Volume 51, Number 2, 1997 APPLIED SPECTROSCOPY 2090003-7028 / 97 / 5102-0209$2.00 / 0q 1997 Society for Applied Spectroscopy

External Re¯ ection Infrared Spectroscopy of AnisotropicAdsorbate Layers on Dielectric Substrates

HELMUT BRUNNER, ULRICH MAYER, and HELMUTH HOFFMANN*Department of Inorganic Chemistry, Technical University of Vienna, Getreidemarkt 9, A-1060 Wien, Austria

Monolayers of octadecylsiloxane were formed on native silicon (Si/SiO2) and glass surfaces by adsorption from dilute solutions of oc-tadecyltrichlorosilane and were investigated by polarization- andangle-dependent external re¯ ection infrared spectroscopy. In con-trast to metal substrates, both the parallel and perpendicular vi-brational components of the adsorbate can be detected on thesedielectric surfaces. The monolayer re¯ ection spectra show signi® -cant changes as a function of the light incidence angle and the po-larization of the infrared radiation, which contain detailed infor-mation on the surface orientation of the ® lm molecules. Spectralsimulations based on classical electromagnetic theory yield an av-erage 108 tilt angle of the hydrocarbon chains with respect to thesurface normal on both silicon and glass surfaces. Despite this ap-parent structural identity of the monolayer ® lms on silicon andglass, signi® cant differences are observed in the monolayer re¯ ec-tion spectra resulting from purely optical effects of the substrate.

Index Headings: External re¯ ection infrared spectroscopy; Self-as-sembling; Alkylsiloxane monolayers; Dielectric substrates.

INTRODUCTION

External re¯ ection infrared spectroscopy (ERIRS) istoday a well-established technique for surface character-ization (a confusingly large number of different namesand acronyms such as IRAS, RAIRS, FT-IRRAS, IR-ERS, etc., are used in the literature for this method),which provides speci® c chemical information about sur-faces and adsorbates as well as structural information onthe surface orientation of adsorbate molecules. Severalcomprehensive review articles are available in the liter-ature dealing with both the theoretical background andvarious practical applications of this method.1± 5 The vastmajority of the hitherto reported studies were carried outon metal substrates with the salient characteristics of highsensitivity at grazing incidence and easily interpretable,`̀ transmission-like’ ’ spectra, whose band intensities aregoverned by the well-known surface selection rules.6 Al-though the basic theory, which was developed almostthree decades ago,6± 9 contained no restriction to metalsubstrates, nonmetals initially appeared barely suitablefor ERIRS because of their general low re¯ ectivity andthe apparent complexity of some of the ® rst experimentaladsorbate spectra on water,10 carbon,11 silicon,12± 14 and ox-ide surfaces,15 which was ascribed to a poorly understoodcombination of orientational effects in the adsorbate lay-ers and optical distortions caused by the substrate. Mean-while, instrumental improvements in FT-IR spectrome-ters and IR detectors, together with more detailed theo-retical treatments considering, in particular, anisotropicadsorbate structures,16,17 have helped to partially over-

Received 5 March 1996; accepted 18 July 1996.* Author to whom correspondence should be sent.

come these problems and to recognize the full potentialof ERIRS for nonmetal surfaces, which is demonstratedin several recent studies.18± 24

In this paper various theoretical and experimental as-pects of external re¯ ection infrared spectroscopy with di-electric substrates are discussed. Dielectrics are common-ly classi® ed as a subgroup of nonmetal materials withrespect to their optical properties, which are characterizedby a vanishingly small absorption coef® cient k within aconsidered frequency interval and a refractive index nranging typically between about 1.3 and 4.0 in the infra-red region.25 We have chosen for this study two sub-stratesÐ namely, glass and siliconÐ which are essentiallynonabsorbing in the region around 3000 cm2 1 and whoserefractive indices are close to the low and high end fordielectric materials (nGlass 5 1.50, nSilicon 5 3.4213,14). Wewill ® rst consider the optimum experimental con® gura-tion (incidence angle and polarization of the IR beam)and compare the theoretically achievable sensitivity withthese dielectric substrates with that of a typical metal sub-strate. We will then discuss the basic parameters whichdetermine the IR band intensities in a re¯ ection spectrumof an adsorbate on a dielectric surface, in particular, therelationship between the intensity of an absorption andthe surface orientation of the corresponding transition di-pole moment, which forms the basis for the quantitativedetermination of molecule orientations in adsorbate ® lms.On the basis of these considerations, we will show for aparticular model system [octadecylsiloxane (ODS) mono-layers on glass and silicon surfaces] that the experimentalre¯ ection spectra measured with different incidence an-gles and polarizations can be accurately reproduced byspectral simulations under the assumption of a certainuniform surface orientation of the hydrocarbon chains.ODS monolayers can be prepared from precursor com-pounds such as octadecyltrichlorosilane via self-assem-bling on a variety of different substrates containing sur-face hydroxyl groups,26 and they exhibit a highly ordered(anisotropic) ® lm structure, which appears to be largelyindependent of the particular substrate (an average tiltangle of about 10 8 of the hydrocarbon chains towards thesurface normal was determined in several recent studiesof ODS monolayers on different substrates24,27,28), there-fore providing a well-characterized model system to dem-onstrate the structural information obtainable from ERIRadsorbate spectra on different nonmetal surfaces.

METHODS

Monolayer Preparation. ODS monolayer ® lms on sil-icon and glass substrates were prepared from dilute so-lutions (c 5 1 mmol/L) of octadecyltrichlorosilane (Al-drich, 96% purity) in toluene, as described previously.24

210 Volume 51, Number 2, 1997

FIG. 1. Calculated re¯ ectivity differences between the clean substrate (R0) and the adsorbate-covered substrate (Rs) for different substrate materialsas a function of the light incidence angle for p-polarized radiation (solid lines) and s-polarized radiation (broken lines) at 3000 cm2 1. A hypotheticaladsorbate layer (thickness d 5 10 AÊ , refractive index n 5 1.5, absorption coef® cient k 5 0.1) was assumed for these calculations, and the followingoptical constants were used for the substrates: silicon (n 5 3.42, k 5 0), glass (n 5 1.5, k 5 0), gold (n 5 3, k 5 30). u B denotes the Brewsterangle for silicon and glass (tan u B 5 n).

P-doped, (100)-oriented, and single-sided polished siliconwafers (Wacker Chemitronic, 25 3 20 3 0.5 mm) andsoda-lime glass slides (70:15:15 weight percent SiO2/Na2O/CaO, 25 3 25 3 3 mm) were used as substratesand were cleaned by sonication in H2SO4/H2O2 (4:1 v/v)solution (caution: H2SO4/H2O2 mixtures react violentlywith organic materials and should be handled with greatcare), followed by extensive rinsing with doubly distilledwater and acetone. Film adsorption was carried out in aglove box ® lled with dry nitrogen by overnight immer-sion of the substrates in the adsorbate solutions.

IR Re¯ ection Spectra. ERIR spectra were measuredon a Mattson RS FT-IR spectrometer using a commercialvariable-angle re¯ ection unit (Seagull, Harrick Scienti® c)and a narrow-band MCT detector (1 mm2 active area,speci® c detectivity D* ; 6 3 1010 cm Hz1/2 W2 1). A wire-grid polarizer (Model PWG U2R, Harrick Scienti® c) lo-cated in front of the detector was used for measuringspectra with either p-polarized (parallel) or s-polarized(perpendicular) radiation with respect to the plane of in-cidence at the sample surface. Double-sided interfero-grams were collected with a mirror speed of 2.5 cm s2 1,a sampling frequency of 80 kHz, and 4 cm2 1 nominalspectral resolution. We averaged 1024 scans in each mea-surement from both the sample and the clean substrate.The interferograms were apodized by triangular apodi-zation and were zero-® lled to yield spectra with one datapoint per wavenumber.

IR Re¯ ection Calculations. Model calculations ofsubstrate re¯ ectivities (R0), sample re¯ ectivities (Rs), andsample absorption intensities ( 2 log Rs/R0) at a single fre-quency as well as full simulations of re¯ ection spectrawere carried out on the basis of a classical three-phasesample model7 consisting of a semi-in® nite substratephase, an adsorbate layer of thickness d, and a semi-in-® nite ambient medium (air) with perfectly ¯ at, plane-par-allel phase boundaries. The mathematical algorithm of

these calculations is based on a rigorous matrix formal-ism described originally by Yeh16 and Allara and Parikh,17

whichÐ in extension of older treatments for isotropic me-dia7 Ð takes into account the direction dependence of theoptical constants of an oriented (anisotropic) adsorbatelayer and therefore allows a calculation of band intensi-ties and a simulation of adsorbate re¯ ection spectra as afunction of the molecule orientation on the surface. De-tails of this procedure are described elsewhere.17,24

RESULTS AND DISCUSSION

Sensitivity Considerations. The intensity of an ab-sorption band in a re¯ ection spectrum is commonly ex-pressed as 2 log Rs/R0, where Rs is the sample (substrateplus adsorbate) re¯ ectivity and R0 is the re¯ ectivity of theclean substrate at the frequency of the absorption maxi-mum. The sensitivity [signal-to-noise ratio (SNR)] is pro-portional to the difference between the background andthe sample signal, R0 2 Rs.15 On metal surfaces, R0 isclose to 1 for both s-polarized and p-polarized radiationregardless of the incidence angle, and the band intensitiesincrease proportionately to the surface electric ® eldstrength, yielding the well-known sensitivity maximumat grazing incidence for p-polarized radiation, while anegligible electric ® eld and therefore no absorption re-sults with s-polarized light.6 A more complex situation isencountered with nonmetallic surfaces. Both R0 and Rs

depend strongly on incidence angle and polarization ofthe probing radiation and on the optical constants of sub-strate and adsorbateÐ which results, for each substratematerial, in a different optimum experimental con® gu-ration (incidence angle, polarization) at which maximumsensitivity is achieved. This pattern is shown in Fig. 1,where for two dielectric substrates (silicon and glass) there¯ ectivity difference R0 2 Rs for a hypothetical adsor-bate vibration at 3000 cm2 1 is plotted against the light

APPLIED SPECTROSCOPY 211

TABLE I. Optimum incidence angles u opt and relative signal-to-noise ratios (SNR) for IR re¯ ection spectra of an adsorbate on dif-ferent metal and dielectric substrates for s-polarized and p-polar-ized radiation.

Substrate Polarization u opt SNR

Gold p 87 100Silicon p 86 1.7Glass p 79 0.5Gold s Ð a Ð a

Silicon s 0 ± 50 1.2Glass s 73 3.6

a Negligibly weak absorption.

FIG. 2. Calculated absorbances 2 log Rs/R0 of a hypothetical adsorbatevibration at 3000 cm2 1 on a silicon and a glass surface as a function ofthe light incidence angle u for p-polarized radiation (Ap, solid lines) ands-polarized radiation (As, broken lines). The thin solid lines denoted withA and A represent the parallel and perpendicular components, respec-x z

p p

tively, of the total absorbance Ap, and u B is the Brewster angle. Thesame optical constants as in Fig. 1 were used for substrates and adsor-bate.

incidence angle for s-polarized and p-polarized radiation.For comparison, the corresponding results for a typicalmetal substrate such as gold are also included in Fig. 1.The optical constants and the thickness of the adsorbatelayer were chosen to simulate a C± H stretching vibrationin a monolayer ® lm of an organic compound. The fol-lowing fundamental differences between metal and di-electric substrates are evident in Fig. 1: First, both p-polarized and s-polarized radiation can be absorbed byan adsorbate on a dielectric substrate, since a sizeableelectric ® eld exists both parallel and perpendicular to thesurface.7± 9 Second, the re¯ ectivity change R0 2 Rs uponabsorption is not always positive (i.e., Rs , R0), as onewould intuitively expect as a consequence of light beingabsorbed by the sample, but can also be negative (i.e., Rs

. R0), resulting in inverted (`̀ negative’’ ) bands in there¯ ection spectrum corresponding to an increase in re-¯ ectivity upon absorption. In fact, the curves for s-po-larized radiation in Fig. 1 on silicon and glass predictinverted bands over the whole incidence angle range,while with p-polarized light `̀ positive’ ’ or `̀ negative’’bands can occur depending on the incidence angle andthe substrate’s refractive index. The relative sensitivitiesachievable with each substrate can be predicted by com-paring the maximum re¯ ectivity change z R0 2 Rs z for eachcurve in Fig. 1. The corresponding values were normal-ized to the maximum re¯ ectivity difference z R0 2 Rsz onthe metal surface, which has been assigned an arbitraryvalue of 100, and are listed in Table I together with thecorresponding optimum incidence angles. On silicon, thebest sensitivity is achieved between 0 8 and 508 incidencefor s-polarized light and at 86 8 incidence for p-polarizedradiation. On glass, the sensitivity maximum is at 73 8 fors-polarization and at 798 for p-polarization (the maximumat 08 incidence for p-polarization is practically irrelevant,since there is no distinction between s-polarized and p-polarized radiation at normal incidence). Compared withthat for a metal substrate, however, the predicted SNR inTable I on either silicon or glass is almost two orders ofmagnitude lower. This seeming contradiction to recentexperimental results, where comparable sensitivitiescould be achieved in monolayer re¯ ection spectra on goldand silicon substrates,29 is resolved by taking into accountthe detector performance at high and low radiation ¯ uxdensities. A typical highly sensitive MCT detector re-quires only a small fraction of the full source intensityre¯ ected off a metal surface to produce an output signal,whose dynamic range will match the resolution capabil-ities of a state-of-the-art analog-to-digital converter.30± 32

At this point, the sensitivity (100% noise level) is limited

by digitization noise and does not further improve withincreasing energy striking the detector. This sensitivitylimit can evidently also be reached with weakly re¯ ect-ing, nonmetallic substrates such as silicon, for which thesame spectral noise levels (typically 2 3 102 5 absorbanceunits for 1024 scans at 4 cm2 1 resolution) were achieved,as with totally re¯ ecting gold substrates.29

Intensities and Directions of Absorption Bands. InFig. 2 the peak intensities 2 log Rs/R0 for the same hy-pothetical adsorbate vibration on silicon and glass areplotted as a function of the incidence angle for s-polar-ized and p-polarized radiation. A random dipole momentdistribution on the surface has been assumed for thesecalculations, resulting in an average orientation withequal x, y, and z surface coordinates of the transition

212 Volume 51, Number 2, 1997

FIG. 3. Geometrical model of the average isotropic orientation of avibrational dipole on a substrate surface in relation to the directions ofthe electric ® eld vectors Ep (p-polarization) and Es (s-polarization) ofthe incident radiation ( u , incidence angle). Equal x, y, and z coordinatesof the transition dipole moment vector result in equal dipole momenttilt angles a 5 54.7 8 with respect to each coordinate axis.

dipole moment vector and an average dipole moment tiltangle of 54.78 towards each of the three coordinate axes,as shown in the geometrical model in Fig. 3. Since thefundamental requirement for absorption is a transition di-pole moment component along the direction of the elec-tric ® eld vector, only the y component contributes to theabsorption with s-polarized light, while both the x and zcomponents can interact with p-polarized radiation. In thelatter case, the net absorption intensity can therefore beresolved into the parallel (x) and perpendicular (z) con-stituents, and the corresponding curves have been includ-ed in Fig. 2. The following general conclusions for di-electric surfaces can be drawn from these results: Ab-sorption of s-polarized light by the adsorbate leads to anincrease in re¯ ectivity, i.e., inverted absorption bands ina re¯ ection spectrum. Their intensities decrease with in-creasing incidence angle and are substantially larger onglass than on silicon. This observation can be explainedqualitatively by the corresponding changes in the surfaceelectric ® eld, which results from the vectorial addition ofthe incident and the re¯ ected electric ® eld vector. Sincefor s-polarized radiation the phase shift upon re¯ ection isessentially 1808 for any substrate regardless of the inci-dence angle, the surface electric ® eld is negligible ontotally re¯ ecting substrates such as metals (due to mutualcancellation of incident and re¯ ected electric ® eld) andincreases with decreasing substrate re¯ ectivity R0. SinceR0 increases with both the incidence angle u and the sub-strate refractive index n, the band intensities decreasewith u and are always larger on glass (n 5 1.50) than onsilicon (n 5 3.42).

p-Polarized radiation shows more complex relation-ships between band intensities, incidence angle, and sub-strate refractive index. A discontinuity is apparent in Fig.2 at the substrate’s Brewster angle u B (73 8 on silicon, 55 8on glass), where (1) the substrate re¯ ectivity R0 is zeroand 2 log Rs/R0 rises to in® nity, and (2) a sudden phasechange upon re¯ ection from 08 (for u , u B) to 1808 (foru . u B) takes place, causing a band inversion at the Brew-ster angle. Therefore two regimesÐ u , u B and u . u BÐmust be distinguished. For u , u B, perpendicular vibra-tional components give regular, positive absorption

bands, and parallel vibrational components give inverted,negative bands, whose intensities increase exponentiallytowards the Brewster angle. The reverseÐ positive bandsfor parallel components, negative bands for perpendicularcomponentsÐ applies for u . u B. Analogous to s-polar-ized radiation, one major difference between glass andsilicon is the stronger parallel electric ® eld on glass and,consequently, an enhancement of the parallel componentrelative to the perpendicular component on glass. Thisobservation explains the different incidence angle depen-dence of the overall band intensities on silicon and glassin Fig. 2 (thick solid lines), which simply represent thesum of the parallel and perpendicular contributions. Oneinteresting consequence of the weaker parallel ® eld onsilicon is the mutual cancellation of the x and z compo-nents at u ; 408 and the resulting net absorbance of zero,whereas on glass the x component dominates over thewhole range of incidence angles (for isotropic dipole mo-ment orientation) and determines the sign of the net ab-sorbance value (Fig. 2). This condition of mutual can-cellation will be discussed below in more detail for an-isotropic adsorbate structures. In this case, one additionalparameter must be taken into account, namely, the sur-face orientation of the transition dipole moment. The lat-ter is unambiguously de® ned by the tilt angle a betweenthe dipole moment vector and the surface normal (z-axis),if a uniaxially symmetric distribution around the surfacenormal, i.e., equal x and y coordinates, is assumedÐ acondition that is nearly always met in practical samplesfor the average orientation over larger surface areas (anarea of roughly 1 cm2 is probed by the IR beam in ourexperiments). The relationship between the intensity ofthe previously chosen model absorption and the tilt anglea of the corresponding transition dipole moment is shownin Fig. 4 for s-polarized and p-polarized radiation at 50 8incidence. s-Polarized light probes only the dipole mo-ment components parallel to the surface and gives in-verted (negative) absorption bands, whose intensities in-crease proportionately to sin2 a . The absolute band inten-sities are always larger on glass than on silicon, as dis-cussed above. The general shape of the curves forp-polarized radiation in Fig. 4 can be explained on thebasis of the results presented in Fig. 2, where it wasshown that absorption bands corresponding to paralleland perpendicular dipole moment components are alwaysopposite in direction. At 508 incidence, positive absorp-tion bands result for perpendicular dipole moment ori-entation ( a 5 08 ) and negative bands occur for parallelorientation ( a 5 908 ) on both silicon and glass surfaces(see Fig. 2). With increasing a , the (positive) perpendic-ular component decreases proportionately to cos2 a andthe (negative) parallel component increases proportion-ately to sin2 a , which results in the overall intensitychange shown in Fig. 4 (solid lines), characterized by aband inversion and a zero, at which point mutual can-cellation of the parallel and perpendicular componentsoccurs. The corresponding tilt angle a , at which the ab-sorption vanishes, shifts from 608 on silicon to 408 onglass because of the different relative intensities of theparallel and perpendicular components on these two sub-strates. We will show in the following discussion an ex-perimental example of such an apparent extinction of anabsorption.

APPLIED SPECTROSCOPY 213

FIG. 4. Calculated absorbances 2 log Rs/R0 of a hypothetical adsorbate vibration at 3000 cm2 1 on a silicon and a glass surface as a function of thetilt angle a of the corresponding transition dipole moment vector towards the surface normal. The same optical constants as in Fig. 1 were usedfor substrates and adsorbate.

Monolayer Re¯ ection Spectra at Different Inci-dence Angles and Polarizations. ODS MonolayerSpectra on Silicon. Figure 5 shows experimental and cal-culated ERIR spectra of a monolayer of ODS on siliconfor s-polarized and p-polarized radiation at incidence an-gles between 20 8 and 808 . Details of the spectral simu-lation procedure for this particular sample have been pre-sented elsewhere.24 In brief, the calculated spectra are de-rived from a transmission spectrum of dioctadecyldisul-® de as a reference compound, assuming a uniform 108tilt angle of the hydrocarbon chains in the monolayerwith respect to the surface normal.24,27,28 In general, goodagreement is found between the measured and calculatedspectra shown in Fig. 5. We will restrict the discussionhere to ® ve main CH stretching absorptions and disregardany of the weaker Fermi resonance or combinationmodes.24 The peak frequencies and band assignments forthese vibrations are listed in Table II together with thecorresponding dipole moment tilt angles a towards thesurface normal, which result for an all-trans extendedhydrocarbon chain with tetrahedral bonding angles (1108 )and a chain axis tilt of 108 with respect to the surfacenormal, as shown in the geometrical model in Fig. 6.Note that, in this simpli® ed model, the alkyl group hasbeen tilted somehow arbitrarily parallel to the moleculeplane de® ned by the C atoms, which leaves the dipolemoment orientations of the vibrations perpendicular tothis plane [n as(CH2), n as(CH3)op] unaffected by the overallchain tilt angle. Strictly speaking, an additional moleculetwist (rotation around the chain axis) would have to betaken into account, which in this particular case, however,affects the individual dipole moment tilt angles a (TableII) only slightly because of the almost perpendicular ori-entation of the chain axis (see Ref. 24).

s-Polarization. The experimental s-polarized spectra in

Fig. 5 are dominated by the n (CH2) stretching absorptionsbecause of their high intrinsic absorption strength (17CH2 groups per molecule) and their essentially paralleldipole moment orientation on the surface (Table II). Allband directions are inverted (pointing downwards), andthe intensities decrease with increasing incidence angle,as theoretically predicted (Fig. 2). The n (CH3) absorp-tions, which are comparatively weak (1 CH3 group permolecule) and haveÐ in partÐ smaller dipole moment tiltangles [n s(CH3), n as(CH3)ip; see Table II], are barely vis-ible in the experimental spectra. A comparison with thesimulated spectra shows a good agreement for the relativeband intensities and an accurate reproduction of the over-all intensity decrease with increasing incidence angle.The absolute band intensities, however, differ by about afactor of 2 between the experimental and the calculatedspectra. This result must be ascribed to uncertainties inseveral parameters needed for the calculations (® lm thick-ness, surface coverage, cone angle of the incident radia-tion), which are not directly determinable and depend oncertain assumptions. All these parameters, however, af-fect only the absolute absorption intensities, while therelative intensities re¯ ect unambiguously the moleculeorientation on the surface.

p-Polarization. The p-polarized spectra in Fig. 5 showmore complex absorption pro® les because of the super-position of parallel and perpendicular components. Werefer to Ref. 24 for a detailed quantitative spectral anal-ysis and shall focus the discussion here on qualitativeinterpretations of spectral changes as a function of thelight incidence angle. At 508 incidence, the absorptionsn s(CH2), n as(CH2), and n as(CH3)op with dipole moment tiltangles of 80 8 , 908 , and 90 8 , respectively (Table II), pointin the negative (downward) direction in Fig. 5, andn s(CH3) and n as(CH3)ip with tilt angles of 45 8 , respectively,

214 Volume 51, Number 2, 1997

FIG. 5. Experimental and calculated IR re¯ ection spectra of an ODSmonolayer on silicon for s-polarized radiation (A) and p-polarized ra-diation (B) at different incidence angles u . A uniform 108 tilt angle ofthe hydrocarbon chains in the monolayer ® lm has been assumed for thesimulated spectra.

TABLE II. CH stretching absorptions and peak frequencies in IRre¯ ection spectra of octadecylsiloxane monolayers together with tiltangles a of the corresponding transition dipole moments towardsthe surface normal for a 108 tilt of the hydrocarbon chain axis.

VibrationPeak frequency

(cm2 1)Tilt angle a

( 8 )

n s(CH2) 2851 80n s(CH3) 2879 45n as(CH2) 2919 90n as(CH3)op

a 2960 90n as(CH3)ip

a 2968 45

a Note: op 5 out-of-plane; ip 5 in-plane.

FIG. 6. Schematic surface orientation of an all-trans hydrocarbonchain tilted by 108 towards the surface normal (A) and the correspond-ing transition dipole moment orientations for the main CH stretchingvibrations (B).

point upwards in accordance with the calculated relation-ship shown in Fig. 4. The calculated spectrum for u 5508 shows a good agreement with the measured spectrumapart from the roughly doubled absolute band intensities,the reasons for which have been discussed above. Withincreasing incidence angle u , the band intensities shouldincrease while retaining the initial band directions, untila sudden band inversion at the Brewster angle ( u 5 73 8 )and a rapid intensity decrease towards u 5 908 shouldoccur. These theoretical predictions derived from Fig. 2are explicitly expressed in the calculated p-polarizedspectra in Fig. 5 (note that the scale of the spectrum at u5 708 has been divided by 10), but are only partly ful-® lled experimentally; the measured spectra do clearly

show a band inversion between u 5 60 8 and u 5 82 8 .However, the band intensities decrease rather than in-crease towards the Brewster angle. This discrepancy canbe explained by the spread of incidence angles aroundthe nominal value for the central incoming ray, whichdepends on the f-number (ratio between focal length andbeam diameter) of the focusing optics; the larger the f-number, the smaller the cone angle of the incident beamand the smaller therefore the incidence angle spread D u(the estimated value for D u in our experiments is about6 10 8 ). Since the experimentally measured band intensi-ties represent the average over this incidence angle rangeD u , they deviate from the intensities calculated in Fig. 5for the idealized case of a single, well-de® ned angle, par-ticularly in regions of strong intensity changes such asthose next to the Brewster angle. This behavior is dem-onstrated in Fig. 7, where the peak intensities of the mod-el vibration in Fig. 2 were recalculated as the averagevalues over an incidence angle spread D u of 6 4 8 and 6 8 8 .Because of the band inversion at the Brewster angle, theaveraged intensities reach a maximum on either side ofu B and then decrease towards u B because of mutual can-cellation of positive and negative contributions from theregions u , u B and u . u B. Therefore, the experimentalband intensities in Fig. 5 at u 5 708 , for instance, are oneorder of magnitude lower than predicted by the calcula-tions for the idealized case of D u 5 0 8 . Since the actualincidence angle spread D u in the experiment can, at best,be estimated, an accurate calculation of absolute band

APPLIED SPECTROSCOPY 215

FIG. 7. Calculated absorbances 2 log Rs/R0 for p-polarized radiation ofa hypothetical adsorbate vibration at 3000 cm2 1 on silicon as a functionof the light incidence angle u for different cone angles k of the incidentradiation. The same optical constants as in Fig. 1 were used for substrateand adsorbate.

FIG. 8. Experimental and calculated IR re¯ ection spectra of an ODSmonolayer on glass for s-polarized radiation (A) and p-polarized radi-ation (B) at different incidence angles u . A uniform 10 8 tilt angle of thehydrocarbon chains in the monolayer ® lm has been assumed for thesimulated spectra.

intensities appears hardly possible in the region closeto u B.

ODS Monolayer Spectra on Glass. Figure 8 shows ex-perimental and calculated IRre¯ ection spectra of an ODSmonolayer on glass. As with silicon, the best ® t of theexperimental spectra is again achieved with an assumed108 tilt angle between the hydrocarbon chains and thesurface normal, which con® rms recent results from wet-ting measurements, ellipsometry, and IR spectroscopy in-dicating that the monolayer structure of these ® lms isessentially independent of the substrate.28 The IR re¯ ec-tion spectra on glass and silicon are qualitatively verysimilar, although certain interesting spectral differencesresult from the different optical properties of silicon andglass.

s-Polarization. Similar to Fig. 5, the s-polarized spec-tra in Fig. 8 show primarily the CH2 stretching absorp-tions at 2919 cm2 1 [n as(CH2)] and 2851 cm2 1 [n s(CH2)]together with a weak CH3 band at 2960 cm2 1 [n as(CH3)op],all of which point in the negative direction (downwards)and decrease in intensity with increasing incidence angle,as predicted by theory. A good agreement in the relativeband intensities, but a deviation of approximately a factorof 2 in the absolute intensities between the experimentaland calculated spectra, is again found, which supports theprevious argument of a combined systematic error insome of the input parameters for the calculations. Theonly major difference between the s-polarized spectra onsilicon and on glass is the absolute band intensities,which are roughly one order of magnitude larger on glassbecause of the stronger surface electric ® eld in compar-ison to silicon (see Fig. 2 and related discussion).

p-Polarization. The same absorption bands at 2919cm2 1 [n as(CH2)], 2851 cm2 1 [n s(CH2)], and 2960 cm2 1

[n as(CH3)op] also appear in the p-polarized spectra in Fig.8, which point in the positive direction for u . u B and inthe negative direction for u , u B ( u B 5 558 for glass) inaccordance with the corresponding dipole moment tilt an-gles (Table II) and the intensity vs. tilt angle relationshipfor a glass substrate shown in Fig. 4. The deviations inthe absolute band intensities between the experimentaland the simulated spectra increase towards the Brewsterangle because of the unknown cone angle of the incidentradiation, as discussed above (see Fig. 7). One interestingdifference in comparison to the corresponding spectra onsilicon (Fig. 5) is that two CH3 absorptions [n s(CH3) at2879 cm2 1 and n as(CH3)ip at 2968 cm2 1] seem to be miss-ing in Fig. 8. This pattern is shown in detail in Fig. 9,

216 Volume 51, Number 2, 1997

FIG. 9. Experimental and calculated IR re¯ ection spectra of an ODS monolayer on silicon and glass for p-polarized light at 508 incidence. Thecalculated sum spectra Ap are resolved into the constituent parallel (A ) and perpendicular (A ) components (see Fig. 2 and related discussion).x z

p p

where the experimental and calculated ODS spectra onsilicon and glass are compared for p-polarized light at508 incidence. The calculated spectra in Fig. 9 have beenresolved into the parallel (A ) and perpendicular (A )x z

p p

component spectra, similar to the results shown in Fig. 2for a single frequency. Considering only the relative bandintensities, the parallel (negative) component spectra Ax

p

on silicon and glass are identical, while the relative con-tribution of the perpendicular (positive) component A isz

p

substantially larger on silicon than on glass (note that theA spectrum on glass in Fig. 9 has been multiplied by 5).z

p

Therefore, positive absorption bands remain in the sumspectrum on silicon for n s(CH3) and n as(CH3)ip, while onglass the parallel and perpendicular components of theseabsorptions are approximately equal and cancel each oth-er. An equivalent, qualitative explanation contains thecurves shown in Fig. 4. For a dipole moment tilt anglea of 458 , which results for both n s(CH3) and n as(CH3)ip fora 10 8 tilted hydrocarbon chain (Table II), the absorptionessentially vanishes on glass but remains as a positiveband on silicon.

CONCLUSION

We have shown in this study that high-quality IRspec-tra of oriented monolayer ® lms on weakly re¯ ecting, di-electric substrates such as silicon and glass can be ob-tained by the external re¯ ection technique. The experi-mental re¯ ection spectra of octadecylsiloxane monolayersprepared by adsorption of octadecyltrichlorosilane on sil-icon and glass surfaces show strong changes as a functionof the polarization and the light incidence angle, whichare accurately reproduced by spectral simulations and al-low a quantitative determination of the surface orienta-tion of the ® lm molecules. An average tilt angle of 108between the surface normal and the hydrocarbon chainaxis of the ® lm molecules has been found on both siliconand glass surfaces, indicating that the structure of these® lms is essentially independent of the substrate. This ob-

servation is in marked contrast to the closely relatedgroup of monolayers of thiol compounds on metal sur-faces, whose structure has been shown to vary signi® -cantly on similar substrates (Au, Ag, Cu)33 and even oncrystallographically different surfaces of the same sub-strate [Au(111), Au(110), Au(100)].34 These ® ndings sug-gest a fundamentally different ® lm formation mechanismof alkyltrichlorosilanes as compared to thiol-based mono-layers, which is currently under further investigation inour laboratory with the use of in situ ellipsometric andIR re¯ ection methods.

ACKNOWLEDGMENTS

This work was supported by the Fonds zur FoÈ rderung der Wissen-schaftlichen Forschung (Proj. No. P 09749) and the Hochschuljubi-laÈ umsstiftung der Stadt Wien.

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