1502 Volume 57, Number 12, 2003 APPLIED SPECTROSCOPY0003-7028 / 03 / 5712-1502$2.00 / 0q 2003 Society for Applied Spectroscopy

Infrared Re� ection Spectroscopy of Thin Films on HighlyOriented Pyrolytic Graphite

THOMAS LEITNER, JURGEN KATTNER, and HELMUTH HOFFMANN*Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, A-1060 Wien, Austria

The properties of highly oriented pyrolytic graphite (HOPG) as asubstrate for external re� ection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physi-sorbed hydrocarbon multilayers, and chemisorbed monolayers ofp-substituted aryl radicals on HOPG were used as samples, and theexperimental spectra were compared and complemented with theresults of spectral simulations. From re� ectivity measurements ofclean HOPG surfaces with polarized light as a function of the lightincidence angle and the frequency, the anisotropic optical constantsn (refractive index) and k (absorption index) were determined forin-plane and out-of-plane directions with respect to the graphitebasal plane. These constants express the semimetallic properties ofHOPG, indicated by an intermediate re� ectivity between a typicalmetal and a dielectric substrate and by asymmetric, distorted peakshapes in adsorbate � lm spectra, which represent a transition statebetween symmetrical, positive absorptions on metals and inverted,negative peaks on dielectric substrates. Regarding spectral sensitiv-ity and surface selection rules, HOPG behaves much like a metaland is therefore an equally suitable substrate for external re� ectioninfrared (IR) measurements.

Index Headings: Graphite surfaces; Infrared re� ection spectroscopy;Chemisorption.

INTRODUCTION

With the advent of highly sensitive infrared spectrom-eters and detectors, external re� ection infrared (IR) spec-troscopy has experienced an enormous growth in the pastdecade both in the number of research groups employingthis technique and in the classes of studied systems.1Whereas highly re� ecting metal substrates were consid-ered for a long time to be the only suitable substratematerials for this technique, a wide range of nonmetalsolids as well as liquids with less than ten percent re� ec-tivity have been used successfully as substrates in IR re-� ection spectroscopic studies of monolayer and sub-monolayer � lms. Today, essentially any solid materialwith a � at surface for infrared light (i.e., with a surfaceroughness in the submicrometer range) can be used as asubstrate for external re� ection IR measurements. De-pending on its optical properties, the resulting re� ectionspectra may be highly complex and dif� cult to analyze.However, with the aid of spectral simulation routines, awealth of chemical and structural information can be ex-tracted.2 Surprisingly, only a few IR spectroscopic studiesof adsorbates on � at carbon surfaces have been reportedto date.3–8 Graphite, in particular, appears to be an almostideal and highly interesting substrate for this purpose:First, graphite is commercially available with a high-quality, almost atomically � at surface. Second, graphiteis a semimetal with optical properties similar to a metal.

Received 21 April 2003; accepted 28 July 2003.* Author to whom correspondence should be sent.

And third, graphite is used in a number of practicallyimportant � elds such as electrochemistry with graphiteelectrodes, tribology of graphite lubrication layers, orgraphite-based energy production and energy storage sys-tems,9 which are controlled by surface processes andwhich would certainly bene� t from the application of anondestructive, composition- and structure-sensitivemethod such as external re� ection infrared spectroscopy.

The aim of the present study was therefore to explorethe properties and characteristics of graphite as a sub-strate material for external re� ection infrared studies, to� nd the optimized measurement conditions, and to com-pare the information content of external re� ection IRspectra on graphite with other, more common substratematerials like metals or semiconductors.

EXPERIMENTAL

Materials. Highly ordered pyrolytic graphite (HOPG,Grade ZYH, 12 3 12 3 2 mm) was purchased fromAdvanced Ceramics (Cleveland, OH). A fresh surfacewas prepared immediately before adsorption by removingthe top layers with adhesive tape. Gold-coated glassslides (25 3 15 3 1 mm) with preferential (111) orien-tation were obtained from Pharmacia (Uppsala, Sweden)and were cleaned by rinsing with acetone and ethanol andblow-drying in high-purity nitrogen. P-doped, (100)-ori-ented , single-s ided polished silicon wafers (WackerChemitronic, test grade, 14–30 W·cm resistivity, 0.5 mmthickness) were cut into pieces of appropriate size (25 315 mm) and were cleaned in the same way as the goldslides.

The solvents acetonitrile (Merck, Uvasol), toluene(Sigma-Aldrich, HPLC-grade, 99.8%), acetone (Sigma-Aldrich, HPLC-grade, 99.9%), and ethanol (Mautner-Markhof, 99.8%), the supporting electrolyte tetraethylam-moniumperchlorate (Acros Organics, 90%), and the ad-sorbate precursor p-nitrobenzenediazonium-tetra� uoro-borate (Sigma-Aldrich, 97%) were used without furtherpuri� cation. The second adsorbate compound, p-hexa-decyloxybenzene diazonium-tetra� uoroborate (HDBDF),was prepared from p-hexadecyloxyaniline (Frinton Lab-oratories, Vineland, NJ) in the following way: A thor-oughly stirred dispersion of 0.5 g nitrosonium-tetra� uo-roborate (Sigma-Aldrich) in 8 mL absolute dichlorometh-ane was cooled to 0–5 8C and a solution of 1.0 g hexa-decyloxyaniline in 5 mL dichloromethane was addeddropwise. After 30 min at 0–5 8C, the mixture was stirredfor a further 30 min at room temperature. After additionof 15 mL diethylether the solid was � ltered, washed withdiethylether, and air-dried. The resulting diazonium salt(1.08 g, 81%) was an off-white solid (1H-NMR, 300MHz, CDCl3): d(ppm) 5 0.88 (t, 3H, CH3), 1.2–1.5 (m,

APPLIED SPECTROSCOPY 1503

FIG. 1. Consecutive cyclic voltammograms of a HOPG electrode im-mersed in a 1 mmol/L solution of p-nitrobenzene-diazoniumtetra� uo-roborate in CH3CN 1 0.1 mol/L Et4NClO4. Scan rate: 200 mV/s; elec-trode area: 0.7 cm 2; reference electrode: Hg/Hg2Cl2.

CH2), 1.8 (t, 2H, CH2), 4.2 (t, 2H, CH2), 7.17 (d, 2H,C6H4), 8.48 (d, 2H, C 6H4).

Film Preparations. Thin � lms of paraf� n oil on gold,silicon, and HOPG substrates were prepared by dispers-ing a calculated amount of a solution of paraf� n (Aldrich)in toluene evenly over the substrate surface. After com-plete evaporation of the solvent, a thin � lm of paraf� noil remained whose thickness was estimated from the vol-ume, the concentration, and the density of the appliedsolution. Electrochemical depositions and cyclic voltam-metry measurements on HOPG substrates were per-formed with a Voltalab t PGZ301 dynamic voltammetrysystem in a standard three-electrode cell, including a plat-inum auxiliary electrode, a calomel (Hg/Hg2Cl2) refer-ence electrode, and the HOPG working electrode. Elec-trical contact to the HOPG substrate was made with asmall alligator clamp and the substrate was immersed intothe electrolyte, yielding an active reaction area of 0.7 cm 2

on the HOPG surface. The cyclovoltammetric scans wereperformed between 20.75 V and 0.75 V at a scan rateof 200 mV/s, using 1 mmol/L solutions of the corre-sponding diazonium salts in acetonitrile with 0.1 mol/Ltetraethylammoniumperchlorate as the supporting electro-lyte. The electrochemical reduction of the diazonium saltsand subsequent adsorption of the intermediate aryl radi-cals could be monitored by the appearance of an irre-versible reduction peak in the cyclic voltammogramswith a peak potential of about 300 mV vs. Hg/Hg2Cl2

(Fig. 1). As shown in Fig. 1, the peak area decreasedrapid ly over subsequent cycles , indicating completemonolayer formation and blocking of the surface after afew cycles.

Infrared Measurements. Infrared spectra were mea-sured on a Mattson RS FTIR spectrometer equipped witha narrow band mercury cadmium telluride (MCT) detec-

tor (1 mm 2 active area, speci� c detectivity D* 5 6 3 1010

cm Hz½ W21). A commercial variable-angle re� ectionunit (Seagull, Harrick Scienti� c) mounted in the internalsample chamber of the spectrometer was used for mea-surements of the HOPG substrate re� ectivity at variableincidence angles. A wire grid polarizer (Model PWGU2R, Harrick Scienti� c) located in front of the detectorwas used to choose between p- (parallel) or s- (perpen-dicular) polarization with respect to the light incidenceplane. At each speci� ed angle, an interferogram of theclean HOPG substrate was measured by coaddition of 16scans, and the peak-to-peak voltage of the centerburstwas evaluated and ratioed against the peak-to-peak volt-age of a gold reference substrate, whose re� ectivity wasassumed to be 100% and independent of incidence angleand polarization. These peak-to-peak voltage ratios mea-sured with s-polarized and p-polarized light were there-fore used as the experimental re� ectivities of HOPG asa function of incidence angle and polarization. Since thismethod yields only average re� ectivities over the mid-infrared wavelength range, the resulting values werecompared to single-wavelength re� ectivities at 2050 cm21

by FT transformation of the measured interferograms intosample (HOPG) and reference (gold) single beam spectraand division of the corresponding single beam intensitiesat 2050 cm21. The resulting single-wavelength re� ectiv-ities were generally identical to the peak-to-peak datawithin the accuracy of these measurements.

External re� ection infrared spectra were measured at a� xed incidence angle of 808 using a custom-made re� ec-tion optical system connected to the Mattson spectrom-eter as described in detail in previous work.10 The beampolarization could be switched between s- and p-orien-tation by means of a wire grid polarizer (Harrick Scien-ti� c). IR re� ection spectra were measured at 4 cm21 res-olution and triangular apodization by coaddition of 512scans per spectrum, using the adsorbate-covered substrateas the sample and the clean substrate as the reference.Reference spectra of the benzenediazonium precursorcompounds in the crystalline state were measured by dif-fuse re� ection (DRIFT) of a sample/KBr mixture (1:8),using the DRIFT accessory of the Seagull re� ection unit.

Spectral Simulations. All calculations were carriedout using a customized computer program written in For-tran 90. The spectral simulation routines are based on atransfer-matrix algorithm, described in detail elsewhere,2

which is capable of handling samples with an arbitrarynumber of adsorbate layers. The sample is modeled as astack of parallel layers separated by abrupt, perfectly � atinterfaces. Each layer is characterized by a complex re-fractive index n 5 n 1 ik (n: real refractive index, k:absorption index) and a thickness d, except for the sub-strate and the incidence medium, which are both treatedas semi-in� nite (d 5 `). Each layer may exhibit eitheruniaxial symmetry with different complex refractive in-dices parallel (in-plane, n ip 1 ikip) and perpendicular (out-of-plane, nop 1 ikop) to the sample surface or an isotropic,randomized structure with mean, nondirectional values(n ip 5 nop 5 n, kip 5 kop 5 k). For an assumed samplemodel, the complex re� ection coef� cients for s-polarized(r s) and p-polarized (r p) radiation can be calculated for agiven incidence angle and wavenumber of the probing

1504 Volume 57, Number 12, 2003

FIG. 2. Experimental and calculated IR re� ectivities of HOPG, relativeto a clean gold surface, for s-polarized and p-polarized radiation as afunction of the light incidence angle.

FIG. 3. Experimental and calculated IR re� ection spectra of HOPG,referenced against a clean gold surface, in the MIR region for s-polar-ized and p-polarized radiation at 808 incidence. Literature data12 for theoptical constants of HOPG were used for the calculations.

FIG. 4. Calculated IR re� ectivities at 2900 cm 21 for s-polarized andp-polarized radiation as a function of the light incidence angle for threedifferent substrates: a typical metal (gold, n 5 2.1, k 5 21.33), a semi-metal (HOPG, nip 5 5.044, kip 5 4.09, nop 5 2.349, kop 5 0.01), and asemiconductor (silicon, n 5 3.433, k 5 0.0).

radiation, from which the re� ectivities R s,p 5 zr s,pz2 couldbe obtained.11

The samples modeled in this paper consisted of eithertwo or three distinct phases, with the substrate (HOPG,gold, or silicon) as phase 1, an optional organic � lm ofthickness df as phase 2, and air as the incidence medium(phase 3, n 5 1.0, k 5 0). The re� ectivity of HOPG asa function of incidence angle (Fig. 2) was calculated asthe ratio between the HOPG re� ectivity and the re� ec-tivity of gold as reference for different incidence anglesand polarizations. In order to simulate the experimentalre� ectivities as average values over the mid-infrared(MIR) range, literature data for the optical constants ofHOPG 12 were averaged over the 800 to 4000 cm21 fre-quency range and were used as starting values for thecalculations. An error function:

N

2G 5 (R 2 R ) (1)O n,e n,cn51

was de� ned with R n ,e and R n ,c being the nth data pointsof the experimental and calculated re� ectivities for a cer-tain wavenumber and polarization. Subsequently, the op-tical constants of HOPG were systematically varied untilG assumed a minimum,13 and the corresponding set ofoptical constants was used as the best � t for the experi-mentally measured, averaged HOPG re� ectivities. Fre-quency-dependent optical constants for HOPG, gold, andsilicon in the MIR region were obtained from the litera-ture.12 These literature data were expanded by linear in-terpolation to obtain a set of equally spaced data pointswith 1 cm21 resolution, which allowed the calculation ofsubstrate re� ectivities as a function of wavenumber (Fig.3). For simulations in the CH stretching region (Figs. 4and 6), the interpolated literature values13 of HOPG, gold,and silicon at 2900 cm21 were used.

RESULTS AND DISCUSSION

Optical Properties of HOPG in the Mid-infraredRegion. Graphite is a semimetallic, crystalline allotropicform of carbon.14 The carbon atoms are arranged in par-allel layers and form a network of regular hexagons in

each layer. The interlayer distance is about three times aslarge as the C–C distance within one layer, resulting in alarge anisotropy of the optical, electrical, and structuralproperties of graphite. Because of the weak interlayerbonding, graphite is often treated as a two-dimensionalsolid. The existence of IR-active vibrations, however, isa direct consequence of the three-dimensional crystal lat-tice, as shown in Scheme I. The two-dimensional graphitelayer has two equivalent carbon atoms per unit cell, re-sulting in two vibrational modes, an in-plane atomic dis-placement E2g and an out-of-plane displacement B 2g, bothof which are IR inactive. In the three-dimensional lattice,

APPLIED SPECTROSCOPY 1505

SCHEME I. Lattice vibrations in a two-dimensional graphite layer andin a three-dimensional graphite crystal. The two IR active vibrationsn1

G and n2G of the three-dimensional lattice yield transition dipole mo-

ments parallel (n1G ) and perpendicular (n2

G ) to the graphite basal plane.

these two carbon atoms are no longer equivalent, becauseone of them has two neighbors directly above and belowin the adjacent layers and the other one has no next-neighbors in the adjacent layers. Consequently, the unitcell consists of four atoms and the single layer vibrationalmodes split into Davydov doublets: the E2g mode turnsinto an IR-active E1u mode and a Raman active E2g mode;and the B2g mode splits into an IR active A2u mode andan inactive B2g mode. Thus, graphite has two IR-activevibrations, the in-plane E1u vibration with a transition di-pole moment parallel to the graphite base plane and theout-of-plane vibration A2u with a dipole moment orien-tation perpendicular to the base plane, as illustrated inScheme I.

The classical method to determine the optical constants(refractive index n, absorption index k) of a substratecomprises re� ectivity measurements at normal incidenceat different frequencies and subsequent Kramers–Kronigtransformation, from which the Fresnel re� ection coef� -cients r and the complex refractive index n 5 n 1 i*k ofthe substrate can be calculated.15 For anisotropic materialslike graphite, however, a total of four optical parametersneed to be determined, two for the in-plane properties n ip,kip parallel to the basal plane and two for the out-of-planeproperties nop, kop perpendicular to the basal plane. In thiscase, re� ectivity measurements at normal incidence aretoo inaccurate because of the dif� culties of preparinghigh-quality surfaces perpendicular to the natural cleav-age plane. A different method was therefore used herebased on re� ectivity measurements of basal plane graph-ite samples with different polarizations and at differentincidence angles in combination with a numerical � ttingprocedure. Figure 2 shows the experimentally measuredre� ectivities for s-polarized and p-polarized radiation asa function of the incidence angle together with the � ttedcurves obtained from a least-squares � tting routine de-scribed in the Experimental section. These curves showthe typical picture for a strongly absorbing substrate witha re� ectivity minimum for p-polarized light at the pseu-do-Brewster angle of about 828. Although the agreement

between experimental and calculated curves is not perfectdue to experimental limitations (the Seagull external re-� ection unit used for these measurements has a cone an-gle of about 6108 of the incident radiation, resulting inan ill-de� ned incidence angle and large light losses athigher angles of incidence), the optical constants extract-ed from these measurements as mean values over theMIR range (nip 5 3.384, kip 5 4.45, nop 5 1.267, kop 50) show good agreement with literature data.12 In partic-ular, the large anisotropy of graphite with almost metallicproperties parallel to the basal plane and insulating prop-erties perpendicular to the plane are apparent from thesevalues.

The frequency dependence of the optical properties ofgraphite as well as its native lattice absorptions can beobserved in external re� ection spectra of a clean graphitesurface.

Figure 3 shows experimental and calculated IR re� ec-tion spectra of a HOPG substrate cleaved along the car-bon base plane, measured at 808 incidence with s-polar-ized and p-polarized radiation. The E-� eld vector of s-polarized light is oriented parallel to the sample surfaceand therefore only the in-plane mode E1u (Scheme I) isseen in the s-polarized spectrum as a weak absorption at1585 cm21. p-polarized light, on the other hand, has E-� eld components both parallel and perpendicular to thesubstrate surface, and both the E1u mode and the out-of-plane A2u mode at 868 cm21 are visible in the spectrum.It is interesting to note that the E1u absorption points up-ward in Fig. 3, i.e., corresponds to an increase of thesubstrate re� ectivity, whereas the A2u mode points down-ward and represents a re� ectivity decrease. This behavioris nicely reproduced in the simulated re� ections, whichwere calculated from the frequency-dependent opticalconstants listed in Ref. 12. Similar observations of ‘‘in-verted’’ or ‘‘negative’’ absorption bands have been re-peatedly described in previous studies and the reader isreferred to Ref. 2 for a detailed discussion of this phe-nomenon, which basically re� ects a sum effect of therelative changes of the absorption index and re� ectionindex across the frequency bandwidth of an absorptionband. Note that both the measured absolute re� ectivitiesfor s-polarized and p-polarized radiation and their wave-number dependence in Fig. 3 agree fairly well with thecalculated results. Also, the absolute intensities of thegraphite absorptions, which depend primarily on the qual-ity of the HOPG crystal (crystallite sizes, edge plane den-sities, etc.), show a good agreement between experimentand theory in Fig. 3.

The overall semimetallic properties of graphite aremost clearly illustrated by its re� ection properties in com-parison to a typical metal such as gold and to a dielectricmaterial like silicon (Fig. 4). First of all, the re� ectivitiesR(08) normal to the surface (a 5 08) are proportional to(n 2 1)2 /(n 1 1) 2 and increase from silicon (R(08) 5 0.31)to HOPG (R(08) 5 0.62) and to gold (R(08) 5 0.99). Froma 5 08, the s-polarized re� ectivity curves rise steadilywith increasing a towards R 5 1 at a 5 908, whereas thep-polarized curves decrease toward a minimum at theBrewster angle (for Si) or at the pseudo-Brewster angle(for HOPG and Au) and then rise sharply toward R 5 1.We have shown previously16 that the signal-to-noise ratio(SNR) of external re� ection infrared measurements of ad-

1506 Volume 57, Number 12, 2003

TABLE I. Optimum incidence angles uopt and relative signal-to-noise ratios (SNR) for IR re� ection spectra of an adsorbate on dif-ferent substrates probed with p-polarized light.

Substrate uopt SNR

GoldHOPGSilicon

887286

1006.21.7

TABLE II. Band assignments and peak wavenumbers (in cm21) forinfrared spectra of p-nitrobenzene-diazoniumtetra� uoroborate(NBDF) and p-hexadecyloxybenzene diazoniumtetra� uoroborate(HDBDF) in the crystalline state (measured in diffuse re� ection)and of the corresponding monolayers p-nitrobenzene (NB) and p-hexadecyloxybenzene (HDB) prepared from the diazonium salts byelectrochemical reduction on HOPG surfaces (measured by externalre� ection).a

VibrationNBDFpowder

NBmonolayer

HDBDFpowder

HDBmonolayer

N(CH)arom

nas(CH 2)ns(CH2)N(N2)ns(C5C)

3120n.e.n.e.

2307n.o.

n.o.n.e.n.e.n.e.n.o.

3120291628492262

1586, 1574

n.o.29332858n.e.n.o.

n2(graphite)nas(NO 2)d(CH2)ns(NO2)n(5C–O)N(BF4)

n.e.1542n.e.

1358n.e.

1057

15901531n.e.

1352n.e.n.e.

n.e.n.e.

1472n.e.

12811059

1590n.e.n.o.n.e.n.o.n.e.

a n.o. 5 not observed; n.e. 5 nonexistent.

FIG. 5. CH stretching absorptions of a thin paraf� n � lm (d 5 30 nm)on HOPG, measured with s-polarized and p-polarized radiation at 808incidence.

sorbate � lms is proportional to the difference R0 2 R s

between the re� ectivities of the clean substrate R0 and thesample Rs (substrate plus adsorbate). The maximum ofR0 2 R s as a function of the incidence angle for anyparticular substrate yields the optimum angle for the mea-surements, and a comparison of these (R0 2 R s)max valuesfor a model adsorbate � lm on different substrates yieldsthe relative sensitivities achievable on these substrates.16

Table I lists calculated SNR values together with the op-timum incidence angles uopt for external re� ection IRmeasurements with p-polarized light on gold, HOPG, andSi, whereby the SNR on gold has been assigned an ar-bitrary value of 100. As a model absorption, a band at2900 cm21 with an absorption index k 5 0.1 and a half-width of 10 cm21 in a 1-nm-thick � lm was used. Al-though the calculated SNR for HOPG is only about 6%of the theoretically achievable SNR on a metal substrate,the large dynamic range of contemporary MCT detectorswill usually compensate for this difference and shouldallow external re� ection IR measurements at or near thedigitization noise limit on HOPG as on metal or dielectricsubstrates.16 A lower, optimum incidence angle of 728 iscalculated for HOPG compared to 888 for metals and 868for silicon. However, the corresponding maximum of theSNR vs. Q curve for HOPG is relatively broad such thatan angle of 808, which is commonly used in most grazingangle optical systems, is a good compromise for all thesubstrates listed in Table I.

Surface Selection Rules, Band Shapes, and Band In-tensities of Adsorbate Films. The surface electric � eldat any particular substrate surface, which results from thesuperposition of incoming and re� ected electromagneticradiation, is generally highly anisotropic because the par-allel and perpendicular components undergo differentphase and amplitude changes upon re� ection. In the ex-treme case of a highly conducting metal substrate, theparallel component essentially disappears (is cancelledupon re� ection) and only a perpendicular surface electric� eld remains. This is the origin of the well-known surfaceselection rules for metal substrates,17 where only perpen-dicular vibrational components give rise to absorptions inIR re� ection spectra. A different type of discriminationbetween perpendicular and parallel vibrations exists onnonabsorbing, dielectric substrates where both orienta-tions yield absorption peaks, which, however, point inopposite directions. 2 Graphite as a semimetal is expectedto show some intermediate properties between these twocategories of metal and dielelectric substrates. Figure 5shows the CH stretching region of a re� ection spectrumof a 30-nm-thick paraf� n � lm on HOPG, measured withp-polarized light and s-polarized light. At � rst glance,graphite seems to behave just like a metal: absorptionsare observed only in the p-polarized spectrum, whereass-polarized light, whose electric � eld vector lies parallel

to the surface, results in no spectral response. A closerlook at the peak shape of the CH stretching bands in Fig.5, however, yields a distinct asymmetry, namely a tail atthe high-frequency side and a dip below the baseline atthe low-frequency side of the overall absorption contour.We have therefore tried to simulate a model absorptionat 2900 cm21 in a hypothetical monolayer � lm (d 5 1nm) on HOPG and to compare the peak shapes and in-tensities of this model absorption between graphite andtwo other substrates, a metal (gold) and a dielectric (sil-icon), as a function of the transition dipole moment tiltangle a with respect to the surface normal. The resultsare shown in Fig. 6. On gold, a symmetric absorptionwith constant peak wavenumber is obtained, whose in-

APPLIED SPECTROSCOPY 1507

FIG. 6. Simulated CH stretching absorption (nmax 5 2900 cm21, k 50.1, FWHH 5 10 cm 21) in a 1-nm-thick organic � lm (n 5 1.50) onthree different substrates ((A) silicon, (B) HOPG, and (C ) gold) fordifferent tilt angles a of the transition dipole moment. The same opticalconstants as in Fig. 4 were used for the substrates and a light incidenceangle u 5 808 was used.

FIG. 7. CH stretching absorptions of thin paraf� n � lms (d 5 30 nm)on silicon, HOPG, and gold, measured with p-polarized radiation at 808incidence.

SCHEME II. Formation of a chemisorbed monolayer on graphite byelectrochemical reduction of an aryldiazonium salt.

tensity decreases from a maximum value at a 5 08 tozero at a 5 908 in accordance with the metal surfaceselection rules. On silicon, a band inversion occurs for aø 678, with upward-pointing, positive peaks for a . 678and downward-pointing, negative peaks for a , 678 con-comitant with a peak frequency shift to higher wavenum-bers. We have analyzed these spectral properties of sili-con substrates in detail in previous publications.2,16,18 No-tably, the absorptions on HOPG display an intermediateshape between gold and silicon: the peak points upwardfor a , 908 and essentially disappears for a 5 908, as ongold, but the onset of a band inversion is clearly visibleby the low-frequency dip and the high-frequency tail ofthe peak pro� le, which is most distinct for perpendiculardipole moment orientation (a 5 08). By variation of thesubstrate’s optical constants it can be shown that the ab-sorption index k (kip for HOPG) is responsible for thepeak shape and the peak direction: for k . 20, metallicbehavior as in Fig. 6C is exhibited; and for k , 1, bandinversion as in Fig. 6A occurs. The region 1 , k , 20with HOPG as a typical representative (kip 5 4) is char-acterized by asymmetric, distorted peaks and a continu-ous changeover with increasing k from symmetrical neg-ative peaks for k , 1 to symmetrical positive peaks fork . 20.

Surface Orientation in Physisorbed and Chemi-sorbed Films. In order to verify the above-predictedband shapes and band intensities experimentally, we haveprepared paraf� n � lms about 30 nm thick on gold,HOPG, and silicon and measured their IR re� ection spec-tra (Fig. 7). Due to the random molecule orientation inthese physisorbed � lms, the average dipole moment tiltangles a assume the isotropic value of 54.78 and the peak

direction is positive on gold and HOPG and negative onsilicon, as predicted in Fig. 6. A slightly asymmetric peakshape (high-frequency tail and low-frequency dip) is alsorecognizable in the spectrum on HOPG, although this ef-fect is rather weak for a 5 54.78 (see Fig. 6B) and isadditionally suppressed in Fig. 7 by the overlap of ad-jacent CH stretching peaks. Chemisorbed monolayerswith a well-de� ned surface coverage and surface orien-tation of the adsorbate molecules are dif� cult to prepareon graphite due to the chemical inertness of carbon. Oneof the few chemisorbed systems described in the litera-ture is based on the electrochemical reduction of aryldi-azonium cations to aryl radicals using a graphite substrateas the anode,19 which results in a stable, covalently bond-ed monolayer of aryl species on the graphite surface(Scheme II). The IR re� ection spectra of two such chem-isorbed monolayers with different compositions are

1508 Volume 57, Number 12, 2003

FIG. 8. Diffuse re� ection IR spectrum of p-nitrobenzene-diazonium-tetra� uoroborate (NBDF) and external re� ection IR spectrum of a p-nitrobenzene monolayer (NB) adsorbed on HOPG. Peak wavenumbersand band assignments are listed in Table II.

FIG. 9. Diffuse re� ection IR spectrum of p-hexadecyloxybenzene-dia-zoniumtetra� uoroborate (HDBDF) and external re� ection IR spectrumof a p-hexadecyloxybenzene monolayer (HDB) adsorbed on HOPG.Peak wavenumbers and band assignments are listed in Table II.

shown in Figs. 8 (p-nitrobenzene) and 9 (p-hexadecyl-oxybenzene). For comparison, reference spectra of thecorresponding precursor compounds p-nitrobenzene-dia-zoniumtetra� uoroborate and p-hexadecyloxybenzene dia-zoniumtetra� uoroborate in the crystalline state are in-cluded in Figs. 8 and 9. The major absorption peaks inboth the monolayer and the reference spectra are listedin Table II. The most prominent absorptions are the NO2

stretching vibrations nas(NO2) and ns(NO2) for the p-ni-trobenzene compound (Fig. 8) and the CH2 stretching vi-brations nas(CH2) and ns(CH2) for the p-hexadecyloxyben-zene compound (Fig. 9). Whereas the n(NO2) peaks areonly slightly shifted to lower wavenumbers in the mono-layer spectrum and essentially retain their peak shapesand relative peak intensities compared to the solid ref-erence spectrum, the n(CH2) absorptions appear at un-usually high wavenumbers (2933 and 2858 cm21) in themonolayer spectrum of Fig. 9 and also show the typicalasymmetric, almost derivative-shaped pro� le (low-fre-quency dip and high-frequency tail) that was discussedbefore as being due to the substrate properties. In general,a shift of the n(CH2) peaks to higher wavenumbers inmonolayer spectra is always an indication of structuraldisorder, i.e., loosely packed, randomly oriented alkylgroups.20 In this particular case, however, part of the highwavenumber shift must be ascribed to a purely geomet-rical effect of the derivative-shaped absorption pro� le,which shifts the apparent peak maximum to even higherwavenumbers. The relative peak intensities of both then(NO2) absorptions in Fig. 8 and the n(CH2) absorptionsin Fig. 9 are fairly similar between the monolayer andthe solid reference spectra. Considering the large discrim-ination between perpendicular and parallel vibrations ongraphite surfaces, this fact indicates a fairly random ori-entation of the corresponding functional groups in themonolayer. This result is in accordance with previousstudies21 about the structure of p-nitrobenzene monolay-ers, for which packing densities of about 1014 molecules/cm 2 have been found, which is only about 15% of thecalculated maximum coverage for a close-packed layer21

and allows the � lm molecules to adopt different, random-ized surface orientations. Long-chain hydrocarbon com-pounds such as HDBDF in Fig. 9, on the other hand, arecapable of forming highly ordered, densely packed � lms,known as self-assembled monolayers,22 on a variety ofsubstrates if the substrate–� lm bonding geometry allowsthe alkyl groups to approach each other to within van derWaals bonding distances and align and orient themselveson the surface. Although the HDB monolayer in Fig. 9shows a random, isotropic structure as well, the goal ofour current studies is to modify the adsorption conditionsin such a way that densely packed, highly ordered mono-layer � lms can be prepared on graphite. This should ul-timately allow a modi� cation and � ne-tuning of the sur-face properties of graphite similar to well-establishedself-assembly processes on other substrates.22

CONCLUSION

We have shown in this study that graphite surfaces andthin � lms adsorbed on it can be investigated by externalre� ection infrared spectroscopy with high sensitivity andyield valuable information about the surface compositionand � lm structure. The optical properties of graphite aresimilar to metals and result in similar surface selectionrules, which essentially suppress the parallel vibrationalcomponents entirely and amplify the perpendicular vibra-tions. A slight but highly characteristic difference frommetal substrates is the asymmetric peak shape in adsor-bate spectra of both physisorbed multilayers and chemi-sorbed monolayer � lms on graphite. This asymmetry isclearly visible also in previously reported IR re� ectionspectra with graphite substrates (e.g., physisorbed CO onHOPG (Ref. 3, Fig. 2) or physisorbed CH3Cl on HOPG(Ref. 4, Fig. 3)), but has not yet been recognized as beingdue to the onset of the nonmetal, dielectric properties ofgraphite. Spectral simulations show that this peak asym-metry depends on the surface orientation of the corre-sponding transition dipole moment and should thereforebe an additional source of information for future inves-

APPLIED SPECTROSCOPY 1509

tigations of adsorbate � lm structures on graphite with in-frared re� ection spectroscopy.

ACKNOWLEDGMENT

This work was supported by the Fonds zur Forderung der Wissen-schaftlichen Forschung (Project P 14769).

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