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Wetting of a self-assembled 1,4-benzenedimethanethiol monolayer on gold and silver: An adhesion force spectroscopy study
Surface Science 600 (2006) 2894–2899 www.elsevier.com/locate/susc
Wetting of a self-assembled 1,4-benzenedimethanethiol monolayer on gold and silver: An adhesion force spectroscopy study Nele Vandamme, Koen Schouteden, Johan Snauwaert, Peter Lievens, Chris Van Haesendonck * Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium Received 27 January 2006; accepted for publication 12 May 2006 Available online 12 June 2006
Abstract Using atomic force microscopy we investigated how local capillary phenomena are affected by the deposition of a self-assembled 1,4benzenedimethanethiol (BDMT) layer on epitaxially grown Au(1 1 1) and Ag(1 1 1) films. Force–distance curves monitored at varying relative humidity show clear differences in the adhesion forces on the different samples, which can be explained in terms of a change in the wetting behavior due to the presence of the molecules. Moreover, we found that not only the chemical structure of the molecules but also their orientation strongly influences the strength of the capillary forces. A detailed analysis of the measurements shows that condensation of water vapor on Au(1 1 1) films is drastically enhanced due to the vertically aligned BDMT molecules, while on Ag(1 1 1) water condensation is reduced due to a parallel molecule orientation. 2006 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Adhesion; Gold; Silver; Water; Alkanes
1. Introduction The large flexibility of self-assembled monolayers (SAMs), their stability and the ease of preparation have made the variety of SAM-based systems immense [1]. In particular, thiolates on gold and silver substrates have drawn considerable attention. The spontaneous chemisorption of such molecules combined with the stability of the metals [2] opens great perspectives in different branches of nanoscience. An appropriate choice of the molecular endgroups and backbones results in an almost unlimited variation of the physical and chemical surface characteristics. In this work we illuminate how the wetting properties of gold and silver films are affected by deposition of 1,4benzenedimethanethiol (BDMT) molecules. Extensive studies on the stability, orientation, chemical reactivity and *
Corresponding author. E-mail address: [email protected] (C. Van Haesendonck). 0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.05.032
electronic behavior of these molecules have been performed [3–5]. It is clear that they are ideally suited for single electron tunnelling configurations, when deposited on gold surfaces [6]. Although knowledge of changes in water adsorption should resolve the problems encountered in reliable scanning probe microscopy visualization of the created configurations [7], the wetting aspect of BDMT molecules has not been addressed in detail yet. When samples are stored and measured under ambient conditions, most surfaces are covered with thin layers of contaminants directly adsorbed from their environment. Because of its abundance and particular chemical properties, water is one of the most important adsorbates. Depending on the affinity of the surface atoms towards water, the thickness of the adsorbed layer strongly differs. Macroscopically, the influence of the surface chemistry on water layer formation can be investigated by measuring the contact angle of a water droplet on the surface. However, to gain microscopic information about the surface wetting, a more advanced local probe technique is required.
N. Vandamme et al. / Surface Science 600 (2006) 2894–2899
Atomic force microscopy (AFM) has proven to be an excellent tool for this purpose, by measuring the forces during the retraction of the tip from the sample. While performing such a force–distance measurement, the total interaction force during the tip-sample contact is the sum of the van der Waals force, the electrostatic interaction and the capillary force. The latter force results from capillary neck formation. This force originates from the selfassociation of water and the strong adhesive properties of water with respect to the surface. For an electrically neutral non-magnetic sample and tip, the force felt during the retraction (Fpull-off) of the tip from the sample can be written as F pull-off ¼ F vdW þ F cap ¼ A
R þ 4pRcw cos h; 6d 2
ð1Þ
where A is the Hamaker constant, R is the radius of the contact area (i.e. the tip radius in case of a flat surface), d is the tip-sample separation, cw is the liquid surface tension of water, and h is the contact angle of a water droplet on the surface. The expression for the capillary force is based on the approximation for a spherically shaped tip in contact with a plane surface [8]. Local information about surface wetting can be gathered by measuring the pull-off force at varying relative humidity (RH). This technique is referred to as adhesion force spectroscopy. While the van der Waals interaction is humidity independent, the capillary force depends on the amount of condensed water, and therefore on the relative humidity [8]. A gradual change in the capillary force is expected in the humidity range where the adsorbed water layer approaches the minimum thickness for capillary neck formation and remains then constant for all higher humidities. Experimental results are in conflict with this prediction. Several groups report three distinct regimes in the humidity dependence of the capillary force [9–15] as illustrated in Fig. 1. In regime I, a small and constant pull-off force is
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measured for low humidities. The adhesion is completely determined by the van der Waals interaction. It is not possible to form a liquid bridge between tip and sample. When increasing the humidity, the thickness of the condensed water accumulates on both tip and sample. At sufficiently high humidity, capillary neck formation becomes possible and the adhesion increases as the meniscus grows, until it reaches a maximum width (regime II) and then starts to decrease (regime III). The exact origin of the latter decrease has been interpreted in various ways, but any transparent interpretation of the experiments is hampered by various unknown factors, including the exact tip geometry, surface corrugation, and contamination. The most common approach describes the decline as an instability of the capillary neck because the pressure outside the neck exceeds the pressure inside the neck [9–11,13]. The meniscus breaks and no capillary force is exerted. Alternatively, the decline at higher humidities can be linked to the fact that the chemical bonding of the liquid in the gap results in an additional force [10,14,16]. Since this extra tip-sample interaction becomes more repulsive with increasing humidity, it finally results in a reduction of the adhesion. 2. Experimental Epitaxially grown 140 nm thick Au(1 1 1) and Ag(1 1 1) films on freshly cleaved mica were prepared by molecular beam epitaxy at elevated temperatures. We refer to Ref. [17] for the exact deposition parameters. The resulting films consist of atomically flat islands (typical root-meansquare (rms) roughness is 0.05 nm) with dimensions up to 500 · 500 nm2. Self-assembled monolayers of 1,4-benzenedimethanethiol (BDMT) were deposited on the films by immersion in a 1 mM solution of BDMT (Sigma–Aldrich) in tetrahydrofuran (THF) for 24 h. After incubation the samples were subsequently copiously rinsed in THF, acetone and distilled water to remove excess molecules, and dried under a nitrogen flow afterwards. The adsorption of the monolayers was characterized by means of ellipsometry and contact angle measurements (Drop Shape Analysis System DSA 10 Mk2). The results are listed in Table 1. The orientation of the molecules (length = 1.1 nm) was derived from the ellipsometric thickness, assuming a length of 0.2 nm for the AuS and AgS bond [2]. On gold the molecules stand upright the surface forming a dense monolayer. The BDMT molecules are tilted under an angle of approximately
Table 1 Ellipsometric thickness and contact angles measured on a bare and BDMT-covered polycrystalline gold/silver film
Fig. 1. Generic sketch of the functional relationship between the pull-off force and relative humidity. The capillary neck formation between tip and sample is illustrated for each of the three regimes I, II and III.
Sample
Ellipsometric SAM thickness
Contact angle ()
Au Au + BDMT Ag Ag + BDMT
– 0.8 ± 0.1 nm – 0.2 ± 0.1 nm
93 ± 2 85 ± 2 83 ± 2 90 ± 2
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100 Au Au + BDMT
Force (nN)
50
0
F pull-off F pull-off
-50
-100 -100 Fig. 2. Schematic representation of the deposited 1,4-benzenedimethanethiol molecules. On Au(1 1 1) the molecules stand upright (tilt angle of approximately 50, see (a)). One thiol group binds to the gold, the second remains free at the surface. On Ag(1 1 1) the molecules are oriented parallel to the surface; both thiol groups bind to the silver (see (b)).
50 with respect to the surface. On silver the molecules are oriented parallel to the surface. Both configurations are illustrated in Fig. 2. Similar results are reported in literature and can be explained in terms of different molecular chemisorption on gold and silver surfaces [3,4]. The different stacking mode results in minor changes of the macroscopic contact angle. With contact angles of approximately 90, all samples are mildly hydrophilic. The contact angle slightly decreases on gold after adsorption of the molecules (more hydrophilic), while on silver it slightly increases (more hydrophobic). Force–distance (F(d)) spectra (500 data points) were measured with a CP AutoProbe microscope (Park Scientific Instruments, now Veeco), using pyramidical silicon cantilevers with a force constant of 3.2 N/m (Ultralevers, Park Scientific Instruments, now Veeco). The measurements are performed in static mode at controlled humidity conditions. The CP setup is enclosed in a glovebox, in which the humidity is regulated by a controlled flow of dry or water-saturated air. A ventilator spreads the air inside the box which contains a hygrometer (HD 8501 H, Delta Ohm, measurable humidity range 5–98% RH) to measure the relative humidity (RH). Before the measurement, the sample stays overnight in a dry air flow to remove all water. During the measurement, the RH is increased in steps of approximately 5% and then stabilized for at least 15 min. 3. Wetting of tilted BDMT molecules In Fig. 3 we present the force–distance curve measured with a naturally oxidized silicon tip, under ambient conditions (50% RH) on bare and BDMT-covered gold, respectively. On bare gold a pull-off force of 40 nN is registered, while after deposition of the dithiol monolayer the pull-off force has increased to 90 nN. This increase is even more pronounced when calculating the energy dissipation as the area confined by the force–distance curve and the zero
-50
0
50
100
Piezo Extension (nm) Fig. 3. Force–distance curve at 50% RH measured with a naturally oxidized silicon tip on a gold sample before (Au, dashed line) and after (Au + BDMT, solid line) deposition of the BDMT monolayer. The curves have been shifted horizontally for clarity. The area confined by the force curve (Au ! light grey; Au + BDMT ! dark grey) and the zero force line corresponds to the dissipation energy.
force line (marked in Fig. 3). The energy needed to break the tip-sample contact has almost increased by a factor of 5 (from 0.62 fJ to 3.0 fJ), due to the presence of the monolayer. Since both measurements are performed under identical experimental conditions, the difference in interaction between the tip and the sample surface may be completely attributed to deposition of the dithiol molecules. However, from the expression for the pull-off force (Eq. (1)), it is clear that two explanations for the increased interaction remain plausible. First, deposition of the molecules may result in an increase of the surface roughness, which then results in a larger contact area and a corresponding increase of the pull-off force. Second, the increase may find its origin in a different type of interaction. Either the van der Waals interaction or the capillary force between tip and sample may have increased due to deposition of the BDMT layer. Based on STM measurements, we found that the surface roughness on bare and BDMT-covered gold are comparable ( 0.1 nm). Therefore, the explanation in terms of an increased contact area can be rejected. In order to determine the interaction type that causes the change, adhesion force spectroscopy measurements were performed on both samples. The humidity dependence of the dissipation energy on bare gold and BDMT-covered gold are presented in Fig. 4. The general tendency on both samples is comparable and corresponds well to the predicted behavior presented in Fig. 1. First, a low interaction regime is measured (regime I, 630% RH). The energy dissipation is negligible and can be completely accounted for by the van der Waals interaction between tip and sample. In regime II (30–55% RH), the energy dissipation increases due to capillary neck formation and reaches a maximum at 55% RH. Finally, the energy dissipation drops down to a low value (regime III, P55 % RH), which is comparable to the value measured at low
N. Vandamme et al. / Surface Science 600 (2006) 2894–2899
Fig. 4. Dissipation energy observed during the contact of a standard AFM tip with a bare (left axis, triangles) and a BDMT-covered gold substrate (right axis, black squares) as a function of the relative humidity. For clarity the results are plotted on an adjusted scale.
humidities. No stable meniscus can be formed and the interaction is determined by the van der Waals interaction between a completely hydrated tip and sample. In spite of the good qualitative agreement, a major difference is observed for the magnitude of the adhesion. The maximum energy dissipation has increased from ED = 0.6 fJ on bare gold to ED = 5.3 fJ on BDMT covered gold (It should be noted that the scale in Fig. 4 was adjusted for both samples). Since the adhesion is comparable for low humidities and obviously humidity dependent, the increased energy dissipation has to originate from a different water layer growth and accompanying capillary forces rather than from a different van der Waals interaction. These results are consistent with the fact that the presence of a BDMT monolayer on gold slightly enhances the macroscopic surface wetting (see contact angles in Table 1). Surprisingly, the relatively small macroscopic enhancement of the surface wetting goes together with pronounced variations of the wetting properties on nanometer scale. The nanoscale properties can be understood in terms of two competing phenomena. On one hand, free thiol groups are exposed at the surface after the molecule deposition (see Fig. 2(a)). The polar character of SH bonds strongly favors the interaction with and condensation of water. This hydrophilic nature induces an enhanced water layer growth, leading to an enhanced adhesion during the tipsample contact. On the other hand, the water layer growth on bare gold samples is suppressed by adsorbed contaminants. Exposure of gold films to ambient conditions results in the condensation of hydrocarbons from the environmental air on the surface, which are known to be apolar and hence tend to dewet the surface [18]. Capillary neck formation between tip an sample is strongly reduced for these hydrophobic samples, resulting in a suppression of the adhesive forces [16]. In case of a complete hydrocarbon coverage of a surface, the water layer of the hydrated silicon oxide tip is simply sandwiched between tip and sample,
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Fig. 5. Dissipation energy for a BDMT covered gold substrate as a function of the relative humidity: (a) for decreasing humidity with a standard oxidized silicon tip, and (b) for increasing humidity with an etched tip. The standard measurement (i.e., increasing humidity and standard oxidized silicon tip) is added for comparison on both graphs (triangles).
without any attraction towards the hydrophobic sample surface. The statement that the increased energy loss indeed originates from capillary forces and not from a direct tip-surface interaction is supported by two additional experiments, for which the results are presented in Fig. 5. In the first experiment, the energy dissipation is determined from F(d) curves measured with a standard oxidized silicon tip for decreasing relative humidity (Fig. 5(a)). The sample is kept at high humidity (90% RH) overnight, and the humidity is then lowered in steps of 5–10% by a dry air flow and stabilized during at least 15 min before the measurement. The general behavior for increasing and decreasing humidities agrees well. Nevertheless, the curve for decreasing RH is less stable (i.e., the measurement error is higher) and a delay of the maximum dissipation occurs. This can be linked to the fact that the hydrophilic character of tip and sample hampers the evaporation of the water from both surfaces. Therefore, a hysteresis occurs between the water layer condensation and evaporation: the maximum in the curve for decreasing humidity is less pronounced and is reached for lower humidities (between 40% and 50% RH) when compared to the curve for increasing humidity (at 55% RH). On the other hand, the good agreement between the shape of both curves indicates that the structure of the BDMT layer is not affected by humidity. After storage in a wet environment for more than 20 h, the wetting properties of the surface remain unaltered. This observation indicates that the thiol groups of the BDMT monolayer are still at the surface. In a second experiment, the apex of a naturally oxidized tip was etched prior to the AFM measurements in order to make it hydrophobic and hence overcome the strong adhesive force. The etching process, which results in the removal of the oxide layer from the tip, is achieved by scanning a mica surface wetted with hydrogen fluoride liquid [7]. After
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the tip modification, the dissipation energy is reduced to a small value ED = 0.3 fJ. Because the tip tends to repel water, the approach and retract process is now completely reversible and determined by the van der Waals interaction for the complete humidity range. 4. Wetting of parallel BDMT molecules Force–distance curves measured under ambient conditions (50% RH) on a bare silver and a BDMT covered silver film are presented in Fig. 6. Although, similar to the situation on gold, a considerable difference in pull-off force and dissipation energy is recorded before and after deposition of the monolayer, the gold and silver based samples clearly behave differently. While on gold deposition of the BDMT molecules results in an increase of the interaction, on silver it gives rise to a decrease from 86 nN (ED = 2.4 fJ) to 54 nN (ED = 0.9 fJ). Again, we may assume that this difference originates from a modified affinity for water adsorption (see Table 1). We now turn to our detailed investigation at varying humidity to identify possible modifications of the wetting behavior due to the monolayer deposition. In Fig. 7 the humidity dependence of the dissipation energy for bare silver and silver covered with BDMT is plotted. The overall shape of both adhesion curves matches well and this shape is consistent with the generic sketch of Fig. 1. First, in regime I (640% RH), a low adhesion level is recorded (ED = 0.2 fJ). At 40% RH, a capillary neck is formed between tip and sample surface, and the adhesion increases when the neck grows (regime II, 40–63% RH). At 63% RH, the energy dissipation reaches its maximum value. A further increase of the humidity after the maximum leads to a degradation (regime III P 63% RH) and finally a breaking of the meniscus. In contrast to the situation on gold, the registered maximum is much more pronounced be-
F pull-off
F
Fig. 7. Dissipation energy observed during the contact of a standard AFM tip with a bare (left axis, triangles) and a BDMT-covered (right axis, black squares) silver substrate as a function of the relative humidity. For clarity the results are plotted on an adjusted scale.
fore (ED = 7.9 fJ) than after (ED = 1.1 fJ) deposition of the BDMT molecules (the scale in Fig. 7 was again adjusted for both samples). The large value on bare silver can be understood in terms of oxidation of the silver surface. Exposure of the silver substrate to air gives rise to the formation of a native oxide layer, which favors the condensation of water. Deposition of a BDMT monolayer results in an orientation of the BDMT molecules parallel to the silver surface (see Fig. 2(b)). For each molecule, a benzene ring and two methane groups are at the surface. The hydrophobic character of the aromatic benzene ring and CH2 groups of methane reduces the adhesion. However, the measured energy loss is still significantly larger than the energy dissipated on a surface that is completely covered by hydrocarbons. For this purpose, we measured on a gold film that was covered by 2-phenylethanethiol molecules (C6H4(CH2)2SH, PEM). Similar to BDMT on gold, the molecules stand upright the surface, forming a dense monolayer. However, in case of PEM the surface consists of CH2 groups instead of thiol groups. The maximum energy dissipation registered on PEM-covered gold was 0.3 fJ, well below the maximum value registered on BDMT-covered silver. This can be understood by the molecular structure of BDMT, which implies in the parallel growth mode a non-dense coverage of hydrocarbons at the surface. Therefore, the hydrophilic silver substrate still participates in the water layer growth and the adhesion.
pull-off
5. Conclusions
Fig. 6. Force–distance curve at 50% RH measured with a SiO2 tip on a silver sample before (Ag, dashed line) and after (Ag + BDMT, solid line). The area confined by the force curve (Ag ! light grey; Ag + BDMT ! dark grey) and the zero force line corresponds to the dissipation energy. The curves have been shifted horizontally for clarity.
Adhesion force spectroscopy measurements at varying humidity were performed on bare and BDMT covered gold and silver films. The measurements turn out to be an excellent tool for probing the wetting characteristics of the samples. The general behavior on all samples is essentially the same, with three distinct adhesion regimes. Transitions in the measured energy dissipation for varying humidity can
N. Vandamme et al. / Surface Science 600 (2006) 2894–2899
be completely attributed to changes in the capillary force. Consequently, differences between the different samples can be accounted for by different wetting behavior. Upon dithiol deposition on gold substrates the maximum tip-sample adhesion drastically increases (by an order of magnitude). The tilted molecule orientation results in a surface of thiol groups, which are more polar than the surface of bare gold. On the BDMT-covered gold a thicker water layer is adsorbed and a strong capillary neck is formed. For silver the opposite effect is observed. After dithiol deposition the adhesion is significantly reduced. Due to the parallel molecule orientation the surface partially consists of hydrocarbons that repel the water. Water condensation remains restricted and the menisci are smaller. Deposition of a 1,4-benzenedimethanethiol layer clearly favors the water adsorption on gold, while it hampers water layer growth on silver. Application of self-assembled monolayers to tune the wetting behavior thus provides important ‘‘remedies’’ for diminishing undesirable effects of capillary water condensation at the surface. Acknowledgements This work has been supported by the Fund for Scientific Research – Flanders (FWO) as well as by the Flemish Concerted Action (GOA) and the Belgian Interuniversity Attraction Poles (IAP) research programs. We are much indebted to Dr. Orlin Blajiev and Prof. Herman Terryn (Department of Chemical Engineering, Free University of Brussels) for performing the ellipsometry measurements
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and to Katleen Boussu (Department of Chemical Engineering, KULeuven) for doing the contact angle measurements. References [1] F. Schreiber, J. Phys.: Condens. Matter 16 (2004) R881. [2] H. Sellers, A. Ulman, Y. Shnidman, J. Eilers, J. Am. Chem. Soc. 115 (1993) 9389. [3] J. Henderson, S. Feng, G. Ferrence, T. Bein, C. Kubiak, Inorg. Chem. Acta 242 (1996) 115. [4] K. Murty, M. Venkataramanan, T. Pradeep, Langmuir 14 (1998) 5446. [5] W. Tian, S. Datta, S. Hong, R. Reifenberger, J. Henderson, C. Kubiak, J. Chem. Phys. 109 (1998) 2874. [6] R. Andres, T. Bein, M. Dorogi, S. Feng, J. Henderson, C. Kubiak, W. Mahoney, R. Osifchin, R. Reifenberger, Science 272 (1996) 1323. [7] N. Vandamme, J. Snauwaert, E. Janssens, E. Vandeweert, P. Lievens, C. Van Haesendonck, Surf. Sci. 558 (2004) 57. [8] J. Israelachvili, Intermolecular and Surface Forces, Second ed., Academic Press, Harcourt Brace and Company, London, 1992. [9] T. Thundat, X. Zhang, G. Chen, S. Sharp, R. Warmack, L. Schonwalter, Appl. Phys. Lett. 63 (1993) 2150. [10] L. Xu, A. Lio, J. Hu, D. Ogletree, M. Salmeron, J. Phys. Chem. B 102 (1998) 540. [11] X. Xiao, L. Qian, Langmuir 16 (2000) 8153. [12] R. Quon, A. Ulman, T. Vanderlick, Langmuir 16 (2000) 8912. [13] D. Sedin, K. Rowlen, Anal. Chem. 72 (2000) 2183. [14] M. He, A. Blum, D. Aston, C. Buenviaje, R.O.R. Luginbiihl, J. Chem. Phys. 114 (2001) 1355. [15] R. Jones, H. Pollock, J. Cleaver, C. Hodges, Langmuir 18 (2002) 8049. [16] M. Binggeli, C. Mate, Appl. Phys. Lett. 65 (1994) 415. [17] N. Vandamme, E. Janssens, F. Vanhoutte, P. Lievens, C. Van Haesendonck, J. Phys.: Cond. Matt. 15 (2003) S2983. [18] K. Hayashi, H. Sugimura, O. Takai, Appl. Surf. Sci. 188 (2002) 513.
Wetting of a self-assembled 1,4-benzenedimethanethiol monolayer on gold and silver: An adhesion force spectroscopy study Nele Vandamme, Koen Schouteden, Johan Snauwaert, Peter Lievens, Chris Van Haesendonck * Laboratorium voor Vaste-Stoffysica en Magnetisme, Katholieke Universiteit Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium Received 27 January 2006; accepted for publication 12 May 2006 Available online 12 June 2006
Abstract Using atomic force microscopy we investigated how local capillary phenomena are affected by the deposition of a self-assembled 1,4benzenedimethanethiol (BDMT) layer on epitaxially grown Au(1 1 1) and Ag(1 1 1) films. Force–distance curves monitored at varying relative humidity show clear differences in the adhesion forces on the different samples, which can be explained in terms of a change in the wetting behavior due to the presence of the molecules. Moreover, we found that not only the chemical structure of the molecules but also their orientation strongly influences the strength of the capillary forces. A detailed analysis of the measurements shows that condensation of water vapor on Au(1 1 1) films is drastically enhanced due to the vertically aligned BDMT molecules, while on Ag(1 1 1) water condensation is reduced due to a parallel molecule orientation. 2006 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Adhesion; Gold; Silver; Water; Alkanes
1. Introduction The large flexibility of self-assembled monolayers (SAMs), their stability and the ease of preparation have made the variety of SAM-based systems immense [1]. In particular, thiolates on gold and silver substrates have drawn considerable attention. The spontaneous chemisorption of such molecules combined with the stability of the metals [2] opens great perspectives in different branches of nanoscience. An appropriate choice of the molecular endgroups and backbones results in an almost unlimited variation of the physical and chemical surface characteristics. In this work we illuminate how the wetting properties of gold and silver films are affected by deposition of 1,4benzenedimethanethiol (BDMT) molecules. Extensive studies on the stability, orientation, chemical reactivity and *
Corresponding author. E-mail address: [email protected] (C. Van Haesendonck). 0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.05.032
electronic behavior of these molecules have been performed [3–5]. It is clear that they are ideally suited for single electron tunnelling configurations, when deposited on gold surfaces [6]. Although knowledge of changes in water adsorption should resolve the problems encountered in reliable scanning probe microscopy visualization of the created configurations [7], the wetting aspect of BDMT molecules has not been addressed in detail yet. When samples are stored and measured under ambient conditions, most surfaces are covered with thin layers of contaminants directly adsorbed from their environment. Because of its abundance and particular chemical properties, water is one of the most important adsorbates. Depending on the affinity of the surface atoms towards water, the thickness of the adsorbed layer strongly differs. Macroscopically, the influence of the surface chemistry on water layer formation can be investigated by measuring the contact angle of a water droplet on the surface. However, to gain microscopic information about the surface wetting, a more advanced local probe technique is required.
N. Vandamme et al. / Surface Science 600 (2006) 2894–2899
Atomic force microscopy (AFM) has proven to be an excellent tool for this purpose, by measuring the forces during the retraction of the tip from the sample. While performing such a force–distance measurement, the total interaction force during the tip-sample contact is the sum of the van der Waals force, the electrostatic interaction and the capillary force. The latter force results from capillary neck formation. This force originates from the selfassociation of water and the strong adhesive properties of water with respect to the surface. For an electrically neutral non-magnetic sample and tip, the force felt during the retraction (Fpull-off) of the tip from the sample can be written as F pull-off ¼ F vdW þ F cap ¼ A
R þ 4pRcw cos h; 6d 2
ð1Þ
where A is the Hamaker constant, R is the radius of the contact area (i.e. the tip radius in case of a flat surface), d is the tip-sample separation, cw is the liquid surface tension of water, and h is the contact angle of a water droplet on the surface. The expression for the capillary force is based on the approximation for a spherically shaped tip in contact with a plane surface [8]. Local information about surface wetting can be gathered by measuring the pull-off force at varying relative humidity (RH). This technique is referred to as adhesion force spectroscopy. While the van der Waals interaction is humidity independent, the capillary force depends on the amount of condensed water, and therefore on the relative humidity [8]. A gradual change in the capillary force is expected in the humidity range where the adsorbed water layer approaches the minimum thickness for capillary neck formation and remains then constant for all higher humidities. Experimental results are in conflict with this prediction. Several groups report three distinct regimes in the humidity dependence of the capillary force [9–15] as illustrated in Fig. 1. In regime I, a small and constant pull-off force is
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measured for low humidities. The adhesion is completely determined by the van der Waals interaction. It is not possible to form a liquid bridge between tip and sample. When increasing the humidity, the thickness of the condensed water accumulates on both tip and sample. At sufficiently high humidity, capillary neck formation becomes possible and the adhesion increases as the meniscus grows, until it reaches a maximum width (regime II) and then starts to decrease (regime III). The exact origin of the latter decrease has been interpreted in various ways, but any transparent interpretation of the experiments is hampered by various unknown factors, including the exact tip geometry, surface corrugation, and contamination. The most common approach describes the decline as an instability of the capillary neck because the pressure outside the neck exceeds the pressure inside the neck [9–11,13]. The meniscus breaks and no capillary force is exerted. Alternatively, the decline at higher humidities can be linked to the fact that the chemical bonding of the liquid in the gap results in an additional force [10,14,16]. Since this extra tip-sample interaction becomes more repulsive with increasing humidity, it finally results in a reduction of the adhesion. 2. Experimental Epitaxially grown 140 nm thick Au(1 1 1) and Ag(1 1 1) films on freshly cleaved mica were prepared by molecular beam epitaxy at elevated temperatures. We refer to Ref. [17] for the exact deposition parameters. The resulting films consist of atomically flat islands (typical root-meansquare (rms) roughness is 0.05 nm) with dimensions up to 500 · 500 nm2. Self-assembled monolayers of 1,4-benzenedimethanethiol (BDMT) were deposited on the films by immersion in a 1 mM solution of BDMT (Sigma–Aldrich) in tetrahydrofuran (THF) for 24 h. After incubation the samples were subsequently copiously rinsed in THF, acetone and distilled water to remove excess molecules, and dried under a nitrogen flow afterwards. The adsorption of the monolayers was characterized by means of ellipsometry and contact angle measurements (Drop Shape Analysis System DSA 10 Mk2). The results are listed in Table 1. The orientation of the molecules (length = 1.1 nm) was derived from the ellipsometric thickness, assuming a length of 0.2 nm for the AuS and AgS bond [2]. On gold the molecules stand upright the surface forming a dense monolayer. The BDMT molecules are tilted under an angle of approximately
Table 1 Ellipsometric thickness and contact angles measured on a bare and BDMT-covered polycrystalline gold/silver film
Fig. 1. Generic sketch of the functional relationship between the pull-off force and relative humidity. The capillary neck formation between tip and sample is illustrated for each of the three regimes I, II and III.
Sample
Ellipsometric SAM thickness
Contact angle ()
Au Au + BDMT Ag Ag + BDMT
– 0.8 ± 0.1 nm – 0.2 ± 0.1 nm
93 ± 2 85 ± 2 83 ± 2 90 ± 2
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100 Au Au + BDMT
Force (nN)
50
0
F pull-off F pull-off
-50
-100 -100 Fig. 2. Schematic representation of the deposited 1,4-benzenedimethanethiol molecules. On Au(1 1 1) the molecules stand upright (tilt angle of approximately 50, see (a)). One thiol group binds to the gold, the second remains free at the surface. On Ag(1 1 1) the molecules are oriented parallel to the surface; both thiol groups bind to the silver (see (b)).
50 with respect to the surface. On silver the molecules are oriented parallel to the surface. Both configurations are illustrated in Fig. 2. Similar results are reported in literature and can be explained in terms of different molecular chemisorption on gold and silver surfaces [3,4]. The different stacking mode results in minor changes of the macroscopic contact angle. With contact angles of approximately 90, all samples are mildly hydrophilic. The contact angle slightly decreases on gold after adsorption of the molecules (more hydrophilic), while on silver it slightly increases (more hydrophobic). Force–distance (F(d)) spectra (500 data points) were measured with a CP AutoProbe microscope (Park Scientific Instruments, now Veeco), using pyramidical silicon cantilevers with a force constant of 3.2 N/m (Ultralevers, Park Scientific Instruments, now Veeco). The measurements are performed in static mode at controlled humidity conditions. The CP setup is enclosed in a glovebox, in which the humidity is regulated by a controlled flow of dry or water-saturated air. A ventilator spreads the air inside the box which contains a hygrometer (HD 8501 H, Delta Ohm, measurable humidity range 5–98% RH) to measure the relative humidity (RH). Before the measurement, the sample stays overnight in a dry air flow to remove all water. During the measurement, the RH is increased in steps of approximately 5% and then stabilized for at least 15 min. 3. Wetting of tilted BDMT molecules In Fig. 3 we present the force–distance curve measured with a naturally oxidized silicon tip, under ambient conditions (50% RH) on bare and BDMT-covered gold, respectively. On bare gold a pull-off force of 40 nN is registered, while after deposition of the dithiol monolayer the pull-off force has increased to 90 nN. This increase is even more pronounced when calculating the energy dissipation as the area confined by the force–distance curve and the zero
-50
0
50
100
Piezo Extension (nm) Fig. 3. Force–distance curve at 50% RH measured with a naturally oxidized silicon tip on a gold sample before (Au, dashed line) and after (Au + BDMT, solid line) deposition of the BDMT monolayer. The curves have been shifted horizontally for clarity. The area confined by the force curve (Au ! light grey; Au + BDMT ! dark grey) and the zero force line corresponds to the dissipation energy.
force line (marked in Fig. 3). The energy needed to break the tip-sample contact has almost increased by a factor of 5 (from 0.62 fJ to 3.0 fJ), due to the presence of the monolayer. Since both measurements are performed under identical experimental conditions, the difference in interaction between the tip and the sample surface may be completely attributed to deposition of the dithiol molecules. However, from the expression for the pull-off force (Eq. (1)), it is clear that two explanations for the increased interaction remain plausible. First, deposition of the molecules may result in an increase of the surface roughness, which then results in a larger contact area and a corresponding increase of the pull-off force. Second, the increase may find its origin in a different type of interaction. Either the van der Waals interaction or the capillary force between tip and sample may have increased due to deposition of the BDMT layer. Based on STM measurements, we found that the surface roughness on bare and BDMT-covered gold are comparable ( 0.1 nm). Therefore, the explanation in terms of an increased contact area can be rejected. In order to determine the interaction type that causes the change, adhesion force spectroscopy measurements were performed on both samples. The humidity dependence of the dissipation energy on bare gold and BDMT-covered gold are presented in Fig. 4. The general tendency on both samples is comparable and corresponds well to the predicted behavior presented in Fig. 1. First, a low interaction regime is measured (regime I, 630% RH). The energy dissipation is negligible and can be completely accounted for by the van der Waals interaction between tip and sample. In regime II (30–55% RH), the energy dissipation increases due to capillary neck formation and reaches a maximum at 55% RH. Finally, the energy dissipation drops down to a low value (regime III, P55 % RH), which is comparable to the value measured at low
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Fig. 4. Dissipation energy observed during the contact of a standard AFM tip with a bare (left axis, triangles) and a BDMT-covered gold substrate (right axis, black squares) as a function of the relative humidity. For clarity the results are plotted on an adjusted scale.
humidities. No stable meniscus can be formed and the interaction is determined by the van der Waals interaction between a completely hydrated tip and sample. In spite of the good qualitative agreement, a major difference is observed for the magnitude of the adhesion. The maximum energy dissipation has increased from ED = 0.6 fJ on bare gold to ED = 5.3 fJ on BDMT covered gold (It should be noted that the scale in Fig. 4 was adjusted for both samples). Since the adhesion is comparable for low humidities and obviously humidity dependent, the increased energy dissipation has to originate from a different water layer growth and accompanying capillary forces rather than from a different van der Waals interaction. These results are consistent with the fact that the presence of a BDMT monolayer on gold slightly enhances the macroscopic surface wetting (see contact angles in Table 1). Surprisingly, the relatively small macroscopic enhancement of the surface wetting goes together with pronounced variations of the wetting properties on nanometer scale. The nanoscale properties can be understood in terms of two competing phenomena. On one hand, free thiol groups are exposed at the surface after the molecule deposition (see Fig. 2(a)). The polar character of SH bonds strongly favors the interaction with and condensation of water. This hydrophilic nature induces an enhanced water layer growth, leading to an enhanced adhesion during the tipsample contact. On the other hand, the water layer growth on bare gold samples is suppressed by adsorbed contaminants. Exposure of gold films to ambient conditions results in the condensation of hydrocarbons from the environmental air on the surface, which are known to be apolar and hence tend to dewet the surface [18]. Capillary neck formation between tip an sample is strongly reduced for these hydrophobic samples, resulting in a suppression of the adhesive forces [16]. In case of a complete hydrocarbon coverage of a surface, the water layer of the hydrated silicon oxide tip is simply sandwiched between tip and sample,
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Fig. 5. Dissipation energy for a BDMT covered gold substrate as a function of the relative humidity: (a) for decreasing humidity with a standard oxidized silicon tip, and (b) for increasing humidity with an etched tip. The standard measurement (i.e., increasing humidity and standard oxidized silicon tip) is added for comparison on both graphs (triangles).
without any attraction towards the hydrophobic sample surface. The statement that the increased energy loss indeed originates from capillary forces and not from a direct tip-surface interaction is supported by two additional experiments, for which the results are presented in Fig. 5. In the first experiment, the energy dissipation is determined from F(d) curves measured with a standard oxidized silicon tip for decreasing relative humidity (Fig. 5(a)). The sample is kept at high humidity (90% RH) overnight, and the humidity is then lowered in steps of 5–10% by a dry air flow and stabilized during at least 15 min before the measurement. The general behavior for increasing and decreasing humidities agrees well. Nevertheless, the curve for decreasing RH is less stable (i.e., the measurement error is higher) and a delay of the maximum dissipation occurs. This can be linked to the fact that the hydrophilic character of tip and sample hampers the evaporation of the water from both surfaces. Therefore, a hysteresis occurs between the water layer condensation and evaporation: the maximum in the curve for decreasing humidity is less pronounced and is reached for lower humidities (between 40% and 50% RH) when compared to the curve for increasing humidity (at 55% RH). On the other hand, the good agreement between the shape of both curves indicates that the structure of the BDMT layer is not affected by humidity. After storage in a wet environment for more than 20 h, the wetting properties of the surface remain unaltered. This observation indicates that the thiol groups of the BDMT monolayer are still at the surface. In a second experiment, the apex of a naturally oxidized tip was etched prior to the AFM measurements in order to make it hydrophobic and hence overcome the strong adhesive force. The etching process, which results in the removal of the oxide layer from the tip, is achieved by scanning a mica surface wetted with hydrogen fluoride liquid [7]. After
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the tip modification, the dissipation energy is reduced to a small value ED = 0.3 fJ. Because the tip tends to repel water, the approach and retract process is now completely reversible and determined by the van der Waals interaction for the complete humidity range. 4. Wetting of parallel BDMT molecules Force–distance curves measured under ambient conditions (50% RH) on a bare silver and a BDMT covered silver film are presented in Fig. 6. Although, similar to the situation on gold, a considerable difference in pull-off force and dissipation energy is recorded before and after deposition of the monolayer, the gold and silver based samples clearly behave differently. While on gold deposition of the BDMT molecules results in an increase of the interaction, on silver it gives rise to a decrease from 86 nN (ED = 2.4 fJ) to 54 nN (ED = 0.9 fJ). Again, we may assume that this difference originates from a modified affinity for water adsorption (see Table 1). We now turn to our detailed investigation at varying humidity to identify possible modifications of the wetting behavior due to the monolayer deposition. In Fig. 7 the humidity dependence of the dissipation energy for bare silver and silver covered with BDMT is plotted. The overall shape of both adhesion curves matches well and this shape is consistent with the generic sketch of Fig. 1. First, in regime I (640% RH), a low adhesion level is recorded (ED = 0.2 fJ). At 40% RH, a capillary neck is formed between tip and sample surface, and the adhesion increases when the neck grows (regime II, 40–63% RH). At 63% RH, the energy dissipation reaches its maximum value. A further increase of the humidity after the maximum leads to a degradation (regime III P 63% RH) and finally a breaking of the meniscus. In contrast to the situation on gold, the registered maximum is much more pronounced be-
F pull-off
F
Fig. 7. Dissipation energy observed during the contact of a standard AFM tip with a bare (left axis, triangles) and a BDMT-covered (right axis, black squares) silver substrate as a function of the relative humidity. For clarity the results are plotted on an adjusted scale.
fore (ED = 7.9 fJ) than after (ED = 1.1 fJ) deposition of the BDMT molecules (the scale in Fig. 7 was again adjusted for both samples). The large value on bare silver can be understood in terms of oxidation of the silver surface. Exposure of the silver substrate to air gives rise to the formation of a native oxide layer, which favors the condensation of water. Deposition of a BDMT monolayer results in an orientation of the BDMT molecules parallel to the silver surface (see Fig. 2(b)). For each molecule, a benzene ring and two methane groups are at the surface. The hydrophobic character of the aromatic benzene ring and CH2 groups of methane reduces the adhesion. However, the measured energy loss is still significantly larger than the energy dissipated on a surface that is completely covered by hydrocarbons. For this purpose, we measured on a gold film that was covered by 2-phenylethanethiol molecules (C6H4(CH2)2SH, PEM). Similar to BDMT on gold, the molecules stand upright the surface, forming a dense monolayer. However, in case of PEM the surface consists of CH2 groups instead of thiol groups. The maximum energy dissipation registered on PEM-covered gold was 0.3 fJ, well below the maximum value registered on BDMT-covered silver. This can be understood by the molecular structure of BDMT, which implies in the parallel growth mode a non-dense coverage of hydrocarbons at the surface. Therefore, the hydrophilic silver substrate still participates in the water layer growth and the adhesion.
pull-off
5. Conclusions
Fig. 6. Force–distance curve at 50% RH measured with a SiO2 tip on a silver sample before (Ag, dashed line) and after (Ag + BDMT, solid line). The area confined by the force curve (Ag ! light grey; Ag + BDMT ! dark grey) and the zero force line corresponds to the dissipation energy. The curves have been shifted horizontally for clarity.
Adhesion force spectroscopy measurements at varying humidity were performed on bare and BDMT covered gold and silver films. The measurements turn out to be an excellent tool for probing the wetting characteristics of the samples. The general behavior on all samples is essentially the same, with three distinct adhesion regimes. Transitions in the measured energy dissipation for varying humidity can
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be completely attributed to changes in the capillary force. Consequently, differences between the different samples can be accounted for by different wetting behavior. Upon dithiol deposition on gold substrates the maximum tip-sample adhesion drastically increases (by an order of magnitude). The tilted molecule orientation results in a surface of thiol groups, which are more polar than the surface of bare gold. On the BDMT-covered gold a thicker water layer is adsorbed and a strong capillary neck is formed. For silver the opposite effect is observed. After dithiol deposition the adhesion is significantly reduced. Due to the parallel molecule orientation the surface partially consists of hydrocarbons that repel the water. Water condensation remains restricted and the menisci are smaller. Deposition of a 1,4-benzenedimethanethiol layer clearly favors the water adsorption on gold, while it hampers water layer growth on silver. Application of self-assembled monolayers to tune the wetting behavior thus provides important ‘‘remedies’’ for diminishing undesirable effects of capillary water condensation at the surface. Acknowledgements This work has been supported by the Fund for Scientific Research – Flanders (FWO) as well as by the Flemish Concerted Action (GOA) and the Belgian Interuniversity Attraction Poles (IAP) research programs. We are much indebted to Dr. Orlin Blajiev and Prof. Herman Terryn (Department of Chemical Engineering, Free University of Brussels) for performing the ellipsometry measurements
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and to Katleen Boussu (Department of Chemical Engineering, KULeuven) for doing the contact angle measurements. References [1] F. Schreiber, J. Phys.: Condens. Matter 16 (2004) R881. [2] H. Sellers, A. Ulman, Y. Shnidman, J. Eilers, J. Am. Chem. Soc. 115 (1993) 9389. [3] J. Henderson, S. Feng, G. Ferrence, T. Bein, C. Kubiak, Inorg. Chem. Acta 242 (1996) 115. [4] K. Murty, M. Venkataramanan, T. Pradeep, Langmuir 14 (1998) 5446. [5] W. Tian, S. Datta, S. Hong, R. Reifenberger, J. Henderson, C. Kubiak, J. Chem. Phys. 109 (1998) 2874. [6] R. Andres, T. Bein, M. Dorogi, S. Feng, J. Henderson, C. Kubiak, W. Mahoney, R. Osifchin, R. Reifenberger, Science 272 (1996) 1323. [7] N. Vandamme, J. Snauwaert, E. Janssens, E. Vandeweert, P. Lievens, C. Van Haesendonck, Surf. Sci. 558 (2004) 57. [8] J. Israelachvili, Intermolecular and Surface Forces, Second ed., Academic Press, Harcourt Brace and Company, London, 1992. [9] T. Thundat, X. Zhang, G. Chen, S. Sharp, R. Warmack, L. Schonwalter, Appl. Phys. Lett. 63 (1993) 2150. [10] L. Xu, A. Lio, J. Hu, D. Ogletree, M. Salmeron, J. Phys. Chem. B 102 (1998) 540. [11] X. Xiao, L. Qian, Langmuir 16 (2000) 8153. [12] R. Quon, A. Ulman, T. Vanderlick, Langmuir 16 (2000) 8912. [13] D. Sedin, K. Rowlen, Anal. Chem. 72 (2000) 2183. [14] M. He, A. Blum, D. Aston, C. Buenviaje, R.O.R. Luginbiihl, J. Chem. Phys. 114 (2001) 1355. [15] R. Jones, H. Pollock, J. Cleaver, C. Hodges, Langmuir 18 (2002) 8049. [16] M. Binggeli, C. Mate, Appl. Phys. Lett. 65 (1994) 415. [17] N. Vandamme, E. Janssens, F. Vanhoutte, P. Lievens, C. Van Haesendonck, J. Phys.: Cond. Matt. 15 (2003) S2983. [18] K. Hayashi, H. Sugimura, O. Takai, Appl. Surf. Sci. 188 (2002) 513.