4116
Langmuir 1994,10, 4116-4130
Structure of Hydrophilic Self-AssembledMonolayers: A Combined Scanning Tunneling Microscopy and Computer Simulation Study M. Sprik,* E. Delamarche, and B. Michel IBM Research Division, Zurich Research Laboratory, CH-8803 Riischlikon, Switzerland
U . Rothlisberger and M. L. Klein Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323
H. Wolf and H. Ringsdorf Institute for Organic Chemistry, Johannes Gutenberg University, Mainz 0-6500, Germany Received April 25, 1994. I n Final Form: August 4, 1994@ Monolayers of thiols self-assembled on Au(ll1) can be made hydrophilic by functionalizingthe surface with polar endgroups. We present scanning tunneling microscopy (STM)images of two such hydrophilic monolayers, one terminated by hydroxyl (mercaptoundecanol) and the other by amino groups (mercaptododecylamine). Both surfaces have a striped appearance but the period of the pattern is different. By comparingto STM images ofnonpolar self-assembled monolayers and the results of molecular dynamics simulation, we examine the role of the formation of hydrogen bonds between the molecules in the layer and with polar coadsorbates(water and solvent). We find that the structure of the amino-terminated layer is compatible with a hydrogen bond induced reconstruction of the clean layer. The more dense stripe pattern observedfor the hydroxylterminated layer is better explained by coadsorption with solvent (ethanol). Simulation shows that the ordered hydrogen bond configurations that might be stable for dry layers are largely dissolved by wetting. It is argued that simular wetting effects may also be responsible for the difficulties of obtaining molecular resolution in STM imaging of hydrophilic layers.
I. Introduction Self-assembly of alkyl mercaptans and disulfides on metal substrates's2 offers a way of modifying and controlling surface properties such as ~ e t t a b i l i t y and ~ - ~adhesion. To mention just one example, the selectivity and degree of protein adsorption on a self-assembledmonolayer (SAM) can be tailored by mixing hydrophobic and hydrophilic terminated alkanethiols.8 Self-assembled monolayers are also of interest since on some substrates (Ag and Au) the mercaptans form dense structures with a high degree of order which can serve as models for biomembrane~.~,'~ Moreover, SAMs,like membranes, are able to immobilize molecules, a feature that can be exploited to measure electronic and mechanical properties of single molecules." The similarity of S A M s to many biological systems of interest and the wide range of applications were the motivation of numerous experimental and theoretical studies in the past decade. One of the major objectives of this research was the detailed understanding of the Abstract published in Advance A C S Abstracts, September 15, 1994. (1)Ullman, A. A n Introduction to Ultrathin Organic Films from Langmuir-Blodgettto Self-Assembly ;AcademicPress: Boston, MA, 1991. (2)Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6, 87. (3)Bain, C. D.; Troughton, E.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J . Am. Chem. SOC.1989,111,321. (4)Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990,6, 682. (5)Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J . A m . Chem. SOC.1991,113,7152. (6)Evans, S.D.; Sharma, R.; Ulman, A. Langmuir 1991,7, 156. (7)Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. ( 8 )Prime, K.L.; Whiteside, G. M. Science 1991,252, 1164. (9)Hausling, L.;Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991,7,1837. (10)Collinson, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992,8, 1247. (11)Michel, B. InHighlights in Condensed Matterphysics and Future Prospects; Esaki, L., Ed.; Plenum: New York, 1991;p 549. @
structure of the simplest SAMs, namely monolayers on gold consisting of alkanethiols (HS(CH&lCH3) of moderate length ( n = 10-20) and their derivatives obtained by replacing the terminal CH3 group by hydrophilic functional groups, such as OH and NH2. The characterization of these systems has been carried out using a variety of techniques, including transmission electron diffraction,12intrared s p e c t r o ~ c o p y , ~optical ~ J ~ ellipsome t ~ y macroscopic ,~ wetting X-ray,15-17 helium beam diffraction18 and scanning tunneling microscopy(STM).19-27The experimental activity stimulated (12)Strong, L.;Whitesides, G. M. Lungmuir 1988,4, 546. (13)Nuzzo, R. G.;Fusco, F. A.; Allara, D. L. J . Am. Chem. SOC.1987, 109,2358. (14)Nuzzo, R. G.; Korenic, E. M.; Dubois, L. A. J . Chem. Phys. 1990, 93,767. (15)Samant, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1991,7, 437. (16)Camillone,N., 111;Chidsey, C. E. D.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K. S.; Liu, G.-Y.; Scoles, G. J . Chem. Phys. 1993,99,744. (17)Fenter, P.; Eisenberger, P.; Liang, K. S.Phys. Rev. Lett. 1993, 70,2447. (18)Camillone, N., 111; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J . Chem. Phys. 1993,98,3503-4234. (19)Hausling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H.Angew. Chem., Int. Ed. Engl. 1991,30,571. (20) Wolf, H. Diplom thesis, Mainz 1991. (21)Widrig,C.A.;Alves,C.A.;Porter,M.D.J.Am.Chem.Soc.1991, 113,2806. (22)Kim, Y.-T.; Bard, A. J. Langmuir 1992,8,1096. (23)Mizutani, W.; Michel, B.; Schierle, R.; Wolf, H.; Rohrer, H.Appl. Phys. Lett. 1993,63,147. (24)Mizutani, W.; Anselmetti, D.; Michel, B. In Computations for the Nanoscale; Blochl, P. E., et al., Eds.; Kluwer: Dordrecht, 1993;pp 43-48. (25)Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994,10,611. (26)Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Guntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994,10,2869. (27)Anselmetti, D.; Baratoff, A.; Guntherodt, H.-J.; Delamarche, E.; Michel, B.; Gerber, Ch.; Kang, H.; Wolf, H.; Ringsdorf, H. submitted to Phys. Rev. Lett.
O7~3-7463/94l2410-~116$Q4.5OlO 0 1994 American Chemical Society
Structure of Hydrophilic SAMs a series of computer simulation s t ~ d i e @ -based ~ ~ on models with varying degrees of sophistication. A detailed a b initio study of the interactions in the monolayer and the chemisorption bond to the metal substrate can be found in ref 34. The basic picture that has emerged from these studies is a (J3xJ3)R3Oo overlayer with the sulfur atoms chemisorbed to every third hollow site on the Au(ll1) surface resulting in a area per molecule of 21.4 k.The conformation of the alkyl chains is predominantly alltrans with a collective tilt of about 30" with respect to the surface normal. The tilt is approximately oriented toward next-nearest neighbors (nnn) of the hexagonal lattice.l7 Recent experiments on the CH3-terminated SAMs indicate that the hexagonal (43x J3)R3Oo layer can only be considered as an approximate description of the structure. Helium diffraction measurements18 of SAMs consisting of octadecanethiol show evidence of tetragonal distortion. The enlarged unit cell is rectangular containing four hydrocarbon chains equivalent to a ~ ( 4 x 2super) lattice. One possible explanation for this superstructure could be the ordering of the orientation of the backbone plane of carbon-carbon bonds (the twist).ls A truely hexagonal symmetry can only exist in a rotator phase with no long range order in the twist angle. Support for this argument is also provided by the observation of a phase transition at elevated temperature.16J7 Progress in the technique of scanning tunneling microscopy has made it possible to obtain a direct real space view of the superstructure. An important improvement is that the tip sample interaction could be reduced to a minimum by performing the scanning in a low current high voltage mode.25,27The new pictures of dodecanethiol on Au(111)26,27 obtained by the high resolution technique reveal large ordered domains of the (43x J3)R3Oo lattice modulated by regular arrangements of individual chains which either protruded or were depressed. The corresponding corrugation of the surface is -0.7 A. Several different types ofc(4x 2) superlattices were observed.Some domains have a rectangular structure while others have a more zigzag stripe like appearanceeZ6 Our knowledge about the outer surface of functionalized SAMs is less complete. What distinguishes the OH and NH2 terminated S A M s from monolayers with CH3 end groups is the potential of hydrogen bonding which may lead to a new type of reconstruction. Earlier STM measurements on mercaptoundecanol layers (SH(CH2)11OH)19,20showed regions with granular arrays running parallel over relatively large distances. The effective spacing was considerably larger than the dimensions of the primitive cell of the (43x1/3)R3O0lattice of the alkyl chains. This superstructure was tentatively explained by a pleating or a dimerization of the chain molecules as a result of hydrogen bonds formed between the end group^.^^!^^ Unfortunately, attempts to refine these images using the new low current techniques have not been (28)Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91,4994,J . Chem. Phys. 1990,93, 7483. (29)Hautman, J.;Klein, M. L.Phys. Rev. Lett. 1991, 68, 2345. (30)Hautman, J.; Bareman, J. P.;Mar, W.; Klein, M. L. J . Chem. Soc., Faraday Trans. 1991, 87,2031. Hautman, J.; Klein, M. L. Mol. Phys. 1992, 75,379. (31)Siepmann, J. I.; McDonald, I. R. Mol. Phys. 1992, 75,255;Mol. Phys. 1993, 79, 457. (32)Siepmann, J.I.; McDonald, I. R. Phys. Reu. Lett. 1993, 70,453. Langmuir 1993, 9, 2351. (33)Mar, W.;Klein, M. L.Langmuir 1994, 10,188. (34)Sellers, H.; Ulman, A.; Shnidman,Y.; Eilers, J. E. J.Am. Chem. SOC.1993, 115,9389. (35)Rijthlisberger, U.;Klein, M. L.;Sprik, M. Proceedings of the NATO ARW on Molecular Recognition and Self Assembly; Kluwer: Dordrecht, 1994;and J . Mater. Chem. 1994,4, 793.
Langmuir, Vol.10,No.11, 1994 4117
as successful as for the CH3-terminated layers because the hydrophilic character increases the affinity to further adsorbates such as water. Under low current conditions the tip partially images the adsorbates on top ofthe S A M , but the gap resistance is not high enough to avoid a large tip-adsorbate interaction. The varying degree of adsorbate imaging and adsorbate manipulation gives the nonregular appearance of the STM images on polar surfaces and prevents reliable molecular resolution over large areas. In the present study, which combines experiment and simulation, we will return to the problem of the structure of SAMs with functional groups with strong hydrogen bonding potential. First, more recent STM images of the undecanol system will be presented along with some results for mercaptododecylamine (SH(CH2)12NH2). These pictures will be discussed using results of molecular dynamics (MD) simulations of realistic models. In previous simulation ~ t u d i e swe ~ ~already ! ~ ~ established that the hydroxyl groups a t the surface of a (clean) undecanol monolayer are lined up in irregular chains by donating a single hydrogen bond to one neighbor and accepting a bond from a second neighbor. These findings are confirmed in the present simulation. More complex but basically similar structures are obtained for the amine system (a preliminary report of this work can be found in ref 35). The first question we want to address is whether the experimentally observed patterns can, in fact, be attributed to such an association due to hydrogen bonding. Our criterion will be direct visual inspection, i.e. we will compare STM images and graphics representation of MD configurations and decide whether the two pictures have a sufficient degree of resemblance. This approach, despite its qualitative character, is helpful and can contribute to the understanding of the STM experiments, especially in cases where the interpretation is ambiguous. Anticipating our results, we find that the answer to our question is negative for the OH system. The STM and simulation pictures ofthe N H 2 system have some features in common. However, a number of uncertainties remain which make a straightforward rationalization in terms of hydrogen bonding induced reconstruction questionable. An important issue is whether the properties of clean SAMs are at all relevant for the experimental conditions. The monolayers are scanned in open air and the hydrophilic surface with its exposed OH and NH2 groups is likely to be saturated with water. In order to determine what effect wetting has on possible surface reconstructions, we performed a series of simulations of wet SH(CHd110H and SH(CH2)12NH2 monolayers. As already noted p r e v i o u ~ l y ,a2large ~ ~ ~ fraction ~ of the hydrogen bonds between alkyl OH groups is broken and replaced by a bond with a water molecule, destroying the ordering that might be stable for clean SAMs. This solvation effect is less severe for the wet NH2 terminated S A M which is related to its reduced wettability in comparison to the hydroxyl system. Another possible coadsorbate that could affect the structure of the outer surface is the solvent used for preparation of the SAMs. The SH(CH&OH monolayers are formed by deposition of alkanethiols on a gold surface immersed in an ethanol solution. This solvent interacts strongly with the surface of the S A M , competing for hydrogen bonds. Hence, we also set up a simulation to investigate what might happen if the solvent is not completely removed after the self-assembly process. It was found that a 1:l coverage of the S A M with ethanol leads to the formation of a bilayer, which is stabilized by hydrogen bonding between the two different alcohol
Sprik et al.
4118 Langmuir, Vol. 10,No. 11, 1994 components. The configuration of the OH groups has a striking resemblance to structures seen in the STM picture.
Topography 0.5 nm 20
40
60
80nm 80nm
1;1c"
11. Experimental Results from STM
k Experimental Techniques. STM data were recorded in constant-current mode under ambient conditions. Au(ll1) surfaces formed by epitaxial growth on freshly cleaved mica exhibit terraces of up to several hundred nanometer^^^ with hexagonal atomic structure. Freshly evaporated Au(111)surfaces were immersed in oxygen-free solutions of the desired thiol. After incubation in tightly sealed and argon flushed vessels, the samples were washed 3 times with ethanol and dried in a stream of nitrogen. Advancing and receding contact angles were then measured on a part of the surface to determine the overall quality of the fresh layers. After that the samples were transferred to an STM and measured under ambient conditions. The presence of organic films was also tested in the STM by their effect on the gap modulation response and on the currentvoltage characteristics. Samples with nonpolar surfaces could be measured over long times whereas measurements on polar surfaces had to be completed within 24 h. The quality of pictures of polar layers exposed to air over longer periods degraded due to unspecified polar adsorbates. Low current detection was achieved with a specially designed current to voltage converter.37 Tips were prepared by mechanical cutting of PMr wires. STM studies of small gold islands evaporated onto SAMs were used to verify that the tip is scanning the surface with little interaction between tip and sample. The characteristic shape of the gold islands could be distinguished well from features of the SAM. Since the gold islands are simply physisorbed on the surface, they can easily be moved around with the scanning tip. Movements of gold islands were very rare when currents less than 5 PA and voltages around 1V were used. Several other techniques were used to verify that the tip is not excessively interacting with the SAM: The most convincing were force gradient measurement^^^ and combined force tunneling experiment^.^^ B. Nonpolar Surfaces. Figure 1 is an STM image of a S A M chemisorbed from a 0.5 mM solution of dodecanethiol (Fluka) during 2 h at room temperature and annealed in the same solution at 50 "C for 48 h. The sample was imaged a t a tunneling current of 9 pA and a bias voltage of 0.927 V (sample positive). It shows two terraces separated a monoatomic gold step of 2.4 and statistically distributed depressions surrounded by several molecular domains and interconnected by domain boundaries (arrow). The gold step starts at the center of Figure 1 in a screw dislocation and extends to the right side. The depressions are missing patches within the gold terraces that are created during the process of molecular selfassembly because thiols can effectively solubilize gold.40 The fraction of missing areas in the gold surface is determined by the ratio of adsorption rate of thiols versus the desorption rate of gold thiolates. The increased interaction between molecules in the SAM eventually
A
(36) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988,200, 45. (37) Michel, B.; Novotny, L.; Durig, U. 1992,4244, 1647. (38) Durig, U.; Zuger, 0.;Michel, B.; Haussling, L.; Ringsdorf, H. Phys. Rev. B 1993,48, 1711. (39)Anselmetti, D.; Gerber, Ch.; Michel, B.; Wolf, H.; Guntherodt, H.J.; Rohrer, H. Europhys. Lett. 1993,23, 421. (40) Edinger, K.; Gijlzhauser,A.; Demota, K.; Woll, Ch.; Grunze, M. Langmuir 1993,9, 4.
60
40
20
Figure 1. STM topview with gray-scale encoding (5 A from black to white) of dodecanethiol self-assembled on Au(ll1). Imaging conditions: P a r tip, 0.927 V sample positive, 9 PA. The surface shows a screw dislocationin the center ofthe image from where a gold terrace with a step light of 0.24 nm extends to the right. The atomically flat gold surface is interrupted by statistically distributed depressions and covered by domains of SAMs. Domains are delimited by straight and well-oriented boundaries (arrows) in the nnn direction of the sulfur lattice.
renders desorption of gold thiolates almost impossible and thereby inhibits further corrosion of the gold surface. The independent origin of domains leads to mismatches of the sulfur lattice as well as of the chain tilt orientation. The observation of larger molecular domains in annealed samples can be explained by a frozen tilt mismatch at room temperature which heals out a t elevated temperature while the sulfur lattice mismatches remain fixed. At the molecular level the predominant feature of the SAMs is the ( 4 3x J3)R3Oo lattice with an intermolecular distance of 5.0 A and angles of 60". The STM image in Figure 2 has been recorded at a current of 80 pA and a bias voltage of 0.885 V (sample positive) on a sample that was prepared using the same procedure as for Figure 1. The image shows two different molecular domains separated by the domain boundary which extends from the lower right to the upper left corner of the image. The monolayer also covers the depressed regions of the gold, but due to the selected gray scale representation, no molecular corrugation is visible in these black regions. The domain boundary (arrow) connects two depressions and is caused by a shiR in the regular sulfur lattice over approximately half a unit. The shiR corresponds to a change to an alternative ( 4 3x d3)R3O0lattice occupying a different subset of hollow sites of the underlying homogeneous hexagonal gold lattice. The molecular corrugation is regular in both domains with all molecules being of approximately the same height and having rotational symmetry. On other samples domains have been found that show superstructures of the (J3x J3)R30° hexagonal In that case four molecules define a rectangular unit mesh which is equivalent to c(4x 2) superlattice. Irrespective of the superstructure all domains show an area density of 21.5 f 0.5 A2 per molecule. With a collective tilt of 33",this density is consistent with a close-packed arrangement of the chains. The origin of the modulation is not understood in all detail but one possible explanation could be a
Structure of Hydrophilic SAMs
Langmuir, Vol. 10, No. 11, 1994 4119
Topography 0.2 nm
5 -e:*-
10 1E
.
-
Topography with illumination 15nm
7
14
--:+ - --.15nm
V ' r t f r
21 nm 21 nm
10
14
5
7
Figure2. STM image with gray-scale encoding(2A from black to white) on same sample as in 1. Imaging conditions: Ptnr tip, 0.885 V sample positive, 80 PA. The surface shows two SAM domains and two depressions. The domain boundary (arrow)extends from one depression to the other and is one of the few defects in Figure 1 that are oriented in the nn direction. It is caused by a mismatch in the sulfur lattice that arises from independent growth of the lattice from several nucleation centers. Consequently the sulfur lattice on the lower domains is shifted partially when compared with the upper domain.
Figure 3. STM topview with simulated illumination of mercaptoundecanol self-assembled on Au( 111). Imaging conditions: P u r tip, 0.975 V sample positive, 72 PA. The surface shows five different gold terraces and several depressions all covered with the SAM. Molecularfeatures can be distinguished on most ofthe surface. Unlike to the hexagonal lattice of Figures 1 and 2 this surface mostly shows parallel lines with a width of 4.2A that frequentlychange direction. The white protrusions (circle) are solvent molecules or contaminations which are readily adsorbed on the polar surface.
variation in the twist angles of the all-trans chains about the molecular axis. Since the chains are tilted from the surface normal, different twist conformations result geometrically in different height levels for the terminal methyl group. The determination ofthe exact height level, however, is difficult since additional electronic effects cannot be excluded and also because the absolute values for the twist angles are unknown. C. Hydroxyl Surface. Changes in the terminal functional group can strongly affect the appearance of the monolayer in a STM and are most noticeable when the nonpolar terminal methyl group is exchanged to a polar terminal group like hydroxyl or amino. The effects are 2-fold: On one hand adjacent hydroxyl groups can interact in a linear fashion and produce a striped appearance of the surface. On the other hand the presence of adsorbates on the surface can change the properties of the tunneling gap and create inhomogeneitie~.~~ The film shown in Figure 3 was adsorbed from a solution of 0.5 mM of mercaptoundecanol in ethanol during 16 h (contrary to the methyl-terminated SAM's,the polar SAM's were not subjected to annealing a t higher temperature, since they tend to accumulate contaminants when heated). Imaging was performed at a current of 6.5 pA and with a bias voltage of 0.920 V (sample positive). On the flat parts of the surface we can discern diagonal lines separated by approximately 4.2 f0.3 A. The lines run parallel to each other over distances and widths of up to 5 nm. They can suddenly change direction but the three nearest neighbor directions of the sulfur adlattice are predominant. In some
regions a hexagonal lattice similar to a CH3-terminated S A M can be observed without apparent modifications due to the functionalization. In addition to the striped areas and the parts of the layer with a flat surface, Figure 3 also shows larger protrusions which could be water, adsorbed solvent molecules (ethanol),or precipitated solvent impurities that form a metastable structure on top of the film (circle). Coadsorption of water and solvent may alter the tunneling current and can lead to instabilities. With the selected current (6.5 PA) and voltage (0.920V) conditions it is clear that the tip scans above the mercaptoundecanol chains. If, however, additional layers are adsorbed, the gap resistance is not high enough to avoid a stronger interaction between this layer and the tip. This large tipmolecule interaction together with the relatively weak bonding forces create the observed unstable structure. If the line patterns in Figure 3 are to be ascribed to the formation of chains of hydrogen bonded OH groups, the spacing d between the chains is the main experimental clue to distinguish between various alternative structures. Three of these structures are schematically depicted in Figure 4. In the layers I and 11, the chains of hydrogen bonds run parallel to a nn direction of the hexagonal lattice of the alkyl groups. In I the hydroxyl groups in a nn row are simply lined up giving a interchain separation of d = b/2 (4.35 A),where b = &is the next nearest neighbor distance. Structure I1 consists of dimerized pairs of nn rows. As a result d = b (8.7 and the density of hydrogen bonded chains is a factor of 2 smaller in comparison to I. Finally in structure I11 the chains are oriented along nnn directions with a spacing d = a (5 A). Of the layers in Figure 4 the geometry of I is the closest to the spacing of d = 4.2 f0.3 A observed in the STM image. Considering the uncertainty in the determination of absolute distances, we cannot definitely exclude structure 111. Structure 11, however, with d = 8.7 can be excluded.
(41) The accumulation of adsorbates is one of the main effects that prevented imaging of polar SAM's under vacuum conditions. Since self-assembled monolayers have to be formed in solution and then brought into vacuum, they are always covered with a water film. I t is almost impossible to remove the water layer(s1 from polar films since desorption would require heating to more than 100 "Cwhich would also desorb the thiol monolayer.
A)
A
4120 Langmuir, Vol. 10,No. 11, 1994
e
I
Sprik et al.
6 6 6
A.
f
Figure4. The three possible ordered arrangementsof hydrogen bonds between the hydroxyl groups at the outer surface of a OH-terminated SAM. a and b are the two basis vectors spanning the rectangular unit cell of the hexagonal lattice ( b = J(3)aof alkyl chains. Topography 0.1 nm 1
2
on the level of the sulfurs are typically 10 times smaller, i.e. the currents are of the order of 3 nA at 0.3 V or smaller. The image of Figure 5 shows several stripes with a distance of 7.5 They are comprised of wide bands of bright structures and narrow gaps (black lines). The darker shade of the gaps implies that they must be viewed as trenches separating the higher parts, which again are subdivided in two maxima, one to the right and one to left extreme and interconnecting ridges. The appearance of the pattern in Figure 5 is similar to structure I1 in Figure 4 and the length scales also a ee within the uncertainties mentioned before. The 7.5- spacing between the stripes is comparable to the nnn distance b. Moreover, the periodicity in the direction of the lines is around 4.5 A which also coincides with the expected molecular separation of hydrogen bonded pairs in structure 11. The monolayer shown in Figure 5 has been adsorbed from a disulfide and not from a thiol like the other samples. Comparison by STM of monolayers adsorbed from thiols and disulfides has shown no differences.20 XPS spectra of the sulfur bond to gold also suggest that both adsorbates result in the same chemical species on the surface, probably a gold thiolate, although the mechanism by which a thiol is converted to a thiolate and loses the S-H proton on adsorption is not clear.43 Despite the lack of evidence of any clear differences in chemisorption characteristics, one could speculate whether the adsorption kinetics might distinguish between possible structures of thiol and disulfide layers. For example, the kinetics of depositing disulfides on the gold surface, prior to the breaking of the S-S bond, might favor a particular modulation of the basic hexagonal structure of the layer. As discussed before, already for a nonpolar surface there seem to be several competing superstructures which could only be detected in high-resolution STM images.26 A second consideration is that the two parts of the amino-terminated disulfide are presumably paired by hydrogen bonding in the nonpolar solvent (chloroform). Hence it is not unlikely that kinetic selectivity might enhance the long range order of the disulfide layers in comparison to layers assembled from thiols.
3
4
5nm .5nm
.3
.2
.1
Figure 5. STM topview with gray-scale encoding (1 A from black to white) of mercaptoundecylamine self-assembled on Au(ll1). Imaging conditions: P a r tip, 0.3 V sample positive, 316 PA. The surface shows stripes with a width of 7.5 A which are composed of tails of dimerized molecules.
D. Amino Surface. The sample shown in Figure 5 has been adsorbed from a 2.0 mM solution of mercaptododecylamine disulfide in chloroform during 42 h.42 The surface shown in Figure 5 has been studied at lower gap resistance than the other samples. In this case the gap voltage was 0.3 V (sample positive) and the current was 316 PA. This implies that the interaction between the tip and the surface is larger. However, we can assume that the perturbation of the molecular order at the surface is still minor since the gap resistances for imaging SAMs (42) Amino groups are known to react with C02 to form carbamate salts, which may affect the surface structure of the NH2-terminated SAM's and the quality of the STM images. We are not sure about the percentage of NH2 converted into carbamates on the surface. Despite this we believe that the STM images are not dominatedby carbamates.
111. Interaction Model and Molecular Dynamics For the choice of a realistic model for the interactions in amphiphilic systems such as the wet monolayers, it is essential to include a sufficient amount of detail to describe the various competing effects without an inordinate increase of the computational cost. The various components of the potential are taken from the l i t e r a t ~ r e ~ ~ - ~ ' and our previous Below we will discuss the aspects of the model which are important for evaluation of the simulation results. For the exact values of the parameter set we refer to the original papers. A. Bonded Interactions and Constraints. Depending on the density, either a full atomic representation can be used (all-atom model)or a simplifiedmodel in which the atoms in a CH2 or CH3 group are fused into a single (43) Bain, C. D.; Biebuyck,H. A.; Whitesides, G. M. Langmuir 1989, 5, 723. (44) Ryckaert, J.-P.; Bellemans, A. J . Chem. SOC., Faraday Trans. 1978, 66, 95. (45) Jorgensen, W. L.; Madura, J. D.; Swenson, C. J. J . Am. Chem. SOC.1984,106,6638. (46) Williams, D. E. J . Phys. Chem. 1967,47, 4680. (47) Ryckaert, J.-P.; McDonald, I. R.; Klein, M. L. Phys. Rev. Lett. 1987,58, 698; Mol. Phys. 1989, 67, 957. (48) van der Ploeg, P.; Berendsen, H. J. C. Mol. Phys. 1983,49,233. (49) Jorgensen, W. L.; Briggs, J. M.; Contreras,M. L. J . Phys. Chem. 1990,94, 1683. (50)Jorgensen, W. L.; Chandrasekhar,W. L.; Madura, J. D.; Impey, R. W.; Klein, M. L. J . Chem. Phys. 79,926. (51) Jorgenson, W. L. J . Phys. Chem. 1986,90,6379; 1978,66,95.
Structure of Hydrophilic SAMs pseudoatom with an enlarged effective radius (unitedatom model). This reduced representation of an alkane is believed to be adequate in liquid^.^,^^ In dense ordered solids on the other hand, an explicit description of H and C atomsa is required in order t o account for the packing of the hydrocarbon chains.47 In earlier MD s t ~ d i e ofs ~ ~ SAMs the united-atom model was applied (see also refs 31 and 32). The justification of this approximation was thatthe areapermolecule0f21.4~~onaAu(lll)substrate is considerably larger than the 18.4 Az available for a chain in the basal plane of the hexagonal alkane solids. With the increase in detail of the recent experimental data on SAMs,16-18325p27 it has become evident that the structure is more solid than liquid-like (see discussion section IV).Indeed, with a tilt angle of 30" the area per molecule in a cross section perpendicular to the carbon backbone is 18.5 A2, close to conditions in dense solids. Hence, the application of an all-atom model is more a p p r ~ p r i a t e All . ~ ~systemsin the present study are treated in the all-atom representation. For the (dry)amino system a second series of runs was carried out using the unitedatom model, in order to verify whether a different modeling of the alkyl group has any qualitative effect on the pattern of hydrogen bonding of the NH2 groups at the outer surface of the film. A further element in the model for which certain approximations can be made is the degree of flexibility of the molecular structure. In the present simulation we have fully constrained the geometry of the CH2, NH2 and OH groups and the water molecules. The S atom was joined with the first CH2 group to a single rigid unit. The C-C, C-0, and C-N bond lengths and C-C-H, C-0-H, and C-N-H bond angles are subject to the standard harmonic potential functions. The force constants for the C-C stretching (2000 kJ/A2)and C-C-C (520 kJ/rad2)48 and C-C-H (375 bending were also used for the bonds with a carbon replaced by an oxygen or nitrogen. The S-C-C bending force constant was equally treated as a C-C-C bend. The forces on the dihedral angles between pairs of C-C bonds are modeled by the potential function proposed by Ryckaert and Bellemaw4 The orientation of the OH or NH2 groups with respect to the carbon backbone was left free. B. Nonbonded Interactions. The atom-atom POtentials are of the 12-6 form (V(r) = 4 ~ [ ( a / r) ~( ~ / r ) ~or] ) the exp-6 form (V(r) = A exp (-Br) - C/r6). Exp-6 alkyl chain parameters for the H-H, H-C, and C-C interactions in the all-atom representation were taken from studies of alkanes by Williams (set lV).46Interactions between atoms in the same chain which are three C-C bonds or less apart were excluded. The 12-6 parameters of ref 45 were used for the united-atom approximation of the methyl and methylene groups with analogous exclusion rules for coupling along the carbon backbone. The all-atom approach is crucial for the modeling of hydrogen bonding between OH or NH2 groups. One of the main advantages of atomic site models is that the geometry and even the energetics of hydrogen bonding in aqueous systems can be reproduced with sufficient accuracy by placing fractional charges on the H,0, and N atoms (the choice ofthe repulsive contribution to the short range interaction is also critical). The charges and the 12-6 parameters for the functional groups are from Jorgensen's work.49When supplementedwith the TIP4PSo water model, this set of potentials provides a consistent treatment for organic molecules in an aqueous environment. For this reason, we have adopted the TIP4P model for the water layer adsorbed on the SAMs. The Lorentz-Berthelot mixing rules aP'e used to determine the 12-6 interactions between atoms of different
Langmuir, Vol. 10, No. 11, 1994 4121 types. Since mixing rules between 12-6 and exp-6 potentials are not available, we replace the all-atom methylene groups by the united-atoms from ref 45 for the evaluation of the coupling of the OH, NH2, and water with the alkyl chains. Finally, the model for the sulfur atom ~ ~ taken from the Jorgensen potential ~ e t , with is ~also ~ ~ p ~ ~ the S-CHZ interaction again treated in the united-atom approximation. C. Chemisorption Potential. Following Hautman and Klein28the surface interaction was modeled by 12-3 potentials depending on the normal distance z only, V(z) =Al(z - zo)12- C/(z- z ~ ) The ~ . metal substrate is described as a geometrically smooth surface with no in-plane (xy) corrugation (model I of ref 28). For the methylene units the attractive l/z3 term is the result of dispersion interaction and is obtained by integrating a l/r6 pair potential over the half space occupied by the metal. For sulfur, however, the l/z3 term accounts for the chemisorption bond, which is intermediate between a bonded and nonbonded interaction. Hence, for the S atom, the coefficients (A, C, and 2 0 ) are fitted to experimental estimates of the desorption energy, vibrational frequency, and chemisorption bond length. With an explicit corrugation missing in this simple approach, the hexagonal lattice of adsorption sites is mimicked by enhancing the effective diameter (a)of the S-S interaction. The value of 4.25 assigned to a,, gives an equilibrium distance close to the hexagonal lattice parameter a = 4.97 A. The (substrate)-CHz-S angle was not subject to any bending potential. D. Molecular Dynamics. All monolayer samples in this study consist of 90 alkanethiol molecules in a rectangular MD cell replicated by 3D periodic boundary conditions. In case of the wet SAMs either the same or double the number of water molecules is added. The xy lane defining the metal substrate has dimensions 43.04 x 44.73 This almost square area is a multiple of the unit cell of the (d3x d3)R30° structure with a n n distance of 4.97 The length of the box in the z (normal)direction is 60 A. The long range forces were treated with a 3D Ewald sum (as usual in a 3D supercell geometry coupling in the z direction cannot be completely eliminated). The equations of motions were integrated using standard constant temperature MD techniques and a time step of 1 fs. For an introduction to MD simulation see ref 52. The temperature in all cases was set to 300 K. Initial configurations and equilibration will be discussed for each system separately in the next sections.
A
x
A.
A.
IV. MD for Coadsorbate-free SAMs A. C& Terminated SAMs. One of the questions concerning the structure of hydrophilic SAMs is to what extent a possible reconstruction of the outer surface by hydrogen bonding is determined by the packing of the alkyl chains below. In the modeling of SAMs this issue is closely related to the choice of the interaction potential for the alkyl chains, since the all-atom model and the computationally less demanding united-atom model are known to give different results for the orientational order.53 Moreover, since the appearance of the early MD simulations, the experimental picture of CH3-terminatedSAMs has become more consistent and complete16-18~25~27 and at the same time our understanding of the models has also increased. In particular, the phase behavior with temperature has been explored.33 The conclusion is that although the all-atom model is a distinct improvement (52) Allen, M. P.; Tildesley, D. J. Computer Simulation ofliquid; Clarendon: Oxford, 1987. (53) Bareman, J. P.; Klein, M. L. J.Phys. Chem. 1990, 94,5202.
4122 Langmuir, Vol. 10, No. 11, 1994 over the united model, neither of the two models is able to reproduce the diffraction experiments7 in all detail. 15) at ambient For chains of moderate length ( n temperature, the united-atom model predicts a monolayer with a collective tilt of about 30", in accord with experiment, but with the wrong tilt directions (nn instead of nnn). The backbone planes of the chains are partially ordered, being equally distributed between two orientations (with twist angles 4 = f90"). The distributions are broad, becoming almost isotropic with decreasing chain length. The all-atom model finds a different high temperature rotator phase33but the collectivetilt angle ( ~ 3 3 " ) and the tilt direction are in agreement with experiment. The system is more solid which is evident from the stronger ordering of the backbone planes: the twist angle distribution has the four discrete maxima familiar from simulations of solid alkanes4' and Langmuir films53using the same all-atom model. Viewed from the top the relatively ordered monolayer as obtained in the all-atom model has a n appearance very similar to the STM image of Figure 2. However, although the evidence is not conclusive, the experiment seems to indicate that even the alkyl groups of the short chain systems of interest here (n = 11,12) are in a crystalline state at room temperature. For these systems also the all-atom model predicts a rotator phase with a decreased tilt angle of approximately 22". This erroneous result would not be considered very serious if the reason for it were an underestimation of the rotator phase transition (the simulation could be simply carried out at a lower temperature to offset the difference). However, the discrepancy also extends to solid phases. The unitedatom model was not designed to be used in the solid phase, and indeed, with the hydrogens missing, the ground state of this model has no clear relation to any structure observed for solid alkanes. The ground state of the all-atom model is the well-known herringbone structure, the equivalent of the orthorombic crystalline phase of solid normal alkanes with and odd number of carbon atoms. As mentioned in the introduction (see also section 11), the experimental structure is more complicated with a t least four molecules per unit cell. In order to gain microscopic understanding of the origin of the experimentally observed superstructure, several hypothetical c(4x2) structures were investigated in ref 33 using the all-atom model described above for a longer alkane thiol ( n = 16). It was confirmed that the proposed structures were energetically unfavorable with respect to the herringbone phase with its two molecule unit cell. In their evaluation of differences between simulation and experiment, the authors ofref33 suggest that the substrate may play a more subtle role in the determination the monolayer structure than is reflected in the simple adsorbate-surface potential used here. For example, in ref 27 it is proposed that the modulation of the surface profile is due to a subtle variation in the ordering of the twist angle (see also section 11). A stabilization of a superstructure of twist angles requires a realistic model of the energetics of the variation of the bond angle of the C-S bond and the substrate. Such a term is absent in the interaction potentials we have used in our previous calculations. Other possible mechanisms could involve an overlayer induced reconstruction of the S-Au interface. Unfortunately, we have not yet refined our models to allow a more complete description of the C-S-Au interaction^^^ and will continue to use the available all-atom model despite its flaws. B. The Alkyl Layer of Hydrophilic SAMs. In view of the observation that the motion of the backbone plane is not completely frozen out in a MD simulation, we first
Sprik et al. verified whether replacing the CH3 groups by OH or NH2 groups destabilizes the rotator phase. For this purpose an initial configuration was set up with the chains perpendicular to the surface and a uniform orientation of the carbon bonds. In response to the increased free volume in this upright configuration, the twist angles disorder with a simultaneous canting of the alkyl chains. A stable collective tilt is established in 20-30 ps accompanied by a gradual reordering of the twist angle. While the chains relax and the density of the alkyl layer increases, the first hydrogen bonds are formed. The monolayer obtained a€ter 50 ps of equilibration has an alkyl chain structure that is very similar to what can be expected for CH3-terminated thiols at 300 K described by the same The average tilt angle of 20" is 30% less than the experimental value, which would correspond to a close-packed crystalline state. The reduced value for the tilt predicted by simulation is typical for partially ordered rotator phases before full orientational disorder sets in at higher temperature. The tilt direction is nnn. The twist angle distribution is also consistent with a structure intermediate between a solid and a fully disordered rotator phase. The characteristic f50",f130" maximima can still be discerned but with considerable broadening. The relaxation from a zero tilt configuration was performed for both the OH- and NH2-terminated systems. No significant differences between the two systems could be observed. The aggregation ofhydrogen bonded clusters at the outer surface is a very slow process that continues after the tilted monolayer of alkyl chains has formed. The structure of the alkyl layer remains stable and essentially unchanged during the slow reconstruction and annealing of the configuration of hydrogen bonds. The only noticeable effect is a modest increase in the number ofgauche defects for the first few carbon bonds attached to the functional group. Hence, in order to accelerate the initial equilibration stage, some of the subsequent MD runs of dry and wet S A M s have been started by setting up the alkyl chains in a herringbone structure uniformly tilted by 20". This ordered initial configuration relaxes relatively quickly to the rotator phase described above. C. OH Hydrogen Bonding. The structure for a OHterminated SAM (n = 11) generated by 300 ps of equilibration is shown in Figure 6. Virtually every OH group participates in the hydrogen bonding, either in small clusters of three or four molecules or chains consisting of a larger number of OH groups. The chain configurations dominate giving the surface a striped appearance. The finite length ofthe chains (typically 10 molecules) and the variationofthe orientation inhibits alignment in a pattern with long range order. In ref 29 the SH(CH&OH layer was studied applying the united-atom model for the alkyl groups. A similar reconstruction was found, despite the difference in tilt direction and distribution of the twist angle (see also ref 30, where the issue of the model for alkyl groups is already briefly discussed). At a first glance, Figure 6 and the STM picture discussed in section I1 (Figure 3) have a certain resemblance. The STM image clearly exhibits parallel stripes which could be interpreted as chains of hydrogen bonds. Moreover, the frequent turns and zigzags appear intrinsic, which suggests that long range order is absent. The very short length of the chains in Figure 6 in comparison to the experimental structure can be easily attributed to insufficient annealing during the limited time of the MD run. However, a straightforward explanation of the stripe pattern in STM picutre in terms of the hydrogen bond induced reconstruction in Figure 6 is not supported by quantative comparison between experiment and simula-
Structure of Hydrophilic SAMs
Langmuir, Vol. 10, No. 11, 1994 4123
Figure 6. Top view of the hydrogen bond induced reconstruction of mercaptoundecanol self-assembled on Au( 111). The picture shows the final configuration of a 0.3-ns MD run at 300 K. Oxygens are red, the hydroxyl hydrogen atoms are light gray. The carbons and hydrogen atoms of the alkyl groups are dark gray and dark blue, respectively. Sulfur atoms are yellow. The gold substrate is not indicated.
tion. In section I1 it was argued that on the basis of the 4.2-A spacingof the lines the STM image must be identified with either structure I or 111. The configuration of hydrogen bonds obtained by simulation, on the other hantl, is a disordered version of structure I1 with a 8.7-A separation between the stripes. Structures I and I11 turn out to be not stable in our simulations. The reason must be related to a basic rule from aqueous solution chemistry: hydrogen bonding is a highly directional and effectively short range interaction with well-defined 0-0 distances of ~2.6-3.0 A. The average distance between nearest neighbor thiols on gold (a = 4.97 A)is considerably larger than this bonding range. Hence, in order to concatenate the OH groups, the tails of the molecules in a stripe have to bend toward each other. Accommodating the mismatch in distance is easier for chains with a larger number of molecules per unit length. Hence the structye I1 with compact chains of two OH groups per a = 5 A is preferred over the more extended arrangements of structures I and 11. The straight chains of structure I are clearly the most unfavorable in this respect. The conclusion of our analysis is that because of the failing quantitative agreement for the length scales, it is doubtful that the hydrogen bonding in Figure 6 offers a credible explanation of the stripe patterns of the STM image. D. N H 2 Hydrogen Bonding. NH2 groups differ in their hydrogen bonding potential from OH groups in two respects: The strength of a bond in terms of binding energy is less. On the other hand, the number of bonds that a single group can form is higher. In bulk liquids, the hydrogen bonding between nitrogen hydrides is in general weaker than between oxygen hydrides (for example, compare ammonia to water or methanol). In order to ascertain if this trend can be extrapolated to hydrophilic SAMs, we have carried out an independent series of simulations for the amino system.
Out of concern for possible long lasting memory effects of the starting configuration, we followed the same procedure as for the OH SAMs. The sample generated from a initial state with molecules standing upright and a 20" tilted herringbone structure were compared. After equilibration times of 0.2-0.4 ns the two systems evolved to a qualitatively similar state. In contrast t o the OH system, the initial order in the herringbone structure seemed to frustrate the formation of hydrogen bonds. As a result, NH2 group association proceeded in the rotationally disordered zero tilt system at a faster rate. The final configuration of the latter system is shown in Figure 7. Since in comparison t o OH groups the hydrogen bonding for NH2 is weaker, the coupling to the organization of the alkyl layer may be stronger. Simplifyingthe modeling of the alkyl chain t o the united-atom approximation and repeating the simulation, we verified that the basic features of the hydrogen bonding are indistinguishable from the results for the all-atom model. The same types of clusters and strings are formed. However, if the order in the system is quantified by some criterion, for example the length of hydrogen bonded chains, it turns out that the amino system is more ordered in the united-atom case. One could speculate that this effect is related to the smaller barriers for rotation in the alkyl layer, which facilitates hydrogen bonding. A similar mechanism may also be the origin of the difference in the relaxation rates we observed for the two initial configurations of the all-atom simulations. The confrontation between simulation and experiment has a more positive outcome in the case of the NH2-coated SAMs than in the one of the OH. In the image of Figure 5 we can discern regularoarrays of spots. The spacing between these rows is 7.5 A. Given the uncert2inty in the STM d9termination of absolute distances ( ~A),1 length of 7.5 A agrees with the nnn parameter b = 8.7 A of the
4124 Langmuir, Vol. 10, No. 11, 1994
Sprik et al.
Figure 7. Top view of the hydrogen bond induced reconstruction of mercaptododecylamine self-assembled on Au( 111)as obtained from a MD run a t 300 K. Nitrogen atoms are light red, the amino hydrogen atoms are light gray. The mercaptan groups (alkyl chain plus sulfur atom) are represented as in Figure 6.
( 4 3x 2/3)R30°lattice. Such a structure is compatible with chains of hydrogen bonds aligned in nn directions (structure I1 in Figure 4). These nn chains spontaneously emerged in the simulation of the OH system (Figure 6) and can also be recognized in Figure 7 for the NHZterminated SAM. In order t o expose the order inherent in Figure 7 more explicitly, the united-atom version of the amino system was slowly cooled down to 50 K. The result is shown in Figure 8. Two features are immediately evident from this picture: the nn tilt which is characteristic for the unitedatom model and the long rows of dimerized amino groups making a 60" angle Yith the tilt direction. The separation of the chains is 8.7 A. If the lack of long range order can be ignored, the resemblance between the experimental picture and the annealed system of Figure 8 seems t o suggest that, unlike the OH case, the STM is indeed probing a hydrogen bonding induced surface reconstruction of the SH(CH&2NH2 layer. It is difficult to understand, however, why the rows in the STM are so remarkably straight, while in simulation and also in the experimental OH pictures the patterns only show local order. In section I1 we discussed the possibility that the enhanced long range order in the amino system might be related to the use of disulfides in the self-assemblyprocess.
Indeed, the contact angles ofNH2-terminated SAMs ( ~ 5 0 ~ ) are surprisingly large considering the hydrophilic nature of the surface.20 The values for OH-coated layers are The experimental data in this system, however, are somewhat ambiguous due to the substantial difference between the measurements with advancing and receding droplets.20 The MD simulation of wet OH- and NH2-terminated SAMs has therefore a double aim: First, we want to investigate whether wetting can give rise t o stable surface reconstructions that can be observed by STM. Second, we want t o see if the modeling is able to discriminate between the wetting of the OH and NHB surfaces and thereby gain some microscopic insight into the origin of the differences in wettability between the two systems. A. Initial States and Equilibration. Before discussing the results of the simulation, we will summarize the procedure for the preparation of the systems. The runs were started from fully ordered layers, utilizing a coincidence for the dimensions of the substrate: The lattice parameter of a Au(ll1) surface (a = 2.87 A) happ$ns to be very close to 00 distances in ice (roo = 2.8-2.9 A). For water molecules directly adsorbed on (111)surfaces of noble metals, the near commensurability with ice leads to stable ordered (slightly distorted) hexagonal layer^.^^?^^ This suggests that a t certain specific coverages, water on a hydrophilic SAM might also condense in an icelike structure with the functional groups anchoring the layer down on the outer surface. With this possibility in mind, the ordered initial states were relaxed with some care at low temperature. The thiol sublayer is again set up in the 20" tilted herringbone structure. The molecules are in all-trans conformation; hence the functional groups at the surface are ordered on a hexagonal lattice with a uniform
V. Effect of Coadsorbates Covering a hydrophilic SAM with a layer of molecules capable of hydrogen bonding may have a drastic effect on the structure of the outer surface. Hydrogen bonds with the coadsorbate may completely disrupt the dimerization of OH and NH2 groups and alter or even inhibit reconstruction. However, bonds formed with the surface complete with the hydrogen bonding in the coadsorbate overlayer itself. In particular, in the case of water, the cohesive forces in a cluster may be so large that waterwater association is preferred. Aggregation of water (54) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. molecules is reflected in a reduced surface ~ e t t a b i l i t y . ~ - ~ ? ~ O ( 5 5 ) Siepmann, J. I.; Sprik, M. Surf. Sei. Lett. 1992,279, L185.
Structure
of
Hydrophilic SAMs
Langmuir, Vol. 10, No. 11, 1994 4125
Figure& Top1riew of an mercaptododecylamine layer obtained by cooling an annealed room temerature MD configuration to 50 K. In contra:;t t o the results shown in other figures, where an all atom model is used, the low temperature structiure shown here was obtaine!d with an united atom model for the alkyl chains which are represented here by the C-C bonds of the backbone.
orientation of OH and NH bonds. The water molecules are placed on the ( d 3 x d 3 ) lattice as follows: With the A sites occupied by OH groups (adopting the terminology of hcp layers), one can insert water molecules in the empty B or C sites o r both the B and C sites and create a dense 1:l or 1:2 structure that satifies the well-defined distance criteria of hydrogen bonding in water. Due to the larger repulsive radius, it is more difficult to fit amino groups in an ice lattice; hence HZO/NH2 bilayers were constructed by raising the water sublattice above the NH2 plane by a small distance. The equilibration was initiated by first eliminating unfavorable orientations of the H atoms with the centers of mass of NH2, OH, H20, and CH2 groups fixed. Next, this constraint was released and the positions of the heavy particles (N, 0) were optimized by annealing a t 100 K. Even at these low temperatures it was not possible to stabilize an icelike structure with long range order. Moreover, upon heating the system the water molecules invariably started clustering and the spatial homogeneity of the coadsorbate distribution is quickly lost. The conclusion is that, on the basis of our simulations, the occurrence of icelike overlayers on hydrophilic SAMs can be ruled out. B. Wet SAMs. The configurations of a 1:l watercovered SH(CH2)llOH SAM after 0-3 ns of equilibration are shown in Figure 9. The most striking feature is the fluctuation in the thickness of the water layer (see side view in Figure 9a). In some places the water molecules are piled up two or even three levels high, while other spots are left almost bare (top view Figure 9b). The tendency of the water layer to segregate into wet and dry regions was already noted in refs 29 and 30 where the MD run for a 1:l system could be extended t o 0.7 ns, applying the computationally less demanding united-atom model for the alkyl system. The results were qualitatively similar. Moreover, when the number of water molecules is doubled (see Figure 9c) the empty regions are not filled up. These two observations suggest that the microscopi-
cally inhomogeneous mode of wetting, intermediate between a uniform layer and aggregation in droplets, is an equilibrium state. Unfortunately, a firm statement about the stability of the structures of Figure 9 is prohibited by the usual limitations in the length of MD runs. Returning to the question whether an ordered hydrogen bonding induced surface reconstruction can coexist with wetting, we show in Figure 10 a view of the OH SAM from below. The alkyl chains in the foreground are rendered in a representation which brings out the hydroxyl groups in the background. Figure 10 clearly reveals that the hydrogen bonded chain structure proposed for clean SAMs is “dissolved”by the water layer. Occasionally, two chain molecules dimerize by direct hydrogen bonding, but the larger fraction of OH groups is fully coordinated by water molecules and hence separated from each other by at least one shell of water. Figure 11shows the final configuration of a 0.3-ns MD simulation of a 1:l water SH(CH&NH2 SAM. The process of water cluster formation has advanced considerably further in comparison to the OH system. The area from which the water has retreated is larger (top view Figure l l a ) and the maximum distance of water molecules from the NH2 surface is also increased (side view Figure l l b ) . The enhanced aggregation of water can be interpreted as an indication of the reduced wettability of the amino SAM. A reliable determination of the relatively small wetting angles for hydrophilic surfaces is beyond the capabilities of current MD technology (see, however, ref 29 for MD estimates of wetting angles of hydrophobic SAMs). In default of this, we can express the difference between the wetting of the OH and NH2 layer in terms of the waterwater coordination as obtained from the 0-0 radial distribution functions. The integral of the first goo peak for the 1:l OH system adds up to n = 1.4, in comparison to n = 2.1 for the NH2 system. The implications for STM samples of SH(CH2)12NH2 SAMs exposed to water vapor are less clear. For the
4126 Langmuir, Vol. 10, No. 11, 1994
Sprik et a2.
Figure 9. Final configurations of a MD simulation of a wet mercaptoundecanol layer with a 1:l ratio of water molecules and hydroxyl groups ((a)side view, (b)top view) and a 2:l layer with twice the number of water molecules ((c)top view). Water molecules are represented by green balls for the oxygens and light gray balls for the hydrogen atoms. Color coding for the atoms of the S A M is as in Figure 6.
Structure of Hydrophilic SAMs
Langmuir, Vol. 10,No. 11, 1994 4127
Figure 10. Same configuration of a 1:lwet mercaptoundecanol layer as in Figure 9 viewed from below. The hydrocarbon chains in front are given in a stick rendition in order t o bring out the details of the hydrogen bonding between water and OH groups in the back.
SH(CH2)110Hlayers, dimerization of the type shown in Figures 6,7, and 8 will be destabilized by water solvation effects provided the surface is fully covered. However, because of the reduced affinity to water, it cannot be excluded that extended reconstructed patches are left exposed. Moreover, in addition to the size, there is a qualitative differencein the nature of open spots in Figures 9 and 11. The OH SAM is nowhere completely dry. There are always one or two isolated water molecules nearby. These molecules may be part of a less dense but uniform sublayer, which is more tightly bound to the OH system than the waters on top. For the NH2 SAM, this phenomenon of a prewetting layer seems to be absent. C. Ethanol as Coadsorbate. Remnants of the ethanol used as a solvent can interact with OH-terminated SAMs in a particularly efficient way because of the structural relation between simple alcohols. Not only is the energetics and geometry of the hydrogen bonds very similar (in fact identical in our model) but the diameters of the alkyl tails also match. An obvious trial configuration for an ethanol covered OH terminated SAM is therefore a 1:l bilayer with the alkyl chains in the 20" tilted herringbone structure and the ethanol upside down in a similar hexagonal arrangement. By displacingthe ethanol with respect to the thiol com onent by 2.87 A (from the A to the B site of the (1/3x&) lattice, we can construct an ordered hydroxyl interface analogous to the H20/0H layers which served as initial state for the wet SAMs. In contrast to the water OH structures we examined before, the ethanol bilayer is stable. When the MD is started at low temperature the hydroxyl groups quickly line up in long chains, with the links alternatingly supplied from below and above. Instead of aggregating upon heating, the coadsorbate layer remains uniform. The most common thermally induced defect is a cross-linking of the chains, leading to characteristic zigzag patterns when two ends of a pair of chains interchange. The final configuration of a 0.2-11s trajectory at 300 K is shown in Figure 12.
From the side view in Figure 12a it can be seen that occasionally an ethanol molecule is lifted slightly above the well-defined hydroxyl interface. These molecules mark the interruption or termination of a chain of hydrogen bonds. None of the ethanol molecules has reversed its orientation. The ethyl groups, as a consequence of the short length, are in a fully disordered rotator phase. The pattern of hydrogen bonds in Figure 12b has a striking resemblance with the STM image of the SH(CH2)110Hlayer. The agreement also holds in quantative respects. Since the density of OH groups is twice higher in comparisonto the clean OH SAM ofFigure 6, the spacing between the hydrogen bonded chains is reduced. The separation between the longer sections of chains is 5 A in agreement with the structure in the STM picture (the separation near turns varies). Is the visual similarity between simulation and experiment sufficiently compelling to conclude that the STM samples are indeed saturated with ethanol? For a truely convincing argument, quantitative information on the thermodynamics of the coadsorption is needed. These data are difficult to obtain from MD simulation. However, the structural features of the bilayer already suggest that ethanol is a particularly persistent solvent and special care must be taken to remove it from the surface after the assembly process. The presence of an ethanol overlayer can also be expected t o affect wetting properties. In fact, the large difference in the advancing (0, = 40") and receding 04 < 3")contact angle for OH functionalized SAMs reported in ref 20 could perhaps be rationalized by the interaction of the ethanol and the droplet used in the measurement of contact angles. Immediately after the droplet is applied and is expanded on the intact bilayer for the determination of the advancing angle, the exposed ethyl groups will give the surface a more hydrophobic character. However, once covered by the relatively large quantity of water, the
4128 Langmuir, Vol. 10, No. 11, 1994
Sprik et a2.
Figure 11. Final configurations of a MD simulation of a wet mercaptododecylamine layer with a 1:1ratio of water molecules and amino groups ((a)gives a top view, (b) a side view).
ethanol top layer will disolve. Hence, only at the subsequent stage of the measurement of the receding angle, when the droplet is retracted, is the layer of OH groups attached to the SAM probed.
VI. Summary and Discussion The aim ofthis combined scanning tunneling microscopy and computer simulation study was to explore the structure of the outer surface of hydrophilic SAMs. The focus was on the effect of hydrogen bonding by the polar end groups of OH- and NHZ-terminated SAMs. Hydrogen bonds within the layer compete with bonding to coadsorbates such as water and polar solvents. By comparison of STM images of functionalized SAMs to high-resolution pictures of non polar CH3 terminated SAMs,it was found that the surface of the polar SAMs shows various possible superstructures that are not observed for the nonpolar surfaces. However, attaining molecular resolution is considerably more difficult for the case of the polar surfaces. Hence, a further objective of this investigation was to try to understand how polar coadsorbates might affect the imaging conditions for hydrophilic SAMs.
The contribution of the molecular dynamics simulations was to help identify the various superstructures. The OH-terminated SAM (mercaptoundecanol) studied here is a good example of the interaction between experiment and simulation. The hypothetical hydrogen bonding induced reconstruction of the dry monolayer, which matches the experimentally observed striped geometry best, turns out to be not stable in the course of a MD run under ambient conditions and must therefore be excluded on thermodynamic grounds. On the other hand, the energetically stable state obtained from a long annealing run at 300 K, although qualitatively similar, fails to reproduce quantitatively the dimensions of the STM pattern. As a solution to this dilemma, we proposed a structure consisting of the SAM coadsorbed with a monolayer of the polar solvent that is used in the process of sample preparation (ethanol). This bilayer satisfies the criterion of structural similarity and remained intact over the length of a long MD run. The STM picture of NHZ-terminated SAM (mercaptododecylamine) was also found to have a striped appearance. The spacing between the stripes is almost twice as large in comparison to the mercaptoundecanol
Structure
of
Hydrophilic SAMs
Langmuir, Vol. 10, No. 11, 1994 4129
I Figure 12. Mercaptoundecanol S A M coadsorbed with a monolayer of ethanol on top in 1:l ratio. A side view is given in (a)and a top view in (b) with stick rendering of the hydrocarbon parts of the alcohols. Oxygen atoms are red and the hydrogen atoms of the OH groups light gray. The chains of hydrogen bonds are formed with hydroxyl groups alternating between top (ethanol) and bottom ( S A M )half of the bilayer.
system, and in better agreement with the configuration of chains of dimerized molecules that were spontaneously formed in the MD simulation. A possibly significant discrepancy between simulation and experiment is the high degree of long range order in the STM image (the remarkably straight lines in Figure 5). As a tentative explanation we suggested the us.e of disulfides for the formation of the monolayer. Even though the sulfursulfur bond is assumed to be broken by the formation of the chemisorption bond, the kinetics of self-assembly of dimers bound in solution on one end by a disulfide bond and on the other by a hydrogen bond between its amino groups could affect the structure of the layer. The modeling of water-covered SAMs confirmed the drastic effect of wetting. Hydrogen bonds between pairs of polar groups belonging to the SAM are almost completely replaced by bonds with the coadsorbate leading to a dissolution of any regular hydrogen bonded pattern that is stable for a dry SAM. Moreover, our simulation results indicate that wetting of the SAM surface is inhomogeneous
with considerable modulation in the thickness of the water adlayer. This effect was more pronounced for the mercaptododecyl S A M ,leading t o aggregation of clusters of water molecules. Partial wetting (on a microscopic scale) might also be an important factor in the apparent deterioriation of molecular resolution of STM imaging of hydrophilic (polar) SAMs in comparison to hydrophobic (nonpolar) surfaces. The various techniques which are used to assure a minimum of tip-sample interaction (see section 11)are far less effective for the kind of roughened wet surfaces that is suggested by the simulation results. Some of the inconsistencies between experiment and simulation must be blamed on the usual limitations inherent in simulation, namely inaccuracies in the modeling and insufficient run times. For example, one of the more serious shortcomings of our modeling is the prediction of a rotator phase for SAM’s under ambient conditions and moderate alkyl chain length. This simulation result conflicts with experiment which finds more solid ordered structures. Another issue is the modeling
4130 Langmuir, Vol. 10, No. 11,1994
of the sulfur-gold interaction which almost certainly is too simple to describe the details of the chemisorption bond. In fact, it is possible that certain aspects of the chemisorption not accounted for in our model are responsible for the experimentally observed superstructures in methyl-terminated SAM’s. In addition, there are differences in the conditions imposed by the environment of the experimental and model systems: (a)The simulationis carried out with the surface either exposed to vacuum or a thin layer of water or solvent whereas the experiment requires the presence of a large metallic tip in the vicinity of the surface and several layers of water with contaminants. (b) The simulation takes place in a field-freeenvironment whereas STM imaging requires a voltage to be applied between tip and sample which generates a huge field of the order of 5 x lo9 V/m. This field is again partly screened by dielectric response of the monolayer. (c) The lateral scanning process can put strain on the surface and cause reordering of the molecules either directly or via the
Sprik et al. omnipresent water or solvent molecules. Finally, without special precautions there is always the possibility that the tip probes structures at a lower level in the SAM or even electronic effects at the sulfur-gold interface in addition to molecular features at the outer surface. As a concluding remark, we want to emphasize that hydrogen bonding (hydrophilicity) which makes SAMs with polar endgroups interesting and potentially useful systems is at the same time a complication for STM imaging. Clearly further developments in scanning and sample preparation techniques are required.
Acknowledgment. This research was supported, in part by the US National Science Foundation and the National Institutes of Health. Some of the calculations were carried out using the facilities at Cornel1 provided under MCA 93-SO20. E.D. acknowledges support from the Swiss National Foundation project 2 4 f . H.W. and H.R.thank DFG/Germany.
