Langmuir 1987, 3, 183-188 been used to any significant extent so far in exploring chromatographic surfaces. Even the limited data presented here show that such studies could lead to detailed understanding of the interfacial region that controls the chromatographic separation.
183
Acknowledgment. We thank Professor F. F. Cantwell for useful discussions and guidance with experimental design and Dr. L. Cunningham for help with data analysis. Registry No. Benzene, 71-43-2; aniline, 62-53-3.
Vibrational Spectroscopic Analysis of Langmuir-Blodgett Multilayers by HREELS: Sampling Depth and Scattering Mechanisms Joseph H. Wandasst and Joseph A. Gardella, Jr.* Department of Chemistry, University at Buffalo, S U N Y , Buffalo, New York 14214 Received June 11, 1986. I n Final Form: November 10, 1986 HREELS analysis of Langmuir-Blodgett mono- and multilayer systems was conducted at various primary electron energies between 1.5 and 6.5 eV and at a specular angle of 60’. Results from the analysis of barium stearate multilayers on polycrystalline Ag indicate that extreme surface sensitivity is being exhibited, as there are no loss features corresponding to carboxylate stretches seen with increasing layer thickness as would be expected if the sampling depth was greater than 40 A; methyl and methylene features dominate the observed spectra. This correlates well with previous work on unsubstituted stearic acid systems analyzed by HREELS. As the layer thickness increases, the elastic peak intensity also increases, supporting a “carpet effect’! where the Langmuir-Blodgett multilayers can shield surface imperfections and create a more specular reflecting surface. Primary energy analysis of a single layer of barium behenate was conducted, and enhancement of several vibrational features was observed at approximately 5.5 eV. This correlates well with resonance with C-H impact scattering modes as evidenced in internal photoemission experiments on hexatricontane, showing that the carbonyl electronic feature is not being excited and further substantiating the limited sampling depth of HREELS in these systems. Vibrational loss features are assigned on the basis of previous work to features seen in IR spectroscopy.
Introduction In two previous papers, we have reported the first high-resolution electron energy loss spectroscopy (HREELS) studies of Langmuir-Blodgett mono- and multilayer systems of fatty acids on various polycrystalline materials.’v2 We have shown it possible to successfully obtain resolved HREELS spectra on such samples and derive information concerning the geometric orientation and functional group identities of the monolayer species from vibrational assignments. Examination of multilayers of stearic acid on Ag (where the layer thickness was 370 A)yielded no doubt that such large assemblies of molecules were amenable to the HREELS technique. In addition, we were able to show differences in relative intensities of various vibrational loss features with respect to the layer thickness, which we feel might have been caused by resonance interaction with electronic spectral features in the multilayers. Another observation was that methyl and methylene vibrations were intensified relative to others such as C-0, indicating that HREELS was exhibiting extreme surface sensitivity in this system; in effect the spectra were being dominated by the topmost -CH3 layers. Finally, we examined the role of substrate interactions with the sample on the character of the observed HREELS spectrum on such substrates as Ag, Au, Ge, and Al. From these studies, we determined that surface roughness factors have much to do with the intensities of both the elastic and loss feature peaks. In this report, we use HREELS to examine barium cation incorporated mono- and mul-
* Author to whom correspondence should be addressed. ‘Current address: Naval Research Laboratory, Chemistry Division, Washington, DC 20375.
tilayer fatty acids deposited on polycrystalline Ag to see if HREELS is sensitive to the incorporation of barium cations into the L-B films and if the resulting deprotonation of the carboxylic acid head can be differentiated from the HREELS spectra of nonbarium-substituted stearic acid multilayers as reported in our last paper. This is important in extending the HREELS technique to biological systems in that cations are readily incorporated into membranes (which L-B systems mimic). We have also performed a primary energy study on a monolayer of barium behenate on Ag which provides evidence for a resonance scattering mechanism for the HREELS technique. HREELS is being used to examine a much broader range of materials than ever before; this includes such samples as polymers, polycrystalline metals and alloys, “large” molecules, and L-B systems. Pireaux et al.3 have studied “bulk massive chunks” of polyethylene and vacuum-evaporated films of hexatriacontane with HREELS using external charge neutralization, comparing their results to transmission infrared data. They assigned the majority of the observed loss features to methyl and methylene vibrations, while some remained unassignable. They also obtained evidence for resonance scattering effects from measured cross section vs. primary electron beam energy data and were able to utilize the surfacesensitive capabilities of HREELS to assess the nature of the polymer ~ u r f a c e . ~Vilar, Heyman, and Schott used backscattering spectroscopy of low-energy electrons (con(1) Wandass, J. H.; Gardella, J. A., Jr. Surf. Sci. 1985, 150, L107. (2) Wandass, J. H.; Gardella, J. A,, Jr. Langmuir 1986, 2, 543. (3) Pireaux, J. J.; Thiry, P. A,; Caudano, R.; Pfluger, P. J . Chem. Phys. 1986,84,6452.
0743-7463/87/2403-0l83~01.50/0 0 1987 American Chemical Society
184 Langmuir, Vol. 3, No. 2, 1987 ceptually similar to HREELS) to examine polydiacetylene single crystals and other organic films on ~ l a t i n u m .They ~ were able to observe loss features corresponding to vibrational, exciton, and plasmon scattering events in the spectra and assign the known features in those various spectral regions. These include the C-C and C-H stretch features at 180 and 360 meV, respectively, in the hexatriacontane films and electronic state features a t 0.9 and 2.0 eV corresponding to the lowest triplet state and singlet state in pentacene. The full spectrum single instrument capabilities of the electron energy loss method are thus aptly demonstrated. Rao et al. has studied the chemisorption of CO on various polycrystalline metals and alloys such as Mn, Ag, and Pt.5 In the realm of large molecule adsorption on single crystals, Nyberg, Surman, et al.6have used HREELS to reexamine the adsorption of benzene and pyridine on Pt(llO), leading to a reassignment of vibrational loss features and generating controversy regarding currently held theoretical views of HREELS sampling mechanisms.6 In particular, they demonstrate evidence that dipole and potential-perturbation rules are inadequate to describe the HREELS scattering mechanisms for benzene. They found that the measured off-specular HREELS loss energy intensities were isotropic for vibrations which were expected to be peaked around the specular direction, thus refuting the dipole theory. The effect of surface reconstruction in single-crystal Pt on the observed loss feature intensity angular distributions is also cited as a possible reason for these results. Demuth et al. used HREELS in combination with UPS measurements to examine the interfacial region of Pd and Cr films over polyimide surface^.^ They were able to see vibrational structures similar to what we have previously reported. In particular, they made use of a downward shift in the C = O stretch loss feature to support formation of a conductive oxide layer in the case of Cr. This effect was not seen in Pd. They ascribe that linebroadening effects were due to excitation of collective surface modes of the metal or to Drude damping. The overall thickness of metal through which they could obtain spectra was 50 monolayers. The effects of resonance scattering in the HREELS spectra of adsorbed molecules have received very little attention. Indeed, for a long time it was doubted that such an effect could be seen on surfaces as the only known observation of resonance scattering was in gaseous systems.E The first experimental evidence of resonance scattering in surface adsorbate systems was made by Andersson and Davenport for OH on Ni0.9 Demuth, Schmeisser, and Avouris saw evidence for resonance scattering of various diatomics adsorbed on &,lo and Ibach studied CO on Pt(ll1) to show that resonance scattering contributed to the observed spectra.l’ The work by Pireaux et al. demonstrated the first evidence of a resonance scattering mechanism being observed for large assemblies of organic molecules. A t approximately 4 and 10 eV, they (4) Vilar, M. R.; Heyman, M.; Schott, M. In Photophysics and Photochemistry above 6 eV; Lahmani, F., Ed.; Elsevier: Amsterdam, 1985;
p 261.
(5) Vishnu, P. K.; Rao, C. N. R. Indian J . Chem., Sect. A 1984, 23A, 973. (6) Nyberg, G. L.; Bare, S. R.; Hofmann, P.; King, D. A.; Surman, M. Appl. Surf. Sci. 1985, 22, 392. (7) Dinardo, N. J.; Demuth, J. E.; Clarke, T. C. Chem. Phys. Lett. 1985, 121, 239. (8) Schulz, G. J. Rev. Mod. Phys. 1973, 45, 378. (9) Andersson, S.; Davenport, J. W. Solid State Commun. 1978, 28, 677. (IO) Demuth, J. E.; Schmeisser, D.; Avouris, Ph. Phys. Reu. Lett. 1981, 47, 1166. (11) Ibach, H. J. Mol. Struct. 1982, 79, 129.
Wandass and Gardella saw anomalous intensity effects which they attribute t o resonance scattering events. The present work was designed to investigate the effects of barium ion addition to the transferred monolayer and resulting evidence in the HREELS spectrum, while also studying the properties of multilayer structure on spectral loss features. To do so, a series of 1, 3, 5 , and 15 layers of barium stearate were deposited on polycrystalline Ag. This allows one to see the influence of deprotonation of the carboxylic acid head which does not normally occur on the Ag substrate. If the carboxylate stretches are not observed in the HREELS spectra of the barium stearate multilayers then this result would correlate well with the absence of carbonyl intensity that we have observed in our previous papers. Coupled with the intensity domination of vibrational features associated with methyl and methylenic stretches, this will show that the sampling depth of HREELS in such organic systems is limited to 20 A or less. In order to see if resonance scattering could be detected, a monolayer of barium behenate was prepared on Ag and subjected to a primary electron energy analysis. These experiments more fully extend the work of our previous papers. Experimental Section The stearic and behenic acids used in these experiments were of chromatographic reference quality (Sigma) and were employed without further purification. They were dissolved in reagent grade benzene to a concentration of approximately 1 mg/mL. The procedures and equipment involved in the production of the Langmuil-Blodgett multilayers have been described el~ewhere.’~J~ Briefly, the L-B trough consisted of a Teflon over aluminum trough with a Wilhelmy plate-magnetic transducer assembly operated in the constant force mode during compression and subsequent transfer. The Ag foil was of 99.999% purity and 0.25 mm thickness (Alfa). As in our previous work, the Ag foil was first washed in detergent to remove oily contaminations, rinsed with triply distilled water, and then plasma glow discharged in air for 5-7 min in a Harrick plasma glow discharge chamber a t 60 W. We have shown that such plasma glow discharging lowers the level of Ag surface carbonaceous contamination by 50% or more.14 This is very important for the successful use of L-B technologies since the presence of surface contaminants can affect the transfer characteristics of adsorbates. In addition, the plasma glow discharge treatment renders a high-energy surface suitable for L-B transfers.15 The trough subphase consisted of a solution of 3 X lob M BaCl, and 4 X 10“ M KHC03 in triply distilled water. This combination of reagents has been demonstrated to facilitate the transfer of fatty acids with a “Y”or alternating type of configuration. In this particular set of experiments, the fatty acid carboxylic “head” was oriented toward the surface in the first acid layer; subsequent layers alternated head-tail with every odd-numbered layer having the acid head toward the metal substrate surface. All multilayers produced for these experiments were transferred a t a surface pressure of 25 dyn/cm which causes the spread film to be in a close-packed solid phase. Film-transfer coefficients were monitored and were close to 1 for each of the samples. Force-area isotherms confirmed that the spread monolayers were being modified by the inclusion of the barium cation; films that include such charged species are more condensed than the uncharged ones. The HREELS spectrometer used to obtain spectra for these experiments was a Leybold Heraeus Model ELS 22 which was part of a multitechnique surface analysis instrument at the Center for Research in Surface Science and Submicron Analysis (CRISS), (12) Baier, R. E.; Zobel, C. R. Nature (London) 1966, 212, 351. (13) Wandass, J. H.; Gardella, J. A,, Jr. J. Am. Chem. SOC.1985, 107, 6192. (14) Wandass, J. H.; Schmitt, R. L.; Gardella, J. A., Jr.; Salvati, L., Jr., unpublished results. (15) Baier, R. E.; DePalma, V. A. Calspan Corporation Report no. 176, 1970; Buffalo, NY.
Langmuir, Vol. 3, No. 2, 1987 185
HREELS Analysis of Langmuir-Blodgett Multilayers Montana State University. The monochromator and analyzer are each based on a dual 127O cylindrical sector design. The Pa and during vacuum system has a base pressure of 5 X these experiments was operated at 5 X 10-l' Pa. All of the samples were mounted on a Cu probe with Ag epoxy cement to ensure a good electrical contact. A mild (100 "C) bakeout was used to obtain UHV conditions, and the samples were maintained at 10 "C above room temperature by flowing a stream of 4 O C water through cooling tubes attached to a thermal path that included the sample holder. The efficiency of this process was monitored by an attached thermocouple. The angle of incidence of the electron beam was 60' to the sample normal. During the examination of the barium stearate multilayers,the primary beam energy was set at 6.45 eV, while during the primary energy studies on barium behenate, the primary beam energy was varied from 1.5 to 6.5 eV. Data were collected by a Tektronix 4052 computer with signal processing capabilities interfaced through a single channel analyzer and an analogue rate meter. No signal averaging was possible. Both slow and fast scans were accumulated on each sample; the data were recorded both digitally and directly out of the ratemeter (analog). For the digital acquisition, either 0.2 or 0.4 meV/s scanning rates were used with a 4- or 1-s counting period. Corresponding time constants for the analog mode were 3 or 10 s. The full width at half-maximum (fwhm) was sample dependent and varied from 22 to 10 meV at typical counting rates of io3to io5 counts/s for the elastic peak. No external charge neutralization was used in these experiments. The energies of the loss feature are accurate in assignment to approximately &2 meV (16.12 cm-').
Results and Discussion The spectra of monolayers and multilayers of each acid exhibit the same characteristic bands: a band centered around 360 meV and a broad band centered around 140 meV with many additional small features superimposed upon this lower loss energy band. This is not only consistent with our previous results, but it also correlates well with the results of other groups such a Pireaux3 and S ~ h o t t Vibrational .~ assignments were made on the basis of our previous work and with reference to infrared studies of the barium salts of stearic and behenic acids and Raman studies of barium stearate multilayers.16-18 In our figures, the millielectronvolt loss assignments merely indicate that possible features could exist there. No attempt is made to correlate every feature observed with others in the IR or Raman spectra of these compounds; t o do so would ignore the obvious signal-to-noise ratio inadequacies and the inherently low resolution of the HREELS technique. Since the resolution of HREELS is not sufficient to determine band shifts upon substitution of the barium cation, the results should be similar to that of the spectra of the unsubstituted acids. The possible assignments fall into the categories of methylene and methyl carbon and hydrogen stretches, twists and rocks and acyl stretches. These similar types of vibrations were observed in our previous stearic acid spectra on Ag. Upon substitution of the doubly charged barium cation into the monolayer, the carbonyl vibration is expected to disappear and to be replaced by the carboxylate stretch whose two bands should appear between 200 and 160 meV.lg The HREELS spectra of the barium stearate multilayers appear in Figure 1. The multilayers of barium stearate vary in thickness from 24 (1monolayer) to 374 (15 layers) A.20 These films (16) Takenaka. T.: Noeami. K. J.Colloid Interface Sci. 1972.40.409. (17) Vogel, C.;'Corset,-J.;Dupeyrat, M. J . C h h . Phys. Phys.-dhim. Biol. 1979. 76. 909. (18) Burns; F. C.; Schlotter, N. E.; Rabolt, J. F.; Swalen, J. D. IBM Infrared Spectroscopy Application Note, 1985; IBM Instruments, San Jose, CA. (19) Bellamy, L. J. In The Infra-Red Spectra of Complex Molecules; Chapman and Hall: London, 1975; Chapter 10.
Analysis of Bo Stearate/Ag Multilayers 15 L a y p 371A
Ijl
0
3 Layers 748
x5 175
400
200
361 C-H
300
400
A€ (meV) Figure 1. HREELS spectra of barium stearate multilayers (1, 5, 15) on Ag using a 6.5-eV primary beam energy.
have been shown to be free from pinhole defects up to 80 monolayers, and they have a smoothly rising dielectric constant vs. the number of layers.21p22 For these samples the elastic peak intensity and fwhm were found to be quite sample dependent; the smallest fwhm and highest sample count rates were found on the 15-layer sample where the fwhm = 12 meV. We hypothesize that sample roughness effects can cause the deterioration of the elastic beam fwhm, and at ca. 400 A, the L-B film begins to remove any surface inhomogeneity that results from the rough oxide overlayer much as a carpet can shield a floor's imperfections. The level of surface irregularities associated with Ag surfaces has recently been investigated by the use of scanning tunneling microscopy by Raether,23 where a statistical root mean square roughness height for Ag vaporized upon a quartz surface is investigated and found to be 15-20 A. Extrapolation of this surface roughness to our systems would probably increase this root mean square value several fold. Thus it is difficult for the "carpet effect" to be seen until >5 layers are transferred. Indeed, this is the case for both barium stearate and stearic acid multi(20) Colton, R. J.; Murday, J. S.;Wyatt, J. R.; DeCorpo, J. J., paper presented at the 25th Meeting of the American Society for Mass Spectrometry. (21) Kapur, V.; Srivastava, V. K. Phys. Status Solidi A 1976,38, K77. (22) Yamamoto, N.; Ohnishi, T.; Hatakeyama, M.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1978,51, 3462. (23) Raether, H. Surf. Sci. 1984, 140, 31-36.
W a n d a s s and Gardella
186 Langmuir, Vol. 3, No. 2, 1987
layers as we have reported previously.2 It is entirely possible that reconstruction of the L-B layers can occur as the multilayer structures are built up around the morphological features resulting in a less specular environment. As an example, if there are gross morphology changes of >lo0 A, the L-B film will deposit with the valley regions of the surface hills; as the multilayers are sequentially deposited, a point will be reached that bridging of the monolayer will “cover” the hill. At this point several factors must be considered: there is the surface pressure applied to the deposited film, deformation to the film due to the covering process, and the possibility of lower energy configurations (such as head-head dimer formation from disruption). If such disruption occurs, this will lead to a less specular surface and increased fwhm. This effect will be a complex combination of several factors, some of which are the surface roughness of the material, the nature and thickness of the L-B film, and the applied surface tension conditions. This can serve as an explanation for the phenomena associated with the increase of fwhm for the 3-layer case. As soon as the thickness increases beyond the “critical” factor, however, the surface is smoother and, as we have shown repeatedly, the fwhm decreases. This result is supported by the observation in our previous work that monolayer films on Ge optical prisms also exhibited extremely high elastic peak count rates while having low elastic fwhm values.2 The deterioration of beam quality is caused by the varying focusing environments of the reflected electrons from various morphologies; the exact opposite is seen on single-crystal substrates where a tightly focused scattering beam is observed. Another related phenomena is the angular distribution of the specular beam elastic peak signal. Over a range of f20° to the sample normal, the specular beam intensity gradually declined to a minimum value. This is also in contrast to single-crystal samples where the beam plane is very rigorously defined to approximately f 5 O and any angular deviations cause marked changes in the resulting signal level. A major feature of these spectra is the C-H stretch located around 360 meV. It showed little position variation except in the case of the 15-monolayer sample, where the peak position is reported as shifted to 369 meV; this observation is explained by the increased fwhm of the C-H stretch feature and the difficulties in establishing the correct peak position. In the region from 80 to 220 meV there are several discernible features: these are present at ea. 95. 130, 155, 170, and 183 meV loss energies. These areas are associated with vibrations due to the following: the -CH3 wag a t 183 meV and the -CH3 deformation at 170 meV, which would be overlapped with the C-0 stretch which would also be located at 170 meV. It is entirely possible that this and other C-0 features overlap those in this C-H region; however, these would be expected to be weak compared to the C-H features because of the surface sensitivity of the HREELS technique. The other major bands can be assigned as the -CH2 stretch which is seen at 93 meV and the C-C stretch a t 130 meV. A shoulder just above 200 meV also appears in these spectra. A possible assignment for this feature is the asymmetric COO- stretch which should appear upon barium substitution into the monolayer. Our previous work with unsubstituted stearic acid/Ag L-B systems demonstrated similar types of loss features. Direct comparison of the loss feature positions shows no difference attributable to the presence of the barium ion. The only difference is in the appearance of the 200-meV shoulder peak assignable to the Carboxylate stretch. In a direct comparison between
Table I. Intensity Ratios for Various Barium Stearate L-B Systems band position sample BalSt Ba3ST Ba5St Bal5ST
IE 1100 800 1200 47000
VCHIIE
6CHZIIE
360 meV 0.093 0.14 0.078 0.0011
180 meV 0.099 0.15 0.084 0.0013
‘CCIIE
130 meV 0.048 0.054 0.034 0.00085
the two sets of samples, in the unsubstituted ones, a carboxylic stretch would be expected to be seen unless the acid reacted with a basic oxide layer on the substrate. Silver has such an oxide layer; however, in our previous study, the HREELS spectra on several different substrates demonstrated no discernible C=O stretch loss feature. These substrates included Au which only forms a thin oxide coating as compared to Ag. Thus, if the acid is deprotonated in the first layer, and the sampling depth is great enough, each successive layer should contribute to carbonyl stretching intensity because the succeeding would not be deprotonated in the L-B multilayer; thus, they would be expected to contribute to the carbonyl stretching intensity if the sampling depth were greater than one molecular unit length. This, however, is not the case. In neither the substituted nor unsubstituted L-B systems is clear evidence of either carbonyl or carboxylate stretching intensities observed. Coupling this with the high levels of methyl and methylenic related spectral features, this indicates that the sampling depth is thus probably less than 40 A in these samples. As one progresses from 1-to 15-layer thickness, several things occur. As previously mentioned, the fwhm decreases while the signal increases. We observed a similar effect for unsubstituted stearic acid/Ag L-B systems in our previous work.2 The fwhm of the C-H stretch also increases. Shown in Table I are the ratios of the intensities of several loss features to the elastic peak for the barium stearate layers. One sees that for the CH vibrational loss feature a t 360 meV, the ratio peaks at the 3-layer sample and then declines slowly. The CH2 and CH3 vibrational ratios also peak at this same thickness then also decline. There is an intensification of the loss feature at 93 meV (which corresponds to the methylene rock) as the layer thickness gets greater. As in our earlier experiments involving stearic acid on Ag, we are able to extract useful information from the multilayer assemblies; such results indicate that HREELS may be applied to more complicated polymer or organic systems. This result is to be expected if the sampling depth of HREELS is less than, say, 40 A. The extreme regularity and smoothness of the L-B systems would render impossible the capability of differentiating between various members of this group on the basis of peak positions alone. The only reproducible differences that we have found between these multilayer films involve the relative intensities of the elastic and loss peaks which serve to indicate that either a “carpet” effect is occurring over a 5-layer thickness or that some sort of resonance mechanism is accounting for the differences. This is possible if the electronic structure of the multilayers was changing along with the layer thickness; specific intensifications of modes would then occur. Another reason for the inability to discriminate between the unsubstituted and substituted stearic acid systems lies in the low S / N ratios in the spectra. Perhaps data averaging could lead to these capabilities. In an attempt to establish whether or not evidence for a resonance scattering mechanism2* could be found, a
Langmuir, Vol. 3, No. 2, 1987 187
HREELS Analysis of Langmuir-Blodgett Multilayers Barium Behenate/Ag 1 Monolayer - Primary Energy Analysis
I YI
Table 11. Primary Energy Study on Barium Behenate Monolayer/AG band Dosition
x67
i79
FE = 5 45eV
energy, eV 1.5 2.5 3.5 4.5 5.5 6.5
IE 3200 2400 1800 1200 8500 5100
VCH/rE
6CH2/IE
360 m e V 0.029 0.024 0.032 0.042 0.019 0.028
180 meV 0.052 0.039 0.048 0.056 0.12 0.055
IL~/IE vs. Primary Beam Energy (eV) for Barium Behenate Monolayer on Ag. PE:
VCClIE
130 meV 0.032 0.032 0.037 0.039 0.089 0.045
hI
4.5eV 008 -
I
007
-
006
-
Y
H
005Y J
H
004
-
003 -
002 v i
0
> :
too
- .
zoo
300
001 -
400
AE (meV) Figure 2. HREELS spectra of barium behenate monolayer on Ag using primary electron beam energies of 1.5,4.5 and 5.5 eV. primary energy analysis of a barium behenate monolayer on Ag was done. Since behenic acid has a longer chain length than stearic acid, it forms even more stable and regular monomolecular films. Primary energies of 1.5, 2.5, 3.5,4.5,5.5, and 6.5 eV were employed for these analyses. Higher electron energies were not used because of the difficulties in obtaining high enough count rates to acquire spectra. Qualitatively, the HREELS spectra for this sample resemble those of the stearate samples with two broad loss features at the same energies (ca. 140 and 360 meV). Again, there are small features discernible on top of the broad base. The spectra of barium behenate monolayers at primary energies of 1.5, 4.5, and 5.45 eV are shown in Figure 2. One can see that there are relative intensity changes occurring between some of the loss features. Since the S / N ratio was so poor, only three large regions were chosen to be compared to one another. The particular loss features that were selected are the vCH at 360 meV, BcHzat 180 meV, and vC4 at 130 meV. The loss feature intensities were scaled to the intensity of their respective elastic peak intensities for the different primary energies. The ratios and the elastic peak intensities for the various samples analyzed are presented in Table 11. The results are presented in Figure 3, which is a graph of the relative intensity ratio of the loss features to their elastic peak intensities vs. the primary energy that was
-
“05
9 15
25
35
45
55
65
Primary Energy (eV)
Figure 3. Graph of the relative intensity of loss features ratioed to their respective elastic peak intensities vs. the primary beam energy for a barium behenate monolayer on Ag. employed. One sees in this graph, that the VC-C and 6CH2 intensities peak at around 5.5 eV rather dramatically. The uCH intensity ratio remains fairly invariant and declines at that same point. This is consistent with a resonance enhancement mechanism. The reason for the decline in CH vibrational intensity may be due to several things. First of all, the CH vibration has a high-impact scattering character associated with it, and this would not allow long residence time of the electron for pronounced resonance scattering effects to occur. Another possibility could be that the spread of data points for the vCH could reflect the experimental error in the technique and as such represent a “base line” to compare the actual intensification of the other features. Since intermediate primary energy data were not collected at small enough intervals to readily determine the resonance peak, our intensification level estimates show that tenfold increase in relative intensity for the B C H p loss feature has occurred at around 5-eV primary energy. In an internal photoemission experiment by Pfluger et al.25performed upon a normal long-chain alkane (n-C36H78),two possible resonance features are exhibited in the intensity vs. energy below the Fermi level. (25) Pfluger, P.; Zeller, H. R.; Bernasconi, J. Phys. Reu. Lett. 1984,53,
(24)
Gadzuk, J. W. Phys. Reu. E : Condens. Matter 1985, 31, 6789.
94.
188 Langmuir, Vol. 3, No. 2, 1987
Such a compound would be expected to have a similar electronic structure to that of the behenic acid. One is due to an increase in the density of states and is located at ca. 4.6 eV while another is due to impact scattering from CH stretches and is a broad feature a t around 5.5 eV. In an experiment involving ultraviolet photoelectron spectroscopic (UPS) examination of vacuum deposited films of stearic acid on various metallic substrates such as Au, Cu, and InO, Taniguchi et al. showed that the dominant ionization potential was above 8 eV.% The broad feature that they saw in this portion of the UPS valence band spectrum is due to II II* transitions associated with the acid carbonyl f ~ n c t i o n a l i t y . ~During ~ our HREELS experiments, it would be impossible to excite resonance with these features because the primary beam energy was removed from their particular energies. Thus, the use of the hexatriacontane results of Pfluger can approximate the electronic structure of the fatty acid L-B layer in our experiment since no resonance with the carbonyl electronic band transitions occurred. Resonance with the C-H impact scattering features then seems to be a cogent explanation for the enhancement phenomena that we have observed in our HREELS spectra of these systems.14 Certainly more efforts at locating the exact peak energy are necessary (along with higher energy primary electron beams), and the effects of structure change on the electronic and HREELS spectra have to be investigated.
-
Conclusions In this paper, we have reported new data on the HREELS investigation of L-B mono- and multilayer (26) Mitsuya, M.; Taniguchi, Y.; Sato, N.; Seki, K.; Inokuchi, H. Chem. Phys. Lett. 1985, 119, 431. (27) Basch, H.; Robin, M. B.; Kuebler, N. A. J. Chem. Phys. 1968,49, 5007.
Wandass and Gardella systems, continuing our original work on unsubstituted fatty acids and barium cation substituted ones. We have shown that vibrational information can be extracted from the spectra corroborating the extreme surface sensitivity of the HREELS technique to methyl and methylene vibrations. In addition, we have shown that L-B multilayers may smooth out imperfections in the underlying substrate surface and thereby provide a more uniform reflecting environment, increasing elastic peak intensities. We have also shown that the differences between the barium ion and non-barium ion systems cannot be differentiated in these experiments but that instrumental limitation may be the source of this problem and can possibly be eliminated. Evidence has also been demonstrated for a resonance scattering mechanism in HREELS of large solidstate organic molecules with accompanying electronic structure information. Future studies already under way will include deeper investigation into the resonance scattering event with the design of specific target molecules with known electronic structures and the HREELS examination of large-scale L-B assemblies involving >lo0 layers. This will help us to see the onset of charging difficulties and to continue to examine the carpet effect of various surfaces.
Acknowledgment. This work was supported by a grant from the Polymers Program of NSF, Grant DMR 8412781. We thank the Center for Research in Surface Science and Submicron Analysis (CRISS) at the Department of Physics, Montana State University, directed by Professor G. J. Lapeyre, Dr. James Anderson, Dr. David Frankel, Milt Jaenig, and Alice Adams, for advice and access to the HREELS instrumentation. The CRISS facility is supported by NSF Grant DMR 8309460. Registry No. Ag, 7440-22-4; barium stearate, 6865-35-6;barium behenate, 2636-16-0.
