Dependence of Patterned Binary Alkanethiolate Self-Assembled

Selective immobilization of Sonic hedgehog on benzylguanine terminated patterned self-assembled monolayers. Chiu-Wai Kwok , Uwe Strähle , Yujie Zhao ...
0 downloads 0 Views 578KB Size
Download PDF
11420

Langmuir 2006, 22, 11420-11425

Dependence of Patterned Binary Alkanethiolate Self-Assembled Monolayers on “UV-Photopatterning” Conditions and Evolution with Time, Terminal Group, and Methylene Chain Length Chuanzhen Zhou and Amy V. Walker* Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, Campus Box 1134, One Brookings DriVe, St. Louis, Missouri 63130 ReceiVed June 27, 2006. In Final Form: September 15, 2006 We have investigated the mechanism of UV photopatterning of binary alkanethiolate self-assembled monolayers (SAMs) adsorbed on Au(111) using time-of-flight secondary ion mass spectrometry. The SAMs were photopatterned using a 500 W Hg arc lamp. The patterning process is strongly dependent on the wavelength of light used. When an unfiltered arc lamp is employed, IR light impinges on the sample and causes considerable sample heating. Methylterminated SAMs with less than 14 carbons in the chain melt at the temperatures reached and become very disordered and so can be easily displaced by a second SAM. This leads to significant pattern degradation (“erosion”). SAMs with greater than 14 carbons undergo a transition to an incommensurate phase but remain stable on the surface, and the pattern is retained. When the IR light is filtered out, a different behavior is observed. UV-photopatterned methylterminated SAMs with 10 carbons in the chain are stable. Terminal group interactions, such as H-bonding, provide extra stabilization energy during photopatterning, so some patterns with shorter carbon chains may also be stable. The displacement of the photooxidized SAMs on the patterned surface follows kinetics similar to that of large-area SAM formation.

Introduction Self-assembled monolayers (SAMs) have been proposed for a wide range of applications from biosensing1 to corrosion inhibition2 to molecular electronics.3,4 SAMs are also employed as model systems for studies of the interaction of metals with organic films5-9 and other systems. This is because SAMs have highly organized, well-defined structures with a uniform density of terminal groups.10,11 SAMs have been patterned using a number of methods including microcontact printing (µCP), nanoimprinting, energetic beam lithography, and scanning probe-based nanolithographies (see refs 12 and 13 and references therein). UV photopatterning has also been shown to be effective.14-16 UV photopatterning is * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (314) 935-8496. Fax: (314) 935-4481. (1) Choi, S. H.; Lee, J. W.; Sim, S. J. Biosens. Bioelectron. 2005, 21, 378383. (2) Jennings, G. K.; Laibinis, P. E. Colloids Surf., A 1996, 116, 105-114. (3) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 43784400. (4) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 17051707. (5) Jung, D. R.; Czanderna, A. W. Crit. ReV. Solid State 1994, 19, 1-54. (6) Fisher, G. L.; Walker, A. V.; Hooper, A. E.; Tighe, T. B.; Bahnck, K. B.; Skriba, H. T.; Reinard, M. D.; Haynie, B. C.; Opila, R. L.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 2002, 124, 5528-5541. (7) Hooper, A.; Fisher, G. L.; Konstadinidis, K.; Jung, D.; Nguyen, H.; Opila, R.; Collins, R. W.; Winograd, N.; Allara, D. L. J. Am. Chem. Soc. 1999, 121, 8052-8064. (8) Walker, A. V.; Tighe, T. B.; Cabarcos, O.; Reinard, M. D.; Haynie, B. C.; Uppili, S.; Allara, D. L.; Winograd, N. J. Am. Chem. Soc. 2004, 126, 3954-3963. (9) Haynie, B. C.; Walker, A. V.; Tighe, T. B.; Allara, D. L.; Winograd, N. Appl. Surf. Sci. 2003, 203-204, 433-436. (10) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (11) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (12) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1-68. (13) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Wilson, C. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1171-1196. (14) Tarlov, M. J.; Burgess, D. R. F., Jr.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (15) Gillen, G.; Bennett, J.; Tarlov, M. J.; Burgess, D. R. F., Jr. Anal. Chem. 1994, 66, 2170-2174.

advantageous for the construction of SAM-based devices since it is parallelizable and can be used to produce complex structures without chemical contamination. In this technique, UV light is shone through a mask onto an alkanethiolate SAM surface. In the areas exposed to UV light, the SAM is photooxidized and can be replaced with a second SAM, creating a patterned surface.14-16 By controlling the SAM terminal group functionality, it is possible to carry out location-specific reactions. For example, it has been recently demonstrated that patterned SAMs can be employed to selectively define areas for metal deposition17 and biochemical functionalization.18 There have been many studies of the UV photooxidation of SAMs.19-27 It has been shown that both the alkyl chain length21,20 and terminal functional groups21 play an important role. However, the mechanism of SAM photooxidation remains controversial. Most of the studies have been carried out using broad-spectrum UV sources, such as Hg arc lamps. A variety of photooxidation mechanisms have been proposed including ozonolysis,23,24 formation of singlet oxygen species,20 and hot electron attachment.27 Recent studies using ozone-free lamps demonstrate that SAMs can still be photooxidized without the presence of ozone, suggesting that other mechanisms are also operative.25,27 (16) Cooper, E.; Leggett, G. Langmuir 1999, 15, 1024-1032. (17) Zhou, C.; Nagy, G.; Walker, A. V. J. Am. Chem. Soc. 2005, 127, 1216012161. (18) Zhou, C.; Walker, A. V. Manuscript in preparation. (19) Huang, J.; Hemminger, J. C. J. Am. Chem. Soc. 1993, 115, 3342-3343. (20) Hutt, D. A.; Leggett, G. J. J. Phys. Chem. 1996, 100, 6657-6662. (21) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795-4801. (22) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174-184. (23) Zhang, Y.; Terrill, R. H.; Tanzer, T. A.; Bohn, P. W. J. Am. Chem. Soc. 1998, 120, 2654-2655. (24) Norrod, K. L.; Rowlen, K. L. J. Am. Chem. Soc. 1998, 120, 2656-2657. (25) Brewer, N. J.; Rawsterne, R. E.; Kothari, S.; Leggett, G. J. J. Am. Chem. Soc. 2001, 123, 4089-4090. (26) Ye, T.; McArthur, E. A.; Borguet, E. J. Phys. Chem. B 2005, 109, 99279938. (27) Brewer, N. J.; Janusz, S.; Critchley, K.; Evans, S. D.; Leggett, G. J. J. Phys. Chem. B 2005, 109, 11247-11256.

10.1021/la0618519 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/01/2006

Patterned Binary Alkanethiol SAMs

Langmuir, Vol. 22, No. 26, 2006 11421

displacement of the photooxidized SAMs on the patterned surface follows kinetics similar to that of large-area SAM formation. Finally, at longer time scales we observe the formation of bilayers on photopatterned SAMs. Experimental Section

Figure 1. A schematic of the UV-photopatterning process and a summary of our results. In step 1, UV light is shown through a mask onto the SAM#1 surface. In the areas exposed to UV light SAM#1 is photooxidized. In step 2 a second SAM (SAM#2) is adsorbed on the surface, forming a patterned surface (step 3). If IR light is present, SAMs shorter than 14 carbons melt and are displaced by the adsorption of the second SAM, leading to pattern degradation. When an IR filter is employed, SAMs with as few as 10 carbon atoms remain stable on the surface and can be photopatterned.

In contrast, there have been fewer studies of the mechanism of UV photopatterning. Using time-of-flight secondary ion mass spectrometry (TOF SIMS) and scanning electron microscopy (SEM), Cooper and Leggett investigated the influence of the terminal group functionality on the patterning of binary alkanethiolate SAM surfaces.16 In this study, it was observed that sharp, well-defined patterned SAM surfaces could be prepared by UV photopatterning 3-mercaptopropanoic acid (MPA) and backfilling the photooxidized areas with methyl-terminated SAMs, such as dodecanethiol (DDT) and octadecanethiol (ODT). The situation was more complicated if the process was performed in the reverse order, i.e., UV photopatterning a methyl-terminated SAM and backfilling with MPA. In this case, for methylterminated SAMs, the pattern was only stable for methylene chain lengths greater than tetradecanethiol (TDT). If the methylene chain length was shorter than TDT, the pattern was eroded by the adsorption of the second SAM. Similar results were obtained using -OH-terminated SAMs, suggesting that hydrogen bonding greatly contributed to the SAM stability.16 In this paper first we demonstrate that the patterning of binary alkanethiolate SAMs is strongly dependent on the wavelengths of light impinging on the sample. To aid in the discussion of the results, a schematic of the UV-photopatterning process and a summary of our results are shown in Figure 1. Hg arc lamps emit a broad range of wavelengths from IR to visible to UV (200 to >2500 nm). When an unfiltered arc lamp is employed for SAM photopatterning, IR light is incident on the sample and causes significant heating. In contrast, when the IR light is filtered out, for example, by using a dichroic mirror, sample heating is greatly reduced. The sample heating causes significant differences in the observed photopatterning behavior. For methyl-terminated SAMs with fewer than 14 carbons in the chain, the sample heating caused by an unfiltered arc lamp leads to the SAM melting during the initial photopatterning step, resulting in a degraded binary patterned SAM upon adsorption of a second SAM. In contrast, when an IR filter is used, SAMs with as few as 10 carbons remain stable on the surface during UV photopatterning and immersion into a second SAM, leading to the formation of patterned SAM surfaces. Patterned surfaces made using SAMs with less than 10 carbons in the chain are not stable because the chain-chain interaction is weaker than the headgroup-substrate interaction, which leads to an increasingly disordered monolayer. This allows adsorption of the second SAM on all areas of the surface. Second, we show that under our experimental conditions (using a 500 W Hg arc lamp fitted with an IR filter) the

Materials. Nonanethiol (NT) (95%), undecanethiol (UDT) (98%), hexadecanethiol (HDT) (92%), 3-mercaptopropanoic acid (g99%), and 11-mercaptoundecanoic acid (MUA) (95%) were purchased from Aldrich and used as received without further purification. 16Mercaptohexadecanoic acid (MHA) was obtained from Prof. D. Allara, Pennsylvania State University, and used as received. Si〈111〉 wafers were obtained from Addison Technologies and were etched using piranha etch before use. Chromium and gold were purchased from Goodfellow and Alfa Aesar and were of g99.99% purity. SAM Preparation. The preparation and characterization of the types of SAMs used in this study have been described in detail previously.28-33 Briefly, Cr (∼50 Å) and Au (∼1000 Å) were deposited sequentially onto Si〈111〉. Self-assembly of well-ordered monolayers was achieved by immersing the resulting Au substrate into a 1 mM ethanolic solution of the relevant alkanethiol solution for 24 h at ambient temperature (21 ( 2 °C). UV Photopatterning of SAMs. Prior to UV exposure the SAMs were characterized using single-wavelength ellipsometry (Gaertner Inc.) and TOF SIMS to ensure that they were well-ordered and had no significant chemical contamination. Tests were also performed to determine the optimum time for UV photooxidation for each of the SAMs used in this study. Samples were placed 50 mm (or 100 mm) away from a 500 W Hg arc lamp equipped with a dichroic mirror and a narrow-band-pass filter (280400 nm) (Thermal Oriel, Spectra Physics Inc.) or a 500 W Hg arc lamp with no filters attached for various lengths of time. The samples were then placed in the TOF SIMS, and the intensities of the SO2-, SO3- and MSO3- ions, where M ) alkanethiolate molecule, were measured. From these measurements the extent of oxidation25 and the optimum time for photooxidation were determined. (An example of the data obtained is given in the Supporting Information.) To UV photopattern a SAM (SAM#1), a mask (a copper TEM grid, Electron Microscopy Inc.) was placed on top of the SAM. In initial experiments the mask/SAM construct was positioned approximately 100 mm away from a 500 W Hg arc lamp either with a dichroic mirror and/or a narrow-band-pass filter or with no filters attached. In later experiments the sample was placed approximately 50 mm away from the 500 W Hg arc lamp, which was equipped with a dichroic mirror (to remove IR light) and a narrow-band-pass UV filter (280 to 400 nm). The SAM surface was exposed to the UV light for the optimum photooxidation time (as determined above), typically in the range 1-2 h, to ensure that the photooxidation was complete. After UV photooxidation the SAM#1 substrate was immersed into a 1 mM ethanolic solution of a second alkanethiol (SAM#2) for various lengths of time. In the areas exposed to UV light, SAM#2 displaced the photoreacted SAM#1, resulting in a patterned SAM#1/SAM#2 surface. The patterned surfaces were rinsed copiously in ethanol to remove any physisorbed SAM molecules and dried with nitrogen gas. Samples were immediately placed in the TOF SIMS instrument for analysis. In all cases at least three SAM samples were prepared and two spots on each sample surface analyzed. The images shown are 500 × 500 µm2 divided into 128 × 128 pixels2. The edge resolution images were obtained with an area of 99.6 × 99.6 µm2 divided into 256 × 256 pixels2. The average (28) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. (29) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570-579. (30) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (31) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167. (32) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663-7676. (33) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148.

11422 Langmuir, Vol. 22, No. 26, 2006

Zhou and Walker Table 1. Estimated Sample Temperature Reached under Various Experimental UV-Photopatterning Conditions surface temperature (°C) exposure dichroic dichroic mirror time lamp-to-sample no mirror only (IR filter) and UV (min) distance (mm) filters (IR filter) narrow-band-pass filter

Figure 2. Negative ion images of AuM2-, where M ) UDT or ODT, for MPA/UDT, MPA/HDT, HDT/MPA, and UDT/MPA. In these experiments SAM#1 was photopatterned by placing it 100 mm away from an unfiltered 500 W Hg arc lamp. Area of analysis: 500 × 500 µm2, 128 × 128 pixels2.

0 10 30 60 0 10 30 50 a

Figure 3. Negative ion images of AuM2-, where M ) UDT or ODT, for MPA/UDT, MPA/HDT, HDT/MPA, and UDT/MPA. In these experiments SAM#1 was photopatterned by placing it 100 mm away from a 500 W Hg arc lamp equipped with a dichroic mirror and a narrow-band-pass UV filter. Area of analysis: 500 × 500 µm2, 128 × 128 pixels2. edge resolution was obtained by determining the lateral distance between 84% and 16% of the maximum ion intensities for each SAM sample and spot (i.e., the average of six measurements). Ions employed to determine the edge resolution include O-, Au2M-, and AuM2- (where M ) SAM molecule) (see the Supporting Information). We note that the image pixel size is 389.1 × 389.1 nm2, so the errors reported are typically (1 pixel. Smaller pixel sizes were not used since we needed to obtain a sufficient ion signal per pixel. Time-of-Flight Secondary Ion Mass Spectrometry. TOF SIMS analyses were conducted using a TOF SIMS IV (ION TOF Inc.) instrument equipped with a time-of-flight analyzer and a Binm+ (n ) 1-7, m ) 1, 2) liquid metal ion gun. The instrument consists of an air lock, a preparation chamber, and an analysis chamber separated by gate valves. The preparation and analysis chambers were maintained at less than 5 × 10-9 mbar to prevent sample contamination. For image acquisition, the primary Bi+ ions had a kinetic energy of 25 keV and were contained within a ∼100 nm diameter probe beam. The total accumulated primary ion dose was less than 1 × 1010 ions/cm2, which is within the static SIMS regime. Secondary ions were extracted into the time-of-flight mass spectrometer and reaccelerated to 10 keV before reaching the detector.

Results and Discussion Figure 2 displays negative ion images of AuM2-, where M ) UDT or HDT, for a patterned SAM surface. In this experiment, the mask/SAM construct was placed approximately 100 mm away from a 500 W Hg arc lamp without either a dichroic mirror (to remove IR light) or a narrow-band-pass UV filter. In agreement with the observations of Leggett and co-workers,16 patterned MPA/UDT, MPA/HDT, and HDT/MPA show clear, sharp edges between the -COOH- and -CH3-terminated SAM areas, but for the UDT/MPA surface no pattern is discernible. Figure 3 displays negative ion images of the same surfaces under the same experimental conditions except that a dichroic mirror and a narrow-band-pass UV filter were placed in front of the 500 W Hg arc lamp. In contrast to Figure 2, it can be clearly seen that all surfaces display well-formed patterns. We note that the pattern

100 100 100 100 50 50 50 50

21 >110a >110a >110a

23 36 37 37

21 22 34 34 21 41 45 47

Thermometer (Ever-Safe N16B) only calibrated to 110 °C.

quality is not significantly altered by removal of the narrowband-pass UV filter (data not shown). The major change in the patterning pathway occurs when there is IR light incident on the SAM surface. IR wavelengths can cause significant heating, so we performed the following experiment to estimate the surface temperature reached during photopatterning. A laboratory thermometer was placed in the sample position and the temperature recorded 0, 10, 30, and 60 min after the arc lamp was switched on (see Table 1). To mimic the reflectivity of the SAM film, the thermometer was covered in a piece of copper foil with a thin polymeric film adsorbed on it. It can clearly be seen that without the IR filter (dichroic mirror) there is significant heating of the sample to at least 110 °C, whereas with the IR filter present the sample temperature is much lower. It therefore seems likely that the surface temperature causes the observed differences in patterning behavior. It is known that at 100 °C methyl-terminated alkanethiolates with n e 14, where n ) number of carbons in the chain, undergo a melting transition to a liquid phase.34 In contrast SAMs with n g 14 undergo a transition to an incommensurate phase at 100 °C before melting at a higher temperature.34 When a SAM melts, it is no longer structurally stable, and it undergoes a slow desorption process from the surface.34 Upon cooling, there are therefore a large number of defects in the film, so a second SAM can readily adsorb in these areas.35 Thus, when there is no IR filter in front of the arc lamp, SAMs with n e 14 melt during the photopatterning process, resulting in a degraded binary patterned SAM. SAMs with n g 14 remain stable (although disordered) and so are not displaced by the adsorption of SAM#2, and a pattern is formed. When an IR and/or UV filter are used, the surface temperature is much lower, so the SAMs do not undergo a phase transition (either melting or forming an incommensurate phase). In this case, SAM#1 remains stable on the surface and is not displaced by SAM#2, resulting in the observed patterned SAM surface. It is therefore critical that an IR filter be employed when photopatterning short-chain (n e 14) alkanethiolates. It is interesting to note that Leggett and co-workers16 determined that photopatterned methyl-terminated SAMs were stable with n g 14, which is the chain length at which the change in the SAM phase diagram is observed. These observations are consistent with the hypothesis that the phase transition is responsible for the observed stability in the SAM photopatterning. However, we note that in a previous study using a similar experimental setup (34) Fenter, P. X-ray and He Atom Diffraction Studies of Self-Assembled Monolayers. In Self-Assembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: San Diego, 1998; Vol. 24, pp 112-147. (35) We note that, after photopatterning, but prior to immersion in the second SAM solution, SAMs with less than 14 carbons exhibit a patterned surface (see for example: Cooper, E.; Leggett, G. J. Langmuir 1999, 15, 1024-1032).

Patterned Binary Alkanethiol SAMs

Langmuir, Vol. 22, No. 26, 2006 11423

Figure 4. Negative TOF SIMS ion images of Au2UDT- (nominal mass m/z ) 571) and Au(UDT)2- (nominal mass m/z ) 581) for a patterned MUA/UDT SAM. Area of analysis: 500 × 500 µm2, 128 × 128 pixels2.

Figure 6. Variation of the SO3- negative ion intensity with immersion time in a 1 mM ethanolic MUA solution for a UDT/ MUA patterned SAM.

Figure 5. (a) Negative ion TOF SIMS images at m/z ) 581 (Au2UDT-) of UV-photopatterned UDT SAMs after various immersion times in a 1 mM MUA ethanolic solution at room temperature (analysis area 500 × 500 µm2, 128 × 128 pixels2). (b) Variation of the lateral edge resolution with immersion time calculated using the Au2UDT- ion intensity.

Hutt and Leggett20 reported a temperature rise to 36 °C, so there may be other factors which affect the quality of photopatterned SAMs. Comparison of -COOH/-CH3 and -CH3/-COOH UV Photopatterned SAMs. In this section we shall discuss the factors that affect UV photopatterning of -CH3-terminated alkanethiolate SAMs. Results for UDT (n ) 11) are presented, which is below the stability limit reported by Leggett and co-workers.16 Similar results are obtained for HDT (see the Supporting Information), which is above the stability limit previously reported. In these experiments the sample-to-lamp distance was approximately 50 mm (to the UV filter). During photopatterning the sample temperature rose to ∼50 °C (Table 1), which is below the temperature at which permanent defects form within SAMs.36 Figure 4 displays negative ion images of Au2UDT- (nominal mass m/z ) 571) and Au(UDT)2- (nominal mass m/z ) 581) for a patterned MUA/UDT SAM surface. In agreement with previous studies the images show sharp edges. The edge resolution (defined as the lateral distance between 84% and 16% of the maximum ion intensities) is 2.7 ( 0.5 µm. The pattern quality of the reverse process, i.e., UDT/MUA, is dependent on the immersion time used. Figure 5a displays images of Au2UDT- (nominal mass m/z ) 571) after different immersion times of the patterned UDT surface in a 1 mM ethanolic solution of MUA. The pattern quality as indicated by the edge resolution (defined as the lateral distance between 84% and 16% of the maximum ion intensities) is shown in Figure 5b. We note that other fragment ions indicative of the acid-terminated SAMs, such as CH2COO- (m/z ) 58) and CH2CHCOO- (m/z ) 73), (36) Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B. J. Vac. Sci. Technol., A 1995, 13, 1331-1336.

exhibit similar behavior (data not shown). The UDT/MUA pattern quality improves during the first 180 min (3 h), then becomes worse at 180-600 min (3-10 h), and improves again. No significant evolution is observed after ∼600 min (10 h). The edge resolution is worse for UDT/MUA than MHA/ODT: after 180 min (3 h) and 1440 min (24 h) of immersion the edge resolution is 5.5 ( 0.5 µm, which is approximately twice that for the MUA/ODT pattern. We note that this edge resolution is similar to that of the photopatterned SAM prior to immersion in MUA. Methyl-terminated SAMs with longer chain lengths exhibit similar behavior. For example, the HDT/MHA pattern quality improves up to a 60 min (1 h) immersion time, then degrades (60-600 min, 1-10 h), and improves again (see the Supporting Information). In summary, there appear to be two “phases” in the photopatterning process. It is likely that at short immersion times (e3 h) a well-ordered SAM monolayer is formed in the UVphotopatterned areas and the improvement in the pattern quality is due to the ordering of this monolayer. The surface diffusion of alkanethiolates on the SAM surface may also play a role in the pattern formation process. Using STM and cyclic voltammetry, Tera´n Arce,37 Imabayashi,38 and co-workers estimated that alkanethiolate molecules have a diffusion rate Ds of (3 ( 2) × 10-17 cm2/s on Au(111) surfaces. Scho¨nenberger et al. observed that a dodecanethiol SAM coalesced at the rate of ∼0.5-1 nm/ min at 90 °C, corresponding to a diffusion rate of (1-4) × 10-17 cm2/s.39 Using these surface diffusion rates, we calculate that the adsorbed alkanethiolate molecules will diffuse ∼20 nm in 24 h. However, since the observed edge resolution changes by several micrometers over the time scales studied, it seems that the surface diffusion of alkanethiolates does not play a significant role. At longer time scales (g3 h) it is likely that a second disordered MUA layer is adsorbed on the patterned SAM surface, causing the edge resolution to become worse. This layer will slowly order on the surface, and thus, the edge resolution improves (3-10 h). First Stages of the Patterning Process. We employed the variation of the SO3- ion intensity to monitor the kinetics of the displacement of the photoreacted UDT and HDT molecules by the -COOH-terminated SAM during the first stages of the patterning process. Figure 6 shows the variation of the SO3- ion (nominal mass m/z ) 80) intensity in the photoreacted UDT (37) Tera´n Arce, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1998, 44, 1053-1067. (38) Imabayashi, S.; Hobara, D.; Kakiuchi, T. Langmuir 2001, 17, 25602563. (39) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259-3271.

11424 Langmuir, Vol. 22, No. 26, 2006

region with immersion time in the -COOH-terminated alkanethiol solution. The SO3- ion intensity decreases sharply in the first minutes and then much more slowly, indicating that there is an initial fast adsorption of the MUA followed by a slower process. During this second phase, the MUA orders on the surface: the lateral edge resolution decreases and the SO3ion intensity slowly decreases as the last of the photooxidized UDT molecules are displaced from the surface. The change in the SO3- ion intensity was modeled using two exponential decays for the first 6 h of immersion in the MUA solution. For UDT the fitted first-order rate coefficients are k1 ) 0.04 min-1 and k2 ) 1.4 × 10-3 min-1. For an HDT/MHA patterned surface the first-order rate coefficients are k1 ) 0.05 min-1 and k2 ) 1.1 × 10-3 min-1. (Data fits are shown in the Supporting Information.) We note that the kinetics observed during the initial SAM#2 adsorption is similar to that observed for large-area SAM formation.10,11,30,33,40 Contact angle measurements30 and nearedge X-ray absorption fine structure (NEXAFS)40 show that there are two distinct adsorption steps in bulk SAM formation: a very fast step which takes several minutes and a slow step lasting several hours. The first step is well-described by diffusioncontrolled Langmuir adsorption in which the akanethiolates adsorb on the Au surface, forming a disordered monolayer. In the second step, the SAM molecules order on the surface, forming a twodimensional crystal. Since the kinetics observed for displacement of the photooxidized SAM by SAM#2 (-COOH-terminated SAM) are similar to those observed for bulk SAM formation, it is likely that the same mechanism is operative. In the first, fast adsorption, step SAM#2 displaces photooxidized SAM#1 molecules and a disordered SAM#2 monolayer forms. It is followed by a slow surface crystallization process in which more photooxidized SAM#1 molecules are displaced and SAM#2 becomes ordered on the surface. Thus, in agreement with experimental observations, upon adsorption of SAM#2 the edge lateral resolution will first increase (due to the formation of a disordered SAM) and then slowly decreases as the SAM orders on the surface (e3 h). Patterning Dynamics on Longer Time Scales. To test whether the worsening of the -CH3/-COOH pattern quality was due to double SAM layer formation on long time scales, we performed the following experiment. Photoreacted HDT SAM surfaces were immersed in a 1 mM ethanolic solution of MHA for only 15 min followed by immersion in absolute ethanol for various times from 0 to 1440 min (from 0 to 24 h). After 15 min of immersion in the MHA solution most of the photooxidized HDT molecules have been displaced from the surface (see the Supporting Information). However the -COOH-terminated SAM is not wellordered. Once the sample has been immersed in absolute ethanol, the influence of continued MHA adsorption is eliminated. We observe that the edge resolution of the HDT improves from ∼8 to 6.3 ( 0.4 µm during the first 30 min of immersion in ethanol and then remains approximately constant at longer immersion times (Figure 7). When the photoreacted HDT is simply left immersed in MHA solution, the pattern quality, as measured by the edge resolution, also improves for the first 60 min (see the Supporting Information). Taking these data together, we conclude that the improvement in the pattern quality is due to the ordering of the MHA monolayer rather than additional MHA adsorption. The absence of a worsening of the pattern quality between 60 and 600 min (between 1 and 10 h) suggests that it is the adsorption of MHA molecules on top of the patterned SAM surface that causes the degradation of the pattern. (40) Ha¨hner, G.; Wo¨ll, C.; Buck, M.; Grunze, M. Langmuir 1993, 9, 19551958.

Zhou and Walker

Figure 7. (a) Negative ion TOF SIMS images centered at m/z ) 651 (Au2HDT-) of UV-photopatterned HDT SAMs after 15 min of immersion in a 1 mM MHA ethanolic solution followed by immersion in absolute ethanol for various times at room temperature (analysis area 500 × 500 µm2, 128 × 128 pixels). (b) Variation of the pattern lateral edge resolution with immersion time in ethanol.

Influence of the Methylene Chain Length on Pattern Quality. It is clear from the above discussion that the terminal group interaction strength greatly affects the pattern quality, with hydrophilic terminal groups such as -COOH providing the bestresolved patterns. However, SAM stability is also governed by interchain interactions, which we now consider. We patterned SAM surfaces using two different chain length methyl-terminated SAMs. Since the terminal groups are the same and do not form hydrogen bonds, the influence of terminal group interactions on the patterning process is eliminated. The SAMs used in this study are nonanethiol (NT, C9, n ) 9), undecanethiol (UDT, C11, n ) 11) and hexadecanethiol (HDT, C16, n ) 16). Figure 8 displays the overlaid molecular ion AuM2- images, where M ) NT, UDT, or HDT, of the resulting SAM surfaces patterned in the following combinations: (a) C16/C9 and C9/ C16, (b) C16/C11 and C11/C16, (c) C11/C9 and C9/C11. For each experiment, the immersion time in the second -CH3terminated alkanethiol solution was 30 min. In contrast to the results of Cooper and Leggett,16 we observe that the C11 photopatterned SAM surfaces are stable most likely due to the different experimental conditions (IR filtering). However, patterned C9 SAMs are not stable, and upon adsorption in a second alkanethiol solution the pattern is degraded. Studies10,11 have shown that there is increasing disorder in alkanethiolate SAMs as the chain length is reduced (typically n e 9, but a sharp change in behavior is not observed). For these short chains, the headgroup-substrate (Au-S) interaction dominates the energetics rather than the chain-chain interaction.11 This suggests that the reason for the change in the patterning behavior is that the C9 SAM is disordered, which allows longer chain SAMs to adsorb in both the photooxidized and unoxidized SAM areas. For longer chains, the interchain interaction is strong enough to maintain a well-ordered SAM throughout the UV-photopatterning process. Then SAM#2 can only adsorb in the photooxidized areas. We can estimate the enthalpic stabilization energies due to hydrogen bonding and the van der Waals interaction energy

Patterned Binary Alkanethiol SAMs

Langmuir, Vol. 22, No. 26, 2006 11425

topropanoic acid was not displaced by longer chain alkanethiols. Using the same analysis, we can estimate the contribution of the -COOH group to be ∆Hsub - (2 × 8.4 kJ mol-1 per -CH2-) ) 73.5 - 16.8 kJ mol-1 ) 56.7 kJ mol-1.42,43 We note that the contribution of the -COOH group in 3-mercaptopropanoic acid SAMs is probably larger than estimated because for very short chain alkanethiols the terminal group-substrate interaction becomes increasingly significant.16 These calculations suggest that the -COOH terminal group hydrogen bonds give a stabilization energy which is equivalent to ∼7 methylene units. Using these values, we predict that decanethiol would have enough stabilizing van der Waals interactions to prevent its displacement by longer chain alkanethiols. We note that this chain length is generally considered to be that required to form a well-ordered monolayer.10,11

Conclusions

Figure 8. Negative ion TOF SIMS images of molecular cluster ions AuM2- of patterned -CH3-terminated SAM surfaces. The masses of the ions are m/z ) 515 (nonanethiol, C9; red), 571 (undecanethiol, C11; green), and 711 (hexadecanethiol, C16; blue). The labels refer to SAM#1/SAM#2. Area of analysis: 500 × 500 µm2, 128 × 128 pixels2.

using the energy of sublimation of analogous compounds.41 In our experiments we observe that when UDT is photopatterned (SAM#1) we always observe a well-defined patterned surface, whereas NT is displaced by both UDT and HDT. This suggests that the number of methylene units required to prevent the displacement of the unphotoreacted SAMs is 9 or 10. The interaction energy between the methylene units in the SAM can be estimated from the heat of sublimation (∆Hsub) of the equivalent n-alkanes.41 A plot of ∆Hsub versus n-alkane chain length yields a slope whose value is approximately the van der Waals interaction per methylene unit (see the Supporting Information and ref 41). Using this plot, we estimate that the contribution per methylene unit is 8.4 kJ mol-1. Thus, the total interaction energy required to stabilize the pattern is ∼[(9 or 10)(8.4 kJ mol-1 per -CH2-)] ≈ 80 kJ mol-1. Cooper and Leggett16 observed that 3-mercap(41) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monnell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119-1122.

The formation and quality of UV-photopatterned binary alkanethiolate SAM surfaces is dependent on many factors including the chemical identity of the SAM and the conditions used. If an unfiltered arc lamp is used, IR light is incident on the surface and causes considerable sample heating. Methylterminated SAMs with less than 14 carbons melt during the photopatterning process and can be easily displaced by the adsorption of a second SAM. This leads to significant pattern erosion or degradation. Longer chain methyl-terminated SAMs do not melt but undergo a transition to an incommensurate phase. These SAMs remain stable on the surface, and the pattern is retained. When the IR light is filtered out using a dichroic mirror, the sample heating is greatly reduced and SAMs with n g 10 carbon atoms can be photopatterned. Shorter chain methylterminated SAMs cannot be used to produce sharp patterns because they form disordered monolayers, and a second SAM can adsorb in these disordered films. Terminal group interactions, such as H-bonding, provide extra stabilization energy, so shorter chain, functionalized alkanethiolate films can be patterned. The displacement of the photooxidized SAM molecules by a second SAM appears to follow the kinetics for large-area SAM formation. Acknowledgment. We acknowledge the financial support of a National Science Foundation grant (CHE-0518063) and an ACS PRF Type G grant (38900-G5S). Supporting Information Available: Variation of the SO2-, SO3-, and MSO3- ion intensities with exposure to UV light on a bare UDT SAM surface, variation of the pattern quality of an HDT/MHA patterned surface with MHA immersion time, variation of the SO3- ion intensity with time during the adsorption of a second SAM, and calculation of the van der Waals interaction energy between methylene units. This material is available free of charge via the Internet at http://pubs.acs.org. LA0618519 (42) http://webbook.nist.gov (accessed Aug 26, 2006). (43) The value of 56.7 kJ mol-1 for the interaction between -COOH groups is reasonable. H-bond strengths for acids range between 4 and 15 kcal mol-1 (between 16.7 and 62.7 kJ mol-1) (An Introduction to Hydrogen Bonding; Jeffrey, G. A.; Oxford University Press: New York, 1997).