Langmuir 1994,10, 4610-4617
4610
Structural Investigation of Molecular Organization in Self-AssembledMonolayers of a Semifluorinated Amidethiol T. J. Lenk,? V. M. Hallmark, C. L. Hoffmann, and J. F. Rabolt* IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099
D. G. Castner Department of Chemical Engineering, BF-10, University of Washington, Seattle, Washington 98195
C . Erdelen and H. Ringsdorf Institute for Organic Chemistry, University of Mainz, J . J . Becher Weg 22, 0-6500, Mainz, Germany Received January 24, 1994. In Final Form: August 15, 1994@ A new fluorocarbon-chain-containing molecule, CF3(CF2)7C(O)N(H)CH2CH2SH,for self-assembly was synthesized using a simple and general technique. Self-assembled monolayers of this molecule were studied using ellipsometry,contact angle measurements, grazing-incidence infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorptionfine structure measurements. The films were found to be uniform and reproducible, presenting a fluorocarbon surface (8,dV(H20)= 114") with predominantly CF3 groups exposed. The C=O and N-H dipoles are found to be oriented parallel to the gold surface, with the fluorocarbon axis oriented perpendicular to the surface.
Introduction The study of self-assembled (SA) monolayers of thiols on gold has grown dramatically in recent years, as these molecules have been observed to form stable, wellorganized f i l m ~ . l -Layers ~ produced in this manner have been used as model substrates for studies of wetting4 and biological a d h e ~ i o nelectrochemical ,~ s t u d i e ~ ,and ~ , ~development of model membranes.** With few exceptions, most studies2a have involved an investigation of long-chain alkanethiols containing either methyl or functionalized end groups. In the case of the latter, a whole series of specifically chosen end groups have been studied as potential candidates for molecular recognition. Recent results5are encouraging and suggest that through this approach low-energy surfaces can be constructed which actually inhibit the binding of proteins from solution. In addition, through the choice of other end groups, completelyhydrophilic surfaceshave also been constructed. Most recently,8bthe molecular assembly of alkanethiol molecules has been extendedto semiconductor t Current address: Raychem Corp., 300 Constitution Drive, Menlo Park, CA 94025-1164. Abstract published inAduance ACSAbstracts, October 1,1994. @
(1)Ulman, A.AnIntroduction to Ultrathin OrganicFiZms;Academic Press: New York, 1991. (2)Bain, C. D.; Troughton, E. B.; Tao, Y.-T.;Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . A m . Chem. Soc. 1986,111, 321-335. (3)Nuzzo, R. G.;Dubois, L. H. Annu. Rev. Phys. Chem. 1992,43, 437-63. (4) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J . Am. Chem. SOC. 1990,112,570-579. ( 5 )Lopez, G. P.; Albers, M. W.; Schreiber, S. I.; Carroll, R.; Peralta, R.;Whitesides, G.M. J . Am. Chem. Soc. 1993,115, 5877-5878. (6)Uosake, K.; Sato, Y.; Eta, H. Electrochim. Acta 1991,36,17991801. (7)Weisshaar, D. E.;Walczak, M. M.; Porter, M. D. Langmuir 1993, 9(U, 323-329. (8)(a) Sheen, C. W.; Shi, J.; Martensson, J.; Parikh, A. N.; Allara, D.L. J.Am. Chem.Soc. 1992,114,1514-1515.(b)Coyle,L.C.;Danilov, Y. N.; Juliano, R. L.; Regen, S. L. Chem. Mater. 1989,I , 606-611.
(GaAs) substrates, providing yet another pathway for device construction. In all cases, knowing both the orientation and order in these SA monolayers has become the quintessential challenge for the surface experimentalist. Fluorinated monolayers have uses as diffusion low-energy contamination-resistant surfaces, and lubricants. There have been a number of attempts to create ordered fluorinated monolayers through the use of Langmuir-Blodgett techniqueslOJ1and self-assembly using fluorinated thiols.12 In the latter case, a similar F8-thiol to the one in this study was used but there was no amide group incorporated into the backbone. The presence of such an amide group which promotes intermolecular hydrogen bonding could influence tilt, packing density, and stability. In this work we describe the synthesis of a fluorinated amidethiol, referred to as F8-thio1, by a simple and general technique and characterize the formation of a stable, uniform monolayer (Figure 1)from this material using IR, XPS,NEXAFS, ellipsometry, and contact angle measurements.
Experimental Section Synthesis. Ethylperfluorononanoate (3 g, 6.1 mM) and 2-mercaptoethylamine(941 mg, 12.1 mhf) were dissolved in 10 mL of diethyl ether. ARer addition of 5 drops of triethylamine, the mixture was stirred overnight. The solvent was then evaporated. The resultingsolid was dissolved in chloroformand washed sequentiallywith2 N hydrochloricacid, saturated sodium bicarbonate solution, and water. The organic phase was then dried with sodium sulfate and evaporated down to a few milliliters. The product was recovered by flashchromatography (9)Shimomura, M.;Song, K.; Rabolt, J. F. Langmuir 1992,8,887893. (10)Naselli, C.;Swalen, J. D.; Rabolt, J. F. J . Chem.Phys. 1989,90, 3855-3860. (11)Schneider, J.;Erdelen, C.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989,22, 3475-3480. (12)Chidsey, C.E. D.; Loiacono, D. Langmuir 1990,6 , 682-691.
0743-7463/94/2410-4610$04.50/00 1994 American Chemical Society
Langmuir, Vol. 10, No. 12, 1994 4611
SAMs of a Semifluorinated Amidethiol
F8-7hiol
and 80". The takeoff angle was defined as the angle between the surface normal and the axis of the analyzer lens system. For determination of a compositional depth profile (CDP), the solid acceptance angle of the analyzer lens was decreased from its normal 30" solid angle to 6" x 12" by placing an aperture over the analyzer lens. This was done to improve the depth resolution a t each takeoff angle.15 The elemental compositions measured a t each takeoff angle were then used to determine a CDP with the regularization method of Tyler et a1.16 The CDP generated in this manner is consistent with, but not necessarily unique to, the experimental angle-dependent XPS data. The mean free paths used in determining the CDPs were calculated from the equations given by Seah and Dench.17 Near-Edge X-ray Absorption Fine Structure. The nearedge X-ray absorption fine structure (NEXAFS)experiments were done on beamline U1A a t the National Synchrotron Light Source (NSLS)located a t Brookhaven National Laboratory. This beam line uses an extended-range grasshopper monochromator that was set for a full-width a t half-maximum resolution of -1 eV a t the carbon K edge. The monochromator energy scale was calibrated by setting the peak for the C1, x* transition in the graphite carbon K NEXAFS spectrum to 285.35 eV.18 The positions of the intensity minima in the incident X-ray beam due to adsorbed carbon on the beamline optics were then calibrated relative to the graphite x* energy and used as an internal energy reference for a! carbon NEXAFS spectra. All NEXAFS spectra were then normalized by the photocurrent from a gold-coated 90% transmission grid placed in the incident X-ray beam and a Au control sample. Total electron yield (EY) spectra were acquired by placing a positive bias on the cone of a channeltron to collect all the secondary, photo, and Auger electrons. The total fluorescence yield (FY) spectra were acquired by using a specially designed proportional counter. l9 The polarizationdependent NEXAFS spectra were obtained by polar rotation of the sample with respect to the incident X-ray beam. The sample position was defined by the angle between the sample surface and the axis of the incident X-ray beam. Thin Film Characterization. Infrared measurements were made using an IBM/Bruker Model 98 evacuable FTIR a t a resolution of 4 cm-l. Typical spectra used coaddition of 8000 scans to obtain a reasonable signal-to-noise ratio. All spectra of the adsorbed films on gold were recorded using grazing angle external reflection a t an incident angle of 81-83', so that only vibrations with components of the change in dipole moment normal to the surface are observed in the spectrum.20121 Ellipsometric measurements were made on a Rudolph Research AutoEL ellipsometer using a 6328 A laser a t a 70' angle of incidence. For thickness calculations a refractive index of 1.36 was assumed, compared to values of 1.38for poly(tetraflu0roethylene) and 1.339 for poly(pentadecafluorocty1 acrylate). Varying this value from 1.3 to 1.45 resulted in about a 2 A difference in the estimated film thickness. Advancing and receding water contact angles were measured using a Rame-Hart Model 100-00 Goniometer and a syringedrive droplet. Contact angles were measured to the nearest 0.5".
-
Figure 1. Schematic diagram showing the structure of the F8-thiol molecule, as well as a model of the monolayer formed by self-assembly onto a gold substrate. The gray shaded region indicates the fluorinated part of the monolayer, with chain axes near normal to the gold surface. Possible hydrogen bonding between adjacent amide groups is suggested by the heavy, hatched bars. using chloroform as the eluant. The yield was 1.42 g (44.5%). The product had the chemical formula CF3(CF2)7C(O>N(H)CH2CHzSH and will be referred to in this work as F8-thiol. Ethylperfluorononanoate was used as supplied by RiedeldHHaen. The 2-mercaptoethylamine was sublimed prior to use. A melting point of 103 "C was measured with a Leitz polarizing microscope equipped with a Mettler hot stage. An NMR spectrum in CDCl3 was recorded on a 200 MHz Bruker AC200 FT-NMR (NH- lH, 6.9 ppm, broad; NHCHz - 2H, 3.57 ppm, multiplet; CH2SH - 2H, 2.73 ppm, multiplet; SH - l H , 1.39 ppm, triplet). Sample Preparation. Glass substrates were cleaned by a sulfuric acid etch, followed by degreasing in isopropyl alcohol vapor and drying under a warm nitrogen flow. Vacuum depositionof 150A of chromium as an adhering layer was followed by deposition of 2000 of gold. Near-edge X-ray absorption spectroscopy(NEXAFS)experimentsused 3000 layers of gold. All depositions were done at a rate of 5-10 k s . Samples were immersed in F8-thiol solution within 1h of removal from vacuum. Gold substrates were placed in a M solution of F8-thiol in ethanol or methylene chloride a t room temperature and left for the desired time (20 h except when kinetics were studied). Samples were rinsed copiously with the adsorption solvent upon removal from solution,blown dry with nitrogen, and then vacuumdried a t room temperature. No residual solvent bands appeared in the infrared spectra. X-ray Photoelectron Spectroscopy. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Surface Science Instruments X-probe spectrometer with a monochromatic.AlKa X-ray source. The binding energy (BE) scales were referenced to the Au 4f7/2 peak at 84.0 eV. Highresolution C l s and S2p spectra were acquired a t an analyzer pass energy of 50 eV, while elemental compositionswere obtained using a pass energy of 150 eV. At this pass energy the transmission function of the spectrometer was assumed to be constant.l3 The peak areas were normalized by the number of scans, points per electrovolt, Scofield's photoionization cross sections,14 and sampling depth. The sampling depth was assumed to vary as "3.7, where KE is the kinetic energy of the photoe1e~trons.l~ The samples did degrade slowly in the X-ray (comparisonof compositions after 5 and 90 min ofX-ray exposure showed a 5% loss in fluorine signal and an 8%increase in the Au signal), so measurements were limited to less than 1h for any particular spot on a sample. To assess the compositional variation with depth, XPS data were acquired at nominal photoelectron takeoff angles of 0,55,
A
(13) Application note from surface Science Instruments, Mountain View, CA, 1987. (14) Scofield, J. H. J . Electro. Spectrosc. Rel. Phenom. 1976,8,129.
Results and Discussion Monolayer Formation. Monolayers of F8-thiol were found to form quickly and reproducibly in both CHZC12 and ethanol. To minimize hazardous solvent exposure, most films were adsorbed from absolute ethanol. For comparison purposes, a previously well-studiedmaterial,2 (15) Tyler, B. J.; Castner, D. G.; Ratner, B. D. Determining Depth Profiles from Angle Dependent X-ray Photoelectron Spectroscopy: The Effects of Analyzer Lens Aperture Size and Geometry. J . Vac. Sci. Technol. 1989, A7, 1646-1654. (16)Tyler, B. J.; Castner, D. G.; Ratner, B. D. Regularization: A Stable and Accurate Method for Generating Depth Profiles from Angle Dependent XPS Data. Surf. Interface Anal. 1989,14,443-450. (17) Seah, M. P.; Dench, W. A. Quantitative Electron Spectroscopy of Surfaces: A Standard Data Base for Electron Inelastic Mean Free Paths in Solids. Surf. Interface Anal. 1979, I , 2-11. (18) Morar, J. F.;Himpsel, F. J.; Hollinger, G.;Jordan, J. L.; Hughes, G.; McFeely, R. Phys. Rev. B 1986,33, 1346. (19) Fischer, D. A.; Colbert, J.; Gland, J. L. Rev. Sci. Znstrum. 1989, 60, 1596. (20) Greenler, R. G. J . Chem. Phys. 1966, 44(1), 310. (21) Rabolt, J. F.;Jurich, M.; Swalen, J. D.App1. Spectrosc. 1985,32, 269.
Lenk et al.
4612 Langmuir, Vol. 10, No. 12, 1994 XPS C1, Spectra
I
, 70'0
5
I
I
I
1
1
I
I 15
10
20
25
Time (hr)
Figure 2. Kinetics of formation of self-assembled monolayers as determined by changesin the water contact angle. All values had essentially reached equilibrium values in less than 1h for M solutions. Symbols are (m) eadv, C18adsorption from thiol; ( 0 )Qreo C184hiol; ( 0 )e&, FS-thiol; (0)Or=, F8-thiol.
I
Table 1. Angular-Dependent XPS Analysis of FB-Thiol on Gold"
composition(atom %) 80"
0"
(glancing) 55" (normal) theory 63.5 64.1 54.8 fluorine (Fls, 688.3 eV) 66.6 31.8 29.8 35.5 carbon (Cls, see Table 4) 29.9 1.9 2.5 3.2 oxygen (018,531.3 eV) 1.1 1.6 2.0 3.2 nitrogen (Nls, 399.8 eV) 1.3 1.2 1.6 3.2 sulfur (s2,3/2, 161.9 ev) 1.1
The gold signal has been subtracted and the results renormalized to give the compositionof the organic material present on the surface. Results are an average of two runs.
300
(22) Patai, S. The Chemistry ofthe Thiol Group; John Wiley & Sons: New York, 1974. (23) Brandup, J., Immergut, E. H., Eds. PolymerHandbook, 3rd ed.; John Wiley & Sons: New York, 1989.
:
0
Figure 3. XPS C l s spectrum for F8-thiol on gold, illustrating the relative abundance of CF3 near the surface and the fact that the NCO, CN, and CS moieties are all located closer to the gold surface than t o the vacuum interface. Table 2. Angular-DependentXPS Analysis of C l s Region of FB-Thiol on Gold" comnosition (atom %)
a
octadecanethiol, was adsorbed under the same conditions. The changes in advancing and receding water contact angle as a function of adsorption time (in both solvents) are shown in Figure 2. For both materials, the film has essentially reached equilibrium in 1h, and little change is observed upon further adsorption. Films adsorbed for longer than 24 h often showed erratic results during characterization, particularly when ethanol was used as the solvent. It is thought that this may be due to slow oxidation of the thiol, possibly forming disulfides1 or oxygen-containing species.22 Average layer properties were determined by preparing four samples of each material using a 20 h adsorption time. Contact angles were determined by measurement of three separate drops on each of the four surfaces. The octadecanethiol reference monolayers showed water convery close tact angles 6adv = 107 f 1" and Or,, = 96 f lo, to previously observed values.2 The F8-thiol water contact angles were measured to be 6adv = 114.5 f 2" and e,,, = 106.5 f 1.5". The advancing contact angle compares favorably with that for poly(tetrafluoroethylene), 116".23 Both films show similar hysteresis, probably due to roughness more than any underlying film properties, since the gold substrate is expected to be rough on a scale much larger than the 10-20 A thickness of the self-assembled monolayers. XPS Characterization. The excellent uniformity and reproducibility of the F8-thiol layers are shown by the low variability in XPS composition measurements. Averages of two spots each on two different samples at a takeoff angle of 55" gave the following atomic percentages and standard deviations: Au, 16.9f0.2; F, 53.6 f0.3; C, 25.3 f 0 . 1 ; 0 , 1 . 5 f 0 . 2 ; S , 1 . 1 f 0 . 1 ; N , 1 . 6 f 0 . 1 . Normalized values for the organic overlayer are given in Table 1, along with the expected atomic compositions (neglecting H).
295 290 285 Binding Energy (eV)
CF3 (293.3 eV) CF2 (291.1 eV) OCN (288.1 eV) CN (285.4 eV) CS (284.3 eV) a
80" (glancing)
0" (normal)
theory
18.8 68.3 4.3 4.4 4.2
12.1 66.7 6.8 7.1 7.3
9.1 63.6 9.1 9.1 9.1
Theoretical values indicate proportions of C existing as given
species.
The results of angular-dependent XPS measurements of F8-thiol layers are also shown in Table 1. The fluorine signal is greatly enhanced over the theoretical value for all angles, indicating that the fluorine is concentrated toward the vacuum interface ofthe film. Oxygen, nitrogen, and sulfur are depleted relative to the theoretical values, particularly a t the glancing angle, where the sampling depth is the smallest. The sulfur, nitrogen, and oxygen concentrations increase with sampling depth. Chemisorption of the thiol to the gold surface is demonstrated by the measured S2p312binding energy of 162 eV. Taken together, these results are consistent with an upright orientation of the amidelhydrocarbon segment of the molecule,with the outer layer composed of the fluorinated sections of the molecule, as schematically shown in Figure 1. Acomparison of the Cls regions at normal and glancing angles supports the elemental analysis. The carbon species closer to the fildvacuum interface will show up as more intense peaks in the 80"(glancing angle) spectrum as compared to the 0" (normal incidence) spectrum. In Figure 3, these two spectra have been normalized to the intensity of the CF2 peak. Comparison of the expected composition with those measured a t glancing and normal incidence (Table 2) reveals that the CN, CS, and NCO carbons are relatively further from the vacuum interface than the fluorinated carbons. Greater enhancement of the CF3 band relative to its CF2 counterpart indicates that the vacuum surfaceis richer in CF3than CF2. Overall, the molecules are packed or arranged so that the exposed surface is dominated by CF3 groups. However, the orientation angle of the fluorocarbon chain cannot be determined from the XPS data. More definitive orien-
SAMs of a Semifluorinated Amidethiol
I
I
.I
3400
Langmuir, Vol. 10,No. 12,1994 4613
1
fa-thiol
3200
5000 2800 2600 Wovenumbers (cm-')
2400
2200
2000
Figure 4. Comparison of grazing incidence infrared spectra (3500-2000 cm-l) of self-assembledfilms of F8-thiol on gold from two different solvents to a transmission spectrum of F8-
thiol in a KBr pellet.
f8-thiol
I
region, amide A (NH stretch) at 3333 cm-l and a weaker amide B (first overtone of NH in plane bending (amide 11)) at 3095 cm-l. It is well known thatz4molecules containing amide groups contain two bands in this region which arise from Fermi resonance between the NH stretch and overtones or combinationsof amide I1vibrations found in the 1500-1600 cm-l region. In order for these two bands to participate in this resonance interaction with one another, theyzsmust have unperturbed frequencies in close proximity and have the same symmetry species. If these two criteria are met, then the extent of interaction will determine the amount of intensity borrowing, the mixing ofvibrational character, and subsequent shift apart which will occur. If the frequency of the overtone or combination occurs at exactly the same frequency as the unperturbed NH stretching fundamental (accidentaldegeneracy),then amide A and amide B are equal mixtures of the unperturbed vibrations, their shift apart from each other is large, and their band intensities would be similar. As seen in Figure 4, the intensity of the amide B vibration at 3095 cm-' is weak relative to that of amide A at 3333 cm-l and can be quantitatively estimated to be in the ratio 1:12. With this ratioz6and the locations of the perturbed amide A and amide B bands (3333 and 3095 cm-l), it is possible to calculate the positions of the unperturbed vibrations using the following expressions derived from a consideration of the anharmonic potential functions:
v, - vb = s
_1 I
I
KBr Pellet
2000
1800
1600
1400 1200 Wavenumbers
1000
800
600
(1)
where s is the frequency difference between perturbed vibrations v, (amide A) and a+, (amide B). The observed intensity ratio of perturbed (observed)amide B to amide A allows one to calculate the splitting, 6 , in cm-l between unperturbed bands, v: and vi, using the expression
Figure 5. Comparison of grazing incidence infrared spectra (2000-600 cm-') of self-assembled films of F8-thiol on gold from two different solvents with a transmission spectrum of
F8-thiol in a KBr pellet.
where 6 = vp - vi. Since the shift apart of the two unperturbed bands due to Fermi resonance is symmetrical, Le., vp shifts up in frequency by the same amount that vi shifts down, the additional condition
v,
Wavenumbers (cm-'
)
Figure 6. Raman spectrum ofbulk F8-thiol. The S-H stretch at 2562 cm-l is clearly visible.
tational information is available from polarized infrared and N E W S measurements of the F8-thiol monolayers. Spectroscopic Measurements. A . IR and Raman Spectroscopy of Bulk F8-Thiol. Before the analysis of polarized IR spectra of F8-thiol on Au is begun, it is important to understand the origin and assignment of all major vibrational bands. To this end, both the IR and Raman spectra of an isotropic sample of F8-thiol were obtained and are shown in Figures 4-6. Since the local symmetry of the FS-thiol molecule is extremely low, one would expect most bands to be both IR and Raman active. This is borne out by comparison of the IR spectra of Figures 4 and 5 with the Raman spectrum of Figure 6. Of initial interest in the IR and Raman spectra of bulk F8-thiol is the 3000-3400 cm-l region which contains bands attributable t o NH stretching vibrations. As seen in the 1R spectrum (Figure 4), two bands appear in this
+ vb = v,0 + vb0
(3)
is also true. Hence application of this analysis to the observed data for F8-thiol gives values for vp and vi of 3314 and 3113 cm-', respectively. These represent the unperturbed values of amideA and amide B in the absence of any Fermi resonance. The frequency (3113 cm-1) of the unperturbed amide B vibration can be compared to that expected (3110 cm-l) for the overtone of the observed amide I1 band at 1555 cm-l. The agreement is quite remarkable and provides a self-consistent argument to this Fermi resonance treatment and the assignment of the amide A and amide B bands. In the 2800-3000 cm-l region are found bands attributable t o the asymmetric (2968 cm-l) and symmetric (2938 cm-l) CHZstretching vibrations. A weak band is also observed at 2934 cm-l in the IR and most likely corresponds to the symmetric stretch observed in the Raman at 2938 cm-l. This coincidence in frequency location can be used to infer information about the conformation of the CC bond adjacent to the amide group (24)Miyazawa, T.J.Mol. Spectrosc. 1960, 4 , 168-172. (25)Moore, W.H.;Krimm, S . Biopolymers 1976, 15, 2439-2464. (26)Rabolt, J.F.;Moore, W. H.; Krimm, S. Macromolecules 1977,10, 1065-1074.
4614 Langmuir, Vol. 10,No. 12, 1994 in the bulk F8-thiol. If the CC bond were trans, there would be no coincidence of the IR and Raman bands in the CH stretching region due to the presence of an inversion center using local symmetry arguments. A gauche CC bond would preclude this mutual exclusion, and both IR and Raman CH2 stretching bands would be coincident as appears to be the case. Further support is provided by the observation of a band at 1080 cm-' in the Raman spectrum of bulk F84hiol. This band has been assigned to a CC stretching vibration of the gauche conformation. It is likely that this gauche CC bond relieves steric constraints and allows the amide groups to participate in intermolecular hydrogen bonding. The SH stretching vibration can be found in the Raman spectrum at 2561 cm-l with an obvious shoulder at 2576 cm-l. This latter band compares favorably with that found for liquid butanethiol reported by Bryant and Pembert0n,2~,~88 suggestingthat the 2561 cm-l band which dominates the doublet in this region is attributable to SH groups found in a crystalline environment. This lower frequency shoulder is present in the spectra of other bulk long-chain thiols and may be indicative of the presence of a second crystal polymorph or the presence of both cis and trans C-S bonds. More will be said about this in a later section. In the 1500-1700 cm-' region, vibrations characteristicZsbof the amide group can be found. The 1693 cm-' band, weak in the Raman and strong in the IR, can be assigned to amide I which has a dominant contribution from C=O stretching motion. The 1555 cm-l band, which has moderate intensity in the IR but is weak in the Raman spectrum, is assignable to amide 11. It has a large component of C-N in-plane bending motion and hence gives rise to a change in dipole moment which is parallel to the F8-thiol molecular axis. Both of these bands are found at higher frequencies than would be expectedzebfor polypeptides having either an a helical (1652and 1546 cm-l) or parallel pleated sheet structure (1630and 1530 cm-l) and suggest an antiparallel arrangement of hydrogen-bonded peptide groups from adjacent chains. Interestingly enough, this can occur in the bulk through a partial interdigitation of chains so that hydrocarbon and fluorocarbon portions of the F84hiol are aligned adjacent to one another in a structure first proposed for a series of semifluorinated n - a l k a n e ~ . ~ ~ " Bands in the 1400-1500 cm-l region originate from bending motions of the CH2 groups. A weak band found at 1446 cm-l in both the IR and Raman can be assigned t o the CHZscissors motion usually found at 1460 cm-'. A second component at 1420 cm-l is also attributable to CH2 bending, its frequency perturbed by the presence of the adjacent C-0 and/or SH groups. The region between 1200 and 1400 cm-' contains a number of bands in both the IR and Raman spectra as shown in Figures 5 and 6. The bands at 1336 and 1373 cm-l are strong in the Raman and of moderate intensity in the IR. As discussed previously,29athey originate from the activation of bands from the interior of the Brillouin zone of the infinite fluorocarbon helix due to the finite length of the CF2 sequence in this F8-thiol. Since, in general, the CF2 plane is tilted relative to the helical axis, these particular symmetric CF2 stretching bands (1336 and 1373 cm-l) will have a relatively strong component of the change in dipole moment along the helical axis, (27) Bryant, M. A,;Pemberton, J. E. J.Am. Chem. SOC.1991,113(lo), 3629-3637. (28) (a) Bryant, M. A.; Pemberton, J. E. J.Am. Chem. SOC.1991, 113(22), 8284-8293. (b) Miyazawa, T.In Polyamino Acids;Fasman, G. D., Ed.; Dekker: New York,1967. (29) (a) Rabolt. J. F.: Russell, T. P.: Twiee. R. J. Macromolecules 1984,17,2786-2794. (b) Masetti, G.; Cabasii, F.; Morelli, G.; Zerbi, G . Macromolecules 1973,6,700.
Lenk et al. while the components perpendicular to the axes will spatially average to zero due to the nature of the helix. Hence these bands will be referred to as "axial CFT stretching vibrations. These bands were not assigned or discussed in the spectra of a fluorinated thiol reported by Chidsey and LoiaconolZeven though in both their spectra and ours these bands dominate the IR reflection data. The dichroism of these bands will again be discussed in a later section when the IR reflection spectra are discussed. In the Raman spectrum there is a cluster of three bands in the vicinity of 1300 cm-' (1313,1299,and 1284 cm-l) which are not observed in the IR and therefore can be confidently assigned to amide I11 (CN stretch NH inplane bend) bands. These vibrations involve stretching of the amide C-N coupled with bending of the N-H group in the amide plane. The strong set of bands observed in the IR at 1230, 1217,1210,and 1143 cm-l with their weak counterparts in the Raman is the characteristic fingerprint of fluorocarbon molecules and involves stretching and bending motions of the CF2 groups. Of these, the 1230,1210,and 1143 cm-l bands all have a significant contribution from the asymmetric CF2 stretching vibration, and in contrast to the axial CF2 stretches at 1330 and 1375 cm-', these vibrations have the resultant change in dipole moment component perpendicular to the helical axis. There has been some confusion associated with the assignment of the polarization ofthe 1210cm-l band due to the prediction by Masetti et al.29bthat an IR-active parallel band should be found at 1213 cm-'. Although experiments by Masetti et aLZgbon stretched poly(tetrafluoroethy1ene) did not support such an assignment, Naselli et al.1° assumed that this band did exhibit parallel dichroism and used it along with a group of six other bands to qualitatively determine the orientation of a semifluorinated fatty acid salt LB film on a solid substrate. This assumption, although incorrect, was but a small contribution to the overall interpretation of the orientation, and hence the effective conclusion of tilt of the fluorocarbon segment in that system remains unchanged. Raman bands observed between 1000 and 1200 cm-' are usually associated with CC stretching vibrations of CH2-CH2 segments and can be used to determine the conformation of the CC bond as mentioned earlier. For a trans bond, two bands can be found at 1060 and 1130 cm-l characteristic of the asymmetric and symmetric CC stretching vibrations of the CH2 backbone. Although there is only one aliphatic CC bond in the F8-thiol, weak bands are observed at 1055 and 1080 cm-' which can be attributed to CC stretching vibrations with the latter characteristic of a gauche bond. The weak IR band observed at 1117 cm-I and the weak Raman bands at 1120 and 1150 cm-' are assigned as progression bands of the v3 dispersion curve as discussed earlier. In the region below 900cm-l, a number of strong Raman bands and several weak IR bands are found. The weak band at 858 cm-l and the strong bands at 766 and 730 cm-l (weak in IR at 730 cm-') all belong to the v3 dispersion curve with the latter corresponding to the zone center CF2 symmetric stretching vibration of the infinite chain poly(tetrafluoroethy1ene) (PTFE). The intense Raman band at 663 cm-l can be confidently assigned to a stretch of the CS bond in the gauche This conformation as shown by Bryant and Pembert~n.~' would indicate that, in the bulk crystalline phase, the preferred conformation of the CS bond is rotated away from the all trans position, most likely to accommodate more efficient packing of the thiol end groups. The remaining IR and Raman bands, assigned to motions of the CF2 bending and rocking vibrations, are listed together with all observed bands in Table 5. Of
+
Langmuir, Vol. 10, No. 12, 1994 4615
SAMs of a Semifluorinated Amidethiol special note is the intense band found at 127 cm-l which can be assigned to the mode associated with accordionlike motion of the chain. This longitudinal acoustic mode (LAM) has been to exist in perfluoro-n-alkanes and its frequency found to be inversely proportional to the chain length. For C8Fl8 the observed LAM frequency is 141 cm-’ while for CgFzo it is 125 cm-l. Hence the observed value for the F8-thiol is closer to that for C~FZO, indicating that the molecule is somewhat longer than C8F18, which it is. However, if all the backbone atoms are considered, the LAM frequency should reflect a chain of 13 atoms. This is certainly not the case and indicates that intermolecular hydrogen bonding of the amide groups on adjacent F8-thiol molecules serves to decouple the accordion-like motion from the remaining part of the molecule as discussed by F a n ~ o n i .Perhaps ~~ a more appropriate model for understanding this perturbative effect would be a rod with one fxed end. B. Grazing Incidence IR Spectrum ofF8-Thiol on Gold. There are several characteristic infrared frequencies in F8-thiol which show interesting polarization properties and hence provide some insight into how the F8-thiol molecule organizes and orients on the gold surface. The amide A (NH stretch) band at 3333 cm-l does not appear to be present in the grazing incidence reflection (E perpendicular to the surface) spectra shown in Figure 5. Although the noise level in this region is quite high, the amide Ais one of the stronger bands in the bulk spectrum, and hence it should be visible if a component ofthe change in dipole moment was perpendicular t o the surface. This indicates that the NH bond is primarily parallel to the surface. Likewise, the same argument can be made for the orientation of the C-0 bond being parallel to the surface using the lack of intensity of amide I (at 1693 cm-’) in Figure 5. Interestingly enough, the amide I1 vibration both increases in relative intensity and shifts from 1554 to 1542 cm-l after chemisorption to the gold surface. The intensity increase is consistent with the orientation ofthe peptide group (Figure 1)with both C=O and NH bonds parallel to the surface. Since amide I1 has a large contribution from NH in-plane (of the peptide group) bending, the change in dipole moment will be in the plane and along the molecule axis which is oriented normal to the surface. The shift to lower frequency indicates that there is different interaction of hydrogenbonded peptide groups in the adsorbed layer than in the bulk material. In the monolayer the F8-thiol molecules are constrained to interact in two-dimensionswhere in the bulk this is not the case. As discussed previously, the observed amide I and amide I1 bands in the bulk are consistent with an antiparallel arrangement of peptide groups, while in the monolayer the lower frequency of amide I1 suggestsz8ba parallel arrangement of hydrogenbonded peptide groups as would be dictated by reaction of the SH groups with the Au surface. Dramatic changes are seen in a number ofcharacteristic C-F stretches. The “axial” CFZbands at 1373 and 1336 cm-l and the 1272 cm-l band all have large intensities in the reflection spectrum. A similar change in the 1373 and 1336 cm-l bands has been observed for both Langmuir-Blodgett films of fluorinated fatty aciddoand a selfassembled film of a fluorinated thiol.12 As discussed previously, these “axial”CFZstretches have their change in dipole moment oriented along the helical axis, and so their high intensity in the reflection spectrum indicates that the fluorocarbon portion of the F8-thiol is oriented almost normal t o the surface. Although the intensity of the 1272cm-l band is large in the reflection spectrum, its origin is unclear. It may correspond to the very weak ~
~
Table 3. Infrared and Raman Vibrations of Bulk and Adsorbed FB-Thiol, 2600-3600 om-’ bulk bulk adsorbed (Raman) (IR,isotropic) (IR,EL) assignment amide AZsb 3332 w 3333 m amide B 3095 vw 2968 m va (CHZ) 2938 s 2934 w vs (CHd 2872 vw v(SH)-ordered 2576 sh v(SH)-disordered 2561 m 2560 vvw a
v-stretch.
band at 1268 cm-l in the isotropic spectrum, but assignment to a vibration of either the fluorocarbon or hydrocarbon portion of the F8-thiol is not possible. Cho et al.3z have studied a series of fluorocarbon oligomers and found a series of nonzone center modes in the region below 1250 cm-l, which they used to trace out portions of the CFZ stretch dispersion curves. No bands were observed above 1250 cm-’. However, this previous work does provide some insight into the assignment of the strong band observed at 1226 cm-l in the reflection spectrum of F8thiol (Figure 5). Because of its observed polarization properties, this band can be confidently assigned to the elusivez9IR-active AZvibration composed of CC stretch and CCC bend which has a change in dipole moment along the molecule axis. The large intensity in the El spectrum further supports the premisethat the fluorocarbon portion of the F8-thiol is oriented normal to the gold surface. Its somewhat large bandwidth (approximately 22 cm-’) is similar to that observed by Cho et al.3zand may be caused by conformationaldisordering(e.g., helix reversals), which may be present due to the looser organization resulting from incommensurate packing of chains on a gold lattice. The asymmetric CFZstretching bands at 1230, 1210, and 1148 cm-l all decrease in intensity in the reflection spectrum of the adsorbed layer as would be expected for vibrations which have their change in dipole moment perpendicular to the fluorocarbon chain axis (Table 3). There are also some large intensity changes in the region near 700 cm-l, with strong bands at 725,688,674,and 643 cm-’. The latter has been assigned to a CF2 wagging vibration, which would have a change in dipole moment parallel to the molecule axis. An identical argument can be used for the 688 cm-’ band, which has also been assigned to a wagging vibration. The strong band at 725 cm-1 is assignable to a CF3 end group stretchingvibration, and as such, the large intensity observed in the El spectrum is expected. Finally, the last band in this region, located at 674 cm-’, can be assigned to a stretch of the CS bond. Its moderate intensity in the reflection spectrum indicates a significant orientation of the CS bond normal to the gold surface. A summary of all bands and their assignments is included in Tables 3-5.
NEXAFS The polarization-dependentcarbon NEXAFS results for the F8-thiol are shown in Figure 7. These results indicate that the perfluoro chains are standing vertically on the Au substrate. The spectra were obtained by polar rotation to vary the angle between the sample surface and the X-ray beam. When the sample was normal to the X-ray beam, no changes in peak intensities were observed upon azimuthal rotation. The peak near 295 eV in the carbon N E W S spectra, which can be assigned to a transition from the C1, orbital
~
(30)Rabolt, J. F.; Fanconi, B. Polymer 1977,18,1258-1264 (31)Fanconi, B. Personal communication.
(32)Cho,H.-G.; Strauss, H. L.; Snyder, R. G . J.Phys. Chem. 1992, 96,5290-5295.
4616 Langmuir, Vol. 10, No. 12, 1994
Lenk et al.
Table 4. Infrared and Raman Vibrations of Bulk and Adsorbed F8-Thiol,1000-2000 cm-' a bulk bulk adsorbed (Raman) (IR, isotropic) (IR, El) assignment 1695m 1694s v(C=O)-amide 128b 1559w 1554m 1542 s d(CNH), v(CN)-amide I1 1446w 1445w d(CH2)"; V,(CFZ)~~ 1425 w d(CH2) 1378s 1373w 1372 s v(CF2) progression; axial CFZstretch 1342m 1336m 1336 s v(CF2) pro&ession; axial CF2 stretch 1313 m v(CN), d(CNH)-amide II128b 1299 m v(CN), d(CNH)-amide I11 1284 w v(CN), d(CNH)-amide I11 1268 vw, sh 1272 s unassigned 1242 vw 1232w 1230s va(CFz),r(CFz)lO; va(CF2),r(CFda 1219vw 1217s 1226 d(CCC), v(CC)29b1S0 1208w 1210vs 1210 sh va(CF2) ~ a ( c F 3 ) ~ ~ 1150w 1143vs 1148 m v.(CF2), d(CF2)10f48 1124vw 1117w 1120 m v(CC), trans planar298 1080w 1084vw v(CC), gauche 1058w 1051vw v(CC), trans planarzga
+
F8-thiol/Au Carbon NEXAFS Sprctro I
I
l
I
I
I
I
I
l
d(C -F)
\-
h
VI
c
1
3
280
290
300
310
320
Photon Energy (eV)
Figure 7. Carbon electron yield N E W S spectra of the F8thiol monolayer with the sample surface at normal and glancing (-20") to the incident X-ray beam.
X-ray beam was varied with respect to the sampleindicates that the FS-thiol is preferentially ~ r i e n t e d . ~For ~,~~ oriented samples, the transitions to a* states have their Table 5. Infrared and Raman Vibrations of Bulk and strongest intensity when the bond orbital (e.g., C-Fa*) Adsorbed FS-Thiol, 500-1000 cm-' that the photoelectron (e.g., CIS)is being excited into is bulk bulk adsorbed aligned parallel to the E vector of the incident X-ray (Raman) (IR, isotropic) (IR, E 3 assignment beam.37 For extended hydrocarbon chains there is some 980 vw 974 w 971 m CF347 disagreement as to whether the transition to the C-Ca* 858 w CF2 progression (v3) orbital is from a virtual C1, orbital oriented along the chain CF2 progression (v3) 766 s axis3*or from the individual C-C bonds.39 To avoid this 745 w r(CH#l disagreement, the intensity variations of the transition 730 s vs(CF2),in phaseZga 730 w,sh 732 sh to the C-Fa* orbital at 292 eV has been used to determine zone center ( v 3 ) 725 w 725 m v(CF3) end group the orientation of the F8-thiol monolayer. With the 700 vw 688 m r(CFd, w(CF2)'l assumption that the X-rays are 85%polarized, the factor 663 s 667 vw 674 m v(C-S) gauche of 3 increase in intensity observed upon changing the X-ray 643 s ~ ( C F ~ ) , ~ ( C F Z ) ~ ~ incident angle from 30 to 90" is consistent with the C-F 621 m 625 w w(CF2MCFz) bonds being oriented parallel to the Au surface.40 Since d(0CN)-amideIVb 610sh,w 610w 612 w the C-F bonds are perpendicular to the chain axis of the 582 Fgthiol, this implies the F8-thiol molecules are perpen554 6(CF2)102s0 532 540 vw w(CF2) or ~ ( C F Z ) ~ ~ dicular to the Au surface. The accuracy of the NEXAFS orientation determination is f10". The major source of v-stretch; &bend; r-rock; w-wag. error in this analysis is the uncertainty of locating the step edge and normalizing the peak areas to an edgejump to the C-Co* orbital,33is enhanced when the sample is of unity due to the presence of the relatively intense C-F rotated, so the X-ray beam strikes the surface at a glancing and C-Ca* transitions. angle. At glancing angles, the electric field vector (E Thickness Results. Thickness results for the control vector) ofthe incident X-ray beam is nearly perpendicular octadecanethiol film give some indication of the accuracy to the Au surface. The peaks near 292 and 299 eV in the of the ellipsometric measurements. Previous studies of carbon N E W S spectra, which can be assigned to these monolayers have discussed the infrared evidence transitions from the CISorbital to the C-Fa* orbitals,33 for a chain tilt of 20-30" from the surface With are enhanced when the sample is normal to the X-ray a maximum extended length for the octadecanethiol beam. In this position the E vector is parallel to the Au molecule of approximately 27 A, the anticipated ellipsosurface. Likewise, a peak near 689 eV in the fluorine metric thickness of the tilted chain film is 23-25 8, NEXAFS spectra, which can be assigned to transitions corresponding favorably with the value of 21.5 f 1.7 A from the F l s orbital to the C-Fa* orbital,33exhibits the measured here. same polarization dependence as the 292 and 299 eV peaks Both grazing incidence IR and NEXAFS measurements in the carbon N E W S spectra. The fluorine spectra are discussed in a previous section conclude that the pernot shown here but were similar to those previously fluorocarbon chains of the FS-thiol are highly oriented observed from fluorocarbon sample^.^^^^^ The same polarization dependence was observed in both the EY and FY NEXAFS spectra, as expected, since the F8-thiol (35)Outka, D.A.;Stohr, J. Springer Ser. Surf. Sci. 1988,10, 201. (36)Stohr, J. In The Structure of Surfaces; Van Hove, M. A., Tong, monolayer is thinner than both the EY and FY sampling S. Y., Eds.; Springer-Verlag: Berlin, 1985;p 140. depths. (37)Zaera, F.; Fischer, D. A.; Carr, R. G.; Gland,J. L. J. Chem. Phys. The observation that the peaks in the NEXAFS spectra 1988,89,5335. (38)Hahner, G.; Kinzler, M.; Woll, Ch.; Grunze, M.; Scheller,M. K.; vary in relative intensity as the orientation ofthe polarized Cederbaum, L. S. Phys. Rev.Lett. 1991,67, 851. v-stretch; d-bend; r-rock; w-wag.
(33)Ohta, T.;Seki, K.; Yokoyama, T.; Morisada, I.; Edamatsu, K. Phys. Scr. 1990,41,150. (34)Castner, D.G.; Lewis, K. B., Jr.; Fischer, D. A.; Ratner, B. D.; Gland, J. L. Langmuir 1993,9,537.
(39)Outka, D.A.;Stohr,J.;Rabe,J. P.; Swalen,J. D. J . Chem. Phys. 1988,88,4076-4087. (40)Stohr, J.; Outka, D. A. Phys. Rev.B 1987,36,7891-7905. (41)Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109,3559.
Langmuir, Vol. 10, No. 12, 1994 4617
SAMs of a Semifluorinated Amidethiol
film. The F8-thioUAuinterfaceappears to be rather broad, as the Au concentration requires -40 A to increase from its minimum to maximum value. The roughness of the Au substrate is probably the cause of this apparent broadness.
XPS CDP for FS-Thiol/Au
1
2 60 e
Conclusions
0
10
20 30 40 50 60 Depth (Angstroms)
70
80
Figure 8. CDP for the F8-thiol monolayer generated from angle-dependent XPS data using the regularization method.
with the chain axis nearly normal to the metal substrate. These observations imply that thicknesses for the monolayer films should be comparable to the maximum extended length of the F8-thiol molecules. If the fluorinated part of the molecule adopts the 1517 helix characteristic of poly(tetrafluoroethy1ene)above 19 "C,the length of an eight-carbon segment would be approximately 9.8 A.42 Assuming that the rest of the molecule is all-trans and oriented at 90" to the surface yields a total calculated thickness of 17.6 A.43 Experimental thicknesses for the F8-thiol films were evaluated by both ellipsometry and XPS. For ellipsometry, the index of refraction of the film was assumed to be that of a typical fluorocarbon chain, 1.36, with no adjustment for the unfluorinated portion of the film nearest to the gold surface. In this case, the measured thickness of 17.0 f 0.8 A agrees well with the expected thickness of 17.6 A. Correction for the unfluorinated portion of the monolayer film would increase the index of refraction slightly, since the hydrocarbon value is nearer to 1.5, somewhat reducing the calculated ellipsometricthickness. However, F8-thiol molecules oriented with the chain axes nearly normal to the surface are consistent with the thickness measurements. A film thickness slightly less than the fully extendedvalue might reflect the presence of a gauche CC bond near the end of the F8-thiol molecule to relieve steric hindrance at the gold surface. Studies of another fluorinated thiol, similar except for the absence of the amide group, also concluded that the measured and maximum thicknesses were comparable,12 again implyingorientation of the perfluoro chains normal to the surface. These two results taken together indicate that the presence of the amide group does not disrupt packing or orientation of the fluorocarbon chains. Such disruption was not anticipated to play a role, since size considerations alone indicate that the 5.6 A diameter of the fluorocarbon helix42is substantially larger than the 3.4 A extended length of the HNCO segment.43 Any appreciable hydrogen bonding present in the amide portion of the monolayer film thus appears to be accommodated without concomitant disorder or tilt of the fluorocarbon chains. An estimate of the film thickness can also be obtained from the CDP generated from the angle-dependent XPS experiments. The CDP for the F8-thiol film shown in Figure 8 indicates the monolayer thickness is -20 A, as determined by the depth where the Au concentration is 50 atom %. This value is within experimental error of the ellipsometric thickness (17 A) measured for the F8-thiol (42) Tadokoro, H. Structure of Crystalline Polymers; Wiley-Interscience: New York, 1979. (43) All bond lengths from Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC: Cleveland, OH, 1978.
An amidethiol with an eight-carbon perfluorinated segment was synthesized, and the self-assembly of this molecule on gold was investigated by FTIR, XPS, NEXAFS, contact angle, and ellipsometric measurements. The layers were found to be uniform and reproducible, exhibiting wetting behavior characteristic of fluorocarbon surfaces with very low energies. Both infrared and N E W S measurements clearly show the fluorocarbon segment to be highly oriented, with the chain axis nearly normal to the gold surface. Such a low chain tilt angle is consistent withkray diffraction results from Langmuir monolayers of perfluorinated amphiphilesU and from close-packed structures of perfluorinated chains predicted from steric arguments45and molecular dynamics simulaof infrared results for the hydrot i o n ~ Interpretation .~~ carbon segments is not as clear, but there are indications (the N-H band and possible C-S stretch intensity changes) that the hydrocarbon segment axis is also oriented near the surface normal. This is also consistent with concentration variations observed in angle-dependent XPS studies and thickness measurements, both from ellipsometric and XPS techniques. The effect of the amide group in the chain is to provide orientational stability through intermolecular hydrogen bonding and should manifest itself in enhanced mechanical integrity. The small amount of tilt which may occur in this segment could result from the incommensuratenature of the gold lattice relative to the intermolecular spacing of fluorocarbon helices.
Acknowledgment. Funding for the synthesis portion of this work was provided by IBM through its Shared University Research(SUR)Program in collaboration with Prof. H. Ringsdorf. This research was funded in part by the National Institutes of Health through Grant RR-01296 (D.G.C.) for the National ESCA and Surface Analysis Center for Biomedical Problems at the University of Washington. The NEXAFS studies were carried out at the NSLS, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. D.G.C. gratefully acknowledges support from the Department of Energy's Division of University and Industry Programs, Office of Energy Research, as a NSLS Faculty Research Support Program participant. J.F.R. acknowledges the support and encouragement of Dr. H. Guard of the Office of Naval Research through Contract No. N00014-93-C0105. (44) Barton, S.W.; Goudot, A.; Boulassa, 0.; Rondelez, F.; Lin, B.; Novak, F.;Acero,A.;Rice, S.A.J. Chem.Phys. 1992,96(2), 1343-1351. (45) Outka, D.A.;Stohr, J.; Rabe, J. B.; Swalen, J. D.; Rotermund, H. H. Phys. Rev.Lett. 1987,59, 1321. (46) Shin, S.;Collazo, N.; Rice, S. A. J. Chem. Phys. 1992, 96(2), 1352-1366. (47) Giegengack, H.; Hinze, D. Phys. Status Solidi A 1971,8,513520. (48) Hsu, S. L.; Reynolds, N.; Bohan, S. P.; Strauss, H. L.; Snyder, R. G.Macromolecules 23,4565-4575. (49) Chau, L.-K.; Porter, M. D. Chem. Phys. Lett. 1990,167,198204. (50)Rabolt, J. F.; Fanconi, B. Macromolecules 1978, 1 1 , 740-745.
