Langmuir 1992,8, 1615-1618
1615
Formation of an Ordered Self-Assembled Monolayer of Docosaneselenol on Gold(111). Structure by Surface X-ray Diffraction 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 December 27, 1991
Docosaneselenol in solution spontaneously forms an ordered (self-assembled (SA)) monolayer on the gold(ll1) surface. The contact wetting angle of hexadecane (53-55O) on the SA monolayer coated Au surface is consistent with a surface exposing methyl groups which are at the end of the Czz chains. The in-plane structure of this SA monolayer was probed by surface X-ray diffraction. The SA monolayer forms an in2ommensurate structure _withan oblique unit cell with a = 5.204 A, b = 4.897 A, and y = 120O. The (6 b) axis is along the Au [2021 direction. The correlation length obtained from the radial scan was approximately60 A. The tilt angle measured from the surface normal was 15" with the chains tilted along the Au [202] direction (i.e. R30"). The oblique structure of the unit cell represents a distorted hexagonal close packed lattice.
+
Introduction Compact, well-defined, organic monolayer films can be formed at solid/liquid interfaces by spontaneous selfassociation and are termed "self-assembled" (SA) monolayers.' In general, the monolayer constituents are long alkyl chain moleculesin which there is a specificinteraction (binding) between a functional terminal group and the solid surface. The well-known examples of the formation of SA monolayers on solid surfaces are alkylsilanes on SiOdSi wafers2and alkyl mercaptans or dialkyl disulfides on gold surfaces.3 The presence of these strongly bound SA monolayers can affect the adhesivity, wetting behavior, lubricity, and reactivity of the surface. They have been proposed as photoresist materials for achieving submicrometer resolution and as active components in optical and electronic devices. In principle, the choice of the functional head group will modify both the organization of molecules and its chemical reactivity, thus allowing us to tailormake surfaces and interfaces. Furthermore, the identity of the exposed tail of the molecule offers the opportunity to modify these characteristics and control surface properties. To make these materials technologically more viable requires understanding in greater detail the parameters which determine the film structure. Although packing density and other measurements suggest an approximately close packed structure, the degree of organization and the arrangement of molecules within these f i i s are matters of speculation. Most studies on these systems utilized optical techniques, e.g. infrared spectroscopy,4ellipsometry,2second harmonic generation? etc., which provide only indirect evidence about the structure. Recent studies have generated adetailed picture of the structure of alkanethiol monolayers on a gold substrate using electron diffraction: helium scattering,' and surface X-ray diffraction.* In this paper, we compare (1) Netzer, L.; Sagiv, J. J. Am. Chem. SOC.1983, 105, 674. (2) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 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. Langmuir 1988, 4, 365. (4) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101, 201. (5) Popovitz, R.; Hill, K.; Landau, E. M.; Lahav, M.; Leiserowitz, L.;
Sagiv, J.; Hsiung, H.; Meredith,G. R.;Vanherzeele, H. J. Am. Chem. SOC. 1988,110, 2672.
the structures of alkanethiols and alkaneselenol SA monolayers prepared under similar conditions to observe the effect of head group chemistry and size. This is the first report on formation of a self-assembled monolayer from an organoselenol on gold. We find that the organoselenol films exhibit a high degree of order and have determined the lattice parameters and tilt angles.
Experimental Section The Au(ll1) substrateswere prepared by epitaxial deposition This of 2000 A of gold on a freshly cleaved mica surfa~e.~ deposition was done at 300 "Cand in a vacuum of 1PTorr. Prior to SA monolayer adsorption,the Au(ll1) on mica was washed with chloroform and then cleaned in a UV/ozone cleaner. The docosaneselenol used for the SA monolayer formation was synthesized in our laboratory from purified docosyl bromide by conventionalmeans. The SA monolayer adsorptionwas found to occur under conditions similar to those used for adsorption of alkanethiolmonolayers. In particular, the gold substrate was immersed in a 1 mM solution of docosaneselenol in n-hexadecane for about 20 h and then was washed with chloroform and dried in flowing argon. The quality of the SA monolayer was determined from the contact angle for both water and hexadecane which was measured with a contact angle goniometer. The surface X-ray diffraction measurements were conducted on beam line X10-B at the National Synchrotron Light Source (NSLS). The samples were mounted in an evacuable cell so that the environment surrounding the sample could be controlled duringthe diffractionmeasurements. The diffractiondata were collected in the symmetricgeometry, where the angle of incidence equals the exit angle, and has been described earlier.1° The Xray beam energy of 7832.3 eV (A = 1.583A) was selected by a bent Ge(111)crystaland calibratedby diffractionfrom a Si(ll1)wafer. The diffractionmeasurements were made with 2-mrad soller slits and scintillation counters were used as detectors. Air scattering was reduced by evacuating the beam path. The sample damage by the X-ray beam was minimized by opening the X-ray shutter (6) Garoff, S.;Deckman, H. W.;Dunsmuir, J. H.;Alvarez, M. S.;Bloch, J. M. J. Phys. (Paris) 1986,47,701. Strong, L.; Whitesides, G. M. Langmuir 1988,4, 546. (7) Chidsey, C. E. D.; Liu, G.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421.
(8)Samant, M. G.; Brown, C. A.; Gordon, J. G., I1 Langmuir 1991, 7, 437.
(9) Pashley, D. W. Philos. Mag. 1959,4,316. Grunbaum, E. Vacuum 1973,24, 153. (10) Samant, M. G.; Toney, M. F.; Borges, G.; Blum, L.; Melroy, 0. R. J . Phys. Chem. 1988,92, 220.
0743-746319212408-1615$03.00/0 0 1992 American Chemical Society
1616 Langmuir, Vol. 8, No. 6, 1992
Samant et al.
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Figure 1. Radial scan on a SA film of docosaneselenol on Au(111). The scan at 6 = 27' and q. = 0.356 A-l. The r axis unit is in reci rocal lattice unit 'h". The conversion is qry = (4r/
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only during the actual data collection period. The azimuthal angle, 6,was defiiedto be zero along the Au(1O) crystal truncation rod (CTR).
Results and Discussion The alkaneselenolfilms were all characterized by contact angle measurements which were consistently in the range of 110-112° for water and 53-55' for hexadecane. These contact angles compare well with those obtained for the SA alkanethiol monolayer on gold surfaces3 and the SA octadecyltrichlorosilane on silicon oxide and quartz,:! suggesting that the alkaneselenol films are of comparable quality. Our X-ray diffraction results, described below, show that docosaneselenol forms a close packed monolayer on Au(ll1). Figure 1 shows the normalized X-ray diffraction intensity obtained from the docosaneselenol monolayer on Au(ll1) during a radial scan (i.e. a scan as a function of qxywhich can be approximated as a 8-28 scan) along the direction 4 = 27' and a t qz = 0.356 A-l. The scattering vector, 4 (q = 47r sin BIX), is defined such that qxyis along the sample surface and qz is perpendicular to the surface. These data show the presence of two peaks a t qxy = 1.394 and 1.482 A-1. The peak positions were determined from the best fit of two Lorenztian line shapes raised to a power as indicated by the solid curve. These peak positions correspond to interrow spacings of 4.506 and 4.241 A, respectively. One is longer and the other is shorter than the value of 4.326 A expected for the commensurate (d3Xd3)R30° structure. From the full width a t halfmaximum (fwhm) of these peaks, we estimate the coherence length of monolayer domains as 75 and 60 A, respectively. In comparison, the Au( 111) substrate has domains with coherence length of 250A. The feature at qxy = 1.6 A-1 (h = 0.367) is not associated with the monolayer as it was absent in radial scans of other equivalent reflections. Figure 2a shows the normalized intensity for an azimuthal scan (i.e. a scan in 4) a t qxy = 1.394 A-1 and qz = 0.356 A-l. Two peaks are observed and the fit shown by solid line through the data indicate their position at 4 = 27 and 33O. The fwhm's of these peaks are 5.5 and 8.5' and are due to the mosaic spread within various domains of the SA monolayer. Since the accuracy of mosaicity is f0.5', these values for mosaic spread are comparable to 6.3' obtained for the Au(ll1) substrate. Figure 2b also represents an azimuthal scan but through the second peak in Figure 1which is at qxy = 1.481 A-l and qr = 0.356 A-l. Here again a doublet is observed positioned at I$ = 27 and 33' with the fit being indicated by the solid curve. The fwhm's of these peaks are 5.6 and 5.9', similar to those obtained before. These four peaks showed 6-fold sym-
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Figure 2. Azimuthal scans on a S A film of docosaneeelenol on A u ( l l 1 ) : (a) scan at qxy= 1.394 A-l and q1 = 0.356 A-l; (b) scan at qxy= 1.481 A-' and q. = 0.356 A-1. peak position
Table I fwhm
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metry similar to that of the Au(ll1) substrate. The experimental results are summarized in Table I. The observation of a cluster of four reflections along a symmetry direction (Au [2021 or R30') having 6-fold symmetry suggests that alkaneselenol forms an incommensurate structure with a definite orientational relationship to the Au(ll1) substrate. To help in interpretation of the data in Figures 1 and 2, we construct a diffraction pattern corresponding to the observed X-ray reflections as shown in Figure 3. The center of the diffraction pattern is marked by The filled circles denote the position of the Au crystal truncation rods (qxy = 2.516 A-l) and are 60' apart due to the 6-fold symmetry of the substrate. The "A" and "0" correspond to the reflections from the SA monolayer at qxy= 1.394 and 1.481 A-l, respectively, and are located at 3 O on the either side of the R30° direction. The reciprocal lattice vectors, A* and 8*,are indicated by the short and long dashed lines with arrow heads, respectively. A* and 8* are 60' apart. From these results we make a real space construction of the structure of the SA monolayer of docosaneselenol on the Au(ll1) substrate. Figure 4 shows the SA monolayer which is incommensurate with the underlying Au(ll1) lattice but has orientational epitaxy. The Au atoms of the substrate are drawn as circles. The chains are shown as ellipses with the major axis of the ellipse along the tilt
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Langmuir, Vol. 8, No. 6, 1992 1617
Docosaneselenol Monolayers on Gold(ll1) e
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Figure 3. Diffraction pattern of the structure of the SA film of docosaneeelenolon Au(ll1). The center of the diffraction pattern is marked by "+.. The filled circles denote the position of the Au crystal truncation rods and are 60' apart. The "A"and "0" correspond to the reflections from the SA monolayer at qIy = 1.394 and 1.481 A-I, respectively, and are located at 3' on the either side of R30'. The asterisk (*) corresponds to reflection at qly = 1.44 A-1 located along R30'.
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Figure 5. Scans along Bragg rods of the SA film of docosaneselenol on Au(ll1): (a) scan at qzy = 1.394 A-l and 4 = 27'; (b) scan at pry = 1.481 and 4 = 27'. following discussion on the tilt angle and tilt direction of the alkyl chains within the SA monolayer. Figure 5a shows a qr scan at 4 = 27' and qxy= 1.394 A-l. The intensity plotted in Figure 5a is corrected for background by subtracting the intensity for qz scans at the same qxybut along 4 = 15O and 45O. The intensity is also corrected for the variations in illuminated sample area and in resolution function as a function of qr following the procedure recommended by Toney et al.ll The intensityvariation with shows a maximum at q2 = 0.340 A-l with a fwhm of 0.19 l. Figure 5b shows normalized intensity, which is corrected as described for data in Figure 5a, as a function of qEat qxy = 1.481 A-l and 4 = 27'. A peak in intensity is observed at qn = 0.324 k1which is quite close in position to that of the peak observed in Figure 5a. The fit to the data also yielded a fwhm of 0.19
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Figure 4. Proposed structure of the SA film of docosaneselenol on Au(ll1). The circles represent the hexagonally closed packed Au atoms of the surface. The lines represent various rows with short dashed lines (qIy = 1.394 A-l), solid lines (qIY = 1.44 A-1), and long dashed lines (qry= 1.481A-1), The ellipse represent the docosaneselenolmolecule and the major axis of the ellipse shows the direction in which organic chains are tilted. direction which we will discuss in detail at a later stage. The two sets of rows which contributed scattered intensity in Figure 1are indicated by short (qxy= 1.394 A-9 and long (qxy = 1.481 A-l) dashed lines which are 120' apart. This structure represents a distorted hexagonal lattice with a distortion of 3% from a perfect hexagonal (d3Xd3)R30° lattice observed in the case of a RSH/Au(lll).68 We also expect to observe diffraction a t the reciprocal lattice vector A*-8* indicated by a solid line with an arrow head in the diffraction pattern shown in Figure 3. A*-B* lies along the R30' symmetry direction of Au and has a magnitude of 1.440 A-l. This magnitude of A*-8* is quite close to the value of 1.452 A-1 and has direction identical to that expected for a hexagonally close packed structure which was observed for alkanethiols on Au(ll1). No reflection was observed a t or close to this position and the reason for this will become clear in the
A-1.
The peak position in the q2 scan is determined by the tilt angle of the alkyl chains. In Figure 5, the observed peak position in both scans is almost identical, suggesting that the component of tilt along each of the two seta of rows is identical. Furthermore, since these two sets of rows (i.e. corresponding to A* and8*) are 120° apart from each other, as seen from the diffraction pattern, the chains are required to be tilted along a direction bisecting these two rows. Thus chain tilt along the third set of rows (Le. corresponding to A*-8*) adequately fits this requirement. This entails the peak corresponding to this set of rows be at qr = 0 A-l. Yet similar to the earlier measurements of X-ray diffraction from alkanemhiols on Au(111),8we were unable to observe this peak at qr = O A-l. The combination of factors such as use of symmetric geometry for data collection, limited fwhm of the peak, and the large critical angle for Au substrate may have caused this peak to be unobservable. An argument based on form factor of a ~~
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(11) Toney, M. F.; Gordon, J. G., 11; Samant, M. G.; Borges, G. I.;
Wiesler, D. G.; Yee, D.; Sorensen, L. B. To be submitted for publication.
Samant et al.
1618 Langmuir, Vol. 8, No. 6,1992 tilted organic chain can also be presented to substantiate the previous statements. Consider a single incommensurate domain of the SA monolayer, we expect six Bragg rods, two each at qxy = 1.394, 1.44, and 1.481 A-1 with Bragg rods at the same qxylocated diametrically opposite of each other. The magnitude and direction of the chain tilt will determine the qz position of the peaks. We can model organic chains in all-trans configuration as rods of finite length, L, but with uniform electron density along its length, such that its form factor is a disk of thickness 2ulL located at the origin with the chain axis perpendicular to the disk. If the chain are not tilted, then the form factor disk intersects all the Bragg rods in the plane of the surface leading to all peaks at qr = 0 A-l. If the chain tilt is along one set of rows (in this case those corresponding t? qxy = 1.44 A-l), then the form factor disk tilts in the direction of these rows. Hence the Bragg rods corresponding to these rows will intersect the form factor disk at qz = 0 A-l. The form factor disk will intersect the other two Bragg rods, one each corresponding to qxy = 1.394 and 1.481 A-1 above and below the sample surface. In particular, the Bragg rods at qxy = 1.394 A-1 will be intersected at qz = +61 and at -61 where 61 = qxytan (tilt angle) cos (27) with the angle of 27' obtained from eometrical considerations. The Bragg rod at qxy= 1.481 will be intersected at qz = + 6 2 and at -62 where 62 = qxytan (tilt angle) cos (33). Hence only two peaks will be observed for this particular domain in reflection experimental geometry. The 6-fold symmetry of the substrate will lead to a total of 12 peaks, and since the structure is incommensurate, we get total of 24 peaks. The tilt angles which we estimate for this structure are 15.3 and 14.6' based on the Bragg rods at qxy= 1.394 and 1.481 A-l. The fwhm of the peak in qz scan gives a measure of the thickness of the monolayer film. We estimate, based on qz scans shown in Figure 5, that the thickness of the SA film in the direction perpendicular to the surface is 33 f 5 A. This compares very well with the estimated thickness of 29.3 A based on chain length and tilt angle. It is interesting that among the two peaks observed we find that peak at qxy= 1.481A-1 is consistently more intense than the peak at qxy= 1.394 A-1. This is due to the difference in length of the monolayer domains in the two directions which are 120' apart. This is supported by the fwhm of these peaks, which for the peak at qry = 1.481 is larger than for the peak at qxy = 1.394 A-l, thus indicating that domains are shorter along rows corresponding to qxy= 1.481A-l leading to a fewer number of rows at an angle to this direction and hence to a less intense peak at qxy = 1.394 A-l. On the other hand larger coherence length along rows corresponding to qr = 1.394 A-1 results in a more intense peak at qxy = 1.481 X-l. The coherence lengths along both these rows issignificantly smaller than that for the Au(ll1) substrate, indicating that it is determined predominantly by the adsorption process of the SA monolayer. From the proposed structure of the SA monolayer of docosaneselenol, the estimated area per alkyl chain is 22.0 A2, which is in good agreement with the area of 21.4 A2 estimated for alkanethiols on Au(111).6 It is also close to the densities observed for the Langmuir-Blodgett layer a t the airlwater interface (19-34 A2).12 For these systems, the structure is sensitive to surface pressure and can be hexagonal, psuedohexagonal, or monoclinic. These systems typically exhibit chain tilting along the expanded direction, whereas we observe chain tilt along the rows
1-1
~~
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(12) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Krayer, P.; Mohwald, H. Thin SolidFilms 1988,159,17. Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopolous,P.; Elrich, S. Phys. Reu. Lett. 1987,58,2228. Barton, S . W.; Thomas, B. N.; Flom, E. B.; Novak, F.; Rice, S. A. Langmuir 1988, 4, 233.
with intermediate value of near neighbor spacing. This tilt direction toward a nearest neighbor was also observed for docosanethiol on Au(lll).* We measure a tilt angle of 15 f 1' for the SA monolayer of docosaneselenol which, though slightly larger than the measured tilt angle of 1 2 O for docosanethiol on Au(ll1) by X-ray diffraction, is definitely smaller than tilt angles of 20 to 30' determined by IR measurements for alkanethiols on Au foil of predominantly (111) texture13 and by prediction from theory.14 In spite of the fact that the SA monolayer of docosaneselenol on Au(ll1) forms an incommensurate structure as opposed to the commensurate structures obtained for docosanethiol on Au(lll1, there is a close resemblance in lattice parameter, area per chain, and tilt angle for both the structures. This suggeststhat the chainchain interaction plays a dominant role in structure determination. The head group-substrate and head grouphead group interactions have a more subtle role. Typically in diatomic systems, bonding of a metal atom with a sulfur atom is significantly stronger than with a selenium atom.16 Hence for sulfur atom as the head group, we expect much stronger substrate-head group interaction resulting in the sulfur atom being confined to a highest coordinated site on the Au substrate (a 3-fold site in the case of Au(ll1)). This leads to a commensurate structure for SA monolayer of alkanethiol on Au(ll1). Relatively weaker interaction between Au-Se allows other interactions to overcome the driving force toward formation of commensurate structure leading to an incommensurate structure. A molecular simulation study of the docosaneselenol system on Au(111)should allow for better understanding of the role of different types of interactions.
Conclusions We have been able to form good quality self-assembled monolayers of docosaneselenol on gold(ll1) surfaces through simple treatment of the surface with a dilute solution of the selenol. The high contact wetting angles observed for both water and hexadecane indicate a well formed layer presenting a dense methyl surface. We have been able to measure X-ray scattering from these surfaces. For docosaneselenol we observe an incommensurate structure with an oblique unit cell which represents a distortion of an ideal hexagonal close packed layer with an interchain spacing of 4.995 A. In this oblique structure, the interchain distance is expanded to 5.204 A along one row and contracted to 4.897 A along the other row. The set of rows with an intermediate spacing (5.039 A) lies along the symmetry direction of Au(ll1). The chains are tilted along the Au 12201 direction at an angle of 15O, and the coherence length is about 70 A. Acknowledgment. We thank Gary Borges for preparingAu(ll1) films and Ofelia Chapa-Perez for synthesis of docosaneselenol and preparation of high-quality films on the Au substrate. We thank Dr. 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. Registry No. Au, 7440-57-5; dodecaneeelenol, 71861-73-6; hexadecane, 544-76-3. (13) 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.1990,112,558. Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem. Phys. 1990, 93, 767. (14) Ulman,A.;Eilers,J.E.;Tillman,N.Langmuir 1989,5,1147. Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994. (15) CRC Handbook of ChemistryandPhysics;CRC Press, Inc.: Boca Raton, FL.
