1082
Langmuir 1993,9, 1082-1085
An Epitaxial Organic Film. The Self -Assembled Monolayer of Docosanoic Acid on Silver(11 1) Mahesh G. Samant,' Charles A. Brown, and Joseph G. Gordon I1
IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received April 27, 1992. In Final Form: January 19, 1993
Docosanoic acid in solution spontaneously forms an ordered self-assembled (SA)monolayer on the silver(ll1) surface, exposing methyl groups to the atmosphere. The contact wetting angles for water and hexadecane are 116" and 55", respectively. Surface X-ray diffraction shows that the in- lane structure of the SA monolayer is ~ ( 2 x 2 ) The . SA monolayer is present in domains of about 215 igwith a mosaic spread of 0.85". The chains within the monolayer are tilted at 27 f 1"from the surface normal toward the near neighbors. We conclude that the carboxylategroup is bound nearly normal to specific sites on the Ag surface and that this determines the interchain spacing. The tilt angle is a consequence of this interchain spacing.
Introduction Ultrathin, well-defined, ordered organic films can be formed on a solid surface by adsorption of amphiphilic organic molecules from solution. These films (one monolayer thick) are called self-assembled (SA) monolayers.' The constraints on the organic molecules are that they contain 10to 24 carbon atoms and have a functional head group capable of chemisorbing on solid surfaces. Wellknown examples are alkylsilanes on SiOdSi wafers2and alkyl mercaptans or dialkyl disulfides on gold surface^.^ Self-assembled monolayer films are expected to be more robust than their corresponding Langmuir-Blodgett films and consequently will be more suitable for proposed applications such as sensors, photoresists, electronic devices, nonlinear optical materials, lubricants, corrosion inhibitors, etc. The functionality and structure of a SA monolayer will be determined by the head group, length of carbon chain, presence of functional groups within the carbon chain, identity of the surface atoms, and substrate structure. The knowledge of the structures of SA monolayers is still in ita infancy and, hence, so is our understanding of the correlation between structure and properties. But rational design of technologically useful materials requires understanding these correlations in greater detail. Recently, structural studies have been reported using transmission electron diffraction: helium diffraction: and surface X-ray diffraction.6 Both head group and substrate were shown to affect the structure (head group), C2zH45S6 vs C22H45Se7on Au(ll1); substrate C22H45Son Au(111P vs Ag(111)8). In this paper we report application of surface X-ray diffraction to study the structure of SA films of docosanoic acid on silver(ll1). We chose this system because the combination of head group and substrate are expected to have predominantly ionic interactions as (1)Netzer, L.; Sagiv, J. J. Am. Chem. SOC.1983,105, 674. (2)Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984,ZOO, 465. (3)Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. SOC.1987, 109,2358.Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. 1988,4,365. (4)Strong, L.;Whitesides, G. M. Langmuir 1988,4,546. ( 5 ) Chidsey, C. E. D.; Liu, G.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421. (6)Samant, M.G.; Brown, C. A.; Gordon, J. G., I1 Langmuir 1991,7 , 437. (7)Samant, M.G.; Brown, C. A.; Gordon, J. G. Langmuir 1992,8, 1615. (8)Fenter, P.; Eisenberger, P.; Li, J.; Camillone,N.; Bernasek, S.;Scoles, G.; Ramanarayanan, T.; Liang, K. S. Langmuir 1991, 7, 2013.
opposed to the predominantly covalent interactions in the previously structurally characterized selenol and thiol films. Earlier studies have established that arachidic acid and 1,32-dotriacontanedioicacid spontaneouslyadsorb on silver?JO Here we report the structure of the SAmonolayer of docosanoic acid and compare it to structures of SA thiol and selenol films.
Experimental Section The Ag(ll1) substrates were prepared by deposition of 2000 A of epitaxial silver on a freshly cleaved mica surface." This deposition was done at substrate temperature of 300 "C and in a vacuum of 10-6 Torr. The docosanoicacid (99.8%purity)was obtained fromAnalabs and was used as received. The SA monolayer was formed by immersing the substrate in a 0.25 mM solution of docosanoic acid in n-hexadecane for 3 days. Following this exposure the sample surface was washed with hexane and dried in flowing argon. The quality of the SA monolayer was determined from the contact anglesfor water and hexadecanewhich were measured with a contact angle goniometer (We-Hart Model A100). The surface X-ray diffraction measurements were conducted on beam line X16-B at the National Synchroton Light Source (NSLS). The samples were mounted in an evacuablecell so that the environment surrounding the sample could be controlled during the diffraction measurements. The diffraction data were collected in the symmetricgeometry,where the angleof incidence equals the exit angle, and were described earlier.l* The X-ray beam energy of 7336.6 eV (A = 1.690 A) was selected by a bent Ge(ll1) crystal and calibrated by diffraction from a Si(ll1) wafer. The detectorwas a scintillation counterand the acceptance angle of the detector was determined by 2 mrad Soller slits. Air scattering was reduced by evacuating the beam path and sample damagewas minimized by opening the X-rayshutteronly during the actual data collection period. The azimuthal angle, 4, was defined to be zero along the Ag(l0) crystal truncation rod (CTR).
Results Three different SA films were used for structure studies. These were characterized by contact angle measurements (9)Schlotter, N. E.;Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986,132,93. (10)Allara. D.L.: Atre. S. V.: Ellineer. C. A.; Snyder, R. G. J. Am. Chem. SOC.1991,113,1852. (11) Pashlev. D.W. Philos. Mag. Grunbaum. E.Vacuum - 4.1959.316. . . 1973,24, 153. ' (12)Samant. M.G.: Tonev. M. F.:. Borees. - . G.:. Blum.. L.:. Melrov, ~. 0. R. J. Phys. Chem.' 1988,92,22b: (13)Toney, M. F.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. To be submitted for publication.
0743-746319312409-1082$04.00/0 0 1993 American Chemical Society
Epitaxial Organic Film
Langmuir, Vol. 9, No.4, 1993 1083
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which were in the range 116117' and 5 5 - 5 6 O for water and hexadecane, respectively, for the samples. These contact angles compare well with those obtained for the SA alkane thiol monolayer on gold surfaces3and the SA octadecyltrichloroeilane on silicon oxide and quartz? suggesting that the docosanoic acid films on silver(ll1)
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are of comparablequality. Our X-ray diffraction resulta, described below, show that docosanoic acid chains form a close packed monolayer on Ag(ll1). Figure 1shows the X-ray diffraction intensity normalized to 10(indicated by diamond symbols) obtained from the docosanoic acid monolayer on Ag(ll1) during a rod
Samant et al.
1084 Langmuir, Vol. 9, No. 4, 1993 L
Ag
Figure 5. Orientation of the docosanoic acid on the Ag surface. Table I position
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domain size thickness mosaic spread tilt angle structure
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intense peak 1.254 0.532 0,60,120 215 29 f 5 0.85 26.2 PW2)
weak peak 1.254 0.209 0,60,120
scan (Le. a scan as a function of qr) at 4 = 60' and at qxy = 1.254 A-1. The scattering vector, Q (q 4~ sin Wh), is defined such that qxyis in the plane of the sample and qr is perpendicular to the surface. The solid line is average background intensity obtained in rod scans at 4 = 55 and 65' and qxy = 1.254 A-l. Three peaks are observed, an intense peak at qr = 0.532 A-l with a fwhm (full width at half maximum) of 0.214 A-l, a weak peak, approximately 8 times less intense, at qr = 0.209 A-l with a fwhm of 0.105 A-l, and a peak at qr = 0.054 A-l. All three samples showed the presence of these three peaks at almost identical positions but with variations in relative intensity of the surface peak with respect to the intense peak. The inplane symmetry of these peaks was probed and was the same for all samples. The results described in Figures 2-5 were obtained on the second sample. Figure 2a shows a radial scan (i.e. a scan as a function of qxywhich closely resembles a 8 - 28 scan) through the intense peak at qr = 0.531 A-l and 4 = 60". This peak has a maximum at qxy = 1.254 A-l and a fwhm of 0.029 A-l. The peak in the 4 scan, Figure 2b, is at 59.96' and the fwhm is 0.89'. A similar set of scans through the equivalent reflections at 4 = ' 0 and 4 = 120' were identical and confirm the 6-fold symmetry of this pattern. No higher order peaks were observed. We searched for the (11)peaks but did not observe them. The (20) peaks overlap with the Ag(10) peaks. The results are summarized in the Table I. The inplane position of both the intense and weak peaks is identical for all azimuthal angles. This position (qxu = 1.254 A-l) is half the value of the Ag(l0) crystal truncation
rod (CTR) (qxy= 2.508 A-l) indicating that the interrow spacing in the monolayer is twice the spacing in the substrate surface. The SA monolayer peaks lie along the same directions as the Ag CTRSand exhibit identical 6-fold symmetry. These observations are consistent with a commensurate~ ( 2 x 2structure. ) From the fwhm of the radial scans, the domain size (2dfwhm) is estimated to be 215 A for the intense peak. As expected, this is emaller than the 570 A estimated for the Ag substrate. From the fwhm of the azimuthal scan, the mosaic spread is 0.85' for the domains contributing to the intense peak. Not surprisingly, this is larger than the mosaic spread, 0.3', for the Ag(ll1) substrate. The tilt angle and tilt direction of the chains and the thickness of the monolayer can be estimated from the positions of peak maxima and the peak widths, respectively, of the rod scan. The estimated thickness of the domains contributing to the intense peak at qt = 0.532 A-l is 29 f 5 A. This is in good agreement with the expected thickness of 26-27 A for the monolayer. The position of this peak, qr = 0.532 A-1, leads to a tilt angle of 26 f 1' between the chain axis and the surface normal. The chains are tilted toward their nearest neighbors. This evaluation is based on arguments presented earlier.s This near neighbor chain tilt requires the presence of a diffraction peak at qz = 0 A-l. This peak is indeed observed at qn = 0.054A-l(Figure 1). Refraction effects shiftthe maximum to near the critical angle =0.054 A-l. The intensity of this surface peak is weak compared to that of the monolayer peak at qr = 0.532 A-l. This we belive is due to surface roughness of the silver(ll1) films. We measured the diffraction intensity of the bulk silver rod through (202) and (311) to understand this effect. These peaks should have comparable intensity for bulk silver but in our case the (202) peak which lies at the surface was almost an order of magnitude lower in intensity than the (311) peak. This is indicative of a rough surface for silver(ll1). The lack of any peak at qn H 1.05 A-l rules out any second neighbor chain tilt. The ~ ( 2 x 2 structure ) leads to a calculated area per docosanoic acid chain of 28.8 A2. Figure 3 shows the radial and 4 scans through the weak eak at q2 = 0.209 A-l. The intensity peaks at qxy= 1.254 X-l and = 60°, the same values as the intense peak. This weak peak is the result of diffraction from a layer of finite thickness. Figure 4 shows the observed data overlaid by the intensity calculated (square of absolute value of structure factor of the molecule) for the model in Figure 5 with a tilt angle of 26.7O. (The maximum of the intense peak is normalized to 1for the purpose of this comparison). The weak peak is reproduced. These model calculations also produce a peak at qz = 0 A-l with the same intensity as the intense peak, at qz = 0.532 A-l. But we have not shown it in Figure 4 because, as we have argued above, surface roughness suppresses its intensity. Discussion The contact angles for water and hexadecane confirm that the head group is toward the silver surface, as is expected from all previous studies of SA monolayers. To demonstrate that the diffraction is from the monolayers, we rinsed the films with chloroform. Figure 6 displays Bragg rod scans through qxy = 1.254 A-l and t$ = 60' as a function of the number of rinses. Chloroform dissolves the SA film of docosanoic acid. A similar conclusion was reached on the basis of the observation that contact angles for both hexadecane and water decreased markedly after the samples were washed in chloroform. Thiols and selenol films are unaffected by
Epitaxial Organic Film I
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Langmuir, Vol. 9, No. 4, 1993 1085
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Figure 6. Bragg rod of the SA monolayer of docosanoic acid on Ag(ll1). The circles show the scan on a fresh sample. The squares show scan on the same sample after a chlorofrom wash. The triangles show scan on the same sample after a subsequent chloroform wash.
washing with chloroform. This suggests that bonding of carboxylate to the silver surface is weak. There was no degradation of observed peak intensity or change in peak position for the full duration of our experiment, which was up to 3 days. Hence we conclude that, under our operating conditions, radiation has negligible effect on the structure. Even after exposure to air and rinsing with chloroform, which partially removes the film,the peak positions are unchanged (although the intensity is, of course, reduced). The ~ ( 2 x 2structure ) observed for the docosanoic acid monolayer on Ag(ll1) is quite different from that of docosane thiol on Ag(ll1) (incommensuratehexagonal) and from docosane thiol and docosane selenol on Au(ll1) ( ( d 3 X d3)R30° and distorted ( 4 3 X d)R30° 7). This is surprising in light of the similarities among the systems. The interatomic spacing between atoms in the Ag and Au lattices is almost identical and the chain length of the molecules is the same. Furthermore, thep(2x2) structure has an area per molecule of 28.8 Az which is much larger than the ~ 2 A2/molecule 1 reported for LB films, where the spacingis generally believed to be determinedbychainchain repulsion. This is not merely due to the lack of external pressure on the SA docosanoic acid film since the area density of SA thiol and selenol films is also 3r( 21 A2/ molecule. Thus head group-substrate and not chain-chain interactions must play the dominant role in the determining the structure of the docosanoic acid SA monolayer. The most likely explanation is that the carboxylate head group is bound to specific sites on the Ag surface. This conclusion is further supported by the commensurate nature of the docosanoic acid film structure. In contrast docosane thiol exhibits an incommensuratestructure and hence is not bound to specific sites on the Ag surface. Specific binding of the carboxylate is supported by surface enhanced Raman spectroscopy (SERS) and infrared (IR) studies of geometry and conformation of a number of organic carboxylic acids adsorbed on silver surfaces. These indicate that the acid is present as the carboxylate14 and that the carboxylate moiety is bound sy"etrically.gJ0J4 Furthermore, the SERS and IR data (14) Moskovits, M.;Suh, J. S. J. Am. Chem. SOC.1985,107, 6826. (15) Porter, M.D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J . Am. Chem. Soc. 1987,109,3559. Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. SOC. 1990,212,558. Nuzzo, R. G.; Korenic, E. M.; Dubois, L.H.J . Chem. Phys. 1990,93, 767.
suggest that the plane of the carboxylate group (COO-) is nearly normal to the surface. This is consistent with the model derived from the X-ray data and shown in Figure 5. This model consists of a fully extended all-trans chain with a tilt angle of 26.7'. This requires an angle of about 6' between the COO- plane and the surface normal (Figure 5), which is not detectably different from zero by vibrational spectroscopy. The dissociation of the acid moiety is reasonable since the Ag surface is exposed to air briefly and probably has some basic hydroxide or oxide which can act as a proton acceptor. Evidence for the presence of an oxide layer was presented by Pockrandet al.16who postulated the existence of approximately 3 A of oxide or hydroxide between Ag and an arachidic acid LB film to obtain a physically meaningful fit to their optical data. However, a complete monolayer is probably not present since prolonged (>1h) exposure to air inhibits SA f i i formation. The tilt angle which we calculate for the docosanoic acid chains, 26.7i1°, is significantly larger than the tilt angles (12') measured by the same technique for docosane thiols and -selenols on gold(ll1). These numbers are actually consistent if we take into account the increase in interchain spacing from 4.995 to 5.779 A. Keeping the structure and chain diameter constant leads to an ex ected tilt angle of 32' for interchain spacing of 5.779 If These results imply that chain packing is similar in these cases. In addition, in the B-solid form of stearic acid the interchain spacing and the tilt of the c-axis along which the organic backbone of the chain lies, are almost identical to values measured here.17 This indicates that the tilt angle is determined by the interchain spacing and the interchain spacing is determinedby the head groupsubstrate interactions.The chain tilt angle of 26.7' for docosanoic acid molecules is smaller than 35-40' expected for packing of all trans chains. The most likely reason is the existence of kinks in the chains. T w o adjacent kinks would increasethe chain cross section at the kink site but leave the chain nearly linear. Increasing the chain cross section decreases the observed tilt angle.
Conclusions X-ray diffraction and contact wetting angles data indicate that the docosanoic acid forms a compact selfassembled monolayer on Ag(ll1) surface exposing a close packed methyl surface to the atmosphere. The in-plane structure of this SA layer isp(2X2) with the chains within the monolayer tilted at 26.7' from the surface normal toward the nearest neighbor. The area per chain within the SA layers is 28.8 A2. These observations lead us to conclude that the carboxylate head group is bound at nearly normal orientation to specific sites on Ag surface and that this determines the intermolecular spacing. The tilt angle is a consequence of the interchain spacing.
Acknowledgment. We thank Gary Borges for preparing Ag(ll1) films and Ofelia Chapa-Perez for preparation of high-quality SA filme on the Ag substrate. We thank Dr. T. Rabedeau,David Wiesler, and Michael Toney for their interest in these results and many useful discussions. This work was done at National Synchrotron Light Source (NSLS)at Brookhaven National Laboratory which is supported by the Department of Energy. (16) Pockrand, I.;Swalen,J. D.; Gordon, J. G., 11; Philpott, M.R. Surf. Sci. 1977, 74, 237. (17) Goto, M.;Asada, E. Bull. Chem. SOC.Jpn. 1978,52, 2456.
