Langmuir 1991, 7,2443-2449
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Electron Energy Loss Spectroscopy of Real Surfaces R.J. Purtell' and M. Pomerantz IBM Research Division, T.J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 Received July 2, 1990. I n Final Form: October 16, 1990 High-resolutionelectron energy loss spectroscopy (EELS) was applied to a sputtered carbon (spC) film onto which stearicacid had been sublimed,but which had been allowed to be exposedto ordinaryatmospheric conditions. This sample is a model 'real" material as it simulates conditions that might be found in practical materials, such as a lubricated surface. In order to apply EELS it was necessary to understand the signals from the substrate and the adsorbate, so various additional diagnostic experiments were made on Ag and ordered graphite surfaces. Comparison with known organic overlayers shows the presence of sp3 bonded hydrocarbons in the sp-C substrates. Despite the interference from such contaminants, it is possible to detect the spectrum of stearic acid that was sublimed onto sp-C at -150 "C under vacuum. The presence in the EELS spectra of stearic acid of Raman and infrared active modes indicates the importance of impact excitation of the EELS. Spectra of stearic acid sublimed onto highly oriented pyrolytic graphite (HOPG) were used to separate chemical versus geometrical orientation effects in the C=O stretch mode of the acid group. Evidence for undissociated acid groups of stearic acid on graphite is indicated by the presence of the C=O stretch at 212 meV. Comparison of the intensity of the C=O stretch mode of stearic acid sublimed onto HOPG to that of a published spectrum of Langmuir-Blodgett films of vertically oriented stearic acid indicates that the molecules are lying down with the acid groups near the surface in the sublimed overlayers of stearic acid on HOPG. A similar effect is not observed for molecules on a sp-C surface where substrate disorder and roughness average out orientational effects in the spectra.
Introduction High-resolution electron energy loss spectroscopy (EELS) is a technique for obtaining information about vibrational modes in the surface and near surface region of a material. A low-energy electron beam of typically 5 eV is directed at the material and the scattered electrons are energy analyzed in a detector which can collect electrons in a specular or nonspecular direction. By measurement of the flux and angular distribution of electrons which have lost energy to the vibrational modes of the material, the type and symmetry of the modes excited can be determined. This information can then be used along with other spectroscopic techniques to deduce what types of molecules or functional groups are present in the surface layers. The electron energy losses due to molecules on surfaces were first observed' in 1967. In order to understand the technique it was first applied under the most idealized conditions possible: under ultrahigh vacuum (UHV) with small molecules on single crystal surfaces.2-5 I t proved to be possible to obtain information about the chemical states and bonding sites of molecules on surfaces by measuring the chemical shifts and symmetry of the vibrational modes of the molecules. Catalytic surfaces were simulated by studying small molecules on metal oxidesas There have also been EELS studies of insulators and semiconductors! as well as metals deposited on semiconductors7 and
* To whom correspondence should be addressed.
(1) Propst, F. M.; Piper, T. C. J. Vac. Sci. Technol. 1967,4, 53. (2) Ibach, H.; Mille,D. L. In Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982. (3) Chestera, M. A. J. Electron Spectrosc. Relat. Phenom. 1986, 38, 123-140. (4)Gadzuk, J. W. In Vibrational Spectroscopy of Molecules on Surfaces Methods of Surface Characterization; Yates, J. T., Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987. (5) Froitzheim, H. Electron Energy Lose Spectroscopy. In Topica in Current Physics Electron Spectroscopy for Surface Analysis; Ibach, H., Ed.;Springer-Verlag: New York, 1977; p 205. (6) Thiry, P. A.; Liehr, M.;Pireaux, J. J.; Caudano, R. Phys. Rev. E Condens. Matter 1984,29,4024.
polymers.s More recently, EELS has been applied to organic surfaces such as polymersgand Langmuir-Blodgett filmslo that were not prepared in UHV. This paper is an investigation of how to extend EELS to the case in which one is unable to carefully choose the surfaces or the molecules in advance. Instead, we presume we are given a sample of an adsorbate, which is a large organic molecule, on a substrate that is complex, and the material has been exposed to ordinary atmospheric conditions. We then seek to identify the materials on the surface. In this sense we are trying to apply EELS to "real" surfaces that are important in technological or scientific work. In order to do this we shall draw upon the information that has been amassed about well-controlled substances and substrates. We report also some new results on stearic acid on ordered graphite and Ag surfaces and ordered films that will help in the analysis of less controlled unknowns. The starting point for understanding EELS is the electromagnetic spectroscopies, infrared absorption (IR) and Raman scattering. One tries to identify an energy loss suffered by the electron with the energy of a vibrational mode that has been observed by electromagnetic interactions. Electrons can excite either dipole or Raman active modes, depending on the interaction mechanism. The principal mechanisms of energy loss by a low-energy electron'' to the vibrational excitations of a molecule can be classified as dipolar or impact. The dipole mechanism arises from the long range electric field of the electron that excites infrared-active modes. Electrons passing within about 60 A of a molecule may excite a vibration and thus lose energy, via this dipolar process. The momentum transfer is relatively small parallel to the (7) Dubois, L. H.; Schwartz, G . P.; Camley, R. E.; Mille, D. L. J. Vac. Sci. Technol., A 1984,2,1086. (8) Dinardo, N. J.; Demuth, J.; Clarke, T. C. Chem. Phys. Lett. 1986, 121, 239. (9) Pireaux, J. J.;Thiry, P. A.; Caudano, R.;Pfluger, P. J. Chem. Phys. 1986, 84, 6452. (10) Wandass, J. H.; Gardella, J. A., Jr. Langmuir 1986,2, 543-548. (11) Reference 4, pp 49-103.
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surface, so that the scattering is approximately specular. If the surface is disordered, however, the scattering may appear diffuse at the macroscopicscale.12 The IR selection rules can also be used to determine the orientation of the functional groups giving rise to the transition. Metallic surfaces enhance absorption by modes perpendicular to the surface and quench those from parallel modes.13 This orientational effect does not occur when the substrate conductivity or polarizability is small. Electrons may also lose energy by an impact scattering mechanism. In one such process the electron effectively enters a molecule, excites it to a negative ion state, and then exits with a loss of energy to the vibrational modes.14 The depth over which this interaction occurs thus depends on the electron mean free path in the material; it tends to be operative through a depth of perhaps 3-25 A. (For some results on mean free paths, see refs 9 and 15.) Both IR- and Raman-active vibrations may cause energy losses of the incident electron. The electron incurs a large momentum loss in an impact excitation so the scattering direction is diffuse. It has also been found by Pireaux et al.9that the probability of a loss process by impact depends on the incident kinetic energy differently from a dipolar excitation. Another mechanism that involves close approach is the excitation of multipole moments higher than dipolar. Thus, in the ideal case of smooth, single crystal metal substrates one has several clues for the characterization of an EELS loss feature: specular (dipole) vs nonspecular (impact). If impact, it may be either a dipole or Ramanactive vibration. If dipolar, the scattering may arise from a greater depth than for impact scattering, so knowledge of the mechanism helps in depth profiling. On metals, the orientation may be obtained because only the perpendicular component of the dipole interaction is activated. In real samples, one faces several problems that might limit the effectiveness of EELS: External Contamination. It may not be possible to control the chemicals that adsorb in addition to those applied intentionally. Roughness and Polycrystallinity. The substrates are disordered so that the scattering is diffuse, even for dipole scattering. This makes it more difficult to determine the mechanism of scattering. Heterogeneity of the Adsorbates and Substrates. There may be a variety of molecules applied as well as reaction products. There may be a variety of binding sites and coverages. (The line shape is known to change with high coverage.) In this report, we investigate the extent to which these difficulties may be ameliorated by a detailed study of the material and by taking advantage of the unique sensitivity of EELS compared to infrared spectroscopyS2 We also compare the results to X-ray photoelectron spectroscopy (XPS) which provides complementary information about functional groups. As a prototype for a "real" sample, we use a surface of sputtered carbon with an adsorbed layer of stearic acid. This kind of surface may be suitable as a protective coating for some applications, and thus has practical interest.'6 (12) Knoll, W.; Philpott, M. R.; Golden, W. G.J. Chem. Phys. 1982, 77,214-225. (13) Ibach, H. Surf. Sci. 1977,66,56-66. See also Ibach and Mills, p 92. (14) Demuth, J. E.; Avouris, P. H.; Schmeiseer,D. J . Electron Spectrosc. Relat. Phenom. 1983,29, 163-174. (15) Cartier, E.; Pfluger, P.; Pireaux,J. J.; Vilar, M. R. Appl. Phys. A 1987, A44, 43-53.
F'urtell and Pomerantz But sputtered C (sp-Chenceforth) has many of the nasty properties one might fear to encounter: it is rough, perhaps on the 100 nm scale; it has intermediate electrical conductivity of about 5 s2 cm; because of the sputtering process a variety of impurities may be incorporated; the substrate C is the same element that is found in organic contaminants and adsorbates, making it difficult to do elemental analysis. The model adsorbate, stearic acid (C&uOOH), is representative of a class of molecules used for 1ubrication.l' Using vibrational spectroscopy, we shall try to assess the chemical composition as well as the orientation of these molecules on the surface, which is likely to be important for the tribological function.18
Experimental Section The data were taken with a Leybold Heraeus surfaceanalysis system which includes an EELS-22 and EA-11 spectrometer in which EELS and XPS measurements, respectively, could be made. Samples were mounted on a manipulator that is capable of both translation and rotation. They were then transferred into an ultrahigh vacuum of 10-loTorr. A very useful feature of our system is that sample handling in the nitrogen atmosphere of a glovebox is available, as a means of reducing contamination. Our instrument also has the capability of cooling the sample to close to -150 O C . EELS analyzer deflection energies were typically 300 meV, with an instrumental resolution of 7 meV. (Experimentalerror in absolute loss energies is 1% ' , based on ramp calibration and comparison to literaturevalues.) The electron beam was incident at 60° from the sample normal and the detector could be rotated to collect electrons as a function of angle. The electron beam irradiates an area of approximately0.3 by 3 mm2with currents to the sample of 5 X 10-11 A. This results in a current density A/mma. at the sample of 4.5 x Carbon films about 250A thick were deposited by dc sputtering from a compressed graphite target, using argon as a sputtering gas. Chemical vapor deposition (CVD) carbon substrates were made by plasma decomposing acetylene with a rf discharge. Highly oriented pyrolytic graphite (HOPG)surfaceswere peeled in a nitrogen atmosphere to expose a fresh surface of the basal plane of graphite. They were then introduced to the UHV environmentand checked for oxygen contamination by XPS. If oxygen was found they were either repeeled or sputtered with argon ions at 3 kV. Silver surfaceswere also cleaned by sputtering in argon until no carbon or oxygen was present. A vacuum sublimation system was set up so that stearic acid could be deposited on the various surfacesunder UHV conditions. The stearic acid used was 99+% pure (mp 67-69 O C ) from Aldrich Chemical Co. The crystalline powder was deposited on a glass slide, which was heated in a glovebox until the stearicacid melted on the surface. The slide was allowed to cool to room temperature and the stearic acid formed a crystallinecoating on it. This slide was then mounted on a manipulator, transferred into the vacuum system,and positioned a centimeter in front of a sp-C substrate cooled to -150 O C . A similar procedure was followed with the silver substrate. The coverage of stearicacid on the silver surface was estimated by monitoring the C Is/(& 3d) XPS ratio as a function of emission angle. These measurements yielded a ratio of thickness to photoelectron escape depth of between 0.77 and 0.28. For average photoelectronenergies of 1100 eV with escape depths of 20 A,le this corresponds to an average overall coverage of stearic acid of between 5 and 15 A, which, if the molecules were standing on end, would be less than a monolayer. The substratewas held at -150 "C during the measurement. ~~
(16) Seki, H.; McClelland, G. MTBullock, D. C. Wear 1987,116,381. (17) Klamann, D. In Lubricants;V e r b Chemie GmbH: Weinheim, FRG,1984. (18) Buckley, D. H. In Surface Effect8 in Adherion, fiction, Wear and Lubrication;Ebevier: Amsterdam, Oxford, New York, 1981. (19) Powell, C. J. Surf. Sci. 1974,44, 29.
Langmuir, Vol. 7, No.11, 1991 2445
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Figure 1. EELS of stearic acid on sp-C and silver. EELS conditions: Eo = 5 eV e-beam incident 60" from normal, specular detection. Spectrometer resolution= 7 meV. Work function change measured by secondarycutoff of sample biased to -45 V. T = -150 "C. Curve A: As-received sp-C. Curve B: Stearic acid/sp-C. In situ dosing. Work function change -0.2 V. Curve C: Same as B, dose to -1.3 V. Curve D: Same as B, except silver substrate. The coverages of stearic acid on carbon versus silver surfaces were estimated by comparing the work function change with stearic acid deposition. Work function changes were monitored by measuring the change in cutoff of the secondary electrons from an X-ray irradiated sample biased to -45 V. The sample was biased negatively to prevent stray electrons in the chamber from interfering with the measurement.
Results and Discussion EELS measurements were made on as-received, lubricated, sputtered carbon films in order to determine what the technique could tell about a complex real world surface. Measurements were then made on pieces of this system and compared to model systems in order to better understand the components. The results are reported in the next three sections. Lubricant on sp-Carbon. In order to sublime a small amount of stearic acid on the sp-C surfaces, the sp-C was exposed under nitrogen to a heated beaker containing stearic acid on its sides. Ellipsometry gave an average thickness of 4 A, before transfer into UHV. The EELS spectrum was virtually identical with that for an uncoated substrate, which is shown in Figure 1A. This could be because the stearic acid was either lost to the vacuum or because such an amount of stearic acid does not produce a visible change in the spectrum relative to the contribution from the substrate. In situ vacuum sublimation of stearic acid on the carbon substrate maintained at -150 "C, however, produces a more complex spectrum than the as-received sp-C samples, with narrow peaks appearing on top of the broad carbon features, as seen in Figure 1B,C. Stearic acid sublimated on to a silver surface is shown in Figure 1D.These spectra show sharp structures due to stearic acid on top of the broad peak at 170 meV and the CH stretch at 366 meV. The major difference between the stearic acid spectra on the sp-C and silver surfaces of Figure 1is the intensity of the C-H stretch. This mode is stronger on carbon substrates than on silver, either because there is an orientational effect of the stearic acid molecule or because the conduction electrons in the substrate screen this mode better on a silver surface. It is expected theoretically that dipoles oriented parallel to a metal surface are screened by the conduction electrons, whereas dipoles in the perpendicular orientation are enhanced by free carriers."
There may also be some contribution to the C-H stretch region from hydrocarbons buried in the underlying carbon layer. This comparison shows the need for separating geometrical orientation and substrate screening from chemical effects in interpreting EELS data. Studies were therefore made of substrates under different conditions and comparisons made to model systems. The lubricants were also studied on model substrates in order to better understand contributions from the lube molecule itself. These results are reported in the next two sections. Analysis of the Substrate. Any attempt to understand spectroscopic transitions due to molecules adsorbed on a complicated real world surface must take into account contributions from the underlying substrate itself. Substrate features may obscure the signals from the overlayer but may also provide some information about the substrate itself. We therefore made measurements on as-received (uncoated) sp-C substrates and compared them to measurements on Raman spectra of sputtered carbon and EELS spectra of carbon as HOPG. We also compared these spectra to those of organic materials of known compositions to further understand the hydrocarbon portion of the substrate. EELS spectra of dc-sputtered carbon and HOPG are shown in Figure 2. The spectra from HOPG and sp-C were scaled to the same intensity at the elastic peak energy; however the intensity of the elastic peak is a factor of 100 lower from the sp-C sample than from an ordered surface such as HOPG. This effect is a combination of the fact that the graphite surface is much smoother and the fact that it is ordered. The ratio of loss features to elastic intensity is also higher on sp-C vs small molecules on metal surfaces. Wandass and Gardellampoint out that this effect may be due to a larger sampling volume in an organic surface which has several atomic layers contributing to the signal. Thus, within the range of surface sensitivity of the technique there are more functional groups giving rise to EELS loss features on these types of samples than there would be in a monolayer of gas on a metal surface. This could result in a larger ratio of loss features to elastic beam intensity. As seen in Figure 2, the EELS spectrum of sp-C has two broad features centered at 176meV and a t 366 meV. These loss peaks have, respectively, the energies of the bending and stretching modes of C-C and C-0 groups and C-H vibrations.21 The feature a t 176 meV also overlaps the Raman spectrum of sp-C taken by Seki et al.,lSas shown in the middle curve of Figure 2. This suggests that Raman-active modes could contribute to the EELS beam, implying an impact mechanism of excitation. Unfortunately a corresponding Fourier transform infrared (FTIR) spectrum is not available for sp-C. The presence of these inherently broader loss features from such chemically complex surfaces as sp-C changes the balance between instrumental resolution and sensitivity needed for optimal signal to noise from that used for small molecules on single crystal surfaces. Wandass and GardellaZ0have shown that increasing the spectrometer sensitivity (from that used in Figure 2) to get better signal to noise will result in lower resolution, which will only obscure features that may appear on these broad peaks. Likewise, improving the analyzer resolution from 7 to 5 meV does not show any additional features. Thus (20) Wanda, J. H.;Gardella, J. A., Jr. Surf. Sci. 1985, 150, L107L114. (21) Shimanouchi, T. Tables of Vibrational Frequencies: J. Phys. Chem. Rev. Data 1977, 6, 993. Consolidated Vol. 1, NSRDS-NBS 39.
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Figure 3. Dashed curve: EELS of acetic acid on silver (2' = -150 "C). Dotted curve: EELS of polyimide spun on silicon, annealed to ca. 250 O C . Solid curve: EELS of stearic acid on silver (T= -150 "C). In situ dosing of acids. Other conditions same as Figure 1.
with these broad lines there is no advantage to run the spectrometer at its highest resolution where the signal would be low. L 384 - CVD Carbon The lowest curve in Figure 2 is our measurement of the Acetic acid EELS spectrum of HOPG, which does not show a mode ....... Stearic acid 366; Polyimide at 197 meV, as in the Raman spectrum of graphite foil seen by Seki et al.,l6 or a t 195 meV as for single-crystal graphite, as seen by Tunistra et a1.22 The absence of a Raman mode in the EELS spectrum of smooth, clean HOPG is puzzling. It may just be disallowed in EELS by a selection rule for Raman modes a t specular ~ c a t t e r i n g . ~ ~ There is however an inelastic background due to electronhole pair excitations in this spectrum, as previously noted by Palmer et al.24 While the EELS features (of deposited carbon films) 200 250 300 350 400 450 500 are too broad to provide a detailed chemical description Energy (mev) of the surface, information about the type of hydrocarbons Figure 4. Blowup of Figure 3 in C-H stretch region compared present in the surface region can be gained by comparing to EELS of CVD carbon. CVD carbon produced by plasma this spectrum to those of known organic materials, as has decomposition of acetylene. been done for molecules on single crystal surfacesa2 Organic layers with different kinds of bonding of carbon I ' I to hydrogen were coated on polycrystalline Ag (again to avoid interference from substrate C for these tests). The ,--. 10 EELS spectra of methyl, aromatic, and methylenic C are . c_ shown in Figure 3. The substrates were held at ca. -150 3 O C . The spectra were taken at 7 meV instrumental resolution. In the case of polyimide, the material was spun $ on silicon and annealed in UHV. The CH2 (methylene) 10 + groups in the stearic acid overlayer appear at 362 meV. C 3 The CH3 (methyl) group of acetic acid appears at 374 meV 0 0 and the aromatic CH groups in polyimide at 384 meV. The locations of these EELS features are consistent with the positions of the centers of the CH stretch modes in the 50 100 150 200 250 300 350 400 450 500 IR spectra2t21of these materials. These data show that Energy (mev) EELS spectra of polycrystalline surfaces still have resolution sufficient to distinguish among different types of Figure 5. EELS of sputtered C, as received and after heating C-H bonds and thus do some chemical analysis. to approximately 100 O C (heat 1) and 200 O C (heat 2). Other conditions same as Figure 1. Figure 4 shows a blowup of the CH region for the above three materials and CVD carbon. The CH groups in the CVD carbon spectrum overlap more strongly with peaks The sensitivity of EELS to the chemical bonding also from the saturated hydrocarbons than with the peak from allows the effect of different methods of deposition on the an aromatic CH group. This is evidence for sp3 bonding chemical nature of the C coating to be seen. Figure 5 of the hydrocarbon portion of the CVD C. This does not shows the EELS spectra of sp-C, and the changes produced exclude the presence of sp2 bonded carbon in the bulk by heating. By comparison, C prepared by chemical vapor material, which is graphitic in nature and does not involve deposition (CVD) and similarly heat treated is shown in bonds to hydrogen. Figure 6. The relative intensity of the CH stretch modes (22) Tuinstra, F.; Koenig, J. L. J. Chem. Phys. 1970,53, 1126. in these two materials at room temperature is virtually (23) Tsang, J. Private communications. identical as seen by EELS. This is surprising because (24) Palmer, R. E.; Annett, J. F.; Willie, R. F. Phys. Reo. Lett. 1987, 58, 2490. measurements by SIMS and nuclear backscattering have .
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Langmuir, Vol. 7,No. 11, 1991 2447
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Figure 7. Solid curve: EELS of as-receivedsp-C. Dotted curve: EELS of sp-Cafter DCM rinse. Other conditionssame as Figure 1.
shown25 that CVD carbon has a higher bulk hydrogen content than sp-C, 20 atom 5% versus ca. 2 atom 9%. EELS seems to be saying that the hydrogen content in the surface is different from that in the bulk. The depth of these C-H stretch modes is most likely 25 A or less since they are probably impact excited.2s Unfortunately, uncertainty in the depth sensitivity of these spectra is worse than what one would like for tribological studies, which makes it difficult to determine if these features are due to the sp-C itself or to adsorbates contaminating the surface. In one instance, however, a dichloromethane rinse prior to transferring into UHV produced a spectrum with a reduction in intensity of the feature at 366 meV, presumably due to reduction in the amount of hydrocarbons adsorbed on the surface. (See Figure 7.) Figures 5 and 6 also show the effects of a heat treatment on the EELS spectra of sp-C and CVD carbon surfaces. The heater power was the same for bothsamples. Although the temperature was not measured directly, we estimate that the temperature reached about 200 "C. I t is evident from the spectra that heating causes greater removal of hydrocarbon groups from the sp-C surface than from the CVD film. This may be because they are less strongly bound or that there is a larger reservoir of hydrocarbons in the bulk of the CVD film to replenish the surface when it is depleted by heating. The power and complementarity of EELS compared to the more traditional XPS measurements are demonstrated ~
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(25) Ruseack, M. Private communications. (26) Backx, C.; deGroot, C.; Biloen, P. Surf. Sci. 1980, 6, 256-272.
Figure 8. X-ray photoelectron spectra of sp-C and stearic acid on sp-C. L-H 11spectrometerat 25 eV pass energy. Magnesium anode. Work function measured by secondary cutoff of sample biased to -45 V.
by comparing the data taken from the sp-C surface by both techniques. The EELS data of Figure 2 show two clear vibrations that can be attributed to C bonded to either C, 0, or H, whereas the XPS spectrum (Figure 8) of the C 1s peak shows some broadening and shift that merely hints at the kind of bonding the C has undergone. XPS could not have seen the changes in hydrocarbon content of the carbon films in Figures 5 and 6 since different kinds of C-H bonds cannot be resolved from C-C bonds. However, in the XPS data there is an indication of a peak at a binding energy of -298 eV, characteristic of the COOH carbon. This vibration was not clearly detected by EELS on this sp-C surface. Likewise, SIMS would have caused destruction of the surface region in order to detect C-H fragments and it would be difficult to deduce what type of hydrocarbon bonding configuration they were originally involved in on the surface. Finally, forward recoil spectroscopy, a technique used in ion beam analysis, could have detected hydrogen with a depth resolution of 800 A but could not verify that it was bonded to carbon.27 Lubricants on Model Substrates. As shown above, EELS measurements of as-received carbon lubricated with stearic acid show the presence of a peak attributable to CH2 and CH3 vibrations but not strong modes, such as the C = O stretch, that distinguish the lubricant molecule from the carbon substrate. T o get a picture of the lube modes, EELS measurements of the lubricant on a noninterfering silver substrate were made. As we now consider the details of the EELS spectrum of stearic acid on the silver surface, we can turn to the literature of infrared and Raman data on stearic acid to determine the source of various peaks. Figure 9 shows the EELS spectrum of stearic acid, (which we measured on a Ag substrate in order to avoid interference from the C-C and C-H losses of the sp-C surface). For comparison we show the Raman spectra of solid stearic acid (taken at 1 cm-l resolution by Lippert, et a1.28 and Verma and Wallach9. The infrared spectrum of stearic acid on silicon was taken by D. Beach (private communication) using a Nicolet 5PC FTIR at 8 cm-I resolution. Since it is not known what the relative sensitivity factors are for these techniques for a given sample, the relative scaling of the spectra in Figure 9 is arbitrary. If the EELS spectrum were sensitive only to dipole active modes, the ~~~
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(27) Green, P.; Mills, P.; Kramer, E. Polymer 1986,27, 1063. (28) Lippett, J. L.; Peticolas, W. L. Biochim. Biophys. Acta 1972,282, 8-27. (29) Verma, S. P.; Wallach, D. F. Biochim. Biophys. Acta 1977,486, 217-227.
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2448 Langmuir, Vol. 7,No. 11, 1991
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Figure 9. Vibrational spectra of stearic acid. Top curve: EELS of 5-10 A stearic acid on polycrystalline silver. Other conditions same as Figure 1. Curve 2: Sum of Raman and FTIR of bulk stearicacid. Curves 3,4,and 5: Raman spectram* of bulk stearic acid at room temperature. Bottom curve: FTIR spectrum of skim of stearic acid on silicon by D. Beach.
EELS and IR spectra should be identical except for resolution differences between the techniques. Clearly, there are major differences in the relative peak intensities in the C-C bending and stretching regions (ca. 180meV) compared to the C-H stretch (360meV), and the C=O stretch a t 212 meV, which is strong in the IR spectrum but is unseen in the EELS. However, the sum of the FTIR and Raman spectra for stearic acid shown in curve 2 of Figure 9 is comparable to the EELS spectrum. Both spectra have the same overall shape, with the broadening of the EELS spectrum relative to the sum of the optical spectra probably due to instrumental broadening. The fact that the EELS spectrum in Figure 7 looks like the sum of the IR and Raman active modes, except for the C-0 stretch at 212 meV, is strong evidence that the excitation mechanism involves both IR and Raman active modes. The mechanism that does this is impact scattering. It could be argued that the large peak in EELS intensity centered at 176 meV is due to beam damage since it does not correspond to any strong IR modes. The existence of Raman peaks in this area, shown in Figure 9, is evidence that this need not be the case. We have abundant evidence of the importance of impact scattering, which could lead to Raman mode excitation. A series of EELS spectra as a function of impact could test this further. We have made additional measurements of stearic acid on an ordered HOPG substrate where the dipole induced peaks should peak along the specular direction, as the elastic beam does. This allows separation of chemical versus geometricalorientation effects of the stearic acid molecule. Stearic acid was therefore sublimated onto HOPG graphite, which has flat layers of relatively pure carbon. This surface should allow specular scattering and also not introduce C-Hvibrations from the substrate. Molecules of stearic acid were deposited for 45min, and the substrates were maintained at a temperature of -150 "C. The data, shown in Figure 10,were taken with electrons incident at 60' from the sample normal, but with two different collection angles. For Figure 10a the scattered electrons were collected at 57' and not at 60' because at specular scattering electron-hole pair excitations dominate the spectrum22and swamp out contributions from the stearic acid molecules. Spectrum 10b was taken with a collection angle of 37.4' from normal. The fact that features in the C-C and C-0 bending and stretching region become ~
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(30)Kesmodel, L.Phys. Rev. Lett. 1984,53, 1001-1004.
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Figure 10. EELS of stearic acid on HOPG graphite at different collection angles. Wf.change -1.3 eV, T = -150 "C. Other conditions same as Figure 1. Peak labeling from Wandass and Gardella.lo
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Figure 11. Angular dependence of 5 eV elastic beam scattered from graphite, silver, and stearic acid on graphite. Angles measured from surface normal. EELS conditions as in Figure 1.
stronger as the detector is moved away from specular emission indicates that these modes are not due to dipole scattering. Impact modes may peak in intensity in an off-specular direction,whereas dipole active modes are maximized along the specular direction because there is only small changes in the momentum parallel to the surface, due to long range dipole interaction. This rule holds only if the elastic electron intensity is peaked along the scattering direction. If this is not so because of a disordered, rough surface, which causes scattering of the incident electron beam in multiple directions,the dipole selection rule does not apply. The angular intensities of the elastic beam for this system as well as from polycrystalline silver and HOPG are shown in Figure 11. It can be seen from this figure that as the thickness of the disordered layer increases, the intensity of the elastic beam decreases. It can be seen that the scattering is more specular from HOPG than from the Ag, as expected because the graphite is ordered. The spectrum taken near specular scattering shows a distinct loss at about 212 meV, corresponding to a C = O vibration. The fact that this feature is weak in the offspecular spectrum (Figure lob) is indicative of the dipolar coupling mechanism for this vibration. The intensity of this mode is relatively weak compared to the rest of the IR spectrum31(Figure 9). In accordance with the above (31)White,R.G. 1nHandbookofIndustrialInfraredAnulysis;Plenum Press: New York, 1964.
EELS on Real Surfaces results, this is further indication that the impact mechanism is the stronger source of interaction for most of the loss features. Off-specular, where only the impact mechanism is expected on this smooth surface, we observe in Figure 10b that the C = O mode is quite weak. This is reasonable, since the impact process should not excite the C-0 any more (or less) strongly than the C-H modes, but there are 35 times more C-H bonds in this molecule than there are C=O, so the C 4 should be relatively weak when excited by impact. The presence of the C=O stretch a t 212 meV in the EELS spectra of Figure 10 shows that at least some of the acid groups are undissociated. The intensity variation between this spectrum and that of a molecule in a known orientation can give information about the orientation of sublimed molecules. In Langmuir-Blodgett filmsit is known that molecules are oriented approximately vertically with the acid groups sandwiched between hydrocarbon chains. In the usual case (y deposition) this places the C=O group about 25 A below the surface.20 In such a case, a Langmuir-Blodgett film of stearic acid on polished Ge taken by Wandass and Gardella,lo the C=O mode is not discernible. The fact that this mode can be seen in spectra from sublimed molecules (Figure 10) can be simply understood if these molecules are randomly oriented on the graphite surface with some of the acid groups quite near the surface of the overlayer. The electrons can then more closely approach the C=O group and excite it. The C=O stretch at 212 meV, whose emission is concentrated along the specular direction in the spectra on the HOPG substrate,is averaged out in all directions on a polycrystalline substrate and tends to be swamped by the other excitations. Thus we cannot use the dipole excited C=O mode to determine experimentally the orientation of sublimed molecules on the disordered, rough sp-C surfaces.
Conclusions This study of a sputtered carbon surface with adsorbed layers of stearic acid proved to be a good simulation of a real world surface situation. The problems we had anticipated appeared contamination, roughness, interference from the substrate, conduction of the substrate affecting the visibility of the adsorbates, effects of the measurement method on the sample. However, we have found ways to overcome some of these obstacles, and point the way for the use of EELS under experimentally challenging conditions. Contamination is of two kinds, intrinsic and adventitious. An example of an intrinsic contaminant is the introduction of H into C by the sputtering process. If one is concerned with organics on the surface of sp-C, these bulk hydrocarbonswil be an interference. The way around this is either to eliminate the H in the sputtering or to label the adsorbate with a group that stands out against the background signal. The accidental contaminants can also sometimes be simply washed off, as in Figure 7. The roughness and disorder of real substrates are
Langmuir, Vol. 7, No. 11, 1991 2449
considerable hindrances. We showed that we could detect the C-0 vibration that identifies our adsorbate, but only at specular reflection of the scattered electrons from a HOPG graphite surface that is ordered. The C=O stretch is strongly excited by the dipolar process, because of the large dipole moment of this bond, and is maximized in the specular direction. This allows us to deduce molecular orientation effects and the existence of undissociated acid groups of stearic acid on a highly oriented pyrolytic graphite surface. On a rough, disordered substrate, this scattering is spread out in angle from the average surface plane. It is then superimposed on the tails of neighboring vibrations and becomes indistinct. Another deduction from our measurements is that the impact mechanism is the dominant one for organic molecules on the variety of substrates we used. As mentioned above, we have evidence of some dipolar excitations but these seem superimposed on the impact excitation. This means that even in the case of smooth surfaces, EELS will not give information about moleclar orientation as easily as might be determined from IR selection rules for molecules on metals. This was already evident in the work of Wandass and Gardella'O who observed a strong C-H absorption by a LangmuirBlodgett film on Ag. If the mechanism were dipolar, the C-H absorptions would have been surpressed because they are parallel to a metal surface. Their strong presence shows that the excitation is by an impact mechanism.The dominance of the impact process in their case was further evidenced by the virtual absence of the C=O absorption even at specular reflection. (In the electromagnetic-IR/ dipole-process this vibration is about as strong as all the C-H absorptions combined.) The advantage of EELS over IR for surface studies is its surface sensitivity. Lastly, the effect of the analysis environment, i.e. UHV with electron beams or X-rays impinging on the sample, is crucial in making comparisonswith what is detected by an electron energy analyzer and what is happening under real conditions. Work in this environment may require precooling the sample to prevent desorption of volatile materials in UHV and monitoring work function or ellipsometry changes to detect changes in the samplesurface. EELS may then be able to identify chemical groups present on surfaces of importance in the real world.
Acknowledgment. We acknowledge the many helpful discussionswith John Baker, Jon Dinardo, and Jim Tsang during the course of this work. Many thanks go to Abe Levi and Art Appel for help in producing the figures. sp-C films were made by Chris Jahnes and Jim Mayerle. CVD carbon films were made by Vish Patel. Registry No. HOPG,7782-42-5;C, 7440-44-0; Ag, 7440-22-4; stearic acid, 57-11-4.
