Interaction of Water with Self-Assembled Monolayers of Alkylsilanes

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Interaction of Water with Self-Assembled Monolayers of Alkylsilanes on Mica Ismael Dı´ez-Pe´rez,† Mo´nica Luna,‡ Fernando Tehera´n,‡ D. Frank Ogletree,‡ Fausto Sanz,† and Miquel Salmeron*,‡ Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Departament de Quı´mica Fı´sica, Universitat de Barcelona, Martı´ i Franque´ s 1, 08028-Barcelona, Spain Received August 11, 2003. In Final Form: November 25, 2003 The interaction of water with self-assembled alkylsilane monolayers on mica substrates has been studied using an atomic force microscope operated in contact, noncontact, and electrical polarization modes. Complete monolayer films were found to be effective in blocking water adsorption. On partially covered surfaces water was found to produce large changes in the conductivity and surface potential of the exposed mica regions. It was also found that water could penetrate films near defects and at island edges.

1. Introduction The increasing interest in self-assembled monolayers (SAMs) is related to their promising applications in fields such as biosensors,1 optoelectronics,2 or tribology.3 From the technologic point of view, the lubricant and antistiction properties of the monolayers are especially important in MEMS and NEMS devices. Despite its scientific importance and technological implications, the interaction of water with surfaces covered by SAMs is poorly understood. Most of the previous work on the effect of humidity on the friction and adhesion properties of SAMs were performed on surfaces completely covered by the SAM film, where the role of defects such as pinholes, partially covered areas, etc. is difficult to determine. In these studies it is normally found that the SAM film constitutes an excellent barrier to water adsorption due to its highly hydrophobic nature.4-7 In this paper we present a study of the properties of alkylsilane monolayers on mica as a function of environmental humidity both in complete films and in films partially covering the surface or defective. Xiao et al.8 found that the driving force for the formation of SAMs of alkylsilanes on mica was the molecule-molecule interaction, which consists of chain-chain van der Waals interactions and the formation of a limited number of intermolecular siloxane bonds. Bonding to the mica surface, on the other hand, is probably electrostatic in nature rather than covalent, because unlike glass or oxidized silicon wafers, there are no OH groups on the surface. We found that water adsorption affects the formation and growth of the alkylsilane layers. Once formed, however, complete alkylsilane monolayers were found to † ‡

Universitat de Barcelona. Lawrence Berkeley National Laboratory.

(1) Schierbaum, K.; Weiss, T.; van Velzen, E. T.; Engbersen, J.; Reinhoudt, D.; Go¨pel, W. Science 1994, 265, 1413. (2) Ulman, A. Thin Films: Self-Assembled Monolayers of Thiols; Academic Press: New York, 1998. (3) Maboudian, R. MRS Bull. 1998, 23, 47. (4) Jun, Y.; Zhu, X. Y. J. Adhes. Sci. Technol. 2003, 17, 593. (5) Ashurst, W. R.; Yau, C.; Carraro, C.; Lee, C.; Kluth, G. J.; Howe, R. T.; Maboudian R. Sens. Actuators, A 2001, 91, 239. (6) Qian, L.; Tian, F.; Xiao, X. D. Tribol. Lett. 2003, 15, 169 (7) Patton, S. T.; Cowan, W. D.; Eapen, K. C.; Zabinski, J. S. Tribol. Lett. 2000, 9, 199. (8) Xiao, X.-D.; Liu, G.-Y.; Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600.

be effective in blocking water adsorption. On defective or incomplete layers however water influences dramatically the dielectric and contact potential properties of the surface. 2. Experimental Section 2.1. Sample Preparation. The alkylsilane molecules used in this study are hexadecylsilanol, CH3(CH2)15Si(OH)3, except in one experiment with CH3(CH2)17Si(OH)3 (Figure 2). The starting material was a triethoxysilane, which was prehydrolyzed to silanol and diluted in cyclohexane, following the procedure described by Xiao et al.8 Freshly cleaved mica was immersed in the solution. The surface coverage was controlled by varying the immersion time and the solution concentration (concentration values varied from 0.2 to 1 mM). To study the influence of humidity, the atomic force microscope (AFM) was enclosed in a plastic box. The relative humidity in the box could be reduced below a few percent by flowing dry N2 for an hour. By bubbling N2 through purified water,9 the relative humidity could be raised above 95% over the course of several hours. 2.2. Operation of the AFM. Our home-built AFM was operated by an RHK electronic control.10 The AFM was operated in a variety of modes, which we now describe briefly, to obtain different and complementary information. Contact Mode. In this mode the tip is in continuous contact with the surface. The cantilever bends as compressive or tensile forces are applied to the tip-sample contact, and twists due to frictional forces during sliding. The cantilevers used in this mode are usually soft, in our case with a nominal force constant of 0.4 N/m and a resonance frequency of 45 kHz.11 The normal deflection of the lever is used for feedback control to produce a topographic image. The lateral resolution is ∼1 nm, depending on the tip radius and applied load. Contact mode imaging is not very suitable for weakly bound adsorbates or liquid films. Attractive Force Modulation Mode (AM). This mode is based on the change in oscillation amplitude of the lever when driven close to its free mechanical resonance frequency.12-15 The oscillation is excited by a small piezoelectric bimorph element attached to the lever. Interaction forces between the tip and (9) Ultrapure Milli-Q water (conductivity 18 Ω‚cm). (10) Bluhm, H.; Pan, S. H.; Xu, L.; Inoue, T.; Ogletree, D. F.; Salmeron, M. Rev. Sci. Instrum. 1998, 69, 1781. (11) Park Scientific Instruments, Sunnyvale, CA. (12) Anczykowski, B.; Kru¨ger, D.; Babcock, K. L.; Fuchs, H. Ultramicroscopy 1996, 66, 251. (13) Ku¨hle, A.; Sørensen, A. H.; Borh, J. J. Appl. Phys. 1997, 81, 6562. (14) Salmeron, M.; Neubauer, G.; Folch, A.; Tomitori, M.; Ogletree, D. F.; Sautet, P. Langmuir 1993, 9, 3600. (15) Salmeron, M. MRS Bull. 1993, 18, 20.

10.1021/la030336x CCC: $27.50 © 2004 American Chemical Society Published on Web 01/23/2004

Interaction of Water with SAMs of Alkylsilanes sample cause changes in the oscillation amplitude, due either to changes in the Q factor or to shifts in resonance frequency. With soft levers and small oscillation amplitudes, when the tip gets sufficiently close to the surface, the oscillation becomes unstable and the tip can easily jump into contact with the surface and stick. In the AM used here, however, the stiffness of the lever and the tip-surface distance are chosen to avoid such instability. The oscillation amplitude was set below 10 nm peak-to-peak and the tip always in the attractive regime. In our work we used tips of ∼1 N/m,16 which are stiffer than those used in contact mode, but softer than those used in tapping mode (TM), where the tip enters in repulsive contact with the sample for a brief time in every oscillation cycle. The phase shift was monitored during imaging to ensure that the AFM was operating in AM. The transition from AM to TM is associated with a significant phase discontinuity.17 Scanning Polarization Force Microscopy (SPFM). SPFM is a noncontact operating mode based on electrostatic forces. Electrostatic methods have been used to study conductive substrates (metals, semiconductors).18-20 SPFM was introduced as a method to image insulating substrates and liquid films.21,22 In this method, a conducting tip is biased at a voltage Vtip ) Vdc + Vac sin(ωt) while the electrode supporting the sample is grounded. The force acting on the tip is F ) -1/2(∂C/∂z)(Vtip - ∆φ)2, where C(r) is the interelectrode capacitance at tip position r, the z axis is perpendicular to the sample surface, and ∆φ is the work function or contact potential difference between electrodes. Lock-in amplifiers are used to detect the first (1ω) and second (2ω) harmonic components of the lever oscillation induced by the electrostatic force. The 2ω component is a pure polarization term which depends only on sample topography and dielectric properties, while the 1ω term has contributions from Vdc, contact potential differences between the electrodes, and any fixed dipoles or charges in the sample. This method was first applied by Schoenenberg et al.23 and later by Yokoyama et al.24 for conductive substrates. When an additional feedback loop adjusts Vdc to minimize the 1ω amplitude, this is known as Kelvin force microscopy.25,26 There are two components to the electrostatic force between the tip and sample, a “local” contribution from the tip apex, which is responsible for virtually all of the image contrast, and a “background” contribution due to the cantilever and tip shaft, which may be several times larger depending on the tip length, opening angle, and imaging distance.27 When the tip-sample distance d is less than the tip radius R, the local attractive force between the tip and a conducting surface is close to (πoV2)R/d.28 For a dielectric slab, the local force also depends on the slab thickness and dielectric constant. When the dielectric slab is thick compared to R, the local attractive force depends only on R, d, and , and remains finite due to the field concentration near the tip. The force gradient (1/F)(∂F/∂d) is smaller than for conducting samples. As d goes to zero, this force approaches a limiting value29 of (πoV2)[1/6( + 1)2 + 2/3[( - 1)/( + 1)] ln(2/( + 1))]. The interpretation of SPFM images can become complex for dielectric samples with mobile charges. Mica, for example, has (16) NANOSENSORS, Neuchatel, Switzerland. (17) Haugstad, G.; Jones, R. R. Ultramicroscopy 1999, 76, 77. (18) Bugg, C. D.; King, P. J. J. Phys. E 1988, 21, 147. (19) Martin, Y.; Abraham, D. W.; Wickramasinghe, H. K. Appl. Phys. Lett. 1988, 52, 1103. (20) Williams, C. C.; Hough, W. P.; Rishton, A. Appl. Phys. Lett. 1989, 55, 203. (21) Salmeron, M.; Xu, L.; Hu, J.; Dai, Q. MRS Bull. 1997, 22(8), 36. (22) Hu, J.; Xiao, X.-D.; Salmeron, M. Appl. Phys. Lett. 1995, 67(4), 476. (23) Scho¨nenberger, C.; Alvarado, S. F. Phys. Rev. Lett. 1990, 65, 3162. (24) Yokohama, U. H.; Inoue, T. Thin Solid Films 1994, 242, 33. (25) Weaver, J. M. R.; Abraham, D. W. J. Vac. Sci. Technol., B 1991, 9, 1559. (26) Nonnenmacher, M.; O’Boyle, P. M.; Wickramasinghe, H. K. Appl. Phys. Lett. 1991, 58, 2921. (27) Colchero, J.; Gil, A.; Baro´, A. M. Phys. Rev. B 2001, 64, 245403. (28) Xu, L.; Salmeron, M. In Nano-Surface Chemistry; Rosoff, M., Ed.; Marcel Dekker: New York, 2001; Nov. 1, Chapter 6, pp 243-287. (29) Go´mez-Mon˜ivas, S.; Froufe-Pere´z, L. S.; Caaman˜o, A. J.; Sa´enz, J. J. Appl. Phy. Lett. 2001, 79, 4048.

Langmuir, Vol. 20, No. 4, 2004 1285 K+ ions at the surface. We have shown that as humidity increases these ions become solvated and become highly mobile. Under these conditions, mica acts as a conductor during SPFM imaging and the electrostatic force is relatively high. At low humidity, the ions are immobile and mica acts as a dielectric with reduced electrostatic force. Between these two limits the forces are frequency-dependent and intermediate in size.30 SPFM imaging is normally performed with a tip-sample distance less than the tip radius. In the present work the distance was between 10 and 20 nm, with a tip radius of ∼30 nm. The ac bias was typically 7 V peak-to-peak at 3 kHz, well below the lever resonance. The “topographic” images, which also depend on the sample dielectric properties, were obtained using the 2ω amplitude as the feedback control signal. In this study the cantilevers used had the same spring constant and resonance frequency as those used for contact mode imaging.

3. Results 3.1. Growth and Structure of Alkylsilane Islands. Contact mode topographic and friction images of the surface with increasing alkylsilane coverage are shown in Figure 1. The mica was immersed for 10, 120, and 300 s for the left, center, and right images, respectively. Immersion times above ∼400 s produced complete layers with very few visible defects. Friction is lower on the silane islands than on the exposed mica, although at the lowest coverage (left panel) there are high-friction regions adjacent to several islands. Since the AFM spatial resolution is limited by the ∼1 nm tip-sample contact area, small defects will not be resolved. Occasionally pinholes can be seen in silane films, as shown in Figure 2 taken from a previous work with octadecylsilanes.31 It is interesting to note that the edges of the ∼15 nm radius pinholes show a slight elevation of ∼0.2 nm surrounding the pinhole extending over a distance of ∼75 nm. We shall discuss this point below. Figure 3 shows AM topographic (a) and phase (b) images of silane islands. The apparent island height in the AM images was 1.9 nm, which is less than the 2.4 nm length of the molecules. This implies that the alkane chains adopt a tilted configuration. There may also be a layer of flatlying molecules surrounding the islands, which could reduce their apparent height. In addition to the 1.9 nm high islands, the images also show regions of 0.7 nm height connecting some of the islands. These regions may be a precursor to island formation, with molecules arranged in a highly tilted or flat-lying configuration. We found that under dry conditions the precursor layer remains stable for several days, while at high humidity (>80% RH) it persists for just a few hours. This indicates that water enhances molecular mobility, so that the low-lying molecules can more rapidly aggregate into energetically more favorable islands of upright molecules. The lack of contrast in the phase image, except for small contributions at the island edges from finite feedback response, is an indication that the scan has indeed been performed out of contact.17,32 In the contact mode images of Figure 1 the island height is only 1.35 ( 0.15 nm, about 30% less than in the AM images. We have shown previously that pressure exerted on the sample by the tip in contact mode can increase the tilt angle in alkane islands.33 The precursor layer cannot be seen directly in contact images, probably due to tip penetration. This (30) Hu, J.; Xiao, X.-D.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 344, 221; Surf. Sci. 1996, 355, 255. (31) Xiao, X.-D.; Liu, G.-Y.;. Charych, D. H.; Salmeron, M. Langmuir 1995, 11, 1600. (32) Anczykowski, B.; Kru¨ger, D.; Babcock, K. L.; Fuchs, H. Ultramicroscopy 1996, 66, 251. (33) Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82(14), 2880.

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Figure 1. Contact AFM images of mica samples covered with hexadecylsilane monolayer islands obtained with three different immersion times into the silanol solution: (a, left) 10 s, (b, middle) 60 s, and (c, right) 300 s. The top images show the topography and the bottom ones the friction force (bright regions correspond to higher friction). The image size is 4.5 × 4.5 µm, and the RH is 37%.

Figure 2. Contact AFM image (1 × 1 µm) of mica covered with a monolayer of octadecylsilane. The monolayer contains numerous pinholes, about 30 nm in diameter. The pinholes are surrounded by regions (slightly brighter areas) of higher elevation (approximately 0.2 nm), which extend up to 80 nm away from the pinhole.

penetration could be expected to lead to higher friction as is indeed observed around many islands in the contact images of Figure 1a. 3.2. Influence of Humidity on the Dielectric Properties of Alkylsilane-Covered Mica. The effects of water adsorption are most clearly visible in the images acquired in SPFM mode, due to the large changes it causes in the dielectric response of mica. This can be shown by measuring the 2ω force amplitude as a function of humidity at a fixed tip-sample distance. Figure 4 shows the results obtained on surfaces covered with complete hexadecylsilane monolayers and with defective monolayers (as in Figure 1c) and on bare mica. In these experiments the

Figure 3. Amplitude modulation AFM topographic and phase images of hexadecylsilane islands on mica. A lower layer, ∼0.7 nm in height, can be seen near the center, adjacent to several monolayer islands. The image size is 2 × 3 µm.

same tip was used for each curve. During measurements the tip height was constant at about 20 nm, as checked by approach curves performed before and after each point. In dry conditions the forces are similar for the three cases. For complete silane layers the 2ω force is nearly independent of humidity. On bare mica the force increased by a factor of about 5 with increasing humidity. In incomplete and defective monolayers the response is

Interaction of Water with SAMs of Alkylsilanes

Figure 4. Amplitude of the second harmonic (2ω) of the cantilever oscillation as a function of humidity. The tip is at an average height of 20 nm. The oscillation is driven by an applied ac bias voltage at 10 kHz and 7 V peak-to-peak amplitude. Bottom curve: complete hexadecylsilane monolayer. Top curve: bare mica. The intermediate points correspond to three different samples with defective monolayers. The lines through the limiting cases are visual guides only.

intermediate, with an initial increase up to 20-30% RH similar to that of bare mica, followed by a much more gradual increase, leading to a force roughly double that for dry conditions. There was a significant sample-to-

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sample variation and scatter in the data for incomplete layers, probably due to sample and spatial variations in the defect concentration. Because the mobility of the hydrated ions is much smaller than that of electrons in a conductor, strong frequency effects are expected. This is indeed shown by measuring the amplitude of the force as a function of frequency ω. The results of experiments using samples with different defect concentrations are shown in Figure 5. In the case of bare mica the result is similar to previously published data.30 As can be seen, a complete alkylsilane layer essentially suppresses ion mobility and the force decreases only very slowly with frequency at any humidity. For defective layers the effect is intermediate between the two extremes and depends on the position of the tip over the surface, as we discuss in more detail in the next section. Notice also that on complete monolayers the contact potential (1ω) remains constant with frequency and humidity (Figure 5B), except for some not reproducible fluctuations at low frequency. 3.3. Topography and Contact Potential of Alkylsilane Islands vs Humidity. Figure 6 shows a sequence of SPFM images of the same silane island, acquired on the same sample as in Figure 1b, for selected values of the humidity. A pair of images is shown for each value of the humidity. The left image corresponds to the topography (2ω feedback) and the right to the local difference in contact potential (1ω). Below 40% RH the alkylsilane islands appear as flat mesas with the same shape as in the contact images (Figure 1b). Their height is 0.9 ( 0.1 nm, smaller than that measured in contact mode or AM (1.3 and 1.9 nm, respectively). Between 30% and 35% RH, the topographic contrast vanishes; i.e., the apparent island elevation becomes zero relative to the surrounding mica,

Figure 5. Amplitude of the 2ω and 1ω components of the cantilever oscillation versus applied bias voltage frequency for different RH values for (A, B) a complete monolayer, (C) a defective monolayer, and (D) bare mica. In these experiments the tip was located 28 nm above the surface.

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Figure 6. Series of SPFM images of hexadecylsilane islands on mica as a function of humidity. The left image of each pair corresponds to the “topographic” image obtained by keeping the 2ω signal constant, and the right image (1ω) is proportional to the tip-sample contact potential difference. The tip-sample distance in these experiments was constant during each pair of images but varied from 14 to 20 nm for RH values up to 41%, and between 20 and 30 nm for RH values between 41% and 90%. It was necessary to increase the average tip-sample distance at high humidity due to the large differences in the 2ω force components between the islands and mica. Notice the contrast reversal in the topographic contrast of the large island, from positive to negative, between 30% and 40% RH.

although the edges are still visible. Above 40% RH, the contrast is reversed and the islands appear as depressions. The zero-contrast point is reached about 2/3 of the way through the “32-40%” RH image. Humidity was increasing slowly from 32% to 40% during acquisition of this image (slow scan direction right to left). Above 40% the negative topographic contrast increased rapidly, reaching the highest value around 80% RH and decreasing afterward. These observations are shown in a more quantitative way in the plots of Figure 7, corresponding to the apparent

height and contact potential differences between the large island and the surrounding mica. There was some variation in the humidity (between 30% and 45% RH) at which the height contrast reversed for different samples, due probably to changes in temperature or the initial conditions of the mica. The qualitative behavior however was reproducible. The contact potential difference between the large island and surrounding mica changed from negative (i.e., the alkylsilane islands are more negative than the mica) to positive around 50% RH. It reached a maximum around 80% RH and then decreased. An

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Figure 8. Schematic model of the water-silane monolayer interaction on a mica substrate. The negative contrast of the island relative to mica in this model is due to the displacement of the K ions by the molecules. The inserted water molecules around the island edge, with their dipole oriented downward, further decrease the surface potential.

4.1. Growth and Structure of Alkylsilane Films. The existence of precursor films of flat-lying molecules has been observed in the past for alkylthiols on gold, both by scanning tunneling microscopy and by atomic force microscopy.34 They form a variety of ordered structures as a function of coverage. With time the molecules diffuse and aggregate into islands of upright molecules forming close-packed assemblies.35 Figure 3 indicates that a similar process takes place on mica. The speed of aggregation depends on humidity, with dry samples retaining the flat precursor layers for a longer time, while the presence of water facilitates diffusion and aggregation. The film islands consist of nearly upright molecules bound together by van der Waals attractive forces, which are responsible for stabilizing the close-packed structures. In previous work with self-assembled alkyl chain molecules (alkylthiols on gold36 and silanes,33 amines,37 and alcohols38 on mica), it was shown that the molecules adopt

configurations favoring the densest possible packing of straight all-trans chains. A tilt angle with respect to the surface normal is often observed. Because of the regular arrangement of CH2 units along the chain, configurations with specific tilt angles are favored. Cross-linking of the headgroups of the molecules to form -Si-O-Si-O- chains of covalent bonds, however, is sterically hindered since the projected Si-O-Si bond length is