-linked. The band contour of the Si-0 modes correlates best with simulation c (Figure 7) which represents partially crosslinked APS. Apparently, thin films of APS cannot crosslink to the same extend as bulk APS (spectrum d, Figure7). This is not surprising as a two-dimensional siloxane network is expected to have a lower degree of cross-linking than a three-dimensional network. Other thin films of organotrialkoxysilanes on metallic surfaces displayed very similar effects. In summary, we observe that freshly deposited silane films correlate with RA simulations based on dilute silane solutions. Hydrolysis and condensation cause a continuous shift to higher frequency, in agreement with the simulations. The degree of condensation for silane thin films appears to be lower than for bulk silanes. Table 2 summarizes the peak positions of the Si-0 modes for trimethoxy- and triethoxysilanes observed in transmission spectra, RA simulations, and RAIR spectra. These data are a compilation of several organosilanes that we studied in our laboratory.
(34) Vapor phase adsorption ofAPS results in films that are already hydrolyzed and partially condensed.
The interpretation of RAIR spectra relies on reference IR data, typically transmission spectra. We can identify
Conclusions
584 Langmuir, Vol. 11, No. 2, 1995
Kurth and Bein
Table 2. Peak Positions and k Values at Maximum of Si-0 Modes (Acetonitrile Mixture, Room Temperature, ca. 40 mM) for Trimethoxy- and Triethoxysilanes Observed in Transmission IR Spectra, RA Simulations (W), and RAIR Spectra for Freshly Prepared Films (gold substrates) ligands at Si
transm
RAS
OMe
1190 1080 1100 1077
1195 1100 1125 1088
OEt
RAIR 1195 1105 1126 1091
k 0.20 1.0" 0.6 0.8
These values were obtained from (3-bromopropy1)trimethoxysilane. The K values depend slightly on the substitution around the silicon.
three cases in which transmission spectra are poor candidates for spectroscopic references: (a)the adsorbate undergoes chemical reactions upon adsorption, (b) the anomalous dispersion of the sample layer contributes significantly to the reflectance of the sample-substrate interface, or ( c ) the phase of the adsorbate is different from the phase that is used to obtain the transmission spectrum. If any ofthese cases apply, a different approach needs to be chosen. We propose the following method: several transmission spectra of the compound are acquired, e.g. spectra of the neat compound and in different solvents, etc.; each transmission spectrum is then transformed into a reflection spectrum. This transformation requires two steps: (1) the optical response function is determined from the transmission spectrum of the compound, and (2)the reflection spectrum is calculated from these data. The calculated reflection spectra serve as spectroscopic references. We demonstrate that the anomalous dispersion of the Si-0 modes in silanes has a significant contribution to the reflectance ofthe film-substrate interface. As a result, peak maxima are shifted to higher frequency and relative peak intensities of RAIR spectra are altered compared to the transmission spectra. We find that the molecular environment of several surface-confined trialkoxyorganosilanes is similar to the
silane in dilute solution. Some intermolecular attractions or associations present in the neat liquid seem to be absent in thin films. Intuitively one would expect intermolecular associations to be diminished in two-dimensional structures, where the silane is attached to the surface and the organic moiety is pointing away from the surface. We can formulate the following general rules for reflection spectroscopy of films much thinner than the wavelength on good conductors (smooth interface): for an angle of incidence below 85",peak shifts are independent of the particular substrate refractive index, the angle of incidence, and the film thickness. The peak shift is a function of the bandwidth and the absorbance, given by M y ) . As a rule of thumb, peak shifts in reflection spectra of thin films become noticeable when lz,, exceeds 0.2. Changes of relative peak intensities are a result of contributions of the anomalous dispersion, n(v), from neighboring bands. In addition, there can be other mechanisms operative that affect the final band contour of the absorbate that were not discussed here.35
Acknowledgment. We greatly appreciate the invaluable help of Mr. S. Esnouf in writing the numerous computer programs for this study. We thank the National Science Foundation for financial support of this work. SupplementaryMaterial Available: Figures showing the calculated absorbance as a function of k(substrate) and n(substrate) in RAIR spectra, absorbance vs wavelength in RAIR spectra, and expressions for the electric fields on thin fYm systems (3 pages). Orderinginformation is given on any current masthead page. ~~940383~ (35) Ueba, H. Prog. Surf Sci. 1986,22, 181. (36) (a) Smith, A. L. Spectrochim. Acta 1960, 16, 87. (b) Anderson, D. R. InAnalysis ofsilicones;Smith, L. A., Ed.; Wiley: NewYork, 1974, Chapter 10. (c) Bellamy, L. J. In The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hill: New York, 1975.
