X-ray photoelectron spectroscopy of partial and stamped thiol-based self-assembled monolayers

Supramokcular

Science 4 (1997) 247-253 F ’ 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved

PII:

SO968-5677(97)00011-4

0968-5677/97/%17.00

ELSEVIER

X-ray photoelectron spectroscopy of partial and stamped thiol-based self-assembled monolayers S. D. Evans*, S. D. Cooper, S. R. Johnson and T. M. Flynn Department

of Physics,

University

of Leeds, Leeds LS2 9JT, UK

and A. Ulmanl

of

Department Chemistry, Polytechnic USA (Received 6 September 1996; revised

University

at Brooklyn,

16 December

Brooklyn,

NY 11201,

1996)

photoelectron spectroscopy was used to explore the local environment of a perfluorinated thiol derivative incorporated into an alkanethiol matrix. The alkanethiol octadecylmercaptan (ODT) was formed via a printing p.rocedure and samples taken at various stages of stamp depletion were investigated. Compared with the spectra obtained from a monolayer of the pertluorinated material adsorbed directly from solution, one can see distinct differences in the shape of the F 1s peak. We present possible interpretations of this behaviour from a molecular perspective. 0 1997 Elsevier Science Ltd. All rights reserved. X-ray

(Keywords: X-ray photoelectron spectroscopy; self-assembled monolayers)

INTRODUCTION One of the most significant breakthroughs in the relatively short history of alkanethiol-derivatized selfassembled monolayers (SAMs) has been the development of patterning techniques to produce multi-functional surfaces. The two techniques being most widely advocated, at present, are: (1) a ‘stamping’ procedure in which an alkylthiol derivative (either in ethanol as a solution or in its neat form) is absorbed into an elastomeric stamp and subsequently transferred by bringing it into contact with a clean gold surface; and (2) a ‘photo-patterning’ procedure in which a pre-formed monolayer is irradiated with UV radiation through a mask to oxidize the thiol (in a specified region) to the sulfoxide, which is subsequently removed by washing with wate?. Both techniques have their relative merits and both significantly widen the range of applications for which SAMs may be used. Monolayers formed by using the ‘stamping’, or ‘printing’, technique tend to yield films with slightly lower average thicknesses than their counterparts formed via direct adsorption from solution5. This probably arises as a result of the significantly shorter contact time used for transfer of the ‘thiol ink’ compared with the much longer adsorption times usually used for the formation of monolayers. If a * To whom correspondence

should

be addressed

substrate patterned in such a way is placed into a solution containing a second alkylthiol derivative, then this derivative will adsorb not only on the regions in which the first monolayer was deliberately not printed but will also penetrate the previously ‘stamped’ region. Such penetration of the second component into the first patterned region may be either a benefit or a hindrance, depending on the desired application and on the form of the penetration; i.e. whether one has island formation (on the meso scale) or homogeneous penetration can be either useful or a nuisance. During a recent study of two-component SAM systems formed by using the ‘stamping’ (or ‘printing’) procedure, we observed that upon placing an octadecylthiol (ODT) stamped monolayer into a solution of perfluorinated thiol (PFl), molecules of PFl penetrated into the ODT region of the film6. This was interesting for two reasons: (1) the F 1s peak (measured by X-ray photoelectron spectroscopy, XPS) from regions of PFl in the ODT film was different from that found for the pure regions of PFl (i.e. adsorbed from solution) and (2) because it showed the usefulness of XPS in its imaging mode for mapping the uniformity of SAMs. It is the former point, however, that we address here. That is, why should there be a difference in the F 1s peak for single-component SAMs of PFl adsorbed from solution compared with the cases where molecules of PFl have penetrated a stamped ODT

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SAM? This is interesting because, in addressing this question, one must consider how such differences could arise and thus may possibly show how XPS can be used to obtain additional information regarding the monolayer quality. The use of XPS for studying monolayers on solid supports has a long history and has largely been used for confirmation of chemical composition. By performing angle-dependent studies one can obtain some structural information; in particular, one can effectively acquire a depth profile of the chemical composition and hence molecular orientation7-’ ’ . In general, XPS measures the kinetic energy of electrons emitted from a surface due to photo-excitation. Careful energy analysis allows one to identify the chemical species from which the electrons originated and hence gives chemical specificity12. Additionally, since the electron mean free path is strongly attenuated due to scattering, only electrons originating close to the surface are collected and this facilitates positional information to be obtained by performing angle-dependent studies7--9. Additional factors that may affect the energy of the emitted electrons arise owing to local field effects, such as band bending, or surface charging’“.“. 13. It is generally thought that the structural properties of a material play an insignificant role in the electron kinetic energy. In view of our previously mentioned results for E-X imaging, we have sought to clarify how the observed differences do indeed occur. We have approached this in two ways. Firstly, we have used the stamping technique to produce a uniform monolayer and then, by repetitive use of the stamp onto clean gold surfaces, produced a sequence of progressively depleted stamped monolayers. We subsequently exposed these depleted monolayers to a perfluorinated thiol solution to allow molecules of PFl to penetrate into the depleted monolayers. Our second approach has been to form monolayers of the perfluorinated derivative for successively longer adsorption times, directly from solution. It was hoped that a comparison of these two experiments would elucidate why the F 1s peak obtained for the perfluorinated molecules incorporated into an ODT matrix is different from that found within single-component monolayers of the perfluorinated thiol derivative. This also raises the possibility of whether such perfluorinated molecules can be used as a label for studying defects within SAMs.

EXPERIMENTAL Materials

All thiol molecules were available from previous studies6 or were purchased from Aldrich Chemicals Ltd. Gold wire (99.999+ %) was obtained from

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Advent Ltd, chromium rods were obtained from Megatech Ltd, ethanol and dichloromethane (HPLC grade) were obtained from Aldrich Chemicals Ltd, and Sylgard elastomer was obtained from Farnell Electronic Components Ltd, UK. For the studies presented here the following molecules were used: ODT, CHs-(CHz)i7-SH, and PFl, CF~-(CFZ)~~-S-C~H~-O-(CH~)~-SH. Substrate preparation

Gold films of thickness N 1200 A were evaporated onto silicon slides primed with a thin (~100A) chromium layer (the chromium layer was used to promote adhesion between the gold and the silicon, to ensure the mechanical stability of the gold film). The metal films were deposited by using an Edwards Auto 306 Turbo evaporator at a pressure of < 3 x10p6 mbar; both the goldOand chromium layers were deposited at a rate of l-2 A s-l. The Au/Cr substrates were cleaned prior to use by immersing for 2 to 3min in a 7:3 mixture of H2S04:H202 (please see precaution given in ref. 13 before using this procedure)14. The slides were then rinsed thoroughly in Millipore water and stored under Millipore water until used. Formation of elastomeric stamps

The use of elastomeric stamps to produce patterned SAMs was first introduced by Kumar et al., who demonstrated that high-resolution patterns could be produced by a simple stamping technique without the need for expensive clean room and lithographic facilitieslm3. A 10: 1 (v/v) mixture of Sylgard silicone elastomer base and Sylgard silicone curing agent was poured over the appropriate stamp master and was allowed to cure for 24 h at room temperature. For the results presented here, a simple ‘planar’ stamp with a cross-sectional area of ca. 1 cm2 was used. The stamp was rinsed thoroughly in ethanol before use. Depleted monolayer formation

The alkanethiol ‘ink’ used in our studies was a 1 mM solution of ODT; a small amount (l-2 ml) of dichloromethane was used to enhance the solubility. Monolayers were formed by placing the ‘inked’ stamp in contact with a clean gold surface (a slight hand pressure was applied to ensure that the pattern was transferred to the gold surface). The sample was then rinsed thoroughly in ethanol before being placed in a 1 mM solution of the second thiol derivative, PFl, to cover the remaining bare gold surface (immersion time: 1 h). The stamp was then placed in contact with a second clean gold surface and the procedure repeated. This whole process was repeated many times until the stamp was effectively devoid of the ‘thiol ink’.

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(a)

Time-dependent monolayer jhrmation

Monolayers of the pure PFl derivative were formed, by direct adsorption from solution, onto clean, goldcoated slides. The monolayers were allowed to form for different time periods between 6 s and 16h. Upon removal from solution, each sample was washed with ethanol and copious amounts of Millipore water.

0.4-

XPS spectra

Conventional XPS spectra were recorded for C Is, F 1s and Au 4f levels by using an X-ray source power of 2.8 kW, and with the analyser slit width and pass energy set to 0.8 mm and 150 eV, respectively. Spectra were obtained for electron take-off angles of 5” and 90”, and the base pressure in the sample chamber was maintained at less than 10p9mbar throughout. Samples were mounted on metallic stubs with doublesided conducting tape and charge compensation from an electron flood gun was found to be unnecessary. All data have been corrected for the number of scans taken and are presented as counts per scan. To account for differences in sample size and position, the data for each sample have been normalized with respect to their respective Au4f signal (note that, since the beam size is smaller than the sample size, this correction is usually unnecessary).

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Figure 1 Advancing (solid symbols) and receding (open symbols) contact angles as a function of stamp number, before (a) and after (b) immersion in the perfluorinated thiol derivative

RESULTS AND DISCUSSION Depleted stamp

(Figure lb). The degree of hysteresis, while larger than

A uniform non-patterned stamp was used to transfer a monolayer of ODT onto clean, gold-coated slides. The stamp was used for a number of times without the ‘thiol ink’ being replenished. Thus the amount of ODT transferred to the surface continuously decreased. Figure I shows the advancing and receding contact angles for the ODT monolayer formed as a function of the number of stamps both before (Figure la) and after (Figure Zb) immersion in the perfluorinated thiol derivative. It can be seen from Figure la that the receding contact angle is the most sensitive to depletion of the monolayer and shows a nearly linear decrease with number of stamps (i.e. linear in cos e)“* 16. The advancing (contact angle, while initially appearing quite sensitive to the depletion of the ODT layer, flattens after about the eighth stamp and thus gives rise to an increasing Idegree of hysteresis which is probably indicative of an increasing degree of chemical heterogeneity of the sample (by the 15th stamp, the difference between the advancing and receding contact angles was approximately 40”). Following immersion in the perfluorinated derivative the contact angles showed a slight increase, with the advancing angles approaching 130” for the most depleted films (i.e. those containing the highest amount of PFl)

that typically found for monolayers adsorbed directly from solution, was approximately constant as a function of the number of stamps. In general, the contact angle data would suggest that the use of the more depleted stamps results in microscopic/ macroscopic inhomogeneities in the chemical composition of the surface15, 16. Figure 2 shows a selection of F 1s spectra obtained after immersion for 1 h in the PFl solution, for progressively greater degrees of depletion (i.e. higher number of stamps), for electron take-off angles of 5” and 90” with respect to the sample surface. In the spectra obtained at normal take-off angle (Figure 2b) there appear to be no dramatic changes as a function of stamp depletion with the exception that the signal due to the PFl increases significantly with increasing number of stamps (depletion). The spectra in Figure 2 are normalized with respect to the main F 1s contribution [lower binding energy (BE)] to allow a better comparison of the ratio of the higher BE component to the lower BE component. A closer inspection of these spectra reveals that there are some definite trends in both the position of the F Is peak and its full-width at half maximum (FWHM). Figure 3 shows a compilation of the integrated intensity, peak

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I

I

1 II (b)

690

695

665

660 695

Binding Energy (eV)

690

665

660

Binding Energy (eV)

Figure 2 Normalized F 1s spectra as a function of stamp number. Spectra in (a) were obtained at an electron take-off angle of 5” while those in (b) were obtained with an electron take-off angle of 90

position and FWHM for the F 1s peak taken for electron take-off angles of 5” and 90”. In particular, we note that in both cases, with increasing PFl content within the monolayer, the peak position

moved slightly towards lower binding energy (by approximately 0.5eV) and at the same time the FWHM decreased from around 1.50 to 1.34eV. We believe that this variation may be associated with increasing uniformity within the PFl regions of the monolayer. Returning to the F 1s spectra taken at an electron take-off angle of 5”, Figure 2a, we find a much more dramatic effect. Here we can clearly see the presence of a second component in the F 1s signal which decreases rapidly with increasing PFl content. This second peak occurs close to 2eV higher in binding energy than the main F 1s contribution. Figure 4 shows the ratio of the area of the higher BE component to that of the lower BE component as a function of stamp depletion. It is evident that the ratio decreases rapidly with increasing stamp depletion. In order to address the origin of this higher BE component and its behaviour with stamp depletion we are drawn to consider the molecular picture. During stamping the ODT is transferred uniformly, on the macroscopic scale, to the clean gold support. Microscopically, however, we have no direct evidence of the order within the monolayer film. In particular, we need to address the question of SAM uniformity: do we have a uniform but less densely packed monolayer, with molecular-scale defects (possibly voids) for the PFl molecules to penetrate into, or are there discrete domains of ODT SAM between which

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regions of PFl monolayer will also be formed in domains? For either scenario, we need to address why we should expect these to give rise to differences in the observed F 1s spectra. Figure 5 shows schematically several simple scenarios which are considered here as the possible origin of the higher binding energy peak. In Figure 5a we show individual molecules of the PFl derivative penetrating the ODT SAM in an isolated fashion. In this scenario the lone PFl molecule is surrounded by a hydrocarbon matrix, and further, since the PFl molecule is slightly longer than the ODT molecules, we expect the tail to protrude above the surface of the

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SAMS:

S. D. Evans et al.

ODT matrix. The principal reasons why this would be expected to give rise to a distinct contribution to the F 1s peak are either: (1) the PFl molecules might sense differences due to local field effects (either from the portion of the molecule surrounded by the hydrocarbon matrix or from the portion extending into the vacuum above the ODT SAM); or (b) there may be some conformational disorder in the perfluorocarbon chain, either in the region protruding from the ODT due to lack of nearest-neighbour interactions or due to the fact that molecules of PFl may not be able to be incorporated into the ODT host environment without the introduction of defects. In Figure 5b we show a scenario for domain formation in this situation; it would only be molecules at the domain boundaries that would contribute to the higher BE peak, either through local field effects or the introduction of conformational defects. In the case of large stamp depletion we would expect both these scenarios to converge to yield the formation of large domains of PFl. In the initial stages of depletion, however, we would expect different behaviour. In the case of the insertion of individual molecules, we would expect no changes in the ratio of these peaks until PFl molecules were surrounded by fellow PFl molecules, . thus leading to changes in the local field or to fewer conformational defects. In the case of domain growth, we would expect a continual decrease in the ratio of the higher BE to the lower BE component with increasing domain size. Indeed, if we assume that the mechanism for the generation of the high BE component (either local field effect or structural defect) is restricted only to nearest-neighbour molecules, then only the molecules on the boundary of a domain would be sensitive to such effects, and the ratio of the high to low BE contributions should decrease with the ratio of the number of molecules in the perimeter to the number contained within the domain (this is approximately l/R, where R is the domain radius, or l/N’/‘, where N is the number of stamps). Unfortunately, the data in Figure 4 are not sufficiently accurate to permit a precise determination of the power-law behaviour of the decay and appear to be lit slightly better by l/N rather than 1/N112. Returning to the wetting behaviour, the increasing contact hysteresis with increased depletion would also be consistent with domain formation. An important observation that must be reconciled with any choice of model is the strong angular dependence of the higher binding energy peak. This observation suggests that the main contribution to this peak originates close to the monolayer/ambient interface.

Kinetic studies Figure 5 Schematic showing of the high BE component

some possible

scenarios

for the origin

In an attempt to gain further information, we performed a second study in parallel with the first in

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which PFl molecules were allowed to adsorb for different lengths of time onto clean, gold-coated substrates. In these experiments the scenario suggested in Figure 5a should not occur, since there is no ODT monolayer for the PFl molecules to penetrate into. Instead, the PFl molecules will be adsorbed on to a clean substrate on which they will form a monolayer, probably via domain growth’7p22. Figure 6 shows the advancing and receding water contact angles as a function of immersion time. It is clear that the adsorption is rapid and essentially complete within the first hour or two. The F 1s spectra taken at 5” and 90“ are shown in Figure 7 for three different adsorption times (the intermediate spectra have been

a

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time (set) Figure 6 Advancing and formed monolayers of PFl, on a log scale)

2

receding contact angles, as a function of adsorption

for partially time (shown

Fls(90")

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omitted for clarity). Although there is a slight asymmetry in the spectra obtained at 5” there is no significant contribution to the high BE side, even for the shortest adsorption times. While this would seem to suggest that the contribution to the higher binding energy component does not result from domain edge effects, but rather from the incorporation of PFl into the ODT matrix, we are at present cautious without further support for this hypothesis since the adsorption of the PFl molecules is very rapid.

CONCLUSIONS Our results clearly show that binding energies, as measured by XPS, may be sensitive to more than just the chemical species present. In particular, the observation of the higher binding energy component for PFl adsorbed within the ODT overlayer suggests that careful analysis of XPS data on SAMs may yield additional useful information. While we have postulated that this high BE contribution could originate from several sources, our data at present are not precise enough to permit an unambiguous determination of the form of the decay (with increasing PFl content) in the ratio of the high to low BE components. The strong angular dependence suggests that this contribution does arise from close to the monolayer/ambient interface, and hence that it is probably due to the protruding portion of the molecules. These experiments highlight the need for further experiments; in particular, we wish to vary the concentration of the alkanethiol solution used for the stamping as well as the length of the perfluorinated and alkylthiol matrix molecules used.

ACKNOWLEDGEMENTS We wish to acknowledge the EPSRC for use of the ESCA facility at Daresbury, UK. One of us (S. R. J.) would like to acknowledge receipt of a Departmental Scholarship.

REFERENCES I. 2. 3. 4.

0

695

690

685

t

Binding Energy (ev)

5.

Binding Energy (ev) 6.

Figure 7 Normalized F 1s spectra as a function of adsorption time. Spectra are shown for electron take-off angles of 5” (left) and 90” (right)

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7.

AND NOTES

Kumar, A. and Whitesides, G. M., Appl. Phys. Let?., 1993, 63, 2002. Kumar, A., Biebuyck, H. A. and Whitesides, G. M., Langmuir. 1994, 10, 1498. Wilbur, J. L., Biebyuck, H. A., MacDonald, J. C. and Whitesides, G. M., Lungmuir, 1995, 11, 825. Tarlov, M. J., Burgess, D. R. F. and Gillan, G., J. Am. Chem. sot., 1993, 115,5305. Evans, S. D., Flynn, T. M. and Ulman, A., Langmuir, 1995, 11, 3811. Evans, S. D., Flynn, T., Ulman, A. and Beamson, G., SIA, 1996, 24, 187. Laibinis, P. E. and Whitesides, G. M., J. Am. Chem. Sot., 1992,114,9022.

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Laibinis, P. E., Whitesides, G. M., Ahara, D. L., Tao, Y. T., Parikh, A. N.andNuzzo,G. R.,J. Am. Chem.Soc., 1991,113,7152. Junn. D. R. and Czanderna. A. W.. Crit. Rev. Solid State hfa;hr. Sci., 1994, 19, I. Fadley, G. S.. Prop. Surf. Sri., 1984, 16, 275. Freeman, T. L., Evans, S. D. and Ulman, A., Lungmuir. 1995, 11.4411. Briggs, D. and Seah, M. P. (eds), Practical Surface Analysis. John Wiley and Sons, New York, 1983. Salneck, E. W., Uvdal. K., Elwing, H., Askendal, A., WelinKlinstrcim, S.. Lundstrom, I. and Salaneck, W. R., J. Co/l. Int.

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Dettre, R. H. and Johnson, R. H., paper presented at Symp. Contact Angle, Bristol, Surface and Colloid Science, Vol. 2, ed. E. Matijevic, Wiley, NY, 1%6, p. 85-154. Ulman, A., Evans, S. D., Shnidman, Y., Sharma, R. and Eilers, J., Adv. Coil. Int. Sci.. 1992, 39, 175. Poirier, G. E. and Tarlov, M. J., Lungmuir. 1994, 10, 2853. Poirier, G. E., Tarlov, M. J. and Rushmeier, H. E., Lungmuir, 1994, 10, 3383.

Camillone, N., Eisenberger, P., Leung, T. Y. B., Shwartz, P., Stoles. G.. Poirier. G. E. and Tarlov. M. J.. J. C/rem. Phvs.. _ 1994, i01,‘1l031. Poirier, G. E. and Tarlov, M. J., J. Phys. Chem.. 1995.99, 10966. Poirier, G. E. and Pylant, E. D., Science, 1996, 272, 1145. Batchelder, D. N., Evans, S. D., Freeman, T. L., Hausshng, L., Wolfe, H. and Ringsdorf, H., J. Am. Chem. Sot.. 1994,116,1050.

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