0 Copyright 1991 American Chemical Society
The ACS Joumal of
Surfaces and Colloids MARCH 1991 VOLUME 7, NUMBER 3
Letters Structure of an Ordered Self-Assembled Monolayer of Docosyl Mercaptan on Gold(11 1) 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 October 29, 1990. In Final Form: December 17, 1990 We report here the first surface X-ray diffraction study of self-assembled monolayers on gold(lll), providing new information about both in-plane and out-of-plane structure, and substantiating the high degree of order, both in the film and in its relation to the gold substrate. Organic monolayer films formed by spontaneous selfassembly during adsorption onto a solid from a liquid phase are a fascinating class of materials. In addition to being a new elementary class of organized assemblies, they present a tool for preparing defined organic chemical surfaces and appear to have intriguing technological applications. The self-assembly process transforms highly flexible molecules with a high number of degrees of conformational freedom into assemblies in which much of this freedom is lost. How much order is achieved is poorly understood. IR studies generally indicate order resembling the solid state. Two recent studies have indicated order. One, using transmission electron diffraction, demonstrated order in chain packing but was limited by irreversible damage to the fi1ms.l The second, usinglow energy helium scattering, indicated ordering of the methyl groups a t the upper surfacee2 Both studies of self-assembled (SA) film of docosyl mercaptan on Au(ll1) are consistent with the formation of hexagonal close packed d 3 X 4 3 R30' structure. Surface X-ray diffraction can provide substantially more information about the structures of SA monolayers since high-resolution measurements can be made of both the in-plane (nature of packing, epitaxy, etc.) and the outof-plane structure (tilt angle, head group-substrate spacing, head group bonding site, etc.). Surprisingly, the SA monolayer films survive an X-ray exposure of several hours (1) Strong, L.; Whitesides, G. M. Langmuir 1988,5, 546. (2) Chidsey, C. E. D.; Liu, G.; Rowntree, P.; Scoles, G. J . Chem. Phys. 1989,91, 4421.
0743-7463/91/2407-0437$02.50/0
in the presence of inert atmosphere a t room temperature, so it was possible to collect considerable data from a single sample. The Au(ll1) substrate was prepared by epitaxial deposition of 2000 A of Au on freshly cleaved mica.3 This evaporative deposition was done a t 300 OC and under a vacuum of lo+ Torr. The Au(ll1) on mica was washed with chloroform and then cleaned in UV/ozone before immersing it in a solution of 1mM docosyl mercaptan in hexadecane. The sample was immersed for 20 h to ensure complete formation of the SA monolayer. The quality of the SA film was checked by measuring the contact angle, which was 50.5' for hexadecane and 112O for water. These values are comparable to those measured for similar systems4 and are indicative of a good quality SA monolayer. The surface X-ray diffraction measurements were conducted on beam line X20-C a t the National Synchrotron Light Source (NSLS). The diffraction geometry used here has been described earlier.5 A Si(ll1) double crystal monochromator selected an X-ray energy of 8046.8 eV (A = 1.5408 A), as determined by calibration by diffraction from a Si(ll1)wafer. The diffraction measurements were made with 1 mrad soller slits before the detector. Radiation damage to the sample was minimized by keeping (3) Pashley, D. W. Philos. Mag. 1959,4,316. Grunbaum, E. Vacuum 1973, 24, 153. (4) Bain, C. D.; Whitesides, G. M. J.Am. Chem. SOC.1988,110,5897. (5) Samant, M. G.; Toney, M. F.; Borges, G.; Blum, L.; Melroy, 0. R. J. Phys. Chem. 1988,92, 220.
0 1991 American Chemical Society
Letters
438 Langmuir, Vol. 7, No. 3, 1991
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Figure 1. The (10) peak from SA film of docosyl mercaptan Au(ll1): (a) the radial scan at 4 = 30' and qr = 0.178 A-1; (b) the azimuthal scan at qxY= 1.454 A-1 and qz = 0.178 A-1; (c) the Bragg rod wan at 6 = 30° and qrr = 1.454 A-1. the X-ray shutter open only during the actual data collection period and by mounting the sample in a cell which was evacuated or purged with nitrogen during the diffraction measurements. The azimuthal angle, 4, is defined to be zero along the Au[Z11] direction. Figure l a shows the normalized scattering intensity obtained from the SA docosyl thiol monolayer on a Au(111)surface from a radial (8-20) scan at 4 = 30" and qz = 0.178 A-l. The scattering vector, 4 (q = 47 sin 8 / h ) , is defined such that qxy is along the sample surface and qz is perpendicular to the surface. This (10) peak shows 6-fold symmetry in 4 and the peak position, qxr = 1.454 A-l, corresponds to an interrow spacing of 4.320 f 0.005 A, in good agreement with 4.326 A expected for a d 3 X 4 3 R30" structure. From the full width at half maximum (fwhm), 0.088 A-l, we estimate the size of monolayer domains to be 70 A,substantially smaller than the 350 A of the Au substrate. Figure l b shows the azimuthal (4) scan through this same peak, at qxr = 1.454 A-' and qz = 0.178 A-I. The
normalized intensity peaks at 4 = 30'. The fwhm of 7.9' is indicative of the mosaic spread in the SA monolayer and is about 50 ?4 larger than the mosaic spread of the Au substrate. This means that the perfection of the SA monolayer is limited by the monolayer growth kinetics and not by the perfection of the substrate. Figure ICshows the rod ( q r )scan at 4 = 30" and qxr = 1.454 A-l. The intensity is corrected for background by subtracting the average intensity collected in qr scans at the same qxybut at 4 = 15" and = 45'. It is also corrected for the variation in exposed sample area and resolution function with qz following a procedure recommended by Toney et a1.6 There is a maximum in intensity at qr = 0.267 A-l. The peak is asymmetric because at very low angles the intensity of scattered X-rays is predominantly determined by the approach to the critical angle. From the sharp increase in scattered intensity from qr = 0 to 0.07 A-l, we estimate the critical angle to be 0.5", which compares favorably with 0.55' expected for gold. The existence of a peak implies that the molecules are tilted, and from position of the peak, we estimate the tilt angle to be 1 2 f 1' along the nearest neighbor direction. We also considered a model in which each molecule is tilted toward its second nearest neighbor. This gave a much poorer fit to the data. From the fwhm of 0.28 A-l, which we estimate from the fall in intensity at qr > 0.267 A-l, we calculate the projected height of individual molecules along the surface normal to be 22 f 5 A. The accuracy of the relationship between the coherence length of the domains and the fwhm is limited for very small domains. Hence although the calculated chain length is somewhat smaller than the expected value of 30.3 A,not much significance can be attached to this difference. The evaluated tilt angle is smaller than previouslyreported values for docosylmercaptan on a Au(ll1) surface as determined by infrared (IR) spectroscopy (20-34°)7-9and as estimated from modeling studies (20-38°).1w11 A reason for this difference could be that X-ray diffraction intensity is determined by coherence between rows of organic chains, whereas in IR every organic chain on the surface (oriented or otherwise) contributes to the absorbance. Thus part of this difference can be explained if we allow the individual chains which are tilted toward their nearest neighbor (R30' direction) to have additional random tilt perpendicular to this direction which results not only in the real tilt being larger but also with the projected height being smaller. A more detailed discussion is presented in the Appendix. In conclusion, our results, verify, in agreement with previous workers but with much higher accuracy, that docosy1 thiol forms a hexagonal close packed structure with d 3 X v'3 R30' symmetry on Au(ll1). We have also been able to show that the coherence length of this thiol structure is about 70 A and the chains are tilted about 12" toward their nearest neighbor (R30' direction). There is considerable disorder in the tilt, however. These results demonstrate that X-ray scattering is a useful technique for characterizing the structure of SA monolayers and means that it can probably be used to study transformations in these layers. (6) Toney, M. F.; Gordon, J. G.; Samant, M. G.;Borges, G. L.; Wiesler, D. G.; Yee, D.; Sorensen, L. B., to be submitted for publication. (7) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.Am. Chem. SOC.1987,109,3559. (8) Nuzzo, R. G.;Dubois, L. H.; Allma, D. L. J.Am. Chem. SOC.1990, 112, 558. (9) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J . Chem. Phye. 1990, 93, 161. (10) Ulman, A.; Eilers, J. E.; Tillman, N. Langmuir 1989,5, 1147. (11) Hautman, J.; Klein, M. L. J . Chem. Phys. 1989, 91, 4994.
Letters
Appendix The tilt angle can be estimated by considering how it will affect the X-ray diffraction peaks. In the case of a hexagonally close-packed monolayer, we expect six symmetrically located (10)Bragg rods which are perpendicular to the surface and at a qxy value determined by the interrow spacing. If we model the organic molecule in alltrans configuration as a rod of finite length, L, but with uniform electron density along its length, the form factor is a disk of thickness 27rlL perpendicular to the molecular axis located at the origin. Diffraction peaks will occur at the intersection of the form factor disk and Bragg rods. If the tilt is along a row of nearest neighbor molecules (R30' direction), then pairs of peaks appear at q, < 0, q, = 0, and qz > 0. The symmetric diffraction geometry only permits detection of the peaks with q, > 0 and a peak at q, = 0.267 A-1 corresponds to a tilt angle of 12". (If the tilt direction were toward the second nearest neighbor (R60" direction), then we would expect one peak at q, = -6, two at q, = -612, two at q, = 612, and one at qr = 6, where 6 = qxy tan (tilt angle). This alternative predicts two peaks in q,. Only one is observed.) The calculated
Langmuir, Vol. 7, No. 3, 1991 439 tilt angle is smaller than the reported values' is a measure of only the coherent component of the total tilt. The other component is random and perpendicular to nearest neighbor direction and hence will not affect theq, position of the diffraction peak. This random tilt may manifest itself as small domains consisting of only a few molecules such that the coherence length is much smaller than the coherence length in the R30" direction. A random tilt of this nature will not affect the position of the peak in q, but will broaden it, resulting in a low estimate for the projection of chain length along i direction.
Acknowledgment. We thank Gary Borges for preparing Au(ll1) films and Ofelia Chapa-Perez for preparation of high-quality SA films. We thank Drs. Jean Jordon-Sweet and Brian Stephenson for their help running the beam line X20-C and Drs. Michael F. Toney and David Wiesler for their interest in these results and for many useful discussions. This work was done at NSLS at Brookhaven National Laboratory, which is supported by the Department of Energy.
