Analysis of Artifacts in Infrared Spectroscopy of Thin Organic Films on

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Analysis of Artifacts in Infrared Spectroscopy of Thin Organic Films on Metallic Substrates Dirk G. Kurth Max-Planck-Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489 Berlin, Germany Received August 11, 1997. In Final Form: September 21, 1998 The influence of experimental parameters on the shape of reflection-absorption infrared (RAIR) spectra of thin organic films on metallic substrates is analyzed in detail. Two main sources for artifacts in RAIR spectroscopy are identified: reproducibility in the angle of incidence and variations in the substrate refractive index. At high angles of incidence, optical artifacts are more prominent. The divergence of the probe beam is of importance if quantitative analysis of intensity contour is intended. In addition, surface cleaning can alter the optical properties of the substrate which will cause spectral artifacts. These difficulties are greatly diminished if the spectral information of the thin film is retrieved from p- and s-polarized spectra.

Introduction Although many analytical techniques afford information on the elemental composition of surface layers, vibrational spectroscopy is unsurpassed in elucidating the nature, structure, and orientation of molecules at interfaces.1 Depending on the particular requirements of the experiment, infrared spectroscopy is employed in many different configurations, with the more common ones being absorbance, diffuse reflectance, attenuated total reflection, and reflection-absorption infrared (RAIR) spectroscopy.2 Both fundamental and applied studies often require investigations of thin organic films on metallic substrates, especially since it was realized that certain classes of organic compounds self-assemble on metallic substrates.3 Because of the high absorptivity of metals, spectra are obtained in reflection techniques. In RAIR spectroscopy, the p-polarized probe beam is incident at the grazing angle on the flat metal substrate. The application of RAIR spectroscopy in surface analysis revealed many details of the molecular structure and will continue to play an important role in developing a comprehensive understanding of the nature of thin organic films.4 Significant changes of the intensity contour compared to transmission spectra of bulk materials occur as a function of the refractive index of the material and the substrate, as well as the orientation and packing of the molecules in the film. A classical electromagnetic theory approach is necessary in order to distinguish optical, chemical, and structural effects.5 A detailed analysis of the intensity contour requires high-quality RAIR spectra free of spectral artifacts. State-of-the-art RAIR spectroscopy uses well-defined atomically clean substrates with in situ techniques in a UHV environment free of atmospheric contaminants.6 The sample remains unaltered in the probe beam for the (1) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211-357. (2) Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; Wiley: New York, 1986. (3) (a) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (c) Grunze, M. Phys. Sci. 1993, T49B, 711. (4) Allara, D. Crit. Rev. Surf. Chem. 1993, 2, 91. (5) Parikh, A. N.; Allara, D. J. Chem. Phys. 1992, 96, 927. (6) Bradshaw, A. M.; Schweizer, E. In Advances in Spectroscopy: Spectroscopy of Surface; Hester, R. E., Ed.; John Wiley: New York, 1988; Vol. 16, pp 413-488.

entire measurement. The result is an accurate RAIR spectrum free of optical artifacts. One of the attractive features of organic thin films is their spontaneous adsorption from solution. Most research done in the field relies on a simple experimental protocol: the substrate is immersed in a solution containing the organic species. In this case, in situ techniques under UHV conditions are not suitable. Interfering liquid prevents RAIR spectroscopy at liquid-solid interfaces. Therefore, the substrate is removed from the spectrometer for film deposition. As a result, the optical path is perturbed, and the RAIR measurement is prone to artifacts. Since organic materials are intrinsically weak absorbers, a variation of 1 in 104 in optical parameters can cause detrimental artifacts. There is no mention made of the causes of spectral artifacts in RAIR spectroscopy in the references given above. While high-quality Fourier transform spectrometers achieve sufficient signal-to-noise ratios, artifacts remain a pervasive issue in RAIR spectroscopy. Because of the importance of thin organic films in fundamental and applied science, this work was undertaken to identify the sources of artifacts. Results and Discussion Model. Reflection-absorption infrared spectroscopy measures the reflectance of the sample interface under oblique illumination with p-polarized radiation. RAIR spectra are reported in absorbance as

A ) -log [RS/RR] RS is the reflectance of the film-substrate interface and RR is the reflectance of the pure substrate (no film). The reflectance is defined as the squared modulus of the ratio of the reflected, Er, and the incident, Ei, electric fields, R ) |Er/Et|2.7 The intensity profile of an oscillator on a metal surface has a maximum at the grazing angle; the maximum absorbance occurs at approximately 88°.8 Reflectance at an interface can be calculated in a (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Kuwana, T. Anal. Chem. 1986, 58, 2461. (8) (a) Lekner, J. Theory of Reflection of Electromagnetic and Particle Waves; Martinus Nijhoff Publishers: Dordrecht, The Netherlands, 1987. (b) Born, M.; Wolf, E. Principles of Optics; Pergamon Press: New York, 1993.

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Figure 1. Effect of misalignment of the angle of incidence on the absorbance, -log[RS/RR], as a function of the angle of incidence in the range from 80° to 88° for blank gold wafers. The angles of incidence of the reference, RR, and the sample, RS, reflectances differ by 0.05°. At the grazing angle, an inaccuracy in the angle of incidence causes excessive baseline distortions.

straightforward fashion by employing standard electrodynamic theory.9 Optical Effects. Optical effects are defined as those features in the RAIR spectrum that are absent in the dielectric function. Compared to transmission spectroscopy, RAIR spectra show the following particularities. The real part of the film refractive index, nF, exceeds that of the ambient medium (nA ) 1). As a result, the absorbance can differ from zero in regions where the film does not absorb (kF ) 0) and the baseline is slightly tilted. The dielectric response of strong oscillators produces an optically induced peak shift toward higher frequency.10 In addition, a Lorentzian oscillator can show an asymmetric line shape if the angle of incidence is high. A variety of optical phenomena are observed in RAIR spectroscopy, such as band shifts, band distortions, waveguide effects, and surface polaritons.11 These effects depend on the angle of incidence, the wavelength, the film thickness, and refractive indices. These optical effects can be accounted for within the theoretical framework and, therefore, do not obscure spectral interpretation. Deviations in the Angle of Incidence. Many experimental protocols require the substrate to be removed from the spectrometer. Most organic materials cannot be delivered through the gas phase, and the interfering absorptivity of the solvent prevents RAIR spectroscopy at liquid-solid interfaces. If the sample is removed from the spectrometer between measurements, the angle of incidence in subsequent measurements may be slightly different. A misalignment of the optical path is troublesome because the reflectance is a sensitive function of the angle of incidence. The effect of a small perturbation of the angle of incidence on the RAIR baseline was computed. The result for a gold substrate is shown in Figure 1. For (9) We use a model consisting of plane parallel layers. Each layer is characterized by an isotropic wavelength-dependent refractive index, N(ν) ) n(ν) - ik(ν). The refractive index of the sample layer is described by a standard dispersion relation (Born, M. Optik; Springer: Berlin, 1985). The refractive index of the substrate is taken from: Palik, E. D. Handbook of Optical Constants of Solids; Academic: Orlando, FL, 1985. (10) Kurth, D. G.; Bein, T. Langmuir 1995, 1, 578. (11) (a) Greenler, R. G.; Rahn, R. R.; Schwartz, J. P. J. Catal. 1971, 23, 42. (b) Harbecke, B.; Heinz, B.; Grosse, P. Appl. Phys. A 1985, 38, 263. (c) Berreman, D. W. Phys. Rev. 1963, 130, 2193.

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Figure 2. Experimental (solid curve) and calculated (dotted curve) RAIR baseline for angles of incidence of 80° and 88° and an angular misalignment of 0.14° and 0.3°, respectively. Referring to Figure 1, these spectra verify the trend that baseline distortions are more prominent at the glancing angle. Smaller variations in the angle of incidence could not be measured accurately in the RAIR sample chamber. The absorption bands for CO2 of around 2300 cm-1 were removed for clarity (evaporated gold substrates).

clarity there is no film present. The angle of incidence of the reference and sample reflectance measurements differ by 0.05°. For angles below 86°, the baseline is tilted and offset which means that the absorbance is not zero in nonabsorbing regions. Above 86°, the baseline distortions (0.02) exceed the signal strength of a common selfassembled monolayer (approximately 0.001). This spectral artifact originates in the interdependence of the reflectance and the angle of incidence. Variations in the angle of incidence can cause additional problems due to photometric errors. To minimize alignment errors, a variety of aids were tried, including a laser-aided alignment and an autocollimator to adjust the sample position. However, best results were obtained by using a very sturdy horizontal sample mount.12 Figure 2 shows examples of experimental RAIR spectra (solid curve) at 80° and 88°, respectively. The errors in the angles of incidence are 0.14° and 0.3°, respectively. Smaller changes in the angles of incidence could not be measured accurately. The calculated spectra (dotted curve), based on the refractive index of gold, the actual angle of incidence, and the error of the angle, describe the baseline distortion fairly accurately. These spectra confirm the trend shown in Figure 1: at the grazing angle, baseline distortions are more prominent. Beam Divergence. The angle of incidence is defined only in the case of a collimated beam. Unless laser or synchrotron radiation is used, the required sample size becomes too large for practical purposes at the glancing angle. Using a focused beam means that the angle of incidence is no longer defined precisely. Rays at the grazing angle accentuate the intensity contour of an absorption band. The intensity contour of the model (12) Other sample-positioning devices were reported by Porter and Arndt. (a) Stole, S. M.; Porter, M. D. Appl. Spectrosc. 1990, 44, 1418. (b) Arndt, T.; Bubeck, C. Thin Solid Films 1988, 159, 443.

Thin Organic Films on Metallic Substrates

Figure 3. Intensity contour of an absorption band (NF ) 1.4i0.24, ν0 ) 1000 cm-1, full width at half-height ) 15 cm-1) for divergence half angles of the incident beam of 0°, 3°, and 6° (from bottom to top, angle of incidence ) 80°, NS ) 10-i50). Clearly, the intensity profile is accentuated if the beam divergence exceeds a few degrees.

oscillator as a function of the beam divergence half angle was computed using the mean-value theorem.13 The calculated intensity profiles of an absorption band at 80° for beam half angles 0°, 3°, and 6° are shown in Figure 3. Contributions from glancing rays of the incident probe beam enhance the signal intensity and produce a slight asymmetry of the band contour. A quantitative analysis of band contours requires an accurate knowledge of the employed optics. For higher angles of incidence, probe beam definition becomes even more critical, since rays exceeding 90° miss the sample entirely. This implies a tradeoff between signal intensity and intensity accuracy. Figure 4 shows the intensity contour of a thin film of a fluorinated silane with beam half angles of 0.6° and 6° at angles of incidence of 80° and 84°, respectively. At 84° a beam half angle of 6° enhances the band intensity by approximately 20%. Substrate Refractive Index. The intensity of the band contour depends only on the imaginary part of the complex substrate refractive index, kS, and the angle of incidence.14 Below 84° the absorbance intensity is fairly independent of kS if kS is large (kS > 20). This condition is met for most metals in the mid-IR region. Under these conditions, the spectral envelop is fairly independent of the underlying substrate. Above 85° the absorbance is also a function of kS.15 This has three consequences. First, a quantitative analysis of spectral contours requires an accurate knowledge of the substrate refractive index. Second, bands at low wavelengths will appear relatively stronger compared to transmission spectra, since kS typically increases toward the far-IR region for conducting substrates.16 Third, the spectral envelope of the absorption (13) Kortu¨m, G. Reflectance Spectroscopy; Springer: Berlin, 1969. (14) Greenler presented calculations for 19 different metals for moderate absorbers in the mid-IR region as a function of the angle of incidence and number of reflections (Greenler, R. G. J. Vac. Sci. Technol. 1975, 12, 1410). (15) Tobin, R. G. Phys. Rev. B 1992, 45, 12110.

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Figure 4. Band contour of RAIR spectra as a function of the beam divergence of the incident beam. The angles of incidence are 80° (bottom) and 84° (top). The beam divergence half angle is 6° (dotted curves) and 0.6° (solid curves). The substrate is evaporated gold, and the film is (3,3,3-trifluoropropyl)trichlorosilane. These spectra confirm the calculated data of Figure 3: the intensity contour is accentuated if the beam divergence exceeds a few degrees.

band can show optically induced shifts depending on the particular substrate.17 The minimum thickness of an evaporated metal film on a solid support should exceed 100 nm for an angle of incidence of 85°; otherwise, there can be additional optical phenomena due to backreflection from the underlying support.18 Heterogeneities of the refractive index across the sample, caused by patches of oxides or different crystallographic faces, affect the reflectance. If the sample is removed from the spectrometer between measurements, the area of illumination can be different after repositioning the sample in the spectrometer. The effect of variations of the real and imaginary parts of the substrate refractive index by 2% on the RAIR baseline as a function of the angle of incidence is shown in Figure 5. At high angles of incidence, minor changes in reflectance cause serious baseline distortions. At small angles, the baseline is tilted and offset. Charge-transfer interactions can also alter the optical properties of the underlying substrate. However, these effects play only a minor role in thin organic films. Overlayers. Contaminants even in submonolayer concentrations will cause unwanted bands in the RAIR spectrum; therefore, various surface cleaning strategies can be employed to eliminate contaminants. Many metal surfaces have naturally occurring oxide overlayers that show Reststrahlen bands in the mid-IR region.19 Such bands introduce a series of optical effects, such as the (16) The refractive index of gold is 2-i20 at 3500 cm-1 and 10-i55 at 1000 cm-1. (17) (a) Porter, M. D.; Bright, T. B.; Allara, D. L. Anal. Chem. 1986, 58, 2461. (b) Wong, J. S.; Yen, Y.-S. Appl. Spectrosc. 1988, 42, 598. (c) Allara, D. L.; Baca, A.; Pride, C. A. Macromolecules 1978, 11, 1215. (d) Kobayashi, Y.; Ogino, T. Appl. Surf. Sci. 1996, 100/101, 407. (18) The transmitted field is a function of the angle of incidence and has a maximum at oblique angle (Fry, T. C. J. Opt. Soc. Am. 1928, 16, 1). (19) Ditchburn, R. W. Light; Dover Publications: New York, 1991.

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before and after plasma cleaning. The bottom spectra show the aluminum oxide Reststrahlen bands of the substrate before (b) and after (a) cleaning (solid curves). A gold reference wafer was used because it shows no absorption in this region. The dotted curves are computed RAIR spectra of the Al/Al2O3 interface. The distorted baseline is a result of a slight change in overlayer morphology induced by sample cleaning. These kinds of samples require careful attention to cleaning strategies to reduce interfering contaminants and to bypass spectral artifacts induced by the cleaning procedure. Static Linear Polarization Modulation. Experimental difficulties discussed above can be largely eliminated if the protocol of the RAIR measurement is modified. One possibility is to take advantage of the fact that only the p-polarized component of the probe beam carries the spectral information of the adsorbate. The absorbance is computed according to Figure 5. Effect of small fluctuations of the substrate refractive index on the RAIR baseline, -log[RS/RR], as a function of the angle of incidence in the range from 80° to 89° (for blank gold wafers, no film is present for clarity). The complex refractive indexes of the reference and sample wafers differ by 2% in the real and imaginary parts. At high angles of incidence distortions are more prominent.

Figure 6. Effect of an overlayer on the RAIR baseline. The top shows the RAIR spectra (solid curve) obtained from single beam spectra of an aluminum substrate before and after cleaning. The two bottom spectra (solid curve) show the aluminum oxide Reststrahlen bands of the substrate before (b) and after (a) cleaning (p-polarized radiation, gold reference substrate, angle of incidence 84°, plasma cleaning). The dotted curves are computed RAIR spectra based on the optical constants of the Al/Al2O3 interface. The baseline distortion of the top spectrum is clearly a result of a slight change in the optical properties induced by sample cleaning.

A ) -log [R|/R⊥] where R| is the reflectance of p-polarized radiation and R⊥ is the reflectance of s-polarized radiation. The spectrum of the adsorbate is recorded by successively accumulating scans with the polarizer azimuth set at 0° and 90°, respectively. Since the linear polarized radiation is used, the method is referred to as static linear polarization modulation. The history and applications of polarization modulation techniques in surface IR analysis have been reviewed by Richmond.21 In this mode the spectral information of the adsorbate is directly obtained in one measurement from the samplesubstrate measurement; no independent reference reflectance is needed. The advantage is that the sample remains in the spectrometer while the p and s reflectances are recorded; therefore, the optical path remains unchanged. The possible spectral range is limited only by the beam splitter and the polarizer material used; the technique is easily implemented by adding a rotating mechanism to the polarizer. The polarization introduced by a Michaelson interferometer can be eliminated in different ways: by using a polarizer with a fixed azimuth of 45° in front of the modulated polarizer, by applying an electronic baseline correction, or by making a reference measurement. In addition, static polarization modulation in short intervals eliminates contributions from atmospheric bands. Figure 7 shows the RAIR spectra of a self-assembled monolayer recorded in the conventional mode (RS/RR) and with static linear polarization modulation. The two methods produce identical spectra. Experimental Section RAIR spectra were recorded with a modified reflection accessory from Harrick in a Bruker IFS 66 spectrometer using a liquid-nitrogen-cooled MCT detector. Evaporated gold on silicon or glass slides were used. Thin films of 1-octadecylthiol and (3,3,3-trifluoropropyl)trichlorosilane were made by absorption from dilute solution. Static-polarization-modulated RAIR spectra were recorded by successively accumulating 20 scans with the polarized set at 0° or 90°. A total of 200 scans were recorded. Aluminum/aluminum oxide wafers were treated in an air plasma.

Berreman effect.20 RAIR spectra with such substrates are sensitive to preparation conditions, overlayer morphology, purity, and surface roughness. Figure 6 shows a typical example encountered with aluminum substrates. An evaporated aluminum film on a glass wafer was cleaned to remove contaminants. The top curve shows the RAIR spectrum obtained from single beam spectra of the aluminum substrate (solid curve)

When RAIR spectroscopy is applied to surface analytical problems, many interesting experiments require that the

(20) Ro¨seler, A. Analytiker-Taschenbuch, B14; Springer: New York, 1996.

(21) Richmond, W. N.; Faguy, P. W.; Jackson, R. S.; Weibel, S. C. Anal. Chem. 1996, 68, 621-628.

Conclusion

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Figure 7. RAIR spectra of a self-assembled monolayer on gold sampled in two different ways: (bottom) -log[RS/RR]; (top) -log[R|/R⊥]. (RS, p-polarized reflectance of the sample; RR, ppolarized reflectance of the reference substrate; R|, reflectance of p-polarized radiation; R⊥, reflectance of s-polarized radiation of the sample-substrate interface.) The angle of incidence is 84°. This section of the spectrum shows that the two methods produce identical results concerning the spectral contour within the experimental errors. In addition, polarization modulation reduces contributions of atmospheric bands (not shown).

sample be removed from the spectrometer between measurements, e.g., for surface derivatization. This mode of operation can result in artifacts that can obscure spectral interpretation. Two conclusions can be drawn from this work. The first is that spectral artifacts depend critically on sample positioning, and the second is that artifacts are more prominent at high angles of incidence. This last point implies a practical limit for the angle of incidence since adequate signal-to-noise ratios for monolayer studies require the highest possible angle. For thin organic films on conducting substrates, an angle of incidence of approximately 84° is an optimum value. The signal is sufficiently strong to achieve an adequate signal-to-noise ratio.

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Variations in the angle of incidence cause baseline distortions because the reflectance is a function of the angle of incidence and because photometric errors are due to beam misalignment. Positioning accuracy is critical if the angle of incidence exceeds 84°. For smaller angles, errors in positioning result in a tilted baseline that can be corrected by an electronic baseline correction. If the sample is removed from the spectrometer, the area of illumination on the sample can vary at different stages in the experiment. Heterogeneities of the substrate refractive index can then introduce spectral artifacts. Quantitative band contour analysis demands an accurate knowledge of the employed optics. If the divergence half angle of the incident probe beam is small, computed band contours based on a single angle of incidence will produce sufficiently accurate results. For high-quality spectra, reference and sample measurements should be taken from the same wafer. This reduces baseline distortions due to variations of the optical constants of the materials. Cleaning procedures require careful attention. Cleaning must successfully remove surface contaminants that cause unwanted bands in the RAIR spectrum, but it should not alter the optical properties of the substrate. In particular, substrates with an overlayer showing a Reststrahlen band can induce severe baseline distortions. Most of these problems can be greatly diminished if the spectral information of the adsorbate is retrieved from static-polarization-modulated spectra. In this mode of operation, successive scans of p-polarized (R|) and spolarized (R⊥) radiation are accumulated. This method takes advantage of the fact that the spectral information sought is in the p-polarized component of the incident radiation; the s-polarized component is used as the reference reflectance. The spectral information is recovered by computing -log[R|/R⊥]. An experimental comparison with conventional RAIR spectra showed that both modes produce identical spectra. Acknowledgment. The author is very grateful to Prof. H. Mo¨hwald for valuable discussions and financial support and to H. Krass for providing the experimental data. LA970903E